Subject: Comments on draft-ietf-ngtrans-mech-v2-01.txt

From: "Fred L. Templin" <[email-masked]>
Date: Fri, 08 Nov 2002 10:50:18 -0800

In section 3.2, the following text appears (actually,
this is unchanged since mech-v2-00.txt):

    Encapsulating nodes that have a large number of tunnels might not be
    able to store the IPv4 Path MTU for all tunnels.  Such nodes can, at
    the expense of additional fragmentation in the network, avoid using
    the IPv4 Path MTU algorithm across the tunnel and instead use the MTU
    of the link layer (under IPv4) in the above algorithm instead of the
    IPv4 path MTU.

    In this case the Don't Fragment bit MUST NOT be set in the
    encapsulating IPv4 header.

When you say: "...and instead use the MTU of the link layer (under IPv4)
in the above algorithm...", it is not clear whether you mean the *full*
MTU of an arbitrary link layer, or the MTU of the link layer *up to
some limit* (e.g., 4400 bytes). If your meaning is the latter, then
it would help make things clearer if you re-iterate the 4400 byte
limit in this text also.

If instead your meaning is to use the full MTU of an arbitrary link
layer, then nodes implementing this specification run the risk of
creating "harmful fragmentation" in the network and/or reassembly
buffer overruns in receivers. So, I suggest this text be made clear.

I believe you should also state somewhere that section 3.2 provides
only a *limited* tunnel MTU configuration mechanism. A more generalized
mechanism is possible if explicit fedback from the decapsulator is
available. For example, see:

  http://www.ietf.org/internet-drafts/draft-templin-v6v4-ndisc-01.txt

Thanks,

Fred Templin
[email-masked]

---

From: Erik Nordmark <[email-masked]>
Date: Wed, 13 Nov 2002 13:02:13 +0100 (CET)

> it would help make things clearer if you re-iterate the 4400 byte

Thanks for catching this inconsistency.
The intent is that this be limited by the magic number as well - and
the magic number (currently) in the document for encapsulators that
don't do path MTU discovery  is 1280. So how about adding:
	In that case the IPv6 MTU for the tunnel SHOULD be limited
	to 1280 and MUST not exceed 4400.

>   http://www.ietf.org/internet-drafts/draft-templin-v6v4-ndisc-01.txt

Hmm - I don't know how to do this without creating a reference
against that draft. Perhaps you can suggest concrete text that we can
look at.

  Erik

---

From: Fred Templin <[email-masked]>
Date: Wed, 13 Nov 2002 06:33:37 -0800 (PST)

> against that draft. Perhaps you can suggest concrete text that we can

Best to wait for the -02 version of that draft. I'd be happy to talk
about what's right and what's wrong with the -01 version at IETF 55.

Fred Templin
[email-masked]

__________________________________________________
Do you Yahoo!?
U2 on LAUNCH - Exclusive greatest hits videos
http://launch.yahoo.com/u2