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

Hangxing Wu,Fadi Farha,Tao Hong,Yang Xu,Tao Zhang

doi:10.1109/access.2019.2956119

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.

Highlights

Traditional TCP is inefficient to use in Data Center Networks (DCN) when TCP-incast phenomenon occurs [1]
The traffic in DCN mainly consists of delay-sensitive short flows and throughput-sensitive long flows
The flows generated by web search, online retail, and social media are usually delay-sensitive short flows, while database updating flows are throughput-sensitive long flows

Summary

INTRODUCTION

Traditional TCP is inefficient to use in Data Center Networks (DCN) when TCP-incast phenomenon occurs [1]. H. Wu et al.: CWND: Coarse, But Simple, Efficient Metric to Reduce Short Flow Completion Time in Data Centers TABLE 1. CWNDs leads to prioritizing the short flows Based on this concept and by using CWND as a coarse metric, LD-DCTCP has been proposed in the paper. Considering the simulation results and the relation between flows which have small CWNDs and short flows, LD-DCTCP is proposed. H. Wu et al.: CWND: Coarse, But Simple, Efficient Metric to Reduce Short Flow Completion Time in Data Centers. Some long flows can have small CWND when they start up, and LD-DCTCP will wrongly identify them as short flows. Because CWND is a coarse metric, a few short flows may be inserted in the low priority queue.

PERFORMANCE EVALUATION

Findings

CONCLUSION