re 5.10 shows a set of example monitor points in grey circles. With monitor points of A and B, we can ensure if there is any failure in any of the services or switches in this part of chain segment. Thus, given this set of monitor points A-E, if any failure occurs in service S1, we can detect a reachability failure from monitor point B. If any failure occurs in service S2 and S3, we will detect it from both monitor C and monitor E. If we observe that both C and E are affected, but B is not affected, then it suggests likely that either S2 or S3, or any of forward- ing elements along this path segment fails. This example shows that Controller Service chains Monitor point Service Figure 5.10 Example to show monitor points for multiple service chains. 5 SDN and NFV in 5G Algorithm 1 Monitor point selection algorithm procedure Select_Monitor_Points(P,G) create a bipartite graph N with two sets of nodes for all elements in P and G for each flow p E P do for each element f e p do add edge f end for end for Sort element f E N according to its out degree for each element f E N do select f to be a monitor point, remove p if p E N = 0 then break; end if end for the paths for multiple service chains may overlap, and we can select a subset of the points in the path to monitor, in order to reduce the load for continuous monitoring. The problem is defined as follows. A service chain is defined as a sequence of forwarding switches and the services, for example, p0=F0,F1,S1,F2,S2,F3,S3,F4, p1=F1,F2,S2,F3,S3,F5. Given multiple service chains, P=p0,p1, ... pk, we need to select a minimum number of monitor points SO that all the path segments are covered. Our key idea is to create a bipartite graph by mapping each monitor point candidates to the service chain it can cover. Then, the monitor points are sorted according to the out degree (the number of service chains it can monitor). We finally greedily select the monitor points until all the services are covered. The detailed algorithm is shown in Algorithm 1. 5.2.3 Optical SFC Optic