NAT v6->v4 and v4->v6 (was Re: WG Action: Conclusion of IP Version 6 )

michael.dillon at michael.dillon at
Fri Sep 28 14:55:24 UTC 2007

> It is also becoming apparent that:
> - the "core internet" (ie the web and any infrastructure 
> server) will take a long time to move to v6 and/or dual stack.
> - new v6-only edges will have to communicate with it. So we 
> need v6->v4 translation in the core

Some companies have implemented MPLS in the core, therefore they can
easily add IPv6 services by configuring 6PE on a couple of PE routers in
each PoP. Beyond the PoP, in the customer's network, they can do pure
IPv6 if that is what they want.

> - legacy v4 PCs (think win95 up to win XP) using RFC1918 
> addresses behind a home gateway  will never be able to 
> upgrade to an IPv6-only environment. So if we provision the 
> home gateway with v6-only (because there will be a point 
> where we do not have any global v4 addresses left for it) 
> those legacy PCs are going to need a double translation, 
> v4->v6 in the home gateway and then v6 back to v4 in the 
> core.

Not if they use an application layer proxy in their gateway. It's not
too late to specify this as a standard function for an IPv6 Internet
gateway device. Also, the "v6 back to v4" conversion could be handled in
an information provider's data center (Google, CNN) not in the core.

> So, no, NAT v4->v6 or v6-v4 does not solve world hunger but 
> solve very real operational problems.

Agreed. Just about every possible transition technique will solve real
operational problems and we should not be purists about this. Whether
the IETF has a specification for it or not, people will build and deploy
NAT and ALGs among other things.

In addition, this transition comes at a time when we have the technology
that allows virtually anyone (high school kids) to build some kind of
network functionality on top of Linux or BSD. If that is useful, anyone
can freely implement this including the manufacturers of Internet
gateway devices who often use Linux or BSD as the foundation of their

Back in 1994 we started to see exponential growth of the Internet
because the barrier to entry suddenly became much lower. It was
financially feasible to buy a bunch of modems, terminal server, Bay or
Cisco routers and a bunch of Linux/BSD servers. The technology was
available cheap enough to encourage many people to take the business
risk. In the interim, technology has advanced somewhat and I expect to
see a flurry of devices as soon as IPv4 exhaustion reaches the general

--Michael Dillon

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