 unrestricted scaling in shared end system domains, and experience severe performance degradation when terminal mobility is introduced [41]. These drawbacks may be mitigated by hybrid approaches, where overlay multicast routing only takes place among selected nodes that are particularly stable and form a virtual infrastructure. The servers of a SIP infrastructure are apt candidates for this, as will be outlined in the following sample architecture. 6.3.1 A Hybrid Architecture for Transparent Group Communication This section introduces a hybrid architecture for SIP proxy-assisted group conferencing, designed to enable global multicast peering at the ISP or enter- prise level, and at the same time, sustain end system transparency. The basic concept preserves multicast routing and lower layer packet transmission within domains, while bridging the inter-domain gap with the help of a structured overlay network to overcome deployment problems. Its focus originates from a customer network or an ISP domain, where multicast services are locally deployed. Multicast service exchange is then implemented like unicast peer- ing by a gateway function, resident on a SIP proxy. The primary call routing function of a SIP server is thereby augmented with a multicast overlay for con- ferencing and media distribution. It interconnects the local multicast routing with the distributed peering on the structured overlay. C 2009 by Taylor & Francis Group, LLC DHT overlay abde004 141.4.50.3 acf43de 21.7.2.4 32.4.3.1 cadb341 154.39.8.8 abcf432 abde004 141.4.50.3 cadb341 154.39.8.8 abcf432 32.4.3.1 abde004 141.4.50.3 acf43de 21.7.2.4 Multicast domain Internet backbone Multicast domain Multicast domain FIGURE 6.7 A hybrid group communication architecture with SIP inter-domain multicast gateways. © 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP The new function, Inter-domain Multicast Gateway (IMG), acts as a gate- way between the overlay it is a member of, and the multicast routing at the intra-domain underlay that i