Re: [rtcweb] Resolution negotiation - mandatory / advisory resolutions

Harald Alvestrand <harald@alvestrand.no> Thu, 19 April 2012 07:46 UTC

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Date: Thu, 19 Apr 2012 09:46:40 +0200
From: Harald Alvestrand <harald@alvestrand.no>
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Subject: Re: [rtcweb] Resolution negotiation - mandatory / advisory resolutions
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When discussing the issue of resolution negotiation locally, we found a 
quandary:

We have two different negotiation needs:
- Maximum resolution that's possible to use in a given context
- The preferred resolution for a given context

The current imageattr= spec (RFC 6236) doesn't specify clearly whether 
the result of the negotiation is to be considered mandatory or advisory 
(it uses the term "wishes" a lot), but the description reads most 
consistently if one assumes that it is a "MUST" type request.

One interpretation is that the highest functionality available in the 
spec is the "limit", while the highest q value is the "preference", thus:

recv [x=320,y=200,q=1.0] [x=200:1024,y=160:768,q=0.1]

would indicate that the receiver is capable of 200x160 to 1024x768, but 
wants 320x200.

This maps reasonably to the capabilities spec's talk about "mandatory" 
vs "optional" capabilities.

Does this interpretation make sense?

                       Harald


On 04/12/2012 10:46 AM, Harald Alvestrand wrote:
> Hello folks,
>
> after the IETF, I've attempted to gather together information about 
> resolution negotiation and what we might need to do there.
>
> I pulled together the paragraphs below - this may want to be 
> incorporated in an I-D on "SDP usage in RTCWEB", or it may want to go 
> somewhere else. Chairs' guidance is appreciated.
>
> At the moment, I have it in xml2rfc format, so it should be easy to do 
> cut-and-paste on it, but don't want to push it as a separate I-D 
> unless that's what the chairs desire.
> Note - there are some QUERY sections in there - I'm unsure what those 
> should be resolved to.
>
> Comments?
>
>                          Harald
>
> 2.  Requirements
>
>    The relevant requirement for video resolution negotiation from the
>    RTCWEB use cases document
>    [I-D.ietf-rtcweb-use-cases-and-requirements] is:
>
>    o  F25 The browser SHOULD use encoding of streams suitable for the
>       current rendering (e.g. video display size) and SHOULD change
>       parameters if the rendering changes during the session.
>
>    The resulting need is:
>
>    o  To negotiate a maximum spatial resolution for all videos at call
>       setup time
>
>    o  To negotiate a maximum temporal resolution ("frame rate") across
>       all videos at call setup time
>
>    o  To indicate the desire of the recipient for a particular spatial
>       or temporal resolution on a particular video source
>
>    o  To indicate the intent of the sender to send a video source in a
>       particular spatial or temporal resolution
>
>    This document does not mention other requirements.