IPv6 Backbone Router
draft-ietf-6lo-backbone-router-13

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Last updated 2019-09-26
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6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6775, 8505 (if approved)                             C. Perkins
Intended status: Standards Track                               Futurewei
Expires: March 29, 2020                                 E. Levy-Abegnoli
                                                           Cisco Systems
                                                      September 26, 2019

                          IPv6 Backbone Router
                   draft-ietf-6lo-backbone-router-13

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 MultiLink 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 March 29, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   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

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  References  . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Updating RFC 6775 and RFC 8505  . . . . . . . . . . . . .   9
     3.2.  Access Link . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Route-Over Mesh . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  The Binding Table . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Primary and Secondary 6BBRs . . . . . . . . . . . . . . .  13
     3.6.  Using Optimistic DAD  . . . . . . . . . . . . . . . . . .  14
   4.  MultiLink Subnet Considerations . . . . . . . . . . . . . . .  14
   5.  Optional 6LBR serving the MultiLink Subnet  . . . . . . . . .  15
   6.  Using IPv6 ND Over the Backbone Link  . . . . . . . . . . . .  15
   7.  Routing Proxy Operations  . . . . . . . . . . . . . . . . . .  16
   8.  Bridging Proxy Operations . . . . . . . . . . . . . . . . . .  17
   9.  Creating and Maintaining a Binding  . . . . . . . . . . . . .  18
     9.1.  Operation on a Binding in Tentative State . . . . . . . .  19
     9.2.  Operation on a Binding in Reachable State . . . . . . . .  20
     9.3.  Operation on a Binding in Stale State . . . . . . . . . .  21
   10. Registering Node Considerations . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. Protocol Constants  . . . . . . . . . . . . . . . . . . . . .  23
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     15.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Possible Future Extensions . . . . . . . . . . . . .  28
   Appendix B.  Applicability and Requirements Served  . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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 node (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.

   Like 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, IPv6
   addresses very rarely conflict because of the entropy of the 64-bit
   Interface IDs, not because address duplications are detected and
   resolved.

   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 broadcasted 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 provide proxy services for IPv6 Neighbor Discovery.  Backbone
   Routers federate multiple LLNs over a Backbone Link to form a
   MultiLink Subnet (MLSN).  Backbone Routers placed along the LLN edge
   of the Backbone handle IPv6 Neighbor Discovery, and forward packets
   on behalf of registered nodes.

   An LLN 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.  Additional benefits are discussed in
   Appendix B.

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.

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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 MultiLink
         Subnet.  The proxy-ND operation of 6BBRs over the Backbone and
         the access links provides the appearance of a subnet for IPv6
         ND.

   Sleeping Proxy

         A 6BBR acts as a Sleeping Proxy if it answers ND Neighbor
         Solicitations over the Backbone on behalf of a Registered Node.

   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 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 6BR 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

   EDAC: Extended Duplicate Address Confirmation

   EDAR: Extended Duplicate Address Request

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:   Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:   Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:   Neighbor Solicitation

   ROVR: Registration Ownership Verifier

   RPL:  IPv6 Routing Protocol for LLNs

   RA:   Router Advertisement

   RS:   Router Solicitation

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   TID:  Transaction ID

2.4.  References

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

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

   o  "Neighbor Discovery Proxies (proxy-ND)" [RFC4389] and "MultiLink
      Subnet Issues" [RFC4903],

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

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

3.  Overview

   Figure 1 illustrates backbone link federating a collection of LLNs as
   a single IPv6 Subnet, with a number of 6BBRs providing proxy-ND
   services to their attached LLNs.

                    |
                 +-----+               +-----+       +-----+
       (default) |     |    (Optional) |     |       |     | IPv6
          Router |     |          6LBR |     |       |     | Node
                 +-----+               +-----+       +-----+
                    |  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

         LLN                        LLN                      LLN

               Figure 1: Backbone Link and Backbone Routers

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   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.

   The main features of a 6BBR are as follows:

   o  Multilink-subnet functions (provided by the 6BBR on the backbone)
      performed on behalf of registered 6LNs, and

   o  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 Nodes 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:

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

   o  Advertise a Registered Address over the Backbone using NA
      messages, asynchronously or as a response to a Neighbor
      Solicitation messages.  This includes participating to the
      solicited-node multicast address associated to the Registered
      Address as specified in section 7.2.1. of [RFC4861] over the
      Backbone.

   o  Either respond using NA messages as a proxy or bridge as a unicast
      frame the IPv6 ND messages (multicast DAD and Address Lookup, and
      unicast NUD) received for the Registered Address over the
      Backbone.  This may include responding on behalf of a sleeping
      node, or checking the liveliness of the Registering Node before
      answering on its behalf.

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

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

   o  Verify liveness for a registration, when needed.

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   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 proxy-ND operation can co-exist with IPv6 ND over the Backbone.

   The 6BBR may co-exist with a proprietary snooping or a traditional
   bridging functionality in an Access Point, in order to support legacy
   nodes that do not support this specification.  In the case, the co-
   existing function may turn multicasts into a series of unicast to the
   legacy nodes.

   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:

   o  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 with 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.

   o  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.

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 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

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   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

   Figure 2 illustrates a flow where 6LN forms an IPv6 Address and
   registers it to a 6BBR acting as a 6LR [RFC8505].  The 6BBRs applies
   ODAD (see Section 3.6) to the registered address to enable
   connectivity while the message flow is still in progress.  In that
   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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          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 2: Initial Registration Flow to a 6BBR acting as Routing Proxy

3.3.  Route-Over Mesh

   Figure 3 illustrates IPv6 signaling that enables a 6LN to form a
   Global or a Unique-Local Address and register it to the 6LBR that
   serves its LLN using [RFC8505].  The 6LBR (acting as Registering
   Node) proxies the registration to the 6BBR, using [RFC8505] to
   register the addresses the 6LN (Registered Node) on its behalf to the
   6BBR, and obtain proxy-ND services from the 6BBR.

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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 3: 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 RPL
   [RFC6550] and collocating the RPL root with a 6LBR that serves the
   LLN, and is either collocated with or connected to the 6BBR over an
   IPv6 Link.

3.4.  The Binding Table

   Addresses in a 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:

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

   o  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.

   o  Identical registrations (same TID, same ROVR) from a 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 MUST be eventually produced, carrying the result of
      the DAD process;

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

   o  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;

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

3.5.  Primary and Secondary 6BBRs

   A 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) as silently ignored.

   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

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   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 2, 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 MUST be
   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 (O) flag [RFC4861] that is not set.  The router can
   then determine the freshest EARO in case of a 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.  MultiLink Subnet Considerations

   The Backbone and the federated LLN Links are considered as different
   links in the MultiLink 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).

   Nodes located inside the subnet do not perform the IPv6 Path MTU
   Discovery [RFC8201].  For that reason, the MTU must have a same value
   on the Backbone and all attached LLNs.  To achieve this, the 6BBR

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   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 MultiLink 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.

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

   This specification enables an address to be registered to more than
   one 6BBR.  It results that a 6LBR MUST be capable to maintain a state
   for each of the 6BBR having registered with a same TID and same ROVR.

   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 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.

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]).

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   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.

   An NA message generated in response to an NS(DAD) MUST have the
   Override flag set and a status of 1 (Duplicate) or 3 (Moved) in the
   EARO.  An NA message generated in response to an NS(Lookup) or an
   NS(NUD) MUST NOT have the Override flag set.

   This specification enables proxy operation for the IPv6 ND resolution
   of LLN devices and a prefix that is used across a MultiLink 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 Neighbor
   Unreachability Detection (NUD) 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 including
   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.

   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.

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   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 a new
   6BBR, the previous 6BBR SHOULD unicast a gratuitous NA with the
   Override flag set 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 with the Override
   flag; this NA will possibly hit all the nodes on the Backbone,
   whether or not they maintain an NCE for the Registered Address.

   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.

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 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.

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   The MultiLink 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 directly to the
   NS(Lookup) message or reinject the message on the backbone, either as
   a Layer-2 unicast or a multicast.

   If the Registering Node owns the Registered Address, then its
   mobility does not impact existing NCEs over the Backbone.  Otherwise,
   when the 6LN selects another Registering Node, the new Registering
   Node SHOULD send a multicast NA with the Override flag set to fix the
   existing NCEs across the Backbone.  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 be discovered and resolved using Neighbor Unreachability
   Detection (NUD) as defined in [RFC4861].

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 on the registering node in
   the LLN interface from which the registration was received.

   The 6LR operation is modified as follows:

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

   o  A Bridging Proxy MAY register Link Local addresses to the 6BBR and
      proxy ND for those addresses over the backbone.

   o  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

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   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, 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 as it was.  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).

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

9.1.  Operation 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:

   o  The Binding MUST be removed if an NA message is received over the
      Backbone for the Registered Address with no EARO, or containing an

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      EARO with a status of 1 (Duplicate) that indicates an existing
      registration owned by a different Registering Node.  In that case,
      an NA MUST be sent back to the Registering Node with a status of 1
      (Duplicate) in the EARO.  This behavior might be overriden 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]).

   o  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.  This behavior might be overriden by
      policy, in particular if the registration is not trusted.

   o  The Binding MUST be removed if an NA message is received over the
      Backbone for the Registered Address containing an EARO with a
      status of 3 (Moved), or an NS(DAD) with an EARO that indicates a
      fresher registration ([RFC8505]) for the same Registered Node
      (i.e. same ROVR).  A status of 3 is returned in the NA(EARO) back
      to the Registering Node.

   o  NS(DAD) and NA messages containing an EARO that indicates a
      registration for the same Registered Node that is not as fresh as
      this SHOULD be answered with an NA message containing an EARO with
      a status of 3 (Moved) in order to clean up the situation
      immediately.

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

   o  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 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 set over the
   backbone (see Section 7 and Section 8).

9.2.  Operation on a Binding in Reachable State

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

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   An NS(DAD) with no EARO or with an EARO that indicates a duplicate 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:

   o  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).  A status of 4 (Removed) is
      returned in an asynchronous NA(EARO) to the Registering Node.
      Based on configuration, an implementation may delay this operation
      by a small timer in order to a allow for a parallel registration
      to arrive to this node, in which case the NA might be ignored.

   o  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.

   o  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).

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

   o  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.

9.3.  Operation 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.

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   For a Binding in Stale state:

   o  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.

   o  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).

   o  If the 6BBR receives an NS(Lookup) or an NS(NUD) message for the
      Registered Address, the 6BBR MUST attempts 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.

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

   When the STALE_DURATION 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 SHOULD 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.

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   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.

11.  Security Considerations

   This specification applies to LLNs in which the link layer is
   protected, either by means of physical or IP security for the
   Backbone Link or MAC-layer security.  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.

   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.
   [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.

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, by default 24
   hours.  In LLNs where addresses are renewed rapidly, e.g. for privacy
   reasons, STALE_DURATION SHOULD be configured with a relatively long
   value, by default 5 minutes.

13.  IANA Considerations

   This document has no request to IANA.

14.  Acknowledgments

   Many thanks to Dorothy Stanley, Thomas Watteyne and Jerome Henry for
   their various contributions.

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15.  References

15.1.  Normative References

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

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

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

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

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

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

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   [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>.

15.2.  Informative References

   [I-D.bi-savi-wlan]
              Bi, J., Wu, J., Wang, Y., and T. Lin, "A SAVI Solution for
              WLAN", draft-bi-savi-wlan-17 (work in progress), May 2019.

   [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", draft-ietf-6lo-ap-nd-12 (work in
              progress), April 2019.

   [I-D.ietf-6man-rs-refresh]
              Nordmark, E., Yourtchenko, A., and S. Krishnan, "IPv6
              Neighbor Discovery Optional RS/RA Refresh", draft-ietf-
              6man-rs-refresh-02 (work in progress), October 2016.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-26 (work
              in progress), August 2019.

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   [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", draft-ietf-mboned-ieee802-mcast-problems-08 (work
              in progress), August 2019.

   [I-D.nordmark-6man-dad-approaches]
              Nordmark, E., "Possible approaches to make DAD more robust
              and/or efficient", draft-nordmark-6man-dad-approaches-02
              (work in progress), October 2015.

   [I-D.thubert-6lo-unicast-lookup]
              Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
              Unicast Lookup", draft-thubert-6lo-unicast-lookup-00 (work
              in progress), January 2019.

   [I-D.yourtchenko-6man-dad-issues]
              Yourtchenko, A. and E. Nordmark, "A survey of issues
              related to IPv6 Duplicate Address Detection", draft-
              yourtchenko-6man-dad-issues-01 (work in progress), March
              2015.

   [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
              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)".

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   [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)".

   [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>.

   [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>.

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

   [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>.

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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
   MultiLink 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, meeting the
   requirements listed in Appendix B.3 of [RFC8505] "Requirements
   Related to Various Low-Power Link Types".

   Each LLN in the subnet is attached at 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.  In 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

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   Appendix B.4 of [RFC8505], "Requirements Related to Proxy
   Operations".

   The IPv6 ND operation is minimized as the number of 6LNs grows in the
   LLN.  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
   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 MultiLink 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].

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Authors' Addresses

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

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

   Charles E. Perkins
   Futurewei
   2330 Central Expressway
   Santa Clara  95050
   United States of America

   Email: charliep@computer.org

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

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

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