don't use a lower RTT than 5ms after receiving a Retry packet

internal/ackhandler/sent_packet_handler.go

@@ -21,6 +21,8 @@ const (
 	packetThreshold = 3
 	// Before validating the client's address, the server won't send more than 3x bytes than it received.
 	amplificationFactor = 3
+	// We use Retry packets to derive an RTT estimate. Make sure we don't set the RTT to a super low value yet.
+	minRTTAfterRetry = 5 * time.Millisecond
 )
 
 type packetNumberSpace struct {
@@ -768,8 +770,9 @@ func (h *sentPacketHandler) ResetForRetry() error {
 	// 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 {
+		// Don't set the RTT to a value lower than 5ms here.
 		now := time.Now()
-		h.rttStats.UpdateRTT(now.Sub(firstPacketSendTime), 0, now)
+		h.rttStats.UpdateRTT(utils.MaxDuration(minRTTAfterRetry, now.Sub(firstPacketSendTime)), 0, now)
 		if h.logger.Debug() {
 			h.logger.Debugf("\tupdated RTT: %s (σ: %s)", h.rttStats.SmoothedRTT(), h.rttStats.MeanDeviation())
 		}

internal/ackhandler/sent_packet_handler_test.go

@@ -1201,6 +1201,21 @@ var _ = Describe("SentPacketHandler", func() {
 			Expect(handler.rttStats.SmoothedRTT()).To(BeNumerically("~", 500*time.Millisecond, 100*time.Millisecond))
 		})
 
+		It("uses a Retry for an RTT estimate, but doesn't set the RTT to a value lower than 5ms", func() {
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    42,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-500 * time.Microsecond),
+			}))
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber:    43,
+				EncryptionLevel: protocol.EncryptionInitial,
+				SendTime:        time.Now().Add(-10 * time.Microsecond),
+			}))
+			Expect(handler.ResetForRetry()).To(Succeed())
+			Expect(handler.rttStats.SmoothedRTT()).To(Equal(minRTTAfterRetry))
+		})
+
 		It("doesn't use a Retry for an RTT estimate, if it was not retransmitted", func() {
 			handler.SentPacket(ackElicitingPacket(&Packet{
 				PacketNumber:    42,