Sierra Wireless Aircard 875 : VPN : Telnet : Packet Loss : WORKAROUND/ANSWER/FIX

[hackhawk]

Hello,

This is my first post on the tech support forum. Please forgive me if I'm in the wrong place. Today I had a difficult problem in which a Google search returned only one potential solution over on that pay-for-the-answer site, so I thought I would post my results somewhere where the answer could be found for free. Keywords added for better finding I hope ;)

I was trying to connect a remote user to our corporate VPN so he could telnet into our primary (ERP) application. From the Laptop, I was using the Cingular Wireless Service (the new AT&T) with a Sierra Wireless AirCard 875 (Licensed by QUALCOMM : 3G HSDPA).

The problem that occurred happened regardless of the VPN technology we used. We tried to VPN directly into a Windows Server on our old T1, and also into our SonicWall Pro 1260 Enhanced using the Global VPN Client on a different T1. Either way, the same problem was encountered.

Basically, every time a telnet session was opened and login initiated the session would freeze up and the connection to the server would be lost (lost connection). When using the same laptop and a dial-up connection the problem was NOT encountered. We believe this pretty much narrowed our problem down to the aircard or wireless service as the culprit.

Additionally, we initiated a sniff (wireshark) of the communication stream behind the firewall. We noted that when the server transmitted a packet where the size was above a certain threshold (say about 1397 bytes), the client/PC would transmit an ACK packet that it had received a zero byte packet. The server would then attempt to retransmit the 1397 sized packet 3 times. An acknowledgment of the retransmission was never received by the server. The client basically disappeared after acknowledging receipt of a zero byte packet.

Based on past experience I decided to play around with the MTU (Maximum Transmission Unit) size of the AirCard on the PC. In the past, reducing the MTU has worked for me.

There are two packet settings on the AirCard that I changed. The default settings of them are IPPacketSize=1450 and MaxMRU=1500. Initially I reduced them to 1300 and 1350 respectively, but failed to get results.

After running PING tests I found that packets were being dropped around the 1370 packet size range. "dos> PING -l 1370 -n 5 #.#.#.#". Now I'm not an expert at TCP/IP, but if I recall correctly, VPN adds bytes to the packets of your transmission. I was thinking, what if it added more than 80 bytes? Perhaps the total packet size was exceeding 1450/1500? So I decided to increase the packet size as follows.

IPPacketSize=1550
MaxMRU=1600

BINGO! No more packet loss, no more lost connection. So my first recommendation to people having these lost connections over AirCards / VPN's is to mess with the packet size settings of the AirCard.

The settings can be found on Windows XP as follows.

. Open My Computer
. Right click on "My Network Places"
. Choose Properties
. Right click on the appropriate "Wireless Network Connection"
In my case, the name was "Sierra Wireless HSDPA Network Adapter"
. Choose Properties
. Click on "Configure" to the right of the adapter
. Click the "Advanced" tab

You should be able to click on the IPPacketSize & MaxMRU settings and change them. Note that you can also get to the same settings via the hardware manager. Right Click on My Computer, choose Manage, then choose Device Manager. You may need to restart your wireless connection, or perhaps even reboot your PC to enforce the changes. In my case the changes were automatic.

DISCLAIMER: Increasing packet sizes can force routers/switches on the Internet to fragment and reassemble your packets. When setting the size above 1500 you may run the risk of slowing down your connection in some cases due to fragmentation. Again, I'm not a TCP/IP expert. I know only enough to make me dangerous. ;)

Good Luck! and I hope this message is helpful to someone in need.

HackHawk aka Richard