  As link speeds increase in datacenter networks, existing congestion control
algorithms become less effective in providing fast convergence. TCP-based
algorithms that probe for bandwidth take a long time to reach the fair-share
and lead to long flow completion times. An ideal congestion control algorithms
for datacenter must provide 1) zero data loss, 2) fast convergence, and 3) low
buffer occupancy. However, these requirements present conflicting goals. For
fast convergence,flows must ramp up quickly, but this risks packet losses and
large queues. Thus, even the state-of-the-art algorithms, such as TIMELY and
DCQCN, rely on link layer flow control (e.g.,Priority-based Flow Control) to
achieve zero loss. This paper presents a new approach, called ExpressPass, an
end-to-end credit-based congestion control algorithm for datacenters.
ExpressPass is inspired by credit-based flow control, but extends it to work
end-to-end. The switches control the amount of credit packets by rate limiting
and ensure data packets flow in the reverse direction without any loss.
ExpressPass leverages this to ramp up aggressively. ExpressPass converges up to
80 times faster than DCTCP at 10Gbps link, and the gap increases as link speeds
become faster. Our simulation with realistic workload shows that ExpressPass
significantly reduces the flow completion time especially for small and medium
size flows compared to DCTCP, HULL, and DX.
