Optimal Relay Placement for Maximizing Path Diversity in Multipath Overlay Networks

Providing a high level of the path diversity is a substantial criterion in designing an overlay network. Given a underlying network infrastructure, the diversity of the overlay paths largely depends on how the overlay nodes, i.e. relays, are placed. In this paper, the problem of optimal relay placement is studied, and the goal is to maximize the diversity of the overlay paths while minimizing the number of relays. To tackle this multi-objective optimization problem, a novel genetic algorithm (GA) is applied hybridizing the standard GA and k-shortest path algorithms. By studying 6 real ISP topologies, we find that there exist non-unique solutions, which provide nearly the same level of the path diversity. In those solutions, only few locations are not replaceable allowing a great flexibility in the overlay network deployment. Perhaps surprisingly, these places do not include all those nodes with the highest number of links. The reason is that those nodes with the highest number of links are usually crossed by the default routing paths. Therefore, the overlay paths tend not to crossing them. We also notice that the number of relays required to maintain a high level of the path diversity is only 5% - 10% of the total number of network nodes depending on the topologies. We believe that our findings are useful for designing and deploying multi-path overlay networks.