dia to P2, and at the same time, P2 sets permissions about whether or not to receive such media. The policy then is evaluated depending on the combined relationship between P1 and P2. Responding to participants regarding changes made in the policy: A policy evaluation report (confirming success or alerting failure of a config- uration request) might be sent to participants via standard XCAP response codes. Sending requests to a media mixer for necessary media mixing: After compiling the media policies, the system determines which media streams need to be mixed and delivered to whom. Using MSCML or some equivalent, the policy server instructs the media mixer to perform the necessary mixing. 14.4.3 Media Mixing and Distribution The media server receives MSCML requests from a policy configuration server. According to the accumulated state, it performs the necessary mixing and delivers these streams to subscribed participants. The maximum number of mixes, the power set of the participants excluding the empty and universal sets, is = 2" - 2. (14.5) Therefore, for n = 3,4,5, the maximum number of mixes would be 6, 14, and 30, respectively. However, depending on participants' media privacy requests, the actual number of mixes might be less. Figure 14.7 illustrates narrowcasting media distribution among four partic- ipants when P1 mutes P2 and deafens P4. All participants send their media C 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control P3+P4 P1+P3+P4 P1+P2+P4 Media from each participant P2+P3 Media submixes FIGURE 14.7 Media mixing and delivery (P1 mutes P2 and deafens P4). to the media server. The media server mixes only the necessary streams and delivers them back to the appropriate recipients. 14.4.4 Sample Mixing Configuration Our prototype environment comprises a SIP server (BEA WebLogic SIP Server), an application server (BEA WebLogic Workshop), a media server (Dialogic/ Cantata Snowshore IP Media Server), and four SIP clients (X-lite). We imple- mented narrowcasting commands