Looking for some program that will do this...

Hi all, I'm looking for a program that can generate "real" looking traffic. It should be able to send TCP/UDP traffic to a large set of hosts (thousands), send traffic to random or semi-random ports and send packets/datagrams that are of random/semi-random size. Ideally it should run on windows or some sort of Perl script would be fine. Anyone have any ideas or should I dust off my programming books? Thanks, Dan