use the receipt of a Retry packet to get a first RTT estimate

internal/ackhandler/sent_packet_handler.go

@@ -772,7 +772,11 @@ func (h *sentPacketHandler) queueFramesForRetransmission(p *Packet) {
 
 func (h *sentPacketHandler) ResetForRetry() error {
 	h.bytesInFlight = 0
+	var firstPacketSendTime time.Time
 	h.initialPackets.history.Iterate(func(p *Packet) (bool, error) {
+		if firstPacketSendTime.IsZero() {
+			firstPacketSendTime = p.SendTime
+		}
 		h.queueFramesForRetransmission(p)
 		return true, nil
 	})
@@ -783,6 +787,18 @@ func (h *sentPacketHandler) ResetForRetry() error {
 		return true, nil
 	})
 
+	// Only use the Retry to estimate the RTT if we didn't send any retransmission for the Initial.
+	// Otherwise, we don't know which Initial the Retry was sent in response to.
+	if h.ptoCount == 0 {
+		now := time.Now()
+		h.rttStats.UpdateRTT(now.Sub(firstPacketSendTime), 0, now)
+		if h.logger.Debug() {
+			h.logger.Debugf("\tupdated RTT: %s (σ: %s)", h.rttStats.SmoothedRTT(), h.rttStats.MeanDeviation())
+		}
+		if h.qlogger != nil {
+			h.qlogger.UpdatedMetrics(h.rttStats, h.congestion.GetCongestionWindow(), h.bytesInFlight, h.packetsInFlight())
+		}
+	}
 	h.initialPackets = newPacketNumberSpace(h.initialPackets.pns.Pop())
 	h.appDataPackets = newPacketNumberSpace(h.appDataPackets.pns.Pop())
 	oldAlarm := h.alarm

internal/ackhandler/sent_packet_handler_test.go

@@ -1149,5 +1149,37 @@ var _ = Describe("SentPacketHandler", func() {
 			// make sure we keep increasing the packet number for 0-RTT packets
 			Expect(handler.PopPacketNumber(protocol.Encryption0RTT)).To(BeNumerically(">", pn))
 		})
+
+		It("uses a Retry for an RTT estimate, if it was not retransmitted", func() {
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    42,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-500 * time.Millisecond),
+			}))
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    43,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-10 * time.Millisecond),
+			}))
+			Expect(handler.ResetForRetry()).To(Succeed())
+			Expect(handler.rttStats.SmoothedRTT()).To(BeNumerically("~", 500*time.Millisecond, 100*time.Millisecond))
+		})
+
+		It("doesn't use a Retry for an RTT estimate, if it was not retransmitted", func() {
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    42,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-800 * time.Millisecond),
+			}))
+			Expect(handler.OnLossDetectionTimeout()).To(Succeed())
+			Expect(handler.SendMode()).To(Equal(SendPTOInitial))
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    43,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-100 * time.Millisecond),
+			}))
+			Expect(handler.ResetForRetry()).To(Succeed())
+			Expect(handler.rttStats.SmoothedRTT()).To(BeZero())
+		})
 	})
 })