fix: BBR scale window for datagram size

https://github.com/apernet/hysteria/commit/61c75a260f935a6c5adf2f3ce0441f117c65fcbb

transport/tuic/congestion_v2/bbr_sender.go

@@ -27,16 +27,13 @@ const (
 
 	invalidPacketNumber            = -1
 	initialCongestionWindowPackets = 32
+	minCongestionWindowPackets     = 4
 
 	// Constants based on TCP defaults.
 	// The minimum CWND to ensure delayed acks don't reduce bandwidth measurements.
 	// Does not inflate the pacing rate.
-	defaultMinimumCongestionWindow = 4 * congestion.ByteCount(congestion.InitialPacketSize)
-
 	// The gain used for the STARTUP, equal to 2/ln(2).
 	defaultHighGain = 2.885
-	// The newly derived gain for STARTUP, equal to 4 * ln(2)
-	derivedHighGain = 2.773
 	// The newly derived CWND gain for STARTUP, 2.
 	derivedHighCWNDGain = 2.0
 )
@@ -63,7 +60,6 @@ const (
 	// Flag.
 	defaultStartupFullLossCount  = 8
 	quicBbr2DefaultLossThreshold = 0.02
-	maxBbrBurstPackets           = 10
 )
 
 type bbrMode int
@@ -267,7 +263,7 @@ func newBbrSender(
 		congestionWindow:             initialCongestionWindow,
 		initialCongestionWindow:      initialCongestionWindow,
 		maxCongestionWindow:          initialMaxCongestionWindow,
-		minCongestionWindow:          defaultMinimumCongestionWindow,
+		minCongestionWindow:          minCongestionWindowForMaxDatagramSize(initialMaxDatagramSize),
 		highGain:                     defaultHighGain,
 		highCwndGain:                 defaultHighGain,
 		drainGain:                    1.0 / defaultHighGain,
@@ -285,19 +281,37 @@ func newBbrSender(
 	}
 	b.pacer = NewPacer(b.bandwidthForPacer)
 
-	/*
-		if b.tracer != nil {
-			b.lastState = logging.CongestionStateStartup
-			b.tracer.UpdatedCongestionState(logging.CongestionStateStartup)
-		}
-	*/
-
 	b.enterStartupMode()
 	b.setHighCwndGain(derivedHighCWNDGain)
 
 	return b
 }
 
+func minCongestionWindowForMaxDatagramSize(maxDatagramSize congestion.ByteCount) congestion.ByteCount {
+	return minCongestionWindowPackets * maxDatagramSize
+}
+
+func scaleByteWindowForDatagramSize(window, oldMaxDatagramSize, newMaxDatagramSize congestion.ByteCount) congestion.ByteCount {
+	if oldMaxDatagramSize == newMaxDatagramSize {
+		return window
+	}
+	return congestion.ByteCount(uint64(window) * uint64(newMaxDatagramSize) / uint64(oldMaxDatagramSize))
+}
+
+func (b *bbrSender) rescalePacketSizedWindows(maxDatagramSize congestion.ByteCount) {
+	oldMaxDatagramSize := b.maxDatagramSize
+	b.maxDatagramSize = maxDatagramSize
+	b.initialCongestionWindow = scaleByteWindowForDatagramSize(b.initialCongestionWindow, oldMaxDatagramSize, maxDatagramSize)
+	b.maxCongestionWindow = scaleByteWindowForDatagramSize(b.maxCongestionWindow, oldMaxDatagramSize, maxDatagramSize)
+	b.minCongestionWindow = minCongestionWindowForMaxDatagramSize(maxDatagramSize)
+	b.cwndToCalculateMinPacingRate = scaleByteWindowForDatagramSize(b.cwndToCalculateMinPacingRate, oldMaxDatagramSize, maxDatagramSize)
+	b.maxCongestionWindowWithNetworkParametersAdjusted = scaleByteWindowForDatagramSize(
+		b.maxCongestionWindowWithNetworkParametersAdjusted,
+		oldMaxDatagramSize,
+		maxDatagramSize,
+	)
+}
+
 func (b *bbrSender) SetRTTStatsProvider(provider congestion.RTTStatsProvider) {
 	b.rttStats = provider
 }
@@ -330,8 +344,6 @@ func (b *bbrSender) OnPacketSent(
 	}
 
 	b.sampler.OnPacketSent(sentTime, packetNumber, bytes, bytesInFlight, isRetransmittable)
-
-	b.maybeAppLimited(bytesInFlight)
 }
 
 // CanSend implements the SendAlgorithm interface.
@@ -364,11 +376,18 @@ func (b *bbrSender) SetMaxDatagramSize(s congestion.ByteCount) {
 	if s < b.maxDatagramSize {
 		panic(fmt.Sprintf("congestion BUG: decreased max datagram size from %d to %d", b.maxDatagramSize, s))
 	}
-	cwndIsMinCwnd := b.congestionWindow == b.minCongestionWindow
-	b.maxDatagramSize = s
-	if cwndIsMinCwnd {
+	oldMinCongestionWindow := b.minCongestionWindow
+	oldInitialCongestionWindow := b.initialCongestionWindow
+	b.rescalePacketSizedWindows(s)
+	switch b.congestionWindow {
+	case oldMinCongestionWindow:
 		b.congestionWindow = b.minCongestionWindow
+	case oldInitialCongestionWindow:
+		b.congestionWindow = b.initialCongestionWindow
+	default:
+		b.congestionWindow = Min(b.maxCongestionWindow, Max(b.congestionWindow, b.minCongestionWindow))
 	}
+	b.recoveryWindow = Min(b.maxCongestionWindow, Max(b.recoveryWindow, b.minCongestionWindow))
 	b.pacer.SetMaxDatagramSize(s)
 }
 
@@ -411,6 +430,8 @@ func (b *bbrSender) OnCongestionEventEx(priorInFlight congestion.ByteCount, even
 	// packet in lost_packets.
 	var lastPacketSendState sendTimeState
 
+	b.maybeAppLimited(priorInFlight)
+
 	// Update bytesInFlight
 	b.bytesInFlight = priorInFlight
 	for _, p := range ackedPackets {
@@ -508,22 +529,6 @@ func (b *bbrSender) PacingRate() Bandwidth {
 	return b.pacingRate
 }
 
-func (b *bbrSender) hasGoodBandwidthEstimateForResumption() bool {
-	return b.hasNonAppLimitedSample()
-}
-
-func (b *bbrSender) hasNonAppLimitedSample() bool {
-	return b.hasNoAppLimitedSample
-}
-
-// Sets the pacing gain used in STARTUP.  Must be greater than 1.
-func (b *bbrSender) setHighGain(highGain float64) {
-	b.highGain = highGain
-	if b.mode == bbrModeStartup {
-		b.pacingGain = highGain
-	}
-}
-
 // Sets the CWND gain used in STARTUP.  Must be greater than 1.
 func (b *bbrSender) setHighCwndGain(highCwndGain float64) {
 	b.highCwndGain = highCwndGain
@@ -532,11 +537,6 @@ func (b *bbrSender) setHighCwndGain(highCwndGain float64) {
 	}
 }
 
-// Sets the gain used in DRAIN.  Must be less than 1.
-func (b *bbrSender) setDrainGain(drainGain float64) {
-	b.drainGain = drainGain
-}
-
 // Get the current bandwidth estimate. Note that Bandwidth is in bits per second.
 func (b *bbrSender) bandwidthEstimate() Bandwidth {
 	return b.maxBandwidth.GetBest()
@@ -697,12 +697,7 @@ func (b *bbrSender) checkIfFullBandwidthReached(lastPacketSendState *sendTimeSta
 }
 
 func (b *bbrSender) maybeAppLimited(bytesInFlight congestion.ByteCount) {
-	congestionWindow := b.GetCongestionWindow()
-	if bytesInFlight >= congestionWindow {
-		return
-	}
-	availableBytes := congestionWindow - bytesInFlight
-	if availableBytes > maxBbrBurstPackets*b.maxDatagramSize {
+	if bytesInFlight < b.getTargetCongestionWindow(1) {
 		b.sampler.OnAppLimited()
 	}
 }
@@ -729,7 +724,7 @@ func (b *bbrSender) maybeEnterOrExitProbeRtt(now monotime.Time, isRoundStart, mi
 		b.pacingGain = 1.0
 		// Do not decide on the time to exit PROBE_RTT until the |bytes_in_flight|
 		// is at the target small value.
-		b.exitProbeRttAt = monotime.Time(0)
+		b.exitProbeRttAt = 0
 	}
 
 	if b.mode == bbrModeProbeRtt {