remove the SSLR (slow start large reduction) experiment

We apparently copied that code over from Chromium's Cubic
implementation, but we certainly don't need it.

internal/congestion/cubic_sender.go

@@ -40,9 +40,6 @@ type cubicSender struct {
 	// Used for stats collection of slowstartPacketsLost
 	lastCutbackExitedSlowstart bool
 
-	// When true, exit slow start with large cutback of congestion window.
-	slowStartLargeReduction bool
-
 	// Congestion window in packets.
 	congestionWindow protocol.ByteCount
 
@@ -63,8 +60,6 @@ type cubicSender struct {
 
 	initialCongestionWindow    protocol.ByteCount
 	initialMaxCongestionWindow protocol.ByteCount
-
-	minSlowStartExitWindow protocol.ByteCount
 }
 
 var _ SendAlgorithm = &cubicSender{}
@@ -167,24 +162,11 @@ func (c *cubicSender) OnPacketLost(
 	// TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
 	// already sent should be treated as a single loss event, since it's expected.
 	if packetNumber <= c.largestSentAtLastCutback {
-		if c.lastCutbackExitedSlowstart {
-			if c.slowStartLargeReduction {
-				// Reduce congestion window by lost_bytes for every loss.
-				c.congestionWindow = utils.MaxByteCount(c.congestionWindow-lostBytes, c.minSlowStartExitWindow)
-				c.slowStartThreshold = c.congestionWindow
-			}
-		}
 		return
 	}
 	c.lastCutbackExitedSlowstart = c.InSlowStart()
 
-	// TODO(chromium): Separate out all of slow start into a separate class.
-	if c.slowStartLargeReduction && c.InSlowStart() {
-		if c.congestionWindow >= 2*c.initialCongestionWindow {
-			c.minSlowStartExitWindow = c.congestionWindow / 2
-		}
-		c.congestionWindow -= maxDatagramSize
-	} else if c.reno {
+	if c.reno {
 		c.congestionWindow = protocol.ByteCount(float32(c.congestionWindow) * c.renoBeta())
 	} else {
 		c.congestionWindow = c.cubic.CongestionWindowAfterPacketLoss(c.congestionWindow)
@@ -295,8 +277,3 @@ func (c *cubicSender) OnConnectionMigration() {
 	c.slowStartThreshold = c.initialMaxCongestionWindow
 	c.maxCongestionWindow = c.initialMaxCongestionWindow
 }
-
-// SetSlowStartLargeReduction allows enabling the SSLR experiment
-func (c *cubicSender) SetSlowStartLargeReduction(enabled bool) {
-	c.slowStartLargeReduction = enabled
-}

internal/congestion/cubic_sender_test.go

@@ -187,84 +187,6 @@ var _ = Describe("Cubic Sender", func() {
 		Expect(sender.hybridSlowStart.Started()).To(BeFalse())
 	})
 
-	It("slow start packet loss with large reduction", func() {
-		sender.SetSlowStartLargeReduction(true)
-
-		sender.SetNumEmulatedConnections(1)
-		const numberOfAcks = 10
-		for i := 0; i < numberOfAcks; i++ {
-			// Send our full send window.
-			SendAvailableSendWindow()
-			AckNPackets(2)
-		}
-		SendAvailableSendWindow()
-		expectedSendWindow := defaultWindowTCP + (maxDatagramSize * 2 * numberOfAcks)
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Lose a packet to exit slow start. We should now have fallen out of
-		// slow start with a window reduced by 1.
-		LoseNPackets(1)
-		expectedSendWindow -= maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Lose 5 packets in recovery and verify that congestion window is reduced
-		// further.
-		LoseNPackets(5)
-		expectedSendWindow -= 5 * maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		packetsInRecoveryWindow := expectedSendWindow / maxDatagramSize
-
-		// Recovery phase. We need to ack every packet in the recovery window before
-		// we exit recovery.
-		numberOfPacketsInWindow := expectedSendWindow / maxDatagramSize
-		AckNPackets(int(packetsInRecoveryWindow))
-		SendAvailableSendWindow()
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// We need to ack the rest of the window before cwnd increases by 1.
-		AckNPackets(int(numberOfPacketsInWindow - 1))
-		SendAvailableSendWindow()
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Next ack should increase cwnd by 1.
-		AckNPackets(1)
-		expectedSendWindow += maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Now RTO and ensure slow start gets reset.
-		Expect(sender.hybridSlowStart.Started()).To(BeTrue())
-		sender.OnRetransmissionTimeout(true)
-		Expect(sender.hybridSlowStart.Started()).To(BeFalse())
-	})
-
-	It("slow start half packet loss with large reduction", func() {
-		sender.SetSlowStartLargeReduction(true)
-
-		sender.SetNumEmulatedConnections(1)
-		const numberOfAcks = 10
-		for i := 0; i < numberOfAcks; i++ {
-			// Send our full send window in half sized packets.
-			SendAvailableSendWindowLen(maxDatagramSize / 2)
-			AckNPackets(2)
-		}
-		SendAvailableSendWindowLen(maxDatagramSize / 2)
-		expectedSendWindow := defaultWindowTCP + (maxDatagramSize * 2 * numberOfAcks)
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Lose a packet to exit slow start. We should now have fallen out of
-		// slow start with a window reduced by 1.
-		LoseNPackets(1)
-		expectedSendWindow -= maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Lose 10 packets in recovery and verify that congestion window is reduced
-		// by 5 packets.
-		LoseNPacketsLen(10, maxDatagramSize/2)
-		expectedSendWindow -= 5 * maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-	})
-
 	It("slow start packet loss PRR", func() {
 		sender.SetNumEmulatedConnections(1)
 		// Test based on the first example in RFC6937.