or one on any topic and sub- mit it to the IETF. There is no formal membership in the IETF; anyone can participate. Every I-D contains the following paragraph on the first page: "Internet-Drafts are documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappro- priate to use Internet-Drafts as reference material or to cite them other than as work in progress." Internet standards are archived by the IETF as the Request for Comments series of numbered documents. As changes are made in a protocol, or new ver- sions come out, a new RFC document with a new number is issued, which "obsoletes" the old RFC. Some I-Ds are cited in this book; I have tried, how- ever, to restrict this to mature documents that are likely to become RFCs by the time this book is published. A standard begins its life as an I-D and then pro- gresses to an RFC once there is consensus and there are working implementa- tions of the protocol. Anyone with Internet access can download any I-D or RFC at no charge using the World Wide Web, ftp, or e-mail. Information on how to do SO can be found on the IETF Web site: http://www.ietf.org. The IETF is organized into working groups, which are chartered to work in a particular area and develop a protocol to solve that particular area. Each SIP and the Internet working group has its own archive and mailing list, which is where most of the work gets done. The IETF also meets three times per year. 1.3 A Brief History of SIP SIP was originally developed by the IETF Multi-Party Multimedia Session Con- trol Working Group, known as MMUSIC. Version 1.0 was submitted as an Internet-Draft in 1997. Significant changes were made to the protocol and resulted in a second version, version 2.0, which was submitted as an Internet- Draft in 1998. The protocol achieved Proposed Standard status in March 1999 and was published as RFC 2543 [3] in April 1999. In September 1999, the SIP working group was established by the IETF to meet the growing interest in t