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IPv6 Maintenance (6man) Working Group                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 4861, 4862 (if approved)                                J. Zorz
Intended status: Standards Track                           Go6 Institute
Expires: August 19, 2020                                    R. Patterson
                                                                  Sky UK
                                                       February 16, 2020


Improving the Robustness of Stateless Address Autoconfiguration (SLAAC)
                      to Flash Renumbering Events
                     draft-gont-6man-slaac-renum-02

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
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   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 August 19, 2020.

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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SLAAC reaction to Flash-renumbering Events  . . . . . . . . .   3
     3.1.  Renumbering without Explicit Signaling  . . . . . . . . .   3
     3.2.  Renumbering with Explicit Signaling . . . . . . . . . . .   4
   4.  Improvements to Stateless Address Autoconfiguration (SLAAC) .   5
     4.1.  More Appropriate Lifetime Values  . . . . . . . . . . . .   6
       4.1.1.  Router Configuration Variables  . . . . . . . . . . .   6
       4.1.2.  Processing of PIO Lifetimes at Hosts  . . . . . . . .   7
     4.2.  Updated Processing of Prefix Information Options  . . . .   7
     4.3.  Interface Initialization  . . . . . . . . . . . . . . . .   8
     4.4.  Recovery from Stale Configuration Information without
           Explicit Signaling  . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Sample Timeline for Host Processing of RAs . . . . .  15
   Appendix B.  Analysis of Some Suggested Workarounds . . . . . . .  16
     B.1.  On a Possible Reaction to ICMPv6 Error Messages . . . . .  16
     B.2.  On a Possible Improvement to Source Address Selection . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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.gont-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.gont-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),



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   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 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 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)



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   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 un-prefer 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 are 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 invalid -- 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.

   Section 4.4 specifies a local policy that SLAAC hosts can implement
   to heuristically infer that network configuration information has
   changed, such that stale configuration 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 option 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 "unprefer" 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]).




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   NOTE:
      Once the number of seconds in the original "Preferred Lifetime"
      have elapsed, all hosts would have "unpreferred" 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 to unprefer/invalidate
   it.

   In scenarios where a router has positive knowledge that a prefix has
   become invalid and thus 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" more
   than to 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 inability to invalidate a stale prefix would prevent
   communication with the new "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 to be employed 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 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
      insist in obsoleting stale information.




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   o  Cap 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 ([RFC4861]
      prevented 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 and also deprecate stale
      information via host-side heuristics (see Section 4.4).

   o  Infer stale network configuration information from received RAs
      (Section 4.4):
      This allows hosts to get rid of stale network configuration
      information, even in the absence of explicit signaling.

4.1.  More Appropriate Lifetime Values

4.1.1.  Router Configuration Variables

   The default value for the "lifetime" parameters in PIOs is updated as
   follows:

      AdvValidLifetime: 48 * AdvDefaultLifetime

      AdvPreferredLifetime: AdvDefaultLifetime

   NOTE:
      [RFC4861] specifies AdvDefaultLifetime as 3 * MaxRtrAdvInterval
      (which defaults to 600 seconds).  This means this document
      specifies AdvPreferredLifetime as 1800 seconds.  This document
      specifies AdvValidLifetime as 48 * AdvDefaultLifetime, resulting
      in a AdvValidLifetime of 86400 seconds (1 day).

   RATIONALE:

      *  The default values for PIO lifetimes should be such that, under
         normal circumstances (including some packet loss), the
         associated timers are refreshed/reset, but in the presence of
         network failures (such as network configuration information
         becoming stale), some fault recovering action (such as un-
         preferring the corresponding addresses and subsequently
         removing them) is triggered.




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      *  In the context of [RFC8028], where it is clear that the use of
         addresses configured for a given prefix is tied to the next-hop
         router that advertised the prefix, the "Preferred Lifetime" of
         a PIO should never be larger than the "Router Lifetime"
         (AdvDefaultLifetime) of Router Advertisement messages.  Some
         leeway should be provided for the "Valid Lifetime" to cope with
         transient network problems.

      *  As a result, this document updates [RFC4861] such that the
         default Valid Lifetime (AdvValidLifetime) and the default
         Preferred Lifetime (AdvPreferredLifetime) of PIOs are specified
         as a function of the default "Router Lifetime"
         (AdvDefaultLifetime) of Router Advertisement messages.

      *  In the absence of RAs that refresh information, addresses
         configured for advertised prefixes become un-preferred in a
         timelier manner, and thus Rule 3 of [RFC6724] will cause other
         configured addresses (if available) to be preferred.

4.1.2.  Processing of PIO Lifetimes at Hosts

   Hosts SHOULD cap the "Valid Lifetime" and "Preferred Lifetime" of
   PIOs as follows:

   o  Valid Lifetime= MIN(Valid Lifetime, 48 * "Router Lifetime")

   o  Preferred Lifetime= MIN(Preferred Lifetime, "Router Lifetime")

   RATIONALE:

      *  Capping the lifetimes in PIOs as suggested will not eliminate
         the problem discussed in this document, but will certainly
         reduce the amount of time it takes for hosts to converge to
         updated network configuration information, even when the SLAAC
         router advertises PIOs with the default values specified in
         [RFC4861] (as opposed to the new default values specified in
         Section 4.1.1) and when the corresponding router ceases to send
         RAs.

4.2.  Updated Processing of Prefix Information Options

   The entire item "e)" (pp. 19-20) from Section 5.5.3 of [RFC4862] is
   replaced with the following text:

      e) If the advertised prefix is equal to the prefix of an address
      configured by stateless autoconfiguration in the list, the valid
      lifetime and the preferred lifetime of the address is reset to the




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      Valid Lifetime and the Preferred Lifetime (respectively) in the
      received advertisement.

   RATIONALE:

      *  This change allows hosts to react to the information provided
         by a router that has positive knowledge that a prefix has
         become invalid.

      *  Attacks aiming at disabling an advertised prefix via a Valid
         Lifetime of 0 are not really more harmful than other attacks
         that can be performed via forged RA messages, such as those
         aiming at completely disabling a next-hop router via an RA that
         advertises a Router Lifetime of 0, or performing a Denial of
         Service (DoS) attack by advertising illegitimate prefixes via
         PIOs.  In scenarios where RA-based attacks are of concern,
         proper mitigations such as RA-Guard [RFC6105] [RFC7113] should
         be implemented.

4.3.  Interface Initialization

   When an interface is initialized, it is paramount that network
   configuration information is spread on the corresponding network
   (particularly in scenarios where an interface has been re-
   initialized, and the conveyed information has changed).  Thus, this
   document replaces the following text from Section 6.2.4 of [RFC4861]:

      In such cases, the router MAY transmit up to
      MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using
      the same rules as when an interface becomes an advertising
      interface.

   with:

      In such cases, the router SHOULD transmit
      MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using
      the same rules as when an interface becomes an advertising
      interface.

   RATIONALE:

      *  Use of stale information can lead to interoperability problems.
         Therefore, it is paramount that new configuration information
         is delivered in a timely manner to all hosts.

   NOTE:
      [I-D.gont-v6ops-cpe-slaac-renum] specifies recommendations for CPE
      routers, including which information should be included in RA



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      messages to deprecate stale network configuration information (if
      any).

4.4.  Recovery from Stale Configuration Information without Explicit
      Signaling

   The goal of the algorithm specified in this section is to allow hosts
   to infer when a previously-advertised prefix has become stale, such
   that previously-configured addresses are "phased-out" and the host
   can transition to the newly-advertised prefixes in a timelier manner.

   Host can normally infer when network configuration information has
   changed.  For example, if a SLAAC router (as identified by its link-
   local address) has ceased to advertise a previously-advertised prefix
   and has also started to advertise new prefixes via PIOs, this should
   be considered an indication that network configuration information
   has changed.  Implementation of this kind of heuristics would allow a
   timelier reaction to network configuration changes even in scenarios
   where there is no explicit signaling from the network -- thus
   improving robustness.

   The basic premise behind this algorithm is that, when a router
   advertises new prefixes for address configuration (PIO with the "A"
   bit set), but fails to advertise the previously-advertised prefixes,
   this is an indication that the previously-advertised prefixes have
   become stale.  Therefore, if this was the only router advertising
   this prefix, configured addresses for the stale prefixes should be
   "un-preferred" (such that they are not employed for new communication
   instances), and they should eventually be removed (if this condition
   persists).

   The algorithm specified in this section updates the state of a
   configured address upon receipt of a number of consecutive RAs that,
   while carrying PIOs, fail to advertise a previously-advertised
   prefix.  This algorithm can accommodate the (theoretical) scenario
   where a router may split PIOs among a number of RA messages.

   Local information maintained for each prefix advertised by each
   router is augmented with one counter named "LTA" (Lifetime Avoidance)
   that defaults to 0, that counts the number of consecutive RAs
   received from the corresponding router that do not advertise the
   corresponding prefix.

   NOTE:
      Hosts are already expected to keep track of which router has
      advertised which prefix in order to be able to properly select the
      first-hop router in multiple-prefix networks [RFC8028] [RFC8504].




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      Throughout this specification, each router is identified by its
      link-local address.

   This algorithm employs two configuration variables:

   LTA_RAS_UNPREFER:  Number of consecutive RAs *not* carrying a given
      prefix from a given router that will cause the prefix to become
      unpreferred/deprecated.  It defaults to LTA_RAS_UNPREFER_DEFAULT,
      which this document specifies as 2.

   LTA_RAS_INVALID:  Number of consecutive RAs *not* carrying a given
      prefix from a given router that will cause the prefix to become
      invalid.  It defaults to LTA_RAS_INVALID_DEFAULT, which this
      document specifies as 4.

   After normal processing of Router Advertisement messages, Router
   Advertisements that contain at least one PIO MUST be processed as
   follows:

   o  The LTA counter for each prefix advertised in the current Router
      Advertisement, and associated with this particular router, should
      be set to 0.

   o  For each prefix that had been previously advertised by this router
      but that is not advertised via a PIO in the received RA, proceed
      as follows:

      *  Increment the LTA counter by one.

      *  IF LTA >= LAT_RAS_INVALID, then:

         +  IF this is the only router advertising this prefix, set the
            "Valid Lifetime" of this prefix to 0.  This will cause the
            removal of all addresses for this prefix and of any routes
            for this prefix associated with the this router.

         +  ELSE IF this prefix has been advertised my multiple
            neighboring routers, simply disassociate this prefix with
            this particular router.  This will cause the fate of this
            prefix to depend on the other routers.

      *  ELSE IF LTA >= LTA_RAS_UNPREFER, then:

         +  IF this is the only router advertising this prefix, set the
            "Preferred Lifetime" of this prefix to 0.  This will cause
            the corresponding addresses to become un-preferred/
            deprecated.




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         +  ELSE IF this prefix has been advertised my multiple
            neighboring routers, simply disassociate this prefix with
            this particular router.  This will cause the fate of this
            prefix to depend on the other routers.

   Appendix A illustrates a possible packet exchange and the operation
   of the algorithm for a typical scenario.

   NOTES:

   o  The processing of RAs that do not contain any PIOs with the "A"
      bit set remains unaffected.

   o  The aforementioned processing assumes that while network
      configuration information might be split among multiple RAs, PIOs
      will be spread among *at most* LTA_RAS_UNPREFER RAs.

   o  If the only prefix that has so far been advertised on the local
      network is the prefix that has become stale, and there is no other
      prefix being advertised, the traditional processing is unaffected
      (the mechanism discussed in this document will *never* be
      triggered because received RAs will not contain other PIOs with
      the "A" bit set).  The rationale here is that it is better to have
      some address, than no address at all.

   o  The specified modification takes the conservative approach of
      first setting the "Preferred Lifetime" to 0 (such that addresses
      become non-preferred), and eventually setting the "Valid Lifetime"
      to 0 (such that addresses are completely removed).  Once the
      addresses for this prefix have been removed, associated routes
      incorporated by the original RA messages SHOULD also be removed.

   o  In cases where this scenario has been triggered by a CPE router
      crashing and rebooting, it would take hosts less than one minute
      to mark the corresponding addresses as "not preferred" (when using
      the default value for LTA_RAS_UNPREFER), and less than five
      minutes to completely remove such addresses from the system (when
      using the default value for LAT_RAS_INVALID).

         Section 6.2.4 of [RFC4861] specifies that when an interface
         becomes an advertising interface, the first few unsolicited RAs
         (up to MAX_INITIAL_RTR_ADVERTISEMENTS, specified as 3) will be
         sent at intervals of at most MAX_INITIAL_RTR_ADVERT_INTERVAL
         (specified as 16 seconds).  This means that, using the default
         value for LTA_RAS_UNPREFER (LTA_RAS_UNPREFER_DEFAULT=2), in the
         worst-case scenario it would take hosts 32 seconds to mark
         stale addresses as "not preferred".  The fourth unsolicited RA
         will be sent after a random amount of time between



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         MinRtrAdvInterval (default: 0.33 * MaxRtrAdvInterval) and
         MaxRtrAdvInterval (default: 600 seconds) has elapsed, thus
         meaning that, when using the default value for LTA_RAS_INVALID
         (LTA_RAS_INVALID_DEFAULT=4) the stale addresses would be
         removed after between 3.3 and 10 minutes of being marked as
         "not preferred".

5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

   An attacker that could impersonate a router could forge multiple RA
   packets that contain PIOs for prefixes that are currently not
   advertised on the local network and that fail to include previously-
   advertised prefixes, to trigger the mechanism specified in this
   document (and thus cause existing addresses to be deprecated, and
   eventually removed).  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.

7.  Acknowledgments

   The authors would lie to thank (in alphabetical order) Mikael
   Abrahamsson, Luis Balbinot, Brian Carpenter, Owen DeLong, Gert
   Doering, Nick Hilliard, Bob Hinden, Philip Homburg, Lee Howard,
   Christian Huitema, Jen Linkova, Albert Manfredi, Jordi Palet
   Martinez, Michael Richardson, Mark Smith, Tarko Tikan, and Ole Troan,
   for providing valuable comments on earlier versions of this document.

   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 has 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].

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>.

   [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>.

   [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>.

8.2.  Informative References

   [I-D.gont-v6ops-cpe-slaac-renum]
              Gont, F., Zorz, J., and R. Patterson, "Improving the
              Reaction of Customer Edge Routers to Renumbering Events",
              draft-gont-v6ops-cpe-slaac-renum-00 (work in progress),
              November 2019.








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   [I-D.gont-v6ops-slaac-renum]
              Gont, F., Zorz, J., and R. Patterson, "Reaction of
              Stateless Address Autoconfiguration (SLAAC) to Flash-
              Renumbering Events", draft-gont-v6ops-slaac-renum-01 (work
              in progress), November 2019.

   [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.

   [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>.








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Appendix A.  Sample Timeline for Host Processing of RAs

   This section shows a sample packet exchange that illustrates the
   algorithm specified in Section 4:

    Router                                           Host
          RA, PIO={2001:DB8:1::/64, L=1, A=1}
        -------------------------------------->
                                                [Host configures addrs
                                                  for this prefix]
                                                     LTA=0

          RA, PIO={2001:DB8:1::/64, L=1, A=1}
        -------------------------------------->
                                                [Normal proc. of RA]
                           .
                           .
   [Router reboots]

          RA, PIO={2001:DB8:2::/64, L=1, A=1}
        -------------------------------------->   {2001:DB8:1::/64,
                                                      LTA=1}
                           .
                           .
          RA, PIO={2001:DB8:2::/64, L=1, A=1}
        -------------------------------------->   {2001:DB8:1::/64,
                                                      LTA=2}
                                               LTA==LTA_RAS_UNPREFER
                                                   Pref. Lftime=0

                           .
                           .
          RA, PIO={2001:DB8:2::/64, L=1, A=1}
        -------------------------------------->  {2001:DB8:1::/64,
                                                      LTA=3}
                           .
                           .
          RA, PIO={2001:DB8:2::/64, L=1, A=1}
        -------------------------------------->  {2001:DB8:1::/64,
                                                      LTA=4}
                                               LTA==LTA_RAS_INVALID
                                                  Valid Lftime=0
                                                  (Addr. Removed!)








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Appendix B.  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.

B.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, un-preferring addresses or removing
      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].





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B.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:

   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:




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      *  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.

   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 4.4 would result in Rule 3 of [RFC6724]
   employing fresh addresses, without leading to non-deterministic
   behaviour.

Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   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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