We would like to thank Pei Cao, Edward W. Felten, and Kai Li for providing us with the traces we used in the experiments. We also would like to thank Yoonho Park, our shepherd, for his help in improving this paper and Donghee Lee for many helpful discussions. Finally, we would like to thank anonymous reviewers for their constructive comments and suggestions. This work was supported in part by the Korea Research Foundation for the program year of 1998.