Stream Control Transmission Protocol

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Stream Control Transmission Protocol
Protocol stack
AbbreviationSCTP
Introduction2000
OSI layerTransport layer (4)
IP number132
RFC(s)Template:IETF RFC

Template:IPstack

The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the protocol provides the message-oriented feature of the User Datagram Protocol (UDP) while ensuring reliable, in-sequence transport of messages with congestion control like the Transmission Control Protocol (TCP). Unlike UDP and TCP, the protocol supports multihoming and redundant paths to increase resilience and reliability.

SCTP is standardized by the Internet Engineering Task Force (IETF) in Template:IETF RFC. The SCTP reference implementation was released as part of FreeBSD version 7 and has since been widely ported to other platforms.

Formal oversight

The IETF Signaling Transport (SIGTRAN) working group defined the protocol (number 132[1]) in October 2000,[2] and the IETF Transport Area (TSVWG) working group maintains it. Template:IETF RFC defines the protocol. Template:IETF RFC provides an introduction.

Message-based multi-streaming

SCTP applications submit data for transmission in messages (groups of bytes) to the SCTP transport layer. SCTP places messages and control information into separate chunks (data chunks and control chunks), each identified by a chunk header. The protocol can fragment a message into multiple data chunks, but each data chunk contains data from only one user message. SCTP bundles the chunks into SCTP packets. The SCTP packet, which is submitted to the Internet Protocol, consists of a packet header, SCTP control chunks (when necessary), followed by SCTP data chunks (when available).

SCTP may be characterized as message-oriented, meaning it transports a sequence of messages (each being a group of bytes), rather than transporting an unbroken stream of bytes as in TCP. As in UDP, in SCTP a sender sends a message in one operation, and that exact message is passed to the receiving application process in one operation. In contrast, TCP is a stream-oriented protocol, transporting streams of bytes reliably and in order. However TCP does not allow the receiver to know how many times the sender application called on the TCP transport passing it groups of bytes to be sent out. At the sender, TCP simply appends more bytes to a queue of bytes waiting to go out over the network, rather than having to keep a queue of individual separate outbound messages which must be preserved as such.

The term multi-streaming refers to the capability of SCTP to transmit several independent streams of chunks in parallel, for example transmitting web page images simultaneously with the web page text. In essence, it involves bundling several connections into a single SCTP association, operating on messages (or chunks) rather than bytes.

TCP preserves byte order in the stream by including a byte sequence number with each segment. SCTP, on the other hand, assigns a sequence number or a message-id[note 1] to each message sent in a stream. This allows independent ordering of messages in different streams. However, message ordering is optional in SCTP; a receiving application may choose to process messages in the order of receipt instead of in the order of sending.

Features

Features of SCTP include:

  • Reliable transmission of both ordered and unordered data streams
  • Multihoming support in which one or both endpoints of a connection can consist of more than one IP address, enabling transparent fail-over between redundant network paths
  • Delivery of chunks within independent streams eliminates unnecessary head-of-line blocking, as opposed to TCP byte-stream delivery.
  • Explicit partial reliability
  • Path selection and monitoring to select a primary data transmission path and test the connectivity of the transmission path
  • Validation and acknowledgment mechanisms protect against flooding attacks and provide notification of duplicated or missing data chunks.
  • Improved error detection suitable for Ethernet jumbo frames

The designers of SCTP originally intended it for the transport of telephony (i.e. Signaling System 7) over Internet Protocol, with the goal of duplicating some of the reliability attributes of the SS7 signaling network in IP. This IETF effort is known as SIGTRAN. In the meantime, other uses have been proposed, for example, the Diameter protocol[3] and Reliable Server Pooling (RSerPool).[4]

Motivation and adoption

TCP has provided the primary means to transfer data reliably across the Internet. However, TCP has imposed limitations on several applications. From Template:IETF RFC:

  • 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 property of TCP causes unnecessary delay.
  • For applications exchanging distinct records or messages, the stream-oriented nature of TCP requires the addition of explicit markers or other encoding to delineate the individual records.
  • In order to avoid sending many small IP packets where one single larger packet would have sufficed, the TCP implementation may delay transmitting data while waiting for possibly more data being queued by the application (Nagle's algorithm). Although many TCP implementations allow the disabling of Nagle's algorithm, this is not required by the specification. SCTP, on the other hand, allows undelayed transmission to be configured as a default for an association, eliminating any undesired delays, but at the cost of higher transfer overhead.[5]
  • The limited scope[<span title="Script error: No such module "decodeEncode".">vague] of TCP sockets complicates the task of providing highly-available data transfer capability using multihomed hosts.
  • TCP is relatively vulnerable to denial-of-service attacks, such as SYN attacks.

Adoption of SCTP has been slowed by lack of awareness, lack of implementations (particularly in Microsoft Windows), lack of application support and lack of network support.[6]

SCTP has seen adoption in the mobile telephony space as the transport protocol for several core network interfaces.[7]

Multihoming

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File:SCTP-Multihoming.png
SCTP multihoming
File:SCTP-LocalMultihoming-RemoteSinglehoming.png
Asymmetric multihoming: local multihoming to remote single homing
File:SCTP-LocalSinglehoming-RemoteMultihoming.png
Asymmetric multihoming: local single homing to remote multihoming

SCTP provides redundant paths to increase reliability.

Each SCTP end point needs to check reachability of the primary and redundant addresses of the remote end point using a heartbeat. Each SCTP end point needs to acknowledge the heartbeats it receives from the remote end point.

When SCTP sends a message to a remote address, the source interface will only be decided by the routing table of the host (and not by SCTP).

In asymmetric multihoming, one of the two endpoints does not support multihoming.

In local multihoming and remote single homing, if the remote primary address is not reachable, the SCTP association fails even if an alternate path is possible.

Packet structure

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An SCTP packet consists of two basic sections:

  1. The common header, which occupies the first 12 bytes and is highlighted in blue.
  2. The data chunks, which occupy the remaining portion of the packet. The first chunk is highlighted in green, and the last of N chunks (Chunk N) is highlighted in red.
Bits 0–7 8–15 16–23 24–31
+0 Source port Destination port
32 Verification tag
64 Checksum
96 Chunk 1 type Chunk 1 flags Chunk 1 length
128 Chunk 1 data
… …
… Chunk N type Chunk N flags Chunk N length
… Chunk N data

Each chunk starts with a one-byte type identifier, with 15 chunk types defined by Template:IETF RFC, and at least 5 more defined by additional RFCs.[note 2] Eight flag bits, a two-byte length field, and the data compose the remainder of the chunk. If the chunk does not form a multiple of 4 bytes (i.e., the length is not a multiple of 4), then it is padded with zeros, which are not included in the chunk length. The two-byte length field limits each chunk to a 65,535-byte length (including the type, flags and length fields).

Security

Although encryption was not part of the original SCTP design, SCTP was designed with features for improved security, such as 4-way handshake (compared to TCP 3-way handshake) to protect against SYN flooding attacks, and large "cookies" for association verification and authenticity.

Reliability was also a key part of the security design of SCTP. Multihoming enables an association to stay open even when some routes and interfaces are down. This is of particular importance for SIGTRAN as it carries SS7 over an IP network using SCTP, and requires strong resilience during link outages to maintain telecommunication service even when enduring network anomalies.

Implementations

The SCTP reference implementation runs on FreeBSD, Mac OS X, Microsoft Windows, and Linux.[8]

The following operating systems implement SCTP:

Third-party drivers:

  • Microsoft Windows:
    • The SctpDrv kernel driver is a port of the BSD SCTP stack to Windows (Abandoned after 2012)[17]
  • MacOS:
    • SCTP Network Kernel Extension for Mac OS X[18]

Userspace library:

The following applications implement SCTP:

Tunneling over UDP

In the absence of native SCTP support in operating systems, it is possible to tunnel SCTP over UDP,[22] as well as to map TCP API calls to SCTP calls so existing applications can use SCTP without modification.[23]

RFCs

  • Template:IETF RFC Stream Control Transmission Protocol
  • Template:IETF RFC Stream Control Transmission Protocol: Errata and Issues in RFC 4960 (obsoleted by RFC 9260)
  • Template:IETF RFC SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol
  • Template:IETF RFC TCP and Stream Control Transmission Protocol (SCTP) RTO Restart
  • Template:IETF RFC Additional Policies for the Partially Reliable Stream Control Transmission Protocol Extension
  • Template:IETF RFC SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol (obsoleted by RFC 9260)
  • Template:IETF RFC UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Stream Reconfiguration
  • Template:IETF RFC Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Chunk Flags Registration (obsoleted by RFC 9260)
  • Template:IETF RFC Security Attacks Found Against the Stream Control Transmission Protocol (SCTP) and Current Countermeasures
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Direct Data Placement (DDP) Adaptation
  • Template:IETF RFC Stream Control Transmission Protocol (obsoleted by RFC 9260)
  • Template:IETF RFC Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)
  • Template:IETF RFC Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Specification Errata and Issues (obsoleted by RFC 9260)
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Management Information Base (MIB)
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Partial Reliability Extension
  • Template:IETF RFC On the Use of Stream Control Transmission Protocol (SCTP) with IPsec
  • Template:IETF RFC Transport Layer Security over Stream Control Transmission Protocol
  • Template:IETF RFC Stream Control Transmission Protocol (SCTP) Checksum Change (obsoleted by RFC 4960)
  • Template:IETF RFC An Introduction to the Stream Control Transmission Protocol
  • Template:IETF RFC Stream Control Transmission Protocol Applicability Statement
  • Template:IETF RFC Stream Control Transmission Protocol (updated by RFC 3309 and obsoleted by RFC 4960)

See also

Notes

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  1. ^ The DATA chunk uses a sequence number for ordered messages, the I-DATA chunk, which solves some problems with the original DATA chunk, uses a message-id for all messages
  2. ^ See SCTP packet structure for more details.

References

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  1. ^ Page Module:Citation/CS1/styles.css has no content."Protocol Numbers". iana.org. IANA. Retrieved 2014-09-09.
  2. ^ Script error: No such module "template wrapper".
  3. ^ Script error: No such module "template wrapper".
  4. ^ Script error: No such module "template wrapper".
  5. ^ RFC 9260, section 1.5.5
  6. ^ Page Module:Citation/CS1/styles.css has no content.Hogg, Scott. "What About Stream Control Transmission Protocol (SCTP)?". Network World. Archived from the original on August 30, 2014. Retrieved 2017-10-04.
  7. ^ Page Module:Citation/CS1/styles.css has no content.Olsson, Magnus; Mulligan, Catherine; Sultana, Shabnam; Rommer, Stefan; Frid, Lars (2013). EPC and 4G packet networks: driving the mobile broadband revolution (2nd ed.). Amsterdam Boston: Elsevier/AP, Academic Press is an imprint of Elsevier. p. 491. ISBN 978-0-12-394595-2.
  8. ^ Page Module:Citation/CS1/styles.css has no content."Reference Implementation for SCTP - RFC4960". GitHub. Retrieved 2013-10-14. This is the reference implementation for SCTP. It is portable and runs on FreeBSD/MAC-OS/Windows and in User Space (including linux).
  9. ^ Page Module:Citation/CS1/styles.css has no content."sys/netinet/sctp.h". BSD Cross Reference. NetBSD. 2017-06-27. Retrieved 2019-01-21.
  10. ^ Page Module:Citation/CS1/styles.css has no content."man4/sctp.4". BSD Cross Reference. NetBSD. 2018-07-31. Retrieved 2019-01-21.
  11. ^ Page Module:Citation/CS1/styles.css has no content."DragonFly Removes SCTP". Lists.dragonflybsd.org. 7 January 2015. Retrieved 2016-04-28.
  12. ^ Page Module:Citation/CS1/styles.css has no content."About FreeBSD's Technological Advances". The FreeBSD Project. 2008-03-09. Retrieved 2008-09-13. SCTP: FreeBSD 7.0 is the reference implementation for the new IETF Stream Control Transmission Protocol (SCTP) protocol, intended to support VoIP, telecommunications, and other applications with strong reliability and variable quality transmission through features such as multi-path delivery, fail-over, and multi-streaming.
  13. ^ Page Module:Citation/CS1/styles.css has no content."Stream Control Transmission Protocol (SCTP)". Hewlett-Packard Development Company.{{cite web}}: CS1 maint: deprecated archival service (link)
  14. ^ Page Module:Citation/CS1/styles.css has no content."TCP/IP Networking". QNX Developer Support. QNX Software Systems. Retrieved 2008-09-13.Page Module:Citation/CS1/styles.css has no content."What's New in this Reference". QNX Library Reference. QNX Software Systems. Retrieved 2012-12-18.
  15. ^ Page Module:Citation/CS1/styles.css has no content."QNX Software Development Platform 6.4.0".
  16. ^ Page Module:Citation/CS1/styles.css has no content."Solaris 10 Operating System Networking — Extreme Network Performance". Sun Microsystems. Retrieved 2008-09-13.
  17. ^ Page Module:Citation/CS1/styles.css has no content."SctpDrv: an SCTP driver for Microsoft Windows". Archived from the original on 2017-10-08. Retrieved 2022-01-04.
  18. ^ Page Module:Citation/CS1/styles.css has no content."SCTP Network Kernel Extension for Mac OS X". GitHub. 23 September 2021.
  19. ^ Page Module:Citation/CS1/styles.css has no content."sctplab/usrsctp". Github. Retrieved 21 September 2021.
  20. ^ Page Module:Citation/CS1/styles.css has no content."sctplib and socketapi: The User-Space SCTP Library (sctplib) and Socket API Library (socketapi)". 2025-07-09. Retrieved 2025-07-09.
  21. ^ Page Module:Citation/CS1/styles.css has no content."Windows SCTP library installer". Retrieved 2011-02-04.
  22. ^ Script error: No such module "template wrapper".
  23. ^ Page Module:Citation/CS1/styles.css has no content.Bickhart, Ryan; Paul D. Amer; Randall R. Stewart (2007). "Transparent TCP-to-SCTP Translation Shim Layer" (PDF). Retrieved 2008-09-13.
  24. ^ Page Module:Citation/CS1/styles.css has no content.D. Wing; A. Yourtchenko (April 2012). "Happy Eyeballs: Success with Dual-Stack Hosts". tools.ietf.org. IETF.
  25. ^ Page Module:Citation/CS1/styles.css has no content.Khademi, Naeem; Brunstrom, Anna; Hurtig, Per; Grinnemo, Karl-Johan (July 21, 2016). "Happy Eyeballs for Transport Selection". tools.ietf.org. IETF. Retrieved 2017-01-09.

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