Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:39 UTC (Fri) by farnz (subscriber, #17727)
In reply to: Should distributors disable IPv4-mapped IPv6? by paulj
Parent article: Should distributors disable IPv4-mapped IPv6?

I actually think that the "IPv4++" approach would have been slower to roll out, not faster. If "routes over IPv4" was a priority for anyone deploying IPv6, we'd see a lot of 6to4 rollout in the wild; RFC 3484 defines address selection policy such that I can advertise 2001:db8::1 and 2002:192.0.2.1::1 in DNS, and have people whose IPv6 support is native communicate over native IPv6, and people who use 6to4 route over the IPv4 network, not depending on intermediate relays.

Empirically, virtually nobody has published 6to4 addresses in DNS along with their native IPv6 addresses, and yet every significant IPv6 stack out there supports RFC 3484 address selection, and has done so for at least the last decade (I don't know what behaviour the pre-Vista Windows stack had). If being able to carry IPv6 traffic over IPv4 routes to people who are stuck routing from behind an IPv4 only AS was operationally beneficial, why are we putting roadblocks in the way of doing that?

Plus, I don't think the IPv6 transition is going at all slowly - I can't think of any large scale, multiple network renumbering exercise that completed quickly in the absence of compulsion; the transition to IPv4 was only quick because NSF said "on this date, the backbone will only carry IPv4" at a time where their backbone was the only choice for long distance routing. Same applies to phone number renumbering - it takes multiple decades to get it to happen (see also UK phone numbers - which are more akin to DNS labels - where the routing is still controlled by moving bits of paper from one operator to another, because 1970s telco kit can't handle automatic routing).

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 10:59 UTC (Fri) by paulj (subscriber, #341) [Link] (3 responses)

You're missing much of the point by arguing in terms of the rollout of 6to4 in a world with an existing deployment of IPv6 on a disjoint address space - due to that being the initial transition strategy. Yes, if you view "extension" solely from that perspective, it doesn't fly. That's great, I fully agree with you. However, the discussion I was having was about the initial transition strategy - discussed and selected in the early to mid 90s.

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 11:03 UTC (Fri) by farnz (subscriber, #17727) [Link] (2 responses)

OK, so what is your point? You're claiming that I'm missing it, but you're not telling me what it is, and when I try to reason based on what happened, you go into "magical sky fairy world", and claim that things would of course have been better.

From where I'm sitting, the only extensions to IPv4 that have succeeded since 1995 are ones that are local-only (like NAT). As soon as you try to push over the general Internet (DCCP, SCTP, IPSec etc), you face unreliable delivery problems due to network admins "knowing" what legit IPv4 traffic looks like. Thus, I think that in a world where the only way to do IPv6 is to do IPv4 plus extension, people without IPv4 would be treated even worse than people with only IPv6 are today - because for everyone else, it's business as usual in IPv4, and we've done the tickybox exercise to show that IPvN could work in theory, but only works in practice as long as you have native IPv4 too.

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 14:06 UTC (Fri) by paulj (subscriber, #341) [Link] (1 responses)

I've explained it several times... A transition strategy that starts with a completely new, disjoint address space and builds a logically distinct Internet has *fundamental* routing issues _if_ it tries to re-use the existing connectivity. And re-use the existing connectivity _was_ /often/ tried, once the sheer scale of the "build a new, logically distinct Internet" became clear - witness the multiple tunneling and 6to4-ish protocols. Except, by that time, it was *too late*.

Had the _initial_ transition strategy - designed and agreed on in the early 90s - been a "re-use the existing connectivity" one, and the disjoint-address-space avoided (at least, till closer to the exhaustion of the old) then that /might/ have allowed a faster rollout, and we might nearly all have had working, efficiently-routed, IPng more than a decade ago. Can't say for sure of course, but it couldn't have been worse.

Would such an approach have been the most aesthetically pleasing? No. Would such an approach have come with packet header overheads? Yes. Might stupid middle-boxes have caused for some at times, yes. But there would have been ways around those with (with additional packet overheads), also stupid middle-boxes will continue to cause problems for some at times, regardless :(.

Also, NAT is *not* local-only. Many hosts connect to lots of sites far away on the Internet through NAT - not local at all. And even NATed hosts can often exchange packets directly with other NATed hosts, using 3rd parties to setup the initial mapping state.

Should distributors disable IPv4-mapped IPv6?

Posted Jun 10, 2016 14:13 UTC (Fri) by farnz (subscriber, #17727) [Link]

Then you're not addressing the points I'm making at all about why any transition was doomed to failure - fundamentally, there's nothing about the transition state that makes it worth people's while taking any pain from IPvN (no matter how minimal) until they cannot get IPv4. Multiply that by the fact that IPvN on its own is not helpful until everyone you wish to communicate with has IPvN, and you get exactly the outcome we see - no-one cares until IANA runs out.

And it absolutely could be much worse than it is - other transitions in network land (e.g. the move to SS7) have taken even longer than the move to IPv6; if IPv6 is a failure, please point to another, faster, global network transition.

You're also misunderstanding what I mean by local-only; NAT is local only in the sense that if I wish to use it, I do not need you to take any action to continue communicating with me. If I want to use IPvN, I need you to understand IPvN, regardless of whether IPvN is an extension atop IPv4 (like MPTCP or SCTP), or whether it's a disjoint network (like IPv6). In other words, I can transition to NAT without any of my peers needing to know or care; the same is definitionally false of a larger address space.