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6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6775, 8505 (if approved)                           C.E. Perkins
Intended status: Standards Track                  Blue Meadow Networking
Expires: 23 August 2020                                 E. Levy-Abegnoli
                                                           Cisco Systems
                                                        20 February 2020


                          IPv6 Backbone Router
                   draft-ietf-6lo-backbone-router-17

Abstract

   This document updates RFC 6775 and RFC 8505 in order to enable proxy
   services for IPv6 Neighbor Discovery by Routing Registrars called
   Backbone Routers.  Backbone Routers are placed along the wireless
   edge of a Backbone, and federate multiple wireless links to form a
   single Multi-Link Subnet.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 23 August 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of 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



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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  References  . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Updating RFC 6775 and RFC 8505  . . . . . . . . . . . . .  10
     3.2.  Access Link . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Route-Over Mesh . . . . . . . . . . . . . . . . . . . . .  12
     3.4.  The Binding Table . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Primary and Secondary 6BBRs . . . . . . . . . . . . . . .  14
     3.6.  Using Optimistic DAD  . . . . . . . . . . . . . . . . . .  15
   4.  Multi-Link Subnet Considerations  . . . . . . . . . . . . . .  15
   5.  Optional 6LBR serving the Multi-Link Subnet . . . . . . . . .  16
   6.  Using IPv6 ND Over the Backbone Link  . . . . . . . . . . . .  17
   7.  Routing Proxy Operations  . . . . . . . . . . . . . . . . . .  18
   8.  Bridging Proxy Operations . . . . . . . . . . . . . . . . . .  20
   9.  Creating and Maintaining a Binding  . . . . . . . . . . . . .  21
     9.1.  Operations on a Binding in Tentative State  . . . . . . .  22
     9.2.  Operations on a Binding in Reachable State  . . . . . . .  23
     9.3.  Operations on a Binding in Stale State  . . . . . . . . .  24
   10. Registering Node Considerations . . . . . . . . . . . . . . .  25
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   12. Protocol Constants  . . . . . . . . . . . . . . . . . . . . .  26
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   16. Informative References  . . . . . . . . . . . . . . . . . . .  28
   Appendix A.  Possible Future Extensions . . . . . . . . . . . . .  31
   Appendix B.  Applicability and Requirements Served  . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   IEEE STD. 802.1 [IEEEstd8021] Ethernet Bridging provides an efficient
   and reliable broadcast service for wired networks; applications and
   protocols have been built that heavily depend on that feature for
   their core operation.  Unfortunately, Low-Power Lossy Networks (LLNs)
   and local wireless networks generally do not provide the broadcast
   capabilities of Ethernet Bridging in an economical fashion.





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   As a result, protocols designed for bridged networks that rely on
   multicast and broadcast often exhibit disappointing behaviours when
   employed unmodified on a local wireless medium (see
   [I-D.ietf-mboned-ieee802-mcast-problems]).

   Wi-Fi [IEEEstd80211] Access Points (APs) deployed in an Extended
   Service Set (ESS) act as Ethernet Bridges [IEEEstd8021], with the
   property that the bridging state is established at the time of
   association.  This ensures connectivity to the end node (the Wi-Fi
   STA) and protects the wireless medium against broadcast-intensive
   Transparent Bridging reactive Lookups.  In other words, the
   association process is used to register the MAC Address of the STA to
   the AP.  The AP subsequently proxies the bridging operation and does
   not need to forward the broadcast Lookups over the radio.

   In the same way as Transparent Bridging, IPv6 [RFC8200] Neighbor
   Discovery [RFC4861] [RFC4862] Protocol (IPv6 ND) is a reactive
   protocol, based on multicast transmissions to locate an on-link
   correspondent and ensure the uniqueness of an IPv6 address.  The
   mechanism for Duplicate Address Detection (DAD) [RFC4862] was
   designed for the efficient broadcast operation of Ethernet Bridging.
   Since broadcast can be unreliable over wireless media, DAD often
   fails to discover duplications [I-D.yourtchenko-6man-dad-issues].  In
   practice, the fact that IPv6 addresses very rarely conflict is mostly
   attributable to the entropy of the 64-bit Interface IDs as opposed to
   the succesful operation of the IPv6 ND duplicate address detection
   and resolution mechanisms.

   The IPv6 ND Neighbor Solicitation (NS) [RFC4861] message is used for
   DAD and address Lookup when a node moves, or wakes up and reconnects
   to the wireless network.  The NS message is targeted to a Solicited-
   Node Multicast Address (SNMA) [RFC4291] and should in theory only
   reach a very small group of nodes.  But in reality, IPv6 multicast
   messages are typically broadcast on the wireless medium, and so they
   are processed by most of the wireless nodes over the subnet (e.g.,
   the ESS fabric) regardless of how few of the nodes are subscribed to
   the SNMA.  As a result, IPv6 ND address Lookups and DADs over a large
   wireless and/or a LowPower Lossy Network (LLN) can consume enough
   bandwidth to cause a substantial degradation to the unicast traffic
   service.

   Because IPv6 ND messages sent to the SNMA group are broadcast at the
   radio MAC Layer, wireless nodes that do not belong to the SNMA group
   still have to keep their radio turned on to listen to multicast NS
   messages, which is a total waste of energy for them.  In order to
   reduce their power consumption, certain battery-operated devices such
   as IoT sensors and smartphones ignore some of the broadcasts, making
   IPv6 ND operations even less reliable.



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   These problems can be alleviated by reducing the IPv6 ND broadcasts
   over wireless access links.  This has been done by splitting the
   broadcast domains and routes between subnets, or even by assigning a
   /64 prefix to each wireless node (see [RFC8273]).

   Another way is to proxy at the boundary of the wired and wireless
   domains the Layer 3 protocols that rely on MAC Layer broadcast
   operations.  For instance, IEEE 802.11 [IEEEstd80211] situates proxy-
   ARP (IPv4) and proxy-ND (IPv6) functions at the Access Points (APs).
   The 6BBR provides a proxy-ND function and can be extended for proxy-
   ARP in a continuation specification.

   Knowledge of which address to proxy for can be obtained by snooping
   the IPV6 ND protocol (see [I-D.bi-savi-wlan]), but it has been found
   to be unreliable.  An IPv6 address may not be discovered immediately
   due to a packet loss, or if a "silent" node is not currently using
   one of its addresses.  A change of state (e.g., due to movement) may
   be missed or misordered, leading to unreliable connectivity and
   incomplete knowledge of the state of the network.

   This specification defines the 6BBR as a Routing Registrar [RFC8505]
   that provides proxy services for IPv6 Neighbor Discovery.  As
   represented in Figure 1, Backbone Routers federate multiple LLNs over
   a Backbone Link to form a Multi-Link Subnet (MLSN).  The MLSN breaks
   the Layer 2 continuity and splits the broadcast domain, in a fashion
   that each Link, including the backbone, is its own broadcast domain.
   This means that devices that rely on a link-scope multicast on the
   backbone will only reach other nodes on the backbone but not LLN
   nodes.  A key property of MLSNs is that link-local traffic and
   traffic with a hop limit of 1 will not transit to nodes in the same
   subnet on a different link, something that may produce unexpected
   behavior in software that expects a subnet to be entirely contained
   within a single link.  In order to enable the continuity of IPv6 ND
   operations beyond the backbone, and enable communication using Global
   or Unique Local Addresses between any pair of nodes in the MLSN,
   Backbone Routers placed along the LLN edge of the Backbone handle
   IPv6 ND on behalf of Registered Nodes and forward IPv6 packets back
   and forth.

   A 6LoWPAN node (6LN) registers all its IPv6 Addresses using an
   NS(EARO) as specified in [RFC8505] to the 6BBR.  The 6BBR is also a
   Border Router that performs IPv6 Neighbor Discovery (IPv6 ND)
   operations on its Backbone interface on behalf of the 6LNs that have
   registered addresses on its LLN interfaces without the need of a
   broadcast over the wireless medium.  A 6LN that resides on the
   backbone does not register to the SNMA groups associated to its
   Registered Addresses and defers to the 6BBR to answer or preferably
   forward to it as unicast the corresponding multicast packets.



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2.  Terminology

2.1.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  New Terms

   This document introduces the following terminology:

   Federated:  A subnet that comprises a Backbone and one or more
      (wireless) access links, is said to be federated into one Multi-
      Link Subnet.  The proxy-ND operation of 6BBRs over the Backbone
      extends IPv6 ND operation over the access links.

   Sleeping Proxy:  A 6BBR acts as a Sleeping Proxy if it answers ND
      Neighbor Solicitations over the Backbone on behalf of the
      Registering Node which might be in a sleep state in a low power
      network.  The Sleeping Proxy that is also a Bridging Proxy will
      preferably forward the relevant messages to the Registering Node
      as unicast frames in accord to the duty cycle of the Registering
      Node and let it respond.

   Routing Proxy:  A Routing Proxy provides IPv6 ND proxy functions and
      enables the MLSN operation over federated links that may not be
      compatible for bridging.  The Routing Proxy advertises its own MAC
      Address as the Target Link Layer Address (TLLA) in the proxied NAs
      over the Backbone, and routes at the Network Layer between the
      federated links.

   Bridging Proxy:  A Bridging Proxy provides IPv6 ND proxy functions
      while preserving forwarding continuity at the MAC Layer.  The
      Bridging Proxy advertises the MAC Address of the Registering Node
      as the TLLA in the proxied NAs over the Backbone.  In that case,
      the MAC Address and the mobility of 6LN is still visible across
      the bridged Backbone, and the 6BBR may be configured to proxy for
      Link Local Addresses.

   Binding Table:  The Binding Table is an abstract database that is
      maintained by the 6BBR to store the state associated with its
      registrations.

   Binding:  A Binding is an abstract state associated to one
      registration, in other words one entry in the Binding Table.



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2.3.  Abbreviations

   This document uses the following abbreviations:

   6BBR:  6LoWPAN Backbone Router
   6LBR:  6LoWPAN Border Router
   6LN:  6LoWPAN Node
   6LR:  6LoWPAN Router
   6CIO:  Capability Indication Option
   ARO:  Address Registration Option
   DAC:  Duplicate Address Confirmation
   DAD:  Duplicate Address Detection
   DAR:  Duplicate Address Request
   EARO:  Extended Address Registration Option
   EDAC:  Extended Duplicate Address Confirmation
   EDAR:  Extended Duplicate Address Request
   DODAG:  Destination-Oriented Directed Acyclic Graph
   ID:  Identifier
   LLN:  Low-Power and Lossy Network
   NA:  Neighbor Advertisement
   MAC:  Medium Access Control
   NCE:  Neighbor Cache Entry
   ND:  Neighbor Discovery
   NDP:  Neighbor Discovery Protocol
   NS:  Neighbor Solicitation
   NS(DAD):  NDP NS message used for the purpose of duplication
      avoidance (multicast)
   NS(Lookup):  NDP NS message used for the purpose of address
      resolution (multicast)
   NS(NUD):  NDP NS message used for the purpose of unreachability
      detection (unicast)
   NUD:  Neighbor Unreachability Detection
   ROVR:  Registration Ownership Verifier
   RPL:  IPv6 Routing Protocol for LLNs
   RA:  Router Advertisement
   RS:  Router Solicitation
   SNMA:  Solicited-Node Multicast Address
   LLA:  Link Layer Address (aka MAC address)
   SLLA:  Source Link Layer Address
   TLLA:  Target Link Layer Address
   TID:  Transaction ID


2.4.  References

   In this document, readers will encounter terms and concepts that are
   discussed in the following documents:




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   *  "Neighbor Discovery for IP version 6" [RFC4861], "IPv6 Stateless
      Address Autoconfiguration" [RFC4862] and "Optimistic Duplicate
      Address Detection" [RFC4429],

   *  "Neighbor Discovery Proxies (proxy-ND)" [RFC4389] and "Multi-Link
      Subnet Issues" [RFC4903],

   *  "Problem Statement and Requirements for IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606], and

   *  Neighbor Discovery Optimization for Low-Power and Lossy Networks
      [RFC6775] and "Registration Extensions for 6LoWPAN Neighbor
      Discovery" [RFC8505].

3.  Overview

   This section and its subsections present a non-normative high level
   view of the operation of the 6BBR.  The following sections cover the
   normative part.  Figure 1 illustrates a backbone link that federates
   a collection of LLNs as a single IPv6 Subnet, with a number of 6BBRs
   providing proxy-ND services to their attached LLNs.

   The LLN may be a hub-and-spoke access link such as (Low-Power) IEEE
   STD. 802.11 (Wi-Fi) [IEEEstd80211] and IEEE STD. 802.15.1 (Bluetooth)
   [IEEEstd802151], or a Mesh-Under or a Route-Over network [RFC8505].
   The proxy state can be distributed across multiple 6BBRs attached to
   the same Backbone.

                    |
                 +-----+               +-----+       +-----+ IPv6
       (default) |     |    (Optional) |     |       |     | Node
          Router |     |          6LBR |     |       |     | or
                 +-----+               +-----+       +-----+ 6LN
                    |  Backbone side      |             |
        ----+-------+-----------------+---+-------------+----+-----
            |                         |                      |
         +------+                 +------+                +------+
         | 6BBR |                 | 6BBR |                | 6BBR |
         |      |                 |      |                |      |
         +------+                 +------+                +------+
            o     Wireless side   o   o  o      o           o o
        o o   o  o  o   o  o  o o   o  o  o   o o  o  o  o o     o   o
       o  o o  o o   o o  o   o   o  o  o  o       o     o  o  o o o
       o   o  o  o  o  o   o  o  o  LLN  o   o  o  o  o   o   o  o   o
         o   o o   o   o   o  o     o  o    o      o     o     o o
        o     o                o

                Figure 1: Backbone Link and Backbone Routers



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   The main features of a 6BBR are as follows:

   *  Multi-Link-subnet functions (provided by the 6BBR on the backbone)
      performed on behalf of registered 6LNs, and

   *  Routing registrar services that reduce multicast within the LLN:

   *  Binding Table management
   *  failover, e.g., due to mobility

   Each Backbone Router (6BBR) maintains a data structure for its
   Registered Addresses called a Binding Table.  The combined Binding
   Tables of all the 6BBRs on a backbone form a distributed database of
   6LNs that reside in the LLNs or on the IPv6 Backbone.

   Unless otherwise configured, a 6BBR does the following:

   *  Create a new entry in a Binding Table for a new Registered Address
      and ensure that the Address is not duplicated over the Backbone.

   *  Advertise a Registered Address over the Backbone using an
      unsolicited NA message, asynchronously or as a response to a NS
      message.  This includes joining the multicast group associated to
      the SNMA derived from the Registered Address as specified in
      section 7.2.1. of [RFC4861] over the Backbone.

   *  The 6BBR may respond immediately as a Proxy in lieu of the
      Registering Node, e.g., if the Registering Node has a sleeping
      cycle that the 6BBR does not want to interrupt, or if the 6BBR has
      a recent state that is deemed fresh enough to permit the proxied
      response.  It is preferred, though, that the 6BBR checks whether
      the Registering Node is still responsive on the Registered
      Address.  To that effect:

      - as a Bridging Proxy:
         the 6BBR forwards the multicast DAD and Address Lookup messages
         as a unicast MAC-Layer frames to the MAC address of the
         Registering Node that matches the Target in the ND message, and
         forwards as is the unicast Neighbor Unreachability Detection
         (NUD) messages, so as to let the Registering Node answer with
         the ND Message and options that it sees fit;
      - as a Routing Proxy:
         the 6BBR checks the liveliness of the Registering Node, e.g.,
         using a NUD verification, before answering on its behalf.

   *  Deliver packets arriving from the LLN, using Neighbor Solicitation
      messages to look up the destination over the Backbone.




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   *  Forward or bridge packets between the LLN and the Backbone.

   *  Verify liveness for a registration, when needed.

   The first of these functions enables the 6BBR to fulfill its role as
   a Routing Registrar for each of its attached LLNs.  The remaining
   functions fulfill the role of the 6BBRs as the border routers
   connecting the Multi-link IPv6 subnet to the Internet.

   The operation of IPv6 ND and of proxy-ND are not mutually exclusive
   on the Backbone, meaning that nodes attached to the Backbone and
   using IPv6 ND can transparently interact with 6LNs that rely on a
   6BBR to proxy ND for them, whether the 6LNs are reachable over an LLN
   or directly attached to the Backbone.

   The [RFC8505] registration mechanism used to learn addresses to be
   proxied for may co-exist in a 6BBR with a proprietary snooping or the
   traditional bridging functionality of an Access Point, in order to
   support legacy LLN nodes that do not support this specification.

   The registration to a proxy service uses an NS/NA(EARO) exchange.
   The 6BBR operation resembles that of a Mobile IPv6 (MIPv6) [RFC6275]
   Home Agent (HA).  The combination of a 6BBR and a MIPv6 HA enables
   full mobility support for 6LNs, inside and outside the links that
   form the subnet.

   The 6BBRs use the Extended Address Registration Option (EARO) defined
   in [RFC8505] as follows:

   *  The EARO is used in the IPv6 ND exchanges over the Backbone
      between the 6BBRs to help distinguish duplication from movement.
      Extended Duplicate Address Messages (EDAR and EDAC) may also be
      used between a 6LBR, if one is present, and the 6BBR.  Address
      duplication is detected using the ROVR field.  Conflicting
      registrations to different 6BBRs for the same Registered Address
      are resolved using the TID field.

   *  The Link Layer Address (LLA) that the 6BBR advertises for the
      Registered Address on behalf of the Registered Node over the
      Backbone can belong to the Registering Node; in that case, the
      6BBR (acting as a Bridging Proxy (see Section 8)) bridges the
      unicast packets.  Alternatively, the LLA can be that of the 6BBR
      on the Backbone interface, in which case the 6BBR (acting as a
      Routing Proxy(see Section 7)) receives the unicast packets at
      Layer 3 and routes over.






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3.1.  Updating RFC 6775 and RFC 8505

   This specification adds the EARO as a possible option in RS, NS(DAD)
   and NA messages over the backbone.  [RFC8505] requires that the
   registration NS(EARO) contains an Source Link Layer Address Option
   (SLLAO).  This specification details the use of those messages over
   the backbone.

   Note: [RFC6775] requires that the registration NS(EARO) contains an
   SLLAO and [RFC4862] that the NS(DAD) is sent from the unspecified
   address for which there cannot be a SLLAO.  Consequently, an NS(DAD)
   cannot be confused with a registration.

   This specification adds the capability to insert IPv6 ND options in
   the EDAR and EDAC messages.  In particular, a 6BBR acting as a 6LR
   for the Registered Address can insert an SLLAO in the EDAR to the
   6LBR in order to avoid a Lookup back.  This enables the 6LBR to store
   the MAC address associated to the Registered Address on a Link and to
   serve as a mapping server as described in
   [I-D.thubert-6lo-unicast-lookup].

3.2.  Access Link

   The simplest Multi-Link Subnet topology from the Layer 3 perspective
   occurs when the wireless network appears as a single hop hub-and-
   spoke network as shown in Figure 2.  The Layer 2 operation may
   effectively be hub-and-spoke (e.g., Wi-Fi) or Mesh-Under, with a
   Layer 2 protocol handling the complex topology.

                    |
                 +-----+               +-----+       +-----+ IPv6
       (default) |     |    (Optional) |     |       |     | Node
          Router |     |          6LBR |     |       |     | or
                 +-----+               +-----+       +-----+ 6LN
                    |  Backbone side      |             |
        ----+-------+-----------------+---+-------------+----+-----
            |                         |                      |
         +------+                 +------+                +------+
         | 6BBR |                 | 6BBR |                | 6BBR |
         | 6LR  |                 | 6LR  |                | 6LR  |
         +------+                 +------+                +------+
      (6LN) (6LN) (6LN)       (6LN) (6LN) (6LN)          (6LN) (6LN)



                       Figure 2: Access Link Use case





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   Figure 3 illustrates a flow where 6LN forms an IPv6 Address and
   registers it to a 6BBR acting as a 6LR [RFC8505].  The 6BBR applies
   ODAD (see Section 3.6) to the registered address to enable
   connectivity while the message flow is still in progress.

          6LN(STA)         6BBR(AP)          6LBR          default GW
            |                 |                |                   |
            | LLN Access Link |  IPv6 Backbone  (e.g., Ethernet)   |
            |                 |                |                   |
            |  RS(multicast)  |                |                   |
            |---------------->|                |                   |
            | RA(PIO, Unicast)|                |                   |
            |<----------------|                |                   |
            |   NS(EARO)      |                |                   |
            |---------------->|                |                   |
            |                 |  Extended DAR  |                   |
            |                 |--------------->|                   |
            |                 |  Extended DAC  |                   |
            |                 |<---------------|                   |
            |                 |                                    |
            |                 |     NS-DAD(EARO, multicast)        |
            |                 |-------->                           |
            |                 |----------------------------------->|
            |                 |                                    |
            |                 |      RS(no SLLAO, for ODAD)        |
            |                 |----------------------------------->|
            |                 | if (no fresher Binding) NS(Lookup) |
            |                 |                   <----------------|
            |                 |<-----------------------------------|
            |                 |      NA(SLLAO, not(O), EARO)       |
            |                 |----------------------------------->|
            |                 |           RA(unicast)              |
            |                 |<-----------------------------------|
            |                 |                                    |
            |           IPv6 Packets in optimistic mode            |
            |<---------------------------------------------------->|
            |                 |                                    |
            |                 |
            |  NA(EARO)       |<DAD timeout>
            |<----------------|
            |                 |

   Figure 3: Initial Registration Flow to a 6BBR acting as Routing Proxy

   In this example, a 6LBR is deployed on the backbone link to serve the
   whole subnet, and EDAR / EDAC messages are used in combination with
   DAD to enable coexistence with IPv6 ND over the backbone.




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   The RS sent initially by the 6LN(STA) is transmitted as a multicast
   but since it is intercepted by the 6BBR, it is never effectively
   broadcast.  The multiple arrows associated to the ND messages on the
   Backbone denote a real Layer 2 broadcast.

3.3.  Route-Over Mesh

   A more complex Multi-Link Subnet topology occurs when the wireless
   network appears as a Layer 3 Mesh network as shown in Figure 4.  A
   so-called Route-Over routing protocol exposes routes between 6LRs
   towards both 6LRs and 6LNs, and a 6LBR acts as Root of the Layer 3
   Mesh network and proxy-registers the LLN addresses to the 6BBR.

                    |
                 +-----+               +-----+       +-----+ IPv6
       (default) |     |    (Optional) |     |       |     | Node
          Router |     |          6LBR |     |       |     | or
                 +-----+               +-----+       +-----+ 6LN
                    |  Backbone side      |             |
        ----+-------+-----------------+---+-------------+----+-----
            |                         |                      |
         +------+                 +------+                +------+
         | 6BBR |                 | 6BBR |                | 6BBR |
         +------+                 +------+                +------+
             |                        |                       |
         +------+                 +------+                +------+
         | 6LBR |                 | 6LBR |                | 6LBR |
         +------+                 +------+                +------+
        (6LN) (6LR) (6LN)       (6LR) (6LN) (6LR)      (6LR) (6LR)(6LN)
     (6LN)(6LR) (6LR) (6LN)   (6LN) (6LR)(6LN) (6LR)  (6LR)  (6LR) (6LN)
       (6LR)(6LR) (6LR)         (6LR)  (6LR)(6LN)    (6LR) (6LR)(6LR)
     (6LR)  (6LR)    (6LR)   (6LR) (6LN)(6LR) (6LR)    (6LR) (6LR) (6LR)
     (6LN) (6LN)(6LN) (6LN) (6LN)       (6LN) (6LN)  (6LN)  (6LN) (6LN)

                     Figure 4: Route-Over Mesh Use case

   Figure 5 illustrates IPv6 signaling that enables a 6LN (the
   Registered Node) to form a Global or a Unique-Local Address and
   register it to the 6LBR that serves its LLN using [RFC8505].  The
   6LBR (the Registering Node) then proxies the [RFC8505] registration
   to the 6BBR to obtain proxy-ND services from the 6BBR.

   As above, the RS sent initially by the 6LN(STA) is a transmitted as a
   multicast but since it is intercepted by the 6BBR, it is never
   effectively broadcast, and the multiple arrows associated to the ND
   messages on the Backbone denote a real Layer 2 broadcast.





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       6LoWPAN Node        6LR             6LBR            6BBR
       (mesh leaf)     (mesh router)   (mesh root)
            |               |               |               |
            |  6LoWPAN ND   |6LoWPAN ND     | 6LoWPAN ND    | IPv6 ND
            |   LLN link    |Route-Over mesh|Ethernet/serial| Backbone
            |               |               |/Internal call |
            |  IPv6 ND RS   |               |               |
            |-------------->|               |               |
            |----------->   |               |               |
            |------------------>            |               |
            |  IPv6 ND RA   |               |               |
            |<--------------|               |               |
            |               |               |               |
            |  NS(EARO)     |               |               |
            |-------------->|               |               |
            | 6LoWPAN ND    | Extended DAR  |               |
            |               |-------------->|               |
            |               |               |  NS(EARO)     |
            |               |               |-------------->|
            |               |               |  (proxied)    | NS-DAD
            |               |               |               |------>
            |               |               |               | (EARO)
            |               |               |               |
            |               |               |  NA(EARO)     |<timeout>
            |               |               |<--------------|
            |               | Extended DAC  |               |
            |               |<--------------|               |
            |  NA(EARO)     |               |               |
            |<--------------|               |               |
            |               |               |               |

          Figure 5: Initial Registration Flow over Route-Over Mesh

   As a non-normative example of a Route-Over Mesh, the 6TiSCH
   architecture [I-D.ietf-6tisch-architecture] suggests using the RPL
   [RFC6550] routing protocol and collocating the RPL root with a 6LBR
   that serves the LLN.  The 6LBR is also either collocated with or
   directly connected to the 6BBR over an IPv6 Link.

3.4.  The Binding Table

   Addresses in an LLN that are reachable from the Backbone by way of
   the 6BBR function must be registered to that 6BBR, using an NS(EARO)
   with the R flag set [RFC8505].  A 6BBR maintains a state for its
   active registrations in an abstract Binding Table.

   An entry in the Binding Table is called a "Binding".  A Binding may
   be in Tentative, Reachable or Stale state.



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   The 6BBR uses a combination of [RFC8505] and IPv6 ND over the
   Backbone to advertise the registration and avoid a duplication.
   Conflicting registrations are solved by the 6BBRs, transparently to
   the Registering Nodes.

   Only one 6LN may register a given Address, but the Address may be
   registered to Multiple 6BBRs for higher availability.

   Over the LLN, Binding Table management is as follows:

   *  De-registrations (newer TID, same ROVR, null Lifetime) are
      accepted with a status of 4 ("Removed"); the entry is deleted;

   *  Newer registrations (newer TID, same ROVR, non-null Lifetime) are
      accepted with a status of 0 (Success); the Binding is updated with
      the new TID, the Registration Lifetime and the Registering Node;
      in Tentative state the EDAC response is held and may be
      overwritten; in other states the Registration Lifetime timer is
      restarted and the entry is placed in Reachable state.

   *  Identical registrations (same TID, same ROVR) from the same
      Registering Node are accepted with a status of 0 (Success).  In
      Tentative state, the response is held and may be overwritten, but
      the response is eventually produced, carrying the result of the
      DAD process;

   *  Older registrations (older TID, same ROVR) from the same
      Registering Node are discarded;

   *  Identical and older registrations (not-newer TID, same ROVR) from
      a different Registering Node are rejected with a status of 3
      (Moved); this may be rate limited to avoid undue interference;

   *  Any registration for the same address but with a different ROVR is
      rejected with a status of 1 (Duplicate).

   The operation of the Binding Table is specified in detail in
   Section 9.

3.5.  Primary and Secondary 6BBRs

   The same address may be successfully registered to more than one
   6BBR, in which case the Registering Node uses the same EARO in all
   the parallel registrations.  To allow for this, ND(DAD) and NA
   messages with an EARO that indicate an identical Binding in another
   6BBR (same Registered address, same TID, same ROVR) are silently
   ignored.




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   A 6BBR may optionally be primary or secondary.  The primary is the
   6BBR that has the highest EUI-64 Address of all the 6BBRs that share
   a registration for the same Registered Address, with the same ROVR
   and same Transaction ID, the EUI-64 Address being considered as an
   unsigned 64bit integer.  A given 6BBR can be primary for a given
   Address and secondary for another Address, regardless of whether or
   not the Addresses belong to the same 6LN.

   In the following sections, is is expected that an NA is sent over the
   backbone only if the node is primary or does not support the concept
   of primary.  More than one 6BBR claiming or defending an address
   generates unwanted traffic but no reachability issue since all 6BBRs
   provide reachability from the Backbone to the 6LN.

3.6.  Using Optimistic DAD

   Optimistic Duplicate Address Detection [RFC4429] (ODAD) specifies how
   an IPv6 Address can be used before completion of Duplicate Address
   Detection (DAD).  ODAD guarantees that this behavior will not cause
   harm if the new Address is a duplicate.

   Support for ODAD avoids delays in installing the Neighbor Cache Entry
   (NCE) in the 6BBRs and the default router, enabling immediate
   connectivity to the registered node.  As shown in Figure 3, if the
   6BBR is aware of the Link-Layer Address (LLA) of a router, then the
   6BBR sends a Router Solicitation (RS), using the Registered Address
   as the IP Source Address, to the known router(s).  The RS is sent
   without a Source LLA Option (SLLAO), to avoid invalidating a
   preexisting NCE in the router.

   Following ODAD, the router may then send a unicast RA to the
   Registered Address, and it may resolve that Address using an
   NS(Lookup) message.  In response, the 6BBR sends an NA with an EARO
   and the Override flag [RFC4861] that is not set.  The router can then
   determine the freshest EARO in case of conflicting NA(EARO) messages,
   using the method described in section 5.2.1 of [RFC8505].  If the
   NA(EARO) is the freshest answer, the default router creates a Binding
   with the SLLAO of the 6BBR (in Routing Proxy mode) or that of the
   Registering Node (in Bridging Proxy mode) so that traffic from/to the
   Registered Address can flow immediately.

4.  Multi-Link Subnet Considerations

   The Backbone and the federated LLN Links are considered as different
   links in the Multi-Link Subnet, even if multiple LLNs are attached to
   the same 6BBR.  ND messages are link-scoped and are not forwarded by
   the 6BBR between the backbone and the LLNs though some packets may be
   reinjected in Bridging Proxy mode (see Section 8).



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   Nodes located inside the subnet do not perform the IPv6 Path MTU
   Discovery [RFC8201].  For that reason, the MTU MUST have the same
   value on the Backbone and all attached LLNs.  As a consequence, the
   6BBR MUST use the same MTU value in RAs over the Backbone and in the
   RAs that it transmits towards the LLN links.

5.  Optional 6LBR serving the Multi-Link Subnet

   A 6LBR can be deployed to serve the whole MLSN.  It may be attached
   to the backbone, in which case it can be discovered by its capability
   advertisement (see section 4.3. of [RFC8505]) in RA messages.

   This specification allows for an address to be registered to more
   than one 6BBR.  Consequently a 6LBR MUST be capable of maintaining
   state for each of the 6BBR having registered with the same TID and
   same ROVR.

   When a 6LBR is present, the 6BBR uses an EDAR/EDAC message exchange
   with the 6LBR to check if the new registration corresponds to a
   duplication or a movement.  This is done prior to the NS(DAD)
   process, which may be avoided if the 6LBR already maintains a
   conflicting state for the Registered Address.

   If this registration is duplicate or not the freshest, then the 6LBR
   replies with an EDAC message with a status code of 1 ("Duplicate
   Address") or 3 ("Moved"), respectively.  If this registration is the
   freshest, then the 6LBR replies with a status code of 0.  In that
   case, if this registration is fresher than an existing registration
   for another 6BBR, then the 6LBR also sends an asynchronous EDAC with
   a status of 4 ("Removed") to that other 6BBR.

   The EDAR message SHOULD carry the SLLAO used in NS messages by the
   6BBR for that Binding, and the EDAC message SHOULD carry the Target
   Link Layer Address Option (TLLAO) associated with the currently
   accepted registration.  This enables a 6BBR to locate the new
   position of a mobile 6LN in the case of a Routing Proxy operation,
   and opens the capability for the 6LBR to serve as a mapping server in
   the future.

   Note that if Link Local addresses are registered, then the scope of
   uniqueness on which the address duplication is checked is the total
   collection of links that the 6LBR serves as opposed to the sole link
   on which the Link Local address is assigned.








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6.  Using IPv6 ND Over the Backbone Link

   On the Backbone side, the 6BBR MUST join the SNMA group corresponding
   to a Registered Address as soon as it creates a Binding for that
   Address, and maintain that SNMA membership as long as it maintains
   the registration.  The 6BBR uses either the SNMA or plain unicast to
   defend the Registered Addresses in its Binding Table over the
   Backbone (as specified in [RFC4862]).  The 6BBR advertises and
   defends the Registered Addresses over the Backbone Link using RS,
   NS(DAD) and NA messages with the Registered Address as the Source or
   Target address, respectively.

   The 6BBR MUST place an EARO in the IPv6 ND messages that it generates
   on behalf of the Registered Node.  Note that an NS(DAD) does not
   contain an SLLAO and cannot be confused with a proxy registration
   such as performed by a 6LBR.

   IPv6 ND operates as follows on the backbone:

   *  Section 7.2.8 of [RFC4861] specifies that an NA message generated
      as a proxy does not have the Override flag set in order to ensure
      that if the real owner is present on the link, its own NA will
      take precedence, and that this NA does not update the NCE for the
      real owner if one exists.

   *  A node that receives multiple NA messages updates an existing NCE
      only if the Override flag is set; otherwise the node will probe
      the cached address.

   *  When an NS(DAD) is received for a tentative address, which means
      that 2 nodes form the same address at nearly the same time,
      section 5.4.3 of [RFC4862] cannot sort out the first come and the
      address is abandoned.

   *  In any fashion, [RFC4862] indicates that a node never responds to
      a Neighbor Solicitation for a tentative address.

   This specification adds information about proxied addresses that
   helps sort out a duplication (different ROVR) from a movement (same
   ROVR, different TID), and in the latter case the older registration
   from the fresher one (by comparing TIDs).

   When a Registering Node moves from one 6BBR to the next, the new 6BBR
   sends NA messages to update the NCE in node over the backbone.  The
   6BBR may set the Override flag in the NA messages if it is known that
   the Registering Node will not connect directly to the backbone (e.g.,
   the Registering Node is attached using a different type of
   interface).



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   A node that supports this specification and that receives multiple NA
   messages with an EARO option and the same ROVR MUST favor the NA with
   the freshest EARO over the others.

   When the Binding is in Tentative state:

   *  an NS(DAD) that indicates a duplication can still not be asserted
      for first come, but the situation can be avoided using a 6LBR on
      the backbone that will serialize the order of appearance of the
      address and ensure first-come/first-serve.

   *  an NS or an NA that denotes an older registration for the same
      Registered Node is not interpreted as a duplication as specified
      in section 5.4.3 and 5.4.4 of [RFC4862], respectively.

   When the Binding is no more in Tentative state:

   *  an NS or an NA with an EARO that denotes a duplicate registration
      (different ROVR) is answered with an NA message that carries an
      EARO with a status of 1 (Duplicate), unless the received message
      is an NA that carries an EARO with a status of 1.

   In any state:

   *  an NS or an NA with an EARO that denotes an older registration
      (same ROVR) is answered with an NA message that carries an EARO
      with a status of 3 (Moved) to ensure that the stale state is
      removed rapidly.

   This behavior is specified in more details in Section 9.

   This specification enables proxy operation for the IPv6 ND resolution
   of LLN devices and a prefix that is used across a Multi-Link Subnet
   MAY be advertised as on-link over the Backbone.  This is done for
   backward compatibility with existing IPv6 hosts by setting the L flag
   in the Prefix Information Option (PIO) of RA messages [RFC4861].

   For movement involving a slow reattachment, the NUD procedure defined
   in [RFC4861] may time out too quickly.  Nodes on the backbone SHOULD
   support [RFC7048] whenever possible.

7.  Routing Proxy Operations

   A Routing Proxy provides IPv6 ND proxy functions for Global and
   Unique Local addresses between the LLN and the backbone, but not for
   Link-Local addresses.  It operates as an IPv6 border router and
   provides a full Link-Layer isolation.




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   In this mode, it is not required that the MAC addresses of the 6LNs
   are visible at Layer 2 over the Backbone.  It is thus useful when the
   messaging over the Backbone that is associated to wireless mobility
   becomes expensive, e.g., when the Layer 2 topology is virtualized
   over a wide area IP underlay.

   This mode is definitely required when the LLN uses a MAC address
   format that is different from that on the Backbone (e.g., EUI-64 vs.
   EUI-48).  Since a 6LN may not be able to resolve an arbitrary
   destination in the MLSN directly, the MLSN prefix MUST NOT be
   advertised as on-link in RA messages sent towards the LLN.

   In order to maintain IP connectivity, the 6BBR installs a connected
   Host route to the Registered Address on the LLN interface, via the
   Registering Node as identified by the Source Address and the SLLA
   option in the NS(EARO) messages.

   When operating as a Routing Proxy, the 6BBR MUST use its Layer 2
   Address on its Backbone Interface in the SLLAO of the RS messages and
   the TLLAO of the NA messages that it generates to advertise the
   Registered Addresses.

   For each Registered Address, multiple peers on the Backbone may have
   resolved the Address with the 6BBR MAC Address, maintaining that
   mapping in their Neighbor Cache.  The 6BBR SHOULD maintain a list of
   the peers on the Backbone which have associated its MAC Address with
   the Registered Address.  If that Registered Address moves to another
   6BBR, the previous 6BBR SHOULD unicast a gratuitous NA to each such
   peer, to supply the LLA of the new 6BBR in the TLLA option for the
   Address.  A 6BBR that does not maintain this list MAY multicast a
   gratuitous NA message; this NA will possibly hit all the nodes on the
   Backbone, whether or not they maintain an NCE for the Registered
   Address.  In either case, the 6BBR MAY set the Override flag if it is
   known that the Registered Node cannot attach to the backbone, so as
   to avoid interruptions and save probing flows in the future.

   If a correspondent fails to receive the gratuitous NA, it will keep
   sending traffic to a 6BBR to which the node was previously
   registered.  Since the previous 6BBR removed its Host route to the
   Registered Address, it will look up the address over the backbone,
   resolve the address with the LLA of the new 6BBR, and forward the
   packet to the correct 6BBR.  The previous 6BBR SHOULD also issue a
   redirect message [RFC4861] to update the cache of the correspondent.








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8.  Bridging Proxy Operations

   A Bridging Proxy provides IPv6 ND proxy functions between the LLN and
   the backbone while preserving the forwarding continuity at the MAC
   Layer.  It acts as a Layer 2 Bridge for all types of unicast packets
   including link-scoped, and appears as an IPv6 Host on the Backbone.

   The Bridging Proxy registers any Binding including for a Link-Local
   address to the 6LBR (if present) and defends it over the backbone in
   IPv6 ND procedures.

   To achieve this, the Bridging Proxy intercepts the IPv6 ND messages
   and may reinject them on the other side, respond directly or drop
   them.  For instance, an ND(Lookup) from the backbone that matches a
   Binding can be responded directly, or turned into a unicast on the
   LLN side to let the 6LN respond.

   As a Bridging Proxy, the 6BBR MUST use the Registering Node's Layer 2
   Address in the SLLAO of the NS/RS messages and the TLLAO of the NA
   messages that it generates to advertise the Registered Addresses.
   The Registering Node's Layer 2 address is found in the SLLA of the
   registration NS(EARO), and maintained in the Binding Table.

   The Multi-Link Subnet prefix SHOULD NOT be advertised as on-link in
   RA messages sent towards the LLN.  If a destination address is seen
   as on-link, then a 6LN may use NS(Lookup) messages to resolve that
   address.  In that case, the 6BBR MUST either answer the NS(Lookup)
   message directly or reinject the message on the backbone, either as a
   Layer 2 unicast or a multicast.

   If the Registering Node owns the Registered Address, meaning that the
   Registering Node is the Registered Node, then its mobility does not
   impact existing NCEs over the Backbone.  In a network where proxy
   registrations are used, meaning that the Registering Node acts on
   behalf of the Registered Node, if the Registered Node selects a new
   Registering Node then the existing NCEs across the Backbone pointing
   at the old Registering Node must be updated.  In that case, the 6BBR
   SHOULD attempt to fix the existing NCEs across the Backbone pointing
   at other 6BBRs using NA messages as described in Section 7.

   This method can fail if the multicast message is not received; one or
   more correspondent nodes on the Backbone might maintain an stale NCE,
   and packets to the Registered Address may be lost.  When this
   condition happens, it is eventually discovered and resolved using NUD
   as defined in [RFC4861].






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9.  Creating and Maintaining a Binding

   Upon receiving a registration for a new Address (i.e., an NS(EARO)
   with the R flag set), the 6BBR creates a Binding and operates as a
   6LR according to [RFC8505], interacting with the 6LBR if one is
   present.

   An implementation of a Routing Proxy that creates a Binding MUST also
   create an associated Host route pointing to the registering node in
   the LLN interface from which the registration was received.

   Acting as a 6BBR, the 6LR operation is modified as follows:

   *  Acting as Bridging Proxy the 6LR MUST proxy ND over the backbone
      for registered Link-Local addresses.

   *  EDAR and EDAC messages SHOULD carry a SLLAO and a TLLAO,
      respectively.

   *  An EDAC message with a status of 9 (6LBR Registry Saturated) is
      assimilated as a status of 0 if a following DAD process protects
      the address against duplication.

   This specification enables nodes on a Backbone Link to co-exist along
   with nodes implementing IPv6 ND [RFC4861] as well as other non-
   normative specifications such as [I-D.bi-savi-wlan].  It is possible
   that not all IPv6 addresses on the Backbone are registered and known
   to the 6LBR, and an EDAR/EDAC echange with the 6LBR might succeed
   even for a duplicate address.  Consequently, and unless
   administratively overridden, the 6BBR still needs to perform IPv6 ND
   DAD over the backbone after an EDAC with a status code of 0 or 9.

   For the DAD operation, the Binding is placed in Tentative state for a
   duration of TENTATIVE_DURATION (Section 12), and an NS(DAD) message
   is sent as a multicast message over the Backbone to the SNMA
   associated with the registered Address [RFC4862].  The EARO from the
   registration MUST be placed unchanged in the NS(DAD) message.

   If a registration is received for an existing Binding with a non-null
   Registration Lifetime and the registration is fresher (same ROVR,
   fresher TID), then the Binding is updated, with the new Registration
   Lifetime, TID, and possibly Registering Node.  In Tentative state
   (see Section 9.1), the current DAD operation continues unaltered.  In
   other states (see Section 9.2 and Section 9.3 ), the Binding is
   placed in Reachable state for the Registration Lifetime, and the 6BBR
   returns an NA(EARO) to the Registering Node with a status of 0
   (Success).




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   Upon a registration that is identical (same ROVR, TID, and
   Registering Node), the 6BBR returns an NA(EARO) back to the
   Registering Node with a status of 0 (Success).  A registration that
   is not as fresh (same ROVR, older TID) is ignored.

   If a registration is received for an existing Binding and a
   registration Lifetime of zero, then the Binding is removed, and the
   6BBR returns an NA(EARO) back to the Registering Node with a status
   of 0 (Success).  An implementation of a Routing Proxy that removes a
   binding MUST remove the associated Host route pointing on the
   registering node.  It MAY preserve a temporary state in order to
   forward packets in flight.  The state may be a NCE formed based on a
   received NA message, or a Binding in Stale state and pointing at the
   new 6BBR on the backbone.

   The old 6BBR SHOULD also use REDIRECT messages as specified in
   [RFC4861] to update the correspondents for the Registered Address,
   pointing to the new 6BBR.

9.1.  Operations on a Binding in Tentative State

   The Tentative state covers a DAD period over the backbone during
   which an address being registered is checked for duplication using
   procedures defined in [RFC4862].

   For a Binding in Tentative state:

   *  The Binding MUST be removed if an NA message is received over the
      Backbone for the Registered Address with no EARO, or containing an
      EARO that indicates an existing registration owned by a different
      Registering Node (different ROVR).  An NA MUST be sent back to the
      Registering Node with a status of 1 (Duplicate).  This behavior
      might be overridden by policy, in particular if the registration
      is trusted, e.g., based on the validation of the ROVR field (see
      [I-D.ietf-6lo-ap-nd]).

   *  The Binding MUST be removed if an NS(DAD) message is received over
      the Backbone for the Registered Address with no EARO, or
      containing an EARO with a different ROVR that indicates a
      tentative registration by a different Registering Node.  In that
      case, an NA MUST be sent back to the Registering Node with a
      status of 1 (Duplicate).  This behavior might be overridden by
      policy, in particular if the registration is trusted, e.g., based
      on the validation of the ROVR field (see [I-D.ietf-6lo-ap-nd]).

   *  The Binding MUST be removed if an NA or an NS(DAD) message is
      received over the Backbone for the Registered Address containing
      an EARO with a that indicates a fresher registration ([RFC8505])



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      for the same Registering Node (same ROVR).  A status of 3 is
      returned in the NA(EARO) back to the Registering Node.

   *  The Binding MUST be kept unchanged if an NA or an NS(DAD) message
      is received over the Backbone for the Registered Address
      containing an EARO with a that indicates an older registration
      ([RFC8505]) for the same Registering Node (same ROVR).  The
      message SHOULD be answered with an NA that carries an EARO with a
      status of 3 (Moved) and the Override flag not set.  This behavior
      might be overridden by policy, in particular if the registration
      is not trusted.

   *  Other NS(DAD) and NA messages from the Backbone are ignored.

   *  NS(Lookup) and NS(NUD) messages SHOULD be optimistically answered
      with an NA message containing an EARO with a status of 0 and the
      Override flag not set (see Section 3.6).  If optimistic DAD is
      disabled, then they SHOULD be queued to be answered when the
      Binding goes to Reachable state.

   When the TENTATIVE_DURATION (Section 12) timer elapses, the Binding
   is placed in Reachable state for the Registration Lifetime, and the
   6BBR returns an NA(EARO) to the Registering Node with a status of 0
   (Success).

   The 6BBR also attempts to take over any existing Binding from other
   6BBRs and to update existing NCEs in backbone nodes.  This is done by
   sending an NA message with an EARO and the Override flag not set over
   the backbone (see Section 7 and Section 8).

9.2.  Operations on a Binding in Reachable State

   The Reachable state covers an active registration after a successful
   DAD process.

   If the Registration Lifetime is of a long duration, an implementation
   might be configured to reassess the availability of the Registering
   Node at a lower period, using a NUD procedure as specified in
   [RFC7048].  If the NUD procedure fails, the Binding SHOULD be placed
   in Stale state immediately.

   For a Binding in Reachable state:

   *  The Binding MUST be removed if an NA or an NS(DAD) message is
      received over the Backbone for the Registered Address containing
      an EARO that indicates a fresher registration ([RFC8505]) for the
      same Registered Node (i.e., same ROVR but fresher TID).  A status
      of 4 (Removed) is returned in an asynchronous NA(EARO) to the



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      Registering Node.  Based on configuration, an implementation may
      delay this operation by a timer with a short setting, e.g., a few
      seconds to a minute, in order to a allow for a parallel
      registration to reach this node, in which case the NA might be
      ignored.

   *  NS(DAD) and NA messages containing an EARO that indicates a
      registration for the same Registered Node that is not as fresh as
      this binding MUST be answered with an NA message containing an
      EARO with a status of 3 (Moved).

   *  An NS(DAD) with no EARO or with an EARO that indicates a duplicate
      registration (i.e., different ROVR) MUST be answered with an NA
      message containing an EARO with a status of 1 (Duplicate) and the
      Override flag not set, unless the received message is an NA that
      carries an EARO with a status of 1, in which case the node
      refrains from answering.

   *  Other NS(DAD) and NA messages from the Backbone are ignored.

   *  NS(Lookup) and NS(NUD) messages SHOULD be answered with an NA
      message containing an EARO with a status of 0 and the Override
      flag not set.  The 6BBR MAY check whether the Registering Node is
      still available using a NUD procedure over the LLN prior to
      answering; this behaviour depends on the use case and is subject
      to configuration.

   When the Registration Lifetime timer elapses, the Binding is placed
   in Stale state for a duration of STALE_DURATION (Section 12).

9.3.  Operations on a Binding in Stale State

   The Stale state enables tracking of the Backbone peers that have a
   NCE pointing to this 6BBR in case the Registered Address shows up
   later.

   If the Registered Address is claimed by another 6LN on the Backbone,
   with an NS(DAD) or an NA, the 6BBR does not defend the Address.

   For a Binding in Stale state:

   *  The Binding MUST be removed if an NA or an NS(DAD) message is
      received over the Backbone for the Registered Address containing
      no EARO or an EARO that indicates either a fresher registration
      for the same Registered Node or a duplicate registration.  A
      status of 4 (Removed) MAY be returned in an asynchronous NA(EARO)
      to the Registering Node.




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   *  NS(DAD) and NA messages containing an EARO that indicates a
      registration for the same Registered Node that is not as fresh as
      this MUST be answered with an NA message containing an EARO with a
      status of 3 (Moved).

   *  If the 6BBR receives an NS(Lookup) or an NS(NUD) message for the
      Registered Address, the 6BBR MUST attempt a NUD procedure as
      specified in [RFC7048] to the Registering Node, targeting the
      Registered Address, prior to answering.  If the NUD procedure
      succeeds, the operation in Reachable state applies.  If the NUD
      fails, the 6BBR refrains from answering.

   *  Other NS(DAD) and NA messages from the Backbone are ignored.

   When the STALE_DURATION (Section 12) timer elapses, the Binding MUST
   be removed.

10.  Registering Node Considerations

   A Registering Node MUST implement [RFC8505] in order to interact with
   a 6BBR (which acts as a routing registrar).  Following [RFC8505], the
   Registering Node signals that it requires IPv6 proxy-ND services from
   a 6BBR by registering the corresponding IPv6 Address using an
   NS(EARO) message with the R flag set.

   The Registering Node may be the 6LN owning the IPv6 Address, or a
   6LBR that performs the registration on its behalf in a Route-Over
   mesh.

   The Registering Node MUST register all of its IPv6 Addresses to its
   6LR, which is the 6BBR when they are connected at Layer 2.  Failure
   to register an address may result in the address being unreachable by
   other parties if the 6BBR cancels the NS(Lookup) over the LLN or to
   selected LLN nodes that are known to register their addresses.

   The Registering Node MUST refrain from using multicast NS(Lookup)
   when the destination is not known as on-link, e.g., if the prefix is
   advertised in a PIO with the L flag that is not set.  In that case,
   the Registering Node sends its packets directly to its 6LR.

   The Registering Node SHOULD also follow [RFC7772] in order to limit
   the use of multicast RAs.  It SHOULD also implement Simple Procedures
   for Detecting Network Attachment in IPv6 [RFC6059] (DNA procedures)
   to detect movements, and support Packet-Loss Resiliency for Router
   Solicitations [RFC7559] in order to improve reliability for the
   unicast RS messages.





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11.  Security Considerations

   This specification applies to LLNs and a backbone in which the
   individual links are protected against rogue access, e.g., by
   authenticating a node that attaches to the network and encrypting at
   the MAC layer the transmissions that may be overheard.  In
   particular, the LLN MAC is required to provide secure unicast to/from
   the Backbone Router and secure Broadcast from the Backbone Router in
   a way that prevents tampering with or replaying the RA messages.

   [I-D.ietf-6lo-ap-nd] guarantees the ownership of a registered address
   based on a proof-of-ownership encoded in the ROVR field and protects
   against address theft and impersonation inside the LLN, because the
   6LR can challenge the Registered Node for a proof-of-ownership.  This
   method does not extend over the backbone since the 6BBR cannot
   provide the proof-of-ownership.  A possible attack over the backbone
   can be done by sending an NS with an EARO and expecting the NA(EARO)
   back to contain the TID and ROVR fields of the existing state.  With
   that information, the attacker can easily increase the TID and take
   over the Binding.

12.  Protocol Constants

   This Specification uses the following constants:

   TENTATIVE_DURATION:  800 milliseconds

   STALE_DURATION:  see below

   In LLNs with long-lived Addresses such as LPWANs, STALE_DURATION
   SHOULD be configured with a relatively long value to cover an
   interval when the address may be reused, and before it is safe to
   expect that the address was definitively released.  A good default
   value can be 24 hours.  In LLNs where addresses are renewed rapidly,
   e.g., for privacy reasons, STALE_DURATION SHOULD be configured with a
   relatively shorter value, by default 5 minutes.

13.  IANA Considerations

   This document has no request to IANA.











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14.  Acknowledgments

   Many thanks to Dorothy Stanley, Thomas Watteyne and Jerome Henry for
   their various contributions.  Also many thanks to Timothy Winters and
   Erik Nordmark for their help, review and support in preparation to
   the IESG cycle, and to Kyle Rose, Elwyn Davies and Dominique Barthel
   for their useful contributions through the IETF last call and IESG
   process.

15.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,
              <https://www.rfc-editor.org/info/rfc6059>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over



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              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7048]  Nordmark, E. and I. Gashinsky, "Neighbor Unreachability
              Detection Is Too Impatient", RFC 7048,
              DOI 10.17487/RFC7048, January 2014,
              <https://www.rfc-editor.org/info/rfc7048>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [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>.

16.  Informative References

   [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>.

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

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",



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              RFC 6606, DOI 10.17487/RFC6606, May 2012,
              <https://www.rfc-editor.org/info/rfc6606>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7559]  Krishnan, S., Anipko, D., and D. Thaler, "Packet-Loss
              Resiliency for Router Solicitations", RFC 7559,
              DOI 10.17487/RFC7559, May 2015,
              <https://www.rfc-editor.org/info/rfc7559>.

   [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>.

   [I-D.yourtchenko-6man-dad-issues]
              Yourtchenko, A. and E. Nordmark, "A survey of issues
              related to IPv6 Duplicate Address Detection", Work in
              Progress, Internet-Draft, draft-yourtchenko-6man-dad-
              issues-01, 3 March 2015, <https://tools.ietf.org/html/
              draft-yourtchenko-6man-dad-issues-01>.

   [I-D.nordmark-6man-dad-approaches]
              Nordmark, E., "Possible approaches to make DAD more robust
              and/or efficient", Work in Progress, Internet-Draft,
              draft-nordmark-6man-dad-approaches-02, 19 October 2015,
              <https://tools.ietf.org/html/draft-nordmark-6man-dad-
              approaches-02>.

   [I-D.ietf-6man-rs-refresh]
              Nordmark, E., Yourtchenko, A., and S. Krishnan, "IPv6
              Neighbor Discovery Optional RS/RA Refresh", Work in
              Progress, Internet-Draft, draft-ietf-6man-rs-refresh-02,
              31 October 2016, <https://tools.ietf.org/html/draft-ietf-
              6man-rs-refresh-02>.





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   [I-D.ietf-6lo-ap-nd]
              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", Work in Progress, Internet-Draft, draft-
              ietf-6lo-ap-nd-19, 6 February 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-ap-nd-19>.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-28>.

   [I-D.ietf-mboned-ieee802-mcast-problems]
              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless
              Media", Work in Progress, Internet-Draft, draft-ietf-
              mboned-ieee802-mcast-problems-11, 11 December 2019,
              <https://tools.ietf.org/html/draft-ietf-mboned-ieee802-
              mcast-problems-11>.

   [I-D.bi-savi-wlan]
              Bi, J., Wu, J., Wang, Y., and T. Lin, "A SAVI Solution for
              WLAN", Work in Progress, Internet-Draft, draft-bi-savi-
              wlan-18, 17 November 2019,
              <https://tools.ietf.org/html/draft-bi-savi-wlan-18>.

   [I-D.thubert-6lo-unicast-lookup]
              Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
              Unicast Lookup", Work in Progress, Internet-Draft, draft-
              thubert-6lo-unicast-lookup-00, 25 January 2019,
              <https://tools.ietf.org/html/draft-thubert-6lo-unicast-
              lookup-00>.

   [IEEEstd8021]
              IEEE standard for Information Technology, "IEEE Standard
              for Information technology -- Telecommunications and
              information exchange between systems Local and
              metropolitan area networks Part 1: Bridging and
              Architecture".

   [IEEEstd80211]
              IEEE standard for Information Technology, "IEEE Standard
              for Information technology -- Telecommunications and
              information exchange between systems Local and
              metropolitan area networks-- Specific requirements Part




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              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications".

   [IEEEstd802151]
              IEEE standard for Information Technology, "IEEE Standard
              for Information Technology - Telecommunications and
              Information Exchange Between Systems - Local and
              Metropolitan Area Networks - Specific Requirements. - Part
              15.1: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Wireless Personal Area
              Networks (WPANs)".

   [IEEEstd802154]
              IEEE standard for Information Technology, "IEEE Standard
              for Local and metropolitan area networks -- Part 15.4:
              Low-Rate Wireless Personal Area Networks (LR-WPANs)".

Appendix A.  Possible Future Extensions

   With the current specification, the 6LBR is not leveraged to avoid
   multicast NS(Lookup) on the Backbone.  This could be done by adding a
   lookup procedure in the EDAR/EDAC exchange.

   By default the specification does not have a trust model, e.g.,
   whereby nodes that associate their address with a proof-of-ownership
   [I-D.ietf-6lo-ap-nd] should be more trusted than nodes that do not.
   Such a trust model and related signaling could be added in the future
   to override the default operation and favor trusted nodes.

   Future documents may extend this specification by allowing the 6BBR
   to redistribute Host routes in routing protocols that would operate
   over the Backbone, or in MIPv6, or FMIP, or the Locator/ID Separation
   Protocol (LISP) [RFC6830] to support mobility on behalf of the 6LNs,
   etc...  LISP may also be used to provide an equivalent to the EDAR/
   EDAC exchange using a Map Server / Map Resolver as a replacement to
   the 6LBR.

Appendix B.  Applicability and Requirements Served

   This document specifies proxy-ND functions that can be used to
   federate an IPv6 Backbone Link and multiple IPv6 LLNs into a single
   Multi-Link Subnet.  The proxy-ND functions enable IPv6 ND services
   for Duplicate Address Detection (DAD) and Address Lookup that do not
   require broadcasts over the LLNs.

   The term LLN is used to cover multiple types of WLANs and WPANs,
   including (Low-Power) Wi-Fi, BLUETOOTH(R) Low Energy, IEEE STD
   802.11ah and IEEE STD.802.15.4 wireless meshes, covering the types of



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   networks listed in Appendix B.3 of [RFC8505] "Requirements Related to
   Various Low-Power Link Types".

   Each LLN in the subnet is attached to an IPv6 Backbone Router (6BBR).
   The Backbone Routers interconnect the LLNs and advertise the
   Addresses of the 6LNs over the Backbone Link using proxy-ND
   operations.

   This specification updates IPv6 ND over the Backbone to distinguish
   Address movement from duplication and eliminate stale state in the
   Backbone routers and Backbone nodes once a 6LN has roamed.  This way,
   mobile nodes may roam rapidly from one 6BBR to the next and
   requirements in Appendix B.1 of [RFC8505] "Requirements Related to
   Mobility" are met.

   A 6LN can register its IPv6 Addresses and thereby obtain proxy-ND
   services over the Backbone, meeting the requirements expressed in
   Appendix B.4 of [RFC8505], "Requirements Related to Proxy
   Operations".

   The impact if the IPv6 ND operation is limited to one of the
   federated LLNs, enabling the number of 6LNs to grow.  The Routing
   Proxy operation avoids the need to expose the MAC addresses of the
   6LNs onto the backbone, keeping the Layer 2 topology simple and
   stable.  This meets the requirements in Appendix B.6 of [RFC8505]
   "Requirements Related to Scalability", as long has the 6BBRs are
   dimensioned for the number of registrations that each needs to
   support.

   In the case of a Wi-Fi access link, a 6BBR may be collocated with the
   Access Point (AP), or with a Fabric Edge (FE) or a CAPWAP [RFC5415]
   Wireless LAN Controller (WLC).  In those cases, the wireless client
   (STA) is the 6LN that makes use of [RFC8505] to register its IPv6
   Address(es) to the 6BBR acting as Routing Registrar.  The 6LBR can be
   centralized and either connected to the Backbone Link or reachable
   over IP.  The 6BBR proxy-ND operations eliminate the need for
   wireless nodes to respond synchronously when a Lookup is performed
   for their IPv6 Addresses.  This provides the function of a Sleep
   Proxy for ND [I-D.nordmark-6man-dad-approaches].

   For the TimeSlotted Channel Hopping (TSCH) mode of [IEEEstd802154],
   the 6TiSCH architecture [I-D.ietf-6tisch-architecture] describes how
   a 6LoWPAN ND host could connect to the Internet via a RPL mesh
   Network, but doing so requires extensions to the 6LOWPAN ND protocol
   to support mobility and reachability in a secure and manageable
   environment.  The extensions detailed in this document also work for
   the 6TiSCH architecture, serving the requirements listed in




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   Appendix B.2 of [RFC8505] "Requirements Related to Routing
   Protocols".

   The registration mechanism may be seen as a more reliable alternate
   to snooping [I-D.bi-savi-wlan].  It can be noted that registration
   and snooping are not mutually exclusive.  Snooping may be used in
   conjunction with the registration for nodes that do not register
   their IPv6 Addresses.  The 6BBR assumes that if a node registers at
   least one IPv6 Address to it, then the node registers all of its
   Addresses to the 6BBR.  With this assumption, the 6BBR can possibly
   cancel all undesirable multicast NS messages that would otherwise
   have been delivered to that node.

   Scalability of the Multi-Link Subnet [RFC4903] requires avoidance of
   multicast/broadcast operations as much as possible even on the
   Backbone [I-D.ietf-mboned-ieee802-mcast-problems].  Although hosts
   can connect to the Backbone using IPv6 ND operations, multicast RAs
   can be saved by using [I-D.ietf-6man-rs-refresh], which also requires
   the support of [RFC7559].

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com


   Charles E. Perkins
   Blue Meadow Networking
   Saratoga,  95070
   United States of America

   Email: charliep@computer.org


   Eric Levy-Abegnoli
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis
   France




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   Phone: +33 497 23 26 20
   Email: elevyabe@cisco.com

















































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