Fallback to primary encoding if redundant block overflow (#2858)

* Fallback to primary encoding if redundant block overflow

* revert mtu change

pkg/sfu/downtrack.go

@@ -406,12 +406,18 @@ func (d *DownTrack) Bind(t webrtc.TrackLocalContext) (webrtc.RTPCodecParameters,
 		return webrtc.RTPCodecParameters{}, err
 	} else if strings.EqualFold(matchedUpstreamCodec.MimeType, "audio/red") {
 		d.isRED = true
-		var primaryPT, secondaryPT int
-		if n, err := fmt.Sscanf(matchedUpstreamCodec.SDPFmtpLine, "%d/%d", &primaryPT, &secondaryPT); err != nil || n != 2 {
-			d.params.Logger.Errorw("failed to parse upstream primary and secondary payload type for RED", err, "matchedCodec", codec)
+		for _, c := range d.upstreamCodecs {
+			// assume upstream primary codec is opus since we only support it for audio now
+			if strings.EqualFold(c.MimeType, "audio/opus") {
+				d.upstreamPrimaryPT = uint8(c.PayloadType)
+				break
+			}
+		}
+		if d.upstreamPrimaryPT == 0 {
+			d.params.Logger.Errorw("failed to find upstream primary opus payload type for RED", nil, "matchedCodec", codec, "upstreamCodec", d.upstreamCodecs)
 		}
-		d.upstreamPrimaryPT = uint8(primaryPT)
 
+		var primaryPT, secondaryPT int
 		if n, err := fmt.Sscanf(codec.SDPFmtpLine, "%d/%d", &primaryPT, &secondaryPT); err != nil || n != 2 {
 			d.params.Logger.Errorw("failed to parse primary and secondary payload type for RED", err, "matchedCodec", codec)
 		}

pkg/sfu/redreceiver.go

@@ -29,13 +29,15 @@ import (
 )
 
 const (
-	maxRedCount = 2
-	mtuSize     = 1500
+	maxRedCount   = 2
+	mtuSize       = 1500
+	maxRedPayload = 1 << 10 // fit into 10 bits length field
 
 	// the RedReceiver is only for chrome / native webrtc now, we always negotiate opus payload to 111 with those clients,
 	// so it is safe to use a fixed payload 111 here for performance(avoid encoding red blocks for each downtrack that
 	// have a different opus payload type).
-	opusPT = 111
+	opusPT    = 111
+	opusRedPT = 63
 )
 
 type RedReceiver struct {
@@ -60,6 +62,14 @@ func (r *RedReceiver) ForwardRTP(pkt *buffer.ExtPacket, spatialLayer int32) int
 	if r.downTrackSpreader.DownTrackCount() == 0 {
 		return 0
 	}
+
+	// fallback to primary codec if payload size exceeds redundant block length
+	if len(pkt.Packet.Payload) >= maxRedPayload {
+		return r.downTrackSpreader.Broadcast(func(dt TrackSender) {
+			_ = dt.WriteRTP(pkt, spatialLayer)
+		})
+	}
+
 	redLen, err := r.encodeRedForPrimary(pkt.Packet, r.redPayloadBuf[:])
 	if err != nil {
 		r.logger.Errorw("red encoding failed", err)
@@ -68,6 +78,7 @@ func (r *RedReceiver) ForwardRTP(pkt *buffer.ExtPacket, spatialLayer int32) int
 
 	pPkt := *pkt
 	redRtpPacket := *pkt.Packet
+	redRtpPacket.PayloadType = 63
 	redRtpPacket.Payload = r.redPayloadBuf[:redLen]
 	pPkt.Packet = &redRtpPacket
 
@@ -117,7 +128,7 @@ func (r *RedReceiver) Close() {
 }
 
 func (r *RedReceiver) ReadRTP(buf []byte, layer uint8, sn uint16) (int, error) {
-	// red encoding don't support nack
+	// red encoding doesn't support nack
 	return 0, bucket.ErrPacketMismatch
 }