[111296] in North American Network Operators' Group
Re: can I ask mtu question
daemon@ATHENA.MIT.EDU (Niels Bakker)
Tue Feb 3 11:09:49 2009
Date: Tue, 3 Feb 2009 17:09:42 +0100
From: Niels Bakker <niels=nanog@bakker.net>
To: nanog@nanog.org
Mail-Followup-To: nanog@nanog.org
In-Reply-To: <498832C6.2070506@spacething.org>
Errors-To: nanog-bounces@nanog.org
* sam_mailinglists@spacething.org (Sam Stickland) [Tue 03 Feb 2009, 13:04 CET]:
>For what it's worth, TCP will negiogate MSS and will work with
>mismatched MTU in a single LAN segment.
No
Machine 1 -- switch with 1500 byte MTU -- switch with smaller MTU --
switch with 1500 byte MTU -- machine 2
Same situation as when you have IP routers with smaller MTUs in the path
that also do not send ICMP Fragmentation Needed errors (or those are
dropped on the way to you)
If you configure one of those machines with an MTU equal to or smaller
than the smallest MTU in the path then yes TCP (assuming MSS option is
used) won't send packets that happen to be too big, but again, same
story as for routers vs on a LAN. The problem isn't that machine 1 and
2 in the above example disagree on MTU, the problem is that equipment in
the path disagrees on the MTU and cannot (in the case of switches) send
notifications of such, or those will not arrive (in the case of stupid
firewall admins in control of networks).
-- Niels.
