tion in the distribution-oriented second setup. In the following, a closer inspection will reveal more characteristic differences. 6.1.1.1 Three-Way Conference Often a three-way conference is initiated spontaneously from a two-party ses- sion. For example, while Charlie and Lucy are on a call, Charlie decides to add Snoopy into the conversation or Snoopy rings him, being aware or unaware of the already established dialog. In either case, Charlie is in the role to act upon Snoopy joining in and thereby turning the session into a conference. As visualized in Figure 6.1, parties are in individual, point-to-point contacts and naturally manage conference negotiations and policy operations between peers. Switching the communication context from a two-party session to a three-party conference raises two additional duties at Charlie's site. The par- allel calls, one with Lucy and the other with Snoopy, need a logical join to form one conference. Also, media data have to be arranged to arrive at all three participants. The simplest solution for this end-point hosted conference would work with- out additional signaling, when realized within the conferencing application. Based on user reactions to calls, Charlie's end system could sort the two ses- sions into a virtual conference, and at the same time, start to mix media SO that each correspondent receives all the information from Charlie within one stream*. Lucy and Snoopy could thus participate without logical or technical awareness of the multi-party situation. There are, however, drawbacks of this simplistic approach. Obviously, all communication relies on the presence of Charlie-his disappearance will ter- minate both calls. In particular, there will be no seamless mechanism to turn * Media mixing is easily achieved for voice, but does likewise work for video with combined pictures or virtual environments with merged update sets. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security the conference into a call between Lucy and Snoopy. 