Denial-of-Service (DoS) attack is an attempt by attacker to prevent legitimate users from using resources. Denial-of-Service denies a victim (host, router, or entire network) from providing or receiving normal services. Distributed Denial of Service (DDoS) Attacks are generated in a \"Many-to-one\" dimension. In DDoS attack model Large number of compromised host are gathered to send useless service requests, packets at the same time. The server manages the user requests and sends the video data to the users. The video data requests are referred as flashcrowds. Sophisticated botmasters attempt to disable detectors by mimicking the traffic patterns of flash crowds. Graphical puzzles are used to differentiate between humans and bots on DDoS attack defending process.