Spoofing source IP addresses is always utilized to perform Distributed Denial-of-Service (DDoS) attacks. Most of current detection and prevention methods against DDoS ignore the innocent side, whose IP is utilized as the spoofed IP by the attacker. In this paper, a novel method has been proposed to against the direct DDoS attacks, which consists of two components: the client detector and the server detector. The cooperation of those two components and their interactive behavior lead to an early stage detection of a DDoS attack. From the result of experiments, the approach presented in this paper yields accurate DDoS alarms at early stage. Furthermore, such approach is insensitive to the false suspect alarms with adopted evaluation functions.