l. SIP gained early support from Level 3 Communications, MCI WorldCom, and the cable TV industry for carrying IP services and interacting with the PSTN. Comcast, Cox Communications, MediaOne, Rogers Cablesystems Limited, Tele- Communications Inc. (now AT&T), Time Warner, and Le Groupe Vidéotron began sponsoring SIP/SDP interoperability efforts in 1999 in conjunction with CableLabs. CableLabs is a research and development consortium of cable television system operators that represents nearly 90% of the cable subscribers in the United States, 80% of cable subscribers in Canada, and 15% of cable subscribers in Mexico. The family is comprised of the following protocols: Session Initiation Protocol (SIP)-For initiating sessions and inviting users. Session Description Protocol (SDP) and Session Announcement Protocol (SAP)-For distributing session descriptions. SAP Security-For providing security for session announcements. Real-Time Stream Protocol (RTSP)-For controlling on-demand delivery of real-time data. Simple Conference Control Protocol (SCCP)-For managing tightly-controlled sessions. Standards for Videoconferencing Session Initiation Protocol (SIP) The Session Initiation Protocol (SIP) is defined in IETF RFP 2543 and was ratified in February 1999. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. It is targeted at Internet multimedia conferences, Internet telephone calls, and multimedia broadcasts. H.323 was designed to extend H.320 to packet-switched local area networks. It depends upon H.245 for control, H.225 for connection establishment, and H.332 for large conferences. Furthermore, it leverages H.450.1-3 for supplementary services, H.235 for security, and H.245 for interoperability with circuit-switched services. The interrelations between these protocols can be quite complex. Call Forward, for instance, relies on components of H.450, H.225.0, and H.245. Consequently, H.323v1 call establishment may require as many as twelve packets a