an be realized in a pairwise unicast or full mesh relationship between n parties. However, due to its n2 signaling complex- ity, this approach remains suitable only for small conferences. Improvements are subject to specialized mesh optimizations. Scalability up to very large groups can be achieved by the use of multicast packet distribution, which - in contrast to the loosely coupled model - can follow a source specific model on the IP layer for both session management and media distribution; see Section 6.2.3 for a detailed discussion. User agent clients may also make use of a gen- eral multicast service resident on the application layer. Subject to its algorith- mic realization, distribution efficiency of application layer multicast commonly increases with a growing number of participants and promotes scalability. © 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP Complementary approaches to distributed conferencing are opened up by structured peer-to-peer technologies. Distributed hash tables (DHTs) offer an efficient, fully distributed persistence layer and can facilitate a decentralized realization of some or all functions of a conference focus. Such a distributed conference coordination scheme raises global routability and NAT traversal issues as major problems to solve. Fundamental work for this purpose is cur- rently chartered in the IETF p2psip working group, complemented by research activities addressed within the Internet Research Task Force (IRTF) p2prg research group. A detailed discussion on peer-to-peer SIP technologies is given in Chapter 9. Beyond signaling, the forwarding capabilities of DHTs give rise to enhanced application layer multicast solutions, which will be reconsidered in more detail in Section 6.2.3. 6.2.2 SIP Extensions Used in Conferencing SIP syntax and semantics require extensions whenever the support of feature- rich conferencing scenarios is desired. Following SIP design principles, invoker- oriented primitives are the common basis to enable a dial-in an