 meeting organized through personal communication or e-mail like in appointed conference calls, or well-known community infor- mation in contexts like gaming. These application scenarios all have in com- mon not only large scaling requirements, but also the need for explicit policy management. In cases where feedback is foreseen and distribution does not remain unidirectional, floor control mechanisms must guide party interactions. Media distribution will scale up to millions of users, if IP multicast is used (see Section 6.2.3). For smaller numbers, powerful multipoint control units (MC'Us) may satisfy the demands, or multicast implementations on the appli- cation layer will enable an optimized stream replication without infrastructure assistance. Distribution-oriented large-scale conferences do not necessarily require SIP. Charlie, for example, can subscribe to an open media channel just by join- ing a multicast group. He may use SIP to inquire on multicast addresses, for authentication and authorization and for enabling feedback control. Con- ference control thereby may be fully decoupled from media distribution and reside on a separate entity as displayed in Figure 6.2. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security INVITE INVITE Contact Contact : sips:dog-matter@ sips :dog-matter@ [SDP] [SDP] Conference controller OK[SDP] OK[SDP] Refer-to snoopy Charlie Snoopy Media source FIGURE 6.2 A multi-party distribution session. Beyond minimal management, SIP supplies a number of optional features, which facilitate interesting interactive enrichments of a basic, reception- oriented application. Charlie could want Snoopy to participate in an ongoing conference. By subscribing to the SIP event package for conferencing states, see Section 6.2.2, he could inquire on Snoopy and discover his absence. To invite Snoopy into the conference, either for raising his awareness or for seam- lessly integrating his conference-unaware user agent client, he could use the REFER method as describ