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

Fernando Gont <> Thu, 18 February 2021 23:10 UTC

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To: David Farmer <>, "Templin (US), Fred L" <>
Cc: IPv6 Operations <>, "" <>
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From: Fernando Gont <>
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Date: Thu, 18 Feb 2021 20:10:05 -0300
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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 18/2/21 19:52, David Farmer wrote:
> So let's be clear "IPv6 Address Scope" does not really have the general 
> meaning of the word "scope", it has a very specific meaning that in my 
> opinion boils down to are Zone Indexes or Identifiers needed to 
> uniquely identify them.

Well, this is a spec inconsistency. You have one spec (RFC4007) defining 
"scope" and "global scope", and another specs:

a) making use of the same terms in an incorrect way, or,

b) employing same terms but with a different definition.

i.e., either the definition in RFC4007 is incorrect, or the use in 
RFC4193 and implicit use in RFC4291 is incorrect.

As you've correctly noted, this applies to other address types, too.

> There is an argument to be made that ULAs should be Scoped, and if they 
> were globally reachable they would have to be Scoped.

Well, you can also argue that they are not globally reachable because 
they are actually scoped ;-)

> Nevertheless, they 
> are not globally reachable, and therefore so while it is possible that 
> there is another user of the same ULA prefix you are using. But, if you 
> follow the algorithm, in RFC4193 for allocation, the probability of that 
> duplicate user ever being reachable by you is very-very small.

This accounts for "why do things do not break in practice". But 
certainly doesn't help with the consistency of the specs.

Fernando Gont
SI6 Networks
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