erlies multicast on CAN. In the other, a distribution tree is erected within the full overlay, to be used as a shared- or source-specific tree. The latter schemes are used in Scribe and SplitStream, where a rendezvous node is chosen from group key ownership, or in Bayeux. The performance of DHT-based multicast has been thoroughly studied in [36] with the comparative focus on tree-based versus flooding approaches "In IPv4 the 232/8 range and the prefix FF3x::/32 in IPv6 are designated as source-specific multicast destination addresses. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Charlie Sally Mixer Linus Marcie Snoopy FIGURE 6.6 Media distribution in a centralized source-specific multicast model. built onto CAN and Pastry. The separate construction of mini-overlays per group as needed for a selective flooding incurred significant overhead. In addi- tion, flooding was found to be outperformed by forwarding along trees, where a shared group tree combined with proximity-aware routing as in SCRIBE could minimize the overlay delay penalty down to a factor of two. For the sake of completeness, we mention that application-layer multicast concepts concur- rently exist for unstructured peer-to-peer approaches. They operate at lower algorithmic complexity, but show significantly higher efforts in coordinative signaling and thereby admit performance measures far below native multicast. From a conceptual point of view, a user agent client can transparently take advantage of its multicast access on the application layer by submitting media data or SIP requests to the DHT. Overlay routing will just transport native packets in an application layer tunnel to the corresponding group of receivers. However, standardization work on peer-to-peer SIP has just begun [37,38] and currently concentrates on distributed user location and NAT traversal. A sig- nificant amount of prestandard work is required until an efficient, globally scal- able, and robust multicast distribution layer becomes ava