Re: [exim] SMTP data timeout

On Fri, Nov 03, 2006 at 09:17:47AM -0800, Tim Wilde wrote:
> Chris Perry wrote:
> > Thanks for elaborating. I suspect the MTU is not the issue at this
> > time. I did have problems with the MTU when I started with DLS, and got
> > that sorted, with assistance from the ISP. Today I was able to send a
> > large email to the affected exim server without issue. The problem
> > seems to centre on some relationship between the sending (UBS) and
> > receiving servers.
>
> It could still be an MTU issue at their end, or a Path MTU Discovery
> issue somewhere on the route between you and them. The symptoms you've
> described definitely sound like MTU issues. Can you disable Path MTU
> Discovery in your TCP stack? On FreeBSD you would do this by setting
> the net.inet.tcp.path_mtu_discovery sysctl variable to 0. I've found
> that this is a good way to find out if PMTUD is the problem or not - if
> the problem messages come through successfully with PMTUD disabled, you
> can further troubleshoot. It seems, though, that the problem may
> actually be at the sending end of things, so you may need to try to get
> them to disable PMTUD on their sending server and see if it works
> better, which may be difficult to arrange.

another obvious thing to try is pings with different sized
packets -- that won't give you a 100% reliable answer re
TCP, but it might tell you something useful. You could
also try a similar trick with tcptraceroute.

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which might well be the company's motto.'' (The Economist, on Nigeria)