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Versions: 00 01

v6ops                                                         J. Linkova
Internet-Draft                                                    Google
Intended status: Informational                              July 5, 2019
Expires: January 6, 2020


Neighbor Cache Entries on First-Hop Routers: Operational Considerations
                  draft-linkova-v6ops-nd-cache-init-01

Abstract

   Neighbor Discovery (RFC4861) is used by IPv6 nodes to determine the
   link-layer addresses of neighboring nodes as well as to discover and
   maintain reachability information.  This document discusses how the
   neighbor discovery state machine on a first-hop router is causing
   user-visible connectivity issues when a new (not being seen on the
   network before) IPv6 address is being used.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 6, 2020.

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
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Potential Solutions . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Do Nothing  . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Change to the Registration-Based Neighbor Discovery . . .   5
     2.3.  Hosts Explicitly Advertizing Their GUAs Using Existing ND
           Mechanisms  . . . . . . . . . . . . . . . . . . . . . . .   6
       2.3.1.  Host Sending Unsolicited NA . . . . . . . . . . . . .   6
         2.3.1.1.  Pros  . . . . . . . . . . . . . . . . . . . . . .   7
         2.3.1.2.  Cons  . . . . . . . . . . . . . . . . . . . . . .   7
       2.3.2.  Host Sending NS to the Router Address from Its GUA  .   7
         2.3.2.1.  Pros  . . . . . . . . . . . . . . . . . . . . . .   7
         2.3.2.2.  Cons  . . . . . . . . . . . . . . . . . . . . . .   7
       2.3.3.  Host Sending Router Solicitation from its GUA . . . .   8
         2.3.3.1.  Pros  . . . . . . . . . . . . . . . . . . . . . .   8
         2.3.3.2.  Cons  . . . . . . . . . . . . . . . . . . . . . .   8
     2.4.  Initiating Hosts2Routers Communication  . . . . . . . . .   8
       2.4.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .   9
       2.4.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .   9
     2.5.  Tweaking Probing Algorithms . . . . . . . . . . . . . . .   9
     2.6.  Routers Buffering More Packets  . . . . . . . . . . . . .   9
       2.6.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .  10
       2.6.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  10
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Avoiding Disruption . . . . . . . . . . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The section 7.2.5 of [RFC4861] states: "When a valid Neighbor
   Advertisement is received (either solicited or unsolicited), the
   Neighbor Cache is searched for the target's entry.  If no entry
   exists, the advertisement SHOULD be silently discarded.  There is no




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   need to create an entry if none exists, since the recipient has
   apparently not initiated any communication with the target."

   This approach is perfectly suitable for host2host communications
   which are in most cases bi-directional and it could be expected that
   if a host A has an ND cache entry for the host B IPv6 address, the
   host B also has the corresponding ND entry for the host A address in
   its cache.  However when a host communicates to off-link destinations
   via its first-hop router that logic does not apply.  Here is the most
   typical scenario when the problem may arise:

   1.  When a host joins the network it receives an RA packet from the
       first-hop router (either a periodic unsolicited RA or a response
       to an RS sent by the host).  The RA contains information the host
       needs to perform SLAAC and to configure its network stack.  Among
       other things the host populates its ND cache with the router
       link-local address and potentially link-layer address (if
       included in the RA Source Link-Layer Address option).

   2.  The host starts opening connections to off-link destinations.
       Very common use case is a mobile device sending probes to detect
       the Internet connectivity and/or the captive portals presence on
       the network.  To speed up that process many implementations are
       using the Optimistic Duplicate Address Detection ([RFC4429])
       which allows them to send probes from their GUA before the DAD
       process is completed.  Imprortant point here is that at that
       moment the device ND cache contains all information required to
       send those probes (such as the default gateway LLA and the link-
       layer address).  The router ND cache, however, might contain an
       entry for the device link-local address (if the device has been
       performing the ND process for the roiter LLA) but there are no
       entries for the device GUA.

   3.  Response packets for the probes (or any other traffic sent by the
       host) are received by the first-hop router.  As the router does
       not have any ND cache entry for the host GUA, the router starts
       the neighbor discovery process by creating an INCOMPLETE cache
       entry and then sending an NS to the Solicited Node Multicast
       Address.  Apparently most of the router implementations buffer
       only one data packet while performing the ND process for its
       destination.  Therefore all packets for the host GUA, except for
       the very first one are dropped until the address resolution
       process is completed.

   4.  As many implementations send multiple probes in parallel it's
       very likely that all probes ex. the first one would be considered
       failed.  If the host implements an exponential backoff for




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       probing it leads to user-noticeable delay in detecting network
       connectivity/reporting the network as usable.

   The above-mentioned scenario illustrates the problem happening when
   the device connects to the network for the first time/after a long
   timeout.  However the same sequence of events happen when the host
   starts using the new (previously unseen by the router or ) GUA (e.g.
   a new privacy address [RFC4941]) or if the router Neighbor Cache has
   been flushed.

   While in dual-stack networks this problem might hidden by Happy
   Eyeballs ([RFC8305]) it manifests itself quite clearly in IPv6-only
   networks, especially wireless ones, leading to poor user experience
   and contributing to negative perception of IPv6-only solutions as
   unstable and non-deployable.

1.1.  Requirements Language

   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.

1.2.  Terminology

   ND: Neighbor Discovery, [RFC4861].

   SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].

   NS: Neighbor Solicitation, [RFC4861].

   NA: Neighbor Advertisement, [RFC4861].

   RS: Router Solicitation, [RFC4861].

   RA: Router Advertisement, [RFC4861].

   SLLA: Source link-layer Address, an option in the ND packets
   containing the link-layer address of the sender of the packet
   ([RFC4861]).

   TLLA: Target link-layer Address, an option in the ND packets
   containing the link-layer address of the target ([RFC4861]).

   GUA: Global Unicast Address ([RFC4291]).

   DAD: Duplicate Address Detection, [RFC4862].



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   Optimistic DAD: a modification of DAD, [RFC4429].

2.  Potential Solutions

   The problem could be addressed from different angles.  Possible
   approaches are:

   o  Just do nothing.

   o  Migrate from the "reactive" Neighbor Discovery ([RFC4861]) to the
      registration-based mechanisms ([RFC8505]).

   o  The host explicitly advertizes its GUAs using Neighbor Discovery
      mechanisms.

   o  The host initiates bidirectional communication to the router using
      the host GUA.

   o  Making the probing logic on hosts more robust.

   o  Increasing the buffer size on routers.

   The following sections discuss those approaches in more detail.

2.1.  Do Nothing

   One of the possible approaches might be to declare that everything is
   working as intended.

2.1.1.  Pros

   o  No work required.

2.1.2.  Cons

   o  Unhappy users.

   o  Many support tickets.

   o  More resistance to deploy IPv6 and IPv6-Only networks.

2.2.  Change to the Registration-Based Neighbor Discovery

   The most radical approach would be to move away from the reactive ND
   as defined in [RFC4861] and expand the registration-based ND
   ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area
   Networks (6LoWPANs) to the rest of IPv6 deployments.




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   This option required some investigation and discussions.  More text
   will be added here in the following revision of the draft.

2.3.  Hosts Explicitly Advertizing Their GUAs Using Existing ND
      Mechanisms

   The Neighbor Discovery is designed to allow IPv6 nodes to discover
   neighboring nodes reachability and learn IPv6 to link-layer addresses
   mapping.  Therefore ND seems to be the most appropriate tool to
   inform the first-hop routers about addresses the host is going to
   use.  The following sections discuss potential apptoaches in more
   detail.

2.3.1.  Host Sending Unsolicited NA

   Section 4.4 of [RFC4861] says:

   "A node sends Neighbor Advertisements in response to Neighbor
   Solicitations and sends unsolicited Neighbor Advertisements in order
   to (unreliably) propagate new information quickly."

   Propagating information about new GUA as quickly as possible is
   exactly what is required to solve the problem outlined in this
   document.  Therefore the host might send an unsolicited NA to
   advertize its GUA as soon as the said address enters Optimistic or
   Preferred state.  The NA should include the target link-layer address
   option.  To ensure that all first-hop routers receive the
   advertisement it could be sent to all-routers multicast address
   (ff02::2).

   As it's been mentioned, [RFC4861] explicitly states that receiving a
   NA should not create a new NC entry.  However the justification for
   that requirement ("There is no need to create an entry if none
   exists, since the recipient has apparently not initiated any
   communication with the target.") clearly does not apply for the case
   discussed.  As per [RFC2119] "there may exist valid reasons in
   particular circumstances to ignore a particular item, but the full
   implications must be understood and carefully weighed before choosing
   a different course.".  Therefore routers creating a new NC entry upon
   receiving an unsolicited NA would still be compliant with [RFC4861].

   It should be noted that some routing and switching platforms have
   implemented such behaviour already.  Administrators could enable
   creating neighbor discovery cache entries based on unsolicited NA
   packets sent from the previously unknown neighbors on that interface.






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2.3.1.1.  Pros

   o  Already implemented on some platforms.

   o  In accordance with [RFC4861].

2.3.1.2.  Cons

   o  Allows a malicious host to execute an ND cache exhaustion attack.
      It's recommended that this functionality is configurable and
      recommendations from [RFC6583] are taken into account.

   o  Requires hosts to send unsolicited NA (changes to the hosts).

   o  Some wireless devices are known to fiddle with ND packets and
      perform various non-obvious forms of ND proxy actions.  In some
      cases unsolicited NAs might not even reach the routers.

2.3.2.  Host Sending NS to the Router Address from Its GUA

   The host could force creating a STALE entry for its GUA in the router
   ND cache by sending the following Neighbor Solicitation message:

   o  The NS source address is the host GUA.

   o  The Source Link-Layer Address option contains the host link-layer
      address.

   o  The target address is the host default gateway address (the
      default router address the host received in the RA).

   The main disadvantage of this approach is that it would not work if
   the GUA the host needs to advertise is still in the Optimistic state.
   The section 2.2 of [RFC4429] explicitly prohibits sending Neighbor
   Solicitations from an Optimistic Address.

2.3.2.1.  Pros

   o  Router implementations which follow recommendations made in
      [RFC6583] might prioritize responding to NS packets to own
      addresses.

2.3.2.2.  Cons

   o  Does not work for Optimistic addresses (see section 2.2 of
      [RFC4429]).





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   o  If first-hop redundancy is deployed in the network, the NS would
      reach the active router only, so all backup routers (or all active
      routers ex. one) would not get their neighbor cache updated.

   o  Some wireless devices are known to fiddle with ND packets and
      perform various non-obvious forms of ND proxy actions.  In some
      cases unsolicited NAs might not even reach the routers.

2.3.3.  Host Sending Router Solicitation from its GUA

   The host could send a router solicitation message to 'all routers'
   multicast address, using its GUA as a source.  If the host link-layer
   address is included in the Source Link-Layer Address option, the
   router would create a STALE entry for the host GUA (see the section
   6.2.6 of [RFC4861]).  However this approach can not be used if the
   GUA is in optimistic state: the section 2.2 of [RFC4429] explicitly
   prohibits using an Optimistic Address as the source address of a
   Router Solicitation with a SLLAO as it might disrupt the rightful
   owner of the address in the case of a collision.  So for the
   optimistic addresses the host can send an RS without SLLAO included.
   In that case the router may respond with either a multicast or a
   unicast RA (only the latter would create a cache entry).

2.3.3.1.  Pros

   o  Unlike NS packets, RS packets would reach all routers on link,
      allowing all routers to update their neighbor caches and
      preventing packet loss in case of asymmetric routing.

2.3.3.2.  Cons

   o  As for the Optimistic addresses SLLAO can not be included into RS
      packets, the cache entry for the optimistic address would be
      created only if the router sends solicited RAs as unicast.  In
      addition, there might be a random delay between receiving an RS
      and sending a unicast RA back (and creating a cache entry) which
      might undermine the idea of creating the cache entry proactively.

   o  Some wireless devices are known to fiddle with ND packets and
      perform various non-obvious forms of ND proxy actions.  In some
      cases RSes might not even reach the routers.

2.4.  Initiating Hosts2Routers Communication

   Every time the host configures a new GUA (when the address enters the
   Optimistic state or, if the optimistic DAD is not used, as soon as it
   changes the state from tentative to preferred) the host can a ping or
   traceroute packet to the default gateway LLA.  As the RTT to the



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   default gateway is lower than RTT to any off-link destinations it's
   quite likely that the router would start the neighbor discovery
   process for the host GUA before the first packet of the returning
   traffic arrives.  There are pretty good chances that the process
   would be completed before the actual data traffic reaches the router.

2.4.1.  Pros

   o  As data packets are involved, there is no potential impact caused
      by smart wireless infrastructure performing ND proxy.

   o  Full compliance with existing standards.

2.4.2.  Cons

   o  Data packets to the router LLA could be blocked by security policy
      or control plane protection mechanism.

   o  Maximum overhead for routers control plane (in addition to
      processing ND packets, the data packet needs to be processed as
      well).

   o  If the first hop redundancy is implemented in the network the host
      ping/traceroute packet would reach the active router only.  All
      backup routers would not receive it and therefore would not start
      populating the cache.  So in the case of asymmetric traffic flow
      (packets leave the network via one router while the return flow is
      going via another) the backup router(s) still would not have the
      cache entry.  (A hacky way to overcome this limitation would be
      sending ping/traceroute packet to 'all routers' ff02::2 multicast
      address).

2.5.  Tweaking Probing Algorithms

   While tweaking the probing logic on devices might make the problem
   less visible it would be still desirable to avoid packet loss
   everytime the new GUA is used by a host.  It would be quite tricky to
   adjust every probing algorithm to find the right balance between
   prompt detection of network connectivity and false positives in
   IPv6-only mode.

2.6.  Routers Buffering More Packets

   Another way to mitigate the issue, at least partially, would be
   increasing the number of packets the router could buffer while
   performing the neighbor discovery process for the INCOMPLETE cache
   entry.  However it would be against recommendations made in the
   section 7.2.2 of [RFC4861] and [RFC6583].



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2.6.1.  Pros

   o  Does not require changes on hosts.

2.6.2.  Cons

   o  This approach makes the routers even more vulnerable to attack
      vectors described in [RFC6583].  In particular, it would amplify
      the impact of any scanning attack.

   o  Against the recommendations from the section 7 of [RFC6583].

   o  Requires router vendors support.

3.  Recommendations

   o  Hosts SHOULD send at least one unsolicited NA packet to all-
      routers multicast address (ff02::2) as soon as one of the
      following events happens:

      *  (if Optimistic DAD is used): a new Optimistic GUA is assigned
         to the host interface.

      *  (if Optimistic DAD is not used): a GUA changes the state from
         tentative to preferred.

   o  Routers SHOULD have a configuration knob to enable creating ND
      cache entry upon receiving unsolicited NAs on a specific
      interface.  This document does not change the behavior if the ND
      cache entry already exists when receiving an unsolicited NA.

3.1.  Avoiding Disruption

   If hosts following the recommendations in this document are using the
   DAD mechanism defined in [RFC4862], they would send unsolicited NA as
   soon as the address changes the state from tentative to preferred
   (after its uniqueness has been verified).  However hosts willing to
   minimize network stack configuration delays might be using optimistic
   addresses, which means there is possibility of the address not being
   unique on the link.  The section 2.2 of [RFC4429] discusses measures
   to ensure that ND packets from the optimistic address do not override
   any existing neighbor cache entries as it would cause traffic
   interruption of the rightful address owner in case of address
   conflict.

   As hosts willing to speed up their network stack configuration are
   most likely to be affected by the problem outlined in this document
   it seems reasonable for such hosts to advertise their optimistic GUAs



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   by sending unsolicited NAs.  The main question to consider is the
   potential risk of overriding the cache entry for the rightful address
   owner if the optimistic address happens to be duplicated.

   As per section 7.2.5 of [RFC4861] if the Neighbor Cache entry for the
   target address already exists and is in in any state other than
   INCOMPLETE then the only change the unsolicited NA could cause is to
   change the entry from REACHABLE to STALE.  It would not cause any
   traffic interruption for the rightful address owner.

   If there is no entry then it would be created/updated with the
   supplied LLA and its state set to STALE.  In that case as soon as the
   entry is used for sending traffic to the host, the entry state will
   be changed to DELAY and the Neighbor Unreachability Detection would
   be started and the rightful owner LLA will be entered in the cache.
   So in the scenario when the rightful owner does not use the address
   for communication then it might be a short (a few seconds) period of
   time when the data packets sent from the outside could reach the host
   with the optimistic address.  However it seems likely that hosts
   using Optimistic DAD would start sending/receiving traffic right
   away, so the first return packet would trigger the NUD process and
   rewrite the cache.

   Another corner case is the INCOMPLETE cache entry for the address.
   If the host sends an unsolicited NA from the Optimistic address it
   would update the entry with the host LLA and set the entry to the
   STALE state.  As the INCOMPLETE entry means that the router has
   started the ND process for the address and the multicast NS has been
   sent, the rightful owner is expected to reply with solicited NA which
   would recover the cache entry and set the LLA to the rightful owner's
   one.  The risk here:

   o  The data packet arrives after the unsolicited NA from the host but
      before the rightful owner responded with the solicited NA.  Those
      packets would be sent to the host with the optimistic address
      instead of its rightful owner.  However without the unsolicited NA
      those packets would have been dropped anyway (as the entry was in
      INCOMPLETE state).

4.  IANA Considerations

   This memo asks the IANA for no new parameters.

5.  Security Considerations

   One of the potential attack vector to consider is a cache spoofing
   when the attacker might try to install a cache entry for the victim's
   IPv6 address and the attacker's Link-Layer address.  However it



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   should be noted that this document does not propose any changes for
   the scenario when the ND cache for the given IPv6 address already
   exists.  Therefore it is not possible for the attacker to override
   any existing cache entry.

   A malicious host could attempt to exhaust the neighbor cache on the
   router by creating a large number of STALE entries.  However this
   attack vector is not new and this document does not increase the risk
   of such attack: the attacker could do it, for example, by sending a
   NS or RS packet with SLLAO included.  All recommendations from
   [RFC6583] still apply.

6.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   review and feedback: Lorenzo Colitti, Tatuya Jinmei, Erik Kline,
   Warren Kumari, Michael Richardson, Pascal Thubert, Loganaden
   Velvindron, Eric Vyncke.

7.  References

7.1.  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>.

   [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>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [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>.






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   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [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>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

7.2.  Informative References

   [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>.

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583,
              DOI 10.17487/RFC6583, March 2012,
              <https://www.rfc-editor.org/info/rfc6583>.

Author's Address

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont, NSW  2009
   AU

   Email: furry@google.com









Linkova                  Expires January 6, 2020               [Page 13]


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