RkN,
Well, since we don't have any commands written specifically for the RTSP protocol, you'd want to use the TCP commands to delve into the packet information to find the specific bits that you're looking for.
What is it that you're trying to do with your rule? All the information of the RTSP request should be available to you in the packet for parsing and logic creation. It's just a matter of extracting what you want and putting it to use with the TCP commands.
The TCP::payload command will likely be invaluable as it returns the accumulated TCP data content. More information on this and other commands that may interest you can be found here:
Click here
Let me know if you have further questions.
-CW