TCP Congestion Control over 3G Communication Systems: An Experimental Evaluation of New Reno, BIC and Westwood+

摘要

One of TCP's key tasks is to react and avoid network congestion episodes which normally arise in packet switched networks. A wide literature is available concerning the behaviour of congestion control algorithms in many different scenarios and several congestion control algorithms have been proposed in order to improve performances in specific scenarios. In this paper we focus on the UMTS wireless scenario and we report a campaign of measurements that involved around 3000 flows and more than 40 hours of measurements using three different TCP stacks: TCP NewReno, which is the congestion control algorithm standardized by IETF, TCP BIC which is the default congestion control algorithm adopted by the Linux operating system, and TCP Westwood+ also available in the Linux kernel. The experimental evaluation has been carried out by accessing the public Internet using an UMTS card. Measurements of goodputs, RTTs over time, packet loss ratios, number of timeouts and Jain Fairness Indices are reported through cumulative distribution functions. Moreover, the efficiency of each TCP version in transferring files has been evaluated by varying the file size in the range from 50 K B up to 500 KB. The cumulative distribution functions reported in the paper show interesting results: 1) a single downlink flow is far from saturating the channel bandwidth; 2) considered TCP stacks provide similar results; 3) 90th (50th) percentile of the goodput of a single downlink flow is less or equal then 230 kbps (120 kbps) compared to a nominal 384 kbps UMTS downlink channel.