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Network Working Group R. Stewart
Request for Comments: 2960 Q. Xie
Category: Standards Track Motorola
K. Morneault
C. Sharp
Cisco
H. Schwarzbauer
Siemens
T. Taylor
Nortel Networks
I. Rytina
Ericsson
M. Kalla
Telcordia
L. Zhang
UCLA
V. Paxson
ACIRI
October 2000
Stream Control Transmission Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the Stream Control Transmission Protocol
(SCTP). SCTP is designed to transport PSTN signaling messages over
IP networks, but is capable of broader applications.
SCTP is a reliable transport protocol operating on top of a
connectionless packet network such as IP. It offers the following
services to its users:
-- acknowledged error-free non-duplicated transfer of user data,
-- data fragmentation to conform to discovered path MTU size,
-- sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual user
messages,
-- optional bundling of multiple user messages into a single SCTP
packet, and
-- network-level fault tolerance through supporting of multi-
homing at either or both ends of an association.
The design of SCTP includes appropriate congestion avoidance behavior
and resistance to flooding and masquerade attacks.
Table of Contents
1. Introduction.................................................. 5
1.1 Motivation.................................................. 6
1.2 Architectural View of SCTP.................................. 6
1.3 Functional View of SCTP..................................... 7
1.3.1 Association Startup and Takedown........................ 8
1.3.2 Sequenced Delivery within Streams....................... 9
1.3.3 User Data Fragmentation................................. 9
1.3.4 Acknowledgement and Congestion Avoidance................ 9
1.3.5 Chunk Bundling ......................................... 10
1.3.6 Packet Validation....................................... 10
1.3.7 Path Management......................................... 11
1.4 Key Terms................................................... 11
1.5 Abbreviations............................................... 15
1.6 Serial Number Arithmetic.................................... 15
2. Conventions.................................................... 16
3. SCTP packet Format............................................ 16
3.1 SCTP Common Header Field Descriptions....................... 17
3.2 Chunk Field Descriptions.................................... 18
3.2.1 Optional/Variable-length Parameter Format............... 20
3.3 SCTP Chunk Definitions...................................... 21
3.3.1 Payload Data (DATA)..................................... 22
3.3.2 Initiation (INIT)....................................... 24
3.3.2.1 Optional or Variable Length Parameters.............. 26
3.3.3 Initiation Acknowledgement (INIT ACK)................... 30
3.3.3.1 Optional or Variable Length Parameters.............. 33
3.3.4 Selective Acknowledgement (SACK)........................ 33
3.3.5 Heartbeat Request (HEARTBEAT)........................... 37
3.3.6 Heartbeat Acknowledgement (HEARTBEAT ACK)............... 38
3.3.7 Abort Association (ABORT)............................... 39
3.3.8 Shutdown Association (SHUTDOWN)......................... 40
3.3.9 Shutdown Acknowledgement (SHUTDOWN ACK)................. 40
3.3.10 Operation Error (ERROR)................................ 41
3.3.10.1 Invalid Stream Identifier.......................... 42
3.3.10.2 Missing Mandatory Parameter........................ 43
3.3.10.3 Stale Cookie Error................................. 43
3.3.10.4 Out of Resource.................................... 44
3.3.10.5 Unresolvable Address............................... 44
3.3.10.6 Unrecognized Chunk Type............................ 44
3.3.10.7 Invalid Mandatory Parameter........................ 45
3.3.10.8 Unrecognized Parameters............................ 45
3.3.10.9 No User Data....................................... 46
3.3.10.10 Cookie Received While Shutting Down............... 46
3.3.11 Cookie Echo (COOKIE ECHO).............................. 46
3.3.12 Cookie Acknowledgement (COOKIE ACK).................... 47
3.3.13 Shutdown Complete (SHUTDOWN COMPLETE).................. 48
4. SCTP Association State Diagram................................. 48
5. Association Initialization..................................... 52
5.1 Normal Establishment of an Association...................... 52
5.1.1 Handle Stream Parameters................................ 54
5.1.2 Handle Address Parameters............................... 54
5.1.3 Generating State Cookie................................. 56
5.1.4 State Cookie Processing................................. 57
5.1.5 State Cookie Authentication............................. 57
5.1.6 An Example of Normal Association Establishment.......... 58
5.2 Handle Duplicate or unexpected INIT, INIT ACK, COOKIE ECHO,
and COOKIE ACK.............................................. 60
5.2.1 Handle Duplicate INIT in COOKIE-WAIT
or COOKIE-ECHOED States................................. 60
5.2.2 Unexpected INIT in States Other than CLOSED,
COOKIE-ECHOED, COOKIE-WAIT and SHUTDOWN-ACK-SENT........ 61
5.2.3 Unexpected INIT ACK..................................... 61
5.2.4 Handle a COOKIE ECHO when a TCB exists.................. 62
5.2.4.1 An Example of a Association Restart................. 64
5.2.5 Handle Duplicate COOKIE ACK............................. 66
5.2.6 Handle Stale COOKIE Error............................... 66
5.3 Other Initialization Issues................................. 67
5.3.1 Selection of Tag Value.................................. 67
6. User Data Transfer............................................. 67
6.1 Transmission of DATA Chunks................................. 69
6.2 Acknowledgement on Reception of DATA Chunks................. 70
6.2.1 Tracking Peer's Receive Buffer Space.................... 73
6.3 Management Retransmission Timer............................. 75
6.3.1 RTO Calculation......................................... 75
6.3.2 Retransmission Timer Rules.............................. 76
6.3.3 Handle T3-rtx Expiration................................ 77
6.4 Multi-homed SCTP Endpoints.................................. 78
6.4.1 Failover from Inactive Destination Address.............. 79
6.5 Stream Identifier and Stream Sequence Number................ 80
6.6 Ordered and Unordered Delivery.............................. 80
6.7 Report Gaps in Received DATA TSNs........................... 81
6.8 Adler-32 Checksum Calculation............................... 82
6.9 Fragmentation............................................... 83
6.10 Bundling .................................................. 84
7. Congestion Control .......................................... 85
7.1 SCTP Differences from TCP Congestion Control................ 85
7.2 SCTP Slow-Start and Congestion Avoidance.................... 87
7.2.1 Slow-Start.............................................. 87
7.2.2 Congestion Avoidance.................................... 89
7.2.3 Congestion Control...................................... 89
7.2.4 Fast Retransmit on Gap Reports.......................... 90
7.3 Path MTU Discovery.......................................... 91
8. Fault Management.............................................. 92
8.1 Endpoint Failure Detection.................................. 92
8.2 Path Failure Detection...................................... 92
8.3 Path Heartbeat.............................................. 93
8.4 Handle "Out of the blue" Packets............................ 95
8.5 Verification Tag............................................ 96
8.5.1 Exceptions in Verification Tag Rules.................... 97
9. Termination of Association..................................... 98
9.1 Abort of an Association..................................... 98
9.2 Shutdown of an Association.................................. 98
10. Interface with Upper Layer....................................101
10.1 ULP-to-SCTP................................................101
10.2 SCTP-to-ULP................................................111
11. Security Considerations.......................................114
11.1 Security Objectives........................................114
11.2 SCTP Responses To Potential Threats........................115
11.2.1 Countering Insider Attacks.............................115
11.2.2 Protecting against Data Corruption in the Network......115
11.2.3 Protecting Confidentiality.............................115
11.2.4 Protecting against Blind Denial of Service Attacks.....116
11.2.4.1 Flooding...........................................116
11.2.4.2 Blind Masquerade...................................118
11.2.4.3 Improper Monopolization of Services................118
11.3 Protection against Fraud and Repudiation...................119
12. Recommended Transmission Control Block (TCB) Parameters.......120
12.1 Parameters necessary for the SCTP instance.................120
12.2 Parameters necessary per association (i.e. the TCB)........120
12.3 Per Transport Address Data.................................122
12.4 General Parameters Needed..................................123
13. IANA Considerations...........................................123
13.1 IETF-defined Chunk Extension...............................123
13.2 IETF-defined Chunk Parameter Extension.....................124
13.3 IETF-defined Additional Error Causes.......................124
13.4 Payload Protocol Identifiers...............................125
14. Suggested SCTP Protocol Parameter Values......................125
15. Acknowledgements..............................................126
16. Authors' Addresses............................................126
17. References....................................................128
18. Bibliography..................................................129
Appendix A .......................................................131
Appendix B .......................................................132
Full Copyright Statement .........................................134
1. Introduction
This section explains the reasoning behind the development of the
Stream Control Transmission Protocol (SCTP), the services it offers,
and the basic concepts needed to understand the detailed description
of the protocol.
1.1 Motivation
TCP [RFC793] has performed immense service as the primary means of
reliable data transfer in IP networks. However, an increasing number
of recent applications have found TCP too limiting, and have
incorporated their own reliable data transfer protocol on top of UDP
[RFC768]. The limitations which users have wished to bypass include
the following:
-- TCP provides both reliable data transfer and strict order-of-
transmission delivery of data. Some applications need reliable
transfer without sequence maintenance, while others would be
satisfied with partial ordering of the data. In both of these
cases the head-of-line blocking offered by TCP causes unnecessary
delay.
-- The stream-oriented nature of TCP is often an inconvenience.
Applications must add their own record marking to delineate their
messages, and must make explicit use of the push facility to
ensure that a complete message is transferred in a reasonable
time.
-- The limited scope of TCP sockets complicates the task of
providing highly-available data transfer capability using multi-
homed hosts.
-- TCP is relatively vulnerable to denial of service attacks, such
as SYN attacks.
Transport of PSTN signaling across the IP network is an application
for which all of these limitations of TCP are relevant. While this
application directly motivated the development of SCTP, other
applications may find SCTP a good match to their requirements.
1.2 Architectural View of SCTP
SCTP is viewed as a layer between the SCTP user application ("SCTP
user" for short) and a connectionless packet network service such as
IP. The remainder of this document assumes SCTP runs on top of IP.
The basic service offered by SCTP is the reliable transfer of user
messages between peer SCTP users. It performs this service within
the context of an association between two SCTP endpoints. Section 10
of this document sketches the API which should exist at the boundary
between the SCTP and the SCTP user layers.
SCTP is connection-oriented in nature, but the SCTP association is a
broader concept than the TCP connection. SCTP provides the means for
each SCTP endpoint (Section 1.4) to provide the other endpoint
(during association startup) with a list of transport addresses
(i.e., multiple IP addresses in combination with an SCTP port)
through which that endpoint can be reached and from which it will
originate SCTP packets. The association spans transfers over all of
the possible source/destination combinations which may be generated
from each endpoint's lists.
_____________ _____________
| SCTP User | | SCTP User |
| Application | | Application |
|-------------| |-------------|
| SCTP | | SCTP |
| Transport | | Transport |
| Service | | Service |
|-------------| |-------------|
| |One or more ---- One or more| |
| IP Network |IP address \/ IP address| IP Network |
| Service |appearances /\ appearances| Service |
|_____________| ---- |_____________|
SCTP Node A |<-------- Network transport ------->| SCTP Node B
Figure 1: An SCTP Association
1.3 Functional View of SCTP
The SCTP transport service can be decomposed into a number of
functions. These are depicted in Figure 2 and explained in the
remainder of this section.
SCTP User Application
-----------------------------------------------------
_____________ ____________________
| | | Sequenced delivery |
| Association | | within streams |
| | |____________________|
| startup |
| | ____________________________
| and | | User Data Fragmentation |
| | |____________________________|
| takedown |
| | ____________________________
| | | Acknowledgement |
| | | and |
| | | Congestion Avoidance |
| | |____________________________|
| |
| | ____________________________
| | | Chunk Bundling |
| | |____________________________|
| |
| | ________________________________
| | | Packet Validation |
| | |________________________________|
| |
| | ________________________________
| | | Path Management |
|_____________| |________________________________|
Figure 2: Functional View of the SCTP Transport Service
1.3.1 Association Startup and Takedown
An association is initiated by a request from the SCTP user (see the
description of the ASSOCIATE (or SEND) primitive in Section 10).
A cookie mechanism, similar to one described by Karn and Simpson in
[RFC2522], is employed during the initialization to provide
protection against security attacks. The cookie mechanism uses a
four-way handshake, the last two legs of which are allowed to carry
user data for fast setup. The startup sequence is described in
Section 5 of this document.
SCTP provides for graceful close (i.e., shutdown) of an active
association on request from the SCTP user. See the description of
the SHUTDOWN primitive in Section 10. SCTP also allows ungraceful
close (i.e., abort), either on request from the user (ABORT
primitive) or as a result of an error condition detected within the
SCTP layer. Section 9 describes both the graceful and the ungraceful
close procedures.
SCTP does not support a half-open state (like TCP) wherein one side
may continue sending data while the other end is closed. When either
endpoint performs a shutdown, the association on each peer will stop
accepting new data from its user and only deliver data in queue at
the time of the graceful close (see Section 9).
1.3.2 Sequenced Delivery within Streams
The term "stream" is used in SCTP to refer to a sequence of user
messages that are to be delivered to the upper-layer protocol in
order with respect to other messages within the same stream. This is
in contrast to its usage in TCP, where it refers to a sequence of
bytes (in this document a byte is assumed to be eight bits).
The SCTP user can specify at association startup time the number of
streams to be supported by the association. This number is
negotiated with the remote end (see Section 5.1.1). User messages
are associated with stream numbers (SEND, RECEIVE primitives, Section
10). Internally, SCTP assigns a stream sequence number to each
message passed to it by the SCTP user. On the receiving side, SCTP
ensures that messages are delivered to the SCTP user in sequence
within a given stream. However, while one stream may be blocked
waiting for the next in-sequence user message, delivery from other
streams may proceed.
SCTP provides a mechanism for bypassing the sequenced delivery
service. User messages sent using this mechanism are delivered to
the SCTP user as soon as they are received.
1.3.3 User Data Fragmentation
When needed, SCTP fragments user messages to ensure that the SCTP
packet passed to the lower layer conforms to the path MTU. On
receipt, fragments are reassembled into complete messages before
being passed to the SCTP user.
1.3.4 Acknowledgement and Congestion Avoidance
SCTP assigns a Transmission Sequence Number (TSN) to each user data
fragment or unfragmented message. The TSN is independent of any
stream sequence number assigned at the stream level. The receiving
end acknowledges all TSNs received, even if there are gaps in the
sequence. In this way, reliable delivery is kept functionally
separate from sequenced stream delivery.
The acknowledgement and congestion avoidance function is responsible
for packet retransmission when timely acknowledgement has not been
received. Packet retransmission is conditioned by congestion
avoidance procedures similar to those used for TCP. See Sections 6
and 7 for a detailed description of the protocol procedures
associated with this function.
1.3.5 Chunk Bundling
As described in Section 3, the SCTP packet as delivered to the lower
layer consists of a common header followed by one or more chunks.
Each chunk may contain either user data or SCTP control information.
The SCTP user has the option to request bundling of more than one
user messages into a single SCTP packet. The chunk bundling function
of SCTP is responsible for assembly of the complete SCTP packet and
its disassembly at the receiving end.
During times of congestion an SCTP implementation MAY still perform
bundling even if the user has requested that SCTP not bundle. The
user's disabling of bundling only affects SCTP implementations that
may delay a small period of time before transmission (to attempt to
encourage bundling). When the user layer disables bundling, this
small delay is prohibited but not bundling that is performed during
congestion or retransmission.
1.3.6 Packet Validation
A mandatory Verification Tag field and a 32 bit checksum field (see
Appendix B for a description of the Adler-32 checksum) are included
in the SCTP common header. The Verification Tag value is chosen by
each end of the association during association startup. Packets
received without the expected Verification Tag value are discarded,
as a protection against blind masquerade attacks and against stale
SCTP packets from a previous association. The Adler-32 checksum
should be set by the sender of each SCTP packet to provide additional
protection against data corruption in the network. The receiver of
an SCTP packet with an invalid Adler-32 checksum silently discards
the packet.
1.3.7 Path Management
The sending SCTP user is able to manipulate the set of transport
addresses used as destinations for SCTP packets through the
primitives described in Section 10. The SCTP path management
function chooses the destination transport address for each outgoing
SCTP packet based on the SCTP user's instructions and the currently
perceived reachability status of the eligible destination set. The
path management function monitors reachability through heartbeats
when other packet traffic is inadequate to provide this information
and advises the SCTP user when reachability of any far-end transport
address changes. The path management function is also responsible
for reporting the eligible set of local transport addresses to the
far end during association startup, and for reporting the transport
addresses returned from the far end to the SCTP user.
At association start-up, a primary path is defined for each SCTP
endpoint, and is used for normal sending of SCTP packets.
On the receiving end, the path management is responsible for
verifying the existence of a valid SCTP association to which the
inbound SCTP packet belongs before passing it for further processing.
Note: Path Management and Packet Validation are done at the same
time, so although described separately above, in reality they cannot
be performed as separate items.
1.4 Key Terms
Some of the language used to describe SCTP has been introduced in the
previous sections. This section provides a consolidated list of the
key terms and their definitions.
o Active destination transport address: A transport address on a
peer endpoint which a transmitting endpoint considers available
for receiving user messages.
o Bundling: An optional multiplexing operation, whereby more than
one user message may be carried in the same SCTP packet. Each
user message occupies its own DATA chunk.
o Chunk: A unit of information within an SCTP packet, consisting of
a chunk header and chunk-specific content.
o Congestion Window (cwnd): An SCTP variable that limits the data,
in number of bytes, a sender can send to a particular destination
transport address before receiving an acknowledgement.
o Cumulative TSN Ack Point: The TSN of the last DATA chunk
acknowledged via the Cumulative TSN Ack field of a SACK.
o Idle destination address: An address that has not had user
messages sent to it within some length of time, normally the
HEARTBEAT interval or greater.
o Inactive destination transport address: An address which is
considered inactive due to errors and unavailable to transport
user messages.
o Message = user message: Data submitted to SCTP by the Upper Layer
Protocol (ULP).
o Message Authentication Code (MAC): An integrity check mechanism
based on cryptographic hash functions using a secret key.
Typically, message authentication codes are used between two
parties that share a secret key in order to validate information
transmitted between these parties. In SCTP it is used by an
endpoint to validate the State Cookie information that is returned
from the peer in the COOKIE ECHO chunk. The term "MAC" has
different meanings in different contexts. SCTP uses this term
with the same meaning as in [RFC2104].
o Network Byte Order: Most significant byte first, a.k.a., Big
Endian.
o Ordered Message: A user message that is delivered in order with
respect to all previous user messages sent within the stream the
message was sent on.
o Outstanding TSN (at an SCTP endpoint): A TSN (and the associated
DATA chunk) that has been sent by the endpoint but for which it
has not yet received an acknowledgement.
o Path: The route taken by the SCTP packets sent by one SCTP
endpoint to a specific destination transport address of its peer
SCTP endpoint. Sending to different destination transport
addresses does not necessarily guarantee getting separate paths.
o Primary Path: The primary path is the destination and source
address that will be put into a packet outbound to the peer
endpoint by default. The definition includes the source address
since an implementation MAY wish to specify both destination and
source address to better control the return path taken by reply
chunks and on which interface the packet is transmitted when the
data sender is multi-homed.
o Receiver Window (rwnd): An SCTP variable a data sender uses to
store the most recently calculated receiver window of its peer, in
number of bytes. This gives the sender an indication of the space
available in the receiver's inbound buffer.
o SCTP association: A protocol relationship between SCTP endpoints,
composed of the two SCTP endpoints and protocol state information
including Verification Tags and the currently active set of
Transmission Sequence Numbers (TSNs), etc. An association can be
uniquely identified by the transport addresses used by the
endpoints in the association. Two SCTP endpoints MUST NOT have
more than one SCTP association between them at any given time.
o SCTP endpoint: The logical sender/receiver of SCTP packets. On a
multi-homed host, an SCTP endpoint is represented to its peers as
a combination of a set of eligible destination transport addresses
to which SCTP packets can be sent and a set of eligible source
transport addresses from which SCTP packets can be received. All
transport addresses used by an SCTP endpoint must use the same
port number, but can use multiple IP addresses. A transport
address used by an SCTP endpoint must not be used by another SCTP
endpoint. In other words, a transport address is unique to an
SCTP endpoint.
o SCTP packet (or packet): The unit of data delivery across the
interface between SCTP and the connectionless packet network
(e.g., IP). An SCTP packet includes the common SCTP header,
possible SCTP control chunks, and user data encapsulated within
SCTP DATA chunks.
o SCTP user application (SCTP user): The logical higher-layer
application entity which uses the services of SCTP, also called
the Upper-layer Protocol (ULP).
o Slow Start Threshold (ssthresh): An SCTP variable. This is the
threshold which the endpoint will use to determine whether to
perform slow start or congestion avoidance on a particular
destination transport address. Ssthresh is in number of bytes.
o Stream: A uni-directional logical channel established from one to
another associated SCTP endpoint, within which all user messages
are delivered in sequence except for those submitted to the
unordered delivery service.
Note: The relationship between stream numbers in opposite directions
is strictly a matter of how the applications use them. It is the
responsibility of the SCTP user to create and manage these
correlations if they are so desired.
o Stream Sequence Number: A 16-bit sequence number used internally
by SCTP to assure sequenced delivery of the user messages within a
given stream. One stream sequence number is attached to each user
message.
o Tie-Tags: Verification Tags from a previous association. These
Tags are used within a State Cookie so that the newly restarting
association can be linked to the original association within the
endpoint that did not restart.
o Transmission Control Block (TCB): An internal data structure
created by an SCTP endpoint for each of its existing SCTP
associations to other SCTP endpoints. TCB contains all the status
and operational information for the endpoint to maintain and
manage the corresponding association.
o Transmission Sequence Number (TSN): A 32-bit sequence number used
internally by SCTP. One TSN is attached to each chunk containing
user data to permit the receiving SCTP endpoint to acknowledge its
receipt and detect duplicate deliveries.
o Transport address: A Transport Address is traditionally defined
by Network Layer address, Transport Layer protocol and Transport
Layer port number. In the case of SCTP running over IP, a
transport address is defined by the combination of an IP address
and an SCTP port number (where SCTP is the Transport protocol).
o Unacknowledged TSN (at an SCTP endpoint): A TSN (and the associated
DATA chunk) which has been received by the endpoint but for which
an acknowledgement has not yet been sent. Or in the opposite case,
for a packet that has been sent but no acknowledgement has been
received.
o Unordered Message: Unordered messages are "unordered" with respect
to any other message, this includes both other unordered messages
as well as other ordered messages. Unordered message might be
delivered prior to or later than ordered messages sent on the same
stream.
o User message: The unit of data delivery across the interface
between SCTP and its user.
o Verification Tag: A 32 bit unsigned integer that is randomly
generated. The Verification Tag provides a key that allows a
receiver to verify that the SCTP packet belongs to the current
association and is not an old or stale packet from a previous
association.
1.5. Abbreviations
MAC - Message Authentication Code [RFC2104]
RTO - Retransmission Time-out
RTT - Round-trip Time
RTTVAR - Round-trip Time Variation
SCTP - Stream Control Transmission Protocol
SRTT - Smoothed RTT
TCB - Transmission Control Block
TLV - Type-Length-Value Coding Format
TSN - Transmission Sequence Number
ULP - Upper-layer Protocol
1.6 Serial Number Arithmetic
It is essential to remember that the actual Transmission Sequence
Number space is finite, though very large. This space ranges from 0
to 2**32 - 1. Since the space is finite, all arithmetic dealing with
Transmission Sequence Numbers must be performed modulo 2**32. This
unsigned arithmetic preserves the relationship of sequence numbers as
they cycle from 2**32 - 1 to 0 again. There are some subtleties to
computer modulo arithmetic, so great care should be taken in
programming the comparison of such values. When referring to TSNs,
the symbol "=<" means "less than or equal"(modulo 2**32).
Comparisons and arithmetic on TSNs in this document SHOULD use Serial
Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.
An endpoint SHOULD NOT transmit a DATA chunk with a TSN that is more
than 2**31 - 1 above the beginning TSN of its current send window.
Doing so will cause problems in comparing TSNs.
Transmission Sequence Numbers wrap around when they reach 2**32 - 1.
That is, the next TSN a DATA chunk MUST use after transmitting TSN =
2*32 - 1 is TSN = 0.
Any arithmetic done on Stream Sequence Numbers SHOULD use Serial
Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.
All other arithmetic and comparisons in this document uses normal
arithmetic.
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[RFC2119].
3. SCTP packet Format
An SCTP packet is composed of a common header and chunks. A chunk
contains either control information or user data.
The SCTP packet format is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Multiple chunks can be bundled into one SCTP packet up to the MTU
size, except for the INIT, INIT ACK, and SHUTDOWN COMPLETE chunks.
These chunks MUST NOT be bundled with any other chunk in a packet.
See Section 6.10 for more details on chunk bundling.
If a user data message doesn't fit into one SCTP packet it can be
fragmented into multiple chunks using the procedure defined in
Section 6.9.
All integer fields in an SCTP packet MUST be transmitted in network
byte order, unless otherwise stated.
3.1 SCTP Common Header Field Descriptions
SCTP Common Header Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port Number | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port Number: 16 bits (unsigned integer)
This is the SCTP sender's port number. It can be used by the
receiver in combination with the source IP address, the SCTP
destination port and possibly the destination IP address to
identify the association to which this packet belongs.
Destination Port Number: 16 bits (unsigned integer)
This is the SCTP port number to which this packet is destined.
The receiving host will use this port number to de-multiplex the
SCTP packet to the correct receiving endpoint/application.
Verification Tag: 32 bits (unsigned integer)
The receiver of this packet uses the Verification Tag to validate
the sender of this SCTP packet. On transmit, the value of this
Verification Tag MUST be set to the value of the Initiate Tag
received from the peer endpoint during the association
initialization, with the following exceptions:
- A packet containing an INIT chunk MUST have a zero Verification
Tag.
- A packet containing a SHUTDOWN-COMPLETE chunk with the T-bit
set MUST have the Verification Tag copied from the packet with
the SHUTDOWN-ACK chunk.
- A packet containing an ABORT chunk may have the verification
tag copied from the packet which caused the ABORT to be sent.
For details see Section 8.4 and 8.5.
An INIT chunk MUST be the only chunk in the SCTP packet carrying it.
Checksum: 32 bits (unsigned integer)
This field contains the checksum of this SCTP packet. Its
calculation is discussed in Section 6.8. SCTP uses the Adler-
32 algorithm as described in Appendix B for calculating the
checksum
3.2 Chunk Field Descriptions
The figure below illustrates the field format for the chunks to be
transmitted in the SCTP packet. Each chunk is formatted with a Chunk
Type field, a chunk-specific Flag field, a Chunk Length field, and a
Value field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Chunk Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Type: 8 bits (unsigned integer)
This field identifies the type of information contained in the
Chunk Value field. It takes a value from 0 to 254. The value of
255 is reserved for future use as an extension field.
The values of Chunk Types are defined as follows:
ID Value Chunk Type
----- ----------
0 - Payload Data (DATA)
1 - Initiation (INIT)
2 - Initiation Acknowledgement (INIT ACK)
3 - Selective Acknowledgement (SACK)
4 - Heartbeat Request (HEARTBEAT)
5 - Heartbeat Acknowledgement (HEARTBEAT ACK)
6 - Abort (ABORT)
7 - Shutdown (SHUTDOWN)
8 - Shutdown Acknowledgement (SHUTDOWN ACK)
9 - Operation Error (ERROR)
10 - State Cookie (COOKIE ECHO)
11 - Cookie Acknowledgement (COOKIE ACK)
12 - Reserved for Explicit Congestion Notification Echo (ECNE)
13 - Reserved for Congestion Window Reduced (CWR)
14 - Shutdown Complete (SHUTDOWN COMPLETE)
15 to 62 - reserved by IETF
63 - IETF-defined Chunk Extensions
64 to 126 - reserved by IETF
127 - IETF-defined Chunk Extensions
128 to 190 - reserved by IETF
191 - IETF-defined Chunk Extensions
192 to 254 - reserved by IETF
255 - IETF-defined Chunk Extensions
Chunk Types are encoded such that the highest-order two bits specify
the action that must be taken if the processing endpoint does not
recognize the Chunk Type.
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
10 - Skip this chunk and continue processing.
11 - Skip this chunk and continue processing, but report in an ERROR
Chunk using the 'Unrecognized Chunk Type' cause of error.
Note: The ECNE and CWR chunk types are reserved for future use of
Explicit Congestion Notification (ECN).
Chunk Flags: 8 bits
The usage of these bits depends on the chunk type as given by the
Chunk Type. Unless otherwise specified, they are set to zero on
transmit and are ignored on receipt.
Chunk Length: 16 bits (unsigned integer)
This value represents the size of the chunk in bytes including the
Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
Therefore, if the Chunk Value field is zero-length, the Length
field will be set to 4. The Chunk Length field does not count any
padding.
Chunk Value: variable length
The Chunk Value field contains the actual information to be
transferred in the chunk. The usage and format of this field is
dependent on the Chunk Type.
The total length of a chunk (including Type, Length and Value fields)
MUST be a multiple of 4 bytes. If the length of the chunk is not a
multiple of 4 bytes, the sender MUST pad the chunk with all zero
bytes and this padding is not included in the chunk length field.
The sender should never pad with more than 3 bytes. The receiver
MUST ignore the padding bytes.
SCTP defined chunks are described in detail in Section 3.3. The
guidelines for IETF-defined chunk extensions can be found in Section
13.1 of this document.
3.2.1 Optional/Variable-length Parameter Format
Chunk values of SCTP control chunks consist of a chunk-type-specific
header of required fields, followed by zero or more parameters. The
optional and variable-length parameters contained in a chunk are
defined in a Type-Length-Value format as shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Parameter Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Parameter Type: 16 bits (unsigned integer)
The Type field is a 16 bit identifier of the type of parameter.
It takes a value of 0 to 65534.
The value of 65535 is reserved for IETF-defined extensions. Values
other than those defined in specific SCTP chunk description are
reserved for use by IETF.
Chunk Parameter Length: 16 bits (unsigned integer)
The Parameter Length field contains the size of the parameter in
bytes, including the Parameter Type, Parameter Length, and
Parameter Value fields. Thus, a parameter with a zero-length
Parameter Value field would have a Length field of 4. The
Parameter Length does not include any padding bytes.
Chunk Parameter Value: variable-length.
The Parameter Value field contains the actual information to be
transferred in the parameter.
The total length of a parameter (including Type, Parameter Length and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is not included in the parameter
length field. A sender SHOULD NOT pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.
The Parameter Types are encoded such that the highest-order two bits
specify the action that must be taken if the processing endpoint does
not recognize the Parameter Type.
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
10 - Skip this parameter and continue processing.
11 - Skip this parameter and continue processing but report the
unrecognized parameter in an 'Unrecognized Parameter Type' (in
either an ERROR or in the INIT ACK).
The actual SCTP parameters are defined in the specific SCTP chunk
sections. The rules for IETF-defined parameter extensions are
defined in Section 13.2.
3.3 SCTP Chunk Definitions
This section defines the format of the different SCTP chunk types.
3.3.1 Payload Data (DATA) (0)
The following format MUST be used for the DATA chunk:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Reserved|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier S | Stream Sequence Number n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data (seq n of Stream S) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved: 5 bits
Should be set to all '0's and ignored by the receiver.
U bit: 1 bit
The (U)nordered bit, if set to '1', indicates that this is an
unordered DATA chunk, and there is no Stream Sequence Number
assigned to this DATA chunk. Therefore, the receiver MUST ignore
the Stream Sequence Number field.
After re-assembly (if necessary), unordered DATA chunks MUST be
dispatched to the upper layer by the receiver without any attempt
to re-order.
If an unordered user message is fragmented, each fragment of the
message MUST have its U bit set to '1'.
B bit: 1 bit
The (B)eginning fragment bit, if set, indicates the first fragment
of a user message.
E bit: 1 bit
The (E)nding fragment bit, if set, indicates the last fragment of
a user message.
An unfragmented user message shall have both the B and E bits set to
'1'. Setting both B and E bits to '0' indicates a middle fragment of
a multi-fragment user message, as summarized in the following table:
B E Description
============================================================
| 1 0 | First piece of a fragmented user message |
+----------------------------------------------------------+
| 0 0 | Middle piece of a fragmented user message |
+----------------------------------------------------------+
| 0 1 | Last piece of a fragmented user message |
+----------------------------------------------------------+
| 1 1 | Unfragmented Message |
============================================================
| Table 1: Fragment Description Flags |
============================================================
When a user message is fragmented into multiple chunks, the TSNs are
used by the receiver to reassemble the message. This means that the
TSNs for each fragment of a fragmented user message MUST be strictly
sequential.
Length: 16 bits (unsigned integer)
This field indicates the length of the DATA chunk in bytes from
the beginning of the type field to the end of the user data field
excluding any padding. A DATA chunk with no user data field will
have Length set to 16 (indicating 16 bytes).
TSN : 32 bits (unsigned integer)
This value represents the TSN for this DATA chunk. The valid
range of TSN is from 0 to 4294967295 (2**32 - 1). TSN wraps back
to 0 after reaching 4294967295.
Stream Identifier S: 16 bits (unsigned integer)
Identifies the stream to which the following user data belongs.
Stream Sequence Number n: 16 bits (unsigned integer)
This value represents the stream sequence number of the following
user data within the stream S. Valid range is 0 to 65535.
When a user message is fragmented by SCTP for transport, the same
stream sequence number MUST be carried in each of the fragments of
the message.
Payload Protocol Identifier: 32 bits (unsigned integer)
This value represents an application (or upper layer) specified
protocol identifier. This value is passed to SCTP by its upper
layer and sent to its peer. This identifier is not used by SCTP
but can be used by certain network entities as well as the peer
application to identify the type of information being carried in
this DATA chunk. This field must be sent even in fragmented DATA
chunks (to make sure it is available for agents in the middle of
the network).
The value 0 indicates no application identifier is specified by
the upper layer for this payload data.
User Data: variable length
This is the payload user data. The implementation MUST pad the
end of the data to a 4 byte boundary with all-zero bytes. Any
padding MUST NOT be included in the length field. A sender MUST
never add more than 3 bytes of padding.
3.3.2 Initiation (INIT) (1)
This chunk is used to initiate a SCTP association between two
endpoints. The format of the INIT chunk is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The INIT chunk contains the following parameters. Unless otherwise
noted, each parameter MUST only be included once in the INIT chunk.
Fixed Parameters Status
----------------------------------------------
Initiate Tag Mandatory
Advertised Receiver Window Credit Mandatory
Number of Outbound Streams Mandatory
Number of Inbound Streams Mandatory
Initial TSN Mandatory
Variable Parameters Status Type Value
-------------------------------------------------------------
IPv4 Address (Note 1) Optional 5
IPv6 Address (Note 1) Optional 6
Cookie Preservative Optional 9
Reserved for ECN Capable (Note 2) Optional 32768 (0x8000)
Host Name Address (Note 3) Optional 11
Supported Address Types (Note 4) Optional 12
Note 1: The INIT chunks can contain multiple addresses that can be
IPv4 and/or IPv6 in any combination.
Note 2: The ECN capable field is reserved for future use of Explicit
Congestion Notification.
Note 3: An INIT chunk MUST NOT contain more than one Host Name
address parameter. Moreover, the sender of the INIT MUST NOT combine
any other address types with the Host Name address in the INIT. The
receiver of INIT MUST ignore any other address types if the Host Name
address parameter is present in the received INIT chunk.
Note 4: This parameter, when present, specifies all the address types
the sending endpoint can support. The absence of this parameter
indicates that the sending endpoint can support any address type.
The Chunk Flags field in INIT is reserved and all bits in it should
be set to 0 by the sender and ignored by the receiver. The sequence
of parameters within an INIT can be processed in any order.
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT (the responding end) records the value of
the Initiate Tag parameter. This value MUST be placed into the
Verification Tag field of every SCTP packet that the receiver of
the INIT transmits within this association.
The Initiate Tag is allowed to have any value except 0. See
Section 5.3.1 for more on the selection of the tag value.
If the value of the Initiate Tag in a received INIT chunk is found
to be 0, the receiver MUST treat it as an error and close the
association by transmitting an ABORT.
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)
This value represents the dedicated buffer space, in number of
bytes, the sender of the INIT has reserved in association with
this window. During the life of the association this buffer space
SHOULD not be lessened (i.e. dedicated buffers taken away from
this association); however, an endpoint MAY change the value of
a_rwnd it sends in SACK chunks.
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT
chunk wishes to create in this association. The value of 0 MUST
NOT be used.
Note: A receiver of an INIT with the OS value set to 0 SHOULD
abort the association.
Number of Inbound Streams (MIS) : 16 bits (unsigned integer)
Defines the maximum number of streams the sender of this INIT
chunk allows the peer end to create in this association. The
value 0 MUST NOT be used.
Note: There is no negotiation of the actual number of streams but
instead the two endpoints will use the min(requested, offered).
See Section 5.1.1 for details.
Note: A receiver of an INIT with the MIS value of 0 SHOULD abort
the association.
Initial TSN (I-TSN) : 32 bits (unsigned integer)
Defines the initial TSN that the sender will use. The valid range
is from 0 to 4294967295. This field MAY be set to the value of
the Initiate Tag field.
3.3.2.1 Optional/Variable Length Parameters in INIT
The following parameters follow the Type-Length-Value format as
defined in Section 3.2.1. Any Type-Length-Value fields MUST come
after the fixed-length fields defined in the previous section.
IPv4 Address Parameter (5)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Address: 32 bits (unsigned integer)
Contains an IPv4 address of the sending endpoint. It is binary
encoded.
IPv6 Address Parameter (6)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Length = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv6 Address: 128 bit (unsigned integer)
Contains an IPv6 address of the sending endpoint. It is binary
encoded.
Note: A sender MUST NOT use an IPv4-mapped IPv6 address [RFC2373]
but should instead use an IPv4 Address Parameter for an IPv4
address.
Combined with the Source Port Number in the SCTP common header,
the value passed in an IPv4 or IPv6 Address parameter indicates a
transport address the sender of the INIT will support for the
association being initiated. That is, during the lifetime of this
association, this IP address can appear in the source address
field of an IP datagram sent from the sender of the INIT, and can
be used as a destination address of an IP datagram sent from the
receiver of the INIT.
More than one IP Address parameter can be included in an INIT
chunk when the INIT sender is multi-homed. Moreover, a multi-
homed endpoint may have access to different types of network, thus
more than one address type can be present in one INIT chunk, i.e.,
IPv4 and IPv6 addresses are allowed in the same INIT chunk.
If the INIT contains at least one IP Address parameter, then the
source address of the IP datagram containing the INIT chunk and
any additional address(es) provided within the INIT can be used as
destinations by the endpoint receiving the INIT. If the INIT does
not contain any IP Address parameters, the endpoint receiving the
INIT MUST use the source address associated with the received IP
datagram as its sole destination address for the association.
Note that not using any IP address parameters in the INIT and
INIT-ACK is an alternative to make an association more likely to
work across a NAT box.
Cookie Preservative (9)
The sender of the INIT shall use this parameter to suggest to the
receiver of the INIT for a longer life-span of the State Cookie.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suggested Cookie Life-span Increment (msec.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Suggested Cookie Life-span Increment: 32 bits (unsigned integer)
This parameter indicates to the receiver how much increment in
milliseconds the sender wishes the receiver to add to its default
cookie life-span.
This optional parameter should be added to the INIT chunk by the
sender when it re-attempts establishing an association with a peer
to which its previous attempt of establishing the association failed
due to a stale cookie operation error. The receiver MAY choose to
ignore the suggested cookie life-span increase for its own security
reasons.
Host Name Address (11)
The sender of INIT uses this parameter to pass its Host Name (in
place of its IP addresses) to its peer. The peer is responsible
for resolving the name. Using this parameter might make it more
likely for the association to work across a NAT box.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Host Name /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Host Name: variable length
This field contains a host name in "host name syntax" per RFC1123
Section 2.1 [RFC1123]. The method for resolving the host name is
out of scope of SCTP.
Note: At least one null terminator is included in the Host Name
string and must be included in the length.
Supported Address Types (12)
The sender of INIT uses this parameter to list all the address
types it can support.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 12 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Type #1 | Address Type #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ......
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Type: 16 bits (unsigned integer)
This is filled with the type value of the corresponding address
TLV (e.g., IPv4 = 5, IPv6 = 6, Hostname = 11).
3.3.3 Initiation Acknowledgement (INIT ACK) (2):
The INIT ACK chunk is used to acknowledge the initiation of an SCTP
association.
The parameter part of INIT ACK is formatted similarly to the INIT
chunk. It uses two extra variable parameters: The State Cookie and
the Unrecognized Parameter:
The format of the INIT ACK chunk is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT ACK records the value of the Initiate Tag
parameter. This value MUST be placed into the Verification Tag
field of every SCTP packet that the INIT ACK receiver transmits
within this association.
The Initiate Tag MUST NOT take the value 0. See Section 5.3.1 for
more on the selection of the Initiate Tag value.
If the value of the Initiate Tag in a received INIT ACK chunk is
found to be 0, the receiver MUST treat it as an error and close
the association by transmitting an ABORT.
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)
This value represents the dedicated buffer space, in number of
bytes, the sender of the INIT ACK has reserved in association with
this window. During the life of the association this buffer space
SHOULD not be lessened (i.e. dedicated buffers taken away from
this association).
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT ACK
chunk wishes to create in this association. The value of 0 MUST
NOT be used.
Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
destroy the association discarding its TCB.
Number of Inbound Streams (MIS) : 16 bits (unsigned integer)
Defines the maximum number of streams the sender of this INIT ACK
chunk allows the peer end to create in this association. The
value 0 MUST NOT be used.
Note: There is no negotiation of the actual number of streams but
instead the two endpoints will use the min(requested, offered).
See Section 5.1.1 for details.
Note: A receiver of an INIT ACK with the MIS value set to 0
SHOULD destroy the association discarding its TCB.
Initial TSN (I-TSN) : 32 bits (unsigned integer)
Defines the initial TSN that the INIT-ACK sender will use. The
valid range is from 0 to 4294967295. This field MAY be set to the
value of the Initiate Tag field.
Fixed Parameters Status
----------------------------------------------
Initiate Tag Mandatory
Advertised Receiver Window Credit Mandatory
Number of Outbound Streams Mandatory
Number of Inbound Streams Mandatory
Initial TSN Mandatory
Variable Parameters Status Type Value
-------------------------------------------------------------
State Cookie Mandatory 7
IPv4 Address (Note 1) Optional 5
IPv6 Address (Note 1) Optional 6
Unrecognized Parameters Optional 8
Reserved for ECN Capable (Note 2) Optional 32768 (0x8000)
Host Name Address (Note 3) Optional 11
Note 1: The INIT ACK chunks can contain any number of IP address
parameters that can be IPv4 and/or IPv6 in any combination.
Note 2: The ECN capable field is reserved for future use of Explicit
Congestion Notification.
Note 3: The INIT ACK chunks MUST NOT contain more than one Host Name
address parameter. Moreover, the sender of the INIT ACK MUST NOT
combine any other address types with the Host Name address in the
INIT ACK. The receiver of the INIT ACK MUST ignore any other address
types if the Host Name address parameter is present.
IMPLEMENTATION NOTE: An implementation MUST be prepared to receive a
INIT ACK that is quite large (more than 1500 bytes) due to the
variable size of the state cookie AND the variable address list. For
example if a responder to the INIT has 1000 IPv4 addresses it wishes
to send, it would need at least 8,000 bytes to encode this in the
INIT ACK.
In combination with the Source Port carried in the SCTP common
header, each IP Address parameter in the INIT ACK indicates to the
receiver of the INIT ACK a valid transport address supported by the
sender of the INIT ACK for the lifetime of the association being
initiated.
If the INIT ACK contains at least one IP Address parameter, then the
source address of the IP datagram containing the INIT ACK and any
additional address(es) provided within the INIT ACK may be used as
destinations by the receiver of the INIT-ACK. If the INIT ACK does
not contain any IP Address parameters, the receiver of the INIT-ACK
MUST use the source address associated with the received IP datagram
as its sole destination address for the association.
The State Cookie and Unrecognized Parameters use the Type-Length-
Value format as defined in Section 3.2.1 and are described below.
The other fields are defined the same as their counterparts in the
INIT chunk.
3.3.3.1 Optional or Variable Length Parameters
State Cookie
Parameter Type Value: 7
Parameter Length: variable size, depending on Size of Cookie
Parameter Value:
This parameter value MUST contain all the necessary state and
parameter information required for the sender of this INIT ACK
to create the association, along with a Message Authentication
Code (MAC). See Section 5.1.3 for details on State Cookie
definition.
Unrecognized Parameters:
Parameter Type Value: 8
Parameter Length: Variable Size.
Parameter Value:
This parameter is returned to the originator of the INIT chunk
when the INIT contains an unrecognized parameter which has a
value that indicates that it should be reported to the sender.
This parameter value field will contain unrecognized parameters
copied from the INIT chunk complete with Parameter Type, Length
and Value fields.
3.3.4 Selective Acknowledgement (SACK) (3):
This chunk is sent to the peer endpoint to acknowledge received DATA
chunks and to inform the peer endpoint of gaps in the received
subsequences of DATA chunks as represented by their TSNs.
The SACK MUST contain the Cumulative TSN Ack and Advertised Receiver
Window Credit (a_rwnd) parameters.
By definition, the value of the Cumulative TSN Ack parameter is the
last TSN received before a break in the sequence of received TSNs
occurs; the next TSN value following this one has not yet been
received at the endpoint sending the SACK. This parameter therefore
acknowledges receipt of all TSNs less than or equal to its value.
The handling of a_rwnd by the receiver of the SACK is discussed in
detail in Section 6.2.1.
The SACK also contains zero or more Gap Ack Blocks. Each Gap Ack
Block acknowledges a subsequence of TSNs received following a break
in the sequence of received TSNs. By definition, all TSNs
acknowledged by Gap Ack Blocks are greater than the value of the
Cumulative TSN Ack.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 |Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #1 Start | Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #N Start | Gap Ack Block #N End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to all zeros on transmit and ignored on receipt.
Cumulative TSN Ack: 32 bits (unsigned integer)
This parameter contains the TSN of the last DATA chunk received in
sequence before a gap.
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)
This field indicates the updated receive buffer space in bytes of
the sender of this SACK, see Section 6.2.1 for details.
Number of Gap Ack Blocks: 16 bits (unsigned integer)
Indicates the number of Gap Ack Blocks included in this SACK.
Number of Duplicate TSNs: 16 bit
This field contains the number of duplicate TSNs the endpoint has
received. Each duplicate TSN is listed following the Gap Ack
Block list.
Gap Ack Blocks:
These fields contain the Gap Ack Blocks. They are repeated for
each Gap Ack Block up to the number of Gap Ack Blocks defined in
the Number of Gap Ack Blocks field. All DATA chunks with TSNs
greater than or equal to (Cumulative TSN Ack + Gap Ack Block
Start) and less than or equal to (Cumulative TSN Ack + Gap Ack
Block End) of each Gap Ack Block are assumed to have been received
correctly.
Gap Ack Block Start: 16 bits (unsigned integer)
Indicates the Start offset TSN for this Gap Ack Block. To
calculate the actual TSN number the Cumulative TSN Ack is added to
this offset number. This calculated TSN identifies the first TSN
in this Gap Ack Block that has been received.
Gap Ack Block End: 16 bits (unsigned integer)
Indicates the End offset TSN for this Gap Ack Block. To calculate
the actual TSN number the Cumulative TSN Ack is added to this
offset number. This calculated TSN identifies the TSN of the last
DATA chunk received in this Gap Ack Block.
For example, assume the receiver has the following DATA chunks newly
arrived at the time when it decides to send a Selective ACK,
----------
| TSN=17 |
----------
| | <- still missing
----------
| TSN=15 |
----------
| TSN=14 |
----------
| | <- still missing
----------
| TSN=12 |
----------
| TSN=11 |
----------
| TSN=10 |
----------
then, the parameter part of the SACK MUST be constructed as follows
(assuming the new a_rwnd is set to 4660 by the sender):
+--------------------------------+
| Cumulative TSN Ack = 12 |
+--------------------------------+
| a_rwnd = 4660 |
+----------------+---------------+
| num of block=2 | num of dup=0 |
+----------------+---------------+
|block #1 strt=2 |block #1 end=3 |
+----------------+---------------+
|block #2 strt=5 |block #2 end=5 |
+----------------+---------------+
Duplicate TSN: 32 bits (unsigned integer)
Indicates the number of times a TSN was received in duplicate
since the last SACK was sent. Every time a receiver gets a
duplicate TSN (before sending the SACK) it adds it to the list of
duplicates. The duplicate count is re-initialized to zero after
sending each SACK.
For example, if a receiver were to get the TSN 19 three times it
would list 19 twice in the outbound SACK. After sending the SACK
if it received yet one more TSN 19 it would list 19 as a duplicate
once in the next outgoing SACK.
3.3.5 Heartbeat Request (HEARTBEAT) (4):
An endpoint should send this chunk to its peer endpoint to probe the
reachability of a particular destination transport address defined in
the present association.
The parameter field contains the Heartbeat Information which is a
variable length opaque data structure understood only by the sender.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Chunk Flags | Heartbeat Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Heartbeat Information TLV (Variable-Length) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
Heartbeat Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the chunk header
and the Heartbeat Information field.
Heartbeat Information: variable length
Defined as a variable-length parameter using the format described
in Section 3.2.1, i.e.:
Variable Parameters Status Type Value
-------------------------------------------------------------
Heartbeat Info Mandatory 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Heartbeat Info Type=1 | HB Info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Sender-specific Heartbeat Info /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Sender-specific Heartbeat Info field should normally include
information about the sender's current time when this HEARTBEAT
chunk is sent and the destination transport address to which this
HEARTBEAT is sent (see Section 8.3).
3.3.6 Heartbeat Acknowledgement (HEARTBEAT ACK) (5):
An endpoint should send this chunk to its peer endpoint as a response
to a HEARTBEAT chunk (see Section 8.3). A HEARTBEAT ACK is always
sent to the source IP address of the IP datagram containing the
HEARTBEAT chunk to which this ack is responding.
The parameter field contains a variable length opaque data structure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Chunk Flags | Heartbeat Ack Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Heartbeat Information TLV (Variable-Length) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
Heartbeat Ack Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the chunk header
and the Heartbeat Information field.
Heartbeat Information: variable length
This field MUST contain the Heartbeat Information parameter of
the Heartbeat Request to which this Heartbeat Acknowledgement is
responding.
Variable Parameters Status Type Value
-------------------------------------------------------------
Heartbeat Info Mandatory 1
3.3.7 Abort Association (ABORT) (6):
The ABORT chunk is sent to the peer of an association to close the
association. The ABORT chunk may contain Cause Parameters to inform
the receiver the reason of the abort. DATA chunks MUST NOT be
bundled with ABORT. Control chunks (except for INIT, INIT ACK and
SHUTDOWN COMPLETE) MAY be bundled with an ABORT but they MUST be
placed before the ABORT in the SCTP packet, or they will be ignored
by the receiver.
If an endpoint receives an ABORT with a format error or for an
association that doesn't exist, it MUST silently discard it.
Moreover, under any circumstances, an endpoint that receives an ABORT
MUST NOT respond to that ABORT by sending an ABORT of its own.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 |Reserved |T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Reserved: 7 bits
Set to 0 on transmit and ignored on receipt.
T bit: 1 bit
The T bit is set to 0 if the sender had a TCB that it destroyed.
If the sender did not have a TCB it should set this bit to 1.
Note: Special rules apply to this chunk for verification, please see
Section 8.5.1 for details.
Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the chunk header
and all the Error Cause fields present.
See Section 3.3.10 for Error Cause definitions.
3.3.8 Shutdown Association (SHUTDOWN) (7):
An endpoint in an association MUST use this chunk to initiate a
graceful close of the association with its peer. This chunk has the
following format.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Chunk Flags | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
Length: 16 bits (unsigned integer)
Indicates the length of the parameter. Set to 8.
Cumulative TSN Ack: 32 bits (unsigned integer)
This parameter contains the TSN of the last chunk received in
sequence before any gaps.
Note: Since the SHUTDOWN message does not contain Gap Ack Blocks,
it cannot be used to acknowledge TSNs received out of order. In a
SACK, lack of Gap Ack Blocks that were previously included
indicates that the data receiver reneged on the associated DATA
chunks. Since SHUTDOWN does not contain Gap Ack Blocks, the
receiver of the SHUTDOWN shouldn't interpret the lack of a Gap Ack
Block as a renege. (see Section 6.2 for information on reneging)
3.3.9 Shutdown Acknowledgement (SHUTDOWN ACK) (8):
This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
chunk at the completion of the shutdown process, see Section 9.2 for
details.
The SHUTDOWN ACK chunk has no parameters.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 8 |Chunk Flags | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
3.3.10 Operation Error (ERROR) (9):
An endpoint sends this chunk to its peer endpoint to notify it of
certain error conditions. It contains one or more error causes. An
Operation Error is not considered fatal in and of itself, but may be
used with an ABORT chunk to report a fatal condition. It has the
following parameters:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Chunk Flags | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ one or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the chunk header
and all the Error Cause fields present.
Error causes are defined as variable-length parameters using the
format described in 3.2.1, i.e.:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code | Cause Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Cause-specific Information /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 16 bits (unsigned integer)
Defines the type of error conditions being reported.
Cause Code
Value Cause Code
--------- ----------------
1 Invalid Stream Identifier
2 Missing Mandatory Parameter
3 Stale Cookie Error
4 Out of Resource
5 Unresolvable Address
6 Unrecognized Chunk Type
7 Invalid Mandatory Parameter
8 Unrecognized Parameters
9 No User Data
10 Cookie Received While Shutting Down
Cause Length: 16 bits (unsigned integer)
Set to the size of the parameter in bytes, including the Cause
Code, Cause Length, and Cause-Specific Information fields
Cause-specific Information: variable length
This field carries the details of the error condition.
Sections 3.3.10.1 - 3.3.10.10 define error causes for SCTP.
Guidelines for the IETF to define new error cause values are
discussed in Section 13.3.
3.3.10.1 Invalid Stream Identifier (1)
Cause of error
---------------
Invalid Stream Identifier: Indicates endpoint received a DATA chunk
sent to a nonexistent stream.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=1 | Cause Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | (Reserved) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Stream Identifier: 16 bits (unsigned integer)
Contains the Stream Identifier of the DATA chunk received in
error.
Reserved: 16 bits
This field is reserved. It is set to all 0's on transmit and
Ignored on receipt.
3.3.10.2 Missing Mandatory Parameter (2)
Cause of error
---------------
Missing Mandatory Parameter: Indicates that one or more mandatory
TLV parameters are missing in a received INIT or INIT ACK.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=2 | Cause Length=8+N*2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of missing params=N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Missing Param Type #1 | Missing Param Type #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Missing Param Type #N-1 | Missing Param Type #N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Number of Missing params: 32 bits (unsigned integer)
This field contains the number of parameters contained in the
Cause-specific Information field.
Missing Param Type: 16 bits (unsigned integer)
Each field will contain the missing mandatory parameter number.
3.3.10.3 Stale Cookie Error (3)
Cause of error
--------------
Stale Cookie Error: Indicates the receipt of a valid State Cookie
that has expired.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=3 | Cause Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measure of Staleness (usec.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Measure of Staleness: 32 bits (unsigned integer)
This field contains the difference, in microseconds, between the
current time and the time the State Cookie expired.
The sender of this error cause MAY choose to report how long past
expiration the State Cookie is by including a non-zero value in
the Measure of Staleness field. If the sender does not wish to
provide this information it should set the Measure of Staleness
field to the value of zero.
3.3.10.4 Out of Resource (4)
Cause of error
---------------
Out of Resource: Indicates that the sender is out of resource. This
is usually sent in combination with or within an ABORT.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=4 | Cause Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.10.5 Unresolvable Address (5)
Cause of error
---------------
Unresolvable Address: Indicates that the sender is not able to
resolve the specified address parameter (e.g., type of address is not
supported by the sender). This is usually sent in combination with
or within an ABORT.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=5 | Cause Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Unresolvable Address /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unresolvable Address: variable length
The unresolvable address field contains the complete Type, Length
and Value of the address parameter (or Host Name parameter) that
contains the unresolvable address or host name.
3.3.10.6 Unrecognized Chunk Type (6)
Cause of error
---------------
Unrecognized Chunk Type: This error cause is returned to the
originator of the chunk if the receiver does not understand the chunk
and the upper bits of the 'Chunk Type' are set to 01 or 11.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=6 | Cause Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Unrecognized Chunk /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unrecognized Chunk: variable length
The Unrecognized Chunk field contains the unrecognized Chunk from
the SCTP packet complete with Chunk Type, Chunk Flags and Chunk
Length.
3.3.10.7 Invalid Mandatory Parameter (7)
Cause of error
---------------
Invalid Mandatory Parameter: This error cause is returned to the
originator of an INIT or INIT ACK chunk when one of the mandatory
parameters is set to a invalid value.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=7 | Cause Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.10.8 Unrecognized Parameters (8)
Cause of error
---------------
Unrecognized Parameters: This error cause is returned to the
originator of the INIT ACK chunk if the receiver does not recognize
one or more Optional TLV parameters in the INIT ACK chunk.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=8 | Cause Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Unrecognized Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unrecognized Parameters: variable length
The Unrecognized Parameters field contains the unrecognized
parameters copied from the INIT ACK chunk complete with TLV. This
error cause is normally contained in an ERROR chunk bundled with
the COOKIE ECHO chunk when responding to the INIT ACK, when the
sender of the COOKIE ECHO chunk wishes to report unrecognized
parameters.
3.3.10.9 No User Data (9)
Cause of error
---------------
No User Data: This error cause is returned to the originator of a
DATA chunk if a received DATA chunk has no user data.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=9 | Cause Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ TSN value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TSN value: 32 bits (+unsigned integer)
The TSN value field contains the TSN of the DATA chunk received
with no user data field.
This cause code is normally returned in an ABORT chunk (see
Section 6.2)
3.3.10.10 Cookie Received While Shutting Down (10)
Cause of error
---------------
Cookie Received While Shutting Down: A COOKIE ECHO was received
While the endpoint was in SHUTDOWN-ACK-SENT state. This error is
usually returned in an ERROR chunk bundled with the retransmitted
SHUTDOWN ACK.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=10 | Cause Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.11 Cookie Echo (COOKIE ECHO) (10):
This chunk is used only during the initialization of an association.
It is sent by the initiator of an association to its peer to complete
the initialization process. This chunk MUST precede any DATA chunk
sent within the association, but MAY be bundled with one or more DATA
chunks in the same packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 10 |Chunk Flags | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Cookie /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bit
Set to zero on transmit and ignored on receipt.
Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the 4 bytes of
the chunk header and the size of the Cookie.
Cookie: variable size
This field must contain the exact cookie received in the State
Cookie parameter from the previous INIT ACK.
An implementation SHOULD make the cookie as small as possible to
insure interoperability.
3.3.12 Cookie Acknowledgement (COOKIE ACK) (11):
This chunk is used only during the initialization of an association.
It is used to acknowledge the receipt of a COOKIE ECHO chunk. This
chunk MUST precede any DATA or SACK chunk sent within the
association, but MAY be bundled with one or more DATA chunks or SACK
chunk in the same SCTP packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 |Chunk Flags | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Set to zero on transmit and ignored on receipt.
3.3.13 Shutdown Complete (SHUTDOWN COMPLETE) (14):
This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
ACK chunk at the completion of the shutdown process, see Section 9.2
for details.
The SHUTDOWN COMPLETE chunk has no parameters.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 14 |Reserved |T| Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Reserved: 7 bits
Set to 0 on transmit and ignored on receipt.
T bit: 1 bit
The T bit is set to 0 if the sender had a TCB that it destroyed.
If the sender did not have a TCB it should set this bit to 1.
Note: Special rules apply to this chunk for verification, please see
Section 8.5.1 for details.
4. SCTP Association State Diagram
During the lifetime of an SCTP association, the SCTP endpoint's
association progress from one state to another in response to various
events. The events that may potentially advance an association's
state include:
o SCTP user primitive calls, e.g., [ASSOCIATE], [SHUTDOWN], [ABORT],
o Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc., control
chunks, or
o Some timeout events.
The state diagram in the figures below illustrates state changes,
together with the causing events and resulting actions. Note that
some of the error conditions are not shown in the state diagram.
Full description of all special cases should be found in the text.
Note: Chunk names are given in all capital letters, while parameter
names have the first letter capitalized, e.g., COOKIE ECHO chunk type
vs. State Cookie parameter. If more than one event/message can occur
which causes a state transition it is labeled (A), (B) etc.
----- -------- (frm any state)
/ \ / rcv ABORT [ABORT]
rcv INIT | | | ---------- or ----------
--------------- | v v delete TCB snd ABORT
generate Cookie \ +---------+ delete TCB
snd INIT ACK ---| CLOSED |
+---------+
/ \ [ASSOCIATE]
/ \ ---------------
| | create TCB
| | snd INIT
| | strt init timer
rcv valid | |
COOKIE ECHO | v
(1) ---------------- | +------------+
create TCB | | COOKIE-WAIT| (2)
snd COOKIE ACK | +------------+
| |
| | rcv INIT ACK
| | -----------------
| | snd COOKIE ECHO
| | stop init timer
| | strt cookie timer
| v
| +--------------+
| | COOKIE-ECHOED| (3)
| +--------------+
| |
| | rcv COOKIE ACK
| | -----------------
| | stop cookie timer
v v
+---------------+
| ESTABLISHED |
+---------------+
(from the ESTABLISHED state only)
|
|
/--------+--------\
[SHUTDOWN] / \
-------------------| |
check outstanding | |
DATA chunks | |
v |
+---------+ |
|SHUTDOWN-| | rcv SHUTDOWN/check
|PENDING | | outstanding DATA
+---------+ | chunks
| |------------------
No more outstanding | |
---------------------| |
snd SHUTDOWN | |
strt shutdown timer | |
v v
+---------+ +-----------+
(4) |SHUTDOWN-| | SHUTDOWN- | (5,6)
|SENT | | RECEIVED |
+---------+ +-----------+
| \ |
(A) rcv SHUTDOWN ACK | \ |
----------------------| \ |
stop shutdown timer | \rcv:SHUTDOWN |
send SHUTDOWN COMPLETE| \ (B) |
delete TCB | \ |
| \ | No more outstanding
| \ |-----------------
| \ | send SHUTDOWN ACK
(B)rcv SHUTDOWN | \ | strt shutdown timer
----------------------| \ |
send SHUTDOWN ACK | \ |
start shutdown timer | \ |
move to SHUTDOWN- | \ |
ACK-SENT | | |
| v |
| +-----------+
| | SHUTDOWN- | (7)
| | ACK-SENT |
| +----------+-
| | (C)rcv SHUTDOWN COMPLETE
| |-----------------
| | stop shutdown timer
| | delete TCB
| |
| | (D)rcv SHUTDOWN ACK
| |--------------
| | stop shutdown timer
| | send SHUTDOWN COMPLETE
| | delete TCB
| |
\ +---------+ /
\-->| CLOSED |<--/
+---------+
Figure 3: State Transition Diagram of SCTP
Notes:
1) If the State Cookie in the received COOKIE ECHO is invalid (i.e.,
failed to pass the integrity check), the receiver MUST silently
discard the packet. Or, if the received State Cookie is expired
(see Section 5.1.5), the receiver MUST send back an ERROR chunk.
In either case, the receiver stays in the CLOSED state.
2) If the T1-init timer expires, the endpoint MUST retransmit INIT
and re-start the T1-init timer without changing state. This MUST
be repeated up to 'Max.Init.Retransmits' times. After that, the
endpoint MUST abort the initialization process and report the
error to SCTP user.
3) If the T1-cookie timer expires, the endpoint MUST retransmit
COOKIE ECHO and re-start the T1-cookie timer without changing
state. This MUST be repeated up to 'Max.Init.Retransmits' times.
After that, the endpoint MUST abort the initialization process and
report the error to SCTP user.
4) In SHUTDOWN-SENT state the endpoint MUST acknowledge any received
DATA chunks without delay.
5) In SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new
send request from its SCTP user.
6) In SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
retransmit data and leave this state when all data in queue is
transmitted.
7) In SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any new
send request from its SCTP user.
The CLOSED state is used to indicate that an association is not
created (i.e., doesn't exist).
5. Association Initialization
Before the first data transmission can take place from one SCTP
endpoint ("A") to another SCTP endpoint ("Z"), the two endpoints must
complete an initialization process in order to set up an SCTP
association between them.
The SCTP user at an endpoint should use the ASSOCIATE primitive to
initialize an SCTP association to another SCTP endpoint.
IMPLEMENTATION NOTE: From an SCTP-user's point of view, an
association may be implicitly opened, without an ASSOCIATE primitive
(see 10.1 B) being invoked, by the initiating endpoint's sending of
the first user data to the destination endpoint. The initiating SCTP
will assume default values for all mandatory and optional parameters
for the INIT/INIT ACK.
Once the association is established, unidirectional streams are open
for data transfer on both ends (see Section 5.1.1).
5.1 Normal Establishment of an Association
The initialization process consists of the following steps (assuming
that SCTP endpoint "A" tries to set up an association with SCTP
endpoint "Z" and "Z" accepts the new association):
A) "A" first sends an INIT chunk to "Z". In the INIT, "A" must
provide its Verification Tag (Tag_A) in the Initiate Tag field.
Tag_A SHOULD be a random number in the range of 1 to 4294967295
(see 5.3.1 for Tag value selection). After sending the INIT, "A"
starts the T1-init timer and enters the COOKIE-WAIT state.
B) "Z" shall respond immediately with an INIT ACK chunk. The
destination IP address of the INIT ACK MUST be set to the source
IP address of the INIT to which this INIT ACK is responding. In
the response, besides filling in other parameters, "Z" must set
the Verification Tag field to Tag_A, and also provide its own
Verification Tag (Tag_Z) in the Initiate Tag field.
Moreover, "Z" MUST generate and send along with the INIT ACK a
State Cookie. See Section 5.1.3 for State Cookie generation.
Note: After sending out INIT ACK with the State Cookie parameter,
"Z" MUST NOT allocate any resources, nor keep any states for the
new association. Otherwise, "Z" will be vulnerable to resource
attacks.
C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-
init timer and leave COOKIE-WAIT state. "A" shall then send the
State Cookie received in the INIT ACK chunk in a COOKIE ECHO
chunk, start the T1-cookie timer, and enter the COOKIE-ECHOED
state.
Note: The COOKIE ECHO chunk can be bundled with any pending
outbound DATA chunks, but it MUST be the first chunk in the packet
and until the COOKIE ACK is returned the sender MUST NOT send any
other packets to the peer.
D) Upon reception of the COOKIE ECHO chunk, Endpoint "Z" will reply
with a COOKIE ACK chunk after building a TCB and moving to the
ESTABLISHED state. A COOKIE ACK chunk may be bundled with any
pending DATA chunks (and/or SACK chunks), but the COOKIE ACK chunk
MUST be the first chunk in the packet.
IMPLEMENTATION NOTE: An implementation may choose to send the
Communication Up notification to the SCTP user upon reception of a
valid COOKIE ECHO chunk.
E) Upon reception of the COOKIE ACK, endpoint "A" will move from the
COOKIE-ECHOED state to the ESTABLISHED state, stopping the T1-
cookie timer. It may also notify its ULP about the successful
establishment of the association with a Communication Up
notification (see Section 10).
An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk.
They MUST be the only chunks present in the SCTP packets that carry
them.
An endpoint MUST send the INIT ACK to the IP address from which it
received the INIT.
Note: T1-init timer and T1-cookie timer shall follow the same rules
given in Section 6.3.
If an endpoint receives an INIT, INIT ACK, or COOKIE ECHO chunk but
decides not to establish the new association due to missing mandatory
parameters in the received INIT or INIT ACK, invalid parameter
values, or lack of local resources, it MUST respond with an ABORT
chunk. It SHOULD also specify the cause of abort, such as the type
of the missing mandatory parameters, etc., by including the error
cause parameters with the ABORT chunk. The Verification Tag field in
the common header of the outbound SCTP packet containing the ABORT
chunk MUST be set to the Initiate Tag value of the peer.
After the reception of the first DATA chunk in an association the
endpoint MUST immediately respond with a SACK to acknowledge the DATA
chunk. Subsequent acknowledgements should be done as described in
Section 6.2.
When the TCB is created, each endpoint MUST set its internal
Cumulative TSN Ack Point to the value of its transmitted Initial TSN
minus one.
IMPLEMENTATION NOTE: The IP addresses and SCTP port are generally
used as the key to find the TCB within an SCTP instance.
5.1.1 Handle Stream Parameters
In the INIT and INIT ACK chunks, the sender of the chunk shall
indicate the number of outbound streams (OS) it wishes to have in the
association, as well as the maximum inbound streams (MIS) it will
accept from the other endpoint.
After receiving the stream configuration information from the other
side, each endpoint shall perform the following check: If the peer's
MIS is less than the endpoint's OS, meaning that the peer is
incapable of supporting all the outbound streams the endpoint wants
to configure, the endpoint MUST either use MIS outbound streams, or
abort the association and report to its upper layer the resources
shortage at its peer.
After the association is initialized, the valid outbound stream
identifier range for either endpoint shall be 0 to min(local OS,
remote MIS)-1.
5.1.2 Handle Address Parameters
During the association initialization, an endpoint shall use the
following rules to discover and collect the destination transport
address(es) of its peer.
A) If there are no address parameters present in the received INIT or
INIT ACK chunk, the endpoint shall take the source IP address from
which the chunk arrives and record it, in combination with the
SCTP source port number, as the only destination transport address
for this peer.
B) If there is a Host Name parameter present in the received INIT or
INIT ACK chunk, the endpoint shall resolve that host name to a
list of IP address(es) and derive the transport address(es) of
this peer by combining the resolved IP address(es) with the SCTP
source port.
The endpoint MUST ignore any other IP address parameters if they
are also present in the received INIT or INIT ACK chunk.
The time at which the receiver of an INIT resolves the host name
has potential security implications to SCTP. If the receiver of
an INIT resolves the host name upon the reception of the chunk,
and the mechanism the receiver uses to resolve the host name
involves potential long delay (e.g. DNS query), the receiver may
open itself up to resource attacks for the period of time while it
is waiting for the name resolution results before it can build the
State Cookie and release local resources.
Therefore, in cases where the name translation involves potential
long delay, the receiver of the INIT MUST postpone the name
resolution till the reception of the COOKIE ECHO chunk from the
peer. In such a case, the receiver of the INIT SHOULD build the
State Cookie using the received Host Name (instead of destination
transport addresses) and send the INIT ACK to the source IP
address from which the INIT was received.
The receiver of an INIT ACK shall always immediately attempt to
resolve the name upon the reception of the chunk.
The receiver of the INIT or INIT ACK MUST NOT send user data
(piggy-backed or stand-alone) to its peer until the host name is
successfully resolved.
If the name resolution is not successful, the endpoint MUST
immediately send an ABORT with "Unresolvable Address" error cause
to its peer. The ABORT shall be sent to the source IP address
from which the last peer packet was received.
C) If there are only IPv4/IPv6 addresses present in the received INIT
or INIT ACK chunk, the receiver shall derive and record all the
transport address(es) from the received chunk AND the source IP
address that sent the INIT or INIT ACK. The transport address(es)
are derived by the combination of SCTP source port (from the
common header) and the IP address parameter(s) carried in the INIT
or INIT ACK chunk and the source IP address of the IP datagram.
The receiver should use only these transport addresses as
destination transport addresses when sending subsequent packets to
its peer.
IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
doesn't control the source IP address that is used for
transmitting), an endpoint might need to include in its INIT or
INIT ACK all possible IP addresses from which packets to the peer
could be transmitted.
After all transport addresses are derived from the INIT or INIT ACK
chunk using the above rules, the endpoint shall select one of the
transport addresses as the initial primary path.
Note: The INIT-ACK MUST be sent to the source address of the INIT.
The sender of INIT may include a 'Supported Address Types' parameter
in the INIT to indicate what types of address are acceptable. When
this parameter is present, the receiver of INIT (initiatee) MUST
either use one of the address types indicated in the Supported
Address Types parameter when responding to the INIT, or abort the
association with an "Unresolvable Address" error cause if it is
unwilling or incapable of using any of the address types indicated by
its peer.
IMPLEMENTATION NOTE: In the case that the receiver of an INIT ACK
fails to resolve the address parameter due to an unsupported type, it
can abort the initiation process and then attempt a re-initiation by
using a 'Supported Address Types' parameter in the new INIT to
indicate what types of address it prefers.
5.1.3 Generating State Cookie
When sending an INIT ACK as a response to an INIT chunk, the sender
of INIT ACK creates a State Cookie and sends it in the State Cookie
parameter of the INIT ACK. Inside this State Cookie, the sender
should include a MAC (see [RFC2104] for an example), a time stamp on
when the State Cookie is created, and the lifespan of the State
Cookie, along with all the information necessary for it to establish
the association.
The following steps SHOULD be taken to generate the State Cookie:
1) Create an association TCB using information from both the received
INIT and the outgoing INIT ACK chunk,
2) In the TCB, set the creation time to the current time of day, and
the lifespan to the protocol parameter 'Valid.Cookie.Life',
3) From the TCB, identify and collect the minimal subset of
information needed to re-create the TCB, and generate a MAC using
this subset of information and a secret key (see [RFC2104] for an
example of generating a MAC), and
4) Generate the State Cookie by combining this subset of information
and the resultant MAC.
After sending the INIT ACK with the State Cookie parameter, the
sender SHOULD delete the TCB and any other local resource related to
the new association, so as to prevent resource attacks.
The hashing method used to generate the MAC is strictly a private
matter for the receiver of the INIT chunk. The use of a MAC is
mandatory to prevent denial of service attacks. The secret key
SHOULD be random ([RFC1750] provides some information on randomness
guidelines); it SHOULD be changed reasonably frequently, and the
timestamp in the State Cookie MAY be used to determine which key
should be used to verify the MAC.
An implementation SHOULD make the cookie as small as possible to
insure interoperability.
5.1.4 State Cookie Processing
When an endpoint (in the COOKIE WAIT state) receives an INIT ACK
chunk with a State Cookie parameter, it MUST immediately send a
COOKIE ECHO chunk to its peer with the received State Cookie. The
sender MAY also add any pending DATA chunks to the packet after the
COOKIE ECHO chunk.
The endpoint shall also start the T1-cookie timer after sending out
the COOKIE ECHO chunk. If the timer expires, the endpoint shall
retransmit the COOKIE ECHO chunk and restart the T1-cookie timer.
This is repeated until either a COOKIE ACK is received or '
Max.Init.Retransmits' is reached causing the peer endpoint to be
marked unreachable (and thus the association enters the CLOSED
state).
5.1.5 State Cookie Authentication
When an endpoint receives a COOKIE ECHO chunk from another endpoint
with which it has no association, it shall take the following
actions:
1) Compute a MAC using the TCB data carried in the State Cookie and
the secret key (note the timestamp in the State Cookie MAY be used
to determine which secret key to use). Reference [RFC2104] can be
used as a guideline for generating the MAC,
2) Authenticate the State Cookie as one that it previously generated
by comparing the computed MAC against the one carried in the State
Cookie. If this comparison fails, the SCTP packet, including the
COOKIE ECHO and any DATA chunks, should be silently discarded,
3) Compare the creation timestamp in the State Cookie to the current
local time. If the elapsed time is longer than the lifespan
carried in the State Cookie, then the packet, including the COOKIE
ECHO and any attached DATA chunks, SHOULD be discarded and the
endpoint MUST transmit an ERROR chunk with a "Stale Cookie" error
cause to the peer endpoint,
4) If the State Cookie is valid, create an association to the sender
of the COOKIE ECHO chunk with the information in the TCB data
carried in the COOKIE ECHO, and enter the ESTABLISHED state,
5) Send a COOKIE ACK chunk to the peer acknowledging reception of the
COOKIE ECHO. The COOKIE ACK MAY be bundled with an outbound DATA
chunk or SACK chunk; however, the COOKIE ACK MUST be the first
chunk in the SCTP packet.
6) Immediately acknowledge any DATA chunk bundled with the COOKIE
ECHO with a SACK (subsequent DATA chunk acknowledgement should
follow the rules defined in Section 6.2). As mentioned in step
5), if the SACK is bundled with the COOKIE ACK, the COOKIE ACK
MUST appear first in the SCTP packet.
If a COOKIE ECHO is received from an endpoint with which the receiver
of the COOKIE ECHO has an existing association, the procedures in
Section 5.2 should be followed.
5.1.6 An Example of Normal Association Establishment
In the following example, "A" initiates the association and then
sends a user message to "Z", then "Z" sends two user messages to "A"
later (assuming no bundling or fragmentation occurs):
Endpoint A Endpoint Z
{app sets association with Z}
(build TCB)
INIT [I-Tag=Tag_A
& other info] --------\
(Start T1-init timer) \
(Enter COOKIE-WAIT state) \---> (compose temp TCB and Cookie_Z)
/--- INIT ACK [Veri Tag=Tag_A,
/ I-Tag=Tag_Z,
(Cancel T1-init timer) <------/ Cookie_Z, & other info]
(destroy temp TCB)
COOKIE ECHO [Cookie_Z] ------\
(Start T1-init timer) \
(Enter COOKIE-ECHOED state) \---> (build TCB enter ESTABLISHED
state)
/---- COOKIE-ACK
/
(Cancel T1-init timer, <-----/
Enter ESTABLISHED state)
{app sends 1st user data; strm 0}
DATA [TSN=initial TSN_A
Strm=0,Seq=1 & user data]--\
(Start T3-rtx timer) \
\->
/----- SACK [TSN Ack=init
TSN_A,Block=0]
(Cancel T3-rtx timer) <------/
...
{app sends 2 messages;strm 0}
/---- DATA
/ [TSN=init TSN_Z
<--/ Strm=0,Seq=1 & user data 1]
SACK [TSN Ack=init TSN_Z,