wise remain unnoticeable in previously established SSM groups. SIP session initiation in conferencing scenarios could facilitate this requirement [30]. The media parameters in tightly coupled or fully distributed conferences are mutually known to all peers. Despite the limitations of the SDP offer/answer model in the multicast context discussed above, each party will be aware of all source addresses of the correspondents and thus be enabled to issue source- specific joins. Without protocol extensions, user agent clients can thus partic- ipate in an SSM-based media session by simply performing IGMP/MLD [31] (S,G) joins for all peers. A simpler model well-suited for medium-size con- ferences has been introduced in the conferencing framework [5]: Each par- ticipant sends its media stream to a central media point or dedicated user agent using unicast. This central point will then redistribute the media using a source-specific multicast address. Whenever a new party joins the confer- ence, the focus will perform the necessary third party call control to assure media reception. This scenario, which may lead to triangular traffic detours, is illustrated in Figure 6.6. Up until now, SIP signaling based on SSM has not been defined. Simple possible extensions for SIP over SSM will be discussed in Section 6.4. 6.2.3.3 Application Layer Multicast Exploiting the novel approach of structured peer-to-peer routing, a collec- tion of group communication services has been developed, with the aim of seamless depoyability as application layer or overlay multicast. Among the most popular approaches are multicast on CAN [32], Bayeux [33], as derived from Tapestry, and Scribe [34] or SplitStream [35], which inherit their dis- tributed indexing from Pastry. Approaches to multicast distribution in the overlay essentially branch in two algorithmic directions. In the first case, dis- tributed hash tables (DHTs) are used to generate a structured sub-overlay of group members, which is then flooded with multicast packets. This mecha- nism und