remove the N connection simulation from the Reno code

internal/congestion/cubic_sender.go

@@ -10,12 +10,12 @@ import (
 const (
 	// maxDatagramSize is the default maximum packet size used in the Linux TCP implementation.
 	// Used in QUIC for congestion window computations in bytes.
-	maxDatagramSize                 = protocol.ByteCount(protocol.MaxPacketSizeIPv4)
-	maxBurstBytes                   = 3 * maxDatagramSize
-	renoBeta                float32 = 0.7 // Reno backoff factor.
-	maxCongestionWindow             = protocol.MaxCongestionWindowPackets * maxDatagramSize
-	minCongestionWindow             = 2 * maxDatagramSize
-	initialCongestionWindow         = 32 * maxDatagramSize
+	maxDatagramSize         = protocol.ByteCount(protocol.MaxPacketSizeIPv4)
+	maxBurstBytes           = 3 * maxDatagramSize
+	renoBeta                = 0.7 // Reno backoff factor.
+	maxCongestionWindow     = protocol.MaxCongestionWindowPackets * maxDatagramSize
+	minCongestionWindow     = 2 * maxDatagramSize
+	initialCongestionWindow = 32 * maxDatagramSize
 )
 
 type cubicSender struct {
@@ -52,9 +52,6 @@ type cubicSender struct {
 	// Slow start congestion window in bytes, aka ssthresh.
 	slowStartThreshold protocol.ByteCount
 
-	// Number of connections to simulate.
-	numConnections int
-
 	// ACK counter for the Reno implementation.
 	numAckedPackets uint64
 
@@ -82,7 +79,6 @@ func newCubicSender(clock Clock, rttStats *RTTStats, reno bool, initialCongestio
 		minCongestionWindow:        minCongestionWindow,
 		slowStartThreshold:         initialMaxCongestionWindow,
 		maxCongestionWindow:        initialMaxCongestionWindow,
-		numConnections:             defaultNumConnections,
 		cubic:                      NewCubic(clock),
 		clock:                      clock,
 		reno:                       reno,
@@ -167,7 +163,7 @@ func (c *cubicSender) OnPacketLost(
 	c.lastCutbackExitedSlowstart = c.InSlowStart()
 
 	if c.reno {
-		c.congestionWindow = protocol.ByteCount(float32(c.congestionWindow) * c.renoBeta())
+		c.congestionWindow = protocol.ByteCount(float64(c.congestionWindow) * renoBeta)
 	} else {
 		c.congestionWindow = c.cubic.CongestionWindowAfterPacketLoss(c.congestionWindow)
 	}
@@ -181,14 +177,6 @@ func (c *cubicSender) OnPacketLost(
 	c.numAckedPackets = 0
 }
 
-func (c *cubicSender) renoBeta() float32 {
-	// kNConnectionBeta is the backoff factor after loss for our N-connection
-	// emulation, which emulates the effective backoff of an ensemble of N
-	// TCP-Reno connections on a single loss event. The effective multiplier is
-	// computed as:
-	return (float32(c.numConnections) - 1. + renoBeta) / float32(c.numConnections)
-}
-
 // Called when we receive an ack. Normal TCP tracks how many packets one ack
 // represents, but quic has a separate ack for each packet.
 func (c *cubicSender) maybeIncreaseCwnd(
@@ -215,9 +203,7 @@ func (c *cubicSender) maybeIncreaseCwnd(
 	if c.reno {
 		// Classic Reno congestion avoidance.
 		c.numAckedPackets++
-		// Divide by num_connections to smoothly increase the CWND at a faster
-		// rate than conventional Reno.
-		if c.numAckedPackets*uint64(c.numConnections) >= uint64(c.congestionWindow)/uint64(maxDatagramSize) {
+		if c.numAckedPackets >= uint64(c.congestionWindow/maxDatagramSize) {
 			c.congestionWindow += maxDatagramSize
 			c.numAckedPackets = 0
 		}
@@ -246,12 +232,6 @@ func (c *cubicSender) BandwidthEstimate() Bandwidth {
 	return BandwidthFromDelta(c.GetCongestionWindow(), srtt)
 }
 
-// SetNumEmulatedConnections sets the number of emulated connections
-func (c *cubicSender) SetNumEmulatedConnections(n int) {
-	c.numConnections = utils.Max(n, 1)
-	c.cubic.SetNumConnections(c.numConnections)
-}
-
 // OnRetransmissionTimeout is called on an retransmission timeout
 func (c *cubicSender) OnRetransmissionTimeout(packetsRetransmitted bool) {
 	c.largestSentAtLastCutback = protocol.InvalidPacketNumber

internal/congestion/cubic_sender_test.go

@@ -145,7 +145,6 @@ var _ = Describe("Cubic Sender", func() {
 	})
 
 	It("slow start packet loss", func() {
-		sender.SetNumEmulatedConnections(1)
 		const numberOfAcks = 10
 		for i := 0; i < numberOfAcks; i++ {
 			// Send our full send window.
@@ -188,7 +187,6 @@ var _ = Describe("Cubic Sender", func() {
 	})
 
 	It("slow start packet loss PRR", func() {
-		sender.SetNumEmulatedConnections(1)
 		// Test based on the first example in RFC6937.
 		// Ack 10 packets in 5 acks to raise the CWND to 20, as in the example.
 		const numberOfAcks = 5
@@ -235,7 +233,6 @@ var _ = Describe("Cubic Sender", func() {
 	})
 
 	It("slow start burst packet loss PRR", func() {
-		sender.SetNumEmulatedConnections(1)
 		// Test based on the second example in RFC6937, though we also implement
 		// forward acknowledgements, so the first two incoming acks will trigger
 		// PRR immediately.
@@ -364,63 +361,7 @@ var _ = Describe("Cubic Sender", func() {
 		Expect(postLossWindow).To(BeNumerically(">", sender.GetCongestionWindow()))
 	})
 
-	It("2 connection congestion avoidance at end of recovery", func() {
-		sender.SetNumEmulatedConnections(2)
-		// Ack 10 packets in 5 acks to raise the CWND to 20.
-		const numberOfAcks = 5
-		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))
-
-		LoseNPackets(1)
-
-		// We should now have fallen out of slow start with a reduced window.
-		expectedSendWindow = protocol.ByteCount(float32(expectedSendWindow) * sender.renoBeta())
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// No congestion window growth should occur in recovery phase, i.e., until the
-		// currently outstanding 20 packets are acked.
-		for i := 0; i < 10; i++ {
-			// Send our full send window.
-			SendAvailableSendWindow()
-			Expect(sender.InRecovery()).To(BeTrue())
-			AckNPackets(2)
-			Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-		}
-		Expect(sender.InRecovery()).To(BeFalse())
-
-		// Out of recovery now. Congestion window should not grow for half an RTT.
-		packetsInSendWindow := expectedSendWindow / maxDatagramSize
-		SendAvailableSendWindow()
-		AckNPackets(int(packetsInSendWindow/2 - 2))
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Next ack should increase congestion window by 1MSS.
-		SendAvailableSendWindow()
-		AckNPackets(2)
-		expectedSendWindow += maxDatagramSize
-		packetsInSendWindow++
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Congestion window should remain steady again for half an RTT.
-		SendAvailableSendWindow()
-		AckNPackets(int(packetsInSendWindow/2 - 1))
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-
-		// Next ack should cause congestion window to grow by 1MSS.
-		SendAvailableSendWindow()
-		AckNPackets(2)
-		expectedSendWindow += maxDatagramSize
-		Expect(sender.GetCongestionWindow()).To(Equal(expectedSendWindow))
-	})
-
 	It("1 connection congestion avoidance at end of recovery", func() {
-		sender.SetNumEmulatedConnections(1)
 		// Ack 10 packets in 5 acks to raise the CWND to 20.
 		const numberOfAcks = 5
 		for i := 0; i < numberOfAcks; i++ {
@@ -465,8 +406,6 @@ var _ = Describe("Cubic Sender", func() {
 	})
 
 	It("no PRR", func() {
-		sender.SetNumEmulatedConnections(1)
-
 		SendAvailableSendWindow()
 		LoseNPackets(9)
 		AckNPackets(1)
@@ -482,7 +421,6 @@ var _ = Describe("Cubic Sender", func() {
 		Expect(sender.slowStartThreshold).To(Equal(MaxCongestionWindow))
 
 		// Starts with slow start.
-		sender.SetNumEmulatedConnections(1)
 		const numberOfAcks = 10
 		for i := 0; i < numberOfAcks; i++ {
 			// Send our full send window.