Stream Control Transmission Protocol

The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol which operates at the transport layer and serves a role similar to the popular protocols TCP and UDP. It is standardized by IETF in RFC 4960. The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol which operates at the transport layer and serves a role similar to the popular protocols TCP and UDP. It is standardized by IETF in RFC 4960. 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 the year 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 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.