CWND: A Coarse, but Simple, Efficient Metric to Reduce Short Flow Completion Time in Data Centers

Abstract

Reducing the delay-sensitive Short Flows Completion Time (SFCT) is a hot topic in data center networks, and prioritizing the short flows is an effective way to do that. However, precisely identifying a short flow is still a challenge, which makes many proposed schemes unfeasible in commodity switch. The short flows often have a small Congestion Window (CWND) due to their small flow sizes, and thus CWND can be used as a coarse metric to identify short flows. Based on this idea, Low Delay Data Center Transport Control Protocol (LD-DCTCP) is proposed. When transferring a data packet whose CWND is less than the CWND threshold, LD-DCTCP sender marks the high priority field in the IP header of the packet. Then, switches can prioritize the high priority packets by a priority-based queue schedule. By simply prioritizing the packets which have small CWND, short flows, in a high probability, can be prioritized, and as well, SFCT can be effectively reduced. Since CWND can be easily obtained from TCP/IP header, LD-DCTCP is implementable in data center networks. The NS2 simulation results show that the average flow completion time of short flows in DCTCP can be reduced by 26%-40% in normal cases. Furthermore, due to the coarseness of CWND metric, some cases in which CWND-based judgment could be wrong are also discussed in the paper, and the simulation results show that no severe problem occurs after using LD-DCTCP.