PMTU and Broken Servers

Leo Bicknell bicknell at
Thu May 8 15:18:27 UTC 2003

In a message written on Thu, May 08, 2003 at 11:12:19AM -0400, Jeff McAdams wrote:
> An only slightly better hack would be to have the tunnel and/or firewall
> twiddle the MSS on outgoing TCP connections to compensate for the lower
> tunnel MTU.  Still pretty gross, but won't have as much of an effect on
> the TCP performance.

I did leave out some details.  We tried Cisco code that mucks with MSS,
and that part worked but the image had other issues for us. :(

Of course, I'm also highly annoyed that you can't raise the MTU on
a Cisco tunnel.  You can raise the MTU on T1's, so you can add the
tunnel overhead, but you can't then raise the MTU of the tunnel
itself.  In a couple cases we could make it so the tunnel had an
MTU of 1500 if we could change it.  *sigh*

       Leo Bicknell - bicknell at - CCIE 3440
        PGP keys at
Read TMBG List - tmbg-list-request at,
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