Chapter 4 - SIP Overview

Abstract

This chapter focuses on the session initiation protocol (SIP), defines some key concepts in the SIP framework, such as sessions and addressing, and describes the various SIP entities in the SIP architecture. It also demonstrates how the SIP entities are involved in a basic SIP call. The term session refers to the media plane aspect of the communication, which enables the exchange of media (e.g., voice, video, and so on) among an association of participants. To create sessions, SIP messages carry session description protocol (SDP) session descriptions that allow participants to agree on a set of parameters needed for the multimedia communication, such as transport addresses or media types. In the SIP architecture, users are usually identified by a SIP URI (universal resource identifier), which may refer to SIP users and SIP servers. SIP basically solves two key aspects in IP multimedia communications, which include session setup, modification, and termination and location of users. The SIP specifications define various SIP elements as part of the SIP architecture, which include user agents (UAs), registrars, proxies, and back-to-back user agents (B2BUA). User agents are typically located at the SIP endpoints and the registrar is a server that accepts registration requests from the user agents. A proxy server is an intermediary entity that makes requests on behalf of other clients and primarily plays the role of routing (SIP routing). A B2BUA is a logical entity that acts as a user agent to both ends of a SIP call and is responsible for handling all SIP signaling between both ends of the call, from call establishment to termination.