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IPv6 Maintenance (6man) Working Group                            F. Gont
Internet-Draft                                              SI6 Networks
Updates: 4861, 4862 (if approved)                                J. Zorz
Intended status: Standards Track                           Go6 Institute
Expires: January 28, 2021                                   R. Patterson
                                                                  Sky UK
                                                           July 27, 2020


Improving the Robustness of Stateless Address Autoconfiguration (SLAAC)
                      to Flash Renumbering Events
                     draft-ietf-6man-slaac-renum-00

Abstract

   In renumbering scenarios where an IPv6 prefix suddenly becomes
   invalid, hosts on the local network will continue using stale
   prefixes for an unacceptably long period of time, thus resulting in
   connectivity problems.  This document improves the reaction of IPv6
   Stateless Address Autoconfiguration to such renumbering scenarios.

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 January 28, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SLAAC reaction to Flash-renumbering Events  . . . . . . . . .   4
     3.1.  Renumbering without Explicit Signaling  . . . . . . . . .   4
     3.2.  Renumbering with Explicit Signaling . . . . . . . . . . .   5
   4.  Improvements to Stateless Address Autoconfiguration (SLAAC) .   6
     4.1.  More Appropriate Lifetime Values  . . . . . . . . . . . .   7
       4.1.1.  Router Configuration Variables  . . . . . . . . . . .   7
       4.1.2.  Processing of PIO Lifetimes at Hosts  . . . . . . . .   7
     4.2.  Honor Small PIO Valid Lifetimes . . . . . . . . . . . . .   7
     4.3.  Interface Initialization  . . . . . . . . . . . . . . . .   7
     4.4.  Conveying Information in Router Advertisement (RA)
           Messages  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Recovery from Stale Configuration Information without
           Explicit Signaling  . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .   7
     6.1.  More Appropriate Lifetime Values  . . . . . . . . . . . .   8
       6.1.1.  Router Configuration Variables  . . . . . . . . . . .   8
       6.1.2.  Processing of PIO Lifetimes at Hosts  . . . . . . . .   8
     6.2.  Honor Small PIO Valid Lifetimes . . . . . . . . . . . . .   9
       6.2.1.  NetworkManager  . . . . . . . . . . . . . . . . . . .   9
     6.3.  Conveying Information in Router Advertisement (RA)
           Messages  . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.4.  Recovery from Stale Configuration Information without
           Explicit Signaling  . . . . . . . . . . . . . . . . . . .   9
       6.4.1.  dhcpcd(8) . . . . . . . . . . . . . . . . . . . . . .   9
     6.5.  Other mitigations implemented in products . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Analysis of Some Suggested Workarounds . . . . . . .  13
     A.1.  On a Possible Reaction to ICMPv6 Error Messages . . . . .  14
     A.2.  On a Possible Improvement to Source Address Selection . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16







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1.  Introduction

   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, there are a number of scenarios that may lead to the so-
   called "flash-renumbering" events, where the prefix being employed on
   a network suddenly becomes invalid and replaced by a new prefix
   [I-D.ietf-v6ops-slaac-renum].  In such scenarios, hosts on the local
   network will continue using stale prefixes for an unacceptably long
   period of time, thus resulting in connectivity problems.
   [I-D.ietf-v6ops-slaac-renum] discusses this problem in detail.

   In some scenarios, the local router producing the network renumbering
   event may try to deprecate the currently-employed prefixes (thus
   explicitly signaling the network about the renumbering event),
   whereas in other scenarios it may be unaware about the renumbering
   event and thus unable signal hosts about it.

   From the perspective of a Stateless Address Autoconfiguration (SLAAC)
   host, there are two different (but related) problems to be solved:

   o  Avoiding the use of stale addresses for new communication
      instances

   o  Performing "garbage collection" for the stale prefixes (and
      related network configuration information)

   Clearly, if a host has both working and stale addresses, it is
   paramount that it employs working addresses for new communication
   instances.  Additionally, a host should also perform garbage
   collection for the stale prefixes/addresses, since they not only tie
   system resources, but also prevent communication with the new
   "owners" of the stale prefixes.

2.  Terminology

   The term "globally reachable" is used in this document as defined in
   [RFC8190].

   The term "Global Unicast Address" (or its acronym "GUA") is used
   throughout this document to refer to "globally reachable" [RFC8190]
   addresses.  That is, when used throughout this document, GUAs do NOT
   include Unique Local Addresses (ULAs) [RFC4193].  Similarly, the term
   "Global Unicast prefix" (or "GUA prefix") is employed throughout this
   document to refer to network prefixes that specify GUAs, and does NOT
   include the ULA prefix (FC00::/7) [RFC4193].




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   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.  SLAAC reaction to Flash-renumbering Events

   As noted in Section 1, in some scenarios the router triggering the
   renumbering event may be able to explicitly signal the network about
   this event, while in other scenarios the renumbered hosts may need to
   infer a renumbering event is taking place.  The following subsections
   analyze specific considerations for each of these scenarios.

3.1.  Renumbering without Explicit Signaling

   In the absence of explicit signalling from SLAAC routers (such as
   sending Prefix Information Options (PIOs) with small lifetimes to
   deprecate the stale prefixes), stale prefixes will remain preferred
   and valid according to the Preferred Lifetime and Valid Lifetime
   values (respectively) of the last received PIO.  IPv6 SLAAC employs
   the following default values for PIOs:

   o  Preferred Lifetime (AdvPreferredLifetime): 604800 seconds (7 days)

   o  Valid Lifetime (AdvValidLifetime): 2592000 seconds (30 days)

   This means that, in the absence of explicit signaling by a SLAAC
   router to deprecate a prefix, it will take a host 7 days (one week)
   to deprecate the corresponding addresses, and 30 days (one month) to
   eventually remove any addresses configured for the stale prefix.
   Clearly, for any practical purposes, employing such long default
   values is the equivalent of not using any timers at all, since taking
   7 days or 30 days (respectively) to recover from a network problem is
   simply unacceptable.

   Use of more appropriate timers in Router Advertisement messages can
   help limit the amount of time that hosts will maintain stale
   configuration information.  Additionally, hosts are normally in a
   position to infer that a prefix has become stale -- for example, if a
   given router ceases to advertise an existing prefix and at the same
   time starts to advertise a new prefix.

   Section 4.1.1 recommends the use of more appropriate lifetimes for
   PIOs, while Section 4.1.2 proposes to cap the accepted Valid Lifetime
   and Preferred Lifetime values at hosts, such that more appropriate
   values are employed even in the presence of legacy routers.




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   Section 4.5 specifies a local policy that SLAAC hosts can implement
   to heuristically infer that network configuration information has
   changed, such that stale configuration information can be phased out.

3.2.  Renumbering with Explicit Signaling

   In scenarios where a local router is aware about the renumbering
   event, it may try to phase out the stale network configuration
   information.  In these scenarios, there are two aspects to be
   considered:

   o  The amount of time during which the router should continue trying
      to deprecate the stale network configuration information

   o  The ability of SLAAC hosts to phase out stale configuration in a
      timelier manner.

   In the absence of Router Advertisements (RAs) that include PIOs that
   would reduce the Valid Lifetime and Preferred Lifetime of a prefix,
   hosts would normally employ the lifetime values from PIO options of
   the last received RA messages.  Since the network could be
   partitioned for an arbitrarily long period of time, a router would
   need to try to deprecate a prefix for the amount of time employed for
   the "Preferred Lifetime", and try to invalidate the prefix for the
   amount of time employed for the "Valid Lifetime" (see Section 12 of
   [RFC4861]).

   NOTE:
      Once the number of seconds in the original "Preferred Lifetime"
      have elapsed, all hosts would have deprecated the corresponding
      addresses anyway, while once the number of seconds in the "Valid
      Lifetime" have elapsed, the corresponding addresses would be
      invalidated and removed.

   Thus, use of more appropriate default lifetimes for PIOs, as proposed
   in Section 4.1.1, would reduce the amount of time a stale prefix
   would need to be announced as such by a router in order to make sure
   that it is deprecated/invalidated.

   In scenarios where a router has positive knowledge that a prefix has
   become invalid and thus could signal this condition to local hosts,
   the current specifications will prevent SLAAC hosts from fully
   recovering from such stale information.  Item "e)" of Section 5.5.3
   of [RFC4862] specifies that an RA may never reduce the
   "RemainingLifetime" to less than two hours.  Additionally, if the
   RemainingLifetime of an address is smaller than 2 hours, then a Valid
   Lifetime smaller than 2 hours will be ignored.  The inability to
   invalidate a stale prefix would prevent communication with the new



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   "owners" of the stale prefix, and thus is highly undesirable.  On the
   other hand, the Preferred Lifetime of an address *can* be reduced to
   any value to avoid the use of a stale prefix for new communications.

   Section 4.2 updates [RFC4862] such that this restriction in removed,
   and hosts react to the advertised "Valid Lifetime" (even if it is
   smaller than 2 hours).

   Finally, Section 4.3 recommends that routers disseminate network
   configuration information when a network interface is initialized,
   such that possibly new configuration information propagates in a
   timelier manner.

4.  Improvements to Stateless Address Autoconfiguration (SLAAC)

   The following subsections update [RFC4861] and [RFC4862], such that
   the problem discussed in this document is mitigated.  The
   aforementioned updates are mostly orthogonal, and mitigate different
   aspects of SLAAC that prevent a timely reaction to flash renumbering
   events.

   o  Reduce the default Valid Lifetime and Preferred Lifetime of PIOs
      (Section 4.1.1):
      This helps limit the amount of time a host will employ stale
      information, and also limits the amount of time a router needs to
      try to obsolete stale information.

   o  Cap the received Valid Lifetime and Preferred Lifetime of PIOs
      (Section 4.1.2):
      This helps limit the amount of time a host will employ stale
      information, even in the presence of legacy ([RFC4861]) routers.

   o  Honor PIOs with small Valid Lifetimes (Section 4.2):
      This allows routers to invalidate stale prefixes, since otherwise
      [RFC4861] prevents hosts from honoring PIOs with a Valid Lifetime
      smaller than two hours.

   o  Recommend routers to retransmit configuration information upon
      interface initialization/reinitialization (Section 4.3):
      This helps spread the new information in a timelier manner, and
      also deprecate stale information via host-side heuristics (see
      Section 4.5).

   o  Recommend routers to always send all options (i.e. the complete
      configuration information) in RA messages, and in the smallest
      possible number of packets (Section 4.4):





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      This helps propagate the same information to all hosts, and also
      allows hosts to better infer that information missing in RA
      messages has become stale (see Section 4.5).

   o  Infer stale network configuration information from received RAs
      (Section 4.5):
      This allows hosts to deprecate stale network configuration
      information, even in the absence of explicit signaling.

4.1.  More Appropriate Lifetime Values

4.1.1.  Router Configuration Variables

   [TBD]

4.1.2.  Processing of PIO Lifetimes at Hosts

   [TBD]

4.2.  Honor Small PIO Valid Lifetimes

   [TBD]

4.3.  Interface Initialization

   [TBD]

4.4.  Conveying Information in Router Advertisement (RA) Messages

   [TBD]

4.5.  Recovery from Stale Configuration Information without Explicit
      Signaling

   [TBD]

5.  IANA Considerations

   This document has no actions for IANA.

6.  Implementation Status

   [NOTE: This section is to be removed by the RFC-Editor before this
   document is published as an RFC.]

   This section summarizes the implementation status of the updates
   proposed in this document.  In some cases, they correspond to
   variants of the mitigations proposed in this document (e.g., use of



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   reduced default lifetimes for PIOs, albeit using different values
   than those recommended in this document).  In such cases, we believe
   these implementations signal the intent to deal with the problems
   described in [I-D.ietf-v6ops-slaac-renum] while lacking any guidance
   on the best possible approach to do it.

6.1.  More Appropriate Lifetime Values

6.1.1.  Router Configuration Variables

6.1.1.1.  rad(8)

   We have produced a patch for OpenBSD's rad(8) [rad] that employs the
   default lifetimes recommended in this document, albeit it has not yet
   been committed to the tree.  The patch is available at:
   <https://www.gont.com.ar/code/fgont-patch-rad-pio-lifetimes.txt>.

6.1.1.2.  radvd(8)

   The radvd(8) daemon [radvd], normally employed by Linux-based router
   implementations, currently employs different default lifetimes than
   those recommended in [RFC4861]. radvd(8) employs the following
   default values [radvd.conf]:

   o  Preferred Lifetime: 14400 seconds (4 hours)

   o  Valid Lifetime: 86400 seconds (1 day)

   This is not following the specific recommendation in this document,
   bu is already a deviation from the current standards.

6.1.2.  Processing of PIO Lifetimes at Hosts

6.1.2.1.  NetworkManager

   NetworkManager [NetworkManager], user-space SLAAC implementation
   employed by some Linux-based operating systems (such as Fedora or
   Ubuntu), caps the lifetimes of the received PIOs as recommended in
   this document.

6.1.2.2.  slaacd(8)

   slaacd(8) [slaacd], a user-space SLAAC implementation employed by
   OpenBSD, caps the lifetimes of the received PIOs as recommended in
   this document.






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6.1.2.3.  systemd-networkd

   systemd-networkd [systemd], a user-space SLAAC implementation
   employed by some Linux-based operating systems, caps the lifetimes of
   the received PIOs as recommended in this document.

6.2.  Honor Small PIO Valid Lifetimes

6.2.1.  NetworkManager

   NetworkManager [NetworkManager] processes RA messages with a Valid
   Lifetime smaller than two hours as recommended in this document.

6.3.  Conveying Information in Router Advertisement (RA) Messages

   We know of no implementation that splits network configuration
   information into multiple RA messages.

6.4.  Recovery from Stale Configuration Information without Explicit
      Signaling

6.4.1.  dhcpcd(8)

   The dhcpcd(8) daemon [dhcpcd], a user-space SLAAC implementation
   employed by some Linux-based and BSD-derived operating systems, will
   set the Preferred Lifetime of addresses corresponding to a given
   prefix to 0 when a single RA from the router that previously
   advertised the prefix fails to advertise the corresponding prefix.
   However, it does not affect the corresponding Valid Lifetime.
   Therefore, it can be considered a partial implementation of this
   feature.

6.5.  Other mitigations implemented in products

   [FRITZ] is a Customer Edge Router that tries to deprecate stale
   prefixes by advertising stale prefixes with a Preferred Lifetime of
   0, and a Valid Lifetime of 2 hours (or less).  There are two things
   to note with respect to this implementation:

   o  Rather than recording prefixes on stable storage (as recommended
      in [I-D.ietf-v6ops-cpe-slaac-renum]), this implementation checks
      the source address of IPv6 packets, and assumes that usage of any
      address that does not correspond to a prefix currently-advertised
      by the Customer Edge Router is the result of stale network
      configuration information.  Hence, upon receipt of a packet that
      employs a source address that does not correspond to a currently-
      advertised prefix, this implementation will start advertising the




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      corresponding prefix with small lifetimes, with the intent of
      deprecating it.

   o  Possibly as a result of item "e)" (pp. 19-20) from Section 5.5.3
      of [RFC4862] (discussed in Section 4.2 of this document), upon
      first occurrence of a stale prefix, this implementation will
      employ a decreasing Valid Lifetime, starting from 2 hours (7200
      seconds), as opposed to a Valid Lifetime of 0.

7.  Security Considerations

   When it comes to the algorithm in Section 4.5, an attacker could
   impersonate the legitimate router and send an RA that does not
   advertise legitimate prefixes being employed in the local network.
   This cause the corresponding addresses to become deprecated.
   However, the addresses would not become invalid since normal
   unsolicited RA messages would refresh the "Preferred Lifetime" and
   "Valid Lifetime" of such addresses.

   However, an attacker that can impersonate a router could more easily
   deprecate addresses by advertising the legitimate prefixes with the
   "Preferred Lifetime" set to 0, or perform a plethora of other
   possible of Denial of Service attacks based on forged RA messages.
   Therefore, when attacks based on forged RA packets are a concern,
   technologies such as RA-Guard [RFC6105] [RFC7113] should be deployed.

   Capping the "Valid Lifetime" and "Preferred Lifetime" at hosts may
   help limit the duration of the effects of non-sustained attacks that
   employ forged RAs with PIOs, since hosts would now recover in a
   timelier manner.

8.  Acknowledgments

   The authors would like to thank (in alphabetical order) Mikael
   Abrahamsson, Tore Anderson, Luis Balbinot, Brian Carpenter, Owen
   DeLong, Gert Doering, Thomas Haller, Nick Hilliard, Bob Hinden,
   Philip Homburg, Lee Howard, Christian Huitema, Erik Kline, Jen
   Linkova, Albert Manfredi, Roy Marples, Florian Obser, Jordi Palet
   Martinez, Michael Richardson, Hiroki Sato, Mark Smith, Hannes
   Frederic Sowa, Tarko Tikan, Ole Troan, and Loganaden Velvindron, for
   providing valuable comments on earlier versions of this document.

   The algorithm specified in Section 4.5 is the result of mailing-list
   discussions over previous versions of this document with Philip
   Homburg.






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   Fernando would like to thank Alejandro D'Egidio and Sander Steffann
   for a discussion of these issues, which led to the publication of
   [I-D.ietf-v6ops-slaac-renum], and eventually to this document.

   Fernando would also like to thank Brian Carpenter who, over the
   years, has answered many questions and provided valuable comments
   that has benefited his protocol-related work.

   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].

9.  References

9.1.  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>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [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>.

   [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>.

   [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>.

   [RFC8190]  Bonica, R., Cotton, M., Haberman, B., and L. Vegoda,
              "Updates to the Special-Purpose IP Address Registries",
              BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017,
              <https://www.rfc-editor.org/info/rfc8190>.



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   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/info/rfc8504>.

9.2.  Informative References

   [dhcpcd]   Marples, R., "dhcpcd - a DHCP client",
              <https://roy.marples.name/projects/dhcpcd/>.

   [FRITZ]    Gont, F., "Quiz: Weird IPv6 Traffic on the Local Network
              (updated with solution)", SI6 Networks Blog, February
              2016, <http://blog.si6networks.com/2016/02/quiz-weird-
              ipv6-traffic-on-local-network.html>.

   [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-03 (work in
              progress), May 2020.

   [I-D.ietf-v6ops-slaac-renum]
              Gont, F., Zorz, J., and R. Patterson, "Reaction of
              Stateless Address Autoconfiguration (SLAAC) to Flash-
              Renumbering Events", draft-ietf-v6ops-slaac-renum-02 (work
              in progress), May 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.

   [NetworkManager]
              NetworkManager, "NetworkManager web site",
              <https://wiki.gnome.org/Projects/NetworkManager>.

   [rad]      Obser, F., "OpenBSD Router Advertisement Daemon - rad(8)",
              <https://cvsweb.openbsd.org/src/usr.sbin/rad/>.

   [radvd]    Hawkins, R. and R. Johnson, "Linux IPv6 Router
              Advertisement Daemon (radvd)",
              <http://www.litech.org/radvd/>.

   [radvd.conf]
              Hawkins, R. and R. Johnson, "radvd.conf - configuration
              file of the router advertisement daemon",
              <https://github.com/reubenhwk/radvd/blob/master/
              radvd.conf.5.man>.




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   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010,
              <https://www.rfc-editor.org/info/rfc5927>.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.

   [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>.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,
              <https://www.rfc-editor.org/info/rfc7113>.

   [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>.

   [slaacd]   Obser, F., "OpenBSD SLAAC Daemon - slaacd(8)",
              <https://cvsweb.openbsd.org/src/usr.sbin/slaacd/>.

   [systemd]  systemd, "systemd web site", <https://systemd.io/>.

Appendix A.  Analysis of Some Suggested Workarounds

   [This section is to be removed before publication of this document as
   an RFC].

   During the discussion of this document, some alternative workarounds
   were suggested on the 6man mailing-list.  The following subsections
   analyze these suggested workarounds, in the hopes of avoiding
   rehashing the same discussions.




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A.1.  On a Possible Reaction to ICMPv6 Error Messages

   It has been suggested that if configured addresses become stale, a
   CPE enforcing ingress/egress filtering (BCP38) ([RFC2827]) could send
   ICMPv6 Type 1 (Destination Unreachable) Code 5 (Source address failed
   ingress/egress policy) error messages to the sending node, and that,
   upon receipt of such error messages, the sending node could perform
   heuristics that might help to mitigate the problem discussed in this
   document.

   The aforementioned proposal has a number of drawbacks and
   limitations:

   o  It assumes that the CPE routers enforce ingress/egress filtering
      [RFC2827].  While this is desirable behaviour, it cannot be relied
      upon.

   o  It assumes that if the CPE enforces ingress/egress filtering, the
      CPE will signal the packet drops to the sending node with ICMPv6
      Type 1 (Destination Unreachable) Code 5 (Source address failed
      ingress/egress policy) error messages.  While this may be
      desirable, [RFC2827] does not suggest signaling the packet drops
      with ICMPv6 error messages, let alone the use of specific error
      messages (such as Type 1 Code 5) as suggested.

   o  ICMPv6 Type 1 Code 5 could be interpreted as the employed address
      being stale, but also as a selected route being inappropriate/
      suboptimal.  If the later, deprecating addresses or invalidating
      addresses upon receipt of these error messages would be
      inappropriate.

   o  Reacting to these error messages would create a new attack vector
      that could be exploited from remote networks.  This is of
      particular concern since ICMP-based attacks do not even require
      that the Source Address of the attack packets be spoofed
      [RFC5927].

A.2.  On a Possible Improvement to Source Address Selection

   [RFC6724] specifies source address selection (SAS) for IPv6.
   Conceptually, it sorts the candidate set of source addresses for a
   given destination, based on a number of pair-wise comparison rules
   that must be successively applied until there is a "winning" address.

   An implementation might improve source address selection, and prefer
   the most-recently advertised information.  In order to incorporate
   the "freshness" of information in source address selection, an
   implementation would be updated as follows:



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   o  The node is assumed to maintain a timer/counter that is updated at
      least once per second.  For example, the time(2) function from
      unix-like systems could be employed for this purpose.

   o  The local information associated with each prefix advertised via
      RAs on the local network is augmented with a "LastAdvertised"
      timestamp value.  Whenever an RA with a PIO with the "A" bit set
      for such prefix is received, the "LastAdvertised" timestamp is
      updated with the current value of the timer/counter.

   o  [RFC6724] is updated such that this rule is incorporated:

      Rule 7.5: Prefer fresh information  If one of the two source
         addresses corresponds to a prefix that has been more recently
         advertised, say LastAdvertised(SA) > LastAdvertised(SA), then
         prefer that address (SA in our case).

   A clear benefit of this approach is that a host will normally prefer
   "fresh" addresses over possibly stale addresses.

   However, there are a number of drawbacks associated with this
   approach:

   o  In scenarios where multiple prefixes are being advertised on the
      same LAN segment, the new SAS rule is *guaranteed* to result in
      non-deterministic behaviour, with hosts frequently changing the
      default source address.  This is certainly not desirable from a
      troubleshooting perspective.

   o  Since the rule must be incorporated before "Rule 8: Use longest
      matching prefix" from [RFC6724], it may lead to suboptimal paths.

   o  This new rule may help to improve the selection of a source
      address, but it does not help with the housekeeping (garbage
      collection) of configured information:

      *  If the stale prefix is re-used in another network, nodes
         employing stale addresses and routes for this prefix will be
         unable to communicate with the new "owner" of the prefix, since
         the stale prefix will most likely be considered "on-link".

      *  Given that the currently recommended default value for the
         "Valid Lifetime" of PIOs is 2592000 seconds (30 days), it would
         take too long for hosts to remove the configured addresses and
         routes for the stale prefix.  While the proposed update in
         Section 4.1 of this document would mitigate this problem, the
         lifetimes advertised by the local SLAAC router are not under
         the control of hosts.



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   As a result, updating IPv6 source address selection does not relieve
   nodes from improving their SLAAC implementations as specified in
   Section 4, if at all desirable.  On the other hand, the algorithm
   specified in Section 4.5 would result in Rule 3 of [RFC6724]
   employing fresh addresses, without leading to non-deterministic
   behaviour.

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
   Go6 Institute
   Frankovo naselje 165
   Skofja Loka  4220
   Slovenia

   Email: jan@go6.si
   URI:   https://www.go6.si


   Richard Patterson
   Sky UK

   Email: richard.patterson@sky.uk


















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