PSA: Simulcast screensharing

[spr...@webrtc.org]

We are currently gradually rolling out a simulcast mode for screensharing, with the intent of launching it in M68.

Any app that meets all of the following requirements might be affected:

* Uses a capture device that indicates screen content

* Has simulcast (multiple SSRCs) configured for a video track

* Has the a=x-google-flag:conference flag set

If any of these requirements are not met, you can ignore this message, though in a future version we aim to make further improvements and remove the conference flag.

With this feature enabled, WebRTC will use two simulcast streams with same resolution but different bitrates and potentially different frame rates.

You might need to make sure that you service can distinguish between the high and low quality streams reliably, as looking at the resolution won't be enough. Alternatively, renegotiate without simulcast or the conference flag.

To test it, you can force enable it by running chrome with the command line argument --force-fieldtrials=WebRTC-SimulcastScreenshare/Enabled/

[Erik Språng]

Short update:

We discovered some unexpected long delays for a small percentage of receivers where the sender had this feature enabled, and so rolled it back.

A few bugs and several optimizations have been put in place, and we now aim to gradually roll this out again starting with M70.

[PhistucK]

Adding Philip and blink-dev.

Could you, please, find another way of indicating multicast is supported, that does not involve adding a prefixed flag ("a=x-google-flag:conference") to the SDP?

New prefixed features are explicitly disallowed...

☆PhistucK

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[Erik Språng]

That's a flag that has been required for layered screencast for a very long time. 

And yes, we're planning on removing the conference flag when the Chrome APIs allows explicitly configuring the things this flag currently triggers.