about parties. Depending on present state, participants may choose to com- municate by short messages, e-mail, voice and video, interact within dedicated collaborative environments or not at all. Presence information not only resolves the physical location of a person or an application, but may indicate the ability and willingness of a user to communicate. Rich presence information conveys the personal and contextual Even though this is merely a reformulation of the layered design principle, it is worth noting that at this occasion we benefit from a long-term resistance to layer violations. C 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP circumstances of a contact, and a caller a priori may judge whether to pursue a contact while the callee is busy at the airport and in a bad mood. This may be particularly useful in larger or extensive conferences as an unpretentious way of feedback from silent parties. Presence indicators follow the SIP event state model, which is outlined in Section 6.2.2. They enrich a community model of "buddies," whose pres- ence is concurrently cultivated and silently enriched by instant message (IM) exchange. SIP enables instant messaging in two ways: Short textual news may be submitted in pager mode via the MESSAGE method [15] aside from session establishment. For more extensive IM exchange, the session mode negotiates media exchange via SDP, where transport in IM sessions is facilitated by the Message Session Relay Protocol (MSRP) [16]. Messages can be exchanged individually or in conferences, realized as client-server or peer-to-peer com- munication. In particular, IM may be integrated in a voice conferencing infras- tructure as a complementary communication format or an orthogonal channel. The latter allows a phoning party to exchange messages with a third person within the same user agent. IM is now part of a large number of tremendously popular personal appli- cations. The technical agreement by AOL, IBM, and Microsoft on presence and IM interoperability, S