Satellites have largely been designed as application-specific and isolated for the past decades. Though with certain benefits, it might lead to resource under utilization and limited satellite applications. As an emerging networking technology, software-defined networking has recently been introduced into satellite networks. In this letter, we propose a software-defined satellite networking (SDSN) architecture, which simplifies networking among versatile satellites and enables new protocols to be easily tested and deployed. In particular, we propose a seamless handover mechanism based on SDSN and conduct physical layer simulation, which shows significant improvement over the existing hard handover and hybrid handover mechanisms in terms of handover latency, throughput, and quality of experience of users.