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The recent tide of Distributed Denial of Service (DDoS) attacks against high-profile web sites demonstrate how devastating DDoS attacks are and how defenseless the Internet is under such attacks. We design a practical DDoS defense system that can protect the availability of web services during severe DDoS attacks. The basic idea behind our system is to isolate and protect legitimate traffic from a huge volume of DDoS traffic when an attack occurs. Traffic that needs to be protected can be recognized and protected using efficient cryptographic techniques. Therefore, by provisioning adequate resource (e.g., bandwidth) to legitimate traffic separated by this process, we are able to provide adequate service to a large percentage of clients during DDoS attacks. The worst-case performance (effectiveness) of the system is evaluated based on a novel game theoretical framework, which characterizes the natural adversarial relationship between a DDoS adversary and the proposed system. We also conduct a simulation study to verify a key assumption used in the game-theoretical analysis and to demonstrate the system dynamics during an attack.