Lucy's media capabilities and the con- ference media distribution policy included [7], Charlie analogously re-invites Lucy into the conference (see Section 6.2.2): INVITE ips:lucy@psychic.org SIP/2.0 Via: SIP/2.0/TLS alpha.brown.com: 5061; branch=z9hG4bknashds Max-Forwards: 70 From: Charlie <sips:charlie@brown.com>;tag=23431 To: Lucy <sips:lucy@psychic.org>;tag=1234567 © 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP Call-ID: 888888@alpha.brown.com CSeq: 1024 INVITE Contact: <sips:dog-matter@alpha.brown.com>;isfocus Content-Type: application/sdp Note that Lucy can explicitly react upon the group context and-if desired- decline the re-invitation. If Snoopy wishes to contact Charlie instead, he may be aware or unaware of an already ongoing conference. In the latter case, he will just call Charlie who will reply with his conference contact information or re-invite Snoopy after their two-party dialog has been established. In the first case, when Snoopy knows about the conference including the identification of one ongoing call, he can explicitly express his will to participate using the join header field (see Section 6.2.2). This simple, end-point hosted conferencing scheme can be extended to an arbitrary number of participants, only limited by scalability issues. Two exten- sions beyond the base specification have been in use, the "isfocus" tag and the join header. Any conference-unaware user agent implementing only RFC 3261 cannot act as a focus or request to join an ongoing conference, but may, nevertheless, participate in a regular client role. The presence of the 'isfocus' tag in a contact header field does not cause interoperability issues, since it will be simply ignored as an unknown header parameter. 6.1.1.2 Large-Scale Conference Large conferencing instances usually go along with some external occasion or scheduling. This may be a program announcement via a Web page, SAP [8], or printed guides like in Internet Protocol Television (IPTV) offers or a streamed real-world conference, a