everyone in a conference except Alice (P2), then Alice will not receive any streams from Bob if Bob deafens Alice. (Transposing the participants suggests an equivalent operation via reciprocity of sources and sinks, P2 mutes P1.) The second column of Table 14.3 shows the media relationship among C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security four participants when deafen is invoked. transitive -1 P - Pj-Pa when P=Po, = reflexive (14.2) when Pa = Po, otherwise. Again in this example, n = 4, a = 1, and 0=2. 14.3.3 Select (Solo) The privacy command select limits received media to particular sources. For instance, students might select a teacher to avoid distractions. P1 will receive media only from P2 if P1 selects P2, implicitly muting the com- plement of the selection. The third column of Table 14.3 shows the media relationships among four participants when select is invoked; two vectors are disabled in this case. not reflexive when Pi=Pa and Pa#Po, , (14.3) otherwise. 14.3.4 Attend Attend is the other including command for narrowcasting, limiting received media to a particular recipient. If Alice attends Bob, only Bob will hear Alice, since other participants are implicitly deafened. The rightmost column of Table 14.3 shows the media relationship among four participants when attend is invoked; again two media vectors are suppressed. not reflexive when P=P, and Pa#Po, , (14.4) otherwise. 14.4 System Design and Implementation The main required functions for media control are policy configuration, policy evaluation, media mixing, and media distribution. The Media Server Compo- nent Model (top right of Table 14.2) selected for our implementation comprises a centralized focus (collocated with the policy server), a centralized mixer, and © 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control Application (policy) server Media server client MSCML / SIP Mixer TCP sockets Legend: User control (signaling) TCP sockets server Media streams Mixer contro