github.com/apernet/quic-go@v0.43.1-0.20240515053213-5e9e635fd9f0/internal/congestion/pacer.go (about)

     1  package congestion
     2  
     3  import (
     4  	"time"
     5  
     6  	"github.com/apernet/quic-go/internal/protocol"
     7  )
     8  
     9  const maxBurstSizePackets = 10
    10  
    11  // The pacer implements a token bucket pacing algorithm.
    12  type pacer struct {
    13  	budgetAtLastSent  protocol.ByteCount
    14  	maxDatagramSize   protocol.ByteCount
    15  	lastSentTime      time.Time
    16  	adjustedBandwidth func() uint64 // in bytes/s
    17  }
    18  
    19  func newPacer(getBandwidth func() Bandwidth) *pacer {
    20  	p := &pacer{
    21  		maxDatagramSize: initialMaxDatagramSize,
    22  		adjustedBandwidth: func() uint64 {
    23  			// Bandwidth is in bits/s. We need the value in bytes/s.
    24  			bw := uint64(getBandwidth() / BytesPerSecond)
    25  			// Use a slightly higher value than the actual measured bandwidth.
    26  			// RTT variations then won't result in under-utilization of the congestion window.
    27  			// Ultimately, this will result in sending packets as acknowledgments are received rather than when timers fire,
    28  			// provided the congestion window is fully utilized and acknowledgments arrive at regular intervals.
    29  			return bw * 5 / 4
    30  		},
    31  	}
    32  	p.budgetAtLastSent = p.maxBurstSize()
    33  	return p
    34  }
    35  
    36  func (p *pacer) SentPacket(sendTime time.Time, size protocol.ByteCount) {
    37  	budget := p.Budget(sendTime)
    38  	if size >= budget {
    39  		p.budgetAtLastSent = 0
    40  	} else {
    41  		p.budgetAtLastSent = budget - size
    42  	}
    43  	p.lastSentTime = sendTime
    44  }
    45  
    46  func (p *pacer) Budget(now time.Time) protocol.ByteCount {
    47  	if p.lastSentTime.IsZero() {
    48  		return p.maxBurstSize()
    49  	}
    50  	budget := p.budgetAtLastSent + (protocol.ByteCount(p.adjustedBandwidth())*protocol.ByteCount(now.Sub(p.lastSentTime).Nanoseconds()))/1e9
    51  	if budget < 0 { // protect against overflows
    52  		budget = protocol.MaxByteCount
    53  	}
    54  	return min(p.maxBurstSize(), budget)
    55  }
    56  
    57  func (p *pacer) maxBurstSize() protocol.ByteCount {
    58  	return max(
    59  		protocol.ByteCount(uint64((protocol.MinPacingDelay+protocol.TimerGranularity).Nanoseconds())*p.adjustedBandwidth())/1e9,
    60  		maxBurstSizePackets*p.maxDatagramSize,
    61  	)
    62  }
    63  
    64  // TimeUntilSend returns when the next packet should be sent.
    65  // It returns the zero value of time.Time if a packet can be sent immediately.
    66  func (p *pacer) TimeUntilSend() time.Time {
    67  	if p.budgetAtLastSent >= p.maxDatagramSize {
    68  		return time.Time{}
    69  	}
    70  	diff := 1e9 * uint64(p.maxDatagramSize-p.budgetAtLastSent)
    71  	bw := p.adjustedBandwidth()
    72  	// We might need to round up this value.
    73  	// Otherwise, we might have a budget (slightly) smaller than the datagram size when the timer expires.
    74  	d := diff / bw
    75  	// this is effectively a math.Ceil, but using only integer math
    76  	if diff%bw > 0 {
    77  		d++
    78  	}
    79  	return p.lastSentTime.Add(max(protocol.MinPacingDelay, time.Duration(d)*time.Nanosecond))
    80  }
    81  
    82  func (p *pacer) SetMaxDatagramSize(s protocol.ByteCount) {
    83  	p.maxDatagramSize = s
    84  }