Direct (or device-to-device, D2D) communications are being investigated in the framework of LTE-Advanced. They allow one-to-one communications between two endpoints, under the control of the eNodeB, which allocates resources for the d2d flow, but does not act as a relay for its traffic. The direct link can also be used for file transfer or proximity-based browsing, i.e. applications running on TCP. In this paper, we evaluate the per- formance of TCP-based traffic transported through the direct link, in several scenarios. We show and explain non-intuitive results, which arise from the interplay of TCP and LTE-A pro- tocol mechanisms, and compare the existing TCP versions in a dynamic environment, where mode switches between the direct and the infrastructure link may induce periodic losses. 

Keywords: {LTE-A, device-to-device, TCP, performance evaluation}