# SDN-OpenFlow Topology Discovery: An Overview of Performance Issues
## 3 Recent SDN Topology Discovery Performance Studies
### Link Discovery Improvement Algorithms
#### 3.1.1 Periodic
However, the ODFP performance is improved by sending a single ETDP packet encapsulated with a Packet_Out message to the SDN-Switch root from the SDN-Controller. This will reduce Packet_Out messages per discovery frequency to just one packet. ETDP also provides the minimum latency path between any two SDN-Switches. In the second suggested application (ETDP-Hybrid), the discovery service is shared between the SDN-Controller and SDN-Switches. SDN-Switches in this application do not depend on the SDN-Controller to start the discovery process, it initiates the discovery process and installs a flow entry in its flow table to send the topology information to the SDN-Controller. In discussing the results, the authors evaluated their application using Mininet in different topologies and different experimental times to compute the mean and standard deviation. The Packet_out count, Packet_In count, and Flow_Mod count messages were used as performance metrics. For ETDP-SDN, the results showed that only one Packet_out message per SDN-Switch. The number of flow_Mod messages was expected to be twice the number of flow_mod messages in standard OFDP, but that was not the case because the locking mechanism (to stop flooding frames and prevent loops) is not fast enough to stop annoying ETDP packets before installing block flow rules. In ETDP-Hybrid, there were no Packet_Out and Flow_Mod messages sent by the SDN-Controller. Similarly, the authors presented the SDN Resource Discovery Protocol (SDN-RDP) in [82] as a solution to sharing network state management between multiple SDN-Controllers. Each SDN-Controller discovers a portion of the network topology to maintain distributed node management and enhance protocol accuracy. The presented approach is asynchronous, does not require full network information, and does not need a global startup step. According to the simulated results, the proposed method effectively reduces the overload of the SDN-Controller. Other topology discovery methods have used centralized techniques, such as the Path Compute Element (PCE). The authors in [83] suggested a centralized topology discovery algorithm called Generalized TOPolgy (GTOP) for PCE. GTOP works by enabling PCE to mechanically create a network topology without employing a global routing protocol such as the Open Short Path protocol (OSPF). The GTOP uses an out-band control channel to proactively collect topology costs from the SDN-Switches. Additionally, it uses the same control channel to update the topology changes in an interactive way. However, concerning the testbed system, the total time for the suggested protocol was 10 ms to update the topology changes. The drawback of this work was using an out-band control channel, which may not be feasible to deploy in large-scale states. In discovering SDN-Optical Network Topology (SONT), the authors in [41] adopted a sequence of signal checking to detect the links one by one. This technique fits into the pre-service category of layer 1 convergence discovery cases and provides a correct links mapping at SDN-Controller despite limiting scalability and time efficiency, especially in large networks. Therefore, the same authors [84] used a parallel mode rather than a sequential mode to overcome their limitations. Likewise, the authors in [85] proposed Hybrid Domain Discovery Protocol (HDDP) to enhance the mechanism of discovery topology between SDN and traditional networks. HDDP is managed by SDN-Controller and works as OFPDv2 through the use of a lightweight agent that implements HDDP and transmits topology information indirectly to the SDN-Controller. HDDP uses a network exploration model based on a controlled flooding mechanism (Packert_Out) to discover non-SDN devices. Subsequently, the authors in [40] developed the HDDP to cover the topology discovery process in different types of wireless networks. Furthermore, other proposed work transfers the topology discovery process from SDN-Controller to layer-2 (SDN-Switch) completely as described in [66].