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TCP vs. TCP: a systematic study of adverse impact of short-lived TCP flows on long-lived TCP flows

This paper describes systematical development of TCP adversarial scenarios where we use short-lived TCP flows to adversely influence long-lived TCP flows. Our scenarios are interesting since, (a) they point out the increased vulnerabilities of recently proposed scheduling, AQM and routing techniques that further favor short-lived TCP flows and (b) they are more difficult to detect when intentionally found to target long-lived TCP flows. We systematically exploit the ability of TCP flows in slow-start to rapidly capture greater proportion of bandwidth compared to long-lived TCP flows in congestion avoidance phase, to a point where they drive long-lived TCP flows into timeout. We use simulations, analysis and experiments to systematically study the dependence of the severity of impact on long-lived TCP flows on key parameters of short-lived TCP flows-including their locations, durations and numbers, as well as the intervals between consecutive flows. We derive characteristics of pattern of short-lived flows that exhibit extreme adverse impact on long-lived TCP flows. Counter to common beliefs, we show that targeting bottleneck links does not always cause maximal performance degradation for the long-lived flows. In particular, our approach illustrates the interactions between TCP flows and multiple bottleneck links and their sensitivities to correlated losses in the absence of 'non-TCP friendly' flows and paves the way for a systematic synthesis of worst-case congestion scenarios. While randomly generated sequences of short-lived TCP flows may provide some reductions (up to 10%) in the throughput of the long-lived flows, the scenarios we generate cause much greater reductions (>85%) for several TCP variants and for different packet drop policies (DropTail, RED).