Re: [homenet] Let's make in-home ULA presence a MUST !?

Brian E Carpenter <> Tue, 14 October 2014 19:44 UTC

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Date: Wed, 15 Oct 2014 08:44:21 +1300
From: Brian E Carpenter <>
Organization: University of Auckland
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To: Ted Lemon <>
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Cc: HOMENET Working Group <>, James Woodyatt <>
Subject: Re: [homenet] Let's make in-home ULA presence a MUST !?
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On 15/10/2014 08:31, Ted Lemon wrote:
> On Oct 14, 2014, at 2:19 PM, James Woodyatt <> wrote:
>> On the topic of the original question, if I were to editorialize here, then I would want to see something like this:
> I get that you have an opinion on this, but you haven't actually stated any argument to support what you think we should do.   And there are some implications in what you are saying that I don't think are necessary.
>> A) An autonomously generated ULA prefix SHOULD be advertised when no other delegated prefix is valid.
> OK, although underspecified.
>> B) Whenever there is any valid delegated prefix, advertisements for an existing autonomously generated ULA prefix MUST be deprecated, i.e. updated with preferred lifetime of zero.
> Why?   What problem does this solve?   Given that it's going to mean additional work, there should be some benefit to doing it.

At a stroke this would destroy the main advantage of ULAs - namely,
invariant addresses for internal traffic. IPv6 assumes multiple
simultaneous addresses; there is no reason whatever to artificially
prevent use of ULAs alongside GUAs.


>> C) A deprecated autonomously generated ULA prefix MUST be withdrawn when it expires, i.e. its valid time reaches zero.
> Okay, given that a prefix expires, it should be withdrawn, whether it's a ULA or a GUA.
>> D) Whenever there is no longer any valid delegated prefix, advertisements for a previously deprecated autonomously generated ULA prefix MUST be updated with non-zero preferred lifetime.
> OK, but seems like unnecessary work.   You're essentially recapitulating the brokenness of IPv4 zeroconf.
>> The idea here is to make sure IPv6 applications can generally rely on home network interior routers to forward traffic among the multiple links in the home, regardless of whether any first-mile Internet services are provisioned, configured and operational, i.e. there shall always be at least one preferred global scope network prefix, and there shall be an autonomously generated local prefix available as a last resort whenever there are no valid delegated prefixes.
> This is where I am just completely puzzled.   We talked about this previously.   I thought the idea was that the homenet ULA should converge: that there should only be one, ultimately, and that when there are two, routing should still work.  You are stating this as if the ULAs are per-subnet of a homenet, and that routing across homenet routers using ULAs isn't supported.
> If you really think that's how this should work, I can see why you want to deprecate them.   But that's not how they should work.
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