Cheating by Duplication: Equilibrium Requires Global Knowledge

TL;DR

This paper investigates the minimal global knowledge about network size needed for rational agents to reach equilibrium in distributed algorithms, highlighting the importance of a-priori knowledge of network size for various problems.

Contribution

It establishes the necessary and sufficient conditions on agents' prior knowledge of network size for equilibrium in multiple distributed problems, introducing new algorithms and impossibility results.

Findings

Equilibrium is impossible without known bounds on network size.

Algorithms are provided for known network size, ensuring equilibrium.

Tight bounds on network size are identified for equilibrium in several problems.

Abstract

The question of what global information must distributed rational agents a-priori know about the network in order for equilibrium to be possible is researched here. Until now, distributed algorithms with rational agents have assumed that $n$, the size of the network, is a-priori known to the participants. We investigate the above question, considering different distributed computing problems and showing how much each agent must a-priori know about $n$ in order for distributed algorithms to be equilibria. The main tool considered throughout the paper is the advantage an agent may gain by duplication- pretending to be more than one agent. We start by proving that when no bound on $n$ is given equilibrium for Coloring and Knowledge Sharing is impossible. %We prove that when agents have no a-priori knowledge on $n$, or even a known bound, equilibrium for both Knowledge Sharing and Coloring is impossible. We provide new algorithms for both problems when $n$ \emph{is} a-priori known to all agents, thus showing that there are algorithms in which the only way for an agent to gain an advantage is duplication. We further show that for each distributed problem there is an a-priori known range, an upper and a lower bound on $n$, such that if the actual $n$ is guaranteed to lay in that range, equilibrium is possible. By providing equilibria for a specific range, and impossibility results for any larger range, we prove the tight range necessary for equilibrium in: Leader Election, Knowledge Sharing, Coloring, Partition and Orientation.