John Levine writes:
FWIW, I also don't think that repurposing 240/4 is a good idea.
As people will be aware, we have a different draft on this issue, so I'm also going to pipe up here. https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-240/ (Our draft offers no specific plan for exactly how to use the address space, arguing that the most important short-term priority is to ensure that implementations stop rejecting it, rather than to decide on a policy for how or when it can be allocated. For example, it might turn out that debogonization appears too daunting a task, in which case there might be a consensus to make it into official private address space in the future.)
To be useful it would require that every host on the Internet update its network stack,
Most hosts other than Windows already made this change following the previous proposal in 2008. So we mostly have to get Windows to make the change now. Routers are a more complicated question, and we would love people to help us obtain some concrete data about this.
which would take on the order of a decade, to free up some space that would likely be depleted in a year or two.
When I presented about this at NANOG 84 and APRICOT recently, I noted that a lot of people's intuitions about rapid exhaustion of IPv4 resources come from RIR allocations that were done with nominal fees. But blocks that became available and were sold for market rates seemed to last longer. We think that, if it does become feasible to allocate historically-reserved space for public Internet use, market-based allocations like auction mechanisms (which can potentially also be done by RIRs) will make people more cautious in their appetite for number resources, while also preventing the price of those resources from rising as quickly as it otherwise would. Someone asked a question about how long we thought it would take for that depletion to occur, and I passed it on to Lee Howard (on account of his experience in the IPv4 secondary markets). I think I remember that his answer was "about seven years" -- I should double-check that it wasn't six years or eight years. I understood the question I was passing on to be something like "how long will these resources last, assuming RIRs allocate them by selling them in a series of auctions or selling them into existing address space markets as though they were newly-recovered previously-allocated address space?".
It's basically the same amount of work as getting everything to work on IPv6.
That's challenging to quantify, but in any case it doesn't appear to be _the same work_ or, necessarily, _work by the same people_. For example, Windows already supports IPv6 quite well, but doesn't support unicast 240/4 at all. My Linux laptop supports both well, but my ISP doesn't give me native IPv6. (I don't know yet whether or not my ISP would have to make changes to support native 240/4, although I look forward to finding out!) I don't want people to work to support 240/4 (and other address ranges we've proposed unreserving) at the expense of supporting IPv6. I agree with the consensus that implementers and operators ought to support IPv6. Still, I haven't seen why one can be expected to substitute for or compete with the other, unless one envisions a very direct conflict between improving IPv4 support or services and improving IPv6 support or services. We also have a new draft (published yesterday) more directly on point about that issue... https://datatracker.ietf.org/doc/draft-schoen-intarea-ietf-maintaining-ipv4/