Today most of the activities like trade, e-commerce are dependent on the availability of Internet. The growing use of internet services in the past few years have facilitated increase in distributed denial of service attack. Due to DDos attacks, caused by malicious hosts secured data communication over the internet is very difficult to achieve and is the need of the hour. DDos attacks are one of the most widely spread problems faced by most of the internet service providers (ISP’s). The work which had already been done was in the direction of detection, prevention and trace-back of DDos attack. Mitigation of these attacks has also gained an utmost importance in the present scenario. A number of techniques have been proposed by various researchers but those techniques produce high collateral Damage so more efforts are needed to be done in the area of mitigation of DDos attacks.This paper focuses on Distributed Denial of Service attack, surveys, classification and also proposed mitigation techniques revealed in literature by various researchers.