Overview

For many Internet applications, two or more hosts form temporary associations for the exchange of data. For example, FTP associates two hosts for the purpose of file transfer, in a file transfer session. In a multicast video streaming session, the video transmission source streams video to multiple recipients using multicasting. In order to establish the associations and the parameters for data exchange, session initiation and management mechanisms are needed. SIP is one such session initiation and management protocol that is general enough to not depend on the type of session it initiates and manages, or on the underlying transport protocol ^[1]. It is a text-based protocol that borrows much of its syntax from HTTP and consists of requests and responses like HTTP.

SIP, despite its name, is used not just for initiation of sessions but also for management and termination of sessions. SIP is used for managing a session in the sense that it can add or drop participants to and from a session, or change parameters such as media encodings during a session. SIP is especially good for initiation, management, and termination of real-time multimedia sessions. For some other kinds of session management protocols, such as file transfer sessions using FTP, only a particular kind of session is supported. The user can specify some parameters related to file transfers (binary or ASCII file transfer), and other IP-based systems like DNS are consulted as needed to locate machines with which a file transfer session is desired. However, SIP...