t these two themes become apparent as you read this book and learn about this exciting new protocol. The text is filled with examples and sample SIP messages. I had to invent a whole set of IP addresses, domain names, and URLs. Please note that they are all fictional-do not try to send anything to them. I would first like to thank the group of current and former engineers at WorldCom who shared their knowledge of this protocol and gave me the opportunity to author my first Internet-Draft document. I particularly thank Henry Sinnreich, Steve Donovan, Dean Willis, and Matt Cannon. I also thank Preface to the First Edition xxiii Robert Sparks, who I first met at the first seminar on SIP that I ever presented. Throughout the whole 3-hour session I kept wondering about the guy with the pony tail who seemed to know more than me about this brand new protocol! Robert and I have spent countless hours discussing fine points of the protocol. In addition, I would like to thank him for his expert review of this manuscript prior to publication-in is a better book due to his thoroughness and attention to detail. I also thank everyone on the IETF SIP list who has assisted me with the protocol and added to my understanding of it. A special thanks to my wife Lisa for the terrific cover artwork and the cool figures throughout the book. Finally, I thank my editor Jon Workman, the series editor and reviewer, and the whole team at Artech for helping me in this, my first adventure in publishing. SIP and the Internet The Session Initiation Protocol (SIP) is a new signaling, presence and instant messaging protocol developed to set up, modify, and tear down multimedia ses- sions, request and deliver presence and instant messages over the Internet [1]. This chapter covers some background for the understanding of the protocol. SIP was developed by the IETF as part of the Internet Multimedia Conferencing Architecture, and was designed to dovetail with other Internet protocols such as Transmission Control Protocol (TCP), Transmission Layer Se