a=imageattr negotiation specification

This change adds a=imageattr generation, parsing, and guidelines for usage.
On 06/10/2014 11:25 PM, Martin Thomson wrote:
>
> In draft-ietf-rtcweb-jsep.xml:
>
> > +            digits, and rounding the maximum up to four decimal digits.</t>
> > +          </section>
> > +
> > +          <section title="Intersection of imageattr values">
> > +            <t>When it is necessary to form an intersection between local
> > +            capabilities and proposed capabilities, as will happen when
> > +            generating answers and when considering an answer the following
> > +            procedure is used, where "result" is the proposal being generated,
> > +            and "proposal" is the imageattr from the corresponding incoming
> > +            m-line.</t>
> > +
> > +            <t>The intention of the intersection algorithm is that the result
> > +            should be a set of capabilities that are acceptable by both sides,
> > +            with priority given to the formats preferred by each side.</t>
> > +
> > +            <t>[[TODO: Specify an actual algorithm]]</t>
>
> I have a proposal: select the highest preference of the receiver that
> the sender is willing to send. Of that, the highest resolution is
> selected that can be fit within the current bandwidth constraints.
>

Yep - the selection of something to send is fairly simple.

When sending an answer, the selection of what you say that you're
willing to receive, given what the other guy says that he's willing to
send .... is a little more complex; if we don't have a strong
preference, we'd like to leave him leeway. I think.

ie if he says in the offer

send 1.0 x=320:640 y=240:480

and we don't care at all what he sends
it seems a bit churlish to send back in the answer

recv 1.0 x=640 y=480

when we could have sent

recv 1.0 x=320:640 y=240:480

giving him the opportunity to step down if he wants to.



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> <https://github.com/rtcweb-wg/jsep/pull/49/files#r13620512>.
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