third_party/lksctp-tools/doc/rfc2960.txt

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:

      -- acknowledged error-free non-duplicated transfer of user data,
      -- data fragmentation to conform to discovered path MTU size,




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      -- 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.









































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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



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   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



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     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.





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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:

      -- 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.

      -- 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.

      -- The limited scope of TCP sockets complicates the task of
      providing highly-available data transfer capability using multi-
      homed hosts.

      -- 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



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   (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.






















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                           SCTP User Application

         -----------------------------------------------------
          _____________                  ____________________
         |             |                | 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



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   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






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   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.









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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.




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   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.




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   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.



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   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.




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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.




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   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.











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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:

      -  A packet containing an INIT chunk MUST have a zero Verification
         Tag.
      -  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.






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   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)



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   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.








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   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.









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   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.





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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.




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   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.




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   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.





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         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.





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      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.




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   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.





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      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.








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   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).







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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.









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   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







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      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.





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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.



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   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.



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   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,















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                        ----------
                        | TSN=17 |
                        ----------
                        |        | <- still missing
                        ----------
                        | TSN=15 |
                        ----------
                        | TSN=14 |
                        ----------
                        |        | <- still missing
                        ----------
                        | TSN=12 |
                        ----------
                        | TSN=11 |
                        ----------
                        | TSN=10 |
                        ----------

   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.





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






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      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









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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.






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



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   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.





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      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.







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   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.



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      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.





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



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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.









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       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.








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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.





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   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.

                       -----          -------- (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  |
                       +---------------+











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                      (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
                          |                   |



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                          |                   | (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).




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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.





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   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.





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   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.



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      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.



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   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.





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   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,





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   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):




















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   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



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   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



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   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.




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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.






















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+------------+------------+---------------+--------------+-------------+
|  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:

      -  Any SCTP DATA Chunks MAY be retained (this is an implementation
         specific option).

      -  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.






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      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):















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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






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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.










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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.





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   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.






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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.






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   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.



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   Acknowledgements MUST be sent in SACK chunks unless shutdown was
   requested by the ULP in which case an endpoint MAY send an
   acknowledgement in the SHUTDOWN chunk.  A SACK chunk can acknowledge
   the reception of multiple DATA chunks.  See Section 3.3.4 for SACK
   chunk format.  In particular, the SCTP endpoint MUST fill in the
   Cumulative TSN Ack field to indicate the latest sequential TSN (of a
   valid DATA chunk) it has received.  Any received DATA chunks with TSN
   greater than the value in the Cumulative TSN Ack field 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



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         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:



















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   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.





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   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.







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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



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       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).





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   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



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       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.



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   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.




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   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.






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   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.
















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      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,



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   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



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      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.








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   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



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   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.








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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.



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   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.






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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





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   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.






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

7.3 Path MTU Discovery

   [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.







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   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.




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   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,




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   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.





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   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,




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   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.









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8.5.1 Exceptions in Verification Tag Rules

   A) Rules for packet carrying INIT:

      -  The sender MUST set the Verification Tag of the packet to 0.

      -  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:

      -  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.

      -  If the ABORT is sent in response to an OOTB packet, the
         endpoint MUST follow the procedure described in Section 8.4.

      -  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:

      -  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.

      -  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

      -  When sending a COOKIE ECHO, the endpoint MUST use the value of
         the Initial Tag received in the INIT ACK.

      -  The receiver of a COOKIE ECHO follows the procedures in Section
         5.





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   E) Rules for packet carrying a SHUTDOWN ACK

      -  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



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   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

   -  enter the SHUTDOWN-RECEIVED state,

   -  stop accepting new data from its SCTP user

   -  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



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   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




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   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






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   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.



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   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




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   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.



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   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.





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   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.




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   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.





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   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.



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   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.






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   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.




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   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.





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   D) COMMUNICATION UP notification

   This notification is used when SCTP becomes ready to send or receive
   user messages, or when a lost communication to an endpoint is
   restored.

   IMPLEMENTATION NOTE: If 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;



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   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.

   -  availability of reliable and timely data transport services
   -  integrity of the user-to-user information carried by SCTP








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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.



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   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:

   -  avoiding commitment of limited resources before determining that
      the request for service is legitimate





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   -  giving priority to completion of processing in progress over the
      acceptance of new work

   -  identification and removal of duplicate or stale queued requests
      for service.

   -  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.










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11.2.4.2 Blind Masquerade

   Masquerade can be used to deny service in several ways:

   -  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.

   -  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.

   -  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.




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   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.





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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.




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   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).






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   Inbound     : An array of structures to track the inbound streams.
   Streams     : Normally including the next sequence number expected
               : and possibly the stream number.

   Outbound    : An array of structures to track the outbound streams.
   Streams     : Normally including the next sequence number to
               : be sent on the stream.

   Reasm Queue : A 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.




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   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:

    -- through definition of additional chunk types,
    -- through definition of additional parameter types, or
    -- 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:




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   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.





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   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.:

   -- first two bytes contain the cause code value
   -- 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.











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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










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   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













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   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.




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   [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.





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   [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.




































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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.





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   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:
















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   &      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);
      }









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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   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
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Acknowledgement

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



















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