 as an extension or dependent on OpenFlow were investigated in [125]. These southbound interface protocols include open virtual switch database management protocol (OVSDB), ForCES, protocol oblivious forwarding (POF), OpFlex, OpenState, and NetConf. All the SDN controllers are adopting the OpenFlow discovery protocol (OFDP) to facilitate the topology discovery by using link layer discovery protocol (LLDP) frames [126]. However, the OFDP has some limitations in terms of reliability and efficiency in heavy-loaded and large networks. The OFDP has been enhanced by OFDPv2, OFDPv2A, and OFDPv2B as discussed in [126] and research works cited therein. Several algorithms have been proposed in the literature for SDN routing in order to reduce latency and save energy as in [127-131]. In [127], a ML algorithm was M. Alfaqawi et al. Table 1.5 Research works to tackle SDN challenges Reference Challenge Proposed algorithm and performance [112] and Topology In [112], OFDPv2 algorithm has been proposed to reduce the [113] discovery number of control messages to be handled by the SDN controller and switches. In comparison with OFDP, it reduces significantly the induced CPU load on the SDN controller for the topology discovery by 40-45%. In [113], a hierarchical distributed topology discovery protocol was investigated for orchestrating multi-domain SDN. It showed a reduced provisioning delay while preserving confidential intra-domain information [114-116] Failover In [114], a greedy incorporation and pre-partitioning strategies duration among controllers to minimize the failover duration have been proposed. The interruption time for the pre-partitioning mechanism was found to be significantly shorter than the greedy algorithm especially with a high number of orphan switches. In [115], fast failover mechanisms of both the control and data channels in the SDN-enabled data centers were presented. In [116], a fast and load-aware controller failover was proposed to resolve the SDN single point failure problem [117-119] In [117], a contr