based on secrecy, anonymity, and solitude [4]. Mute is a popular feature for media privacy. It has three different varieties, juxtaposed in Table 14.1: self-mute, PBX-mute, and narrowcasting mute. Self- mute allows a user to withhold his media streams from other participants. In PBX-mute, a moderator disables a participant's outgoing media to other participants. Narrowcasting mute refers to P2P control with which a partic- ipant (controller) can select another participant (controllee) to disallow the controllee's media towards the controller. A Call Whisper [5] feature allows a participant to talk privately to one or more participants in a group. This walkie-talkie-like feature creates a one-way voice or video communication channel. The session terminates when the con- troller releases the push-to-talk (PTT) button, SO such a system is not practical when two-way communication is necessary. Voice Chat [6] allows participants to create one or more private audio conferences: although the communica- tion channel in the private voice chat group provides duplex communication, participants can hear the main conference at low volume. Private Conver- sation [7] offers a private video, voice, and text conversation session inside a main conference. It is similar to a Call Whisper feature, but adds duplex C 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control Supervisor Mixer Agent Customer Agent + Supervisor + customer customer Agent Call center Supervisor Agent Customer (a) Customer support (b) Selective media FIGURE 14.1 Media privacy: A call center application scenario. communication capability and text messaging. In a WebEx audio conference, a conference chairperson can selectively disable microphones to allow only cer- tain attendees to speak. An "audio-only" option allows a moderator to revoke and restore speaking privileges to attendees, SO that muted attendees can only lurk, listen but not speak. WebEx participants can have a private chat with someone during a meeting. Whisper Coachi