A node-capacitated Okamura-Seymour theorem

TL;DR

This paper proves an approximate flow/cut theorem for node-capacitated planar graphs, showing that demands can be fractionally routed under certain cut conditions, extending classical results to more general network settings.

Contribution

It establishes the first approximate flow/cut theorem for node-capacitated planar graphs, answering an open question and extending to polymatroid networks.

Findings

Feasible routing of a constant fraction of demands under node cut conditions

Extension of flow/cut theorems to node-capacitated and polymatroid networks

Introduction of a new random metric embedding technique for rounding convex programs

Abstract

The classical Okamura-Seymour theorem states that for an edge-capacitated, multi-commodity flow instance in which all terminals lie on a single face of a planar graph, there exists a feasible concurrent flow if and only if the cut conditions are satisfied. Simple examples show that a similar theorem is impossible in the node-capacitated setting. Nevertheless, we prove that an approximate flow/cut theorem does hold: For some universal c > 0, if the node cut conditions are satisfied, then one can simultaneously route a c-fraction of all the demands. This answers an open question of Chekuri and Kawarabayashi. More generally, we show that this holds in the setting of multi-commodity polymatroid networks introduced by Chekuri, et. al. Our approach employs a new type of random metric embedding in order to round the convex programs corresponding to these more general flow problems.