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rfc:rfc2960

Network Working Group R. Stewart Request for Comments: 2960 Q. Xie Category: Standards Track Motorola

                                                          K. Morneault
                                                              C. Sharp
                                                                 Cisco
                                                       H. Schwarzbauer
                                                               Siemens
                                                             T. Taylor
                                                       Nortel Networks
                                                             I. Rytina
                                                              Ericsson
                                                              M. Kalla
                                                             Telcordia
                                                              L. Zhang
                                                                  UCLA
                                                             V. Paxson
                                                                 ACIRI
                                                          October 2000
                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.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 This document describes the Stream Control Transmission Protocol
 (SCTP).  SCTP is designed to transport 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,
  2. - data fragmentation to conform to discovered path MTU size,

Stewart, et al. Standards Track [Page 1] RFC 2960 Stream Control Transmission Protocol October 2000

  1. - sequenced delivery of user messages within multiple streams,

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

       messages,
    -- optional bundling of multiple user messages into a single SCTP
       packet, and
    -- 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, et al. Standards Track [Page 2] RFC 2960 Stream Control Transmission Protocol October 2000

Table of Contents

 1.  Introduction..................................................  5
   1.1 Motivation..................................................  6
   1.2 Architectural View of SCTP..................................  6
   1.3 Functional View of SCTP.....................................  7
     1.3.1 Association Startup and Takedown........................  8
     1.3.2 Sequenced Delivery within Streams.......................  9
     1.3.3 User Data Fragmentation.................................  9
     1.3.4 Acknowledgement and Congestion Avoidance................  9
     1.3.5 Chunk Bundling ......................................... 10
     1.3.6 Packet Validation....................................... 10
     1.3.7 Path Management......................................... 11
   1.4 Key Terms................................................... 11
   1.5 Abbreviations............................................... 15
   1.6 Serial Number Arithmetic.................................... 15
 2. Conventions.................................................... 16
 3.  SCTP packet Format............................................ 16
   3.1 SCTP Common Header Field Descriptions....................... 17
   3.2 Chunk Field Descriptions.................................... 18
     3.2.1 Optional/Variable-length Parameter Format............... 20
   3.3 SCTP Chunk Definitions...................................... 21
     3.3.1 Payload Data (DATA)..................................... 22
     3.3.2 Initiation (INIT)....................................... 24
       3.3.2.1 Optional or Variable Length Parameters.............. 26
     3.3.3 Initiation Acknowledgement (INIT ACK)................... 30
       3.3.3.1 Optional or Variable Length Parameters.............. 33
     3.3.4 Selective Acknowledgement (SACK)........................ 33
     3.3.5 Heartbeat Request (HEARTBEAT)........................... 37
     3.3.6 Heartbeat Acknowledgement (HEARTBEAT ACK)............... 38
     3.3.7 Abort Association (ABORT)............................... 39
     3.3.8 Shutdown Association (SHUTDOWN)......................... 40
     3.3.9 Shutdown Acknowledgement (SHUTDOWN ACK)................. 40
     3.3.10 Operation Error (ERROR)................................ 41
       3.3.10.1 Invalid Stream Identifier.......................... 42
       3.3.10.2 Missing Mandatory Parameter........................ 43
       3.3.10.3 Stale Cookie Error................................. 43
       3.3.10.4 Out of Resource.................................... 44
       3.3.10.5 Unresolvable Address............................... 44
       3.3.10.6 Unrecognized Chunk Type............................ 44
       3.3.10.7 Invalid Mandatory Parameter........................ 45
       3.3.10.8 Unrecognized Parameters............................ 45
       3.3.10.9 No User Data....................................... 46
       3.3.10.10 Cookie Received While Shutting Down............... 46
     3.3.11 Cookie Echo (COOKIE ECHO).............................. 46
     3.3.12 Cookie Acknowledgement (COOKIE ACK).................... 47
     3.3.13 Shutdown Complete (SHUTDOWN COMPLETE).................. 48
 4. SCTP Association State Diagram................................. 48

Stewart, et al. Standards Track [Page 3] RFC 2960 Stream Control Transmission Protocol October 2000

 5. Association Initialization..................................... 52
   5.1 Normal Establishment of an Association...................... 52
     5.1.1 Handle Stream Parameters................................ 54
     5.1.2 Handle Address Parameters............................... 54
     5.1.3 Generating State Cookie................................. 56
     5.1.4 State Cookie Processing................................. 57
     5.1.5 State Cookie Authentication............................. 57
     5.1.6 An Example of Normal Association Establishment.......... 58
   5.2 Handle Duplicate or unexpected INIT, INIT ACK, COOKIE ECHO,
       and COOKIE ACK.............................................. 60
     5.2.1 Handle Duplicate INIT in COOKIE-WAIT
           or COOKIE-ECHOED States................................. 60
     5.2.2 Unexpected INIT in States Other than CLOSED,
           COOKIE-ECHOED, COOKIE-WAIT and SHUTDOWN-ACK-SENT........ 61
     5.2.3 Unexpected INIT ACK..................................... 61
     5.2.4 Handle a COOKIE ECHO when a TCB exists.................. 62
       5.2.4.1 An Example of a Association Restart................. 64
     5.2.5 Handle Duplicate COOKIE ACK............................. 66
     5.2.6 Handle Stale COOKIE Error............................... 66
   5.3 Other Initialization Issues................................. 67
     5.3.1 Selection of Tag Value.................................. 67
 6. User Data Transfer............................................. 67
   6.1 Transmission of DATA Chunks................................. 69
   6.2 Acknowledgement on Reception of DATA Chunks................. 70
     6.2.1 Tracking Peer's Receive Buffer Space.................... 73
   6.3 Management Retransmission Timer............................. 75
     6.3.1 RTO Calculation......................................... 75
     6.3.2 Retransmission Timer Rules.............................. 76
     6.3.3 Handle T3-rtx Expiration................................ 77
   6.4 Multi-homed SCTP Endpoints.................................. 78
     6.4.1 Failover from Inactive Destination Address.............. 79
   6.5 Stream Identifier and Stream Sequence Number................ 80
   6.6 Ordered and Unordered Delivery.............................. 80
   6.7 Report Gaps in Received DATA TSNs........................... 81
   6.8 Adler-32 Checksum Calculation............................... 82
   6.9 Fragmentation............................................... 83
   6.10 Bundling .................................................. 84
 7. Congestion Control   .......................................... 85
   7.1 SCTP Differences from TCP Congestion Control................ 85
   7.2 SCTP Slow-Start and Congestion Avoidance.................... 87
     7.2.1 Slow-Start.............................................. 87
     7.2.2 Congestion Avoidance.................................... 89
     7.2.3 Congestion Control...................................... 89
     7.2.4 Fast Retransmit on Gap Reports.......................... 90
   7.3 Path MTU Discovery.......................................... 91
 8.  Fault Management.............................................. 92
   8.1 Endpoint Failure Detection.................................. 92
   8.2 Path Failure Detection...................................... 92

Stewart, et al. Standards Track [Page 4] RFC 2960 Stream Control Transmission Protocol October 2000

   8.3 Path Heartbeat.............................................. 93
   8.4 Handle "Out of the blue" Packets............................ 95
   8.5 Verification Tag............................................ 96
     8.5.1 Exceptions in Verification Tag Rules.................... 97
 9. Termination of Association..................................... 98
   9.1 Abort of an Association..................................... 98
   9.2 Shutdown of an Association.................................. 98
 10. Interface with Upper Layer....................................101
   10.1 ULP-to-SCTP................................................101
   10.2 SCTP-to-ULP................................................111
 11. Security Considerations.......................................114
   11.1 Security Objectives........................................114
   11.2 SCTP Responses To Potential Threats........................115
     11.2.1 Countering Insider Attacks.............................115
     11.2.2 Protecting against Data Corruption in the Network......115
     11.2.3 Protecting Confidentiality.............................115
     11.2.4 Protecting against Blind Denial of Service Attacks.....116
       11.2.4.1 Flooding...........................................116
       11.2.4.2 Blind Masquerade...................................118
       11.2.4.3 Improper Monopolization of Services................118
   11.3 Protection against Fraud and Repudiation...................119
 12. Recommended Transmission Control Block (TCB) Parameters.......120
   12.1 Parameters necessary for the SCTP instance.................120
   12.2 Parameters necessary per association (i.e. the TCB)........120
   12.3 Per Transport Address Data.................................122
   12.4 General Parameters Needed..................................123
 13. IANA Considerations...........................................123
   13.1 IETF-defined Chunk Extension...............................123
   13.2 IETF-defined Chunk Parameter Extension.....................124
   13.3 IETF-defined Additional Error Causes.......................124
   13.4 Payload Protocol Identifiers...............................125
 14. Suggested SCTP Protocol Parameter Values......................125
 15. Acknowledgements..............................................126
 16. Authors' Addresses............................................126
 17. References....................................................128
 18. Bibliography..................................................129
 Appendix A .......................................................131
 Appendix B .......................................................132
 Full Copyright Statement .........................................134

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.

Stewart, et al. Standards Track [Page 5] RFC 2960 Stream Control Transmission Protocol October 2000

1.1 Motivation

 TCP [RFC793] 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
 [RFC768].  The limitations which 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.

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 which 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 1.4) to provide the other endpoint

Stewart, et al. Standards Track [Page 6] RFC 2960 Stream Control Transmission Protocol October 2000

 (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 which 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 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, et al. Standards Track [Page 7] RFC 2960 Stream Control Transmission Protocol October 2000

                         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

1.3.1 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 security 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, et al. Standards Track [Page 8] RFC 2960 Stream Control Transmission Protocol October 2000

 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).

1.3.2 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 eight 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.

1.3.3 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.

1.3.4 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

Stewart, et al. Standards Track [Page 9] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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 Sections 6
 and 7 for a detailed description of the protocol procedures
 associated with this function.

1.3.5 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 messages 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.

1.3.6 Packet Validation

 A mandatory Verification Tag field and a 32 bit checksum field (see
 Appendix B for a description of the Adler-32 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 Adler-32 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 Adler-32 checksum silently discards
 the packet.

Stewart, et al. Standards Track [Page 10] RFC 2960 Stream Control Transmission Protocol October 2000

1.3.7 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
 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 start-up, 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.

1.4 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 which a transmitting endpoint considers available
    for receiving user messages.
 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.

Stewart, et al. Standards Track [Page 11] RFC 2960 Stream Control Transmission Protocol October 2000

 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 which 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 the
    message was sent on.
 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.
 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.

Stewart, et al. Standards Track [Page 12] RFC 2960 Stream Control Transmission Protocol October 2000

 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).
 o  Slow Start Threshold (ssthresh): An SCTP variable.  This is the
    threshold which 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 uni-directional 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.

Stewart, et al. Standards Track [Page 13] RFC 2960 Stream Control Transmission Protocol October 2000

 o  Stream Sequence Number: A 16-bit sequence number used internally
    by SCTP to assure sequenced delivery of the user messages within a
    given stream.  One stream sequence number is attached to each user
    message.
 o  Tie-Tags: Verification Tags from a previous association.  These
    Tags are used within a State Cookie so that the newly restarting
    association can be linked to the original association within the
    endpoint that did not restart.
 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 Network Layer address, Transport Layer protocol and 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) which 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.
 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.  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.

Stewart, et al. Standards Track [Page 14] RFC 2960 Stream Control Transmission Protocol October 2000

1.5. Abbreviations

 MAC    - Message Authentication Code [RFC2104]
 RTO    - Retransmission Time-out
 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

1.6 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.

Stewart, et al. Standards Track [Page 15] RFC 2960 Stream Control Transmission Protocol October 2000

 All other arithmetic and comparisons in this document uses normal
 arithmetic.

2. Conventions

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
 they appear in this document, are to be interpreted as described in
 [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, et al. Standards Track [Page 16] RFC 2960 Stream Control Transmission Protocol October 2000

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.
 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.
 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.
    -  A packet containing an ABORT chunk may have the verification
       tag copied from the packet which caused the ABORT to be sent.
       For details see Section 8.4 and 8.5.
 An INIT chunk MUST be the only chunk in the SCTP packet carrying it.

Stewart, et al. Standards Track [Page 17] RFC 2960 Stream Control Transmission Protocol October 2000

 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 Adler-
       32 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)
 12         - Reserved for Explicit Congestion Notification Echo (ECNE)
 13         - Reserved for Congestion Window Reduced (CWR)

Stewart, et al. Standards Track [Page 18] RFC 2960 Stream Control Transmission Protocol October 2000

 14         - Shutdown Complete (SHUTDOWN COMPLETE)
 15 to 62   - reserved by IETF
 63         - IETF-defined Chunk Extensions
 64 to 126  - reserved by IETF
 127        - IETF-defined Chunk Extensions
 128 to 190 - reserved by IETF
 191        - IETF-defined Chunk Extensions
 192 to 254 - reserved by IETF
 255        - IETF-defined Chunk Extensions
 Chunk Types are encoded such that the highest-order two 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
      parameter in an 'Unrecognized Parameter Type' (in either an
      ERROR or in the INIT ACK).
 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).
 Chunk Flags: 8 bits
    The usage of these bits depends on the chunk type as given by the
    Chunk Type.  Unless otherwise specified, they are set to zero 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
    padding.

Stewart, et al. Standards Track [Page 19] RFC 2960 Stream Control Transmission Protocol October 2000

 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 should never 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
 13.1 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                         /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 description are
    reserved for use by IETF.

Stewart, et al. Standards Track [Page 20] RFC 2960 Stream Control Transmission Protocol October 2000

 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 SHOULD NOT pad with more than 3 bytes.  The
 receiver MUST ignore the padding bytes.
 The Parameter Types are encoded such that the highest-order two bits
 specify the action that must be taken if the processing endpoint does
 not recognize the Parameter 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
      parameter in an 'Unrecognized Parameter Type' (in either an
      ERROR or in the INIT ACK).
 10 - Skip this parameter and continue processing.
 11 - Skip this parameter and continue processing but report the
      unrecognized parameter in an 'Unrecognized Parameter Type' (in
      either an ERROR or in the INIT ACK).
 The actual SCTP parameters are defined in the specific SCTP chunk
 sections.  The rules for IETF-defined parameter extensions are
 defined in Section 13.2.

3.3 SCTP Chunk Definitions

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

Stewart, et al. Standards Track [Page 21] RFC 2960 Stream Control Transmission Protocol October 2000

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 re-assembly (if necessary), unordered DATA chunks MUST be
    dispatched to the upper layer by the receiver without any attempt
    to re-order.
    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, et al. Standards Track [Page 22] RFC 2960 Stream Control Transmission Protocol October 2000

 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 no user data field will
    have Length set to 16 (indicating 16 bytes).
 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.
    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.

Stewart, et al. Standards Track [Page 23] RFC 2960 Stream Control Transmission Protocol October 2000

 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 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).
    The value 0 indicates 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 a SCTP association between two
 endpoints.  The format of the INIT 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 = 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.

Stewart, et al. Standards Track [Page 24] RFC 2960 Stream Control Transmission Protocol October 2000

       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.
 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.
 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.

Stewart, et al. Standards Track [Page 25] RFC 2960 Stream Control Transmission Protocol October 2000

    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.
 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.

Stewart, et al. Standards Track [Page 26] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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 bit (unsigned integer)
    Contains an IPv6 address of the sending endpoint.  It is binary
    encoded.
    Note: A sender MUST NOT use an IPv4-mapped IPv6 address [RFC2373]
    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 lifetime 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.

Stewart, et al. Standards Track [Page 27] RFC 2960 Stream Control Transmission Protocol October 2000

    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.
 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 re-attempts 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.

Stewart, et al. Standards Track [Page 28] RFC 2960 Stream Control Transmission Protocol October 2000

 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 RFC1123
    Section 2.1 [RFC1123].  The method for resolving the host name is
    out of scope of SCTP.
    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, Hostname = 11).

Stewart, et al. Standards Track [Page 29] RFC 2960 Stream Control Transmission Protocol October 2000

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:
 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 treat it as an error and close
    the association by transmitting an ABORT.

Stewart, et al. Standards Track [Page 30] RFC 2960 Stream Control Transmission Protocol October 2000

 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
    NOT be used.
    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

Stewart, et al. Standards Track [Page 31] RFC 2960 Stream Control Transmission Protocol October 2000

    Variable Parameters                  Status     Type Value
    -------------------------------------------------------------
    State Cookie                        Mandatory   7
    IPv4 Address (Note 1)               Optional    5
    IPv6 Address (Note 1)               Optional    6
    Unrecognized Parameters             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.
 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 a
 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.
 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 lifetime 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.

Stewart, et al. Standards Track [Page 32] RFC 2960 Stream Control Transmission Protocol October 2000

3.3.3.1 Optional or Variable Length Parameters

 State Cookie
    Parameter Type Value: 7
    Parameter Length:  variable size, depending on Size of Cookie
    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 Parameters:
    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 which has a
       value that indicates that 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 and Advertised Receiver
 Window Credit (a_rwnd) parameters.
 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.

Stewart, et al. Standards Track [Page 33] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
     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 zeros 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.
 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.

Stewart, et al. Standards Track [Page 34] RFC 2960 Stream Control Transmission Protocol October 2000

 Number of Gap Ack Blocks: 16 bits (unsigned integer)
    Indicates the number of Gap Ack Blocks included in this SACK.
 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.
 For example, assume the receiver has the following DATA chunks newly
 arrived at the time when it decides to send a Selective ACK,

Stewart, et al. Standards Track [Page 35] RFC 2960 Stream Control Transmission Protocol October 2000

  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 re-initialized 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, et al. Standards Track [Page 36] RFC 2960 Stream Control Transmission Protocol October 2000

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 zero 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               /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart, et al. Standards Track [Page 37] RFC 2960 Stream Control Transmission Protocol October 2000

    The Sender-specific Heartbeat Info field should normally include
    information about the sender's current time when this HEARTBEAT
    chunk is sent and the destination transport address to which this
    HEARTBEAT is sent (see Section 8.3).

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 zero 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, et al. Standards Track [Page 38] RFC 2960 Stream Control Transmission Protocol October 2000

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 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 for an
 association that doesn't exist, 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 had a TCB that it destroyed.
    If the sender did not have a TCB it should set this bit to 1.
 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, et al. Standards Track [Page 39] RFC 2960 Stream Control Transmission Protocol October 2000

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 zero 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.
     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               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Stewart, et al. Standards Track [Page 40] RFC 2960 Stream Control Transmission Protocol October 2000

 Chunk Flags:  8 bits
    Set to zero 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 zero 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.
 Error causes are defined as variable-length parameters using the
 format described in 3.2.1, i.e.:
     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.

Stewart, et al. Standards Track [Page 41] RFC 2960 Stream Control Transmission Protocol October 2000

    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
 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.
 Sections 3.3.10.1 - 3.3.10.10 define error causes for SCTP.
 Guidelines for the IETF to define new error cause values are
 discussed in Section 13.3.

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.

Stewart, et al. Standards Track [Page 42] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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.

Stewart, et al. Standards Track [Page 43] RFC 2960 Stream Control Transmission Protocol October 2000

    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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

Stewart, et al. Standards Track [Page 44] RFC 2960 Stream Control Transmission Protocol October 2000

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     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.

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 a 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, et al. Standards Track [Page 45] RFC 2960 Stream Control Transmission Protocol October 2000

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 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          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

Stewart, et al. Standards Track [Page 46] RFC 2960 Stream Control Transmission Protocol October 2000

     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 zero 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.
 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
    insure interoperability.

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 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 zero on transmit and ignored on receipt.

Stewart, et al. Standards Track [Page 47] RFC 2960 Stream Control Transmission Protocol October 2000

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
 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 had a TCB that it destroyed.
    If the sender did not have a TCB it should set this bit to 1.
 Note: Special rules apply to this chunk for verification, please see
 Section 8.5.1 for details.

4. SCTP Association State Diagram

 During the lifetime of an SCTP association, the SCTP endpoint's
 association progress 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 description of all special cases should be found in the text.

Stewart, et al. Standards Track [Page 48] RFC 2960 Stream Control Transmission Protocol October 2000

 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
 which causes a state transition it is labeled (A), (B) etc.
  1. —- ——– (frm 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, et al. Standards Track [Page 49] RFC 2960 Stream Control Transmission Protocol October 2000

                    (from the ESTABLISHED state only)
                                  |
                                  |
                         /--------+--------\
     [SHUTDOWN]         /                   \
     -------------------|                   |
     check outstanding  |                   |
     DATA chunks        |                   |
                        v                   |
                   +---------+              |
                   |SHUTDOWN-|              | rcv SHUTDOWN/check
                   |PENDING  |              | 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, et al. Standards Track [Page 50] RFC 2960 Stream Control Transmission Protocol October 2000

                        |                   | (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 re-start 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 SCTP user.
 3) If the T1-cookie timer expires, the endpoint MUST retransmit
    COOKIE ECHO and re-start 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 SCTP user.
 4) In SHUTDOWN-SENT state the endpoint MUST acknowledge any received
    DATA chunks without delay.
 5) In SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new
    send request from its SCTP user.
 6) In SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
    retransmit data and leave this state when all data in queue is
    transmitted.
 7) In SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any new
    send request from its SCTP user.
 The CLOSED state is used to indicate that an association is not
 created (i.e., doesn't exist).

Stewart, et al. Standards Track [Page 51] RFC 2960 Stream Control Transmission Protocol October 2000

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 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 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, nor keep any states for the
    new association.  Otherwise, "Z" will be vulnerable to resource
    attacks.

Stewart, et al. Standards Track [Page 52] RFC 2960 Stream Control Transmission Protocol October 2000

 C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-
    init timer and leave 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 MUST 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, et al. Standards Track [Page 53] RFC 2960 Stream Control Transmission Protocol October 2000

 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 shall
 indicate the number of outbound streams (OS) it wishes to have in the
 association, as well as the maximum inbound streams (MIS) it will
 accept from the other endpoint.
 After receiving the stream configuration information from the other
 side, each endpoint shall 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 either use MIS outbound streams, or
 abort the association and report to its upper layer the resources
 shortage at its peer.
 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.
 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.

Stewart, et al. Standards Track [Page 54] RFC 2960 Stream Control Transmission Protocol October 2000

    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 shall derive and record all the
    transport address(es) from the received chunk AND the source IP
    address that sent the INIT or INIT ACK.  The transport address(es)
    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.
    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.

Stewart, et al. Standards Track [Page 55] RFC 2960 Stream Control Transmission Protocol October 2000

 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 (initiatee) 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 re-initiation by
 using a 'Supported Address Types' parameter in the new INIT to
 indicate what types of address it prefers.

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 time stamp 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',
 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.

Stewart, et al. Standards Track [Page 56] RFC 2960 Stream Control Transmission Protocol October 2000

 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 ([RFC1750] 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
 insure interoperability.

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' 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).  Reference [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,

Stewart, et al. Standards Track [Page 57] RFC 2960 Stream Control Transmission Protocol October 2000

 3) 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,
 4) 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,
 5) Send a COOKIE ACK chunk to the peer acknowledging reception 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.
 6) 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
    5), 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.

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):

Stewart, et al. Standards Track [Page 58] RFC 2960 Stream Control Transmission Protocol October 2000

 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=1 & 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=1 & user data 1]
 SACK [TSN Ack=init TSN_Z,      /---- DATA
       Block=0]     --------\  /        [TSN=init TSN_Z +1,
                             \/          Strm=0,Seq=2 & user data 2]
                      <------/\
                               \
                                \------>
                   Figure 4: INITiation Example
 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

Stewart, et al. Standards Track [Page 59] RFC 2960 Stream Control Transmission Protocol October 2000

 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 6.3 to determine the proper timer value.

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

 COOKIE ACK
 During the lifetime 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 a SCTP transport address and is from a 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, re-started 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.

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 or COOKIE-ECHOED state, an
 endpoint MUST respond with an INIT ACK using the same parameters it

Stewart, et al. Standards Track [Page 60] RFC 2960 Stream Control Transmission Protocol October 2000

 sent in its original INIT chunk (including its Initiation Tag,
 unchanged).  These 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 with the Tag information of itself and its peer (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 reception of an unexpected INIT for
 this association, the endpoint shall generate an INIT ACK with a
 State Cookie.  In the outbound INIT ACK the endpoint MUST copy its
 current Verification Tag and peer's Verification Tag into a reserved
 place within the state cookie.  We shall refer to these locations as
 the Peer's-Tie-Tag and the Local-Tie-Tag.  The outbound SCTP packet
 containing this INIT ACK MUST carry a Verification Tag value equal to
 the Initiation Tag found in the unexpected INIT.  And the INIT ACK
 MUST contain a new Initiation 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, 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 state are the Tie-Tags populated.  For a normal association INIT
 (i.e. the endpoint is in a COOKIE-WAIT state), the Tie-Tags MUST be
 set to 0 (indicating that no previous TCB existed).  The INIT ACK and
 State Cookie are populated as specified in section 5.2.1.

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.

Stewart, et al. Standards Track [Page 61] RFC 2960 Stream Control Transmission Protocol October 2000

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, 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 of 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.

Stewart, et al. Standards Track [Page 62] RFC 2960 Stream Control Transmission Protocol October 2000

+————+————+—————+————–+————-+

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:
  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.

Stewart, et al. Standards Track [Page 63] RFC 2960 Stream Control Transmission Protocol October 2000

    If the endpoint is in the SHUTDOWN-ACK-SENT state and recognizes
    the peer has restarted (Action A), it MUST NOT setup 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
    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 always
    enter the ESTABLISHED state, if it has not already done so. It
    should stop any init or cookie timers 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, et al. Standards Track [Page 64] RFC 2960 Stream Control Transmission Protocol October 2000

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 a 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=1 & user data]--\

(Start T3-rtx timer) \

                               \->
                            /----- SACK [TSN Ack=init TSN_A,Block=0]

(Cancel T3-rtx timer) ←—–/

                Figure 5: A Restart Example

Stewart, et al. Standards Track [Page 65] RFC 2960 Stream Control Transmission Protocol October 2000

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) That 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 setup, 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 re-attempt the setup procedure.
 2) Discard the TCB and report to the upper layer the inability to
    setup the association.
 3) Send a new INIT chunk to the endpoint, adding a Cookie
    Preservative parameter requesting an extension to the lifetime 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
    lifetimes making the endpoint more subject to a replay attack.

Stewart, et al. Standards Track [Page 66] RFC 2960 Stream Control Transmission Protocol October 2000

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 [RFC1750] 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 lifetime of an
 association.  A new Verification Tag value MUST be used each time the
 endpoint tears-down and then re-establishes an association to the
 same peer.

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 COOKIE-WAIT state.
 DATA chunks MUST only be received according to the rules below in
 ESTABLISHED, SHUTDOWN-PENDING, SHUTDOWN-SENT.  A DATA chunk received
 in CLOSED is out of the blue and SHOULD be handled per 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 8.4.  A SACK chunk received in
 any other state SHOULD be discarded.
 A SCTP receiver MUST be able to receive a minimum of 1500 bytes in
 one SCTP packet.  This means that a 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.

Stewart, et al. Standards Track [Page 67] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
               +--------------------------+
               |      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 Sections
 6.9 and 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.

Stewart, et al. Standards Track [Page 68] RFC 2960 Stream Control Transmission Protocol October 2000

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.
 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 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.
 C) When the time comes for the sender to transmit, before sending new
    DATA chunks, the sender MUST first transmit any outstanding DATA
    chunks which are marked for retransmission (limited by the current
    cwnd).
 D) Then, the sender can send out as many new DATA chunks as Rule A
    and Rule B above 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 a 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.

Stewart, et al. Standards Track [Page 69] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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.
 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.
 A 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 500ms but MUST NOT raise it above 500ms.

Stewart, et al. Standards Track [Page 70] RFC 2960 Stream Control Transmission Protocol October 2000

 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 SHOULD also be
 reported in the Gap Ack Block fields.
 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.
    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

Stewart, et al. Standards Track [Page 71] RFC 2960 Stream Control Transmission Protocol October 2000

       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 circumstance 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.
 The following example illustrates the use of delayed
 acknowledgements:

Stewart, et al. Standards Track [Page 72] RFC 2960 Stream Control Transmission Protocol October 2000

 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 Acknowledgment 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
 order, the data sender can develop an incorrect view of the peer's
 receive buffer space.

Stewart, et al. Standards Track [Page 73] RFC 2960 Stream Control Transmission Protocol October 2000

 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 (via either
    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 mark the corresponding DATA chunk as
       available for retransmit:  Mark it as missing for 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.

Stewart, et al. Standards Track [Page 74] RFC 2960 Stream Control Transmission Protocol October 2000

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 follows 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'| 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.
 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

Stewart, et al. Standards Track [Page 75] RFC 2960 Stream Control Transmission Protocol October 2000

     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 packet or a
     later instance).
 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.

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 T3-rtx
     timer for that address with its current RTO (if there is still
     outstanding data on that address).

Stewart, et al. Standards Track [Page 76] RFC 2960 Stream Control Transmission Protocol October 2000

 R4) Whenever a SACK is received missing a TSN that was previously
     acknowledged via a Gap Ack Block, start 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
 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)
                                  \
                                 / \
 (Re-start 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

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

Stewart, et al. Standards Track [Page 77] RFC 2960 Stream Control Transmission Protocol October 2000

     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):
 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 Sections 5.1.2 and 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.

Stewart, et al. Standards Track [Page 78] RFC 2960 Stream Control Transmission Protocol October 2000

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

6.4.1 Failover from 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 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, if the endpoint is multi-homed, it should
 consider each source-destination address pair in its retransmission
 selection policy.  When retransmitting the endpoint should attempt to
 pick the most divergent source-destination pair from the original
 source-destination pair to which the packet was transmitted.

Stewart, et al. Standards Track [Page 79] RFC 2960 Stream Control Transmission Protocol October 2000

 Note: Rules for picking the most divergent source-destination pair
 are an implementation decision and is 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 shall start from 0 when
 the association is established.  Also, when the stream sequence
 number reaches the value 65535 the next stream sequence number shall
 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
 their stream sequence number, the endpoint MUST hold the received
 DATA chunks from delivery to the ULP until they are re-ordered.
 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 re-assembly 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.

Stewart, et al. Standards Track [Page 80] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 Upon the reception of a SACK, the endpoint MUST remove all DATA
 chunks which 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.

Stewart, et al. Standards Track [Page 81] RFC 2960 Stream Control Transmission Protocol October 2000

    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,
                                   /             Strt=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 can not 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 Adler-32 Checksum Calculation

 When sending an SCTP packet, the endpoint MUST strengthen the data
 integrity of the transmission by including the Adler-32 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 shall:
 1) Fill in the proper Verification Tag in the SCTP common header and
    initialize the checksum field to 0's.
 2) Calculate the Adler-32 checksum of the whole packet, including the
    SCTP common header and all the chunks.  Refer to appendix B for
    details of the Adler-32 algorithm.  And,
 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
 Adler-32 checksum:
 1) Store the received Adler-32 checksum value aside,

Stewart, et al. Standards Track [Page 82] RFC 2960 Stream Control Transmission Protocol October 2000

 2) Replace the 32 bits of the checksum field in the received SCTP
    packet with all '0's and calculate an Adler-32 checksum value of
    the whole received packet.  And,
 3) Verify that the calculated Adler-32 checksum is the same as the
    received Adler-32 checksum.  If 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.

6.9 Fragmentation and Reassembly

 An endpoint MAY support fragmentation when sending DATA chunks, but
 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.
 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.
 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

Stewart, et al. Standards Track [Page 83] RFC 2960 Stream Control Transmission Protocol October 2000

    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 re-assembly.  Once the user message is
 reassembled, SCTP shall pass the re-assembled user message to the
 specific stream for possible re-ordering and final dispatching.
 Note: If the data receiver runs out of buffer space while still
 waiting for more fragments to complete the re-assembly 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 or equal to
 the current Path MTU.
 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 a 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.

Stewart, et al. Standards Track [Page 84] RFC 2960 Stream Control Transmission Protocol October 2000

 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 assure 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.
 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, are 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

Stewart, et al. Standards Track [Page 85] RFC 2960 Stream Control Transmission Protocol October 2000

 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 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
    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.

Stewart, et al. Standards Track [Page 86] RFC 2960 Stream Control Transmission Protocol October 2000

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.
 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.

Stewart, et al. Standards Track [Page 87] RFC 2960 Stream Control Transmission Protocol October 2000

 o  The initial cwnd before DATA transmission or after a sufficiently
    long idle period MUST be <= 2*MTU.
 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 (assuming the
    current congestion window is being fully utilized).  If an
    incoming SACK advances the Cumulative TSN Ack Point, 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 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
 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, 2*MTU) per RTO.

Stewart, et al. Standards Track [Page 88] RFC 2960 Stream Control Transmission Protocol October 2000

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, 2*MTU) per RTO.
 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, 2*MTU)
    cwnd = ssthresh
 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, 2*MTU)
    cwnd = 1*MTU

Stewart, et al. Standards Track [Page 89] RFC 2960 Stream Control Transmission Protocol October 2000

 and assure 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 some TSN(s)
 missing, it SHOULD wait for 3 further miss indications (via
 subsequent SACK's) on the same TSN(s) before taking action with
 regard to Fast Retransmit.
 When the TSN(s) is reported as missing in the fourth consecutive
 SACK, the data sender shall:
 1) Mark the missing DATA chunk(s) for retransmission,
 2) 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.
 4) Restart 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.
 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 Sections 7.2.1 and 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 4 and
 starting the fast retransmit procedure, the counter resets to 0.

Stewart, et al. Standards Track [Page 90] RFC 2960 Stream Control Transmission Protocol October 2000

 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

 [RFC1191] specifies "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
 which exceed the MTU, other than occasional attempts to probe for a
 change in the Path MTU (PMTU).  RFC 1191 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.  [RFC1981] specifies the same mechanisms for
 IPv6.  An SCTP sender using IPv6 MUST use Path MTU Discovery unless
 all packets are less than the minimum IPv6 MTU [RFC2460].
 An endpoint SHOULD apply these techniques, and SHOULD do so on a
 per-destination-address basis.
 There are 4 ways in which SCTP differs from the description in RFC
 1191 of applying MTU discovery to TCP:
 1) SCTP associations can span multiple addresses.  An endpoint MUST
    maintain separate MTU estimates for each destination address of
    its peer.
 2) Elsewhere in this document, when the term "MTU" is discussed, it
    refers to the MTU associated with the destination address
    corresponding to the context of the discussion.
 3) Unlike TCP, SCTP does not have a notion of "Maximum Segment Size".
    Accordingly, the MTU for each destination address SHOULD be
    initialized to a value no larger than the link MTU for the local
    interface to which packets for that remote destination address
    will be routed.
 4) Since data transmission in SCTP is naturally structured in terms
    of TSNs rather than bytes (as is the case for TCP), the discussion
    in Section 6.5 of RFC 1191 applies: When retransmitting an IP
    datagram to a remote address for which the IP datagram appears too
    large for the path MTU to that address, the IP datagram SHOULD be
    retransmitted without the DF bit set, allowing it to possibly be
    fragmented.  Transmissions of new IP datagrams MUST have DF set.

Stewart, et al. Standards Track [Page 91] RFC 2960 Stream Control Transmission Protocol October 2000

 5) The sender should track an association PMTU which 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.
 Other than these differences, the discussion of TCP's use of MTU
 discovery in RFCs 1191 and 1981 applies to SCTP on a per-
 destination-address basis.
 Note: For IPv6 destination addresses the DF bit does not exist,
 instead the IP datagram must be fragmented as described in [RFC2460].

8. Fault Management

8.1 Endpoint Failure Detection

 An endpoint shall keep a counter on the total number of consecutive
 retransmissions to its peer (including retransmissions to all the
 destination transport addresses of the peer if it is multi-homed).
 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 shall 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.
 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 a
 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 a 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.

Stewart, et al. Standards Track [Page 92] RFC 2960 Stream Control Transmission Protocol October 2000

 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 the 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.

8.3 Path Heartbeat

 By default, an SCTP endpoint shall 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).
 A destination transport address is considered "idle" if no new chunk
 which 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,

Stewart, et al. Standards Track [Page 93] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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 field copied
 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.

Stewart, et al. Standards Track [Page 94] RFC 2960 Stream Control Transmission Protocol October 2000

 On an idle destination address that is allowed to heartbeat, a
 HEARTBEAT chunk is RECOMMENDED to be sent once per RTO of that
 destination address plus the protocol parameter 'HB.interval' , with
 jittering of +/- 50%, and exponential back-off 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 a 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
 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 Adler-32 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, 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.  Otherwise,
 4) If the packet contains a COOKIE ECHO in the first chunk, process
    it as described in Section 5.1.  Otherwise,

Stewart, et al. Standards Track [Page 95] RFC 2960 Stream Control Transmission Protocol October 2000

 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 no TCB was
    found. 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,
 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 no
    TCB was found.  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 which 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.

Stewart, et al. Standards Track [Page 96] RFC 2960 Stream Control Transmission Protocol October 2000

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 shall 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.

  1. The receiver MUST accept the packet if the Verification Tag

matches either its own tag, OR the tag of its peer. 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.  Only where no TCB exists should the sender use the
       Verification Tag from the SHUTDOWN ACK.
  1. The receiver of a SHUTDOWN COMPLETE shall accept the packet if

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

       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 Initial Tag received in the INIT ACK.

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

5.

Stewart, et al. Standards Track [Page 97] RFC 2960 Stream Control Transmission Protocol October 2000

 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
 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 shall
 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.
 An endpoint MUST NOT respond to any received packet that contains an
 ABORT chunk (also see Section 8.4).
 An endpoint receiving an ABORT shall apply the special Verification
 Tag check rules described in Section 8.5.1.
 After checking the Verification Tag, the receiving endpoint shall
 remove the association from its record, and shall report the
 termination to its 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 SHUTDOWN-PENDING state and remains there until all
 outstanding data has been acknowledged by its peer.  The endpoint

Stewart, et al. Standards Track [Page 98] RFC 2960 Stream Control Transmission Protocol October 2000

 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 re-send 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.
 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 the reception of the SHUTDOWN, the peer endpoint shall
  1. enter the SHUTDOWN-RECEIVED state,
  1. stop accepting new data from its SCTP user
  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 as 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 shall 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 SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
 respond to each received packet containing one or more DATA chunk(s)
 with a SACK, a SHUTDOWN chunk, and restart the T2-shutdown timer. If

Stewart, et al. Standards Track [Page 99] RFC 2960 Stream Control Transmission Protocol October 2000

 it has no more outstanding DATA chunks, the SHUTDOWN receiver shall
 send a SHUTDOWN ACK and start a T2-shutdown timer of its own,
 entering the SHUTDOWN-ACK-SENT state.  If the timer expires, the
 endpoint must re-send 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 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 assure 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 SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED, or
 SHUTDOWN-ACK-SENT state.
 If an endpoint is in 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

Stewart, et al. Standards Track [Page 100] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 If a SHUTDOWN is received in COOKIE WAIT or COOKIE ECHOED states the
 SHUTDOWN chunk SHOULD be silently discarded.
 If an endpoint is in 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 a 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 (ULP) shall request for 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 the
 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

Stewart, et al. Standards Track [Page 101] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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 which defines the endpoint (see Section 1.4).  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.

Stewart, et al. Standards Track [Page 102] RFC 2960 Stream Control Transmission Protocol October 2000

 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 address
 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
 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

Stewart, et al. Standards Track [Page 103] RFC 2960 Stream Control Transmission Protocol October 2000

 Optional attributes:
 None.
 D) Abort
 Format: ABORT(association id [, cause code])
 -> 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 abortion 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  cause code - 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]
         [,unorder 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.

Stewart, et al. Standards Track [Page 104] RFC 2960 Stream Control Transmission Protocol October 2000

 o  stream id - to indicate which stream to send the data on.  If not
    specified, stream 0 will be used.
 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 lifetime
 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 lifetime 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  unorder 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 primary path for sending packets.

Stewart, et al. Standards Track [Page 105] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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 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.

Stewart, et al. Standards Track [Page 106] RFC 2960 Stream Control Transmission Protocol October 2000

 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.
 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.

Stewart, et al. Standards Track [Page 107] RFC 2960 Stream Control Transmission Protocol October 2000

 I) Change Heartbeat
 Format: CHANGEHEARTBEAT(association id, destination transport address,
         new state [,interval])
 -> result
 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, effects 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.

Stewart, et al. Standards Track [Page 108] RFC 2960 Stream Control Transmission Protocol October 2000

 K) Get SRTT Report
 Format: GETSRTTREPORT(association id, destination transport address)
 -> srtt result
 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)
 -> 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.

Stewart, et al. Standards Track [Page 109] RFC 2960 Stream Control Transmission Protocol October 2000

 Mandatory attributes:
 o  association id - local handle to the SCTP association
 o  protocol parameter list - The specific names and values of the
    protocol parameters (e.g., Association.Max.Retrans [see Section
    14]) 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.

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 O)  Receive unacknowledged message
 Format: RECEIVE_UNACKED(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.
 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 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 an ULP process, certain information
 is passed to the ULP.

Stewart, et al. Standards Track [Page 111] RFC 2960 Stream Control Transmission Protocol October 2000

 IMPLEMENTATION NOTE: In some cases this may be done through a
 separate socket or error channel.
 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 be 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 can not be delivered SCTP shall invoke this notification
 on the ULP.
 The following may be 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.

Stewart, et al. Standards Track [Page 112] RFC 2960 Stream Control Transmission Protocol October 2000

 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 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 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.
 o  last-acked - the TSN last acked by that peer endpoint;

Stewart, et al. Standards Track [Page 113] RFC 2960 Stream Control Transmission Protocol October 2000

 o  last-sent - the TSN last sent to that peer endpoint;
 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
  2. integrity of the user-to-user information carried by SCTP

Stewart, et al. Standards Track [Page 114] RFC 2960 Stream Control Transmission Protocol October 2000

11.2 SCTP Responses To Potential Threats

 SCTP may potentially be used in a wide variety of risk situations.
 It is important for operator(s) 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.
 In order to promote software code-reuse, to avoid re-inventing the
 wheel, and to avoid gratuitous complexity to SCTP, the IP
 Authentication Header [RFC2402] SHOULD be used when the threat
 environment requires stronger integrity protections, but does not
 require confidentiality.
 A widely implemented BSD Sockets API extension exists for
 applications to request IP security services, such as AH or ESP from
 an operating system kernel.  Applications can use such an API to
 request AH whenever AH use is appropriate.

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.

Stewart, et al. Standards Track [Page 115] RFC 2960 Stream Control Transmission Protocol October 2000

 Alternately, the user application MAY use an implementation-specific
 API to request that the IP Encapsulating Security Payload (ESP)
 [RFC2406] be used to provide confidentiality and integrity.
 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 ISAKMP [RFC2408]
 and the Internet Key Exchange (IKE) [RFC2409] SHOULD be used for key
 management.
 Operators should consult [RFC2401] 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

Stewart, et al. Standards Track [Page 116] RFC 2960 Stream Control Transmission Protocol October 2000

  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.

  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 start-up 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 INIT's 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.

Stewart, et al. Standards Track [Page 117] RFC 2960 Stream Control Transmission Protocol October 2000

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

provoking counter-measures which 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.  Man-in-the-middle
 masquerade attacks are discussed in Section 11.3 below.  Because the
 initial exchange is memoryless, 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.

Stewart, et al. Standards Track [Page 118] RFC 2960 Stream Control Transmission Protocol October 2000

 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 Protection against Fraud and Repudiation

 The objective of fraud is to obtain services without authorization
 and specifically without paying for them.  In order to achieve this
 objective, the attacker must induce the SCTP user application at the
 target SCTP node to provide the desired service while accepting
 invalid billing data or failing to collect it.  Repudiation is a
 related problem, since it may occur as a deliberate act of fraud or
 simply because the repudiating party kept inadequate records of
 service received.
 Potential fraudulent attacks include interception and misuse of
 authorizing information such as credit card numbers, blind masquerade
 and replay, and man-in-the middle attacks which modify the packets
 passing through a target SCTP association in real time.
 The interception attack is countered by the confidentiality measures
 discussed in Section 11.2.3 above.
 Section 11.2.4.2 describes how SCTP is resistant to blind masquerade
 attacks, as a result of the four-way startup handshake and the
 Verification Tag.  The Verification Tag and TSN together are
 protections against blind replay attacks, where the replay is into an
 existing association.
 However, SCTP does not protect against man-in-the-middle attacks
 where the attacker is able to intercept and alter the packets sent
 and received in an association.  For example, the INIT ACK will have
 sufficient information sent on the wire for an adversary in the
 middle to hijack an existing SCTP association.  Where a significant
 possibility of such attacks is seen to exist, or where possible
 repudiation is an issue, the use of the IPSEC AH service is
 recommended to ensure both the integrity and the authenticity of the
 SCTP packets passed.
 SCTP also provides no protection against attacks originating at or
 beyond the SCTP node and taking place within the context of an
 existing association.  Prevention of such attacks should be covered
 by appropriate security policies at the host site, as discussed in
 Section 11.2.1.

Stewart, et al. Standards Track [Page 119] RFC 2960 Stream Control Transmission Protocol October 2000

12. 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.

12.1 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 [RFC1750] 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 the endpoint is bound to.

12.2 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.

Stewart, et al. Standards Track [Page 120] RFC 2960 Stream Control Transmission Protocol October 2000

 Peer        : A list of SCTP transport addresses that the peer is
 Transport   : bound to.  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 TSN's 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, SACK's are not delayed (see Section
             : 6).

Stewart, et al. Standards Track [Page 121] RFC 2960 Stream Control Transmission Protocol October 2000

 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 re-assembly 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.

12.3 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
 needs 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 6.2.2)
 state       : The current state of this destination, i.e. DOWN, UP,
             : ALLOW-HB, NO-HEARTBEAT, etc.
 PMTU        : The current known path MTU.

Stewart, et al. Standards Track [Page 122] RFC 2960 Stream Control Transmission Protocol October 2000

 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 a
               RTT.  If this flag is 0, the next DATA chunk sent to this
               destination should be used to compute a 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 this destination was last sent to.  This can be
 used        : used to determine if a HEARTBEAT is needed.

12.4 General Parameters Needed

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

13. IANA Considerations

 This protocol will require port reservation like TCP for the use of
 "well known" servers within the Internet.  All current TCP ports
 shall be automatically reserved in the SCTP port address space.  New
 requests should follow IANA's current mechanisms for TCP.
 This protocol may also be extended through IANA in three ways:
  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

 In the case where a particular ULP using SCTP desires to have its own
 ports, the ULP should be responsible for registering with IANA for
 getting its ports assigned.

13.1 IETF-defined Chunk Extension

 The definition and use of new chunk types is an integral part of
 SCTP.  Thus, new chunk types are assigned by IANA through an IETF
 Consensus action as defined in [RFC2434].
 The documentation for a new chunk code type must include the
 following information:

Stewart, et al. Standards Track [Page 123] RFC 2960 Stream Control Transmission Protocol October 2000

 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;
 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.

13.2 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.

13.3 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 which receives an ERROR (or
    ABORT) chunk containing this cause code.

Stewart, et al. Standards Track [Page 124] RFC 2960 Stream Control Transmission Protocol October 2000

 d) Detailed description of the structure and content of data fields
    which accompany this cause code.
 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 two bytes contain the cause code value
  2. - last two bytes contain length of the Cause Parameter.

13.4 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. Suggested SCTP Protocol Parameter Values

 The following protocol parameters are RECOMMENDED:
 RTO.Initial              - 3  seconds
 RTO.Min                  - 1  second
 RTO.Max                 -  60 seconds
 RTO.Alpha                - 1/8
 RTO.Beta                 - 1/4
 Valid.Cookie.Life        - 60  seconds
 Association.Max.Retrans  - 10 attempts
 Path.Max.Retrans         - 5  attempts (per destination address)
 Max.Init.Retransmits     - 8  attempts
 HB.interval              - 30 seconds
 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.

Stewart, et al. Standards Track [Page 125] RFC 2960 Stream Control Transmission Protocol October 2000

15. Acknowledgements

 The authors wish to thank 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.

16. Authors' Addresses

 Randall R. Stewart
 24 Burning Bush Trail.
 Crystal Lake, IL 60012
 USA
 Phone: +1-815-477-2127
 EMail: rrs@cisco.com
 Qiaobing Xie
 Motorola, Inc.
 1501 W. Shure Drive, #2309
 Arlington Heights, IL 60004
 USA
 Phone: +1-847-632-3028
 EMail: qxie1@email.mot.com
 Ken Morneault
 Cisco Systems Inc.
 13615 Dulles Technology Drive
 Herndon, VA. 20171
 USA
 Phone: +1-703-484-3323
 EMail: kmorneau@cisco.com

Stewart, et al. Standards Track [Page 126] RFC 2960 Stream Control Transmission Protocol October 2000

 Chip Sharp
 Cisco Systems Inc.
 7025 Kit Creek Road
 Research Triangle Park, NC  27709
 USA
 Phone: +1-919-392-3121
 EMail: chsharp@cisco.com
 Hanns Juergen Schwarzbauer
 SIEMENS AG
 Hofmannstr. 51
 81359 Munich
 Germany
 Phone: +49-89-722-24236
 EMail: HannsJuergen.Schwarzbauer@icn.siemens.de
 Tom Taylor
 Nortel Networks
 1852 Lorraine Ave.
 Ottawa, Ontario
 Canada K1H 6Z8
 Phone: +1-613-736-0961
 EMail: taylor@nortelnetworks.com
 Ian Rytina
 Ericsson Australia
 37/360 Elizabeth Street
 Melbourne, Victoria 3000
 Australia
 Phone: +61-3-9301-6164
 EMail: ian.rytina@ericsson.com

Stewart, et al. Standards Track [Page 127] RFC 2960 Stream Control Transmission Protocol October 2000

 Malleswar Kalla
 Telcordia Technologies
 3 Corporate Place
 PYA-2J-341
 Piscataway, NJ  08854
 USA
 Phone: +1-732-699-3728
 EMail: mkalla@telcordia.com
 Lixia Zhang
 UCLA Computer Science Department
 4531G Boelter Hall
 Los Angeles, CA 90095-1596
 USA
 Phone: +1-310-825-2695
 EMail: lixia@cs.ucla.edu
 Vern Paxson
 ACIRI
 1947 Center St., Suite 600,
 Berkeley, CA 94704-1198
 USA
 Phone: +1-510-666-2882
 EMail: vern@aciri.org

17. References

 [RFC768]   Postel, J. (ed.), "User Datagram Protocol", STD 6, RFC
            768, August 1980.
 [RFC793]   Postel, J. (ed.), "Transmission Control Protocol", STD 7,
            RFC 793, September 1981.
 [RFC1123]  Braden, R., "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.
 [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
            1700, October 1994.
 [RFC1981]  McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
            for IP version 6", RFC 1981, August 1996.

Stewart, et al. Standards Track [Page 128] RFC 2960 Stream Control Transmission Protocol October 2000

 [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
            August 1996.
 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401,  November 1998.
 [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC
            2402, November 1998.
 [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.
 [RFC2408]  Maughan, D., Schertler, M., Schneider, M. and J. Turner,
            "Internet Security Association and Key Management
            Protocol", RFC 2408, November 1998.
 [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998.
 [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.

18. Bibliography

 [ALLMAN99] Allman, M. and Paxson, V., "On Estimating End-to-End
            Network Path Properties", Proc. SIGCOMM'99, 1999.
 [FALL96]   Fall, K. and Floyd, S., Simulation-based Comparisons of
            Tahoe, Reno, and SACK TCP, Computer Communications Review,
            V. 26 N. 3, July 1996, pp. 5-21.
 [RFC1750]  Eastlake, D. (ed.), "Randomness Recommendations for
            Security", RFC 1750, December 1994.

Stewart, et al. Standards Track [Page 129] RFC 2960 Stream Control Transmission Protocol October 2000

 [RFC1950]  Deutsch P. and J. Gailly, "ZLIB Compressed Data Format
            Specification version 3.3", RFC 1950, May 1996.
 [RFC2104]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:  Keyed-
            Hashing for Message Authentication", RFC 2104, March 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.
 [SAVAGE99] Savage, S., Cardwell, N., Wetherall, D., and Anderson, T.,
            "TCP Congestion Control with a Misbehaving Receiver",  ACM
            Computer Communication Review, 29(5), October 1999.

Stewart, et al. Standards Track [Page 130] RFC 2960 Stream Control Transmission Protocol October 2000

Appendix A: Explicit Congestion Notification

 ECN (Ramakrishnan, K., Floyd, S., "Explicit Congestion Notification",
 RFC 2481, January 1999) 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 RFC 2481 should be followed with the
 following exceptions.
 Negotiation:
 RFC2481 details negotiation of ECN during the SYN and SYN-ACK stages
 of a TCP connection.  The sender of the SYN sets two bits in the TCP
 flags, and the sender of the SYN-ACK sets only 1 bit.  The reasoning
 behind this is to assure 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:
 RFC 2481 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, et al. Standards Track [Page 131] RFC 2960 Stream Control Transmission Protocol October 2000

 CWR:
 RFC 2481 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 Alder 32 bit checksum calculation

 The Adler-32 checksum calculation given in this appendix is copied from
 [RFC1950].
 Adler-32 is composed of two sums accumulated per byte: s1 is the sum
 of all bytes, s2 is the sum of all s1 values.  Both sums are done
 modulo 65521.  s1 is initialized to 1, s2 to zero.  The Adler-32
 checksum is stored as s2*65536 + s1 in network byte order.
 The following C code computes the Adler-32 checksum of a data buffer.
 It is written for clarity, not for speed.  The sample code is in the
 ANSI C programming language.  Non C users may find it easier to read
 with these hints:

Stewart, et al. Standards Track [Page 132] RFC 2960 Stream Control Transmission Protocol October 2000

 &      Bitwise AND operator.
 >>     Bitwise right shift operator.  When applied to an
        unsigned quantity, as here, right shift inserts zero bit(s)
        at the left.
 <<     Bitwise left shift operator.  Left shift inserts zero
        bit(s) at the right.
 ++     "n++" increments the variable n.
 %      modulo operator: a % b is the remainder of a divided by b.
  #define BASE 65521 /* largest prime smaller than 65536 */
  /*
    Update a running Adler-32 checksum with the bytes buf[0..len-1]
    and return the updated checksum.  The Adler-32 checksum should be
    initialized to 1.
     Usage example:
       unsigned long adler = 1L;
       while (read_buffer(buffer, length) != EOF) {
         adler = update_adler32(adler, buffer, length);
       }
       if (adler != original_adler) error();
    */
    unsigned long update_adler32(unsigned long adler,
       unsigned char *buf, int len)
    {
      unsigned long s1 = adler & 0xffff;
      unsigned long s2 = (adler >> 16) & 0xffff;
      int n;
      for (n = 0; n < len; n++) {
        s1 = (s1 + buf[n]) % BASE;
        s2 = (s2 + s1)     % BASE;
      }
      return (s2 << 16) + s1;
    }
    /* Return the adler32 of the bytes buf[0..len-1] */
    unsigned long adler32(unsigned char *buf, int len)
    {
      return update_adler32(1L, buf, len);
    }

Stewart, et al. Standards Track [Page 133] RFC 2960 Stream Control Transmission Protocol October 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Stewart, et al. Standards Track [Page 134]

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