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

David Farmer <> Thu, 18 February 2021 23:41 UTC

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From: David Farmer <>
Date: Thu, 18 Feb 2021 17:41:27 -0600
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To: Fernando Gont <>
Cc: "Templin (US), Fred L" <>, IPv6 Operations <>, "" <>
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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)
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On Thu, Feb 18, 2021 at 5:10 PM Fernando Gont <> wrote:

> On 18/2/21 19:52, David Farmer wrote:
> > So let's be clear "IPv6 Address Scope" does not really have the general
> > meaning of the word "scope", it has a very specific meaning that in my
> > opinion boils down to are Zone Indexes or Identifiers needed to
> > uniquely identify them.
> Well, this is a spec inconsistency. You have one spec (RFC4007) defining
> "scope" and "global scope", and another specs:
> a) making use of the same terms in an incorrect way, or,
> b) employing same terms but with a different definition.
> i.e., either the definition in RFC4007 is incorrect, or the use in
> RFC4193 and implicit use in RFC4291 is incorrect.
> As you've correctly noted, this applies to other address types, too.
> > There is an argument to be made that ULAs should be Scoped, and if they
> > were globally reachable they would have to be Scoped.
> Well, you can also argue that they are not globally reachable because
> they are actually scoped ;-)
> > Nevertheless, they
> > are not globally reachable, and therefore so while it is possible that
> > there is another user of the same ULA prefix you are using. But, if you
> > follow the algorithm, in RFC4193 for allocation, the probability of that
> > duplicate user ever being reachable by you is very-very small.
> This accounts for "why do things do not break in practice". But
> certainly doesn't help with the consistency of the specs.

I agree there are inconsistencies, how fatal those inconsistencies are, I
think we disagree. I believe adding a single word to the definition in
RFC4007 will align RFC4007 and RFC4193,  "uniquely identifying interfaces
anywhere *reachable* on the Internet".

Further, I think when we eventually get around to revising RFC4291, every
remaining use of the word "scope" needs to be examined, and probably a
reference to the updated definition in RFC4007 or its replacement should be
added. Note most of the words "scope" in RFC4291 are in reference to local
and global IIDs, which basically go away with RFC7136. But even after that
cleanup, I think there is some confusion around "scope" and its meaning, at
least for the uninitiated reader of RFC4291.


David Farmer     
Networking & Telecommunication Services
Office of Information Technology
University of Minnesota
2218 University Ave SE        Phone: 612-626-0815
Minneapolis, MN 55414-3029   Cell: 612-812-9952