Fragmentation / MTU Discovery

From: Douglas Cowan (news@RemovEThisBITdacowan.co.uk)
Date: 02/24/03


From: "Douglas Cowan" <news@RemovEThisBITdacowan.co.uk>
Date: Sun, 23 Feb 2003 23:34:29 -0000

I had been receiving occasional 1500 byte ICMP echo request packets to my
Linux-box router, and had been wondering what they were. It appears they are
MTU path discovery attempts.

The MTU for my DSL connection is 1458 (PPPoA), and this means that receiving
a 1500 byte echo-request with DF (don't fragment) set should result in my
router sending back a destination-unreachable fragmentation needed with the
MTU attached (code 3 / 4)?

Instead, it appears just to send an echo-reply back, which is indeed
fragmented -- so it's completely ignoring the DF request.

I've checked this a number of times by logging in to a remote machine and
running:

ping -s 1472 -M do <my ip>

Every time my system replies happily to the 1500 byte packet without any
sign of a destination-unreachable message to aid with MTU path discovery.

Surely some mistake?

Douglas