DDoS means a distributed denial-of-service attack under which a multitude of compromised systems attack a single target, thereby causing users of the targeted system to be deprived of the services of the targeted system. Since the incoming traffic flooding the victim originates from many different sources, it is impossible to stop the attack simply by using ingress filtering. It also makes it very difficult to distinguish legitimate user traffic from the attack traffic when spread across so many points of origin. The use of spoofing IP sender addresses further complicates the issue.