RTP AV profile, streaming H.264+s24b

[tvtalkshowshigh]

I have a hardware device can send and receive streams of H.264 video and 24-bit PCM audio using the RTP Audio Video Profile. I can receive streams from it with VLC by passing VLC the SDP file that the device produces. I'd now like to send data back to the device, but I'm struggling to work out the command line to stream such data from VLC.

VLC can play the MKV file, and the SDP file looks fine so I'm confident that VLC supports RTP/AVP output. Is that correct?

This is my command line:
vlc sb.mkv :sout='#rtp{dst=224.2.2.4,port=6748,ttl=1,sdp="file:///tmp/test.sdp"}' :sout-all :sout-keep -L

If VLC can do this, where am I going wrong?

Many thanks.