  Increasingly stringent throughput and latency requirements in datacenter
networks demand fast and accurate congestion control. We observe that the
reaction time and accuracy of existing datacenter congestion control schemes
are inherently limited. They either rely only on explicit feedback about the
network state (e.g., queue lengths in DCTCP) or only on variations of state
(e.g., RTT gradient in TIMELY). To overcome these limitations, we propose a
novel congestion control algorithm, PowerTCP, which achieves much more
fine-grained congestion control by adapting to the bandwidth-window product
(henceforth called power). PowerTCP leverages in-band network telemetry to
react to changes in the network instantaneously without loss of throughput and
while keeping queues short. Due to its fast reaction time, our algorithm is
particularly well-suited for dynamic network environments and bursty traffic
patterns. We show analytically and empirically that PowerTCP can significantly
outperform the state-of-the-art in both traditional datacenter topologies and
emerging reconfigurable datacenters where frequent bandwidth changes make
congestion control challenging. In traditional datacenter networks, PowerTCP
reduces tail flow completion times of short flows by 80% compared to DCQCN and
TIMELY, and by 33% compared to HPCC even at 60% network load. In reconfigurable
datacenters, PowerTCP achieves 85% circuit utilization without incurring
additional latency and cuts tail latency by at least 2x compared to existing
approaches.
