ACK pushout to achieve TCP fairness under the existence of bandwidth asymmetry

Abstract

On a bandwidth asymmetric link, such as an ADSL cable or a wireless link over an IEEE802.11-based wireless LAN, TCP flows are likely either data bottlenecked or ACK bottlenecked, where a data (ACK) bottlenecked flow means the one whose ACK (data) segments are rarely lost but data (ACK) segments are frequently lost. The bandwidth asymmetry causes two types of unfairness problems. One is the unfairness between data- and ACK-bottlenecked flows; ACK bottlenecked flows obtain larger bandwidths than data bottlenecked flows. The other is the unfairness among ACK bottlenecked flows; some of ACK bottlenecked flows consume most of bandwidths. In this article, we propose a buffer management scheme, called the ACK pushout, in order to cope with the two types of unfairness problems simultaneously. To improve the unfairness between data- and ACK-bottlenecked flows, the ACK pushout simply removes an ACK segment from the buffer when a data segment arriving at the buffer finds that it is fully occupied. To improve the unfairness among ACK-bottlenecked flows, the ACK pushout discards one of ACK segments of the flow that has the largest number of ACKs in the buffer when an ACK segment arriving at the buffer finds that it is fully occupied. Through extensive simulation experiments by ns2, we verify that the ACK pushout greatly improves the fairness.