TCP vs. TCP: a Systematic Study of Adverse Impact

Abstract — While earlier studies have pointed out that short-lived TCP flows (mice) may hurt long-lived TCP flows (elephants) in the long term, they provide insufficient insight for developing scenarios leading to drastic drop in throughputs of long-lived TCP flows. We have systematically developed 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 longlived 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 (Tahoe, Reno, New Reno, Sack), and for different packet drop policies (DropTail, RED). Keywords-TCP, short-lived flow, long-lived flow, Do