Perpetual exploration in anonymous synchronous networks with a Byzantine black hole

TL;DR

This paper explores how mobile agents can perpetually explore unknown networks despite the presence of a malicious stationary node, called a Byzantine black hole, which can destroy agents, and determines the minimum number of agents needed.

Contribution

It introduces the first study of perpetual exploration in arbitrary networks with a Byzantine black hole, providing optimal algorithms and bounds on the number of agents required.

Findings

Optimal algorithms for trees with 4 and 6 agents.

Lower bounds of 2Δ-1 agents for general exploration.

Upper bounds of 3Δ+3 agents for home exploration.

Abstract

In this paper, we investigate: ``How can a group of initially co-located mobile agents perpetually explore an unknown graph, when one stationary node occasionally behaves maliciously, under an adversary's control?'' We call this node a ``Byzantine black hole (BBH)'' and at any given round it may choose to destroy all visiting agents, or none. This subtle power can drastically undermine classical exploration strategies designed for an always active black hole. We study this perpetual exploration problem in the presence of at most one BBH, without initial knowledge of the network size. Since the underlying graph may be 1-connected, perpetual exploration of the entire graph may be infeasible. We thus define two variants: \pbmPerpExpl\ and \pbmPerpExplHome. In the former, the agents are tasked to perform perpetual exploration of at least one component, obtained after the exclusion of the BBH. In the latter, the agents are tasked to perform perpetual exploration of the component which contains the \emph{home} node, where agents are initially co-located. Naturally, \pbmPerpExplHome\ is a special case of \pbmPerpExpl. Agents operate under a synchronous scheduler and communicate in a face-to-face model. Our goal is to determine the minimum number of agents necessary and sufficient to solve these problems. In acyclic networks, we obtain optimal algorithms that solve \pbmPerpExpl\ with $4$ agents, and \pbmPerpExplHome\ with $6$ agents in trees. The lower bounds hold even in path graphs. In general graphs, we give a non-trivial lower bound of $2\Delta-1$ agents for \pbmPerpExpl, and an upper bound of $3\Delta+3$ agents for \pbmPerpExplHome. To our knowledge, this is the first study of a black-hole variant in arbitrary networks without initial topological knowledge.