Virtualization of network functions and centralized management are anticipated to provide 5G mobile networks with flexibility, lower end-to-end latency and reduced cost. Based on the concept of emerging Software Defined Network (SDN) and Network Function Virtualization (NFV) techniques, we propose Software Defined Protocol (SDP) technique to facilitate a flexible service-oriented protocol stack deployment under centralized network control. The design objective of SDP is to provide high-throughput, low-latency and elastic mobile services by making data-plane protocol programmable. In this paper, we first elaborate the SDP mechanisms and then address one of the most important issues in SDP, namely protocol stack mapping (PSM). We formulate the PSM problem as a 0–1 quadratic programming for selecting the optimal SDP servers to balance network load. We employ the legacy LTE data-plane processing as a benchmark for validating the effectiveness of the SDP and PSM algorithm. Numerical results show that SDP is effective to provide elastic low-latency mobile services and the proposed PSM algorithm significantly outperforms the benchmark in stack processing delay, mapping cost and resource utilization.