Rethinking the timescales at which congestion-control operates

The efficiency of TCP congestion-control in achieving high throughput is quite poor in high-speed, lossy, and dynamic-bandwidth environments. The main culprit is the slow bandwidth-search process used by TCP, which may take up to several thousands of round-trip times (RTTs) in searching for and acquiring the end-to-end spare bandwidth. While several alternate protocols have been proposed to speed up the search process, these still take hundreds of RTTs for doing so. In this paper, we argue that the sluggishness of existing protocols stems from two limiting design decisions that help a transfer remain non-intrusive to competing transfers. We argue that these legacy design decisions can be done away with if we limit the impact of probing for spare bandwidth. We use this idea to design a new approach for congestion-control that allows TCP connections to boldly search for, and adapt to, the available bandwidth within a single RTT. Our approach relies on carefully orchestrated packet sending times and estimates the available bandwidth based on the delays experienced by these. We instantiate our new protocol, referred to as RAPID, using mechanisms that promote efficiency as well as queue-friendliness. Our experimental evaluations indicate that RAPID: (i) converges to an updated value of bandwidth within 1-2 RTTs; (ii) helps maintain fairly small queues even in high-speed networks; and (iii) has negligible impact on regular TCP traffic. The benefits of our approach are especially significant on lossy links and those with rapidly-changing bandwidth.