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Versions: 00 01

IPv6 Maintenance (6man) Working Group                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 4861, 4862 (if approved)                                J. Zorz
Intended status: Standards Track                       February 18, 2019
Expires: August 22, 2019


 Reaction of Stateless Address Autoconfiguration (SLAAC) to Renumbering
                                 Events
                     draft-gont-6man-slaac-renum-01

Abstract

   In scenarios where network configuration information related to IPv6
   prefixes becomes invalid without any explicit signaling of that
   condition (such as when a CPE crashes and reboots without knowledge
   of the previously-employed prefixes), nodes on the local network will
   continue using stale prefixes for an unacceptably long period of
   time, thus resulting in connectivity problems.  This document
   analyzes these problem scenarios, and proposes workarounds to improve
   network robustness.  This document updates RFC4861 and RFC4862 to
   allow for a more robust reaction to network configuration changes.

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 August 22, 2019.

Copyright Notice

   Copyright (c) 2019 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Analysis of the Problem . . . . . . . . . . . . . . . . . . .   4
     3.1.  Inappropriate Default Timer Values in IPv6 Stateless
           Address Autoconfiguration (SLAAC) . . . . . . . . . . . .   4
     3.2.  Inability of SLAAC Hosts to Recover from Stale Network
           Configuration Information . . . . . . . . . . . . . . . .   5
     3.3.  Lack of Explicit Signaling about Stale Information  . . .   6
     3.4.  Under-specified Interaction Between DHCPv6-PD and SLAAC .   6
   4.  Use of Dynamic Prefixes . . . . . . . . . . . . . . . . . . .   7
   5.  Possible workarounds  . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Improvements to SLAAC . . . . . . . . . . . . . . . . . .   8
       5.1.1.  Default Timer Values  . . . . . . . . . . . . . . . .   8
       5.1.2.  Signaling Stale Configuration Information . . . . . .   9
       5.1.3.  Recovery from Stale Configuration Information . . . .  10
     5.2.  Interaction Between DHCPv6-PD and SLAAC . . . . . . . . .  13
     5.3.  Improved CPE behavior . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Flowchart for Host Processing of RAs . . . . . . . .  17
   Appendix B.  Sample Timeline for Host Processing of RAs . . . . .  18
   Appendix C.  Analysis of Some Suggested Workarounds . . . . . . .  19
     C.1.  On a Possible Reaction to ICMPv6 Error Messages . . . . .  20
     C.2.  On a Possible Improvement to Source Address Selection . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   In scenarios where network configuration information related to IPv6
   prefixes becomes invalid without any explicit signaling of that
   condition, nodes on the local network will continue using stale
   prefixes for an unacceptably long period of time, thus resulting in
   connectivity problems.





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   There are a number of scenarios where this problem may arise.  For
   example, the most common deployment scenario for IPv6 in home
   networks is that in which a CPE router employs DHCPv6 Prefix
   Delegation (DHCPv6-PD) [RFC8415] to request a prefix from the
   Internet Service Provider (ISP), and a prefix belonging to the leased
   prefix is advertised on the LAN-side of the CPE via Stateless Address
   Autoconfiguration (SLAAC) [RFC4862].  In scenarios where the CPE
   router crashes and reboots, the CPE may be leased (via DHCPv6-PD) a
   different prefix from the one previously leased, and will therefore
   advertise (via SLAAC) the new prefix on the LAN side.  Hosts will
   normally configure an address for the new prefix, but will normally
   retain and actively employ the previously-configured addresses, since
   their associated Preferred Lifetime and Valid Lifetime allow them to
   do so.

   Lacking any explicit signaling to "obsolete" the previously-
   configured addresses (for the now invalid/stale prefix), hosts may
   continue employing the previously-configured addresses which will
   typically result in packets being blackholed -- whether because of
   egress-filtering by the CPE or ISP, or because responses to such
   packets will be discarded or routed elsewhere.

   The default values for the "Valid Lifetime" and "Preferred Lifetime"
   of Prefix Information Options (PIOs), as specified in [RFC4861], are:

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

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

   This means that in the aforementioned scenarios, the stale addresses
   would be retained for unacceptably long period of time, thus leading
   to interoperability problems, instead of nodes transitioning to the
   newly-advertised prefix(es) in a timelier manner.

   Some devices have implemented mechanisms to address this problem,
   such as sending RAs to invalidate the apparently stale prefixes when
   the device receive any packets employing a source address from a
   prefix not being advertised for address configuration [FRITZ].
   However, this may introduce other interoperability problems,
   particularly in multihomed scenarios.  This is yet another indication
   that advice in this area is warranted.

   This unresponsiveness is caused by the both the inability of the
   network to deprecate stale information, as well as by the inability
   of hosts to react to network configuration changes in a timelier
   manner.  Clearly, it is desirable that behave in a way that hosts are
   explicitly notified when configuration information has become stale.
   However, for robustness reasons it is paramount for hosts to be able



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   to recover from stale configuration information even if somehow the
   network is unable to explictly notify hosts about such condition.

   Section 3 analysis this problem in more detail.  Section 5 proposes
   workarounds to improve network robustness.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Analysis of the Problem

   As noted in Section 1, the problem discussed in this document is
   associated with sub-optimal behaviour and policies of different
   entities.  Each of the following sections analyze each of them in
   detail.

3.1.  Inappropriate Default Timer Values in IPv6 Stateless Address
      Autoconfiguration (SLAAC)

   Many protocols, from different layers, normally employ timers.  The
   general logic is as follows:

   o  A timer is set with a value that, under normal conditions, the
      timer does *not* go off.

   o  Whenever a fault condition arises, the timer goes off, and the
      protocol can perform fault recovery

   One common example for the use of timers is when implementing
   reliability mechanisms where a packet is transmitted, and unless a
   response is received, a retransmission timer will go off to trigger
   retransmission of the original packet.

   For obvious reasons, the whole point of using timers is that in
   problematic scenarios, they will go off, and trigger some recovery
   action.

   However, IPv6 SLAAC employs, for PIOs, these default values:

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

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

   Under normal network conditions, these timers will be reset/refreshed
   to the default values.  However, under problematic circumstances such



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   as where the corresponding network information has become stale
   without any explicit signal from the network (as described in
   Section 1), 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 is 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.

   The default values for these timers should be such that, under normal
   circumstances (including some packet loss), the timers will be
   refreshed/rest, but in the presence of network faults (such as
   network configuration information becoming stale without explicit
   signaling), the timers go off and trigger some fault recovering
   action (such as un-preferring the corresponding addresses and
   subsequently removing them).

   In the context of [RFC8028], where it is clear that use of addresses
   configured for a given prefix is mostly tied to using the router that
   advertised the prefix as a next hop, one could tell that neither the
   "Preferred Lifetime" or the "Valid Lifetime" of a PIO should ever be
   longer than the default value for the "Router Lifetime" of Router
   Advertisement packets.  This means that since [RFC4861] specifies the
   default value for the "Router Lifetime" as: AdvDefaultLifetime: 3 *
   MaxRtrAdvInterval, and MaxRtrAdvInterval defaults to 600 seconds, the
   values employed for the "Preferred Lifetime" (AdvPreferredLifetime)
   and "Valid Lifetime" (AdvValidLifetime) of PIOs should never be
   larger than 1800 seconds (30 minutes).  Capping the lifetimes in PIOs
   as suggested would not eliminate the problem discussed in this
   document, but certainly reduces the amount of time it takes for hosts
   to converge to updated network configuration information.

   Section 5.1.1 of this document updates the SLAAC specification to
   employ more appropriate timer values.

3.2.  Inability of SLAAC Hosts to Recover from Stale Network
      Configuration Information

   In the absence of explicit signalling from SLAAC routers (such as
   sending PIOs with a "Valid Lifetime" set to 0), SLAAC hosts fail to
   recover from stale configuration information in a timely manner.
   This is exacerbated by the inappropriate timers employed for the
   lifetime of PIOs, as discussed in Section 3.1.

   It is possible to heuristically infer that network configuration
   information has changed.  For example, if the same SLAAC router (as



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   identified by its link-local address) is advertising new prefixes via
   PIOs, but has ceased to advertise the previously-advertised prefixes,
   this should be considered an indication that network configuration
   information has changed.  Implementing 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.

   Section 5.1.3 of this document specifies a local policy that SLAAC
   hosts can implement to recover from stale prefixes.

3.3.  Lack of Explicit Signaling about Stale Information

   Whenever prefix information has changed, a SLAAC router should not
   only advertise the new information, but should also advertise the
   stale information with appropriate lifetime values (both "Preferred
   Lifetime" and "Valid Lifetime" set to 0), such that there is explicit
   signaling to SLAAC hosts to remove the stale information (including
   configured addresses and routes).

   In order to cope with the possibility of a SLAAC router crashing and
   rebooting without any state of the previously-advertised prefixes, a
   SLAAC router should record on stable storage the information of which
   prefixes were being advertised on which interfaces, such that upon
   reboots, such prefixes may be advertised with appropriate lifetimes
   (both "Preferred Lifetime" and "Valid Lifetime" set to 0) to cause
   hosts to remove any configuration information derived from previous
   announcements of such prefixes.

   Explicit signaling of network configuration changes would eliminate
   the problem discussed in this document.  However, since it is
   impossible for a host to know whether such explicit signals will be
   received, they are not relieved from inferring changes in network
   configuration information, as discussed in Section 3.2.

   Section 5.1.2 updates the SLAAC specification such that routers
   explicitly signal stale configuration to SLAAC hosts.  Section 5.3
   specifies the corresponding requirements for CPE routers.

3.4.  Under-specified Interaction Between DHCPv6-PD and SLAAC

   While DHCPv6-PD is normally employed along with SLAAC, the
   interaction between the two protocols is largely unspecified.  Not
   unusually, the two protocols are specified in two different software
   components with the interface between the two implemented by some
   sort of script that takes feeds the SLAAC implementation with values
   learned from DHCPv6-PD.




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   Quite frequently, the prefix lease time is fed as a constant value to
   the SLAAC router implementation, meaning that eventually the prefix
   lifetime advertised on the LAN side will span *past* the DHCPv6-PD
   lease time.  This is clearly incorrect, since the SLAAC router
   implementation would be allowing the use of such prefixes for a
   longer time than it has been granted usage of those prefixes via
   DHCPv6-PD.  The lifetime values employed for the "Preferred Lifetime"
   (AdvPreferredLifetime) and "Valid Lifetime" (AdvValidLifetime) should
   never be larger than the remaining lease time for the corresponding
   prefix (as learned via DHCPv6-PD).

   Section 5.2 of this document specifies this aspect of the interaction
   between DHCPv6-PD and SLAAC.

4.  Use of Dynamic Prefixes

   The problem discussed in this document would be avoided if DHCPv6-PD
   would lease "stable" prefixes.  However, a recent survey [UK-NOF]
   indicates that 37% of the responding ISPs employ dynamic prefixes.
   That is, dynamic IPv6 prefixes are an operational reality.

   Deployment reality aside, there are a number of possible issues
   associated with stable prefixes:

   o  Provisioning systems may be unable to deliver stable IPv6
      prefixes.

   o  While there is a range of information that may be employed to
      correlate network activity [RFC7721], the use of stable prefixes
      clearly simplifies network activity correlation, and may
      essentially render features such as "temporary addresses"
      [RFC4941] irrelevant.

   o  Applicable legislation may require the ISP to deliver dynamic IPv6
      prefixes *by default* (see e.g.  [GERMAN-DP]).

   The authors of this document understand that, for a number of reasons
   (such as the ones stated above), IPv6 deployments may employ dynamic
   prefixes, or there might be scenarios in which the dynamics of a
   network are such that the network exhibits the behaviour of dynamic
   prefixes.  Rather than try to preclude how operators may run their
   networks, this document aims at improving network robustness in the
   deployed Internet.








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5.  Possible workarounds

   The following subsections discuss possible workarounds for the
   aforementioned problem.  Section 5.1 specifies modifications to SLAAC
   which include the use of more appropriate lifetime values and a
   mechanism for hosts to infer when a previously-advertised prefix has
   become stale.  This modification leads to more robust behaviour even
   for existing deployments.

   Section 5.3 specifies the interaction between DHCPv6-PD and SLAAC,
   such that devices such as CPEs may be in a better position to convey
   current network information to hosts on the LAN-side.  For obvious
   reasons (legacy CPEs, etc.), this improved behaviour cannot be relied
   upon for mitigating the problem described in Section 1.  However, it
   does contribute to more robust IPv6 networks.

   Finally, Section 4 analyzes the trade-offs of employing stable vs.
   dynamic network prefixes.

5.1.  Improvements to SLAAC

5.1.1.  Default Timer Values

5.1.1.1.  Router Configuration Variables

   The "default" value for the router configuration variable
   "MaxRtrAdvInterval" (Section 6.2.1 of [RFC4861] is changed to 300
   seconds (5 minutes).

   As a result of this change, the default values for two other
   configuration variables are indirectly modified (since they are
   specified in relation with MaxRtrAdvInterval):

   o  MinRtrAdvInterval: 0.33 * MaxRtrAdvInterval = 99 seconds

   o  AdvDefaultLifetime: 3 * MaxRtrAdvInterval = 900 seconds (15
      minutes)

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

      AdvValidLifetime: AdvDefaultLifetime (which according to this
      specification defaults to 900 seconds / 15 minutes)

      AdvPreferredLifetime: 0.50 * AdvValidLifetime (which would result
      in 450 seconds, that is, 7.5 minutes)

   The motivation of this update is as follows:



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   o  Link the lifetimes associated with prefixes to the lifetime of the
      router advertising the prefixes, since use of advertised prefixes
      is closely tied to the router that has advertised them (as per
      [RFC8028]).

   o  Lacking RAs that refresh information, addresses configured for
      such prefixes would become un-preferred, and thus Rule 3 of
      [RFC6724] would cause other configured addresses (if available) to
      be preferred and used instead.

   o  Limit the amount of time that a stale prefix needs to be
      advertised with a lifetime od "0" on the local network (see
      Section 12 of [RFC4861]).

   o  Employ default router lifetimes that improve the ability of hosts
      to recover from fault scenarios.

5.1.1.2.  Processing of PIO Lifetimes at Hosts

   Hosts should cap the "Valid Lifetime" and "Preferred Lifetime" of
   PIOs to the "Router Lifetime" value in the received Router
   Advertisement.  That is, if the "Valid Lifetime" or "Preferred
   Lifetime" of a PIO is larger than the "Router Lifetime" value of the
   Router Advertisement carrying the PIO, the corresponding value should
   be capped to that of the "Router Lifetime" value of the received RA.

   The motivation for this update is as follows:

   o  Limits the amount of time a host is required to maintain possibly
      stale information.

5.1.2.  Signaling Stale Configuration Information

   In order to phase-out stale configuration information, the SLAAC
   specification is updated as follows:

   o  A router sending RAs that advertise dynamically-learned prefixes
      (e.g. via DHCPv6-PD) on an interface MUST record, on stable
      storage, the list of prefixes being advertised on each network
      segment.

   o  Upon changes to the advertised prefixes, and after bootstrapping,
      the router advertising prefix information via SLAAC should proceed
      as follows:

      *  Any prefixes that were previously advertised via SLAAC, but
         that are not currently intended for address configuration, MUST




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         be advertised with a PIO option with the "A" bit set to 1 and
         the "Valid Lifetime" and a "Preferred Lifetime" set to 0.

      *  Any prefixes that were previously advertised via SLAAC as "on-
         link", but that are not currently not considered "on-link",
         MUST be advertised with a PIO option with the "L" bit set to 1
         and the "Valid Lifetime" and a "Preferred Lifetime" set to 0.

      *  If both of the previous conditions are met (a prefix was
         previously advertised with both the "A" and "L" bits set, but
         is currently *not* intended for address configuration and is
         *not* considered on-link), the prefix MUST be advertised with a
         PIO option with both the "A" and "L" bits set to 1 and the
         "Valid Lifetime" and a "Preferred Lifetime" set to 0.  That is.
         the advertisements of the previous two steps can be coalesced
         into a single one with both the "A" and "L" bits set.

      *  The aforementioned advertisement SHOULD be performed for at
         least the "Valid Lifetime" previously employed for such prefix.

   Additionally, 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.

   to:

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

   The rationale for this update is:

   o  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.

5.1.3.  Recovery from Stale Configuration Information

   The goal of the mechanism 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.




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   The basic premise behind the mechanism specified in this section 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, configured
   addresses for the stale prefixes are initially "un-preferred" (such
   that they are not employed for new communication instances), and they
   are subsequently removed (if this condition persists).

   Local information maintained for each prefix advertised by each
   router is augmented with one boolean flag named "LTA" (Lifetime
   Avoidance) that defaults to "FALSE".  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].

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

   o  The LTA flag for each prefix advertised in the current Router
      Advertisement should be set to "FALSE".

   o  For each prefix that had been previously advertised by this router
      but that does not have a corresponding PIO with the "A" flag set
      in the received RA, proceed as follows:

      *  IF the LTA flag is "TRUE", then:

         +  IF there is any address configured for this prefix with a
            "Preferred Lifetime" larger than 0, set its "Preferred
            Lifetime" to 0, and the LTA flag of this prefix to "FALSE".

         +  ELSE (all addresses for this prefix have a "Preferred
            Lifetime" of 0), set the "Valid Lifetime" of all addresses
            configured for this prefix to 0, and the LTA flag of this
            prefix to "FALSE".  This will cause removal of all addresses
            for this prefix.  Additionally, if the corresponding prefix
            had been advertised as on-link ("L"=1) by this router,
            remove any routes to this prefix associated with the network
            interface card on which the RA packet was received.

      *  ELSE (the LTA flag is "FALSE"):

         +  Set the LTA flag of the corresponding prefix to "TRUE".






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   Appendix B illustrates the packet exchange and operation of the
   algorithm for a typical scenario.  Appendix A provides a flowchart
   for this algorithm.

   NOTES:

   o  The aforementioned processing assumes that while network
      configuration information might be split into multiple RAs, PIOs
      will be spread among *at most* two RAs.  This assumption avoids
      the use of any timers for this specific purpose.

   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 new
      prefix being advertised, the traditional processing is unaffected
      (the mechanism discussed in this document will *never* be
      triggered because no packets with PIOs with the "A" flag will be
      received).  The logic here is that it is better to have some
      address, than no address at all.

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

   o  The specified modification takes the conservative approach of
      first setting the "Preferred Lifetime" to 0 (such that addresses
      become non-preferred), and subsequently setting the "Valid
      Lifetime" to 0 (such as the addresses are completely removed).
      Once the addresses for this prefix have been removed, routes to
      this prefix associated with the network interface on which the RA
      packets were received are also 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", and less
      than five minutes to completely remove such addresses from the
      system.  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, 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
      MinRtrAdvInterval (default: 0.33 * MaxRtrAdvInterval) and
      MaxRtrAdvInterval (default: 600 seconds) has elapsed, thus meaning
      that the stale addresses would be removed between 3.3 and 10
      minutes of being marked as "not preferred".





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5.2.  Interaction Between DHCPv6-PD and SLAAC

   The "Preferred Lifetime" and "Valid Lifetime" of PIOs corresponding
   to prefixes learned via DHCPv6-PD MUST NOT span past the lease time
   of the DHCPv6-PD prefixes.  This means that the advertised "Preferred
   Lifetime" and "Valid Lifetime" MUST be dynamically adjusted such that
   the advertised lifetimes never span past the lease time of the
   prefixes delegated via DHCPv6-PD.

   This is in line with these existing requirements from other
   specifications, which we reference here for clarity:

   o  [RFC8415] specifies, in Section 6.3, that "if the delegated prefix
      or a prefix derived from it is advertised for stateless address
      autoconfiguration [RFC4862], the advertised preferred and valid
      lifetimes MUST NOT exceed the corresponding remaining lifetimes of
      the delegated prefix."

5.3.  Improved CPE behavior

   This section specifies and clarifies requirements for CPE routers
   (particularly when they advertise prefixes learned via DHCPv6-PD)
   that can help mitigate the problem discussed in Section 1.  This
   improves the situation for hosts that do not implement the
   modification specified in Section 5.1 but would obviously make
   robustness dependent on the CPE (on which the user or ISP may have no
   control), as opposed to the host itself.  This mechanism is mostly
   orthogonal to the improved host behaviour specified in Section 5.1.

   The updated behaviour is as follows:

   o  The CPE MUST signal stale configuration information as specified
      in Section 5.1.2

   o  The CPE MUST implement the DHCPv6-PD/SLAAC interface specified in
      Section 5.2.

   The aforementioned improved behaviour assumes compliance with the
   following existing requirements from other specifications, which we
   reference here for clarity:

   o  [RFC7084] specifies (requirement LE-13, in Section 4.3) that when
      the delegated prefix changes (i.e., the current prefix is replaced
      with a new prefix without any overlapping time period), "the IPv6
      CE router MUST immediately advertise the old prefix with a
      Preferred Lifetime of zero and a Valid Lifetime of either a) zero
      or b) the lower of the current Valid Lifetime and two hours (which




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      must be decremented in real time) in a Router Advertisement
      message as described in Section 5.5.3, (e) of [RFC4862]"

6.  IANA Considerations

   This document has no actions for IANA.

7.  Security Considerations

   This document discusses a a problem that may arise in scenarios where
   dynamic IPv6 prefixes are employed, and proposes workarounds that
   enable such usage while avoiding interoperability problems.  The
   security and privacy implications of IPv6 addresses are discussed in
   [RFC7721].

   An attacker that could impersonate a router could forge multiple RA
   packets that contain PIOs of prefixes that are currently not
   advertised on the local network, to trigger the mechanism specified
   in this document to cause addresses currently configured for the
   legitimate prefixes to be removed.  However, an attacker that can
   impersonate a router could more easily achieve the same goal by
   advertising the legitimate prefixes with both the "Preferred
   Lifetime" and "Valid Lifetime" set to 0.

   Capping the "Valid Lifetime" and "Preferred Lifetime" at hosts limit
   the length of the effects of a non-sustained attack, since hosts
   would now be able to recover in a timelier manner.

   Attacks based on forged RA packed can be mitigated with technologies
   such as RA-Guard [RFC6105] [RFC7113].

8.  Acknowledgments

   The authors would lie to thank (in alphabetical order) Mikael
   Abrahamsson, Luis Balbinot, Brian Carpenter, Gert Doering, Nick
   Hilliard, Philip Homburg, Lee Howard, Christian Huitema, Albert
   Manfredi, Jordi Palet Martinez, Richard Patterson, 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.

   The problem discussed in this document has been previously documented
   in [RIPE-690].






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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>.

   [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>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [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

   [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>.






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   [GERMAN-DP]
              BFDI, "Einfuhrung von IPv6 Hinweise fur Provider im
              Privatkundengeschaft und Herstellere", Entschliessung der
              84. Konferenz der Datenschutzbeauftragten des Bundes und
              der Lander am 7./8. November 2012 in Frankfurt (Oder),
              November 2012,
              <http://www.bfdi.bund.de/SharedDocs/Publikationen/
              Entschliessungssammlung/DSBundLaender/84DSK_EinfuehrungIPv
              6.pdf?__blob=publicationFile>.

   [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>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <https://www.rfc-editor.org/info/rfc7084>.

   [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>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.








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   [RIPE-690]
              Zorz, J., Zorz, S., Drazumeric, P., Townsley, M., Alston,
              J., Doering, G., Palet, J., Linkova, J., Balbinot, L.,
              Meynell, K., and L. Howard, "Best Current Operational
              Practice for Operators: IPv6 prefix assignment for end-
              users - persistent vs non-persistent, and what size to
              choose", RIPE 690, October 2017,
              <https://www.ripe.net/publications/docs/ripe-690>.

   [UK-NOF]   Palet, J., "IPv6 Deployment Survey (Residential/Household
              Services) How IPv6 is being deployed?", UK NOF 39, January
              2018,
              <https://indico.uknof.org.uk/event/41/contributions/542/
              attachments/712/866/bcop-ipv6-prefix-v9.pdf>.

Appendix A.  Flowchart for Host Processing of RAs

   Conceptually, the mechanism operates as follows:

































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   +----------------+
   |  Normal proc.  |
   |  of RA packet  |
   +---------+------+
           |
           v
   +----------------+
   |   RA has PIO   |  No
   |   with A==1?   |------>  DONE
   +----------------+
           |
           |  Yes
           v
   +----------------+
   | (For each PIO) |
   |    LTA=TRUE    |
   +----------------+
           |
           v
   +----------------+      +-----------------+
   | (For each Pref |      |                 |
   |   assoc. with  | Yes  |                 |
   |   this router) |----->|    LTA=FALSE    |
   | Is there corr. |      |                 |
   | PIO in the RA? |      |                 |
   +----------------+      +-----------------+
           |
           | No
           v
   +----------------+      +-----------------+      +----------------+
   |                |      | (For each addr. |      | Valid Lftime=0 |
   |                | Yes  | for this Pref.) | Yes  |   LTA=FALSE    |
   |   LTA==TRUE?   |----->|                 |----->|  (REM. ADDR!)  |
   |                |      | Pref Lftime==0? |      |                !
   +----------------+      +-----------------+      +----------------+
           |                        |
           | No                     | No
           v                        v
   +----------------+      +-----------------+
   |                |      |  Pref. Lftime=0 |
   |   LTA=TRUE     |      |    LTA=FALSE    |
   +----------------+      +-----------------+

Appendix B.  Sample Timeline for Host Processing of RAs

   The following example illustrates a sample packet exchange that
   illustrates the algorithm specified in Section 5.1:




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

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

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

Appendix C.  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 (e.g. on the 6man list).  The following subsections
   analyze these suggested workarounds, in the hopes of avoiding
   rehashing discussions of such workarounds.






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C.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]) might 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 might 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 enforces 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
      to be 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.
      This is of particular concern since ICMP-based attack do not even
      require that the Source Address of the attack packets be spoofed
      [RFC5927].

C.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.

   It has been suggested that 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 3.5: 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).

   There are a number of limitations and drawbacks associated with this
   approach:

   o  It may help to improve the selection of a source address, but it
      does not help with the housekeeping of configured information:

      *  If the stale prefix is re-used in another network, nodes
         employing stale addresses and routes for this prefixes will be
         unable to communicate with the new "owners" of the prefix.

      *  Given that 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.

   o  The earlier the above rule is incorporated into the [RFC6724]
      rules, the more it could lead to suboptimal address selection.
      For example, if incorporated as Rule 3.5 (before Rule #4, but
      after Rule 3), an implementation may end up choosing a source
      address from a "fresher" prefix rather than one with a longest-
      matching prefix, thus leading to suboptimal routing.  On the other
      hand, the later the rule is incorporated into the [RFC6724] rules,
      the more irrelevant the rule becomes (since existing rules might
      prefer the stale addresses).

   o  In scenarios where multiple prefixes are being advertised (whether
      by a single router via multiple RAs, multiple routers on the same
      LAN segment, or different routers on different LAN segments (when
      a host has multiple interfaces)), the new SAS rule is guaranteed
      to result in non-deterministic behaviour, with hosts frequently




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      changing the selected address.  This is certainly not desirable
      from a troubleshooting perspective.

   As a result, updating IPv6 source address selection does not relieve
   nodes from improving their SLAAC implementations as specified in
   Section 5.1, if at all desirable.  On the other hand, employing
   appropriate timers as specified in Section 5.1.1 would result in Rule
   3 of [RFC6724] employing fresh addresses, without leading to
   undeterministic behaviour.

Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   Segurola y Habana 4310, 7mo Piso
   Villa Devoto, Ciudad Autonoma de Buenos Aires
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com


   Jan Zorz

   Email: jan@go6.si

























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