llocated with an IP core location. There are two core routers, A and B, per core loca- tion. All unicast traffic originates/terminates at the edge routers and each such router is connected Controller FIGURE 16.2 Example IP/optical network with backbone (B) and edge (E) locations. Network Measurements to at least two core routers (same or different locations) using physically diverse paths. In addition, there may also be point-to-multipoint multicast traffic and all the endpoints of such traffic are also edge routers (it is of course possible for the same router to perform both edge and core functions). A subset of all possible pairs of IP routers is connected via IP links to form an IP network. An IP link between two different core locations needs to be routed over the optical network. As an example, the IP router A in location B2 may be connected to the IP link B in location B4 over the sequence of optical nodes 02-07-06-04. The SDN controller can control the edge routers, the core routers, and the optical nodes. The SDN controller is logically shown as a single centralized entity but it may be functionally separated into one controlling the IP network and one controlling the optical network as shown in Figure 16.1. Furthermore, for the purpose of reliability and disaster recovery, it makes sense to have one active SDN controller and one or more standby SDN controllers located geographically in different places. 16.2.2 WHERE AND How TO MEASURE REAL-TIME NETWORK DATA The key measurement types and locations are the following: Edge-to-edge measurements: It is possible to measure and report on all edge-to-edge unicast and multicast flows in a small network. However, this is not scalable for large ISP networks since there may be hundreds of millions of flows in such a network. Instead, one can aggre- gate all traffic flows by type between a specific source-destination pair of edge routers to a small number of TE tunnels (point-to-multipoint tunnels in case of multicast flows). Studies have shown that typically we do 