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Cubic pacing rate tolerates a zero smoothed RTT #109

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Cubic pacing rate tolerates a zero smoothed RTT #109

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`Cubic` pacing rate tolearates a zero smoothed RTT
rnro Aug 20, 2026
d8443aa
review comments
rnro Aug 25, 2026
67ef7d2
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rnro Aug 25, 2026
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23 changes: 14 additions & 9 deletions Sources/SwiftNetwork/QUIC/Cubic.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -226,16 +226,21 @@ struct Cubic: CongestionControlProtocol, CubicLikeProtocol {
guard let path, path.pacer.enabled else {
return
}
var rate = congestionWindow

// A short RTT can round to zero: `RTT.processNewSample` stores the sample as whole
// microseconds, so an ack duration under 500ns becomes 0µs and dividing by it below would
// trap. Fall back to the initial estimate.
let smoothedRTTInMicroseconds =
smoothedRTT.microseconds == 0 ? pacingInitialRTT.microseconds : smoothedRTT.microseconds

// Use 200% rate when in slow start
if congestionWindow < slowStartThreshold {
rate *= 2
}
// Multiply by USEC_PER_SEC as srtt is in microseconds

@rpaulo rpaulo Aug 21, 2026

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Keep this comment?

@rnro rnro Aug 25, 2026

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So I've taken a slightly different approach here. I think the underlying issue once I called it smoothedRTTInMicroseconds is that the units of rate were not clear. I've changed that and I think then the comment is not needed because the code is self-evident.

rate = (rate * System.Time.USEC_PER_SEC) / UInt64(smoothedRTT.microseconds)
let burst = rate >> burstQueueShift
path.pacer.setRate(rate: rate)
path.pacer.setBurstSize(burstSize: UInt32(truncatingIfNeeded: burst))
let pacedWindow = congestionWindow < slowStartThreshold ? congestionWindow * 2 : congestionWindow
let rateInBytesPerSecond =
pacedWindow * System.Time.USEC_PER_SEC / UInt64(smoothedRTTInMicroseconds)
let burstSize = rateInBytesPerSecond >> burstQueueShift

path.pacer.setRate(rate: rateInBytesPerSecond)
path.pacer.setBurstSize(burstSize: UInt32(truncatingIfNeeded: burstSize))
}

@discardableResult
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26 changes: 12 additions & 14 deletions Sources/SwiftNetwork/QUIC/Prague.swift
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Expand Up @@ -300,23 +300,21 @@ struct Prague: CongestionControlProtocol, CubicLikeProtocol {
guard let path, path.pacer.enabled else {
return
}
var sRTT = smoothedRTT.microseconds
if sRTT == 0 {
sRTT = pacingInitialRTT.microseconds
}
var rate = congestionWindow

// Use 200% rate when in slow start
if congestionWindow < slowStartThreshold {
rate *= 2
}
// A short RTT can round to zero: `RTT.processNewSample` stores the sample as whole
// microseconds, so an ack duration under 500ns becomes 0µs and dividing by it below would
// trap. Fall back to the initial estimate.
let smoothedRTTInMicroseconds =
smoothedRTT.microseconds == 0 ? pacingInitialRTT.microseconds : smoothedRTT.microseconds

// Multiply by USEC_PER_SEC as sRTT is in microseconds
rate = (rate * System.Time.USEC_PER_SEC) / UInt64(sRTT)
let burst = rate >> burstQueueShift
// Use 200% rate when in slow start
let pacedWindow = congestionWindow < slowStartThreshold ? congestionWindow * 2 : congestionWindow
let rateInBytesPerSecond =
pacedWindow * System.Time.USEC_PER_SEC / UInt64(smoothedRTTInMicroseconds)
let burstSize = rateInBytesPerSecond >> burstQueueShift

path.pacer.setRate(rate: rate)
path.pacer.setBurstSize(burstSize: UInt32(truncatingIfNeeded: burst))
path.pacer.setRate(rate: rateInBytesPerSecond)
path.pacer.setBurstSize(burstSize: UInt32(truncatingIfNeeded: burstSize))
}

private func packetInRecovery(sentTime: NetworkClock.Instant) -> Bool {
Expand Down
26 changes: 26 additions & 0 deletions Tests/QUICTests/CubicTests.swift
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Expand Up @@ -470,6 +470,32 @@ final class CubicTests: XCTestCase {
== Constants.maxBurstIntervalKernelPacing.milliseconds
)
}

/// A smoothed RTT that rounds to zero microseconds must not reach the pacing-rate division; it
/// traps there.
func testCubicPacerSurvivesASubMicrosecondSmoothedRTT() {
let connection = QUICConnection(context: NetworkContext.implicitContext)
let path = QUICPath(parent: connection)
path.pacePackets = true
path.set(interface: nil, priority: 1, isInitial: true)
path.pacer.setInitialState(10_000_000, 10000)
path.pacer.reset()

rtt.smoothedRTT = .nanoseconds(400)

// double-check that the rounding happens as expected
XCTAssertEqual(rtt.smoothedRTT.microseconds, 0, "smoothedRTT is expected to round to zero microseconds")

let time = NetworkClock.Instant.now
path.congestionControlPacketsSent(bytesSent: 1000)
path.congestionControlAckBegin()
path.congestionControlPacketsAcked(bytesAcked: 1000, sentTime: time)
path.congestionControlAckEnd(rtt: rtt, path: path, mss: path.mss, packetsLost: false)

// Now that we haven't trapped, assert the rate is as expected. One packet
// acked takes the window to 13000, slow start doubles it, and the 100ms fallback divides.
XCTAssertEqual(path.pacer.rate, 26000 * System.Time.USEC_PER_SEC / 100_000)
}
}

#endif