Re: [IPv6] Second Working Group Last Call for <draft-ietf-6man-rfc6724-update>

[Ole Troan <[e-mail]>]

> On 11 Apr 2024, at 05:30, Lorenzo Colitti <lorenzo=[e-mail]> wrote:
> 
> On Thu, Apr 11, 2024 at 1:04 AM Ted Lemon <[e-mail]> wrote:
> I continue to think that section 3,  "Operational Issues Regarding Preference for IPv4 addresses over ULAs," should make the new proposed ULA behavior mandatory rather than optional. I don't see a downside to making it mandatory. Hosts will come into compliance when they can; older implementations will not implement this new behavior, but I don't see any point in perpetuating that.
> 
> Absolutely agree. This document should not proceed without that MUST. Preferring non-local ULA over IPv4 is incorrect because IPv4 implies global reachability, and ULA does not offer global reachability. So publishing this document without the MUST is harmful: an implementation that does not implement the SHOULD will cause regressions and break use cases that work today.

A host should not make those assumptions.
A RFC1918 IPv4 address may or may not have global reachability.
A ULA may (or may not) have global reachability.

In essence SA/DA combination can be assumed to provide reachability. It has to be probed.
The _only_ thing SAS/DAS selection should be used for is ordering of the candidate list.

> Also, MUST allows us to make ULA more useful than it is today. It is *desirable* to be able to publish non-local ULAs and have hosts know what is local and what is not. As a simple example: once all hosts implement the MUST, it will be safe to publish local ULAs in the global DNS, because hosts won't try to use them unless they are local.

That’s likely a simplification. As they are certainly going to be networks where there will not be possible to signal all ULA prefixes to every host.
The IETF conviction that as long as we make something a MUST then every implementor will implement it is flawed. The only thing it does is to water out the value of the MUST. Any MUST/SHOULD debate motivated by this (as opposed to a real interoperability breaking issue) is bike-shedding.

O.