Hi...

what you want to check through the simulation .... for example are you testing the bandwidth ? TCP connection? or what ? 

Please check this link it may help you 

http://serverfault.com/questions/515908/tools-for-simulating-ddos-attacks 

Hi; 

Java Programming has many threads to simulate DDoS attack for cloud, I would recommend you to use Java threading to design and implement DDoS tool. 

if you need a good tool, you can use [can and able] to simulate DDoS attack in Cloud.

Regards  

you can follow "Cloud Computing Vulnerability: DDoS as its main Security Threat, and

Analysis of IDS as a Solution Model" Rajni Goel

I would like to prefer cloudsim I have my hands on ns2 and its working good but I am not getting enough tutorials

Hi

Plz have a look at

https://sourceforge.net/projects/ddosim/

https://sourceforge.net/projects/ddosim/