mtu Size

Sometimes devices in the path from you to your destination may have restrictions in place that make full size packets fail to get through but work fine with smaller ones. This can be seen if an attempt is made to run a plain VNC or RDP session or a session via an SSH tunnel; the connection can be made and authenticated but when a large amount of data is transferred (initial screen state for example) packets can be lost and the connection will hang. To correct the issue the TCP MTU size can be reduced.

Discovery of the MTU size in Windows

netsh interface ipv4 show subinterfaces

The MTU is shown under the MTU column heading for each interface

Changing the MTU size in Windows

To set the MTU to 1200 on "Local Area Connection" (ethernet default MTU is 1500):

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1200

To set the MTU to 1480 on "Ethernet":

netsh interface ipv4 set subinterface "Ethernet" mtu=1480

Discovery of MTU size in Linux

Changing the MTU size in Linux

ip link set dev <DEVICE> mtu <NEWMTU>
ip link set dev tun0 mtu 1480

References

• Become The Solution Windows MTU Discovery and Change with Pictures
• Task-centred iproute2 user guide