Lower Bounds for $k$-Set Agreement in Fault-Prone Networks

2025-08-21

AI Summary

This paper establishes a lower bound on the round complexity of the $k$-set agreement problem in synchronous message-passing systems with arbitrary directed communication topologies and up to $t$ crash failures. To overcome prior limitations—where topological lower bounds were restricted to complete or undirected graphs—the authors introduce shellable carrier map sequences and a generalized Sperner-type argument, along with a novel protocol cost metric based on the radius of the communication graph. They prove that any correct $k$-set agreement protocol requires at least $lceil t/k ceil + 1$ rounds, strictly improving the classical lower bound. Moreover, they reduce the size of the input complex from exponential to polynomial, and uniformly handle initial failures. The core innovation lies in extending topological techniques to directed networks, achieving breakthroughs in lower-bound construction, analytical tools, and complexity measurement.

Technology Category

• Multiagent Systems: Agreement, Argumentation & Negotiation
• Knowledge Representation and Reasoning: Computational Complexity of Reasoning
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty

Application Category

• Security and Privacy: Large-scale security measurements
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems

Abstract

We develop a new lower bound for k-set agreement in synchronous message-passing systems connected by an arbitrary directed communication network, where up to t processes may crash. Our result thus generalizes the t/k+1 lower bound for complete networks in the t-resilient model by Chaudhuri, Herlihy, Lynch, and Tuttle [JACM'00]. Moreover, it generalizes two lower bounds for oblivious algorithms in synchronous systems connected by an arbitrary undirected communication network known to the processes, namely, the domination number-based lower bound by Castaneda, Fraigniaud, Paz, Rajsbaum, Roy, and Travers [TCS'21] for failure-free processes, and the radius-based lower bound in the t-resilient model by Fraigniaud, Nguyen, and Paz [STACS'24]. Our topological proof non-trivially generalizes and extends the connectivity-based approach for the complete network, as presented in the book by Herlihy, Kozlov, and Rajsbaum (2013). It is based on a sequence of shellable carrier maps that, starting from a shellable input complex, determine the evolution of the protocol complex: During the first t/k rounds, carrier maps that crash exactly k processes per round are used, ensuring high connectivity of their images. A Sperner's lemma style argument is used to prove that k-set agreement is still impossible by that round. From round t/k+1 up to our lower bound, we employ a novel carrier map that maintains high connectivity. Our proof also provides a strikingly simple lower bound for k-set agreement in synchronous systems with an arbitrary communication network with initial crashes. We express the resulting additional agreement overhead via an appropriately defined radius of the communication graphs. Finally, we prove that the usual input pseudosphere complex for k-set agreement can be replaced by an exponentially smaller input complex based on Kuhn triangulations, which we prove to be also shellable.

Problem

• Generalizing lower bounds for k-set agreement in fault-prone networks
• Extending connectivity-based topological proofs for arbitrary directed graphs
• Establishing impossibility results under process crash failures

Innovation

• Shellable carrier maps for protocol complex evolution
• Sperner's lemma argument for k-set impossibility
• Kuhn triangulations for smaller input complexes