chore: add new bbr implementation

transport/tuic/congestion_v2/bbr_sender.go

@@ -0,0 +1,927 @@
+package congestion
+
+// src from https://github.com/google/quiche/blob/e7872fc9e12bb1d46a118949c3d4da36de58aa44/quiche/quic/core/congestion_control/bbr_sender.cc
+
+import (
+	"fmt"
+	"net"
+	"time"
+
+	"github.com/metacubex/quic-go/congestion"
+
+	"github.com/zhangyunhao116/fastrand"
+)
+
+// BbrSender implements BBR congestion control algorithm.  BBR aims to estimate
+// the current available Bottleneck Bandwidth and RTT (hence the name), and
+// regulates the pacing rate and the size of the congestion window based on
+// those signals.
+//
+// BBR relies on pacing in order to function properly.  Do not use BBR when
+// pacing is disabled.
+//
+
+const (
+	invalidPacketNumber            = -1
+	initialCongestionWindowPackets = 32
+
+	// Constants based on TCP defaults.
+	// The minimum CWND to ensure delayed acks don't reduce bandwidth measurements.
+	// Does not inflate the pacing rate.
+	defaultMinimumCongestionWindow = 4 * congestion.ByteCount(congestion.InitialPacketSizeIPv4)
+
+	// The gain used for the STARTUP, equal to 2/ln(2).
+	defaultHighGain = 2.885
+	// The newly derived gain for STARTUP, equal to 4 * ln(2)
+	derivedHighGain = 2.773
+	// The newly derived CWND gain for STARTUP, 2.
+	derivedHighCWNDGain = 2.0
+)
+
+// The cycle of gains used during the PROBE_BW stage.
+var pacingGain = [...]float64{1.25, 0.75, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0}
+
+const (
+	// The length of the gain cycle.
+	gainCycleLength = len(pacingGain)
+	// The size of the bandwidth filter window, in round-trips.
+	bandwidthWindowSize = gainCycleLength + 2
+
+	// The time after which the current min_rtt value expires.
+	minRttExpiry = 10 * time.Second
+	// The minimum time the connection can spend in PROBE_RTT mode.
+	probeRttTime = 200 * time.Millisecond
+	// If the bandwidth does not increase by the factor of |kStartupGrowthTarget|
+	// within |kRoundTripsWithoutGrowthBeforeExitingStartup| rounds, the connection
+	// will exit the STARTUP mode.
+	startupGrowthTarget                         = 1.25
+	roundTripsWithoutGrowthBeforeExitingStartup = int64(3)
+
+	// Flag.
+	defaultStartupFullLossCount  = 8
+	quicBbr2DefaultLossThreshold = 0.02
+	maxBbrBurstPackets           = 3
+)
+
+type bbrMode int
+
+const (
+	// Startup phase of the connection.
+	bbrModeStartup = iota
+	// After achieving the highest possible bandwidth during the startup, lower
+	// the pacing rate in order to drain the queue.
+	bbrModeDrain
+	// Cruising mode.
+	bbrModeProbeBw
+	// Temporarily slow down sending in order to empty the buffer and measure
+	// the real minimum RTT.
+	bbrModeProbeRtt
+)
+
+// Indicates how the congestion control limits the amount of bytes in flight.
+type bbrRecoveryState int
+
+const (
+	// Do not limit.
+	bbrRecoveryStateNotInRecovery = iota
+	// Allow an extra outstanding byte for each byte acknowledged.
+	bbrRecoveryStateConservation
+	// Allow two extra outstanding bytes for each byte acknowledged (slow
+	// start).
+	bbrRecoveryStateGrowth
+)
+
+type bbrSender struct {
+	rttStats congestion.RTTStatsProvider
+	clock    Clock
+	pacer    *Pacer
+
+	mode bbrMode
+
+	// Bandwidth sampler provides BBR with the bandwidth measurements at
+	// individual points.
+	sampler *bandwidthSampler
+
+	// The number of the round trips that have occurred during the connection.
+	roundTripCount roundTripCount
+
+	// The packet number of the most recently sent packet.
+	lastSentPacket congestion.PacketNumber
+	// Acknowledgement of any packet after |current_round_trip_end_| will cause
+	// the round trip counter to advance.
+	currentRoundTripEnd congestion.PacketNumber
+
+	// Number of congestion events with some losses, in the current round.
+	numLossEventsInRound uint64
+
+	// Number of total bytes lost in the current round.
+	bytesLostInRound congestion.ByteCount
+
+	// The filter that tracks the maximum bandwidth over the multiple recent
+	// round-trips.
+	maxBandwidth *WindowedFilter[Bandwidth, roundTripCount]
+
+	// Minimum RTT estimate.  Automatically expires within 10 seconds (and
+	// triggers PROBE_RTT mode) if no new value is sampled during that period.
+	minRtt time.Duration
+	// The time at which the current value of |min_rtt_| was assigned.
+	minRttTimestamp time.Time
+
+	// The maximum allowed number of bytes in flight.
+	congestionWindow congestion.ByteCount
+
+	// The initial value of the |congestion_window_|.
+	initialCongestionWindow congestion.ByteCount
+
+	// The largest value the |congestion_window_| can achieve.
+	maxCongestionWindow congestion.ByteCount
+
+	// The smallest value the |congestion_window_| can achieve.
+	minCongestionWindow congestion.ByteCount
+
+	// The pacing gain applied during the STARTUP phase.
+	highGain float64
+
+	// The CWND gain applied during the STARTUP phase.
+	highCwndGain float64
+
+	// The pacing gain applied during the DRAIN phase.
+	drainGain float64
+
+	// The current pacing rate of the connection.
+	pacingRate Bandwidth
+
+	// The gain currently applied to the pacing rate.
+	pacingGain float64
+	// The gain currently applied to the congestion window.
+	congestionWindowGain float64
+
+	// The gain used for the congestion window during PROBE_BW.  Latched from
+	// quic_bbr_cwnd_gain flag.
+	congestionWindowGainConstant float64
+	// The number of RTTs to stay in STARTUP mode.  Defaults to 3.
+	numStartupRtts int64
+
+	// Number of round-trips in PROBE_BW mode, used for determining the current
+	// pacing gain cycle.
+	cycleCurrentOffset int
+	// The time at which the last pacing gain cycle was started.
+	lastCycleStart time.Time
+
+	// Indicates whether the connection has reached the full bandwidth mode.
+	isAtFullBandwidth bool
+	// Number of rounds during which there was no significant bandwidth increase.
+	roundsWithoutBandwidthGain int64
+	// The bandwidth compared to which the increase is measured.
+	bandwidthAtLastRound Bandwidth
+
+	// Set to true upon exiting quiescence.
+	exitingQuiescence bool
+
+	// Time at which PROBE_RTT has to be exited.  Setting it to zero indicates
+	// that the time is yet unknown as the number of packets in flight has not
+	// reached the required value.
+	exitProbeRttAt time.Time
+	// Indicates whether a round-trip has passed since PROBE_RTT became active.
+	probeRttRoundPassed bool
+
+	// Indicates whether the most recent bandwidth sample was marked as
+	// app-limited.
+	lastSampleIsAppLimited bool
+	// Indicates whether any non app-limited samples have been recorded.
+	hasNoAppLimitedSample bool
+
+	// Current state of recovery.
+	recoveryState bbrRecoveryState
+	// Receiving acknowledgement of a packet after |end_recovery_at_| will cause
+	// BBR to exit the recovery mode.  A value above zero indicates at least one
+	// loss has been detected, so it must not be set back to zero.
+	endRecoveryAt congestion.PacketNumber
+	// A window used to limit the number of bytes in flight during loss recovery.
+	recoveryWindow congestion.ByteCount
+	// If true, consider all samples in recovery app-limited.
+	isAppLimitedRecovery bool // not used
+
+	// When true, pace at 1.5x and disable packet conservation in STARTUP.
+	slowerStartup bool // not used
+	// When true, disables packet conservation in STARTUP.
+	rateBasedStartup bool // not used
+
+	// When true, add the most recent ack aggregation measurement during STARTUP.
+	enableAckAggregationDuringStartup bool
+	// When true, expire the windowed ack aggregation values in STARTUP when
+	// bandwidth increases more than 25%.
+	expireAckAggregationInStartup bool
+
+	// If true, will not exit low gain mode until bytes_in_flight drops below BDP
+	// or it's time for high gain mode.
+	drainToTarget bool
+
+	// If true, slow down pacing rate in STARTUP when overshooting is detected.
+	detectOvershooting bool
+	// Bytes lost while detect_overshooting_ is true.
+	bytesLostWhileDetectingOvershooting congestion.ByteCount
+	// Slow down pacing rate if
+	// bytes_lost_while_detecting_overshooting_ *
+	// bytes_lost_multiplier_while_detecting_overshooting_ > IW.
+	bytesLostMultiplierWhileDetectingOvershooting uint8
+	// When overshooting is detected, do not drop pacing_rate_ below this value /
+	// min_rtt.
+	cwndToCalculateMinPacingRate congestion.ByteCount
+
+	// Max congestion window when adjusting network parameters.
+	maxCongestionWindowWithNetworkParametersAdjusted congestion.ByteCount // not used
+
+	// Params.
+	maxDatagramSize congestion.ByteCount
+	// Recorded on packet sent. equivalent |unacked_packets_->bytes_in_flight()|
+	bytesInFlight congestion.ByteCount
+}
+
+var _ congestion.CongestionControl = &bbrSender{}
+
+func NewBbrSender(
+	clock Clock,
+	initialMaxDatagramSize congestion.ByteCount,
+	initialCongestionWindowPackets congestion.ByteCount,
+) *bbrSender {
+	return newBbrSender(
+		clock,
+		initialMaxDatagramSize,
+		initialCongestionWindowPackets*initialMaxDatagramSize,
+		congestion.MaxCongestionWindowPackets*initialMaxDatagramSize,
+	)
+}
+
+func newBbrSender(
+	clock Clock,
+	initialMaxDatagramSize,
+	initialCongestionWindow,
+	initialMaxCongestionWindow congestion.ByteCount,
+) *bbrSender {
+	b := &bbrSender{
+		clock:                        clock,
+		mode:                         bbrModeStartup,
+		sampler:                      newBandwidthSampler(roundTripCount(bandwidthWindowSize)),
+		lastSentPacket:               invalidPacketNumber,
+		currentRoundTripEnd:          invalidPacketNumber,
+		maxBandwidth:                 NewWindowedFilter(roundTripCount(bandwidthWindowSize), MaxFilter[Bandwidth]),
+		congestionWindow:             initialCongestionWindow,
+		initialCongestionWindow:      initialCongestionWindow,
+		maxCongestionWindow:          initialMaxCongestionWindow,
+		minCongestionWindow:          defaultMinimumCongestionWindow,
+		highGain:                     defaultHighGain,
+		highCwndGain:                 defaultHighGain,
+		drainGain:                    1.0 / defaultHighGain,
+		pacingGain:                   1.0,
+		congestionWindowGain:         1.0,
+		congestionWindowGainConstant: 2.0,
+		numStartupRtts:               roundTripsWithoutGrowthBeforeExitingStartup,
+		recoveryState:                bbrRecoveryStateNotInRecovery,
+		endRecoveryAt:                invalidPacketNumber,
+		recoveryWindow:               initialMaxCongestionWindow,
+		bytesLostMultiplierWhileDetectingOvershooting:    2,
+		cwndToCalculateMinPacingRate:                     initialCongestionWindow,
+		maxCongestionWindowWithNetworkParametersAdjusted: initialMaxCongestionWindow,
+		maxDatagramSize: initialMaxDatagramSize,
+	}
+	b.pacer = NewPacer(func() congestion.ByteCount {
+		// Pacer wants bytes per second, but Bandwidth is in bits per second.
+		return congestion.ByteCount(float64(b.bandwidthEstimate()) * b.congestionWindowGain / float64(BytesPerSecond))
+	})
+
+	/*
+		if b.tracer != nil {
+			b.lastState = logging.CongestionStateStartup
+			b.tracer.UpdatedCongestionState(logging.CongestionStateStartup)
+		}
+	*/
+
+	b.enterStartupMode(b.clock.Now())
+	b.setHighCwndGain(derivedHighCWNDGain)
+
+	return b
+}
+
+func (b *bbrSender) SetRTTStatsProvider(provider congestion.RTTStatsProvider) {
+	b.rttStats = provider
+}
+
+// TimeUntilSend implements the SendAlgorithm interface.
+func (b *bbrSender) TimeUntilSend(bytesInFlight congestion.ByteCount) time.Time {
+	return b.pacer.TimeUntilSend()
+}
+
+// HasPacingBudget implements the SendAlgorithm interface.
+func (b *bbrSender) HasPacingBudget(now time.Time) bool {
+	return b.pacer.Budget(now) >= b.maxDatagramSize
+}
+
+// OnPacketSent implements the SendAlgorithm interface.
+func (b *bbrSender) OnPacketSent(
+	sentTime time.Time,
+	bytesInFlight congestion.ByteCount,
+	packetNumber congestion.PacketNumber,
+	bytes congestion.ByteCount,
+	isRetransmittable bool,
+) {
+	b.pacer.SentPacket(sentTime, bytes)
+
+	b.lastSentPacket = packetNumber
+	b.bytesInFlight = bytesInFlight
+
+	if bytesInFlight == 0 {
+		b.exitingQuiescence = true
+	}
+
+	b.sampler.OnPacketSent(sentTime, packetNumber, bytes, bytesInFlight, isRetransmittable)
+}
+
+// CanSend implements the SendAlgorithm interface.
+func (b *bbrSender) CanSend(bytesInFlight congestion.ByteCount) bool {
+	return bytesInFlight < b.GetCongestionWindow()
+}
+
+// MaybeExitSlowStart implements the SendAlgorithm interface.
+func (b *bbrSender) MaybeExitSlowStart() {
+	// Do nothing
+}
+
+// OnPacketAcked implements the SendAlgorithm interface.
+func (b *bbrSender) OnPacketAcked(number congestion.PacketNumber, ackedBytes, priorInFlight congestion.ByteCount, eventTime time.Time) {
+	// Do nothing.
+}
+
+// OnPacketLost implements the SendAlgorithm interface.
+func (b *bbrSender) OnPacketLost(number congestion.PacketNumber, lostBytes, priorInFlight congestion.ByteCount) {
+	// Do nothing.
+}
+
+// OnRetransmissionTimeout implements the SendAlgorithm interface.
+func (b *bbrSender) OnRetransmissionTimeout(packetsRetransmitted bool) {
+	// Do nothing.
+}
+
+// SetMaxDatagramSize implements the SendAlgorithm interface.
+func (b *bbrSender) SetMaxDatagramSize(s congestion.ByteCount) {
+	if s < b.maxDatagramSize {
+		panic(fmt.Sprintf("congestion BUG: decreased max datagram size from %d to %d", b.maxDatagramSize, s))
+	}
+	cwndIsMinCwnd := b.congestionWindow == b.minCongestionWindow
+	b.maxDatagramSize = s
+	if cwndIsMinCwnd {
+		b.congestionWindow = b.minCongestionWindow
+	}
+	b.pacer.SetMaxDatagramSize(s)
+}
+
+// InSlowStart implements the SendAlgorithmWithDebugInfos interface.
+func (b *bbrSender) InSlowStart() bool {
+	return b.mode == bbrModeStartup
+}
+
+// InRecovery implements the SendAlgorithmWithDebugInfos interface.
+func (b *bbrSender) InRecovery() bool {
+	return b.recoveryState != bbrRecoveryStateNotInRecovery
+}
+
+// GetCongestionWindow implements the SendAlgorithmWithDebugInfos interface.
+func (b *bbrSender) GetCongestionWindow() congestion.ByteCount {
+	if b.mode == bbrModeProbeRtt {
+		return b.probeRttCongestionWindow()
+	}
+
+	if b.InRecovery() {
+		return Min(b.congestionWindow, b.recoveryWindow)
+	}
+
+	return b.congestionWindow
+}
+
+func (b *bbrSender) OnCongestionEvent(number congestion.PacketNumber, lostBytes, priorInFlight congestion.ByteCount) {
+	// Do nothing.
+}
+
+func (b *bbrSender) OnCongestionEventEx(priorInFlight congestion.ByteCount, eventTime time.Time, ackedPackets []congestion.AckedPacketInfo, lostPackets []congestion.LostPacketInfo) {
+	totalBytesAckedBefore := b.sampler.TotalBytesAcked()
+	totalBytesLostBefore := b.sampler.TotalBytesLost()
+
+	var isRoundStart, minRttExpired bool
+	var excessAcked, bytesLost congestion.ByteCount
+
+	// The send state of the largest packet in acked_packets, unless it is
+	// empty. If acked_packets is empty, it's the send state of the largest
+	// packet in lost_packets.
+	var lastPacketSendState sendTimeState
+
+	b.maybeApplimited(priorInFlight)
+
+	// Update bytesInFlight
+	b.bytesInFlight = priorInFlight
+	for _, p := range ackedPackets {
+		b.bytesInFlight -= p.BytesAcked
+	}
+	for _, p := range lostPackets {
+		b.bytesInFlight -= p.BytesLost
+	}
+
+	if len(ackedPackets) != 0 {
+		lastAckedPacket := ackedPackets[len(ackedPackets)-1].PacketNumber
+		isRoundStart = b.updateRoundTripCounter(lastAckedPacket)
+		b.updateRecoveryState(lastAckedPacket, len(lostPackets) != 0, isRoundStart)
+	}
+
+	sample := b.sampler.OnCongestionEvent(eventTime,
+		ackedPackets, lostPackets, b.maxBandwidth.GetBest(), infBandwidth, b.roundTripCount)
+	if sample.lastPacketSendState.isValid {
+		b.lastSampleIsAppLimited = sample.lastPacketSendState.isAppLimited
+		b.hasNoAppLimitedSample = b.hasNoAppLimitedSample || !b.lastSampleIsAppLimited
+	}
+	// Avoid updating |max_bandwidth_| if a) this is a loss-only event, or b) all
+	// packets in |acked_packets| did not generate valid samples. (e.g. ack of
+	// ack-only packets). In both cases, sampler_.total_bytes_acked() will not
+	// change.
+	if totalBytesAckedBefore != b.sampler.TotalBytesAcked() {
+		if !sample.sampleIsAppLimited || sample.sampleMaxBandwidth > b.maxBandwidth.GetBest() {
+			b.maxBandwidth.Update(sample.sampleMaxBandwidth, b.roundTripCount)
+		}
+	}
+
+	if sample.sampleRtt != infRTT {
+		minRttExpired = b.maybeUpdateMinRtt(eventTime, sample.sampleRtt)
+	}
+	bytesLost = b.sampler.TotalBytesLost() - totalBytesLostBefore
+
+	excessAcked = sample.extraAcked
+	lastPacketSendState = sample.lastPacketSendState
+
+	if len(lostPackets) != 0 {
+		b.numLossEventsInRound++
+		b.bytesLostInRound += bytesLost
+	}
+
+	// Handle logic specific to PROBE_BW mode.
+	if b.mode == bbrModeProbeBw {
+		b.updateGainCyclePhase(eventTime, priorInFlight, len(lostPackets) != 0)
+	}
+
+	// Handle logic specific to STARTUP and DRAIN modes.
+	if isRoundStart && !b.isAtFullBandwidth {
+		b.checkIfFullBandwidthReached(&lastPacketSendState)
+	}
+
+	b.maybeExitStartupOrDrain(eventTime)
+
+	// Handle logic specific to PROBE_RTT.
+	b.maybeEnterOrExitProbeRtt(eventTime, isRoundStart, minRttExpired)
+
+	// Calculate number of packets acked and lost.
+	bytesAcked := b.sampler.TotalBytesAcked() - totalBytesAckedBefore
+
+	// After the model is updated, recalculate the pacing rate and congestion
+	// window.
+	b.calculatePacingRate(bytesLost)
+	b.calculateCongestionWindow(bytesAcked, excessAcked)
+	b.calculateRecoveryWindow(bytesAcked, bytesLost)
+
+	// Cleanup internal state.
+	if len(lostPackets) != 0 {
+		lastLostPacket := lostPackets[len(lostPackets)-1].PacketNumber
+		b.sampler.RemoveObsoletePackets(lastLostPacket)
+	}
+	if isRoundStart {
+		b.numLossEventsInRound = 0
+		b.bytesLostInRound = 0
+	}
+}
+
+func (b *bbrSender) PacingRate() Bandwidth {
+	if b.pacingRate == 0 {
+		return Bandwidth(b.highGain * float64(
+			BandwidthFromDelta(b.initialCongestionWindow, b.getMinRtt())))
+	}
+
+	return b.pacingRate
+}
+
+func (b *bbrSender) hasGoodBandwidthEstimateForResumption() bool {
+	return b.hasNonAppLimitedSample()
+}
+
+func (b *bbrSender) hasNonAppLimitedSample() bool {
+	return b.hasNoAppLimitedSample
+}
+
+// Sets the pacing gain used in STARTUP.  Must be greater than 1.
+func (b *bbrSender) setHighGain(highGain float64) {
+	b.highGain = highGain
+	if b.mode == bbrModeStartup {
+		b.pacingGain = highGain
+	}
+}
+
+// Sets the CWND gain used in STARTUP.  Must be greater than 1.
+func (b *bbrSender) setHighCwndGain(highCwndGain float64) {
+	b.highCwndGain = highCwndGain
+	if b.mode == bbrModeStartup {
+		b.congestionWindowGain = highCwndGain
+	}
+}
+
+// Sets the gain used in DRAIN.  Must be less than 1.
+func (b *bbrSender) setDrainGain(drainGain float64) {
+	b.drainGain = drainGain
+}
+
+// What's the current estimated bandwidth in bytes per second.
+func (b *bbrSender) bandwidthEstimate() Bandwidth {
+	return b.maxBandwidth.GetBest()
+}
+
+// Returns the current estimate of the RTT of the connection.  Outside of the
+// edge cases, this is minimum RTT.
+func (b *bbrSender) getMinRtt() time.Duration {
+	if b.minRtt != 0 {
+		return b.minRtt
+	}
+	// min_rtt could be available if the handshake packet gets neutered then
+	// gets acknowledged. This could only happen for QUIC crypto where we do not
+	// drop keys.
+	minRtt := b.rttStats.MinRTT()
+	if minRtt == 0 {
+		return 100 * time.Millisecond
+	} else {
+		return minRtt
+	}
+}
+
+// Computes the target congestion window using the specified gain.
+func (b *bbrSender) getTargetCongestionWindow(gain float64) congestion.ByteCount {
+	bdp := bdpFromRttAndBandwidth(b.getMinRtt(), b.bandwidthEstimate())
+	congestionWindow := congestion.ByteCount(gain * float64(bdp))
+
+	// BDP estimate will be zero if no bandwidth samples are available yet.
+	if congestionWindow == 0 {
+		congestionWindow = congestion.ByteCount(gain * float64(b.initialCongestionWindow))
+	}
+
+	return Max(congestionWindow, b.minCongestionWindow)
+}
+
+// The target congestion window during PROBE_RTT.
+func (b *bbrSender) probeRttCongestionWindow() congestion.ByteCount {
+	return b.minCongestionWindow
+}
+
+func (b *bbrSender) maybeUpdateMinRtt(now time.Time, sampleMinRtt time.Duration) bool {
+	// Do not expire min_rtt if none was ever available.
+	minRttExpired := b.minRtt != 0 && now.After(b.minRttTimestamp.Add(minRttExpiry))
+	if minRttExpired || sampleMinRtt < b.minRtt || b.minRtt == 0 {
+		b.minRtt = sampleMinRtt
+		b.minRttTimestamp = now
+	}
+
+	return minRttExpired
+}
+
+// Enters the STARTUP mode.
+func (b *bbrSender) enterStartupMode(now time.Time) {
+	b.mode = bbrModeStartup
+	// b.maybeTraceStateChange(logging.CongestionStateStartup)
+	b.pacingGain = b.highGain
+	b.congestionWindowGain = b.highCwndGain
+}
+
+// Enters the PROBE_BW mode.
+func (b *bbrSender) enterProbeBandwidthMode(now time.Time) {
+	b.mode = bbrModeProbeBw
+	// b.maybeTraceStateChange(logging.CongestionStateProbeBw)
+	b.congestionWindowGain = b.congestionWindowGainConstant
+
+	// Pick a random offset for the gain cycle out of {0, 2..7} range. 1 is
+	// excluded because in that case increased gain and decreased gain would not
+	// follow each other.
+	b.cycleCurrentOffset = int(fastrand.Int31n(congestion.PacketsPerConnectionID)) % (gainCycleLength - 1)
+	if b.cycleCurrentOffset >= 1 {
+		b.cycleCurrentOffset += 1
+	}
+
+	b.lastCycleStart = now
+	b.pacingGain = pacingGain[b.cycleCurrentOffset]
+}
+
+// Updates the round-trip counter if a round-trip has passed.  Returns true if
+// the counter has been advanced.
+func (b *bbrSender) updateRoundTripCounter(lastAckedPacket congestion.PacketNumber) bool {
+	if b.currentRoundTripEnd == invalidPacketNumber || lastAckedPacket > b.currentRoundTripEnd {
+		b.roundTripCount++
+		b.currentRoundTripEnd = b.lastSentPacket
+		return true
+	}
+	return false
+}
+
+// Updates the current gain used in PROBE_BW mode.
+func (b *bbrSender) updateGainCyclePhase(now time.Time, priorInFlight congestion.ByteCount, hasLosses bool) {
+	// In most cases, the cycle is advanced after an RTT passes.
+	shouldAdvanceGainCycling := now.After(b.lastCycleStart.Add(b.getMinRtt()))
+	// If the pacing gain is above 1.0, the connection is trying to probe the
+	// bandwidth by increasing the number of bytes in flight to at least
+	// pacing_gain * BDP.  Make sure that it actually reaches the target, as long
+	// as there are no losses suggesting that the buffers are not able to hold
+	// that much.
+	if b.pacingGain > 1.0 && !hasLosses && priorInFlight < b.getTargetCongestionWindow(b.pacingGain) {
+		shouldAdvanceGainCycling = false
+	}
+
+	// If pacing gain is below 1.0, the connection is trying to drain the extra
+	// queue which could have been incurred by probing prior to it.  If the number
+	// of bytes in flight falls down to the estimated BDP value earlier, conclude
+	// that the queue has been successfully drained and exit this cycle early.
+	if b.pacingGain < 1.0 && b.bytesInFlight <= b.getTargetCongestionWindow(1) {
+		shouldAdvanceGainCycling = true
+	}
+
+	if shouldAdvanceGainCycling {
+		b.cycleCurrentOffset = (b.cycleCurrentOffset + 1) % gainCycleLength
+		b.lastCycleStart = now
+		// Stay in low gain mode until the target BDP is hit.
+		// Low gain mode will be exited immediately when the target BDP is achieved.
+		if b.drainToTarget && b.pacingGain < 1 &&
+			pacingGain[b.cycleCurrentOffset] == 1 &&
+			b.bytesInFlight > b.getTargetCongestionWindow(1) {
+			return
+		}
+		b.pacingGain = pacingGain[b.cycleCurrentOffset]
+	}
+}
+
+// Tracks for how many round-trips the bandwidth has not increased
+// significantly.
+func (b *bbrSender) checkIfFullBandwidthReached(lastPacketSendState *sendTimeState) {
+	if b.lastSampleIsAppLimited {
+		return
+	}
+
+	target := Bandwidth(float64(b.bandwidthAtLastRound) * startupGrowthTarget)
+	if b.bandwidthEstimate() >= target {
+		b.bandwidthAtLastRound = b.bandwidthEstimate()
+		b.roundsWithoutBandwidthGain = 0
+		if b.expireAckAggregationInStartup {
+			// Expire old excess delivery measurements now that bandwidth increased.
+			b.sampler.ResetMaxAckHeightTracker(0, b.roundTripCount)
+		}
+		return
+	}
+
+	b.roundsWithoutBandwidthGain++
+	if b.roundsWithoutBandwidthGain >= b.numStartupRtts ||
+		b.shouldExitStartupDueToLoss(lastPacketSendState) {
+		b.isAtFullBandwidth = true
+	}
+}
+
+func (b *bbrSender) maybeApplimited(bytesInFlight congestion.ByteCount) {
+	congestionWindow := b.GetCongestionWindow()
+	if bytesInFlight >= congestionWindow {
+		return
+	}
+	availableBytes := congestionWindow - bytesInFlight
+	drainLimited := b.mode == bbrModeDrain && bytesInFlight > congestionWindow/2
+	if !drainLimited || availableBytes > maxBbrBurstPackets*b.maxDatagramSize {
+		b.sampler.OnAppLimited()
+	}
+}
+
+// Transitions from STARTUP to DRAIN and from DRAIN to PROBE_BW if
+// appropriate.
+func (b *bbrSender) maybeExitStartupOrDrain(now time.Time) {
+	if b.mode == bbrModeStartup && b.isAtFullBandwidth {
+		b.mode = bbrModeDrain
+		// b.maybeTraceStateChange(logging.CongestionStateDrain)
+		b.pacingGain = b.drainGain
+		b.congestionWindowGain = b.highCwndGain
+	}
+	if b.mode == bbrModeDrain && b.bytesInFlight <= b.getTargetCongestionWindow(1) {
+		b.enterProbeBandwidthMode(now)
+	}
+}
+
+// Decides whether to enter or exit PROBE_RTT.
+func (b *bbrSender) maybeEnterOrExitProbeRtt(now time.Time, isRoundStart, minRttExpired bool) {
+	if minRttExpired && !b.exitingQuiescence && b.mode != bbrModeProbeRtt {
+		b.mode = bbrModeProbeRtt
+		// b.maybeTraceStateChange(logging.CongestionStateProbRtt)
+		b.pacingGain = 1.0
+		// Do not decide on the time to exit PROBE_RTT until the |bytes_in_flight|
+		// is at the target small value.
+		b.exitProbeRttAt = time.Time{}
+	}
+
+	if b.mode == bbrModeProbeRtt {
+		b.sampler.OnAppLimited()
+		// b.maybeTraceStateChange(logging.CongestionStateApplicationLimited)
+
+		if b.exitProbeRttAt.IsZero() {
+			// If the window has reached the appropriate size, schedule exiting
+			// PROBE_RTT.  The CWND during PROBE_RTT is kMinimumCongestionWindow, but
+			// we allow an extra packet since QUIC checks CWND before sending a
+			// packet.
+			if b.bytesInFlight < b.probeRttCongestionWindow()+congestion.MaxPacketBufferSize {
+				b.exitProbeRttAt = now.Add(probeRttTime)
+				b.probeRttRoundPassed = false
+			}
+		} else {
+			if isRoundStart {
+				b.probeRttRoundPassed = true
+			}
+			if now.Sub(b.exitProbeRttAt) >= 0 && b.probeRttRoundPassed {
+				b.minRttTimestamp = now
+				if !b.isAtFullBandwidth {
+					b.enterStartupMode(now)
+				} else {
+					b.enterProbeBandwidthMode(now)
+				}
+			}
+		}
+	}
+
+	b.exitingQuiescence = false
+}
+
+// Determines whether BBR needs to enter, exit or advance state of the
+// recovery.
+func (b *bbrSender) updateRecoveryState(lastAckedPacket congestion.PacketNumber, hasLosses, isRoundStart bool) {
+	// Disable recovery in startup, if loss-based exit is enabled.
+	if !b.isAtFullBandwidth {
+		return
+	}
+
+	// Exit recovery when there are no losses for a round.
+	if hasLosses {
+		b.endRecoveryAt = b.lastSentPacket
+	}
+
+	switch b.recoveryState {
+	case bbrRecoveryStateNotInRecovery:
+		if hasLosses {
+			b.recoveryState = bbrRecoveryStateConservation
+			// This will cause the |recovery_window_| to be set to the correct
+			// value in CalculateRecoveryWindow().
+			b.recoveryWindow = 0
+			// Since the conservation phase is meant to be lasting for a whole
+			// round, extend the current round as if it were started right now.
+			b.currentRoundTripEnd = b.lastSentPacket
+		}
+	case bbrRecoveryStateConservation:
+		if isRoundStart {
+			b.recoveryState = bbrRecoveryStateGrowth
+		}
+		fallthrough
+	case bbrRecoveryStateGrowth:
+		// Exit recovery if appropriate.
+		if !hasLosses && lastAckedPacket > b.endRecoveryAt {
+			b.recoveryState = bbrRecoveryStateNotInRecovery
+		}
+	}
+}
+
+// Determines the appropriate pacing rate for the connection.
+func (b *bbrSender) calculatePacingRate(bytesLost congestion.ByteCount) {
+	if b.bandwidthEstimate() == 0 {
+		return
+	}
+
+	targetRate := Bandwidth(b.pacingGain * float64(b.bandwidthEstimate()))
+	if b.isAtFullBandwidth {
+		b.pacingRate = targetRate
+		return
+	}
+
+	// Pace at the rate of initial_window / RTT as soon as RTT measurements are
+	// available.
+	if b.pacingRate == 0 && b.rttStats.MinRTT() != 0 {
+		b.pacingRate = BandwidthFromDelta(b.initialCongestionWindow, b.rttStats.MinRTT())
+		return
+	}
+
+	if b.detectOvershooting {
+		b.bytesLostWhileDetectingOvershooting += bytesLost
+		// Check for overshooting with network parameters adjusted when pacing rate
+		// > target_rate and loss has been detected.
+		if b.pacingRate > targetRate && b.bytesLostWhileDetectingOvershooting > 0 {
+			if b.hasNoAppLimitedSample ||
+				b.bytesLostWhileDetectingOvershooting*congestion.ByteCount(b.bytesLostMultiplierWhileDetectingOvershooting) > b.initialCongestionWindow {
+				// We are fairly sure overshoot happens if 1) there is at least one
+				// non app-limited bw sample or 2) half of IW gets lost. Slow pacing
+				// rate.
+				b.pacingRate = Max(targetRate, BandwidthFromDelta(b.cwndToCalculateMinPacingRate, b.rttStats.MinRTT()))
+				b.bytesLostWhileDetectingOvershooting = 0
+				b.detectOvershooting = false
+			}
+		}
+	}
+
+	// Do not decrease the pacing rate during startup.
+	b.pacingRate = Max(b.pacingRate, targetRate)
+}
+
+// Determines the appropriate congestion window for the connection.
+func (b *bbrSender) calculateCongestionWindow(bytesAcked, excessAcked congestion.ByteCount) {
+	if b.mode == bbrModeProbeRtt {
+		return
+	}
+
+	targetWindow := b.getTargetCongestionWindow(b.congestionWindowGain)
+	if b.isAtFullBandwidth {
+		// Add the max recently measured ack aggregation to CWND.
+		targetWindow += b.sampler.MaxAckHeight()
+	} else if b.enableAckAggregationDuringStartup {
+		// Add the most recent excess acked.  Because CWND never decreases in
+		// STARTUP, this will automatically create a very localized max filter.
+		targetWindow += excessAcked
+	}
+
+	// Instead of immediately setting the target CWND as the new one, BBR grows
+	// the CWND towards |target_window| by only increasing it |bytes_acked| at a
+	// time.
+	if b.isAtFullBandwidth {
+		b.congestionWindow = Min(targetWindow, b.congestionWindow+bytesAcked)
+	} else if b.congestionWindow < targetWindow ||
+		b.sampler.TotalBytesAcked() < b.initialCongestionWindow {
+		// If the connection is not yet out of startup phase, do not decrease the
+		// window.
+		b.congestionWindow += bytesAcked
+	}
+
+	// Enforce the limits on the congestion window.
+	b.congestionWindow = Max(b.congestionWindow, b.minCongestionWindow)
+	b.congestionWindow = Min(b.congestionWindow, b.maxCongestionWindow)
+}
+
+// Determines the appropriate window that constrains the in-flight during recovery.
+func (b *bbrSender) calculateRecoveryWindow(bytesAcked, bytesLost congestion.ByteCount) {
+	if b.recoveryState == bbrRecoveryStateNotInRecovery {
+		return
+	}
+
+	// Set up the initial recovery window.
+	if b.recoveryWindow == 0 {
+		b.recoveryWindow = b.bytesInFlight + bytesAcked
+		b.recoveryWindow = Max(b.minCongestionWindow, b.recoveryWindow)
+		return
+	}
+
+	// Remove losses from the recovery window, while accounting for a potential
+	// integer underflow.
+	if b.recoveryWindow >= bytesLost {
+		b.recoveryWindow = b.recoveryWindow - bytesLost
+	} else {
+		b.recoveryWindow = b.maxDatagramSize
+	}
+
+	// In CONSERVATION mode, just subtracting losses is sufficient.  In GROWTH,
+	// release additional |bytes_acked| to achieve a slow-start-like behavior.
+	if b.recoveryState == bbrRecoveryStateGrowth {
+		b.recoveryWindow += bytesAcked
+	}
+
+	// Always allow sending at least |bytes_acked| in response.
+	b.recoveryWindow = Max(b.recoveryWindow, b.bytesInFlight+bytesAcked)
+	b.recoveryWindow = Max(b.minCongestionWindow, b.recoveryWindow)
+}
+
+// Return whether we should exit STARTUP due to excessive loss.
+func (b *bbrSender) shouldExitStartupDueToLoss(lastPacketSendState *sendTimeState) bool {
+	if b.numLossEventsInRound < defaultStartupFullLossCount || !lastPacketSendState.isValid {
+		return false
+	}
+
+	inflightAtSend := lastPacketSendState.bytesInFlight
+
+	if inflightAtSend > 0 && b.bytesLostInRound > 0 {
+		if b.bytesLostInRound > congestion.ByteCount(float64(inflightAtSend)*quicBbr2DefaultLossThreshold) {
+			return true
+		}
+		return false
+	}
+	return false
+}
+
+func bdpFromRttAndBandwidth(rtt time.Duration, bandwidth Bandwidth) congestion.ByteCount {
+	return congestion.ByteCount(rtt) * congestion.ByteCount(bandwidth) / congestion.ByteCount(BytesPerSecond) / congestion.ByteCount(time.Second)
+}
+
+func GetInitialPacketSize(addr net.Addr) congestion.ByteCount {
+	// If this is not a UDP address, we don't know anything about the MTU.
+	// Use the minimum size of an Initial packet as the max packet size.
+	if udpAddr, ok := addr.(*net.UDPAddr); ok {
+		if udpAddr.IP.To4() != nil {
+			return congestion.InitialPacketSizeIPv4
+		} else {
+			return congestion.InitialPacketSizeIPv6
+		}
+	} else {
+		return congestion.MinInitialPacketSize
+	}
+}