The AT&T global network supports all of AT&T's network-connectivity services and application services for business and consumer customers worldwide. Due to the network's very large-scale Building the Network of the Future SDN Network Controller Fabric Switch Overlay Access NE Optical NE MAN/WAN NE ("Underlay") Controllers Controllers Controllers Controllers Controllers vS/vR Access Optical Fabric Switches MAN/WAN NEs (PNFs-VNFs) FIGURE 6.1 SDN network control architecture. (in terms of the number of network elements) and global geographic footprint, and very high avail- ability/reliability performance requirements, the SDN control architecture is based on a hybrid design of centralized and distributed controls, which is shown in Figure 6.1. For smaller networks, a simpler framework may suffice. The distributed controls are provided by a set of distributed local controllers, which are responsible for the traffic forwarding (including failure detection and recovery) operations of individual network elements in their respective local domain. The centralized controls are provided by the global SDN net- work controller for the purpose of end-to-end service management and network resource optimization. 6.1.1 NETWORK CONTROL Network control is responsible for the management, deployment, operation, and coordination of net- work connectivity services implemented on underlying network resources, which comprises virtu- alized network functions (VNFs) and physical network functions (PNFs). Network control includes the following: The establishment and management of network connectivity Maintaining network-wide resource inventory Running network-wide data collection and analysis processes Running a policy engine using network-wide set of policies Handling network-wide exceptions In the remainder of this chapter, the following short-hand terminology is used, unless explicitly noted: Network services-refers to network connectivity services, which are services providing layer 0-3 communications or closely related functions Networ