Unifying Partial Synchrony

TL;DR

This paper proves that in distributed systems, any time-agnostic property achievable in the unknown latency model can also be achieved in the global stabilization time model, unifying two partial synchrony models.

Contribution

It establishes the equivalence of the UL and GST models for time-agnostic properties, clarifying their relative computational power.

Findings

UL model can simulate GST model for time-agnostic properties

Properties achievable in UL are also achievable in GST

The two models are equally demanding for time-agnostic properties

Abstract

The distributed computing literature considers multiple options for modeling communication. Most simply, communication is categorized as either synchronous or asynchronous. Synchronous communication assumes that messages get delivered within a publicly known timeframe and that parties' clocks are synchronized. Asynchronous communication, on the other hand, only assumes that messages get delivered eventually. A more nuanced approach, or a middle ground between the two extremes, is given by the partially synchronous model, which is arguably the most realistic option. This model comes in two commonly considered flavors: (i) The Global Stabilization Time (GST) model: after an (unknown) amount of time, the network becomes synchronous. This captures scenarios where network issues are transient. (ii) The Unknown Latency (UL) model: the network is, in fact, synchronous, but the message delay bound is unknown. This work formally establishes that any time-agnostic property that can be achieved by a protocol in the UL model can also be achieved by a (possibly different) protocol in the GST model. By time-agnostic, we mean properties that can depend on the order in which events happen but not on time as measured by the parties. Most properties considered in distributed computing are time-agnostic. The converse was already known, even without the time-agnostic requirement, so our result shows that the two network conditions are, under one sensible assumption, equally demanding.