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

[Erik Auerswald <[e-mail]>]

Hi Eduard,

On Mon, Apr 25, 2022 at 07:57:05AM +0000, Vasilenko Eduard wrote:
> Hi Erik,
> See below
> Eduard

I have added the quoting level you omitted to help differentiate who
wrote what, and trimmed the message below.

> -----Original Message-----
> From: Erik Auerswald [mailto:[e-mail]] 
> Sent: Monday, April 25, 2022 10:48 AM
> > On Sun, Apr 24, 2022 at 08:12:29PM +0000, Vasilenko Eduard wrote:
> > > [...]
> > > 2. Attach a special Label (redundant action because FC/7 already has 
> > > unique Label=13)
> >
> > The different label to differentiate between locally used ULA
> > prefix(es) and the remaining ULA space is intentional.
> 
> [EV] Why?
> I do not understand the advantage to populate more prefixes into the host
> If the site would have many ULA prefixes.

This is required to prefer locally used ULA over IPv4 and possibly
IPv6 GUA, but prefer IPv6 GUA and IPv4 over non-local ULA.
Non-local ULA prefixes are generally not reachable.

In a special situation, where ULA is used as a replacement of GUA,
say a large private network, or a NAT66 setup, ULA should be treated
just as GUA, as was done before RFC 6724.  This can be achieved with
RFC 6724 compliant systems by changing the fc00::/7 policy table
entry to use identical preference and label as the ::/0 entry.
This expresses the local policy to treat ULA identical to GUA,
just as private IPv4 space is treated identical to public IPv6
space for address selection.

> > > Imagine the situation where only one router on the link
> > > announces ULA PIO, but the other one - GUA PIO.  (Routers are
> > > not in sync for announcements).

It seems that RFC 8028 as mentioned by Brian would provide a solution
for this.

> > Using different first hop gateways for GUA and ULA is another
> > problem.  It would be nice if hosts would (optionally) not
> > consider a gateway announcing a ULA prefix as default gateway,
> > but only for ULA reachability.
> 
> [EV] Nope. It is better to revert the logic: decide about the
>      source address first (needed anyway for MHMP) only then to
>      decide about the next-hop.

If a router announcing only a ULA prefix would not be considered as
a default router, the host could detect that GUA is not reachable,
and fall back to IPv4.  I think this is a different use case from
what you have in mind.

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