Allow layer selection for publisher

[dreamerns]

Related to: https://github.com/pion/ion-sfu/pull/477

Description

• Export Layer type
• Allow tweaking encoding params per layer by using optional layer param. This can be used to disable certain layer (for example keep the high layer disabled and enable it only for the main speaker)
• Added API to select which layer should be enabled.
• Allow sending a message to 'ion-sfu' channel for the publisher to communicate layer selection
• Support new message type and layer selection callback

Reference issue

Fixes #...

[tarrencev]

Just to make sure i understand, is this so the sfu can tell the client which layers to send?

[dreamerns]

Just to make sure i understand, is this so the sfu can tell the client which layers to send?

This change allows the publisher to select which layer(s) should be enabled. Right now if I select FHD, I will send 3 layers all the time. If I want to change that, I'll have to renegotiate. With this change, the publisher can enable/disable the layer on the fly. An additional message is needed because there is no (easy) way for ion-sfu to detect that layer has been disabled. Once we send message of new layer selection, ion-sfu will check all the down tracks and update accordingly. New channel message activeLayer is for informational purposes only, so that subscribers are aware.

[adwpc]

This force subscriber to use one layer？

[tarrencev]

Just to make sure i understand, is this so the sfu can tell the client which layers to send?

This change allows the publisher to select which layer(s) should be enabled. Right now if I select FHD, I will send 3 layers all the time. If I want to change that, I'll have to renegotiate. With this change, the publisher can enable/disable the layer on the fly. An additional message is needed because there is no (easy) way for ion-sfu to detect that layer has been disabled. Once we send message of new layer selection, ion-sfu will check all the down tracks and update accordingly. New channel message activeLayer is for informational purposes only, so that subscribers are aware.

I think the SFU needs to do active track detection. Even without this change it is necessary since the browser can automatically toggle layers without user input. I think @OrlandoCo has plans to implement support for it at some point. My preference would be to wait for that since it avoids a lot of clientside/signaling complexity