Subject: [AVT] Question from FECFRAME about RTP Payload Format Parameters

From: Mark Watson <[email-masked]>
Date: Mon, 17 Nov 2008 12:21:30 -0600

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 Œopaque¹ 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
_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www.ietf.org/mailman/listinfo/avt