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

"STARK, BARBARA H" <bs7652@att.com> Mon, 25 February 2019 14:36 UTC

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From: "STARK, BARBARA H" <bs7652@att.com>
To: 'Richard Patterson' <richard@helix.net.nz>, Fernando Gont <fernando@gont.com.ar>
CC: IPv6 Operations <v6ops@ietf.org>, "6man@ietf.org" <6man@ietf.org>
Thread-Topic: [v6ops] A common problem with SLAAC in "renumbering" scenarios
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Date: Mon, 25 Feb 2019 12:42:18 +0000
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Subject: Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios
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> > * As per RFC4862, it turns out that you cannot remove a stale prefix
> > vy sending an RA wiht a "Prefix Lifetime" of 0. SO, with current
> > standards, not even the CPEs can (even if they wanted to) do something
> > to fix the problem.
> 
> 
> The Valid Lifetime cannot be zeroed or shortened below 2 hours, but the
> Preferred Lifetime can.  So we can't invalidate the prefix, but we
> can deprecate it so it's not used for new outbound sessions.   This is
> what we've implemented in our CPEs, after an unavoidable change in prefix,
> and it seems to have mitigated (or reduced the impact of) the issue.
> 
> RFC4862 §5.5.3
> 
> e)  If the advertised prefix is equal to the prefix of an address
>       configured by stateless autoconfiguration in the list, the
>       preferred lifetime of the address is reset to the Preferred
>       Lifetime in the received advertisement.
> 
> ....
> 
>       Note that the preferred lifetime of the corresponding address is
>       always reset to the Preferred Lifetime in the received Prefix
>       Information option, regardless of whether the valid lifetime is
>       also reset or ignored.  The difference comes from the fact that
>       the possible attack for the preferred lifetime is relatively
>       minor.  Additionally, it is even undesirable to ignore the
>       preferred lifetime when a valid administrator wants to deprecate a
>       particular address by sending a short preferred lifetime (and the
>       valid lifetime is ignored by accident).

Per RFC 7084:
   L-13:  If the delegated prefix changes, i.e., the current prefix is
          replaced with a new prefix without any overlapping time
          period, then the IPv6 CE router MUST immediately advertise the
          old prefix with a Preferred Lifetime of zero and a Valid
          Lifetime of either a) zero or b) the lower of the current
          Valid Lifetime and two hours (which must be decremented in
          real time) in a Router Advertisement message as described in
          Section 5.5.3, (e) of [RFC4862].

Barbara