1.过去20年中可编程网络的发展可以分为几个阶段？每个阶段的贡献是什么？
We divide the history into three stages
历史发展可以分为三个阶段
（1) active networks (from the mid-1990s to the early 2000s), which introduced programmable functions in the network to enable greater to innovation
他在网络中引入了可编程功能
（2) control and data plane separation (from around 2001 to 2007), which developed open interfaces between the control and data planes
控制层和数据层相分离
（3) the OpenFlow API and network operating systems (from 2007 to around 2010), which represented the first instance of widespread adoption of an open interface and developed ways to make control-data plane separation scalable and practical.
开发了使控制数据平面分离可扩展且实用的方法
2.网络虚拟化与SDN的关系？
（1）SDN as an enabling technology for network virtualization.
多个客户共享同一网络基础设施和SDN一个控制器控制多个交换机的概念十分接近。
（2) Network virtualization for evaluating and testing SDNs.
为SDN在部署到实际网络之前提供实验平台。
（3) Virtualizing (“slicing”) an SDN.
虚拟化SDN交换机更简单。传统交换机的虚拟化要把各自的配置做也一起配置，而SDN交换机的虚拟化采用一个控制器和一片交换机的方式，简化了虚拟的过程。