feat: support publishing average loss rate

This change exposes an API for reading the loss rate.

pkg/cc/interceptor.go

@@ -24,7 +24,9 @@ type BandwidthEstimator interface {
 	AddStream(*interceptor.StreamInfo, interceptor.RTPWriter) interceptor.RTPWriter
 	WriteRTCP([]rtcp.Packet, interceptor.Attributes) error
 	GetTargetBitrate() int
+	GetLossRate() float64
 	OnTargetBitrateChange(f func(bitrate int))
+	OnLossRateChange(f func(loss float64))
 	GetStats() map[string]interface{}
 	Close() error
 }

pkg/gcc/send_side_bwe.go

@@ -43,6 +43,7 @@ type SendSideBWE struct {
 	feedbackAdapter *cc.FeedbackAdapter
 
 	onTargetBitrateChange func(bitrate int)
+	onLossRateChange func(loss float64)
 
 	lock          sync.Mutex
 	latestStats   Stats
@@ -164,6 +165,14 @@ func (e *SendSideBWE) GetTargetBitrate() int {
 	return e.latestBitrate
 }
 
+// GetLossRate returns the current packet loss rate in the range [0, 1].
+func (e *SendSideBWE) GetLossRate() float64 {
+	e.lossController.lock.Lock()
+	defer e.lossController.lock.Unlock()
+
+	return e.lossController.averageLoss
+}
+
 // GetStats returns some internal statistics of the bandwidth estimator
 func (e *SendSideBWE) GetStats() map[string]interface{} {
 	e.lock.Lock()
@@ -188,6 +197,12 @@ func (e *SendSideBWE) OnTargetBitrateChange(f func(bitrate int)) {
 	e.onTargetBitrateChange = f
 }
 
+// OnLossRateChange sets the callback that is called when the packet loss
+// rate changes
+func (e *SendSideBWE) OnLossRateChange(f func(loss float64)) {
+	e.onLossRateChange = f
+}
+
 // isClosed returns true if SendSideBWE is closed
 func (e *SendSideBWE) isClosed() bool {
 	select {
@@ -227,6 +242,11 @@ func (e *SendSideBWE) onDelayUpdate(delayStats DelayStats) {
 		go e.onTargetBitrateChange(bitrate)
 	}
 
+	// in principle this could update more frequently but this is a convenient place to put this hook.
+	if e.latestStats.LossStats.AverageLoss != lossStats.AverageLoss && e.onLossRateChange != nil {
+		go e.onLossRateChange(lossStats.AverageLoss)
+	}
+
 	e.latestStats = Stats{
 		LossStats:  lossStats,
 		DelayStats: delayStats,

pkg/gcc/send_side_bwe_test.go

@@ -70,6 +70,7 @@ func TestSendSideBWE(t *testing.T) {
 	twccSender.BindRTCPWriter(m.twccResponder)
 
 	require.Equal(t, latestBitrate, bwe.GetTargetBitrate())
+	require.Equal(t, 0.0, bwe.GetLossRate())
 	require.NotEqual(t, 0, len(bwe.GetStats()))
 
 	rtpWriter := bwe.AddStream(streamInfo, m)
@@ -89,6 +90,7 @@ func TestSendSideBWE(t *testing.T) {
 
 	// Sending a stream with zero loss and no RTT should increase estimate
 	require.Less(t, latestBitrate, bwe.GetTargetBitrate())
+	require.Equal(t, 0.0, bwe.GetLossRate())
 }
 
 func TestSendSideBWE_ErrorOnWriteRTCPAtClosedState(t *testing.T) {