## 3 Design of the Proposed Solution  
The proposed Route Path Selection Optimization (RPSO) scheme selects a path with optimized links quality and switches resources from source to destination in SDN. Quality links are selected considering multi-constraints (link latency, link delivery ratio), and a minimal set of critical switches are considered from the switches point of view. Figure 1 presents the architecture view of the proposed RPSO. The network topology maintained by the SDN-Controller is entirely dependent on OpenFlow Discovery Protocol (OFDP) [7]. Any falsification in network topology can directly affect the SDN controller services. Therefore, the SDN controller uses a Link-Layer Discovery Protocol (LLDP) message to discover the network forwarding element such as switches, links, and host at time \(t_{i}\). In this regard, the SDN controller generates a _Packet-Out event_ to send LLDP advertisements to each active switch. In return, the switch will send the link information by encapsulating the LLDP message through _the Packet-In event_. Similarly, the arrival of new flows or table-mis entry, switches generate _Packet-In event_ to consult the SDN controller for further action. In this study, the controller generates special crafted _Eco_Ping_ (ECP) packets through special Internet Protocol (IP) and Medium Access Control (MAC) address from the controller and back while measuring the amount of time it took to do so. This way, the ethernet frame was broadcasted with a payload containing the port number and a timestamp of the packet's creation time toward adjacent switches S\({}_{1}\) via _Packet-Out_. Conversely, entry-miss will occur for the ethernet type at S\({}_{2}\) and ECP will be sent back to the controller through _the Packet-In event_. This way, RPSO retrieves the packet and deduce how long it took to complete the journey by subtracting the received time from sending timestamp. The RPSO scheme consists of two main phases (i) an initial routing path set-up which includes topology discovery, multi-link constrained aware routing phase, and (ii) critical switch detection. The subsequent subsection describes the details of each phase.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Schemes** & **Aim** & **Algorithm/ Scheme** & **Link Latency/ Delay** & **Switch Update Operation** & **Limitation** \\ \hline [PERSON] [16] & Path selection aimed at accepting large video request & Video path selection algorithm & X & X & Discovery of previous and current link bandwidth \\ \hline [PERSON] et al. [10] & To prevent congestion or avoid link overload and achieve load balancing & Path selection mechanism (PSM) & ✓ & X & result in trade-off b/w topo discovery period and learning which result in path setup latency \\ \hline [PERSON] et al. [31, 32] & minimizing the total path switching time of all paths in the network & The path switching time model & X & ✓ & The solution was tested in small network and frequent network changes can affect the quality of links on a selected path \\ \hline [PERSON] et al. [33] & Investigate the impact of link utilization and latency on network performance & ILP Model & ✓ & X & ILP solution required frequent trigger when a network change \\ \hline
[28] & path selection high-quality transmission service & QoS-driven path selection scheme & ✓ & X & Bandwidth based path selection cannot guarantee optimal path setup latency \\ \hline \hline \end{tabular}
\end{table}
Table 1: \(Cont.\)