## 3 DenseNet-Hybrid Control Plane Architecture
### Control Logic Switching Algorithm
For low values of CPLR, the centralized operation performed well, as lower retransmissions and small delays were experienced by the control packets, and there was a lower convergence time as the network graph was already known by the SDN controller. However, for high CPLR values, due to the high loss of control packets, the exchange of control messages substantially affected (due to higher retransmissions and consequent delay) the TCP-based control communication between the SDN controller and forwarding nodes. For this reason, if the CPLR of the links increased, the performance of the centralized operation substantially degraded, as it depends on a centralized SDN controller, which becomes a single point of failure in high-loss conditions. On the other hand, as LSAs are flooded, if a node fails to receive a copy of the LSA from a node due to the degraded channel, it might still receive a copy of that LSA from another node. In this way, the distributed link-state routing can converge during topology changes and shows better performance than the centralized operation under high control packet loss conditions. Figure 4:0 Aggregated throughput behavior of distributed network operation, during data plane link failures for various times under unreliable channel conditions in a 16-node grid network. Figure 4:1 Average latency behavior of distributed network operation, during data plane link failures for various times under unreliable channel conditions in a 16-node grid network. ### Hybrid-SDN Operation

In the previous section, the performance evaluation of the canonical-SDN and distributed network during unreliable conditions has been analyzed for different network sizes and several injected flows. From our evaluated scenarios, by examining the performance of the centralized and distributed modes during unreliable control communication channel conditions, we characterized the switching point between centralized and distributed operation to the CPLR value of 15% and set it as \(CPLR_{max}\). From the previous section that below this point, centralized operation performed better, and, above it, distributed operation outperformed the centralized operation (see Figures 4-7). In this section, we quantify the gains that hSDN operation offers. In this direction, we emulate the same network topologies by injecting the same number of flows; however, the difference is that the forwarding nodes are hybrid nodes, where centralized and distributed operations coexist and the nodes can take autonomous switching decision between both modes. #### 4.4.1 Centralized to Distributed: Recovery Operation

Initially, the network was operated in centralized mode. The local agent that was integrated into nodes (see Figure 2) periodically monitored the CPLR of the links every 10 s. Then, we began increasing the CPLR, and when the CPLR of the links increased, the decision module integrated with the algorithm made a network operation switching decision, and the local agent performed switching from centralized to distributed operation, as described in Section 3.3.1. From Figures 4-7, during high CPLR (e.g., 20%), the canonical-SDN showed lower throughput and higher latency due to high control packet losses, whereas Figures 16-19 report that hSDN--by switching the network logic to distributed--was able to obtain a throughput improvement of up to 28%, and the improvement was much higher when CPLR increased even more. distributed switching back and forth) in our hSDN approach by measuring the CPLR value of the control communication channel. Initially, all the nodes in the 16-node grid network were in centralized mode. We began increasing impairments to the links and, at around 60 s, a high CPLR was measured, and the decision module of the local agent made a decision based on the integrated algorithm to perform network mode switching and performed switching to the distributed operation.