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

Ted Lemon <> Sat, 13 February 2021 05:47 UTC

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From: Ted Lemon <>
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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)
Date: Sat, 13 Feb 2021 00:46:54 -0500
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Cc: Brian E Carpenter <>, Michael Richardson <>, IPv6 Operations <>, "" <>
To: Fernando Gont <>
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Okay. Consider the clarifications I suggested a while back:

ULA addresses are, in principle, VALID in any scope.

They are not, in principle, UNIQUE to a particular link: it’s entirely possible to have two instances of the same ULA referring to different interfaces connected to different links.

In principle, the set of all networks which can route a packet to a particular instance of a ULA /48 MUST be DISJOINT from the set of all networks which can route a packet to some other instance of that ULA /48.

In practice, the randomness of ULAs gives us some reasonable assurance that the principle will hold.

However, users of ULAs that are routed beyond an individual site had better have some policies and procedures in place to make sure that this is true.

Internet backbone routers should never accept BGP advertisements for ULA prefixes.

Sites connecting to the Internet should never, by default, route ULAs northbound of their connection to their ISP.

The last four lines are points of practice, not points of definition of terms.

But the bottom line is that if the term “global” is confusing as it applies to ULAs, it shouldn’t be that hard to clarify what we mean by global.

Do you think anything I’ve said here is wrong, not in the sense of contradicting RFC 4007, but in the sense that it is incorrect?

Is there anything missing?

I’m not trying to win an argument here—the reason I wrote the above is that I think it’s correct, and I was trying to figure out whether it was in any way consistent with the problem you have.

I think Brian has said that the “scoped architecture” is just not how things actually work in real life, with which I agree, so the fact that you can’t explain it to anyone is not a big shock. I think RFC 4007 says some interesting and useful things. It might be worthwhile to write a new document that’s a sort of Talmudic commentary on RFC 4007.

What I do not want to see is some kind of effort to rationalize ULAs into something other than what they are at present, which is quite useful.