Impact of TCP ACK Losses on TCP Fairness in Wireless Mesh Networks

This paper analyzes TCP fairness in IEEE 802.11-based wireless mesh networks (WMNs). Fairness of bandwidth sharing is important for equitable network access by various users. With our Indoor WMN testbed, we observe that even though TCP flows have similar round trip time and loss probability, they encounter an unfairness problem. Especially, when TCP flows experience high TCP ACK losses (around 20%), unfairness happens in a relatively long period (1~100 seconds). We denote this phenomenon as a semi-long-term unfairness in the same path (SLU-SP) problem because it happens even when TCP flows share the same routing path. Additionally, we find out that the extended hidden terminal problem boosts up high TCP ACK losses and leads to a serious SLU-SP problem. Also, by carefully observing TCP dynamics through an active monitoring tool, PaPMo (packet-accurate protocol monitor), we find out that the cause of SLU-SP problem comes from the cumulative ACK mechanism of TCP. Cumulative ACK makes TCP flows tolerable to few ACK losses, but when the TCP flows have different sizes of congestion window, the TCP flows have unfair bandwidth sharing in the relatively long period.