ters like never before. Distributed data centers have been widely used in today's large-scale enterprise networks, such as Google B4 network [79]. In such scenarios, the inter-data center wide area network (WAN) providers become a very important component in efficient and cost-effective connectivity and operation of distributed data centers. Figure 4.4 illustrates this. The static provisioning mechanisms used currently by operators to provision resources on the WAN will not only degrade the performance of distributed data centers but also increase the cost of operation. Thus, it is necessary that the inter-data center WAN evolves into a network- ing resource that can be allocated and provisioned with the same agility and flexibility that is possible within data centers. SDN is a very good candidate to enable such agile and flexible WAN operations the data center operators are expecting. Centralized control of the WAN 4.2 SDN Controller Intra-DC WAN (SDN network) Figure 4.4 Inter-data center WAN: a use case of SDN. using an SDN controller provides its northbound Open API to enable intelligent traffic engineering and complex policy management. It not only provides a single unified control of the WAN resources but also enables on-demand provisioning of network resources for multiple clients based on their policies and SLAs, in a seamless fashion. Another advantage of a centralized SDN controller in the inter-data center WAN is that it facilitates unified control along with data center resources. Data center operators are increasingly demanding a sin- gle unified mechanism to manage all the data centers in their domain instead of a separate, distributed management. SDN on the WAN is one of the mechanisms to achieve this. SDN Controller There are multiple SDN controllers in the industry today, supporting different deployment scenarios, each of which has pros and cons. The network controller provides a uniform and centralized programmatic interface to the entire network. It provides the ability to observe and control a