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Re: PMTU-D: remember, your load balancer is broken
From: Valdis.Kletnieks () vt edu
Date: Wed, 14 Jun 2000 00:05:30 -0400


On Tue, 13 Jun 2000 23:50:55 EDT, "Steven M. Bellovin" said:
There are two places where it's very important.  First, some server 
farms are on FDDI rings, so they have a higher MTU.  Second -- and this 

Yes, but I think I covered that in (a) - if you know there's a
bigger MTU, nail it down.  I dread to think of a load-balancer
in the middle of a FDDI ring in a server farm ;)

one is growing in importance -- tunnels, for IPsec, PPTP, etc. -- 
generally have smaller MTUs.  This very reply will travel over a tunnel 
with an MTU of, I believe, 1480.

Good point.  It's been a long day, I wasn't QUITE thinking
straight.  Another respondent commented that Windows98 apparently
nails an MTU of 576 on a dialup - Apparently I've not run into
any Windows98 people setting their clocks off the server I got
the numbers from.  Also, he said that ADSL uses just under 1500.
I don't have a Win98 or ADSL handy to check. ;)

In any case, it's good fodder for an operational debate. ;)

                                Valdis Kletnieks
                                Operating Systems Analyst
                                Virginia Tech



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