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List: ipng
Subject: draft-ietf-6man-slaac-renum: Router Configuration Variables
From: Fernando Gont <fgont () si6networks ! com>
Date: 2020-09-03 13:09:58
Message-ID: 2c9a9258-2a2a-b67f-5189-a034b1b67cab () si6networks ! com
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Folks,
We continue discussing each of the mitigation proposals that we had in
draft-gont-6man-slaac-renum.
The next one is about the default values to use by routers for the
Preferred and Valid Lifetime of the PIOs they advertise.
The text below proposes to set the PIO lifetimes at routers as a
function of the advertised Router Lifetime.
---- cut here ----
4.1.1. Router Configuration Variables
The default value for the "lifetime" parameters in PIOs is updated as
follows:
AdvPreferredLifetime: max(AdvDefaultLifetime, 3 *
MaxRtrAdvInterval)
AdvValidLifetime: 2 * max(AdvDefaultLifetime, 3 *
MaxRtrAdvInterval)
where:
AdvPreferredLifetime:
Value to be included in the "Preferred Lifetime" field of the PIO.
AdvValidLifetime:
Value to be included in the "Valid Lifetime" field of the PIO.
AdvDefaultLifetime:
Value of the "Router Lifetime" field of the Router Advertisement
message that will carry the PIO.
max():
A function that outputs the maximum of its arguments.
NOTE:
[RFC4861] specifies AdvDefaultLifetime as 3 * MaxRtrAdvInterval
(which defaults to 600 seconds). This means that, when employing
default values for MaxRtrAdvInterval, the Router Lifetime would be
set to AdvPreferredLifetime (1800 seconds). Thus, when employing
the default values, or when manually setting AdvDefaultLifetime to
a value smaller than 1800 seconds, AdvPreferredLifetime and
AdvValidLifetime would be set to 1800 seconds (30 minutes) and
3600 seconds (1 hour), respectively. We note that when
implementing BCP202 [RFC7772] AdvDefaultLifetime will typically be
in the range of 45-90 minutes, and therefore AdvPreferredLifetime
will be in the range 45-90 minutes, while AdvValidLifetime will be
in the range of 90-180 minutes.
RATIONALE:
* The default values for PIO lifetimes should be such that, under
normal circumstances (including some packet loss), the
associated timers are refreshed/reset, but in the presence of
network failures (such as network configuration information
becoming stale), some fault recovering action (such as
deprecating the corresponding addresses and subsequently
removing them) is triggered.
* In the context of [RFC8028], where it is clear that the use of
addresses configured for a given prefix is tied to the next-hop
router that advertised the prefix, the "Preferred Lifetime" of
a PIO should never be larger than the "Router Lifetime" of
Router Advertisement messages. Some leeway should be provided
for the "Valid Lifetime" to cope with transient network
problems. As a result, this document updates [RFC4861] such
that the default Valid Lifetime (AdvValidLifetime) and the
default Preferred Lifetime (AdvPreferredLifetime) of PIOs are
specified as a function of the default "Router Lifetime"
(AdvDefaultLifetime) of Router Advertisement messages. In the
absence of RAs that refresh information, addresses configured
for advertised prefixes become deprecated in a timelier manner,
and thus Rule 3 of [RFC6724] will cause other configured
addresses (if available) to be preferred.
* The expression above computes the maximum between
AdvDefaultLifetime and "3 * MaxRtrAdvInterval" (the default
value for AdvDefaultLifetime, as per [RFC4861]) to cope with
the case where an operator might simply want to disable one
local router for maintenance, without disabling the use of the
corresponding prefixes on the local network (e.g., on a multi-
router network). [RFC4862] implementations would otherwise
deprecate the corresponding prefixes. Similarly, [RFC8028]
would likely behave in the same way.
---- cut here ----
Note: I have tweaked the text we had in draft-gont-6man-slaac-renum-08
to reflect the discussion we had in v6ops, and to point to BCP202/RFC7772.
Please do let us know what you think about the text above.
Thanks!
Regards,
--
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
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