拒绝服务攻击是一类最难对付的网络安全问题.近来,人们提出了多种对策.其中由Savage等人提出的一类基于概率的包标记方案比较有研究价值.这里先对拒绝服务攻击的对策作一简述,然后分析了几种包标记方案,指出了它们的一些缺陷,并提出了一些改进措施.其中,对基本型概率包标记方案的一个修改使得计算量大大减少. Denial of service attacks are one of the most difficult cyber-security problems.Recently, many countermeasures have been put forward, among which a kind of probability-based packet labeling scheme proposed by Savage et al has more research value.At first, Then, a brief description of the countermeasures is given, and then several package marking schemes are analyzed, some of them are pointed out and some improvement measures are put forward, in which a modification of the basic probability labeling scheme greatly reduces the computational cost.