Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios

Fernando Gont <fgont@si6networks.com> Sun, 03 February 2019 09:43 UTC

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To: Philip Homburg <pch-ipv6-ietf-6@u-1.phicoh.com>, ipv6@ietf.org
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From: Fernando Gont <fgont@si6networks.com>
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Date: Sun, 03 Feb 2019 01:26:58 -0300
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Subject: Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios
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Hello, Philip,

On 2/2/19 08:32, Philip Homburg wrote:
>> FWIW, our first initial version only changed the "Preferred Lifetime",
>> and never modified the "Valid Lifetime". Then we realized it was easy to
>> update the "Valid Lifetime" with the same "mechanism" and incorporated
>> that. THe most important bit is, indeed, updating the "Preferred Lifetime".
>>
>> Question:
>> How about making the update to the "Valid Lifetime" a SHOULD or MAY,
>> instead?  Or, maybe, at least, set it to the current "Valid Lifetime"?
>> Otherwise, with the default values for the "Valid Lifetime", the
>> addresses might lie around for 1 month....
> 
> In think it is better to solve two separate problems independently and only
> merge the solutions if the same solution happens to apply to both.
> 
> When router reboots and switches to a new prefix, hosts should switch to the
> new prefix as soon as possible. 
> 
> On the other hand, removing addresses that have preferred lifetime of zero
> is a bookkeeping task that can take more time.

The main concern is certainly "Preferred" addresses. However, the issue
of lagging addresses is related.



> One question is whether it makes sense for routers to have valid lifetimes of
> more than a day for prefixes that are obtained using DHCP-PD.

In principle, I guess that as long as you cap the "Valid" and
"Preferred" lifetimes when you announce such prefixes on the LAN side
according to the lease time, you are kind of "okish".

Anyway: do you suggest that we split the "book-keeping stuff" (i.e.,
affecting the Valid Lifetime) into a separate section?



> Another is whether general purpose hosts should accept lifetimes of more
> than a day. Maybe hosts should just truncate.
> 
> If that doesn't solve the issue, then for the valid lifetime it make sense for
> the host to wait multiple RA intervals to make sure the router really doesn't
> advertise the prefix.

What we do in the I-D is equivalent, so to speak: if you do hear RAs
from the router, but the prefix is not advertised, and a different
prefix is, then that's a clear indication that the router has ceased to
advertise the original prefix.

Just waiting for RA intervals (without checking anything else) might
mean the RAs were simply lost, whereas receiving RAs with PIOs that
don't include the previous prefixes is an indication that RAs are
getting through, but the previous prefixes are not advertised anymore...




>> The mechanism in the document tries to accommodate the case where the
>> information conveyed in an RA is split among two RA messages -- so it's
>> the second RA with the PIO that causes the update, rather than the first
>> one. In all current practical cases, all info is normally conveyed into
>> a single RA. However, the mechanism that's currently in the I-D allows
>> for cases where it can be split in two.
> 
> What happens if a router would advertise 4 prefixes in 4 separate RAs?

The addresses would oscillate between Preferred <-> non-preferred:

    Router                                           Host
          RA, PIO={2001:DB8:1::/64, L=1, A=1}
        -------------------------------------->
                                                [Host configures addrs
                                                  for this prefix]

                           .
                           .


          RA, PIO={2001:DB8:2::/64, L=1, A=1}
        -------------------------------------->
                                                  deprecate=TRUE

          RA, PIO={2001:DB8:3::/64, L=1, A=1}
        -------------------------------------->
                                                  Pref. Lftime=0
                                                  deprecate=FALSE

          RA, PIO={2001:DB8:4::/64, L=1, A=1}
        -------------------------------------->
                                                  deprecate=TRUE

          RA, PIO={2001:DB8:1::/64, L=1, A=1}
        -------------------------------------->
                                                  Valid and Preferred
                                                  timers are reset
                                                  deprecate=FALSE



The address would only get removed if you advertise five different
prefixes over five different RAs. I haven't seen any router
implementation that is so broken, though -- All I've encountered follow
the practice of "put as much info into each RA as possible".

That said, I wouldn't mind providing alternative mechanisms for
un-preferring and removing the addresses that involve timers. We went
ahead with a mechanism like the one we documented because it makes sense
engineering-wise: it's simple, encompasses realistic scenarios, and
avoid timers.

Thanks!

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