229 lines
9 KiB
Go
229 lines
9 KiB
Go
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package congestion
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import (
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"math"
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"time"
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"github.com/lucas-clemente/quic-go/internal/utils"
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"github.com/lucas-clemente/quic-go/protocol"
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)
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// This cubic implementation is based on the one found in Chromiums's QUIC
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// implementation, in the files net/quic/congestion_control/cubic.{hh,cc}.
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// Constants based on TCP defaults.
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// The following constants are in 2^10 fractions of a second instead of ms to
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// allow a 10 shift right to divide.
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// 1024*1024^3 (first 1024 is from 0.100^3)
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// where 0.100 is 100 ms which is the scaling
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// round trip time.
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const cubeScale = 40
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const cubeCongestionWindowScale = 410
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const cubeFactor protocol.PacketNumber = 1 << cubeScale / cubeCongestionWindowScale
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const defaultNumConnections = 2
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// Default Cubic backoff factor
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const beta float32 = 0.7
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// Additional backoff factor when loss occurs in the concave part of the Cubic
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// curve. This additional backoff factor is expected to give up bandwidth to
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// new concurrent flows and speed up convergence.
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const betaLastMax float32 = 0.85
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// If true, Cubic's epoch is shifted when the sender is application-limited.
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const shiftQuicCubicEpochWhenAppLimited = true
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const maxCubicTimeInterval = 30 * time.Millisecond
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// Cubic implements the cubic algorithm from TCP
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type Cubic struct {
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clock Clock
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// Number of connections to simulate.
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numConnections int
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// Time when this cycle started, after last loss event.
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epoch time.Time
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// Time when sender went into application-limited period. Zero if not in
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// application-limited period.
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appLimitedStartTime time.Time
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// Time when we updated last_congestion_window.
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lastUpdateTime time.Time
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// Last congestion window (in packets) used.
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lastCongestionWindow protocol.PacketNumber
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// Max congestion window (in packets) used just before last loss event.
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// Note: to improve fairness to other streams an additional back off is
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// applied to this value if the new value is below our latest value.
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lastMaxCongestionWindow protocol.PacketNumber
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// Number of acked packets since the cycle started (epoch).
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ackedPacketsCount protocol.PacketNumber
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// TCP Reno equivalent congestion window in packets.
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estimatedTCPcongestionWindow protocol.PacketNumber
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// Origin point of cubic function.
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originPointCongestionWindow protocol.PacketNumber
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// Time to origin point of cubic function in 2^10 fractions of a second.
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timeToOriginPoint uint32
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// Last congestion window in packets computed by cubic function.
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lastTargetCongestionWindow protocol.PacketNumber
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}
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// NewCubic returns a new Cubic instance
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func NewCubic(clock Clock) *Cubic {
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c := &Cubic{
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clock: clock,
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numConnections: defaultNumConnections,
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}
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c.Reset()
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return c
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}
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// Reset is called after a timeout to reset the cubic state
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func (c *Cubic) Reset() {
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c.epoch = time.Time{}
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c.appLimitedStartTime = time.Time{}
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c.lastUpdateTime = time.Time{}
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c.lastCongestionWindow = 0
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c.lastMaxCongestionWindow = 0
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c.ackedPacketsCount = 0
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c.estimatedTCPcongestionWindow = 0
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c.originPointCongestionWindow = 0
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c.timeToOriginPoint = 0
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c.lastTargetCongestionWindow = 0
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}
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func (c *Cubic) alpha() float32 {
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// TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
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// beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
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// We derive the equivalent alpha for an N-connection emulation as:
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b := c.beta()
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return 3 * float32(c.numConnections) * float32(c.numConnections) * (1 - b) / (1 + b)
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}
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func (c *Cubic) beta() float32 {
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// kNConnectionBeta is the backoff factor after loss for our N-connection
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// emulation, which emulates the effective backoff of an ensemble of N
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// TCP-Reno connections on a single loss event. The effective multiplier is
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// computed as:
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return (float32(c.numConnections) - 1 + beta) / float32(c.numConnections)
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}
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// OnApplicationLimited is called on ack arrival when sender is unable to use
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// the available congestion window. Resets Cubic state during quiescence.
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func (c *Cubic) OnApplicationLimited() {
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if shiftQuicCubicEpochWhenAppLimited {
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// When sender is not using the available congestion window, Cubic's epoch
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// should not continue growing. Record the time when sender goes into an
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// app-limited period here, to compensate later when cwnd growth happens.
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if c.appLimitedStartTime.IsZero() {
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c.appLimitedStartTime = c.clock.Now()
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}
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} else {
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// When sender is not using the available congestion window, Cubic's epoch
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// should not continue growing. Reset the epoch when in such a period.
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c.epoch = time.Time{}
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}
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}
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// CongestionWindowAfterPacketLoss computes a new congestion window to use after
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// a loss event. Returns the new congestion window in packets. The new
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// congestion window is a multiplicative decrease of our current window.
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func (c *Cubic) CongestionWindowAfterPacketLoss(currentCongestionWindow protocol.PacketNumber) protocol.PacketNumber {
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if currentCongestionWindow < c.lastMaxCongestionWindow {
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// We never reached the old max, so assume we are competing with another
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// flow. Use our extra back off factor to allow the other flow to go up.
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c.lastMaxCongestionWindow = protocol.PacketNumber(betaLastMax * float32(currentCongestionWindow))
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} else {
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c.lastMaxCongestionWindow = currentCongestionWindow
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}
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c.epoch = time.Time{} // Reset time.
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return protocol.PacketNumber(float32(currentCongestionWindow) * c.beta())
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}
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// CongestionWindowAfterAck computes a new congestion window to use after a received ACK.
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// Returns the new congestion window in packets. The new congestion window
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// follows a cubic function that depends on the time passed since last
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// packet loss.
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func (c *Cubic) CongestionWindowAfterAck(currentCongestionWindow protocol.PacketNumber, delayMin time.Duration) protocol.PacketNumber {
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c.ackedPacketsCount++ // Packets acked.
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currentTime := c.clock.Now()
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// Cubic is "independent" of RTT, the update is limited by the time elapsed.
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if c.lastCongestionWindow == currentCongestionWindow && (currentTime.Sub(c.lastUpdateTime) <= maxCubicTimeInterval) {
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return utils.MaxPacketNumber(c.lastTargetCongestionWindow, c.estimatedTCPcongestionWindow)
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}
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c.lastCongestionWindow = currentCongestionWindow
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c.lastUpdateTime = currentTime
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if c.epoch.IsZero() {
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// First ACK after a loss event.
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c.epoch = currentTime // Start of epoch.
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c.ackedPacketsCount = 1 // Reset count.
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// Reset estimated_tcp_congestion_window_ to be in sync with cubic.
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c.estimatedTCPcongestionWindow = currentCongestionWindow
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if c.lastMaxCongestionWindow <= currentCongestionWindow {
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c.timeToOriginPoint = 0
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c.originPointCongestionWindow = currentCongestionWindow
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} else {
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c.timeToOriginPoint = uint32(math.Cbrt(float64(cubeFactor * (c.lastMaxCongestionWindow - currentCongestionWindow))))
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c.originPointCongestionWindow = c.lastMaxCongestionWindow
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}
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} else {
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// If sender was app-limited, then freeze congestion window growth during
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// app-limited period. Continue growth now by shifting the epoch-start
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// through the app-limited period.
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if shiftQuicCubicEpochWhenAppLimited && !c.appLimitedStartTime.IsZero() {
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shift := currentTime.Sub(c.appLimitedStartTime)
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c.epoch = c.epoch.Add(shift)
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c.appLimitedStartTime = time.Time{}
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}
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}
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// Change the time unit from microseconds to 2^10 fractions per second. Take
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// the round trip time in account. This is done to allow us to use shift as a
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// divide operator.
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elapsedTime := int64((currentTime.Add(delayMin).Sub(c.epoch)/time.Microsecond)<<10) / 1000000
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offset := int64(c.timeToOriginPoint) - elapsedTime
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// Right-shifts of negative, signed numbers have
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// implementation-dependent behavior. Force the offset to be
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// positive, similar to the kernel implementation.
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if offset < 0 {
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offset = -offset
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}
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deltaCongestionWindow := protocol.PacketNumber((cubeCongestionWindowScale * offset * offset * offset) >> cubeScale)
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var targetCongestionWindow protocol.PacketNumber
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if elapsedTime > int64(c.timeToOriginPoint) {
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targetCongestionWindow = c.originPointCongestionWindow + deltaCongestionWindow
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} else {
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targetCongestionWindow = c.originPointCongestionWindow - deltaCongestionWindow
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}
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// With dynamic beta/alpha based on number of active streams, it is possible
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// for the required_ack_count to become much lower than acked_packets_count_
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// suddenly, leading to more than one iteration through the following loop.
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for {
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// Update estimated TCP congestion_window.
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requiredAckCount := protocol.PacketNumber(float32(c.estimatedTCPcongestionWindow) / c.alpha())
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if c.ackedPacketsCount < requiredAckCount {
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break
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}
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c.ackedPacketsCount -= requiredAckCount
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c.estimatedTCPcongestionWindow++
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}
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// We have a new cubic congestion window.
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c.lastTargetCongestionWindow = targetCongestionWindow
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// Compute target congestion_window based on cubic target and estimated TCP
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// congestion_window, use highest (fastest).
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if targetCongestionWindow < c.estimatedTCPcongestionWindow {
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targetCongestionWindow = c.estimatedTCPcongestionWindow
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}
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return targetCongestionWindow
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}
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// SetNumConnections sets the number of emulated connections
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func (c *Cubic) SetNumConnections(n int) {
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c.numConnections = n
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}
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