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

James Woodyatt <jhw@nestlabs.com> Fri, 17 October 2014 22:16 UTC

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Date: Fri, 17 Oct 2014 15:16:44 -0700
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From: James Woodyatt <jhw@nestlabs.com>
To: HOMENET Working Group <homenet@ietf.org>
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Subject: Re: [homenet] Let's make in-home ULA presence a MUST !?
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On Tue, Oct 14, 2014 at 3:28 PM, Ted Lemon <mellon@fugue.com> wrote:

> On Oct 14, 2014, at 5:14 PM, James Woodyatt <jhw@nestlabs.com> wrote:
> > But there is a problem with only deprecating prefixes without expiring
> them. If they never expire, then they accumulate without limit within
> existing networks as they join with newly commissioned networks over the
> course of their lifetimes.
>
> Ah, sorry, I didn't mean to say that we deprecate them but don't ever get
> rid of them.   I think once a deprecated ULA has expired, it should be
> gc'd.   If the homenet is partitioned, the two options are for the
> partitions to continue using one ULA and try to keep prefixes stable, in
> anticipation of the partition being healed later, or for both partitions to
> switch to new ULAs, or for one homenet router to "own" the ULA and get to
> keep it for use in whichever partition it winds up in, while the other
> partition has to choose a new ULA.
>
> Personally I think keeping the ULA stable across partitions is preferable,
> but I'm not sure it's possible to do it without the risk of flash
> renumbering.
>
> > So what's the problem? My language above ensures that home network hosts
> always have at least one gracefully renumbered IPv6 address routable
> throughout the entire network. If we need a further guarantee that hosts
> always have an *invariant* address— which is an objective you've said above
> that you think we don't actually have— only then are we faced with the
> problem of prefix accumulation through network joins, which is a problem
> I'm not sure we know how to solve effectively. My proposal avoids that
> trouble.
>
> I understood your language to be trying to get rid of all ULAs if any GUAs
> are present.   Did I misunderstand?
>

Mr. Lemon, this is the only message in this thread where I can find you
saying anything about the expiration of locally generate ULA prefixes.

p1. It looks like you agree that locally generate ULA prefixes should be
allowed to expire. What I don't see is any conceptual outline for deciding,
in a distributed methodology, which prefixes to renew and which to release
when their valid lifetime expires. Without seeing that, I can't agree that
you've proposed anything that solves the problem I keep yammering about,
much less offered a better solution than the one I proposed earlier in the
thread.

p2. I also remain confused about the reasoning behind calling for a
persistent locally-generated ULA prefix. In a previous message you said
that it's okay for locally-generated ULA prefixes to expire, because there
is no need for hosts on home networks to have any guarantee that at least
one of its interface addresses is invariant over time, just that at least
one of their addresses is never flash renumbered when a delegated prefix
changes. As Lorenzo has demonstrated earlier today, this quality of never
being flash-renumbered is easily met by delegated ULA and ordinary GUA
prefixes.

Returning to my question: why do we always need a locally-generated ULA
prefix?  If it's to provide a time-invariant locally routable address to
hosts, then locally generated ULA prefixes cannot ever be permitted to
expire for any reason.  If they are ever allowed to expire, then they don't
provide the time-invariant property.  However, if we don't actually need
the time-invariant property, then what does a locally-generated ULA prefix
do for us whenever one or more delegated prefixes is also present? It's not
clear to me they are anything but absolutely redundant and unnecessary in
that situation.

-- 
james woodyatt <jhw@nestlabs.com>
Nest Labs, Communications Engineering