Packet Guide to Voice over IP

Chapter 3. Session Initiation Protocol

Introduction

It seems that competition has always been a part of network protocols as vendors, products, or protocols vie for dominance in the market. Appletalk versus TCP/IP and 802.11g versus 802.11a are examples of this conflict. Local area network protocols such as Token Ring, Ethernet, and Fiber Distributed Data Interchange battled it out for quite some time. Smaller examples, such as the selection of a trunking protocol, are sprinkled throughout our decision processes. Usually the competition goes on until one becomes the de facto standard or drops out of the race.

Voice over IP is no different. This book covers several different signaling protocols, all of them handling the same set of functions. But which one is the right choice? A couple of years ago, the choice would not have been clear due to the variety of equipment being deployed, vendors, and legacy equipment. Today, most professionals agree that the Session Initialization Protocol (SIP) will be the right choice for the future.

SIP is an Internet Engineering Task Force protocol standardized in RFC 3261, though there are several companion RFCs. It is a nonproprietary signaling protocol that is now supported by almost all vendors in the VoIP industry. Although there is a significant installed base of systems running Skinny and H.323, newer network builds are likely to adopt SIP over the others. Like the other signaling protocols, SIP relies on the Real-Time Transport Protocol (RTP) ...