TCP Acknowledgement Compression for Fairness Among Uplink TCP Flows in IEEE 802.11n WLANs

Abstract

This paper deals with the problem of unfairness among uplink TCP (Transmission Control Protocol) flows associated with frame aggregation employed in IEEE 802.11n WLANs (Wireless Local Area Networks). When multiple stations have uplink TCP flows and transmit TCP data packets to an AP (Access Point), the AP has to compete for channel access with stations for the transmission of TCP ACK (acknowledgement) packets to the stations. Due to this contention-based channel access, TCP ACKs tend to be accumulated in the AP's downlink buffer. We show that the frame aggregation in the MAC (Medium Access Control) layer increases TCP ACK losses in the AP and leads to the serious unfair operation of TCP congestion control. To resolve this problem, we propose the TAC (TCP ACK Compression) mechanism operating at the top of the AP's interface queue. By exploiting the properties of cumulative TCP ACK and frame aggregation, TAC serves only the representative TCP ACK without serving redundant TCP ACKs. Therefore, TAC reduces queue occupancy and prevents ACK losses due to buffer overflow, which significantly contributes to fairness among uplink TCP flows. Also, TAC enhances the channel efficiency by not transmitting unnecessary TCP ACKs. The simulation results show that TAC tightly assures fairness under various network conditions while increasing the aggregate throughput, compared to the existing schemes. The unfairness problem has been addressed in the literatures [2][3][4][5]. The works in [2] and [3] aim to prevent buffer overflow in the AP by giving the AP more channel access chances. This can be accomplished by providing high priority to the AP with the aid of differentiated channel access mechanism of IEEE 802.11e [2], or by assuring bi-directional channel access for the STA and AP [3]. However, this approach has a drawback, i.e., the aggregate throughput may be decreased, because the prioritized channel access by the AP increases the probability of collision with STAs [2], or the bi-directional channel access opportunity given to the AP may be wasted due to lack of backlogged TCP ACK [3]. In [4], the generation of TCP ACK is regulated to assure fairness by intentionally dropping the received TCP data packet in the AP based on its queue occupancy. This is effective to assure fairness, but it may decrease throughput since the TCP data packets that were successfully delivered to the AP are dropped. To mitigate the unfairness among uplink TCP flows, a specific scheduling algorithm is proposed in MAC layer transfers to the lower layer. Also, there are two kinds of ACKs, TCP ACK and MAC ACK, we distinguish them by indicating the terms "TCP" or "MAC" in front of ACK. To our best knowledge, none of the existing studies considers the effect of the IEEE 802.11n frame aggregation on the unfairness problem among uplink TCP flows. This is the very first study revealing that the unfairness problem becomes remarkably exacerbated due to an unintended interaction between MAC frame aggregation and TCP congestion control. Moreover, the ...

Coupled IEEE 802.11ac and TCP Goodput Improvement Using Aggregation and Reverse Direction

“…The issue of TCP performance over IEEE 802.11 networks has been investigated in many papers in the past, e.g. [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In this past research however, it is assumed that stations compete to get access to the channel using the contention based CSMA/CA access method.…”

“…In [14] it is argued that aggregation increases the discrepancy among upload TCP connections. The model is an AP with several stations that initiate TCP upload connections.…”

“…In relation to IEEE 802.11n/ac where aggregation is introduced, we are aware of only three research papers that handle the Throughput performance of TCP in the various aggregation schemes [14,15,25]. In [25] the authors also assume the model of the AP and a single station that maintain a TCP connection.…”