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

[Sergio Garcia Murillo <[e-mail]>]

On 05/11/2018 13:11, Iñaki Baz Castillo wrote:
> On Mon, 5 Nov 2018 at 12:45, Harald Alvestrand <[e-mail]> wrote:
>> On 11/05/2018 10:03 AM, Sergio Garcia Murillo wrote:
>>> I am not talking about replacing a transceiver but adding a new one
>>> and renegotiate. The initial O/A will drop the simulcast attribute on
>>> the answer and create a recvonly transceiver, and then create a new
>>> transceiver with simulcast encoding that will require a O/A.
>> OK, so we have the same picture in mind of what will happen in your
>> proposal. We end up with an useless transceiver and a new transceiver
>> that is suggested by the browser end, in a new media section; it's
>> impossible to use the media section that the (initiating) server proposed.
>>
>> That seems ugly.
> So:
>
> - receive a remote offer with a single m=video section requesting
> local simulcast
> - reply with a=recvonly (fxxx you, conference server)
> - reoffer with a new m=video with "correlated" simulcast settings
> - have the conference server like that
>
> And... all this party instead of just telling the client (via custom
> messaging before it "joins" the conference) the desired simulcast
> settings? :)
>
Not sure if you are responding to  Harald or to me.  If it is to me, my 
"do nothing" proposal to enable simulcast when the offer is sent by the 
SFU (just in case anyone wants to do it):

-SFU sends an offer with all the available streams/tracks (no simulcast 
m-line)
-Browser reply with recvonly on all m-lines
-Browsers reoffer with an additional m-line (sendonly?) with simulcast
-SFU answers

No change to the spec at the cost of an additional round trip. Note that 
this is almost the same case than if the browser sends and initial offer 
with simulcast, sfu answers with recvonly, and then re-offers with all 
the available streams/track.

Also, note that many of SFU devs (at minimum Iñaki and me)  are not even 
sending SDP on the wire but doing renegotiations locally.

Best regards
Sergio