IP multi- cast in SIP is defined by the maddr parameter in the via header field. All SIP users of the community dog. net listening to the multicast address 239.12.11.10 could thus be contacted using the URI: sip: *@dog net;maddr=239.12.11.10;ttl=3 Syntax specification of RFC 3261 suffers from the nit of allowing only IPv4 address arguments with the ttl field serving as scoping limiter, while the semantic is bound to ASM use. 6.2.3.1 Any-Source Multicast Deering's early host group model [27], which enables single packet submission to an unrestricted group of delocalized receivers, is the most general form of group communication. It is also known as any-source multicast (ASM), since any sender is entitled to issue data to a group of potentially unknown receivers. Receivers simply join to a group address, more specifically a (*, G) channel, and the multicast routing will provide a transparent delivery service. ASM is fundamental to rendezvous processes and service discovery-SIP takes advan- tage for registrar access, accordingly. A user agent client may send register messages to the well-known "ALL SIP Servers" URI sip:sip.mcast.net, mapped to the IPv4 address 224.0.1.75, without being aware of any indi- vidual registrar. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security INVITE[SDP] Media streams OK[SDP] FIGURE 6.5 SIP call initiation based on ASM - a callee negotiates with a previously defined multicast group. For multicast conference management, SIP defines only a limited "discovery- like" service, delivering a request to a group of homogeneous servers. A client wishing to initiate or join into a multi-party conference sends its INVITE request to a multicast group by employing the maddr attribute in the SIP VIA header. Group members subsequently indicate their presence by responding to the same group (see Figure 6.5). The transactional nature of SIP dialogs is preserved in the sense that the inviting party interprets the first arriv- ing OK as the regular completion, while ad