  This paper introduces Nimbus, a robust technique to detect whether the cross
traffic competing with a flow is "elastic", and shows that this elasticity
detector improves congestion control. If cross traffic is inelastic, then a
sender can control queueing delays while achieving high throughput, but in the
presence of elastic traffic, it may lose throughput if it attempts to control
packet delay. To estimate elasticity, Nimbus modulates the flow's sending rate
with sinusoidal pulses that create small traffic fluctuations at the bottleneck
link, and measures the frequency response of the rate of the cross traffic. Our
results on emulated and real-world paths show that congestion control using
elasticity detection achieves throughput comparable to Cubic, but with delays
that are 50-70 ms lower when cross traffic is inelastic. Nimbus detects the
nature of the cross traffic more accurately than Copa, and is usable as a
building block by other end-to-end algorithms.
