Fix nack issue for svc codecs (#1856)

* Fix nack issue for svc codecs

* Fix test

pkg/sfu/forwarder.go

@@ -1599,8 +1599,10 @@ func (f *Forwarder) getTranslationParamsVideo(extPkt *buffer.ExtPacket, layer in
 	if !result.IsSelected {
 		tp.shouldDrop = true
 		if f.started && result.IsRelevant {
-			f.rtpMunger.UpdateAndGetSnTs(extPkt) // call to update highest incoming sequence number and other internal structures
-			f.rtpMunger.PacketDropped(extPkt)
+			// call to update highest incoming sequence number and other internal structures
+			if _, err := f.rtpMunger.UpdateAndGetSnTs(extPkt); err == nil {
+				f.rtpMunger.PacketDropped(extPkt)
+			}
 		}
 		return tp, nil
 	}

pkg/sfu/videolayerselector/dependencydescriptor.go

@@ -46,6 +46,9 @@ func (d *DependencyDescriptor) IsOvershootOkay() bool {
 }
 
 func (d *DependencyDescriptor) Select(extPkt *buffer.ExtPacket, _layer int32) (result VideoLayerSelectorResult) {
+	// a packet is always relevant for the svc codec
+	result.IsRelevant = true
+
 	ddwdt := extPkt.DependencyDescriptor
 	if ddwdt == nil {
 		// packet doesn't have dependency descriptor
@@ -54,9 +57,6 @@ func (d *DependencyDescriptor) Select(extPkt *buffer.ExtPacket, _layer int32) (r
 
 	dd := ddwdt.Descriptor
 
-	// a packet is relevant as long as it has DD extension
-	result.IsRelevant = true
-
 	frameNum := d.frameNum.Update(dd.FrameNumber)
 	extFrameNum := frameNum.ExtendedVal
 

pkg/sfu/videolayerselector/dependencydescriptor_test.go

@@ -4,11 +4,12 @@ import (
 	"sort"
 	"testing"
 
+	"github.com/pion/rtp"
+	"github.com/stretchr/testify/require"
+
 	"github.com/livekit/livekit-server/pkg/sfu/buffer"
 	dd "github.com/livekit/livekit-server/pkg/sfu/dependencydescriptor"
 	"github.com/livekit/protocol/logger"
-	"github.com/pion/rtp"
-	"github.com/stretchr/testify/require"
 )
 
 func TestDecodeTarget(t *testing.T) {
@@ -124,7 +125,7 @@ func TestDependencyDescriptor(t *testing.T) {
 	// no dd ext, dropped
 	ret := ddSelector.Select(&buffer.ExtPacket{}, 0)
 	require.False(t, ret.IsSelected)
-	require.False(t, ret.IsRelevant)
+	require.True(t, ret.IsRelevant)
 
 	// non key frame, dropped
 	ret = ddSelector.Select(&buffer.ExtPacket{