23.1.1 Brief History

SIP, an IETF standard, is an ASCII-based application layer control/signaling protocol for creating, modifying, maintaining, and terminating sessions with one or more participating terminals on an IP network. A session, in contrast to a connection in circuit-switched telephone networks, consists of a set of data streams that flow from a sender to one or more receivers. The data stream can be carried over a reliable TCP or unreliable UDP layer. SIP is an alternative protocol and architecture to H.323 for providing multimedia applications over IP networks.

SIP is designed to provide signaling and session management capabilities on packet networks. Signaling allows call information to be carried across network boundaries. Session management provides the ability to control the attributes of an end-to-end call.

Since the draft standard of SIP as RFC 2543 (Handley et al. 1999) was first published, additional service features have been added on a continuous basis. A revision to the original SIP was completed in mid-year 2002 (Rosenberg, Schulzrinne et al., 2002)

SIP is considered Internet friendly in many respects. Its simplicity and exclusive focus on the Internet have helped the protocol achieve wide acceptance in VoIP application despite the fact that it is a relatively late comer compared with the H.323 standards. The fact that the SIP client is incorporated...