Knowledgebase: WinGate VPN
MTU Issues
Posted by Adrien de Croy (Import) on 28 April 2004 11:06 AM
An issue can sometimes arise (often related to WinGate VPN) where although computers can communicate with each other (e.g. with ping) or make connections, they are unable to browse large directories or transfer files. This can be caused by MTU issues.

MTU (Maximum Transmission Unit) is the largest size of an Ethernet packet (data packet) that can be sent between any 2 computers. It can be different for every combination of 2 computers. 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.

You can test the size of the MTU and largest packet allowable by using the Ping command in conjunction with the "-l" (lower case L) switch followed by the packet size in bytes.

Usually the MTU is set to 1500 bytes by default so it is best to start with a figure around 1300 and work upwards. In most MTU test scenarios it is recommended that the –f (don’t fragment packet) switch is used as in most cases the connection will attempt to fragment packets that are larger then the MTU.

However WinGate VPN will attempt to fragment the packets anyway so it is not needed. The aim of this particular test will be is to see that even after fragmentation, what are the largest packets that WinGate VPN can handle before packets are lost.

To do this:
  1. Initiate a WinGate VPN connection.
  2. On the machine joining the VPN open up a command prompt.
  3. From the command line type ping (Ip address of the Host VPN machine) –l 1300.
  4. e.g . Ping –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 bytes 1300 time <10ms TTL=128
Reply from bytes 1300 time <10ms TTL=128
Reply from bytes 1300 time <10ms TTL=128

Ping statistics for :
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.

WinGate version 6 and WinGate VPN version 2 allow you to change the MTU from within GateKeeper:
  1. Open GateKeeper.
  2. Select the Network tab.
  3. In the Network Connections pane, double click the approriate network interface and select the Advanced tab from the network connection properties.
  4. Select the Overide MTU setting and adjust the MTU to the appropriate value.
  5. Click OK.

MTU Override

Network Interfaces will often allow you to set the MTU through the registry (refer to MS 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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