[dhcwg] PD lifetimes

[JINMEI Tatuya / 神明達哉 <[e-mail]>]

I have several comments about lifetimes of prefix delegation (PD).  In
this message, I'm talking about
draft-ietf-dhc-dhcpv6-opt-prefix-delegation-01.txt.

0. (I once discussed this point privately, but I could not help
   raising this again.  Please forgive me for this repetition.)

I cannot understand the reason why the prefix has the notion of
the preferred lifetime, whose role is not clear.  The role of the
valid lifetime is very clear; the period that the delegated site can
use the prefix.  However, the preferred lifetime only controls a loose
relationship with the preferred lifetime in router advertisements
within the site, and roughly controls the T1/T2 values.

It would be much simpler to have a single "lifetime" only, which
controls T1 and T2, and loosely affects the RA lifetimes.

In the following comments, however, I'll assume the current valid +
preferred scheme.

1. Section 9 says

     In a message sent by a delegating router the preferred and valid
     lifetimes should be set to the values specified in section "Router
     Configuration Variables" of RFC2461 [3], unless administratively
     configured.

Technically, the wording 'values specified in section "Router
Configuration Variables"' is not clear, because the router
configuration variables of RFC 2461 do not contain the valid and
preferred lifetimes.  The intended variables should probably be
AdvValidLifetime and AdvPreferredLifetime, respectively.  Even so,
however, I still suspect these are appropriate values.
AdvXXXLifetimes are for each end host (in a site), but the lifetimes
given by PD affect the entire site.  In general (and IMO), the latter
should be larger than the former.

Today, on typical IPv6 links, we see fixed values of the valid and
preferred lifetimes in Router Advertisements, i.e., AdvValidLifetime
and AdvPreferredLifetime.  However, according to the default values of
the PD lifetimes and to the specified relationship between the PD
lifetimes and RA lifetimes described in the last paragraph of Section
11.1, it is highly possible to see smaller RA lifetimes than the
current typical cases.  Moreover, if the requesting router simply uses
the PD valid (or preferred) lifetime for the RA valid (or preferred)
lifetime, we'll see the RA lifetime being decremented.

I believe the default values of PD lifetimes should keep the default
values of RA lifetimes in the typical configuration.  Thus, I would
recommend the following values:

The default value for the PD preferred lifetime is
(5 / 4) * AdvPreferredLifetime.
The default value for the PD valid lifetime can be an arbitrary value,
but should not be smaller than AdvValidLifetime + PD preferred lifetime.

The rationale of the default is that we won't see "abnormal" RA
lifetimes unless the requesting router fails renew/reply exchanges and
starts rebinding, assuming that "T2" for the prefix is (equal to or
smaller than) the PD preferred lifetime * 0.8.

2. Section 9 also says

   In a message sent by a delegating router to a requesting router, the
   requesting router MUST use the value in the valid lifetime field and
   MAY use the value in the preferred lifetime field.

This sentence is not clear to me.  Where and how does the requesting
router "use" the lifetimes?  This ambiguity also makes the term "MUST"
and "MAY" unclear.  Perhaps the sentence intends to specify the
relationship between RA lifetimes and PD lifetimes described in
Section 11.1, but I cannot be sure without any reference.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[e-mail]
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