## 3 DenseNet-Hybrid Control Plane Architecture
### Control Logic Switching Algorithm
Therefore, the loss of control packets (e.g., _PortStatusReq_) may have adverse impacts on ABW measurements. ## 7 Conclusions and Future Work

Since the SDN paradigm generally assumes full reliability of the control plane, this paper set its goal to evaluate what happens when this is not the case, and we also evaluated a possible solution. This scenario is becoming more likely in 5G when deployed at higher frequencies through massive small cell deployments served through wireless transport links. Given the criticality of control plane message losses or SDN controller failure due to network performance, we quantitatively evaluated the impacts on overall network performance as well as the improvements achieved when a hSDN approach is deployed. By detecting control plane unreliability conditions, this scheme decided when to switch from centralized SDN operation to a distributed one. This way of working resulted in a substantial aggregated throughput improvement of the flows traversing the network (e.g., around 28% for CPLR = 20% and much more for higher CPLR) at the cost of a slight increase in the average latency in the evaluated scenarios. However, the latency gap between the optimal operation (centralized in good network conditions and distributed in bad network conditions) decreased as the network conditions worsened (i.e., a higher CPLR and more links experiencing problems). Based on these results, we conclude that such hSDN approaches are promising in SDN-based deployments over unreliable transmission media. Our future work plans to explore to what extent and under what conditions Artificial Intelligence (AI) and Machine Learning (ML)-based techniques can improve hSDN network operation without adding much complexity to the nodes. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Schemes** & **A** & **B** & **C** & **Score** \\ \hline HyperFlow [15] & + & - & - & 1 \\ ResilientFlow [16] & - & + & - & 1 \\ Ravana [17] & + & - & - & 1 \\ wmSDN [18] & + & - & - & 1 \\ Proposed & + & + & + & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative analysis of hSDN schemes. **Author Contributions:** [PERSON] was responsible for conceptualization, methodology, formal analysis, investigation, designing experiment and writing original draft. [PERSON] was responsible for supervision, review and editing the article. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was partially funded by Spanish MINECO grant TEC2017-88373-R (5G-REFINE) and also the research leading to these results was supported by Generalitat de Catalunya grant FI-DGR 2015 and 2017 SGR 1195. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data underlying this article will be shared on reasonable request from the corresponding author. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. ## References

* [1] [PERSON] Small cell backhaul requirements. In _NGMN White Paper_, NGMN Alliance: Frankfurt, Germany 2012; pp. 1-40. * [2] Small Cell Millimeter Wave Mesh Backhaul. In _InterDigital White Paper_, InterDigital, Inc.: Wilmington, DE, USA, 2013. * [3] Small Cells Drive Microwave Backhaul Boom. Small Cells 2019-2026. In _RAN Research Market Report_; Rethink Technology Research: Bristol, UK, 2020. * [4] ETSI NFV. Report on SDN Usage in NFV Architectural Framework. In _ETSI GS NFV-EVE 005 v1.1.1_; Network Functions Virtualisation (NFV) ETSI Industry Specification Group (ISG): Sophia Antipolis, France, 2015. * [5] Open Networking Foundation. OpenFlow v1.3.