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prr_sender.go
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prr_sender.go
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package congestion
import (
"github.com/lucas-clemente/quic-go/internal/protocol"
)
// PrrSender implements the Proportional Rate Reduction (PRR) per RFC 6937
type PrrSender struct {
bytesSentSinceLoss protocol.ByteCount
bytesDeliveredSinceLoss protocol.ByteCount
ackCountSinceLoss protocol.ByteCount
bytesInFlightBeforeLoss protocol.ByteCount
}
// OnPacketSent should be called after a packet was sent
func (p *PrrSender) OnPacketSent(sentBytes protocol.ByteCount) {
p.bytesSentSinceLoss += sentBytes
}
// OnPacketLost should be called on the first loss that triggers a recovery
// period and all other methods in this class should only be called when in
// recovery.
func (p *PrrSender) OnPacketLost(priorInFlight protocol.ByteCount) {
p.bytesSentSinceLoss = 0
p.bytesInFlightBeforeLoss = priorInFlight
p.bytesDeliveredSinceLoss = 0
p.ackCountSinceLoss = 0
}
// OnPacketAcked should be called after a packet was acked
func (p *PrrSender) OnPacketAcked(ackedBytes protocol.ByteCount) {
p.bytesDeliveredSinceLoss += ackedBytes
p.ackCountSinceLoss++
}
// CanSend returns if packets can be sent
func (p *PrrSender) CanSend(congestionWindow, bytesInFlight, slowstartThreshold protocol.ByteCount) bool {
// Return QuicTime::Zero In order to ensure limited transmit always works.
if p.bytesSentSinceLoss == 0 || bytesInFlight < protocol.DefaultTCPMSS {
return true
}
if congestionWindow > bytesInFlight {
// During PRR-SSRB, limit outgoing packets to 1 extra MSS per ack, instead
// of sending the entire available window. This prevents burst retransmits
// when more packets are lost than the CWND reduction.
// limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
return p.bytesDeliveredSinceLoss+p.ackCountSinceLoss*protocol.DefaultTCPMSS > p.bytesSentSinceLoss
}
// Implement Proportional Rate Reduction (RFC6937).
// Checks a simplified version of the PRR formula that doesn't use division:
// AvailableSendWindow =
// CEIL(prr_delivered * ssthresh / BytesInFlightAtLoss) - prr_sent
return p.bytesDeliveredSinceLoss*slowstartThreshold > p.bytesSentSinceLoss*p.bytesInFlightBeforeLoss
}