DCTCP with Front Marking: Decouple ECN Control Loop From The Queuing Delay

Datacenter transports should provide low flow completion times (FCT) to achieve desired application performance for short flows, and it should also ensure largely sustained throughput for long flows. DCTCP is designed for datacenter to achieve those goals. As an end-to-end congestion algorithm, DCTCP marks packets, those exceed the threshold utilizing Explicit Congestion Notification (ECN) and feedbacks congestion extent to the sender for accurate congestion control. However, DCTCP marks the packet at the tail of the queue, which causes the ECN signal to experience hundreds of microseconds queue delay. In this paper, an enhanced DCTCP called DCTCP-FM (DCTCP with front marking) is designed based on the notion of ECN. DCTCP-FM ensures ECN marking on the front of queue. Thus, the mark always conveys the up-to-date information about the state of congestion at the time of packet departure, eliminating packets queuing delay. As a result, the sender can cut down its congestion window timely to alleviate congestion. Our preliminary simulation results show that compared to DCTCP, DCTCP-FM can: 1)put off the throughput collapse caused by Incast traffic; 2)reduce the buffer size requirement at the switch and avoid packet losses when the buffer is limited; 3)suppress queue oscillation to reduce end-to-end latency; 4) reduce 99th percentile FCTs of short flows in a real datacenter simulation.