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

Ted Lemon <> Sun, 14 February 2021 20:39 UTC

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From: Ted Lemon <>
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Date: Sun, 14 Feb 2021 15:39:02 -0500
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Cc: Brian E Carpenter <>, David Farmer <>, Fred Baker <>, IPv6 Operations <>, "" <>
To: Fernando Gont <>
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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 Feb 14, 2021, at 3:27 PM, Fernando Gont <> wrote:
> But ULAs, and several other prefixes raised by David ar considered "global scope" by RFC4291. Isn't the inconsistency clear?

No. I think you’re getting confused between theory and practice.

In theory, a GUA is globally unique. But there’s no mechanism for ensuring that this is in fact the case. I could configure a GUA prefix on my local network that’s also in use somewhere else. The “architecture” does not guarantee that I have not done so. The mechanism for ensuring that GUAs are unique is delegation; this is a fairly solid mechanism, but what actually makes GUAs unique is the practicality that if you screw up the allocation and wind up with two GUAs identifying two different interfaces on two different links, you may experience communication difficulties.

In theory, a ULA is unique, because 40 bits of randomness is a lot, and the likelihood of a collision is small. There is no mechanism to enforce uniqueness other than the requirement that ULA prefixes be generated using a random number generator; uniqueness is assumed, just as it is with GUAs.

So in fact in terms of architecture there is no difference between ULAs and GUAs. They are both global in scope. It’s true that our system for ensuring the uniqueness of GUAs is less likely to produce a collision than our system for ensuring the uniqueness of ULAs, but the meaning of “global” is the same in both cases.

The architectural globalness of ULAs and GUAs does not provide any guarantee. Rather, it is a statement of intent: this is what we intend to be true about GUAs and ULAs.

The reason that RFC 6724 solves this for me is that it explains how, practically, to manage ULAs and GUAs so that the architectural intention of the appearance of global uniqueness is not violated.