On the impossibility of efficient self-stabilization in virtual overlays with churn

Abstract

Virtual overlays generate topologies for greedy routing, like rings or hypercubes, on connectivity restricted networks. They have been proposed to achieve efficient content discovery in the Darknet mode of Freenet, for instance, which provides a private and secure communication platform for dissidents and whistle-blowers. Virtual overlays create tunnels between nodes with neighboring addresses in the topology. The routing performance hence is directly related to the length of the tunnels, which have to be set up and maintained at the cost of communication overhead in the absence of an underlying routing protocol. In this paper, we show the impossibility to efficiently maintain sufficiently short tunnels. Specifically, we prove that in a dynamic network either the maintenance or the routing eventually exceeds polylog cost in the number of participants. Our simulations additionally show that the length of the tunnels increases fast if standard maintenance protocols are applied. Thus, we show that virtual overlays can only offer efficient routing at the price of high maintenance costs.