[76796] in North American Network Operators' Group
Re: Smallest Transit MTU
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Wed Dec 29 16:59:29 2004
In-Reply-To: <6.2.0.14.2.20041229125633.06020c50@mira-sjc5-b.cisco.com>
Cc: NANOG list <nanog@merit.edu>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Wed, 29 Dec 2004 22:58:48 +0100
To: Fred Baker <fred@cisco.com>
Errors-To: owner-nanog-outgoing@merit.edu
On 29-dec-04, at 22:25, Fred Baker wrote:
> That said, RFC 1042 ("Standard for the transmission of IP datagrams
> over IEEE 802 networks.") notes that
> Note that the MTU for the Ethernet allows a 1500 octet IP datagram,
> with the MTU for the 802.3 network allows only a 1492 octet IP
> datagram.
> For an RFC to require an MTU of 1500 octets without fragmentation
> would imply requiring it to not use IEEE 802.3 framing,
:-) You've succeeded in making me laugh out loud, Fred.
Thinking that IP over Ethernet has _anything_ to do with 802.3 is such
a rookie mistake... Does your employer even make gear that supports
this? If they do, I've never seen it.
(For the record, the IP over Ethernet that we all know and love is is
RFC 894, (don't let the april first publication date fool you) which
specifies IP over "Ethernet II", a pre-IEEE incarnation of Ethernet. On
a Cisco router this shows up as "Encapsulation ARPA". There is some
stuff on MTU issues in this RFC too, but it's completely outdated of
course.)
> To be honest, I think we should be carefully considering Mathis' newer
> approach to Path MTU, described in
> http://www.psc.edu/~mathis/MTU/pmtud/draft-mathis-pmtud-method-00.txt
> and a more recent but expired internet draft.
Why reinvent a broken wheel?
Implementations just shouldn't send ALL packets with the DF bit set.
Doing this for a few percent of all packets accomplishes PMTUD just as
well without the nasty side effects. Alternatively, the right way to do
PMTUD would be for the receiver to notify the sender of any
fragmentation and how big the largest fragments or unfragmented packets
were.
Regardless of this, it's probably a good idea to obsolete the original
meaning of the DF bit.
