Re: Non routable IPv6 registry proposal

David Farmer <> Thu, 11 March 2021 20:52 UTC

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From: David Farmer <>
Date: Thu, 11 Mar 2021 14:51:43 -0600
Message-ID: <>
Subject: Re: Non routable IPv6 registry proposal
To: Brian E Carpenter <>
Cc: Keith Moore <>, IETF-Discussion Discussion <>
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On Thu, Mar 11, 2021 at 2:16 PM Brian E Carpenter <> wrote:

> David,
> On 12-Mar-21 08:19, David Farmer wrote:
> > On Thu, Mar 11, 2021 at 10:49 AM Keith Moore <
> <>> wrote:
> >
> >     On 3/11/21 5:22 AM, Nico Schottelius wrote:
> >
> >>>     Another question I have is whether such ULA allocations
> >>>     will realistically remain local.
> >>     ULAs are unlikely staying local, as we have seen with radio
> networks in
> >>     Germany. Tunnels are being used to interconnect remote cities and
> >>     non-collision (not necessarily public routing) are a primary
> concern.
> >
> >     Despite the name, there's no reason that ULAs should stay local.
> As long as they are properly chosen, it's perfectly reasonable to route
> them privately between cooperating networks, and IMO this is part of their
> design.   One of the problems with RFC 1918 addresses in IPv4 was that
> enterprises had a need to route traffic between networks each using that
> space.   The resulting address collisions generally required explicit NAT
> configurations to work around, and these were failure-prone and difficult
> to manage.  ULAs were intended in part to remedy this problem.
> >
> >     Keith
> >
> > The "L" for Local isn't intended to have a strict definition of Local.
> However, similarly, the "U" for Unique isn't intended to have a strict
> definition of Unique either, especially a mathematical definition of
> Unique.
> >
> > You can easily interconnect thousands or even tens of thousands of ULA
> prefixes without much chance of an address collision, as long as the random
> assignment process is actually used. Whereas, if you try to interconnect
> billions of ULA prefixes, you will probably start running into the birthday
> paradox.
> >
> > So the interconnection of ULA prefixes, the route-ability of them, is
> not intended to be unlimited. There are limits to the number of ULA
> prefixes that SHOULD be interconnected to each other; nevertheless, this
> limit is extremely generous for the intended use cases.
> >
> > If you disregard the intended use cases and use them outside the
> intended use cases, then address collisions could become an issue.
> I'm not sure where you get your "intended" from. I don't think we've ever
> really written done the intended use cases in such detail. (Except for the
> abandoned
> )
>     Brian

The first sentence of the Abstract for RFC4193 says;

This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications, usually inside of a site.

This is expanded upon in the first paragraph of the Introduction to RFC4193;

This document defines an IPv6 unicast address format that is
globally unique and is intended for local communications [IPV6]. ...
They are routable inside of a more limited area such as a site.  They may
also be routed between a limited set of sites.

Those sound a lot like intended use cases to me, the key phrases in that
for me are, "local communications", "usually ... a site", and "a limited
set of sites."

Yes, that's pretty vague, but I don't see a reasonable interpretation of
those phrases that include every site on the Internet, or even every site
in a country or state, maybe it could include every site in a small to
modest city, but even that's a bit of a stretch in my opinion.


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