Distributed Coordination Function (DCF) is the basis of the IEEE 802.11 WLAN MAC protocols. This paper proposes a scheme named DCF+, which can be regarded as an option to DCF, to enhance the performance of reliable transport protocol over WLAN. To analyze the performance of DCF and DCF+, this paperalso introduces an analytical model to compute the saturated throughput performance of WLAN. Compared with other models, this model is more accurate, which is verified by elaborate simulations. Moreover, DCF+ is shown to be able to improve the performance of TCP over WLAN by both modeling and simulations.