[111297] in North American Network Operators' Group
Re: can I ask mtu question
daemon@ATHENA.MIT.EDU (Sam Stickland)
Tue Feb 3 11:42:29 2009
Date: Tue, 03 Feb 2009 16:42:19 +0000
From: Sam Stickland <sam_mailinglists@spacething.org>
To: nanog@nanog.org
In-Reply-To: <20090203160942.GP56621@burnout.tpb.net>
Errors-To: nanog-bounces@nanog.org
Niels Bakker wrote:
> * 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).
Sorry, I should had clarified. I meant mismatched host MTUs within a
jumbo MTU supporting L2 domain.
Sam
