Reducing the TCP ACK packet backlog at the WLAN access point

Abstract

Modern loss based Transmission Control Protocols take aggressive congestion window (CWND) control strategies in order to gain better throughput, but such strategies may cause a large number of packets to be backlogged and eventually dropped at the entry point to the wireless access network. This problem applies not only to the downstream TCP sessions but also to the upstream TCP sessions when the terminal is connected via a Wireless Local Area Network (WLAN), which disregards the size of packets in its scheduling. This paper focuses on the ACK packet backlog problem with the upstream TCP sessions, and proposes a CUBIC based CWND control mechanism as part of the middleware for the Android terminals. It utilizes the Round Trip Time (RTT) as an indication for the TCP ACK backlog condition at the WLAN AP, and controls the upper and lower bounds of its CWND size to suppress excessive transmissions of own TCP DATA packets. An experimental study with up to 10 Android terminals shows that the proposed mechanism can improve both aggregate throughput and fairness of the WLAN, and that it is highly effective particularly for cases where very long RTTs are observed.