5.3.4. NFD

Due to the hybrid SDN topologies where N-SDN nodes cannot be selected as IAP, not every combination of S, D and L can successfully deploy intercept access point. We count the total number of failures to deploy IEP (i.e., NFD), to evaluate the performance of SDN interception models. The statistical results are shown in Figure 11a–c. We can clearly observe from the figures that in the three hybrid SDN topologies, the total number of failures to deploy IAPs (NFD) in Fermat-point model is the least compared with NFD in T model and ECMP-T model, which means that Fermat-point model has a high success rate to deploy intercept access point. More specifically, compared with T model and ECMP-T model, Fermat-point model decreases NFD on average by 88.21%, 86.87% in CRN topology, 76.9%, 74.68% in COST 239, and 67.53%, 66.26% in NSFNet. To sum up, the performance of Fermat-point model outperforms T model and ECMP-T model and thus Fermat-point model is the best interception model in terms of NFD.

**Figure 11.** The impact of SDN interception models on NFD under three topologies. NFD. The lower, the better.
