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

Mark Smith <> Wed, 06 January 2021 02:01 UTC

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From: Mark Smith <>
Date: Wed, 6 Jan 2021 13:00:36 +1100
Message-ID: <>
To: Fernando Gont <>
Cc: 6MAN <>, 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 Wed, 6 Jan 2021, 12:21 Fernando Gont, <> wrote:

> Folks,
> Based on the recent discussion on the v6ops list
> (,
> I've posted this new I-D, meant to discuss the scope of ULAs:
> Title: "Scope of Unique Local IPv6 Unicast Addresses"
> I-D:
> Short version of the story:
> ULAs are formally part of the GUA space. However, the characteristics of
> ULAs do not seem to match the definition of global scope from RFC4007
> (IPv6 Scope Addr Architecture). ULA seem to have a scope of
> scope(link-local) < scope(ULA) < scope(GUA).
> This is not only a terminology thing (which I think is nevertheless
> important to get right) but also has practical implications. For
> example, there's a python library that considers ULAs as "not global",
> and "private" -- contradicting the current RFC4291/RFC4193 specs.
> Prior to posting this document, we had some on-list discussion (on the
> v6ops list) and also some off-list discussion with some of you (bcc'ed).
> The opinions have been in one of these camps:
> 1) the current specs are coherent and there's no problem
> 2) There's a problem with the definition of "global scope" -- so ULAs
> *are* global scope, but global scope does not really stand for the
> definition in RFC4007.

> 3) The definitions in RFC4007 are correct, and thus the scope of ULAs is
> not really global, but scopee(link-local) < scope(ULAs) < scope(global)

The thing that is really missing from "global scope" is what scope or
domain is being described? Forwarding scope? Uniqueness scope? Some other
scope (DNS visibility is probably another one). All of them?

ULAs are intended to be globally unique addresses, but not to be globally
(Internet) forwardable. Their forwarding scope is limited to non-global,
either within a single local network, or between a set of local networks
that have agreed to forward their respective ULA /48 prefixes between each
other, overriding the default of local networks only forwarding scope.
(Ethernet addresses are a similar example, globally unique addresses, link
only forwarding scope.)

GUAs also are intended to be globally unique addresses, but are intended to
be globally (Internet) forwardable.

There isn't really a ULA equivalent in IPv4, although I think a lot of the
arguments in RFC1627, "Network 10 Considered Harmful (Some Practices
Shouldn't be Codified)" would have been arguments for one e.g, "The lesson
that we learned was that every IP address ought to be globally unique,
independent of its attachment to the Internet." (There is a statement about
effectively everything attached to the Internet too, however that's 1994
Internet naivety, before DDoSes and Internet wide scanning for and
exploiting of vulnerabilities were much of a thing.)


> While this document does propose a way out (it assumes #3 above, and
> acts accordingly), I believe the first step is to agree on what "global
> scope" means and, subsequently, whether ULAs are really "global scope"
> or not. Since opinions on the topic have vary a lot (as noted above),
> I've posted this I-D and I'm sending this note for further input from
> the WG.
> Thanks!
> Regards,
> Fernando
> -------- Forwarded Message --------
> Subject: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt
> Date: Tue, 05 Jan 2021 17:02:20 -0800
> From:
> To: Fernando Gont <>
> A new version of I-D, draft-gont-6man-ipv6-ula-scope-00.txt
> has been successfully submitted by Fernando Gont and posted to the
> IETF repository.
> Name:           draft-gont-6man-ipv6-ula-scope
> Revision:       00
> Title:          Scope of Unique Local IPv6 Unicast Addresses
> Document date:  2021-01-05
> Group:          Individual Submission
> Pages:          8
> URL:
> Status:
> Htmlized:
> Htmlized:
> Abstract:
>     Unique Local IPv6 Unicast Addresses (ULAs) are formally part of the
>     IPv6 Global Unicast address space.  However, the semantics of ULAs
>     clearly contradict the definition of "global scope".  This document
>     discusses the why the terminology employed for the specification of
>     ULAs is problematic, along with some practical consequences of the
>     current specification of ULAs.  Finally, it formally updates RFC4291
>     and RFC4193 such that the scope of ULAs is defined as "local".
> Please note that it may take a couple of minutes from the time of
> submission
> until the htmlized version and diff are available at
> The IETF Secretariat
> _______________________________________________
> v6ops mailing list