With the world passing 10% IPv6 penetration over the weekend, we see the same old debates coming up again; people claiming IPv6 will never happen (despite several years now of exponential growth!), and that if they had only designed it differently, it would have been all over by now.
In particular, people like to point to a 20023 article by D. J. Bernstein, complete with rants about how Google would never set up useless IPv6 addresses (and then they did that in 2007I was involved). It's difficult to understand exactly what the article proposes since it's heavy on calling people idiots and light on actual implementation details (as opposed to when DJB's gotten involved in other fields; e.g. thanks to him we now have elliptical curve crypto that doesn't suck, even if the reference implementation was sort of a pain to build), but I will try to go through it nevertheless and show how I cannot find any way it would work well in practice.
One thing first, though: Sorry, guys, the ship has sailed. Whatever genius solution DJB may have thought up that I'm missing, and whatever IPv6's shortcomings (they're certainly there), IPv6 is what we have. By now, you can not expect anything else to arise and take over the momentum; we will either live with IPv6 or die with IPv4.
So, let's see what DJB says. As far as I can see, his primary call is for a version of IPv6 where the address space is an extension of the IPv4 space. For sake of discussion, let's call that IPv4+, although it would share a number of properties with IPv6. In particular, his proposal requires changing the OS and other software on every single end host out there, just as IPv6; he readily admits that and outlines how it's done in rough terms (change all structs, change all configuration files, change all databases, change all OS APIs, etc.). From what I can see, he also readily admits that IPv4 and IPv4+ hosts cannot talk to each other, or more clearly, we cannot start using the extended address space before almost everybody has IPv4+ capable software. (E.g., quote: Once these software upgrades have been done on practically every Internet computer, we'll have reached the magic moment: people can start relying on public IPv6 addresses as replacements for public IPv4 addresses.)
So, exactly how does the IPv4 address space fit into the IPv4+ address space? The article doesn't really say anything about this, but I can imagine only two strategies: Build the IPv4+ space around the IPv4 space (ie., the IPv4 space occupies a little corner of the IPv4+ space, similar to how v4-mapped addresses are used within softwarebut not on the wiretoday, to let applications do unified treatment of IPv4 addresses as a sort of special IPv6 address), or build it as a hierarchical extension.
Let's look at the former first; one IPv4 address gives you one IPv4+ addresses. Somehow this seems to give you all the disadvantages of IPv4 and all the disadvantages of IPv6. The ISP is not supposed to give you any more IPv4+ addresses (or at least DJB doesn't want to contact his ISP about morealso saying that the fact that automatic address distribution does not change his argument), so if you have one, you're stuck with one. So you still need NAT. (DJB talks about proxies, but I guess that the way things evolved, this either actually means NAT, or it talks about the practice of application-level proxies such as Squid or SOCKS proxies to reach the Internet, which really isn't commonplace anymore, so I'll assume for the sake of discussion it means NAT.)
However, we already do NAT. The IPv4 crunch happened despite ubiquitous NAT everywhere; we're actually pretty empty. So we will need to hand out IPv4+ addresses at the very least to new deployments, and also probably reconfigure every site that wants to expand and is out of IPv4 addresses. (Site here could mean any organizational unit, such as if your neighborhood gets too many new subscribers for your ISP's local addressing scheme to have enough addresses for you.)
A much more difficult problem is that we now need to route these addresses on the wire. Ironically, the least clear part of DJB's plan is step 1, saying we will extend the format of IP packets to allow 16-byte addresses; how exactly will this happen? For this scheme, I can only assume some sort of IPv4 option that says the stuff in the dstaddr field is just the start and doesn't make sense as an IPv4 address on its own; here are the remaining 12 bytes to complete the IPv4+ address. But now your routers need to understand that format, so you cannot do with only upgrading the end hosts; you also need to upgrade every single router out there, not just the end hosts. (Note that many of these do routing in hardware, so you can't just upgrade the software and call it a day.) And until that's done, you're exactly in the same situation as with IPv4/IPv6 today; it's incompatible.
I do believe this option is what DJB talks about. However, I fail to see exactly how it is much better than the IPv6 we got ourselves into; you still need to upgrade all software on the planet and all routers on the planet. The benefit is supposedly that a company or user that doesn't care can just keep doing nothing, but they do need to care, since they need to upgrade 100% of their stuff to understand IPv4+ before we can start even deploying it alongside IPv4 (in contrast with IPv6, where we now have lots of experience in running production networks). The single benefit is that they won't have to renumberuntil they need to grow, at which point they need to anyway.
However, let me also discuss the other possible interpretation, namely that of the IPv4+ address space being an extension of IPv4, ie. if you have 188.8.131.52 in IPv4, you have 184.108.40.206.x.x.x.x or similar in IPv4+. (DJB's article mentions 128-bit addresses and not 64-bit, though; we'll get to that in a moment.) People keep bringing this up, too; it's occasionally been called BangIP (probably jokingly, as in this April Fool's joke) due to the similarity with how explicit mail routing would work before SMTP became commonplace. I'll use that name, even though others have been proposed.
The main advantage of BangIP is that you can keep your Internet core routing infrastructure. One way or the other, they will keep seeing IPv4 addresses and IPv4 packets; you need no new peering arrangements etc.. The exact details are unclear, though; I've seen people suggest GRE tunneling, ignoring problems they have through NAT, and I've seen suggestions of IPv4 options for source/destination addresses, also ignoring that someting as innocious as setting the ECN bits has been known to break middleboxes left and right.
But let's assume you can pull that off, because your middlebox will almost certainly need to be the point that decapsulates BangIP anyway and converts it to IPv4 on the inside, presumably with a 10.0.0.0/8 address space so that your internal routing can keep using IPv4 without an IPv4+ forklift upgrade. (Note that you now lose the supposed security benefit of NAT, by the way, although you could probably encrypt the address.) Of course, your hosts will need to support IPv4+ still, and you will need some way of communicating that you are on the inside of the BangIP boundary. And you will need to know what the inside is, so that when you communicate on this side, you'll send IPv4 and not IPv4+. (For a home network with no routing, you could probably even just do IPv4+ on the inside, although I can imagine complications.)
But like I wrote above, experience has shown us that 32 extra bits isn't enough. One layer of NAT isn't doing it, we need two. You could imagine the inter-block routability of BangIP helping a fair bit here (e.g., a company with too many machines for 10.0.0.0/8 could probably easily get more addresses for more external IPv4 addresses, yielding 10.0.0.0/8 blocks), but ultimately, it is a problem that you chop the Internet off in two distinct halves that work very differently. My ISP will probably want to use BangIP for itself, meaning I'm on the outside of the core; how many of those extra bits will they allocate for me? Any at all?
Having multiple levels of bang sounds like pain; effectively we're creating a variable-length address. Does anyone ever want that? From experience, when we're creating protocols with variable-length addresses, people just tend to use the maximum level anyway, so why not design it with 128-bit to begin with? (The original IP protocol proposals actually had variable-length addresses, by the way.) So we can create our 32/96 BangIP, where the first 32 bits are for the existing public Internet, and then every IPv4 address gives you a 2^96 addresses to play with. (In a sense, it reminds me of 6to4, which never worked very well and is now thankfully dead.)
However, this makes the inside/outside-core problem even worse. I now need two very different wire protocols coexisting on the Internet; IPv4+ (which looks like regular IPv4 to the core) for the core, and a sort of IPv4+-for-the-outside (similar to IPv6) outside it. If I build a company network, I need to make sure all of my routers are IPv4+-for-the-outside and talk that, while if I build the Internet core, I need to make sure all of my connections are IPv4 since I have no guarantee that I will be routable on the Internet otherwise. Furthermore, I have a fixed prefix that I cannot really get out of, defined by my IPv4 address(es). This is called hierarchical routing, and the IPv6 world gave it up relatively early despite it sounding like a great idea at first, because it makes multihoming a complete pain: If I have an address 220.127.116.11 from ISP A and 18.104.22.168 from ISP B, which one do I use as the first 32 bits of my IPv4+ network if I want it routable on the public Internet? You could argue that the solution for me is to get an IPv4 PI netblock (supposedly a /24, since we're not changing the Internet core), but we're already out of those, which is why we started this thing to begin with. Furthermore, if the IPv4/IPv4+ boundary is above my immediate connection to the Internet (say, ISP A doesn't have an IPv4 address, just IPv4+), I'm pretty hosed; I cannot announce an IPv4 netblock in BGP. The fact that the Internet runs on largely the same protocol everywhere is a very nice thing; in contrast, what is described here really would be a mess!
So, well. I honestly don't think it's as easy to just do extension instead of alternative when it comes to the address spaces. We'll just need to deal with the pain and realize that upgrading the equipment and software is the larger part of the job anyway, and we'll need to do that no matter what solution we go with.
Congrats on reaching 10%! Now get to work with the remaining 90%.