l FIGURE 14.6 Media Server Component Model with collaborative virtual environment integration. distributed participants. The architecture, elaborated in Figure 14.6, embeds policy configuration, media mixing, and a collaborative virtual environment (CVE) interface within a SIP framework. All the components in this architec- ture are standard SIP UAS extended with additional user interfaces needed for media policy configuration and control. Communication protocols XCAP (Extensible Markup Language Configuration Access Protocol) and MSCML (Media Server Control Markup Language) [16] are IETF standards, considered in the following descriptions. 14.4.1 Policy Configuration In an extended SIP framework, conference participants could configure privacy by sending requests to the policy server using XCAP [15], a standardized way to use HTTP to store, retrieve, and manipulate configuration and application data in XML format. In our proof-of-concept, participants set policies using GUIS to invoke narrowcasting commands on specified controllees, and control is distributed via TCP sockets or HTTP directly (without XCAP). 14.4.2 Policy Evaluation An application server performs three major functions to evaluate policy: Evaluating policies configured by each participant: The policy from each participant can be logically compiled into a matrix, as shown in Table 14.4, where entry Cij of the matrix represents connectivity of source i to sink j, and the main diagonal is populated by "don't care"s. Each participant (P1, P2, , Pn), where n is the total number of participants, logically sets permissions in authorized cells. Since a media relationship ultimately factors at least two participants, a source and a sink, each cell contains policies from both. For example, P1 P2, i.e., media sourced at P1 and sunk at P2, has C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security TABLE 14.4 Policy Matrix P = =[Pij] P2(P2 policy involvement of both P1 and P2: P1 sets permissions about whether or not to send me