4.4.4.4 Client Requests FEC Stream from Server
From the SDP description sequence, the client can select audio or video streams, and for each of these it can select associated FEC streams.
The following example illustrates a request/response exchange in which the client requests FEC using a packet span of 24 and 4 FEC packets per span (FecSpan=4). The third parameter, the FecBurstMargin field, is used to buffer a set of packet spans and associated FEC packets in the form of a Vandermonde matrix (section 2.2.2.2), which is a calculation commonly applied to error correction problems.
The example also shows another FEC parameter, known as the FEC burst margin (FecBurstMargin=6), used to recover from burst losses.
Client to server:
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SETUP rtsp://myserver.com/mycontent.wmv/stream=5/fec98 RTSP/1.0 Transport: RTP/AVP/UDP;unicast;client_port=2408;ssrc=6dded651;mode=PLAY;FecSpan=4; FecPerSpan=1;FecBurstMargin=6, RTP/AVP/TCP;unicast;interleaved=0-1; ssrc=6dded651;mode=PLAY If-Match: "{83A04BD0-FD30-1984-4994-0A22CA116ED3}" Date: Fri, 23 Mar 2001 04:28:14 GMT CSeq: 8 Session: 1077055954 User-Agent: WMPlayer/9.0.0.197 guid/CB131790-CC16-4CCE-A234-6D29BEE21FCE Accept-Language: en-us, *;q=0.1 Accept-Charset: UTF-8, *;q=0.1 X-Accept-Authentication: NTLM, Digest
Server to client:
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RTSP/1.0 200 OK Transport: RTP/AVP/UDP;unicast;source=157.56.216.159;server_port=2410;client_port=2408; ssrc=3874dd27;mode=PLAY;FecSpan=4;FecPerSpan=1;FecBurstMargin=6 Date: Fri, 23 Mar 2001 04:28:14 GMT CSeq: 8 Timestamp: 1 0.031 Session:1077055954;timeout=60 Server: WMServer/ 9.0.0.197 Cache-Control: must-revalidate, proxy-revalidate