Maintain time offsets on the sender side.

conn.go

@@ -41,4 +41,5 @@ type downTrack interface {
 	WriteRTP(packat *rtp.Packet) error
 	Accumulate(bytes uint32)
 	GetMaxBitrate(now uint64) uint64
+	setTimeOffset(ntp uint64, rtp uint32)
 }

disk.go

@@ -204,6 +204,9 @@ func newDiskConn(directory, label string, up upConnection, remoteTracks []upTrac
 	return &conn, nil
 }
 
+func (t *diskTrack) setTimeOffset(ntp uint64, rtp uint32) {
+}
+
 func clonePacket(packet *rtp.Packet) *rtp.Packet {
 	buf, err := packet.Marshal()
 	if err != nil {

rtpconn.go

@@ -78,6 +78,8 @@ type rtpDownTrack struct {
 	stats          *receiverStats
 	srTime         uint64
 	srNTPTime      uint64
+	remoteNTPTime  uint64
+	remoteRTPTime  uint32
 	rtt            uint64
 }
 
@@ -98,6 +100,11 @@ func (down *rtpDownTrack) GetMaxBitrate(now uint64) uint64 {
 	return br2
 }
 
+func (down *rtpDownTrack) setTimeOffset(ntp uint64, rtp uint32) {
+	atomic.StoreUint64(&down.remoteNTPTime, ntp)
+	atomic.StoreUint32(&down.remoteRTPTime, rtp)
+}
+
 type rtpDownConnection struct {
 	id            string
 	pc            *webrtc.PeerConnection
@@ -554,11 +561,19 @@ func writeLoop(conn *rtpUpConnection, track *rtpUpTrack, ch <-chan packetIndex)
 			if action.add {
 				local = append(local, action.track)
 				firSent = false
+				track.mu.Lock()
+				ntp := track.srNTPTime
+				rtp := track.srRTPTime
+				track.mu.Unlock()
+				if ntp != 0 {
+					action.track.setTimeOffset(ntp, rtp)
+				}
 			} else {
 				found := false
 				for i, t := range local {
 					if t == action.track {
-						local = append(local[:i], local[i+1:]...)
+						local = append(local[:i],
+							local[i+1:]...)
 						found = true
 						break
 					}
@@ -754,6 +769,10 @@ func rtcpUpListener(conn *rtpUpConnection, track *rtpUpTrack, r *webrtc.RTPRecei
 				track.srNTPTime = p.NTPTime
 				track.srRTPTime = p.RTPTime
 				track.mu.Unlock()
+				local := track.getLocal()
+				for _, l := range local {
+					l.setTimeOffset(p.NTPTime, p.RTPTime)
+				}
 			case *rtcp.SourceDescription:
 			}
 		}
@@ -848,37 +867,22 @@ func sendSR(conn *rtpDownConnection) error {
 
 	for _, t := range conn.tracks {
 		clockrate := t.track.Codec().ClockRate
-		remote := t.remote
 
 		var nowRTP uint32
 
-		switch r := remote.(type) {
+		remoteNTP := atomic.LoadUint64(&t.remoteNTPTime)
-		case *rtpUpTrack:
+		remoteRTP := atomic.LoadUint32(&t.remoteRTPTime)
-			r.mu.Lock()
+		if remoteNTP == 0 {
-			lastTime := r.srTime
+			// we never got a remote SR for this track
-			srNTPTime := r.srNTPTime
-			srRTPTime := r.srRTPTime
-			r.mu.Unlock()
-			if lastTime == 0 {
-				// we never got a remote SR, skip this track
 			continue
 		}
-			if srNTPTime != 0 {
+		srTime := rtptime.NTPToTime(remoteNTP)
-				srTime := rtptime.NTPToTime(srNTPTime)
 		d := now.Sub(srTime)
 		if d > 0 && d < time.Hour {
 			delay := rtptime.FromDuration(
 				d, clockrate,
 			)
-					nowRTP = srRTPTime + uint32(delay)
+			nowRTP = remoteRTP + uint32(delay)
-				}
-			}
-		default:
-			ts, ok := remote.getTimestamp()
-			if !ok {
-				continue
-			}
-			nowRTP = ts
 		}
 
 		p, b := t.rate.Totals()