## 5 Implemented Scenarios and Experimental Results
Although there is a slightly higher difference in the mean values, another cluster can be identified, as in Figure 9, for the middle set of tunnels illustrated in Figure 7. Also, the mean and standard deviations are mostly comparable in this case. Figure 8: Clustered RTT distribution trend for the first six tunnels (one refers just to a clean IP connection) of Figure 7.

Figure 7: The trend of the average RTT for the considered connections. The same considerations for Figures 8 and 9 can be made for Figure 10, where the last tunnels of Figure 7 (right side) have been clustered. We also show how the RTT is distributed over time with MTU = 1500B and how the distributions can also be clustered in this case: Figures 11-13. At this point, we can state that for a Clean (no tunneling), FOU, GENEVE, GENEVE IPsec, GREtap, GREtap IPsec, and IPsec (Cluster 1) the average RTT with MTU = 9000B is \(\mu_{RTT}\) = 1.4231 s, with a standard deviation of \(\sigma_{RTT}\) = 0.4821 s. For GRE IPsec, IP in IP, IP in IPPsec, MAC VLAN, VTI IPsec, VXLAN, VXLAN IPsec, and XFRMi (Cluster 2) the average RTT with MTU = 9000B is \(\mu_{RTT}\) = 2.2439 s, with \(\sigma_{RTT}\) = 0.6458 s. For Open Connect, Open VPN, Tinc, and WG (Cluster 3), the average RTT with MTU = 9000B is \(\mu_{RTT}\) = 4.4304 s, and \(\sigma_{RTT}\) = 0.9753 s.

Figure 10: Clustered RTT distribution trend for the central group of tunnels illustrated in ${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $0

${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $1 Clustered RTT distribution trend for the central group of tunnels illustrated in ${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $2

As regards the MTU = 1500B, we found that for Clean, FOU, GENEVE, GENEVE IPsec, GREIPsec, GREtap, GREtap IPsec, GUE, IPsec, MACVLAN, MACsec, VTI IPsec, VXLAN, and XFRMi (Cluster 1) \(\mu_{RTT}\) = 1.5247 s and \(\sigma_{RTT}\) = 0.43023 s, for IP in IP, IP in IPsec, Tinc, VXLAN IPsec, WireGuard and MACsec (Cluster 2), \(\mu_{RTT}\) = 2.12041 s and \(\sigma_{RTT}\) = 0.4868 s and for Nebula, OpenConnect and OpenVPN (Cluster 3), \(\mu_{RTT}\) = 3.22462 s and \(\sigma_{RTT}\) = 0.52786 s. Bold fonts are used to connect different clusters in the MTU function. From the data illustrated above, we can conclude that, independently from the chosen MTU, in terms of RTT, Clean (obvious), FOU, GENEVE, GENEVE IPsec, GREtap, and GREtap IPsec offer the best performance (the needed header length is low and their reaction time is the minimum for MTUs of both 1500B and 9000B). Out of Cluster 1, the RTT worsens in the needed header dimension function. ${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $3 Clustered RTT distribution trend for the central group of tunnels illustrated in ${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $4

${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $5 Clustered RTT distribution trend for the central group of tunnels illustrated in ${IFACE_MTU} ip a a 10.0.0.1 peer 10.0.0.2 dev fou0 ip r a $6

Let us examine how MTU sizes affect network performance in overlay channels and VPNs. Using larger MTU sizes enables the operating system to send fewer but larger packets while maintaining the same network speed. This significantly reduces the processing required by the operating system, provided that the workload allows for the sending of larger messages. However, larger MTU sizes will not have a significant impact if the workload primarily consists of small messages. Using the largest MTU size supported by the adapter and the network is advisable. For example, with ATM adapters, the default MTU size of 9180 bytes is much more efficient than using an MTU size of 1500, which is commonly used by LAN Emulation. In Gigabit and 10 Gigabit Ethernet networks, if all machines have Gigabit Ethernet adapters and no 10/100 adapters are present, it is best to use jumbo frame mode. For instance, connections from server to server within a computer lab can typically be optimized using jumbo frames.