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


   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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   This document is subject to BCP 78 and the IETF Trust's Legal
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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

   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

   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

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

   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

   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

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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

   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.

      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

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)


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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].


      *  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

   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

   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

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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

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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [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,

   [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,

   [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,

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-

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              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,

              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.

              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.

              Linkova, J., "Default Address Selection and Subnet
              Renumbering", draft-linkova-6man-default-addr-selection-
              update-00 (work in progress), March 2017.

              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,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [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,

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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,

   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy
              Consumption of Router Advertisements", BCP 202, RFC 7772,
              DOI 10.17487/RFC7772, February 2016,

              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,

   [UK-NOF]   Palet, J., "IPv6 Deployment Survey (Residential/Household
              Services) How IPv6 is being deployed?", UK NOF 39, January

Authors' Addresses

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

   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com

   Jan Zorz

   Email: jan@connect.com

   Richard Patterson
   Sky UK

   Email: richard.patterson@sky.uk

Gont, et al.               Expires May 6, 2021                 [Page 13]