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

Paul Kyzivat <pkyzivat@alum.mit.edu> Tue, 29 October 2013 18:53 UTC

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Date: Tue, 29 Oct 2013 14:53:22 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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To: Martin Thomson <martin.thomson@gmail.com>
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Subject: Re: [rtcweb] inactive m-lines in draft-ietf-rtcweb-jsep-05
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On 10/29/13 2:27 PM, Martin Thomson wrote:
> On 29 October 2013 08:05, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>> How is this expected to work? Is the browser supposed to do this in the
>> absence of a MediaStreamTrack?
>
> As I understand it, and my understanding is probably imperfect [22],
> the idea is that at all points where an offer or answer is
> constructed, the using application has access to MediaStreamTrack
> objects that relate to the various m-lines.  Mostly.

My question only pertains to those cases where it doesn't.

> That is, prior to creating an offer, you have the MediaStreamTrack
> instances that relate to what you are sending.  And Prior to creating
> an answer, you have both the tracks you want to send and the ones that
> you want to receive.  Adam has proposed [2] that enabling or disabling
> these tracks causes the appropriate a=sendonly/etc. attribute to be
> set.

This confuses me a little, probably because I am not well informed about 
JSEP. The above implies that for a sendrecv m-line there will be two 
MediaStreamTracks, one for sending and one for receiving. But section 
5.3.1 of JSEP seems inconsistent with that:

    For each non-rejected m= section of a given media type, if there is a
    local MediaStreamTrack of the specified type which has been added to
    the PeerConnection via addStream and not yet associated with a m=
    section, the MediaStreamTrack is associated with the m= section at
    this time.  If there are more m= sections of a certain type than
    MediaStreamTracks, some m= sections will not have an associated
    MediaStreamTrack.  If there are more MediaStreamTracks of a certain
    type than m= sections, only the first N MediaStreamTracks will be
    able to be associated in the constructed answer.  The remainder will
    need to be associated in a subsequent offer.

This maps each MediaStreamTracks of a given type to a different m-line. 
I *thought* the use of "specified type" above meant audio/video. Am I 
missing something?

> The only case that is a little strange is the one relating to the
> received streams when creating an initial offer, where you use the
> OfferToReceiveAudio or OfferToReceiveVideo constraint in order to
> indirectly control the send/recv attributes.

I know that is one way of creating the situation I was wondering about. 
It isn't clear to me if that is the only way.

	Thanks,
	Paul