# SDN-OpenFlow Topology Discovery: An Overview of Performance Issues
## 1 Introduction
Therefore, the topology discovery protocol consists of three main processes: SDN-Switch discovery, link discovery (between SDN-Switches), and host discovery [34]. The SDN-Controller uses link discovery to discover links between forwarding devices in the infrastructure layer. Therefore, the OFDP will be the mediator between SDN-Controller and network devices in the infrastructure layer to discover these links. OFDP uses the Link Layer  
Figure 1: Standard SDN architecture [7]. Discovery Protocol (LLDP) message format for that purpose and the SDN-Controller sends a large number of LLDP advertisements at relatively large, fixed intervals to each active SDN-Switch port in the network to discover links between SDN-Switches. The SDN-Controller uses Packet_Out OpenFlow message to send the LLDP advertisement to each active SDN-Switch port, and the SDN-Switch on the other side (data layer) will send the link information by encapsulating the LLDP packet in a Packet_In OpenFlow message. This process is cyclic every 10 s, which means that every 10 s the SDN-Controller will send several Packet_Out messages equal to the number of active SDN-Switch ports [34; 35; 37; 38; 39]. The results of the previous processes are a burden on the resources of the SDN-Controller and are inefficient in detecting network topology changes in a timely manner. This leads to many studies in the literature that OFDP has critical limitations in terms of performance which makes it inefficient especially for large SDN networks. Another issue related to topology discovery between SDN and non-SDN networks [40; 41]. Therefore, attention to improving the performance of ODFP is an important priority that must be discussed to improve the performance of SDN technology. Different studies have been working to improve OFDP protocol in [38; 42; 43; 44; 45; 46], but still, this domain needs more attention and research. Moreover, in the literature, many surveys discuss SDN in general [13; 47; 48], and others focus on the features that have been added to SDN [49; 50]. Another group discussed the adoption of SDN architecture [51; 52; 53]. Furthermore, most of the SDN OpenFlow surveys have discussed the security and scalability [3; 15; 29; 31; 51; 54; 55], and the authors in [56] had worked on the OpenFlow in term of performance but did not discuss the details of the topology discovery process in the SDN. Therefore, this survey focuses on the performance issues in SDN-OFDP networks. The main contributions of this paper can be summarized as follows:  
* Explain in-depth how OFDP works, the factors that affect its performance, and OFDP limitations. * A survey of the recent existing techniques found in the literature in response to enhance the performance of OFDP. The pros and cons of each technique also are highlighted. * OFDP's open challenges and future research solutions. The rest of the paper is organized as follows. Section 2 introduces a brief background about SDN architecture and OpenFlow southbound interface. Section 3 presents the topology discovery in SDN networks and its de facto OFDP, performance issues in OFDP, and the current proposals to enhance the performance of OFDP. Section 4 discusses the pros and cons of the proposals for OFDP and Section 5 concludes the paper.