The answer is based on the paper given by my teacher, Miss Zhu.
The Chinese version of questions is:
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？
2.网络虚拟化与SDN的关系？
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How to divide the development of programmable network into different phases over the last 20 years? What are contributions at each phase?
(1) active networks (from the mid-1990s to the early 2000s)
• introduced programmable functions in the network to enable greater to innovation;
• Programmable functions in the network to lower the barrier to innovation;
• Network virtualization, and the ability to demultiplex to software programs based on packet headers；;
• The vision of a unified architecture for middlebox orchestration.
(2) control and data plane separation (from around 2001 to 2007)
• developed open interfaces between the control and data planes;
• Logically centralized control using an open interface to the data plane;
• Distributed state management;
(3) the OpenFlow API and network operating systems (from 2007 to around 2010)
• represented the first instance of widespread adoption of an open interface and developed ways to make control-data plane separation scalable and practical.
• Generalizing network devices and functions
• The vision of a network operating system
• Distributed state management techniques
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What is the relationship between network virtualization and SDN?
• SDN as an enabling technology for network virtualization
• Network virtualization for evaluating and testing SDNs
• Virtualizing (“slicing”) an SDN