[RTW] Review of draft-holmberg-rtcweb-ucreqs-00 (Web Real-Time Communication Use-cases and Requirements)

[Ted Hardie]

On Thu, Mar 10, 2011 at 4:57 AM, Christer Holmberg

>>A5: The web application MUST be able to control the media
>>format (codec) to be used for the streams sent to a peer. I
>>think the MUST is that the sender and recipient need to be
>>able to find a common codec, if one exists; I'm not sure I
>>see a MUST for the webapp actually picking one.
>
> First, the sender and recipient of need to be able to perform
> codec negotiation, in order to find the common codecs.
>
> If the codec negotiation is handled by the web application
> (i.e. JavaScript based) the API must support this.
>
> If the codec negotiation is handled by the browser, then the app
> might not need to not have as much control.
>
> We try to cover that in the note associated with A5.
>
So, I think we're all in agreement that rtc-web must specify a
mechanism that allows for codec negotiation.  But I think we may need
some more discussion on the expected mechanics.  The options include:

Web app queries browser/host system via API for available codecs and
sends selected codecs to rendezvous server, which runs the selection
algorithm.  All peers acknowledge the selection.

Web app queries browser/host system via API for available codecs and
sends selected codecs to peers, which answer the offer.  The original
app acknowledges the answer, and things move on from there.

Web app requests browser/host system to select candidate codecs based
on some set of characteristics; it then sends the selected codecs to
rendezvous server, which runs the selection algorithm.  All peers
acknowledge the selection.

Web app requests browser/host system to select candidate codecs based
on some set of characteristics; it sends selected codecs to peers,
which answer the offer.  The original app acknowledges the answer, and
things move on from there.

The two axes which vary in that set of choices are:  whether the web
app makes the selection from among candidate codecs or the
browser/host system makes the selection based on info provided;
whether the negotiation takes place in the rendezvous server or in a
peer-base offer/answer/acknowledgement set.   An obvious consequence
of these choices is that the logic for condec selection moves around.
An additional consequence of these choices will be what element in the
system needs to know about the possibility of network-provided
transcoding.

I think some discussion of which negotiation method is expected would
be useful.  If, for example, we rule out the negotiation server acting
as the agent for negotiation, we can re-use the same protocol
mechanics for offer-answer-acknowledgement, no matter whether the web
app or browser/host system provides the codec selections.

regards,

Ted Hardie