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 08:16 UTC

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From: Mark Smith <>
Date: Wed, 6 Jan 2021 19:16:27 +1100
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To: Christopher Morrow <>
Cc: Lorenzo Colitti <>, Fernando Gont <>, IPv6 Operations <>, 6MAN <>
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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, 18:30 Christopher Morrow, <>

> On Wed, Jan 6, 2021 at 2:09 AM Lorenzo Colitti
> <> wrote:
> >
> > On Wed, Jan 6, 2021 at 11:01 AM Mark Smith <>
> 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. :-(
> option 4, deprecate ULA.
> the best option (tm).

If you want to destroy IPv6 by causing enterprises to replicate RFC1918s
via site-locals and use NAT66, do that.

ULAs exist because site-locals would have created that problem.

People are already deploying ULAs mostly correctly - I know of 4
electricity smart meter networks that have more than 2 million meters
attached that are using ULA addressing. The only relatively minor mistake
made was that the smartmeter vendor took the 4 network's ULA spaces out of
a single /48, using /52s if I recall correctly, rather than generating a
/48 for each one.

While probably prohibited by government, if any if those networks merged,
routing between them will be as simple as linking them together and trading
/52 routes.

If they'd used site-locals, merging would instead be either as complex as
renumbering, or as bad as NAT66, the latter being most likely.