## 3 DenseNet-Hybrid Control Plane Architecture
### Control Logic Switching Algorithm
However, in the case of high values of CPLR (e.g., 25%), although dropped control packets were retransmitted again, the high loss of the control packets affected the restoration of a new path at the data plane, which rendered the network unusable, as the impact was the same as if the SDN controller had failed. The impact on the aggregated throughput and latency were also highly noticeable. Figure 4: Data plane performance metric (aggregated throughput) comparison between centralized and distributed operations, during unreliable control plane conditions in a 9-node grid network. Figure 5: Data plane performance metric (average latency) comparison between centralized and distributed operations, during unreliable control plane conditions in a 9-node grid network. Figure 6: Data plane performance metric (aggregated throughput) comparison between centralized and distributed operations, during unreliable control plane conditions in a 16-node grid network. To evaluate the impact of control message loss over data plane performance, we generated different numbers of link failures at the data plane with the intention of redirecting control messages to the SDN controller over lossy control communication channels. Figures 8-11 report the data plane performance of canonical-SDN - while the network was experiencing an increasing number of data plane link impairments under a certain CPLR value. We repeated the experiments for different network topologies by injecting seven TCP flows in each topology. We observe that, for both topologies, the aggregated throughput at the data plane substantially degraded, and the latency increased with the increasing number of broken links at the data plane. This was evaluated for various values of CPLR to assess its dependency on varying degrees of data plane link impairments. As CPLR increased, so did the restoration time due to control packet loss and the consequent retransmission of control packets between the network nodes and the centralized SDN controller. For both network topologies, for CPLR values of 25%, the degradation of the aggregated throughput at the data plane was remarkable, and the latency was substantially higher due to the high loss of control packets, which caused several retransmissions that affected the proper communication between the SDN controller and the forwarding nodes. As latency is very high during a CPLR of 25%, for both cases, this is not represented in the figures. The network topology is a grid, and the connectedness of the graph is maintained, which may not be the case in a real deployment with lower nodal degrees of the graph. In this sense, the network performance presented in the figures may be considered the best possible case in terms of the path diversity. Figure 7: Data plane performance metric (average latency) comparison between centralized and distributed operations, during unreliable control plane conditions in a 16-node grid network. Figure 8: Aggregated throughput behavior of centralized network operation during data plane link failures for various times under unreliable control plane conditions in a 9-node grid network. Figure 10: Aggregated throughput behavior of centralized network operation during data plane link failures for various times under unreliable control plane conditions in a 16-node grid network. Figure 9: Average latency behavior of centralized network operation during data plane link failures for various times under unreliable control plane conditions in a 9-node grid network. ### Distributed Operation: Unreliable Condition

We evaluated the same network topologies with the same number of TCP connections in the distributed network. We conducted our experiments in the distributed case where all the links were degraded communication links, and, in a controlled way, we increased the loss of control messages exchanged between two neighbor nodes. In the case of our evaluated distributed scenario, Hello and LSA (Link State Advertisement) messages are periodically exchanged among the neighbor nodes.