Pion WebRTC: Video not received in Sub SFU when using ReplaceTrack in layered SFU architecture

Question:

I'm implementing a layered SFU architecture using Pion WebRTC in Go. The setup is as follows:

Architecture

• Master SFU:

  • Receives a single upstream client stream (audio + video).
  • Only forwards tracks to Sub SFUs. No mixing or transcoding.
  • Also allows downstream clients to pull the stream directly.

• Sub SFU(s):

  • Receives the forwarded streams from Master SFU.
  • Can provide streams to other clients.
  • Main purpose: scale to more users or geographic distribution.

• Single upstream client pushing a stream.

Flow:

Client P (push) ---> Master SFU ---> Sub SFU ---> Client(s) pull
                   |
                   |--> Client(s) pull

Problem

I use AddTransceiverFromKind during Sub SFU initialization and then ReplaceTrack in the OnTrackHandler when receiving tracks from the upstream stream.

Issue:

• Audio works fine.
• Video track never reaches the Sub SFU; the Sub SFU receives only audio.

I also tried setting the transceiver to RecvOnly or SendRecv on the Sub SFU side, but the video track still doesn't propagate.

Logs

• Master SFU

2025-11-05T18:04:20.353+0800    INFO    sfu/ontrack.go:55   Received remote audio track
2025-11-05T18:04:20.355+0800    INFO    sfu/ontrack.go:34   Received remote video track
2025-11-05T18:04:20.355+0800    INFO    sfu/ontrack.go:51   Successfully replaced sub SFU video track   {"subAddr": "127.0.0.1:20001"}
2025-11-05T18:04:20.355+0800    INFO    sfu/ontrack.go:80   Starting RTP forwarding {"TrackKind": "video"}
2025-11-05T18:04:20.355+0800    INFO    sfu/ontrack.go:70   Successfully replaced sub SFU audio track   {"subAddr": "127.0.0.1:20001"}
2025-11-05T18:04:20.355+0800    INFO    sfu/ontrack.go:80   Starting RTP forwarding {"TrackKind": "audio"}

• Sub SFU

2025-11-05T18:04:18.210+0800    INFO    gotest/main.go:136      waiting for video and audio tracks
2025-11-05T18:04:20.356+0800    INFO    sfu/ontrack.go:55       Received remote audio track

Relevant code snippets

1. Sub SFU initialization (AddTransceiverFromKind)

// Create PeerConnection
peerConn, err := api.NewPeerConnection(webrtc.Configuration{})
if err != nil {
    return nil, fmt.Errorf("failed to create peerConnection: %w", err)
}
peerConn.OnICEConnectionStateChange(func(state webrtc.ICEConnectionState) {
    log.Logger.Info("sub SFU ICEConnectionState:", zap.String("state", state.String()), zap.String("subAddr", subAddr))
})
// Add transceiver for receiving
if t, err := peerConn.AddTransceiverFromKind(webrtc.RTPCodecTypeVideo, webrtc.RTPTransceiverInit{
    Direction: webrtc.RTPTransceiverDirectionSendrecv,
}); err != nil {
    return nil, fmt.Errorf("failed to add transceiver for receiving Video: %w", err)
} else {
    sfu.videoRTPTransceiver = t
}
if t, err := peerConn.AddTransceiverFromKind(webrtc.RTPCodecTypeAudio, webrtc.RTPTransceiverInit{
    Direction: webrtc.RTPTransceiverDirectionSendrecv,
}); err != nil {
    return nil, fmt.Errorf("failed to add transceiver for receiving Audio: %w", err)
} else {
    sfu.audioRTPTransceiver = t
}
// And Create offer、Wait for ICE gathering to complete、Exchange SDP...

2. OnTrackHandler (ReplaceTrack + localTrack)

localTrack, err = webrtc.NewTrackLocalStaticRTP(
    remoteTrack.Codec().RTPCodecCapability, "video", "pion",
)
if err != nil {
    log.Logger.Fatal("Failed to create video track", zap.Error(err))
}
localVideoTrackChan <- localTrack
// Replace all sub SFU tracks
lock.Lock()
for subAddr, subSFU := range SubSFUMap {
    if err := subSFU.videoRTPTransceiver.Sender().ReplaceTrack(localTrack); err != nil {
        log.Logger.Error("Failed to replace sub SFU video track", zap.String("addr", subAddr), zap.Error(err), zap.String("subAddr", subAddr))
        continue
    }
    log.Logger.Info("Successfully replaced sub SFU video track", zap.String("subAddr", subAddr))
}
lock.Unlock()

3. RTP forwarding loop

go func() {
    log.Logger.Info("Starting RTP forwarding", zap.Any("TrackKind", remoteTrack.Kind()))
    rtpBuf := make([]byte, 1400)
    for {
        n, _, err := remoteTrack.Read(rtpBuf)
        if err != nil {
            log.Logger.Warn("Failed to read RTP", zap.Error(err))
            return
        }
        if _, err := localTrack.Write(rtpBuf[:n]); err != nil {
            log.Logger.Warn("Failed to write RTP", zap.Error(err))
        }
    }
}()

Questions

1. Could this issue be related to asynchronous RTP forwarding or buffering, like discussed in this issue?
2. Are there better patterns for single-push layered SFU with Pion WebRTC to forward video to Sub SFUs reliably?

Additional info

• Go version: 1.24.0
• Pion WebRTC version: v4.1.6
• Single upstream client (no simulcast / multiple tracks)
• Master SFU does not process tracks, just forwards to Sub SFU

What I've tried

• Changed transceiver direction (RecvOnly / SendRecv).
• Verified RTPSender is not nil before calling ReplaceTrack.
• Created separate localTrack per kind (video/audio).
• Checked that audio works, video does not.

Goal

• Master SFU: forward video + audio to Sub SFUs
• Sub SFUs: allow downstream clients to pull video + audio
• Avoid unnecessary SDP renegotiation if possible

Request

I would like to know what the issue is that is causing me to only receive audio and not be able to transmit video.