North American Network Operators Group

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Re: PMTU-D: remember, your load balancer is broken

  • From: Steven M. Bellovin
  • Date: Tue Jun 13 23:53:56 2000

In message <[email protected]>, [email protected]
.edu writes:

>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.
>Is anybody seeing any documentable wins by using PMTU-D?

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 
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.

		--Steve Bellovin