On Dynamics in Selfish Network Creation

We consider the dynamic behavior of several variants of the Network Creation Game, introduced by Fabrikant et al. [PODC'03]. Equilibrium networks in these models have desirable properties like low social cost and small diameter, which makes them attractive for the decentralized creation of overlay-networks. Unfortunately, due to the non-constructiveness of the Nash equilibrium, no distributed algorithm for finding such networks is known. We treat these games as sequential-move games and analyze whether (uncoordinated) selfish play eventually converges to an equilibrium state. Thus, we shed light on one of the most natural algorithms for this problem: distributed local search, where in each step some agent performs a myopic selfish improving move. We show that fast convergence is guaranteed for all versions of Swap Games, introduced by Alon et al. [SPAA'10], if the initial network is a tree, and show that this process can be sped up to an almost optimal number of moves. For non-tree networks we show the surprising result that even one non-tree edge suffices to destroy the convergence guarantee and no move policy can enforce convergence. This answers an open problem from Ehsani et al. [SPAA'11] in the negative. We extend our negative results to the well-studied original version and prove that there is no convergence guarantee -- even if all agents play optimally. Furthermore, we show the quite surprising result that employing cost-sharing yields even worse dynamic behavior. Finally, we contrast our mostly negative theoretical results by a careful empirical study. Our simulations indicate two positive facts: (1) The non-convergent behavior seems to be confined to a small set of pathological instances and is unlikely to show up in practice. (2) In all our simulations we observed a remarkably fast convergence towards a stable network in O(n) steps, where n is the number of agents.