> Small (say, under 50,000 customer) ISPs in my experience have a planning 
> horizon which is less than five years from now. Anything further out than 
> that is not "foreseeable" in the sense that I meant it. I have much less 
> first-hand experience with large, carrier-sized ISPs and what I have is a 
> decade old, so perhaps the small ISP experience is not universal, but I'd be 
> somewhat surprised giving the velocity of the target and what I perceive as 
> substantial inertia in carrier-sized ISPs whether there's much practical 
> difference.
> 
Ready or not, IPv6-only (or reasonably IPv6-only) residential customers are 
less than 2 years out, so, well within
your 5-year planning horizon, whether those ISPs see that or not. Denial is an 
impressive human phenomenon.

> So, what's a reasonable target for the next five years?
> 
In five years we should be just about ready to start deprecating IPv4, if not 
already beginning to do so.

> 1. Deployed dual-stack access which interact nicely with consumer CPEs and 
> electronics, the IPv4 side of the stack deployed through increased use of NAT 
> when ISPs run out of numbers.
> 
Icky, but, probably necessary for some fraction of users. Ideally, we try to 
avoid these multi-NAT areas being
done in such a way that the end user in question doesn't also have reasonably 
clean IPv6 connectivity.

> 2. IPv6-only access, CPE and hosts, with some kind of transition mechanism to 
> deliver v4-only content (from content providers and v4-only peers) to the 
> v6-only customers.
> 
This is, IMHO, preferable to option 1.

> Perhaps it's because I've never seen a NAT-PT replacement that was any 
> prettier than NAT-PT, but I don't see (2) being anything that a residential 
> customer would buy before 2016. Perhaps I'm wrong, but I don't hear a lot of 
> people shouting about their success.
> 
Personally, I think DS-Lite is the cleanest solution, but, it isn't without its 
issues. The reality is that post-runout
IPv4 is going to be ugly, regardless of whether it's NAT64 ugliness, LSN 
ugliness, or DS-LITE ugliness.

IPv4 is all about which flavor of bitter you prefer at this point. The 
sweetness is all on IPv6.

> Note, I'm not talking about the ISPs who have already invested time, capex 
> and opex in deploying dual-stack environments. I'm talking about what I see 
> as the majority of the problem space, namely ISPs who have not.
> 
The majority of the problem space IMHO is end-user-space at ISPs that have put 
at least some dual-stack
research effort in, but, haven't come to solutions for end-users.

However, we're less than 2 years away from seeing what happens in those 
environments without IPv4.

Owen


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