Re: [rtcweb] JSEP question: How to set up simulcast for server-originated calls?

[Iñaki Baz Castillo <[e-mail]>]

On Mon, 5 Nov 2018 at 15:21, Sergio Garcia Murillo
<[e-mail]> wrote:
>
> By the way, according to the simulcast draft that should not be a problem at all:
>
>    An answerer that receives an offer with simulcast that lists a number
>    of simulcast streams, MAY reduce the number of simulcast streams in
>    the answer, but MUST NOT add simulcast streams.

Not sure if that also covers "recv" simulcast items in the offer.
Probably yes, so ok.

However still useless IMHO. Imagine an SFU that wants to receive, at
least those bitrates (because somehow it knows that all devices can
handle at least 3 simulcast streams):

- 150kbps
- 400kbps
- 1000kbps

Imagine there are also some devices supporting up to 5 streams, so the
SFU produces this simulcast settings in the offer for receiving:

- 150kbps
- 120kbps
- 400kbps
- 750kbps
- 1000kbps

If a device that just supports 3 simulcast streams receives such an
offer, it would discard 2. Which ones? ok, the SDP can include info
about the desired max-bitrate per rid, and we may think: oh, we can
get the list of rids and bitrates  after pc.setRemoteDescription() by
calling pc.getTransceivers()[1].receiver.getParameters().encodings, so
the app can decide which ones to enable.

But that's not true since receiver.getParameters() should not contain
more rids than those supported by the device. Well, this constraint is
really about sender.getParameters(). So you may say: ok we somehow
read the maximum simulcast streams that our browser/device can send
and then choose the appropriate ones from the receiver can call
transceiver.sender.setParameters() with them.. Error! there is no API
for knowing the max number of simulcast streams the browser can
produce.

Too many assumptions and non existing features in this text to make
this scenario really possible.

-- 
Iñaki Baz Castillo
<[e-mail]>