Abstract: Distributed Denial of Service (DDoS) attacks are a relatively new type of attack on the availability of Internet services and resources. A Denial of service attack is with the purpose of preventing legitimate users from using victim system resources. DDoS is a large scale coordinated attack on the availability of services of a victim system (or) network resource. Large-scale distributed nature of Internet makes DDoS attack stealthy and difficult to counter. As attack traffic is indistinguishable from normal traffic, it would be desirable to develop comprehensive DDoS solution. As various attack tools become widely available, automated anti-DdoS systems become increasingly important. This study proposes taxonomy of various mitigation mechanisms against DDoS attack. We studied the different mitigation mechanisms like IP-Traceback,Change point monitoring method and perimeter based defense. Finally we drafted a taxonomy which helps to understand the advantages and drawbacks of various mitigation mechanisms and scope of the DDoS problem.