Chapter 7. Session Initiation Protocol and Related Protocols

This chapter provides a technological overview of the Session Initiation Protocol (SIP) that focuses on its ability to enable VoIP. If you take the time to read the SIP standard, Request For Comments (RFC) 3261, you will notice that SIP can do much more than set up basic calls. Rather than writing an exhaustive reinterpretation of the normative statements found within the standard, it seems more useful to explain SIP in its simplest form. After reading this chapter, you should be able to read through a SIP call flow, interpret a log of the message text, and gain some new insight into some of the activity that transpires within VOCAL when users pick up their IP phones and call one another. If you are already familiar with SIP, you may want to skip ahead to Chapter 8 where we discuss the data structures found within the Vovida SIP stack.

While most of the facts covered in this chapter will remain static for a long time, there is always a potential for change. The Internet Engineering Task Force (IETF)’s web site,, is your best source for keeping up with the latest RFCs and drafts. Refer to RFC 2026 for an explanation of the Internet standards process.

What Is SIP?

SIP is a rendezvous protocol for finding users and setting up and modifying multimedia sessions. VoIP is only one of many possible applications for SIP. In our initial discussion of SIP message flows, we attempt to describe SIP generically. Later ...

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