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package congestion
import (
"time"
"github.com/quic-go/quic-go/internal/protocol"
)
const maxBurstSizePackets = 10
// The pacer implements a token bucket pacing algorithm.
type pacer struct {
budgetAtLastSent protocol.ByteCount
maxDatagramSize protocol.ByteCount
lastSentTime time.Time
adjustedBandwidth func() uint64 // in bytes/s
}
func newPacer(getBandwidth func() Bandwidth) *pacer {
p := &pacer{
maxDatagramSize: initialMaxDatagramSize,
adjustedBandwidth: func() uint64 {
// Bandwidth is in bits/s. We need the value in bytes/s.
bw := uint64(getBandwidth() / BytesPerSecond)
// Use a slightly higher value than the actual measured bandwidth.
// RTT variations then won't result in under-utilization of the congestion window.
// Ultimately, this will result in sending packets as acknowledgments are received rather than when timers fire,
// provided the congestion window is fully utilized and acknowledgments arrive at regular intervals.
return bw * 5 / 4
},
}
p.budgetAtLastSent = p.maxBurstSize()
return p
}
func (p *pacer) SentPacket(sendTime time.Time, size protocol.ByteCount) {
budget := p.Budget(sendTime)
if size >= budget {
p.budgetAtLastSent = 0
} else {
p.budgetAtLastSent = budget - size
}
p.lastSentTime = sendTime
}
func (p *pacer) Budget(now time.Time) protocol.ByteCount {
if p.lastSentTime.IsZero() {
return p.maxBurstSize()
}
budget := p.budgetAtLastSent + (protocol.ByteCount(p.adjustedBandwidth())*protocol.ByteCount(now.Sub(p.lastSentTime).Nanoseconds()))/1e9
if budget < 0 { // protect against overflows
budget = protocol.MaxByteCount
}
return min(p.maxBurstSize(), budget)
}
func (p *pacer) maxBurstSize() protocol.ByteCount {
return max(
protocol.ByteCount(uint64((protocol.MinPacingDelay+protocol.TimerGranularity).Nanoseconds())*p.adjustedBandwidth())/1e9,
maxBurstSizePackets*p.maxDatagramSize,
)
}
// TimeUntilSend returns when the next packet should be sent.
// It returns the zero value of time.Time if a packet can be sent immediately.
func (p *pacer) TimeUntilSend() time.Time {
if p.budgetAtLastSent >= p.maxDatagramSize {
return time.Time{}
}
diff := 1e9 * uint64(p.maxDatagramSize-p.budgetAtLastSent)
bw := p.adjustedBandwidth()
// We might need to round up this value.
// Otherwise, we might have a budget (slightly) smaller than the datagram size when the timer expires.
d := diff / bw
// this is effectively a math.Ceil, but using only integer math
if diff%bw > 0 {
d++
}
return p.lastSentTime.Add(max(protocol.MinPacingDelay, time.Duration(d)*time.Nanosecond))
}
func (p *pacer) SetMaxDatagramSize(s protocol.ByteCount) {
p.maxDatagramSize = s
}
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