However?, you breach the question of scope. What's the scope of the layerId? I was assuming it was per RtpSender/RtpReceiver. But now you're suggesting that they have basically a global scope (across all RtpSender/RtpReceivers). That's an important question to answer. One one hand, it the JS more power. On the other, it is more complex. I'm usually inclined to stick with simple, not complex. How compelling do you think having dependent layers on different ports really is?
[BA] This will only arise with Multi-Session Transport. It will not arise with Single-Session Transport, with either single or multiple sources. As I understand it, VP8/9 is Single-Session Transport (and single source) and all H.264/SVC implementations I am aware of are Single-Session (some are single source, others multiple source). So maybe we can say Multiple Session Transport (though not multiple source) is out of scope?