SCTP: new transport protocol for TCP/IP - Internet Computing, IEEE

T he transport layer’s primary role is to provide end-to-end communications service between two or more applications running on different hosts. It isolates the applications from the specifics of the underlying network connecting the hosts and provides a simple interface for applications developers. The transport layer can also perform sophisticated actions such as flow control, error recovery, and reliable delivery, which might be necessary for the communicating applications to run properly with reasonable performance.1 For the past 20 years, applications and end users of the TCP/IP suite have employed one of two protocols: the transmission control protocol or the user datagram protocol. Yet some applications already require greater functionality than what either TCP or UDP has to offer, and future applications might require even more. To extend transport layer functionality, the Internet Engineering Task Force approved the stream control transmission protocol (SCTP) as a proposed standard in October 2000.2 SCTP was spawned from an effort started in the IETF Signaling Transport (Sigtrans) working group to develop a specialized transport protocol for call control signaling in voice-over-IP (VoIP) networks.3 Recognizing that other applications could use some of the new protocol’s capabilities, the IETF now embraces SCTP as a general-purpose transport layer protocol, joining TCP and UDP above the IP layer. Like TCP, SCTP offers a point-to-point, connection-oriented, reliable delivery transport service for applications communicating over an IP network. It inherits many of the functions developed for TCP over the past two decades, including powerful congestion control and packet loss recovery functions. Indeed, any application running over TCP can be ported to run over SCTP without loss of function, but the many similarities between the two soon give way to several differences. The most interesting of these differences revolve around SCTP’s support for multihoming and partial ordering. Multihoming enables an SCTP host to establish a “session” with another SCTP host over multiple interfaces identified by separate IP addresses. Partial ordering lets SCTP provide in-order delivery of one or more related sequences of messages flowing between two hosts. Thus, SCTP can benefit applications that require reliable delivery and fast processing of multiple, unrelated data streams.