orts a large variety of adapters SO that it can configure and control a large number of physical and virtual devices through established and emerging protocols (BGP, PCEP [4], NetConf, OpenFlow). OpenDaylight's modeling capability is the strongest of any of the open source SDN controllers. The dynamic binding of YANG model-based java objects, makes it easy to create new services and a great fit to the configuration phase of NetConf devices that use YANG models. OpenContrail [6] is used in AT&T as a local controller. Primarily because of its unique approach to managing the virtual router (vRouter) on the compute nodes, the controller provides a large data center scaled solution for dealing with the overlay network, at scale. It does not have as good a modeling capability as OpenDaylight but it is a good fit for service chaining in the overlay networks. ONOS [7] is not directly used by AT&T but it is positioned as the embedded controller that would best fit into the control of merchant silicon switches. In this mode, the SDN controller needs to be focused on switch-control interfaces and in efficiently managing the OpenFlow-based protocols through high-level abstractions like network intents. 6.10 TOPICS FOR FURTHER STUDY In this section, we identify and describe some topics for further exploration. Network closed-loop control with telemetry lag: There is measurable lag in collection of telem- etry from large-scale networks. There is also a massive amount of data to be analyzed to determine both the current state and whether a new state is more optimal. Between collection lag and analytic lag, centralized closed-loop control can be challenging and usually puts the centralized algorithm into the space of optimizing after distributed algorithms have already reacted. It would be interest- ing to look at this problem and determine whether there were better algorithms to manage the two types of lag and come up with more optimal centralized control-loop paradigms. Model to execution software techniques: Today, the YANG m