6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     cisco
Intended status: Standards Track                        January 11, 2017
Expires:                           July 15, 17, 2017
Expires: January 18, 2018

                          IPv6 Backbone Router


   This specification proposes an update to IPv6 Neighbor Discovery, to
   enhance the operation of IPv6 over wireless links that exhibit lossy
   multicast support, and enable a large degree of scalability by
   splitting the broadcast domains.  A higher speed broadcast-efficient backbone
   running classical IPv6 Neighbor Discovery federates multiple wireless
   links to form a large MultiLink Subnet. Subnet, but the broadcast domain does
   not need to extend to the wireless links for the purpose of ND
   operation.  Backbone Routers acting as Layer-3 Access Point placed at the wireless edge of the
   backbone proxy the ND operation and route packets to from/to registered
   nodes, where an classical Layer-2 Access Point would bridge.
   Conversely, and wireless nodes register or are proxy-registered to the
   Backbone Router to setup routing proxy services in a fashion that is
   essentially similar to a classical 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 http://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 July 15, 2017. January 18, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Applicability and Requirements Served . . . . . . . . . . . .   5   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   8   7
   5.  Backbone Router Routing Operations  . . . . . . . . . . . . .  10   9
     5.1.  Over the Backbone Link  . . . . . . . . . . . . . . . . .  10
     5.2.  Over the LLN Link . . . . . . . . . . . . . . . . . . . .  12  11
   6.  BackBone Router Proxy Operations  . . . . . . . . . . . . . .  13
     6.1.  Registration and Binding State Creation . . . . . . . . .  16  15
     6.2.  Defending Addresses . . . . . . . . . . . . . . . . . . .  17  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  Protocol Constants  . . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19  18
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
     11.3.  External Informative References  . . . . . . . . . . . .  24  23
   Appendix A.  Requirements . . . . . . . . . . . . . . . . . . . .  24
     A.1.  Requirements Related to Mobility  . . . . . . . . . . . .  24
     A.2.  Requirements Related to Routing Protocols . . . . . . . .  25
     A.3.  Requirements Related to the Variety of Low-Power Link
           types . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     A.4.  Requirements Related to Proxy Operations  . . . . . . . .  26
     A.5.  Requirements Related to Security  . . . . . . . . . . . .  27
     A.6.  Requirements Related to Scalability . . . . . . . . . . .  28
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  28  29

1.  Introduction

   Classical IPv6 Neighbor Discovery [RFC4862] operations are reactive

   One of the key services provided by IEEE std. 802.1 [IEEEstd8021]
   Ethernet Bridging is an efficient and rely reliable broadcast service, and
   multiple applications and protocols have been built that heavily
   depends on multicast operations to locate a correspondent.
   When this was designed, it was a natural match that feature for their core operation.  But a wide range
   of wireless networks do not provide the transparent
   bridging operation solid and cheap broadcast
   capabilities of Ethernet.  Access Points defined by Ethernet Bridging, and protocols designed for bridged
   networks that rely on broadcast often exhibit disappointing
   behaviours when applied unmodified to a wireless medium.

   IEEE std std. 802.11 [IEEEstd80211] Access Points (APs) deployed in an
   Extended Service Set (ESS) effectively act as bridges, but, in order
   to ensure a solid connectivity to the devices and protect the medium, medium
   against harmful broadcasts, they do not implement transparent bridging. refrain from relying on broadcast-
   intensive protocols such as Transparent Bridging on the wireless
   side.  Instead, a so-called an association process is used to register
   proactively the MAC addresses of the wireless STAs device (STA) to the APs.
   Sadly, AP,
   and then the IPv6 ND APs proxy the bridging operation was not adapted to match that model.

   Though in most cases, including Low-Power ones, IEEE std 802.11 is
   operated as a wireless extension to an Ethernet bridged domain, and cancel the
   impact of radio broadcasts for

   Classical IPv6 [RFC2460] [RFC8200] Neighbor Discovery [RFC4862] Protocol (NDP)
   operations are reactive and rely heavily on multicast operations,
   in particular related operations to
   locate an on-link correspondent and ensure address uniqueness, which
   is a pillar that sustains the power consumption of battery-operated
   devices, lead whole IP architecture.  When the community to rethink the plain layer-2 approach and
   consider splitting
   Duplicate Address Detection [RFC4862] (DAD) mechanism was designed,
   it was a natural match with the efficient broadcast domain between the wired and operation of
   Ethernet Bridging, but with the
   wireless access links.  To unreliable broadcast that effect, the current IEEE std 802.11
   specifications require is typical
   of wireless media, DAD is bound to fail to discover duplications
   [I-D.yourtchenko-6man-dad-issues].  In other words, because the capability
   broadcast service is unreliable, DAD appears to perform ARP work on wireless
   media not because address duplication is detected and ND proxy
   [RFC4389] functions at the Access Points (APs), solved as
   designed, but rely on snooping
   for acquiring because the related state, which duplication is unsatisfactory in a lossy
   and mobile environments.

   Without a proxy, any IP multicast that circulates in the bridged
   domain ends up broadcasted by the Access Points to all STAs,
   including Low-Power battery-operated ones.  With an incorrect or
   missing state in the proxy, very rare event as a packet may not be delivered to the
   destination, which may have operational impacts depending on the
   criticality side
   effect of the packet.

   Some messages are lost for the lack of retries, regardless sheer amount of their
   degree of criticality; it results for instance that Duplicate Address
   Detection (DAD) as defined entropy in [RFC4862] is mostly broken over Wi-Fi

   On 64-bits Interface IDs.

   In the other hand, real world, IPv6 multicast messages are effectively broadcast,
   so they are processed by most if not all wireless nodes over the ESS
   fabric even when very few if any of the nodes is effectively
   listening to the multicast address.  It results that a simple
   Neighbor Solicitation (NS) lookup message [RFC4861], that is
   supposedly targeted to a very small group of nodes, ends up polluting
   the whole wireless bandwidth across the fabric

   It appears that  In other words, the reactive
   IPv6 ND operation leads to undesirable power consumption in a variety battery-
   operated devices.

   The inefficiencies of Wireless Local Area Networks (WLANs)
   and Wireless Personal Area Networks (WPANs), the decision using radio broadcasts to leverage
   the broadcast support of a particular link should be left IPv6 NDP lead
   the community to Layer-3
   based on consider (again) splitting the criticality of broadcast domain
   between the message wired and the number of interested
   listeners on that link, for the lack of capability to indicate that
   criticality wireless access links.  One classical way
   to the lower layer.  To achieve this, this is to split the operation subnet in multiple ones, and at the Access Point cannot be a Layer-2 bridge operation, but that of
   extreme provide a /64 per wireless device.  Another is to proxy the
   Layer-3 router; protocols that rely on broadcast operation at the concept boundary of MultiLink Subnet (MLSN) must be
   reintroduced, with IPv6 backbone routers (6BBRs) interconnecting
   various links and routing within the subnet.  For link-scope
   multicast operations, a 6BBR participates to MLD on its access links wired and a multicast routing protocol is setup between the 6BBRs over the
   backbone of wireless domains, effectively emulating the MLSN.

   As Layer-2
   association at layer-3.  To that effect, the network scales up, none of current IEEE std. 802.11
   specifications require the approaches of using either
   broadcast or N*unicast for a multicast packet is really satisfying capability to perform ARP and ND proxy
   [RFC4389] functions at the protocols themselves need to be adapted to reduce their use
   of multicast.

   One degree of improvement can be achieved by changing Access Points (APs).

   But for the tuning of lack a comprehensive specification for the protocol parameters ND proxy and operational practices, such as suggested
   in Reducing energy consumption of Router Advertisements [RFC7772]
   (RA).  This works enables to lower particular the rate lack of RA messages but does
   not solve an equivalent to an association process,
   implementations have to rely on snooping for acquiring the problem associated with multicast NS and NA messages, related
   state, which are a lot more frequent is unsatisfactory in large-scale radio environments with
   mobile devices which exhibit intermittent access patterns a lossy and short-
   lived mobile conditions.
   With snooping, a state (e.g. a new IPv6 addresses. address) may not be
   discovered or a change of state (e.g. a movement) may be missed,
   leading to unreliable connectivity.

   In the context of IEEE std std. 802.15.4 [IEEEstd802154], the more drastic step of
   considering the radio as a medium that is different from Ethernet because of the impact of multicast, was
   already taken with the adoption publication of Neighbor Discovery Optimization
   for IPv6 over Low-
   Power Low-Power Wireless Personal Area Networks (6LoWPANs)
   [RFC6775].  RFC 6775 is updated as [I-D.ietf-6lo-rfc6775-update]; the
   update includes changes that are required by this document.

   This specification applies that same thinking to other wireless links
   such as Low-Power IEEE std std. 802.11 (Wi-Fi) and IEEE std std. 802.15.1
   (Bluetooth) [IEEEstd802151], and extends [RFC6775] to enable proxy
   operation by the 6BBR so as to decouple the broadcast domain in the
   backbone from the wireless links.  The proxy operation can be
   maintained asynchronous so that low-power nodes or nodes that are
   deep in a mesh do not need to be bothered synchronously when a lookup
   is performed for their addresses, effectively implementing the ND
   contribution to the concept of a Sleep Proxy

   RFC 6775 is updated as [I-D.ietf-6lo-rfc6775-update]; the update
   includes changes that are required by this document, so it is a

   DHCPv6 [RFC3315] is still a viable option in Low power and Lossy
   Network (LLN) to assign IPv6 global addresses.  However, the IETF
   standard that supports address assignment specifically for LLNs is
   6LoWPAN ND [RFC6775], which is a mix of IPv6 stateless
   autoconfiguration mechanism (SLAAC) [RFC4862] and a new registration
   process for ND.  This specification introduces a Layer-3 association
   process based on 6LoWPAN ND that maintains a proxy state in the 6BBR
   to keep the LLN nodes reachable and protect their addresses through
   sleeping periods.

   A number of use cases, including the Industrial Internet, require a
   large scale deployment of monitoring sensors that can only be
   realized in a cost-effective fashion with wireless technologies.
   Mesh networks are deployed when simpler hub-and-spoke topologies are
   not sufficient for the expected size, throughput, and density.
   Meshes imply the routing of packets, operated at either Layer-2 or
   Layer-3.  For routing over a mesh at Layer-3, the IETF has designed
   the IPv6 Routing Protocol over LLN (RPL) [RFC6550].  6LoWPAN ND was
   designed as a stand-alone mechanism separately from RPL, and the
   interaction between the 2 protocols was not defined.  This
   specification details how periodic updates from RPL can be used by
   the RPL root to renew the association of the RPL node to the 6BBR on
   its behalf so as to maintain the proxy operation active for nodes or nodes that

   This document suggests are
   deep in a limited evolution to [RFC6775] so as mesh do not need to
   allow operation of a 6LoWPAN ND node while be bothered synchronously when a routing protocol (in
   first instance RPL) lookup
   is present and operational.  It also suggests a
   more generalized use of the information in the ARO option of performed for their addresses, effectively implementing the ND
   messages outside
   contribution to the strict LLN domain, for instance over concept of a
   federating backbone. Sleep Proxy

2.  Applicability and Requirements Served

   Efficiency aware IPv6 Neighbor Discovery Optimizations
   [I-D.chakrabarti-nordmark-6man-efficient-nd] suggests that 6LoWPAN ND
   [RFC6775] can be extended to other types of links beyond IEEE std std.
   802.15.4 for which it was defined.  The registration technique is
   beneficial when the Link-Layer technique used to carry IPv6 multicast
   packets is not sufficiently efficient in terms of delivery ratio or
   energy consumption in the end devices, in particular to enable
   energy-constrained sleeping nodes.  The value of such extension is
   especially apparent in the case of mobile wireless nodes, to reduce
   the multicast operations that are related to classical ND ([RFC4861],
   [RFC4862]) and plague the wireless medium.

   This specification updates and generalizes 6LoWPAN ND to a broader
   range of Low power and Lossy Networks (LLNs) with a solid 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 std.
   802.11AH and IEEE std std. 802.15.4 wireless meshes, so as to address the
   requirements listed in Appendix A.3
   The scope of this draft is a Backbone Link that federates 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 over the Backbone Link and emulate that the LLN
   nodes are present on the Backbone using proxy-ND operations.  This
   specification extends IPv6 ND over the backbone to discriminate
   address movement from duplication and eliminate stale state in the
   backbone routers and backbone nodes once a LLN node has roamed.  This
   way, mobile nodes may roam rapidly from a 6BBR to the next and
   requirements in Appendix A.1 are met.

   This specification can be used by any wireless node to associate at
   Layer-3 with a 6BBR and register its IPv6 addresses to obtain routing
   services including proxy-ND operations over the backbone, effectively
   providing a solution to the requirements expressed in Appendix A.4.

   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 which 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).  In the
   latter case, the 6BBR may maintain the list of correspondents to
   which it has advertised its own MAC address on behalf of the LLN node
   and the IPv6 ND operation is minimized as the number of nodes scale
   up in the LLN.  This enables to meet the requirements in Appendix A.6
   as long has the 6BBRs are dimensioned for the number of registration
   that each needs to support.

   In the context of the the TimeSlotted Channel Hopping (TSCH) mode of
   [IEEEstd802154], the 6TiSCH architecture
   [I-D.ietf-6tisch-architecture] introduces how a 6LoWPAN ND host could
   connect to the Internet via a RPL mesh Network, but this requires
   additions to the 6LOWPAN ND protocol to support mobility and
   reachability in a secured and manageable environment.  This
   specification details the new operations that are required to
   implement the 6TiSCH architecture and serves the requirements listed
   in Appendix A.2.

   In the case of Low-Power IEEE std std. 802.11, a 6BBR may be collocated
   with a standalone AP or a CAPWAP [RFC5415] wireless controller, and
   the wireless client (STA) leverages this specification to register
   its IPv6 address(es) to the 6BBR over the wireless medium.  In the
   case of a 6TiSCH LLN mesh, 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 leverages this specification to
   register the LLN nodes on their behalf to the 6BBR.  In the case of
   BTLE, the 6BBR is collocated with the router that implements the BTLE
   central role as discussed in section 2.2 of [RFC7668].

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in "Neighbor Discovery for IP version 6"
   [RFC4861], "IPv6 Stateless Address Autoconfiguration" [RFC4862],
   "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
   Overview, Assumptions, Problem Statement, and Goals" [RFC4919],
   Neighbor Discovery Optimization for Low-power and Lossy Networks
   [RFC6775] and "Multi-link Subnet Support in IPv6"

   Readers would benefit from reading "Multi-Link Subnet Issues"
   [RFC4903], ,"Mobility Support in IPv6" [RFC6275], "Neighbor Discovery
   Proxies (ND Proxy)" [RFC4389] and "Optimistic Duplicate Address
   Detection" [RFC4429] prior to this specification for a clear
   understanding of the art in ND-proxying and binding.

   Additionally, this document uses terminology from [RFC7102],
   [I-D.ietf-6lo-rfc6775-update] and [I-D.ietf-6tisch-terminology], 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 whenever possible.  This is the default mode
         for this specification but it may be overridden, for instance
         by configuration, into Unicasting Proxy.

   Unicasting  Proxy  As a Unicasting Proxy, the 6BBR forwards NS
         messages to the Registering Node, transforming Layer-2
         multicast into unicast whenever possible.

   Routing proxy  A 6BBR acts as a routing proxy if it advertises its
         own MAC address, as opposed to that of the node that performs
         the registration, as the TLLA in the proxied NAs over the
         backbone.  In that case, the MAC address of the node is not
         visible at Layer-2 over the backbone and the bridging fabric is
         not aware of the addresses of the LLN devices and their
         mobility.  The 6BBR installs a connected host route towards the
         registered node over the interface to the node, and acts as a
         Layer-3 router for unicast packets to the node.  The 6BBR
         updates the ND Neighbor Cache Entries (NCE) in correspondent
         nodes if the wireless node moves and registers to another 6BBR,
         either with a single broadcast, or with a series of unicast
         NA(O) messages, indicating the TLLA of the new router.

   Bridging proxy  A 6BBR acts as a bridging proxy if it 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 the node is still visible
         across the bridged backbone fabric, as is traditionally the
         case with Layer-2 APs.  The 6BBR acts as a Layer-2 bridge for
         unicast packets to the registered node.  The MAC address
         exposed in the S/TLLA is that of the Registering Node, which is
         not necessarily the Registered Device.  When a device moves
         within a LLN mesh, it may end up attached to a different 6LBR
         acting as Registering Node, and the LLA that is exposed over
         the backbone will change.

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

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

4.  Overview

   An LLN node can move freely from an LLN anchored at a Backbone Router
   to an LLN anchored at another Backbone Router on the same backbone
   and conserve any of the IPv6 addresses that it has formed,

               |     | Other (default) Router
               |     |
                  |      Backbone Link
            |                    |                  |
         +-----+             +-----+             +-----+
         |     | Backbone    |     | Backbone    |     | Backbone
         |     | router      |     | router      |     | router
         +-----+             +-----+             +-----+
            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

   The Backbone Routers maintain an abstract Binding Table of their
   Registered Nodes.  The Binding Table operates as a distributed
   database of all the wireless Nodes whether they reside on the LLNs or
   on the backbone, and use an extension to the Neighbor Discovery
   Protocol to exchange that information across the Backbone in the
   classical ND reactive fashion.

   The Extended Address Registration Option (ARO) defined in
   [I-D.ietf-6lo-rfc6775-update] is used to enable the registration for
   routing and proxy Neighbor Discovery operations by the 6BBR, and the
   Extended ARO (EARO) option is included in the ND exchanges over the
   backbone between the 6BBRs to sort out duplication from movement.

   Address duplication is sorted out with the Owner Unique-ID field in
   the EARO, which is a generalization of the EUI-64 that allows
   different types of unique IDs beyond the name space derived from the
   MAC addresses.  First-Come First-Serve rules apply, whether the
   duplication happens between LLN nodes as represented by their
   respective 6BBRs, or between an LLN node and a classical node that
   defends its address over the backbone with classical ND and does not
   include the EARO option.

   In case of conflicting registrations to multiple 6BBRs from a same
   node, a sequence counter called Transaction ID (TID) is introduced
   that enables 6BBRs to sort out the latest anchor for that node.
   Registrations with a same TID are compatible and maintained, but, in
   case of different TIDs, only the freshest registration is maintained
   and the stale state is eliminated.

   With this specification, Backbone Routers perform ND proxy over the
   Backbone Link on behalf of their Registered Nodes.  The Backbone
   Router operation is essentially similar to that of a Mobile IPv6
   (MIPv6) [RFC6275] Home Agent.  This enables mobility support for LLN
   nodes that would move outside of the network delimited by the
   Backbone link attach to a Home Agent from that point on.  This also
   enables collocation of Home Agent functionality within Backbone
   Router functionality on the same backbone interface of a router.
   Further specification may extend this be allowing the 6BBR to
   redistribute host routes in routing protocols that would operate over
   the backbone, or in MIPv6 or the Locator/ID Separation Protocol
   (LISP) [RFC6830] to support mobility on behalf of the nodes, etc...

   The Optimistic Duplicate Address Detection [RFC4429] (ODAD)
   specification details how an address can be used before a Duplicate
   Address Detection (DAD) is complete, and insists that an address that
   is TENTATIVE should not be associated to a Source Link-Layer Address
   Option in a Neighbor Solicitation message.  This specification
   leverages ODAD to create a temporary proxy state in the 6BBR till DAD
   is completed over the backbone.  This way, the specification enables
   to distribute proxy states across multiple 6BBR and co-exist with
   classical ND over the backbone.

5.  Backbone Router Routing Operations
               |     | Other (default) Router
               |     |
                  | /64
                  |      Backbone Link
            | /64               | /64               | /64
         +-----+             +-----+             +-----+
         |     | Backbone    |     | Backbone    |     | Backbone
         |     | router      |     | router      |     | router
         +-----+             +-----+             +-----+
            o N*/128       o o  o M*/128          o o P*/128
        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 2: Routing Configuration in the ML Subnet

5.1.  Over the Backbone Link

   The Backbone Router is a specific kind of Border Router that performs
   proxy Neighbor Discovery on its backbone interface on behalf of the
   nodes that it has discovered on its LLN interfaces.

   The backbone is expected to be a high speed, reliable Backbone link,
   with affordable and reliable multicast capabilities, such as a
   bridged Ethernet Network, and to allow a full support of classical ND
   as specified in [RFC4861] and subsequent RFCs.  In other words, the
   backbone is not a LLN.

   Still, some restrictions of the attached LLNs will apply to the
   backbone.  In particular, it is expected that the MTU is set to the
   same value on the backbone and all attached LLNs, and the scalability
   of the whole subnet requires that broadcast operations are avoided as
   much as possible on the backbone as well.  Unless configured
   otherwise, the Backbone Router MUST echo the MTU that it learns in
   RAs over the backbone in the RAs that it sends towards the LLN links.

   As a router, the Backbone Router behaves like any other IPv6 router
   on the backbone side.  It has a connected route installed towards the
   backbone for the prefixes that are present on that backbone and that
   it proxies for on the LLN interfaces.

   As a proxy, the 6BBR uses an EARO option in the NS-DAD and the
   multicast NA messages that it generates on behalf of a Registered
   Node, and it places an EARO in its unicast NA messages if and only if
   the NS/NA that stimulates it had an EARO in it.

   When possible, the 6BBR SHOULD use unicast or solicited-node
   multicast address (SNMA) [RFC4291] to defend its Registered Addresses
   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 as long as it maintains that entry,
   whatever the state of the entry.  The expectation is that it is
   possible to get a message delivered to all the nodes on the backbone
   that listen to a particular address and support this specification -
   which includes all the 6BBRs in the MultiLink Subnet - by sending a
   multicast message to the associated SNMA over the backbone.

   The support of Optimistic DAD (ODAD) [RFC4429] is recommended for all
   nodes in the backbone and followed by the 6BBRs in their proxy
   activity over the backbone.  With ODAD, any optimistic node MUST join
   the SNMA of a Tentative address, which interacts better with this

   This specification allows the 6BBR in Routing Proxy mode to advertise
   the Registered IPv6 Address with the 6BBR Link Layer Address, and
   attempts to update Neighbor Cache Entries (NCE) in correspondent
   nodes over the backbone, using gratuitous NA(Override).  This method
   may fail of the multicast message is not properly received, and
   correspondent nodes may maintain an incorrect neighbor state, which
   they will eventually discover through Neighbor Unreachability
   Detection (NUD).  Because mobility may be slow, the NUD procedure
   defined in [RFC4861] may be too impatient, and the support of
   [RFC7048] is recommended in all nodes in the network.

   Since the MultiLink Subnet may grow very large in terms of individual
   IPv6 addresses, multicasts should be avoided as much as possible even
   on the backbone.  Though it is possible for plain hosts to
   participate with legacy IPv6 ND support, the support by all nodes
   connected to the backbone of [I-D.ietf-6man-rs-refresh] is
   recommended, and this implies the support of [RFC7559] as well.

5.2.  Over the LLN Link

   As a router, the Nodes and Backbone Router operation on the LLN
   follows [RFC6775].  Per that specification, LLN Hosts generally do
   not depend on multicast RAs to discover routers.  It is still
   generally required for LLN nodes to accept multicast RAs [RFC7772],
   but those are rare on the LLN link.  Nodes are expected to follow the
   Simple Procedures for Detecting Network Attachment in IPv6 [RFC6059]
   (DNA procedures) to assert movements, and to support the Packet-Loss
   Resiliency for Router Solicitations [RFC7559] to make the unicast RS
   more reliable.

   The Backbone Router acquires its states about the addresses on the
   LLN side through a registration process from either the nodes
   themselves, or from a node such as a RPL root / 6LBR (the Registering
   Node) that performs the registration on behalf of the address owner
   (the Registered Node).

   When operating as a Routing Proxy, the router installs hosts routes
   (/128) to the Registered Addresses over the LLN links, via the
   Registering Node as identified by the Source Address and the SLLAO
   option in the NS(EARO) messages.

   In that mode, the 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 proxyed NS and NA messages.  It results that for
   each Registered Address, a number of peer Nodes on the backbone have
   resolved the address with the 6BBR MAC address and keep that mapping
   stored in their Neighbor cache.

   The 6BBR SHOULD maintain, per Registered Address, the list of the
   peers on the backbone to which it answered with it MAC address, and
   when a binding moves to a different 6BBR, it SHOULD send a unicast
   gratuitous NA(O) individually to each of them to inform them that the
   address has moved and pass the MAC address of the new 6BBR in the
   TLLAO option.  If the 6BBR can not maintain that list, then it SHOULD
   remember whether that list is empty or not and if not, send a
   multicast NA(O) to all nodes to update the impacted Neighbor Caches
   with the information from the new 6BBR.

   The Bridging Proxy is a variation where the BBR function is
   implemented in a Layer-3 switch or an wireless Access Point that acts
   as a Host from the IPv6 standpoint, and, in particular, does not
   operate the routing of IPv6 packets.  In that case, the SLLAO in the
   proxied NA messages is that of the Registering Node and classical
   bridging operations take place on data frames.

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

6.  BackBone Router Proxy Operations

   This specification enables a Backbone Router to proxy Neighbor
   Discovery operations over the backbone on behalf of the nodes that
   are registered to it, allowing any node on the backbone to reach a
   Registered Node as if it was on-link.  The backbone and the LLNs are
   considered different Links in a MultiLink subnet but the prefix that
   is used may still be advertised as on-link on the backbone to support
   legacy nodes; multicast ND messages are link-scoped and not forwarded
   across the backbone routers.

   ND Messages on the backbone side that do not match to a registration
   on the LLN side are not acted upon on the LLN side, which stands
   protected.  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.

   The default operation in this specification is Sleeping proxy which

   o  creating a new entry in an abstract Binding Table for a new
      Registered Address and validating that the address is not a
      duplicate over the backbone

   o  defending a Registered Address over the backbone using NA messages
      with the Override bit set on behalf of the sleeping node whenever

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

   o  Looking up a destination over the backbone in order to deliver
      packets arriving from the LLN using Neighbor Solicitation

   o  Forwarding packets from the LLN over the backbone, and the other
      way around.

   o  Eventually triggering a liveliness verification of a stale

   A 6BBR may act as a Sleeping Proxy only if the state of the binding
   entry is REACHABLE, or TENTATIVE in which case the answer is delayed.
   In any other state, the Sleeping Proxy operates as a Unicasting

   As a Unicasting Proxy, the 6BBR forwards NS messages to the
   Registering Node, transforming Layer-2 multicast into unicast
   whenever possible.  This is not possible in UNREACHABLE state, so the
   NS messages are multicasted, and rate-limited to protect the medium
   with an exponential back-off.  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

   The draft introduces the optional concept of primary and secondary
   BBRs.  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 Owner Unique ID and same
   Transaction ID, the EUI-64 address being considered as an unsigned
   64bit integer.  The concept is defined with the granularity of an
   address, that is a given 6BBR can be primary for a given address and
   secondary or another one, regardless on whether the addresses belong
   to the same node or not.  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 LLN node, and
   the address appears on the backbone as an anycast address.

   The Backbone Routers maintain a distributed binding table, using
   classical ND over the backbone to detect duplication.  This
   specification requires that:

   1.  All addresses that can be reachable from the backbone, including
       IPv6 addresses based on burn-in EUI64 addresses MUST be
       registered to the 6BBR.

   2.  A Registered Node MUST include the EARO option in an NS message
       that used to register an addresses to a 6LR; the 6LR MUST
       propagate that option unchanged to the 6LBR in the DAR/DAC
       exchange, and the 6LBR MUST propagate that option unchanged in
       proxy registrations.

   3.  The 6LR MUST echo the same EARO option in the NA that it uses to
       respond, but for the status filed which is not used in NS
       messages, and significant in NA.

   A false positive duplicate detection may arise over the backbone, for
   instance if the Registered Address is registered to more than one
   LBR, or if the node has moved.  Both situations are handled
   gracefully unbeknownst to the node.  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 back and forth.

   In the latter case the LBR that receives the newest registration wins
   and becomes primary.

   The expectation in this specification is that there is a single
   Registering Node at a time per Backbone Router for a given Registered
   Address, but that a Registered Address may be registered to Multiple
   6BBRs for higher availability.

   Over the LLN, and for any given Registered Address, it is REQUIRED

      de-registrations (newer TID, same OUID, null Lifetime) are
      accepted and responded immediately with a status of 4; the entry
      is deleted;

      newer registrations (newer TID, same OUID, non-null Lifetime) are
      accepted and responded with a status of 0 (success); the entry is
      updated with the new TID, the new Registration Lifetime and the
      new Registering Node, if any has changed; in TENTATIVE state the
      response is held and may be overwritten; in other states the
      Registration-Lifetime timer is restarted and the entry is placed
      in REACHABLE state.

      identical registrations (same TID, same OUID) from a same
      Registering Node are not processed but responded with a status of
      0 (success); they are expected to be identical and an error may be
      logged if not; 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 (not(newer or equal) TID, same OUID) from a
      same Registering Node are ignored;

      identical and older registrations (not-newer TID, same OUID) from
      a different Registering Node are responded immediately with a
      status of 3 (moved); this may be rate limited to protect the

      and any registration for a different Registered Node (different
      OUID) are responded immediately with a status of 1 (duplicate).

6.1.  Registration and Binding State Creation

   Upon a registration for a new address with an NS(EARO), the 6BBR
   performs a DAD operation 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 entry is created in
   TENTATIVE state for a duration of TENTATIVE_DURATION.  The NS-DAD
   message is sent multicast over the backbone to the SNMA address
   associated with the registered address.  If that operation is known
   to be costly, and the 6BBR has an indication from another source
   (such as a NCE) that the Registered Address was present on the
   backbone, that information may be leveraged to send the NS-DAD
   message as a Layer-2 unicast to the MAC that was associated with the
   Registered Address.

   In TENTATIVE state:

   o  the entry is removed if an NA is received over the backbone for
      the Registered Address with no EARO option, or with an EARO option
      with a status of 1 (duplicate) that indicates an existing
      registration for another LLN node.  The OUID and TID fields in the
      EARO option received over the backbone are ignored.  A status of 1
      is returned in the EARO option of the NA back to the Registering

   o  the entry is also removed if an NA with an ARO option with a
      status of 3 (moved), or a NS-DAD 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;

   o  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;

   o  Other NS (including DAD with no EARO option) and NA from the
      backbone are not responded in TENTATIVE state, but the list of
      their origins may be kept in memory and if so, the 6BBR may send
      them each a unicast NA with eventually an EARO option when the
      TENTATIVE_DURATION timer elapses, so as to cover legacy nodes that
      do not support ODAD.

   o  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
      DAD process is successful and 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.

6.2.  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 concept, it MUST
      defend that address over the backbone upon an incoming NS-DAD,
      either if the NS does not carry an EARO, or if an EARO is present
      that indicates a different Registering Node (different OUID).  The
      6BBR sends a NA message with the Override bit set and the NA
      carries an EARO option if and only if the NS-DAD did so.  When
      present, the EARO in the NA(O) that is sent in response to the NS-
      DAD(EARO) carries a status of 1 (duplicate), and the OUID and TID
      fields in the EARO option are obfuscated with null or random
      values to avoid network scanning and impersonation attacks.

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

   o  If the 6BBR receives an NA(EARO) that reflect a newer
      registration, the 6BBR removes its entry and sends a NA(AERO) with
      a status of 3 (moved) to the Registering Node, if the Registering
      Node is different from the Registered Node.  If necessary, the
      6BBR cleans up ND cache in peers nodes as discussed in
      Section 5.1, by sending a series of unicast to the impacted nodes,
      or one broadcast NA(O) to all-nodes.

   o  If the 6BBR received 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

   The STALE state is a chance to keep track of the backbone peers that
   may have an ND cache pointing on this 6BBR in case the Registered
   Address shows back up on this or a different 6BBR at a later time.
   In STALE state:

   o  If the Registered Address is claimed by another node on the
      backbone, with an NS-DAD or an NA, the 6BBR does not defend the
      address.  Upon an NA(O), or the stale time elapses, the 6BBR
      removes its entry and sends a NA(AERO) with a status of 4
      (removed) to the Registering Node.

   o  If the 6BBR received 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 expected to be mapped by the Registering Node
      into a liveliness validation of the Registered Node if they are in
      fact different nodes.

7.  Security Considerations

   This specification expects that the link layer is sufficiently
   protected, either by means of physical or IP security for the
   Backbone Link or MAC sublayer cryptography.  In particular, it is
   expected that the LLN MAC provides secure unicast to/from the
   Backbone Router and secure Broadcast from the Backbone Router in a
   way that prevents tempering 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.  This specification RECOMMENDS the use of
   additional protection against address theft such as provided by
   [I-D.ietf-6lo-ap-nd], which guarantees the ownership of the OUID.

   When the ownership of the OUID cannot be assessed, this specification
   limits the cases where the OUID 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


   DEFAULT_NS_POLLING:        3 times

9.  IANA Considerations

   This document has no request to IANA.

10.  Acknowledgments

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

11.  References

11.1.  Normative References

              Thubert, P., Nordmark, E., and S. Chakrabarti, "An Update
              to 6LoWPAN ND", draft-ietf-6lo-rfc6775-update-00 draft-ietf-6lo-rfc6775-update-06 (work in
              progress), December 2016. June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

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

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,

   [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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,

   [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,

   [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,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

11.2.  Informative References

              Chakrabarti, S., Nordmark, E., Thubert, P., and M.
              Wasserman, "IPv6 Neighbor Discovery Optimizations for
              Wired and Wireless Networks", draft-chakrabarti-nordmark-
              6man-efficient-nd-07 (work in progress), February 2015.

              Vega, L., Robles, I., and R. Morabito, "IPv6 over
              802.11ah", draft-delcarpio-6lo-wlanah-01 (work in
              progress), October 2015.

              Lynn, K., Martocci, J., Neilson, C., and S. Donaldson,
              "Transmission of IPv6 over MS/TP Networks", draft-ietf-
              6lo-6lobac-06 (work in progress), October 2016.

              Sarikaya, B., Thubert, P., and M. Sethi, "Address
              Protected Neighbor Discovery for Low-power and Lossy
              Networks", draft-ietf-6lo-ap-nd-00 (work in progress),
              November 2016.

              Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D.
              Barthel, "Transmission of IPv6 Packets over DECT Ultra Low
              Energy", draft-ietf-6lo-dect-ule-09 draft-ietf-6lo-ap-nd-02 (work in progress),
              December 2016. May

              Choi, Y., Hong, Y., Youn, J., Kim, D., and Y. Hong, J. Choi,
              "Transmission of IPv6 Packets over Near Field
              Communication", draft-ietf-6lo-
              nfc-05 draft-ietf-6lo-nfc-07 (work in progress), October 2016.
              June 2017.

              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.

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-10 draft-ietf-6tisch-architecture-11 (work
              in progress), June 2016. January 2017.

              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-08 draft-ietf-6tisch-terminology-09 (work in
              progress), December 2016. June 2017.

              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-05 draft-ietf-bier-architecture-07 (work in
              progress), October 2016. June 2017.

              Thaler, D. and C. Huitema, "Multi-link Subnet Support in
              IPv6", draft-ietf-ipv6-multilink-subnets-00 (work in
              progress), July 2002.

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

              Popa, D. and J. Hui, "6LoPLC: Transmission of IPv6 Packets
              over IEEE 1901.2 Narrowband Powerline Communication
              Networks", draft-popa-6lo-6loplc-ipv6-over-
              ieee19012-networks-00 (work in progress), March 2014.

              Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
              Yourtchenko, "Why Network-Layer Multicast is Not Always
              Efficient At Datalink Layer", draft-vyncke-6man-mcast-not-
              efficient-01 (work in progress), February 2014.

              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

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <http://www.rfc-editor.org/info/rfc4389>.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,

   [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,

   [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,

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

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

   [RFC7048]  Nordmark, E. and I. Gashinsky, "Neighbor Unreachability
              Detection Is Too Impatient", RFC 7048,
              DOI 10.17487/RFC7048, January 2014,

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

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,

   [RFC7559]  Krishnan, S., Anipko, D., and D. Thaler, "Packet-Loss
              Resiliency for Router Solicitations", RFC 7559,
              DOI 10.17487/RFC7559, May 2015,

   [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,

   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy
              Consumption of Router Advertisements", BCP 202, RFC 7772,
              DOI 10.17487/RFC7772, February 2016,

   [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
              M., and D. Barthel, "Transmission of IPv6 Packets over
              Digital Enhanced Cordless Telecommunications (DECT) Ultra
              Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
              2017, <http://www.rfc-editor.org/info/rfc8105>.

   [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
              Donaldson, "Transmission of IPv6 over Master-Slave/Token-
              Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
              May 2017, <http://www.rfc-editor.org/info/rfc8163>.

11.3.  External Informative References

              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

              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".

              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)".

              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.  Requirements

   This section lists requirements that were discussed at 6lo for an
   update to 6LoWPAN ND.  This specification meets most of them, but
   those listed in Appendix A.5 which are deferred to a different
   specification such as [I-D.ietf-6lo-ap-nd].

A.1.  Requirements Related to Mobility

   Due to the unstable nature of LLN links, even in a LLN of immobile
   nodes a 6LoWPAN Node may change its point of attachment to a 6LR, say
   6LR-a, and may not be able to notify 6LR-a.  Consequently, 6LR-a may
   still attract traffic that it cannot deliver any more.  When links to
   a 6LR change state, there is thus a need to identify stale states in
   a 6LR and restore reachability in a timely fashion.

   Req1.1: Upon a change of point of attachment, connectivity via a new
   6LR MUST be restored timely without the need to de-register from the
   previous 6LR.

   Req1.2: For that purpose, the protocol MUST enable to differentiate
   between multiple registrations from one 6LoWPAN Node and
   registrations from different 6LoWPAN Nodes claiming the same address.

   Req1.3: Stale states MUST be cleaned up in 6LRs.

   Req1.4: A 6LoWPAN Node SHOULD also be capable to register its Address
   to multiple 6LRs, and this, concurrently.

A.2.  Requirements Related to Routing Protocols

   The point of attachment of a 6LoWPAN Node may be a 6LR in an LLN
   mesh.  IPv6 routing in a LLN can be based on RPL, which is the
   routing protocol that was defined at the IETF for this particular
   purpose.  Other routing protocols than RPL are also considered by
   Standard Defining Organizations (SDO) on the basis of the expected
   network characteristics.  It is required that a 6LoWPAN Node attached
   via ND to a 6LR would need to participate in the selected routing
   protocol to obtain reachability via the 6LR.

   Next to the 6LBR unicast address registered by ND, other addresses
   including multicast addresses are needed as well.  For example a
   routing protocol often uses a multicast address to register changes
   to established paths.  ND needs to register such a multicast address
   to enable routing concurrently with discovery.

   Multicast is needed for groups.  Groups MAY be formed by device type
   (e.g. routers, street lamps), location (Geography, RPL sub-tree), or

   The Bit Index Explicit Replication (BIER) Architecture
   [I-D.ietf-bier-architecture] proposes an optimized technique to
   enable multicast in a LLN with a very limited requirement for routing
   state in the nodes.

   Related requirements are:

   Req2.1: The ND registration method SHOULD be extended in such a
   fashion that the 6LR MAY advertise the Address of a 6LoWPAN Node over
   the selected routing protocol and obtain reachability to that Address
   using the selected routing protocol.

   Req2.2: Considering RPL, the Address Registration Option that is used
   in the ND registration SHOULD be extended to carry enough information
   to generate a DAO message as specified in [RFC6550] section 6.4, in
   particular the capability to compute a Path Sequence and, as an
   option, a RPLInstanceID.

   Req2.3: Multicast operations SHOULD be supported and optimized, for
   instance using BIER or MPL.  Whether ND is appropriate for the
   registration to the 6BBR is to be defined, considering the additional
   burden of supporting the Multicast Listener Discovery Version 2
   [RFC3810] (MLDv2) for IPv6.

A.3.  Requirements Related to the Variety of Low-Power Link types

   6LoWPAN ND [RFC6775] was defined with a focus on IEEE std std. 802.15.4
   and in particular the capability to derive a unique Identifier from a
   globally unique MAC-64 address.  At this point, the 6lo Working Group
   is extending the 6LoWPAN Header Compression (HC) [RFC6282] technique
   to other link types ITU-T G.9959 [RFC7428], Master-Slave/Token-
   Passing [I-D.ietf-6lo-6lobac], [RFC8163], DECT Ultra Low Energy
   [I-D.ietf-6lo-dect-ule], [RFC8105], Near Field
   Communication [I-D.ietf-6lo-nfc], IEEE std std. 802.11ah
   [I-D.delcarpio-6lo-wlanah], as well as IEEE1901.2 Narrowband
   Powerline Communication Networks
   [I-D.popa-6lo-6loplc-ipv6-over-ieee19012-networks] and BLUETOOTH(R)
   Low Energy [RFC7668].

   Related requirements are:

   Req3.1: The support of the registration mechanism SHOULD be extended
   to more LLN links than IEEE 802.15.4, matching at least the LLN links
   for which an "IPv6 over foo" specification exists, as well as Low-
   Power Wi-Fi.

   Req3.2: As part of this extension, a mechanism to compute a unique
   Identifier should be provided, with the capability to form a Link-
   Local Address that SHOULD be unique at least within the LLN connected
   to a 6LBR discovered by ND in each node within the LLN.

   Req3.3: The Address Registration Option used in the ND registration
   SHOULD be extended to carry the relevant forms of unique Identifier.

   Req3.4: The Neighbour Discovery should specify the formation of a
   site-local address that follows the security recommendations from

A.4.  Requirements Related to Proxy Operations

   Duty-cycled devices may not be able to answer themselves to a lookup
   from a node that uses classical ND on a backbone and may need a
   proxy.  Additionally, the duty-cycled device may need to rely on the
   6LBR to perform registration to the 6BBR.

   The ND registration method SHOULD defend the addresses of duty-cycled
   devices that are sleeping most of the time and not capable to defend
   their own Addresses.

   Related requirements are:

   Req4.1: The registration mechanism SHOULD enable a third party to
   proxy register an Address on behalf of a 6LoWPAN node that may be
   sleeping or located deeper in an LLN mesh.

   Req4.2: The registration mechanism SHOULD be applicable to a duty-
   cycled device regardless of the link type, and enable a 6BBR to
   operate as a proxy to defend the registered Addresses on its behalf.

   Req4.3: The registration mechanism SHOULD enable long sleep
   durations, in the order of multiple days to a month.

A.5.  Requirements Related to Security

   In order to guarantee the operations of the 6LoWPAN ND flows, the
   spoofing of the 6LR, 6LBR and 6BBRs roles should be avoided.  Once a
   node successfully registers an address, 6LoWPAN ND should provide
   energy-efficient means for the 6LBR to protect that ownership even
   when the node that registered the address is sleeping.

   In particular, the 6LR and the 6LBR then should be able to verify
   whether a subsequent registration for a given Address comes from the
   original node.

   In a LLN it makes sense to base security on layer-2 security.  During
   bootstrap of the LLN, nodes join the network after authorization by a
   Joining Assistant (JA) or a Commissioning Tool (CT).  After joining
   nodes communicate with each other via secured links.  The keys for
   the layer-2 security are distributed by the JA/CT.  The JA/CT can be
   part of the LLN or be outside the LLN.  In both cases it is needed
   that packets are routed between JA/CT and the joining node.

   Related requirements are:

   Req5.1: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for
   the 6LR, 6LBR and 6BBR to authenticate and authorize one another for
   their respective roles, as well as with the 6LoWPAN Node for the role
   of 6LR.

   Req5.2: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for
   the 6LR and the 6LBR to validate new registration of authorized
   nodes.  Joining of unauthorized nodes MUST be impossible.

   Req5.3: 6LoWPAN ND security mechanisms SHOULD lead to small packet
   sizes.  In particular, the NS, NA, DAR and DAC messages for a re-
   registration flow SHOULD NOT exceed 80 octets so as to fit in a
   secured IEEE std std. 802.15.4 frame.

   Req5.4: Recurrent 6LoWPAN ND security operations MUST NOT be
   computationally intensive on the LoWPAN Node CPU.  When a Key hash
   calculation is employed, a mechanism lighter than SHA-1 SHOULD be

   Req5.5: The number of Keys that the 6LoWPAN Node needs to manipulate
   SHOULD be minimized.

   Req5.6: The 6LoWPAN ND security mechanisms SHOULD enable CCM* for use
   at both Layer 2 and Layer 3, and SHOULD enable the reuse of security
   code that has to be present on the device for upper layer security
   such as TLS.

   Req5.7: Public key and signature sizes SHOULD be minimized while
   maintaining adequate confidentiality and data origin authentication
   for multiple types of applications with various degrees of

   Req5.8: Routing of packets should continue when links pass from the
   unsecured to the secured state.

   Req5.9: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for
   the 6LR and the 6LBR to validate whether a new registration for a
   given address corresponds to the same 6LoWPAN Node that registered it
   initially, and, if not, determine the rightful owner, and deny or
   clean-up the registration that is duplicate.

A.6.  Requirements Related to Scalability

   Use cases from Automatic Meter Reading (AMR, collection tree
   operations) and Advanced Metering Infrastructure (AMI, bi-directional
   communication to the meters) indicate the needs for a large number of
   LLN nodes pertaining to a single RPL DODAG (e.g. 5000) and connected
   to the 6LBR over a large number of LLN hops (e.g. 15).

   Related requirements are:

   Req6.1: The registration mechanism SHOULD enable a single 6LBR to
   register multiple thousands of devices.

   Req6.2: The timing of the registration operation should allow for a
   large latency such as found in LLNs with ten and more hops.

Author's Address

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

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