This work is partially sponsored by a USENIX Scholars Research Grant; and by the Defense Advanced Research Projects Agency (DARPA) and Rome Laboratory, Air Force Materiel Command, USAF, under agreement numbers F30602-96-1-0320 and F30602-99-1-0517; and by the National Science Foundation under grant number CCR-9411590. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of DARPA, Rome Laboratory, or the U.S. Government.