RFC 2960
Stream Control Transmission Protocol
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 INIT received in COOKIE-WAIT or COOKIE-ECHOED State (Item B)......................... 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 Processing a Received SACK.............................. 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 Protocol1.6 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].
(page 16 continued on part 2)
