Re: [rtcweb] Fwd: I-D Action: draft-alvestrand-one-rtp-00.txt

On 08/11/11 20:47, Paul Kyzivat wrote:
> On 8/11/11 7:32 AM, Harald Alvestrand wrote:
>> I have taken some of the information I learned from the discussions in
>> Quebec City about the issues of putting video and audio into the same
>> RTP session and created a draft from them outlining a solution to the
>> problem of signalling this configuration using SDP.
>>
>> Comments welcome, including the recommended context for processing the
>> document; in a few days I'll send the same note to AVTCORE and/or 
>> MMUSIC.
>
> Harald,
>
> A few questions/comments:
>
> 1) If ICE is being used, how do you expect it to be handled on the 2nd 
> m-line? Since one of the goals of multiplexing on a single rtp session 
> is to minimize ICE processing, one would hope only to do ICE on the 
> first of the grouped m-lines. But if the answerer doesn't support this 
> grouping, then the ICE will be needed on the others.
I tried to be explicit that if negotiation is successful, only the ICE 
parameters from the first section listed in the A=group:TOGETHER would 
be used:

    The following parameters are taken from the first section only:

    o  Port number from the m= line

    o  All media-level attributes defined in RFC 5245 section 15.1 - this
       includes "candidate", "remote-candidates", "ice-mismatch", "ice-
       ufrag", "ice-pwd"

Suggestions for improving this language?

> 2) The draft only shows this mechanism being used to combine one audio 
> and one video. Do you also imagine it being used to represent multiple 
> audio and/or multiple video streams that might otherwise be handled 
> independently? (I see no reason why it couldn't be used that way.) 
> This would allow description of different properties for each one.
I certainly think that any number of m= sections could be combined this 
way, but in other discussions, I've seen people tend towards thinking of 
sections as "one m= section for all the audio channels, one m= section 
for all the video channels". Can you give an example of the usage you're 
thinking of?

I'm worried that we may get too many special rules about how parameters 
from sections are to be combined - multiple codecs with different PT 
numbers are fine, but other parameters could be confusing, so I'd like 
to see examples.
>
> 3) In the example, for an entity that doesn't understand TOGETHER, you 
> still show the a=mid lines. This is appropriate for an answerer that 
> *does* understand a=group. However an answerer that doesn't implement 
> rfc3388 would presumably omit the a=mid lines from the answer. So the 
> offerer had better be prepared for that as well.
Yes, that's a reasonable point. I'll add a third example answer.
>
>     Thanks,
>     Paul
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