Abstract: DDoS attack is a big problem to the Internet community due to its high-profile, severe damage, and the difficulty in defending against it. Several countermeasures are proposed for it in the literature, among which, probabilistic packet marking (PPM) first developed by Savage et al is promising. However, the PPM marking schemes have the limitations in two main aspects: high computation overhead and large number of false positives. An IP traceback scheme with packet marking in blocks is proposed, which is more practical because of higher precision, and computationally more efficient compared with the PPM scheme proposed by Savage.