RTSP/UDP: RTCP NACK/RTX (RFC 4585/4588)

[benp-dev]

Question

Hi team,
Our encoder publishes to MediaMTX via RTSP/TCP (GStreamer rtspclientsink), and downstream NVR/VMS clients pull via RTSP/UDP on a congested LAN. To keep latency low while improving recovery from packet loss, especially IDR/keyframe loss, we’re looking for RTP/AVPF + RTCP NACK/RTX on the RTSP path. The NVR/VMS supports RTCP NACK.

Could you please confirm whether MediaMTX currently supports this? If so, how can we enable/configure it (e.g., retransmission cache/window, rtx-time)? If not, is support planned on the roadmap?

Example SDP lines:

a=rtcp-fb:<pt> nack
a=rtpmap:<rtx-pt> rtx/90000
a=fmtp:<rtx-pt> apt=<pt>;rtx-time=120

Thank you.

[aler9]

i've created a feature request here: bluenviron/gortsplib#901