LVS/TUN MTU problem, missing hints in documentation

[Joseph Mack NA3T]

On Thu, 7 Dec 2006, jarol1 at seznam.cz wrote:

> Today I ran into problem of MTU in LVS/Tunneling.
>
> Small packets were forwarded to real servers without problems, but the bigger 
> ones weren't and TCP retransmissions occurred. I noticed the problem 
> dissapeared when I switched to LVS/DR so this gave me hint to where the 
> problem might be. MTU 1480 had to be set on the outgoing interface of 
> realservers with tunl0 having standard 1500. Directors have 1500 on all 
> interfaces. This way TCP syn ack contained correct MTU and client didn't send 
> big packets that were discarded on director anymore. IP header is 20 bytes 
> long so 1480 is the maximum value that works.
>
> It isn't mentioned in documentation that this problem will occur if tunneling 
> is used

http://www.austintek.com/LVS/LVS-HOWTO/HOWTO/LVS-HOWTO.LVS-Tun.html#MTU

This was a hard problem to figure out and solve. People 
worked on this for about 2yrs before finding a satisfactory 
solution. The problem is that the ipip code in Linux doesn't 
handle fragmentation (or handle it properly).

You did well to come up with a solution in one go.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!