ription begins with the session-level information and the media-level information, if any is present, comes after. The session- level section always starts off with v=0, where U is the type and 0 is the value. This line indicates that the protocol version is zero (SDP version 0). Chapter 3 Ensuing lines, up until the first media-level section or the end of the session description as the case may be, provide information about the whole ses- sion. Media-level sections begin with an m line. The lines below it, until the next m line occurs or until the end of the session description, provide infor- mation about that particular media stream. The following is an example of an SDP session description: o=Bob 2890844526 2890842807 IN IP4 131.160.1.112 s=SIP seminar i=A Seminar on the Session Initiation Protocol =http://www.cs.columbia.edu/sip e=bob@university.edu C=IN IP4 224.2.17.12/127 t=2873397496 2873404696 a=recvonly m=audio 49170 RTP/AVP 0 a=rtpmap: PCMU/8000 m=video 51372 RTP/AVP 31 a=rtpmap 31 H261/90000 m=video 53000 RTP/AVP 32 a=rtpmap: 32 MPV/90000 In this example, the session-level section consists of the first nine lines, from v=0 to a=recvonly, and has three media-level sections: one audio stream and two video streams. The o line indicates the creator of the session (in this case, Bob) and the IP address of his site. The S line contains the name of the session and the i line contains general information about the session. The U line provides a Uniform Resource Locator (URL) where more information about the topic of the session can be retrieved. The e line con- tains the e-mail address of the contact person for this session. The multicast address where the session can be received is described in the C line and the t line indicates when the session is active. The last line of the session-level section, the a line, indicates that this is not an interactive session; it's The Internet Multimedia Conferencing Architecture receive only. The format of the m lines is especially important. An m line begins with the me