Multicriteria Spanners -- A New Tool for Network Design

Abstract: Designing sparse directed spanners, which are subgraphs that approximately maintain distance constraints, has attracted sustained interest in TCS, especially due to their wide applicability, as well as the difficulty to obtain tight results. However, a significant drawback of the notion of spanners is that it cannot capture multiple distance-like constraints for the same demand pair. In this paper we initiate the study of directed multicriteria spanners, in which the notion of edge lengths is replaced by the notion of resource consumption vectors, where each entry corresponds to the consumption of the respective resource on that edge. The goal is to find a minimum-cost routing solution that satisfies the multiple constraints. To the best of our knowledge, we obtain the first approximation algorithms for the directed multicriteria spanners problems, under natural assumptions. Our results match the state-of-the-art approximation ratios in special cases of ours. We also establish reductions from other natural network connectivity problems to the directed multicriteria spanners problems, including Group Steiner Distances, introduced in the undirected setting by Bil`o, Gual`a, Leucci and Straziota (ESA 2024), and Edge-Avoiding spanners. Our reductions imply approximation algorithms for these problems and illustrate that the notion of directed multicriteria spanners is an appropriate abstraction and generalization of natural special cases from the literature. Our main technical tool is a delicate generalization of the minimum-density junction tree framework of Chekuri, Even, Gupta, and Segev (SODA 2008, TALG 2011) to the notion of minimum-density resource-constrained junction trees, which also extends ideas from Chlamt\'a\v{c}, Dinitz, Kortsarz, and Laekhanukit (SODA 2017, TALG 2020).