中文摘要：

Software defined networking (SDN) has been applied increasingly in practical networks. Currently, SDN is mainly used to improve the flexibility and efficiency of datacenter networks, enterprise networks and wide?area networks (WAN). There also emerge some studies that try to deploy SDN to inter?domain settings. In this article, we introduce the progress stages of inter?domain SDN and studies related to each stage. Finally, we discuss the applications and challenges of inter?domain SDN.