Internet DRAFT - draft-ietf-v6ops-cpe-slaac-renum
draft-ietf-v6ops-cpe-slaac-renum
IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft SI6 Networks
Updates: 7084 (if approved) J. Zorz
Intended status: Best Current Practice 6connect
Expires: November 28, 2021 R. Patterson
Sky UK
B. Volz
Cisco
May 27, 2021
Improving the Reaction of Customer Edge Routers to IPv6 Renumbering
Events
draft-ietf-v6ops-cpe-slaac-renum-08
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 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 November 28, 2021.
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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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . . . 2
3. Improved Customer Edge Router Behavior . . . . . . . . . . . 3
3.1. Automatic DHCPv6 RELEASEs . . . . . . . . . . . . . . . . 4
3.2. Stability of IAIDs . . . . . . . . . . . . . . . . . . . 4
3.3. Interface Between WAN-side and LAN-side . . . . . . . . . 4
3.4. LAN-side Option Lifetimes . . . . . . . . . . . . . . . . 5
3.5. Signaling Stale Configuration Information . . . . . . . . 7
4. Recommended Option Lifetimes Configuration Values . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
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 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 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.
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
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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 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 prefix-delegation requirements to
those specified in [RFC7084]:
o WPD-9: CE routers SHOULD NOT automatically send DHCPv6-PD RELEASE
messages upon reboot events. See Section 3.1 for further details.
o WPD-10: CE Routers MUST by default use a WAN-side 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:
o 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]:
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 3.3.
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 3.4.
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3.1. Automatic DHCPv6 RELEASEs
Some CE Routers are known to automatically send DHCPv6-PD RELEASE
messages upon reboot 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
reboot 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. everytime the underlying PPP
session is established. 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 failing 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 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 "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 leased 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:
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* 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."
* 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 to ND_PREFERRED_LIMIT.
CE Routers SHOULD also set the PIO Preferred Lifetime to the lesser
of the remaining preferred lifetime (see Section 3.3) 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.
NOTES: In scenarios where the valid-lifetime and the preferred-
lifetime of the prefix leased 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.
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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 DHCPv6 IA Address Option preferred-lifetime to the lesser of
the remaining preferred lifetime (see Section 3.3) 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 3.3) 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:
+ 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 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) WAN-side
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, 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 Option employed by
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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:
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 via PIOs 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 by the CE Router
via PIOs in RA messages, but that have now become stale, MUST
be advertised with a PIO that has the "Valid Lifetime" and the
"Preferred Lifetime" set to 0, and the "A" and "L" bits
unchanged.
* The aforementioned advertisement MUST be performed for at least
the "Valid Lifetime" previously employed for such prefix. The
CE Router MUST advertise this information with unsolicited
Router Advertisements as described in Section 6.2.4 of
[RFC4861], and MAY advertise this information via unicast
Router Advertisements when possible and applicable.
+ Note: If requirement L-16 (Section 3.4) is followed, the
Valid Lifetime need not be saved and the stale 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 3.4 are employed.
When a CE Router provides LAN-side DHCPv6 (address assignment or
prefix delegation), then:
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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, and Rebind messages, send
IA Address Options or IA Prefix Options (as appropriate) for
any address assignments or prefix delegations for the
deprecated 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 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 [RFC8978]. One such scenario is when a DHCPv6
server employs dynamic prefixes and the Customer Edge Router
crashes and reboots. The requirements in 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], and Section 6.3 of [RFC8415].
* 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.
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* The requirement in this section is conveyed as a "SHOULD" (as
opposed to a "MUST"), since 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.
* 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
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 [RFC8978].
ND_VALID_LIMIT is set to 2 * ND_PREFERRED_LIMIT to provide some
additional leeway before configuration information is finally
discarded by the host.
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
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security issues, and thus the same security considerations as for
[RFC4861], [RFC4862], [RFC7084], and [RFC8415] apply.
7. 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,
Tim Chown, Brian Carpenter, 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, Richard Patterson, Pete Resnick, Michael
Richardson, Mark Smith, Job Snijders, Sander Steffann, Tarko Tikan,
Ole Troan, Loganaden Velvindron, Eric Vyncke, Robert Wilton, Timothy
Winters, Christopher Wood, and Chongfeng Xie, for providing valuable
comments on earlier 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.
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>.
[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>.
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[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>.
8.2. Informative References
[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-02 (work in progress), January 2021.
[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., Zorz, 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>.
Authors' Addresses
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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@6connect.com
Richard Patterson
Sky UK
Email: richard.patterson@sky.uk
Bernie Volz
Cisco Systems, Inc.
300 Beaver Brook Rd
Boxborough, MA 01719
USA
Email: volz@cisco.com
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