Communication Modalities

TL;DR

This paper explores the role of communication modalities in epistemic logic for distributed systems, especially addressing challenges posed by Byzantine agents who may lie, by introducing new modalities to represent message content.

Contribution

It introduces two novel modalities, hope and creed, to better model trust and information content in Byzantine and non-uniform communication scenarios.

Findings

Hope modality for fully Byzantine agents

Creed modality for non-uniform protocols

Enhanced epistemic analysis of faulty communication

Abstract

Epistemic analysis of distributed systems is one of the biggest successes among applications of logic in computer science. The reason for that is that agents' actions are necessarily guided by their knowledge. Thus, epistemic modal logic, with its knowledge and belief modalities (and group versions thereof), has played a vital role in establishing both impossibility results and necessary conditions for solvable distributed tasks. In distributed systems, knowledge is largely attained via communication. It has been standard in both distributed systems and dynamic epistemic logic to treat incoming messages as trustworthy, thus, creating difficulties in the epistemic analysis of byzantine distributed systems where faulty agents may lie. In this paper, we argue that handling such communication scenarios calls for additional modalities representing the informational content of messages that should not be taken at face value. We present two such modalities: hope for the case of fully byzantine agents and creed for non-uniform communication protocols in general.