Classical covert channel analysas has focused on channels available on a single computer: timing channels and storage channels. In this paper, we characterize network covert channels. 130tenlial network covert channels are exploited b y modulating transmission characteristics. We distinguish between spatial covert channels, caused due to vara'ation in thc relative volume of communication between nodes in the network, and temporal covert charnels caused due t o variation in transmission characteristics over time, extending the work of Gzrling [5]. A ,model for obtaan-,ing spatially neutral transmissaon schedule is given in /16, 1' 71. Temporally neutral transmisszons are characterized and scheduling policies to generate temporally neutral transmission schedules are gaven in [23]. In thas paper, we estzmate the covert channel ixpacity using an adaptive scheduling policy, modeling the system as a mode secure system [l]. Rased on our measurements on University of Florida campus-wide backbone network (UFNET), we discuss the auditability of network coved channels and suggest somc handling policies to reduce the capacity of fhese covert channels to TCSEG acceptable lewis.

Ensemble is an X-Windows based, object-oriented graphics editor based on the tgif graphics editor from UCLA.It relies on Unix* 4.3bsd sockets and can be used as a stand-alone program or as an application in the University of Florida's distributed conferencing system (DCS). It uses implicitly placed write locks for concurrency control, with locks placed when an object is selected and removed when it is deselected. Multiple users may read or edit a file concurrently, with all users receiving updates whenever a lock is removed. Pointers are shared by mutual consent, so that users may collaborate to the degree desired. Ensemble is a prototype lock-based approach to object-oriented concurrent graphics editing.

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