This work was supported in part by the New York State Science and Technology Foundation's Center for Advanced Technology in Computers and Information Systems; by a National Science Foundation CISE Institutional Infrastructure grant, number CDA-90-24735; and by the Center for Telecommunications Research, an NSF Engineering Research Center supported by grant number ECD-88-11111.
We thank an anonymous member of the Usenix program committee for the suggestion to use file checksums. We thank the Usenix program committee, especially David Rosenthal and Matt Blaze, for valuable advice about presentation and emphasis.