Implement packet pacing.

We used to send all of the packets corresponding to a received packet
as a single burst, which led to persistent packet drops for clients
at the end of the queue.  Pace the writer, which introduces up to 1ms
extra jitter.

rtpconn.go

@@ -480,6 +480,7 @@ func newUpConn(c client, id string) (*rtpUpConnection, error) {
 type packetIndex struct {
 	seqno uint16
 	index uint16
+	delay uint32
 }
 
 func readLoop(conn *rtpUpConnection, track *rtpUpTrack) {
@@ -531,15 +532,34 @@ func readLoop(conn *rtpUpConnection, track *rtpUpTrack) {
 			continue
 		}
 
+		_, rate := track.rate.Estimate()
+		delay := uint32(rtptime.JiffiesPerSec / 1024)
+		if rate > 512 {
+			delay = rtptime.JiffiesPerSec / rate / 2
+		}
+
+		pi := packetIndex{packet.SequenceNumber, index, delay}
 		select {
-		case ch <- packetIndex{packet.SequenceNumber, index}:
+		case ch <- pi:
 		default:
+			// the writer is congested
 			if isvideo {
-				// the writer is congested.  Drop until
-				// the end of the frame.
+				// keep dropping until the end of the frame
 				if isvideo && !packet.Marker {
 					drop = 7
 				}
+			} else {
+				// try again with half the delay on our side
+				timer := time.NewTimer(rtptime.ToDuration(
+					uint64(delay/2),
+					rtptime.JiffiesPerSec,
+				))
+				pi.delay = delay / 2
+				select {
+				case ch <- pi:
+					timer.Stop()
+				case <-timer.C:
+				}
 			}
 		}
 	}
@@ -598,8 +618,15 @@ func writeLoop(conn *rtpUpConnection, track *rtpUpTrack, ch <-chan packetIndex)
 				continue
 			}
 
-			kfNeeded := false
+			var delay time.Duration
+			if len(local) > 0 {
+				delay = rtptime.ToDuration(
+					uint64(pi.delay / uint32(len(local))),
+					rtptime.JiffiesPerSec,
+				)
+			}
 
+			kfNeeded := false
 			for _, l := range local {
 				err := l.WriteRTP(&packet)
 				if err != nil {
@@ -611,6 +638,9 @@ func writeLoop(conn *rtpUpConnection, track *rtpUpTrack, ch <-chan packetIndex)
 					continue
 				}
 				l.Accumulate(uint32(bytes))
+				if delay > 0 {
+					time.Sleep(delay)
+				}
 			}
 
 			if kfNeeded {