Configure WebRTC simulcast with multiple spatial layers for VP8 or H.264

Steps

1. When calling addTransceiver() or addTrack(), pass an encodings array with multiple entries to RTCPeerConnection — each entry specifies rid (e.g., 'low', 'mid', 'hi'), scaleResolutionDownBy (e.g., 4, 2, 1), and maxBitrate.
2. Ensure your SDP offer contains multiple a=ssrc lines and a=rid lines with the corresponding restriction identifiers, plus an a=simulcast line listing the send layers; most browsers generate this automatically from the encodings array.
3. On the receiving SFU side, match each RID to its SSRC and configure the SFU to forward the appropriate layer to each subscriber based on available bandwidth.
4. Use RTCRtpSender.setParameters() at runtime to change the active flag on an encoding object to disable a layer without renegotiation, or adjust maxBitrate to throttle a layer.
5. Subscribe to the RTCPeerConnection 'iceconnectionstatechange' and RTCPeerConnection.getStats() to monitor per-layer packet loss and round-trip time, using them to drive adaptive layer selection on the SFU.
6. Note that H.264 and VP8 support only simulcast over distinct SSRCs (no SVC spatial layers); VP9 and AV1 additionally support in-band SVC via scalabilityMode such as L3T3.