An issue can sometimes arise within WinGate VPN where, although machines and network shares are visible and users can connect, they are unable to browse large directories or transfer files. This can be caused by MTU issues.
The MTU (Maximum Transmission Unit) is the largest size of an Ethernet packet (data packet) that can be sent across a point to point connection, and if you are experiencing the types of issues outlined above then loss of these packets (due to their size) can often be the cause.
By using the Ping utility from the command line you can establish the largest MTU size that can be used before packets begin to be lost.
Using Ping, you can send packets of different sizes. WinGate VPN fragments packets (if allowed) when it transfers them across the VPN. Therefore you should be able to send large ping packets successfully across the VPN if everything is working properly. If not, then once you get to a certain size, they will stop working.
To send a packet of a certain size, use the -l switch on the ping command. e.g.
This will send a ping packet with a 1422 byte ICMP payload. It is important to note that the actual packet size of the ping packet is 28 bytes larger than this since the IP plus ICMP headers use 26 bytes. Therefore the example above will send a packet of 1450 bytes (not including the Ethernet header). The Ethernet header is not counted because this is stripped off and not transmitted over the VPN.
By working out the ping size that works vs the size that doesn't you can calculate what the effective MTU really is. For dialup connections and some network interfaces, it is then possible to modify the MTU so that your client machines will no longer send packets that are too big.
From the command line type ping IP address of the Host VPN machine -l 1300
e.g . ping 192.168.4.55 -l 1300
This will send a ping packet to the Host VPN machine with a packet size of 1300.
With a successful reply you should see a response like:
Reply from 192.168.4.55 bytes 1300 time <10ms TTL=128>
Reply from 192.168.4.55 bytes 1300 time <10ms TTL=128>
Reply from 192.168.4.55 bytes 1300 time <10ms TTL=128>
Ping statistics for 192.168.4.55 :
Packets: Sent = 4 Received = 4, Lost = 0 <0%loss>
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
The purpose of discovering the MTU size is to determine the largest size for a data packet that WinGate VPN can send without it being lost. Once this packet size has been established, then the MTU setting on the network interface will need to be changed.
The WinGate VPN allows you to change the MTU from within the WinGate Management console:
Navigate to Control Panel > Network Connections to open the Network Connections panel.
In the Network Connections panel, double click the appropriate network interface and select the Advanced tab from the network connection properties.
Network interfaces will often allow you to set the MTU through the registry (refer to Microsoft's knowledgebase for instructions for your particular operating system) or alternatively there is third party software available that will provide a GUI that will let you change the MTU in an easy and safe manner.
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