IPv6 day and tunnels

Mon Jun 4 05:41:03 UTC 2012

Joe Maimon wrote:

> So IPv6 fixes the fragmentation and MTU issues of IPv4 by how exactly?

Completely wrongly.

> 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.

For further details, see my presentation at APNIC32:

	http://meetings.apnic.net/32/program/apops
	How Path MTU Discovery Doesn't work
	Masataka Ohta

> In IPv4 I can make tunneling just work nearly all of the time. So I have
> to munge a tcp mss header, or clear a df-bit, or fragment the
> encapsulated packet when all else fails, but at least the tools are
> there. And on the host, /proc/sys/net

FYI, IETF is trying to inhibit clearing DF bit explicitly with

	draft-ietf-intarea-ipv4-id-update-05.txt
	>> IPv4 datagram transit devices MUST NOT clear the DF bit.

which is now under the last call.

						Masataka Ohta