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rfc:rfc4960

Network Working Group R. Stewart, Ed. Request for Comments: 4960 September 2007 Obsoletes: 2960, 3309 Category: Standards Track

                Stream Control Transmission Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document obsoletes RFC 2960 and RFC 3309.  It describes the
 Stream Control Transmission Protocol (SCTP).  SCTP is designed to
 transport Public Switched Telephone Network (PSTN) signaling messages
 over IP networks, but is capable of broader applications.
 SCTP is a reliable transport protocol operating on top of a
 connectionless packet network such as IP.  It offers the following
 services to its users:
  1. - acknowledged error-free non-duplicated transfer of user data,
  1. - data fragmentation to conform to discovered path MTU size,
  1. - sequenced delivery of user messages within multiple streams, with

an option for order-of-arrival delivery of individual user

     messages,
  1. - optional bundling of multiple user messages into a single SCTP

packet, and

  1. - network-level fault tolerance through supporting of multi-homing

at either or both ends of an association.

 The design of SCTP includes appropriate congestion avoidance behavior
 and resistance to flooding and masquerade attacks.

Stewart Standards Track [Page 1] RFC 4960 Stream Control Transmission Protocol September 2007

Table of Contents

 1. Introduction ....................................................5
    1.1. Motivation .................................................5
    1.2. Architectural View of SCTP .................................6
    1.3. Key Terms ..................................................6
    1.4. Abbreviations .............................................10
    1.5. Functional View of SCTP ...................................10
         1.5.1. Association Startup and Takedown ...................11
         1.5.2. Sequenced Delivery within Streams ..................12
         1.5.3. User Data Fragmentation ............................12
         1.5.4. Acknowledgement and Congestion Avoidance ...........12
         1.5.5. Chunk Bundling .....................................13
         1.5.6. Packet Validation ..................................13
         1.5.7. Path Management ....................................13
    1.6. Serial Number Arithmetic ..................................14
    1.7. Changes from RFC 2960 .....................................15
 2. Conventions ....................................................15
 3. SCTP Packet Format .............................................15
    3.1. SCTP Common Header Field Descriptions .....................16
    3.2. Chunk Field Descriptions ..................................17
         3.2.1. Optional/Variable-Length Parameter Format ..........19
         3.2.2. Reporting of Unrecognized Parameters ...............21
    3.3. SCTP Chunk Definitions ....................................21
         3.3.1. Payload Data (DATA) (0) ............................22
         3.3.2. Initiation (INIT) (1) ..............................24
                3.3.2.1. Optional/Variable-Length
                         Parameters in INIT ........................27
         3.3.3. Initiation Acknowledgement (INIT ACK) (2) ..........30
                3.3.3.1. Optional or Variable-Length Parameters ....33
         3.3.4. Selective Acknowledgement (SACK) (3) ...............34
         3.3.5. Heartbeat Request (HEARTBEAT) (4) ..................38
         3.3.6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5) ......39
         3.3.7. Abort Association (ABORT) (6) ......................40
         3.3.8. Shutdown Association (SHUTDOWN) (7) ................41
         3.3.9. Shutdown Acknowledgement (SHUTDOWN ACK) (8) ........41
         3.3.10. Operation Error (ERROR) (9) .......................42
                3.3.10.1. Invalid Stream Identifier (1) ............44
                3.3.10.2. Missing Mandatory Parameter (2) ..........44
                3.3.10.3. Stale Cookie Error (3) ...................45
                3.3.10.4. Out of Resource (4) ......................45
                3.3.10.5. Unresolvable Address (5) .................46
                3.3.10.6. Unrecognized Chunk Type (6) ..............46
                3.3.10.7. Invalid Mandatory Parameter (7) ..........47
                3.3.10.8. Unrecognized Parameters (8) ..............47
                3.3.10.9. No User Data (9) .........................48
                3.3.10.10. Cookie Received While Shutting
                           Down (10) ...............................48

Stewart Standards Track [Page 2] RFC 4960 Stream Control Transmission Protocol September 2007

                3.3.10.11. Restart of an Association with
                           New Addresses (11) ......................49
                3.3.10.12. User-Initiated Abort (12) ...............49
                3.3.10.13. Protocol Violation (13) .................50
         3.3.11. Cookie Echo (COOKIE ECHO) (10) ....................50
         3.3.12. Cookie Acknowledgement (COOKIE ACK) (11) ..........51
         3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14) ........51
 4. SCTP Association State Diagram .................................52
 5. Association Initialization .....................................56
    5.1. Normal Establishment of an Association ....................56
         5.1.1. Handle Stream Parameters ...........................58
         5.1.2. Handle Address Parameters ..........................58
         5.1.3. Generating State Cookie ............................61
         5.1.4. State Cookie Processing ............................62
         5.1.5. State Cookie Authentication ........................62
         5.1.6. An Example of Normal Association Establishment .....64
    5.2. Handle Duplicate or Unexpected INIT, INIT ACK,
         COOKIE ECHO, and ..........................................65
         5.2.1. INIT Received in COOKIE-WAIT or
                COOKIE-ECHOED State (Item B) .......................66
         5.2.2. Unexpected INIT in States Other than
                CLOSED, COOKIE-ECHOED, .............................66
         5.2.3. Unexpected INIT ACK ................................67
         5.2.4. Handle a COOKIE ECHO when a TCB Exists .............67
                5.2.4.1. An Example of a Association Restart .......69
         5.2.5. Handle Duplicate COOKIE-ACK. .......................71
         5.2.6. Handle Stale COOKIE Error ..........................71
    5.3. Other Initialization Issues ...............................72
         5.3.1. Selection of Tag Value .............................72
    5.4. Path Verification .........................................72
 6. User Data Transfer .............................................73
    6.1. Transmission of DATA Chunks ...............................75
    6.2. Acknowledgement on Reception of DATA Chunks ...............78
         6.2.1. Processing a Received SACK .........................81
    6.3. Management of Retransmission Timer ........................83
         6.3.1. RTO Calculation ....................................83
         6.3.2. Retransmission Timer Rules .........................85
         6.3.3. Handle T3-rtx Expiration ...........................86
    6.4. Multi-Homed SCTP Endpoints ................................87
         6.4.1. Failover from an Inactive Destination Address ......88
    6.5. Stream Identifier and Stream Sequence Number ..............88
    6.6. Ordered and Unordered Delivery ............................88
    6.7. Report Gaps in Received DATA TSNs .........................89
    6.8. CRC32c Checksum Calculation ...............................90
    6.9. Fragmentation and Reassembly ..............................91
    6.10. Bundling .................................................92
 7. Congestion Control .............................................93
    7.1. SCTP Differences from TCP Congestion Control ..............94

Stewart Standards Track [Page 3] RFC 4960 Stream Control Transmission Protocol September 2007

    7.2. SCTP Slow-Start and Congestion Avoidance ..................95
         7.2.1. Slow-Start .........................................96
         7.2.2. Congestion Avoidance ...............................97
         7.2.3. Congestion Control .................................98
         7.2.4. Fast Retransmit on Gap Reports .....................98
    7.3. Path MTU Discovery .......................................100
 8. Fault Management ..............................................100
    8.1. Endpoint Failure Detection ...............................100
    8.2. Path Failure Detection ...................................101
    8.3. Path Heartbeat ...........................................102
    8.4. Handle "Out of the Blue" Packets .........................104
    8.5. Verification Tag .........................................105
         8.5.1. Exceptions in Verification Tag Rules ..............105
 9. Termination of Association ....................................106
    9.1. Abort of an Association ..................................107
    9.2. Shutdown of an Association ...............................107
 10. Interface with Upper Layer ...................................110
    10.1. ULP-to-SCTP .............................................110
    10.2. SCTP-to-ULP .............................................120
 11. Security Considerations ......................................123
    11.1. Security Objectives .....................................123
    11.2. SCTP Responses to Potential Threats .....................124
         11.2.1. Countering Insider Attacks .......................124
         11.2.2. Protecting against Data Corruption in the
                 Network ..........................................124
         11.2.3. Protecting Confidentiality .......................124
         11.2.4. Protecting against Blind
                 Denial-of-Service Attacks ........................125
                11.2.4.1. Flooding ................................125
                11.2.4.2. Blind Masquerade ........................126
                11.2.4.3. Improper Monopolization of Services .....127
    11.3. SCTP Interactions with Firewalls ........................127
    11.4. Protection of Non-SCTP-Capable Hosts ....................128
 12. Network Management Considerations ............................128
 13. Recommended Transmission Control Block (TCB) Parameters ......129
    13.1. Parameters Necessary for the SCTP Instance ..............129
    13.2. Parameters Necessary per Association (i.e., the TCB) ....129
    13.3. Per Transport Address Data ..............................131
    13.4. General Parameters Needed ...............................132
 14. IANA Considerations ..........................................132
    14.1. IETF-defined Chunk Extension ............................132
    14.2. IETF-Defined Chunk Parameter Extension ..................133
    14.3. IETF-Defined Additional Error Causes ....................133
    14.4. Payload Protocol Identifiers ............................134
    14.5. Port Numbers Registry ...................................134
 15. Suggested SCTP Protocol Parameter Values .....................136
 16. Acknowledgements .............................................137
 Appendix A. Explicit Congestion Notification .....................139

Stewart Standards Track [Page 4] RFC 4960 Stream Control Transmission Protocol September 2007

 Appendix B. CRC32c Checksum Calculation ..........................140
 Appendix C. ICMP Handling ........................................142
 References .......................................................149
    Normative References ..........................................149
    Informative References ........................................150

1. Introduction

 This section explains the reasoning behind the development of the
 Stream Control Transmission Protocol (SCTP), the services it offers,
 and the basic concepts needed to understand the detailed description
 of the protocol.
 This document obsoletes [RFC2960] and [RFC3309].

1.1. Motivation

 TCP [RFC0793] has performed immense service as the primary means of
 reliable data transfer in IP networks.  However, an increasing number
 of recent applications have found TCP too limiting, and have
 incorporated their own reliable data transfer protocol on top of UDP
 [RFC0768].  The limitations that users have wished to bypass include
 the following:
  1. - TCP provides both reliable data transfer and strict order-of-

transmission delivery of data. Some applications need reliable

    transfer without sequence maintenance, while others would be
    satisfied with partial ordering of the data.  In both of these
    cases, the head-of-line blocking offered by TCP causes unnecessary
    delay.
  1. - The stream-oriented nature of TCP is often an inconvenience.

Applications must add their own record marking to delineate their

    messages, and must make explicit use of the push facility to
    ensure that a complete message is transferred in a reasonable
    time.
  1. - The limited scope of TCP sockets complicates the task of providing

highly-available data transfer capability using multi-homed hosts.

  1. - TCP is relatively vulnerable to denial-of-service attacks, such as

SYN attacks.

 Transport of PSTN signaling across the IP network is an application
 for which all of these limitations of TCP are relevant.  While this
 application directly motivated the development of SCTP, other
 applications may find SCTP a good match to their requirements.

Stewart Standards Track [Page 5] RFC 4960 Stream Control Transmission Protocol September 2007

1.2. Architectural View of SCTP

 SCTP is viewed as a layer between the SCTP user application ("SCTP
 user" for short) and a connectionless packet network service such as
 IP.  The remainder of this document assumes SCTP runs on top of IP.
 The basic service offered by SCTP is the reliable transfer of user
 messages between peer SCTP users.  It performs this service within
 the context of an association between two SCTP endpoints.  Section 10
 of this document sketches the API that should exist at the boundary
 between the SCTP and the SCTP user layers.
 SCTP is connection-oriented in nature, but the SCTP association is a
 broader concept than the TCP connection.  SCTP provides the means for
 each SCTP endpoint (Section 1.3) to provide the other endpoint
 (during association startup) with a list of transport addresses
 (i.e., multiple IP addresses in combination with an SCTP port)
 through which that endpoint can be reached and from which it will
 originate SCTP packets.  The association spans transfers over all of
 the possible source/destination combinations that may be generated
 from each endpoint's lists.
     _____________                                      _____________
    |  SCTP User  |                                    |  SCTP User  |
    | Application |                                    | Application |
    |-------------|                                    |-------------|
    |    SCTP     |                                    |    SCTP     |
    |  Transport  |                                    |  Transport  |
    |   Service   |                                    |   Service   |
    |-------------|                                    |-------------|
    |             |One or more    ----      One or more|             |
    | IP Network  |IP address      \/        IP address| IP Network  |
    |   Service   |appearances     /\       appearances|   Service   |
    |_____________|               ----                 |_____________|
      SCTP Node A |<-------- Network transport ------->| SCTP Node B
                       Figure 1: An SCTP Association

1.3. Key Terms

 Some of the language used to describe SCTP has been introduced in the
 previous sections.  This section provides a consolidated list of the
 key terms and their definitions.
 o  Active destination transport address: A transport address on a
    peer endpoint that a transmitting endpoint considers available for
    receiving user messages.

Stewart Standards Track [Page 6] RFC 4960 Stream Control Transmission Protocol September 2007

 o  Bundling: An optional multiplexing operation, whereby more than
    one user message may be carried in the same SCTP packet.  Each
    user message occupies its own DATA chunk.
 o  Chunk: A unit of information within an SCTP packet, consisting of
    a chunk header and chunk-specific content.
 o  Congestion window (cwnd): An SCTP variable that limits the data,
    in number of bytes, a sender can send to a particular destination
    transport address before receiving an acknowledgement.
 o  Cumulative TSN Ack Point: The TSN of the last DATA chunk
    acknowledged via the Cumulative TSN Ack field of a SACK.
 o  Idle destination address: An address that has not had user
    messages sent to it within some length of time, normally the
    HEARTBEAT interval or greater.
 o  Inactive destination transport address: An address that is
    considered inactive due to errors and unavailable to transport
    user messages.
 o  Message = user message: Data submitted to SCTP by the Upper Layer
    Protocol (ULP).
 o  Message Authentication Code (MAC): An integrity check mechanism
    based on cryptographic hash functions using a secret key.
    Typically, message authentication codes are used between two
    parties that share a secret key in order to validate information
    transmitted between these parties.  In SCTP, it is used by an
    endpoint to validate the State Cookie information that is returned
    from the peer in the COOKIE ECHO chunk.  The term "MAC" has
    different meanings in different contexts.  SCTP uses this term
    with the same meaning as in [RFC2104].
 o  Network Byte Order: Most significant byte first, a.k.a., big
    endian.
 o  Ordered Message: A user message that is delivered in order with
    respect to all previous user messages sent within the stream on
    which the message was sent.
 o  Outstanding TSN (at an SCTP endpoint): A TSN (and the associated
    DATA chunk) that has been sent by the endpoint but for which it
    has not yet received an acknowledgement.

Stewart Standards Track [Page 7] RFC 4960 Stream Control Transmission Protocol September 2007

 o  Path: The route taken by the SCTP packets sent by one SCTP
    endpoint to a specific destination transport address of its peer
    SCTP endpoint.  Sending to different destination transport
    addresses does not necessarily guarantee getting separate paths.
 o  Primary Path: The primary path is the destination and source
    address that will be put into a packet outbound to the peer
    endpoint by default.  The definition includes the source address
    since an implementation MAY wish to specify both destination and
    source address to better control the return path taken by reply
    chunks and on which interface the packet is transmitted when the
    data sender is multi-homed.
 o  Receiver Window (rwnd): An SCTP variable a data sender uses to
    store the most recently calculated receiver window of its peer, in
    number of bytes.  This gives the sender an indication of the space
    available in the receiver's inbound buffer.
 o  SCTP association: A protocol relationship between SCTP endpoints,
    composed of the two SCTP endpoints and protocol state information
    including Verification Tags and the currently active set of
    Transmission Sequence Numbers (TSNs), etc.  An association can be
    uniquely identified by the transport addresses used by the
    endpoints in the association.  Two SCTP endpoints MUST NOT have
    more than one SCTP association between them at any given time.
 o  SCTP endpoint: The logical sender/receiver of SCTP packets.  On a
    multi-homed host, an SCTP endpoint is represented to its peers as
    a combination of a set of eligible destination transport addresses
    to which SCTP packets can be sent and a set of eligible source
    transport addresses from which SCTP packets can be received.  All
    transport addresses used by an SCTP endpoint must use the same
    port number, but can use multiple IP addresses.  A transport
    address used by an SCTP endpoint must not be used by another SCTP
    endpoint.  In other words, a transport address is unique to an
    SCTP endpoint.
 o  SCTP packet (or packet): The unit of data delivery across the
    interface between SCTP and the connectionless packet network
    (e.g., IP).  An SCTP packet includes the common SCTP header,
    possible SCTP control chunks, and user data encapsulated within
    SCTP DATA chunks.
 o  SCTP user application (SCTP user): The logical higher-layer
    application entity which uses the services of SCTP, also called
    the Upper-Layer Protocol (ULP).

Stewart Standards Track [Page 8] RFC 4960 Stream Control Transmission Protocol September 2007

 o  Slow-Start Threshold (ssthresh): An SCTP variable.  This is the
    threshold that the endpoint will use to determine whether to
    perform slow start or congestion avoidance on a particular
    destination transport address.  Ssthresh is in number of bytes.
 o  Stream: A unidirectional logical channel established from one to
    another associated SCTP endpoint, within which all user messages
    are delivered in sequence except for those submitted to the
    unordered delivery service.
 Note: The relationship between stream numbers in opposite directions
 is strictly a matter of how the applications use them.  It is the
 responsibility of the SCTP user to create and manage these
 correlations if they are so desired.
 o  Stream Sequence Number: A 16-bit sequence number used internally
    by SCTP to ensure sequenced delivery of the user messages within a
    given stream.  One Stream Sequence Number is attached to each user
    message.
 o  Tie-Tags: Two 32-bit random numbers that together make a 64-bit
    nonce.  These tags are used within a State Cookie and TCB so that
    a newly restarting association can be linked to the original
    association within the endpoint that did not restart and yet not
    reveal the true Verification Tags of an existing association.
 o  Transmission Control Block (TCB): An internal data structure
    created by an SCTP endpoint for each of its existing SCTP
    associations to other SCTP endpoints.  TCB contains all the status
    and operational information for the endpoint to maintain and
    manage the corresponding association.
 o  Transmission Sequence Number (TSN): A 32-bit sequence number used
    internally by SCTP.  One TSN is attached to each chunk containing
    user data to permit the receiving SCTP endpoint to acknowledge its
    receipt and detect duplicate deliveries.
 o  Transport address: A transport address is traditionally defined by
    a network-layer address, a transport-layer protocol, and a
    transport-layer port number.  In the case of SCTP running over IP,
    a transport address is defined by the combination of an IP address
    and an SCTP port number (where SCTP is the transport protocol).
 o  Unacknowledged TSN (at an SCTP endpoint): A TSN (and the
    associated DATA chunk) that has been received by the endpoint but
    for which an acknowledgement has not yet been sent.  Or in the
    opposite case, for a packet that has been sent but no
    acknowledgement has been received.

Stewart Standards Track [Page 9] RFC 4960 Stream Control Transmission Protocol September 2007

 o  Unordered Message: Unordered messages are "unordered" with respect
    to any other message; this includes both other unordered messages
    as well as other ordered messages.  An unordered message might be
    delivered prior to or later than ordered messages sent on the same
    stream.
 o  User message: The unit of data delivery across the interface
    between SCTP and its user.
 o  Verification Tag: A 32-bit unsigned integer that is randomly
    generated.  The Verification Tag provides a key that allows a
    receiver to verify that the SCTP packet belongs to the current
    association and is not an old or stale packet from a previous
    association.

1.4. Abbreviations

 MAC    -  Message Authentication Code [RFC2104]
 RTO    -  Retransmission Timeout
 RTT    -  Round-Trip Time
 RTTVAR -  Round-Trip Time Variation
 SCTP   -  Stream Control Transmission Protocol
 SRTT   -  Smoothed RTT
 TCB    -  Transmission Control Block
 TLV    -  Type-Length-Value coding format
 TSN    -  Transmission Sequence Number
 ULP    -  Upper-Layer Protocol

1.5. Functional View of SCTP

 The SCTP transport service can be decomposed into a number of
 functions.  These are depicted in Figure 2 and explained in the
 remainder of this section.

Stewart Standards Track [Page 10] RFC 4960 Stream Control Transmission Protocol September 2007

                         SCTP User Application
  1. —————————————————-

_

          |             |                | Sequenced Delivery |
          | Association |                |   within Streams   |
          |             |                |____________________|
          |   Startup   |
          |             |         ____________________________
          |     and     |        |    User Data Fragmentation |
          |             |        |____________________________|
          |   Takedown  |
          |             |         ____________________________
          |             |        |     Acknowledgement        |
          |             |        |          and               |
          |             |        |    Congestion Avoidance    |
          |             |        |____________________________|
          |             |
          |             |         ____________________________
          |             |        |       Chunk Bundling       |
          |             |        |____________________________|
          |             |
          |             |     ________________________________
          |             |    |      Packet Validation         |
          |             |    |________________________________|
          |             |
          |             |     ________________________________
          |             |    |     Path Management            |
          |_____________|    |________________________________|
            Figure 2: Functional View of the SCTP Transport Service

1.5.1. Association Startup and Takedown

 An association is initiated by a request from the SCTP user (see the
 description of the ASSOCIATE (or SEND) primitive in Section 10).
 A cookie mechanism, similar to one described by Karn and Simpson in
 [RFC2522], is employed during the initialization to provide
 protection against synchronization attacks.  The cookie mechanism
 uses a four-way handshake, the last two legs of which are allowed to
 carry user data for fast setup.  The startup sequence is described in
 Section 5 of this document.
 SCTP provides for graceful close (i.e., shutdown) of an active
 association on request from the SCTP user.  See the description of
 the SHUTDOWN primitive in Section 10.  SCTP also allows ungraceful
 close (i.e., abort), either on request from the user (ABORT

Stewart Standards Track [Page 11] RFC 4960 Stream Control Transmission Protocol September 2007

 primitive) or as a result of an error condition detected within the
 SCTP layer.  Section 9 describes both the graceful and the ungraceful
 close procedures.
 SCTP does not support a half-open state (like TCP) wherein one side
 may continue sending data while the other end is closed.  When either
 endpoint performs a shutdown, the association on each peer will stop
 accepting new data from its user and only deliver data in queue at
 the time of the graceful close (see Section 9).

1.5.2. Sequenced Delivery within Streams

 The term "stream" is used in SCTP to refer to a sequence of user
 messages that are to be delivered to the upper-layer protocol in
 order with respect to other messages within the same stream.  This is
 in contrast to its usage in TCP, where it refers to a sequence of
 bytes (in this document, a byte is assumed to be 8 bits).
 The SCTP user can specify at association startup time the number of
 streams to be supported by the association.  This number is
 negotiated with the remote end (see Section 5.1.1).  User messages
 are associated with stream numbers (SEND, RECEIVE primitives, Section
 10).  Internally, SCTP assigns a Stream Sequence Number to each
 message passed to it by the SCTP user.  On the receiving side, SCTP
 ensures that messages are delivered to the SCTP user in sequence
 within a given stream.  However, while one stream may be blocked
 waiting for the next in-sequence user message, delivery from other
 streams may proceed.
 SCTP provides a mechanism for bypassing the sequenced delivery
 service.  User messages sent using this mechanism are delivered to
 the SCTP user as soon as they are received.

1.5.3. User Data Fragmentation

 When needed, SCTP fragments user messages to ensure that the SCTP
 packet passed to the lower layer conforms to the path MTU.  On
 receipt, fragments are reassembled into complete messages before
 being passed to the SCTP user.

1.5.4. Acknowledgement and Congestion Avoidance

 SCTP assigns a Transmission Sequence Number (TSN) to each user data
 fragment or unfragmented message.  The TSN is independent of any
 Stream Sequence Number assigned at the stream level.  The receiving
 end acknowledges all TSNs received, even if there are gaps in the
 sequence.  In this way, reliable delivery is kept functionally
 separate from sequenced stream delivery.

Stewart Standards Track [Page 12] RFC 4960 Stream Control Transmission Protocol September 2007

 The acknowledgement and congestion avoidance function is responsible
 for packet retransmission when timely acknowledgement has not been
 received.  Packet retransmission is conditioned by congestion
 avoidance procedures similar to those used for TCP.  See Section 6
 and Section 7 for a detailed description of the protocol procedures
 associated with this function.

1.5.5. Chunk Bundling

 As described in Section 3, the SCTP packet as delivered to the lower
 layer consists of a common header followed by one or more chunks.
 Each chunk may contain either user data or SCTP control information.
 The SCTP user has the option to request bundling of more than one
 user message into a single SCTP packet.  The chunk bundling function
 of SCTP is responsible for assembly of the complete SCTP packet and
 its disassembly at the receiving end.
 During times of congestion, an SCTP implementation MAY still perform
 bundling even if the user has requested that SCTP not bundle.  The
 user's disabling of bundling only affects SCTP implementations that
 may delay a small period of time before transmission (to attempt to
 encourage bundling).  When the user layer disables bundling, this
 small delay is prohibited but not bundling that is performed during
 congestion or retransmission.

1.5.6. Packet Validation

 A mandatory Verification Tag field and a 32-bit checksum field (see
 Appendix B for a description of the CRC32c checksum) are included in
 the SCTP common header.  The Verification Tag value is chosen by each
 end of the association during association startup.  Packets received
 without the expected Verification Tag value are discarded, as a
 protection against blind masquerade attacks and against stale SCTP
 packets from a previous association.  The CRC32c checksum should be
 set by the sender of each SCTP packet to provide additional
 protection against data corruption in the network.  The receiver of
 an SCTP packet with an invalid CRC32c checksum silently discards the
 packet.

1.5.7. Path Management

 The sending SCTP user is able to manipulate the set of transport
 addresses used as destinations for SCTP packets through the
 primitives described in Section 10.  The SCTP path management
 function chooses the destination transport address for each outgoing
 SCTP packet based on the SCTP user's instructions and the currently
 perceived reachability status of the eligible destination set.  The
 path management function monitors reachability through heartbeats

Stewart Standards Track [Page 13] RFC 4960 Stream Control Transmission Protocol September 2007

 when other packet traffic is inadequate to provide this information
 and advises the SCTP user when reachability of any far-end transport
 address changes.  The path management function is also responsible
 for reporting the eligible set of local transport addresses to the
 far end during association startup, and for reporting the transport
 addresses returned from the far end to the SCTP user.
 At association startup, a primary path is defined for each SCTP
 endpoint, and is used for normal sending of SCTP packets.
 On the receiving end, the path management is responsible for
 verifying the existence of a valid SCTP association to which the
 inbound SCTP packet belongs before passing it for further processing.
 Note: Path Management and Packet Validation are done at the same
 time, so although described separately above, in reality they cannot
 be performed as separate items.

1.6. Serial Number Arithmetic

 It is essential to remember that the actual Transmission Sequence
 Number space is finite, though very large.  This space ranges from 0
 to 2**32 - 1.  Since the space is finite, all arithmetic dealing with
 Transmission Sequence Numbers must be performed modulo 2**32.  This
 unsigned arithmetic preserves the relationship of sequence numbers as
 they cycle from 2**32 - 1 to 0 again.  There are some subtleties to
 computer modulo arithmetic, so great care should be taken in
 programming the comparison of such values.  When referring to TSNs,
 the symbol "=<" means "less than or equal"(modulo 2**32).
 Comparisons and arithmetic on TSNs in this document SHOULD use Serial
 Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.
 An endpoint SHOULD NOT transmit a DATA chunk with a TSN that is more
 than 2**31 - 1 above the beginning TSN of its current send window.
 Doing so will cause problems in comparing TSNs.
 Transmission Sequence Numbers wrap around when they reach 2**32 - 1.
 That is, the next TSN a DATA chunk MUST use after transmitting TSN =
 2*32 - 1 is TSN = 0.
 Any arithmetic done on Stream Sequence Numbers SHOULD use Serial
 Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.
 All other arithmetic and comparisons in this document use normal
 arithmetic.

Stewart Standards Track [Page 14] RFC 4960 Stream Control Transmission Protocol September 2007

1.7. Changes from RFC 2960

 SCTP was originally defined in [RFC2960], which this document
 obsoletes.  Readers interested in the details of the various changes
 that this document incorporates are asked to consult [RFC4460].

2. Conventions

 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. SCTP Packet Format

 An SCTP packet is composed of a common header and chunks.  A chunk
 contains either control information or user data.
 The SCTP packet format is shown below:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Common Header                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Chunk #1                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           ...                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Chunk #n                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Multiple chunks can be bundled into one SCTP packet up to the MTU
 size, except for the INIT, INIT ACK, and SHUTDOWN COMPLETE chunks.
 These chunks MUST NOT be bundled with any other chunk in a packet.
 See Section 6.10 for more details on chunk bundling.
 If a user data message doesn't fit into one SCTP packet it can be
 fragmented into multiple chunks using the procedure defined in
 Section 6.9.
 All integer fields in an SCTP packet MUST be transmitted in network
 byte order, unless otherwise stated.

Stewart Standards Track [Page 15] RFC 4960 Stream Control Transmission Protocol September 2007

3.1. SCTP Common Header Field Descriptions

                     SCTP Common Header Format
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Source Port Number        |     Destination Port Number   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Verification Tag                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Checksum                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Source Port Number: 16 bits (unsigned integer)
    This is the SCTP sender's port number.  It can be used by the
    receiver in combination with the source IP address, the SCTP
    destination port, and possibly the destination IP address to
    identify the association to which this packet belongs.  The port
    number 0 MUST NOT be used.
 Destination Port Number: 16 bits (unsigned integer)
    This is the SCTP port number to which this packet is destined.
    The receiving host will use this port number to de-multiplex the
    SCTP packet to the correct receiving endpoint/application.  The
    port number 0 MUST NOT be used.
 Verification Tag: 32 bits (unsigned integer)
    The receiver of this packet uses the Verification Tag to validate
    the sender of this SCTP packet.  On transmit, the value of this
    Verification Tag MUST be set to the value of the Initiate Tag
    received from the peer endpoint during the association
    initialization, with the following exceptions:
  1. A packet containing an INIT chunk MUST have a zero Verification

Tag.

  1. A packet containing a SHUTDOWN COMPLETE chunk with the T bit

set MUST have the Verification Tag copied from the packet with

       the SHUTDOWN ACK chunk.
  1. A packet containing an ABORT chunk may have the verification

tag copied from the packet that caused the ABORT to be sent.

       For details see Section 8.4 and Section 8.5.

Stewart Standards Track [Page 16] RFC 4960 Stream Control Transmission Protocol September 2007

 An INIT chunk MUST be the only chunk in the SCTP packet carrying it.
 Checksum: 32 bits (unsigned integer)
    This field contains the checksum of this SCTP packet.  Its
    calculation is discussed in Section 6.8.  SCTP uses the CRC32c
    algorithm as described in Appendix B for calculating the checksum.

3.2. Chunk Field Descriptions

 The figure below illustrates the field format for the chunks to be
 transmitted in the SCTP packet.  Each chunk is formatted with a Chunk
 Type field, a chunk-specific Flag field, a Chunk Length field, and a
 Value field.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Chunk Type  | Chunk  Flags  |        Chunk Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                          Chunk Value                          /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Type: 8 bits (unsigned integer)
    This field identifies the type of information contained in the
    Chunk Value field.  It takes a value from 0 to 254.  The value of
    255 is reserved for future use as an extension field.
    The values of Chunk Types are defined as follows:
 ID Value    Chunk Type
 -----       ----------
 0          - Payload Data (DATA)
 1          - Initiation (INIT)
 2          - Initiation Acknowledgement (INIT ACK)
 3          - Selective Acknowledgement (SACK)
 4          - Heartbeat Request (HEARTBEAT)
 5          - Heartbeat Acknowledgement (HEARTBEAT ACK)
 6          - Abort (ABORT)
 7          - Shutdown (SHUTDOWN)
 8          - Shutdown Acknowledgement (SHUTDOWN ACK)
 9          - Operation Error (ERROR)
 10         - State Cookie (COOKIE ECHO)
 11         - Cookie Acknowledgement (COOKIE ACK)

Stewart Standards Track [Page 17] RFC 4960 Stream Control Transmission Protocol September 2007

 12         - Reserved for Explicit Congestion Notification Echo
              (ECNE)
 13         - Reserved for Congestion Window Reduced (CWR)
 14         - Shutdown Complete (SHUTDOWN COMPLETE)
 15 to 62   - available
 63         - reserved for IETF-defined Chunk Extensions
 64 to 126  - available
 127        - reserved for IETF-defined Chunk Extensions
 128 to 190 - available
 191        - reserved for IETF-defined Chunk Extensions
 192 to 254 - available
 255        - reserved for IETF-defined Chunk Extensions
    Chunk Types are encoded such that the highest-order 2 bits specify
    the action that must be taken if the processing endpoint does not
    recognize the Chunk Type.
    00 -  Stop processing this SCTP packet and discard it, do not
          process any further chunks within it.
    01 -  Stop processing this SCTP packet and discard it, do not
          process any further chunks within it, and report the
          unrecognized chunk in an 'Unrecognized Chunk Type'.
    10 -  Skip this chunk and continue processing.
    11 -  Skip this chunk and continue processing, but report in an
          ERROR chunk using the 'Unrecognized Chunk Type' cause of
          error.
    Note: The ECNE and CWR chunk types are reserved for future use of
    Explicit Congestion Notification (ECN); see Appendix A.
 Chunk Flags: 8 bits
    The usage of these bits depends on the Chunk type as given by the
    Chunk Type field.  Unless otherwise specified, they are set to 0
    on transmit and are ignored on receipt.
 Chunk Length: 16 bits (unsigned integer)
    This value represents the size of the chunk in bytes, including
    the Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
    Therefore, if the Chunk Value field is zero-length, the Length
    field will be set to 4.  The Chunk Length field does not count any
    chunk padding.

Stewart Standards Track [Page 18] RFC 4960 Stream Control Transmission Protocol September 2007

    Chunks (including Type, Length, and Value fields) are padded out
    by the sender with all zero bytes to be a multiple of 4 bytes
    long.  This padding MUST NOT be more than 3 bytes in total.  The
    Chunk Length value does not include terminating padding of the
    chunk.  However, it does include padding of any variable-length
    parameter except the last parameter in the chunk.  The receiver
    MUST ignore the padding.
    Note: A robust implementation should accept the chunk whether or
    not the final padding has been included in the Chunk Length.
 Chunk Value: variable length
    The Chunk Value field contains the actual information to be
    transferred in the chunk.  The usage and format of this field is
    dependent on the Chunk Type.
 The total length of a chunk (including Type, Length, and Value
 fields) MUST be a multiple of 4 bytes.  If the length of the chunk is
 not a multiple of 4 bytes, the sender MUST pad the chunk with all
 zero bytes, and this padding is not included in the Chunk Length
 field.  The sender MUST NOT pad with more than 3 bytes.  The receiver
 MUST ignore the padding bytes.
 SCTP-defined chunks are described in detail in Section 3.3.  The
 guidelines for IETF-defined chunk extensions can be found in Section
 14.1 of this document.

3.2.1. Optional/Variable-Length Parameter Format

 Chunk values of SCTP control chunks consist of a chunk-type-specific
 header of required fields, followed by zero or more parameters.  The
 optional and variable-length parameters contained in a chunk are
 defined in a Type-Length-Value format as shown below.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Parameter Type       |       Parameter Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                       Parameter Value                         /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart Standards Track [Page 19] RFC 4960 Stream Control Transmission Protocol September 2007

 Chunk Parameter Type: 16 bits (unsigned integer)
    The Type field is a 16-bit identifier of the type of parameter.
    It takes a value of 0 to 65534.
    The value of 65535 is reserved for IETF-defined extensions.
    Values other than those defined in specific SCTP chunk
    descriptions are reserved for use by IETF.
 Chunk Parameter Length: 16 bits (unsigned integer)
    The Parameter Length field contains the size of the parameter in
    bytes, including the Parameter Type, Parameter Length, and
    Parameter Value fields.  Thus, a parameter with a zero-length
    Parameter Value field would have a Length field of 4.  The
    Parameter Length does not include any padding bytes.
 Chunk Parameter Value: variable length
    The Parameter Value field contains the actual information to be
    transferred in the parameter.
    The total length of a parameter (including Type, Parameter Length,
    and Value fields) MUST be a multiple of 4 bytes.  If the length of
    the parameter is not a multiple of 4 bytes, the sender pads the
    parameter at the end (i.e., after the Parameter Value field) with
    all zero bytes.  The length of the padding is not included in the
    Parameter Length field.  A sender MUST NOT pad with more than 3
    bytes.  The receiver MUST ignore the padding bytes.
    The Parameter Types are encoded such that the highest-order 2 bits
    specify the action that must be taken if the processing endpoint
    does not recognize the Parameter Type.
    00 -  Stop processing this parameter; do not process any further
          parameters within this chunk.
    01 -  Stop processing this parameter, do not process any further
          parameters within this chunk, and report the unrecognized
          parameter in an 'Unrecognized Parameter', as described in
          Section 3.2.2.
    10 -  Skip this parameter and continue processing.
    11 -  Skip this parameter and continue processing but report the
          unrecognized parameter in an 'Unrecognized Parameter', as
          described in Section 3.2.2.

Stewart Standards Track [Page 20] RFC 4960 Stream Control Transmission Protocol September 2007

 Please note that in all four cases, an INIT ACK or COOKIE ECHO chunk
 is sent.  In the 00 or 01 case, the processing of the parameters
 after the unknown parameter is canceled, but no processing already
 done is rolled back.
 The actual SCTP parameters are defined in the specific SCTP chunk
 sections.  The rules for IETF-defined parameter extensions are
 defined in Section 14.2.  Note that a parameter type MUST be unique
 across all chunks.  For example, the parameter type '5' is used to
 represent an IPv4 address (see Section 3.3.2.1).  The value '5' then
 is reserved across all chunks to represent an IPv4 address and MUST
 NOT be reused with a different meaning in any other chunk.

3.2.2. Reporting of Unrecognized Parameters

 If the receiver of an INIT chunk detects unrecognized parameters and
 has to report them according to Section 3.2.1, it MUST put the
 'Unrecognized Parameter' parameter(s) in the INIT ACK chunk sent in
 response to the INIT chunk.  Note that if the receiver of the INIT
 chunk is NOT going to establish an association (e.g., due to lack of
 resources), an 'Unrecognized Parameter' would NOT be included with
 any ABORT being sent to the sender of the INIT.
 If the receiver of an INIT ACK chunk detects unrecognized parameters
 and has to report them according to Section 3.2.1, it SHOULD bundle
 the ERROR chunk containing the 'Unrecognized Parameters' error cause
 with the COOKIE ECHO chunk sent in response to the INIT ACK chunk.
 If the receiver of the INIT ACK cannot bundle the COOKIE ECHO chunk
 with the ERROR chunk, the ERROR chunk MAY be sent separately but not
 before the COOKIE ACK has been received.
 Note: Any time a COOKIE ECHO is sent in a packet, it MUST be the
 first chunk.

3.3. SCTP Chunk Definitions

 This section defines the format of the different SCTP chunk types.

Stewart Standards Track [Page 21] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.1. Payload Data (DATA) (0)

 The following format MUST be used for the DATA chunk:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 0    | Reserved|U|B|E|    Length                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              TSN                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Stream Identifier S      |   Stream Sequence Number n    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Payload Protocol Identifier                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                 User Data (seq n of Stream S)                 /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Reserved: 5 bits
    Should be set to all '0's and ignored by the receiver.
 U bit: 1 bit
    The (U)nordered bit, if set to '1', indicates that this is an
    unordered DATA chunk, and there is no Stream Sequence Number
    assigned to this DATA chunk.  Therefore, the receiver MUST ignore
    the Stream Sequence Number field.
    After reassembly (if necessary), unordered DATA chunks MUST be
    dispatched to the upper layer by the receiver without any attempt
    to reorder.
    If an unordered user message is fragmented, each fragment of the
    message MUST have its U bit set to '1'.
 B bit: 1 bit
    The (B)eginning fragment bit, if set, indicates the first fragment
    of a user message.
 E bit: 1 bit
    The (E)nding fragment bit, if set, indicates the last fragment of
    a user message.

Stewart Standards Track [Page 22] RFC 4960 Stream Control Transmission Protocol September 2007

 An unfragmented user message shall have both the B and E bits set to
 '1'.  Setting both B and E bits to '0' indicates a middle fragment of
 a multi-fragment user message, as summarized in the following table:
             B E                  Description
          ============================================================
          |  1 0 | First piece of a fragmented user message          |
          +----------------------------------------------------------+
          |  0 0 | Middle piece of a fragmented user message         |
          +----------------------------------------------------------+
          |  0 1 | Last piece of a fragmented user message           |
          +----------------------------------------------------------+
          |  1 1 | Unfragmented message                              |
          ============================================================
          |             Table 1: Fragment Description Flags          |
          ============================================================
 When a user message is fragmented into multiple chunks, the TSNs are
 used by the receiver to reassemble the message.  This means that the
 TSNs for each fragment of a fragmented user message MUST be strictly
 sequential.
 Length: 16 bits (unsigned integer)
    This field indicates the length of the DATA chunk in bytes from
    the beginning of the type field to the end of the User Data field
    excluding any padding.  A DATA chunk with one byte of user data
    will have Length set to 17 (indicating 17 bytes).
    A DATA chunk with a User Data field of length L will have the
    Length field set to (16 + L) (indicating 16+L bytes) where L MUST
    be greater than 0.
 TSN: 32 bits (unsigned integer)
    This value represents the TSN for this DATA chunk.  The valid
    range of TSN is from 0 to 4294967295 (2**32 - 1).  TSN wraps back
    to 0 after reaching 4294967295.
 Stream Identifier S: 16 bits (unsigned integer)
    Identifies the stream to which the following user data belongs.
 Stream Sequence Number n: 16 bits (unsigned integer)
    This value represents the Stream Sequence Number of the following
    user data within the stream S.  Valid range is 0 to 65535.

Stewart Standards Track [Page 23] RFC 4960 Stream Control Transmission Protocol September 2007

    When a user message is fragmented by SCTP for transport, the same
    Stream Sequence Number MUST be carried in each of the fragments of
    the message.
 Payload Protocol Identifier: 32 bits (unsigned integer)
    This value represents an application (or upper layer) specified
    protocol identifier.  This value is passed to SCTP by its upper
    layer and sent to its peer.  This identifier is not used by SCTP
    but can be used by certain network entities, as well as by the
    peer application, to identify the type of information being
    carried in this DATA chunk.  This field must be sent even in
    fragmented DATA chunks (to make sure it is available for agents in
    the middle of the network).  Note that this field is NOT touched
    by an SCTP implementation; therefore, its byte order is NOT
    necessarily big endian.  The upper layer is responsible for any
    byte order conversions to this field.
    The value 0 indicates that no application identifier is specified
    by the upper layer for this payload data.
 User Data: variable length
    This is the payload user data.  The implementation MUST pad the
    end of the data to a 4-byte boundary with all-zero bytes.  Any
    padding MUST NOT be included in the Length field.  A sender MUST
    never add more than 3 bytes of padding.

3.3.2. Initiation (INIT) (1)

 This chunk is used to initiate an SCTP association between two
 endpoints.  The format of the INIT chunk is shown below:

Stewart Standards Track [Page 24] RFC 4960 Stream Control Transmission Protocol September 2007

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 1    |  Chunk Flags  |      Chunk Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Initiate Tag                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Advertised Receiver Window Credit (a_rwnd)          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Number of Outbound Streams   |  Number of Inbound Streams    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Initial TSN                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /              Optional/Variable-Length Parameters              /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The INIT chunk contains the following parameters.  Unless otherwise
 noted, each parameter MUST only be included once in the INIT chunk.
          Fixed Parameters                     Status
          ----------------------------------------------
          Initiate Tag                        Mandatory
          Advertised Receiver Window Credit   Mandatory
          Number of Outbound Streams          Mandatory
          Number of Inbound Streams           Mandatory
          Initial TSN                         Mandatory
        Variable Parameters                  Status     Type Value
        -------------------------------------------------------------
        IPv4 Address (Note 1)               Optional    5 IPv6 Address
        (Note 1)               Optional    6 Cookie Preservative
        Optional    9 Reserved for ECN Capable (Note 2)   Optional
        32768 (0x8000) Host Name Address (Note 3)          Optional
        11 Supported Address Types (Note 4)    Optional    12
 Note 1: The INIT chunks can contain multiple addresses that can be
 IPv4 and/or IPv6 in any combination.
 Note 2: The ECN Capable field is reserved for future use of Explicit
 Congestion Notification.
 Note 3: An INIT chunk MUST NOT contain more than one Host Name
 Address parameter.  Moreover, the sender of the INIT MUST NOT combine
 any other address types with the Host Name Address in the INIT.  The
 receiver of INIT MUST ignore any other address types if the Host Name
 Address parameter is present in the received INIT chunk.

Stewart Standards Track [Page 25] RFC 4960 Stream Control Transmission Protocol September 2007

 Note 4: This parameter, when present, specifies all the address types
 the sending endpoint can support.  The absence of this parameter
 indicates that the sending endpoint can support any address type.
 IMPLEMENTATION NOTE: If an INIT chunk is received with known
 parameters that are not optional parameters of the INIT chunk, then
 the receiver SHOULD process the INIT chunk and send back an INIT ACK.
 The receiver of the INIT chunk MAY bundle an ERROR chunk with the
 COOKIE ACK chunk later.  However, restrictive implementations MAY
 send back an ABORT chunk in response to the INIT chunk.
 The Chunk Flags field in INIT is reserved, and all bits in it should
 be set to 0 by the sender and ignored by the receiver.  The sequence
 of parameters within an INIT can be processed in any order.
 Initiate Tag: 32 bits (unsigned integer)
    The receiver of the INIT (the responding end) records the value of
    the Initiate Tag parameter.  This value MUST be placed into the
    Verification Tag field of every SCTP packet that the receiver of
    the INIT transmits within this association.
    The Initiate Tag is allowed to have any value except 0.  See
    Section 5.3.1 for more on the selection of the tag value.
    If the value of the Initiate Tag in a received INIT chunk is found
    to be 0, the receiver MUST treat it as an error and close the
    association by transmitting an ABORT.
 Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
 integer)
    This value represents the dedicated buffer space, in number of
    bytes, the sender of the INIT has reserved in association with
    this window.  During the life of the association, this buffer
    space SHOULD NOT be lessened (i.e., dedicated buffers taken away
    from this association); however, an endpoint MAY change the value
    of a_rwnd it sends in SACK chunks.
 Number of Outbound Streams (OS): 16 bits (unsigned integer)
    Defines the number of outbound streams the sender of this INIT
    chunk wishes to create in this association.  The value of 0 MUST
    NOT be used.
    Note: A receiver of an INIT with the OS value set to 0 SHOULD
    abort the association.

Stewart Standards Track [Page 26] RFC 4960 Stream Control Transmission Protocol September 2007

 Number of Inbound Streams (MIS): 16 bits (unsigned integer)
    Defines the maximum number of streams the sender of this INIT
    chunk allows the peer end to create in this association.  The
    value 0 MUST NOT be used.
    Note: There is no negotiation of the actual number of streams but
    instead the two endpoints will use the min(requested, offered).
    See Section 5.1.1 for details.
    Note: A receiver of an INIT with the MIS value of 0 SHOULD abort
    the association.
 Initial TSN (I-TSN): 32 bits (unsigned integer)
    Defines the initial TSN that the sender will use.  The valid range
    is from 0 to 4294967295.  This field MAY be set to the value of
    the Initiate Tag field.

3.3.2.1. Optional/Variable-Length Parameters in INIT

 The following parameters follow the Type-Length-Value format as
 defined in Section 3.2.1.  Any Type-Length-Value fields MUST come
 after the fixed-length fields defined in the previous section.
 IPv4 Address Parameter (5)
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Type = 5               |      Length = 8               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        IPv4 Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IPv4 Address: 32 bits (unsigned integer)
    Contains an IPv4 address of the sending endpoint.  It is binary
    encoded.

Stewart Standards Track [Page 27] RFC 4960 Stream Control Transmission Protocol September 2007

 IPv6 Address Parameter (6)
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Type = 6           |          Length = 20          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 Address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IPv6 Address: 128 bits (unsigned integer)
    Contains an IPv6 [RFC2460] address of the sending endpoint.  It is
    binary encoded.
    Note: A sender MUST NOT use an IPv4-mapped IPv6 address [RFC4291],
    but should instead use an IPv4 Address parameter for an IPv4
    address.
    Combined with the Source Port Number in the SCTP common header,
    the value passed in an IPv4 or IPv6 Address parameter indicates a
    transport address the sender of the INIT will support for the
    association being initiated.  That is, during the life time of
    this association, this IP address can appear in the source address
    field of an IP datagram sent from the sender of the INIT, and can
    be used as a destination address of an IP datagram sent from the
    receiver of the INIT.
    More than one IP Address parameter can be included in an INIT
    chunk when the INIT sender is multi-homed.  Moreover, a multi-
    homed endpoint may have access to different types of network;
    thus, more than one address type can be present in one INIT chunk,
    i.e., IPv4 and IPv6 addresses are allowed in the same INIT chunk.
    If the INIT contains at least one IP Address parameter, then the
    source address of the IP datagram containing the INIT chunk and
    any additional address(es) provided within the INIT can be used as
    destinations by the endpoint receiving the INIT.  If the INIT does
    not contain any IP Address parameters, the endpoint receiving the
    INIT MUST use the source address associated with the received IP
    datagram as its sole destination address for the association.
    Note that not using any IP Address parameters in the INIT and INIT
    ACK is an alternative to make an association more likely to work
    across a NAT box.

Stewart Standards Track [Page 28] RFC 4960 Stream Control Transmission Protocol September 2007

 Cookie Preservative (9)
 The sender of the INIT shall use this parameter to suggest to the
 receiver of the INIT for a longer life-span of the State Cookie.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Type = 9             |          Length = 8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Suggested Cookie Life-Span Increment (msec.)          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Suggested Cookie Life-Span Increment: 32 bits (unsigned integer)
    This parameter indicates to the receiver how much increment in
    milliseconds the sender wishes the receiver to add to its default
    cookie life-span.
    This optional parameter should be added to the INIT chunk by the
    sender when it reattempts establishing an association with a peer
    to which its previous attempt of establishing the association
    failed due to a stale cookie operation error.  The receiver MAY
    choose to ignore the suggested cookie life-span increase for its
    own security reasons.
 Host Name Address (11)
 The sender of INIT uses this parameter to pass its Host Name (in
 place of its IP addresses) to its peer.  The peer is responsible for
 resolving the name.  Using this parameter might make it more likely
 for the association to work across a NAT box.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Type = 11            |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                          Host Name                            /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Host Name: variable length
    This field contains a host name in "host name syntax" per RFC 1123
    Section 2.1 [RFC1123].  The method for resolving the host name is
    out of scope of SCTP.

Stewart Standards Track [Page 29] RFC 4960 Stream Control Transmission Protocol September 2007

    Note: At least one null terminator is included in the Host Name
    string and must be included in the length.
 Supported Address Types (12)
 The sender of INIT uses this parameter to list all the address types
 it can support.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Type = 12            |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Address Type #1        |        Address Type #2        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ......                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+
 Address Type: 16 bits (unsigned integer)
    This is filled with the type value of the corresponding address
    TLV (e.g., IPv4 = 5, IPv6 = 6, Host name = 11).

3.3.3. Initiation Acknowledgement (INIT ACK) (2)

 The INIT ACK chunk is used to acknowledge the initiation of an SCTP
 association.
 The parameter part of INIT ACK is formatted similarly to the INIT
 chunk.  It uses two extra variable parameters: The State Cookie and
 the Unrecognized Parameter:

Stewart Standards Track [Page 30] RFC 4960 Stream Control Transmission Protocol September 2007

 The format of the INIT ACK chunk is shown below:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 2    |  Chunk Flags  |      Chunk Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Initiate Tag                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Advertised Receiver Window Credit                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Number of Outbound Streams   |  Number of Inbound Streams    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Initial TSN                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /              Optional/Variable-Length Parameters              /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiate Tag: 32 bits (unsigned integer)
    The receiver of the INIT ACK records the value of the Initiate Tag
    parameter.  This value MUST be placed into the Verification Tag
    field of every SCTP packet that the INIT ACK receiver transmits
    within this association.
    The Initiate Tag MUST NOT take the value 0.  See Section 5.3.1 for
    more on the selection of the Initiate Tag value.
    If the value of the Initiate Tag in a received INIT ACK chunk is
    found to be 0, the receiver MUST destroy the association
    discarding its TCB.  The receiver MAY send an ABORT for debugging
    purpose.
 Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
 integer)
    This value represents the dedicated buffer space, in number of
    bytes, the sender of the INIT ACK has reserved in association with
    this window.  During the life of the association, this buffer
    space SHOULD NOT be lessened (i.e., dedicated buffers taken away
    from this association).
 Number of Outbound Streams (OS): 16 bits (unsigned integer)
    Defines the number of outbound streams the sender of this INIT ACK
    chunk wishes to create in this association.  The value of 0 MUST

Stewart Standards Track [Page 31] RFC 4960 Stream Control Transmission Protocol September 2007

    NOT be used, and the value MUST NOT be greater than the MIS value
    sent in the INIT chunk.
    Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
    destroy the association discarding its TCB.
 Number of Inbound Streams (MIS): 16 bits (unsigned integer)
    Defines the maximum number of streams the sender of this INIT ACK
    chunk allows the peer end to create in this association.  The
    value 0 MUST NOT be used.
    Note: There is no negotiation of the actual number of streams but
    instead the two endpoints will use the min(requested, offered).
    See Section 5.1.1 for details.
    Note: A receiver of an INIT ACK with the MIS value set to 0 SHOULD
    destroy the association discarding its TCB.
 Initial TSN (I-TSN): 32 bits (unsigned integer)
    Defines the initial TSN that the INIT ACK sender will use.  The
    valid range is from 0 to 4294967295.  This field MAY be set to the
    value of the Initiate Tag field.
       Fixed Parameters                     Status
       ----------------------------------------------
       Initiate Tag                        Mandatory
       Advertised Receiver Window Credit   Mandatory
       Number of Outbound Streams          Mandatory
       Number of Inbound Streams           Mandatory
       Initial TSN                         Mandatory
       Variable Parameters                  Status     Type Value
       -------------------------------------------------------------
       State Cookie                        Mandatory   7
       IPv4 Address (Note 1)               Optional    5
       IPv6 Address (Note 1)               Optional    6
       Unrecognized Parameter              Optional    8
       Reserved for ECN Capable (Note 2)   Optional    32768 (0x8000)
       Host Name Address (Note 3)          Optional    11
 Note 1: The INIT ACK chunks can contain any number of IP address
 parameters that can be IPv4 and/or IPv6 in any combination.
 Note 2: The ECN Capable field is reserved for future use of Explicit
 Congestion Notification.

Stewart Standards Track [Page 32] RFC 4960 Stream Control Transmission Protocol September 2007

 Note 3: The INIT ACK chunks MUST NOT contain more than one Host Name
 Address parameter.  Moreover, the sender of the INIT ACK MUST NOT
 combine any other address types with the Host Name Address in the
 INIT ACK.  The receiver of the INIT ACK MUST ignore any other address
 types if the Host Name Address parameter is present.
 IMPLEMENTATION NOTE: An implementation MUST be prepared to receive an
 INIT ACK that is quite large (more than 1500 bytes) due to the
 variable size of the State Cookie AND the variable address list.  For
 example if a responder to the INIT has 1000 IPv4 addresses it wishes
 to send, it would need at least 8,000 bytes to encode this in the
 INIT ACK.
 IMPLEMENTATION NOTE: If an INIT ACK chunk is received with known
 parameters that are not optional parameters of the INIT ACK chunk,
 then the receiver SHOULD process the INIT ACK chunk and send back a
 COOKIE ECHO.  The receiver of the INIT ACK chunk MAY bundle an ERROR
 chunk with the COOKIE ECHO chunk.  However, restrictive
 implementations MAY send back an ABORT chunk in response to the INIT
 ACK chunk.
 In combination with the Source Port carried in the SCTP common
 header, each IP Address parameter in the INIT ACK indicates to the
 receiver of the INIT ACK a valid transport address supported by the
 sender of the INIT ACK for the life time of the association being
 initiated.
 If the INIT ACK contains at least one IP Address parameter, then the
 source address of the IP datagram containing the INIT ACK and any
 additional address(es) provided within the INIT ACK may be used as
 destinations by the receiver of the INIT ACK.  If the INIT ACK does
 not contain any IP Address parameters, the receiver of the INIT ACK
 MUST use the source address associated with the received IP datagram
 as its sole destination address for the association.
 The State Cookie and Unrecognized Parameters use the Type-Length-
 Value format as defined in Section 3.2.1 and are described below.
 The other fields are defined the same as their counterparts in the
 INIT chunk.

3.3.3.1. Optional or Variable-Length Parameters

 State Cookie
 Parameter Type Value: 7
    Parameter Length: Variable size, depending on size of Cookie.

Stewart Standards Track [Page 33] RFC 4960 Stream Control Transmission Protocol September 2007

 Parameter Value:
    This parameter value MUST contain all the necessary state and
    parameter information required for the sender of this INIT ACK to
    create the association, along with a Message Authentication Code
    (MAC).  See Section 5.1.3 for details on State Cookie definition.
 Unrecognized Parameter:
    Parameter Type Value: 8
 Parameter Length: Variable size.
 Parameter Value:
    This parameter is returned to the originator of the INIT chunk
    when the INIT contains an unrecognized parameter that has a value
    that indicates it should be reported to the sender.  This
    parameter value field will contain unrecognized parameters copied
    from the INIT chunk complete with Parameter Type, Length, and
    Value fields.

3.3.4. Selective Acknowledgement (SACK) (3)

 This chunk is sent to the peer endpoint to acknowledge received DATA
 chunks and to inform the peer endpoint of gaps in the received
 subsequences of DATA chunks as represented by their TSNs.
 The SACK MUST contain the Cumulative TSN Ack, Advertised Receiver
 Window Credit (a_rwnd), Number of Gap Ack Blocks, and Number of
 Duplicate TSNs fields.
 By definition, the value of the Cumulative TSN Ack parameter is the
 last TSN received before a break in the sequence of received TSNs
 occurs; the next TSN value following this one has not yet been
 received at the endpoint sending the SACK.  This parameter therefore
 acknowledges receipt of all TSNs less than or equal to its value.
 The handling of a_rwnd by the receiver of the SACK is discussed in
 detail in Section 6.2.1.
 The SACK also contains zero or more Gap Ack Blocks.  Each Gap Ack
 Block acknowledges a subsequence of TSNs received following a break
 in the sequence of received TSNs.  By definition, all TSNs
 acknowledged by Gap Ack Blocks are greater than the value of the
 Cumulative TSN Ack.

Stewart Standards Track [Page 34] RFC 4960 Stream Control Transmission Protocol September 2007

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 3    |Chunk  Flags   |      Chunk Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Cumulative TSN Ack                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Advertised Receiver Window Credit (a_rwnd)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Number of Gap Ack Blocks = N  |  Number of Duplicate TSNs = X |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Gap Ack Block #1 Start       |   Gap Ack Block #1 End        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Gap Ack Block #N Start      |  Gap Ack Block #N End         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN 1                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN X                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to all '0's on transmit and ignored on receipt.
 Cumulative TSN Ack: 32 bits (unsigned integer)
    This parameter contains the TSN of the last DATA chunk received in
    sequence before a gap.  In the case where no DATA chunk has been
    received, this value is set to the peer's Initial TSN minus one.
 Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
 integer)
    This field indicates the updated receive buffer space in bytes of
    the sender of this SACK; see Section 6.2.1 for details.
 Number of Gap Ack Blocks: 16 bits (unsigned integer)
    Indicates the number of Gap Ack Blocks included in this SACK.

Stewart Standards Track [Page 35] RFC 4960 Stream Control Transmission Protocol September 2007

 Number of Duplicate TSNs: 16 bit
    This field contains the number of duplicate TSNs the endpoint has
    received.  Each duplicate TSN is listed following the Gap Ack
    Block list.
 Gap Ack Blocks:
    These fields contain the Gap Ack Blocks.  They are repeated for
    each Gap Ack Block up to the number of Gap Ack Blocks defined in
    the Number of Gap Ack Blocks field.  All DATA chunks with TSNs
    greater than or equal to (Cumulative TSN Ack + Gap Ack Block
    Start) and less than or equal to (Cumulative TSN Ack + Gap Ack
    Block End) of each Gap Ack Block are assumed to have been received
    correctly.
 Gap Ack Block Start: 16 bits (unsigned integer)
    Indicates the Start offset TSN for this Gap Ack Block.  To
    calculate the actual TSN number the Cumulative TSN Ack is added to
    this offset number.  This calculated TSN identifies the first TSN
    in this Gap Ack Block that has been received.
 Gap Ack Block End: 16 bits (unsigned integer)
    Indicates the End offset TSN for this Gap Ack Block.  To calculate
    the actual TSN number, the Cumulative TSN Ack is added to this
    offset number.  This calculated TSN identifies the TSN of the last
    DATA chunk received in this Gap Ack Block.

Stewart Standards Track [Page 36] RFC 4960 Stream Control Transmission Protocol September 2007

 For example, assume that the receiver has the following DATA chunks
 newly arrived at the time when it decides to send a Selective ACK,
  1. ———

| TSN=17 |

  1. ———

| | ← still missing

  1. ———

| TSN=15 |

  1. ———

| TSN=14 |

  1. ———

| | ← still missing

  1. ———

| TSN=12 |

  1. ———

| TSN=11 |

  1. ———

| TSN=10 |

  1. ———
 then the parameter part of the SACK MUST be constructed as follows
 (assuming the new a_rwnd is set to 4660 by the sender):
                   +--------------------------------+
                   |   Cumulative TSN Ack = 12      |
                   +--------------------------------+
                   |        a_rwnd = 4660           |
                   +----------------+---------------+
                   | num of block=2 | num of dup=0  |
                   +----------------+---------------+
                   |block #1 strt=2 |block #1 end=3 |
                   +----------------+---------------+
                   |block #2 strt=5 |block #2 end=5 |
                   +----------------+---------------+
 Duplicate TSN: 32 bits (unsigned integer)
    Indicates the number of times a TSN was received in duplicate
    since the last SACK was sent.  Every time a receiver gets a
    duplicate TSN (before sending the SACK), it adds it to the list of
    duplicates.  The duplicate count is reinitialized to zero after
    sending each SACK.
 For example, if a receiver were to get the TSN 19 three times it
 would list 19 twice in the outbound SACK.  After sending the SACK, if
 it received yet one more TSN 19 it would list 19 as a duplicate once
 in the next outgoing SACK.

Stewart Standards Track [Page 37] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.5. Heartbeat Request (HEARTBEAT) (4)

 An endpoint should send this chunk to its peer endpoint to probe the
 reachability of a particular destination transport address defined in
 the present association.
 The parameter field contains the Heartbeat Information, which is a
 variable-length opaque data structure understood only by the sender.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 4    | Chunk  Flags  |      Heartbeat Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /            Heartbeat Information TLV (Variable-Length)        /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.
 Heartbeat Length: 16 bits (unsigned integer)
    Set to the size of the chunk in bytes, including the chunk header
    and the Heartbeat Information field.
 Heartbeat Information: variable length
    Defined as a variable-length parameter using the format described
    in Section 3.2.1, i.e.:
       Variable Parameters                  Status     Type Value
       -------------------------------------------------------------
       Heartbeat Info                       Mandatory   1
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Heartbeat Info Type=1      |         HB Info Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Sender-Specific Heartbeat Info               /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The Sender-Specific Heartbeat Info field should normally include
    information about the sender's current time when this HEARTBEAT

Stewart Standards Track [Page 38] RFC 4960 Stream Control Transmission Protocol September 2007

    chunk is sent and the destination transport address to which this
    HEARTBEAT is sent (see Section 8.3).  This information is simply
    reflected back by the receiver in the HEARTBEAT ACK message (see
    Section 3.3.6).  Note also that the HEARTBEAT message is both for
    reachability checking and for path verification (see Section 5.4).
    When a HEARTBEAT chunk is being used for path verification
    purposes, it MUST hold a 64-bit random nonce.

3.3.6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5)

 An endpoint should send this chunk to its peer endpoint as a response
 to a HEARTBEAT chunk (see Section 8.3).  A HEARTBEAT ACK is always
 sent to the source IP address of the IP datagram containing the
 HEARTBEAT chunk to which this ack is responding.
 The parameter field contains a variable-length opaque data structure.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 5    | Chunk  Flags  |    Heartbeat Ack Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /            Heartbeat Information TLV (Variable-Length)        /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.
 Heartbeat Ack Length: 16 bits (unsigned integer)
    Set to the size of the chunk in bytes, including the chunk header
    and the Heartbeat Information field.
 Heartbeat Information: variable length
    This field MUST contain the Heartbeat Information parameter of the
    Heartbeat Request to which this Heartbeat Acknowledgement is
    responding.
       Variable Parameters                  Status     Type Value
       -------------------------------------------------------------
       Heartbeat Info                       Mandatory   1

Stewart Standards Track [Page 39] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.7. Abort Association (ABORT) (6)

 The ABORT chunk is sent to the peer of an association to close the
 association.  The ABORT chunk may contain Cause Parameters to inform
 the receiver about the reason of the abort.  DATA chunks MUST NOT be
 bundled with ABORT.  Control chunks (except for INIT, INIT ACK, and
 SHUTDOWN COMPLETE) MAY be bundled with an ABORT, but they MUST be
 placed before the ABORT in the SCTP packet or they will be ignored by
 the receiver.
 If an endpoint receives an ABORT with a format error or no TCB is
 found, it MUST silently discard it.  Moreover, under any
 circumstances, an endpoint that receives an ABORT MUST NOT respond to
 that ABORT by sending an ABORT of its own.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 6    |Reserved     |T|           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                   zero or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Reserved: 7 bits
    Set to 0 on transmit and ignored on receipt.
    T bit: 1 bit
    The T bit is set to 0 if the sender filled in the Verification Tag
    expected by the peer.  If the Verification Tag is reflected, the T
    bit MUST be set to 1.  Reflecting means that the sent Verification
    Tag is the same as the received one.
    Note: Special rules apply to this chunk for verification; please
    see Section 8.5.1 for details.
 Length: 16 bits (unsigned integer)
    Set to the size of the chunk in bytes, including the chunk header
    and all the Error Cause fields present.
    See Section 3.3.10 for Error Cause definitions.

Stewart Standards Track [Page 40] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.8. Shutdown Association (SHUTDOWN) (7)

 An endpoint in an association MUST use this chunk to initiate a
 graceful close of the association with its peer.  This chunk has the
 following format.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 7    | Chunk  Flags  |      Length = 8               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Cumulative TSN Ack                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.
 Length: 16 bits (unsigned integer)
    Indicates the length of the parameter.  Set to 8.
 Cumulative TSN Ack: 32 bits (unsigned integer)
    This parameter contains the TSN of the last chunk received in
    sequence before any gaps.
    Note: Since the SHUTDOWN message does not contain Gap Ack Blocks,
    it cannot be used to acknowledge TSNs received out of order.  In a
    SACK, lack of Gap Ack Blocks that were previously included
    indicates that the data receiver reneged on the associated DATA
    chunks.  Since SHUTDOWN does not contain Gap Ack Blocks, the
    receiver of the SHUTDOWN shouldn't interpret the lack of a Gap Ack
    Block as a renege.  (See Section 6.2 for information on reneging.)

3.3.9. Shutdown Acknowledgement (SHUTDOWN ACK) (8)

 This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
 chunk at the completion of the shutdown process; see Section 9.2 for
 details.
 The SHUTDOWN ACK chunk has no parameters.

Stewart Standards Track [Page 41] RFC 4960 Stream Control Transmission Protocol September 2007

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 8    |Chunk  Flags   |      Length = 4               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.

3.3.10. Operation Error (ERROR) (9)

 An endpoint sends this chunk to its peer endpoint to notify it of
 certain error conditions.  It contains one or more error causes.  An
 Operation Error is not considered fatal in and of itself, but may be
 used with an ABORT chunk to report a fatal condition.  It has the
 following parameters:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 9    | Chunk  Flags  |           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                    one or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.
 Length: 16 bits (unsigned integer)
    Set to the size of the chunk in bytes, including the chunk header
    and all the Error Cause fields present.

Stewart Standards Track [Page 42] RFC 4960 Stream Control Transmission Protocol September 2007

 Error causes are defined as variable-length parameters using the
 format described in Section 3.2.1, that is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Cause Code          |       Cause Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Cause-Specific Information                 /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Cause Code: 16 bits (unsigned integer)
    Defines the type of error conditions being reported.
       Cause Code
       Value           Cause Code
       ---------      ----------------
        1              Invalid Stream Identifier
        2              Missing Mandatory Parameter
        3              Stale Cookie Error
        4              Out of Resource
        5              Unresolvable Address
        6              Unrecognized Chunk Type
        7              Invalid Mandatory Parameter
        8              Unrecognized Parameters
        9              No User Data
       10              Cookie Received While Shutting Down
       11              Restart of an Association with New Addresses
       12              User Initiated Abort
       13              Protocol Violation
 Cause Length: 16 bits (unsigned integer)
    Set to the size of the parameter in bytes, including the Cause
    Code, Cause Length, and Cause-Specific Information fields.
 Cause-Specific Information: variable length
    This field carries the details of the error condition.
 Section 3.3.10.1 - Section 3.3.10.13 define error causes for SCTP.
 Guidelines for the IETF to define new error cause values are
 discussed in Section 14.3.

Stewart Standards Track [Page 43] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.1. Invalid Stream Identifier (1)

 Cause of error
 ---------------
 Invalid Stream Identifier: Indicates endpoint received a DATA chunk
 sent to a nonexistent stream.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=1              |      Cause Length=8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Stream Identifier      |         (Reserved)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Stream Identifier: 16 bits (unsigned integer)
    Contains the Stream Identifier of the DATA chunk received in
    error.
 Reserved: 16 bits
    This field is reserved.  It is set to all 0's on transmit and
    ignored on receipt.

3.3.10.2. Missing Mandatory Parameter (2)

 Cause of error
 ---------------
 Missing Mandatory Parameter: Indicates that one or more mandatory TLV
 parameters are missing in a received INIT or INIT ACK.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=2              |      Cause Length=8+N*2       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Number of missing params=N                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Missing Param Type #1       |   Missing Param Type #2       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Missing Param Type #N-1     |   Missing Param Type #N       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Number of Missing params: 32 bits (unsigned integer)
    This field contains the number of parameters contained in the
    Cause-Specific Information field.

Stewart Standards Track [Page 44] RFC 4960 Stream Control Transmission Protocol September 2007

 Missing Param Type: 16 bits (unsigned integer)
    Each field will contain the missing mandatory parameter number.

3.3.10.3. Stale Cookie Error (3)

 Cause of error
 --------------
 Stale Cookie Error: Indicates the receipt of a valid State Cookie
 that has expired.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=3              |       Cause Length=8          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Measure of Staleness (usec.)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Measure of Staleness: 32 bits (unsigned integer)
    This field contains the difference, in microseconds, between the
    current time and the time the State Cookie expired.
    The sender of this error cause MAY choose to report how long past
    expiration the State Cookie is by including a non-zero value in
    the Measure of Staleness field.  If the sender does not wish to
    provide this information, it should set the Measure of Staleness
    field to the value of zero.

3.3.10.4. Out of Resource (4)

 Cause of error
 ---------------
 Out of Resource: Indicates that the sender is out of resource.  This
 is usually sent in combination with or within an ABORT.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=4              |      Cause Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart Standards Track [Page 45] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.5. Unresolvable Address (5)

 Cause of error
 ---------------
 Unresolvable Address: Indicates that the sender is not able to
 resolve the specified address parameter (e.g., type of address is not
 supported by the sender).  This is usually sent in combination with
 or within an ABORT.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=5              |      Cause Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Unresolvable Address                         /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Unresolvable Address: variable length
    The Unresolvable Address field contains the complete Type, Length,
    and Value of the address parameter (or Host Name parameter) that
    contains the unresolvable address or host name.

3.3.10.6. Unrecognized Chunk Type (6)

 Cause of error
 ---------------
 Unrecognized Chunk Type: This error cause is returned to the
 originator of the chunk if the receiver does not understand the chunk
 and the upper bits of the 'Chunk Type' are set to 01 or 11.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=6              |      Cause Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Unrecognized Chunk                           /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Unrecognized Chunk: variable length
    The Unrecognized Chunk field contains the unrecognized chunk from
    the SCTP packet complete with Chunk Type, Chunk Flags, and Chunk
    Length.

Stewart Standards Track [Page 46] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.7. Invalid Mandatory Parameter (7)

 Cause of error
 ---------------
 Invalid Mandatory Parameter: This error cause is returned to the
 originator of an INIT or INIT ACK chunk when one of the mandatory
 parameters is set to an invalid value.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=7              |      Cause Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.10.8. Unrecognized Parameters (8)

 Cause of error
 ---------------
 Unrecognized Parameters: This error cause is returned to the
 originator of the INIT ACK chunk if the receiver does not recognize
 one or more Optional TLV parameters in the INIT ACK chunk.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=8              |      Cause Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Unrecognized Parameters                      /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Unrecognized Parameters: variable length
    The Unrecognized Parameters field contains the unrecognized
    parameters copied from the INIT ACK chunk complete with TLV.  This
    error cause is normally contained in an ERROR chunk bundled with
    the COOKIE ECHO chunk when responding to the INIT ACK, when the
    sender of the COOKIE ECHO chunk wishes to report unrecognized
    parameters.

Stewart Standards Track [Page 47] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.9. No User Data (9)

 Cause of error
 ---------------
 No User Data: This error cause is returned to the originator of a
 DATA chunk if a received DATA chunk has no user data.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=9              |      Cause Length=8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  TSN value                                    /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 TSN value: 32 bits (unsigned integer)
    The TSN value field contains the TSN of the DATA chunk received
    with no user data field.
    This cause code is normally returned in an ABORT chunk (see
    Section 6.2).

3.3.10.10. Cookie Received While Shutting Down (10)

 Cause of error
 ---------------
 Cookie Received While Shutting Down: A COOKIE ECHO was received while
 the endpoint was in the SHUTDOWN-ACK-SENT state.  This error is
 usually returned in an ERROR chunk bundled with the retransmitted
 SHUTDOWN ACK.
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=10              |      Cause Length=4          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart Standards Track [Page 48] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.11. Restart of an Association with New Addresses (11)

 Cause of error
 --------------
 Restart of an association with new addresses: An INIT was received on
 an existing association.  But the INIT added addresses to the
 association that were previously NOT part of the association.  The
 new addresses are listed in the error code.  This ERROR is normally
 sent as part of an ABORT refusing the INIT (see Section 5.2).
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=11         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                       New Address TLVs                        /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note: Each New Address TLV is an exact copy of the TLV that was found
 in the INIT chunk that was new, including the Parameter Type and the
 Parameter Length.

3.3.10.12. User-Initiated Abort (12)

 Cause of error
 --------------
 This error cause MAY be included in ABORT chunks that are sent
 because of an upper-layer request.  The upper layer can specify an
 Upper Layer Abort Reason that is transported by SCTP transparently
 and MAY be delivered to the upper-layer protocol at the peer.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=12         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Upper Layer Abort Reason                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart Standards Track [Page 49] RFC 4960 Stream Control Transmission Protocol September 2007

3.3.10.13. Protocol Violation (13)

 Cause of error
 --------------
 This error cause MAY be included in ABORT chunks that are sent
 because an SCTP endpoint detects a protocol violation of the peer
 that is not covered by the error causes described in Section 3.3.10.1
 to Section 3.3.10.12.  An implementation MAY provide additional
 information specifying what kind of protocol violation has been
 detected.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=13         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Additional Information                     /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.11. Cookie Echo (COOKIE ECHO) (10)

 This chunk is used only during the initialization of an association.
 It is sent by the initiator of an association to its peer to complete
 the initialization process.  This chunk MUST precede any DATA chunk
 sent within the association, but MAY be bundled with one or more DATA
 chunks in the same packet.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 10   |Chunk  Flags   |         Length                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                     Cookie                                    /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bit
    Set to 0 on transmit and ignored on receipt.
 Length: 16 bits (unsigned integer)
    Set to the size of the chunk in bytes, including the 4 bytes of
    the chunk header and the size of the cookie.

Stewart Standards Track [Page 50] RFC 4960 Stream Control Transmission Protocol September 2007

 Cookie: variable size
    This field must contain the exact cookie received in the State
    Cookie parameter from the previous INIT ACK.
    An implementation SHOULD make the cookie as small as possible to
    ensure interoperability.
    Note: A Cookie Echo does NOT contain a State Cookie parameter;
    instead, the data within the State Cookie's Parameter Value
    becomes the data within the Cookie Echo's Chunk Value.  This
    allows an implementation to change only the first 2 bytes of the
    State Cookie parameter to become a COOKIE ECHO chunk.

3.3.12. Cookie Acknowledgement (COOKIE ACK) (11)

 This chunk is used only during the initialization of an association.
 It is used to acknowledge the receipt of a COOKIE ECHO chunk.  This
 chunk MUST precede any DATA or SACK chunk sent within the
 association, but MAY be bundled with one or more DATA chunks or SACK
 chunk's in the same SCTP packet.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 11   |Chunk  Flags   |     Length = 4                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits
    Set to 0 on transmit and ignored on receipt.

3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14)

 This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
 ACK chunk at the completion of the shutdown process; see Section 9.2
 for details.
 The SHUTDOWN COMPLETE chunk has no parameters.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 14   |Reserved     |T|      Length = 4               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Chunk Flags: 8 bits

Stewart Standards Track [Page 51] RFC 4960 Stream Control Transmission Protocol September 2007

    Reserved: 7 bits
       Set to 0 on transmit and ignored on receipt.
       T bit: 1 bit
    The T bit is set to 0 if the sender filled in the Verification Tag
    expected by the peer.  If the Verification Tag is reflected, the T
    bit MUST be set to 1.  Reflecting means that the sent Verification
    Tag is the same as the received one.
 Note: Special rules apply to this chunk for verification, please see
 Section 8.5.1 for details.

4. SCTP Association State Diagram

 During the life time of an SCTP association, the SCTP endpoint's
 association progresses from one state to another in response to
 various events.  The events that may potentially advance an
 association's state include:
 o  SCTP user primitive calls, e.g., [ASSOCIATE], [SHUTDOWN], [ABORT],
 o  Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc., control
    chunks, or
 o  Some timeout events.
 The state diagram in the figures below illustrates state changes,
 together with the causing events and resulting actions.  Note that
 some of the error conditions are not shown in the state diagram.
 Full descriptions of all special cases are found in the text.
 Note: Chunk names are given in all capital letters, while parameter
 names have the first letter capitalized, e.g., COOKIE ECHO chunk type
 vs. State Cookie parameter.  If more than one event/message can occur
 that causes a state transition, it is labeled (A), (B), etc.

Stewart Standards Track [Page 52] RFC 4960 Stream Control Transmission Protocol September 2007

  1. —- ——– (from any state)

/ \ / rcv ABORT [ABORT]

 rcv INIT        |         |    |   ----------  or ----------
 --------------- |         v    v   delete TCB     snd ABORT
 generate Cookie  \    +---------+                 delete TCB
 snd INIT ACK       ---|  CLOSED |
                       +---------+
                        /      \      [ASSOCIATE]
                       /        \     ---------------
                      |          |    create TCB
                      |          |    snd INIT
                      |          |    strt init timer
       rcv valid      |          |
     COOKIE  ECHO     |          v
 (1) ---------------- |      +------------+
     create TCB       |      | COOKIE-WAIT| (2)
     snd COOKIE ACK   |      +------------+
                      |          |
                      |          |    rcv INIT ACK
                      |          |    -----------------
                      |          |    snd COOKIE ECHO
                      |          |    stop init timer
                      |          |    strt cookie timer
                      |          v
                      |      +--------------+
                      |      | COOKIE-ECHOED| (3)
                      |      +--------------+
                      |          |
                      |          |    rcv COOKIE ACK
                      |          |    -----------------
                      |          |    stop cookie timer
                      v          v
                    +---------------+
                    |  ESTABLISHED  |
                    +---------------+

Stewart Standards Track [Page 53] RFC 4960 Stream Control Transmission Protocol September 2007

                  (from the ESTABLISHED state only)
                                |
                                |
                       /--------+--------\
   [SHUTDOWN]         /                   \
   -------------------|                   |
   check outstanding  |                   |
   DATA chunks        |                   |
                      v                   |
                 +---------+              |
                 |SHUTDOWN-|              | rcv SHUTDOWN
                 |PENDING  |              |------------------
                 +---------+              | check outstanding
                      |                   | DATA chunks
 No more outstanding  |                   |
 ---------------------|                   |
 snd SHUTDOWN         |                   |
 strt shutdown timer  |                   |
                      v                   v
                 +---------+        +-----------+
             (4) |SHUTDOWN-|        | SHUTDOWN- |  (5,6)
                 |SENT     |        | RECEIVED  |
                 +---------+        +-----------+
                      |  \                |
 (A) rcv SHUTDOWN ACK  |   \               |
 ----------------------|    \              |
 stop shutdown timer   |     \rcv:SHUTDOWN |
 send SHUTDOWN COMPLETE|      \  (B)       |
 delete TCB            |       \           |
                       |        \          | No more outstanding
                       |         \         |-----------------
                       |          \        | send SHUTDOWN ACK
 (B)rcv SHUTDOWN       |           \       | strt shutdown timer
 ----------------------|            \      |
 send SHUTDOWN ACK     |             \     |
 start shutdown timer  |              \    |
 move to SHUTDOWN-     |               \   |
 ACK-SENT              |                |  |
                       |                v  |
                       |             +-----------+
                       |             | SHUTDOWN- | (7)
                       |             | ACK-SENT  |
                       |             +----------+-
                       |                   | (C)rcv SHUTDOWN COMPLETE
                       |                   |-----------------
                       |                   | stop shutdown timer
                       |                   | delete TCB
                       |                   |

Stewart Standards Track [Page 54] RFC 4960 Stream Control Transmission Protocol September 2007

                       |                   | (D)rcv SHUTDOWN ACK
                       |                   |--------------
                       |                   | stop shutdown timer
                       |                   | send SHUTDOWN COMPLETE
                       |                   | delete TCB
                       |                   |
                       \    +---------+    /
                        \-->| CLOSED  |<--/
                            +---------+
              Figure 3: State Transition Diagram of SCTP
 Notes:
 1)  If the State Cookie in the received COOKIE ECHO is invalid (i.e.,
     failed to pass the integrity check), the receiver MUST silently
     discard the packet.  Or, if the received State Cookie is expired
     (see Section 5.1.5), the receiver MUST send back an ERROR chunk.
     In either case, the receiver stays in the CLOSED state.
 2)  If the T1-init timer expires, the endpoint MUST retransmit INIT
     and restart the T1-init timer without changing state.  This MUST
     be repeated up to 'Max.Init.Retransmits' times.  After that, the
     endpoint MUST abort the initialization process and report the
     error to the SCTP user.
 3)  If the T1-cookie timer expires, the endpoint MUST retransmit
     COOKIE ECHO and restart the T1-cookie timer without changing
     state.  This MUST be repeated up to 'Max.Init.Retransmits' times.
     After that, the endpoint MUST abort the initialization process
     and report the error to the SCTP user.
 4)  In the SHUTDOWN-SENT state, the endpoint MUST acknowledge any
     received DATA chunks without delay.
 5)  In the SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any
     new send requests from its SCTP user.
 6)  In the SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
     retransmit data and leave this state when all data in queue is
     transmitted.
 7)  In the SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any
     new send requests from its SCTP user.
 The CLOSED state is used to indicate that an association is not
 created (i.e., doesn't exist).

Stewart Standards Track [Page 55] RFC 4960 Stream Control Transmission Protocol September 2007

5. Association Initialization

 Before the first data transmission can take place from one SCTP
 endpoint ("A") to another SCTP endpoint ("Z"), the two endpoints must
 complete an initialization process in order to set up an SCTP
 association between them.
 The SCTP user at an endpoint should use the ASSOCIATE primitive to
 initialize an SCTP association to another SCTP endpoint.
 IMPLEMENTATION NOTE: From an SCTP user's point of view, an
 association may be implicitly opened, without an ASSOCIATE primitive
 (see Section 10.1 B) being invoked, by the initiating endpoint's
 sending of the first user data to the destination endpoint.  The
 initiating SCTP will assume default values for all mandatory and
 optional parameters for the INIT/INIT ACK.
 Once the association is established, unidirectional streams are open
 for data transfer on both ends (see Section 5.1.1).

5.1. Normal Establishment of an Association

 The initialization process consists of the following steps (assuming
 that SCTP endpoint "A" tries to set up an association with SCTP
 endpoint "Z" and "Z" accepts the new association):
 A) "A" first sends an INIT chunk to "Z".  In the INIT, "A" must
    provide its Verification Tag (Tag_A) in the Initiate Tag field.
    Tag_A SHOULD be a random number in the range of 1 to 4294967295
    (see Section 5.3.1 for Tag value selection).  After sending the
    INIT, "A" starts the T1-init timer and enters the COOKIE-WAIT
    state.
 B) "Z" shall respond immediately with an INIT ACK chunk.  The
    destination IP address of the INIT ACK MUST be set to the source
    IP address of the INIT to which this INIT ACK is responding.  In
    the response, besides filling in other parameters, "Z" must set
    the Verification Tag field to Tag_A, and also provide its own
    Verification Tag (Tag_Z) in the Initiate Tag field.
    Moreover, "Z" MUST generate and send along with the INIT ACK a
    State Cookie.  See Section 5.1.3 for State Cookie generation.
    Note: After sending out INIT ACK with the State Cookie parameter,
    "Z" MUST NOT allocate any resources or keep any states for the new
    association.  Otherwise, "Z" will be vulnerable to resource
    attacks.

Stewart Standards Track [Page 56] RFC 4960 Stream Control Transmission Protocol September 2007

 C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-
    init timer and leave the COOKIE-WAIT state.  "A" shall then send
    the State Cookie received in the INIT ACK chunk in a COOKIE ECHO
    chunk, start the T1-cookie timer, and enter the COOKIE-ECHOED
    state.
    Note: The COOKIE ECHO chunk can be bundled with any pending
    outbound DATA chunks, but it MUST be the first chunk in the packet
    and until the COOKIE ACK is returned the sender MUST NOT send any
    other packets to the peer.
 D) Upon reception of the COOKIE ECHO chunk, endpoint "Z" will reply
    with a COOKIE ACK chunk after building a TCB and moving to the
    ESTABLISHED state.  A COOKIE ACK chunk may be bundled with any
    pending DATA chunks (and/or SACK chunks), but the COOKIE ACK chunk
    MUST be the first chunk in the packet.
    IMPLEMENTATION NOTE: An implementation may choose to send the
    Communication Up notification to the SCTP user upon reception of a
    valid COOKIE ECHO chunk.
 E) Upon reception of the COOKIE ACK, endpoint "A" will move from the
    COOKIE-ECHOED state to the ESTABLISHED state, stopping the T1-
    cookie timer.  It may also notify its ULP about the successful
    establishment of the association with a Communication Up
    notification (see Section 10).
 An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk.
 They MUST be the only chunks present in the SCTP packets that carry
 them.
 An endpoint MUST send the INIT ACK to the IP address from which it
 received the INIT.
 Note: T1-init timer and T1-cookie timer shall follow the same rules
 given in Section 6.3.
 If an endpoint receives an INIT, INIT ACK, or COOKIE ECHO chunk but
 decides not to establish the new association due to missing mandatory
 parameters in the received INIT or INIT ACK, invalid parameter
 values, or lack of local resources, it SHOULD respond with an ABORT
 chunk.  It SHOULD also specify the cause of abort, such as the type
 of the missing mandatory parameters, etc., by including the error
 cause parameters with the ABORT chunk.  The Verification Tag field in
 the common header of the outbound SCTP packet containing the ABORT
 chunk MUST be set to the Initiate Tag value of the peer.

Stewart Standards Track [Page 57] RFC 4960 Stream Control Transmission Protocol September 2007

 Note that a COOKIE ECHO chunk that does NOT pass the integrity check
 is NOT considered an 'invalid parameter' and requires special
 handling; see Section 5.1.5.
 After the reception of the first DATA chunk in an association the
 endpoint MUST immediately respond with a SACK to acknowledge the DATA
 chunk.  Subsequent acknowledgements should be done as described in
 Section 6.2.
 When the TCB is created, each endpoint MUST set its internal
 Cumulative TSN Ack Point to the value of its transmitted Initial TSN
 minus one.
 IMPLEMENTATION NOTE: The IP addresses and SCTP port are generally
 used as the key to find the TCB within an SCTP instance.

5.1.1. Handle Stream Parameters

 In the INIT and INIT ACK chunks, the sender of the chunk MUST
 indicate the number of outbound streams (OSs) it wishes to have in
 the association, as well as the maximum inbound streams (MISs) it
 will accept from the other endpoint.
 After receiving the stream configuration information from the other
 side, each endpoint MUST perform the following check: If the peer's
 MIS is less than the endpoint's OS, meaning that the peer is
 incapable of supporting all the outbound streams the endpoint wants
 to configure, the endpoint MUST use MIS outbound streams and MAY
 report any shortage to the upper layer.  The upper layer can then
 choose to abort the association if the resource shortage is
 unacceptable.
 After the association is initialized, the valid outbound stream
 identifier range for either endpoint shall be 0 to min(local OS,
 remote MIS)-1.

5.1.2. Handle Address Parameters

 During the association initialization, an endpoint shall use the
 following rules to discover and collect the destination transport
 address(es) of its peer.
 A) If there are no address parameters present in the received INIT or
    INIT ACK chunk, the endpoint shall take the source IP address from
    which the chunk arrives and record it, in combination with the
    SCTP source port number, as the only destination transport address
    for this peer.

Stewart Standards Track [Page 58] RFC 4960 Stream Control Transmission Protocol September 2007

 B) If there is a Host Name parameter present in the received INIT or
    INIT ACK chunk, the endpoint shall resolve that host name to a
    list of IP address(es) and derive the transport address(es) of
    this peer by combining the resolved IP address(es) with the SCTP
    source port.
    The endpoint MUST ignore any other IP Address parameters if they
    are also present in the received INIT or INIT ACK chunk.
    The time at which the receiver of an INIT resolves the host name
    has potential security implications to SCTP.  If the receiver of
    an INIT resolves the host name upon the reception of the chunk,
    and the mechanism the receiver uses to resolve the host name
    involves potential long delay (e.g., DNS query), the receiver may
    open itself up to resource attacks for the period of time while it
    is waiting for the name resolution results before it can build the
    State Cookie and release local resources.
    Therefore, in cases where the name translation involves potential
    long delay, the receiver of the INIT MUST postpone the name
    resolution till the reception of the COOKIE ECHO chunk from the
    peer.  In such a case, the receiver of the INIT SHOULD build the
    State Cookie using the received Host Name (instead of destination
    transport addresses) and send the INIT ACK to the source IP
    address from which the INIT was received.
    The receiver of an INIT ACK shall always immediately attempt to
    resolve the name upon the reception of the chunk.
    The receiver of the INIT or INIT ACK MUST NOT send user data
    (piggy-backed or stand-alone) to its peer until the host name is
    successfully resolved.
    If the name resolution is not successful, the endpoint MUST
    immediately send an ABORT with "Unresolvable Address" error cause
    to its peer.  The ABORT shall be sent to the source IP address
    from which the last peer packet was received.
 C) If there are only IPv4/IPv6 addresses present in the received INIT
    or INIT ACK chunk, the receiver MUST derive and record all the
    transport addresses from the received chunk AND the source IP
    address that sent the INIT or INIT ACK.  The transport addresses
    are derived by the combination of SCTP source port (from the
    common header) and the IP Address parameter(s) carried in the INIT
    or INIT ACK chunk and the source IP address of the IP datagram.
    The receiver should use only these transport addresses as
    destination transport addresses when sending subsequent packets to
    its peer.

Stewart Standards Track [Page 59] RFC 4960 Stream Control Transmission Protocol September 2007

 D) An INIT or INIT ACK chunk MUST be treated as belonging to an
    already established association (or one in the process of being
    established) if the use of any of the valid address parameters
    contained within the chunk would identify an existing TCB.
 IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
 doesn't control the source IP address that is used for transmitting),
 an endpoint might need to include in its INIT or INIT ACK all
 possible IP addresses from which packets to the peer could be
 transmitted.
 After all transport addresses are derived from the INIT or INIT ACK
 chunk using the above rules, the endpoint shall select one of the
 transport addresses as the initial primary path.
 Note: The INIT ACK MUST be sent to the source address of the INIT.
 The sender of INIT may include a 'Supported Address Types' parameter
 in the INIT to indicate what types of address are acceptable.  When
 this parameter is present, the receiver of INIT (initiate) MUST
 either use one of the address types indicated in the Supported
 Address Types parameter when responding to the INIT, or abort the
 association with an "Unresolvable Address" error cause if it is
 unwilling or incapable of using any of the address types indicated by
 its peer.
 IMPLEMENTATION NOTE: In the case that the receiver of an INIT ACK
 fails to resolve the address parameter due to an unsupported type, it
 can abort the initiation process and then attempt a reinitiation by
 using a 'Supported Address Types' parameter in the new INIT to
 indicate what types of address it prefers.
 IMPLEMENTATION NOTE: If an SCTP endpoint that only supports either
 IPv4 or IPv6 receives IPv4 and IPv6 addresses in an INIT or INIT ACK
 chunk from its peer, it MUST use all the addresses belonging to the
 supported address family.  The other addresses MAY be ignored.  The
 endpoint SHOULD NOT respond with any kind of error indication.
 IMPLEMENTATION NOTE: If an SCTP endpoint lists in the 'Supported
 Address Types' parameter either IPv4 or IPv6, but uses the other
 family for sending the packet containing the INIT chunk, or if it
 also lists addresses of the other family in the INIT chunk, then the
 address family that is not listed in the 'Supported Address Types'
 parameter SHOULD also be considered as supported by the receiver of
 the INIT chunk.  The receiver of the INIT chunk SHOULD NOT respond
 with any kind of error indication.

Stewart Standards Track [Page 60] RFC 4960 Stream Control Transmission Protocol September 2007

5.1.3. Generating State Cookie

 When sending an INIT ACK as a response to an INIT chunk, the sender
 of INIT ACK creates a State Cookie and sends it in the State Cookie
 parameter of the INIT ACK.  Inside this State Cookie, the sender
 should include a MAC (see [RFC2104] for an example), a timestamp on
 when the State Cookie is created, and the lifespan of the State
 Cookie, along with all the information necessary for it to establish
 the association.
 The following steps SHOULD be taken to generate the State Cookie:
 1)  Create an association TCB using information from both the
     received INIT and the outgoing INIT ACK chunk,
 2)  In the TCB, set the creation time to the current time of day, and
     the lifespan to the protocol parameter 'Valid.Cookie.Life' (see
     Section 15),
 3)  From the TCB, identify and collect the minimal subset of
     information needed to re-create the TCB, and generate a MAC using
     this subset of information and a secret key (see [RFC2104] for an
     example of generating a MAC), and
 4)  Generate the State Cookie by combining this subset of information
     and the resultant MAC.
 After sending the INIT ACK with the State Cookie parameter, the
 sender SHOULD delete the TCB and any other local resource related to
 the new association, so as to prevent resource attacks.
 The hashing method used to generate the MAC is strictly a private
 matter for the receiver of the INIT chunk.  The use of a MAC is
 mandatory to prevent denial-of-service attacks.  The secret key
 SHOULD be random ([RFC4086] provides some information on randomness
 guidelines); it SHOULD be changed reasonably frequently, and the
 timestamp in the State Cookie MAY be used to determine which key
 should be used to verify the MAC.
 An implementation SHOULD make the cookie as small as possible to
 ensure interoperability.

Stewart Standards Track [Page 61] RFC 4960 Stream Control Transmission Protocol September 2007

5.1.4. State Cookie Processing

 When an endpoint (in the COOKIE-WAIT state) receives an INIT ACK
 chunk with a State Cookie parameter, it MUST immediately send a
 COOKIE ECHO chunk to its peer with the received State Cookie.  The
 sender MAY also add any pending DATA chunks to the packet after the
 COOKIE ECHO chunk.
 The endpoint shall also start the T1-cookie timer after sending out
 the COOKIE ECHO chunk.  If the timer expires, the endpoint shall
 retransmit the COOKIE ECHO chunk and restart the T1-cookie timer.
 This is repeated until either a COOKIE ACK is received or
 'Max.Init.Retransmits' (see Section 15) is reached causing the peer
 endpoint to be marked unreachable (and thus the association enters
 the CLOSED state).

5.1.5. State Cookie Authentication

 When an endpoint receives a COOKIE ECHO chunk from another endpoint
 with which it has no association, it shall take the following
 actions:
 1)  Compute a MAC using the TCB data carried in the State Cookie and
     the secret key (note the timestamp in the State Cookie MAY be
     used to determine which secret key to use).  [RFC2104] can be
     used as a guideline for generating the MAC,
 2)  Authenticate the State Cookie as one that it previously generated
     by comparing the computed MAC against the one carried in the
     State Cookie.  If this comparison fails, the SCTP packet,
     including the COOKIE ECHO and any DATA chunks, should be silently
     discarded,
 3)  Compare the port numbers and the Verification Tag contained
     within the COOKIE ECHO chunk to the actual port numbers and the
     Verification Tag within the SCTP common header of the received
     packet.  If these values do not match, the packet MUST be
     silently discarded.
 4)  Compare the creation timestamp in the State Cookie to the current
     local time.  If the elapsed time is longer than the lifespan
     carried in the State Cookie, then the packet, including the
     COOKIE ECHO and any attached DATA chunks, SHOULD be discarded,
     and the endpoint MUST transmit an ERROR chunk with a "Stale
     Cookie" error cause to the peer endpoint.

Stewart Standards Track [Page 62] RFC 4960 Stream Control Transmission Protocol September 2007

 5)  If the State Cookie is valid, create an association to the sender
     of the COOKIE ECHO chunk with the information in the TCB data
     carried in the COOKIE ECHO and enter the ESTABLISHED state.
 6)  Send a COOKIE ACK chunk to the peer acknowledging receipt of the
     COOKIE ECHO.  The COOKIE ACK MAY be bundled with an outbound DATA
     chunk or SACK chunk; however, the COOKIE ACK MUST be the first
     chunk in the SCTP packet.
 7)  Immediately acknowledge any DATA chunk bundled with the COOKIE
     ECHO with a SACK (subsequent DATA chunk acknowledgement should
     follow the rules defined in Section 6.2).  As mentioned in step
     6, if the SACK is bundled with the COOKIE ACK, the COOKIE ACK
     MUST appear first in the SCTP packet.
 If a COOKIE ECHO is received from an endpoint with which the receiver
 of the COOKIE ECHO has an existing association, the procedures in
 Section 5.2 should be followed.

Stewart Standards Track [Page 63] RFC 4960 Stream Control Transmission Protocol September 2007

5.1.6. An Example of Normal Association Establishment

 In the following example, "A" initiates the association and then
 sends a user message to "Z", then "Z" sends two user messages to "A"
 later (assuming no bundling or fragmentation occurs):
  Endpoint A                                          Endpoint Z
  {app sets association with Z}
  (build TCB)
  INIT [I-Tag=Tag_A
        & other info]  ------\
  (Start T1-init timer)       \
  (Enter COOKIE-WAIT state)    \---> (compose temp TCB and Cookie_Z)
                                  /-- INIT ACK [Veri Tag=Tag_A,
                                 /             I-Tag=Tag_Z,
  (Cancel T1-init timer) <------/              Cookie_Z, & other info]
                                       (destroy temp TCB)
  COOKIE ECHO [Cookie_Z] ------\
  (Start T1-init timer)         \
  (Enter COOKIE-ECHOED state)    \---> (build TCB enter ESTABLISHED
                                        state)
                                 /---- COOKIE-ACK
                                /
  (Cancel T1-init timer, <-----/
   Enter ESTABLISHED state)
  {app sends 1st user data; strm 0}
  DATA [TSN=initial TSN_A
      Strm=0,Seq=0 & user data]--\
  (Start T3-rtx timer)            \
                                   \->
                                 /----- SACK [TSN Ack=init
                                /           TSN_A,Block=0]
  (Cancel T3-rtx timer) <------/
                                        ...
                                       {app sends 2 messages;strm 0}
                                 /---- DATA
                                /        [TSN=init TSN_Z
                            <--/          Strm=0,Seq=0 & user data 1]
  SACK [TSN Ack=init TSN_Z,      /---- DATA
        Block=0]     --------\  /        [TSN=init TSN_Z +1,
                              \/          Strm=0,Seq=1 & user data 2]
                       <------/\
                                \
                                 \------>
                      Figure 4: INITIATION Example

Stewart Standards Track [Page 64] RFC 4960 Stream Control Transmission Protocol September 2007

 If the T1-init timer expires at "A" after the INIT or COOKIE ECHO
 chunks are sent, the same INIT or COOKIE ECHO chunk with the same
 Initiate Tag (i.e., Tag_A) or State Cookie shall be retransmitted and
 the timer restarted.  This shall be repeated Max.Init.Retransmits
 times before "A" considers "Z" unreachable and reports the failure to
 its upper layer (and thus the association enters the CLOSED state).
 When retransmitting the INIT, the endpoint MUST follow the rules
 defined in Section 6.3 to determine the proper timer value.

5.2. Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE ECHO, and

    COOKIE ACK
 During the life time of an association (in one of the possible
 states), an endpoint may receive from its peer endpoint one of the
 setup chunks (INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK).  The
 receiver shall treat such a setup chunk as a duplicate and process it
 as described in this section.
 Note: An endpoint will not receive the chunk unless the chunk was
 sent to an SCTP transport address and is from an SCTP transport
 address associated with this endpoint.  Therefore, the endpoint
 processes such a chunk as part of its current association.
 The following scenarios can cause duplicated or unexpected chunks:
 A) The peer has crashed without being detected, restarted itself, and
    sent out a new INIT chunk trying to restore the association,
 B) Both sides are trying to initialize the association at about the
    same time,
 C) The chunk is from a stale packet that was used to establish the
    present association or a past association that is no longer in
    existence,
 D) The chunk is a false packet generated by an attacker, or
 E) The peer never received the COOKIE ACK and is retransmitting its
    COOKIE ECHO.
 The rules in the following sections shall be applied in order to
 identify and correctly handle these cases.

Stewart Standards Track [Page 65] RFC 4960 Stream Control Transmission Protocol September 2007

5.2.1. INIT Received in COOKIE-WAIT or COOKIE-ECHOED State (Item B)

 This usually indicates an initialization collision, i.e., each
 endpoint is attempting, at about the same time, to establish an
 association with the other endpoint.
 Upon receipt of an INIT in the COOKIE-WAIT state, an endpoint MUST
 respond with an INIT ACK using the same parameters it sent in its
 original INIT chunk (including its Initiate Tag, unchanged).  When
 responding, the endpoint MUST send the INIT ACK back to the same
 address that the original INIT (sent by this endpoint) was sent.
 Upon receipt of an INIT in the COOKIE-ECHOED state, an endpoint MUST
 respond with an INIT ACK using the same parameters it sent in its
 original INIT chunk (including its Initiate Tag, unchanged), provided
 that no NEW address has been added to the forming association.  If
 the INIT message indicates that a new address has been added to the
 association, then the entire INIT MUST be discarded, and NO changes
 should be made to the existing association.  An ABORT SHOULD be sent
 in response that MAY include the error 'Restart of an association
 with new addresses'.  The error SHOULD list the addresses that were
 added to the restarting association.
 When responding in either state (COOKIE-WAIT or COOKIE-ECHOED) with
 an INIT ACK, the original parameters are combined with those from the
 newly received INIT chunk.  The endpoint shall also generate a State
 Cookie with the INIT ACK.  The endpoint uses the parameters sent in
 its INIT to calculate the State Cookie.
 After that, the endpoint MUST NOT change its state, the T1-init timer
 shall be left running, and the corresponding TCB MUST NOT be
 destroyed.  The normal procedures for handling State Cookies when a
 TCB exists will resolve the duplicate INITs to a single association.
 For an endpoint that is in the COOKIE-ECHOED state, it MUST populate
 its Tie-Tags within both the association TCB and inside the State
 Cookie (see Section 5.2.2 for a description of the Tie-Tags).

5.2.2. Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED,

      COOKIE-WAIT, and SHUTDOWN-ACK-SENT
 Unless otherwise stated, upon receipt of an unexpected INIT for this
 association, the endpoint shall generate an INIT ACK with a State
 Cookie.  Before responding, the endpoint MUST check to see if the
 unexpected INIT adds new addresses to the association.  If new
 addresses are added to the association, the endpoint MUST respond
 with an ABORT, copying the 'Initiate Tag' of the unexpected INIT into
 the 'Verification Tag' of the outbound packet carrying the ABORT.  In

Stewart Standards Track [Page 66] RFC 4960 Stream Control Transmission Protocol September 2007

 the ABORT response, the cause of error MAY be set to 'restart of an
 association with new addresses'.  The error SHOULD list the addresses
 that were added to the restarting association.  If no new addresses
 are added, when responding to the INIT in the outbound INIT ACK, the
 endpoint MUST copy its current Tie-Tags to a reserved place within
 the State Cookie and the association's TCB.  We shall refer to these
 locations inside the cookie as the Peer's-Tie-Tag and the Local-Tie-
 Tag.  We will refer to the copy within an association's TCB as the
 Local Tag and Peer's Tag.  The outbound SCTP packet containing this
 INIT ACK MUST carry a Verification Tag value equal to the Initiate
 Tag found in the unexpected INIT.  And the INIT ACK MUST contain a
 new Initiate Tag (randomly generated; see Section 5.3.1).  Other
 parameters for the endpoint SHOULD be copied from the existing
 parameters of the association (e.g., number of outbound streams) into
 the INIT ACK and cookie.
 After sending out the INIT ACK or ABORT, the endpoint shall take no
 further actions; i.e., the existing association, including its
 current state, and the corresponding TCB MUST NOT be changed.
 Note: Only when a TCB exists and the association is not in a COOKIE-
 WAIT or SHUTDOWN-ACK-SENT state are the Tie-Tags populated with a
 value other than 0.  For a normal association INIT (i.e., the
 endpoint is in the CLOSED state), the Tie-Tags MUST be set to 0
 (indicating that no previous TCB existed).

5.2.3. Unexpected INIT ACK

 If an INIT ACK is received by an endpoint in any state other than the
 COOKIE-WAIT state, the endpoint should discard the INIT ACK chunk.
 An unexpected INIT ACK usually indicates the processing of an old or
 duplicated INIT chunk.

5.2.4. Handle a COOKIE ECHO when a TCB Exists

 When a COOKIE ECHO chunk is received by an endpoint in any state for
 an existing association (i.e., not in the CLOSED state) the following
 rules shall be applied:
 1)  Compute a MAC as described in step 1 of Section 5.1.5,
 2)  Authenticate the State Cookie as described in step 2 of Section
     5.1.5 (this is case C or D above).
 3)  Compare the timestamp in the State Cookie to the current time.
     If the State Cookie is older than the lifespan carried in the
     State Cookie and the Verification Tags contained in the State
     Cookie do not match the current association's Verification Tags,

Stewart Standards Track [Page 67] RFC 4960 Stream Control Transmission Protocol September 2007

     the packet, including the COOKIE ECHO and any DATA chunks, should
     be discarded.  The endpoint also MUST transmit an ERROR chunk
     with a "Stale Cookie" error cause to the peer endpoint (this is
     case C or D in Section 5.2).
     If both Verification Tags in the State Cookie match the
     Verification Tags of the current association, consider the State
     Cookie valid (this is case E in Section 5.2) even if the lifespan
     is exceeded.
 4)  If the State Cookie proves to be valid, unpack the TCB into a
     temporary TCB.
 5)  Refer to Table 2 to determine the correct action to be taken.

+————+————+—————+————–+————-+

Local Tag Peer's Tag Local-Tie-Tag Peer's-Tie-Tag Action/
Description

+————+————+—————+————–+————-+

X X M M (A)

+————+————+—————+————–+————-+

M X A A (B)

+————+————+—————+————–+————-+

M 0 A A (B)

+————+————+—————+————–+————-+

X M 0 0 (C)

+————+————+—————+————–+————-+

M M A A (D)

+======================================================================+

Table 2: Handling of a COOKIE ECHO when a TCB Exists

+======================================================================+

 Legend:
    X - Tag does not match the existing TCB.
    M - Tag matches the existing TCB.
    0 - No Tie-Tag in cookie (unknown).
    A - All cases, i.e., M, X, or 0.
 Note: For any case not shown in Table 2, the cookie should be
 silently discarded.
 Action
 A) In this case, the peer may have restarted.  When the endpoint
    recognizes this potential 'restart', the existing session is
    treated the same as if it received an ABORT followed by a new
    COOKIE ECHO with the following exceptions:

Stewart Standards Track [Page 68] RFC 4960 Stream Control Transmission Protocol September 2007

  1. Any SCTP DATA chunks MAY be retained (this is an

implementation-specific option).

  1. A notification of RESTART SHOULD be sent to the ULP instead of

a "COMMUNICATION LOST" notification.

    All the congestion control parameters (e.g., cwnd, ssthresh)
    related to this peer MUST be reset to their initial values (see
    Section 6.2.1).
    After this, the endpoint shall enter the ESTABLISHED state.
    If the endpoint is in the SHUTDOWN-ACK-SENT state and recognizes
    that the peer has restarted (Action A), it MUST NOT set up a new
    association but instead resend the SHUTDOWN ACK and send an ERROR
    chunk with a "Cookie Received While Shutting Down" error cause to
    its peer.
 B) In this case, both sides may be attempting to start an association
    at about the same time, but the peer endpoint started its INIT
    after responding to the local endpoint's INIT.  Thus, it may have
    picked a new Verification Tag, not being aware of the previous tag
    it had sent this endpoint.  The endpoint should stay in or enter
    the ESTABLISHED state, but it MUST update its peer's Verification
    Tag from the State Cookie, stop any init or cookie timers that may
    be running, and send a COOKIE ACK.
 C) In this case, the local endpoint's cookie has arrived late.
    Before it arrived, the local endpoint sent an INIT and received an
    INIT ACK and finally sent a COOKIE ECHO with the peer's same tag
    but a new tag of its own.  The cookie should be silently
    discarded.  The endpoint SHOULD NOT change states and should leave
    any timers running.
 D) When both local and remote tags match, the endpoint should enter
    the ESTABLISHED state, if it is in the COOKIE-ECHOED state.  It
    should stop any cookie timer that may be running and send a COOKIE
    ACK.
 Note: The "peer's Verification Tag" is the tag received in the
 Initiate Tag field of the INIT or INIT ACK chunk.

5.2.4.1. An Example of a Association Restart

 In the following example, "A" initiates the association after a
 restart has occurred.  Endpoint "Z" had no knowledge of the restart
 until the exchange (i.e., Heartbeats had not yet detected the failure
 of "A") (assuming no bundling or fragmentation occurs):

Stewart Standards Track [Page 69] RFC 4960 Stream Control Transmission Protocol September 2007

 Endpoint A                                          Endpoint Z
 <-------------- Association is established---------------------->
 Tag=Tag_A                                             Tag=Tag_Z
 <--------------------------------------------------------------->
 {A crashes and restarts}
 {app sets up a association with Z}
 (build TCB)
 INIT [I-Tag=Tag_A'
       & other info]  --------\
 (Start T1-init timer)         \
 (Enter COOKIE-WAIT state)      \---> (find an existing TCB
                                       compose temp TCB and Cookie_Z
                                       with Tie-Tags to previous
                                       association)
                                 /--- INIT ACK [Veri Tag=Tag_A',
                                /               I-Tag=Tag_Z',
 (Cancel T1-init timer) <------/                Cookie_Z[TieTags=
                                                Tag_A,Tag_Z
                                                 & other info]
                                      (destroy temp TCB,leave original
                                       in place)
 COOKIE ECHO [Veri=Tag_Z',
              Cookie_Z
              Tie=Tag_A,
              Tag_Z]----------\
 (Start T1-init timer)         \
 (Enter COOKIE-ECHOED state)    \---> (Find existing association,
                                       Tie-Tags match old tags,
                                       Tags do not match, i.e.,
                                       case X X M M above,
                                       Announce Restart to ULP
                                       and reset association).
                                /---- COOKIE ACK
 (Cancel T1-init timer, <------/
  Enter ESTABLISHED state)
 {app sends 1st user data; strm 0}
 DATA [TSN=initial TSN_A
     Strm=0,Seq=0 & user data]--\
 (Start T3-rtx timer)            \
                                  \->
                               /--- SACK [TSN Ack=init TSN_A,Block=0]
 (Cancel T3-rtx timer) <------/
                      Figure 5: A Restart Example

Stewart Standards Track [Page 70] RFC 4960 Stream Control Transmission Protocol September 2007

5.2.5. Handle Duplicate COOKIE-ACK.

 At any state other than COOKIE-ECHOED, an endpoint should silently
 discard a received COOKIE ACK chunk.

5.2.6. Handle Stale COOKIE Error

 Receipt of an ERROR chunk with a "Stale Cookie" error cause indicates
 one of a number of possible events:
 A) The association failed to completely setup before the State Cookie
    issued by the sender was processed.
 B) An old State Cookie was processed after setup completed.
 C) An old State Cookie is received from someone that the receiver is
    not interested in having an association with and the ABORT chunk
    was lost.
 When processing an ERROR chunk with a "Stale Cookie" error cause an
 endpoint should first examine if an association is in the process of
 being set up, i.e., the association is in the COOKIE-ECHOED state.
 In all cases, if the association is not in the COOKIE-ECHOED state,
 the ERROR chunk should be silently discarded.
 If the association is in the COOKIE-ECHOED state, the endpoint may
 elect one of the following three alternatives.
 1)  Send a new INIT chunk to the endpoint to generate a new State
     Cookie and reattempt the setup procedure.
 2)  Discard the TCB and report to the upper layer the inability to
     set up the association.
 3)  Send a new INIT chunk to the endpoint, adding a Cookie
     Preservative parameter requesting an extension to the life time
     of the State Cookie.  When calculating the time extension, an
     implementation SHOULD use the RTT information measured based on
     the previous COOKIE ECHO / ERROR exchange, and should add no more
     than 1 second beyond the measured RTT, due to long State Cookie
     life times making the endpoint more subject to a replay attack.

Stewart Standards Track [Page 71] RFC 4960 Stream Control Transmission Protocol September 2007

5.3. Other Initialization Issues

5.3.1. Selection of Tag Value

 Initiate Tag values should be selected from the range of 1 to 2**32 -
 1.  It is very important that the Initiate Tag value be randomized to
 help protect against "man in the middle" and "sequence number"
 attacks.  The methods described in [RFC4086] can be used for the
 Initiate Tag randomization.  Careful selection of Initiate Tags is
 also necessary to prevent old duplicate packets from previous
 associations being mistakenly processed as belonging to the current
 association.
 Moreover, the Verification Tag value used by either endpoint in a
 given association MUST NOT change during the life time of an
 association.  A new Verification Tag value MUST be used each time the
 endpoint tears down and then reestablishes an association to the same
 peer.

5.4. Path Verification

 During association establishment, the two peers exchange a list of
 addresses.  In the predominant case, these lists accurately represent
 the addresses owned by each peer.  However, it is possible that a
 misbehaving peer may supply addresses that it does not own.  To
 prevent this, the following rules are applied to all addresses of the
 new association:
 1)  Any address passed to the sender of the INIT by its upper layer
    is automatically considered to be CONFIRMED.
 2)  For the receiver of the COOKIE ECHO, the only CONFIRMED address
    is the one to which the INIT-ACK was sent.
 3)  All other addresses not covered by rules 1 and 2 are considered
    UNCONFIRMED and are subject to probing for verification.
 To probe an address for verification, an endpoint will send
 HEARTBEATs including a 64-bit random nonce and a path indicator (to
 identify the address that the HEARTBEAT is sent to) within the
 HEARTBEAT parameter.
 Upon receipt of the HEARTBEAT ACK, a verification is made that the
 nonce included in the HEARTBEAT parameter is the one sent to the
 address indicated inside the HEARTBEAT parameter.  When this match
 occurs, the address that the original HEARTBEAT was sent to is now
 considered CONFIRMED and available for normal data transfer.

Stewart Standards Track [Page 72] RFC 4960 Stream Control Transmission Protocol September 2007

 These probing procedures are started when an association moves to the
 ESTABLISHED state and are ended when all paths are confirmed.
 In each RTO, a probe may be sent on an active UNCONFIRMED path in an
 attempt to move it to the CONFIRMED state.  If during this probing
 the path becomes inactive, this rate is lowered to the normal
 HEARTBEAT rate.  At the expiration of the RTO timer, the error
 counter of any path that was probed but not CONFIRMED is incremented
 by one and subjected to path failure detection, as defined in Section
 8.2.  When probing UNCONFIRMED addresses, however, the association
 overall error count is NOT incremented.
 The number of HEARTBEATS sent at each RTO SHOULD be limited by the
 HB.Max.Burst parameter.  It is an implementation decision as to how
 to distribute HEARTBEATS to the peer's addresses for path
 verification.
 Whenever a path is confirmed, an indication MAY be given to the upper
 layer.
 An endpoint MUST NOT send any chunks to an UNCONFIRMED address, with
 the following exceptions:
  1. A HEARTBEAT including a nonce MAY be sent to an UNCONFIRMED

address.

  1. A HEARTBEAT ACK MAY be sent to an UNCONFIRMED address.
  1. A COOKIE ACK MAY be sent to an UNCONFIRMED address, but it MUST be

bundled with a HEARTBEAT including a nonce. An implementation

    that does NOT support bundling MUST NOT send a COOKIE ACK to an
    UNCONFIRMED address.
  1. A COOKIE ECHO MAY be sent to an UNCONFIRMED address, but it MUST

be bundled with a HEARTBEAT including a nonce, and the packet MUST

    NOT exceed the path MTU.  If the implementation does NOT support
    bundling or if the bundled COOKIE ECHO plus HEARTBEAT (including
    nonce) would exceed the path MTU, then the implementation MUST NOT
    send a COOKIE ECHO to an UNCONFIRMED address.

6. User Data Transfer

 Data transmission MUST only happen in the ESTABLISHED, SHUTDOWN-
 PENDING, and SHUTDOWN-RECEIVED states.  The only exception to this is
 that DATA chunks are allowed to be bundled with an outbound COOKIE
 ECHO chunk when in the COOKIE-WAIT state.

Stewart Standards Track [Page 73] RFC 4960 Stream Control Transmission Protocol September 2007

 DATA chunks MUST only be received according to the rules below in
 ESTABLISHED, SHUTDOWN-PENDING, and SHUTDOWN-SENT.  A DATA chunk
 received in CLOSED is out of the blue and SHOULD be handled per
 Section 8.4.  A DATA chunk received in any other state SHOULD be
 discarded.
 A SACK MUST be processed in ESTABLISHED, SHUTDOWN-PENDING, and
 SHUTDOWN-RECEIVED.  An incoming SACK MAY be processed in COOKIE-
 ECHOED.  A SACK in the CLOSED state is out of the blue and SHOULD be
 processed according to the rules in Section 8.4.  A SACK chunk
 received in any other state SHOULD be discarded.
 An SCTP receiver MUST be able to receive a minimum of 1500 bytes in
 one SCTP packet.  This means that an SCTP endpoint MUST NOT indicate
 less than 1500 bytes in its initial a_rwnd sent in the INIT or INIT
 ACK.
 For transmission efficiency, SCTP defines mechanisms for bundling of
 small user messages and fragmentation of large user messages.  The
 following diagram depicts the flow of user messages through SCTP.
 In this section, the term "data sender" refers to the endpoint that
 transmits a DATA chunk and the term "data receiver" refers to the
 endpoint that receives a DATA chunk.  A data receiver will transmit
 SACK chunks.

Stewart Standards Track [Page 74] RFC 4960 Stream Control Transmission Protocol September 2007

               +--------------------------+
               |      User Messages       |
               +--------------------------+
     SCTP user        ^  |
    ==================|==|=======================================
                      |  v (1)
           +------------------+    +--------------------+
           | SCTP DATA Chunks |    |SCTP Control Chunks |
           +------------------+    +--------------------+
                      ^  |             ^  |
                      |  v (2)         |  v (2)
                   +--------------------------+
                   |      SCTP packets        |
                   +--------------------------+
     SCTP                      ^  |
    ===========================|==|===========================
                               |  v
           Connectionless Packet Transfer Service (e.g., IP)
 Notes:
    1) When converting user messages into DATA chunks, an endpoint
       will fragment user messages larger than the current association
       path MTU into multiple DATA chunks.  The data receiver will
       normally reassemble the fragmented message from DATA chunks
       before delivery to the user (see Section 6.9 for details).
    2) Multiple DATA and control chunks may be bundled by the sender
       into a single SCTP packet for transmission, as long as the
       final size of the packet does not exceed the current path MTU.
       The receiver will unbundle the packet back into the original
       chunks.  Control chunks MUST come before DATA chunks in the
       packet.
              Figure 6: Illustration of User Data Transfer
 The fragmentation and bundling mechanisms, as detailed in Section 6.9
 and Section 6.10, are OPTIONAL to implement by the data sender, but
 they MUST be implemented by the data receiver, i.e., an endpoint MUST
 properly receive and process bundled or fragmented data.

6.1. Transmission of DATA Chunks

 This document is specified as if there is a single retransmission
 timer per destination transport address, but implementations MAY have
 a retransmission timer for each DATA chunk.

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 The following general rules MUST be applied by the data sender for
 transmission and/or retransmission of outbound DATA chunks:
 A) At any given time, the data sender MUST NOT transmit new data to
    any destination transport address if its peer's rwnd indicates
    that the peer has no buffer space (i.e., rwnd is 0; see Section
    6.2.1).  However, regardless of the value of rwnd (including if it
    is 0), the data sender can always have one DATA chunk in flight to
    the receiver if allowed by cwnd (see rule B, below).  This rule
    allows the sender to probe for a change in rwnd that the sender
    missed due to the SACK's having been lost in transit from the data
    receiver to the data sender.
    When the receiver's advertised window is zero, this probe is
    called a zero window probe.  Note that a zero window probe SHOULD
    only be sent when all outstanding DATA chunks have been
    cumulatively acknowledged and no DATA chunks are in flight.  Zero
    window probing MUST be supported.
    If the sender continues to receive new packets from the receiver
    while doing zero window probing, the unacknowledged window probes
    should not increment the error counter for the association or any
    destination transport address.  This is because the receiver MAY
    keep its window closed for an indefinite time.  Refer to Section
    6.2 on the receiver behavior when it advertises a zero window.
    The sender SHOULD send the first zero window probe after 1 RTO
    when it detects that the receiver has closed its window and SHOULD
    increase the probe interval exponentially afterwards.  Also note
    that the cwnd SHOULD be adjusted according to Section 7.2.1.  Zero
    window probing does not affect the calculation of cwnd.
    The sender MUST also have an algorithm for sending new DATA chunks
    to avoid silly window syndrome (SWS) as described in [RFC0813].
    The algorithm can be similar to the one described in Section
    4.2.3.4 of [RFC1122].
    However, regardless of the value of rwnd (including if it is 0),
    the data sender can always have one DATA chunk in flight to the
    receiver if allowed by cwnd (see rule B below).  This rule allows
    the sender to probe for a change in rwnd that the sender missed
    due to the SACK having been lost in transit from the data receiver
    to the data sender.
 B) At any given time, the sender MUST NOT transmit new data to a
    given transport address if it has cwnd or more bytes of data
    outstanding to that transport address.

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 C) When the time comes for the sender to transmit, before sending new
    DATA chunks, the sender MUST first transmit any outstanding DATA
    chunks that are marked for retransmission (limited by the current
    cwnd).
 D) When the time comes for the sender to transmit new DATA chunks,
    the protocol parameter Max.Burst SHOULD be used to limit the
    number of packets sent.  The limit MAY be applied by adjusting
    cwnd as follows:
    if((flightsize + Max.Burst*MTU) < cwnd) cwnd = flightsize +
    Max.Burst*MTU
    Or it MAY be applied by strictly limiting the number of packets
    emitted by the output routine.
 E) Then, the sender can send out as many new DATA chunks as rule A
    and rule B allow.
 Multiple DATA chunks committed for transmission MAY be bundled in a
 single packet.  Furthermore, DATA chunks being retransmitted MAY be
 bundled with new DATA chunks, as long as the resulting packet size
 does not exceed the path MTU.  A ULP may request that no bundling is
 performed, but this should only turn off any delays that an SCTP
 implementation may be using to increase bundling efficiency.  It does
 not in itself stop all bundling from occurring (i.e., in case of
 congestion or retransmission).
 Before an endpoint transmits a DATA chunk, if any received DATA
 chunks have not been acknowledged (e.g., due to delayed ack), the
 sender should create a SACK and bundle it with the outbound DATA
 chunk, as long as the size of the final SCTP packet does not exceed
 the current MTU.  See Section 6.2.
 IMPLEMENTATION NOTE: When the window is full (i.e., transmission is
 disallowed by rule A and/or rule B), the sender MAY still accept send
 requests from its upper layer, but MUST transmit no more DATA chunks
 until some or all of the outstanding DATA chunks are acknowledged and
 transmission is allowed by rule A and rule B again.
 Whenever a transmission or retransmission is made to any address, if
 the T3-rtx timer of that address is not currently running, the sender
 MUST start that timer.  If the timer for that address is already
 running, the sender MUST restart the timer if the earliest (i.e.,
 lowest TSN) outstanding DATA chunk sent to that address is being
 retransmitted.  Otherwise, the data sender MUST NOT restart the
 timer.

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 When starting or restarting the T3-rtx timer, the timer value must be
 adjusted according to the timer rules defined in Sections 6.3.2 and
 6.3.3.
 Note: The data sender SHOULD NOT use a TSN that is more than 2**31 -
 1 above the beginning TSN of the current send window.

6.2. Acknowledgement on Reception of DATA Chunks

 The SCTP endpoint MUST always acknowledge the reception of each valid
 DATA chunk when the DATA chunk received is inside its receive window.
 When the receiver's advertised window is 0, the receiver MUST drop
 any new incoming DATA chunk with a TSN larger than the largest TSN
 received so far.  If the new incoming DATA chunk holds a TSN value
 less than the largest TSN received so far, then the receiver SHOULD
 drop the largest TSN held for reordering and accept the new incoming
 DATA chunk.  In either case, if such a DATA chunk is dropped, the
 receiver MUST immediately send back a SACK with the current receive
 window showing only DATA chunks received and accepted so far.  The
 dropped DATA chunk(s) MUST NOT be included in the SACK, as they were
 not accepted.  The receiver MUST also have an algorithm for
 advertising its receive window to avoid receiver silly window
 syndrome (SWS), as described in [RFC0813].  The algorithm can be
 similar to the one described in Section 4.2.3.3 of [RFC1122].
 The guidelines on delayed acknowledgement algorithm specified in
 Section 4.2 of [RFC2581] SHOULD be followed.  Specifically, an
 acknowledgement SHOULD be generated for at least every second packet
 (not every second DATA chunk) received, and SHOULD be generated
 within 200 ms of the arrival of any unacknowledged DATA chunk.  In
 some situations, it may be beneficial for an SCTP transmitter to be
 more conservative than the algorithms detailed in this document
 allow.  However, an SCTP transmitter MUST NOT be more aggressive than
 the following algorithms allow.
 An SCTP receiver MUST NOT generate more than one SACK for every
 incoming packet, other than to update the offered window as the
 receiving application consumes new data.
 IMPLEMENTATION NOTE: The maximum delay for generating an
 acknowledgement may be configured by the SCTP administrator, either
 statically or dynamically, in order to meet the specific timing
 requirement of the protocol being carried.
 An implementation MUST NOT allow the maximum delay to be configured
 to be more than 500 ms.  In other words, an implementation MAY lower
 this value below 500 ms but MUST NOT raise it above 500 ms.

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 Acknowledgements MUST be sent in SACK chunks unless shutdown was
 requested by the ULP, in which case an endpoint MAY send an
 acknowledgement in the SHUTDOWN chunk.  A SACK chunk can acknowledge
 the reception of multiple DATA chunks.  See Section 3.3.4 for SACK
 chunk format.  In particular, the SCTP endpoint MUST fill in the
 Cumulative TSN Ack field to indicate the latest sequential TSN (of a
 valid DATA chunk) it has received.  Any received DATA chunks with TSN
 greater than the value in the Cumulative TSN Ack field are reported
 in the Gap Ack Block fields.  The SCTP endpoint MUST report as many
 Gap Ack Blocks as can fit in a single SACK chunk limited by the
 current path MTU.
 Note: The SHUTDOWN chunk does not contain Gap Ack Block fields.
 Therefore, the endpoint should use a SACK instead of the SHUTDOWN
 chunk to acknowledge DATA chunks received out of order.
 When a packet arrives with duplicate DATA chunk(s) and with no new
 DATA chunk(s), the endpoint MUST immediately send a SACK with no
 delay.  If a packet arrives with duplicate DATA chunk(s) bundled with
 new DATA chunks, the endpoint MAY immediately send a SACK.  Normally,
 receipt of duplicate DATA chunks will occur when the original SACK
 chunk was lost and the peer's RTO has expired.  The duplicate TSN
 number(s) SHOULD be reported in the SACK as duplicate.
 When an endpoint receives a SACK, it MAY use the duplicate TSN
 information to determine if SACK loss is occurring.  Further use of
 this data is for future study.
 The data receiver is responsible for maintaining its receive buffers.
 The data receiver SHOULD notify the data sender in a timely manner of
 changes in its ability to receive data.  How an implementation
 manages its receive buffers is dependent on many factors (e.g.,
 operating system, memory management system, amount of memory, etc.).
 However, the data sender strategy defined in Section 6.2.1 is based
 on the assumption of receiver operation similar to the following:
 A) At initialization of the association, the endpoint tells the peer
    how much receive buffer space it has allocated to the association
    in the INIT or INIT ACK.  The endpoint sets a_rwnd to this value.
 B) As DATA chunks are received and buffered, decrement a_rwnd by the
    number of bytes received and buffered.  This is, in effect,
    closing rwnd at the data sender and restricting the amount of data
    it can transmit.

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 C) As DATA chunks are delivered to the ULP and released from the
    receive buffers, increment a_rwnd by the number of bytes delivered
    to the upper layer.  This is, in effect, opening up rwnd on the
    data sender and allowing it to send more data.  The data receiver
    SHOULD NOT increment a_rwnd unless it has released bytes from its
    receive buffer.  For example, if the receiver is holding
    fragmented DATA chunks in a reassembly queue, it should not
    increment a_rwnd.
 D) When sending a SACK, the data receiver SHOULD place the current
    value of a_rwnd into the a_rwnd field.  The data receiver SHOULD
    take into account that the data sender will not retransmit DATA
    chunks that are acked via the Cumulative TSN Ack (i.e., will drop
    from its retransmit queue).
 Under certain circumstances, the data receiver may need to drop DATA
 chunks that it has received but hasn't released from its receive
 buffers (i.e., delivered to the ULP).  These DATA chunks may have
 been acked in Gap Ack Blocks.  For example, the data receiver may be
 holding data in its receive buffers while reassembling a fragmented
 user message from its peer when it runs out of receive buffer space.
 It may drop these DATA chunks even though it has acknowledged them in
 Gap Ack Blocks.  If a data receiver drops DATA chunks, it MUST NOT
 include them in Gap Ack Blocks in subsequent SACKs until they are
 received again via retransmission.  In addition, the endpoint should
 take into account the dropped data when calculating its a_rwnd.
 An endpoint SHOULD NOT revoke a SACK and discard data.  Only in
 extreme circumstances should an endpoint use this procedure (such as
 out of buffer space).  The data receiver should take into account
 that dropping data that has been acked in Gap Ack Blocks can result
 in suboptimal retransmission strategies in the data sender and thus
 in suboptimal performance.

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 The following example illustrates the use of delayed
 acknowledgements:
  Endpoint A                                      Endpoint Z
  {App sends 3 messages; strm 0}
  DATA [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
  (Start T3-rtx timer)
  DATA [TSN=8,Strm=0,Seq=4] ------------> (send ack)
                                /------- SACK [TSN Ack=8,block=0]
  (cancel T3-rtx timer)  <-----/
  DATA [TSN=9,Strm=0,Seq=5] ------------> (ack delayed)
  (Start T3-rtx timer)
                                         ...
                                         {App sends 1 message; strm 1}
                                         (bundle SACK with DATA)
                                  /----- SACK [TSN Ack=9,block=0] \
                                 /         DATA [TSN=6,Strm=1,Seq=2]
  (cancel T3-rtx timer)  <------/        (Start T3-rtx timer)
  (ack delayed)
  (send ack)
  SACK [TSN Ack=6,block=0] -------------> (cancel T3-rtx timer)
         Figure 7:  Delayed Acknowledgement Example
 If an endpoint receives a DATA chunk with no user data (i.e., the
 Length field is set to 16), it MUST send an ABORT with error cause
 set to "No User Data".
 An endpoint SHOULD NOT send a DATA chunk with no user data part.

6.2.1. Processing a Received SACK

 Each SACK an endpoint receives contains an a_rwnd value.  This value
 represents the amount of buffer space the data receiver, at the time
 of transmitting the SACK, has left of its total receive buffer space
 (as specified in the INIT/INIT ACK).  Using a_rwnd, Cumulative TSN
 Ack, and Gap Ack Blocks, the data sender can develop a representation
 of the peer's receive buffer space.
 One of the problems the data sender must take into account when
 processing a SACK is that a SACK can be received out of order.  That
 is, a SACK sent by the data receiver can pass an earlier SACK and be
 received first by the data sender.  If a SACK is received out of

Stewart Standards Track [Page 81] RFC 4960 Stream Control Transmission Protocol September 2007

 order, the data sender can develop an incorrect view of the peer's
 receive buffer space.
 Since there is no explicit identifier that can be used to detect
 out-of-order SACKs, the data sender must use heuristics to determine
 if a SACK is new.
 An endpoint SHOULD use the following rules to calculate the rwnd,
 using the a_rwnd value, the Cumulative TSN Ack, and Gap Ack Blocks in
 a received SACK.
 A) At the establishment of the association, the endpoint initializes
    the rwnd to the Advertised Receiver Window Credit (a_rwnd) the
    peer specified in the INIT or INIT ACK.
 B) Any time a DATA chunk is transmitted (or retransmitted) to a peer,
    the endpoint subtracts the data size of the chunk from the rwnd of
    that peer.
 C) Any time a DATA chunk is marked for retransmission, either via
    T3-rtx timer expiration (Section 6.3.3) or via Fast Retransmit
    (Section 7.2.4), add the data size of those chunks to the rwnd.
    Note: If the implementation is maintaining a timer on each DATA
    chunk, then only DATA chunks whose timer expired would be marked
    for retransmission.
 D) Any time a SACK arrives, the endpoint performs the following:
      i) If Cumulative TSN Ack is less than the Cumulative TSN Ack
         Point, then drop the SACK.  Since Cumulative TSN Ack is
         monotonically increasing, a SACK whose Cumulative TSN Ack is
         less than the Cumulative TSN Ack Point indicates an out-of-
         order SACK.
     ii) Set rwnd equal to the newly received a_rwnd minus the number
         of bytes still outstanding after processing the Cumulative
         TSN Ack and the Gap Ack Blocks.
    iii) If the SACK is missing a TSN that was previously acknowledged
         via a Gap Ack Block (e.g., the data receiver reneged on the
         data), then consider the corresponding DATA that might be
         possibly missing: Count one miss indication towards Fast
         Retransmit as described in Section 7.2.4, and if no
         retransmit timer is running for the destination address to
         which the DATA chunk was originally transmitted, then T3-rtx
         is started for that destination address.

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     iv) If the Cumulative TSN Ack matches or exceeds the Fast
         Recovery exitpoint (Section 7.2.4), Fast Recovery is exited.

6.3. Management of Retransmission Timer

 An SCTP endpoint uses a retransmission timer T3-rtx to ensure data
 delivery in the absence of any feedback from its peer.  The duration
 of this timer is referred to as RTO (retransmission timeout).
 When an endpoint's peer is multi-homed, the endpoint will calculate a
 separate RTO for each different destination transport address of its
 peer endpoint.
 The computation and management of RTO in SCTP follow closely how TCP
 manages its retransmission timer.  To compute the current RTO, an
 endpoint maintains two state variables per destination transport
 address: SRTT (smoothed round-trip time) and RTTVAR (round-trip time
 variation).

6.3.1. RTO Calculation

 The rules governing the computation of SRTT, RTTVAR, and RTO are as
 follows:
 C1)  Until an RTT measurement has been made for a packet sent to the
      given destination transport address, set RTO to the protocol
      parameter 'RTO.Initial'.
 C2)  When the first RTT measurement R is made, set
      SRTT <- R,
      RTTVAR <- R/2, and
      RTO <- SRTT + 4 * RTTVAR.
 C3)  When a new RTT measurement R' is made, set
      RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'|
      and
      SRTT <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'
      Note: The value of SRTT used in the update to RTTVAR is its
      value before updating SRTT itself using the second assignment.
      After the computation, update RTO <- SRTT + 4 * RTTVAR.

Stewart Standards Track [Page 83] RFC 4960 Stream Control Transmission Protocol September 2007

 C4)  When data is in flight and when allowed by rule C5 below, a new
      RTT measurement MUST be made each round trip.  Furthermore, new
      RTT measurements SHOULD be made no more than once per round trip
      for a given destination transport address.  There are two
      reasons for this recommendation: First, it appears that
      measuring more frequently often does not in practice yield any
      significant benefit [ALLMAN99]; second, if measurements are made
      more often, then the values of RTO.Alpha and RTO.Beta in rule C3
      above should be adjusted so that SRTT and RTTVAR still adjust to
      changes at roughly the same rate (in terms of how many round
      trips it takes them to reflect new values) as they would if
      making only one measurement per round-trip and using RTO.Alpha
      and RTO.Beta as given in rule C3.  However, the exact nature of
      these adjustments remains a research issue.
 C5)  Karn's algorithm: RTT measurements MUST NOT be made using
      packets that were retransmitted (and thus for which it is
      ambiguous whether the reply was for the first instance of the
      chunk or for a later instance)
      IMPLEMENTATION NOTE: RTT measurements should only be made using
      a chunk with TSN r if no chunk with TSN less than or equal to r
      is retransmitted since r is first sent.
 C6)  Whenever RTO is computed, if it is less than RTO.Min seconds
      then it is rounded up to RTO.Min seconds.  The reason for this
      rule is that RTOs that do not have a high minimum value are
      susceptible to unnecessary timeouts [ALLMAN99].
 C7)  A maximum value may be placed on RTO provided it is at least
      RTO.max seconds.
 There is no requirement for the clock granularity G used for
 computing RTT measurements and the different state variables, other
 than:
 G1) Whenever RTTVAR is computed, if RTTVAR = 0, then adjust RTTVAR <-
 G.
 Experience [ALLMAN99] has shown that finer clock granularities (<=
 100 msec) perform somewhat better than more coarse granularities.

Stewart Standards Track [Page 84] RFC 4960 Stream Control Transmission Protocol September 2007

6.3.2. Retransmission Timer Rules

 The rules for managing the retransmission timer are as follows:
 R1)  Every time a DATA chunk is sent to any address (including a
      retransmission), if the T3-rtx timer of that address is not
      running, start it running so that it will expire after the RTO
      of that address.  The RTO used here is that obtained after any
      doubling due to previous T3-rtx timer expirations on the
      corresponding destination address as discussed in rule E2 below.
 R2)  Whenever all outstanding data sent to an address have been
      acknowledged, turn off the T3-rtx timer of that address.
 R3)  Whenever a SACK is received that acknowledges the DATA chunk
      with the earliest outstanding TSN for that address, restart the
      T3-rtx timer for that address with its current RTO (if there is
      still outstanding data on that address).
 R4)  Whenever a SACK is received missing a TSN that was previously
      acknowledged via a Gap Ack Block, start the T3-rtx for the
      destination address to which the DATA chunk was originally
      transmitted if it is not already running.
 The following example shows the use of various timer rules (assuming
 that the receiver uses delayed acks).
 Endpoint A                                         Endpoint Z
 {App begins to send}
 Data [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
 (Start T3-rtx timer)
                                         {App sends 1 message; strm 1}
                                         (bundle ack with data)
 DATA [TSN=8,Strm=0,Seq=4] ----\     /-- SACK [TSN Ack=7,Block=0]
                                \   /      DATA [TSN=6,Strm=1,Seq=2]
                                 \ /     (Start T3-rtx timer)
                                  \
                                 / \
 (Restart T3-rtx timer)  <------/   \--> (ack delayed)
 (ack delayed)
 {send ack}
 SACK [TSN Ack=6,Block=0] --------------> (Cancel T3-rtx timer)
                                         ..
                                         (send ack)
 (Cancel T3-rtx timer)  <-------------- SACK [TSN Ack=8,Block=0]
                     Figure 8: Timer Rule Examples

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6.3.3. Handle T3-rtx Expiration

 Whenever the retransmission timer T3-rtx expires for a destination
 address, do the following:
 E1)  For the destination address for which the timer expires, adjust
      its ssthresh with rules defined in Section 7.2.3 and set the
      cwnd <- MTU.
 E2)  For the destination address for which the timer expires, set RTO
      <- RTO * 2 ("back off the timer").  The maximum value discussed
      in rule C7 above (RTO.max) may be used to provide an upper bound
      to this doubling operation.
 E3)  Determine how many of the earliest (i.e., lowest TSN)
      outstanding DATA chunks for the address for which the T3-rtx has
      expired will fit into a single packet, subject to the MTU
      constraint for the path corresponding to the destination
      transport address to which the retransmission is being sent
      (this may be different from the address for which the timer
      expires; see Section 6.4).  Call this value K.  Bundle and
      retransmit those K DATA chunks in a single packet to the
      destination endpoint.
 E4)  Start the retransmission timer T3-rtx on the destination address
      to which the retransmission is sent, if rule R1 above indicates
      to do so.  The RTO to be used for starting T3-rtx should be the
      one for the destination address to which the retransmission is
      sent, which, when the receiver is multi-homed, may be different
      from the destination address for which the timer expired (see
      Section 6.4 below).
 After retransmitting, once a new RTT measurement is obtained (which
 can happen only when new data has been sent and acknowledged, per
 rule C5, or for a measurement made from a HEARTBEAT; see Section
 8.3), the computation in rule C3 is performed, including the
 computation of RTO, which may result in "collapsing" RTO back down
 after it has been subject to doubling (rule E2).
 Note: Any DATA chunks that were sent to the address for which the
 T3-rtx timer expired but did not fit in one MTU (rule E3 above)
 should be marked for retransmission and sent as soon as cwnd allows
 (normally, when a SACK arrives).
 The final rule for managing the retransmission timer concerns
 failover (see Section 6.4.1):

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 F1)  Whenever an endpoint switches from the current destination
      transport address to a different one, the current retransmission
      timers are left running.  As soon as the endpoint transmits a
      packet containing DATA chunk(s) to the new transport address,
      start the timer on that transport address, using the RTO value
      of the destination address to which the data is being sent, if
      rule R1 indicates to do so.

6.4. Multi-Homed SCTP Endpoints

 An SCTP endpoint is considered multi-homed if there are more than one
 transport address that can be used as a destination address to reach
 that endpoint.
 Moreover, the ULP of an endpoint shall select one of the multiple
 destination addresses of a multi-homed peer endpoint as the primary
 path (see Section 5.1.2 and Section 10.1 for details).
 By default, an endpoint SHOULD always transmit to the primary path,
 unless the SCTP user explicitly specifies the destination transport
 address (and possibly source transport address) to use.
 An endpoint SHOULD transmit reply chunks (e.g., SACK, HEARTBEAT ACK,
 etc.) to the same destination transport address from which it
 received the DATA or control chunk to which it is replying.  This
 rule should also be followed if the endpoint is bundling DATA chunks
 together with the reply chunk.
 However, when acknowledging multiple DATA chunks received in packets
 from different source addresses in a single SACK, the SACK chunk may
 be transmitted to one of the destination transport addresses from
 which the DATA or control chunks being acknowledged were received.
 When a receiver of a duplicate DATA chunk sends a SACK to a multi-
 homed endpoint, it MAY be beneficial to vary the destination address
 and not use the source address of the DATA chunk.  The reason is that
 receiving a duplicate from a multi-homed endpoint might indicate that
 the return path (as specified in the source address of the DATA
 chunk) for the SACK is broken.
 Furthermore, when its peer is multi-homed, an endpoint SHOULD try to
 retransmit a chunk that timed out to an active destination transport
 address that is different from the last destination address to which
 the DATA chunk was sent.
 Retransmissions do not affect the total outstanding data count.
 However, if the DATA chunk is retransmitted onto a different
 destination address, both the outstanding data counts on the new

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 destination address and the old destination address to which the data
 chunk was last sent shall be adjusted accordingly.

6.4.1. Failover from an Inactive Destination Address

 Some of the transport addresses of a multi-homed SCTP endpoint may
 become inactive due to either the occurrence of certain error
 conditions (see Section 8.2) or adjustments from the SCTP user.
 When there is outbound data to send and the primary path becomes
 inactive (e.g., due to failures), or where the SCTP user explicitly
 requests to send data to an inactive destination transport address,
 before reporting an error to its ULP, the SCTP endpoint should try to
 send the data to an alternate active destination transport address if
 one exists.
 When retransmitting data that timed out, if the endpoint is multi-
 homed, it should consider each source-destination address pair in its
 retransmission selection policy.  When retransmitting timed-out data,
 the endpoint should attempt to pick the most divergent source-
 destination pair from the original source-destination pair to which
 the packet was transmitted.
 Note: Rules for picking the most divergent source-destination pair
 are an implementation decision and are not specified within this
 document.

6.5. Stream Identifier and Stream Sequence Number

 Every DATA chunk MUST carry a valid stream identifier.  If an
 endpoint receives a DATA chunk with an invalid stream identifier, it
 shall acknowledge the reception of the DATA chunk following the
 normal procedure, immediately send an ERROR chunk with cause set to
 "Invalid Stream Identifier" (see Section 3.3.10), and discard the
 DATA chunk.  The endpoint may bundle the ERROR chunk in the same
 packet as the SACK as long as the ERROR follows the SACK.
 The Stream Sequence Number in all the streams MUST start from 0 when
 the association is established.  Also, when the Stream Sequence
 Number reaches the value 65535 the next Stream Sequence Number MUST
 be set to 0.

6.6. Ordered and Unordered Delivery

 Within a stream, an endpoint MUST deliver DATA chunks received with
 the U flag set to 0 to the upper layer according to the order of
 their Stream Sequence Number.  If DATA chunks arrive out of order of

Stewart Standards Track [Page 88] RFC 4960 Stream Control Transmission Protocol September 2007

 their Stream Sequence Number, the endpoint MUST hold the received
 DATA chunks from delivery to the ULP until they are reordered.
 However, an SCTP endpoint can indicate that no ordered delivery is
 required for a particular DATA chunk transmitted within the stream by
 setting the U flag of the DATA chunk to 1.
 When an endpoint receives a DATA chunk with the U flag set to 1, it
 must bypass the ordering mechanism and immediately deliver the data
 to the upper layer (after reassembly if the user data is fragmented
 by the data sender).
 This provides an effective way of transmitting "out-of-band" data in
 a given stream.  Also, a stream can be used as an "unordered" stream
 by simply setting the U flag to 1 in all DATA chunks sent through
 that stream.
 IMPLEMENTATION NOTE: When sending an unordered DATA chunk, an
 implementation may choose to place the DATA chunk in an outbound
 packet that is at the head of the outbound transmission queue if
 possible.
 The 'Stream Sequence Number' field in a DATA chunk with U flag set to
 1 has no significance.  The sender can fill it with arbitrary value,
 but the receiver MUST ignore the field.
 Note: When transmitting ordered and unordered data, an endpoint does
 not increment its Stream Sequence Number when transmitting a DATA
 chunk with U flag set to 1.

6.7. Report Gaps in Received DATA TSNs

 Upon the reception of a new DATA chunk, an endpoint shall examine the
 continuity of the TSNs received.  If the endpoint detects a gap in
 the received DATA chunk sequence, it SHOULD send a SACK with Gap Ack
 Blocks immediately.  The data receiver continues sending a SACK after
 receipt of each SCTP packet that doesn't fill the gap.
 Based on the Gap Ack Block from the received SACK, the endpoint can
 calculate the missing DATA chunks and make decisions on whether to
 retransmit them (see Section 6.2.1 for details).
 Multiple gaps can be reported in one single SACK (see Section 3.3.4).
 When its peer is multi-homed, the SCTP endpoint SHOULD always try to
 send the SACK to the same destination address from which the last
 DATA chunk was received.

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 Upon the reception of a SACK, the endpoint MUST remove all DATA
 chunks that have been acknowledged by the SACK's Cumulative TSN Ack
 from its transmit queue.  The endpoint MUST also treat all the DATA
 chunks with TSNs not included in the Gap Ack Blocks reported by the
 SACK as "missing".  The number of "missing" reports for each
 outstanding DATA chunk MUST be recorded by the data sender in order
 to make retransmission decisions.  See Section 7.2.4 for details.
 The following example shows the use of SACK to report a gap.
     Endpoint A                                    Endpoint Z {App
     sends 3 messages; strm 0} DATA [TSN=6,Strm=0,Seq=2] ----------
     -----> (ack delayed) (Start T3-rtx timer)
     DATA [TSN=7,Strm=0,Seq=3] --------> X (lost)
     DATA [TSN=8,Strm=0,Seq=4] ---------------> (gap detected,
                                                 immediately send ack)
                                     /----- SACK [TSN Ack=6,Block=1,
                                    /             Start=2,End=2]
                             <-----/ (remove 6 from out-queue,
      and mark 7 as "1" missing report)
                Figure 9: Reporting a Gap using SACK
 The maximum number of Gap Ack Blocks that can be reported within a
 single SACK chunk is limited by the current path MTU.  When a single
 SACK cannot cover all the Gap Ack Blocks needed to be reported due to
 the MTU limitation, the endpoint MUST send only one SACK, reporting
 the Gap Ack Blocks from the lowest to highest TSNs, within the size
 limit set by the MTU, and leave the remaining highest TSN numbers
 unacknowledged.

6.8. CRC32c Checksum Calculation

 When sending an SCTP packet, the endpoint MUST strengthen the data
 integrity of the transmission by including the CRC32c checksum value
 calculated on the packet, as described below.
 After the packet is constructed (containing the SCTP common header
 and one or more control or DATA chunks), the transmitter MUST
 1)  fill in the proper Verification Tag in the SCTP common header and
     initialize the checksum field to '0's,
 2)  calculate the CRC32c checksum of the whole packet, including the
     SCTP common header and all the chunks (refer to Appendix B for
     details of the CRC32c algorithm); and

Stewart Standards Track [Page 90] RFC 4960 Stream Control Transmission Protocol September 2007

 3)  put the resultant value into the checksum field in the common
     header, and leave the rest of the bits unchanged.
 When an SCTP packet is received, the receiver MUST first check the
 CRC32c checksum as follows:
 1)  Store the received CRC32c checksum value aside.
 2)  Replace the 32 bits of the checksum field in the received SCTP
     packet with all '0's and calculate a CRC32c checksum value of the
     whole received packet.
 3)  Verify that the calculated CRC32c checksum is the same as the
     received CRC32c checksum.  If it is not, the receiver MUST treat
     the packet as an invalid SCTP packet.
 The default procedure for handling invalid SCTP packets is to
 silently discard them.
 Any hardware implementation SHOULD be done in a way that is
 verifiable by the software.

6.9. Fragmentation and Reassembly

 An endpoint MAY support fragmentation when sending DATA chunks, but
 it MUST support reassembly when receiving DATA chunks.  If an
 endpoint supports fragmentation, it MUST fragment a user message if
 the size of the user message to be sent causes the outbound SCTP
 packet size to exceed the current MTU.  If an implementation does not
 support fragmentation of outbound user messages, the endpoint MUST
 return an error to its upper layer and not attempt to send the user
 message.
 Note: If an implementation that supports fragmentation makes
 available to its upper layer a mechanism to turn off fragmentation,
 it may do so.  However, in so doing, it MUST react just like an
 implementation that does NOT support fragmentation, i.e., it MUST
 reject sends that exceed the current Path MTU (P-MTU).
 IMPLEMENTATION NOTE: In this error case, the Send primitive discussed
 in Section 10.1 would need to return an error to the upper layer.
 If its peer is multi-homed, the endpoint shall choose a size no
 larger than the association Path MTU.  The association Path MTU is
 the smallest Path MTU of all destination addresses.

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 Note: Once a message is fragmented, it cannot be re-fragmented.
 Instead, if the PMTU has been reduced, then IP fragmentation must be
 used.  Please see Section 7.3 for details of PMTU discovery.
 When determining when to fragment, the SCTP implementation MUST take
 into account the SCTP packet header as well as the DATA chunk
 header(s).  The implementation MUST also take into account the space
 required for a SACK chunk if bundling a SACK chunk with the DATA
 chunk.
 Fragmentation takes the following steps:
 1)  The data sender MUST break the user message into a series of DATA
     chunks such that each chunk plus SCTP overhead fits into an IP
     datagram smaller than or equal to the association Path MTU.
 2)  The transmitter MUST then assign, in sequence, a separate TSN to
     each of the DATA chunks in the series.  The transmitter assigns
     the same SSN to each of the DATA chunks.  If the user indicates
     that the user message is to be delivered using unordered
     delivery, then the U flag of each DATA chunk of the user message
     MUST be set to 1.
 3)  The transmitter MUST also set the B/E bits of the first DATA
     chunk in the series to '10', the B/E bits of the last DATA chunk
     in the series to '01', and the B/E bits of all other DATA chunks
     in the series to '00'.
 An endpoint MUST recognize fragmented DATA chunks by examining the
 B/E bits in each of the received DATA chunks, and queue the
 fragmented DATA chunks for reassembly.  Once the user message is
 reassembled, SCTP shall pass the reassembled user message to the
 specific stream for possible reordering and final dispatching.
 Note: If the data receiver runs out of buffer space while still
 waiting for more fragments to complete the reassembly of the message,
 it should dispatch part of its inbound message through a partial
 delivery API (see Section 10), freeing some of its receive buffer
 space so that the rest of the message may be received.

6.10. Bundling

 An endpoint bundles chunks by simply including multiple chunks in one
 outbound SCTP packet.  The total size of the resultant IP datagram,
 including the SCTP packet and IP headers, MUST be less that or equal
 to the current Path MTU.

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 If its peer endpoint is multi-homed, the sending endpoint shall
 choose a size no larger than the latest MTU of the current primary
 path.
 When bundling control chunks with DATA chunks, an endpoint MUST place
 control chunks first in the outbound SCTP packet.  The transmitter
 MUST transmit DATA chunks within an SCTP packet in increasing order
 of TSN.
 Note: Since control chunks must be placed first in a packet and since
 DATA chunks must be transmitted before SHUTDOWN or SHUTDOWN ACK
 chunks, DATA chunks cannot be bundled with SHUTDOWN or SHUTDOWN ACK
 chunks.
 Partial chunks MUST NOT be placed in an SCTP packet.  A partial chunk
 is a chunk that is not completely contained in the SCTP packet; i.e.,
 the SCTP packet is too short to contain all the bytes of the chunk as
 indicated by the chunk length.
 An endpoint MUST process received chunks in their order in the
 packet.  The receiver uses the Chunk Length field to determine the
 end of a chunk and beginning of the next chunk taking account of the
 fact that all chunks end on a 4-byte boundary.  If the receiver
 detects a partial chunk, it MUST drop the chunk.
 An endpoint MUST NOT bundle INIT, INIT ACK, or SHUTDOWN COMPLETE with
 any other chunks.

7. Congestion Control

 Congestion control is one of the basic functions in SCTP.  For some
 applications, it may be likely that adequate resources will be
 allocated to SCTP traffic to ensure prompt delivery of time-critical
 data -- thus, it would appear to be unlikely, during normal
 operations, that transmissions encounter severe congestion
 conditions.  However, SCTP must operate under adverse operational
 conditions, which can develop upon partial network failures or
 unexpected traffic surges.  In such situations, SCTP must follow
 correct congestion control steps to recover from congestion quickly
 in order to get data delivered as soon as possible.  In the absence
 of network congestion, these preventive congestion control algorithms
 should show no impact on the protocol performance.
 IMPLEMENTATION NOTE: As far as its specific performance requirements
 are met, an implementation is always allowed to adopt a more
 conservative congestion control algorithm than the one defined below.

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 The congestion control algorithms used by SCTP are based on
 [RFC2581].  This section describes how the algorithms defined in
 [RFC2581] are adapted for use in SCTP.  We first list differences in
 protocol designs between TCP and SCTP, and then describe SCTP's
 congestion control scheme.  The description will use the same
 terminology as in TCP congestion control whenever appropriate.
 SCTP congestion control is always applied to the entire association,
 and not to individual streams.

7.1. SCTP Differences from TCP Congestion Control

 Gap Ack Blocks in the SCTP SACK carry the same semantic meaning as
 the TCP SACK.  TCP considers the information carried in the SACK as
 advisory information only.  SCTP considers the information carried in
 the Gap Ack Blocks in the SACK chunk as advisory.  In SCTP, any DATA
 chunk that has been acknowledged by SACK, including DATA that arrived
 at the receiving end out of order, is not considered fully delivered
 until the Cumulative TSN Ack Point passes the TSN of the DATA chunk
 (i.e., the DATA chunk has been acknowledged by the Cumulative TSN Ack
 field in the SACK).  Consequently, the value of cwnd controls the
 amount of outstanding data, rather than (as in the case of non-SACK
 TCP) the upper bound between the highest acknowledged sequence number
 and the latest DATA chunk that can be sent within the congestion
 window.  SCTP SACK leads to different implementations of Fast
 Retransmit and Fast Recovery than non-SACK TCP.  As an example, see
 [FALL96].
 The biggest difference between SCTP and TCP, however, is multi-
 homing.  SCTP is designed to establish robust communication
 associations between two endpoints each of which may be reachable by
 more than one transport address.  Potentially different addresses may
 lead to different data paths between the two endpoints; thus, ideally
 one may need a separate set of congestion control parameters for each
 of the paths.  The treatment here of congestion control for multi-
 homed receivers is new with SCTP and may require refinement in the
 future.  The current algorithms make the following assumptions:
 o  The sender usually uses the same destination address until being
    instructed by the upper layer to do otherwise; however, SCTP may
    change to an alternate destination in the event an address is
    marked inactive (see Section 8.2).  Also, SCTP may retransmit to a
    different transport address than the original transmission.
 o  The sender keeps a separate congestion control parameter set for
    each of the destination addresses it can send to (not each
    source-destination pair but for each destination).  The parameters

Stewart Standards Track [Page 94] RFC 4960 Stream Control Transmission Protocol September 2007

    should decay if the address is not used for a long enough time
    period.
 o  For each of the destination addresses, an endpoint does slow start
    upon the first transmission to that address.
 Note: TCP guarantees in-sequence delivery of data to its upper-layer
 protocol within a single TCP session.  This means that when TCP
 notices a gap in the received sequence number, it waits until the gap
 is filled before delivering the data that was received with sequence
 numbers higher than that of the missing data.  On the other hand,
 SCTP can deliver data to its upper-layer protocol even if there is a
 gap in TSN if the Stream Sequence Numbers are in sequence for a
 particular stream (i.e., the missing DATA chunks are for a different
 stream) or if unordered delivery is indicated.  Although this does
 not affect cwnd, it might affect rwnd calculation.

7.2. SCTP Slow-Start and Congestion Avoidance

 The slow-start and congestion avoidance algorithms MUST be used by an
 endpoint to control the amount of data being injected into the
 network.  The congestion control in SCTP is employed in regard to the
 association, not to an individual stream.  In some situations, it may
 be beneficial for an SCTP sender to be more conservative than the
 algorithms allow; however, an SCTP sender MUST NOT be more aggressive
 than the following algorithms allow.
 Like TCP, an SCTP endpoint uses the following three control variables
 to regulate its transmission rate.
 o  Receiver advertised window size (rwnd, in bytes), which is set by
    the receiver based on its available buffer space for incoming
    packets.
    Note: This variable is kept on the entire association.
 o  Congestion control window (cwnd, in bytes), which is adjusted by
    the sender based on observed network conditions.
    Note: This variable is maintained on a per-destination-address
    basis.
 o  Slow-start threshold (ssthresh, in bytes), which is used by the
    sender to distinguish slow-start and congestion avoidance phases.
    Note: This variable is maintained on a per-destination-address
    basis.

Stewart Standards Track [Page 95] RFC 4960 Stream Control Transmission Protocol September 2007

 SCTP also requires one additional control variable,
 partial_bytes_acked, which is used during congestion avoidance phase
 to facilitate cwnd adjustment.
 Unlike TCP, an SCTP sender MUST keep a set of these control variables
 cwnd, ssthresh, and partial_bytes_acked for EACH destination address
 of its peer (when its peer is multi-homed).  Only one rwnd is kept
 for the whole association (no matter if the peer is multi-homed or
 has a single address).

7.2.1. Slow-Start

 Beginning data transmission into a network with unknown conditions or
 after a sufficiently long idle period requires SCTP to probe the
 network to determine the available capacity.  The slow-start
 algorithm is used for this purpose at the beginning of a transfer, or
 after repairing loss detected by the retransmission timer.
 o  The initial cwnd before DATA transmission or after a sufficiently
    long idle period MUST be set to min(4*MTU, max (2*MTU, 4380
    bytes)).
 o  The initial cwnd after a retransmission timeout MUST be no more
    than 1*MTU.
 o  The initial value of ssthresh MAY be arbitrarily high (for
    example, implementations MAY use the size of the receiver
    advertised window).
 o  Whenever cwnd is greater than zero, the endpoint is allowed to
    have cwnd bytes of data outstanding on that transport address.
 o  When cwnd is less than or equal to ssthresh, an SCTP endpoint MUST
    use the slow-start algorithm to increase cwnd only if the current
    congestion window is being fully utilized, an incoming SACK
    advances the Cumulative TSN Ack Point, and the data sender is not
    in Fast Recovery.  Only when these three conditions are met can
    the cwnd be increased; otherwise, the cwnd MUST not be increased.
    If these conditions are met, then cwnd MUST be increased by, at
    most, the lesser of 1) the total size of the previously
    outstanding DATA chunk(s) acknowledged, and 2) the destination's
    path MTU.  This upper bound protects against the ACK-Splitting
    attack outlined in [SAVAGE99].
 In instances where its peer endpoint is multi-homed, if an endpoint
 receives a SACK that advances its Cumulative TSN Ack Point, then it
 should update its cwnd (or cwnds) apportioned to the destination
 addresses to which it transmitted the acknowledged data.  However, if

Stewart Standards Track [Page 96] RFC 4960 Stream Control Transmission Protocol September 2007

 the received SACK does not advance the Cumulative TSN Ack Point, the
 endpoint MUST NOT adjust the cwnd of any of the destination
 addresses.
 Because an endpoint's cwnd is not tied to its Cumulative TSN Ack
 Point, as duplicate SACKs come in, even though they may not advance
 the Cumulative TSN Ack Point an endpoint can still use them to clock
 out new data.  That is, the data newly acknowledged by the SACK
 diminishes the amount of data now in flight to less than cwnd, and so
 the current, unchanged value of cwnd now allows new data to be sent.
 On the other hand, the increase of cwnd must be tied to the
 Cumulative TSN Ack Point advancement as specified above.  Otherwise,
 the duplicate SACKs will not only clock out new data, but also will
 adversely clock out more new data than what has just left the
 network, during a time of possible congestion.
 o  When the endpoint does not transmit data on a given transport
    address, the cwnd of the transport address should be adjusted to
    max(cwnd/2, 4*MTU) per RTO.

7.2.2. Congestion Avoidance

 When cwnd is greater than ssthresh, cwnd should be incremented by
 1*MTU per RTT if the sender has cwnd or more bytes of data
 outstanding for the corresponding transport address.
 In practice, an implementation can achieve this goal in the following
 way:
 o  partial_bytes_acked is initialized to 0.
 o  Whenever cwnd is greater than ssthresh, upon each SACK arrival
    that advances the Cumulative TSN Ack Point, increase
    partial_bytes_acked by the total number of bytes of all new chunks
    acknowledged in that SACK including chunks acknowledged by the new
    Cumulative TSN Ack and by Gap Ack Blocks.
 o  When partial_bytes_acked is equal to or greater than cwnd and
    before the arrival of the SACK the sender had cwnd or more bytes
    of data outstanding (i.e., before arrival of the SACK, flightsize
    was greater than or equal to cwnd), increase cwnd by MTU, and
    reset partial_bytes_acked to (partial_bytes_acked - cwnd).
 o  Same as in the slow start, when the sender does not transmit DATA
    on a given transport address, the cwnd of the transport address
    should be adjusted to max(cwnd / 2, 4*MTU) per RTO.

Stewart Standards Track [Page 97] RFC 4960 Stream Control Transmission Protocol September 2007

 o  When all of the data transmitted by the sender has been
    acknowledged by the receiver, partial_bytes_acked is initialized
    to 0.

7.2.3. Congestion Control

 Upon detection of packet losses from SACK (see Section 7.2.4), an
 endpoint should do the following:
    ssthresh = max(cwnd/2, 4*MTU)
    cwnd = ssthresh
    partial_bytes_acked = 0
 Basically, a packet loss causes cwnd to be cut in half.
 When the T3-rtx timer expires on an address, SCTP should perform slow
 start by:
    ssthresh = max(cwnd/2, 4*MTU)
    cwnd = 1*MTU
 and ensure that no more than one SCTP packet will be in flight for
 that address until the endpoint receives acknowledgement for
 successful delivery of data to that address.

7.2.4. Fast Retransmit on Gap Reports

 In the absence of data loss, an endpoint performs delayed
 acknowledgement.  However, whenever an endpoint notices a hole in the
 arriving TSN sequence, it SHOULD start sending a SACK back every time
 a packet arrives carrying data until the hole is filled.
 Whenever an endpoint receives a SACK that indicates that some TSNs
 are missing, it SHOULD wait for two further miss indications (via
 subsequent SACKs for a total of three missing reports) on the same
 TSNs before taking action with regard to Fast Retransmit.
 Miss indications SHOULD follow the HTNA (Highest TSN Newly
 Acknowledged) algorithm.  For each incoming SACK, miss indications
 are incremented only for missing TSNs prior to the highest TSN newly
 acknowledged in the SACK.  A newly acknowledged DATA chunk is one not
 previously acknowledged in a SACK.  If an endpoint is in Fast
 Recovery and a SACK arrives that advances the Cumulative TSN Ack
 Point, the miss indications are incremented for all TSNs reported
 missing in the SACK.
 When the third consecutive miss indication is received for a TSN(s),
 the data sender shall do the following:

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 1)  Mark the DATA chunk(s) with three miss indications for
     retransmission.
 2)  If not in Fast Recovery, adjust the ssthresh and cwnd of the
     destination address(es) to which the missing DATA chunks were
     last sent, according to the formula described in Section 7.2.3.
 3)  Determine how many of the earliest (i.e., lowest TSN) DATA chunks
     marked for retransmission will fit into a single packet, subject
     to constraint of the path MTU of the destination transport
     address to which the packet is being sent.  Call this value K.
     Retransmit those K DATA chunks in a single packet.  When a Fast
     Retransmit is being performed, the sender SHOULD ignore the value
     of cwnd and SHOULD NOT delay retransmission for this single
     packet.
 4)  Restart the T3-rtx timer only if the last SACK acknowledged the
     lowest outstanding TSN number sent to that address, or the
     endpoint is retransmitting the first outstanding DATA chunk sent
     to that address.
 5)  Mark the DATA chunk(s) as being fast retransmitted and thus
     ineligible for a subsequent Fast Retransmit.  Those TSNs marked
     for retransmission due to the Fast-Retransmit algorithm that did
     not fit in the sent datagram carrying K other TSNs are also
     marked as ineligible for a subsequent Fast Retransmit.  However,
     as they are marked for retransmission they will be retransmitted
     later on as soon as cwnd allows.
 6)  If not in Fast Recovery, enter Fast Recovery and mark the highest
     outstanding TSN as the Fast Recovery exit point.  When a SACK
     acknowledges all TSNs up to and including this exit point, Fast
     Recovery is exited.  While in Fast Recovery, the ssthresh and
     cwnd SHOULD NOT change for any destinations due to a subsequent
     Fast Recovery event (i.e., one SHOULD NOT reduce the cwnd further
     due to a subsequent Fast Retransmit).
 Note: Before the above adjustments, if the received SACK also
 acknowledges new DATA chunks and advances the Cumulative TSN Ack
 Point, the cwnd adjustment rules defined in Section 7.2.1 and Section
 7.2.2 must be applied first.
 A straightforward implementation of the above keeps a counter for
 each TSN hole reported by a SACK.  The counter increments for each
 consecutive SACK reporting the TSN hole.  After reaching 3 and
 starting the Fast-Retransmit procedure, the counter resets to 0.

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 Because cwnd in SCTP indirectly bounds the number of outstanding
 TSN's, the effect of TCP Fast Recovery is achieved automatically with
 no adjustment to the congestion control window size.

7.3. Path MTU Discovery

 [RFC4821], [RFC1981], and [RFC1191] specify "Packetization Layer Path
 MTU Discovery", whereby an endpoint maintains an estimate of the
 maximum transmission unit (MTU) along a given Internet path and
 refrains from sending packets along that path that exceed the MTU,
 other than occasional attempts to probe for a change in the Path MTU
 (PMTU).  [RFC4821] is thorough in its discussion of the MTU discovery
 mechanism and strategies for determining the current end-to-end MTU
 setting as well as detecting changes in this value.
 An endpoint SHOULD apply these techniques, and SHOULD do so on a
 per-destination-address basis.
 There are two important SCTP-specific points regarding Path MTU
 discovery:
 1)  SCTP associations can span multiple addresses.  An endpoint MUST
     maintain separate MTU estimates for each destination address of
     its peer.
 2)  The sender should track an association PMTU that will be the
     smallest PMTU discovered for all of the peer's destination
     addresses.  When fragmenting messages into multiple parts this
     association PMTU should be used to calculate the size of each
     fragment.  This will allow retransmissions to be seamlessly sent
     to an alternate address without encountering IP fragmentation.

8. Fault Management

8.1. Endpoint Failure Detection

 An endpoint shall keep a counter on the total number of consecutive
 retransmissions to its peer (this includes retransmissions to all the
 destination transport addresses of the peer if it is multi-homed),
 including unacknowledged HEARTBEAT chunks.  If the value of this
 counter exceeds the limit indicated in the protocol parameter
 'Association.Max.Retrans', the endpoint shall consider the peer
 endpoint unreachable and shall stop transmitting any more data to it
 (and thus the association enters the CLOSED state).  In addition, the
 endpoint MAY report the failure to the upper layer and optionally
 report back all outstanding user data remaining in its outbound
 queue.  The association is automatically closed when the peer
 endpoint becomes unreachable.

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 The counter shall be reset each time a DATA chunk sent to that peer
 endpoint is acknowledged (by the reception of a SACK) or a HEARTBEAT
 ACK is received from the peer endpoint.

8.2. Path Failure Detection

 When its peer endpoint is multi-homed, an endpoint should keep an
 error counter for each of the destination transport addresses of the
 peer endpoint.
 Each time the T3-rtx timer expires on any address, or when a
 HEARTBEAT sent to an idle address is not acknowledged within an RTO,
 the error counter of that destination address will be incremented.
 When the value in the error counter exceeds the protocol parameter
 'Path.Max.Retrans' of that destination address, the endpoint should
 mark the destination transport address as inactive, and a
 notification SHOULD be sent to the upper layer.
 When an outstanding TSN is acknowledged or a HEARTBEAT sent to that
 address is acknowledged with a HEARTBEAT ACK, the endpoint shall
 clear the error counter of the destination transport address to which
 the DATA chunk was last sent (or HEARTBEAT was sent).  When the peer
 endpoint is multi-homed and the last chunk sent to it was a
 retransmission to an alternate address, there exists an ambiguity as
 to whether or not the acknowledgement should be credited to the
 address of the last chunk sent.  However, this ambiguity does not
 seem to bear any significant consequence to SCTP behavior.  If this
 ambiguity is undesirable, the transmitter may choose not to clear the
 error counter if the last chunk sent was a retransmission.
 Note: When configuring the SCTP endpoint, the user should avoid
 having the value of 'Association.Max.Retrans' larger than the
 summation of the 'Path.Max.Retrans' of all the destination addresses
 for the remote endpoint.  Otherwise, all the destination addresses
 may become inactive while the endpoint still considers the peer
 endpoint reachable.  When this condition occurs, how SCTP chooses to
 function is implementation specific.
 When the primary path is marked inactive (due to excessive
 retransmissions, for instance), the sender MAY automatically transmit
 new packets to an alternate destination address if one exists and is
 active.  If more than one alternate address is active when the
 primary path is marked inactive, only ONE transport address SHOULD be
 chosen and used as the new destination transport address.

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8.3. Path Heartbeat

 By default, an SCTP endpoint SHOULD monitor the reachability of the
 idle destination transport address(es) of its peer by sending a
 HEARTBEAT chunk periodically to the destination transport
 address(es).  HEARTBEAT sending MAY begin upon reaching the
 ESTABLISHED state and is discontinued after sending either SHUTDOWN
 or SHUTDOWN-ACK.  A receiver of a HEARTBEAT MUST respond to a
 HEARTBEAT with a HEARTBEAT-ACK after entering the COOKIE-ECHOED state
 (INIT sender) or the ESTABLISHED state (INIT receiver), up until
 reaching the SHUTDOWN-SENT state (SHUTDOWN sender) or the SHUTDOWN-
 ACK-SENT state (SHUTDOWN receiver).
 A destination transport address is considered "idle" if no new chunk
 that can be used for updating path RTT (usually including first
 transmission DATA, INIT, COOKIE ECHO, HEARTBEAT, etc.) and no
 HEARTBEAT has been sent to it within the current heartbeat period of
 that address.  This applies to both active and inactive destination
 addresses.
 The upper layer can optionally initiate the following functions:
 A) Disable heartbeat on a specific destination transport address of a
    given association,
 B) Change the HB.interval,
 C) Re-enable heartbeat on a specific destination transport address of
    a given association, and
 D) Request an on-demand HEARTBEAT on a specific destination transport
    address of a given association.
 The endpoint should increment the respective error counter of the
 destination transport address each time a HEARTBEAT is sent to that
 address and not acknowledged within one RTO.
 When the value of this counter reaches the protocol parameter
 'Path.Max.Retrans', the endpoint should mark the corresponding
 destination address as inactive if it is not so marked, and may also
 optionally report to the upper layer the change of reachability of
 this destination address.  After this, the endpoint should continue
 HEARTBEAT on this destination address but should stop increasing the
 counter.
 The sender of the HEARTBEAT chunk should include in the Heartbeat
 Information field of the chunk the current time when the packet is
 sent out and the destination address to which the packet is sent.

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 IMPLEMENTATION NOTE: An alternative implementation of the heartbeat
 mechanism that can be used is to increment the error counter variable
 every time a HEARTBEAT is sent to a destination.  Whenever a
 HEARTBEAT ACK arrives, the sender SHOULD clear the error counter of
 the destination that the HEARTBEAT was sent to.  This in effect would
 clear the previously stroked error (and any other error counts as
 well).
 The receiver of the HEARTBEAT should immediately respond with a
 HEARTBEAT ACK that contains the Heartbeat Information TLV, together
 with any other received TLVs, copied unchanged from the received
 HEARTBEAT chunk.
 Upon the receipt of the HEARTBEAT ACK, the sender of the HEARTBEAT
 should clear the error counter of the destination transport address
 to which the HEARTBEAT was sent, and mark the destination transport
 address as active if it is not so marked.  The endpoint may
 optionally report to the upper layer when an inactive destination
 address is marked as active due to the reception of the latest
 HEARTBEAT ACK.  The receiver of the HEARTBEAT ACK must also clear the
 association overall error count as well (as defined in Section 8.1).
 The receiver of the HEARTBEAT ACK should also perform an RTT
 measurement for that destination transport address using the time
 value carried in the HEARTBEAT ACK chunk.
 On an idle destination address that is allowed to heartbeat, it is
 recommended that a HEARTBEAT chunk is sent once per RTO of that
 destination address plus the protocol parameter 'HB.interval', with
 jittering of +/- 50% of the RTO value, and exponential backoff of the
 RTO if the previous HEARTBEAT is unanswered.
 A primitive is provided for the SCTP user to change the HB.interval
 and turn on or off the heartbeat on a given destination address.  The
 heartbeat interval set by the SCTP user is added to the RTO of that
 destination (including any exponential backoff).  Only one heartbeat
 should be sent each time the heartbeat timer expires (if multiple
 destinations are idle).  It is an implementation decision on how to
 choose which of the candidate idle destinations to heartbeat to (if
 more than one destination is idle).
 Note: When tuning the heartbeat interval, there is a side effect that
 SHOULD be taken into account.  When this value is increased, i.e.,
 the HEARTBEAT takes longer, the detection of lost ABORT messages
 takes longer as well.  If a peer endpoint ABORTs the association for
 any reason and the ABORT chunk is lost, the local endpoint will only
 discover the lost ABORT by sending a DATA chunk or HEARTBEAT chunk
 (thus causing the peer to send another ABORT).  This must be

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 considered when tuning the HEARTBEAT timer.  If the HEARTBEAT is
 disabled, only sending DATA to the association will discover a lost
 ABORT from the peer.

8.4. Handle "Out of the Blue" Packets

 An SCTP packet is called an "out of the blue" (OOTB) packet if it is
 correctly formed (i.e., passed the receiver's CRC32c check; see
 Section 6.8), but the receiver is not able to identify the
 association to which this packet belongs.
 The receiver of an OOTB packet MUST do the following:
 1)  If the OOTB packet is to or from a non-unicast address, a
     receiver SHOULD silently discard the packet.  Otherwise,
 2)  If the OOTB packet contains an ABORT chunk, the receiver MUST
     silently discard the OOTB packet and take no further action.
     Otherwise,
 3)  If the packet contains an INIT chunk with a Verification Tag set
     to '0', process it as described in Section 5.1.  If, for whatever
     reason, the INIT cannot be processed normally and an ABORT has to
     be sent in response, the Verification Tag of the packet
     containing the ABORT chunk MUST be the Initiate Tag of the
     received INIT chunk, and the T bit of the ABORT chunk has to be
     set to 0, indicating that the Verification Tag is NOT reflected.
 4)  If the packet contains a COOKIE ECHO in the first chunk, process
     it as described in Section 5.1.  Otherwise,
 5)  If the packet contains a SHUTDOWN ACK chunk, the receiver should
     respond to the sender of the OOTB packet with a SHUTDOWN
     COMPLETE.  When sending the SHUTDOWN COMPLETE, the receiver of
     the OOTB packet must fill in the Verification Tag field of the
     outbound packet with the Verification Tag received in the
     SHUTDOWN ACK and set the T bit in the Chunk Flags to indicate
     that the Verification Tag is reflected.  Otherwise,
 6)  If the packet contains a SHUTDOWN COMPLETE chunk, the receiver
     should silently discard the packet and take no further action.
     Otherwise,
 7)  If the packet contains a "Stale Cookie" ERROR or a COOKIE ACK,
     the SCTP packet should be silently discarded.  Otherwise,

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 8)  The receiver should respond to the sender of the OOTB packet with
     an ABORT.  When sending the ABORT, the receiver of the OOTB
     packet MUST fill in the Verification Tag field of the outbound
     packet with the value found in the Verification Tag field of the
     OOTB packet and set the T bit in the Chunk Flags to indicate that
     the Verification Tag is reflected.  After sending this ABORT, the
     receiver of the OOTB packet shall discard the OOTB packet and
     take no further action.

8.5. Verification Tag

 The Verification Tag rules defined in this section apply when sending
 or receiving SCTP packets that do not contain an INIT, SHUTDOWN
 COMPLETE, COOKIE ECHO (see Section 5.1), ABORT, or SHUTDOWN ACK
 chunk.  The rules for sending and receiving SCTP packets containing
 one of these chunk types are discussed separately in Section 8.5.1.
 When sending an SCTP packet, the endpoint MUST fill in the
 Verification Tag field of the outbound packet with the tag value in
 the Initiate Tag parameter of the INIT or INIT ACK received from its
 peer.
 When receiving an SCTP packet, the endpoint MUST ensure that the
 value in the Verification Tag field of the received SCTP packet
 matches its own tag.  If the received Verification Tag value does not
 match the receiver's own tag value, the receiver shall silently
 discard the packet and shall not process it any further except for
 those cases listed in Section 8.5.1 below.

8.5.1. Exceptions in Verification Tag Rules

 A) Rules for packet carrying INIT:
  1. The sender MUST set the Verification Tag of the packet to 0.
  1. When an endpoint receives an SCTP packet with the Verification

Tag set to 0, it should verify that the packet contains only an

     INIT chunk.  Otherwise, the receiver MUST silently discard the
     packet.
 B) Rules for packet carrying ABORT:
  1. The endpoint MUST always fill in the Verification Tag field of

the outbound packet with the destination endpoint's tag value, if

     it is known.
  1. If the ABORT is sent in response to an OOTB packet, the endpoint

MUST follow the procedure described in Section 8.4.

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  1. The receiver of an ABORT MUST accept the packet if the

Verification Tag field of the packet matches its own tag and the

     T bit is not set OR if it is set to its peer's tag and the T bit
     is set in the Chunk Flags.  Otherwise, the receiver MUST silently
     discard the packet and take no further action.
 C) Rules for packet carrying SHUTDOWN COMPLETE:
  1. When sending a SHUTDOWN COMPLETE, if the receiver of the SHUTDOWN

ACK has a TCB, then the destination endpoint's tag MUST be used,

     and the T bit MUST NOT be set.  Only where no TCB exists should
     the sender use the Verification Tag from the SHUTDOWN ACK, and
     MUST set the T bit.
  1. The receiver of a SHUTDOWN COMPLETE shall accept the packet if

the Verification Tag field of the packet matches its own tag and

     the T bit is not set OR if it is set to its peer's tag and the T
     bit is set in the Chunk Flags.  Otherwise, the receiver MUST
     silently discard the packet and take no further action.  An
     endpoint MUST ignore the SHUTDOWN COMPLETE if it is not in the
     SHUTDOWN-ACK-SENT state.
 D) Rules for packet carrying a COOKIE ECHO
  1. When sending a COOKIE ECHO, the endpoint MUST use the value of

the Initiate Tag received in the INIT ACK.

  1. The receiver of a COOKIE ECHO follows the procedures in Section

5.

 E) Rules for packet carrying a SHUTDOWN ACK
  1. If the receiver is in COOKIE-ECHOED or COOKIE-WAIT state the

procedures in Section 8.4 SHOULD be followed; in other words, it

     should be treated as an Out Of The Blue packet.

9. Termination of Association

 An endpoint should terminate its association when it exits from
 service.  An association can be terminated by either abort or
 shutdown.  An abort of an association is abortive by definition in
 that any data pending on either end of the association is discarded
 and not delivered to the peer.  A shutdown of an association is
 considered a graceful close where all data in queue by either
 endpoint is delivered to the respective peers.  However, in the case
 of a shutdown, SCTP does not support a half-open state (like TCP)
 wherein one side may continue sending data while the other end is
 closed.  When either endpoint performs a shutdown, the association on

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 each peer will stop accepting new data from its user and only deliver
 data in queue at the time of sending or receiving the SHUTDOWN chunk.

9.1. Abort of an Association

 When an endpoint decides to abort an existing association, it MUST
 send an ABORT chunk to its peer endpoint.  The sender MUST fill in
 the peer's Verification Tag in the outbound packet and MUST NOT
 bundle any DATA chunk with the ABORT.  If the association is aborted
 on request of the upper layer, a User-Initiated Abort error cause
 (see Section 3.3.10.12) SHOULD be present in the ABORT chunk.
 An endpoint MUST NOT respond to any received packet that contains an
 ABORT chunk (also see Section 8.4).
 An endpoint receiving an ABORT MUST apply the special Verification
 Tag check rules described in Section 8.5.1.
 After checking the Verification Tag, the receiving endpoint MUST
 remove the association from its record and SHOULD report the
 termination to its upper layer.  If a User-Initiated Abort error
 cause is present in the ABORT chunk, the Upper Layer Abort Reason
 SHOULD be made available to the upper layer.

9.2. Shutdown of an Association

 Using the SHUTDOWN primitive (see Section 10.1), the upper layer of
 an endpoint in an association can gracefully close the association.
 This will allow all outstanding DATA chunks from the peer of the
 shutdown initiator to be delivered before the association terminates.
 Upon receipt of the SHUTDOWN primitive from its upper layer, the
 endpoint enters the SHUTDOWN-PENDING state and remains there until
 all outstanding data has been acknowledged by its peer.  The endpoint
 accepts no new data from its upper layer, but retransmits data to the
 far end if necessary to fill gaps.
 Once all its outstanding data has been acknowledged, the endpoint
 shall send a SHUTDOWN chunk to its peer including in the Cumulative
 TSN Ack field the last sequential TSN it has received from the peer.
 It shall then start the T2-shutdown timer and enter the SHUTDOWN-SENT
 state.  If the timer expires, the endpoint must resend the SHUTDOWN
 with the updated last sequential TSN received from its peer.
 The rules in Section 6.3 MUST be followed to determine the proper
 timer value for T2-shutdown.  To indicate any gaps in TSN, the
 endpoint may also bundle a SACK with the SHUTDOWN chunk in the same
 SCTP packet.

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 An endpoint should limit the number of retransmissions of the
 SHUTDOWN chunk to the protocol parameter 'Association.Max.Retrans'.
 If this threshold is exceeded, the endpoint should destroy the TCB
 and MUST report the peer endpoint unreachable to the upper layer (and
 thus the association enters the CLOSED state).  The reception of any
 packet from its peer (i.e., as the peer sends all of its queued DATA
 chunks) should clear the endpoint's retransmission count and restart
 the T2-shutdown timer, giving its peer ample opportunity to transmit
 all of its queued DATA chunks that have not yet been sent.
 Upon reception of the SHUTDOWN, the peer endpoint shall
  1. enter the SHUTDOWN-RECEIVED state,
  1. stop accepting new data from its SCTP user, and
  1. verify, by checking the Cumulative TSN Ack field of the chunk,

that all its outstanding DATA chunks have been received by the

    SHUTDOWN sender.
 Once an endpoint has reached the SHUTDOWN-RECEIVED state, it MUST NOT
 send a SHUTDOWN in response to a ULP request, and should discard
 subsequent SHUTDOWN chunks.
 If there are still outstanding DATA chunks left, the SHUTDOWN
 receiver MUST continue to follow normal data transmission procedures
 defined in Section 6, until all outstanding DATA chunks are
 acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
 from its SCTP user.
 While in the SHUTDOWN-SENT state, the SHUTDOWN sender MUST
 immediately respond to each received packet containing one or more
 DATA chunks with a SHUTDOWN chunk and restart the T2-shutdown timer.
 If a SHUTDOWN chunk by itself cannot acknowledge all of the received
 DATA chunks (i.e., there are TSNs that can be acknowledged that are
 larger than the cumulative TSN, and thus gaps exist in the TSN
 sequence), or if duplicate TSNs have been received, then a SACK chunk
 MUST also be sent.
 The sender of the SHUTDOWN MAY also start an overall guard timer
 'T5-shutdown-guard' to bound the overall time for the shutdown
 sequence.  At the expiration of this timer, the sender SHOULD abort
 the association by sending an ABORT chunk.  If the 'T5-shutdown-
 guard' timer is used, it SHOULD be set to the recommended value of 5
 times 'RTO.Max'.
 If the receiver of the SHUTDOWN has no more outstanding DATA chunks,
 the SHUTDOWN receiver MUST send a SHUTDOWN ACK and start a T2-

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 shutdown timer of its own, entering the SHUTDOWN-ACK-SENT state.  If
 the timer expires, the endpoint must resend the SHUTDOWN ACK.
 The sender of the SHUTDOWN ACK should limit the number of
 retransmissions of the SHUTDOWN ACK chunk to the protocol parameter
 'Association.Max.Retrans'.  If this threshold is exceeded, the
 endpoint should destroy the TCB and may report the peer endpoint
 unreachable to the upper layer (and thus the association enters the
 CLOSED state).
 Upon the receipt of the SHUTDOWN ACK, the SHUTDOWN sender shall stop
 the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its peer,
 and remove all record of the association.
 Upon reception of the SHUTDOWN COMPLETE chunk, the endpoint will
 verify that it is in the SHUTDOWN-ACK-SENT state; if it is not, the
 chunk should be discarded.  If the endpoint is in the SHUTDOWN-ACK-
 SENT state, the endpoint should stop the T2-shutdown timer and remove
 all knowledge of the association (and thus the association enters the
 CLOSED state).
 An endpoint SHOULD ensure that all its outstanding DATA chunks have
 been acknowledged before initiating the shutdown procedure.
 An endpoint should reject any new data request from its upper layer
 if it is in the SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED,
 or SHUTDOWN-ACK-SENT state.
 If an endpoint is in the SHUTDOWN-ACK-SENT state and receives an INIT
 chunk (e.g., if the SHUTDOWN COMPLETE was lost) with source and
 destination transport addresses (either in the IP addresses or in the
 INIT chunk) that belong to this association, it should discard the
 INIT chunk and retransmit the SHUTDOWN ACK chunk.
 Note: Receipt of an INIT with the same source and destination IP
 addresses as used in transport addresses assigned to an endpoint but
 with a different port number indicates the initialization of a
 separate association.
 The sender of the INIT or COOKIE ECHO should respond to the receipt
 of a SHUTDOWN ACK with a stand-alone SHUTDOWN COMPLETE in an SCTP
 packet with the Verification Tag field of its common header set to
 the same tag that was received in the SHUTDOWN ACK packet.  This is
 considered an Out of the Blue packet as defined in Section 8.4.  The
 sender of the INIT lets T1-init continue running and remains in the
 COOKIE-WAIT or COOKIE-ECHOED state.  Normal T1-init timer expiration
 will cause the INIT or COOKIE chunk to be retransmitted and thus
 start a new association.

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 If a SHUTDOWN is received in the COOKIE-WAIT or COOKIE ECHOED state,
 the SHUTDOWN chunk SHOULD be silently discarded.
 If an endpoint is in the SHUTDOWN-SENT state and receives a SHUTDOWN
 chunk from its peer, the endpoint shall respond immediately with a
 SHUTDOWN ACK to its peer, and move into the SHUTDOWN-ACK-SENT state
 restarting its T2-shutdown timer.
 If an endpoint is in the SHUTDOWN-ACK-SENT state and receives a
 SHUTDOWN ACK, it shall stop the T2-shutdown timer, send a SHUTDOWN
 COMPLETE chunk to its peer, and remove all record of the association.

10. Interface with Upper Layer

 The Upper Layer Protocols (ULPs) shall request services by passing
 primitives to SCTP and shall receive notifications from SCTP for
 various events.
 The primitives and notifications described in this section should be
 used as a guideline for implementing SCTP.  The following functional
 description of ULP interface primitives is shown for illustrative
 purposes.  Different SCTP implementations may have different ULP
 interfaces.  However, all SCTPs must provide a certain minimum set of
 services to guarantee that all SCTP implementations can support the
 same protocol hierarchy.

10.1. ULP-to-SCTP

 The following sections functionally characterize a ULP/SCTP
 interface.  The notation used is similar to most procedure or
 function calls in high-level languages.
 The ULP primitives described below specify the basic functions that
 SCTP must perform to support inter-process communication.  Individual
 implementations must define their own exact format, and may provide
 combinations or subsets of the basic functions in single calls.
 A) Initialize
    Format: INITIALIZE ([local port],[local eligible address list])->
    local SCTP instance name
 This primitive allows SCTP to initialize its internal data structures
 and allocate necessary resources for setting up its operation
 environment.  Once SCTP is initialized, ULP can communicate directly
 with other endpoints without re-invoking this primitive.
 SCTP will return a local SCTP instance name to the ULP.

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 Mandatory attributes:
 None.
 Optional attributes:
 The following types of attributes may be passed along with the
 primitive:
 o  local port - SCTP port number, if ULP wants it to be specified.
 o  local eligible address list - an address list that the local SCTP
    endpoint should bind.  By default, if an address list is not
    included, all IP addresses assigned to the host should be used by
    the local endpoint.
 IMPLEMENTATION NOTE: If this optional attribute is supported by an
 implementation, it will be the responsibility of the implementation
 to enforce that the IP source address field of any SCTP packets sent
 out by this endpoint contains one of the IP addresses indicated in
 the local eligible address list.
 B) Associate
    Format: ASSOCIATE(local SCTP instance name,
            destination transport addr, outbound stream count)
    -> association id [,destination transport addr list]
          [,outbound stream count]
 This primitive allows the upper layer to initiate an association to a
 specific peer endpoint.
 The peer endpoint shall be specified by one of the transport
 addresses that defines the endpoint (see Section 1.3).  If the local
 SCTP instance has not been initialized, the ASSOCIATE is considered
 an error.
 An association id, which is a local handle to the SCTP association,
 will be returned on successful establishment of the association.  If
 SCTP is not able to open an SCTP association with the peer endpoint,
 an error is returned.
 Other association parameters may be returned, including the complete
 destination transport addresses of the peer as well as the outbound
 stream count of the local endpoint.  One of the transport addresses
 from the returned destination addresses will be selected by the local
 endpoint as default primary path for sending SCTP packets to this
 peer.  The returned "destination transport addr list" can be used by

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 the ULP to change the default primary path or to force sending a
 packet to a specific transport address.
 IMPLEMENTATION NOTE: If ASSOCIATE primitive is implemented as a
 blocking function call, the ASSOCIATE primitive can return
 association parameters in addition to the association id upon
 successful establishment.  If ASSOCIATE primitive is implemented as a
 non-blocking call, only the association id shall be returned and
 association parameters shall be passed using the COMMUNICATION UP
 notification.
 Mandatory attributes:
 o  local SCTP instance name - obtained from the INITIALIZE operation.
 o  destination transport addr - specified as one of the transport
    addresses of the peer endpoint with which the association is to be
    established.
 o  outbound stream count - the number of outbound streams the ULP
    would like to open towards this peer endpoint.
 Optional attributes:
 None.
 C) Shutdown
    Format: SHUTDOWN(association id)
    -> result
 Gracefully closes an association.  Any locally queued user data will
 be delivered to the peer.  The association will be terminated only
 after the peer acknowledges all the SCTP packets sent.  A success
 code will be returned on successful termination of the association.
 If attempting to terminate the association results in a failure, an
 error code shall be returned.
 Mandatory attributes:
 o association id - local handle to the SCTP association.
 Optional attributes:
 None.

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 D) Abort
    Format: ABORT(association id [, Upper Layer Abort Reason]) ->
    result
 Ungracefully closes an association.  Any locally queued user data
 will be discarded, and an ABORT chunk is sent to the peer.  A success
 code will be returned on successful abort of the association.  If
 attempting to abort the association results in a failure, an error
 code shall be returned.
 Mandatory attributes:
 o association id - local handle to the SCTP association.
 Optional attributes:
 o Upper Layer Abort Reason - reason of the abort to be passed to the
 peer.
 None.
 E) Send
  Format: SEND(association id, buffer address, byte count [,context]
          [,stream id] [,life time] [,destination transport address]
          [,unordered flag] [,no-bundle flag] [,payload protocol-id] )
  -> result
 This is the main method to send user data via SCTP.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  buffer address - the location where the user message to be
    transmitted is stored.
 o  byte count - the size of the user data in number of bytes.
 Optional attributes:
 o  context - an optional 32-bit integer that will be carried in the
    sending failure notification to the ULP if the transportation of
    this user message fails.
 o  stream id - to indicate which stream to send the data on.  If not
    specified, stream 0 will be used.

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 o  life time - specifies the life time of the user data.  The user
    data will not be sent by SCTP after the life time expires.  This
    parameter can be used to avoid efforts to transmit stale user
    messages.  SCTP notifies the ULP if the data cannot be initiated
    to transport (i.e., sent to the destination via SCTP's send
    primitive) within the life time variable.  However, the user data
    will be transmitted if SCTP has attempted to transmit a chunk
    before the life time expired.
 IMPLEMENTATION NOTE: In order to better support the data life time
 option, the transmitter may hold back the assigning of the TSN number
 to an outbound DATA chunk to the last moment.  And, for
 implementation simplicity, once a TSN number has been assigned the
 sender should consider the send of this DATA chunk as committed,
 overriding any life time option attached to the DATA chunk.
 o  destination transport address - specified as one of the
    destination transport addresses of the peer endpoint to which this
    packet should be sent.  Whenever possible, SCTP should use this
    destination transport address for sending the packets, instead of
    the current primary path.
 o  unordered flag - this flag, if present, indicates that the user
    would like the data delivered in an unordered fashion to the peer
    (i.e., the U flag is set to 1 on all DATA chunks carrying this
    message).
 o  no-bundle flag - instructs SCTP not to bundle this user data with
    other outbound DATA chunks.  SCTP MAY still bundle even when this
    flag is present, when faced with network congestion.
 o  payload protocol-id - a 32-bit unsigned integer that is to be
    passed to the peer indicating the type of payload protocol data
    being transmitted.  This value is passed as opaque data by SCTP.
 F) Set Primary
    Format: SETPRIMARY(association id, destination transport address,
                       [source transport address] )
    -> result
 Instructs the local SCTP to use the specified destination transport
 address as the primary path for sending packets.
 The result of attempting this operation shall be returned.  If the
 specified destination transport address is not present in the
 "destination transport address list" returned earlier in an associate
 command or communication up notification, an error shall be returned.

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 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  destination transport address - specified as one of the transport
    addresses of the peer endpoint, which should be used as the
    primary address for sending packets.  This overrides the current
    primary address information maintained by the local SCTP endpoint.
 Optional attributes:
 o  source transport address - optionally, some implementations may
    allow you to set the default source address placed in all outgoing
    IP datagrams.
 G) Receive
  Format: RECEIVE(association id, buffer address, buffer size
          [,stream id])
  -> byte count [,transport address] [,stream id] [,stream sequence
     number] [,partial flag] [,delivery number] [,payload protocol-id]
 This primitive shall read the first user message in the SCTP in-queue
 into the buffer specified by ULP, if there is one available.  The
 size of the message read, in bytes, will be returned.  It may,
 depending on the specific implementation, also return other
 information such as the sender's address, the stream id on which it
 is received, whether there are more messages available for retrieval,
 etc.  For ordered messages, their Stream Sequence Number may also be
 returned.
 Depending upon the implementation, if this primitive is invoked when
 no message is available the implementation should return an
 indication of this condition or should block the invoking process
 until data does become available.
 Mandatory attributes:
 o  association id - local handle to the SCTP association
 o  buffer address - the memory location indicated by the ULP to store
    the received message.
 o  buffer size - the maximum size of data to be received, in bytes.
 Optional attributes:
 o  stream id - to indicate which stream to receive the data on.

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 o  Stream Sequence Number - the Stream Sequence Number assigned by
    the sending SCTP peer.
 o  partial flag - if this returned flag is set to 1, then this
    Receive contains a partial delivery of the whole message.  When
    this flag is set, the stream id and Stream Sequence Number MUST
    accompany this receive.  When this flag is set to 0, it indicates
    that no more deliveries will be received for this Stream Sequence
    Number.
 o  payload protocol-id - a 32-bit unsigned integer that is received
    from the peer indicating the type of payload protocol of the
    received data.  This value is passed as opaque data by SCTP.
 H) Status
    Format: STATUS(association id)
    -> status data
 This primitive should return a data block containing the following
 information:
    association connection state,
    destination transport address list,
    destination transport address reachability states,
    current receiver window size,
    current congestion window sizes,
    number of unacknowledged DATA chunks,
    number of DATA chunks pending receipt,
    primary path,
    most recent SRTT on primary path,
    RTO on primary path,
    SRTT and RTO on other destination addresses, etc.
 Mandatory attributes:
 o association id - local handle to the SCTP association.
 Optional attributes:
 None.
 I) Change Heartbeat
    Format: CHANGE HEARTBEAT(association id,
            destination transport address, new state [,interval])
    -> result

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 Instructs the local endpoint to enable or disable heartbeat on the
 specified destination transport address.
 The result of attempting this operation shall be returned.
 Note: Even when enabled, heartbeat will not take place if the
 destination transport address is not idle.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  destination transport address - specified as one of the transport
    addresses of the peer endpoint.
 o  new state - the new state of heartbeat for this destination
    transport address (either enabled or disabled).
 Optional attributes:
 o  interval - if present, indicates the frequency of the heartbeat if
    this is to enable heartbeat on a destination transport address.
    This value is added to the RTO of the destination transport
    address.  This value, if present, affects all destinations.
 J) Request HeartBeat
    Format: REQUESTHEARTBEAT(association id, destination transport
            address)
    -> result
 Instructs the local endpoint to perform a HeartBeat on the specified
 destination transport address of the given association.  The returned
 result should indicate whether the transmission of the HEARTBEAT
 chunk to the destination address is successful.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  destination transport address - the transport address of the
    association on which a heartbeat should be issued.
 K) Get SRTT Report
    Format: GETSRTTREPORT(association id,
                          destination transport address)
    -> srtt result

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 Instructs the local SCTP to report the current SRTT measurement on
 the specified destination transport address of the given association.
 The returned result can be an integer containing the most recent SRTT
 in milliseconds.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  destination transport address - the transport address of the
    association on which the SRTT measurement is to be reported.
 L) Set Failure Threshold
    Format: SETFAILURETHRESHOLD(association id, destination transport
            address, failure threshold)
  1. > result
 This primitive allows the local SCTP to customize the reachability
 failure detection threshold 'Path.Max.Retrans' for the specified
 destination address.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.
 o  destination transport address - the transport address of the
    association on which the failure detection threshold is to be set.
 o  failure threshold - the new value of 'Path.Max.Retrans' for the
    destination address.
 M) Set Protocol Parameters
    Format: SETPROTOCOLPARAMETERS(association id,
            [,destination transport address,]
            protocol parameter list)
    -> result
 This primitive allows the local SCTP to customize the protocol
 parameters.
 Mandatory attributes:
 o  association id - local handle to the SCTP association.

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 o  protocol parameter list - the specific names and values of the
    protocol parameters (e.g., Association.Max.Retrans; see Section
    15) that the SCTP user wishes to customize.
 Optional attributes:
 o  destination transport address - some of the protocol parameters
    may be set on a per destination transport address basis.
 N) Receive Unsent Message
    Format: RECEIVE_UNSENT(data retrieval id, buffer address, buffer
            size [,stream id] [, stream sequence number] [,partial
            flag] [,payload protocol-id])
 o  data retrieval id - the identification passed to the ULP in the
    failure notification.
 o  buffer address - the memory location indicated by the ULP to store
    the received message.
 o  buffer size - the maximum size of data to be received, in bytes.
 Optional attributes:
 o  stream id - this is a return value that is set to indicate which
    stream the data was sent to.
 o  Stream Sequence Number - this value is returned indicating the
    Stream Sequence Number that was associated with the message.
 o  partial flag - if this returned flag is set to 1, then this
    message is a partial delivery of the whole message.  When this
    flag is set, the stream id and Stream Sequence Number MUST
    accompany this receive.  When this flag is set to 0, it indicates
    that no more deliveries will be received for this Stream Sequence
    Number.
 o  payload protocol-id - The 32 bit unsigned integer that was sent to
    be sent to the peer indicating the type of payload protocol of the
    received data.
 o  Receive Unacknowledged Message
    Format: RECEIVE_UNACKED(data retrieval id, buffer address, buffer
            size, [,stream id] [, stream sequence number] [,partial
            flag] [,payload protocol-id])

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 o  data retrieval id - the identification passed to the ULP in the
    failure notification.
 o  buffer address - the memory location indicated by the ULP to store
    the received message.
 o  buffer size - the maximum size of data to be received, in bytes.
 Optional attributes:
 o  stream id - this is a return value that is set to indicate which
    stream the data was sent to.
 o  Stream Sequence Number - this value is returned indicating the
    Stream Sequence Number that was associated with the message.
 o  partial flag - if this returned flag is set to 1, then this
    message is a partial delivery of the whole message.  When this
    flag is set, the stream id and Stream Sequence Number MUST
    accompany this receive.  When this flag is set to 0, it indicates
    that no more deliveries will be received for this Stream Sequence
    Number.
 o  payload protocol-id - the 32-bit unsigned integer that was sent to
    the peer indicating the type of payload protocol of the received
    data.
 P) Destroy SCTP Instance
    Format: DESTROY(local SCTP instance name)
 o  local SCTP instance name - this is the value that was passed to
    the application in the initialize primitive and it indicates which
    SCTP instance is to be destroyed.

10.2. SCTP-to-ULP

 It is assumed that the operating system or application environment
 provides a means for the SCTP to asynchronously signal the ULP
 process.  When SCTP does signal a ULP process, certain information is
 passed to the ULP.
 IMPLEMENTATION NOTE: In some cases, this may be done through a
 separate socket or error channel.

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 A) DATA ARRIVE notification
 SCTP shall invoke this notification on the ULP when a user message is
 successfully received and ready for retrieval.
 The following may optionally be passed with the notification:
 o  association id - local handle to the SCTP association.
 o  stream id - to indicate which stream the data is received on.
 B) SEND FAILURE notification
 If a message cannot be delivered, SCTP shall invoke this notification
 on the ULP.
 The following may optionally be passed with the notification:
 o  association id - local handle to the SCTP association.
 o  data retrieval id - an identification used to retrieve unsent and
    unacknowledged data.
 o  cause code - indicating the reason of the failure, e.g., size too
    large, message life time expiration, etc.
 o  context - optional information associated with this message (see D
    in Section 10.1).
 C) NETWORK STATUS CHANGE notification
 When a destination transport address is marked inactive (e.g., when
 SCTP detects a failure) or marked active (e.g., when SCTP detects a
 recovery), SCTP shall invoke this notification on the ULP.
 The following shall be passed with the notification:
 o  association id - local handle to the SCTP association.
 o  destination transport address - this indicates the destination
    transport address of the peer endpoint affected by the change.
 o  new-status - this indicates the new status.

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 D) COMMUNICATION UP notification
 This notification is used when SCTP becomes ready to send or receive
 user messages, or when a lost communication to an endpoint is
 restored.
 IMPLEMENTATION NOTE: If the ASSOCIATE primitive is implemented as a
 blocking function call, the association parameters are returned as a
 result of the ASSOCIATE primitive itself.  In that case,
 COMMUNICATION UP notification is optional at the association
 initiator's side.
 The following shall be passed with the notification:
 o  association id -  local handle to the SCTP association.
 o  status -  This indicates what type of event has occurred.
 o  destination transport address list -  the complete set of
    transport addresses of the peer.
 o  outbound stream count -  the maximum number of streams allowed to
    be used in this association by the ULP.
 o  inbound stream count -  the number of streams the peer endpoint
    has requested with this association (this may not be the same
    number as 'outbound stream count').
 E) COMMUNICATION LOST notification
 When SCTP loses communication to an endpoint completely (e.g., via
 Heartbeats) or detects that the endpoint has performed an abort
 operation, it shall invoke this notification on the ULP.
 The following shall be passed with the notification:
 o  association id -  local handle to the SCTP association.
 o  status -  this indicates what type of event has occurred; the
              status may indicate that a failure OR a normal
              termination event occurred in response to a shutdown or
              abort request.
 The following may be passed with the notification:
 o  data retrieval id -  an identification used to retrieve unsent and
    unacknowledged data.

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 o  last-acked -  the TSN last acked by that peer endpoint.
 o  last-sent -  the TSN last sent to that peer endpoint.
 o  Upper Layer Abort Reason -  the abort reason specified in case of
    a user-initiated abort.
 F) COMMUNICATION ERROR notification
 When SCTP receives an ERROR chunk from its peer and decides to notify
 its ULP, it can invoke this notification on the ULP.
 The following can be passed with the notification:
 o  association id -  local handle to the SCTP association.
 o  error info -  this indicates the type of error and optionally some
    additional information received through the ERROR chunk.
 G) RESTART notification
 When SCTP detects that the peer has restarted, it may send this
 notification to its ULP.
 The following can be passed with the notification:
 o  association id -  local handle to the SCTP association.
 H) SHUTDOWN COMPLETE notification
 When SCTP completes the shutdown procedures (Section 9.2), this
 notification is passed to the upper layer.
 The following can be passed with the notification:
 o  association id -  local handle to the SCTP association.

11. Security Considerations

11.1. Security Objectives

 As a common transport protocol designed to reliably carry time-
 sensitive user messages, such as billing or signaling messages for
 telephony services, between two networked endpoints, SCTP has the
 following security objectives.
  1. availability of reliable and timely data transport services

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  1. integrity of the user-to-user information carried by SCTP

11.2. SCTP Responses to Potential Threats

 SCTP may potentially be used in a wide variety of risk situations.
 It is important for operators of systems running SCTP to analyze
 their particular situations and decide on the appropriate counter-
 measures.
 Operators of systems running SCTP should consult [RFC2196] for
 guidance in securing their site.

11.2.1. Countering Insider Attacks

 The principles of [RFC2196] should be applied to minimize the risk of
 theft of information or sabotage by insiders.  Such procedures
 include publication of security policies, control of access at the
 physical, software, and network levels, and separation of services.

11.2.2. Protecting against Data Corruption in the Network

 Where the risk of undetected errors in datagrams delivered by the
 lower-layer transport services is considered to be too great,
 additional integrity protection is required.  If this additional
 protection were provided in the application layer, the SCTP header
 would remain vulnerable to deliberate integrity attacks.  While the
 existing SCTP mechanisms for detection of packet replays are
 considered sufficient for normal operation, stronger protections are
 needed to protect SCTP when the operating environment contains
 significant risk of deliberate attacks from a sophisticated
 adversary.
 The SCTP Authentication extension SCTP-AUTH [RFC4895] MAY be used
 when the threat environment requires stronger integrity protections,
 but does not require confidentiality.

11.2.3. Protecting Confidentiality

 In most cases, the risk of breach of confidentiality applies to the
 signaling data payload, not to the SCTP or lower-layer protocol
 overheads.  If that is true, encryption of the SCTP user data only
 might be considered.  As with the supplementary checksum service,
 user data encryption MAY be performed by the SCTP user application.
 Alternately, the user application MAY use an implementation-specific
 API to request that the IP Encapsulating Security Payload (ESP)
 [RFC4303] be used to provide confidentiality and integrity.

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 Particularly for mobile users, the requirement for confidentiality
 might include the masking of IP addresses and ports.  In this case,
 ESP SHOULD be used instead of application-level confidentiality.  If
 ESP is used to protect confidentiality of SCTP traffic, an ESP
 cryptographic transform that includes cryptographic integrity
 protection MUST be used, because if there is a confidentiality threat
 there will also be a strong integrity threat.
 Whenever ESP is in use, application-level encryption is not generally
 required.
 Regardless of where confidentiality is provided, the Internet Key
 Exchange Protocol version 2 (IKEv2) [RFC4306] SHOULD be used for key
 management.
 Operators should consult [RFC4301] for more information on the
 security services available at and immediately above the Internet
 Protocol layer.

11.2.4. Protecting against Blind Denial-of-Service Attacks

 A blind attack is one where the attacker is unable to intercept or
 otherwise see the content of data flows passing to and from the
 target SCTP node.  Blind denial-of-service attacks may take the form
 of flooding, masquerade, or improper monopolization of services.

11.2.4.1. Flooding

 The objective of flooding is to cause loss of service and incorrect
 behavior at target systems through resource exhaustion, interference
 with legitimate transactions, and exploitation of buffer-related
 software bugs.  Flooding may be directed either at the SCTP node or
 at resources in the intervening IP Access Links or the Internet.
 Where the latter entities are the target, flooding will manifest
 itself as loss of network services, including potentially the breach
 of any firewalls in place.
 In general, protection against flooding begins at the equipment
 design level, where it includes measures such as:
  1. avoiding commitment of limited resources before determining that

the request for service is legitimate.

  1. giving priority to completion of processing in progress over the

acceptance of new work.

  1. identification and removal of duplicate or stale queued requests

for service.

Stewart Standards Track [Page 125] RFC 4960 Stream Control Transmission Protocol September 2007

  1. not responding to unexpected packets sent to non-unicast

addresses.

 Network equipment should be capable of generating an alarm and log if
 a suspicious increase in traffic occurs.  The log should provide
 information such as the identity of the incoming link and source
 address(es) used, which will help the network or SCTP system operator
 to take protective measures.  Procedures should be in place for the
 operator to act on such alarms if a clear pattern of abuse emerges.
 The design of SCTP is resistant to flooding attacks, particularly in
 its use of a four-way startup handshake, its use of a cookie to defer
 commitment of resources at the responding SCTP node until the
 handshake is completed, and its use of a Verification Tag to prevent
 insertion of extraneous packets into the flow of an established
 association.
 The IP Authentication Header and Encapsulating Security Payload might
 be useful in reducing the risk of certain kinds of denial-of-service
 attacks.
 The use of the host name feature in the INIT chunk could be used to
 flood a target DNS server.  A large backlog of DNS queries, resolving
 the host name received in the INIT chunk to IP addresses, could be
 accomplished by sending INITs to multiple hosts in a given domain.
 In addition, an attacker could use the host name feature in an
 indirect attack on a third party by sending large numbers of INITs to
 random hosts containing the host name of the target.  In addition to
 the strain on DNS resources, this could also result in large numbers
 of INIT ACKs being sent to the target.  One method to protect against
 this type of attack is to verify that the IP addresses received from
 DNS include the source IP address of the original INIT.  If the list
 of IP addresses received from DNS does not include the source IP
 address of the INIT, the endpoint MAY silently discard the INIT.
 This last option will not protect against the attack against the DNS.

11.2.4.2. Blind Masquerade

 Masquerade can be used to deny service in several ways:
  1. by tying up resources at the target SCTP node to which the

impersonated node has limited access. For example, the target

    node may by policy permit a maximum of one SCTP association with
    the impersonated SCTP node.  The masquerading attacker may attempt
    to establish an association purporting to come from the
    impersonated node so that the latter cannot do so when it requires
    it.

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  1. by deliberately allowing the impersonation to be detected, thereby

provoking counter-measures that cause the impersonated node to be

    locked out of the target SCTP node.
  1. by interfering with an established association by inserting

extraneous content such as a SHUTDOWN request.

 SCTP reduces the risk of blind masquerade attacks through IP spoofing
 by use of the four-way startup handshake.  Because the initial
 exchange is memory-less, no lockout mechanism is triggered by blind
 masquerade attacks.  In addition, the INIT ACK containing the State
 Cookie is transmitted back to the IP address from which it received
 the INIT.  Thus, the attacker would not receive the INIT ACK
 containing the State Cookie.  SCTP protects against insertion of
 extraneous packets into the flow of an established association by use
 of the Verification Tag.
 Logging of received INIT requests and abnormalities such as
 unexpected INIT ACKs might be considered as a way to detect patterns
 of hostile activity.  However, the potential usefulness of such
 logging must be weighed against the increased SCTP startup processing
 it implies, rendering the SCTP node more vulnerable to flooding
 attacks.  Logging is pointless without the establishment of operating
 procedures to review and analyze the logs on a routine basis.

11.2.4.3. Improper Monopolization of Services

 Attacks under this heading are performed openly and legitimately by
 the attacker.  They are directed against fellow users of the target
 SCTP node or of the shared resources between the attacker and the
 target node.  Possible attacks include the opening of a large number
 of associations between the attacker's node and the target, or
 transfer of large volumes of information within a legitimately
 established association.
 Policy limits should be placed on the number of associations per
 adjoining SCTP node.  SCTP user applications should be capable of
 detecting large volumes of illegitimate or "no-op" messages within a
 given association and either logging or terminating the association
 as a result, based on local policy.

11.3. SCTP Interactions with Firewalls

 It is helpful for some firewalls if they can inspect just the first
 fragment of a fragmented SCTP packet and unambiguously determine
 whether it corresponds to an INIT chunk (for further information,
 please refer to [RFC1858]).  Accordingly, we stress the requirements,
 stated in Section 3.1, that (1) an INIT chunk MUST NOT be bundled

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 with any other chunk in a packet, and (2) a packet containing an INIT
 chunk MUST have a zero Verification Tag.  Furthermore, we require
 that the receiver of an INIT chunk MUST enforce these rules by
 silently discarding an arriving packet  with an INIT chunk that is
 bundled with other chunks or has a non-zero verification tag and
 contains an INIT-chunk.

11.4. Protection of Non-SCTP-Capable Hosts

 To provide a non-SCTP-capable host with the same level of protection
 against attacks as for SCTP-capable ones, all SCTP stacks MUST
 implement the ICMP handling described in Appendix C.
 When an SCTP stack receives a packet containing multiple control or
 DATA chunks and the processing of the packet requires the sending of
 multiple chunks in response, the sender of the response chunk(s) MUST
 NOT send more than one packet.  If bundling is supported, multiple
 response chunks that fit into a single packet MAY be bundled together
 into one single response packet.  If bundling is not supported, then
 the sender MUST NOT send more than one response chunk and MUST
 discard all other responses.  Note that this rule does NOT apply to a
 SACK chunk, since a SACK chunk is, in itself, a response to DATA and
 a SACK does not require a response of more DATA.
 An SCTP implementation SHOULD abort the association if it receives a
 SACK acknowledging a TSN that has not been sent.
 An SCTP implementation that receives an INIT that would require a
 large packet in response, due to the inclusion of multiple ERROR
 parameters, MAY (at its discretion) elect to omit some or all of the
 ERROR parameters to reduce the size of the INIT ACK.  Due to a
 combination of the size of the COOKIE parameter and the number of
 addresses a receiver of an INIT may be indicating to a peer, it is
 always possible that the INIT ACK will be larger than the original
 INIT.  An SCTP implementation SHOULD attempt to make the INIT ACK as
 small as possible to reduce the possibility of byte amplification
 attacks.

12. Network Management Considerations

 The MIB module for SCTP defined in [RFC3873] applies for the version
 of the protocol specified in this document.

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13. Recommended Transmission Control Block (TCB) Parameters

 This section details a recommended set of parameters that should be
 contained within the TCB for an implementation.  This section is for
 illustrative purposes and should not be deemed as requirements on an
 implementation or as an exhaustive list of all parameters inside an
 SCTP TCB.  Each implementation may need its own additional parameters
 for optimization.

13.1. Parameters Necessary for the SCTP Instance

 Associations: A list of current associations and mappings to the data
               consumers for each association.  This may be in the
               form of a hash table or other implementation-dependent
               structure.  The data consumers may be process
               identification information such as file descriptors,
               named pipe pointer, or table pointers dependent on how
               SCTP is implemented.
 Secret Key:   A secret key used by this endpoint to compute the MAC.
               This SHOULD be a cryptographic quality random number
               with a sufficient length.  Discussion in RFC 4086 can
               be helpful in selection of the key.
 Address List: The list of IP addresses that this instance has bound.
               This information is passed to one's peer(s) in INIT and
               INIT ACK chunks.
 SCTP Port:    The local SCTP port number to which the endpoint is
               bound.

13.2. Parameters Necessary per Association (i.e., the TCB)

 Peer        : Tag value to be sent in every packet and is received
 Verification: in the INIT or INIT ACK chunk.
 Tag         :
 My          : Tag expected in every inbound packet and sent in the
 Verification: INIT or INIT ACK chunk.
 Tag         :
 State       : A state variable indicating what state the association
             : is in, i.e., COOKIE-WAIT, COOKIE-ECHOED, ESTABLISHED,
             : SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED,
             : SHUTDOWN-ACK-SENT.
               Note: No "CLOSED" state is illustrated since if a
               association is "CLOSED" its TCB SHOULD be removed.

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 Peer        : A list of SCTP transport addresses to which the peer
 Transport   : is bound.  This information is derived from the INIT or
 Address     : INIT ACK and is used to associate an inbound packet
 List        : with a given association.  Normally, this information
             : is hashed or keyed for quick lookup and access of the
             : TCB.
 Primary     : This is the current primary destination transport
 Path        : address of the peer endpoint.  It may also specify a
             : source transport address on this endpoint.
 Overall     : The overall association error count.
 Error Count :
 Overall     : The threshold for this association that if the Overall
 Error       : Error Count reaches will cause this association to be
 Threshold   : torn down.
 Peer Rwnd   : Current calculated value of the peer's rwnd.
 Next TSN    : The next TSN number to be assigned to a new DATA chunk.
             : This is sent in the INIT or INIT ACK chunk to the peer
             : and incremented each time a DATA chunk is assigned a
             : TSN (normally just prior to transmit or during
             : fragmentation).
 Last Rcvd   : This is the last TSN received in sequence.  This value
 TSN         : is set initially by taking the peer's initial TSN,
             : received in the INIT or INIT ACK chunk, and
             : subtracting one from it.
 Mapping     : An array of bits or bytes indicating which out-of-
 Array       : order TSNs have been received (relative to the
             : Last Rcvd TSN).  If no gaps exist, i.e., no out-of-
             : order packets have been received, this array will
             : be set to all zero.  This structure may be in the
             : form of a circular buffer or bit array.
 Ack State   : This flag indicates if the next received packet
             : is to be responded to with a SACK.  This is initialized
             : to 0.  When a packet is received it is incremented.
             : If this value reaches 2 or more, a SACK is sent and the
             : value is reset to 0.  Note: This is used only when no
             : DATA chunks are received out of order.  When DATA
             : chunks are out of order, SACKs are not delayed (see
             : Section 6).

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 Inbound     : An array of structures to track the inbound streams,
 Streams     : normally including the next sequence number expected
             : and possibly the stream number.
 Outbound    : An array of structures to track the outbound streams,
 Streams     : normally including the next sequence number to
             : be sent on the stream.
 Reasm Queue : A reassembly queue.
 Local       : The list of local IP addresses bound in to this
 Transport   : association.
 Address     :
 List        :
 Association : The smallest PMTU discovered for all of the
 PMTU        : peer's transport addresses.

13.3. Per Transport Address Data

 For each destination transport address in the peer's address list
 derived from the INIT or INIT ACK chunk, a number of data elements
 need to be maintained including:
 Error Count : The current error count for this destination.
 Error       : Current error threshold for this destination, i.e.,
 Threshold   : what value marks the destination down if error count
             : reaches this value.
 cwnd        : The current congestion window.
 ssthresh    : The current ssthresh value.
 RTO         : The current retransmission timeout value.
 SRTT        : The current smoothed round-trip time.
 RTTVAR      : The current RTT variation.
 partial     : The tracking method for increase of cwnd when in
 bytes acked : congestion avoidance mode (see Section 7.2.2).
 state       : The current state of this destination, i.e., DOWN, UP,
             : ALLOW-HB, NO-HEARTBEAT, etc.

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 PMTU        : The current known path MTU.
 Per         : A timer used by each destination.
 Destination :
 Timer       :
 RTO-Pending : A flag used to track if one of the DATA chunks sent to
             : this address is currently being used to compute an
             : RTT.  If this flag is 0, the next DATA chunk sent to
             : this destination should be used to compute an RTT and
             : this flag should be set.  Every time the RTT
             : calculation completes (i.e., the DATA chunk is SACK'd),
             : clear this flag.
 last-time   : The time to which this destination was last sent.
             : This can be to determine if a HEARTBEAT is needed.

13.4. General Parameters Needed

 Out Queue : A queue of outbound DATA chunks.
 In Queue  : A queue of inbound DATA chunks.

14. IANA Considerations

 SCTP defines three registries that IANA maintains:
  1. through definition of additional chunk types,
  2. through definition of additional parameter types, or
  3. through definition of additional cause codes within ERROR chunks.
 SCTP requires that the IANA Port Numbers registry be opened for SCTP
 port registrations, Section 14.5 describes how.  An IESG-appointed
 Expert Reviewer supports IANA in evaluating SCTP port allocation
 requests.

14.1. IETF-Defined Chunk Extension

 The assignment of new chunk parameter type codes is done through an
 IETF Consensus action, as defined in [RFC2434].  Documentation of the
 chunk parameter MUST contain the following information:
 a) A long and short name for the new chunk type.
 b) A detailed description of the structure of the chunk, which MUST
    conform to the basic structure defined in Section 3.2.

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 c) A detailed definition and description of intended use of each
    field within the chunk, including the chunk flags if any.
 d) A detailed procedural description of the use of the new chunk type
    within the operation of the protocol.
 The last chunk type (255) is reserved for future extension if
 necessary.

14.2. IETF-Defined Chunk Parameter Extension

 The assignment of new chunk parameter type codes is done through an
 IETF Consensus action as defined in [RFC2434].  Documentation of the
 chunk parameter MUST contain the following information:
 a) Name of the parameter type.
 b) Detailed description of the structure of the parameter field.
    This structure MUST conform to the general Type-Length-Value
    format described in Section 3.2.1.
 c) Detailed definition of each component of the parameter value.
 d) Detailed description of the intended use of this parameter type,
    and an indication of whether and under what circumstances multiple
    instances of this parameter type may be found within the same
    chunk.
 e) Each parameter type MUST be unique across all chunks.

14.3. IETF-Defined Additional Error Causes

 Additional cause codes may be allocated in the range 11 to 65535
 through a Specification Required action as defined in [RFC2434].
 Provided documentation must include the following information:
 a) Name of the error condition.
 b) Detailed description of the conditions under which an SCTP
    endpoint should issue an ERROR (or ABORT) with this cause code.
 c) Expected action by the SCTP endpoint that receives an ERROR (or
    ABORT) chunk containing this cause code.
 d) Detailed description of the structure and content of data fields
    that accompany this cause code.

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 The initial word (32 bits) of a cause code parameter MUST conform to
 the format shown in Section 3.3.10, i.e.:
  1. first 2 bytes contain the cause code value
  2. last 2 bytes contain the length of the cause parameter.

14.4. Payload Protocol Identifiers

 Except for value 0, which is reserved by SCTP to indicate an
 unspecified payload protocol identifier in a DATA chunk, SCTP will
 not be responsible for standardizing or verifying any payload
 protocol identifiers; SCTP simply receives the identifier from the
 upper layer and carries it with the corresponding payload data.
 The upper layer, i.e., the SCTP user, SHOULD standardize any specific
 protocol identifier with IANA if it is so desired.  The use of any
 specific payload protocol identifier is out of the scope of SCTP.

14.5. Port Numbers Registry

 SCTP services may use contact port numbers to provide service to
 unknown callers, as in TCP and UDP.  IANA is therefore requested to
 open the existing Port Numbers registry for SCTP using the following
 rules, which we intend to mesh well with existing Port Numbers
 registration procedures.  An IESG-appointed Expert Reviewer supports
 IANA in evaluating SCTP port allocation requests, according to the
 procedure defined in [RFC2434].
 Port numbers are divided into three ranges.  The Well Known Ports are
 those from 0 through 1023, the Registered Ports are those from 1024
 through 49151, and the Dynamic and/or Private Ports are those from
 49152 through 65535.  Well Known and Registered Ports are intended
 for use by server applications that desire a default contact point on
 a system.  On most systems, Well Known Ports can only be used by
 system (or root) processes or by programs executed by privileged
 users, while Registered Ports can be used by ordinary user processes
 or programs executed by ordinary users.  Dynamic and/or Private Ports
 are intended for temporary use, including client-side ports, out-of-
 band negotiated ports, and application testing prior to registration
 of a dedicated port; they MUST NOT be registered.
 The Port Numbers registry should accept registrations for SCTP ports
 in the Well Known Ports and Registered Ports ranges.  Well Known and
 Registered Ports SHOULD NOT be used without registration.  Although
 in some cases -- such as porting an application from TCP to SCTP --
 it may seem natural to use an SCTP port before registration
 completes, we emphasize that IANA will not guarantee registration of

Stewart Standards Track [Page 134] RFC 4960 Stream Control Transmission Protocol September 2007

 particular Well Known and Registered Ports.  Registrations should be
 requested as early as possible.
 Each port registration SHALL include the following information:
 o  A short port name, consisting entirely of letters (A-Z and a-z),
    digits (0-9), and punctuation characters from "-_+./*" (not
    including the quotes).
 o  The port number that is requested for registration.
 o  A short English phrase describing the port's purpose.
 o  Name and contact information for the person or entity performing
    the registration, and possibly a reference to a document defining
    the port's use.  Registrations coming from IETF working groups
    need only name the working group, but indicating a contact person
    is recommended.
 Registrants are encouraged to follow these guidelines when submitting
 a registration.
 o  A port name SHOULD NOT be registered for more than one SCTP port
    number.
 o  A port name registered for TCP MAY be registered for SCTP as well.
    Any such registration SHOULD use the same port number as the
    existing TCP registration.
 o  Concrete intent to use a port SHOULD precede port registration.
    For example, existing TCP ports SHOULD NOT be registered in
    advance of any intent to use those ports for SCTP.
    This document registers the following ports.  (These registrations
    should be considered models to follow for future allocation
    requests.)
       discard    9/sctp  Discard  # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          The discard service, which accepts SCTP connections on port
          9, discards all incoming application data and sends no data
          in response.  Thus, SCTP's discard port is analogous to
          TCP's discard port, and might be used to check the health
          of an SCTP stack.

Stewart Standards Track [Page 135] RFC 4960 Stream Control Transmission Protocol September 2007

       ftp-data  20/sctp  FTP      # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
       ftp       21/sctp  FTP      # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          File Transfer Protocol (FTP) data (20) and control ports
          (21).
       ssh       22/sctp  SSH      # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          The Secure Shell (SSH) remote login service, which allows
          secure shell logins to a host.
       http      80/sctp  HTTP     # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          World Wide Web HTTP over SCTP.
       bgp      179/sctp  BGP      # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          Border Gateway Protocol over SCTP.
       https    443/sctp  HTTPS    # IETF TSVWG
                                   # Randall Stewart <rrs@cisco.com>
                                   # [RFC4960]
          World Wide Web HTTP over TLS/SSL over SCTP.

15. Suggested SCTP Protocol Parameter Values

 The following protocol parameters are RECOMMENDED:
    RTO.Initial - 3 seconds
    RTO.Min - 1 second
    RTO.Max - 60 seconds
    Max.Burst - 4
    RTO.Alpha - 1/8
    RTO.Beta - 1/4
    Valid.Cookie.Life - 60 seconds
    Association.Max.Retrans - 10 attempts

Stewart Standards Track [Page 136] RFC 4960 Stream Control Transmission Protocol September 2007

    Path.Max.Retrans - 5 attempts (per destination address)
    Max.Init.Retransmits - 8 attempts
    HB.interval - 30 seconds
    HB.Max.Burst - 1
 IMPLEMENTATION NOTE: The SCTP implementation may allow ULP to
 customize some of these protocol parameters (see Section 10).
 Note: RTO.Min SHOULD be set as recommended above.

16. Acknowledgements

 An undertaking represented by this updated document is not a small
 feat and represents the summation of the initial authors of RFC 2960:
 Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor, I.
 Rytina, M. Kalla, L. Zhang, and V. Paxson.
 Add to that, the comments from everyone who contributed to the
 original RFC:
 Mark Allman, R.J. Atkinson, Richard Band, Scott Bradner, Steve
 Bellovin, Peter Butler, Ram Dantu, R. Ezhirpavai, Mike Fisk, Sally
 Floyd, Atsushi Fukumoto, Matt Holdrege, Henry Houh, Christian
 Huitema, Gary Lehecka, Jonathan Lee, David Lehmann, John Loughney,
 Daniel Luan, Barry Nagelberg, Thomas Narten, Erik Nordmark, Lyndon
 Ong, Shyamal Prasad, Kelvin Porter, Heinz Prantner, Jarno Rajahalme,
 Raymond E. Reeves, Renee Revis, Ivan Arias Rodriguez, A. Sankar, Greg
 Sidebottom, Brian Wyld, La Monte Yarroll, and many others for their
 invaluable comments.
 Then, add the authors of the SCTP implementor's guide, I. Arias-
 Rodriguez, K. Poon, A. Caro, and M. Tuexen.
 Then add to these the efforts of all the subsequent seven SCTP
 interoperability tests and those who commented on RFC 4460 as shown
 in its acknowledgements:
 Barry Zuckerman, La Monte Yarroll, Qiaobing Xie, Wang Xiaopeng,
 Jonathan Wood, Jeff Waskow, Mike Turner, John Townsend, Sabina
 Torrente, Cliff Thomas, Yuji Suzuki, Manoj Solanki, Sverre Slotte,
 Keyur Shah, Jan Rovins, Ben Robinson, Renee Revis, Ian Periam, RC
 Monee, Sanjay Rao, Sujith Radhakrishnan, Heinz Prantner, Biren Patel,
 Nathalie Mouellic, Mitch Miers, Bernward Meyknecht, Stan McClellan,
 Oliver Mayor, Tomas Orti Martin, Sandeep Mahajan, David Lehmann,
 Jonathan Lee, Philippe Langlois, Karl Knutson, Joe Keller, Gareth
 Keily, Andreas Jungmaier, Janardhan Iyengar, Mutsuya Irie, John
 Hebert, Kausar Hassan, Fred Hasle, Dan Harrison, Jon Grim, Laurent
 Glaude, Steven Furniss, Atsushi Fukumoto, Ken Fujita, Steve Dimig,

Stewart Standards Track [Page 137] RFC 4960 Stream Control Transmission Protocol September 2007

 Thomas Curran, Serkan Cil, Melissa Campbell, Peter Butler, Rob
 Brennan, Harsh Bhondwe, Brian Bidulock, Caitlin Bestler, Jon Berger,
 Robby Benedyk, Stephen Baucke, Sandeep Balani, and Ronnie Sellar.
 A special thanks to Mark Allman, who should actually be a co-author
 for his work on the max-burst, but managed to wiggle out due to a
 technicality.  Also, we would like to acknowledge Lyndon Ong and Phil
 Conrad for their valuable input and many contributions.
 And finally, you have this document, and those who have commented
 upon that including Alfred Hoenes and Ronnie Sellars.
 My thanks cannot be adequately expressed to all of you who have
 participated in the coding, testing, and updating process of this
 document.  All I can say is, Thank You!
 Randall Stewart - Editor

Stewart Standards Track [Page 138] RFC 4960 Stream Control Transmission Protocol September 2007

Appendix A. Explicit Congestion Notification

 ECN [RFC3168] describes a proposed extension to IP that details a
 method to become aware of congestion outside of datagram loss.  This
 is an optional feature that an implementation MAY choose to add to
 SCTP.  This appendix details the minor differences implementers will
 need to be aware of if they choose to implement this feature.  In
 general, [RFC3168] should be followed with the following exceptions.
 Negotiation:
 [RFC3168] details negotiation of ECN during the SYN and SYN-ACK
 stages of a TCP connection.  The sender of the SYN sets 2 bits in the
 TCP flags, and the sender of the SYN-ACK sets only 1 bit.  The
 reasoning behind this is to ensure that both sides are truly ECN
 capable.  For SCTP, this is not necessary.  To indicate that an
 endpoint is ECN capable, an endpoint SHOULD add to the INIT and or
 INIT ACK chunk the TLV reserved for ECN.  This TLV contains no
 parameters, and thus has the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Parameter Type = 32768      |     Parameter Length = 4      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ECN-Echo:
 [RFC3168] details a specific bit for a receiver to send back in its
 TCP acknowledgements to notify the sender of the Congestion
 Experienced (CE) bit having arrived from the network.  For SCTP, this
 same indication is made by including the ECNE chunk.  This chunk
 contains one data element, i.e., the lowest TSN associated with the
 IP datagram marked with the CE bit, and looks as follows:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Chunk Type=12 | Flags=00000000|    Chunk Length = 8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Lowest TSN Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Note: The ECNE is considered a Control chunk.

Stewart Standards Track [Page 139] RFC 4960 Stream Control Transmission Protocol September 2007

 CWR:
 [RFC3168] details a specific bit for a sender to send in the header
 of its next outbound TCP segment to indicate to its peer that it has
 reduced its congestion window.  This is termed the CWR bit.  For
 SCTP, the same indication is made by including the CWR chunk.  This
 chunk contains one data element, i.e., the TSN number that was sent
 in the ECNE chunk.  This element represents the lowest TSN number in
 the datagram that was originally marked with the CE bit.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Chunk Type=13 | Flags=00000000|    Chunk Length = 8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Lowest TSN Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Note: The CWR is considered a Control chunk.

Appendix B. CRC32c Checksum Calculation

 We define a 'reflected value' as one that is the opposite of the
 normal bit order of the machine.  The 32-bit CRC (Cyclic Redundancy
 Check) is calculated as described for CRC32c and uses the polynomial
 code 0x11EDC6F41 (Castagnoli93) or x^32+x^28+x^27+x^26+x^25
 +x^23+x^22+x^20+x^19+x^18+ x^14+x^13+x^11+x^10+x^9+x^8+x^6+x^0.  The
 CRC is computed using a procedure similar to ETHERNET CRC [ITU32],
 modified to reflect transport-level usage.
 CRC computation uses polynomial division.  A message bit-string M is
 transformed to a polynomial, M(X), and the CRC is calculated from
 M(X) using polynomial arithmetic.
 When CRCs are used at the link layer, the polynomial is derived from
 on-the-wire bit ordering: the first bit 'on the wire' is the high-
 order coefficient.  Since SCTP is a transport-level protocol, it
 cannot know the actual serial-media bit ordering.  Moreover,
 different links in the path between SCTP endpoints may use different
 link-level bit orders.
 A convention must therefore be established for mapping SCTP transport
 messages to polynomials for purposes of CRC computation.  The bit-
 ordering for mapping SCTP messages to polynomials is that bytes are
 taken most-significant first, but within each byte, bits are taken
 least-significant first.  The first byte of the message provides the
 eight highest coefficients.  Within each byte, the least-significant
 SCTP bit gives the most-significant polynomial coefficient within

Stewart Standards Track [Page 140] RFC 4960 Stream Control Transmission Protocol September 2007

 that byte, and the most-significant SCTP bit is the least-significant
 polynomial coefficient in that byte.  (This bit ordering is sometimes
 called 'mirrored' or 'reflected' [WILLIAMS93].)  CRC polynomials are
 to be transformed back into SCTP transport-level byte values, using a
 consistent mapping.
 The SCTP transport-level CRC value should be calculated as follows:
  1. CRC input data are assigned to a byte stream, numbered from 0 to

N-1.

  1. The transport-level byte stream is mapped to a polynomial value.

An N-byte PDU with j bytes numbered 0 to N-1 is considered as

    coefficients of a polynomial M(x) of order 8N-1, with bit 0 of
    byte j being coefficient x^(8(N-j)-8), and bit 7 of byte j being
    coefficient x^(8(N-j)-1).
  1. The CRC remainder register is initialized with all 1s and the CRC

is computed with an algorithm that simultaneously multiplies by

    x^32 and divides by the CRC polynomial.
  1. The polynomial is multiplied by x^32 and divided by G(x), the

generator polynomial, producing a remainder R(x) of degree less

    than or equal to 31.
  1. The coefficients of R(x) are considered a 32-bit sequence.
  1. The bit sequence is complemented. The result is the CRC

polynomial.

  1. The CRC polynomial is mapped back into SCTP transport-level bytes.

The coefficient of x^31 gives the value of bit 7 of SCTP byte 0,

    and the coefficient of x^24 gives the value of bit 0 of byte 0.
    The coefficient of x^7 gives bit 7 of byte 3, and the coefficient
    of x^0 gives bit 0 of byte 3.  The resulting 4-byte transport-
    level sequence is the 32-bit SCTP checksum value.
 IMPLEMENTATION NOTE: Standards documents, textbooks, and vendor
 literature on CRCs often follow an alternative formulation, in which
 the register used to hold the remainder of the long-division
 algorithm is initialized to zero rather than all-1s, and instead the
 first 32 bits of the message are complemented.  The long-division
 algorithm used in our formulation is specified such that the initial
 multiplication by 2^32 and the long-division are combined into one
 simultaneous operation.  For such algorithms, and for messages longer
 than 64 bits, the two specifications are precisely equivalent.  That
 equivalence is the intent of this document.

Stewart Standards Track [Page 141] RFC 4960 Stream Control Transmission Protocol September 2007

 Implementors of SCTP are warned that both specifications are to be
 found in the literature, sometimes with no restriction on the long-
 division algorithm.  The choice of formulation in this document is to
 permit non-SCTP usage, where the same CRC algorithm may be used to
 protect messages shorter than 64 bits.
 There may be a computational advantage in validating the association
 against the Verification Tag, prior to performing a checksum, as
 invalid tags will result in the same action as a bad checksum in most
 cases.  The exceptions for this technique would be INIT and some
 SHUTDOWN-COMPLETE exchanges, as well as a stale COOKIE ECHO.  These
 special-case exchanges must represent small packets and will minimize
 the effect of the checksum calculation.

Appendix C. ICMP Handling

 Whenever an ICMP message is received by an SCTP endpoint, the
 following procedures MUST be followed to ensure proper utilization of
 the information being provided by layer 3.
 ICMP1) An implementation MAY ignore all ICMPv4 messages where the
        type field is not set to "Destination Unreachable".
 ICMP2) An implementation MAY ignore all ICMPv6 messages where the
        type field is not "Destination Unreachable", "Parameter
        Problem",, or "Packet Too Big".
 ICMP3) An implementation MAY ignore any ICMPv4 messages where the
        code does not indicate "Protocol Unreachable" or
        "Fragmentation Needed".
 ICMP4) An implementation MAY ignore all ICMPv6 messages of type
        "Parameter Problem" if the code is not "Unrecognized Next
        Header Type Encountered".
 ICMP5) An implementation MUST use the payload of the ICMP message (v4
        or v6) to locate the association that sent the message to
        which ICMP is responding.  If the association cannot be found,
        an implementation SHOULD ignore the ICMP message.
 ICMP6) An implementation MUST validate that the Verification Tag
        contained in the ICMP message matches the Verification Tag of
        the peer.  If the Verification Tag is not 0 and does NOT
        match, discard the ICMP message.  If it is 0 and the ICMP
        message contains enough bytes to verify that the chunk type is
        an INIT chunk and that the Initiate Tag matches the tag of the

Stewart Standards Track [Page 142] RFC 4960 Stream Control Transmission Protocol September 2007

        peer, continue with ICMP7.  If the ICMP message is too short
        or the chunk type or the Initiate Tag does not match, silently
        discard the packet.
 ICMP7) If the ICMP message is either a v6 "Packet Too Big" or a v4
        "Fragmentation Needed", an implementation MAY process this
        information as defined for PATH MTU discovery.
 ICMP8) If the ICMP code is an "Unrecognized Next Header Type
        Encountered" or a "Protocol Unreachable", an implementation
        MUST treat this message as an abort with the T bit set if it
        does not contain an INIT chunk.  If it does contain an INIT
        chunk and the association is in the COOKIE-WAIT state, handle
        the ICMP message like an ABORT.
 ICMP9) If the ICMPv6 code is "Destination Unreachable", the
        implementation MAY mark the destination into the unreachable
        state or alternatively increment the path error counter.
 Note that these procedures differ from [RFC1122] and from its
 requirements for processing of port-unreachable messages and the
 requirements that an implementation MUST abort associations in
 response to a "protocol unreachable" message.  Port-unreachable
 messages are not processed, since an implementation will send an
 ABORT, not a port unreachable.  The stricter handling of the
 "protocol unreachable" message is due to security concerns for hosts
 that do NOT support SCTP.
 The following non-normative sample code is taken from an open-source
 CRC generator [WILLIAMS93], using the "mirroring" technique and
 yielding a lookup table for SCTP CRC32c with 256 entries, each 32
 bits wide.  While neither especially slow nor especially fast, as
 software table-lookup CRCs go, it has the advantage of working on
 both big-endian and little-endian CPUs, using the same (host-order)
 lookup tables, and using only the predefined ntohl() and htonl()
 operations.  The code is somewhat modified from [WILLIAMS93], to
 ensure portability between big-endian and little-endian
 architectures.  (Note that if the byte endian-ness of the target
 architecture is known to be little-endian, the final bit-reversal and
 byte-reversal steps can be folded into a single operation.)
 /*************************************************************/
 /* Note Definition for Ross Williams table generator would   */
 /* be: TB_WIDTH=4, TB_POLLY=0x1EDC6F41, TB_REVER=TRUE        */
 /* For Mr. Williams direct calculation code use the settings */
 /* cm_width=32, cm_poly=0x1EDC6F41, cm_init=0xFFFFFFFF,      */
 /* cm_refin=TRUE, cm_refot=TRUE, cm_xorort=0x00000000        */
 /*************************************************************/

Stewart Standards Track [Page 143] RFC 4960 Stream Control Transmission Protocol September 2007

 /* Example of the crc table file */
 #ifndef __crc32cr_table_h__
 #define __crc32cr_table_h__
 #define CRC32C_POLY 0x1EDC6F41
 #define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])
 unsigned long  crc_c[256] =
 {
 0x00000000L, 0xF26B8303L, 0xE13B70F7L, 0x1350F3F4L,
 0xC79A971FL, 0x35F1141CL, 0x26A1E7E8L, 0xD4CA64EBL,
 0x8AD958CFL, 0x78B2DBCCL, 0x6BE22838L, 0x9989AB3BL,
 0x4D43CFD0L, 0xBF284CD3L, 0xAC78BF27L, 0x5E133C24L,
 0x105EC76FL, 0xE235446CL, 0xF165B798L, 0x030E349BL,
 0xD7C45070L, 0x25AFD373L, 0x36FF2087L, 0xC494A384L,
 0x9A879FA0L, 0x68EC1CA3L, 0x7BBCEF57L, 0x89D76C54L,
 0x5D1D08BFL, 0xAF768BBCL, 0xBC267848L, 0x4E4DFB4BL,
 0x20BD8EDEL, 0xD2D60DDDL, 0xC186FE29L, 0x33ED7D2AL,
 0xE72719C1L, 0x154C9AC2L, 0x061C6936L, 0xF477EA35L,
 0xAA64D611L, 0x580F5512L, 0x4B5FA6E6L, 0xB93425E5L,
 0x6DFE410EL, 0x9F95C20DL, 0x8CC531F9L, 0x7EAEB2FAL,
 0x30E349B1L, 0xC288CAB2L, 0xD1D83946L, 0x23B3BA45L,
 0xF779DEAEL, 0x05125DADL, 0x1642AE59L, 0xE4292D5AL,
 0xBA3A117EL, 0x4851927DL, 0x5B016189L, 0xA96AE28AL,
 0x7DA08661L, 0x8FCB0562L, 0x9C9BF696L, 0x6EF07595L,
 0x417B1DBCL, 0xB3109EBFL, 0xA0406D4BL, 0x522BEE48L,
 0x86E18AA3L, 0x748A09A0L, 0x67DAFA54L, 0x95B17957L,
 0xCBA24573L, 0x39C9C670L, 0x2A993584L, 0xD8F2B687L,
 0x0C38D26CL, 0xFE53516FL, 0xED03A29BL, 0x1F682198L,
 0x5125DAD3L, 0xA34E59D0L, 0xB01EAA24L, 0x42752927L,
 0x96BF4DCCL, 0x64D4CECFL, 0x77843D3BL, 0x85EFBE38L,
 0xDBFC821CL, 0x2997011FL, 0x3AC7F2EBL, 0xC8AC71E8L,
 0x1C661503L, 0xEE0D9600L, 0xFD5D65F4L, 0x0F36E6F7L,
 0x61C69362L, 0x93AD1061L, 0x80FDE395L, 0x72966096L,
 0xA65C047DL, 0x5437877EL, 0x4767748AL, 0xB50CF789L,
 0xEB1FCBADL, 0x197448AEL, 0x0A24BB5AL, 0xF84F3859L,
 0x2C855CB2L, 0xDEEEDFB1L, 0xCDBE2C45L, 0x3FD5AF46L,
 0x7198540DL, 0x83F3D70EL, 0x90A324FAL, 0x62C8A7F9L,
 0xB602C312L, 0x44694011L, 0x5739B3E5L, 0xA55230E6L,
 0xFB410CC2L, 0x092A8FC1L, 0x1A7A7C35L, 0xE811FF36L,
 0x3CDB9BDDL, 0xCEB018DEL, 0xDDE0EB2AL, 0x2F8B6829L,
 0x82F63B78L, 0x709DB87BL, 0x63CD4B8FL, 0x91A6C88CL,
 0x456CAC67L, 0xB7072F64L, 0xA457DC90L, 0x563C5F93L,
 0x082F63B7L, 0xFA44E0B4L, 0xE9141340L, 0x1B7F9043L,
 0xCFB5F4A8L, 0x3DDE77ABL, 0x2E8E845FL, 0xDCE5075CL,
 0x92A8FC17L, 0x60C37F14L, 0x73938CE0L, 0x81F80FE3L,
 0x55326B08L, 0xA759E80BL, 0xB4091BFFL, 0x466298FCL,

Stewart Standards Track [Page 144] RFC 4960 Stream Control Transmission Protocol September 2007

 0x1871A4D8L, 0xEA1A27DBL, 0xF94AD42FL, 0x0B21572CL,
 0xDFEB33C7L, 0x2D80B0C4L, 0x3ED04330L, 0xCCBBC033L,
 0xA24BB5A6L, 0x502036A5L, 0x4370C551L, 0xB11B4652L,
 0x65D122B9L, 0x97BAA1BAL, 0x84EA524EL, 0x7681D14DL,
 0x2892ED69L, 0xDAF96E6AL, 0xC9A99D9EL, 0x3BC21E9DL,
 0xEF087A76L, 0x1D63F975L, 0x0E330A81L, 0xFC588982L,
 0xB21572C9L, 0x407EF1CAL, 0x532E023EL, 0xA145813DL,
 0x758FE5D6L, 0x87E466D5L, 0x94B49521L, 0x66DF1622L,
 0x38CC2A06L, 0xCAA7A905L, 0xD9F75AF1L, 0x2B9CD9F2L,
 0xFF56BD19L, 0x0D3D3E1AL, 0x1E6DCDEEL, 0xEC064EEDL,
 0xC38D26C4L, 0x31E6A5C7L, 0x22B65633L, 0xD0DDD530L,
 0x0417B1DBL, 0xF67C32D8L, 0xE52CC12CL, 0x1747422FL,
 0x49547E0BL, 0xBB3FFD08L, 0xA86F0EFCL, 0x5A048DFFL,
 0x8ECEE914L, 0x7CA56A17L, 0x6FF599E3L, 0x9D9E1AE0L,
 0xD3D3E1ABL, 0x21B862A8L, 0x32E8915CL, 0xC083125FL,
 0x144976B4L, 0xE622F5B7L, 0xF5720643L, 0x07198540L,
 0x590AB964L, 0xAB613A67L, 0xB831C993L, 0x4A5A4A90L,
 0x9E902E7BL, 0x6CFBAD78L, 0x7FAB5E8CL, 0x8DC0DD8FL,
 0xE330A81AL, 0x115B2B19L, 0x020BD8EDL, 0xF0605BEEL,
 0x24AA3F05L, 0xD6C1BC06L, 0xC5914FF2L, 0x37FACCF1L,
 0x69E9F0D5L, 0x9B8273D6L, 0x88D28022L, 0x7AB90321L,
 0xAE7367CAL, 0x5C18E4C9L, 0x4F48173DL, 0xBD23943EL,
 0xF36E6F75L, 0x0105EC76L, 0x12551F82L, 0xE03E9C81L,
 0x34F4F86AL, 0xC69F7B69L, 0xD5CF889DL, 0x27A40B9EL,
 0x79B737BAL, 0x8BDCB4B9L, 0x988C474DL, 0x6AE7C44EL,
 0xBE2DA0A5L, 0x4C4623A6L, 0x5F16D052L, 0xAD7D5351L,
 };
 #endif
  /* Example of table build routine */
 #include <stdio.h>
 #include <stdlib.h>
 #define OUTPUT_FILE   "crc32cr.h"
 #define CRC32C_POLY    0x1EDC6F41L
 FILE *tf;
 unsigned long
 reflect_32 (unsigned long b)
 {
   int i;
   unsigned long rw = 0L;
   for (i = 0; i < 32; i++){
       if (b & 1)
         rw |= 1 << (31 - i);

Stewart Standards Track [Page 145] RFC 4960 Stream Control Transmission Protocol September 2007

       b >>= 1;
   }
   return (rw);
 }
 unsigned long
 build_crc_table (int index)
 {
   int i;
   unsigned long rb;
   rb = reflect_32 (index);
   for (i = 0; i < 8; i++){
       if (rb & 0x80000000L)
        rb = (rb << 1) ^ CRC32C_POLY;
       else
        rb <<= 1;
   }
   return (reflect_32 (rb));
 }
 main ()
 {
   int i;
   printf ("\nGenerating CRC-32c table file <%s>\n",
   OUTPUT_FILE);
   if ((tf = fopen (OUTPUT_FILE, "w")) == NULL){
       printf ("Unable to open %s\n", OUTPUT_FILE);
       exit (1);
   }
   fprintf (tf, "#ifndef __crc32cr_table_h__\n");
   fprintf (tf, "#define __crc32cr_table_h__\n\n");
   fprintf (tf, "#define CRC32C_POLY 0x%08lX\n",
   CRC32C_POLY);
   fprintf (tf,
   "#define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])\n");
   fprintf (tf, "\nunsigned long  crc_c[256] =\n{\n");
   for (i = 0; i < 256; i++){
       fprintf (tf, "0x%08lXL, ", build_crc_table (i));
       if ((i & 3) == 3)
         fprintf (tf, "\n");
   }
   fprintf (tf, "};\n\n#endif\n");
   if (fclose (tf) != 0)
     printf ("Unable to close <%s>." OUTPUT_FILE);

Stewart Standards Track [Page 146] RFC 4960 Stream Control Transmission Protocol September 2007

   else
     printf ("\nThe CRC-32c table has been written to <%s>.\n",
       OUTPUT_FILE);
 }
 /* Example of crc insertion */
 #include "crc32cr.h"
 unsigned long
 generate_crc32c(unsigned char *buffer, unsigned int length)
 {
   unsigned int i;
   unsigned long crc32 = ~0L;
   unsigned long result;
   unsigned char byte0,byte1,byte2,byte3;
   for (i = 0; i < length; i++){
       CRC32C(crc32, buffer[i]);
   }
   result = ~crc32;
   /*  result now holds the negated polynomial remainder;
    *  since the table and algorithm is "reflected" [williams95].
    *  That is, result has the same value as if we mapped the message
    *  to a polynomial, computed the host-bit-order polynomial
    *  remainder, performed final negation, then did an end-for-end
    *  bit-reversal.
    *  Note that a 32-bit bit-reversal is identical to four inplace
    *  8-bit reversals followed by an end-for-end byteswap.
    *  In other words, the bytes of each bit are in the right order,
    *  but the bytes have been byteswapped.  So we now do an explicit
    *  byteswap.  On a little-endian machine, this byteswap and
    *  the final ntohl cancel out and could be elided.
    */
   byte0 = result & 0xff;
   byte1 = (result>>8) & 0xff;
   byte2 = (result>>16) & 0xff;
   byte3 = (result>>24) & 0xff;
   crc32 = ((byte0 << 24) |
            (byte1 << 16) |
            (byte2 << 8)  |
            byte3);
   return ( crc32 );
 }

Stewart Standards Track [Page 147] RFC 4960 Stream Control Transmission Protocol September 2007

 int
 insert_crc32(unsigned char *buffer, unsigned int length)
 {
   SCTP_message *message;
   unsigned long crc32;
   message = (SCTP_message *) buffer;
   message->common_header.checksum = 0L;
   crc32 = generate_crc32c(buffer,length);
   /* and insert it into the message */
   message->common_header.checksum = htonl(crc32);
   return 1;
 }
 int
 validate_crc32(unsigned char *buffer, unsigned int length)
 {
   SCTP_message *message;
   unsigned int i;
   unsigned long original_crc32;
   unsigned long crc32 = ~0L;
   /* save and zero checksum */
   message = (SCTP_message *) buffer;
   original_crc32 = ntohl(message->common_header.checksum);
   message->common_header.checksum = 0L;
   crc32 = generate_crc32c(buffer,length);
   return ((original_crc32 == crc32)? 1 : -1);
 }

Stewart Standards Track [Page 148] RFC 4960 Stream Control Transmission Protocol September 2007

References

Normative References

 [ITU32]      "ITU-T Recommendation V.42, "Error-correcting procedures
              for DCEs using asynchronous-to-synchronous
              conversion".", ITU-T section 8.1.1.6.2.
 [RFC0768]    Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.
 [RFC0793]    Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.
 [RFC1122]    Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1123]    Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123, October 1989.
 [RFC1191]    Mogul, J. and S. Deering, "Path MTU discovery", RFC
              1191, November 1990.
 [RFC1981]    McCann, J., Deering, S., and J. Mogul, "Path MTU
              Discovery for IP version 6", RFC 1981, August 1996.
 [RFC1982]    Elz, R. and R. Bush, "Serial Number Arithmetic", RFC
              1982, August 1996.
 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.
 [RFC2460]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.
 [RFC2581]    Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.
 [RFC3873]    Pastor, J. and M. Belinchon, "Stream Control
              Transmission Protocol (SCTP) Management Information Base
              (MIB)", RFC 3873, September 2004.
 [RFC4291]    Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

Stewart Standards Track [Page 149] RFC 4960 Stream Control Transmission Protocol September 2007

 [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.
 [RFC4306]    Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, December 2005.
 [RFC4821]    Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

Informative References

 [FALL96]     Fall, K. and S. Floyd, "Simulation-based Comparisons of
              Tahoe, Reno, and SACK TCP", SIGCOMM'99 V. 26 N. 3 pp 5-
              21, July 1996.
 [SAVAGE99]   Savage, S., Cardwell, N., Wetherall, D., and T.
              Anderson, "TCP Congestion Control with a Misbehaving
              Receiver", ACM Computer Communications Review 29(5),
              October 1999.
 [ALLMAN99]   Allman, M. and V. Paxson, "On Estimating End-to-End
              Network Path Properties", SIGCOMM'99 , 1999.
 [WILLIAMS93] Williams, R., "A PAINLESS GUIDE TO CRC ERROR DETECTION
              ALGORITHMS", Internet publication,
              http://www.geocities.com/SiliconValley/Pines/
              8659/crc.htm, August 1993.
 [RFC0813]    Clark, D., "Window and Acknowledgement Strategy in TCP",
              RFC 813, July 1982.
 [RFC1858]    Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.
 [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
              Keyed-Hashing for Message Authentication", RFC 2104,
              February 1997.
 [RFC2196]    Fraser, B., "Site Security Handbook", FYI 8, RFC 2196,
              September 1997.
 [RFC2522]    Karn, P. and W. Simpson, "Photuris: Session-Key
              Management Protocol", RFC 2522, March 1999.

Stewart Standards Track [Page 150] RFC 4960 Stream Control Transmission Protocol September 2007

 [RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.
 [RFC3309]    Stone, J., Stewart, R., and D. Otis, "Stream Control
              Transmission Protocol (SCTP) Checksum Change", RFC 3309,
              September 2002.
 [RFC3168]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.
 [RFC4086]    Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC
              4086, June 2005.
 [RFC4460]    Stewart, R., Arias-Rodriguez, I., Poon, K., Caro, A.,
              and M. Tuexen, "Stream Control Transmission Protocol
              (SCTP) Specification Errata and Issues", RFC 4460, April
              2006.
 [RFC4895]    Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

Editor's Address

 Randall R. Stewart
 4875 Forest Drive
 Suite 200
 Columbia, SC  29206
 US
 EMail: rrs@cisco.com

Stewart Standards Track [Page 151] RFC 4960 Stream Control Transmission Protocol September 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Stewart Standards Track [Page 152]

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