Re: Non routable IPv6 registry proposal

Fred Baker <> Thu, 11 March 2021 22:25 UTC

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From: Fred Baker <>
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Subject: Re: Non routable IPv6 registry proposal
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Date: Thu, 11 Mar 2021 14:25:47 -0800
Cc: Brian E Carpenter <>, Keith Moore <>, IETF-Discussion Discussion <>
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To: David Farmer <>
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I can tell you definitively that the DNS Root Servers use IPv6 addresses for anycast, but they are indistinguishable from unicast addresses in format. The anycast magic is in BGP.

Sent using a machine that autocorrects in interesting ways...

> On Mar 11, 2021, at 12:53 PM, David Farmer <> wrote:
>> 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.
> Thanks
> -- 
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> David Farmer     
> Networking & Telecommunication Services
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> University of Minnesota   
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