North American Network Operators Group
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Re: PMTU-D: remember, your load balancer is broken

  • From: Brett Frankenberger
  • Date: Wed Jun 14 08:25:26 2000 
----- Original Message -----
From: <[email protected]>
>
> 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. ;)

Small MTUs at the ends don't matter.  If I dial up with a Windows 98
machine and negotiate an MTU of 576 bytes, the MSS will be set
accordingly in the TCP SYN and SYN ACK frames that I send, and the far
end will start with 576 byte frames.  No PMTU Discovery required.

Same thing with ADSL or end-user VPN stuff.

PMTU Discovery is important when you have larger MTUs on the ends and
small MTUs in the middle.  For example, a tunnel (VPN or otherwise)
between two routers or VPN servers, for a WAN link with a small MTU, or
...

It's a real problem, and the Load Balancer manufacturers need to handle
the ICMPs properly.  But it's not so bad that everyone with a 576 byte
Windwos 98 PPP dial-up would be unable to reach Load Balanced sites.
(Arguably, it would be better if it were a problem for such users,
because that would guarantee that the problem would get fixed quickly
...)

     -- Brett