Internet Engineering Task Force (IETF) F. Gont

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 9096 SI6 Networks
BCP: 234 J. Žorž
Updates: 7084 6connect
Category: Best Current Practice R. Patterson
ISSN: 2070-1721 Sky UK
B. Volz
Individual Contributor
August 2021

Improving the Reaction of Customer Edge Routers to IPv6 Renumbering
Events

Abstract

This document specifies improvements to Customer Edge routers that
help mitigate the problems that may arise when network configuration
information becomes invalid without any explicit signaling of that
condition to the local nodes. This document updates RFC 7084.

Status of This Memo

This memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9096.

Copyright Notice

Copyright (c) 2021 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
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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. Requirements Language
3. Improved Customer Edge Router Behavior
3.1. Automatic DHCPv6 RELEASEs
3.2. Stability of IAIDs
3.3. Interface between the WAN Side and LAN Side
3.4. LAN-Side Option Lifetimes
3.5. Signaling Stale Configuration Information
4. Recommended Option Lifetimes Configuration Values
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

In scenarios where network configuration information becomes invalid
without any explicit signaling of that condition (such as when a
Customer Edge (CE) router crashes and reboots without knowledge of
the previously employed configuration information), hosts 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 [RFC8978].

This document specifies improvements to CE routers that help mitigate
the aforementioned problem for residential and small office
scenarios. It specifies recommendations for the default behavior of
CE routers but 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 RFC 7084.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Improved Customer Edge Router Behavior

This section specifies and clarifies requirements for CE 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 CE 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 [6MAN-SLAAC-RENUM].

This document specifies additional WAN-side prefix-delegation (WPD)
requirements to those specified in [RFC7084]:

WPD-9: CE routers SHOULD NOT automatically send DHCPv6-PD RELEASE
messages upon restart events. See Section 3.1 for further
details.

WPD-10: CE routers MUST by default use a WAN-side Identity
Association IDentifier (IAID) value that is stable between CE
router restarts, DHCPv6 client restarts, or interface state
changes (e.g., transient PPP interfaces), unless the CE router
employs the IAID techniques discussed in Section 4.5 of [RFC7844].
See Section 3.2 for further details.

This document also replaces LAN-side requirement L-13 from [RFC7084]
with:

L-13: CE routers MUST signal stale configuration information as
specified in Section 3.5.

Finally, this document specifies the following additional LAN-side
requirements to those from [RFC7084]:

L-15: CE routers MUST NOT advertise prefixes via SLAAC or assign
addresses or delegate prefixes via DHCPv6 on the LAN side using
lifetimes that exceed the remaining lifetimes of the corresponding
prefixes learned on the WAN side via DHCPv6-PD. For more details,
see Section 3.3.

L-16: CE routers SHOULD advertise capped SLAAC option lifetimes,
capped DHCPv6 IA Address option lifetimes, and capped IA Prefix
option lifetimes, as specified in Section 3.4.

3.1. Automatic DHCPv6 RELEASEs

Some CE routers are known to automatically send DHCPv6-PD RELEASE
messages upon restart events. However, this may inadvertently
trigger a flash-renumbering scenario, along with the associated
problems discussed in [RFC8978], that this document attempts to
mitigate.

As a result, requirement WPD-9 from Section 3 specifies that CE
routers SHOULD NOT automatically send DHCPv6-PD RELEASE messages upon
restart events.

3.2. Stability of IAIDs

[RFC8415] requires that the IAID for an IA MUST be consistent across
restarts of the DHCP client. However, some popular CE routers are
known to select new random IAIDs, e.g., every time the underlying PPP
session is established or when the device is rebooted. This could be
the result of extrapolating the behavior described in [RFC7844] or
simply a consequence of not storing IAIDs on stable storage along
with failure to employ an algorithm that consistently generates the
same IAID upon reboots. Thus, requirement WPD-10 from Section 3
prevents CE routers from inadvertently triggering flash-renumbering
events on the local network.

3.3. Interface between the WAN Side and LAN Side

The "Preferred Lifetime" and "Valid Lifetime" of Prefix Information
Options (PIOs) [RFC4861] corresponding to prefixes learned via
DHCPv6-PD on the WAN side MUST NOT span past the remaining preferred
and valid lifetimes of the corresponding DHCPv6-PD prefixes. This
means that the "Preferred Lifetime" and the "Valid Lifetime"
advertised in PIOs by the CE router 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 learned via DHCPv6-PD on the
WAN side. This means that the "preferred-lifetime" and "valid-
lifetime" of DHCPv6 IA Address options and DHCPv6 IA Prefix options
employed with DHCPv6 on the LAN side MUST be dynamically adjusted
such that they 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.

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 of the corresponding prefixes (as learned via
DHCPv6-PD on the WAN side). This is in line with the requirement
from Section 6.3 of [RFC8415], which states:

| In particular, 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.

* 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.

3.4. LAN-Side Option Lifetimes

CE routers SHOULD override the default lifetime values of Neighbor
Discovery options that depend in any way on changes in the prefix
employed for address configuration on the LAN side, and employ
shorter lifetime values to improve the robustness to renumbering
events, while complying with the requirements from Section 3.3 of
this document and the recommendations in [RFC7772].

CE routers SHOULD set the "Router Lifetime" of Router Advertisement
(RA) messages to ND_PREFERRED_LIMIT.

CE routers SHOULD also set the PIO "Preferred Lifetime" to the lesser
of the remaining preferred lifetime of the corresponding prefix (see
Section 3.3) and ND_PREFERRED_LIMIT, and set the PIO "Valid Lifetime"
to the lesser of the remaining valid lifetime of the corresponding
prefix and ND_VALID_LIMIT. Additionally, the "Route Lifetime" of
Route Information Options (RIOs) [RFC4191], the "Lifetime" of
Recursive DNS Server (RDNSS) options [RFC8106], and the "Lifetime" of
DNS Search List (DNSSL) options [RFC8106] SHOULD be set to the lesser
of the longest remaining valid lifetime of a prefix (leased via
DHCPv6 on the WAN side) and ND_VALID_LIMIT, if any of these options
are included in Router Advertisement messages.

NOTE: In scenarios where the valid lifetime and the preferred
lifetime of prefixes learned via DHCPv6 on the WAN side are always
larger than ND_VALID_LIMIT and ND_PREFERRED_LIMIT, respectively,
the lifetime values advertised on the LAN side will not experience
actual changes.

The above text refers to the Neighbor Discovery options that are
typically employed by CE routers. A CE router may need to apply the
same policy for setting the lifetime of other Neighbor Discovery
options it employs, if and where applicable.

CE routers providing stateful address configuration via DHCPv6 SHOULD
set the "preferred-lifetime" of a DHCPv6 IA Address option to the
lesser of the remaining preferred lifetime of the corresponding
prefix (see Section 3.3) and ND_PREFERRED_LIMIT, and set the "valid-
lifetime" of the same option to the lesser of the remaining valid
lifetime of the corresponding prefix and ND_VALID_LIMIT.

CE routers providing DHCPv6-PD on the LAN side SHOULD set the
"preferred-lifetime" of a DHCPv6 IA Prefix option to the lesser of
the remaining preferred lifetime of the corresponding prefix (see
Section 3.3) and ND_PREFERRED_LIMIT, and set the "valid-lifetime" of
the same option to the lesser of the remaining valid lifetime of the
corresponding prefix and ND_VALID_LIMIT.

RATIONALE:

* The "Valid Lifetime" and "Preferred Lifetime" of PIOs have a
direct impact on three different aspects:

- The amount of time hosts may end up employing stale network
configuration information (see [RFC8978]).

- The amount of time CE routers need to persist trying to
deprecate stale network configuration information (e.g., to
handle cases where hosts miss Router Advertisement messages 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 time
span represented by the option lifetimes employed on the LAN
side.

* CE routers need not employ the (possibly long) WAN-side DHCPv6-PD
lifetimes for the "Valid Lifetime" and "Preferred Lifetime" of
PIOs sent in Router Advertisement messages to advertise sub-
prefixes of the leased prefix. Instead, CE 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) WAN-side
DHCPv6-PD lifetimes for the "valid-lifetime" and "preferred-
lifetime" of IA Address options and IA Prefix options 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.

3.5. Signaling Stale Configuration Information

When a CE router provides LAN-side address-configuration information
via SLAAC:

* A CE router sending RAs that advertise prefixes belonging to a
dynamically learned prefix (e.g., via DHCPv6-PD) SHOULD record, on
stable storage, the list of prefixes being advertised via PIOs on
each network segment and the state of the "A" and "L" flags of the
corresponding PIOs.

* 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 by the CE router
via PIOs in RA messages, but that have now become stale, MUST
be advertised with PIOs that have the "Valid Lifetime" and the
"Preferred Lifetime" set to 0 and the "A" and "L" bits
unchanged.

- The aforementioned advertisements MUST be performed for at
least the "Valid Lifetime" previously employed for such
prefixes. The CE router MUST advertise this information with
unsolicited Router Advertisement messages, as described in
Section 6.2.4 of [RFC4861], and MAY advertise this information
via unicast Router Advertisement messages when possible and
applicable.

NOTE: If requirement L-16 (Section 3) is followed, the
"Valid Lifetime" need not be saved, and the stale prefix can
simply be advertised for a period of ND_VALID_LIMIT.

* CE routers receiving DHCPv6 IA Prefix options 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 PIOs 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 3.4 are employed.

When a CE router provides LAN-side DHCPv6 (address assignment or
prefix delegation), then:

* The CE router SHOULD record, on stable storage, the DHCPv6 address
and delegated-prefix bindings corresponding to the LAN side.

* 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 IA Prefix
options with 0 lifetimes, the CE router MUST:

- In Replies to DHCPv6 Request, Renew, and Rebind messages, send
IA Address options or IA Prefix options (as appropriate) for
any address assignments or prefix delegations for the stale
prefixes. The aforementioned options MUST be sent with both
the "valid-lifetime" and the "preferred-lifetime" set to 0, for
at least the "valid-lifetime" originally employed for them, or
for a period of ND_VALID_LIMIT if the recommended lifetimes
from Section 3.4 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 stale 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 a prefix being employed
on a network suddenly becomes invalid and replaced by a new prefix
[RFC8978]. One such scenario is when an Internet Service Provider
(ISP) employs dynamic prefixes and the CE router crashes and
reboots. The requirements in this section are meant to allow CE
routers to deprecate stale information in such scenarios.

* The recommendations in this section expand from requirement L-13
in Section 4.3 of [RFC7084] and Section 6.3 of [RFC8415].

* Hosts 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 PIOs of the
last received Router Advertisement messages. Since Router
Advertisement messages may be lost or fail to be received for
various reasons, CE 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 requirements in this section to store information on stable
storage are conveyed as "SHOULD" (as opposed to "MUST"), since
they 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.

* The above text does not include DHCPv6 Advertise messages sent in
response to DHCPv6 Solicit messages, since Section 18.3.9 of
[RFC8415] requires that a DHCPv6 server that is not going to
assign an address or delegated prefix received as a hint in the
Solicit message MUST NOT include that address or delegated prefix
in the Advertise message. Additionally, any subsequent Request
messages will trigger the response specified in this section and
therefore cause the address or prefix to be deprecated.

4. Recommended Option Lifetimes Configuration Values

* ND_PREFERRED_LIMIT: 2700 seconds (45 minutes)

* 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 the "Router Lifetime", following the rationale from
Section 3.2 of [RFC8978].

* ND_VALID_LIMIT is set to 2 * ND_PREFERRED_LIMIT to provide some
additional leeway before configuration information is finally
discarded by the hosts.

5. IANA Considerations

This document has no IANA actions.

6. Security Considerations

This document discusses a problem that may arise, e.g., in scenarios
where dynamic IPv6 prefixes are employed, and it proposes
improvements to CE routers [RFC7084] to mitigate the problem for
residential or small office scenarios. It does not introduce new
security issues; thus, the same security considerations as for
[RFC4861], [RFC4862], [RFC7084], and [RFC8415] apply.

7. References

7.1. 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>.

[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>.

[RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC7844, May 2016,
<https://www.rfc-editor.org/info/rfc7844>.

[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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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>.

7.2. Informative References

[6MAN-SLAAC-RENUM]
Gont, F., Zorz, J., and R. Patterson, "Improving the
Robustness of Stateless Address Autoconfiguration (SLAAC)
to Flash Renumbering Events", Work in Progress, Internet-
Draft, draft-ietf-6man-slaac-renum-02, 19 January 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
slaac-renum-02>.

[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>.

[RFC8978] Gont, F., Žorž, J., and R. Patterson, "Reaction of IPv6
Stateless Address Autoconfiguration (SLAAC) to Flash-
Renumbering Events", RFC 8978, DOI 10.17487/RFC8978, March
2021, <https://www.rfc-editor.org/info/rfc8978>.

Acknowledgments

The authors would like to thank Owen DeLong, Philip Homburg, Erik
Kline, and Ted Lemon for their valuable help in improving this
document via successive detailed reviews.

The authors would like to thank Mikael Abrahamsson, Luis Balbinot,
Brian Carpenter, Tim Chown, Lorenzo Colitti, Alejandro D'Egidio, Gert
Doering, Fernando Frediani, Guillermo Gont, Steinar Haug, Nick
Hilliard, Lee Howard, Christian Huitema, Sheng Jiang, Benjamin Kaduk,
Suresh Krishnan, Warren Kumari, Albert Manfredi, Olorunloba Olopade,
Jordi Palet Martinez, Pete Resnick, Michael Richardson, Mark Smith,
Job Snijders, Sander Steffann, Tarko Tikan, Ole Trøan, Loganaden
Velvindron, Éric Vyncke, Robert Wilton, Timothy Winters, Christopher
Wood, and Chongfeng Xie for providing valuable comments on earlier
draft versions of this document.

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.

Authors' Addresses

Fernando Gont
SI6 Networks
Segurola y Habana 4310, 7mo Piso
Villa Devoto
Ciudad Autonoma de Buenos Aires
Argentina

Email: [email protected]
URI: https://www.si6networks.com

Jan Žorž
6connect

Email: [email protected]

Richard Patterson
Sky UK

Email: [email protected]

Bernie Volz
Individual Contributor
116 Hawkins Pond Road
Center Harbor, NH 03226
United States of America

Email: [email protected]