sions, including multicast and point-to-point sessions. SIP, as we know it today, was not designed from scratch, but resulted from the merger of two IETF protocols proposed for the same purpose. SIP picked up the best features from each protocol and from then on, all efforts within the community converged on it. The Session Initiation Protocol: SIP Figure 4-1 SIP enables us to invite users to This is a funny movie! bet Laura sessions. would love to watch it... How could I tell her about it? The Internet Session Invitation Protocol: SIPv1 Although the first voice transmissions over packet-switched networks took place around 1974, the first multimedia conference systems appeared in the early 1990s. Thierry Turletti developed the INRIA Videoconferencing Sys- tem (IVS), a system for audio and video transmissions over the Internet. An IVS user could call another user and they could establish a unicast session. IVS could also be used in multicast sessions. Capitalizing on H.261 video coding over the Internet, the work on IVS was instrumental in developing the Real-time Transport Protocol (RTP) payload format for H.261 video streams [RFC 2032]. Soon thereafter, Eve Schooler developed the Multimedia Conference Con- trol (MMCC). MMCC software provided point-to-point and multipoint tele- conferences, with audio, video, and whiteboard tools. To connect various users, MMCC used the Connection Control Protocol (CCP), a transaction-oriented protocol. A typical transaction consists of one request (from the user) and one response (from the remote user). For trans- port, CCP used User Datagram Protocol (UDP) SO it implemented time-outs and retransmissions to ensure the reliable delivery of protocol messages. These two first multimedia systems gave way to the design of the Session Invitation Protocol created by Mark Handley and Eve Schooler. The first version of SIP, SIPv1, was submitted to the IETF as an Internet draft on February 22, 1996. SIPv1 used Session Description Protocol (SDP) to describe sessions and UDP as a transport.