nt with SIP (a) Ring (b) Polygon (c) Full mesh FIGURE 6.8 Peer-to-peer routing topologies on the overlay. From the perspective of the conference, super peers form a distributed focus. To keep distribution transparent to leaves, each super peer needs to provide full conferencing service functions, e.g., synchronized policing and event noti- fication, and most likely assistance in media mixing and redistribution. Focus nodes consequently require signaling relationships among each other, estab- lished on top of an application layer routing of sufficient performance for a simultaneous distribution of media streams. The design of a routing will admit critical impact on scalability, application performance, as well as forwarding and maintanance load of the super peers. The three characteristic topologies for routing between N super peers as displayed in Figure 6.8 explore the prob- lem space: On one extreme, routing on a ring will minimize neighbor states and the forwarding load of each peer, but requires O(N) hops and thus induces large, varying delays. A full mesh, on the other extreme, places the burden of N - 1 neighbor states to be fed in replicated forwarding, but guarantees a rigid three-hop forwarding limit and minimal delays. A polygonal mesh of dimension d keeps replication load constant (but dependent on d), while its corresponding path lengths grow as O(N). Forwarding on a polygonal mesh will require routing intelligence, which is neither needed on a ring or in a full mesh topology. As routing paths in conferencing scenarios are equivalent to the signaling relationships, mesh robustness or scheme redundancy is equivalent to the number of neighbor states at each peer. Alternative routing topologies may be deployed on the basis of DHTs, admitting logarithmic scaling in all measures on the price of a higher algorithmic complexity. Refocussing on the problem of moderately sized peer-to-peer conferences of simple and robust nature, a favorable routing scheme is easily identified. The full mesh topology outperfor