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

[Erik Auerswald <[e-mail]>]

Hi Lorenzo,

On Mon, Apr 25, 2022 at 09:16:14AM +0900, Lorenzo Colitti wrote:
> 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
> .

Looking at the slides, the optional rule 5.5 ("[p]refer addresses
in a prefix advertised by the next-hop") from RFC 6724 would have
resulted in preferring the GUA address over the ULA address when
initiating the connection to "Host 2" from "Host 1."

But rule 6 ("[p]refer matching label") would have chosen the ULA
source address already before considering rule 8 ("[u]se longest
matching prefix").

Thus I do not think that just adding an exception for ULA to rule
8 would suffice.  In a way each possible /48 ULA prefix would need
to be treated as having a different label, before a modification to
disregard the longest matching prefix for ULA from different /48 in
rule 8 would have the intended effect.  Additionally, now ULA and
GUA were equally valid choices as source address in this scenario.
Thus now a new rule 9 would need to be added to prefer GUA over
ULA, e.g., based on the preference value in the address selection
policy table (i.e., use highest precedence).

And all that would probably not preclude constructing a different
scenario where choosing GUA over ULA as source address would break
connectivity.

Thus I would rather suggest to consider if making tracking which
gateways advertised which prefixes mandatory would be a better
approach.

> [...]

Kind regards,
Erik
-- 
Dipl.-Inform. Erik Auerswald
Gesellschaft für Fundamental Generic Networking mbH
Geschäftsführung: Volker Bauer, Jörg Mayer
Gerichtsstand: Amtsgericht Kaiserslautern - HRB: 3630