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

Lorenzo Colitti <lorenzo@google.com> Wed, 06 January 2021 07:08 UTC

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From: Lorenzo Colitti <lorenzo@google.com>
Date: Wed, 6 Jan 2021 16:07:51 +0900
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To: Mark Smith <markzzzsmith@gmail.com>
Cc: Fernando Gont <fgont@si6networks.com>, IPv6 Operations <v6ops@ietf.org>, 6MAN <6man@ietf.org>
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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, Jan 6, 2021 at 11:01 AM Mark Smith <markzzzsmith@gmail.com> wrote:

> 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.)
>

IMO defining ULAs as they are was a mistake. Global scope implies unique.
But probabilistic uniqueness doesn't work because humans choose ULAs
instead of generating them manually. Registry-based uniqueness doesn't work
(and, to be fair, was never tried by the IETF) because there is no registry
that has jurisdiction. Even if there were, there is no reason to keep
addresses unique if they don't have global reachability.

So I guess I'm somewhere between 1) and 3). The specs are consistent but
they fail to consider human behaviour, so they don't actually work in
practice. I don't know what to do about this though. If we say they're
non-global scope, then they are going to be the exact equivalent of RFC1918
addresses, with all the problems that that causes. If we continue to say
they're global scope, then the specs don't match reality. :-(