add exponential backoff to RTO calculation

fixes #112

ackhandler/sent_packet_handler.go

@@ -42,6 +42,8 @@ type sentPacketHandler struct {
 
 	rttStats   *congestion.RTTStats
 	congestion congestion.SendAlgorithm
+
+	consecutiveRTOCount uint32
 }
 
 // NewSentPacketHandler creates a new sentPacketHandler
@@ -227,6 +229,11 @@ func (h *sentPacketHandler) ReceivedAck(ackFrame *frames.AckFrame, withPacketNum
 		}
 	}
 
+	if rttUpdated {
+		// Reset counter if a new packet was acked
+		h.consecutiveRTOCount = 0
+	}
+
 	h.garbageCollectSkippedPackets()
 
 	h.stopWaitingManager.ReceivedAck(ackFrame)
@@ -300,6 +307,7 @@ func (h *sentPacketHandler) MaybeQueueRTOs() {
 		h.queuePacketForRetransmission(el)
 		// Reset the RTO timer here, since it's not clear that this packet contained any retransmittable frames
 		h.lastSentPacketTime = time.Now()
+		h.consecutiveRTOCount++
 		return
 	}
 }
@@ -309,7 +317,10 @@ func (h *sentPacketHandler) getRTO() time.Duration {
 	if rto == 0 {
 		rto = protocol.DefaultRetransmissionTime
 	}
-	return utils.MaxDuration(rto, protocol.MinRetransmissionTime)
+	rto = utils.MaxDuration(rto, protocol.MinRetransmissionTime)
+	// Exponential backoff
+	rto *= 1 << h.consecutiveRTOCount
+	return utils.MinDuration(rto, protocol.MaxRetransmissionTime)
 }
 
 func (h *sentPacketHandler) TimeOfFirstRTO() time.Time {

ackhandler/sent_packet_handler_test.go

@@ -786,6 +786,21 @@ var _ = Describe("SentPacketHandler", func() {
 			handler.rttStats.UpdateRTT(rtt, 0, time.Now())
 			Expect(handler.getRTO()).To(Equal(protocol.MinRetransmissionTime))
 		})
+
+		It("limits RTO max", func() {
+			rtt := time.Hour
+			handler.rttStats.UpdateRTT(rtt, 0, time.Now())
+			Expect(handler.getRTO()).To(Equal(protocol.MaxRetransmissionTime))
+		})
+
+		It("implements exponential backoff", func() {
+			handler.consecutiveRTOCount = 0
+			Expect(handler.getRTO()).To(Equal(protocol.DefaultRetransmissionTime))
+			handler.consecutiveRTOCount = 1
+			Expect(handler.getRTO()).To(Equal(2 * protocol.DefaultRetransmissionTime))
+			handler.consecutiveRTOCount = 2
+			Expect(handler.getRTO()).To(Equal(4 * protocol.DefaultRetransmissionTime))
+		})
 	})
 
 	Context("RTO retransmission", func() {

protocol/protocol.go

@@ -59,5 +59,8 @@ const DefaultRetransmissionTime = 500 * time.Millisecond
 // MinRetransmissionTime is the minimum RTO time
 const MinRetransmissionTime = 200 * time.Millisecond
 
+// MaxRetransmissionTime is the maximum RTO time
+const MaxRetransmissionTime = 60 * time.Second
+
 // ClientHelloMinimumSize is the minimum size the server expects an inchoate CHLO to have.
 const ClientHelloMinimumSize = 1024