ers that will be used. The Session Initiation Protocol: SIP Figure 4-3 SIP core specification SIP extensions The SIP toolkit consists of the core specification and several extensions. SIP is independent of the type of multimedia session handled and of the mechanism used to describe the session. It is equally useful for videocon- ferences, audio calls, shared whiteboards, and gaming sessions. Sessions consisting of RTP streams carrying audio and video are usually described using SDP, but some types of session can be described with other descrip- tion protocols. Assuming Bob wants to play chess with Laura, he has the option of using a chess-specific session, which will be described by a chess- specific description protocol instead of SDP. If Bob and Laura were to play a video game over the Internet, they also would probably use a protocol other than SDP to describe their gaming session. In short, SIP is used to distribute session descriptions among potential participants (Figure 4-4). Once the session description is distributed, SIP can be used to negotiate and modify the parameters of the session and ter- minate the session. The following example illustrates all of these functions. Bob wants to have an audio-video session with Laura and plans to use a Pulse Code Mod- ulation (PCM) codec to encode voice. In this example, the session distribu- tion part consists of Bob sending Laura a session description with a PCM codec for the voice component of the session. Laura prefers to use a Global System for Mobile Communications (GSM) codec because it consumes less bandwidth, SO she persuades Bob to do it her way. Both finally settle on a GSM audio codec, but the session cannot be established until this negotia- tion is concluded. Chapter 4 Figure 4-4 Bob invites Laura and I would like to speak to Laura and Tom. I will use SIP to establish a voice session Tom to a voice among us. session. Would you like to join this session? Session description SIP distributes SIP message carrying the session a session description descript