ivery of Provisional Responses Mid-session Transactions That Do Not Change the State of the Session Multiple Message Bodies Instant Messages Automatic Configuration of UAs Preconditions to Be Fulfilled Before Alerting Caller Preferences Asynchronous Notification of Events Contents Third-party Call Control Session Transfer Sending Commands SIP Security Chapter 7 Building Applications with the SIP Toolkit Third-generation Mobile Systems Network Domains Call Flow Examples Instant Messages and Presence SIMPLE Working Group Presence Architecture Instant Messaging PacketCable Architecture Call Flow Example PSTN to SIP Interworking Low-Capacity Gateways High-Capacity Gateways SIP Extensions for PSTN Interworking The PINT Service Protocol SIP for Conferencing Multicast Conferences End User Mixing Model Multipoint Control Unit (MCU) Decentralized Multipoint Conference Control of Networked Appliances Appendix Finding Futher Information on SIP IETF Web site Henning Schulzrinne's SIP Web page Dean Willis' Web Pages The SIP forum RFC example Acronyms References Index PREFACE The Session Initiation Protocol (SIP) has gained a lot of attention over the last few years. Lately, the decision to use SIP as the signalling protocol to provide IP multimedia services in the third generation of mobile systems has dramatically increased the number of people interested in knowing about SIP. SIP is the protocol that will merge together the cellular and the Internet worlds. It will provide ubiquitous access to all the services that have made the Internet SO successful Users will be able to combine traditional Internet services such as e-mail and the Web with newer ser- vices such as multimedia and instant messaging. Although the services that SIP can provide are relatively well known, there is a lack of knowledge about the protocol itself. SIP is seen by many people as a protocol that can resolve every problem one could imagine, when in reality, SIP has a limited well-defined scope. During all the years that I have been working on SIP stand