Tweak adaptation to increase in propagation delay. (#2598)

* Tweak adaptation to increase in propagation delay.

A couple of issues
- RTCP Sender Reports rate will vary based on underying track bitrate.
  (at least in theory, not all entities will do it though, for example
  SFU does standard rate of one per three seconds irrespective of track
  bit rate). So, adapt the long term estimate of propagation delay delta
  based on spacing of reports.
- Re-init of propagation delay to adapt to path change was taking the
  last value before the switch. But, that one value could have been an
  outlier and accepting it is not great. So, adapt spike time
  propagation delay in a smoother fashion to ensure that all values
  during spike contribute to the final value.

* clean up

pkg/sfu/buffer/rtpstats_receiver.go

@@ -41,6 +41,8 @@ const (
 	cPropagationDelayFallFactor = float64(0.95)
 	cPropagationDelayRiseFactor = float64(0.05)
 
+	cPropagationDelaySpikeAdaptationFactor = float64(0.5)
+
 	// do not adapt to small OR large (outlier) changes
 	cPropagationDelayDeltaThresholdMin       = 5 * time.Millisecond
 	cPropagationDelayDeltaThresholdMaxFactor = 2
@@ -51,7 +53,7 @@ const (
 	// A long term version of delta of propagation delay is maintained and delta propagation delay exceeding
 	// a factor of the long term version is considered a sharp increase. That will trigger the start of the
 	// path change condition and if it persists, propagation delay will be reset.
-	cPropagationDelayDeltaAdaptationFactor    = float64(0.05)
+	cPropagationDelayDeltaMaxInterval         = 10 * time.Second
 	cPropagationDelayDeltaHighResetNumReports = 3
 	cPropagationDelayDeltaHighResetWait       = 10 * time.Second
 )
@@ -83,6 +85,7 @@ type RTPStatsReceiver struct {
 	longTermDeltaPropagationDelay      time.Duration
 	propagationDelayDeltaHighCount     int
 	propagationDelayDeltaHighStartTime time.Time
+	propagationDelaySpike              time.Duration
 
 	clockSkewCount               int
 	outOfOrderSsenderReportCount int
@@ -370,11 +373,17 @@ func (r *RTPStatsReceiver) SetRtcpSenderReportData(srData *RTCPSenderReportData)
 			"current", &srDataCopy,
 		}
 	}
-	initPropagationDelay := func(pd time.Duration) {
-		r.propagationDelay = pd
-		r.longTermDeltaPropagationDelay = 0
+	resetDelta := func() {
 		r.propagationDelayDeltaHighCount = 0
 		r.propagationDelayDeltaHighStartTime = time.Time{}
+		r.propagationDelaySpike = 0
+	}
+	initPropagationDelay := func(pd time.Duration) {
+		r.propagationDelay = pd
+
+		r.longTermDeltaPropagationDelay = 0
+
+		resetDelta()
 	}
 
 	ntpTime := srDataCopy.NTPTimestamp.Time()
@@ -392,14 +401,18 @@ func (r *RTPStatsReceiver) SetRtcpSenderReportData(srData *RTCPSenderReportData)
 				if r.propagationDelayDeltaHighStartTime.IsZero() {
 					r.propagationDelayDeltaHighStartTime = time.Now()
 				}
+				if r.propagationDelaySpike == 0 {
+					r.propagationDelaySpike = propagationDelay
+				} else {
+					r.propagationDelaySpike += time.Duration(cPropagationDelaySpikeAdaptationFactor * float64(propagationDelay-r.propagationDelaySpike))
+				}
 
 				if r.propagationDelayDeltaHighCount >= cPropagationDelayDeltaHighResetNumReports && time.Since(r.propagationDelayDeltaHighStartTime) >= cPropagationDelayDeltaHighResetWait {
 					r.logger.Debugw("re-initializing propagation delay", append(getPropagationFields(), "newPropagationDelay", propagationDelay.String())...)
-					initPropagationDelay(propagationDelay)
+					initPropagationDelay(r.propagationDelaySpike)
 				}
 			} else {
-				r.propagationDelayDeltaHighCount = 0
-				r.propagationDelayDeltaHighStartTime = time.Time{}
+				resetDelta()
 
 				if deltaPropagationDelay.Abs() > cPropagationDelayDeltaThresholdMin {
 					factor := cPropagationDelayFallFactor
@@ -421,7 +434,9 @@ func (r *RTPStatsReceiver) SetRtcpSenderReportData(srData *RTCPSenderReportData)
 		if r.longTermDeltaPropagationDelay == 0 {
 			r.longTermDeltaPropagationDelay = deltaPropagationDelay
 		} else {
-			r.longTermDeltaPropagationDelay += time.Duration(cPropagationDelayDeltaAdaptationFactor * float64(deltaPropagationDelay-r.longTermDeltaPropagationDelay))
+			sinceLastReport := srDataCopy.NTPTimestamp.Time().Sub(r.srNewest.NTPTimestamp.Time())
+			adaptationFactor := min(1.0, float64(sinceLastReport)/float64(cPropagationDelayDeltaMaxInterval))
+			r.longTermDeltaPropagationDelay += time.Duration(adaptationFactor * float64(deltaPropagationDelay-r.longTermDeltaPropagationDelay))
 		}
 	}
 	// adjust receive time to estimated propagation delay