Re: [rtcweb] I-D Action: draft-ietf-rtcweb-audio-02.txt

Bernard Aboba <bernard_aboba@hotmail.com> Thu, 12 September 2013 19:43 UTC

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From: Bernard Aboba <bernard_aboba@hotmail.com>
To: Tim Panton <tim@phonefromhere.com>, "stephane.proust@orange.com" <stephane.proust@orange.com>
Date: Thu, 12 Sep 2013 12:43:08 -0700
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Subject: Re: [rtcweb] I-D Action: draft-ietf-rtcweb-audio-02.txt
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Tim said: 

> - Of course this whole discussion highlights the need for a set of constraints (or an API) that let the web developer 
> express her preferences wrt codec selection.

[BA] This - and the need to specify the default ordering of codecs within the API, so that by default the MTI codecs will be offered at a higher priority. 

> How about: 
> 
> To ensure a baseline level of interoperability between WebRTC clients, 
> a minimum set of required codecs are specified below.
> If other suitable audio codecs are available to the browser to use, it 
> is advised that they are also included in the offer at a lower priority than the required codecs in order to 
> maximize the possibility to establish the session without the need for 
> audio transcoding when the required codecs are unavailable to one party.
 
[BA] On reading this I'm unsure whether by "offer" you mean the SDP used in the API, of if you're talking about an offer sent over the wire.   The text makes more sense to me if you're talking about the API. 
 
If the developer decides they want to offer amr-nb as the preferred codec in an Offer sent over the wire, that's their choice, as long as they don't delete the MTI codecs from the offer, which could potentially cause an interoperability failure.   However, if the developer is just trusting the API to do something sensible, then by default I'd say that one of the MTI codecs should be negotiated by default.  In particular, in the browser-browser case I'd advocate that Opus by negotiated by default, for audio.