Whilst working for a client in Chippenham recently, I was using the local Starbucks to do some preparatory work. As I per normal, I logged into the T-Mobile hotpot there to get online, collect mail etc. Part of the work I needed to do required me to use the Windows PPTP VPN Client. It connected fine however I was not able to actually connect to anything. Some web pages worked, replication would start but not complete, remote desktop would connect but not login. All symptoms of TCP/IP MTU issues. (See Wikipedia "Maximum Transmission Unit - MTU")
MTU within windows can be manipulated for LAN Interfaces fairly easily - this quite often has to happen for ADSL Broadband connections etc - see Google.
However, manipulating this for PPTP VPN connections was slightly more tricky to work out however a big of searching found the details. The registry key requiring modification is :
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters\Protocols\0]
"TunnelMTU"=dword:000004b0
"ProtocolType"=dword:00000800
"PPPProtocolType"=dword:00000021
Full details at : http://support.microsoft.com/default.aspx?scid=kb;en-us;826159&Product=winxp
I found an MTU of 576 (decimal) was what was needed for PPTP at T-Mobile hotspots. I then also found a reference to this at "The Air Source" (although in relation to Linux configuration)
Attached is a .reg file that creates the above key and sets the MTU to 576.
Steve Seymour June 20th, 2007 11:00:00