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switch to a byte-based congestion controller

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Chrome removed the packet-based congestion controller and switched to
byte-based as well.
This commit is contained in:
Marten Seemann 2018-04-06 14:54:20 +07:00
parent 2127e2f1de
commit 2b97fb41c9
13 changed files with 442 additions and 459 deletions
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124
internal/congestion/cubic_sender.go
View file

@ -8,9 +8,9 @@ import (
)
const (
maxBurstBytes = 3 * protocol.DefaultTCPMSS
defaultMinimumCongestionWindow protocol.PacketNumber = 2
renoBeta float32 = 0.7 // Reno backoff factor.
maxBurstBytes = 3 * protocol.DefaultTCPMSS
renoBeta float32 = 0.7 // Reno backoff factor.
defaultMinimumCongestionWindow protocol.ByteCount = 2 * protocol.DefaultTCPMSS
)
type cubicSender struct {
@ -31,12 +31,6 @@ type cubicSender struct {
// Track the largest packet number outstanding when a CWND cutback occurs.
largestSentAtLastCutback protocol.PacketNumber
// Congestion window in packets.
congestionWindow protocol.PacketNumber
// Slow start congestion window in packets, aka ssthresh.
slowstartThreshold protocol.PacketNumber
// Whether the last loss event caused us to exit slowstart.
// Used for stats collection of slowstartPacketsLost
lastCutbackExitedSlowstart bool
@ -44,24 +38,35 @@ type cubicSender struct {
// When true, exit slow start with large cutback of congestion window.
slowStartLargeReduction bool
// Minimum congestion window in packets.
minCongestionWindow protocol.PacketNumber
// Congestion window in packets.
congestionWindow protocol.ByteCount
// Maximum number of outstanding packets for tcp.
maxTCPCongestionWindow protocol.PacketNumber
// 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
// Number of connections to simulate.
numConnections int
// ACK counter for the Reno implementation.
congestionWindowCount protocol.ByteCount
numAckedPackets uint64
initialCongestionWindow protocol.PacketNumber
initialMaxCongestionWindow protocol.PacketNumber
initialCongestionWindow protocol.ByteCount
initialMaxCongestionWindow protocol.ByteCount
minSlowStartExitWindow protocol.ByteCount
}
var _ SendAlgorithm = &cubicSender{}
var _ SendAlgorithmWithDebugInfo = &cubicSender{}
// NewCubicSender makes a new cubic sender
func NewCubicSender(clock Clock, rttStats *RTTStats, reno bool, initialCongestionWindow, initialMaxCongestionWindow protocol.PacketNumber) SendAlgorithmWithDebugInfo {
func NewCubicSender(clock Clock, rttStats *RTTStats, reno bool, initialCongestionWindow, initialMaxCongestionWindow protocol.ByteCount) SendAlgorithmWithDebugInfo {
return &cubicSender{
rttStats: rttStats,
initialCongestionWindow: initialCongestionWindow,
@ -69,7 +74,7 @@ func NewCubicSender(clock Clock, rttStats *RTTStats, reno bool, initialCongestio
congestionWindow: initialCongestionWindow,
minCongestionWindow: defaultMinimumCongestionWindow,
slowstartThreshold: initialMaxCongestionWindow,
maxTCPCongestionWindow: initialMaxCongestionWindow,
maxCongestionWindow: initialMaxCongestionWindow,
numConnections: defaultNumConnections,
cubic: NewCubic(clock),
reno: reno,
@ -80,21 +85,26 @@ func NewCubicSender(clock Clock, rttStats *RTTStats, reno bool, initialCongestio
func (c *cubicSender) TimeUntilSend(bytesInFlight protocol.ByteCount) time.Duration {
if c.InRecovery() {
// PRR is used when in recovery.
if c.prr.TimeUntilSend(c.GetCongestionWindow(), bytesInFlight, c.GetSlowStartThreshold()) == 0 {
if c.prr.CanSend(c.GetCongestionWindow(), bytesInFlight, c.GetSlowStartThreshold()) {
return 0
}
}
delay := c.rttStats.SmoothedRTT() / time.Duration(2*c.GetCongestionWindow()/protocol.DefaultTCPMSS)
delay := c.rttStats.SmoothedRTT() / time.Duration(2*c.GetCongestionWindow())
if !c.InSlowStart() { // adjust delay, such that it's 1.25*cwd/rtt
delay = delay * 8 / 5
}
return delay
}
func (c *cubicSender) OnPacketSent(sentTime time.Time, bytesInFlight protocol.ByteCount, packetNumber protocol.PacketNumber, bytes protocol.ByteCount, isRetransmittable bool) bool {
// Only update bytesInFlight for data packets.
func (c *cubicSender) OnPacketSent(
sentTime time.Time,
bytesInFlight protocol.ByteCount,
packetNumber protocol.PacketNumber,
bytes protocol.ByteCount,
isRetransmittable bool,
) {
if !isRetransmittable {
return false
return
}
if c.InRecovery() {
// PRR is used when in recovery.
@ -102,7 +112,6 @@ func (c *cubicSender) OnPacketSent(sentTime time.Time, bytesInFlight protocol.By
}
c.largestSentPacketNumber = packetNumber
c.hybridSlowStart.OnPacketSent(packetNumber)
return true
}
func (c *cubicSender) InRecovery() bool {
@ -114,18 +123,18 @@ func (c *cubicSender) InSlowStart() bool {
}
func (c *cubicSender) GetCongestionWindow() protocol.ByteCount {
return protocol.ByteCount(c.congestionWindow) * protocol.DefaultTCPMSS
return c.congestionWindow
}
func (c *cubicSender) GetSlowStartThreshold() protocol.ByteCount {
return protocol.ByteCount(c.slowstartThreshold) * protocol.DefaultTCPMSS
return c.slowstartThreshold
}
func (c *cubicSender) ExitSlowstart() {
c.slowstartThreshold = c.congestionWindow
}
func (c *cubicSender) SlowstartThreshold() protocol.PacketNumber {
func (c *cubicSender) SlowstartThreshold() protocol.ByteCount {
return c.slowstartThreshold
}
@ -135,20 +144,29 @@ func (c *cubicSender) MaybeExitSlowStart() {
}
}
func (c *cubicSender) OnPacketAcked(ackedPacketNumber protocol.PacketNumber, ackedBytes protocol.ByteCount, bytesInFlight protocol.ByteCount) {
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() {
// PRR is used when in recovery.
c.prr.OnPacketAcked(ackedBytes)
return
}
c.maybeIncreaseCwnd(ackedPacketNumber, ackedBytes, bytesInFlight)
c.maybeIncreaseCwnd(ackedPacketNumber, ackedBytes, priorInFlight, eventTime)
if c.InSlowStart() {
c.hybridSlowStart.OnPacketAcked(ackedPacketNumber)
}
}
func (c *cubicSender) OnPacketLost(packetNumber protocol.PacketNumber, lostBytes protocol.ByteCount, bytesInFlight protocol.ByteCount) {
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 {
@ -156,10 +174,8 @@ func (c *cubicSender) OnPacketLost(packetNumber protocol.PacketNumber, lostBytes
c.stats.slowstartPacketsLost++
c.stats.slowstartBytesLost += lostBytes
if c.slowStartLargeReduction {
if c.stats.slowstartPacketsLost == 1 || (c.stats.slowstartBytesLost/protocol.DefaultTCPMSS) > (c.stats.slowstartBytesLost-lostBytes)/protocol.DefaultTCPMSS {
// Reduce congestion window by 1 for every mss of bytes lost.
c.congestionWindow = utils.MaxPacketNumber(c.congestionWindow-1, c.minCongestionWindow)
}
// Reduce congestion window by lost_bytes for every loss.
c.congestionWindow = utils.MaxByteCount(c.congestionWindow-lostBytes, c.minSlowStartExitWindow)
c.slowstartThreshold = c.congestionWindow
}
}
@ -170,17 +186,19 @@ func (c *cubicSender) OnPacketLost(packetNumber protocol.PacketNumber, lostBytes
c.stats.slowstartPacketsLost++
}
c.prr.OnPacketLost(bytesInFlight)
c.prr.OnPacketLost(priorInFlight)
// TODO(chromium): Separate out all of slow start into a separate class.
if c.slowStartLargeReduction && c.InSlowStart() {
c.congestionWindow = c.congestionWindow - 1
if c.congestionWindow >= 2*c.initialCongestionWindow {
c.minSlowStartExitWindow = c.congestionWindow / 2
}
c.congestionWindow = c.congestionWindow - protocol.DefaultTCPMSS
} else if c.reno {
c.congestionWindow = protocol.PacketNumber(float32(c.congestionWindow) * c.RenoBeta())
c.congestionWindow = protocol.ByteCount(float32(c.congestionWindow) * c.RenoBeta())
} else {
c.congestionWindow = c.cubic.CongestionWindowAfterPacketLoss(c.congestionWindow)
}
// Enforce a minimum congestion window.
if c.congestionWindow < c.minCongestionWindow {
c.congestionWindow = c.minCongestionWindow
}
@ -188,7 +206,7 @@ func (c *cubicSender) OnPacketLost(packetNumber protocol.PacketNumber, lostBytes
c.largestSentAtLastCutback = c.largestSentPacketNumber
// reset packet count from congestion avoidance mode. We start
// counting again when we're out of recovery.
c.congestionWindowCount = 0
c.numAckedPackets = 0
}
func (c *cubicSender) RenoBeta() float32 {
@ -201,32 +219,38 @@ func (c *cubicSender) RenoBeta() float32 {
// 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(ackedPacketNumber protocol.PacketNumber, ackedBytes protocol.ByteCount, bytesInFlight protocol.ByteCount) {
func (c *cubicSender) maybeIncreaseCwnd(
ackedPacketNumber 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(bytesInFlight) {
if !c.isCwndLimited(priorInFlight) {
c.cubic.OnApplicationLimited()
return
}
if c.congestionWindow >= c.maxTCPCongestionWindow {
if c.congestionWindow >= c.maxCongestionWindow {
return
}
if c.InSlowStart() {
// TCP slow start, exponential growth, increase by one for each ACK.
c.congestionWindow++
c.congestionWindow += protocol.DefaultTCPMSS
return
}
// Congestion avoidance
if c.reno {
// Classic Reno congestion avoidance.
c.congestionWindowCount++
c.numAckedPackets++
// Divide by num_connections to smoothly increase the CWND at a faster
// rate than conventional Reno.
if protocol.PacketNumber(c.congestionWindowCount*protocol.ByteCount(c.numConnections)) >= c.congestionWindow {
c.congestionWindow++
c.congestionWindowCount = 0
if c.numAckedPackets*uint64(c.numConnections) >= uint64(c.congestionWindow)/uint64(protocol.DefaultTCPMSS) {
c.congestionWindow += protocol.DefaultTCPMSS
c.numAckedPackets = 0
}
} else {
c.congestionWindow = utils.MinPacketNumber(c.maxTCPCongestionWindow, c.cubic.CongestionWindowAfterAck(c.congestionWindow, c.rttStats.MinRTT()))
c.congestionWindow = utils.MinByteCount(c.maxCongestionWindow, c.cubic.CongestionWindowAfterAck(ackedBytes, c.congestionWindow, c.rttStats.MinRTT(), eventTime))
}
}
@ -282,10 +306,10 @@ func (c *cubicSender) OnConnectionMigration() {
c.largestSentAtLastCutback = 0
c.lastCutbackExitedSlowstart = false
c.cubic.Reset()
c.congestionWindowCount = 0
c.numAckedPackets = 0
c.congestionWindow = c.initialCongestionWindow
c.slowstartThreshold = c.initialMaxCongestionWindow
c.maxTCPCongestionWindow = c.initialMaxCongestionWindow
c.maxCongestionWindow = c.initialMaxCongestionWindow
}
// SetSlowStartLargeReduction allows enabling the SSLR experiment