Re: Happy St Nicholas Day: Re-Launching the IPv6 ULA registry

Brian E Carpenter <brian.e.carpenter@gmail.com> Tue, 08 December 2020 01:58 UTC

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Subject: Re: Happy St Nicholas Day: Re-Launching the IPv6 ULA registry
To: Fernando Gont <fgont@si6networks.com>, Nico Schottelius <nico.schottelius@ungleich.ch>, Lorenzo Colitti <lorenzo@google.com>
Cc: IPv6 List <ipv6@ietf.org>
References: <87r1o3deni.fsf@ungleich.ch> <CAKD1Yr3ptRjewThToEgERUOKwehTwdqNUAq14acc_nHLFqf3bg@mail.gmail.com> <87im9ds0z9.fsf@ungleich.ch> <6763ff23-f33d-1e45-a495-22c7ae3e741b@si6networks.com>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Message-ID: <2aa92872-8251-cd1a-8269-6b268ed378dc@gmail.com>
Date: Tue, 08 Dec 2020 14:58:46 +1300
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On 08-Dec-20 14:07, Fernando Gont wrote:
> On 7/12/20 19:30, Nico Schottelius wrote:
> [....]
>> fd00::/48 is actually an example entry created during the Hackathon,
>> which led us to implement the random generation instead as the primary
>> option instead of focussing on submission of existing prefixes.
>>
>>> In fact, I just configured it on my network, so if
>>> there was no duplicate before, there is one now. :-)
>>
>> With that argument even GUA is not GUA - I can also configure 2600::/32
>> or even 2001:db8::/32 in my network, it does not make me "own" 2600::/32
>> nor remove the documentation character from 2001:db8::/32.
> 
> FWIW, for all practical purposes, ULAs are non-global-scope addresses -- 
> the only reason (?) they are considered global is that RFC4291 still 
> marks the ULA "space" as GUA, and RFC4193 hasn't updated that.

It's also a bit of a historical accident, too. ULAs were invented at
the same time that site-local was deprecated [1]. They are routeable
(unlike link-locals) so RAs, PIOs and routing protocols treat them
exactly like any other routeable address. So, given that site-local
scope was abolished, they could only be called global-scope.
They are only special in domain boundary routers, which should
block them.

Yes, this has led to confusion in the terminology, only partly
resolved by https://tools.ietf.org/html/rfc8190#section-2.1 
 
> So, an ULA is not meant to be globally usable, and only make sense 
> within an organization (whatever that means) -- and the same ULA prefix 
> being re-used is just fine.

There were two arguments for making re-use highly unlikely. Merging two
ULA networks, as I already mentioned, or VPNs interconnecting two ULA
networks. We were told that both of those were real, active operational
problems with RFC1918 addresses.

> OTOH, GUAs are globally-meaningful -- and while you can indeed configure 
> any GUA prefix you want, you will not get the return traffic (which you 
> *would* if you really owned the prefix).

Right. In that sense a ULA prefix really is a GUA subset - it should never
be advertised in transit networks or the mythical DFZ.

Regards
    Brian

[1] https://tools.ietf.org/html/rfc3879