limit the exponential PTO backoff to 60s (#3595)

internal/ackhandler/sent_packet_handler.go

@@ -23,6 +23,8 @@ const (
 	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
+	// The PTO duration uses exponential backoff, but is truncated to a maximum value, as allowed by RFC 8961, section 4.4.
+	maxPTODuration = 60 * time.Second
 )
 
 type packetNumberSpace struct {
@@ -457,6 +459,14 @@ func (h *sentPacketHandler) getLossTimeAndSpace() (time.Time, protocol.Encryptio
 	return lossTime, encLevel
 }
 
+func (h *sentPacketHandler) getScaledPTO(includeMaxAckDelay bool) time.Duration {
+	pto := h.rttStats.PTO(includeMaxAckDelay) << h.ptoCount
+	if pto > maxPTODuration || pto <= 0 {
+		return maxPTODuration
+	}
+	return pto
+}
+
 // same logic as getLossTimeAndSpace, but for lastAckElicitingPacketTime instead of lossTime
 func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protocol.EncryptionLevel, ok bool) {
 	// We only send application data probe packets once the handshake is confirmed,
@@ -465,7 +475,7 @@ func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protoc
 		if h.peerCompletedAddressValidation {
 			return
 		}
-		t := time.Now().Add(h.rttStats.PTO(false) << h.ptoCount)
+		t := time.Now().Add(h.getScaledPTO(false))
 		if h.initialPackets != nil {
 			return t, protocol.EncryptionInitial, true
 		}
@@ -475,18 +485,18 @@ func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protoc
 	if h.initialPackets != nil {
 		encLevel = protocol.EncryptionInitial
 		if t := h.initialPackets.lastAckElicitingPacketTime; !t.IsZero() {
-			pto = t.Add(h.rttStats.PTO(false) << h.ptoCount)
+			pto = t.Add(h.getScaledPTO(false))
 		}
 	}
 	if h.handshakePackets != nil && !h.handshakePackets.lastAckElicitingPacketTime.IsZero() {
-		t := h.handshakePackets.lastAckElicitingPacketTime.Add(h.rttStats.PTO(false) << h.ptoCount)
+		t := h.handshakePackets.lastAckElicitingPacketTime.Add(h.getScaledPTO(false))
 		if pto.IsZero() || (!t.IsZero() && t.Before(pto)) {
 			pto = t
 			encLevel = protocol.EncryptionHandshake
 		}
 	}
 	if h.handshakeConfirmed && !h.appDataPackets.lastAckElicitingPacketTime.IsZero() {
-		t := h.appDataPackets.lastAckElicitingPacketTime.Add(h.rttStats.PTO(true) << h.ptoCount)
+		t := h.appDataPackets.lastAckElicitingPacketTime.Add(h.getScaledPTO(true))
 		if pto.IsZero() || (!t.IsZero() && t.Before(pto)) {
 			pto = t
 			encLevel = protocol.Encryption1RTT

internal/ackhandler/sent_packet_handler_test.go

@@ -687,6 +687,14 @@ var _ = Describe("SentPacketHandler", func() {
 			handler.ptoCount = 2
 			handler.setLossDetectionTimer()
 			Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(4 * timeout))
+			// truncated when the exponential gets too large
+			handler.ptoCount = 20
+			handler.setLossDetectionTimer()
+			Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(maxPTODuration))
+			// protected from rollover
+			handler.ptoCount = 100
+			handler.setLossDetectionTimer()
+			Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(maxPTODuration))
 		})
 
 		It("reset the PTO count when receiving an ACK", func() {
@@ -1036,7 +1044,7 @@ var _ = Describe("SentPacketHandler", func() {
 		})
 
 		It("correctly sets the timer after the Initial packet number space has been dropped", func() {
-			handler.SentPacket(initialPacket(&Packet{PacketNumber: 1, SendTime: time.Now().Add(-42 * time.Second)}))
+			handler.SentPacket(initialPacket(&Packet{PacketNumber: 1, SendTime: time.Now().Add(-19 * time.Second)}))
 			_, err := handler.ReceivedAck(
 				&wire.AckFrame{AckRanges: []wire.AckRange{{Smallest: 1, Largest: 1}}},
 				protocol.EncryptionInitial,
@@ -1048,6 +1056,8 @@ var _ = Describe("SentPacketHandler", func() {
 
 			pto := handler.rttStats.PTO(false)
 			Expect(pto).ToNot(BeZero())
+			// pto is approximately 19 * 3. Using a number > 19 above will
+			// run into the maxPTODuration limit
 			Expect(handler.GetLossDetectionTimeout()).To(BeTemporally("~", time.Now().Add(pto), 10*time.Millisecond))
 		})