On the unfairness among TCP flows in IEEE 802.11 multi-hop ad-hoc networks

The problem of unfairness among competing flows in TCP multihop wireless networks is well known, and has often been traced to the misbehaviour of dynamic routing protocols in case of link failure. We show, through simulations, that the competition between TCP flows in a multihop wireless network leads to an unfair outcome, independently of the routing protocol involved. We focus on the impact of hidden node collisions on the performance of multi-hop TCP flows, and trace the source of the problem to collisions between RTS frames belonging to the starved flow and long data frames sent by the active flow. Such collisions lead to a rapid increase of the MAC contention window at the TCP sender of the starved flow; this, in conjunction with TCP's congestion control algorithm, allows the running flow to hold the channel for a long time, until the situation is reversed. We also show that a simple multi-channel MAC can ease the competition among flows.