A Combinatorial Characterization of Self-Stabilizing Population Protocols

TL;DR

This paper provides a complete characterization of which functions can be computed in a self-stabilizing manner within population protocols on complete graphs, using set-theoretic conditions and Dickson's Lemma.

Contribution

It establishes necessary and sufficient conditions for self-stabilization and constructs protocols for all functions satisfying these conditions, introducing the concept of the root set.

Findings

Functions without certain set-theoretic properties cannot be self-stabilizing.

A protocol is constructed for all functions meeting the characterization.

Dickson's Lemma and the root set are key tools in the characterization.

Abstract

We fully characterize self-stabilizing functions in population protocols for complete interaction graphs. In particular, we investigate self-stabilization in systems of $n$ finite state agents in which a malicious scheduler selects an arbitrary sequence of pairwise interactions under a global fairness condition. We show a necessary and sufficient condition for self-stabilization. Specifically we show that functions without certain set-theoretic conditions are impossible to compute in a self-stabilizing manner. Our main contribution is in the converse, where we construct a self-stabilizing protocol for all other functions that meet this characterization. Our positive construction uses Dickson's Lemma to develop the notion of the root set, a concept that turns out to fundamentally characterize self-stabilization in this model. We believe it may lend to characterizing self-stabilization in more general models as well.