Wireless local area networks, in particular the ones based in the IEEE 802.11 standard, are nowadays used around the world to deliver Internet access, and are becoming increasingly prevalent. One of the crucial performance issues that these networks introduce is the possibility of having multiple transmission rates in the physical layer. In this paper, we use the Network Utility Maximization framework to characterize the cross-layer interaction between the transport protocols such as TCP and the underlying MAC level rate adaptation.We describe the resource allocation imposed by current wireless networks in this framework, and characterize its equilibrium.Moreover, we propose alternative resource allocations that overcome the inefficiencies found in current protocols, and show simple mechanisms to impose more efficient equilibria in single cell scenarios. We also present simulations of these mechanisms in action, and discuss further generalizations to more complex networks.