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Versions: (draft-thubert-6lo-backbone-router) 00 01 02 03 04 05 06 07 08 09 10 11

6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     cisco
Intended status: Standards Track                              C. Perkins
Expires: April 25, 2019                                        Futurewei
                                                        October 22, 2018


                          IPv6 Backbone Router
                   draft-ietf-6lo-backbone-router-08

Abstract

   Backbone Routers running IPv6 Neighbor Discovery can manage multiple
   wireless links to form a large MultiLink Subnet, but it is more
   efficient if IPv6 Neighbor Discovery packets are not broadcast over
   the wireless links.  This specification specifies proxy operations
   for IPv6 Neighbor Discovery on behalf of devices located on
   broadcast-inefficient wireless networks.  Backbone Routers placed
   along the wireless edge of the backbone handle IPv6 Neighbor
   Discovery, and route packets on behalf of registered nodes.  Wireless
   nodes register, or are registered by proxy, to a Backbone Router to
   establish proxy services in a fashion similar to layer-2 association.

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 April 25, 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include 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.  Applicability and Requirements Served . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Backbone Router Routing Operations  . . . . . . . . . . . . .   8
     5.1.  Over the Backbone Link  . . . . . . . . . . . . . . . . .   8
     5.2.  Proxy Operations Over the LLN Interface . . . . . . . . .   9
       5.2.1.  Routing Proxy Operations  . . . . . . . . . . . . . .  10
       5.2.2.  Bridging Proxy Operations . . . . . . . . . . . . . .  10
   6.  Backbone Router Proxy Operations  . . . . . . . . . . . . . .  11
     6.1.  Primary and Secondary BBRs  . . . . . . . . . . . . . . .  12
     6.2.  Binding Table . . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Registration and Binding Table Entry Creation . . . . . .  13
     6.4.  Defending Addresses . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  Protocol Constants  . . . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
     12.3.  External Informative References  . . . . . . . . . . . .  19
   Appendix A.  Changes from revision 07 to revision 08  . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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, many wireless networks do not economically provide the
   broadcast capabilities of Ethernet Bridging; protocols designed for
   bridged networks that rely on broadcast often exhibit disappointing
   behaviours when applied unmodified to a wireless medium (see
   [I-D.ietf-mboned-ieee802-mcast-problems]).

   WiFi [IEEEstd80211] Access Points (APs) deployed in an Extended
   Service Set (ESS) act as bridges.  In order to ensure a solid



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   connectivity to the devices and protect the medium against harmful
   broadcasts, they refrain from relying on broadcast-intensive
   protocols such as Transparent Bridging on the wireless side.
   Instead, an association process is used to register the MAC addresses
   of the wireless device (STA) to the AP.  The APs subsequently proxy
   the bridging operation and eliminate the broadcasts.

   The IPv6 [RFC8200] Neighbor Discovery [RFC4861] [RFC4862] Protocol
   (IPv6 ND) operations are reactive and rely heavily on multicast
   transmissions to locate an on-link correspondent and ensure address
   uniqueness.  Duplicate Address Detection [RFC4862] (DAD) mechanism
   was 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].  DAD usually appears
   to work on wireless media, not because address duplication is
   detected and solved as designed, but because the use of 64-bit
   Interface IDs makes duplication into a very rare event.

   IPv6 multicast messages are typically broadcast over the wireless
   medium.  They are processed by most if not all wireless nodes over
   the ESS fabric even when very few if any of them are subscribed to
   the multicast address.  A simple Neighbor Solicitation (NS)
   [RFC4861], that is supposedly targeted to a small group of nodes, can
   congest the wireless bandwidth
   [I-D.ietf-mboned-ieee802-mcast-problems].  The IPv6 ND operation
   leads to undesirable power consumption in battery-operated devices.

   These problems suggest restricting IPv6 ND broadcasts over wireless
   access links, which can be done by dividing up the subnet.  Another
   way is to take over (proxy) the Layer-3 protocols that rely on
   broadcast operation at the boundary of the wired and wireless
   domains, emulating the Layer-2 association at layer-3.  For instance,
   IEEE 802.11 [IEEEstd80211] specifies ARP and ND proxy [RFC4389]
   services at the Access Points (APs).

   Current devices rely on snooping for detecting association state,
   which is failure-prone in lossy and mobile conditions.  With
   snooping, a state (e.g. a new IPv6 address) may not be discovered, or
   a change of state (e.g. a movement) may be missed, leading to
   unreliable connectivity.

   WPAN devices (i.e., those implementing IEEE STD. 802.15.4
   [IEEEstd802154]) can make use of Neighbor Discovery Optimization for
   IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)
   [RFC6775] which treats the wireless medium as different from
   Ethernet.  RFC 6775 is updated as [I-D.ietf-6lo-rfc6775-update]; the
   update includes changes that are required by this document.



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2.  Applicability and Requirements Served

   This specification updates and generalizes 6LoWPAN ND to a broader
   range of Low power and Lossy Networks (LLNs) with support for
   Duplicate Address Detection (DAD) and address lookup that does not
   require broadcasts over the LLNs.  The term LLN is used loosely in
   this specification 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 [I-D.ietf-6lo-rfc6775-update]
   "Requirements Related to the Variety of Low-Power Link types".

   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 [I-D.ietf-6lo-rfc6775-update] "Requirements Related
   to Routing Protocols".

   This specification also applies to wireless links such as Low-Power
   IEEE STD. 802.11 (Wi-Fi) and IEEE STD. 802.15.1 (Bluetooth)
   [IEEEstd802151].  It makes use of extensions to [RFC6775] to enable
   proxy operation by the 6BBR, as specified in
   [I-D.ietf-6lo-rfc6775-update].  The BBR proxy operations eliminate
   the need for wireless nodes to respond synchronously when a lookup is
   performed for their addresses.  This provides the function of a Sleep
   Proxy for ND [I-D.nordmark-6man-dad-approaches].

   This draft establishes a Backbone that treats multiple LLNs as a
   single IPv6 MultiLink Subnet.  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 using proxy-ND
   operations.  This specification extends 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
   [I-D.ietf-6lo-rfc6775-update] "Requirements Related to Mobility" are
   met.

   This specification enables any 6LN to register its IPv6 addresses and
   thereby obtain routing services including proxy-ND operations over
   the backbone, providing a solution to the requirements expressed in
   Appendix B.4 of [I-D.ietf-6lo-rfc6775-update] "Requirements Related
   to Proxy Operations".



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   The Link Layer Address (LLA) that is returned as Target LLA (TLLA) in
   Neighbor Advertisements (NA) messages by the 6BBR 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), or that of the 6BBR on the backbone, in which case
   the 6BBRs needs to route the unicast packets (Routing proxy).  The
   IPv6 ND operation is minimized as the number of 6LNs grows in the
   LLN.  This meets the requirements in Appendix B.6 of
   [I-D.ietf-6lo-rfc6775-update] "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 Low-Power IEEE STD. 802.11, a 6BBR may be collocated
   with a standalone AP or a CAPWAP [RFC5415] wireless controller.  Then
   the wireless client (STA) makes use of this specification to register
   its IPv6 address(es) to the 6BBR over the wireless medium.  In the
   case RPL, the RPL root is collocated with a 6LoWPAN Border Router
   (6LBR), and either collocated with or connected to the 6BBR over an
   IPv6 Link.  The 6LBR makes use of this specification to register the
   6LNs on their behalf to the 6BBR.

3.  Terminology

   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] .

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

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

   o  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

   o  "Multi-Link 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  "Neighbor Discovery Proxies (ND Proxy)" [RFC4389]

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



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   o  "Registration Extensions for 6LoWPAN Neighbor Discovery"
      [I-D.ietf-6lo-rfc6775-update]

   This document also uses terminology from [RFC7102] and
   [I-D.ietf-6lo-rfc6775-update], and introduces the following
   terminology:

   Sleeping Proxy

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

   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 BBR

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

   Secondary BBR

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

4.  Overview

   The services specified in this document assist a 6LN to move freely
   from an LLN anchored at one 6BBR to an LLN anchored at another 6BBR
   on the same backbone and keep any or all of the IPv6 addresses that
   the 6LN has formed.



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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 a Binding Table of its
   Registered Nodes.  The Binding Tables form a distributed database of
   wireless 6LNs that reside on the LLNs or on the backbone, and use an
   extension to IPv6 ND to exchange that information across the Backbone
   as described below.

   The Extended Address Registration Option (EARO) defined in
   [I-D.ietf-6lo-rfc6775-update] is used in the ND exchanges over the
   backbone between the 6BBRs to enable the registration for routing and
   proxy services, as well as distinguish duplication from movement.

   Address duplication is detected using the ROVR field in the EARO.  In
   case of conflicting registrations to multiple 6BBRs from the same
   node, the Transaction ID (TID) in the EARO enables 6BBRs to determine
   the latest registration for that 6LN.

   6BBRs perform ND proxy operations over the backbone, on behalf of
   their Registered Nodes.  Registration to a proxy service is done via
   a NS/NA(EARO) exchange.  6BBR operation resembles that of a Mobile
   IPv6 (MIPv6) [RFC6275] Home Agent.  This enables mobility support for
   6LNs; if they move outside of the network delimited by the Backbone
   link, then they make use of a Home Agent.  Home Agent functionality
   can easily be collocated with a 6BBR on the same backbone interface
   of a router.



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   The Optimistic Duplicate Address Detection [RFC4429] (ODAD)
   specification details how an address can be used before a Duplicate
   Address Detection (DAD) is complete, and mandates that an address
   that is TENTATIVE should not be associated to a Source Link-Layer
   Address Option in a Neighbor Solicitation message.  This
   specification makes use of ODAD to create a temporary proxy state in
   the 6BBR until DAD is completed over the backbone.  This way, the
   specification allows proxy state distribution across multiple 6BBR
   and co-existence with IPv6 ND over the backbone.

5.  Backbone Router Routing Operations

                  |
               +-----+
               |     | Gateway (default) Router
               |     |
               +-----+
                  | /64
                  |      Backbone Link
            +-------------------+-------------------+
            | /64               | /64               | /64
         +------+            +------+           +------+
         | 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: N*/128        LLN: M*/128       LLN: P*/128

    Figure 2: Example Routing Configuration for 3 LLNs in the ML Subnet

5.1.  Over the Backbone Link

   A 6BBR is a specific kind of Border Router that performs proxy
   Neighbor Discovery on its backbone interface on behalf of registered
   6LNs on its LLN interfaces.

   On the backbone side, the 6BBR advertises the prefixes of the LLNs
   for which it serves as a proxy.  Some restrictions of the attached
   LLNs will apply to the backbone.  In particular, the MTU SHOULD be
   set to the same value on the backbone and all attached LLNs.  The
   scalability of the multilink subnet [RFC4903] requires that broadcast
   operations are avoided as much as possible on the backbone as well.




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   The 6BBR uses an EARO in the NS-DAD and the multicast NA messages
   that it generates over the Backbone Link on behalf of a Registered
   Node.  The 6BBR places an EARO in its unicast NA messages, if and
   only if the NS/NA that stimulates it had an EARO in it and the 'R'
   bit set.

   The 6BBR SHOULD use unicast or the solicited-node multicast address
   (SNMA) [RFC4291] to defend its Registered Addresses in its Binding
   Table over the backbone.  In particular, the 6BBR MUST join the SNMA
   group that corresponds to a Registered Address as soon as it creates
   an entry for that address, and maintain its SNMA membership as long
   as it maintains that entry.

   Optimistic DAD (ODAD) [RFC4429] SHOULD be supported by the 6BBRs in
   their proxy activity over the backbone.  A 6BBR supporting ODAD MUST
   join the SNMA of a Tentative address.

   A 6BBR in Routing Proxy mode MAY advertise the Registered IPv6
   Address with the 6BBR Link Layer Address, and update Neighbor Cache
   Entries (NCE) in correspondent nodes over the backbone, using
   gratuitous NA(Override).  This method may fail if the multicast
   message is not received, and correspondent nodes may maintain an
   incorrect neighbor state, which they will eventually discover through
   Neighbor Unreachability Detection (NUD).  For slow movements, the NUD
   procedure defined in [RFC4861] may time out too quickly, and the
   support of [RFC7048] is recommended in all 6LNs in the network.

   Multicast should be avoided as much as possible even on the backbone
   [I-D.ietf-mboned-ieee802-mcast-problems].  Although hosts can
   participate using legacy IPv6 ND, all 6LNs connected to the backbone
   SHOULD support [I-D.ietf-6man-rs-refresh], which also requires the
   support of [RFC7559].

5.2.  Proxy Operations Over the LLN Interface

   6LNs on the LLN follow [RFC6775] and do not depend on multicast RAs
   to discover routers.  6LNs SHOULD accept multicast RAs [RFC7772], but
   those are expected to be rare within in the LLN.  Nodes SHOULD follow
   the Simple Procedures for Detecting Network Attachment in IPv6
   [RFC6059] (DNA procedures) to assert movements, and support Packet-
   Loss Resiliency for Router Solicitations [RFC7559] to make the
   unicast RS more reliable.

   A 6LN signals that it requires IPv6 ND proxy services from a 6BBR by
   registering the corresponding IPv6 Address with an NS(EARO) message
   with the 'R' flag set.  The 6LN that performs the registration (the
   Registering Node) may be the owner of the IPv6 Address (the




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   Registered Node) or a 6LBR that performs the registration on its
   behalf.

5.2.1.  Routing Proxy Operations

   When operating as a Routing Proxy, the BBR installs host routes
   (/128) to the Registered Addresses within the LLN, via the
   Registering Node as identified by the Source Address and the SLLA
   option in the NS(EARO) messages.  In that case, the MAC address of
   the 6LN is not visible at Layer-2 over the backbone.  The 6BBR
   installs a host route towards the Registered Node over the interface
   toward the 6LN, and routes unicast packets to the 6LN.

   The Routing Proxy 6BBR handles the ND protocol over the backbone on
   behalf of the Registered Nodes, using its own MAC address in the TLLA
   and SLLA options in proxied NS and NA messages.  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.

   For each Registered Address, 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 different
   6BBR, the first 6BBR SHOULD unicast a gratuitous NA(Override) to each
   such peer, to supply the MAC address of the new 6BBR in the TLLA
   option for the Address.

5.2.2.  Bridging Proxy Operations

   A Bridging Proxy can be implemented in a Layer-3 switch, or in a
   wireless Access Point that acts as an IPv6 Host.  In the latter case,
   the SLLA option in the proxied NA messages is that of the Registering
   Node, and the 6BBR acts as a Layer-2 bridge for unicast packets to
   the Registered Address.  The MAC address in the S/TLLA is that of the
   Registering Node, which is not necessarily the Registered Node.  When
   a 6LN moves within a LLN mesh, it may attach to a different 6LBR
   acting as Registering Node, and the MAC address advertised over the
   backbone might change.

   If a registration moves from one 6BBR to the next, but the
   Registering Node does not change, as indicated by the S/TLLA option
   in the ND exchanges, there is no need to update the Neighbor Caches
   of the peer's Nodes on the backbone.  On the other hand, if the LLA
   changes, the 6BBR SHOULD inform all the relevant peers as described
   above, to update the affected Neighbor Caches.  In the same fashion,
   if the Registering Node changes with a new registration, the 6BBR
   SHOULD also update the affected Neighbor Caches over the backbone.




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6.  Backbone Router Proxy Operations

   The LLNs attached to each 6BBR are considered different Links in a
   multi-link subnet.  The prefix that is used may still be advertised
   as on-link on the backbone to support legacy 6LNs.  Multicast ND
   messages are link-scoped and not forwarded across the backbone
   routers.

   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.

   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.





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6.1.  Primary and Secondary BBRs

   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
   [I-D.ietf-6lo-rfc6775-update] 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
   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




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      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
   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




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       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
      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 5.1,
      by unicasting to each such peer, or one broadcast NA(Override).




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   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.

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.






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   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

   Kudos to Eric Levy-Abegnoli who designed the First Hop Security
   infrastructure at Cisco.

12.  References

12.1.  Normative References

   [I-D.ietf-6lo-rfc6775-update]
              Thubert, P., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for 6LoWPAN Neighbor
              Discovery", draft-ietf-6lo-rfc6775-update-21 (work in
              progress), June 2018.

   [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>.




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   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [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.





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   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-15 (work
              in progress), October 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-02 (work
              in progress), August 2018.

   [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>.







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   [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>.

   [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>.

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

   [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>.

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".









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   [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)".

   [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.  Changes from revision 07 to revision 08

   This section lists the changes between draft-ietf-6lo-backbone-router
   revisions ...-07.txt and ...-08.txt.

   o  Reorganized the order of presentation of some sections so that
      related material is closer together.

   o  Added "Future Work" section.

   o  Added this section detailing recent changes.

   o  Used '6LN' when LLN node is meant.

   o  Updated bibliographic citations.

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

































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