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Re: ReplaceTrack and track.id (Re: ReplaceTrack - need to evaluate alternatives)

From: Justin Uberti <juberti@google.com>
Date: Wed, 15 Apr 2015 18:57:23 -0700
Message-ID: <CAOJ7v-22+5GMRLYYEQmsS5U-mqdVUBxma4=pgaNgBcjNXzgzew@mail.gmail.com>
To: Peter Thatcher <pthatcher@google.com>
Cc: Martin Thomson <martin.thomson@gmail.com>, Harald Alvestrand <harald@alvestrand.no>, "public-webrtc@w3.org" <public-webrtc@w3.org>
The two arguments (that I know of) against using mid by itself:
1) addTrack followed by removeTrack followed by addTrack will result in the
same mid for the second track, due to the quirks of SDP. This means that
sometimes addTrack gives you a new mid, sometimes it won't. This is
unfortunate, although if we wanted to go down this path, we could prohibit
recycling m= sections without a corresponding mid change. IOW, you could
only recycle rejected m= sections, and so the example here would result in
two m= lines.
2) without a=msid, there is no way to detect a recyclable m= line
(currently we can look for a=msid to determine this).

Specifically, if you have a remote description without a=msid in a m=
section, and you stop your local track for that m= section, you don't know
whether to set port 0 (i.e. dead m= section) or not in subsequent offers,
because you can't tell if the remote side is still using that m= section.

On Wed, Apr 15, 2015 at 5:12 PM, Peter Thatcher <pthatcher@google.com>

> Ah, I see.  It's true that if we didn't allow JS to choose the value, it
> would be the same as now where the JS cannot choose the track ID.  So we
> wouldn't be losing anything compared to the status quo.
> But isn't the JS always able to set it anyway via SDP munging?
> On Wed, Apr 15, 2015 at 3:24 PM, Martin Thomson <martin.thomson@gmail.com>
> wrote:
>> On 15 April 2015 at 15:19, Peter Thatcher <pthatcher@google.com> wrote:
>> > And te question isn't whether to allow the JS to choose the MID.  It's
>> > whether to have two IDs/labels (MID + something else), or just one
>> (MID).
>> > If one will work, I prefer one.
>> Like I said, I am OK with just one and with that one being a=mid; I
>> was pushing back at your suggestion that we let the JS set it, that's
>> all.
Received on Thursday, 16 April 2015 01:58:11 UTC

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