ssage Bodies," RFC 2045, 1996. Related Protocols The Session Initiation Protocol (SIP) is one part of the protocol suite that makes up the Internet Multimedia Conferencing architecture, as shown in Figure 1.1. In this chapter, other related Internet protocols mentioned or referenced in other sections are introduced, along with details on the use of the protocol with SIP. This is by no means a complete discussion of multimedia communication protocols over the Internet. First, SDP, the media description language, will be discussed. Then the RTP and RTCP media transport protocols will be dis- cussed. The application of RTP/AVP profiles that link SDP and RTP will then be then covered. The chapter concludes with a brief discussion of signaling pro- tocols in the PSTN. The H.323 protocol will be discussed and compared to SIP in the next chapter. The chapter concludes with a discussion of SIP-T and UPnP protocols. 7.1 SDP-Session Description Protocol The Session Description Protocol, defined by RFC 2327 [1], was developed by the IETF MMUSIC working group. It is more of a description syntax than a protocol in that it does not provide a full-range media negotiation capability. The original purpose of SDP was to describe multicast sessions set up over the Internet's multicast backbone, the MBONE. The first application of SDP was by the experimental Session Announcement Protocol (SAP) [2] used to post and retrieve announcements of MBONE sessions. SAP messages carry a SDP mes- sage body, and was the template for SIP's use of SDP. Even though it was SIP: Understanding the Session Initiation Protocol designed for multicast, SDP has been applied to the more general problem of describing general multimedia sessions established using SIP. As seen in the examples of Chapter 3, SDP contains the following infor- mation about the media session: IP Address (IPv4 address or host name); Port number (used by UDP or TCP for transport); Media type (audio, video, interactive whiteboard, and SO forth); Media encoding scheme (PCM A-Law, MPEG 