## 3 DenseNet-Hybrid Control Plane Architecture
### Control Logic Switching Algorithm
Every 2 s, Hello messages are exchanged between two neighbor nodes that confirm the availability of the neighboring nodes. On the other hand, every 5 s, LSAs are exchanged by the nodes in order to learn about the topology of the network. During any change in the network topology that may be caused by link failure or the unavailability of nodes, updated LSAs are exchanged by the nodes to learn about the changes in the topology. In our evaluated scenario, we generated losses of these control messages in order to investigate the throughput and latency performance of distributed operation during unreliable conditions. In a way, we degraded the control communication channel by dropping control messages, and we also changed the network topology by breaking links in the network. We did so in order to exchange control messages during unreliable conditions. When there is a change of topology (e.g., link failure happens), the nodes exchange the updated LSAs to learn about topology changes. However, due to the degraded control communication channels, some control messages are dropped, which, in turn, affects the network convergence time. The dropped control messages are retransmitted again; however, such losses affect the throughput and latency performance, as there is some delay in the reestablishment of routes caused by control packet loss. From Figures 4-7, it can be observed that, in distributed network operation (green curve for three flows and purple for five flows in distributed case) and for low CPLR values (e.g., 5%), the aggregated throughput was higher, and the latency was lower because of lower control packet loss and also due to the lower retransmission rate of lost control messages. As CPLR increased, there was a degradation of the aggregated throughput while the latency went upward. With the increase of CPLR values, the loss of control messages increased, which also increased the rate of retransmissions. For this reason



Due to a high loss of control messages, during topology changes for several times, the exchange of the updated LSAs between nodes suffered from losses, which caused more retransmissions and the consequent delay of control messages that affected the throughput and latency of the network. ### Performance Comparison: Canonical-SDN and Distributed Network

To observe the performance of centralized canonical-SDN and distributed operation under unreliable conditions, we generated controlled losses of: (1) control messages exchanged between the forwarding nodes and the SDN controller in the centralized mode and (2) messages exchanged between nodes that keep track of neighbor availability in the distributed mode of operation. From Figure 4-7, for both network sizes and for low CPLR values, centralized operation performed better (in terms of the throughput and latency) than distributed operation due to lower retransmissions, and the delay of control packets to reach the SDN controller is lower. When the CPLR increased above 15%, distributed operation outperformed centralized operation by maintaining higher throughput and lower latency as higher retransmission of control packets was required for centralized operation, and also delay was incurred by the control packets. Moreover, Figures 8-11 illustrate that, for both network sizes and for high

Figure 12: Aggregated throughput behavior of distributed network operation, during data plane link failures for various times under unreliable channel conditions in a 9-node grid network. Figure 13: Average latency behavior of distributed network operation, during data plane link failures for various times under unreliable channel conditions in a 9-node grid network. CPLR values (e.g., 25%), while the topology of the network changed very often, centralized operation failed to operate correctly. On the other hand, distributed operation kept the network operational (see Figures 12-15).