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6MAN                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                               May 2, 2019
Expires: November 3, 2019

              IPv6 Neighbor Discovery on Wireless Networks


   This document describes how the original IPv6 Neighbor Discovery and
   Wireless ND (WiND) can be applied on various abstractions of wireless

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 November 3, 2019.

Copyright Notice

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

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  IP Models . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Physical Broadcast Domain . . . . . . . . . . . . . . . .   6
     3.2.  MAC-Layer Broadcast Emulations  . . . . . . . . . . . . .   7
     3.3.  Mapping the IPv6 Link Abstraction . . . . . . . . . . . .   8
     3.4.  Mapping the IPv6 Subnet Abstraction . . . . . . . . . . .   9
   4.  Wireless ND . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Introduction to WiND  . . . . . . . . . . . . . . . . . .  10
     4.2.  Links and Link-Local Addresses  . . . . . . . . . . . . .  11
     4.3.  Subnets and Global Addresses  . . . . . . . . . . . . . .  11
   5.  WiND Applicability  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Case of LPWANs  . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Case of Infrastructure BSS and ESS  . . . . . . . . . . .  13
     5.3.  Case of Mesh Under Technologies . . . . . . . . . . . . .  14
     5.4.  Case of DMC radios  . . . . . . . . . . . . . . . . . . .  14
       5.4.1.  Using IPv6 ND only  . . . . . . . . . . . . . . . . .  14
       5.4.2.  Using Wireless ND . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

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

   As a result, protocols designed for bridged networks that rely on
   multicast and broadcast often exhibit disappointing behaviours when
   employed unmodified on a local wireless medium (see

   Wi-Fi [IEEEstd80211] Access Points (APs) deployed in an Extended
   Service Set (ESS) act as Ethernet Bridges [IEEEstd8021], with the
   property that the bridging state is established at the time of
   association.  This ensures connectivity to the node (STA) and
   protects the wireless medium against broadcast-intensive Transparent
   Bridging reactive Lookups.

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   In other words, the association process is used to register the MAC
   Address of the STA to the AP.  The AP subsequently proxies the
   bridging operation and does not need to forward the broadcast Lookups
   over the radio.

   Like Transparent Bridging, IPv6 [RFC8200] Neighbor Discovery
   [RFC4861] [RFC4862] Protocol (IPv6 ND) is a reactive protocol, based
   on multicast transmissions to locate an on-link correspondent and
   ensure the uniqueness of an IPv6 address.  The mechanism for
   Duplicate Address Detection (DAD) [RFC4862] was designed for the
   efficient broadcast operation of Ethernet Bridging.  Since broadcast
   can be unreliable over wireless media, DAD often fails to discover
   duplications [I-D.yourtchenko-6man-dad-issues].  In practice, IPv6
   addresses very rarely conflict because of the entropy of the 64-bit
   Interface IDs, not because address duplications are detected and

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

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

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

   Another way is to proxy at the boundary of the wired and wireless
   domains the Layer-3 protocols that rely on MAC Layer broadcast
   operations.  For instance, IEEE 802.11 [IEEEstd80211] situates proxy-
   ARP (IPv4) and proxy-ND (IPv6) functions at the Access Points (APs).

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   But proxying ND requires a perfect knowledge of the peer IPv6
   addresses for which proxying is provided.  In a generic fashion,
   radio connectivity changes with movements and variations in the
   environment, which makes forming and maintaining that knowledge a
   hard problem in the general case.

   Discovering peer addresses by snooping the IPV6 ND protocol as
   proposed for SAVI [I-D.bi-savi-wlan] was found to be unreliable.  An
   IPv6 address may not be discovered immediately due to a packet loss,
   or if a "silent" node is not currently using one of its addresses,
   e.g., a node that waits in wake-on-lan state.  A change of state,
   e.g. due to a movement, may be missed or misordered, leading to
   unreliable connectivity and an incomplete knowledge of the set of

   Wireless ND (WiND) introduces a new approach to IPv6 ND that is
   designed to apply to the WLANs and WPANs types of networks.  On the
   one hand, WiND avoids the use of broadcast operation for Address
   Lookup and Duplicate Address Detection, and on the other hand, WiND
   supports use cases where Subnet and MAC-level domains are not
   congruent, which is common in those types of networks unless a
   specific MAC-Level emulation is provided.

   To achieve this, WiND applies routing inside the Subnets, which
   enables MultiLink Subnets.  Hosts register their addresses to their
   serving routers with [RFC8505].  With the registration, routers have
   a complete knowledge of the hosts they serve and in return, hosts
   obtain routing services for their registered addresses.  The
   registration is abstract to the routing protocol, and it can be
   protected to prevent impersonation attacks with [I-D.ietf-6lo-ap-nd].

   The routing service can be a simple reflexion in a Hub-and-Spoke
   Subnet that emulates an IEEE Std 802.11 Infrastructure BSS at Layer
   3.  It can also be a full-fledge routing protocol, in particular RPL
   [RFC6550] that was designed to adapt to various LLNs such as WLAN and
   WPAN radio meshes with the concept of Objective Function.  Finally,
   the routing service can also be ND proxy that emulates an IEEE Std
   802.11 Infrastructure ESS at Layer 3.  WiND specifies the IPv6
   Backbone Router for that purpose in [I-D.ietf-6lo-backbone-router].

   More details on WiND can be found in Section 4.1.

2.  Acronyms

   This document uses the following abbreviations:

   6BBR: 6LoWPAN Backbone Router

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   6LBR: 6LoWPAN Border Router

   6LN:  6LoWPAN Node

   6LR:  6LoWPAN Router

   6CIO: Capability Indication Option

   ARO:  Address Registration Option

   DAC:  Duplicate Address Confirmation

   DAD:  Duplicate Address Detection

   DAR:  Duplicate Address Request

   EDAC: Extended Duplicate Address Confirmation

   EDAR: Extended Duplicate Address Request

   DODAG:  Destination-Oriented Directed Acyclic Graph

   MLSN: Multi-Link Subnet

   LLN:  Low-Power and Lossy Network

   NA:   Neighbor Advertisement

   NBMA: Non-Broadcast Multi-Access

   NCE:  Neighbor Cache Entry

   ND:   Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:   Neighbor Solicitation

   ROVR: Registration Ownership Verifier

   RPL:  IPv6 Routing Protocol for LLNs

   RA:   Router Advertisement

   RS:   Router Solicitation

   TID:  Transaction ID

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   WiND: Wireless Neighbor Discovery

   WLAN: Wireless Local Area Network

   WPAN: Wireless Personal Area Network

3.  IP Models

3.1.  Physical Broadcast Domain

   At the physical (PHY) Layer, a broadcast domain is the set of all
   peers that may receive a datagram that one sends over an interface.
   This set can comprise a single peer on a serial cable used as point-
   to-point (P2P) link.  It may also comprise multiple peer nodes on a
   broadcast radio or a shared physical resource such as the legacy
   Ethernet shared wire.

   On WLAN and WPAN radios, the physical broadcast domain is defined by
   a particular transmitter, as the set of nodes that can receive what
   this transmitter is sending.  Litterally every datagram defines its
   own broadcast domain since the chances of reception of a given
   datagram are statistical.  In average and in stable conditions, the
   broadcast domain of a particular node can be still be seen as mostly
   constant and can be used to define a closure of nodes on which an
   upper-layer abstraction can be built.

   A PHY-layer communication can be established between 2 nodes if their
   physical broadcast domains overlap.

   On WLAN and WPAN radios, this property is usually reflexive, meaning
   that if B can receive a datagram from A, then A can receive a
   datagram from B.  But there can be asymmetries due to power levels,
   interferers near one of the receivers, or differences in the quality
   of the hardware (e.g., cristals, PAs and antennas) that may affect
   the balance to the point that the connectivity becomes mostly uni-
   directional, e.g., A to B but not practically not B to A.  It takes a
   particular effort to place a set of devices in a fashion that all
   their physical broadcast domains fully overlap, and it can not be
   assumed in the general case.  In other words, the property of radio
   connectivity is generally not transitive, meaning that A may talk to
   B and B may talk to C does not necessarily imply that A can talk to

   We define MAC-Layer Direct Broadcast (DMC) a transmission mode where
   the broadcast domain that is usable at the MAC layer is directly the
   physical broadcast domain.  IEEE 802.15.4 [IEEE802154] and IEEE
   802.11 [IEEEstd80211] OCB (for Out of the Context of a BSS) are

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   examples of DMC radios.  This constrasts with a number of MAC-layer
   Broadcast Emulation schemes that are described in the next section.

3.2.  MAC-Layer Broadcast Emulations

   While a physical broadcast domain is constrained to a single shared
   wire, Ethernet Briging emulates the broadcast properties of that wire
   over a whole physical mesh of Ethernet links.  For the upper layer,
   the qualities of the shared wire are essentially conserved, with a
   reliable and cheap broadcast operation over a closure of nodes
   defined by their connectivity to the emulated wire.

   In large switched fabrics, overlay techniques enable a limited
   connectivity between nodes that are known to a mapping server.  The
   emulated broadcast domain is configured to the system, e.g., with a
   VXLAN network identifier (VNID).  Broadcast operations on the overlay
   can be emulated but can become very expensive, and it makes sense to
   proactively install the relevant state in the mapping server as
   opposed to rely on reactive broadcast lookups.

   An IEEE Std 802.11 Infrastructure Basic Service Set (BSS) also
   provides a closure of nodes as defined by the broadcast domain of a
   central Access Point (AP).  The AP relays both unicast and broadcast
   packets and ensures a reflexive and transitive emulation of the
   shared wire between the associated nodes, with the capability to
   signal link-up/link-down to the upper layer.  Within an
   Infrastructure BSS, the physical broadcast domain of the AP serves as
   emulated broadcast domain for all the nodes that are associated to
   the AP.  Broadcast packets are relayed by the AP and are not
   acknowledged.  For that reason, special efforts are made to ensure
   that all nodes in the BSS receive the broadcast transmission.  To
   achieve this, the transmission is sent at the highest power and
   slowest PHY speed.  This translates into maximum co-channel
   interferences for others and longest occupancy of the medium, for a
   duration that can be 100 times that of a unicast.  For that reason,
   upper layer protocols should tend to avoid the use of broadcast when
   operating over Wi-Fi.

   In an IEEE Std 802.11 Infrastructure Extended Service Set (ESS), the
   process of the association also prepares a bridging state proactively
   at the AP, so as to avoid the reactive broadcast lookup that takes
   place in the process of transparent bridging over a spanning tree.
   This model provides a more reliable operation than the reactive
   transparent bridging and avoid the need of multicast, and it is only
   logical that IPv6 ND evolved towards proposes similar methods at
   Layer-3 for its operation.

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   in some cases of WLAN and WPAN radios, a mesh-under technology (e.g.,
   a IEEE 802.11s or IEEE 802.15.10) provides meshing services that are
   similar to bridgeing, and the broadcast domain is well defined by the
   membership of the mesh.  Mesh-Under emulates a broadcast domain by
   flooding the broadcast packets at Layer-2.  When operating on a
   single frequency, this operation is known to interfere with itself,
   forcing deployment to introduce delays that dampen the collisions.
   All in all, the mechanism is slow, inefficient and expensive.

   Going down the list of cases above, the cost of a broadcast
   transmissions becomes increasingly expensive, and there is a push to
   rethink the upper-layer protocols so as to reduce the depency on
   broadcast operations.

   There again, a MAC-layer communication can be established between 2
   nodes if their MAC-layer broadcast domains overlap.  In the absence
   of a MAC-layer emulation such as a mesh-under or an Infrastructure
   BSS, the MAC-layer broadcast domain is congruent with that of the
   PHY-layer and inherits its properties for reflexivity and
   transitivity.  IEEE 802.11p, which operates Out of the Context of a
   BSS (DMC radios) is an example of a network that does not have a MAC-
   Layer broadcast domain emulation, which means that it will exhibit
   mostly reflexive and mostly non-transitive transmission properties.

3.3.  Mapping the IPv6 Link Abstraction

   IPv6 defines a concept of Link, Link Scope and Link-Local Addresses
   (LLA), an LLA being unique and usable only within the Scope of a
   Link.  The IPv6 Neighbor Discovery (ND) [RFC4861][RFC4862] Duplicate
   Adress Detection (DAD) process leverages a multicast transmission to
   ensure that an IPv6 address is unique as long as the owner of the
   address is connected to the broadcast domain.  It must be noted that
   in all the cases in this specification, the Layer-3 multicast
   operation is always a MAC_Layer broadcast for the lack of a Layer-2
   multicast operation that could handle a possibly very large number of
   groups in order to make the unicast efficient.  This means that for
   every multicast packet regardless of the destination group, all nodes
   will receive the packet and process it all the way to Layer-3.

   On wired media, the Link is often confused with the physical
   broadcast domain because both are determined by the serial cable or
   the Ethernet shared wire.  Ethernet Bridging reinforces that illusion
   by provising a MAC-Layer broadcast domain that emulates a physical
   broadcast domain over the mesh of wires.  But the difference shows on
   legacy Non-Broadcast Multi-Access (NBMA) such as ATM and Frame-Relay,
   on shared links and on newer types of NBMA networks such as radio and
   composite radio-wires networks.  It also shows when private VLANs or

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   Layer-2 cryptography restrict the capability to read a frame to a
   subset of the connected nodes.

   In mesh-under and Infrastructure BSS, the IP Link extends beyond the
   physical broadcast domain to the emulated MAC-Layer broadcast domain.
   Relying on Multicast for the ND operation remains feasible but
   becomes detrimental to unicast traffic, energy-inefficient and
   unreliable, and its use is discouraged.

   On DMC radios, IP Links between peers come and go as the individual
   physical broadcast domains of the transmitters meet and overlap.  The
   DAD operation cannot provide once and for all guarantees on the
   broadcast domain defined by one radio transmitter if that transmitter
   keeps meeting new peers on the go.  The nodes may need to form new
   LLAs to talk to one another and the scope where LLA uniqueness can be
   dynamically checked is that pair of nodes.  As long as there's no
   conflict a node may use the same LLA with multiple peers but it has
   to revalidate DAD with every new peer node.  In practice, each pair
   of nodes defines a temporary P2P link, which can be modeled as a sub-
   interface of the radio interface.

3.4.  Mapping the IPv6 Subnet Abstraction

   IPv6 also defines a concept of Subnet for Glocal and Unique Local
   Addresses.  Addresses in a same Subnet share a same prefix and by
   extension, a node belongs to a Subnet if it has an interface with an
   address on that Subnet.  A Subnet prefix is Globally Unique so it is
   sufficient to validate that an address that is formed from a Subnet
   prefix is unique within that Subnet to guarantee that it is globally
   unique.  IPv6 aggregation relies on the property that a packet from
   the outside of a Subnet can be routed to any router that belongs to
   the Subnet, and that this router will be able to either resolve the
   destination MAC address and deliver the packet, or route the packet
   to the destination within the Subnet.  If the Subnet is known as
   onlink, then any node may also resolve the destination MAC address
   and deliver the packet, but if the Subnet is not onlink, then a host
   that does not have an NCE for the destination will need to pass the
   packet to a router.

   On IEEE Std. 802.3, a Subnet is often congruent with an IP Link
   because both are determined by the physical attachment to an Ethernet
   shared wire or an IEEE Std. 802.1 bridged broadcast domain.  In that
   case, the connectivity over the Link is transitive, the Subnet can
   appear as onlink, and any node can resolve a destination MAC address
   of any other node directly using IPv6 Neighbor Discovery.

   But an IP Link and an IP Subnet are not always congruent.  In a
   shared Link situation, a Subnet may encompass only a subset of the

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   nodes connected to the Link.  In Route-Over Multi-Link Subnets (MLSN)
   [RFC4903], routers federate the Links between nodes that belong to
   the Subnet, the Subnet is not onlink and it extends beyond any of the
   federated Links.

   The DAD and lookup procedures in IPv6 ND expects that a node in a
   Subnet is reachable within the broadcast domain of any other node in
   the Subnet when that other node attempts to form an address that
   would be a duplicate or attempts to resolve the MAC address of this
   node.  This is why ND is only applicable for P2P and transit links,
   and requires extensions for other topologies.

4.  Wireless ND

4.1.  Introduction to WiND

   Wireless Neighbor Discovery (WiND)
   defines a new ND operation that is based on 2 major paradigm changes,
   proactive address registration by hosts to their attachment routers
   and routing to host routes (/128) within the subnet.  This allows
   WiND to avoid the classical ND expectations of transit links and
   Subnet-wide broadcast domains.

   The proactive address registration is performed with a new option in
   NS/NA messages, the Extended Address Registration Option (EARO)
   defiend in [RFC8505].  This method allows to prepare and maintain the
   host routes in the routers and avoids the reactive NS(Lookup) found
   in IPv6 ND.  This is a direct benefit for wireless Links since it
   avoids the MAC level broadcasts that are associated to NS(Lookup).

   The EARO provides information to the router that is independent to
   the routing protocol and routing can take multiple forms, from a
   traditional IGP to a collapsed ub-and-Spoke model where only one
   router owns and advertises the prefix.  [RFC8505] is already
   referenced for RIFT [I-D.ietf-rift-rift], RPL [RFC6550] with
   [I-D.thubert-roll-unaware-leaves] and IPv6 ND proxy

   WiND does not change IPv6 addressing [RFC4291] or the current
   practices of assigning prefixes to subnets.  It is still typical to
   assign a /64 to a subnet and to use interface IDs of 64 bits.
   Duplicate Address detection within the Subnet is performed with a
   central registrar, using new ND Extended Duplicate Address messages
   (EDAR and EDAC) [RFC8505].  This operation modernizes ND for
   application in overlays with Map Resolvers and enables unicast
   lookups [I-D.thubert-6lo-unicast-lookup] for addresses registered to
   the resolver.

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   WiND also enables to extend a legacy /64 on Ethernet with ND proxy
   over the wireless.  This way nodes can form any address the want and
   move freely from an L3-AP (that is really a backbone router in
   bridging mode, more in [I-D.ietf-6lo-backbone-router]) to another,
   without renumbering.  Backbone Routers federate multiple LLNs over a
   Backbone Link to form a MultiLink Subnet (MLSN).  Backbone Routers
   placed along the LLN edge of the Backbone handle IPv6 Neighbor
   Discovery, and forward packets on behalf of registered nodes.

   An LLN node (6LN) registers all its IPv6 Addresses using an NS(EARO)
   as specified in [RFC8505] to the 6BBR.  The 6BBR is also a Border
   Router that performs IPv6 Neighbor Discovery (IPv6 ND) operations on
   its Backbone interface on behalf of the 6LNs that have registered
   addresses on its LLN interfaces without the need of a broadcast over
   the wireless medium.

   WiND is also compatible with DHCPv6 and other forms of address
   assignment in which case it can still be used for DAD.

4.2.  Links and Link-Local Addresses

   For Link-Local Addresses, DAD is performed between communicating
   pairs of nodes.  It is carried out as part of a registration process
   that is based on a NS/NA exchange that transports an EARO.  During
   that process, the DAD is validated and a Neighbor Cache Entry (NCE)
   is populated with a single unicast exchange.

   Ior instance, in the case of a Bluetooth Low Energy (BLE)
   [RFC7668][IEEEstd802151] Hub-and Spoke configuration, Uniqueness of
   Link local Addresses need only to be verified between the pairs of
   communicating nodes, a central router and a peripheral host.  In that
   example, 2 peripheral hosts connected to the same central router can
   not have the same Link Local Address because the Binding Cache
   Entries (BCEs) would collide at the central router which could not
   talk to both over the same interface.  The WiND operation is
   appropriate for that DAD operation, but the one from ND is not,
   because peripheral hosts are not on the same broadcast domain.  On
   the other hand, Global and ULA DAD is validated at the Subnet Level,
   using a registrar hosted by the central router.

4.3.  Subnets and Global Addresses

   WiND extends IPv6 ND for Hub-and-Spoke (e.g., BLE) and Route-Over
   (e.g., RPL) Multi-Link Subnets (MLSNs).

   In the Hub-and-Spoke case, each Hub-Spoke pair is a distinct IP Link,
   and a Subnet can be mapped on a collection of Links that are
   connected to the Hub. The Subnet prefix is associated to the Hub.

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   Acting as 6LR, the Hub advertises the prefix as not-onlink to the
   spokes in RA messages Prefix Information Options (PIO).  Acting as
   6LNs, the Spokes autoconfigure addresses from that prefix and
   register them to the Hub with a corresponding lifetime.  Acting as a
   6LBR, the Hub maintains a binding table of all the registered IP
   addresses and rejects duplicate registrations, thus ensuring a DAD
   protection for a registered address even if the registering node is
   sleeping.  Acting as 6LR, the Hub also maintains an NCE for the
   registered addresses and can deliver a packet to any of them for
   their respective lifetimes.  It can be observed that this design
   builds a form of Layer-3 Infrastructure BSS.

   A Route-Over MLSN is considered as a collection of Hub-and-Spoke
   where the Hubs form a connected dominating set of the member nodes of
   the Subnet, and IPv6 routing takes place between the Hubs within the
   Subnet.  A single logical 6LBR is deployed to serve the whole mesh.
   The registration in [RFC8505] is abstract to the routing protocol and
   provides enough information to feed a routing protocol such as RPL as
   specified in [I-D.thubert-roll-unaware-leaves].  In a degraded mode,
   all the Hubs are connected to a same high speed backbone such as an
   Ethernet bridging domain where IPv6 ND is operated.  In that case, it
   is possible to federate the Hub, Spoke and Backbone nodes as a single
   Subnet, operating IPv6 ND proxy operations
   [I-D.ietf-6lo-backbone-router] at the Hubs, acting as 6BBRs.  It can
   be observed that this latter design builds a form of Layer-3
   Infrastructure ESS.

5.  WiND Applicability

   WiND allows P2P, P2MP hub-and spoke, MAC-level broadcast domain
   emulation such as mesh-under and Wi-Fi BSS, and route over meshes.^

   There is an intersection where Link and Subnet are congruent and
   where both ND and WiND could apply.  These includes P2P, the MAC
   emulation of a PHY broadcast domain, and the particular case of
   always on, fully overlapping physical radio broadcast domain.  But
   even in those cases where both are possible, WiND is preferable vs.
   ND because it reduces the need of broadcast ( this is discussed in
   the introduction of [I-D.ietf-6lo-backbone-router]).

   There are also numerous practical use cases in the wireless world
   where Links and Subnets are not P2P and not congruent:

   o  IEEE std 802.11 infrastructure BSS enables one subnet per AP, and
      emulates a broadcast domain at L2.  Infra ESS extends that and
      recommends to use an IPv6 ND proxy [IEEEstd80211] to coexist with
      Ethernet connected nodes.  WiND incorporates an ND proxy to serve
      that need and that was missing so far.

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   o  BlueTooth is Hub-and-Spoke at the MAC layer.  It would make little
      sense to configure a different subnet between the central and each
      individual peripheral node (e.g., sensor).  Rather, [RFC7668]
      allocates a prefix to the central node acting as router (6LR), and
      each peripheral host (acting as a host (6LR) forms one or more
      address(es) from that same prefix and registers it.

   o  A typical Smartgrid networks puts together Route-Over MLSNs that
      comprise thousands of IPv6 nodes.  The 6TiSCH architecture
      [I-D.ietf-6tisch-architecture] presents the Route-Over model over
      a [IEEEstd802154] Time-Slotted Channel-Hopping mesh, and
      generalizes it for multiple other applications.  Each node in a
      Smartgrid network may have tens to a hundred others nodes in
      range.  A key problem for the routing protocol is which other
      node(s) should this node peer with, because most of the possible
      peers do not provide added routing value.  When both energy and
      bandwidth are constrained,talking to them is a bad idea and most
      of the possible P2P links are not even used.  Peerings that are
      actually used come and go with the dynamics of radio signal
      propagation.  It results that allocating prefixes to all the
      possible P2P Links and maintain as many addresses in all nodes is
      not even considered.

5.1.  Case of LPWANs

   LPWANs are by nature so constrained that the addresses and Subnets
   are fully pre-configured and operate as P2P or Hub-and-Spoke.  This
   saves the steps of neighbor Discovery and enables a very efficient
   stateful compression of the IPv6 header.

5.2.  Case of Infrastructure BSS and ESS

   In contrast to IPv4, IPv6 enables a node to form multiple addresses,
   some of them temporary to elusive, and with a particular attention
   paid to privacy.  Addresses may be formed and deprecated
   asynchronously to the association.  Even if the knowledge of IPv6
   addresses used by a STA can be obtained by snooping protocols such as
   IPv6 ND and DHCPv6, or by observing data traffic sourced at the STA,
   such methods provide only an imperfect knowledge of the state of the
   STA at the AP.  This may result in a loss of connectivity for some
   IPv6 addresses, in particular for addresses rarely used and in a
   situation of mobility.  This may also result in undesirable remanent
   state in the AP when a STA ceases to use an IPv6 address.  It results
   that snooping protocols is not a recommended technique and that it
   should only be used as last resort.

   The recommended alternate is to use the IPv6 Registration method
   speficied in p.  By that method, the AP exposes its capability to

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   proxy ND to the STA in Router Advertisement messages.  In turn, the
   STA may request proxy ND services from the AP for one or more IPv6
   addresses, using an Address Registration Option.  The Registration
   state has a lifetime that limits unwanted state remanence in the
   network.  The registration is optionally secured using
   [I-D.ietf-6lo-ap-nd] to prevent address theft and impersonation.  The
   registration carries a sequence number, which enables a fast mobility
   without a loss of connectivity.

   The ESS mode requires a proxy ND operation at the AP.  The proxy ND
   operation must cover Duplicate Address Detection, Neighbor
   Unreachability Detection, Address Resolution and Address Mobility to
   transfer a role of ND proxy to the AP where a STA is associated
   following the mobility of the STA.  The proxy ND specification
   associated to the address registration is
   [I-D.ietf-6lo-backbone-router].  With that specification, the AP
   participates to the protocol as a Backbone Router, typically
   operating as a bridging proxy though the routing proxy operation is
   also possible.  As a bridging proxy, the proxy replies to NS lookups
   with the MAC address of the STA, and then bridges packets to the STA
   normally; as a routing proxy, it replies with its own MAC address and
   then routes to the STA at the IP layer.  The routing proxy reduces
   the need to expose the MAC address of the STA on the wired side, for
   a better stability and scalability of the bridged fabric.

5.3.  Case of Mesh Under Technologies

   The Mesh-Under provides a broadcast domain emulation with reflexive
   and Transitive properties and defines a transit Link for IPv6
   operations.  It results that the model for IPv6 operation is similar
   to that of a BSS, with the root of the mesh operating an Access Point
   does in a BSS/ESS.  While it is still possible to operate IPv6 ND,
   the inefficiencies of the flooding operation make the IPv6 ND
   operations even less desirable than in a BSS, and the use of WiND is
   highly recommended.

5.4.  Case of DMC radios

   IPv6 over DMC radios uses P2P Links that can be formed and maintained
   when a pair of DMC radios transmitters are in range from one another.

5.4.1.  Using IPv6 ND only

   DMC radios do not provide MAC level broadcast emulation.  An example
   of that is OCB (outside the context of a BSS), which uses IEEE Std.
   802.11 transmissions but does not provide the BSS functions.

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   It is possible to form P2P IP Links between each individual pairs of
   nodes and operate IPv6 ND over those Links with Link Local addresses.
   DAD must be performed for all addresses on all P2P IP Links.

   If special deployment care is taken so that the physical broadcast
   domains of a collection of the nodes fully overlap, then it is also
   possible to build an IP Subnet within that collection of nodes and
   operate IPv6 ND.

   The model can be stretched beyond the scope of IPv6 ND if an external
   mechanism avoids duplicate addresses and if the deployment ensures
   the connectivity between peers.  This can be achieved for instance in
   a Hub-and-Spoke deployment if the Hub is the only router in the
   Subnet and the Prefix is advertised as not onlink.

5.4.2.  Using Wireless ND

   Though this can be achieved with IPv6 ND, WiND is the recommended
   approach since it uses more unicast communications which are more
   reliable and less impacting for other users of the medium.

   Router and Hosts respectively send a compressed RA/NA with a SLLAO at
   a regular period.  The period can be indicated in a RA as in an RA-
   Interval Option [RFC6275].  If available, the message can be
   transported in a compressed form in a beacon, e.g., in OCB Basic
   Safety Messages (BSM) that are nominally sent every 100ms.  An active
   beaconing mode is possible whereby the Host sends broadcast RS
   messages to which a router can answer with a unicast RA.

   A router that has Internet connectivity and is willing to serve as an
   Internet Access may advertise itself as a default router [RFC4191] in
   its RA.  The NA/RA is sent over an Unspecified Link where it does not
   conflict to anyone, so DAD is not necessary at that stage.

   The receiver instantiates a Link where the sender's address is not a
   duplicate.  To achieve this, it forms an LLA that does not conflict
   with that of the sender and registers to the sender using [RFC8505].
   If the sender sent an RA(PIO) the receiver can also autoconfigure an
   address from the advertised prefix and register it.

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       6LoWPAN Node        6LR
        (RPL leaf)       (router)
            |               |
            |   LLN link    |
            |               |
            |  IPv6 ND RS   |
            |----------->   |
            |  IPv6 ND RA   |
            |               |
            |  NS(EARO)     |
            |               |
            |  NA(EARO)     |
            |               |

                    Figure 1: Initial Registration Flow

   The lifetime in the registration should start with a small value
   (X=RMin, TBD), and exponentially grow with each reregistration to a
   mlarger value (X=Rmax, TBD).  The IP Link is considered down when
   (X=NbBeacons, TDB) expected messages are not received in a row.  It
   must be noted that the Link flapping does not affect the state of the
   registration and when a Link comes back up, the active -lifetime not
   elapsed- registrations are still usable.  Packets should be held or
   destroyed when the Link is down.

   P2P Links may be federated in Hub-and-Spoke and then in Route-Over
   MLSNs as described above.  More details on the operation of WiND and
   RPL over the MLSN can be found in section 3.1, 3.2, 4.1 and 4.2.2 of

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

        Figure 2: Initial Registration Flow over Multi-Link Subnet

   An example Hub-and-Spoke is an OCB Road-Side Unit (RSU) that owns a
   prefix, provides Internet connectivity using that prefix to On-Board
   Units (OBUs) within its physical broadcast domain.  An example of
   Route-Over MLSN is a collection of cars in a parking lot operating
   RPL to extend the connectivity provided by the RSU beyond its
   physical broadcast domain.  Cars may then operate NEMO [RFC3963] for
   their own prefix using their address derived from the prefix of the
   RSU as CareOf Address.

6.  IANA Considerations

   This specification does not require IANA action.

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7.  Security Considerations

   This specification refers to the security sections of IPv6 ND and
   WiND, respectively.

8.  Acknowledgments

   Many thanks to the participants of the 6lo WG where a lot of the work
   discussed here happened.  Also ROLL, 6TiSCH, and 6LoWPAN.

9.  References

9.1.  Normative References

              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", draft-ietf-6lo-ap-nd-12 (work in
              progress), April 2019.

              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", draft-ietf-6lo-backbone-router-11 (work
              in progress), February 2019.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, DOI 10.17487/RFC3963, January 2005,

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [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,

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

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

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,

9.2.  Informative References

              Bi, J., Wu, J., Wang, Y., and T. Lin, "A SAVI Solution for
              WLAN", draft-bi-savi-wlan-16 (work in progress), November

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
              in progress), March 2019.

              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless
              Media", draft-ietf-mboned-ieee802-mcast-problems-05 (work
              in progress), April 2019.

              Team, T., "RIFT: Routing in Fat Trees", draft-ietf-rift-
              rift-05 (work in progress), April 2019.

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

              Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
              unaware-leaves-07 (work in progress), April 2019.

              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

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              IEEE standard for Information Technology, "IEEE Std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks".

              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)".

   [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>.

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

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   [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,

   [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,

   [RFC8273]  Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
              per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,

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

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