112 lines
3.9 KiB
Go
112 lines
3.9 KiB
Go
package congestion
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import (
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"time"
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"github.com/metacubex/quic-go/congestion"
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)
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// Note(pwestin): the magic clamping numbers come from the original code in
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// tcp_cubic.c.
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const hybridStartLowWindow = congestion.ByteCount(16)
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// Number of delay samples for detecting the increase of delay.
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const hybridStartMinSamples = uint32(8)
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// Exit slow start if the min rtt has increased by more than 1/8th.
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const hybridStartDelayFactorExp = 3 // 2^3 = 8
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// The original paper specifies 2 and 8ms, but those have changed over time.
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const (
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hybridStartDelayMinThresholdUs = int64(4000)
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hybridStartDelayMaxThresholdUs = int64(16000)
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)
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// HybridSlowStart implements the TCP hybrid slow start algorithm
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type HybridSlowStart struct {
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endPacketNumber congestion.PacketNumber
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lastSentPacketNumber congestion.PacketNumber
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started bool
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currentMinRTT time.Duration
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rttSampleCount uint32
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hystartFound bool
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}
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// StartReceiveRound is called for the start of each receive round (burst) in the slow start phase.
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func (s *HybridSlowStart) StartReceiveRound(lastSent congestion.PacketNumber) {
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s.endPacketNumber = lastSent
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s.currentMinRTT = 0
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s.rttSampleCount = 0
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s.started = true
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}
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// IsEndOfRound returns true if this ack is the last packet number of our current slow start round.
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func (s *HybridSlowStart) IsEndOfRound(ack congestion.PacketNumber) bool {
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return s.endPacketNumber < ack
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}
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// ShouldExitSlowStart should be called on every new ack frame, since a new
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// RTT measurement can be made then.
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// rtt: the RTT for this ack packet.
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// minRTT: is the lowest delay (RTT) we have seen during the session.
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// congestionWindow: the congestion window in packets.
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func (s *HybridSlowStart) ShouldExitSlowStart(latestRTT time.Duration, minRTT time.Duration, congestionWindow congestion.ByteCount) bool {
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if !s.started {
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// Time to start the hybrid slow start.
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s.StartReceiveRound(s.lastSentPacketNumber)
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}
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if s.hystartFound {
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return true
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}
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// Second detection parameter - delay increase detection.
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// Compare the minimum delay (s.currentMinRTT) of the current
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// burst of packets relative to the minimum delay during the session.
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// Note: we only look at the first few(8) packets in each burst, since we
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// only want to compare the lowest RTT of the burst relative to previous
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// bursts.
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s.rttSampleCount++
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if s.rttSampleCount <= hybridStartMinSamples {
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if s.currentMinRTT == 0 || s.currentMinRTT > latestRTT {
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s.currentMinRTT = latestRTT
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}
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}
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// We only need to check this once per round.
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if s.rttSampleCount == hybridStartMinSamples {
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// Divide minRTT by 8 to get a rtt increase threshold for exiting.
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minRTTincreaseThresholdUs := int64(minRTT / time.Microsecond >> hybridStartDelayFactorExp)
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// Ensure the rtt threshold is never less than 2ms or more than 16ms.
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minRTTincreaseThresholdUs = Min(minRTTincreaseThresholdUs, hybridStartDelayMaxThresholdUs)
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minRTTincreaseThreshold := time.Duration(Max(minRTTincreaseThresholdUs, hybridStartDelayMinThresholdUs)) * time.Microsecond
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if s.currentMinRTT > (minRTT + minRTTincreaseThreshold) {
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s.hystartFound = true
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}
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}
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// Exit from slow start if the cwnd is greater than 16 and
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// increasing delay is found.
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return congestionWindow >= hybridStartLowWindow && s.hystartFound
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}
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// OnPacketSent is called when a packet was sent
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func (s *HybridSlowStart) OnPacketSent(packetNumber congestion.PacketNumber) {
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s.lastSentPacketNumber = packetNumber
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}
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// OnPacketAcked gets invoked after ShouldExitSlowStart, so it's best to end
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// the round when the final packet of the burst is received and start it on
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// the next incoming ack.
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func (s *HybridSlowStart) OnPacketAcked(ackedPacketNumber congestion.PacketNumber) {
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if s.IsEndOfRound(ackedPacketNumber) {
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s.started = false
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}
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}
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// Started returns true if started
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func (s *HybridSlowStart) Started() bool {
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return s.started
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}
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// Restart the slow start phase
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func (s *HybridSlowStart) Restart() {
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s.started = false
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s.hystartFound = false
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}
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