Memory Requirements for Local Geometric Routing and Traversal in Digraphs

Local route discovery in geometric, connected, undirected plane graphs is guaranteed by the Face Routing algorithm. The algorithm is local and geometric in the sense that it is executed by an agent moving along a network and using at each node only information about the current node (incl. its position) and a finite number of others (independent of graph size). Local geometric traversal algorithms also exist for undirected plane graphs. In this paper we show that no comparable routing or traversal algorithms exist for the class of strongly connected plane directed graphs (digraphs). We construct a class of digraphs embedded in the plane for which either local routing or local traversal requires Ω(n) memory bits, where n is the order of the graph. We discuss these results in light of finding a suitable model for mobile ad hoc networks with uni-directional edges, showing in the extended version of this paper that digraphs for which the Ω(n) lower bound holds occur even in the class of embedded digraphs arising out of a very conservative model.