Linksys → Re: MTU, PPPoE, Servers and LinkSys Routers

Bill, I read this thread with great interest.

My question is this... You mentioned that in some applications such as FTP, the client actually acts as a server. Is VPN one of those scenarios? From what I've read, I think it may be.

The reason I ask is that I've noticed when I'm connected over VPN, especially when running Outlook, that I get disconnected fairly frequently but I never knew why. I've band-aided it by setting the automatic-redial-on-disconnect flag on the WinXP connectoid but that's kind of a hack and it gets Outlook all bent out of shape.

This looks like it might be a possible explanation, and if so, you'd have to set MTU on the VPN clients too.

said by billbliss:

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You mentioned that in some applications such as FTP, the client actually acts as a server. Is VPN one of those scenarios? From what I've read, I think it may be.

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I'm not up on all the VPNs but I believe you're right. But some VPNs set their own MTU separately or even have their own characteristic MTU. And it's usually much lower than 1492 as I recall. If it tries to be higher, problems will occur (lower is fine).

Aza has the best way to determine what your MTU is. It can't identify what's limiting it but sure is handy:

ping -f -l 1472 yahoo.com
(edit: -l is a lower-case L)

1472 is a number 28 less than MTU so 1472 will work on a 1500 MTU but 1473 will not. pass/fail for 1492 MTU will be 1464/1465. Connect VPN and try it!

Then there's my Telocity modem/gateway. I can measure 1492 all day yet a TCP-SYN gets reduced by the darn thing to 1362!!! Ping test says they're wasting MTU in this case (PPPoE underneath a fixed-IP kludge system it is!).
[text was edited by author 2001-11-14 22:25:16]