TUN-4597: Add a QUIC server skeleton

- Added a QUIC server to accept streams
- Unit test for this server also tests ALPN
- Temporary echo capability for HTTP ConnectionType

vendor/github.com/lucas-clemente/quic-go/internal/congestion/bandwidth.go

@@ -0,0 +1,25 @@
+package congestion
+
+import (
+	"math"
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// Bandwidth of a connection
+type Bandwidth uint64
+
+const infBandwidth Bandwidth = math.MaxUint64
+
+const (
+	// BitsPerSecond is 1 bit per second
+	BitsPerSecond Bandwidth = 1
+	// BytesPerSecond is 1 byte per second
+	BytesPerSecond = 8 * BitsPerSecond
+)
+
+// BandwidthFromDelta calculates the bandwidth from a number of bytes and a time delta
+func BandwidthFromDelta(bytes protocol.ByteCount, delta time.Duration) Bandwidth {
+	return Bandwidth(bytes) * Bandwidth(time.Second) / Bandwidth(delta) * BytesPerSecond
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/clock.go

@@ -0,0 +1,18 @@
+package congestion
+
+import "time"
+
+// A Clock returns the current time
+type Clock interface {
+	Now() time.Time
+}
+
+// DefaultClock implements the Clock interface using the Go stdlib clock.
+type DefaultClock struct{}
+
+var _ Clock = DefaultClock{}
+
+// Now gets the current time
+func (DefaultClock) Now() time.Time {
+	return time.Now()
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/cubic.go

@@ -0,0 +1,214 @@
+package congestion
+
+import (
+	"math"
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+	"github.com/lucas-clemente/quic-go/internal/utils"
+)
+
+// This cubic implementation is based on the one found in Chromiums's QUIC
+// implementation, in the files net/quic/congestion_control/cubic.{hh,cc}.
+
+// Constants based on TCP defaults.
+// The following constants are in 2^10 fractions of a second instead of ms to
+// allow a 10 shift right to divide.
+
+// 1024*1024^3 (first 1024 is from 0.100^3)
+// where 0.100 is 100 ms which is the scaling round trip time.
+const (
+	cubeScale                                    = 40
+	cubeCongestionWindowScale                    = 410
+	cubeFactor                protocol.ByteCount = 1 << cubeScale / cubeCongestionWindowScale / maxDatagramSize
+	// TODO: when re-enabling cubic, make sure to use the actual packet size here
+	maxDatagramSize = protocol.ByteCount(protocol.InitialPacketSizeIPv4)
+)
+
+const defaultNumConnections = 1
+
+// Default Cubic backoff factor
+const beta float32 = 0.7
+
+// Additional backoff factor when loss occurs in the concave part of the Cubic
+// curve. This additional backoff factor is expected to give up bandwidth to
+// new concurrent flows and speed up convergence.
+const betaLastMax float32 = 0.85
+
+// Cubic implements the cubic algorithm from TCP
+type Cubic struct {
+	clock Clock
+
+	// Number of connections to simulate.
+	numConnections int
+
+	// Time when this cycle started, after last loss event.
+	epoch time.Time
+
+	// Max congestion window used just before last loss event.
+	// Note: to improve fairness to other streams an additional back off is
+	// applied to this value if the new value is below our latest value.
+	lastMaxCongestionWindow protocol.ByteCount
+
+	// Number of acked bytes since the cycle started (epoch).
+	ackedBytesCount protocol.ByteCount
+
+	// TCP Reno equivalent congestion window in packets.
+	estimatedTCPcongestionWindow protocol.ByteCount
+
+	// Origin point of cubic function.
+	originPointCongestionWindow protocol.ByteCount
+
+	// Time to origin point of cubic function in 2^10 fractions of a second.
+	timeToOriginPoint uint32
+
+	// Last congestion window in packets computed by cubic function.
+	lastTargetCongestionWindow protocol.ByteCount
+}
+
+// NewCubic returns a new Cubic instance
+func NewCubic(clock Clock) *Cubic {
+	c := &Cubic{
+		clock:          clock,
+		numConnections: defaultNumConnections,
+	}
+	c.Reset()
+	return c
+}
+
+// Reset is called after a timeout to reset the cubic state
+func (c *Cubic) Reset() {
+	c.epoch = time.Time{}
+	c.lastMaxCongestionWindow = 0
+	c.ackedBytesCount = 0
+	c.estimatedTCPcongestionWindow = 0
+	c.originPointCongestionWindow = 0
+	c.timeToOriginPoint = 0
+	c.lastTargetCongestionWindow = 0
+}
+
+func (c *Cubic) alpha() float32 {
+	// TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
+	// beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
+	// We derive the equivalent alpha for an N-connection emulation as:
+	b := c.beta()
+	return 3 * float32(c.numConnections) * float32(c.numConnections) * (1 - b) / (1 + b)
+}
+
+func (c *Cubic) beta() 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 + beta) / float32(c.numConnections)
+}
+
+func (c *Cubic) betaLastMax() float32 {
+	// betaLastMax is the additional backoff factor after loss for our
+	// N-connection emulation, which emulates the additional 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 + betaLastMax) / float32(c.numConnections)
+}
+
+// OnApplicationLimited is called on ack arrival when sender is unable to use
+// the available congestion window. Resets Cubic state during quiescence.
+func (c *Cubic) OnApplicationLimited() {
+	// When sender is not using the available congestion window, the window does
+	// not grow. But to be RTT-independent, Cubic assumes that the sender has been
+	// using the entire window during the time since the beginning of the current
+	// "epoch" (the end of the last loss recovery period). Since
+	// application-limited periods break this assumption, we reset the epoch when
+	// in such a period. This reset effectively freezes congestion window growth
+	// through application-limited periods and allows Cubic growth to continue
+	// when the entire window is being used.
+	c.epoch = time.Time{}
+}
+
+// CongestionWindowAfterPacketLoss computes a new congestion window to use after
+// a loss event. Returns the new congestion window in packets. The new
+// congestion window is a multiplicative decrease of our current window.
+func (c *Cubic) CongestionWindowAfterPacketLoss(currentCongestionWindow protocol.ByteCount) protocol.ByteCount {
+	if currentCongestionWindow+maxDatagramSize < c.lastMaxCongestionWindow {
+		// We never reached the old max, so assume we are competing with another
+		// flow. Use our extra back off factor to allow the other flow to go up.
+		c.lastMaxCongestionWindow = protocol.ByteCount(c.betaLastMax() * float32(currentCongestionWindow))
+	} else {
+		c.lastMaxCongestionWindow = currentCongestionWindow
+	}
+	c.epoch = time.Time{} // Reset time.
+	return protocol.ByteCount(float32(currentCongestionWindow) * c.beta())
+}
+
+// CongestionWindowAfterAck computes a new congestion window to use after a received ACK.
+// Returns the new congestion window in packets. The new congestion window
+// follows a cubic function that depends on the time passed since last
+// packet loss.
+func (c *Cubic) CongestionWindowAfterAck(
+	ackedBytes protocol.ByteCount,
+	currentCongestionWindow protocol.ByteCount,
+	delayMin time.Duration,
+	eventTime time.Time,
+) protocol.ByteCount {
+	c.ackedBytesCount += ackedBytes
+
+	if c.epoch.IsZero() {
+		// First ACK after a loss event.
+		c.epoch = eventTime            // Start of epoch.
+		c.ackedBytesCount = ackedBytes // Reset count.
+		// Reset estimated_tcp_congestion_window_ to be in sync with cubic.
+		c.estimatedTCPcongestionWindow = currentCongestionWindow
+		if c.lastMaxCongestionWindow <= currentCongestionWindow {
+			c.timeToOriginPoint = 0
+			c.originPointCongestionWindow = currentCongestionWindow
+		} else {
+			c.timeToOriginPoint = uint32(math.Cbrt(float64(cubeFactor * (c.lastMaxCongestionWindow - currentCongestionWindow))))
+			c.originPointCongestionWindow = c.lastMaxCongestionWindow
+		}
+	}
+
+	// Change the time unit from microseconds to 2^10 fractions per second. Take
+	// the round trip time in account. This is done to allow us to use shift as a
+	// divide operator.
+	elapsedTime := int64(eventTime.Add(delayMin).Sub(c.epoch)/time.Microsecond) << 10 / (1000 * 1000)
+
+	// Right-shifts of negative, signed numbers have implementation-dependent
+	// behavior, so force the offset to be positive, as is done in the kernel.
+	offset := int64(c.timeToOriginPoint) - elapsedTime
+	if offset < 0 {
+		offset = -offset
+	}
+
+	deltaCongestionWindow := protocol.ByteCount(cubeCongestionWindowScale*offset*offset*offset) * maxDatagramSize >> cubeScale
+	var targetCongestionWindow protocol.ByteCount
+	if elapsedTime > int64(c.timeToOriginPoint) {
+		targetCongestionWindow = c.originPointCongestionWindow + deltaCongestionWindow
+	} else {
+		targetCongestionWindow = c.originPointCongestionWindow - deltaCongestionWindow
+	}
+	// Limit the CWND increase to half the acked bytes.
+	targetCongestionWindow = utils.MinByteCount(targetCongestionWindow, currentCongestionWindow+c.ackedBytesCount/2)
+
+	// Increase the window by approximately Alpha * 1 MSS of bytes every
+	// time we ack an estimated tcp window of bytes.  For small
+	// congestion windows (less than 25), the formula below will
+	// increase slightly slower than linearly per estimated tcp window
+	// of bytes.
+	c.estimatedTCPcongestionWindow += protocol.ByteCount(float32(c.ackedBytesCount) * c.alpha() * float32(maxDatagramSize) / float32(c.estimatedTCPcongestionWindow))
+	c.ackedBytesCount = 0
+
+	// We have a new cubic congestion window.
+	c.lastTargetCongestionWindow = targetCongestionWindow
+
+	// Compute target congestion_window based on cubic target and estimated TCP
+	// congestion_window, use highest (fastest).
+	if targetCongestionWindow < c.estimatedTCPcongestionWindow {
+		targetCongestionWindow = c.estimatedTCPcongestionWindow
+	}
+	return targetCongestionWindow
+}
+
+// SetNumConnections sets the number of emulated connections
+func (c *Cubic) SetNumConnections(n int) {
+	c.numConnections = n
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/cubic_sender.go

@@ -0,0 +1,316 @@
+package congestion
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+	"github.com/lucas-clemente/quic-go/internal/utils"
+	"github.com/lucas-clemente/quic-go/logging"
+)
+
+const (
+	// maxDatagramSize is the default maximum packet size used in the Linux TCP implementation.
+	// Used in QUIC for congestion window computations in bytes.
+	initialMaxDatagramSize     = protocol.ByteCount(protocol.InitialPacketSizeIPv4)
+	maxBurstPackets            = 3
+	renoBeta                   = 0.7 // Reno backoff factor.
+	minCongestionWindowPackets = 2
+	initialCongestionWindow    = 32
+)
+
+type cubicSender struct {
+	hybridSlowStart HybridSlowStart
+	rttStats        *utils.RTTStats
+	cubic           *Cubic
+	pacer           *pacer
+	clock           Clock
+
+	reno bool
+
+	// Track the largest packet that has been sent.
+	largestSentPacketNumber protocol.PacketNumber
+
+	// Track the largest packet that has been acked.
+	largestAckedPacketNumber protocol.PacketNumber
+
+	// Track the largest packet number outstanding when a CWND cutback occurs.
+	largestSentAtLastCutback protocol.PacketNumber
+
+	// Whether the last loss event caused us to exit slowstart.
+	// Used for stats collection of slowstartPacketsLost
+	lastCutbackExitedSlowstart bool
+
+	// Congestion window in packets.
+	congestionWindow protocol.ByteCount
+
+	// Slow start congestion window in bytes, aka ssthresh.
+	slowStartThreshold protocol.ByteCount
+
+	// ACK counter for the Reno implementation.
+	numAckedPackets uint64
+
+	initialCongestionWindow    protocol.ByteCount
+	initialMaxCongestionWindow protocol.ByteCount
+
+	maxDatagramSize protocol.ByteCount
+
+	lastState logging.CongestionState
+	tracer    logging.ConnectionTracer
+}
+
+var (
+	_ SendAlgorithm               = &cubicSender{}
+	_ SendAlgorithmWithDebugInfos = &cubicSender{}
+)
+
+// NewCubicSender makes a new cubic sender
+func NewCubicSender(
+	clock Clock,
+	rttStats *utils.RTTStats,
+	initialMaxDatagramSize protocol.ByteCount,
+	reno bool,
+	tracer logging.ConnectionTracer,
+) *cubicSender {
+	return newCubicSender(
+		clock,
+		rttStats,
+		reno,
+		initialMaxDatagramSize,
+		initialCongestionWindow*initialMaxDatagramSize,
+		protocol.MaxCongestionWindowPackets*initialMaxDatagramSize,
+		tracer,
+	)
+}
+
+func newCubicSender(
+	clock Clock,
+	rttStats *utils.RTTStats,
+	reno bool,
+	initialMaxDatagramSize,
+	initialCongestionWindow,
+	initialMaxCongestionWindow protocol.ByteCount,
+	tracer logging.ConnectionTracer,
+) *cubicSender {
+	c := &cubicSender{
+		rttStats:                   rttStats,
+		largestSentPacketNumber:    protocol.InvalidPacketNumber,
+		largestAckedPacketNumber:   protocol.InvalidPacketNumber,
+		largestSentAtLastCutback:   protocol.InvalidPacketNumber,
+		initialCongestionWindow:    initialCongestionWindow,
+		initialMaxCongestionWindow: initialMaxCongestionWindow,
+		congestionWindow:           initialCongestionWindow,
+		slowStartThreshold:         protocol.MaxByteCount,
+		cubic:                      NewCubic(clock),
+		clock:                      clock,
+		reno:                       reno,
+		tracer:                     tracer,
+		maxDatagramSize:            initialMaxDatagramSize,
+	}
+	c.pacer = newPacer(c.BandwidthEstimate)
+	if c.tracer != nil {
+		c.lastState = logging.CongestionStateSlowStart
+		c.tracer.UpdatedCongestionState(logging.CongestionStateSlowStart)
+	}
+	return c
+}
+
+// TimeUntilSend returns when the next packet should be sent.
+func (c *cubicSender) TimeUntilSend(_ protocol.ByteCount) time.Time {
+	return c.pacer.TimeUntilSend()
+}
+
+func (c *cubicSender) HasPacingBudget() bool {
+	return c.pacer.Budget(c.clock.Now()) >= c.maxDatagramSize
+}
+
+func (c *cubicSender) maxCongestionWindow() protocol.ByteCount {
+	return c.maxDatagramSize * protocol.MaxCongestionWindowPackets
+}
+
+func (c *cubicSender) minCongestionWindow() protocol.ByteCount {
+	return c.maxDatagramSize * minCongestionWindowPackets
+}
+
+func (c *cubicSender) OnPacketSent(
+	sentTime time.Time,
+	_ protocol.ByteCount,
+	packetNumber protocol.PacketNumber,
+	bytes protocol.ByteCount,
+	isRetransmittable bool,
+) {
+	c.pacer.SentPacket(sentTime, bytes)
+	if !isRetransmittable {
+		return
+	}
+	c.largestSentPacketNumber = packetNumber
+	c.hybridSlowStart.OnPacketSent(packetNumber)
+}
+
+func (c *cubicSender) CanSend(bytesInFlight protocol.ByteCount) bool {
+	return bytesInFlight < c.GetCongestionWindow()
+}
+
+func (c *cubicSender) InRecovery() bool {
+	return c.largestAckedPacketNumber != protocol.InvalidPacketNumber && c.largestAckedPacketNumber <= c.largestSentAtLastCutback
+}
+
+func (c *cubicSender) InSlowStart() bool {
+	return c.GetCongestionWindow() < c.slowStartThreshold
+}
+
+func (c *cubicSender) GetCongestionWindow() protocol.ByteCount {
+	return c.congestionWindow
+}
+
+func (c *cubicSender) MaybeExitSlowStart() {
+	if c.InSlowStart() &&
+		c.hybridSlowStart.ShouldExitSlowStart(c.rttStats.LatestRTT(), c.rttStats.MinRTT(), c.GetCongestionWindow()/c.maxDatagramSize) {
+		// exit slow start
+		c.slowStartThreshold = c.congestionWindow
+		c.maybeTraceStateChange(logging.CongestionStateCongestionAvoidance)
+	}
+}
+
+func (c *cubicSender) OnPacketAcked(
+	ackedPacketNumber protocol.PacketNumber,
+	ackedBytes protocol.ByteCount,
+	priorInFlight protocol.ByteCount,
+	eventTime time.Time,
+) {
+	c.largestAckedPacketNumber = utils.MaxPacketNumber(ackedPacketNumber, c.largestAckedPacketNumber)
+	if c.InRecovery() {
+		return
+	}
+	c.maybeIncreaseCwnd(ackedPacketNumber, ackedBytes, priorInFlight, eventTime)
+	if c.InSlowStart() {
+		c.hybridSlowStart.OnPacketAcked(ackedPacketNumber)
+	}
+}
+
+func (c *cubicSender) OnPacketLost(packetNumber protocol.PacketNumber, lostBytes, priorInFlight protocol.ByteCount) {
+	// 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 {
+		return
+	}
+	c.lastCutbackExitedSlowstart = c.InSlowStart()
+	c.maybeTraceStateChange(logging.CongestionStateRecovery)
+
+	if c.reno {
+		c.congestionWindow = protocol.ByteCount(float64(c.congestionWindow) * renoBeta)
+	} else {
+		c.congestionWindow = c.cubic.CongestionWindowAfterPacketLoss(c.congestionWindow)
+	}
+	if minCwnd := c.minCongestionWindow(); c.congestionWindow < minCwnd {
+		c.congestionWindow = minCwnd
+	}
+	c.slowStartThreshold = c.congestionWindow
+	c.largestSentAtLastCutback = c.largestSentPacketNumber
+	// reset packet count from congestion avoidance mode. We start
+	// counting again when we're out of recovery.
+	c.numAckedPackets = 0
+}
+
+// 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(
+	_ protocol.PacketNumber,
+	ackedBytes protocol.ByteCount,
+	priorInFlight protocol.ByteCount,
+	eventTime time.Time,
+) {
+	// Do not increase the congestion window unless the sender is close to using
+	// the current window.
+	if !c.isCwndLimited(priorInFlight) {
+		c.cubic.OnApplicationLimited()
+		c.maybeTraceStateChange(logging.CongestionStateApplicationLimited)
+		return
+	}
+	if c.congestionWindow >= c.maxCongestionWindow() {
+		return
+	}
+	if c.InSlowStart() {
+		// TCP slow start, exponential growth, increase by one for each ACK.
+		c.congestionWindow += c.maxDatagramSize
+		c.maybeTraceStateChange(logging.CongestionStateSlowStart)
+		return
+	}
+	// Congestion avoidance
+	c.maybeTraceStateChange(logging.CongestionStateCongestionAvoidance)
+	if c.reno {
+		// Classic Reno congestion avoidance.
+		c.numAckedPackets++
+		if c.numAckedPackets >= uint64(c.congestionWindow/c.maxDatagramSize) {
+			c.congestionWindow += c.maxDatagramSize
+			c.numAckedPackets = 0
+		}
+	} else {
+		c.congestionWindow = utils.MinByteCount(c.maxCongestionWindow(), c.cubic.CongestionWindowAfterAck(ackedBytes, c.congestionWindow, c.rttStats.MinRTT(), eventTime))
+	}
+}
+
+func (c *cubicSender) isCwndLimited(bytesInFlight protocol.ByteCount) bool {
+	congestionWindow := c.GetCongestionWindow()
+	if bytesInFlight >= congestionWindow {
+		return true
+	}
+	availableBytes := congestionWindow - bytesInFlight
+	slowStartLimited := c.InSlowStart() && bytesInFlight > congestionWindow/2
+	return slowStartLimited || availableBytes <= maxBurstPackets*c.maxDatagramSize
+}
+
+// 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 infBandwidth
+	}
+	return BandwidthFromDelta(c.GetCongestionWindow(), srtt)
+}
+
+// OnRetransmissionTimeout is called on an retransmission timeout
+func (c *cubicSender) OnRetransmissionTimeout(packetsRetransmitted bool) {
+	c.largestSentAtLastCutback = protocol.InvalidPacketNumber
+	if !packetsRetransmitted {
+		return
+	}
+	c.hybridSlowStart.Restart()
+	c.cubic.Reset()
+	c.slowStartThreshold = c.congestionWindow / 2
+	c.congestionWindow = c.minCongestionWindow()
+}
+
+// OnConnectionMigration is called when the connection is migrated (?)
+func (c *cubicSender) OnConnectionMigration() {
+	c.hybridSlowStart.Restart()
+	c.largestSentPacketNumber = protocol.InvalidPacketNumber
+	c.largestAckedPacketNumber = protocol.InvalidPacketNumber
+	c.largestSentAtLastCutback = protocol.InvalidPacketNumber
+	c.lastCutbackExitedSlowstart = false
+	c.cubic.Reset()
+	c.numAckedPackets = 0
+	c.congestionWindow = c.initialCongestionWindow
+	c.slowStartThreshold = c.initialMaxCongestionWindow
+}
+
+func (c *cubicSender) maybeTraceStateChange(new logging.CongestionState) {
+	if c.tracer == nil || new == c.lastState {
+		return
+	}
+	c.tracer.UpdatedCongestionState(new)
+	c.lastState = new
+}
+
+func (c *cubicSender) SetMaxDatagramSize(s protocol.ByteCount) {
+	if s < c.maxDatagramSize {
+		panic(fmt.Sprintf("congestion BUG: decreased max datagram size from %d to %d", c.maxDatagramSize, s))
+	}
+	cwndIsMinCwnd := c.congestionWindow == c.minCongestionWindow()
+	c.maxDatagramSize = s
+	if cwndIsMinCwnd {
+		c.congestionWindow = c.minCongestionWindow()
+	}
+	c.pacer.SetMaxDatagramSize(s)
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/hybrid_slow_start.go

@@ -0,0 +1,113 @@
+package congestion
+
+import (
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+	"github.com/lucas-clemente/quic-go/internal/utils"
+)
+
+// Note(pwestin): the magic clamping numbers come from the original code in
+// tcp_cubic.c.
+const hybridStartLowWindow = protocol.ByteCount(16)
+
+// Number of delay samples for detecting the increase of delay.
+const hybridStartMinSamples = uint32(8)
+
+// Exit slow start if the min rtt has increased by more than 1/8th.
+const hybridStartDelayFactorExp = 3 // 2^3 = 8
+// The original paper specifies 2 and 8ms, but those have changed over time.
+const (
+	hybridStartDelayMinThresholdUs = int64(4000)
+	hybridStartDelayMaxThresholdUs = int64(16000)
+)
+
+// HybridSlowStart implements the TCP hybrid slow start algorithm
+type HybridSlowStart struct {
+	endPacketNumber      protocol.PacketNumber
+	lastSentPacketNumber protocol.PacketNumber
+	started              bool
+	currentMinRTT        time.Duration
+	rttSampleCount       uint32
+	hystartFound         bool
+}
+
+// StartReceiveRound is called for the start of each receive round (burst) in the slow start phase.
+func (s *HybridSlowStart) StartReceiveRound(lastSent protocol.PacketNumber) {
+	s.endPacketNumber = lastSent
+	s.currentMinRTT = 0
+	s.rttSampleCount = 0
+	s.started = true
+}
+
+// IsEndOfRound returns true if this ack is the last packet number of our current slow start round.
+func (s *HybridSlowStart) IsEndOfRound(ack protocol.PacketNumber) bool {
+	return s.endPacketNumber < ack
+}
+
+// ShouldExitSlowStart should be called on every new ack frame, since a new
+// RTT measurement can be made then.
+// rtt: the RTT for this ack packet.
+// minRTT: is the lowest delay (RTT) we have seen during the session.
+// congestionWindow: the congestion window in packets.
+func (s *HybridSlowStart) ShouldExitSlowStart(latestRTT time.Duration, minRTT time.Duration, congestionWindow protocol.ByteCount) bool {
+	if !s.started {
+		// Time to start the hybrid slow start.
+		s.StartReceiveRound(s.lastSentPacketNumber)
+	}
+	if s.hystartFound {
+		return true
+	}
+	// Second detection parameter - delay increase detection.
+	// Compare the minimum delay (s.currentMinRTT) of the current
+	// burst of packets relative to the minimum delay during the session.
+	// Note: we only look at the first few(8) packets in each burst, since we
+	// only want to compare the lowest RTT of the burst relative to previous
+	// bursts.
+	s.rttSampleCount++
+	if s.rttSampleCount <= hybridStartMinSamples {
+		if s.currentMinRTT == 0 || s.currentMinRTT > latestRTT {
+			s.currentMinRTT = latestRTT
+		}
+	}
+	// We only need to check this once per round.
+	if s.rttSampleCount == hybridStartMinSamples {
+		// Divide minRTT by 8 to get a rtt increase threshold for exiting.
+		minRTTincreaseThresholdUs := int64(minRTT / time.Microsecond >> hybridStartDelayFactorExp)
+		// Ensure the rtt threshold is never less than 2ms or more than 16ms.
+		minRTTincreaseThresholdUs = utils.MinInt64(minRTTincreaseThresholdUs, hybridStartDelayMaxThresholdUs)
+		minRTTincreaseThreshold := time.Duration(utils.MaxInt64(minRTTincreaseThresholdUs, hybridStartDelayMinThresholdUs)) * time.Microsecond
+
+		if s.currentMinRTT > (minRTT + minRTTincreaseThreshold) {
+			s.hystartFound = true
+		}
+	}
+	// Exit from slow start if the cwnd is greater than 16 and
+	// increasing delay is found.
+	return congestionWindow >= hybridStartLowWindow && s.hystartFound
+}
+
+// OnPacketSent is called when a packet was sent
+func (s *HybridSlowStart) OnPacketSent(packetNumber protocol.PacketNumber) {
+	s.lastSentPacketNumber = packetNumber
+}
+
+// OnPacketAcked gets invoked after ShouldExitSlowStart, so it's best to end
+// the round when the final packet of the burst is received and start it on
+// the next incoming ack.
+func (s *HybridSlowStart) OnPacketAcked(ackedPacketNumber protocol.PacketNumber) {
+	if s.IsEndOfRound(ackedPacketNumber) {
+		s.started = false
+	}
+}
+
+// Started returns true if started
+func (s *HybridSlowStart) Started() bool {
+	return s.started
+}
+
+// Restart the slow start phase
+func (s *HybridSlowStart) Restart() {
+	s.started = false
+	s.hystartFound = false
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/interface.go

@@ -0,0 +1,28 @@
+package congestion
+
+import (
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// A SendAlgorithm performs congestion control
+type SendAlgorithm interface {
+	TimeUntilSend(bytesInFlight protocol.ByteCount) time.Time
+	HasPacingBudget() bool
+	OnPacketSent(sentTime time.Time, bytesInFlight protocol.ByteCount, packetNumber protocol.PacketNumber, bytes protocol.ByteCount, isRetransmittable bool)
+	CanSend(bytesInFlight protocol.ByteCount) bool
+	MaybeExitSlowStart()
+	OnPacketAcked(number protocol.PacketNumber, ackedBytes protocol.ByteCount, priorInFlight protocol.ByteCount, eventTime time.Time)
+	OnPacketLost(number protocol.PacketNumber, lostBytes protocol.ByteCount, priorInFlight protocol.ByteCount)
+	OnRetransmissionTimeout(packetsRetransmitted bool)
+	SetMaxDatagramSize(protocol.ByteCount)
+}
+
+// A SendAlgorithmWithDebugInfos is a SendAlgorithm that exposes some debug infos
+type SendAlgorithmWithDebugInfos interface {
+	SendAlgorithm
+	InSlowStart() bool
+	InRecovery() bool
+	GetCongestionWindow() protocol.ByteCount
+}

vendor/github.com/lucas-clemente/quic-go/internal/congestion/pacer.go

@@ -0,0 +1,77 @@
+package congestion
+
+import (
+	"math"
+	"time"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+	"github.com/lucas-clemente/quic-go/internal/utils"
+)
+
+const maxBurstSizePackets = 10
+
+// The pacer implements a token bucket pacing algorithm.
+type pacer struct {
+	budgetAtLastSent     protocol.ByteCount
+	maxDatagramSize      protocol.ByteCount
+	lastSentTime         time.Time
+	getAdjustedBandwidth func() uint64 // in bytes/s
+}
+
+func newPacer(getBandwidth func() Bandwidth) *pacer {
+	p := &pacer{
+		maxDatagramSize: initialMaxDatagramSize,
+		getAdjustedBandwidth: func() uint64 {
+			// Bandwidth is in bits/s. We need the value in bytes/s.
+			bw := uint64(getBandwidth() / BytesPerSecond)
+			// Use a slightly higher value than the actual measured bandwidth.
+			// RTT variations then won't result in under-utilization of the congestion window.
+			// Ultimately, this will  result in sending packets as acknowledgments are received rather than when timers fire,
+			// provided the congestion window is fully utilized and acknowledgments arrive at regular intervals.
+			return bw * 5 / 4
+		},
+	}
+	p.budgetAtLastSent = p.maxBurstSize()
+	return p
+}
+
+func (p *pacer) SentPacket(sendTime time.Time, size protocol.ByteCount) {
+	budget := p.Budget(sendTime)
+	if size > budget {
+		p.budgetAtLastSent = 0
+	} else {
+		p.budgetAtLastSent = budget - size
+	}
+	p.lastSentTime = sendTime
+}
+
+func (p *pacer) Budget(now time.Time) protocol.ByteCount {
+	if p.lastSentTime.IsZero() {
+		return p.maxBurstSize()
+	}
+	budget := p.budgetAtLastSent + (protocol.ByteCount(p.getAdjustedBandwidth())*protocol.ByteCount(now.Sub(p.lastSentTime).Nanoseconds()))/1e9
+	return utils.MinByteCount(p.maxBurstSize(), budget)
+}
+
+func (p *pacer) maxBurstSize() protocol.ByteCount {
+	return utils.MaxByteCount(
+		protocol.ByteCount(uint64((protocol.MinPacingDelay+protocol.TimerGranularity).Nanoseconds())*p.getAdjustedBandwidth())/1e9,
+		maxBurstSizePackets*p.maxDatagramSize,
+	)
+}
+
+// TimeUntilSend returns when the next packet should be sent.
+// It returns the zero value of time.Time if a packet can be sent immediately.
+func (p *pacer) TimeUntilSend() time.Time {
+	if p.budgetAtLastSent >= p.maxDatagramSize {
+		return time.Time{}
+	}
+	return p.lastSentTime.Add(utils.MaxDuration(
+		protocol.MinPacingDelay,
+		time.Duration(math.Ceil(float64(p.maxDatagramSize-p.budgetAtLastSent)*1e9/float64(p.getAdjustedBandwidth())))*time.Nanosecond,
+	))
+}
+
+func (p *pacer) SetMaxDatagramSize(s protocol.ByteCount) {
+	p.maxDatagramSize = s
+}