You can have look into:

• http://sourceforge.net/projects/ddosim/
• https://code.google.com/p/bonesi/

But if I were you, I would write a real one to attack a real system deployed by myself. Only in that way, can you really feel what is DDoS; and how to defence it.

Dear Friend, if you know how to capture and analyze network traffic then my opinion is you use snort or suricata IDS to protect DDOS attack practically. First you attack using various DDOS tools and then analyse the traffics. Then you make your own rule to defend DDOS attack. Its easy, realtime, and effective. 

Hey Rup, first of all you should define which type of DDoS attack do you want to simulate/defend (layer 3&4 or application layer). Second, you should define in which scenario you want to experiment. If you really want to do everything in a cloud environment I recommend you to take a look on the Amazon (http://aws.amazon.com/ec2/), but it is paid. However if you want to make things easier you don't need necessarily to perform your experiment on the cloud, and a set o VMs is enough. Third, to generate traffic you need a tool depending on the type of attack such as http://sourceforge.net/projects/ddosim/ (application layer attack) and https://code.google.com/p/bonesi/ (layer 3 and 4 attacks), as pointed out by Shiping. Finally, you need a defence mechanism (i.e., Intrusion Detection System - IDS) such as pointed out by Santosh: SNORT (https://www.snort.org) or Suricata (http://suricata-ids.org). If you have any question I will be glad to help you. Just send me a email!

I recommend you to use resources of a second cloud to mount the DDoS against the first cloud. Otherwise, it will be hard to achieve the necessary computing power.

Take a look at some GPSS implementation (maybe Minuteman Software's version). It is very good for simulation services' times, system load, clients (requests), etc.

You can also emulate network environment trough, for example, GNS3 platform. Put the VM's at the both ends and analyze traffic flow, traffic characteristics and everything else you need. 

I am working on DDoS attack detection in cloud will any one suggest me the implementation options available that are acceptable to SCI journals. I had not found any SCI paper having CloudSim as implementation tool for cloud security.

What could be the possible ways how DoS attacks can be launched in in-vehicle network Controller area network (CAN) bus and possible attack surfaces ? I would be grateful for your response.

You can look at several DOS Attack simulations options in NetSim provided in https://tetcos.com/file-exchange.html