SCTP: A Proposed Standard for Robust Internet Data Transport

M ost Internet protocol-based networks employ either the transmission control protocol (TCP) or the user datagram protocol (UDP) for data transfer. However, these two general-purpose protocols provide disjointed services and do not ideally satisfy all application needs. The general-purpose stream control transmission protocol is designed to expand the scope beyond TCP and UDP. SCTP evolved from a telephony sig-naling protocol for IP networks. 1 Today it is a proposed Internet Engineering Task Force standard (RFC 2960). 2 Like TCP, SCTP provides a reliable, full-duplex connection and mechanisms to control network congestion. Unlike both TCP and UDP, however, SCTP offers new delivery options that are particularly desirable for telephony signaling and multimedia applications. Table 1 compares SCTP's services and features with those of TCP and UDP. An SCTP connection, called an association, provides novel services such as multihoming, which allows the end points of a single association to have multiple IP addresses, and multistreaming, which allows for independent delivery among data streams. SCTP also features a four-way handshake to establish an association, which makes it resistant to blind denial-of-service attacks and thus improves overall protocol security. TCP provides a byte-stream data delivery service, whereas SCTP provides a message-oriented data delivery service. Figure 1 illustrates a generalization of the SCTP packet format. The packets always begin with an SCTP common header, a minimal structure that provides three basic functions: • Source and destination ports. Together with the IP addresses in the IP header, the port numbers identify the association to which an SCTP packet belongs. • Verification tags. Vtags ensure that the packet belongs to the current incarnation of an association. • Checksum. This computed value maintains the entire packet's data integrity. The remainder of an SCTP packet consists of one or more chunks, concatenated building blocks that contain either control or data information. This format differs from TCP and UDP packets, which include control information in the header and offer only a single optional data field. SCTP control chunks transfer information needed for association functionality, while data chunks carry application-layer data. The current specification defines 14 different control chunks for association establishment, association termination, data acknowledgment (ACK), destination failure detection and recovery, explicit congestion notification (ECN), and error reporting. SCTP is exten-sible, allowing new control chunk types to be defined in the future. Each chunk has a chunk header that identifies its length, type, and any special flags the type needs. …