, and services anywhere they are connected to the Internet. SIP applications provide support for presence-the ability to find out the status or location of a user without attempting to set up a session. Another major change in the past few years is the adoption of wireless SIP to enable multimedia IP communications. As described in the chapter on wire- less, SIP is now being used both in its standard form over 802.11 wireless net- works and in planned commercial Third Generation Partnership Project (3GPP) rollouts in the coming years. SIP is ideally suited for this key application. SIP: Understanding the Session Initiation Protocol Since 2001 SIP has also grown in terms of the specification itself. Initially, SIP was described by a single RFC with a few related RFCs and a couple of RFC extensions. In Chapter 6 alone, more than 20 SIP-related RFCs are referenced. This book attempts to put all those documents together and provide a single ref- erence for the protocol and all its extensions. Even SIP headers and responses that were standardized in the past but are now removed (deprecated) are listed in this text, providing useful context and background. Many others are discussed that are in the final stages of standardization prior to publication as RFCs, pro- viding an up-to-date insider's view of the future of the protocol. In closing, I again thank my colleagues in the Internet Engineering Task Force (IETF) and at MCI for all their contributions to the development of this protocol-it has been a privilege to be a part of a group of people that have created the SIP industry. Finally, I'd like to tip my hat to two of the key inven- tors of SIP who continue to develop and propel its implementation: Henning Schulzrinne and Jonathan Rosenberg. Preface to the First Edition When I began looking into the Session Initiation Protocol (SIP) in October 1998, I had prepared a list of a half dozen protocols relating to Voice over IP and Next Generation Networking. It was only a few days into my study that my list narrowed to ju