Network Working Group                                     R. R. Stewart
INTERNET-DRAFT                                                   Q. Xie
                                                               Motorola
                                                           K. Morneault
                                                               C. Sharp
                                                                  Cisco
                                                     H. J. Schwarzbauer
                                                                Siemens
                                                              T. Taylor
                                                        Nortel Networks
                                                              I. Rytina
                                                               Ericsson
                                                               M. Kalla
                                                              Telcordia
                                                               L. Zhang
                                                                   UCLA
                                                              V. Paxson
                                                                  ACIRI

expires in six months                                      July 3,2000 10,2000

                 Stream Control Transmission Protocol
                   <draft-ietf-sigtran-sctp-11.txt>
                   <draft-ietf-sigtran-sctp-12.txt>

Status of This Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of [RFC2026]. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Internet draft      Stream Control Transmission Protocol       July 2000

Abstract

This document describes the Stream Control Transmission Protocol
(SCTP). SCTP is designed to transport PSTN signaling messages over
IP networks, but is capable of broader applications.

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

  -- acknowledged error-free non-duplicated transfer of user data,
  -- data fragmentation to conform to discovered path MTU size,
  -- sequenced delivery of user messages within multiple streams,
     with an option for order-of-arrival delivery of individual
     user messages,
  -- optional bundling of multiple user messages into a single SCTP
     packet, and
  -- network-level fault tolerance through supporting of multi-homing
     at either or both ends of an association.

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

                        TABLE OF CONTENTS

1.  Introduction..................................................  4  5
  1.1 Motivation..................................................  5
  1.2 Architectural View of SCTP..................................  5
  1.3 Functional View of SCTP.....................................  6
    1.3.1 Association Startup and Takedown........................  7
    1.3.2 Sequenced Delivery within Streams.......................  8  7
    1.3.3 User Data Fragmentation.................................  8
    1.3.4 Acknowledgement and Congestion Avoidance................  8
    1.3.5 Chunk Bundling .........................................  8
    1.3.6 Packet Validation.......................................  8
    1.3.7 Path Management.........................................  9
  1.4 Key Terms...................................................  9
  1.5 Abbreviations............................................... 12
  1.6 Serial Number Arithmetic.................................... 12 13
2. Conventions.................................................... 13
3.  SCTP packet Format............................................ 13
  3.1 SCTP Common Header Field Descriptions....................... 14
  3.2 Chunk Field Descriptions.................................... 15
    3.2.1 Optional/Variable-length Parameter Format............... 17
  3.3 SCTP Chunk Definitions...................................... 18
    3.3.1 Payload Data (DATA)..................................... 18
    3.3.2 Initiation (INIT)....................................... 20
      3.3.2.1 Optional or Variable Length Parameters.............. 22 23
    3.3.3 Initiation Acknowledgement (INIT ACK)................... 25
      3.3.3.1 Optional or Variable Length Parameters.............. 28

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    3.3.4 Selective Acknowledgement (SACK)........................ 28
    3.3.5 Heartbeat Request (HEARTBEAT)........................... 31
    3.3.6 Heartbeat Acknowledgement (HEARTBEAT ACK)............... 32
    3.3.7 Abort Association (ABORT)............................... 33
    3.3.8 Shutdown Association (SHUTDOWN)......................... 33 34
    3.3.9 Shutdown Acknowledgement (SHUTDOWN ACK)................. 34
    3.3.10 Operation Error (ERROR)................................ 34 35
      3.3.10.1 Invalid Stream Identifier.......................... 36
      3.3.10.2 Missing Mandatory Parameter........................ 36
      3.3.10.3 Stale Cookie Error................................. 37
      3.3.10.4 Out of Resource.................................... 37
      3.3.10.5 Unresolvable Address............................... 37
      3.3.10.6 Unrecognized Chunk Type............................ 38
      3.3.10.7 Invalid Mandatory Parameter........................ 38
      3.3.10.8 Unrecognized Parameters............................ 38
      3.3.10.9 No User Data....................................... 39
      3.3.10.10 Cookie Received While Shutting Down............... 39
    3.3.11 Cookie Echo (COOKIE ECHO).............................. 39 40
    3.3.12 Cookie Acknowledgement (COOKIE ACK).................... 40
    3.3.13 Shutdown Complete (SHUTDOWN COMPLETE).................. 40 41
4. SCTP Association State Diagram................................. 41
5. Association Initialization..................................... 44
  5.1 Normal Establishment of an Association...................... 44
    5.1.1 Handle Stream Parameters................................ 46
    5.1.2 Handle Address Parameters............................... 46
    5.1.3 Generating State Cookie................................. 48
    5.1.4 State Cookie Processing................................. 48 49
    5.1.5 State Cookie Authentication............................. 49
    5.1.6 An Example of Normal Association Establishment.......... 50
  5.2 Handle Duplicate or unexpected INIT, INIT ACK, COOKIE ECHO,
      and COOKIE ACK.............................................. 51
    5.2.1 Handle Duplicate INIT in COOKIE-WAIT
          or COOKIE-ECHOED States................................. 52
    5.2.2 Unexpected INIT in States Other than CLOSED,
          COOKIE-ECHOED and COOKIE-WAIT........................... 52
    5.2.3 Unexpected INIT ACK..................................... 52
    5.2.4 Handle a COOKIE ECHO when a TCB exists.................. 52
      5.2.4.1 An Example of a Association Restart................. 55
    5.2.5 Handle Duplicate COOKIE ACK............................. 54 56
    5.2.6 Handle Stale COOKIE Error............................... 55 56
  5.3 Other Initialization Issues................................. 55 56
    5.3.1 Selection of Tag Value.................................. 55 56
6. User Data Transfer............................................. 56 57
  6.1 Transmission of DATA Chunks................................. 57 58
  6.2 Acknowledgement on Reception of DATA Chunks................. 58 59
    6.2.1 Tracking Peer's Receive Buffer Space.................... 61 62
  6.3 Management Retransmission Timer............................. 62 63
    6.3.1 RTO Calculation......................................... 62 63
    6.3.2 Retransmission Timer Rules.............................. 63 65
    6.3.3 Handle T3-rtx Expiration................................ 64 65
  6.4 Multi-homed SCTP Endpoints.................................. 65 67
    6.4.1 Failover from Inactive Destination Address.............. 66 67
  6.5 Stream Identifier and Stream Sequence Number................ 67 68
  6.6 Ordered and Unordered Delivery.............................. 67 68

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  6.7 Report Gaps in Received DATA TSNs........................... 68 69
  6.8 Adler-32 Checksum Calculation............................... 69 70
  6.9 Fragmentation............................................... 69 70
  6.10 Bundling .................................................. 70 71
7. Congestion Control	.......................................... 71 72
  7.1 SCTP Differences from TCP Congestion Control................ 71 73
  7.2 SCTP Slow-Start and Congestion Avoidance.................... 72 74
    7.2.1 Slow-Start.............................................. 73 74
    7.2.2 Congestion Avoidance.................................... 74 75
    7.2.3 Congestion Control...................................... 75 76
    7.2.4 Fast Retransmit on Gap Reports.......................... 75 76
  7.3 Path MTU Discovery.......................................... 76 77
8.  Fault Management.............................................. 77 78
  8.1 Endpoint Failure Detection.................................. 77 78
  8.2 Path Failure Detection...................................... 77 78
  8.3 Path Heartbeat.............................................. 78 79
  8.4 Handle "Out of the blue" Packets............................ 80 81
  8.5 Verification Tag............................................ 81 82
    8.5.1 Exceptions in Verification Tag Rules.................... 81 82
9. Termination of Association..................................... 82 83
  9.1 Abort of an Association..................................... 82 83
  9.2 Shutdown of an Association.................................. 83 84
10. Interface with Upper Layer.................................... 85 86
  10.1 ULP-to-SCTP................................................ 85 86
  10.2 SCTP-to-ULP................................................ 94 95
11. Security Considerations....................................... 96 98
  11.1 Security Objectives........................................ 96 98
  11.2 SCTP Responses To Potential Threats........................ 97 98
    11.2.1 Countering Insider Attacks............................. 97 98
    11.2.2 Protecting against Data Corruption in the Network...... 97 98
    11.2.3 Protecting Confidentiality............................. 97 99
    11.2.4 Protecting against Blind Denial of Service Attacks..... 98 99
      11.2.4.1 Flooding........................................... 98
      11.2.4.2 Masquerade......................................... 99
      11.2.4.2 Blind Masquerade...................................100
      11.2.4.3 Improper Monopolization of Services................100 Services................101
  11.3 Protection against Fraud and Repudiation...................100 Repudiation...................101
12. Recommended Transmission Control Block (TCB) Parameters.......101 Parameters.......102
  12.1 Parameters necessary for the SCTP instance.................101 instance.................102
  12.2 Parameters necessary per association (i.e. the TCB)........101 TCB)........103
  12.3 Per Transport Address Data.................................102 Data.................................104
  12.4 General Parameters Needed..................................104 Needed..................................105
13. IANA Consideration............................................104 Consideration............................................105
  13.1 IETF-defined Chunk Extension...............................104 Extension...............................105
  13.2 IETF-defined Additional Error Causes.......................104 Causes.......................106
  13.3 Payload Protocol Identifiers...............................105 Identifiers...............................106
14. Suggested SCTP Protocol Parameter Values......................105 Values......................107
15. Acknowledgements..............................................106 Acknowledgements..............................................107
16. Authors' Addresses............................................106 Addresses............................................107
17. References....................................................107 References....................................................109
18. Bibliography..................................................108 Bibliography..................................................110
Appendix A .......................................................109 .......................................................110
Appendix B .......................................................110 .......................................................111

Internet draft      Stream Control Transmission Protocol       July 2000

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.

1.1 Motivation

TCP [RFC793] has performed immense service as the primary means of
reliable data transfer in IP networks. However, an increasing number of
recent applications have found TCP too limiting, and have incorporated
their own reliable data transfer protocol on top of UDP [RFC768]. The
limitations which users have wished to bypass include the following:

     -- TCP provides both reliable data transfer and strict order-
     of-transmission delivery of data. Some applications need reliable
     transfer without sequence maintenance, while others would be
     satisfied with partial ordering of the data. In both of these
     cases the head-of-line blocking offered by TCP causes
     unnecessary delay.

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

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

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

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

1.2 Architectural View of SCTP

SCTP is viewed as a layer between the SCTP user application ("SCTP
user" for short) and a connectionless packet network service such
as IP. The remainder of this document assumes SCTP runs on top of IP.
The basic service offered by SCTP is the reliable transfer of
user messages between peer SCTP users. It performs this service
within the context of an association between two SCTP endpoints.
Section 10 of this document sketches the API which should exist at the
boundary between the SCTP and the SCTP user layers.

SCTP is connection-oriented in nature, but the SCTP association is a
broader concept than the TCP connection. SCTP provides the means for

Internet draft      Stream Control Transmission Protocol       July 2000

each SCTP endpoint (Section 1.4) to provide the other endpoint (during
association startup) with a list of transport addresses (i.e., multiple
IP addresses in combination with an SCTP port) through which that
endpoint can be reached and from which it will originate SCTP packets.
The association spans transfers over all of the possible
source/destination combinations which may be generated from each
endpoint's lists.

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

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

                    Figure 1: An SCTP Association

1.3 Functional View of SCTP

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

                   SCTP User Application

  ..-----------------------------------------------------
  .. _____________                  ____________________
    |             |                | Sequenced delivery |
    | Association |                |   within streams   |
    |             |                |____________________|
    |   startup   |
  ..|             |         ____________________________
    |     and     |        |    User Data Fragmentation |
    |             |        |____________________________|
    |   takedown  |
  ..|             |         ____________________________
    |             |        |     Acknowledgement        |
    |             |        |          and               |
    |             |        |    Congestion Avoidance    |
  ..|             |        |____________________________|
    |             |
    |             |         ____________________________
    |             |        |       Chunk Bundling       |
    |             |        |____________________________|
    |             |

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    |             |     ________________________________
    |             |    |      Packet Validation         |
    |             |    |________________________________|
    |             |
    |             |     ________________________________
    |             |    |     Path Management            |
    |______________    |________________________________|

   Figure 2: Functional View of the SCTP Transport Service

1.3.1 Association Startup and Takedown

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

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

SCTP provides for graceful close (i.e., shutdown) of an active
association on request from the SCTP user. See the description of the
SHUTDOWN primitive in Section 10. SCTP also allows ungraceful close
(i.e., abort), either on request from the user (ABORT primitive) or as
a result of an error condition detected within the SCTP layer. Section
9 describes both the graceful and the ungraceful close procedures.

SCTP does not support a half-open state (like TCP) wherein one side
may continue sending data while the other end is closed. When either

endpoint performs a shutdown, the association on each peer will stop
accepting new data from its user and only deliver data in queue at the
time of the graceful close (see Section 9).

1.3.2 Sequenced Delivery within Streams

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

1.3.3 User Data Fragmentation

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

1.3.4 Acknowledgement and Congestion Avoidance

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

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

1.3.5 Chunk Bundling

As described in Section 3, the SCTP packet as delivered to the lower
layer consists of a common header followed by one or more chunks. Each
chunk may contain either user data or SCTP control information. The

SCTP user has the option to request bundling of more than one user
messages into a single SCTP packet. The chunk bundling function of SCTP
is responsible for assembly of the complete SCTP packet and its
disassembly at the receiving end.

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

1.3.6 Packet Validation

A mandatory Verification Tag field and a 32 bit checksum field (see
Appendix B for a description of the Adler-32 checksum) are included in
the SCTP common header. The Verification Tag value is chosen by each
end of the association during association startup. Packets received
without the expected Verification Tag value are discarded, as a

Internet draft      Stream Control Transmission Protocol       July 2000

protection against blind masquerade attacks and against stale SCTP
packets from a previous association. The Adler-32 checksum should be
set by the sender of each SCTP packet to provide additional protection
against data corruption in the network.  The receiver of an SCTP packet
with an invalid Adler-32 checksum silently discards the packet.

1.3.7 Path Management

The sending SCTP user is able to manipulate the set of transport
addresses used as destinations for SCTP packets through the
primitives described in Section 10. The SCTP path management function
chooses the destination transport address for each outgoing SCTP
packet based on the SCTP user's instructions and the currently
perceived reachability status of the eligible destination set.
The path management function monitors reachability through heartbeats
when other packet traffic is inadequate to provide this information
and advises the SCTP user when reachability of any far-end transport
address changes. The path management function is also responsible for
reporting the eligible set of local transport addresses to the far end
during association startup, and for reporting the transport addresses
returned from the far end to the SCTP user.

At association start-up, a primary path is defined for each SCTP
endpoint, and is used for normal sending of SCTP packets.

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

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

1.4 Key Terms

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

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

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

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

 o Cumulative TSN Ack Point: The TSN of the last DATA chunk
   acknowledged via the Cumulative TSN Ack field of a SACK.

 o Idle destination address: An address that has not had user messages
   sent to it within some length of time, normally the HEARTBEAT
   interval or greater.

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

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

 o Message Authentication Code (MAC):  An integrity check mechanism
   based on cryptographic hash functions using a secret key.
   Typically, message authentication codes are used between two
   parties that share a secret key in order to validate information
   transmitted between these parties. In SCTP it is used by an
   endpoint to validate the State Cookie information that is
   returned from the peer in the COOKIE ECHO chunk.  The term "MAC"
   has different meanings in different contexts.  SCTP uses this
   term with the same meaning as in [RFC2104].

 o Network Byte Order: Most significant byte first, a.k.a., Big Endian.

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

 o Outstanding TSN (at an SCTP endpoint): A TSN (and the associated
   DATA chunk) that has been sent by the endpoint but for which it has
   not yet received an acknowledgement.

 o Path: The route taken by the SCTP packets sent by one SCTP
   endpoint to a specific destination transport address of its peer
   SCTP endpoint. Sending to different destination transport
   addresses does not necessarily guarantee getting separate paths.

 o Primary Path: The primary path is the destination and
   source address that will be put into a packet outbound
   to the peer endpoint by default. The definition includes
   the source address since an implementation MAY wish to
   specify both destination and source address to better
   control the return path taken by reply chunks and on which
   interface the packet is transmitted when the data sender
   is multi-homed.

 o Receiver Window (rwnd): An SCTP variable a data sender uses to store
   the most recently calculated receiver window of its peer, in number
   of bytes. This gives the sender an indication of the space available
   in the receiver's inbound buffer.

 o SCTP association: A protocol relationship between SCTP endpoints,

Internet draft      Stream Control Transmission Protocol       July 2000

   composed of the two SCTP endpoints and protocol state information
   including Verification Tags and the currently active set of
   Transmission Sequence Numbers (TSNs), etc.  An association can be
   uniquely identified by the transport addresses used by the endpoints
   in the association. Two SCTP endpoints MUST NOT have more than one
   SCTP association between them at any given time.

 o SCTP endpoint: The logical sender/receiver of SCTP packets. On a
   multi-homed host, an SCTP endpoint is represented to its peers as a
   combination of a set of eligible destination transport addresses to
   which SCTP packets can be sent and a set of eligible source
   transport addresses from which SCTP packets can be received.
   All transport addresses used by an SCTP endpoint must use the
   same port number, but can use multiple IP addresses. A transport
   address used by an SCTP endpoint must not be used by another
   SCTP endpoint. In other words, a transport address is unique
   to an SCTP endpoint.

 o SCTP packet (or packet): The unit of data delivery across the
   interface between SCTP and the connectionless packet network (e.g.,
   IP).  An SCTP packet includes the common SCTP header, possible SCTP
   control chunks, and user data encapsulated within SCTP DATA chunks.

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

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

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

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

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

 o Tie-Tags: Verification Tags from a previous association. These
   Tags are used within a State Cookie so that the newly restarting
   association can be linked to the original association within
   the endpoint that did NOT restart.

 o Transmission Control Block (TCB): An internal data structure
   created by an SCTP endpoint for each of its existing SCTP

Internet draft      Stream Control Transmission Protocol       July 2000

   associations to other SCTP endpoints. TCB contains all the status
   and operational information for the endpoint to maintain and manage
   the corresponding association.

 o Transmission Sequence Number (TSN): A 32-bit sequence number used
   internally by SCTP. One TSN is attached to each chunk containing
   user data to permit the receiving SCTP endpoint to acknowledge its
   receipt and detect duplicate deliveries.

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

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

 o Unordered Message: Unordered messages are "unordered" with respect
   to any other message, this includes both other unordered messages
   as well as other ordered messages. Unordered message might be
   delivered prior to or later than ordered messages sent on the
   same stream.

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

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

1.5. Abbreviations

MAC    - Message Authentication Code [RFC2104]

RTO    - Retransmission Time-out

RTT    - Round-trip Time

RTTVAR - Round-trip Time Variation

SCTP   - Stream Control Transmission Protocol

SRTT   - Smoothed RTT

TCB    - Transmission Control Block

TLV    - Type-Length-Value Coding Format

Internet draft      Stream Control Transmission Protocol       July 2000

TSN    - Transmission Sequence Number

ULP    - Upper-layer Protocol

1.6 Serial Number Arithmetic

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

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

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

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

Any arithmetic done on Stream Sequence Numbers SHOULD use Serial Number
Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.

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

2. Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [RFC2119].

3.  SCTP packet Format

An SCTP packet is composed of a common header and chunks. A chunk
contains either control information or user data.

The SCTP packet format is shown below:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Common Header                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  |                          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.

3.1 SCTP Common Header Field Descriptions

                     SCTP Common Header Format

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Source Port Number        |     Destination Port Number   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Verification Tag                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           Checksum                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Port Number: 16 bits (unsigned integer)

  This is the SCTP sender's port number. It can be used by the
  receiver in combination with the source IP address, the
  SCTP destination port and possibly the destination IP address
  to identify the association to which this packet belongs.

Destination Port Number: 16 bits (unsigned integer)

  This is the SCTP port number to which this packet is destined. The
  receiving host will use this port number to de-multiplex the
  SCTP packet to the correct receiving endpoint/application.

Verification Tag: 32 bits (unsigned integer)

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

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   - A packet containing an INIT chunk MUST have a zero
     Verification Tag.
   - A packet containing a SHUTDOWN-COMPLETE chunk with the T-bit
     set MUST have the Verification Tag copied from the packet
     with the SHUTDOWN-ACK chunk.
   - A packet containing an ABORT chunk may have the verification
     tag copied from the packet which caused the ABORT to be sent.
     For details see Section 8.4 and 8.5.

 An INIT chunk MUST be the only chunk in the SCTP packet carrying it.

Checksum: 32 bits (unsigned integer)

  This field contains the checksum of this SCTP packet. Its calculation
  is discussed in Section 6.8.  SCTP uses the Adler-32 algorithm as
  described in Appendix B for calculating the checksum

3.2  Chunk Field Descriptions

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

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

Chunk Type: 8 bits (unsigned integer)

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

  The values of Chunk Types are defined as follows:

  ID Value    Chunk Type
  -----       ----------
  0          - Payload Data (DATA)
  1          - Initiation (INIT)
  2          - Initiation Acknowledgement (INIT ACK)
  3          - Selective Acknowledgement (SACK)
  4          - Heartbeat Request (HEARTBEAT)
  5          - Heartbeat Acknowledgement (HEARTBEAT ACK)
  6          - Abort (ABORT)
  7          - Shutdown (SHUTDOWN)

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  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)
  14         - Shutdown Complete (SHUTDOWN COMPLETE)
  15 to 62   - reserved by IETF
  63         - IETF-defined Chunk Extensions
  64 to 126  - reserved by IETF
  127        - IETF-defined Chunk Extensions
  128 to 190 - reserved by IETF
  191        - IETF-defined Chunk Extensions
  192 to 254 - reserved by IETF
  255        - IETF-defined Chunk Extensions

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

  00 - Stop processing this SCTP packet and discard it, do NOT process any
       further chunks within it.

  01 - Stop processing this SCTP packet and discard it, do NOT process any
       further chunks within it, and report in an Operation Error Chunk
       using the 'Unrecognized Chunk Type' cause of error.

  10 - Skip this chunk and continue processing.

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

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

Chunk Flags: 8 bits

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

Chunk Length: 16 bits (unsigned integer)

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

Chunk Value: variable length

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

The total length of a chunk (including Type, Length and Value fields)
MUST be a multiple of 4 bytes. If the length of the chunk is not a
multiple of 4 bytes, the sender MUST pad the chunk with all zero bytes
and this padding is NOT included in the chunk length field. The sender
should never pad with more than 3 bytes. The receiver MUST ignore the
padding bytes.

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

3.2.1  Optional/Variable-length Parameter Format

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

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

Chunk Parameter Type:  16 bits (unsigned integer)

  The Type field is a 16 bit identifier of the type of parameter. It
  takes a value of 0 to 65534.

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

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.

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The total length of a parameter (including Type, Parameter Length and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is NOT included in the parameter
length field. A sender should NEVER pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.

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

00 - Stop processing this SCTP packet and discard it, do NOT process any
     further chunks within it.

01 - Stop processing this SCTP packet and discard it, do NOT process any
     further chunks within it, and report the unrecognized parameter in
     an 'Unrecognized Parameter Type' (in either a Operational Error or
     in the  INIT ACK).

10 - Skip this parameter and continue processing.

11 - Skip this parameter and continue processing but report the
     the unrecognized parameter in an 'Unrecognized Parameter Type'
     (in either a Operational Error or in the  INIT ACK).

The actual SCTP parameters are defined in the specific SCTP chunk
sections. The rules for IETF-defined parameter extensions are
defined in Section 13.2.

3.3 SCTP Chunk Definitions

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

3.3.1 Payload Data (DATA) (0)

The following format MUST be used for the DATA chunk:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 0    | Reserved|U|B|E|    Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              TSN                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Stream Identifier S      |   Stream Sequence Number n    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Payload Protocol Identifier                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                 User Data (seq n of Stream S)                 /
   \                                                               \

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved: 5 bits
  Should be set to all '0's and ignored by the receiver.

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

  After re-assembly (if necessary), unordered DATA chunks MUST be
  dispatched to the upper layer by the receiver without any attempt to
  re-order.

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

B bit: 1 bit

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

E bit:  1 bit
  The (E)nding fragment bit, if set, indicates the last fragment of a
  user message.

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

       B E                  Description
    ============================================================
    |  1 0 | First piece of a fragmented user message          |
    +----------------------------------------------------------+
    |  0 0 | Middle piece of a fragmented user message         |
    +----------------------------------------------------------+
    |  0 1 | Last piece of a fragmented user message           |
    +----------------------------------------------------------+
    |  1 1 | Unfragmented Message                              |
    ============================================================
    |             Table 1: Fragment Description Flags          |
    ============================================================

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

Length:  16 bits (unsigned integer)

  This field indicates the length of the DATA chunk in bytes from the

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  beginning of the type field to the end of the user data field
  excluding any padding.  A DATA chunk with no user data field will
  have Length set to 16 (indicating 16 bytes).

TSN : 32 bits (unsigned integer)

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

Stream Identifier S: 16 bits (unsigned integer)

  Identifies the stream to which the following user data belongs.

Stream Sequence Number n: 16 bits (unsigned integer)

  This value represents the stream sequence number of the following
  user data within the stream S. Valid range is 0 to 65535.

  When a user message is fragmented by SCTP for transport, the
  same stream sequence number MUST be carried in each of the fragments
  of the message.

Payload Protocol Identifier: 32 bits (unsigned integer)

  This value represents an application (or upper layer) specified
  protocol identifier. This value is passed to SCTP by its upper layer
  and sent to its peer. This identifier is not used by SCTP but can be
  used by certain network entities as well as the peer application to
  identify the type of information being carried in this DATA chunk.
  This field must be sent even in fragmented DATA chunks (to make
  sure it is available for agents in the middle of the network).

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

User Data: variable length

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

3.3.2 Initiation (INIT) (1)

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

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

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

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

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

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

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

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

Note 3: An INIT chunk MUST NOT contain more than one Host Name address
parameter. Moreover, the sender of the INIT MUST NOT combine any other
address types with the Host Name address in the INIT. The receiver
of INIT MUST ignore any other address types if the Host Name address
parameter is present in the received INIT chunk.

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

The Chunk Flags field in INIT is reserved and all bits in it should be
set to 0 by the sender and ignored by the receiver. The sequence of

Internet draft      Stream Control Transmission Protocol       July 2000

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.

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.

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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)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Type = 6           |          Length = 20          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         IPv6 Address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  IPv6 Address: 128 bit (unsigned integer)

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

  Note: A sender MUST NOT use an IPv4-mapped IPv6 address [RFC2373]
  but should instead use an IPv4 Address Parameter for an IPv4 address.

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

  More than one IP Address parameter can be included in an INIT

Internet draft      Stream Control Transmission Protocol       July 2000

  chunk when the INIT sender is multi-homed. Moreover, a multi-homed
  endpoint may have access to different types of network, thus more
  than one address type can be present in one INIT chunk, i.e., IPv4
  and IPv6 addresses are allowed in the same INIT chunk.

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

  Note that not using any IP address parameters in the INIT and INIT-ACK
  is an alternative to make an association more likely to work across
  a NAT box.

Cookie Preservative (9)

  The sender of the INIT shall use this parameter to suggest to the
  receiver of the INIT for a longer life-span of the State Cookie.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Type = 9             |          Length = 8           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         Suggested Cookie Life-span Increment (msec.)          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Suggested Cookie Life-span Increment: 32 bits (unsigned integer)

    This parameter indicates to the receiver how much increment in
    milliseconds the sender wishes the receiver to add to its default
    cookie life-span.

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

Host Name Address (11)

  The sender of INIT uses this parameter to pass its Host Name (in
  place of its IP addresses) to its peer. The peer is responsible for
  resolving the name. Using this parameter might make it more likely
  for the association to work across a NAT box.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Type = 11            |          Length               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  /                          Host Name                            /
  \                                                               \
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Host Name: variable length

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

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

Supported Address Types (12)

  The sender of INIT uses this parameter to list all the address types
  it can support.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Type = 12            |          Length               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        Address Type #1        |        Address Type #2        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        ......
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Address Type: 16 bits (unsigned integer)

    This is filled with the type value of the corresponding address
    TLV (e.g., IPv4 = 5, IPv6 = 6, Hostname = 11).

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

Internet draft      Stream Control Transmission Protocol       July 2000

   |              Advertised Receiver Window Credit                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Number of Outbound Streams   |  Number of Inbound Streams    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Initial TSN                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /              Optional/Variable-Length Parameters              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Initiate Tag: 32 bits (unsigned integer)

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

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

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

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

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

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

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

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

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

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

  Note: There is no negotiation of the actual number of streams but

Internet draft      Stream Control Transmission Protocol       July 2000

  instead the two endpoints will use the min(requested,
  offered). See Section 5.1.1 for details.

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

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

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

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

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

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

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

  Note 3: The INIT ACK chunks MUST NOT contain more than one Host Name
  address parameter. Moreover, the sender of the INIT ACK MUST NOT
  combine any other address types with the Host Name address in the
  INIT ACK. The receiver of the INIT ACK MUST ignore any other
  address types if the Host Name address parameter is present.

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

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 Parameters:
    Parameter Type Value: 8

    Parameter Length:  Variable Size.

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

3.3.4 Selective Acknowledgement (SACK) (3):

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

The SACK MUST contain the Cumulative TSN Ack and Advertised Receiver
Window Credit (a_rwnd) parameters.

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

The SACK also contains zero or more Gap Ack Blocks. Each
Gap Ack Block acknowledges a subsequence of TSNs received following
a break in the sequence of received TSNs.  By definition, all TSNs
acknowledged by Gap Ack Blocks are greater than the value of the
Cumulative TSN Ack.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type = 3    |Chunk  Flags   |      Chunk Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Cumulative TSN Ack                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Advertised Receiver Window Credit (a_rwnd)           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Number of Gap Ack Blocks = N  |  Number of Duplicate TSNs = X |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Gap Ack Block #1 Start       |   Gap Ack Block #1 End        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                              ...                              \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Gap Ack Block #N Start      |  Gap Ack Block #N End         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Duplicate TSN 1                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                              ...                              \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Duplicate TSN X                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk Flags: 8 bits

  Set to all zeros on transmit and ignored on receipt.

Cumulative TSN Ack: 32 bits (unsigned integer)

  This parameter contains the TSN of the last DATA chunk received in
  sequence before a gap.

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

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

Number of Duplicate TSNs: 16 bit

  This field contains the number of duplicate TSNs the endpoint
  has received. Each duplicate TSN is listed following the Gap Ack
  Block list.

Gap Ack Blocks:

  These fields contain the Gap Ack Blocks. They are repeated for each
  Gap Ack Block up to the number of Gap Ack Blocks defined in the
  Number of Gap Ack Blocks field. All DATA chunks with TSNs greater
  than or equal to (Cumulative TSN Ack + Gap Ack Block Start) and less
  than or equal to (Cumulative TSN Ack + Gap Ack Block End) of each Gap
  Ack Block are assumed to have been received correctly.

Gap Ack Block Start: 16 bits (unsigned integer)

  Indicates the Start offset TSN for this Gap Ack Block. To calculate
  the actual TSN number the Cumulative TSN Ack is added to this
  offset number. This calculated TSN identifies the first TSN in this
  Gap Ack Block that has been received.

Gap Ack Block End:  16 bits (unsigned integer)

  Indicates the End offset TSN for this Gap Ack Block. To calculate the
  actual TSN number the Cumulative TSN Ack is added to this
  offset number. This calculated TSN identifies the TSN of the last
  DATA chunk received in this Gap Ack Block.

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

                 ----------
                 | 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):

Internet draft      Stream Control Transmission Protocol       July 2000

        +--------------------------------+
        |   Cumulative TSN Ack = 12      |
        +--------------------------------+
        |        a_rwnd = 4660           |
        +----------------+---------------+
        | num of block=2 | num of dup=0  |
        +----------------+---------------+
        |block #1 strt=2 |block #1 end=3 |
        +----------------+---------------+
        |block #2 strt=5 |block #2 end=5 |
        +----------------+---------------+

Duplicate TSN: 32 bits (unsigned integer)

  Indicates the number of times a TSN was received in duplicate since
  the last SACK was sent. Every time a receiver gets a duplicate TSN
  (before sending the SACK) it adds it to the list of duplicates. The
  duplicate count is re-initialized to zero after sending each SACK.

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

3.3.5 Heartbeat Request (HEARTBEAT) (4):

An endpoint should send this chunk to its peer endpoint to probe the
reachability of a particular destination transport address defined in
the present association.

The parameter field contains the Heartbeat Information which is a
variable length opaque data structure understood only by the sender.

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

Chunk Flags: 8 bits

  Set to zero on transmit and ignored on receipt.

Heartbeat Length: 16 bits (unsigned integer)

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

Internet draft      Stream Control Transmission Protocol       July 2000

Heartbeat Information: variable length

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

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

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

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

3.3.6 Heartbeat Acknowledgement (HEARTBEAT ACK) (5):

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

The parameter field contains a variable length opaque data structure.

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

Chunk Flags: 8 bits

  Set to zero on transmit and ignored on receipt.

Heartbeat Ack Length:  16 bits (unsigned integer)

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

3.3.7 Abort Association (ABORT) (6):

The ABORT chunk is sent to the peer of an association to close the
association. The ABORT chunk may contain Cause Parameters to inform
the receiver the reason of the abort. DATA chunks MUST NOT be bundled
with ABORT. Control chunks (except for INIT, INIT ACK and SHUTDOWN
COMPLETE) MAY be bundled with an ABORT but they MUST be placed before
the ABORT in the SCTP packet, or they will be ignored by the receiver.

If an endpoint receives an ABORT with a format error or for an
association that doesn't exist, it MUST silently discard it.
Moreover, under any circumstances, an endpoint that receives an ABORT
MUST NOT respond to that ABORT by sending an ABORT of its own.

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

Chunk Flags: 8 bits

  Reserved:  7 bits
    Set to 0 on transmit and ignored on receipt.

  T bit:  1 bit
    The T bit is set to 0 if the sender had a TCB that it destroyed. If
    the sender did NOT have a TCB it should set this bit to 1.

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

Length:  16 bits (unsigned integer)

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

Internet draft      Stream Control Transmission Protocol       July 2000

See Section 3.3.10 for Error Cause definitions.

3.3.8 Shutdown Association (SHUTDOWN) (7):

An endpoint in an association MUST use this chunk to initiate a
graceful close of the association with its peer.  This chunk has
the following format.

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

Chunk Flags: 8 bits

  Set to zero on transmit and ignored on receipt.

Length:  16 bits (unsigned integer)
  Indicates the length of the parameter.  Set to 8.

Cumulative TSN Ack: 32 bits (unsigned integer)

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

  Note:  Since the SHUTDOWN message does not contain Gap Ack Blocks, it
  cannot be used to acknowledge TSNs received out of order.  In a SACK,
  lack of Gap Ack Blocks that were previously included indicates that
  the data receiver reneged on the associated DATA chunks.  Since
  SHUTDOWN does not contain Gap Ack Blocks, the receiver of the
  SHUTDOWN shouldn't interpret the lack of a Gap Ack Block as a renege.
  (see Section 6.2 for information on reneging)

3.3.9 Shutdown Acknowledgement (SHUTDOWN ACK) (8):

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

The SHUTDOWN ACK chunk has no parameters.

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

Chunk Flags:  8 bits

  Set to zero on transmit and ignored on receipt.

Internet draft      Stream Control Transmission Protocol       July 2000

3.3.10 Operation Error (ERROR) (9):

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

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

Chunk Flags:  8 bits

  Set to zero on transmit and ignored on receipt.

Length:  16 bits (unsigned integer)

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

Error causes are defined as variable-length parameters using the
format described in 3.2.1, i.e.:

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

Cause Code: 16 bits (unsigned integer)

  Defines the type of error conditions being reported.

  Cause Code
  Value           Cause Code
  ---------      ----------------
   1              Invalid Stream Identifier
   2              Missing Mandatory Parameter
   3              Stale Cookie Error
   4              Out of Resource
   5              Unresolvable Address
   6              Unrecognized Chunk Type
   7              Invalid Mandatory Parameter

Internet draft      Stream Control Transmission Protocol       July 2000

   8              Unrecognized Parameters
   9              No User Data
  10              Cookie Received While Shutting Down

Cause Length: 16 bits (unsigned integer)

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

Cause-specific Information: variable length

  This field carries the details of the error condition.

Sections 3.3.10.1 - 3.3.10.8 define error causes for SCTP.  Guidelines
for the IETF to define new error cause values are discussed in Section
13.3.

3.3.10.1 Invalid Stream Identifier (1)

  Cause of error
  ---------------
  Invalid Stream Identifier:  Indicates endpoint received a DATA chunk
  sent to a nonexistent stream.

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

   Stream Identifier: 16 bits (unsigned integer)
     Contains the Stream Identifier of the DATA chunk received in
     error.

   Reserved: 16 bits
    This field is reserved.  It is set to all 0's on transmit and
    Ignored on receipt.

3.3.10.2 Missing Mandatory Parameter (2)

  Cause of error
  ---------------
  Missing Mandatory Parameter:  Indicates that one or more
  mandatory TLV parameters are missing in a received INIT or INIT ACK.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=2              |      Cause Length=8+N*2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Number of missing params=N                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Missing Param Type #1       |   Missing Param Type #2       |

Internet draft      Stream Control Transmission Protocol       July 2000

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

  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

Internet draft      Stream Control Transmission Protocol       July 2000

  not supported by the sender). This is usually sent in combination
  with or within an ABORT.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=5              |      Cause Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                  Unresolvable Address                         /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Unresolvable Address:  variable length
    The unresolvable address field contains the complete Type, Length
    and Value of the address parameter (or Host Name parameter) that
    contains the unresolvable address or host name.

3.3.10.6 Unrecognized Chunk Type (6)

  Cause of error
  ---------------
  Unrecognized Chunk Type:  This error cause is returned to the
  originator of the chunk if the receiver does not understand
  the chunk and the upper bit of the 'Chunk Type' is set to one.

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

  Unrecognized Chunk:  variable length

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

3.3.10.7 Invalid Mandatory Parameter (7)

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

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

3.3.10.8 Unrecognized Parameters (8)

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

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

  Unrecognized Parameters:  variable length
    The Unrecognized Parameters field contains the unrecognized
    parameters copied from the INIT ACK chunk complete with TLV. This
    error cause is normally contained in an ERROR chunk bundled with
    the COOKIE ECHO chunk when responding to the INIT ACK, when the
    sender of the COOKIE ECHO chunk wishes to report unrecognized
    parameters.

3.3.10.9 No User Data (9)

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

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

  TSN value:  32 bits (+unsigned integer)
    The TSN value field contains the TSN of the DATA chunk received
    with no user data field.

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

3.3.10.10 Cookie Received While Shutting Down (10)

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

3.3.11 Cookie Echo (COOKIE ECHO) (10):

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

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

Chunk Flags: 8 bit

  Set to zero on transmit and ignored on receipt.

Length: 16 bits (unsigned integer)

  Set to the size of the chunk in bytes, including the 4 bytes of
  the chunk header and the size of the Cookie.

Cookie: variable size

  This field must contain the exact cookie received in the
  State Cookie parameter from the previous INIT ACK.

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

3.3.12 Cookie Acknowledgement (COOKIE ACK) (11):

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

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

Chunk Flags: 8 bits

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  Set to zero 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
    The T bit is set to 0 if the sender had a TCB that it destroyed. If
    the sender did NOT have a TCB it should set this bit to 1.

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

4. SCTP Association State Diagram

During the lifetime of an SCTP association, the SCTP endpoint's association
progress from one state to another in response to various events. The
events that may potentially advance an association's state include:

  o SCTP user primitive calls, e.g., [ASSOCIATE], [SHUTDOWN], [ABORT],

  o Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc. control
    chunks, or

  o Some timeout events.

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

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

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

                   (from the ESTABLISHED state only)
                                 |
                                 |
                        /--------+--------\
    [SHUTDOWN]         /                   \
    -------------------|                   |
    check outstanding  |                   |
    DATA chunks        |                   |
                       v                   |
                  +---------+              |
                  |SHUTDOWN-|              | rcv SHUTDOWN/check
                  |PENDING  |              | outstanding DATA
                  +---------+              | chunks
                       |                   |------------------
  No more outstanding  |                   |
  ---------------------|                   |

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  snd SHUTDOWN         |                   |
  strt shutdown timer  |                   |
                       v                   v
                  +---------+        +-----------+
              (4) |SHUTDOWN-|        | SHUTDOWN- |  (5,6)
                  |SENT     |        | RECEIVED  |
                  +---------+        +-----------+
                       |  \                |
 (A) rcv SHUTDOWN ACK  |   \               |
 ----------------------|    \              |
 stop shutdown timer   |     \rcv:SHUTDOWN |
 send SHUTDOWN COMPLETE|      \  (B)       |
 delete TCB            |       \           |
                       |        \          | No more outstanding
                       |         \         |-----------------
                       |          \        | send SHUTDOWN ACK
 (B)rcv SHUTDOWN       |           \       | strt shutdown timer
 ----------------------|            \      |
 send SHUTDOWN ACK     |             \     |
 start shutdown timer  |              \    |
 move to SHUTDOWN-     |               \   |
 ACK-SENT              |                |  |
                       |                v  |
                       |             +-----------+
                       |             | SHUTDOWN- | (7)
                       |             | ACK-SENT  |
                       |             +-----------+
                       |                   | (C)rcv SHUTDOWN COMPLETE
                       |                   |-----------------
                       |                   | stop shutdown timer
                       |                   | delete TCB
                       |                   |
                       |                   | (D)rcv SHUTDOWN ACK
                       |                   |--------------
                       |                   | stop shutdown timer
                       |                   | send SHUTDOWN COMPLETE
                       |                   | delete TCB
                       |                   |
                       \    +---------+    /
                        \-->| CLOSED  |<--/
                            +---------+

           Figure 3: State Transition Diagram of SCTP

Notes:
(1) If the State Cookie in the received COOKIE ECHO is invalid (i.e.,
    failed to pass the integrity check), the receiver MUST silently
    discard the packet. Or, if the received State Cookie is expired
    (see Section 5.1.5), the receiver MUST send back an ERROR chunk.
    In either case, the receiver stays in the CLOSED state.

(2) If the T1-init timer expires, the endpoint MUST retransmit INIT
    and re-start the T1-init timer without changing state. This MUST be
    repeated up to 'Max.Init.Retransmits' times. After that, the

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    endpoint MUST abort the initialization process and report the
    error to SCTP user.

(3) If the T1-cookie timer expires, the endpoint MUST retransmit
    COOKIE ECHO and re-start the T1-cookie timer without changing
    state. This MUST be repeated up to 'Max.Init.Retransmits'
    times. After that, the endpoint MUST abort the initialization
    process and report the error to SCTP user.

(4) In SHUTDOWN-SENT state the endpoint MUST acknowledge any received
    DATA chunks without delay.

(5) In SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new
    send request from its SCTP user.

(6) In SHUTDOWN-RECEIVED state, the endpoint MUST transmit or retransmit
    data and leave this state when all data inqueue is transmitted.

(7) In SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any new
    send request from its SCTP user.

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

5. Association Initialization

Before the first data transmission can take place from one SCTP
endpoint ("A") to another SCTP endpoint ("Z"), the two endpoints must
complete an initialization process in order to set up an SCTP
association between them.

The SCTP user at an endpoint should use the ASSOCIATE primitive to
initialize an SCTP association to another SCTP endpoint.

  IMPLEMENTATION NOTE: From an SCTP-user's point of view, an
  association may be implicitly opened, without an ASSOCIATE primitive
  (see 10.1 B) being invoked, by the initiating endpoint's sending of
  the first user data to the destination endpoint. The initiating SCTP
  will assume default values for all mandatory and optional parameters
  for the INIT/INIT ACK.

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

5.1 Normal Establishment of an Association

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

A) "A" first sends an INIT chunk to "Z". In the INIT, "A" must
   provide its Verification Tag (Tag_A) in the Initiate Tag field.
   Tag_A SHOULD be a random number in the range of 1 to 4294967295

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   (see 5.3.1 for Tag value selection). After sending the INIT, "A"
   starts the T1-init timer and enters the COOKIE-WAIT state.

B) "Z" shall respond immediately with an INIT ACK chunk.  The
   destination IP address of the INIT ACK MUST be set to the source
   IP address of the INIT to which this INIT ACK is responding.  In
   the response, besides filling in other parameters, "Z" must set the
   Verification Tag field to Tag_A, and also provide its own
   Verification Tag (Tag_Z) in the Initiate Tag field.

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

   Note: After sending out INIT ACK with the State Cookie parameter,
   "Z" MUST NOT allocate any resources, nor keep any states for the new
   association. Otherwise, "Z" will be vulnerable to resource attacks.

C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-init
   timer and leave COOKIE-WAIT state. "A" shall then send the State
   Cookie received in the INIT ACK chunk in a COOKIE ECHO chunk, start
   the T1-cookie timer, and enter the COOKIE-ECHOED state.

   Note: The COOKIE ECHO chunk can be bundled with any pending outbound
   DATA chunks, but it MUST be the first chunk in the packet and
   until the COOKIE ACK is returned the sender MUST NOT send any
   other packets to the peer.

D) Upon reception of the COOKIE ECHO chunk, Endpoint "Z" will reply
   with a COOKIE ACK chunk after building a TCB and moving to
   the ESTABLISHED state. A COOKIE ACK chunk may be bundled with
   any pending DATA chunks (and/or SACK chunks), but the COOKIE ACK
   chunk MUST be the first chunk in the packet.

  IMPLEMENTATION NOTE: An implementation may choose to send the
  Communication Up notification to the SCTP user upon reception
  of a valid COOKIE ECHO chunk.

E) Upon reception of the COOKIE ACK, endpoint "A" will move from the
   COOKIE-ECHOED state to the ESTABLISHED state, stopping the T1-cookie
   timer. It may also notify its ULP about the successful
   establishment of the association with a Communication Up
   notification (see Section 10).

An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk.

They MUST be the only chunks present in the SCTP packets that carry
them.

An endpoint MUST send the INIT ACK to the IP address from which it
received the INIT.

  Note: T1-init timer and T1-cookie timer shall follow the same rules
  given in Section 6.3.

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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 MUST respond with an ABORT chunk. It
SHOULD also specify the cause of abort, such as the type of the
missing mandatory parameters, etc., by including the error cause
parameters with the ABORT chunk.  The Verification Tag field in the
common header of the outbound SCTP packet containing the ABORT chunk
MUST be set to the Initiate Tag value of the peer.

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

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

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

5.1.1 Handle Stream Parameters

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

After receiving the stream configuration information from the other
side, each endpoint shall perform the following check:  If the peer's
MIS is less than the endpoint's OS, meaning that the peer is incapable
of supporting all the outbound streams the endpoint wants to
configure, the endpoint MUST either use MIS outbound streams,
or abort the association and report to its upper layer the resources
shortage at its peer.

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

5.1.2 Handle Address Parameters

During the association initialization, an endpoint shall use the
following rules to discover and collect the destination transport
address(es) of its peer.

  A) If there are no address parameters present in the received INIT
  or INIT ACK chunk, the endpoint shall take the source IP address
  from which the chunk arrives and record it, in combination with
  the SCTP source port number, as the only destination transport
  address for this peer.

  B) If there is a Host Name parameter present in the received INIT or

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  INIT ACK chunk, the endpoint shall resolve that host name to a
  list of IP address(es) and derive the transport address(es) of this
  peer by combining the resolved IP address(es) with the SCTP source
  port.

  The endpoint MUST ignore any other IP address parameters if
  they are also present in the received INIT or INIT ACK chunk.

  The time at which the receiver of an INIT resolves the host
  name has potential security implications to SCTP. If the receiver of
  an INIT resolves the host name upon the reception of the chunk, and
  the mechanism the receiver uses to resolve the host name involves
  potential long delay (e.g. DNS query), the receiver may open itself
  up to resource attacks for the period of time while it is waiting for
  the name resolution results before it can build the State Cookie and
  release local resources.

  Therefore, in cases where the name translation involves potential
  long delay, the receiver of the INIT MUST postpone the name
  resolution till the reception of the COOKIE ECHO chunk from the
  peer. In such a case, the receiver of the INIT SHOULD build the
  State Cookie using the received Host Name (instead of destination
  transport addresses) and send the INIT ACK to the source IP
  address from which the INIT was received.

  The receiver of an INIT ACK shall always immediately attempt to
  resolve the name upon the reception of the chunk.

  The receiver of the INIT or INIT ACK MUST NOT send user data
  (piggy-backed or stand-alone) to its peer until the host name is
  successfully resolved.

  If the name resolution is not successful, the endpoint MUST
  immediately send an ABORT with "Unresolvable Address" error cause to
  its peer. The ABORT shall be sent to the source IP address from which
  the last peer packet was received.

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

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

After all transport addresses are derived from the INIT or INIT ACK

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chunk using the above rules, the endpoint shall select one of the
transport addresses as the initial primary path.

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

The sender of INIT may include a 'Supported Address Types'
parameter in the INIT to indicate what types of address are
acceptable. When this parameter is present, the receiver of INIT
(initiatee) MUST either use one of the address types indicated in the
Supported Address Types parameter when responding to the INIT, or
abort the association with an "Unresolvable Address" error cause if it
is unwilling or incapable of using any of the address types indicated
by its peer.

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

5.1.3 Generating State Cookie

When sending an INIT ACK as a response to an INIT chunk, the sender
of INIT ACK creates a State Cookie and sends it in the State Cookie
parameter of the INIT ACK. Inside this State Cookie, the sender should
include a MAC (see [RFC2104] for an example), a time stamp on when the
State Cookie is created, and the lifespan of the State Cookie, along
with all the information necessary for it to establish the association.

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

1) Create an association TCB using information from both the received
   INIT and the outgoing INIT ACK chunk,

2) In the TCB, set the creation time to the current time of day, and
   the lifespan to the protocol parameter 'Valid.Cookie.Life',

3) From the TCB, identify and collect the minimal subset of
   information needed to re-create the TCB, and generate a MAC using
   this subset of information and a secret key (see [RFC2104] for an
   example of generating a MAC), and

4) Generate the State Cookie by combining this subset of information
   and the resultant MAC.

After sending the INIT ACK with the State Cookie parameter, the sender
SHOULD delete the TCB and any other local resource related to the new
association, so as to prevent resource attacks.

The hashing method used to generate the MAC is strictly a
private matter for the receiver of the INIT chunk. The use of a MAC
is mandatory to prevent denial of service attacks. The secret key
SHOULD be random ([RFC1750] provides some information on randomness

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guidelines); it SHOULD be changed reasonably frequently, and the
timestamp in the State Cookie MAY be used to determine which key should
be used to verify the MAC.

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

5.1.4 State Cookie Processing

When an endpoint receives an INIT ACK chunk with a State Cookie
parameter, it MUST immediately send a COOKIE ECHO chunk to its peer
with the received State Cookie.  The sender MAY also add any pending
DATA chunks to the packet after the COOKIE ECHO chunk.

The endpoint shall also start the T1-cookie timer after sending out the
COOKIE ECHO chunk. If the timer expires, the endpoint shall retransmit
the COOKIE ECHO chunk and restart the T1-cookie timer. This is repeated
until either a COOKIE ACK is received or 'Max.Init.Retransmits' is
reached causing the peer endpoint to be marked unreachable (and thus
the association enters the CLOSED state).

5.1.5 State Cookie Authentication

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

1) Compute a MAC using the TCB data carried in the State
   Cookie and the secret key (note the timestamp in the State Cookie
   MAY be used to determine which secret key to use).  Reference
   [RFC2104] can be used as a guideline for generating the MAC,

2) Authenticate the State Cookie as one that it previously generated by
   comparing the computed MAC against the one carried in the
   State Cookie. If this comparison fails, the SCTP packet, including
   the COOKIE ECHO and any DATA chunks, should be silently discarded,

3) Compare the creation timestamp in the State Cookie to the current
   local time. If the elapsed time is longer than the lifespan carried
   in the State Cookie, then the packet, including the COOKIE ECHO and
   any attached DATA chunks, SHOULD be discarded and the endpoint MUST
   transmit an ERROR chunk with a "Stale Cookie" error cause to the
   peer endpoint,

4) If the State Cookie is valid, create an association to the sender of
   the COOKIE ECHO chunk with the information in the TCB data carried
   in the COOKIE ECHO, and enter the ESTABLISHED state,

5) Send a COOKIE ACK chunk to the peer acknowledging reception of
   the COOKIE ECHO. The COOKIE ACK MAY be bundled with an outbound
   DATA chunk or SACK chunk; however, the COOKIE ACK MUST be the first
   chunk in the SCTP packet.

6) Immediately acknowledge any DATA chunk bundled with the COOKIE ECHO

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   with a SACK (subsequent DATA chunk acknowledgement should follow the
   rules defined in Section 6.2).  As mentioned in step 5), if the SACK
   is bundled with the COOKIE ACK, the COOKIE ACK MUST appear first in
   the SCTP packet.

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

5.1.6 An Example of Normal Association Establishment

In the following example, "A" initiates the association and then sends
a user message to "Z", then "Z" sends two user messages to "A" later
(assuming no bundling or fragmentation occurs):

Endpoint A                                          Endpoint Z
{app sets association with Z}
(build TCB)
INIT [I-Tag=Tag_A
      & other info]  --------\
(Start T1-init timer)         \
(Enter COOKIE-WAIT state)      \---> (compose temp TCB and Cookie_Z)

                                /--- INIT ACK [Veri Tag=Tag_A,
                               /               I-Tag=Tag_Z,
(Cancel T1-init timer) <------/                Cookie_Z, & other info]
                                     (destroy temp TCB)
COOKIE ECHO [Cookie_Z] ------\
(Start T1-init timer)         \
(Enter COOKIE-ECHOED state)    \---> (build TCB enter ESTABLISHED
                                      state)

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

                                     ...
                                     {app sends 2 messages;strm 0}
                               /---- DATA
                              /        [TSN=init TSN_Z
                          <--/          Strm=0,Seq=1 & user data 1]
SACK [TSN Ack=init TSN_Z,      /---- DATA
      Block=0]     --------\  /        [TSN=init TSN_Z +1,

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                            \/          Strm=0,Seq=2 & user data 2]
                     <------/\
                              \
                               \------>

                  Figure 4: INITiation Example

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

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

During the lifetime of an association (in one of the possible
states), an endpoint may receive from its peer endpoint one of the
setup chunks (INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK). The
receiver shall treat such a setup chunk as a duplicate and process it
as described in this section.
  Note:  An endpoint will not receive the chunk unless the chunk was
  sent to a SCTP transport address and is from a SCTP transport address
  associated with this endpoint.  Therefore, the endpoint processes
  such a chunk as part of its current association.

The following scenarios can cause duplicated or unexpected chunks:

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

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

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

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

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

The rules in the following sections shall be applied in order to
identify and correctly handle these cases.

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

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This usually indicates an initialization collision, i.e., each
endpoint is attempting, at about the same time, to establish an
association with the other endpoint.

Upon receipt of an INIT in the COOKIE-WAIT or COOKIE-ECHOED state, an
endpoint MUST respond with an INIT ACK using the same parameters it
sent in its original INIT chunk (including its Verification Tag,
unchanged). These original parameters are combined with those from the
newly received INIT chunk. The endpoint shall also generate a State
Cookie with the INIT ACK. The endpoint uses the parameters sent in its
INIT to calculate the State Cookie.

After that, the endpoint MUST NOT change its state, the T1-init
timer shall be left running and the corresponding TCB MUST NOT be
destroyed. The normal procedures for handling State Cookies when
a TCB exists will resolve the duplicate INITs to a single association.

For an endpoint that is in the COOKIE-ECHOED state it MUST populate
its Tie-Tags with the Tag information of itself and its peer (see
section 5.2.2 for a description of the Tie-Tags).

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

Unless otherwise stated, upon reception of an unexpected INIT for this
association, the endpoint shall generate an INIT ACK with a State
Cookie. In the outbound INIT ACK the endpoint MUST copy its current
Verification Tag and Peers Verification tag into a reserved place
within the state cookie. We shall refer to these locations as the
Peers-Tie-Tag and the Local-Tie-Tag. The INIT ACK MUST contain a new
Verification Tag (randomly generated see Section 5.3.1). Other
parameters for the endpoint SHOULD be copied from the existing
parameters of the association (e.g. number of outbound streams) into
the INIT ACK and cookie.

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

Note: Only when a TCB exists and the association is NOT in a
COOKIE-WAIT state are the Tie-Tags populated. For a normal association
INIT (i.e. the endpoint is in a COOKIE-WAIT state), the Tie-Tags MUST
be set to 0 (indicating that no previous TCB existed). The INIT ACK
and State Cookie are populated as specified in section 5.2.1.

5.2.3 Unexpected INIT ACK

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

5.2.4 Handle a COOKIE ECHO when a TCB exists

Internet draft      Stream Control Transmission Protocol       July 2000

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

1) Compute a MAC as described in Step 1 of Section 5.1.5,

2) Authenticate the State Cookie as described in Step 2 of Section
   5.1.5 (this is case C or D above).

3) Compare the timestamp in the State Cookie to the current time. If
   the State Cookie is older than the lifespan carried in the State
   Cookie and the Verification Tags contained in the State Cookie do
   not match the current association's Verification Tags, the packet,
   including the COOKIE ECHO and any DATA chunks, should be discarded.
   The endpoint also MUST transmit an ERROR chunk with a "Stale Cookie"
   error cause to the peer endpoint (this is case C or D above).

   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) even if the lifespan is exceeded.

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

5) If the Verification Tags in the Temporary TCB match the
   Verification Tags in the existing TCB, the State Cookie is a
   duplicate cookie. A COOKIE ACK should be sent to the peer
   endpoint but no update should be made to the existing
   TCB (only the local Verification Tag needs to be compared if
   the peer's Verification Tag is not yet available).

   The endpoint doesn't leave the current state and all timers
   remain running.

6) If either of the Verification Tags do NOT match, refer to the following
   table to determine the correct action to be taken.

  +------------+------------+---------------+--------------+-------------+
  |  Local Tag |  Peers Tag | Local-Tie-Tag | Peers-Tie-Tag|   Action/   |
  |            |            |               |              | Description |
  +------------+------------+---------------+--------------+-------------+
  |    X       |     X      |      M        |      M       |     (A)     |
  +------------+------------+---------------+--------------+-------------+
  |    M       |     A      |      A        |      A       |     (B)     |
  +------------+------------+---------------+--------------+-------------+
  |    X       |     M      |      0        |      0       |     (C)     |
  +------------+------------+---------------+--------------+-------------+
  |    M       |     M      |      A        |      A       |     (D)     |
  +======================================================================+
  |       Table 2: Handling of a Cookie when a TCB exists                |
  +======================================================================+

  Legend:

Internet draft      Stream Control Transmission Protocol       July 2000

     X - Tag does not match the existing TCB
     M - Tag matches the existing TCB.
     0 - No Tie-Tag in Cookie. Cookie (unknown).
     A - All cases match, no-match or unknown i.e. 0. unknown.

Note: For any case not shown in Table 2, the cookie should be
      silently discarded.

Action

(A)In this case, the peer may have restarted. When the endpoint
   recognizes this potential 'restart', the existing session is
   treated the same as if it received an ABORT followed by a new
   Cookie Echo with the following exceptions:

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

    - A notification of RESTART SHOULD be sent to the ULP instead
      of a "COMMUNICATION LOST" notification.

   All the congestion control parameters (e.g., cwnd, ssthresh) related
   to this peer MUST be reset to their initial values (see Section
   6.2.1).

   After this the endpoint shall enter the ESTABLISHED state.

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

(B)In this case, both sides may be attempting to start an
   association at about the same time but the peer endpoint
   started its INIT after responding to the local endpoints
   INIT. Thus it may have picked a new Verification Tag not being aware
   of the previous Tag it had sent this endpoint. The endpoint
   should stay in or enter the Established state but it MUST update
   its peers Verification Tag from the Cookie, stop any init
   or cookie timers that may running and send a Cookie Ack.

(C)In this case, the local endpoints cookie has arrived
   late. Before it arrived the local endpoint, sent
   a INIT and received a INIT-ACK and finally sent a
   Cookie with the peers same tag but a new tag of
   its own. The cookie should be silently discarded.
   The endpoint should NOT change states and should
   leave any timers running.

(D)When both local and remote tags match the endpoint should
   always enter the Established state. It should stop any init
   or cookie timers that may running and send a Cookie Ack.

Internet draft      Stream Control Transmission Protocol       July 2000

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 occured. 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):

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

Internet draft      Stream Control Transmission Protocol       July 2000

5.2.5 Handle Duplicate COOKIE-ACK.

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

5.2.6 Handle Stale COOKIE Error

Receipt of an Operational ERROR chunk with a "Stale Cookie" error
cause indicates one of a number of possible events:

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

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

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

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

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

1) Send a new INIT chunk to the endpoint to generate a new State
   Cookie and re-attempt the setup procedure.

2) Discard the TCB and report to the upper layer the inability to
   setup the association.

3) Send a new INIT chunk to the endpoint, adding a Cookie
   Preservative parameter requesting an extension to the lifetime of
   the State Cookie. When calculating the time extension, an
   implementation SHOULD use the RTT information measured based on the
   previous COOKIE ECHO / ERROR exchange, and should add no more
   than 1 second beyond the measured RTT, due to long State Cookie
   lifetimes making the endpoint more subject to a replay attack.

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

Internet draft      Stream Control Transmission Protocol       July 2000

randomized to help protect against "man in the middle" and "sequence
number" attacks.  The methods described in [RFC1750] can be used for
the Initiate Tag randomization.  Careful selection of Initiate Tags is
also necessary to prevent old duplicate packets from previous
associations being mistakenly processed as belonging to the current
association.

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

6. User Data Transfer

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

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

A SACK MUST be processed in ESTABLISHED, SHUTDOWN-PENDING, and
SHUTDOWN-RECEIVED. An incoming SACK MAY be processed in
COOKIE-ECHOED. A SACK in the CLOSED state is out of the blue
and SHOULD be processed according to the rules in 8.4. A SACK
chunk received in any other state SHOULD be discarded.

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

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

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        ^  |
   ==================|==|=======================================

Internet draft      Stream Control Transmission Protocol       July 2000

                     |  v (1)
          +------------------+    +--------------------+
          | SCTP DATA Chunks |    |SCTP Control Chunks |
          +------------------+    +--------------------+
                     ^  |             ^  |
                     |  v (2)         |  v (2)
                  +--------------------------+
                  |      SCTP packets        |
                  +--------------------------+
    SCTP                      ^  |
   ===========================|==|===========================
                              |  v
          Connectionless Packet Transfer Service (e.g., IP)

   Notes:
   (1) When converting user messages into DATA chunks, an endpoint
       will fragment user messages larger than the current association
       path MTU into multiple DATA chunks. The data receiver will
       normally reassemble the fragmented message from DATA chunks
       before delivery to the user (see Section 6.9 for details).

   (2) Multiple DATA and control chunks may be bundled by the
       sender into a single SCTP packet for transmission, as long as
       the final size of the packet does not exceed the current path
       MTU. The receiver will unbundle the packet back into
       the original chunks. Control chunks MUST come before
       DATA chunks in the packet.

          Figure 5: 6: Illustration of User Data Transfer

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

6.1  Transmission of DATA Chunks

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

The following general rules MUST be applied by the data sender for
transmission and/or retransmission of outbound DATA chunks:

A) At any given time, the data sender MUST NOT transmit new data to any
   destination transport address if its peer's rwnd indicates that the
   peer has no buffer space (i.e. rwnd is 0, see Section 6.2.1).
   However, regardless of the value of rwnd (including if it is 0),
   the data sender can always have one DATA chunk in flight to the
   receiver if allowed by cwnd (see rule B below). This rule
   allows the sender to probe for a change in rwnd that the sender
   missed due to the SACK having been lost in transit from
   the data receiver to the data sender.

Internet draft      Stream Control Transmission Protocol       July 2000

B) At any given time, the sender MUST NOT transmit new data to a
   given transport address if it has cwnd or more bytes of data
   outstanding to that transport address.

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

D) Then, the sender can send out as many new DATA chunks as Rule A and
   Rule B above allow.

Multiple DATA chunks committed for transmission MAY be
bundled in a single packet. Furthermore, DATA chunks being
retransmitted MAY be bundled with new DATA chunks, as long as the
resulting packet size does not exceed the path MTU. A ULP
may request that no bundling is performed but this should only turn off
any delays that a SCTP implementation may be using to increase
bundling efficiency.  It does not in itself stop all bundling
from occurring (i.e. in case of congestion or retransmission).

Before an endpoint transmits a DATA chunk, if any received DATA
chunks have not been acknowledged (e.g., due to delayed ack), the
sender should create a SACK and bundle it with the outbound DATA
chunk, as long as the size of the final SCTP packet does not exceed
the current MTU. See Section 6.2.

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

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

When starting or restarting the T3-rtx timer, the timer value must be
adjusted according to the timer rules defined in Sections 6.3.2,
and 6.3.3.

  Note: The data sender SHOULD NOT use a TSN that is more than
  2**31 - 1 above the beginning TSN of the current send window.

6.2  Acknowledgement on Reception of DATA Chunks

The SCTP endpoint MUST always acknowledge the reception of each valid
DATA chunk.

The guidelines on delayed acknowledgement algorithm specified in

Internet draft      Stream Control Transmission Protocol       July 2000

Section 4.2 of [RFC2581] SHOULD be followed. Specifically, an
acknowledgement SHOULD be generated for at least every second packet
(not every second DATA chunk) received, and SHOULD be generated within
200 ms of the arrival of any unacknowledged DATA chunk. In some
situations it may be beneficial for an SCTP transmitter to be more
conservative than the algorithms detailed in this document allow.
However, an SCTP transmitter MUST NOT be more aggressive than the
following algorithms allow.

A SCTP receiver MUST NOT generate more than one SACK for every
incoming packet, other than to update the offered window as the
receiving application consumes new data.

  IMPLEMENTATION NOTE: The maximum delay for generating an
  acknowledgement may be configured by the SCTP administrator, either
  statically or dynamically, in order to meet the specific
  timing requirement of the protocol being carried.

An implementation MUST NOT allow the maximum delay to be configured to
be more than 500 ms. In other words an implementation MAY lower this
value below 500ms but MUST NOT raise it above 500ms.

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

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

When a packet arrives with duplicate DATA chunk(s) and with no new
DATA chunk(s), the endpoint MUST immediately send a SACK with no
delay. If a packet arrives with duplicate DATA chunk(s) bundled with
new DATA chunks, the endpoint MAY immediately send a SACK.  Normally
receipt of duplicate DATA chunks will occur when the original SACK
chunk was lost and the peer's RTO has expired. The duplicate TSN
number(s) SHOULD be reported in the SACK as duplicate.

When an endpoint receives a SACK, it MAY use the Duplicate TSN
information to determine if SACK loss is occurring. Further use of
this data is for future study.

The data receiver is responsible for maintaining its receive buffers.
The data receiver SHOULD notify the data sender in a timely manner of
changes in its ability to receive data.  How an implementation manages
its receive buffers is dependent on many factors (e.g., Operating
System, memory management system, amount of memory, etc.).  However,

Internet draft      Stream Control Transmission Protocol       July 2000

the data sender strategy defined in Section 6.2.1 is based on the
assumption of receiver operation similar to the following:

      A) At initialization of the association, the endpoint tells the
      peer how much receive buffer space it has allocated to the
      association in the INIT or INIT ACK.  The endpoint sets a_rwnd
      to this value.

      B) As DATA chunks are received and buffered, decrement a_rwnd by
      the number of bytes received and buffered.  This is, in effect,
      closing rwnd at the data sender and restricting the amount of
      data it can transmit.

      C) As DATA chunks are delivered to the ULP and released from the
      receive buffers, increment a_rwnd by the number of bytes
      delivered to the upper layer.  This is, in effect, opening up
      rwnd on the data sender and allowing it to send more data.  The
      data receiver SHOULD NOT increment a_rwnd unless it has released
      bytes from its receive buffer.  For example, if the receiver is
      holding fragmented DATA chunks in a reassembly queue, it should
      not increment a_rwnd.

      D)  When sending a SACK, the data receiver SHOULD place the
      current value of a_rwnd into the a_rwnd field.  The data
      receiver SHOULD take into account that the data sender will not
      retransmit DATA chunks that are acked via the Cumulative TSN Ack
      (i.e., will drop from its retransmit queue).

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

An endpoint SHOULD NOT revoke a SACK and discard data. Only in extreme
circumstance should an endpoint use this procedure (such as out of buffer
space).  The data receiver should take into account that dropping data that
has been acked in Gap Ack Blocks can result in suboptimal retransmission
strategies in the data sender and thus in suboptimal performance.

The following example illustrates the use of delayed acknowledgements:

Endpoint A                                      Endpoint Z

{App sends 3 messages; strm 0}
DATA [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
(Start T3-rtx timer)

Internet draft      Stream Control Transmission Protocol       July 2000

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 5: 7:  Delayed Acknowledgment Example

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

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

6.2.1  Processing a Received SACK

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

One of the problems the data sender must take into account when processing
a SACK is that a SACK can be received out of order.  That is, a SACK sent
by the data receiver can pass an earlier SACK and be received first by the
data sender.  If a SACK is received out of order, the data sender can
develop an incorrect view of the peer's receive buffer space.

Since there is no explicit identifier that can be used to detect
out-of-order SACKs, the data sender must use heuristics to determine if a
SACK is new.

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

A) At the establishment of the association, the endpoint
    initializes the rwnd to the Advertised Receiver Window

Internet draft      Stream Control Transmission Protocol       July 2000

    Credit (a_rwnd) the peer specified in the INIT or INIT ACK.

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

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

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

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

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

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

    iii) If the SACK is missing a TSN that was previously
    acknowledged via a Gap Ack Block (e.g., the data receiver
    reneged on the data), then mark the corresponding DATA chunk
    as available for retransmit:  Mark it as missing for fast
    retransmit as described in Section 7.2.4 and if no retransmit
    timer is running for the destination address to which the DATA
    chunk was originally transmitted, then T3-rtx is started for
    that destination address.

6.3 Management of Retransmission Timer

An SCTP endpoint uses a retransmission timer T3-rtx to ensure data
delivery in the absence of any feedback from its peer. The duration of
this timer is referred to as RTO (retransmission timeout).

When an endpoint's peer is multi-homed, the endpoint will calculate a
separate RTO for each different destination transport address of its
peer endpoint.

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

6.3.1 RTO Calculation

Internet draft      Stream Control Transmission Protocol       July 2000

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

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

C2) When the first RTT measurement R is made, set SRTT <- R,
    RTTVAR <- R/2, and RTO <- SRTT + 4 * RTTVAR.

C3) When a new RTT measurement R' is made, set

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

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

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

C4) When data is in flight and when allowed by rule C5 below, a new
    RTT measurement MUST be made each round trip.  Furthermore, new RTT
    measurements SHOULD be made no more than once per round-trip for a
    given destination transport address. There are two reasons for this
    recommendation:  First, it appears that measuring more frequently
    often does not in practice yield any significant benefit
    [ALLMAN99]; second, if measurements are made more often, then the
    values of RTO.Alpha and RTO.Beta in rule C3 above should be
    adjusted so that SRTT and RTTVAR still adjust to changes at roughly
    the same rate (in terms of how many round trips it takes them to
    reflect new values) as they would if making only one measurement
    per round-trip and using RTO.Alpha and RTO.Beta as given in rule
    C3. However, the exact nature of these adjustments remains a
    research issue.

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

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

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

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

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

Internet draft      Stream Control Transmission Protocol       July 2000

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

6.3.2 Retransmission Timer Rules

The rules for managing the retransmission timer are as follows:

R1) Every time a DATA chunk is sent to any address (including
    a retransmission), if the T3-rtx timer of that address is not
    running, start it running so that it will expire after the RTO of
    that address. The RTO used here is that obtained after any doubling
    due to previous T3-rtx timer expirations on the corresponding
    destination address as discussed in rule E2 below.

R2) Whenever all outstanding data sent to an address have been
    acknowledged, turn off the T3-rtx timer of that address.

R3) Whenever a SACK is received that acknowledges the DATA chunk with
    the earliest outstanding TSN for that address, restart T3-rtx timer
    for that address with its current RTO.

(R4) Whenever a SACK is received missing a TSN that was previously acknowledged
     via a Gap Ack Block, start T3-rtx for the destination address to which
     the DATA chunk was originally transmitted if it is not already running.

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

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

              Figure 6 8 - Timer Rule Examples

6.3.3 Handle T3-rtx Expiration

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Whenever the retransmission timer T3-rtx expires for a destination
address, do the following:

E1) For the destination address for which the timer expires, adjust its
    ssthresh with rules defined in Section 7.2.3 and set the
    cwnd <- MTU.

E2) For the destination address for which the timer expires, set
    RTO <- RTO * 2 ("back off the timer"). The maximum value discussed
    in rule C7 above (RTO.max) may be used to provide an upper bound
    to this doubling operation.

E3) Determine how many of the earliest (i.e., lowest TSN) outstanding
    DATA chunks for the address for which the T3-rtx has expired will
    fit into a single packet, subject to the MTU constraint for the
    path corresponding to the destination transport address to which
    the retransmission is being sent (this may be different from the
    address for which the timer expires [see Section 6.4]). Call this
    value K. Bundle and retransmit those K DATA chunks in a single
    packet to the destination endpoint.

E4) Start the retransmission timer T3-rtx on the destination address
    to which the retransmission is sent, if rule R1 above indicates to
    do so.  The RTO to be used for starting T3-rtx should be the
    one for the destination address to which the retransmission is
    sent, which, when the receiver is multi-homed, may be different
    from the destination address for which the timer expired (see
    Section 6.4 below).

After retransmitting, once a new RTT measurement is obtained
(which can happen only when new data has been sent and acknowledged,
per rule C5, or for a measurement made from a HEARTBEAT [see Section
8.3]), the computation in rule C3 is performed, including the
computation of RTO, which may result in "collapsing" RTO back down
after it has been subject to doubling (rule E2).

  Note: Any DATA chunks that were sent to the address for which the
  T3-rtx timer expired but did not fit in one MTU (rule E3 above),
  should be marked for retransmission and sent as soon as cwnd allows
  (normally when a SACK arrives).

The final rule for managing the retransmission timer concerns failover
(see Section 6.4.1):

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.

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6.4 Multi-homed SCTP Endpoints

An SCTP endpoint is considered multi-homed if there are more than one
transport address that can be used as a destination address to reach
that endpoint.

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

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

An endpoint SHOULD transmit reply chunks (e.g., SACK, HEARTBEAT ACK,
etc.) to the same destination transport address from which it received
the DATA or control chunk to which it is replying. This rule should
also be followed if the endpoint is bundling DATA chunks together
with the reply chunk.

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

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

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

Retransmissions do not affect the total outstanding data
count. However, if the DATA chunk is retransmitted onto a different
destination address, both the outstanding data counts on the new
destination address and the old destination address to which the data
chunk was last sent shall be adjusted accordingly.

6.4.1 Failover from Inactive Destination Address

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

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

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send the data to an alternate active destination transport address if
one exists.

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

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

6.5 Stream Identifier and Stream Sequence Number

Every DATA chunk MUST carry a valid stream identifier. If an endpoint
receives a DATA chunk with an invalid stream identifier, it shall
acknowledge the reception of the DATA chunk following the normal
procedure, immediately send an ERROR chunk with cause set to "Invalid
Stream Identifier" (see Section 3.3.10) and discard the DATA chunk.
The endpoint may bundle the ERROR chunk in the same packet as the SACK
as long as the ERROR follows the SACK.

The stream sequence number in all the streams shall start from 0
when the association is established. Also, when the stream sequence
number reaches the value 65535 the next stream sequence number shall
be set to 0.

6.6 Ordered and Unordered Delivery

Within a stream, an endpoint MUST deliver DATA chunks received with the
U flag set to 0 to the upper layer according to the order of their
stream sequence number. If DATA chunks arrive out of order of their
stream sequence number, the endpoint MUST hold the received DATA chunks
from delivery to the ULP until they are re-ordered.

However, an SCTP endpoint can indicate that no ordered delivery is
required for a particular DATA chunk transmitted within the stream by
setting the U flag of the DATA chunk to 1.

When an endpoint receives a DATA chunk with the U flag set to 1, it
must bypass the ordering mechanism and immediately deliver the data to
the upper layer (after re-assembly if the user data is fragmented by
the data sender).

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

  IMPLEMENTATION NOTE: When sending an unordered DATA chunk, an
  implementation may choose to place the DATA chunk in an outbound

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  packet that is at the head of the outbound transmission queue if
  possible.

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

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

6.7 Report Gaps in Received DATA TSNs

Upon the reception of a new DATA chunk, an endpoint shall examine
the continuity of the TSNs received. If the endpoint detects a gap
in the received DATA chunk sequence, it SHOULD send a SACK with Gap Ack
Blocks immediately. The data receiver continues sending a SACK after
receipt of each SCTP packet that doesn't fill the gap.

Based on the Gap Ack Block from the received SACK, the endpoint
can calculate the missing DATA chunks and make decisions on whether to
retransmit them (see Section 6.2.1 for details).

Multiple gaps can be reported in one single SACK (see Section 3.3.4).

When its peer is multi-homed, the SCTP endpoint SHOULD always
try to send the SACK to the same destination address from which the
last DATA chunk was received.

Upon the reception of a SACK, the endpoint MUST remove all DATA
chunks which have been acknowledged by the SACK's Cumulative TSN Ack
from its transmit queue. The endpoint MUST also treat all the DATA
chunks with TSNs not included in the Gap Ack Blocks reported by the
SACK as "missing". The number of "missing" reports for each outstanding
DATA chunk MUST be recorded by the data sender in order to make
retransmission decisions.  See Section 7.2.4 for details.

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

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

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

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

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           Figure 8 9 - Reporting a Gap using SACK

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

6.8 Adler-32 Checksum Calculation

When sending an SCTP packet, the endpoint MUST strengthen the data
integrity of the transmission by including the Adler-32 checksum
value calculated on the packet, as described below.

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

1) Fill in the proper Verification Tag in the SCTP common header and
   initialize the checksum field to 0's.

2) Calculate the Adler-32 checksum of the whole packet, including the
   SCTP common header and all the chunks. Refer to appendix B
   for details of the Adler-32 algorithm. And,

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

When an SCTP packet is received, the receiver MUST first check the
Adler-32 checksum:

1) Store the received Adler-32 checksum value aside,

2) Replace the 32 bits of the checksum field in the received
   SCTP packet with all '0's and calculate an Adler-32 checksum
   value of the whole received packet. And,

3) Verify that the calculated Adler-32 checksum is the same as the
   received Adler-32 checksum, If not, the receiver MUST treat the
   packet as an invalid SCTP packet.

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

6.9 Fragmentation and Reassembly

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

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

If its peer is multi-homed, the endpoint shall choose a
size no larger than the association Path MTU. The association Path
MTU is the smallest Path MTU of all destination addresses.

  Note: Once a message is fragmented it cannot be re-fragmented.
  Instead if the PMTU has been reduced, then IP fragmentation must be
  used.  Please see Section 7.3 for details of PMTU discovery.

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

Fragmentation takes the following steps:

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

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

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

An endpoint MUST recognize fragmented DATA chunks by examining the B/E
bits in each of the received DATA chunks, and queue the fragmented DATA
chunks for re-assembly. Once the user message is reassembled, SCTP
shall pass the re-assembled user message to the specific stream for
possible re-ordering and final dispatching.

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

6.10 Bundling

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

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

When bundling control chunks with DATA chunks, an endpoint MUST place
control chunks first in the outbound SCTP packet. The transmitter
MUST transmit DATA chunks within a SCTP packet in increasing order of
TSN.
  Note:  Since control chunks must be placed first in a packet and
  since DATA chunks must be transmitted before SHUTDOWN or SHUTDOWN ACK
  chunks, DATA chunks cannot be bundled with SHUTDOWN or SHUTDOWN ACK
  chunks.

Partial chunks MUST NOT be placed in an SCTP packet.

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

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

7. Congestion control

Congestion control is one of the basic functions in SCTP.
For some applications, it may be likely that adequate resources will
be allocated to SCTP traffic to assure prompt delivery of
time-critical data - thus it would appear to be unlikely, during
normal operations, that transmissions encounter severe congestion
conditions. However SCTP must operate under adverse operational
conditions, which can develop upon partial network failures or
unexpected traffic surges.  In such situations SCTP must follow correct
congestion control steps to recover from congestion quickly in order
to get data delivered as soon as possible.  In the absence of network
congestion, these preventive congestion control algorithms should show
no impact on the protocol performance.

  IMPLEMENTATION NOTE: As far as its specific performance requirements
  are met, an implementation is always allowed to adopt a more
  conservative congestion control algorithm than the one defined
  below.

The congestion control algorithms used by SCTP are based on
[RFC2581].  This section describes how the algorithms defined in
RFC2581 are adapted for use in SCTP.  We first list differences in
protocol designs between TCP and SCTP, and then describe SCTP's
congestion control scheme.  The description will use the same
terminology as in TCP congestion control whenever appropriate.

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

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7.1 SCTP Differences from TCP Congestion control

Gap Ack Blocks in the SCTP SACK carry the same semantic meaning as the
TCP SACK. TCP considers the information carried in the SACK as advisory
information only. SCTP considers the information carried in the Gap Ack
Blocks in the SACK chunk as advisory.  In SCTP, any DATA chunk that has
been acknowledged by SACK, including DATA that arrived at the receiving
end out of order, are NOT considered fully delivered until the
Cumulative TSN Ack Point passes the TSN of the DATA chunk (i.e., the

DATA chunk has been acknowledged by the Cumulative TSN Ack field in the
SACK). Consequently, the value of cwnd controls the amount of
outstanding data, rather than (as in the case of non-SACK TCP) the
upper bound between the highest acknowledged sequence number and the
latest DATA chunk that can be sent within the congestion window. SCTP
SACK leads to different implementations of fast-retransmit and fast-
recovery than non-SACK TCP. As an example see [FALL96].

The biggest difference between SCTP and TCP, however, is multi-homing.
SCTP is designed to establish robust communication associations
between two endpoints each of which may be reachable by more than one
transport address.  Potentially different addresses may lead to
different data paths between the two endpoints, thus ideally one may
need a separate set of congestion control parameters for each of the
paths.  The treatment here of congestion control for multi-homed
receivers is new with SCTP and may require refinement in the
future. The current algorithms make the following assumptions:

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

o The sender keeps a separate congestion control parameter set for each
  of the destination addresses it can send to (NOT each
  source-destination pair but for each destination) . The parameters
  should decay if the address is not used for a long enough
  time period.

o For each of the destination addresses, an endpoint does slow-start
  upon the first transmission to that address.

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

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7.2 SCTP Slow-Start and Congestion Avoidance

The slow start and congestion avoidance algorithms MUST be used by an
endpoint to control the amount of data being injected into the network.
The congestion control in SCTP is employed in regard to the
association, not to an individual stream.  In some situations it
may be beneficial for an SCTP sender to be more conservative than the
algorithms allow; however, an SCTP sender MUST NOT be more aggressive
than the following algorithms allow.

Like TCP, an SCTP endpoint uses the following three control variables
to regulate its transmission rate.

o Receiver advertised window size (rwnd, in bytes), which is set by
  the receiver based on its available buffer space for incoming
  packets.

  Note: This variable is kept on the entire association.

o Congestion control window (cwnd, in bytes), which is adjusted by
  the sender based on observed network conditions.

  Note: This variable is maintained on a per-destination address basis.

o Slow-start threshold (ssthresh, in bytes), which is used by the
  sender to distinguish slow start and congestion avoidance phases.

  Note: This variable is maintained on a per-destination address basis.

SCTP also requires one additional control variable,
partial_bytes_acked, which is used during congestion avoidance phase to
facilitate cwnd adjustment.

Unlike TCP, an SCTP sender MUST keep a set of these control variables
for EACH destination address of its peer (when its peer is multi-
homed).

7.2.1 Slow-Start

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

o The initial cwnd before data transmission or after a sufficiently
  long idle period MUST be <= 2*MTU.

o The initial cwnd after a retransmission timeout MUST be no more
  than 1*MTU.

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o The initial value of ssthresh MAY be arbitrarily high (for example,
  implementations MAY use the size of the receiver advertised window).

o Whenever cwnd is greater than zero, the endpoint is allowed to have
  cwnd bytes of data outstanding on that transport address.

o When cwnd is less than or equal to ssthresh an SCTP endpoint MUST use
  the slow start algorithm to increase cwnd (assuming the current
  congestion window is being fully utilized). If an incoming SACK
  advances the Cumulative TSN Ack Point, cwnd MUST be increased by at

  most the lesser of 1) the total size of the previously outstanding
  DATA chunk(s) acknowledged, and 2) the destination's path MTU.
  This protects against the ACK-Splitting attack outlined in
  [SAVAGE99].

  In instances where its peer endpoint is multi-homed, if an endpoint
  receives a SACK that advances its Cumulative TSN Ack Point, then it
  should update its cwnd (or cwnds) apportioned to the destination
  addresses to which it transmitted the acknowledged data. However if
  the received SACK does not advance the Cumulative TSN Ack Point, the
  endpoint MUST NOT adjust the cwnd of any of the destination
  addresses.

  Because an endpoint's cwnd is not tied to its Cumulative TSN Ack
  Point, as duplicate SACKs come in, even though they may not advance
  the Cumulative TSN Ack Point an endpoint can still use them to clock
  out new data.  That is, the data newly acknowledged by the SACK
  diminishes the amount of data now in flight to less than cwnd; and so
  the current, unchanged value of cwnd now allows new data to be sent.
  On the other hand, the increase of cwnd must be tied to the
  Cumulative TSN Ack Point advancement as specified above.  Otherwise
  the duplicate SACKs will not only clock out new data, but also will
  adversely clock out more new data than what has just left the
  network, during a time of possible congestion.

o When the endpoint does not transmit data on a given transport
  address, the cwnd of the transport address should be adjusted to
  max(cwnd/2, 2*MTU) per RTO.

7.2.2 Congestion Avoidance

When cwnd is greater than ssthresh, cwnd should be incremented
by 1*MTU per RTT if the sender has cwnd or more bytes of data
outstanding for the corresponding transport address.

In practice an implementation can achieve this goal in the
following way:

o partial_bytes_acked is initialized to 0.

o Whenever cwnd is greater than ssthresh, upon each SACK arrival that
  advances the Cumulative TSN Ack Point, increase partial_bytes_acked

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  by the total number of bytes of all new chunks acknowledged in that
  SACK including chunks acknowledged by the new Cumulative TSN Ack and
  by Gap Ack Blocks.

o When partial_bytes_acked is equal to or greater than cwnd and before
  the arrival of the SACK the sender had cwnd or more bytes of data
  outstanding (i.e., before arrival of the SACK, flightsize was greater
  than or equal to cwnd), increase cwnd by MTU, and reset
  partial_bytes_acked to (partial_bytes_acked - cwnd).

o Same as in the slow start, when the sender does not transmit data on
  a given transport address, the cwnd of the transport address should
  be adjusted to max(cwnd / 2, 2*MTU) per RTO.

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

7.2.3 Congestion Control

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

  ssthresh = max(cwnd/2, 2*MTU)
  cwnd = ssthresh

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

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

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

and assure that no more than one DATA chunk will be in flight for that
address until the endpoint receives acknowledgement for successful
delivery of data to that address.

7.2.4 Fast Retransmit on Gap Reports

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

Whenever an endpoint receives a SACK that indicates some TSN(s)
missing, it SHOULD wait for 3 further miss indications (via subsequent
SACK's) on the same TSN(s) before taking action with regard to Fast
Retransmit.

When the TSN(s) is reported as missing in the fourth consecutive SACK,
the data sender shall:

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1) Mark the missing DATA chunk(s) for retransmission,

2) Adjust the ssthresh and cwnd of the destination address(es) to which
   the missing DATA chunks were last sent, according to the formula
   described in Section 7.2.3.

3) Determine how many of the earliest (i.e., lowest TSN) DATA
   chunks marked for retransmission will fit into a single packet,
   subject to constraint of the path MTU of the destination transport
   address to which the packet is being sent. Call this value K.
   Retransmit those K DATA chunks in a single packet.

4) Restart T3-rtx timer only if the last SACK acknowledged the lowest
   outstanding TSN number sent to that address, or the endpoint is
   retransmitting the first outstanding DATA chunk sent to that
   address.

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

A straightforward implementation of the above keeps a counter for each
TSN hole reported by a SACK. The counter increments for each
consecutive SACK reporting the TSN hole.  After reaching 4 and starting
the fast retransmit procedure, the counter resets to 0.

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

7.3 Path MTU Discovery

[RFC1191] specifies "Path MTU Discovery", whereby an endpoint
maintains an estimate of the maximum transmission unit (MTU) along a
given Internet path and refrains from sending packets along that path
which exceed the MTU, other than occasional attempts to probe for a
change in the Path MTU (PMTU).  RFC 1191 is thorough in its discussion
of the MTU discovery mechanism and strategies for determining the
current end-to-end MTU setting as well as detecting changes in this
value.  [RFC1981] specifies the same mechanisms for IPv6. An SCTP
sender using IPv6 MUST use Path MTU Discovery unless all packets are
less than the minimum IPv6 MTU [RFC2460].

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

There are 4 ways in which SCTP differs from the description in RFC 1191
of applying MTU discovery to TCP:

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

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2)  Elsewhere in this document, when the term "MTU" is discussed,
    it refers to the MTU associated with the destination address
    corresponding to the context of the discussion.

3)  Unlike TCP, SCTP does not have a notion of "Maximum Segment
    Size".  Accordingly, the MTU for each destination address
    SHOULD be initialized to a value no larger than the link MTU
    for the local interface to which packets for that remote
    destination address will be routed.

4)  Since data transmission in SCTP is naturally structured in
    terms of TSNs rather than bytes (as is the case for TCP), the
    discussion in Section 6.5 of RFC 1191 applies: When retransmitting
    an IP datagram to a remote address for which the IP datagram
    appears too large for the path MTU to that address, the IP datagram
    SHOULD be retransmitted without the DF bit set, allowing it to
    possibly be fragmented. Transmissions of new IP datagrams MUST have
    DF set.

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

Other than these differences, the discussion of TCP's use of MTU
discovery in RFCs 1191 and 1981 applies to SCTP on a
per-destination-address basis.

  Note: For IPv6 destination addresses the DF bit does not exist,
  instead the IP datagram must be fragmented as described in [RFC2460].

8.  Fault Management

8.1 Endpoint Failure Detection

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

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

8.2 Path Failure Detection

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

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

When an outstanding TSN is acknowledged or a HEARTBEAT sent to that
address is acknowledged with a HEARTBEAT ACK, the endpoint shall
clear the error counter of the destination transport address
to which the DATA chunk was last sent (or HEARTBEAT was sent). When the
peer endpoint is multi-homed and the last chunk sent to it was a
retransmission to an alternate address, there exists an ambiguity as to
whether or not the acknowledgement should be credited to the address of
the last chunk sent. However, this ambiguity does not seem to bear any
significant consequence to SCTP behavior. If this ambiguity is
undesirable, the transmitter may choose not to clear the
error counter if the last chunk sent was a retransmission.

  Note: When configuring the SCTP endpoint, the user should avoid
  having the value of 'Association.Max.Retrans' larger than the
  summation of the 'Path.Max.Retrans' of all the destination addresses
  for the remote endpoint. Otherwise, all the destination addresses may
  become inactive while the endpoint still considers the peer endpoint
  reachable. When this condition occurs, how the SCTP chooses to
  function is implementation specific.

When the primary path is marked inactive (due to excessive
retransmissions, for instance), the sender MAY automatically transmit
new packets to an alternate destination address if one exists and is
active. If more than one alternate address is active when the primary
path is marked inactive only ONE transport address SHOULD be chosen
and used as the new destination transport address.

8.3 Path Heartbeat

By default, an SCTP endpoint shall monitor the reachability of the
idle destination transport address(es) of its peer by sending a
HEARTBEAT chunk periodically to the destination transport
address(es).

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

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addresses.

The upper layer can optionally initiate the following functions:

A) Disable heartbeat on a specific destination transport address of a
   given association,
B) Change the HB.interval,
C) Re-enable heartbeat on a specific destination transport address of
   a given association, and,
D) Request an on-demand HEARTBEAT on a specific destination transport
   address of a given association.

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

When the value of this counter reaches the protocol parameter
'Path.Max.Retrans', the endpoint should mark the corresponding
destination address as inactive if it is not so marked, and may also
optionally report to the upper layer the change of reachability of
this destination address. After this, the endpoint should continue
HEARTBEAT on this destination address but should stop increasing the
counter.

The sender of the HEARTBEAT chunk should include in the Heartbeat
Information field of the chunk the current time when the packet is
sent out and the destination address to which the packet is sent.

   IMPLEMENTATION NOTE: An alternative implementation of the heartbeat
   mechanism that can be used is to increment the error counter
   variable every time a HEARTBEAT is sent to a destination. Whenever
   a HEARTBEAT ACK arrives, the sender SHOULD clear the
   error counter of the destination that the HEARTBEAT was
   sent to. This in effect would clear the previously stroked
   error (and any other error counts as well).

The receiver of the HEARTBEAT should immediately respond with a
HEARTBEAT ACK that contains the Heartbeat Information field copied
from the received HEARTBEAT chunk.

Upon the receipt of the HEARTBEAT ACK, the sender of the HEARTBEAT
should clear the error counter of the destination transport
address to which the HEARTBEAT was sent, and mark the destination
transport address as active if it is not so marked. The endpoint may
optionally report to the upper layer when an inactive destination
address is marked as active due to the reception of the latest
HEARTBEAT ACK. The receiver of the HEARTBEAT ACK must also
clear the association overall error count as well (as defined
in section 8.1).

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

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

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

Note: When tuning the heartbeat interval, there is a side effect that
SHOULD be taken into account. When this value is increased, i.e.  the
HEARTBEAT takes longer, the detection of lost ABORT messages takes
longer as well. If a peer endpoint ABORTs the association for
any reason and the ABORT chunk is lost, the local endpoint will only
discover the lost ABORT by sending a DATA chunk or HEARTBEAT chunk
(thus causing the peer to send another ABORT). This must be considered
when tuning the HEARBEAT timer. If the HEARTBEAT is disabled only
sending DATA to the association will discover a lost ABORT from the
peer.

8.4 Handle "Out of the blue" Packets

An SCTP packet is called an "out of the blue" (OOTB) packet if it
is correctly formed, i.e., passed the receiver's Adler-32 check (see
Section 6.8), but the receiver is not able to identify the association
to which this packet belongs.

The receiver of an OOTB packet MUST do the following:

1) If the OOTB packet is to or from a non-unicast address, silently
   discard the packet. Otherwise,

2) If the OOTB packet contains an ABORT chunk, the receiver MUST
   silently discard the OOTB packet and take no further action.
   Otherwise,

3) If the packet contains an INIT chunk with a Verification Tag set to
   '0', process it as described in Section 5.1. Otherwise,

4) If the packet contains a COOKIE ECHO in the first chunk, process it
   as described in Section 5.1. Otherwise,

5) If the packet contains a SHUTDOWN ACK chunk, the receiver should
   respond to the sender of the OOTB packet with a SHUTDOWN COMPLETE.
   When sending the SHUTDOWN COMPLETE, the receiver of the OOTB packet
   must fill in the Verification Tag field of the outbound packet with
   the Verification Tag received in the SHUTDOWN ACK and set the
   T-bit in the Chunk Flags to indicate that no TCB was found.

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   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 receiver is in COOKIE-ECHOED or COOKIE-WAIT state the procedures
   in section 5 SHOULD be followed. Otherwise,

8) If the packet contains a "Stale cookie" ERROR or a COOKIE ACK
   the SCTP Packet should be silently discarded. Otherwise,

9)

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

8.5 Verification Tag

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

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

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

8.5.1 Exceptions in Verification Tag Rules

A) Rules for packet carrying INIT:

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

 - When an endpoint receives an SCTP packet with the Verification Tag
   set to 0, it should verify that the packet contains only an INIT
   chunk. Otherwise, the receiver MUST silently discard the packet.

B) Rules for packet carrying ABORT:

 - The endpoint shall always fill in the Verification Tag field of the
   outbound packet with the destination endpoint's tag value if it
   is known.

 - If the ABORT is sent in response to an OOTB packet, the endpoint

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   MUST follow the procedure described in Section 8.4.

 - The receiver MUST accept the packet if the Verification Tag
   matches either its own tag, OR the tag of its peer. Otherwise, the
   receiver MUST silently discard the packet and take no further
   action.

C) Rules for packet carrying SHUTDOWN COMPLETE:

 - When sending a SHUTDOWN COMPLETE, if the receiver of the SHUTDOWN
   ACK has a TCB then the destination endpoint's tag MUST be used. Only
   where no TCB exists should the sender use the Verification Tag from
   the SHUTDOWN ACK.

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

D) Rules for packet carrying a COOKIE ECHO

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

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

E) Rules for packet carrying a SHUTDOWN ACK

- If the receiver is in COOKIE-ECHOED or COOKIE-WAIT state the procedures
  in section 8.4 SHOULD be followed, in other words it should be
  treated as an Out Of The Blue packet.

9. Termination of Association

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

9.1 Abort of an Association

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When an endpoint decides to abort an existing association, it
shall send an ABORT chunk to its peer endpoint. The sender MUST fill
in the peer's Verification Tag in the outbound packet and MUST NOT
bundle any DATA chunk with the ABORT.

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

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

After checking the Verification Tag, the receiving endpoint shall
remove the association from its record, and shall report the
termination to its upper layer.

9.2 Shutdown of an Association

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

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

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

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

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

Upon the reception of the SHUTDOWN, the peer endpoint shall
  - enter the SHUTDOWN-RECEIVED state,

  - stop accepting new data from its SCTP user

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

Once an endpoint as reached the SHUTDOWN-RECEIVED state it MUST NOT
send a SHUTDOWN in response to a ULP request. And should discard
subsequent SHUTDOWN chunks.

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

While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
respond to each received packet containing one or more DATA chunk(s)
with a SACK, a SHUTDOWN chunk, and restart the T2-shutdown timer.
If it has no more outstanding DATA chunks, the SHUTDOWN receiver shall
send a SHUTDOWN ACK and start a T2-shutdown timer of its own, entering
the SHUTDOWN-ACK-SENT state. If the timer expires, the endpoint must
re-send the SHUTDOWN ACK.

The sender of the SHUTDOWN ACK should limit the number of
retransmissions of the SHUTDOWN ACK chunk to the protocol parameter
'Association.Max.Retrans'. If this threshold is exceeded the endpoint
should destroy the TCB and may report the peer endpoint unreachable to
the upper layer (and thus the association enters the CLOSED state).

Upon the receipt of the SHUTDOWN ACK, the SHUTDOWN sender shall stop
the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its
peer, and remove all record of the association.

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

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

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

If an endpoint is in SHUTDOWN-ACK-SENT state and receives an INIT chunk
(e.g., if the SHUTDOWN COMPLETE was lost) with source and destination
transport addresses (either in the IP addresses or in the INIT chunk)
that belong to this association, it should discard the INIT chunk and
retransmit the SHUTDOWN ACK chunk.
  Note: Receipt of an INIT with the same source and destination IP
  addresses as used in transport addresses assigned to an endpoint but
  with a different port number indicates the initialization of a
  separate association.

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The sender of the INIT or COOKIE 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.

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

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

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

10. Interface with Upper Layer

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

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

10.1 ULP-to-SCTP

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

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

A) Initialize

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

This primitive allows SCTP to initialize its internal data structures

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and allocate necessary resources for setting up its operation
environment. Once SCTP is initialized, ULP can communicate
directly with other endpoints without re-invoking this primitive.

SCTP will return a local SCTP instance name to the ULP.

Mandatory attributes:

None.

Optional attributes:

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

 o local port - SCTP port number, if ULP wants it to be specified;

 o local eligible address list - An address list that the local SCTP
   endpoint should bind. By default, if an address list is not
   included, all IP addresses assigned to the host should be used by
   the local endpoint.

   IMPLEMENTATION NOTE: If this optional attribute is supported by an
   implementation, it will be the responsibility of the implementation
   to enforce that the IP source address field of any SCTP packets
   sent out by this endpoint contains one of the IP addresses
   indicated in the local eligible address list.

B) Associate

Format: ASSOCIATE(local SCTP instance name, destination transport addr,
        outbound stream count)
-> association id [,destination transport addr list] [,outbound stream
   count]

This primitive allows the upper layer to initiate an association to a
specific peer endpoint.

The peer endpoint shall be specified by one of the transport addresses
which defines the endpoint (see Section 1.4).  If the local SCTP
instance has not been initialized, the ASSOCIATE is considered an
error.

An association id, which is a local handle to the SCTP association,
will be returned on successful establishment of the association. If
SCTP is not able to open an SCTP association with the peer endpoint,
an error is returned.

Other association parameters may be returned, including the complete
destination transport addresses of the peer as well as the outbound
stream count of the local endpoint. One of the transport address from
the returned destination addresses will be selected by the local
endpoint as default primary path for sending SCTP
packets to this peer.  The returned "destination transport addr

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list" can be used by the ULP to change the default primary path or to
force sending a packet to a specific transport address.

  IMPLEMENTATION NOTE: If ASSOCIATE primitive is implemented as a
  blocking function call, the ASSOCIATE primitive can return
  association parameters in addition to the association id upon
  successful establishment. If ASSOCIATE primitive is implemented as a
  non-blocking call, only the association id shall be returned and
  association parameters shall be passed using the COMMUNICATION UP
  notification.

Mandatory attributes:

 o local SCTP instance name - obtained from the INITIALIZE operation.

 o destination transport addr - specified as one of the transport
   addresses of the peer endpoint with which the association is to be
   established.

 o outbound stream count - the number of outbound streams the ULP
   would like to open towards this peer endpoint.

Optional attributes:

None.

C) Shutdown

Format: SHUTDOWN(association id)
-> result

Gracefully closes an association. Any locally queued user data
will be delivered to the peer. The association will be terminated only
after the peer acknowledges all the SCTP packets sent.  A success code
will be returned on successful termination of the association. If
attempting to terminate the association results in a failure, an error
code shall be returned.

Mandatory attributes:

 o association id - local handle to the SCTP association

Optional attributes:

None.

D) Close

Format: ABORT(association id [, cause code])
-> result

Ungracefully closes an association. Any locally queued user data
will be discarded and an ABORT chunk is sent to the peer. A success
code will be returned on successful abortion of the association. If

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attempting to abort the association results in a failure, an error
code shall be returned.

Mandatory attributes:

 o association id - local handle to the SCTP association

Optional attributes:

 o cause code - reason of the abort to be passed to the peer.

None.

E) Send

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

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

Mandatory attributes:

 o association id - local handle to the SCTP association

 o buffer address - the location where the user message to be
   transmitted is stored;

 o byte count - The size of the user data in number of bytes;

Optional attributes:

 o context - an optional 32 bit integer that will be carried in the
   sending failure notification to the ULP if the transportation of
   this User Message fails.

 o stream id - to indicate which stream to send the data on. If not
   specified, stream 0 will be used.

 o life time - specifies the life time of the user data. The user data
   will not be sent by SCTP after the life time expires. This
   parameter can be used to avoid efforts to transmit stale
   user messages. SCTP notifies the ULP if the data cannot be
   initiated to transport (i.e. sent to the destination via SCTP's
   send primitive) within the life time variable. However, the
   user data will be transmitted if SCTP has attempted to transmit a
   chunk before the life time expired.

  IMPLEMENTATION NOTE: In order to better support the data lifetime
  option, the transmitter may hold back the assigning of the TSN
  number to an outbound DATA chunk to the last moment. And, for
  implementation simplicity, once a TSN number has been assigned the
  sender should consider the send of this DATA chunk as committed,

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  overriding any lifetime option attached to the DATA chunk.

 o destination transport address - specified as one of the destination
   transport addresses of the peer endpoint to which this packet
   should be sent. Whenever possible, SCTP should use this destination
   transport address for sending the packets, instead of the current
   primary path.

 o unorder flag - this flag, if present, indicates that the user
   would like the data delivered in an unordered fashion to the peer
   (i.e., the U flag is set to 1 on all DATA chunks carrying this
   message).

 o no-bundle flag - instructs SCTP not to bundle this user data with
   other outbound DATA chunks. SCTP MAY still bundle even when
   this flag is present, when faced with network congestion.

 o payload protocol-id - A 32 bit unsigned integer that is to be
   passed to the peer indicating the type of payload protocol data
   being transmitted. This value is passed as opaque data by SCTP.

F) Set Primary

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

Instructs the local SCTP to use the specified destination transport
address as primary path for sending packets.

The result of attempting this operation shall be returned. If the
specified destination transport address is not present in the
"destination transport address list" returned earlier in an associate
command or communication up notification, an error shall be returned.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o destination transport address - specified as one of the transport
   addresses of the peer endpoint, which should be used as primary
   address for sending packets. This overrides the current primary
   address information maintained by the local SCTP endpoint.

Optional attributes:

 o source transport address - optionally, some implementations may
   allow you to set the default source address placed in all
   outgoing IP datagrams.

G) Receive

Format: RECEIVE(association id, buffer address, buffer size
        [,stream id])

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-> byte count [,transport address] [,stream id] [,stream sequence
   number] [,partial flag] [,delivery number] [,payload protocol-id]

This primitive shall read the first user message in the SCTP in-queue
into the buffer specified by ULP, if there is one available. The size
of the message read, in bytes, will be returned. It may, depending on
the specific implementation, also return other information such as the
sender's address, the stream id on which it is received, whether there
are more messages available for retrieval, etc. For ordered messages,
their stream sequence number may also be returned.

Depending upon the implementation, if this primitive is invoked when
no message is available the implementation should return an indication
of this condition or should block the invoking process until data does
become available.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o buffer address - the memory location indicated by the ULP to store
   the received message.

 o buffer size - the maximum size of data to be received, in bytes.

Optional attributes:

 o stream id - to indicate which stream to receive the data on.

 o stream sequence number - the stream sequence number assigned by the
   sending SCTP peer.

 o partial flag - if this returned flag is set to 1, then this
   Receive contains  a partial delivery of the whole message. When
   this flag is set, the stream id and stream sequence number MUST
   accompany this receive. When this flag is set to 0, it indicates
   that no more deliveries will be received for this stream sequence
   number.

 o payload protocol-id - A 32 bit unsigned integer that is received
   from the peer indicating the type of payload protocol of the
   received data. This value is passed as opaque data by SCTP.

H) Status

Format: STATUS(association id)
-> status data

This primitive should return a data block containing the following
information:
  association connection state,
  destination transport address list,
  destination transport address reachability states,
  current receiver window size,

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  current congestion window sizes,
  number of  unacknowledged DATA chunks,
  number of DATA chunks pending receipt,
  primary path,
  most recent SRTT on primary path,
  RTO on primary path,
  SRTT and RTO on other destination addresses, etc.

Mandatory attributes:

 o association id - local handle to the SCTP association

Optional attributes:

 None.

I) Change Heartbeat

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

Instructs the local endpoint to enable or disable heartbeat on the
specified destination transport address.

The result of attempting this operation shall be returned.

  Note: Even when enabled, heartbeat will not take place if the
  destination transport address is not idle.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o destination transport address - specified as one of the transport
   addresses of the peer endpoint.

 o new state - the new state of heartbeat for this destination
   transport address (either enabled or disabled).

Optional attributes:

 o interval - if present, indicates the frequency of the heartbeat if
   this is to enable heartbeat on a destination transport
   address. Default interval is the RTO of the destination address.

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

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result should indicate whether the transmission of the HEARTBEAT
chunk to the destination address is successful.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o destination transport address - the transport address of the
   association on which a heartbeat should be issued.

K) Get SRTT Report

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

Instructs the local SCTP to report the current SRTT measurement on the
specified destination transport address of the given association. The
returned result can be an integer containing the most recent SRTT in
milliseconds.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o destination transport address - the transport address of the
   association on which the SRTT measurement is to be reported.

L) Set Failure Threshold

Format: SETFAILURETHRESHOLD(association id, destination transport
        address, failure threshold)
-> result

This primitive allows the local SCTP to customize the reachability
failure detection threshold 'Path.Max.Retrans' for the specified
destination address.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o destination transport address - the transport address of the
   association on which the failure detection threshold is to be set.

 o failure threshold - the new value of 'Path.Max.Retrans' for the
   destination address.

M) Set Protocol Parameters

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

This primitive allows the local SCTP to customize the protocol

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parameters.

Mandatory attributes:

 o association id - local handle to the SCTP association

 o protocol parameter list - The specific names and values of the
   protocol parameters (e.g., Association.Max.Retrans [see Section 14])
   that the SCTP user wishes to customize.

Optional attributes:

 o destination transport address - some of the protocol parameters may
   be set on a per destination transport address basis.

N) Receive unsent message

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

 o data retrieval id - The identification passed to the ULP in the
   failure notification.

 o buffer address - the memory location indicated by the ULP to store
   the received message.

 o buffer size - the maximum size of data to be received, in bytes.

Optional attributes:

 o stream id - this is a return value that is set to  indicate
   which stream the data was sent to.

 o stream sequence number - this value is returned indicating
   the stream sequence number that was associated with the message.

 o partial flag - if this returned flag is set to 1, then this
   message is a partial delivery of the whole message. When
   this flag is set, the stream id and stream sequence number MUST
   accompany this receive. When this flag is set to 0, it indicates
   that no more deliveries will be received for this stream sequence
   number.

 o payload protocol-id - The 32 bit unsigned integer that was sent to
   be sent to the peer indicating the type of payload protocol of the
   received data.

O) Receive unacknowledged message

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

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 o data retrieval id - The identification passed to the ULP in the
   failure notification.

 o buffer address - the memory location indicated by the ULP to store
   the received message.

 o buffer size - the maximum size of data to be received, in bytes.

Optional attributes:

 o stream id - this is a return value that is set to  indicate
   which stream the data was sent to.

 o stream sequence number - this value is returned indicating
   the stream sequence number that was associated with the message.

 o partial flag - if this returned flag is set to 1, then this
   message is a partial delivery of the whole message. When
   this flag is set, the stream id and stream sequence number MUST
   accompany this receive. When this flag is set to 0, it indicates
   that no more deliveries will be received for this stream sequence
   number.

 o payload protocol-id - The 32 bit unsigned integer that was sent to
   be sent to the peer indicating the type of payload protocol of the
   received data.

P) Destroy SCTP instance

Format: DESTROY(local SCTP instance name)

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

10.2 SCTP-to-ULP

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

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

A) DATA ARRIVE notification

SCTP shall invoke this notification on the ULP when a user message is
successfully received and ready for retrieval.

The following may be optionally be passed with the notification:

 o association id - local handle to the SCTP association

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 o stream id - to indicate which stream the data is received on.

B) SEND FAILURE notification

If a message can not be delivered SCTP shall invoke this notification
on the ULP.

The following may be optionally be passed with the notification:

 o association id - local handle to the SCTP association

 o data retrieval id - an identification used to retrieve
   unsent and unacknowledged data.

 o cause code - indicating the reason of the failure, e.g., size too
   large, message life-time expiration, etc.

 o context - optional information associated with this message (see
   D in Section 10.1).

C) NETWORK STATUS CHANGE notification

When a destination transport address is marked inactive (e.g., when
SCTP detects a failure), or marked active (e.g., when SCTP detects a
recovery), SCTP shall invoke this notification on the ULP.

The following shall be passed with the notification:

 o association id - local handle to the SCTP association

 o destination transport address - This indicates the destination
   transport address of the peer endpoint affected by the change;

 o new-status - This indicates the new status.

D) COMMUNICATION UP notification

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

  IMPLEMENTATION NOTE: If ASSOCIATE primitive is implemented as a
  blocking function call, the association parameters are returned as a
  result of the ASSOCIATE primitive itself. In that case,
  COMMUNICATION UP notification is optional at the association
  initiator's side.

The following shall be passed with the notification:

 o association id - local handle to the SCTP association

 o status - This indicates what type of event has occurred

 o destination transport address list - the complete set of transport

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   addresses of the peer

 o outbound stream count - the maximum number of streams allowed to be
   used in this association by the ULP

 o inbound stream count - the number of streams the peer endpoint
   has requested with this association (this may not be the same
   number as 'outbound stream count').

E) COMMUNICATION LOST notification

When SCTP loses communication to an endpoint completely (e.g., via
Heartbeats) or detects that the endpoint has performed an abort
operation, it shall invoke this notification on the ULP.

The following shall be passed with the notification:

 o association id - local handle to the SCTP association

 o status - This indicates what type of event has occurred;
            The status may indicate a failure OR a normal
            termination event occurred in response to a
            shutdown or abort request.

The following may be passed with the notification:

 o data retrieval id - an identification used to retrieve
   unsent and unacknowledged data.

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

 o last-sent - the TSN last sent to that peer endpoint;

F) COMMUNICATION ERROR notification

When SCTP receives an ERROR chunk from its peer and decides to notify
its ULP, it can invoke this notification on the ULP.

The following can be passed with the notification:

 o association id - local handle to the SCTP association

 o error info - this indicates the type of error and optionally some
   additional information received through the ERROR chunk.

G) RESTART notification

When SCTP detects that the peer has restarted, it may send
this notification to its ULP.

The following can be passed with the notification:

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 o association id - local handle to the SCTP association

H) SHUTDOWN COMPLETE notification

When SCTP completes the shutdown procedures (section 9.2) this
notification is passed to the upper layer.

The following can be passed with the notification:

 o association id - local handle to the SCTP association

11. Security Considerations

11.1 Security Objectives

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

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

11.2 SCTP Responses To Potential Threats

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

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

11.2.1 Countering Insider Attacks

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

11.2.2 Protecting against Data Corruption in the Network

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

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attacks from a sophisticated adversary.

In order to promote software code-reuse, to avoid re-inventing the
wheel, and to avoid gratuitous complexity to SCTP, the IP
Authentication Header [RFC2402] SHOULD be used when the threat
environment requires stronger integrity protections, but does not
require confidentiality.

A widely implemented BSD Sockets API extension exists for applications
to request IP security services, such as AH or ESP from an operating
system kernel.  Applications can use such an API to request AH whenever
AH use is appropriate.

11.2.3 Protecting Confidentiality

In most cases, the risk of breach of confidentiality applies to the
signaling data payload, not to the SCTP or lower-layer protocol
overheads. If that is true, encryption of the SCTP user data only might
be considered. As with the supplementary checksum service, user data
encryption MAY be performed by the SCTP user application.  Alternately,
the user application MAY use an implementation-specific API to request
that the IP Encapsulating Security Payload (ESP) [RFC2406] be used to
provide confidentiality and integrity.

Particularly for mobile users, the requirement for confidentiality
might include the masking of IP addresses and ports.  In this case ESP
SHOULD be used instead of application-level confidentiality. If ESP is
used to protect confidentiality of SCTP traffic, an ESP cryptographic
transform that includes cryptographic integrity protection MUST be
used, because if there is a confidentiality threat there will also be a
strong integrity threat.

Whenever ESP is in use, application-level encryption is not generally
required.

Regardless of where confidentiality is provided, the ISAKMP [RFC2408]
and the Internet Key Exchange (IKE) [RFC2409] SHOULD be used for key
management.

Operators should consult [RFC2401] for more information on the security
services available at and immediately above the Internet Protocol
layer.

11.2.4 Protecting against Blind Denial of Service Attacks

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

11.2.4.1 Flooding

The objective of flooding is to cause loss of service and incorrect
behavior at target systems through resource exhaustion, interference

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with legitimate transactions, and exploitation of buffer-related
software bugs.  Flooding may be directed either at the SCTP node or at
resources in the intervening IP Access Links or the Internet.
Where the latter entities are the target, flooding will manifest
itself as loss of network services, including potentially the breach
of any firewalls in place.

In general, protection against flooding begins at the equipment
design level, where it includes measures such as:

 - avoiding commitment of limited resources before determining that
   the request for service is legitimate
 - giving priority to completion of processing in progress over the
   acceptance of new work
 - identification and removal of duplicate or stale queued requests
   for service.
 - not responding to unexpected packets sent to non-unicast
   addresses.

Network equipment should be capable of generating an alarm and log
if a suspicious increase in traffic occurs.  The log should provide
information such as the identity of the incoming link and source
address(es) used which will help the network or SCTP system operator
to take protective measures.  Procedures should be in place for the
operator to act on such alarms if a clear pattern of abuse emerges.

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

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

The use of the Host Name feature in the INIT chunk could be used to
flood a target DNS server. A large backlog of DNS queries, resolving
the Host Name received in the INIT chunk to IP addresses, could be
accomplished by sending INIT's to multiple hosts in a given domain.
In addition, an attacker could use the Host Name feature in an indirect
attack on a third party by sending large numbers of INITs to random
hosts containing the host name of the target.  In addition to the
strain on DNS resources, this could also result in large numbers of
INIT ACKs being sent to the target.  One method to protect against this
type of attack is to verify that the IP addresses received from DNS
include the source IP address of the original INIT.  If the list of IP
addresses received from DNS does not include the source IP address of
the INIT, the endpoint MAY silently discard the INIT.  This last option
will not protect against the attack against the DNS.

11.2.4.2 Blind Masquerade

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Masquerade can be used to deny service in several ways:

 - by tying up resources at the target SCTP node to which the
   impersonated node has limited access.  For example, the target node
   may by policy permit a maximum of one SCTP association with the
   impersonated SCTP node. The masquerading attacker may attempt to
   establish an association purporting to come from the impersonated
   node so that the latter cannot do so when it requires it.
 - by deliberately allowing the impersonation to be detected,
   thereby provoking counter-measures which cause the impersonated node
   to be locked out of the target SCTP node.
 - by interfering with an established association by inserting
   extraneous content such as a SHUTDOWN request.

SCTP reduces the risk of blind masquerade attacks through IP spoofing
by use of the four-way startup handshake. Man-in-the-middle masqurade
attacks are discussed in Section 11.3 below. Because the initial exchange
is memoryless, no lockout mechanism is triggered by blind masquerade attacks.
In addition, the INIT ACK containing the State Cookie is transmitted
back to the IP address from which it received the INIT.  Thus the
attacker would not receive the INIT ACK containing the State Cookie.
SCTP protects against insertion of extraneous packets into the flow of
an established association by use of the Verification Tag.

Logging of received INIT requests and abnormalities such as
unexpected INIT ACKs might be considered as a way to detect patterns
of hostile activity.  However, the potential usefulness of such
logging must be weighed against the increased SCTP startup
processing it implies, rendering the SCTP node more vulnerable to
flooding attacks.  Logging is pointless without the establishment of
operating procedures to review and analyze the logs on a routine
basis.

11.2.4.3 Improper Monopolization of Services

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

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

11.3 Protection against Fraud and Repudiation

The objective of fraud is to obtain services without authorization
and specifically without paying for them.  In order to achieve this
objective, the attacker must induce the SCTP user application at the

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target SCTP node to provide the desired service while accepting
invalid billing data or failing to collect it.  Repudiation is a
related problem, since it may occur as a deliberate act of fraud or
simply because the repudiating party kept inadequate records of
service received.

Potential fraudulent attacks include interception and misuse of
authorizing information such as credit card numbers, blind
masquerade and replay, and man-in-the middle attacks which modify
the packets passing through a target SCTP association in real time.

The interception attack is countered by the confidentiality measures
discussed in Section 11.2.3 above.

Section 11.2.4.2 describes how SCTP is resistant to blind masquerade
attacks, as a result of the four-way startup handshake and the
Verification Tag.  The Verification Tag and TSN together are
protections against blind replay attacks, where the replay is into an
existing association.

However, SCTP does not protect against man-in-the-middle attacks
where the attacker is able to intercept and alter the packets sent
and received in an association.  For example, the INIT ACK will have
sufficient information sent on the wire for an adversary in the middle
to hijack an existing SCTP association. Where a significant possibility
of such attacks is seen to exist, or where possible repudiation is an
issue, the use of the IPSEC AH service is recommended to ensure both
the integrity and the authenticity of the SCTP packets passed.

SCTP also provides no protection against attacks originating at or
beyond the SCTP node and taking place within the context of an
existing association.  Prevention of such attacks should be covered
by appropriate security policies at the host site, as discussed in
Section 11.2.1.

12. Recommended Transmission Control Block (TCB) Parameters

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

12.1 Parameters necessary for the SCTP instance

Associations: A list of current associations and mappings to the
              data consumers for each association. This may be in
              the form of a hash table or other implementation
              dependent structure. The data consumers may be process
              identification information such as file descriptors,
              named pipe pointer, or table pointers dependent on how
              SCTP is implemented.

Secret Key:   A secret key used by this endpoint to compute the MAC.

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              This SHOULD be a cryptographic quality random number with
              a sufficient length. Discussion in [RFC1750] can be
              helpful in selection of the key.

Address List: The list of IP addresses that this instance has bound.
              This information is passed to one's peer(s) in INIT and
              INIT ACK chunks.

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

12.2 Parameters necessary per association (i.e. the TCB)

Peer        : Tag value to be sent in every packet and is received
Verification: in the INIT or INIT ACK chunk.
Tag         :

My          : Tag expected in every inbound packet and sent in the
Verification: INIT or INIT ACK chunk.
Tag         :

State       : A state variable indicating what state the association is
            : in, i.e. COOKIE-WAIT, COOKIE-ECHOED, ESTABLISHED,
            : SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED,
            : SHUTDOWN-ACK-SENT.

              Note: No "CLOSED" state is illustrated since if a
              association is "CLOSED" its TCB SHOULD be removed.

Peer        : A list of SCTP transport addresses that the peer is
Transport   : bound to. This information is derived from the INIT or
Address     : INIT ACK and is used to associate an inbound packet
List        : with a given association. Normally this information is
            : hashed or keyed for quick lookup and access of the TCB.

Primary     : This is the current primary destination transport
Path        : address of the peer endpoint.  It may also specify a
            : source transport address on this endpoint.

Overall     : The overall association error count.
Error Count :

Overall     : The threshold for this association that if the Overall
Error       : Error Count reaches will cause this association to be
Threshold   : torn down.

Peer Rwnd   : Current calculated value of the peer's rwnd.

Next TSN    : The next TSN number to be assigned to a new DATA chunk.
            : This is sent in the INIT or INIT ACK chunk to the peer
            : and incremented each time a DATA chunk is assigned a
            : TSN (normally just prior to transmit or during
            : fragmentation).

Last Rcvd   : This is the last TSN received in sequence. This value is

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TSN         : set initially by taking the peer's Initial TSN,
            : received in the INIT or INIT ACK chunk, and
            : subtracting one from it.

Mapping     : An array of bits or bytes indicating which out of
Array       : order TSN's have been received (relative to the
            : Last Rcvd TSN). If no gaps exist, i.e. no out of order
            : packets have been received, this array will be set to all
            : zero. This structure may be in the form of a circular
            : buffer or bit array.

Ack State   : This flag indicates if the next received packet
            : is to be responded to with a SACK. This is initialized
            : to 0.  When a packet is received it is incremented.
            : If this value reaches 2 or more, a SACK is sent and the
            : value is reset to 0. Note: This is used only when no DATA
            : chunks are received out of order. When DATA chunks are
            : out of order, SACK's are not delayed (see Section 6).

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

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

Reasm Queue : A re-assembly queue.

Local       : The list of local IP addresses bound in to this
Transport   : association.
Address     :
List        :

Association : The smallest PMTU discovered for all of the
PMTU        : peer's transport addresses.

12.3 Per Transport Address Data

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

Error count : The current error count for this destination.

Error       : Current error threshold for this destination i.e.
Threshold   : what value marks the destination down if Error count
            : reaches this value.

cwnd        : The current congestion window.

ssthresh    : The current ssthresh value.

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RTO         : The current retransmission timeout value.

SRTT        : The current smoothed round trip time.

RTTVAR      : The current RTT variation.

partial     : The tracking method for increase of cwnd when in
bytes acked : congestion avoidance mode (see Section 6.2.2)

state       : The current state of this destination, i.e. DOWN, UP,
            : ALLOW-HB, NO-HEARTBEAT, etc.

PMTU       : The current known path MTU.

Per         : A timer used by each destination.
Destination :
Timer       :

RTO-Pending : A flag used to track if one of the DATA chunks sent to
             this address is currently being used to compute a RTT. If
             this flag is 0, the next DATA chunk sent to this
             destination should be used to compute a RTT and this flag
             should be set. Every time the RTT calculation
             completes (i.e. the DATA chunk is SACK'd) clear this flag.

last-time   : The time this destination was last sent to. This can be
used        : used to determine if a HEARTBEAT is needed.

12.4 General Parameters Needed

Out Queue   : A queue of outbound DATA chunks.

In Queue    : A queue of inbound DATA chunks.

13. IANA Consideration

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

This protocol may also be extended through IANA in three ways:
 -- through definition of additional chunk types,
 -- through definition of additional parameter types, or
 -- through definition of additional cause codes within
    ERROR chunks

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

13.1 IETF-defined Chunk Extension

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The definition and use of new chunk types is an integral part of
SCTP.  Thus, new chunk types are assigned by IANA through an
IETF Consensus action as defined in [RFC2434].

The documentation for a new chunk code type must include the following
information:
(a) A long and short name for the new chunk type;
(b) A detailed description of the structure of the chunk, which MUST
    conform to the basic structure defined in Section 3.2;
(c) A detailed definition and description of intended use of each field
    within the chunk, including the chunk flags if any;
(d) A detailed procedural description of the use of the new chunk type
    within the operation of the protocol.

The last chunk type (255) is reserved for future extension if
necessary.

13.2 IETF-defined Chunk Parameter Extension

The assignment of new chunk parameter type codes is done through an
IETF Consensus action as defined in [RFC2434]. Documentation of the
chunk parameter MUST contain the following information:

(a) Name of the parameter type.
(b) Detailed description of the structure of the parameter field. This
    structure MUST conform to the general type-length-value format
    described in Section 3.2.1.
(c) Detailed definition of each component of the parameter value.
(d) Detailed description of the intended use of this parameter type,
    and an indication of whether and under what circumstances
    multiple instances of this parameter type may be found within the
    same chunk.

13.3 IETF-defined Additional Error Causes

Additional cause codes may be allocated in the range 11 to 65535
through a Specification Required action as defined in [RFC2434].
Provided documentation must include the following information:

(a) Name of the error condition.
(b) Detailed description of the conditions under which an SCTP
    endpoint should issue an ERROR (or ABORT) with this cause code.
(c) Expected action by the SCTP endpoint which receives an ERROR
    (or ABORT) chunk containing this cause code.
(d) Detailed description of the structure and content of data fields
    which accompany this cause code.

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

13.3 Payload Protocol Identifiers

Internet draft      Stream Control Transmission Protocol       July 2000

Except for value 0 which is reserved by SCTP to indicate an
unspecified payload protocol identifier in a DATA chunk, SCTP will
not be responsible for standardizing or verifying any payload protocol
identifiers; SCTP simply receives the identifier from the upper layer
and carries it with the corresponding payload data.

The upper layer, i.e., the SCTP user, SHOULD standardize any specific
protocol identifier with IANA if it is so desired. The use of any
specific payload protocol identifier is out of the scope of SCTP.

14. Suggested SCTP Protocol Parameter Values

The following protocol parameters are RECOMMENDED:

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

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

  Note: RTO.Min SHOULD be set as recommended above.

15. Acknowledgements

The authors wish to thank Mark Allman, R.J.Atkinson, Richard Band,
Scott Bradner, Steve Bellovin, 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, Thomas Narten, Erik Nordmark, Lyndon Ong, Shyamal Prasad,
Kelvin Porter, Heinz Prantner, Jarno Rajahalme, Raymond E. Reeves,
Renee Revis, Ivan Arias Rodriguez, A. Sankar, Greg Sidebottom, Brian
Wyld, La Monte Yarroll, and many others for their invaluable comments.

16.  Authors' Addresses

Randall R. Stewart                      Tel: +1-815-479-8536
24 Burning Bush Trail.                  EMail: rstewart@flashcom.net
Crystal Lake, IL 60012
USA

Qiaobing Xie                            Tel: +1-847-632-3028
Motorola, Inc.                          EMail: qxie1@email.mot.com
1501 W. Shure Drive, #2309

Internet draft      Stream Control Transmission Protocol       July 2000

Arlington Heights, IL 60004
USA

Ken Morneault                           Tel: +1-703-484-3323
Cisco Systems Inc.                      EMail: kmorneau@cisco.com
13615 Dulles Technology Drive
Herndon, VA. 20171
USA

Chip Sharp                              Tel: +1-919-392-3121
Cisco Systems Inc.                      EMail:chsharp@cisco.com
7025 Kit Creek Road
Research Triangle Park, NC  27709
USA

Hanns Juergen Schwarzbauer              Tel: +49-89-722-24236
SIEMENS AG
Hofmannstr. 51
81359 Munich
Germany
EMail: HannsJuergen.Schwarzbauer@icn.siemens.de

Tom Taylor                              Tel: +1-613-736-0961
Nortel Networks
1852 Lorraine Ave.
Ottawa, Ontario
Canada K1H 6Z8
EMail:taylor@nortelnetworks.com

Ian Rytina                              Tel: +61-3-9301-6164
Ericsson Australia                      EMail:ian.rytina@ericsson.com
37/360 Elizabeth Street
Melbourne, Victoria 3000
Australia

Malleswar Kalla                         Tel: +1-973-829-5212
Telcordia Technologies
MCC 1J211R
445 South Street
Morristown, NJ 07960
USA
EMail: kalla@research.telcordia.com

Lixia Zhang                             Tel: +1-310-825-2695
UCLA Computer Science Department        EMail: lixia@cs.ucla.edu
4531G Boelter Hall
Los Angeles, CA 90095-1596
USA

Vern Paxson                             Tel: +1-510-642-4274 x 302
ACIRI                                   EMail: vern@aciri.org
1947 Center St., Suite 600,
Berkeley, CA 94704-1198
USA

Internet draft      Stream Control Transmission Protocol       July 2000

17. References

[RFC768]  Postel, J. (ed.), "User Datagram Protocol", RFC 768, August
          1980.

[RFC793]  Postel, J. (ed.), "Transmission Control Protocol", RFC 793,
          September 1981.

[RFC1123] Braden, R., "Requirements for Internet hosts - application
          and support.", RFC 1123, October 1989.

[RFC1191] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191,
          November 1990.

[RFC1700] Reynolds, J., and Postel, J. (ed.), "Assigned Numbers",
          RFC 1700,

[RFC1981] McCann, J., Deering, S., and Mogul, J., "Path MTU Discovery
          for IP version 6", RFC 1981, August 1996.

[RFC1982] Elz, R., Bush, R., "Serial Number Arithmetic", RFC 1982,
          August 1996.

[RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3",
          RFC 2026, October 1996.

[RFC2119] Bradner, S. "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2401] Kent, S., and Atkinson, R., "Security Architecture for the
          Internet Protocol", RFC 2401,  November 1998.

[RFC2402] S. Kent, R. Atkinson., "IP Authentication Header.",
          RFC 2402, November 1998.

[RFC2406] S. Kent, R. Atkinson., "IP Encapsulating Security Payload
          (ESP)." RFC-2406, November 1998.

[RFC2408] D. Maughan, M. Schertler, M. Schneider, J. Turner.,
          "Internet Security Association and Key Management Protocol"
          RFC 2408, November 1998.

[RFC2409] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
          RFC 2409, November 1998.

[RFC2434] T. Narten, and H. Avestrand, "Guidelines for Writing an IANA
          Considerations Section in RFCs.", RFC2434,  October 1998.

[RFC2460] Deering, S., and R. Hinden, "Internet Protocol, Version
          6 (IPv6) Specification", RFC 2460, December 1998.

[RFC2581] Allman, M., Paxson, V., and Stevens, W., "TCP Congestion

Internet draft      Stream Control Transmission Protocol       July 2000

          Control", RFC 2581, April 1999. October 1994.

18. Bibliography

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

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

[RFC1750]  Eastlake , D. (ed.), "Randomness Recommendations for
           Security", RFC 1750, December 1994.

[RFC1950]  Deutsch P., Gailly J-L., "ZLIB Compressed Data Format
           Specification version 3.3" , RFC1950, May 1996.

[RFC2104]  Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
           for Message Authentication", RFC 2104, March 1997.

[RFC2196]  Fraser, B. (ed.), "Site Security Handbook", RFC 2196,
           September 1997.

[RFC2522]  Karn, P., and Simpson, W., "Photuris: Session-Key Management
           Protocol", RFC 2522, March 1999.

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

Appendix A: Explicit Congestion Notification

ECN (Ramakrishnan, k., Floyd, S., "Explicit Congestion Notification",
RFC 2481, January 1999) describes a proposed extension to IP that
details a method to become aware of congestion outside of datagram
loss. This is an optional feature that an implementation MAY choose to
add to SCTP. This appendix details the minor differences implementers
will need to be aware of if they choose to implement this feature.
In general RFC 2481 should be followed with the following exceptions.

Negotiation:

RFC2481 details negotiation of ECN during the SYN and SYN-ACK stages
of a TCP connection. The sender of the SYN sets two bits in the
TCP flags, and the sender of the SYN-ACK sets only 1 bit. The reasoning
behind this is to assure both sides are truly ECN capable. For SCTP
this is not necessary. To indicate that an endpoint is ECN capable
an endpoint SHOULD add to the INIT and or INIT ACK chunk the TLV
reserved for ECN. This TLV contains no parameters, and thus has
the following format:

   0                   1                   2                   3

Internet draft      Stream Control Transmission Protocol       July 2000

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

ECN-Echo:

RFC 2481 details a specific bit for a receiver to send back in its
TCP acknowledgements to notify the sender of the Congestion Experienced
(CE) bit having arrived from the network. For SCTP this same indication
is made by including the ECNE chunk. This chunk contains one data
element, i.e. the lowest TSN associated with the IP datagram marked
with the CE bit, and looks as follows:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Chunk Type=12 | Flags=00000000|    Chunk Length = 8           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Lowest TSN Number                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Note: The ECNE is considered a Control chunk.

CWR:

RFC 2481 details a specific bit for a sender to send in the header of
its next outbound TCP segment to indicate to its peer that it has
reduced its congestion window. This is termed the CWR bit. For
SCTP the same indication is made by including the CWR chunk.
This chunk contains one data element, i.e. the TSN number that
was sent in the ECN-Echo. This element represents the lowest
TSN number in the datagram that was originally marked with the
CE bit.

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

  Note: The CWR is considered a Control chunk.

Appendix B Alder 32 bit checksum calculation

The Adler-32 checksum calculation given in this appendix is copied from
[RFC1950].

Adler-32 is composed of two sums accumulated per byte: s1 is the sum

Internet draft      Stream Control Transmission Protocol       July 2000

of all bytes, s2 is the sum of all s1 values. Both sums are done
modulo 65521. s1 is initialized to 1, s2 to zero.  The Adler-32
checksum is stored as s2*65536 + s1 in network byte order.

The following C code computes the Adler-32 checksum of a data buffer.
It is written for clarity, not for speed.  The sample code is in the
ANSI C programming language. Non C users may find it easier to read
with these hints:

   &      Bitwise AND operator.
   >>     Bitwise right shift operator. When applied to an
          unsigned quantity, as here, right shift inserts zero bit(s)
          at the left.
   <<     Bitwise left shift operator. Left shift inserts zero
          bit(s) at the right.
   ++     "n++" increments the variable n.
   %      modulo operator: a % b is the remainder of a divided by b.
    #define BASE 65521 /* largest prime smaller than 65536 */
    /*
      Update a running Adler-32 checksum with the bytes buf[0..len-1]
      and return the updated checksum. The Adler-32 checksum should be

      initialized to 1.

       Usage example:

         unsigned long adler = 1L;

         while (read_buffer(buffer, length) != EOF) {
           adler = update_adler32(adler, buffer, length);
         }
         if (adler != original_adler) error();
      */
      unsigned long update_adler32(unsigned long adler,
         unsigned char *buf, int len)
      {
        unsigned long s1 = adler & 0xffff;
        unsigned long s2 = (adler >> 16) & 0xffff;
        int n;

        for (n = 0; n < len; n++) {
          s1 = (s1 + buf[n]) % BASE;
          s2 = (s2 + s1)     % BASE;
        }
        return (s2 << 16) + s1;
      }

      /* Return the adler32 of the bytes buf[0..len-1] */
      unsigned long adler32(unsigned char *buf, int len)
      {
        return update_adler32(1L, buf, len);
      }

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