The fat tree topology with multipath capability has been used in many recent data center networks (DCNs) for increased bandwidth and fault tolerance. Traditional routing protocols have only limited support for multipath routing, and cannot fully utilize the available bandwidth in such networks. In this paper, we study multipath routing for fat tree networks. We formulate the problem as a linear program and prove its NP-completeness. We propose a practical solution, which takes advantage of the emerging software-defined networking paradigm. Our algorithm relies on a central controller to collect necessary network state information in order to make optimized routing decisions. We implemented the algorithm as an OpenFlow controller module and validated it with Mininet emulation. We also developed a fluid-based DCN simulator and conducted experiments, which show that our algorithm outperforms the traditional multipath algorithm based on random assignments, both in terms of increased throughput and in reduced end-to-end delay.
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