Optimal Computation in Anonymous Dynamic Networks

TL;DR

This paper characterizes functions computable by anonymous processes in dynamic networks, introduces efficient deterministic algorithms with optimal running times, and employs a novel 'history tree' structure to advance fundamental problems like consensus and counting.

Contribution

It provides the first deterministic algorithms with linear time complexity in anonymous dynamic networks, using a new combinatorial structure called 'history tree'.

Findings

Algorithms are asymptotically optimal for fixed leaders.

First deterministic algorithms with linear scaling in processes and network disconnectivity.

Progress on leaderless average consensus and multi-leader counting.

Abstract

We give a simple characterization of the functions that can be computed deterministically by anonymous processes in dynamic networks, depending on the number of leaders in the network. In addition, we provide efficient distributed algorithms for computing all such functions assuming minimal or no knowledge about the network. Each of our algorithms comes in two versions: one that terminates with the correct output and a faster one that stabilizes on the correct output without explicit termination. Notably, these are the first deterministic algorithms whose running times scale linearly with both the number of processes and a parameter of the network which we call "dynamic disconnectivity" (meaning that our dynamic networks do not necessarily have to be connected at all times). We also provide matching lower bounds, showing that all our algorithms are asymptotically optimal for any fixed number of leaders. While most of the existing literature on anonymous dynamic networks relies on classic mass-distribution techniques, our work makes use of a novel combinatorial structure called "history tree", which is of independent interest. Among other contributions, our results make conclusive progress on two popular fundamental problems for anonymous dynamic networks: leaderless Average Consensus (i.e., computing the mean value of input numbers distributed among the processes) and multi-leader Counting (i.e., determining the exact number of processes in the network). Our contribution not only opens a promising line of research on applications of history trees, but also demonstrates that computation in anonymous dynamic networks is practically feasible and far less demanding than previously conjectured.