[137537] in North American Network Operators' Group
Re: NIST and SP800-119
daemon@ATHENA.MIT.EDU (Joe Abley)
Wed Feb 16 09:58:18 2011
From: Joe Abley <jabley@hopcount.ca>
In-Reply-To: <4D5B804B.7020408@mail-abuse.org>
Date: Wed, 16 Feb 2011 09:57:23 -0500
To: Douglas Otis <dotis@mail-abuse.org>
X-SA-Exim-Mail-From: jabley@hopcount.ca
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On 2011-02-16, at 02:44, Douglas Otis wrote:
> Routers indicate local MTUs, but minimum MTUs are not assured to have =
1280 octets when IPv4 translation is involved.
> See Section 5 in rfc2460.
I've heard that interpretation of 2460 before from Bill Manning, but I =
still don't see it myself. The text seems fairly clear that 1280 is the =
minimum MTU for any interface, regardless of the type of interface =
(tunnel, PPP, whatever). In particular,
Links that have a configurable MTU (for example, PPP links [RFC-
1661]) must be configured to have an MTU of at least 1280 octets; it
is recommended that they be configured with an MTU of 1500 octets or
greater, to accommodate possible encapsulations (i.e., tunneling)
without incurring IPv6-layer fragmentation.
That same section indicates that pMTUd is strongly recommended in IPv6 =
rather than mandatory, but in the context of embedded devices that can =
avoid implementing pMTUd by never sending a packet larger than the =
minimum MTU. Such devices would break if there was an interface (of any =
kind) in the path with a sub-1280 MTU.
Joe
