Need to set reference layer when starting with dummy packets. (#2034)

Dummy packets are used at start to trigger Pion's OnTrack.

pkg/sfu/buffer/rtpstats.go

@@ -644,8 +644,6 @@ func (r *RTPStats) UpdateFromReceiverReport(rr rtcp.ReceptionReport) (rtt uint32
 	if extHighestSNOverridden < r.sequenceNumber.GetExtendedStart() {
 		// it is possible that the `LastSequenceNumber` in the receiver report is before the starting
 		// sequence number when dummy packets are used to trigger Pion's OnTrack path.
-		r.lastRRTime = time.Now()
-		r.lastRR = rr
 		return
 	}
 
@@ -1211,7 +1209,7 @@ func (r *RTPStats) DeltaInfo(snapshotId uint32) *RTPDeltaInfo {
 }
 
 func (r *RTPStats) DeltaInfoOverridden(snapshotId uint32) *RTPDeltaInfo {
-	if !r.params.IsReceiverReportDriven {
+	if !r.params.IsReceiverReportDriven || r.lastRRTime.IsZero() {
 		return nil
 	}
 

pkg/sfu/downtrack.go

@@ -1786,8 +1786,11 @@ func (d *DownTrack) sendSilentFrameOnMuteForOpus() {
 				AbsSendTimeExtID:   uint8(d.absSendTimeExtID),
 				TransportWideExtID: uint8(d.transportWideExtID),
 				WriteStream:        d.writeStream,
-				Metadata:           sendPacketMetadata{},
-				OnSent:             d.packetSent,
+				Metadata: sendPacketMetadata{
+					// although this is using empty frames, mark as padding as these are used to trigger Pion OnTrack only
+					isPadding: true,
+				},
+				OnSent: d.packetSent,
 			})
 		}
 

pkg/sfu/forwarder.go

@@ -1454,6 +1454,15 @@ func (f *Forwarder) processSourceSwitch(extPkt *buffer.ExtPacket, layer int32) e
 			"referenceLayerSpatial", f.referenceLayerSpatial,
 		)
 		return nil
+	} else if f.referenceLayerSpatial == buffer.InvalidLayerSpatial {
+		f.referenceLayerSpatial = layer
+		f.logger.Debugw(
+			"catch up forwarding",
+			"sequenceNumber", extPkt.Packet.SequenceNumber,
+			"timestamp", extPkt.Packet.Timestamp,
+			"layer", layer,
+			"referenceLayerSpatial", f.referenceLayerSpatial,
+		)
 	}
 
 	logTransition := func(message string, extExpectedTS, extRefTS, extLastTS uint64, diffSeconds float64) {