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Versions: (draft-thubert-6lo-backbone-router) 00 01 02 03 04 05 06 07 08 09

6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 4861, 8505 (if approved)                             C. Perkins
Intended status: Standards Track                               Futurewei
Expires: June 8, 2019                                   E. Levy-Abegnoli
                                                           Cisco Systems
                                                        December 5, 2018


                          IPv6 Backbone Router
                   draft-ietf-6lo-backbone-router-09

Abstract

   Backbone Routers are RFC8505 Routing Registrars that provide proxy
   services for IPv6 Neighbor Discovery.  Backbone Routers federate
   multiple wireless Links over a Backbone Link to form a MultiLink
   Subnet.  Backbone Routers placed along the wireless edge of the
   Backbone handle IPv6 Neighbor Discovery, and route packets on behalf
   of registered nodes.

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 June 8, 2019.

Copyright Notice

   Copyright (c) 2018 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



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Acronym Definitions . . . . . . . . . . . . . . . . . . .   6
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Access Link . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Route-Over Mesh . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  MultiLink Subnet Consistency  . . . . . . . . . . . . . .  11
     3.4.  Registering Node  . . . . . . . . . . . . . . . . . . . .  11
     3.5.  Using IPv6 ND Over the Backbone Link  . . . . . . . . . .  12
     3.6.  Routing Proxy Operations  . . . . . . . . . . . . . . . .  13
     3.7.  Bridging Proxy Operations . . . . . . . . . . . . . . . .  14
     3.8.  Leveraging Optimistic DAD . . . . . . . . . . . . . . . .  14
   4.  Updating RFC 4861 . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Updating RFC 8505 . . . . . . . . . . . . . . . . . . . . . .  15
   6.  6BBR detailed Operations  . . . . . . . . . . . . . . . . . .  15
     6.1.  Primary and Secondary  6BBRs  . . . . . . . . . . . . . .  16
     6.2.  Binding Table . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Registration and Binding Table Entry Creation . . . . . .  17
     6.4.  Defending Addresses . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  Protocol Constants  . . . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   10. Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  20
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     12.2.  Informative References . . . . . . . . . . . . . . . . .  22
     12.3.  External Informative References  . . . . . . . . . . . .  24
   Appendix A.  Applicability and Requirements Served  . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   IEEE STD. 802.1 [IEEEstd8021] Ethernet Bridging provides an efficient
   and reliable broadcast service; 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



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   Ethernet Bridging in an economical fashion; 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
   interesting caveat that the bridging state is populated proactively
   at the association time.  This ensures a solid connectivity to the
   node (STA) and protects the wireless medium against the 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 APs subsequently proxies the bridging operation and does
   not need to forward the broadcast lookups over the radio.

   Like Transparent Bridging, the operations of the IPv6 [RFC8200]
   Neighbor Discovery [RFC4861] [RFC4862] Protocol (IPv6 ND) are
   reactive and rely heavily on multicast transmissions to locate an on-
   link correspondent and ensure the uniqueness of an Address.  The
   mechanism for Duplicate Address Detection (DAD) [RFC4862] was also
   designed as a natural match with the efficient broadcast operation of
   Ethernet Bridging.  However, since broadcast can be unreliable over
   wireless media, DAD often fails to discover duplications
   [I-D.yourtchenko-6man-dad-issues].  A conflict of IPv6 Address is
   still a very rare event, not because Address duplications are
   detected and solved as designed, but because of the sheer entropy of
   the 64-bit Interface IDs.

   IPv6 multicast messages are typically broadcast over the wireless
   medium; they are processed by most if not all the wireless nodes over
   the subnet - e.g., the ESS fabric - even when very few if any of the
   nodes is subscribed to the multicast flow.  The IPv6 ND Neighbor
   Solicitation (NS) [RFC4861] is such a message; NS messages are used
   for DAD and Address lookup, and are frequently observed in a
   situation of mobility and when a node wakes up and reconnects to the
   wireless network.  The NS message is targeted to a Sollicitated-Node
   Multicast Address (SNMA) [RFC4291] and should in theory only reach a
   very small group of nodes; but since Layer-3 multicast messages are
   effectively broadcasted at Layer-2, the volume of Address lookups and
   DADs over a large fabric can effectively consume bandwidth to the
   point that it becomes detrimental to unicast traffic (see
   [I-D.ietf-mboned-ieee802-mcast-problems]).

   Additionally, wireless nodes that do not belong to the SNMA group
   still have to keep their radio awake and listen to broadcasted NS
   messages, which is a total waste of energy for them.  In order to
   control their power consumption, battery-operated nodes such as IOT
   sensors and smartphones may then elect to blindly ignore a portion of



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   the broadcasts, which tends to make the Layer-3 protocol operations
   even less reliable.

   These problems can be alleviated by a reduction of IPv6 ND broadcasts
   over wireless access links.  One classical way to achieve this to
   split the broadcast domains and route between subnets, possibly by
   assigning a /64 prefix to each wireless node (see [RFC8273]).

   Another way is to proxy the Layer-3 protocols that rely on broadcast
   operations at the boundary of the wired and wireless domains, in a
   fashion similar to the Layer-2 association but at layer-3.  To that
   effect, IEEE 802.11 [IEEEstd80211] requires ARP and proxy-ND
   [RFC4389] services at the Access Points (APs), and this specification
   is a possible response to that requirement.

   IPv6 proxy-ND services can be obtained automatically by snooping the
   IPV6 ND protocol (see [I-D.bi-savi-wlan]).  Proprietary techniques
   for IPv6 ND and DHCP snooping are effectively deployed, and though
   snooping is really useful to cancel undesirable broadcast
   transmissions, it has also proven to be unreliable; An IPv6 Address
   may not be discovered immediately due to a packet loss, or a silent
   node that does not use the Address for a while; a change of state
   (e.g. due a movement) may be missed or misordered, leading to
   unreliable connectivity and a partial knowledge of the state of the
   network.

   With this specification, a wireless node proactively registers its
   IPv6 Addresses using a NS(EARO) as specified in [RFC8505] to an IPv6
   Backbone Router (6BBR).  The 6BBR is a Routing Registrar per
   [RFC8505].  It is also a Border Router that performs the IPv6 proxy
   Neighbor Discovery operations on its Backbone interface on behalf of
   the 6LNs that are registered on its LLN interfaces.  This effectively
   recreates at Layer-3 the equivalent of an association such as found
   in IEEE STD. 802.11 for the purpose of providing reachability to the
   registered Addresses without the need of a broadcast lookup over the
   wireless medium.  Additional benefits are discussed in Appendix A.

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.  References

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

   o  "Neighbor Discovery Proxies (proxy-ND)" [RFC4389]

   o  "Optimistic Duplicate Address Detection" [RFC4429], and

   o  "Neighbor Discovery for IP version 6" [RFC4861],

   o  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

   o  "MultiLink Subnet Issues" [RFC4903],

   o  "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
      Overview, Assumptions, Problem Statement, and Goals" [RFC4919],

   o  Neighbor Discovery Optimization for Low-Power and Lossy Networks
      [RFC6775],

   o  ,"Mobility Support in IPv6" [RFC6275],

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

   o  "Registration Extensions for 6LoWPAN Neighbor Discovery"
      [RFC8505].

2.3.  New Terms

   This document also introduces the following terminology:

   Federated

         A subnet that is partitionned over a Backbone and one or more
         (wireless) access links, is said to be federated into one
         MultiLink Subnet by the proxy-ND operation of 6BBRs located at
         the edge of the Backbone and the access links and providing a
         semblance of a non-partitionned subnet for IPv6 ND over the
         Backbone.

   Sleeping Proxy

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



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   Unicasting  Proxy

         A Unicasting Proxy forwards NS messages to the Registering
         Node, transforming Layer-2 multicast into unicast.

   Routing Proxy

         A Routing Proxy advertises its own MAC Address as the TLLA in
         the proxied NAs over the Backbone, as opposed to that of the
         node that performs the registration.

   Bridging Proxy

         A Bridging Proxy advertises the MAC Address of the node that
         performs the registration 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 fabric.

   Primary  6BBR

         The 6BBR that will defend a Registered Address for the purpose
         of DAD over the Backbone.

   Secondary  6BBR

         A 6BBR other than the Primary 6BBR to which an Address is
         registered.  A Secondary Router MAY advertise the Address over
         the Backbone and proxy for it.

2.4.  Acronym Definitions

   This document uses the following acronyms:

   6BBR: 6LoWPAN Backbone Router

   6LBR: 6LoWPAN Border Router

   6LN:  6LoWPAN Node

   6LR:  6LoWPAN Router

   6CIO: Capability Indication Option

   EARO: (Extended) Address Registration Option -- (E)ARO

   EDAR: (Extended) Duplicate Address Request -- (E)DAR

   EDAC: (Extended) Duplicate Address Confirmation -- (E)DAC



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   DAD:  Duplicate Address Detection

   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 (pronounced rover)

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple) [RFC6550]

   RA:   Router Advertisement

   RS:   Router Solicitation

   TID:  Transaction ID (a sequence counter in the EARO)

3.  Overview

   A 6BBR provides proxy-ND services to 6LNs attached to an LLN that is
   anchored at the 6BBR; this way, a subnet that is located on a
   Backbone can be extended in the LLN as a MultiLink Subnet.  The LLN
   may be a hub-and-spoke network, a mesh-under or a route-over network.

   The proxy-ND operation can co-exist with IPv6 ND over the Backbone.
   The proxy state can be distributed across multiple 6BBR attached to a
   same Backbone.  A 6LN may move freely from an LLN anchored at one
   6BBR to an LLN anchored at another 6BBR on the same Backbone and
   retain any or all of the IPv6 Addresses that the 6LN has formed.

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







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                 |
               +-----+
               |     | Gateway (default) Router
               |     |
               +-----+
                  |
                  |           Backbone Link
            +-------------------------+----------------------+
            |                         |                      |
         +------+                 +------+                +------+
         | 6BBR |                 | 6BBR |                | 6BBR |
         |      |                 |      |                |      |
         +------+                 +------+                +------+
            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                        LLN                      LLN


               Figure 1: Backbone Link and Backbone Routers

   Each Backbone Router (6BBR) maintains an abstract Binding Table of
   its Registered Nodes.  The Binding Tables form a distributed database
   of 6LNs that reside on the LLNs or on the IPv6 Backbone, and use an
   extension to IPv6 ND to exchange that information across the
   Backbone.  In that process:

      The Extended Address Registration Option (EARO) defined in
      [RFC8505] is used in the ND exchanges over the Backbone between
      the 6BBRs to help distinguish duplication from movement.
      Optionally, Extended Duplicate Address Messages (EDAR and EDAC)
      can also be used between the 6BBR and a 6LBR if one is present on
      the Backbone.  Address duplication is detected using the ROVR
      field, and conflicting registrations to different 6BBRs by a same
      owner 6LR 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 may be that of the Registering Node; in that case, the
      6BBR needs to bridge the unicast packets (Bridging Proxy).
      Alternatively, the LLA can be that of the 6BBR on the Backbone
      interface, in which case the 6BBRs receives at Layer-2 and and
      needs to route at Layer-3 the unicast packets (Routing Proxy).
      This is discussed in more details in Section 3.6 and Section 3.7,
      respectively.



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3.1.  Access Link

   This specification also applies to (hub-and-spoke) Access Links such
   as (Low-Power) IEEE STD. 802.11 (Wi-Fi) [IEEEstd80211] and IEEE STD.
   802.15.1 (Bluetooth) [IEEEstd802151].  Figure 2 illustrates an ODAD-
   complient (see Section 3.8) example of a 6LN that forms an IPv6
   Address and registers it to a 6BBR acting as a 6LR [RFC8505].

       6LoWPAN Node        6BBR          6LBR            default
          (STA)            (AP)                           Router
            |(Wireless) LLN |       IPv6 ND Backbone        |
            |               |         (Ethernet)            |
            |       RS      |              |                |
            |-------------->|              |                |
            |  (multicast)  |              |                |
            |               |              |                |
            |  RA(PIO)      |              |                |
            |<--------------|              |                |
            | (L2 unicast)  |              |                |
            |               |              |                |
            |  NS(EARO)     |              |                |
            |-------------->|              |                |
            | (optimistic)  |              |                |
            |               | Extended DAR |                |
            |               |------------->|                |
            |               | Extended DAC |                |
            |               |<-------------|                |
            |               |         NS-DAD(EARO)          |
            |               |------------------------------>|
            |               |------->   (multicast)         |
            |               |--------------------->         |
            |               |   RS(no SLLAO, for ODAD)      |
            |               |------------------------------>|
            |               |   (if no BCE) 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



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3.2.  Route-Over Mesh

   In the case of a Route-Over Mesh, e.g., using RPL [RFC6550], the
   6TiSCH architecture [I-D.ietf-6tisch-architecture] suggests to
   collocate the RPL root with a 6LoWPAN Border Router (6LBR), which is
   either collocated with or connected to the 6BBR over an IPv6 Link.

   Figure 3 illustrates the initial IPv6 signaling that enables a 6LN to
   form a Global or a Unique-Local Address and register it to the 6LBR
   using [RFC8505].  The 6LBR also leverages [RFC8505] to register the
   6LNs on their behalf to the 6BBR and obtain proxy-ND services.


       6LoWPAN Node        6LR             6LBR            6BBR
       (mesh leaf)     (mesh router)   (mesh root)
            |               |               |               |
            |  6LoWPAN ND   |6LoWPAN ND+RPL | 6LoWPAN ND    | IPv6 ND
            |   LLN link    |Route-Over mesh|Ethernet/serial| Backbone
            |               |               |/Internal call |
            |  IPv6 ND RS   |               |               |
            |-------------->|               |               |
            |----------->   |               |               |
            |------------------>            |               |
            |  IPv6 ND RA   |               |               |
            |<--------------|               |               |
            |               |    <once>     |               |
            |  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





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3.3.  MultiLink Subnet Consistency

   The Backbone and the federated LLN Links are considered as different
   Links in the MultiLink Subnet, even if multiple LLNs are attached to
   a same 6BBR.  Multicast ND messages are link-scoped and MUST NOT be
   forwarded across the Backbone Routers.

   A prefix that is used across a MultiLink Subnet may still be
   advertised as on-link over the Backbone, by setting the "L" bit in
   the Prefix Information Option (PIO) in RA messages ([RFC4861]), in
   order to support classical IPv6 hosts; but the MultiLink Subnet
   prefix MUST be advertised as not-onlink in RAs sent towards the LLN.

   Nodes located inside the subnet will not perform the IPv6 Path MTU
   Discovery [RFC8201] between one another.  For that reason, the MTU
   must have a same value on the Backbone and all attached LLNs.  To
   achieve this, the 6BBR MUST use the same MTU value that is used in
   RAs over the Backbone in the RAs that it transmits towards the LLN
   links.

3.4.  Registering Node

   A Registering Node MUST implement [RFC6775] as updated by [RFC8505]
   in order to interact with a 6BBR.  As such, it does not depend on
   multicast RAs to discover the 6LR(s).

   The Registering Node MUST accept multicast RAs, but those are
   expected to be rare within in the LLN is the best practices
   ([RFC7772]) are followed.

   The Registering Node SHOULD comply with the Simple Procedures for
   Detecting Network Attachment in IPv6 [RFC6059] (DNA procedures) to
   assert movements, and support Packet-Loss Resiliency for Router
   Solicitations [RFC7559] in order to make the unicast RS messages more
   reliable.

   The Registering Node signals that it requires IPv6 proxy-ND services
   from a 6BBR by registering the corresponding IPv6 Address with an
   NS(EARO) message with the 'R' flag set ([RFC8505]).  It may be the
   actual owner of 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 Global Unicast and
   Unique-Local IPv6 Addresses to the 6BBRs.  Failure to register a
   subset of Addresses may result in those Addresses 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.




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3.5.  Using IPv6 ND Over the Backbone Link

   On the Backbone side, the 6BBR MUST join the SNMA group that
   corresponds to a Registered Address as soon as it creates an entry
   for that Address, and conserve its SNMA membership as long as it
   maintains the associated entry.  The 6BBR uses either the SNMA or
   plain unicast to defend the Registered Addresses in its Binding
   Table over the Backbone.

   The 6BBR advertises and defends the Registered Addresses over the
   Backbone using the IPv6 ND protocol [RFC4861].  It MUST uses an EARO
   in the NS(DAD) and NA messages that it generates over the Backbone
   Link for the Registered Address.  A NA message generated in response
   to a NS(LOOKUP) MUST NOT have the override (O) bit set.  A proxied NS
   MUST NOT contain an SLLAO to avoid the confusion with a registration.

   A 6BBR may asynchronously update the NCEs in correspondent nodes over
   the Backbone, e.g., in case of a movement.  This is achieved using a
   gratuitous NA with the override (O) bit set, that may be sent unicast
   to each individual correspondent, or multicast to all nodes (more in
   Section 3.7 and Section 3.6).

   A 6LBR may optionally be deployed over the Backbone.  When that is
   the case, the 6BBR uses an EDAR/EDAC echange to check for duplication
   or movement as prescribed in [RFC8505].  If this registration is
   duplicate or not the freshest, then the 6LBR replies 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 there was an existing registration
   on an old 6BBR, then the 6LBR also sends an asynchronous EDAC with a
   status of 4 ("Removed") to the old 6BBR.  Note that an alternate
   protocol such as LISP [RFC6830] may be used to provide an equivalent
   service.

   Nodes implementing this specification is expected to co-exist on a
   same Backbone Link with nodes implementing classical IPv6 ND
   [RFC4861] and snooping [I-D.bi-savi-wlan].  It results that the fact
   that there is a 6LBR or an alternate protocol that is deployed on the
   Backbone does not mean that all IPv6 addresses are known there; the
   fact that a unicast DAD succeeds with the 6LBR does not mean that the
   address is not duplicate, and, unless administratively overridden,
   6BBRs must still perform classical IPv6 ND DAD after an EDAC with a
   status code of 0.

   For slow movements, the Neighbor Unreachability Detection (NUD)
   procedure defined in [RFC4861] may time out too quickly, and the
   support of [RFC7048] is recommended for all nodes in the subnet.




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3.6.  Routing Proxy Operations

   When operating as a Routing Proxy, the 6BBR MUST use the Layer-2
   Address on its Backbone Interface in the TLLA and SLLA options, when
   present, of the RS, NS and NA messages that it generates to advertise
   the Registered Addresses.  In that case, the MAC Addresses of the
   6LNs do not need to be visible at Layer-2 over the Backbone to
   maintain end-to-end IP connectivity, but the NCEs of the
   correspondents must be updated when the owner registers to a
   different 6BBR.

   This technique is useful when the churn on the Backbone fabric
   associated to wireless mobility becomes expensive, e.g., when the
   Layer-2 topology is virtualized over a wide area IP underlay.  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.

   This technique is also useful when the LLN uses a MAC address format
   that is different from that on the Backbone (e.g., EUI-64 vs. EUI-
   48).

   For each Registered Address, multiple peer Nodes 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 from an old
   to a new 6BBR, the old 6BBR SHOULD unicast a gratuitous NA with the
   Override (O) bit set to each such peer, to supply the LLA of the new
   6BBR in the TLLA option for the Address.

   If the 6BBR fails to maintain this list, then it MAY send the
   gratuitous NA with the Override (O) bit set as a multicast message
   that will possibly hit all the nodes on the Backbone, whether they
   maintain an NCE or not 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.  That old 6BBR having removed its host route to the
   Registered Address, it will look it up over the backbone, resolve the
   with the LLA of the new 6BBR, and forward the packet to the correct
   6BBR.  The old 6BBR SHOULD also issue a redirect message [RFC4861] is
   order to update the cache of the correspondent.







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3.7.  Bridging Proxy Operations

   A Bridging Proxy can be implemented in a Layer-3 switch, or in a
   wireless Access Point or wireless Controller that acts as a Layer-2
   Bridge for unicast packets from/to the Registered Address.  The
   Bridging Proxy appears as an IPv6 Host on the Backbone whereas the
   Routing Proxy described in Section 3.6 is an IPv6 router operating as
   a border router between Links of a MultiLink Subnet.

   When operating as a Bridging Proxy, the 6BBR MUST use the Registering
   Node's Layer-2 Address in the TLLA and SLLA options, when present,
   of, respectively, the RS, NS and 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 abstract Binding Table.

   If the Registering Node is the owner of the Registered Address, then
   its mobility does not impact existing NCEs over the Backbone.  If it
   is not, then when the 6LN selects another Registering Node, the new
   Registering Node SHOULD send a multicast NA with the Override (O) bit
   set to fix the existing NCEs across the Backbone.  This method may
   fail if the multicast message is not received, in which case one or
   more correspondent nodes on the Backbone may maintain an obsolete NCE
   and traffic to the Registered Address may be lost for a while.  When
   this condition happens, it is eventually be discovered and solved
   through the Neighbor Unreachability Detection (NUD) procedure defined
   in [RFC4861].

3.8.  Leveraging Optimistic DAD

   The Optimistic Duplicate Address Detection [RFC4429] (ODAD)
   specification details how an IPv6 Address can be used before a
   Duplicate Address Detection (DAD) is complete.

   ODAD provides a set of rules that guarantee that this behavior may
   not harm an existing state should the new Address effectively be a
   duplicate.  This specification leverages ODAD to avoid delays in
   installing the Neighbor Cache Entry (NCE) in the 6BBRs and the
   default router in order to obtain immediate connectivity to the
   registered node.

   This specification RECOMMENDS to support ODAD to create an optimistic
   proxy state in the 6BBR until DAD is completed over the Backbone.  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 Sollicitation (RS),
   sourced with the Registered Address, to the known router(s).  The RS
   MUST be sent without a Source LLA Option (SLLAO), to ensure that a
   preexisting NCE in the router is not affected.



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   Following the ODAD flows, the router may then send a unicast RA to
   the Registered Address, and in the process of doing so, it may
   resolve it using an NS(LOOKUP) message.  In response, the 6BBR sends
   a NA with the override (O) bit that is not set (per [RFC4429]), and
   an EARO option.  If the router supports this specification, then it
   can determine the freshest EARO option in case of a conflicting
   NA(EARO) messages, using section 5.2.1 of [RFC8505].  If the NA(EARO)
   is the freshest or only answer then the default router creates a BCE
   with the SLLAO of the 6BBR (in Routing Proxy mode) or that of the
   Registering Node (in Bridging Proxy mode) and traffic from/to the
   Registered Address can flow immediately.

4.  Updating RFC 4861

   This specification adds the EARO as a possible option in RS, NS(DAD)
   and NA messages over the backbone.  Note that [RFC8505] requires that
   the registration NS(EARO) contains an SLLAO.  Note that an NS(DAD)
   does not contain an SLLAO and thus cannot be confused with a
   registration.

5.  Updating RFC 8505

   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.

6.  6BBR detailed Operations

   By default, a 6BBR operates as a Sleeping Proxy, as follows:

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

   o  Defend a Registered Address over the Backbone using NA messages
      with the Override bit set on behalf of the sleeping 6LN

   o  Advertise a Registered Address over the Backbone using NA
      messages, asynchronously or as a response to a Neighbor
      Solicitation messages.

   o  To deliver packets arriving from the LLN, use Neighbor
      Solicitation messages to look up the destination over the
      Backbone.

   o  Forward packets between the LLN and the Backbone.

   o  Verify liveliness when needed for a stale registration.



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   A 6BBR may act as a Sleeping Proxy only for a Registered Address that
   is REACHABLE, or TENTATIVE in which case the answer is delayed.  In
   any other state, the Sleeping Proxy operates as a Unicasting Proxy.

   The 6BBR does not act on ND Messages over the Backbone unless they
   are relevant to a Registered Node on the LLN side, saving wireless
   interference.  On the LLN side, the prefixes associated to the
   MultiLink Subnet are presented as not on-link, so Address resolution
   for other hosts do not occur.

   As a Unicasting Proxy, the 6BBR forwards NS lookup messages to the
   Registering Node, transforming Layer-2 multicast into unicast.  This
   is not possible in UNREACHABLE state, so the NS messages are
   multicasted, and rate-limited.  Retries are possible, using an
   exponential back-off to protect the medium.  In other states, the
   messages are forwarded to the Registering Node as unicast Layer-2
   messages.  In TENTATIVE state, the NS message is either held till DAD
   completes, or dropped if DAD does not complete.

6.1.  Primary and Secondary 6BBRs

   A 6BBR MAY be primary or secondary.  The primary is the Backbone
   router that has the highest EUI-64 Address of all the 6BBRs that
   share a registration for a 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.  The primary Backbone
   Router is in charge of protecting the Address for DAD over the
   Backbone.  Any of the Primary and Secondary 6BBR may claim the
   Address over the Backbone, since they are all capable to route from
   the Backbone to the 6LN; the Address appears on the Backbone as an
   anycast Address.

6.2.  Binding Table

   Each 6BBR maintains a Binding Table, using IPv6 ND over the Backbone
   to detect duplication.  Another document [RFC8505] provides details
   about how the EARO is used between 6LRs and 6LBRs by way of DAR/DAC
   messages within the LLN.  Addresses in a LLN that can be reachable
   from the Backbone by way of a 6BBR MUST be registered to that 6BBR.

   A false positive duplicate detection may arise over the Backbone, for
   instance if a 6LN's Registered Address is registered to more than one
   LBR, or if the 6LN has moved.  Both situations are handled by the
   6BBR transparently to the 6LN.  In the former case, one LBR becomes
   primary to defend the Address over the Backbone while the others
   become secondary and may still forward packets.  In the latter case



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   the LBR that receives the newest registration becomes primary because
   of the TID.

   Only one 6LN may register a given Address at a particular 6BBR.
   However, that Registered 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 and acknowledged with a status of 4 (TBD); the entry is
      deleted;

      Newer registrations (newer TID, same ROVR, non-null Lifetime) are
      acknowledged 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 acknowledgement 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 a same
      Registering Node are acknowledged with a status of 0 (success).
      If they are not identical, an error SHOULD be logged.  In
      TENTATIVE state, the response is held and may be overwritten, but
      it MUST be eventually produced and it carries the result of the
      DAD process;

      Older registrations (older TID, same ROVR) from a Registering Node
      are ignored;

      Identical and older registrations (not-newer TID, same ROVR) from
      a different Registering Node are acknowledged with a status of 3
      (moved); this may be rate limited to protect the medium;

      Any registration for a different Registered Node (different ROVR)
      are acknowledged with a status of 1 (duplicate).

6.3.  Registration and Binding Table Entry Creation

   Upon receiving a registration for a new Address with an NS(EARO) with
   the 'R' bit set, the 6BBR performs DAD over the Backbone, placing the
   new Address as target in the NS(DAD) message.  The EARO from the
   registration MUST be placed unchanged in the NS(DAD) message, and an
   Neighbor Cache entry created in TENTATIVE state for a duration of
   TENTATIVE_DURATION.  The NS(DAD) message is sent multicast over the
   Backbone to the SNMA associated with the registered Address, unless
   that operation is known to be costly, and the 6BBR has an indication
   from another source (such as a Neighbor Cache entry) that the



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   Registered Address was known on the Backbone; in the latter case, an
   NS(DAD) message may be sent as a Layer-2 unicast to the MAC Address
   that was associated with the Registered Address.

   In TENTATIVE state after EARO with 'R' bit set:

   1.  The entry is removed if an NA is received over the Backbone for
       the Registered Address with no EARO, or containing an EARO with a
       status of 1 (duplicate) that indicates an existing registration
       for another 6LN.  The ROVR and TID fields in the EARO received
       over the Backbone are ignored.  A status of 1 is returned in the
       EARO of the NA back to the Registering Node;

   2.  The entry is also removed if an NA with an ARO option with a
       status of 3 (moved), or a NS with an ARO option that indicates a
       newer registration for the same Registered Node, is received over
       the Backbone for the Registered Address.  A status of 3 is
       returned in the NA(EARO) back to the Registering Node;

   3.  When a registration is updated but not deleted, e.g. from a newer
       registration, the DAD process on the Backbone continues and the
       running timers are not restarted;

   4.  Other NS (including DAD with no EARO) and NA from the Backbone
       are not acknowledged in TENTATIVE state.  To cover legacy 6LNs
       that do not support ODAD, the list of their origins MAY be stored
       and then, if the TENTATIVE_DURATION timer elapses, the 6BBR MAY
       send each such legacy 6LN a unicast NA.

   5.  When the TENTATIVE_DURATION timer elapses, a status 0 (success)
       is returned in a NA(EARO) back to the Registering Node(s), and
       the entry goes to REACHABLE state for the Registration Lifetime.
       The 6BBR MUST send a multicast NA(EARO) to the SNMA associated to
       the Registered Address over the Backbone with the Override bit
       set so as to take over the binding from other 6BBRs.

6.4.  Defending Addresses

   If a 6BBR has an entry in REACHABLE state for a Registered Address:

   o  If the 6BBR is primary, or does not support the function of
      primary, it MUST defend that Address over the Backbone upon
      receiving NS, either if the NS does not carry an EARO, or if an
      EARO is present that indicates a different Registering Node
      (different ROVR).  The 6BBR sends a NA message with the Override
      bit set and the NA carries an EARO if and only if the NS(DAD) did
      so.  When present, the EARO in the NA(Override) that is sent in
      response to the NS(EARO) carries a status of 1 (duplicate), and



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      the ROVR and TID fields in the EARO are obfuscated with null or
      random values to avoid network scanning and impersonation attacks.

   o  If the 6BBR receives an NS(EARO) for a newer registration, the
      6BBR updates the entry and the routing state to forward packets to
      the new 6BBR, but keeps the entry REACHABLE.  Afterwards, the 6BBR
      MAY use REDIRECT messages to reroute traffic for the Registered
      Address to the new 6BBR.

   o  If the 6BBR receives an NA(EARO) for a newer registration, the
      6BBR removes its entry and sends a NA(EARO) with a status of 3
      (MOVED) to the Registering Node, if the Registering Node is
      different from the Registered Node.  The 6BBR cleans up existing
      Neighbor Cache entries in peer nodes as discussed in Section 3.5,
      by unicasting to each such peer, or one broadcast NA(Override).

   o  If the 6BBR receives a NS(LOOKUP) for a Registered Address, it
      answers immediately with an NA on behalf of the Registered Node,
      without polling it.  There is no need of an EARO in that exchange.

   o  When the Registration-Lifetime timer elapses, the entry goes to
      STALE state for a duration of STABLE_STALE_DURATION in LLNs that
      keep stable Addresses such as LWPANs, and UNSTABLE_STALE_DURATION
      in LLNs where Addresses are renewed rapidly, e.g. for privacy
      reasons.

   The STALE state enables tracking of the Backbone peers that have a
   Neighbor Cache entry 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.  In STALE state:

   o  If STALE_DURATION elapses, the 6BBR removes the entry.

   o  Upon receiving an NA(Override) the 6BBR removes its entry and
      sends a NA(EARO) with a status of 4 (removed) to the Registering
      Node.

   o  If the 6BBR receives a NS(LOOKUP) for a Registered Address, the
      6BBR MUST send an NS(NUD) following rules in [RFC7048] to the
      Registering Node targeting the Registered Address prior to
      answering.  If the NUD succeeds, the operation in REACHABLE state
      applies.  If the NUD fails, the 6BBR refrains from answering the
      lookup.  The NUD SHOULD be used by the Registering Node to
      indicate liveness of the Registered Node, if they are different
      nodes.





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7.  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 sublayer cryptography.  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.

   The use of EUI-64 for forming the Interface ID in the link local
   Address prevents the usage of Secure ND ([RFC3971] and [RFC3972]) and
   Address privacy techniques.  Additional protection against Address
   theft is provided by [I-D.ietf-6lo-ap-nd], which guarantees the
   ownership of the ROVR.

   When the ownership of the ROVR cannot be assessed, this specification
   limits the cases where the ROVR and the TID are multicasted, and
   obfuscates them in responses to attempts to take over an Address.

8.  Protocol Constants

   This Specification uses the following constants:

   TENTATIVE_DURATION:        800 milliseconds

   STABLE_STALE_DURATION:     24 hours

   UNSTABLE_STALE_DURATION:   5 minutes

   DEFAULT_NS_POLLING:        3 times

9.  IANA Considerations

   This document has no request to IANA.

10.  Future Work

   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...

11.  Acknowledgments

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




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12.  References

12.1.  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>.

   [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>.



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   [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>.

12.2.  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-16 (work in progress), November
              2018.

   [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-08 (work in
              progress), October 2018.

   [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-17 (work
              in progress), November 2018.

   [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-04 (work
              in progress), November 2018.







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   [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.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.

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

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

   [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>.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,
              <https://www.rfc-editor.org/info/rfc4919>.

   [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>.








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   [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>.

   [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>.

   [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>.

12.3.  External Informative References

   [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".





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   [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.  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 loosely 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, so as to
   address the requirements listed in Appendix B.3 of [RFC8505]
   "Requirements Related to Various Low-Power Link Types".

   Each LLN in the subnet is anchored 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.

   Any 6LN may register its IPv6 Addresses and thereby obtain proxy-ND
   services over the Backbone, providing a solution to the requirements
   expressed in 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



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   dimensioned for the number of registrations that each needs to
   support.

   In the case of a (Low-Power) 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 that case, the
   wireless client (STA) is the 6LN [RFC8505] that makes use of this
   specification 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.

   The scalability of the MultiLink Subnet [RFC4903] requires that
   multicast/broadcast operations are avoided as much as possible even
   on the Backbone [I-D.ietf-mboned-ieee802-mcast-problems].  Although
   hosts can connect to the Backbone using classical 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








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   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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