Re: [v6ops] SLAAC renum: Problem Statement & Operational workarounds

Fernando Gont <fgont@si6networks.com> Thu, 24 October 2019 11:28 UTC

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To: Mark Smith <markzzzsmith@gmail.com>
Cc: Philip Homburg <pch-v6ops-9@u-1.phicoh.com>, v6ops list <v6ops@ietf.org>
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From: Fernando Gont <fgont@si6networks.com>
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Date: Thu, 24 Oct 2019 08:22:52 -0300
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Subject: Re: [v6ops] SLAAC renum: Problem Statement & Operational workarounds
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On 24/10/19 03:17, Mark Smith wrote:
> 
> 
> On Thu, 24 Oct 2019, 03:08 Fernando Gont, <fgont@si6networks.com
> <mailto:fgont@si6networks.com>> wrote:
> 
>     On 23/10/19 10:08, Philip Homburg wrote:
>     >> The document is available at:
>     >> https://tools.ietf.org/html/draft-gont-v6ops-slaac-renum
>     >
>     > I think this is an important subject that is worth fixing.
>     >
>     > In this reply I'll focus on the preferred and valid lifetimes.
>     >
>     > On a host, the preferred and valid lifetimes play quite different
>     roles.
>     >
>     > The preferred lifetime plays a role in setting up new flows. I.e.
>     if the
>     > application does not provide a source address then the IPv6 layer uses
>     > the preferred lifetime to select the best source address among
>     multiple
>     > candidates. 
> 
> 
> 
> Actually it doesn't, according to SA selection in RFC 6724.>
> It's easy to think that, which is why I have in the past.

Using the preferred lifetime for SAS makes the source address flap if
there are multiple prefixes on the local network. This is not nice for
troubleshooting.



> I've even thought about proposing it, however when you think about it,
> you realise there are cases where it wouldn't work. The largest
> preferred lifetime address isn't always the best SA to use.

Indeed. And while there are recommended defaults, multiple routers might
be using different preferred lifetimes for the prefixes they advertise.



> I thought about this in the exact context of the scenario of this draft.
> Although the design of the broadband deployment I worked on always
> planned to give out stable prefixes, I worked through this scenario and
> realised that so many complex issues disappear if you provide stable
> prefixes.

FWIW, we're not against stable prefixes. We recognize there are
scenarios in which they will not be stable, and we want the network to
be robust in such scenarios.

Thanks!

Cheers,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492