From 1d7e761aad19f4c7d0ef9f5e9df57a134baf84de Mon Sep 17 00:00:00 2001
From: Raja Subramanian <raja.gobi@tutanota.com>
Date: Tue, 6 Oct 2026 09:57:40 +0530
Subject: [PATCH] fix: switch VP8 temporal layer up only at a layer sync frame
 (#4947)

---
 pkg/sfu/forwarder_test.go                     | 42 +++++++++++++++++++
 .../temporallayerselector/vp8.go              | 11 ++++-
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/pkg/sfu/forwarder_test.go b/pkg/sfu/forwarder_test.go
index 931c79e7e..146662802 100644
--- a/pkg/sfu/forwarder_test.go
+++ b/pkg/sfu/forwarder_test.go
@@ -1933,6 +1933,48 @@ func TestForwarderGetTranslationParamsVideo(t *testing.T) {
 	require.Equal(t, f.lastSSRC, params.SSRC)
 }
 
+func TestForwarderVP8TemporalUpSwitchAtLayerSync(t *testing.T) {
+	f := newForwarder(testutils.TestVP8Codec, webrtc.RTPCodecTypeVideo)
+	f.vls.SetCurrent(buffer.VideoLayer{Spatial: 0, Temporal: 0})
+	f.vls.SetTarget(buffer.VideoLayer{Spatial: 0, Temporal: 2})
+
+	// a frame without Y may reference a dropped frame of its layer
+	steps := []struct {
+		name            string
+		tid             uint8
+		y               bool
+		expectedThis    int32
+		expectedCurrent int32
+	}{
+		{name: "TL2 without Y", tid: 2, expectedThis: 0, expectedCurrent: 0},
+		{name: "TL1 without Y", tid: 1, expectedThis: 0, expectedCurrent: 0},
+		{name: "TL0", tid: 0, expectedThis: 0, expectedCurrent: 0},
+		{name: "TL2 with Y before TL1 sync", tid: 2, y: true, expectedThis: 0, expectedCurrent: 0},
+		{name: "TL1 with Y", tid: 1, y: true, expectedThis: 1, expectedCurrent: 1},
+		{name: "TL2 without Y after TL1 switch", tid: 2, expectedThis: 1, expectedCurrent: 1},
+		{name: "TL2 with Y", tid: 2, y: true, expectedThis: 2, expectedCurrent: 2},
+	}
+	for i, step := range steps {
+		extPkt, err := testutils.GetTestExtPacketVP8(
+			&testutils.TestExtPacketParams{SequenceNumber: uint16(i), PayloadSize: 20},
+			&codec.VP8{S: true, I: true, PictureID: uint16(i), T: true, TID: step.tid, Y: step.y},
+		)
+		require.NoError(t, err)
+		require.Equal(t, step.expectedThis, f.vls.SelectTemporal(extPkt), step.name)
+		require.Equal(t, step.expectedCurrent, f.vls.GetCurrent().Temporal, step.name)
+	}
+
+	// key frame refreshes all buffers, up-switch at it even without Y
+	f.vls.SetCurrent(buffer.VideoLayer{Spatial: 0, Temporal: 0})
+	extPkt, err := testutils.GetTestExtPacketVP8(
+		&testutils.TestExtPacketParams{SequenceNumber: 10, PayloadSize: 20},
+		&codec.VP8{S: true, I: true, PictureID: 10, T: true, TID: 0, IsKeyFrame: true},
+	)
+	require.NoError(t, err)
+	require.Equal(t, int32(2), f.vls.SelectTemporal(extPkt))
+	require.Equal(t, int32(2), f.vls.GetCurrent().Temporal)
+}
+
 func TestForwarderGetSnTsForPadding(t *testing.T) {
 	f := newForwarder(testutils.TestVP8Codec, webrtc.RTPCodecTypeVideo)
 
diff --git a/pkg/sfu/videolayerselector/temporallayerselector/vp8.go b/pkg/sfu/videolayerselector/temporallayerselector/vp8.go
index 39480c439..bc5679c58 100644
--- a/pkg/sfu/videolayerselector/temporallayerselector/vp8.go
+++ b/pkg/sfu/videolayerselector/temporallayerselector/vp8.go
@@ -44,7 +44,16 @@ func (v *VP8) Select(extPkt *buffer.ExtPacket, current int32, target int32) (thi
 
 	tid := extPkt.Temporal
 	if current < target {
-		if tid > current && tid <= target && vp8.S {
+		// Up-switch only where nothing dropped can be referenced: a layer sync
+		// (Y) frame depends only on TL0 (RFC 7741), a key frame refreshes all
+		// reference buffers. Key frames also cover encoders that never set Y.
+		// A sync frame raises one layer at a time, the layers in between would
+		// otherwise forward frames that reference their dropped frames.
+		switch {
+		case vp8.IsKeyFrame && tid <= target:
+			this = target
+			next = target
+		case tid == current+1 && tid <= target && vp8.S && vp8.Y:
 			this = tid
 			next = tid
 		}