Due to the growing popularity of mobile user applications, cellular network operators are increasingly interested in offloading their scarce licensed spectrum resources. To this aim, using unlicensed-band radio technologies is attractive owing to its reduced costs. This work designs a system of direct D2D (device-to-device) connections to offload mobile traffic that is served by LTE (long-term evolution) cellular networks. In particular, a network assistance protocol over LTE architecture is proposed for D2D connections, and its system-level performance is evaluated to quantify the resulting traffic-steering efficiency. In addition, practical operation of D2D communication in a test LTE network is described, which allows making substantiated conclusions about the actual feasibility of the proposed solution in real-life conditions.