Distributed Denial of Service(DDoS)attack is among the hardest network security problems to address.Recently,several countermeasures are proposed,among which,PPM(probabilistic packet marking)pioneered by Savage et al.is promising.In this paper,a brief review of countermeasures to DDoS is given and then an analysis on some of the packet marking schemes is provided.Some modifications are further provided.One modification to the basic PPM scheme can reduce its computation remarkably.