icast peering realized by multi- homed network connectivity. Replication operations will be seamlessly empowered by the self-organization capabilities of the DHT overlay. Active coordination between gateway peers may be achieved by SIP or non-SIP means. Peer-Managed Conferences An increasing number of applications aim for simple, flexible, and cost-efficient ad-hoc conferencing functions, which scale appropriately, but avoid any infras- tructure assistance. Such solutions require group session management and media distribution at peers. Commonly implemented as pure software on stan- dard personal devices, user agent peers are exposed to severe restrictions in real-world deployments: Often they are located behind NATs and firewalls © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security with network capacities confined to asymmetric DSL or wireless links. Mul- ticast routing may be unsupported or only available in parts of the network facilities spanned by the conference. Clients may run on handhelds or other truly mobile devices that admit processing resources too scarce to serve for mixing and redistribution purposes. Peers may join or leave a conference in an unpredictable manner, advising other members not to rely on its relaying ser- vice. Nevertheless, real-time constraints apply to data processing and packet forwarding, whenever voice, video, or interactive elements are the purpose of the conference. Capacity constraints and resilience to node failures require peer-managed ad-hoc conferences to organize in a distributed multi-party model. As a key component, the heterogenity of clients must be accounted for. Ranges of scal- ability, however, may vary. From an application point of view, many unmod- erated systems are designed for only about one dozen participants, which in particular holds for dialog-oriented video conferences. Other applications, equally built of lightweight peers, may foresee media streams to reach large numbers of receivers. Two examples of SIP initiation,