## 7 Performance Evaluation
### Performance Evaluation of SeFLOOD  
Performance of SeFLOOD has been evaluated in terms of overhead that security solutions add to FLOOD. Our evaluation encompasses both the network discovery phase and the reconfiguration phase. Such an evaluation has been carried out through a simulative approach using the SimPy simulator [37]. In the simulations we have used realistic results obtained during the field experimental campaign of the UAN project [5]. In particular, we have used an average Round Trip Time equal to about 19 s when the maximum underwater acoustic transmission rate is 500 bits/s. We have considered a network composed of seven fixed nodes, the gateway GW, and one mobile node. For each node we have assumed a maximum transmission power of 189 dB re \(\mu\)Pa, a transmission range  
Figure 11: Average round-trip time when security is enabled/disabled. Figure 10: Average Delivery Ratio when security is enabled/disabled. of 2 km, and a working depth of 200 m. Furthermore, we have considered two deployment scenarios aimed at evaluating performance in two different network topologies: (i) a _sparse scenario_, where each node has two neighbours; and (ii) a _dense scenario_, where each node has five neighbours (see Figure 12). The HELLO and CLUSTER_KEY messages in the Cluster Key Distribution Protocol (CKDP) have been implemented as follows. The HELLO message is composed of the following fields: (i) _msg-type_ (8 bits), that specifies the message type; (ii) _node-id_ (16 bits), that specifies the node identifier; (iii) _nonce_ (16 bit); and, the (iv) _additional information_ (24 bit), that specifies additional information (e.g., sea conditions). The CLUSTER_KEY message is composed of the following fields: (i) _msg-type_ (8 bits); (ii) _node-id_ (16 bits); (iii) _nonce_ (16 bit); the (iv) _key_ (128 bit), that specifies the cluster key; and, the (v) _digest_ (24 bit), that specifies the digest. It follows that the size in bits of an HELLO message is 64 (8 bytes) whereas the size of a CLUSTER_KEY message is 192 (24 bytes). We have evaluated the performance of SeFLOOD with respect to two metrics: (a) the _number of messages (NoM)_, and (b) the _number of bits_ (_NoB_). The _NoM_ and _NoB_ metrics measure how much communication overhead SeFLOOD adds to FLOOD in terms of total number of sent messages and total number of sent bits, respectively. Each metric \(M\) specifies the percentage increase of SeFLOOD with respect to FLOOD:  
\[M=\frac{M_{S}-M_{F}}{M_{F}}\times 100 \tag{1}\]  
where \(M_{S}\) represents the value of the metric for the SeFLOOD protocol, while \(M_{F}\) represents the same metric for the FLOOD protocol. Each metric has been computed as an average over fifteen runs, each lasting 5,000 s. Table 2 resumes the simulation parameters. Figure 13 shows the _NoM_ in both the dense and sparse scenario. As to _NoM_, we only consider the discovery messages and neglect the add and delete messages because SeFLOOD does not add messages to the Reconfiguration Phase and thus SeFLOOD and FLOOD have the same number of messages in this phase. Figure 14 shows _NoB_, which, instead, considers all types of messages. As to the Discovery Phase, the additional messages introduced by SeFLOOD derive from the Cluster Key Distribution Protocol (CKDP). \begin{table}
\begin{tabular}{|l|c|} \hline
**Parameter** & **Value** \\ \hline Duration & 5,000 s. \\ Number of runs & 15 \\ Average transmission range & 2 km \\ Maximum transmission power & 189 dB re \(\mu\)Pa \\ Depth & 200 m \\ Number of nodes & 9 \\ DIFS (silent period before transmission is considered) & 0.25 s \\ SIFS (Wait time between DATA and ACK) & 0.01 s \\ Max number of retransmissions & 7 \\ \hline  & **Sparse Scenario** & **Dense Scenario** \\ \hline Average number of Neighbors & 2 & 5 \\ \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters. In particular, FLOOD sends 17 messages in the dense scenario and 13 messages in the sparse scenario. SeFLOOD, instead, sends 73 messages in the dense scenario and 40 messages in the sparse scenario.