Using A VPN With A Tethered Wireless Broadband Connection

Posted by jerry as technology

I have a Blackberry 8703 from Verizon Wireless, and use it extensively.  I opted to buy the plan to use the Blackberry as a tethered wireless modem for a PC, for an extra $10/month.

I’ve used the blackberry as a wireless connection on many occasions, and it generally works pretty well, though often seems pretty slow.  One thing I had never been able to get working was the VPN client to my employer.  I hadn’t spent a lot of time thinking about it, but one day I really needed to connect and had no other options.

Now, the VPN connection would establish, but I could not reach any systems on the company network.  I had spent a good amount of time as a network engineer in a former life, so I’m fairly adept at troubleshooting such things, despite my management lobotomy.

I opened up a DOS shell, and tried pinging the intranet web server  Sure enough, the pings were getting through.  But, I still could not get get a web browser to connect to the site.  After pondering it for a minute, I knew the answer.  I tried the ping again, but this time I set the packet size to 1500 bytes, the normal TCP maximum size.  Viola!  The pings did not get through.  I retried the ping, decreasing by 100 bytes each time, until I got to 1100 bytes.  At 1100, the pings worked again.

Next, I went out and found this document on resetting MTU size in Windows.  I followed the instructions in the section labelled “Change the MTU Settings for VPN Connections”.  I followed the directions to reset the VPN MTU to 1100 bytes.  A requisite reboot, and I was connected.

I have since observed that web browsing in general is much faster as well.  Originally, I had thought that the overhead of the VPN client was causing an overrun in the size of the packets, forcing them to be fragmented, which is generally not handled elegantly.

Hopefully someone is able to find this tidbit useful.