Most of the other criticism is not relevant anymore, since we now have a lot of transition technologies that allow IPv6 clients to interoperate with IPv4 servers (this way around is possible since IPv6 is a superset of IPv4). Overall we are now much further into the IPv6 migration than djb ever envisioned.
The only way to remain incentive compatible is to remain administratively compatible, and that is where IPv6 as presently constituted fails dramatically by requiring two independent network configurations to be maintained for the better part of a century, without giving anyone an incremental incentive to maintain the second one, leading to a hold out problem.
The public switched phone network has gone through major upgrades and yet at no point did someone say we are going to throw out all your existing phone numbers and require you to get new ones, or require you to have two independent and incompatible phone numbers that you put on your business cards, have two phones on your desk, or a phone with a mode selection button depending on whether you wanted to call a new style phone number or an old style phone number.
And that - from an administrative point of view - is the fundamental problem with the deployment of IPv6 as we know it. Dual stack now and for decades to come. Dual stack anything is not incentive compatible and should never have been done. The proper solution is single stack everything with capabilities that are dormant until they are deployed on a global level as part of the normal upgrade process in an administratively compatible fashion so that no large scale administrative intervention is required now or at any time in the future.
Other changes were made in the early days, like the introduction of dialing, then long distance dialing.
SLACC, interface-specific link-local address both look good in paper but cause lots real life headache.
When it works, it works; when it don't, you have to unlearn and relearn everything network before you could possibly understand the problem, let alone fixing them.
Let me summarize my understanding of what he's saying, because I don't quite see why/how you disagree. I think you (or I) might be misunderstanding his claim.
Imagine this topology: C (client, IPv4-only) <=> R (intermediate router) <=> S (server)
My understanding of djb is he's saying that IPv6 could have been designed such that S could still serve C via only simple software updates -- this means, crucially, without the need for S to separately obtain a public IPv6 address through R, because its IPv4 address would be automatically valid for IPv6.
How can this work? Well, there are two scenarios:
1. If R is IPv4-only, then S could figure that out during some startup/negotiation process, and send only IPv4 packets to R. R only lets IPv4 clients connect to S anyway, so any response (even from IPv6 applications) on S must be going back to an IPv4 address. So the kernel can transparently translate those IPv6 addresses into IPv4 before passing them along to R, and vice-versa.
2. If R supports IPv6, then R can do the same thing S would've done in the previous^ scenario. (In fact, I think S could become IPv6-communication-only in that case, reserving IPv4 for just address leasing? I'm not sure, but in any case, I don't think that matters here.)
Notice that all of this is almost completely stateless. (I think the only state S needs to track here is 1 bit, indicating whether R supports IPv6 or not.) So, S and R can be independently and (importantly, rather trivially) upgraded to support the IPv6 protocol, without losing the ability to talk to any clients within the IPv4 address range.
This is easy and requires no explicit leasing of IPv6 addresses. That step can be implemented and have support for it added later, whenever S is ready to serve clients beyond the IPv4 address space.
Does this make sense? If so, then it seems to show how IPv4-only clients could talk to IPv6 servers without modification. If not, then I'd love to see where I'm mistaken (I very well might be).
The inverse situation (IPv6-only client but IPv4-only server) is not really an issue, since for that situation NAT64 works, since you can embed IPv4 addresses into IPv6.
The only way C (ipv4-only) could communicate with S (ipv6-only) is by either allocating a dedicated ipv4-address to S (doesn't have to be directly connected to S - it can be sent to some translation box that does SIIT) or by upgrading C to support ipv6 and tunneling it (6to4, 6rd, teredo, etc.).
If S is IPv6-only, then either the address is from the IPv4 range, or it's not. If it is, then R can translate between them. If not, then they can't connect. I don't see him suggesting otherwise anywhere.
Even ignoring translation possibility, his point here isn't to claim its magically possible for a server outside the IPv4 address range to talk to an IPv4 client. His point is that this can move everyone to IPv6 for communication, without ISPs also having to deal with leasing addresses outside the IPv4 range, which he believes would help adoption.
But maybe I should reread the article a bit more generously and less literally, maybe he is actually advocating for the IPv6 we have today? Because there are transition technologies like NAT64, SIIT, 6to4, 6rd, etc. that sort of allow some of the things he suggested? "If the IPv6 configuration isn't automatic, it won't happen." sounds like he is actually advocating SLAAC to me?
How are R and S negotiating when R cannot even name S on the network? Its stack only allows for 32-bit addresses and S can't have a 32-bit address.
That post was written 20 years ago. I would hope that the migration would be more than "much further along", I'd have hoped it had been completed, like a decade ago.
> is based on the same fundamental misunderstanding that one somehow can extend IPv4 in a way somehow, but remain compatible with IPv4-only clients
I'm not a network engineer, but I've seen loads of commentary from knowledgeable sources that it would have been quite possible to have extended the ipv4 address space without requiring 2 completely separate network stacks.
I think the simple fact that ipv6 includes so many other parts beyond just extending the address space shows what a foolish endeavor it was in the first place. I'm not saying the other bits aren't good ideas, but the only immovable factor that has people wringing their hands about ipv4 is the address limitation. If they had just focused on that, we probably wouldn't be in a situation where we're still running 2 network stacks virtually everywhere, and will be for the foreseeable future. The famous XKCD "Standards" meme says it best: https://xkcd.com/927/
It may sound like a great plan as long as one doesn't look too closely at the details. IPv4 has fixed 32-bit addresses and one cannot cram more than 32-bit of information into a fixed 32-bit field. But one would need to do that for it to be forward compatible, since how would a IPv4-only client open communications with a expanded address space server?
One idea is to only upgrade the client and server and tunnel the expanded address space packets over IPv4. IPv6 has that - that's how it was bootstrapped before native IPv6 connectivity was a thing.
I don't know that I believe this claim at all, but it is at least coherent and possible (unlike claims that N-bit addresses could have been added to IPv4 in a backwards compatible manner). Perhaps SLAAC, the focus on routable end user devices etc were indeed major distractions that pulled focus away from a move to a new larger address space - which is the only thing people actually wanted from IPv6.
I'm not sure this is true. There's a lot of warts in the way IPv4 was put together (ARP is a tire-fire of badness for example), and the opportunity was taken to implement those in a better way. Lots of network engineers are very happy about that.
I think what annoys people is everything else that changed with it, if DHCP (I know), subnets, NAT if you wanted it, etc. was all just the same, if the model was the same the IPs were just longer now, that they wouldn't really complain.
I'm not even sure it would be necessary to put it in unaddressable v4 block, since it would be too long and a different version anyway. Obviously people have thought about this a lot more than me so know a lot more about it, I'm not naïve about that.
https://www.google.com/intl/en/ipv6/statistics.html
Yes, the IPv6 migration has taken much longer than anyone expected. But this argument would have made more sense in 2015 when we were looking at 5% IPv6 deployment and very erratic growth. But it's not, we've been looking at 10% of the market gaining IPv6 support for the last 3 years and are now at 45%. Now granted, this is likely to be largely "new" devices, e.g. in mobile networks and in countries like India where these were hidden behind CGNAT before. But these are exactly the type of devices that an IPv4 extension header couldn't have reached either.
