rresponding re-INVITE procedure. If a new source Snew contacts an established SSM group conference, it will do SO analogously by inviting some member S. If S decides to accept the caller, it will redistribute its INVITE to the SSM group and acknowledge the © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security INVITE[SDP Snew] INVITE[SDP OK[SDPs11 Snew] OK[SDPs21 Join(Snew G) Initiate Join(S, G) re-INVITE Join(S,G) Join(Snew G) Join(S,G) INVITE INVITE CONTACT: G Join(S, G) CONTACT: G FIGURE 6.9 Call flow of switching from unicast to SSM within re-INVITE. initial call by placing the group conference URI in the CONTACT header field. As displayed in Figure 6.10, all group members will immediately add Snew to their source-specific multicast filters. Snew subsequently will learn about all group members from (unicast) OK messages as needed for its own multicast subscriptions. Note that call redistribution will remain a point-to- point user transaction of Snew at the SIP layer, but a transmission from source S at the network layer, and therefore compliant with previous SSM group establishment. Proceeding along this incremental way, a callee will never be required to redistribute messages to more than one party or group. This scheme thus remains fully scalable and fairly transparent to group sizes. Multicast initia- tion of media sessions may be led correspondingly. 6.5 Summary and Conclusions The concepts and mechanisms in this chapter describe how group conferences can be realized with SIP. They apply equally to voice and video, messaging and chat, gaming and collaborative environments, as well as to many other application areas of this lively and growing field. Starting from elementary operations to initiate a multi-party conversation, an overview of basic SIP concepts and service primitives is presented for C 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP INVITE[SDP Join(Snew G) Join( (S Snew] Join(S, G) INVITE[SDP OK[SDP s] Snew] Contact: Join(S), G) OK[SDP