In-Order Packet Delivery in Interconnection Networks using Adaptive Routing

Most commercial switch-based network technologies for PC clusters use deterministic routing. Alternatively, adaptive routing could be used to improve network performance. In this case, switches decide the path to reach the destination by using local information about the state of the possible outgoing links. However, there are two drawbacks that discourage adaptive routing from being applied to commercial interconnects. The first one concerns the possible switch complexity increase with respect to deterministic routing. The second drawback is due to the fact that adaptive routing may introduce out-of-order packet delivery, which is not acceptable for some applications. For the best of our knowledge, there are no works that analyze the degree of out-of-order packet delivery caused by different network and traffic conditions. In this paper, we take on such a challenge. We show that only for high traffic conditions (reaching saturation) out-of-order delivery is introduced. Moreover, by using small buffers and simple sorting mechanisms at destination, we show that high network throughput can be obtained at the same time packets are delivered in order. Thus, the paper demonstrates that it is possible to use adaptive routing, while still guaranteeing in-order packet delivery, without using large buffer resources nor degrading significantly its performance.