II video, and SO forth). In addition, SDP contains information about the following: Subject of the session; Start and stop times; Contact information about the session. Like SIP, SDP uses text coding. An SDP message is composed of a series of lines, called fields, whose names are abbreviated by a single lower-case letter, and are in a required order to simplify parsing. The set of mandatory SDP fields is shown in Table 2.1. The complete set is shown in Table 7.1. SDP was not designed to be easily extensible, and parsing rules are strict. The only way to extend or add new capabilities to SDP is to define a new attrib- ute type. However, unknown attribute types can be silently ignored. A SDP parser must not ignore an unknown field, a missing mandatory field, or an out- of-sequence line. An example SDP message containing many of the optional fields is shown here: o=johnston 2890844526 2890844526 IN IP4 43.32.1.5 s=SIP Tutorial i=This broadcast will cover this new IETF protocol u=http://www.digitalari.com/sip e=Alan Johnston alan@mci.com p=+1-314-555-3333 (Daytime Only) C=IN IP4 225.45.3.56/236 b=CT: 144 t=2877631875 2879633673 m=audio 49172 RTP/AVP 0 a=rtpmap 0 PCMU/8000 m=video 23422 RTP/AVP 31 a=rtpmap 31 H261/90000 The general form of a SDP message is: x=parameter1 parameter2 parameterN Related Protocols Table 7.1 SDP Field List in Their Required Order Mandatory/ Field Name Optional Protocol version number Owner/creator and session identifier Session name Session information Uniform Resource Identifer Email address Phone number Connection information Bandwidth information Time session starts and stops Repeat times Time zone corrections Encryption key Attribute lines Media information Media attributes The line begins with a single lower-case letter X. There are never any spaces between the letter and the =, and there is exactly one space between each parameter. Each field has a defined number of parameters. Each line ends with a CRLF. The individual fields will now be discussed in detail. 7.1.1 Protocol Version Th