IPv6 Neighbor Discovery on Wireless Networks
draft-thubert-6man-ipv6-over-wireless-06

Versions: 00 01 02 03 04 05 06                                          
6MAN                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                             1 June 2020
Expires: 3 December 2020


              IPv6 Neighbor Discovery on Wireless Networks
                draft-thubert-6man-ipv6-over-wireless-06

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 December 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  ND-Classic, Wireless ND and ND-Proxies  . . . . . . . . . . .   4
   4.  IP Models . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Physical Broadcast Domain . . . . . . . . . . . . . . . .   6
     4.2.  Link Layer Broadcast Emulations . . . . . . . . . . . . .   7
     4.3.  Mapping the IPv6 link Abstraction . . . . . . . . . . . .   9
     4.4.  Mapping the IPv6 subnet Abstraction . . . . . . . . . . .  10
   5.  Wireless Neighbor Discovery . . . . . . . . . . . . . . . . .  10
     5.1.  Introduction to Wireless ND . . . . . . . . . . . . . . .  11
     5.2.  links and Link-Local Addresses  . . . . . . . . . . . . .  12
     5.3.  subnets and Global Addresses  . . . . . . . . . . . . . .  12
   6.  WiND Applicability  . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Case of LPWANs  . . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Case of Infrastructure BSS and ESS  . . . . . . . . . . .  14
     6.3.  Case of Mesh Under Technologies . . . . . . . . . . . . .  15
     6.4.  Case of DMB radios  . . . . . . . . . . . . . . . . . . .  16
       6.4.1.  Using ND-Classic only . . . . . . . . . . . . . . . .  16
       6.4.2.  Using Wireless ND . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  19
   11. Informative References  . . . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   [IEEE Std. 802.1] 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 Wireless Local Area Networks (WLANs) generally do not benefit
   from the same reliable and cheap broadcast capabilities as Ethernet
   links.

   As opposed to unicast transmissions, the broadcast transmissions over
   wireless links are not subject to automatic retries (ARQ) and can be
   very unreliable.  Reducing the speed at the physical (PHY) layer for
   broadcast transmissions can increase the reliability, at the expense
   of a higher relative cost of broadcast on the overall available
   bandwidth.  As a result, protocols designed for bridged networks that
   rely on broadcast transmissions often exhibit disappointing
   behaviours when employed unmodified on a local wireless medium (see
   [MCAST PROBLEMS]).




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   Like Transparent Bridging, the IPv6 [RFC8200] Neighbor Discovery
   [RFC4861] [RFC4862] Protocol (ND-Classic) is reactive, and relies on
   on-demand Network Layer multicast to locate an on-link correspondent
   (Address Resolution, AR) and ensure the uniqueness of an IPv6 address
   (Duplicate Address Detection, DAD).  On Ethernet LANs and most WLANs
   and Low-Power Personal Area Networks (LoWPANs), the Network Layer
   multicast operation is typically implemented as a Link Layer
   broadcast for the lack of an adapted and scalable Link Layer
   multicast operation.

   It results that on wireless, an ND-Classic multicast message is
   typically broadcasted.  So even though there are very few nodes
   subscribed to the Network Layer multicast group, and there is at most
   one intended Target, the broadcast is received by many wireless nodes
   over the whole subnet (e.g., the ESS fabric).  And yet, the broadcast
   transmission being unreliable, the intended Target may effectively
   have missed the packet.

   On paper, a Wi-Fi station must keep its radio turned on to listen to
   the periodic series of broadcast frames, which for the most part will
   be dropped when they reach Network Layer.  In order to avoid this
   waste of energy and increase its battery life, a typical battery-
   operated device such as an IoT sensor or a smartphone will blindly
   ignore a ratio of the broadcasts, making ND-Classic operations even
   less reliable.

   Wi-Fi [IEEE Std. 802.11] Access Points (APs) deployed in an Extended
   Service Set (ESS) act as [IEEE Std. 802.1] bridges between the
   wireless stations (STA) and the wired backbone.  As opposed to the
   classical Transparent (aka Learning) Bridge operation that installs
   the forwarding state reactively to traffic, the bridging state in the
   AP is established proactively, at the time of association.  This
   protects the wireless medium against broadcast-intensive Transparent
   Bridging lookups.  The association process registers the Link Layer
   (MAC) Address (LLA) of the STA to the AP proactively, i.e., before it
   is needed.  The AP maintains the list of the associated addresses and
   blocks the lookups for destinations that are not registered.  This
   solves the broadcast issue for the Link Layer lookups, but the
   Network Layer problem remains.

   Though ND-Classic was the state of the art when designed for an
   Ethernet wire at the end of the twentieth century, it must be
   reevaluated for the new technologies, such as wireless and overlays,
   that evolved since then.  This document discusses the applicability
   of ND-Classic over wireless links, as compared with routing-based
   alternatives such as prefix-per node and multi-link subnets (MLSN),
   and with Wireless ND (WiND), that is similar to the Wi-Fi association
   and reduces the need for Network Layer multicast.



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2.  Acronyms

   This document uses the following abbreviations:

   6BBR:  6LoWPAN Backbone Router
   6LN:  6LoWPAN Node
   6LR:  6LoWPAN Router
   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
   MLSN:  Multi-link subnet
   LLN:  Low-Power and Lossy Network
   LoWPAN:  Low-Power Wireless Personal Area Network
   NA:  Neighbor Advertisement
   NBMA:  Non-Broadcast Multi-Access
   NCE:  Neighbor Cache Entry
   ND:  Neighbor Discovery
   NDP:  Neighbor Discovery Protocol
   NS:  Neighbor Solicitation
   RPL:  IPv6 Routing Protocol for LLNs
   RA:  Router Advertisement
   RS:  Router Solicitation
   VLAN:  Virtual Local Area Network
   WiND:  Wireless Neighbor Discovery
   WLAN:  Wireless Local Area Network
   WPAN:  Wireless Personal Area Network

3.  ND-Classic, Wireless ND and ND-Proxies

   The ND-Classic Neighbor Solicitation (NS) [RFC4861] message is used
   as a multicast IP packet for Address Resolution (AR) and Duplicate
   Address Setection (DAD) [RFC4862].  In those cases, the NS message is
   sent at the Network Layer to a Solicited-Node Multicast Address
   (SNMA) [RFC4291] and should in theory only reach a very small group
   of nodes.  Those messages are generated individually for each
   address, and may occur when a node joins the network, moves, or wakes
   up and reconnects to the network.

   DAD was designed for the efficient broadcast operation of Ethernet.
   Experiments show that DAD often fails to discover the duplication of
   IPv6 addresses in large wireless access networks [DAD ISSUES].  In
   practice, IPv6 addresses very rarely conflict, not because the
   address duplications are detected and resolved by the DAD operation,
   but thanks to the entropy of the 64-bit Interface IDs (IIDs) that
   makes a collision quasi-impossible for randomized IIDs.



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   ND-Classic Address Lookups and DADs over a very large fabric can
   generate hundreds of broadcasts per second.  If the broadcasts were
   blindly copied over Wi-Fi, the ND-related multicast traffic could
   consume enough bandwidth to cause a substantial degradation to the
   unicast service [MCAST EFFICIENCY].  To protect their bandwidth, some
   networks throttle ND-related broadcasts, which reduces the capability
   for the ND protocol to operate as expected.

   This problem can be alleviated by reducing the size of the broadcast
   domain that encompasses wireless access links.  This has been done in
   the art of IP subnetting by partitioning the subnets and by routing
   between them, at the extreme by assigning a /64 prefix to each
   wireless node (see [RFC8273]).

   Another way to split the broadcast domain within a subnet is to proxy
   at the boundary of the wired and wireless domains the Network Layer
   protocols that rely on Link Layer broadcast operations.  For
   instance, [IEEE Std. 802.11] recommends to deploy proxies for the
   IPv4 Address Resolution Protocl (ARP)) and IPv6 Neighbor Discosvery
   functions at the Access Points (APs).  But proxying ND requires the
   full list of the IPv6 addresses for which proxying is provided.
   Forming and maintaining that knowledge a hard problem in the general
   case of radio connectivity, which keeps changing with movements and
   other variations in the environment.

   [SAVI] suggests to discover the addresses by snooping the ND-Classic
   protocol, but that can also be unreliable.  An IPv6 address may not
   be discovered immediately due to a packet loss.  It may never be
   discovered in the case of a "silent" node that is not currently using
   one of its addresses, e.g., a printer that waits in wake-on-lan
   state.  A change of anchor, e.g. due to a movement, may be missed or
   misordered, leading to unreliable connectivity and an incomplete list
   of IPv6 addresses to be proxied for.

   Wireless ND (WiND) introduces a new approach to IPv6 Neighbor
   Discovery that is designed to apply to the WLANs and LoWPANs types of
   networks, as well as other Non-Broadcast Multi-Access (NBMA) networks
   such as Data-Center overlays.  WiND applies routing inside the
   subnets, which enables to form potentially large MLSNs without
   creating a large broadcast domain at the Link Layer.

   In a fashion similar to a Wi-Fi Association, IPv6 Hosts register
   their addresses to their serving router(s), using [RFC8505].  With
   the registration, the 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
   service, and it can be protected to prevent impersonation attacks
   with [ADDR PROTECT].



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   The routing service can be a simple reflexion in a Hub-and-Spoke
   subnet that emulates an IEEE Std. 802.11 Infrastructure BSS at the
   Network Layer.  It can also be a full-fledge routing protocol, in
   particular RPL [RFC6550], which is designed to adapt to various LLNs
   such as WLAN and WPAN radio meshes.  Finally, the routing service can
   also be an ND proxy that emulates an IEEE Std. 802.11 Infrastructure
   ESS at the Network Layer, as specified in the IPv6 Backbone Router
   [BB ROUTER].

   On the one hand, WiND avoids the use of broadcast operation for DAD
   and AR, and on the other hand, WiND supports use cases where subnet
   and Link Layer domains are not congruent, which is common in wireless
   networks unless a specific Link Layer emulation is provided.  More
   details on WiND can be found in Section 5.1.

4.  IP Models

4.1.  Physical Broadcast Domain

   At the physical (PHY) Layer, a broadcast domain is the set of nodes
   that may receive a transmission that one sends over an interface, in
   other words the set of nodes in range of the radio transmission.
   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 Ethernet
   wires and hubs for which ND-Classic was initially designed.

   On WLAN and LoWPAN radios, the physical broadcast domain is defined
   relative to a particular transmitter, as the set of nodes that can
   receive what this transmitter is sending.  Literally every frame
   defines its own broadcast domain since the chances of reception of a
   given frame 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 two nodes if the
   physical broadcast domains of their unicast transmissions overlap.
   On WLAN and LoWPAN radios, that relation is usually not reflexive,
   since nodes disable the reception when they transmit; still they may
   retain a copy of the transmitted frame, so it can be seen as
   reflexive at the MAC Layer.  It is often symmetric, meaning that if B
   can receive a frame from A, then A can receive a frame 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.,
   crystals, PAs and antennas) that may affect the balance to the point
   that the connectivity becomes mostly uni-directional, e.g., A to B
   but practically not B to A.



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   It takes a particular effort to place a set of devices in a fashion
   that all their physical broadcast domains fully overlap, and that
   specific situation can not be assumed in the general case.  In other
   words, the relation of radio connectivity is generally not
   transitive, meaning that A in range with B and B in range with C does
   not necessarily imply that A is in range with C.

4.2.  Link Layer Broadcast Emulations

   We call Direct MAC Broadcast (DMB) the transmission mode where the
   broadcast domain that is usable at the MAC layer is directly the
   physical broadcast domain.  [IEEE Std. 802.15.4] and [IEEE Std.
   802.11] OCB (for Out of the Context of a BSS) are examples of DMB
   radios.  DMB networks provide mostly symmetric and non-transitive
   transmission.  This contrasts with a number of Link Layer Broadcast
   Emulation (LLBE) schemes that are described in this section.

   In the case of Ethernet, while a physical broadcast domain is
   constrained to a single shared wire, the [IEEE Std. 802.1] bridging
   function 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 transitive 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 Map Resolver.  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 to do so.

   An [IEEE Std. 802.11] Infrastructure Basic Service Set (BSS) also
   provides a transitive closure of nodes as defined by the broadcast
   domain of a central AP.  The AP relays both unicast and broadcast
   packets and provides the symmetric and transitive emulation of a
   shared wire between the associated nodes, with the capability to
   signal link-up/link-down to the upper layer.  Within a 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.  To increase
   the chances that all nodes in the BSS receive the broadcast
   transmission, AP transmits at the slowest PHY speed.  This translates
   into maximum co-channel interferences for others and the longest
   occupancy of the medium, for a duration that can be a hundred times
   that of the unicast transmission of a frame of the same size.




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   For that reason, upper layer protocols should tend to avoid the use
   of broadcast when operating over Wi-Fi.  To cope with this problems,
   APs may implement strategies such as turn a broadcast into a series
   of unicast transmissions, or drop the message altogether, which may
   impact the upper layer protocols.  For instance, some APs may not
   copy Router Solicitation (RS) messages under the assumption that
   there is no router across the wireless interface.  This assumption
   may be correct at some point of time and may become incorrect in the
   future.  Another strategy used in Wi-Fi APS is to proxy protocols
   that heavily rely on broadcast, such as the Address Resolution in ARP
   and ND-Classic, and either respond on behalf or preferably forward
   the broadcast frame as a unicast to the intended Target.

   In an [IEEE Std. 802.11] Infrastructure Extended Service Set (ESS),
   infrastructure BSSes are interconnected by a bridged network,
   typically running Transparent Bridging and the Spanning tree Protocol
   or a more advanced Layer 2 Routing (L2R) scheme.  In the original
   model of learning bridges, the forwarding state is set by observing
   the source MAC address of the frames.  When a state is missing for a
   destination MAC address, the frame is broadcasted with the
   expectation that the response will populate the state on the reverse
   path.  This is a reactive operation, meaning that the state is
   populated reactively to the need to reach a destination.  It is also
   possible in the original model to broadcast a gratuitous frame to
   advertise self throughout the bridged network, and that is also a
   broadcast.

   The process of the Wi-Fi association prepares a bridging state
   proactively at the AP, which avoids the need for a reactive broadcast
   lookup over the wireless access.  In an ESS, the AP may also generate
   a gratuitous broadcast sourced at the MAC address of the STA to
   prepare or update the state in the learning bridges so they point
   towards the AP for the MAC address of the STA.  WiND emulates that
   proactive method at the Network Layer for the operations of AR, DAD
   and ND proxy.

   In some instances of WLANs and LoWPANs, a Mesh-Under technology
   (e.g., a IEEE Std. 802.11s or IEEE Std. 802.15.10) provides meshing
   services that are similar to bridging, 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 the Link Layer.
   When operating on a single frequency, this operation is known to
   interfere with itself, and requires inter-frame gaps to dampen the
   collisions, which reduces further the amount of available bandwidth.







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   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.

4.3.  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 ND-Classic [RFC4861] DAD [RFC4862] process uses a
   multicast transmission to detect a duplicate address, which requires
   that the owner of the address is connected to the Link Layer
   broadcast domain of the sender.

   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
   with a Link 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) networks 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 Link Layer 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 Link Layer broadcast
   domain.  Relying on Multicast for the ND operation remains feasible
   but becomes highly detrimental to the unicast traffic, and becomes
   less and less energy-efficient and reliable as the network grows.

   On DMB 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 over the
   broadcast domain defined by one radio transmitter if that transmitter
   keeps meeting new peers on the go.

   The scope on which the uniqueness of an LLA must be checked is each
   new pair of nodes for the duration of their conversation.  As long as
   there's no conflict, a node may use the same LLA with multiple peers
   but it has to perform DAD again with each new peer.  A node may need
   to form a new LLA to talk to a new peer, and multiple LLAs may be
   present in the same radio interface to talk to different peers.  In
   practice, each pair of nodes defines a temporary P2P link, which can
   be modeled as a sub-interface of the radio interface.






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4.4.  Mapping the IPv6 subnet Abstraction

   IPv6 also defines the concept of a subnet for Global and Unique Local
   Addresses (GLA and ULA).  All the addresses in a subnet share the
   same prefix, and by extension, a node belongs to a subnet if it has
   with an address in that subnet.

   Unless intently replicated in different locations for very specific
   purposes, a subnet prefix is unique within a routing system; for
   ULAs, the routing system is typically a limited domain, whereas for
   GLAs, it is the whole Internet.

   For that reason, it is sufficient to validate that an address that is
   formed from a subnet prefix is unique within the scope of that subnet
   to guarantee that it is globally unique within the whole routing
   system.  Note that a subnet may become partitioned due to the loss of
   a wired or wireless link, so even that operation is not necessarily
   obvious, more in [DAD APPROACHES].

   The IPv6 aggregation model 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 Link Layer address and deliver the packet, or, in the
   case of an MLSN, route the packet to the destination within the
   subnet.

   If the subnet is known as on-link, then any node may also resolve the
   destination Link Layer address and deliver the packet, but if the
   subnet is not on-link, then a host in the subnet that does not have a
   Neighbor Cache Entry (NCE) for the destination will also need to pass
   the packet to a router, more in [RFC5942].

   On Ethernet, an IP subnet is often congruent with an IP link because
   both are determined by the physical attachment to a shared wire or an
   IEEE Std. 802.1 bridged domain.  In that case, the connectivity over
   the link is both symmetric and transitive, the subnet can appear as
   on-link, and any node can resolve a destination MAC address of any
   other node directly using ND-Classic.

   But an IP link and an IP subnet are not always congruent.  In the
   case of a Shared Link, individual subnets may each encompass only a
   subset of the nodes connected to the link.  Conversely, in Route-Over
   Multi-link subnets (MLSN) [RFC4903], routers federate the links
   between nodes that belong to the subnet, the subnet is not on-link
   and it extends beyond any of the federated links.

5.  Wireless Neighbor Discovery




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5.1.  Introduction to Wireless ND

   The DAD and AR procedures in ND-Classic expect 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 applicable for P2P and transit links, but
   requires extensions for more complex topologies.

   WiND [RFC6775][RFC8505][BB ROUTER][ADDR PROTECT] defines a new
   operation for ND 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 expectations of transit links and subnet-wide broadcast domains.

   WiND is agnostic to the method used for Address Assignment, e.g.,
   Stateless Address Autoconfiguration (SLAAC) [RFC4862] or DHCPv6
   [RFC8415].  It does not change the IPv6 addressing [RFC4291] or the
   current practices of assigning prefixes, typically a /64, to a
   subnet.  But the DAD operation is performed as a unicast exchange
   with a central registrar, using new ND Extended Duplicate Address
   messages (EDAR and EDAC) [RFC6775][RFC8505].  This modernizes ND for
   application in overlays with Map Resolvers and enables unicast
   lookups [UNICAST AR] for addresses registered to the resolver.

   The proactive address registration is performed with a new option in
   NS/NA messages, the Extended Address Registration Option (EARO)
   defined in [RFC8505].  This method allows to prepare and maintain the
   host routes in the routers and avoids the reactive Address Resolution
   in ND-Classic and the associated Link Layer broadcasts transmissions.

   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 Hub-and-Spoke model where only one
   router owns and advertises the prefix.  [RFC8505] is already
   referenced as the registrtaion interface to "RIFT: Routing in Fat
   Trees" [I-D.ietf-rift-rift] and "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550] with [RPL UNAWARE LEAVES].

   WiND also enables to span a subnet over an MLSN that federates edge
   wireless links with a high-speed, typically Ethernet, backbone.  This
   way, nodes can form any address they want and move freely from a
   wireless edge link to another, without renumbering.  Backbone Routers
   (6BBRs) placed along the wireless edge of the Backbone handle IPv6
   Neighbor Discovery and forward packets over the backbone on behalf of
   the registered nodes on the wireless edge.





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   For instance, a 6BBR in bridging proxy mode (more in [BB ROUTER]) can
   operate as a Layer-3 AP to serve wireless IPv6 hosts that are Wi-Fi
   STAs and maintain the reachability for Global Unicast and Link-LOcal
   Addresses within the federated MLSN.

5.2.  links and Link-Local Addresses

   For Link-Local Addresses, DAD is typically performed between
   communicating pairs of nodes and an NCE can be populated with a
   single unicast exchange.  In the case of a bridging proxies, though,
   the Link-Local traffic is bridged over the backbone and the DAD must
   proxied there as well.

   For instance, in the case of Bluetooth Low Energy (BLE)
   [RFC7668][IEEEstd802151], the uniqueness of Link-Local Addresses
   needs only to be verified between the pair of communicating nodes,
   the central router and the peripheral host.  In that example, 2
   peripheral hosts connected to the same central router can not have
   the same Link-Local Address because the addresses would collision at
   the central router which could not talk to both over the same
   interface.  The DAD operation from WiND is appropriate for that use
   case, but the one from ND is not, because the peripheral hosts are
   not on the same broadcast domain.

   On the other hand, the uniqueness of Global and Unique-Local
   Addresses is validated at the subnet Level, using a logical registrar
   that is global to the subnet.

5.3.  subnets and Global Addresses

   WiND extends ND-Classic 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.

   Acting as routers, the Hub advertises the prefix as not-on-link to
   the spokes in RA messages Prefix Information Options (PIO).  Acting
   as hosts, the Spokes autoconfigure addresses from that prefix and
   register them to the Hub with a corresponding lifetime.  Acting as a
   registrar, 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.  The Hub also maintains an NCE for the registered addresses
   and can deliver a packet to any of them during their respective
   lifetimes.  It can be observed that this design builds a form of
   Network Layer Infrastructure BSS.



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   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 registrar 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 [RPL UNAWARE LEAVES].  In a degraded mode, all the Hubs
   are connected to a same high speed backbone such as an Ethernet
   bridging domain where ND-Classic is operated.  In that case, it is
   possible to federate the Hub, Spoke and Backbone nodes as a single
   subnet, operating ND proxy operations [BB ROUTER] at the Hubs, acting
   as 6BBRs.  It can be observed that this latter design builds a form
   of Network Layer Infrastructure ESS.

6.  WiND Applicability

   WiND applies equally to P2P links, P2MP Hub-and-Spoke, Link Layer
   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 more details in the introduction of [BB ROUTER].

   There are also a number of practical use cases in the wireless world
   where links and subnets are not congruent:

   *  The IEEE Std. 802.11 infrastructure BSS enables one subnet per AP,
      and emulates a broadcast domain at the Link Layer.  The
      Infrastructure ESS extends that model over a backbone and
      recommends the use of an ND proxy [IEEE Std. 802.11] to
      interoperate with Ethernet-connected nodes.  WiND incorporates an
      ND proxy to serve that need, which was missing so far.

   *  BlueTooth is Hub-and-Spoke at the Link 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,
      and each peripheral host (acting as a host) forms one or more
      address(es) from that same prefix and registers it.




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   *  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
      an IEEE Std. 802.15.4 Time-Slotted Channel-Hopping (TSCH)
      [IEEEstd802154] 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 waste
      of resources 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.

6.1.  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.

6.2.  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.

   Snooping protocols such as ND-Classic and DHCPv6 and observing data
   traffic sourced at the STA provides an imperfect knowledge of the
   state of the STA at the AP.  Missing a state or a transition may
   result in the loss of connectivity for some of the addresses, in
   particular for an address that is rarely used, belongs to a sleeping
   node, or one in a situation of mobility.  This may also result in
   undesirable remanent state in the AP when the STA ceases to use an
   IPv6 address while remaining associated.  It results that snooping
   protocols is not a recommended technique and that it should only be
   used as last resort, when the WiND registration is not available to
   populate the state.








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   The recommended alternative method is to use the WiND Registration
   for IPv6 Addresses.  This way, the AP exposes its capability to proxy
   ND to the STA in Router Advertisement messages.  In turn, the STA may
   request proxy ND services from the AP for all of its IPv6 addresses,
   using the Extended Address Registration Option, which provides the
   following elements:

   *  The registration state has a lifetime that limits unwanted state
      remanence in the network.

   *  The registration is optionally secured using [ADDR PROTECT] to
      prevent address theft and impersonation.

   *  The registration carries a sequence number, which enables to
      figure the order of events in a fast mobility scenario without
      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 WiND proxy ND specification that associated to the Address
   Registration is [BB 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 backbone router either
   replies to NS lookups with the MAC address of the STA, or preferably
   forwards the lookups to the STA as Link Layer unicast frames to let
   the STA answer.  For the data plane, the backbone router acts as a
   normal AP and bridges the packets to the STA as usual.  As a routing
   proxy, the backbone router 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.

6.3.  Case of Mesh Under Technologies

   The Mesh-Under provides a broadcast domain emulation with symmetric
   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 as an Access
   Point does in a BSS/ESS.

   While it is still possible to operate ND-Classic, the inefficiencies
   of the flooding operation make the associated operations even less
   desirable than in a BSS, and the use of WiND is highly recommended.



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6.4.  Case of DMB radios

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

6.4.1.  Using ND-Classic only

   DMB radios do not provide MAC level broadcast emulation.  An example
   of that is IEEE Std. 802.11 OCB which uses IEEE Std. 802.11 MAC/PHYs
   but does not provide the BSS functions.

   It is possible to form P2P IP links between each individual pairs of
   nodes and operate ND-Classic 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 ND-Classic.

   If an external mechanism avoids duplicate addresses and if the
   deployment ensures the connectivity between peers, a non-transit Hub-
   and-Spoke deployment is also possible where the Hub is the only
   router in the subnet and the Prefix is advertised as not on-link.

6.4.2.  Using Wireless ND

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

   The routers send RAs with a SLLAO at a regular period.  The period
   can be indicated in the 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 messages.  The RA is sent over an unspecified link where it
   does not conflict to anyone, so DAD is not necessary at that stage.






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

         (host)          (router)
            |               |
            |   DMB 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 re-registration to a
   larger 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 registrations
   (i.e., registrations for which lifetime is not elapsed) 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 illustrated in Figure 2.  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 [I-D.ietf-6tisch-architecture].












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       6LoWPAN Node        6LR             6LBR            6BBR
        (RPL leaf)       (router)         (root)
            |               |               |               |
            |  6LoWPAN ND   |6LoWPAN ND+RPL | 6LoWPAN ND    | ND-Classic
            |   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.

7.  IANA Considerations

   This specification does not require IANA action.






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8.  Security Considerations

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

9.  Acknowledgments

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

10.  Normative References

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, DOI 10.17487/RFC3963, January 2005,
              <https://www.rfc-editor.org/info/rfc3963>.

   [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,
              <https://www.rfc-editor.org/info/rfc4861>.

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

   [RFC5942]  Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
              Model: The Relationship between Links and Subnet
              Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
              <https://www.rfc-editor.org/info/rfc5942>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.







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   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

   [ADDR PROTECT]
              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", Work in Progress, Internet-Draft, draft-
              ietf-6lo-ap-nd-23, 30 April 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-ap-nd-23>.

   [BB ROUTER]
              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", Work in Progress, Internet-Draft, draft-
              ietf-6lo-backbone-router-20, 23 March 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-backbone-
              router-20>.

11.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc4903>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [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,
              <https://www.rfc-editor.org/info/rfc7668>.




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   [RFC8273]  Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
              per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,
              <https://www.rfc-editor.org/info/rfc8273>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [I-D.ietf-rift-rift]
              Przygienda, T., Sharma, A., Thubert, P., Rijsman, B., and
              D. Afanasiev, "RIFT: Routing in Fat Trees", Work in
              Progress, Internet-Draft, draft-ietf-rift-rift-12, 26 May
              2020,
              <https://tools.ietf.org/html/draft-ietf-rift-rift-12>.

   [RPL UNAWARE LEAVES]
              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              Work in Progress, Internet-Draft, draft-ietf-roll-unaware-
              leaves-15, 15 April 2020, <https://tools.ietf.org/html/
              draft-ietf-roll-unaware-leaves-15>.

   [DAD ISSUES]
              Yourtchenko, A. and E. Nordmark, "A survey of issues
              related to IPv6 Duplicate Address Detection", Work in
              Progress, Internet-Draft, draft-yourtchenko-6man-dad-
              issues-01, 3 March 2015, <https://tools.ietf.org/html/
              draft-yourtchenko-6man-dad-issues-01>.

   [MCAST EFFICIENCY]
              Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
              Yourtchenko, "Why Network-Layer Multicast is Not Always
              Efficient At Datalink Layer", Work in Progress, Internet-
              Draft, draft-vyncke-6man-mcast-not-efficient-01, 14
              February 2014, <https://tools.ietf.org/html/draft-vyncke-
              6man-mcast-not-efficient-01>.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-28>.

   [MCAST PROBLEMS]
              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless



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              Media", Work in Progress, Internet-Draft, draft-ietf-
              mboned-ieee802-mcast-problems-11, 11 December 2019,
              <https://tools.ietf.org/html/draft-ietf-mboned-ieee802-
              mcast-problems-11>.

   [SAVI]     Bi, J., Wu, J., Wang, Y., and T. Lin, "A SAVI Solution for
              WLAN", Work in Progress, Internet-Draft, draft-bi-savi-
              wlan-19, 14 May 2020,
              <https://tools.ietf.org/html/draft-bi-savi-wlan-19>.

   [UNICAST AR]
              Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
              Unicast Lookup", Work in Progress, Internet-Draft, draft-
              thubert-6lo-unicast-lookup-00, 25 January 2019,
              <https://tools.ietf.org/html/draft-thubert-6lo-unicast-
              lookup-00>.

   [DAD APPROACHES]
              Nordmark, E., "Possible approaches to make DAD more robust
              and/or efficient", Work in Progress, Internet-Draft,
              draft-nordmark-6man-dad-approaches-02, 19 October 2015,
              <https://tools.ietf.org/html/draft-nordmark-6man-dad-
              approaches-02>.

   [IEEE Std. 802.15.4]
              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 Std. 802.11]
              IEEE standard for Information Technology, "IEEE Standard
              for Information technology -- Telecommunications and
              information exchange between systems Local and
              metropolitan area networks-- Specific requirements Part
              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications".

   [IEEEstd802151]
              IEEE standard for Information Technology, "IEEE Standard
              for Information Technology - Telecommunications and
              Information Exchange Between Systems - Local and
              Metropolitan Area Networks - Specific Requirements. - Part
              15.1: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Wireless Personal Area
              Networks (WPANs)".





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   [IEEEstd802154]
              IEEE standard for Information Technology, "IEEE Standard
              for Local and metropolitan area networks -- Part 15.4:
              Low-Rate Wireless Personal Area Networks (LR-WPANs)".

   [IEEE Std. 802.1]
              IEEE standard for Information Technology, "IEEE Standard
              for Information technology -- Telecommunications and
              information exchange between systems Local and
              metropolitan area networks Part 1: Bridging and
              Architecture".

Author's Address

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

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




























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