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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
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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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