snapshot of https://github.com/livekit/livekit.git @ 08c5433449c106943231037d0b892895f6e92d3b (2026-09-15)

pkg/sfu/bwe/sendsidebwe/twcc_feedback.go

@@ -0,0 +1,127 @@
+// Copyright 2023 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sendsidebwe
+
+import (
+	"time"
+
+	"github.com/livekit/protocol/logger"
+	"github.com/pion/rtcp"
+	"go.uber.org/zap/zapcore"
+)
+
+// ------------------------------------------------------
+
+const (
+	cOutlierReportFactor            = 3
+	cEstimatedFeedbackIntervalAlpha = float64(0.9)
+
+	cReferenceTimeMask       = (1 << 24) - 1
+	cReferenceTimeResolution = 64 // 64 ms
+)
+
+// ------------------------------------------------------
+
+type twccFeedbackParams struct {
+	Logger logger.Logger
+}
+
+type twccFeedback struct {
+	params twccFeedbackParams
+
+	lastFeedbackTime          time.Time
+	estimatedFeedbackInterval time.Duration
+	numReports                int
+	numReportsOutOfOrder      int
+
+	highestFeedbackCount uint8
+
+	cycles               int64
+	highestReferenceTime uint32
+}
+
+func newTWCCFeedback(params twccFeedbackParams) *twccFeedback {
+	return &twccFeedback{
+		params: params,
+	}
+}
+
+func (t *twccFeedback) ProcessReport(report *rtcp.TransportLayerCC, at time.Time) (int64, bool) {
+	t.numReports++
+	if t.lastFeedbackTime.IsZero() {
+		t.lastFeedbackTime = at
+		t.highestReferenceTime = report.ReferenceTime
+		t.highestFeedbackCount = report.FbPktCount
+		return (t.cycles + int64(report.ReferenceTime)) * cReferenceTimeResolution * 1000, false
+	}
+
+	isOutOfOrder := false
+	if (report.FbPktCount - t.highestFeedbackCount) > (1 << 7) {
+		t.numReportsOutOfOrder++
+		isOutOfOrder = true
+		t.params.Logger.Infow(
+			"send side bwe: received out-of-order feedback report",
+			"highestFeedbackCount", t.highestFeedbackCount,
+			"fbPktCount", report.FbPktCount,
+			"numReportsOutOfOrder", t.numReportsOutOfOrder,
+		)
+	}
+
+	// reference time wrap around handling
+	var referenceTime int64
+	if (report.ReferenceTime-t.highestReferenceTime)&cReferenceTimeMask < (1 << 23) {
+		if report.ReferenceTime < t.highestReferenceTime {
+			t.cycles += (1 << 24)
+		}
+		t.highestReferenceTime = report.ReferenceTime
+		referenceTime = t.cycles + int64(report.ReferenceTime)
+	} else {
+		cycles := t.cycles
+		if report.ReferenceTime > t.highestReferenceTime && cycles >= (1<<24) {
+			cycles -= (1 << 24)
+		}
+		referenceTime = cycles + int64(report.ReferenceTime)
+	}
+
+	if !isOutOfOrder {
+		sinceLast := at.Sub(t.lastFeedbackTime)
+		if t.estimatedFeedbackInterval == 0 {
+			t.estimatedFeedbackInterval = sinceLast
+		} else {
+			// filter out outliers from estimate
+			if sinceLast > t.estimatedFeedbackInterval/cOutlierReportFactor && sinceLast < cOutlierReportFactor*t.estimatedFeedbackInterval {
+				// smoothed version of inter feedback interval
+				t.estimatedFeedbackInterval = time.Duration(cEstimatedFeedbackIntervalAlpha*float64(t.estimatedFeedbackInterval) + (1.0-cEstimatedFeedbackIntervalAlpha)*float64(sinceLast))
+			}
+		}
+		t.lastFeedbackTime = at
+		t.highestFeedbackCount = report.FbPktCount
+	}
+
+	return referenceTime * cReferenceTimeResolution * 1000, isOutOfOrder
+}
+
+func (t *twccFeedback) MarshalLogObject(e zapcore.ObjectEncoder) error {
+	if t == nil {
+		return nil
+	}
+
+	e.AddTime("lastFeedbackTime", t.lastFeedbackTime)
+	e.AddDuration("estimatedFeedbackInterval", t.estimatedFeedbackInterval)
+	e.AddInt("numReports", t.numReports)
+	e.AddInt("numReportsOutOfOrder", t.numReportsOutOfOrder)
+	e.AddInt64("cycles", t.cycles/(1<<24))
+	return nil
+}