The Real Data Transport protocol is used by realPlayer and other players to transport media samples. RDT frames can either be interleaved within the controlling RTSP connection or can be setup over a separate UDP stream.
RTSP is used by the client (player) and media server to set up RDT sessions.
The RDT dissector is very nearly complete.
Show stream setup information (default yes)
Register default UDP client port (default no)
Default UDP client port (default 6970, the port used by realPlayer)
A complete list of RDT display filter fields can be found in the display filter reference
Show only the RDT based traffic: rdt
You cannot directly filter RDT protocols while capturing. However, if you know the RTSP port used (tcp port 554 by default), you can filter on that (this will work at least for the case where RDT frames are interleaved within the RTSP connection)
There seems not to be full specification of RDT available, but the source for a player and server that implement the protocol are at Helix Community