[ixp1200] Packet Generator

Hi, I'm working of L3fwd16_RFC1812. I would like to check my program with packets in different sizes (random if possible) Is any of you can explain me how can I do that? or maybe you have more complicate packet generator that you can send me? Another question - How do I change the dependency of the receive and transmit in the project ? (I want the the receive microengines will send packets all the time, without depend in the transmit rate) Thanks a lot!!