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Selectively Applying Host Isolation to Simplify IPv6 First-hop Deployment
draft-ietf-v6ops-nd-considerations-00

Document Type Active Internet-Draft (v6ops WG)
Authors XiPeng Xiao , Eduard V , Eduard Metz , Gyan Mishra , Nick Buraglio
Last updated 2022-10-24
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draft-ietf-v6ops-nd-considerations-00
IPv6 Operations (v6ops) Working Group                           X. Xiao
Internet Draft                                             E. Vasilenko
Intended status: Informational                      Huawei Technologies
Expires: April 2023                                              E. Metz
                                                                    KPN
                                                              G. Mishra
                                                           Verizon Inc.
                                                            N. Buraglio
                                                Energy Sciences Network
                                                           October 2022

      Selectively Applying Host Isolation to Simplify IPv6 First-hop
                                Deployment
                   draft-ietf-v6ops-nd-considerations-00

Abstract

   Neighbor Discovery (ND) is a key protocol of IPv6 first-hop. ND uses
   multicast extensively and trusts all hosts. In some scenarios like
   wireless networks, multicast can be inefficient. In other scenarios
   like public access networks, hosts may not be trustable.
   Consequently, ND has potential issues in various scenarios. The
   issues and the solutions for them are documented in more than 30
   RFCs. It is difficult to keep track of all these issues and
   solutions. Therefore, an overview and some guidelines are useful.

   This document firstly summarizes the known ND issues and
   optimization solutions into a one-stop reference. Analyzing these
   solutions reveals an insight: isolating hosts is effective in
   preventing ND issues. Four isolation methods are proposed and their
   applicability is discussed. Guidelines are then described for
   selecting a suitable isolation method based on the deployment
   scenario.

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents

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   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 21 April 2023.

Copyright Notice

   Copyright (c) 2022 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ................................................. 3
      1.1. Terminology ............................................. 4
   2. Review of ND Issues .......................................... 5
      2.1. Multicast Causes Performance and Reliability Issues ..... 5
      2.2. Trusting-all-hosts Causes On-link Security Issues ....... 6
      2.3. Router-NCE-on-Demand Causes Performance, NCE Exhaustion and
      Lack of Subscriber Management Issues ......................... 6
      2.4. Summary of ND Issue ..................................... 7
   3. Review of ND Solutions ....................................... 8
      3.1. ND Solution in Mobile Broadband IPv6 .................... 8
      3.2. ND Solution in Fixed Broadband IPv6 ..................... 9
      3.3. Unique IPv6 Prefix per Host ............................ 10
      3.4. Wireless ND ............................................ 10
      3.5. Scalable Address Resolution Protocol ................... 11
      3.6. ARP and ND Optimization for Transparent Interconnection of
      Lots of Links (TRILL): ...................................... 12
      3.7. Proxy ARP/ND in EVPN ................................... 12
      3.8. Gratuitous Neighbor Discovery .......................... 12
      3.9. Reducing Router Advertisements ......................... 12
      3.10. Source Address Validation Improvement and Router
      Advertisement Guard ......................................... 13
      3.11. Dealing with Off-link Attack that May Cause Router NCE
      Exhaustion .................................................. 13
      3.12. Enhanced DAD .......................................... 14

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      3.13. ND Mediation for IP Interworking of Layer 2 VPNs ...... 14
      3.14. ND Solutions Defined before the Latest Versions of ND . 14
         3.14.1. SeND ............................................. 14
         3.14.2. Cryptographically Generated Addresses (CGA)....... 15
         3.14.3. ND Proxy ......................................... 15
         3.14.4. Optimistic DAD  .................................. 16
      3.15. Observations on the Solutions ......................... 16
   4. Isolating Hosts to Simplify First-hop Deployments ........... 19
      4.1. Applicability of P2P Link and Subnet Isolation.......... 20
      4.2. Applicability of P2MP Link and Subnet Isolation ........ 21
      4.3. Applicability of GUA Isolation ......................... 21
      4.4. Applicability of Proxy Isolation ....................... 22
      4.5. ND Deployment Guidelines ............................... 22
      4.6. Impact of Host Isolation on Other Protocols in IPv6 First-
      hops ........................................................ 25
   5. Security Considerations ..................................... 26
   6. IANA Considerations ......................................... 26
   7. References .................................................. 26
      7.1. Informative References ................................. 26
   8. Acknowledgments ............................................. 29

1. Introduction

   Neighbor Discovery [ND] is specified in RFC 4861. It defines how
   hosts and routers in the link interact with each other. ND contains
   seven main procedures:

      1.Hosts generate Link Local Addresses (LLAs) and use multicast
        Neighbor Solicitations (NSs) for Duplicate Address Detection
        (DAD).
      2.Hosts send multicast Router Solicitations (RSs) to discover
        first-hop routers. Routers respond with unicast Router
        Advertisements (RAs) with subnet prefixes for the link and
        other information. Routers also send unsolicited multicast RAs
        from time to time.
      3.Hosts form Global Unicast Address (GUA) or Unique Local Address
        (ULA) and use multicast Neighbor Solicitations (NSs) for DAD.
      4.When a packet is to be sent, routers use multicast NSs to
        perform address resolution for the destination host.
      5.When a packet is to be sent, hosts use multicast NSs to perform
        address resolution for the destination host.
      6.Hosts/routers use unicast NS for Node Unreachability Detection
        (NUD).
      7.Hosts may use multicast NSs to announce link layer address
        changes.

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   Due to multicast, trusting all hosts, etc, ND has potential issues
   in some scenarios. Various ND issues and solutions for them have
   been described in more than 30 RFCs. These include: ND Trust Models
   and Threats [RFC3756], Secure ND [SeND], Cryptographically Generated
   Addresses [CGA], ND Proxy [RFC4389], Optimistic ND [RFC4429], ND for
   mobile broadband [RFC6459][RFC7066], ND for fixed broadband [TR177],
   ND Mediation [RFC6575], Operational ND Problems [RFC6583], Wireless
   ND (WiND) [RFC6775][RFC8505][RFC8928][RFC8929], DAD Proxy [RFC6957],
   Source Address Validation Improvement [SAVI], Router Advertisement
   Guard [RA-Guard][RA-Guard+], Enhanced Duplicate Address Detection
   [RFC7527], Scalable ARP [RFC7586], Reducing Router Advertisements
   [RFC7772], Unique Prefix Per Host [RFC8273], ND Optimization for
   TRILL [RFC8302], Gratuitous Neighbor Discovery [GRAND], Proxy ARP/ND
   for EVPN [RFC9161]. It is difficult to understand all these issues
   and solutions, and how they fit together. Consequently, people may
   feel that ND is complicated and fragile, and become reluctant to
   deploy IPv6. This document summarizes the potential issues and
   solutions to provide a clear picture. It also provides guidelines
   for preventing the issues as much as possible and for selecting
   solutions for the remaining issues.

1.1. Terminology

   Some important terms are defined in this section.

   MAC -    To avoid confusion with link local address, link layer
            address is called MAC in this document.

   Link isolation for hosts - isolating hosts in L2. This includes 2
            flavors: P2P link isolation and P2MP link isolation.

   P2P link isolation - connecting each host in a P2P link to the
            router. The router has a separate interface for each host.
            Consequently, any L2 message from a host can only reach the
            router, and any L2 message from the router can only reach
            one host.

   P2MP link isolation - connecting multiple hosts in a P2MP link to
            the router. The router has a single interface for all hosts.
            Example P2MP links are Private VLAN [PVLAN] and Wi-Fi with
            Wireless Isolation [W-Iso].  Consequently, any L2 message
            from a host can only reach the router, not other hosts. But
            an L2 multicast message from the router can reach multiple
            hosts simultaneously.

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   Subnet isolation for hosts - assigning a unique prefix per host so
            each host is in its own subnet [RFC8273].

   GUA/ULA isolation for hosts - setting PIO L-bit=0 so that other
            hosts appear off-link [ND]. There will be no GUA/ULA
            resolution for other hosts in the link, and all GUA/ULA
            traffic will be sent via the router. Therefore hosts appear
            isolated from a GUA/ULA perspective. To be simple, this is
            also called GUA Isolation in this document.

   Proxy isolation for hosts - using an ND proxy device to represent
            the hosts behind it, and effectively isolate such hosts from
            other hosts.

2. Review of ND Issues

2.1. Multicast Causes Performance and Reliability Issues

   ND uses multicast extensively for Node Solicitations (NSs), Router
   Solicitations (RSs) and Router Advertisements (RAs). Multicast can
   be inefficient in some scenarios, e.g. large L2 networks or wireless
   networks.

   In large L2 networks, e.g. DC networks involving many Virtual
   Machines (VMs), ND multicast can create a large amount of protocol
   traffic. This can consume network bandwidth, create a processing
   burden, and reduce network performance [RFC7342].

   In wireless networks, to ensure that the multicast messages reach
   even the remotest hosts, multicast messages are sent at the lowest
   modulation rate. This prolongs receiving time and consumes more
   power of the hosts. Some low-power or remote hosts may not receive
   or respond to multicast messages. In addition, multicast messages
   are not acknowledged at L2. Consequently, multicast in wireless
   networks reduces not only network performance but also protocol
   reliability [RFC9119]. For example, ND uses no response as an
   indication of no duplication in Duplicate Address Detection (DAD).
   If the DAD multicast messages are lost, DAD will not work properly.

   ND uses the following multicast messages. Their impact on
   performance and reliability is summarized below:

      o Hosts' LLA DAD: may cause a performance issue, and a
        reliability issue in wireless networks.
      o Router's periodic unsolicited RAs: may cause performance issue
        if it is sent frequently [RFC7772].

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      o Hosts' GUA (or ULA) DAD: may cause a performance issue, and a
        reliability issue in wireless networks.
      o Router's address resolution for hosts: in a large network of N
        hosts, there can be N such multicast messages. This may cause a
        performance issue.
      o Hosts address resolution for hosts: in a large network of N
        hosts, there can be N-square such multicast messages. This may
        cause the largest performance issue.
      o Hosts' MAC change NAs: this type of multicast messages is rare
        and will not cause a performance issue. It will not be further
        discussed.

   Multicast originated from hosts and routers will be called host
   multicast and router multicast hereafter.

2.2. Trusting-all-hosts Causes On-link Security Issues

   ND trusts all hosts. In some scenarios like public access networks,
   some hosts may not be trustable. An attacker host in the link can
   cause the following security issues [RFC3756][RFC9099]:

      o Source IP address spoofing: an attacker can use a victim host's
        IP address as the source address of its ND message to pretend
        to be the victim. The attacker can then launch Redirect or
        Denial of Service (DoS) attacks on the victim.
      o DAD denial: an attacker can repeatedly reply to a victim's DAD
        messages, causing the victim's address configuration procedure
        to fail, and resulting in a denial of service to the victim
        host.
      o Forged RAs: an attacker can send RAs to other hosts to claim to
        be a router and preempt the real router, resulting in a
        Redirect attack.
      0 Forged Redirects: an attacker can pretend to be the router and
        send Redirects to other hosts to redirect their traffic to the
        router to itself, resulting in a Redirect attack.
      o Replay attacks: an attacker captures valid ND messages and
        replays them later.

2.3. Router-NCE-on-Demand Causes Performance, NCE Exhaustion and Lack
     of Subscriber Management Issues

   In ND, the router does not maintain (IP, MAC) binding (i.e. Neighbor
   Cache Entry or NCE) for a host until it is needed. This is called
   Router-NCE-on-Demand. When the router is to forward a packet to an
   on-link host, it will use address resolution to find out the MAC of
   the host. This can cause three issues:

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      o The packet has to be queued before the router finds out the MAC
        of the destination host. This reduces forwarding performance
        and may be an issue in high-performance computing environment,
        e.g. DCs.  This is called "Router-NCE-on-Demand Performance
        Degradation" in this document.
      o The way ND performs address resolution is the node will create
        an NCE entry first and set its state to INCOMPLETE, the node
        will then multicast NSs to all the hosts and wait for the
        destination host to reply with its MAC. This creates a security
        vulnerability.  If an attacker sends a large number of packets
        destined to non-existing IP addresses to the router, the router
        will create a large amount of NCEs with INCOMPLETE state while
        trying to resolve the MACs. The router may run out of resources
        and stop functioning. This is called "Router NCE Exhaustion" in
        this document. Note that in this case, the attacker can be off-
        link. So this is different from the on-link security issues.
      o Without an NCE, a router does not know the IP address of a host
        when SLAAC is used rather than [DHCPv6]. In a service provider
        network, subscribers are generally managed by their IP
        addresses, because MAC addresses are only present in the first-
        hop. Consequently, if the router does not know a host's IP
        address, the service provider cannot manage the subscriber.
        This is an issue for public access networks.

2.4. Summary of ND Issue

   The ND issues discussed in Sections 2.1 to 2.3 are summarized below.
   It is worth noting that these issues originate from three causes:
   multicast, trusting all hosts and Router-NCE-on-Demand. If a cause
   can be eliminated, the corresponding issues will also be eliminated.
   This points out the directions for ND optimization.

       Performance issues caused by multicast
           o LLA DAD degrading performance
           o Unsolicited RA degrading performance
           o GUA (or ULA) DAD degrading performance
           o Router address resolution for hosts degrading performance
           o Host Address resolution for other hosts degrading
             performance
           o Host MAC change announcement degrading performance (minor
             issue, no further discussion)
       Reliability issues caused by multicast
           o LLA DAD not reliable for wireless networks
           o GUA (or ULA) DAD not reliable for wireless networks
       On-link security issues caused by trusting all hosts
           o Source IP address spoofing

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           o DAD denial
           o Forged RAs
           o Forged Redirects
           o Replay attacks
       Off-link security issues caused by Router-NCE-on-Demand
           o Router NCE exhaustion
       Performance issue caused by Router-NCE-on-Demand
           o NCE on demand degrading performance
       Subscriber management issue caused by Router-NCE-on-Demand
           o Lack of subscriber management using ND with SLAAC

   It is worth noting again that these are just potential issues.
   Depending on the usage scenarios, they may not actually arise, and
   when they do arise, they may not necessarily cause harm. But it is
   advisable to be aware of these potential issues and the solutions
   for them.

3. Review of ND Solutions

   This section reviews the ND optimization solutions developed over
   the years so that network administrators can get an idea of what
   solutions are available for which issues. The solutions are reviewed
   in an order that helps to reveal a theme: isolating hosts to avoid
   ND issues. This theme will be further analyzed in Section 3.15 after
   all the solutions are reviewed.

3.1. ND Solution in Mobile Broadband IPv6

   Mobile Broadband IPv6 (MBBv6) is defined in "IPv6 in 3GPP EPS"
   [RFC6459] and "IPv6 for 3GPP Cellular Hosts" [RFC7066]. The solution
   key points are:

      o Putting every host, i.e. the mobile User Equipment (UE), in a
        P2P link with the router, i.e. the mobile gateway. MBBv6 also
        simplifies ND to take advantage of this P2P architecture. As a
        result:
           o All multicast is effectively turned into unicast
           o The P2P links in MBB do not have link layer address.
             Therefore, Router-NCE-on-Demand is not needed.
           o Trusting-all-host is only relevant to the router. By
             applying some filtering at the router, e.g. dropping RAs
             from the host, even malicious hosts cannot cause security
             harm.
      o Assigning a unique /64 prefix to each host, as each host is a
        separate link and subnet.

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      o Maintaining (prefix, interface) binding at the router for
        forwarding purpose.

   Since all the three causes of ND issues are addressed, MBBv6 solves
   all ND issues.

3.2. ND Solution in Fixed Broadband IPv6

   FBBv6 is defined in "IPv6 in the context of TR-101" [TR177]. FBBv6
   has two flavors:

      o P2P: every host, i.e. the Residential Gateway (RG), is in a P2P
        link with the router, i.e. the Broadband Network Gateway (BNG).
        In this case, the solution is essentially the same as MBBv6.
        All ND issues are solved.
      o P2MP: all hosts on an access device, e.g. the Optical Line
        Terminal (OLT), are in a P2MP link with the router.  This is
        implemented by aggregating all hosts into a single VLAN at the
        router and implementing Split Horizon at the access device to
        prevent direct host communication.

   The solution key points of FBBv6-P2MP [TR177] are:

      o Putting all hosts in a P2MP link with the router, and
        implementing DAD Proxy. P2MP architecture with Split Horizon
        breaks normal ND's DAD procedure. Because all hosts are in the
        same interface from the router's perspective, the router must
        ensure that the hosts have different LLAs and GUAs. Otherwise,
        the router will not be able to distinguish them. But because
        hosts cannot reach each other, normal DAD will not work.
        Therefore, the router must participate in the hosts' DAD
        process and help hosts resolve duplication. This is called DAD
        Proxy [RFC6957]. With P2MP link and DAD Proxy:
           o All host multicast to the router is effectively turned into
             unicast, as every host can only reach the router.
           o Trusting-all-host is only relevant to the router. By
             applying some simple filtering at the router, e.g.
             dropping RAs from the host, even malicious hosts cannot
             cause security harm.
      o Assigning a unique /64 prefix to each host. As a result:
           o When a prefix is assigned to the host, the router can
             proactively create (IP prefix, MAC) binding and use it for
             forwarding.  There is no need for Router-NCE-on-Demand.
           o Since different hosts are in different subnets, hosts will
             send traffic to other hosts via the router. There is no
             address resolution for other hosts.

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           o Without address resolution, router multicast to hosts
             consists only of unsolicited RAs. Because every host is in
             its own subnet, unsolicited RAs will be sent individually
             to each host with the "host's MAC replacing the multicast
             MAC" approach specified in [RFC6085]. Therefore, router
             multicast is turned into unicast.

   Since all the three causes of ND issues are addressed, FBBv6-P2MP
   addresses all ND issues.

3.3. Unique IPv6 Prefix per Host

   Unique IPv6 Prefix per Host is specified in [RFC8273]. The purpose
   is to "improve host isolation and enhanced subscriber management on
   shared network segments" such as Wi-Fi or Ethernet. The solution key
   points are:

       Assigning a unique prefix to each host with SLAAC. As a result:
           o When a prefix is assigned to the host, the router can
             proactively create (Prefix, MAC) binding and use it for
             forwarding.  There is no need for Router-NCE-on-Demand.
           o 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.
           o Without address resolution, downstream multicast to hosts
             consists only of unsolicited RAs. They will be sent host
             by host in unicast because the prefix for every host is
             different.

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

   RFC 8273 believes 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". But this may not be true
   when hosts are on a certain shared medium like Ethernet. In that
   case, hosts may still reach each other in L2 with their LLAs via
   upgream multicast. So issues caused by host multicast and Trusting-
   all-hosts may happen, e.g. on-link security issues.

3.4. Wireless ND

   Wireless ND (WiND) is specified in a series of RFCs
   [RFC6775][RFC8505][RFC8928][RFC8929]. WiND defines a new ND solution
   for Low-Power and Lossy Networks (LLNs) [RFC7102]. WiND changes host

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   and router behaviors to use multicast only for router discovery. The
   solution key points are:

      o Hosts use unicast to proactively register their addresses at
        the routers. Routers use unicast to communicate with hosts and
        become the central address register and arbitrator for the
        hosts.
      o The router also proactively installs Neighbor Cache Entries
        (NCEs) for the hosts. This avoids the need for address
        resolution for the hosts.
      o The router sets PIO L-bit to 0. Each host communicates only
        with the router.
      o Other functionalities that are relevant only to LLNs.

   WiND is a totally new ND solution. It addresses all ND issues in
   LLNs. If it is used outside LLNs, it avoids ND issues caused by
   Router-NCE-on-Demand and router multicast. But if it is used with
   shared media, its use of host multicast for router discovery means
   that host-multicast related issues and Trusting-all-host related
   issues may happen.

3.5. Scalable Address Resolution Protocol

   Scalable Address Resolution Protocol (SARP) is specified in
   [RFC7586]. It is an Experimental solution. The usage scenario is
   Data Centers (DCs) where large L2 domains spanned 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 native or overlay L2 network.

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

   Unlike MBBv6, FBBv6 and RFC 8372 which try to address all ND issues,
   SARP focuses on reducing address resolution multicast to improve
   performance and scalability of large L2 domains in DCs.

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3.6. ARP and ND Optimization for Transparent Interconnection of Lots of
   Links (TRILL):

   ARP and ND Optimization for TRILL is specified in [RFC8302]. The
   solution is very similar to SARP discussed in Section 3.5.  It can
   be considered as an application of SARP in the TRILL environment.

   Like SARP, ARP and ND Optimization for TRILL focuses on reducing
   address resolution multicast.

3.7. Proxy ARP/ND in EVPN

   Proxy ARP/ND in EVPN is specified in [RFC9161]. The usage scenario
   is Data Centers (DCs) where large L2 domains spanned across multiple
   sites. In each site, multiple hosts are connected to a Provider Edge
   (PE) router acting as a switch.  The PEs are interconnected by an
   overlay network.

   PE of each site snoops the local address resolution NAs to build
   (IP, MAC) Proxy ND table entries. PEs then propagate such Proxy ND
   entries to other PEs via BGP EVPN. Each PE also snoops address
   resolution NSs from its hosts. If an entry exists in its Proxy ND
   table for the specified destination IP address, 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. Gratuitous Neighbor Discovery

   Gratuitous Neighbor Discovery is specified in [GRAND]. GRAND changes
   router and host behaviors to allow routers to proactively create an
   NCE when a new IPv6 address is assigned to a host, and to recommend
   that hosts send unsolicited Neighbor Advertisements upon having a
   new IPv6 address. It can be considered as the IPv6 equivalent of
   Gratuitous ARP in IPv4.

   GRAND prevents the Router-NCE-on-Demand issue.

3.9. Reducing Router Advertisements

   [RFC7772] specifies a solution for reducing RAs. The key points are:

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      o The router should respond to RS with unicast RA if the host's
        source IP address is not unspecified (i.e. the RS is not the
        first RS before GUA DAD) and the host's MAC is valid.
      o The router should reduce multicast RA frequency.
        Sleeping hosts that process unicast packets while asleep must
        also process multicast RAs while asleep.
      o Sleeping hosts that do not intend to maintain IPv6 connectivity
        while asleep should either disconnect from the network and
        clear all IPv6 configuration, or perform Detecting Network
        Attachment in IPv6 (DNAv6) procedures [RFC6059] when waking up.

   RFC 7772 alleviates the multicast RA issue.

3.10. Source Address Validation Improvement and Router Advertisement
      Guard

   Source Address Validation Improvement is specified in [SAVI]. Router
   Advertisement Guard is specified in [RA-Guard][RA-Guard+]. SAVI
   binds an address to a port and rejects claims from other ports for
   that address. Therefore, a node cannot spoof the IP address of
   another node.  RA-Guard and RA-Guard+ only allow RAs from a port
   that a router is connected to. Therefore, nodes on other ports
   cannot pretend to be a router.

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

3.11. Dealing with Off-link Attack that May Cause Router NCE Exhaustion

   Router NCE Exhaustion handling is described in [RFC6583]. This is to
   deal with the off-link security issue discussed in Section 2.3. The
   solution key points are:

       For operators:
           o Filtering of unused address space so that messages to such
             addresses can be dropped rather than triggering NCE
             creation;
           o Minimizing subnet size so that there are fewer potential
             NCEs to create;
           o Rate-limiting the NDP queue to avoid CPU/memory overflow.
       For vendors:
           o Prioritizing NDP processing for existing NCEs over creating
             new NCEs

   RFC 6583 acknowledges that "some of these options are 'kludges',
   and can be operationally difficult to manage". RFC 6583 partially
   addresses the Router NCE Exhaustion issue.

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3.12. Enhanced DAD

   Enhanced DAD is specified in [RFC7527]. Enhanced DAD addresses a DAD
   failure issue in a specific situation: 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 (defined in [SeND]) 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 of the ND issues discussed in
   Section 2. It extends ND to work in a new scenario: looped back
   interface.  It is reviewed here for completeness but will not be
   further discussed.

3.13. ND Mediation for IP Interworking of Layer 2 VPNs

   ND mediation is specified in [RFC6575]. When two Attachment Circuits
   (ACs) are interconnected by a Virtual Private Wired Service (VPWS),
   and the two ACs are of different medium (e.g. one is Ethernet while
   the other is Frame Relay), the two Provider Edges (PEs) must
   interwork to provide mediation service so that a Customer Edge (CE)
   can resolve the link layer address of the remote end. RFC 6575
   specifies such a solution.

   ND Mediation does not address any of the ND issues discussed in
   Section 2. It extends ND to work in a new scenario: two ACs of
   different media interconnected by a VPWS. It is reviewed here for
   completeness but will not be further discussed.

3.14. ND Solutions Defined before the Latest Versions of ND

   The latest versions of [ND] and [SLAAC] are specified in RFCs 4861
   and 4862. Several ND optimization solutions are based on the older
   version of ND and SLAAC. They are reviewed in this section for
   completeness but will not be further discussed.

3.14.1. SeND

   Secure Neighbor Discovery [SeND] is specified in RFC 3971. The
   purpose is to ensure that hosts and routers are trustable. SeND
   defined three new ND options (i.e. Cryptographically Generated
   Addresses [CGA], RSA public-key cryptosystem, Timestamp/Nonce), an
   authorization delegation discovery process, an address ownership

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   proof mechanism, and requirements for the use of these components in
   NDP.

   SeND addresses the Trusting-all-hosts issues. But it has high
   requirements on the hosts and routers, especially to maintain the
   keys.  SeND is rarely deployed and will not be further discussed in
   this document.

3.14.2. Cryptographically Generated Addresses (CGA)

   Cryptographically Generated Addresses [CGA] is specified in RFC
   3972. The purpose is to associate a cryptographic public key with an
   IPv6 address in [SeND]. The solution key point is to generate the
   Interface Identifier (IID) of the 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 uses the fact that a legitimate host does not care about the bit
   combination of IID that would be created as a result of 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 that is a strong mathematical challenge.

   CGA is part of SeND. It is rarely deployed and will not be further
   discussed in this document.

3.14.3. ND Proxy

   ND Proxy is specified in [RFC4389]. It is an Experimental solution.
   The purpose is to enable multiple links joined by an ND-Proxy device
   to work as a single link. The ND-Proxy acts like a bridge. The
   solution key points are:

      o When it receives an ND request from a host in a link, it will
        "proxy" the message out from the "best" outgoing interface. How
        to determine the "best" interface is explained later. 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 to
        the ND-Proxy's outgoing interface's IP and MAC, and create an
        NCE entry at the outgoing interface accordingly.
      o When ND-Proxy receives an ND reply, it will act as if the ND
        message is destined to itself, and update the NCE entry state
        at the receiving interface. Based on such state information,

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        the ND-Proxy can determine the "best" outgoing interface for
        future ND requests. The ND-Proxy then "proxy" the ND message
        back to the requesting host.

   ND Proxy does not solve any of the ND issues discussed in Section 2.
   It extends ND to work in a new scenario: multiple links joined by a
   device that is not a bridge but acting like a bridge.

   The idea of ND Proxy is widely used in SARP, ND Optimization for
   TRILL and Proxy ARP/ND in EVPN which are discussed in Sections 3.4
   to 3.6.

3.14.4. Optimistic DAD

   Optimistic DAD is specified in [RFC4429]. The purpose is to minimize
   address configuration delays in the successful case and to reduce
   disruption as far as possible in the failure case. Optimistic DAD
   modified the original ND (RFC 2461) and SLAAC (RFC 2462) but the
   solution was not incorporated into the latest specification of [ND]
   and [SLAAC].

   Optimistic DAD does not solve any of the ND issues discussed in
   Section 2. It tries to enhance ND's performance for DAD. But the
   changes are big and the benefits are not significant. Optimistic DAD
   has not been widely deployed.

3.15. Observations on the Solutions

   Which ND solution solving which ND issue is tabulated below, for the
   fifteen ND issues summarized in Section 2.4:

       Performance issues caused by multicast
           o I1: LLA DAD degrading performance
           o I2: Unsolicited RA degrading performance
           o I3: GUA (or ULA) DAD degrading performance
           o I4: Router address resolution for hosts degrading
             performance
           o I5: Host Address resolution for other hosts degrading
             performance
       Reliability issues caused by multicast
           o I6: LLA DAD not reliable for wireless networks
           o I7: GUA DAD not reliable for wireless networks
       On-link security issues caused by trusting all hosts
           o I8: Source IP address spoofing
           o I9: DAD denial
           o I10: Fake RAs

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           o I11: Fake Redirect
           o I12: Replay attacks
       Off-link security issues caused by Router-NCE-on-Demand
           o I13: Router NCE exhaustion
       Performance issue caused by Router-NCE-on-Demand
           o I14: NCE on demand degrading performance
       Subscriber management issue caused by Router-NCE-on-Demand
           o I15: Lack of subscriber management using ND with SLAAC

   +-----+-------------------+--------+--------+--------+------+-----+

   |     |     Multicast     | Reli-  |On-link |Off-link|NCE on|Sub  |

   |     |     performance   | ability|security|security|Demand|Mgmt.|

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |Issue| 1 | 2 | 3 | 4 | 5 | 6 |  7 |  8-12  |   13   |  14  | 15  |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |MBBv6|               All issues solved                           |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |FBBv6|               All issues solved                           |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |8273 |   | X | X | X | X |   |  X |        |    X   |   X  |  X  |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |WiND |               All issues solved for LLNs                  |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |SARP |   |   |   |   | X |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |ND   |   |   |   |   | X |   |    |        |        |      |     |

   |TRILL|   |   |   |   |   |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

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   |ND   |   |   |   |   | X |   |    |        |        |      |     |

   |EVPN |   |   |   |   |   |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |7772 |   | X |   |   |   |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |GRAND|   |   |   | X |   |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |SAVI/|   |   |   |   |   |   |    |        |        |      |     |

   |RAG  |   |   |   |   |   |   |    |   X    |        |      |     |

   |G+   |   |   |   |   |   |   |    |        |        |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

   |6583 |   |   |   |   |   |   |    |        |    X   |      |     |

   +-----+---+---+---+---+---+---+----+--------+--------+------+-----+

               Table 1. Which solution solves which issue(s)

   It is worth noting again that SARP is an Experimental solution.
   Caution should be applied whether to choose this solution.

   Although the various ND solutions look unrelated, dividing them into
   four groups will help to reveal a common theme: isolating hosts to
   prevent issues.

   The first group contains MBBv6, FBBv6, Unique Prefix Per Host and
   WiND. These solutions all isolate hosts individually in some way,
   and they prevent all or most ND issues.

   The second group contains SARP, ND Optimization for TRILL, and Proxy
   ND in EVPN. They use a proxy device to represent the hosts behind
   it, and effectively isolate such hosts from other hosts. The
   solutions alleviate the biggest ND issue - address resolution among
   hosts.

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   The third group contains Reducing RAs, SAVI, RA-Guard, RA-Guard+,
   and Dealing with Off-link Security Issue. They do not try to isolate
   hosts. They focus on solving a specific ND issue instead.

   The fourth group contains the solutions designated "will not be
   further discussed".  They are not relevant to the discussion here.

   This theme reveals an insight: isolating hosts is effective in
   solving ND issues. The stronger hosts are isolated, the more ND
   issues can be prevented. This is natural because isolating hosts
   reduces multicast and hosts to trust, two of the causes of ND
   issues.

   This insight can be used to formulate guidelines to simplify IPv6
   first-hop deployments where ND plays a key role.

4. Isolating Hosts to Simplify First-hop Deployments

   This section describes how to isolate hosts and the advantages and
   disadvantages of doing so. It also provides some guidelines on how
   to select a suitable isolation method based on the deployment
   scenario.

   The solution review in Section 3 reveals four different host
   isolation methods:

       Link isolation, with 2 flavors:
           o P2P link isolation, used in MBBv6 and FBBv6-PPPoE
           o P2MP link isolation, used in FBBv6-IPoE
       Subnet isolation (i.e. Unique Prefix Per Host), used in MBBv6,
             FBBv6 and RFC 8273
       GUA isolation (i.e. setting PIO L-bit=0), used in WiND and RFC
             8273
           o GUA isolation is different from link isolation in that
             there can be multiple hosts in the link. It is just that
             each host treats other hosts as off-link and does not
             perform address resolution for other hosts' GUA. The host
             will send messages with GUA via the router instead. But
             all messages with LLA can still reach other hosts. In link
             isolation, there is only one host in each link. A host
             cannot send messages with LLAs to other hosts.
       Proxy isolation, used in SARP, ND Optimization for TRILL, Proxy
             ND in EVPN

   These different isolation methods are not independent:

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   First, [RFC4291] stated that "IPv6 continues the IPv4 model in that
   a subnet prefix is associated with one link". Therefore, link
   isolation and subnet isolation should be used together, so that the
   link and the subnet are congruent in scope: both have just one host.
   Otherwise, additional ND issues will appear and the solution will be
   more complicated:

      o L2 isolation without subnet isolation, which creates a Multi-
        Link SubNet (MLSN), violates [RFC4291] by making the subnet
        associate with multiple links (i.e., the subnet is bigger than
        each link). The consequence is GUA DAD may not work unless the
        router provides DAD Proxy. [RFC4903] documented the concerns
        about MLSN.
      o Subnet isolation without L2 isolation, when used for multiple
        hosts on a shared medium, also violates [RFC4291] by making the
        subnet not associate with any link (i.e., the subnet with just
        one host is smaller than the link with multiple hosts).  The
        consequence is, on-link security issues may happen. For
        example, LLA DAD denial may happen.

   Second, link isolation and subnet isolation automatically imply GUA
   isolation. When there is only one host in a link/subnet, setting PIO
   L-bit to 1 has the same effect as setting it to 0, because all
   communication will go through the router.

   Third, proxy isolation and other isolation methods are mutually
   exclusive. Proxy isolation uses a proxy to represent multiple hosts
   at a site. In other words, hosts are not isolated individually like
   in link/subnet/GUA isolation.

   Therefore, these different isolation methods only produce four
   meaningful combinations:

      o P2P Link and Subnet Isolation
      o P2MP Link and Subnet Isolation
      o GUA Isolation (without link or subnet isolation)
      o Proxy Isolation

   These isolation methods are listed from the highest degree of
   isolation to the lowest. Their applicability is discussed below.

4.1. Applicability of P2P Link and Subnet Isolation

   The advantages of applying P2P link and subnet isolation are:

   o All ND issues are prevented.

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   The disadvantages are:

   o The hosts must be able to set up P2P links with the router.

   o Many interfaces will be needed at the router, one per host.

   o Many prefixes will be needed, one per host.

        o This is probably not an issue for IPv6. Today, any company can
          get a /29 from a Regional Internet Registry (RIR) [RIPE738].
          This contains 32 billion /64 prefixes and should be
          sufficient for any scenarios. The fact that MBBv6 assigns a
          /64 to every mobile UE [RFC6459], and FBBv6 assigns a /56 to
          every routed RG [TR177] is evidence.

   o All hosts will communicate through the router, and the router may
      become a bottleneck. So this cannot be used in a high-performance
      computing environment like DCs.

   o Services relying on multicast, e.g. mDNS, will not work.

4.2. Applicability of P2MP Link and Subnet Isolation

   The applicability of P2MP Link and Subnet Isolation is the same as
   P2P, except that:

   o DAD Proxy is required in P2MP.

   o Hosts do not need the capability to set up P2P links with the
      router. [PVLAN] or Wireless Isolation [W-Iso] must be configured
      to enable the P2MP link instead.

   o Only one interface is needed at the router

4.3. Applicability of GUA Isolation

   The advantages of GUA Isolation are:

   o No address resolution for GUA or ULA among hosts. This eliminates
      the largest source of multicast in ND.

   o This is normal ND behavior. No ND optimization solution is
      needed.

   The disadvantages are:

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   o Only multicast address resolution for GUA or ULA among hosts is
      eliminated. All other ND issues may still happen depending on the
      usage scenarios.  Consequently, other solutions may be needed to
      solve such issues.

   o All host communication with GUA or ULA will go through the
      router, and the router may become a bottleneck. So this cannot be
      used in a high-performance computing environment like DCs.

4.4. Applicability of Proxy Isolation

   The advantages of Proxy Isolation are:

   o Reduced address resolution for GUA among hosts behind different
      proxies. This reduces the largest source of multicast in ND.

   o Hosts can communicate directly without going through the router.
      This can be used in a high-performance computing environment like
      DCs.

   The disadvantages are:

   o Only multicast address resolution for GUA among hosts behind
      different proxies is reduced. All other ND issues may happen.
      Consequently, other solutions may be needed to solve such issues.

4.5. ND Deployment Guidelines

   Given the applicability analysis above, network administrators can
   decide where to apply which isolation method.

   The guidelines below start from the strongest isolation method. This
   prevents the most ND issues, and therefore, requires fewest
   additional solutions for the remaining issues. The overall solution
   will likely be the simplest. But the strongest isolation also has
   the highest entry requirements and the fewest applicable scenarios.
   If the strongest isolation is not possible, the next level of
   isolation is tried, until no isolation is applied.  Therefore,
   network administrators can likely find the most suitable isolation
   method for their deployment scenarios.

   1. If P2P Link and Subnet Isolation is feasible:

        a) Applicable scenarios:

             1) Direct host to host communication is not required.

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             2) A P2P architecture is feasible.

             3) Multicast is not desirable (implying mDNS is not needed)
               for performance or reliability reasons, or

             4) Hosts may not be trustable, or

             5) Subscriber management is needed.

             Examples are public access networks such as MBBv6 or FBBv6
             PPPoE.

        b) Entry requirements:

             1) Hosts must be able to set up P2P links with the router.

             2) The router must have an optimized ND solution that
               avoids router multicast (i.e. DADs, unsolicited RAs,
               address resolution for hosts), like MBBv6 or FBBv6 or
               RFC 8273.

        c) Remaining issues and solutions:

             1) All ND issues are prevented, except Trusting-all-host at
               the router.

             2) Filtering may be needed at the router to discard
               malicious/erroneous ND messages from hosts, e.g. RAs, in
               case the hosts are not trustable.

   2. Otherwise, if P2MP Link and Subnet Isolation is feasible

        a) Applicable scenarios:

             1) Same as the P2P scenarios, except that a P2P
               architecture is not possible while a P2MP architecture
               is possible.

             Examples are FBBv6 IPoE or public Wi-Fi access.

        b) Entry requirements:

             1) The L2 media supports a P2MP architecture (e.g. with
               PVLAN on Ethernet, or with wireless isolation on Wi-Fi).

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             2) DAD Proxy must be added on top of the P2P-aware ND
               optimization solution.

        c) Remaining issues and solutions

             1) Same as the P2P case

   3. Otherwise, if GUA Isolation (i.e. setting PIO L-bit=0) is feasible

        a) Applicable scenarios:

             1) Direct host to host communication is not required.

             2) Multicast is needed, or the L2 medium is not feasible to
               support P2P/P2MP architecture

             Examples are [HomeNet] where mDNS is desired or LLNs.

        b) Entry requirements:

             1) For LLNs, WiND is required.

             2) For other scenarios, no other requirement than ND.

        c) Remaining issues and solutions:

             1) If WiND is used, all ND issues are prevented, as WiND
               has modified ND to handle the issues.

             2) If normal ND is used, only multicast address resolution
               for GUA among hosts is eliminated. All other ND issues
               may happen. Depending on the specific deployment
               scenario, only a subset of issues may actually happen.

             3) Use Table 1 to pick the solutions for the issues that
               will actually happen.

   4. Otherwise, if Proxy Isolation is feasible

        a) Applicable scenarios:

             1) Direct host to host communication is required.

             Examples are large scale DC involving a large number of
             VMs, and the link spanned across multiple sites
             interconnected by PEs.

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        b) Entry requirements:

             1) A Proxy Isolation solution like SARP, ND Optimization
               for TRILL or Proxy ND in EVPN.

        c) Remaining issues and solutions:

             1) Only multicast address resolution for GUA among hosts is
               reduced. All other ND issues may happen. Depending on
               the specific deployment scenario, only a subset of
               issues may actually happen.

             2) Use Table 1 to pick the solutions for the issues that
               will actually happen.

   5. Otherwise, no isolation to apply

        a) Applicable scenarios:

             1) Small scale and non-critical scenarios.

        b) Entry requirements:

             1) None.

        c) Remaining issues and solutions

             1) All ND issues may happen. Depending on the specific
               deployment scenario, only some issues may actually
               happen, and even fewer issues may be of concern, because
               this is a small scale and non-critical scenario.

             2) Use Table 1 to pick the solutions for the issues that
               are of concern.

4.6. Impact of Host Isolation on Other Protocols in IPv6 First-hops

   The impact (i.e. the disadvantages) of various isolation methods to
   the IPv6 first-hop has been discussed in the applicability sections.
   The guidelines have considered such applicability in selecting a
   suitable isolation method and given the network adminstrators the
   deciding power not to use any isolation method at all. Therefore, if
   an isolation method is indeed selected, its benefits likely outweigh
   its cost.

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5. Security Considerations

   This document provides guidelines on how to select a suitable
   isolation method depending on the deployment scenario. When an
   isolation method is selected, the security considerations of the
   used solutions which are defined in the corresponding RFCs apply.
   This document does not introduce any new solutions. Therefore, it
   does not introduce new security issues.

6. IANA Considerations

   This document has no request to IANA.

7. References

7.1. Informative References

   [CGA]     T. Aura, "Cryptographically Generated Addresses (CGA)",
             RFC3972

   [DHCPv6]  T. Mrugalski M. Siodelski B. Volz A. Yourtchenko M.
             Richardson S. Jiang T. Lemon T. Winters, "Dynamic Host
             Configuration Protocol for IPv6 (DHCPv6)", RFC 8415.

   [GRAND]   J. Linkova, "Gratuitous Neighbor Discovery: Creating
             Neighbor Cache Entries on First-Hop Routers", RFC 9131

   [HomeNet] T. Chown, J. Arkko, A. Brandt, O. Troan, J. Weil, "IPv6
             Home Networking Architecture Principles", RFC 7368, DOI
             10.17487/RFC7368, October 2014, <https://www.rfc-
             editor.org/info/rfc7368>.

   [mDNS]    S. Cheshire, M. Krochmal, "Multicast DNS", RFC 6762.

   [ND]      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>.

   [PVLAN]  https://en.wikipedia.org/wiki/Private_VLAN

   [RA-Guard] E. Levy-Abegnoli, G. Van de Velde, C. Popoviciu, J.
             Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, DOI
             10.17487/RFC6105, February 2011, <https://www.rfc-
             editor.org/info/rfc6105>.

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   [RA-Guard+] F. Gont, "Implementation Advice for IPv6 Router
             Advertisement Guard (RA-Guard)", RFC 7113, DOI
             10.17487/RFC7113, February 2014, <https://www.rfc-
             editor.org/info/rfc7113>.

   [RFC3756] P. Nikander, J. Kempf, E. Nordmark, "IPv6 Neighbor
             Discovery (ND) Trust Models and Threats", RFC 3756.

   [RFC4291] R. Hinden, S.Deering, "IP Version 6 Addressing
             Architecture", RFC 4291.

   [RFC4389] D. Thaler, M. Talwar, C. Patel, "Neighbor Discovery
             Proxies (ND Proxy)", RFC 4389.

   [RFC4429] N. Moore, "Optimistic Duplicate Address Detection (DAD)
             for IPv6", RFC 4429.

   [RFC4903] D. Thaler, "Multi-Link Subnet Issues", RFC 4903.

   [RFC6459] J. Korhonen, J. Soininen, B. Patil, T. Savolainen, G.
             Bajko, K. Iisakkila, "IPv6 in 3rd Generation Partnership
             Project (3GPP) Evolved Packet System (EPS)", RFC 6459.

   [RFC6059] S. Krishnan, G. Daley, "Simple Procedures for Detecting
             Network Attachment in IPv6", RFC 6059.

   [RFC6085] S. Gundavelli, M. Townsley, O. Troan, W. Dec, "Address
             Mapping of IPv6 Multicast Packets on Ethernet", RFC 6085.

   [RFC6575] H. Shah, E. Rosen, G. Heron, V. Kompella, "Address
             Resolution Protocol (ARP) Mediation for IP Interworking of
             Layer 2 VPNs", RFC 6575.

   [RFC6583] I. Gashinsky, J. Jaeggli, W. Kumari, "Operational Neighbor
             Discovery Problems", RFC 6583.

   [RFC6775] Z. Shelby, S. Chakrabarti, E. Nordmark, C. Bormann,
             "Neighbor Discovery Optimization for IPv6 over Low-Power
             Wireless Personal Area Networks (6LoWPANs)", RFC 6775.

   [RFC6957] F. Costa, J-M. Combes, X. Pougnard, H. Li, "Duplicate
             Address Detection Proxy", RFC 6957

   [RFC7066] J. Korhonen, J. Arkko, T. Savolainen, S. Krishnan, "IPv6
             for Third Generation Partnership Project (3GPP) Cellular
             Hosts", RFC 7066.

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   [RFC7102] JP. Vasseur, "Terms Used in Routing for Low-Power and
             Lossy Networks", RFC 7102.

   [RFC7342] L. Dunbar, W. Kumari, I. Gashinsky, "Practices for Scaling
             ARP and Neighbor Discovery (ND) in Large Data Centers",
             RFC 7342.

   [RFC7527] R. Asati, H. Singh, W. Beebee, C. Pignataro, E. Dart, W.
             George, "Enhanced Duplicate Address Detection", RFC 7527.

   [RFC7586] Y. Nachum, L. Dunbar, I. Yerushalmi, T. Mizrahi, "The
             Scalable Address Resolution Protocol (SARP) for Large Data
             Centers", RFC7586.

   [RFC7772] A. Yourtchenko, L. Colitti, "Reducing Energy Consumption
             of Router Advertisements", RFC 7772.

   [RFC8273] J. Brzozowski, G. Van de Velde, "Unique IPv6 Prefix per
             Host", RFC 8273.

   [RFC8302] Y. Li, D. Eastlake 3rd, L. Dunbar, R. Perlman, M. Umair,
             "Transparent Interconnection of Lots of Links (TRILL): ARP
             and Neighbor Discovery (ND) Optimization", RFC 8302.

   [RFC8505] P. Thubert, E. Nordmark, S. Chakrabarti, C. Perkins,
             "Registration Extensions for IPv6 over  Low-Power Wireless
             Personal Area Network (6LoWPAN) Neighbor Discovery", RFC
             8505.

   [RFC8928] P. Thubert, B. Sarikaya, M. Sethi, R. Struik, "Address-
             Protected Neighbor Discovery for Low-Power and Lossy
             Networks", RFC 8928.

   [RFC8929] P. Thubert, C.E. Perkins, E. Levy-Abegnoli, "IPv6 Backbone
             Router", RFC 8929.

   [RFC9099] E. Vyncke, K. Chittimaneni, M. Kaeo, E. Rey, "Operational
             Security Considerations for IPv6 Networks", RFC 9099.

   [RFC9119] C. Perkins, M. McBride, D. Stanley, W. Kumari, JC. Zuniga,
             "Multicast Considerations over IEEE 802 Wireless Media",
             RFC 9119.

   [RFC9161] J. Rabadan, S. Sathappan, K. Nagaraj, G. Hankins, T. King,
             "Operational Aspects of Proxy ARP/ND in Ethernet Virtual
             Private Networks", RFC 9161

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   [RIPE738] IPv6 Address Allocation and Assignment Policy,
             https://www.ripe.net/publications/docs/ripe-738

   [SAVI]    J. Wu, J. Bi, M. Bagnulo, F. Baker, C. Vogt, "Source
             Address Validation Improvement (SAVI) Framework", RFC 7039

   [SeND]    J. Arkko, J. Kempf, B. Zill, P. Nikander, "SEcure Neighbor
             Discovery (SEND)", RFC3971

   [SLAAC]   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>.

   [TR177]   S. Ooghe, B. Varga, W. Dec, D. Allan, "IPv6 in the context
             of TR-101", Broadband Forum, TR-177.

   [W-Iso]   Wireless Isolation, https://www.quora.com/What-is-
             wireless-isolation

8. Acknowledgments

   The authors would like to thank Pascal Thubert, Ole Troan, Brian
   Carpenter, David Thaler, Jen Linkova, Eric Vyncke, Lorenzo Colitti,
   Philipp Tiesel, Chongfeng Xie, Mike Ackermann for the discussion and
   input.

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Authors' Addresses

   XiPeng Xiao
   Huawei Technologies Dusseldorf
   Hansaallee 205, 40549 Dusseldorf, Germany

   Email: xipengxiao@huawei.com

   Eduard Vasilenko
   Huawei Technologies
   17/4 Krylatskaya st, Moscow, Russia 121614

   Email: vasilenko.eduard@huawei.com

   Eduard Metz
   KPN N.V.
   Maanplein 55, 2516CK  The Hague, The Netherlands

   Email: eduard.metz@kpn.com

   Gyan Mishra
   Verizon Inc.

   Email: gyan.s.mishra@verizon.com

   Nick Buraglio
   Energy Sciences Network

   Email: buraglio@es.net

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