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Re: [Openvpn-users] mssfix default

  • Subject: Re: [Openvpn-users] mssfix default
  • From: Peter Rabbitson <rabbit@xxxxxxxxx>
  • Date: Wed, 04 Apr 2007 16:28:28 +0200

Giampaolo Carmagnani wrote:
> (snip)
> want to kow the openvpn's overhead.  Then I found this page:
> http://mia.ece.uic.edu/~papers/volans/openvpn1.html

I don't know how you found this page, I have been searching all over for 
this kind of information. It sheds some light on the subject, but still 
it contradicts with what I observed in my original post. I use the 
encryption and compression as the tester, and for some packet sizes I 
should be getting an overhead of 45 bytes just like in his tables. 
However when I set mssfix to 1464, the resulting tunneled mss is 1382, 
which allows for at most 42 bytes of overhead. So somehow OpenVPN knows 
that it will never cross this boundary, as it is not possible to change 
the mss of a connection "in flight". Maybe it is guaranteed for large 
packets only, who knows. My C is not nearly up to the task to understand 
how it is actually done.

Either way, it was a pretty interesting reading. Thanks for the input.

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