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Versions: (draft-gont-v6ops-cpe-slaac-renum)
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In: IESG_Evaluation
IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft SI6 Networks
Updates: 7084 (if approved) J. Zorz
Intended status: Informational 6connect
Expires: April 1, 2021 R. Patterson
Sky UK
B. Volz
Cisco
September 28, 2020
Improving the Reaction of Customer Edge Routers to Renumbering Events
draft-ietf-v6ops-cpe-slaac-renum-05
Abstract
In scenarios where network configuration information becomes invalid
without any explicit signaling of that condition (such as when a
Customer Edge Router crashes and reboots without knowledge of the
previously-employed configuration information), hosts on the local
network will continue using stale network configuration information
for an unacceptably long period of time, thus resulting in
connectivity problems. This document specifies improvements to
Customer Edge Routers that help mitigate the aforementioned problem
for typical residential and small office scenarios. This document
updates RFC7084.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 1, 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Improved Customer Edge Router Behavior . . . . . . . . . . . 3
3.1. Interface Between WAN-side and LAN-side . . . . . . . . . 4
3.2. LAN-side Option Lifetimes . . . . . . . . . . . . . . . . 5
3.3. Signaling Stale Configuration Information . . . . . . . . 6
4. Recommended Option Lifetimes Configuration Values . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In scenarios where network configuration information becomes invalid
without any explicit signaling of that condition, nodes on the local
network will continue using stale information for an unacceptably
long period of time, thus resulting in connectivity problems. This
problem is documented in detail in [I-D.ietf-v6ops-slaac-renum].
This document specifies improvements to Customer Edge (CE) Routers
that help mitigate the aforementioned problem for residential and
small office scenarios. It specifies recommendations for the default
behavior of CE Routers, and does not preclude the availability of
configuration knobs that might allow an operator or user to manually-
configure the CE Router to deviate from these recommendations. This
document updates RFC7084.
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2. Requirements Language
Take careful note: Unlike other IETF documents, the key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are not used as
described in [RFC2119]. This document uses these keywords not
strictly for the purpose of interoperability, but rather for the
purpose of establishing industry-common baseline functionality. As
such, the document points to several other specifications (preferable
in RFC or stable form) to provide additional guidance to implementers
regarding any protocol implementation required to produce a
successful CE router that interoperates successfully with a
particular subset of currently deploying and planned common IPv6
access networks.
Note: the aforementioned terms are used in exactly the same way as in
[RFC7084], with the above explanation copied verbatim from
Section 1.1 of [RFC7084].
3. Improved Customer Edge Router Behavior
This section specifies and clarifies requirements for Customer Edge
Routers that can help mitigate the problem discussed in Section 1,
particularly when they employ prefixes learned via DHCPv6-Prefix
Delegation (DHCPv6-PD) [RFC8415] on the WAN-side with Stateless
Address Autoconfiguration (SLAAC) [RFC4862] or DHCPv6 [RFC8415] on
the LAN-side. The recommendations in this document help improve
robustness at the Customer Edge Router (on which the user or ISP may
have no control), and do not preclude implementation of host-side
improvements such as those specified in [I-D.ietf-6man-slaac-renum].
This document specifies additional LAN-side requirements to
requirements L-1 through L-14 specified in [RFC7084]:
o L-15: CE routers MUST NOT advertise prefixes via SLAAC or assign
addresses or delegate prefixes via DHCPv6 on the LAN-side,
employing lifetimes that exceed the remaining lifetimes of the
corresponding prefixes learned from the WAN-side via DHCPv6-PD.
For more details, see Section 3.1.
o L-16: CE routers SHOULD advertise capped SLAAC option lifetimes
and capped DHCPv6 IA Address Option and IA Prefix Option
lifetimes, as specified in Section 3.2.
o L-17: CE routers MUST signal stale configuration information as
specified in Section 3.3.
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o L-18: CE routers SHOULD NOT automatically send DHCPv6-PD RELEASE
messages upon reboot events.
3.1. Interface Between WAN-side and LAN-side
The "Preferred Lifetime" and "Valid Lifetime" of Prefix Information
Options (PIOs) [RFC4861] corresponding to prefixes learned via
DHCPv6-PD MUST NOT span past the remaining preferred and valid
lifetimes of the corresponding DHCPv6-PD prefixes. This means that
the advertised "Preferred Lifetime" and "Valid Lifetime" MUST be
dynamically adjusted such that they never span past the remaining
preferred and valid lifetimes of the corresponding prefixes delegated
via DHCPv6-PD on the WAN-side.
Similarly, the "preferred-lifetime" and "valid-lifetime" of DHCPv6 IA
Address Options and DHCPv6 IA Prefix Options employed with DHCPv6 on
the LAN-side MUST NOT span past the remaining preferred and valid
lifetimes of the corresponding prefixes leased via DHCPv6-PD on the
WAN-side. This means that the advertised "Preferred Lifetime" and
"Valid Lifetime" MUST be dynamically adjusted such that the
advertised lifetimes never span past the remaining preferred and
valid lifetimes of the corresponding prefixes delegated to the CE
Router on the WAN-side via DHCPv6-PD.
CE Routers providing stateful address configuration via DHCPv6 SHOULD
set the DHCPv6 IA Address Option preferred-lifetime to the lesser of
the remaining preferred lifetime and ND_PREFERRED_LIMIT, and the
valid-lifetime of the same option to the lesser of the remaining
valid lifetime and ND_VALID_LIMIT.
CE Routers providing DHCPv6-PD on the LAN-side SHOULD set the DHCPv6
IA Prefix Option preferred-lifetime to the lesser of the remaining
preferred lifetime and ND_PREFERRED_LIMIT, and the valid-lifetime of
the same option to the lesser of the remaining valid lifetime and
ND_VALID_LIMIT.
RATIONALE:
* The lifetime values employed for the "Preferred Lifetime"
(AdvPreferredLifetime) and "Valid Lifetime" (AdvValidLifetime)
of SLAAC Prefix Information Options must never be larger than
the remaining lifetimes for the corresponding prefix (as
learned via DHCPv6-PD on the WAN-side). This is in line with
the requirement from Section 6.3 of [RFC8415], which states
that "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."
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* The lifetime values of prefixes advertised on the LAN-side via
SLAAC must be dynamically updated (rather than static values),
otherwise the advertised lifetimes would eventually span past
the DHCPv6-PD lifetimes.
* The same considerations apply for the valid-lifetime and
preferred-lifetime of IA Address Options and IA Prefix Options
employed with DHCPv6 on the LAN-side.
3.2. LAN-side Option Lifetimes
CE Routers SHOULD override the default PIO "Preferred Lifetime" and
"Valid Lifetime" values from [RFC4861], and employ shorter lifetime
values to improve the robustness to renumbering events, while
complying with the requirements from Section 2.1 of this document and
the recommendations in [RFC7772].
CE routers SHOULD set the Router Lifetime to ND_PREFERRED_LIMIT. CE
routers SHOULD also set the PIO Preferred Lifetime to the lesser of
the remaining preferred lifetime (see Section 3.1) and
ND_PREFERRED_LIMIT, and the PIO Valid Lifetime to the lesser of the
remaining valid lifetime and ND_VALID_LIMIT. Additionally, the Route
Lifetime of Route Information Options (RIOs) [RFC4191], the Lifetime
of Recursive DNS Search Options (RDNSSO) [RFC8106], and the Lifetime
of DNS Search List Options (DNSSLO) [RFC8106] SHOULD be set to the
lesser of the longest valid-lifetime in a DHCPv6 IA Prefix Option
(received via DHCPv6 on the WAN-side) and ND_VALID_LIMIT, if any of
these options are included in Router Advertisement messages.
CE Routers providing stateful address configuration via DHCPv6 SHOULD
set the DHCPv6 IA Address Option preferred-lifetime to the lesser of
the remaining preferred lifetime (see Section 3.1) and
ND_PREFERRED_LIMIT, and the valid-lifetime of the same option to the
lesser of the remaining valid lifetime and ND_VALID_LIMIT.
CE Routers providing DHCPv6-PD on the LAN-side SHOULD set the DHCPv6
IA Prefix Option preferred-lifetime to the lesser of the remaining
preferred lifetime (see Section 3.1) and ND_PREFERRED_LIMIT, and the
valid-lifetime of the same option to the lesser of the remaining
valid lifetime and ND_VALID_LIMIT.
RATIONALE:
* The Valid Lifetime and Preferred Lifetime of PIOs have a direct
impact on three different aspects:
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+ The amount of time hosts may end up employing stale network
configuration information (see
[I-D.ietf-v6ops-slaac-renum]).
+ The amount of time CE Routers need to persist trying to
deprecate stale network configuration information (e.g. to
handle cases where nodes miss Router Advertisements and thus
still consider the stale information as valid).
+ The amount of information that CE Routers need to maintain
when e.g. multiple crash-and-reboot events occur in the
timespan represented by the option lifetimes employed on the
LAN-side.
* CE Routers need not employ the (possibly long) DHCPv6-PD
lifetimes for 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 WAN-side DHCPv6-PD
lifetimes.
* Similarly, CE Routers need not employ the (possibly long)
DHCPv6-PD lifetimes for the valid-lifetime and preferred-
lifetime of IA Address Options and IA Prefix Option employed by
DHCPv6 on the LAN-side, since the renewal of bindings by DHCPv6
clients will lead to the same effective lifetimes as specified
by the WAN-side DHCPv6-PD lifetimes.
3.3. Signaling Stale Configuration Information
In order to phase-out stale SLAAC configuration information:
o A CE router sending RAs that advertise dynamically-learned
prefixes (e.g. via DHCPv6-PD) SHOULD record, on stable storage,
the list of prefixes being advertised on each network segment, and
the state of the "A" and "L" flags of the corresponding PIOs.
o Upon changes to the advertised prefixes, and after bootstrapping,
the CE Router advertising prefix information via SLAAC proceeds as
follows:
* Any prefixes that were previously advertised via Router
Advertisement (RA) messages, but that have now become stale,
MUST be advertised with a "Valid Lifetime" and a "Preferred
Lifetime" set to 0, and the "A" and "L" bits unchanged.
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* The aforementioned advertisement SHOULD be performed for at
least the "Valid Lifetime" previously employed for such prefix.
Note: If requirement L-16 (Section 3.2) is followed, the Valid
Lifetime need not be saved and the prefix can simply be
advertised for a period of ND_VALID_LIMIT.
o CE Routers receiving DHCPv6 Prefix Delegations with a 0 valid-
lifetime MUST advertise the corresponding sub-prefixes (as they
would be generated for the same leased prefix with a non-zero
lifetime) with a PIO with both the Preferred Lifetime and the
Valid Lifetime set to 0, for at least the WAN-side DHCPv6-PD
valid-lifetime, or for a period of ND_VALID_LIMIT if the
recommended lifetimes from Section 3.2 are employed.
If a CE Router provides LAN-side DHCPv6 (address assignment or prefix
delegation), then:
o The CE Router SHOULD record, on stable storage, the DHCPv6 address
and delegated-prefix bindings corresponding to the LAN-side.
o If the CE Router finds that the prefix to be employed for address
assignment and/or prefix delegation has changed (e.g., upon a
crash-and-reboot event) or the CE Router receives DHCPv6 Prefix
Delegations with 0 lifetimes, the CE Router MUST:
* In Replies to DHCPv6 Request, Renew, Rebind messages, send 0
lifetimes for any address assignments or prefix delegations for
the deprecated prefixes for at least the valid-lifetime
previously employed for them, or for a period of ND_VALID_LIMIT
if the recommended lifetimes from Section 3.2 are employed.
* Initiate sending Reconfigure messages (if possible - i.e.,
client requests Reconfigure support and the CE Router offers
it) to those clients with address assignments or prefix
delegations for the deprecated prefixes.
RATIONALE:
* IPv6 network renumbering is expected to take place in a planned
manner, with old/stale prefixes being phased-out via reduced
prefix lifetimes while new prefixes (with normal lifetimes) are
introduced. However, a number of scenarios may lead to the so-
called "flash-renumbering" events, where the prefix being
employed on a network suddenly becomes invalid and replaced by
a new prefix [I-D.ietf-v6ops-slaac-renum]. One such scenario
is when a DHCPv6 server employs dynamic prefixes and the
Customer Edge Router crashes and reboots. The requirements in
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this section are meant to allow Customer Edge Routers to
deprecate stale information in such scenarios.
* The recommendations in this section expand from requirement
L-13 in Section 4.3 of [RFC7084].
* Host configuring addresses via SLAAC on the local network may
employ addresses configured for the previously advertised
prefixes for at most the "Valid Lifetime" of the corresponding
PIO of the last received Router Advertisement message. Since
Router Advertisement messages may be lost or fail to be
received for various reasons, Customer Edge Routers need to try
to deprecate stale prefixes for a period of time equal to the
"Valid Lifetime" of the PIO employed when originally
advertising the prefix.
* The requirement in this section is conveyed as a "SHOULD" (as
opposed to a "MUST"), since we acknowledge that the requirement
to store information on stable storage may represent a
challenge for some implementations.
* Advertising DHCPv6-leased prefixes with zero lifetimes on the
LAN-side would handle the case where a CE Router has no stable
storage but receives the prefixes via DHCPv6 with 0 lifetimes.
4. Recommended Option Lifetimes Configuration Values
o ND_PREFERRED_LIMIT: 2700 seconds (45 minutes)
o ND_VALID_LIMIT: 5400 seconds (90 minutes)
RATIONALE:
These values represent a trade-off among a number of factors,
including responsiveness and possible impact on the battery life
of connected devices [RFC7772].
ND_PREFERRED_LIMIT is set according to the recommendations in
[RFC7772] for Router Lifetime, following the rationale from
Section 3.2 of [I-D.ietf-v6ops-slaac-renum].
ND_VALID_LIMIT is set to 2 * ND_PREFERRED_LIMIT to provide some
additional leeway before configuration information is finally
discarded by the host.
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5. IANA Considerations
This document has no actions for IANA.
6. Security Considerations
This document discusses a problem that may arise in scenarios where
dynamic IPv6 prefixes are employed, and proposes improvements to
Customer Edge Routers [RFC7084] to mitigate the problem for
residential or small office scenarios. It does not introduce new
security issues.
7. Acknowledgments
The authors would like to thank Owen DeLong, Philip Homburg, and Ted
Lemon, for their valuable help in improving this document via
successive detailed reviews.
The authors would like to thank Mikael Abrahamsson, Brian Carpenter,
Lorenzo Colitti, Alejandro D'Egidio, Fernando Frediani, Guillermo
Gont, Nick Hilliard, Erik Kline, Warren Kumari, Olorunloba Olopade,
Pete Resnick, Mark Smith, Job Snijders, Sander Steffann, Ole Troan,
Loganaden Velvindron, Timothy Winters, Christopher Wood, and
Chongfeng Xie, for providing valuable comments on earlier versions of
this document.
The authors would lie to thank Mikael Abrahamsson, Luis Balbinot, Tim
Chown, Brian Carpenter, Owen DeLong, Gert Doering, Steinar Haug, Nick
Hilliard, Philip Homburg, Lee Howard, Christian Huitema, Ted Lemon,
Albert Manfredi, Jordi Palet Martinez, Richard Patterson, Michael
Richardson, Mark Smith, Job Snijders, Tarko Tikan, and Ole Troan, for
providing valuable comments on [I-D.gont-6man-slaac-renum], on which
this document is based.
Fernando would also like to thank Brian Carpenter who, over the
years, has answered many questions and provided valuable comments
that have benefited his protocol-related work.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC7772] Yourtchenko, A. and L. Colitti, "Reducing Energy
Consumption of Router Advertisements", BCP 202, RFC 7772,
DOI 10.17487/RFC7772, February 2016,
<https://www.rfc-editor.org/info/rfc7772>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
8.2. Informative References
[I-D.gont-6man-slaac-renum]
Gont, F., Zorz, J., and R. Patterson, "Improving the
Robustness of Stateless Address Autoconfiguration (SLAAC)
to Flash Renumbering Events", draft-gont-6man-slaac-
renum-08 (work in progress), May 2020.
[I-D.ietf-6man-slaac-renum]
Gont, F., Zorz, J., and R. Patterson, "Improving the
Robustness of Stateless Address Autoconfiguration (SLAAC)
to Flash Renumbering Events", draft-ietf-6man-slaac-
renum-01 (work in progress), August 2020.
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[I-D.ietf-v6ops-slaac-renum]
Gont, F., Zorz, J., and R. Patterson, "Reaction of
Stateless Address Autoconfiguration (SLAAC) to Flash-
Renumbering Events", draft-ietf-v6ops-slaac-renum-03 (work
in progress), August 2020.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
Authors' Addresses
Fernando Gont
SI6 Networks
Segurola y Habana 4310, 7mo Piso
Villa Devoto, Ciudad Autonoma de Buenos Aires
Argentina
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Jan Zorz
6connect
Email: jan@connect.com
Richard Patterson
Sky UK
Email: richard.patterson@sky.uk
Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
USA
Email: volz@cisco.com
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