There are provided measures for enabling/realizing interface functionality for RAN-WLAN radio aggregation, i.e. radio level integration/aggregation between a cellular radio access network and a wireless area network. Such measures exemplarily comprise user-plane data forwarding from a cellular radio access network over a wireless local area network to a terminal, both the cellular radio access network and the wireless local area network providing radio level connectivity for the terminal, wherein in the user-plane data forwarding user-plane data packets according to a cellular data convergence protocol are transported, at least partly, using a reliable transport protocol over the wireless local area network to the terminal, providing feedback on performance of user-plane data packets transport using the reliable transport protocol, at least partly, from the wireless local area network to the cellular radio access network, and executing flow control on the basis of the performance feedback.