SDP. Session description and CODECs negotiation video channel, a shared virtual white board, ... 200 OK message. Thus after one roundtrip, common pairs are chosen. For this purpose, SIP uses the Session Description Protocol [17] (SDP). SDP is a textual protocol designed to describe multimedia sessions. SDP provides information about the type of media (audio, video, ...), the CODEC (MPEG, PCM µ-law, ...), the transport protocol for media stream (RTP, ...), and port numbers. It consists of several fields, among which are: • Version field (v:) carries the SDP protocol version • Origin field (o:) carries information on the user proposing the session (name, network address, ...) • Session Name field (s:) gives a title for the session • E-mail address field (e:) may provide the e-mail address of the caller • Phone Number field (p:) may provide the phone number of the caller • Time field (t:) describes when the session description is valid • Connection Data field (c:) gives information about the network con- nection to establish for the session • Media Announcements field (m:) describes the type of media used (audio/video, CODEC, ...) • Attribute field (a:) gives additional properties for the session or one of the media streams, such as the aspect ratio. A typical SDP description for a call established with SIP may look as follows: v=0 o=3344 3344 IN IP4 130.237.251.200 s=Minisip Session c=IN IP4 130.237.251.200 t=0 0 m=audio 1061 RTP/AVP 0 a=rtpmap:0 PCMU/8000/1 m=video 1062 RTP/AVP 31 a=rtpmap:31 H261/90000 Each media stream is given an Media Announcement field which assigns it one or more CODECs and a network port. When describing his own capabilities, the responder puts a 0 in the network port of the streams he does not want to establish. In our case, the responder does not want to establish a video stream, so his description could be: v=0 o=3344 3344 IN IP4 130.237.251.200 s=Minisip Session c=IN IP4 130.237.251.200 t=0 0 m=audio 1061 RTP/AVP 0 a=rtpmap:0 PCMU/8000/1 m=video 0 RTP/AVP 31
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