ACTION-62, Priority API

ACTION-62, http://www.w3.org/2011/04/webrtc/track/actions/62, “Propose 
Priority API” was assigned to me at Lyon.

There are several way to do this, e.g.
* Constraints at addStream time
* Fortran style, e.g.
** pc.setPriority(track, priority);
* Fortran style with constraints
** pc.applyConstraints(track, constraints); //constraint for priority 
included
* Follow what we did for DTMF, allow the creation of a separate object
** pc.createTransportController(track);
** Operate on the “TransporController”.

After some thinking, I think I prefer the last solution (i.e. enable the 
creation of a separate object to handle transport related things) in 
combination with re-using constraints in the way Travis proposed in v6 [1].

There are a couple of reasons for this:
* Constraints at addStream time can’t handle tracks added to a stream at 
a later time, nor does it allow for changes
* I think we will not only want to change priority, but also things like 
bit-rate, video codec operation (CBR, VBR), DTX on/off, … - this means 
the Fortral style design would make the PeerConnection API grow a lot
* Fortran style with Constraints is quite OK, but gives no natural place 
for reporting if during the session a constraint can (temporarily) not 
be met

So what I propose is basically:

* Add one new method to PeerConnection:
** createTransportHandler - takes a track (must be in a MediaStream in 
localStreams - otherwise there will be an error) as argument and returns 
a RTCTransportHandler object

* The RTCTransportHandler (and please propose better names!) uses 
constraints in the same way is outlined in section 6.2 of [1]
** Initial constraints are priority and bitrate - we can add later as we 
see need

This design is very similar to the one selected for DTMF, re-uses 
constraints and how they are proposed to be used with MediaStreamTracks.

I have attached a pdf with a more complete proposal.

Is this a reasonable approach?

Stefan

[1] 
http://dvcs.w3.org/hg/dap/raw-file/tip/media-stream-capture/proposals/SettingsAPI_proposal_v6.html