[v6ops] cpe-slaac-renum: Proposed text for prefix lifetimes

[Fernando Gont <[e-mail]>]

Folks,

In this recent thread on cpe-slaac-renum, there was a proposal to add 
text to the draft, and I promised I would start a new thread just in 
case somebody missed the chance to review in the rather long thread.

This text is meant to reflect the comments from Philip, Lorenzo, and 
others (please do let me know if I misunderstood what you mean). The 
text is:

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The Valid Lifetime and Preferred Lifetime of PIOs have direct impact on 
two different aspects:

* The amount of time hosts may end up employing stale network 
configuration information (see [draft-ietf-v6ops-slaac-renum]).

* The amount of time Customer Edge routers would need to persist trying 
to deprecate stale network configuration information (to handle cases 
where nodes miss Router Advertisements and thus still consider the stale 
information as valid).

As a result, CPE Routers SHOULD override the default "Preferred 
Lifetime" and "Valid Lifetime" values from [RFC4861], and employ shorter 
values to mitigate the aforementioned effect, while complying with the 
requirement from Section 2.1 of this document, and the recommendations 
in RFC7772. This document RECOMMENDS that Customer Edge Router set the 
Router Lifetime and PIO Preferred Lifetime to 2700 seconds (45 minutes) 
and the PIO Valid Lifetime to 86400 seconds (1 day).

We note that a CPE Router need not employ the (possibly long) DHCPv6-PD 
lease times in the Valid Lifetime and Preferred Lifetime of PIOs sent in 
Router Advertisements messages to advertise sub-prefixes of the leased 
prefix. Instead, CPE Routers SHOULD use shorter values for the Valid 
Lifetime and Preferred Lifetime of PIOs, since subsequent Router 
Advertisement messages will nevertheless refresh the associated 
lifetimes, leading to the same effective lifetimes as specified by the 
DHCPv6-lease time.
--- cut here ----


The above change tackles to different issues:

* Saner lifetimes that at least allow for proper garbage collection in a 
timelier manner.

* In the typical scenario (but depending on DHCPv6 server parameters), 
the DHCPv6-PD leased will be longer than the PIO lifetimes advertised by 
the CE Router, and the CE Router will renew the prefix lease well before 
the PIO lifetimes need to be adjusted/reduced (to avoid having PIO 
lifetimes that span past the dhcpv6-pd lease). The result is that, in 
practice, the CE Routers would send RAs that always contain the same 
lifetime values, thus avoiding the battery consumption problem mentioned 
by Lorenzo.

Thoughts?

Thanks!

Cheers,
-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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