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rfc4960-sctp.pdf
Table of Contents
1. Introduction
1.1. Motivation
1.2. Architectural View of SCTP
1.3. Key Terms
1.4. Abbreviations
1.5. Functional View of SCTP
1.5.1. Association Startup and Takedown
1.5.2. Sequenced Delivery within Streams
1.5.3. User Data Fragmentation
1.5.4. Acknowledgement and Congestion Avoidance
1.5.5. Chunk Bundling
1.5.6. Packet Validation
1.5.7. Path Management
1.6. Serial Number Arithmetic
1.7. Changes from RFC 2960
2. Conventions
3. SCTP Packet Format
3.1. SCTP Common Header Field Descriptions
3.2. Chunk Field Descriptions
3.2.1. Optional/Variable-Length Parameter Format
3.2.2. Reporting of Unrecognized Parameters
3.3. SCTP Chunk Definitions
3.3.1. Payload Data (DATA) (0)
3.3.2. Initiation (INIT) (1)
3.3.2.1. Optional/Variable-Length Parameters in INIT
3.3.3. Initiation Acknowledgement (INIT ACK) (2)
3.3.3.1. Optional or Variable-Length Parameters
3.3.4. Selective Acknowledgement (SACK) (3)
3.3.5. Heartbeat Request (HEARTBEAT) (4)
3.3.6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5)
3.3.7. Abort Association (ABORT) (6)
3.3.8. Shutdown Association (SHUTDOWN) (7)
3.3.9. Shutdown Acknowledgement (SHUTDOWN ACK) (8)
3.3.10. Operation Error (ERROR) (9)
3.3.10.1. Invalid Stream Identifier (1)
3.3.10.2. Missing Mandatory Parameter (2)
3.3.10.3. Stale Cookie Error (3)
3.3.10.4. Out of Resource (4)
3.3.10.5. Unresolvable Address (5)
3.3.10.6. Unrecognized Chunk Type (6)
3.3.10.7. Invalid Mandatory Parameter (7)
3.3.10.8. Unrecognized Parameters (8)
3.3.10.9. No User Data (9)
3.3.10.10. Cookie Received While Shutting Down (10)
3.3.10.11. Restart of an Association with New Addresses (11)
3.3.10.12. User-Initiated Abort (12)
3.3.10.13. Protocol Violation (13)
3.3.11. Cookie Echo (COOKIE ECHO) (10)
3.3.12. Cookie Acknowledgement (COOKIE ACK) (11)
3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14)
4. SCTP Association State Diagram
5. Association Initialization
5.1. Normal Establishment of an Association
5.1.1. Handle Stream Parameters
5.1.2. Handle Address Parameters
5.1.3. Generating State Cookie
5.1.4. State Cookie Processing
5.1.5. State Cookie Authentication
5.1.6. An Example of Normal Association Establishment
5.2. Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK
5.2.1. INIT Received in COOKIE-WAIT or COOKIE-ECHOED State (Item B)
5.2.2. Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED, COOKIE-WAIT, and SHUTDOWN-ACK-SENT
5.2.3. Unexpected INIT ACK
5.2.4. Handle a COOKIE ECHO when a TCB Exists
5.2.4.1. An Example of a Association Restart
5.2.5. Handle Duplicate COOKIE-ACK.
5.2.6. Handle Stale COOKIE Error
5.3. Other Initialization Issues
5.3.1. Selection of Tag Value
5.4. Path Verification
6. User Data Transfer
6.1. Transmission of DATA Chunks
6.2. Acknowledgement on Reception of DATA Chunks
6.2.1. Processing a Received SACK
6.3. Management of Retransmission Timer
6.3.1. RTO Calculation
6.3.2. Retransmission Timer Rules
6.3.3. Handle T3-rtx Expiration
6.4. Multi-Homed SCTP Endpoints
6.4.1. Failover from an Inactive Destination Address
6.5. Stream Identifier and Stream Sequence Number
6.6. Ordered and Unordered Delivery
6.7. Report Gaps in Received DATA TSNs
6.8. CRC32c Checksum Calculation
6.9. Fragmentation and Reassembly
6.10. Bundling
7. Congestion Control
7.1. SCTP Differences from TCP Congestion Control
7.2. SCTP Slow-Start and Congestion Avoidance
7.2.1. Slow-Start
7.2.2. Congestion Avoidance
7.2.3. Congestion Control
7.2.4. Fast Retransmit on Gap Reports
7.3. Path MTU Discovery
8. Fault Management
8.1. Endpoint Failure Detection
8.2. Path Failure Detection
8.3. Path Heartbeat
8.4. Handle "Out of the Blue" Packets
8.5. Verification Tag
8.5.1. Exceptions in Verification Tag Rules
9. Termination of Association
9.1. Abort of an Association
9.2. Shutdown of an Association
10. Interface with Upper Layer
10.1. ULP-to-SCTP
10.2. SCTP-to-ULP
11. Security Considerations
11.1. Security Objectives
11.2. SCTP Responses to Potential Threats
11.2.1. Countering Insider Attacks
11.2.2. Protecting against Data Corruption in the Network
11.2.3. Protecting Confidentiality
11.2.4. Protecting against Blind Denial-of-Service Attacks
11.2.4.1. Flooding
11.2.4.2. Blind Masquerade
11.2.4.3. Improper Monopolization of Services
11.3. SCTP Interactions with Firewalls
11.4. Protection of Non-SCTP-Capable Hosts
12. Network Management Considerations
13. Recommended Transmission Control Block (TCB) Parameters
13.1. Parameters Necessary for the SCTP Instance
13.2. Parameters Necessary per Association (i.e., the TCB)
13.3. Per Transport Address Data
13.4. General Parameters Needed
14. IANA Considerations
14.1. IETF-Defined Chunk Extension
14.2. IETF-Defined Chunk Parameter Extension
14.3. IETF-Defined Additional Error Causes
14.4. Payload Protocol Identifiers
14.5. Port Numbers Registry
15. Suggested SCTP Protocol Parameter Values
16. Acknowledgements
Appendix A. Explicit Congestion Notification
Appendix B. CRC32c Checksum Calculation
Appendix C. ICMP Handling
References
Normative References
Informative References
Network Working Group R. Stewart, Ed.
Request for Comments: 4960 September 2007
Obsoletes: 2960, 3309
Category: Standards Track
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.
Abstract
This document obsoletes RFC 2960 and RFC 3309. It describes the
Stream Control Transmission Protocol (SCTP). SCTP is designed to
transport Public Switched Telephone Network (PSTN) signaling messages
over IP networks, but is capable of broader applications.
SCTP is a reliable transport protocol operating on top of a
connectionless packet network such as IP. It offers the following
services to its users:
-- 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.
Stewart Standards Track [Page 1]
RFC 4960 Stream Control Transmission Protocol September 2007
Table of Contents
1. Introduction ....................................................5
1.1. Motivation .................................................5
1.2. Architectural View of SCTP .................................6
1.3. Key Terms ..................................................6
1.4. Abbreviations .............................................10
1.5. Functional View of SCTP ...................................10
1.5.1. Association Startup and Takedown ...................11
1.5.2. Sequenced Delivery within Streams ..................12
1.5.3. User Data Fragmentation ............................12
1.5.4. Acknowledgement and Congestion Avoidance ...........12
1.5.5. Chunk Bundling .....................................13
1.5.6. Packet Validation ..................................13
1.5.7. Path Management ....................................13
1.6. Serial Number Arithmetic ..................................14
1.7. Changes from RFC 2960 .....................................15
2. Conventions ....................................................15
3. SCTP Packet Format .............................................15
3.1. SCTP Common Header Field Descriptions .....................16
3.2. Chunk Field Descriptions ..................................17
3.2.1. Optional/Variable-Length Parameter Format ..........19
3.2.2. Reporting of Unrecognized Parameters ...............21
3.3. SCTP Chunk Definitions ....................................21
3.3.1. Payload Data (DATA) (0) ............................22
3.3.2. Initiation (INIT) (1) ..............................24
3.3.2.1. Optional/Variable-Length
Parameters in INIT ........................27
3.3.3. Initiation Acknowledgement (INIT ACK) (2) ..........30
3.3.3.1. Optional or Variable-Length Parameters ....33
3.3.4. Selective Acknowledgement (SACK) (3) ...............34
3.3.5. Heartbeat Request (HEARTBEAT) (4) ..................38
3.3.6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5) ......39
3.3.7. Abort Association (ABORT) (6) ......................40
3.3.8. Shutdown Association (SHUTDOWN) (7) ................41
3.3.9. Shutdown Acknowledgement (SHUTDOWN ACK) (8) ........41
3.3.10. Operation Error (ERROR) (9) .......................42
3.3.10.1. Invalid Stream Identifier (1) ............44
3.3.10.2. Missing Mandatory Parameter (2) ..........44
3.3.10.3. Stale Cookie Error (3) ...................45
3.3.10.4. Out of Resource (4) ......................45
3.3.10.5. Unresolvable Address (5) .................46
3.3.10.6. Unrecognized Chunk Type (6) ..............46
3.3.10.7. Invalid Mandatory Parameter (7) ..........47
3.3.10.8. Unrecognized Parameters (8) ..............47
3.3.10.9. No User Data (9) .........................48
3.3.10.10. Cookie Received While Shutting
Down (10) ...............................48
Stewart Standards Track [Page 2]
RFC 4960 Stream Control Transmission Protocol September 2007
3.3.10.11. Restart of an Association with
New Addresses (11) ......................49
3.3.10.12. User-Initiated Abort (12) ...............49
3.3.10.13. Protocol Violation (13) .................50
3.3.11. Cookie Echo (COOKIE ECHO) (10) ....................50
3.3.12. Cookie Acknowledgement (COOKIE ACK) (11) ..........51
3.3.13. Shutdown Complete (SHUTDOWN COMPLETE) (14) ........51
4. SCTP Association State Diagram .................................52
5. Association Initialization .....................................56
5.1. Normal Establishment of an Association ....................56
5.1.1. Handle Stream Parameters ...........................58
5.1.2. Handle Address Parameters ..........................58
5.1.3. Generating State Cookie ............................61
5.1.4. State Cookie Processing ............................62
5.1.5. State Cookie Authentication ........................62
5.1.6. An Example of Normal Association Establishment .....64
5.2. Handle Duplicate or Unexpected INIT, INIT ACK,
COOKIE ECHO, and ..........................................65
5.2.1. INIT Received in COOKIE-WAIT or
COOKIE-ECHOED State (Item B) .......................66
5.2.2. Unexpected INIT in States Other than
CLOSED, COOKIE-ECHOED, .............................66
5.2.3. Unexpected INIT ACK ................................67
5.2.4. Handle a COOKIE ECHO when a TCB Exists .............67
5.2.4.1. An Example of a Association Restart .......69
5.2.5. Handle Duplicate COOKIE-ACK. .......................71
5.2.6. Handle Stale COOKIE Error ..........................71
5.3. Other Initialization Issues ...............................72
5.3.1. Selection of Tag Value .............................72
5.4. Path Verification .........................................72
6. User Data Transfer .............................................73
6.1. Transmission of DATA Chunks ...............................75
6.2. Acknowledgement on Reception of DATA Chunks ...............78
6.2.1. Processing a Received SACK .........................81
6.3. Management of Retransmission Timer ........................83
6.3.1. RTO Calculation ....................................83
6.3.2. Retransmission Timer Rules .........................85
6.3.3. Handle T3-rtx Expiration ...........................86
6.4. Multi-Homed SCTP Endpoints ................................87
6.4.1. Failover from an Inactive Destination Address ......88
6.5. Stream Identifier and Stream Sequence Number ..............88
6.6. Ordered and Unordered Delivery ............................88
6.7. Report Gaps in Received DATA TSNs .........................89
6.8. CRC32c Checksum Calculation ...............................90
6.9. Fragmentation and Reassembly ..............................91
6.10. Bundling .................................................92
7. Congestion Control .............................................93
7.1. SCTP Differences from TCP Congestion Control ..............94
Stewart Standards Track [Page 3]
RFC 4960 Stream Control Transmission Protocol September 2007
7.2. SCTP Slow-Start and Congestion Avoidance ..................95
7.2.1. Slow-Start .........................................96
7.2.2. Congestion Avoidance ...............................97
7.2.3. Congestion Control .................................98
7.2.4. Fast Retransmit on Gap Reports .....................98
7.3. Path MTU Discovery .......................................100
8. Fault Management ..............................................100
8.1. Endpoint Failure Detection ...............................100
8.2. Path Failure Detection ...................................101
8.3. Path Heartbeat ...........................................102
8.4. Handle "Out of the Blue" Packets .........................104
8.5. Verification Tag .........................................105
8.5.1. Exceptions in Verification Tag Rules ..............105
9. Termination of Association ....................................106
9.1. Abort of an Association ..................................107
9.2. Shutdown of an Association ...............................107
10. Interface with Upper Layer ...................................110
10.1. ULP-to-SCTP .............................................110
10.2. SCTP-to-ULP .............................................120
11. Security Considerations ......................................123
11.1. Security Objectives .....................................123
11.2. SCTP Responses to Potential Threats .....................124
11.2.1. Countering Insider Attacks .......................124
11.2.2. Protecting against Data Corruption in the
Network ..........................................124
11.2.3. Protecting Confidentiality .......................124
11.2.4. Protecting against Blind
Denial-of-Service Attacks ........................125
11.2.4.1. Flooding ................................125
11.2.4.2. Blind Masquerade ........................126
11.2.4.3. Improper Monopolization of Services .....127
11.3. SCTP Interactions with Firewalls ........................127
11.4. Protection of Non-SCTP-Capable Hosts ....................128
12. Network Management Considerations ............................128
13. Recommended Transmission Control Block (TCB) Parameters ......129
13.1. Parameters Necessary for the SCTP Instance ..............129
13.2. Parameters Necessary per Association (i.e., the TCB) ....129
13.3. Per Transport Address Data ..............................131
13.4. General Parameters Needed ...............................132
14. IANA Considerations ..........................................132
14.1. IETF-defined Chunk Extension ............................132
14.2. IETF-Defined Chunk Parameter Extension ..................133
14.3. IETF-Defined Additional Error Causes ....................133
14.4. Payload Protocol Identifiers ............................134
14.5. Port Numbers Registry ...................................134
15. Suggested SCTP Protocol Parameter Values .....................136
16. Acknowledgements .............................................137
Appendix A. Explicit Congestion Notification .....................139
Stewart Standards Track [Page 4]
RFC 4960 Stream Control Transmission Protocol September 2007
Appendix B. CRC32c Checksum Calculation ..........................140
Appendix C. ICMP Handling ........................................142
References .......................................................149
Normative References ..........................................149
Informative References ........................................150
1. Introduction
This section explains the reasoning behind the development of the
Stream Control Transmission Protocol (SCTP), the services it offers,
and the basic concepts needed to understand the detailed description
of the protocol.
This document obsoletes [RFC2960] and [RFC3309].
1.1. Motivation
TCP [RFC0793] has performed immense service as the primary means of
reliable data transfer in IP networks. However, an increasing number
of recent applications have found TCP too limiting, and have
incorporated their own reliable data transfer protocol on top of UDP
[RFC0768]. The limitations that users have wished to bypass include
the following:
-- 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.
Stewart Standards Track [Page 5]
RFC 4960 Stream Control Transmission Protocol September 2007
1.2. Architectural View of SCTP
SCTP is viewed as a layer between the SCTP user application ("SCTP
user" for short) and a connectionless packet network service such as
IP. The remainder of this document assumes SCTP runs on top of IP.
The basic service offered by SCTP is the reliable transfer of user
messages between peer SCTP users. It performs this service within
the context of an association between two SCTP endpoints. Section 10
of this document sketches the API that should exist at the boundary
between the SCTP and the SCTP user layers.
SCTP is connection-oriented in nature, but the SCTP association is a
broader concept than the TCP connection. SCTP provides the means for
each SCTP endpoint (Section 1.3) to provide the other endpoint
(during association startup) with a list of transport addresses
(i.e., multiple IP addresses in combination with an SCTP port)
through which that endpoint can be reached and from which it will
originate SCTP packets. The association spans transfers over all of
the possible source/destination combinations that may be generated
from each endpoint’s lists.
_____________ _____________
| SCTP User | | SCTP User |
| Application | | Application |
|-------------| |-------------|
| SCTP | | SCTP |
| Transport | | Transport |
| Service | | Service |
|-------------| |-------------|
| |One or more ---- One or more| |
| IP Network |IP address \/ IP address| IP Network |
| Service |appearances /\ appearances| Service |
|_____________| ---- |_____________|
SCTP Node A |<-------- Network transport ------->| SCTP Node B
Figure 1: An SCTP Association
1.3. Key Terms
Some of the language used to describe SCTP has been introduced in the
previous sections. This section provides a consolidated list of the
key terms and their definitions.
o Active destination transport address: A transport address on a
peer endpoint that a transmitting endpoint considers available for
receiving user messages.
Stewart Standards Track [Page 6]
RFC 4960 Stream Control Transmission Protocol September 2007
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 that 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 on
which the message was sent.
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.
Stewart Standards Track [Page 7]
RFC 4960 Stream Control Transmission Protocol September 2007
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).
Stewart Standards Track [Page 8]
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