The TCP Outcast Problem: Exposing Unfairness in Data Center Networks

In this paper, the authors observe that bandwidth sharing via TCP in commodity data center networks organized in multi-rooted tree topologies can lead to severe unfairness, which they term as the TCP Outcast problem, under many common traffic patterns. When many flows and a few flows arrive at two ports of a switch destined to one common output port, the small set of flows lose out on their throughput share significantly (almost by an order of magnitude sometimes). The Outcast problem occurs mainly in taildrop queues that commodity switches use.

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