Epistemic Reasoning with Byzantine-Faulty Agents
We introduce a novel comprehensive framework for epistemic reasoning in multi-agent systems where agents may behave asynchronously and may be byzantine faulty. Extending Fagin et al.’s classic runs-and-systems framework to agents who may arbitrarily deviate from their protocols, it combines epistemic and temporal logic and incorporates fine-grained mechanisms for specifying distributed protocols and their behaviors. Besides our framework’s ability to express any type of faulty behavior, from fully byzantine to fully benign, it allows to specify arbitrary timing and synchronization properties. As a consequence, it can be adapted to any message-passing distributed computing model we are aware of, including synchronous processes and communication, (un-)reliable uni-/multi-/broadcast communication, and even coordinated action. The utility of our framework is demonstrated by formalizing the brain-in-a-vat scenario, which exposes the substantial limitations of what can be known by asynchronous agents in fault-tolerant distributed systems. Given the knowledge of preconditions principle, this restricts preconditions that error-prone agents can use in their protocols. In particular, it is usually necessary to relativize preconditions with respect to the correctness of the acting agent.
We are grateful to Hans van Ditmarsch and Yoram Moses for extensive helpful comments on earlier versions of this paper. We also thank the anonymous reviewers for their comments and suggestions on related research.
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