SCTP

In computer networking, the Stream Control Transmission Protocol (SCTP) is a transport-layer , serving in a similar role to the popular protocols and . It is standardized by IETF in RFC 4960.

SCTP provides some of the same service 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 these in 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 SCTP over UDP, as well as mapping TCP API calls to SCTP ones. The reference implementation was released as part of FreeBSD version 7. It has subsequently 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 , 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.

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