Consider first packet when reading Simulcast IDs

peerconnection.go

@@ -1784,7 +1784,20 @@
 		return err
 	}
 
+	// try to read simulcast IDs from the packet we already have
 	var mid, rid, rsid string
+	if _, err = handleUnknownRTPPacket(
+		b[:i], uint8(midExtensionID), //nolint:gosec // G115
+		uint8(streamIDExtensionID),       //nolint:gosec // G115
+		uint8(repairStreamIDExtensionID), //nolint:gosec // G115
+		&mid,
+		&rid,
+		&rsid,
+	); err != nil {
+		return err
+	}
+
+	// if the first packet didn't contain simuilcast IDs, then probe more packets
 	var paddingOnly bool
 	for readCount := 0; readCount <= simulcastProbeCount; readCount++ {
 		if mid == "" || (rid == "" && rsid == "") {
@@ -1798,7 +1811,7 @@
 				return err
 			}
 
-			if _, paddingOnly, err = handleUnknownRTPPacket(
+			if paddingOnly, err = handleUnknownRTPPacket(
 				b[:i], uint8(midExtensionID), //nolint:gosec // G115
 				uint8(streamIDExtensionID),       //nolint:gosec // G115
 				uint8(repairStreamIDExtensionID), //nolint:gosec // G115

peerconnection_media_test.go

@@ -1072,7 +1072,7 @@
 	closePairNow(t, pcOffer, pcAnswer)
 }
 
 func TestPeerConnection_Simulcast_Probe(t *testing.T) { //nolint:cyclop
 	lim := test.TimeOut(time.Second * 30) //nolint
 	defer lim.Stop()
 
@@ -1132,6 +1132,114 @@
 		close(testFinished)
 	})
 
+	// Assert that we can send just one packet with Simulcast IDs (using extensions) and they will be properly received
+	t.Run("ExtractIDs", func(t *testing.T) {
+		offerer, answerer, err := newPair()
+		assert.NoError(t, err)
+
+		rids := []string{"layer_1", "layer_2", "layer_3"}
+		ridSelected := rids[0]
+
+		onTrackCalled := &atomic.Bool{}
+		answerer.OnTrack(func(remote *TrackRemote, receiver *RTPReceiver) {
+			assert.Equal(t, remote.rid, ridSelected)
+			onTrackCalled.Store(true)
+		})
+
+		vp8WriterA, err := NewTrackLocalStaticRTP(
+			RTPCodecCapability{MimeType: MimeTypeVP8}, "video", "pion1", WithRTPStreamID(rids[0]),
+		)
+		assert.NoError(t, err)
+
+		vp8WriterB, err := NewTrackLocalStaticRTP(
+			RTPCodecCapability{MimeType: MimeTypeVP8}, "video", "pion1", WithRTPStreamID(rids[1]),
+		)
+		assert.NoError(t, err)
+
+		vp8WriterC, err := NewTrackLocalStaticRTP(
+			RTPCodecCapability{MimeType: MimeTypeVP8}, "video", "pion1", WithRTPStreamID(rids[2]),
+		)
+		assert.NoError(t, err)
+
+		sender, err := offerer.AddTrack(vp8WriterA)
+		assert.NoError(t, err)
+		assert.NotNil(t, sender)
+
+		assert.NoError(t, sender.AddEncoding(vp8WriterB))
+		assert.NoError(t, sender.AddEncoding(vp8WriterC))
+
+		assert.NoError(t, signalPair(offerer, answerer))
+
+		peerConnectionConnected := untilConnectionState(PeerConnectionStateConnected, offerer, answerer)
+		peerConnectionConnected.Wait()
+
+		parameters := sender.GetParameters()
+
+		var midID, ridID uint8
+		for _, extension := range parameters.HeaderExtensions {
+			switch extension.URI {
+			case sdp.SDESMidURI:
+				midID = uint8(extension.ID) //nolint:gosec // G115
+			case sdp.SDESRTPStreamIDURI:
+				ridID = uint8(extension.ID) //nolint:gosec // G115
+			}
+		}
+		assert.NotZero(t, midID)
+		assert.NotZero(t, ridID)
+
+		ticker := time.NewTicker(time.Millisecond * 20)
+		defer ticker.Stop()
+		testFinished := make(chan struct{})
+		seenOneStream, seenOneStreamCancel := context.WithCancel(context.Background())
+
+		go func() {
+			sentOnePacket := false
+
+			senderTrack := vp8WriterA
+
+			for {
+				select {
+				case <-testFinished:
+					return
+				case <-ticker.C:
+					answerer.dtlsTransport.lock.Lock()
+					if len(answerer.dtlsTransport.simulcastStreams) >= 1 {
+						seenOneStreamCancel()
+					}
+					answerer.dtlsTransport.lock.Unlock()
+
+					senderTrack.mu.Lock()
+
+					// We send just one packet with the RID, that's the point of this test
+					if !sentOnePacket && len(senderTrack.bindings) > 0 {
+						sentOnePacket = true
+
+						header := &rtp.Header{
+							Version: 2,
+							SSRC:    util.RandUint32(),
+						}
+						header.Extension = true
+						header.ExtensionProfile = 0x1000
+						assert.NoError(t, header.SetExtension(midID, []byte("0")))
+						assert.NoError(t, header.SetExtension(ridID, []byte(ridSelected)))
+
+						_, err = senderTrack.bindings[0].writeStream.WriteRTP(header, []byte{0, 1, 2, 3, 4, 5})
+						assert.NoError(t, err)
+					}
+
+					senderTrack.mu.Unlock()
+				}
+			}
+		}()
+
+		<-seenOneStream.Done()
+
+		assert.Equal(t, true, onTrackCalled.Load())
+
+		closePairNow(t, offerer, answerer)
+		close(testFinished)
+	})
+
 	// Assert that NonSimulcast Traffic isn't incorrectly broken by the probe
 	t.Run("Break NonSimulcast", func(t *testing.T) {
 		unhandledSimulcastError := make(chan struct{})

rtptransceiver.go

@@ -291,21 +291,20 @@
 	streamIDExtensionID,
 	repairStreamIDExtensionID uint8,
 	mid, rid, rsid *string,
-) (payloadType PayloadType, paddingOnly bool, err error) {
+) (paddingOnly bool, err error) {
 	rp := &rtp.Packet{}
 	if err = rp.Unmarshal(buf); err != nil {
-		return 0, false, err
+		return false, err
 	}
 
 	if rp.Padding && len(rp.Payload) == 0 {
 		paddingOnly = true
 	}
 
 	if !rp.Header.Extension {
-		return payloadType, paddingOnly, nil
+		return paddingOnly, nil
 	}
 
-	payloadType = PayloadType(rp.PayloadType)
 	if payload := rp.GetExtension(midExtensionID); payload != nil {
 		*mid = string(payload)
 	}
@@ -318,5 +317,5 @@
 		*rsid = string(payload)
 	}
 
-	return payloadType, paddingOnly, nil
+	return paddingOnly, nil
 }