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rfc:rfc7143

Internet Engineering Task Force (IETF) M. Chadalapaka Request for Comments: 7143 Microsoft Obsoletes: 3720, 3980, 4850, 5048 J. Satran Updates: 3721 Infinidat Ltd. Category: Standards Track K. Meth ISSN: 2070-1721 IBM

                                                              D. Black
                                                                   EMC
                                                            April 2014
     Internet Small Computer System Interface (iSCSI) Protocol
                           (Consolidated)

Abstract

 This document describes a transport protocol for SCSI that works on
 top of TCP.  The iSCSI protocol aims to be fully compliant with the
 standardized SCSI Architecture Model (SAM-2).  RFC 3720 defined the
 original iSCSI protocol.  RFC 3721 discusses iSCSI naming examples
 and discovery techniques.  Subsequently, RFC 3980 added an additional
 naming format to the iSCSI protocol.  RFC 4850 followed up by adding
 a new public extension key to iSCSI.  RFC 5048 offered a number of
 clarifications as well as a few improvements and corrections to the
 original iSCSI protocol.
 This document obsoletes RFCs 3720, 3980, 4850, and 5048 by
 consolidating them into a single document and making additional
 updates to the consolidated specification.  This document also
 updates RFC 3721.  The text in this document thus supersedes the text
 in all the noted RFCs wherever there is a difference in semantics.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7143.

Chadalapaka, et al. Standards Track [Page 1] RFC 7143 iSCSI (Consolidated) April 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ...................................................11
 2. Acronyms, Definitions, and Document Summary ....................11
    2.1. Acronyms ..................................................11
    2.2. Definitions ...............................................13
    2.3. Summary of Changes ........................................19
    2.4. Conventions ...............................................20
 3. UML Conventions ................................................20
    3.1. UML Conventions Overview ..................................20
    3.2. Multiplicity Notion .......................................21
    3.3. Class Diagram Conventions .................................22
    3.4. Class Diagram Notation for Associations ...................23
    3.5. Class Diagram Notation for Aggregations ...................24
    3.6. Class Diagram Notation for Generalizations ................25
 4. Overview .......................................................25
    4.1. SCSI Concepts .............................................25
    4.2. iSCSI Concepts and Functional Overview ....................26
         4.2.1. Layers and Sessions ................................27
         4.2.2. Ordering and iSCSI Numbering .......................28
                4.2.2.1. Command Numbering and Acknowledging .......28
                4.2.2.2. Response/Status Numbering and
                         Acknowledging .............................32
                4.2.2.3. Response Ordering .........................32
                         4.2.2.3.1. Need for Response Ordering .....32
                         4.2.2.3.2. Response Ordering Model
                                    Description ....................33
                         4.2.2.3.3. iSCSI Semantics with
                                    the Interface Model ............33
                         4.2.2.3.4. Current List of Fenced
                                    Response Use Cases .............34
                4.2.2.4. Data Sequencing ...........................35

Chadalapaka, et al. Standards Track [Page 2] RFC 7143 iSCSI (Consolidated) April 2014

         4.2.3. iSCSI Task Management ..............................36
                4.2.3.1. Task Management Overview ..................36
                4.2.3.2. Notion of Affected Tasks ..................36
                4.2.3.3. Standard Multi-Task Abort Semantics .......37
                4.2.3.4. FastAbort Multi-Task Abort Semantics ......38
                4.2.3.5. Affected Tasks Shared across
                         Standard and FastAbort Sessions ...........40
                4.2.3.6. Rationale behind the FastAbort Semantics ..41
         4.2.4. iSCSI Login ........................................42
         4.2.5. iSCSI Full Feature Phase ...........................44
                4.2.5.1. Command Connection Allegiance .............44
                4.2.5.2. Data Transfer Overview ....................45
                4.2.5.3. Tags and Integrity Checks .................46
                4.2.5.4. SCSI Task Management during iSCSI
                         Full Feature Phase ........................47
         4.2.6. iSCSI Connection Termination .......................47
         4.2.7. iSCSI Names ........................................47
                4.2.7.1. iSCSI Name Properties .....................48
                4.2.7.2. iSCSI Name Encoding .......................50
                4.2.7.3. iSCSI Name Structure ......................51
                4.2.7.4. Type "iqn." (iSCSI Qualified Name) ........52
                4.2.7.5. Type "eui." (IEEE EUI-64 Format) ..........53
                4.2.7.6. Type "naa." (Network Address Authority) ...54
         4.2.8. Persistent State ...................................55
         4.2.9. Message Synchronization and Steering ...............55
                4.2.9.1. Sync/Steering and iSCSI PDU Length ........56
    4.3. iSCSI Session Types .......................................56
    4.4. SCSI-to-iSCSI Concepts Mapping Model ......................57
         4.4.1. iSCSI Architecture Model ...........................58
         4.4.2. SCSI Architecture Model ............................59
         4.4.3. Consequences of the Model ..........................61
                4.4.3.1. I_T Nexus State ...........................62
                4.4.3.2. Reservations ..............................63
    4.5. iSCSI UML Model ...........................................64
    4.6. Request/Response Summary ..................................66
         4.6.1. Request/Response Types Carrying SCSI Payload .......66
                4.6.1.1. SCSI Command ..............................66
                4.6.1.2. SCSI Response .............................66
                4.6.1.3. Task Management Function Request ..........67
                4.6.1.4. Task Management Function Response .........68
                4.6.1.5. SCSI Data-Out and SCSI Data-In ............68
                4.6.1.6. Ready To Transfer (R2T) ...................69
         4.6.2. Requests/Responses Carrying SCSI and iSCSI
                Payload ............................................69
                4.6.2.1. Asynchronous Message ......................69

Chadalapaka, et al. Standards Track [Page 3] RFC 7143 iSCSI (Consolidated) April 2014

         4.6.3. Requests/Responses Carrying iSCSI-Only Payload .....69
                4.6.3.1. Text Requests and Text Responses ..........69
                4.6.3.2. Login Requests and Login Responses ........70
                4.6.3.3. Logout Requests and Logout Responses ......71
                4.6.3.4. SNACK Request .............................71
                4.6.3.5. Reject ....................................71
                4.6.3.6. NOP-Out Request and NOP-In Response .......71
 5. SCSI Mode Parameters for iSCSI .................................72
 6. Login and Full Feature Phase Negotiation .......................72
    6.1. Text Format ...............................................73
    6.2. Text Mode Negotiation .....................................76
         6.2.1. List Negotiations ..................................80
         6.2.2. Simple-Value Negotiations ..........................80
    6.3. Login Phase ...............................................81
         6.3.1. Login Phase Start ..................................84
         6.3.2. iSCSI Security Negotiation .........................87
         6.3.3. Operational Parameter Negotiation during
                the Login Phase ....................................87
         6.3.4. Connection Reinstatement ...........................88
         6.3.5. Session Reinstatement, Closure, and Timeout ........89
                6.3.5.1. Loss of Nexus Notification ................90
         6.3.6. Session Continuation and Failure ...................90
    6.4. Operational Parameter Negotiation outside the
         Login Phase ...............................................90
 7. iSCSI Error Handling and Recovery ..............................92
    7.1. Overview ..................................................92
         7.1.1. Background .........................................92
         7.1.2. Goals ..............................................92
         7.1.3. Protocol Features and State Expectations ...........93
         7.1.4. Recovery Classes ...................................94
                7.1.4.1. Recovery Within-command ...................95
                7.1.4.2. Recovery Within-connection ................96
                7.1.4.3. Connection Recovery .......................96
                7.1.4.4. Session Recovery ..........................97
         7.1.5. Error Recovery Hierarchy ...........................97
    7.2. Retry and Reassign in Recovery ............................99
         7.2.1. Usage of Retry .....................................99
         7.2.2. Allegiance Reassignment ...........................100
    7.3. Usage of Reject PDU in Recovery ..........................101
    7.4. Error Recovery Considerations for Discovery Sessions .....102
         7.4.1. ErrorRecoveryLevel for Discovery Sessions .........102
         7.4.2. Reinstatement Semantics for Discovery Sessions ....102
                7.4.2.1. Unnamed Discovery Sessions ...............103
                7.4.2.2. Named Discovery Sessions .................103
         7.4.3. Target PDUs during Discovery ......................103

Chadalapaka, et al. Standards Track [Page 4] RFC 7143 iSCSI (Consolidated) April 2014

    7.5. Connection Timeout Management ............................104
         7.5.1. Timeouts on Transport Exception Events ............104
         7.5.2. Timeouts on Planned Decommissioning ...............104
    7.6. Implicit Termination of Tasks ............................104
    7.7. Format Errors ............................................105
    7.8. Digest Errors ............................................106
    7.9. Sequence Errors ..........................................107
    7.10. Message Error Checking ..................................108
    7.11. SCSI Timeouts ...........................................108
    7.12. Negotiation Failures ....................................109
    7.13. Protocol Errors .........................................110
    7.14. Connection Failures .....................................110
    7.15. Session Errors ..........................................111
 8. State Transitions .............................................112
    8.1. Standard Connection State Diagrams .......................112
         8.1.1. State Descriptions for Initiators and Targets .....112
         8.1.2. State Transition Descriptions for
                Initiators and Targets ............................114
         8.1.3. Standard Connection State Diagram for an
                Initiator .........................................118
         8.1.4. Standard Connection State Diagram for a Target ....120
    8.2. Connection Cleanup State Diagram for Initiators
         and Targets ..............................................122
         8.2.1. State Descriptions for Initiators and Targets .....124
         8.2.2. State Transition Descriptions for
                Initiators and Targets ............................124
    8.3. Session State Diagrams ...................................126
         8.3.1. Session State Diagram for an Initiator ............126
         8.3.2. Session State Diagram for a Target ................127
         8.3.3. State Descriptions for Initiators and Targets .....129
         8.3.4. State Transition Descriptions for
                Initiators and Targets ............................129
 9. Security Considerations .......................................131
    9.1. iSCSI Security Mechanisms ................................132
    9.2. In-Band Initiator-Target Authentication ..................132
         9.2.1. CHAP Considerations ...............................134
         9.2.2. SRP Considerations ................................136
         9.2.3. Kerberos Considerations ...........................136
    9.3. IPsec ....................................................137
         9.3.1. Data Authentication and Integrity .................137
         9.3.2. Confidentiality ...................................138
         9.3.3. Policy, Security Associations, and
                Cryptographic Key Management ......................139
    9.4. Security Considerations for the X#NodeArchitecture Key ...141
    9.5. SCSI Access Control Considerations .......................143

Chadalapaka, et al. Standards Track [Page 5] RFC 7143 iSCSI (Consolidated) April 2014

 10. Notes to Implementers ........................................143
    10.1. Multiple Network Adapters ...............................143
         10.1.1. Conservative Reuse of ISIDs ......................143
         10.1.2. iSCSI Name, ISID, and TPGT Use ...................144
    10.2. Autosense and Auto Contingent Allegiance (ACA) ..........146
    10.3. iSCSI Timeouts ..........................................146
    10.4. Command Retry and Cleaning Old Command Instances ........147
    10.5. Sync and Steering Layer, and Performance ................147
    10.6. Considerations for State-Dependent Devices and
          Long-Lasting SCSI Operations ............................147
         10.6.1. Determining the Proper ErrorRecoveryLevel ........148
    10.7. Multi-Task Abort Implementation Considerations ..........149
 11. iSCSI PDU Formats ............................................150
    11.1. iSCSI PDU Length and Padding ............................150
    11.2. PDU Template, Header, and Opcodes .......................150
         11.2.1. Basic Header Segment (BHS) .......................152
                11.2.1.1. I (Immediate) Bit .......................152
                11.2.1.2. Opcode ..................................152
                11.2.1.3. F (Final) Bit ...........................154
                11.2.1.4. Opcode-Specific Fields ..................154
                11.2.1.5. TotalAHSLength ..........................154
                11.2.1.6. DataSegmentLength .......................154
                11.2.1.7. LUN .....................................154
                11.2.1.8. Initiator Task Tag ......................154
         11.2.2. Additional Header Segment (AHS) ..................155
                11.2.2.1. AHSType .................................155
                11.2.2.2. AHSLength ...............................155
                11.2.2.3. Extended CDB AHS ........................156
                11.2.2.4. Bidirectional Read Expected Data
                          Transfer Length AHS .....................156
         11.2.3. Header Digest and Data Digest ....................156
         11.2.4. Data Segment .....................................157
    11.3. SCSI Command ............................................158
         11.3.1. Flags and Task Attributes (Byte 1) ...............159
         11.3.2. CmdSN - Command Sequence Number ..................159
         11.3.3. ExpStatSN ........................................160
         11.3.4. Expected Data Transfer Length ....................160
         11.3.5. CDB - SCSI Command Descriptor Block ..............160
         11.3.6. Data Segment - Command Data ......................161
    11.4. SCSI Response ...........................................161
         11.4.1. Flags (Byte 1) ...................................162
         11.4.2. Status ...........................................163
         11.4.3. Response .........................................163
         11.4.4. SNACK Tag ........................................164

Chadalapaka, et al. Standards Track [Page 6] RFC 7143 iSCSI (Consolidated) April 2014

         11.4.5. Residual Count ...................................164
                11.4.5.1. Field Semantics .........................164
                11.4.5.2. Residuals Concepts Overview .............164
                11.4.5.3. SCSI REPORT LUNS Command and
                          Residual Overflow .......................165
         11.4.6. Bidirectional Read Residual Count ................166
         11.4.7. Data Segment - Sense and Response Data Segment ...167
                11.4.7.1. SenseLength .............................167
                11.4.7.2. Sense Data ..............................168
         11.4.8. ExpDataSN ........................................168
         11.4.9. StatSN - Status Sequence Number ..................168
         11.4.10. ExpCmdSN - Next Expected CmdSN from This
                  Initiator .......................................169
         11.4.11. MaxCmdSN - Maximum CmdSN from This Initiator ....169
    11.5. Task Management Function Request ........................170
         11.5.1. Function .........................................170
         11.5.2. TotalAHSLength and DataSegmentLength .............173
         11.5.3. LUN ..............................................173
         11.5.4. Referenced Task Tag ..............................173
         11.5.5. RefCmdSN .........................................174
         11.5.6. ExpDataSN ........................................174
    11.6. Task Management Function Response .......................175
         11.6.1. Response .........................................176
         11.6.2. TotalAHSLength and DataSegmentLength .............177
    11.7. SCSI Data-Out and SCSI Data-In ..........................178
         11.7.1. F (Final) Bit ....................................180
         11.7.2. A (Acknowledge) Bit ..............................180
         11.7.3. Flags (Byte 1) ...................................181
         11.7.4. Target Transfer Tag and LUN ......................181
         11.7.5. DataSN ...........................................182
         11.7.6. Buffer Offset ....................................182
         11.7.7. DataSegmentLength ................................182
    11.8. Ready To Transfer (R2T) .................................183
         11.8.1. TotalAHSLength and DataSegmentLength .............184
         11.8.2. R2TSN ............................................184
         11.8.3. StatSN ...........................................185
         11.8.4. Desired Data Transfer Length and Buffer Offset ...185
         11.8.5. Target Transfer Tag ..............................185
    11.9. Asynchronous Message ....................................186
         11.9.1. AsyncEvent .......................................187
         11.9.2. AsyncVCode .......................................189
         11.9.3. LUN ..............................................189
         11.9.4. Sense Data and iSCSI Event Data ..................190
                11.9.4.1. SenseLength .............................190

Chadalapaka, et al. Standards Track [Page 7] RFC 7143 iSCSI (Consolidated) April 2014

    11.10. Text Request ...........................................191
         11.10.1. F (Final) Bit ...................................192
         11.10.2. C (Continue) Bit ................................192
         11.10.3. Initiator Task Tag ..............................192
         11.10.4. Target Transfer Tag .............................192
         11.10.5. Text ............................................193
    11.11. Text Response ..........................................194
         11.11.1. F (Final) Bit ...................................194
         11.11.2. C (Continue) Bit ................................195
         11.11.3. Initiator Task Tag ..............................195
         11.11.4. Target Transfer Tag .............................195
         11.11.5. StatSN ..........................................196
         11.11.6. Text Response Data ..............................196
    11.12. Login Request ..........................................196
         11.12.1. T (Transit) Bit .................................197
         11.12.2. C (Continue) Bit ................................197
         11.12.3. CSG and NSG .....................................198
         11.12.4. Version .........................................198
                11.12.4.1. Version-max ............................198
                11.12.4.2. Version-min ............................198
         11.12.5. ISID ............................................199
         11.12.6. TSIH ............................................200
         11.12.7. Connection ID (CID) .............................200
         11.12.8. CmdSN ...........................................201
         11.12.9. ExpStatSN .......................................201
         11.12.10. Login Parameters ...............................201
    11.13. Login Response .........................................202
         11.13.1. Version-max .....................................202
         11.13.2. Version-active ..................................203
         11.13.3. TSIH ............................................203
         11.13.4. StatSN ..........................................203
         11.13.5. Status-Class and Status-Detail ..................203
         11.13.6. T (Transit) Bit .................................206
         11.13.7. C (Continue) Bit ................................206
         11.13.8. Login Parameters ................................207
    11.14. Logout Request .........................................207
         11.14.1. Reason Code .....................................209
         11.14.2. TotalAHSLength and DataSegmentLength ............209
         11.14.3. CID .............................................210
         11.14.4. ExpStatSN .......................................210
         11.14.5. Implicit Termination of Tasks ...................210
    11.15. Logout Response ........................................211
         11.15.1. Response ........................................212
         11.15.2. TotalAHSLength and DataSegmentLength ............212
         11.15.3. Time2Wait .......................................212
         11.15.4. Time2Retain .....................................212

Chadalapaka, et al. Standards Track [Page 8] RFC 7143 iSCSI (Consolidated) April 2014

    11.16. SNACK Request ..........................................213
         11.16.1. Type ............................................214
         11.16.2. Data Acknowledgment .............................215
         11.16.3. Resegmentation ..................................215
         11.16.4. Initiator Task Tag ..............................216
         11.16.5. Target Transfer Tag or SNACK Tag ................216
         11.16.6. BegRun ..........................................216
         11.16.7. RunLength .......................................216
    11.17. Reject .................................................217
         11.17.1. Reason ..........................................218
         11.17.2. DataSN/R2TSN ....................................219
         11.17.3. StatSN, ExpCmdSN, and MaxCmdSN ..................219
         11.17.4. Complete Header of Bad PDU ......................219
    11.18. NOP-Out ................................................220
         11.18.1. Initiator Task Tag ..............................221
         11.18.2. Target Transfer Tag .............................221
         11.18.3. Ping Data .......................................221
    11.19. NOP-In .................................................222
         11.19.1. Target Transfer Tag .............................223
         11.19.2. StatSN ..........................................223
         11.19.3. LUN .............................................223
 12. iSCSI Security Text Keys and Authentication Methods ..........223
    12.1. AuthMethod ..............................................224
         12.1.1. Kerberos .........................................226
         12.1.2. Secure Remote Password (SRP) .....................226
         12.1.3. Challenge Handshake Authentication
                 Protocol (CHAP) ..................................228
 13. Login/Text Operational Text Keys .............................229
    13.1. HeaderDigest and DataDigest .............................230
    13.2. MaxConnections ..........................................232
    13.3. SendTargets .............................................232
    13.4. TargetName ..............................................232
    13.5. InitiatorName ...........................................233
    13.6. TargetAlias .............................................233
    13.7. InitiatorAlias ..........................................234
    13.8. TargetAddress ...........................................234
    13.9. TargetPortalGroupTag ....................................235
    13.10. InitialR2T .............................................236
    13.11. ImmediateData ..........................................236
    13.12. MaxRecvDataSegmentLength ...............................237
    13.13. MaxBurstLength .........................................238
    13.14. FirstBurstLength .......................................238
    13.15. DefaultTime2Wait .......................................239
    13.16. DefaultTime2Retain .....................................239
    13.17. MaxOutstandingR2T ......................................239
    13.18. DataPDUInOrder .........................................240
    13.19. DataSequenceInOrder ....................................240
    13.20. ErrorRecoveryLevel .....................................241

Chadalapaka, et al. Standards Track [Page 9] RFC 7143 iSCSI (Consolidated) April 2014

    13.21. SessionType ............................................241
    13.22. The Private Extension Key Format .......................242
    13.23. TaskReporting ..........................................242
    13.24. iSCSIProtocolLevel Negotiation .........................243
    13.25. Obsoleted Keys .........................................243
    13.26. X#NodeArchitecture .....................................244
         13.26.1. Definition ......................................244
         13.26.2. Implementation Requirements .....................244
 14. Rationale for Revised IANA Considerations ....................245
 15. IANA Considerations ..........................................246
 16. References ...................................................248
    16.1. Normative References ....................................248
    16.2. Informative References ..................................251
 Appendix A. Examples .............................................254
   A.1. Read Operation Example ....................................254
   A.2. Write Operation Example ...................................255
   A.3. R2TSN/DataSN Use Examples .................................256
        A.3.1. Output (Write) Data DataSN/R2TSN Example ...........256
        A.3.2. Input (Read) Data DataSN Example ...................257
        A.3.3. Bidirectional DataSN Example .......................258
        A.3.4. Unsolicited and Immediate Output (Write) Data
               with DataSN Example ................................259
   A.4. CRC Examples ..............................................259
 Appendix B. Login Phase Examples .................................261
 Appendix C. SendTargets Operation ................................268
 Appendix D. Algorithmic Presentation of Error Recovery
             Classes ..............................................272
   D.1. General Data Structure and Procedure Description ..........273
   D.2. Within-command Error Recovery Algorithms ..................274
        D.2.1. Procedure Descriptions .............................274
        D.2.2. Initiator Algorithms ...............................275
        D.2.3. Target Algorithms ..................................277
   D.3. Within-connection Recovery Algorithms .....................279
        D.3.1. Procedure Descriptions .............................279
        D.3.2. Initiator Algorithms ...............................280
        D.3.3. Target Algorithms ..................................283
   D.4. Connection Recovery Algorithms ............................283
        D.4.1. Procedure Descriptions .............................283
        D.4.2. Initiator Algorithms ...............................284
        D.4.3. Target Algorithms ..................................286
 Appendix E. Clearing Effects of Various Events on Targets ........288
   E.1. Clearing Effects on iSCSI Objects .........................288
   E.2. Clearing Effects on SCSI Objects ..........................293
 Acknowledgments ..................................................294

Chadalapaka, et al. Standards Track [Page 10] RFC 7143 iSCSI (Consolidated) April 2014

1. Introduction

 The Small Computer System Interface (SCSI) is a popular family of
 protocols for communicating with I/O devices, especially storage
 devices.  SCSI is a client-server architecture.  Clients of a SCSI
 interface are called "initiators".  Initiators issue SCSI "commands"
 to request services from components -- logical units of a server
 known as a "target".  A "SCSI transport" maps the client-server SCSI
 protocol to a specific interconnect.  An initiator is one endpoint of
 a SCSI transport, and a target is the other endpoint.
 The SCSI protocol has been mapped over various transports, including
 Parallel SCSI, Intelligent Peripheral Interface (IPI), IEEE 1394
 (FireWire), and Fibre Channel.  These transports are I/O-specific and
 have limited distance capabilities.
 The iSCSI protocol defined in this document describes a means of
 transporting SCSI packets over TCP/IP, providing for an interoperable
 solution that can take advantage of existing Internet infrastructure,
 Internet management facilities, and address distance limitations.

2. Acronyms, Definitions, and Document Summary

2.1. Acronyms

 Acronym     Definition
 --------------------------------------------------------------
 3DES        Triple Data Encryption Standard
 ACA         Auto Contingent Allegiance
 AEN         Asynchronous Event Notification
 AES         Advanced Encryption Standard
 AH          Additional Header (not the IPsec AH!)
 AHS         Additional Header Segment
 API         Application Programming Interface
 ASC         Additional Sense Code
 ASCII       American Standard Code for Information Interchange
 ASCQ        Additional Sense Code Qualifier
 ATA         AT Attachment
 BHS         Basic Header Segment
 CBC         Cipher Block Chaining
 CD          Compact Disk
 CDB         Command Descriptor Block
 CHAP        Challenge Handshake Authentication Protocol
 CID         Connection ID
 CO          Connection Only
 CRC         Cyclic Redundancy Check
 CRL         Certificate Revocation List
 CSG         Current Stage

Chadalapaka, et al. Standards Track [Page 11] RFC 7143 iSCSI (Consolidated) April 2014

 CSM         Connection State Machine
 DES         Data Encryption Standard
 DNS         Domain Name Server
 DOI         Domain of Interpretation
 DVD         Digital Versatile Disk
 EDTL        Expected Data Transfer Length
 ESP         Encapsulating Security Payload
 EUI         Extended Unique Identifier
 FFP         Full Feature Phase
 FFPO        Full Feature Phase Only
 HBA         Host Bus Adapter
 HMAC        Hashed Message Authentication Code
 I_T         Initiator_Target
 I_T_L       Initiator_Target_LUN
 IANA        Internet Assigned Numbers Authority
 IB          InfiniBand
 ID          Identifier
 IDN         Internationalized Domain Name
 IEEE        Institute of Electrical and Electronics Engineers
 IETF        Internet Engineering Task Force
 IKE         Internet Key Exchange
 I/O         Input-Output
 IO          Initialize Only
 IP          Internet Protocol
 IPsec       Internet Protocol Security
 IPv4        Internet Protocol Version 4
 IPv6        Internet Protocol Version 6
 IQN         iSCSI Qualified Name
 iSCSI       Internet SCSI
 iSER        iSCSI Extensions for RDMA (see [RFC7145])
 ISID        Initiator Session ID
 iSNS        Internet Storage Name Service (see [RFC4171])
 ITN         iSCSI Target Name
 ITT         Initiator Task Tag
 KRB5        Kerberos V5
 LFL         Lower Functional Layer
 LTDS        Logical-Text-Data-Segment
 LO          Leading Only
 LU          Logical Unit
 LUN         Logical Unit Number
 MAC         Message Authentication Code
 NA          Not Applicable
 NAA         Network Address Authority
 NIC         Network Interface Card
 NOP         No Operation
 NSG         Next Stage
 OCSP        Online Certificate Status Protocol
 OS          Operating System

Chadalapaka, et al. Standards Track [Page 12] RFC 7143 iSCSI (Consolidated) April 2014

 PDU         Protocol Data Unit
 PKI         Public Key Infrastructure
 R2T         Ready To Transfer
 R2TSN       Ready To Transfer Sequence Number
 RDMA        Remote Direct Memory Access
 RFC         Request For Comments
 SA          Security Association
 SAM         SCSI Architecture Model
 SAM-2       SCSI Architecture Model - 2
 SAN         Storage Area Network
 SAS         Serial Attached SCSI
 SATA        Serial AT Attachment
 SCSI        Small Computer System Interface
 SLP         Service Location Protocol
 SN          Sequence Number
 SNACK       Selective Negative Acknowledgment - also
             Sequence Number Acknowledgement for data
 SPDTL       SCSI-Presented Data Transfer Length
 SPKM        Simple Public-Key Mechanism
 SRP         Secure Remote Password
 SSID        Session ID
 SW          Session-Wide
 TCB         Task Control Block
 TCP         Transmission Control Protocol
 TMF         Task Management Function
 TPGT        Target Portal Group Tag
 TSIH        Target Session Identifying Handle
 TTT         Target Transfer Tag
 UA          Unit Attention
 UFL         Upper Functional Layer
 ULP         Upper Level Protocol
 URN         Uniform Resource Name
 UTF         Universal Transformation Format
 WG          Working Group

2.2. Definitions

  1. Alias: An alias string can also be associated with an iSCSI node.

The alias allows an organization to associate a user-friendly

   string with the iSCSI name.  However, the alias string is not a
   substitute for the iSCSI name.
  1. CID (connection ID): Connections within a session are identified by

a connection ID. It is a unique ID for this connection within the

   session for the initiator.  It is generated by the initiator and
   presented to the target during Login Requests and during logouts
   that close connections.

Chadalapaka, et al. Standards Track [Page 13] RFC 7143 iSCSI (Consolidated) April 2014

  1. Connection: A connection is a TCP connection. Communication

between the initiator and target occurs over one or more TCP

   connections.  The TCP connections carry control messages, SCSI
   commands, parameters, and data within iSCSI Protocol Data Units
   (iSCSI PDUs).
  1. I/O Buffer: An I/O Buffer is a buffer that is used in a SCSI read

or write operation so SCSI data may be sent from or received into

   that buffer.  For a read or write data transfer to take place for a
   task, an I/O Buffer is required on the initiator and at least one
   is required on the target.
  1. INCITS: "INCITS" stands for InterNational Committee for Information

Technology Standards. The INCITS has a broad standardization scope

   within the field of Information and Communications Technologies
   (ICT), encompassing storage, processing, transfer, display,
   management, organization, and retrieval of information.  INCITS
   serves as ANSI's Technical Advisory Group for the ISO/IEC Joint
   Technical Committee 1 (JTC 1).  See <http://www.incits.org>.
  1. InfiniBand: InfiniBand is an I/O architecture originally intended

to replace Peripheral Component Interconnect (PCI) and address

   high-performance server interconnectivity [IB].
  1. iSCSI Device: An iSCSI device is a SCSI device using an iSCSI

service delivery subsystem. The Service Delivery Subsystem is

   defined by [SAM2] as a transport mechanism for SCSI commands and
   responses.
  1. iSCSI Initiator Name: The iSCSI Initiator Name specifies the

worldwide unique name of the initiator.

  1. iSCSI Initiator Node: An iSCSI initiator node is the "initiator"

device. The word "initiator" has been appropriately qualified as

   either a port or a device in the rest of the document when the
   context is ambiguous.  All unqualified usages of "initiator" refer
   to an initiator port (or device), depending on the context.
  1. iSCSI Layer: This layer builds/receives iSCSI PDUs and

relays/receives them to/from one or more TCP connections that form

   an initiator-target "session".
  1. iSCSI Name: This is the name of an iSCSI initiator or iSCSI target.
  1. iSCSI Node: The iSCSI node represents a single iSCSI initiator or

iSCSI target, or a single instance of each. There are one or more

   iSCSI nodes within a Network Entity.  The iSCSI node is accessible
   via one or more Network Portals.  An iSCSI node is identified by

Chadalapaka, et al. Standards Track [Page 14] RFC 7143 iSCSI (Consolidated) April 2014

   its iSCSI name.  The separation of the iSCSI name from the
   addresses used by and for the iSCSI node allows multiple iSCSI
   nodes to use the same address and the same iSCSI node to use
   multiple addresses.
  1. iSCSI Target Name: The iSCSI Target Name specifies the worldwide

unique name of the target.

  1. iSCSI Target Node: The iSCSI target node is the "target" device.

The word "target" has been appropriately qualified as either a port

   or a device in the rest of the document when the context is
   ambiguous.  All unqualified usages of "target" refer to a target
   port (or device), depending on the context.
  1. iSCSI Task: An iSCSI task is an iSCSI request for which a response

is expected.

  1. iSCSI Transfer Direction: The iSCSI transfer direction is defined

with regard to the initiator. Outbound or outgoing transfers are

   transfers from the initiator to the target, while inbound or
   incoming transfers are from the target to the initiator.
  1. ISID: The ISID is the initiator part of the session identifier. It

is explicitly specified by the initiator during login.

  1. I_T Nexus: According to [SAM2], the I_T nexus is a relationship

between a SCSI initiator port and a SCSI target port. For iSCSI,

   this relationship is a session, defined as a relationship between
   an iSCSI initiator's end of the session (SCSI initiator port) and
   the iSCSI target's portal group.  The I_T nexus can be identified
   by the conjunction of the SCSI port names; that is, the I_T nexus
   identifier is the tuple (iSCSI Initiator Name + ',i,' + ISID, iSCSI
   Target Name + ',t,' + Target Portal Group Tag).
  1. I_T_L Nexus: An I_T_L nexus is a SCSI concept and is defined as the

relationship between a SCSI initiator port, a SCSI target port, and

   a Logical Unit (LU).
  1. NAA: "NAA" refers to Network Address Authority, a naming format

defined by the INCITS T11 Fibre Channel protocols [FC-FS3].

  1. Network Entity: The Network Entity represents a device or gateway

that is accessible from the IP network. A Network Entity must have

   one or more Network Portals, each of which can be used to gain
   access to the IP network by some iSCSI nodes contained in that
   Network Entity.

Chadalapaka, et al. Standards Track [Page 15] RFC 7143 iSCSI (Consolidated) April 2014

  1. Network Portal: The Network Portal is a component of a Network

Entity that has a TCP/IP network address and that may be used by an

   iSCSI node within that Network Entity for the connection(s) within
   one of its iSCSI sessions.  A Network Portal in an initiator is
   identified by its IP address.  A Network Portal in a target is
   identified by its IP address and its listening TCP port.
  1. Originator: In a negotiation or exchange, the originator is the

party that initiates the negotiation or exchange.

  1. PDU (Protocol Data Unit): The initiator and target divide their

communications into messages. The term "iSCSI Protocol Data Unit"

   (iSCSI PDU) is used for these messages.
  1. Portal Groups: iSCSI supports multiple connections within the same

session; some implementations will have the ability to combine

   connections in a session across multiple Network Portals.  A portal
   group defines a set of Network Portals within an iSCSI Network
   Entity that collectively supports the capability of coordinating a
   session with connections spanning these portals.  Not all Network
   Portals within a portal group need participate in every session
   connected through that portal group.  One or more portal groups may
   provide access to an iSCSI node.  Each Network Portal, as utilized
   by a given iSCSI node, belongs to exactly one portal group within
   that node.
  1. Portal Group Tag: This 16-bit quantity identifies a portal group

within an iSCSI node. All Network Portals with the same Portal

   Group Tag in the context of a given iSCSI node are in the same
   portal group.
  1. Recovery R2T: A recovery R2T is an R2T generated by a target upon

detecting the loss of one or more Data-Out PDUs through one of the

   following means: a digest error, a sequence error, or a sequence
   reception timeout.  A recovery R2T carries the next unused R2TSN
   but requests all or part of the data burst that an earlier R2T
   (with a lower R2TSN) had already requested.
  1. Responder: In a negotiation or exchange, the responder is the party

that responds to the originator of the negotiation or exchange.

  1. SAS: The Serial Attached SCSI (SAS) standard contains both a

physical layer compatible with Serial ATA, and protocols for

   transporting SCSI commands to SAS devices and ATA commands to SATA
   devices [SAS] [SPL].

Chadalapaka, et al. Standards Track [Page 16] RFC 7143 iSCSI (Consolidated) April 2014

  1. SCSI Device: This is the SAM-2 term for an entity that contains one

or more SCSI ports that are connected to a service delivery

   subsystem and supports a SCSI application protocol.  For example, a
   SCSI initiator device contains one or more SCSI initiator ports and
   zero or more application clients.  A target device contains one or
   more SCSI target ports and one or more device servers and
   associated LUs.  For iSCSI, the SCSI device is the component within
   an iSCSI node that provides the SCSI functionality.  As such, there
   can be at most one SCSI device within a given iSCSI node.  Access
   to the SCSI device can only be achieved in an iSCSI Normal
   operational session.  The SCSI device name is defined to be the
   iSCSI name of the node.
  1. SCSI Layer: This builds/receives SCSI CDBs (Command Descriptor

Blocks) and relays/receives them with the remaining Execute Command

   [SAM2] parameters to/from the iSCSI Layer.
  1. Session: The group of TCP connections that link an initiator with a

target form a session (loosely equivalent to a SCSI I_T nexus).

   TCP connections can be added and removed from a session.  Across
   all connections within a session, an initiator sees one and the
   same target.
  1. SCSI Port: This is the SAM-2 term for an entity in a SCSI device

that provides the SCSI functionality to interface with a service

   delivery subsystem.  For iSCSI, the definitions of the SCSI
   initiator port and the SCSI target port are different.
  1. SCSI Initiator Port: This maps to the endpoint of an iSCSI Normal

operational session. An iSCSI Normal operational session is

   negotiated through the login process between an iSCSI initiator
   node and an iSCSI target node.  At successful completion of this
   process, a SCSI initiator port is created within the SCSI initiator
   device.  The SCSI initiator port name and SCSI initiator port
   identifier are both defined to be the iSCSI Initiator Name together
   with (a) a label that identifies it as an initiator port
   name/identifier and (b) the ISID portion of the session identifier.
  1. SCSI Port Name: This is a name consisting of UTF-8 [RFC3629]

encoding of Unicode [UNICODE] characters and includes the iSCSI

   name + 'i' or 't' + ISID or Target Portal Group Tag.
  1. SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the aggregate

data length of the data that the SCSI layer logically "presents" to

   the iSCSI layer for a Data-In or Data-Out transfer in the context
   of a SCSI task.  For a bidirectional task, there are two SPDTL
   values -- one for Data-In and one for Data-Out.  Note that the
   notion of "presenting" includes immediate data per the data

Chadalapaka, et al. Standards Track [Page 17] RFC 7143 iSCSI (Consolidated) April 2014

   transfer model in [SAM2] and excludes overlapping data transfers,
   if any, requested by the SCSI layer.
  1. SCSI Target Port: This maps to an iSCSI target portal group.
  1. SCSI Target Port Name and SCSI Target Port Identifier: These are

both defined to be the iSCSI Target Name together with (a) a label

   that identifies it as a target port name/identifier and (b) the
   Target Portal Group Tag.
  1. SSID (Session ID): A session between an iSCSI initiator and an

iSCSI target is defined by a session ID that is a tuple composed of

   an initiator part (ISID) and a target part (Target Portal Group
   Tag).  The ISID is explicitly specified by the initiator at session
   establishment.  The Target Portal Group Tag is implied by the
   initiator through the selection of the TCP endpoint at connection
   establishment.  The TargetPortalGroupTag key must also be returned
   by the target as a confirmation during connection establishment.
  1. T10: T10 is a technical committee within INCITS that develops

standards and technical reports on I/O interfaces, particularly the

   series of SCSI (Small Computer System Interface) standards.  See
   <http://www.t10.org>.
  1. T11: T11 is a technical committee within INCITS responsible for

standards development in the areas of Intelligent Peripheral

   Interface (IPI), High-Performance Parallel Interface (HIPPI), and
   Fibre Channel (FC).  See <http://www.t11.org>.
  1. Target Portal Group Tag: This is a numerical identifier (16-bit)

for an iSCSI target portal group.

  1. Target Transfer Tag (TTT): The TTT is an iSCSI protocol field used

in a few iSCSI PDUs (e.g., R2T, NOP-In) that is always sent from

   the target to the initiator first and then quoted as a reference in
   initiator-sent PDUs back to the target relating to the same
   task/exchange.  Therefore, the TTT effectively acts as an opaque
   handle to an existing task/exchange to help the target associate
   the incoming PDUs from the initiator to the proper execution
   context.
  1. Third-party: This term is used in this document as a qualifier to

nexus objects (I_T or I_T_L) and iSCSI sessions, to indicate that

   these objects and sessions reap the side effects of actions that
   take place in the context of a separate iSCSI session.  One example
   of a third-party session is an iSCSI session discovering that its
   I_T_L nexus to a LU got reset due to a LU reset operation
   orchestrated via a separate I_T nexus.

Chadalapaka, et al. Standards Track [Page 18] RFC 7143 iSCSI (Consolidated) April 2014

  1. TSIH (Target Session Identifying Handle): This is a target-assigned

tag for a session with a specific named initiator. The target

   generates it during session establishment.  Other than defining it
   as a 16-bit binary string, its internal format and content are not
   defined by this protocol but for the value with all bits set to 0
   that is reserved and used by the initiator to indicate a new
   session.  It is given to the target during additional connection
   establishment for the same session.

2.3. Summary of Changes

 1)  Consolidated RFCs 3720, 3980, 4850, and 5048, and made the
     necessary editorial changes.
 2)  Specified iSCSIProtocolLevel as "1" in Section 13.24 and added a
     related normative reference to [RFC7144].
 3)  Removed markers and related keys.
 4)  Removed SPKM authentication and related keys.
 5)  Added a new Section 13.25 on responding to obsoleted keys.
 6)  Have explicitly allowed initiator+target implementations
     throughout the text.
 7)  Clarified in Section 4.2.7 that implementations SHOULD NOT rely
     on SLP-based discovery.
 8)  Added Unified Modeling Language (UML) diagrams and related
     conventions in Section 3.
 9)  Made FastAbort implementation a "SHOULD" requirement in
     Section 4.2.3.4, rather than the previous "MUST" requirement.
 10) Required in Section 4.2.7.1 that iSCSI Target Name be the same as
     iSCSI Initiator Name for SCSI (composite) devices with both
     roles.
 11) Changed the "MUST NOT" to "should be avoided" in Section 4.2.7.2
     regarding usage of characters such as punctuation marks in iSCSI
     names.
 12) Updated Section 9.3 to require the following: MUST implement
     IPsec, 2400-series RFCs (IPsec v2, IKEv1); and SHOULD implement
     IPsec, 4300-series RFCs (IPsec v3, IKEv2).

Chadalapaka, et al. Standards Track [Page 19] RFC 7143 iSCSI (Consolidated) April 2014

 13) Clarified in Section 10.2 that ACA is a "SHOULD" only for iSCSI
     targets.
 14) Prohibited usage of X# name prefix for new public keys in
     Section 6.2.
 15) Prohibited usage of Y# name prefix for new digest extensions in
     Section 13.1 and Z# name prefix for new authentication method
     extensions in Section 12.1.
 16) Added a "SHOULD" in Section 6.2 that initiators and targets
     support at least six (6) exchanges during text negotiation.
 17) Added a clarification that Appendix C is normative.
 18) Added a normative requirement on [RFC7146] and made a few related
     changes in Section 9.3 to align the text in this document with
     that of [RFC7146].
 19) Added a new Section 9.2.3 covering Kerberos authentication
     considerations.
 20) Added text in Section 9.3.3 noting that OCSP is now allowed for
     checking certificates used with IPsec in addition to the use
     of CRLs.
 21) Added text in Section 9.3.1 specifying that extended sequence
     numbers (ESNs) are now required for ESPv2 (part of IPsec v2).

2.4. Conventions

 In examples, "I->" and "T->" show iSCSI PDUs sent by the initiator
 and target, respectively.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

3. UML Conventions

3.1. UML Conventions Overview

 The SCSI Architecture Model (SAM) uses class diagrams and object
 diagrams with notation that is based on the Unified Modeling Language
 [UML].  Therefore, this document also uses UML to model the
 relationships for SCSI and iSCSI objects.

Chadalapaka, et al. Standards Track [Page 20] RFC 7143 iSCSI (Consolidated) April 2014

 A treatise on the graphical notation used in UML is beyond the scope
 of this document.  However, given the use of ASCII drawing for UML
 static class diagrams, a description of the notational conventions
 used in this document is included in the remainder of this section.

3.2. Multiplicity Notion

 Not specified   The number of instances of an attribute is not
                 specified.
             1   One instance of the class or attribute exists.
          0..*   Zero or more instances of the class or attribute
                 exist.
          1..*   One or more instances of the class or attribute
                 exist.
          0..1   Zero or one instance of the class or attribute
                 exists.
          n..m   n to m instances of the class or attribute exist
                 (e.g., 2..8).
       x, n..m   Multiple disjoint instances of the class or
                 attribute exist (e.g., 2, 8..15).

Chadalapaka, et al. Standards Track [Page 21] RFC 7143 iSCSI (Consolidated) April 2014

3.3. Class Diagram Conventions

   +--------------+    +--------------+       +--------------+
   |  Class Name  |    |  Class Name  |       |  Class Name  |
   +--------------+    +--------------+       +--------------+
   |              |    |              |
   +--------------+    +--------------+
   |              |
   +--------------+
   The previous three diagrams are examples of a class with no
   attributes and with no operations.
   +-------------------+    +-------------------+
   |    Class Name     |    |    Class Name     |
   +-------------------+    +-------------------+
   | attribute 01[1]   |    |   attribute 01[1] |
   | attribute 02[1]   |    |   attribute 02[1] |
   +-------------------+    +-------------------+
   |                   |
   +-------------------+
   The preceding two diagrams are examples of a class with attributes
   and with no operations.
   +------------------------+
   |      Class Name        |
   +------------------------+
   |    attribute 01[1..*]  |
   |    attribute 02[1]     |
   +------------------------+
   |    operation 01()      |
   |    operation 02()      |
   +------------------------+
   The preceding diagram is an example of a class with attributes
   that have a specified multiplicity and operations.

Chadalapaka, et al. Standards Track [Page 22] RFC 7143 iSCSI (Consolidated) April 2014

3.4. Class Diagram Notation for Associations

   +-----------------+
   |     Class A     |
   +-----------------+ association_name   +-----------------+
   | attribute 01[1] |<------------------>|     Class B     |
   | attribute 02[1] | 1..*          0..1 +-----------------+
   +-----------------+                    | attribute 03[1] |
   | operation 1()   |                    +-----------------+
   +-----------------+
   The preceding diagram is an example where Class A knows about
   Class B (i.e., read as "Class A association_name Class B") and
   Class B knows about Class A (i.e., read as "Class B
   association_name Class A").  The use of association_name is
   optional.  The multiplicity notation (1..* and 0..1) indicates the
   number of instances of the object.
   +--------------------+
   |      Class A       |
   +--------------------+              +--------------------+
   | attribute 01[1]    |<-------------|      Class B       |
   | attribute 02[1]    | 1      0..1  +--------------------+
   +--------------------+              | attribute 03[1]    |
   | operation 1()      |              +--------------------+
   +--------------------+
   The preceding diagram is an example where Class B knows about
   Class A (i.e., read as "Class B knows about Class A") but Class A
   does not know about Class B.
   +----------------------+
   |       Class A        |
   +----------------------+            +--------------------+
   |   attribute 01[1]    |----------->|      Class B       |
   |   attribute 02[1]    | 0..*     1 +--------------------+
   +----------------------+            | attribute 03[1]    |
   |    operation 1()     |            +--------------------+
   +----------------------+
   The preceding diagram is an example where Class A knows about
   Class B (i.e., read as "Class A knows about Class B") but Class B
   does not know about Class A.

Chadalapaka, et al. Standards Track [Page 23] RFC 7143 iSCSI (Consolidated) April 2014

3.5. Class Diagram Notation for Aggregations

   +---------------+             +--------------+
   |  Class whole  |o------------|  Class part  |
   +---------------+             +--------------+
   The preceding diagram is an example where Class whole is an
   aggregate that contains Class part and where Class part may
   continue to exist even if Class whole is removed (i.e., read as
   "the whole contains the part").
   +---------------+             +--------------+
   |  Class whole  |@------------|  Class part  |
   +---------------+             +--------------+
   The preceding diagram is an example where Class whole is an
   aggregate that contains Class part where Class part only belongs
   to one Class whole, and the Class part does not continue to exist
   if the Class whole is removed (i.e., read as "the whole contains
   the part").
   +-------------+
   |             |
   +-------------+
      |       |
      + =(a)= +
      |       |
   The preceding diagram is an example where there is a constraint
   between the associations, where the (a) footnote describes the
   constraint.

Chadalapaka, et al. Standards Track [Page 24] RFC 7143 iSCSI (Consolidated) April 2014

3.6. Class Diagram Notation for Generalizations

   +---------------+
   |  Superclass   |
   +-------^-------+
          /_\
           |
   +---------------+
   |    Subclass   |
   +---------------+
   The preceding diagram is an example where the subclass is a kind
   of superclass.  A subclass shares all the attributes and
   operations of the superclass (i.e., the subclass inherits from the
   superclass).

4. Overview

4.1. SCSI Concepts

 The SCSI Architecture Model - 2 [SAM2] describes in detail the
 architecture of the SCSI family of I/O protocols.  This section
 provides a brief background of the SCSI architecture and is intended
 to familiarize readers with its terminology.
 At the highest level, SCSI is a family of interfaces for requesting
 services from I/O devices, including hard drives, tape drives, CD and
 DVD drives, printers, and scanners.  In SCSI terminology, an
 individual I/O device is called a "logical unit" (LU).
 SCSI is a client-server architecture.  Clients of a SCSI interface
 are called "initiators".  Initiators issue SCSI "commands" to request
 services from components -- LUs of a server known as a "target".  The
 "device server" on the LU accepts SCSI commands and processes them.
 A "SCSI transport" maps the client-server SCSI protocol to a specific
 interconnect.  The initiator is one endpoint of a SCSI transport.
 The "target" is the other endpoint.  A target can contain multiple
 LUs.  Each LU has an address within a target called a Logical Unit
 Number (LUN).
 A SCSI task is a SCSI command or possibly a linked set of SCSI
 commands.  Some LUs support multiple pending (queued) tasks, but the
 queue of tasks is managed by the LU.  The target uses an initiator-
 provided "task tag" to distinguish between tasks.  Only one command
 in a task can be outstanding at any given time.

Chadalapaka, et al. Standards Track [Page 25] RFC 7143 iSCSI (Consolidated) April 2014

 Each SCSI command results in an optional data phase and a required
 response phase.  In the data phase, information can travel from the
 initiator to the target (e.g., write), from the target to the
 initiator (e.g., read), or in both directions.  In the response
 phase, the target returns the final status of the operation,
 including any errors.
 Command Descriptor Blocks (CDBs) are the data structures used to
 contain the command parameters that an initiator sends to a target.
 The CDB content and structure are defined by [SAM2] and device-type
 specific SCSI standards.

4.2. iSCSI Concepts and Functional Overview

 The iSCSI protocol is a mapping of the SCSI command, event, and task
 management model (see [SAM2]) over the TCP protocol.  SCSI commands
 are carried by iSCSI requests, and SCSI responses and status are
 carried by iSCSI responses.  iSCSI also uses the request-response
 mechanism for iSCSI protocol mechanisms.
 For the remainder of this document, the terms "initiator" and
 "target" refer to "iSCSI initiator node" and "iSCSI target node",
 respectively (see iSCSI), unless otherwise qualified.
 As its title suggests, Section 4 presents an overview of the iSCSI
 concepts, and later sections in the rest of the specification contain
 the normative requirements -- in many cases covering the same
 concepts discussed in Section 4.  Such normative requirements text
 overrides the overview text in Section 4 if there is a disagreement
 between the two.
 In keeping with similar protocols, the initiator and target divide
 their communications into messages.  This document uses the term
 "iSCSI Protocol Data Unit" (iSCSI PDU) for these messages.
 For performance reasons, iSCSI allows a "phase-collapse".  A command
 and its associated data may be shipped together from initiator to
 target, and data and responses may be shipped together from targets.
 The iSCSI transfer direction is defined with respect to the
 initiator.  Outbound or outgoing transfers are transfers from an
 initiator to a target, while inbound or incoming transfers are from a
 target to an initiator.
 An iSCSI task is an iSCSI request for which a response is expected.

Chadalapaka, et al. Standards Track [Page 26] RFC 7143 iSCSI (Consolidated) April 2014

 In this document, "iSCSI request", "iSCSI command", request, or
 (unqualified) command have the same meaning.  Also, unless otherwise
 specified, status, response, or numbered response have the same
 meaning.

4.2.1. Layers and Sessions

 The following conceptual layering model is used to specify initiator
 and target actions and the way in which they relate to transmitted
 and received Protocol Data Units:
  1. The SCSI layer builds/receives SCSI CDBs (Command Descriptor

Blocks) and passes/receives them with the remaining Execute

      Command [SAM2] parameters to/from
  1. the iSCSI layer that builds/receives iSCSI PDUs and

relays/receives them to/from one or more TCP connections; the

      group of connections form an initiator-target "session".
 Communication between the initiator and target occurs over one or
 more TCP connections.  The TCP connections carry control messages,
 SCSI commands, parameters, and data within iSCSI Protocol Data Units
 (iSCSI PDUs).  The group of TCP connections that link an initiator
 with a target form a session (equivalent to a SCSI I_T nexus; see
 Section 4.4.2).  A session is defined by a session ID that is
 composed of an initiator part and a target part.  TCP connections can
 be added and removed from a session.  Each connection within a
 session is identified by a connection ID (CID).
 Across all connections within a session, an initiator sees one
 "target image".  All target-identifying elements, such as a LUN, are
 the same.  A target also sees one "initiator image" across all
 connections within a session.  Initiator-identifying elements, such
 as the Initiator Task Tag, are global across the session, regardless
 of the connection on which they are sent or received.
 iSCSI targets and initiators MUST support at least one TCP connection
 and MAY support several connections in a session.  For error recovery
 purposes, targets and initiators that support a single active
 connection in a session SHOULD support two connections during
 recovery.

Chadalapaka, et al. Standards Track [Page 27] RFC 7143 iSCSI (Consolidated) April 2014

4.2.2. Ordering and iSCSI Numbering

 iSCSI uses command and status numbering schemes and a data sequencing
 scheme.
 Command numbering is session-wide and is used for ordered command
 delivery over multiple connections.  It can also be used as a
 mechanism for command flow control over a session.
 Status numbering is per connection and is used to enable missing
 status detection and recovery in the presence of transient or
 permanent communication errors.
 Data sequencing is per command or part of a command (R2T-triggered
 sequence) and is used to detect missing data and/or R2T PDUs due to
 header digest errors.
 Typically, fields in the iSCSI PDUs communicate the sequence numbers
 between the initiator and target.  During periods when traffic on a
 connection is unidirectional, iSCSI NOP-Out/NOP-In PDUs may be
 utilized to synchronize the command and status ordering counters of
 the target and initiator.
 The iSCSI session abstraction is equivalent to the SCSI I_T nexus,
 and the iSCSI session provides an ordered command delivery from the
 SCSI initiator to the SCSI target.  For detailed design
 considerations that led to the iSCSI session model as it is defined
 here and how it relates the SCSI command ordering features defined in
 SCSI specifications to the iSCSI concepts, see [RFC3783].

4.2.2.1. Command Numbering and Acknowledging

 iSCSI performs ordered command delivery within a session.  All
 commands (initiator-to-target PDUs) in transit from the initiator to
 the target are numbered.
 iSCSI considers a task to be instantiated on the target in response
 to every request issued by the initiator.  A set of task management
 operations, including abort and reassign (see Section 11.5), may be
 performed on an iSCSI task; however, an abort operation cannot be
 performed on a task management operation, and usage of reassign
 operations has certain constraints.  See Section 11.5.1 for details.
 Some iSCSI tasks are SCSI tasks, and many SCSI activities are related
 to a SCSI task ([SAM2]).  In all cases, the task is identified by the
 Initiator Task Tag for the life of the task.

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 The command number is carried by the iSCSI PDU as the CmdSN (command
 sequence number).  The numbering is session-wide.  Outgoing iSCSI
 PDUs carry this number.  The iSCSI initiator allocates CmdSNs with a
 32-bit unsigned counter (modulo 2**32).  Comparisons and arithmetic
 on CmdSNs use Serial Number Arithmetic as defined in [RFC1982] where
 SERIAL_BITS = 32.
 Commands meant for immediate delivery are marked with an immediate
 delivery flag; they MUST also carry the current CmdSN.  The CmdSN
 MUST NOT advance after a command marked for immediate delivery is
 sent.
 Command numbering starts with the first Login Request on the first
 connection of a session (the leading login on the leading
 connection), and the CmdSN MUST be incremented by 1 in a Serial
 Number Arithmetic sense, as defined in [RFC1982], for every
 non-immediate command issued afterwards.
 If immediate delivery is used with task management commands, these
 commands may reach the target before the tasks on which they are
 supposed to act.  However, their CmdSN serves as a marker of their
 position in the stream of commands.  The initiator and target MUST
 ensure that the SCSI task management functions specified in [SAM2]
 act in accordance with the [SAM2] specification.  For example, both
 commands and responses appear as if delivered in order.  Whenever the
 CmdSN for an outgoing PDU is not specified by an explicit rule, the
 CmdSN will carry the current value of the local CmdSN variable (see
 later in this section).
 The means by which an implementation decides to mark a PDU for
 immediate delivery or by which iSCSI decides by itself to mark a PDU
 for immediate delivery are beyond the scope of this document.
 The number of commands used for immediate delivery is not limited,
 and their delivery to execution is not acknowledged through the
 numbering scheme.  An iSCSI target MAY reject immediate commands,
 e.g., due to lack of resources to accommodate additional commands.
 An iSCSI target MUST be able to handle at least one immediate task
 management command and one immediate non-task-management iSCSI
 command per connection at any time.
 In this document, delivery for execution means delivery to the SCSI
 execution engine or an iSCSI protocol-specific execution engine
 (e.g., for Text Requests with public or private extension keys
 involving an execution component).  With the exception of the
 commands marked for immediate delivery, the iSCSI target layer MUST
 deliver the commands for execution in the order specified by the
 CmdSN.  Commands marked for immediate delivery may be delivered by

Chadalapaka, et al. Standards Track [Page 29] RFC 7143 iSCSI (Consolidated) April 2014

 the iSCSI target layer for execution as soon as detected.  iSCSI may
 avoid delivering some commands to the SCSI target layer if required
 by a prior SCSI or iSCSI action (e.g., a CLEAR TASK SET task
 management request received before all the commands on which it was
 supposed to act).
 On any connection, the iSCSI initiator MUST send the commands in
 increasing order of CmdSN, except for commands that are retransmitted
 due to digest error recovery and connection recovery.
 For the numbering mechanism, the initiator and target maintain the
 following three variables for each session:
  1. CmdSN: the current command sequence number, advanced by 1 on

each command shipped except for commands marked for immediate

      delivery as discussed above.  The CmdSN always contains the
      number to be assigned to the next command PDU.
  1. ExpCmdSN: the next expected command by the target. The target

acknowledges all commands up to, but not including, this number.

      The initiator treats all commands with a CmdSN less than the
      ExpCmdSN as acknowledged.  The target iSCSI layer sets the
      ExpCmdSN to the largest non-immediate CmdSN that it can deliver
      for execution "plus 1" per [RFC1982].  There MUST NOT be any
      holes in the acknowledged CmdSN sequence.
  1. MaxCmdSN: the maximum number to be shipped. The queuing

capacity of the receiving iSCSI layer is

      MaxCmdSN - ExpCmdSN + 1.
 The initiator's ExpCmdSN and MaxCmdSN are derived from target-to-
 initiator PDU fields.  Comparisons and arithmetic on the ExpCmdSN and
 MaxCmdSN MUST use Serial Number Arithmetic as defined in [RFC1982]
 where SERIAL_BITS = 32.
 The target MUST NOT transmit a MaxCmdSN that is less than
 ExpCmdSN - 1.  For non-immediate commands, the CmdSN field can take
 any value from the ExpCmdSN to the MaxCmdSN inclusive.  The target
 MUST silently ignore any non-immediate command outside of this range
 or non-immediate duplicates within the range.  The CmdSN carried by
 immediate commands may lie outside the ExpCmdSN-to-MaxCmdSN range.
 For example, if the initiator has previously sent a non-immediate
 command carrying the CmdSN equal to the MaxCmdSN, the target window
 is closed.  For group task management commands issued as immediate
 commands, the CmdSN indicates the scope of the group action (e.g., an
 ABORT TASK SET indicates which commands are to be aborted).

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 MaxCmdSN and ExpCmdSN fields are processed by the initiator as
 follows:
  1. If the PDU MaxCmdSN is less than the PDU ExpCmdSN - 1 (in a

Serial Number Arithmetic sense), they are both ignored.

  1. If the PDU MaxCmdSN is greater than the local MaxCmdSN (in a

Serial Number Arithmetic sense), it updates the local MaxCmdSN;

      otherwise, it is ignored.
  1. If the PDU ExpCmdSN is greater than the local ExpCmdSN (in a

Serial Number Arithmetic sense), it updates the local ExpCmdSN;

      otherwise, it is ignored.
 This sequence is required because updates may arrive out of order
 (e.g., the updates are sent on different TCP connections).
 iSCSI initiators and targets MUST support the command numbering
 scheme.
 A numbered iSCSI request will not change its allocated CmdSN,
 regardless of the number of times and circumstances in which it is
 reissued (see Section 7.2.1).  At the target, the CmdSN is only
 relevant while the command has not created any state related to its
 execution (execution state); afterwards, the CmdSN becomes
 irrelevant.  Testing for the execution state (represented by
 identifying the Initiator Task Tag) MUST precede any other action at
 the target.  If no execution state is found, it is followed by
 ordering and delivery.  If an execution state is found, it is
 followed by delivery if it has not already been delivered.
 If an initiator issues a command retry for a command with CmdSN R on
 a connection when the session CmdSN value is Q, it MUST NOT advance
 the CmdSN past R + 2**31 - 1 unless
  1. the connection is no longer operational (i.e., it has returned

to the FREE state; see Section 8.1.3),

  1. the connection has been reinstated (see Section 6.3.4), or
  1. a non-immediate command with a CmdSN equal to or greater than Q

was issued subsequent to the command retry on the same

      connection and the reception of that command is acknowledged by
      the target (see Section 10.4).
 A target command response or Data-In PDU with status MUST NOT precede
 the command acknowledgment.  However, the acknowledgment MAY be
 included in the response or the Data-In PDU.

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4.2.2.2. Response/Status Numbering and Acknowledging

 Responses in transit from the target to the initiator are numbered.
 The StatSN (status sequence number) is used for this purpose.  The
 StatSN is a counter maintained per connection.  The ExpStatSN is used
 by the initiator to acknowledge status.  The status sequence number
 space is 32-bit unsigned integers, and the arithmetic operations are
 the regular mod(2**32) arithmetic.
 Status numbering starts with the Login Response to the first Login
 Request of the connection.  The Login Response includes an initial
 value for status numbering (any initial value is valid).
 To enable command recovery, the target MAY maintain enough state
 information for data and status recovery after a connection failure.
 A target doing so can safely discard all of the state information
 maintained for recovery of a command after the delivery of the status
 for the command (numbered StatSN) is acknowledged through the
 ExpStatSN.
 A large absolute difference between the StatSN and the ExpStatSN may
 indicate a failed connection.  Initiators MUST undertake recovery
 actions if the difference is greater than an implementation-defined
 constant that MUST NOT exceed 2**31 - 1.
 Initiators and targets MUST support the response-numbering scheme.

4.2.2.3. Response Ordering

4.2.2.3.1. Need for Response Ordering

 Whenever an iSCSI session is composed of multiple connections, the
 Response PDUs (task responses or TMF Responses) originating in the
 target SCSI layer are distributed onto the multiple connections by
 the target iSCSI layer according to iSCSI connection allegiance
 rules.  This process generally may not preserve the ordering of the
 responses by the time they are delivered to the initiator SCSI layer.
 Since ordering is not expected across SCSI Response PDUs anyway, this
 approach works fine in the general case.  However, to address the
 special cases where some ordering is desired by the SCSI layer, we
 introduce the notion of a "Response Fence": a Response Fence is
 logically the attribute/property of a SCSI response message handed
 off to a target iSCSI layer that indicates that there are special
 SCSI-level ordering considerations associated with this particular
 response message.  Whenever a Response Fence is set or required on a

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 SCSI response message, we define the semantics in Section 4.2.2.3.2
 with respect to the target iSCSI layer's handling of such SCSI
 response messages.

4.2.2.3.2. Response Ordering Model Description

 The target SCSI protocol layer hands off the SCSI response messages
 to the target iSCSI layer by invoking the "Send Command Complete"
 protocol data service ([SAM2], Clause 5.4.2) and "Task Management
 Function Executed" ([SAM2], Clause 6.9) service.  On receiving the
 SCSI response message, the iSCSI layer exhibits the Response Fence
 behavior for certain SCSI response messages (Section 4.2.2.3.4
 describes the specific instances where the semantics must be
 realized).
 Whenever the Response Fence behavior is required for a SCSI response
 message, the target iSCSI layer MUST ensure that the following
 conditions are met in delivering the response message to the
 initiator iSCSI layer:
  1. A response with a Response Fence MUST be delivered

chronologically after all the "preceding" responses on the I_T_L

      nexus, if the preceding responses are delivered at all, to the
      initiator iSCSI layer.
  1. A response with a Response Fence MUST be delivered

chronologically prior to all the "following" responses on the

      I_T_L nexus.
 The notions of "preceding" and "following" refer to the order of
 handoff of a response message from the target SCSI protocol layer to
 the target iSCSI layer.

4.2.2.3.3. iSCSI Semantics with the Interface Model

 Whenever the TaskReporting key (Section 13.23) is negotiated to
 ResponseFence or FastAbort for an iSCSI session and the Response
 Fence behavior is required for a SCSI response message, the target
 iSCSI layer MUST perform the actions described in this section for
 that session.
    a) If it is a single-connection session, no special processing is
       required.  The standard SCSI Response PDU build and dispatch
       process happens.
    b) If it is a multi-connection session, the target iSCSI layer
       takes note of the last-sent and unacknowledged StatSN on each
       of the connections in the iSCSI session, and waits for an

Chadalapaka, et al. Standards Track [Page 33] RFC 7143 iSCSI (Consolidated) April 2014

       acknowledgment (NOP-In PDUs MAY be used to solicit
       acknowledgments as needed in order to accelerate this process)
       of each such StatSN to clear the fence.  The SCSI Response PDU
       requiring the Response Fence behavior MUST NOT be sent to the
       initiator before acknowledgments are received for each of the
       unacknowledged StatSNs.
    c) The target iSCSI layer must wait for an acknowledgment of the
       SCSI Response PDU that carried the SCSI response requiring the
       Response Fence behavior.  The fence MUST be considered cleared
       only after receiving the acknowledgment.
    d) All further status processing for the LU is resumed only after
       clearing the fence.  If any new responses for the I_T_L nexus
       are received from the SCSI layer before the fence is cleared,
       those Response PDUs MUST be held and queued at the iSCSI layer
       until the fence is cleared.

4.2.2.3.4. Current List of Fenced Response Use Cases

 This section lists the situations in which fenced response behavior
 is REQUIRED in iSCSI target implementations.  Note that the following
 list is an exhaustive enumeration as currently identified -- it is
 expected that as SCSI protocol specifications evolve, the
 specifications will enumerate when response fencing is required on a
 case-by-case basis.
 Whenever the TaskReporting key (Section 13.23) is negotiated to
 ResponseFence or FastAbort for an iSCSI session, the target iSCSI
 layer MUST assume that the Response Fence is required for the
 following SCSI completion messages:
    a) The first completion message carrying the UA after the multi-
       task abort on issuing and third-party sessions.  See
       Section 4.2.3.2 for related TMF discussion.
    b) The TMF Response carrying the multi-task TMF Response on the
       issuing session.
    c) The completion message indicating ACA establishment on the
       issuing session.
    d) The first completion message carrying the ACA ACTIVE status
       after ACA establishment on issuing and third-party sessions.

Chadalapaka, et al. Standards Track [Page 34] RFC 7143 iSCSI (Consolidated) April 2014

    e) The TMF Response carrying the CLEAR ACA response on the issuing
       session.
    f) The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
       command.
 Notes:
  1. Due to the absence of ACA-related fencing requirements in

[RFC3720], initiator implementations SHOULD NOT use ACA on

      multi-connection iSCSI sessions with targets complying only with
      [RFC3720].  This can be determined via TaskReporting key
      (Section 13.23) negotiation -- when the negotiation results in
      either "RFC3720" or "NotUnderstood".
  1. Initiators that want to employ ACA on multi-connection iSCSI

sessions SHOULD first assess response-fencing behavior via

      negotiating for the "ResponseFence" or "FastAbort" value for the
      TaskReporting (Section 13.23) key.

4.2.2.4. Data Sequencing

 Data and R2T PDUs transferred as part of some command execution MUST
 be sequenced.  The DataSN field is used for data sequencing.  For
 input (read) data PDUs, the DataSN starts with 0 for the first data
 PDU of an input command and advances by 1 for each subsequent data
 PDU.  For output data PDUs, the DataSN starts with 0 for the first
 data PDU of a sequence (the initial unsolicited sequence or any data
 PDU sequence issued to satisfy an R2T) and advances by 1 for each
 subsequent data PDU.  R2Ts are also sequenced per command.  For
 example, the first R2T has an R2TSN of 0 and advances by 1 for each
 subsequent R2T.  For bidirectional commands, the target uses the
 DataSN/R2TSN to sequence Data-In and R2T PDUs in one continuous
 sequence (undifferentiated).  Unlike command and status, data PDUs
 and R2Ts are not acknowledged by a field in regular outgoing PDUs.
 Data-In PDUs can be acknowledged on demand by a special form of the
 SNACK PDU.  Data and R2T PDUs are implicitly acknowledged by status
 for the command.  The DataSN/R2TSN field enables the initiator to
 detect missing data or R2T PDUs.
 For any read or bidirectional command, a target MUST issue less than
 2**32 combined R2T and Data-In PDUs.  Any output data sequence MUST
 contain less than 2**32 Data-Out PDUs.

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4.2.3. iSCSI Task Management

4.2.3.1. Task Management Overview

 iSCSI task management features allow an initiator to control the
 active iSCSI tasks on an operational iSCSI session that it has with
 an iSCSI target.  Section 11.5 defines the task management function
 types that this specification defines -- ABORT TASK, ABORT TASK SET,
 CLEAR ACA, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET,
 TARGET COLD RESET, and TASK REASSIGN.
 Out of these function types, ABORT TASK and TASK REASSIGN functions
 manage a single active task, whereas ABORT TASK SET, CLEAR TASK SET,
 LOGICAL UNIT RESET, TARGET WARM RESET, and TARGET COLD RESET
 functions can each potentially affect multiple active tasks.

4.2.3.2. Notion of Affected Tasks

 This section defines the notion of "affected tasks" in multi-task
 abort scenarios.  Scope definitions in this section apply to both the
 standard multi-task abort semantics (Section 4.2.3.3) and the
 FastAbort multi-task abort semantics behavior (Section 4.2.3.4).
 ABORT TASK SET: All outstanding tasks for the I_T_L nexus identified
    by the LUN field in the ABORT TASK SET TMF Request PDU.
 CLEAR TASK SET: All outstanding tasks in the task set for the LU
    identified by the LUN field in the CLEAR TASK SET TMF Request PDU.
    See [SPC3] for the definition of a "task set".
 LOGICAL UNIT RESET: All outstanding tasks from all initiators for the
    LU identified by the LUN field in the LOGICAL UNIT RESET
    Request PDU.
 TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks from all
    initiators across all LUs to which the TMF-issuing session has
    access on the SCSI target device hosting the iSCSI session.
 Usage: An "ABORT TASK SET TMF Request PDU" in the preceding text is
    an iSCSI TMF Request PDU with the "Function" field set to "ABORT
    TASK SET" as defined in Section 11.5.  Similar usage is employed
    for other scope descriptions.

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4.2.3.3. Standard Multi-Task Abort Semantics

 All iSCSI implementations MUST support the protocol behavior defined
 in this section as the default behavior.  The execution of ABORT TASK
 SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET, and
 TARGET COLD RESET TMF Requests consists of the following sequence of
 actions in the specified order on the specified party.
 The initiator iSCSI layer:
    a) MUST continue to respond to each TTT received for the affected
       tasks.
    b) SHOULD process any responses received for affected tasks in the
       normal fashion.  This is acceptable because the responses are
       guaranteed to have been sent prior to the TMF Response.
    c) SHOULD receive the TMF Response concluding all the tasks in the
       set of affected tasks, unless the initiator has done something
       (e.g., LU reset, connection drop) that may prevent the TMF
       Response from being sent or received.  The initiator MUST thus
       conclude all affected tasks as part of this step in either case
       and MUST discard any TMF Response received after the affected
       tasks are concluded.
 The target iSCSI layer:
    a) MUST wait for responses on currently valid Target Transfer Tags
       of the affected tasks from the issuing initiator.  MAY wait for
       responses on currently valid Target Transfer Tags of the
       affected tasks from third-party initiators.
    b) MUST wait (concurrent with the wait in Step a) for all commands
       of the affected tasks to be received based on the CmdSN
       ordering.  SHOULD NOT wait for new commands on third-party
       affected sessions -- only the instantiated tasks have to be
       considered for the purpose of determining the affected tasks.
       However, in the case of target-scoped requests (i.e., TARGET
       WARM RESET and TARGET COLD RESET), all of the commands that are
       not yet received on the issuing session in the command stream
       can be considered to have been received with no command waiting
       period -- i.e., the entire CmdSN space up to the CmdSN of the
       task management function can be "plugged".
    c) MUST propagate the TMF Request to, and receive the response
       from, the target SCSI layer.

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    d) MUST provide the Response Fence behavior for the TMF Response
       on the issuing session as specified in Section 4.2.2.3.2.
    e) MUST provide the Response Fence behavior on the first post-TMF
       Response on third-party sessions as specified in
       Section 4.2.2.3.3.  If some tasks originate from non-iSCSI
       I_T_L nexuses, then the means by which the target ensures that
       all affected tasks have returned their status to the initiator
       are defined by the specific non-iSCSI transport protocol(s).
 Technically, the TMF servicing is complete in Step d).  Data
 transfers corresponding to terminated tasks may, however, still be in
 progress on third-party iSCSI sessions even at the end of Step e).
 The TMF Response MUST NOT be sent by the target iSCSI layer before
 the end of Step d) and MAY be sent at the end of Step d) despite
 these outstanding data transfers until after Step e).

4.2.3.4. FastAbort Multi-Task Abort Semantics

 Protocol behavior defined in this section SHOULD be implemented by
 all iSCSI implementations complying with this document, noting that
 some steps below may not be compatible with [RFC3720] semantics.
 However, protocol behavior defined in this section MUST be exhibited
 by iSCSI implementations on an iSCSI session when they negotiate the
 TaskReporting (Section 13.23) key to "FastAbort" on that session.
 The execution of ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET,
 TARGET WARM RESET, and TARGET COLD RESET TMF Requests consists of the
 following sequence of actions in the specified order on the specified
 party.
 The initiator iSCSI layer:
    a) MUST NOT send any more Data-Out PDUs for affected tasks on the
       issuing connection of the issuing iSCSI session once the TMF is
       sent to the target.
    b) SHOULD process any responses received for affected tasks in the
       normal fashion.  This is acceptable because the responses are
       guaranteed to have been sent prior to the TMF Response.
    c) MUST respond to each Async Message PDU with a Task Termination
       AsyncEvent (5) as defined in Section 11.9.

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    d) MUST treat the TMF Response as terminating all affected tasks
       for which responses have not been received and MUST discard any
       responses for affected tasks received after the TMF Response is
       passed to the SCSI layer (although the semantics defined in
       this section ensure that such an out-of-order scenario will
       never happen with a compliant target implementation).
 The target iSCSI layer:
    a) MUST wait for all commands of the affected tasks to be received
       based on the CmdSN ordering on the issuing session.  SHOULD NOT
       wait for new commands on third-party affected sessions -- only
       the instantiated tasks have to be considered for the purpose of
       determining the affected tasks.  In the case of target-scoped
       requests (i.e., TARGET WARM RESET and TARGET COLD RESET), all
       the commands that are not yet received on the issuing session
       in the command stream can be considered to have been received
       with no command waiting period -- i.e., the entire CmdSN space
       up to the CmdSN of the task management function can be
       "plugged".
    b) MUST propagate the TMF Request to, and receive the response
       from, the target SCSI layer.
    c) MUST leave all active "affected TTTs" (i.e., active TTTs
       associated with affected tasks) valid.
    d) MUST send an Asynchronous Message PDU with AsyncEvent=5
       (Section 11.9) on:
       1) each connection of each third-party session to which at
          least one affected task is allegiant if
          TaskReporting=FastAbort is operational on that third-party
          session, and
       2) each connection except the issuing connection of the issuing
          session that has at least one allegiant affected task.
          If there are multiple affected LUs (say, due to a target
          reset), then one Async Message PDU MUST be sent for each
          such LU on each connection that has at least one allegiant
          affected task.  The LUN field in the Asynchronous Message
          PDU MUST be set to match the LUN for each such LU.
    e) MUST address the Response Fence flag on the TMF Response on the
       issuing session as defined in Section 4.2.2.3.3.

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    f) MUST address the Response Fence flag on the first post-TMF
       Response on third-party sessions as defined in
       Section 4.2.2.3.3.  If some tasks originate from non-iSCSI
       I_T_L nexuses, then the means by which the target ensures that
       all affected tasks have returned their status to the initiator
       are defined by the specific non-iSCSI transport protocol(s).
    g) MUST free up the affected TTTs (and STags for iSER, if
       applicable) and the corresponding buffers, if any, once it
       receives each associated NOP-Out acknowledgment that the
       initiator generated in response to each Async Message.
 Technically, the TMF servicing is complete in Step e).  Data
 transfers corresponding to terminated tasks may, however, still be in
 progress even at the end of Step f).  A TMF Response MUST NOT be sent
 by the target iSCSI layer before the end of Step e) and MAY be sent
 at the end of Step e) despite these outstanding Data transfers until
 Step g).  Step g) specifies an event to free up any such resources
 that may have been reserved to support outstanding data transfers.

4.2.3.5. Affected Tasks Shared across Standard and FastAbort Sessions

 If an iSCSI target implementation is capable of supporting
 TaskReporting=FastAbort functionality (Section 13.23), it may end up
 in a situation where some sessions have TaskReporting=RFC3720
 operational (RFC 3720 sessions) while some other sessions have
 TaskReporting=FastAbort operational (FastAbort sessions) even while
 accessing a shared set of affected tasks (Section 4.2.3.2).  If the
 issuing session is an RFC 3720 session, the iSCSI target
 implementation is FastAbort-capable, and the third-party affected
 session is a FastAbort session, the following behavior SHOULD be
 exhibited by the iSCSI target layer:
    a) Between Steps c) and d) of the target behavior in
       Section 4.2.3.3, send an Asynchronous Message PDU with
       AsyncEvent=5 (Section 11.9) on each connection of each third-
       party session to which at least one affected task is allegiant.
       If there are multiple affected LUs, then send one Async Message
       PDU for each such LU on each connection that has at least one
       allegiant affected task.  When sent, the LUN field in the
       Asynchronous Message PDU MUST be set to match the LUN for each
       such LU.
    b) After Step e) of the target behavior in Section 4.2.3.3, free
       up the affected TTTs (and STags for iSER, if applicable) and
       the corresponding buffers, if any, once each associated NOP-Out
       acknowledgment is received that the third-party initiator
       generated in response to each Async Message sent in Step a).

Chadalapaka, et al. Standards Track [Page 40] RFC 7143 iSCSI (Consolidated) April 2014

 If the issuing session is a FastAbort session, the iSCSI target
 implementation is FastAbort-capable, and the third-party affected
 session is an RFC 3720 session, the iSCSI target layer MUST NOT send
 Asynchronous Message PDUs on the third-party session to prompt the
 FastAbort behavior.
 If the third-party affected session is a FastAbort session and the
 issuing session is a FastAbort session, the initiator in the third-
 party role MUST respond to each Async Message PDU with AsyncEvent=5
 as defined in Section 11.9.  Note that an initiator MAY thus receive
 these Async Messages on a third-party affected session even if the
 session is a single-connection session.

4.2.3.6. Rationale behind the FastAbort Semantics

 There are fundamentally three basic objectives behind the semantics
 specified in Sections 4.2.3.3 and 4.2.3.4.
    a) Maintaining an ordered command flow I_T nexus abstraction to
       the target SCSI layer even with multi-connection sessions.
  1. Target iSCSI processing of a TMF Request must maintain the

single flow illusion. The target behavior in Step b) of

         Section 4.2.3.3 and the target behavior in Step a) of
         Section 4.2.3.4 correspond to this objective.
    b) Maintaining a single ordered response flow I_T nexus
       abstraction to the initiator SCSI layer even with multi-
       connection sessions when one response (i.e., TMF Response)
       could imply the status of other unfinished tasks from the
       initiator's perspective.
  1. The target must ensure that the initiator does not see "old"

task responses (that were placed on the wire chronologically

         earlier than the TMF Response) after seeing the TMF Response.
         The target behavior in Step d) of Section 4.2.3.3 and the
         target behavior in Step e) of Section 4.2.3.4 correspond to
         this objective.
  1. Whenever the result of a TMF action is visible across

multiple I_T_L nexuses, [SAM2] requires the SCSI device

         server to trigger a UA on each of the other I_T_L nexuses.
         Once an initiator is notified of such a UA, the application
         client on the receiving initiator is required to clear its
         task state (Clause 5.5 of [SAM2]) for the affected tasks.  It
         would thus be inappropriate to deliver a SCSI Response for a
         task after the task state is cleared on the initiator, i.e.,
         after the UA is notified.  The UA notification contained in

Chadalapaka, et al. Standards Track [Page 41] RFC 7143 iSCSI (Consolidated) April 2014

         the first SCSI Response PDU on each affected third-party
         I_T_L nexus after the TMF action thus MUST NOT pass the
         affected task responses on any of the iSCSI sessions
         accessing the LU.  The target behavior in Step e) of
         Section 4.2.3.3 and the target behavior in Step f) of
         Section 4.2.3.4 correspond to this objective.
    c) Draining all active TTTs corresponding to affected tasks in a
       deterministic fashion.
  1. Data-Out PDUs with stale TTTs arriving after the tasks are

terminated can create a buffer management problem even for

         traditional iSCSI implementations and is fatal for the
         connection for iSCSI/iSER implementations.  Either the
         termination of affected tasks should be postponed until the
         TTTs are retired (as in Step a) of Section 4.2.3.3), or the
         TTTs and the buffers should stay allocated beyond task
         termination to be deterministically freed up later (as in
         Steps c) and g) of Section 4.2.3.4).
 The only other notable optimization is the plugging.  If all tasks on
 an I_T nexus will be aborted anyway (as with a target reset), there
 is no need to wait to receive all commands to plug the CmdSN holes.
 The target iSCSI layer can simply plug all missing CmdSN slots and
 move on with TMF processing.  The first objective (maintaining a
 single ordered command flow) is still met with this optimization
 because the target SCSI layer only sees ordered commands.

4.2.4. iSCSI Login

 The purpose of the iSCSI login is to enable a TCP connection for
 iSCSI use, authentication of the parties, negotiation of the
 session's parameters, and marking of the connection as belonging to
 an iSCSI session.
 A session is used to identify to a target all the connections with a
 given initiator that belong to the same I_T nexus.  (For more details
 on how a session relates to an I_T nexus, see Section 4.4.2.)
 The targets listen on a well-known TCP port or other TCP port for
 incoming connections.  The initiator begins the login process by
 connecting to one of these TCP ports.
 As part of the login process, the initiator and target SHOULD
 authenticate each other and MAY set a security association protocol
 for the session.  This can occur in many different ways and is
 subject to negotiation; see Section 12.

Chadalapaka, et al. Standards Track [Page 42] RFC 7143 iSCSI (Consolidated) April 2014

 To protect the TCP connection, an IPsec security association MAY be
 established before the Login Request.  For information on using IPsec
 security for iSCSI, see Section 9, [RFC3723], and [RFC7146].
 The iSCSI Login Phase is carried through Login Requests and
 Responses.  Once suitable authentication has occurred and operational
 parameters have been set, the session transitions to the Full Feature
 Phase and the initiator may start to send SCSI commands.  The
 security policy for whether and by what means a target chooses to
 authorize an initiator is beyond the scope of this document.  For a
 more detailed description of the Login Phase, see Section 6.
 The login PDU includes the ISID part of the session ID (SSID).  The
 target portal group that services the login is implied by the
 selection of the connection endpoint.  For a new session, the TSIH is
 zero.  As part of the response, the target generates a TSIH.
 During session establishment, the target identifies the SCSI
 initiator port (the "I" in the "I_T nexus") through the value pair
 (InitiatorName, ISID).  We describe InitiatorName later in this
 section.  Any persistent state (e.g., persistent reservations) on the
 target that is associated with a SCSI initiator port is identified
 based on this value pair.  Any state associated with the SCSI target
 port (the "T" in the "I_T nexus") is identified externally by the
 TargetName and Target Portal Group Tag (see Section 4.4.1).  The ISID
 is subject to reuse restrictions because it is used to identify a
 persistent state (see Section 4.4.3).
 Before the Full Feature Phase is established, only Login Request and
 Login Response PDUs are allowed.  Login Requests and Responses MUST
 be used exclusively during login.  On any connection, the Login Phase
 MUST immediately follow TCP connection establishment, and a
 subsequent Login Phase MUST NOT occur before tearing down the
 connection.
 A target receiving any PDU except a Login Request before the Login
 Phase is started MUST immediately terminate the connection on which
 the PDU was received.  Once the Login Phase has started, if the
 target receives any PDU except a Login Request, it MUST send a Login
 reject (with Status "invalid during login") and then disconnect.  If
 the initiator receives any PDU except a Login Response, it MUST
 immediately terminate the connection.

Chadalapaka, et al. Standards Track [Page 43] RFC 7143 iSCSI (Consolidated) April 2014

4.2.5. iSCSI Full Feature Phase

 Once the two sides successfully conclude the login on the first --
 also called the leading -- connection in the session, the iSCSI
 session is in the iSCSI Full Feature Phase.  A connection is in the
 Full Feature Phase if the session is in the Full Feature Phase and
 the connection login has completed successfully.  An iSCSI connection
 is not in the Full Feature Phase when
    a) it does not have an established transport connection, or
    b) when it has a valid transport connection, but a successful
       login was not performed or the connection is currently
       logged out.
 In a normal Full Feature Phase, the initiator may send SCSI commands
 and data to the various LUs on the target by encapsulating them in
 iSCSI PDUs that go over the established iSCSI session.

4.2.5.1. Command Connection Allegiance

 For any iSCSI request issued over a TCP connection, the corresponding
 response and/or other related PDU(s) MUST be sent over the same
 connection.  We call this "connection allegiance".  If the original
 connection fails before the command is completed, the connection
 allegiance of the command may be explicitly reassigned to a different
 transport connection as described in detail in Section 7.2.
 Thus, if an initiator issues a read command, the target MUST send the
 requested data, if any, followed by the status, to the initiator over
 the same TCP connection that was used to deliver the SCSI command.
 If an initiator issues a write command, the initiator MUST send the
 data, if any, for that command over the same TCP connection that was
 used to deliver the SCSI command.  The target MUST return Ready To
 Transfer (R2T), if any, and the status over the same TCP connection
 that was used to deliver the SCSI command.  Retransmission requests
 (SNACK PDUs), and the data and status that they generate, MUST also
 use the same connection.
 However, consecutive commands that are part of a SCSI linked command-
 chain task (see [SAM2]) MAY use different connections.  Connection
 allegiance is strictly per command and not per task.  During the
 iSCSI Full Feature Phase, the initiator and target MAY interleave
 unrelated SCSI commands, their SCSI data, and responses over the
 session.

Chadalapaka, et al. Standards Track [Page 44] RFC 7143 iSCSI (Consolidated) April 2014

4.2.5.2. Data Transfer Overview

 Outgoing SCSI data (initiator-to-target user data or command
 parameters) is sent as either solicited data or unsolicited data.
 Solicited data are sent in response to R2T PDUs.  Unsolicited data
 can be sent as part of an iSCSI Command PDU ("immediate data") or in
 separate iSCSI data PDUs.
 Immediate data are assumed to originate at offset 0 in the initiator
 SCSI write-buffer (outgoing data buffer).  All other data PDUs have
 the buffer offset set explicitly in the PDU header.
 An initiator may send unsolicited data up to FirstBurstLength (see
 Section 13.14) as immediate (up to the negotiated maximum PDU
 length), in a separate PDU sequence, or both.  All subsequent data
 MUST be solicited.  The maximum length of an individual data PDU or
 the immediate-part of the first unsolicited burst MAY be negotiated
 at login.
 The maximum amount of unsolicited data that can be sent with a
 command is negotiated at login through the FirstBurstLength (see
 Section 13.14) key.  A target MAY separately enable immediate data
 (through the ImmediateData key) without enabling the more general
 (separate data PDUs) form of unsolicited data (through the
 InitialR2T key).
 Unsolicited data for a write are meant to reduce the effect of
 latency on throughput (no R2T is needed to start sending data).  In
 addition, immediate data is meant to reduce the protocol overhead
 (both bandwidth and execution time).
 An iSCSI initiator MAY choose not to send unsolicited data, only
 immediate data or FirstBurstLength bytes of unsolicited data with a
 command.  If any non-immediate unsolicited data is sent, the total
 unsolicited data MUST be either FirstBurstLength or all of the data,
 if the total amount is less than the FirstBurstLength.
 It is considered an error for an initiator to send unsolicited data
 PDUs to a target that operates in R2T mode (only solicited data are
 allowed).  It is also an error for an initiator to send more
 unsolicited data, whether immediate or as separate PDUs, than
 FirstBurstLength.
 An initiator MUST honor an R2T data request for a valid outstanding
 command (i.e., carrying a valid Initiator Task Tag) and deliver all
 the requested data, provided the command is supposed to deliver

Chadalapaka, et al. Standards Track [Page 45] RFC 7143 iSCSI (Consolidated) April 2014

 outgoing data and the R2T specifies data within the command bounds.
 The initiator action is unspecified for receiving an R2T request that
 specifies data, all or in part, outside of the bounds of the command.
 A target SHOULD NOT silently discard data and then request
 retransmission through R2T.  Initiators SHOULD NOT keep track of the
 data transferred to or from the target (scoreboarding).  SCSI targets
 perform residual count calculation to check how much data was
 actually transferred to or from the device by a command.  This may
 differ from the amount the initiator sent and/or received for reasons
 such as retransmissions and errors.  Read or bidirectional commands
 implicitly solicit the transmission of the entire amount of data
 covered by the command.  SCSI data packets are matched to their
 corresponding SCSI commands by using tags specified in the protocol.
 In addition, iSCSI initiators and targets MUST enforce some ordering
 rules.  When unsolicited data is used, the order of the unsolicited
 data on each connection MUST match the order in which the commands on
 that connection are sent.  Command and unsolicited data PDUs may be
 interleaved on a single connection as long as the ordering
 requirements of each are maintained (e.g., command N + 1 MAY be sent
 before the unsolicited Data-Out PDUs for command N, but the
 unsolicited Data-Out PDUs for command N MUST precede the unsolicited
 Data-Out PDUs of command N + 1).  A target that receives data out of
 order MAY terminate the session.

4.2.5.3. Tags and Integrity Checks

 Initiator tags for pending commands are unique initiator-wide for a
 session.  Target tags are not strictly specified by the protocol.  It
 is assumed that target tags are used by the target to tag (alone or
 in combination with the LUN) the solicited data.  Target tags are
 generated by the target and "echoed" by the initiator.
 These mechanisms are designed to accomplish efficient data delivery
 along with a large degree of control over the data flow.
 As the Initiator Task Tag is used to identify a task during its
 execution, the iSCSI initiator and target MUST verify that all other
 fields used in task-related PDUs have values that are consistent with
 the values used at the task instantiation, based on the Initiator
 Task Tag (e.g., the LUN used in an R2T PDU MUST be the same as the
 one used in the SCSI Command PDU used to instantiate the task).
 Using inconsistent field values is considered a protocol error.

Chadalapaka, et al. Standards Track [Page 46] RFC 7143 iSCSI (Consolidated) April 2014

4.2.5.4. SCSI Task Management during iSCSI Full Feature Phase

 SCSI task management assumes that individual tasks and task groups
 can be aborted based solely on the task tags (for individual tasks)
 or the timing of the task management command (for task groups) and
 that the task management action is executed synchronously -- i.e., no
 message involving an aborted task will be seen by the SCSI initiator
 after receiving the task management response.  In iSCSI, initiators
 and targets interact asynchronously over several connections.  iSCSI
 specifies the protocol mechanism and implementation requirements
 needed to present a synchronous SCSI view while using an asynchronous
 iSCSI infrastructure.

4.2.6. iSCSI Connection Termination

 An iSCSI connection may be terminated via a transport connection
 shutdown or a transport reset.  A transport reset is assumed to be an
 exceptional event.
 Graceful TCP connection shutdowns are done by sending TCP FINs.  A
 graceful transport connection shutdown SHOULD only be initiated by
 either party when the connection is not in the iSCSI Full Feature
 Phase.  A target MAY terminate a Full Feature Phase connection on
 internal exception events, but it SHOULD announce the fact through an
 Asynchronous Message PDU.  Connection termination with outstanding
 commands may require recovery actions.
 If a connection is terminated while in the Full Feature Phase,
 connection cleanup (see Section 7.14) is required prior to recovery.
 By doing connection cleanup before starting recovery, the initiator
 and target will avoid receiving stale PDUs after recovery.

4.2.7. iSCSI Names

 Both targets and initiators require names for the purpose of
 identification.  In addition, names enable iSCSI storage resources to
 be managed, regardless of location (address).  An iSCSI Node Name is
 also the SCSI device name contained in the iSCSI node.  The iSCSI
 name of a SCSI device is the principal object used in authentication
 of targets to initiators and initiators to targets.  This name is
 also used to identify and manage iSCSI storage resources.
 iSCSI names must be unique within the operation domain of the end
 user.  However, because the operation domain of an IP network is
 potentially worldwide, the iSCSI name formats are architected to be
 worldwide unique.  To assist naming authorities in the construction
 of worldwide unique names, iSCSI provides three name formats for
 different types of naming authorities.

Chadalapaka, et al. Standards Track [Page 47] RFC 7143 iSCSI (Consolidated) April 2014

 iSCSI names are associated with iSCSI nodes, and not iSCSI network
 adapter cards, to ensure that the replacement of network adapter
 cards does not require reconfiguration of all SCSI and iSCSI resource
 allocation information.
 Some SCSI commands require that protocol-specific identifiers be
 communicated within SCSI CDBs.  See Section 2.2 for the definition of
 the SCSI port name/identifier for iSCSI ports.
 An initiator may discover the iSCSI Target Names to which it has
 access, along with their addresses, using the SendTargets Text
 Request, or other techniques discussed in [RFC3721].
 iSCSI equipment that needs discovery functions beyond SendTargets
 SHOULD implement iSNS (see [RFC4171]) for extended discovery
 management capabilities and interoperability.  Although [RFC3721]
 implies an SLP ([RFC2608]) implementation requirement, SLP has not
 been widely implemented or deployed for use with iSCSI in practice.
 iSCSI implementations therefore SHOULD NOT rely on SLP-based
 discovery interoperability.

4.2.7.1. iSCSI Name Properties

 Each iSCSI node, whether it is an initiator, a target, or both, MUST
 have an iSCSI name.  Whenever an iSCSI node contains an iSCSI
 initiator node and an iSCSI target node, the iSCSI Initiator Name
 MUST be the same as the iSCSI Target Name for the contained Nodes
 such that there is only one iSCSI Node Name for the iSCSI node
 overall.  Note the related requirements in Section 9.2.1 on how to
 map CHAP names to iSCSI names in such a scenario.
 Initiators and targets MUST support the receipt of iSCSI names of up
 to the maximum length of 223 bytes.
 The initiator MUST present both its iSCSI Initiator Name and the
 iSCSI Target Name to which it wishes to connect in the first Login
 Request of a new session or connection.  The only exception is if a
 Discovery session (see Section 4.3) is to be established.  In this
 case, the iSCSI Initiator Name is still required, but the iSCSI
 Target Name MAY be omitted.
 iSCSI names have the following properties:
  1. iSCSI names are globally unique. No two initiators or targets

can have the same name.

  1. iSCSI names are permanent. An iSCSI initiator node or target

node has the same name for its lifetime.

Chadalapaka, et al. Standards Track [Page 48] RFC 7143 iSCSI (Consolidated) April 2014

  1. iSCSI names do not imply a location or address. An iSCSI

initiator or target can move or have multiple addresses. A

      change of address does not imply a change of name.
  1. iSCSI names do not rely on a central name broker; the naming

authority is distributed.

  1. iSCSI names support integration with existing unique naming

schemes.

  1. iSCSI names rely on existing naming authorities. iSCSI does not

create any new naming authority.

 The encoding of an iSCSI name has the following properties:
  1. iSCSI names have the same encoding method, regardless of the

underlying protocols.

  1. iSCSI names are relatively simple to compare. The algorithm for

comparing two iSCSI names for equivalence does not rely on an

      external server.
  1. iSCSI names are composed only of printable ASCII and Unicode

characters. iSCSI names allow the use of international

      character sets, but uppercase characters are prohibited.  The
      iSCSI stringprep profile [RFC3722] maps uppercase characters to
      lowercase and SHOULD be used to prepare iSCSI names from input
      that may include uppercase characters.  No whitespace characters
      are used in iSCSI names; see [RFC3722] for details.
  1. iSCSI names may be transported using both binary and ASCII-based

protocols.

 An iSCSI name really names a logical software entity and is not tied
 to a port or other hardware that can be changed.  For instance, an
 Initiator Name should name the iSCSI initiator node, not a particular
 NIC or HBA.  When multiple NICs are used, they should generally all
 present the same iSCSI Initiator Name to the targets, because they
 are simply paths to the same SCSI layer.  In most operating systems,
 the named entity is the operating system image.
 Similarly, a target name should not be tied to hardware interfaces
 that can be changed.  A target name should identify the logical
 target and must be the same for the target, regardless of the
 physical portion being addressed.  This assists iSCSI initiators in
 determining that the two targets it has discovered are really two
 paths to the same target.

Chadalapaka, et al. Standards Track [Page 49] RFC 7143 iSCSI (Consolidated) April 2014

 The iSCSI name is designed to fulfill the functional requirements for
 Uniform Resource Names (URNs) [RFC1737].  For example, it is required
 that the name have a global scope, be independent of address or
 location, and be persistent and globally unique.  Names must be
 extensible and scalable with the use of naming authorities.  The name
 encoding should be both human and machine readable.  See [RFC1737]
 for further requirements.

4.2.7.2. iSCSI Name Encoding

 An iSCSI name MUST be a UTF-8 (see [RFC3629]) encoding of a string of
 Unicode characters with the following properties:
  1. It is in Normalization Form C (see "Unicode Normalization Forms"

[UNICODE]).

  1. It only contains characters allowed by the output of the iSCSI

stringprep template (described in [RFC3722]).

  1. The following characters are used for formatting iSCSI names:
         dash ('-'=U+002d)
         dot ('.'=U+002e)
         colon (':'=U+003a)
  1. The UTF-8 encoding of the name is not larger than 223 bytes.
 The stringprep process is described in [RFC3454]; iSCSI's use of the
 stringprep process is described in [RFC3722].  The stringprep process
 is a method designed by the Internationalized Domain Name (IDN)
 working group to translate human-typed strings into a format that can
 be compared as opaque strings.  iSCSI names are expected to be used
 by administrators for purposes such as system configuration; for this
 reason, characters that may lead to human confusion among different
 iSCSI names (e.g., punctuation, spacing, diacritical marks) should be
 avoided, even when such characters are allowed as stringprep
 processing output by [RFC3722].  The stringprep process also converts
 strings into equivalent strings of lowercase characters.
 The stringprep process does not need to be implemented if the names
 are generated using only characters allowed as output by the
 stringprep processing specified in [RFC3722].  Those allowed
 characters include all ASCII lowercase and numeric characters, as
 well as lowercase Unicode characters as specified in [RFC3722].  Once
 iSCSI names encoded in UTF-8 are "normalized" as described in this
 section, they may be safely compared byte for byte.

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4.2.7.3. iSCSI Name Structure

 An iSCSI name consists of two parts -- a type designator followed by
 a unique name string.
 iSCSI uses three existing naming authorities in constructing globally
 unique iSCSI names.  The type designator in an iSCSI name indicates
 the naming authority on which the name is based.  The three iSCSI
 name formats are the following:
    a) iSCSI-Qualified Name: based on domain names to identify a
       naming authority
    b) NAA format Name: based on a naming format defined by [FC-FS3]
       for constructing globally unique identifiers, referred to as
       the Network Address Authority (NAA)
    c) EUI format Name: based on EUI names, where the IEEE
       Registration Authority assists in the formation of worldwide
       unique names (EUI-64 format)
 The corresponding type designator strings currently defined are:
    a) iqn. - iSCSI Qualified name
    b) naa. - Remainder of the string is an INCITS T11-defined Network
       Address Authority identifier, in ASCII-encoded hexadecimal
    c) eui. - Remainder of the string is an IEEE EUI-64 identifier, in
       ASCII-encoded hexadecimal
 These three naming authority designators were considered sufficient
 at the time of writing this document.  The creation of additional
 naming type designators for iSCSI may be considered by the IETF and
 detailed in separate RFCs.

Chadalapaka, et al. Standards Track [Page 51] RFC 7143 iSCSI (Consolidated) April 2014

 The following table summarizes the current SCSI transport protocols
 and their naming formats.
      SCSI Transport Protocol       Naming Format
   +----------------------------+-------+-----+----+
   |                            | EUI-64| NAA |IQN |
   |----------------------------|-------|-----|----|
   | iSCSI (Internet SCSI)      |   X   |  X  | X  |
   |----------------------------|-------|-----|----|
   | FCP (Fibre Channel)        |       |  X  |    |
   |----------------------------|-------|-----|----|
   | SAS (Serial Attached SCSI) |       |  X  |    |
   +----------------------------+-------+-----+----+

4.2.7.4. Type "iqn." (iSCSI Qualified Name)

 This iSCSI name type can be used by any organization that owns a
 domain name.  This naming format is useful when an end user or
 service provider wishes to assign iSCSI names for targets and/or
 initiators.
 To generate names of this type, the person or organization generating
 the name must own a registered domain name.  This domain name does
 not have to resolve to an address; it just needs to be reserved to
 prevent others from generating iSCSI names using the same
 domain name.
 Since a domain name can expire, be acquired by another entity, or may
 be used to generate iSCSI names by both owners, the domain name must
 be additionally qualified by a date during which the naming authority
 owned the domain name.  A date code is provided as part of the "iqn."
 format for this reason.
 The iSCSI qualified name string consists of:
  1. The string "iqn.", used to distinguish these names from "eui."

formatted names.

  1. A date code, in yyyy-mm format. This date MUST be a date during

which the naming authority owned the domain name used in this

      format and SHOULD be the first month in which the domain name
      was owned by this naming authority at 00:01 GMT of the first day
      of the month.  This date code uses the Gregorian calendar.  All
      four digits in the year must be present.  Both digits of the
      month must be present, with January == "01" and December ==
      "12".  The dash must be included.
  1. A dot "."

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  1. The reverse domain name of the naming authority (person or

organization) creating this iSCSI name.

  1. An optional, colon (:)-prefixed string within the character set

and length boundaries that the owner of the domain name deems

      appropriate.  This may contain product types, serial numbers,
      host identifiers, or software keys (e.g., it may include colons
      to separate organization boundaries).  With the exception of the
      colon prefix, the owner of the domain name can assign everything
      after the reverse domain name as desired.  It is the
      responsibility of the entity that is the naming authority to
      ensure that the iSCSI names it assigns are worldwide unique.
      For example, "Example Storage Arrays, Inc." might own the domain
      name "example.com".
 The following are examples of iSCSI qualified names that might be
 generated by "EXAMPLE Storage Arrays, Inc."
                  Naming     String defined by
    Type  Date     Auth      "example.com" naming authority
    +--++-----+ +---------+ +--------------------------------+
    | ||      | |         | |                                |
    iqn.2001-04.com.example:storage:diskarrays-sn-a8675309
    iqn.2001-04.com.example
    iqn.2001-04.com.example:storage.tape1.sys1.xyz
    iqn.2001-04.com.example:storage.disk2.sys1.xyz

4.2.7.5. Type "eui." (IEEE EUI-64 Format)

 The IEEE Registration Authority provides a service for assigning
 globally unique identifiers [EUI].  The EUI-64 format is used to
 build a global identifier in other network protocols.  For example,
 Fibre Channel defines a method of encoding it into a WorldWideName.
 For more information on registering for EUI identifiers, see [OUI].
 The format is "eui." followed by an EUI-64 identifier (16 ASCII-
 encoded hexadecimal digits).
    Example iSCSI name:
       Type   EUI-64 identifier (ASCII-encoded hexadecimal)
       +--++--------------+
       |  ||              |
       eui.02004567A425678D

Chadalapaka, et al. Standards Track [Page 53] RFC 7143 iSCSI (Consolidated) April 2014

 The IEEE EUI-64 iSCSI name format might be used when a manufacturer
 is already registered with the IEEE Registration Authority and uses
 EUI-64 formatted worldwide unique names for its products.
 More examples of name construction are discussed in [RFC3721].

4.2.7.6. Type "naa." (Network Address Authority)

 The INCITS T11 Framing and Signaling Specification [FC-FS3] defines a
 format called the Network Address Authority (NAA) format for
 constructing worldwide unique identifiers that use various identifier
 registration authorities.  This identifier format is used by the
 Fibre Channel and SAS SCSI transport protocols.  As FC and SAS
 constitute a large fraction of networked SCSI ports, the NAA format
 is a widely used format for SCSI transports.  The objective behind
 iSCSI supporting a direct representation of an NAA format Name is to
 facilitate construction of a target device name that translates
 easily across multiple namespaces for a SCSI storage device
 containing ports served by different transports.  More specifically,
 this format allows implementations wherein one NAA identifier can be
 assigned as the basis for the SCSI device name for a SCSI target with
 both SAS ports and iSCSI ports.
 The iSCSI NAA naming format is "naa.", followed by an NAA identifier
 represented in ASCII-encoded hexadecimal digits.
 An example of an iSCSI name with a 64-bit NAA value follows:
    Type  NAA identifier (ASCII-encoded hexadecimal)
    +--++--------------+
    |  ||              |
    naa.52004567BA64678D
 An example of an iSCSI name with a 128-bit NAA value follows:
    Type  NAA identifier (ASCII-encoded hexadecimal)
    +--++------------------------------+
    |  ||                              |
    naa.62004567BA64678D0123456789ABCDEF
 The iSCSI NAA naming format might be used in an implementation when
 the infrastructure for generating NAA worldwide unique names is
 already in place because the device contains both SAS and iSCSI SCSI
 ports.

Chadalapaka, et al. Standards Track [Page 54] RFC 7143 iSCSI (Consolidated) April 2014

 The NAA identifier formatted in an ASCII-hexadecimal representation
 has a maximum size of 32 characters (128-bit NAA format).  As a
 result, there is no issue with this naming format exceeding the
 maximum size for iSCSI Node Names.

4.2.8. Persistent State

 iSCSI does not require any persistent state maintenance across
 sessions.  However, in some cases, SCSI requires persistent
 identification of the SCSI initiator port name (see Sections 4.4.2
 and 4.4.3.)
 iSCSI sessions do not persist through power cycles and boot
 operations.
 All iSCSI session and connection parameters are reinitialized on
 session and connection creation.
 Commands persist beyond connection termination if the session
 persists and command recovery within the session is supported.
 However, when a connection is dropped, command execution, as
 perceived by iSCSI (i.e., involving iSCSI protocol exchanges for the
 affected task), is suspended until a new allegiance is established by
 the "TASK REASSIGN" task management function.  See Section 11.5.

4.2.9. Message Synchronization and Steering

 iSCSI presents a mapping of the SCSI protocol onto TCP.  This
 encapsulation is accomplished by sending iSCSI PDUs of varying
 lengths.  Unfortunately, TCP does not have a built-in mechanism for
 signaling message boundaries at the TCP layer.  iSCSI overcomes this
 obstacle by placing the message length in the iSCSI message header.
 This serves to delineate the end of the current message as well as
 the beginning of the next message.
 In situations where IP packets are delivered in order from the
 network, iSCSI message framing is not an issue and messages are
 processed one after the other.  In the presence of IP packet
 reordering (i.e., frames being dropped), legacy TCP implementations
 store the "out of order" TCP segments in temporary buffers until the
 missing TCP segments arrive, at which time the data must be copied to
 the application buffers.  In iSCSI, it is desirable to steer the SCSI
 data within these out-of-order TCP segments into the preallocated
 SCSI buffers rather than store them in temporary buffers.  This
 decreases the need for dedicated reassembly buffers as well as the
 latency and bandwidth related to extra copies.

Chadalapaka, et al. Standards Track [Page 55] RFC 7143 iSCSI (Consolidated) April 2014

 Relying solely on the "message length" information from the iSCSI
 message header may make it impossible to find iSCSI message
 boundaries in subsequent TCP segments due to the loss of a TCP
 segment that contains the iSCSI message length.  The missing TCP
 segment(s) must be received before any of the following segments can
 be steered to the correct SCSI buffers (due to the inability to
 determine the iSCSI message boundaries).  Since these segments cannot
 be steered to the correct location, they must be saved in temporary
 buffers that must then be copied to the SCSI buffers.
 Different schemes can be used to recover synchronization.  The
 details of any such schemes are beyond this protocol specification,
 but it suffices to note that [RFC4297] provides an overview of the
 direct data placement problem on IP networks, and [RFC5046] specifies
 a protocol extension for iSCSI that facilitates this direct data
 placement objective.  The rest of this document refers to any such
 direct data placement protocol usage as an example of a "Sync and
 Steering layer".
 Under normal circumstances (no PDU loss or data reception out of
 order), iSCSI data steering can be accomplished by using the
 identifying tag and the data offset fields in the iSCSI header in
 addition to the TCP sequence number from the TCP header.  The
 identifying tag helps associate the PDU with a SCSI buffer address,
 while the data offset and TCP sequence number are used to determine
 the offset within the buffer.

4.2.9.1. Sync/Steering and iSCSI PDU Length

 When a large iSCSI message is sent, the TCP segment(s) that contains
 the iSCSI header may be lost.  The remaining TCP segment(s) up to the
 next iSCSI message must be buffered (in temporary buffers) because
 the iSCSI header that indicates to which SCSI buffers the data are to
 be steered was lost.  To minimize the amount of buffering, it is
 recommended that the iSCSI PDU length be restricted to a small value
 (perhaps a few TCP segments in length).  During login, each end of
 the iSCSI session specifies the maximum iSCSI PDU length it will
 accept.

4.3. iSCSI Session Types

 iSCSI defines two types of sessions:
    a) Normal operational session - an unrestricted session.

Chadalapaka, et al. Standards Track [Page 56] RFC 7143 iSCSI (Consolidated) April 2014

    b) Discovery session - a session only opened for target discovery.
       The target MUST ONLY accept Text Requests with the SendTargets
       key and a Logout Request with reason "close the session".  All
       other requests MUST be rejected.
 The session type is defined during login with the SessionType=value
 parameter in the login command.

4.4. SCSI-to-iSCSI Concepts Mapping Model

 The following diagram shows an example of how multiple iSCSI nodes
 (targets in this case) can coexist within the same Network Entity and
 can share Network Portals (IP addresses and TCP ports).  Other more
 complex configurations are also possible.  For detailed descriptions
 of the components of these diagrams, see Section 4.4.1.
               +-----------------------------------+
               | Network Entity (iSCSI Client)     |
               |                                   |
               |          +-------------+          |
               |          | iSCSI Node  |          |
               |          | (Initiator) |          |
               |          +-------------+          |
               |              |      |             |
               | +--------------+ +--------------+ |
               | |Network Portal| |Network Portal| |
               | |   192.0.2.4  | |   192.0.2.5  | |
               +-+--------------+-+--------------+-+
                        |                  |
                        |   IP Networks    |
                        |                  |
               +-+--------------+-+--------------+-+
               | |Network Portal| |Network Portal| |
               | |198.51.100.21 | |198.51.100.3  | |
               | | TCP Port 3260| | TCP Port 3260| |
               | +--------------+ +--------------+ |
               |        |                  |       |
               |         ------------------        |
               |            |          |           |
               | +-------------+ +--------------+  |
               | | iSCSI Node  | | iSCSI Node   |  |
               | | (Target)    | | (Target)     |  |
               | +-------------+ +--------------+  |
               |                                   |
               |   Network Entity (iSCSI Server)   |
               +-----------------------------------+

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4.4.1. iSCSI Architecture Model

 This section describes the part of the iSCSI Architecture Model that
 has the most bearing on the relationship between iSCSI and the SCSI
 Architecture Model.
  1. Network Entity - represents a device or gateway that is

accessible from the IP network. A Network Entity must have one

      or more Network Portals (see the "Network Portal" item below),
      each of which can be used by some iSCSI nodes (see the next
      item) contained in that Network Entity to gain access to the IP
      network.
  1. iSCSI Node - represents a single iSCSI initiator or iSCSI

target, or an instance of each. There are one or more iSCSI

      nodes within a Network Entity.  The iSCSI node is accessible via
      one or more Network Portals (see below).  An iSCSI node is
      identified by its iSCSI name (see Sections 4.2.7 and 13).  The
      separation of the iSCSI name from the addresses used by and for
      the iSCSI node allows multiple iSCSI nodes to use the same
      addresses and allows the same iSCSI node to use multiple
      addresses.
  1. An alias string may also be associated with an iSCSI node. The

alias allows an organization to associate a user-friendly string

      with the iSCSI name.  However, the alias string is not a
      substitute for the iSCSI name.
  1. Network Portal - a component of a Network Entity that has a

TCP/IP network address and that may be used by an iSCSI node

      within that Network Entity for the connection(s) within one of
      its iSCSI sessions.  In an initiator, it is identified by its IP
      address.  In a target, it is identified by its IP address and
      its listening TCP port.
  1. Portal Groups - iSCSI supports multiple connections within the

same session; some implementations will have the ability to

      combine connections in a session across multiple Network
      Portals.  A portal group defines a set of Network Portals within
      an iSCSI node that collectively supports the capability of
      coordinating a session with connections that span these portals.
      Not all Network Portals within a portal group need to
      participate in every session connected through that portal
      group.  One or more portal groups may provide access to an iSCSI
      node.  Each Network Portal, as utilized by a given iSCSI node,
      belongs to exactly one portal group within that node.  Portal
      groups are identified within an iSCSI node by a Portal Group
      Tag, a simple unsigned integer between 0 and 65535 (see

Chadalapaka, et al. Standards Track [Page 58] RFC 7143 iSCSI (Consolidated) April 2014

      Section 13.9).  All Network Portals with the same Portal Group
      Tag in the context of a given iSCSI node are in the same portal
      group.
      Both iSCSI initiators and iSCSI targets have portal groups,
      though only the iSCSI target portal groups are used directly in
      the iSCSI protocol (e.g., in SendTargets).  For references to
      the initiator portal Groups, see Section 10.1.2.
  1. Portals within a portal group should support similar session

parameters, because they may participate in a common session.

 The following diagram shows an example of one such configuration on a
 target and how a session that shares Network Portals within a portal
 group may be established.
  1. —————————IP Network———————

| | |

       +----|----------------|----+        +----|---------+
       | +---------+ +---------+  |        | +---------+  |
       | | Network | | Network |  |        | | Network |  |
       | | Portal  | | Portal  |  |        | | Portal  |  |
       | +---------+ +---------+  |        | +---------+  |
       |    |                |    |        |    |         |
       |    |    Portal      |    |        |    | Portal  |
       |    |    Group 1     |    |        |    | Group 2 |
       +--------------------------+        +--------------+
            |                |                  |
   +--------|----------------|------------------|------------------+
   |        |                |                  |                  |
   | +----------------------------+ +----------------------------+ |
   | | iSCSI Session (Target side)| | iSCSI Session (Target side)| |
   | |                            | |                            | |
   | |        (TSIH = 56)         | |        (TSIH = 48)         | |
   | +----------------------------+ +----------------------------+ |
   |                                                               |
   |                      iSCSI Target Node                        |
   |             (within Network Entity, not shown)                |
   +---------------------------------------------------------------+

4.4.2. SCSI Architecture Model

 This section describes the relationship between the SCSI Architecture
 Model [SAM2] and constructs of the SCSI device, SCSI port and I_T
 nexus, and the iSCSI constructs described in Section 4.4.1.
 This relationship implies implementation requirements in order to
 conform to the SAM-2 model and other SCSI operational functions.

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 These requirements are detailed in Section 4.4.3.
 The following list outlines mappings of SCSI architectural elements
 to iSCSI.
    a) SCSI Device - This is the SAM-2 term for an entity that
       contains one or more SCSI ports that are connected to a service
       delivery subsystem and supports a SCSI application protocol.
       For example, a SCSI initiator device contains one or more SCSI
       initiator ports and zero or more application clients.  A SCSI
       target device contains one or more SCSI target ports and one or
       more LUs.  For iSCSI, the SCSI device is the component within
       an iSCSI node that provides the SCSI functionality.  As such,
       there can be at most one SCSI device within an iSCSI node.
       Access to the SCSI device can only be achieved in an iSCSI
       Normal operational session (see Section 4.3).  The SCSI device
       name is defined to be the iSCSI name of the node and MUST be
       used in the iSCSI protocol.
    b) SCSI Port - This is the SAM-2 term for an entity in a SCSI
       device that provides the SCSI functionality to interface with a
       service delivery subsystem or transport.  For iSCSI, the
       definitions of the SCSI initiator port and the SCSI target port
       are different.
       SCSI initiator port: This maps to one endpoint of an iSCSI
       Normal operational session (see Section 4.3).  An iSCSI Normal
       operational session is negotiated through the login process
       between an iSCSI initiator node and an iSCSI target node.  At
       successful completion of this process, a SCSI initiator port is
       created within the SCSI initiator device.  The SCSI initiator
       port Name and SCSI initiator port Identifier are both defined
       to be the iSCSI Initiator Name together with (a) a label that
       identifies it as an initiator port name/identifier and (b) the
       ISID portion of the session identifier.
       SCSI target port: This maps to an iSCSI target portal group.
       The SCSI Target Port Name and the SCSI Target Port Identifier
       are both defined to be the iSCSI Target Name together with (a)
       a label that identifies it as a target port name/identifier and
       (b) the Target Portal Group Tag.
       The SCSI port name MUST be used in iSCSI.  When used in SCSI
       parameter data, the SCSI port name MUST be encoded as:
       1) the iSCSI name in UTF-8 format, followed by
       2) a comma separator (1 byte), followed by

Chadalapaka, et al. Standards Track [Page 60] RFC 7143 iSCSI (Consolidated) April 2014

       3) the ASCII character 'i' (for SCSI initiator port) or the
          ASCII character 't' (for SCSI target port) (1 byte),
          followed by
       4) a comma separator (1 byte), followed by
       5) a text encoding as a hex-constant (see Section 6.1) of the
          ISID (for SCSI initiator port) or the Target Portal Group
          Tag (for SCSI target port), including the initial 0X or 0x
          and the terminating null (15 bytes for iSCSI initiator port,
          7 bytes for iSCSI target port).
          The ASCII character 'i' or 't' is the label that identifies
          this port as either a SCSI initiator port or a SCSI target
          port.
    c) I_T nexus - This indicates a relationship between a SCSI
       initiator port and a SCSI target port, according to [SAM2].
       For iSCSI, this relationship is a session, defined as a
       relationship between an iSCSI initiator's end of the session
       (SCSI initiator port) and the iSCSI target's portal group.  The
       I_T nexus can be identified by the conjunction of the SCSI port
       names or by the iSCSI session identifier (SSID).  iSCSI defines
       the I_T nexus identifier to be the tuple (iSCSI Initiator Name
       + ",i,0x" + ISID in text format, iSCSI Target Name + ",t,0x" +
       Target Portal Group Tag in text format).  An uppercase hex
       prefix "0X" may alternatively be used in place of "0x".
       NOTE: The I_T nexus identifier is not equal to the SSID.

4.4.3. Consequences of the Model

 This section describes implementation and behavioral requirements
 that result from the mapping of SCSI constructs to the iSCSI
 constructs defined above.  Between a given SCSI initiator port and a
 given SCSI target port, only one I_T nexus (session) can exist.  No
 more than one nexus relationship (parallel nexus) is allowed by
 [SAM2].  Therefore, at any given time, only one session with the same
 SSID can exist between a given iSCSI initiator node and an iSCSI
 target node.
 These assumptions lead to the following conclusions and requirements:
 ISID RULE: Between a given iSCSI initiator and iSCSI target portal
 group (SCSI target port), there can only be one session with a given
 value for the ISID that identifies the SCSI initiator port.  See
 Section 11.12.5.

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 The structure of the ISID that contains a naming authority component
 (see Section 11.12.5 and [RFC3721]) provides a mechanism to
 facilitate compliance with the ISID RULE.  See Section 10.1.1.
 The iSCSI initiator node should manage the assignment of ISIDs prior
 to session initiation.  The "ISID RULE" does not preclude the use of
 the same ISID from the same iSCSI initiator with different target
 portal groups on the same iSCSI target or on other iSCSI targets (see
 Section 10.1.1).  Allowing this would be analogous to a single SCSI
 initiator port having relationships (nexus) with multiple SCSI target
 ports on the same SCSI target device or SCSI target ports on other
 SCSI target devices.  It is also possible to have multiple sessions
 with different ISIDs to the same target portal group.  Each such
 session would be considered to be with a different initiator even
 when the sessions originate from the same initiator device.  The same
 ISID may be used by a different iSCSI initiator because it is the
 iSCSI name together with the ISID that identifies the SCSI initiator
 port.
 NOTE: A consequence of the ISID RULE and the specification for the
 I_T nexus identifier is that two nexuses with the same identifier
 should never exist at the same time.
 TSIH RULE: The iSCSI target selects a non-zero value for the TSIH at
 session creation (when an initiator presents a 0 value at login).
 After being selected, the same TSIH value MUST be used whenever the
 initiator or target refers to the session and a TSIH is required.

4.4.3.1. I_T Nexus State

 Certain nexus relationships contain an explicit state (e.g.,
 initiator-specific mode pages) that may need to be preserved by the
 device server [SAM2] in a LU through changes or failures in the iSCSI
 layer (e.g., session failures).  In order for that state to be
 restored, the iSCSI initiator should reestablish its session
 (re-login) to the same target portal group using the previous ISID.
 That is, it should reinstate the session via iSCSI session
 reinstatement (Section 6.3.5) or continue via session continuation
 (Section 6.3.6).  This is because the SCSI initiator port identifier
 and the SCSI target port identifier (or relative target port) form
 the datum that the SCSI LU device server uses to identify the I_T
 nexus.

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4.4.3.2. Reservations

 There are two reservation management methods defined in the SCSI
 standards: reserve/release reservations, based on the RESERVE and
 RELEASE commands [SPC2]; and persistent reservations, based on the
 PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT commands [SPC3].
 Reserve/release reservations are obsolete [SPC3] and should not be
 used.  Persistent reservations are suggested as an alternative; see
 Annex B of [SPC4].
 State for persistent reservations is required to persist through
 changes and failures at the iSCSI layer that result in I_T nexus
 failures; see [SPC3] for details and specific requirements.
 In contrast, [SPC2] does not specify detailed persistence
 requirements for reserve/release reservation state after an I_T nexus
 failure.  Nonetheless, when reserve/release reservations are
 supported by an iSCSI target, the preferred implementation approach
 is to preserve reserve/release reservation state for iSCSI session
 reinstatement (see Section 6.3.5) or session continuation (see
 Section 6.3.6).
 Two additional caveats apply to reserve/release reservations:
  1. Retention of a failed session's reserve/release reservation

state by an iSCSI target, even after that failed iSCSI session

      is not reinstated or continued, may require an initiator to
      issue a reset (e.g., LOGICAL UNIT RESET; see Section 11.5) in
      order to remove that reservation state.
  1. Reserve/release reservations may not behave as expected when

persistent reservations are also used on the same LU; see the

      discussion of "Exceptions to SPC-2 RESERVE and RELEASE behavior"
      in [SPC4].

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4.5. iSCSI UML Model

 This section presents the application of the UML modeling concepts
 discussed in Section 3 to the iSCSI and SCSI Architecture Model
 discussed in Section 4.4.
                     +----------------+
                     | Network Entity |
                     +----------------+
                          @ 1     @ 1
                          |       |
   +----------------------+       |
   |                              |
   |                              | 0..*
   |                   +------------------+
   |                   | iSCSI Node       |
   |                   +------------------+
   |                       @       @
   |                       |       |
   |           +-----------+ =(a)= +-----------+
   |           |                               |
   |           | 0..1                          | 0..1
   | +------------------------+       +----------------------+
   | |    iSCSI Target Node   |       | iSCSI Initiator Node |
   | +------------------------+       +----------------------+
   |             @ 1                            @ 1
   |             +---------------+              |
   |                        1..* |              | 1..*
   |                    +-----------------------------+
   |                    |         Portal Group        |
   |                    +-----------------------------+
   |                                     O 1
   |                                     |
   |                                     | 1..*
   |               1..* +------------------------+
   +--------------------|        Network Portal  |
                        +------------------------+
 (a) Each instance of an iSCSI node class MUST contain one iSCSI
     target node instance, one iSCSI initiator node instance, or both.

Chadalapaka, et al. Standards Track [Page 64] RFC 7143 iSCSI (Consolidated) April 2014

                  +----------------+
                  | Network Entity |
                  +----------------+
                       @ 1         @ 1
                       |           |              +------------------+
 +---------------------+           |              |   iSCSI Session  |
 |                                 |              +------------------+
 |                                 | 0..*         |     SSID[1]      |
 |                  +--------------------+        |     ISID[1]      |
 |                  |      iSCSI Node    |        +------------------+
 |                  +--------------------+                   @ 1
 |                  | iSCSI Node Name[1] |                   |
 |                  |    Alias [0..1]    |                   | 0..*
 |                  +--------------------+        +------------------+
 |                  |                    |        | iSCSI Connection |
 |                  +--------------------+        +------------------+
 |                         @ 1         @ 1        |      CID[1]      |
 |                         |           |          +------------------+
 |           +-------------+ ==(b)==   +---------+              0..* |
 |           | 1                                 | 1                 |
 | +------------------------+             +------------------------+ |
 | |   iSCSI Target Node    |             | iSCSI Initiator Node   | |
 | +------------------------+             +------------------------+ |
 | | iSCSI Target Name [1]  |             |iSCSI Initiator Name [1]| |
 | +------------------------+             +------------------------+ |
 |            @ 1                                    @ 1             |
 |            | 1..*                                 | 1..*          |
 | +--------------------------+           +------------------------+ |
 | |   Target Portal Group    |           | Initiator Portal Group | |
 | +--------------------------+           +------------------------+ |
 | |Target Portal Group Tag[1]|           | Portal Group Tag[1]    | |
 | +--------------------------+           +------------------------+ |
 |            o 1                                    o 1             |
 |            +------------+              +----------+               |
 |                    1..* |              | 1..*                     |
 |                +-------------------------+                        |
 |                |          Network Portal |                        |
 |                +-------------------------+                        |
 |          1..*  |         IP Address [1]  | 1                      |
 +----------------|         TCP Port [0..1] |<-----------------------+
                  +-------------------------+
 (b) Each instance of an iSCSI node class MUST contain one iSCSI
     target node instance, one iSCSI initiator node instance, or both.
     However, in all scenarios, note that an iSCSI node MUST only have
     a single iSCSI name.  Note the related requirement in
     Section 4.2.7.1.

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4.6. Request/Response Summary

 This section lists and briefly describes all the iSCSI PDU types
 (requests and responses).
 All iSCSI PDUs are built as a set of one or more header segments
 (basic and auxiliary) and zero or one data segments.  The header
 group and the data segment may each be followed by a CRC (digest).
 The basic header segment has a fixed length of 48 bytes.

4.6.1. Request/Response Types Carrying SCSI Payload

4.6.1.1. SCSI Command

 This request carries the SCSI CDB and all the other SCSI Execute
 Command [SAM2] procedure call IN arguments, such as task attributes,
 Expected Data Transfer Length for one or both transfer directions
 (the latter for bidirectional commands), and a task tag (as part of
 the I_T_L_x nexus).  The I_T_L nexus is derived by the initiator and
 target from the LUN field in the request, and the I_T nexus is
 implicit in the session identification.
 In addition, the SCSI Command PDU carries information required for
 the proper operation of the iSCSI protocol -- the command sequence
 number (CmdSN) and the expected status sequence number (ExpStatSN) on
 the connection it is issued.
 All or part of the SCSI output (write) data associated with the SCSI
 command may be sent as part of the SCSI Command PDU as a data
 segment.

4.6.1.2. SCSI Response

 The SCSI Response carries all the SCSI Execute Command procedure call
 (see [SAM2]) OUT arguments and the SCSI Execute Command procedure
 call return value.
 The SCSI Response contains the residual counts from the operation, if
 any; an indication of whether the counts represent an overflow or an
 underflow; and the SCSI status if the status is valid or a response
 code (a non-zero return value for the Execute Command procedure call)
 if the status is not valid.
 For a valid status that indicates that the command has been processed
 but resulted in an exception (e.g., a SCSI CHECK CONDITION), the PDU
 data segment contains the associated sense data.  The use of
 Autosense ([SAM2]) is REQUIRED by iSCSI.

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 Some data segment content may also be associated (in the data
 segment) with a non-zero response code.
 In addition, the SCSI Response PDU carries information required for
 the proper operation of the iSCSI protocol:
  1. ExpDataSN - the number of Data-In PDUs that a target has sent

(to enable the initiator to check that all have arrived)

  1. StatSN - the status sequence number on this connection
  1. ExpCmdSN - the next expected command sequence number at the

target

  1. MaxCmdSN - the maximum CmdSN acceptable at the target from this

initiator

4.6.1.3. Task Management Function Request

 The Task Management Function Request provides an initiator with a way
 to explicitly control the execution of one or more SCSI tasks or
 iSCSI functions.  The PDU carries a function identifier (i.e., which
 task management function to perform) and enough information to
 unequivocally identify the task or task set on which to perform the
 action, even if the task(s) to act upon has not yet arrived or has
 been discarded due to an error.
 The referenced tag identifies an individual task if the function
 refers to an individual task.
 The I_T_L nexus identifies task sets.  In iSCSI, the I_T_L nexus is
 identified by the LUN and the session identification (the session
 identifies an I_T nexus).
 For task sets, the CmdSN of the Task Management Function Request
 helps identify the tasks upon which to act, namely all tasks
 associated with a LUN and having a CmdSN preceding the Task
 Management Function Request CmdSN.
 For a task management function, the coordination between responses to
 the tasks affected and the Task Management Function Response is done
 by the target.

Chadalapaka, et al. Standards Track [Page 67] RFC 7143 iSCSI (Consolidated) April 2014

4.6.1.4. Task Management Function Response

 The Task Management Function Response carries an indication of
 function completion for a Task Management Function Request, including
 how it completed (response and qualifier) and additional information
 for failure responses.
 After the Task Management Function Response indicates task management
 function completion, the initiator will not receive any additional
 responses from the affected tasks.

4.6.1.5. SCSI Data-Out and SCSI Data-In

 SCSI Data-Out and SCSI Data-In are the main vehicles by which SCSI
 data payload is carried between the initiator and target.  Data
 payload is associated with a specific SCSI command through the
 Initiator Task Tag.  For target convenience, outgoing solicited data
 also carries a Target Transfer Tag (copied from R2T) and the LUN.
 Each PDU contains the payload length and the data offset relative to
 the buffer address contained in the SCSI Execute Command procedure
 call.
 In each direction, the data transfer is split into "sequences".  An
 end-of-sequence is indicated by the F bit.
 An outgoing sequence is either unsolicited (only the first sequence
 can be unsolicited) or consists of all the Data-Out PDUs sent in
 response to an R2T.
 Input sequences enable the switching of direction for bidirectional
 commands as required.
 For input, the target may request positive acknowledgment of input
 data.  This is limited to sessions that support error recovery and is
 implemented through the A bit in the SCSI Data-In PDU header.
 Data-In and Data-Out PDUs also carry the DataSN to enable the
 initiator and target to detect missing PDUs (discarded due to an
 error).
 In addition, the StatSN is carried by the Data-In PDUs.
 To enable a SCSI command to be processed while involving a minimum
 number of messages, the last SCSI Data-In PDU passed for a command
 may also contain the status if the status indicates termination with
 no exceptions (no sense or response involved).

Chadalapaka, et al. Standards Track [Page 68] RFC 7143 iSCSI (Consolidated) April 2014

4.6.1.6. Ready To Transfer (R2T)

 R2T is the mechanism by which the SCSI target "requests" the
 initiator for output data.  R2T specifies to the initiator the offset
 of the requested data relative to the buffer address from the Execute
 Command procedure call and the length of the solicited data.
 To help the SCSI target associate the resulting Data-Out with an R2T,
 the R2T carries a Target Transfer Tag that will be copied by the
 initiator in the solicited SCSI Data-Out PDUs.  There are no
 protocol-specific requirements with regard to the value of these
 tags, but it is assumed that together with the LUN, they will enable
 the target to associate data with an R2T.
 R2T also carries information required for proper operation of the
 iSCSI protocol, such as:
  1. R2TSN (to enable an initiator to detect a missing R2T)
  1. StatSN
  1. ExpCmdSN
  1. MaxCmdSN

4.6.2. Requests/Responses Carrying SCSI and iSCSI Payload

4.6.2.1. Asynchronous Message

 Asynchronous Message PDUs are used to carry SCSI asynchronous event
 notifications (AENs) and iSCSI asynchronous messages.
 When carrying an AEN, the event details are reported as sense data in
 the data segment.

4.6.3. Requests/Responses Carrying iSCSI-Only Payload

4.6.3.1. Text Requests and Text Responses

 Text Requests and Responses are designed as a parameter negotiation
 vehicle and as a vehicle for future extension.
 In the data segment, Text Requests/Responses carry text information
 using a simple "key=value" syntax.

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 Text Requests/Responses may form extended sequences using the same
 Initiator Task Tag.  The initiator uses the F (Final) flag bit in the
 Text Request header to indicate its readiness to terminate a
 sequence.  The target uses the F bit in the Text Response header to
 indicate its consent to sequence termination.
 Text Requests and Responses also use the Target Transfer Tag to
 indicate continuation of an operation or a new beginning.  A target
 that wishes to continue an operation will set the Target Transfer Tag
 in a Text Response to a value different from the default 0xffffffff.
 An initiator willing to continue will copy this value into the Target
 Transfer Tag of the next Text Request.  If the initiator wants to
 restart the current target negotiation (start fresh), it will set the
 Target Transfer Tag to 0xffffffff.
 Although a complete exchange is always started by the initiator,
 specific parameter negotiations may be initiated by the initiator or
 target.

4.6.3.2. Login Requests and Login Responses

 Login Requests and Responses are used exclusively during the Login
 Phase of each connection to set up the session and connection
 parameters.  (The Login Phase consists of a sequence of Login
 Requests and Responses carrying the same Initiator Task Tag.)
 A connection is identified by an arbitrarily selected connection ID
 (CID) that is unique within a session.
 Similar to the Text Requests and Responses, Login Requests/Responses
 carry key=value text information with a simple syntax in the data
 segment.
 The Login Phase proceeds through several stages (security
 negotiation, operational parameter negotiation) that are selected
 with two binary coded fields in the header -- the Current Stage (CSG)
 and the Next Stage (NSG) -- with the appearance of the latter being
 signaled by the "Transit" flag (T).
 The first Login Phase of a session plays a special role, called the
 leading login, which determines some header fields (e.g., the version
 number, the maximum number of connections, and the session
 identification).
 The CmdSN initial value is also set by the leading login.
 The StatSN for each connection is initiated by the connection login.

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 A Login Request may indicate an implied logout (cleanup) of the
 connection to be logged in (a connection restart) by using the same
 connection ID (CID) as an existing connection as well as the same
 session-identifying elements of the session to which the old
 connection was associated.

4.6.3.3. Logout Requests and Logout Responses

 Logout Requests and Responses are used for the orderly closing of
 connections for recovery or maintenance.  The Logout Request may be
 issued following a target prompt (through an Asynchronous Message) or
 at an initiator's initiative.  When issued on the connection to be
 logged out, no other request may follow it.
 The Logout Response indicates that the connection or session cleanup
 is completed and no other responses will arrive on the connection (if
 received on the logging-out connection).  In addition, the Logout
 Response indicates how long the target will continue to hold
 resources for recovery (e.g., command execution that continues on a
 new connection) in the Time2Retain field and how long the initiator
 must wait before proceeding with recovery in the Time2Wait field.

4.6.3.4. SNACK Request

 With the SNACK Request, the initiator requests retransmission of
 numbered responses or data from the target.  A single SNACK Request
 covers a contiguous set of missing items, called a run, of a given
 type of items.  The type is indicated in a type field in the PDU
 header.  The run is composed of an initial item (StatSN, DataSN,
 R2TSN) and the number of missed Status, Data, or R2T PDUs.  For long
 Data-In sequences, the target may request (at predefined minimum
 intervals) a positive acknowledgment for the data sent.  A SNACK
 Request with a type field that indicates ACK and the number of
 Data-In PDUs acknowledged conveys this positive acknowledgment.

4.6.3.5. Reject

 Reject enables the target to report an iSCSI error condition (e.g.,
 protocol, unsupported option) that uses a Reason field in the PDU
 header and includes the complete header of the bad PDU in the Reject
 PDU data segment.

4.6.3.6. NOP-Out Request and NOP-In Response

 This request/response pair may be used by an initiator and target as
 a "ping" mechanism to verify that a connection/session is still
 active and all of its components are operational.  Such a ping may be

Chadalapaka, et al. Standards Track [Page 71] RFC 7143 iSCSI (Consolidated) April 2014

 triggered by the initiator or target.  The triggering party indicates
 that it wants a reply by setting a value different from the default
 0xffffffff in the corresponding Initiator/Target Transfer Tag.
 NOP-In/NOP-Out may also be used in "unidirectional" fashion to convey
 to the initiator/target command, status, or data counter values when
 there is no other "carrier" and there is a need to update the
 initiator/target.

5. SCSI Mode Parameters for iSCSI

 There are no iSCSI-specific mode pages.

6. Login and Full Feature Phase Negotiation

 iSCSI parameters are negotiated at session or connection
 establishment by using Login Requests and Responses (see
 Section 4.2.4) and during the Full Feature Phase (Section 4.2.5) by
 using Text Requests and Responses.  In both cases, the mechanism used
 is an exchange of iSCSI-text-key=value pairs.  For brevity,
 iSCSI-text-keys are called just "keys" in the rest of this document.
 Keys are either declarative or require negotiation, and the key
 description indicates whether the key is declarative or requires
 negotiation.
 For the declarative keys, the declaring party sets a value for the
 key.  The key specification indicates whether the key can be declared
 by the initiator, the target, or both.
 For the keys that require negotiation, one of the parties (the
 proposing party) proposes a value or set of values by including the
 key=value in the data part of a Login or Text Request or Response.
 The other party (the accepting party) makes a selection based on the
 value or list of values proposed and includes the selected value in a
 key=value in the data part of the following Login or Text Response or
 Request.  For most of the keys, both the initiator and target can be
 proposing parties.
 The login process proceeds in two stages -- the security negotiation
 stage and the operational parameter negotiation stage.  Both stages
 are optional, but at least one of them has to be present to enable
 setting some mandatory parameters.
 If present, the security negotiation stage precedes the operational
 parameter negotiation stage.

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 Progression from stage to stage is controlled by the T (Transit) bit
 in the Login Request/Response PDU header.  Through the T bit set
 to 1, the initiator indicates that it would like to transition.  The
 target agrees to the transition (and selects the next stage) when
 ready.  A field in the Login PDU header indicates the current stage
 (CSG), and during transition, another field indicates the next stage
 (NSG) proposed (initiator) and selected (target).
 The text negotiation process is used to negotiate or declare
 operational parameters.  The negotiation process is controlled by the
 F (Final) bit in the PDU header.  During text negotiations, the F bit
 is used by the initiator to indicate that it is ready to finish the
 negotiation and by the target to acquiesce the end of negotiation.
 Since some key=value pairs may not fit entirely in a single PDU, the
 C (Continue) bit is used (both in Login and Text) to indicate that
 "more follows".
 The text negotiation uses an additional mechanism by which a target
 may deliver larger amounts of data to an inquiring initiator.  The
 target sets a Target Task Tag to be used as a bookmark that, when
 returned by the initiator, means "go on".  If reset to a "neutral
 value", it means "forget about the rest".
 This section details the types of keys and values used, the syntax
 rules for parameter formation, and the negotiation schemes to be used
 with different types of parameters.

6.1. Text Format

 The initiator and target send a set of key=value pairs encoded in
 UTF-8 Unicode.  All the text keys and text values specified in this
 document are case sensitive; they are to be presented and interpreted
 as they appear in this document without change of case.
 The following character symbols are used in this document for text
 items (the hexadecimal values represent Unicode code points):
 (a-z, A-Z) (0x61-0x7a, 0x41-0x5a) - letters
                 (0-9) (0x30-0x39) - digits
                        " " (0x20) - space
                        "." (0x2e) - dot
                        "-" (0x2d) - minus
                        "+" (0x2b) - plus
                        "@" (0x40) - commercial at
                        "_" (0x5f) - underscore
                        "=" (0x3d) - equal
                        ":" (0x3a) - colon

Chadalapaka, et al. Standards Track [Page 73] RFC 7143 iSCSI (Consolidated) April 2014

                        "/" (0x2f) - solidus or slash
                        "[" (0x5b) - left bracket
                        "]" (0x5d) - right bracket
                       null (0x00) - null separator
                        "," (0x2c) - comma
                        "~" (0x7e) - tilde
 Key=value pairs may span PDU boundaries.  An initiator or target that
 sends partial key=value text within a PDU indicates that more text
 follows by setting the C bit in the Text or Login Request or the Text
 or Login Response to 1.  Data segments in a series of PDUs that have
 the C bit set to 1 and end with a PDU that has the C bit set to 0, or
 that include a single PDU that has the C bit set to 0, have to be
 considered as forming a single logical-text-data-segment (LTDS).
 Every key=value pair, including the last or only pair in a LTDS, MUST
 be followed by one null (0x00) delimiter.
 A key-name is whatever precedes the first "=" in the key=value pair.
 The term "key" is used frequently in this document in place of
 "key-name".
 A value is whatever follows the first "=" in the key=value pair up to
 the end of the key=value pair, but not including the null delimiter.
 The following definitions will be used in the rest of this document:
  1. standard-label: A string of one or more characters that consists

of letters, digits, dot, minus, plus, commercial at, or

      underscore.  A standard-label MUST begin with a capital letter
      and must not exceed 63 characters.
  1. key-name: A standard-label.
  1. text-value: A string of zero or more characters that consists of

letters, digits, dot, minus, plus, commercial at, underscore,

      slash, left bracket, right bracket, or colon.
  1. iSCSI-name-value: A string of one or more characters that

consists of minus, dot, colon, or any character allowed by the

      output of the iSCSI stringprep template as specified in
      [RFC3722] (see also Section 4.2.7.2).
  1. iSCSI-local-name-value: A UTF-8 string; no null characters are

allowed in the string. This encoding is to be used for

      localized (internationalized) aliases.
  1. boolean-value: The string "Yes" or "No".

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  1. hex-constant: A hexadecimal constant encoded as a string that

starts with "0x" or "0X" followed by one or more digits or the

      letters a, b, c, d, e, f, A, B, C, D, E, or F.  Hex-constants
      are used to encode numerical values or binary strings.  When
      used to encode numerical values, the excessive use of leading 0
      digits is discouraged.  The string following 0X (or 0x)
      represents a base16 number that starts with the most significant
      base16 digit, followed by all other digits in decreasing order
      of significance and ending with the least significant base16
      digit.  When used to encode binary strings, hexadecimal
      constants have an implicit byte-length that includes four bits
      for every hexadecimal digit of the constant, including leading
      zeroes.  For example, a hex-constant of n hexadecimal digits has
      a byte-length of (the integer part of) (n + 1)/2.
  1. decimal-constant: An unsigned decimal number with the digit 0 or

a string of one or more digits that starts with a non-zero

      digit.  Decimal-constants are used to encode numerical values or
      binary strings.  Decimal-constants can only be used to encode
      binary strings if the string length is explicitly specified.
      There is no implicit length for decimal strings.
      Decimal-constants MUST NOT be used for parameter values if the
      values can be equal to or greater than 2**64 (numerical) or for
      binary strings that can be longer than 64 bits.
  1. base64-constant: Base64 constant encoded as a string that starts

with "0b" or "0B" followed by 1 or more digits, letters, plus

      sign, slash, or equals sign.  The encoding is done according to
      [RFC4648].
  1. numerical-value: An unsigned integer always less than 264 encoded as a decimal-constant or a hex-constant. Unsigned integer arithmetic applies to numerical-values. - large-numerical-value: An unsigned integer that can be larger than or equal to 264 encoded as a hex-constant or

base64-constant. Unsigned integer arithmetic applies to large-

      numerical-values.
  1. numerical-range: Two numerical-values separated by a tilde,

where the value to the right of the tilde must not be lower than

      the value to the left.
  1. regular-binary-value: A binary string not longer than 64 bits

encoded as a decimal-constant, hex-constant, or base64-constant.

      The length of the string is either specified by the key
      definition or is the implicit byte-length of the encoded string.

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  1. large-binary-value: A binary string longer than 64 bits encoded

as a hex-constant or base64-constant. The length of the string

      is either specified by the key definition or is the implicit
      byte-length of the encoded string.
  1. binary-value: A regular-binary-value or a large-binary-value.

Operations on binary values are key-specific.

  1. simple-value: Text-value, iSCSI-name-value, boolean-value,

numerical-value, a numerical-range, or a binary-value.

  1. list-of-values: A sequence of text-values separated by a comma.
 If not otherwise specified, the maximum length of a simple-value (not
 its encoded representation) is 255 bytes, not including the delimiter
 (comma or zero byte).
 Any iSCSI target or initiator MUST support receiving at least
 8192 bytes of key=value data in a negotiation sequence.  When
 proposing or accepting authentication methods that explicitly require
 support for very long authentication items, the initiator and target
 MUST support receiving at least 64 kilobytes of key=value data.

6.2. Text Mode Negotiation

 During login, and thereafter, some session or connection parameters
 are either declared or negotiated through an exchange of textual
 information.
 The initiator starts the negotiation and/or declaration through a
 Text or Login Request and indicates when it is ready for completion
 (by setting the F bit to 1 and keeping it at 1 in a Text Request, or
 the T bit in the Login Request).  As negotiation text may span PDU
 boundaries, a Text or Login Request or a Text or Login Response PDU
 that has the C bit set to 1 MUST NOT have the F bit or T bit set
 to 1.
 A target receiving a Text or Login Request with the C bit set to 1
 MUST answer with a Text or Login Response with no data segment
 (DataSegmentLength 0).  An initiator receiving a Text or Login
 Response with the C bit set to 1 MUST answer with a Text or Login
 Request with no data segment (DataSegmentLength 0).
 A target or initiator SHOULD NOT use a Text or Login Response or a
 Text or Login Request with no data segment (DataSegmentLength 0)
 unless explicitly required by a general or a key-specific negotiation
 rule.

Chadalapaka, et al. Standards Track [Page 76] RFC 7143 iSCSI (Consolidated) April 2014

 There MUST NOT be more than one outstanding Text Request, or Text
 Response PDU on an iSCSI connection.  An outstanding PDU in this
 context is one that has not been acknowledged by the remote iSCSI
 side.
 The format of a declaration is:
    Declarer-> <key>=<valuex>
 The general format of text negotiation is:
    Proposer-> <key>=<valuex>
    Acceptor-> <key>={<valuey>|NotUnderstood|Irrelevant|Reject}
 Thus, a declaration is a one-way textual exchange (unless the key is
 not understood by the receiver), while a negotiation is a two-way
 exchange.
 The proposer or declarer can be either the initiator or the target,
 and the acceptor can be either the target or initiator, respectively.
 Targets are not limited to respond to key=value pairs as proposed by
 the initiator.  The target may propose key=value pairs of its own.
 All negotiations are explicit (i.e., the result MUST only be based on
 newly exchanged or declared values).  There are no implicit
 proposals.  If a proposal is not made, then a reply cannot be
 expected.  Conservative design also requires that default values
 should not be relied upon when the use of some other value has
 serious consequences.
 The value proposed or declared can be a numerical-value, a numerical-
 range defined by the lower and upper value with both integers
 separated by a tilde, a binary value, a text-value, an iSCSI-name-
 value, an iSCSI-local-name-value, a boolean-value (Yes or No), or a
 list of comma-separated text-values.  A range, a large-numerical-
 value, an iSCSI-name-value, and an iSCSI-local-name-value MAY ONLY be
 used if explicitly allowed.  An accepted value can be a numerical-
 value, a large-numerical-value, a text-value, or a boolean-value.
 If a specific key is not relevant for the current negotiation, the
 acceptor may answer with the constant "Irrelevant" for all types of
 negotiations.  However, the negotiation is not considered to have
 failed if the answer is "Irrelevant".  The "Irrelevant" answer is
 meant for those cases in which several keys are presented by a
 proposing party but the selection made by the acceptor for one of the

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 keys makes other keys irrelevant.  The following example illustrates
 the use of "Irrelevant":
    I->T InitialR2T=No,ImmediateData=Yes,FirstBurstLength=4192
    T->I InitialR2T=Yes,ImmediateData=No,FirstBurstLength=Irrelevant
    I->T X-rdname-vkey1=(bla,alb,None), X-rdname-vkey2=(bla,alb)
    T->I X-rdname-vkey1=None, X-rdname-vkey2=Irrelevant
 Any key not understood by the acceptor may be ignored by the acceptor
 without affecting the basic function.  However, the answer for a key
 that is not understood MUST be key=NotUnderstood.  Note that
 NotUnderstood is a valid answer for both declarative and negotiated
 keys.  The general iSCSI philosophy is that comprehension precedes
 processing for any iSCSI key.  A proposer of an iSCSI key, negotiated
 or declarative, in a text key exchange MUST thus be able to properly
 handle a NotUnderstood response.
 The proper way to handle a NotUnderstood response depends on where
 the key is specified and whether the key is declarative or
 negotiated.  An iSCSI implementation MUST comprehend all text keys
 defined in this document.  Returning a NotUnderstood response on any
 of these text keys therefore MUST be considered a protocol error and
 handled accordingly.  For all other "later" keys, i.e., text keys
 defined in later specifications, a NotUnderstood answer concludes the
 negotiation for a negotiated key, whereas for a declarative key a
 NotUnderstood answer simply informs the declarer of a lack of
 comprehension by the receiver.
 In either case, a NotUnderstood answer always requires that the
 protocol behavior associated with that key not be used within the
 scope of the key (connection/session) by either side.
 The constants "None", "Reject", "Irrelevant", and "NotUnderstood" are
 reserved and MUST ONLY be used as described here.  Violation of this
 rule is a protocol error (in particular, the use of "Reject",
 "Irrelevant", and "NotUnderstood" as proposed values).
 "Reject" or "Irrelevant" are legitimate negotiation options where
 allowed, but their excessive use is discouraged.  A negotiation is
 considered complete when the acceptor has sent the key value pair
 even if the value is "Reject", "Irrelevant", or "NotUnderstood".
 Sending the key again would be a renegotiation and is forbidden for
 many keys.

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 If the acceptor sends "Reject" as an answer, the negotiated key is
 left at its current value (or default if no value was set).  If the
 current value is not acceptable to the proposer on the connection or
 to the session in which it is sent, the proposer MAY choose to
 terminate the connection or session.
 All keys in this document MUST be supported by iSCSI initiators and
 targets when used as specified here.  If used as specified, these
 keys MUST NOT be answered with NotUnderstood.
 Implementers may introduce new private keys by prefixing them with X-
 followed by their (reverse) domain name, or with new public keys
 registered with IANA.  For example, the entity owning the domain
 example.com can issue:
    X-com.example.bar.foo.do_something=3
 Each new public key in the course of standardization MUST define the
 acceptable responses to the key, including NotUnderstood as
 appropriate.  Unlike [RFC3720], note that this document prohibits the
 X# prefix for new public keys.  Based on iSCSI implementation
 experience, we know that there is no longer a need for a standard
 name prefix for keys that allow a NotUnderstood response.  Note that
 NotUnderstood will generally have to be allowed for new public keys
 for backwards compatibility, as well as for private X- keys.  Thus,
 the name prefix "X#" in new public key-names does not carry any
 significance.  To avoid confusion, new public key-names MUST NOT
 begin with an "X#" prefix.
 Implementers MAY also introduce new values, but ONLY for new keys or
 authentication methods (see Section 12) or digests (see
 Section 13.1).
 Whenever parameter actions or acceptance are dependent on other
 parameters, the dependency rules and parameter sequence must be
 specified with the parameters.
 In the Login Phase (see Section 6.3), every stage is a separate
 negotiation.  In the Full Feature Phase, a Text Request/Response
 sequence is a negotiation.  Negotiations MUST be handled as atomic
 operations.  For example, all negotiated values go into effect after
 the negotiation concludes in agreement or are ignored if the
 negotiation fails.
 Some parameters may be subject to integrity rules (e.g., parameter-x
 must not exceed parameter-y, or parameter-u not 1 implies that
 parameter-v be Yes).  Whenever required, integrity rules are
 specified with the keys.  Checking for compliance with the integrity

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 rule must only be performed after all the parameters are available
 (the existent and the newly negotiated).  An iSCSI target MUST
 perform integrity checking before the new parameters take effect.  An
 initiator MAY perform integrity checking.
 An iSCSI initiator or target MAY terminate a negotiation that does
 not terminate within an implementation-specific reasonable time or
 number of exchanges but SHOULD allow at least six (6) exchanges.

6.2.1. List Negotiations

 In list negotiation, the originator sends a list of values (which may
 include "None"), in order of preference.
 The responding party MUST respond with the same key and the first
 value that it supports (and is allowed to use for the specific
 originator) selected from the originator list.
 The constant "None" MUST always be used to indicate a missing
 function.  However, "None" is only a valid selection if it is
 explicitly proposed.  When "None" is proposed as a selection item in
 a negotiation for a key, it indicates to the responder that not
 supporting any functionality related to that key is legal, and if
 "None" is the negotiation result for such a key, it means that key-
 specific semantics are not operational for the negotiation scope
 (connection or session) of that key.
 If an acceptor does not understand any particular value in a list, it
 MUST ignore it.  If an acceptor does not support, does not
 understand, or is not allowed to use any of the proposed options with
 a specific originator, it may use the constant "Reject" or terminate
 the negotiation.  The selection of a value not proposed MUST be
 handled by the originator as a protocol error.

6.2.2. Simple-Value Negotiations

 For simple-value negotiations, the accepting party MUST answer with
 the same key.  The value it selects becomes the negotiation result.
 Proposing a value not admissible (e.g., not within the specified
 bounds) MAY be answered with the constant "Reject"; otherwise, the
 acceptor MUST select an admissible value.
 The selection, by the acceptor, of a value not admissible under the
 selection rules is considered a protocol error.  The selection rules
 are key-specific.

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 For a numerical range, the value selected MUST be an integer within
 the proposed range or "Reject" (if the range is unacceptable).
 For Boolean negotiations (i.e., keys taking the values "Yes" or
 "No"), the accepting party MUST answer with the same key and the
 result of the negotiation when the received value does not determine
 that result by itself.  The last value transmitted becomes the
 negotiation result.  The rules for selecting the value with which to
 answer are expressed as Boolean functions of the value received, and
 the value that the accepting party would have selected if given a
 choice.
 Specifically, the two cases in which answers are OPTIONAL are:
  1. The Boolean function is "AND" and the value "No" is received.

The outcome of the negotiation is "No".

  1. The Boolean function is "OR" and the value "Yes" is received.

The outcome of the negotiation is "Yes".

 Responses are REQUIRED in all other cases, and the value chosen and
 sent by the acceptor becomes the outcome of the negotiation.

6.3. Login Phase

 The Login Phase establishes an iSCSI connection between an initiator
 and a target; it also creates a new session or associates the
 connection to an existing session.  The Login Phase sets the iSCSI
 protocol parameters and security parameters, and authenticates the
 initiator and target to each other.
 The Login Phase is only implemented via Login Requests and Responses.
 The whole Login Phase is considered as a single task and has a single
 Initiator Task Tag (similar to the linked SCSI commands).
 There MUST NOT be more than one outstanding Login Request or Login
 Response on an iSCSI connection.  An outstanding PDU in this context
 is one that has not been acknowledged by the remote iSCSI side.
 The default MaxRecvDataSegmentLength is used during login.

Chadalapaka, et al. Standards Track [Page 81] RFC 7143 iSCSI (Consolidated) April 2014

 The Login Phase sequence of requests and responses proceeds as
 follows:
  1. Login initial request
  1. Login partial response (optional)
  1. More Login Requests and Responses (optional)
  1. Login Final-Response (mandatory)
 The initial Login Request of any connection MUST include the
 InitiatorName key=value pair.  The initial Login Request of the first
 connection of a session MAY also include the SessionType key=value
 pair.  For any connection within a session whose type is not
 "Discovery", the first Login Request MUST also include the TargetName
 key=value pair.
 The Login Final-Response accepts or rejects the Login Request.
 The Login Phase MAY include a SecurityNegotiation stage and a
 LoginOperationalNegotiation stage and MUST include at least one of
 them, but the included stage MAY be empty except for the mandatory
 names.
 The Login Requests and Responses contain a field (CSG) that indicates
 the current negotiation stage (SecurityNegotiation or
 LoginOperationalNegotiation).  If both stages are used, the
 SecurityNegotiation MUST precede the LoginOperationalNegotiation.
 Some operational parameters can be negotiated outside the login
 through Text Requests and Responses.
 Authentication-related security keys (Section 12) MUST be completely
 negotiated within the Login Phase.  The use of underlying IPsec
 security is specified in Section 9.3, in [RFC3723], and in [RFC7146].
 iSCSI support for security within the protocol only consists of
 authentication in the Login Phase.
 In some environments, a target or an initiator is not interested in
 authenticating its counterpart.  It is possible to bypass
 authentication through the Login Request and Response.
 The initiator and target MAY want to negotiate iSCSI authentication
 parameters.  Once this negotiation is completed, the channel is
 considered secure.

Chadalapaka, et al. Standards Track [Page 82] RFC 7143 iSCSI (Consolidated) April 2014

 Most of the negotiation keys are only allowed in a specific stage.
 The keys used during the SecurityNegotiation stage are listed in
 Section 12, and the keys used during the LoginOperationalNegotiation
 stage are discussed in Section 13.  Only a limited set of keys
 (marked as Any-Stage in Section 13) may be used in either of the two
 stages.
 Any given Login Request or Response belongs to a specific stage; this
 determines the negotiation keys allowed with the request or response.
 Sending a key that is not allowed in the current stage is considered
 a protocol error.
 Stage transition is performed through a command exchange
 (request/response) that carries the T bit and the same CSG code.
 During this exchange, the next stage is selected by the target via
 the Next Stage code (NSG).  The selected NSG MUST NOT exceed the
 value stated by the initiator.  The initiator can request a
 transition whenever it is ready, but a target can only respond with a
 transition after one is proposed by the initiator.
 In a negotiation sequence, the T bit settings in one Login Request-
 Login Response pair have no bearing on the T bit settings of the next
 pair.  An initiator that has the T bit set to 1 in one pair and is
 answered with a T bit setting of 0 may issue the next request with
 the T bit set to 0.
 When a transition is requested by the initiator and acknowledged by
 the target, both the initiator and target switch to the selected
 stage.
 Targets MUST NOT submit parameters that require an additional
 initiator Login Request in a Login Response with the T bit set to 1.
 Stage transitions during login (including entering and exit) are only
 possible as outlined in the following table:
   +-----------------------------------------------------------+
   |From      To ->  | Security    | Operational | FullFeature |
   | |               |             |             |             |
   | V               |             |             |             |
   +-----------------------------------------------------------+
   | (start)         | yes         | yes         | no          |
   +-----------------------------------------------------------+
   | Security        | no          | yes         | yes         |
   +-----------------------------------------------------------+
   | Operational     | no          | no          | yes         |
   +-----------------------------------------------------------+

Chadalapaka, et al. Standards Track [Page 83] RFC 7143 iSCSI (Consolidated) April 2014

 The Login Final-Response that accepts a Login Request can only come
 as a response to a Login Request with the T bit set to 1, and both
 the request and response MUST indicate FullFeaturePhase as the next
 phase via the NSG field.
 Neither the initiator nor the target should attempt to declare or
 negotiate a parameter more than once during login, except for
 responses to specific keys that explicitly allow repeated key
 declarations (e.g., TargetAddress).  An attempt to
 renegotiate/redeclare parameters not specifically allowed MUST be
 detected by the initiator and target.  If such an attempt is detected
 by the target, the target MUST respond with a Login reject (initiator
 error); if detected by the initiator, the initiator MUST drop the
 connection.

6.3.1. Login Phase Start

 The Login Phase starts with a Login Request from the initiator to the
 target.  The initial Login Request includes:
  1. Protocol version supported by the initiator
  1. iSCSI Initiator Name and iSCSI Target Name
  1. ISID, TSIH, and connection IDs
  1. Negotiation stage that the initiator is ready to enter

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 A login may create a new session, or it may add a connection to an
 existing session.  Between a given iSCSI initiator node (selected
 only by an InitiatorName) and a given iSCSI target defined by an
 iSCSI TargetName and a Target Portal Group Tag, the login results are
 defined by the following table:
  +----------------------------------------------------------------+
  |ISID    | TSIH        | CID    |   Target Action                |
  +----------------------------------------------------------------+
  |new     | non-zero    | any    |   fail the login               |
  |        |             |        |   ("session does not exist")   |
  +----------------------------------------------------------------+
  |new     | zero        | any    |   instantiate a new session    |
  +----------------------------------------------------------------+
  |existing| zero        | any    |   do session reinstatement     |
  |        |             |        |   (see Section 6.3.5)          |
  +----------------------------------------------------------------+
  |existing| non-zero    | new    |   add a new connection to      |
  |        | existing    |        |   the session                  |
  +----------------------------------------------------------------+
  |existing| non-zero    |existing|   do connection reinstatement  |
  |        | existing    |        |   (see Section 7.1.4.3)        |
  +----------------------------------------------------------------+
  |existing| non-zero    | any    |   fail the login               |
  |        | new         |        |   ("session does not exist")   |
  +----------------------------------------------------------------+
 The determination of "existing" or "new" is made by the target.
 Optionally, the Login Request may include:
  1. Security parameters OR
  1. iSCSI operational parameters AND/OR
  1. The next negotiation stage that the initiator is ready to

enter

 The target can answer the login in the following ways:
  1. Login Response with Login reject. This is an immediate

rejection from the target that causes the connection to

      terminate and the session to terminate if this is the first (or
      only) connection of a new session.  The T bit, the CSG field,
      and the NSG field are reserved.

Chadalapaka, et al. Standards Track [Page 85] RFC 7143 iSCSI (Consolidated) April 2014

  1. Login Response with Login accept as the Final-Response (T bit

set to 1 and the NSG in both request and response is set to

      FullFeaturePhase).  The response includes the protocol version
      supported by the target and the session ID and may include iSCSI
      operational or security parameters (that depend on the current
      stage).
  1. Login Response with Login accept as a partial response (NSG not

set to FullFeaturePhase in both request and response) that

      indicates the start of a negotiation sequence.  The response
      includes the protocol version supported by the target and either
      security or iSCSI parameters (when no security mechanism is
      chosen) supported by the target.
 If the initiator decides to forego the SecurityNegotiation stage, it
 issues the Login with the CSG set to LoginOperationalNegotiation, and
 the target may reply with a Login Response that indicates that it is
 unwilling to accept the connection (see Section 11.13) without
 SecurityNegotiation and will terminate the connection with a response
 of Authentication failure (see Section 11.13.5).
 If the initiator is willing to negotiate iSCSI security, but is
 unwilling to make the initial parameter proposal and may accept a
 connection without iSCSI security, it issues the Login with the T bit
 set to 1, the CSG set to SecurityNegotiation, and the NSG set to
 LoginOperationalNegotiation.  If the target is also ready to skip
 security, the Login Response only contains the TargetPortalGroupTag
 key (see Section 13.9), the T bit set to 1, the CSG set to
 SecurityNegotiation, and the NSG set to LoginOperationalNegotiation.
 An initiator that chooses to operate without iSCSI security and with
 all the operational parameters taking the default values issues the
 Login with the T bit set to 1, the CSG set to
 LoginOperationalNegotiation, and the NSG set to FullFeaturePhase.  If
 the target is also ready to forego security and can finish its
 LoginOperationalNegotiation, the Login Response has the T bit set to
 1, the CSG set to LoginOperationalNegotiation, and the NSG set to
 FullFeaturePhase in the next stage.
 During the Login Phase, the iSCSI target MUST return the
 TargetPortalGroupTag key with the first Login Response PDU with which
 it is allowed to do so (i.e., the first Login Response issued after
 the first Login Request with the C bit set to 0) for all session
 types.  The TargetPortalGroupTag key value indicates the iSCSI portal
 group servicing the Login Request PDU.  If the reconfiguration of
 iSCSI portal groups is a concern in a given environment, the iSCSI
 initiator should use this key to ascertain that it had indeed
 initiated the Login Phase with the intended target portal group.

Chadalapaka, et al. Standards Track [Page 86] RFC 7143 iSCSI (Consolidated) April 2014

6.3.2. iSCSI Security Negotiation

 The security exchange sets the security mechanism and authenticates
 the initiator and the target to each other.  The exchange proceeds
 according to the authentication method chosen in the negotiation
 phase and is conducted using the key=value parameters carried in the
 Login Requests and Responses.
 An initiator-directed negotiation proceeds as follows:
  1. The initiator sends a Login Request with an ordered list of the

options it supports (authentication algorithm). The options are

      listed in the initiator's order of preference.  The initiator
      MAY also send private or public extension options.
  1. The target MUST reply with the first option in the list it

supports and is allowed to use for the specific initiator,

      unless it does not support any, in which case it MUST answer
      with "Reject" (see Section 6.2).  The parameters are encoded in
      UTF-8 as key=value.  For security parameters, see Section 12.
  1. When the initiator considers itself ready to conclude the

SecurityNegotiation stage, it sets the T bit to 1 and the NSG to

      what it would like the next stage to be.  The target will then
      set the T bit to 1 and set the NSG to the next stage in the
      Login Response when it finishes sending its security keys.  The
      next stage selected will be the one the target selected.  If the
      next stage is FullFeaturePhase, the target MUST reply with a
      Login Response with the TSIH value.
 If the security negotiation fails at the target, then the target MUST
 send the appropriate Login Response PDU.  If the security negotiation
 fails at the initiator, the initiator SHOULD close the connection.
 It should be noted that the negotiation might also be directed by the
 target if the initiator does support security but is not ready to
 direct the negotiation (propose options); see Appendix B for an
 example.

6.3.3. Operational Parameter Negotiation during the Login Phase

 Operational parameter negotiation during the Login Phase MAY be done:
  1. starting with the first Login Request if the initiator does not

propose any security/integrity option.

  1. starting immediately after the security negotiation if the

initiator and target perform such a negotiation.

Chadalapaka, et al. Standards Track [Page 87] RFC 7143 iSCSI (Consolidated) April 2014

 Operational parameter negotiation MAY involve several Login Request-
 Login Response exchanges started and terminated by the initiator.
 The initiator MUST indicate its intent to terminate the negotiation
 by setting the T bit to 1; the target sets the T bit to 1 on the last
 response.
 Even when the initiator indicates its intent to switch stages by
 setting the T bit to 1 in a Login Request, the target MAY respond
 with a Login Response with the T bit set to 0.  In that case, the
 initiator SHOULD continue to set the T bit to 1 in subsequent Login
 Requests (even empty requests) that it sends, until the target sends
 a Login Response with the T bit set to 1 or sends a key that requires
 the initiator to set the T bit to 0.
 Some session-specific parameters can only be specified during the
 Login Phase of the first connection of a session (i.e., begun by a
 Login Request that contains a zero-valued TSIH) -- the leading Login
 Phase (e.g., the maximum number of connections that can be used for
 this session).
 A session is operational once it has at least one connection in the
 Full Feature Phase.  New or replacement connections can only be added
 to a session after the session is operational.
 For operational parameters, see Section 13.

6.3.4. Connection Reinstatement

 Connection reinstatement is the process of an initiator logging in
 with an ISID-TSIH-CID combination that is possibly active from the
 target's perspective, which causes the implicit logging out of the
 connection corresponding to the CID and reinstatement of a new Full
 Feature Phase iSCSI connection in its place (with the same CID).
 Thus, the TSIH in the Login Request PDU MUST be non-zero, and the CID
 does not change during a connection reinstatement.  The Login Request
 performs the logout function of the old connection if an explicit
 logout was not performed earlier.  In sessions with a single
 connection, this may imply the opening of a second connection with
 the sole purpose of cleaning up the first.  Targets MUST support
 opening a second connection even when they do not support multiple
 connections in the Full Feature Phase if ErrorRecoveryLevel is 2 and
 SHOULD support opening a second connection if ErrorRecoveryLevel is
 less than 2.
 If the operational ErrorRecoveryLevel is 2, connection reinstatement
 enables future task reassignment.  If the operational
 ErrorRecoveryLevel is less than 2, connection reinstatement is the

Chadalapaka, et al. Standards Track [Page 88] RFC 7143 iSCSI (Consolidated) April 2014

 replacement of the old CID without enabling task reassignment.  In
 this case, all the tasks that were active on the old CID must be
 immediately terminated without further notice to the initiator.
 The initiator connection state MUST be CLEANUP_WAIT (Section 8.1.3)
 when the initiator attempts a connection reinstatement.
 In practical terms, in addition to the implicit logout of the old
 connection, reinstatement is equivalent to a new connection login.

6.3.5. Session Reinstatement, Closure, and Timeout

 Session reinstatement is the process of an initiator logging in with
 an ISID that is possibly active from the target's perspective for
 that initiator, thus implicitly logging out the session that
 corresponds to the ISID and reinstating a new iSCSI session in its
 place (with the same ISID).  Therefore, the TSIH in the Login PDU
 MUST be zero to signal session reinstatement.  Session reinstatement
 causes all the tasks that were active on the old session to be
 immediately terminated by the target without further notice to the
 initiator.
 The initiator session state MUST be FAILED (Section 8.3) when the
 initiator attempts a session reinstatement.
 Session closure is an event defined to be one of the following:
  1. a successful "session close" logout.
  1. a successful "connection close" logout for the last Full Feature

Phase connection when no other connection in the session is

      waiting for cleanup (Section 8.2) and no tasks in the session
      are waiting for reassignment.
 Session timeout is an event defined to occur when the last connection
 state timeout expires and no tasks are waiting for reassignment.
 This takes the session to the FREE state (see the session state
 diagrams in Section 8.3).

Chadalapaka, et al. Standards Track [Page 89] RFC 7143 iSCSI (Consolidated) April 2014

6.3.5.1. Loss of Nexus Notification

 The iSCSI layer provides the SCSI layer with the "I_T nexus loss"
 notification when any one of the following events happens:
  1. successful completion of session reinstatement
  1. session closure event
  1. session timeout event
 Certain SCSI object clearing actions may result due to the
 notification in the SCSI end nodes, as documented in Appendix E.

6.3.6. Session Continuation and Failure

 Session continuation is the process by which the state of a
 preexisting session continues to be used by connection reinstatement
 (Section 6.3.4) or by adding a connection with a new CID.  Either of
 these actions associates the new transport connection with the
 session state.
 Session failure is an event where the last Full Feature Phase
 connection reaches the CLEANUP_WAIT state (Section 8.2) or completes
 a successful recovery logout, thus causing all active tasks (that are
 formerly allegiant to the connection) to start waiting for task
 reassignment.

6.4. Operational Parameter Negotiation outside the Login Phase

 Some operational parameters MAY be negotiated outside (after) the
 Login Phase.
 Parameter negotiation in the Full Feature Phase is done through Text
 Requests and Responses.  Operational parameter negotiation MAY
 involve several Text Request-Text Response exchanges, all of which
 use the same Initiator Task Tag; the initiator always starts and
 terminates each of these exchanges.  The initiator MUST indicate its
 intent to finish the negotiation by setting the F bit to 1; the
 target sets the F bit to 1 on the last response.
 If the target responds to a Text Request with the F bit set to 1 with
 a Text Response with the F bit set to 0, the initiator should keep
 sending the Text Request (even empty requests) with the F bit set to
 1 while it still wants to finish the negotiation, until it receives
 the Text Response with the F bit set to 1.  Responding to a Text
 Request with the F bit set to 1 with an empty (no key=value pairs)
 response with the F bit set to 0 is discouraged.

Chadalapaka, et al. Standards Track [Page 90] RFC 7143 iSCSI (Consolidated) April 2014

 Even when the initiator indicates its intent to finish the
 negotiation by setting the F bit to 1 in a Text Request, the target
 MAY respond with a Text Response with the F bit set to 0.  In that
 case, the initiator SHOULD continue to set the F bit to 1 in
 subsequent Text Requests (even empty requests) that it sends, until
 the target sends the final Text Response with the F bit set to 1.
 Note that in the same case of a Text Request with the F bit set to 1,
 the target SHOULD NOT respond with an empty (no key=value pairs) Text
 Response with the F bit set to 0, because such a response may cause
 the initiator to abandon the negotiation.
 Targets MUST NOT submit parameters that require an additional
 initiator Text Request in a Text Response with the F bit set to 1.
 In a negotiation sequence, the F bit settings in one Text Request-
 Text Response pair have no bearing on the F bit settings of the next
 pair.  An initiator that has the F bit set to 1 in a request and is
 being answered with an F bit setting of 0 may issue the next request
 with the F bit set to 0.
 Whenever the target responds with the F bit set to 0, it MUST set the
 Target Transfer Tag to a value other than the default 0xffffffff.
 An initiator MAY reset an operational parameter negotiation by
 issuing a Text Request with the Target Transfer Tag set to the value
 0xffffffff after receiving a response with the Target Transfer Tag
 set to a value other than 0xffffffff.  A target may reset an
 operational parameter negotiation by answering a Text Request with a
 Reject PDU.
 Neither the initiator nor the target should attempt to declare or
 negotiate a parameter more than once during any negotiation sequence,
 except for responses to specific keys that explicitly allow repeated
 key declarations (e.g., TargetAddress).  If such an attempt is
 detected by the target, the target MUST respond with a Reject PDU
 with a reason of "Protocol Error".  The initiator MUST reset the
 negotiation as outlined above.
 Parameters negotiated by a text exchange negotiation sequence only
 become effective after the negotiation sequence is completed.

Chadalapaka, et al. Standards Track [Page 91] RFC 7143 iSCSI (Consolidated) April 2014

7. iSCSI Error Handling and Recovery

7.1. Overview

7.1.1. Background

 The following two considerations prompted the design of much of the
 error recovery functionality in iSCSI:
  1. An iSCSI PDU may fail the digest check and be dropped, despite

being received by the TCP layer. The iSCSI layer must

      optionally be allowed to recover such dropped PDUs.
  1. A TCP connection may fail at any time during the data transfer.

All the active tasks must optionally be allowed to be continued

      on a different TCP connection within the same session.
 Implementations have considerable flexibility in deciding what degree
 of error recovery to support, when to use it, and by which mechanisms
 to achieve the required behavior.  Only the externally visible
 actions of the error recovery mechanisms must be standardized to
 ensure interoperability.
 This section describes a general model for recovery in support of
 interoperability.  See Appendix D for further details on how the
 described model may be implemented.  Compliant implementations do not
 have to match the implementation details of this model as presented,
 but the external behavior of such implementations must correspond to
 the externally observable characteristics of the presented model.

7.1.2. Goals

 The major design goals of the iSCSI error recovery scheme are as
 follows:
  1. Allow iSCSI implementations to meet different requirements by

defining a collection of error recovery mechanisms from which

      implementations may choose.
  1. Ensure interoperability between any two implementations

supporting different sets of error recovery capabilities.

  1. Define the error recovery mechanisms to ensure command ordering

even in the face of errors, for initiators that demand ordering.

  1. Do not make additions in the fast path, but allow moderate

complexity in the error recovery path.

Chadalapaka, et al. Standards Track [Page 92] RFC 7143 iSCSI (Consolidated) April 2014

  1. Prevent both the initiator and target from attempting to recover

the same set of PDUs at the same time. For example, there must

      be a clear "error recovery functionality distribution" between
      the initiator and target.

7.1.3. Protocol Features and State Expectations

 The initiator mechanisms defined in connection with error recovery
 are:
    a) NOP-Out to probe sequence numbers of the target (Section 11.18)
    b) Command retry (Section 7.2.1)
    c) Recovery R2T support (Section 7.8)
    d) Requesting retransmission of status/data/R2T using the SNACK
       facility (Section 11.16)
    e) Acknowledging the receipt of the data (Section 11.16)
    f) Reassigning the connection allegiance of a task to a different
       TCP connection (Section 7.2.2)
    g) Terminating the entire iSCSI session to start afresh
       (Section 7.1.4.4)
 The target mechanisms defined in connection with error recovery are:
    a) NOP-In to probe sequence numbers of the initiator
       (Section 11.19)
    b) Requesting retransmission of data using the recovery R2T
       feature (Section 7.8)
    c) SNACK support (Section 11.16)
    d) Requesting that parts of read data be acknowledged
       (Section 11.7.2)
    e) Allegiance reassignment support (Section 7.2.2)
    f) Terminating the entire iSCSI session to force the initiator to
       start over (Section 7.1.4.4)
 For any outstanding SCSI command, it is assumed that iSCSI, in
 conjunction with SCSI at the initiator, is able to keep enough
 information to be able to rebuild the command PDU and that outgoing

Chadalapaka, et al. Standards Track [Page 93] RFC 7143 iSCSI (Consolidated) April 2014

 data is available (in host memory) for retransmission while the
 command is outstanding.  It is also assumed that at the target,
 incoming data (read data) MAY be kept for recovery, or it can be
 reread from a device server.
 It is further assumed that a target will keep the "status and sense"
 for a command it has executed if it supports status retransmission.
 A target that agrees to support data retransmission is expected to be
 prepared to retransmit the outgoing data (i.e., Data-In) on request
 until either the status for the completed command is acknowledged or
 the data in question has been separately acknowledged.

7.1.4. Recovery Classes

 iSCSI enables the following classes of recovery (in the order of
 increasing scope of affected iSCSI tasks):
  1. within a command (i.e., without requiring command restart)
  1. within a connection (i.e., without requiring the connection to

be rebuilt, but perhaps requiring command restart)

  1. connection recovery (i.e., perhaps requiring connections to be

rebuilt and commands to be reissued)

  1. session recovery
 The recovery scenarios detailed in the rest of this section are
 representative rather than exclusive.  In every case, they detail the
 lowest recovery class that MAY be attempted.  The implementer is left
 to decide under which circumstances to escalate to the next recovery
 class and/or what recovery classes to implement.  Both the iSCSI
 target and initiator MAY escalate the error handling to an error
 recovery class, which impacts a larger number of iSCSI tasks in any
 of the cases identified in the following discussion.
 In all classes, the implementer has the choice of deferring errors to
 the SCSI initiator (with an appropriate response code), in which case
 the task, if any, has to be removed from the target and all the side
 effects, such as ACA, must be considered.
 The use of within-connection and within-command recovery classes MUST
 NOT be attempted before the connection is in the Full Feature Phase.

Chadalapaka, et al. Standards Track [Page 94] RFC 7143 iSCSI (Consolidated) April 2014

 In the detailed description of the recovery classes, the mandating
 terms (MUST, SHOULD, MAY, etc.) indicate normative actions to be
 executed if the recovery class is supported (see Section 7.1.5 for
 the related negotiation semantics) and used.

7.1.4.1. Recovery Within-command

 At the target, the following cases lend themselves to within-command
 recovery:
    Lost data PDU - realized through one of the following:
    a) Data digest error - dealt with as specified in Section 7.8,
       using the option of a recovery R2T
    b) Sequence reception timeout (no data or partial-data-and-no-
       F-bit) - considered an implicit sequence error and dealt with
       as specified in Section 7.9, using the option of a recovery R2T
    c) Header digest error, which manifests as a sequence reception
       timeout or a sequence error - dealt with as specified in
       Section 7.9, using the option of a recovery R2T
 At the initiator, the following cases lend themselves to within-
 command recovery:
    Lost data PDU or lost R2T - realized through one of the following:
    a) Data digest error - dealt with as specified in Section 7.8,
       using the option of a SNACK
    b) Sequence reception timeout (no status) or response reception
       timeout - dealt with as specified in Section 7.9, using the
       option of a SNACK
    c) Header digest error, which manifests as a sequence reception
       timeout or a sequence error - dealt with as specified in
       Section 7.9, using the option of a SNACK
 To avoid a race with the target, which may already have a recovery
 R2T or a termination response on its way, an initiator SHOULD NOT
 originate a SNACK for an R2T based on its internal timeouts (if any).
 Recovery in this case is better left to the target.
 The timeout values used by the initiator and target are outside the
 scope of this document.  A sequence reception timeout is generally a
 large enough value to allow the data sequence transfer to be
 complete.

Chadalapaka, et al. Standards Track [Page 95] RFC 7143 iSCSI (Consolidated) April 2014

7.1.4.2. Recovery Within-connection

 At the initiator, the following cases lend themselves to within-
 connection recovery:
    a) Requests not acknowledged for a long time.  Requests are
       acknowledged explicitly through the ExpCmdSN or implicitly by
       receiving data and/or status.  The initiator MAY retry
       non-acknowledged commands as specified in Section 7.2.
    b) Lost iSCSI numbered response.  It is recognized by either
       identifying a data digest error on a Response PDU or a Data-In
       PDU carrying the status, or receiving a Response PDU with a
       higher StatSN than expected.  In the first case, digest error
       handling is done as specified in Section 7.8, using the option
       of a SNACK.  In the second case, sequence error handling is
       done as specified in Section 7.9, using the option of a SNACK.
 At the target, the following cases lend themselves to within-
 connection recovery:
  1. Status/Response not acknowledged for a long time. The target

MAY issue a NOP-In (with a valid Target Transfer Tag or

      otherwise) that carries the next status sequence number it is
      going to use in the StatSN field.  This helps the initiator
      detect any missing StatSN(s) and issue a SNACK for the status.
 The timeout values used by the initiator and the target are outside
 the scope of this document.

7.1.4.3. Connection Recovery

 At an iSCSI initiator, the following cases lend themselves to
 connection recovery:
    a) TCP connection failure: The initiator MUST close the
       connection.  It then MUST either implicitly or explicitly log
       out the failed connection with the reason code "remove the
       connection for recovery" and reassign connection allegiance for
       all commands still in progress associated with the failed
       connection on one or more connections (some or all of which MAY
       be newly established connections) using the "TASK REASSIGN"
       task management function (see Section 11.5.1).  For an
       initiator, a command is in progress as long as it has not
       received a response or a Data-In PDU including status.

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       Note: The logout function is mandatory.  However, a new
       connection establishment is only mandatory if the failed
       connection was the last or only connection in the session.
    b) Receiving an Asynchronous Message that indicates that one or
       all connections in a session have been dropped.  The initiator
       MUST handle it as a TCP connection failure for the
       connection(s) referred to in the message.
 At an iSCSI target, the following cases lend themselves to connection
 recovery:
  1. TCP connection failure: The target MUST close the connection

and, if more than one connection is available, the target SHOULD

      send an Asynchronous Message that indicates that it has dropped
      the connection.  Then, the target will wait for the initiator to
      continue recovery.

7.1.4.4. Session Recovery

 Session recovery should be performed when all other recovery attempts
 have failed.  Very simple initiators and targets MAY perform session
 recovery on all iSCSI errors and rely on recovery on the SCSI layer
 and above.
 Session recovery implies the closing of all TCP connections,
 internally aborting all executing and queued tasks for the given
 initiator at the target, terminating all outstanding SCSI commands
 with an appropriate SCSI service response at the initiator, and
 restarting a session on a new set of connection(s) (TCP connection
 establishment and login on all new connections).
 For possible clearing effects of session recovery on SCSI and iSCSI
 objects, refer to Appendix E.

7.1.5. Error Recovery Hierarchy

 The error recovery classes described so far are organized into a
 hierarchy for ease in understanding and to limit the complexity of
 the implementation.  With a few well-defined recovery levels,
 interoperability is easier to achieve.  The attributes of this
 hierarchy are as follows:
    a) Each level is a superset of the capabilities of the previous
       level.  For example, Level 1 support implies supporting all
       capabilities of Level 0 and more.

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    b) As a corollary, supporting a higher error recovery level means
       increased sophistication and possibly an increase in resource
       requirements.
    c) Supporting error recovery level "n" is advertised and
       negotiated by each iSCSI entity by exchanging the text key
       "ErrorRecoveryLevel=n".  The lower of the two exchanged values
       is the operational ErrorRecoveryLevel for the session.
 The following diagram represents the error recovery hierarchy.
                          +
                         / \
                        / 2 \      <-- Connection recovery
                       +-----+
                      /   1   \    <-- Digest failure recovery
                     +---------+
                    /     0     \  <-- Session failure recovery
                   +-------------+
 The following table lists the error recovery (ER) capabilities
 expected from the implementations that support each error recovery
 level.
  +-------------------+--------------------------------------------+
  |ErrorRecoveryLevel | Associated Error Recovery Capabilities     |
  +-------------------+--------------------------------------------+
  |        0          | Session recovery class                     |
  |                   | (Session Recovery)                         |
  +-------------------+--------------------------------------------+
  |        1          | Digest failure recovery (see Note below)   |
  |                   | plus the capabilities of ER Level 0        |
  +-------------------+--------------------------------------------+
  |        2          | Connection recovery class                  |
  |                   | (Connection Recovery)                      |
  |                   | plus the capabilities of ER Level 1        |
  +-------------------+--------------------------------------------+
 Note: Digest failure recovery is comprised of two recovery classes:
 the Within-connection recovery class (recovery within-connection) and
 the Within-command recovery class (recovery within-command).
 When a defined value of ErrorRecoveryLevel is proposed by an
 originator in a text negotiation, the originator MUST support the
 functionality defined for the proposed value and, additionally,
 functionality corresponding to any defined value numerically less
 than the proposed value.  When a defined value of ErrorRecoveryLevel

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 is returned by a responder in a text negotiation, the responder MUST
 support the functionality corresponding to the ErrorRecoveryLevel it
 is accepting.
 When either party attempts to use error recovery functionality beyond
 what is negotiated, the recovery attempts MAY fail, unless an
 a priori agreement outside the scope of this document exists between
 the two parties to provide such support.
 Implementations MUST support error recovery level "0", while the rest
 are OPTIONAL to implement.  In implementation terms, the above
 striation means that the following incremental sophistication with
 each level is required:
  +-------------------+--------------------------------------------+
  | Level Transition  | Incremental Requirement                    |
  +-------------------+--------------------------------------------+
  |        0->1       | PDU retransmissions on the same connection |
  +-------------------+--------------------------------------------+
  |        1->2       | Retransmission across connections and      |
  |                   | allegiance reassignment                    |
  +-------------------+--------------------------------------------+

7.2. Retry and Reassign in Recovery

 This section summarizes two important and somewhat related iSCSI
 protocol features used in error recovery.

7.2.1. Usage of Retry

 By resending the same iSCSI Command PDU ("retry") in the absence of a
 command acknowledgment (by way of an ExpCmdSN update) or a response,
 an initiator attempts to "plug" (what it thinks are) the
 discontinuities in CmdSN ordering on the target end.  Discarded
 command PDUs, due to digest errors, may have created these
 discontinuities.
 Retry MUST NOT be used for reasons other than plugging command
 sequence gaps and, in particular, cannot be used for requesting PDU
 retransmissions from a target.  Any such PDU retransmission requests
 for a currently allegiant command in progress may be made using the
 SNACK mechanism described in Section 11.16, although the usage of
 SNACK is OPTIONAL.

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 If initiators, as part of plugging command sequence gaps as described
 above, inadvertently issue retries for allegiant commands already in
 progress (i.e., targets did not see the discontinuities in CmdSN
 ordering), the duplicate commands are silently ignored by targets as
 specified in Section 4.2.2.1.
 When an iSCSI command is retried, the command PDU MUST carry the
 original Initiator Task Tag and the original operational attributes
 (e.g., flags, function names, LUN, CDB, etc.) as well as the original
 CmdSN.  The command being retried MUST be sent on the same connection
 as the original command, unless the original connection was already
 successfully logged out.

7.2.2. Allegiance Reassignment

 By issuing a "TASK REASSIGN" task management request
 (Section 11.5.1), the initiator signals its intent to continue an
 already active command (but with no current connection allegiance) as
 part of connection recovery.  This means that a new connection
 allegiance is requested for the command, which seeks to associate it
 to the connection on which the task management request is being
 issued.  Before the allegiance reassignment is attempted for a task,
 an implicit or explicit Logout with the reason code "remove the
 connection for recovery" (see Section 11.14.1) MUST be successfully
 completed for the previous connection to which the task was
 allegiant.
 In reassigning connection allegiance for a command, the target SHOULD
 continue the command from its current state.  For example, when
 reassigning read commands, the target SHOULD take advantage of the
 ExpDataSN field provided by the Task Management Function Request
 (which must be set to 0 if there was no data transfer) and bring the
 read command to completion by sending the remaining data and sending
 (or resending) the status.  The ExpDataSN acknowledges all data sent
 up to, but not including, the Data-In PDU and/or R2T with the DataSN
 (or R2TSN) equal to the ExpDataSN.  However, targets may choose to
 send/receive all unacknowledged data or all of the data on a
 reassignment of connection allegiance if unable to recover or
 maintain accurate state.  Initiators MUST NOT subsequently request
 data retransmission through Data SNACK for PDUs numbered less than
 the ExpDataSN (i.e., prior to the acknowledged sequence number).  For
 all types of commands, a reassignment request implies that the task
 is still considered in progress by the initiator, and the target must
 conclude the task appropriately if the target returns the "Function
 complete" response to the reassignment request.  This might possibly
 involve retransmission of data/R2T/status PDUs as necessary but MUST
 involve the (re)transmission of the status PDU.

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 It is OPTIONAL for targets to support the allegiance reassignment.
 This capability is negotiated via the ErrorRecoveryLevel text key
 during the login time.  When a target does not support allegiance
 reassignment, it MUST respond with a task management response code of
 "Task allegiance reassignment not supported".  If allegiance
 reassignment is supported by the target but the task is still
 allegiant to a different connection, or a successful recovery Logout
 of the previously allegiant connection was not performed, the target
 MUST respond with a task management response code of "Task still
 allegiant".
 If allegiance reassignment is supported by the target, the task
 management response to the reassignment request MUST be issued before
 the reassignment becomes effective.
 If a SCSI command that involves data input is reassigned, any SNACK
 Tag it holds for a final response from the original connection is
 deleted, and the default value of 0 MUST be used instead.

7.3. Usage of Reject PDU in Recovery

 Targets MUST NOT implicitly terminate an active task by sending a
 Reject PDU for any PDU exchanged during the life of the task.  If the
 target decides to terminate the task, a Response PDU (SCSI, Text,
 Task, etc.) must be returned by the target to conclude the task.  If
 the task had never been active before the Reject (i.e., the Reject is
 on the command PDU), targets should not send any further responses
 because the command itself is being discarded.
 The above rule means that the initiator can eventually expect a
 response on receiving Rejects, if the received Reject is for a PDU
 other than the command PDU itself.  The non-command Rejects only have
 diagnostic value in logging the errors, and they can be used for
 retransmission decisions by the initiators.
 The CmdSN of the rejected command PDU (if it is a non-immediate
 command) MUST NOT be considered received by the target (i.e., a
 command sequence gap must be assumed for the CmdSN), even though the
 CmdSN of the rejected command PDU may be reliably ascertained.  Upon
 receiving the Reject, the initiator MUST plug the CmdSN gap in order
 to continue to use the session.  The gap may be plugged by either
 transmitting a command PDU with the same CmdSN or aborting the task
 (see Section 7.11 for information regarding how an abort may plug a
 CmdSN gap).

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 When a data PDU is rejected and its DataSN can be ascertained, a
 target MUST advance the ExpDataSN for the current data burst if a
 recovery R2T is being generated.  The target MAY advance its
 ExpDataSN if it does not attempt to recover the lost data PDU.

7.4. Error Recovery Considerations for Discovery Sessions

7.4.1. ErrorRecoveryLevel for Discovery Sessions

 The negotiation of the key ErrorRecoveryLevel is not required for
 Discovery sessions -- i.e., for sessions that negotiated
 "SessionType=Discovery" -- because the default value of 0 is
 necessary and sufficient for Discovery sessions.  It is, however,
 possible that some legacy iSCSI implementations might attempt to
 negotiate the ErrorRecoveryLevel key on Discovery sessions.  When
 such a negotiation attempt is made by the remote side, a compliant
 iSCSI implementation MUST propose a value of 0 (zero) in response.
 The operational ErrorRecoveryLevel for Discovery sessions thus MUST
 be 0.  This naturally follows from the functionality constraints that
 Section 4.3 imposes on Discovery sessions.

7.4.2. Reinstatement Semantics for Discovery Sessions

 Discovery sessions are intended to be relatively short-lived.
 Initiators are not expected to establish multiple Discovery sessions
 to the same iSCSI Network Portal.  An initiator may use the same
 iSCSI Initiator Name and ISID when establishing different unique
 sessions with different targets and/or different portal groups.  This
 behavior is discussed in Section 10.1.1 and is, in fact, encouraged
 as conservative reuse of ISIDs.
 The ISID RULE in Section 4.4.3 states that there must not be more
 than one session with a matching 4-tuple: <InitiatorName, ISID,
 TargetName, TargetPortalGroupTag>.  While the spirit of the ISID RULE
 applies to Discovery sessions the same as it does for Normal
 sessions, note that some Discovery sessions differ from the Normal
 sessions in two important aspects:
    a) Because Appendix C allows a Discovery session to be established
       without specifying a TargetName key in the Login Request PDU
       (let us call such a session an "Unnamed" Discovery session),
       there is no target node context to enforce the ISID RULE.
    b) Portal groups are defined only in the context of a target node.
       When the TargetName key is NULL-valued (i.e., not specified),
       the TargetPortalGroupTag thus cannot be ascertained to enforce
       the ISID RULE.

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 The following two sections describe Unnamed Discovery sessions and
 Named Discovery sessions, respectively.

7.4.2.1. Unnamed Discovery Sessions

 For Unnamed Discovery sessions, neither the TargetName nor the
 TargetPortalGroupTag is available to the targets in order to enforce
 the ISID RULE.  Therefore, the following rule applies.
 UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
 following 4-tuple for Unnamed Discovery sessions: <InitiatorName,
 ISID, NULL, TargetAddress>.  The following semantics are implied by
 this uniqueness requirement.
 Targets SHOULD allow concurrent establishment of one Discovery
 session with each of its Network Portals by the same initiator port
 with a given iSCSI Node Name and an ISID.  Each of the concurrent
 Discovery sessions, if established by the same initiator port to
 other Network Portals, MUST be treated as independent sessions --
 i.e., one session MUST NOT reinstate the other.
 A new Unnamed Discovery session that has a matching <InitiatorName,
 ISID, NULL, TargetAddress> to an existing Discovery session MUST
 reinstate the existing Unnamed Discovery session.  Note thus that
 only an Unnamed Discovery session may reinstate another Unnamed
 Discovery session.

7.4.2.2. Named Discovery Sessions

 For Named Discovery sessions, the TargetName key is specified by the
 initiator, and thus the target can unambiguously ascertain the
 TargetPortalGroupTag as well.  Since all the four elements of the
 4-tuple are known, the ISID RULE MUST be enforced by targets with no
 changes from Section 4.4.3 semantics.  A new session with a matching
 <InitiatorName, ISID, TargetName, TargetPortalGroupTag> thus will
 reinstate an existing session.  Note in this case that any new iSCSI
 session (Discovery or Normal) with the matching 4-tuple may reinstate
 an existing Named Discovery iSCSI session.

7.4.3. Target PDUs during Discovery

 Targets SHOULD NOT send any responses other than a Text Response and
 Logout Response on a Discovery session, once in the Full Feature
 Phase.
 Implementation Note: A target may simply drop the connection in a
 Discovery session when it would have requested a Logout via an Async
 Message on Normal sessions.

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7.5. Connection Timeout Management

 iSCSI defines two session-global timeout values (in seconds) --
 Time2Wait and Time2Retain -- that are applicable when an iSCSI Full
 Feature Phase connection is taken out of service either intentionally
 or by an exception.  Time2Wait is the initial "respite time" before
 attempting an explicit/implicit Logout for the CID in question or
 task reassignment for the affected tasks (if any).  Time2Retain is
 the maximum time after the initial respite interval that the task
 and/or connection state(s) is/are guaranteed to be maintained on the
 target to cater to a possible recovery attempt.  Recovery attempts
 for the connection and/or task(s) SHOULD NOT be made before
 Time2Wait seconds but MUST be completed within Time2Retain seconds
 after that initial Time2Wait waiting period.

7.5.1. Timeouts on Transport Exception Events

 A transport connection shutdown or a transport reset without any
 preceding iSCSI protocol interactions informing the endpoints of the
 fact causes a Full Feature Phase iSCSI connection to be abruptly
 terminated.  The timeout values to be used in this case are the
 negotiated values of DefaultTime2Wait (Section 13.15) and
 DefaultTime2Retain (Section 13.16) text keys for the session.

7.5.2. Timeouts on Planned Decommissioning

 Any planned decommissioning of a Full Feature Phase iSCSI connection
 is preceded by either a Logout Response PDU or an Async Message PDU.
 The Time2Wait and Time2Retain field values (Section 11.15) in a
 Logout Response PDU, and the Parameter2 and Parameter3 fields of an
 Async Message (AsyncEvent types "drop the connection" or "drop all
 the connections"; see Section 11.9.1), specify the timeout values to
 be used in each of these cases.
 These timeout values are only applicable for the affected connection
 and the tasks active on that connection.  These timeout values have
 no bearing on initiator timers (if any) that are already running on
 connections or tasks associated with that session.

7.6. Implicit Termination of Tasks

 A target implicitly terminates the active tasks due to iSCSI protocol
 dynamics in the following cases:
    a) When a connection is implicitly or explicitly logged out with
       the reason code "close the connection" and there are active
       tasks allegiant to that connection.

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    b) When a connection fails and eventually the connection state
       times out (state transition M1 in Section 8.2.2), and there are
       active tasks allegiant to that connection.
    c) When a successful Logout with the reason code "remove the
       connection for recovery" is performed while there are active
       tasks allegiant to that connection, and those tasks eventually
       time out after the Time2Wait and Time2Retain periods without
       allegiance reassignment.
    d) When a connection is implicitly or explicitly logged out with
       the reason code "close the session" and there are active tasks
       in that session.
 If the tasks terminated in cases a), b), c), and d) above are SCSI
 tasks, they must be internally terminated as if with CHECK CONDITION
 status.  This status is only meaningful for appropriately handling
 the internal SCSI state and SCSI side effects with respect to
 ordering, because this status is never communicated back as a
 terminating status to the initiator.  However, additional actions may
 have to be taken at the SCSI level, depending on the SCSI context as
 defined by the SCSI standards (e.g., queued commands and ACA; UA for
 the next command on the I_T nexus in cases a), b), and c); etc. --
 see [SAM2] and [SPC3]).

7.7. Format Errors

 The following two explicit violations of PDU layout rules are format
 errors:
    a) Illegal contents of any PDU header field except the Opcode
       (legal values are specified in Section 11).
    b) Inconsistent field contents (consistent field contents are
       specified in Section 11).
 Format errors indicate a major implementation flaw in one of the
 parties.
 When a target or an initiator receives an iSCSI PDU with a format
 error, it MUST immediately terminate all transport connections in the
 session with either a connection close or a connection reset, and
 escalate the format error to session recovery (see Section 7.1.4.4).
 All initiator-detected PDU construction errors MUST be considered as
 format errors.  Some examples of such errors are:
  1. NOP-In with a valid TTT but an invalid LUN

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  1. NOP-In with a valid ITT (i.e., a NOP-In response) and also a

valid TTT

  1. SCSI Response PDU with Status=CHECK CONDITION, but

DataSegmentLength = 0

7.8. Digest Errors

 The discussion below regarding the legal choices in handling digest
 errors excludes session recovery as an explicit option, but either
 party detecting a digest error may choose to escalate the error to
 session recovery.
 When a target or an initiator receives any iSCSI PDU with a header
 digest error, it MUST either discard the header and all data up to
 the beginning of a later PDU or close the connection.  Because the
 digest error indicates that the length field of the header may have
 been corrupted, the location of the beginning of a later PDU needs to
 be reliably ascertained by other means, such as the operation of a
 Sync and Steering layer.
 When a target receives any iSCSI PDU with a payload digest error, it
 MUST answer with a Reject PDU with a reason code of Data-Digest-Error
 and discard the PDU.
  1. If the discarded PDU is a solicited or unsolicited iSCSI data PDU

(for immediate data in a command PDU, the non-data PDU rule below

   applies), the target MUST do one of the following:
   a) Request retransmission with a recovery R2T.
   b) Terminate the task with a SCSI Response PDU with a CHECK
      CONDITION Status and an iSCSI Condition of "Protocol Service CRC
      error" (Section 11.4.7.2).  If the target chooses to implement
      this option, it MUST wait to receive all the data (signaled by a
      data PDU with the Final bit set for all outstanding R2Ts) before
      sending the SCSI Response PDU.  A task management command (such
      as an ABORT TASK) from the initiator during this wait may also
      conclude the task.
  1. No further action is necessary for targets if the discarded PDU is

a non-data PDU. In the case of immediate data being present on a

   discarded command, the immediate data is implicitly recovered when
   the task is retried (see Section 7.2.1), followed by the entire
   data transfer for the task.
 When an initiator receives any iSCSI PDU with a payload digest error,
 it MUST discard the PDU.

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  1. If the discarded PDU is an iSCSI data PDU, the initiator MUST do

one of the following:

      a) Request the desired data PDU through SNACK.  In response to
         the SNACK, the target MUST either resend the data PDU or
         reject the SNACK with a Reject PDU with a reason code of
         "SNACK reject", in which case:
         a.1) If the status has not already been sent for the command,
              the target MUST terminate the command with a CHECK
              CONDITION Status and an iSCSI Condition of "SNACK
              rejected" (Section 11.4.7.2).
         a.2) If the status was already sent, no further action is
              necessary for the target.  The initiator in this case
              MUST wait for the status to be received and then discard
              it, so as to internally signal the completion with CHECK
              CONDITION Status and an iSCSI Condition of "Protocol
              Service CRC error" (Section 11.4.7.2).
      b) Abort the task and terminate the command with an error.
  1. If the discarded PDU is a response PDU or an unsolicited PDU

(e.g., Async, Reject), the initiator MUST do one of the

      following:
      a) Request PDU retransmission with a status of SNACK.
      b) Log out the connection for recovery, and continue the tasks
         on a different connection instance as described in
         Section 7.2.
      c) Log out to close the connection (abort all the commands
         associated with the connection).
    Note that an unsolicited PDU carries the next StatSN value on an
    iSCSI connection, thereby advancing the StatSN.  When an initiator
    discards one of these PDUs due to a payload digest error, the
    entire PDU, including the header, MUST be discarded.
    Consequently, the initiator MUST treat the exception like a loss
    of any other solicited response PDU.

7.9. Sequence Errors

 When an initiator receives an iSCSI R2T/data PDU with an out-of-order
 R2TSN/DataSN or a SCSI Response PDU with an ExpDataSN that implies
 missing data PDU(s), it means that the initiator must have detected a
 header or payload digest error on one or more earlier R2T/data PDUs.

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 The initiator MUST address these implied digest errors as described
 in Section 7.8.  When a target receives a data PDU with an out-of-
 order DataSN, it means that the target must have hit a header or
 payload digest error on at least one of the earlier data PDUs.  The
 target MUST address these implied digest errors as described in
 Section 7.8.
 When an initiator receives an iSCSI status PDU with an out-of-order
 StatSN that implies missing responses, it MUST address the one or
 more missing status PDUs as described in Section 7.8.  As a side
 effect of receiving the missing responses, the initiator may discover
 missing data PDUs.  If the initiator wants to recover the missing
 data for a command, it MUST NOT acknowledge the received responses
 that start from the StatSN of the relevant command until it has
 completed receiving all the data PDUs of the command.
 When an initiator receives duplicate R2TSNs (due to proactive
 retransmission of R2Ts by the target) or duplicate DataSNs (due to
 proactive SNACKs by the initiator), it MUST discard the duplicates.

7.10. Message Error Checking

 In iSCSI implementations to date, there has been some uncertainty
 regarding the extent to which incoming messages have to be checked
 for protocol errors, beyond what is strictly required for processing
 the inbound message.  This section addresses this question.
 Unless this document requires it, an iSCSI implementation is not
 required to do an exhaustive protocol conformance check on an
 incoming iSCSI PDU.  The iSCSI implementation in particular is not
 required to double-check the remote iSCSI implementation's
 conformance to protocol requirements.

7.11. SCSI Timeouts

 An iSCSI initiator MAY attempt to plug a command sequence gap on the
 target end (in the absence of an acknowledgment of the command by way
 of the ExpCmdSN) before the ULP timeout by retrying the
 unacknowledged command, as described in Section 7.2.
 On a ULP timeout for a command (that carried a CmdSN of n), if the
 iSCSI initiator intends to continue the session it MUST abort the
 command by using either an appropriate Task Management Function
 Request for the specific command or a "close the connection" logout.

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 When using an ABORT TASK, if the ExpCmdSN is still less than (n + 1),
 the target may see the abort request while missing the original
 command itself, due to one of the following reasons:
  1. The original command was dropped due to digest error.
  1. The connection on which the original command was sent was

successfully logged out. On logout, the unacknowledged commands

      issued on the connection being logged out are discarded.
 If the abort request is received and the original command is missing,
 targets MUST consider the original command with that RefCmdSN as
 received and issue a task management response with the response code
 "Function complete".  This response concludes the task on both ends.
 If the abort request is received and the target can determine (based
 on the Referenced Task Tag) that the command was received and
 executed, and also that the response was sent prior to the abort,
 then the target MUST respond with the response code "Task Does Not
 Exist".

7.12. Negotiation Failures

 Text Request and Response sequences, when used to set/negotiate
 operational parameters, constitute the negotiation/parameter setting.
 A negotiation failure is considered to be one or more of the
 following:
  1. For a negotiated key, none of the choices are acceptable to one

of the sides in the negotiation.

  1. For a declarative key, the declared value is not acceptable to

the other side in the negotiation.

  1. The Text Request timed out and possibly terminated.
  1. The Text Request was answered with a Reject PDU.
 The following two rules should be used to address negotiation
 failures:
    a) During login, any failure in negotiation MUST be considered a
       login process failure; the Login Phase, along with the
       connection, MUST be terminated.  If the target detects the
       failure, it must terminate the login with the appropriate Login
       response code.

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    b) A failure in negotiation during the Full Feature Phase will
       terminate the entire negotiation sequence, which may consist of
       a series of Text Requests that use the same Initiator Task Tag.
       The operational parameters of the session or the connection
       MUST continue to be the values agreed upon during an earlier
       successful negotiation (i.e., any partial results of this
       unsuccessful negotiation MUST NOT take effect and MUST be
       discarded).

7.13. Protocol Errors

 Mapping framed messages over a "streaming" connection such as TCP
 makes the proposed mechanisms vulnerable to simple software framing
 errors.  On the other hand, the introduction of framing mechanisms to
 limit the effects of these errors may be onerous on performance for
 simple implementations.  Command sequence numbers and the mechanisms
 for dropping and reestablishing connections (discussed earlier in
 Section 7 and its subsections) help handle this type of mapping
 errors.
 All violations of iSCSI PDU exchange sequences specified in this
 document are also protocol errors.  This category of errors can only
 be addressed by fixing the implementations; iSCSI defines Reject and
 response codes to enable this.

7.14. Connection Failures

 iSCSI can keep a session in operation if it is able to keep/establish
 at least one TCP connection between the initiator and the target in a
 timely fashion.  Targets and/or initiators may recognize a failing
 connection by either transport-level means (TCP), a gap in the
 command sequence number, a response stream that is not filled for a
 long time, or a failing iSCSI NOP (acting as a ping).  The latter MAY
 be used periodically to increase the speed and likelihood of
 detecting connection failures.  As an example for transport-level
 means, initiators and targets MAY also use the keep-alive option (see
 [RFC1122]) on the TCP connection to enable early link failure
 detection on otherwise idle links.

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 On connection failure, the initiator and target MUST do one of the
 following:
    a) Attempt connection recovery within the session (Connection
       Recovery).
    b) Log out the connection with the reason code "close the
       connection" (Section 11.14.5), reissue missing commands, and
       implicitly terminate all active commands.  This option requires
       support for the Within-connection recovery class (recovery
       within-connection).
    c) Perform session recovery (Session Recovery).
 Either side may choose to escalate to session recovery (via the
 initiator dropping all the connections or via an Async Message that
 announces the similar intent from a target), and the other side MUST
 give it precedence.  On a connection failure, a target MUST terminate
 and/or discard all of the active immediate commands, regardless of
 which of the above options is used (i.e., immediate commands are not
 recoverable across connection failures).

7.15. Session Errors

 If all of the connections of a session fail and cannot be
 reestablished in a short time, or if initiators detect protocol
 errors repeatedly, an initiator may choose to terminate a session and
 establish a new session.
 In this case, the initiator takes the following actions:
  1. Resets or closes all the transport connections.
  1. Terminates all outstanding requests with an appropriate response

before initiating a new session. If the same I_T nexus is

      intended to be reestablished, the initiator MUST employ session
      reinstatement (see Section 6.3.5).
 When the session timeout (the connection state timeout for the last
 failed connection) happens on the target, it takes the following
 actions:
  1. Resets or closes the TCP connections (closes the session).
  1. Terminates all active tasks that were allegiant to the

connection(s) that constituted the session.

Chadalapaka, et al. Standards Track [Page 111] RFC 7143 iSCSI (Consolidated) April 2014

 A target MUST also be prepared to handle a session reinstatement
 request from the initiator that may be addressing session errors.

8. State Transitions

 iSCSI connections and iSCSI sessions go through several well-defined
 states from the time they are created to the time they are cleared.
 The connection state transitions are described in two separate but
 dependent sets of state diagrams for ease in understanding.  The
 first set of diagrams, "standard connection state diagrams",
 describes the connection state transitions when the iSCSI connection
 is not waiting for, or undergoing, a cleanup by way of an explicit or
 implicit logout.  The second set, "connection cleanup state diagram",
 describes the connection state transitions while performing the iSCSI
 connection cleanup.  While the first set has two diagrams -- one each
 for initiator and target -- the second set has a single diagram
 applicable to both initiators and targets.
 The "session state diagram" describes the state transitions an iSCSI
 session would go through during its lifetime, and it depends on the
 states of possibly multiple iSCSI connections that participate in the
 session.
 States and transitions are described in text, tables, and diagrams.
 The diagrams are used for illustration.  The text and the tables are
 the governing specification.

8.1. Standard Connection State Diagrams

8.1.1. State Descriptions for Initiators and Targets

 State descriptions for the standard connection state diagram are as
 follows:
 S1: FREE
  1. initiator: State on instantiation, or after successful

connection closure.

  1. target: State on instantiation, or after successful

connection closure.

Chadalapaka, et al. Standards Track [Page 112] RFC 7143 iSCSI (Consolidated) April 2014

 S2: XPT_WAIT
  1. initiator: Waiting for a response to its transport

connection establishment request.

  1. target: Illegal.
 S3: XPT_UP
  1. initiator: Illegal.
  1. target: Waiting for the login process to commence.
 S4: IN_LOGIN
  1. initiator: Waiting for the login process to conclude,

possibly involving several PDU exchanges.

  1. target: Waiting for the login process to conclude,

possibly involving several PDU exchanges.

 S5: LOGGED_IN
  1. initiator: In the Full Feature Phase, waiting for all

internal, iSCSI, and transport events.

  1. target: In the Full Feature Phase, waiting for all internal,

iSCSI, and transport events.

 S6: IN_LOGOUT
  1. initiator: Waiting for a Logout Response.
  1. target: Waiting for an internal event signaling completion

of logout processing.

 S7: LOGOUT_REQUESTED
  1. initiator: Waiting for an internal event signaling

readiness to proceed with Logout.

  1. target: Waiting for the Logout process to start after

having requested a Logout via an Async Message.

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 S8: CLEANUP_WAIT
  1. initiator: Waiting for the context and/or resources to

initiate the cleanup processing for this CSM.

  1. target: Waiting for the cleanup process to start for this CSM.

8.1.2. State Transition Descriptions for Initiators and Targets

 T1:
  1. initiator: Transport connect request was made (e.g., TCP SYN

sent).

  1. target: Illegal.
 T2:
  1. initiator: Transport connection request timed out, a

transport reset was received, or an internal event of

       receiving a Logout Response (success) on another connection
       for a "close the session" Logout Request was received.
  1. target: Illegal.
 T3:
  1. initiator: Illegal.
  1. target: Received a valid transport connection request that

establishes the transport connection.

 T4:
  1. initiator: Transport connection established, thus

prompting the initiator to start the iSCSI Login.

  1. target: Initial iSCSI Login Request was received.
 T5:
  1. initiator: The final iSCSI Login Response with a Status-Class

of zero was received.

  1. target: The final iSCSI Login Request to conclude the

Login Phase was received, thus prompting the target to send

       the final iSCSI Login Response with a Status-Class of zero.

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 T6:
  1. initiator: Illegal.
  1. target: Timed out waiting for an iSCSI Login, transport

disconnect indication was received, transport reset was

       received, or an internal event indicating a transport
       timeout was received.  In all these cases, the connection is
       to be closed.
 T7:
  1. initiator: One of the following events caused the transition:
       a) The final iSCSI Login Response was received with a
          non-zero Status-Class.
       b) Login timed out.
       c) A transport disconnect indication was received.
       d) A transport reset was received.
       e) An internal event indicating a transport timeout was
          received.
       f) An internal event of receiving a Logout Response
          (success) on another connection for a "close the
          session" Logout Request was received.
     In all these cases, the transport connection is closed.
  1. target: One of the following events caused the transition:
       a) The final iSCSI Login Request to conclude the Login
          Phase was received, prompting the target to send the
          final iSCSI Login Response with a non-zero Status-Class.
       b) Login timed out.
       c) A transport disconnect indication was received.
       d) A transport reset was received.
       e) An internal event indicating a transport timeout was
          received.

Chadalapaka, et al. Standards Track [Page 115] RFC 7143 iSCSI (Consolidated) April 2014

       f) On another connection, a "close the session" Logout Request
          was received.
     In all these cases, the connection is to be closed.
 T8:
  1. initiator: An internal event of receiving a Logout

Response (success) on another connection for a "close the

       session" Logout Request was received, thus closing this
       connection and requiring no further cleanup.
  1. target: An internal event of sending a Logout Response

(success) on another connection for a "close the session"

       Logout Request was received, or an internal event of a
       successful connection/session reinstatement was received,
       thus prompting the target to close this connection cleanly.
 T9, T10:
  1. initiator: An internal event that indicates the readiness

to start the Logout process was received, thus prompting an

       iSCSI Logout to be sent by the initiator.
  1. target: An iSCSI Logout Request was received.
 T11, T12:
  1. initiator: An Async PDU with AsyncEvent "Request Logout"

was received.

  1. target: An internal event that requires the decommissioning

of the connection was received, thus causing an Async PDU with

       an AsyncEvent "Request Logout" to be sent.
 T13:
  1. initiator: An iSCSI Logout Response (success) was received,

or an internal event of receiving a Logout Response (success)

       on another connection for a "close the session" Logout Request
       was received.
  1. target: An internal event was received that indicates

successful processing of the Logout, which prompts an iSCSI

       Logout Response (success) to be sent; an internal event of
       sending a Logout Response (success) on another connection
       for a "close the session" Logout Request was received; or

Chadalapaka, et al. Standards Track [Page 116] RFC 7143 iSCSI (Consolidated) April 2014

       an internal event of a successful connection/session
       reinstatement was received.  In all these cases, the
       transport connection is closed.
 T14:
  1. initiator: An Async PDU with AsyncEvent "Request Logout"

was received again.

  1. target: Illegal.
 T15, T16:
  1. initiator: One or more of the following events caused this

transition:

       a) An internal event that indicates a transport connection
          timeout was received, thus prompting a transport reset
          or transport connection closure.
       b) A transport reset was received.
       c) A transport disconnect indication was received.
       d) An Async PDU with AsyncEvent "Drop connection" (for this
          CID) was received.
       e) An Async PDU with AsyncEvent "Drop all connections" was
          received.
  1. target: One or more of the following events caused this

transition:

       a) Internal event that indicates that a transport connection
          timeout was received, thus prompting a transport reset
          or transport connection closure.
       b) An internal event of a failed connection/session
          reinstatement was received.
       c) A transport reset was received.
       d) A transport disconnect indication was received.
       e) An internal emergency cleanup event was received, which
          prompts an Async PDU with AsyncEvent "Drop connection" (for
          this CID), or event "Drop all connections".

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 T17:
  1. initiator: One or more of the following events caused this

transition:

       a) A Logout Response (failure, i.e., a non-zero status)
          was received, or Logout timed out.
       b) Any of the events specified for T15 and T16 occurred.
  1. target: One or more of the following events caused this

transition:

       a) An internal event that indicates a failure of the
          Logout processing was received, which prompts a
          Logout Response (failure, i.e., a non-zero status)
          to be sent.
       b) Any of the events specified for T15 and T16 occurred.
 T18:
  1. initiator: An internal event of receiving a Logout

Response (success) on another connection for a "close the

       session" Logout Request was received.
  1. target: An internal event of sending a Logout Response

(success) on another connection for a "close the session"

       Logout Request was received, or an internal event of a
       successful connection/session reinstatement was received.
       In both these cases, the connection is closed.
 The CLEANUP_WAIT state (S8) implies that there are possible iSCSI
 tasks that have not reached conclusion and are still considered
 busy.

8.1.3. Standard Connection State Diagram for an Initiator

 Symbolic names for states:
    S1: FREE
    S2: XPT_WAIT
    S4: IN_LOGIN
    S5: LOGGED_IN

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    S6: IN_LOGOUT
    S7: LOGOUT_REQUESTED
    S8: CLEANUP_WAIT
 States S5, S6, and S7 constitute the Full Feature Phase operation of
 the connection.
 The state diagram is as follows:
  1. ——←————+

+———>/ S1 \←—+ |

       T13|       +->\       /<-+   \      |
          |      /    ---+---    \   \     |
          |     /        |     T2 \   |    |
          |  T8 |        |T1       |  |    |
          |     |        |        /   |T7  |
          |     |        |       /    |    |
          |     |        |      /     |    |
          |     |        V     /     /     |
          |     |     ------- /     /      |
          |     |    / S2    \     /       |
          |     |    \       /    /        |
          |     |     ---+---    /         |
          |     |        |T4    /          |
          |     |        V     /           | T18
          |     |     ------- /            |
          |     |    / S4    \             |
          |     |    \       /             |
          |     |     ---+---              |         T15
          |     |        |T5      +--------+---------+
          |     |        |       /T16+-----+------+  |
          |     |        |      /   -+-----+--+   |  |
          |     |        |     /   /  S7   \  |T12|  |
          |     |        |    / +->\       /<-+   V  V
          |     |        |   / /    -+-----       -------
          |     |        |  / /T11   |T10        /  S8   \
          |     |        V / /       V  +----+   \       /
          |     |      ---+-+-      ----+--  |    -------
          |     |     / S5    \T9  / S6    \<+      ^
          |     +-----\       /--->\       / T14    |
          |            -------      --+---+---------+T17
          +---------------------------+

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 The following state transition table represents the above diagram.
 Each row represents the starting state for a given transition, which,
 after taking a transition marked in a table cell, would end in the
 state represented by the column of the cell.  For example, from
 state S1, the connection takes the T1 transition to arrive at
 state S2.  The fields marked "-" correspond to undefined transitions.
    +----+---+---+---+---+----+---+
    |S1  |S2 |S4 |S5 |S6 |S7  |S8 |
 ---+----+---+---+---+---+----+---+
  S1| -  |T1 | - | - | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S2|T2  |-  |T4 | - | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S4|T7  |-  |-  |T5 | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S5|T8  |-  |-  | - |T9 |T11 |T15|
 ---+----+---+---+---+---+----+---+
  S6|T13 |-  |-  | - |T14|-   |T17|
 ---+----+---+---+---+---+----+---+
  S7|T18 |-  |-  | - |T10|T12 |T16|
 ---+----+---+---+---+---+----+---+
  S8| -  |-  |-  | - | - | -  | - |
 ---+----+---+---+---+---+----+---+

8.1.4. Standard Connection State Diagram for a Target

 Symbolic names for states:
    S1: FREE
    S3: XPT_UP
    S4: IN_LOGIN
    S5: LOGGED_IN
    S6: IN_LOGOUT
    S7: LOGOUT_REQUESTED
    S8: CLEANUP_WAIT
 States S5, S6, and S7 constitute the Full Feature Phase operation of
 the connection.

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 The state diagram is as follows:
  1. ——←————+

+———>/ S1 \←—+ |

          T13|       +->\       /<-+   \      |
             |      /    ---+---    \   \     |
             |     /        |     T6 \   |    |
             |  T8 |        |T3       |  |    |
             |     |        |        /   |T7  |
             |     |        |       /    |    |
             |     |        |      /     |    |
             |     |        V     /     /     |
             |     |     ------- /     /      |
             |     |    / S3    \     /       |
             |     |    \       /    /        | T18
             |     |     ---+---    /         |
             |     |        |T4    /          |
             |     |        V     /           |
             |     |     ------- /            |
             |     |    / S4    \             |
             |     |    \       /             |
             |     |     ---+---         T15  |
             |     |        |T5      +--------+---------+
             |     |        |       /T16+-----+------+  |
             |     |        |      /  -+-----+---+   |  |
             |     |        |     /   /  S7   \  |T12|  |
             |     |        |    / +->\       /<-+   V  V
             |     |        |   / /    -+-----       -------
             |     |        |  / /T11   |T10        /  S8   \
             |     |        V / /       V           \       /
             |     |      ---+-+-      -------       -------
             |     |     / S5    \T9  / S6    \        ^
             |     +-----\       /--->\       /        |
             |            -------      --+---+---------+T17
             +---------------------------+

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 The following state transition table represents the above diagram and
 follows the conventions described for the initiator diagram.
    +----+---+---+---+---+----+---+
    |S1  |S3 |S4 |S5 |S6 |S7  |S8 |
 ---+----+---+---+---+---+----+---+
  S1| -  |T3 | - | - | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S3|T6  |-  |T4 | - | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S4|T7  |-  |-  |T5 | - | -  | - |
 ---+----+---+---+---+---+----+---+
  S5|T8  |-  |-  | - |T9 |T11 |T15|
 ---+----+---+---+---+---+----+---+
  S6|T13 |-  |-  | - |-  |-   |T17|
 ---+----+---+---+---+---+----+---+
  S7|T18 |-  |-  | - |T10|T12 |T16|
 ---+----+---+---+---+---+----+---+
  S8| -  |-  |-  | - | - | -  | - |
 ---+----+---+---+---+---+----+---+

8.2. Connection Cleanup State Diagram for Initiators and Targets

 Symbolic names for states:
    R1: CLEANUP_WAIT (same as S8)
    R2: IN_CLEANUP
    R3: FREE (same as S1)
 Whenever a connection state machine in cleanup (let's call it CSM-C)
 enters the CLEANUP_WAIT state (S8), it must go through the state
 transitions described in the connection cleanup state diagram, using
 either a) a separate Full Feature Phase connection (let's call it
 CSM-E, for explicit) in the LOGGED_IN state in the same session or
 b) a new transport connection (let's call it CSM-I, for implicit) in
 the FREE state that is to be added to the same session.  In the CSM-E
 case, an explicit logout for the CID that corresponds to CSM-C (as
 either a connection or session logout) needs to be performed to
 complete the cleanup.  In the CSM-I case, an implicit logout for the
 CID that corresponds to CSM-C needs to be performed by way of
 connection reinstatement (Section 6.3.4) for that CID.  In either
 case, the protocol exchanges on CSM-E or CSM-I determine the state
 transitions for CSM-C.  Therefore, this cleanup state diagram is only
 applicable to the instance of the connection in cleanup (i.e.,
 CSM-C).  In the case of an implicit logout, for example, CSM-C

Chadalapaka, et al. Standards Track [Page 122] RFC 7143 iSCSI (Consolidated) April 2014

 reaches FREE (R3) at the time CSM-I reaches LOGGED_IN.  In the case
 of an explicit logout, CSM-C reaches FREE (R3) when CSM-E receives a
 successful Logout Response while continuing to be in the LOGGED_IN
 state.
 An initiator must initiate an explicit or implicit connection logout
 for a connection in the CLEANUP_WAIT state, if the initiator intends
 to continue using the associated iSCSI session.
 The following state diagram applies to both initiators and targets.
 (M1, M2, M3, and M4 are defined in Section 8.2.2.)
  1. ——–

/ R1 \

                    +---\         /<-+
                   /     ----+----    \
                  /          |         \ M3
               M1 |          |M2        |
                  |          |         /
                  |          |        /
                  |          |       /
                  |          V      /
                  |       ---------/
                  |      / R2      \
                  |      \         /
                  |       ---------
                  |          |
                  |          |M4
                  |          |
                  |          |
                  |          |
                  |          V
                  |       --------
                  |      / R3     \
                  +----->\        /
                          --------

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 The following state transition table represents the above diagram and
 follows the same conventions as in earlier sections.
      +----+----+----+
      |R1  |R2  |R3  |
 -----+----+----+----+
  R1  | -  |M2  |M1  |
 -----+----+----+----+
  R2  |M3  | -  |M4  |
 -----+----+----+----+
  R3  | -  | -  | -  |
 -----+----+----+----+

8.2.1. State Descriptions for Initiators and Targets

 R1: CLEANUP_WAIT (same as S8)
  1. initiator: Waiting for the internal event to initiate the

cleanup processing for CSM-C.

  1. target: Waiting for the cleanup process to start for CSM-C.
 R2: IN_CLEANUP
  1. initiator: Waiting for the connection cleanup process to

conclude for CSM-C.

  1. target: Waiting for the connection cleanup process to conclude

for CSM-C.

 R3: FREE (same as S1)
  1. initiator: End state for CSM-C.
  1. target: End state for CSM-C.

8.2.2. State Transition Descriptions for Initiators and Targets

 M1: One or more of the following events was received:
  1. initiator:
  • An internal event that indicates connection state timeout.
  • An internal event of receiving a successful Logout Response

on a different connection for a "close the session" Logout.

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  1. target:
  • An internal event that indicates connection state timeout.
  • An internal event of sending a Logout Response (success) on a

different connection for a "close the session" Logout

         Request.
 M2: An implicit/explicit logout process was initiated by the
     initiator.
  1. In CSM-I usage:
  • initiator: An internal event requesting the connection (or

session) reinstatement was received, thus prompting a

         connection (or session) reinstatement Login to be sent,
         transitioning CSM-I to state IN_LOGIN.
  • target: A connection/session reinstatement Login was received

while in state XPT_UP.

  1. In CSM-E usage:
  • initiator: An internal event was received that indicates that

an explicit logout was sent for this CID in state LOGGED_IN.

  • target: An explicit logout was received for this CID in state

LOGGED_IN.

 M3: Logout failure was detected.
  1. In CSM-I usage:
  • initiator: CSM-I failed to reach LOGGED_IN and arrived into

FREE instead.

  • target: CSM-I failed to reach LOGGED_IN and arrived into FREE

instead.

  1. In CSM-E usage:
  • initiator: either CSM-E moved out of LOGGED_IN, or Logout

timed out and/or aborted, or Logout Response (failure) was

         received.

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  • target: either CSM-E moved out of LOGGED_IN, Logout timed out

and/or aborted, or an internal event that indicates that a

         failed Logout processing was received.  A Logout Response
         (failure) was sent in the last case.
 M4: Successful implicit/explicit logout was performed.
  1. In CSM-I usage:
  • initiator: CSM-I reached state LOGGED_IN, or an internal

event of receiving a Logout Response (success) on another

         connection for a "close the session" Logout Request was
         received.
  • target: CSM-I reached state LOGGED_IN, or an internal event

of sending a Logout Response (success) on a different

         connection for a "close the session" Logout Request was
         received.
  1. In CSM-E usage:
  • initiator: CSM-E stayed in LOGGED_IN and received a Logout

Response (success), or an internal event of receiving a

         Logout Response (success) on another connection for a "close
         the session" Logout Request was received.
  • target: CSM-E stayed in LOGGED_IN and an internal event

indicating a successful Logout processing was received, or an

         internal event of sending a Logout Response (success) on a
         different connection for a "close the session" Logout Request
         was received.

8.3. Session State Diagrams

8.3.1. Session State Diagram for an Initiator

 Symbolic names for states:
    Q1: FREE
    Q3: LOGGED_IN
    Q4: FAILED
 State Q3 represents the Full Feature Phase operation of the session.

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 The state diagram is as follows.  (N1, N3, N4, N5, and N6 are defined
 in Section 8.3.4.)
  1. ——–

/ Q1 \

                    +---------->\         /<-+
                   /             ----+----   |
                  /                  |       |N3
              N6  |                  |N1     |
                  |                  |       |
                  |       N4         |       |
                  | +------------+   |      /
                  | |            |   |     /
                  | |            |   |    /
                  | |            V   V   /
                --+-+---         -------+-
               / Q4     \ N5    / Q3      \
               \        /<------\         /
                --------         ---------
 The state transition table is as follows:
      +---+---+---+
      |Q1 |Q3 |Q4 |
 -----+---+---+---+
  Q1  | - |N1 | - |
 -----+---+---+---+
  Q3  |N3 | - |N5 |
 -----+---+---+---+
  Q4  |N6 |N4 | - |
 -----+---+---+---+

8.3.2. Session State Diagram for a Target

 Symbolic names for states:
    Q1: FREE
    Q2: ACTIVE
    Q3: LOGGED_IN
    Q4: FAILED
    Q5: IN_CONTINUE
 State Q3 represents the Full Feature Phase operation of the session.

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 The state diagram is as follows:
  1. ——–

+——————→/ Q1 \

                  /     +-------------->\         /<-+
                  |     |                ---+-----   |
                  |     |                 ^ |        |N3
               N6 |     |N11            N9| V N1     |
                  |     |                 +--------  |
                  |     |                / Q2      \ |
                  |     |                \         / |
                  |  ---+-----            +--+-----  |
                  | / Q5      \              |       |
                  | \         / N10          |       |
                  |  -+-+----+-----------+   | N2   /
                  |   ^ |                |   |     /
                  | N7| |N8              |   |    /
                  |   | |                |   V   /
                --+---+-V                V------+-
               / Q4      \ N5           / Q3      \
               \         /<-------------\         /
                ---------                ---------
 The state transition table is as follows:
      +----+----+----+----+----+
      |Q1  |Q2  |Q3  |Q4  |Q5  |
 -----+----+----+----+----+----+
  Q1  | -  |N1  | -  | -  | -  |
 -----+----+----+----+----+----+
  Q2  |N9  | -  |N2  | -  | -  |
 -----+----+----+----+----+----+
  Q3  |N3  | -  | -  |N5  | -  |
 -----+----+----+----+----+----+
  Q4  |N6  | -  | -  | -  |N7  |
 -----+----+----+----+----+----+
  Q5  |N11 | -  |N10 |N8  | -  |
 -----+----+----+----+----+----+

Chadalapaka, et al. Standards Track [Page 128] RFC 7143 iSCSI (Consolidated) April 2014

8.3.3. State Descriptions for Initiators and Targets

 Q1: FREE
  1. initiator: State on instantiation or after cleanup.
  1. target: State on instantiation or after cleanup.
 Q2: ACTIVE
  1. initiator: Illegal.
  1. target: The first iSCSI connection in the session transitioned

to IN_LOGIN, waiting for it to complete the login process.

 Q3: LOGGED_IN
  1. initiator: Waiting for all session events.
  1. target: Waiting for all session events.
 Q4: FAILED
  1. initiator: Waiting for session recovery or session

continuation.

  1. target: Waiting for session recovery or session continuation.
 Q5: IN_CONTINUE
  1. initiator: Illegal.
  1. target: Waiting for session continuation attempt to reach a

conclusion.

8.3.4. State Transition Descriptions for Initiators and Targets

 N1:
  1. initiator: At least one transport connection reached the

LOGGED_IN state.

  1. target: The first iSCSI connection in the session had reached

the IN_LOGIN state.

Chadalapaka, et al. Standards Track [Page 129] RFC 7143 iSCSI (Consolidated) April 2014

 N2:
  1. initiator: Illegal.
  1. target: At least one iSCSI connection reached the LOGGED_IN

state.

 N3:
  1. initiator: Graceful closing of the session via session closure

(Section 6.3.6).

  1. target: Graceful closing of the session via session closure

(Section 6.3.6) or a successful session reinstatement cleanly

       closed the session.
 N4:
  1. initiator: A session continuation attempt succeeded.
  1. target: Illegal.
 N5:
     - initiator: Session failure (Section 6.3.6) occurred.
  1. target: Session failure (Section 6.3.6) occurred.
 N6:
  1. initiator: Session state timeout occurred, or a session

reinstatement cleared this session instance. This results in

       the freeing of all associated resources, and the session state
       is discarded.
  1. target: Session state timeout occurred, or a session

reinstatement cleared this session instance. This results in

       the freeing of all associated resources, and the session state
       is discarded.
 N7:
  1. initiator: Illegal.
  1. target: A session continuation attempt was initiated.

Chadalapaka, et al. Standards Track [Page 130] RFC 7143 iSCSI (Consolidated) April 2014

 N8:
  1. initiator: Illegal.
  1. target: The last session continuation attempt failed.
 N9:
  1. initiator: Illegal.
  1. target: Login attempt on the leading connection failed.
 N10:
  1. initiator: Illegal.
  1. target: A session continuation attempt succeeded.
 N11:
  1. initiator: Illegal.
  1. target: A successful session reinstatement cleanly closed the

session.

9. Security Considerations

 Historically, native storage systems have not had to consider
 security, because their environments offered minimal security risks.
 That is, these environments consisted of storage devices either
 directly attached to hosts or connected via a Storage Area Network
 (SAN) distinctly separate from the communications network.  The use
 of storage protocols, such as SCSI, over IP networks requires that
 security concerns be addressed.  iSCSI implementations must provide
 means of protection against active attacks (e.g., pretending to be
 another identity; message insertion, deletion, modification, and
 replaying) and passive attacks (e.g., eavesdropping, gaining
 advantage by analyzing the data sent over the line).
 Although technically possible, iSCSI SHOULD NOT be configured without
 security, specifically in-band authentication; see Section 9.2.
 iSCSI configured without security should be confined to closed
 environments that have very limited and well-controlled security
 risks.  [RFC3723] specifies the mechanisms that must be used in order
 to mitigate risks fully described in that document.
 The following section describes the security mechanisms provided by
 an iSCSI implementation.

Chadalapaka, et al. Standards Track [Page 131] RFC 7143 iSCSI (Consolidated) April 2014

9.1. iSCSI Security Mechanisms

 The entities involved in iSCSI security are the initiator, target,
 and the IP communication endpoints.  iSCSI scenarios in which
 multiple initiators or targets share a single communication endpoint
 are expected.  To accommodate such scenarios, iSCSI supports two
 separate security mechanisms: in-band authentication between the
 initiator and the target at the iSCSI connection level (carried out
 by exchange of iSCSI Login PDUs), and packet protection (integrity,
 authentication, and confidentiality) by IPsec at the IP level.  The
 two security mechanisms complement each other.  The in-band
 authentication provides end-to-end trust (at login time) between the
 iSCSI initiator and the target, while IPsec provides a secure channel
 between the IP communication endpoints.  iSCSI can be used to access
 sensitive information for which significant security protection is
 appropriate.  As further specified in the rest of this security
 considerations section, both iSCSI security mechanisms are mandatory
 to implement (MUST).  The use of in-band authentication is strongly
 recommended (SHOULD).  In contrast, the use of IPsec is optional
 (MAY), as the security risks that it addresses may only be present
 over a subset of the networks used by an iSCSI connection or a
 session; a specific example is that when an iSCSI session spans data
 centers, IPsec VPN gateways at the data center boundaries to protect
 the WAN connectivity between data centers may be appropriate in
 combination with in-band iSCSI authentication.
 Further details on typical iSCSI scenarios and the relationship
 between the initiators, targets, and the communication endpoints can
 be found in [RFC3723].

9.2. In-Band Initiator-Target Authentication

 During login, the target MAY authenticate the initiator and the
 initiator MAY authenticate the target.  The authentication is
 performed on every new iSCSI connection by an exchange of iSCSI Login
 PDUs using a negotiated authentication method.
 The authentication method cannot assume an underlying IPsec
 protection, because IPsec is optional to use.  An attacker should
 gain as little advantage as possible by inspecting the authentication
 phase PDUs.  Therefore, a method using cleartext (or equivalent)
 passwords MUST NOT be used; on the other hand, identity protection is
 not strictly required.
 The authentication mechanism protects against an unauthorized login
 to storage resources by using a false identity (spoofing).  Once the
 authentication phase is completed, if the underlying IPsec is not
 used, all PDUs are sent and received in the clear.  The

Chadalapaka, et al. Standards Track [Page 132] RFC 7143 iSCSI (Consolidated) April 2014

 authentication mechanism alone (without underlying IPsec) should only
 be used when there is no risk of eavesdropping or of message
 insertion, deletion, modification, and replaying.
 Section 12 defines several authentication methods and the exact steps
 that must be followed in each of them, including the iSCSI-text-keys
 and their allowed values in each step.  Whenever an iSCSI initiator
 gets a response whose keys, or their values, are not according to the
 step definition, it MUST abort the connection.
 Whenever an iSCSI target gets a request or response whose keys, or
 their values, are not according to the step definition, it MUST
 answer with a Login reject with the "Initiator Error" or "Missing
 Parameter" status.  These statuses are not intended for
 cryptographically incorrect values such as the CHAP response, for
 which the "Authentication Failure" status MUST be specified.  The
 importance of this rule can be illustrated in CHAP with target
 authentication (see Section 12.1.3), where the initiator would have
 been able to conduct a reflection attack by omitting its response key
 (CHAP_R), using the same CHAP challenge as the target and reflecting
 the target's response back to the target.  In CHAP, this is prevented
 because the target must answer the missing CHAP_R key with a
 Login reject with the "Missing Parameter" status.
 For some of the authentication methods, a key specifies the identity
 of the iSCSI initiator or target for authentication purposes.  The
 value associated with that key MAY be different from the iSCSI name
 and SHOULD be configurable (CHAP_N: see Section 12.1.3; SRP_U: see
 Section 12.1.2).  For this reason, iSCSI implementations SHOULD
 manage authentication in a way that impersonation across iSCSI names
 via these authentication identities is not possible.  Specifically,
 implementations SHOULD allow configuration of an authentication
 identity for a Name if different, and authentication credentials for
 that identity.  During the login time, implementations SHOULD verify
 the Name-to-identity relationship in addition to authenticating the
 identity through the negotiated authentication method.
 When an iSCSI session has multiple TCP connections, either
 concurrently or sequentially, the authentication method and
 identities should not vary among the connections.  Therefore, all
 connections in an iSCSI session SHOULD use the same authentication
 method, iSCSI name, and authentication identity (for authentication
 methods that use an authentication identity).  Implementations SHOULD
 check this and cause an authentication failure on a new connection
 that uses a different authentication method, iSCSI name, or
 authentication identity from those already used in the session.  In

Chadalapaka, et al. Standards Track [Page 133] RFC 7143 iSCSI (Consolidated) April 2014

 addition, implementations SHOULD NOT support both authenticated and
 unauthenticated TCP connections in the same iSCSI session, added
 either concurrently or sequentially to the session.

9.2.1. CHAP Considerations

 Compliant iSCSI initiators and targets MUST implement the CHAP
 authentication method [RFC1994] (according to Section 12.1.3,
 including the target authentication option).
 When CHAP is performed over a non-encrypted channel, it is vulnerable
 to an off-line dictionary attack.  Implementations MUST support the
 use of up to 128-bit random CHAP secrets, including the means to
 generate such secrets and to accept them from an external generation
 source.  Implementations MUST NOT provide secret generation (or
 expansion) means other than random generation.
 An administrative entity of an environment in which CHAP is used with
 a secret that has less than 96 random bits MUST enforce IPsec
 encryption (according to the implementation requirements in
 Section 9.3.2) to protect the connection.  Moreover, in this case,
 IKE authentication with group pre-shared cryptographic keys SHOULD
 NOT be used unless it is not essential to protect group members
 against off-line dictionary attacks by other members.
 CHAP secrets MUST be an integral number of bytes (octets).  A
 compliant implementation SHOULD NOT continue with the login step in
 which it should send a CHAP response (CHAP_R; see Section 12.1.3)
 unless it can verify that the CHAP secret is at least 96 bits or that
 IPsec encryption is being used to protect the connection.
 Any CHAP secret used for initiator authentication MUST NOT be
 configured for authentication of any target, and any CHAP secret used
 for target authentication MUST NOT be configured for authentication
 of any initiator.  If the CHAP response received by one end of an
 iSCSI connection is the same as the CHAP response that the receiving
 endpoint would have generated for the same CHAP challenge, the
 response MUST be treated as an authentication failure and cause the
 connection to close (this ensures that the same CHAP secret is not
 used for authentication in both directions).  Also, if an iSCSI
 implementation can function as both initiator and target, different
 CHAP secrets and identities MUST be configured for these two roles.
 The following is an example of the attacks prevented by the above
 requirements:
    a) "Rogue" wants to impersonate "Storage" to Alice and knows that
       a single secret is used for both directions of Storage-Alice
       authentication.

Chadalapaka, et al. Standards Track [Page 134] RFC 7143 iSCSI (Consolidated) April 2014

    b) Rogue convinces Alice to open two connections to itself and
       identifies itself as Storage on both connections.
    c) Rogue issues a CHAP challenge on Connection 1, waits for Alice
       to respond, and then reflects Alice's challenge as the initial
       challenge to Alice on Connection 2.
    d) If Alice doesn't check for the reflection across connections,
       Alice's response on Connection 2 enables Rogue to impersonate
       Storage on Connection 1, even though Rogue does not know the
       Alice-Storage CHAP secret.
 Originators MUST NOT reuse the CHAP challenge sent by the responder
 for the other direction of a bidirectional authentication.
 Responders MUST check for this condition and close the iSCSI TCP
 connection if it occurs.
 The same CHAP secret SHOULD NOT be configured for authentication of
 multiple initiators or multiple targets, as this enables any of them
 to impersonate any other one of them, and compromising one of them
 enables the attacker to impersonate any of them.  It is recommended
 that iSCSI implementations check for the use of identical CHAP
 secrets by different peers when this check is feasible and take
 appropriate measures to warn users and/or administrators when this is
 detected.
 When an iSCSI initiator or target authenticates itself to
 counterparts in multiple administrative domains, it SHOULD use a
 different CHAP secret for each administrative domain to avoid
 propagating security compromises across domains.
 Within a single administrative domain:
  1. A single CHAP secret MAY be used for authentication of an

initiator to multiple targets.

  1. A single CHAP secret MAY be used for an authentication of a

target to multiple initiators when the initiators use an

      external server (e.g., RADIUS [RFC2865]) to verify the target's
      CHAP responses and do not know the target's CHAP secret.
 If an external response verification server (e.g., RADIUS) is not
 used, employing a single CHAP secret for authentication of a target
 to multiple initiators requires that all such initiators know that
 target's secret.  Any of these initiators can impersonate the target
 to any other such initiator, and compromise of such an initiator
 enables an attacker to impersonate the target to all such initiators.
 Targets SHOULD use separate CHAP secrets for authentication to each

Chadalapaka, et al. Standards Track [Page 135] RFC 7143 iSCSI (Consolidated) April 2014

 initiator when such risks are of concern; in this situation, it may
 be useful to configure a separate logical iSCSI target with its own
 iSCSI Node Name for each initiator or group of initiators among which
 such separation is desired.
 The above requirements strengthen the security properties of CHAP
 authentication for iSCSI by comparison to the basic CHAP
 authentication mechanism [RFC1994].  It is very important to adhere
 to these requirements, especially the requirements for strong (large
 randomly generated) CHAP secrets, as iSCSI implementations and
 deployments that fail to use strong CHAP secrets are likely to be
 highly vulnerable to off-line dictionary attacks on CHAP secrets.
 Replacement of CHAP with a better authentication mechanism is
 anticipated in a future version of iSCSI.  The FC-SP-2 standard
 [FC-SP-2] has specified the Extensible Authentication Protocol -
 Generalized Pre-Shared Key (EAP-GPSK) authentication mechanism
 [RFC5433] as an alternative to (and possible future replacement for)
 Fibre Channel's similar usage of strengthened CHAP.  Another possible
 replacement for CHAP is a secure password mechanism, e.g., an updated
 version of iSCSI's current SRP authentication mechanism.

9.2.2. SRP Considerations

 The strength of the SRP authentication method (specified in
 [RFC2945]) is dependent on the characteristics of the group being
 used (i.e., the prime modulus N and generator g).  As described in
 [RFC2945], N is required to be a Sophie Germain prime (of the form
 N = 2q + 1, where q is also prime) and the generator g is a primitive
 root of GF(N).  In iSCSI authentication, the prime modulus N MUST be
 at least 768 bits.
 The list of allowed SRP groups is provided in [RFC3723].

9.2.3. Kerberos Considerations

 iSCSI uses raw Kerberos V5 [RFC4120] for authenticating a client
 (iSCSI initiator) principal to a service (iSCSI target) principal.
 Note that iSCSI does not use the Generic Security Service Application
 Program Interface (GSS-API) [RFC2743] or the Kerberos V5 GSS-API
 security mechanism [RFC4121].  This means that iSCSI implementations
 supporting the KRB5 AuthMethod (Section 12.1) are directly involved
 in the Kerberos protocol.  When Kerberos V5 is used for
 authentication, the following actions MUST be performed as specified
 in [RFC4120]:
  1. The target MUST validate KRB_AP_REQ to ensure that the initiator

can be trusted.

Chadalapaka, et al. Standards Track [Page 136] RFC 7143 iSCSI (Consolidated) April 2014

  1. When mutual authentication is selected, the initiator MUST

validate KRB_AP_REP to determine the outcome of mutual

      authentication.
 As Kerberos V5 is capable of providing mutual authentication,
 implementations SHOULD support mutual authentication by default for
 login authentication.
 Note, however, that Kerberos authentication only assures that the
 server (iSCSI target) can be trusted by the Kerberos client
 (initiator) and vice versa; an initiator should employ appropriately
 secured service discovery techniques (e.g., iSNS; see Section 4.2.7)
 to ensure that it is talking to the intended target principal.
 iSCSI does not use Kerberos v5 for either integrity or
 confidentiality protection of the iSCSI protocol.  iSCSI uses IPsec
 for those purposes as specified in Section 9.3.

9.3. IPsec

 iSCSI uses the IPsec mechanism for packet protection (cryptographic
 integrity, authentication, and confidentiality) at the IP level
 between the iSCSI communicating endpoints.  The following sections
 describe the IPsec protocols that must be implemented for data
 authentication and integrity; confidentiality; and cryptographic key
 management.
 An iSCSI initiator or target may provide the required IPsec support
 fully integrated or in conjunction with an IPsec front-end device.
 In the latter case, the compliance requirements with regard to IPsec
 support apply to the "combined device".  Only the "combined device"
 is to be considered an iSCSI device.
 Detailed considerations and recommendations for using IPsec for iSCSI
 are provided in [RFC3723] as updated by [RFC7146].  The IPsec
 requirements are reproduced here for convenience and are intended to
 match those in [RFC7146]; in the event of a discrepancy, the
 requirements in [RFC7146] apply.

9.3.1. Data Authentication and Integrity

 Data authentication and integrity are provided by a cryptographic
 keyed Message Authentication Code in every sent packet.  This code
 protects against message insertion, deletion, and modification.
 Protection against message replay is realized by using a sequence
 counter.

Chadalapaka, et al. Standards Track [Page 137] RFC 7143 iSCSI (Consolidated) April 2014

 An iSCSI-compliant initiator or target MUST provide data
 authentication and integrity by implementing IPsec v2 [RFC2401] with
 ESPv2 [RFC2406] in tunnel mode, SHOULD provide data authentication
 and integrity by implementing IPsec v3 [RFC4301] with ESPv3 [RFC4303]
 in tunnel mode, and MAY provide data authentication and integrity by
 implementing either IPsec v2 or v3 with the appropriate version of
 ESP in transport mode.  The IPsec implementation MUST fulfill the
 following iSCSI-specific requirements:
  1. HMAC-SHA1 MUST be implemented in the specific form of

HMAC-SHA-1-96 [RFC2404].

  1. AES CBC MAC with XCBC extensions using 128-bit keys SHOULD be

implemented [RFC3566].

  1. Implementations that support IKEv2 [RFC5996] SHOULD also

implement AES Galois Message Authentication Code (GMAC)

      [RFC4543] using 128-bit keys.
 The ESP anti-replay service MUST also be implemented.
 At the high speeds at which iSCSI is expected to operate, a single
 IPsec SA could rapidly exhaust the ESP 32-bit sequence number space,
 requiring frequent rekeying of the SA, as rollover of the ESP
 sequence number within a single SA is prohibited for both ESPv2
 [RFC2406] and ESPv3 [RFC4303].  In order to provide the means to
 avoid this potentially undesirable frequent rekeying, implementations
 that are capable of operating at speeds of 1 gigabit/second or higher
 MUST implement extended (64-bit) sequence numbers for ESPv2 (and
 ESPv3, if supported) and SHOULD use extended sequence numbers for all
 iSCSI traffic.  Extended sequence number negotiation as part of
 security association establishment is specified in [RFC4304] for
 IKEv1 and [RFC5996] for IKEv2.

9.3.2. Confidentiality

 Confidentiality is provided by encrypting the data in every packet.
 When confidentiality is used, it MUST be accompanied by data
 authentication and integrity to provide comprehensive protection
 against eavesdropping and against message insertion, deletion,
 modification, and replaying.
 An iSCSI-compliant initiator or target MUST provide confidentiality
 by implementing IPsec v2 [RFC2401] with ESPv2 [RFC2406] in tunnel
 mode, SHOULD provide confidentiality by implementing IPsec v3
 [RFC4301] with ESPv3 [RFC4303] in tunnel mode, and MAY provide

Chadalapaka, et al. Standards Track [Page 138] RFC 7143 iSCSI (Consolidated) April 2014

 confidentiality by implementing either IPsec v2 or v3 with the
 appropriate version of ESP in transport mode, with the following
 iSCSI-specific requirements that apply to IPsec v2 and IPsec v3:
  1. 3DES in CBC mode MAY be implemented [RFC2451].
  1. AES in CBC mode with 128-bit keys MUST be implemented [RFC3602];

other key sizes MAY be supported.

  1. AES in Counter mode MAY be implemented [RFC3686].
  1. Implementations that support IKEv2 [RFC5996] SHOULD also

implement AES Galois/Counter Mode (GCM) with 128-bit keys

      [RFC4106]; other key sizes MAY be supported.
 Due to its inherent weakness, DES in CBC mode MUST NOT be used.
 The NULL encryption algorithm MUST also be implemented.

9.3.3. Policy, Security Associations, and Cryptographic Key Management

 A compliant iSCSI implementation MUST meet the cryptographic key
 management requirements of the IPsec protocol suite.  Authentication,
 security association negotiation, and cryptographic key management
 MUST be provided by implementing IKE [RFC2409] using the IPsec DOI
 [RFC2407] and SHOULD be provided by implementing IKEv2 [RFC5996],
 with the following iSCSI-specific requirements:
    a) Peer authentication using a pre-shared cryptographic key MUST
       be supported.  Certificate-based peer authentication using
       digital signatures MAY be supported.  For IKEv1 ([RFC2409]),
       peer authentication using the public key encryption methods
       outlined in Sections 5.2 and 5.3 of [RFC2409] SHOULD NOT be
       used.
    b) When digital signatures are used to achieve authentication, an
       IKE negotiator SHOULD use IKE Certificate Request Payload(s) to
       specify the certificate authority.  IKE negotiators SHOULD
       check certificate validity via the pertinent Certificate
       Revocation List (CRL) or via the use of the Online Certificate
       Status Protocol (OCSP) [RFC6960] before accepting a PKI
       certificate for use in IKE authentication procedures.  OCSP
       support within the IKEv2 protocol is specified in [RFC4806].
       These checks may not be needed in environments where a small
       number of certificates are statically configured as trust
       anchors.

Chadalapaka, et al. Standards Track [Page 139] RFC 7143 iSCSI (Consolidated) April 2014

    c) Conformant iSCSI implementations of IKEv1 MUST support Main
       Mode and SHOULD support Aggressive Mode.  Main Mode with a
       pre-shared key authentication method SHOULD NOT be used when
       either the initiator or the target uses dynamically assigned
       addresses.  While in many cases pre-shared keys offer good
       security, situations in which dynamically assigned addresses
       are used force the use of a group pre-shared key, which creates
       vulnerability to a man-in-the-middle attack.
    d) In the IKEv1 Phase 2 Quick Mode, in exchanges for creating the
       Phase 2 SA, the Identification Payload MUST be present.
    e) The following identification type requirements apply to IKEv1:
       ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack supports
       IPv6), and ID_FQDN Identification Types MUST be supported;
       ID_USER_FQDN SHOULD be supported.  The IP Subnet, IP Address
       Range, ID_DER_ASN1_DN, and ID_DER_ASN1_GN Identification Types
       SHOULD NOT be used.  The ID_KEY_ID Identification Type MUST NOT
       be used.
    f) If IKEv2 is supported, the following identification
       requirements apply:  ID_IPV4_ADDR, ID_IPV6_ADDR (if the
       protocol stack supports IPv6), and ID_FQDN Identification Types
       MUST be supported; ID_RFC822_ADDR SHOULD be supported.  The
       ID_DER_ASN1_DN and ID_DER_ASN1_GN Identification Types SHOULD
       NOT be used.  The ID_KEY_ID Identification Type MUST NOT be
       used.
 The reasons for the "MUST NOT" and "SHOULD NOT" for identification
 type requirements in preceding bullets e) and f) are:
  1. IP Subnet and IP Address Range are too broad to usefully

identify an iSCSI endpoint.

  1. The DN and GN types are X.500 identities; it is usually better

to use an identity from subjectAltName in a PKI certificate.

  1. ID_KEY_ID is not interoperable as specified.
 Manual cryptographic keying MUST NOT be used, because it does not
 provide the necessary rekeying support.
 When Diffie-Hellman (DH) groups are used, a DH group of at least
 2048 bits SHOULD be offered as a part of all proposals to create
 IPsec security associations to protect iSCSI traffic, with both IKEv1
 and IKEv2.

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 When IPsec is used, the receipt of an IKEv1 Phase 2 delete message or
 an IKEv2 INFORMATIONAL exchange that deletes the SA SHOULD NOT be
 interpreted as a reason for tearing down the iSCSI TCP connection.
 If additional traffic is sent on it, a new IKE SA will be created to
 protect it.
 The method used by the initiator to determine whether the target
 should be connected using IPsec is regarded as an issue of IPsec
 policy administration and thus not defined in the iSCSI standard.
 The method used by an initiator that supports both IPsec v2 and v3 to
 determine which versions of IPsec are supported by the target is also
 regarded as an issue of IPsec policy administration and thus not
 defined in the iSCSI standard.  If both IPsec v2 and v3 are supported
 by both the initiator and target, the use of IPsec v3 is recommended.
 If an iSCSI target is discovered via a SendTargets request in a
 Discovery session not using IPsec, the initiator should assume that
 it does not need IPsec to establish a session to that target.  If an
 iSCSI target is discovered using a Discovery session that does use
 IPsec, the initiator SHOULD use IPsec when establishing a session to
 that target.

9.4. Security Considerations for the X#NodeArchitecture Key

 The security considerations in this section are specific to the
 X#NodeArchitecture discussed in Section 13.26.
 This extension key transmits specific implementation details about
 the node that sends it; such details may be considered sensitive in
 some environments.  For example, if a certain software or firmware
 version is known to contain security weaknesses, announcing the
 presence of that version via this key may not be desirable.  The
 countermeasures for this security concern are:
    a) sending less detailed information in the key values,
    b) not sending the extension key, or
    c) using IPsec ([RFC4303]) to provide confidentiality for the
       iSCSI connection on which the key is sent.
 To support the first and second countermeasures, all implementations
 of this extension key MUST provide an administrative mechanism to
 disable sending the key.  In addition, all implementations SHOULD
 provide an administrative mechanism to configure a verbosity level of
 the key value, thereby controlling the amount of information sent.

Chadalapaka, et al. Standards Track [Page 141] RFC 7143 iSCSI (Consolidated) April 2014

 For example, a lower verbosity level might enable transmission of
 node architecture component names only, but no version numbers.  The
 choice of which countermeasure is most appropriate depends on the
 environment.  However, sending less detailed information in the key
 values may be an acceptable countermeasure in many environments,
 since it provides a compromise between sending too much information
 and the other more complete countermeasures of not sending the key at
 all or using IPsec.
 In addition to security considerations involving transmission of the
 key contents, any logging method(s) used for the key values MUST keep
 the information secure from intruders.  For all implementations, the
 requirements to address this security concern are as follows:
    a) Display of the log MUST only be possible with administrative
       rights to the node.
    b) Options to disable logging to disk and to keep logs for a fixed
       duration SHOULD be provided.
 Finally, it is important to note that different nodes may have
 different levels of risk, and these differences may affect the
 implementation.  The components of risk include assets, threats, and
 vulnerabilities.  Consider the following example iSCSI nodes, which
 demonstrate differences in assets and vulnerabilities of the nodes,
 and, as a result, differences in implementation:
    a) One iSCSI target based on a special-purpose operating system:
       Since the iSCSI target controls access to the data storage
       containing company assets, the asset level is seen as very
       high.  Also, because of the special-purpose operating system,
       in which vulnerabilities are less well known, the vulnerability
       level is viewed as low.
    b) Multiple iSCSI initiators in a blade farm, each running a
       general-purpose operating system: The asset level of each node
       is viewed as low, since blades are replaceable and low cost.
       However, the vulnerability level is viewed as high, since there
       may be many well-known vulnerabilities to that general-purpose
       operating system.  For this target, an appropriate
       implementation might be the logging of received key values but
       no transmission of the key.  For this initiator, an appropriate
       implementation might be transmission of the key but no logging
       of received key values.

Chadalapaka, et al. Standards Track [Page 142] RFC 7143 iSCSI (Consolidated) April 2014

9.5. SCSI Access Control Considerations

 iSCSI is a SCSI transport protocol and as such does not apply any
 access controls on SCSI-level operations such as SCSI task management
 functions (e.g., LU reset; see Section 11.5.1).  SCSI-level access
 controls (e.g., ACCESS CONTROL OUT; see [SPC3]) have to be
 appropriately deployed in practice to address SCSI-level security
 considerations, in addition to security via iSCSI connection and
 packet protection mechanisms that were already discussed in preceding
 sections.

10. Notes to Implementers

 This section notes some of the performance and reliability
 considerations of the iSCSI protocol.  This protocol was designed to
 allow efficient silicon and software implementations.  The iSCSI task
 tag mechanism was designed to enable Direct Data Placement (DDP -- a
 DMA form) at the iSCSI level or lower.
 The guiding assumption made throughout the design of this protocol is
 that targets are resource constrained relative to initiators.
 Implementers are also advised to consider the implementation
 consequences of the iSCSI-to-SCSI mapping model as outlined in
 Section 4.4.3.

10.1. Multiple Network Adapters

 The iSCSI protocol allows multiple connections, not all of which need
 to go over the same network adapter.  If multiple network connections
 are to be utilized with hardware support, the iSCSI protocol command-
 data-status allegiance to one TCP connection ensures that there is no
 need to replicate information across network adapters or otherwise
 require them to cooperate.
 However, some task management commands may require some loose form of
 cooperation or replication at least on the target.

10.1.1. Conservative Reuse of ISIDs

 Historically, the SCSI model (and implementations and applications
 based on that model) has assumed that SCSI ports are static, physical
 entities.  Recent extensions to the SCSI model have taken advantage
 of persistent worldwide unique names for these ports.  In iSCSI,
 however, the SCSI initiator ports are the endpoints of dynamically
 created sessions, so the presumptions of "static and physical" do not
 apply.  In any case, the "model" sections (particularly,

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 Section 4.4.1) provide for persistent, reusable names for the
 iSCSI-type SCSI initiator ports even though there does not need to be
 any physical entity bound to these names.
 To both minimize the disruption of legacy applications and better
 facilitate the SCSI features that rely on persistent names for SCSI
 ports, iSCSI implementations SHOULD attempt to provide a stable
 presentation of SCSI initiator ports (both to the upper OS layers and
 the targets to which they connect).  This can be achieved in an
 initiator implementation by conservatively reusing ISIDs.  In other
 words, the same ISID should be used in the login process to multiple
 target portal groups (of the same iSCSI target or different iSCSI
 targets).  The ISID RULE (Section 4.4.3) only prohibits reuse to the
 same target portal group.  It does not "preclude" reuse to other
 target portal groups.  The principle of conservative reuse
 "encourages" reuse to other target portal groups.  When a SCSI target
 device sees the same (InitiatorName, ISID) pair in different sessions
 to different target portal groups, it can identify the underlying
 SCSI initiator port on each session as the same SCSI port.  In
 effect, it can recognize multiple paths from the same source.

10.1.2. iSCSI Name, ISID, and TPGT Use

 The designers of the iSCSI protocol are aware that legacy SCSI
 transports rely on initiator identity to assign access to storage
 resources.  Although newer techniques that simplify access control
 are available, support for configuration and authentication schemes
 that are based on initiator identity is deemed important in order to
 support legacy systems and administration software.  iSCSI thus
 supports the notion that it should be possible to assign access to
 storage resources based on "initiator device" identity.
 When there are multiple hardware or software components coordinated
 as a single iSCSI node, there must be some (logical) entity that
 represents the iSCSI node that makes the iSCSI Node Name available to
 all components involved in session creation and login.  Similarly,
 this entity that represents the iSCSI node must be able to coordinate
 session identifier resources (the ISID for initiators) to enforce
 both the ISID RULE and the TSIH RULE (see Section 4.4.3).
 For targets, because of the closed environment, implementation of
 this entity should be straightforward.  However, vendors of iSCSI
 hardware (e.g., NICs or HBAs) intended for targets SHOULD provide
 mechanisms for configuration of the iSCSI Node Name across the portal
 groups instantiated by multiple instances of these components within
 a target.

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 However, complex targets making use of multiple Target Portal Group
 Tags may reconfigure them to achieve various quality goals.  The
 initiators have two mechanisms at their disposal to discover and/or
 check reconfiguring targets -- the Discovery session type and a key
 returned by the target during login to confirm the TPGT.  An
 initiator should attempt to "rediscover" the target configuration
 whenever a session is terminated unexpectedly.
 For initiators, in the long term, it is expected that operating
 system vendors will take on the role of this entity and provide
 standard APIs that can inform components of their iSCSI Node Name and
 can configure and/or coordinate ISID allocation, use, and reuse.
 Recognizing that such initiator APIs are not available today, other
 implementations of the role of this entity are possible.  For
 example, a human may instantiate the (common) node name as part of
 the installation process of each iSCSI component involved in session
 creation and login.  This may be done by pointing the component to
 either a vendor-specific location for this datum or a system-wide
 location.  The structure of the ISID namespace (see Section 11.12.5
 and [RFC3721]) facilitates implementation of the ISID coordination by
 allowing each component vendor to independently (of other vendor's
 components) coordinate allocation, use, and reuse of its own
 partition of the ISID namespace in a vendor-specific manner.
 Partitioning of the ISID namespace within initiator portal groups
 managed by that vendor allows each such initiator portal group to act
 independently of all other portal groups when selecting an ISID for a
 login; this facilitates enforcement of the ISID RULE (see
 Section 4.4.3) at the initiator.
 A vendor of iSCSI hardware (e.g., NICs or HBAs) intended for use in
 initiators MUST implement a mechanism for configuring the iSCSI Node
 Name.  Vendors and administrators must ensure that iSCSI Node Names
 are worldwide unique.  It is therefore important that when one
 chooses to reuse the iSCSI Node Name of a disabled unit one does not
 reassign that name to the original unit unless its worldwide
 uniqueness can be ascertained again.
 In addition, a vendor of iSCSI hardware must implement a mechanism to
 configure and/or coordinate ISIDs for all sessions managed by
 multiple instances of that hardware within a given iSCSI node.  Such
 configuration might be either permanently preassigned at the factory
 (in a necessarily globally unique way), statically assigned (e.g.,
 partitioned across all the NICs at initialization in a locally unique
 way), or dynamically assigned (e.g., on-line allocator, also in a
 locally unique way).  In the latter two cases, the configuration may

Chadalapaka, et al. Standards Track [Page 145] RFC 7143 iSCSI (Consolidated) April 2014

 be via public APIs (perhaps driven by an independent vendor's
 software, such as the OS vendor) or private APIs driven by the
 vendor's own software.
 The process of name assignment and coordination has to be as
 encompassing and automated as possible, as years of legacy usage have
 shown that it is highly error-prone.  It should be mentioned that
 today SCSI has alternative schemes of access control that can be used
 by all transports, and their security is not dependent on strict
 naming coordination.

10.2. Autosense and Auto Contingent Allegiance (ACA)

 "Autosense" refers to the automatic return of sense data to the
 initiator in cases where a command did not complete successfully.
 iSCSI initiators and targets MUST support and use Autosense.
 ACA helps preserve ordered command execution in the presence of
 errors.  As there can be many commands in-flight between an initiator
 and a target, SCSI initiator functionality in some operating systems
 depends on ACA to enforce ordered command execution during error
 recovery, and hence iSCSI initiator implementations for those
 operating systems need to support ACA.  In order to support error
 recovery for these operating systems and iSCSI initiators, iSCSI
 targets SHOULD support ACA.

10.3. iSCSI Timeouts

 iSCSI recovery actions are often dependent on iSCSI timeouts being
 recognized and acted upon before SCSI timeouts.  Determining the
 right timeouts to use for various iSCSI actions (command
 acknowledgments expected, status acknowledgments, etc.) is very much
 dependent on infrastructure (e.g., hardware, links, TCP/IP stack,
 iSCSI driver).  As a guide, the implementer may use an average
 NOP-Out/NOP-In turnaround delay multiplied by a "safety factor"
 (e.g., 4) as a good estimate for the basic delay of the iSCSI stack
 for a given connection.  The safety factor should account for network
 load variability.  For connection teardown, the implementer may want
 to also consider TCP common practice for the given infrastructure.
 Text negotiations MAY also be subject to either time limits or limits
 in the number of exchanges.  Those limits SHOULD be generous enough
 to avoid affecting interoperability (e.g., allowing each key to be
 negotiated on a separate exchange).
 The relationship between iSCSI timeouts and SCSI timeouts should also
 be considered.  SCSI timeouts should be longer than iSCSI timeouts
 plus the time required for iSCSI recovery whenever iSCSI recovery is

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 planned.  Alternatively, an implementer may choose to interlock iSCSI
 timeouts and recovery with SCSI timeouts so that SCSI recovery will
 become active only where iSCSI is not planned to, or failed to,
 recover.
 The implementer may also want to consider the interaction between
 various iSCSI exception events -- such as a digest failure -- and
 subsequent timeouts.  When iSCSI error recovery is active, a digest
 failure is likely to result in discovering a missing command or data
 PDU.  In these cases, an implementer may want to lower the timeout
 values to enable faster initiation for recovery procedures.

10.4. Command Retry and Cleaning Old Command Instances

 To avoid having old, retried command instances appear in a valid
 command window after a command sequence number wraparound, the
 protocol requires (see Section 4.2.2.1) that on every connection on
 which a retry has been issued a non-immediate command be issued and
 acknowledged within an interval of 2**31 - 1 commands from the CmdSN
 of the retried command.  This requirement can be fulfilled by an
 implementation in several ways.
 The simplest technique to use is to send a (non-retry) non-immediate
 SCSI command (or a NOP if no SCSI command is available for a while)
 after every command retry on the connection on which the retry was
 attempted.  Because errors are deemed rare events, this technique is
 probably the most effective, as it does not involve additional checks
 at the initiator when issuing commands.

10.5. Sync and Steering Layer, and Performance

 While a Sync and Steering layer is optional, an initiator/target that
 does not have it working against a target/initiator that demands sync
 and steering may experience performance degradation caused by packet
 reordering and loss.  Providing a sync and steering mechanism is
 recommended for all high-speed implementations.

10.6. Considerations for State-Dependent Devices and Long-Lasting SCSI

     Operations
 Sequential access devices operate on the principle that the position
 of the device is based on the last command processed.  As such,
 command processing order, and knowledge of whether or not the
 previous command was processed, are of the utmost importance to
 maintain data integrity.  For example, inadvertent retries of SCSI
 commands when it is not known if the previous SCSI command was
 processed is a potential data integrity risk.

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 For a sequential access device, consider the scenario in which a SCSI
 SPACE command to backspace one filemark is issued and then reissued
 due to no status received for the command.  If the first SPACE
 command was actually processed, the reissued SPACE command, if
 processed, will cause the position to change.  Thus, a subsequent
 write operation will write data to the wrong position, and any
 previous data at that position will be overwritten.
 For a medium changer device, consider the scenario in which an
 EXCHANGE MEDIUM command (the SOURCE ADDRESS and DESTINATION ADDRESS
 are the same, thus performing a swap) is issued and then reissued due
 to no status received for the command.  If the first EXCHANGE MEDIUM
 command was actually processed, the reissued EXCHANGE MEDIUM command,
 if processed, will perform the swap again.  The net effect is that no
 swap was performed, thus putting data integrity at risk.
 All commands that change the state of the device (e.g., SPACE
 commands for sequential access devices and EXCHANGE MEDIUM commands
 for medium changer devices) MUST be issued as non-immediate commands
 for deterministic and ordered delivery to iSCSI targets.
 For many of those state-changing commands, the execution model also
 assumes that the command is executed exactly once.  Devices
 implementing READ POSITION and LOCATE provide a means for SCSI-level
 command recovery, and new tape-class devices should support those
 commands.  In their absence, a retry at the SCSI level is difficult,
 and error recovery at the iSCSI level is advisable.
 Devices operating on long-latency delivery subsystems and performing
 long-lasting SCSI operations may need mechanisms that enable
 connection replacement while commands are running (e.g., during an
 extended copy operation).

10.6.1. Determining the Proper ErrorRecoveryLevel

 The implementation and use of a specific ErrorRecoveryLevel should be
 determined based on the deployment scenarios of a given iSCSI
 implementation.  Generally, the following factors must be considered
 before deciding on the proper level of recovery:
    a) Application resilience to I/O failures.
    b) Required level of availability in the face of transport
       connection failures.

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    c) Probability of transport-layer "checksum escape" (message error
       undetected by TCP checksum -- see [RFC3385] for related
       discussion).  This in turn decides the iSCSI digest failure
       frequency and thus the criticality of iSCSI-level error
       recovery.  The details of estimating this probability are
       outside the scope of this document.
 A consideration of the above factors for SCSI tape devices as an
 example suggests that implementations SHOULD use ErrorRecoveryLevel=1
 when transport connection failure is not a concern and SCSI-level
 recovery is unavailable, and ErrorRecoveryLevel=2 when there is a
 high likelihood of connection failure during a backup/retrieval.
 For extended copy operations, implementations SHOULD use
 ErrorRecoveryLevel=2 whenever there is a relatively high likelihood
 of connection failure.

10.7. Multi-Task Abort Implementation Considerations

 Multi-task abort operations are typically issued in emergencies, such
 as clearing a device lock-up, HA failover/failback, etc.  In these
 circumstances, it is desirable to rapidly go through the error-
 handling process as opposed to the target waiting on multiple third-
 party initiators that may not even be functional anymore --
 especially if this emergency is triggered because of one such
 initiator failure.  Therefore, both iSCSI target and initiator
 implementations SHOULD support FastAbort multi-task abort semantics
 (Section 4.2.3.4).
 Note that in both standard semantics (Section 4.2.3.3) and FastAbort
 semantics (Section 4.2.3.4) there may be outstanding data transfers
 even after the TMF completion is reported on the issuing session.  In
 the case of iSCSI/iSER [RFC7145], these would be tagged data
 transfers for STags not owned by any active tasks.  Whether or not
 real buffers support these data transfers is implementation
 dependent.  However, the data transfers logically MUST be silently
 discarded by the target iSCSI layer in all cases.  A target MAY, on
 an implementation-defined internal timeout, also choose to drop the
 connections on which it did not receive the expected Data-Out
 sequences (Section 4.2.3.3) or NOP-Out acknowledgments
 (Section 4.2.3.4) so as to reclaim the associated buffer, STag, and
 TTT resources as appropriate.

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11. iSCSI PDU Formats

 All multi-byte integers that are specified in formats defined in this
 document are to be represented in network byte order (i.e.,
 big-endian).  Any field that appears in this document assumes that
 the most significant byte is the lowest numbered byte and the most
 significant bit (within byte or field) is the lowest numbered bit
 unless specified otherwise.
 Any compliant sender MUST set all bits not defined and all reserved
 fields to 0, unless specified otherwise.  Any compliant receiver MUST
 ignore any bit not defined and all reserved fields unless specified
 otherwise.  Receipt of reserved code values in defined fields MUST be
 reported as a protocol error.
 Reserved fields are marked by the word "reserved", some abbreviation
 of "reserved", or by "." for individual bits when no other form of
 marking is technically feasible.

11.1. iSCSI PDU Length and Padding

 iSCSI PDUs are padded to the closest integer number of 4-byte words.
 The padding bytes SHOULD be sent as 0.

11.2. PDU Template, Header, and Opcodes

 All iSCSI PDUs have one or more header segments and, optionally, a
 data segment.  After the entire header segment group, a header digest
 MAY follow.  The data segment MAY also be followed by a data digest.
 The Basic Header Segment (BHS) is the first segment in all of the
 iSCSI PDUs.  The BHS is a fixed-length 48-byte header segment.  It
 MAY be followed by Additional Header Segments (AHS), a Header-Digest,
 a Data Segment, and/or a Data-Digest.

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 The overall structure of an iSCSI PDU is as follows:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0/ Basic Header Segment (BHS)                                    /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48/ Additional Header Segment 1 (AHS) (optional)                  /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   / Additional Header Segment 2 (AHS) (optional)                  /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   +---------------+---------------+---------------+---------------+
   / Additional Header Segment n (AHS) (optional)                  /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  k/ Header-Digest (optional)                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  l/ Data Segment (optional)                                       /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  m/ Data-Digest (optional)                                        /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 All PDU segments and digests are padded to the closest integer number
 of 4-byte words.  For example, all PDU segments and digests start at
 a 4-byte word boundary, and the padding ranges from 0 to 3 bytes.
 The padding bytes SHOULD be sent as 0.
 iSCSI Response PDUs do not have AH Segments.

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11.2.1. Basic Header Segment (BHS)

 The BHS is 48 bytes long.  The Opcode and DataSegmentLength fields
 appear in all iSCSI PDUs.  In addition, when used, the Initiator Task
 Tag and Logical Unit Number always appear in the same location in the
 header.
 The format of the BHS is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| Opcode    |F| Opcode-specific fields                      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Opcode-specific fields                                 |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20/ Opcode-specific fields                                        /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48

11.2.1.1. I (Immediate) Bit

 For Request PDUs, the I bit set to 1 is an immediate delivery marker.

11.2.1.2. Opcode

 The Opcode indicates the type of iSCSI PDU the header encapsulates.
 The Opcodes are divided into two categories: initiator Opcodes and
 target Opcodes.  Initiator Opcodes are in PDUs sent by the initiator
 (Request PDUs).  Target Opcodes are in PDUs sent by the target
 (Response PDUs).
 Initiators MUST NOT use target Opcodes, and targets MUST NOT use
 initiator Opcodes.

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 Initiator Opcodes defined in this specification are:
    0x00 NOP-Out
    0x01 SCSI Command (encapsulates a SCSI Command Descriptor
         Block)
    0x02 SCSI Task Management Function Request
    0x03 Login Request
    0x04 Text Request
    0x05 SCSI Data-Out (for write operations)
    0x06 Logout Request
    0x10 SNACK Request
    0x1c-0x1e Vendor-specific codes
 Target Opcodes are:
    0x20 NOP-In
    0x21 SCSI Response - contains SCSI status and possibly sense
         information or other response information
    0x22 SCSI Task Management Function Response
    0x23 Login Response
    0x24 Text Response
    0x25 SCSI Data-In (for read operations)
    0x26 Logout Response
    0x31 Ready To Transfer (R2T) - sent by target when it is ready
         to receive data
    0x32 Asynchronous Message - sent by target to indicate certain
         special conditions
    0x3c-0x3e Vendor-specific codes
    0x3f Reject

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 All other Opcodes are unassigned.

11.2.1.3. F (Final) Bit

 When set to 1 it indicates the final (or only) PDU of a sequence.

11.2.1.4. Opcode-Specific Fields

 These fields have different meanings for different Opcode types.

11.2.1.5. TotalAHSLength

 This is the total length of all AHS header segments in units of
 4-byte words, including padding, if any.
 The TotalAHSLength is only used in PDUs that have an AHS and MUST be
 0 in all other PDUs.

11.2.1.6. DataSegmentLength

 This is the data segment payload length in bytes (excluding padding).
 The DataSegmentLength MUST be 0 whenever the PDU has no data segment.

11.2.1.7. LUN

 Some Opcodes operate on a specific LU.  The Logical Unit Number (LUN)
 field identifies which LU.  If the Opcode does not relate to a LU,
 this field is either ignored or may be used in an Opcode-specific
 way.  The LUN field is 64 bits and should be formatted in accordance
 with [SAM2].  For example, LUN[0] from [SAM2] is BHS byte 8 and so on
 up to LUN[7] from [SAM2], which is BHS byte 15.

11.2.1.8. Initiator Task Tag

 The initiator assigns a task tag to each iSCSI task it issues.  While
 a task exists, this tag MUST uniquely identify the task session-wide.
 SCSI may also use the Initiator Task Tag as part of the SCSI task
 identifier when the timespan during which an iSCSI Initiator Task Tag
 must be unique extends over the timespan during which a SCSI task tag
 must be unique.  However, the iSCSI Initiator Task Tag must exist and
 be unique even for untagged SCSI commands.
 An ITT value of 0xffffffff is reserved and MUST NOT be assigned for a
 task by the initiator.  The only instance in which it may be seen on
 the wire is in a target-initiated NOP-In PDU (Section 11.19) and in
 the initiator response to that PDU, if necessary.

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11.2.2. Additional Header Segment (AHS)

 The general format of an AHS is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0| AHSLength                     | AHSType       | AHS-Specific  |
   +---------------+---------------+---------------+---------------+
  4/ AHS-Specific                                                  /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  x

11.2.2.1. AHSType

 The AHSType field is coded as follows:
    bit 0-1 - Reserved
    bit 2-7 - AHS code
    0 - Reserved
    1 - Extended CDB
    2 - Bidirectional Read Expected Data Transfer Length
    3 - 63 Reserved

11.2.2.2. AHSLength

 This field contains the effective length in bytes of the AHS,
 excluding AHSType and AHSLength and padding, if any.  The AHS is
 padded to the smallest integer number of 4-byte words (i.e., from 0
 up to 3 padding bytes).

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11.2.2.3. Extended CDB AHS

 The format of the Extended CDB AHS is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0| AHSLength (CDBLength - 15)    | 0x01          |  Reserved     |
   +---------------+---------------+---------------+---------------+
  4/ ExtendedCDB...+padding                                        /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  x
 This type of AHS MUST NOT be used if the CDBLength is less than 17.
 The length includes the reserved byte 3.

11.2.2.4. Bidirectional Read Expected Data Transfer Length AHS

 The format of the Bidirectional Read Expected Data Transfer Length
 AHS is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0| AHSLength (0x0005)            | 0x02          | Reserved      |
   +---------------+---------------+---------------+---------------+
  4| Bidirectional Read Expected Data Transfer Length              |
   +---------------+---------------+---------------+---------------+
  8

11.2.3. Header Digest and Data Digest

 Optional header and data digests protect the integrity of the header
 and data, respectively.  The digests, if present, are located,
 respectively, after the header and PDU-specific data and cover,
 respectively, the header and the PDU data, each including the padding
 bytes, if any.
 The existence and type of digests are negotiated during the Login
 Phase.

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 The separation of the header and data digests is useful in iSCSI
 routing applications, in which only the header changes when a message
 is forwarded.  In this case, only the header digest should be
 recalculated.
 Digests are not included in data or header length fields.
 A zero-length Data Segment also implies a zero-length Data-Digest.

11.2.4. Data Segment

 The (optional) Data Segment contains PDU-associated data.  Its
 payload effective length is provided in the BHS field --
 DataSegmentLength.  The Data Segment is also padded to an integer
 number of 4-byte words.

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11.3. SCSI Command

 The format of the SCSI Command PDU is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| 0x01      |F|R|W|. .|ATTR | Reserved                      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| Logical Unit Number (LUN)                                     |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Expected Data Transfer Length                                 |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32/ SCSI Command Descriptor Block (CDB)                           /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48/ AHS (optional)                                                /
   +---------------+---------------+---------------+---------------+
  x/ Header-Digest (optional)                                      /
   +---------------+---------------+---------------+---------------+
  y/ (DataSegment, Command Data) (optional)                        /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
  z/ Data-Digest (optional)                                        /
   +---------------+---------------+---------------+---------------+

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11.3.1. Flags and Task Attributes (Byte 1)

 The flags for a SCSI Command PDU are:
    bit 0    (F) is set to 1 when no unsolicited SCSI Data-Out PDUs
             follow this PDU.  When F = 1 for a write and if Expected
             Data Transfer Length is larger than the
             DataSegmentLength, the target may solicit additional data
             through R2T.
    bit 1    (R) is set to 1 when the command is expected to input
             data.
    bit 2    (W) is set to 1 when the command is expected to output
             data.
    bit 3-4  Reserved.
    bit 5-7  contains Task Attributes.
 Task Attributes (ATTR) have one of the following integer values (see
 [SAM2] for details):
      0 - Untagged
      1 - Simple
      2 - Ordered
      3 - Head of queue
      4 - ACA
    5-7 - Reserved
 At least one of the W and F bits MUST be set to 1.
 Either or both of R and W MAY be 1 when the Expected Data Transfer
 Length and/or the Bidirectional Read Expected Data Transfer Length
 are 0, but they MUST NOT both be 0 when the Expected Data Transfer
 Length and/or Bidirectional Read Expected Data Transfer Length are
 not 0 (i.e., when some data transfer is expected, the transfer
 direction is indicated by the R and/or W bit).

11.3.2. CmdSN - Command Sequence Number

 The CmdSN enables ordered delivery across multiple connections in a
 single session.

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11.3.3. ExpStatSN

 Command responses up to ExpStatSN - 1 (modulo 2**32) have been
 received (acknowledges status) on the connection.

11.3.4. Expected Data Transfer Length

 For unidirectional operations, the Expected Data Transfer Length
 field contains the number of bytes of data involved in this SCSI
 operation.  For a unidirectional write operation (W flag set to 1 and
 R flag set to 0), the initiator uses this field to specify the number
 of bytes of data it expects to transfer for this operation.  For a
 unidirectional read operation (W flag set to 0 and R flag set to 1),
 the initiator uses this field to specify the number of bytes of data
 it expects the target to transfer to the initiator.  It corresponds
 to the SAM-2 byte count.
 For bidirectional operations (both R and W flags are set to 1), this
 field contains the number of data bytes involved in the write
 transfer.  For bidirectional operations, an additional header segment
 MUST be present in the header sequence that indicates the
 Bidirectional Read Expected Data Transfer Length.  The Expected Data
 Transfer Length field and the Bidirectional Read Expected Data
 Transfer Length field correspond to the SAM-2 byte count.
 If the Expected Data Transfer Length for a write and the length of
 the immediate data part that follows the command (if any) are the
 same, then no more data PDUs are expected to follow.  In this case,
 the F bit MUST be set to 1.
 If the Expected Data Transfer Length is higher than the
 FirstBurstLength (the negotiated maximum amount of unsolicited data
 the target will accept), the initiator MUST send the maximum amount
 of unsolicited data OR ONLY the immediate data, if any.
 Upon completion of a data transfer, the target informs the initiator
 (through residual counts) of how many bytes were actually processed
 (sent and/or received) by the target.

11.3.5. CDB - SCSI Command Descriptor Block

 There are 16 bytes in the CDB field to accommodate the commonly used
 CDBs.  Whenever the CDB is larger than 16 bytes, an Extended CDB AHS
 MUST be used to contain the CDB spillover.

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11.3.6. Data Segment - Command Data

 Some SCSI commands require additional parameter data to accompany the
 SCSI command.  This data may be placed beyond the boundary of the
 iSCSI header in a data segment.  Alternatively, user data (e.g., from
 a write operation) can be placed in the data segment (both cases are
 referred to as immediate data).  These data are governed by the rules
 for solicited vs. unsolicited data outlined in Section 4.2.5.2.

11.4. SCSI Response

 The format of the SCSI Response PDU is:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x21      |1|. .|o|u|O|U|.| Response      | Status        |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| Reserved                                                      |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| SNACK Tag or Reserved                                         |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| ExpDataSN or Reserved                                         |
   +---------------+---------------+---------------+---------------+
 40| Bidirectional Read Residual Count or Reserved                 |
   +---------------+---------------+---------------+---------------+
 44| Residual Count or Reserved                                    |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / Data Segment (optional)                                       /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+

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11.4.1. Flags (Byte 1)

 bit 1-2     Reserved.
 bit 3 - (o) set for Bidirectional Read Residual Overflow.  In this
             case, the Bidirectional Read Residual Count indicates the
             number of bytes that were not transferred to the
             initiator because the initiator's Bidirectional Read
             Expected Data Transfer Length was not sufficient.
 bit 4 - (u) set for Bidirectional Read Residual Underflow.  In this
             case, the Bidirectional Read Residual Count indicates the
             number of bytes that were not transferred to the
             initiator out of the number of bytes expected to be
             transferred.
 bit 5 - (O) set for Residual Overflow.  In this case, the Residual
             Count indicates the number of bytes that were not
             transferred because the initiator's Expected Data
             Transfer Length was not sufficient.  For a bidirectional
             operation, the Residual Count contains the residual for
             the write operation.
 bit 6 - (U) set for Residual Underflow.  In this case, the Residual
             Count indicates the number of bytes that were not
             transferred out of the number of bytes that were expected
             to be transferred.  For a bidirectional operation, the
             Residual Count contains the residual for the write
             operation.
 bit 7 - (0) Reserved.
 Bits O and U and bits o and u are mutually exclusive (i.e., having
 both o and u or O and U set to 1 is a protocol error).
 For a response other than "Command Completed at Target", bits 3-6
 MUST be 0.

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11.4.2. Status

 The Status field is used to report the SCSI status of the command (as
 specified in [SAM2]) and is only valid if the response code is
 Command Completed at Target.
 Some of the status codes defined in [SAM2] are:
    0x00 GOOD
    0x02 CHECK CONDITION
    0x08 BUSY
    0x18 RESERVATION CONFLICT
    0x28 TASK SET FULL
    0x30 ACA ACTIVE
    0x40 TASK ABORTED
 See [SAM2] for the complete list and definitions.
 If a SCSI device error is detected while data from the initiator is
 still expected (the command PDU did not contain all the data and the
 target has not received a data PDU with the Final bit set), the
 target MUST wait until it receives a data PDU with the F bit set in
 the last expected sequence before sending the Response PDU.

11.4.3. Response

 This field contains the iSCSI service response.
 iSCSI service response codes defined in this specification are:
    0x00 - Command Completed at Target
    0x01 - Target Failure
    0x80-0xff - Vendor specific
 All other response codes are reserved.
 The Response field is used to report a service response.  The mapping
 of the response code into a SCSI service response code value, if
 needed, is outside the scope of this document.  However, in symbolic
 terms, response value 0x00 maps to the SCSI service response (see

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 [SAM2] and [SPC3]) of TASK COMPLETE or LINKED COMMAND COMPLETE.  All
 other Response values map to the SCSI service response of SERVICE
 DELIVERY OR TARGET FAILURE.
 If a SCSI Response PDU does not arrive before the session is
 terminated, the SCSI service response is SERVICE DELIVERY OR TARGET
 FAILURE.
 A non-zero response field indicates a failure to execute the command,
 in which case the Status and Flag fields are undefined and MUST be
 ignored on reception.

11.4.4. SNACK Tag

 This field contains a copy of the SNACK Tag of the last SNACK Tag
 accepted by the target on the same connection and for the command for
 which the response is issued.  Otherwise, it is reserved and should
 be set to 0.
 After issuing a R-Data SNACK, the initiator must discard any SCSI
 status unless contained in a SCSI Response PDU carrying the same
 SNACK Tag as the last issued R-Data SNACK for the SCSI command on the
 current connection.
 For a detailed discussion on R-Data SNACK, see Section 11.16.3.

11.4.5. Residual Count

11.4.5.1. Field Semantics

 The Residual Count field MUST be valid in the case where either the U
 bit or the O bit is set.  If neither bit is set, the Residual Count
 field MUST be ignored on reception and SHOULD be set to 0 when
 sending.  Targets may set the residual count, and initiators may use
 it when the response code is Command Completed at Target (even if the
 status returned is not GOOD).  If the O bit is set, the Residual
 Count indicates the number of bytes that were not transferred because
 the initiator's Expected Data Transfer Length was not sufficient.  If
 the U bit is set, the Residual Count indicates the number of bytes
 that were not transferred out of the number of bytes expected to be
 transferred.

11.4.5.2. Residuals Concepts Overview

 "SCSI-Presented Data Transfer Length (SPDTL)" is the term this
 document uses (see Section 2.2 for definition) to represent the
 aggregate data length that the target SCSI layer attempts to transfer
 using the local iSCSI layer for a task.  "Expected Data Transfer

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 Length (EDTL)" is the iSCSI term that represents the length of data
 that the iSCSI layer expects to transfer for a task.  EDTL is
 specified in the SCSI Command PDU.
 When SPDTL = EDTL for a task, the target iSCSI layer completes the
 task with no residuals.  Whenever SPDTL differs from EDTL for a task,
 that task is said to have a residual.
 If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
 SCSI Response PDU as specified in Section 11.4.5.1.  The Residual
 Count MUST be set to the numerical value of (SPDTL - EDTL).
 If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
 SCSI Response PDU as specified in Section 11.4.5.1.  The Residual
 Count MUST be set to the numerical value of (EDTL - SPDTL).
 Note that the Overflow and Underflow scenarios are independent of
 Data-In and Data-Out.  Either scenario is logically possible in
 either direction of data transfer.

11.4.5.3. SCSI REPORT LUNS Command and Residual Overflow

 This section discusses the residual overflow issues, citing the
 example of the SCSI REPORT LUNS command.  Note, however, that there
 are several SCSI commands (e.g., INQUIRY) with ALLOCATION LENGTH
 fields following the same underlying rules.  The semantics in the
 rest of the section apply to all such SCSI commands.
 The specification of the SCSI REPORT LUNS command requires that the
 SCSI target limit the amount of data transferred to a maximum size
 (ALLOCATION LENGTH) provided by the initiator in the REPORT LUNS CDB.
 If the Expected Data Transfer Length (EDTL) in the iSCSI header of
 the SCSI Command PDU for a REPORT LUNS command is set to at least as
 large as that ALLOCATION LENGTH, the SCSI-layer truncation prevents
 an iSCSI Residual Overflow from occurring.  A SCSI initiator can
 detect that such truncation has occurred via other information at the
 SCSI layer.  The rest of the section elaborates on this required
 behavior.
 The SCSI REPORT LUNS command requests a target SCSI layer to return a
 LU inventory (LUN list) to the initiator SCSI layer (see Clause 6.21
 of [SPC3]).  The size of this LUN list may not be known to the
 initiator SCSI layer when it issues the REPORT LUNS command; to avoid
 transferring more LUN list data than the initiator is prepared for,
 the REPORT LUNS CDB contains an ALLOCATION LENGTH field to specify
 the maximum amount of data to be transferred to the initiator for
 this command.  If the initiator SCSI layer has underestimated the

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 number of LUs at the target, it is possible that the complete LU
 inventory does not fit in the specified ALLOCATION LENGTH.  In this
 situation, Clause 4.3.4.6 of [SPC3] requires that the target SCSI
 layer "shall terminate transfers to the Data-In Buffer" when the
 number of bytes specified by the ALLOCATION LENGTH field have been
 transferred.
 Therefore, in response to a REPORT LUNS command, the SCSI layer at
 the target presents at most ALLOCATION LENGTH bytes of data (LU
 inventory) to iSCSI for transfer to the initiator.  For a REPORT LUNS
 command, if the iSCSI EDTL is at least as large as the ALLOCATION
 LENGTH, the SCSI truncation ensures that the EDTL will accommodate
 all of the data to be transferred.  If all of the LU inventory data
 presented to the iSCSI layer -- i.e., the data remaining after any
 SCSI truncation -- is transferred to the initiator by the iSCSI
 layer, an iSCSI Residual Overflow has not occurred and the iSCSI (O)
 bit MUST NOT be set in the SCSI Response or final SCSI Data-Out PDU.
 Note that this behavior is implied in Section 11.4.5.1, along with
 the specification of the REPORT LUNS command in [SPC3].  However, if
 the iSCSI EDTL is larger than the ALLOCATION LENGTH in this scenario,
 note that the iSCSI Underflow MUST be signaled in the SCSI Response
 PDU.  An iSCSI Underflow MUST also be signaled when the iSCSI EDTL is
 equal to the ALLOCATION LENGTH but the LU inventory data presented to
 the iSCSI layer is smaller than the ALLOCATION LENGTH.
 The LUN LIST LENGTH field in the LU inventory (the first field in the
 inventory) is not affected by truncation of the inventory to fit in
 ALLOCATION LENGTH; this enables a SCSI initiator to determine that
 the received inventory is incomplete by noticing that the LUN LIST
 LENGTH in the inventory is larger than the ALLOCATION LENGTH that was
 sent in the REPORT LUNS CDB.  A common initiator behavior in this
 situation is to reissue the REPORT LUNS command with a larger
 ALLOCATION LENGTH.

11.4.6. Bidirectional Read Residual Count

 The Bidirectional Read Residual Count field MUST be valid in the case
 where either the u bit or the o bit is set.  If neither bit is set,
 the Bidirectional Read Residual Count field is reserved.  Targets may
 set the Bidirectional Read Residual Count, and initiators may use it
 when the response code is Command Completed at Target.  If the o bit
 is set, the Bidirectional Read Residual Count indicates the number of
 bytes that were not transferred to the initiator because the
 initiator's Bidirectional Read Expected Data Transfer Length was not
 sufficient.  If the u bit is set, the Bidirectional Read Residual
 Count indicates the number of bytes that were not transferred to the
 initiator out of the number of bytes expected to be transferred.

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11.4.7. Data Segment - Sense and Response Data Segment

 iSCSI targets MUST support and enable Autosense.  If Status is CHECK
 CONDITION (0x02), then the data segment MUST contain sense data for
 the failed command.
 For some iSCSI responses, the response data segment MAY contain some
 response-related information (e.g., for a target failure, it may
 contain a vendor-specific detailed description of the failure).
 If the DataSegmentLength is not 0, the format of the data segment is
 as follows:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|SenseLength                    | Sense Data                    |
   +---------------+---------------+---------------+---------------+
  x/ Sense Data                                                    /
   +---------------+---------------+---------------+---------------+
  y/ Response Data                                                 /
   /                                                               /
   +---------------+---------------+---------------+---------------+

11.4.7.1. SenseLength

 This field indicates the length of Sense Data.

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11.4.7.2. Sense Data

 The Sense Data contains detailed information about a CHECK CONDITION.
 [SPC3] specifies the format and content of the Sense Data.
 Certain iSCSI conditions result in the command being terminated at
 the target (response code of Command Completed at Target) with a SCSI
 CHECK CONDITION Status as outlined in the next table:
 +--------------------------+-----------+---------------------------+
 | iSCSI Condition          |Sense      | Additional Sense Code and |
 |                          |Key        | Qualifier                 |
 +--------------------------+-----------+---------------------------+
 | Unexpected unsolicited   |Aborted    | ASC = 0x0c ASCQ = 0x0c    |
 | data                     |Command-0B | Write Error               |
 +--------------------------+-----------+---------------------------+
 | Incorrect amount of data |Aborted    | ASC = 0x0c ASCQ = 0x0d    |
 |                          |Command-0B | Write Error               |
 +--------------------------+-----------+---------------------------+
 | Protocol Service CRC     |Aborted    | ASC = 0x47 ASCQ = 0x05    |
 | error                    |Command-0B | CRC Error Detected        |
 +--------------------------+-----------+---------------------------+
 | SNACK rejected           |Aborted    | ASC = 0x11 ASCQ = 0x13    |
 |                          |Command-0B | Read Error                |
 +--------------------------+-----------+---------------------------+
 The target reports the "Incorrect amount of data" condition if,
 during data output, the total data length to output is greater than
 FirstBurstLength and the initiator sent unsolicited non-immediate
 data but the total amount of unsolicited data is different than
 FirstBurstLength.  The target reports the same error when the amount
 of data sent as a reply to an R2T does not match the amount
 requested.

11.4.8. ExpDataSN

 This field indicates the number of Data-In (read) PDUs the target has
 sent for the command.
 This field MUST be 0 if the response code is not Command Completed at
 Target or the target sent no Data-In PDUs for the command.

11.4.9. StatSN - Status Sequence Number

 The StatSN is a sequence number that the target iSCSI layer generates
 per connection and that in turn enables the initiator to acknowledge
 status reception.  The StatSN is incremented by 1 for every
 response/status sent on a connection, except for responses sent as a

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 result of a retry or SNACK.  In the case of responses sent due to a
 retransmission request, the StatSN MUST be the same as the first time
 the PDU was sent, unless the connection has since been restarted.

11.4.10. ExpCmdSN - Next Expected CmdSN from This Initiator

 The ExpCmdSN is a sequence number that the target iSCSI returns to
 the initiator to acknowledge command reception.  It is used to update
 a local variable with the same name.  An ExpCmdSN equal to
 MaxCmdSN + 1 indicates that the target cannot accept new commands.

11.4.11. MaxCmdSN - Maximum CmdSN from This Initiator

 The MaxCmdSN is a sequence number that the target iSCSI returns to
 the initiator to indicate the maximum CmdSN the initiator can send.
 It is used to update a local variable with the same name.  If the
 MaxCmdSN is equal to ExpCmdSN - 1, this indicates to the initiator
 that the target cannot receive any additional commands.  When the
 MaxCmdSN changes at the target while the target has no pending PDUs
 to convey this information to the initiator, it MUST generate a
 NOP-In to carry the new MaxCmdSN.

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11.5. Task Management Function Request

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| 0x02      |1| Function    | Reserved                      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| Logical Unit Number (LUN) or Reserved                         |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Referenced Task Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32| RefCmdSN or Reserved                                          |
   +---------------+---------------+---------------+---------------+
 36| ExpDataSN or Reserved                                         |
   +---------------+---------------+---------------+---------------+
 40/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+

11.5.1. Function

 The task management functions provide an initiator with a way to
 explicitly control the execution of one or more tasks (SCSI and iSCSI
 tasks).  The task management function codes are listed below.  For a
 more detailed description of SCSI task management, see [SAM2].
    1  ABORT TASK - aborts the task identified by the Referenced Task
       Tag field.
    2  ABORT TASK SET - aborts all tasks issued via this session on
       the LU.
    3  CLEAR ACA - clears the Auto Contingent Allegiance condition.

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    4  CLEAR TASK SET - aborts all tasks in the appropriate task set
       as defined by the TST field in the Control mode page
       (see [SPC3]).
    5  LOGICAL UNIT RESET
    6  TARGET WARM RESET
    7  TARGET COLD RESET
    8  TASK REASSIGN - reassigns connection allegiance for the task
       identified by the Initiator Task Tag field to this connection,
       thus resuming the iSCSI exchanges for the task.
 Values 9-12 are assigned in [RFC7144].  All other possible values for
 the Function field are unassigned.
 For all these functions, the Task Management Function Response MUST
 be returned as detailed in Section 11.6.  All these functions apply
 to the referenced tasks, regardless of whether they are proper SCSI
 tasks or tagged iSCSI operations.  Task management requests must act
 on all the commands from the same session having a CmdSN lower than
 the task management CmdSN.  LOGICAL UNIT RESET, TARGET WARM RESET,
 and TARGET COLD RESET may affect commands from other sessions or
 commands from the same session, regardless of their CmdSN value.
 If the task management request is marked for immediate delivery, it
 must be considered immediately for execution, but the operations
 involved (all or part of them) may be postponed to allow the target
 to receive all relevant tasks.  According to [SAM2], for all the
 tasks covered by the task management response (i.e., with a CmdSN
 lower than the task management command CmdSN), except for the task
 management response to a TASK REASSIGN, additional responses MUST NOT
 be delivered to the SCSI layer after the task management response.
 The iSCSI initiator MAY deliver to the SCSI layer all responses
 received before the task management response (i.e., it is a matter of
 implementation if the SCSI responses that are received before the
 task management response but after the task management request was
 issued are delivered to the SCSI layer by the iSCSI layer in the
 initiator).  The iSCSI target MUST ensure that no responses for the
 tasks covered by a task management function are delivered to the
 iSCSI initiator after the task management response, except for a task
 covered by a TASK REASSIGN.
 For ABORT TASK SET and CLEAR TASK SET, the issuing initiator MUST
 continue to respond to all valid Target Transfer Tags (received via
 R2T, Text Response, NOP-In, or SCSI Data-In PDUs) related to the
 affected task set, even after issuing the task management request.

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 The issuing initiator SHOULD, however, terminate (i.e., by setting
 the F bit to 1) these response sequences as quickly as possible.  The
 target for its part MUST wait for responses on all affected Target
 Transfer Tags before acting on either of these two task management
 requests.  If all or part of the response sequence is not received
 (due to digest errors) for a valid TTT, the target MAY treat it as a
 case of a within-command error recovery class (see Section 7.1.4.1)
 if it is supporting ErrorRecoveryLevel >= 1 or, alternatively, may
 drop the connection to complete the requested task set function.
 If an ABORT TASK is issued for a task created by an immediate
 command, then the RefCmdSN MUST be that of the task management
 request itself (i.e., the CmdSN and RefCmdSN are equal); otherwise,
 the RefCmdSN MUST be set to the CmdSN of the task to be aborted
 (lower than the CmdSN).
 If the connection is still active (i.e., it is not undergoing an
 implicit or explicit logout), an ABORT TASK MUST be issued on the
 same connection to which the task to be aborted is allegiant at the
 time the task management request is issued.  If the connection is
 implicitly or explicitly logged out (i.e., no other request will be
 issued on the failing connection and no other response will be
 received on the failing connection), then an ABORT TASK function
 request may be issued on another connection.  This task management
 request will then establish a new allegiance for the command to be
 aborted as well as abort it (i.e., the task to be aborted will not
 have to be retried or reassigned, and its status, if sent but not
 acknowledged, will be resent followed by the task management
 response).
 At the target, an ABORT TASK function MUST NOT be executed on a task
 management request; such a request MUST result in a task management
 response of "Function rejected".
 For the LOGICAL UNIT RESET function, the target MUST behave as
 dictated by the Logical Unit Reset function in [SAM2].
 The implementation of the TARGET WARM RESET function and the TARGET
 COLD RESET function is OPTIONAL and, when implemented, should act as
 described below.  The TARGET WARM RESET is also subject to SCSI
 access controls on the requesting initiator as defined in [SPC3].
 When authorization fails at the target, the appropriate response as
 described in Section 11.6.1 MUST be returned by the target.  The
 TARGET COLD RESET function is not subject to SCSI access controls,
 but its execution privileges may be managed by iSCSI mechanisms such
 as login authentication.

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 When executing the TARGET WARM RESET and TARGET COLD RESET functions,
 the target cancels all pending operations on all LUs known by the
 issuing initiator.  Both functions are equivalent to the TARGET RESET
 function specified by [SAM2].  They can affect many other initiators
 logged in with the servicing SCSI target port.
 Additionally, the target MUST treat the TARGET COLD RESET function as
 a power-on event, thus terminating all of its TCP connections to all
 initiators (all sessions are terminated).  For this reason, the
 service response (defined by [SAM2]) for this SCSI task management
 function may not be reliably delivered to the issuing initiator port.
 For the TASK REASSIGN function, the target should reassign the
 connection allegiance to this new connection (and thus resume iSCSI
 exchanges for the task).  TASK REASSIGN MUST ONLY be received by the
 target after the connection on which the command was previously
 executing has been successfully logged out.  The task management
 response MUST be issued before the reassignment becomes effective.
 For additional usage semantics, see Section 7.2.
 At the target, a TASK REASSIGN function request MUST NOT be executed
 to reassign the connection allegiance of a Task Management Function
 Request, an active text negotiation task, or a Logout task; such a
 request MUST result in a task management response of "Function
 rejected".
 TASK REASSIGN MUST be issued as an immediate command.

11.5.2. TotalAHSLength and DataSegmentLength

 For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.5.3. LUN

 This field is required for functions that address a specific LU
 (ABORT TASK, CLEAR TASK SET, ABORT TASK SET, CLEAR ACA, LOGICAL UNIT
 RESET) and is reserved in all others.

11.5.4. Referenced Task Tag

 This is the Initiator Task Tag of the task to be aborted for the
 ABORT TASK function or reassigned for the TASK REASSIGN function.
 For all the other functions, this field MUST be set to the reserved
 value 0xffffffff.

Chadalapaka, et al. Standards Track [Page 173] RFC 7143 iSCSI (Consolidated) April 2014

11.5.5. RefCmdSN

 If an ABORT TASK is issued for a task created by an immediate
 command, then the RefCmdSN MUST be that of the task management
 request itself (i.e., the CmdSN and RefCmdSN are equal).
 For an ABORT TASK of a task created by a non-immediate command, the
 RefCmdSN MUST be set to the CmdSN of the task identified by the
 Referenced Task Tag field.  Targets must use this field as described
 in Section 11.6.1 when the task identified by the Referenced Task Tag
 field is not with the target.
 Otherwise, this field is reserved.

11.5.6. ExpDataSN

 For recovery purposes, the iSCSI target and initiator maintain a data
 acknowledgment reference number -- the first input DataSN number
 unacknowledged by the initiator.  When issuing a new command, this
 number is set to 0.  If the function is TASK REASSIGN, which
 establishes a new connection allegiance for a previously issued read
 or bidirectional command, the ExpDataSN will contain an updated data
 acknowledgment reference number or the value 0; the latter indicates
 that the data acknowledgment reference number is unchanged.  The
 initiator MUST discard any data PDUs from the previous execution that
 it did not acknowledge, and the target MUST transmit all Data-In PDUs
 (if any) starting with the data acknowledgment reference number.  The
 number of retransmitted PDUs may or may not be the same as the
 original transmission, depending on if there was a change in
 MaxRecvDataSegmentLength in the reassignment.  The target MAY also
 send no more Data-In PDUs if all data has been acknowledged.
 The value of ExpDataSN MUST be 0 or higher than the DataSN of the
 last acknowledged Data-In PDU, but not larger than DataSN + 1 of the
 last Data-IN PDU sent by the target.  Any other value MUST be ignored
 by the target.
 For other functions, this field is reserved.

Chadalapaka, et al. Standards Track [Page 174] RFC 7143 iSCSI (Consolidated) April 2014

11.6. Task Management Function Response

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x22      |1| Reserved    | Response      | Reserved      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------------------------------------------------------+
  8/ Reserved                                                      /
   /                                                               /
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
 For the functions ABORT TASK, ABORT TASK SET, CLEAR ACA, CLEAR TASK
 SET, LOGICAL UNIT RESET, TARGET COLD RESET, TARGET WARM RESET, and
 TASK REASSIGN, the target performs the requested task management
 function and sends a task management response back to the initiator.
 For TASK REASSIGN, the new connection allegiance MUST ONLY become
 effective at the target after the target issues the task management
 response.

Chadalapaka, et al. Standards Track [Page 175] RFC 7143 iSCSI (Consolidated) April 2014

11.6.1. Response

 The target provides a response, which may take on the following
 values:
     0 - Function complete
     1 - Task does not exist
     2 - LUN does not exist
     3 - Task still allegiant
     4 - Task allegiance reassignment not supported
     5 - Task management function not supported
     6 - Function authorization failed
   255 - Function rejected
 In addition to the above values, the value 7 is defined by [RFC7144].
 For a discussion on the usage of response codes 3 and 4, see
 Section 7.2.2.
 For the TARGET COLD RESET and TARGET WARM RESET functions, the target
 cancels all pending operations across all LUs known to the issuing
 initiator.  For the TARGET COLD RESET function, the target MUST then
 close all of its TCP connections to all initiators (terminates all
 sessions).
 The mapping of the response code into a SCSI service response code
 value, if needed, is outside the scope of this document.  However, in
 symbolic terms, Response values 0 and 1 map to the SCSI service
 response of FUNCTION COMPLETE.  Response value 2 maps to the SCSI
 service response of INCORRECT LOGICAL UNIT NUMBER.  All other
 Response values map to the SCSI service response of FUNCTION
 REJECTED.  If a Task Management Function Response PDU does not arrive
 before the session is terminated, the SCSI service response is
 SERVICE DELIVERY OR TARGET FAILURE.
 The response to ABORT TASK SET and CLEAR TASK SET MUST only be issued
 by the target after all of the commands affected have been received
 by the target, the corresponding task management functions have been
 executed by the SCSI target, and the delivery of all responses
 delivered until the task management function completion has been
 confirmed (acknowledged through the ExpStatSN) by the initiator on
 all connections of this session.  For the exact timeline of events,
 refer to Sections 4.2.3.3 and 4.2.3.4.

Chadalapaka, et al. Standards Track [Page 176] RFC 7143 iSCSI (Consolidated) April 2014

 For the ABORT TASK function,
    a) if the Referenced Task Tag identifies a valid task leading to a
       successful termination, then targets must return the "Function
       complete" response.
    b) if the Referenced Task Tag does not identify an existing task
       but the CmdSN indicated by the RefCmdSN field in the Task
       Management Function Request is within the valid CmdSN window
       and less than the CmdSN of the Task Management Function Request
       itself, then targets must consider the CmdSN as received and
       return the "Function complete" response.
    c) if the Referenced Task Tag does not identify an existing task
       and the CmdSN indicated by the RefCmdSN field in the Task
       Management Function Request is outside the valid CmdSN window,
       then targets must return the "Task does not exist" response.
 For response semantics on function types that can potentially impact
 multiple active tasks on the target, see Section 4.2.3.

11.6.2. TotalAHSLength and DataSegmentLength

 For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

Chadalapaka, et al. Standards Track [Page 177] RFC 7143 iSCSI (Consolidated) April 2014

11.7. SCSI Data-Out and SCSI Data-In

 The SCSI Data-Out PDU for write operations has the following format:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x05      |F| Reserved                                    |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 36| DataSN                                                        |
   +---------------+---------------+---------------+---------------+
 40| Buffer Offset                                                 |
   +---------------+---------------+---------------+---------------+
 44| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment                                                   /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+

Chadalapaka, et al. Standards Track [Page 178] RFC 7143 iSCSI (Consolidated) April 2014

 The SCSI Data-In PDU for read operations has the following format:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x25      |F|A|0 0 0|O|U|S| Reserved      |Status or Rsvd |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| StatSN or Reserved                                            |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| DataSN                                                        |
   +---------------+---------------+---------------+---------------+
 40| Buffer Offset                                                 |
   +---------------+---------------+---------------+---------------+
 44| Residual Count                                                |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment                                                   /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 Status can accompany the last Data-In PDU if the command did not end
 with an exception (i.e., the status is "good status" -- GOOD,
 CONDITION MET, or INTERMEDIATE-CONDITION MET).  The presence of
 status (and of a residual count) is signaled via the S flag bit.
 Although targets MAY choose to send even non-exception status in
 separate responses, initiators MUST support non-exception status in
 Data-In PDUs.

Chadalapaka, et al. Standards Track [Page 179] RFC 7143 iSCSI (Consolidated) April 2014

11.7.1. F (Final) Bit

 For outgoing data, this bit is 1 for the last PDU of unsolicited data
 or the last PDU of a sequence that answers an R2T.
 For incoming data, this bit is 1 for the last input (read) data PDU
 of a sequence.  Input can be split into several sequences, each
 having its own F bit.  Splitting the data stream into sequences does
 not affect DataSN counting on Data-In PDUs.  It MAY be used as a
 "change direction" indication for bidirectional operations that need
 such a change.
 DataSegmentLength MUST NOT exceed MaxRecvDataSegmentLength for the
 direction it is sent, and the total of all the DataSegmentLength of
 all PDUs in a sequence MUST NOT exceed MaxBurstLength (or
 FirstBurstLength for unsolicited data).  However, the number of
 individual PDUs in a sequence (or in total) may be higher than the
 ratio of MaxBurstLength (or FirstBurstLength) to
 MaxRecvDataSegmentLength (as PDUs may be limited in length by the
 capabilities of the sender).  Using a DataSegmentLength of 0 may
 increase beyond what is reasonable for the number of PDUs and should
 therefore be avoided.
 For bidirectional operations, the F bit is 1 for both the end of the
 input sequences and the end of the output sequences.

11.7.2. A (Acknowledge) Bit

 For sessions with ErrorRecoveryLevel=1 or higher, the target sets
 this bit to 1 to indicate that it requests a positive acknowledgment
 from the initiator for the data received.  The target should use the
 A bit moderately; it MAY only set the A bit to 1 once every
 MaxBurstLength bytes, or on the last Data-In PDU that concludes the
 entire requested read data transfer for the task from the target's
 perspective, and it MUST NOT do so more frequently.  The target MUST
 NOT set to 1 the A bit for sessions with ErrorRecoveryLevel=0.  The
 initiator MUST ignore the A bit set to 1 for sessions with
 ErrorRecoveryLevel=0.
 On receiving a Data-In PDU with the A bit set to 1 on a session with
 ErrorRecoveryLevel greater than 0, if there are no holes in the read
 data until that Data-In PDU, the initiator MUST issue a SNACK of type
 DataACK, except when it is able to acknowledge the status for the
 task immediately via the ExpStatSN on other outbound PDUs if the
 status for the task is also received.  In the latter case
 (acknowledgment through the ExpStatSN), sending a SNACK of type
 DataACK in response to the A bit is OPTIONAL, but if it is done, it
 must not be sent after the status acknowledgment through the

Chadalapaka, et al. Standards Track [Page 180] RFC 7143 iSCSI (Consolidated) April 2014

 ExpStatSN.  If the initiator has detected holes in the read data
 prior to that Data-In PDU, it MUST postpone issuing the SNACK of type
 DataACK until the holes are filled.  An initiator also MUST NOT
 acknowledge the status for the task before those holes are filled.  A
 status acknowledgment for a task that generated the Data-In PDUs is
 considered by the target as an implicit acknowledgment of the Data-In
 PDUs if such an acknowledgment was requested by the target.

11.7.3. Flags (Byte 1)

 The last SCSI data packet sent from a target to an initiator for a
 SCSI command that completed successfully (with a status of GOOD,
 CONDITION MET, INTERMEDIATE, or INTERMEDIATE-CONDITION MET) may also
 optionally contain the Status for the data transfer.  In this case,
 Sense Data cannot be sent together with the Command Status.  If the
 command is completed with an error, then the response and sense data
 MUST be sent in a SCSI Response PDU (i.e., MUST NOT be sent in a SCSI
 data packet).  For bidirectional commands, the status MUST be sent in
 a SCSI Response PDU.
    bit 2-4          - Reserved.
    bit 5-6          - used the same as in a SCSI Response.  These
                       bits are only valid when S is set to 1.  For
                       details, see Section 11.4.1.
    bit 7 S (status) - set to indicate that the Command Status field
                       contains status.  If this bit is set to 1, the
                       F bit MUST also be set to 1.
 The fields StatSN, Status, and Residual Count only have meaningful
 content if the S bit is set to 1.  The values for these fields are
 defined in Section 11.4.

11.7.4. Target Transfer Tag and LUN

 On outgoing data, the Target Transfer Tag is provided to the target
 if the transfer is honoring an R2T.  In this case, the Target
 Transfer Tag field is a replica of the Target Transfer Tag provided
 with the R2T.
 On incoming data, the Target Transfer Tag and LUN MUST be provided by
 the target if the A bit is set to 1; otherwise, they are reserved.
 The Target Transfer Tag and LUN are copied by the initiator into the
 SNACK of type DataACK that it issues as a result of receiving a SCSI
 Data-In PDU with the A bit set to 1.

Chadalapaka, et al. Standards Track [Page 181] RFC 7143 iSCSI (Consolidated) April 2014

 The Target Transfer Tag values are not specified by this protocol,
 except that the value 0xffffffff is reserved and means that the
 Target Transfer Tag is not supplied.  If the Target Transfer Tag is
 provided, then the LUN field MUST hold a valid value and be
 consistent with whatever was specified with the command; otherwise,
 the LUN field is reserved.

11.7.5. DataSN

 For input (read) or bidirectional Data-In PDUs, the DataSN is the
 input PDU number within the data transfer for the command identified
 by the Initiator Task Tag.
 R2T and Data-In PDUs, in the context of bidirectional commands, share
 the numbering sequence (see Section 4.2.2.4).
 For output (write) data PDUs, the DataSN is the Data-Out PDU number
 within the current output sequence.  Either the current output
 sequence is identified by the Initiator Task Tag (for unsolicited
 data) or it is a data sequence generated for one R2T (for data
 solicited through R2T).

11.7.6. Buffer Offset

 The Buffer Offset field contains the offset of this PDU payload data
 within the complete data transfer.  The sum of the buffer offset and
 length should not exceed the expected transfer length for the
 command.
 The order of data PDUs within a sequence is determined by
 DataPDUInOrder.  When set to Yes, it means that PDUs have to be in
 increasing buffer offset order and overlays are forbidden.
 The ordering between sequences is determined by DataSequenceInOrder.
 When set to Yes, it means that sequences have to be in increasing
 buffer offset order and overlays are forbidden.

11.7.7. DataSegmentLength

 This is the data payload length of a SCSI Data-In or SCSI Data-Out
 PDU.  The sending of 0-length data segments should be avoided, but
 initiators and targets MUST be able to properly receive 0-length data
 segments.
 The data segments of Data-In and Data-Out PDUs SHOULD be filled to
 the integer number of 4-byte words (real payload), unless the F bit
 is set to 1.

Chadalapaka, et al. Standards Track [Page 182] RFC 7143 iSCSI (Consolidated) April 2014

11.8. Ready To Transfer (R2T)

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x31      |1| Reserved                                    |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN                                                           |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag                                           |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| R2TSN                                                         |
   +---------------+---------------+---------------+---------------+
 40| Buffer Offset                                                 |
   +---------------+---------------+---------------+---------------+
 44| Desired Data Transfer Length                                  |
   +---------------------------------------------------------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
 When an initiator has submitted a SCSI command with data that passes
 from the initiator to the target (write), the target may specify
 which blocks of data it is ready to receive.  The target may request
 that the data blocks be delivered in whichever order is convenient
 for the target at that particular instant.  This information is
 passed from the target to the initiator in the Ready To Transfer
 (R2T) PDU.
 In order to allow write operations without an explicit initial R2T,
 the initiator and target MUST have negotiated the key InitialR2T to
 No during login.
 An R2T MAY be answered with one or more SCSI Data-Out PDUs with a
 matching Target Transfer Tag.  If an R2T is answered with a single
 Data-Out PDU, the buffer offset in the data PDU MUST be the same as

Chadalapaka, et al. Standards Track [Page 183] RFC 7143 iSCSI (Consolidated) April 2014

 the one specified by the R2T, and the data length of the data PDU
 MUST be the same as the Desired Data Transfer Length specified in the
 R2T.  If the R2T is answered with a sequence of data PDUs, the buffer
 offset and length MUST be within the range of those specified by the
 R2T, and the last PDU MUST have the F bit set to 1.  If the last PDU
 (marked with the F bit) is received before the Desired Data Transfer
 Length is transferred, a target MAY choose to reject that PDU with
 the "Protocol Error" reason code.  DataPDUInOrder governs the
 Data-Out PDU ordering.  If DataPDUInOrder is set to Yes, the buffer
 offsets and lengths for consecutive PDUs MUST form a continuous
 non-overlapping range, and the PDUs MUST be sent in increasing offset
 order.
 The target may send several R2T PDUs.  It therefore can have a number
 of pending data transfers.  The number of outstanding R2T PDUs is
 limited by the value of the negotiated key MaxOutstandingR2T.  Within
 a task, outstanding R2Ts MUST be fulfilled by the initiator in the
 order in which they were received.
 R2T PDUs MAY also be used to recover Data-Out PDUs.  Such an R2T
 (Recovery-R2T) is generated by a target upon detecting the loss of
 one or more Data-Out PDUs due to:
  1. Digest error
  1. Sequence error
  1. Sequence reception timeout
 A Recovery-R2T carries the next unused R2TSN but requests part of or
 the entire data burst that an earlier R2T (with a lower R2TSN) had
 already requested.
 DataSequenceInOrder governs the buffer offset ordering in consecutive
 R2Ts.  If DataSequenceInOrder is Yes, then consecutive R2Ts MUST
 refer to continuous non-overlapping ranges, except for Recovery-R2Ts.

11.8.1. TotalAHSLength and DataSegmentLength

 For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.8.2. R2TSN

 R2TSN is the R2T PDU input PDU number within the command identified
 by the Initiator Task Tag.
 For bidirectional commands, R2T and Data-In PDUs share the input PDU
 numbering sequence (see Section 4.2.2.4).

Chadalapaka, et al. Standards Track [Page 184] RFC 7143 iSCSI (Consolidated) April 2014

11.8.3. StatSN

 The StatSN field will contain the next StatSN.  The StatSN for this
 connection is not advanced after this PDU is sent.

11.8.4. Desired Data Transfer Length and Buffer Offset

 The target specifies how many bytes it wants the initiator to send
 because of this R2T PDU.  The target may request the data from the
 initiator in several chunks, not necessarily in the original order of
 the data.  The target therefore also specifies a buffer offset that
 indicates the point at which the data transfer should begin, relative
 to the beginning of the total data transfer.  The Desired Data
 Transfer Length MUST NOT be 0 and MUST NOT exceed MaxBurstLength.

11.8.5. Target Transfer Tag

 The target assigns its own tag to each R2T request that it sends to
 the initiator.  This tag can be used by the target to easily identify
 the data it receives.  The Target Transfer Tag and LUN are copied in
 the outgoing data PDUs and are only used by the target.  There is no
 protocol rule about the Target Transfer Tag except that the value
 0xffffffff is reserved and MUST NOT be sent by a target in an R2T.

Chadalapaka, et al. Standards Track [Page 185] RFC 7143 iSCSI (Consolidated) April 2014

11.9. Asynchronous Message

 An Asynchronous Message may be sent from the target to the initiator
 without corresponding to a particular command.  The target specifies
 the reason for the event and sense data.
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x32      |1| Reserved                                    |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| 0xffffffff                                                    |
   +---------------+---------------+---------------+---------------+
 20| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| AsyncEvent    | AsyncVCode    | Parameter1 or Reserved        |
   +---------------+---------------+---------------+---------------+
 40| Parameter2 or Reserved        | Parameter3 or Reserved        |
   +---------------+---------------+---------------+---------------+
 44| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment - Sense Data and iSCSI Event Data                 /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 Some Asynchronous Messages are strictly related to iSCSI, while
 others are related to SCSI [SAM2].
 The StatSN counts this PDU as an acknowledgeable event (the StatSN is
 advanced), which allows for initiator and target state
 synchronization.

Chadalapaka, et al. Standards Track [Page 186] RFC 7143 iSCSI (Consolidated) April 2014

11.9.1. AsyncEvent

 The codes used for iSCSI Asynchronous Messages (events) are:
      0 (SCSI Async Event) - a SCSI asynchronous event is reported in
        the sense data.  Sense Data that accompanies the report, in
        the data segment, identifies the condition.  The sending of a
        SCSI event ("asynchronous event reporting" in SCSI
        terminology) is dependent on the target support for SCSI
        asynchronous event reporting (see [SAM2]) as indicated in the
        standard INQUIRY data (see [SPC3]).  Its use may be enabled by
        parameters in the SCSI Control mode page (see [SPC3]).
      1 (Logout Request) - the target requests Logout.  This Async
        Message MUST be sent on the same connection as the one
        requesting to be logged out.  The initiator MUST honor this
        request by issuing a Logout as early as possible but no later
        than Parameter3 seconds.  The initiator MUST send a Logout
        with a reason code of "close the connection" OR "close the
        session" to close all the connections.  Once this message is
        received, the initiator SHOULD NOT issue new iSCSI commands on
        the connection to be logged out.  The target MAY reject any
        new I/O requests that it receives after this message with the
        reason code "Waiting for Logout".  If the initiator does not
        log out in Parameter3 seconds, the target should send an Async
        PDU with iSCSI event code "Dropped the connection" if possible
        or simply terminate the transport connection.  Parameter1 and
        Parameter2 are reserved.
      2 (Connection Drop Notification) - the target indicates that it
        will drop the connection.
        The Parameter1 field indicates the CID of the connection that
        is going to be dropped.
        The Parameter2 field (Time2Wait) indicates, in seconds, the
        minimum time to wait before attempting to reconnect or
        reassign.
        The Parameter3 field (Time2Retain) indicates the maximum time
        allowed to reassign commands after the initial wait (in
        Parameter2).
        If the initiator does not attempt to reconnect and/or reassign
        the outstanding commands within the time specified by
        Parameter3, or if Parameter3 is 0, the target will terminate

Chadalapaka, et al. Standards Track [Page 187] RFC 7143 iSCSI (Consolidated) April 2014

        all outstanding commands on this connection.  In this case, no
        other responses should be expected from the target for the
        outstanding commands on this connection.
        A value of 0 for Parameter2 indicates that reconnect can be
        attempted immediately.
      3 (Session Drop Notification) - the target indicates that it
        will drop all the connections of this session.
        The Parameter1 field is reserved.
        The Parameter2 field (Time2Wait) indicates, in seconds, the
        minimum time to wait before attempting to reconnect.
        The Parameter3 field (Time2Retain) indicates the maximum time
        allowed to reassign commands after the initial wait (in
        Parameter2).
        If the initiator does not attempt to reconnect and/or reassign
        the outstanding commands within the time specified by
        Parameter3, or if Parameter3 is 0, the session is terminated.
        In this case, the target will terminate all outstanding
        commands in this session; no other responses should be
        expected from the target for the outstanding commands in this
        session.  A value of 0 for Parameter2 indicates that reconnect
        can be attempted immediately.
      4 (Negotiation Request) - the target requests parameter
        negotiation on this connection.  The initiator MUST honor this
        request by issuing a Text Request (that can be empty) on the
        same connection as early as possible, but no later than
        Parameter3 seconds, unless a Text Request is already pending
        on the connection, or by issuing a Logout Request.  If the
        initiator does not issue a Text Request, the target may
        reissue the Asynchronous Message requesting parameter
        negotiation.

Chadalapaka, et al. Standards Track [Page 188] RFC 7143 iSCSI (Consolidated) April 2014

      5 (Task Termination) - all active tasks for a LU with a matching
        LUN field in the Async Message PDU are being terminated.  The
        receiving initiator iSCSI layer MUST respond to this message
        by taking the following steps, in order:
  1. Stop Data-Out transfers on that connection for all active

TTTs for the affected LUN quoted in the Async Message PDU.

  1. Acknowledge the StatSN of the Async Message PDU via a

NOP-Out PDU with ITT=0xffffffff (i.e., non-ping flavor),

          while copying the LUN field from the Async Message to
          NOP-Out.
        This value of AsyncEvent, however, MUST NOT be used on an
        iSCSI session unless the new TaskReporting text key defined in
        Section 13.23 was negotiated to FastAbort on the session.
  248-255 (Vendor-unique) - vendor-specific iSCSI event.  The
        AsyncVCode details the vendor code, and data MAY accompany the
        report.
 All other event codes are unassigned.

11.9.2. AsyncVCode

 AsyncVCode is a vendor-specific detail code that is only valid if the
 AsyncEvent field indicates a vendor-specific event.  Otherwise, it is
 reserved.

11.9.3. LUN

 The LUN field MUST be valid if AsyncEvent is 0.  Otherwise, this
 field is reserved.

Chadalapaka, et al. Standards Track [Page 189] RFC 7143 iSCSI (Consolidated) April 2014

11.9.4. Sense Data and iSCSI Event Data

 For a SCSI event, this data accompanies the report in the data
 segment and identifies the condition.
 For an iSCSI event, additional vendor-unique data MAY accompany the
 Async event.  Initiators MAY ignore the data when not understood,
 while processing the rest of the PDU.
 If the DataSegmentLength is not 0, the format of the DataSegment is
 as follows:
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|SenseLength                    | Sense Data                    |
   +---------------+---------------+---------------+---------------+
  x/ Sense Data                                                    /
   +---------------+---------------+---------------+---------------+
  y/ iSCSI Event Data                                              /
   /                                                               /
   +---------------+---------------+---------------+---------------+
  z|

11.9.4.1. SenseLength

 This is the length of Sense Data.  When the Sense Data field is empty
 (e.g., the event is not a SCSI event), SenseLength is 0.

Chadalapaka, et al. Standards Track [Page 190] RFC 7143 iSCSI (Consolidated) April 2014

11.10. Text Request

 The Text Request is provided to allow for the exchange of information
 and for future extensions.  It permits the initiator to inform a
 target of its capabilities or request some special operations.
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| 0x04      |F|C| Reserved                                  |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment (Text)                                            /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 An initiator MUST NOT have more than one outstanding Text Request on
 a connection at any given time.
 On a connection failure, an initiator must either explicitly abort
 any active allegiant text negotiation task or cause such a task to be
 implicitly terminated by the target.

Chadalapaka, et al. Standards Track [Page 191] RFC 7143 iSCSI (Consolidated) April 2014

11.10.1. F (Final) Bit

 When set to 1, this bit indicates that this is the last or only Text
 Request in a sequence of Text Requests; otherwise, it indicates that
 more Text Requests will follow.

11.10.2. C (Continue) Bit

 When set to 1, this bit indicates that the text (set of key=value
 pairs) in this Text Request is not complete (it will be continued on
 subsequent Text Requests); otherwise, it indicates that this Text
 Request ends a set of key=value pairs.  A Text Request with the C bit
 set to 1 MUST have the F bit set to 0.

11.10.3. Initiator Task Tag

 This is the initiator-assigned identifier for this Text Request.  If
 the command is sent as part of a sequence of Text Requests and
 responses, the Initiator Task Tag MUST be the same for all the
 requests within the sequence (similar to linked SCSI commands).  The
 I bit for all requests in a sequence also MUST be the same.

11.10.4. Target Transfer Tag

 When the Target Transfer Tag is set to the reserved value 0xffffffff,
 it tells the target that this is a new request, and the target resets
 any internal state associated with the Initiator Task Tag (resets the
 current negotiation state).
 The target sets the Target Transfer Tag in a Text Response to a value
 other than the reserved value 0xffffffff whenever it indicates that
 it has more data to send or more operations to perform that are
 associated with the specified Initiator Task Tag.  It MUST do so
 whenever it sets the F bit to 0 in the response.  By copying the
 Target Transfer Tag from the response to the next Text Request, the
 initiator tells the target to continue the operation for the specific
 Initiator Task Tag.  The initiator MUST ignore the Target Transfer
 Tag in the Text Response when the F bit is set to 1.
 This mechanism allows the initiator and target to transfer a large
 amount of textual data over a sequence of text-command/text-response
 exchanges or to perform extended negotiation sequences.
 If the Target Transfer Tag is not 0xffffffff, the LUN field MUST be
 sent by the target in the Text Response.

Chadalapaka, et al. Standards Track [Page 192] RFC 7143 iSCSI (Consolidated) April 2014

 A target MAY reset its internal negotiation state if an exchange is
 stalled by the initiator for a long time or if it is running out of
 resources.
 Long Text Responses are handled as shown in the following example:
    I->T Text SendTargets=All (F = 1, TTT = 0xffffffff)
    T->I Text <part 1> (F = 0, TTT = 0x12345678)
    I->T Text <empty> (F = 1, TTT = 0x12345678)
    T->I Text <part 2> (F = 0, TTT = 0x12345678)
    I->T Text <empty> (F = 1, TTT = 0x12345678)
    ...
    T->I Text <part n> (F = 1, TTT = 0xffffffff)

11.10.5. Text

 The data lengths of a Text Request MUST NOT exceed the iSCSI target
 MaxRecvDataSegmentLength (a parameter that is negotiated per
 connection and per direction).  The text format is specified in
 Section 6.2.
 Sections 12 and 13 list some basic Text key=value pairs, some of
 which can be used in Login Requests/Responses and some in Text
 Requests/Responses.
 A key=value pair can span Text Request or Text Response boundaries.
 A key=value pair can start in one PDU and continue on the next.  In
 other words, the end of a PDU does not necessarily signal the end of
 a key=value pair.
 The target responds by sending its response back to the initiator.
 The response text format is similar to the request text format.  The
 Text Response MAY refer to key=value pairs presented in an earlier
 Text Request, and the text in the request may refer to earlier
 responses.
 Section 6.2 details the rules for the Text Requests and Responses.
 Text operations are usually meant for parameter setting/negotiations
 but can also be used to perform some long-lasting operations.

Chadalapaka, et al. Standards Track [Page 193] RFC 7143 iSCSI (Consolidated) April 2014

 Text operations that take a long time should be placed in their own
 Text Request.

11.11. Text Response

 The Text Response PDU contains the target's responses to the
 initiator's Text Request.  The format of the Text field matches that
 of the Text Request.
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x24      |F|C| Reserved                                  |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment (Text)                                            /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+

11.11.1. F (Final) Bit

 When set to 1, in response to a Text Request with the Final bit set
 to 1, the F bit indicates that the target has finished the whole
 operation.  Otherwise, if set to 0 in response to a Text Request with
 the Final Bit set to 1, it indicates that the target has more work to

Chadalapaka, et al. Standards Track [Page 194] RFC 7143 iSCSI (Consolidated) April 2014

 do (invites a follow-on Text Request).  A Text Response with the
 F bit set to 1 in response to a Text Request with the F bit set to 0
 is a protocol error.
 A Text Response with the F bit set to 1 MUST NOT contain key=value
 pairs that may require additional answers from the initiator.
 A Text Response with the F bit set to 1 MUST have a Target Transfer
 Tag field set to the reserved value 0xffffffff.
 A Text Response with the F bit set to 0 MUST have a Target Transfer
 Tag field set to a value other than the reserved value 0xffffffff.

11.11.2. C (Continue) Bit

 When set to 1, this bit indicates that the text (set of key=value
 pairs) in this Text Response is not complete (it will be continued on
 subsequent Text Responses); otherwise, it indicates that this Text
 Response ends a set of key=value pairs.  A Text Response with the
 C bit set to 1 MUST have the F bit set to 0.

11.11.3. Initiator Task Tag

 The Initiator Task Tag matches the tag used in the initial Text
 Request.

11.11.4. Target Transfer Tag

 When a target has more work to do (e.g., cannot transfer all the
 remaining text data in a single Text Response or has to continue the
 negotiation) and has enough resources to proceed, it MUST set the
 Target Transfer Tag to a value other than the reserved value
 0xffffffff.  Otherwise, the Target Transfer Tag MUST be set to
 0xffffffff.
 When the Target Transfer Tag is not 0xffffffff, the LUN field may be
 significant.
 The initiator MUST copy the Target Transfer Tag and LUN in its next
 request to indicate that it wants the rest of the data.
 When the target receives a Text Request with the Target Transfer Tag
 set to the reserved value 0xffffffff, it resets its internal
 information (resets state) associated with the given Initiator Task
 Tag (restarts the negotiation).

Chadalapaka, et al. Standards Track [Page 195] RFC 7143 iSCSI (Consolidated) April 2014

 When a target cannot finish the operation in a single Text Response
 and does not have enough resources to continue, it rejects the Text
 Request with the appropriate Reject code.
 A target may reset its internal state associated with an Initiator
 Task Tag (the current negotiation state) as expressed through the
 Target Transfer Tag if the initiator fails to continue the exchange
 for some time.  The target may reject subsequent Text Requests with
 the Target Transfer Tag set to the "stale" value.

11.11.5. StatSN

 The target StatSN variable is advanced by each Text Response sent.

11.11.6. Text Response Data

 The data lengths of a Text Response MUST NOT exceed the iSCSI
 initiator MaxRecvDataSegmentLength (a parameter that is negotiated
 per connection and per direction).
 The text in the Text Response Data is governed by the same rules as
 the text in the Text Request Data (see Section 11.11.2).
 Although the initiator is the requesting party and controls the
 request-response initiation and termination, the target can offer
 key=value pairs of its own as part of a sequence and not only in
 response to the initiator.

11.12. Login Request

 After establishing a TCP connection between an initiator and a
 target, the initiator MUST start a Login Phase to gain further access
 to the target's resources.
 The Login Phase (see Section 6.3) consists of a sequence of Login
 Requests and Login Responses that carry the same Initiator Task Tag.
 Login Requests are always considered as immediate.

Chadalapaka, et al. Standards Track [Page 196] RFC 7143 iSCSI (Consolidated) April 2014

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|1| 0x03      |T|C|.|.|CSG|NSG| Version-max   | Version-min   |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| ISID                                                          |
   +                               +---------------+---------------+
 12|                               | TSIH                          |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| CID                           | Reserved                      |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN or Reserved                                         |
   +---------------+---------------+---------------+---------------+
 32| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 36| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 40/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48/ DataSegment - Login Parameters in Text Request Format         /
  +/                                                               /
   +---------------+---------------+---------------+---------------+

11.12.1. T (Transit) Bit

 When set to 1, this bit indicates that the initiator is ready to
 transit to the next stage.
 If the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
 this also indicates that the initiator is ready for the Login
 Final-Response (see Section 6.3).

11.12.2. C (Continue) Bit

 When set to 1, this bit indicates that the text (set of key=value
 pairs) in this Login Request is not complete (it will be continued on
 subsequent Login Requests); otherwise, it indicates that this Login
 Request ends a set of key=value pairs.  A Login Request with the
 C bit set to 1 MUST have the T bit set to 0.

Chadalapaka, et al. Standards Track [Page 197] RFC 7143 iSCSI (Consolidated) April 2014

11.12.3. CSG and NSG

 Through these fields -- Current Stage (CSG) and Next Stage (NSG) --
 the Login negotiation requests and responses are associated with a
 specific stage in the session (SecurityNegotiation,
 LoginOperationalNegotiation, FullFeaturePhase) and may indicate the
 next stage to which they want to move (see Section 6.3).  The Next
 Stage value is only valid when the T bit is 1; otherwise, it is
 reserved.
 The stage codes are:
    0 - SecurityNegotiation
    1 - LoginOperationalNegotiation
    3 - FullFeaturePhase
 All other codes are reserved.

11.12.4. Version

 The version number for this document is 0x00.  Therefore, both
 Version-min and Version-max MUST be set to 0x00.

11.12.4.1. Version-max

 Version-max indicates the maximum version number supported.
 All Login Requests within the Login Phase MUST carry the same
 Version-max.
 The target MUST use the value presented with the first Login Request.

11.12.4.2. Version-min

 All Login Requests within the Login Phase MUST carry the same
 Version-min.  The target MUST use the value presented with the first
 Login Request.

Chadalapaka, et al. Standards Track [Page 198] RFC 7143 iSCSI (Consolidated) April 2014

11.12.5. ISID

 This is an initiator-defined component of the session identifier and
 is structured as follows (see Section 10.1.1 for details):
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  8| T |     A     |              B                |      C        |
   +---------------+---------------+---------------+---------------+
 12|               D               |
   +---------------+---------------+
 The T field identifies the format and usage of A, B, C, and D as
 indicated below:
    T
    00b    OUI-Format
           A and B: 22-bit OUI
           (the I/G and U/L bits are omitted)
           C and D: 24-bit Qualifier
    01b    EN: Format (IANA Enterprise Number)
           A: Reserved
           B and C: EN (IANA Enterprise Number)
           D: Qualifier
    10b    "Random"
           A: Reserved
           B and C: Random
           D: Qualifier
    11b    A, B, C, and D: Reserved
 For the T field values 00b and 01b, a combination of A and B (for
 00b) or B and C (for 01b) identifies the vendor or organization whose
 component (software or hardware) generates this ISID.  A vendor or

Chadalapaka, et al. Standards Track [Page 199] RFC 7143 iSCSI (Consolidated) April 2014

 organization with one or more OUIs, or one or more Enterprise
 Numbers, MUST use at least one of these numbers and select the
 appropriate value for the T field when its components generate ISIDs.
 An OUI or EN MUST be set in the corresponding fields in network byte
 order (byte big-endian).
 If the T field is 10b, B and C are set to a random 24-bit unsigned
 integer value in network byte order (byte big-endian).  See [RFC3721]
 for how this affects the principle of "conservative reuse".
 The Qualifier field is a 16-bit or 24-bit unsigned integer value that
 provides a range of possible values for the ISID within the selected
 namespace.  It may be set to any value within the constraints
 specified in the iSCSI protocol (see Sections 4.4.3 and 10.1.1).
 The T field value of 11b is reserved.
 If the ISID is derived from something assigned to a hardware adapter
 or interface by a vendor as a preset default value, it MUST be
 configurable to a value assigned according to the SCSI port behavior
 desired by the system in which it is installed (see Sections 10.1.1
 and 10.1.2).  The resultant ISID MUST also be persistent over power
 cycles, reboot, card swap, etc.

11.12.6. TSIH

 The TSIH must be set in the first Login Request.  The reserved value
 0 MUST be used on the first connection for a new session.  Otherwise,
 the TSIH sent by the target at the conclusion of the successful login
 of the first connection for this session MUST be used.  The TSIH
 identifies to the target the associated existing session for this new
 connection.
 All Login Requests within a Login Phase MUST carry the same TSIH.
 The target MUST check the value presented with the first Login
 Request and act as specified in Section 6.3.1.

11.12.7. Connection ID (CID)

 The CID provides a unique ID for this connection within the session.
 All Login Requests within the Login Phase MUST carry the same CID.
 The target MUST use the value presented with the first Login Request.

Chadalapaka, et al. Standards Track [Page 200] RFC 7143 iSCSI (Consolidated) April 2014

 A Login Request with a non-zero TSIH and a CID equal to that of an
 existing connection implies a logout of the connection followed by a
 login (see Section 6.3.4).  For details regarding the implicit Logout
 Request, see Section 11.14.

11.12.8. CmdSN

 The CmdSN is either the initial command sequence number of a session
 (for the first Login Request of a session -- the "leading" login) or
 the command sequence number in the command stream if the login is for
 a new connection in an existing session.
 Examples:
  1. Login on a leading connection: If the leading login carries the

CmdSN 123, all other Login Requests in the same Login Phase carry

   the CmdSN 123, and the first non-immediate command in the Full
   Feature Phase also carries the CmdSN 123.
  1. Login on other than a leading connection: If the current CmdSN at

the time the first login on the connection is issued is 500, then

   that PDU carries CmdSN=500.  Subsequent Login Requests that are
   needed to complete this Login Phase may carry a CmdSN higher than
   500 if non-immediate requests that were issued on other connections
   in the same session advance the CmdSN.
 If the Login Request is a leading Login Request, the target MUST use
 the value presented in the CmdSN as the target value for the
 ExpCmdSN.

11.12.9. ExpStatSN

 For the first Login Request on a connection, this is the ExpStatSN
 for the old connection, and this field is only valid if the Login
 Request restarts a connection (see Section 6.3.4).
 For subsequent Login Requests, it is used to acknowledge the Login
 Responses with their increasing StatSN values.

11.12.10. Login Parameters

 The initiator MUST provide some basic parameters in order to enable
 the target to determine if the initiator may use the target's
 resources and the initial text parameters for the security exchange.
 All the rules specified in Section 11.10.5 for Text Requests also
 hold for Login Requests.  Keys and their explanations are listed in
 Section 12 (security negotiation keys) and in Section 13 (operational

Chadalapaka, et al. Standards Track [Page 201] RFC 7143 iSCSI (Consolidated) April 2014

 parameter negotiation keys).  All keys listed in Section 13, except
 for the X extension formats, MUST be supported by iSCSI initiators
 and targets.  Keys listed in Section 12 only need to be supported
 when the function to which they refer is mandatory to implement.

11.13. Login Response

 The Login Response indicates the progress and/or end of the Login
 Phase.
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x23      |T|C|.|.|CSG|NSG| Version-max   |Version-active |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| ISID                                                          |
   +                               +---------------+---------------+
 12|                               | TSIH                          |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| Status-Class  | Status-Detail | Reserved                      |
   +---------------+---------------+---------------+---------------+
 40/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48/ DataSegment - Login Parameters in Text Request Format         /
  +/                                                               /
   +---------------+---------------+---------------+---------------+

11.13.1. Version-max

 This is the highest version number supported by the target.
 All Login Responses within the Login Phase MUST carry the same
 Version-max.

Chadalapaka, et al. Standards Track [Page 202] RFC 7143 iSCSI (Consolidated) April 2014

 The initiator MUST use the value presented as a response to the first
 Login Request.

11.13.2. Version-active

 Version-active indicates the highest version supported by the target
 and initiator.  If the target does not support a version within the
 range specified by the initiator, the target rejects the login and
 this field indicates the lowest version supported by the target.
 All Login Responses within the Login Phase MUST carry the same
 Version-active.
 The initiator MUST use the value presented as a response to the first
 Login Request.

11.13.3. TSIH

 The TSIH is the target-assigned session-identifying handle.  Its
 internal format and content are not defined by this protocol, except
 for the value 0, which is reserved.  With the exception of the Login
 Final-Response in a new session, this field should be set to the TSIH
 provided by the initiator in the Login Request.  For a new session,
 the target MUST generate a non-zero TSIH and ONLY return it in the
 Login Final-Response (see Section 6.3).

11.13.4. StatSN

 For the first Login Response (the response to the first Login
 Request), this is the starting status sequence number for the
 connection.  The next response of any kind -- including the next
 Login Response, if any, in the same Login Phase -- will carry this
 number + 1.  This field is only valid if the Status-Class is 0.

11.13.5. Status-Class and Status-Detail

 The Status returned in a Login Response indicates the execution
 status of the Login Phase.  The status includes:
    Status-Class
    Status-Detail
 A Status-Class of 0 indicates success.
 A non-zero Status-Class indicates an exception.  In this case,
 Status-Class is sufficient for a simple initiator to use when
 handling exceptions, without having to look at the Status-Detail.

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 The Status-Detail allows finer-grained exception handling for more
 sophisticated initiators and for better information for logging.
 The Status-Classes are as follows:
    0  Success - indicates that the iSCSI target successfully
       received, understood, and accepted the request.  The numbering
       fields (StatSN, ExpCmdSN, MaxCmdSN) are only valid if Status-
       Class is 0.
    1  Redirection - indicates that the initiator must take further
       action to complete the request.  This is usually due to the
       target moving to a different address.  All of the redirection
       Status-Class responses MUST return one or more text key
       parameters of the type "TargetAddress", which indicates the
       target's new address.  A redirection response MAY be issued by
       a target prior to or after completing a security negotiation if
       a security negotiation is required.  A redirection SHOULD be
       accepted by an initiator, even without having the target
       complete a security negotiation if any security negotiation is
       required, and MUST be accepted by the initiator after the
       completion of the security negotiation if any security
       negotiation is required.
    2  Initiator Error (not a format error) - indicates that the
       initiator most likely caused the error.  This MAY be due to a
       request for a resource for which the initiator does not have
       permission.  The request should not be tried again.
    3  Target Error - indicates that the target sees no errors in the
       initiator's Login Request but is currently incapable of
       fulfilling the request.  The initiator may retry the same Login
       Request later.

Chadalapaka, et al. Standards Track [Page 204] RFC 7143 iSCSI (Consolidated) April 2014

 The table below shows all of the currently allocated status codes.
 The codes are in hexadecimal; the first byte is the Status-Class, and
 the second byte is the status detail.
  1. —————————————————————-

Status | Code | Description

                 |(hex) |
   -----------------------------------------------------------------
   Success       | 0000 | Login is proceeding OK (*1).
   -----------------------------------------------------------------
   Target moved  | 0101 | The requested iSCSI Target Name (ITN)
   temporarily   |      | has temporarily moved
                 |      | to the address provided.
   -----------------------------------------------------------------
   Target moved  | 0102 | The requested ITN has permanently moved
   permanently   |      | to the address provided.
   -----------------------------------------------------------------
   Initiator     | 0200 | Miscellaneous iSCSI initiator
   error         |      | errors.
   -----------------------------------------------------------------
   Authentication| 0201 | The initiator could not be
   failure       |      | successfully authenticated or target
                 |      | authentication is not supported.
   -----------------------------------------------------------------
   Authorization | 0202 | The initiator is not allowed access
   failure       |      | to the given target.
   -----------------------------------------------------------------
   Not found     | 0203 | The requested ITN does not
                 |      | exist at this address.
   -----------------------------------------------------------------
   Target removed| 0204 | The requested ITN has been removed, and
                 |      | no forwarding address is provided.
   -----------------------------------------------------------------
   Unsupported   | 0205 | The requested iSCSI version range is
   version       |      | not supported by the target.
   -----------------------------------------------------------------
   Too many      | 0206 | Too many connections on this SSID.
   connections   |      |
   -----------------------------------------------------------------
   Missing       | 0207 | Missing parameters (e.g., iSCSI
   parameter     |      | Initiator Name and/or Target Name).
   -----------------------------------------------------------------
   Can't include | 0208 | Target does not support session
   in session    |      | spanning to this connection (address).
   -----------------------------------------------------------------
   Session type  | 0209 | Target does not support this type of
   not supported |      | session or not from this initiator.
   -----------------------------------------------------------------

Chadalapaka, et al. Standards Track [Page 205] RFC 7143 iSCSI (Consolidated) April 2014

   Session does  | 020a | Attempt to add a connection
   not exist     |      | to a non-existent session.
   -----------------------------------------------------------------
   Invalid during| 020b | Invalid request type during login.
   login         |      |
   -----------------------------------------------------------------
   Target error  | 0300 | Target hardware or software error.
   -----------------------------------------------------------------
   Service       | 0301 | The iSCSI service or target is not
   unavailable   |      | currently operational.
   -----------------------------------------------------------------
   Out of        | 0302 | The target has insufficient session,
   resources     |      | connection, or other resources.
   -----------------------------------------------------------------
 (*1) If the response T bit is set to 1 in both the request and the
      matching response, and the NSG is set to FullFeaturePhase in
      both the request and the matching response, the Login Phase is
      finished, and the initiator may proceed to issue SCSI commands.
 If the Status-Class is not 0, the initiator and target MUST close the
 TCP connection.
 If the target wishes to reject the Login Request for more than one
 reason, it should return the primary reason for the rejection.

11.13.6. T (Transit) Bit

 The T bit is set to 1 as an indicator of the end of the stage.  If
 the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
 this is also the Login Final-Response (see Section 6.3).  A T bit of
 0 indicates a "partial" response, which means "more negotiation
 needed".
 A Login Response with the T bit set to 1 MUST NOT contain key=value
 pairs that may require additional answers from the initiator within
 the same stage.
 If the Status-Class is 0, the T bit MUST NOT be set to 1 if the T bit
 in the request was set to 0.

11.13.7. C (Continue) Bit

 When set to 1, this bit indicates that the text (set of key=value
 pairs) in this Login Response is not complete (it will be continued
 on subsequent Login Responses); otherwise, it indicates that this
 Login Response ends a set of key=value pairs.  A Login Response with
 the C bit set to 1 MUST have the T bit set to 0.

Chadalapaka, et al. Standards Track [Page 206] RFC 7143 iSCSI (Consolidated) April 2014

11.13.8. Login Parameters

 The target MUST provide some basic parameters in order to enable the
 initiator to determine if it is connected to the correct port and the
 initial text parameters for the security exchange.
 All the rules specified in Section 11.11.6 for Text Responses also
 hold for Login Responses.  Keys and their explanations are listed in
 Section 12 (security negotiation keys) and in Section 13 (operational
 parameter negotiation keys).  All keys listed in Section 13, except
 for the X extension formats, MUST be supported by iSCSI initiators
 and targets.  Keys listed in Section 12 only need to be supported
 when the function to which they refer is mandatory to implement.

11.14. Logout Request

 The Logout Request is used to perform a controlled closing of a
 connection.
 An initiator MAY use a Logout Request to remove a connection from a
 session or to close an entire session.
 After sending the Logout Request PDU, an initiator MUST NOT send any
 new iSCSI requests on the closing connection.  If the Logout Request
 is intended to close the session, new iSCSI requests MUST NOT be sent
 on any of the connections participating in the session.
 When receiving a Logout Request with the reason code "close the
 connection" or "close the session", the target MUST terminate all
 pending commands, whether acknowledged via the ExpCmdSN or not, on
 that connection or session, respectively.
 When receiving a Logout Request with the reason code "remove the
 connection for recovery", the target MUST discard all requests not
 yet acknowledged via the ExpCmdSN that were issued on the specified
 connection and suspend all data/status/R2T transfers on behalf of
 pending commands on the specified connection.
 The target then issues the Logout Response and half-closes the TCP
 connection (sends FIN).  After receiving the Logout Response and
 attempting to receive the FIN (if still possible), the initiator MUST
 completely close the logging-out connection.  For the terminated
 commands, no additional responses should be expected.
 A Logout for a CID may be performed on a different transport
 connection when the TCP connection for the CID has already been
 terminated.  In such a case, only a logical "closing" of the iSCSI
 connection for the CID is implied with a Logout.

Chadalapaka, et al. Standards Track [Page 207] RFC 7143 iSCSI (Consolidated) April 2014

 All commands that were not terminated or not completed (with status)
 and acknowledged when the connection is closed completely can be
 reassigned to a new connection if the target supports connection
 recovery.
 If an initiator intends to start recovery for a failing connection,
 it MUST use the Logout Request to "clean up" the target end of a
 failing connection and enable recovery to start, or use the Login
 Request with a non-zero TSIH and the same CID on a new connection for
 the same effect.  In sessions with a single connection, the
 connection can be closed and then a new connection reopened.  A
 connection reinstatement login can be used for recovery (see
 Section 6.3.4).
 A successful completion of a Logout Request with the reason code
 "close the connection" or "remove the connection for recovery"
 results at the target in the discarding of unacknowledged commands
 received on the connection being logged out.  These are commands that
 have arrived on the connection being logged out but that have not
 been delivered to SCSI because one or more commands with a smaller
 CmdSN have not been received by iSCSI.  See Section 4.2.2.1.  The
 resulting holes in the command sequence numbers will have to be
 handled by appropriate recovery (see Section 7), unless the session
 is also closed.
 The entire logout discussion in this section is also applicable for
 an implicit Logout realized by way of a connection reinstatement or
 session reinstatement.  When a Login Request performs an implicit
 Logout, the implicit Logout is performed as if having the reason
 codes specified below:
   Reason Code     Type of Implicit Logout
   -------------------------------------------------------------
        0          session reinstatement
        1          connection reinstatement when the operational
                   ErrorRecoveryLevel < 2
        2          connection reinstatement when the operational
                   ErrorRecoveryLevel = 2

Chadalapaka, et al. Standards Track [Page 208] RFC 7143 iSCSI (Consolidated) April 2014

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| 0x06      |1| Reason Code | Reserved                      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------------------------------------------------------+
  8/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| CID or Reserved               | Reserved                      |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+

11.14.1. Reason Code

 The Reason Code field indicates the reason for Logout as follows:
    0 - close the session.  All commands associated with the
        session (if any) are terminated.
    1 - close the connection.  All commands associated with the
        connection (if any) are terminated.
    2 - remove the connection for recovery.  The connection is
        closed, and all commands associated with it, if any, are
        to be prepared for a new allegiance.
 All other values are reserved.

11.14.2. TotalAHSLength and DataSegmentLength

 For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

Chadalapaka, et al. Standards Track [Page 209] RFC 7143 iSCSI (Consolidated) April 2014

11.14.3. CID

 This is the connection ID of the connection to be closed (including
 closing the TCP stream).  This field is only valid if the reason code
 is not "close the session".

11.14.4. ExpStatSN

 This is the last ExpStatSN value for the connection to be closed.

11.14.5. Implicit Termination of Tasks

 A target implicitly terminates the active tasks due to the iSCSI
 protocol in the following cases:
    a) When a connection is implicitly or explicitly logged out with
       the reason code "close the connection" and there are active
       tasks allegiant to that connection.
    b) When a connection fails and eventually the connection state
       times out (state transition M1 in Section 8.2.2) and there are
       active tasks allegiant to that connection.
    c) When a successful recovery Logout is performed while there are
       active tasks allegiant to that connection and those tasks
       eventually time out after the Time2Wait and Time2Retain periods
       without allegiance reassignment.
    d) When a connection is implicitly or explicitly logged out with
       the reason code "close the session" and there are active tasks
       in that session.
 If the tasks terminated in any of the above cases are SCSI tasks,
 they must be internally terminated as if with CHECK CONDITION status.
 This status is only meaningful for appropriately handling the
 internal SCSI state and SCSI side effects with respect to ordering,
 because this status is never communicated back as a terminating
 status to the initiator.  However, additional actions may have to be
 taken at the SCSI level, depending on the SCSI context as defined by
 the SCSI standards (e.g., queued commands and ACA; UA for the next
 command on the I_T nexus in cases a), b), and c) above).  After the
 tasks are terminated, the target MUST report a Unit Attention
 condition on the next command processed on any connection for each
 affected I_T_L nexus with the status of CHECK CONDITION, the ASC/ASCQ
 value of 47h/7Fh ("SOME COMMANDS CLEARED BY ISCSI PROTOCOL EVENT"),
 etc.; see [SPC3].

Chadalapaka, et al. Standards Track [Page 210] RFC 7143 iSCSI (Consolidated) April 2014

11.15. Logout Response

 The Logout Response is used by the target to indicate if the cleanup
 operation for the connection(s) has completed.
 After Logout, the TCP connection referred by the CID MUST be closed
 at both ends (or all connections must be closed if the logout reason
 was session close).
 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x26      |1| Reserved    | Response      | Reserved      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------------------------------------------------------+
  8/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag                                            |
   +---------------+---------------+---------------+---------------+
 20| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| Reserved                                                      |
   +---------------------------------------------------------------+
 40| Time2Wait                     | Time2Retain                   |
   +---------------+---------------+---------------+---------------+
 44| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+

Chadalapaka, et al. Standards Track [Page 211] RFC 7143 iSCSI (Consolidated) April 2014

11.15.1. Response

 Response field settings are as follows:
    0 - connection or session closed successfully.
    1 - CID not found.
    2 - connection recovery is not supported (i.e., the Logout reason
        code was "remove the connection for recovery" and the target
        does not support it as indicated by the operational
        ErrorRecoveryLevel).
    3 - cleanup failed for various reasons.

11.15.2. TotalAHSLength and DataSegmentLength

 For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.15.3. Time2Wait

 If the Logout response code is 0 and the operational
 ErrorRecoveryLevel is 2, this is the minimum amount of time, in
 seconds, to wait before attempting task reassignment.  If the Logout
 response code is 0 and the operational ErrorRecoveryLevel is less
 than 2, this field is to be ignored.
 This field is invalid if the Logout response code is 1.
 If the Logout response code is 2 or 3, this field specifies the
 minimum time to wait before attempting a new implicit or explicit
 logout.
 If Time2Wait is 0, the reassignment or a new Logout may be attempted
 immediately.

11.15.4. Time2Retain

 If the Logout response code is 0 and the operational
 ErrorRecoveryLevel is 2, this is the maximum amount of time, in
 seconds, after the initial wait (Time2Wait) that the target waits for
 the allegiance reassignment for any active task, after which the task
 state is discarded.  If the Logout response code is 0 and the
 operational ErrorRecoveryLevel is less than 2, this field is to be
 ignored.
 This field is invalid if the Logout response code is 1.

Chadalapaka, et al. Standards Track [Page 212] RFC 7143 iSCSI (Consolidated) April 2014

 If the Logout response code is 2 or 3, this field specifies the
 maximum amount of time, in seconds, after the initial wait
 (Time2Wait) that the target waits for a new implicit or explicit
 logout.
 If it is the last connection of a session, the whole session state is
 discarded after Time2Retain.
 If Time2Retain is 0, the target has already discarded the connection
 (and possibly the session) state along with the task states.  No
 reassignment or Logout is required in this case.

11.16. SNACK Request

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x10      |1|.|.|.| Type  | Reserved                      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag or 0xffffffff                              |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or SNACK Tag or 0xffffffff                |
   +---------------+---------------+---------------+---------------+
 24| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 40| BegRun                                                        |
   +---------------------------------------------------------------+
 44| RunLength                                                     |
   +---------------------------------------------------------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
 If the implementation supports ErrorRecoveryLevel greater than zero,
 it MUST support all SNACK types.

Chadalapaka, et al. Standards Track [Page 213] RFC 7143 iSCSI (Consolidated) April 2014

 The SNACK is used by the initiator to request the retransmission of
 numbered responses, data, or R2T PDUs from the target.  The SNACK
 Request indicates the numbered responses or data "runs" whose
 retransmission is requested, where the run starts with the first
 StatSN, DataSN, or R2TSN whose retransmission is requested and
 indicates the number of Status, Data, or R2T PDUs requested,
 including the first.  0 has special meaning when used as a starting
 number and length:
  1. When used in RunLength, it means all PDUs starting with the

initial.

  1. When used in both BegRun and RunLength, it means all

unacknowledged PDUs.

 The numbered response(s) or R2T(s) requested by a SNACK MUST be
 delivered as exact replicas of the ones that the target transmitted
 originally, except for the fields ExpCmdSN, MaxCmdSN, and ExpDataSN,
 which MUST carry the current values.  R2T(s)requested by SNACK MUST
 also carry the current value of the StatSN.
 The numbered Data-In PDUs requested by a Data SNACK MUST be delivered
 as exact replicas of the ones that the target transmitted originally,
 except for the fields ExpCmdSN and MaxCmdSN, which MUST carry the
 current values; and except for resegmentation (see Section 11.16.3).
 Any SNACK that requests a numbered response, data, or R2T that was
 not sent by the target or was already acknowledged by the initiator
 MUST be rejected with a reason code of "Protocol Error".

11.16.1. Type

 This field encodes the SNACK function as follows:
    0 - Data/R2T SNACK: requesting retransmission of one or more
        Data-In or R2T PDUs.
    1 - Status SNACK: requesting retransmission of one or more
        numbered responses.
    2 - DataACK: positively acknowledges Data-In PDUs.
    3 - R-Data SNACK: requesting retransmission of Data-In PDUs with
        possible resegmentation and status tagging.
 All other values are reserved.

Chadalapaka, et al. Standards Track [Page 214] RFC 7143 iSCSI (Consolidated) April 2014

 Data/R2T SNACK, Status SNACK, or R-Data SNACK for a command MUST
 precede status acknowledgment for the given command.

11.16.2. Data Acknowledgment

 If an initiator operates at ErrorRecoveryLevel 1 or higher, it MUST
 issue a SNACK of type DataACK after receiving a Data-In PDU with the
 A bit set to 1.  However, if the initiator has detected holes in the
 input sequence, it MUST postpone issuing the SNACK of type DataACK
 until the holes are filled.  An initiator MAY ignore the A bit if it
 deems that the bit is being set aggressively by the target (i.e.,
 before the MaxBurstLength limit is reached).
 The DataACK is used to free resources at the target and not to
 request or imply data retransmission.
 An initiator MUST NOT request retransmission for any data it had
 already acknowledged.

11.16.3. Resegmentation

 If the initiator MaxRecvDataSegmentLength changed between the
 original transmission and the time the initiator requests
 retransmission, the initiator MUST issue a R-Data SNACK (see
 Section 11.16.1).  With R-Data SNACK, the initiator indicates that it
 discards all the unacknowledged data and expects the target to resend
 it.  It also expects resegmentation.  In this case, the retransmitted
 Data-In PDUs MAY be different from the ones originally sent in order
 to reflect changes in MaxRecvDataSegmentLength.  Their DataSN starts
 with the BegRun of the last DataACK received by the target if any was
 received; otherwise, it starts with 0 and is increased by 1 for each
 resent Data-In PDU.
 A target that has received a R-Data SNACK MUST return a SCSI Response
 that contains a copy of the SNACK Tag field from the R-Data SNACK in
 the SCSI Response SNACK Tag field as its last or only Response.  For
 example, if it has already sent a response containing another value
 in the SNACK Tag field or had the status included in the last Data-In
 PDU, it must send a new SCSI Response PDU.  If a target sends more
 than one SCSI Response PDU due to this rule, all SCSI Response PDUs
 must carry the same StatSN (see Section 11.4.4).  If an initiator
 attempts to recover a lost SCSI Response (with a Status-SNACK; see
 Section 11.16.1) when more than one response has been sent, the
 target will send the SCSI Response with the latest content known to
 the target, including the last SNACK Tag for the command.

Chadalapaka, et al. Standards Track [Page 215] RFC 7143 iSCSI (Consolidated) April 2014

 For considerations in allegiance reassignment of a task to a
 connection with a different MaxRecvDataSegmentLength, refer to
 Section 7.2.2.

11.16.4. Initiator Task Tag

 For a Status SNACK and DataACK, the Initiator Task Tag MUST be set to
 the reserved value 0xffffffff.  In all other cases, the Initiator
 Task Tag field MUST be set to the Initiator Task Tag of the
 referenced command.

11.16.5. Target Transfer Tag or SNACK Tag

 For a R-Data SNACK, this field MUST contain a value that is different
 from 0 or 0xffffffff and is unique for the task (identified by the
 Initiator Task Tag).  This value MUST be copied by the iSCSI target
 in the last or only SCSI Response PDU it issues for the command.
 For DataACK, the Target Transfer Tag MUST contain a copy of the
 Target Transfer Tag and LUN provided with the SCSI Data-In PDU with
 the A bit set to 1.
 In all other cases, the Target Transfer Tag field MUST be set to the
 reserved value 0xffffffff.

11.16.6. BegRun

 This field indicates the DataSN, R2TSN, or StatSN of the first PDU
 whose retransmission is requested (Data/R2T and Status SNACK), or the
 next expected DataSN (DataACK SNACK).
 A BegRun of 0, when used in conjunction with a RunLength of 0, means
 "resend all unacknowledged Data-In, R2T or Response PDUs".
 BegRun MUST be 0 for a R-Data SNACK.

11.16.7. RunLength

 This field indicates the number of PDUs whose retransmission is
 requested.
 A RunLength of 0 signals that all Data-In, R2T, or Response PDUs
 carrying the numbers equal to or greater than BegRun have to be
 resent.
 The RunLength MUST also be 0 for a DataACK SNACK in addition to a
 R-Data SNACK.

Chadalapaka, et al. Standards Track [Page 216] RFC 7143 iSCSI (Consolidated) April 2014

11.17. Reject

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x3f      |1| Reserved    | Reason        | Reserved      |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 16| 0xffffffff                                                    |
   +---------------+---------------+---------------+---------------+
 20| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36| DataSN/R2TSN or Reserved                                      |
   +---------------+---------------+---------------+---------------+
 40| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 44| Reserved                                                      |
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
 xx/ Complete Header of Bad PDU                                    /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 yy/Vendor-specific data (if any)                                  /
   /                                                               /
   +---------------+---------------+---------------+---------------+
 zz| Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 Reject is used to indicate an iSCSI error condition (protocol,
 unsupported option, etc.).

Chadalapaka, et al. Standards Track [Page 217] RFC 7143 iSCSI (Consolidated) April 2014

11.17.1. Reason

 The reject Reason is coded as follows:
 +------+----------------------------------------+----------------+
 | Code | Explanation                            |Can the original|
 | (hex)|                                        |PDU be resent?  |
 +------+----------------------------------------+----------------+
 | 0x01 | Reserved                               | no             |
 |      |                                        |                |
 | 0x02 | Data (payload) digest error            | yes (Note 1)   |
 |      |                                        |                |
 | 0x03 | SNACK Reject                           | yes            |
 |      |                                        |                |
 | 0x04 | Protocol Error (e.g., SNACK Request for| no             |
 |      | a status that was already acknowledged)|                |
 |      |                                        |                |
 | 0x05 | Command not supported                  | no             |
 |      |                                        |                |
 | 0x06 | Immediate command reject - too many    | yes            |
 |      | immediate commands                     |                |
 |      |                                        |                |
 | 0x07 | Task in progress                       | no             |
 |      |                                        |                |
 | 0x08 | Invalid data ack                       | no             |
 |      |                                        |                |
 | 0x09 | Invalid PDU field                      | no (Note 2)    |
 |      |                                        |                |
 | 0x0a | Long op reject - Can't generate Target | yes            |
 |      | Transfer Tag - out of resources        |                |
 |      |                                        |                |
 | 0x0b | Deprecated; MUST NOT be used           | N/A (Note 3)   |
 |      |                                        |                |
 | 0x0c | Waiting for Logout                     | no             |
 +------+----------------------------------------+----------------+
 Note 1: For iSCSI, Data-Out PDU retransmission is only done if the
         target requests retransmission with a recovery R2T.  However,
         if this is the data digest error on immediate data, the
         initiator may choose to retransmit the whole PDU, including
         the immediate data.
 Note 2: A target should use this reason code for all invalid values
         of PDU fields that are meant to describe a task, a response,
         or a data transfer.  Some examples are invalid TTT/ITT,
         buffer offset, LUN qualifying a TTT, and an invalid sequence
         number in a SNACK.

Chadalapaka, et al. Standards Track [Page 218] RFC 7143 iSCSI (Consolidated) April 2014

 Note 3: Reason code 0x0b ("Negotiation Reset") as defined in
         Section 10.17.1 of [RFC3720] is deprecated and MUST NOT be
         used by implementations.  An implementation receiving reason
         code 0x0b MUST treat it as a negotiation failure that
         terminates the Login Phase and the TCP connection, as
         specified in Section 7.12.
 All other values for Reason are unassigned.
 In all the cases in which a pre-instantiated SCSI task is terminated
 because of the reject, the target MUST issue a proper SCSI command
 response with CHECK CONDITION as described in Section 11.4.3.  In
 these cases in which a status for the SCSI task was already sent
 before the reject, no additional status is required.  If the error is
 detected while data from the initiator is still expected (i.e., the
 command PDU did not contain all the data and the target has not
 received a Data-Out PDU with the Final bit set to 1 for the
 unsolicited data, if any, and all outstanding R2Ts, if any), the
 target MUST wait until it receives the last expected Data-Out PDUs
 with the F bit set to 1 before sending the Response PDU.
 For additional usage semantics of the Reject PDU, see Section 7.3.

11.17.2. DataSN/R2TSN

 This field is only valid if the rejected PDU is a Data/R2T SNACK and
 the Reject reason code is "Protocol Error" (see Section 11.16).  The
 DataSN/R2TSN is the next Data/R2T sequence number that the target
 would send for the task, if any.

11.17.3. StatSN, ExpCmdSN, and MaxCmdSN

 These fields carry their usual values and are not related to the
 rejected command.  The StatSN is advanced after a Reject.

11.17.4. Complete Header of Bad PDU

 The target returns the header (not including the digest) of the PDU
 in error as the data of the response.

Chadalapaka, et al. Standards Track [Page 219] RFC 7143 iSCSI (Consolidated) April 2014

11.18. NOP-Out

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|I| 0x00      |1| Reserved                                    |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag or 0xffffffff                              |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| CmdSN                                                         |
   +---------------+---------------+---------------+---------------+
 28| ExpStatSN                                                     |
   +---------------+---------------+---------------+---------------+
 32/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment - Ping Data (optional)                            /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 NOP-Out may be used by an initiator as a "ping request" to verify
 that a connection/session is still active and all its components are
 operational.  The NOP-In response is the "ping echo".
 A NOP-Out is also sent by an initiator in response to a NOP-In.
 A NOP-Out may also be used to confirm a changed ExpStatSN if another
 PDU will not be available for a long time.
 Upon receipt of a NOP-In with the Target Transfer Tag set to a valid
 value (not the reserved value 0xffffffff), the initiator MUST respond
 with a NOP-Out.  In this case, the NOP-Out Target Transfer Tag MUST
 contain a copy of the NOP-In Target Transfer Tag.  The initiator

Chadalapaka, et al. Standards Track [Page 220] RFC 7143 iSCSI (Consolidated) April 2014

 SHOULD NOT send a NOP-Out in response to any other received NOP-In,
 in order to avoid lengthy sequences of NOP-In and NOP-Out PDUs sent
 in response to each other.

11.18.1. Initiator Task Tag

 The NOP-Out MUST have the Initiator Task Tag set to a valid value
 only if a response in the form of a NOP-In is requested (i.e., the
 NOP-Out is used as a ping request).  Otherwise, the Initiator Task
 Tag MUST be set to 0xffffffff.
 When a target receives the NOP-Out with a valid Initiator Task Tag,
 it MUST respond with a NOP-In Response (see Section 4.6.3.6).
 If the Initiator Task Tag contains 0xffffffff, the I bit MUST be set
 to 1, and the CmdSN is not advanced after this PDU is sent.

11.18.2. Target Transfer Tag

 The Target Transfer Tag is a target-assigned identifier for the
 operation.
 The NOP-Out MUST only have the Target Transfer Tag set if it is
 issued in response to a NOP-In with a valid Target Transfer Tag.  In
 this case, it copies the Target Transfer Tag from the NOP-In PDU.
 Otherwise, the Target Transfer Tag MUST be set to 0xffffffff.
 When the Target Transfer Tag is set to a value other than 0xffffffff,
 the LUN field MUST also be copied from the NOP-In.

11.18.3. Ping Data

 Ping data is reflected in the NOP-In Response.  The length of the
 reflected data is limited to MaxRecvDataSegmentLength.  The length of
 ping data is indicated by the DataSegmentLength.  0 is a valid value
 for the DataSegmentLength and indicates the absence of ping data.

Chadalapaka, et al. Standards Track [Page 221] RFC 7143 iSCSI (Consolidated) April 2014

11.19. NOP-In

 Byte/     0       |       1       |       2       |       3       |
    /              |               |               |               |
   |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
   +---------------+---------------+---------------+---------------+
  0|.|.| 0x20      |1| Reserved                                    |
   +---------------+---------------+---------------+---------------+
  4|TotalAHSLength | DataSegmentLength                             |
   +---------------+---------------+---------------+---------------+
  8| LUN or Reserved                                               |
   +                                                               +
 12|                                                               |
   +---------------+---------------+---------------+---------------+
 16| Initiator Task Tag or 0xffffffff                              |
   +---------------+---------------+---------------+---------------+
 20| Target Transfer Tag or 0xffffffff                             |
   +---------------+---------------+---------------+---------------+
 24| StatSN                                                        |
   +---------------+---------------+---------------+---------------+
 28| ExpCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 32| MaxCmdSN                                                      |
   +---------------+---------------+---------------+---------------+
 36/ Reserved                                                      /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
 48| Header-Digest (optional)                                      |
   +---------------+---------------+---------------+---------------+
   / DataSegment - Return Ping Data                                /
  +/                                                               /
   +---------------+---------------+---------------+---------------+
   | Data-Digest (optional)                                        |
   +---------------+---------------+---------------+---------------+
 NOP-In is sent by a target as either a response to a NOP-Out, a
 "ping" to an initiator, or a means to carry a changed ExpCmdSN and/or
 MaxCmdSN if another PDU will not be available for a long time (as
 determined by the target).
 When a target receives the NOP-Out with a valid Initiator Task Tag
 (not the reserved value 0xffffffff), it MUST respond with a NOP-In
 with the same Initiator Task Tag that was provided in the NOP-Out
 request.  It MUST also duplicate up to the first
 MaxRecvDataSegmentLength bytes of the initiator-provided Ping Data.
 For such a response, the Target Transfer Tag MUST be 0xffffffff.  The

Chadalapaka, et al. Standards Track [Page 222] RFC 7143 iSCSI (Consolidated) April 2014

 target SHOULD NOT send a NOP-In in response to any other received
 NOP-Out in order to avoid lengthy sequences of NOP-In and NOP-Out
 PDUs sent in response to each other.
 Otherwise, when a target sends a NOP-In that is not a response to a
 NOP-Out received from the initiator, the Initiator Task Tag MUST be
 set to 0xffffffff, and the data segment MUST NOT contain any data
 (DataSegmentLength MUST be 0).

11.19.1. Target Transfer Tag

 If the target is responding to a NOP-Out, this field is set to the
 reserved value 0xffffffff.
 If the target is sending a NOP-In as a ping (intending to receive a
 corresponding NOP-Out), this field is set to a valid value (not the
 reserved value 0xffffffff).
 If the target is initiating a NOP-In without wanting to receive a
 corresponding NOP-Out, this field MUST hold the reserved value
 0xffffffff.

11.19.2. StatSN

 The StatSN field will always contain the next StatSN.  However, when
 the Initiator Task Tag is set to 0xffffffff, the StatSN for the
 connection is not advanced after this PDU is sent.

11.19.3. LUN

 A LUN MUST be set to a correct value when the Target Transfer Tag is
 valid (not the reserved value 0xffffffff).

12. iSCSI Security Text Keys and Authentication Methods

 Only the following keys are used during the SecurityNegotiation stage
 of the Login Phase:
    SessionType
    InitiatorName
    TargetName
    TargetAddress
    InitiatorAlias

Chadalapaka, et al. Standards Track [Page 223] RFC 7143 iSCSI (Consolidated) April 2014

    TargetAlias
    TargetPortalGroupTag
    AuthMethod and the keys used by the authentication methods
       specified in Section 12.1, along with all of their associated
       keys, as well as Vendor-Specific Authentication Methods.
 Other keys MUST NOT be used.
 SessionType, InitiatorName, TargetName, InitiatorAlias, TargetAlias,
 and TargetPortalGroupTag are described in Section 13 as they can be
 used in the OperationalNegotiation stage as well.
 All security keys have connection-wide applicability.

12.1. AuthMethod

 Use: During Login - Security Negotiation
 Senders: Initiator and target
 Scope: connection
 AuthMethod = <list-of-values>
 The main item of security negotiation is the authentication method
 (AuthMethod).
 The authentication methods that can be used (appear in the list-of-
 values) are either vendor-unique methods or those listed in the
 following table:
  +--------------------------------------------------------------+
  | Name         | Description                                   |
  +--------------------------------------------------------------+
  | KRB5         | Kerberos V5 - defined in [RFC4120]            |
  +--------------------------------------------------------------+
  | SRP          | Secure Remote Password -                      |
  |              | defined in [RFC2945]                          |
  +--------------------------------------------------------------+
  | CHAP         | Challenge Handshake Authentication Protocol - |
  |              | defined in [RFC1994]                          |
  +--------------------------------------------------------------+
  | None         | No authentication                             |
  +--------------------------------------------------------------+
 The AuthMethod selection is followed by an "authentication exchange"
 specific to the authentication method selected.

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 The authentication method proposal may be made by either the
 initiator or the target.  However, the initiator MUST make the first
 step specific to the selected authentication method as soon as it is
 selected.  It follows that if the target makes the authentication
 method proposal, the initiator sends the first key(s) of the exchange
 together with its authentication method selection.
 The authentication exchange authenticates the initiator to the target
 and, optionally, the target to the initiator.  Authentication is
 OPTIONAL to use but MUST be supported by the target and initiator.
 The initiator and target MUST implement CHAP.  All other
 authentication methods are OPTIONAL.
 Private or public extension algorithms MAY also be negotiated for
 authentication methods.  Whenever a private or public extension
 algorithm is part of the default offer (the offer made in the absence
 of explicit administrative action), the implementer MUST ensure that
 CHAP is listed as an alternative in the default offer and "None" is
 not part of the default offer.
 Extension authentication methods MUST be named using one of the
 following two formats:
    1) Z-reversed.vendor.dns_name.do_something=
    2) New public key with no name prefix constraints
 Authentication methods named using the Z- format are used as private
 extensions.  New public keys must be registered with IANA using the
 IETF Review process ([RFC5226]).  New public extensions for
 authentication methods MUST NOT use the Z# name prefix.
 For all of the public or private extension authentication methods,
 the method-specific keys MUST conform to the format specified in
 Section 6.1 for standard-label.
 To identify the vendor for private extension authentication methods,
 we suggest using the reversed DNS-name as a prefix to the proper
 digest names.
 The part of digest-name following Z- MUST conform to the format for
 standard-label specified in Section 6.1.
 Support for public or private extension authentication methods is
 OPTIONAL.

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 The following subsections define the specific exchanges for each of
 the standardized authentication methods.  As mentioned earlier, the
 first step is always done by the initiator.

12.1.1. Kerberos

 For KRB5 (Kerberos V5) [RFC4120] [RFC1964], the initiator MUST use:
    KRB_AP_REQ=<KRB_AP_REQ>
 where KRB_AP_REQ is the client message as defined in [RFC4120].
 The default principal name assumed by an iSCSI initiator or target
 (prior to any administrative configuration action) MUST be the iSCSI
 Initiator Name or iSCSI Target Name, respectively, prefixed by the
 string "iscsi/".
 If the initiator authentication fails, the target MUST respond with a
 Login reject with "Authentication Failure" status.  Otherwise, if the
 initiator has selected the mutual authentication option (by setting
 MUTUAL-REQUIRED in the ap-options field of the KRB_AP_REQ), the
 target MUST reply with:
    KRB_AP_REP=<KRB_AP_REP>
 where KRB_AP_REP is the server's response message as defined in
 [RFC4120].
 If mutual authentication was selected and target authentication
 fails, the initiator MUST close the connection.
 KRB_AP_REQ and KRB_AP_REP are binary-values, and their binary length
 (not the length of the character string that represents them in
 encoded form) MUST NOT exceed 65536 bytes.  Hex or Base64 encoding
 may be used for KRB_AP_REQ and KRB_AP_REP; see Section 6.1.

12.1.2. Secure Remote Password (SRP)

 For SRP [RFC2945], the initiator MUST use:
    SRP_U=<U> TargetAuth=Yes     /* or TargetAuth=No */
 The target MUST answer with a Login reject with the "Authorization
 Failure" status or reply with:
    SRP_GROUP=<G1,G2...> SRP_s=<s>
 where G1,G2... are proposed groups, in order of preference.

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 The initiator MUST either close the connection or continue with:
    SRP_A=<A> SRP_GROUP=<G>
 where G is one of G1,G2... that were proposed by the target.
 The target MUST answer with a Login reject with the "Authentication
 Failure" status or reply with:
    SRP_B=<B>
 The initiator MUST close the connection or continue with:
    SRP_M=<M>
 If the initiator authentication fails, the target MUST answer with a
 Login reject with "Authentication Failure" status.  Otherwise, if the
 initiator sent TargetAuth=Yes in the first message (requiring target
 authentication), the target MUST reply with:
    SRP_HM=<H(A | M | K)>
 If the target authentication fails, the initiator MUST close the
 connection:
 where U, s, A, B, M, and H(A | M | K) are defined in [RFC2945] (using
 the SHA1 hash function, such as SRP-SHA1)
 and
 G,Gn ("Gn" stands for G1,G2...) are identifiers of SRP groups
 specified in [RFC3723].
 G, Gn, and U are text strings; s,A,B,M, and H(A | M | K) are
 binary-values.  The length of s,A,B,M and H(A | M | K) in binary form
 (not the length of the character string that represents them in
 encoded form) MUST NOT exceed 1024 bytes.  Hex or Base64 encoding may
 be used for s,A,B,M and H(A | M | K); see Section 6.1.
 See Appendix B for the related login example.
 For the SRP_GROUP, all the groups specified in [RFC3723] up to
 1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) must be
 supported by initiators and targets.  To guarantee interoperability,
 targets MUST always offer "SRP-1536" as one of the proposed groups.

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12.1.3. Challenge Handshake Authentication Protocol (CHAP)

 For CHAP [RFC1994], the initiator MUST use:
    CHAP_A=<A1,A2...>
 where A1,A2... are proposed algorithms, in order of preference.
 The target MUST answer with a Login reject with the "Authentication
 Failure" status or reply with:
    CHAP_A=<A> CHAP_I=<I> CHAP_C=<C>
 where A is one of A1,A2... that were proposed by the initiator.
 The initiator MUST continue with:
    CHAP_N=<N> CHAP_R=<R>
 or, if it requires target authentication, with:
    CHAP_N=<N> CHAP_R=<R> CHAP_I=<I> CHAP_C=<C>
 If the initiator authentication fails, the target MUST answer with a
 Login reject with "Authentication Failure" status.  Otherwise, if the
 initiator required target authentication, the target MUST either
 answer with a Login reject with "Authentication Failure" or reply
 with:
    CHAP_N=<N> CHAP_R=<R>
 If the target authentication fails, the initiator MUST close the
 connection:
 where N, (A,A1,A2), I, C, and R are (correspondingly) the Name,
 Algorithm, Identifier, Challenge, and Response as defined in
 [RFC1994].
 N is a text string; A,A1,A2, and I are numbers; C and R are
 binary-values.  Their binary length (not the length of the character
 string that represents them in encoded form) MUST NOT exceed
 1024 bytes.  Hex or Base64 encoding may be used for C and R; see
 Section 6.1.
 See Appendix B for the related login example.

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 For the Algorithm, as stated in [RFC1994], one value is required to
 be implemented:
    5     (CHAP with MD5)
 To guarantee interoperability, initiators MUST always offer it as one
 of the proposed algorithms.

13. Login/Text Operational Text Keys

 Some session-specific parameters MUST only be carried on the leading
 connection and cannot be changed after the leading connection login
 (e.g., MaxConnections -- the maximum number of connections).  This
 holds for a single connection session with regard to connection
 restart.  The keys that fall into this category have the "use: LO"
 (Leading Only).
 Keys that can only be used during login have the "use: IO"
 (Initialize Only), while those that can be used in both the Login
 Phase and Full Feature Phase have the "use: ALL".
 Keys that can only be used during the Full Feature Phase use FFPO
 (Full Feature Phase Only).
 Keys marked as Any-Stage may also appear in the SecurityNegotiation
 stage, while all other keys described in this section are
 operational keys.
 Keys that do not require an answer are marked as Declarative.
 Key scope is indicated as session-wide (SW) or connection-only (CO).
 "Result function", wherever mentioned, states the function that can
 be applied to check the validity of the responder selection.
 "Minimum" means that the selected value cannot exceed the offered
 value.  "Maximum" means that the selected value cannot be lower than
 the offered value.  "AND" means that the selected value must be a
 possible result of a Boolean "and" function with an arbitrary Boolean
 value (e.g., if the offered value is No the selected value must be
 No).  "OR" means that the selected value must be a possible result of
 a Boolean "or" function with an arbitrary Boolean value (e.g., if the
 offered value is Yes the selected value must be Yes).

Chadalapaka, et al. Standards Track [Page 229] RFC 7143 iSCSI (Consolidated) April 2014

13.1. HeaderDigest and DataDigest

 Use: IO
 Senders: Initiator and target
 Scope: CO
 HeaderDigest = <list-of-values>
 DataDigest = <list-of-values>
 Default is None for both HeaderDigest and DataDigest.
 Digests enable the checking of end-to-end, non-cryptographic data
 integrity beyond the integrity checks provided by the link layers and
 the covering of the whole communication path, including all elements
 that may change the network-level PDUs, such as routers, switches,
 and proxies.
 The following table lists cyclic integrity checksums that can be
 negotiated for the digests and MUST be implemented by every iSCSI
 initiator and target.  These digest options only have error detection
 significance.
   +---------------------------------------------+
   | Name          | Description     | Generator |
   +---------------------------------------------+
   | CRC32C        | 32-bit CRC      |0x11edc6f41|
   +---------------------------------------------+
   | None          | no digest                   |
   +---------------------------------------------+
 The generator polynomial G(x) for this digest is given in hexadecimal
 notation (e.g., "0x3b" stands for 0011 1011, and the polynomial is
 x**5 + x**4 + x**3 + x + 1).
 When the initiator and target agree on a digest, this digest MUST be
 used for every PDU in the Full Feature Phase.
 Padding bytes, when present in a segment covered by a CRC, SHOULD be
 set to 0 and are included in the CRC.
 The CRC MUST be calculated by a method that produces the same results
 as the following process:
  1. The PDU bits are considered as the coefficients of a polynomial

M(x) of degree n - 1; bit 7 of the lowest numbered byte is

   considered the most significant bit (x**n - 1), followed by bit 6
   of the lowest numbered byte through bit 0 of the highest numbered
   byte (x**0).

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  1. The most significant 32 bits are complemented.
  1. The polynomial is multiplied by x32, then divided by G(x). The generator polynomial produces a remainder R(x) of degree ⇐ 31. - The coefficients of R(x) are formed into a 32-bit sequence. - The bit sequence is complemented, and the result is the CRC. - The CRC bits are mapped into the digest word. The x31

coefficient is mapped to bit 7 of the lowest numbered byte of the

   digest, and the mapping continues with successive coefficients and
   bits so that the x**24 coefficient is mapped to bit 0 of the lowest
   numbered byte.  The mapping continues further with the x**23
   coefficient mapped to bit 7 of the next byte in the digest until
   the x**0 coefficient is mapped to bit 0 of the highest numbered
   byte of the digest.
  1. Computing the CRC over any segment (data or header) extended to

include the CRC built using the generator 0x11edc6f41 will always

   get the value 0x1c2d19ed as its final remainder (R(x)).  This value
   is given here in its polynomial form (i.e., not mapped as the
   digest word).
 For a discussion about selection criteria for the CRC, see [RFC3385].
 For a detailed analysis of the iSCSI polynomial, see [Castagnoli93].
 Private or public extension algorithms MAY also be negotiated for
 digests.  Whenever a private or public digest extension algorithm is
 part of the default offer (the offer made in the absence of explicit
 administrative action), the implementer MUST ensure that CRC32C is
 listed as an alternative in the default offer and "None" is not part
 of the default offer.
 Extension digest algorithms MUST be named using one of the following
 two formats:
    1) Y-reversed.vendor.dns_name.do_something=
    2) New public key with no name prefix constraints
 Digests named using the Y- format are used for private purposes
 (unregistered).  New public keys must be registered with IANA using
 the IETF Review process ([RFC5226]).  New public extensions for
 digests MUST NOT use the Y# name prefix.
 For private extension digests, to identify the vendor we suggest
 using the reversed DNS-name as a prefix to the proper digest names.

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 The part of digest-name following Y- MUST conform to the format for
 standard-label specified in Section 6.1.
 Support for public or private extension digests is OPTIONAL.

13.2. MaxConnections

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 MaxConnections=<numerical-value-from-1-to-65535>
 Default is 1.
 Result function is Minimum.
 The initiator and target negotiate the maximum number of connections
 requested/acceptable.

13.3. SendTargets

 Use: FFPO
 Senders: Initiator
 Scope: SW
 For a complete description, see Appendix C.

13.4. TargetName

 Use: IO by initiator, FFPO by target -- only as response to a
    SendTargets, Declarative, Any-Stage
 Senders: Initiator and target
 Scope: SW
 TargetName=<iSCSI-name-value>
 Examples:
    TargetName=iqn.1993-11.com.disk-vendor:diskarrays.sn.45678
    TargetName=eui.020000023B040506
    TargetName=naa.62004567BA64678D0123456789ABCDEF

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 The initiator of the TCP connection MUST provide this key to the
 remote endpoint in the first Login Request if the initiator is not
 establishing a Discovery session.  The iSCSI Target Name specifies
 the worldwide unique name of the target.
 The TargetName key may also be returned by the SendTargets Text
 Request (which is its only use when issued by a target).
 The TargetName MUST NOT be redeclared within the Login Phase.

13.5. InitiatorName

 Use: IO, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW
 InitiatorName=<iSCSI-name-value>
 Examples:
    InitiatorName=iqn.1992-04.com.os-vendor.plan9:cdrom.12345
    InitiatorName=iqn.2001-02.com.ssp.users:customer235.host90
    InitiatorName=naa.52004567BA64678D
 The initiator of the TCP connection MUST provide this key to the
 remote endpoint at the first login of the Login Phase for every
 connection.  The InitiatorName key enables the initiator to identify
 itself to the remote endpoint.
 The InitiatorName MUST NOT be redeclared within the Login Phase.

13.6. TargetAlias

 Use: ALL, Declarative, Any-Stage
 Senders: Target
 Scope: SW
 TargetAlias=<iSCSI-local-name-value>
 Examples:
    TargetAlias=Bob-s Disk
    TargetAlias=Database Server 1 Log Disk
    TargetAlias=Web Server 3 Disk 20

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 If a target has been configured with a human-readable name or
 description, this name SHOULD be communicated to the initiator during
 a Login Response PDU if SessionType=Normal (see Section 13.21).  This
 string is not used as an identifier, nor is it meant to be used for
 authentication or authorization decisions.  It can be displayed by
 the initiator's user interface in a list of targets to which it is
 connected.

13.7. InitiatorAlias

 Use: ALL, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW
 InitiatorAlias=<iSCSI-local-name-value>
 Examples:
    InitiatorAlias=Web Server 4
    InitiatorAlias=spyalley.nsa.gov
    InitiatorAlias=Exchange Server
 If an initiator has been configured with a human-readable name or
 description, it SHOULD be communicated to the target during a Login
 Request PDU.  If not, the host name can be used instead.  This string
 is not used as an identifier, nor is it meant to be used for
 authentication or authorization decisions.  It can be displayed by
 the target's user interface in a list of initiators to which it is
 connected.

13.8. TargetAddress

 Use: ALL, Declarative, Any-Stage
 Senders: Target
 Scope: SW
 TargetAddress=domainname[:port][,portal-group-tag]
 The domainname can be specified as either a DNS host name, a dotted-
 decimal IPv4 address, or a bracketed IPv6 address as specified in
 [RFC3986].
 If the TCP port is not specified, it is assumed to be the IANA-
 assigned default port for iSCSI (see Section 14).

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 If the TargetAddress is returned as the result of a redirect status
 in a Login Response, the comma and portal-group-tag MUST be omitted.
 If the TargetAddress is returned within a SendTargets response, the
 portal-group-tag MUST be included.
 Examples:
    TargetAddress=10.0.0.1:5003,1
    TargetAddress=[1080:0:0:0:8:800:200C:417A],65
    TargetAddress=[1080::8:800:200C:417A]:5003,1
    TargetAddress=computingcenter.example.com,23
 The use of the portal-group-tag is described in Appendix C.  The
 formats for the port and portal-group-tag are the same as the one
 specified in TargetPortalGroupTag.

13.9. TargetPortalGroupTag

 Use: IO by target, Declarative, Any-Stage
 Senders: Target
 Scope: SW
 TargetPortalGroupTag=<16-bit-binary-value>
 Example:
    TargetPortalGroupTag=1
 The TargetPortalGroupTag key is a 16-bit binary-value that uniquely
 identifies a portal group within an iSCSI target node.  This key
 carries the value of the tag of the portal group that is servicing
 the Login Request.  The iSCSI target returns this key to the
 initiator in the Login Response PDU to the first Login Request PDU
 that has the C bit set to 0 when TargetName is given by the
 initiator.
 [SAM2] notes in its informative text that the TPGT value should be
 non-zero; note that this is incorrect.  A zero value is allowed as a
 legal value for the TPGT.  This discrepancy currently stands
 corrected in [SAM4].
 For the complete usage expectations of this key, see Section 6.3.

Chadalapaka, et al. Standards Track [Page 235] RFC 7143 iSCSI (Consolidated) April 2014

13.10. InitialR2T

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 InitialR2T=<boolean-value>
 Examples:
    I->InitialR2T=No
    T->InitialR2T=No
 Default is Yes.
 Result function is OR.
 The InitialR2T key is used to turn off the default use of R2T for
 unidirectional operations and the output part of bidirectional
 commands, thus allowing an initiator to start sending data to a
 target as if it has received an initial R2T with Buffer
 Offset=Immediate Data Length and Desired Data Transfer
 Length=(min(FirstBurstLength, Expected Data Transfer Length) -
 Received Immediate Data Length).
 The default action is that R2T is required, unless both the initiator
 and the target send this key-pair attribute specifying InitialR2T=No.
 Only the first outgoing data burst (immediate data and/or separate
 PDUs) can be sent unsolicited (i.e., not requiring an explicit R2T).

13.11. ImmediateData

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 ImmediateData=<boolean-value>
 Default is Yes.
 Result function is AND.
 The initiator and target negotiate support for immediate data.  To
 turn immediate data off, the initiator or target must state its
 desire to do so.  ImmediateData can be turned on if both the
 initiator and target have ImmediateData=Yes.

Chadalapaka, et al. Standards Track [Page 236] RFC 7143 iSCSI (Consolidated) April 2014

 If ImmediateData is set to Yes and InitialR2T is set to Yes
 (default), then only immediate data are accepted in the first burst.
 If ImmediateData is set to No and InitialR2T is set to Yes, then the
 initiator MUST NOT send unsolicited data and the target MUST reject
 unsolicited data with the corresponding response code.
 If ImmediateData is set to No and InitialR2T is set to No, then the
 initiator MUST NOT send unsolicited immediate data but MAY send one
 unsolicited burst of Data-OUT PDUs.
 If ImmediateData is set to Yes and InitialR2T is set to No, then the
 initiator MAY send unsolicited immediate data and/or one unsolicited
 burst of Data-OUT PDUs.
 The following table is a summary of unsolicited data options:
   +----------+-------------+------------------+-------------+
   |InitialR2T|ImmediateData|    Unsolicited   |ImmediateData|
   |          |             |   Data-Out PDUs  |             |
   +----------+-------------+------------------+-------------+
   | No       | No          | Yes              | No          |
   +----------+-------------+------------------+-------------+
   | No       | Yes         | Yes              | Yes         |
   +----------+-------------+------------------+-------------+
   | Yes      | No          | No               | No          |
   +----------+-------------+------------------+-------------+
   | Yes      | Yes         | No               | Yes         |
   +----------+-------------+------------------+-------------+

13.12. MaxRecvDataSegmentLength

 Use: ALL, Declarative
 Senders: Initiator and target
 Scope: CO
 MaxRecvDataSegmentLength=<numerical-value-512-to-(2**24 - 1)>
 Default is 8192 bytes.
 The initiator or target declares the maximum data segment length in
 bytes it can receive in an iSCSI PDU.
 The transmitter (initiator or target) is required to send PDUs with a
 data segment that does not exceed MaxRecvDataSegmentLength of the
 receiver.

Chadalapaka, et al. Standards Track [Page 237] RFC 7143 iSCSI (Consolidated) April 2014

 A target receiver is additionally limited by MaxBurstLength for
 solicited data and FirstBurstLength for unsolicited data.  An
 initiator MUST NOT send solicited PDUs exceeding MaxBurstLength nor
 unsolicited PDUs exceeding FirstBurstLength (or FirstBurstLength-
 Immediate Data Length if immediate data were sent).

13.13. MaxBurstLength

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 MaxBurstLength=<numerical-value-512-to-(2**24 - 1)>
 Default is 262144 (256 KB).
 Result function is Minimum.
 The initiator and target negotiate the maximum SCSI data payload in
 bytes in a Data-In or a solicited Data-Out iSCSI sequence.  A
 sequence consists of one or more consecutive Data-In or Data-Out PDUs
 that end with a Data-In or Data-Out PDU with the F bit set to 1.

13.14. FirstBurstLength

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 Irrelevant when: ( InitialR2T=Yes and ImmediateData=No )
 FirstBurstLength=<numerical-value-512-to-(2**24 - 1)>
 Default is 65536 (64 KB).
 Result function is Minimum.
 The initiator and target negotiate the maximum amount in bytes of
 unsolicited data an iSCSI initiator may send to the target during the
 execution of a single SCSI command.  This covers the immediate data
 (if any) and the sequence of unsolicited Data-Out PDUs (if any) that
 follow the command.
 FirstBurstLength MUST NOT exceed MaxBurstLength.

Chadalapaka, et al. Standards Track [Page 238] RFC 7143 iSCSI (Consolidated) April 2014

13.15. DefaultTime2Wait

 Use: LO
 Senders: Initiator and target
 Scope: SW
 DefaultTime2Wait=<numerical-value-0-to-3600>
 Default is 2.
 Result function is Maximum.
 The initiator and target negotiate the minimum time, in seconds, to
 wait before attempting an explicit/implicit logout or an active task
 reassignment after an unexpected connection termination or a
 connection reset.
 A value of 0 indicates that logout or active task reassignment can be
 attempted immediately.

13.16. DefaultTime2Retain

 Use: LO
 Senders: Initiator and target
 Scope: SW
 DefaultTime2Retain=<numerical-value-0-to-3600>
 Default is 20.
 Result function is Minimum.
 The initiator and target negotiate the maximum time, in seconds,
 after an initial wait (Time2Wait), before which an active task
 reassignment is still possible after an unexpected connection
 termination or a connection reset.
 This value is also the session state timeout if the connection in
 question is the last LOGGED_IN connection in the session.
 A value of 0 indicates that connection/task state is immediately
 discarded by the target.

13.17. MaxOutstandingR2T

 Use: LO
 Senders: Initiator and target
 Scope: SW
 MaxOutstandingR2T=<numerical-value-from-1-to-65535>

Chadalapaka, et al. Standards Track [Page 239] RFC 7143 iSCSI (Consolidated) April 2014

 Irrelevant when: SessionType=Discovery
 Default is 1.
 Result function is Minimum.
 The initiator and target negotiate the maximum number of outstanding
 R2Ts per task, excluding any implied initial R2T that might be part
 of that task.  An R2T is considered outstanding until the last data
 PDU (with the F bit set to 1) is transferred or a sequence reception
 timeout (Section 7.1.4.1) is encountered for that data sequence.

13.18. DataPDUInOrder

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 DataPDUInOrder=<boolean-value>
 Default is Yes.
 Result function is OR.
 "No" is used by iSCSI to indicate that the data PDUs within sequences
 can be in any order.  "Yes" is used to indicate that data PDUs within
 sequences have to be at continuously increasing addresses and
 overlays are forbidden.

13.19. DataSequenceInOrder

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 DataSequenceInOrder=<boolean-value>
 Default is Yes.
 Result function is OR.
 A data sequence is a sequence of Data-In or Data-Out PDUs that end
 with a Data-In or Data-Out PDU with the F bit set to 1.  A Data-Out
 sequence is sent either unsolicited or in response to an R2T.
 Sequences cover an offset-range.
 If DataSequenceInOrder is set to No, data PDU sequences may be
 transferred in any order.

Chadalapaka, et al. Standards Track [Page 240] RFC 7143 iSCSI (Consolidated) April 2014

 If DataSequenceInOrder is set to Yes, data sequences MUST be
 transferred using continuously non-decreasing sequence offsets (R2T
 buffer offset for writes, or the smallest SCSI Data-In buffer offset
 within a read data sequence).
 If DataSequenceInOrder is set to Yes, a target may retry at most the
 last R2T, and an initiator may at most request retransmission for the
 last read data sequence.  For this reason, if ErrorRecoveryLevel is
 not 0 and DataSequenceInOrder is set to Yes, then MaxOutstandingR2T
 MUST be set to 1.

13.20. ErrorRecoveryLevel

 Use: LO
 Senders: Initiator and target
 Scope: SW
 ErrorRecoveryLevel=<numerical-value-0-to-2>
 Default is 0.
 Result function is Minimum.
 The initiator and target negotiate the recovery level supported.
 Recovery levels represent a combination of recovery capabilities.
 Each recovery level includes all the capabilities of the lower
 recovery levels and adds some new ones to them.
 In the description of recovery mechanisms, certain recovery classes
 are specified.  Section 7.1.5 describes the mapping between the
 classes and the levels.

13.21. SessionType

 Use: LO, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW
 SessionType=<Discovery|Normal>
 Default is Normal.
 The initiator indicates the type of session it wants to create.  The
 target can either accept it or reject it.

Chadalapaka, et al. Standards Track [Page 241] RFC 7143 iSCSI (Consolidated) April 2014

 A Discovery session indicates to the target that the only purpose of
 this session is discovery.  The only requests a target accepts in
 this type of session are a Text Request with a SendTargets key and a
 Logout Request with reason "close the session".
 The Discovery session implies MaxConnections = 1 and overrides both
 the default and an explicit setting.  As Section 7.4.1 states,
 ErrorRecoveryLevel MUST be 0 (zero) for Discovery sessions.
 Depending on the type of session, a target may decide on resources to
 allocate, the security to enforce, etc., for the session.  If the
 SessionType key is thus going to be offered as "Discovery", it SHOULD
 be offered in the initial Login Request by the initiator.

13.22. The Private Extension Key Format

 Use: ALL
 Senders: Initiator and target
 Scope: specific key dependent
 X-reversed.vendor.dns_name.do_something=
 Keys with this format are used for private extension purposes.  These
 keys always start with X- if unregistered with IANA (private).  New
 public keys (if registered with IANA via an IETF Review [RFC5226]) no
 longer have an X# name prefix requirement; implementers may propose
 any intuitive unique name.
 For unregistered keys, to identify the vendor we suggest using the
 reversed DNS-name as a prefix to the key-proper.
 The part of key-name following X- MUST conform to the format for
 key-name specified in Section 6.1.
 Vendor-specific keys MUST ONLY be used in Normal sessions.
 Support for public or private extension keys is OPTIONAL.

13.23. TaskReporting

 Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 TaskReporting=<list-of-values>
 Default is RFC3720.

Chadalapaka, et al. Standards Track [Page 242] RFC 7143 iSCSI (Consolidated) April 2014

 This key is used to negotiate the task completion reporting semantics
 from the SCSI target.  The following table describes the semantics
 that an iSCSI target MUST support for respective negotiated key
 values.  Whenever this key is negotiated, at least the RFC3720 and
 ResponseFence values MUST be offered as options by the negotiation
 originator.
   +--------------+------------------------------------------+
   | Name         |             Description                  |
   +--------------+------------------------------------------+
   | RFC3720      | RFC 3720-compliant semantics.  Response  |
   |              | fencing is not guaranteed, and fast      |
   |              | completion of multi-task aborting is not |
   |              | supported.                               |
   +--------------+------------------------------------------+
   | ResponseFence| Response Fence (Section 4.2.2.3.3)       |
   |              | semantics MUST be supported in reporting |
   |              | task completions.                        |
   +--------------+------------------------------------------+
   | FastAbort    | Updated fast multi-task abort semantics  |
   |              | defined in Section 4.2.3.4 MUST be       |
   |              | supported.  Support for the Response     |
   |              | Fence is implied -- i.e., semantics as   |
   |              | described in Section 4.2.2.3.3 MUST be   |
   |              | supported as well.                       |
   +--------------+------------------------------------------+
 When TaskReporting is not negotiated to FastAbort, the standard
 multi-task abort semantics in Section 4.2.3.3 MUST be used.

13.24. iSCSIProtocolLevel Negotiation

 The iSCSIProtocolLevel associated with this document is "1".  As a
 responder or an originator in a negotiation of this key, an iSCSI
 implementation compliant to this document alone, without any future
 protocol extensions, MUST use this value as defined by [RFC7144].

13.25. Obsoleted Keys

 This document obsoletes the following keys defined in [RFC3720]:
 IFMarker, OFMarker, OFMarkInt, and IFMarkInt.  However, iSCSI
 implementations compliant to this document may still receive these
 obsoleted keys -- i.e., in a responder role -- in a text negotiation.
 When an IFMarker or OFMarker key is received, a compliant iSCSI
 implementation SHOULD respond with the constant "Reject" value.  The
 implementation MAY alternatively respond with a "No" value.

Chadalapaka, et al. Standards Track [Page 243] RFC 7143 iSCSI (Consolidated) April 2014

 However, the implementation MUST NOT respond with a "NotUnderstood"
 value for either of these keys.
 When an IFMarkInt or OFMarkInt key is received, a compliant iSCSI
 implementation MUST respond with the constant "Reject" value.  The
 implementation MUST NOT respond with a "NotUnderstood" value for
 either of these keys.

13.26. X#NodeArchitecture

13.26.1. Definition

 Use: LO, Declarative
 Senders: Initiator and target
 Scope: SW
 X#NodeArchitecture=<list-of-values>
 Default is None.
 Examples:
    X#NodeArchitecture=ExampleOS/v1234,ExampleInc_SW_Initiator/1.05a
    X#NodeArchitecture=ExampleInc_HW_Initiator/4010,Firmware/2.0.0.5
    X#NodeArchitecture=ExampleInc_SW_Initiator/2.1,CPU_Arch/i686
 This document does not define the structure or content of the list of
 values.
 The initiator or target declares the details of its iSCSI node
 architecture to the remote endpoint.  These details may include, but
 are not limited to, iSCSI vendor software, firmware, or hardware
 versions; the OS version; or hardware architecture.  This key may be
 declared on a Discovery session or a Normal session.
 The length of the key value (total length of the list-of-values) MUST
 NOT be greater than 255 bytes.
 X#NodeArchitecture MUST NOT be redeclared during the Login Phase.

13.26.2. Implementation Requirements

 Functional behavior of the iSCSI node (this includes the iSCSI
 protocol logic -- the SCSI, iSCSI, and TCP/IP protocols) MUST NOT
 depend on the presence, absence, or content of the X#NodeArchitecture
 key.  The key MUST NOT be used by iSCSI nodes for interoperability or

Chadalapaka, et al. Standards Track [Page 244] RFC 7143 iSCSI (Consolidated) April 2014

 for exclusion of other nodes.  To ensure proper use, key values
 SHOULD be set by the node itself, and there SHOULD NOT be provisions
 for the key values to contain user-defined text.
 Nodes implementing this key MUST choose one of the following
 implementation options:
  1. only transmit the key,
  1. only log the key values received from other nodes, or
  1. both transmit and log the key values.
 Each node choosing to implement transmission of the key values MUST
 be prepared to handle the response of iSCSI nodes that do not
 understand the key.
 Nodes that implement transmission and/or logging of the key values
 may also implement administrative mechanisms that disable and/or
 change the logging and key transmission details (see Section 9.4).
 Thus, a valid behavior for this key may be that a node is completely
 silent (the node does not transmit any key value and simply discards
 any key values it receives without issuing a NotUnderstood response).

14. Rationale for Revised IANA Considerations

 This document makes rather significant changes in this area, and this
 section outlines the reasons behind the changes.  As previously
 specified in [RFC3720], iSCSI had used text string prefixes, such as
 X- and X#, to distinguish extended login/text keys, digest
 algorithms, and authentication methods from their standardized
 counterparts.  Based on experience with other protocols, [RFC6648],
 however, strongly recommends against this practice, in large part
 because extensions that use such prefixes may become standard over
 time, at which point it can be infeasible to change their text string
 names due to widespread usage under the existing text string name.
 iSCSI's experience with public extensions supports the
 recommendations in [RFC6648], as the only extension item ever
 registered with IANA, the X#NodeArchitecture key, was specified as a
 standard key in a Standards Track RFC [RFC4850] and hence did not
 require the X# prefix.  In addition, that key is the only public
 iSCSI extension that has been registered with IANA since RFC 3720 was
 originally published, so there has been effectively no use of the X#,
 Y#, and Z# public extension formats.

Chadalapaka, et al. Standards Track [Page 245] RFC 7143 iSCSI (Consolidated) April 2014

 Therefore, this document makes the following changes to the IANA
 registration procedures for iSCSI:
    1) The separate registries for X#, Y#, and Z# public extensions
       are removed.  The single entry in the registry for X#
       login/text keys (X#NodeArchitecture) is transferred to the main
       "iSCSI Login/Text Keys" registry.  IANA has never created the
       latter two registries because there have been no registration
       requests for them.  These public extension formats (X#, Y#, Z#)
       MUST NOT be used, with the exception of the existing
       X#NodeArchitecture key.
    2) The registration procedures for the main "iSCSI Login/Text
       Keys", "iSCSI digests", and "iSCSI authentication methods" IANA
       registries are changed to IETF Review [RFC5226] for possible
       future extensions to iSCSI.  This change includes a deliberate
       decision to remove the possibility of specifying an IANA-
       registered iSCSI extension in an RFC published via an RFC
       Editor Independent Submission, as the level of review in that
       process is insufficient for iSCSI extensions.
    3) The restriction against registering items using the private
       extension formats (X-, Y-, Z-) in the main IANA registries is
       removed.  Extensions using these formats MAY be registered
       under the IETF Review registration procedures, but each format
       is restricted to the type of extension for which it is
       specified in this RFC and MUST NOT be used for other types.
       For example, the X- extension format for extension login/text
       keys MUST NOT be used for digest algorithms or authentication
       methods.

15. IANA Considerations

 The well-known TCP port number for iSCSI connections assigned by IANA
 is 3260, and this is the default iSCSI port.  Implementations needing
 a system TCP port number may use port 860, the port assigned by IANA
 as the iSCSI system port; however, in order to use port 860, it MUST
 be explicitly specified -- implementations MUST NOT default to the
 use of port 860, as 3260 is the only allowed default.
 IANA has replaced the references for ports 860 and 3260, both TCP and
 UDP, with references to this document.  Please see
 http://www.iana.org/assignments/service-names-port-numbers.
 IANA has updated all references to RFC 3720, RFC 4850, and RFC 5048
 to instead reference this RFC in all of the iSCSI registries that are
 part of the "Internet Small Computer System Interface (iSCSI)
 Parameters" set of registries.  This change reflects the fact that

Chadalapaka, et al. Standards Track [Page 246] RFC 7143 iSCSI (Consolidated) April 2014

 those three RFCs are obsoleted by this RFC.  References to other RFCs
 that are not being obsoleted (e.g., RFC 3723, RFC 5046) should not be
 changed.
 IANA has performed the following actions on the "iSCSI Login/Text
 Keys" registry:
  1. Changed the registration procedure to IETF Review from Standard

Required.

  1. Changed the RFC 5048 reference for the registry to reference

this RFC.

  1. Added the X#NodeArchitecture key from the "iSCSI extended key"

registry, and changed its reference to this RFC.

  1. Changed all references to RFC 3720 and RFC 5048 to instead

reference this RFC.

 IANA has changed the registration procedures for the "iSCSI
 authentication methods" and "iSCSI digests" registries to IETF Review
 from RFC Required.
 IANA has removed the "iSCSI extended key" registry, as its one entry
 has been added to the "iSCSI Login/Text Keys" registry.
 IANA has marked as obsolete the values 4 and 5 for SPKM1 and SPKM2,
 respectively, in the "iSCSI authentication methods" subregistry of
 the "Internet Small Computer System Interface (iSCSI) Parameters" set
 of registries.
 IANA has added this document to the "iSCSI Protocol Level" registry
 with value 1, as mentioned in Section 13.24.
 All the other IANA considerations stated in [RFC3720] and [RFC5048]
 remain unchanged.  The assignments contained in the following
 subregistries are not repeated in this document:
  1. iSCSI authentication methods (from Section 13 of [RFC3720])
  1. iSCSI digests (from Section 13 of [RFC3720])
 This document obsoletes the SPKM1 and SPKM2 key values for the
 AuthMethod text key.  Consequently, the SPKM_ text key prefix MUST be
 treated as obsolete and not be reused.

Chadalapaka, et al. Standards Track [Page 247] RFC 7143 iSCSI (Consolidated) April 2014

16. References

16.1. Normative References

 [EUI]      "Guidelines for 64-bit Global Identifier (EUI-64(TM))",
            <http://standards.ieee.org/regauth/oui/tutorials/
            EUI64.html>.
 [FC-FS3]   INCITS Technical Committee T11, "Fibre Channel - Framing
            and Signaling - 3 (FC-FS-3)", ANSI INCITS 470-2011, 2011.
 [OUI]      "IEEE OUI and "company_id" Assignments",
            <http://standards.ieee.org/regauth/oui>.
 [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
            RFC 1964, June 1996.
 [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
            August 1996.
 [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
            Protocol (CHAP)", RFC 1994, August 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
            ESP and AH", RFC 2404, November 1998.
 [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.
 [RFC2451]  Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
            Algorithms", RFC 2451, November 1998.
 [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",
            RFC 2945, September 2000.
 [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
            Internationalized Strings ("stringprep")", RFC 3454,
            December 2002.
 [RFC3566]  Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm
            and Its Use With IPsec", RFC 3566, September 2003.

Chadalapaka, et al. Standards Track [Page 248] RFC 7143 iSCSI (Consolidated) April 2014

 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
            ISO 10646", STD 63, RFC 3629, November 2003.
 [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
            Counter Mode With IPsec Encapsulating Security Payload
            (ESP)", RFC 3686, January 2004.
 [RFC3722]  Bakke, M., "String Profile for Internet Small Computer
            Systems Interface (iSCSI) Names", RFC 3722, April 2004.
 [RFC3723]  Aboba, B., Tseng, J., Walker, J., Rangan, V., and F.
            Travostino, "Securing Block Storage Protocols over IP",
            RFC 3723, April 2004.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.
 [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
            (GCM) in IPsec Encapsulating Security Payload (ESP)",
            RFC 4106, June 2005.
 [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
            Kerberos Network Authentication Service (V5)", RFC 4120,
            July 2005.
 [RFC4171]  Tseng, J., Gibbons, K., Travostino, F., Du Laney, C., and
            J. Souza, "Internet Storage Name Service (iSNS)",
            RFC 4171, September 2005.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, February 2006.
 [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, December 2005.
 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
            RFC 4303, December 2005.
 [RFC4304]  Kent, S., "Extended Sequence Number (ESN) Addendum to
            IPsec Domain of Interpretation (DOI) for Internet Security
            Association and Key Management Protocol (ISAKMP)",
            RFC 4304, December 2005.
 [RFC4543]  McGrew, D. and J. Viega, "The Use of Galois Message
            Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
            May 2006.

Chadalapaka, et al. Standards Track [Page 249] RFC 7143 iSCSI (Consolidated) April 2014

 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
            "Internet Key Exchange Protocol Version 2 (IKEv2)",
            RFC 5996, September 2010.
 [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
            Galperin, S., and C. Adams, "X.509 Internet Public Key
            Infrastructure Online Certificate Status Protocol - OCSP",
            RFC 6960, June 2013.
 [RFC7144]  Knight, F. and M. Chadalapaka, "Internet Small Computer
            System Interface (iSCSI) SCSI Features Update", RFC 7144,
            April 2014.
 [RFC7145]  Ko, M. and A. Nezhinsky, "Internet Small Computer System
            Interface (iSCSI) Extensions for the Remote Direct Memory
            Access (RDMA) Specification", RFC 7145, April 2014.
 [RFC7146]  Black, D. and P. Koning, "Securing Block Storage Protocols
            over IP: RFC 3723 Requirements Update for IPsec v3",
            RFC 7146, April 2014.
 [SAM2]     INCITS Technical Committee T10, "SCSI Architecture Model -
            2 (SAM-2)", ANSI INCITS 366-2003, ISO/IEC 14776-412, 2003.
 [SAM4]     INCITS Technical Committee T10, "SCSI Architecture Model -
            4 (SAM-4)", ANSI INCITS 447-2008, ISO/IEC 14776-414, 2008.
 [SPC2]     INCITS Technical Committee T10, "SCSI Primary Commands -
            2", ANSI INCITS 351-2001, ISO/IEC 14776-452, 2001.
 [SPC3]     INCITS Technical Committee T10, "SCSI Primary Commands -
            3", ANSI INCITS 408-2005, ISO/IEC 14776-453, 2005.
 [UML]      ISO, "Unified Modeling Language (UML) Version 1.4.2",
            ISO/IEC 19501:2005.
 [UNICODE]  The Unicode Consortium, "Unicode Standard Annex #15:
            Unicode Normalization Forms", 2013,
            <http://www.unicode.org/unicode/reports/tr15>.

Chadalapaka, et al. Standards Track [Page 250] RFC 7143 iSCSI (Consolidated) April 2014

16.2. Informative References

 [Castagnoli93]
            Castagnoli, G., Brauer, S., and M. Herrmann, "Optimization
            of Cyclic Redundancy-Check Codes with 24 and 32 Parity
            Bits", IEEE Transact. on Communications, Vol. 41, No. 6,
            June 1993.
 [FC-SP-2]  INCITS Technical Committee T11, "Fibre Channel Security
            Protocols 2", ANSI INCITS 496-2012, 2012.
 [IB]       InfiniBand, "InfiniBand(TM) Architecture Specification",
            Vol. 1, Rel. 1.2.1, InfiniBand Trade Association,
            <http://www.infinibandta.org>.
 [RFC1737]  Sollins, K. and L. Masinter, "Functional Requirements for
            Uniform Resource Names", RFC 1737, December 1994.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [RFC2407]  Piper, D., "The Internet IP Security Domain of
            Interpretation for ISAKMP", RFC 2407, November 1998.
 [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998.
 [RFC2608]  Guttman, E., Perkins, C., Veizades, J., and M. Day,
            "Service Location Protocol, Version 2", RFC 2608,
            June 1999.
 [RFC2743]  Linn, J., "Generic Security Service Application Program
            Interface Version 2, Update  ", RFC 2743, January 2000.
 [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)",
            RFC 2865, June 2000.
 [RFC3385]  Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
            "Internet Protocol Small Computer System Interface (iSCSI)
            Cyclic Redundancy Check (CRC)/Checksum Considerations",
            RFC 3385, September 2002.
 [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
            Algorithm and Its Use with IPsec", RFC 3602,
            September 2003.

Chadalapaka, et al. Standards Track [Page 251] RFC 7143 iSCSI (Consolidated) April 2014

 [RFC3720]  Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M.,
            and E. Zeidner, "Internet Small Computer Systems Interface
            (iSCSI)", RFC 3720, April 2004.
 [RFC3721]  Bakke, M., Hafner, J., Hufferd, J., Voruganti, K., and M.
            Krueger, "Internet Small Computer Systems Interface
            (iSCSI) Naming and Discovery", RFC 3721, April 2004.
 [RFC3783]  Chadalapaka, M. and R. Elliott, "Small Computer Systems
            Interface (SCSI) Command Ordering Considerations with
            iSCSI", RFC 3783, May 2004.
 [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
            Version 5 Generic Security Service Application Program
            Interface (GSS-API) Mechanism: Version 2", RFC 4121,
            July 2005.
 [RFC4297]  Romanow, A., Mogul, J., Talpey, T., and S. Bailey, "Remote
            Direct Memory Access (RDMA) over IP Problem Statement",
            RFC 4297, December 2005.
 [RFC4806]  Myers, M. and H. Tschofenig, "Online Certificate Status
            Protocol (OCSP) Extensions to IKEv2", RFC 4806,
            February 2007.
 [RFC4850]  Wysochanski, D., "Declarative Public Extension Key for
            Internet Small Computer Systems Interface (iSCSI) Node
            Architecture", RFC 4850, April 2007.
 [RFC5046]  Ko, M., Chadalapaka, M., Hufferd, J., Elzur, U., Shah, H.,
            and P. Thaler, "Internet Small Computer System Interface
            (iSCSI) Extensions for Remote Direct Memory Access
            (RDMA)", RFC 5046, October 2007.
 [RFC5048]  Chadalapaka, M., Ed., "Internet Small Computer System
            Interface (iSCSI) Corrections and Clarifications",
            RFC 5048, October 2007.
 [RFC5433]  Clancy, T. and H. Tschofenig, "Extensible Authentication
            Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method",
            RFC 5433, February 2009.
 [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
            "Deprecating the "X-" Prefix and Similar Constructs in
            Application Protocols", BCP 178, RFC 6648, June 2012.
 [SAS]      INCITS Technical Committee T10, "Serial Attached SCSI -
            2.1 (SAS-2.1)", ANSI INCITS 457-2010, 2010.

Chadalapaka, et al. Standards Track [Page 252] RFC 7143 iSCSI (Consolidated) April 2014

 [SBC2]     INCITS Technical Committee T10, "SCSI Block Commands - 2
            (SBC-2)", ANSI INCITS 405-2005, ISO/IEC 14776-322, 2005.
 [SPC4]     INCITS Technical Committee T10, "SCSI Primary Commands -
            4", ANSI INCITS 513-201x.
 [SPL]      INCITS Technical Committee T10, "SAS Protocol Layer - 2
            (SPL-2)", ANSI INCITS 505-2013, ISO/IEC 14776-262, 2013.

Chadalapaka, et al. Standards Track [Page 253] RFC 7143 iSCSI (Consolidated) April 2014

Appendix A. Examples

A.1. Read Operation Example

 +------------------+-----------------------+---------------------+
 |Initiator Function|       PDU Type        |   Target Function   |
 +------------------+-----------------------+---------------------+
 | Command request  |SCSI Command (read)>>> |                     |
 | (read)           |                       |                     |
 +------------------+-----------------------+---------------------+
 |                  |                       |Prepare Data Transfer|
 +------------------+-----------------------+---------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data         |
 +------------------+-----------------------+---------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data         |
 +------------------+-----------------------+---------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data         |
 +------------------+-----------------------+---------------------+
 |                  |   <<< SCSI Response   |Send Status and Sense|
 +------------------+-----------------------+---------------------+
 | Command Complete |                       |                     |
 +------------------+-----------------------+---------------------+

Chadalapaka, et al. Standards Track [Page 254] RFC 7143 iSCSI (Consolidated) April 2014

A.2. Write Operation Example

 +------------------+-----------------------+---------------------+
 |Initiator Function|       PDU Type        |   Target Function   |
 +------------------+-----------------------+---------------------+
 | Command request  |SCSI Command (write)>>>| Receive command     |
 | (write)          |                       | and queue it        |
 +------------------+-----------------------+---------------------+
 |                  |                       | Process old commands|
 +------------------+-----------------------+---------------------+
 |                  |                       | Ready to process    |
 |                  |   <<< R2T             | write command       |
 +------------------+-----------------------+---------------------+
 |   Send Data      |   SCSI Data-Out >>>   |   Receive Data      |
 +------------------+-----------------------+---------------------+
 |                  |   <<< R2T             | Ready for data      |
 +------------------+-----------------------+---------------------+
 |                  |   <<< R2T             | Ready for data      |
 +------------------+-----------------------+---------------------+
 |   Send Data      |   SCSI Data-Out >>>   |   Receive Data      |
 +------------------+-----------------------+---------------------+
 |   Send Data      |   SCSI Data-Out >>>   |   Receive Data      |
 +------------------+-----------------------+---------------------+
 |                  |   <<< SCSI Response   |Send Status and Sense|
 +------------------+-----------------------+---------------------+
 | Command Complete |                       |                     |
 +------------------+-----------------------+---------------------+

Chadalapaka, et al. Standards Track [Page 255] RFC 7143 iSCSI (Consolidated) April 2014

A.3. R2TSN/DataSN Use Examples

A.3.1. Output (Write) Data DataSN/R2TSN Example

 +-------------------+------------------------+---------------------+
 |Initiator Function |  PDU Type and Content  |   Target Function   |
 +-------------------+------------------------+---------------------+
 | Command request   |SCSI Command (write)>>> | Receive command     |
 | (write)           |                        | and queue it        |
 +-------------------+------------------------+---------------------+
 |                   |                        | Process old commands|
 +-------------------+------------------------+---------------------+
 |                   |   <<< R2T              | Ready for data      |
 |                   |   R2TSN = 0            |                     |
 +-------------------+------------------------+---------------------+
 |                   |   <<< R2T              | Ready for more data |
 |                   |   R2TSN = 1            |                     |
 +-------------------+------------------------+---------------------+
 | Send Data         |   SCSI Data-Out >>>    |   Receive Data      |
 | for R2TSN 0       |   DataSN = 0, F = 0    |                     |
 +-------------------+------------------------+---------------------+
 | Send Data         |   SCSI Data-Out >>>    |   Receive Data      |
 | for R2TSN 0       |   DataSN = 1, F = 1    |                     |
 +-------------------+------------------------+---------------------+
 | Send Data         |   SCSI Data >>>        |   Receive Data      |
 | for R2TSN 1       |   DataSN = 0, F = 1    |                     |
 +-------------------+------------------------+---------------------+
 |                   |   <<< SCSI Response    |Send Status and Sense|
 |                   |   ExpDataSN = 0        |                     |
 +-------------------+------------------------+---------------------+
 | Command Complete  |                        |                     |
 +-------------------+------------------------+---------------------+

Chadalapaka, et al. Standards Track [Page 256] RFC 7143 iSCSI (Consolidated) April 2014

A.3.2. Input (Read) Data DataSN Example

 +------------------+-----------------------+----------------------+
 |Initiator Function|        PDU Type       |    Target Function   |
 +------------------+-----------------------+----------------------+
 | Command request  |SCSI Command (read)>>> |                      |
 | (read)           |                       |                      |
 +------------------+-----------------------+----------------------+
 |                  |                       |Prepare Data Transfer |
 +------------------+-----------------------+----------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data          |
 |                  |   DataSN = 0, F = 0   |                      |
 +------------------+-----------------------+----------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data          |
 |                  |   DataSN = 1, F = 0   |                      |
 +------------------+-----------------------+----------------------+
 |   Receive Data   |   <<< SCSI Data-In    |   Send Data          |
 |                  |   DataSN = 2, F = 1   |                      |
 +------------------+-----------------------+----------------------+
 |                  |   <<< SCSI Response   |Send Status and Sense |
 |                  |   ExpDataSN = 3       |                      |
 +------------------+-----------------------+----------------------+
 | Command Complete |                       |                      |
 +------------------+-----------------------+----------------------+

Chadalapaka, et al. Standards Track [Page 257] RFC 7143 iSCSI (Consolidated) April 2014

A.3.3. Bidirectional DataSN Example

 +------------------+-----------------------+---------------------+
 |Initiator Function|       PDU Type        |   Target Function   |
 +------------------+-----------------------+---------------------+
 | Command request  |SCSI Command >>>       |                     |
 | (Read-Write)     | Read-Write            |                     |
 +------------------+-----------------------+---------------------+
 |                  |                       | Process old commands|
 +------------------+-----------------------+---------------------+
 |                  |   <<< R2T             | Ready to process    |
 |                  |   R2TSN = 0           | write command       |
 +------------------+-----------------------+---------------------+
 | * Receive Data   |   <<< SCSI Data-In    |   Send Data         |
 |                  |   DataSN = 0, F = 0   |                     |
 +------------------+-----------------------+---------------------+
 | * Receive Data   |   <<< SCSI Data-In    |   Send Data         |
 |                  |   DataSN = 1, F = 1   |                     |
 +------------------+-----------------------+---------------------+
 | * Send Data      |   SCSI Data-Out >>>   |   Receive Data      |
 | for R2TSN 0      |   DataSN = 0, F = 1   |                     |
 +------------------+-----------------------+---------------------+
 |                  |   <<< SCSI Response   |Send Status and Sense|
 |                  |   ExpDataSN = 2       |                     |
 +------------------+-----------------------+---------------------+
 | Command Complete |                       |                     |
 +------------------+-----------------------+---------------------+
  • Send Data and Receive Data may be transferred simultaneously as in

an atomic Read-Old-Write-New or sequentially as in an atomic

   Read-Update-Write (in the latter case, the R2T may follow the
   received data).

Chadalapaka, et al. Standards Track [Page 258] RFC 7143 iSCSI (Consolidated) April 2014

A.3.4. Unsolicited and Immediate Output (Write) Data with DataSN

      Example
 +------------------+------------------------+----------------------+
 |Initiator Function|  PDU Type and Content  |   Target Function    |
 +------------------+------------------------+----------------------+
 | Command request  |SCSI Command (write)>>> | Receive command      |
 | (write)          |F = 0                   | and data             |
 |+ immediate data  |                        | and queue it         |
 +------------------+------------------------+----------------------+
 | Send Unsolicited |    SCSI Write Data >>> | Receive more Data    |
 | Data             |    DataSN = 0, F = 1   |                      |
 +------------------+------------------------+----------------------+
 |                  |                        | Process old commands |
 +------------------+------------------------+----------------------+
 |                  |    <<< R2T             | Ready for more data  |
 |                  |    R2TSN = 0           |                      |
 +------------------+------------------------+----------------------+
 | Send Data        |    SCSI Write Data >>> |   Receive Data       |
 | for R2TSN 0      |    DataSN = 0, F = 1   |                      |
 +------------------+------------------------+----------------------+
 |                  |    <<< SCSI Response   |Send Status and Sense |
 |                  |                        |                      |
 +------------------+------------------------+----------------------+
 | Command Complete |                        |                      |
 +------------------+------------------------+----------------------+

A.4. CRC Examples

 Note: All values are hexadecimal.
 32 bytes of zeroes:
    Byte:        0  1  2  3
       0:       00 00 00 00
     ...
      28:       00 00 00 00
     CRC:       aa 36 91 8a

Chadalapaka, et al. Standards Track [Page 259] RFC 7143 iSCSI (Consolidated) April 2014

 32 bytes of ones:
    Byte:        0  1  2  3
       0:       ff ff ff ff
     ...
      28:       ff ff ff ff
     CRC:       43 ab a8 62
 32 bytes of incrementing 00..1f:
    Byte:        0  1  2  3
       0:       00 01 02 03
     ...
      28:       1c 1d 1e 1f
     CRC:       4e 79 dd 46
 32 bytes of decrementing 1f..00:
    Byte:        0  1  2  3
       0:       1f 1e 1d 1c
     ...
      28:       03 02 01 00
     CRC:       5c db 3f 11
 An iSCSI - SCSI Read (10) Command PDU:
   Byte:        0     1    2    3
      0:       01    c0   00   00
      4:       00    00   00   00
      8:       00    00   00   00
     12:       00    00   00   00
     16:       14    00   00   00
     20:       00    00   04   00
     24:       00    00   00   14
     28:       00    00   00   18
     32:       28    00   00   00
     36:       00    00   00   00
     40:       02    00   00   00
     44:       00    00   00   00
    CRC:       56    3a   96   d9

Chadalapaka, et al. Standards Track [Page 260] RFC 7143 iSCSI (Consolidated) April 2014

Appendix B. Login Phase Examples

 In the first example, the initiator and target authenticate each
 other via Kerberos:
    I-> Login (CSG,NSG=0,1 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,SRP,None
    T-> Login (CSG,NSG=0,0 T=0)
        AuthMethod=KRB5
    I-> Login (CSG,NSG=0,1 T=1)
        KRB_AP_REQ=<krb_ap_req>
 (krb_ap_req contains the Kerberos V5 ticket and authenticator with
 MUTUAL-REQUIRED set in the ap-options field)
 If the authentication is successful, the target proceeds with:
    T-> Login (CSG,NSG=0,1 T=1)
        KRB_AP_REP=<krb_ap_rep>
 (krb_ap_rep is the Kerberos V5 mutual authentication reply)
 If the authentication is successful, the initiator may proceed
 with:
    I-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=8192
    T-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=4096
        MaxBurstLength=8192
    I-> Login (CSG,NSG=1,0 T=0) MaxBurstLength=8192
        ... more iSCSI Operational Parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... more iSCSI Operational Parameters
    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"

Chadalapaka, et al. Standards Track [Page 261] RFC 7143 iSCSI (Consolidated) April 2014

 If the initiator's authentication by the target is not successful,
 the target responds with:
    T-> Login "login reject"
 instead of the Login KRB_AP_REP message, and it terminates the
 connection.
 If the target's authentication by the initiator is not successful,
 the initiator terminates the connection (without responding to the
 Login KRB_AP_REP message).
 In the next example, only the initiator is authenticated by the
 target via Kerberos:
    I-> Login (CSG,NSG=0,1 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=SRP,KRB5,None
    T-> Login-PR (CSG,NSG=0,0 T=0)
        AuthMethod=KRB5
    I-> Login (CSG,NSG=0,1 T=1)
        KRB_AP_REQ=krb_ap_req
 (MUTUAL-REQUIRED not set in the ap-options field of krb_ap_req)
 If the authentication is successful, the target proceeds with:
    T-> Login (CSG,NSG=0,1 T=1)
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    . . .
    T-> Login (CSG,NSG=1,3 T=1)"login accept"

Chadalapaka, et al. Standards Track [Page 262] RFC 7143 iSCSI (Consolidated) April 2014

 In the next example, the initiator and target authenticate each other
 via SRP:
    I-> Login (CSG,NSG=0,1 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,SRP,None
    T-> Login-PR (CSG,NSG=0,0 T=0)
        AuthMethod=SRP
    I-> Login (CSG,NSG=0,0 T=0)
        SRP_U=<user>
        TargetAuth=Yes
    T-> Login (CSG,NSG=0,0 T=0)
        SRP_N=<N>
        SRP_g=<g>
        SRP_s=<s>
    I-> Login (CSG,NSG=0,0 T=0)
        SRP_A=<A>
    T-> Login (CSG,NSG=0,0 T=0)
        SRP_B=<B>
    I-> Login (CSG,NSG=0,1 T=1)
        SRP_M=<M>
 If the initiator authentication is successful, the target proceeds
 with:
    T-> Login (CSG,NSG=0,1 T=1)
        SRP_HM=<H(A | M | K)>
 where N, g, s, A, B, M, and H(A | M | K) are defined in [RFC2945].
 If the target authentication is not successful, the initiator
 terminates the connection; otherwise, it proceeds.
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters

Chadalapaka, et al. Standards Track [Page 263] RFC 7143 iSCSI (Consolidated) April 2014

    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"
 If the initiator authentication is not successful, the target
 responds with:
    T-> Login "login reject"
 instead of the T-> Login SRP_HM=<H(A | M | K)> message, and it
 terminates the connection.
 In the next example, only the initiator is authenticated by the
 target via SRP:
    I-> Login (CSG,NSG=0,1 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,SRP,None
    T-> Login-PR (CSG,NSG=0,0 T=0)
        AuthMethod=SRP
    I-> Login (CSG,NSG=0,0 T=0)
        SRP_U=<user>
        TargetAuth=No
    T-> Login (CSG,NSG=0,0 T=0)
        SRP_N=<N>
        SRP_g=<g>
        SRP_s=<s>
    I-> Login (CSG,NSG=0,0 T=0)
        SRP_A=<A>
    T-> Login (CSG,NSG=0,0 T=0)
        SRP_B=<B>
    I-> Login (CSG,NSG=0,1 T=1)
         SRP_M=<M>

Chadalapaka, et al. Standards Track [Page 264] RFC 7143 iSCSI (Consolidated) April 2014

 If the initiator authentication is successful, the target proceeds
 with:
    T-> Login (CSG,NSG=0,1 T=1)
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"
 In the next example, the initiator and target authenticate each other
 via CHAP:
    I-> Login (CSG,NSG=0,0 T=0)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,CHAP,None
    T-> Login-PR (CSG,NSG=0,0 T=0)
        AuthMethod=CHAP
    I-> Login (CSG,NSG=0,0 T=0)
        CHAP_A=<A1,A2>
    T-> Login (CSG,NSG=0,0 T=0)
        CHAP_A=<A1>
        CHAP_I=<I>
        CHAP_C=<C>
    I-> Login (CSG,NSG=0,1 T=1)
        CHAP_N=<N>
        CHAP_R=<R>
        CHAP_I=<I>
        CHAP_C=<C>

Chadalapaka, et al. Standards Track [Page 265] RFC 7143 iSCSI (Consolidated) April 2014

 If the initiator authentication is successful, the target proceeds
 with:
    T-> Login (CSG,NSG=0,1 T=1)
        CHAP_N=<N>
        CHAP_R=<R>
 If the target authentication is not successful, the initiator aborts
 the connection; otherwise, it proceeds.
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"
 If the initiator authentication is not successful, the target
 responds with:
    T-> Login "login reject"
 instead of the Login CHAP_R=<response> "proceed and change stage"
 message, and it terminates the connection.
 In the next example, only the initiator is authenticated by the
 target via CHAP:
    I-> Login (CSG,NSG=0,1 T=0)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,CHAP,None
    T-> Login-PR (CSG,NSG=0,0 T=0)
        AuthMethod=CHAP
    I-> Login (CSG,NSG=0,0 T=0)
        CHAP_A=<A1,A2>

Chadalapaka, et al. Standards Track [Page 266] RFC 7143 iSCSI (Consolidated) April 2014

    T-> Login (CSG,NSG=0,0 T=0)
        CHAP_A=<A1>
        CHAP_I=<I>
        CHAP_C=<C>
    I-> Login (CSG,NSG=0,1 T=1)
        CHAP_N=<N>
        CHAP_R=<R>
 If the initiator authentication is successful, the target proceeds
 with:
    T-> Login (CSG,NSG=0,1 T=1)
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"
 In the next example, the initiator does not offer any security
 parameters.  It therefore may offer iSCSI parameters on the Login PDU
 with the T bit set to 1, and the target may respond with a final
 Login Response PDU immediately:
    I-> Login (CSG,NSG=1,3 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"
        ... ISCSI parameters
 In the next example, the initiator does offer security parameters on
 the Login PDU, but the target does not choose any (i.e., chooses the
 "None" values):
    I-> Login (CSG,NSG=0,1 T=1)
        InitiatorName=iqn.1999-07.com.os:hostid.77
        TargetName=iqn.1999-07.com.example:diskarray.sn.88
        AuthMethod=KRB5,SRP,None

Chadalapaka, et al. Standards Track [Page 267] RFC 7143 iSCSI (Consolidated) April 2014

    T-> Login-PR (CSG,NSG=0,1 T=1)
        AuthMethod=None
    I-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    T-> Login (CSG,NSG=1,0 T=0)
        ... iSCSI parameters
    And at the end:
    I-> Login (CSG,NSG=1,3 T=1)
        optional iSCSI parameters
    T-> Login (CSG,NSG=1,3 T=1) "login accept"

Appendix C. SendTargets Operation

 The text in this appendix is a normative part of this document.
 To reduce the amount of configuration required on an initiator, iSCSI
 provides the SendTargets Text Request.  The initiator uses the
 SendTargets request to get a list of targets to which it may have
 access, as well as the list of addresses (IP address and TCP port) on
 which these targets may be accessed.
 To make use of SendTargets, an initiator must first establish one of
 two types of sessions.  If the initiator establishes the session
 using the key "SessionType=Discovery", the session is a Discovery
 session, and a target name does not need to be specified.  Otherwise,
 the session is a Normal operational session.  The SendTargets command
 MUST only be sent during the Full Feature Phase of a Normal or
 Discovery session.
 A system that contains targets MUST support Discovery sessions on
 each of its iSCSI IP address-port pairs and MUST support the
 SendTargets command on the Discovery session.  In a Discovery
 session, a target MUST return all path information (IP address-port
 pairs and Target Portal Group Tags) for the targets on the target
 Network Entity that the requesting initiator is authorized to access.
 A target MUST support the SendTargets command on operational
 sessions; these will only return path information about the target to
 which the session is connected and do not need to return information
 about other target names that may be defined in the responding
 system.
 An initiator MAY make use of the SendTargets command as it sees fit.

Chadalapaka, et al. Standards Track [Page 268] RFC 7143 iSCSI (Consolidated) April 2014

 A SendTargets command consists of a single Text Request PDU.  This
 PDU contains exactly one text key and value.  The text key MUST be
 SendTargets.  The expected response depends upon the value, as well
 as whether the session is a Discovery session or an operational
 session.
 The value must be one of:
    All
       The initiator is requesting that information on all relevant
       targets known to the implementation be returned.  This value
       MUST be supported on a Discovery session and MUST NOT be
       supported on an operational session.
    <iSCSI-target-name>
       If an iSCSI Target Name is specified, the session should
       respond with addresses for only the named target, if possible.
       This value MUST be supported on Discovery sessions.  A
       Discovery session MUST be capable of returning addresses for
       those targets that would have been returned had value=All been
       designated.
    <nothing>
       The session should only respond with addresses for the target
       to which the session is logged in.  This MUST be supported on
       operational sessions and MUST NOT return targets other than the
       one to which the session is logged in.
 The response to this command is a Text Response that contains a list
 of zero or more targets and, optionally, their addresses.  Each
 target is returned as a target record.  A target record begins with
 the TargetName text key, followed by a list of TargetAddress text
 keys, and bounded by the end of the Text Response or the next
 TargetName key, which begins a new record.  No text keys other than
 TargetName and TargetAddress are permitted within a SendTargets
 response.
 For the format of the TargetName, see Section 13.4.
 A Discovery session MAY respond to a SendTargets request with its
 complete list of targets, or with a list of targets that is based on
 the name of the initiator logged in to the session.
 A SendTargets response MUST NOT contain target names if there are no
 targets for the requesting initiator to access.

Chadalapaka, et al. Standards Track [Page 269] RFC 7143 iSCSI (Consolidated) April 2014

 Each target record returned includes zero or more TargetAddress
 fields.
 Each target record starts with one text key of the form:
    TargetName=<target-name-goes-here>
 followed by zero or more address keys of the form:
 TargetAddress=<hostname-or-ipaddress>[:<tcp-port>],
    <portal-group-tag>
 The hostname-or-ipaddress contains a domain name, IPv4 address, or
 IPv6 address ([RFC4291]), as specified for the TargetAddress key.
 A hostname-or-ipaddress duplicated in TargetAddress responses for a
 given node (the port is absent or equal) would probably indicate that
 multiple address families are in use at once (IPv6 and IPv4).
 Each TargetAddress belongs to a portal group, identified by its
 numeric Target Portal Group Tag (see Section 13.9).  The iSCSI Target
 Name, together with this tag, constitutes the SCSI port identifier;
 the tag only needs to be unique within a given target's name list of
 addresses.
 Multiple-connection sessions can span iSCSI addresses that belong to
 the same portal group.
 Multiple-connection sessions cannot span iSCSI addresses that belong
 to different portal groups.
 If a SendTargets response reports an iSCSI address for a target, it
 SHOULD also report all other addresses in its portal group in the
 same response.
 A SendTargets Text Response can be longer than a single Text Response
 PDU and makes use of the long Text Responses as specified.
 After obtaining a list of targets from the Discovery session, an
 iSCSI initiator may initiate new sessions to log in to the discovered
 targets for full operation.  The initiator MAY keep the Discovery
 session open and MAY send subsequent SendTargets commands to discover
 new targets.

Chadalapaka, et al. Standards Track [Page 270] RFC 7143 iSCSI (Consolidated) April 2014

 Examples:
 This example is the SendTargets response from a single target that
 has no other interface ports.
 The initiator sends a Text Request that contains:
    SendTargets=All
 The target sends a Text Response that contains:
    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309
 All the target had to return in this simple case was the target name.
 It is assumed by the initiator that the IP address and TCP port for
 this target are the same as those used on the current connection to
 the default iSCSI target.
 The next example has two internal iSCSI targets, each accessible via
 two different ports with different IP addresses.  The following is
 the Text Response:
    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309
    TargetAddress=10.1.0.45:3000,1
    TargetAddress=10.1.1.45:3000,2
    TargetName=iqn.1993-11.com.example:diskarray.sn.1234567
    TargetAddress=10.1.0.45:3000,1
    TargetAddress=10.1.1.45:3000,2
 Both targets share both addresses; the multiple addresses are likely
 used to provide multi-path support.  The initiator may connect to
 either target name on either address.  Each of the addresses has its
 own Target Portal Group Tag; they do not support spanning multiple-
 connection sessions with each other.  Keep in mind that the Target
 Portal Group Tags for the two named targets are independent of one
 another; portal group "1" on the first target is not necessarily the
 same as portal group "1" on the second target.
 In the above example, a DNS host name or an IPv6 address could have
 been returned instead of an IPv4 address.

Chadalapaka, et al. Standards Track [Page 271] RFC 7143 iSCSI (Consolidated) April 2014

 The next Text Response shows a target that supports spanning sessions
 across multiple addresses and further illustrates the use of the
 Target Portal Group Tags:
    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309
    TargetAddress=10.1.0.45:3000,1
    TargetAddress=10.1.1.46:3000,1
    TargetAddress=10.1.0.47:3000,2
    TargetAddress=10.1.1.48:3000,2
    TargetAddress=10.1.1.49:3000,3
 In this example, any of the target addresses can be used to reach the
 same target.  A single-connection session can be established to any
 of these TCP addresses.  A multiple-connection session could span
 addresses .45 and .46 or .47 and .48 but cannot span any other
 combination.  A TargetAddress with its own tag (.49) cannot be
 combined with any other address within the same session.
 This SendTargets response does not indicate whether .49 supports
 multiple connections per session; it is communicated via the
 MaxConnections text key upon login to the target.

Appendix D. Algorithmic Presentation of Error Recovery Classes

 This appendix illustrates the error recovery classes using a
 pseudo-programming language.  The procedure names are chosen to be
 obvious to most implementers.  Each of the recovery classes described
 has initiator procedures as well as target procedures.  These
 algorithms focus on outlining the mechanics of error recovery classes
 and do not exhaustively describe all other aspects/cases.  Examples
 of this approach are as follows:
  1. Handling for only certain Opcode types is shown.
  1. Only certain reason codes (e.g., Recovery in Logout command) are

outlined.

  1. Resultant cases, such as recovery of Synchronization on a header

digest error, are considered out of scope in these algorithms.

      In this particular example, a header digest error may lead to
      connection recovery if some type of Sync and Steering layer is
      not implemented.

Chadalapaka, et al. Standards Track [Page 272] RFC 7143 iSCSI (Consolidated) April 2014

 These algorithms strive to convey the iSCSI error recovery concepts
 in the simplest terms and are not designed to be optimal.

D.1. General Data Structure and Procedure Description

 This section defines the procedures and data structures that are
 commonly used by all the error recovery algorithms.  The structures
 may not be the exhaustive representations of what is required for a
 typical implementation.
 Data structure definitions:
 struct TransferContext {
         int TargetTransferTag;
         int ExpectedDataSN;
 };
 struct TCB {              /* task control block */
         Boolean SoFarInOrder;
         int ExpectedDataSN; /* used for both R2Ts and Data */
         int MissingDataSNList[MaxMissingDPDU];
         Boolean FbitReceived;
         Boolean StatusXferd;
         Boolean CurrentlyAllegiant;
         int ActiveR2Ts;
         int Response;
         char *Reason;
         struct TransferContext
                     TransferContextList[MaxOutstandingR2T];
         int InitiatorTaskTag;
         int CmdSN;
         int SNACK_Tag;
 };
 struct Connection {
         struct Session SessionReference;
         Boolean SoFarInOrder;
         int CID;
         int State;
         int CurrentTimeout;
         int ExpectedStatSN;
         int MissingStatSNList[MaxMissingSPDU];
         Boolean PerformConnectionCleanup;
 };

Chadalapaka, et al. Standards Track [Page 273] RFC 7143 iSCSI (Consolidated) April 2014

 struct Session {
         int NumConnections;
         int CmdSN;
         int Maxconnections;
         int ErrorRecoveryLevel;
         struct iSCSIEndpoint OtherEndInfo;
         struct Connection ConnectionList[MaxSupportedConns];
 };
 Procedure descriptions:
 Receive-an-In-PDU(transport connection, inbound PDU);
 check-basic-validity(inbound PDU);
 Start-Timer(timeout handler, argument, timeout value);
 Build-And-Send-Reject(transport connection, bad PDU, reason code);

D.2. Within-command Error Recovery Algorithms

D.2.1. Procedure Descriptions

 Recover-Data-if-Possible(last required DataSN, task control block);
 Build-And-Send-DSnack(task control block);
 Build-And-Send-RDSnack(task control block);
 Build-And-Send-Abort(task control block);
 SCSI-Task-Completion(task control block);
 Build-And-Send-A-Data-Burst(transport connection, data-descriptor,
    task control block);
 Build-And-Send-R2T(transport connection, data-descriptor,
    task control block);
 Build-And-Send-Status(transport connection, task control block);
 Transfer-Context-Timeout-Handler(transfer context);
 Notes:
  1. One procedure used in this section: the Handle-Status-SNACK-request

is defined in Appendix D.3.

  1. The response-processing pseudocode shown in the target algorithms

applies to all solicited PDUs that carry the StatSN – SCSI

   Response, Text Response, etc.

Chadalapaka, et al. Standards Track [Page 274] RFC 7143 iSCSI (Consolidated) April 2014

D.2.2. Initiator Algorithms

 Recover-Data-if-Possible(LastRequiredDataSN, TCB)
 {
     if (operational ErrorRecoveryLevel > 0) {
          if (# of missing PDUs is trackable) {
                Note the missing DataSNs in TCB.
                if (the task spanned a change in
                           MaxRecvDataSegmentLength) {
                     if (TCB.StatusXferd is TRUE)
                         drop the status PDU;
                     Build-And-Send-RDSnack(TCB);
                } else {
                     Build-And-Send-DSnack(TCB);
                }
          } else {
              TCB.Reason = "Protocol Service CRC error";
                   }
     } else {
           TCB.Reason = "Protocol Service CRC error";
     }
     if (TCB.Reason == "Protocol Service CRC error") {
           Clear the missing PDU list in the TCB.
           if (TCB.StatusXferd is not TRUE)
              Build-And-Send-Abort(TCB);
     }
 }
 Receive-an-In-PDU(Connection, CurrentPDU)
 {
  check-basic-validity(CurrentPDU);
  if (Header-Digest-Bad) discard, return;
  Retrieve TCB for CurrentPDU.InitiatorTaskTag.
  if ((CurrentPDU.type == Data)
              or (CurrentPDU.type = R2T)) {
     if (Data-Digest-Bad for Data) {
               send-data-SNACK = TRUE;
       LastRequiredDataSN = CurrentPDU.DataSN;
             } else {
           if (TCB.SoFarInOrder = TRUE) {
               if (current DataSN is expected) {
                    Increment TCB.ExpectedDataSN.
               } else {
                       TCB.SoFarInOrder = FALSE;
                       send-data-SNACK = TRUE;
                      }

Chadalapaka, et al. Standards Track [Page 275] RFC 7143 iSCSI (Consolidated) April 2014

           } else {
                   if (current DataSN was considered missing) {
                      remove current DataSN from missing PDU list.
                  } else if (current DataSN is higher than expected) {
                              send-data-SNACK = TRUE;
                       } else {
                             discard, return;
                       }
                       Adjust TCB.ExpectedDataSN if appropriate.
              }
              LastRequiredDataSN = CurrentPDU.DataSN - 1;
                }
                if (send-data-SNACK is TRUE and
                  task is not already considered failed) {
              Recover-Data-if-Possible(LastRequiredDataSN, TCB);
     }
             if (missing data PDU list is empty) {
                TCB.SoFarInOrder = TRUE;
             }
     if (CurrentPDU.type == R2T) {
        Increment ActiveR2Ts for this task.
        Create a data-descriptor for the data burst.
        Build-And-Send-A-Data-Burst(Connection, data-descriptor, TCB);
      }
   } else if (CurrentPDU.type == Response) {
      if (Data-Digest-Bad) {
                 send-status-SNACK = TRUE;
              } else {
         TCB.StatusXferd = TRUE;
         Store the status information in TCB.
         if (ExpDataSN does not match) {
              TCB.SoFarInOrder = FALSE;
              Recover-Data-if-Possible(current DataSN, TCB);
         }
                 if (missing data PDU list is empty) {
                      TCB.SoFarInOrder = TRUE;
                 }
      }
   } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT SHOWN */
   }
   if ((TCB.SoFarInOrder == TRUE) and
                         (TCB.StatusXferd == TRUE)) {
           SCSI-Task-Completion(TCB);
    }
 }

Chadalapaka, et al. Standards Track [Page 276] RFC 7143 iSCSI (Consolidated) April 2014

D.2.3. Target Algorithms

 Receive-an-In-PDU(Connection, CurrentPDU)
 {
   check-basic-validity(CurrentPDU);
   if (Header-Digest-Bad) discard, return;
   Retrieve TCB for CurrentPDU.InitiatorTaskTag.
   if (CurrentPDU.type == Data) {
       Retrieve TContext from CurrentPDU.TargetTransferTag;
       if (Data-Digest-Bad) {
                   Build-And-Send-Reject(Connection, CurrentPDU,
                                Payload-Digest-Error);
          Note the missing data PDUs in MissingDataRange[].
                   send-recovery-R2T = TRUE;
                } else {
          if (current DataSN is not expected) {
              Note the missing data PDUs in MissingDataRange[].
                       send-recovery-R2T = TRUE;
                   }
          if (CurrentPDU.Fbit == TRUE) {
              if (current PDU is solicited) {
                      Decrement TCB.ActiveR2Ts.
              }
              if ((current PDU is unsolicited and
                      data received is less than I/O length and
                        data received is less than FirstBurstLength)
                   or (current PDU is solicited and the length of
                        this burst is less than expected)) {
                   send-recovery-R2T = TRUE;
                   Note the missing data in MissingDataRange[].
              }
                   }
                }
                Increment TContext.ExpectedDataSN.
       if (send-recovery-R2T is TRUE and
                 task is not already considered failed) {
          if (operational ErrorRecoveryLevel > 0) {
              Increment TCB.ActiveR2Ts.
              Create a data-descriptor for the data burst
                         from MissingDataRange.
              Build-And-Send-R2T(Connection, data-descriptor, TCB);
          } else {
               if (current PDU is the last unsolicited)
                   TCB.Reason = "Not enough unsolicited data";
               else
                   TCB.Reason = "Protocol Service CRC error";
          }
       }

Chadalapaka, et al. Standards Track [Page 277] RFC 7143 iSCSI (Consolidated) April 2014

       if (TCB.ActiveR2Ts == 0) {
          Build-And-Send-Status(Connection, TCB);
       }
   } else if (CurrentPDU.type == SNACK) {
       snack-failure = FALSE;
       if (operational ErrorRecoveryLevel > 0) {
          if (CurrentPDU.type == Data/R2T) {
              if (the request is satisfiable) {
                 if (request for Data) {
                    Create a data-descriptor for the data burst
                        from BegRun and RunLength.
                    Build-And-Send-A-Data-Burst(Connection,
                       data-descriptor, TCB);
                 } else { /* R2T */
                    Create a data-descriptor for the data burst
                        from BegRun and RunLength.
                    Build-And-Send-R2T(Connection, data-descriptor,
                       TCB);
                  }
               } else {
                     snack-failure = TRUE;
               }
          } else if (CurrentPDU.type == status) {
               Handle-Status-SNACK-request(Connection, CurrentPDU);
          } else if (CurrentPDU.type == DataACK) {
                 Consider all data up to CurrentPDU.BegRun as
                 acknowledged.
                 Free up the retransmission resources for that data.
            } else if (CurrentPDU.type == R-Data SNACK) {
                          Create a data descriptor for a data burst
                          covering all unacknowledged data.
                Build-And-Send-A-Data-Burst(Connection,
                   data-descriptor, TCB);
                TCB.SNACK_Tag = CurrentPDU.SNACK_Tag;
                if (there's no more data to send) {
                   Build-And-Send-Status(Connection, TCB);
                }
          }
       } else { /* operational ErrorRecoveryLevel = 0 */
                snack-failure = TRUE;
       }
       if (snack-failure == TRUE) {
            Build-And-Send-Reject(Connection, CurrentPDU,
                SNACK-Reject);
            if (TCB.StatusXferd != TRUE) {
                TCB.Reason = "SNACK rejected";
                Build-And-Send-Status(Connection, TCB);
            }

Chadalapaka, et al. Standards Track [Page 278] RFC 7143 iSCSI (Consolidated) April 2014

       }
   } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT SHOWN */
   }
 }
 Transfer-Context-Timeout-Handler(TContext)
 {
   Retrieve TCB and Connection from TContext.
   Decrement TCB.ActiveR2Ts.
   if (operational ErrorRecoveryLevel > 0 and
                 task is not already considered failed) {
       Note the missing data PDUs in MissingDataRange[].
       Create a data-descriptor for the data burst
                         from MissingDataRange[].
       Build-And-Send-R2T(Connection, data-descriptor, TCB);
     } else {
         TCB.Reason = "Protocol Service CRC error";
         if (TCB.ActiveR2Ts = 0) {
            Build-And-Send-Status(Connection, TCB);
         }
     }
 }

D.3. Within-connection Recovery Algorithms

D.3.1. Procedure Descriptions

 Procedure descriptions:
 Recover-Status-if-Possible(transport connection,
    currently received PDU);
 Evaluate-a-StatSN(transport connection, currently received PDU);
 Retransmit-Command-if-Possible(transport connection, CmdSN);
 Build-And-Send-SSnack(transport connection);
 Build-And-Send-Command(transport connection,
    task control block);
 Command-Acknowledge-Timeout-Handler(task control block);
 Status-Expect-Timeout-Handler(transport connection);
 Build-And-Send-NOP-Out(transport connection);
 Handle-Status-SNACK-request(transport connection,
    Status SNACK PDU);
 Retransmit-Status-Burst(Status SNACK, task control block);
 Is-Acknowledged(beginning StatSN, run length);

Chadalapaka, et al. Standards Track [Page 279] RFC 7143 iSCSI (Consolidated) April 2014

 Implementation-specific parameters that are tunable:
 InitiatorProactiveSNACKEnabled
 Notes:
  1. The initiator algorithms only deal with unsolicited NOP-In PDUs for

generating Status SNACKs. A solicited NOP-In PDU has an assigned

   StatSN that, when out of order, could trigger the out-of-order
   StatSN handling in within-command algorithms, again leading to
   Recover-Status-if-Possible.
  1. The pseudocode shown may result in the retransmission of

unacknowledged commands in more cases than necessary. This will

   not, however, affect the correctness of the operation because the
   target is required to discard the duplicate CmdSNs.
  1. The procedure Build-And-Send-Async is defined in the connection

recovery algorithms.

  1. The procedure Status-Expect-Timeout-Handler describes how

initiators may proactively attempt to retrieve the Status if they

   so choose.  This procedure is assumed to be triggered much before
   the standard ULP timeout.

D.3.2. Initiator Algorithms

   Recover-Status-if-Possible(Connection, CurrentPDU)
   {
       if ((Connection.state == LOGGED_IN) and
                   connection is not already considered failed) {
          if (operational ErrorRecoveryLevel > 0) {
             if (# of missing PDUs is trackable) {
                   Note the missing StatSNs in Connection
                   that were not already requested with SNACK;
               Build-And-Send-SSnack(Connection);
                     } else {
                       Connection.PerformConnectionCleanup = TRUE;
             }
          } else {
                     Connection.PerformConnectionCleanup = TRUE;
          }
          if (Connection.PerformConnectionCleanup == TRUE) {
             Start-Timer(Connection-Cleanup-Handler, Connection, 0);
                   }
       }
   }

Chadalapaka, et al. Standards Track [Page 280] RFC 7143 iSCSI (Consolidated) April 2014

   Retransmit-Command-if-Possible(Connection, CmdSN)
   {
       if (operational ErrorRecoveryLevel > 0) {
          Retrieve the InitiatorTaskTag, and thus TCB for the CmdSN.
          Build-And-Send-Command(Connection, TCB);
       }
   }
   Evaluate-a-StatSN(Connection, CurrentPDU)
   {
       send-status-SNACK = FALSE;
       if (Connection.SoFarInOrder == TRUE) {
          if (current StatSN is the expected) {
               Increment Connection.ExpectedStatSN.
          } else {
                        Connection.SoFarInOrder = FALSE;
                        send-status-SNACK = TRUE;
                   }
       } else {
          if (current StatSN was considered missing) {
               remove current StatSN from the missing list.
          } else {
                        if (current StatSN is higher than expected){
                            send-status-SNACK = TRUE;
                        } else {
                            send-status-SNACK = FALSE;
                    discard the PDU;
               }
          }
          Adjust Connection.ExpectedStatSN if appropriate.
          if (missing StatSN list is empty) {
               Connection.SoFarInOrder = TRUE;
                   }
       }
       return send-status-SNACK;
   }
   Receive-an-In-PDU(Connection, CurrentPDU)
   {
       check-basic-validity(CurrentPDU);
       if (Header-Digest-Bad) discard, return;
       Retrieve TCB for CurrentPDU.InitiatorTaskTag.
       if (CurrentPDU.type == NOP-In) {
             if (the PDU is unsolicited) {
                   if (current StatSN is not expected) {
                        Recover-Status-if-Possible(Connection,
                                     CurrentPDU);
                   }

Chadalapaka, et al. Standards Track [Page 281] RFC 7143 iSCSI (Consolidated) April 2014

                   if (current ExpCmdSN is not Session.CmdSN) {
                        Retransmit-Command-if-Possible(Connection,
                                     CurrentPDU.ExpCmdSN);
                   }
             }
       } else if (CurrentPDU.type == Reject) {
             if (it is a data digest error on immediate data) {
                   Retransmit-Command-if-Possible(Connection,
                                     CurrentPDU.BadPDUHeader.CmdSN);
             }
       } else if (CurrentPDU.type == Response) {
            send-status-SNACK = Evaluate-a-StatSN(Connection,
                                           CurrentPDU);
            if (send-status-SNACK == TRUE)
                Recover-Status-if-Possible(Connection, CurrentPDU);
       } else { /* REST UNRELATED TO WITHIN-CONNECTION-RECOVERY,
                 * NOT SHOWN */
       }
   }
   Command-Acknowledge-Timeout-Handler(TCB)
   {
       Retrieve the Connection for TCB.
       Retransmit-Command-if-Possible(Connection, TCB.CmdSN);
   }
   Status-Expect-Timeout-Handler(Connection)
   {
       if (operational ErrorRecoveryLevel > 0) {
           Build-And-Send-NOP-Out(Connection);
       } else if (InitiatorProactiveSNACKEnabled){
           if ((Connection.state == LOGGED_IN) and
                        connection is not already considered failed) {
                Build-And-Send-SSnack(Connection);
           }
       }
   }

Chadalapaka, et al. Standards Track [Page 282] RFC 7143 iSCSI (Consolidated) April 2014

D.3.3. Target Algorithms

 Handle-Status-SNACK-request(Connection, CurrentPDU)
   {
       if (operational ErrorRecoveryLevel > 0) {
          if (request for an acknowledged run) {
              Build-And-Send-Reject(Connection, CurrentPDU,
                                            Protocol-Error);
          } else if (request for an untransmitted run) {
              discard, return;
          } else {
              Retransmit-Status-Burst(CurrentPDU, TCB);
          }
       } else {
          Build-And-Send-Async(Connection, DroppedConnection,
                                DefaultTime2Wait, DefaultTime2Retain);
       }
   }

D.4. Connection Recovery Algorithms

D.4.1. Procedure Descriptions

 Build-And-Send-Async(transport connection, reason code,
    minimum time, maximum time);
 Pick-A-Logged-In-Connection(session);
 Build-And-Send-Logout(transport connection,
    logout connection identifier, reason code);
 PerformImplicitLogout(transport connection,
    logout connection identifier, target information);
 PerformLogin(transport connection, target information);
 CreateNewTransportConnection(target information);
 Build-And-Send-Command(transport connection, task control block);
 Connection-Cleanup-Handler(transport connection);
 Connection-Resource-Timeout-Handler(transport connection);
 Quiesce-And-Prepare-for-New-Allegiance(session, task control block);
 Build-And-Send-Logout-Response(transport connection,
    CID of connection in recovery, reason code);
 Build-And-Send-TaskMgmt-Response(transport connection,
    task mgmt command PDU, response code);
 Establish-New-Allegiance(task control block, transport connection);
 Schedule-Command-To-Continue(task control block);

Chadalapaka, et al. Standards Track [Page 283] RFC 7143 iSCSI (Consolidated) April 2014

 Note:
  1. Transport exception conditions such as unexpected connection

termination, connection reset, and hung connection while the

   connection is in the Full Feature Phase are all assumed to be
   asynchronously signaled to the iSCSI layer using the
   Transport_Exception_Handler procedure.

D.4.2. Initiator Algorithms

   Receive-an-In-PDU(Connection, CurrentPDU)
   {
       check-basic-validity(CurrentPDU);
       if (Header-Digest-Bad) discard, return;
       Retrieve TCB from CurrentPDU.InitiatorTaskTag.
       if (CurrentPDU.type == Async) {
           if (CurrentPDU.AsyncEvent == ConnectionDropped) {
              Retrieve the AffectedConnection for
                 CurrentPDU.Parameter1.
              AffectedConnection.CurrentTimeout =
                 CurrentPDU.Parameter3;
             AffectedConnection.State = CLEANUP_WAIT;
             Start-Timer(Connection-Cleanup-Handler,
                          AffectedConnection, CurrentPDU.Parameter2);
           } else if (CurrentPDU.AsyncEvent == LogoutRequest)) {
             AffectedConnection = Connection;
             AffectedConnection.State = LOGOUT_REQUESTED;
             AffectedConnection.PerformConnectionCleanup = TRUE;
                      AffectedConnection.CurrentTimeout =
                         CurrentPDU.Parameter3;
             Start-Timer(Connection-Cleanup-Handler,
                           AffectedConnection, 0);
           } else if (CurrentPDU.AsyncEvent == SessionDropped)) {
             for (each Connection) {
                 Connection.State = CLEANUP_WAIT;
                 Connection.CurrentTimeout = CurrentPDU.Parameter3;
                 Start-Timer(Connection-Cleanup-Handler,
                           Connection, CurrentPDU.Parameter2);
             }
             Session.state = FAILED;
           }
       } else if (CurrentPDU.type == LogoutResponse) {
           Retrieve the CleanupConnection for CurrentPDU.CID.
           if (CurrentPDU.Response = failure) {
              CleanupConnection.State = CLEANUP_WAIT;

Chadalapaka, et al. Standards Track [Page 284] RFC 7143 iSCSI (Consolidated) April 2014

           } else {
               CleanupConnection.State = FREE;
           }
       } else if (CurrentPDU.type == LoginResponse) {
            if (this is a response to an implicit Logout) {
               Retrieve the CleanupConnection.
               if (successful) {
                   CleanupConnection.State = FREE;
                   Connection.State = LOGGED_IN;
               } else {
                    CleanupConnection.State = CLEANUP_WAIT;
                    DestroyTransportConnection(Connection);
               }
            }
       } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
                 * NOT SHOWN */
       }
       if (CleanupConnection.State == FREE) {
          for (each command that was active on CleanupConnection) {
          /* Establish new connection allegiance */
               NewConnection = Pick-A-Logged-In-Connection(Session);
               Build-And-Send-Command(NewConnection, TCB);
           }
       }
   }
   Connection-Cleanup-Handler(Connection)
   {
       Retrieve Session from Connection.
       if (Connection can still exchange iSCSI PDUs) {
           NewConnection = Connection;
       } else {
           Start-Timer(Connection-Resource-Timeout-Handler,
                 Connection, Connection.CurrentTimeout);
           if (there are other logged-in connections) {
                NewConnection = Pick-A-Logged-In-Connection(Session);
           } else {
                NewConnection =
                   CreateTransportConnection(Session.OtherEndInfo);
                Initiate an implicit Logout on NewConnection for
                   Connection.CID.
                return;
           }
       }
       Build-And-Send-Logout(NewConnection, Connection.CID,
                                           RecoveryRemove);
   }

Chadalapaka, et al. Standards Track [Page 285] RFC 7143 iSCSI (Consolidated) April 2014

   Transport_Exception_Handler(Connection)
   {
       Connection.PerformConnectionCleanup = TRUE;
       if (the event is an unexpected transport disconnect) {
           Connection.State = CLEANUP_WAIT;
           Connection.CurrentTimeout = DefaultTime2Retain;
           Start-Timer(Connection-Cleanup-Handler, Connection,
                          DefaultTime2Wait);
       } else {
           Connection.State = FREE;
       }
   }

D.4.3. Target Algorithms

   Receive-an-In-PDU(Connection, CurrentPDU)
   {
       check-basic-validity(CurrentPDU);
       if (Header-Digest-Bad) discard, return;
       else if (Data-Digest-Bad) {
                 Build-And-Send-Reject(Connection, CurrentPDU,
                                          Payload-Digest-Error);
                 discard, return;
       }
       Retrieve TCB and Session.
       if (CurrentPDU.type == Logout) {
          if (CurrentPDU.ReasonCode = RecoveryRemove) {
              Retrieve the CleanupConnection from CurrentPDU.CID).
              for (each command active on CleanupConnection) {
                   Quiesce-And-Prepare-for-New-Allegiance(Session,
                      TCB);
                   TCB.CurrentlyAllegiant = FALSE;
              }
              Cleanup-Connection-State(CleanupConnection);
              if ((quiescing successful) and (cleanup successful))
   {
                   Build-And-Send-Logout-Response(Connection,
                                     CleanupConnection.CID, Success);
              } else {
                   Build-And-Send-Logout-Response(Connection,
                                     CleanupConnection.CID, Failure);
              }
           }

Chadalapaka, et al. Standards Track [Page 286] RFC 7143 iSCSI (Consolidated) April 2014

       } else if ((CurrentPDU.type == Login) and
                            operational ErrorRecoveryLevel == 2) {
               Retrieve the CleanupConnection from CurrentPDU.CID).
               for (each command active on CleanupConnection) {
                     Quiesce-And-Prepare-for-New-Allegiance(Session,
                        TCB);
                     TCB.CurrentlyAllegiant = FALSE;
               }
               Cleanup-Connection-State(CleanupConnection);
               if ((quiescing successful) and (cleanup successful))
   {
                     Continue with the rest of the login processing;
               } else {
                     Build-And-Send-Login-Response(Connection,
                                CleanupConnection.CID, Target Error);
               }
           }
       } else if (CurrentPDU.type == TaskManagement) {
             if (CurrentPDU.function == "TaskReassign") {
                   if (Session.ErrorRecoveryLevel < 2) {
                       Build-And-Send-TaskMgmt-Response(Connection,
                          CurrentPDU,
                             "Task allegiance reassignment not
                                                 supported");
                   } else if (task is not found) {
                       Build-And-Send-TaskMgmt-Response(Connection,
                          CurrentPDU, "Task not in task set");
                   } else if (task is currently allegiant) {
                       Build-And-Send-TaskMgmt-Response(Connection,
                          CurrentPDU, "Task still allegiant");
                   } else {
                       Establish-New-Allegiance(TCB, Connection);
                       TCB.CurrentlyAllegiant = TRUE;
                       Schedule-Command-To-Continue(TCB);
                   }
             }
       } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
                 * NOT SHOWN */
       }
   }

Chadalapaka, et al. Standards Track [Page 287] RFC 7143 iSCSI (Consolidated) April 2014

   Transport_Exception_Handler(Connection)
   {
       Connection.PerformConnectionCleanup = TRUE;
       if (the event is an unexpected transport disconnect) {
           Connection.State = CLEANUP_WAIT;
            Start-Timer(Connection-Resource-Timeout-Handler,
               Connection, (DefaultTime2Wait+DefaultTime2Retain));
             if (this Session has Full Feature Phase connections
                   left) {
                 DifferentConnection =
                    Pick-A-Logged-In-Connection(Session);
                  Build-And-Send-Async(DifferentConnection,
                        DroppedConnection, DefaultTime2Wait,
                          DefaultTime2Retain);
           }
       } else {
             Connection.State = FREE;
       }
   }

Appendix E. Clearing Effects of Various Events on Targets

E.1. Clearing Effects on iSCSI Objects

 The following tables describe the target behavior on receiving the
 events specified in the rows of the table.  The second table is an
 extension of the first table and defines clearing actions for more
 objects on the same events.  The legend is:
  Y = Yes (cleared/discarded/reset on the event specified in the row).
      Unless otherwise noted, the clearing action is only applicable
      for the issuing initiator port.
  N = No (not affected on the event specified in the row, i.e., stays
      at previous value).
 NA = Not Applicable or Not Defined.

Chadalapaka, et al. Standards Track [Page 288] RFC 7143 iSCSI (Consolidated) April 2014

                          +------+------+------+------+------+
                          |IT (1)|IC (2)|CT (5)|ST (6)|PP (7)|
   +----------------------+------+------+------+------+------+
   |connection failure (8)|Y     |Y     |N     |N     |Y     |
   +----------------------+------+------+------+------+------+
   |connection state      |NA    |NA    |Y     |N     |NA    |
   |timeout (9)           |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |session timeout/      |Y     |Y     |Y     |Y     |Y (14)|
   |closure/reinstatement |      |      |      |      |      |
   |(10)                  |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |session continuation  |NA    |NA    |N (11)|N     |NA    |
   |(12)                  |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |successful connection |Y     |Y     |Y     |N     |Y (13)|
   |close logout          |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |session failure (18)  |Y     |Y     |N     |N     |Y     |
   +----------------------+------+------+------+------+------+
   |successful recovery   |Y     |Y     |N     |N     |Y (13)|
   |Logout                |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |failed Logout         |Y     |Y     |N     |N     |Y     |
   +----------------------+------+------+------+------+------+
   |connection Login      |NA    |NA    |NA    |Y (15)|NA    |
   |(leading)             |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |connection Login      |NA    |NA    |N (11)|N     |Y     |
   |(non-leading)         |      |      |      |      |      |
   +----------------------+------+------+------+------+------+
   |TARGET COLD RESET (16)|Y (20)|Y     |Y     |Y     |Y     |
   +----------------------+------+------+------+------+------+
   |TARGET WARM RESET (16)|Y (20)|Y     |Y     |Y     |Y     |
   +----------------------+------+------+------+------+------+
   |LU reset (19)         |Y (20)|Y     |Y     |Y     |Y     |
   +----------------------+------+------+------+------+------+
   |power cycle (16)      |Y     |Y     |Y     |Y     |Y     |
   +----------------------+------+------+------+------+------+
   (1)  Incomplete TTTs (IT) are Target Transfer Tags on which the
        target is still expecting PDUs to be received.  Examples
        include TTTs received via R2T, NOP-In, etc.
   (2)  Immediate Commands (IC) are immediate commands, but waiting
        for execution on a target (for example, ABORT TASK SET).

Chadalapaka, et al. Standards Track [Page 289] RFC 7143 iSCSI (Consolidated) April 2014

   (5)  Connection Tasks (CT) are tasks that are active on the iSCSI
        connection in question.
   (6)  Session Tasks (ST) are tasks that are active on the entire
        iSCSI session.  A union of "connection tasks" on all
        participating connections.
   (7)  Partial PDUs (PP) (if any) are PDUs that are partially sent
        and waiting for transport window credit to complete the
        transmission.
   (8)  Connection failure is a connection exception condition - one
        of the transport connections shut down, transport connections
        reset, or transport connections timed out, which abruptly
        terminated the iSCSI Full Feature Phase connection.  A
        connection failure always takes the connection state machine
        to the CLEANUP_WAIT state.
   (9)  Connection state timeout happens if a connection spends more
        time than agreed upon during login negotiation in the
        CLEANUP_WAIT state, and this takes the connection to the FREE
        state (M1 transition in connection cleanup state diagram; see
        Section 8.2).
   (10) Session timeout, closure, and reinstatement are defined in
        Section 6.3.5.
   (11) This clearing effect is "Y" only if it is a connection
        reinstatement and the operational ErrorRecoveryLevel is less
        than 2.
   (12) Session continuation is defined in Section 6.3.6.
   (13) This clearing effect is only valid if the connection is being
        logged out on a different connection and when the connection
        being logged out on the target may have some partial PDUs
        pending to be sent.  In all other cases, the effect is "NA".
   (14) This clearing effect is only valid for a "close the session"
        logout in a multi-connection session.  In all other cases, the
        effect is "NA".
   (15) Only applicable if this leading connection login is a session
        reinstatement.  If this is not the case, it is "NA".
   (16) This operation affects all logged-in initiators.
   (18) Session failure is defined in Section 6.3.6.

Chadalapaka, et al. Standards Track [Page 290] RFC 7143 iSCSI (Consolidated) April 2014

   (19) This operation affects all logged-in initiators, and the
        clearing effects are only applicable to the LU being reset.
   (20) With standard multi-task abort semantics (Section 4.2.3.3), a
        TARGET WARM RESET or a TARGET COLD RESET or a LU reset would
        clear the active TTTs upon completion.  However, the FastAbort
        multi-task abort semantics defined by Section 4.2.3.4 do not
        guarantee that the active TTTs are cleared by the end of the
        reset operations.  In fact, the FastAbort semantics are
        designed to allow clearing the TTTs in a "lazy" fashion after
        the TMF Response is delivered.  Thus, when
        TaskReporting=FastAbort (Section 13.23) is operational on a
        session, the clearing effects of reset operations on
        "Incomplete TTTs" is "N".

Chadalapaka, et al. Standards Track [Page 291] RFC 7143 iSCSI (Consolidated) April 2014

                         +------+-------+------+------+-------+
                         |DC (1)|DD (2) |SS (3)|CS (4)|DS (5) |
   +---------------------+------+-------+------+------+-------+
   |connection failure   |N     |Y      |N     |N     |N      |
   +---------------------+------+-------+------+------+-------+
   |connection state     |Y     |NA     |Y     |N     |NA     |
   |timeout              |      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |session timeout/     |Y     |Y      |Y (7) |Y     |NA     |
   |closure/reinstatement|      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |session continuation |N (11)|NA (12)|NA    |N     |NA (13)|
   +---------------------+------+-------+------+------+-------+
   |successful connection|Y     |Y      |Y     |N     |NA     |
   |close Logout         |      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |session failure      |N     |Y      |N     |N     |N      |
   +---------------------+------+-------+------+------+-------+
   |successful recovery  |Y     |Y      |Y     |N     |N      |
   |Logout               |      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |failed Logout        |N     |Y (9)  |N     |N     |N      |
   +---------------------+------+-------+------+------+-------+
   |connection Login     |NA    |NA     |N (8) |N (8) |NA     |
   |(leading             |      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |connection Login     |N (11)|NA (12)|N (8) |N     |NA (13)|
   |(non-leading)        |      |       |      |      |       |
   +---------------------+------+-------+------+------+-------+
   |TARGET COLD RESET    |Y     |Y      |Y     |Y (10)|NA     |
   +---------------------+------+-------+------+------+-------+
   |TARGET WARM RESET    |Y     |Y      |N     |N     |NA     |
   +---------------------+------+-------+------+------+-------+
   |LU reset             |N     |Y      |N     |N     |N      |
   +---------------------+------+-------+------+------+-------+
   |power cycle          |Y     |Y      |Y     |Y (10)|NA     |
   +---------------------+------+-------+------+------+-------+
   (1)  Discontiguous Commands (DC) are commands allegiant to the
        connection in question and waiting to be reordered in the
        iSCSI layer.  All "Y"s in this column assume that the task
        causing the event (if indeed the event is the result of a
        task) is issued as an immediate command, because the
        discontiguities can be ahead of the task.
   (2)  Discontiguous Data (DD) are data PDUs received for the task in
        question and waiting to be reordered due to prior
        discontiguities in the DataSN.

Chadalapaka, et al. Standards Track [Page 292] RFC 7143 iSCSI (Consolidated) April 2014

   (3)  "SS" refers to the StatSN.
   (4)  "CS" refers to the CmdSN.
   (5)  "DS" refers to the DataSN.
   (7)  This action clears the StatSN on all the connections.
   (8)  This sequence number is instantiated on this event.
   (9)  A logout failure drives the connection state machine to the
        CLEANUP_WAIT state, similar to the connection failure event.
        Hence, it has a similar effect on this and several other
        protocol aspects.
   (10) This is cleared by virtue of the fact that all sessions with
        all initiators are terminated.
   (11) This clearing effect is "Y" if it is a connection
        reinstatement.
   (12) This clearing effect is "Y" only if it is a connection
        reinstatement and the operational ErrorRecoveryLevel is 2.
   (13) This clearing effect is "N" only if it is a connection
        reinstatement and the operational ErrorRecoveryLevel is 2.

E.2. Clearing Effects on SCSI Objects

 The only iSCSI protocol action that can effect clearing actions on
 SCSI objects is the "I_T nexus loss" notification (Section 6.3.5.1
 ("Loss of Nexus Notification")).  [SPC3] describes the clearing
 effects of this notification on a variety of SCSI attributes.  In
 addition, SCSI standards documents (such as [SAM2] and [SBC2]) define
 additional clearing actions that may take place for several SCSI
 objects on SCSI events such as LU resets and power-on resets.
 Since iSCSI defines a TARGET COLD RESET as a "protocol-equivalent" to
 a target power-cycle, the iSCSI TARGET COLD RESET must also be
 considered as the power-on reset event in interpreting the actions
 defined in the SCSI standards.
 When the iSCSI session is reconstructed (between the same SCSI ports
 with the same nexus identifier) reestablishing the same I_T nexus,
 all SCSI objects that are defined to not clear on the "I_T nexus
 loss" notification event, such as persistent reservations, are
 automatically associated to this new session.

Chadalapaka, et al. Standards Track [Page 293] RFC 7143 iSCSI (Consolidated) April 2014

Acknowledgments

 Several individuals on the original IPS Working Group made
 significant contributions to the original RFCs 3720, 3980, 4850,
 and 5048.
 Specifically, the authors of the original RFCs -- which herein are
 consolidated into a single document -- were the following:
    RFC 3720: Julian Satran, Kalman Meth, Costa Sapuntzakis,
    Mallikarjun Chadalapaka, Efri Zeidner
    RFC 3980: Marjorie Krueger, Mallikarjun Chadalapaka, Rob Elliott
    RFC 4850: David Wysochanski
    RFC 5048: Mallikarjun Chadalapaka
 Many thanks to Fred Knight for contributing to the UML notations and
 drawings in this document.
 We would in addition like to acknowledge the following individuals
 who contributed to this revised document: David Harrington, Paul
 Koning, Mark Edwards, Rob Elliott, and Martin Stiemerling.
 Thanks to Yi Zeng and Nico Williams for suggesting and/or reviewing
 Kerberos-related security considerations text.
 The authors gratefully acknowledge the valuable feedback during the
 Last Call review process from a number of individuals; their feedback
 significantly improved this document.  The individuals were Stephen
 Farrell, Brian Haberman, Barry Leiba, Pete Resnick, Sean Turner,
 Alexey Melnikov, Kathleen Moriarty, Fred Knight, Mike Christie, Qiang
 Wang, Shiv Rajpal, and Andy Banta.
 Finally, this document also benefited from significant review
 contributions from the Storm Working Group at large.
 Comments may be sent to Mallikarjun Chadalapaka.

Chadalapaka, et al. Standards Track [Page 294] RFC 7143 iSCSI (Consolidated) April 2014

Authors' Addresses

 Mallikarjun Chadalapaka
 Microsoft
 One Microsoft Way
 Redmond, WA  98052
 USA
 EMail: cbm@chadalapaka.com
 Julian Satran
 Infinidat Ltd.
 EMail: julians@infinidat.com, julian@satran.net
 Kalman Meth
 IBM Haifa Research Lab
 Haifa University Campus - Mount Carmel
 Haifa 31905, Israel
 Phone +972.4.829.6341
 EMail: meth@il.ibm.com
 David L. Black
 EMC Corporation
 176 South St.
 Hopkinton, MA  01748
 USA
 Phone +1 (508) 293-7953
 EMail: david.black@emc.com

Chadalapaka, et al. Standards Track [Page 295]

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