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cubic.go
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cubic.go
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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
const cubeCongestionWindowScale = 410
const cubeFactor protocol.ByteCount = 1 << cubeScale / cubeCongestionWindowScale / protocol.DefaultTCPMSS
const defaultNumConnections = 2
// 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+protocol.DefaultTCPMSS < 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) * protocol.DefaultTCPMSS >> 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(protocol.DefaultTCPMSS) / 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
}