  Congestion control algorithms rely on a variety of congestion signals (packet
loss, Explicit Congestion Notification, delay, etc.) to achieve fast
convergence, high utilization, and fairness among flows. A key limitation of
these congestion signals is that they are either late in feedback or they incur
significant overheads. An ideal congestion control must discover any available
bandwidth in the network, detect congestion as soon as link utilization
approaches full capacity, and react timely to avoid queuing and packet drops,
without significant overheads. To this end, this work proposes Scout service
that leverages priority queues to infer bandwidth availability and link
busyness at the host. The key observation here is that as the high priority
queue (HPQ) gets busier, the low priority queue (LPQ) is served less.
Therefore, the state of the link can be observed from the LPQ and any
congestion can be detected several RTTs earlier than observing the HPQ. We
propose a new transport protocol, Double-Window Transmission Control Protocol
(DWTCP) that builds upon the Scout service to dynamically adjust its congestion
window. Our testbed and simulation-based evaluation demonstrates that Scout
enables a data center transport to achieve high throughput, near-zero queues,
lower latency, and high fairness.
