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cubic_sender.go
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cubic_sender.go
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package congestion
import (
"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.
maxDatagramSize = protocol.ByteCount(protocol.MaxPacketSizeIPv4)
maxBurstBytes = 3 * maxDatagramSize
renoBeta = 0.7 // Reno backoff factor.
maxCongestionWindow = protocol.MaxCongestionWindowPackets * maxDatagramSize
minCongestionWindow = 2 * maxDatagramSize
initialCongestionWindow = 32 * maxDatagramSize
)
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
// Minimum congestion window in packets.
minCongestionWindow protocol.ByteCount
// Maximum congestion window.
maxCongestionWindow 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
lastState logging.CongestionState
tracer logging.ConnectionTracer
}
var (
_ SendAlgorithm = &cubicSender{}
_ SendAlgorithmWithDebugInfos = &cubicSender{}
)
// NewCubicSender makes a new cubic sender
func NewCubicSender(clock Clock, rttStats *utils.RTTStats, reno bool, tracer logging.ConnectionTracer) *cubicSender {
return newCubicSender(clock, rttStats, reno, initialCongestionWindow, maxCongestionWindow, tracer)
}
func newCubicSender(clock Clock, rttStats *utils.RTTStats, reno bool, 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,
minCongestionWindow: minCongestionWindow,
slowStartThreshold: initialMaxCongestionWindow,
maxCongestionWindow: initialMaxCongestionWindow,
cubic: NewCubic(clock),
clock: clock,
reno: reno,
tracer: tracer,
}
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()) >= maxDatagramSize
}
func (c *cubicSender) OnPacketSent(
sentTime time.Time,
bytesInFlight 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()/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 protocol.ByteCount,
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 c.congestionWindow < c.minCongestionWindow {
c.congestionWindow = c.minCongestionWindow
}
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 += 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/maxDatagramSize) {
c.congestionWindow += 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 <= maxBurstBytes
}
// 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
c.maxCongestionWindow = c.initialMaxCongestionWindow
}
func (c *cubicSender) maybeTraceStateChange(new logging.CongestionState) {
if c.tracer == nil || new == c.lastState {
return
}
c.tracer.UpdatedCongestionState(new)
c.lastState = new
}