Subject: Re: Path MTU for draft-ietf-v6ops-mech-v2-00.txt

From: Pekka Savola <[email-masked]>
Date: Fri, 2 May 2003 17:22:00 +0300 (EEST)

On Wed, 30 Apr 2003 [email-masked] wrote:
> >I fail to see your point; how is 1280 "simpler" than 1380? It's
> >just a static MTU assignment in either case. You will need to
> >define "simplicity" for your arguement to even make sense.
> 
> 	1280 is the only link MTU requirement value appears in RFC2460, and it
> 	is the minimum required value for any IPv6 links (which does not
> 	require wacky fragmentation header rule at the end of 2460 section 5).
> 	i dunno what word other than "simple" would capture this.

Correct me if wrong, but it appears the fragmentation header rule must be 
included also for the case of MTU 1280 when the IPv4 IP-IP tunnels have DF 
bit set.  Which appears to be the typical case, e.g. one major router 
vendor doesn't even support anything else.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

---

From: [email-masked]
Date: Sat, 03 May 2003 10:15:56 +0900

>vendor doesn't even support anything else.

	i don't understand your scenario.  even if there's IPv4-over-IPv4
	tunnel with outer DF bit set, ICMPv4 too big won't get propagated
	to IPv6-over-IPv4 endpoint, nor IPv6 source.

itojun

---

From: Pekka Savola <[email-masked]>
Date: Sat, 3 May 2003 13:49:28 +0300 (EEST)

> 	to IPv6-over-IPv4 endpoint, nor IPv6 source.

What I'm trying to say is that there are implementations of IP-IP 
tunneling (subsequently, IPv6-over-IPv4), which always set DF bit in the 
IPv4 header.

In case that IPv4 path MTU is below 1300 bytes, the wacky fragmentation
header rule needs to stay anyway -- unless these implementations are
deemed non-compliant.

So, in practice, PMTUD may be necessary (to lower the MTU) in these cases
even if IPv6 nodes only send packets of 1280 bytes or smaller.

---

From: [email-masked]
Date: Sun, 04 May 2003 02:44:53 +0900

>tunneling (subsequently, IPv6-over-IPv4), which always set DF bit in the 

	RFC2893 is rather clear about DF bit handling - it must not be set
	for tunnel with non-dynamic MTU.  so i would consider the above example
	as "buggy".

itojun