stead, with the aid of the optimized routing algorithm, the SDN controller will monitor and, if necessary, step in to manage these LSPs to react to traffic and net- work conditions while maintaining high reliability. Separate from the data plane, a global SDN controller communicates with the routers and ROADMs through various interfaces to perform monitoring and control tasks. For the optical layer, the key control capability is to establish/release wavelength circuits to adjust the layer 3 logical topology. The controller performs such operations through a vendor-provided control module or an open RESTful interface to an open ROADM controller, which then proceeds with light path computation and ROADM configuration. For the IP/MPLS layer, the key control function is to monitor traffic load and link state to perform globally optimized LSP operations, such as establish- ing/releasing LSPs, adjusting the paths/bandwidth of LSPs, and multipath routing. The controller provides interfaces to support customized policies and various applications, such as centralized TE, dynamic service provisioning, and optimized failure restoration. The logically centralized control- ler could be implemented in a physically distributed way to achieve scalability and resilience. The specific controller design and deployment are beyond the scope of this chapter. Interested readers could refer to related materials, such as References 2-4. The interfaces between the control and data plane are extensible to support multiple protocols. While OpenFlow is frequently discussed in the SDN community [5], SO far it is not the most criti- cal protocol in the carrier network architecture. Figure 16.1 illustrates a few indispensable proto- cols and data: Border Gateway Protocol-Link State (BGP-LS) for link status, capacity and per-class bandwidth usage; Path Computation Element Communication Protocol (PCEP) supports interactions between the controller and routers in terms of reporting and readjusting route and bandwidth of LSPs; Simple Network Managem