 Starting in 1976 he has been on the MIT EECS department faculty, at Xerox PARC, and at the DEC Systems Research Center.At MIT he was involved with Multics, where his contributions included a seminal work on security architecture for shared information systems.Some other systems he has built are Grapevine (distributed system), the filesystem of Cedar, Topaz (distributed OS), Autonet (LAN) and Pachyderm (web based email).