ng Cascade mixers Distributed conference server Note: P = participant; F = focus; M = media mixer. In the last model, PF = primary focus. Dotted lines = signaling; dashed lines = media transmission; solid lines : mixer control. Modification: A participant or focus can modify a session in a confer- ence using a re-INVITE. For instance, when an audio conference extends to video, the focus re-INVITES each participant adding a video media stream. A participant or focus may also put media streams on hold or take them off hold. Narrowcasting commands are applied to a session by selectively enabling the media streams. Termination: A privileged participant (typically a moderator or confer- ence creator) closes a session by sending a BYE request to the focus. The focus then distributes a BYE request to all other participants in the conference, terminating the session. 14.2.3 Conference Privacy Privacy has two interpretations. The first association, with sources, is that of avoiding "leaks," protecting secrets. But a second interpretation, with sinks, means freedom from disturbance; in the sense of solitude, not being both- ered by irrelevance or interruption, as suggested by Figure 14.3. Our dis- tributed interface features narrowcasting operations that manage privacy in both senses, by filtering duplex media flow through an articulated conferenc- ing model that limits and focuses information streams. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security PRIVATE PRIVATE S.GROSS FIGURE 14.3 Privacy: Freedom from disturbance. (© The New Yorker Collection 1996 Sam Gross from cartoonbank.com. All rights reserved.) 14.3 Media Privacy: Narrowcasting Concept In traditional conferencing systems, participants have little or no privacy, as their voices are by default shared with all others in a session. Such systems can- not offer participants options for muting and deafening other members. The concept of narrowcasting can be applied to make these kinds of filters available in multimedia conferenc