Cubic implementation for Netstack.

This CL implements CUBIC as described in https://tools.ietf.org/html/rfc8312.

PiperOrigin-RevId: 207353142
Change-Id: I329cbf3277f91127e99e488f07d906f6779c6603

8 files changed, 478 insertions, 19 deletions

diff --git a/pkg/tcpip/transport/tcp/BUILD b/pkg/tcpip/transport/tcp/BUILD
index 9ebae6cc7..8b911c295 100644
--- a/pkg/tcpip/transport/tcp/BUILD
+++ b/pkg/tcpip/transport/tcp/BUILD
@@ -19,6 +19,7 @@ go_library(
     srcs = [
         "accept.go",
         "connect.go",
+        "cubic.go",
         "endpoint.go",
         "endpoint_state.go",
         "forwarder.go",
diff --git a/pkg/tcpip/transport/tcp/cubic.go b/pkg/tcpip/transport/tcp/cubic.go
new file mode 100644
index 000000000..cdb85598d
--- /dev/null
+++ b/pkg/tcpip/transport/tcp/cubic.go
@@ -0,0 +1,230 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcp
+
+import (
+	"math"
+	"time"
+)
+
+// cubicState stores the variables related to TCP CUBIC congestion
+// control algorithm state.
+//
+// See: https://tools.ietf.org/html/rfc8312.
+type cubicState struct {
+	// wLastMax is the previous wMax value.
+	wLastMax float64
+
+	// wMax is the value of the congestion window at the
+	// time of last congestion event.
+	wMax float64
+
+	// t denotes the time when the current congestion avoidance
+	// was entered.
+	t time.Time
+
+	// numCongestionEvents tracks the number of congestion events since last
+	// RTO.
+	numCongestionEvents int
+
+	// c is the cubic constant as specified in RFC8312. It's fixed at 0.4 as
+	// per RFC.
+	c float64
+
+	// k is the time period that the above function takes to increase the
+	// current window size to W_max if there are no further congestion
+	// events and is calculated using the following equation:
+	//
+	// K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2)
+	k float64
+
+	// beta is the CUBIC multiplication decrease factor. that is, when a
+	// congestion event is detected, CUBIC reduces its cwnd to
+	// W_cubic(0)=W_max*beta_cubic.
+	beta float64
+
+	// wC is window computed by CUBIC at time t. It's calculated using the
+	// formula:
+	//
+	//  W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
+	wC float64
+
+	// wEst is the window computed by CUBIC at time t+RTT i.e
+	// W_cubic(t+RTT).
+	wEst float64
+
+	s *sender
+}
+
+// newCubicCC returns a partially initialized cubic state with the constants
+// beta and c set and t set to current time.
+func newCubicCC(s *sender) *cubicState {
+	return &cubicState{
+		t:    time.Now(),
+		beta: 0.7,
+		c:    0.4,
+		s:    s,
+	}
+}
+
+// enterCongestionAvoidance is used to initialize cubic in cases where we exit
+// SlowStart without a real congestion event taking place. This can happen when
+// a connection goes back to slow start due to a retransmit and we exceed the
+// previously lowered ssThresh without experiencing packet loss.
+//
+// Refer: https://tools.ietf.org/html/rfc8312#section-4.8
+func (c *cubicState) enterCongestionAvoidance() {
+	// See: https://tools.ietf.org/html/rfc8312#section-4.7 &
+	// https://tools.ietf.org/html/rfc8312#section-4.8
+	if c.numCongestionEvents == 0 {
+		c.k = 0
+		c.t = time.Now()
+		c.wLastMax = c.wMax
+		c.wMax = float64(c.s.sndCwnd)
+	}
+}
+
+// updateSlowStart will update the congestion window as per the slow-start
+// algorithm used by NewReno. If after adjusting the congestion window we cross
+// the ssThresh then it will return the number of packets that must be consumed
+// in congestion avoidance mode.
+func (c *cubicState) updateSlowStart(packetsAcked int) int {
+	// Don't let the congestion window cross into the congestion
+	// avoidance range.
+	newcwnd := c.s.sndCwnd + packetsAcked
+	enterCA := false
+	if newcwnd >= c.s.sndSsthresh {
+		newcwnd = c.s.sndSsthresh
+		c.s.sndCAAckCount = 0
+		enterCA = true
+	}
+
+	packetsAcked -= newcwnd - c.s.sndCwnd
+	c.s.sndCwnd = newcwnd
+	if enterCA {
+		c.enterCongestionAvoidance()
+	}
+	return packetsAcked
+}
+
+// Update updates cubic's internal state variables. It must be called on every
+// ACK received.
+// Refer: https://tools.ietf.org/html/rfc8312#section-4
+func (c *cubicState) Update(packetsAcked int) {
+	if c.s.sndCwnd < c.s.sndSsthresh {
+		packetsAcked = c.updateSlowStart(packetsAcked)
+		if packetsAcked == 0 {
+			return
+		}
+	} else {
+		c.s.sndCwnd = c.getCwnd(packetsAcked, c.s.sndCwnd, c.s.srtt)
+	}
+}
+
+// cubicCwnd computes the CUBIC congestion window after t seconds from last
+// congestion event.
+func (c *cubicState) cubicCwnd(t float64) float64 {
+	return c.c*math.Pow(t, 3.0) + c.wMax
+}
+
+// getCwnd returns the current congestion window as computed by CUBIC.
+// Refer: https://tools.ietf.org/html/rfc8312#section-4
+func (c *cubicState) getCwnd(packetsAcked, sndCwnd int, srtt time.Duration) int {
+	elapsed := time.Since(c.t).Seconds()
+
+	// Compute the window as per Cubic after 'elapsed' time
+	// since last congestion event.
+	c.wC = c.cubicCwnd(elapsed - c.k)
+
+	// Compute the TCP friendly estimate of the congestion window.
+	c.wEst = c.wMax*c.beta + (3.0*((1.0-c.beta)/(1.0+c.beta)))*(elapsed/srtt.Seconds())
+
+	// Make sure in the TCP friendly region CUBIC performs at least
+	// as well as Reno.
+	if c.wC < c.wEst && float64(sndCwnd) < c.wEst {
+		// TCP Friendly region of cubic.
+		return int(c.wEst)
+	}
+
+	// In Concave/Convex region of CUBIC, calculate what CUBIC window
+	// will be after 1 RTT and use that to grow congestion window
+	// for every ack.
+	tEst := (time.Since(c.t) + srtt).Seconds()
+	wtRtt := c.cubicCwnd(tEst - c.k)
+	// As per 4.3 for each received ACK cwnd must be incremented
+	// by (w_cubic(t+RTT) - cwnd/cwnd.
+	cwnd := float64(sndCwnd)
+	for i := 0; i < packetsAcked; i++ {
+		// Concave/Convex regions of cubic have the same formulas.
+		// See: https://tools.ietf.org/html/rfc8312#section-4.3
+		cwnd += (wtRtt - cwnd) / cwnd
+	}
+	return int(cwnd)
+}
+
+// HandleNDupAcks implements congestionControl.HandleNDupAcks.
+func (c *cubicState) HandleNDupAcks() {
+	// See: https://tools.ietf.org/html/rfc8312#section-4.5
+	c.numCongestionEvents++
+	c.t = time.Now()
+	c.wLastMax = c.wMax
+	c.wMax = float64(c.s.sndCwnd)
+
+	c.fastConvergence()
+	c.reduceSlowStartThreshold()
+}
+
+// HandleRTOExpired implements congestionContrl.HandleRTOExpired.
+func (c *cubicState) HandleRTOExpired() {
+	// See: https://tools.ietf.org/html/rfc8312#section-4.6
+	c.t = time.Now()
+	c.numCongestionEvents = 0
+	c.wLastMax = c.wMax
+	c.wMax = float64(c.s.sndCwnd)
+
+	c.fastConvergence()
+
+	// We lost a packet, so reduce ssthresh.
+	c.reduceSlowStartThreshold()
+
+	// Reduce the congestion window to 1, i.e., enter slow-start. Per
+	// RFC 5681, page 7, we must use 1 regardless of the value of the
+	// initial congestion window.
+	c.s.sndCwnd = 1
+}
+
+// fastConvergence implements the logic for Fast Convergence algorithm as
+// described in https://tools.ietf.org/html/rfc8312#section-4.6.
+func (c *cubicState) fastConvergence() {
+	if c.wMax < c.wLastMax {
+		c.wLastMax = c.wMax
+		c.wMax = c.wMax * (1.0 + c.beta) / 2.0
+	} else {
+		c.wLastMax = c.wMax
+	}
+	// Recompute k as wMax may have changed.
+	c.k = math.Cbrt(c.wMax * (1 - c.beta) / c.c)
+}
+
+// PostRecovery implemements congestionControl.PostRecovery.
+func (c *cubicState) PostRecovery() {
+	c.t = time.Now()
+}
+
+// reduceSlowStartThreshold returns new SsThresh as described in
+// https://tools.ietf.org/html/rfc8312#section-4.7.
+func (c *cubicState) reduceSlowStartThreshold() {
+	c.s.sndSsthresh = int(math.Max(float64(c.s.sndCwnd)*c.beta, 2.0))
+}
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index de1883d84..8bfb68f91 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -187,6 +187,10 @@ type endpoint struct {
 	sndWaker      sleep.Waker `state:"manual"`
 	sndCloseWaker sleep.Waker `state:"manual"`
 
+	// cc stores the name of the Congestion Control algorithm to use for
+	// this endpoint.
+	cc CongestionControlOption
+
 	// The following are used when a "packet too big" control packet is
 	// received. They are protected by sndBufMu. They are used to
 	// communicate to the main protocol goroutine how many such control
@@ -254,6 +258,11 @@ func newEndpoint(stack *stack.Stack, netProto tcpip.NetworkProtocolNumber, waite
 		e.rcvBufSize = rs.Default
 	}
 
+	var cs CongestionControlOption
+	if err := stack.TransportProtocolOption(ProtocolNumber, &cs); err == nil {
+		e.cc = cs
+	}
+
 	if p := stack.GetTCPProbe(); p != nil {
 		e.probe = p
 	}
diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index cbe0e564e..194d3f41d 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -69,6 +69,11 @@ type ReceiveBufferSizeOption struct {
 	Max     int
 }
 
+const (
+	ccReno  = "reno"
+	ccCubic = "cubic"
+)
+
 // CongestionControlOption sets the current congestion control algorithm.
 type CongestionControlOption string
 
@@ -227,8 +232,8 @@ func init() {
 		return &protocol{
 			sendBufferSize:             SendBufferSizeOption{minBufferSize, DefaultBufferSize, maxBufferSize},
 			recvBufferSize:             ReceiveBufferSizeOption{minBufferSize, DefaultBufferSize, maxBufferSize},
-			congestionControl:          "reno",
-			availableCongestionControl: []string{"reno"},
+			congestionControl:          ccReno,
+			availableCongestionControl: []string{ccReno, ccCubic},
 		}
 	})
 }
diff --git a/pkg/tcpip/transport/tcp/reno.go b/pkg/tcpip/transport/tcp/reno.go
index 03ae8d747..feb593234 100644
--- a/pkg/tcpip/transport/tcp/reno.go
+++ b/pkg/tcpip/transport/tcp/reno.go
@@ -96,3 +96,8 @@ func (r *renoState) HandleRTOExpired() {
 	// initial congestion window.
 	r.s.sndCwnd = 1
 }
+
+// PostRecovery implements congestionControl.PostRecovery.
+func (r *renoState) PostRecovery() {
+	// noop.
+}
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index 376e81846..568bd7024 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -51,6 +51,11 @@ type congestionControl interface {
 	// number of packet's that were acked by the most recent cumulative
 	// acknowledgement.
 	Update(packetsAcked int)
+
+	// PostRecovery is invoked when the sender is exiting a fast retransmit/
+	// recovery phase. This provides congestion control algorithms a way
+	// to adjust their state when exiting recovery.
+	PostRecovery()
 }
 
 // sender holds the state necessary to send TCP segments.
@@ -174,7 +179,7 @@ func newSender(ep *endpoint, iss, irs seqnum.Value, sndWnd seqnum.Size, mss uint
 		},
 	}
 
-	s.cc = newRenoCC(s)
+	s.cc = s.initCongestionControl(ep.cc)
 
 	// A negative sndWndScale means that no scaling is in use, otherwise we
 	// store the scaling value.
@@ -189,6 +194,17 @@ func newSender(ep *endpoint, iss, irs seqnum.Value, sndWnd seqnum.Size, mss uint
 	return s
 }
 
+func (s *sender) initCongestionControl(congestionControlName CongestionControlOption) congestionControl {
+	switch congestionControlName {
+	case ccCubic:
+		return newCubicCC(s)
+	case ccReno:
+		fallthrough
+	default:
+		return newRenoCC(s)
+	}
+}
+
 // updateMaxPayloadSize updates the maximum payload size based on the given
 // MTU. If this is in response to "packet too big" control packets (indicated
 // by the count argument), it also reduces the number of outstanding packets and
@@ -409,6 +425,7 @@ func (s *sender) sendData() {
 }
 
 func (s *sender) enterFastRecovery() {
+	s.fr.active = true
 	// Save state to reflect we're now in fast recovery.
 	// See : https://tools.ietf.org/html/rfc5681#section-3.2 Step 3.
 	// We inflat the cwnd by 3 to account for the 3 packets which triggered
@@ -417,7 +434,6 @@ func (s *sender) enterFastRecovery() {
 	s.fr.first = s.sndUna
 	s.fr.last = s.sndNxt - 1
 	s.fr.maxCwnd = s.sndCwnd + s.outstanding
-	s.fr.active = true
 }
 
 func (s *sender) leaveFastRecovery() {
@@ -429,12 +445,13 @@ func (s *sender) leaveFastRecovery() {
 
 	// Deflate cwnd. It had been artificially inflated when new dups arrived.
 	s.sndCwnd = s.sndSsthresh
+	s.cc.PostRecovery()
 }
 
 // checkDuplicateAck is called when an ack is received. It manages the state
 // related to duplicate acks and determines if a retransmit is needed according
 // to the rules in RFC 6582 (NewReno).
-func (s *sender) checkDuplicateAck(seg *segment) bool {
+func (s *sender) checkDuplicateAck(seg *segment) (rtx bool) {
 	ack := seg.ackNumber
 	if s.fr.active {
 		// We are in fast recovery mode. Ignore the ack if it's out of
@@ -474,6 +491,7 @@ func (s *sender) checkDuplicateAck(seg *segment) bool {
 		//
 		// N.B. The retransmit timer will be reset by the caller.
 		s.fr.first = ack
+		s.dupAckCount = 0
 		return true
 	}
 
@@ -508,16 +526,11 @@ func (s *sender) checkDuplicateAck(seg *segment) bool {
 	return true
 }
 
-// updateCwnd updates the congestion window based on the number of packets that
-// were acknowledged.
-func (s *sender) updateCwnd(packetsAcked int) {
-}
-
 // handleRcvdSegment is called when a segment is received; it is responsible for
 // updating the send-related state.
 func (s *sender) handleRcvdSegment(seg *segment) {
 	// Check if we can extract an RTT measurement from this ack.
-	if s.rttMeasureSeqNum.LessThan(seg.ackNumber) {
+	if !s.ep.sendTSOk && s.rttMeasureSeqNum.LessThan(seg.ackNumber) {
 		s.updateRTO(time.Now().Sub(s.rttMeasureTime))
 		s.rttMeasureSeqNum = s.sndNxt
 	}
@@ -534,10 +547,25 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 	// Ignore ack if it doesn't acknowledge any new data.
 	ack := seg.ackNumber
 	if (ack - 1).InRange(s.sndUna, s.sndNxt) {
+		s.dupAckCount = 0
 		// When an ack is received we must reset the timer. We stop it
 		// here and it will be restarted later if needed.
 		s.resendTimer.disable()
 
+		// See : https://tools.ietf.org/html/rfc1323#section-3.3.
+		// Specifically we should only update the RTO using TSEcr if the
+		// following condition holds:
+		//
+		//    A TSecr value received in a segment is used to update the
+		//    averaged RTT measurement only if the segment acknowledges
+		//    some new data, i.e., only if it advances the left edge of
+		//    the send window.
+		if s.ep.sendTSOk && seg.parsedOptions.TSEcr != 0 {
+			// TSVal/Ecr values sent by Netstack are at a millisecond
+			// granularity.
+			elapsed := time.Duration(s.ep.timestamp()-seg.parsedOptions.TSEcr) * time.Millisecond
+			s.updateRTO(elapsed)
+		}
 		// Remove all acknowledged data from the write list.
 		acked := s.sndUna.Size(ack)
 		s.sndUna = ack
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 45ebca5b1..11410b050 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -17,6 +17,7 @@ package tcp_test
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"testing"
 	"time"
 
@@ -2005,7 +2006,7 @@ func TestCongestionAvoidance(t *testing.T) {
 
 		// Check we don't receive any more packets on this iteration.
 		// The timeout can't be too high or we'll trigger a timeout.
-		c.CheckNoPacketTimeout("More packets received than expected for this cwnd.", 50*time.Millisecond)
+		c.CheckNoPacketTimeout("More packets received than expected for this cwnd (slow start phase).", 50*time.Millisecond)
 	}
 
 	// Don't acknowledge the first packet of the last packet train. Let's
@@ -2043,7 +2044,7 @@ func TestCongestionAvoidance(t *testing.T) {
 
 		// Check we don't receive any more packets on this iteration.
 		// The timeout can't be too high or we'll trigger a timeout.
-		c.CheckNoPacketTimeout("More packets received than expected for this cwnd.", 50*time.Millisecond)
+		c.CheckNoPacketTimeout("More packets received than expected for this cwnd (congestion avoidance phase).", 50*time.Millisecond)
 
 		// Acknowledge all the data received so far.
 		c.SendAck(790, bytesRead)
@@ -2054,6 +2055,130 @@ func TestCongestionAvoidance(t *testing.T) {
 	}
 }
 
+// cubicCwnd returns an estimate of a cubic window given the
+// originalCwnd, wMax, last congestion event time and sRTT.
+func cubicCwnd(origCwnd int, wMax int, congEventTime time.Time, sRTT time.Duration) int {
+	cwnd := float64(origCwnd)
+	// We wait 50ms between each iteration so sRTT as computed by cubic
+	// should be close to 50ms.
+	elapsed := (time.Since(congEventTime) + sRTT).Seconds()
+	k := math.Cbrt(float64(wMax) * 0.3 / 0.7)
+	wtRTT := 0.4*math.Pow(elapsed-k, 3) + float64(wMax)
+	cwnd += (wtRTT - cwnd) / cwnd
+	return int(cwnd)
+}
+
+func TestCubicCongestionAvoidance(t *testing.T) {
+	maxPayload := 10
+	c := context.New(t, uint32(header.TCPMinimumSize+header.IPv4MinimumSize+maxPayload))
+	defer c.Cleanup()
+
+	enableCUBIC(t, c)
+
+	c.CreateConnected(789, 30000, nil)
+
+	const iterations = 7
+	data := buffer.NewView(2 * maxPayload * (tcp.InitialCwnd << (iterations + 1)))
+
+	for i := range data {
+		data[i] = byte(i)
+	}
+
+	// Write all the data in one shot. Packets will only be written at the
+	// MTU size though.
+	if _, err := c.EP.Write(tcpip.SlicePayload(data), tcpip.WriteOptions{}); err != nil {
+		t.Fatalf("Unexpected error from Write: %v", err)
+	}
+
+	// Do slow start for a few iterations.
+	expected := tcp.InitialCwnd
+	bytesRead := 0
+	for i := 0; i < iterations; i++ {
+		expected = tcp.InitialCwnd << uint(i)
+		if i > 0 {
+			// Acknowledge all the data received so far if not on
+			// first iteration.
+			c.SendAck(790, bytesRead)
+		}
+
+		// Read all packets expected on this iteration. Don't
+		// acknowledge any of them just yet, so that we can measure the
+		// congestion window.
+		for j := 0; j < expected; j++ {
+			c.ReceiveAndCheckPacket(data, bytesRead, maxPayload)
+			bytesRead += maxPayload
+		}
+
+		// Check we don't receive any more packets on this iteration.
+		// The timeout can't be too high or we'll trigger a timeout.
+		c.CheckNoPacketTimeout("More packets received than expected for this cwnd (during slow-start phase).", 50*time.Millisecond)
+	}
+
+	// Don't acknowledge the first packet of the last packet train. Let's
+	// wait for them to time out, which will trigger a restart of slow
+	// start, and initialization of ssthresh to cwnd * 0.7.
+	rtxOffset := bytesRead - maxPayload*expected
+	c.ReceiveAndCheckPacket(data, rtxOffset, maxPayload)
+
+	// Acknowledge all pending data.
+	c.SendAck(790, bytesRead)
+
+	// Store away the time we sent the ACK and assuming a 200ms RTO
+	// we estimate that the sender will have an RTO 200ms from now
+	// and go back into slow start.
+	packetDropTime := time.Now().Add(200 * time.Millisecond)
+
+	// This part is tricky: when the timeout happened, we had "expected"
+	// packets pending, cwnd reset to 1, and ssthresh set to expected * 0.7.
+	// By acknowledging "expected" packets, the slow-start part will
+	// increase cwnd to expected/2 essentially putting the connection
+	// straight into congestion avoidance.
+	wMax := expected
+	// Lower expected as per cubic spec after a congestion event.
+	expected = int(float64(expected) * 0.7)
+	cwnd := expected
+	for i := 0; i < iterations; i++ {
+		// Cubic grows window independent of ACKs. Cubic Window growth
+		// is a function of time elapsed since last congestion event.
+		// As a result the congestion window does not grow
+		// deterministically in response to ACKs.
+		//
+		// We need to roughly estimate what the cwnd of the sender is
+		// based on when we sent the dupacks.
+		cwnd := cubicCwnd(cwnd, wMax, packetDropTime, 50*time.Millisecond)
+
+		packetsExpected := cwnd
+		for j := 0; j < packetsExpected; j++ {
+			c.ReceiveAndCheckPacket(data, bytesRead, maxPayload)
+			bytesRead += maxPayload
+		}
+		t.Logf("expected packets received, next trying to receive any extra packets that may come")
+
+		// If our estimate was correct there should be no more pending packets.
+		// We attempt to read a packet a few times with a short sleep in between
+		// to ensure that we don't see the sender send any unexpected packets.
+		packetsUnexpected := 0
+		for {
+			gotPacket := c.ReceiveNonBlockingAndCheckPacket(data, bytesRead, maxPayload)
+			if !gotPacket {
+				break
+			}
+			bytesRead += maxPayload
+			packetsUnexpected++
+			time.Sleep(1 * time.Millisecond)
+		}
+		if packetsUnexpected != 0 {
+			t.Fatalf("received %d unexpected packets for iteration %d", packetsUnexpected, i)
+		}
+		// Check we don't receive any more packets on this iteration.
+		// The timeout can't be too high or we'll trigger a timeout.
+		c.CheckNoPacketTimeout("More packets received than expected for this cwnd(congestion avoidance)", 5*time.Millisecond)
+
+		// Acknowledge all the data received so far.
+		c.SendAck(790, bytesRead)
+	}
+}
+
 func TestFastRecovery(t *testing.T) {
 	maxPayload := 10
 	c := context.New(t, uint32(header.TCPMinimumSize+header.IPv4MinimumSize+maxPayload))
@@ -2864,8 +2989,9 @@ func TestSetCongestionControl(t *testing.T) {
 		mustPass bool
 	}{
 		{"reno", true},
-		{"cubic", false},
+		{"cubic", true},
 	}
+
 	for _, tc := range testCases {
 		t.Run(fmt.Sprintf("SetTransportProtocolOption(.., %v)", tc.cc), func(t *testing.T) {
 			c := context.New(t, 1500)
@@ -2881,7 +3007,7 @@ func TestSetCongestionControl(t *testing.T) {
 			if err := s.TransportProtocolOption(tcp.ProtocolNumber, &cc); err != nil {
 				t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &cc, err)
 			}
-			if got, want := cc, tcp.CongestionControlOption("reno"); got != want {
+			if got, want := cc, tc.cc; got != want {
 				t.Fatalf("unexpected value for congestion control got: %v, want: %v", got, want)
 			}
 		})
@@ -2899,7 +3025,7 @@ func TestAvailableCongestionControl(t *testing.T) {
 	if err := s.TransportProtocolOption(tcp.ProtocolNumber, &aCC); err != nil {
 		t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &aCC, err)
 	}
-	if got, want := aCC, tcp.AvailableCongestionControlOption("reno"); got != want {
+	if got, want := aCC, tcp.AvailableCongestionControlOption("reno cubic"); got != want {
 		t.Fatalf("unexpected value for AvailableCongestionControlOption: got: %v, want: %v", got, want)
 	}
 }
@@ -2917,11 +3043,19 @@ func TestSetAvailableCongestionControl(t *testing.T) {
 	}
 
 	// Verify that we still get the expected list of congestion control options.
-	var cc tcp.CongestionControlOption
+	var cc tcp.AvailableCongestionControlOption
 	if err := s.TransportProtocolOption(tcp.ProtocolNumber, &cc); err != nil {
 		t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &cc, err)
 	}
-	if got, want := cc, tcp.CongestionControlOption("reno"); got != want {
-		t.Fatalf("unexpected value for congestion control got: %v, want: %v", got, want)
+	if got, want := cc, tcp.AvailableCongestionControlOption("reno cubic"); got != want {
+		t.Fatalf("unexpected value for available congestion control got: %v, want: %v", got, want)
+	}
+}
+
+func enableCUBIC(t *testing.T, c *context.Context) {
+	t.Helper()
+	opt := tcp.CongestionControlOption("cubic")
+	if err := c.Stack().SetTransportProtocolOption(tcp.ProtocolNumber, opt); err != nil {
+		t.Fatalf("c.s.SetTransportProtocolOption(tcp.ProtocolNumber, %v = %v", opt, err)
 	}
 }
diff --git a/pkg/tcpip/transport/tcp/testing/context/context.go b/pkg/tcpip/transport/tcp/testing/context/context.go
index e44979527..6b5786140 100644
--- a/pkg/tcpip/transport/tcp/testing/context/context.go
+++ b/pkg/tcpip/transport/tcp/testing/context/context.go
@@ -242,6 +242,27 @@ func (c *Context) GetPacket() []byte {
 	return nil
 }
 
+// GetPacketNonBlocking reads a packet from the link layer endpoint
+// and verifies that it is an IPv4 packet with the expected source
+// and destination address. If no packet is available it will return
+// nil immediately.
+func (c *Context) GetPacketNonBlocking() []byte {
+	select {
+	case p := <-c.linkEP.C:
+		if p.Proto != ipv4.ProtocolNumber {
+			c.t.Fatalf("Bad network protocol: got %v, wanted %v", p.Proto, ipv4.ProtocolNumber)
+		}
+		b := make([]byte, len(p.Header)+len(p.Payload))
+		copy(b, p.Header)
+		copy(b[len(p.Header):], p.Payload)
+
+		checker.IPv4(c.t, b, checker.SrcAddr(StackAddr), checker.DstAddr(TestAddr))
+		return b
+	default:
+		return nil
+	}
+}
+
 // SendICMPPacket builds and sends an ICMPv4 packet via the link layer endpoint.
 func (c *Context) SendICMPPacket(typ header.ICMPv4Type, code uint8, p1, p2 []byte, maxTotalSize int) {
 	// Allocate a buffer data and headers.
@@ -355,6 +376,32 @@ func (c *Context) ReceiveAndCheckPacket(data []byte, offset, size int) {
 	}
 }
 
+// ReceiveNonBlockingAndCheckPacket reads a packet from the link layer endpoint
+// and verifies that the packet packet payload of packet matches the slice of
+// data indicated by offset & size. It returns true if a packet was received and
+// processed.
+func (c *Context) ReceiveNonBlockingAndCheckPacket(data []byte, offset, size int) bool {
+	b := c.GetPacketNonBlocking()
+	if b == nil {
+		return false
+	}
+	checker.IPv4(c.t, b,
+		checker.PayloadLen(size+header.TCPMinimumSize),
+		checker.TCP(
+			checker.DstPort(TestPort),
+			checker.SeqNum(uint32(c.IRS.Add(seqnum.Size(1+offset)))),
+			checker.AckNum(uint32(seqnum.Value(testInitialSequenceNumber).Add(1))),
+			checker.TCPFlagsMatch(header.TCPFlagAck, ^uint8(header.TCPFlagPsh)),
+		),
+	)
+
+	pdata := data[offset:][:size]
+	if p := b[header.IPv4MinimumSize+header.TCPMinimumSize:]; bytes.Compare(pdata, p) != 0 {
+		c.t.Fatalf("Data is different: expected %v, got %v", pdata, p)
+	}
+	return true
+}
+
 // CreateV6Endpoint creates and initializes c.ep as a IPv6 Endpoint. If v6Only
 // is true then it sets the IP_V6ONLY option on the socket to make it a IPv6
 // only endpoint instead of a default dual stack socket.