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Re: [Openvpn-users] MTU, link-mtu and tun-mtu

  • Subject: Re: [Openvpn-users] MTU, link-mtu and tun-mtu
  • From: Phillip Vandry <vandry@xxxxxxxxx>
  • Date: Sun, 23 Oct 2005 16:26:07 -0400

On Sun, Oct 23, 2005 at 11:51:58AM +0200, Mathias Sundman wrote:
> I don't know the exact details of this so I'll let someone else like James 
> comment on this but my first guess is that there is a missunderstanding 
> somewhere about what sizes that is to be refered. If it is "data payload 
> size", IP packet size, ethernet frame size and so on. For example when 


Thank you for pointing out that possible source of confusion. To make
matters worse, not all ping programs report the same way: many report
the ICMP payload size (must add 8 for ICMP header and 20 for IP header
to get the IP packet size) but I think Cisco IOS ping reports the
total IP packet size.

So let me clarify: all sizes I made reference to in my original post
were IP packet size, no datalink header.

> details by head, I just think there is a logical explaination to what you 
> are seeing rather than a bug in the software.

I agree, that's why I'm hoping to understand OpenVPN a little better.

> The only way you can workaround this without going back to the old PtP 
> mode is setting up "groups" of users that can live with the same MTU 
> settings and run multiple instances of OpenVPN in server mode on your 

That's a good idea.

Thank you for your quick response.


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