  Congestion control plays a pivotal role in large-scale data centers,
facilitating ultra-low latency, high bandwidth, and optimal utilization. Even
with the deployment of data center congestion control mechanisms such as DCQCN
and HPCC, these algorithms often respond to congestion sluggishly. This
sluggishness is primarily due to the slow notification of congestion. It takes
almost one round-trip time (RTT) for the congestion information to reach the
sender. In this paper, we introduce the Fast Notification Congestion Control
(FNCC) mechanism, which achieves sub-RTT notification. FNCC leverages the
acknowledgment packet (ACK) from the return path to carry in-network telemetry
(INT) information of the request path, offering the sender more timely and
accurate INT. To further accelerate the responsiveness of last-hop congestion
control, we propose that the receiver notifies the sender of the number of
concurrent congested flows, which can be used to adjust the congested flows to
a fair rate quickly. Our experimental results demonstrate that FNCC reduces
flow completion time by 27.4% and 88.9% compared to HPCC and DCQCN,
respectively. Moreover, FNCC triggers minimal pause frames and maintains high
utilization even at 400Gbps.
