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Re: [Openvpn-users] path-mtu 1460 : message too long

  • Subject: Re: [Openvpn-users] path-mtu 1460 : message too long
  • From: richard <rvenne@xxxxxxxxxxxxxxxxx>
  • Date: Tue, 04 Jan 2005 11:07:14 +0100

yeah sure,I agree.
which means:
a routing table had been changed, (ospf, bgb who knows), and my packets are too long for one of them, which make those errors. so, how can I guesse the right mtu value in order to be sure, that my packets could pass through the isp's equipments? should I try about a lot of mtu values?each of my vpn clients could have a diffrent isp with diffrent wan link:

(x)dsl, satelite, isdn, wifi, radio,frame relay..........and they could be in diffrent countries.so, I figure that (mtu error) may happen...
so my question is:

why my mtu errors were only visible in openvpn console? they may be a copy of 4th(tcp/udp)layer error message. why don't the mtu-discovry mechanisme do its job, in order to adjust?does openvpn have its own mtu-discovry mechanisme inside the tunnels?
James Yonan wrote:
On Mon, 3 Jan 2005, richard wrote:

aah, thanks a lot, that works well

but why is it necessary? that worked well before until last week.

Here's a metaphor. Suppose you are driving a truck (lorry) across your country and it's 5 metres tall. Now a natural disaster occurs and the roads which you usually drive on are closed, forcing you to detour onto smaller roads which have lower bridges and tunnels. Maybe some of the tunnels give you less than 5 metres of safe clearance!

In this metaphor each internet router is like a bridge or tunnel which constrains the height of your vehicle, i.e. the packet size.

Using an mssfix value of 1200 is sort of like using a truck that's only 4 metres tall instead of 5. You can't carry as much payload, so it's less efficient, but at least you know that you will get through.


James Yonan wrote:

On Fri, 31 Dec 2004, richard wrote:

I get this openvpn(beta 15) error message when I put in "ls /etc/", or "less $A_BIG_PLAIN_TEXT_FILE". my session freeze and the only way to get it back is killing the ssh process (openvpn still running).

linux client------->fbsd server--------->linux client

Try "mssfix 1200".


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