Subject: Re: ipv6 mtu
To: None <tech-net@NetBSD.org>
From: Ignatios Souvatzis <is@netbsd.org>
List: tech-net
Date: 12/02/2004 21:00:43
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On Thu, Dec 02, 2004 at 08:34:50PM +0100, Konstantin KABASSANOV wrote:
> Well, I've probably taken the wrong way to debug but:
>=20
> >ping6 -s 1300 my-ipv6-address
>=20
> gives me icmp packets fragmented to payload 1232 and additional fragments
> of 76 bytes...

I can reproduce this with a 1.6.2/arm32 pinging 1.6.2/i386.

However, when I produce UDP packets (using traceroute6 ) fragmentation
starts at "datasize" 1428 bytes, I think as expected.

Now, for the ICMPv6 handling:=20

what _I_ am seeing is that the _reply_ is fragmented, but not the request.
(Konstantin: is this what you are seeing, too?)

So I think the ICMPv6 reply machinery limits to the minimum IPv6 MTU of=20
1280 bytes, always, which might conform to RFC2463 on a quick reading.
What looks odd is that it fragments, instead of discarding the data
going over the limit (which I'd expect for ICMPv6 special handling).

Does anybody know?

Regards,
	-is

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