A Constraint-Based Approach to Diagnosing Con guration Problems

A prototype system has been developed for diagnosing problems in software that controls computer networks. Techniques for model-based diagnosis that have been used successfully in physical systems are applied and extended to computer network software. Our approach divides this software into its natural hierarchy of layers, subdividing each layer into three separately modeled components: the interface to the layer above on the same machine, the protocol to the same layer on a remote machine, and the connguration. This paper presents a case study illustrating the diagnosis of some connguration problems involving the widely used FTP and DNS network software. Our approach considers diagnosis as a dynamic partial constraint satisfaction problem. Activity constraints are used to interface the model with the "real world" of the network, allowing the model to dynamically obtain data from the network and to use that data to change the problem as the search for a solution progresses. The partial solutions discovered by our system constitute the diagnosis. This work concentrates on modeling the network infrastructure itself. We believe it gives us a solid foundation for understanding basic problems that occur at all levels in information networks, and that the techniques we have developed are applicable at higher levels as well.