dia type. In the previous example, the media types are audio for the first media stream and video for the second and the third. m=<media type> <port number> <transport protocol> <media formats> The port number indicates where the media can be received. The trans- port protocol field usually takes the value of RTP/AVP, but can take another value if a protocol other than RTP is used. RTP/AVP refers to the audio/video profile for RTP; in this example, encoded audio and video are transported using RTP over UDP. The media format depends on the type of media transported. For audio, it's the codec being used. In this example, a value of zero means that the audio is encoded in a single channel using PCM u-law and sampled at 8 The a=rtpmap lines convey information, such as the clock rate or num- ber of channels, about the media formats used. In the second media stream of this example, the media format number 31 is referred to as H.261 and it uses a clock rate of 90 KHz. Table 3-1 contains all the types defined by SDP and their meaning. Extending SDP The media attribute lines, the a lines, provide a means to extend SDP. When an application needs a feature missing in SDP, it can add an a line containing it. For example, if the creator of a multicast ses- sion wanted the receivers to play the audio at a certain volume, he or she could define a new media attribute and add it at the end of the media-level section. m=audio 49170 RTP/AVP 0 a=volume : 8 Applications that understand this new a line will play the audio at vol- ume 8. When an application finds an a line that it doesn't understand, it simply ignores the line and proceeds as if no line had been encountered. The application that failed to understand our new a=volume line could still receive media properly, although it would not be able to play back at the proper volume. Chapter 3 Protocol version Table 3-1 Bandwidth information SDP Types Owner of the session and session identifier Time zone adjustments Name of the session Encryption key Information about the session At