AFCC: ACK-Based Fast Congestion Control in Lossless Networks

Congestion control is vital for achieving ultra-low latency and ultra-high throughput in large-scale data centers. However, existing mechanisms like DCQCN and HPCC often respond slowly and inaccurately to congestion, leading to extended queuing delays and reduced throughput. The notification delay of nearly one round-trip time (RTT) exacerbates congestion issues. Furthermore, these algorithms fail to identify queue growth caused by priority flow control (PFC) pause frames from the downstream device in lossless networks, resulting in unnecessary rate reductions on innocent flows. In this paper, we introduce the ACK-based Fast Congestion Control (AFCC) mechanism, designed to achieve sub-RTT notification delays by leveraging ACK packets. To enhance responsiveness to last-hop congestion, AFCC incorporates the number of concurrent congested flows within each ACK. Furthermore, AFCC utilizes ACK’s In-band Network Telemetry (INT) to identify the paused phase, thereby facilitating accurate congestion detection. Experimental results show that AFCC effectively maintains high utilization and significantly reduces flow completion time by 31.2% compared to HPCC and 78.4% compared to DCQCN.