ence Work References Appendix A: Changes in the SIP Specification from RFC 2543 to RFC 3261 About the Author Index Foreword to the First Edition The Internet now challenges the close to $1 trillion world telecom industry. A renaissance in communications is taking place on the Internet. At its source are new communication protocols that would be impractical on the centralized con- trol systems of circuit-switched networks used in telecommunications The Internet and the World Wide Web can be technically defined only by their protocols. Similarly, IP telephony and the wider family of IP communica- tions are defined by several key protocols, most notably by the Session Initiation Protocol, or SIP. The previously closed door of telecommunications is now wide open to web developers because of SIP and its relation to the web HTTP 1.1 protocol and the e-mail SMTP protocol. IP communications include voice/video, pres- ence, instant messaging, mobility, conferencing, and even games. We believe many other communication areas are yet to be invented. The integration of all types of communications on the Internet may represent the next "killer applica- tion" and generate yet another wave of Internet growth. As explained in this book, SIP is a close relative of the HTTP 1.1 and SMTP protocols. This represents a revolution in communications because it abandons the telecom signaling and control models developed for telephony over many years in favor of Internet and web-based protocols. Users and service providers obtain not only seamless integration of telephony and conferencing with many other World Wide Web and messaging applications, but also benefit from new forms of communications, such as presence and instant messaging. Mobility can also be managed across various networks and devices using SIP. Location management is now under user control, SO that incoming "calls" can be routed to any network and device that the called party may prefer. Users xviii SIP: Understanding the Session Initiation Protocol may even move across the