Abstract. DoS(Denial of Service) / DDoS(Distributed Denial of Service) attacks threaten Internet security nowadays. However, the current Internet protocol and backbone network do not support traceback to know attacker's real location. Many methods to defend DoS/DDoS attack have been proposed. However these kinds of methods cause network overhead because they use many packets to reconstruct an attack path. In this paper we propose effective probability marking methods and a pushback algorithm to reduce network overhead.