Re: IPv6 day and tunnels

[Jeroen Massar <jeroen () unfix org>]

On 3 Jun 2012, at 22:41, Masataka Ohta <mohta () necom830 hpcl titech ac jp> wrote:

> Joe Maimon wrote:
>
> > So IPv6 fixes the fragmentation and MTU issues of IPv4 by how exactly?
>
> Completely wrongly.

Got a better solution? ;)

> > Or was the fix incorporating the breakage into the basic design?
>
> Yes.
>
> Because IPv6 requires ICMP packet too big generated against
> multicast, it is designed to cause ICMP implosions, which
> means ISPs must filter ICMP packet too big at least against
> multicast packets and, as distinguishing them from unicast
> ones is not very easy, often against unicast ones.

I do not see the problem that you are seeing, to adress the two issues in your slides:
 - for multicast just set your max packetsize to 1280, no need for pmtu and thus this "implosion"
    You think might happen. The sender controls the packetsize anyway and one does not want
    to frag packets for multicast thus 1280 solves all of it.

 - when doing IPv6 inside IPv6 the outer path has to be 1280+tunneloverhead, if it is not then
    you need to use a tunneling protocol that knows how to frag and reassemble as is acting as a
    medium with an mtu less than the minimum of 1280

Greets,
 Jeroen