It's hard to argue that there was no transition plan. There were in fact at least three transition plans for the selected approach (dual stack, 6to4, and tunneling) some of which have been discarded along the way; while others came to be based on operational experience. Moreover, the only way to really know that a transition mechanism is really going to work is to let it out of the lab. And ALL of the proposals would have suffered the very same transition pains, because just as Jeroen has pointed out, the pain stretched all the way to the application.

I don't think it's reasonable to argue that we should have waited for some other mythical better proposal to come along. I don't recall anyone arguing for that at the time, and there's no reason to believe that such a mythical proposal would have ever come to be in any foreseeable time frame. In fact Erik Fair, Dave Crocker, Tom Kessler and I argued the very opposite, that we were digging ourselves a hole with NAT. Your argument at the time (Interop '95, Vegas) was that the IETF didn't have the right to dictate address usage to deployments. True enough, but then people shouldn't hang their woes on the IETF.

As I mentioned earlier, the fundamental issue was that there were no ng proposals that were in fact substitutable resources for v4, and NAT was. From there, economics has ruled, arguments be damned.

* I did in fact posit an approach in 1992 that would have allowed for orderly transition such that IPv4 could continue, and that was to define class E addresses as extension blocks that were in fact name space prefixes for another IPv4 header. It wasn't taken seriously, and perhaps rightly so. This actually borrowed a page from the PSTN - most people communicate locally and so you would rarely use those extended name spaces. This was before Paul (Tsuchiya) Francis offered PIP, which had a notion of Landmark Routing that also went nowhere.

On 14 Sep 2021, at 3:46 AM, Eliot Lear <lear@ofcourseimright.com> wrote:

….

There is no evidence that any other design choices on the table at the time would have gotten us transitioned any faster, and a lot of evidence and analysis that the exact opposite is more likely.

Elliot -

If by “design choices” you mean the criteria that we set forth for the new protocol (IPng), then that’s potentially true - it’s fairly challenging to hypothecate what impact different technical criteria would have had on the outcome.

If by “design choices” you mean the tradeoffs accepted in selecting a particular candidate protocol and declaring victory, then I’d strongly disagree. I believe that we had the appropriate technical criteria for IPng (very nicely compiled and edited by Craig Patridge and Frank Kastenholz in RFC1726) and then made conscious decisions to disregard those very criteria in order to “make a decision” & “move forward.”

All of the IPng proposals where completely deficient with respect to transition capabilities. In the rush to make a IPng decision, the actual IPng Transition Criteria [1] that mandated a straightforward transition plan from IPv4 was simply acknowledged and then declared as “resolved" because we would also simultaneously form some working groups to study all of the transition requirements and made good on the transition criteria via future deliverables...(deliverables that were subsequently not delivered on)

The right answer would have been to formally and critically evaluate each of the candidate protocols against the requirements and not make any selection until candidate presented itself that actually met the required technical criteria. Instead, IPv6 transition was left as an afterthought for the operator community to solve, and thus the battles with the IETF on NAT-based transition for nearly two decades to get this basic technical requirement met.

FYI,
/John

Disclaimer: my views alone - made from 100% recycled electrons.

=== [1] The actual IPng Transition criteria (per RFC 1726) are as follows -

"
5.5 Transition

  CRITERION
     The protocol must have a straightforward transition plan from the
     current IPv4.

  DISCUSSION
     A smooth, orderly, transition from IPv4 to IPng is needed. If we
     can't transition to the new protocol, then no matter how wonderful
     it is, we'll never get to it.

     We believe that it is not possible to have a "flag-day" form of
     transition in which all hosts and routers must change over at
     once. The size, complexity, and distributed administration of the
     Internet make such a cutover impossible.

     Rather, IPng will need to co-exist with IPv4 for some period of
     time. There are a number of ways to achieve this co-existence
     such as requiring hosts to support two stacks, converting between
     protocols, or using backward compatible extensions to IPv4. Each
     scheme has its strengths and weaknesses, which have to be weighed.

     Furthermore, we note that, in all probability, there will be IPv4
     hosts on the Internet effectively forever. IPng must provide
     mechanisms to allow these hosts to communicate, even after IPng
     has become the dominant network layer protocol in the Internet.

     The absence of a rational and well-defined transition plan is not
     acceptable. Indeed, the difficulty of running a network that is
     transitioning from IPv4 to IPng must be minimized. (A good target
     is that running a mixed IPv4-IPng network should be no more and
     preferably less difficult than running IPv4 in parallel with
     existing non-IP protocols).
"

In short:

  1) The protocol must have a straightforward transition plan
  2) A number of ways to achieve this which are to be explored
  3) IPng must provide backward-compatibility to IPv4-only hosts
  4) The absence of a well-defined transition plan is not acceptable

===