v6ops                                                         J. Linkova
Internet-Draft                                                    Google
Intended status: Informational                          October 27,                         December 10, 2019
Expires: April 29, June 12, 2020

Neighbor Cache Entries on First-Hop Routers: Operational Considerations
                   draft-ietf-v6ops-nd-cache-init-00
                   draft-ietf-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
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   This Internet-Draft will expire on April 29, June 12, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Potential Solutions . . . .  Proposed Solution . . . . . . . . . . . . . . . . .   5
     2.1.  Do Nothing  . . . . . . . . .   4
     2.1.  Solution Requirements . . . . . . . . . . . . . .   5
       2.1.1.  Pros . . . .   4
     2.2.  Solution Overview . . . . . . . . . . . . . . . . . . . .   5
       2.1.2.  Cons  . . . . . . . . . . . . . .
   3.  Solutions Considered but Discarded  . . . . . . . . . .   5
     2.2.  Change to the Registration-Based Neighbor Discovery . . .   6
     2.3.  Hosts Explicitly Advertizing Their GUAs Using Existing ND
           Mechanisms  . . . . . . . . . . . . .
     3.1.  Do Nothing  . . . . . . . . . .   6
       2.3.1.  Host Sending Unsolicited NA . . . . . . . . . . . . .   6
         2.3.1.1.  Pros  . . . . . . . . . . . . . . . . . . . . . .   7
         2.3.1.2.  Cons  . . . . . . . . . . . . . . . . . . .
     3.2.  Change to the Registration-Based Neighbor Discovery . . .   7
       2.3.2.
     3.3.  Host Sending NS to the Router Address from Its GUA  .   7
         2.3.2.1.  Pros  . . . . . . . . . . . . . . . . . . . . . .   8
         2.3.2.2.  Cons  . . . . . . . . . . . . . . . . . . . . . .   8
       2.3.3.   7
     3.4.  Host Sending Router Solicitation from its GUA . . . .   8
         2.3.3.1.  Pros . .   7
     3.5.  Routers Populating Their Caches by Gleaning From Neighbor
           Discovery Packets . . . . . . . . . . . . . . . . . . . .   8
         2.3.3.2.  Cons  . . . . . . . . . . . . .
     3.6.  Initiating Hosts2Routers Communication  . . . . . . . . .   8
     2.4.  Routers Populating Their Caches by Gleaning   9
     3.7.  Transit Dataplane Traffic From Neighbor
           Discovery Packets . a New Address Triggering
           Address Resolution  . . . . . . . . . . . . . . . . . . .   9
       2.4.1.  Pros  . . .
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
       2.4.2.  Cons  . . . . .
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     2.5.  Initiating Hosts2Routers Communication  . . . . . . .
   6.  Acknowledgements  . .   9
       2.5.1.  Pros . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . .  10
       2.5.2.  Cons . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . .  10
     2.6.  Tweaking Probing Algorithms . . . . . . . . . . . . . . .  10
     2.7.  Routers Buffering More Packets  . .
     7.2.  Informative References  . . . . . . . . . . .  10
       2.7.1.  Pros . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . .  10
       2.7.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  11
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   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
   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  The most typical
   scenario when the problem may arise:

   1.  When arise is a host joining the network,
   forming a new address and using that address for accessing the
   Internet:

   1.  A host joins the network it and receives an RA a Router Advertisement
       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  As in most cases the host populates its ND cache with RA also contains the router
       link-local address and potentially Source
       link-layer address (if
       included in of the RA Source Link-Layer Address option). router, the host can populate its
       Neighbor Cache with the router link-local and link-layer
       addresses.

   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 use
       the Optimistic Duplicate Address Detection ([RFC4429]) [RFC4429] which allows
       them to send probes from their GUA before the DAD process is
       completed.  Imprortant point here is that at  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 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  Return traffic sent by the
       host) are is received by the first-hop router.  As the
       router does not have any ND cache entry for the host GUA, GUA yet, 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  Most router implementations buffer only
       one data packet while performing the ND process neighbor discovery for its
       destination.  Therefore the
       packet destination address, so it would drop all subsequent
       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  If the host sends multiple probes in parallel it's
       very likely that all probes ex. the first one it would be considered
       all but one of them failed.  If the host implements an exponential backoff for
       probing it  It leads to user-noticeable user-visible delay in detecting network
       connectivity/reporting
       connecting to the network network, especially if the host implements some
       form of backoff mechanism and does not retransmit the porbes as usable.

   The above-mentioned
       soon as possible.

   This scenario illustrates the problem happening when the device
   connects to the network for the first time/after time or after a timeout long
   timeout.
   enough for the device address to be removed from the router neighbor
   cache.  However the same sequence of events happen when the host
   starts using the new (previously GUA previously unseen by the router or ) GUA (e.g. router, such as a
   new privacy address [RFC4941]) [RFC4941] or if the router Neighbor Cache has
   been flushed.

   While in dual-stack networks this problem might hidden by Happy
   Eyeballs ([RFC8305]) [RFC8305] it manifests itself manifest quite clearly in IPv6-only
   networks,
   environments, 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 outer 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]). packet,
   [RFC4861].

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

   GUA: Global Unicast Address ([RFC4291]). Address, [RFC4291].

   DAD: Duplicate Address Detection, [RFC4862].

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

2.  Potential Solutions

   The problem could  Proposed Solution

2.1.  Solution Requirements

   It would be addressed from different angles.  Possible
   approaches are:

   o  Just do nothing.

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

   o  The host explicitly advertizes its GUAs using Neighbor Discovery
      mechanisms.
   mechanics so routers have a usable cache entry for a host address by
   the time the first packet for that address is received by the router.
   In particular,
   o  The  If the router creates new entries in its does not have a Neighbor Cache by gleaning
      from Neighbor Discovery DAD messages. entry for the
      address, a STALE entry needs to be created.

   o  The host initiates bidirectional communication solution needs to work for Optimistic addresses as well.
      Devices implementing the router using Optimistic DAD usually attempt to
      minimize the host GUA.

   o  Making delay in connecting to the probing logic on hosts network and therefore are
      more robust.

   o  Increasing likely to be affected by the buffer size on routers.

   The following sections discuss those approaches problem described in more detail.

2.1.  Do Nothing

   One this
      document.

   o  In case of duplicate addresses present in 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 network the Registration-Based Neighbor Discovery

   The most radical approach would be to move away from proposed
      solution MUST NOT override the reactive ND
   as defined in [RFC4861] and expand existing entry.

   o  In topologies with multiple first hop routers the registration-based ND
   ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area
   Networks (6LoWPANs) cache needs to the rest
      be updated on all of IPv6 deployments.

   This option required some investigation and discussions and seems to them, as traffic might be an overkill for asymmetric:
      outgoing flows leaving the problem described in this document..

2.3.  Hosts Explicitly Advertizing Their GUAs Using Existing ND
      Mechanisms network via one router while the return
      traffic enters the segment via another one.

2.2.  Solution Overview

   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 with the
   target link-layer address option 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 proposed solution is discussed in [I-D.linkova-6man-grand].  In
   summary the
   advertisement it could be sent follwing changes to all-routers multicast address
   (ff02::2).

   As it's been mentioned, [RFC4861] explicitly states that receiving a are suggested:

   o  Hosts SHOULD send at least one unsolicited NA should not create packet to all-
      routers multicast address (ff02::2) as soon as one of the
      following events happens:

      *  (if Optimistic DAD is used): a new NC entry.  However the justification for
   that requirement ("There Optimistic GUA is no need assigned
         to create an entry if none
   exists, since the recipient has apparently not initiated any
   communication with the target.") clearly does host interface.

      *  (if Optimistic DAD is not apply for the case
   discussed.  As per [RFC2119] "there may exist valid reasons in
   particular circumstances to ignore used): a particular item, but GUA changes the full
   implications must be understood and carefully weighed before choosing
   a different course.".  Therefore routers creating state from
         tentative to preferred.

   o  Routers SHOULD create a new NC ND cache entry upon receiving an
      unsolicited NA would still be compliant with [RFC4861]. NAs.

   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.

2.3.1.1.  Pros

3.  Solutions Considered but Discarded

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

   o  Already implemented on some platforms.  Just do nothing.

   o  In accordance with [RFC4861].

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

   o  Allows a malicious  The router creates new entries in its Neighbor Cache by gleaning
      from Neighbor Discovery DAD messages.

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

   o  Making the probing logic on hosts more robust.

   o  Increasing the buffer size on routers.

   o  Transit dataplane traffic from an ND unknown address (an address w/o
      the corresponding neighbor cache exhaustion attack.
      It's recommended entry) triggers an address
      resolution process on the router.

   The following sections discuss those approaches in more detail.

3.1.  Do Nothing

   One of the possible approaches might be to declare that this functionality everything is configurable
   working as intended and
      recommendations from [RFC6583] are taken into account. let the upper-layer protocols to deal with
   packet loss.  The obvious drawbacks include:

   o  Requires hosts  Unhappy users.

   o  Many support tickets.

   o  More resistance to send unsolicited NA (changes deploy IPv6 and IPv6-Only networks.

3.2.  Change to the Registration-Based Neighbor Discovery

   The most radical approach would be to move away from the hosts).

   o  Some wireless devices are known to fiddle with reactive ND packets
   as defined in [RFC4861] and
      perform various non-obvious forms of expand the registration-based ND proxy actions.  In
   ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area
   Networks (6LoWPANs) to the rest of IPv6 deployments.  This option
   required some
      cases unsolicited NAs might not even reach investigation and discussions and seems to be an
   overkill for the routers.

2.3.2. problem described in this document.

3.3.  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 disadvantages of this approach is that it would are:

   o  Would not work if
   the GUA the host needs to advertise is still in the for Optimistic state.
   The addresses as 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]).

   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.

3.4.  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 as per the section
   6.2.6 of [RFC4861]). [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

   This approach has the following drawbacks:

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

2.3.3.2.  Cons

   o  As for the Optimistic addresses SLLAO state the RS can not be included into RS
      packets, the cache entry for contain
      SLLAO.  As a result the optimistic address router would be
      created only create a cache entry if
      the router sends solicited RAs is sent as as a unicast.  In
      addition, there  Routers sending
      solicited RAs as multicast would not create a new cache entry as
      they do not need to send a unicast packet back to the host.

   o  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.

3.5.  Routers Populating Their Caches by Gleaning From Neighbor
      Discovery Packets

   If hosts do not send unsolicited NAs upon configuring new addresses
   as described above the routers

   Routers may be able to learn about new address addresses by gleaning from the
   DAD Neighbor Solicitation messages.  The router could listen to all
   solicited node multicast address groups and upon receiving a Neighbor
   Solicitation from the unspecified address search its Neighbor Cache
   for the solicitation's Target Address.  If no entry exists the router
   may create an entry entry, set its reachability state to 'INCOMPLETE' and
   start the address resolution for that entry.

   The same solution was proposed in
   [I-D.halpern-6man-nd-pre-resolve-addr].  Some routing vendors support
   such optimization already.  However this approach has a number of
   drawbacks and set it's
   reachability state to 'INCOMPLETE'.  Then the router can start the
   address resolution for therefore should not be used as the new entry.

2.4.1.  Pros

   o  No changes required on hosts.

2.4.2.  Cons only solution:

   o  Routers would need to receive all multicast Neighbor Discovery packets
      which might negatively impact the routers CPU.

   o  If the router starts the address resolution as soon as it receives
      the DAD Neighbor Solicitation the host might be still performing
      the DAD and the target address might be tentative.  In that case
      the host SHOULD silently ignore the received Neighbor Solicitation
      from the router as per the Section 5.4.3 of [RFC4862].  Such race
      condition scenario would prevent  As a
      result the router might not be able to learn complete the new
      address.

2.5. address
      resolution before the return traffic arrives.

3.6.  Initiating Hosts2Routers Communication

   Every time the

   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 may trigger the state from tentative router to preferred) start the host can address resolution by
   sending a data packet such as ping or traceroute packet to the its default gateway LLA.
   router link-local address, using the GUA as a source address.  As the
   RTT to the 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.

2.5.1.  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.

2.5.2.  Cons

   The downside of this approach includes:

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

   o  Maximum  Additional 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  Unless the host
      ping/traceroute data 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 sent to 'all routers' ff02::2 multicast
      address).

2.6.  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
      address, if the right balance between
   prompt detection of network connectivity and false positives in
   IPv6-only mode.

2.7.  Routers Buffering More Packets

   Another way to mitigate the issue, at least partially, would be
   increasing the number of packets provides a first-hop redundancy then only
      the active router could buffer while
   performing the neighbor discovery process for the INCOMPLETE would create a new cache entry.  However it would be against recommendations made in the
   section 7.2.2 of [RFC4861] and [RFC6583].

2.7.1.  Pros

   o  Does not require changes on hosts.

2.7.2.  Cons

   o  This approach makes the routers even more vulnerable to attack
      vectors described in [RFC6583].  In particular,

3.7.  Transit Dataplane Traffic From a New Address Triggering Address
      Resolution

   When a router receives a transit packet it would amplify might check the impact presence
   of any scanning attack.

   o  Against the recommendations from neighbor cache entry for 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 source address (ff02::2) as soon as one of and if the
      following events happens:

      *  (if Optimistic DAD is used):
   entry does not exist start address resolution process.  This approach
   does ensure that a new Optimistic GUA is assigned
         to the host interface.

      *  (if Optimistic DAD Neighbor Cache entry is not used): proactively created every
   time a new, previously unseen GUA changes the state from
         tentative is used for sending offlink
   traffic.  However this functionality needs to preferred.

   o  Routers SHOULD have a configuration knob be limited to enable creating ND
      cache entry upon receiving unsolicited NAs on a specific
      interface.  This document does not change the behavior if
   explicitly configured networks/interfaces, as the ND
      cache entry already exists when receiving an unsolicited NA.

   As router needs to
   distinguish between onlink addresses (ones the recommendations include modification router needs to have
   Neighbor Discovery
   state machine defined in [RFC4861] Cache entries for) and hosts behaviour, they are
   discussed in a separate Standart track document draft-linkova-6man-
   grand. the rest of the address space.  In
   addition, implementing such functionality is much more complicated
   than all other solutions as it would involve complex data-control
   planes interaction.

4.  IANA Considerations

   This memo asks the IANA for no new parameters.

5.  Security Considerations

   See

   This memo documents the operational issue and does not introduce any
   new security considerations.  Security Considerations considerations of the proposed
   solution are discussed in the corresponding section of draft-linkova-6man-grand.
   [I-D.linkova-6man-grand].

6.  Acknowledgements

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

7.  References

7.1.  Normative References

   [I-D.linkova-6man-grand]
              Linkova, J., "Gratuitous Neighbor Discovery: Creating
              Neighbor Cache Entries on First-Hop Routers", draft-
              linkova-6man-grand-01 (work in progress), November 2019.

   [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>.

   [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

   [I-D.halpern-6man-nd-pre-resolve-addr]
              Chen, I. and J. Halpern, "Triggering ND Address Resolution
              on Receiving DAD-NS", draft-halpern-6man-nd-pre-resolve-
              addr-00 (work in progress), January 2014.

   [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