无线自组网络中TCP流公平性的分析与改进

研究了TCP(transmission control protocol)流在多跳无线自组网络中的公平性问题,发现IEEE802.11DCF协议在此环境下会导致严重的不公平性,即部分节点垄断了网络带宽而其他节点被饿死.首先,通过仿真分析了产生TCP流不公平性的原因,指出其根源在于MAC(media access and control)协议的不公平性,同时,TCP的超时机制加剧了不公平性的产生;然后,利用概率模型定量分析了TCP不公平性与MAC协议参数之间的关系,发现TCP流的公平性与TCP报文长度直接相关,并且增加MAC协议初始竞争窗口的大小能够有效提高公平性.据此,提出了一种根据TCP报文长度动态调节初始回退窗口大小的自适应回退MAC协议改进算法.理论分析和仿真表明,该算法在很大程度上可以有效缓解不公平性问题的产生,并且不会引起网络吞吐量的严重降低.

Analyzing and Improving the TCP Flow Fairness in Wireless Ad Hoc Networks

The TCP(transmission control protocol) flow fairness problem in wireless multi-hop ad hoc networks is investigated. It is identified that the IEEE 802.11 DCF protocol can lead to severe unfairness, i.e., some nodes seize the whole channel capacity while others are starved. The TCP flow unfairness problem is first analyzed by simulation and it is found that the main reason lies in the unfairness of MAC(media access and control) protocol, while the TCP timeout mechanism makes the unfairness more severe. Then a probability model is used to quantitatively analyze the relation between the TCP unfairness and MAC parameters, which shows that the TCP flow fairness heavily correlates with the TCP packet length, and increasing the initial contention window of the MAC protocol can improve the fairness effectively. Based on these observations, a novel adaptive backoff algorithm is proposed, which dynamically adjusts the initial contention window according to the TCP packet length. Both theoretic analysis and simulation results show that the proposed algorithm can relieve the fairness problem to a large extent without significantly impairing aggregate throughput in wireless ad hoc networks.