SCTP provides some of the features of both UDP and TCP: it is message-oriented like UDP and ensures reliable, in-sequence transport of messages with congestion control like TCP. It differs from those protocols by providing multi-homing and redundant paths to increase resilience and reliability.
In the absence of native SCTP support in operating systems, it is possible to tunnel SCTP over UDP, as well as to map TCP API calls to SCTP calls so existing applications can use SCTP without modification.
The reference implementation was released as part of FreeBSD version 7. It has since been widely ported.
The IETF Signaling Transport (SIGTRAN) working group defined the protocol (number 132) in 2000, and the IETF Transport Area (TSVWG) working group maintains it. RFC 4960 defines the protocol. RFC 3286 provides an introduction.
SCTP applications submit their data to be transmitted 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 a number of 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).
One can characterize SCTP 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 does 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 together 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 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 eliminate 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.
The designers of SCTP originally intended it for the transport of telephony (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 and Reliable Server Pooling (RSerPool).
Motivations and adoption
TCP has provided the primary means to transfer data reliably across the Internet. However, TCP has imposed limitations on several applications. From RFC 4960:
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). If and when such a small delay is undesirable, the application must explicitly request undelayed transmission on a case-by-case basis using the push facility (i.e. by setting the PSH flag in the TCP packet header). 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.
The limited scope[vague] 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.
Adoption has been slowed by lack of awareness, lack of implementations (particularly in Microsoft Windows), lack of application support and lack of network support.
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 ack 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).
Asymmetric multi homing
In asymmetric multi homing, one of the two end points does not support multi homing.
Local multi homing - Remote single homing
In Local multi homing and Remote single homing, if the remote primary address is not reachable, the SCTP association fails even if an alternate path is possible.
Asymmetric Multi homing : Local Multi homing - Remote Single homing
Local single homing - Remote multi homing
Asymmetric multi homing : Local Single homing - Remote multi homing
Chunk 1 type
Chunk 1 flags
Chunk 1 length
Chunk 1 data
Chunk N type
Chunk N flags
Chunk N length
Chunk N data
An SCTP packet consists of two basic sections:
The common header, which occupies the first 12 bytes and is highlighted in blue, and
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.
Each chunk starts with a one byte type identifier, with 15 chunk types defined by RFC 4960, and at least 5 more defined by additional RFCs. 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).
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.
SCTP is sometimes a good fingerprinting candidate. Some operating systems ship with SCTP support enabled, and, as it is not as well known as TCP or UDP, it is sometimes overlooked in firewall and intrusion detection configurations, thus often permitting probing traffic.
The SCTP reference implementation runs on FreeBSD, Mac OS X, Microsoft Windows, and Linux.
^"About FreeBSD's Technological Advances". The FreeBSD Project. 2008-03-09. Retrieved . 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.
^Seggelmann, R.; Tuxen, M.; Rathgeb, E.P. (18-20 July 2012). "SSH over SCTP -- Optimizing a multi-channel protocol by adapting it to SCTP". Communication Systems, Networks & Digital Signal Processing (CSNDSP), 2012 8th International Symposium on: 1-6. doi:10.1109/CSNDSP.2012.6292659. ISBN978-1-4577-1473-3.
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