there is no way to control which port an RTMFP client will use. RTMFP clients bind to a random UDP port. and of course if you go through a NAT, then the translated port will be a different and totally random port.
the server exerts no influence over the choice of port.
RTMFP's NAT/firewall traversal techniques make it so that if you allow "outbound UDP" to any port (and allow replies to come back in), you should be able to do P2P in most cases.
some of the tricks "other applications" use wouldn't be appropriate for Flash Player (in particular, relaying/tunneling via other peers, and using well-known ports that are often let through firewalls for their well-known "real" applications, (such as UDP port 53/DNS)).
Thanks, that's what I needed to know.
For what it's worth, I continue to think that it would be worthwhile to allow ActionScript to tell the RTMFP connection object that it should only try to establish a connection over a given range of ports. That way, folks in our position could tell network administrators, "You only need to open up ports x-y", not, "You need to open up every single port."
Excuse my ignorance, but isn't this similar to how things like Oracle TNS work? The initial handshake for database connection is (usually) port 1521, after which the session connection is maintian on some very high port number, albeit TCP. I can't imagine there'd be a rule(s) on firewalls for all these high number ports.
Also, check out this link for RTMFP connectivity test - it checks/reports on whether you can access an RTMFP server from wherever you are: