Choose a correct marking position: ECN should be freed from tail mark

Abstract

Explicit Congestion Notification (ECN) has been widely employed in production datacenter networks (DCNs) to deliver high throughput and low latency communications. Despite being successful, the drawback of ECN-based transports is that they always mark the packets in the tail of the queue. We call this ECN marking scheme as TM-ECN (tail mark ECN). TM-ECN leads to long ECN feedback delay, significant queue oscillation, and poor short-flow friendliness. In this paper, we reveal these problems by leveraging extensive simulations to explore the intrinsic drawback of TM-ECN. Using the guidelines learned from the exploration, we design FM-ECN (front mark ECN) and RM-ECN (random mark ECN), two simple yet effective ECN marking schemes to mitigate the drawbacks. We theoretically analyze the TM-ECN, FM-ECN, and RM-ECN in the ECN feedback delay, queue stability, and short-flow friendliness. Finally, the conclusions from the analysis inspire us to propose a mixed mark ECN (MM-ECN) marking scheme. With the advantages of FM-ECN and RM-ECN, MM-ECN can maximize the effect of ECN. We further implement these ECN marking schemes in Linux kernel and ns-3 to evaluate them through testbed experiments and large-scale simulations. Our experimental results show that DCTCP with an appropriate ECN marking scheme achieves up to 55% higher concurrency tolerance of incast, and achieves up to 16% (25%) lower average (99th percentile) flow completion time (FCT) of short flows while delivering similar FCT for mid/large flows under production workloads.