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Callback on track added? (Re: Semantics question for onaddstream() callback)

From: Harald Alvestrand <harald@alvestrand.no>
Date: Mon, 14 Nov 2011 07:50:50 +0800
Message-ID: <4EC057DA.3020106@alvestrand.no>
To: Justin Uberti <juberti@google.com>
CC: Stefan Håkansson <stefan.lk.hakansson@ericsson.com>, "public-webrtc@w3.org" <public-webrtc@w3.org>
(top-posting, apologies - but we're now perhaps discussing overarching 

It seems to me that onaddstream() is logically a MediaStream level event 
while onmediastarted() is logically a MediaStreamTrack level event.

Perhaps the correct interface is to define an onmediastarted() event on 
the MediaStream, and have that be triggered when the underlying 
implementation has been convinced that enough media has arrived to do 
something with? (Yes, leaving wiggle room to do both first-frame and to 
do first-packet).

At the moment I don't think we have an event that can be watched for to 
catch streams as they are defined, and in Santa Clara, we decided that 
the tracklist is NOT immutable - which means that we have to define how 
people are supposed to watch it for changes, too.


On 11/14/2011 12:35 AM, Justin Uberti wrote:
> On Sun, Nov 13, 2011 at 9:59 AM, Stefan Håkansson 
> <stefan.lk.hakansson@ericsson.com 
> <mailto:stefan.lk.hakansson@ericsson.com>> wrote:
>     On 11/13/2011 03:33 PM, Justin Uberti wrote:
>         On Mon, Nov 7, 2011 at 11:51 AM, Stefan Håkansson LK
>         <stefan.lk.hakansson@ericsson.com
>         <mailto:stefan.lk.hakansson@ericsson.com>
>         <mailto:stefan.lk.hakansson@ericsson.com
>         <mailto:stefan.lk.hakansson@ericsson.com>>> wrote:
>         >On Mon, Nov 7, 2011 at 7:15 AM, Harald Alvestrand
>         <harald@alvestrand.no <mailto:harald@alvestrand.no>
>         <mailto:harald@alvestrand.no <mailto:harald@alvestrand.no>>>
>         wrote:
>         >> 1) We know that this stream is expected. The callback occurs as
>            soon as we
>         >> have the OFFER or ANSWER containing information about this
>            stream. Move the
>         >> last onaddstream() up to just after onconnecting().
>         >>
>         >> 2) We know that media is arriving on the stream. The callback
>            occurs as soon
>         >> as the first media packet arrives. Move the first onaddstream()
>            down below
>         >> the "ICE Completes" line
>         >>
>         >>
>         >> My order of preference is 1) followed by 2). But I think we
>         need
>            to pick one
>         >> and only one, and make sure the API spec says exactly that.
>         >
>         >My vote would be (1) and to add a onmediastarted() callback to
>         >indicate the first
>         >packet for a given media stream.
>            I share this view - you need both. //Stefan
>         Agree regarding (1). Regarding (2), what do we expect the
>         application
>         would do when it gets an onmediastarted()?
>     That's the indication that things worked out. (1) only indicates
>     that signaling has been carried out, (2) tells the application
>     that media is actually getting through. There can be problems with
>     getting media through; there may FWs or NATs or other reasons for
>     not getting it through.
>     The application could e.g. wait with displaying the video surface
>     until something is actually received. (Sure there are other ways -
>     paint a canvas&analyze - but (2) is quite convenient)
> I agree this is valuable, but typically in this case you want to wait 
> until the first frame is received, not just the first packet; you 
> don't want to start display of video until you actually have something 
> to display.
> Failure to receive media also seems like it might be a general problem 
> for the <video/> tag, so perhaps any notification of errors should (or 
> perhaps already do) happen there.
>     As the API is currently defined, feeding back to the sender that
>     (2) has happened can also be used as an indicator to the sender
>     that the streams sent are received be the receiver (of course we
>     could add error events at the addStream level to accomplish the
>     same thing - but it is already there in some sense with (2)). So
>     it can be part of error handling.
>     If I had to select, (2) would be the one to keep; but both make a
>     lot of sense.
>     my $0.02
Received on Sunday, 13 November 2011 23:51:19 UTC

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