re is drawn from the IEEE Communications Magazine article [1] authored by some of the contributors to this chapter. The data plane is an IP/MPLS over a wave division multiplexing (WDM) optical network. The opti- cal layer uses flexible ROADMs that provide open interfaces to allow wavelength reconfiguration. Such interfaces enable centralized control of the optical network to dynamically and remotely estab- lish/release wavelength circuits, thus adjusting the layer 3 logical topology and capacity. Extending SDN control from layer 3 to the optical layer is important to automating and optimizing network provisioning and operation tasks, such as capacity augmentation and failure restoration. Many ven- dors support such control interfaces in their new products. This figure highlights the explicit use of real-time data coupled with efficient and intelligent optimization algorithms to dynamically manage a large IP network, both logical and physical layers. The IP/MPLS routers support traditional distributed routing. They also provide open interfaces enabling centralized SDN control. MPLS still plays a critical role to ISPs in many operations, includ- ing routing, traffic engineering (TE), priority control, service provisioning, and failure restoration. Network Measurements Apps and Algorithms Traffic Service Failure Policies Engineering Provisioning Restoration SDN Controller Collection, Cleansing, Centralized Control Logic Analysis, Storage Protocol Plug-ins BGP-LS NETCONF Vendor Plug-in Vendor-Provided WDM Network Controller IP/MPLS Network WDM Network FIGURE 16.1 SDN network architecture. Accordingly, the SDN control in this architecture works at the level of label-switched paths (LSPs). Some prior works perform fine-grain traffic control based on 5-tuple flows identified by source/des- tination IP addresses, protocol, and source/destination ports. While such an approach is beneficial to certain systems (such as data-center networks), it is unnecessary and technically challenging for large-scale backbone networks. In