Internet DRAFT - draft-thubert-6man-ipv6-over-wireless
draft-thubert-6man-ipv6-over-wireless
6MAN P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational M. Richardson
Expires: 9 September 2023 Sandelman
8 March 2023
Architecture and Framework for IPv6 over Non-Broadcast Access
draft-thubert-6man-ipv6-over-wireless-15
Abstract
This document presents an architecture for IPv6 access networks that
decouples the network-layer concepts of Links, Interface, and Subnets
from the link-layer concepts of links, ports, and broadcast domains,
and limits the reliance on link-layer broadcasts. This architecture
is suitable for IPv6 over any network, including non-broadcast
networks. A study of the issues with ND-Classic over wireless media
is presented, and a framework to solve those issues within the new
architecture is proposed.
Status of This Memo
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This Internet-Draft will expire on 9 September 2023.
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extracted from this document must include Revised BSD License text as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Issues with ND-Classic-Based Access . . . . . . . . . . . . . 4
2.1. ND-Classic and ND-Proxies . . . . . . . . . . . . . . . . 4
2.2. The case of Wireless . . . . . . . . . . . . . . . . . . 6
2.3. The case of Overlays . . . . . . . . . . . . . . . . . . 8
2.4. Power and Sustainability . . . . . . . . . . . . . . . . 9
2.5. Security and Privacy . . . . . . . . . . . . . . . . . . 10
2.6. More Middleboxes . . . . . . . . . . . . . . . . . . . . 10
2.7. Summary of Issues . . . . . . . . . . . . . . . . . . . . 12
3. An Architecture for IPv6 over Non-Broadcast Networks . . . . 13
3.1. Basic Concepts . . . . . . . . . . . . . . . . . . . . . 13
3.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.1. IP Links . . . . . . . . . . . . . . . . . . . . . . 16
3.3.2. IP Interfaces . . . . . . . . . . . . . . . . . . . . 18
3.3.3. IP Subnets . . . . . . . . . . . . . . . . . . . . . 19
3.3.4. ND Proxies . . . . . . . . . . . . . . . . . . . . . 21
3.3.5. Subnet Gateway Protocols . . . . . . . . . . . . . . 21
3.4. IP Models . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4.1. Physical Broadcast Domain . . . . . . . . . . . . . . 22
3.4.2. Link-layer Broadcast Emulations . . . . . . . . . . . 23
3.4.3. Mapping the IP Link Abstraction . . . . . . . . . . . 25
3.4.4. Mapping the IPv6 Subnet Abstraction . . . . . . . . . 26
3.5. Stateful address Autoconfiguration and Subnet Routing . . 27
4. A Framework for Stateful address Autoconfiguration and Subnet
Routing . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.1. Implementing Stateful address Autoconfiguration . . . . . 28
4.2. links and link-local Addresses . . . . . . . . . . . . . 29
4.3. Subnets and Global Addresses . . . . . . . . . . . . . . 29
4.4. Anycast and Multicast Addresses . . . . . . . . . . . . . 30
4.5. Advertising Prefixes in the SGP . . . . . . . . . . . . . 31
5. WiND Applicability . . . . . . . . . . . . . . . . . . . . . 31
5.1. Case of LPWANs . . . . . . . . . . . . . . . . . . . . . 32
5.2. Case of Infrastructure BSS and ESS . . . . . . . . . . . 32
5.3. Case of Mesh Under Technologies . . . . . . . . . . . . . 34
5.4. Case of DMB radios . . . . . . . . . . . . . . . . . . . 34
5.4.1. Using ND-Classic only . . . . . . . . . . . . . . . . 34
5.4.2. Using Wireless ND . . . . . . . . . . . . . . . . . . 34
6. Coexistence with ND-Classic . . . . . . . . . . . . . . . . . 40
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 43
8. Security Considerations . . . . . . . . . . . . . . . . . . . 44
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
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10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 44
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
12. Normative References . . . . . . . . . . . . . . . . . . . . 45
13. Informative References . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
IEEE Std. 802.1 [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 legacy Ethernet yellow wires, and protocols that rely
on broadcasts are less suited in those environments.
Similarly, the use of broadcast is discouraged in large Data Center
(DC) fabrics and DC Interconnect (DCI) that extend the lower-layer
links in large and physically distributed topologies, e.g., as meshes
of point-to-point (P2P) tunnels. In such case, an overlay broadcast
service is typically emulated as ingress (or reflector) replication
and generates massive amounts of underlay unicast messages, possibly
over expensive Wide Area Network (WAN) links.
All in all, as IPv6 [RFC8200] networks migrate from a physical wires
to virtual or intangible media, the common requirement shows to
decouple the abstractions that are manipulated at the network layer
from physical properties such as broadcast capabilities and
transitivity that are handled at the lower layers.
The original IPv6 Neighbor Discovery Protocol [RFC4861], [RFC4862]
(ND-Classic) relies heavily on broadcast operation for Router
Advertisement (RA), address Resolution (AR) and Duplicate address
Detection (DAD). In modern networks, this may be inefficient (due to
many replications over constrained links), unreliable (broadcast may
be lost in transmission), counterproductive to network operations
(broadcast storms), prone to impersonation and multiplication attacks
(a unicast from the outside may cause a broadcast inside), and may be
detrimental to privacy (an observer inside the network can discover
other onlink addresses).
This document presents an architecture for IPv6 access networks that
1) decouples the network-layer concepts of Links, Interface, and
Subnets from the link-layer concepts of links, ports, and broadcast
domains, and 2) limits the reliance on (and impact thereof) link-
layer broadcasts inside the Subnet. This architecture is suitable
for IPv6 over any network, including modern Non-Broadcast MultiAccess
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(NBMA) and non-transitive Point-to-Multipoint (P2MP) networks. A
study of the issues with ND-Classic over wireless media is presented,
and a framework to solve those issues within the new architecture is
proposed.
2. Issues with ND-Classic-Based Access
2.1. ND-Classic and ND-Proxies
Though ND-Classic was the state of the art when designed for early
Ethernet links at the end of the twentieth century, it is less
appropriate for modern networks such as wireless and overlays that
cannot provide the same cheap and reliable broadcast as a shared
yellow wire. The reactive AR operation was designed to limit the
amount of memory that is needed for the ND cache, at times where
memory was scarce in the adapters. This trade-off, broadcast
bandwidth vs. memory in the adapters, should be reevaluated for
networks where router memory is aplenty but broadcast has become
expensive.
The ND-Classic Neighbor Solicitation (NS) [RFC4861] message is used
as a multicast IP packet for AR and DAD [RFC4862]. While the AR
message is intended for one node that owns the Target address, the
expectation for DAD is that there's no node at all with that address.
A message that is intended for at most one node is certainly a poor
match for a broadcast operation.
The NS message for AR and DAD are 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. But to support SNMA,
the host must also support multicast Listener Discovery (MLD)
[RFC3810], which may be an additional burden to a constrained stack,
and the switches should support their own multicast routing and
state, which is pushing the real problems to the lower layers.
Also, if implemented, the SNMA proceure would entail close to one
state per address is every switch since there is often only one
address with the same SNMA in the network - though the birthday
paradox applies. This amount of memory may not have been available
in early switches, and still makes little economical sense today when
a complete ND cache requires one state per address in every router
only, as opposed to one in every switch, when the Subnet prefix is
advertised as not-onlink.
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This is why, in practice, ND-Classic messages to a SNMA are mapped to
link-layer broadcast on Ethernet and Wireless networks, and to full
ingress replication in overlays. multicast NS transmissions may
occur when a node joins the network, moves, or wakes up and
reconnects to the network. Over a very large fabric, this can
generate hundreds of broadcasts per second.
If the broadcasts were blindly copied over Wi-Fi links, the link-
layer broadcast traffic associated to ND network-layer multicast
could consume enough bandwidth to cause a substantial degradation to
the unicast service [MCAST EFFICIENCY]. This is why ND Proxies are
deployed and charged to reduce the resulting flood; sadly, ND-Proxies
are not fully reliable for the lack of a deterministic state on all
existing addresses, which leads to unpredictable failures in ND-
Classic operations.
The ND-Classic Neighbor Advertisement (NA) [RFC4861] message can also
be sent as a multicast to all nodes, as a gratuitous information that
can be used to override the address mapping in nodes with an existing
Neighbor Cache Entry (NCE) for the Target address. If this is done,
all nodes in the broadcast domain are impected though there's
probably none or very few with an NCE. When it is not done, nodes
with an NCE will be unable to reach the Target IP address until
Neighbor Unreachability Detection (NUD) discovers the issue. Both
alternatives are unsatisfactory, meaning that the whole approach
should be revisited.
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 prefix, say a /64, to
each wireless node (see [RFC8273]).
Another way to split the broadcast domain within a Subnet is to proxy
the network-layer protocols that rely on link-layer broadcast
operations at the boundary of the split broadcast domains. As an
example, [IEEE Std. 802.11] recommends deploying an "ARP proxy" for
the IPv4 address Resolution Protocol (ARP) and ND-Classic at the APs.
The correct operation of the proxy requires the exhaustive list of
the IP addresses for which proxying is provided. Forming and
maintaining that knowledge is a hard problem in the general case of
radio connectivity, which keeps changing with movements and
variations in the environment that alter the range of transmissions.
It is achieved in Wi-Fi networks through the proactive method of the
wireless association, which is akin to the registration procedure in
this architecture.
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[SAVI] suggests discovering 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 addresses. Bottom line: snooping ND-Classic is not appropriate to
form and maintain a deterministic knowledge of the IPv6 addresses of
all the neighbors that are reachable over a network port.
2.2. The case of Wireless
Like Transparent Bridging, the ND-Classic operation is reactive, and
relies on IP multicast for the AR and DAD procedures. As discussed
in Section 2, 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 on most WLANs and Low-Power
Personal Area Networks (LoWPANs). It results that on wireless, ND-
Classic multicast messages are typically broadcasted.
As opposed to unicast transmissions, the broadcast transmissions over
wireless links are not subject to automatic retries (ARQ) and
therefore are not reliable. 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.
Excessive use of broadcast by protocols such as ND-Classic and
Bonjour led network administrators to install multicast rate limiting
to protect the network. Experimentally, this proved to have a
dramatic effect on ND performance in large wireless networks. From
some testing done at an IETF meeting a few years ago, it seemed that
up to 90% of IPv6 link-local multicasts were dropped. The impact on
user experience is usually limited since for the most part, the users
will connect to addresses outside the Subnet and will not attempt to
locate one another. The impact on the NOC might still be
significant, since a failure related to ND operations might be
transient and difficult to debunk after the fact.
Another experiment conducted during an IETF meeting consisted in
manually forcing address duplicates to observe the DAD behavior.
This experiment showed that DAD often (up to 80% of times was
observed) fails to discover the duplication of IPv6 addresses, at
least in a large wireless access networks, see [DAD ISSUES] for more.
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 typically 64-bit Interface IDs
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(IIDs) that makes a collision quasi-impossible for randomized IIDs.
This is why, even when DAD fails, the user experience is rarely
affected.
Excessive use of broadcast also places a toll on the battery of
wireless devices such as IoT sensors and smartphones. On paper, a
Wi-Fi station must keep its radio turned on to listen to the periodic
series of broadcast frames. Most of those broadcasts are dropped at
the network layer when the receiving node finds it is not interested,
as is the case of NS messages when the node is not the Target. In
order to protect the battery lifetime, a typical smartphone will
listen at a multiple of the broadcast period, blindly ignoring a
large ratio of the broadcasts, and making ND-Classic operations even
less reliable.
Net-net: broadcast transmissions are not reliable on wireless.
Protocols designed for bridged networks that rely on broadcast
transmissions often exhibit disappointing behaviors when employed
unmodified on a local wireless medium (more in [MCAST PROBLEMS]).
Even though there is at most one intended Target for a broadcast AR
or DAD message, the broadcast impacts many wireless nodes over the
whole Subnet (e.g., the ESS fabric), and yet the chances that
intended Target receives the packet are limited. The fact that the
user experience for classic-ND is not so dramatically affected only
shows that those broadcasts are, for a large part, a useless waste of
expensive resources.
Wi-Fi Access Points [IEEE Std. 802.11] (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. The
association process registers the link-layer (MAC) address of the STA
to the AP proactively, i.e., before it is needed. Based on that
information, the AP maintains the exhaustive list of the associated
MAC addresses and blocks the link-layer lookups for destination MAC
addresses that are not associated to this AP. The association
procedure protects the wireless medium against broadcast-intensive
Transparent Bridging lookups, but the network-layer problem remains
for the lack of a similar procedure at the network layer.
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2.3. The case of Overlays
link-layer Overlays (VLANs) reduce the broadcast domain from the
physical one, whereas network-layer overlays (e.g., VxLAN) can extend
the Subnet beyond the limits of the physical network, enabling to
deploy a Subnet over large physical domains. network-layer overlays
are a clear indication of the need to decouple the Subnet from the
limits of the physical network.
A network-layer overlay is typically a partial or full mesh of point
to point tunnels between routers. In case of a full mesh, BUM
(Broadcast, Unknown, and multicast) forwarding across the overlay can
be implemented as a replication at the ingress router or at a
dedicated reflector, but either way entails a growing congestion and
latency as the overlay grows in size. BUM forwarding has become so
detrimental to the network operations that some operators decide to
turn it off. This means that a silent node, whose location is
unknown to the fabric control plane and possibly forgotten, cannot be
rediscovered by AR procedures, and will not be reachable again until
it volunteers its own sign of life. To avoid this, modern protocols
should be designed such that the use of overlay-wide broadcasts are
limited to operations where a real distribution operation is desired,
e.g., every node is interested in receiving the packet.
While a multicast ND-Classic RA message may be of interest within a
site or a subsite to all local nodes, it is probably of little
interest to other sites that are served by other routers, even when
the overlay spans across both sites. The same goes for a local
printer, the broadcast mDNS lookup should reach local printers but
not faraway ones. This is another indication of the need to decouple
the span of the Subnet from the lower layer broadcast domain, and
dedicate the broadcast service to local operations such as discovery.
As discussed in Section 2.2, multicast ND-Classic messages are in
fact broadcasted across the overlay, meaning that they contribute to
the BUM traffic and are treated as full broadcast. Yet, those
messages are used for AR and DAD and intend to reach at most one
node, the owner of the Target address, if any. Using a BUM
transmission that reaches every nodes in the overlay to communicate
with at most one is a misuse of the overlay resources and should be
replaced by unicast-based methods.
In data centers, overlays are typically combined with server
multihoming at the edge. An advanced Network Interface Card (NIC) is
equipped with more than one Ethernet ports for redundancy, which
connect to different leaf switches (aka ToR) to the same cloud
network. The server (say, using Kubernetes) needs a single address
and would rather use that address on both ports to the same Subnet,
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and use either network port for its own reasons, e.g., load
balancing, independently of IP addressing considerations. This means
that the IP Interface abstraction that the server needs is a logical
construct, decoupled from the network ports, and capable to encompass
more than one.
2.4. Power and Sustainability
In the wireless case, broadcasts are sent at the slowest speed
available, which can be a hundred time slower than unicast
transmissions, in order to maximize the chances that all nodes in the
BSS will receive the frame. For that extra long duration, the
broadcast transmission holds the spectrum locally, which adds to the
unicast latency, and generates interferences remotely, which may
cascade in losses and retries in adjacent networks. In the process,
the broadcast transmission consumes up to a hundred time more power
than unicast transmission of equivalent payload size. Power is also
wasted when replicating a multicast packet to span an overlay, each
time the packet is ultimately dropped by the recipient.
Constrained IoT devices conserve their power by placing themselves in
deep sleep for most of the time. While a device is sleeping, it
cannot answer ND-Classic messages for AR and DAD. This makes ND-
Classic unsuitable to IoT devices. The 6LoWPAN WG has determined
that the most appropriate model for a constrained network is a pull
model where the device wakes up, negotiates with its router(s) for
addresses and connectivity for some amount of time in the future, and
goes back to sleep. The router can then perform a role of sleep
proxy that defends the address(es) and holds traffic for the device
till the device wakes up and pulls it from the router. Additionally,
when the constrained network grows into a mesh, broadcast operations
become rapidly inacceptable in terms of power and bandwidth, and the
not-onlink model whereby for the most part hosts do not lookup one
another, is mandatory.
In all cases, to make the internet greener, we must reconsider the
use of broadcast over large access networks. To maintain the
capability to build the Subnets we want, the IPv6 architecture must
enable to decouple the Subnet from the broadcast domain, make the
broadcast domain small and local, and refocus the use of broadcast to
the cases where all nodes are interested. Additionally, the IPv6
architecture must enable a model where the network serves and
protects low-power devices that sleep most of the time and wake up on
their own schedule.
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2.5. Security and Privacy
Broadcasting ND_Classic messages expose the source and the Target
addresses to the whole network, including nodes that do not need to
know that those addresses are present in the network. A passive
listener may discover the addresses without any observable action or
possibility of control by the source. Once it has discovered the
presence of neighbor addresses, the onlink attacker can impersonate
any host in the network, either by sourcing packets with a stolen
address, or by overriding the neighbor caches with a NA that
indicates the attacker's link-layer address.
It is thus desirable to avoid exposing the host IPv6 address in
broadcast ND messages. A more private approach has each node see and
connect to only a subset of the routers using the not-onlink model.
The source may still shortcut to the destination when the destination
is effectively on link, based on redirect messages from the router,
when desirable. Additionally, it is desirable that only the address
owner can source packets with that address, and that another party
may not be able to claim and use that address.
The reactive NS lookup method can also be leveraged from the outside
of the network to perform DOS attacks on constrained resources in the
network. The attacker just needs to forge any random address from
the Subnet prefix and send one packet to that random address. The
Subnet ingress router will have to store that packet for the duration
of the lookup time, and broadcast an NS to lookup the forged address.
This both locks memory in the router and consumes bandwidth and
energy, impacting all nodes in the Subnet.
To avoid the lookup delays and associated attacks, it makes sense to
use a proactive method whereby the router knows all the address
mappings in advance. When that is achieved, if the destination
address of an incoming packet is not present in the router tables,
then the destination does not exist in the network and the packet can
safely be dropped by the forwarding engine, e.g., in hardware.
2.6. More Middleboxes
The above problems have been observed at least since the early 2000s.
A number of actions were taken. For instance, IEEE std 802.11 [IEEE
Std. 802.11] mandates the support of a middlebox operation called
"ARP proxy" for IPv4 and IPv6 in the Access Point (AP). The "ARP
Proxy" cancels broadcasts over a BSS when the IP Target of the ARP/ND
message is not owned by a STA associated to the AP. With IPv4, the
expectation is that the STA owns exactly one IP address, and that the
address is obtained via DHCP right after the association, so it is
simple and deterministic to snoop the address in the DHCP exchange
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(as long as it remains in the clear) and cancel undesirable ARP
lookups.
In contrast to IPv4, IPv6 enables a node to form multiple addresses,
some of them temporary and with a particular attention paid to
privacy. Addresses may be formed and deprecated asynchronously to
the association. Even if the knowledge of IPv6 addresses used by a
STA can be obtained by snooping protocols such as ND-Classic and
DHCPv6, or by observing data traffic sourced at the STA, such methods
provide only an imperfect knowledge of the state of the STA at the
AP. This may result in a loss of connectivity for some IPv6
addresses, in particular for addresses rarely used and in a situation
of mobility. This may also result in undesirable state persistence
in the AP when a STA ceases to use an IPv6 address. It follows that
snooping protocols is not a recommended technique and that it should
only be used as last resort.
Because ND-Classic is so easy to attack, some vendors have deployed
undocumented proprietary counter measures as middlebox operation in
the switches and routers. Those middleboxes snoop the ND-Classic
messages, filter them or modify them, for instance to change their
link-layer scope from broadcast to unicast. Based on the snooped
information, the middlebox may for instance drop an RA message that
appears to be coming from a host (e.g., as inferred because it is
received on a wireless adapter), or an NA message coming from a
device that does not appear to be the owner. But, for the lack of an
explicit contract between the host and the middlebox, the middlebox
cannot determine who the real owner is, and it may deny a rightful
user.
In a managed network, IP addresses are an expensive and thus a
limited resource. To ensure a fair use and protect against DOS
attacks, the middlebox may also block Stateless address
Autoconfiguration (SLAAC) from a host above a fixed number of
addresses. When that happens, the host believes that it can use the
address but fails to connect with it. This might happen even if the
host has ceased to use other addresses and is now within the allowed
quota. Classic-ND lacks both a method for the host to know how many
addresses it can own, and a method for the router to know which
addresses the host uses at any point of time. The infrastructure
needs a deterministic knowledge of the addresses in use, and for that
a contract must be passed between the host and the network to ensure
that the all the addresses are known and usable.
Maybe the most insidious side effect of those middleboxes is that as
opposed to NAT, their operation is obfuscated and proprietary. From
one vendor to the next, and from one product generation to the next,
their behavior may evolve and affect ND-Classic in different
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fashions. In the short term, this may only impact some specific
deployments, which may have to work around the issues. In the longer
term, this may affect the capability we have to evolve the protocol,
like firewalls impact our capability to develop new transports in
parallel to TCP and UDP. We must either standardize (e.g., for ND
proxy) or eliminate those middlebox activities, and for that, the
IPv6 ND protocol must evolve to a model where proprietary middleboxes
are not needed anymore. This demands a model that minimizes the use
of broadcasts, and where a contract provides mutual guarantees for
the host that need IPv6 addresses and the routers that provide
reachability and protection for these addresses.
2.7. Summary of Issues
ND-Classic inherited 2 majors design points from IPv4, a strong
coupling of logical and physical concepts, which creates unacceptable
constraints on modern deployments with virtual and intangible links,
and a reactive operation for AR and DAD that requires an extensive
use of broadcast spanning the Subnet. While IPv4 and IPv6 behaviors
are similar for addresses obtained via DHCP, the cost of AR and DAD
makes IPv6 significantly more expensive than IPv4 when SLAAC is
enabled.
And while IPv4 supported NBMA and P2MP models (e.g., on Frame Relay
leveraging OSPFv2), the IPv6 promise to support NBMA (for ATM)
remains unmet to this day, as only P2P and Transit links are properly
supported by ND-Classic. For those reasons, as well as inherent
complexity and unpredictability, IPv6 with SLAAC can be significantly
less attractive than IPv4 to some network administrators.
ND-Classic exposes all addresses to all nodes in the network, which
is unfit for privacy. It is prone to DOS attacks from outside the
network and impersonation attacks from the inside, with no method to
prove the address ownership and perform Source address Validation
(SAVI) later on the data traffic. To protect against such threats,
the vendors had to introduce middleboxes that interfere with the
protocol operation and affect the capability to evolve the protocol
in the future.
ND-Classic lacks a support for mobility (which typically entails a
sequence counter maintained by the host and the deprecation of state
that is based on older sequences) and for anycast. This makes it
very hard for the network to defend the addresses on behalf of the
owner, e.g., when the owner is temporarily disconnected. It results
that the operation of the middleboxes is unsatisfactory and may cause
discontinuities in connectivity.
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The extensive use of broadcast operations in ND-Classic is not only
detrimental to bandwidth, it is also an issue for energy conservation
and sustainability. Devices must be always attached and always
powered on to answer NS messages, which makes ND-Classic inapplicable
to power-conserving devices such as IoT sensors that sleep for the
vast majority of their time.
3. An Architecture for IPv6 over Non-Broadcast Networks
3.1. Basic Concepts
This document introduces an alternate architecture for IPv6 access
networks that is designed to apply to the WLANs and LoWPANs types of
networks as well as other NBMA networks such as Data-Center overlays
and P2MP networks such as IoT radio meshes. It may be used as a
replacement to the ND-Classic reactive model in any network where the
issues discussed in Section 2 are detrimental to the network
operation.
The key design points in this architecture derive from the original
observations made at the 6LoWPAN WG for constrained devices and
networks, and focus on avoiding waste of limited resources such as
spectrum and energy, by using broadcasts only when broadcast is
really needed, and decoupling the IP abstraction of a Subnet from the
broadcast domains to avoid Subnet-wide broadcast storms. To that
effect, this architecture leverages the not-onlink model and routing
inside the Subnet, which enables to form potentially large MLSNs
without creating a large broadcast domain at the link layer.
To support the deployment agility that virtual (e.g., VxLAN overlays
and pseudowires) and intangible (e.g., wireless, laser, and quantum)
links enable, the IP abstractions of Interface, Link, and Subnet are
decoupled from their classical physical counterparts of port, link,
and broadcast domains. The Subnet is defined by a prefix length
called Subnet Prefix Length (SPL), as the longest aggregation that
can be advertised in the IGP. An SPL of 64 is typical though the
architecture does not mandate it. Host routes and prefixes longer
than SPL are advertised inside the Subnet only, using a separate
Subnet Gateway protocol (SGP).
Any device that owns an address within the Subnet prefix belongs to
the Subnet, this is now decoupled from the physical connectivity and
broadcast domain. Instead, the IPv6 routers that serve a Subnet must
form a connected dominating set such that every host in the Subnet is
connected to at least one router and the routers are connected to one
another directly (classical NBMA, aka full mesh) or indirectly via
other routers (Point to MultiPoint, P2MP, aka partial mesh). The
not-onlink model is used throughout, so hosts do not look each other
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up, saving all the associated broadcast. Instead, they rely on the
routers to forward the packets inside and outside the Subnet. This
way, the Subnet can have any structure needed for the deployment,
where hosts can move from router to router in the Subnet, or anywhere
in the Internet provided they can lay a mobility tunnel to one of the
routers for use as IP Link to the Subnet.
All IP Links are abstracted as Point-to-Point, though a lower-layer
broadcast service may be used by the router to send RAs to a subset
of local hosts in the Subnet, or by the host to send an RS message to
a subset of the routers. An IP Interface bundles one or more
subInterfaces, one per Subnet that can be reached through that
Interface. A Global IPv6 address is installed on the subInterface
that connects to the Subnet from which the address derives. The IP
Interface connects to one or more IP Links (to different neighbors)
over the same or over different physical ports (they are decoupled).
A link-local address is associated to the IP Link directly. Each
SubInterface connects to the subset of those IP Links that reach
other nodes in the Subnet.
In a fashion similar to a IEEE std 802.11 [IEEE Std. 802.11]
Association, IPv6 nodes register their addresses to one or more
neighbor router(s), which may reject the registration, e.g., in case
of a duplication. With the registration, the routers collectively
build a complete knowledge of the hosts they serve and in return,
hosts obtain guaranteed routing services for their registered
addresses for a contractual lifetime.
To support distributed routers in the Subnet, an abstract registrar
service maintains the state of all active registrations in the Subnet
and answers queries to lookup mappings, validate ownership, and avoid
duplications. The registration is abstract to the routing service
and the registrar service, and it can be protected to prevent
impersonation attacks. The registration enables the network to know
deterministically all the IPv6 addresses and link-layer address
mapping currently in use, and eliminates the need for lookups and
DAD, and for the associated broadcasts.
The abstract routing service allows an ingress router to find a path
to the destination address within the Subnet. It can be a simple
reflection 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, e.g., RPL (see [RFC9010]), which is
designed to adapt to various LLNs such as WLAN and WPAN radio meshes,
or RIFT (see [I-D.ietf-rift-rift]) or BGP/EVPN (see [RFC8365]), for
application in data centers. It can be based on overlay tunnels
between ingress router and egress router leveraging a resolver
service such as LISP, see [RFC7834] for more. Finally, the routing
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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 [RFC8929].
The abstract registrar service maintains the mapping between the
registered node link-layer address and the registered IPv6 address.
It contains meta data that enables to ascertain that the second
registration for the same address is performed for the same
registered node, so it also binds the registered node with the
registered IPv6 address. The registrar service provides APIs to look
up a link-layer address for an IPv6 address as well as validate IPv6
address ownership. The registrar can be implemented as a mapping
server ala LISP [RFC6830], a distributed state ala ND proxy
[RFC8929], or a synchronized state ala EVPN [RFC7432]. In the former
case, this enables the reactive lookups to be performed as unicast
requests to the map resolver. In the latter, the address mapping is
synchronized by the routing protocol and known to all the routers for
all nodes in the IP Subnet, so there is never a need for a reactive
lookup.
On the one hand, the Architecture proposed in this document avoids
the use of broadcast operation for DAD and AR, and on the other hand,
it 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.
The address registration establishes a contract between the nodes and
the routers where nodes can ask for addresses which will be
guaranteed to be operational for a contractual lifetime, and the
network may accept or refuse granting additional addresses based on
state (e.g., duplicate address) as well as policy (e.g., quota).
This ways hosts and routers agree deterministically on which
addresses will be served to which nodes in the Subnet. The
registration is agnostic to the router to router and router to
registrar interfaces. The latter interface can be implemented in
various fashions that can blend in existing technologies such as
legacy ND-Classic network through ND proxy, as well as EVPN-based and
LISP-based overlays.
3.2. Acronyms
This document uses the following abbreviations:
6BBR: 6LoWPAN Backbone Router
6LN: 6LoWPAN Node
6LR: 6LoWPAN Router
ARO: address Registration Option
BGP: Border Gateway Protocol
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DAC: Duplicate address Confirmation (message)
DAD: Duplicate address Detection
DAR: Duplicate address Request (message)
EDAC: Extended Duplicate address Confirmation
EDAR: Extended Duplicate address Request
EVPN: Ethernet VPN
IGP: Interior Gateway Protocol
LAN: Local Area Network
LISP: Locator/ID Separation Protocol
LLN: Low-Power and Lossy Network
LLA: link-local address
LoWPAN: Low-Power WPAN
MAC: Medium Access Control
MLSN: Multi-link Subnet
MLD: multicast Listener Discovery
NA: Neighbor Advertisement (message)
NBMA: Non-Broadcast Multi-Access
NCE: Neighbor Cache Entry
ND: Neighbor Discovery (protocol)
NDP: Neighbor Discovery Protocol
NS: Neighbor Solicitation (message)
P2P: Point-to-Point
P2MP: Point-to-Multipoint
RPL: IPv6 Routing Protocol for LLNs
RA: Router Advertisement (message)
RS: Router Solicitation (message)
SGP: Subnet Gateway Protocol
SPL: Subnet Prefix Length
ULP: Upper-Layer Protocol
VLAN: Virtual LAN
VxLAN: Virtual Extensible LAN
VPN: Virtual Private Network
WAN: Wide Area Network
WiND: Wireless Neighbor Discovery (protocol)
WLAN: Wireless Local Area Network
WPAN: Wireless Personal Area Network
3.3. Terminology
3.3.1. IP Links
The term "link" refers to layer 2 (comprising MAC and link layers)
communication medium that can be leveraged at layer 3 (aka IP layer,
aka network layer) to instantiate one IP hop (see section 2 of
[RFC8200]. In this document we conserve that term (lowercase) but
differentiate it from an IP Link, which is a network-layer
abstraction that represents the link but is not the link, like the
map is not the country.
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With IPv6, IP has moved to network-layer abstractions for its
operations, e.g., with the use of a link-local address (LLA), and
that of IP multicast for link-scoped operations. At the same time,
the concept of an IP Link emerged as an abstraction that represents
how the network layer considers the link:
* An IP Link connects an IP node to one or more other IP nodes using
a lower-layer subnetwork. The lower-layer subnetwork may comprise
multiple links, e.g., in the case of a switched fabric or a mesh-
under LLN.
* an IP Link defines the scope of an LLA, and defines the domain in
which the LLA must be unique
* An IP Link is attached to a physical port, and one link-local
address is associated to the IP Link.
* an IP Link provides a subset of the connectivity that is offered
by the physical link at the lower layer; if the IP Link is
narrower than the link-layer reachable domain, then network-layer
filters must restrict the link-scoped communication to remain
between peers on a same IP Link. More than one IP Link may be
installed on the same network port to connect to different peers.
* an IP Link can be Point to Point (P2P), Point to Point (P2MP,
forming a partial mesh and non-transitive), NBMA (non-broadcast
multi-access, fully meshed), or transit (broadcast-capable and
any-to-any).
It is a network design decision to use one IP Link model or another
over a given lower-layer subnetwork, e.g., to map a Frame Relay
network as a P2MP IP Link, or as a collection of P2P IP Links. As
another example, an Ethernet fabric may be bridged, in which case the
nodes that interconnect the lower-layer links are L2 switches, and
the fabric can be abstracted as a single transit IP Link; or the
fabric can be routed, in which case the P2P IP Links are congruent
with the link-layer links, and the nodes that interconnect the links
are routers.
This architecture only uses P2P Link abstractions as shown in
Figure 1, where an IP Link is identified by a pair of local and
remote link-layer (MAC) address. A network port may enable to reach
to more than one peer at the link layer; in that case, this
architecture maps each peer relationship as a different IP Link. A
link-local address only needs to be unique within that peer to peer
relationship.
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+--------------------
|
| IP Link 1 ---------------------------> Node 1
link-layer address MAC1 link-layer address MAC3
| link-local address LLA1 link-local address LLAN1
|
| IP Link 2 ---------------------------> Node 2
link-layer address MAC2 link-layer address MAC4
| link-local address LLA2 link-local address LLAN2
| ...
|
| (LLA 1 may be the same as LLA 2)
+--------------------
network port
Figure 1: P2P Link Abstraction
If only the network port but not the link-layer address of the peer
is visible from the network layer when processing a message, then the
network layer cannot discriminate the IP Link of packets arriving on
the same network port, and for that reason, it will reject a second
registration for the same link-local address by a second peer,
meaning that a link-local address will have to be unique on a network
port across IP Links. In that case, the link-local address of the
peer is used to identify the IP Link, and all the addresses
registered to this node with the same peer link-local address as
source will be associated to the same IP Link to that peer.
3.3.2. IP Interfaces
As is the case for links, the term interface has been historically
confused between the network port that provides physical
connectivity, and the network-layer abstraction that connects the
host with the IP Link:
* an IP Interface is an abstraction that connects the host with a
collection of IP Links (for the purpose of link-local
communication) and bundles the interfaces for each IP Subnet as
subInterfaces. The host installs at least one link-local address
on an IP Interface for each IP Link that is connected through that
Interface, and one subInterface per Subnet. The same link-local
address may be reused over different IP Links as long as it is not
a collision for the peer on that IP Link. Similarly, the host
installs one or more global scope unicast address(es) on an IP
subInterface for the associated Subnet, and the address is
advertised over each IP Link in the SubInterface.
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* an IP Interface can be P2P, in which case it connects to a single
IP Link, or P2MP, in which case it aggregates multiple IP Links.
In a multihomed host, a single IP Interface can be installed to
connect to the IP Links associated to different network ports, in
which case the same IPv6 address may be advertised on more than
one network port. Conversely, when more than one Subnet is
reachable over a network port, more than one IP Interface may
leverage that network port for transmission.
+--------------------
|
| IP SubInterface a ------------------> IP Link A
| IP Subnet a::/64 IP Link B
| IP Addresses a::1 .. a::n ...
| IP Link N
|
| IP SubInterface b ------------------> IP Link A
| IP Subnet b::/64 IP Link D
| IP Addresses b::1 .. b::n ...
| IP Link P
| ...
|
| (Link A and B may be attached to different network ports)
| (Link A may belong to both subInterfaces a and b)
|
+--------------------
IP Interface, using SPL=64
Figure 2: Interface Abstraction
3.3.3. IP Subnets
IPv6 builds another abstraction, the IP Subnet, over one shared IP
Link or over a collection IP Links, forming a MLSN in the latter
case. An MLSN is formed over IP Links (e.g., P2P or P2MP) that are
interconnected by routers that either inject hosts routes in an SGP,
in which case the topology can be anything, or perform ND proxy
operations, in which case the structure of links must be strictly
hierarchical to avoid loops.
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+--------------------
| router 4
| ^ ^
| +----------------------------------+ / |
| | L2 broadcast domain IP Link IP Link
| | / | |
| | v | v
| | router1 <--IP Link--> router 2 <--IP Link--> router 3
| | ^ ^ ^ ^ | ^
| | | \ / | | |
| | IP Link IP Link IP Link IP Link | IP Link
| | | \ / | | |
| | v v v v | v
| | host 1 host 2 host 3 | host 4
| | |
| +----------------------------------+
|
| (Different IP Links may be sustained by different media)
|
+--------------------
IP Subnet
Figure 3: Subnet Abstraction
It is a network design decision to use one IP Subnet model or another
over a given lower-layer network. A switched fabric can host one or
more IP subnets, in which case the IP Links can reach all and beyond
one Subnet. On the other hand, a Subnet can encompass a collection
of links; in that case, the scope of the link-local addresses, which
is the IP Link, is narrower than the span of the Subnet.
A Subnet prefix is associated with the IP Subnet, and a node is a
member of an IP Subnet when it has an IP address that derives from
that prefix. The IP address has Global Unicast scope (in the formal
sense of [RFC4291]), and, as opposed to link-local Addresses, the
scope of the address is not limited to the IP Link.
The switched and routed fabric above could be the exact same network
of physical links and boxes, what changes is the way the networking
abstractions are mapped onto the system, and the implication of such
decision include the capability to reach another node at the link
layer, and the size of the broadcast domain and related broadcast
storms.
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3.3.4. ND Proxies
[RFC8929] defines bridging and routing ND-Classic proxies for
registering nodes / registered addresses. Both forms of ND proxies
interconnect IP Links and enable to isolate the link-layer broadcast
domains. But in the case of a bridging proxy, the link-layer unicast
communication can still exist between the link-layer domains that are
covered by network-layer links, whereas in the base of a routing
proxy, they are isolated, and packets must be routed back and forth.
Bridging proxies are possible between compatible technologies and
translational bridges (e.g., Wi-Fi to Ethernet), whereas routing
proxies are required between non-bridgeable technologies and
desirable to avoid exposing the link-layer addresses across, e.g.,
for reasons of stability and scalability.
ND proxies can also serve IPv6 nodes that still rely on ND-Classic in
a coexistence scenario. The ND proxy intercepts (snoops) the
multicast NS messages from the nodes and, in case or AR or DAD, polls
the registrar to lookup whether an active mapping exists for the
Target. When that is the case, the ND proxy may forward the NS
message as a link-layer unicast to the node that owns the binding,
else it may either drop the multicast or broadcast it at the link
layer. Once the node formed an address, the ND proxy fills the
registrar to associate the IPv6 address with the node. The method is
brittle, since there is no contract with the node to guarantee the
ownership, no "contract", as discussed in Section 2.6, so for those
addresses, the registrar may be inaccurate.
3.3.5. Subnet Gateway Protocols
The SPL boundary creates a wall between the traditional Interior
Gateway Protocols (IGP) that operate between Subnets and manipulate
shorter than SPL prefixes, and Subnet Gateway Protocols (SGP) that
operate inside a Subnet and manipulate longer than SPL prefixes,
typically /128 host routes, and possibly more specific data like
link-layer address mappings and address Proof of Ownership.
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As opposed to classical IGPs, an SGP must support rapid mobility of
addresses to cope with wireless devices and virtual machines
mobility. In that regard, an SGP operates mores as a MANET protocol
than as a classical IGP. Ideally, there should be no stale route,
and no microloop. A classical method in MANETs to achieve this is to
sequence the movements and advertise the sequence in the routing
protocol, so only routes with the most recent sequence can be
followed, and once a packet starts following a route with a certain
sequence, it must be discarded rather to have to follow a path with
an older sequence. To support this approach, the node that registers
an address must be the owner of a mobility sequence number and update
that sequence when it moves.
Multihoming being a classical requirement in DC environments, the SGP
must be able to differentiate not only address duplication from
movement, but also from anycast addresses, which can be advertised
from multiple places in a coordinated (same mobility sequence) or
uncoordinated fashion. For unicast addresses, an token that
identifies the address owner can be used for address duplication
avoidance, and if that token is cryptographic, it can be used as
registration ownership verifier as well.
3.4. IP Models
3.4.1. Physical Broadcast Domain
At the physical (PHY) layer, a node's broadcast domain is the set of
nodes that may receive a transmission that the node sends over a
network port, for instance 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 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 physical-layer communication can be established between two nodes
if the physical broadcast domains of their unicast transmissions
include one another. 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
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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.
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 of B and B in range of C does not
necessarily imply that A is in range of C.
3.4.2. 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 [IEEE Std. 802.15.4]
and IEEE Std. 802.11 [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 [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 [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
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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.
For that reason, upper-layer protocols (ULPs) should tend to avoid
the use of broadcast when operating over IEEE std 802.11 [IEEE Std.
802.11] as they already typically do over IEEE std 802.15.4
[IEEEstd802154]. To cope with these 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 network. 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 [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 link-layer 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. This framework
emulates that proactive method at the network layer for the
operations of AR, DAD and ND proxy.
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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.
As the cost of broadcast transmissions becomes increasingly
expensive, there is a push to rethink the upper-layer protocols to
reduce the dependency on broadcast operations.
3.4.3. Mapping the IP Link Abstraction
As introduced in Section 3.3.1, IPv6 defines a concept of IP Link,
link scope and link-local Addresses (LLA), an LLA being unique and
usable only within the scope of an IP 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 a wired medium, the IP 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
P2MP and 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
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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 network to talk to different peers. In
this framework, each pair of nodes defines a P2P IP Link, and define
the domain where an LLA must be unique.
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.
3.4.4. Mapping the IPv6 Subnet Abstraction
As introduced in Section 3.3.3, IPv6 also defines the concept of a IP
Subnet for IPv6 unicast addresses with a global scope, Global and
Unique Local Addresses (GUA and ULA). All the addresses in the same
Subnet share the same prefix, and by extension, a node belongs to an
IP Subnet if it has an address that derives from the prefix of the
Subnet. That address must be topologically correct, meaning that it
must be installed on a sub-Interface that connects to the Subnet, for
use with routers that expose the Subnet in their RA messages (see
[RFC5942]).
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
GUAs, 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.
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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 IP 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.
3.5. Stateful address Autoconfiguration and Subnet Routing
This Architecture defines a new operation for ND that is based on 2
major paradigm changes, a proactive address registration by hosts to
their attachment routers and routing to host routes (/128) within the
Subnet. This allows ND to avoid the expectations of transit links
and Subnet-wide broadcast domains.
The proactive address registration, called Stateful address
Autoconfiguration (SFAAC) by opposition to SLAAC, is agnostic to the
method used for address Assignment, e.g., Manual, Semantically Opaque
Autoconfiguration [RFC7217], randomized [RFC8981], or DHCPv6
[RFC8415]. It does not change the IPv6 addressing [RFC4291] or the
current practices of assigning prefixes, with typically a SPL of 64,
to a Subnet. But the DAD operation is performed as a unicast
exchange with the abstract egistrar service.
This Architecture combines SFAAC with the not-onlink model on the IP
Interfaces. Hosts do not expect the IP Subnet to be reachable over
the L2 broadcast domain and rely on their routers to forward the
packets inside and outside the Subnet. In turn, the router expose to
each other all the IPv6 addresses that are either owned or registered
to it as host routes over a Subnet Gateway Protocol, a routing
protocol that is specialized in routing inside the Subnet and can be
decoupled with the IGP, that is the routing protocol used between
subnets.
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4. A Framework for Stateful address Autoconfiguration and Subnet
Routing
4.1. Implementing Stateful address Autoconfiguration
SFAAC was initially standardized for IoT and wireless links as
[RFC6775], [RFC8505], and [RFC8928]. A new option in NS/NA messages,
the Extended address Registration Option (EARO) signals that the
Target address is being registered and provides the registration
parameters [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 broadcast transmissions.
The EARO provides information to the router that is independent to
the routing protocol and routing can take multiple forms, from an SGP
to a collapsed Hub-and-Spoke model where only one router owns and
advertises the prefix. [RFC8505] is already referenced as the
registration 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 [RFC9010].
Wireless ND (WiND) is an example instantiation of the Architecture
presented in Section 3; it combines SFAAC with a Backbone Router
(6BBR) ND proxy function (more in [RFC8929]) operating as a network-
layer Access Point. Multiple 6BBRs placed along the wireless edge of
a Backbone link handle IPv6 Neighbor Discovery and forward packets
over the backbone on behalf of the registered nodes on the wireless
edge. This enables to span a Subnet over an MLSN that federates edge
wireless links with a high-speed, typically Ethernet, backbone (as a
network-layer ESS). The ND proxy maintains the reachability for
Global Unicast and link-local Addresses within the federated MLSN,
either as a routing proxy where it replies with its own MAC address
or as a bridging proxy that typically forwards the multicast ND
messages as unicast link-layer frames to their target. The wireless
nodes can form any address they want and move freely from a wireless
edge link to another, without renumbering. In that case, the
registrar is distributed between the 6BBR, each 6BBR maintaining only
a state for the subset of the addresses that were registered to it
and for which it is authoritative. When the 6BBR is not currently
authoritative for a new address being registered to it, it relies on
ND-Classic that is used reactively over the backbone to obtain an
existing registration state in the disaggragated registrar that the
6BBRs form collectively.
This framework allows other implementations of the abstract concept
of the registrar. For instance, [EVPN-SFAAC] allows to distribute
the registrar in every router, and leverages EVPN as the method to
synchronize the registrar state between routers. In that case, BGP
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acts both as the SGP to announce the reachability of the addresses
and as the synchronization protocol between the distributed
registrar. All the routers know proactively the mapping for all the
addresses, and there is no need for a reactive lookup as is the case
for WiND. As another example, a Locator/ID Separation Protocol
(LISP) Map-Resolver [RFC6830] could support the EDAR/EDAC exchange
either directly or via a proxy, and serve as registrar.
The framework allows for mixed environments with registrations and
ND-Classic, using [RFC8929] to perform ND proxy operations on behalf
of registered address and respond to DAD and lookups from legacy
nodes, and prevent registering nodes from autoconfiguring addresses
that exist in legacy nodes by performing DAD on behalf of the
registering nodes, more in Section 6.
4.2. 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 network
port, unless it can separate the IP Links, e.g., based on the remote
MAC address. The DAD operation from SFAAC 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 GUAs and ULAs is validated at
the Subnet Level, using a logical registrar that is global to the
Subnet.
4.3. Subnets and Global Addresses
SFAAC 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.
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Acting as a router, 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.
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 [RFC9010]. 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 [RFC8929] at the Hubs, acting as 6BBRs.
It can be observed that this latter design builds a form of network-
layer Infrastructure ESS.
4.4. Anycast and Multicast Addresses
While ND-Classic is defined for unicast addresses only,
[I-D.ietf-6lo-multicast-registration] extends [RFC8505] for anycast
and multicast IPv6 addresses. Though RPL [RFC6550], which is
extended in that document, is the SGP of choice in a Low-power Lossy
Network (LLN), the registration is agnostic to the SGP and the same
model applies to any SGP that is capable of advertising multicast
and/or anycast addresses as well as unicast.
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[I-D.ietf-6lo-multicast-registration] can be used as a replacement
for MLD [RFC3810] for use cases where broadcast are not desirable,
and when a device push model such as SFAAC is preferred over a
network pull such as MLD and ND-Classic. With [RFC8505], the host
does not need to define SNMAs for its unicast addresses and does not
perform the associated MLDv2 operation. With
[I-D.ietf-6lo-multicast-registration], MLDv2 and its extensive use of
broadcast can be totally eliminated.
In the case of anycast, the signal enables the 6BBRs to accept more
than one registration for the same address, and collectively elect
the registering host receives a packet for a given anycast address.
4.5. Advertising Prefixes in the SGP
By definition, prefixes longer than SPL are inside a Subnet and do
not leak outside the SGP. Still, it is valid for a node to register
a prefix of any size longer than SPL, and for the router to advertise
the registered prefix in the SGP. This can be useful for instance to
expose a /96 Prefix that is used to transport IPv4 mapped traffic
[RFC6052].
[PREFIX REGISTRATION] extends [RFC8505] to enable a node that owns or
is directly connected to a Prefix to register that Prefix to neighbor
routers. The registration indicates that the registered Prefix can
be reached via the advertising node without a loop. The prefix
registration also provides a protocol-independent interface for the
node to request the router to redistribute the prefix in the SGP.
5. WiND Applicability
WiND applies equally to physical links that are P2P, transit, P2MP
Hub-and-Spoke, to links that provide link-layer Broadcast Domain
Emulation such as Mesh-Under and Wi-Fi BSS, and to Route-Over meshes.
In either cases, the IP Link abstraction in WiND is always P2P.
There is an intersection where The IP Link and the IP 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; for more details,
see the introduction of [RFC8929].
There are also a number of practical use cases in the wireless world
where links and subnets are not congruent:
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* 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.
* A typical SmartGrid networks puts together Route-Over MLSNs that
comprise thousands of IPv6 nodes. The 6TiSCH architecture
[RFC9030] 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.
5.1. Case of LPWANs
LPWANs are by nature so constrained that the addresses and subnets
are fully pre-configured and operate as P2P or Hub-and-Spoke. This
saves the steps of neighbor Discovery and enables a very efficient
stateful compression of the IPv6 header. So neither Classic-ND nor
WiND is really used in that space.
5.2. Case of Infrastructure BSS and ESS
In contrast to IPv4, IPv6 enables a node to form multiple addresses,
some of them temporary to elusive, and with a particular attention
paid to privacy. Addresses may be formed and deprecated
asynchronously to the association.
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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.
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 [RFC8928] 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 "ARP-Proxy" operation at the AP. This
includes a proxy ND operation that 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 [RFC8929]. 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 network 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.
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5.3. 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.
5.4. 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.
5.4.1. 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.
5.4.2. 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.
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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 IP Link where it
does not conflict to anyone, so DAD is not necessary at that stage.
The host instantiates an IP Link where the router's address is not a
duplicate. To achieve this, it forms a link-local address 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 |
| |
| ND-Classic RS |
|----------------->|
|------------> |
|------------------------>
| ND-Classic RA |
|<-----------------|
| |
| NS(EARO) |
|----------------->|
| |
| NA(EARO) |
|<-----------------|
| |
...
| NS(EARO) |
|----------------->|
| |
| NA(EARO) |
|<-----------------|
| |
...
Figure 4: RFC 8505 Registration Flow for a link-local address
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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 physical link flapping does not affect the
state of the registration and when a physical 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 IP Link is down.
P2P links may be federated in Hub-and-Spoke by edge routers, and the
Subnet may comprise multiple edge routers, in which case each
advertises its registered addresses over the SGP as illustrated in
Figure 5. Note that the Extended DAR/DAC exchange can be omitted if
it can be replaced with the information that is distributed in the
SGP, see for instance [RFC9010] which applies to IoT environments,
which needs only the first EDAR/EDAC exchange, and [EVPN-SFAAC], for
EVPN-based wireless deployments in enterprise and campus, which does
not use EDAR/EDAC at all.
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(host) (router) (registrar)
| | |
| | |
| ND-Classic RS | |
|----------------->| |
|-----------> | |
|---------------------> |
| ND-Classic RA | |
|<-----------------| |
| | |
| NS(EARO) | |
|----------------->| |
| | Extended DAR |
| |-------------->|
| | |
| | Extended DAC |
| |<--------------|
| NA(EARO) |
|<-----------------|<inject in SGP> ->
...
| | |
| NS(EARO) | |
|----------------->| |
| | Extended DAR |
| |-------------->|
| | |
| | Extended DAC |
| |<--------------|
| NA(EARO) |
|<-----------------|<maintain in SGP> ->
| | |
...
Figure 5: RFC 8505 Registration Flow for a Global address
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.
As opposed to unicast addresses, there is no concept of duplication
with multicast and anycast addresses, and there might be multiple
registrations from multiple parties for the same address. The router
conserves one registration per party per multicast or anycast
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address, but injects the route into the SGP only once for each
address, asynchronously to the registration. On the other hand, the
validation exchange with the registrar is still needed if the router
checks the right for the host to listen to the anycast or multicast
address.
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6LoWPAN Node 6LR 6LBR
(host1) (router) (registrar)
| | |
| DMB link | |
| | |
| ND-Classic RS | |
|----------------->| |
|------------> | |
|------------------------> |
| ND-Classic RA | |
|<-----------------| |
| | |
| NS(EARO) | |
|----------------->| |
| | Extended DAR |
| |-------------->|
| | |
| | Extended DAC |
| |<--------------|
| NA(EARO) |
|<-----------------|<inject in SGP> ->
| |
...
(host2) (router) 6LBR
| NS(EARO) | |
|----------------->| |
| | |
| | Extended DAR |
| |-------------->|
| | |
| | Extended DAC |
| |<--------------|
| NA(EARO) | |
|<-----------------| |
...
(host1) (router)
| NS(EARO) | |
|----------------->| |
| | |
| NA(EARO) | |
|<-----------------| |
...
| |<maintain in SGP> ->
...
Figure 6: Registration Flow for an anycast or multicast address
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6. Coexistence with ND-Classic
The framework allows for a mixed environment with both models, ND-
Classic and SFAAC, coexist. With [RFC8929], an ethernet backbone
link operating ND-Classic federates a MultiLink Subnet (MLSN) of
wireless links and/or meshes, and routers called Backbone Routers
(6BBR) operate as ND proxies.
In a wireless deployments, the Backbone Routers are placed along the
wireless edge of a backbone (e.g., in Access Points) and federate
multiple wireless links to form a single MLSN, echoing the Wi-Fi ESS
structure but at the network layer, as shown in Figure 7. In that
example, Optimistic Duplicate address Detection (ODAD) [RFC4429]
allows the IPv6 address to be used before completion of DAD, so the
whole flow below can happen in the milliseconds that follow the Wi-Fi
association.
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6LN(STA) 6BBR(AP) 6LBR default GW
| | | |
| Wi-Fi Access BSS | IPv6 Backbone (e.g., Ethernet) |
| | | |
| RS(multicast) | | |
|----------------->| | |
| RA(PIO, Unicast) | | |
|<-----------------| | |
| NS(EARO) | | |
|----------------->| | |
| | Extended DAR | |
| |--------------->| |
| | Extended DAC | |
| |<---------------| |
| | |
| | NS-DAD(EARO, multicast) |
| |--------> |
| |-------------------> |
| |----------------------------------->|
| | |
| | RS(no SLLAO, for ODAD) |
| |----------------------------------->|
| | if (no fresher Binding) NS(Lookup) |
| | <----------------|
| | <-------------------------|
| |<-----------------------------------|
| | NA(SLLAO, not(O), EARO) |
| |----------------------------------->|
| | RA(unicast) |
| |<-----------------------------------|
| | |
| IPv6 Packets in Optimistic Mode |
|<----------------------------------------------------->|
| | |
| |
| NA(EARO) | <DAD timeout>
|<--------- -------|
| |
Figure 7: Initial Registration Flow to a 6BBR Acting as a ND Proxy
In use cases such as overlays, a Map Resolver acting as 6LBR may be
deployed on the Backbone Link to serve the whole Subnet, and EDAR/
EDAC messages (or equivalent alternates, e.g., using LISP) can be
used in combination with DAD to enable coexistence with ND-Classic
over the backbone. The 6LBR proactive operations will then coexist
on the Backbone with the reactive ND-Classic operation. Nodes that
support [UNICAST AR] may query the mappings they look up with the
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6LBR before attempting the reactive operation, which may be avoided
if the 6LBR is conclusive, either detecting a duplication or
returning a mapping. This model also enables a snooping switch
acting as ND proxy to intercept Ar and DAD NS messages and perform
unicast lookups to the resolver and only broadcast the original NS
messgae when the unicast lookup fails.
Note that the RS sent initially by the 6LN (e.g., a Wi-Fi STA) is
transmitted as a multicast, but since it is intercepted by the 6BBR,
it is never effectively broadcast at link layer. The multiple arrows
in Figure 7 associated to the ND messages on the backbone denote a
real link-layer broadcast.
It is not necessary to isolate the registering nodes in separate
physical links, but it is preferred with wireless links as it enables
to isolate the broadcast domain on the ethernet link from the
wireless links at the Access Points. In other words, the 6BBRs
collectively form a global registrar for the Subnet that aggregates
the information in each local registrar in the 6LBR. The global
registrar is distributed between the 6BBRs, which leverage ND-Classic
(AR and DAD) to lookup information that they do not have locally from
the other 6BBRs and from nodes that are connected to the backbone.
In the case of wireless meshes, RPL may be used as local SGP in each
mesh as shown in Figure 8. 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
[RFC9030].
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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
| | | |
| ND-Classic RS | | |
|-------------->| | |
|-----------> | | |
|------------------> | |
| ND-Classic RA | | |
|<--------------| | |
| | <once> | |
| NS(EARO) | | |
|-------------->| | |
| 6LoWPAN ND | Extended DAR | |
| |-------------->| |
| | | NS(EARO) |
| | |-------------->|
| | | proxy registration
| | | |
| | | NS-DAD (EARO)
| | | |------>
| | | ND proxy
| | | |
| | | NA(EARO) |<timeout>
| | |<--------------|
| | Extended DAC | |
| |<--------------| |
| NA(EARO) | | |
|<--------------| | |
| | | |
Figure 8: Initial Registration Flow with 6BBR ND-Proxy
7. Privacy Considerations
ND-Classic exposes all addresses to all nodes in the Subnet, which is
a privacy issue and makes impersonation attacks easier. In contrast,
in switched and wireless networks, a host is not on-path of the
unicast packets for registration and for data for other hosts, so it
cannot snoop the other addresses in the network. A rogue host can
only discover the existence of an addresses by trying and failing to
register that address, but for that it would need to fathom which
address to try and that can be very hard in, say, a SPL=64 address
space that is used wisely. For that reason, this framework limits
that knowledge to on-path snooping switches, to the routers and to
the abstract registrar, which are typically more controlled / harder
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to hack than the common host. When ND-Classic and SFAAC coexist
within the same Subnet, all addresses in the Subnet, including
registered addresses, can be snooped in the broadcast domain where
ND-Classic is operated. It makes sense to reduce that domain to the
maximum and control which device connect to it.
The exposure of addresses can be further reduced if the exchanges
with the registrar (e.g., EDAR and EDAC) are encrypted, e.g., using a
public key associated with the registrar. The registration and
routing exchanges could also be encrypted to avoid leaking the
addresses to snopping switches, but this is typically not done inside
a physical site where the networking gear is tightly controlled. In
a DCI environment, the inter-side (SD-WAN) links are typically
encrypted, to the exchanges are obfuscated from an on-path listener.
8. Security Considerations
The registration model [RFC8505] implemented by this framework allows
for a model where the ingress routers have a full knowledge of all
the addresses in the Subnet. The ingress router can thus discard any
packet which destination appears to be in the Subnet from its prefix,
but is not known, meaning that it does not exist. This mostly
defeats the traditional DoS scanning attacks against ND whereby the
remote attacker sends volumes of packets to as many non-existent
addresses to saturate the Neighbor Cache and clog the Subnet internal
bandwidth in broadcasts.
When the ownership verifier is cryptographic, this framework enables
a zerotrust model whereby only the address owner can advertise an
address in ND and as source of data packets, more in [RFC8928]. This
defeats the classical impersonation attacks against ND-Classic and
allows to disable the proprietary middlebox software aimed at
protecting the address ownership against onlink rogues.
9. IANA Considerations
This specification does not require IANA action.
10. Contributors
Brian Carpenter Provided support, hints, and text snippets
throughout the lifetime of the I-Draft
11. Acknowledgments
Many thanks to the participants of the 6lo WG where a lot of the work
discussed here happened, following work at ROLL, 6TiSCH, and mainly
6LoWPAN.
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Special thanks to Brian Carpenter and Eric Levy-Abegnoli who provided
support and useful comments throughout the development of this
architecture, and to Erik Nordmark and Zach Shelby with whom this
work really started during IETF 72 in Dublin.
Also many thanks to Eduard Vasilenko, XiPeng Xiao, Behcet Sarikaya
for their contributions and support to this work at 6MAN, v6Ops, and
ETSI IPE.
12. Normative References
[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>.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.
[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
2011, <https://www.rfc-editor.org/info/rfc6275>.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
DOI 10.17487/RFC6830, January 2013,
<https://www.rfc-editor.org/info/rfc6830>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
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[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
[RFC8928] Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
"Address-Protected Neighbor Discovery for Low-Power and
Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
2020, <https://www.rfc-editor.org/info/rfc8928>.
[RFC8929] Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
"IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
November 2020, <https://www.rfc-editor.org/info/rfc8929>.
13. Informative 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>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
<https://www.rfc-editor.org/info/rfc4429>.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
DOI 10.17487/RFC4903, June 2007,
<https://www.rfc-editor.org/info/rfc4903>.
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[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<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>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC7834] Saucez, D., Iannone, L., Cabellos, A., and F. Coras,
"Locator/ID Separation Protocol (LISP) Impact", RFC 7834,
DOI 10.17487/RFC7834, April 2016,
<https://www.rfc-editor.org/info/rfc7834>.
[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>.
[RFC8365] Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
Uttaro, J., and W. Henderickx, "A Network Virtualization
Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
DOI 10.17487/RFC8365, March 2018,
<https://www.rfc-editor.org/info/rfc8365>.
[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>.
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[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.
[I-D.ietf-rift-rift]
Przygienda, T., Sharma, A., Thubert, P., Rijsman, B.,
Afanasiev, D., and J. Head, "RIFT: Routing in Fat Trees",
Work in Progress, Internet-Draft, draft-ietf-rift-rift-16,
12 September 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rift-rift-16>.
[RFC9010] Thubert, P., Ed. and M. Richardson, "Routing for RPL
(Routing Protocol for Low-Power and Lossy Networks)
Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
<https://www.rfc-editor.org/info/rfc9010>.
[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://datatracker.ietf.org/doc/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://datatracker.ietf.org/doc/html/
draft-vyncke-6man-mcast-not-efficient-01>.
[RFC9030] Thubert, P., Ed., "An Architecture for IPv6 over the Time-
Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
RFC 9030, DOI 10.17487/RFC9030, May 2021,
<https://www.rfc-editor.org/info/rfc9030>.
[MCAST PROBLEMS]
Perkins, C. E., McBride, M., Stanley, D., Kumari, W. A.,
and J. C. Zúñiga, "Multicast Considerations over IEEE 802
Wireless Media", Work in Progress, Internet-Draft, draft-
ietf-mboned-ieee802-mcast-problems-15, 28 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-mboned-
ieee802-mcast-problems-15>.
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[SAVI] Bi, J., Wu, J., Lin, T., Wang, Y., and L. He, "A SAVI
Solution for WLAN", Work in Progress, Internet-Draft,
draft-bi-savi-wlan-24, 13 November 2022,
<https://datatracker.ietf.org/doc/html/draft-bi-savi-wlan-
24>.
[UNICAST AR]
Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
Unicast Lookup", Work in Progress, Internet-Draft, draft-
thubert-6lo-unicast-lookup-02, 8 November 2021,
<https://datatracker.ietf.org/doc/html/draft-thubert-6lo-
unicast-lookup-02>.
[PREFIX REGISTRATION]
Thubert, P., "IPv6 Neighbor Discovery Prefix
Registration", Work in Progress, Internet-Draft, draft-
thubert-6lo-prefix-registration-02, 3 January 2023,
<https://datatracker.ietf.org/doc/html/draft-thubert-6lo-
prefix-registration-02>.
[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://datatracker.ietf.org/doc/html/draft-nordmark-
6man-dad-approaches-02>.
[EVPN-SFAAC]
Thubert, P., Przygienda, T., and J. Tantsura, "Secure EVPN
MAC Signaling", Work in Progress, Internet-Draft, draft-
thubert-bess-secure-evpn-mac-signaling-03, 31 January
2022, <https://datatracker.ietf.org/doc/html/draft-
thubert-bess-secure-evpn-mac-signaling-03>.
[I-D.ietf-6lo-multicast-registration]
Thubert, P., "IPv6 Neighbor Discovery Multicast and
Anycast Address Listener Subscription", Work in Progress,
Internet-Draft, draft-ietf-6lo-multicast-registration-14,
8 March 2023,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-6lo-multicast-registration/>.
[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".
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[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)".
[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".
Authors' Addresses
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
Michael C. Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/
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