North American Network Operators Group

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

  • From: Marc Slemko
  • Date: Fri Jun 16 00:58:00 2000

On Thu, 15 Jun 2000, Greg A. Woods wrote:

> So, how about it everyone?  Can we please all disable PMTU everywhere

I assume you Mean PMTU-D, not PMTU.

> and try just increasing our default MSS where necessary?  I.e. even if
> you're using a load balancer or not?  Pretty please?  The extra
> fragmentation is only going to be a problem for those people who live
> behind tunnels of one sort or another.  I certainly don't mind paying
> for a bit of extra fragmentation in order to use my low-cost
> high-bandwidth tunnel!


If PMTU-D is causing problems, then get whoever has a broken network to
fix it.  Is it always practical?  Of course not.  But education is the
key.  PMTU-D is not the problem here, and it is very shortsighted to
say "oh, we just know better and can manually tune things to work
well".  That is not a wise "solution".  If even 5% of people are in a
situation where broken networks cause PMTU-D to not work, then such
broken networks will be fixed, period.  If you want to work around it on
your systems, then lower your MTUs.  But the solution is not for everyone
to go disable PMTU-D because there are some broken networks; after all,
the people that would listen to disable it are the same people who would 
just fix their broken networks.  And in 99% of the cases, the broken 
network will be at their end or at the user's end, it will very seldom
be in some network in the middle providing transit.