[29174] in North American Network Operators' Group
Re: PMTU-D: remember, your load balancer is broken
daemon@ATHENA.MIT.EDU (Brett Frankenberger)
Wed Jun 14 08:24:40 2000
Message-ID: <06c901bfd5fb$34239d00$a34784a7@isdn.omhq.uprr.com>
From: "Brett Frankenberger" <rbf@rbfnet.com>
To: <Valdis.Kletnieks@vt.edu>
Cc: <nanog@merit.edu>
Date: Wed, 14 Jun 2000 07:21:54 -0500
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----- Original Message -----
From: <Valdis.Kletnieks@vt.edu>
>
> 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
