why IP, and not other network layer protocols, is used to implement packet based services in modern networks. The remainder of Chapter 2 and Chapter 3, "The Internet Multimedia Conferencing Architecture and How Protocols Mature," set SIP in its con- text. They describe how SIP interacts with other protocols (the Internet multimedia conferencing architecture), and how SIP standardization is carried out within the Internet Engineering Task Force (IETF). This gives the reader an idea of the different maturity levels of the different SIP extensions and what they mean. Knowing the Internet multimedia con- ferencing architecture is useful in order to understand the scope of SIP and of the rest of the protocols that belong to the architecture. All these protocols interact among them to provide multimedia services to the users. Chapter 4, "The Session Initiation Protocol," through Chapter 6, "Extending SIP: The SIP Toolkit," deal more with what and how, without forgetting about why. However, these two concepts are kept separate as much as possible. It is important to distinguish between the functional- ity provided by SIP, and the protocol details of how this functionality is achieved. Understanding first what the protocol does, it is easier to study how it is implemented. Chapter 4 deals with what SIP does wheras Chap- ter 5, "SIP: Protocol Operation," explains the protocol syntax. This dis- tinction is also present in Chapter 6, where several SIP extensions are explained. Every extension is clearly divided into two sections: the first one explains what the extension does, and the second deals with its imple- mentation. Finally, Chapter 7, "Building Applications with the SIP Tookit," pro- vides examples of architectures that have chosen SIP as a signaling protocol, such as 3G or PacketCable. Preface After reading this book you will have a good understanding of the three aspects of SIP: what, how, and why. You will be able to understand its role in different architectures and its interactions with other protocols. Fur- t