# SDN-OpenFlow Topology Discovery: An Overview of Performance Issues
## 2 Topology Discovery in SDN Networks
### Performance of the Link Discovery (OFDP)
#### 2.6.2 OFDP Performance Metrics  
As we discussed earlier, three main entities participate with each other in the OFPD protocol to get work done: the SDN-Controller, SDN-Switches, and control channels between the SDN-Controller and SDN-Switches. The SDN-Controller will send, receive,and process messages related to the discovery process. Likewise, SDN-Switches will receive, send, process messages, and all of these messages will use the control channel as arguments. Ultimately, these processes are an overhead on all participating entities. The overhead incurred by OFDP consists of the OFDP-connection to the connector (control channel) from one side and processing overheads on the SDN-Controller and SDN-Switches on the other side. Until this moment, there are no performance metrics that are adopted by any of the standard organizations to measure the performance of the topology discovery service in SDN, but there is some research suggesting and using some of the commonly recognized performance metrics. Therefore, we will discuss the performance metrics used in these studies and suggest some others for their importance as follows:  
1. The number of packets sent and received by the SDN-Controller  
The authors in [39; 42; 45; 76] used the number of packets sent and received by the SDN-Controller as a performance metric. To discover the links between SDN-Switches, in each discovery round the SDN-Controller sends several LLDP Packet_Out (\(P_{OUT}\)) messages equals to the number of active SDN-Switch ports in the network and will receive a number of Packet. In (\(P_{IN}\)) messages equals to twice of the number of links as shown in Equations (1) and (2). \[P_{OUT}=\sum_{i=1}^{N}P_{i} \tag{1}\]  
\[P_{IN}=2L \tag{2}\]  
where \(N\) is the number of SDN-Switches and \(P_{i}\) is the number of ports of DS-Switch \(i\), \(L\) is the number of links between the SDN-Switches in the network. As illustrated from Figure 6, the SDN-Controller will send three LLDP Packet_Out messages equal to the number of active ports on SDN-Switch (s1) and it will receive one LLDP Packet_In message to discover the one-way link from s1 and s2. The SDN-Controller also needs to send another three LLDP Packet_Out messages to s2 to discover the one-way link from s2 to s1 and it also needs to receive an LLDP Packet_In message. In total, the SDN-Controller will send six LLDP Packet_Out messages (i.e., number of active ports on s1 and s2) and two LLDP Packet_In (i.e., double of the number of links) messages to discover the link between s1 and s2. Moreover, the authors in [39] performed three experiments with different topologies as shown in Table 2. The letters 'd' and 'f' donate the depth and fan-out parameters in tree topologies as well as the letter'm' refers to the number of SDN-Switches in linear topologies. The experimental results reveal that the number of LLDP Packet_Out messages is equal to the number of active ports regardless of the topology type as previously mentioned in Equation (1). There are two types of SDN network topologies; Linear and tree [77]. In a linear structure, each SDN-Switch is associated with a single host, and in a tree, the SDN-Switches are arranged like tree branches as well as terminal branches associated with hosts. It is the only structure without loops, and the tree topology has the highest throughput with OpenDaylight, which is approximately 13 gigabits per second (Gbps) [77]. However, the drawbacks of the linear structure are the occurrence of disturbances in the network if the network element fails or smashes, and the troubleshooting difficulty is high and time-consuming. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Topology Number** & **Topology Type** & **Topology Parameters** & **Number of SDN-Switches** & **Number of SDN-Switches** & **Number of Packet\_Out** \\ \hline Topology 1 & Tree & d = 4, f = 4 & 85 & 424 & 424 \\ Topology 2 & Tree & d = 7, f = 2 & 127 & 380 & 380 \\ Topology 3 & Linear & m = 100 & 100 & 298 & 298 \\ \hline \hline \end{tabular}
\end{table}