MTU settings changing because of VPN software - question

[Sorcier11]

Hello.

The company I work for is offering us access to their network via a VPN with the ability to use our own ISP. I have an ADSL connection using PPPOE. I have it tweaked with an MTU of 1492 and an RWIN of 255552. As soon as I install the VPN software on my win 98 OS, it sets up a service which automatically downgrades my MTU to 1392 (at least, that's what the analyser on the main page of this site states). I read somewhere that this somewhat normal for VPN softwares to do this but my question is this. Leaving the RWIN setting at 255552 and having the MTU set at 1392 for regular browsing, how big an impact will this create on my connection on downloads for example? I see that the MTU went down by 6% (100/1492) so would I expect my downloads of kilobits per seconds to go down by 6% as well? The RWIN stays the same so I'm not sure of the relationship between the MTU and RWIN and how they translate in kbps download values.
If it does indeed have an impact, I'm going to look into starting the 'service' that the VPN software installed through a batch file instead so that I can start and stop it at will.

What are your thoughts?

Thanks.

[Lobo]

I would set MTU back to 1492 and :

Try Tcp Recieve Window of 31944 or 63888, scaling unchecked if RWIN is below 65535:)

[Sorcier11]

The thing is.... my MTU within win 98 is set at 1492. For some reason, the VPN software will not allow any communications to go beyond 1392. When I have the VPN software running, and go to the TCP/IP Analyzer, it shows my MTU as 1392 yet in the win 98 registry, my MTU are set at 1492.

[Lobo]

Click on PPPoE problems in my signature, Bob will help you:)

[Sorcier11]

Thanks Lobo.

I wasn't really concerned (yet anyway) on getting the VPN software to run up to 1492 for the MTU. I was more curious on the impact to my downloads in the MTU being set at 1392 with the regular rwin.

Thanks again. I'll post a note on Bob's forum and see where it leads from there.

[Bob Carrick]

Do not set the MTU up to 1492 as it will break your VPN. I replied back on my forums. I'm actually happy to see a VPN application which did over ride your MTU so the connection will work. Does the MTU stay over rided even when your VPN software is not connected?

[Sorcier11]

Hi Bob.

Well, with the VPN software installed on my win 98, it set up a 'service' program which runs in the background and starts up as soon as I boot up windows. That program is not started through the start programs group and doesn't show up when I check on alt-ctrl-del. If I close the VPN dialer software to disconnect me from my employer's VPN and attempt to browse on my own, the MTU being recognized is still set at 1392 even though my registry settings state that they are 1492. Only when I remove the VPN software completely does the 1492 get recognized. I'm assuming that the 'service' program is what is controlling the MTU and I am going to try to 'batch' it so that it starts only when I need to connect to the VPN then I can close it when I'm done so that my regular MTU comes back.

What are your thoughts on that?

Thanks to everyone who replied so far.

[Bob Carrick]

Very interesting. You can probably stop it from running at boot, by going in to msconfig, then the stratup tab.

[Sorcier11]

Yes it is interesting. The only place I see the 'service' started is under msconfig/start up. I was thinking of creating a .bat file which would call the same program that's under msconfig/start up with the same options so that I can then close it at will. To stop it and restart it in msconfig/start up requires rebooting all of the time which is a pain.... I'm hoping my idea will work as I think it will...

[Bob Carrick]

Good luck.