implement cubic "exponential slow start" test

congestion/cubic_sender.go

@@ -199,3 +199,13 @@ func (c *cubicSender) isCwndLimited(bytesInFlight uint64) bool {
 	slowStartLimited := c.InSlowStart() && bytesInFlight > congestionWindow/2
 	return slowStartLimited || availableBytes <= maxBurstBytes
 }
+
+// BandwidthEstimate returns the current bandwidth estimate
+func (c *cubicSender) BandwidthEstimate() Bandwidth {
+	srtt := c.rttStats.SmoothedRTT()
+	if srtt == 0 {
+		// If we haven't measured an rtt, the bandwidth estimate is unknown.
+		return 0
+	}
+	return BandwidthFromDelta(c.GetCongestionWindow(), srtt)
+}

congestion/cubic_sender_test.go

@@ -100,4 +100,23 @@ var _ = Describe("Cubic Sender", func() {
 		// half the CWND.
 		Expect(bytesToSend).To(Equal(defaultWindowTCP + protocol.DefaultTCPMSS*2*2))
 	})
+
+	It("exponential slow start", func() {
+		const kNumberOfAcks = 20
+		// At startup make sure we can send.
+		Expect(sender.TimeUntilSend(clock.Now(), 0)).To(BeZero())
+		Expect(sender.BandwidthEstimate()).To(BeZero())
+		// Make sure we can send.
+		Expect(sender.TimeUntilSend(clock.Now(), 0)).To(BeZero())
+
+		for i := 0; i < kNumberOfAcks; i++ {
+			// Send our full send window.
+			SendAvailableSendWindow(protocol.DefaultTCPMSS)
+			AckNPackets(2)
+		}
+		cwnd := sender.GetCongestionWindow()
+		Expect(cwnd).To(Equal(defaultWindowTCP + protocol.DefaultTCPMSS*2*kNumberOfAcks))
+		Expect(sender.BandwidthEstimate()).To(Equal(congestion.BandwidthFromDelta(cwnd, rttStats.SmoothedRTT())))
+	})
+
 })

congestion/interface.go

@@ -11,4 +11,5 @@ type SendAlgorithm interface {
 	OnPacketSent(sentTime time.Time, bytesInFlight uint64, packetNumber protocol.PacketNumber, bytes uint64, isRetransmittable bool) bool
 	GetCongestionWindow() uint64
 	OnCongestionEvent(rttUpdated bool, bytesInFlight uint64, ackedPackets PacketVector, lostPackets PacketVector)
+	BandwidthEstimate() Bandwidth
 }