Re: [v6ops] Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)

[Mark Smith <[e-mail]>]

On Fri, 19 Feb 2021 at 14:22, Fernando Gont <[e-mail]> wrote:
>
> On 18/2/21 22:21, Mark Smith wrote:
> >
> >
> > On Fri, 19 Feb 2021, 11:37 Brian E Carpenter,
> > <[e-mail] <mailto:[e-mail]>> wrote:
> >
> >     So, my thought (and it belongs on this thread OR the 'IPv6
> >     addressing: Gaps?' one) is something like:
> >
> >     We should abolish, delete, expunge and deprecate the word "scope"
> >     from all IPv6 documents. It clearly doesn't have an agreed meaning,
> >     so it is worse than useless.
> >
> >
> > I think the word scope is fine.
> >
> > The problem is that by itself it doesn't specify what thing the scope is
> > applying to, and people aren't explicitly saying what that scope they're
> > referring to. "Global scope" - scope of what?
>
> As per RFC4007, scope refers to address uniqueness.
>
>

RFC4007 doesn't apply to ULAs:

"Though the current address architecture specification [1] defines
unicast site-local addresses, the IPv6 working group decided to
deprecate the syntax and the usage [5] and is now investigating other
forms of local IPv6 addressing.  The usage of any new forms of
local addresses will be documented elsewhere in the future.  Thus,
this document intentionally focuses on link-local and multicast
scopes only."

RFC4193 is of course this future document.

>
> > ULAs (and addresses in general) have at least two different scopes:
> >
> > - scope or domain of intended uniqueness
>
> For ULAs, this is "set of interconnected ULA-based networks" -- i.e.,
> some sort of limited domain/scope.
>

RFC4193 explicitly says that the scope of uniqueness is global:

"Local IPv6 unicast addresses have the following characteristics:

      - Globally unique prefix (with high probability of uniqueness)."

>
>
> > - scope or domain of intended forwarding within a network or across a
> > set of networks
>
> In this case, same as before.
>
> Normally, both of these are equal:

In ULAs they aren't.

Link-Local addresses generated using RFC7271s aren't either.

The IID is likely to be globally unique, even though the Link-Local
prefix isn't. Combining the globally unique IID with the Link-Local
prefix results in a globally unique Link-Local address - the
forwarding scope of the packets with those addresses is limited to a
link, however the RFC7217 LL address's scope of uniqueness is global.


and address uniqueness will limit
> reachability.
>

I'm not sure I understand what you're saying.

I am wondering if my definition of "reachability" is different from
yours. I see it as describing the likely current forwarding capability
of the network, because the main use of the term I've encountered is
in the context of BGP.

So the "forwarding domain" is what is designed to be the case, whereas
actual reachability depends on route tables, paths and links being
available, ACLs or other security policies etc, intentional or not
duplicate addresses (i.e. anycast).


Regards,
Mark.

>
>
> > "Scope" and "domain" seem to almost be synonyms. "Global domain",
> > without specifying what "domain" refers to, would have the same problems
> > as the term "global scope".
>
> Per RFC4007, it's global uniqueness it refers to.
>
> That's why https://www.rfc-editor.org/rfc/rfc8190.txt introduced the
> flag "globally reachable" to the "IANA IPv6 Special-Purpose Address
> Registry".
>
> Thanks,
> --
> Fernando Gont
> SI6 Networks
> e-mail: [e-mail]
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
>
>
>
>