[76807] in North American Network Operators' Group
Re: Smallest Transit MTU
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Wed Dec 29 19:19:03 2004
In-Reply-To: <OFFF8B4537.89708A6B-ON88256F79.0080B823-88256F79.0082BD88@us.ibm.com>
Cc: nanog@merit.edu
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Thu, 30 Dec 2004 01:17:02 +0100
To: Tony Rall <trall@almaden.ibm.com>
Errors-To: owner-nanog-outgoing@merit.edu
On 30-dec-04, at 0:48, Tony Rall wrote:
> Remember that the DF bit is in the IP header - it can be on in any
> protocol. I know that AIX and my old RH Linux (at least) defaults to
> PMTUD enabled for tcp and udp. You can even see it in dns lookups.
I'm interested to learn what a poor unsuspecting UDP application does
when it sends out packets that turn out to be too large? The UDP
protocol is in no position to limit the packet size. So if the
application doesn't do it either, the IP layer has to fragment the
packets. This is exactly what happens with IPv6, but since routers must
be prepared to fragment in IPv4 anyway, the whole exercise becomes
fairly pointless...
