In message <20041202200043.GA1005@beverly.kleinbus.org>, Ignatios Souvatzis wri
tes:

>
>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).
>

IPv6 doesn't have fragmentation in the network; it relies on Path MTU 
and fragmentation by the sender.  But Path MTU is difficult for 
protocols that aren't connection-oriented.  I'm thus not surprised to 
hear that ICMP or UDP fragments intentionally at the 1280 limit.

		--Steve Bellovin, http://www.research.att.com/~smb