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Versions: 00 01 02 03 04

Network Working Group                                         R. Stewart
Internet-Draft                                             Netflix, Inc.
Obsoletes: 4960 (if approved)                                  M. Tuexen
Intended status: Standards Track        Muenster Univ. of Appl. Sciences
Expires: January 27, 2020                                     K. Nielsen
                                                            Kamstrup A/S
                                                           July 26, 2019


                  Stream Control Transmission Protocol
                    draft-ietf-tsvwg-rfc4960-bis-04

Abstract

   This document obsoletes RFC 4960, if approved.  It describes the
   Stream Control Transmission Protocol (SCTP).  SCTP is designed to
   transport Public Switched Telephone Network (PSTN) signaling messages
   over IP networks, but is capable of broader applications.

   SCTP is a reliable transport protocol operating on top of a
   connectionless packet network such as IP.  It offers the following
   services to its users:

   o  acknowledged error-free non-duplicated transfer of user data,

   o  data fragmentation to conform to discovered path MTU size,

   o  sequenced delivery of user messages within multiple streams, with
      an option for order-of-arrival delivery of individual user
      messages,

   o  optional bundling of multiple user messages into a single SCTP
      packet, and

   o  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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute




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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 27, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   6
     1.2.  Architectural View of SCTP  . . . . . . . . . . . . . . .   6
     1.3.  Key Terms . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.4.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .  11
     1.5.  Functional View of SCTP . . . . . . . . . . . . . . . . .  11
       1.5.1.  Association Startup and Takedown  . . . . . . . . . .  12
       1.5.2.  Sequenced Delivery within Streams . . . . . . . . . .  13
       1.5.3.  User Data Fragmentation . . . . . . . . . . . . . . .  13



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       1.5.4.  Acknowledgement and Congestion Avoidance  . . . . . .  13
       1.5.5.  Chunk Bundling  . . . . . . . . . . . . . . . . . . .  14
       1.5.6.  Packet Validation . . . . . . . . . . . . . . . . . .  14
       1.5.7.  Path Management . . . . . . . . . . . . . . . . . . .  14
     1.6.  Serial Number Arithmetic  . . . . . . . . . . . . . . . .  15
     1.7.  Changes from RFC 4960 . . . . . . . . . . . . . . . . . .  16
   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
       3.2.2.  Reporting of Unrecognized Parameters  . . . . . . . .  22
     3.3.  SCTP Chunk Definitions  . . . . . . . . . . . . . . . . .  22
       3.3.1.  Payload Data (DATA) (0) . . . . . . . . . . . . . . .  22
       3.3.2.  Initiation (INIT) (1) . . . . . . . . . . . . . . . .  25
         3.3.2.1.  Optional/Variable-Length Parameters in INIT . . .  28
       3.3.3.  Initiation Acknowledgement (INIT ACK) (2) . . . . . .  31
         3.3.3.1.  Optional or Variable-Length Parameters  . . . . .  34
       3.3.4.  Selective Acknowledgement (SACK) (3)  . . . . . . . .  35
       3.3.5.  Heartbeat Request (HEARTBEAT) (4) . . . . . . . . . .  39
       3.3.6.  Heartbeat Acknowledgement (HEARTBEAT ACK) (5) . . . .  40
       3.3.7.  Abort Association (ABORT) (6) . . . . . . . . . . . .  41
       3.3.8.  Shutdown Association (SHUTDOWN) (7) . . . . . . . . .  42
       3.3.9.  Shutdown Acknowledgement (SHUTDOWN ACK) (8) . . . . .  42
       3.3.10. Operation Error (ERROR) (9) . . . . . . . . . . . . .  43
         3.3.10.1.  Invalid Stream Identifier (1)  . . . . . . . . .  44
         3.3.10.2.  Missing Mandatory Parameter (2)  . . . . . . . .  45
         3.3.10.3.  Stale Cookie Error (3) . . . . . . . . . . . . .  45
         3.3.10.4.  Out of Resource (4)  . . . . . . . . . . . . . .  46
         3.3.10.5.  Unresolvable Address (5) . . . . . . . . . . . .  46
         3.3.10.6.  Unrecognized Chunk Type (6)  . . . . . . . . . .  46
         3.3.10.7.  Invalid Mandatory Parameter (7)  . . . . . . . .  47
         3.3.10.8.  Unrecognized Parameters (8)  . . . . . . . . . .  47
         3.3.10.9.  No User Data (9) . . . . . . . . . . . . . . . .  48
         3.3.10.10. Cookie Received While Shutting Down (10) . . . .  48
         3.3.10.11. Restart of an Association with New Addresses
                    (11) . . . . . . . . . . . . . . . . . . . . . .  49
         3.3.10.12. User-Initiated Abort (12)  . . . . . . . . . . .  49
         3.3.10.13. Protocol Violation (13)  . . . . . . . . . . . .  49
       3.3.11. Cookie Echo (COOKIE ECHO) (10)  . . . . . . . . . . .  50
       3.3.12. Cookie Acknowledgement (COOKIE ACK) (11)  . . . . . .  51
       3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14)  . . . . .  51
   4.  SCTP Association State Diagram  . . . . . . . . . . . . . . .  52
   5.  Association Initialization  . . . . . . . . . . . . . . . . .  55
     5.1.  Normal Establishment of an Association  . . . . . . . . .  55
       5.1.1.  Handle Stream Parameters  . . . . . . . . . . . . . .  57
       5.1.2.  Handle Address Parameters . . . . . . . . . . . . . .  58
       5.1.3.  Generating State Cookie . . . . . . . . . . . . . . .  59



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       5.1.4.  State Cookie Processing . . . . . . . . . . . . . . .  60
       5.1.5.  State Cookie Authentication . . . . . . . . . . . . .  60
       5.1.6.  An Example of Normal Association Establishment  . . .  61
     5.2.  Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE
           ECHO, and COOKIE ACK  . . . . . . . . . . . . . . . . . .  63
       5.2.1.  INIT Received in COOKIE-WAIT or COOKIE-ECHOED State
               (Item B)  . . . . . . . . . . . . . . . . . . . . . .  63
       5.2.2.  Unexpected INIT in States Other than CLOSED, COOKIE-
               ECHOED, COOKIE-WAIT, and SHUTDOWN-ACK-SENT  . . . . .  64
       5.2.3.  Unexpected INIT ACK . . . . . . . . . . . . . . . . .  65
       5.2.4.  Handle a COOKIE ECHO when a TCB Exists  . . . . . . .  65
         5.2.4.1.  An Example of a Association Restart . . . . . . .  67
       5.2.5.  Handle Duplicate COOKIE ACK . . . . . . . . . . . . .  69
       5.2.6.  Handle Stale COOKIE Error . . . . . . . . . . . . . .  69
     5.3.  Other Initialization Issues . . . . . . . . . . . . . . .  69
       5.3.1.  Selection of Tag Value  . . . . . . . . . . . . . . .  69
     5.4.  Path Verification . . . . . . . . . . . . . . . . . . . .  70
   6.  User Data Transfer  . . . . . . . . . . . . . . . . . . . . .  71
     6.1.  Transmission of DATA Chunks . . . . . . . . . . . . . . .  73
     6.2.  Acknowledgement on Reception of DATA Chunks . . . . . . .  76
       6.2.1.  Processing a Received SACK  . . . . . . . . . . . . .  79
     6.3.  Management of Retransmission Timer  . . . . . . . . . . .  81
       6.3.1.  RTO Calculation . . . . . . . . . . . . . . . . . . .  81
       6.3.2.  Retransmission Timer Rules  . . . . . . . . . . . . .  82
       6.3.3.  Handle T3-rtx Expiration  . . . . . . . . . . . . . .  83
     6.4.  Multi-Homed SCTP Endpoints  . . . . . . . . . . . . . . .  84
       6.4.1.  Failover from an Inactive Destination Address . . . .  86
     6.5.  Stream Identifier and Stream Sequence Number  . . . . . .  86
     6.6.  Ordered and Unordered Delivery  . . . . . . . . . . . . .  86
     6.7.  Report Gaps in Received DATA TSNs . . . . . . . . . . . .  87
     6.8.  CRC32c Checksum Calculation . . . . . . . . . . . . . . .  88
     6.9.  Fragmentation and Reassembly  . . . . . . . . . . . . . .  89
     6.10. Bundling  . . . . . . . . . . . . . . . . . . . . . . . .  90
   7.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  91
     7.1.  SCTP Differences from TCP Congestion Control  . . . . . .  92
     7.2.  SCTP Slow-Start and Congestion Avoidance  . . . . . . . .  93
       7.2.1.  Slow-Start  . . . . . . . . . . . . . . . . . . . . .  94
       7.2.2.  Congestion Avoidance  . . . . . . . . . . . . . . . .  95
       7.2.3.  Congestion Control  . . . . . . . . . . . . . . . . .  96
       7.2.4.  Fast Retransmit on Gap Reports  . . . . . . . . . . .  96
       7.2.5.  Making Changes to Differentiated Services Code Points  97
     7.3.  Path MTU Discovery  . . . . . . . . . . . . . . . . . . .  98
   8.  Fault Management  . . . . . . . . . . . . . . . . . . . . . .  98
     8.1.  Endpoint Failure Detection  . . . . . . . . . . . . . . .  98
     8.2.  Path Failure Detection  . . . . . . . . . . . . . . . . .  99
     8.3.  Path Heartbeat  . . . . . . . . . . . . . . . . . . . . . 100
     8.4.  Handle "Out of the Blue" Packets  . . . . . . . . . . . . 102
     8.5.  Verification Tag  . . . . . . . . . . . . . . . . . . . . 103



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       8.5.1.  Exceptions in Verification Tag Rules  . . . . . . . . 103
   9.  Termination of Association  . . . . . . . . . . . . . . . . . 104
     9.1.  Abort of an Association . . . . . . . . . . . . . . . . . 105
     9.2.  Shutdown of an Association  . . . . . . . . . . . . . . . 105
   10. Interface with Upper Layer  . . . . . . . . . . . . . . . . . 108
     10.1.  ULP-to-SCTP  . . . . . . . . . . . . . . . . . . . . . . 108
     10.2.  SCTP-to-ULP  . . . . . . . . . . . . . . . . . . . . . . 117
   11. Security Considerations . . . . . . . . . . . . . . . . . . . 119
     11.1.  Security Objectives  . . . . . . . . . . . . . . . . . . 119
     11.2.  SCTP Responses to Potential Threats  . . . . . . . . . . 119
       11.2.1.  Countering Insider Attacks . . . . . . . . . . . . . 120
       11.2.2.  Protecting against Data Corruption in the Network  . 120
       11.2.3.  Protecting Confidentiality . . . . . . . . . . . . . 120
       11.2.4.  Protecting against Blind Denial-of-Service Attacks . 121
         11.2.4.1.  Flooding . . . . . . . . . . . . . . . . . . . . 121
         11.2.4.2.  Blind Masquerade . . . . . . . . . . . . . . . . 122
         11.2.4.3.  Improper Monopolization of Services  . . . . . . 123
     11.3.  SCTP Interactions with Firewalls . . . . . . . . . . . . 123
     11.4.  Protection of Non-SCTP-Capable Hosts . . . . . . . . . . 123
   12. Network Management Considerations . . . . . . . . . . . . . . 124
   13. Recommended Transmission Control Block (TCB) Parameters . . . 124
     13.1.  Parameters Necessary for the SCTP Instance . . . . . . . 124
     13.2.  Parameters Necessary per Association (i.e., the TCB) . . 125
     13.3.  Per Transport Address Data . . . . . . . . . . . . . . . 126
     13.4.  General Parameters Needed  . . . . . . . . . . . . . . . 127
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 127
     14.1.  IETF-Defined Chunk Extension . . . . . . . . . . . . . . 127
     14.2.  IETF Chunk Flags Registration  . . . . . . . . . . . . . 128
     14.3.  IETF-Defined Chunk Parameter Extension . . . . . . . . . 128
     14.4.  IETF-Defined Additional Error Causes . . . . . . . . . . 128
     14.5.  Payload Protocol Identifiers . . . . . . . . . . . . . . 129
     14.6.  Port Numbers Registry  . . . . . . . . . . . . . . . . . 129
   15. Suggested SCTP Protocol Parameter Values  . . . . . . . . . . 131
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 131
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . . 132
     17.1.  Normative References . . . . . . . . . . . . . . . . . . 132
     17.2.  Informative References . . . . . . . . . . . . . . . . . 134
   Appendix A.  Explicit Congestion Notification . . . . . . . . . . 136
   Appendix B.  CRC32c Checksum Calculation  . . . . . . . . . . . . 137
   Appendix C.  ICMP Handling  . . . . . . . . . . . . . . . . . . . 144
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 146

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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   This document obsoletes [RFC4960], if approved.

1.1.  Motivation

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

   o  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.

   o  The stream-oriented nature of TCP is often an inconvenience.
      Applications add their own record marking to delineate their
      messages, and make explicit use of the push facility to ensure
      that a complete message is transferred in a reasonable time.

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

   o  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 might 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 that exists at the boundary between
   the SCTP and the SCTP user layers.

   SCTP is connection-oriented in nature, but the SCTP association is a
   broader concept than the TCP connection.  SCTP provides the means for
   each SCTP endpoint (Section 1.3) to provide the other endpoint



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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 that can be generated
   from each endpoint's lists.


     _____________                                      _____________
    |  SCTP User  |                                    |  SCTP User  |
    | Application |                                    | Application |
    |-------------|                                    |-------------|
    |    SCTP     |                                    |    SCTP     |
    |  Transport  |                                    |  Transport  |
    |   Service   |                                    |   Service   |
    |-------------|                                    |-------------|
    |             |One or more    ----      One or more|             |
    | IP Network  |IP address      \/        IP address| IP Network  |
    |   Service   |appearances     /\       appearances|   Service   |
    |_____________|               ----                 |_____________|

      SCTP Node A |<-------- Network transport ------->| SCTP Node B


                       Figure 1: An SCTP Association

1.3.  Key Terms

   Some of the language used to describe SCTP has been introduced in the
   previous sections.  This section provides a consolidated list of the
   key terms and their definitions.

   Active destination transport address:  A transport address on a peer
      endpoint that a transmitting endpoint considers available for
      receiving user messages.

   Bundling:  An optional multiplexing operation, whereby more than one
      user message might be carried in the same SCTP packet.  Each user
      message occupies its own DATA chunk.

   Chunk:  A unit of information within an SCTP packet, consisting of a
      chunk header and chunk-specific content.

   Congestion window (cwnd):  An SCTP variable that limits outstanding
      data, in number of bytes, that a sender can send to a particular
      destination transport address before receiving an acknowledgement.





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   Cumulative TSN Ack Point:  The Transmission Sequence Number (TSN) of
      the last DATA chunk acknowledged via the Cumulative TSN Ack field
      of a SACK.

   Flightsize:  The amount of bytes of outstanding data to a particular
      destination transport address at any given time.

   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.

   Inactive destination transport address:  An address that is
      considered inactive due to errors and unavailable to transport
      user messages.

   Message = user message:  Data submitted to SCTP by the Upper Layer
      Protocol (ULP).

   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].

   Network Byte Order:  Most significant byte first, a.k.a., big endian.

   Ordered Message:  A user message that is delivered in order with
      respect to all previous user messages sent within the stream on
      which the message was sent.

   Outstanding data (or "data outstanding" or "data in flight"):  The
      total amount of the DATA chunks associated with outstanding TSNs.
      A retransmitted DATA chunk is counted once in outstanding data.  A
      DATA chunk that is classified as lost but that has not yet been
      retransmitted is not in outstanding data.

   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.

   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.



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   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.

   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.

   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.

   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.

   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.

   SCTP user application (SCTP user):  The logical higher-layer
      application entity which uses the services of SCTP, also called
      the Upper-Layer Protocol (ULP).

   Slow-Start Threshold (ssthresh):  An SCTP variable.  This is the
      threshold that the endpoint will use to determine whether to
      perform slow start or congestion avoidance on a particular
      destination transport address.  Ssthresh is in number of bytes.





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   Stream:  A unidirectional logical channel established from one to
      another associated SCTP endpoint, within which all user messages
      are delivered in sequence except for those submitted to the
      unordered delivery service.

      Note: The relationship between stream numbers in opposite
      directions is strictly a matter of how the applications use them.
      It is the responsibility of the SCTP user to create and manage
      these correlations if they are so desired.

   Stream Sequence Number:  A 16-bit sequence number used internally by
      SCTP to ensure sequenced delivery of the user messages within a
      given stream.  One Stream Sequence Number is attached to each user
      message.

   Tie-Tags:  Two 32-bit random numbers that together make a 64-bit
      nonce.  These tags are used within a State Cookie and TCB so that
      a newly restarting association can be linked to the original
      association within the endpoint that did not restart and yet not
      reveal the true Verification Tags of an existing association.

   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.

   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.

   Transport address:  A transport address is traditionally defined by a
      network-layer address, a transport-layer protocol, and a
      transport-layer port number.  In the case of SCTP running over IP,
      a transport address is defined by the combination of an IP address
      and an SCTP port number (where SCTP is the transport protocol).

   Unacknowledged TSN (at an SCTP endpoint):  A TSN (and the associated
      DATA chunk) that has been received by the endpoint but for which
      an acknowledgement has not yet been sent.  Or in the opposite
      case, for a packet that has been sent but no acknowledgement has
      been received.

   Unordered Message:  Unordered messages are "unordered" with respect
      to any other message; this includes both other unordered messages
      as well as other ordered messages.  An unordered message might be




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      delivered prior to or later than ordered messages sent on the same
      stream.

   User message:  The unit of data delivery across the interface between
      SCTP and its user.

   Verification Tag:  A 32-bit unsigned integer that is randomly
      generated.  The Verification Tag provides a key that allows a
      receiver to verify that the SCTP packet belongs to the current
      association and is not an old or stale packet from a previous
      association.

1.4.  Abbreviations

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

1.5.  Functional View of SCTP

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





















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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

1.5.1.  Association Startup and Takedown

   An association is initiated by a request from the SCTP user (see the
   description of the ASSOCIATE (or SEND) primitive in Section 10).

   A cookie mechanism, similar to one described by Karn and Simpson in
   [RFC2522], is employed during the initialization to provide
   protection against synchronization attacks.  The cookie mechanism
   uses a four-way handshake, the last two legs of which are allowed to
   carry user data for fast setup.  The startup sequence is described in
   Section 5 of this document.

   SCTP provides for graceful close (i.e., shutdown) of an active
   association on request from the SCTP user.  See the description of
   the SHUTDOWN primitive in Section 10.  SCTP also allows ungraceful



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   close (i.e., abort), either on request from the user (ABORT
   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
   continues sending data while the other end is closed.  When either
   endpoint performs a shutdown, the association on each peer will stop
   accepting new data from its user and only deliver data in queue at
   the time of the graceful close (see Section 9).

1.5.2.  Sequenced Delivery within Streams

   The term "stream" is used in SCTP to refer to a sequence of user
   messages that are to be delivered to the upper-layer protocol in
   order with respect to other messages within the same stream.  This is
   in contrast to its usage in TCP, where it refers to a sequence of
   bytes (in this document, a byte is assumed to be 8 bits).

   The SCTP user can specify at association startup time the number of
   streams to be supported by the association.  This number is
   negotiated with the remote end (see Section 5.1.1).  User messages
   are associated with stream numbers (SEND, RECEIVE primitives,
   Section 10).  Internally, SCTP assigns a Stream Sequence Number to
   each message passed to it by the SCTP user.  On the receiving side,
   SCTP ensures that messages are delivered to the SCTP user in sequence
   within a given stream.  However, while one stream might be blocked
   waiting for the next in-sequence user message, delivery from other
   streams might proceed.

   SCTP provides a mechanism for bypassing the sequenced delivery
   service.  User messages sent using this mechanism are delivered to
   the SCTP user as soon as they are received.

1.5.3.  User Data Fragmentation

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

1.5.4.  Acknowledgement and Congestion Avoidance

   SCTP assigns a Transmission Sequence Number (TSN) to each user data
   fragment or unfragmented message.  The TSN is independent of any
   Stream Sequence Number assigned at the stream level.  The receiving
   end acknowledges all TSNs received, even if there are gaps in the




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

1.5.5.  Chunk Bundling

   As described in Section 3, the SCTP packet as delivered to the lower
   layer consists of a common header followed by one or more chunks.
   Each chunk might contain either user data or SCTP control
   information.  The SCTP user has the option to request bundling of
   more than one user message into a single SCTP packet.  The chunk
   bundling function of SCTP is responsible for assembly of the complete
   SCTP packet and its disassembly at the receiving end.

   During times of congestion, an SCTP implementation MAY still perform
   bundling even if the user has requested that SCTP not bundle.  The
   user's disabling of bundling only affects SCTP implementations that
   might delay a small period of time before transmission (to attempt to
   encourage bundling).  When the user layer disables bundling, this
   small delay is prohibited but not bundling that is performed during
   congestion or retransmission.

1.5.6.  Packet Validation

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

1.5.7.  Path Management

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



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   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 startup, a primary path is defined for each SCTP
   endpoint, and is used for normal sending of SCTP packets.

   On the receiving end, the path management is responsible for
   verifying the existence of a valid SCTP association to which the
   inbound SCTP packet belongs before passing it for further processing.

   Note: Path Management and Packet Validation are done at the same
   time, so although described separately above, in reality they cannot
   be performed as separate items.

1.6.  Serial Number Arithmetic

   It is essential to remember that the actual Transmission Sequence
   Number space is finite, though very large.  This space ranges from 0
   to 2**32 - 1.  Since the space is finite, all arithmetic dealing with
   Transmission Sequence Numbers MUST be performed modulo 2**32.  This
   unsigned arithmetic preserves the relationship of sequence numbers as
   they cycle from 2**32 - 1 to 0 again.  There are some subtleties to
   computer modulo arithmetic, so great care has to be taken in
   programming the comparison of such values.  When referring to TSNs,
   the symbol "=<" means "less than or equal"(modulo 2**32).

   Comparisons and arithmetic on TSNs in this document SHOULD use Serial
   Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.

   An endpoint SHOULD NOT transmit a DATA chunk with a TSN that is more
   than 2**31 - 1 above the beginning TSN of its current send window.
   Doing so will cause problems in comparing TSNs.

   Transmission Sequence Numbers wrap around when they reach 2**32 - 1.
   That is, the next TSN a DATA chunk MUST use after transmitting TSN =
   2**32 - 1 is TSN = 0.

   Any arithmetic done on Stream Sequence Numbers SHOULD use Serial
   Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.
   All other arithmetic and comparisons in this document use normal
   arithmetic.




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1.7.  Changes from RFC 4960

   SCTP was originally defined in [RFC4960], which this document
   obsoletes, if approved.  Readers interested in the details of the
   various changes that this document incorporates are asked to consult
   [RFC8540].

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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.  The port
      number 0 MUST NOT be used.

   Destination Port Number: 16 bits (unsigned integer)

      This is the SCTP port number to which this packet is destined.
      The receiving host will use this port number to de-multiplex the
      SCTP packet to the correct receiving endpoint/application.  The
      port number 0 MUST NOT be used.

   Verification Tag: 32 bits (unsigned integer)

      The receiver of this packet uses the Verification Tag to validate
      the sender of this SCTP packet.  On transmit, the value of this
      Verification Tag MUST be set to the value of the Initiate Tag
      received from the peer endpoint during the association
      initialization, with the following exceptions:

      *  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 that caused the ABORT to be sent.
         For details see Section 8.4 and Section 8.5.



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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 CRC32c
      algorithm as described in Appendix B for calculating the checksum.

3.2.  Chunk Field Descriptions

   The figure below illustrates the field format for the chunks to be
   transmitted in the SCTP packet.  Each chunk is formatted with a Chunk
   Type field, a chunk-specific Flag field, a Chunk Length field, and a
   Value field.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Chunk Type  | Chunk  Flags  |        Chunk Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                          Chunk Value                          /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Type: 8 bits (unsigned integer)

      This field identifies the type of information contained in the
      Chunk Value field.  It takes a value from 0 to 254.  The value of
      255 is reserved for future use as an extension field.

      The values of Chunk Types are defined as follows:

      ID Value      Chunk Type
      -----         ----------
      0          -  Payload Data (DATA)
      1          -  Initiation (INIT)
      2          -  Initiation Acknowledgement (INIT ACK)
      3          -  Selective Acknowledgement (SACK)
      4          -  Heartbeat Request (HEARTBEAT)
      5          -  Heartbeat Acknowledgement (HEARTBEAT ACK)
      6          -  Abort (ABORT)
      7          -  Shutdown (SHUTDOWN)
      8          -  Shutdown Acknowledgement (SHUTDOWN ACK)
      9          -  Operation Error (ERROR)
      10         -  State Cookie (COOKIE ECHO)
      11         -  Cookie Acknowledgement (COOKIE ACK)
      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   -  available
      63         -  reserved for IETF-defined Chunk Extensions
      64 to 126  -  available
      127        -  reserved for IETF-defined Chunk Extensions
      128 to 190 -  available
      191        -  reserved for IETF-defined Chunk Extensions
      192 to 254 -  available
      255        -  reserved for IETF-defined Chunk Extensions

      Chunk Types are encoded such that the highest-order 2 bits specify
      the action that is taken if the processing endpoint does not
      recognize the Chunk Type.

      00 - Stop processing this SCTP packet; discard the unrecognized
           chunk and all further chunks.

      01 - Stop processing this SCTP packet, discard the unrecognized
           chunk and all further chunks, and report the unrecognized
           chunk in an 'Unrecognized Chunk Type'.

      10 - Skip this chunk and continue processing.

      11 - Skip this chunk and continue processing, but report in an
           ERROR chunk using the 'Unrecognized Chunk Type' cause of
           error.

      Note: The ECNE and CWR chunk types are reserved for future use of
      Explicit Congestion Notification (ECN); see Appendix A.

   Chunk Flags: 8 bits

      The usage of these bits depends on the Chunk type as given by the
      Chunk Type field.  Unless otherwise specified, they are set to 0
      on transmit and are ignored on receipt.

   Chunk Length: 16 bits (unsigned integer)

      This value represents the size of the chunk in bytes, including
      the Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
      Therefore, if the Chunk Value field is zero-length, the Length
      field will be set to 4.  The Chunk Length field does not count any
      chunk padding.

      Chunks (including Type, Length, and Value fields) are padded out
      by the sender with all zero bytes to be a multiple of 4 bytes
      long.  This padding MUST NOT be more than 3 bytes in total.  The
      Chunk Length value does not include terminating padding of the



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      chunk.  However, it does include padding of any variable-length
      parameter except the last parameter in the chunk.  The receiver
      MUST ignore the padding.

      Note: A robust implementation is expected to accept the chunk
      whether or not the final padding has been included in the Chunk
      Length.

   Chunk Value: variable length

      The Chunk Value field contains the actual information to be
      transferred in the chunk.  The usage and format of this field is
      dependent on the Chunk Type.

   The total length of a chunk (including Type, Length, and Value
   fields) MUST be a multiple of 4 bytes.  If the length of the chunk is
   not a multiple of 4 bytes, the sender MUST pad the chunk with all
   zero bytes, and this padding is not included in the Chunk Length
   field.  The sender MUST NOT pad with more than 3 bytes.  The receiver
   MUST ignore the padding bytes.

   SCTP-defined chunks are described in detail in Section 3.3.  The
   guidelines for IETF-defined chunk extensions can be found in
   Section 14.1 of this document.

3.2.1.  Optional/Variable-Length Parameter Format

   Chunk values of SCTP control chunks consist of a chunk-type-specific
   header of required fields, followed by zero or more parameters.  The
   optional and variable-length parameters contained in a chunk are
   defined in a Type-Length-Value format as shown below.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Parameter Type       |       Parameter Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                       Parameter Value                         /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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.





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      The value of 65535 is reserved for IETF-defined extensions.
      Values other than those defined in specific SCTP chunk
      descriptions are reserved for use by IETF.

   Chunk Parameter Length: 16 bits (unsigned integer)

      The Parameter Length field contains the size of the parameter in
      bytes, including the Parameter Type, Parameter Length, and
      Parameter Value fields.  Thus, a parameter with a zero-length
      Parameter Value field would have a Length field of 4.  The
      Parameter Length does not include any padding bytes.

   Chunk Parameter Value: variable length

      The Parameter Value field contains the actual information to be
      transferred in the parameter.

   The total length of a parameter (including Type, Parameter Length,
   and Value fields) MUST be a multiple of 4 bytes.  If the length of
   the parameter is not a multiple of 4 bytes, the sender pads the
   parameter at the end (i.e., after the Parameter Value field) with all
   zero bytes.  The length of the padding is not included in the
   Parameter Length field.  A sender MUST NOT pad with more than 3
   bytes.  The receiver MUST ignore the padding bytes.

   The Parameter Types are encoded such that the highest-order 2 bits
   specify the action that is taken if the processing endpoint does not
   recognize the Parameter Type.

   00 - Stop processing this parameter; do not process any further
        parameters within this chunk.

   01 - Stop processing this parameter, do not process any further
        parameters within this chunk, and report the unrecognized
        parameter in an 'Unrecognized Parameter', as described in
        Section 3.2.2.

   10 - Skip this parameter and continue processing.

   11 - Skip this parameter and continue processing but report the
        unrecognized parameter in an 'Unrecognized Parameter', as
        described in Section 3.2.2.

   Please note that in all four cases, an INIT ACK or COOKIE ECHO chunk
   is sent.  In the 00 or 01 case, the processing of the parameters
   after the unknown parameter is canceled, but no processing already
   done is rolled back.




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   The actual SCTP parameters are defined in the specific SCTP chunk
   sections.  The rules for IETF-defined parameter extensions are
   defined in Section 14.3.  Note that a parameter type MUST be unique
   across all chunks.  For example, the parameter type '5' is used to
   represent an IPv4 address (see Section 3.3.2.1).  The value '5' then
   is reserved across all chunks to represent an IPv4 address and MUST
   NOT be reused with a different meaning in any other chunk.

3.2.2.  Reporting of Unrecognized Parameters

   If the receiver of an INIT chunk detects unrecognized parameters and
   has to report them according to Section 3.2.1, it MUST put the
   'Unrecognized Parameter' parameter(s) in the INIT ACK chunk sent in
   response to the INIT chunk.  Note that if the receiver of the INIT
   chunk is NOT going to establish an association (e.g., due to lack of
   resources), an 'Unrecognized Parameter' would NOT be included with
   any ABORT being sent to the sender of the INIT.

   If the receiver of an INIT ACK chunk detects unrecognized parameters
   and has to report them according to Section 3.2.1, it SHOULD bundle
   the ERROR chunk containing the 'Unrecognized Parameters' error cause
   with the COOKIE ECHO chunk sent in response to the INIT ACK chunk.
   If the receiver of the INIT ACK cannot bundle the COOKIE ECHO chunk
   with the ERROR chunk, the ERROR chunk MAY be sent separately but not
   before the COOKIE ACK has been received.

   Note: Any time a COOKIE ECHO is sent in a packet, it MUST be the
   first chunk.

3.3.  SCTP Chunk Definitions

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

3.3.1.  Payload Data (DATA) (0)

   The following format MUST be used for the DATA chunk:















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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 = 0    |  Res  |I|U|B|E|    Length                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              TSN                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Stream Identifier S      |   Stream Sequence Number n    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Payload Protocol Identifier                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                 User Data (seq n of Stream S)                 /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Res: 4 bits

      Should be set to all '0's and ignored by the receiver.

   I bit: 1 bit

      The (I)mmediate bit MAY be set by the sender whenever the sender
      of a DATA chunk can benefit from the corresponding SACK chunk
      being sent back without delay.  See Section 4 of [RFC7053] for a
      discussion of the benefits.

   U bit: 1 bit

      The (U)nordered bit, if set to '1', indicates that this is an
      unordered DATA chunk, and there is no Stream Sequence Number
      assigned to this DATA chunk.  Therefore, the receiver MUST ignore
      the Stream Sequence Number field.

      After reassembly (if necessary), unordered DATA chunks MUST be
      dispatched to the upper layer by the receiver without any attempt
      to reorder.

      If an unordered user message is fragmented, each fragment of the
      message MUST have its U bit set to '1'.

   B bit: 1 bit

      The (B)eginning fragment bit, if set, indicates the first fragment
      of a user message.

   E bit: 1 bit




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      The (E)nding fragment bit, if set, indicates the last fragment of
      a user message.

   An unfragmented user message MUST have both the B and E bits set to
   '1'.  Setting both B and E bits to '0' indicates a middle fragment of
   a multi-fragment user message, as summarized in the following table:

           +---+---+-------------------------------------------+
           | B | E |                Description                |
           +---+---+-------------------------------------------+
           | 1 | 0 |  First piece of a fragmented user message |
           +---+---+-------------------------------------------+
           | 0 | 0 | Middle piece of a fragmented user message |
           +---+---+-------------------------------------------+
           | 0 | 1 |  Last piece of a fragmented user message  |
           +---+---+-------------------------------------------+
           | 1 | 1 |            Unfragmented message           |
           +---+---+-------------------------------------------+

                    Table 1: Fragment Description Flags

   When a user message is fragmented into multiple chunks, the TSNs are
   used by the receiver to reassemble the message.  This means that the
   TSNs for each fragment of a fragmented user message MUST be strictly
   sequential.

   Length: 16 bits (unsigned integer)

      This field indicates the length of the DATA chunk in bytes from
      the beginning of the type field to the end of the User Data field
      excluding any padding.  A DATA chunk with one byte of user data
      will have Length set to 17 (indicating 17 bytes).

      A DATA chunk with a User Data field of length L will have the
      Length field set to (16 + L) (indicating 16+L bytes) where L MUST
      be greater than 0.

   TSN: 32 bits (unsigned integer)

      This value represents the TSN for this DATA chunk.  The valid
      range of TSN is from 0 to 4294967295 (2**32 - 1).  TSN wraps back
      to 0 after reaching 4294967295.

   Stream Identifier S: 16 bits (unsigned integer)

      Identifies the stream to which the following user data belongs.

   Stream Sequence Number n: 16 bits (unsigned integer)



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      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.

   Payload Protocol Identifier: 32 bits (unsigned integer)

      This value represents an application (or upper layer) specified
      protocol identifier.  This value is passed to SCTP by its upper
      layer and sent to its peer.  This identifier is not used by SCTP
      but can be used by certain network entities, as well as by the
      peer application, to identify the type of information being
      carried in this DATA chunk.  This field MUST be sent even in
      fragmented DATA chunks (to make sure it is available for agents in
      the middle of the network).  Note that this field is NOT touched
      by an SCTP implementation; therefore, its byte order is NOT
      necessarily big endian.  The upper layer is responsible for any
      byte order conversions to this field.

      The value 0 indicates that no application identifier is specified
      by the upper layer for this payload data.

   User Data: variable length

      This is the payload user data.  The implementation MUST pad the
      end of the data to a 4-byte boundary with all-zero bytes.  Any
      padding MUST NOT be included in the Length field.  A sender MUST
      never add more than 3 bytes of padding.

3.3.2.  Initiation (INIT) (1)

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
















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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 = 1    |  Chunk Flags  |      Chunk Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Initiate Tag                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Advertised Receiver Window Credit (a_rwnd)          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Number of Outbound Streams   |  Number of Inbound Streams    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Initial TSN                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /              Optional/Variable-Length Parameters              /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The INIT chunk contains the following parameters.  Unless otherwise
   noted, each parameter MUST only be included once in the INIT chunk.

              Fixed Parameters                     Status
              ----------------------------------------------
              Initiate Tag                        Mandatory
              Advertised Receiver Window Credit   Mandatory
              Number of Outbound Streams          Mandatory
              Number of Inbound Streams           Mandatory
              Initial TSN                         Mandatory

      Variable Parameters                  Status     Type Value
      -------------------------------------------------------------
      IPv4 Address (Note 1)               Optional    5
      IPv6 Address (Note 1)               Optional    6
      Cookie Preservative                 Optional    9
      Reserved for ECN Capable (Note 2)   Optional    32768 (0x8000)
      Host Name Address (Note 3)          Optional    11
      Supported Address Types (Note 4)    Optional    12

   Note 1: The INIT chunks can contain multiple addresses that can be
   IPv4 and/or IPv6 in any combination.

   Note 2: The ECN Capable field is reserved for future use of Explicit
   Congestion Notification.

   Note 3: An INIT chunk MUST NOT contain the Host Name Address
   parameter.  The receiver of an INIT chunk containing a Host Name
   Address parameter MUST send an ABORT and MAY include an "Unresolvable
   Address" error cause.



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   Note 4: This parameter, when present, specifies all the address types
   the sending endpoint can support.  The absence of this parameter
   indicates that the sending endpoint can support any address type.

   IMPLEMENTATION NOTE: If an INIT chunk is received with known
   parameters that are not optional parameters of the INIT chunk, then
   the receiver SHOULD process the INIT chunk and send back an INIT ACK.
   The receiver of the INIT chunk MAY bundle an ERROR chunk with the
   COOKIE ACK chunk later.  However, restrictive implementations MAY
   send back an ABORT chunk in response to the INIT chunk.

   The Chunk Flags field in INIT is reserved, and all bits in it SHOULD
   be set to 0 by the sender and ignored by the receiver.  The sequence
   of parameters within an INIT can be processed in any order.

   Initiate Tag: 32 bits (unsigned integer)

      The receiver of the INIT (the responding end) records the value of
      the Initiate Tag parameter.  This value MUST be placed into the
      Verification Tag field of every SCTP packet that the receiver of
      the INIT transmits within this association.

      The Initiate Tag is allowed to have any value except 0.  See
      Section 5.3.1 for more on the selection of the tag value.

      If the value of the Initiate Tag in a received INIT chunk is found
      to be 0, the receiver MUST treat it as an error and close the
      association by transmitting an ABORT.

   Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
   integer)

      This value represents the dedicated buffer space, in number of
      bytes, the sender of the INIT has reserved in association with
      this window.  During the life of the association, this buffer
      space SHOULD NOT be lessened (i.e., dedicated buffers taken away
      from this association); however, an endpoint MAY change the value
      of a_rwnd it sends in SACK chunks.

   Number of Outbound Streams (OS): 16 bits (unsigned integer)

      Defines the number of outbound streams the sender of this INIT
      chunk wishes to create in this association.  The value of 0 MUST
      NOT be used.

      Note: A receiver of an INIT with the OS value set to 0 SHOULD
      abort the association.




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   Number of Inbound Streams (MIS): 16 bits (unsigned integer)

      Defines the maximum number of streams the sender of this INIT
      chunk allows the peer end to create in this association.  The
      value 0 MUST NOT be used.

      Note: There is no negotiation of the actual number of streams but
      instead the two endpoints will use the min(requested, offered).
      See Section 5.1.1 for details.

      Note: A receiver of an INIT with the MIS value of 0 SHOULD abort
      the association.

   Initial TSN (I-TSN): 32 bits (unsigned integer)

      Defines the initial TSN that the sender will use.  The valid range
      is from 0 to 4294967295.  This field MAY be set to the value of
      the Initiate Tag field.

3.3.2.1.  Optional/Variable-Length Parameters in INIT

   The following parameters follow the Type-Length-Value format as
   defined in Section 3.2.1.  Any Type-Length-Value fields MUST come
   after the fixed-length fields defined in the previous section.

   IPv4 Address Parameter (5)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Type = 5               |      Length = 8               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        IPv4 Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 Address: 32 bits (unsigned integer)

      Contains an IPv4 address of the sending endpoint.  It is binary
      encoded.

   IPv6 Address Parameter (6)










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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 = 6           |          Length = 20          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 Address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv6 Address: 128 bits (unsigned integer)

      Contains an IPv6 [RFC8200] address of the sending endpoint.  It is
      binary encoded.

      Note: A sender MUST NOT use an IPv4-mapped IPv6 address [RFC4291],
      but SHOULD instead use an IPv4 Address parameter for an IPv4
      address.

   Combined with the Source Port Number in the SCTP common header, the
   value passed in an IPv4 or IPv6 Address parameter indicates a
   transport address the sender of the INIT will support for the
   association being initiated.  That is, during the life time of this
   association, this IP address can appear in the source address field
   of an IP datagram sent from the sender of the INIT, and can be used
   as a destination address of an IP datagram sent from the receiver of
   the INIT.

   More than one IP Address parameter can be included in an INIT chunk
   when the INIT sender is multi-homed.  Moreover, a multi-homed
   endpoint might 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)



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   The sender of the INIT SHOULD 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 MAY be added to the INIT chunk by the
      sender when it reattempts establishing an association with a peer
      to which its previous attempt of establishing the association
      failed due to a stale cookie operation error.  The receiver MAY
      choose to ignore the suggested cookie life-span increase for its
      own security reasons.

   Host Name Address (11)

   The sender of an INIT chunk MUST NOT include this parameter.  The
   usage of the Host Name Address parameter is deprecated.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Type = 11            |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                          Host Name                            /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Host Name: variable length

      This field contains a host name in "host name syntax" per RFC 1123
      Section 2.1 [RFC1123].  The method for resolving the host name is
      out of scope of SCTP.

      Note: At least one null terminator is included in the Host Name
      string and MUST be included in the length.

   Supported Address Types (12)



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   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).  The value indicating the Host
      Name Address parameter (Host name = 11) MUST NOT be used.

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




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   Initiate Tag: 32 bits (unsigned integer)

      The receiver of the INIT ACK records the value of the Initiate Tag
      parameter.  This value MUST be placed into the Verification Tag
      field of every SCTP packet that the INIT ACK receiver transmits
      within this association.

      The Initiate Tag MUST NOT take the value 0.  See Section 5.3.1 for
      more on the selection of the Initiate Tag value.

      If the value of the Initiate Tag in a received INIT ACK chunk is
      found to be 0, the receiver MUST destroy the association
      discarding its TCB.  The receiver MAY send an ABORT for debugging
      purpose.

   Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
   integer)

      This value represents the dedicated buffer space, in number of
      bytes, the sender of the INIT ACK has reserved in association with
      this window.  During the life of the association, this buffer
      space SHOULD NOT be lessened (i.e., dedicated buffers taken away
      from this association).

   Number of Outbound Streams (OS): 16 bits (unsigned integer)

      Defines the number of outbound streams the sender of this INIT ACK
      chunk wishes to create in this association.  The value of 0 MUST
      NOT be used, and the value MUST NOT be greater than the MIS value
      sent in the INIT chunk.

      Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
      destroy the association discarding its TCB.

   Number of Inbound Streams (MIS): 16 bits (unsigned integer)

      Defines the maximum number of streams the sender of this INIT ACK
      chunk allows the peer end to create in this association.  The
      value 0 MUST NOT be used.

      Note: There is no negotiation of the actual number of streams but
      instead the two endpoints will use the min(requested, offered).
      See Section 5.1.1 for details.

      Note: A receiver of an INIT ACK with the MIS value set to 0 SHOULD
      destroy the association discarding its TCB.

   Initial TSN (I-TSN): 32 bits (unsigned integer)



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      Defines the initial TSN that the INIT ACK sender will use.  The
      valid range is from 0 to 4294967295.  This field MAY be set to the
      value of the Initiate Tag field.

              Fixed Parameters                     Status
              ----------------------------------------------
              Initiate Tag                        Mandatory
              Advertised Receiver Window Credit   Mandatory
              Number of Outbound Streams          Mandatory
              Number of Inbound Streams           Mandatory
              Initial TSN                         Mandatory

      Variable Parameters                  Status     Type Value
      -------------------------------------------------------------
      State Cookie                        Mandatory   7
      IPv4 Address (Note 1)               Optional    5
      IPv6 Address (Note 1)               Optional    6
      Unrecognized Parameter              Optional    8
      Reserved for ECN Capable (Note 2)   Optional    32768 (0x8000)
      Host Name Address (Note 3)          Optional    11

   Note 1: The INIT ACK chunks can contain any number of IP address
   parameters that can be IPv4 and/or IPv6 in any combination.

   Note 2: The ECN Capable field is reserved for future use of Explicit
   Congestion Notification.

   Note 3: An INIT ACK chunk MUST NOT contain the Host Name Address
   parameter.  The receiver of INIT ACK chunks containing a Host Name
   Address parameter MUST send an ABORT and MAY include an "Unresolvable
   Address" error cause.

   IMPLEMENTATION NOTE: An implementation MUST be prepared to receive an
   INIT ACK that is quite large (more than 1500 bytes) due to the
   variable size of the State Cookie AND the variable address list.  For
   example if a responder to the INIT has 1000 IPv4 addresses it wishes
   to send, it would need at least 8,000 bytes to encode this in the
   INIT ACK.

   IMPLEMENTATION NOTE: If an INIT ACK chunk is received with known
   parameters that are not optional parameters of the INIT ACK chunk,
   then the receiver SHOULD process the INIT ACK chunk and send back a
   COOKIE ECHO.  The receiver of the INIT ACK chunk MAY bundle an ERROR
   chunk with the COOKIE ECHO chunk.  However, restrictive
   implementations MAY send back an ABORT chunk in response to the INIT
   ACK chunk.





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   In combination with the Source Port carried in the SCTP common
   header, each IP Address parameter in the INIT ACK indicates to the
   receiver of the INIT ACK a valid transport address supported by the
   sender of the INIT ACK for the life time of the association being
   initiated.

   If the INIT ACK contains at least one IP Address parameter, then the
   source address of the IP datagram containing the INIT ACK and any
   additional address(es) provided within the INIT ACK MAY be used as
   destinations by the receiver of the INIT ACK.  If the INIT ACK does
   not contain any IP Address parameters, the receiver of the INIT ACK
   MUST use the source address associated with the received IP datagram
   as its sole destination address for the association.

   The State Cookie and Unrecognized Parameters use the Type-Length-
   Value format as defined in Section 3.2.1 and are described below.
   The other fields are defined the same as their counterparts in the
   INIT chunk.

3.3.3.1.  Optional or Variable-Length Parameters

   State Cookie

   Parameter Type Value:  7

   Parameter Length:  Variable size, depending on size of Cookie.

   Parameter Value:

      This parameter value MUST contain all the necessary state and
      parameter information required for the sender of this INIT ACK to
      create the association, along with a Message Authentication Code
      (MAC).  See Section 5.1.3 for details on State Cookie definition.

   Unrecognized Parameter:

   Parameter Type Value:  8

   Parameter Length:  Variable size.

   Parameter Value:

      This parameter is returned to the originator of the INIT chunk
      when the INIT contains an unrecognized parameter that has a value
      that indicates it SHOULD be reported to the sender.  This
      parameter value field will contain unrecognized parameters copied
      from the INIT chunk complete with Parameter Type, Length, and
      Value fields.



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3.3.4.  Selective Acknowledgement (SACK) (3)

   This chunk is sent to the peer endpoint to acknowledge received DATA
   chunks and to inform the peer endpoint of gaps in the received
   subsequences of DATA chunks as represented by their TSNs.

   The SACK MUST contain the Cumulative TSN Ack, Advertised Receiver
   Window Credit (a_rwnd), Number of Gap Ack Blocks, and Number of
   Duplicate TSNs fields.

   By definition, the value of the Cumulative TSN Ack parameter is the
   last TSN received before a break in the sequence of received TSNs
   occurs; the next TSN value following this one has not yet been
   received at the endpoint sending the SACK.  This parameter therefore
   acknowledges receipt of all TSNs less than or equal to its value.

   The handling of a_rwnd by the receiver of the SACK is discussed in
   detail in Section 6.2.1.

   The SACK also contains zero or more Gap Ack Blocks.  Each Gap Ack
   Block acknowledges a subsequence of TSNs received following a break
   in the sequence of received TSNs.  The Gap Ack Blocks SHOULD be
   isolated.  This means that the TSN just before each Gap Ack Block and
   the TSN just after each Gap Ack Block have not been received.  By
   definition, all TSNs acknowledged by Gap Ack Blocks are greater than
   the value of the Cumulative TSN Ack.

























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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 = 3    |Chunk  Flags   |      Chunk Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Cumulative TSN Ack                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Advertised Receiver Window Credit (a_rwnd)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Number of Gap Ack Blocks = N  |  Number of Duplicate TSNs = X |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Gap Ack Block #1 Start       |   Gap Ack Block #1 End        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Gap Ack Block #N Start      |  Gap Ack Block #N End         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN 1                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN X                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Set to all '0's on transmit and ignored on receipt.

   Cumulative TSN Ack: 32 bits (unsigned integer)

      This parameter contains the TSN of the last DATA chunk received in
      sequence before a gap.  In the case where no DATA chunk has been
      received, this value is set to the peer's Initial TSN minus one.

   Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
   integer)

      This field indicates the updated receive buffer space in bytes of
      the sender of this SACK; see Section 6.2.1 for details.

   Number of Gap Ack Blocks: 16 bits (unsigned integer)

      Indicates the number of Gap Ack Blocks included in this SACK.




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   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 Blocks SHOULD be isolated.  This means that
      the DATA chunks with TSNs equal to (Cumulative TSN Ack + Gap Ack
      Block Start - 1) and (Cumulative TSN Ack + Gap Ack Block End + 1)
      have not been received.

   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 that 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 reinitialized to zero after
      sending each SACK.

   For example, if a receiver were to get the TSN 19 three times it
   would list 19 twice in the outbound SACK.  After sending the SACK, if
   it received yet one more TSN 19 it would list 19 as a duplicate once
   in the next outgoing SACK.





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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 0 on transmit and ignored on receipt.

   Heartbeat Length: 16 bits (unsigned integer)

      Set to the size of the chunk in bytes, including the chunk header
      and the Heartbeat Information field.

   Heartbeat Information: variable length

      Defined as a variable-length parameter using the format described
      in Section 3.2.1, i.e.:

        Variable Parameters                  Status     Type Value
        -------------------------------------------------------------
        Heartbeat Info                       Mandatory   1

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Heartbeat Info Type=1      |         HB Info Length        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                  Sender-Specific Heartbeat Info               /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Sender-Specific Heartbeat Info field SHOULD include
      information about the sender's current time when this HEARTBEAT



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

3.3.6.  Heartbeat Acknowledgement (HEARTBEAT ACK) (5)

   An endpoint MUST send this chunk to its peer endpoint as a response
   to a HEARTBEAT chunk (see Section 8.3).  A HEARTBEAT ACK is always
   sent to the source IP address of the IP datagram containing the
   HEARTBEAT chunk to which this ack is responding.

   The parameter field contains a variable-length opaque data structure.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 5    | Chunk  Flags  |    Heartbeat Ack Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /            Heartbeat Information TLV (Variable-Length)        /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Set to 0 on transmit and ignored on receipt.

   Heartbeat Ack Length: 16 bits (unsigned integer)

      Set to the size of the chunk in bytes, including the chunk header
      and the Heartbeat Information field.

   Heartbeat Information: variable length

      This field MUST contain the Heartbeat Information parameter of the
      Heartbeat Request to which this Heartbeat Acknowledgement is
      responding.

        Variable Parameters                  Status     Type Value
        -------------------------------------------------------------
        Heartbeat Info                       Mandatory   1






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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 about the reason of the abort.  DATA chunks MUST NOT be
   bundled with ABORT.  Control chunks (except for INIT, INIT ACK, and
   SHUTDOWN COMPLETE) MAY be bundled with an ABORT, but they MUST be
   placed before the ABORT in the SCTP packet or they will be ignored by
   the receiver.

   If an endpoint receives an ABORT with a format error or no TCB is
   found, it MUST silently discard it.  Moreover, under any
   circumstances, an endpoint that receives an ABORT MUST NOT respond to
   that ABORT by sending an ABORT of its own.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 6    |Reserved     |T|           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                   zero or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Reserved: 7 bits

         Set to 0 on transmit and ignored on receipt.

      T bit: 1 bit

         The T bit is set to 0 if the sender filled in the Verification
         Tag expected by the peer.  If the Verification Tag is
         reflected, the T bit MUST be set to 1.  Reflecting means that
         the sent Verification Tag is the same as the received one.


   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.

   Note: Special rules apply to this chunk for verification; please see
   Section 8.5.1 for details.



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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 0 on transmit and ignored on receipt.

   Length: 16 bits (unsigned integer)

      Indicates the length of the parameter.  Set to 8.

   Cumulative TSN Ack: 32 bits (unsigned integer)

      This parameter contains the TSN of the last chunk received in
      sequence before any gaps.

      Note: Since the SHUTDOWN message does not contain Gap Ack Blocks,
      it cannot be used to acknowledge TSNs received out of order.  In a
      SACK, lack of Gap Ack Blocks that were previously included
      indicates that the data receiver reneged on the associated DATA
      chunks.  Since SHUTDOWN does not contain Gap Ack Blocks, the
      receiver of the SHUTDOWN MUST NOT 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 0 on transmit and ignored on receipt.

3.3.10.  Operation Error (ERROR) (9)

   An endpoint sends this chunk to its peer endpoint to notify it of
   certain error conditions.  It contains one or more error causes.  An
   Operation Error is not considered fatal in and of itself, but MAY be
   used with an ABORT chunk to report a fatal condition.  It has the
   following parameters:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 9    | Chunk  Flags  |           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                    one or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Set to 0 on transmit and ignored on receipt.

   Length: 16 bits (unsigned integer)

      Set to the size of the chunk in bytes, including the chunk header
      and all the Error Cause fields present.

   Error causes are defined as variable-length parameters using the
   format described in Section 3.2.1, that is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Cause Code          |       Cause Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Cause-Specific Information                 /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Cause Code: 16 bits (unsigned integer)

      Defines the type of error conditions being reported.

      Cause Code



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      Value             Cause Code
      ---------         ----------------
       1                Invalid Stream Identifier
       2                Missing Mandatory Parameter
       3                Stale Cookie Error
       4                Out of Resource
       5                Unresolvable Address
       6                Unrecognized Chunk Type
       7                Invalid Mandatory Parameter
       8                Unrecognized Parameters
       9                No User Data
      10                Cookie Received While Shutting Down
      11                Restart of an Association with New Addresses
      12                User Initiated Abort
      13                Protocol Violation
   Cause Length: 16 bits (unsigned integer)

      Set to the size of the parameter in bytes, including the Cause
      Code, Cause Length, and Cause-Specific Information fields.
   Cause-Specific Information: variable length

      This field carries the details of the error condition.

   Section 3.3.10.1 - Section 3.3.10.13 define error causes for SCTP.
   Guidelines for the IETF to define new error cause values are
   discussed in Section 14.4.

3.3.10.1.  Invalid Stream Identifier (1)

   Cause of error
   ---------------

   Invalid Stream Identifier: Indicates endpoint received a DATA chunk
   sent to a nonexistent stream.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=1              |      Cause Length=8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Stream Identifier      |         (Reserved)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Stream Identifier: 16 bits (unsigned integer)

      Contains the Stream Identifier of the DATA chunk received in
      error.

   Reserved: 16 bits




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      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
   ---------------






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   Unrecognized Chunk Type: This error cause is returned to the
   originator of the chunk if the receiver does not understand the chunk
   and the upper bits of the 'Chunk Type' are set to 01 or 11.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=6              |      Cause Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Unrecognized Chunk                           /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Unrecognized Chunk: variable length

      The Unrecognized Chunk field contains the unrecognized chunk from
      the SCTP packet complete with Chunk Type, Chunk Flags, and Chunk
      Length.

3.3.10.7.  Invalid Mandatory Parameter (7)

   Cause of error
   ---------------

   Invalid Mandatory Parameter: This error cause is returned to the
   originator of an INIT or INIT ACK chunk when one of the mandatory
   parameters is set to an invalid value.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=7              |      Cause Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.10.8.  Unrecognized Parameters (8)

   Cause of error
   ---------------

   Unrecognized Parameters: This error cause is returned to the
   originator of the INIT ACK chunk if the receiver does not recognize
   one or more Optional TLV parameters in the INIT ACK chunk.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=8              |      Cause Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  Unrecognized Parameters                      /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Unrecognized Parameters: variable length




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      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.

3.3.10.9.  No User Data (9)

   Cause of error
   ---------------

   No User Data: This error cause is returned to the originator of a
   DATA chunk if a received DATA chunk has no user data.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=9              |      Cause Length=8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                  TSN value                                    /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   TSN value: 32 bits (unsigned integer)

      The TSN value field contains the TSN of the DATA chunk received
      with no user data field.

   This cause code is normally returned in an ABORT chunk (see
   Section 6.2).

3.3.10.10.  Cookie Received While Shutting Down (10)

   Cause of error
   ---------------

   Cookie Received While Shutting Down: A COOKIE ECHO was received while
   the endpoint was in the SHUTDOWN-ACK-SENT state.  This error is
   usually returned in an ERROR chunk bundled with the retransmitted
   SHUTDOWN ACK.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Cause Code=10              |      Cause Length=4          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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

   Cause of error
   --------------

   Restart of an association with new addresses: An INIT was received on
   an existing association.  But the INIT added addresses to the
   association that were previously NOT part of the association.  The
   new addresses are listed in the error code.  This ERROR is normally
   sent as part of an ABORT refusing the INIT (see Section 5.2).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=11         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                       New Address TLVs                        /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note: Each New Address TLV is an exact copy of the TLV that was found
   in the INIT chunk that was new, including the Parameter Type and the
   Parameter Length.

3.3.10.12.  User-Initiated Abort (12)

   Cause of error
   --------------

   This error cause MAY be included in ABORT chunks that are sent
   because of an upper-layer request.  The upper layer can specify an
   Upper Layer Abort Reason that is transported by SCTP transparently
   and MAY be delivered to the upper-layer protocol at the peer.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=12         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Upper Layer Abort Reason                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.10.13.  Protocol Violation (13)

   Cause of error
   --------------




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   This error cause MAY be included in ABORT chunks that are sent
   because an SCTP endpoint detects a protocol violation of the peer
   that is not covered by the error causes described in Section 3.3.10.1
   to Section 3.3.10.12.  An implementation MAY provide additional
   information specifying what kind of protocol violation has been
   detected.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Cause Code=13         |      Cause Length=Variable    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                    Additional Information                     /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.11.  Cookie Echo (COOKIE ECHO) (10)

   This chunk is used only during the initialization of an association.
   It is sent by the initiator of an association to its peer to complete
   the initialization process.  This chunk MUST precede any DATA chunk
   sent within the association, but MAY be bundled with one or more DATA
   chunks in the same packet.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 10   |Chunk  Flags   |         Length                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                     Cookie                                    /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Set to 0 on transmit and ignored on receipt.

   Length: 16 bits (unsigned integer)

      Set to the size of the chunk in bytes, including the 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.





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      An implementation SHOULD make the cookie as small as possible to
      ensure interoperability.

      Note: A Cookie Echo does NOT contain a State Cookie parameter;
      instead, the data within the State Cookie's Parameter Value
      becomes the data within the Cookie Echo's Chunk Value.  This
      allows an implementation to change only the first 2 bytes of the
      State Cookie parameter to become a COOKIE ECHO chunk.

3.3.12.  Cookie Acknowledgement (COOKIE ACK) (11)

   This chunk is used only during the initialization of an association.
   It is used to acknowledge the receipt of a COOKIE ECHO chunk.  This
   chunk MUST precede any DATA or SACK chunk sent within the
   association, but MAY be bundled with one or more DATA chunks or SACK
   chunk's in the same SCTP packet.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 11   |Chunk  Flags   |     Length = 4                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Set to 0 on transmit and ignored on receipt.

3.3.13.  Shutdown Complete (SHUTDOWN COMPLETE) (14)

   This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
   ACK chunk at the completion of the shutdown process; see Section 9.2
   for details.

   The SHUTDOWN COMPLETE chunk has no parameters.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 14   |Reserved     |T|      Length = 4               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Chunk Flags: 8 bits

      Reserved: 7 bits

         Set to 0 on transmit and ignored on receipt.

      T bit: 1 bit



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         The T bit is set to 0 if the sender filled in the Verification
         Tag expected by the peer.  If the Verification Tag is
         reflected, the T bit MUST be set to 1.  Reflecting means that
         the sent Verification Tag is the same as the received one.

   Note: Special rules apply to this chunk for verification, please see
   Section 8.5.1 for details.

4.  SCTP Association State Diagram

   During the life time of an SCTP association, the SCTP endpoint's
   association progresses from one state to another in response to
   various events.  The events that might potentially advance an
   association's state include:

   o  SCTP user primitive calls, e.g., [ASSOCIATE], [SHUTDOWN], [ABORT],
   o  Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc., control
      chunks, or
   o  Some timeout events.

   The state diagram in the figures below illustrates state changes,
   together with the causing events and resulting actions.  Note that
   some of the error conditions are not shown in the state diagram.
   Full descriptions of all special cases are found in the text.

   Note: Chunk names are given in all capital letters, while parameter
   names have the first letter capitalized, e.g., COOKIE ECHO chunk type
   vs. State Cookie parameter.  If more than one event/message can occur
   that causes a state transition, it is labeled (A), (B), etc.


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



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                        |          |
                        |          |    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  |
                      +---------------+
                               |
                               |
                          /----+------------\
      [SHUTDOWN]         /                   \
      -------------------|                   |
      check outstanding  |                   |
      DATA chunks        |                   |
                         v                   |
                    +---------+              |
                    |SHUTDOWN-|              | rcv SHUTDOWN
                    |PENDING  |              |------------------
                    +---------+              | check outstanding
                         |                   | DATA chunks
    No more outstanding  |                   |
    ---------------------|                   |
    snd SHUTDOWN         |                   |
    strt shutdown timer  |                   |
                         v                   v
                    +---------+        +-----------+
                (4) |SHUTDOWN-|        | SHUTDOWN- |  (5,6)
                    |SENT     |        | RECEIVED  |
                    +---------+        +-----------+
                         |  \                |
   (A) rcv SHUTDOWN ACK  |   \               |
   ----------------------|    \              |
   stop shutdown timer   |     \rcv:SHUTDOWN |
   send SHUTDOWN COMPLETE|      \  (B)       |
   delete TCB            |       \           |
                         |        \          | No more outstanding
                         |         \         |-----------------



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                         |          \        | 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
                         |                   |
                         |                   | (D)rcv SHUTDOWN ACK
                         |                   |--------------
                         |                   | stop shutdown timer
                         |                   | send SHUTDOWN COMPLETE
                         |                   | delete TCB
                         |                   |
                         \    +---------+    /
                          \-->| CLOSED  |<--/
                              +---------+


                Figure 3: State Transition Diagram of SCTP

   Notes:

   1)  If the State Cookie in the received COOKIE ECHO is invalid (i.e.,
       failed to pass the integrity check), the receiver MUST silently
       discard the packet.  Or, if the received State Cookie is expired
       (see Section 5.1.5), the receiver MUST send back an ERROR chunk.
       In either case, the receiver stays in the CLOSED state.
   2)  If the T1-init timer expires, the endpoint MUST retransmit INIT
       and restart the T1-init timer without changing state.  This MUST
       be repeated up to 'Max.Init.Retransmits' times.  After that, the
       endpoint MUST abort the initialization process and report the
       error to the SCTP user.
   3)  If the T1-cookie timer expires, the endpoint MUST retransmit
       COOKIE ECHO and restart the T1-cookie timer without changing
       state.  This MUST be repeated up to 'Max.Init.Retransmits' times.
       After that, the endpoint MUST abort the initialization process
       and report the error to the SCTP user.




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   4)  In the SHUTDOWN-SENT state, the endpoint MUST acknowledge any
       received DATA chunks without delay.
   5)  In the SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any
       new send requests from its SCTP user.
   6)  In the SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
       retransmit data and leave this state when all data in queue is
       transmitted.
   7)  In the SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any
       new send requests from its SCTP user.

   The CLOSED state is used to indicate that an association is not
   created (i.e., doesn't exist).

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 can 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 might be implicitly opened, without an ASSOCIATE
   primitive (see Section 10.1 B) being invoked, by the initiating
   endpoint's sending of the first user data to the destination
   endpoint.  The initiating SCTP will assume default values for all
   mandatory and optional parameters for the INIT/INIT ACK.

   Once the association is established, unidirectional streams are open
   for data transfer on both ends (see Section 5.1.1).

5.1.  Normal Establishment of an Association

   The initialization process consists of the following steps (assuming
   that SCTP endpoint "A" tries to set up an association with SCTP
   endpoint "Z" and "Z" accepts the new association):

   A)  "A" first sends an INIT chunk to "Z".  In the INIT, "A" MUST
       provide its Verification Tag (Tag_A) in the Initiate Tag field.
       Tag_A SHOULD be a random number in the range of 1 to 4294967295
       (see Section 5.3.1 for Tag value selection).  After sending the
       INIT, "A" starts the T1-init timer and enters the COOKIE-WAIT
       state.

   B)  "Z" responds immediately with an INIT ACK chunk.  The destination
       IP address of the INIT ACK MUST be set to the source IP address



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       of the INIT to which this INIT ACK is responding.  In the
       response, besides filling in other parameters, "Z" MUST set the
       Verification Tag field to Tag_A, and also provide its own
       Verification Tag (Tag_Z) in the Initiate Tag field.

       Moreover, "Z" MUST generate and send along with the INIT ACK a
       State Cookie.  See Section 5.1.3 for State Cookie generation.

       Note: After sending out INIT ACK with the State Cookie parameter,
       "Z" MUST NOT allocate any resources or keep any states for the
       new association.  Otherwise, "Z" will be vulnerable to resource
       attacks.

   C)  Upon reception of the INIT ACK from "Z", "A" stops the T1-init
       timer and leave the COOKIE-WAIT state.  "A" then sends the State
       Cookie received in the INIT ACK chunk in a COOKIE ECHO chunk,
       starts the T1-cookie timer, and enters the COOKIE-ECHOED state.

       Note: The COOKIE ECHO chunk MAY 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 can 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 can 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 SHOULD follow the same rules
   given in Section 6.3.



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   If an endpoint receives an INIT, INIT ACK, or COOKIE ECHO chunk but
   decides not to establish the new association due to missing mandatory
   parameters in the received INIT or INIT ACK, invalid parameter
   values, or lack of local resources, it SHOULD respond with an ABORT
   chunk.  It SHOULD also specify the cause of abort, such as the type
   of the missing mandatory parameters, etc., by including the error
   cause parameters with the ABORT chunk.  The Verification Tag field in
   the common header of the outbound SCTP packet containing the ABORT
   chunk MUST be set to the Initiate Tag value of the peer.

   Note that a COOKIE ECHO chunk that does NOT pass the integrity check
   is NOT considered an 'invalid parameter' and requires special
   handling; see Section 5.1.5.

   After the reception of the first DATA chunk in an association the
   endpoint MUST immediately respond with a SACK to acknowledge the DATA
   chunk.  Subsequent acknowledgements SHOULD be done as described in
   Section 6.2.

   When the TCB is created, each endpoint MUST set its internal
   Cumulative TSN Ack Point to the value of its transmitted Initial TSN
   minus one.

   IMPLEMENTATION NOTE: The IP addresses and SCTP port are generally
   used as the key to find the TCB within an SCTP instance.

5.1.1.  Handle Stream Parameters

   In the INIT and INIT ACK chunks, the sender of the chunk MUST
   indicate the number of outbound streams (OSs) it wishes to have in
   the association, as well as the maximum inbound streams (MISs) it
   will accept from the other endpoint.

   After receiving the stream configuration information from the other
   side, each endpoint MUST perform the following check: If the peer's
   MIS is less than the endpoint's OS, meaning that the peer is
   incapable of supporting all the outbound streams the endpoint wants
   to configure, the endpoint MUST use MIS outbound streams and MAY
   report any shortage to the upper layer.  The upper layer can then
   choose to abort the association if the resource shortage is
   unacceptable.

   After the association is initialized, the valid outbound stream
   identifier range for either endpoint MUST be 0 to min(local OS,
   remote MIS)-1.






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5.1.2.  Handle Address Parameters

   During the association initialization, an endpoint uses 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 MUST 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 Address parameter present in the received
       INIT or INIT ACK chunk, the endpoint MUST immediately send an
       ABORT and MAY include an "Unresolvable Address" error cause to
       its peer.  The ABORT SHOULD be sent to the source IP address from
       which the last peer packet was received.

   C)  If there are only IPv4/IPv6 addresses present in the received
       INIT or INIT ACK chunk, the receiver MUST derive and record all
       the transport addresses from the received chunk AND the source IP
       address that sent the INIT or INIT ACK.  The transport addresses
       are derived by the combination of SCTP source port (from the
       common header) and the IP Address parameter(s) carried in the
       INIT or INIT ACK chunk and the source IP address of the IP
       datagram.  The receiver SHOULD use only these transport addresses
       as destination transport addresses when sending subsequent
       packets to its peer.

   D)  An INIT or INIT ACK chunk MUST be treated as belonging to an
       already established association (or one in the process of being
       established) if the use of any of the valid address parameters
       contained within the chunk would identify an existing TCB.

   IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
   doesn't control the source IP address that is used for transmitting),
   an endpoint might need to include in its INIT or INIT ACK all
   possible IP addresses from which packets to the peer could be
   transmitted.

   After all transport addresses are derived from the INIT or INIT ACK
   chunk using the above rules, the endpoint selects one of the
   transport addresses as the initial primary path.

   Note: The INIT ACK MUST be sent to the source address of the INIT.






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   The sender of INIT chunks MAY include a 'Supported Address Types'
   parameter in the INIT to indicate what types of addresses are
   acceptable.

   IMPLEMENTATION NOTE: In the case that the receiver of an INIT ACK
   fails to resolve the address parameter due to an unsupported type, it
   can abort the initiation process and then attempt a reinitiation by
   using a 'Supported Address Types' parameter in the new INIT to
   indicate what types of address it prefers.

   IMPLEMENTATION NOTE: If an SCTP endpoint that only supports either
   IPv4 or IPv6 receives IPv4 and IPv6 addresses in an INIT or INIT ACK
   chunk from its peer, it MUST use all the addresses belonging to the
   supported address family.  The other addresses MAY be ignored.  The
   endpoint SHOULD NOT respond with any kind of error indication.

   IMPLEMENTATION NOTE: If an SCTP endpoint lists in the 'Supported
   Address Types' parameter either IPv4 or IPv6, but uses the other
   family for sending the packet containing the INIT chunk, or if it
   also lists addresses of the other family in the INIT chunk, then the
   address family that is not listed in the 'Supported Address Types'
   parameter SHOULD also be considered as supported by the receiver of
   the INIT chunk.  The receiver of the INIT chunk SHOULD NOT respond
   with any kind of error indication.

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 can
   include a MAC (see [RFC2104] for an example), a timestamp on when the
   State Cookie is created, and the lifespan of the State Cookie, along
   with all the information necessary for it to establish the
   association.

   The following steps SHOULD be taken to generate the State Cookie:

   1)  Create an association TCB using information from both the
       received INIT and the outgoing INIT ACK chunk,
   2)  In the TCB, set the creation time to the current time of day, and
       the lifespan to the protocol parameter 'Valid.Cookie.Life' (see
       Section 15),
   3)  From the TCB, identify and collect the minimal subset of
       information needed to re-create the TCB, and generate a MAC using
       this subset of information and a secret key (see [RFC2104] for an
       example of generating a MAC), and
   4)  Generate the State Cookie by combining this subset of information
       and the resultant MAC.



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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 ([RFC4086] provides some information on randomness
   guidelines); it SHOULD be changed reasonably frequently, and the
   timestamp in the State Cookie MAY be used to determine which key is
   used to verify the MAC.

   An implementation SHOULD make the cookie as small as possible to
   ensure 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 MUST also start the T1-cookie timer after sending out
   the COOKIE ECHO chunk.  If the timer expires, the endpoint MUST
   retransmit the COOKIE ECHO chunk and restart the T1-cookie timer.
   This is repeated until either a COOKIE ACK is received or
   'Max.Init.Retransmits' (see Section 15) is reached causing the peer
   endpoint to be marked unreachable (and thus the association enters
   the CLOSED state).

5.1.5.  State Cookie Authentication

   When an endpoint receives a COOKIE ECHO chunk from another endpoint
   with which it has no association, it takes the following actions:

   1)  Compute a MAC using the TCB data carried in the State Cookie and
       the secret key (note the timestamp in the State Cookie MAY be
       used to determine which secret key to use).  [RFC2104] can be
       used as a guideline for generating the MAC,
   2)  Authenticate the State Cookie as one that it previously generated
       by comparing the computed MAC against the one carried in the
       State Cookie.  If this comparison fails, the SCTP packet,
       including the COOKIE ECHO and any DATA chunks, SHOULD be silently
       discarded,
   3)  Compare the port numbers and the Verification Tag contained
       within the COOKIE ECHO chunk to the actual port numbers and the
       Verification Tag within the SCTP common header of the received



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       packet.  If these values do not match, the packet MUST be
       silently discarded.
   4)  Compare the creation timestamp in the State Cookie to the current
       local time.  If the elapsed time is longer than the lifespan
       carried in the State Cookie, then the packet, including the
       COOKIE ECHO and any attached DATA chunks, SHOULD be discarded,
       and the endpoint MUST transmit an ERROR chunk with a "Stale
       Cookie" error cause to the peer endpoint.
   5)  If the State Cookie is valid, create an association to the sender
       of the COOKIE ECHO chunk with the information in the TCB data
       carried in the COOKIE ECHO and enter the ESTABLISHED state.
   6)  Send a COOKIE ACK chunk to the peer acknowledging receipt of the
       COOKIE ECHO.  The COOKIE ACK MAY be bundled with an outbound DATA
       chunk or SACK chunk; however, the COOKIE ACK MUST be the first
       chunk in the SCTP packet.
   7)  Immediately acknowledge any DATA chunk bundled with the COOKIE
       ECHO with a SACK (subsequent DATA chunk acknowledgement SHOULD
       follow the rules defined in Section 6.2).  As mentioned in step
       6, if the SACK is bundled with the COOKIE ACK, the COOKIE ACK
       MUST appear first in the SCTP packet.

   If a COOKIE ECHO is received from an endpoint with which the receiver
   of the COOKIE ECHO has an existing association, the procedures in
   Section 5.2 SHOULD be followed.

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-cookie timer)       \
   (Enter COOKIE-ECHOED state)    \---> (build TCB, enter ESTABLISHED
                                         state)
                                  /---- COOKIE ACK
                                 /
   (Cancel T1-cookie timer, <---/
    enter ESTABLISHED state)
   {app sends 1st user data; strm 0}
   DATA [TSN=initial TSN_A
       Strm=0,Seq=0 & user data]--\
   (Start T3-rtx timer)            \
                                    \->
                                  /----- SACK [TSN Ack=init
                                 /           TSN_A,Block=0]
   (Cancel T3-rtx timer) <------/
                                         ...
                                        {app sends 2 messages;strm 0}
                                  /---- DATA
                                 /        [TSN=init TSN_Z
                             <--/          Strm=0,Seq=0 & user data 1]
   SACK [TSN Ack=init TSN_Z,      /---- DATA
         Block=0]     --------\  /        [TSN=init TSN_Z +1,
                               \/          Strm=0,Seq=1 & user data 2]
                        <------/\
                                 \
                                  \------>


                       Figure 4: INITIATION Example

   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 is retransmitted and the
   timer restarted.  This is 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).



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   When retransmitting the INIT, the endpoint MUST follow the rules
   defined in Section 6.3 to determine the proper timer value.

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

   During the life time of an association (in one of the possible
   states), an endpoint can receive from its peer endpoint one of the
   setup chunks (INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK).  The
   receiver treats such a setup chunk as a duplicate and process it as
   described in this section.

   Note: An endpoint will not receive the chunk unless the chunk was
   sent to an SCTP transport address and is from an SCTP transport
   address associated with this endpoint.  Therefore, the endpoint
   processes such a chunk as part of its current association.

   The following scenarios can cause duplicated or unexpected chunks:

   A)  The peer has crashed without being detected, restarted itself,
       and sent out a new INIT chunk trying to restore the association,

   B)  Both sides are trying to initialize the association at about the
       same time,

   C)  The chunk is from a stale packet that was used to establish the
       present association or a past association that is no longer in
       existence,

   D)  The chunk is a false packet generated by an attacker, or

   E)  The peer never received the COOKIE ACK and is retransmitting its
       COOKIE ECHO.

   The rules in the following sections are 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 state, an endpoint MUST
   respond with an INIT ACK using the same parameters it sent in its
   original INIT chunk (including its Initiate Tag, unchanged).  When
   responding, the following rules MUST be applied:




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   1)  The INIT ACK MUST only be sent to an address passed by the upper
       layer in the request to initialize the association.

   2)  The INIT ACK MUST only be sent to an address reported in the
       incoming INIT.

   3)  The INIT ACK SHOULD be sent to the source address of the received
       INIT.

   Upon receipt of an INIT in the COOKIE-ECHOED state, an endpoint MUST
   respond with an INIT ACK using the same parameters it sent in its
   original INIT chunk (including its Initiate Tag, unchanged), provided
   that no NEW address has been added to the forming association.  If
   the INIT message indicates that a new address has been added to the
   association, then the entire INIT MUST be discarded, and SHOULD NOT
   do any changes to the existing association.  An ABORT SHOULD be sent
   in response that MAY include the error 'Restart of an association
   with new addresses'.  The error SHOULD list the addresses that were
   added to the restarting association.

   When responding in either state (COOKIE-WAIT or COOKIE-ECHOED) with
   an INIT ACK, the original parameters are combined with those from the
   newly received INIT chunk.  The endpoint MUST 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
   MUST be left running, and the corresponding TCB MUST NOT be
   destroyed.  The normal procedures for handling State Cookies when a
   TCB exists will resolve the duplicate INITs to a single association.

   For an endpoint that is in the COOKIE-ECHOED state, it MUST populate
   its Tie-Tags within both the association TCB and inside the State
   Cookie (see Section 5.2.2 for a description of the Tie-Tags).

5.2.2.  Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED,
        COOKIE-WAIT, and SHUTDOWN-ACK-SENT

   Unless otherwise stated, upon receipt of an unexpected INIT for this
   association, the endpoint MUST generate an INIT ACK with a State
   Cookie.  Before responding, the endpoint MUST check to see if the
   unexpected INIT adds new addresses to the association.  If new
   addresses are added to the association, the endpoint MUST respond
   with an ABORT, copying the 'Initiate Tag' of the unexpected INIT into
   the 'Verification Tag' of the outbound packet carrying the ABORT.  In
   the ABORT response, the cause of error MAY be set to 'restart of an
   association with new addresses'.  The error SHOULD list the addresses
   that were added to the restarting association.  If no new addresses



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   are added, when responding to the INIT in the outbound INIT ACK, the
   endpoint MUST copy its current Tie-Tags to a reserved place within
   the State Cookie and the association's TCB.  We refer to these
   locations inside the cookie as the Peer's-Tie-Tag and the Local-Tie-
   Tag.  We will refer to the copy within an association's TCB as the
   Local Tag and Peer's Tag.  The outbound SCTP packet containing this
   INIT ACK MUST carry a Verification Tag value equal to the Initiate
   Tag found in the unexpected INIT.  And the INIT ACK MUST contain a
   new Initiate Tag (randomly generated; see Section 5.3.1).  Other
   parameters for the endpoint SHOULD be copied from the existing
   parameters of the association (e.g., number of outbound streams) into
   the INIT ACK and cookie.

   After sending out the INIT ACK or ABORT, the endpoint MUST take no
   further actions; i.e., the existing association, including its
   current state, and the corresponding TCB MUST NOT be changed.

   Note: Only when a TCB exists and the association is not in a COOKIE-
   WAIT or SHUTDOWN-ACK-SENT state are the Tie-Tags populated with a
   value other than 0.  For a normal association INIT (i.e., the
   endpoint is in the CLOSED state), the Tie-Tags MUST be set to 0
   (indicating that no previous TCB existed).

5.2.3.  Unexpected INIT ACK

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

5.2.4.  Handle a COOKIE ECHO when a TCB Exists

   When a COOKIE ECHO chunk is received by an endpoint in any state for
   an existing association (i.e., not in the CLOSED state) the following
   rules are 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




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       with a "Stale Cookie" error cause to the peer endpoint (this is
       case C or D in Section 5.2).

       If both Verification Tags in the State Cookie match the
       Verification Tags of the current association, consider the State
       Cookie valid (this is case E in Section 5.2) even if the lifespan
       is exceeded.

   4)  If the State Cookie proves to be valid, unpack the TCB into a
       temporary TCB.

   5)  Refer to Table 2 to determine the correct action to be taken.

   +-----------+------------+---------------+----------------+--------+
   | Local Tag | Peer's Tag | Local-Tie-Tag | Peer's-Tie-Tag | Action |
   +-----------+------------+---------------+----------------+--------+
   |     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 might 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).



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       *  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 enters the ESTABLISHED state.

       If the endpoint is in the SHUTDOWN-ACK-SENT state and recognizes
       that the peer has restarted (Action A), it MUST NOT set up a new
       association but instead resend the SHUTDOWN ACK and send an ERROR
       chunk with a "Cookie Received While Shutting Down" error cause to
       its peer.

   B)  In this case, both sides might 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
       might 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 might be running, and send a COOKIE ACK.

   C)  In this case, the local endpoint's cookie has arrived late.
       Before it arrived, the local endpoint sent an INIT and received
       an INIT ACK and finally sent a COOKIE ECHO with the peer's same
       tag but a new tag of its own.  The cookie SHOULD be silently
       discarded.  The endpoint SHOULD NOT change states and SHOULD
       leave any timers running.

   D)  When both local and remote tags match, the endpoint SHOULD enter
       the ESTABLISHED state, if it is in the COOKIE-ECHOED state.  It
       SHOULD stop any cookie timer that is 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 an existing TCB
                                         compose temp TCB and Cookie_Z
                                         with Tie-Tags to previous
                                         association)
                                   /--- INIT ACK [Veri Tag=Tag_A',
                                  /               I-Tag=Tag_Z',
   (Cancel T1-init timer) <------/                Cookie_Z[TieTags=
                                                  Tag_A,Tag_Z
                                                   & other info]
                                        (destroy temp TCB,leave original
                                         in place)
   COOKIE ECHO [Veri=Tag_Z',
                Cookie_Z
                Tie=Tag_A,
                Tag_Z]----------\
   (Start T1-init timer)         \
   (Enter COOKIE-ECHOED state)    \---> (Find existing association,
                                         Tie-Tags match old tags,
                                         Tags do not match, i.e.,
                                         case X X M M above,
                                         Announce Restart to ULP
                                         and reset association).
                                  /---- COOKIE ACK
   (Cancel T1-init timer, <------/
    Enter ESTABLISHED state)
   {app sends 1st user data; strm 0}
   DATA [TSN=initial TSN_A
       Strm=0,Seq=0 & user data]--\
   (Start T3-rtx timer)            \
                                    \->
                                 /--- SACK [TSN Ack=init TSN_A,Block=0]
   (Cancel T3-rtx timer) <------/


                        Figure 5: A Restart Example






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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)  The association failed to completely setup before the State
       Cookie issued by the sender was processed.

   B)  An old State Cookie was processed after setup completed.

   C)  An old State Cookie is received from someone that the receiver is
       not interested in having an association with and the ABORT chunk
       was lost.

   When processing an ERROR chunk with a "Stale Cookie" error cause an
   endpoint SHOULD first examine if an association is in the process of
   being set up, i.e., the association is in the COOKIE-ECHOED state.
   In all cases, if the association is not in the COOKIE-ECHOED state,
   the ERROR chunk SHOULD be silently discarded.

   If the association is in the COOKIE-ECHOED state, the endpoint MAY
   elect one of the following three alternatives.

   1)  Send a new INIT chunk to the endpoint to generate a new State
       Cookie and reattempt the setup procedure.
   2)  Discard the TCB and report to the upper layer the inability to
       set up the association.
   3)  Send a new INIT chunk to the endpoint, adding a Cookie
       Preservative parameter requesting an extension to the life time
       of the State Cookie.  When calculating the time extension, an
       implementation SHOULD use the RTT information measured based on
       the previous COOKIE ECHO / ERROR exchange, and SHOULD add no more
       than 1 second beyond the measured RTT, due to long State Cookie
       life times making the endpoint more subject to a replay attack.

5.3.  Other Initialization Issues

5.3.1.  Selection of Tag Value

   Initiate Tag values SHOULD be selected from the range of 1 to 2**32 -
   1.  It is very important that the Initiate Tag value be randomized to
   help protect against "man in the middle" and "sequence number"
   attacks.  The methods described in [RFC4086] can be used for the



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   Initiate Tag randomization.  Careful selection of Initiate Tags is
   also necessary to prevent old duplicate packets from previous
   associations being mistakenly processed as belonging to the current
   association.

   Moreover, the Verification Tag value used by either endpoint in a
   given association MUST NOT change during the life time of an
   association.  A new Verification Tag value MUST be used each time the
   endpoint tears down and then reestablishes an association to the same
   peer.

5.4.  Path Verification

   During association establishment, the two peers exchange a list of
   addresses.  In the predominant case, these lists accurately represent
   the addresses owned by each peer.  However, it is possible that a
   misbehaving peer might supply addresses that it does not own.  To
   prevent this, the following rules are applied to all addresses of the
   new association:

   1)  Any addresses passed to the sender of the INIT by its upper layer
       in the request to initialize an association are automatically
       considered to be CONFIRMED.

   2)  For the receiver of the COOKIE ECHO, the only CONFIRMED address
       is the address to which the INIT ACK was sent.

   3)  All other addresses not covered by rules 1 and 2 are considered
       UNCONFIRMED and are subject to probing for verification.

   To probe an address for verification, an endpoint will send
   HEARTBEATs including a 64-bit random nonce and a path indicator (to
   identify the address that the HEARTBEAT is sent to) within the
   HEARTBEAT parameter.

   Upon receipt of the HEARTBEAT ACK, a verification is made that the
   nonce included in the HEARTBEAT parameter is the one sent to the
   address indicated inside the HEARTBEAT parameter.  When this match
   occurs, the address that the original HEARTBEAT was sent to is now
   considered CONFIRMED and available for normal data transfer.

   These probing procedures are started when an association moves to the
   ESTABLISHED state and are ended when all paths are confirmed.

   In each RTO, a probe MAY be sent on an active UNCONFIRMED path in an
   attempt to move it to the CONFIRMED state.  If during this probing
   the path becomes inactive, this rate is lowered to the normal
   HEARTBEAT rate.  At the expiration of the RTO timer, the error



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   counter of any path that was probed but not CONFIRMED is incremented
   by one and subjected to path failure detection, as defined in
   Section 8.2.  When probing UNCONFIRMED addresses, however, the
   association overall error count is NOT incremented.

   The number of HEARTBEATS sent at each RTO SHOULD be limited by the
   HB.Max.Burst parameter.  It is an implementation decision as to how
   to distribute HEARTBEATS to the peer's addresses for path
   verification.

   Whenever a path is confirmed, an indication MAY be given to the upper
   layer.

   An endpoint MUST NOT send any chunks to an UNCONFIRMED address, with
   the following exceptions:

   o  A HEARTBEAT including a nonce MAY be sent to an UNCONFIRMED
      address.

   o  A HEARTBEAT ACK MAY be sent to an UNCONFIRMED address.

   o  A COOKIE ACK MAY be sent to an UNCONFIRMED address, but it MUST be
      bundled with a HEARTBEAT including a nonce.  An implementation
      that does NOT support bundling MUST NOT send a COOKIE ACK to an
      UNCONFIRMED address.

   o  A COOKIE ECHO MAY be sent to an UNCONFIRMED address, but it MUST
      be bundled with a HEARTBEAT including a nonce, and the packet MUST
      NOT exceed the path MTU.  If the implementation does NOT support
      bundling or if the bundled COOKIE ECHO plus HEARTBEAT (including
      nonce) would exceed the path MTU, then the implementation MUST NOT
      send a COOKIE ECHO to an UNCONFIRMED address.

6.  User Data Transfer

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

   DATA chunks MUST only be received according to the rules below in
   ESTABLISHED, SHUTDOWN-PENDING, and SHUTDOWN-SENT.  A DATA chunk
   received in CLOSED is out of the blue and SHOULD be handled per
   Section 8.4.  A DATA chunk received in any other state SHOULD be
   discarded.

   A SACK MUST be processed in ESTABLISHED, SHUTDOWN-PENDING, and
   SHUTDOWN-RECEIVED.  An incoming SACK MAY be processed in COOKIE-



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   ECHOED.  A SACK in the CLOSED state is out of the blue and SHOULD be
   processed according to the rules in Section 8.4.  A SACK chunk
   received in any other state SHOULD be discarded.

   An SCTP receiver MUST be able to receive a minimum of 1500 bytes in
   one SCTP packet.  This means that an SCTP endpoint MUST NOT indicate
   less than 1500 bytes in its initial a_rwnd sent in the INIT or INIT
   ACK.

   For transmission efficiency, SCTP defines mechanisms for bundling of
   small user messages and fragmentation of large user messages.  The
   following diagram depicts the flow of user messages through SCTP.

   In this section, the term "data sender" refers to the endpoint that
   transmits a DATA chunk and the term "data receiver" refers to the
   endpoint that receives a DATA chunk.  A data receiver will transmit
   SACK chunks.


                  +--------------------------+
                  |      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)


               Figure 6: Illustration of User Data Transfer

   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).



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   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.

   The fragmentation and bundling mechanisms, as detailed in Section 6.9
   and Section 6.10, are OPTIONAL to implement by the data sender, but
   they MUST be implemented by the data receiver, i.e., an endpoint MUST
   properly receive and process bundled or fragmented data.

6.1.  Transmission of DATA Chunks

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

   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 smaller than the
       size of the next DATA chunk; see Section 6.2.1).

       When the receiver has no buffer space, this probe is called a
       zero window probe.  Note that a zero window probe SHOULD only be
       sent when all outstanding DATA chunks have been cumulatively
       acknowledged and no DATA chunks are in flight.  Zero window
       probing MUST be supported.

       If the sender continues to receive SACKs from the peer while
       doing zero window probing, the unacknowledged window probes
       SHOULD NOT increment the error counter for the association or any
       destination transport address.  This is because the receiver
       could keep its window closed for an indefinite time.  Section 6.2
       describes the receiver behavior when it advertises a zero window.
       The sender SHOULD send the first zero window probe after 1 RTO
       when it detects that the receiver has closed its window and
       SHOULD increase the probe interval exponentially afterwards.
       Also note that the cwnd SHOULD be adjusted according to
       Section 7.2.1.  Zero window probing does not affect the
       calculation of cwnd.

       The sender MUST also have an algorithm for sending new DATA
       chunks to avoid silly window syndrome (SWS) as described in
       [RFC1122].  The algorithm can be similar to the one described in
       Section 4.2.3.4 of [RFC1122].



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       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 + (PMTU - 1) or more bytes
       of data outstanding to that transport address.  If data is
       available, the sender SHOULD exceed cwnd by up to (PMTU - 1)
       bytes on a new data transmission if the flightsize does not
       currently reach cwnd.  The breach of cwnd MUST constitute one
       packet only.

   C)  When the time comes for the sender to transmit, before sending
       new DATA chunks, the sender MUST first transmit any DATA chunks
       that are marked for retransmission (limited by the current cwnd).

   D)  When the time comes for the sender to transmit new DATA chunks,
       the protocol parameter Max.Burst SHOULD be used to limit the
       number of packets sent.  The limit MAY be applied by adjusting
       cwnd temporarily, as follows:

                if ((flightsize + Max.Burst * MTU) < cwnd)
                    cwnd = flightsize + Max.Burst * MTU;

       Or, it MAY be applied by strictly limiting the number of packets
       emitted by the output routine.  When calculating the number of
       packets to transmit, and particularly when using the formula
       above, cwnd SHOULD NOT be changed permanently.

   E)  Then, the sender can send out as many new DATA chunks as rule A
       and rule B allow.

   Multiple DATA chunks committed for transmission MAY be bundled in a
   single packet.  Furthermore, DATA chunks being retransmitted MAY be
   bundled with new DATA chunks, as long as the resulting packet size
   does not exceed the path MTU.  A ULP can request that no bundling is
   performed, but this only turns off any delays that an SCTP
   implementation might 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




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   chunk, as long as the size of the final SCTP packet does not exceed
   the current MTU.  See Section 6.2.

   IMPLEMENTATION NOTE: When the window is full (i.e., transmission is
   disallowed by rule A and/or rule B), the sender MAY still accept send
   requests from its upper layer, but MUST transmit no more DATA chunks
   until some or all of the outstanding DATA chunks are acknowledged and
   transmission is allowed by rule A and rule B again.

   Whenever a transmission or retransmission is made to any address, if
   the T3-rtx timer of that address is not currently running, the sender
   MUST start that timer.  If the timer for that address is already
   running, the sender MUST restart the timer if the earliest (i.e.,
   lowest TSN) outstanding DATA chunk sent to that address is being
   retransmitted.  Otherwise, the data sender MUST NOT restart the
   timer.

   When starting or restarting the T3-rtx timer, the timer value SHOULD
   be adjusted according to the timer rules defined in Section 6.3.2 and
   Section 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.

   Note: For each stream, the data sender SHOULD NOT have more than
   2**16 - 1 ordered user messages in the current send window.

   Whenever the sender of a DATA chunk can benefit from the
   corresponding SACK chunk being sent back without delay, the sender
   MAY set the I bit in the DATA chunk header.  Please note that why the
   sender has set the I bit is irrelevant to the receiver.

   Reasons for setting the I bit include, but are not limited to, the
   following (see Section 4 of [RFC7053] for a discussion of the
   benefits):

   o  The application requests that the I bit of the last DATA chunk of
      a user message be set when providing the user message to the SCTP
      implementation (see Section 10.1).
   o  The sender is in the SHUTDOWN-PENDING state.
   o  The sending of a DATA chunk fills the congestion or receiver
      window.

   /







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6.2.  Acknowledgement on Reception of DATA Chunks

   The SCTP endpoint MUST always acknowledge the reception of each valid
   DATA chunk when the DATA chunk received is inside its receive window.

   When the receiver's advertised window is 0, the receiver MUST drop
   any new incoming DATA chunk with a TSN larger than the largest TSN
   received so far.  If the new incoming DATA chunk holds a TSN value
   less than the largest TSN received so far, then the receiver SHOULD
   drop the largest TSN held for reordering and accept the new incoming
   DATA chunk.  In either case, if such a DATA chunk is dropped, the
   receiver MUST immediately send back a SACK with the current receive
   window showing only DATA chunks received and accepted so far.  The
   dropped DATA chunk(s) MUST NOT be included in the SACK, as they were
   not accepted.  The receiver MUST also have an algorithm for
   advertising its receive window to avoid receiver silly window
   syndrome (SWS), as described in [RFC1122].  The algorithm can be
   similar to the one described in Section 4.2.3.3 of [RFC1122].

   The guidelines on delayed acknowledgement algorithm specified in
   Section 4.2 of [RFC5681] 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 might be beneficial for an SCTP transmitter to be
   more conservative than the algorithms detailed in this document
   allow.  However, an SCTP transmitter MUST NOT be more aggressive than
   the following algorithms allow.

   An SCTP receiver MUST NOT generate more than one SACK for every
   incoming packet, other than to update the offered window as the
   receiving application consumes new data.  When the window opens up,
   an SCTP receiver SHOULD send additional SACK chunks to update the
   window even if no new data is received.  The receiver MUST avoid
   sending a large number of window updates -- in particular, large
   bursts of them.  One way to achieve this is to send a window update
   only if the window can be increased by at least a quarter of the
   receive buffer size of the association.

   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 (protocol
   parameter 'SACK.Delay') to be configured to be more than 500 ms.  In
   other words, an implementation MAY lower the value of SACK.Delay
   below 500 ms but MUST NOT raise it above 500 ms.



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   Acknowledgements MUST be sent in SACK chunks unless shutdown was
   requested by the ULP, in which case an endpoint MAY send an
   acknowledgement in the SHUTDOWN chunk.  A SACK chunk can acknowledge
   the reception of multiple DATA chunks.  See Section 3.3.4 for SACK
   chunk format.  In particular, the SCTP endpoint MUST fill in the
   Cumulative TSN Ack field to indicate the latest sequential TSN (of a
   valid DATA chunk) it has received.  Any received DATA chunks with TSN
   greater than the value in the Cumulative TSN Ack field are reported
   in the Gap Ack Block fields.  The SCTP endpoint MUST report as many
   Gap Ack Blocks as can fit in a single SACK chunk limited by the
   current path MTU.

   Note: The SHUTDOWN chunk does not contain Gap Ack Block fields.
   Therefore, the endpoint SHOULD use a SACK instead of the SHUTDOWN
   chunk to acknowledge DATA chunks received out of order.

   Upon receipt of an SCTP packet containing a DATA chunk with the I bit
   set, the receiver SHOULD NOT delay the sending of the corresponding
   SACK chunk, i.e., the receiver SHOULD immediately respond with the
   corresponding SACK chunk.

   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,




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       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
       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 drop DATA chunks
   that it has received but hasn't released from its receive buffers
   (i.e., delivered to the ULP).  These DATA chunks might have been
   acked in Gap Ack Blocks.  For example, the data receiver might be
   holding data in its receive buffers while reassembling a fragmented
   user message from its peer when it runs out of receive buffer space.
   It MAY drop these DATA chunks even though it has acknowledged them in
   Gap Ack Blocks.  If a data receiver drops DATA chunks, it MUST NOT
   include them in Gap Ack Blocks in subsequent SACKs until they are
   received again via retransmission.  In addition, the endpoint SHOULD
   take into account the dropped data when calculating its a_rwnd.

   An endpoint SHOULD NOT revoke a SACK and discard data.  Only in
   extreme circumstances might 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 Acknowledgement Example

   If an endpoint receives a DATA chunk with no user data (i.e., the
   Length field is set to 16), it MUST send an ABORT with error cause
   set to "No User Data".

   An endpoint SHOULD NOT send a DATA chunk with no user data part.

6.2.1.  Processing a Received SACK

   Each SACK an endpoint receives contains an a_rwnd value.  This value
   represents the amount of buffer space the data receiver, at the time
   of transmitting the SACK, has left of its total receive buffer space
   (as specified in the INIT/INIT ACK).  Using a_rwnd, Cumulative TSN
   Ack, and Gap Ack Blocks, the data sender can develop a representation
   of the peer's receive buffer space.

   One of the problems the data sender takes 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 uses heuristics to determine if a
   SACK is new.

   An endpoint SHOULD use the following rules to calculate the rwnd,
   using the a_rwnd value, the Cumulative TSN Ack, and Gap Ack Blocks in
   a received SACK.

   A)  At the establishment of the association, the endpoint initializes
       the rwnd to the Advertised Receiver Window Credit (a_rwnd) the
       peer specified in the INIT or INIT ACK.

   B)  Any time a DATA chunk is transmitted (or retransmitted) to a
       peer, the endpoint subtracts the data size of the chunk from the
       rwnd of that peer.

   C)  Any time a DATA chunk is marked for retransmission, either via
       T3-rtx timer expiration (Section 6.3.3) or via Fast Retransmit
       (Section 7.2.4), add the data size of those chunks to the rwnd.

       Note: If the implementation is maintaining a timer on each DATA
       chunk, then only DATA chunks whose timer expired would be marked
       for retransmission.

   D)  Any time a SACK arrives, the endpoint performs the following:

       i)    If Cumulative TSN Ack is less than the Cumulative TSN Ack
             Point, then drop the SACK.  Since Cumulative TSN Ack is
             monotonically increasing, a SACK whose Cumulative TSN Ack
             is less than the Cumulative TSN Ack Point indicates an out-
             of-order SACK.

       ii)   Set rwnd equal to the newly received a_rwnd minus the
             number of bytes still outstanding after processing the
             Cumulative TSN Ack and the Gap Ack Blocks.

       iii)  If the SACK is missing a TSN that was previously
             acknowledged via a Gap Ack Block (e.g., the data receiver
             reneged on the data), then consider the corresponding DATA
             that might be possibly missing: Count one miss indication
             towards Fast Retransmit as described in Section 7.2.4, and
             if no retransmit timer is running for the destination
             address to which the DATA chunk was originally transmitted,
             then T3-rtx is started for that destination address.

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



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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 follow closely how TCP
   manages its retransmission timer.  To compute the current RTO, an
   endpoint maintains two state variables per destination transport
   address: SRTT (smoothed round-trip time) and RTTVAR (round-trip time
   variation).

6.3.1.  RTO Calculation

   The rules governing the computation of SRTT, RTTVAR, and RTO are as
   follows:

   C1)  Until an RTT measurement has been made for a packet sent to the
        given destination transport address, set RTO to the protocol
        parameter 'RTO.Initial'.
   C2)  When the first RTT measurement R is made, set

        SRTT <- R,

        RTTVAR <- R/2, and

        RTO <- SRTT + 4 * RTTVAR.
   C3)  When a new RTT measurement R' is made, set

        RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'|

        and

        SRTT <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'

        Note: The value of SRTT used in the update to RTTVAR is its
        value before updating SRTT itself using the second assignment.

        After the computation, update RTO <- SRTT + 4 * RTTVAR.
   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



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        measuring more frequently often does not in practice yield any
        significant benefit [ALLMAN99]; second, if measurements are made
        more often, then the values of RTO.Alpha and RTO.Beta in rule C3
        above SHOULD be adjusted so that SRTT and RTTVAR still adjust to
        changes at roughly the same rate (in terms of how many round
        trips it takes them to reflect new values) as they would if
        making only one measurement per round-trip and using RTO.Alpha
        and RTO.Beta as given in rule C3.  However, the exact nature of
        these adjustments remains a research issue.

   C5)  Karn's algorithm: RTT measurements MUST NOT be made using
        packets that were retransmitted (and thus for which it is
        ambiguous whether the reply was for the first instance of the
        chunk or for a later instance)

        IMPLEMENTATION NOTE: RTT measurements SHOULD only be made using
        a chunk with TSN r if no chunk with TSN less than or equal to r
        is retransmitted since r is first sent.

   C6)  Whenever RTO is computed, if it is less than RTO.Min seconds
        then it is rounded up to RTO.Min seconds.  The reason for this
        rule is that RTOs that do not have a high minimum value are
        susceptible to unnecessary timeouts [ALLMAN99].

   C7)  A maximum value MAY be placed on RTO provided it is at least
        RTO.max seconds.

   There is no requirement for the clock granularity G used for
   computing RTT measurements and the different state variables, other
   than:

   G1)  Whenever RTTVAR is computed, if RTTVAR = 0, then adjust RTTVAR
        <- G.

   Experience [ALLMAN99] has shown that finer clock granularities (<=
   100 msec) perform somewhat better than more coarse granularities.

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.




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   R2)  Whenever all outstanding data sent to an address have been
        acknowledged, turn off the T3-rtx timer of that address.
   R3)  Whenever a SACK is received that acknowledges the DATA chunk
        with the earliest outstanding TSN for that address, restart the
        T3-rtx timer for that address with its current RTO (if there is
        still outstanding data on that address).
   R4)  Whenever a SACK is received missing a TSN that was previously
        acknowledged via a Gap Ack Block, start the T3-rtx for the
        destination address to which the DATA chunk was originally
        transmitted if it is not already running.

   The following example shows the use of various timer rules (assuming
   that the receiver uses delayed acks).


   Endpoint A                                         Endpoint Z
   {App begins to send}
   Data [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
   (Start T3-rtx timer)
                                           {App sends 1 message; strm 1}
                                           (bundle ack with data)
   DATA [TSN=8,Strm=0,Seq=4] ----\     /-- SACK [TSN Ack=7,Block=0]
                                  \   /      DATA [TSN=6,Strm=1,Seq=2]
                                   \ /     (Start T3-rtx timer)
                                    \
                                   / \
   (Restart T3-rtx timer)  <------/   \--> (ack delayed)
   (ack delayed)
   {send ack}
   SACK [TSN Ack=6,Block=0] --------------> (Cancel T3-rtx timer)
                                           ..
                                           (send ack)
   (Cancel T3-rtx timer)  <-------------- SACK [TSN Ack=8,Block=0]


                       Figure 8: Timer Rule Examples

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




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        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 might be different from the address for which the timer
        expires; see Section 6.4).  Call this value K.  Bundle and
        retransmit those K DATA chunks in a single packet to the
        destination endpoint.
   E4)  Start the retransmission timer T3-rtx on the destination address
        to which the retransmission is sent, if rule R1 above indicates
        to do so.  The RTO to be used for starting T3-rtx SHOULD be the
        one for the destination address to which the retransmission is
        sent, which, when the receiver is multi-homed, might 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 might 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.




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   Moreover, the ULP of an endpoint selects one of the multiple
   destination addresses of a multi-homed peer endpoint as the primary
   path (see Section 5.1.2 and Section 10.1 for details).

   By default, an endpoint SHOULD always transmit to the primary path,
   unless the SCTP user explicitly specifies the destination transport
   address (and possibly source transport address) to use.

   An endpoint SHOULD transmit reply chunks (e.g., INIT ACK, COOKIE ACK,
   HEARTBEAT ACK) in response to control chunks to the same destination
   transport address from which it received the control chunk to which
   it is replying.

   The selection of the destination transport address for packets
   containing SACK chunks is implementation dependent.  However, an
   endpoint SHOULD NOT vary the destination transport address of a SACK
   when it receives DATA chunks coming from the same source address.

   When acknowledging multiple DATA chunks received in packets from
   different source addresses in a single SACK, the SACK chunk MAY be
   transmitted to one of the destination transport addresses from which
   the DATA or control chunks being acknowledged were received.

   When a receiver of a duplicate DATA chunk sends a SACK to a multi-
   homed endpoint, it MAY be beneficial to vary the destination address
   and not use the source address of the DATA chunk.  The reason is that
   receiving a duplicate from a multi-homed endpoint might indicate that
   the return path (as specified in the source address of the DATA
   chunk) for the SACK is broken.

   Furthermore, when its peer is multi-homed, an endpoint SHOULD try to
   retransmit a chunk that timed out to an active destination transport
   address that is different from the last destination address to which
   the DATA chunk was sent.

   When its peer is multi-homed, an endpoint SHOULD send fast
   retransmissions to the same destination transport address to which
   the original data was sent.  If the primary path has been changed and
   the original data was sent to the old primary path before the Fast
   Retransmit, the implementation MAY send it to the new primary path.

   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 is adjusted accordingly.





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6.4.1.  Failover from an Inactive Destination Address

   Some of the transport addresses of a multi-homed SCTP endpoint might
   become inactive due to either the occurrence of certain error
   conditions (see Section 8.2) or adjustments from the SCTP user.

   When there is outbound data to send and the primary path becomes
   inactive (e.g., due to failures), or where the SCTP user explicitly
   requests to send data to an inactive destination transport address,
   before reporting an error to its ULP, the SCTP endpoint SHOULD try to
   send the data to an alternate active destination transport address if
   one exists.

   When retransmitting data that timed out, if the endpoint is multi-
   homed, it needs to consider each source-destination address pair in
   its retransmission selection policy.  When retransmitting timed-out
   data, the endpoint SHOULD attempt to pick the most divergent source-
   destination pair from the original source-destination pair to which
   the packet was transmitted.

   Note: Rules for picking the most divergent source-destination pair
   are an implementation decision and are not specified within this
   document.

6.5.  Stream Identifier and Stream Sequence Number

   Every DATA chunk MUST carry a valid stream identifier.  If an
   endpoint receives a DATA chunk with an invalid stream identifier, it
   SHOULD 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 and the SACK
   chunk in the same packet.

   The Stream Sequence Number in all the streams MUST start from 0 when
   the association is established.  Also, when the Stream Sequence
   Number reaches the value 65535 the next Stream Sequence Number MUST
   be set to 0.

6.6.  Ordered and Unordered Delivery

   Within a stream, an endpoint MUST deliver DATA chunks received with
   the U flag set to 0 to the upper layer according to the order of
   their Stream Sequence Number.  If DATA chunks arrive out of order of
   their Stream Sequence Number, the endpoint MUST hold the received
   DATA chunks from delivery to the ULP until they are reordered.





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   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
   bypasses the ordering mechanism and immediately deliver the data to
   the upper layer (after reassembly if the user data is fragmented by
   the data sender).

   This provides an effective way of transmitting "out-of-band" data in
   a given stream.  Also, a stream can be used as an "unordered" stream
   by simply setting the U flag to 1 in all DATA chunks sent through
   that stream.

   IMPLEMENTATION NOTE: When sending an unordered DATA chunk, an
   implementation MAY choose to place the DATA chunk in an outbound
   packet that is at the head of the outbound transmission queue if
   possible.

   The 'Stream Sequence Number' field in a DATA chunk with U flag set to
   1 has no significance.  The sender can fill it with arbitrary value,
   but the receiver MUST ignore the field.

   Note: When transmitting ordered and unordered data, an endpoint does
   not increment its Stream Sequence Number when transmitting a DATA
   chunk with U flag set to 1.

6.7.  Report Gaps in Received DATA TSNs

   Upon the reception of a new DATA chunk, an endpoint examines 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 that have been acknowledged by the SACK's Cumulative TSN Ack
   from its transmit queue.  The endpoint MUST also treat all the DATA



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   chunks with TSNs not included in the Gap Ack Blocks reported by the
   SACK as "missing".  The number of "missing" reports for each
   outstanding DATA chunk MUST be recorded by the data sender in order
   to make retransmission decisions.  See Section 7.2.4 for details.

   The following example shows the use of SACK to report a gap.


     Endpoint A                                    Endpoint Z
     {App sends 3 messages; strm 0}
     DATA [TSN=6,Strm=0,Seq=2] ---------------> (ack delayed)
     (Start T3-rtx timer)

     DATA [TSN=7,Strm=0,Seq=3] --------> X (lost)

     DATA [TSN=8,Strm=0,Seq=4] ---------------> (gap detected,
                                                 immediately send ack)
                                     /----- SACK [TSN Ack=6,Block=1,
                                    /             Start=2,End=2]
                             <-----/
     (remove 6 from out-queue,
      and mark 7 as "1" missing report)


                   Figure 9: Reporting a Gap using SACK

   The maximum number of Gap Ack Blocks that can be reported within a
   single SACK chunk is limited by the current path MTU.  When a single
   SACK cannot cover all the Gap Ack Blocks needed to be reported due to
   the MTU limitation, the endpoint MUST send only one SACK, reporting
   the Gap Ack Blocks from the lowest to highest TSNs, within the size
   limit set by the MTU, and leave the remaining highest TSN numbers
   unacknowledged.

6.8.  CRC32c Checksum Calculation

   When sending an SCTP packet, the endpoint MUST strengthen the data
   integrity of the transmission by including the CRC32c checksum value
   calculated on the packet, as described below.

   After the packet is constructed (containing the SCTP common header
   and one or more control or DATA chunks), the transmitter MUST

   1)  fill in the proper Verification Tag in the SCTP common header and
       initialize the checksum field to '0's,
   2)  calculate the CRC32c checksum of the whole packet, including the
       SCTP common header and all the chunks (refer to Appendix B for
       details of the CRC32c algorithm); and



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   3)  put the resultant value into the checksum field in the common
       header, and leave the rest of the bits unchanged.

   When an SCTP packet is received, the receiver MUST first check the
   CRC32c checksum as follows:

   1)  Store the received CRC32c checksum value aside.
   2)  Replace the 32 bits of the checksum field in the received SCTP
       packet with all '0's and calculate a CRC32c checksum value of the
       whole received packet.
   3)  Verify that the calculated CRC32c checksum is the same as the
       received CRC32c checksum.  If it is not, the receiver MUST treat
       the packet as an invalid SCTP packet.

   The default procedure for handling invalid SCTP packets is to
   silently discard them.

   Any hardware implementation SHOULD be done in a way that is
   verifiable by the software.

6.9.  Fragmentation and Reassembly

   An endpoint MAY support fragmentation when sending DATA chunks, but
   it MUST support reassembly when receiving DATA chunks.  If an
   endpoint supports fragmentation, it MUST fragment a user message if
   the size of the user message to be sent causes the outbound SCTP
   packet size to exceed the current MTU.  If an implementation does not
   support fragmentation of outbound user messages, the endpoint MUST
   return an error to its upper layer and not attempt to send the user
   message.

   Note: If an implementation that supports fragmentation makes
   available to its upper layer a mechanism to turn off fragmentation,
   it might do so.  However, in so doing, it MUST react just like an
   implementation that does NOT support fragmentation, i.e., it MUST
   reject sends that exceed the current Path MTU (P-MTU).

   IMPLEMENTATION NOTE: In this error case, the Send primitive discussed
   in Section 10.1 would need to return an error to the upper layer.

   If its peer is multi-homed, the endpoint SHOULD 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.




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   When determining when to fragment, the SCTP implementation MUST take
   into account the SCTP packet header as well as the DATA chunk
   header(s).  The implementation MUST also take into account the space
   required for a SACK chunk if bundling a SACK chunk with the DATA
   chunk.

   Fragmentation takes the following steps:

   1)  The data sender MUST break the user message into a series of DATA
       chunks such that each chunk plus SCTP overhead fits into an IP
       datagram smaller than or equal to the association Path MTU.

   2)  The transmitter MUST then assign, in sequence, a separate TSN to
       each of the DATA chunks in the series.  The transmitter assigns
       the same SSN to each of the DATA chunks.  If the user indicates
       that the user message is to be delivered using unordered
       delivery, then the U flag of each DATA chunk of the user message
       MUST be set to 1.

   3)  The transmitter MUST also set the B/E bits of the first DATA
       chunk in the series to '10', the B/E bits of the last DATA chunk
       in the series to '01', and the B/E bits of all other DATA chunks
       in the series to '00'.

   An endpoint MUST recognize fragmented DATA chunks by examining the B/
   E bits in each of the received DATA chunks, and queue the fragmented
   DATA chunks for reassembly.  Once the user message is reassembled,
   SCTP passes the reassembled user message to the specific stream for
   possible reordering and final dispatching.

   Note: If the data receiver runs out of buffer space while still
   waiting for more fragments to complete the reassembly of the message,
   it SHOULD dispatch part of its inbound message through a partial
   delivery API (see Section 10), freeing some of its receive buffer
   space so that the rest of the message can be received.

6.10.  Bundling

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

   If its peer endpoint is multi-homed, the sending endpoint SHOULD
   choose a size no larger than the latest MTU of the current primary
   path.





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   When bundling control chunks with DATA chunks, an endpoint MUST place
   control chunks first in the outbound SCTP packet.  The transmitter
   MUST transmit DATA chunks within an SCTP packet in increasing order
   of TSN.

   Note: Since control chunks are placed first in a packet and since
   DATA chunks are transmitted before SHUTDOWN or SHUTDOWN ACK chunks,
   DATA chunks cannot be bundled with SHUTDOWN or SHUTDOWN ACK chunks.

   Partial chunks MUST NOT be placed in an SCTP packet.  A partial chunk
   is a chunk that is not completely contained in the SCTP packet; i.e.,
   the SCTP packet is too short to contain all the bytes of the chunk as
   indicated by the chunk length.

   An endpoint MUST process received chunks in their order in the
   packet.  The receiver uses the Chunk Length field to determine the
   end of a chunk and beginning of the next chunk taking account of the
   fact that all chunks end on a 4-byte boundary.  If the receiver
   detects a partial chunk, it MUST drop the chunk.

   An endpoint MUST NOT bundle INIT, INIT ACK, or SHUTDOWN COMPLETE with
   any other chunks.

7.  Congestion Control

   Congestion control is one of the basic functions in SCTP.  For some
   applications, it might be likely that adequate resources will be
   allocated to SCTP traffic to ensure prompt delivery of time-critical
   data -- thus, it would appear to be unlikely, during normal
   operations, that transmissions encounter severe congestion
   conditions.  However, SCTP operates under adverse operational
   conditions, which can develop upon partial network failures or
   unexpected traffic surges.  In such situations, SCTP follows 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 are
   expected to 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
   [RFC5681].  This section describes how the algorithms defined in
   [RFC5681] 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.



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   SCTP congestion control is always applied to the entire association,
   and not to individual streams.

7.1.  SCTP Differences from TCP Congestion Control

   Gap Ack Blocks in the SCTP SACK carry the same semantic meaning as
   the TCP SACK.  TCP considers the information carried in the SACK as
   advisory information only.  SCTP considers the information carried in
   the Gap Ack Blocks in the SACK chunk as advisory.  In SCTP, any DATA
   chunk that has been acknowledged by SACK, including DATA that arrived
   at the receiving end out of order, is not considered fully delivered
   until the Cumulative TSN Ack Point passes the TSN of the DATA chunk
   (i.e., the DATA chunk has been acknowledged by the Cumulative TSN Ack
   field in the SACK).  Consequently, the value of cwnd controls the
   amount of outstanding data, rather than (as in the case of non-SACK
   TCP) the upper bound between the highest acknowledged sequence number
   and the latest DATA chunk that can be sent within the congestion
   window.  SCTP SACK leads to different implementations of Fast
   Retransmit and Fast Recovery than non-SACK TCP.  As an example, see
   [FALL96].

   The biggest difference between SCTP and TCP, however, is multi-
   homing.  SCTP is designed to establish robust communication
   associations between two endpoints each of which might be reachable
   by more than one transport address.  Potentially different addresses
   might lead to different data paths between the two endpoints; thus,
   ideally one needs 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 might require refinement
   in the future.  The current algorithms make the following
   assumptions:

   o  The sender usually uses the same destination address until being
      instructed by the upper layer to do otherwise; however, SCTP MAY
      change to an alternate destination in the event an address is
      marked inactive (see Section 8.2).  Also, SCTP MAY retransmit to a
      different transport address than the original transmission.

   o  The sender keeps a separate congestion control parameter set for
      each of the destination addresses it can send to (not each source-
      destination pair but for each destination).  The parameters 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



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   notices a gap in the received sequence number, it waits until the gap
   is filled before delivering the data that was received with sequence
   numbers higher than that of the missing data.  On the other hand,
   SCTP can deliver data to its upper-layer protocol even if there is a
   gap in TSN if the Stream Sequence Numbers are in sequence for a
   particular stream (i.e., the missing DATA chunks are for a different
   stream) or if unordered delivery is indicated.  Although this does
   not affect cwnd, it might affect rwnd calculation.

7.2.  SCTP Slow-Start and Congestion Avoidance

   The slow-start and congestion avoidance algorithms MUST be used by an
   endpoint to control the amount of data being injected into the
   network.  The congestion control in SCTP is employed in regard to the
   association, not to an individual stream.  In some situations, it
   might 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).







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7.2.1.  Slow-Start

   Beginning data transmission into a network with unknown conditions or
   after a sufficiently long idle period requires SCTP to probe the
   network to determine the available capacity.  The slow-start
   algorithm is used for this purpose at the beginning of a transfer, or
   after repairing loss detected by the retransmission timer.

   o  The initial cwnd before data transmission MUST be set to
      min(4*MTU, max (2*MTU, 4380 bytes)).
   o  The initial cwnd after a retransmission timeout MUST be no more
      than 1*MTU, and only one packet is allowed to be in flight until
      successful acknowledgement.
   o  The initial value of ssthresh SHOULD be arbitrarily high (e.g.,
      the size of the largest possible advertised window).
   o  Whenever cwnd is greater than zero, the endpoint is allowed to
      have cwnd bytes of data outstanding on that transport address.  A
      limited overbooking as described in Section 6.1 B) SHOULD be
      supported.
   o  When cwnd is less than or equal to ssthresh, an SCTP endpoint MUST
      use the slow-start algorithm to increase cwnd only if the current
      congestion window is being fully utilized, an incoming SACK
      advances the Cumulative TSN Ack Point, and the data sender is not
      in Fast Recovery.  Only when these three conditions are met can
      the cwnd be increased; otherwise, the cwnd MUST NOT be increased.
      If these conditions are met, then cwnd MUST be increased by, at
      most, the lesser of 1) the total size of the previously
      outstanding DATA chunk(s) acknowledged, and 2) the destination's
      path MTU.  This upper bound protects against the ACK-Splitting
      attack outlined in [SAVAGE99].

   In instances where its peer endpoint is multi-homed, if an endpoint
   receives a SACK that advances its Cumulative TSN Ack Point, then it
   SHOULD update its cwnd (or cwnds) apportioned to the destination
   addresses to which it transmitted the acknowledged data.  However, if
   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 might 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



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   adversely clock out more new data than what has just left the
   network, during a time of possible congestion.

   o  While the endpoint does not transmit data on a given transport
      address, the cwnd of the transport address SHOULD be adjusted to
      max(cwnd/2, 4*MTU) once per RTO.  Before the first cwnd
      adjustment, the ssthresh of the transport address SHOULD be set to
      the cwnd.

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.  The basic
   guidelines for incrementing cwnd during congestion avoidance are as
   follows:

   o  SCTP MAY increment cwnd by 1*MTU.

   o  SCTP SHOULD increment cwnd by 1*MTU once per RTT when the sender
      has cwnd or more bytes of data outstanding for the corresponding
      transport address.

   o  SCTP MUST NOT increment cwnd by more than 1*MTU per RTT.

   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,
      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, by Gap Ack Blocks, and
      by the number of bytes of duplicated chunks reported in Duplicate
      TSNs.
   o  (1) when partial_bytes_acked is greater than cwnd and (2) before
      the arrival of the SACK the sender had less than cwnd bytes of
      data outstanding (i.e., before the arrival of the SACK, flightsize
      was less than cwnd), reset partial_bytes_acked to cwnd.
   o  (1) when partial_bytes_acked is equal to or greater than cwnd and
      (2) before the arrival of the SACK the sender had cwnd or more
      bytes of data outstanding (i.e., before the arrival of the SACK,
      flightsize was greater than or equal to cwnd), partial_bytes_acked
      is reset to (partial_bytes_acked - cwnd).  Next, cwnd is increased
      by 1*MTU.
   o  Same as in the slow start, when the sender does not transmit DATA
      on a given transport address, the cwnd of the transport address
      SHOULD be adjusted to max(cwnd / 2, 4*MTU) per RTO.



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

7.2.3.  Congestion Control

   Upon detection of packet losses from SACK (see Section 7.2.4), an
   endpoint SHOULD do the following:

      ssthresh = max(cwnd/2, 4*MTU)
      cwnd = ssthresh
      partial_bytes_acked = 0

   Basically, a packet loss causes cwnd to be cut in half.

   When the T3-rtx timer expires on an address, SCTP SHOULD perform slow
   start by:

      ssthresh = max(cwnd/2, 4*MTU)
      cwnd = 1*MTU
      partial_bytes_acked = 0

   and ensure that no more than one SCTP packet will be in flight for
   that address until the endpoint receives acknowledgement for
   successful delivery of data to that address.

7.2.4.  Fast Retransmit on Gap Reports

   In the absence of data loss, an endpoint performs delayed
   acknowledgement.  However, whenever an endpoint notices a hole in the
   arriving TSN sequence, it SHOULD start sending a SACK back every time
   a packet arrives carrying data until the hole is filled.

   Whenever an endpoint receives a SACK that indicates that some TSNs
   are missing, it SHOULD wait for two further miss indications (via
   subsequent SACKs for a total of three missing reports) on the same
   TSNs before taking action with regard to Fast Retransmit.

   Miss indications SHOULD follow the HTNA (Highest TSN Newly
   Acknowledged) algorithm.  For each incoming SACK, miss indications
   are incremented only for missing TSNs prior to the highest TSN newly
   acknowledged in the SACK.  A newly acknowledged DATA chunk is one not
   previously acknowledged in a SACK.  If an endpoint is in Fast
   Recovery and a SACK arrives that advances the Cumulative TSN Ack
   Point, the miss indications are incremented for all TSNs reported
   missing in the SACK.





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   When the third consecutive miss indication is received for a TSN(s),
   the data sender does the following:

   1)  Mark the DATA chunk(s) with three miss indications for
       retransmission.
   2)  If not in Fast Recovery, adjust the ssthresh and cwnd of the
       destination address(es) to which the missing DATA chunks were
       last sent, according to the formula described in Section 7.2.3.
   3)  If not in Fast Recovery, 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 PMTU of the
       destination transport address to which the packet is being sent.
       Call this value K.  Retransmit those K DATA chunks in a single
       packet.  When a Fast Retransmit is being performed, the sender
       SHOULD ignore the value of cwnd and SHOULD NOT delay
       retransmission for this single packet.
   4)  Restart the T3-rtx timer only if the last SACK acknowledged the
       lowest outstanding TSN number sent to that address, or the
       endpoint is retransmitting the first outstanding DATA chunk sent
       to that address.
   5)  Mark the DATA chunk(s) as being fast retransmitted and thus
       ineligible for a subsequent Fast Retransmit.  Those TSNs marked
       for retransmission due to the Fast-Retransmit algorithm that did
       not fit in the sent datagram carrying K other TSNs are also
       marked as ineligible for a subsequent Fast Retransmit.  However,
       as they are marked for retransmission they will be retransmitted
       later on as soon as cwnd allows.
   6)  If not in Fast Recovery, enter Fast Recovery and mark the highest
       outstanding TSN as the Fast Recovery exit point.  When a SACK
       acknowledges all TSNs up to and including this exit point, Fast
       Recovery is exited.  While in Fast Recovery, the ssthresh and
       cwnd SHOULD NOT change for any destinations due to a subsequent
       Fast Recovery event (i.e., one SHOULD NOT reduce the cwnd further
       due to a subsequent Fast Retransmit).

   Note: Before the above adjustments, if the received SACK also
   acknowledges new DATA chunks and advances the Cumulative TSN Ack
   Point, the cwnd adjustment rules defined in Section 7.2.1 and
   Section 7.2.2 MUST be applied first.

7.2.5.  Making Changes to Differentiated Services Code Points

   SCTP implementations MAY allow an application to configure the
   Differentiated Services Code Point (DSCP) used for sending packets.
   If a DSCP change might result in outgoing packets being queued in
   different queues, the congestion control parameters for all affected
   destination addresses MUST be reset to their initial values.




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7.3.  Path MTU Discovery

   [RFC4821], [RFC8201], and [RFC1191] specify "Packetization Layer Path
   MTU Discovery", whereby an endpoint maintains an estimate of the
   maximum transmission unit (MTU) along a given Internet path and
   refrains from sending packets along that path that exceed the MTU,
   other than occasional attempts to probe for a change in the Path MTU
   (PMTU).  [RFC4821] is thorough in its discussion of the MTU discovery
   mechanism and strategies for determining the current end-to-end MTU
   setting as well as detecting changes in this value.

   An endpoint SHOULD apply these techniques, and SHOULD do so on a per-
   destination-address basis.

   There are two important SCTP-specific points regarding Path MTU
   discovery:

   1)  SCTP associations can span multiple addresses.  An endpoint MUST
       maintain separate MTU estimates for each destination address of
       its peer.

   2)  The sender SHOULD track an association PMTU that will be the
       smallest PMTU discovered for all of the peer's destination
       addresses.  When fragmenting messages into multiple parts this
       association PMTU SHOULD be used to calculate the size of each
       fragment.  This will allow retransmissions to be seamlessly sent
       to an alternate address without encountering IP fragmentation.

8.  Fault Management

8.1.  Endpoint Failure Detection

   An endpoint SHOULD keep a counter on the total number of consecutive
   retransmissions to its peer (this includes data retransmissions to
   all the destination transport addresses of the peer if it is multi-
   homed), including the number of unacknowledged HEARTBEAT chunks
   observed on the path that is currently used for data transfer.
   Unacknowledged HEARTBEAT chunks observed on paths different from the
   path currently used for data transfer SHOULD NOT increment the
   association error counter, as this could lead to association closure
   even if the path that is currently used for data transfer is
   available (but idle).  If the value of this counter exceeds the limit
   indicated in the protocol parameter 'Association.Max.Retrans', the
   endpoint SHOULD 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 SHOULD report the failure
   to the upper layer and optionally report back all outstanding user




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   data remaining in its outbound queue.  The association is
   automatically closed when the peer endpoint becomes unreachable.

   The counter MUST be reset each time a DATA chunk sent to that peer
   endpoint is acknowledged (by the reception of a SACK).  When a
   HEARTBEAT ACK is received from the peer endpoint, the counter SHOULD
   also be reset.  The receiver of the HEARTBEAT ACK MAY choose not to
   clear the counter if there is outstanding data on the association.
   This allows for handling the possible difference in reachability
   based on DATA chunks and HEARTBEAT chunks.

8.2.  Path Failure Detection

   When its peer endpoint is multi-homed, an endpoint SHOULD keep an
   error counter for each of the destination transport addresses of the
   peer endpoint.

   Each time the T3-rtx timer expires on any address, or when a
   HEARTBEAT sent to an idle address is not acknowledged within an RTO,
   the error counter of that destination address will be incremented.
   When the value in the error counter exceeds the protocol parameter
   'Path.Max.Retrans' of that destination address, the endpoint SHOULD
   mark the destination transport address as inactive, and a
   notification SHOULD be sent to the upper layer.

   When an outstanding TSN is acknowledged or a HEARTBEAT sent to that
   address is acknowledged with a HEARTBEAT ACK, the endpoint SHOULD
   clear the error counter of the destination transport address to which
   the DATA chunk was last sent (or HEARTBEAT was sent) and SHOULD also
   report to the upper layer when an inactive destination address is
   marked as active.  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 could be
   credited to the address of the last chunk sent.  However, this
   ambiguity does not seem to have significant consequences for 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
   might become inactive while the endpoint still considers the peer
   endpoint reachable.  When this condition occurs, how SCTP chooses to
   function is implementation specific.





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   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 SHOULD monitor the reachability of the
   idle destination transport address(es) of its peer by sending a
   HEARTBEAT chunk periodically to the destination transport
   address(es).  HEARTBEAT sending MAY begin upon reaching the
   ESTABLISHED state and is discontinued after sending either SHUTDOWN
   or SHUTDOWN ACK.  A receiver of a HEARTBEAT MUST respond to a
   HEARTBEAT with a HEARTBEAT ACK after entering the COOKIE-ECHOED state
   (INIT sender) or the ESTABLISHED state (INIT receiver), up until
   reaching the SHUTDOWN-SENT state (SHUTDOWN sender) or the SHUTDOWN-
   ACK-SENT state (SHUTDOWN receiver).

   A destination transport address is considered "idle" if no new chunk
   that can be used for updating path RTT (usually including first
   transmission DATA, INIT, COOKIE ECHO, HEARTBEAT, etc.) and no
   HEARTBEAT has been sent to it within the current heartbeat period of
   that address.  This applies to both active and inactive destination
   addresses.

   The upper layer can optionally initiate the following functions:

   A)  Disable heartbeat on a specific destination transport address of
       a given association,

   B)  Change the HB.interval,

   C)  Re-enable heartbeat on a specific destination transport address
       of a given association, and

   D)  Request an on-demand HEARTBEAT on a specific destination
       transport address of a given association.

   The endpoint SHOULD increment the respective error counter of the
   destination transport address each time a HEARTBEAT is sent to that
   address and not acknowledged within one RTO.

   When the value of this counter exceeds the protocol parameter
   'Path.Max.Retrans', the endpoint SHOULD mark the corresponding
   destination address as inactive if it is not so marked and SHOULD
   also report to the upper layer the change in reachability of this



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   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 TLV, together
   with any other received TLVs, copied unchanged from the received
   HEARTBEAT chunk.

   Upon the receipt of the HEARTBEAT ACK, the sender of the HEARTBEAT
   SHOULD clear the error counter of the destination transport address
   to which the HEARTBEAT was sent and mark the destination transport
   address as active if it is not so marked.  The endpoint SHOULD 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 SHOULD also clear the association
   overall error count (as defined in Section 8.1).

   The receiver of the HEARTBEAT ACK SHOULD also perform an RTT
   measurement for that destination transport address using the time
   value carried in the HEARTBEAT ACK chunk.

   On an idle destination address that is allowed to heartbeat, it is
   RECOMMENDED that a HEARTBEAT chunk is sent once per RTO of that
   destination address plus the protocol parameter 'HB.interval', with
   jittering of +/- 50% of the RTO value, and exponential backoff of the
   RTO if the previous HEARTBEAT is unanswered.

   A primitive is provided for the SCTP user to change the HB.interval
   and turn on or off the heartbeat on a given destination address.  The
   heartbeat interval set by the SCTP user is added to the RTO of that
   destination (including any exponential backoff).  Only one heartbeat
   SHOULD be sent each time the heartbeat timer expires (if multiple
   destinations are idle).  It is an implementation decision on how to
   choose which of the candidate idle destinations to heartbeat to (if
   more than one destination is idle).



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   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 is to 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 CRC32c check; see
   Section 6.8), but the receiver is not able to identify the
   association to which this packet belongs.

   The receiver of an OOTB packet MUST do the following:

   1)  If the OOTB packet is to or from a non-unicast address, a
       receiver SHOULD silently discard the packet.  Otherwise,
   2)  If the OOTB packet contains an ABORT chunk, the receiver MUST
       silently discard the OOTB packet and take no further action.
       Otherwise,
   3)  If the packet contains an INIT chunk with a Verification Tag set
       to '0', process it as described in Section 5.1.  If, for whatever
       reason, the INIT cannot be processed normally and an ABORT has to
       be sent in response, the Verification Tag of the packet
       containing the ABORT chunk MUST be the Initiate Tag of the
       received INIT chunk, and the T bit of the ABORT chunk has to be
       set to 0, indicating that the Verification Tag is NOT reflected.
   4)  If the packet contains a COOKIE ECHO in the first chunk, process
       it as described in Section 5.1.  Otherwise,
   5)  If the packet contains a SHUTDOWN ACK chunk, the receiver SHOULD
       respond to the sender of the OOTB packet with a SHUTDOWN
       COMPLETE.  When sending the SHUTDOWN COMPLETE, the receiver of
       the OOTB packet MUST fill in the Verification Tag field of the
       outbound packet with the Verification Tag received in the
       SHUTDOWN ACK and set the T bit in the Chunk Flags to indicate
       that the Verification Tag is reflected.  Otherwise,
   6)  If the packet contains a SHUTDOWN COMPLETE chunk, the receiver
       SHOULD silently discard the packet and take no further action.
       Otherwise,
   7)  If the packet contains a "Stale Cookie" ERROR or a COOKIE ACK,
       the SCTP packet SHOULD be silently discarded.  Otherwise,
   8)  The receiver SHOULD respond to the sender of the OOTB packet with
       an ABORT.  When sending the ABORT, the receiver of the OOTB



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       packet MUST fill in the Verification Tag field of the outbound
       packet with the value found in the Verification Tag field of the
       OOTB packet and set the T bit in the Chunk Flags to indicate that
       the Verification Tag is reflected.  After sending this ABORT, the
       receiver of the OOTB packet MUST discard the OOTB packet and MUST
       NOT take any further action.

8.5.  Verification Tag

   The Verification Tag rules defined in this section apply when sending
   or receiving SCTP packets that do not contain an INIT, SHUTDOWN
   COMPLETE, COOKIE ECHO (see Section 5.1), ABORT, or SHUTDOWN ACK
   chunk.  The rules for sending and receiving SCTP packets containing
   one of these chunk types are discussed separately in Section 8.5.1.

   When sending an SCTP packet, the endpoint MUST fill in the
   Verification Tag field of the outbound packet with the tag value in
   the Initiate Tag parameter of the INIT or INIT ACK received from its
   peer.

   When receiving an SCTP packet, the endpoint MUST ensure that the
   value in the Verification Tag field of the received SCTP packet
   matches its own tag.  If the received Verification Tag value does not
   match the receiver's own tag value, the receiver MUST silently
   discard the packet and MUST NOT process it any further except for
   those cases listed in Section 8.5.1 below.

8.5.1.  Exceptions in Verification Tag Rules

   A) Rules for packet carrying INIT:

      *  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 MUST 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.





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      *  The receiver of an ABORT MUST accept the packet if the
         Verification Tag field of the packet matches its own tag and
         the T bit is not set OR if it is set to its peer's tag and the
         T bit is set in the Chunk Flags.  Otherwise, the receiver MUST
         silently discard the packet and take no further action.

   C) Rules for packet carrying SHUTDOWN COMPLETE:

      *  When sending a SHUTDOWN COMPLETE, if the receiver of the
         SHUTDOWN ACK has a TCB, then the destination endpoint's tag
         MUST be used, and the T bit MUST NOT be set.  Only where no TCB
         exists SHOULD the sender use the Verification Tag from the
         SHUTDOWN ACK, and MUST set the T bit.

      *  The receiver of a SHUTDOWN COMPLETE accepts the packet if the
         Verification Tag field of the packet matches its own tag and
         the T bit is not set OR if it is set to its peer's tag and the
         T bit is set in the Chunk Flags.  Otherwise, the receiver MUST
         silently discard the packet and take no further action.  An
         endpoint MUST ignore the SHUTDOWN COMPLETE if it is not in the
         SHUTDOWN-ACK-SENT state.

   D) Rules for packet carrying a COOKIE ECHO:

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

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

   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 is 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 might continue sending data while the other end is
   closed.  When either endpoint performs a shutdown, the association on



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

9.1.  Abort of an Association

   When an endpoint decides to abort an existing association, it MUST
   send an ABORT chunk to its peer endpoint.  The sender MUST fill in
   the peer's Verification Tag in the outbound packet and MUST NOT
   bundle any DATA chunk with the ABORT.  If the association is aborted
   on request of the upper layer, a User-Initiated Abort error cause
   (see Section 3.3.10.12) SHOULD be present in the ABORT chunk.

   An endpoint MUST NOT respond to any received packet that contains an
   ABORT chunk (also see Section 8.4).

   An endpoint receiving an ABORT MUST apply the special Verification
   Tag check rules described in Section 8.5.1.

   After checking the Verification Tag, the receiving endpoint MUST
   remove the association from its record and SHOULD report the
   termination to its upper layer.  If a User-Initiated Abort error
   cause is present in the ABORT chunk, the Upper Layer Abort Reason
   SHOULD be made available to the upper layer.

9.2.  Shutdown of an Association

   Using the SHUTDOWN primitive (see Section 10.1), the upper layer of
   an endpoint in an association can gracefully close the association.
   This will allow all outstanding DATA chunks from the peer of the
   shutdown initiator to be delivered before the association terminates.

   Upon receipt of the SHUTDOWN primitive from its upper layer, the
   endpoint enters the SHUTDOWN-PENDING state and remains there until
   all outstanding data has been acknowledged by its peer.  The endpoint
   accepts no new data from its upper layer, but retransmits data to the
   far end if necessary to fill gaps.

   Once all its outstanding data has been acknowledged, the endpoint
   sends a SHUTDOWN chunk to its peer including in the Cumulative TSN
   Ack field the last sequential TSN it has received from the peer.  It
   SHOULD then start the T2-shutdown timer and enter the SHUTDOWN-SENT
   state.  If the timer expires, the endpoint MUST resend the SHUTDOWN
   with the updated last sequential TSN received from its peer.

   The rules in Section 6.3 MUST be followed to determine the proper
   timer value for T2-shutdown.  To indicate any gaps in TSN, the
   endpoint MAY also bundle a SACK with the SHUTDOWN chunk in the same
   SCTP packet.



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   An endpoint SHOULD limit the number of retransmissions of the
   SHUTDOWN chunk to the protocol parameter 'Association.Max.Retrans'.
   If this threshold is exceeded, the endpoint SHOULD destroy the TCB
   and SHOULD report the peer endpoint unreachable to the upper layer
   (and thus the association enters the CLOSED state).  The reception of
   any packet from its peer (i.e., as the peer sends all of its queued
   DATA chunks) SHOULD clear the endpoint's retransmission count and
   restart the T2-shutdown timer, giving its peer ample opportunity to
   transmit all of its queued DATA chunks that have not yet been sent.

   Upon reception of the SHUTDOWN, the peer endpoint does the following:

   o  enter the SHUTDOWN-RECEIVED state,

   o  stop accepting new data from its SCTP user, and

   o  verify, by checking the Cumulative TSN Ack field of the chunk,
      that all its outstanding DATA chunks have been received by the
      SHUTDOWN sender.

   Once an endpoint has reached the SHUTDOWN-RECEIVED state, it MUST
   ignore ULP shutdown requests but MUST continue responding to SHUTDOWN
   chunks from its peer.

   If there are still outstanding DATA chunks left, the SHUTDOWN
   receiver MUST continue to follow normal data transmission procedures
   defined in Section 6, until all outstanding DATA chunks are
   acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
   from its SCTP user.

   While in the SHUTDOWN-SENT state, the SHUTDOWN sender MUST
   immediately respond to each received packet containing one or more
   DATA chunks with a SHUTDOWN chunk and restart the T2-shutdown timer.
   If a SHUTDOWN chunk by itself cannot acknowledge all of the received
   DATA chunks (i.e., there are TSNs that can be acknowledged that are
   larger than the cumulative TSN, and thus gaps exist in the TSN
   sequence), or if duplicate TSNs have been received, then a SACK chunk
   MUST also be sent.

   The sender of the SHUTDOWN MAY also start an overall guard timer 'T5-
   shutdown-guard' to bound the overall time for the shutdown sequence.
   At the expiration of this timer, the sender SHOULD abort the
   association by sending an ABORT chunk.  If the 'T5-shutdown- guard'
   timer is used, it SHOULD be set to the RECOMMENDED value of 5 times
   'RTO.Max'.

   If the receiver of the SHUTDOWN has no more outstanding DATA chunks,
   the SHUTDOWN receiver MUST send a SHUTDOWN ACK and start a T2-



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   shutdown timer of its own, entering the SHUTDOWN-ACK-SENT state.  If
   the timer expires, the endpoint MUST resend the SHUTDOWN ACK.

   The sender of the SHUTDOWN ACK SHOULD limit the number of
   retransmissions of the SHUTDOWN ACK chunk to the protocol parameter
   'Association.Max.Retrans'.  If this threshold is exceeded, the
   endpoint SHOULD destroy the TCB and SHOULD 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 MUST stop
   the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its peer,
   and remove all record of the association.

   Upon reception of the SHUTDOWN COMPLETE chunk, the endpoint will
   verify that it is in the SHUTDOWN-ACK-SENT state; if it is not, the
   chunk SHOULD be discarded.  If the endpoint is in the SHUTDOWN-ACK-
   SENT state, the endpoint SHOULD stop the T2-shutdown timer and remove
   all knowledge of the association (and thus the association enters the
   CLOSED state).

   An endpoint SHOULD ensure that all its outstanding DATA chunks have
   been acknowledged before initiating the shutdown procedure.

   An endpoint SHOULD reject any new data request from its upper layer
   if it is in the SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED,
   or SHUTDOWN-ACK-SENT state.

   If an endpoint is in the SHUTDOWN-ACK-SENT state and receives an INIT
   chunk (e.g., if the SHUTDOWN COMPLETE was lost) with source and
   destination transport addresses (either in the IP addresses or in the
   INIT chunk) that belong to this association, it SHOULD discard the
   INIT chunk and retransmit the SHUTDOWN ACK chunk.

   Note: Receipt of an INIT with the same source and destination IP
   addresses as used in transport addresses assigned to an endpoint but
   with a different port number indicates the initialization of a
   separate association.

   The sender of the INIT or COOKIE ECHO SHOULD respond to the receipt
   of a SHUTDOWN ACK with a stand-alone SHUTDOWN COMPLETE in an SCTP
   packet with the Verification Tag field of its common header set to
   the same tag that was received in the SHUTDOWN ACK packet.  This is
   considered an Out of the Blue packet as defined in Section 8.4.  The
   sender of the INIT lets T1-init continue running and remains in the
   COOKIE-WAIT or COOKIE-ECHOED state.  Normal T1-init timer expiration
   will cause the INIT or COOKIE chunk to be retransmitted and thus
   start a new association.



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   If a SHUTDOWN is received in the COOKIE-WAIT or COOKIE ECHOED state,
   the SHUTDOWN chunk SHOULD be silently discarded.

   If an endpoint is in the SHUTDOWN-SENT state and receives a SHUTDOWN
   chunk from its peer, the endpoint SHOULD respond immediately with a
   SHUTDOWN ACK to its peer, and move into the SHUTDOWN-ACK-SENT state
   restarting its T2-shutdown timer.

   If an endpoint is in the SHUTDOWN-ACK-SENT state and receives a
   SHUTDOWN ACK, it MUST stop the T2-shutdown timer, send a SHUTDOWN
   COMPLETE chunk to its peer, and remove all record of the association.

10.  Interface with Upper Layer

   The Upper Layer Protocols (ULPs) request services by passing
   primitives to SCTP and receive notifications from SCTP for various
   events.

   The primitives and notifications described in this section can be
   used as a guideline for implementing SCTP.  The following functional
   description of ULP interface primitives is shown for illustrative
   purposes.  Different SCTP implementations can have different ULP
   interfaces.  However, all SCTPs aer expected to provide a certain
   minimum set of services to guarantee that all SCTP implementations
   can support the same protocol hierarchy.

10.1.  ULP-to-SCTP

   The following sections functionally characterize a ULP/SCTP
   interface.  The notation used is similar to most procedure or
   function calls in high-level languages.

   The ULP primitives described below specify the basic functions that
   SCTP performs to support inter-process communication.  Individual
   implementations define their own exact format, and provide
   combinations or subsets of the basic functions in single calls.

   A)  Initialize

       Format: INITIALIZE ([local port],[local eligible address list])
               -> local SCTP instance name

       This primitive allows SCTP to initialize its internal data
       structures and allocate necessary resources for setting up its
       operation environment.  Once SCTP is initialized, ULP can
       communicate directly with other endpoints without re-invoking
       this primitive.
       SCTP will return a local SCTP instance name to the ULP.



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       Mandatory attributes:

       *  None.

       Optional attributes:
       The following types of attributes can be passed along with the
       primitive:

       *  local port - SCTP port number, if ULP wants it to be
          specified.
       *  local eligible address list - an address list that the local
          SCTP endpoint binds.  By default, if an address list is not
          included, all IP addresses assigned to the host are 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 is specified by one of the transport addresses
       that defines the endpoint (see Section 1.3).  If the local SCTP
       instance has not been initialized, the ASSOCIATE is considered an
       error.
       An association id, which is a local handle to the SCTP
       association, will be returned on successful establishment of the
       association.  If SCTP is not able to open an SCTP association
       with the peer endpoint, an error is returned.
       Other association parameters can be returned, including the
       complete destination transport addresses of the peer as well as
       the outbound stream count of the local endpoint.  One of the
       transport addresses from the returned destination addresses will
       be selected by the local endpoint as default primary path for
       sending SCTP packets to this peer.  The returned "destination
       transport addr list" can be used by 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



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       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 is returned and
       association parameters are passed using the COMMUNICATION UP
       notification.
       Mandatory attributes:

       *  local SCTP instance name - obtained from the INITIALIZE
          operation.
       *  destination transport addr - specified as one of the transport
          addresses of the peer endpoint with which the association is
          to be established.
       *  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 is returned.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.

       Optional attributes:

       *  None.
   D)  Abort

       Format: ABORT(association id [, Upper Layer Abort Reason]) ->
               result

       Ungracefully closes an association.  Any locally queued user data
       will be discarded, and an ABORT chunk is sent to the peer.  A
       success code will be returned on successful abort of the
       association.  If attempting to abort the association results in a
       failure, an error code is returned.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.



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       Optional attributes:

       *  Upper Layer Abort Reason - reason of the abort to be passed to
          the peer.
       *  None.
   E)  Send

       Format: SEND(association id, buffer address, byte count
               [,context] [,stream id] [,life time] [,destination
               transport address] [,unordered flag] [,no-bundle flag]
               [,payload protocol-id] [,sack-immediately flag] ) ->
               result

       This is the main method to send user data via SCTP.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.
       *  buffer address - the location where the user message to be
          transmitted is stored.
       *  byte count - the size of the user data in number of bytes.

       Optional attributes:

       *  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.
       *  stream id - to indicate which stream to send the data on.  If
          not specified, stream 0 will be used.
       *  life time - specifies the life time of the user data.  The
          user data will not be sent by SCTP after the life time
          expires.  This parameter can be used to avoid efforts to
          transmit stale user messages.  SCTP notifies the ULP if the
          data cannot be initiated to transport (i.e., sent to the
          destination via SCTP's send primitive) within the life time
          variable.  However, the user data will be transmitted if SCTP
          has attempted to transmit a chunk before the life time
          expired.
          IMPLEMENTATION NOTE: In order to better support the data life
          time option, the transmitter can 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 considers the send of this DATA chunk as
          committed, overriding any life time option attached to the
          DATA chunk.
       *  destination transport address - specified as one of the
          destination transport addresses of the peer endpoint to which
          this packet is sent.  Whenever possible, SCTP uses this




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          destination transport address for sending the packets, instead
          of the current primary path.
       *  unordered flag - this flag, if present, indicates that the
          user would like the data delivered in an unordered fashion to
          the peer (i.e., the U flag is set to 1 on all DATA chunks
          carrying this message).
       *  no-bundle flag - instructs SCTP not to bundle this user data
          with other outbound DATA chunks.  When faced with network
          congestion, SCTP might still bundle the data, even when this
          flag is present.
       *  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.
       *  sack-immediately flag - set the I bit on the last DATA chunk
          used for the user message to be transmitted.
   F)  Set Primary

       Format: SETPRIMARY(association id, destination transport address,
               [source transport address]) -> result

       Instructs the local SCTP to use the specified destination
       transport address as the primary path for sending packets.
       The result of attempting this operation is 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 is
       returned.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.
       *  destination transport address - specified as one of the
          transport addresses of the peer endpoint, which is used as the
          primary address for sending packets.  This overrides the
          current primary address information maintained by the local
          SCTP endpoint.

       Optional attributes:

       *  source transport address - optionally, some implementations
          can 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] [,payload
               protocol-id]



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       This primitive reads 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 might,
       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 might also be returned.
       Depending upon the implementation, if this primitive is invoked
       when no message is available the implementation returns an
       indication of this condition or blocks the invoking process until
       data does become available.
       Mandatory attributes:

       *  association id - local handle to the SCTP association
       *  buffer address - the memory location indicated by the ULP to
          store the received message.
       *  buffer size - the maximum size of data to be received, in
          bytes.

       Optional attributes:

       *  stream id - to indicate which stream to receive the data on.
       *  stream sequence number - the Stream Sequence Number assigned
          by the sending SCTP peer.
       *  partial flag - if this returned flag is set to 1, then this
          primitive contains a partial delivery of the whole message.
          When this flag is set, the stream id and stream sequence
          number accompanies this primitive.  When this flag is set to
          0, it indicates that no more deliveries will be received for
          this stream sequence number.
       *  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 returns 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,



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          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:

       *  association id - local handle to the SCTP association.

       Optional attributes:

       *  None.
   I)  Change Heartbeat

       Format: CHANGE HEARTBEAT(association id, destination transport
               address, new state [,interval]) -> result

       Instructs the local endpoint to enable or disable heartbeat on
       the specified destination transport address.
       The result of attempting this operation is returned.
       Note: Even when enabled, heartbeat will not take place if the
       destination transport address is not idle.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.
       *  destination transport address - specified as one of the
          transport addresses of the peer endpoint.
       *  new state - the new state of heartbeat for this destination
          transport address (either enabled or disabled).

       Optional attributes:

       *  interval - if present, indicates the frequency of the
          heartbeat if this is to enable heartbeat on a destination
          transport address.  This value is added to the RTO of the
          destination transport address.  This value, if present,
          affects all destinations.
   J)  Request HeartBeat

       Format: REQUESTHEARTBEAT(association id, destination transport
               address) -> result

       Instructs the local endpoint to perform a HeartBeat on the
       specified destination transport address of the given association.
       The returned result indicates whether the transmission of the
       HEARTBEAT chunk to the destination address is successful.
       Mandatory attributes:



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       *  association id - local handle to the SCTP association.
       *  destination transport address - the transport address of the
          association on which a heartbeat is issued.
   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:

       *  association id - local handle to the SCTP association.
       *  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:

       *  association id - local handle to the SCTP association.
       *  destination transport address - the transport address of the
          association on which the failure detection threshold is to be
          set.
       *  failure threshold - the new value of 'Path.Max.Retrans' for
          the destination address.
   M)  Set Protocol Parameters

       Format: SETPROTOCOLPARAMETERS(association id, [destination
               transport address,] protocol parameter list) -> result

       This primitive allows the local SCTP to customize the protocol
       parameters.
       Mandatory attributes:

       *  association id - local handle to the SCTP association.
       *  protocol parameter list - the specific names and values of the
          protocol parameters (e.g., Association.Max.Retrans (see
          Section 15), or other parameters like the DSCP) that the SCTP
          user wishes to customize.




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       Optional attributes:

       *  destination transport address - some of the protocol
          parameters might 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])

       *  data retrieval id - the identification passed to the ULP in
          the failure notification.
       *  buffer address - the memory location indicated by the ULP to
          store the received message.
       *  buffer size - the maximum size of data to be received, in
          bytes.

       Optional attributes:

       *  stream id - this is a return value that is set to indicate
          which stream the data was sent to.
       *  stream sequence number - this value is returned indicating the
          Stream Sequence Number that was associated with the message.
       *  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
          accompanies this primitive.  When this flag is set to 0, it
          indicates that no more deliveries will be received for this
          stream sequence number.
       *  payload protocol-id - The 32 bit unsigned integer that was
          sent to be sent to the peer indicating the type of payload
          protocol of the received data.
   O)  Receive Unacknowledged Message

       Format: RECEIVE_UNACKED(data retrieval id, buffer address, buffer
               size, [,stream id] [,stream sequence number] [,partial
               flag] [,payload protocol-id])

       *  data retrieval id - the identification passed to the ULP in
          the failure notification.
       *  buffer address - the memory location indicated by the ULP to
          store the received message.
       *  buffer size - the maximum size of data to be received, in
          bytes.

       Optional attributes:




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       *  stream id - this is a return value that is set to indicate
          which stream the data was sent to.
       *  stream sequence number - this value is returned indicating the
          Stream Sequence Number that was associated with the message.
       *  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
          accompanies this primitive.  When this flag is set to 0, it
          indicates that no more deliveries will be received for this
          stream sequence number.
       *  payload protocol-id - the 32-bit unsigned integer that was
          sent to the peer indicating the type of payload protocol of
          the received data.
   P)  Destroy SCTP Instance

       Format: DESTROY(local SCTP instance name)

       *  local SCTP instance name - this is the value that was passed
          to the application in the initialize primitive and it
          indicates which SCTP instance is to be destroyed.

10.2.  SCTP-to-ULP

   It is assumed that the operating system or application environment
   provides a means for the SCTP to asynchronously signal the ULP
   process.  When SCTP does signal a ULP process, certain information is
   passed to the ULP.

   IMPLEMENTATION NOTE: In some cases, this might be done through a
   separate socket or error channel.

   A)  DATA ARRIVE notification
       SCTP invokes this notification on the ULP when a user message is
       successfully received and ready for retrieval.
       The following might optionally be passed with the notification:

       *  association id - local handle to the SCTP association.
       *  stream id - to indicate which stream the data is received on.
   B)  SEND FAILURE notification
       If a message cannot be delivered, SCTP invokes this notification
       on the ULP.
       The following might optionally be passed with the notification:

       *  association id - local handle to the SCTP association.
       *  data retrieval id - an identification used to retrieve unsent
          and unacknowledged data.
       *  cause code - indicating the reason of the failure, e.g., size
          too large, message life time expiration, etc.



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       *  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 invokes this notification on the ULP.
       The following is passed with the notification:

       *  association id - local handle to the SCTP association.
       *  destination transport address - this indicates the destination
          transport address of the peer endpoint affected by the change.
       *  new-status - this indicates the new status.
   D)  COMMUNICATION UP notification
       This notification is used when SCTP becomes ready to send or
       receive user messages, or when a lost communication to an
       endpoint is restored.
       IMPLEMENTATION NOTE: If the ASSOCIATE primitive is implemented as
       a blocking function call, the association parameters are returned
       as a result of the ASSOCIATE primitive itself.  In that case,
       COMMUNICATION UP notification is optional at the association
       initiator's side.
       The following is passed with the notification:

       *  association id - local handle to the SCTP association.
       *  status - This indicates what type of event has occurred.
       *  destination transport address list - the complete set of
          transport addresses of the peer.
       *  outbound stream count - the maximum number of streams allowed
          to be used in this association by the ULP.
       *  inbound stream count - the number of streams the peer endpoint
          has requested with this association (this might 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 invokes this notification on the ULP.
       The following is passed with the notification:

       *  association id - local handle to the SCTP association.
       *  status - this indicates what type of event has occurred; the
          status might indicate that a failure OR a normal termination
          event occurred in response to a shutdown or abort request.

       The following might be passed with the notification:

       *  data retrieval id - an identification used to retrieve unsent
          and unacknowledged data.
       *  last-acked - the TSN last acked by that peer endpoint.



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       *  last-sent - the TSN last sent to that peer endpoint.
       *  Upper Layer Abort Reason - the abort reason specified in case
          of a user-initiated abort.
   F)  COMMUNICATION ERROR notification
       When SCTP receives an ERROR chunk from its peer and decides to
       notify its ULP, it can invoke this notification on the ULP.
       The following can be passed with the notification:

       *  association id - local handle to the SCTP association.
       *  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 might send this
       notification to its ULP.
       The following can be passed with the notification:

       *  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:

       *  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.

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

11.2.  SCTP Responses to Potential Threats

   SCTP could potentially be used in a wide variety of risk situations.
   It is important for operators of systems running SCTP to analyze
   their particular situations and decide on the appropriate counter-
   measures.

   Operators of systems running SCTP might consult [RFC2196] for
   guidance in securing their site.






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11.2.1.  Countering Insider Attacks

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

11.2.2.  Protecting against Data Corruption in the Network

   Where the risk of undetected errors in datagrams delivered by the
   lower-layer transport services is considered to be too great,
   additional integrity protection is required.  If this additional
   protection were provided in the application layer, the SCTP header
   would remain vulnerable to deliberate integrity attacks.  While the
   existing SCTP mechanisms for detection of packet replays are
   considered sufficient for normal operation, stronger protections are
   needed to protect SCTP when the operating environment contains
   significant risk of deliberate attacks from a sophisticated
   adversary.

   The SCTP Authentication extension SCTP-AUTH [RFC4895] MAY be used
   when the threat environment requires stronger integrity protections,
   but does not require confidentiality.

11.2.3.  Protecting Confidentiality

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

   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.






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   Regardless of where confidentiality is provided, the Internet Key
   Exchange Protocol version 2 (IKEv2) [RFC7296] SHOULD be used for key
   management.

   Operators might consult [RFC4301] for more information on the
   security services available at and immediately above the Internet
   Protocol layer.

11.2.4.  Protecting against Blind Denial-of-Service Attacks

   A blind attack is one where the attacker is unable to intercept or
   otherwise see the content of data flows passing to and from the
   target SCTP node.  Blind denial-of-service attacks can 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 can 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:

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

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

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

   o  not responding to unexpected packets sent to non-unicast
      addresses.

   Network equipment is expected to be capable of generating an alarm
   and log if a suspicious increase in traffic occurs.  The log provides
   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 are expected to be in place
   for the operator to act on such alarms if a clear pattern of abuse
   emerges.



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   The design of SCTP is resistant to flooding attacks, particularly in
   its use of a four-way startup handshake, its use of a cookie to defer
   commitment of resources at the responding SCTP node until the
   handshake is completed, and its use of a Verification Tag to prevent
   insertion of extraneous packets into the flow of an established
   association.

   The IP Authentication Header and Encapsulating Security Payload might
   be useful in reducing the risk of certain kinds of denial-of-service
   attacks.

   Support for the Host Name Address parameter has been removed from the
   protocol.  Endpoints receiving INIT or INIT ACK chunks containing the
   Host Name Address parameter MUST send an ABORT chunk in response and
   MAY include an "Unresolvable Address" error cause.

11.2.4.2.  Blind Masquerade

   Masquerade can be used to deny service in several ways:

   o  by tying up resources at the target SCTP node to which the
      impersonated node has limited access.  For example, the target
      node can by policy permit a maximum of one SCTP association with
      the impersonated SCTP node.  The masquerading attacker can attempt
      to establish an association purporting to come from the
      impersonated node so that the latter cannot do so when it requires
      it.

   o  by deliberately allowing the impersonation to be detected, thereby
      provoking counter-measures that cause the impersonated node to be
      locked out of the target SCTP node.

   o  by interfering with an established association by inserting
      extraneous content such as a SHUTDOWN request.

   SCTP reduces the risk of blind masquerade attacks through IP spoofing
   by use of the four-way startup handshake.  Because the initial
   exchange is memory-less, no lockout mechanism is triggered by blind
   masquerade attacks.  In addition, the INIT ACK containing the State
   Cookie is transmitted back to the IP address from which it received
   the INIT.  Thus, the attacker would not receive the INIT ACK
   containing the State Cookie.  SCTP protects against insertion of
   extraneous packets into the flow of an established association by use
   of the Verification Tag.

   Logging of received INIT requests and abnormalities such as
   unexpected INIT ACKs might be considered as a way to detect patterns
   of hostile activity.  However, the potential usefulness of such



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   logging has to be weighed against the increased SCTP startup
   processing it implies, rendering the SCTP node more vulnerable to
   flooding attacks.  Logging is pointless without the establishment of
   operating procedures to review and analyze the logs on a routine
   basis.

11.2.4.3.  Improper Monopolization of Services

   Attacks under this heading are performed openly and legitimately by
   the attacker.  They are directed against fellow users of the target
   SCTP node or of the shared resources between the attacker and the
   target node.  Possible attacks include the opening of a large number
   of associations between the attacker's node and the target, or
   transfer of large volumes of information within a legitimately
   established association.

   Policy limits are expected to be placed on the number of associations
   per adjoining SCTP node.  SCTP user applications are expected to be
   capable of detecting large volumes of illegitimate or "no-op"
   messages within a given association and either logging or terminating
   the association as a result, based on local policy.

11.3.  SCTP Interactions with Firewalls

   It is helpful for some firewalls if they can inspect just the first
   fragment of a fragmented SCTP packet and unambiguously determine
   whether it corresponds to an INIT chunk (for further information,
   please refer to [RFC1858]).  Accordingly, we stress the requirements,
   as stated in Section 3.1, that (1) an INIT chunk MUST NOT be bundled
   with any other chunk in a packet and (2) a packet containing an INIT
   chunk MUST have a zero Verification Tag.  The receiver of an INIT
   chunk MUST silently discard the INIT chunk and all further chunks if
   the INIT chunk is bundled with other chunks or the packet has a non-
   zero Verification Tag.

11.4.  Protection of Non-SCTP-Capable Hosts

   To provide a non-SCTP-capable host with the same level of protection
   against attacks as for SCTP-capable ones, all SCTP stacks MUST
   implement the ICMP handling described in Appendix C.

   When an SCTP stack receives a packet containing multiple control or
   DATA chunks and the processing of the packet requires the sending of
   multiple chunks in response, the sender of the response chunk(s) MUST
   NOT send more than one packet.  If bundling is supported, multiple
   response chunks that fit into a single packet MAY be bundled together
   into one single response packet.  If bundling is not supported, then
   the sender MUST NOT send more than one response chunk and MUST



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   discard all other responses.  Note that this rule does NOT apply to a
   SACK chunk, since a SACK chunk is, in itself, a response to DATA and
   a SACK does not require a response of more DATA.

   An SCTP implementation SHOULD abort the association if it receives a
   SACK acknowledging a TSN that has not been sent.

   An SCTP implementation that receives an INIT that would require a
   large packet in response, due to the inclusion of multiple ERROR
   parameters, MAY (at its discretion) elect to omit some or all of the
   ERROR parameters to reduce the size of the INIT ACK.  Due to a
   combination of the size of the COOKIE parameter and the number of
   addresses a receiver of an INIT indicates to a peer, it is always
   possible that the INIT ACK will be larger than the original INIT.  An
   SCTP implementation SHOULD attempt to make the INIT ACK as small as
   possible to reduce the possibility of byte amplification attacks.

12.  Network Management Considerations

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

13.  Recommended Transmission Control Block (TCB) Parameters

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

13.1.  Parameters Necessary for the SCTP Instance

   Associations: A list of current associations and mappings to the data
                 consumers for each association.  This might be in the
                 form of a hash table or other implementation-dependent
                 structure.  The data consumers might 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 [RFC4086] 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.




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   SCTP Port:    The local SCTP port number to which the endpoint is
                 bound.

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

   Peer Verification Tag:  Tag value to be sent in every packet and is
                 received in the INIT or INIT ACK chunk.
   My Verification Tag:  Tag expected in every inbound packet and sent
                 in the INIT or INIT ACK chunk.
   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.
   Peer Transport Address List:  A list of SCTP transport addresses to
                 which the peer is bound.  This information is derived
                 from the INIT or INIT ACK and is used to associate an
                 inbound packet with a given association.  Normally,
                 this information is hashed or keyed for quick lookup
                 and access of the TCB.
   Primary Path: This is the current primary destination transport
                 address of the peer endpoint.  It might also specify a
                 source transport address on this endpoint.
   Overall Error Count:  The overall association error count.
   Overall Error Threshold:  The threshold for this association that if
                 the Overall Error Count reaches will cause this
                 association to be 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 TSN:  This is the last TSN received in sequence.  This
                 value is set initially by taking the peer's initial
                 TSN, received in the INIT or INIT ACK chunk, and
                 subtracting one from it.
   Mapping Array:  An array of bits or bytes indicating which out-of-
                 order TSNs have been received (relative to the Last
                 Rcvd TSN).  If no gaps exist, i.e., no out-of- order
                 packets have been received, this array will be set to
                 all zero.  This structure might 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



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                 is reset to 0.  Note: This is used only when no DATA
                 chunks are received out of order.  When DATA chunks are
                 out of order, SACKs are not delayed (see Section 6).
   Inbound Streams:  An array of structures to track the inbound
                 streams, normally including the next sequence number
                 expected and possibly the stream number.
   Outbound Streams:  An array of structures to track the outbound
                 streams, normally including the next sequence number to
                 be sent on the stream.
   Reasm Queue:  A reassembly queue.
   Local Transport Address List:  The list of local IP addresses bound
                 in to this association.
   Association PMTU:  The smallest PMTU discovered for all of the peer's
                 transport addresses.

13.3.  Per Transport Address Data

   For each destination transport address in the peer's address list
   derived from the INIT or INIT ACK chunk, a number of data elements
   need to be maintained including:

   Error Count:  The current error count for this destination.
   Error Threshold:  Current error threshold for this destination, i.e.,
                 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 bytes acked:  The tracking method for increase of cwnd when
                 in congestion avoidance mode (see Section 7.2.2).
   state:        The current state of this destination, i.e., DOWN, UP,
                 ALLOW-HB, NO-HEARTBEAT, etc.
   PMTU:         The current known path MTU.
   Per Destination Timer:  A timer used by each destination.
   RTO-Pending:  A flag used to track if one of the DATA chunks sent to
                 this address is currently being used to compute an RTT.
                 If this flag is 0, the next DATA chunk sent to this
                 destination is expected to be used to compute an RTT
                 and this flag is expected to be set.  Every time the
                 RTT calculation completes (i.e., the DATA chunk is
                 SACK'd), clear this flag.
   last-time:    The time to which this destination was last sent.  This
                 can be to determine if a HEARTBEAT is needed.






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13.4.  General Parameters Needed

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

14.  IANA Considerations

   SCTP defines three registries that IANA maintains:

   o  through definition of additional chunk types,
   o  through definition of additional parameter types, or
   o  through definition of additional cause codes within ERROR chunks.

   SCTP requires that the IANA Port Numbers registry be opened for SCTP
   port registrations, Section 14.6 describes how.  An IESG-appointed
   Expert Reviewer supports IANA in evaluating SCTP port allocation
   requests.

14.1.  IETF-Defined Chunk Extension

   The assignment of new chunk type codes is done through an IETF Review
   action, as defined in [RFC8126].  Documentation for a new chunk MUST
   contain the following information:

   a)  A long and short name for the new chunk type.

   b)  A detailed description of the structure of the chunk, which MUST
       conform to the basic structure defined in Section 3.2.

   c)  A detailed definition and description of intended use of each
       field within the chunk, including the chunk flags if any.
       Defined chunk flags will be used as initial entries in the chunk
       flags table for the new chunk type.

   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.

   For each new chunk type, IANA creates a registration table for the
   chunk flags of that type.  The procedure for registering particular
   chunk flags is described in Section 14.2.








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14.2.  IETF Chunk Flags Registration

   The assignment of new chunk flags is done through an RFC Required
   action, as defined in [RFC8126].  Documentation for the chunk flags
   MUST contain the following information:

   a)  A name for the new chunk flag.

   b)  A detailed procedural description of the use of the new chunk
       flag within the operation of the protocol.  It MUST be considered
       that implementations not supporting the flag will send '0' on
       transmit and just ignore it on receipt.


   IANA selects a chunk flags value.  This MUST be one of 0x01, 0x02,
   0x04, 0x08, 0x10, 0x20, 0x40, or 0x80, which MUST be unique within
   the chunk flag values for the specific chunk type.

14.3.  IETF-Defined Chunk Parameter Extension

   The assignment of new chunk parameter type codes is done through an
   IETF Review action as defined in [RFC8126].  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 can be found within the
       same chunk.

   e)  Each parameter type MUST be unique across all chunks.

14.4.  IETF-Defined Additional Error Causes

   Additional cause codes can be allocated in the range 11 to 65535
   through a Specification Required action as defined in [RFC8126].
   Provided documentation MUST include the following information:

   a)  Name of the error condition.





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   b)  Detailed description of the conditions under which an SCTP
       endpoint issues an ERROR (or ABORT) with this cause code.

   c)  Expected action by the SCTP endpoint that receives an ERROR (or
       ABORT) chunk containing this cause code.

   d)  Detailed description of the structure and content of data fields
       that accompany this cause code.

   The initial word (32 bits) of a cause code parameter MUST conform to
   the format shown in Section 3.3.10, i.e.:

   o  first 2 bytes contain the cause code value
   o  last 2 bytes contain the length of the cause parameter.

14.5.  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.

14.6.  Port Numbers Registry

   SCTP services can use contact port numbers to provide service to
   unknown callers, as in TCP and UDP.  IANA is therefore requested to
   open the existing "Service Name and Transport Protocol Port Number
   Registry" for SCTP using the following rules, which we intend to mesh
   well with existing port-number registration procedures.  An IESG-
   appointed expert reviewer supports IANA in evaluating SCTP port
   allocation requests, according to the procedure defined in [RFC8126].
   The details of this process are defined in [RFC6335].

   This document registers the following ports.  (These registrations
   are to be considered models to follow for future allocation
   requests.)










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      discard    9/sctp  Discard  # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         The discard service, which accepts SCTP connections on port
         9, discards all incoming application data and sends no data
         in response.  Thus, SCTP's discard port is analogous to
         TCP's discard port, and might be used to check the health
         of an SCTP stack.

      ftp-data  20/sctp  FTP      # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

      ftp       21/sctp  FTP      # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         File Transfer Protocol (FTP) data (20) and control ports
         (21).

      ssh       22/sctp  SSH      # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         The Secure Shell (SSH) remote login service, which allows
         secure shell logins to a host.

      http      80/sctp  HTTP     # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         World Wide Web HTTP over SCTP.

      bgp      179/sctp  BGP      # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         Border Gateway Protocol over SCTP.

      https    443/sctp  HTTPS    # IETF TSVWG
                                  # Randall Stewart <rrs@cisco.com>
                                  # [RFC4960]

         World Wide Web HTTP over TLS/SSL over SCTP.






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15.  Suggested SCTP Protocol Parameter Values

   The following protocol parameters are RECOMMENDED:

   RTO.Initial:  1 second
   RTO.Min:  1 second
   RTO.Max:  60 seconds
   Max.Burst:  4
   RTO.Alpha:  1/8
   RTO.Beta:  1/4
   Valid.Cookie.Life:  60 seconds
   Association.Max.Retrans:  10 attempts
   Path.Max.Retrans:  5 attempts (per destination address)
   Max.Init.Retransmits:  8 attempts
   HB.interval:  30 seconds
   HB.Max.Burst:  1
   SACK.Delay:  200 milliseconds

   IMPLEMENTATION NOTE: The SCTP implementation can allow ULP to
   customize some of these protocol parameters (see Section 10).

   Note: RTO.Min SHOULD be set as described above in this section.

16.  Acknowledgements

   An undertaking represented by this updated document is not a small
   feat and represents the summation of the initial authors of RFC 2960:
   Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor,
   I. Rytina, M. Kalla, L. Zhang, and V. Paxson.

   Add to that, the comments from everyone who contributed to the
   original RFC:

   Mark Allman, R.J. Atkinson, Richard Band, Scott Bradner, Steve
   Bellovin, Peter Butler, Ram Dantu, R. Ezhirpavai, Mike Fisk, Sally
   Floyd, Atsushi Fukumoto, Matt Holdrege, Henry Houh, Christian
   Huitema, Gary Lehecka, Jonathan Lee, David Lehmann, John Loughney,
   Daniel Luan, Barry Nagelberg, Thomas Narten, Erik Nordmark, Lyndon
   Ong, Shyamal Prasad, Kelvin Porter, Heinz Prantner, Jarno Rajahalme,
   Raymond E. Reeves, Renee Revis, Ivan Arias Rodriguez, A. Sankar, Greg
   Sidebottom, Brian Wyld, La Monte Yarroll, and many others for their
   invaluable comments.

   Then, add the authors of the SCTP implementor's guide, I. Arias-
   Rodriguez, K. Poon, A. Caro, and M. Tuexen.






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   Then add to these the efforts of all the subsequent seven SCTP
   interoperability tests and those who commented on RFC 4460 as shown
   in its acknowledgements:

   Barry Zuckerman, La Monte Yarroll, Qiaobing Xie, Wang Xiaopeng,
   Jonathan Wood, Jeff Waskow, Mike Turner, John Townsend, Sabina
   Torrente, Cliff Thomas, Yuji Suzuki, Manoj Solanki, Sverre Slotte,
   Keyur Shah, Jan Rovins, Ben Robinson, Renee Revis, Ian Periam, RC
   Monee, Sanjay Rao, Sujith Radhakrishnan, Heinz Prantner, Biren Patel,
   Nathalie Mouellic, Mitch Miers, Bernward Meyknecht, Stan McClellan,
   Oliver Mayor, Tomas Orti Martin, Sandeep Mahajan, David Lehmann,
   Jonathan Lee, Philippe Langlois, Karl Knutson, Joe Keller, Gareth
   Keily, Andreas Jungmaier, Janardhan Iyengar, Mutsuya Irie, John
   Hebert, Kausar Hassan, Fred Hasle, Dan Harrison, Jon Grim, Laurent
   Glaude, Steven Furniss, Atsushi Fukumoto, Ken Fujita, Steve Dimig,
   Thomas Curran, Serkan Cil, Melissa Campbell, Peter Butler, Rob
   Brennan, Harsh Bhondwe, Brian Bidulock, Caitlin Bestler, Jon Berger,
   Robby Benedyk, Stephen Baucke, Sandeep Balani, and Ronnie Sellar.

   A special thanks to Mark Allman, who should actually be a co-author
   for his work on the max-burst, but managed to wiggle out due to a
   technicality.  Also, we would like to acknowledge Lyndon Ong and Phil
   Conrad for their valuable input and many contributions.

   And finally, you have this document, and those who have commented
   upon that including Alfred Hoenes and Ronnie Sellars.

   My thanks cannot be adequately expressed to all of you who have
   participated in the coding, testing, and updating process of this
   document.  All I can say is, Thank You!

   Randall Stewart - Editor

17.  References

17.1.  Normative References

   [ITU.V42.1994]
              International Telecommunications Union, "Error-correcting
              Procedures for DCEs Using Asynchronous-to-Synchronous
              Conversion", ITU-T Recommendation V.42, 1994.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.






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   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3873]  Pastor, J. and M. Belinchon, "Stream Control Transmission
              Protocol (SCTP) Management Information Base (MIB)",
              RFC 3873, DOI 10.17487/RFC3873, September 2004,
              <https://www.rfc-editor.org/info/rfc3873>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.




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   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

17.2.  Informative References

   [ALLMAN99]
              Allman, M. and V. Paxson, "On Estimating End-to-End
              Network Path Properties", SIGCOM 99, 1999.

   [FALL96]   Fall, K. and S. Floyd, "Simulation-based Comparisons of
              Tahoe, Reno, and SACK TCP", SIGCOM 99, V. 26, N. 3,
              pp 5-21, July 1996.







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   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              DOI 10.17487/RFC1858, October 1995,
              <https://www.rfc-editor.org/info/rfc1858>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2196]  Fraser, B., "Site Security Handbook", FYI 8, RFC 2196,
              DOI 10.17487/RFC2196, September 1997,
              <https://www.rfc-editor.org/info/rfc2196>.

   [RFC2522]  Karn, P. and W. Simpson, "Photuris: Session-Key Management
              Protocol", RFC 2522, DOI 10.17487/RFC2522, March 1999,
              <https://www.rfc-editor.org/info/rfc2522>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
              2007, <https://www.rfc-editor.org/info/rfc4895>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC7053]  Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
              IMMEDIATELY Extension for the Stream Control Transmission
              Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013,
              <https://www.rfc-editor.org/info/rfc7053>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.





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   [RFC8540]  Stewart, R., Tuexen, M., and M. Proshin, "Stream Control
              Transmission Protocol: Errata and Issues in RFC 4960",
              RFC 8540, DOI 10.17487/RFC8540, February 2019,
              <https://www.rfc-editor.org/info/rfc8540>.

   [SAVAGE99]
              Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
              "TCP Congestion Control with a Misbehaving Receiver", ACM
              Computer Communications Review 29(5), October 1999.

   [WILLIAMS93]
              Williams, R., "A PAINLESS GUIDE TO CRC ERROR DETECTION
              ALGORITHMS", SIGCOM 99, August 1993,
              <http://www.geocities.com/SiliconValley/Pines/8659/
              crc.htm>.

Appendix A.  Explicit Congestion Notification

   ECN as specified in [RFC3168] (updated by [RFC8311]) describes an
   extension to IP that details a method for becoming aware of
   congestion outside of datagram loss.  This is a feature that an
   implementation MAY choose to add to SCTP.  This appendix details the
   minor differences implementers will need to be aware of if they
   choose to implement this feature.  In general, [RFC3168] (updated by
   [RFC8311]) SHOULD be followed, with the following exceptions.

   Negotiation:

   [RFC3168] (updated by [RFC8311]) details the negotiation of ECN
   during the SYN and SYN-ACK stages of a TCP connection.  The sender of
   the SYN sets 2 bits in the TCP flags, and the sender of the SYN-ACK
   sets only 1 bit.  The reasoning behind this is to ensure that both
   sides are truly ECN capable.  For SCTP, this is not necessary.  To
   indicate that an endpoint is ECN capable, an endpoint SHOULD add to
   the INIT and or INIT ACK chunk the TLV reserved for ECN.  This TLV
   contains no parameters, and thus has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Parameter Type = 32768      |     Parameter Length = 4      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   ECN-Echo:

   [RFC3168] (updated by [RFC8311]) details a specific bit for a
   receiver to send back in its TCP acknowledgements to notify the
   sender of the Congestion Experienced (CE) bit that the CE bit has



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   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.

   CWR:

   [RFC3168] (updated by [RFC8311]) details a specific bit for a sender
   to send in the header of its next outbound TCP segment to indicate to
   its peer that it has reduced its congestion window.  This is termed
   the CWR bit.  For SCTP, the same indication is made by including the
   CWR chunk.  This chunk contains one data element, i.e., the TSN
   number that was sent in the ECNE chunk.  This element represents the
   lowest TSN number in the datagram that was originally marked with the
   CE bit.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Chunk Type=13 | Flags=00000000|    Chunk Length = 8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Lowest TSN Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note: The CWR is considered a Control chunk.

Appendix B.  CRC32c Checksum Calculation

   We define a 'reflected value' as one that is the opposite of the
   normal bit order of the machine.  The 32-bit CRC (Cyclic Redundancy
   Check) is calculated as described for CRC32c and uses the polynomial
   code 0x11EDC6F41 (Castagnoli93) or
   x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+
   x^14+x^13+x^11+x^10+x^9+x^8+x^6+x^0.  The CRC is computed using a
   procedure similar to ETHERNET CRC [ITU.V42.1994], modified to reflect
   transport-level usage.





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   CRC computation uses polynomial division.  A message bit-string M is
   transformed to a polynomial, M(X), and the CRC is calculated from
   M(X) using polynomial arithmetic.

   When CRCs are used at the link layer, the polynomial is derived from
   on-the-wire bit ordering: the first bit 'on the wire' is the high-
   order coefficient.  Since SCTP is a transport-level protocol, it
   cannot know the actual serial-media bit ordering.  Moreover,
   different links in the path between SCTP endpoints can use different
   link-level bit orders.

   A convention therefore is established for mapping SCTP transport
   messages to polynomials for purposes of CRC computation.  The bit-
   ordering for mapping SCTP messages to polynomials is that bytes are
   taken most-significant first, but within each byte, bits are taken
   least-significant first.  The first byte of the message provides the
   eight highest coefficients.  Within each byte, the least-significant
   SCTP bit gives the most-significant polynomial coefficient within
   that byte, and the most-significant SCTP bit is the least-significant
   polynomial coefficient in that byte.  (This bit ordering is sometimes
   called 'mirrored' or 'reflected' [WILLIAMS93].)  CRC polynomials are
   to be transformed back into SCTP transport-level byte values, using a
   consistent mapping.

   The SCTP transport-level CRC value can be calculated as follows:

   o  CRC input data are assigned to a byte stream, numbered from 0 to
      N-1.

   o  The transport-level byte stream is mapped to a polynomial value.
      An N-byte PDU with j bytes numbered 0 to N-1 is considered as
      coefficients of a polynomial M(x) of order 8N-1, with bit 0 of
      byte j being coefficient x^(8(N-j)-8), and bit 7 of byte j being
      coefficient x^(8(N-j)-1).

   o  The CRC remainder register is initialized with all 1s and the CRC
      is computed with an algorithm that simultaneously multiplies by
      x^32 and divides by the CRC polynomial.

   o  The polynomial is multiplied by x^32 and divided by G(x), the
      generator polynomial, producing a remainder R(x) of degree less
      than or equal to 31.

   o  The coefficients of R(x) are considered a 32-bit sequence.

   o  The bit sequence is complemented.  The result is the CRC
      polynomial.




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   o  The CRC polynomial is mapped back into SCTP transport-level bytes.
      The coefficient of x^31 gives the value of bit 7 of SCTP byte 0,
      and the coefficient of x^24 gives the value of bit 0 of byte 0.
      The coefficient of x^7 gives bit 7 of byte 3, and the coefficient
      of x^0 gives bit 0 of byte 3.  The resulting 4-byte transport-
      level sequence is the 32-bit SCTP checksum value.

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

   Implementors of SCTP are warned that both specifications are to be
   found in the literature, sometimes with no restriction on the long-
   division algorithm.  The choice of formulation in this document is to
   permit non-SCTP usage, where the same CRC algorithm can be used to
   protect messages shorter than 64 bits.

   There can be a computational advantage in validating the association
   against the Verification Tag, prior to performing a checksum, as
   invalid tags will result in the same action as a bad checksum in most
   cases.  The exceptions for this technique would be INIT and some
   SHUTDOWN-COMPLETE exchanges, as well as a stale COOKIE ECHO.  These
   special-case exchanges represent small packets and will minimize the
   effect of the checksum calculation.

   The following non-normative sample code is taken from an open-source
   CRC generator [WILLIAMS93], using the "mirroring" technique and
   yielding a lookup table for SCTP CRC32c with 256 entries, each 32
   bits wide.  While neither especially slow nor especially fast, as
   software table-lookup CRCs go, it has the advantage of working on
   both big-endian and little-endian CPUs, using the same (host-order)
   lookup tables, and using only the predefined ntohl() and htonl()
   operations.  The code is somewhat modified from [WILLIAMS93], to
   ensure portability between big-endian and little-endian
   architectures.  (Note that if the byte endian-ness of the target
   architecture is known to be little-endian, the final bit-reversal and
   byte-reversal steps can be folded into a single operation.)


   <CODE BEGINS>
   /****************************************************************/



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   /* Note: The definitions for Ross Williams's table generator    */
   /* would be TB_WIDTH=4, TB_POLY=0x1EDC6F41, TB_REVER=TRUE.      */
   /* For Mr. Williams's direct calculation code, use the settings */
   /* cm_width=32, cm_poly=0x1EDC6F41, cm_init=0xFFFFFFFF,         */
   /* cm_refin=TRUE, cm_refot=TRUE, cm_xorot=0x00000000.           */
   /****************************************************************/

   /* Example of the crc table file */
   #ifndef __crc32cr_h__
   #define __crc32cr_h__

   #define CRC32C_POLY 0x1EDC6F41UL
   #define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])

   uint32_t crc_c[256] = {
     0x00000000UL, 0xF26B8303UL, 0xE13B70F7UL, 0x1350F3F4UL,
     0xC79A971FUL, 0x35F1141CUL, 0x26A1E7E8UL, 0xD4CA64EBUL,
     0x8AD958CFUL, 0x78B2DBCCUL, 0x6BE22838UL, 0x9989AB3BUL,
     0x4D43CFD0UL, 0xBF284CD3UL, 0xAC78BF27UL, 0x5E133C24UL,
     0x105EC76FUL, 0xE235446CUL, 0xF165B798UL, 0x030E349BUL,
     0xD7C45070UL, 0x25AFD373UL, 0x36FF2087UL, 0xC494A384UL,
     0x9A879FA0UL, 0x68EC1CA3UL, 0x7BBCEF57UL, 0x89D76C54UL,
     0x5D1D08BFUL, 0xAF768BBCUL, 0xBC267848UL, 0x4E4DFB4BUL,
     0x20BD8EDEUL, 0xD2D60DDDUL, 0xC186FE29UL, 0x33ED7D2AUL,
     0xE72719C1UL, 0x154C9AC2UL, 0x061C6936UL, 0xF477EA35UL,
     0xAA64D611UL, 0x580F5512UL, 0x4B5FA6E6UL, 0xB93425E5UL,
     0x6DFE410EUL, 0x9F95C20DUL, 0x8CC531F9UL, 0x7EAEB2FAUL,
     0x30E349B1UL, 0xC288CAB2UL, 0xD1D83946UL, 0x23B3BA45UL,
     0xF779DEAEUL, 0x05125DADUL, 0x1642AE59UL, 0xE4292D5AUL,
     0xBA3A117EUL, 0x4851927DUL, 0x5B016189UL, 0xA96AE28AUL,
     0x7DA08661UL, 0x8FCB0562UL, 0x9C9BF696UL, 0x6EF07595UL,
     0x417B1DBCUL, 0xB3109EBFUL, 0xA0406D4BUL, 0x522BEE48UL,
     0x86E18AA3UL, 0x748A09A0UL, 0x67DAFA54UL, 0x95B17957UL,
     0xCBA24573UL, 0x39C9C670UL, 0x2A993584UL, 0xD8F2B687UL,
     0x0C38D26CUL, 0xFE53516FUL, 0xED03A29BUL, 0x1F682198UL,
     0x5125DAD3UL, 0xA34E59D0UL, 0xB01EAA24UL, 0x42752927UL,
     0x96BF4DCCUL, 0x64D4CECFUL, 0x77843D3BUL, 0x85EFBE38UL,
     0xDBFC821CUL, 0x2997011FUL, 0x3AC7F2EBUL, 0xC8AC71E8UL,
     0x1C661503UL, 0xEE0D9600UL, 0xFD5D65F4UL, 0x0F36E6F7UL,
     0x61C69362UL, 0x93AD1061UL, 0x80FDE395UL, 0x72966096UL,
     0xA65C047DUL, 0x5437877EUL, 0x4767748AUL, 0xB50CF789UL,
     0xEB1FCBADUL, 0x197448AEUL, 0x0A24BB5AUL, 0xF84F3859UL,
     0x2C855CB2UL, 0xDEEEDFB1UL, 0xCDBE2C45UL, 0x3FD5AF46UL,
     0x7198540DUL, 0x83F3D70EUL, 0x90A324FAUL, 0x62C8A7F9UL,
     0xB602C312UL, 0x44694011UL, 0x5739B3E5UL, 0xA55230E6UL,
     0xFB410CC2UL, 0x092A8FC1UL, 0x1A7A7C35UL, 0xE811FF36UL,
     0x3CDB9BDDUL, 0xCEB018DEUL, 0xDDE0EB2AUL, 0x2F8B6829UL,
     0x82F63B78UL, 0x709DB87BUL, 0x63CD4B8FUL, 0x91A6C88CUL,



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     0x456CAC67UL, 0xB7072F64UL, 0xA457DC90UL, 0x563C5F93UL,
     0x082F63B7UL, 0xFA44E0B4UL, 0xE9141340UL, 0x1B7F9043UL,
     0xCFB5F4A8UL, 0x3DDE77ABUL, 0x2E8E845FUL, 0xDCE5075CUL,
     0x92A8FC17UL, 0x60C37F14UL, 0x73938CE0UL, 0x81F80FE3UL,
     0x55326B08UL, 0xA759E80BUL, 0xB4091BFFUL, 0x466298FCUL,
     0x1871A4D8UL, 0xEA1A27DBUL, 0xF94AD42FUL, 0x0B21572CUL,
     0xDFEB33C7UL, 0x2D80B0C4UL, 0x3ED04330UL, 0xCCBBC033UL,
     0xA24BB5A6UL, 0x502036A5UL, 0x4370C551UL, 0xB11B4652UL,
     0x65D122B9UL, 0x97BAA1BAUL, 0x84EA524EUL, 0x7681D14DUL,
     0x2892ED69UL, 0xDAF96E6AUL, 0xC9A99D9EUL, 0x3BC21E9DUL,
     0xEF087A76UL, 0x1D63F975UL, 0x0E330A81UL, 0xFC588982UL,
     0xB21572C9UL, 0x407EF1CAUL, 0x532E023EUL, 0xA145813DUL,
     0x758FE5D6UL, 0x87E466D5UL, 0x94B49521UL, 0x66DF1622UL,
     0x38CC2A06UL, 0xCAA7A905UL, 0xD9F75AF1UL, 0x2B9CD9F2UL,
     0xFF56BD19UL, 0x0D3D3E1AUL, 0x1E6DCDEEUL, 0xEC064EEDUL,
     0xC38D26C4UL, 0x31E6A5C7UL, 0x22B65633UL, 0xD0DDD530UL,
     0x0417B1DBUL, 0xF67C32D8UL, 0xE52CC12CUL, 0x1747422FUL,
     0x49547E0BUL, 0xBB3FFD08UL, 0xA86F0EFCUL, 0x5A048DFFUL,
     0x8ECEE914UL, 0x7CA56A17UL, 0x6FF599E3UL, 0x9D9E1AE0UL,
     0xD3D3E1ABUL, 0x21B862A8UL, 0x32E8915CUL, 0xC083125FUL,
     0x144976B4UL, 0xE622F5B7UL, 0xF5720643UL, 0x07198540UL,
     0x590AB964UL, 0xAB613A67UL, 0xB831C993UL, 0x4A5A4A90UL,
     0x9E902E7BUL, 0x6CFBAD78UL, 0x7FAB5E8CUL, 0x8DC0DD8FUL,
     0xE330A81AUL, 0x115B2B19UL, 0x020BD8EDUL, 0xF0605BEEUL,
     0x24AA3F05UL, 0xD6C1BC06UL, 0xC5914FF2UL, 0x37FACCF1UL,
     0x69E9F0D5UL, 0x9B8273D6UL, 0x88D28022UL, 0x7AB90321UL,
     0xAE7367CAUL, 0x5C18E4C9UL, 0x4F48173DUL, 0xBD23943EUL,
     0xF36E6F75UL, 0x0105EC76UL, 0x12551F82UL, 0xE03E9C81UL,
     0x34F4F86AUL, 0xC69F7B69UL, 0xD5CF889DUL, 0x27A40B9EUL,
     0x79B737BAUL, 0x8BDCB4B9UL, 0x988C474DUL, 0x6AE7C44EUL,
     0xBE2DA0A5UL, 0x4C4623A6UL, 0x5F16D052UL, 0xAD7D5351UL,
   };

   #endif


   /* Example of table build routine */

   #include <stdio.h>
   #include <stdlib.h>

   #define OUTPUT_FILE   "crc32cr.h"
   #define CRC32C_POLY    0x1EDC6F41UL

   static FILE *tf;

   static uint32_t
   reflect_32(uint32_t b)



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   {
     int i;
     uint32_t rw = 0UL;

     for (i = 0; i < 32; i++) {
       if (b & 1)
         rw |= 1 << (31 - i);
       b >>= 1;
     }
     return (rw);
   }

   static uint32_t
   build_crc_table (int index)
   {
     int i;
     uint32_t rb;

     rb = reflect_32(index);

     for (i = 0; i < 8; i++) {
       if (rb & 0x80000000UL)
         rb = (rb << 1) ^ (uint32_t)CRC32C_POLY;
       else
         rb <<= 1;
     }
     return (reflect_32(rb));
   }

   int
   main (void)
   {
     int i;

     printf("\nGenerating CRC32c table file <%s>.\n",
     OUTPUT_FILE);
     if ((tf = fopen(OUTPUT_FILE, "w")) == NULL) {
       printf("Unable to open %s.\n", OUTPUT_FILE);
       exit (1);
     }
     fprintf(tf, "#ifndef __crc32cr_h__\n");
     fprintf(tf, "#define __crc32cr_h__\n\n");
     fprintf(tf, "#define CRC32C_POLY 0x%08XUL\n",
       (uint32_t)CRC32C_POLY);
     fprintf(tf,
       "#define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])\n");
     fprintf(tf, "\nuint32_t crc_c[256] =\n{\n");
     for (i = 0; i < 256; i++) {



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       fprintf(tf, "0x%08XUL,", build_crc_table (i));
       if ((i & 3) == 3)
         fprintf(tf, "\n");
       else
         fprintf(tf, " ");
     }
     fprintf(tf, "};\n\n#endif\n");

     if (fclose(tf) != 0)
       printf("Unable to close <%s>.\n", OUTPUT_FILE);
     else
       printf("\nThe CRC32c table has been written to <%s>.\n",
         OUTPUT_FILE);
     return (0);
   }

   /* Example of crc insertion */

   #include "crc32cr.h"

   uint32_t
   generate_crc32c(unsigned char *buffer, unsigned int length)
   {
     unsigned int i;
     uint32_t crc32 = 0xffffffffUL;
     uint32_t result;
     uint8_t byte0, byte1, byte2, byte3;

     for (i = 0; i < length; i++) {
       CRC32C(crc32, buffer[i]);
     }

     result = ~crc32;

     /*  result now holds the negated polynomial remainder,
      *  since the table and algorithm are "reflected" [williams95].
      *  That is, result has the same value as if we mapped the message
      *  to a polynomial, computed the host-bit-order polynomial
      *  remainder, performed final negation, and then did an
      *  end-for-end bit-reversal.
      *  Note that a 32-bit bit-reversal is identical to four in-place
      *  8-bit bit-reversals followed by an end-for-end byteswap.
      *  In other words, the bits of each byte are in the right order,
      *  but the bytes have been byteswapped.  So, we now do an explicit
      *  byteswap.  On a little-endian machine, this byteswap and
      *  the final ntohl cancel out and could be elided.
      */




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     byte0 = result & 0xff;
     byte1 = (result>>8) & 0xff;
     byte2 = (result>>16) & 0xff;
     byte3 = (result>>24) & 0xff;
     crc32 = ((byte0 << 24) |
              (byte1 << 16) |
              (byte2 << 8)  |
              byte3);
     return (crc32);
   }

   int
   insert_crc32(unsigned char *buffer, unsigned int length)
   {
     SCTP_message *message;
     uint32_t crc32;
     message = (SCTP_message *) buffer;
     message->common_header.checksum = 0UL;
     crc32 = generate_crc32c(buffer,length);
     /* and insert it into the message */
     message->common_header.checksum = htonl(crc32);
     return (1);
   }

   int
   validate_crc32(unsigned char *buffer, unsigned int length)
   {
     SCTP_message *message;
     unsigned int i;
     uint32_t original_crc32;
     uint32_t crc32;

     /* save and zero checksum */
     message = (SCTP_message *)buffer;
     original_crc32 = ntohl(message->common_header.checksum);
     message->common_header.checksum = 0L;
     crc32 = generate_crc32c(buffer, length);
     return ((original_crc32 == crc32) ? 1 : -1);
   }
   <CODE ENDS>


Appendix C.  ICMP Handling

   Whenever an ICMP message is received by an SCTP endpoint, the
   following procedures MUST be followed to ensure proper utilization of
   the information being provided by layer 3.




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   ICMP1)  An implementation MAY ignore all ICMPv4 messages where the
           type field is not set to "Destination Unreachable".

   ICMP2)  An implementation MAY ignore all ICMPv6 messages where the
           type field is not "Destination Unreachable", "Parameter
           Problem", or "Packet Too Big".

   ICMP3)  An implementation SHOULD ignore any ICMP messages where the
           code indicates "Port Unreachable".

   ICMP4)  An implementation MAY ignore all ICMPv6 messages of type
           "Parameter Problem" if the code is not "Unrecognized Next
           Header Type Encountered".

   ICMP5)  An implementation MUST use the payload of the ICMP message
           (v4 or v6) to locate the association that sent the message to
           which ICMP is responding.  If the association cannot be
           found, an implementation SHOULD ignore the ICMP message.

   ICMP6)  An implementation MUST validate that the Verification Tag
           contained in the ICMP message matches the Verification Tag of
           the peer.  If the Verification Tag is not 0 and does NOT
           match, discard the ICMP message.  If it is 0 and the ICMP
           message contains enough bytes to verify that the chunk type
           is an INIT chunk and that the Initiate Tag matches the tag of
           the peer, continue with ICMP7.  If the ICMP message is too
           short or the chunk type or the Initiate Tag does not match,
           silently discard the packet.

   ICMP7)  If the ICMP message is either a v6 "Packet Too Big" or a v4
           "Fragmentation Needed", an implementation MAY process this
           information as defined for PMTU discovery.

   ICMP8)  If the ICMP code is an "Unrecognized Next Header Type
           Encountered" or a "Protocol Unreachable", an implementation
           MUST treat this message as an abort with the T bit set if it
           does not contain an INIT chunk.  If it does contain an INIT
           chunk and the association is in the COOKIE-WAIT state, handle
           the ICMP message like an ABORT.

   ICMP9)  If the ICMP type is "Destination Unreachable", the
           implementation MAY move the destination to the unreachable
           state or, alternatively, increment the path error counter.
           SCTP MAY provide information to the upper layer indicating
           the reception of ICMP messages when reporting a network
           status change.





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   Note that these procedures differ from [RFC1122] and from its
   requirements for processing of port-unreachable messages and the
   requirements that an implementation MUST abort associations in
   response to a "protocol unreachable" message.  Port-unreachable
   messages are not processed, since an implementation will send an
   ABORT, not a port unreachable.  The stricter handling of the
   "protocol unreachable" message is due to security concerns for hosts
   that do NOT support SCTP.

Authors' Addresses

   Randall R. Stewart
   Netflix, Inc.
   2455 Heritage Green Ave
   Davenport, FL  33837
   United States

   Email: randall@lakerest.net


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   Steinfurt  48565
   Germany

   Email: tuexen@fh-muenster.de


   Karen E. E. Nielsen
   Kamstrup A/S
   Industrivej 28
   Skanderborg  DK-8660
   Denmark

   Email: kee@kamstrup.com















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