Subject: Re: (ngtrans) more comments on draft-ietf-ngtrans-mech-v2-00.txt

From: Erik Nordmark <[email-masked]>
Date: Fri, 20 Sep 2002 15:28:34 +0200 (CEST)

>   | 3. State that when the dynamic PMTU is not implemented the sender MUST
> NOT
>   |    by default send IPv6 packets larger than "max MTU" into the tunnel.
>   |    We can debate this number.
> 
> I'm failing to understand the intent.

The first paragraph in Section 3.2 in the spec can be interpreted
as it being ok to threat the tunnel interface as having a MTU (from IPv6's
perspective) of 65535-20 bytes. Thus IPv4 fragmentation could be freely used
up to 65535 bytes of IPv4 packets.
Given than receivers might not want to dedicate that much reassembly buffer
it would make sense to pick a smaller number. That is the intent
of the above point.

Note that implementations which use the optional dynamic MTU stuff
would only use IPv4 fragmentation for IPv6 packets that are 1279 bytes
and smaller. 

   Erik

---

From: Robert Elz <[email-masked]>
Date: Fri, 20 Sep 2002 20:51:58 +0700

Date:        Fri, 20 Sep 2002 15:28:34 +0200 (CEST)
    From:        Erik Nordmark <[email-masked]>
    Message-ID:  <[email-masked]>

Did you mean to send this only to me, or did you get bitten by the
dumb Reply-To that the list is adding, the same way I constantly am?

  | The first paragraph in Section 3.2 in the spec can be interpreted
  | as it being ok to threat the tunnel interface as having a MTU (from IPv6's
  | perspective) of 65535-20 bytes. Thus IPv4 fragmentation could be freely used
  | up to 65535 bytes of IPv4 packets.
  | Given than receivers might not want to dedicate that much reassembly buffer
  | it would make sense to pick a smaller number. That is the intent
  | of the above point.

Yes, all that's fine, but I thought that was point 1.

  | Note that implementations which use the optional dynamic MTU stuff
  | would only use IPv4 fragmentation for IPv6 packets that are 1279 bytes
  | and smaller. 

Something more like that was what I was assuming point 3 was about, but
I just didn't know how to deduce it from the language.

What's this "max MTU", or is that just the "min MRU" in other language?

In any case, I'm not sure I would want to require PMTUD before exceeding
1280 as the MTU of the link.   That's certainly one option, but these days
I think a MTU of 1480 (or perhaps 1440 to allow a couple of IPv4 tunnels
to exist inside each other - perish the thought) would be just fine.  That
is, for practical purposes, the safe PMTU of today's IPv4 internet is 1500
(there are still some smaller links, but they're not worth worrying about).

So, I don't thing we need to require PMTUD (from the 6to4 box - ie: router)
before allowing the MTU to be set to some number around there.

Similarly, in the future, if the practical safe PMTU of the internet
grows, there's no need to have a spec that says not to use it...   as long
as the MTU isn't higher than the required MRU, all is safe.   It might
not be optimal, but safe is good enough, and if a larger MTU turns out to
be sub-optimal enough, it can just get reduced again.

kre