Joining a Multicast Stream via Rtsp Using Ffplay

I have a RTSP server that provides access to a multicast stream. I can play it fine in VLC and various other players, however in ffmpeg / ffplay I get no data back.

After investigation this seems to be caused by the RTSP SETUP listing only unicast as a transport method.

If I use:

ffplay -rtsp_transport udp_multicast rtsp://<blah>

then it works fine.

How does ffmpeg decide which transport methods to put into its SETUP call? Is it perhaps some part of the SDP returned by the DESCRIBE call? Ideally I'd like to find a way to allow ffplay to play the multicast stream without the need to force the transport mode.