A Cross-layer Approach to Enhance TCP Fairness in Wireless Ad-hoc Networks

摘要

Wireless ad-hoc networks represent an emerging technology, well suited for providing temporary network connectivity where a wired infrastructure cannot be easily set up. The very popular 802.11 standard enables wireless ad-hoc networking, using the Distributed Coordination Function (DCF) for multiple access to the shared radio channel. Unfortunately, the interaction between TCP dynamics, driven by the Additive Increase Multiplicative Decrease (AIMD) paradigm, and DCF channel access rules, which are based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithm, leads to an inefficient spatial channel usage. As a consequence, 802.11 ad-hoc networks provide an unfair service to TCP flows. In order to achieve a reasonable trade-off between throughput and fairness, this paper proposes a cross-layer algorithm that dynamically limits the number of in flight segments in a TCP connection by taking into account measurements of frame collision probability, collected at the MAC layer along the path. Performance of the proposed algorithm have been evaluated by ns-2 simulations; results have shown that the developed cross-layer scheme provides the same goodput, improves fairness in bandwidth sharing, and reduces segment retransmission ratios with respect to the standard TCP over 802.11 MAC.