Performance of TCP in Multi-Hop Access Networks

Wireless multi-hop access networks are an increasingly popular option to provide cost-efficient last-mile Internet access. However, despite extensive research, performance of even basic communication services, such as TCP, is still problematic. Measurements collected on a wireless testbed indicate that the poor performance of multi-hop access networks is caused by poor interactions between TCP congestion control and link- layer bit-rate adaptation resulting in severely reduced network efficiency even over short wireless paths (< 6 hops). However, bit-rate adaptation improves fairness across TCP flows. The same principal observations hold for hybrid wireless/wireline paths. To investigate approaches to improve TCP performance, we present a simple model that captures the cause for the inefficiency of TCP over autorate links. We then examine several techniques at both the TCP level and the link layer (TCP Vegas, clamping, limiting the buffer size at the wireless routers) to alleviate contention. None of these techniques works for all scenarios, but the simple approach to limit the buffer size is attractive in many settings that include four or more wireless hops.