DDoS attack represents a big problem to the Internet community for its high profile,severe damage,and difficult defending.Several countermeasures are proposed for it in the literature,among which,Probabilistic Packet Marking (PPM) is promising.However, all the existing marking schemes are bearing limitations in some aspects.In this paper, a new packet marking scheme is proposed,which is more prompt because of fewer packets needed,more scalable and more efficient in computation compared with other schemes.Furthermore,this scheme limits attackers' ability in spoofing trace message.