IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft SI6 Networks
Intended status: Informational J. Zorz
Expires: May 6, 2021 6connect
R. Patterson
Sky UK
November 2, 2020
Reaction of Stateless Address Autoconfiguration (SLAAC) to Flash-
Renumbering Events
draft-ietf-v6ops-slaac-renum-05
Abstract
In scenarios where network configuration information related to IPv6
prefixes becomes invalid without any explicit and reliable signaling
of that condition (such as when a Customer Edge router crashes and
reboots without knowledge of the previously-employed prefixes), nodes
on the local network may continue using stale prefixes for an
unacceptably long time (on the order of several days), thus resulting
in connectivity problems. This document describes this issue and
discusses operational workarounds that may help to improve network
robustness. Additionally, it highlights areas where further work may
be needed.
Status of This Memo
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Analysis of the Problem . . . . . . . . . . . . . . . . . . . 5
2.1. Use of Dynamic Prefixes . . . . . . . . . . . . . . . . . 5
2.2. Default Timer Values in IPv6 Stateless Address
Autoconfiguration (SLAAC) . . . . . . . . . . . . . . . . 6
2.3. Recovering from Stale Network Configuration Information . 7
2.4. Lack of Explicit Signaling about Stale Information . . . 7
2.5. Interaction Between DHCPv6-PD and SLAAC . . . . . . . . . 8
3. Operational Mitigations . . . . . . . . . . . . . . . . . . . 8
3.1. Stable Prefixes . . . . . . . . . . . . . . . . . . . . . 8
3.2. SLAAC Parameter Tweaking . . . . . . . . . . . . . . . . 8
4. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
IPv6 Stateless address autoconfiguration (SLAAC) [RFC4862] conveys
information about prefixes to be employed for address configuration
via Prefix Information Options (PIOs) sent in Router Advertisement
(RA) messages. IPv6 largely assumes prefix stability, with network
renumbering only taking place in a planned manner, with old/stale
prefixes being phased-out via reduced prefix lifetimes, and new
prefixes (with longer lifetimes) being introduced at the same time.
However, there are several scenarios that may lead to the so-called
"flash-renumbering" events, where the prefix employed by a network
suddenly becomes invalid and replaced by a new prefix. In some of
these scenarios, the local router producing the network renumbering
event may try to deprecate the currently-employed prefixes (by
explicitly signaling the network about the renumbering event),
whereas in other scenarios it may be unable to do so.
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In scenarios where network configuration information related to IPv6
prefixes becomes invalid without any explicit and reliable signaling
of that condition, nodes on the local network may continue using
stale prefixes for an unacceptably long period of time, thus
resulting in connectivity problems.
Scenarios where this problem may arise include, but are not limited
to, the following:
o The most common IPv6 deployment scenario for residential or small
office networks, where a Customer Edge (CE) router employs DHCPv6
Prefix Delegation (DHCPv6-PD) [RFC8415] to request a prefix from
an Internet Service Provider (ISP), and a sub-prefix of the leased
prefix is advertised on the LAN-side of the CE router via
Stateless Address Autoconfiguration (SLAAC) [RFC4862]. In
scenarios where the CE router crashes and reboots, the CE may
obtain (via DHCPv6-PD) a different prefix from the one previously
leased, and therefore advertise (via SLAAC) the new prefix on the
LAN side. Hosts will typically configure addresses for the new
prefix, but will normally retain and may actively employ the
addresses configured for the previously-advertised prefix, since
their associated Preferred Lifetime and Valid Lifetime allow them
to do so.
o A router (e.g. Customer Edge router) advertises autoconfiguration
prefixes corresponding to prefixes learned via DHCPv6-PD with
constant PIO lifetimes that are not synchronized with the
DHCPv6-PD lease time (even though Section 6.3 of [RFC8415]
requires such synchronization). While this behavior violates the
aforementioned requirement from [RFC8415], it is not an unusual
behavior, particularly when e.g. DHCPv6-PD is implemented in a
different software module than the SLAAC router component.
o A switch-port the host is connected to is moved to another subnet
(VLAN) as a result of manual switch-port reconfiguration or 802.1x
re-authentication. There has been evidence that some 802.1x
supplicants do not reset network settings after successful 802.1x
authentication. So if a host fails 802.1x authentication for some
reason, is placed in a "quarantine" VLAN and is successfully
authenticated later on, it might end up having IPv6 addresses from
both the old ("quarantine") and the new VLANs.
o During the planned network renumbering, a router is configured to
send an RA with the Preferred Lifetime for the "old" Prefix
Information Option (PIO) set to zero and the new PIO with a non-
zero Preferred Lifetime. However, due to unsolicited RAs being
sent to a multicast destination address, and multicast being
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rather unreliable on busy wifi networks, the RA might not be
received by local hosts.
o Automated device config management system performs periodic config
pushes to network devices. In these scenarios, network devices
may simply immediately forget their previous configuration, rather
than withdrawing it gracefully. If such a push results in
changing the subnet configured on a particular network, hosts
attached to that network would not get notified about the subnet
change, and their addresses from the "old" prefix will not be
deprecated. A related scenario is the incorrect network
renumbering where a network administrator renumbers a network by
simply removing the "old" prefix from the configuration and
configuring a new prefix instead.
Lacking any explicit and reliable signaling to deprecate the
previously-advertised prefixes, hosts may continue to employ the
previously-configured addresses, which will typically result in
packets being blackholed (whether because of egress-filtering by the
CE router or ISP) or the return traffic being discarded or routed
elsewhere.
The default values for the "Preferred Lifetime" and "Valid Lifetime"
of PIOs specified in [RFC4861] mean that, in the aforementioned
scenarios, the stale addresses would be retained, and could be
actively employed for new communications instances, for an
unacceptably long period of time (one month, and one week,
respectively). This could lead to interoperability problems, instead
of hosts transitioning to the newly-advertised prefix(es) in a more
timely manner.
Some devices have implemented ad-hoc mechanisms to address this
problem, such as sending RAs to deprecate apparently-stale prefixes
when the device receives any packets employing a source address from
a prefix not currently advertised for address configuration on the
local network [FRITZ]. However, this may introduce other
interoperability problems, particularly in multihomed/multiprefix
scenarios. This is a clear indication that advice in this area is
warranted.
Unresponsiveness to these "flash-renumbering" events is caused by the
inability of the network to deprecate stale information, as well as
by the inability of hosts to react to network configuration changes
in a more timely manner. Clearly, it would be desirable that these
flash-renumbering scenarios do not occur, and that, when they do
occur, that hosts are explicitly and reliably notified of their
occurrence. However, for robustness reasons, it is paramount for
hosts to be able to recover from stale configuration information even
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when these flash-renumbering events occur and the network is unable
to explicitly and reliably notify hosts about such conditions.
Section 2 analyzes this problem in more detail. Section 3 describes
possible operational mitigations. Section 4 describes possible
future work to mitigate the aforementioned problem.
2. Analysis of the Problem
As noted in Section 1, the problems discussed in this document are
exacerbated by the default values of some protocol parameters and
other factors. The following sections analyze each of them in
detail.
2.1. Use of Dynamic Prefixes
In network scenarios where dynamic prefixes are employed, renumbering
events lead to updated network configuration information being
propagated through the network, such that the renumbering events are
gracefully handled. However, if the renumbering event happens along
with e.g. loss of configuration state by some of the devices involved
in the renumbering procedure (e.g., a router crashes, reboots, and
gets leased a new prefix), this may result in a flash-renumbering
event, where new prefixes are introduced without properly phasing out
the old ones.
In simple residential or small office scenario, the problem discussed
in this document would be avoided if DHCPv6-PD would lease "stable"
prefixes. However, a recent survey [UK-NOF] indicates that 37% of
the responding ISPs employ dynamic prefixes. That is, dynamic IPv6
prefixes are an operational reality.
Deployment reality aside, there are a number of possible issues
associated with stable prefixes:
o Provisioning systems may be unable to deliver stable IPv6
prefixes.
o While an ISP might lease stable prefixes to the home or small
office, the Customer Edge router might in turn lease sub-prefixes
of these prefixes to other internal network devices. Unless the
associated lease databases are stored on non-volatile memory,
these internal devices might be leased dynamic sub-prefixes of the
stable prefix leased by the ISP. In other words, every time a
prefix is leased there is the potential for the resulting prefixes
to become dynamic, even if the device leasing sub-prefixes has
been leased a stable prefix by its upstream router.
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o While there is a range of information that may be employed to
correlate network activity [RFC7721], the use of stable prefixes
clearly simplifies network activity correlation, and may
essentially render features such as "temporary addresses"
[RFC4941] irrelevant.
o There may be existing advice for ISPs to deliver dynamic IPv6
prefixes *by default* (see e.g. [GERMAN-DP]) over privacy
concerns associated with stable prefixes.
For a number of reasons (such as the ones stated above), IPv6
deployments may employ dynamic prefixes (even at the expense of the
issues discussed in this document), and that there might be scenarios
in which the dynamics of a network are such that the network exhibits
the behaviour of dynamic prefixes. Rather than trying to regulate
how operators may run their networks, this document aims at improving
network robustness in the deployed Internet.
2.2. Default Timer Values in IPv6 Stateless Address Autoconfiguration
(SLAAC)
The impact of the issue discussed in this document is a function of
the lifetime values employed for the PIO lifetimes, since these
values determine for how long the corresponding addresses will be
preferred and considered valid. Thus, when the problem discussed in
this document is experienced, the longer the PIO lifetimes, the
higher the impact.
[RFC4861] specifies the following default PIO lifetime values:
o Preferred Lifetime (AdvPreferredLifetime): 604800 seconds (7 days)
o Valid Lifetime (AdvValidLifetime): 2592000 seconds (30 days)
Under problematic circumstances, such as where the corresponding
network information has become stale without any explicit and
reliable signal from the network (as described in Section 1), it
could take hosts up to 7 days (one week) to deprecate the
corresponding addresses, and up to 30 days (one month) to eventually
invalidate and remove any addresses configured for the stale prefix.
This means that it will typically take hosts an unacceptably long
period of time (on the order of several days) to recover from these
scenarios.
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2.3. Recovering from Stale Network Configuration Information
SLAAC hosts are unable to recover from stale network configuration
information for a number of reasons:
o Item "e)" of Section 5.5.3 of [RFC4862] specifies that an
unauthenticated RA may never reduce the "RemainingLifetime" to
less than two hours. If the RemainingLifetime of an address is
smaller than 2 hours, then a Valid Lifetime smaller than 2 hours
will be ignored. The Preferred Lifetime of an address can be
reduced to any value to avoid using a stale prefix for new
communications.
o In the absence of explicit signalling from SLAAC routers (such as
sending PIOs with a "Preferred Lifetime" set to 0), SLAAC hosts
fail to recover from stale configuration information in a timely
manner. However, when a network element is able to explicitly
signal the renumbering event, it will only be able to deprecate
the stale prefix, but not to invalidate the prefix in question.
Therefore, communication with the new "owners" of the stale prefix
will not be possible, since the stale prefix will still be
considered "on-link".
2.4. Lack of Explicit Signaling about Stale Information
Whenever prefix information has changed, a SLAAC router should not
only advertise the new information, but should also advertise the
stale information with appropriate lifetime values (both "Preferred
Lifetime" and "Valid Lifetime" set to 0). This would provide
explicit signaling to SLAAC hosts to remove the stale information
(including configured addresses and routes). However, in scenarios
such as when a CE router crashes and reboots, the CE router may have
no knowledge about the previously-advertised prefixes, and thus may
be unable to advertise them with appropriate lifetimes (in order to
deprecate them).
However, we note that, as discussed in Section 2.3, PIOs with small
Valid Lifetimes in unauthenticated RAs will not lower the Valid
Lifetime to any value shorter than two hours (as per [RFC4862]).
Therefore, even if a SLAAC router tried to explicitly signal the
network about the stale configuration information via unauthenticated
RAs, implementations compliant with [RFC4862] would deprecate the
corresponding prefixes, but would fail to invalidate them.
NOTE:
Some implementations have been updated to honor small PIO
lifetimes values, as proposed in [I-D.ietf-6man-slaac-renum]. For
example, please see [Linux-update].
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2.5. Interaction Between DHCPv6-PD and SLAAC
While DHCPv6-PD is normally employed along with SLAAC, the
interaction between the two protocols is largely unspecified. Not
unusually, the two protocols are implemented in two different
software components with the interface between the two implemented by
some sort of script that feeds the SLAAC implementation with values
learned from DHCPv6-PD.
At times, the prefix lease time is fed as a constant value to the
SLAAC router implementation, meaning that, eventually, the prefix
lifetime advertised on the LAN side will span *past* the DHCPv6-PD
lease time. This is clearly incorrect, since the SLAAC router
implementation would be allowing the use of such prefixes for a
longer time than it has been granted usage of those prefixes via
DHCPv6-PD.
3. Operational Mitigations
The following subsections discuss possible operational workarounds
for the aforementioned problems.
3.1. Stable Prefixes
As noted in Section 2.1, the use of stable prefixes would eliminate
the issue in *some* of the scenarios discussed in Section 1 of this
document, such as the typical home network deployment. However, even
in such scenarios, there might be reasons for which an administrator
may want or may need to employ dynamic prefixes
3.2. SLAAC Parameter Tweaking
An operator may wish to override some SLAAC parameters such that,
under normal circumstances, the timers will be refreshed/reset, but
in the presence of network faults (such as the one discussed in this
document), the timers go off and trigger some fault recovering action
(e.g. deprecate and subsequently invalidate stale addresses).
The following router configuration variables from [RFC4861]
(corresponding to the "lifetime" parameters of PIOs) could be
overridden as follows:
AdvPreferredLifetime: 2700 seconds (45 minutes)
AdvValidLifetime: 5400 seconds (90 minutes)
NOTES:
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The aforementioned values for AdvPreferredLifetime and
AdvValidLifetime are expected to be appropriate for most networks.
In some networks, particularly where the operator has complete
control of prefix allocation and where hosts on the network may
spend long periods sleeping (e.g., sensors with limited battery),
longer values may be appropriate.
A CE router advertising a sub-prefix of a prefix leased via
DHCPv6-PD will periodically refresh the Preferred Lifetime and the
Valid Lifetime of an advertised prefix to AdvPreferredLifetime and
AdvValidLifetime, respectively, as long as the resulting lifetime
of the corresponding prefixes does not extend past the DHCPv6-PD
lease time [I-D.ietf-v6ops-cpe-slaac-renum].
RATIONALE:
* In the context of [RFC8028], where it is clear that use of
addresses configured for a given prefix is tied to using the
next-hop router that advertised the prefix, it does not make
sense for the "Preferred Lifetime" of a PIO to be larger than
the "Router Lifetime" (AdvDefaultLifetime) of the corresponding
Router Advertisement messages. The "Valid Lifetime" is set to
a much larger value to cope with transient network problems.
* Lacking RAs that refresh information, addresses configured for
advertised prefixes become deprecated in a more timely manner,
and thus Rule 3 of [RFC6724] causes other configured addresses
(if available) to be used instead.
* We note that lowering the default values for the "Valid
Lifetime" helps reduce the amount of time a host may maintain
stale information and the amount of time an advertising router
would need to advertise stale prefixes to deprecate them, while
reducing the default "Preferred Lifetime" would reduce the
amount of time it takes for a host to prefer other working
prefixes (see Section 12 of [RFC4861]). However, while the
values suggested in this section are an improvement over the
default values specified in [RFC4861], they represent a trade-
off among a number of factors, including responsiveness,
possible impact on the battery life of connected devices
[RFC7772], etc. Thus, they may or may not provide sufficient
mitigation to the problem discussed in this document.
4. Future Work
Improvement in Customer Edge Routers [RFC7084] such that they can
signal the network about stale prefixes and deprecate them
accordingly can help mitigate the problem discussed in this document
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for the "home network" scenario. Such work is currently being
pursued in [I-D.ietf-v6ops-cpe-slaac-renum].
Improvements in the SLAAC protocol [RFC4862] and other algorithms
such as "Default Address Selection for IPv6" [RFC6724] would help
improve network robustness. Such work is currently being pursued in
[I-D.ietf-6man-slaac-renum].
The aforementioned work is considered out of the scope of this
present document, which only focuses on documenting the problem and
discussing operational mitigations.
5. IANA Considerations
This document has no actions for IANA.
6. Security Considerations
This document discusses a problem that may arise in scenarios where
flash-renumbering events occur, and proposes workarounds to mitigate
the aforementioned problems. This document does not introduce any
new security issues, and thus the same security considerations as for
[RFC4861] and [RFC4862] apply.
7. Acknowledgments
The authors would like to thank (in alphabetical order) Brian
Carpenter, Alissa Cooper, Roman Danyliw, Owen DeLong, Martin Duke,
Guillermo Gont, Philip Homburg, Sheng Jiang, Benjamin Kaduk, Erik
Kline, Murray Kucherawy, Warren Kumari, Ted Lemon, Juergen
Schoenwaelder, Eric Vyncke, Klaas Wierenga, Robert Wilton, and Dale
Worley, for providing valuable comments on earlier versions of this
document.
The authors would like to thank (in alphabetical order) Mikael
Abrahamsson, Luis Balbinot, Brian Carpenter, Tassos Chatzithomaoglou,
Uesley Correa, Owen DeLong, Gert Doering, Martin Duke, Fernando
Frediani, Steinar Haug, Nick Hilliard, Philip Homburg, Lee Howard,
Christian Huitema, Ted Lemon, Albert Manfredi, Jordi Palet Martinez,
Michael Richardson, Mark Smith, Tarko Tikan, and Ole Troan, for
providing valuable comments on a previous document on which this
document is based.
Fernando would like to thank Alejandro D'Egidio and Sander Steffann
for a discussion of these issues. 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.
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The problem discussed in this document has been previously documented
by Jen Linkova in [I-D.linkova-6man-default-addr-selection-update],
and also in [RIPE-690]. Section 1 borrows text from
[I-D.linkova-6man-default-addr-selection-update], authored by Jen
Linkova.
8. References
8.1. Normative References
[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>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by
Hosts in a Multi-Prefix Network", RFC 8028,
DOI 10.17487/RFC8028, November 2016,
<https://www.rfc-editor.org/info/rfc8028>.
[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
[FRITZ] Gont, F., "Quiz: Weird IPv6 Traffic on the Local Network
(updated with solution)", SI6 Networks Blog, February
2016, <https://www.si6networks.com/2016/02/16/quiz-weird-
ipv6-traffic-on-the-local-network-updated-with-solution/>.
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[GERMAN-DP]
BFDI, "Einfuhrung von IPv6 Hinweise fur Provider im
Privatkundengeschaft und Herstellere", Entschliessung der
84. Konferenz der Datenschutzbeauftragten des Bundes und
der Lander am 7./8. November 2012 in Frankfurt (Oder),
November 2012,
<http://www.bfdi.bund.de/SharedDocs/Publikationen/
Entschliessungssammlung/DSBundLaender/84DSK_EinfuehrungIPv
6.pdf?__blob=publicationFile>.
[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.
[I-D.ietf-v6ops-cpe-slaac-renum]
Gont, F., Zorz, J., Patterson, R., and B. Volz, "Improving
the Reaction of Customer Edge Routers to Renumbering
Events", draft-ietf-v6ops-cpe-slaac-renum-05 (work in
progress), September 2020.
[I-D.linkova-6man-default-addr-selection-update]
Linkova, J., "Default Address Selection and Subnet
Renumbering", draft-linkova-6man-default-addr-selection-
update-00 (work in progress), March 2017.
[Linux-update]
Gont, F., "[net-next] ipv6: Honor all IPv6 PIO Valid
Lifetime values", Post to the netdev mailing-list
http://vger.kernel.org/vger-lists.html, April 2020,
<https://patchwork.ozlabs.org/project/netdev/
patch/20200419122457.GA971@archlinux-
current.localdomain/>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[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>.
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[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[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>.
[RIPE-690]
Zorz, J., Zorz, S., Drazumeric, P., Townsley, M., Alston,
J., Doering, G., Palet, J., Linkova, J., Balbinot, L.,
Meynell, K., and L. Howard, "Best Current Operational
Practice for Operators: IPv6 prefix assignment for end-
users - persistent vs non-persistent, and what size to
choose", RIPE 690, October 2017,
<https://www.ripe.net/publications/docs/ripe-690>.
[UK-NOF] Palet, J., "IPv6 Deployment Survey (Residential/Household
Services) How IPv6 is being deployed?", UK NOF 39, January
2018,
<https://indico.uknof.org.uk/event/41/contributions/542/
attachments/712/866/bcop-ipv6-prefix-v9.pdf>.
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
Gont, et al. Expires May 6, 2021 [Page 13]
