Future pervasive networks will be constructed by various access networks, which are managed by different operators. Wireless LAN hotspot services will be available in numerous locations, and cellular services, which provide a wide coverage area, will also be available. In order to realize efficient mobile computing in such an environment, mobile hosts must have multiple network media as well as functions to select the optimum network among them based upon the current situation without the disconnecting communications. In order to provide mobile hosts with such a function, we propose an association layer, which is inserted between the transport and network layers. The association layer selects the appropriate network medium for each flow according to the network conditions and flow characteristics, providing end-to-end QoS in multihomed networks. Furthermore, we show the effectiveness of the proposed scheme through simulation results and demonstrate proof of concept based on empirical evaluations of a prototype implementation.