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quic: add RTT estimator
Implement the round-trip time estimation algorithm from RFC 9002, Section 5. For golang/go#58547 Change-Id: I494e692e710f77270c9ad28354366f384feb4ac7 Reviewed-on: https://go-review.googlesource.com/c/net/+/499286 TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Damien Neil <dneil@google.com> Reviewed-by: Jonathan Amsterdam <jba@google.com>
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‎internal/quic/math.go‎

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// Copyright 2023 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build go1.21
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package quic
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func abs[T ~int | ~int64](a T) T {
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if a < 0 {
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return -a
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}
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return a
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}

‎internal/quic/rtt.go‎

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// Copyright 2023 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build go1.21
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package quic
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import (
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"time"
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)
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type rttState struct {
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minRTT time.Duration
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latestRTT time.Duration
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smoothedRTT time.Duration
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rttvar time.Duration // RTT variation
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firstSampleTime time.Time // time of first RTT sample
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}
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func (r *rttState) init() {
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r.minRTT = -1 // -1 indicates the first sample has not been taken yet
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// "[...] the initial RTT SHOULD be set to 333 milliseconds."
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-6.2.2-1
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const initialRTT = 333 * time.Millisecond
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.3-12
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r.smoothedRTT = initialRTT
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r.rttvar = initialRTT / 2
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}
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func (r *rttState) establishPersistentCongestion() {
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// "Endpoints SHOULD set the min_rtt to the newest RTT sample
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// after persistent congestion is established."
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// https://www.rfc-editor.org/rfc/rfc9002#section-5.2-5
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r.minRTT = r.latestRTT
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}
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// updateRTTSample is called when we generate a new RTT sample.
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5
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func (r *rttState) updateSample(now time.Time, handshakeConfirmed bool, spaceID numberSpace, latestRTT, ackDelay, maxAckDelay time.Duration) {
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r.latestRTT = latestRTT
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if r.minRTT < 0 {
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// First RTT sample.
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// "min_rtt MUST be set to the latest_rtt on the first RTT sample."
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.2-2
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r.minRTT = latestRTT
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.3-14
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r.smoothedRTT = latestRTT
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r.rttvar = latestRTT / 2
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r.firstSampleTime = now
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return
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}
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// "min_rtt MUST be set to the lesser of min_rtt and latest_rtt [...]
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// on all other samples."
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.2-2
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r.minRTT = min(r.minRTT, latestRTT)
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.3-16
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if handshakeConfirmed {
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ackDelay = min(ackDelay, maxAckDelay)
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}
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adjustedRTT := latestRTT - ackDelay
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if adjustedRTT < r.minRTT {
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adjustedRTT = latestRTT
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}
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r.smoothedRTT = ((7 * r.smoothedRTT) + adjustedRTT) / 8
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rttvarSample := abs(r.smoothedRTT - adjustedRTT)
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r.rttvar = (3*r.rttvar + rttvarSample) / 4
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}

‎internal/quic/rtt_test.go‎

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// Copyright 2023 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build go1.21
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package quic
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import (
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"testing"
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"time"
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)
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func TestRTTMinRTT(t *testing.T) {
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var (
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handshakeConfirmed = false
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ackDelay = 0 * time.Millisecond
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maxAckDelay = 25 * time.Millisecond
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now = time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC)
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)
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rtt := &rttState{}
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rtt.init()
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// "min_rtt MUST be set to the latest_rtt on the first RTT sample."
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.2-2
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 10*time.Millisecond, ackDelay, maxAckDelay)
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if got, want := rtt.latestRTT, 10*time.Millisecond; got != want {
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t.Errorf("on first sample: latest_rtt = %v, want %v", got, want)
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}
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if got, want := rtt.minRTT, 10*time.Millisecond; got != want {
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t.Errorf("on first sample: min_rtt = %v, want %v", got, want)
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}
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// "min_rtt MUST be set to the lesser of min_rtt and latest_rtt [...]
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// on all other samples."
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 20*time.Millisecond, ackDelay, maxAckDelay)
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if got, want := rtt.latestRTT, 20*time.Millisecond; got != want {
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t.Errorf("on increasing sample: latest_rtt = %v, want %v", got, want)
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}
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if got, want := rtt.minRTT, 10*time.Millisecond; got != want {
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t.Errorf("on increasing sample: min_rtt = %v, want %v (no change)", got, want)
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}
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 5*time.Millisecond, ackDelay, maxAckDelay)
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if got, want := rtt.latestRTT, 5*time.Millisecond; got != want {
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t.Errorf("on new minimum: latest_rtt = %v, want %v", got, want)
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}
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if got, want := rtt.minRTT, 5*time.Millisecond; got != want {
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t.Errorf("on new minimum: min_rtt = %v, want %v", got, want)
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}
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// "Endpoints SHOULD set the min_rtt to the newest RTT sample
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// after persistent congestion is established."
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// https://www.rfc-editor.org/rfc/rfc9002.html#section-5.2-5
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 15*time.Millisecond, ackDelay, maxAckDelay)
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if got, want := rtt.latestRTT, 15*time.Millisecond; got != want {
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t.Errorf("on increasing sample: latest_rtt = %v, want %v", got, want)
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}
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if got, want := rtt.minRTT, 5*time.Millisecond; got != want {
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t.Errorf("on increasing sample: min_rtt = %v, want %v (no change)", got, want)
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}
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rtt.establishPersistentCongestion()
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if got, want := rtt.minRTT, 15*time.Millisecond; got != want {
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t.Errorf("after persistent congestion: min_rtt = %v, want %v", got, want)
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}
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}
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func TestRTTInitialRTT(t *testing.T) {
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var (
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handshakeConfirmed = false
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ackDelay = 0 * time.Millisecond
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maxAckDelay = 25 * time.Millisecond
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now = time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC)
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)
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rtt := &rttState{}
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rtt.init()
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// "When no previous RTT is available,
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// the initial RTT SHOULD be set to 333 milliseconds."
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// https://www.rfc-editor.org/rfc/rfc9002#section-6.2.2-1
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if got, want := rtt.smoothedRTT, 333*time.Millisecond; got != want {
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t.Errorf("initial smoothed_rtt = %v, want %v", got, want)
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}
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if got, want := rtt.rttvar, 333*time.Millisecond/2; got != want {
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t.Errorf("initial rttvar = %v, want %v", got, want)
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}
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 10*time.Millisecond, ackDelay, maxAckDelay)
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smoothedRTT := 10 * time.Millisecond
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if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
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t.Errorf("after first rtt sample of 10ms, smoothed_rtt = %v, want %v", got, want)
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}
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rttvar := 5 * time.Millisecond
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if got, want := rtt.rttvar, rttvar; got != want {
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t.Errorf("after first rtt sample of 10ms, rttvar = %v, want %v", got, want)
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}
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// "[...] MAY ignore the acknowledgment delay for Initial packets [...]"
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// https://www.rfc-editor.org/rfc/rfc9002#section-5.3-7.1
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ackDelay = 1 * time.Millisecond
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rtt.updateSample(now, handshakeConfirmed, initialSpace, 10*time.Millisecond, ackDelay, maxAckDelay)
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adjustedRTT := 10 * time.Millisecond
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smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
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if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
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t.Errorf("smoothed_rtt = %v, want %v", got, want)
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}
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rttvarSample := abs(smoothedRTT - adjustedRTT)
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rttvar = (3*rttvar + rttvarSample) / 4
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if got, want := rtt.rttvar, rttvar; got != want {
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t.Errorf("rttvar = %v, want %v", got, want)
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}
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// "[...] SHOULD ignore the peer's max_ack_delay until the handshake is confirmed [...]"
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// https://www.rfc-editor.org/rfc/rfc9002#section-5.3-7.2
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ackDelay = 30 * time.Millisecond
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maxAckDelay = 25 * time.Millisecond
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rtt.updateSample(now, handshakeConfirmed, handshakeSpace, 40*time.Millisecond, ackDelay, maxAckDelay)
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adjustedRTT = 10 * time.Millisecond // latest_rtt (40ms) - ack_delay (30ms)
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smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
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if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
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t.Errorf("smoothed_rtt = %v, want %v", got, want)
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}
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rttvarSample = abs(smoothedRTT - adjustedRTT)
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rttvar = (3*rttvar + rttvarSample) / 4
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if got, want := rtt.rttvar, rttvar; got != want {
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t.Errorf("rttvar = %v, want %v", got, want)
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}
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// "[...] MUST use the lesser of the acknowledgment delay and
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// the peer's max_ack_delay after the handshake is confirmed [...]"
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// https://www.rfc-editor.org/rfc/rfc9002#section-5.3-7.3
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ackDelay = 30 * time.Millisecond
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maxAckDelay = 25 * time.Millisecond
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handshakeConfirmed = true
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rtt.updateSample(now, handshakeConfirmed, handshakeSpace, 40*time.Millisecond, ackDelay, maxAckDelay)
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adjustedRTT = 15 * time.Millisecond // latest_rtt (40ms) - max_ack_delay (25ms)
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smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
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if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
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t.Errorf("smoothed_rtt = %v, want %v", got, want)
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}
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rttvarSample = abs(smoothedRTT - adjustedRTT)
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rttvar = (3*rttvar + rttvarSample) / 4
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if got, want := rtt.rttvar, rttvar; got != want {
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t.Errorf("rttvar = %v, want %v", got, want)
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}
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// "[...] MUST NOT subtract the acknowledgment delay from
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// the RTT sample if the resulting value is smaller than the min_rtt."
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// https://www.rfc-editor.org/rfc/rfc9002#section-5.3-7.4
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ackDelay = 25 * time.Millisecond
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maxAckDelay = 25 * time.Millisecond
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handshakeConfirmed = true
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rtt.updateSample(now, handshakeConfirmed, handshakeSpace, 30*time.Millisecond, ackDelay, maxAckDelay)
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if got, want := rtt.minRTT, 10*time.Millisecond; got != want {
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t.Errorf("min_rtt = %v, want %v", got, want)
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}
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// latest_rtt (30ms) - ack_delay (25ms) = 5ms, which is less than min_rtt (10ms)
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adjustedRTT = 30 * time.Millisecond // latest_rtt
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smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
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if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
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t.Errorf("smoothed_rtt = %v, want %v", got, want)
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}
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rttvarSample = abs(smoothedRTT - adjustedRTT)
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rttvar = (3*rttvar + rttvarSample) / 4
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if got, want := rtt.rttvar, rttvar; got != want {
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t.Errorf("rttvar = %v, want %v", got, want)
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}
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}

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