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Mark,
RTP payload formats are named using media subtypes, which have associated media subtype parameters. Those parameters are conveyed in SDP using the "a=fmtp:" line. If you're designing an RTP payload format, it should use that standard mechanism for naming the format and conveying its parameters. The FEC-scheme-specific information would be one of those parameters, I assume, carried in the "a=fmtp:" line.
Colin
On 17 Nov 2008, at 18:21, Mark Watson wrote: All,
The following question was discussed at FECFRAME and it was suggested to take this to AVT as well.
FECFRAME has defined an SDP attribute to carry information about FEC repair flows which is specific to the particular FEC Scheme being used. This attribute contains an ‘opaque’ container which contains FEC-Scheme-specific information that is supposed to be handled by the FEC scheme. This is a generic mechanism which could be applied whether the FEC repair data is carried over RTP or directly over UDP.
In the case that RTP is used for repair data, then RTP Payload Format must be defined for the FEC Scheme in the usual way. In this context we have the possibility to define RTP Payload Format parameters which would then be carried in the a=fmtp SDP parameter.
The question under discussion is whether we should define RTP Payload Format parameters at all, or whether we can use the FEC Framework SDP attribute in the RTP context just as it is used for repair data carried over UDP.
The advantage of defining RTP Payload Format Parameters is that this is the standard and recognised way of communicating parameters which are needed to process data carried in an RTP Payload.
The disadvantage of defining RTP Payload Format Parameters is that we essentially end up with two mechanisms for carrying the same information and this causes additional implementation work and potential for confusion about which mechanism should be used when.
An example of SDP using the FECFRAME attribute would be:
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
t=0 0
a=group:FEC S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 raptorfec/90000
a=fec-repair-flow: encoding-id=0; fssi=4W5S6X
a=repair-window: 200 ms
a=mid:R1
And an example using RTP Payload Format Parameters would be:
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
t=0 0
a=group:FEC S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 raptorfec/90000
a=fmtp:110 sbl=1231; symbol-size=96; repair-window=200ms
a=mid:R1
FECFRAME would like to hear opinions from AVT on this question.
Regards,
Mark Watson_______________________________________________ Fecframe mailing list |