Hi. What we did was to simulate the rapid refresh of a page on the target server launched by the attacking system, to simulate a DDOS. The servers and systems were virtualized using VMWare. A paper detailing the theory and framework of our work is published at http://www.jacn.net/papers/86-S3006.pdf

You can find a helpful dataset provided by CAIDA here: http://www.caida.org/data/passive/ddos-20070804_dataset.xml

It gives you traffic traces from a real DDoS attack (in 2007). You can use these to drive your simulations to ensure realistic results.