Re: A common problem with SLAAC in "renumbering" scenarios

[Jen Linkova <[e-mail]>]

On Tue, Feb 19, 2019 at 10:41 AM Fernando Gont <[e-mail]> wrote:
> > I might be biased here indeed but I think that using the "most
> > recently updated" (however we define that) address would be the
> > simpler solution than
> > deprecating the whole prefix (and both solution requires hosts to keep
> > the PIO <> advertising router mapping).
>
> Say the Stale prefix is 2001:Db8:1::/64, for which you configured
> 2001:db8:1::1.
>
> Say that Prefix(2001:Db8:1::/64) has now become stale, and a new prefix
> 2001:db8:2::/64 is being announced, for which you have 2001:db8:2::1.
>
> Say you now need to send packets to 2001:db8:1::/64.
>
> You will use the new addresses (from 2001:db8:2::/64), but will send the
> packets assuming nodes are "on-link", instead of sending them to the
> default router.

True. I'm not convinced that this use case is worth solving by
introducing the complexity (and impact on other scenarios)  the draft
is suggesting

What I'm not a big fun of is assuming that absence of evidence is
actually evidence of absence. When a router stops advertising a prefix
it does not necessary mean
that hosts shall stop using the addresses. It does indeed in the
scenario the draft describes but the host can not know the
topology/scenario.
The proposed host changes would solve  one particular case (DHCPv6-PD
router crash/reboot) while multiprefix/multihomed networks.

Lets say there are two routers (R1 and R2) on the LAN.
R1 advertised 2001:db8:1::/64 and 2001:db8:3::/64
R2 advertised 2001:db8::2::/64 and 2001:db8:3::/64.

Hosts have addresses from 3 prefixes.
Now R2 stops advertising 2001:db8:3::/64.

There is no reason for hosts to deprecate addresses in 2001:db8:3::/64.
What's going to happen is every time an RA from R1 is received, hosts
would get preferred addresses in 2001:db8:3::/64 and every RA from R2
would deprecate them.

I do believe that the default preferred lifetime is a bit longer that
it should be (and I have to tweak it all the time). So maybe we shall
change that.

> Similarly, if you have multiple interfaces, Rule 5 might override your
> rule 8.

Sorry, I must be very slow today. Could you please elaborate on this
scenario?  I'm having difficulties figuring it out..
The draft is talking about deprecating prefixes on *another*
interface? I hope not..

Actually I've just realized that I could not find any clear definition
of what 'the same router' means. The same address installed as
'next-hop'?
The same combination of IPv6 address and L2 address?

-- 
SY, Jen Linkova aka Furry