Constant Congestion Routing of Symmetric Demands in Planar Directed Graphs

We study the problem of routing symmetric demand pairs in planar digraphs. The input consists of a directed planar graph G = (V, E) and a collection of k source-destination pairs M = {s1t1, . . . , sktk}. The goal is to maximize the number of pairs that are routed along disjoint paths. A pair siti is routed in the symmetric setting if there is a directed path connecting si to ti and a directed path connecting ti to si. In this paper we obtain a randomized poly-logarithmic approximation with constant congestion for this problem in planar digraphs. The main technical contribution is to show that a planar digraph with directed treewidth h contains a constant congestion crossbar of size Ω(h/polylog(h)).