1 Introduction
Denial-of-service(DoS) attacks have been threatening the utility of the Internet severely [1]. In 2002, a coordinated attack on the Internet name service infrastructure showed the possibility and potential impact of such dedicated attacks [2]. More recently, it was reported that DoS attacks have been used as a means of extortion and have become the subject of lawsuits [3]. Defending against DoS attacks requires not only measures for mitigating the effects of the attacks but also mechanisms for identifying the entities accountable for such attacks.