 conference management tasks. Note that this model allows for cascading conferences, expressed by the sidebar conference elements. An overview of the conference state information is given in Table 6.1. Following the general SIP event notification model, a user agent client will receive conference event state changes from the state agent via the NOTIFY method after having issued a SUBSCRIBE with reference to the package name "conference." The corresponding call flow is displayed in Figure 6.4. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security TABLE 6.1 The Conference State Event Groups Conference-info Identifies this event set by a conference URI and a version. Conference-description Describes the content of the conference by textual meta-data like subject, keywords, additional (service) URIs and available media. Host-info Contains information about the host of the conference. Conference-state Expresses the conference state (active, locked) and current user count. Users This large container block enumerates individual users. A user record consists of a display name, user URI, role and a comprehensive characterization of the employed devices. sidebars-by-ref A reference pointer to a sidebar conference, given by its conference URI. sidebars-by-val Full representation of a sidebar conference, optionally including all conference state events. Charlie Conference focus Snoopy Subscribe (conference) NOTIFY (full) INVITE NOTIFY (user snoopy) FIGURE 6.4 Call flow for SIP conference event state subscription and notification. Event notification is performed incrementally on changes except for the initial NOTIFY response to subscription. Due to the expected amount of data in large conferences, a user agent client can receive partial notification about those compound, volatile elements assembled in container groups. 6.2.3 SIP with Multicast Conferencing requires some basic group communication mechanisms for dis- tributing media data to all parties and for mutual signaling, if applicable. 