erface with the network. 4.1.2.3 Southbound Control/Data Plane Protocol A secure and extensible control plane signaling protocol is an impor- tant component for the success of SDN. It should enable efficient and flexible control of network resources by the centralized control plane. OpenFlow [13] is a well-known control plane signaling protocol that is standardized and increasingly being made extensible and flexible. There are other southbound protocols such as OVSDB, which is more backward compatible with existing protocols. 4 Software-Defined Networks Principles and Applications 4.1.2.4 Applications Multiple applications can be built across the control and data plane, such as network virtualization, QoS, service function chaining, and resilience. For example, SDN can be used to implement virtualization, where virtualization of resources is required at the control plane, con- trol channel, and the data plane levels. 4.1.3 Use Cases We discuss two important use cases of SDN: access/aggregation domains of public telecommunication networks and data center networks. Access/aggregation network domains include mobile- backhaul, where the virtualization principle of SDN could play a critical role in isolating different service groups and establishing a shared infrastructure that could be used by multiple operators. On the other hand, using SDN in data center can improve load balancing and dynamic resource allocation (hence, increased network and server utilization) and the support of session/server migration through dynamic coordination and switch configuration. The decoupling principle of SDN could provide the possibility to develop data-center solutions based on commodity switches instead of typically high-end expensive commercial equipments. We will discuss more about the mobile network use case in a later chapter, SO we focus on its data center use cases here. Data centers are an important area of interest for service providers. Applications such as "cloud bursting" are bringing together private and public data cen