



Internet Engineering Task Force (IETF)                           X. Xiao
Request for Comments: 9898                                  E. Vasilenko
Category: Informational                              Huawei Technologies
ISSN: 2070-1721                                                  E. Metz
                                                                KPN N.V.
                                                               G. Mishra
                                                            Verizon Inc.
                                                             N. Buraglio
                                                 Energy Sciences Network
                                                           November 2025


         Neighbor Discovery Considerations in IPv6 Deployments

Abstract

   The Neighbor Discovery (ND) protocol is a critical component of the
   IPv6 architecture.  The protocol uses multicast in many messages.  It
   also assumes a security model where all nodes on a link are trusted.
   Such a design might be inefficient in some scenarios (e.g., use of
   multicast in wireless networks) or when nodes are not trustworthy
   (e.g., public access networks).  These security and operational
   issues and the associated mitigation solutions are documented in more
   than twenty RFCs.  There is a need to track these issues and
   solutions in a single document.

   To that aim, this document summarizes the published ND issues and
   then describes how all these issues originate from three causes.
   Addressing the issues is made simpler by addressing the causes.  This
   document also analyzes the mitigation solutions and demonstrates that
   isolating hosts into different subnets and links can help to address
   the three causes.  Guidance is provided for selecting a suitable
   isolation method to prevent potential ND issues.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9898.

Copyright Notice

   Copyright (c) 2025 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
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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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Review of Inventoried ND Issues
     2.1.  Multicast May Cause Performance and Reliability Issues
     2.2.  Trusting-All-Nodes May Cause On-Link Security Issues
     2.3.  Router-NCE-on-Demand May Cause Forwarding Delay, NCE
           Exhaustion, and Address Accountability Issues
     2.4.  Summary of ND Issues
   3.  Review of ND Mitigation Solutions
     3.1.  Mobile Broadband IPv6 (MBBv6)
     3.2.  Fixed Broadband IPv6 (FBBv6)
     3.3.  Unique Prefix per Host (UPPH)
     3.4.  Wireless ND (WiND)
     3.5.  Scalable Address Resolution Protocol (SARP)
     3.6.  ND Optimization for TRILL
     3.7.  Proxy ND in Ethernet Virtual Private Networks (ND EVPN)
     3.8.  Reducing Router Advertisements per RFC 7772
     3.9.  Gratuitous Neighbor Discovery (GRAND)
     3.10. Source Address Validation Improvement (SAVI) and Router
            Advertisement Guard (RA-Guard)
     3.11. Dealing with NCE Exhaustion Attacks per RFC 6583
     3.12. Registering Self-Generated IPv6 Addresses Using DHCPv6 per
            RFC 9686
     3.13. Enhanced DAD
     3.14. ND Mediation for IP Interworking of Layer 2 VPNs
     3.15. ND Solutions Defined Before the Latest Versions of ND
       3.15.1.  Secure Neighbor Discovery (SEND)
       3.15.2.  Cryptographically Generated Addresses (CGA)
       3.15.3.  ND Proxy
       3.15.4.  Optimistic DAD
   4.  Guidelines for Prevention of Potential ND Issues
     4.1.  Learning Host Isolation from the Existing Solutions
     4.2.  Applicability of Various Isolation Methods
       4.2.1.  Applicability of L3+L2 Isolation
       4.2.2.  Applicability of L3 Isolation
       4.2.3.  Applicability of Partial L2 Isolation
     4.3.  Guidelines for Applying Isolation Methods
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Neighbor Discovery (ND) [RFC4861] specifies the mechanisms that IPv6
   nodes (hosts and routers) on the same link use to communicate and
   learn about each other.  Stateless Address Autoconfiguration (SLAAC)
   [RFC4862] builds on those ND mechanisms to let nodes configure their
   own IPv6 addresses.  When analyzing the issues nodes may encounter
   with ND, it helps to view the ND messages they exchange throughout
   their life cycle, taking SLAAC into consideration.

   For a host, the overall procedure is as follows:

   1.  LLA DAD: The host forms a Link-Local Address (LLA) and performs
       Duplicate Address Detection (DAD) using multicast Neighbor
       Solicitations (NSs).

   2.  Router discovery: The host sends multicast Router Solicitations
       (RSs) to discover a router on the link.  The router responds with
       Router Advertisements (RAs), providing subnet prefixes and other
       information.  The host installs a Neighbor Cache Entry (NCE) for
       that router upon receiving the RAs.  In contrast, the router
       cannot install an NCE for the host at this moment of the exchange
       because the host's global IP address is still unknown.  When the
       router later needs to forward a packet to the host's global
       address, it will perform address resolution and install an NCE
       for the host.

   3.  GUA DAD: The host forms a Global Unicast Address (GUA) [RFC3587]
       or a Unique Local Address (ULA) [RFC4193] and uses multicast NSs
       for DAD.  For simplicity of description, this document will not
       further distinguish GUA and ULA.

   4.  Next-hop determination and address resolution: When the host
       needs to send a packet, it will first determine whether the next
       hop is a router or an on-link host (which is the destination).
       If the next hop is a router, the host already has the NCE for
       that router.  If the next hop is an on-link host, it will use
       multicast NSs to perform address resolution for the destination
       host.  As a result, the source host installs an NCE for the
       destination host.

   5.  Node Unreachability Detection (NUD): The host uses unicast NSs to
       determine whether another node with an NCE is still reachable.

   6.  Link-layer address change announcement: If a host's link-layer
       address changes, it may use multicast Neighbor Advertisements
       (NAs) to announce its new link-layer address to other nodes.

   For a router, the procedure is similar except that there is no router
   discovery.  Instead, routers perform a Redirect procedure that hosts
   do not have.  A router sends a Redirect to inform a node of a better
   next hop for the node's traffic.

   ND uses multicast in many messages and trusts messages from all
   nodes; in addition, routers may install NCEs for hosts on demand when
   they are to forward packets to these hosts.  These may lead to
   issues.  Concretely, various ND issues and mitigation solutions have
   been published in more than 20 RFCs, including:

   *  "IPv6 Neighbor Discovery (ND) Trust Models and Threats" [RFC3756]

   *  "SEcure Neighbor Discovery (SEND)" [RFC3971]

   *  "Cryptographically Generated Addresses (CGA)" [RFC3972]

   *  "Neighbor Discovery Proxies (ND Proxy)" [RFC4389]

   *  "Optimistic Duplicate Address Detection (DAD) for IPv6" [RFC4429]

   *  "IPv6 in 3rd Generation Partnership Project (3GPP) Evolved Packet
      System (EPS)" [RFC6459]

   *  "IPv6 for Third Generation Partnership Project (3GPP) Cellular
      Hosts" [RFC7066]

   *  "IPv6 in the context of TR-101" [TR177]

   *  "Address Resolution Protocol (ARP) Mediation for IP Interworking
      of Layer 2 VPNs" [RFC6575]

   *  "Operational Neighbor Discovery Problems" [RFC6583]

   *  "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
      Personal Area Networks (6LoWPANs)" [RFC6775]

   *  "Registration Extensions for IPv6 over Low-Power Wireless Personal
      Area Network (6LoWPAN) Neighbor Discovery" [RFC8505]

   *  "Address-Protected Neighbor Discovery for Low-Power and Lossy
      Networks" [RFC8928]

   *  "IPv6 Backbone Router" [RFC8929]

   *  "Architecture and Framework for IPv6 over Non-Broadcast Access"
      [SND]

   *  "Duplicate Address Detection Proxy" [RFC6957]

   *  "Source Address Validation Improvement (SAVI) Framework" [RFC7039]

   *  "IPv6 Router Advertisement Guard" [RFC6105]

   *  "Implementation Advice for IPv6 Router Advertisement Guard (RA-
      Guard)" [RFC7113]

   *  "Enhanced Duplicate Address Detection" [RFC7527]

   *  "The Scalable Address Resolution Protocol (SARP) for Large Data
      Centers" [RFC7586]

   *  "Reducing Energy Consumption of Router Advertisements" [RFC7772]

   *  "Unique IPv6 Prefix per Host" [RFC8273]

   *  "Transparent Interconnection of Lots of Links (TRILL): ARP and
      Neighbor Discovery (ND) Optimization" [RFC8302]

   *  "Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on
      First-Hop Routers" [RFC9131]

   *  "Operational Aspects of Proxy ARP/ND in Ethernet Virtual Private
      Networks" [RFC9161]

   *  "Using DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
      IPv6 Prefixes per Client in Large Broadcast Networks" [RFC9663]

   This document summarizes these RFCs into a one-stop reference (as of
   the time of writing) for easier access.  This document also
   identifies three causes of the issues and defines three host
   isolation methods to address the causes and prevent potential ND
   issues.

1.1.  Terminology

   This document uses the terms defined in [RFC4861].  Additional terms
   are defined in this section.

   MAC:  Media Access Control.  To avoid confusion with link-local
      addresses, link-layer addresses are referred to as "MAC addresses"
      in this document.

   Host Isolation:  Separating hosts into different subnets or links.

   L3 Isolation:  Allocating a Unique Prefix per Host (UPPH) [RFC8273]
      [RFC9663] so that every host is in a different subnet.  Given that
      a unique prefix can be allocated per host on shared media, hosts
      in different subnets may be on the same link.

   L2 Isolation:  Taking measures to prevent a host from reaching other
      hosts directly in Layer 2 (L2) so that every host is in a
      different link.  Due to the existence of Multi-Link Subnet
      [RFC4903], hosts in different links may be in the same subnet.
      Therefore, L2 Isolation does not imply L3 Isolation, and L3
      Isolation does not imply L2 Isolation either.

   L3+L2 Isolation:  Applying L3 Isolation and L2 Isolation
      simultaneously so that every host is in a different subnet and on
      a different link.

   Partial L2 Isolation:  Using an L3 ND Proxy [RFC4389] device to
      represent the hosts behind it to other hosts in the same subnet.
      Within the subnet, ND multicast exchange is segmented into
      multiple smaller scopes, each represented by an ND Proxy device.

2.  Review of Inventoried ND Issues

2.1.  Multicast May Cause Performance and Reliability Issues

   In some cases, ND uses multicast for NSs, NAs, RSs, and RAs.  While
   multicast can be highly efficient in certain scenarios (e.g., in
   wired networks), multicast can also be inefficient in other scenarios
   (e.g., in large L2 networks or wireless networks).

   Typically, multicast can create a large amount of protocol traffic in
   large L2 networks.  This can consume network bandwidth, increase
   processing overhead, and degrade network performance [RFC7342].

   In wireless networks, multicast can be inefficient or even unreliable
   due to a higher probability of transmission interference, lower data
   rate, and lack of acknowledgements (Section 3.1 of [RFC9119]).

   Multicast-related performance issues of the various ND messages are
   summarized below:

   *  Issue 1: LLA DAD degrades performance

      In an L2 network of N addresses (which can be much larger than the
      number of hosts, as each host can have multiple addresses), there
      can be N such multicast messages.  This may cause performance
      issues when N is large.

   *  Issue 2: Router's periodic unsolicited RAs drain host's battery

      Multicast RAs are generally limited to one packet every
      MIN_DELAY_BETWEEN_RAS (3 seconds), and there are usually only one
      or two routers on the link, so it is unlikely to cause a
      performance issue.  However, for battery-powered hosts, such
      messages may wake them up and drain their batteries [RFC7772].

   *  Issue 3: GUA DAD degrades performance

      This is the same as in Issue 1.

   *  Issue 4: Router's address resolution for hosts degrades
      performance

      This is the same as in Issue 1.

   *  Issue 5: Host's address resolution for hosts degrades performance

      This is the same as in Issue 1.

   *  Issue for further study: Multicast NAs for host's MAC address
      changes may degrade performance

      With randomized and changing MAC addresses [MADINAS], there may be
      many such multicast messages.

   In wireless networks, multicast is more likely to cause packet loss.
   Because DAD treats no response as no duplicate address detected,
   packet loss may cause duplicate addresses to be undetected.
   Multicast reliability issues are summarized below:

   *  Issue 6: LLA DAD not completely reliable in wireless networks

   *  Issue 7: GUA DAD not completely reliable in wireless networks

   Note: IPv6 address collisions are extremely unlikely.  As a result,
   these two issues are largely theoretical rather than practical.

2.2.  Trusting-All-Nodes May Cause On-Link Security Issues

   In scenarios such as public access networks, some nodes may not be
   trustworthy.  An attacker on the link can cause the following on-link
   security issues [RFC3756] [RFC9099]:

   *  Issue 8: Source IP address spoofing

      An attacker can use another node's IP address as the source
      address of its ND message to pretend to be that node.  The
      attacker can then launch various Redirect or Denial-of-Service
      (DoS) attacks.

   *  Issue 9: Denial of DAD

      An attacker can repeatedly reply to a victim's DAD messages,
      causing the victim's address configuration procedure to fail,
      resulting in a DoS to the victim.

   *  Issue 10: Rogue RAs

      An attacker can send RAs to victim hosts to pretend to be a
      router.  The attacker can then launch various Redirect or DoS
      attacks.

   *  Issue 11: Spoofed redirects

      An attacker can send forged Redirects to victim hosts to redirect
      their traffic to the legitimate router itself.

   *  Issue 12: Replay attacks

      An attacker can capture valid ND messages and replay them later.

2.3.  Router-NCE-on-Demand May Cause Forwarding Delay, NCE Exhaustion,
      and Address Accountability Issues

   When a router needs to forward a packet to a node but does not yet
   have a Neighbor-Cache Entry (NCE) for that node, it first creates an
   NCE in the INCOMPLETE state.  The router then multicasts an NS to the
   node's solicited-node multicast address.  When the destination
   replies with an NA containing its MAC address, the router updates the
   NCE with that address and changes its state to REACHABLE, thereby
   completing the entry.  This process is referred to as
   "Router-NCE-on-Demand" in this document.

   Router-NCE-on-Demand can cause the following issues:

   *  Issue 13: NCE exhaustion

      An attacker can send a high volume of packets targeting non-
      existent IP addresses, causing the router to create numerous NCEs
      in the INCOMPLETE state.  The resulting resource exhaustion may
      cause the router to malfunction.  This vulnerability, described as
      "NCE exhaustion" in this document, does not require the attacker
      to be on-link.

   *  Issue 14: Router forwarding delay

      When a packet arrives at a router, the router buffers it while
      attempting to determine the host's MAC address.  This buffering
      delays forwarding and, depending on the router's buffer size, may
      lead to packet loss.  This delay is referred to as
      "Router-NCE-on-Demand forwarding delay" in this document.

   *  Issue 15: Lack of address accountability

      With SLAAC, hosts generate their IP addresses.  The router does
      not become aware of a host's IP address until an NCE entry is
      created.  With DHCPv6 [RFC8415], the router may not know the
      host's addresses unless it performs DHCPv6 snooping.  In public
      access networks, where subscriber management often relies on IP
      address (or prefix) identification, this lack of address
      accountability poses a challenge [AddrAcc].  Without knowledge of
      the host's IP address, network administrators are unable to
      effectively manage subscribers, which is particularly problematic
      in public access networks.  Moreover, once a router has created
      its NCEs, ND [RFC4861] provides no mechanism to retrieve them for
      management or monitoring, as noted in Section 2.6.1 of [RFC9099].

2.4.  Summary of ND Issues

   The ND issues, as discussed in Sections 2.1, 2.2, and 2.3, are
   summarized below.  These issues stem from three primary causes:
   multicast, Trusting-all-nodes, and Router-NCE-on-Demand.  Eliminating
   any of these causes would also mitigate the corresponding issues.
   These observations provide guidance for addressing and preventing ND-
   related issues.

   1.  Multicast-related issues:

       *  Performance issues:

          -  Issue 1: LLA DAD degrades performance

          -  Issue 2: Router's periodic unsolicited RAs drain host's
             battery

          -  Issue 3: GUA DAD degrades performance

          -  Issue 4: Router's address resolution for hosts degrades
             performance

          -  Issue 5: Host's address resolution for hosts degrades
             performance

       *  Reliability issues:

          -  Issue 6: LLA DAD not completely reliable in wireless
             networks

          -  Issue 7: GUA DAD not completely reliable in wireless
             networks

   2.  Trusting-all-nodes related issues:

       *  Issue 8: Source IP address spoofing

       *  Issue 9: Denial of DAD

       *  Issue 10: Rogue RAs

       *  Issue 11: Spoofed redirects

       *  Issue 12: Replay attacks

   3.  Router-NCE-on-Demand related issues:

       *  Issue 13: NCE exhaustion

       *  Issue 14: Router forwarding delay

       *  Issue 15: Lack of address accountability

   These issues are potential vulnerabilities and may not manifest in
   all usage scenarios.

   When these issues may occur in a specific deployment, it is advisable
   to consider the mitigation solutions available.  They are described
   in the following section.

3.  Review of ND Mitigation Solutions

   Table 1 summarizes ND mitigation solutions available for Issues 1-15
   described in Section 2.4.  Similar solutions are grouped, beginning
   with those that address the most issues.  Unrelated solutions are
   ordered based on the issues (listed in Section 2.4) they address.
   Each solution in the table will be explained in a sub-section later,
   where abbreviations in the table are described.

   In Table 1, a letter code indicates the RFC category of the
   mitigation solution (see BCP 9 [RFC2026] for an explanation of these
   categories):

   S:   Standards Track (Proposed Standard or Internet Standard)
   E:   Experimental
   I:   Informational
   B:   Best Current Practice
   N/A:  Not Applicable (not an RFC)

   The abbreviations in Table 1 correspond to Section 2.4 as follows:

   On-link sec.:  Trusting-all-nodes related issues
   NCE exh.:  NCE exhaustion
   Fwd. delay:  Router forwarding delay
   No addr. acc.:  Lack of address accountability

   +==========+====+===========+=======+=========+====+=======+=======+
   | ND       |RFC |Multicast  |Reliabi| On-link |NCE | Fwd.  | No    |
   | solution |cat.|performance|lity   | sec.    |exh.| delay | addr. |
   |          |    |           |       |         |    |       | acc.  |
   |          |    +===+=+=+=+=+=====+=+=========+====+=======+=======+
   |          |    | 1 |2|3|4|5|  6  |7|   8-12  | 13 |   14  |   15  |
   +==========+====+===+=+=+=+=+=====+=+=========+====+=======+=======+
   | MBBv6    | I  |           All identified issues solved           |
   +----------+----+--------------------------------------------------+
   | FBBv6    |N/A |           All identified issues solved           |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | UPPH     | I  |   |X| |X|X|     |X|         | X  |   X   |   X   |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | WiND     | S  |All issues solved for Low-Power and Lossy Networks|
   |          |    |                      (LLNs)                      |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | SARP     | E  |   | | | |X|     | |         |    |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | ND TRILL | S  |   | | | |X|     | |         |    |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | ND EVPN  | S  |   | | | |X|     | |         |    |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | RFC 7772 | B  |   |X| | | |     | |         |    |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | GRAND    | S  |   | | |X| |     | |         |    |   X   |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | SAVI/    | I  |   | | | | |     | |    X    |    |       |       |
   | RA-G     |    |   | | | | |     | |         |    |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | RFC 6583 | I  |   | | | | |     | |         | X  |       |       |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+
   | RFC 9686 | S  |   | | | | |     | |         |    |       |   X   |
   +----------+----+---+-+-+-+-+-----+-+---------+----+-------+-------+

                 Table 1: Solutions for Identified Issues

3.1.  Mobile Broadband IPv6 (MBBv6)

   The IPv6 solution defined in "IPv6 in 3rd Generation Partnership
   Project (3GPP) Evolved Packet System (EPS)" [RFC6459], "IPv6 for
   Third Generation Partnership Project (3GPP) Cellular Hosts"
   [RFC7066], and "Extending an IPv6 /64 Prefix from a Third Generation
   Partnership Project (3GPP) Mobile Interface to a LAN Link" [RFC7278]
   is called Mobile Broadband IPv6 (MBBv6) in this document.  They are
   Informational RFCs.  The key points are:

   *  Putting every host (e.g., the mobile User Equipment (UE)) in a
      Point-to-Point (P2P) link with the router (e.g., the mobile
      gateway) has the following outcomes:

      -  All multicast is effectively turned into unicast.

      -  The P2P links do not have a MAC address.  Therefore, Router-
         NCE-on-Demand is not needed.

      -  Trusting-all-nodes is only relevant to the router.  By applying
         filtering at the router (e.g., dropping RAs from the hosts),
         even malicious hosts cannot cause harm.

   *  Assigning a unique /64 prefix to each host.  Together with the P2P
      link, this puts each host on a separate link and subnet.

   *  Maintaining (prefix, interface) binding at the router for
      forwarding purposes.

   Since all the three causes of ND issues are addressed, all the issues
   discussed in Section 2.4 are addressed.

3.2.  Fixed Broadband IPv6 (FBBv6)

   The IPv6 solution defined in "IPv6 in the context of TR-101" [TR177]
   is called Fixed Broadband IPv6 (FBBv6) in this document.  FBBv6 has
   two flavors:

   *  P2P: Every host (e.g., the Residential Gateway (RG)) is in a P2P
      link with the router (e.g., the Broadband Network Gateway (BNG)).
      In this case, the solution is functionally similar to MBBv6.  All
      ND issues discussed in Section 2.4 are solved.

   *  Point to Multipoint (P2MP): All hosts (e.g., the RGs) connected to
      an access device (e.g., the Optical Line Terminal (OLT)) are in a
      P2MP link with the router (e.g., the BNG).  This is achieved by
      placing all hosts in a single VLAN on the router and configuring
      the OLT to block any frame from being forwarded between its access
      ports; traffic from each host can travel only up toward the
      router, not sideways to another host, thereby preventing direct
      host-to-host communication.

   The following list summarizes the two key aspects of the FBBv6-P2MP
   architecture as described in [TR177] and the associated benefits:

   *  Implementing DAD proxy [RFC6957]:

      In a P2MP architecture described above, the normal ND DAD
      procedure will break down because hosts cannot exchange NSs with
      one another.  To address this, the router participates in the DAD
      process as a DAD Proxy to resolve address duplication.

      The benefits are:

      -  Multicast traffic from all hosts to the router is effectively
         converted into unicast, as hosts can only communicate directly
         with the router.

      -  The Trusting-all-nodes model is limited to the router.  By
         applying simple filtering (e.g., dropping RAs from hosts), the
         router can mitigate security risks, even from malicious hosts.

   *  Assigning a unique /64 prefix to each host:

      Assigning each host a unique /64 prefix results in several
      operational improvements:

      -  The router can proactively install a forwarding entry for that
         prefix towards the host, eliminating the need for Router-NCE-
         on-Demand.

      -  Since each host resides in a different subnet, traffic between
         hosts is routed through the router, eliminating the need for
         hosts to perform address resolution for one another.

      -  Without address resolution, router multicast to hosts is
         limited to unsolicited RAs.  As each host resides in its own
         subnet, these RAs are sent as unicast packets to individual
         hosts.  This follows the approach specified in [RFC6085], where
         the host's MAC address replaces the multicast MAC address in
         the RA.

   Since all three causes of ND issues are addressed, all ND issues
   (Section 2.4) are also addressed.

3.3.  Unique Prefix per Host (UPPH)

   Unique Prefix per Host (UPPH) solutions are described in [RFC8273]
   and [RFC9663].  Both are Informational RFCs.  [RFC8273] relies on
   SLAAC for unique prefix allocation while [RFC9663] relies on DHCPv6
   Prefix Delegation (DHCPv6-PD).  That difference in allocation
   mechanism does not change the discussion on ND issues, because every
   IPv6 node is still required to run SLAAC, even when it receives its
   prefix via DHCPv6-PD.  Therefore, discussing [RFC8273] alone is
   sufficient.

   [RFC8273] "improves host isolation and enhanced subscriber management
   on shared network segments" such as Wi-Fi or Ethernet.  The key
   points are:

   *  When a prefix is allocated to the host, the router can proactively
      install a forwarding entry for that prefix towards the host.
      There is no more Router-NCE-on-Demand.

   *  Without address resolution, router multicast to hosts consists
      only of unsolicited RAs.  They will be sent to hosts one by one in
      unicast because the prefix for every host is different.

   *  Since different hosts are in different subnets, hosts will send
      traffic to other hosts via the router.  There is no host-to-host
      address resolution.

   Therefore, ND issues caused by Router-NCE-on-Demand and router
   multicast to hosts are prevented.

   [RFC8273] indicates that a "network implementing a unique IPv6 prefix
   per host can simply ensure that devices cannot send packets to each
   other except through the first-hop router".  However, when hosts are
   on a shared medium like Ethernet, ensuring "devices cannot send
   packets to each other except through the first-hop router" requires
   additional measures like Private VLAN [RFC5517].  Without such
   additional measures, on a shared medium, hosts can still reach each
   other in L2 as they belong to the same Solicited-Node Multicast
   Group.  Therefore, Trusting-all-nodes and host multicast to routers
   may cause issues.  Of the host multicast issues (i.e., Issues 1, 3,
   5, 6, and 7), UPPH prevents Issues 5 and 7, because there is no need
   for address resolution among hosts (Issue 5), and there is no
   possibility of GUA duplication (Issue 7).  However, Issues 1, 3, and
   6 may occur.

3.4.  Wireless ND (WiND)

   The term "Wireless ND (WiND)" is used in this document to describe
   the fundamentally different ND solution for Low-Power and Lossy
   Networks (LLNs) [RFC7102] that is defined in [RFC6775], [RFC8505],
   [RFC8928], and [RFC8929] (Standards Track).  WiND changes host and
   router behaviors to use multicast only for router discovery.  The key
   points are:

   *  Hosts use unicast to proactively register their addresses at the
      routers.  Routers use unicast to communicate with hosts and become
      an abstract registrar and arbitrator for address ownership.

   *  The router also proactively installs NCEs for the hosts.  This
      avoids the need for address resolution for the hosts.

   *  The router sets the Prefix Information Option (PIO) L-bit to 0.
      Each host communicates only with the router (Section 6.3.4 of
      [RFC4861]).

   *  Other functionalities that are relevant only to LLNs.

   WiND addresses all ND issues (Section 2.4) in LLNs.  However, WiND
   support is not mandatory for general-purpose hosts.  Therefore, it
   cannot be relied upon as a deployment option without imposing
   additional constraints on the participating nodes.

3.5.  Scalable Address Resolution Protocol (SARP)

   The Scalable Address Resolution Protocol (SARP) [RFC7586] was an
   Experimental solution.  That experiment ended in 2017, two years
   after the RFC was published.  Because the idea has been used in
   mitigation solutions for more specific scenarios (described in
   Sections 3.6 and 3.7), it is worth describing here.  The usage
   scenario is Data Centers (DCs), where large L2 domains span across
   multiple sites.  In each site, multiple hosts are connected to a
   switch.  The hosts can be Virtual Machines (VMs), so the number can
   be large.  The switches are interconnected by a pure or overlay L2
   network.

   The switch will snoop and install a (IP, MAC address) proxy table for
   the local hosts.  The switch will also reply to address resolution
   requests from other sites to its hosts with its own MAC address.  In
   doing so, all hosts within a site will appear to have a single MAC
   address to other sites.  As such, a switch only needs to build a MAC
   address table for the local hosts and the remote switches, not for
   all the hosts in the L2 domain.  Consequently, the MAC address table
   size of the switches is significantly reduced.  A switch will also
   add the (IP, MAC address) replies from remote switches to its proxy
   ND table so that it can reply to future address resolution requests
   from local hosts for such IPs directly.  This greatly reduces the
   number of address resolution multicast in the network.

   Unlike MBBv6, FBBv6, and UPPH, which try to address all ND issues
   discussed in Section 2.4, SARP focuses on reducing address resolution
   multicast to improve the performance and scalability of large L2
   domains in DCs.

3.6.  ND Optimization for TRILL

   ARP and ND optimization for Transparent Interconnection of Lots of
   Links (TRILL) [RFC8302] (Standards Track) is similar to SARP
   (Section 3.5).  It can be considered an application of SARP in the
   TRILL environment.

   Like SARP, ND optimization for TRILL focuses on reducing multicast
   address resolution.  That is, it addresses Issue 5 (Section 2.1).

3.7.  Proxy ND in Ethernet Virtual Private Networks (ND EVPN)

   Proxy ARP/ND in EVPN is specified in [RFC9161] (Standards Track).
   The usage scenario is DCs where large L2 domains span across multiple
   sites.  In each site, multiple hosts are connected to a Provider Edge
   (PE) router.  The PEs are interconnected by EVPN tunnels.

   The PE of each site snoops the local address resolution NAs to build
   (IP, MAC address) Proxy ND table entries.  PEs then propagate such
   Proxy ND entries to other PEs via the Border Gateway Protocol (BGP).
   Each PE also snoops local hosts' address resolution NSs for remote
   hosts.  If an entry exists in its Proxy ND table for the remote
   hosts, the PE will reply directly.  Consequently, the number of
   multicast address resolution messages is significantly reduced.

   Like SARP, Proxy ARP/ND in EVPN also focuses on reducing address
   resolution multicast.

3.8.  Reducing Router Advertisements per RFC 7772

   Maintaining IPv6 connectivity requires that hosts be able to receive
   periodic multicast RAs [RFC4861].  Hosts that process unicast packets
   while they are asleep must also process multicast RAs while they are
   asleep.  An excessive number of RAs can significantly reduce the
   battery life of mobile hosts.  [RFC7772] (Best Current Practice)
   specifies a solution to reduce RAs:

   *  The router should respond to RS with unicast RA (rather than the
      normal multicast RA) if the host's source IP address is specified
      and the host's MAC address is valid.  This way, other hosts will
      not receive this RA.

   *  The router should reduce the multicast RA frequency.

   [RFC7772] addresses Issue 2 (Section 2.1).

3.9.  Gratuitous Neighbor Discovery (GRAND)

   GRAND [RFC9131] (Standards Track) changes ND in the following ways:

   *  A node sends unsolicited NAs upon assigning a new IPv6 address to
      its interface.

   *  A router creates a new NCE for the node and sets its state to
      STALE.

   When a packet for the host later arrives, the router can use the
   existing STALE NCE to forward it immediately ([RFC4861],
   Section 7.2.2).  It then verifies reachability by sending a unicast
   NS rather than a multicast one for address resolution.  In this way,
   GRAND eliminates the router forwarding delay, but it does not solve
   other Router-NCE-on-Demand issues.  For example, NCE exhaustion can
   still happen.

3.10.  Source Address Validation Improvement (SAVI) and Router
       Advertisement Guard (RA-Guard)

   Source Address Validation Improvement (SAVI) [RFC7039]
   (Informational) binds an address to a port on an L2 switch and
   rejects claims from other ports for that address.  Therefore, a node
   cannot spoof the IP address of another node.

   Router Advertisement Guard (RA-Guard) [RFC6105] [RFC7113]
   (Informational) only allows RAs from a port that a router is
   connected to.  Therefore, nodes on other ports cannot pretend to be a
   router.

   SAVI and RA-Guard address the on-link security issues.

3.11.  Dealing with NCE Exhaustion Attacks per RFC 6583

   [RFC6583] (Informational) deals with the NCE exhaustion attack issue
   (Section 2.3).  It recommends that:

   *  Operators should:

      -  Filter unused address space so that messages to such addresses
         can be dropped rather than triggering NCE creation.

      -  Implement rate-limiting mechanisms for ND message processing to
         prevent CPU and memory resources from being overwhelmed.

   *  Vendors should:

      -  Prioritize NDP processing for existing NCEs over creating new
         NCEs.

   [RFC6583] acknowledges that "some of these options are 'kludges', and
   can be operationally difficult to manage".  [RFC6583] partially
   addresses the Router NCE exhaustion issue.  In practice, router
   vendors cap the number of NCEs per interface to prevent cache
   exhaustion.  If the link has more addresses than that cap, the router
   cannot keep an entry for every address, and packets destined for
   addresses without an NCE are simply dropped [RFC9663].

3.12.  Registering Self-Generated IPv6 Addresses Using DHCPv6 per RFC
       9686

   In IPv4, network administrators can retrieve a host's IP address from
   the DHCP server and use it for subscriber management.  In IPv6 and
   SLAAC, this is not possible (Section 2.3).

   [RFC9686] (Standards Track) defines a method for informing a DHCPv6
   server that a host has one or more self-generated or statically
   configured addresses.  This enables network administrators to
   retrieve the IPv6 addresses for each host from the DHCPv6 server.
   [RFC9686] provides a solution for Issue 15 (Section 2.3).

3.13.  Enhanced DAD

   Enhanced DAD [RFC7527] (Standards Track) addresses a DAD failure
   issue in a specific situation: a looped-back interface.  DAD will
   fail in a looped-back interface because the sending host will receive
   the DAD message back and will interpret it as another host is trying
   to use the same address.  The solution is to include a Nonce option
   [RFC3971] in each DAD message so that the sending host can detect
   that the looped-back DAD message is sent by itself.

   Enhanced DAD does not solve any ND issue.  It extends ND to work in a
   new scenario: a looped-back interface.  It is reviewed here only for
   completeness.

3.14.  ND Mediation for IP Interworking of Layer 2 VPNs

   ND mediation is specified in [RFC6575] (Standards Track).  When two
   Attachment Circuits (ACs) are interconnected by a Virtual Private
   Wired Service (VPWS), and the two ACs are of different media (e.g.,
   one is Ethernet while the other is Frame Relay), the two PEs must
   interwork to provide mediation service so that a Customer Edge (CE)
   can resolve the MAC address of the remote end.  [RFC6575] specifies
   such a solution.

   ND mediation does not address any ND issue.  It extends ND to work in
   a new scenario: two ACs of different media interconnected by a VPWS.
   It is reviewed here only for completeness.

3.15.  ND Solutions Defined Before the Latest Versions of ND

   The latest versions of ND and SLAAC are specified in [RFC4861] and
   [RFC4862].  Several ND mitigation solutions were published before
   [RFC4861].  They are reviewed in this section only for completeness.

3.15.1.  Secure Neighbor Discovery (SEND)

   The purpose of SEND [RFC3971] (Standards Track) is to ensure that
   hosts and routers are trustworthy.  SEND defined three new ND
   options: Cryptographically Generated Addresses (CGA) [RFC3972]
   (Standards Track), RSA public-key cryptosystem, and Timestamp/Nonce.
   In addition, SEND also defined an authorization delegation discovery
   process, an address ownership proof mechanism, and requirements for
   the use of these components in the ND protocol.

3.15.2.  Cryptographically Generated Addresses (CGA)

   The purpose of CGA is to associate a cryptographic public key with an
   IPv6 address in the SEND protocol.  The key point is to generate the
   Interface Identifier (IID) of an IPv6 address by computing a
   cryptographic hash of the public key.  The resulting IPv6 address is
   called a CGA.  The corresponding private key can then be used to sign
   messages sent from the address.

   CGA assumes that a legitimate host does not care about the bit
   combination of the IID that would be created by some hash procedure.
   The attacker needs an exact IID to impersonate the legitimate hosts,
   but then the attacker is challenged to do a reverse hash calculation,
   which is a strong mathematical challenge.

   CGA is part of SEND.  There is no reported deployment.

3.15.3.  ND Proxy

   ND Proxy [RFC4389] (Experimental) aims to enable multiple links
   joined by an ND Proxy device to work as a single link.

   *  When an ND Proxy receives an ND request from a host on a link, it
      will proxy the message out the "best" (defined in the next
      paragraph) outgoing interface.  If there is no best interface, the
      ND Proxy will proxy the message to all other links.  Here, proxy
      means acting as if the ND message originates from the ND Proxy
      itself.  That is, the ND Proxy will change the ND message's source
      IP and source MAC address to the ND Proxy's outgoing interface's
      IP and MAC address, and create an NCE entry at the outgoing
      interface accordingly.

   *  When ND Proxy receives an ND reply, it will act as if the ND
      message is destined for itself, and update the NCE entry state at
      the receiving interface.  Based on such state information, the ND
      Proxy can determine the "best" outgoing interface for future ND
      requests.  The ND Proxy then proxies the ND message back to the
      requesting host.

   ND Proxy is widely used in SARP (Section 3.5), ND optimization for
   TRILL (Section 3.6), and Proxy ARP/ND in EVPN (Section 3.7).

3.15.4.  Optimistic DAD

   Optimistic DAD [RFC4429] (Standards Track) seeks to minimize address
   configuration delays in the successful case and to reduce disruption
   as far as possible in the failure case.  That is, Optimistic DAD lets
   hosts immediately use the newly formed address to communicate before
   DAD completes, assuming that DAD will succeed anyway.  If the address
   turns out to be duplicate, Optimistic DAD provides a set of
   mechanisms to minimize the impact.  Optimistic DAD modified the
   original ND [RFC2461] and original SLAAC [RFC2462] (both of which are
   obsolete), but the solution was not incorporated into the latest
   specifications of ND [RFC4861] and SLAAC [RFC4862].  However,
   implementations of Optimistic DAD exist.

   Optimistic DAD does not solve any ND issue (Section 2).  It is
   reviewed here only for completeness.

4.  Guidelines for Prevention of Potential ND Issues

   By knowing the potential ND issues and associated mitigation
   solutions, network administrators of existing IPv6 deployments can
   assess whether these issues may occur in their networks and, if so,
   whether to deploy the mitigation solutions proactively.  Deploying
   these solutions may take time and additional resources.  Therefore,
   it is advisable to plan.

   Network administrators planning to start their IPv6 deployments can
   use the issue-solution information to help plan their deployments.
   Moreover, they can take proactive action to prevent potential ND
   issues.

4.1.  Learning Host Isolation from the Existing Solutions

   While various ND solutions may initially appear unrelated,
   categorizing them into four distinct groups highlights an important
   observation: host isolation is an effective strategy for mitigating
   ND-related issues.

   *  Group 1: L3 and L2 Isolation

      This group includes MBBv6 and FBBv6, which isolate hosts at both
      L3 and L2 by placing each host within its subnet and link.  This
      prevents ND issues caused by multicast and Trusting-all-nodes, as
      each host operates within its isolated domain.  Furthermore, since
      routers can route packets to a host based on its unique prefix,
      the need for Router-NCE-on-Demand is also eliminated.  Therefore,
      L3 and L2 Isolation prevent all ND issues.

   *  Group 2: L3 Isolation

      This group includes UPPH solutions like [RFC8273] and [RFC9663],
      which isolate hosts into separate subnets while potentially
      leaving them on the same shared medium.  This approach mitigates
      ND issues caused by router multicast to hosts and eliminates the
      need for Router-NCE-on-Demand, as detailed in Section 3.3.

   *  Group 3: Partial L2 Isolation

      This group encompasses solutions such as WiND, SARP, ND
      optimization for TRILL, and Proxy ND in EVPN.  These solutions use
      a proxy device to represent the hosts behind it, effectively
      isolating those hosts into distinct multicast domains.  While
      hosts are still located within the same subnet, their separation
      into different multicast domains reduces the scope of ND issues
      related to multicast-based address resolution.

   *  Group 4: Non-Isolating Solutions

      The final group includes remaining solutions that do not implement
      host isolation.  These solutions do not prevent ND issues but
      instead focus on addressing specific ND problems.

   The analysis demonstrates that the stronger the isolation of hosts,
   the more ND issues can be mitigated.  This correlation is intuitive,
   as isolating hosts reduces the multicast scope, minimizes the number
   of nodes that must be trusted, and may eliminate the need for Router-
   NCE-on-Demand, the three primary causes of ND issues.

   This understanding can be used to prevent ND issues.

4.2.  Applicability of Various Isolation Methods

4.2.1.  Applicability of L3+L2 Isolation

   Benefits:

   *  All ND issues (Section 2.4) can be effectively mitigated.

   Constraints:

   1.  L2 Isolation:

       Actions must be taken to isolate hosts in L2.  The required
       effort varies by the chosen method and deployment context.  For
       example, the P2P method [RFC7066] is heavyweight, while the
       Private VLAN method [RFC5517] is more manageable.

   2.  Unique prefix allocation:

       A large number of prefixes will be required, with one prefix
       assigned per host.  This is generally not a limitation for IPv6.
       For instance, members of a Regional Internet Registry (RIR) can
       obtain a /29 prefix allocation [RIPE738], which provides 32
       billion /64 prefixes -- sufficient for any foreseeable deployment
       scenarios.  Practical implementations, such as MBBv6 assigning
       /64 prefixes to billions of mobile UEs [RFC6459], and FBBv6
       assigning /56 prefixes to hundreds of millions of routed RGs
       [TR177], demonstrate the feasibility of this approach.

   3.  Privacy issue from unique prefix identifiability:

       Assigning a unique prefix to each host may theoretically reduce
       privacy, as hosts can be directly identified by their assigned
       prefix.  However, alternative host identification methods, such
       as cookies, are commonly used.  Therefore, unique prefix
       identifiability may not make much difference.  The actual impact
       on privacy is therefore likely to be limited.

   4.  Router support for L3 Isolation:

       The router must support an L3 Isolation solution, e.g., [RFC8273]
       or [RFC9663].

   5.  A large number of router interfaces may be needed:

       If the P2P method is used, the router must instantiate a separate
       logical interface for every attached host.  In this case, a large
       number of interfaces will be needed at the router.

   6.  Router as a bottleneck:

       Since all communication between hosts is routed through the
       router, the router may become a performance bottleneck in high-
       traffic scenarios.

   7.  Incompatibility with host-based multicast services:

       Services that rely on multicast communication among hosts, such
       as the Multicast Domain Name System [RFC6762], will be disrupted.

4.2.2.  Applicability of L3 Isolation

   Benefits:

   *  All ND issues (Section 2.4) are mitigated, with the exception of:

      -  LLA DAD multicast degrades performance,

      -  LLA DAD not reliable in wireless networks, and

      -  on-link security.

      These remaining issues depend on the characteristics of the shared
      medium:

      -  If the shared medium is Ethernet, the issues related to LLA DAD
         multicast are negligible.

      -  If nodes can be trusted, such as in private networks, on-link
         security concerns are not significant.

   *  There is no need for L2 Isolation.  Consequently, this method can
      be applied in a wide range of scenarios, making it possibly the
      most practical host isolation method.

   Constraints (as discussed in Section 4.2.1):

   1.  Unique prefix allocation

   2.  Router support for L3 Isolation

   3.  Router as a bottleneck

   4.  Privacy issue from unique prefix identifiability

4.2.3.  Applicability of Partial L2 Isolation

   Benefit:

   *  Reduced multicast traffic: This method reduces multicast traffic,
      particularly for address resolution, by dividing the subnet into
      multiple multicast domains.

   Constraint:

   *  Router support for Partial L2 Isolation: The router must implement
      a Partial L2 Isolation solution such as WiND, SARP, ND
      optimization for TRILL, and Proxy ND in EVPN to support this
      method.

4.3.  Guidelines for Applying Isolation Methods

   Based on the applicability analysis provided in the preceding
   sections, network administrators can determine whether to implement
   an isolation method and, if so, which method is most appropriate for
   their specific deployment.

   A simple guideline is to consider the isolation methods in the order
   listed in the preceding sections, progressing from the strongest
   isolation to the weakest:

   *  Stronger isolation methods can prevent more ND issues, but may
      also impose higher entry requirements.

   *  Weaker isolation methods have fewer entry requirements but may
      leave some ND issues unmitigated.

   The choice between L3+L2 Isolation and L3 Isolation often depends on
   the cost of implementing L2 Isolation:

   *  If the cost is acceptable, L3+L2 Isolation is preferable because
      it eliminates every ND issue listed in Section 2.4.

   *  Otherwise, L3 Isolation addresses most of those issues while
      keeping the implementation effort reasonable.

   Selecting an isolation method that is either too strong or too weak
   does not result in serious consequences:

   *  Choosing an overly strong isolation method may require the network
      administrator to meet higher entry requirements initially, such as
      measures for L2 Isolation, additional prefixes, or additional
      router capabilities.

   *  Choosing a weaker isolation method may necessitate deploying
      supplemental ND mitigation techniques to address any unresolved ND
      issues.

   In either case, the resulting solution can be functional and
   effective.

5.  Security Considerations

   This document is a review of known ND issues and solutions, including
   security.  It does not introduce any new solutions.  Therefore, it
   does not introduce new security issues.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [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>.

7.2.  Informative References

   [AddrAcc]  Chown, T., Cummings, C., and D. W. Carder, "IPv6 Address
              Accountability Considerations", Work in Progress,
              Internet-Draft, draft-ccc-v6ops-address-accountability-00,
              21 October 2024, <https://datatracker.ietf.org/doc/html/
              draft-ccc-v6ops-address-accountability-00>.

   [MADINAS]  Henry, J. and Y. Lee, "Randomized and Changing Media
              Access Control (MAC) Addresses: Context, Network Impacts,
              and Use Cases", RFC 9797, DOI 10.17487/RFC9797, June 2025,
              <https://www.rfc-editor.org/info/rfc9797>.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              DOI 10.17487/RFC2461, December 1998,
              <https://www.rfc-editor.org/info/rfc2461>.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, DOI 10.17487/RFC2462,
              December 1998, <https://www.rfc-editor.org/info/rfc2462>.

   [RFC3587]  Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
              Unicast Address Format", RFC 3587, DOI 10.17487/RFC3587,
              August 2003, <https://www.rfc-editor.org/info/rfc3587>.

   [RFC3756]  Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
              Neighbor Discovery (ND) Trust Models and Threats",
              RFC 3756, DOI 10.17487/RFC3756, May 2004,
              <https://www.rfc-editor.org/info/rfc3756>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <https://www.rfc-editor.org/info/rfc4389>.

   [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>.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,
              <https://www.rfc-editor.org/info/rfc4903>.

   [RFC5517]  HomChaudhuri, S. and M. Foschiano, "Cisco Systems' Private
              VLANs: Scalable Security in a Multi-Client Environment",
              RFC 5517, DOI 10.17487/RFC5517, February 2010,
              <https://www.rfc-editor.org/info/rfc5517>.

   [RFC6085]  Gundavelli, S., Townsley, M., Troan, O., and W. Dec,
              "Address Mapping of IPv6 Multicast Packets on Ethernet",
              RFC 6085, DOI 10.17487/RFC6085, January 2011,
              <https://www.rfc-editor.org/info/rfc6085>.

   [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>.

   [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
              T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, DOI 10.17487/RFC6459, January 2012,
              <https://www.rfc-editor.org/info/rfc6459>.

   [RFC6575]  Shah, H., Ed., Rosen, E., Ed., Heron, G., Ed., and V.
              Kompella, Ed., "Address Resolution Protocol (ARP)
              Mediation for IP Interworking of Layer 2 VPNs", RFC 6575,
              DOI 10.17487/RFC6575, June 2012,
              <https://www.rfc-editor.org/info/rfc6575>.

   [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>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [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>.

   [RFC6957]  Costa, F., Combes, J., Ed., Pougnard, X., and H. Li,
              "Duplicate Address Detection Proxy", RFC 6957,
              DOI 10.17487/RFC6957, June 2013,
              <https://www.rfc-editor.org/info/rfc6957>.

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,
              <https://www.rfc-editor.org/info/rfc7039>.

   [RFC7066]  Korhonen, J., Ed., Arkko, J., Ed., Savolainen, T., and S.
              Krishnan, "IPv6 for Third Generation Partnership Project
              (3GPP) Cellular Hosts", RFC 7066, DOI 10.17487/RFC7066,
              November 2013, <https://www.rfc-editor.org/info/rfc7066>.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <https://www.rfc-editor.org/info/rfc7102>.

   [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>.

   [RFC7278]  Byrne, C., Drown, D., and A. Vizdal, "Extending an IPv6
              /64 Prefix from a Third Generation Partnership Project
              (3GPP) Mobile Interface to a LAN Link", RFC 7278,
              DOI 10.17487/RFC7278, June 2014,
              <https://www.rfc-editor.org/info/rfc7278>.

   [RFC7342]  Dunbar, L., Kumari, W., and I. Gashinsky, "Practices for
              Scaling ARP and Neighbor Discovery (ND) in Large Data
              Centers", RFC 7342, DOI 10.17487/RFC7342, August 2014,
              <https://www.rfc-editor.org/info/rfc7342>.

   [RFC7527]  Asati, R., Singh, H., Beebee, W., Pignataro, C., Dart, E.,
              and W. George, "Enhanced Duplicate Address Detection",
              RFC 7527, DOI 10.17487/RFC7527, April 2015,
              <https://www.rfc-editor.org/info/rfc7527>.

   [RFC7586]  Nachum, Y., Dunbar, L., Yerushalmi, I., and T. Mizrahi,
              "The Scalable Address Resolution Protocol (SARP) for Large
              Data Centers", RFC 7586, DOI 10.17487/RFC7586, June 2015,
              <https://www.rfc-editor.org/info/rfc7586>.

   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy
              Consumption of Router Advertisements", BCP 202, RFC 7772,
              DOI 10.17487/RFC7772, February 2016,
              <https://www.rfc-editor.org/info/rfc7772>.

   [RFC8273]  Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
              per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,
              <https://www.rfc-editor.org/info/rfc8273>.

   [RFC8302]  Li, Y., Eastlake 3rd, D., Dunbar, L., Perlman, R., and M.
              Umair, "Transparent Interconnection of Lots of Links
              (TRILL): ARP and Neighbor Discovery (ND) Optimization",
              RFC 8302, DOI 10.17487/RFC8302, January 2018,
              <https://www.rfc-editor.org/info/rfc8302>.

   [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>.

   [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>.

   [RFC8928]  Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
              "Address-Protected Neighbor Discovery for Low-Power and
              Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
              2020, <https://www.rfc-editor.org/info/rfc8928>.

   [RFC8929]  Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
              "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
              November 2020, <https://www.rfc-editor.org/info/rfc8929>.

   [RFC9099]  Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
              "Operational Security Considerations for IPv6 Networks",
              RFC 9099, DOI 10.17487/RFC9099, August 2021,
              <https://www.rfc-editor.org/info/rfc9099>.

   [RFC9119]  Perkins, C., McBride, M., Stanley, D., Kumari, W., and JC.
              Zúñiga, "Multicast Considerations over IEEE 802 Wireless
              Media", RFC 9119, DOI 10.17487/RFC9119, October 2021,
              <https://www.rfc-editor.org/info/rfc9119>.

   [RFC9131]  Linkova, J., "Gratuitous Neighbor Discovery: Creating
              Neighbor Cache Entries on First-Hop Routers", RFC 9131,
              DOI 10.17487/RFC9131, October 2021,
              <https://www.rfc-editor.org/info/rfc9131>.

   [RFC9161]  Rabadan, J., Ed., Sathappan, S., Nagaraj, K., Hankins, G.,
              and T. King, "Operational Aspects of Proxy ARP/ND in
              Ethernet Virtual Private Networks", RFC 9161,
              DOI 10.17487/RFC9161, January 2022,
              <https://www.rfc-editor.org/info/rfc9161>.

   [RFC9663]  Colitti, L., Linkova, J., Ed., and X. Ma, Ed., "Using
              DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
              IPv6 Prefixes per Client in Large Broadcast Networks",
              RFC 9663, DOI 10.17487/RFC9663, October 2024,
              <https://www.rfc-editor.org/info/rfc9663>.

   [RFC9686]  Kumari, W., Krishnan, S., Asati, R., Colitti, L., Linkova,
              J., and S. Jiang, "Registering Self-Generated IPv6
              Addresses Using DHCPv6", RFC 9686, DOI 10.17487/RFC9686,
              December 2024, <https://www.rfc-editor.org/info/rfc9686>.

   [RIPE738]  RIPE, "IPv6 Address Allocation and Assignment Policy",
              RIPE-738, March 2020,
              <https://www.ripe.net/publications/docs/ripe-738>.

   [SND]      Thubert, P. and M. Richardson, "Architecture and Framework
              for IPv6 over Non-Broadcast Access", Work in Progress,
              Internet-Draft, draft-ietf-6man-ipv6-over-wireless-09, 9
              November 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-6man-ipv6-over-wireless-09>.

   [TR177]    Broadband Forum, "IPv6 in the context of TR-101", TR-177,
              November 2017,
              <https://www.broadband-forum.org/pdfs/tr-177-1-0-1.pdf>.

Acknowledgements

   The authors would like to thank Éric Vyncke, Gunter Van de Velde,
   Lorenzo Colitti, Erik Kline, Warren Kumari, Mohamed Boucadair, Gorry
   Fairhurst, Pascal Thubert, Jen Linkova, Brian Carpenter, Mike
   Ackermann, Nalini Elkins, Ed Horley, Ole Troan, David Thaler,
   Chongfeng Xie, Chris Cummings, Dale Carder, Tim Chown, Priyanka
   Sinha, Aijun Wang, Ines Robles, Magnus Westerlund, Barry Leiba, Deb
   Cooley, and Paul Wouters for their reviews and comments.  The authors
   would also like to thank Tim Winters for being the document shepherd.

Authors' Addresses

   XiPeng Xiao
   Huawei Technologies
   Email: xipengxiao@huawei.com


   Eduard Vasilenko
   Huawei Technologies
   Email: vasilenko.eduard@huawei.com


   Eduard Metz
   KPN N.V.
   Email: eduard.metz@kpn.com


   Gyan Mishra
   Verizon Inc.
   Email: gyan.s.mishra@verizon.com


   Nick Buraglio
   Energy Sciences Network
   Email: buraglio@es.net
