curity (TLS), User Datagram Protocol (UDP), Internet Protocol (IP), Domain Name System (DNS), and others. This organization and these related protocols will be briefly introduced. Related background topics such as Internet uniform resource indi- cators (URIs) and uniform resource locators (URLs), IP multicast routing, and Augmented Backus-Naur Format (ABNF) representations of protocol messages will also be covered. 1.1 Signaling Protocols This book is about the Session Initiation Protocol. As the name implies, the pro- tocol allows two end points to establish media sessions with each other. The main signaling functions of the protocol are as follows: Location of an end point; Contacting an end point to determine willingness to establish a session; Exchange of media information to allow session to be established; Modification of existing media sessions; SIP: Understanding the Session Initiation Protocol Tear-down of existing media sessions. SIP has also been extended to request and deliver presence information (on-line/off-line status and location information such as that contained in a "buddy" list) as well as instant message sessions. These functions include: Publishing and uploading of presence information; Requesting delivery of presence information; Presence and other event notification; Transporting of instant messages. While some of the examples discuss SIP from a telephony perspective, there will be many nontelephony uses for SIP. SIP will likely be used to establish a whole set of session types that bear almost no resemblance to a telephone call. 1.2 The Internet Engineering Task Force SIP was developed by the IETF. To quote "The Tao of the IETF" [2]: "The Internet Engineering Task Force is a loosely self-organized group of people who make technical and other contributions to the engineering and evolution of the Internet and its technologies." The two document types used within the IETF are Internet-Drafts (I-Ds) and Request for Comments (RFCs). I-Ds are the working documents of the group; anyone can auth