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

Philip Homburg <> Wed, 23 October 2019 15:08 UTC

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Cc: Fernando Gont <>
From: Philip Homburg <>
In-reply-to: Your message of "Wed, 23 Oct 2019 03:51:32 -0500 ." <>
Date: Wed, 23 Oct 2019 17:08:15 +0200
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Subject: Re: [v6ops] SLAAC renum: Problem Statement & Operational workarounds
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>The document is available at:

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. Mistakenly setting the preferred lifetime of an address to zero 
may cause problems for new flows but has no impact on existing flows

In contrast setting the valid lifetime to zero immediately signals an error
condition to existing flows. This could cause a lot of trouble for long 
lived flows.

The other side of the coin is that if a host ends up with stale prefixes,
then the user maybe willing to wait for a few minutes for the situation to
resolve itself, but probably not much longer.

So the suggestion that
	AdvPreferredLifetime: AdvDefaultLifetime
is likely not sufficient to be responsive to a user. And at the same time
	AdvValidLifetime: 1.5 * AdvDefaultLifetime
is very risky if AdvDefaultLifetime is low.

Note that RFC 4861 allows MaxRtrAdvInterval to be a low as 4 seconds, 
AdvDefaultLifetime can be as low as MaxRtrAdvInterval, so also 4 seconds.

This creates a very fragile system.

At the same time, AdvDefaultLifetime is commonly 1800 seconds. Which is way
too long to wait for stale information to expire.

I agree that valid and preferred lifetimes of resp. 30 days and 7 days 
are excessive. But I'm not at all convinced that tying them to 
AdvDefaultLifetime will have a positive effect. I'm also not convinced
that there are sensible values for these lifetimes that are robust enough 
and at the same time responsive enough to deal with the issue at hand.

Reducing the lifetimes would help against build up of stale prefixes. If
they are set to a couple of hours.