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

Fernando Gont <fgont@si6networks.com> Tue, 26 February 2019 17:35 UTC

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To: 神明達哉 <jinmei@wide.ad.jp>
Cc: Ole Troan <otroan@employees.org>, IPv6 Operations <v6ops@ietf.org>, "6man@ietf.org" <6man@ietf.org>, Fernando Gont <fernando@gont.com.ar>
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From: Fernando Gont <fgont@si6networks.com>
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Date: Tue, 26 Feb 2019 14:35:20 -0300
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Subject: Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios
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Hello, Jinmei,

On 26/2/19 14:16, 神明達哉 wrote:
> At Tue, 26 Feb 2019 07:43:00 -0300,
> Fernando Gont <fgont@si6networks.com <mailto:fgont@si6networks.com>> wrote:
> 
>> > In the DHCP PD case they should also be counted down in real time.
>> > And the lifetimes can be dynamically adjusted.
>>
>> They should. However, quite frequently the DHCPv6-PD part is a different
>> piece of software from the ra{d,dvd} part. Both pieces are glued by some
>> script, and the lifetime is *not* dynamically adjusted.
> 
> It's a clear violation of Section 18.2.10.1 of RFC8415:
> 
>       If the client uses a delegated prefix to configure addresses on
>       interfaces on itself or other nodes behind it, the preferred and
>       valid lifetimes of those addresses MUST be no longer than the
>       remaining preferred and valid lifetimes, respectively, for the
>       delegated prefix at any time.  In particular, if the delegated
>       prefix or a prefix derived from it is advertised for stateless
>       address autoconfiguration [RFC4862], the advertised preferred and
>       valid lifetimes MUST NOT exceed the corresponding remaining
>       lifetimes of the delegated prefix.
> 
> while we cannot completely ignore non-compliant implementations (if
> any) in practice, IMO the existence of violators shouldn't be the
> primary reason for tweaking the protocol.

Which specific tweak are you referring to?



> Besides, I'm actually not so sure if it's so difficult to comply with
> today's tools.  RFC4861 has the concept of dynamically decrementing
> the advertised lifetimes as a protocol parameter (with a MAY):
> 
>                         AdvValidLifetime
> [...]
>                                - a time that decrements in real time,
>                                  that is, one that will result in a
>                                  Lifetime of zero at the specified time
>                                  in the future, or
> [...]
>                         AdvPreferredLifetime
> [...]
>                                - a time that decrements in real time,
>                                  that is, one that will result in a
>                                  Lifetime of zero at a specified time in
>                                  the future, or
> 
> and I suspect that non-negligible number of implementations support
> this concept.  BSD's rtadvd does, apparently so does Linux radvd.
> For these implementations, it's just a matter of configuration, even
> if the DHCP-PD client and RA advertisement are implemented as
> different pieces of software: a "script" takes the lifetimes from
> DHCPv6 PD and dumps the RA configuration by setting AdvXXXLifetimes to
> that values that decrement in real time.

I haven't been following developments in this area... but I do remember
 scripts to put the two pieces together, and others doing the same.



> I don't necessarily oppose to the idea of revising the default prefix
> lifetimes currently specified in RFC4861.  The growing deployment of
> mobile environments may justify an update, for example.  But the
> discussion should be based on accurate understanding of what's
> possible with the current specification and actual implementations,
> rather than speculative "red herring".

Tee only thing that I mentioned is that the CPE should be prepared to
"deprecate" a prefix for "Valid Lifetime". -- a local ISP recently
reported that they employ DHCPv6-PD times of over a month. IN those
cases, the spec you reference is complied with, and what I mention above
is the case: -- if there-s a carsh-and-reboot scenario, the CPE should
(in theory) be prepared to deprecate a prefix for "Valid LIfetime". --
whether in practice you can get away with firing a few RAs is a
different thing.


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