Re: [rtcweb] draft-ietf-rtcweb-jsep-05 - Recycling m-lines

Harald Alvestrand <harald@alvestrand.no> Wed, 06 November 2013 20:59 UTC

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Date: Wed, 06 Nov 2013 21:59:13 +0100
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Subject: Re: [rtcweb] draft-ietf-rtcweb-jsep-05 - Recycling m-lines
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On 11/06/2013 09:36 PM, Paul Kyzivat wrote:
> On 11/3/13 11:09 PM, Justin Uberti wrote:
>>
>> On Fri, Nov 1, 2013 at 2:45 PM, Paul Kyzivat <pkyzivat@alum.mit.edu
>> <mailto:pkyzivat@alum.mit.edu>> wrote:
> ...
>>     Later in 5.2.2:
>>
>>         o  If a m= section exists in the current local description,
>> but has
>>            its state set to inactive or recvonly, and a new
>> MediaStreamTrack
>>            is added, the previously existing m= section MUST be recycled
>>            instead of creating a new m= section.  [OPEN ISSUE: Nail down
>>            exactly what this means.  Should the codecs remain the same?
>>            (No.)  Should ICE restart?  (No.)  Can the "a=mid"
>> attribute be
>>            changed?  (Yes?)]
>>
>>     This seems to assume that there is no existing MediaStreamTrack for
>>     these m-lines.
>>
>> Yes. The text should state that more clearly.
>>
>>     If there is, then
>>     - the m-line shouldn't be recycled.
>>     If there isn't, then
>>     - where do its address and port come from?
>>     - what is managing the RTCP?
>>
>>
>> The m= section will still have completed ICE negotiation, despite being
>> marked as inactive. So the address/port information should already be
>> present.
>
> *Why* would you do this if there is no MediaStreamTrack and hence no
> possibility of using the port?
>
> IOW, why not use port zero in these cases?
>
>> I don't follow your question about who is managing RTCP.
>
> It may be a misunderstanding on my part.
>
> My thinking is that the MediaStreamTrack is a surrogate for the
> resource that manages the RTP/RTCP for that track. If so, then no
> track -> no RTCP.
>
> Of course, with bundling, that resource gets spread across multiple
> MediaStreamTracks.

That's why thinking of it in the way you suggest is likely to lead to
misunderstandings and confusion all around.

I've had greater success thinking of a MediaStreamTrack as an SSRC; the
resource that manages the RTP/RTCP is then a container that contains MSTs.