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

Fernando Gont <> Mon, 15 February 2021 05:38 UTC

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To: "Manfredi (US), Albert E" <>
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From: Fernando Gont <>
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Date: Mon, 15 Feb 2021 02:03:17 -0300
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Subject: Re: [v6ops] [EXTERNAL] Re: 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 14/2/21 19:25, Manfredi (US), Albert E wrote:
> -----Original Message-----
> From: ipv6 <> On Behalf Of Fernando Gont
>> ULAs can't be global.
> I think we've been going around and around on this.
> Maybe it's good to compare the ULA situation with RFC 1918 private IPv4 addresses. In such a comparison, it is clear that RFC 1918 addresses "can't be global," and must be used only within some admin domain. But it is also clear that the intention, or hope, of ULAs, is that they be globally unique.

The intention is that, *given a subset of ULAs* the probability of 
collision is low.

> Yes, in practice, we know that there is a non-zero probability that ULAs won’t be globally unique, but at the same time, it is false to claim that ULAs "can’t be" globally unique. 

Please compute the birthday paradox assuming that each CPE in the world 
locally-generates a ULA prefix, and share the math with us. (that's what 
global scope means as per RFC4007)

Spoiler: P~1

> No one is telling us, "Go ahead and duplicate your ULAs, among admin domains, because they will be filtered out at border routers anyway."
> At most, I'd add somewhere a notion of "only guaranteed within an administrative domain," and be done with it. I just don’t see how that is so confusing for anyone.

As per RFC4007:

scope means: topological span where the address is unique.
global means: Internet-wide span

Then we have this notion of "ULAs are 'global scope'", in which "global" 
meaning something different than what "global scope" means as per 
RFC4007. -- Hence the incongruence.

e.g., if "global" means "an administrative domain", then that's not 
global -- unless you're also keen to argue that link-locals can be 
considered "global scope".

Fernando Gont
SI6 Networks
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