# SDN-OpenFlow Topology Discovery: An Overview of Performance Issues
## 3 Recent SDN Topology Discovery Performance Studies
### Control Channel Improvement Algorithms  
Furthermore, in Section 2, we mentioned the importance and how the OpenFlow protocol control channel works. Where it is the hot line of communication between the controller and the plane layer devices. There are a number of constraints for both out-of-band and in-band controls. On the one hand, out-of-band control is costly and inconvenient. This is due to the fact that each data plane device requires additional physical interfaces and cabling to link to the controller. Furthermore, data plane devices may be physically positioned far from the controller, necessitating a significant additional cost to construct a network protocol for out-of-band control. In this subsection, we will discuss the proposed solutions that are related to control channel failure detection and recovery, where sustaining communication between the SDN-Controller and SDN-Switches becomes the main difficulty during network disruptions. Moreover, a comparison of these proposals is provided in Table 6 as well. As the control channel is used to establish in-band and out-band connection between the SDN-Controller layer and data layer in OpenFlow-SDN, the control channel failure threatens the network availability. According to [98; 99] a lossy control channel dramatically reduces data layer throughput and response time. Therefore, several recent works have been proposed to help sustain OpenFlow availability. The authors in [100] proposed control channel recovery for in-band and out-band control links. To enable local recovery from failure, the author integrated logical ring topology with source-routed forwarding. The SDN-Switch-to-SDN-Controller communication was made more robust using a ring-based local recovery technique. A source forwarding method was employed to ensure the SDN-Controller-to-SDN-Switch communication was robust, as a robust SDN-Switch-to-SDN-Controller communication channel allows the SDN-Controller to be updated of the complete topology. However, while keeping the functionality of data plane devices, this recovery does not necessitate SDN-Controller involvement. Moreover, [98] protected the in-band control channel from failure by finding a set of ideal pathways. To recover from SDN-Switch failure in an SDN, this research used a bypass and non-intervention backup recovery methodology. When an SDN-Switch fails, the goal is to deal with it as quickly as possible, especially if it is part of an SDN-Switch group. If a group of dependent SDN-Switches is affected by the failure, all control traffic from the impacted SDN-Switches is re-routed to a different recovery path. Using a full cartellized control channel, the authors in [99] suggested a centralized trust management system for selecting the most trustworthy data transfer pathways (in-band control) in the SDN. The authors viewed the network as a multi-agent system, with SDN-Switches and routers serving as the agents. The authors thought that their centralized unit had complete control over the route choices made by the agents. Fully centralized systems, in most circumstances, necessitate highly complex resources for the central unit, while also introducing a single point of failure. The central unit must be completely operational, tamper-resistant, and physically secure. It is also necessary to keep a close eye on all of the system's agents. The cost of applying such systems rises dramatically when all of the above-described qualities are combined. In contrast, the authors in [101] proposed a new method called hybrid-SDN to operate the network regardless of the level of control vulnerability by  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  
shifting network control from central to distributed. This approach depends on the Control Packet Loss Ratio (CPLR). The OpenFlow-SDN is converted to a hybrid-SDN (distributed control channel to SDN-Switches) approach when the CPLR value reaches an undesirable level.