rfc4960.nroff

.pl 10.0i
.po 0
.ll 7.2i
.lt 7.2i
.nr LL 7.2i
.nr LT 7.2i
.ds LF Stewart
.ds RF FORMFEED[Page %]
.ds CF Standards Track
.ds LH RFC 4960
.ds RH September 2007
.ds CH Stream Control Transmission Protocol
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Network Working Group                                    R. Stewart, Ed.
Request for Comments: 4960                                September 2007
Obsoletes: 2960, 3309
Category: Standards Track                                 


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Stream Control Transmission Protocol

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Status of This Memo

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

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Abstract

This document obsoletes RFC 2960 and RFC 3309.  It
describes the Stream Control Transmission Protocol (SCTP).  SCTP is
designed to transport Public Switched Telephone Network (PSTN)
signaling messages over IP networks, but is capable of broader
applications. 

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

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

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--  data fragmentation to conform to discovered path MTU size,

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

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

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--  network-level fault tolerance through supporting of multi-homing
at either or both ends of an association.

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

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   1. Introduction ....................................................5
      1.1. Motivation .................................................5
      1.2. Architectural View of SCTP .................................6
      1.3. Key Terms ..................................................6
      1.4. Abbreviations .............................................10
      1.5. Functional View of SCTP ...................................10
           1.5.1. Association Startup and Takedown ...................11
           1.5.2. Sequenced Delivery within Streams ..................12
           1.5.3. User Data Fragmentation ............................12
           1.5.4. Acknowledgement and Congestion Avoidance ...........12
           1.5.5. Chunk Bundling .....................................13
           1.5.6. Packet Validation ..................................13
           1.5.7. Path Management ....................................13
      1.6. Serial Number Arithmetic ..................................14
      1.7. Changes from RFC 2960 .....................................15
   2. Conventions ....................................................15
   3. SCTP Packet Format .............................................15
      3.1. SCTP Common Header Field Descriptions .....................16
      3.2. Chunk Field Descriptions ..................................17
           3.2.1. Optional/Variable-Length Parameter Format ..........19
           3.2.2. Reporting of Unrecognized Parameters ...............21
      3.3. SCTP Chunk Definitions ....................................21
           3.3.1. Payload Data (DATA) (0) ............................22
           3.3.2. Initiation (INIT) (1) ..............................24
                  3.3.2.1. Optional/Variable-Length 
                           Parameters in INIT ........................27
           3.3.3. Initiation Acknowledgement (INIT ACK) (2) ..........30
                  3.3.3.1. Optional or Variable-Length Parameters ....33
           3.3.4. Selective Acknowledgement (SACK) (3) ...............34
           3.3.5. Heartbeat Request (HEARTBEAT) (4) ..................38
           3.3.6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5) ......39
           3.3.7. Abort Association (ABORT) (6) ......................40
           3.3.8. Shutdown Association (SHUTDOWN) (7) ................41
           3.3.9. Shutdown Acknowledgement (SHUTDOWN ACK) (8) ........41
           3.3.10. Operation Error (ERROR) (9) .......................42
                  3.3.10.1. Invalid Stream Identifier (1) ............44
                  3.3.10.2. Missing Mandatory Parameter (2) ..........44
                  3.3.10.3. Stale Cookie Error (3) ...................45
                  3.3.10.4. Out of Resource (4) ......................45
                  3.3.10.5. Unresolvable Address (5) .................46
                  3.3.10.6. Unrecognized Chunk Type (6) ..............46
                  3.3.10.7. Invalid Mandatory Parameter (7) ..........47
                  3.3.10.8. Unrecognized Parameters (8) ..............47
                  3.3.10.9. No User Data (9) .........................48
                  3.3.10.10. Cookie Received While Shutting 
                             Down (10) ...............................48
                  3.3.10.11. Restart of an Association with 
                             New Addresses (11) ......................49
                  3.3.10.12. User-Initiated Abort (12) ...............49
                  3.3.10.13. Protocol Violation (13) .................50
           3.3.11. Cookie Echo (COOKIE ECHO) (10) ....................50
           3.3.12. Cookie Acknowledgement (COOKIE ACK) (11) ..........51
           3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14) ........51
   4. SCTP Association State Diagram .................................52
   5. Association Initialization .....................................56
      5.1. Normal Establishment of an Association ....................56
           5.1.1. Handle Stream Parameters ...........................58
           5.1.2. Handle Address Parameters ..........................58
           5.1.3. Generating State Cookie ............................61
           5.1.4. State Cookie Processing ............................62
           5.1.5. State Cookie Authentication ........................62
           5.1.6. An Example of Normal Association Establishment .....64
      5.2. Handle Duplicate or Unexpected INIT, INIT ACK, 
           COOKIE ECHO, and ..........................................65
           5.2.1. INIT Received in COOKIE-WAIT or 
                  COOKIE-ECHOED State (Item B) .......................66
           5.2.2. Unexpected INIT in States Other than 
                  CLOSED, COOKIE-ECHOED, .............................66
           5.2.3. Unexpected INIT ACK ................................67
           5.2.4. Handle a COOKIE ECHO when a TCB Exists .............67
                  5.2.4.1. An Example of a Association Restart .......69
           5.2.5. Handle Duplicate COOKIE-ACK. .......................71
           5.2.6. Handle Stale COOKIE Error ..........................71
      5.3. Other Initialization Issues ...............................72
           5.3.1. Selection of Tag Value .............................72
      5.4. Path Verification .........................................72
   6. User Data Transfer .............................................73
      6.1. Transmission of DATA Chunks ...............................75
      6.2. Acknowledgement on Reception of DATA Chunks ...............78
           6.2.1. Processing a Received SACK .........................81
      6.3. Management of Retransmission Timer ........................83
           6.3.1. RTO Calculation ....................................83
           6.3.2. Retransmission Timer Rules .........................85
           6.3.3. Handle T3-rtx Expiration ...........................86
      6.4. Multi-Homed SCTP Endpoints ................................87
           6.4.1. Failover from an Inactive Destination Address ......88
      6.5. Stream Identifier and Stream Sequence Number ..............88
      6.6. Ordered and Unordered Delivery ............................88
      6.7. Report Gaps in Received DATA TSNs .........................89
      6.8. CRC32c Checksum Calculation ...............................90
      6.9. Fragmentation and Reassembly ..............................91
      6.10. Bundling .................................................92
   7. Congestion Control .............................................93
      7.1. SCTP Differences from TCP Congestion Control ..............94
      7.2. SCTP Slow-Start and Congestion Avoidance ..................95
           7.2.1. Slow-Start .........................................96
           7.2.2. Congestion Avoidance ...............................97
           7.2.3. Congestion Control .................................98
           7.2.4. Fast Retransmit on Gap Reports .....................98
      7.3. Path MTU Discovery .......................................100
   8. Fault Management ..............................................100
      8.1. Endpoint Failure Detection ...............................100
      8.2. Path Failure Detection ...................................101
      8.3. Path Heartbeat ...........................................102
      8.4. Handle "Out of the Blue" Packets .........................104
      8.5. Verification Tag .........................................105
           8.5.1. Exceptions in Verification Tag Rules ..............105
   9. Termination of Association ....................................106
      9.1. Abort of an Association ..................................107
      9.2. Shutdown of an Association ...............................107
   10. Interface with Upper Layer ...................................110
      10.1. ULP-to-SCTP .............................................110
      10.2. SCTP-to-ULP .............................................120
   11. Security Considerations ......................................123
      11.1. Security Objectives .....................................123
      11.2. SCTP Responses to Potential Threats .....................124
           11.2.1. Countering Insider Attacks .......................124
           11.2.2. Protecting against Data Corruption in the 
                   Network ..........................................124
           11.2.3. Protecting Confidentiality .......................124
           11.2.4. Protecting against Blind 
                   Denial-of-Service Attacks ........................125
                  11.2.4.1. Flooding ................................125
                  11.2.4.2. Blind Masquerade ........................126
                  11.2.4.3. Improper Monopolization of Services .....127
      11.3. SCTP Interactions with Firewalls ........................127
      11.4. Protection of Non-SCTP-Capable Hosts ....................128
   12. Network Management Considerations ............................128
   13. Recommended Transmission Control Block (TCB) Parameters ......129
      13.1. Parameters Necessary for the SCTP Instance ..............129
      13.2. Parameters Necessary per Association (i.e., the TCB) ....129
      13.3. Per Transport Address Data ..............................131
      13.4. General Parameters Needed ...............................132
   14. IANA Considerations ..........................................132
      14.1. IETF-defined Chunk Extension ............................132
      14.2. IETF-Defined Chunk Parameter Extension ..................133
      14.3. IETF-Defined Additional Error Causes ....................133
      14.4. Payload Protocol Identifiers ............................134
      14.5. Port Numbers Registry ...................................134
   15. Suggested SCTP Protocol Parameter Values .....................136
   16. Acknowledgements .............................................137
   Appendix A. Explicit Congestion Notification .....................139
   Appendix B. CRC32c Checksum Calculation ..........................140
   Appendix C. ICMP Handling ........................................142
   References .......................................................149
      Normative References ..........................................149
      Informative References ........................................150

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1.  Introduction

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

This document obsoletes [RFC2960] and [RFC3309].

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1.1.  Motivation

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

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-- 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.

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-- 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.

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

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

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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.

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1.2.  Architectural View of SCTP

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

SCTP is connection-oriented in nature, but the SCTP association is a
broader concept than the TCP connection.  SCTP provides the means for
each SCTP endpoint (Section 1.3) to provide the other endpoint
(during association startup) with a list of transport addresses
(i.e., multiple IP addresses in combination with an SCTP port)
through which that endpoint can be reached and from which it will
originate SCTP packets.  The association spans transfers over all of
the possible source/destination combinations that may be generated
from each endpoint's lists.

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 _____________                                      _____________
|  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   |
|_____________|               ----                 |_____________|

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

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Figure 1: An SCTP Association

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1.3.  Key Terms

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

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

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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.

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

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

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

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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.

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

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

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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].

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

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

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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.

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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.

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

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

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

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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.

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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.

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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).

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

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

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

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

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

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o  Transmission Control Block (TCB): An internal data structure
created by an SCTP endpoint for each of its existing SCTP
associations to other SCTP endpoints.  TCB contains all the status
and operational information for the endpoint to maintain and
manage the corresponding association.

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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.

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

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

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

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

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

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1.4.  Abbreviations

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MAC    -  Message Authentication Code [RFC2104]

RTO    -  Retransmission Timeout

RTT    -  Round-Trip Time

RTTVAR -  Round-Trip Time Variation

SCTP   -  Stream Control Transmission Protocol

SRTT   -  Smoothed RTT

TCB    -  Transmission Control Block

TLV    -  Type-Length-Value coding format

TSN    -  Transmission Sequence Number

ULP    -  Upper-Layer Protocol

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1.5.  Functional View of SCTP

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

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SCTP User Application

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-----------------------------------------------------
 _____________                  ____________________
|             |                | Sequenced Delivery |
| Association |                |   within Streams   |
|             |                |____________________|
|   Startup   |
|             |         ____________________________
|     and     |        |    User Data Fragmentation |
|             |        |____________________________|
|   Takedown  |
|             |         ____________________________
|             |        |     Acknowledgement        |
|             |        |          and               |
|             |        |    Congestion Avoidance    |
|             |        |____________________________|
|             |
|             |         ____________________________
|             |        |       Chunk Bundling       |
|             |        |____________________________|
|             |
|             |     ________________________________
|             |    |      Packet Validation         |
|             |    |________________________________|
|             |
|             |     ________________________________
|             |    |     Path Management            |
|_____________|    |________________________________|

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Figure 2: Functional View of the SCTP Transport Service

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1.5.1.  Association Startup and Takedown

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

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

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

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1.5.2.  Sequenced Delivery within Streams

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

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

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

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1.5.3.  User Data Fragmentation

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

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1.5.4.  Acknowledgement and Congestion Avoidance

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

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

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1.5.5.  Chunk Bundling

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

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

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1.5.6.  Packet Validation

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

.ti 0
1.5.7.  Path Management

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

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

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

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

.ti 0
1.6.  Serial Number Arithmetic

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

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

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

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

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

.ne 5
.ti 0
1.7.  Changes from RFC 2960

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

.ti 0
2.  Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

.ti 0
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:

.in  7
.nf
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Common Header                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Chunk #1                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           ...                                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Chunk #n                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
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.

.ne 10
.in  0
3.1.  SCTP Common Header Field Descriptions

.ce
SCTP Common Header Format

.in  7
.nf
 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                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
Source Port Number: 16 bits (unsigned integer)

This is the SCTP sender's port number.  It can be used by the
receiver in combination with the source IP address, the SCTP
destination port, and possibly the destination IP address to
identify the association to which this packet belongs.  The port
number 0 MUST NOT be used.

.ti 3
Destination Port Number: 16 bits (unsigned integer)

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

.in  3
Verification Tag: 32 bits (unsigned integer)

.in  6
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:

.fi
.in 9
.ti 6
-  A packet containing an INIT chunk MUST have a zero Verification
   Tag.

.ti  6
-  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.

.ti  6
-  A packet containing an ABORT chunk may have the verification
tag copied from the packet that caused the ABORT to be sent.
For details see Section 8.4 and Section 8.5.

.in 3
An INIT chunk MUST be the only chunk in the SCTP packet carrying
it.

.fi
.in 6
.ti 3
Checksum: 32 bits (unsigned integer)

This field contains the checksum of this SCTP packet.  Its
calculation is discussed in Section 6.8.  SCTP uses the CRC32c
algorithm as described in Appendix B for calculating the checksum.

.in  3
.ti 0
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.

.in  7
.nf
 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                          /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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:

.nf
.in 3
ID Value    Chunk Type
-----       ----------
0          - Payload Data (DATA)
1          - Initiation (INIT)
2          - Initiation Acknowledgement (INIT ACK)
3          - Selective Acknowledgement (SACK)
4          - Heartbeat Request (HEARTBEAT)
5          - Heartbeat Acknowledgement (HEARTBEAT ACK)
6          - Abort (ABORT)
7          - Shutdown (SHUTDOWN)
8          - Shutdown Acknowledgement (SHUTDOWN ACK)
9          - Operation Error (ERROR)
10         - State Cookie (COOKIE ECHO)
11         - Cookie Acknowledgement (COOKIE ACK)
.bp
12         - Reserved for Explicit Congestion Notification Echo 
             (ECNE)
13         - Reserved for Congestion Window Reduced (CWR)
14         - Shutdown Complete (SHUTDOWN COMPLETE)
15 to 62   - available
63         - reserved for IETF-defined Chunk Extensions
64 to 126  - available
127        - reserved for IETF-defined Chunk Extensions
128 to 190 - available
191        - reserved for IETF-defined Chunk Extensions
192 to 254 - available
255        - reserved for IETF-defined Chunk Extensions

.fi
.in  6
Chunk Types are encoded such that the highest-order 2 bits
specify the action that must be taken if the processing endpoint
does not recognize the Chunk Type.

.in  12
.ti  6
00 -  Stop processing this SCTP packet and discard it, do not
process any further chunks within it.

.ti  6
01 -  Stop processing this SCTP packet and discard it, do not
process any further chunks within it, and report the
unrecognized chunk in an \'Unrecognized Chunk Type'.

.ti  6
10 -  Skip this chunk and continue processing.

.ti 6
11 -  Skip this chunk and continue processing, but report in an
ERROR chunk using the \'Unrecognized Chunk Type' cause of
error.

.in 6
Note: The ECNE and CWR chunk types are reserved for future use of
Explicit Congestion Notification (ECN); see Appendix A.

.fi
.ti 3
Chunk Flags: 8 bits

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

.ti 3
Chunk Length: 16 bits (unsigned integer)

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

.ne 3
Chunks (including Type, Length, and Value fields) are padded out
by the sender with all zero bytes to be a multiple of 4 bytes
long.  This padding MUST NOT be more than 3 bytes in total.  The
Chunk Length value does not include terminating padding of the
chunk.  However, it does include padding of any variable-length
parameter except the last parameter in the chunk.  The receiver
MUST ignore the padding.

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

.ti 3
Chunk Value: variable length

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

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

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

.ti 0
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.

.in  7
.nf
 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                         /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.ne 4
.in  6
.ti 3
Chunk Parameter Type: 16 bits (unsigned integer)

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

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

.ti 3
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.

.ti 3
Chunk Parameter Value: variable length

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

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

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

.fi
.in 12
.ti 6
00 -  Stop processing this parameter; do not process any further
parameters within this chunk.

.ti  6
01 -  Stop processing this parameter, do not process any further
parameters within this chunk, and report the unrecognized
parameter in an \'Unrecognized Parameter', as described in
Section 3.2.2.

.ti  6
10 -  Skip this parameter and continue processing.

.ti  6
11 -  Skip this parameter and continue processing but report the
unrecognized parameter in an \'Unrecognized Parameter', as
described in Section 3.2.2.

.in  3
Please note that in all four cases, an INIT ACK or COOKIE ECHO chunk
is sent.  In the 00 or 01 case, the processing of the parameters after
the unknown parameter is canceled, but no processing already done is
rolled back.

The actual SCTP parameters are defined in the specific SCTP chunk
sections.  The rules for IETF-defined parameter extensions are
defined in Section 14.2.  Note that a parameter type MUST be unique
across all chunks.  For example, the parameter type \'5' is used to
represent an IPv4 address (see Section 3.3.2.1).  The value \'5' then is
reserved across all chunks to represent an IPv4 address and MUST NOT
be reused with a different meaning in any other chunk.

.ti 0
3.2.2.  Reporting of Unrecognized Parameters

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

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

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

.in  0
3.3.  SCTP Chunk Definitions

.in  3
This section defines the format of the different SCTP chunk types.

.ne 15
.in  0
3.3.1.  Payload Data (DATA) (0)

.in  3
The following format MUST be used for the DATA chunk:

.in  7
.nf
 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)                 /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Reserved: 5 bits

.in  6
Should be set to all \'0's and ignored by the receiver.

.ti 3
U bit: 1 bit

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

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

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

.ti 3
B bit: 1 bit

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

.ti 3
E bit: 1 bit

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

.in  3
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:

.in  12
.nf
   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          |
============================================================

.fi
.in  3
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.

.in  6
.ti 3
Length: 16 bits (unsigned integer)

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

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

.ti 3
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.

.in  3
Stream Identifier S: 16 bits (unsigned integer)

.in  6
Identifies the stream to which the following user data belongs.

.ti 3
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.

.ti 3
Payload Protocol Identifier: 32 bits (unsigned integer)

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

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

.ti 3
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.

.in  3
.ti 0
3.3.2.  Initiation (INIT) (1)

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

.ne 20
.in  7
.nf
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Type = 1    |  Chunk Flags  |      Chunk Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Initiate Tag                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Advertised Receiver Window Credit (a_rwnd)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Number of Outbound Streams   |  Number of Inbound Streams    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Initial TSN                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\\                                                               \\
/              Optional/Variable-Length Parameters              /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
The INIT chunk contains the following parameters.  Unless otherwise
noted, each parameter MUST only be included once in the INIT chunk.

.in  12
.nf
Fixed Parameters                     Status
----------------------------------------------
Initiate Tag                        Mandatory
Advertised Receiver Window Credit   Mandatory
Number of Outbound Streams          Mandatory
Number of Inbound Streams           Mandatory
Initial TSN                         Mandatory

.fi
.in  10
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

.in  3
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.

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

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

.in  6
.ti 3
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.

.in  3
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)

.in  6
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.

.ti 3
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.

.ti 3
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.

.ti 3
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.

.in  3
.ti 0
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)

.in  7
.nf
 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                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
IPv4 Address: 32 bits (unsigned integer)

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

.ne 10
.in  3
IPv6 Address Parameter (6)

.in  7
.nf
 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                          |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
IPv6 Address: 128 bits (unsigned integer)

Contains an IPv6 [RFC2460] address of the sending endpoint.  It is binary
encoded.

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

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

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

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

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

.in  3
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.

.in  7
.nf
 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.)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
Suggested Cookie Life-Span Increment: 32 bits (unsigned integer)

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

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

.in  3
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.

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

.fi
.in  6
.ti 3
Host Name: variable length

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

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

.in  3
Supported Address Types (12)

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

.in  7
.nf
 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        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            ......                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+

.fi
.in  6
.ti 3
Address Type: 16 bits (unsigned integer)

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

.in  3
.ti 0
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:
.bp
The format of the INIT ACK chunk is shown below:

.in  7
.nf
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Type = 2    |  Chunk Flags  |      Chunk Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Initiate Tag                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Advertised Receiver Window Credit                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Number of Outbound Streams   |  Number of Inbound Streams    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Initial TSN                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\\                                                               \\
/              Optional/Variable-Length Parameters              /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
Initiate Tag: 32 bits (unsigned integer)

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

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

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

.in  3
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)

.in  6
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).

.ti 3
Number of Outbound Streams (OS): 16 bits (unsigned integer)

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

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

.ti 3
Number of Inbound Streams (MIS): 16 bits (unsigned integer)

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

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

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

.ti 3
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.

.in  9
.nf
Fixed Parameters                     Status
----------------------------------------------
Initiate Tag                        Mandatory
Advertised Receiver Window Credit   Mandatory
Number of Outbound Streams          Mandatory
Number of Inbound Streams           Mandatory
Initial TSN                         Mandatory

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

.fi
.in  3
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 an
INIT ACK that is quite large (more than 1500 bytes) due to the
variable size of the State Cookie AND the variable address list.  For
example if a responder to the INIT has 1000 IPv4 addresses it wishes
to send, it would need at least 8,000 bytes to encode this in the
INIT ACK.

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

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

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

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

.in  0
3.3.3.1.  Optional or Variable-Length Parameters

.in  3
State Cookie

Parameter Type Value: 7

.in  6
Parameter Length: Variable size, depending on size of Cookie.

.ti 3
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.

.in  3
Unrecognized Parameter:

.in  6
Parameter Type Value: 8

.ti 3
Parameter Length: Variable size.

.ti 3
Parameter Value:

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

.in  3
.ti 0
3.3.4.  Selective Acknowledgement (SACK) (3)

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

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

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

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

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

.ne 10 
.in  7
.nf
 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                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to all \'0's on transmit and ignored on receipt.

.ti 3
Cumulative TSN Ack: 32 bits (unsigned integer)

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

.in  3
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)

.in  6
This field indicates the updated receive buffer space in bytes of
the sender of this SACK; see Section 6.2.1 for details.

.ne 3
.in  3
Number of Gap Ack Blocks: 16 bits (unsigned integer)

.in  6
Indicates the number of Gap Ack Blocks included in this SACK.

.ne 5
.ti 3
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.

.ti 3
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.

.ti 3
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.

.ti 3
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.

.bp
.in  3
For example, assume that the receiver has the following DATA chunks newly
arrived at the time when it decides to send a Selective ACK,

.in  27
.nf
----------
| TSN=17 |
----------
|        | <- still missing
----------
| TSN=15 |
----------
| TSN=14 |
----------
|        | <- still missing
----------
| TSN=12 |
----------
| TSN=11 |
----------
| TSN=10 |
----------

.fi
.in  3
then the parameter part of the SACK MUST be constructed as follows
(assuming the new a_rwnd is set to 4660 by the sender):

.in  21
.nf
+--------------------------------+
|   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 |
+----------------+---------------+

.fi
.in  6
.ti 3
Duplicate TSN: 32 bits (unsigned integer)

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

.in  3
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.

.ti 0
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.

.in  7
.nf
 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)        /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.ti 3
Heartbeat Length: 16 bits (unsigned integer)

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

.ti 3
Heartbeat Information: variable length

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

.in  9
Variable Parameters                  Status     Type Value
-------------------------------------------------------------
Heartbeat Info                       Mandatory   1

.in  7
.nf
 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               /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
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).  This information is simply
reflected back by the receiver in the HEARTBEAT ACK message (see
Section 3.3.6).  Note also that the HEARTBEAT message is both for
reachability checking and for path verification (see Section 5.4).
When a HEARTBEAT chunk is being used for path verification
purposes, it MUST hold a 64-bit random nonce.

.in  3
.ti 0
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.

.in  7
.nf
 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)        /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.ti 3
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.

.ti 3
Heartbeat Information: variable length

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

.in  9
Variable Parameters                  Status     Type Value
-------------------------------------------------------------
Heartbeat Info                       Mandatory   1

.ne 5
.in  3
.ti 0
3.3.7.  Abort Association (ABORT) (6)

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

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

.in  7
.nf
 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                   /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Reserved: 7 bits

Set to 0 on transmit and ignored on receipt.

T bit: 1 bit

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

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

.ti 3
Length: 16 bits (unsigned integer)

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

See Section 3.3.10 for Error Cause definitions.

.ne 4
.in  3
.ti 0
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.

.in  7
.nf
 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                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.in  3
Length: 16 bits (unsigned integer)

.in  6
Indicates the length of the parameter.  Set to 8.

.ti 3
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.)

.in  3
.ti 0
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.

.ne 10
.in  7
.nf
 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               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.in  3
.ti 0
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:

.in  7
.nf
 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                   /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.ti 3
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.

.bp
.in  3
Error causes are defined as variable-length parameters using the
format described in Section 3.2.1, that is:

.in  7
.nf
 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                 /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Cause Code: 16 bits (unsigned integer)

.in  6
Defines the type of error conditions being reported.

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

.fi
.in  6
.ti 3
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.

.in  3
Cause-Specific Information: variable length

.in  6
This field carries the details of the error condition.

.in  3
Section 3.3.10.1 - Section 3.3.10.13 define error causes for SCTP.
Guidelines for the IETF to define new error cause values are
discussed in Section 14.3.

.ne 5
.in  0
3.3.10.1.  Invalid Stream Identifier (1)

.in  3
Cause of error
.br
---------------

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

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=1              |      Cause Length=8           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Stream Identifier      |         (Reserved)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
Stream Identifier: 16 bits (unsigned integer)

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

.ti 3
Reserved: 16 bits

This field is reserved.  It is set to all 0's on transmit and
ignored on receipt.

.in  0
3.3.10.2.  Missing Mandatory Parameter (2)

.in  3
Cause of error
.br
---------------

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

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=2              |      Cause Length=8+N*2       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Number of missing params=N                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Missing Param Type #1       |   Missing Param Type #2       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Missing Param Type #N-1     |   Missing Param Type #N       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
Number of Missing params: 32 bits (unsigned integer)

This field contains the number of parameters contained in the
Cause-Specific Information field.

.bp
.in  3
Missing Param Type: 16 bits (unsigned integer)

.in  6
Each field will contain the missing mandatory parameter number.

.in  0
3.3.10.3.  Stale Cookie Error (3)

.in  3
Cause of error
.br
--------------

Stale Cookie Error: Indicates the receipt of a valid State Cookie
that has expired.

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=3              |       Cause Length=8          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Measure of Staleness (usec.)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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.

.in  0
3.3.10.4.  Out of Resource (4)

.in  3
Cause of error
.br
---------------

Out of Resource: Indicates that the sender is out of resource.  This
is usually sent in combination with or within an ABORT.

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=4              |      Cause Length=4           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.bp
.fi
.in  0
3.3.10.5.  Unresolvable Address (5)

.in  3
Cause of error
.br
---------------

Unresolvable Address: Indicates that the sender is not able to
resolve the specified address parameter (e.g., type of address is not
supported by the sender).  This is usually sent in combination with
or within an ABORT.

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=5              |      Cause Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                  Unresolvable Address                         /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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.

.in  0
3.3.10.6.  Unrecognized Chunk Type (6)

.in  3
Cause of error
.br
---------------

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

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=6              |      Cause Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                  Unrecognized Chunk                           /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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.

.bp
.in  0
3.3.10.7.  Invalid Mandatory Parameter (7)

.in  3
.nf
Cause of error
.br
---------------

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

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=7              |      Cause Length=4           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  0
3.3.10.8.  Unrecognized Parameters (8)

.in  3
.nf
Cause of error
.br
---------------

.fi
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.

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=8              |      Cause Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                  Unrecognized Parameters                      /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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.
.bp
.in  0
3.3.10.9.  No User Data (9)

.in  3
Cause of error
.br
---------------

No User Data: This error cause is returned to the originator of a

DATA chunk if a received DATA chunk has no user data.

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=9              |      Cause Length=8           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                  TSN value                                    /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  6
.ti 3
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).

.in  0
3.3.10.10.  Cookie Received While Shutting Down (10)

.in  3
Cause of error
.br
---------------

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

.in  7
.nf
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Cause Code=10              |      Cause Length=4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.bp
.fi
.in  0
3.3.10.11.  Restart of an Association with New Addresses (11)

.in  3
Cause of error
.br
--------------

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

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

.fi
.in  3
Note: Each New Address TLV is an exact copy of the TLV that was found
in the INIT chunk that was new, including the Parameter Type and the
Parameter Length.

.in  0
3.3.10.12.  User-Initiated Abort (12)

.in  3
Cause of error
.br
--------------

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

.in  7
.nf
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Cause Code=12         |      Cause Length=Variable    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                    Upper Layer Abort Reason                   /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.bp
.fi
.in  0
3.3.10.13.  Protocol Violation (13)

.in  3
Cause of error
.br
--------------

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

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

.fi
.in  3
.ti 0
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.

.in  7
.nf
 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                                    /
\\                                                               \\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bit

.in  6
Set to 0 on transmit and ignored on receipt.

.ti 3
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.

.ne 5
.ti 3
Cookie: variable size

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

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

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

.in  3
.ti 0
3.3.12.  Cookie Acknowledgement (COOKIE ACK) (11)

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

.in  7
.nf
 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                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Set to 0 on transmit and ignored on receipt.

.in  3
.ti 0
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.

.in  7
.nf
 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               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
.in  3
Chunk Flags: 8 bits

.in  6
Reserved: 7 bits

.in  9
Set to 0 on transmit and ignored on receipt.

T bit: 1 bit

.in  6
The T bit is set to 0 if the sender filled in the Verification Tag
expected by the peer.  If the Verification Tag is reflected, the T
bit MUST be set to 1.  Reflecting means that the sent Verification
Tag is the same as the received one.

.in  3
Note: Special rules apply to this chunk for verification, please see
Section 8.5.1 for details.

.ti 0
4.  SCTP Association State Diagram

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

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

.in  6
.ti  3
o  Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc., control
chunks, or

.in  3
o  Some timeout events.

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

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

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

.bp
.in  3
.nf
                 (from the ESTABLISHED state only)
                               |
                               |
                      /--------+--------\\
  [SHUTDOWN]         /                   \\
  -------------------|                   |
  check outstanding  |                   |
  DATA chunks        |                   |
                     v                   |
                +---------+              |
                |SHUTDOWN-|              | rcv SHUTDOWN
                |PENDING  |              |------------------
                +---------+              | check outstanding
                     |                   | DATA chunks
No more outstanding  |                   |
---------------------|                   |
snd SHUTDOWN         |                   |
strt shutdown timer  |                   |
                     v                   v
                +---------+        +-----------+
            (4) |SHUTDOWN-|        | SHUTDOWN- |  (5,6)
                |SENT     |        | RECEIVED  |
                +---------+        +-----------+
                     |  \\                |
(A) rcv SHUTDOWN ACK  |   \\               |
----------------------|    \\              |
stop shutdown timer   |     \\rcv:SHUTDOWN |
send SHUTDOWN COMPLETE|      \\  (B)       |
delete TCB            |       \\           |
                      |        \\          | No more outstanding
                      |         \\         |-----------------
                      |          \\        | send SHUTDOWN ACK
(B)rcv SHUTDOWN       |           \\       | strt shutdown timer
----------------------|            \\      |
send SHUTDOWN ACK     |             \\     |
start shutdown timer  |              \\    |
move to SHUTDOWN-     |               \\   |
ACK-SENT              |                |  |
                      |                v  |
                      |             +-----------+
                      |             | SHUTDOWN- | (7)
                      |             | ACK-SENT  |
                      |             +----------+-
                      |                   | (C)rcv SHUTDOWN COMPLETE
                      |                   |-----------------
                      |                   | stop shutdown timer
                      |                   | delete TCB
                      |                   |
                      |                   | (D)rcv SHUTDOWN ACK
                      |                   |--------------
                      |                   | stop shutdown timer
                      |                   | send SHUTDOWN COMPLETE
                      |                   | delete TCB
                      |                   |
                      \\    +---------+    /
                       \\-->| CLOSED  |<--/
                           +---------+

.fi
.ce
Figure 3: State Transition Diagram of SCTP

.in 3
Notes:

.in  7
.ti  3
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.

.ti  3
2)  If the T1-init timer expires, the endpoint MUST retransmit INIT
and restart the T1-init timer without changing state.  This MUST
be repeated up to \'Max.Init.Retransmits' times.  After that, the
endpoint MUST abort the initialization process and report the
error to the SCTP user.

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

.ti  3
4)  In the SHUTDOWN-SENT state, the endpoint MUST acknowledge any received
DATA chunks without delay.

.ti  3
5)  In the SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new
send requests from its SCTP user.

.ti  3
6)  In the SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
retransmit data and leave this state when all data in queue is
transmitted.

.ti  3
7)  In the SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any new
send requests from its SCTP user.

.in  3
The CLOSED state is used to indicate that an association is not
created (i.e., doesn't exist).

.ti 0
5.  Association Initialization

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

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

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

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

.ti 0
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):

.in  6
.ti  3
A) "A" first sends an INIT chunk to "Z".  In the INIT, "A" must
provide its Verification Tag (Tag_A) in the Initiate Tag field.
Tag_A SHOULD be a random number in the range of 1 to 4294967295
(see Section 5.3.1 for Tag value selection).  After sending the
INIT, "A" starts the T1-init timer and enters the COOKIE-WAIT
state.

.ti  3
B) "Z" shall respond immediately with an INIT ACK chunk.  The
destination IP address of the INIT ACK MUST be set to the source
IP address of the INIT to which this INIT ACK is responding.  In
the response, besides filling in other parameters, "Z" must set
the Verification Tag field to Tag_A, and also provide its own
Verification Tag (Tag_Z) in the Initiate Tag field.

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

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

.ti  3
\# FIXME MT: T1- init -> T1-init
C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1- init
timer and leave the COOKIE-WAIT state.  "A" shall then send the
State Cookie received in the INIT ACK chunk in a COOKIE ECHO
chunk, start the T1-cookie timer, and enter the COOKIE-ECHOED
state.

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

.ti  3
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.

.ti  3
E) Upon reception of the COOKIE ACK, endpoint "A" will move from the
\# FIXME MT: T1- cookie -> T1-cookie
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).

.in  3
An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk.
They MUST be the only chunks present in the SCTP packets that carry
them.

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

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

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

.ne 3
Note that a COOKIE ECHO chunk that does NOT pass the integrity check
is NOT considered an \'invalid parameter' and requires special
handling; see Section 5.1.5.

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

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

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

.ti 0
5.1.1.  Handle Stream Parameters

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

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

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

.ti 0
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.

.in  6
.ti  3
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.

.ne 3
.ti  3
B) If there is a Host Name parameter present in the received INIT or
INIT ACK chunk, the endpoint shall resolve that host name to a
list of IP address(es) and derive the transport address(es) of
this peer by combining the resolved IP address(es) with the SCTP
source port.

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

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

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

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

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

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

.in 6
.ti 3
C) If there are only IPv4/IPv6 addresses present in the received INIT
or INIT ACK chunk, the receiver MUST derive and record all the
transport addresses from the received chunk AND the source IP
address that sent the INIT or INIT ACK.  The transport addresses
are derived by the combination of SCTP source port (from the
common header) and the IP Address parameter(s) carried in the INIT
or INIT ACK chunk and the source IP address of the IP datagram.
The receiver should use only these transport addresses as
destination transport addresses when sending subsequent packets to
its peer.

.ti 3
D) An INIT or INIT ACK chunk MUST be treated as belonging to an
already established association (or one in the process of being
established) if the use of any of the valid address parameters
contained within the chunk would identify an existing TCB.

.in 3
.fi
IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
doesn't control the source IP address that is used for transmitting),
an endpoint might need to include in its INIT or INIT ACK all
possible IP addresses from which packets to the peer could be
transmitted.

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

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

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

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

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

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

.ne 5
.ti 0
5.1.3.  Generating State Cookie

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

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

.in  7
.ti  3
1)  Create an association TCB using information from both the
received INIT and the outgoing INIT ACK chunk,

.ti  3
2)  In the TCB, set the creation time to the current time of day, and
the lifespan to the protocol parameter \'Valid.Cookie.Life' (see
Section 15),

.ti  3
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

.ti  3
4)  Generate the State Cookie by combining this subset of information
and the resultant MAC.

.in  3
After sending the INIT ACK with the State Cookie parameter, the
sender SHOULD delete the TCB and any other local resource related to
the new association, so as to prevent resource attacks.

The hashing method used to generate the MAC is strictly a private
matter for the receiver of the INIT chunk.  The use of a MAC is
mandatory to prevent denial-of-service attacks.  The secret key
SHOULD be random ([RFC4086] provides some information on randomness
guidelines); it SHOULD be changed reasonably frequently, and the
timestamp in the State Cookie MAY be used to determine which key
should be used to verify the MAC.

An implementation SHOULD make the cookie as small as possible to
ensure interoperability.
.bp
.ti 0
5.1.4.  State Cookie Processing

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

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

.ti 0
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:

.in  7
.ti  3
1)  Compute a MAC using the TCB data carried in the State Cookie and
the secret key (note the timestamp in the State Cookie MAY be
used to determine which secret key to use).  [RFC2104]
can be used as a guideline for generating the MAC,

.ti  3
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,

.ti  3
3)  Compare the port numbers and the Verification Tag contained
within the COOKIE ECHO chunk to the actual port numbers and the
Verification Tag within the SCTP common header of the received
packet.  If these values do not match, the packet MUST be
silently discarded.

.ti  3
4)  Compare the creation timestamp in the State Cookie to the current
local time.  If the elapsed time is longer than the lifespan
carried in the State Cookie, then the packet, including the
COOKIE ECHO and any attached DATA chunks, SHOULD be discarded,
and the endpoint MUST transmit an ERROR chunk with a "Stale
Cookie" error cause to the peer endpoint.
.bp
.ti  3
5)  If the State Cookie is valid, create an association to the sender
of the COOKIE ECHO chunk with the information in the TCB data
carried in the COOKIE ECHO and enter the ESTABLISHED state.

.ti  3
6)  Send a COOKIE ACK chunk to the peer acknowledging receipt of the
COOKIE ECHO.  The COOKIE ACK MAY be bundled with an outbound DATA
chunk or SACK chunk; however, the COOKIE ACK MUST be the first
chunk in the SCTP packet.

.ti  3
7)  Immediately acknowledge any DATA chunk bundled with the COOKIE
ECHO with a SACK (subsequent DATA chunk acknowledgement should
follow the rules defined in Section 6.2).  As mentioned in step
6, if the SACK is bundled with the COOKIE ACK, the COOKIE ACK
MUST appear first in the SCTP packet.

.in  3
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.
.bp
.ti 0
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):

.in  4
.nf
Endpoint A                                          Endpoint Z
{app sets association with Z}
(build TCB)
INIT [I-Tag=Tag_A
      & other info]  ------\\
(Start T1-init timer)       \\
(Enter COOKIE-WAIT state)    \\---> (compose temp TCB and Cookie_Z)
                                /-- INIT ACK [Veri Tag=Tag_A,
                               /             I-Tag=Tag_Z,
(Cancel T1-init timer) <------/              Cookie_Z, & other info]
                                     (destroy temp TCB)
COOKIE ECHO [Cookie_Z] ------\\
(Start T1-init timer)         \\
(Enter COOKIE-ECHOED state)    \\---> (build TCB enter ESTABLISHED
                                      state)
                               /---- COOKIE-ACK
                              /
(Cancel T1-init timer, <-----/
 Enter ESTABLISHED state)
{app sends 1st user data; strm 0}
DATA [TSN=initial TSN_A
    Strm=0,Seq=0 & user data]--\\
(Start T3-rtx timer)            \\
                                 \\->
                               /----- SACK [TSN Ack=init
                              /           TSN_A,Block=0]
(Cancel T3-rtx timer) <------/
                                      ...
                                     {app sends 2 messages;strm 0}
                               /---- DATA
                              /        [TSN=init TSN_Z
                          <--/          Strm=0,Seq=0 & user data 1]
SACK [TSN Ack=init TSN_Z,      /---- DATA
      Block=0]     --------\\  /        [TSN=init TSN_Z +1,
                            \\/          Strm=0,Seq=1 & user data 2]
                     <------/\\
                              \\
                               \\------>

.fi
.ce
Figure 4: INITIATION Example

.ne 3
.in  3
If the T1-init timer expires at "A" after the INIT or COOKIE ECHO
chunks are sent, the same INIT or COOKIE ECHO chunk with the same
Initiate Tag (i.e., Tag_A) or State Cookie shall be retransmitted and
the timer restarted.  This shall be repeated Max.Init.Retransmits
times before "A" considers "Z" unreachable and reports the failure to
its upper layer (and thus the association enters the CLOSED state).

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

.in  6
.ti  0
5.2.  Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE ECHO, and
COOKIE ACK

.in  3
During the life time of an association (in one of the possible
states), an endpoint may receive from its peer endpoint one of the
setup chunks (INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK).  The
receiver shall treat such a setup chunk as a duplicate and process it
as described in this section.

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

The following scenarios can cause duplicated or unexpected chunks:

.in  6
.ti  3
A) The peer has crashed without being detected, restarted itself, and
sent out a new INIT chunk trying to restore the association,

.ti  3
B) Both sides are trying to initialize the association at about the
same time,

.ti  3
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,

.in  3
D) The chunk is a false packet generated by an attacker, or

.in  6
.ti  3
E) The peer never received the COOKIE ACK and is retransmitting its
COOKIE ECHO.

.in  3
The rules in the following sections shall be applied in order to
identify and correctly handle these cases.

.ne 5
.ti 0
5.2.1.  INIT Received in COOKIE-WAIT or COOKIE-ECHOED State (Item B)

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

Upon receipt of an INIT in the COOKIE-WAIT state, an endpoint MUST
respond with an INIT ACK using the same parameters it sent in its
original INIT chunk (including its Initiate Tag, unchanged).  When
responding, the endpoint MUST send the INIT ACK back to the same
address that the original INIT (sent by this endpoint) was sent.

Upon receipt of an INIT in the COOKIE-ECHOED state, an endpoint MUST
respond with an INIT ACK using the same parameters it sent in its
original INIT chunk (including its Initiate Tag, unchanged),
provided that no NEW address has been added to the forming
association.  If the INIT message indicates that a new address has
been added to the association, then the entire INIT MUST be
discarded, and NO changes should be made to the existing association.
An ABORT SHOULD be sent in response that MAY include the error
\'Restart of an association with new addresses'.  The error SHOULD
list the addresses that were added to the restarting association.

When responding in either state (COOKIE-WAIT or COOKIE-ECHOED) with
an INIT ACK, the original parameters are combined with those from the
newly received INIT chunk.  The endpoint shall also generate a State
Cookie with the INIT ACK.  The endpoint uses the parameters sent in
its INIT to calculate the State Cookie.

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

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

.in  8
.ti  0
5.2.2.  Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED,
COOKIE-WAIT, and SHUTDOWN-ACK-SENT

.in  3
Unless otherwise stated, upon receipt of an unexpected INIT for this
association, the endpoint shall generate an INIT ACK with a State
Cookie.  Before responding, the endpoint MUST check to see if the
unexpected INIT adds new addresses to the association.  If new
addresses are added to the association, the endpoint MUST respond
with an ABORT, copying the \'Initiate Tag' of the unexpected INIT
into the \'Verification Tag' of the outbound packet carrying the
ABORT.  In the ABORT response, the cause of error MAY be set to
\'restart of an association with new addresses'.  The error SHOULD
list the addresses that were added to the restarting association.  If
no new addresses are added, when responding to the INIT in the
outbound INIT ACK, the endpoint MUST copy its current Tie-Tags to a
reserved place within the State Cookie and the association's TCB.  We
shall refer to these locations inside the cookie as the Peer's-Tie-Tag
and the Local-Tie-Tag.  We will refer to the copy within an
association's TCB as the Local Tag and Peer's Tag.  The outbound SCTP
packet containing this INIT ACK MUST carry a Verification Tag value
equal to the Initiate Tag found in the unexpected INIT.  And the
INIT ACK MUST contain a new Initiate Tag (randomly generated; see
Section 5.3.1).  Other parameters for the endpoint SHOULD be copied
from the existing parameters of the association (e.g., number of
outbound streams) into the INIT ACK and cookie.

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

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

.ti 0
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.

.ti 0
5.2.4.  Handle a COOKIE ECHO when a TCB Exists

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

.in 7
.ti 3
1)  Compute a MAC as described in step 1 of Section 5.1.5,

.ti  3
2)  Authenticate the State Cookie as described in step 2 of
Section 5.1.5 (this is case C or D above).

.ti  3
3)  Compare the timestamp in the State Cookie to the current time.
If the State Cookie is older than the lifespan carried in the
State Cookie and the Verification Tags contained in the State
Cookie do not match the current association's Verification Tags,
the packet, including the COOKIE ECHO and any DATA chunks, should
be discarded.  The endpoint also MUST transmit an ERROR chunk
with a "Stale Cookie" error cause to the peer endpoint (this is
case C or D in Section 5.2).

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

.in 7
.ti 3
4)  If the State Cookie proves to be valid, unpack the TCB into a
temporary TCB.

.ti 3
5)  Refer to Table 2 to determine the correct action to be taken.

.fi
.in  0
.nf
+------------+------------+---------------+--------------+-------------+
|  Local Tag | Peer's Tag | Local-Tie-Tag |Peer's-Tie-Tag|   Action/   |
|            |            |               |              | Description |
+------------+------------+---------------+--------------+-------------+
|    X       |     X      |      M        |      M       |     (A)     |
+------------+------------+---------------+--------------+-------------+
|    M       |     X      |      A        |      A       |     (B)     |
+------------+------------+---------------+--------------+-------------+
|    M       |     0      |      A        |      A       |     (B)     |
+------------+------------+---------------+--------------+-------------+
|    X       |     M      |      0        |      0       |     (C)     |
+------------+------------+---------------+--------------+-------------+
|    M       |     M      |      A        |      A       |     (D)     |
+======================================================================+
|       Table 2: Handling of a COOKIE ECHO when a TCB Exists           |
+======================================================================+

.fi
.in  3
Legend:

.in  6
.nf
X - Tag does not match the existing TCB.
M - Tag matches the existing TCB.
0 - No Tie-Tag in cookie (unknown).
A - All cases, i.e., M, X, or 0.

.fi
.in  3
Note: For any case not shown in Table 2, the cookie should be
silently discarded.

Action

.in  6
.ti  3
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:

.in  9
.ti  6
-  Any SCTP DATA chunks MAY be retained (this is an
implementation-specific option).

.ti  6
-  A notification of RESTART SHOULD be sent to the ULP instead of
a "COMMUNICATION LOST" notification.

.in  6
All the congestion control parameters (e.g., cwnd, ssthresh)
related to this peer MUST be reset to their initial values (see
Section 6.2.1).

After this, the endpoint shall enter the ESTABLISHED state.

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

.ti  3
B) In this case, both sides may be attempting to start an association
at about the same time, but the peer endpoint started its INIT
after responding to the local endpoint's INIT.  Thus, it may have
picked a new Verification Tag, not being aware of the previous tag
it had sent this endpoint.  The endpoint should stay in or enter
the ESTABLISHED state, but it MUST update its peer's Verification
Tag from the State Cookie, stop any init or cookie timers that may be
running, and send a COOKIE ACK.

.ti  3
C) In this case, the local endpoint's cookie has arrived late.
Before it arrived, the local endpoint sent an INIT and received an
INIT ACK and finally sent a COOKIE ECHO with the peer's same tag
but a new tag of its own.  The cookie should be silently
discarded.  The endpoint SHOULD NOT change states and should leave
any timers running.

.ti  3
D) When both local and remote tags match, the endpoint should enter
the ESTABLISHED state, if it is in the COOKIE-ECHOED state.  It
should stop any cookie timer that may be running and send a COOKIE
ACK.

.in  3
Note: The "peer's Verification Tag" is the tag received in the
Initiate Tag field of the INIT or INIT ACK chunk.

.ti 0
5.2.4.1.  An Example of a Association Restart

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

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

.fi
.ce
Figure 5: A Restart Example

.bp
.ti 0
5.2.5.  Handle Duplicate COOKIE-ACK.

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

.ti 0
5.2.6.  Handle Stale COOKIE Error

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

.in  6
.ti  3
A) The association failed to completely setup before the State
Cookie issued by the sender was processed.

.in  3
B) An old State Cookie was processed after setup completed.

.in  6
.ti  3
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.

.in  3
When processing an ERROR chunk with a "Stale Cookie" error cause an
endpoint should first examine if an association is in the process of
being set up, i.e., the association is in the COOKIE-ECHOED state.  In
all cases, if the association is not in the COOKIE-ECHOED state, the
ERROR chunk should be silently discarded.

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

.in  7
.ti  3
1)  Send a new INIT chunk to the endpoint to generate a new State
Cookie and reattempt the setup procedure.

.ti  3
2)  Discard the TCB and report to the upper layer the inability to
set up the association.

.ti  3
3)  Send a new INIT chunk to the endpoint, adding a Cookie
Preservative parameter requesting an extension to the life time of
the State Cookie.  When calculating the time extension, an
implementation SHOULD use the RTT information measured based on
the previous COOKIE ECHO / ERROR exchange, and should add no more
than 1 second beyond the measured RTT, due to long State Cookie
life times making the endpoint more subject to a replay attack.
.bp
.in  0
5.3.  Other Initialization Issues

.in  3
.ti 0
5.3.1.  Selection of Tag Value

Initiate Tag values should be selected from the range of 1 to 2**32 -
1.  It is very important that the Initiate Tag value be randomized to
help protect against "man in the middle" and "sequence number"
attacks.  The methods described in [RFC4086] can be used for the
Initiate Tag randomization.  Careful selection of Initiate Tags is
also necessary to prevent old duplicate packets from previous
associations being mistakenly processed as belonging to the current
association.

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

.in  0
5.4.  Path Verification

.in  3
During association establishment, the two peers exchange a list of
addresses.  In the predominant case, these lists accurately represent
the addresses owned by each peer.  However, it is possible that a
misbehaving peer may supply addresses that it does not own.  To
prevent this, the following rules are applied to all addresses of the
new association:

.in 6 
.ti 3
1)  Any address passed to the sender of the INIT by its upper layer
is automatically considered to be CONFIRMED.

.ti 3
2)  For the receiver of the COOKIE ECHO, the only CONFIRMED address is
the one to which the INIT-ACK was sent.

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

.in 3
.fi
To probe an address for verification, an endpoint will send
HEARTBEATs including a 64-bit random nonce and a path indicator (to
identify the address that the HEARTBEAT is sent to) within the
HEARTBEAT parameter.

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

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

In each RTO, a probe may be sent on an active UNCONFIRMED path in an
attempt to move it to the CONFIRMED state.  If during this probing
the path becomes inactive, this rate is lowered to the normal
HEARTBEAT rate.  At the expiration of the RTO timer, the error
counter of any path that was probed but not CONFIRMED is incremented
by one and subjected to path failure detection, as defined in
Section 8.2.  When probing UNCONFIRMED addresses, however, the
association overall error count is NOT incremented.

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

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

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

.in 6
.ti 3 
-  A HEARTBEAT including a nonce MAY be sent to an UNCONFIRMED
address.

.ti 3
-  A HEARTBEAT ACK MAY be sent to an UNCONFIRMED address.

.ti 3
-  A COOKIE ACK MAY be sent to an UNCONFIRMED address, but it MUST be
bundled with a HEARTBEAT including a nonce.  An implementation
that does NOT support bundling MUST NOT send a COOKIE ACK to an
UNCONFIRMED address.

.ti 3
-  A COOKIE ECHO MAY be sent to an UNCONFIRMED address, but it MUST be
bundled with a HEARTBEAT including a nonce, and the packet MUST
NOT exceed the path MTU.  If the implementation does NOT support
bundling or if the bundled COOKIE ECHO plus HEARTBEAT (including
nonce) would exceed the path MTU, then the implementation MUST NOT
send a COOKIE ECHO to an UNCONFIRMED address.

.fi
.in 3
.ti 0
6.  User Data Transfer

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

.ne 3
DATA chunks MUST only be received according to the rules below in
ESTABLISHED, SHUTDOWN-PENDING, and SHUTDOWN-SENT.  A DATA chunk received
in CLOSED is out of the blue and SHOULD be handled per Section 8.4.  A DATA
chunk received in any other state SHOULD be discarded.

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

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

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

In this section, the term "data sender" refers to the endpoint that
transmits a DATA chunk and the term "data receiver" refers to the
endpoint that receives a DATA chunk.  A data receiver will transmit
SACK chunks.
.bp
.in  6
.nf
           +--------------------------+
           |      User Messages       |
           +--------------------------+
 SCTP user        ^  |
==================|==|=======================================
                  |  v (1)
       +------------------+    +--------------------+
       | SCTP DATA Chunks |    |SCTP Control Chunks |
       +------------------+    +--------------------+
                  ^  |             ^  |
                  |  v (2)         |  v (2)
               +--------------------------+
               |      SCTP packets        |
               +--------------------------+
 SCTP                      ^  |
===========================|==|===========================
                           |  v
       Connectionless Packet Transfer Service (e.g., IP)

.fi
.in  3
Notes:

.in  9
.ti  6
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).

.ti  6
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.

.in  16
Figure 6: Illustration of User Data Transfer

.in  3
The fragmentation and bundling mechanisms, as detailed in Section 6.9
and Section 6.10, are OPTIONAL to implement by the data sender, but
they MUST be implemented by the data receiver, i.e., an endpoint MUST
properly receive and process bundled or fragmented data.

.ti 0
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.

.ne 2
The following general rules MUST be applied by the data sender for
transmission and/or retransmission of outbound DATA chunks:

.in  6
.ti  3
A) At any given time, the data sender MUST NOT transmit new data to
any destination transport address if its peer's rwnd indicates
that the peer has no buffer space (i.e., rwnd is 0; see
Section 6.2.1).  However, regardless of the value of rwnd
(including if it is 0), the data sender can always have one DATA
chunk in flight to the receiver if allowed by cwnd (see rule B,
below).  This rule allows the sender to probe for a change in rwnd
that the sender missed due to the SACK's having been lost in
transit from the data receiver to the data sender.

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

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

The sender MUST also have an algorithm for sending new DATA chunks
to avoid silly window syndrome (SWS) as described in [RFC0813].
The algorithm can be similar to the one described in Section
4.2.3.4 of [RFC1122].

However, regardless of the value of rwnd (including if it is 0),
the data sender can always have one DATA chunk in flight to the
receiver if allowed by cwnd (see rule B below).  This rule allows
the sender to probe for a change in rwnd that the sender missed
due to the SACK having been lost in transit from the data receiver
to the data sender.

.ti  3
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.

.ne 4
.ti  3
C) When the time comes for the sender to transmit, before sending new
DATA chunks, the sender MUST first transmit any outstanding DATA
chunks that are marked for retransmission (limited by the current
cwnd).

.ti  3
D) When the time comes for the sender to transmit new DATA chunks,
the protocol parameter Max.Burst SHOULD be used to limit the
number of packets sent.  The limit MAY be applied by adjusting
cwnd as follows:

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

Or it MAY be applied by strictly limiting the number of packets
emitted by the output routine.

.ti  3
E) Then, the sender can send out as many new DATA chunks as rule A
and rule B allow.

.in  3
Multiple DATA chunks committed for transmission MAY be bundled in a
single packet.  Furthermore, DATA chunks being retransmitted MAY be
bundled with new DATA chunks, as long as the resulting packet size
does not exceed the path MTU.  A ULP may request that no bundling is
performed, but this should only turn off any delays that an SCTP
implementation may be using to increase bundling efficiency.  It does
not in itself stop all bundling from occurring (i.e., in case of
congestion or retransmission).

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

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

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

.ne 3
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.

.ti 0
6.2.  Acknowledgement on Reception of DATA Chunks

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

When the receiver's advertised window is 0, the receiver MUST drop
any new incoming DATA chunk with a TSN larger than the largest TSN
received so far.  If the new incoming DATA chunk holds a TSN value
less than the largest TSN received so far, then the receiver SHOULD
drop the largest TSN held for reordering and accept the new incoming
DATA chunk.  In either case, if such a DATA chunk is dropped, the
receiver MUST immediately send back a SACK with the current receive
window showing only DATA chunks received and accepted so far.  The
dropped DATA chunk(s) MUST NOT be included in the SACK, as they were
not accepted.  The receiver MUST also have an algorithm for
advertising its receive window to avoid receiver silly window
syndrome (SWS), as described in [RFC0813].  The algorithm can be
similar to the one described in Section 4.2.3.3 of [RFC1122].

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

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

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

.ne 3
An implementation MUST NOT allow the maximum delay to be configured
to be more than 500 ms.  In other words, an implementation MAY lower
this value below 500 ms but MUST NOT raise it above 500 ms.

.ne 3
Acknowledgements MUST be sent in SACK chunks unless shutdown was
requested by the ULP, in which case an endpoint MAY send an
acknowledgement in the SHUTDOWN chunk.  A SACK chunk can acknowledge
the reception of multiple DATA chunks.  See Section 3.3.4 for SACK
chunk format.  In particular, the SCTP endpoint MUST fill in the
Cumulative TSN Ack field to indicate the latest sequential TSN (of a
valid DATA chunk) it has received.  Any received DATA chunks with TSN
greater than the value in the Cumulative TSN Ack field are reported
in the Gap Ack Block fields.  The SCTP endpoint MUST report as many
Gap Ack Blocks as can fit in a single SACK chunk limited by the
current path MTU.

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

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

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

The data receiver is responsible for maintaining its receive buffers.
The data receiver SHOULD notify the data sender in a timely manner of
changes in its ability to receive data.  How an implementation
manages its receive buffers is dependent on many factors (e.g.,
operating system, memory management system, amount of memory, etc.).
However, the data sender strategy defined in Section 6.2.1 is based
on the assumption of receiver operation similar to the following:

.in  6
.ti  3
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.

.ti  3
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.

.ne 5
.ti  3
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.

.ti  3
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).

.in  3
Under certain circumstances, the data receiver may need to drop DATA
chunks that it has received but hasn't released from its receive
buffers (i.e., delivered to the ULP).  These DATA chunks may have
been acked in Gap Ack Blocks.  For example, the data receiver may be
holding data in its receive buffers while reassembling a fragmented
user message from its peer when it runs out of receive buffer space.
It may drop these DATA chunks even though it has acknowledged them in
Gap Ack Blocks.  If a data receiver drops DATA chunks, it MUST NOT
include them in Gap Ack Blocks in subsequent SACKs until they are
received again via retransmission.  In addition, the endpoint should
take into account the dropped data when calculating its a_rwnd.

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

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

.in  4
.nf
Endpoint A                                      Endpoint Z

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

DATA [TSN=8,Strm=0,Seq=4] ------------> (send ack)
                              /------- SACK [TSN Ack=8,block=0]
(cancel T3-rtx timer)  <-----/

DATA [TSN=9,Strm=0,Seq=5] ------------> (ack delayed)
(Start T3-rtx timer)
                                       ...
                                       {App sends 1 message; strm 1}
                                       (bundle SACK with DATA)
                                /----- SACK [TSN Ack=9,block=0] \\
                               /         DATA [TSN=6,Strm=1,Seq=2]
(cancel T3-rtx timer)  <------/        (Start T3-rtx timer)

(ack delayed)
(send ack)
SACK [TSN Ack=6,block=0] -------------> (cancel T3-rtx timer)

.in  11
Figure 7:  Delayed Acknowledgement Example

.fi
.in  3
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.

.ti 0
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
.bp
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.

.in  6
.ti  3
A) At the establishment of the association, the endpoint initializes
the rwnd to the Advertised Receiver Window Credit (a_rwnd) the
peer specified in the INIT or INIT ACK.

.ti  3
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.

.ti  3
C) Any time a DATA chunk is marked for retransmission, either via
T3-rtx timer expiration (Section 6.3.3) or via Fast Retransmit
(Section 7.2.4), add the data size of those chunks to the rwnd.

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

.ti 3
D) Any time a SACK arrives, the endpoint performs the following:

.in 11
.ti 6
  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.

.ti 6
 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.

.ti 6
iii) If the SACK is missing a TSN that was previously
acknowledged via a Gap Ack Block (e.g., the data receiver
reneged on the data), then consider the corresponding DATA that
might be possibly missing: Count one miss indication towards
Fast Retransmit as described in Section 7.2.4, and if no
retransmit timer is running for the destination address to
which the DATA chunk was originally transmitted, then T3-rtx is
started for that destination address.

.ne 2
.ti 6
 iv) If the Cumulative TSN Ack matches or exceeds the Fast
Recovery exitpoint (Section 7.2.4), Fast Recovery is exited.

.fi
.in  3
.ti 0
6.3.  Management of Retransmission Timer

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

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

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

.ti 0
6.3.1.  RTO Calculation

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

.in  8
.ti  3
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'.

.in  3
C2)  When the first RTT measurement R is made, set

.in  8
SRTT <- R,

RTTVAR <- R/2, and

RTO <- SRTT + 4 * RTTVAR.

.in  3
C3)  When a new RTT measurement R' is made, set

.in  8
RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'|

and

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

.ne 2
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.

.ti  3
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.

.ti  3
C5)  Karn's algorithm: RTT measurements MUST NOT be made using
packets that were retransmitted (and thus for which it is
ambiguous whether the reply was for the first instance of the
chunk or for a later instance)

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

.ti  3
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].

.ti  3
C7)  A maximum value may be placed on RTO provided it is at least
RTO.max seconds.

.in  3
There is no requirement for the clock granularity G used for
computing RTT measurements and the different state variables, other
than:

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

Experience [ALLMAN99] has shown that finer clock granularities (<=
100 msec) perform somewhat better than more coarse granularities.
.bp
.in  0
6.3.2.  Retransmission Timer Rules

.in  3
The rules for managing the retransmission timer are as follows:

.in  8
.ti  3
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.

.ti  3
R2)  Whenever all outstanding data sent to an address have been
acknowledged, turn off the T3-rtx timer of that address.

.ti  3
R3)  Whenever a SACK is received that acknowledges the DATA chunk
with the earliest outstanding TSN for that address, restart the T3-rtx
timer for that address with its current RTO (if there is still
outstanding data on that address). 

.ti  3
R4)  Whenever a SACK is received missing a TSN that was previously
acknowledged via a Gap Ack Block, start the T3-rtx for the
destination address to which the DATA chunk was originally
transmitted if it is not already running.

.in  3
The following example shows the use of various timer rules (assuming that
the receiver uses delayed acks).

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

.ce
Figure 8: Timer Rule Examples

.fi
.in  3
.ti 0
6.3.3.  Handle T3-rtx Expiration

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

.in  8
.ti  3
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.

.ti 3
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.

.ti  3
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.

.ti  3
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).

.in  3
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):

.ne 5
.in  8
.ti  3
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.

.in  3
.ti 0
6.4.  Multi-Homed SCTP Endpoints

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

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

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

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

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

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

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

.ne 3
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.

.ti 0
6.4.1.  Failover from an Inactive Destination Address

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

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

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

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

.ti 0
6.5.  Stream Identifier and Stream Sequence Number

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

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

.ne 5
.ti 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
.bp
their Stream Sequence Number, the endpoint MUST hold the received
DATA chunks from delivery to the ULP until they are reordered.

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

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

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

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

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

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

.ti 0
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).

.ne 3
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.

.ne 3
Upon the reception of a SACK, the endpoint MUST remove all DATA
chunks that have been acknowledged by the SACK's Cumulative TSN Ack
from its transmit queue.  The endpoint MUST also treat all the DATA
chunks with TSNs not included in the Gap Ack Blocks reported by the
SACK as "missing".  The number of "missing" reports for each
outstanding DATA chunk MUST be recorded by the data sender in order
to make retransmission decisions.  See Section 7.2.4 for details.

.in  7
.ti 3
The following example shows the use of SACK to report a gap.

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

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

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

.fi
.in  18
Figure 9: Reporting a Gap using SACK

.in  3
The maximum number of Gap Ack Blocks that can be reported within a
single SACK chunk is limited by the current path MTU.  When a single
SACK cannot cover all the Gap Ack Blocks needed to be reported due
to the MTU limitation, the endpoint MUST send only one SACK,
reporting the Gap Ack Blocks from the lowest to highest TSNs, within
the size limit set by the MTU, and leave the remaining highest TSN
numbers unacknowledged.

.ti 0
6.8.  CRC32c Checksum Calculation

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

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

.ne 2
.in  7
.ti  3
1)  fill in the proper Verification Tag in the SCTP common header and
initialize the checksum field to \'0's,

.ne 3
.ti  3
2)  calculate the CRC32c checksum of the whole packet, including the
SCTP common header and all the chunks (refer to Appendix B for
details of the CRC32c algorithm); and

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

.in  3
When an SCTP packet is received, the receiver MUST first check the
CRC32c checksum as follows:

1)  Store the received CRC32c checksum value aside.

.in  7
.ti  3
2)  Replace the 32 bits of the checksum field in the received SCTP
packet with all \'0's and calculate a CRC32c checksum value of the
whole received packet.

.ti  3
3)  Verify that the calculated CRC32c checksum is the same as the
received CRC32c checksum.  If it is not, the receiver MUST treat
the packet as an invalid SCTP packet.

.in  3
The default procedure for handling invalid SCTP packets is to
silently discard them.

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

.ti 0
6.9.  Fragmentation and Reassembly

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

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

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

.ne 3
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.

.ne 3
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:

.in  7
.ti  3
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.

.ti  3
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.

.ti  3
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'.

.in  3
An endpoint MUST recognize fragmented DATA chunks by examining the
B/E bits in each of the received DATA chunks, and queue the
fragmented DATA chunks for reassembly.  Once the user message is
reassembled, SCTP shall pass the reassembled user message to the
specific stream for possible reordering and final dispatching.

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

.ti 0
6.10.  Bundling

An endpoint bundles chunks by simply including multiple chunks in one
outbound SCTP packet.  The total size of the resultant IP datagram,

including the SCTP packet and IP headers, MUST be less that or equal to
the current Path MTU.

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

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

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

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

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

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

.ti 0
7.  Congestion Control

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

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

.ne 3
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.

.ti 0
7.1.  SCTP Differences from TCP Congestion Control

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

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

.in  6
.ti  3
o  The sender usually uses the same destination address until being
instructed by the upper layer to do otherwise; however, SCTP may
change to an alternate destination in the event an address is
marked inactive (see Section 8.2).  Also, SCTP may retransmit to a
different transport address than the original transmission.

.ne 3
.ti  3
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
.bp
should decay if the address is not used for a long enough time period.

.ti  3
o  For each of the destination addresses, an endpoint does slow start
upon the first transmission to that address.

.in  3
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.

.ti 0
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.

.in  6
.ti  3
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.

.ti  3
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.

.ti  3
o  Slow-start threshold (ssthresh, in bytes), which is used by the
sender to distinguish slow-start and congestion avoidance phases.

.ne 2
Note: This variable is maintained on a per-destination-address
basis.

.ne 3
.in  3
SCTP also requires one additional control variable,
partial_bytes_acked, which is used during congestion avoidance phase
to facilitate cwnd adjustment.

Unlike TCP, an SCTP sender MUST keep a set of these control variables
cwnd, ssthresh, and partial_bytes_acked for EACH destination address
of its peer (when its peer is multi-homed).  Only one rwnd is kept
for the whole association (no matter if the peer is multi-homed or
has a single address).

.ti 0
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.

.in  6
.ti  3
o  The initial cwnd before DATA transmission or after a sufficiently
long idle period MUST be set to min(4*MTU, max (2*MTU, 4380
bytes)).

.ti  3
o  The initial cwnd after a retransmission timeout MUST be no more
than 1*MTU.

.ti  3
o  The initial value of ssthresh MAY be arbitrarily high (for
example, implementations MAY use the size of the receiver
advertised window).

.ti  3
o  Whenever cwnd is greater than zero, the endpoint is allowed to
have cwnd bytes of data outstanding on that transport address.

.ti  3
o  When cwnd is less than or equal to ssthresh, an SCTP endpoint MUST
use the slow-start algorithm to increase cwnd only if the current
congestion window is being fully utilized, an incoming SACK
advances the Cumulative TSN Ack Point, and the data sender is not
in Fast Recovery.  Only when these three conditions are met can
the cwnd be increased; otherwise, the cwnd MUST not be increased.
If these conditions are met, then cwnd MUST be increased by, at
most, the lesser of 1) the total size of the previously
outstanding DATA chunk(s) acknowledged, and 2) the destination's
path MTU.  This upper bound protects against the ACK-Splitting
attack outlined in [SAVAGE99].

.ne 3
.in  3
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.

.in  6
.ti  3
o  When the endpoint does not transmit data on a given transport
address, the cwnd of the transport address should be adjusted to
max(cwnd/2, 4*MTU) per RTO.

.in  3
.ti 0
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.

.in  6
.ti  3
o  Whenever cwnd is greater than ssthresh, upon each SACK arrival
that advances the Cumulative TSN Ack Point, increase
partial_bytes_acked by the total number of bytes of all new chunks
acknowledged in that SACK including chunks acknowledged by the new
Cumulative TSN Ack and by Gap Ack Blocks.

.ti  3
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).

.ti  3
o  Same as in the slow start, when the sender does not transmit DATA
on a given transport address, the cwnd of the transport address
should be adjusted to max(cwnd / 2, 4*MTU) per RTO.

.ne 3
.ti  3
o  When all of the data transmitted by the sender has been
acknowledged by the receiver, partial_bytes_acked is initialized
to 0.

.ne 4
.in  0
7.2.3.  Congestion Control

.in  3
Upon detection of packet losses from SACK (see Section 7.2.4), an
endpoint should do the following:

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

.fi
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:

.nf
   ssthresh = max(cwnd/2, 4*MTU)
   cwnd = 1*MTU

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

.ti 0
7.2.4.  Fast Retransmit on Gap Reports

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

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

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

When the third consecutive miss indication is received for a TSN(s),
the data sender shall do the following:
.bp
.in  7
.ti  3
1)  Mark the DATA chunk(s) with three miss indications for
retransmission.

.ti  3
2)  If not in Fast Recovery, adjust the ssthresh and cwnd of the
destination address(es) to which the missing DATA chunks were
last sent, according to the formula described in Section 7.2.3.

.ti  3
3)  Determine how many of the earliest (i.e., lowest TSN) DATA chunks
marked for retransmission will fit into a single packet, subject
to constraint of the path MTU of the destination transport
address to which the packet is being sent.  Call this value K.
Retransmit those K DATA chunks in a single packet.  When a Fast
Retransmit is being performed, the sender SHOULD ignore the value
of cwnd and SHOULD NOT delay retransmission for this single
packet.

.ti  3
4)  Restart the T3-rtx timer only if the last SACK acknowledged the
lowest outstanding TSN number sent to that address, or the
endpoint is retransmitting the first outstanding DATA chunk sent
to that address.

.ti  3
5)  Mark the DATA chunk(s) as being fast retransmitted and thus
ineligible for a subsequent Fast Retransmit.  Those TSNs marked
for retransmission due to the Fast-Retransmit algorithm that did
not fit in the sent datagram carrying K other TSNs are also
marked as ineligible for a subsequent Fast Retransmit.  However,
as they are marked for retransmission they will be retransmitted
later on as soon as cwnd allows.

.ti  3
6)  If not in Fast Recovery, enter Fast Recovery and mark the highest
outstanding TSN as the Fast Recovery exit point.  When a SACK
acknowledges all TSNs up to and including this exit point, Fast
Recovery is exited.  While in Fast Recovery, the ssthresh and
cwnd SHOULD NOT change for any destinations due to a subsequent
Fast Recovery event (i.e., one SHOULD NOT reduce the cwnd further
due to a subsequent Fast Retransmit).

.in  3
Note: Before the above adjustments, if the received SACK also
acknowledges new DATA chunks and advances the Cumulative TSN Ack
Point, the cwnd adjustment rules defined in Section 7.2.1 and
Section 7.2.2 must be applied first.

A straightforward implementation of the above keeps a counter for
each TSN hole reported by a SACK.  The counter increments for each
consecutive SACK reporting the TSN hole.  After reaching 3 and
starting the Fast-Retransmit procedure, the counter resets to 0.
.bp
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.

.ti 0
7.3.  Path MTU Discovery

[RFC4821], [RFC1981], and [RFC1191] specify "Packetization Layer Path MTU Discovery", whereby
an endpoint maintains an estimate of the maximum transmission unit
(MTU) along a given Internet path and refrains from sending packets
along that path that exceed the MTU, other than occasional attempts
to probe for a change in the Path MTU (PMTU).  [RFC4821] is thorough
in its discussion of the MTU discovery mechanism and strategies for
determining the current end-to-end MTU setting as well as detecting
changes in this value.

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

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

.in  7
.ti  3
1)  SCTP associations can span multiple addresses.  An endpoint MUST
maintain separate MTU estimates for each destination address of
its peer.

.ti  3
2)  The sender should track an association PMTU that will be the
smallest PMTU discovered for all of the peer's destination
addresses.  When fragmenting messages into multiple parts this
association PMTU should be used to calculate the size of each
fragment.  This will allow retransmissions to be seamlessly sent
to an alternate address without encountering IP fragmentation.

.in  0
8.  Fault Management

.in  3
.ti 0
8.1.  Endpoint Failure Detection

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

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.

.ti 0
8.2.  Path Failure Detection

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

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

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

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

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

.ne 5
.ti 0
8.3.  Path Heartbeat

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

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

The upper layer can optionally initiate the following functions:

.in  6
.ti  3
A) Disable heartbeat on a specific destination transport address of a
given association,

.in  3
B) Change the HB.interval,

.in  6
.ti  3
C) Re-enable heartbeat on a specific destination transport address of
a given association, and

.ti  3
D) Request an on-demand HEARTBEAT on a specific destination transport
address of a given association.

.in  3
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.

.ne 3
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.

.ne 5
IMPLEMENTATION NOTE: An alternative implementation of the heartbeat
mechanism that can be used is to increment the error counter variable
every time a HEARTBEAT is sent to a destination.  Whenever a
HEARTBEAT ACK arrives, the sender SHOULD clear the error counter of
the destination that the HEARTBEAT was sent to.  This in effect would
clear the previously stroked error (and any other error counts as
well).

The receiver of the HEARTBEAT should immediately respond with a
HEARTBEAT ACK that contains the Heartbeat Information TLV, together
with any other received TLVs, copied unchanged from the received
HEARTBEAT chunk.

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

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

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

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

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

.ti 0
8.4.  Handle "Out of the Blue" Packets

An SCTP packet is called an "out of the blue" (OOTB) packet if it is
correctly formed (i.e., passed the receiver's CRC32c check; see
Section 6.8), but the receiver is not able to identify the
association to which this packet belongs.

The receiver of an OOTB packet MUST do the following:

.in  7
.ti  3
1)  If the OOTB packet is to or from a non-unicast address, a
receiver SHOULD silently discard the packet.  Otherwise,

.ti  3
2)  If the OOTB packet contains an ABORT chunk, the receiver MUST
silently discard the OOTB packet and take no further action.
Otherwise,

.ti  3
3)  If the packet contains an INIT chunk with a Verification Tag set
to \'0', process it as described in Section 5.1.  If, for whatever
reason, the INIT cannot be processed normally and an ABORT has to
be sent in response, the Verification Tag of the packet
containing the ABORT chunk MUST be the Initiate Tag of the
received INIT chunk, and the T bit of the ABORT chunk has to be
set to 0, indicating that the Verification Tag is NOT reflected.

.ti  3
4)  If the packet contains a COOKIE ECHO in the first chunk, process
it as described in Section 5.1.  Otherwise,

.ti  3
5)  If the packet contains a SHUTDOWN ACK chunk, the receiver should
respond to the sender of the OOTB packet with a SHUTDOWN
COMPLETE.  When sending the SHUTDOWN COMPLETE, the receiver of
the OOTB packet must fill in the Verification Tag field of the
outbound packet with the Verification Tag received in the
SHUTDOWN ACK and set the T bit in the Chunk Flags to indicate
that the Verification Tag is reflected.  Otherwise,

.ti  3
6)  If the packet contains a SHUTDOWN COMPLETE chunk, the receiver
should silently discard the packet and take no further action.
Otherwise,

.ti  3
7)  If the packet contains a "Stale Cookie" ERROR or a COOKIE ACK, the
SCTP packet should be silently discarded.  Otherwise,

.ne 4
.ti  3
8)  The receiver should respond to the sender of the OOTB packet with
an ABORT.  When sending the ABORT, the receiver of the OOTB
packet MUST fill in the Verification Tag field of the outbound
packet with the value found in the Verification Tag field of the
OOTB packet and set the T bit in the Chunk Flags to indicate that
the Verification Tag is reflected.  After sending this ABORT, the
receiver of the OOTB packet shall discard the OOTB packet and
take no further action.

.in  3
.ti 0
8.5.  Verification Tag

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

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

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

.in  0
8.5.1.  Exceptions in Verification Tag Rules

.in  3
A) Rules for packet carrying INIT:

.in 7
.ti 3
-   The sender MUST set the Verification Tag of the packet to 0.

.ti  3
-   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.

.ti 3
B) Rules for packet carrying ABORT:

.ti  3
-   The endpoint MUST always fill in the Verification Tag field of
the outbound packet with the destination endpoint's tag value, if
it is known.

.ti  3
-   If the ABORT is sent in response to an OOTB packet, the endpoint
MUST follow the procedure described in Section 8.4.

.ti  3
-   The receiver of an ABORT MUST accept the packet if the
Verification Tag field of the packet matches its own tag and the
T bit is not set OR if it is set to its peer's tag and the T bit
is set in the Chunk Flags.  Otherwise, the receiver MUST silently
discard the packet and take no further action.

.in  3
C) Rules for packet carrying SHUTDOWN COMPLETE:

.in  7
.ti  3
-   When sending a SHUTDOWN COMPLETE, if the receiver of the SHUTDOWN
ACK has a TCB, then the destination endpoint's tag MUST be used,
and the T bit MUST NOT be set.  Only where no TCB exists should
the sender use the Verification Tag from the SHUTDOWN ACK, and
MUST set the T bit.

.ti  3
-   The receiver of a SHUTDOWN COMPLETE shall accept the packet if
the Verification Tag field of the packet matches its own tag and
the T bit is not set OR if it is set to its peer's tag and the T
bit is set in the Chunk Flags.  Otherwise, the receiver MUST
silently discard the packet and take no further action.  An
endpoint MUST ignore the SHUTDOWN COMPLETE if it is not in the
SHUTDOWN-ACK-SENT state.

.in  3
D) Rules for packet carrying a COOKIE ECHO

.in  7
.ti  3
-   When sending a COOKIE ECHO, the endpoint MUST use the value of
the Initiate Tag received in the INIT ACK.

.ti  3
-   The receiver of a COOKIE ECHO follows the procedures in
Section 5.

.in  3
E) Rules for packet carrying a SHUTDOWN ACK

.in  7
.ti  3
-   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.

.in  3
.ti 0
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.

.ti 0
9.1.  Abort of an Association

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

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

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

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

.ti 0
9.2.  Shutdown of an Association

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

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

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

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

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

Upon reception of the SHUTDOWN, the peer endpoint shall

-  enter the SHUTDOWN-RECEIVED state,

-  stop accepting new data from its SCTP user, and

.in  6
.ti  3
-  verify, by checking the Cumulative TSN Ack field of the chunk,
that all its outstanding DATA chunks have been received by the
SHUTDOWN sender.

.in  3
Once an endpoint has reached the SHUTDOWN-RECEIVED state, it MUST NOT
send a SHUTDOWN in response to a ULP request, and should discard
subsequent SHUTDOWN chunks.

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

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

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

If the receiver of the SHUTDOWN has no more outstanding DATA chunks,
\# FIXME MT: T2- shutdown -> T2-shutdown
the SHUTDOWN receiver MUST 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 resend the SHUTDOWN ACK.

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

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

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

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

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

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

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

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

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

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

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

.ti 0
10.  Interface with Upper Layer

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

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

.ti 0
10.1.  ULP-to-SCTP

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

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

A) Initialize

.in  6
Format: INITIALIZE ([local port],[local eligible address list])->
local SCTP instance name

.in  3
This primitive allows SCTP to initialize its internal data structures
and allocate necessary resources for setting up its operation
environment.  Once SCTP is initialized, ULP can communicate directly
with other endpoints without re-invoking this primitive.

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

Mandatory attributes:

None.

Optional attributes:

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

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

.in  6
.ti  3
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.

.in  3
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

.in  6
.nf
Format: ASSOCIATE(local SCTP instance name,
        destination transport addr, outbound stream count)
-> association id [,destination transport addr list]
      [,outbound stream count]

.fi
.in  3
This primitive allows the upper layer to initiate an association to a
specific peer endpoint.

The peer endpoint shall be specified by one of the transport
addresses that defines the endpoint (see Section 1.3).  If the local
SCTP instance has not been initialized, the ASSOCIATE is considered
an error.

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

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

.in  6
.ti  3
o  destination transport addr - specified as one of the transport
addresses of the peer endpoint with which the association is to be
established.

.ti  3
o  outbound stream count - the number of outbound streams the ULP
would like to open towards this peer endpoint.

.in  3
Optional attributes:

None.

C) Shutdown

.in  6
.nf
Format: SHUTDOWN(association id)
-> result

.fi
.in  3
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.
.bp
.in  6
.ti 3
D) Abort

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

.in  3
Ungracefully closes an association.  Any locally queued user data
will be discarded, and an ABORT chunk is sent to the peer.  A success
code will be returned on successful abort of the association.  If
attempting to abort the association results in a failure, an error
code shall be returned.

Mandatory attributes:

o association id - local handle to the SCTP association.

Optional attributes:

o Upper Layer Abort Reason - reason of the abort to be passed to the
peer.

None.

E) Send

.in  4
.nf
Format: SEND(association id, buffer address, byte count [,context]
        [,stream id] [,life time] [,destination transport address]
        [,unordered flag] [,no-bundle flag] [,payload protocol-id] )
-> result

.fi
.in  3
This is the main method to send user data via SCTP.

Mandatory attributes:

o  association id - local handle to the SCTP association.

.in  6
.ti  3
o  buffer address - the location where the user message to be
transmitted is stored.

.in  3
o  byte count - the size of the user data in number of bytes.

Optional attributes:

.in  6
.ti  3
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.

.ti  3
o  stream id - to indicate which stream to send the data on.  If not
specified, stream 0 will be used.

.ti  3
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.

.in  3
IMPLEMENTATION NOTE: In order to better support the data life time
option, the transmitter may hold back the assigning of the TSN number
to an outbound DATA chunk to the last moment.  And, for
implementation simplicity, once a TSN number has been assigned the
sender should consider the send of this DATA chunk as committed,
overriding any life time option attached to the DATA chunk.

.in  6
.ti  3
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.

.ti  3
o  unordered flag - this flag, if present, indicates that the user
would like the data delivered in an unordered fashion to the peer
(i.e., the U flag is set to 1 on all DATA chunks carrying this
message).

.ti  3
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.

.ti  3
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.

.in  3
F) Set Primary

.in  6
.nf
Format: SETPRIMARY(association id, destination transport address,
                   [source transport address] )
-> result

.fi
.in  3
Instructs the local SCTP to use the specified destination transport
address as the primary path for sending packets.

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

Mandatory attributes:

o  association id - local handle to the SCTP association.

.in  6
.ti  3
o  destination transport address - specified as one of the transport
addresses of the peer endpoint, which should be used as the primary
address for sending packets.  This overrides the current primary
address information maintained by the local SCTP endpoint.

.in  3
Optional attributes:

.in  6
.ti  3
o  source transport address - optionally, some implementations may
allow you to set the default source address placed in all outgoing
IP datagrams.

.in  3
G) Receive

.in  4
.nf
Format: RECEIVE(association id, buffer address, buffer size
        [,stream id])
-> byte count [,transport address] [,stream id] [,stream sequence
   number] [,partial flag] [,delivery number] [,payload protocol-id]

.fi
.in  3
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

.in  6
.ti  3
o  buffer address - the memory location indicated by the ULP to store
the received message.

.in  3
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.

.in  6
.ti  3
o  Stream Sequence Number - the Stream Sequence Number assigned by
the sending SCTP peer.

.ti  3
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.

.ti  3
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.

.in  3
H) Status

.in  6
.nf
Format: STATUS(association id)
-> status data

.fi
.in  3
This primitive should return a data block containing the following
information:

.nf
   association connection state,
   destination transport address list,
   destination transport address reachability states,
   current receiver window size,
   current congestion window sizes,
   number of unacknowledged DATA chunks,
   number of DATA chunks pending receipt,
   primary path,
   most recent SRTT on primary path,
   RTO on primary path,
   SRTT and RTO on other destination addresses, etc.

.fi
.in  3
Mandatory attributes:

o association id - local handle to the SCTP association.

Optional attributes:

None.

I) Change Heartbeat

.in  6
.nf
Format: CHANGE HEARTBEAT(association id,
        destination transport address, new state [,interval])
-> result

.fi
.in  3
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.

.in  6
.ti  3
o  destination transport address - specified as one of the transport
addresses of the peer endpoint.

.ti  3
o  new state - the new state of heartbeat for this destination
transport address (either enabled or disabled).

.in  3
Optional attributes:

.in  6
.ti  3
o  interval - if present, indicates the frequency of the heartbeat if
this is to enable heartbeat on a destination transport address.
This value is added to the RTO of the destination transport
address.  This value, if present, affects all destinations.

.in  3
J) Request HeartBeat

.in  6
.nf
Format: REQUESTHEARTBEAT(association id, destination transport
        address)
-> result

.fi
.in  3
Instructs the local endpoint to perform a HeartBeat on the specified
destination transport address of the given association.  The returned
result should indicate whether the transmission of the HEARTBEAT
chunk to the destination address is successful.

Mandatory attributes:

o  association id - local handle to the SCTP association.

.in  6
.ti  3
o  destination transport address - the transport address of the
association on which a heartbeat should be issued.

.in  3
K) Get SRTT Report

.in  6
.nf
Format: GETSRTTREPORT(association id,
                      destination transport address)
-> srtt result

.fi
.in  3
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.

.in  6
.ti  3
o  destination transport address - the transport address of the
association on which the SRTT measurement is to be reported.

.in  3
L) Set Failure Threshold

.in  14
.ti  6
Format: SETFAILURETHRESHOLD(association id, destination transport
address, failure threshold)

.in  6
-> result

.in  3
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.

.in  6
.ti  3
o  destination transport address - the transport address of the
association on which the failure detection threshold is to be set.

.ti  3
o  failure threshold - the new value of \'Path.Max.Retrans' for the
destination address.

.in  3
M) Set Protocol Parameters

.in  6
.nf
Format: SETPROTOCOLPARAMETERS(association id,
        [,destination transport address,]
        protocol parameter list)
-> result

.fi
.in  3
This primitive allows the local SCTP to customize the protocol
parameters.

Mandatory attributes:

o  association id - local handle to the SCTP association.

.bp
.in  6
.ti  3
o  protocol parameter list - the specific names and values of the
protocol parameters (e.g., Association.Max.Retrans; see Section 15)
that the SCTP user wishes to customize. 

.ti 3
Optional attributes:

.ti  3
o  destination transport address - some of the protocol parameters
may be set on a per destination transport address basis.

.in  3
N) Receive Unsent Message

.in  14
.ti  6
Format: RECEIVE_UNSENT(data retrieval id, buffer address,
buffer size [,stream id] [, stream sequence number]
[,partial flag] [,payload protocol-id])

.in  6
.ti  3
o  data retrieval id - the identification passed to the ULP in the
failure notification.

.ti  3
o  buffer address - the memory location indicated by the ULP to store
the received message.

.in  3
o  buffer size - the maximum size of data to be received, in bytes.

Optional attributes:

.in  6
.ti  3
o  stream id - this is a return value that is set to indicate which
stream the data was sent to.

.ti  3
o  Stream Sequence Number - this value is returned indicating the
Stream Sequence Number that was associated with the message.

.ti  3
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.

.ti  3
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.

.in  3
o  Receive Unacknowledged Message

.in  14
.ti  6
Format: RECEIVE_UNACKED(data retrieval id, buffer address,
buffer size, [,stream id] [, stream sequence number]
[,partial flag] [,payload protocol-id])
.bp
.in  6
.ti  3
o  data retrieval id - the identification passed to the ULP in the
failure notification.

.ti  3
o  buffer address - the memory location indicated by the ULP to store
the received message.

.in  3
o  buffer size - the maximum size of data to be received, in bytes.

Optional attributes:

.in  6
.ti  3
o  stream id - this is a return value that is set to indicate which
stream the data was sent to.

.ti  3
o  Stream Sequence Number - this value is returned indicating the
Stream Sequence Number that was associated with the message.

.ti  3
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.

.ti  3
o  payload protocol-id - the 32-bit unsigned integer that was sent to
the peer indicating the type of payload protocol of the
received data.

.in  3
P) Destroy SCTP Instance

.in  6
Format: DESTROY(local SCTP instance name)

.ti  3
o  local SCTP instance name - this is the value that was passed to
the application in the initialize primitive and it indicates which
SCTP instance is to be destroyed.

.in  3
.ti 0
10.2.  SCTP-to-ULP

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

IMPLEMENTATION NOTE: In some cases, this may be done through a
separate socket or error channel.
.bp
A) DATA ARRIVE notification

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

The following may optionally be passed with the notification:

o  association id - local handle to the SCTP association.

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

B) SEND FAILURE notification

If a message cannot be delivered, SCTP shall invoke this notification
on the ULP.

The following may optionally be passed with the notification:

o  association id - local handle to the SCTP association.

.in  6
.ti  3
o  data retrieval id - an identification used to retrieve unsent and
unacknowledged data.

.ti  3
o  cause code - indicating the reason of the failure, e.g., size too
large, message life time expiration, etc.

.ti  3
o  context - optional information associated with this message (see D
in Section 10.1).

.in  3
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.

.in  6
.ti  3
o  destination transport address - this indicates the destination
transport address of the peer endpoint affected by the change.

.ti 3
o  new-status - this indicates the new status.
.bp
.in  3
D) COMMUNICATION UP notification

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

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

The following shall be passed with the notification:

o  association id -  local handle to the SCTP association.

o  status -  This indicates what type of event has occurred.

.in  6
.ti  3
o  destination transport address list -  the complete set of transport
addresses of the peer.

.ti  3
o  outbound stream count -  the maximum number of streams allowed to
be used in this association by the ULP.

.ti  3
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').

.in  3
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.

.in  16
.ti  3
o  status -  this indicates what type of event has occurred; the
status may indicate that a failure OR a normal termination
event occurred in response to a shutdown or abort request.

.in  3
The following may be passed with the notification:

.in  6
.ti  3
o  data retrieval id -  an identification used to retrieve unsent and
unacknowledged data.

.in  3
o  last-acked -  the TSN last acked by that peer endpoint.

o  last-sent -  the TSN last sent to that peer endpoint.

.in  6
.ti  3
o  Upper Layer Abort Reason -  the abort reason specified in case of a
user-initiated abort.

.in  3
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.

.in  6
.ti  3
o  error info -  this indicates the type of error and optionally some
additional information received through the ERROR chunk.

.in  3
G) RESTART notification

When SCTP detects that the peer has restarted, it may send this
notification to its ULP.

The following can be passed with the notification:

o  association id -  local handle to the SCTP association.

H) SHUTDOWN COMPLETE notification

When SCTP completes the shutdown procedures (Section 9.2), this
notification is passed to the upper layer.

The following can be passed with the notification:

o  association id -  local handle to the SCTP association.

.in  0
11.  Security Considerations

.in  3
.ti 0
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

.ti 0
11.2.  SCTP Responses to Potential Threats

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

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

.ti 0
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.

.ti 0
11.2.2.  Protecting against Data Corruption in the Network

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

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

.ti 0
11.2.3.  Protecting Confidentiality

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

.ne 3
Particularly for mobile users, the requirement for confidentiality
might include the masking of IP addresses and ports.  In this case,
ESP SHOULD be used instead of application-level confidentiality.  If
ESP is used to protect confidentiality of SCTP traffic, an ESP
cryptographic transform that includes cryptographic integrity
protection MUST be used, because if there is a confidentiality threat
there will also be a strong integrity threat.

Whenever ESP is in use, application-level encryption is not generally
required.

Regardless of where confidentiality is provided, the Internet Key
Exchange Protocol version 2 (IKEv2) [RFC4306] SHOULD be used for key management.

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

.ti 0
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.

.ti 0
11.2.4.1.  Flooding

The objective of flooding is to cause loss of service and incorrect
behavior at target systems through resource exhaustion, interference
with legitimate transactions, and exploitation of buffer-related
software bugs.  Flooding may be directed either at the SCTP node or
at resources in the intervening IP Access Links or the Internet.
Where the latter entities are the target, flooding will manifest
itself as loss of network services, including potentially the breach
of any firewalls in place.

In general, protection against flooding begins at the equipment
design level, where it includes measures such as:

.in  6
.ti  3
-  avoiding commitment of limited resources before determining that
the request for service is legitimate.

.ti  3
-  giving priority to completion of processing in progress over the
acceptance of new work.

.ne 2
.ti  3
-  identification and removal of duplicate or stale queued requests
for service.

.ne 2
.ti  3
-  not responding to unexpected packets sent to non-unicast
addresses.

.in  3
Network equipment should be capable of generating an alarm and log if
a suspicious increase in traffic occurs.  The log should provide
information such as the identity of the incoming link and source
address(es) used, which will help the network or SCTP system operator
to take protective measures.  Procedures should be in place for the
operator to act on such alarms if a clear pattern of abuse emerges.

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

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

The use of the host name feature in the INIT chunk could be used to
flood a target DNS server.  A large backlog of DNS queries, resolving
the host name received in the INIT chunk to IP addresses, could be
accomplished by sending INITs to multiple hosts in a given domain.
In addition, an attacker could use the host name feature in an
indirect attack on a third party by sending large numbers of INITs to
random hosts containing the host name of the target.  In addition to
the strain on DNS resources, this could also result in large numbers
of INIT ACKs being sent to the target.  One method to protect against
this type of attack is to verify that the IP addresses received from
DNS include the source IP address of the original INIT.  If the list
of IP addresses received from DNS does not include the source IP
address of the INIT, the endpoint MAY silently discard the INIT.
This last option will not protect against the attack against the DNS.

.in  0
11.2.4.2.  Blind Masquerade

.in  3
Masquerade can be used to deny service in several ways:

.in  6
.ti  3
-  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.

.ti  3
-  by deliberately allowing the impersonation to be detected, thereby
provoking counter-measures that cause the impersonated node to be
locked out of the target SCTP node.

.ti  3
-  by interfering with an established association by inserting
extraneous content such as a SHUTDOWN request.

.in  3
SCTP reduces the risk of blind masquerade attacks through IP spoofing
by use of the four-way startup handshake.  Because the initial
exchange is memory-less, no lockout mechanism is triggered by blind
masquerade attacks.  In addition, the INIT ACK containing the State
Cookie is transmitted back to the IP address from which it received
the INIT.  Thus, the attacker would not receive the INIT ACK
containing the State Cookie.  SCTP protects against insertion of
extraneous packets into the flow of an established association by use
of the Verification Tag.

Logging of received INIT requests and abnormalities such as
unexpected INIT ACKs might be considered as a way to detect patterns
of hostile activity.  However, the potential usefulness of such
logging must be weighed against the increased SCTP startup processing
it implies, rendering the SCTP node more vulnerable to flooding
attacks.  Logging is pointless without the establishment of operating
procedures to review and analyze the logs on a routine basis.

.ti 0
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.

.ti 0
11.3.  SCTP Interactions with Firewalls

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

.ti 0
11.4.  Protection of Non-SCTP-Capable Hosts

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

When an SCTP stack receives a packet containing multiple control or
DATA chunks and the processing of the packet requires the sending of
multiple chunks in response, the sender of the response chunk(s) MUST
NOT send more than one packet.  If bundling is supported, multiple
response chunks that fit into a single packet MAY be bundled together
into one single response packet.  If bundling is not supported, then
the sender MUST NOT send more than one response chunk and MUST
discard all other responses.  Note that this rule does NOT apply to a
SACK chunk, since a SACK chunk is, in itself, a response to DATA and
a SACK does not require a response of more DATA.

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

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

.ti 0
12.  Network Management Considerations

The MIB module for SCTP defined in [RFC3873] applies for the version
of the protocol specified in this document.
.bp
.ti 0
13.  Recommended Transmission Control Block (TCB) Parameters

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

.in  0
13.1.  Parameters Necessary for the SCTP Instance

.in  17
.ti  3
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.

.ti  3
Secret Key:   A secret key used by this endpoint to compute the MAC.
This SHOULD be a cryptographic quality random number
with a sufficient length.  Discussion in RFC 4086 can
be helpful in selection of the key.

.ti  3
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.

.ti 3
SCTP Port:    The local SCTP port number to which the endpoint is bound.

.in 3
.ti 0
13.2.  Parameters Necessary per Association (i.e., the TCB)

.nf
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         :

.ti  3
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.

.fi
.in  17
Note: No "CLOSED" state is illustrated since if a
association is "CLOSED" its TCB SHOULD be removed.
.bp
.nf
.in  3
Peer        : A list of SCTP transport addresses to which the peer
Transport   : is bound.  This information is derived from the INIT or
Address     : INIT ACK and is used to associate an inbound packet
List        : with a given association.  Normally, this information
            : is hashed or keyed for quick lookup and access of the 
            : TCB.

Primary     : This is the current primary destination transport
Path        : address of the peer endpoint.  It may also specify a
            : source transport address on this endpoint.

Overall     : The overall association error count.
Error Count :

Overall     : The threshold for this association that if the Overall
Error       : Error Count reaches will cause this association to be
Threshold   : torn down.

Peer Rwnd   : Current calculated value of the peer's rwnd.

Next TSN    : The next TSN number to be assigned to a new DATA chunk.
            : This is sent in the INIT or INIT ACK chunk to the peer
            : and incremented each time a DATA chunk is assigned a
            : TSN (normally just prior to transmit or during
            : fragmentation).

Last Rcvd   : This is the last TSN received in sequence.  This value
TSN         : is set initially by taking the peer's initial TSN,
            : received in the INIT or INIT ACK chunk, and
            : subtracting one from it.

Mapping     : An array of bits or bytes indicating which out-of-
Array       : order TSNs have been received (relative to the
            : Last Rcvd TSN).  If no gaps exist, i.e., no out-of-
            : order packets have been received, this array will 
            : be set to all zero.  This structure may be in the 
            : form of a circular buffer or bit array.

Ack State   : This flag indicates if the next received packet
            : is to be responded to with a SACK.  This is initialized
            : to 0.  When a packet is received it is incremented.
            : If this value reaches 2 or more, a SACK is sent and the
            : value is reset to 0.  Note: This is used only when no
            : DATA chunks are received out of order.  When DATA 
            : chunks are out of order, SACKs are not delayed (see
            : Section 6).
.bp
Inbound     : An array of structures to track the inbound streams,
Streams     : normally including the next sequence number expected
            : and possibly the stream number.

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

Reasm Queue : A reassembly queue.

Local       : The list of local IP addresses bound in to this
Transport   : association.
Address     :
List        :

Association : The smallest PMTU discovered for all of the
PMTU        : peer's transport addresses.

.fi
.ti 0
13.3.  Per Transport Address Data

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

.nf
Error Count : The current error count for this destination.

Error       : Current error threshold for this destination, i.e.,
Threshold   : what value marks the destination down if error count
            : reaches this value.

cwnd        : The current congestion window.

ssthresh    : The current ssthresh value.

RTO         : The current retransmission timeout value.

SRTT        : The current smoothed round-trip time.

RTTVAR      : The current RTT variation.

partial     : The tracking method for increase of cwnd when in
bytes acked : congestion avoidance mode (see Section 7.2.2).

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

Per         : A timer used by each destination.
Destination :
Timer       :

RTO-Pending : A flag used to track if one of the DATA chunks sent to
            : this address is currently being used to compute an
            : RTT.  If this flag is 0, the next DATA chunk sent to
            : this destination should be used to compute an RTT and
            : this flag should be set.  Every time the RTT
            : calculation completes (i.e., the DATA chunk is SACK'd),
            : clear this flag.

last-time   : The time to which this destination was last sent. 
            : This can be to determine if a HEARTBEAT is needed.

.in  0
13.4.  General Parameters Needed

.in  3
Out Queue : A queue of outbound DATA chunks.

In Queue  : A queue of inbound DATA chunks.

.fi
.ti 0
14.  IANA Considerations

SCTP defines three registries that IANA maintains:

.in 6
.ti 3
-  through definition of additional chunk types,
.ti 3
-  through definition of additional parameter types, or
.ti 3
-  through definition of additional cause codes within ERROR chunks. 

.in 3
.fi
SCTP requires that the IANA Port Numbers registry be opened for
SCTP port registrations, Section 14.5 describes how.  An IESG-appointed
Expert Reviewer supports IANA in evaluating SCTP port allocation
requests.

.ti 0
14.1.  IETF-Defined Chunk Extension

The assignment of new chunk parameter type codes is done through an
IETF Consensus action, as defined in [RFC2434].  Documentation of the
chunk parameter MUST contain the following information:

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

.in  6
.ti  3
b) A detailed description of the structure of the chunk, which MUST
conform to the basic structure defined in Section 3.2.

.ne 2
.ti  3
c) A detailed definition and description of intended use of each
field within the chunk, including the chunk flags if any.

.ti  3
d) A detailed procedural description of the use of the new chunk type
within the operation of the protocol.

.in  3
The last chunk type (255) is reserved for future extension if
necessary.

.ti 0
14.2.  IETF-Defined Chunk Parameter Extension

The assignment of new chunk parameter type codes is done through an
IETF Consensus action as defined in [RFC2434].  Documentation of the
chunk parameter MUST contain the following information:

a) Name of the parameter type.

.in  6
.ti  3
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.

.ti 3
c) Detailed definition of each component of the parameter value.

.ti 3
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.

.ti 3
e) Each parameter type MUST be unique across all chunks.

.fi
.in 3
.ti 0
14.3.  IETF-Defined Additional Error Causes

Additional cause codes may be allocated in the range 11 to 65535
through a Specification Required action as defined in [RFC2434].
Provided documentation must include the following information:

a) Name of the error condition.

.in  6
.ti  3
b) Detailed description of the conditions under which an SCTP
endpoint should issue an ERROR (or ABORT) with this cause code.

.ti  3
c) Expected action by the SCTP endpoint that receives an ERROR (or
ABORT) chunk containing this cause code.

.ti  3
d) Detailed description of the structure and content of data fields
that accompany this cause code.

.ne 3
.in  3
The initial word (32 bits) of a cause code parameter MUST conform to
the format shown in Section 3.3.10, i.e.:

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

.fi
.ti 0
14.4.  Payload Protocol Identifiers

Except for value 0, which is reserved by SCTP to indicate an
unspecified payload protocol identifier in a DATA chunk, SCTP will
not be responsible for standardizing or verifying any payload
protocol identifiers; SCTP simply receives the identifier from the
upper layer and carries it with the corresponding payload data.

The upper layer, i.e., the SCTP user, SHOULD standardize any specific
protocol identifier with IANA if it is so desired.  The use of any
specific payload protocol identifier is out of the scope of SCTP.

.ti 0
14.5.  Port Numbers Registry

SCTP services may use contact port numbers to provide service to
unknown callers, as in TCP and UDP.  IANA is therefore requested to
open the existing Port Numbers registry for SCTP using the following
rules, which we intend to mesh well with existing Port Numbers
registration procedures.  An IESG-appointed Expert Reviewer supports
IANA in evaluating SCTP port allocation requests, according to the
procedure defined in [RFC2434].

Port numbers are divided into three ranges.  The Well Known Ports are
those from 0 through 1023, the Registered Ports are those from 1024
through 49151, and the Dynamic and/or Private Ports are those from
49152 through 65535.  Well Known and Registered Ports are intended
for use by server applications that desire a default contact point on
a system.  On most systems, Well Known Ports can only be used by
system (or root) processes or by programs executed by privileged
users, while Registered Ports can be used by ordinary user processes
or programs executed by ordinary users.  Dynamic and/or Private Ports
are intended for temporary use, including client-side ports,
out-of-band negotiated ports, and application testing prior to registration
of a dedicated port; they MUST NOT be registered.

The Port Numbers registry should accept registrations for SCTP ports
in the Well Known Ports and Registered Ports ranges.  Well Known and
Registered Ports SHOULD NOT be used without registration.  Although
in some cases -- such as porting an application from TCP to SCTP --
it may seem natural to use an SCTP port before registration completes,
we emphasize that IANA will not guarantee registration of 
.bp
particular
Well Known and Registered Ports.  Registrations should be requested
as early as possible.

Each port registration SHALL include the following information:

.in  6
.ti 3
o  A short port name, consisting entirely of letters (A-Z and a-z),
digits (0-9), and punctuation characters from "-_+./*" (not
including the quotes).

.ti 3
o  The port number that is requested for registration.

.ti 3
o  A short English phrase describing the port's purpose.

.ti 3
o  Name and contact information for the person or entity performing
the registration, and possibly a reference to a document defining
the port's use.  Registrations coming from IETF working groups
need only name the working group, but indicating a contact person
is recommended.

.in 3
Registrants are encouraged to follow these guidelines when submitting
a registration.

.in 6
.ti 3
o  A port name SHOULD NOT be registered for more than one SCTP port
number.

.ti 3
o  A port name registered for TCP MAY be registered for SCTP as well.
Any such registration SHOULD use the same port number as the
existing TCP registration.

.ti 3
o  Concrete intent to use a port SHOULD precede port registration.
For example, existing TCP ports SHOULD NOT be registered in
advance of any intent to use those ports for SCTP.

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

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

   The discard service, which accepts SCTP connections on port
   9, discards all incoming application data and sends no data
   in response.  Thus, SCTP's discard port is analogous to 
   TCP's discard port, and might be used to check the health 
   of an SCTP stack. 
.bp
ftp-data  20/sctp  FTP      # IETF TSVWG
                            # Randall Stewart <rrs@cisco.com>
                            # [RFC4960]

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

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

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

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

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

   World Wide Web HTTP over SCTP.

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

   Border Gateway Protocol over SCTP.

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

   World Wide Web HTTP over TLS/SSL over SCTP.

.fi
.in  0
15.  Suggested SCTP Protocol Parameter Values

.in  6
.ti 3
The following protocol parameters are RECOMMENDED:

.nf
RTO.Initial - 3 seconds
RTO.Min - 1 second
RTO.Max - 60 seconds
Max.Burst - 4
RTO.Alpha - 1/8
RTO.Beta - 1/4
Valid.Cookie.Life - 60 seconds
Association.Max.Retrans - 10 attempts
Path.Max.Retrans - 5 attempts (per destination address)
Max.Init.Retransmits - 8 attempts
HB.interval - 30 seconds
HB.Max.Burst - 1

.fi
.in  3
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.

.ti 0
16.  Acknowledgements

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

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

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

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

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

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

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

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

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

Randall Stewart - Editor
.bp
.in  3
.ti 0
Appendix A.  Explicit Congestion Notification

ECN [RFC3168] describes a proposed extension to IP that
details a method to become aware of congestion outside of datagram
loss.  This is an optional feature that an implementation MAY choose
to add to SCTP.  This appendix details the minor differences
implementers will need to be aware of if they choose to implement
this feature.  In general, [RFC3168] should be followed with the
following exceptions.

Negotiation:

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

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

.fi
.in  3
ECN-Echo:

[RFC3168] details a specific bit for a receiver to send back in its
TCP acknowledgements to notify the sender of the Congestion
Experienced (CE) bit having arrived from the network.  For SCTP, this
same indication is made by including the ECNE chunk.  This chunk
contains one data element, i.e., the lowest TSN associated with the IP
datagram marked with the CE bit, and looks as follows:

.in  7
.nf
 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                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
Note: The ECNE is considered a Control chunk.

.bp
.in  3
CWR:

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

.in  7
.nf
 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                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.fi
Note: The CWR is considered a Control chunk.

.in  3
.ti 0
Appendix B.  CRC32c Checksum Calculation

We define a \'reflected value' as one that is the opposite of the
normal bit order of the machine.  The 32-bit CRC (Cyclic Redundancy Check) is calculated as
described for CRC32c and uses the polynomial code 0x11EDC6F41
(Castagnoli93) or x^32+x^28+x^27+x^26+x^25 +x^23+x^22+x^20+x^19+x^18+
x^14+x^13+x^11+x^10+x^9+x^8+x^6+x^0.  The CRC is computed using a
procedure similar to ETHERNET CRC [ITU32], modified to reflect
transport-level usage.

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

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

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

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

.in 6
.ti 3
-  CRC input data are assigned to a byte stream, numbered from 0 to
N-1.

.ti 3
-  The transport-level byte stream is mapped to a polynomial value.
An N-byte PDU with j bytes numbered 0 to N-1 is considered as
coefficients of a polynomial M(x) of order 8N-1, with bit 0 of
byte j being coefficient x^(8(N-j)-8), and bit 7 of byte j being
coefficient x^(8(N-j)-1).

.ti 3 
-  The CRC remainder register is initialized with all 1s and the CRC
is computed with an algorithm that simultaneously multiplies by
x^32 and divides by the CRC polynomial.

.ti 3
-  The polynomial is multiplied by x^32 and divided by G(x), the
generator polynomial, producing a remainder R(x) of degree less
than or equal to 31.

.ti 3
-  The coefficients of R(x) are considered a 32-bit sequence.

.ti 3
-  The bit sequence is complemented.  The result is the CRC
polynomial.

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

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

.ne 3
Implementors of SCTP are warned that both specifications are to be
found in the literature, sometimes with no restriction on the long-
division algorithm.  The choice of formulation in this document is to
permit non-SCTP usage, where the same CRC algorithm may be used to
protect messages shorter than 64 bits.

There may be a computational advantage in validating the association
against the Verification Tag, prior to performing a checksum, as
invalid tags will result in the same action as a bad checksum in most
cases.  The exceptions for this technique would be INIT and some
SHUTDOWN-COMPLETE exchanges, as well as a stale COOKIE ECHO.  These
special-case exchanges must represent small packets and will minimize
the effect of the checksum calculation.

.ti 0
Appendix C.  ICMP Handling

.in  3
Whenever an ICMP message is received by an SCTP endpoint, the
following procedures MUST be followed to ensure proper utilization of
the information being provided by layer 3.

.in 10
.ti 3
ICMP1) An implementation MAY ignore all ICMPv4 messages where the
type field is not set to "Destination Unreachable".

.ti 3
ICMP2) An implementation MAY ignore all ICMPv6 messages where the
type field is not "Destination Unreachable", "Parameter Problem",, or
"Packet Too Big".

.ti 3
ICMP3) An implementation MAY ignore any ICMPv4 messages where the
code does not indicate "Protocol Unreachable" or "Fragmentation
Needed".

.ti 3
ICMP4) An implementation MAY ignore all ICMPv6 messages of type
"Parameter Problem" if the code is not "Unrecognized Next Header
Type Encountered".

.ti 3
ICMP5) An implementation MUST use the payload of the ICMP message
(v4 or v6) to locate the association that sent the message to which
ICMP is responding.  If the association cannot be found, an
implementation SHOULD ignore the ICMP message.

.ti 3
ICMP6) An implementation MUST validate that the Verification Tag
contained in the ICMP message matches the Verification Tag of the
peer.  If the Verification Tag is not 0 and does NOT match,
discard the ICMP message.  If it is 0 and the ICMP message
contains enough bytes to verify that the chunk type is an INIT
chunk and that the Initiate Tag matches the tag of the 
.bp
peer,
continue with ICMP7.  If the ICMP message is too short or the
chunk type or the Initiate Tag does not match, silently discard
the packet.

.ne 3
.ti 3
ICMP7) If the ICMP message is either a v6 "Packet Too Big" or a v4
"Fragmentation Needed", an implementation MAY process this
information as defined for PATH MTU discovery.

.ti 3
ICMP8) If the ICMP code is an "Unrecognized Next Header Type
Encountered" or a "Protocol Unreachable", an implementation MUST
treat this message as an abort with the T bit set if it does not
contain an INIT chunk.  If it does contain an INIT chunk and the
association is in the COOKIE-WAIT state, handle the ICMP message like
an ABORT.

.ti 3
ICMP9) If the ICMPv6 code is "Destination Unreachable", the
implementation MAY mark the destination into the unreachable state
or alternatively increment the path error counter.

.in 3
.fi
Note that these procedures differ from [RFC1122] and from its
requirements for processing of port-unreachable messages and the
requirements that an implementation MUST abort associations in
response to a "protocol unreachable" message.  Port-unreachable
messages are not processed, since an implementation will send an
ABORT, not a port unreachable.  The stricter handling of the
"protocol unreachable" message is due to security concerns for hosts
that do NOT support SCTP.

The following non-normative sample code is taken from an open-source
CRC generator [WILLIAMS93], using the "mirroring" technique and
yielding a lookup table for SCTP CRC32c with 256 entries, each 32
bits wide.  While neither especially slow nor especially fast, as
software table-lookup CRCs go, it has the advantage of working on
both big-endian and little-endian CPUs, using the same (host-order)
lookup tables, and using only the predefined ntohl() and htonl()
operations.  The code is somewhat modified from [WILLIAMS93], to
ensure portability between big-endian and little-endian
architectures.  (Note that if the byte endian-ness of the target
architecture is known to be little-endian, the final bit-reversal and
byte-reversal steps can be folded into a single operation.)

.nf
/*************************************************************/
/* Note Definition for Ross Williams table generator would   */
/* be: TB_WIDTH=4, TB_POLLY=0x1EDC6F41, TB_REVER=TRUE        */
/* For Mr. Williams direct calculation code use the settings */
/* cm_width=32, cm_poly=0x1EDC6F41, cm_init=0xFFFFFFFF,      */
/* cm_refin=TRUE, cm_refot=TRUE, cm_xorort=0x00000000        */
/*************************************************************/

/* Example of the crc table file */
#ifndef __crc32cr_table_h__
#define __crc32cr_table_h__

#define CRC32C_POLY 0x1EDC6F41
#define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])

unsigned long  crc_c[256] =
{
0x00000000L, 0xF26B8303L, 0xE13B70F7L, 0x1350F3F4L,
0xC79A971FL, 0x35F1141CL, 0x26A1E7E8L, 0xD4CA64EBL,
0x8AD958CFL, 0x78B2DBCCL, 0x6BE22838L, 0x9989AB3BL,
0x4D43CFD0L, 0xBF284CD3L, 0xAC78BF27L, 0x5E133C24L,
0x105EC76FL, 0xE235446CL, 0xF165B798L, 0x030E349BL,
0xD7C45070L, 0x25AFD373L, 0x36FF2087L, 0xC494A384L,
0x9A879FA0L, 0x68EC1CA3L, 0x7BBCEF57L, 0x89D76C54L,
0x5D1D08BFL, 0xAF768BBCL, 0xBC267848L, 0x4E4DFB4BL,
0x20BD8EDEL, 0xD2D60DDDL, 0xC186FE29L, 0x33ED7D2AL,
0xE72719C1L, 0x154C9AC2L, 0x061C6936L, 0xF477EA35L,
0xAA64D611L, 0x580F5512L, 0x4B5FA6E6L, 0xB93425E5L,
0x6DFE410EL, 0x9F95C20DL, 0x8CC531F9L, 0x7EAEB2FAL,
0x30E349B1L, 0xC288CAB2L, 0xD1D83946L, 0x23B3BA45L,

0xF779DEAEL, 0x05125DADL, 0x1642AE59L, 0xE4292D5AL,
0xBA3A117EL, 0x4851927DL, 0x5B016189L, 0xA96AE28AL,
0x7DA08661L, 0x8FCB0562L, 0x9C9BF696L, 0x6EF07595L,
0x417B1DBCL, 0xB3109EBFL, 0xA0406D4BL, 0x522BEE48L,
0x86E18AA3L, 0x748A09A0L, 0x67DAFA54L, 0x95B17957L,
0xCBA24573L, 0x39C9C670L, 0x2A993584L, 0xD8F2B687L,
0x0C38D26CL, 0xFE53516FL, 0xED03A29BL, 0x1F682198L,
0x5125DAD3L, 0xA34E59D0L, 0xB01EAA24L, 0x42752927L,
0x96BF4DCCL, 0x64D4CECFL, 0x77843D3BL, 0x85EFBE38L,
0xDBFC821CL, 0x2997011FL, 0x3AC7F2EBL, 0xC8AC71E8L,
0x1C661503L, 0xEE0D9600L, 0xFD5D65F4L, 0x0F36E6F7L,
0x61C69362L, 0x93AD1061L, 0x80FDE395L, 0x72966096L,
0xA65C047DL, 0x5437877EL, 0x4767748AL, 0xB50CF789L,
0xEB1FCBADL, 0x197448AEL, 0x0A24BB5AL, 0xF84F3859L,
0x2C855CB2L, 0xDEEEDFB1L, 0xCDBE2C45L, 0x3FD5AF46L,
0x7198540DL, 0x83F3D70EL, 0x90A324FAL, 0x62C8A7F9L,
0xB602C312L, 0x44694011L, 0x5739B3E5L, 0xA55230E6L,
0xFB410CC2L, 0x092A8FC1L, 0x1A7A7C35L, 0xE811FF36L,
0x3CDB9BDDL, 0xCEB018DEL, 0xDDE0EB2AL, 0x2F8B6829L,
0x82F63B78L, 0x709DB87BL, 0x63CD4B8FL, 0x91A6C88CL,
0x456CAC67L, 0xB7072F64L, 0xA457DC90L, 0x563C5F93L,
0x082F63B7L, 0xFA44E0B4L, 0xE9141340L, 0x1B7F9043L,
0xCFB5F4A8L, 0x3DDE77ABL, 0x2E8E845FL, 0xDCE5075CL,
0x92A8FC17L, 0x60C37F14L, 0x73938CE0L, 0x81F80FE3L,
0x55326B08L, 0xA759E80BL, 0xB4091BFFL, 0x466298FCL,
0x1871A4D8L, 0xEA1A27DBL, 0xF94AD42FL, 0x0B21572CL,
0xDFEB33C7L, 0x2D80B0C4L, 0x3ED04330L, 0xCCBBC033L,
0xA24BB5A6L, 0x502036A5L, 0x4370C551L, 0xB11B4652L,
0x65D122B9L, 0x97BAA1BAL, 0x84EA524EL, 0x7681D14DL,
0x2892ED69L, 0xDAF96E6AL, 0xC9A99D9EL, 0x3BC21E9DL,
0xEF087A76L, 0x1D63F975L, 0x0E330A81L, 0xFC588982L,
0xB21572C9L, 0x407EF1CAL, 0x532E023EL, 0xA145813DL,
0x758FE5D6L, 0x87E466D5L, 0x94B49521L, 0x66DF1622L,
0x38CC2A06L, 0xCAA7A905L, 0xD9F75AF1L, 0x2B9CD9F2L,
0xFF56BD19L, 0x0D3D3E1AL, 0x1E6DCDEEL, 0xEC064EEDL,
0xC38D26C4L, 0x31E6A5C7L, 0x22B65633L, 0xD0DDD530L,
0x0417B1DBL, 0xF67C32D8L, 0xE52CC12CL, 0x1747422FL,
0x49547E0BL, 0xBB3FFD08L, 0xA86F0EFCL, 0x5A048DFFL,
0x8ECEE914L, 0x7CA56A17L, 0x6FF599E3L, 0x9D9E1AE0L,
0xD3D3E1ABL, 0x21B862A8L, 0x32E8915CL, 0xC083125FL,
0x144976B4L, 0xE622F5B7L, 0xF5720643L, 0x07198540L,
0x590AB964L, 0xAB613A67L, 0xB831C993L, 0x4A5A4A90L,
0x9E902E7BL, 0x6CFBAD78L, 0x7FAB5E8CL, 0x8DC0DD8FL,
0xE330A81AL, 0x115B2B19L, 0x020BD8EDL, 0xF0605BEEL,
0x24AA3F05L, 0xD6C1BC06L, 0xC5914FF2L, 0x37FACCF1L,
0x69E9F0D5L, 0x9B8273D6L, 0x88D28022L, 0x7AB90321L,
0xAE7367CAL, 0x5C18E4C9L, 0x4F48173DL, 0xBD23943EL,
0xF36E6F75L, 0x0105EC76L, 0x12551F82L, 0xE03E9C81L,

0x34F4F86AL, 0xC69F7B69L, 0xD5CF889DL, 0x27A40B9EL,
0x79B737BAL, 0x8BDCB4B9L, 0x988C474DL, 0x6AE7C44EL,
0xBE2DA0A5L, 0x4C4623A6L, 0x5F16D052L, 0xAD7D5351L,
};

#endif

.in  4
/* Example of table build routine */

.in  3
#include <stdio.h>
#include <stdlib.h>

#define OUTPUT_FILE   "crc32cr.h"
#define CRC32C_POLY    0x1EDC6F41L
FILE *tf;
unsigned long
reflect_32 (unsigned long b)
{
  int i;
  unsigned long rw = 0L;

  for (i = 0; i < 32; i++){
      if (b & 1)
        rw |= 1 << (31 - i);
      b >>= 1;
  }
  return (rw);
}

unsigned long
build_crc_table (int index)
{
  int i;
  unsigned long rb;

.in  5
rb = reflect_32 (index);

.in  3
  for (i = 0; i < 8; i++){
      if (rb & 0x80000000L)
       rb = (rb << 1) ^ CRC32C_POLY;
      else
       rb <<= 1;
  }
  return (reflect_32 (rb));
}

main ()
{
  int i;

.in  5
printf ("\\nGenerating CRC-32c table file <%s>\\n",
OUTPUT_FILE);
if ((tf = fopen (OUTPUT_FILE, "w")) == NULL){
    printf ("Unable to open %s\\n", OUTPUT_FILE);
    exit (1);
}
fprintf (tf, "#ifndef __crc32cr_table_h__\\n");
fprintf (tf, "#define __crc32cr_table_h__\\n\\n");
fprintf (tf, "#define CRC32C_POLY 0x%08lX\\n",
CRC32C_POLY);
fprintf (tf,
"#define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])\\n");
fprintf (tf, "\\nunsigned long  crc_c[256] =\\n{\\n");
for (i = 0; i < 256; i++){
    fprintf (tf, "0x%08lXL, ", build_crc_table (i));
    if ((i & 3) == 3)
      fprintf (tf, "\\n");
}
fprintf (tf, "};\\n\\n#endif\\n");

.in  3
  if (fclose (tf) != 0)
    printf ("Unable to close <%s>." OUTPUT_FILE);
  else
    printf ("\\nThe CRC-32c table has been written to <%s>.\\n",
      OUTPUT_FILE);
}

/* Example of crc insertion */

#include "crc32cr.h"

unsigned long
generate_crc32c(unsigned char *buffer, unsigned int length)
{
  unsigned int i;
  unsigned long crc32 = ~0L;
  unsigned long result;
  unsigned char byte0,byte1,byte2,byte3;

.in  5
for (i = 0; i < length; i++){
    CRC32C(crc32, buffer[i]);
}

result = ~crc32;

.in  6
.ti  5
/*  result now holds the negated polynomial remainder;
*  since the table and algorithm is "reflected" [williams95].
*  That is, result has the same value as if we mapped the message
*  to a polynomial, computed the host-bit-order polynomial
*  remainder, performed final negation, then did an end-for-end
*  bit-reversal.
*  Note that a 32-bit bit-reversal is identical to four inplace
*  8-bit reversals followed by an end-for-end byteswap.
*  In other words, the bytes of each bit are in the right order,
*  but the bytes have been byteswapped.  So we now do an explicit
*  byteswap.  On a little-endian machine, this byteswap and
*  the final ntohl cancel out and could be elided.
*/

.in  3
  byte0 = result & 0xff;
  byte1 = (result>>8) & 0xff;
  byte2 = (result>>16) & 0xff;
  byte3 = (result>>24) & 0xff;
  crc32 = ((byte0 << 24) |
           (byte1 << 16) |
           (byte2 << 8)  |
           byte3);
  return ( crc32 );
}

int
insert_crc32(unsigned char *buffer, unsigned int length)
{
  SCTP_message *message;
  unsigned long crc32;
  message = (SCTP_message *) buffer;
  message->common_header.checksum = 0L;
  crc32 = generate_crc32c(buffer,length);
  /* and insert it into the message */
  message->common_header.checksum = htonl(crc32);
  return 1;
}

int
validate_crc32(unsigned char *buffer, unsigned int length)
{
  SCTP_message *message;
  unsigned int i;
  unsigned long original_crc32;
  unsigned long crc32 = ~0L;

  /* save and zero checksum */
  message = (SCTP_message *) buffer;
  original_crc32 = ntohl(message->common_header.checksum);
  message->common_header.checksum = 0L;
  crc32 = generate_crc32c(buffer,length);
  return ((original_crc32 == crc32)? 1 : -1);
}

.bp
.fi
.ti 0
References

.ti 0
Normative References

.in  16
.ti  3
[ITU32]      "ITU-T Recommendation V.42, "Error-correcting procedures
for DCEs using asynchronous-to-synchronous conversion".",
ITU-T section 8.1.1.6.2.

.ti  3
[RFC0768]    Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.

.ti  3
[RFC0793]    Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.

.ti  3
[RFC1122]    Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.

.ti  3
[RFC1123]    Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.

.ti  3
[RFC1191]    Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.

.ti  3
[RFC1981]    McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.

.ti  3
[RFC1982]    Elz, R. and R. Bush, "Serial Number Arithmetic", RFC
1982, August 1996.

.ti  3
[RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

.ti  3
[RFC2434]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

.ti  3
[RFC2460]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

.ti  3
[RFC2581]    Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.

.ti  3
[RFC3873]    Pastor, J. and M. Belinchon, "Stream Control Transmission
Protocol (SCTP) Management Information Base (MIB)", RFC 3873,
September 2004.

.ti  3
[RFC4291]    Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.

.ti  3
[RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

.ti  3
[RFC4303]    Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.

.ti  3
[RFC4306]    Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.

.ti  3
[RFC4821]    Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.

.in  0
Informative References

.in  16
.ti  3
[FALL96]     Fall, K. and S. Floyd, "Simulation-based Comparisons of
\# FIXME MT: 5- 21 -> 5-21
Tahoe, Reno, and SACK TCP", SIGCOMM'99 V. 26 N. 3 pp 5- 21,
July 1996.

.ne 3
.ti  3
[SAVAGE99]   Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP Congestion Control with a Misbehaving Receiver", ACM
Computer Communications Review 29(5), October 1999.

.ti  3
[ALLMAN99]   Allman, M. and V. Paxson, "On Estimating End-to-End
Network Path Properties", SIGCOMM'99 , 1999.

.ti  3
[WILLIAMS93] Williams, R., "A PAINLESS GUIDE TO CRC ERROR DETECTION
ALGORITHMS", Internet publication,
http://www.geocities.com/SiliconValley/Pines/
.br
8659/crc.htm, August 1993.

.ti  3
[RFC0813]    Clark, D., "Window and Acknowledgement Strategy in TCP",
RFC 813, July 1982.

.ti  3
[RFC1858]    Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858, October 1995.

.ti  3
[RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
\%Keyed-Hashing for Message Authentication", RFC 2104, February 1997.

.ti  3
[RFC2196]    Fraser, B., "Site Security Handbook", FYI 8, RFC 2196,
September 1997.

.ti  3
[RFC2522]    Karn, P. and W. Simpson, "Photuris: Session-Key
Management Protocol", RFC 2522, March 1999.

.ti  3
[RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and
V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October
2000.

.ti  3
[RFC3309]    Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309, September 2002.

.ti  3
[RFC3168]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC 3168, September
2001.

.ne 3
.ti  3
[RFC4086]    Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.

.ti  3
[RFC4460]    Stewart, R., Arias-Rodriguez, I., Poon, K., Caro, A., and
M. Tuexen, "Stream Control Transmission Protocol (SCTP) Specification
Errata and Issues", RFC 4460, April 2006.

.ti  3
[RFC4895]    Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for Stream Control Transmission Protocol
(SCTP)", RFC 4895, August 2007.

.in  0
Editor's Address

.in  3
.nf
Randall R. Stewart
4875 Forest Drive
Suite 200
Columbia, SC  29206
US

EMail: rrs@cisco.com

.bp
.in  3
.fi
.ti 0
Full Copyright Statement

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.ti 0
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