l m= field contains information about the type of media session. The field contains: m=media port transport format-list The media parameter is either audio, video, application, data, telephone-event, or control. The port parameter contains the port number. The transport parameter contains the transport protocol, which is either RTP/AVP or udp. (RTP/AVP stands for Real-time Transport Protocol [4] / audio video profiles [5], which is described in Section 7.3.) The format-list contains more information about the media. Usually, it con- tains media payload types defined in RTP audio video profiles. More than one media payload type can be listed, allowing multiple alternative codecs for the media session. For example, the following media field lists three codecs: m=audio 49430 RTP/AVP 0 6 8 99 One of these three codecs can be used for the audio media session. If the intention is to establish three audio channels, three separate media fields would be used. For non-RTP media, Internet media types should be listed in the format-list. For example, m=application 52341 udp wb could be used to specify the application/wb media type. 7.1.11 Attributes The optional a= field contains attributes of the preceding media session. This field can be used to extend SDP to provide more information about the media. If not fully understood by a SDP user, the attribute field can be ignored. There can be one or more attribute fields for each media payload type listed in the media field. For the RTP/AVP example in Section 7.1.10, the following three attribute fields could follow the media field: a=rtpmap : 0 PCMU/8000 a=rtpmap : 6 DVI4/16000 a=rtpmap : 8 PCMA/8000 a=rtpmap : 99 iLBC Related Protocols Other attributes are shown in Table 7.2. Full details of the use of these attributes are in the standard document [1]. The details of the iLBC (Internet Low Bit Rate) Codec are in [6]. 7.1.12 Use of SDP in SIP The use of SDP with SIP is given in the SDP Offer Answer RFC 3264 [7]. The default message body type in SIP is application/sdp. The calli