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Re: PMTU-D: remember, your load balancer is broken
From: Ryan O`Connell <nemesis () eh org>
Date: Wed, 14 Jun 2000 10:57:35 +0100 (BST)



On 14-Jun-2000 Valdis.Kletnieks () vt edu wrote:
b) If you're a webserver or something else providing service Out
There to random users, just nail the MTU at 1500, which will
work for any Ethernet/PPP/SLIP out there.  And if you're load
balancing to geographically disparate servers, then your users
are probably Out There, with an MTU almost guaranteed to be 1500.

I assert that the chances of PMTU-D helping are in direct ratio to the
number of end users who have connections with MTU>1500 - it's almost
a sure thing that you probably won't have users with an MTU on their
last-hop that's bigger than their campus backbone and/or Internet
connection's MTU.

www.bt.com drops (Or at least used to) all ICMP silently, and this can
cause problems - one of our ISPs (U-Net) runs a Frame Relay network
internally from some customers that had an MTU of 1496, (The default MTU
for FR on some equipment, including (earlier?) Cisco IOSes, apparently)

Symptom - web site unreachable. Complained to bt.com, go the usual
"everything is fine here" response. :-( Similar symptoms accessing
other sites, although it was intermittent. Apparently, the problem
is more often seen on NT servers (No surprise there, then) as they
set the DF bit on outbound packets.

Managed to persuade U-Net to change their Frame Relay network to
have an MTU of 1500, which was quite nice of them as it wasn't really
their system that was broken! Improved performance noticably however.

-- 
Ryan O'Connell - http://www.complicity.co.uk/ - <nemesis () eh org>

You are the Dancing Queen, young and sweet, only seventeen 
Dancing Queen, feel the beat from the tambourine 
You can dance, you can jive, having the time of your life 
See that girl, watch that scene, dig in the Dancing Queen 




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