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

Fernando Gont <fgont@si6networks.com> Sat, 13 February 2021 02:47 UTC

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Subject: Re: [v6ops] Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)
To: Ted Lemon <mellon@fugue.com>, Fred Baker <fredbaker.ietf@gmail.com>
Cc: IPv6 Operations <v6ops@ietf.org>, "6man@ietf.org" <6man@ietf.org>
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From: Fernando Gont <fgont@si6networks.com>
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Date: Fri, 12 Feb 2021 23:46:33 -0300
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On 12/2/21 23:13, Ted Lemon wrote:
> On Feb 12, 2021, at 8:55 PM, Fred Baker <fredbaker.ietf@gmail.com>
> wrote:
>>> On Feb 12, 2021, at 4:04 PM, Ted Lemon <mellon@fugue.com> wrote: 
>>> Global scope is “you can forward it to the default route.”
>> No. Global scope, per the RFC  you're quoting, is "Global scope,
>> for uniquely identifying interfaces anywhere in the Internet.”
> 
> 
> If you’re referring to RFC 4007, and talking about unicast addresses,
> then the document is unequivocal on this point: an address is either
> global or link-local. Those are the only two options. 

RFC4007 does not limit the possible scopes to only two options. For 
instance, there's a definition of scope:

       "[the] topological span within which the address may be used as a
       unique identifier for an interface or set of interfaces"

and of "global scope" in particular:

       "uniquely identifying interfaces anywhere in the Internet"


ULAs certainly have a scope larger than link-local, but also certainly 
do not comply with the above definition of "global scope".



> I think here
> “global” means “there is no context in which this address is, by
> definition, out of scope.”

But that's a circular definition ;-)   So, what's the definition of 
"scope", in the first place?



> I agree that there’s a terminology question here, and perhaps that is
> a problem to solve, but I don’t see any way to solve it. I think the
> following things are all true:
> 
> ULA addresses are, in principle, VALID in any scope.
> 
> They are not, in principle, UNIQUE to a particular link: it’s
> entirely possible to have two instances of the same ULA referring to
> different interfaces connected to different links.
> 
> In principle, the set of all networks which can route a packet to a
> particular instance of a ULA /48 MUST be DISJOINT from the set of all
> networks which can route a packet to some other instance of that ULA
> /48.

These last two are essentially at odds with the definition of "global 
scope" from RFC4007.

OTOH, these two items apply perfectly fine for real GUAs.



> But the bottom line is that if the term “global” is confusing as it
> applies to ULAs, it shouldn’t be that hard to clarify what we mean by
> global.

My pov is that the definitions in RFC4007, are sensible. However, these 
are at odds with the definition of ULAs as global addresses.

(Bottom-line here is that this thread has shown that the terminology 
employed is confusing -- if not incorrect -- even for us involved with 
the stds.  So I'd expect that to be even more the case for "newcomers". 
  In fact, this issue partly came up while explaining the ipv6 
addressing architecture to other folks... )

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492