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Versions: 00 01 02 03 04 05 07 draft-ietf-6man-slaac-renum

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: October 28, 2020                                   R. Patterson
                                                                  Sky UK
                                                          April 26, 2020


Improving the Robustness of Stateless Address Autoconfiguration (SLAAC)
                      to Flash Renumbering Events
                     draft-gont-6man-slaac-renum-07

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 October 28, 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SLAAC reaction to Flash-renumbering Events  . . . . . . . . .   3
     3.1.  Renumbering without Explicit Signaling  . . . . . . . . .   4
     3.2.  Renumbering with Explicit Signaling . . . . . . . . . . .   4
   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  . . . . . . . .   8
     4.2.  Honor Small PIO Valid Lifetimes . . . . . . . . . . . . .   9
     4.3.  Interface Initialization  . . . . . . . . . . . . . . . .   9
     4.4.  Conveying Information in Router Advertisement (RA)
           Messages  . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Recovery from Stale Configuration Information without
           Explicit Signaling  . . . . . . . . . . . . . . . . . . .  11
       4.5.1.  Counter-based Algorithm (Algorithm #1)  . . . . . . .  12
       4.5.2.  Timer-based Algorithm (Algorithm #2)  . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  19
     6.1.  More Appropriate Lifetime Values  . . . . . . . . . . . .  20
       6.1.1.  Router Configuration Variables  . . . . . . . . . . .  20
       6.1.2.  Processing of PIO Lifetimes at Hosts  . . . . . . . .  20
     6.2.  Honor Small PIO Valid Lifetimes . . . . . . . . . . . . .  21
       6.2.1.  NetworkManager  . . . . . . . . . . . . . . . . . . .  21
     6.3.  Conveying Information in Router Advertisement (RA)
           Messages  . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.4.  Recovery from Stale Configuration Information without
           Explicit Signaling  . . . . . . . . . . . . . . . . . . .  21
       6.4.1.  dhcpcd(8) . . . . . . . . . . . . . . . . . . . . . .  21
     6.5.  Other mitigations implemented in products . . . . . . . .  21
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Appendix A.  Sample Timeline for Algorithm #1 . . . . . . . . . .  25
   Appendix B.  Analysis of Some Suggested Workarounds . . . . . . .  26
     B.1.  On a Possible Reaction to ICMPv6 Error Messages . . . . .  27
     B.2.  On a Possible Improvement to Source Address Selection . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29




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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 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



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   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.

   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 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:




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   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
   "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,




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   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):
      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.







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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:

      AdvPreferredLifetime: max(AdvDefaultLifetime, 3 *
      MaxRtrAdvInterval)

      AdvValidLifetime: 48 * max(AdvDefaultLifetime, 3 *
      MaxRtrAdvInterval)

   where:

   AdvPreferredLifetime:
      Value to be included in the "Preferred Lifetime" field of the PIO.

   AdvValidLifetime:
      Value to be included in the "Valid Lifetime" field of the PIO.

   AdvDefaultLifetime:
      Value of the "Router Lifetime" field of the Router Advertisement
      message that will carry the PIO.

   max():
      A function that outputs the maximum of its arguments.

   NOTE:
      [RFC4861] specifies AdvDefaultLifetime as 3 * MaxRtrAdvInterval
      (which defaults to 600 seconds).  This means that, when employing
      default values for MaxRtrAdvInterval, the Router Lifetime would be
      set to AdvPreferredLifetime (1800 seconds).  Thus, when employing
      the default values, or when manually setting AdvDefaultLifetime to
      a value smaller than 1800 seconds, AdvPreferredLifetime and
      AdvValidLifetime would be set to 1800 seconds (30 minutes) and
      86400 seconds (1 day), respectively.

   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
         deprecating 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" 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 deprecated in a timelier manner,
         and thus Rule 3 of [RFC6724] will cause other configured
         addresses (if available) to be preferred.

      *  The expression above computes the maximum between
         AdvDefaultLifetime and "3 * MaxRtrAdvInterval" (the default
         value for AdvDefaultLifetime, as per [RFC4861]) to cope with
         the case where an operator might simply want to disable one
         local router for maintenance, without disabling the use of the
         corresponding prefixes on the local network (e.g., on a multi-
         router network).  [RFC4862] implementations would otherwise
         deprecate the corresponding prefixes.  Similarly, [RFC8028]
         would likely behave in the same way.

4.1.2.  Processing of PIO Lifetimes at Hosts

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

   o  IF (Router Lifetime != 0) AND (Preferred Lifetime != 0xffffffff)
      AND (Valid Lifetime != 0xffffffff), then:

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

         Valid Lifetime= MIN(Valid Lifetime, 48 * "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) or when the corresponding router ceases to send
         RAs.



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      *  A Router Lifetime of 0 has the special meaning of "this router
         is not to be employed as a default router", and may be employed
         only to advertise prefixes via SLAAC (but not as a default
         router).  As a result, PIO lifetimes are not capped when Router
         Lifetime == 0.

      *  A PIO lifetime of 0xffffffff has the special meaning of
         "infinity", which means that these prefixes (and their
         corresponding addresses) should never time out.  As a result,
         PIO lifetimes are not capped when the PIO Valid Lifetime ==
         0xffffffff or the PIO Preferred Lifetime == 0xffffffff.

4.2.  Honor Small PIO Valid Lifetimes

   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 should be
      updated by processing the Valid Lifetime and the Preferred
      Lifetime (respectively) in the received advertisement.

   NOTE:  "Processing" the Valid Lifetime and Preferred Lifetime
      includes capping the received values as specified in Section 4.1.2
      of this document.

   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-



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   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
         propagates in a timelier manner to all hosts.

   NOTE:
      [I-D.ietf-v6ops-cpe-slaac-renum] specifies recommendations for CPE
      routers to deprecate any stale network configuration information.

4.4.  Conveying Information in Router Advertisement (RA) Messages

   Intentionally omitting information in Router Advertisements may
   prevent the propagation of such information.  To the best of the
   authors' knowledge, SLAAC routers always send all options in the
   smallest possible number of packets, so this section simply more
   clearly aligns the existing specifications with existing
   implementations.

   This document replaces the following text from Section 6.2.3 of
   [RFC4861]:

      A router MAY choose not to include some or all options when
      sending unsolicited Router Advertisements.  For example, if prefix
      lifetimes are much longer than AdvDefaultLifetime, including them
      every few advertisements may be sufficient.  However, when
      responding to a Router Solicitation or while sending the first few
      initial unsolicited advertisements, a router SHOULD include all
      options so that all information (e.g., prefixes) is propagated
      quickly during system initialization.






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      If including all options causes the size of an advertisement to
      exceed the link MTU, multiple advertisements can be sent, each
      containing a subset of the options.

   with:

      When sending Router Advertisements, a router SHOULD include all
      options.

      If including all options causes the size of an advertisement to
      exceed the link MTU, multiple advertisements can be sent, each
      containing a subset of the options.  In all cases, routers SHOULD
      convey all information using the smallest possible number of
      packets.

   RATIONALE:

      *  Sending information in the smallest possible number of packets
         was somewhat already implied from the original text in
         [RFC4861], and in this respect the proposed update just adds
         clarity.  Including all options when sending RAs both leads to
         simpler code (as opposed to dealing with special cases where
         specific information is intentionally omitted), and also helps
         hosts infer network configuration changes in a timelier manner.
         Note that while [RFC4861] allowed some RAs to omit some
         options, the authors of this document know of no implementation
         of such behavior.  Therefore, the proposed change simply
         reflects existing practice.

4.5.  Recovery from Stale Configuration Information without Explicit
      Signaling

   [NOTE: Based on recent mailing-list discussions, the authors have
   kept the algorithm from previous versions of the document (in
   Section 4.5.1) and have also added a new alternative algorithm (in
   Section 4.5.2).  It is up to the WG to pick one, or keep specifying
   both alternative algorithms].

   The following subsections specify two alternative algorithms that
   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.  Most of the value of these algorithms is in being
   able to mitigate the problem discussed in this document at hosts
   themselves, without requiring updates on local routers.

   Host can normally infer when network configuration information has
   changed.  For example, if a SLAAC router (as identified by its link-



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   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 these algorithms is that, when a router
   advertises new prefixes for address configuration (i.e., PIOs 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 the prefix, configured addresses for the stale prefixes
   should be deprecated (such that they are not employed for new
   communication instances), and they should eventually be removed (if
   this condition persists).  If other routers were advertising the same
   prefix, the prefix should simply be dis-associated with the router
   that ceased to advertise it, and the fate of the corresponding
   addresses should depend on the routers that continue advertising the
   prefix.

   The two alternative algorithms specified in the following subsections
   aim at producing the same outcome, but differ in some details:

   o  Algorithm #1 (Section 4.5.1) employs counters to keep track on the
      number of RAs that have failed to advertise a given prefix, and
      reacts based on the number of such packets.  On the other hand,
      Algorithm #2 (Section 4.5.2 reacts to a PIO that fails to
      advertise a prefix by simply reducing the Preferred and Valid
      Lifetime of a prefix; such timers will be refresh if the prefix is
      not stale, but will otherwise time out and cause the corresponding
      addresses to be deprecated and eventually invalidated.

   o  Algorithm #1 assumes that SLAAC information (at least PIOs) is
      conveyed in the smallest possible number of packets, or at least
      in at most two packets (as per the default values for the
      configuration variables in the algorithm).  While this reflects
      the behavior of real-world implementations, this is not a formal
      requirement in [RFC4861].  On the other hand, Algorithm #2 does
      not require this assumption.

4.5.1.  Counter-based Algorithm (Algorithm #1)

   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




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

   This algorithm employs two configuration variables:

   LTA_RAS_DEPRECATE:
      Number of consecutive RAs *not* carrying a given prefix from a
      given router that will cause the prefix to become deprecated.  It
      defaults to LTA_RAS_DEPRECATE_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
      MUST be larger than LTA_RAS_DEPRECATE.  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  If the packet contains at least one PIO that advertises a Global
      Unicast [RFC4291] prefix then, for each Global Unicast 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 corresponding LTA counter by one.

      *  IF LTA >= LAT_RAS_INVALID, then:





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         +  IF this is the only router advertising this prefix, set the
            "Valid Lifetime" of IPv6 addresses corresponding to this
            prefix to 0, and the "Valid Lifetime" for this prefix (as
            used for on-link determination) 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_DEPRECATE, then:

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

         +  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.

   o  If the packet contains at least one PIO that advertises a Unique
      Local [RFC4193] prefix then, for each Unique Local 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 corresponding LTA counter by one.

      *  IF LTA >= LAT_RAS_INVALID, then:

         +  IF this is the only router advertising this prefix, set the
            "Valid Lifetime" of addresses corresponding to this prefix
            to 0, and the "Valid Lifetime" for this prefix (as used for
            on-link determination) 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_DEPRECATE, then:





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

         +  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 PIOs will be spread
      among *at most* LTA_RAS_DEPRECATE RAs.

         We know of no implementations that intentionally spread network
         configuration information among multiple RAs, and know of no
         real deployment scenarios where the amount of SLAAC information
         requires that it be split into multiple Router Advertisement
         Messages.  Therefore, while we have been conservative in
         selecting the default values for LTA_RAS_DEPRECATE and
         LTA_RAS_INVALID as 2 and 4 (respectively), we believe that all
         real-world scenarios would be able to set them to 1 and 2
         (respectively), without any impact on robustness.  Section 4.4
         provides additional clarifications regarding how information is
         conveyed in Router Advertisement messages.

   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  Only RAs that advertise Global Unicast prefixes may deprecate
      Global Unicast Addresses (GUAs), while only RAs that advertise
      Unique Local prefixes may deprecate Unique Local Addresses (ULAs).

   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"



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      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_DEPRECATE), 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_DEPRECATE (LTA_RAS_DEPRECATE_DEFAULT=2), in
         the worst-case scenario it would take hosts 32 seconds to mark
         stale addresses as "deprecated".  The fourth unsolicited RA
         will be sent after a random amount of time between
         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".

4.5.2.  Timer-based Algorithm (Algorithm #2)

   The algorithm specified in this section updates the state of a
   configured address upon receipt of an RA that, while carrying PIOs,
   fails to advertise a previously-advertised prefix.  Namely, such an
   RA reduces the "Preferred Lifetime" of the corresponding addresses,
   to cause such addresses to be quickly deprecated, while accommodating
   the case where the advertising router might be sending SLAAC options
   in multiple separate packets.  Similarly, the "Valid Lifetime" of
   such addresses is reduced, such that the addresses are invalidated in
   a timelier manner, while still providing some leeway for the local
   router to re-advertise the corresponding prefix.

   Local information maintained for each prefix advertised by each
   router is augmented with one variable named "LTA_LA" (Lifetime
   Avoidance_Last Advertised), that records the last time a given prefix
   has been advertised by a given router.

   NOTE:
      While not strictly required, we note that existing implementations
      may already record the last time a prefix has been advertised by a



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      given router as a possible implementation approach to be able to
      compute the remaining lifetime of an address.

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

   This algorithm employs two configuration variables:

   LTA_DEPRECATE:
      A time value (in seconds) to set the "Preferred Lifetime" of
      addresses corresponding to a given prefix, when a received RA
      suggests that such addresses might have become stale.  It defaults
      to LTA_DEPRECATE_DEFAULT, which this document specifies as 5
      seconds.  This value is a rough estimate of the maximum amount of
      time to send a "batch" of RA messages that advertise the complete
      set of SLAAC information.  [NOTE: We believe this variable could
      be set to a value even smaller than this]

   LTA_INVALID:
      A time value (in seconds) to set the "Valid Lifetime" of addresses
      corresponding to a given prefix, and the "Valid Lifetime" of a
      prefix (for on-link determination), when a received RA suggests
      that such addresses and prefix might have become stale.  It
      defaults to LTA_INVALID_DEFAULT, which this document specifies as
      1800 seconds (which corresponds to the largest possible value for
      MaxRtrAdvInterval [RFC4861]).  [NOTE: We believe that it would be
      possible to set this variable to smaller values, but just opted
      for the most conservative setting].

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

   o  For each prefix prefix advertised by a PIO with the "A" flag set,
      proceed as follows:

      *  LTA_LA = current_time()

   o  If the RA advertises at least one Global Unicast Prefix then, for
      each Global Unicast prefix that had been previously advertised by
      this router but that is not advertised by a PIO in the received
      RA, proceed as follows:

      *  IF current_time() >= (LTA_LA + LTA_DEPRECATE) &&




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         Preferred Lifetime > LTA_DEPRECATE && Valid Lifetime >
         LTA_INVALID, then:

         +  IF this is the only router advertising this prefix, set the
            "Preferred Lifetime" and the "Valid Lifetime" of IPv6
            addresses corresponding to this prefix to LTA_DEPRECATE and
            LTA_INVALID, respectively.  Additionally, set the "Valid
            Lifetime" associated with this prefix (for on-link
            determination) to LTA_INVALID.

         +  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.

   o  If the RA advertises at least one Unique Local [RFC4193] prefix
      then, for each Unique Local prefix that had been previously
      advertised by this router but that is not advertised by a PIO in
      the received RA, proceed as follows:

      *  IF current_time() >= (LTA_LA + LTA_DEPRECATE) &&
         Preferred Lifetime > LTA_DEPRECATE && Valid Lifetime >
         LTA_INVALID, then:

         +  IF this is the only router advertising this prefix, set the
            "Preferred Lifetime" and the "Valid Lifetime" of IPv6
            addresses corresponding to this prefix to LTA_DEPRECATE and
            LTA_INVALID, respectively.  Additionally, set the "Valid
            Lifetime" associated with this prefix (for on-link
            determination) to LTA_INVALID.

         +  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.

   NOTES:

   o  current_time() is a monotonically-increasing counter that is
      incremented once per second, and is employed to measure time.

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

   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



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      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  Only RAs that advertise Global Unicast prefixes may deprecate
      Global Unicast Addresses (GUAs), while only RAs that advertise
      Unique Local prefixes may deprecate Unique Local Addresses (ULAs).

   o  The specified modification takes the conservative approach of
      setting the "Preferred Lifetime" to LTA_DEPRECATE to allow for
      SLAAC information to be conveyed in multiple RA messages (that can
      be sent during a window of LTA_DEPRECATE seconds), and setting the
      "Valid Lifetime" to LTA_INVALID (to accommodate for possible
      packet loss, and transient problems).  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 LTA_DEPRECATE seconds
      to mark the corresponding addresses as "not preferred", and
      LTA_INVALID to completely remove such addresses from the system --
      that is, 5 seconds and 600 seconds, respectively.

   o  The pseudo-code above checks that "Preferred Lifetime >
      LTA_DEPRECATE && Valid Lifetime > LTA_INVALID" to prevent
      subsequent RA packets that do not contain a required PIO from
      resetting the Preferred Lifetime and Valid Lifetime to
      LTA_DEPRECATE and LTA_INVALID (respectively) once they have
      already been reduced by this algorithm.  Otherwise, the Preferred
      Lifetime and Valid Lifetime might never get decremented to 0 as
      expected.

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
   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




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   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.

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.



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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
      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




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      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 4.5.1, 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).  On the other hand, when it comes to the algorithm in
   Section 4.5.2, 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 4.5.2 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>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.




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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., and R. Patterson, "Improving the
              Reaction of Customer Edge Routers to Renumbering Events",
              draft-ietf-v6ops-cpe-slaac-renum-01 (work in progress),
              March 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-01 (work
              in progress), March 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.  Sample Timeline for Algorithm #1

   This section shows a sample packet exchange that illustrates the
   algorithm specified in Section 4.5.1 (Algorithm #1):









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    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_DEPRECATE
                                                   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!)

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.



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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, 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].

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:



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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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