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

Title: Re: [AVT] [Fecframe] Question from FECFRAME about RTP Payload Format Parameters
Roni, Colin,

I guess I should clarify that the FECFRAME SDP attributes will be defined in any case because they are needed for flows which are signaled in SDP but which do not use RTP.

In the case that RTP is used, then, we have the choice of using the FECFRAME attributes or using RTP Payload Format parameters.

To be honest, my feeling is that it is not very clean either way: either we break the RTP convention (or rule?) that all the required information to interpret the payload is available in the RTP Payload Format Parameters or we have duplicate ways to signal the same information.

Colin’s mail suggests we could define an RTP Payload Format parameter to carry the scheme-specific information in the same way that the FECFRAME SDP attribute does (as a base64-encoded octet string) and then all FECFRAME RTP Payload Formats could use the same parameter. But I wonder whether the advantage of this is marginal compared to the advantage of the parameters being clearly visible in the SDP.

Regards,

Mark

On 12/9/08 12:03 PM, "Roni Even" <ron.even.tlv at gmail.com> wrote:

Hi,
Just to emphasis Colin email.
If raptorfec is a the payload format you want to standardize than it can have parameters specified for this payload type and conveyed in fmtp line.
 
The first example is using the generic SDP parameters approach and you will need to specify the semantics for session and media level usage of the parameters. I noticed that it is different from draft-begen-fecframe-sdp-elements-00. This solution will need to register the parameter and I assume they will relate to a registered scheme id as specified in the begen draft.
 
Both will work and it depends what approach you are using.
 
Roni Even

 

From: avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of Colin Perkins
Sent: Tuesday, December 09, 2008 8:18 PM
To: Mark Watson
Cc: avt at ietf.org; fecframe at ietf.org
Subject: Re: [AVT] [Fecframe] Question from FECFRAME about RTP Payload Format Parameters
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

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