Re: [v6ops] ULA precedence [Thoughts about wider operational input]

[Lorenzo Colitti <[e-mail]>]

On Mon, Apr 25, 2022 at 2:28 AM Erik Auerswald <[e-mail]>
wrote:

>  "Since ULAs are defined to have a /48 site prefix, an implementation
>   might choose to add such a row automatically on a machine with
>   a ULA."
>
> The result is that only the local ULA prefix, i.e., exactly the
> local IPv6 addresses, are preferred over IPv4 (and IPv6 GUA).
> This should be exactly what is needed to use ULA addresses inside
> an organization, or for a lab.
> [...]
> Implementing the non-normative suggestion from Section 10.6 of RFC
> 6724 would in all likelihood result in making ULA usable for local
> tests and even first steps in deploying IPv6.  ULA addresses would
> only be used locally.  Existing IPv4 based Internet access would not
> be impaired by adding IPv6 ULA.
>

That does seem like it might make ULA more useful, yes.

Additionally, maybe we could clarify that the longest-prefix match rule
does not apply to ULAs outside the same /48? I think that would fix the
issue observed by +Ted Lemon <[e-mail]> in home networks:
https://datatracker.ietf.org/meeting/113/materials/slides-113-6man-source-address-selection-for-foreign-ulas-00
.


> In order to keep IPv6 deployment similar to IPv4, IPv6 NAT could be
> considered.  To make this work as intended, the address selection
> policy table could be adjusted to contain the local ULA prefix
> with precedence greater or equal to GUA and the same label as GUA.
>

This seems like it would encourage the use of IPv6 NAT. I think there is
reasonably strong consensus within the IETF that that is not the right way
to go, because it pushes problems on to application developers. This adds
costs for NAT traversal software development and maintenance, and requires
devices to implement NAT keepalives, increasing battery usage.