of each other. Session Initialization Protocol (SIP) [1] as presented by the Internet Engineering Task Force (IETF) forms a flexible, comprehensive signaling solution and allows for a lightweight deployment of many, but not all of the well-known service features. Since media sessions are only negotiated by, but not tight to SIP control streams, calls may be arranged in a variable, adaptive fashion. This degree of freedom is of particu- lar importance in large conferences, where media transmission and processing can easily rise beyond the capacities of single devices. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security The original development of SIP was inspired by connection-oriented tele- phone services, whence its nature derives from a point-to-point model. It is designed as a multi-layered application protocol that interacts between com- ponents in a transactional way. Each (asynchronous) request initiates an open transaction state and requires completion by at least one response. Group communication complicates this process significantly. A newly joining mem- ber faces an entire group, which requires appropriate addressing and trans- actional state management. Negotiations on media parameters grow complex as common parameter intersections may have to be evaluated for many mem- bers. Extensions to perform scalable group session management are not easy to achieve, while schemes refrain from central control. In multimedia conference scenarios, each member commonly operates as receiver and as sender on a group communication layer. In addition, real-time communication such as voice or video over Internet Protocol (IP) places severe Quality of Service (QoS) requirements: Seamless distribution services need to limit disruptions or delays to less than 100 ms. Jitter disturbances should not exceed 50 ms. Note that 100 ms is about the duration of a spoken syllable in real-time audio. As an extensible protocol, SIP is open for the creation of new methods, header fields and protocol 