Sierra Wireless - Heart of the Wireless Machine (sierrawireless.com)

Using Ping to Troubleshoot VPN Issues

This document assumes that you have no other connectivity issues while using Sierra Wireless products.

This document applies to all Sierra Wireless AirCard devices.

Symptoms

When trying to connect to your VPN server the connection may:

1) Be unable to authenticate
2) Be able to authenticate but not pull internal web pages nor access internal mail servers
3) Be able to authenticate and pull internal web pages but not access internal mail servers

Resolution

If the issue is MTU (Maximum Transmission Unit) related, you can determine this by using the ping utility that is available with all versions of MS Windows (excluding CE versions). You will need to send a packet to an IP (Internet Protocol) address, using the ping command and specifying the send buffer size. This will enable you to determine if your AirCard is sending packets that are too big for the accepting host.

In Windows open a MS-DOS Prompt or a Command Prompt and type the following command:

Ping -l -f

For additional help with ping, at a command prompt, type:

Ping /?

As an example, you may type:

C:\>ping 216.115.102.75 -l 1000 -f

If the packet that you are sending is not too big you should see the following:

C:\>ping 216.115.102.75 -l 1000 -f

Pinging 216.115.102.75 with 1000 bytes of data:

Reply from 216.115.102.75: bytes=1000 time=68ms TTL=51
Reply from 216.115.102.75: bytes=1000 time=69ms TTL=51
Reply from 216.115.102.75: bytes=1000 time=62ms TTL=51
Reply from 216.115.102.75: bytes=1000 time=148ms TTL=51

Ping statistics for 216.115.102.75:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 62ms, Maximum = 148ms, Average = 86ms

C:\>

If the packet you are sending is too big then you may see the following:

C:\>ping 216.115.102.75 -l 2000 -f

Pinging 216.115.102.75 with 2000 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 216.115.102.75:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\>

If you see the above message (Packet needs to be fragmented but DF set) then you will have to acquire a third party MTU modification package to change the size to something that works. Each time you change the MTU size, you'd need reboot your machine.

Additional readings:

1) How to Change the Default Maximum Transmission Unit (MTU) Size:

    http://support.microsoft.com/default.aspx?scid=kb;en-us;826159

    http://www.winguides.com/registry/display.php/280/

    http://www.dslreports.com/faq/tweaks/4.%20MTU

2) Third party software to change MTU:

http://www.paraben.com/html/ijolt.html#

Disclaimer: Modifying the registry can cause serious problems that may require you to reinstall your operating system. We cannot guarantee that problems resulting from modifications to the registry can be solved. Use the information provided at your own risk.