Access to the IPv4 net for IPv6-only systems, was: Re: WG Action: Conclusion of IP Version 6 (ipv6)

Mon Oct 1 18:15:58 UTC 2007

Thus spake "Iljitsch van Beijnum" <iljitsch at muada.com>
> For the purpose of this particular discussion, NAT in IPv4 is  basically a 
> given: coming up with an IPv4-IPv6 transition
> mechanism that only works with if no IPv4 NAT is present both
> defeats the purpose (if we had that kind of address space we
> wouldn't have a problem in the first place) and it's completely
> unrealistic.
>
> The issue is that introducing NAT in IPv6, even if it's only in the 
> context of translating IPv6 to IPv4, for a number of protocols,  requires 
> ALGs in the middle and/or application awareness. These  things don't exist 
> in IPv6, but they do exist in IPv4. So it's a  better engineering choice 
> to have IPv4 NAT than IPv6 NAT.

Of course ALGs will exist in IPv6: they'll be needed for stateful firewalls, 
which aren't going away in even the most optimistic ideas of what an 
IPv6-only network will look like.

> I don't see the problem with proxying, except that it only works for  TCP. 
> Yes, you need a box in the middle, but that's true of any  solution where 
> you have an IPv6-only host talk to an IPv4-only
> host.  If both sides use a dual stack proxy, it's even possible to
> use address-based referrals. E.g., the IPv4 host asks the proxy
> to set up a session towards 2001:db8:31::1 and voila, the IPv4
> host can talk to the IPv6 internet. Not possible with a NAT-PT
> like solution.

Only one side needs to proxy/translate; if both sides have a device to do 
it, one of them will not be used.  Better, if both sides support the same 
version (either v4 or v6), that would be used without any proxying or 
translating at all.

> Tunneling IPv4 over IPv6 is a lot cleaner than translating between  the 
> two. It preserves IPv4 end-to-end.  :-)

And when we run out of v4 addresses in a few years, what do you propose we 
do?  It makes little sense to tunnel v4 over v6 until v6 packets become the 
majority on the backbones -- and the only way that'll happen is if everyone 
dual-stacks or is v6-only.  If everyone has v6 connectivity, then why do we 
need to route v4 anymore, even over tunnels?

S

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