[109287] in North American Network Operators' Group
NAT66 and the subscriber prefix length
daemon@ATHENA.MIT.EDU (michael.dillon@bt.com)
Fri Nov 14 12:08:02 2008
Date: Fri, 14 Nov 2008 17:07:40 -0000
From: <michael.dillon@bt.com>
To: <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
Not long ago, ARIN changed the IPv6 policy so that
residential subscribers could be issued with a /56
instead of the normal /48 assignment. This was done
so that ISPs with large numbers of subscriber sites
would not exhaust their /32 (or larger) allocations
too soon. Since these ISPs are allowed to assign
a /56 to residential subscriber sites, their initial
IPv6 allocation will last a lot longer and they won't
have to apply for an additional allocation while=20
everyone is getting up to speed with an IPv6 Internet.
Now, however, the IETF is discussing a form of NAT
for IPv6 called NAT66. See this draft for details
<http://www.ietf.org/internet-drafts/draft-mrw-behave-nat66-01.txt>
Part of this new NAT is that they are checksum neutral.
They do this by modifying bits in the address that are
not needed. Specifically, they assume that the=20
end site has a /48 allocation, and that the next
16 bits up to the /64 boundary, are non-essential
information outside the end-site boundary. These
bits are then twiddled to preserve the IPv6 header
checksum. Of course, these are the same bits that=20
an ISP relies on for reducing the assignment size
to /56.
I see a potential conflict here. If we assume that NAT66
will be widely used by consumers, then it follows that
consumer end-sites will need a /48 assignment in order
for IPv6 to work. But some ISPs want to reduce the end
site assignment to /56 meaning that NAT66 won't work for
those consumers.
Of course, it's not all set in stone yet which is why I
am posting this to NANOG. If ISP's who intend to use
/56 allocations could join in the discussions, then perhaps
we could develop some form of NAT66 that works with /56
prefix lengths.
Personally, I would be happy to just see every site
consistently use a /48 assignment. Corporate campus or
one-room studio apartment; it's all the same to me.
--Michael Dillon
