eful for participants to be informed of who is control- ling media using a narrowcasting command (mute, deafen, select, attend). For example, teachers may want to be notified whether any stu- dents have muted them during a lecture. Depending on the conditions, such role-based transparency could be appropriate or not. A parent might insist upon the ability to override a teenager's 'ignore' filter: "How dare you mute me?" In the current narrowcasting implementation, a notification mechanism has not been incorporated. This section proposes such an event notification mechanism. The SIP event notification framework (RFC 3265) [22] defines general mech- anisms for subscribing to, and receiving notifications of, events within SIP networks. The framework uses standard SIP methods PUBLISH, SUBSCRIBE, and NOTIFY to deliver event-related information. Figure 14.12 shows an event information exchange flow. In order to allow conference participants to be notified of narrowcasting control status, an event template package can be introduced. A specialized template event package, which might be called "NARROWCAST" or "NC," needs to be standardized within the IETF community. Such a template event package could be used in combination with the "CONFERENCE" event package (defined in RFC 4575 [23]) to inherit the state of a conference. As shown in Figure 14.13, the publisher (a narrowcasting conference participant), the notifier (presence server), and the subscriber (another narrowcasting conference participant) must support "CONFERENCE.NO," a combination of the CONFERENCE event and NARROWCAST template event packages. In this example, P2 subscribes to P1 and mentions CONFERENCE. in the "Event" header in the subscription body. As a result, P2 will be notified of the narrowcasting control informa- tion of P. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Publisher Notifier Subscriber Participant P1 Presence server Participant P2 PUBLISH Status: Joined SUBSCRIBE, P1 Event: conference.nc NOTIFY St