This work was supported in part by the Advanced Research Projects Agency, ARPA order number B094, under contract N00014-94-1-0719, monitored by the Office of Naval Research; and in part by the Center for Telecommunications Research, an NSF Engineering Research Center supported by grant number ECD-88-11111.
We wish to thank Carl Tait for many constructive conversations and insightful comments. We also wish to thank the anonymous reviewers and our USENIX shepherd, Carl Staelin, for their remarks.