## 3 DenseNet-Hybrid Control Plane Architecture
### Control Logic Switching Algorithm
At around 120 s, the performance of the control communication channel became fair (i.e., low loss), and, as a consequence, the CPLR became low, and the local agent restored the centralized operation based on the decision taken by the decision module. By doing so, the SDN controller gained control of the network, and the network was managed in a centralized fashion. Figure 4:2 Data plane performance (average latency) of hSDN during data plane link failures for various times under unreliable control plane conditions in a 9-node grid network. Figure 4:3 Data plane performance (aggregated throughput) of hSDN during data plane link failures for various times under unreliable control plane conditions in a 16-node grid network. ## 5 Comparison of hSDN Schemes

Our proposed hSDN approach brings novelty in the way the control plane failure is handled. We considered the metric Control Plane Loss Ratio (CPLR) to investigate reliability of control communication channel and also argue that the investigation should be done from node points of view. By doing so, failure of the controller, control communication channel, and even degraded control communication channel can be detected, which brings huge advantages in network management. Moreover, our approach introduces a network switching algorithm that predicts the quality of the control communication channel by detecting trends of the control packet loss curve (i.e., slope of the CPLR vs. t curve) and makes an autonomous decision to switch the network operation. A comparative analysis between the proposed approach and the existing approaches is presented in Figure 4:4 The comparison was made based on control plane reliability. We considered the following fault detection criteria: A-SDN controller failure, B-control communication channel failure, and a C-degraded control communication channel. The \"+\" sign indicates the presence of the fault detection criteria and scores 1 point. The \"-\" sign defines the absence of the criteria and no score (i.e., 0 (zero) point). Figure 4:5 Data plane performance (average latency) of hSDN during data plane link failures for various times under unreliable control plane conditions in a 16-node grid network. Figure 4:6 Aggregated throughput behavior over time during network operation switching in hSDN. ## 6 Discussion

Generally, the centralized SDN is assumed to have an uninterruptible communication between forwarding devices and the SDN controller. However, in a multi-hop wireless network scenario, the performance of centralized SDN may degrade as the in-band channels are responsible for carrying control packets from forwarding devices to the SDN controller and the wireless channels are more likely to be experiencing impairments due to environmental conditions. In such a scenario, the performance (in terms of data plane throughput and latency) of SDN may degrade or even become nonoperational during critical conditions (i.e., high control packet loss), which was analyzed in our work. Apart from the throughput and latency metrics, the Available Bandwidth (ABW) [28] measurement is one of the significant metrics in SDN to obtain information about the current load on the links as well as on the network. In the SDN scenario, to measure ABW, the controller periodically performs polling of the measurement report from the forwarding devices using _PortStatusReq_ messages. As presented in [28], while delay increases in communication between the controller and the forwarding devices, the ABW estimation error also increases, which has an adverse impact on the path selection for any services. For in-band SDN deployment in a wireless multi-hop scenario where control packets are sent over the wireless links, due to environmental conditions, the communication between the controller and the forwarding devices may be hampered. The degraded control communication link may incur delay on control packets or even the loss of control packets, which was investigated in our work.