In recent years, the high frequent activities of Denial of Service ( DoS ) attacks have been bringing large loss to Internet Service Provider ( ISP ), as DoS attack is a mode of attack more difficult to be detected and prevented than others. 近年来拒绝服务攻击活动频繁,对网络供应商造成不小损失。研究表明,拒绝服务攻击是众多网络攻击中较难防御的一种攻击方式。