Use an improved RTT calculation

Previously each ack was assigned an RTT estimation using the time
between packet departure and ack arrival. That method is inaccurate as
multiple acks are aggregated in a single feedback packet. Instead, use
the min round trip time between packet departure and ack in a feedback
packet and subtracting the pending time between packet arrival and
feedback departure.

Author: mengelbart

internal/cc/acknowledgment.go

@@ -15,7 +15,6 @@ type Acknowledgment struct {
 	Size           int
 	Departure      time.Time
 	Arrival        time.Time
-	RTT            time.Duration
 	ECN            rtcp.ECN
 }
 
@@ -25,6 +24,5 @@ func (a Acknowledgment) String() string {
 	s += fmt.Sprintf("\tSIZE:\t%v\n", a.Size)
 	s += fmt.Sprintf("\tDEPARTURE:\t%v\n", int64(float64(a.Departure.UnixNano())/1e+6))
 	s += fmt.Sprintf("\tARRIVAL:\t%v\n", int64(float64(a.Arrival.UnixNano())/1e+6))
-	s += fmt.Sprintf("\tRTT:\t%v\n", a.RTT)
 	return s
 }

internal/cc/feedback_adapter.go

@@ -43,7 +43,6 @@ func (f *FeedbackAdapter) onSentRFC8888(ts time.Time, header *rtp.Header, size i
 		Size:           size,
 		Departure:      ts,
 		Arrival:        time.Time{},
-		RTT:            0,
 		ECN:            0,
 	})
 	return nil
@@ -65,7 +64,6 @@ func (f *FeedbackAdapter) onSentTWCC(ts time.Time, extID uint8, header *rtp.Head
 		Size:           header.MarshalSize() + size,
 		Departure:      ts,
 		Arrival:        time.Time{},
-		RTT:            0,
 		ECN:            0,
 	})
 	return nil
@@ -83,7 +81,7 @@ func (f *FeedbackAdapter) OnSent(ts time.Time, header *rtp.Header, size int, att
 	return f.onSentRFC8888(ts, header, size)
 }
 
-func (f *FeedbackAdapter) unpackRunLengthChunk(ts time.Time, start uint16, refTime time.Time, chunk *rtcp.RunLengthChunk, deltas []*rtcp.RecvDelta) (consumedDeltas int, nextRef time.Time, acks []Acknowledgment, err error) {
+func (f *FeedbackAdapter) unpackRunLengthChunk(start uint16, refTime time.Time, chunk *rtcp.RunLengthChunk, deltas []*rtcp.RecvDelta) (consumedDeltas int, nextRef time.Time, acks []Acknowledgment, err error) {
 	result := make([]Acknowledgment, chunk.RunLength)
 	deltaIndex := 0
 
@@ -101,7 +99,6 @@ func (f *FeedbackAdapter) unpackRunLengthChunk(ts time.Time, start uint16, refTi
 				}
 				refTime = refTime.Add(time.Duration(deltas[deltaIndex].Delta) * time.Microsecond)
 				ack.Arrival = refTime
-				ack.RTT = ts.Sub(ack.Departure)
 				deltaIndex++
 			}
 			result[resultIndex] = ack
@@ -111,7 +108,7 @@ func (f *FeedbackAdapter) unpackRunLengthChunk(ts time.Time, start uint16, refTi
 	return deltaIndex, refTime, result, nil
 }
 
-func (f *FeedbackAdapter) unpackStatusVectorChunk(ts time.Time, start uint16, refTime time.Time, chunk *rtcp.StatusVectorChunk, deltas []*rtcp.RecvDelta) (consumedDeltas int, nextRef time.Time, acks []Acknowledgment, err error) {
+func (f *FeedbackAdapter) unpackStatusVectorChunk(start uint16, refTime time.Time, chunk *rtcp.StatusVectorChunk, deltas []*rtcp.RecvDelta) (consumedDeltas int, nextRef time.Time, acks []Acknowledgment, err error) {
 	result := make([]Acknowledgment, len(chunk.SymbolList))
 	deltaIndex := 0
 	resultIndex := 0
@@ -127,7 +124,6 @@ func (f *FeedbackAdapter) unpackStatusVectorChunk(ts time.Time, start uint16, re
 				}
 				refTime = refTime.Add(time.Duration(deltas[deltaIndex].Delta) * time.Microsecond)
 				ack.Arrival = refTime
-				ack.RTT = ts.Sub(ack.Departure)
 				deltaIndex++
 			}
 			result[resultIndex] = ack
@@ -152,7 +148,7 @@ func (f *FeedbackAdapter) OnTransportCCFeedback(ts time.Time, feedback *rtcp.Tra
 	for _, chunk := range feedback.PacketChunks {
 		switch chunk := chunk.(type) {
 		case *rtcp.RunLengthChunk:
-			n, nextRefTime, acks, err := f.unpackRunLengthChunk(ts, index, refTime, chunk, recvDeltas)
+			n, nextRefTime, acks, err := f.unpackRunLengthChunk(index, refTime, chunk, recvDeltas)
 			if err != nil {
 				return nil, err
 			}
@@ -161,7 +157,7 @@ func (f *FeedbackAdapter) OnTransportCCFeedback(ts time.Time, feedback *rtcp.Tra
 			recvDeltas = recvDeltas[n:]
 			index = uint16(int(index) + len(acks))
 		case *rtcp.StatusVectorChunk:
-			n, nextRefTime, acks, err := f.unpackStatusVectorChunk(ts, index, refTime, chunk, recvDeltas)
+			n, nextRefTime, acks, err := f.unpackStatusVectorChunk(index, refTime, chunk, recvDeltas)
 			if err != nil {
 				return nil, err
 			}
@@ -196,7 +192,6 @@ func (f *FeedbackAdapter) OnRFC8888Feedback(ts time.Time, feedback *rtcp.CCFeedb
 				if mb.Received {
 					delta := time.Duration((float64(mb.ArrivalTimeOffset) / 1024.0) * float64(time.Second))
 					ack.Arrival = referenceTime.Add(-delta)
-					ack.RTT = ts.Sub(ack.Departure)
 					ack.ECN = mb.ECN
 				}
 				result = append(result, ack)