Internet Engineering Task Force (IETF) L. Colitti

Internet Engineering Task Force (IETF) L. Colitti
Request for Comments: 9663 Google, LLC
Category: Informational J. Linkova, Ed.
ISSN: 2070-1721 X. Ma, Ed.
Google
October 2024

Using DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique IPv6
Prefixes per Client in Large Broadcast Networks

Abstract

This document discusses an IPv6 deployment scenario when individual
nodes connected to large broadcast networks (such as enterprise
networks or public Wi-Fi networks) are allocated unique prefixes via
DHCPv6 Prefix Delegation (DHCPv6-PD), as specified in RFC 8415.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9663.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
2. Requirements Language
3. Terminology
4. Design Principles
5. Applicability and Limitations
6. Routing and Addressing Considerations
6.1. Prefix Pool Allocation
6.2. First-Hop Router Requirements
6.3. Topologies with Multiple First-Hop Routers
6.4. On-Link Communication
7. DHCPv6-PD Server Considerations
8. Prefix Length Considerations
9. Client Mobility
10. Antispoofing and SAVI Interaction
11. Migration Strategies and Co-existence with SLAAC Using Prefixes
from the PIO
12. Benefits
13. Privacy Considerations
14. IANA Considerations
15. Security Considerations
16. References
16.1. Normative References
16.2. Informative References
Appendix A. Multiple Addresses Considerations
Acknowledgements
Authors' Addresses

1. Introduction

Often, broadcast networks such as enterprise or public Wi-Fi
deployments place many devices on a shared link with a single on-link
prefix. This document describes an alternative deployment model
where individual devices obtain prefixes from the network. This
provides two important advantages.

First, it offers better scalability. Unlike IPv4, IPv6 allows hosts
to have multiple addresses, and this is the case in most deployments
(see Appendix A for more details). However, increasing the number of
addresses introduces scalability issues on the network
infrastructure. Network devices need to maintain various types of
tables and hashes (Neighbor Cache on first-hop routers, Neighbor
Discovery Proxy caches on Layer 2 devices, etc.). On Virtual
eXtensible Local Area Network (VXLAN) networks [RFC7348], each
address might be represented as a route. This means, for example,
that if every client has 10 addresses instead of one, the network
must support 10 times more routes, etc. If an infrastructure
device's resources are exhausted, the device might drop some IPv6
addresses from the corresponding tables, while the address owner
might still be using the address to send traffic. This leads to
traffic being discarded and a degraded customer experience.
Providing every host with one prefix allows the network to maintain
only one entry per device, while still providing the device the
ability to use an arbitrary number of addresses.

Second, this deployment model provides the ability to extend the
network. In IPv4, a device that connects to the network can provide
connectivity to subtended devices by using NAT. With DHCPv6 Prefix
Delegation (DHCPv6-PD) [RFC8415], such a device can similarly extend
the network, but unlike IPv4 NAT, it can provide its subtended
devices with full end-to-end connectivity.

Another method of deploying unique prefixes per device is documented
in [RFC8273]. Similarly, the standard deployment model in cellular
IPv6 networks [RFC6459] provides a unique prefix to every device.
However, providing a unique prefix per device is very uncommon in
enterprise-style networks, where nodes are usually connected to
broadcast segments such as VLANs and each link has a single on-link
prefix assigned. This document takes a similar approach to
[RFC8273], but allocates the prefix using DHCPv6-PD.

This document focuses on the behavior of the network. Host behavior
is not defined in this document.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Terminology

Node: a device that implements IPv6 [RFC8200]

Host: any node that is not a router [RFC8200]

Client: a node that connects to a network and acquires addresses.
The node may wish to obtain addresses for its own use, or it may
be a router that wishes to extend the network to its physical or
virtual subsystems, or both. It may be either a host or a router
as defined by [RFC8200].

AP: (wireless) Access Point

DHCPv6 IA_NA: Identity Association for Non-temporary Addresses
(Section 21.4 of [RFC8415])

DHCPv6 IA_PD: Identity Association for Prefix Delegation
(Section 21.21 of [RFC8415])

DHCPv6-PD: DHCPv6 Prefix Delegation [RFC8415]; a mechanism to
delegate IPv6 prefixes to clients.

ND: Neighbor Discovery [RFC4861]

NUD: Neighbor Unreachability Detection [RFC4861]

PIO: Prefix Information Option [RFC4862]

SLAAC: IPv6 Stateless Address Autoconfiguration [RFC4862]

4. Design Principles

Instead of all clients on a given link forming addresses from the
same shared prefix assigned to that link, this deployment model
operates as described below:

* A device acts as a DHCPv6-PD client and requests a prefix via
DHCPv6-PD by sending an IA_PD request.

* The server delegates a prefix to the client and the delegated
prefix is installed into the routing table of the first-hop router
as a route pointing to the client's link-local address. The
first-hop router can act as a DHCPv6 relay and snoop DHCPv6 Reply
messages from an off-link DHCPv6 server, or it can act as a DHCPv6
server itself. In both cases, it can install the route locally,
and if the network is running a dynamic routing protocol,
distribute the route or the entire prefix pool into the protocol.

* For the router and all other infrastructure devices, the delegated
prefix is considered off-link, so traffic to that prefix does not
trigger any ND packets, other than the minimum ND required to
sustain Neighbor Unreachability Detection (NUD) for the client's
link-local address.

* The device can use the delegated prefix in various ways. For
example, it can form addresses, as described in requirement WAA-7
of [RFC7084]. It can also extend the network, as described in
[RFC7084] or [RFC7278].

An example scenario is shown in Figure 1. Note that the prefix
lengths used in the example are /64 because that is the prefix length
currently supported by SLAAC and is not otherwise required by the
proposed deployment model.

+------------------------------------------------------------------+
| DHCPv6 Servers |
| Pool 3fff:0:d::/48 for clients on 2001:db8:ff::/64 link |
+------------+---------------------+----------------------------+--+
^ | | ^ |
| | | | |
+-------+------+---------------------+--------------------+-------+---+
| DHCPv6| | IPv6 Network DHCPv6 | | |
|Relay-Forward | Relay-Forward | |
| ^ v Route for 3fff:0:d::/48 ^ v |
| | DHCPv6 | | | DHCPv6 |
| | Relay-Reply | | | Relay-Reply|
| | | | | | | |
+------+-------+--+----------+------------+------+-------+--------+---+
| | | | | | | |
| v | v v | | v
+----+----------+---------------+ +---------+-------+------------+
| First-hop router/DHCPv6 relay | | First-hop Router/DHCPv6 relay|
| 3fff:0:d:1::/64 -> fe80::aa | | 3fff:0:d:1::/64 -> fe80::aa |
| 3fff:0:d:2::/64 -> fe80::cc | | 3fff:0:d:2::/64 -> fe80::cc |
+------------+----------+-------+ +--------+----------------+----+
^ | | Shared IPv6 link | ^ |
| | | 2001:db8:ff::/64 | | |
| | -+-----+-----------+---------+-----+- | |
| | | | | | |
| | | +---------------+-------------+ | DHCPv6 |
DHCPv6 | | | Client B (no DHCPv6-PD) | | Request v
Request | | |link-local address fe80::b | | ^ DHCPv6
^ | | |global address 2001:db8:ff::b| | | Reply
| | | +-----------------------------+ | | |
| v | | | v
| DHCPv6 | +--------------------+--+----------+
| Reply | | Client C |
| | | | link-local address fe80::cc |
| | | | delegated prefix 3fff:0:d:2::/64 |
| | | +------------+-------------------+-+
| v | | |
+---+-------------+----------------------+ | Router |
| Client A | | Advertisement |
| link-local address: fe80::aa | | containing PIO v
| delegated prefix: 3fff:0:d:1::/64 | | 3fff:0:d:2::/64
| +----------------+ +----------------+ | -+---------+-----------
| | virtual system | | virtual system | | |
| | 3fff:0:d:1::de | | 3fff:0:d:1::ad | | +------+----------+
| | 3fff:0:d:1::ca | | 3fff:0:d:1::fe | | | Tethered device |
| +----------------+ +----------------+ | | 3fff:0:d:2::66 |
| | +-----------------+
+----------------------------------------+

Figure 1: An Example Topology with Two First-Hop Routers

5. Applicability and Limitations

Delegating a unique prefix per client provides all the benefits of
both SLAAC and DHCPv6 address allocation, but at the cost of greater
address-space usage. This design would substantially benefit some
networks (see Section 12) in which the additional cost of an
additional service (such as DHCPv6 Prefix Delegation) and allocation
of a larger amount of address space can easily be justified.
Examples of such networks include but are not limited to:

* Large-scale networks where even three to five addresses per client
might introduce scalability issues.

* Networks with a high number of virtual hosts, so physical devices
require multiple addresses.

* Managed networks where extensive troubleshooting, device traffic
logging, or forensics might be required.

In smaller networks, such as home networks or small enterprises with
smaller address space and a lower number of clients, SLAAC is a
simpler and often preferred option.

6. Routing and Addressing Considerations

6.1. Prefix Pool Allocation

One simple deployment model is to assign a dedicated prefix pool to
each link. The prefixes from each link's pool are only issued to
requesting clients on the link; if clients move to another link, they
will obtain a prefix from the pool associated with the new link (see
Section 9).

This is very similar to how address pools are allocated when using
DHCP to assign individual addresses (e.g., DHCPv4 or DHCPv6 IA_NA),
where each link has a dedicated pool of addresses, and clients on the
link obtain addresses from the pool. In this model, the network can
route the entire pool to the link's first-hop routers, and the
routers do not need to advertise individual delegated prefixes into
the network's dynamic routing protocol.

Other deployment models, such as prefix pools shared over multiple
links or routers, are possible but are not described in this
document.

6.2. First-Hop Router Requirements

In large networks, DHCPv6 servers are usually centralized and reached
via DHCPv6 relays co-located with the first-hop routers. To delegate
IPv6 prefixes to clients, the first hop routers need to implement
DHCPv6 relay functions and meet the requirements defined in
[RFC8987]. In particular, per Section 4.2 of [RFC8987], the first-
hop router must maintain a local routing table that contains all
prefixes delegated to clients.

With the first-hop routers performing DHCPv6 relay functions, the
proposed design neither requires any subsequent relays in the path
nor introduces any requirements (e.g., snooping) for such subsequent
relays, if they are deployed.

To ensure that routes to the delegated prefixes are preserved even if
a relay is rebooted or replaced, the operator MUST ensure that all
relays in the network infrastructure support DHCPv6 Bulk Leasequery
as defined in [RFC5460]. While Section 4.3 of [RFC8987] lists
keeping active prefix delegations in persistent storage as an
alternative to DHCPv6 Bulk Leasequery, relying on persistent storage
has the following drawbacks:

* In a network with multiple relays, network state can change
significantly while the relay is rebooting (new prefixes might be
delegated or some prefixes might be expiring, etc).

* Persistent storage might not be preserved if the router is
physically replaced.

Another mechanism for first-hop routers to obtain information about
delegated prefixes is by using Active Leasequery [RFC7653], though
this is not yet widely supported.

6.3. Topologies with Multiple First-Hop Routers

In a topology with redundant first-hop routers, all the routers need
to relay DHCPv6 traffic, install the delegated prefixes into their
routing tables and, if needed, advertise those prefixes to the
network.

If the first-hop routers obtain information about delegated prefixes
by snooping DHCPv6 Reply messages sent by the server, then all the
first-hop routers must be able to snoop these messages. This is
possible if the client multicasts the DHCPv6 messages it sends to the
server. The server will receive one copy of the client message
through each first-hop relay, and will reply unicast to each of them
via the relay (or chain of relays) from which it received the
message. Thus, all first-hop relays will be able to snoop the
replies. Per Section 14 of [RFC8415], clients always use multicast
unless the server uses the Server Unicast option to explicitly allow
unicast communication ([RFC8415], Section 21.12). Therefore, in
topologies with multiple first-hop routers, the DHCPv6 servers MUST
be configured not to use the Server Unicast option. It should be
noted that [RFC8415bis] deprecates the Server Unicast option
precisely because it is not compatible with topologies with multiple
first-hop relays.

To recover from crashes or reboots, relays can use Bulk Leasequery or
Active Leasequery to issue a QUERY_BY_RELAY_ID with the ID(s) of the
other relay(s), as configured by the operator. Additionally, some
vendors provide vendor-specific mechanisms to synchronize state
between DHCP relays.

6.4. On-Link Communication

For security reasons, some networks block on-link device-to-device
traffic at Layer 2 to prevent communication between clients on the
same link. In this case, delegating a prefix to each client doesn't
affect traffic flows, as all traffic is sent to the first-hop router
anyway. Depending on the network security policy, the router may
allow or drop the traffic.

If the network does allow peer-to-peer communication, the PIO for the
on-link prefix is usually advertised with the L-bit set to 1
[RFC4861]. As a result, all addresses from that prefix are
considered on-link, and traffic to those destinations is sent
directly (not via routers). If such a network delegates prefixes to
clients (as described in this document), then each client will
consider other client's destination addresses to be off-link, because
those addresses are from the delegated prefixes and are no longer
within the on-link prefix. When a client sends traffic to another
client, packets will initially be sent to the default router. The
router will respond with an ICMPv6 redirect message (Section 4.5 of
[RFC4861]). If the client receives and accepts the redirect, then
traffic can flow directly from device to device. Therefore, the
administrator deploying the solution described in this document
SHOULD ensure that the first-hop routers can send ICMPv6 redirects
(the routers are configured to do so and the security policies permit
those messages).

7. DHCPv6-PD Server Considerations

This document does not introduce any changes to the DHCPv6 protocol
itself. However, for the proposed solution to work correctly, the
DHCPv6-PD server needs to be configured as follows:

* The server MUST follow recommendations from [RFC8168] on
processing prefix-length hints.

* The server MUST provide a prefix short enough for the client to
extend the network to at least one interface and allow nodes on
that interface to obtain addresses via SLAAC. The server MAY
provide more address space to clients that ask for it, either by
delegating multiple such prefixes, or by delegating a single
prefix of a shorter length. It should be noted that [RFC8168]
allows the server to provide a prefix shorter than the prefix-
length hint value received from the client.

* If the server receives the same Solicit message from the same
client multiple times through multiple relays, it MUST reply to
all of them with the same prefix(es). This ensures that all the
relays will correctly configure routes to the delegated prefixes.

* The server MUST NOT send the Server Unicast option to the client
unless the network topology guarantees that no client is connected
to a link with multiple relays (see Section 6.3).

* In order to ensure uninterrupted connectivity when a first-hop
router crashes or reboots, the server MUST support Bulk Leasequery
or Active Leasequery.

As most operators have some experience with IPv4, they can use a
similar approach for choosing the pool size and the timers (such as
T1 and T2 timers). In particular, the following factors should be
taken into account:

* the expected maximum number of clients;

* the average duration of client connections;

* how mobile the clients are (a network where all clients are
connected to a single wired VLAN might choose longer timers than a
network where clients can switch between multiple wireless
networks);

* how often clients are expected to reconnect to the network (for
example, a corporate authenticated Wi-Fi network might be using
longer timers than an open public Wi-Fi).

DHCPv6 servers that delegate prefixes can interface with Dynamic DNS
infrastructure to automatically populate reverse DNS using wildcard
records, similarly to what is described in Section 2.2 of [RFC8501].
Networks that also wish to populate forward DNS cannot do so
automatically based only on DHCPv6 prefix delegation transactions,
but they can do so in other ways, such as by supporting DHCPv6
address registration as described in [ADDR-NOTIFICATION].

Some additional recommendations driven by security and privacy
considerations are discussed in Section 15 and Section 13.

8. Prefix Length Considerations

Delegating a prefix of sufficient size to use SLAAC allows the client
to extend the network, providing limitless addresses to IPv6 nodes
connected to it (e.g., virtual machines or tethered devices), because
all IPv6 hosts are required to support SLAAC [RFC8504].
Additionally, even clients that support other forms of address
assignment require SLAAC for some functions, such as forming
dedicated addresses for the use of 464XLAT (see Section 6.3 of
[RFC6877]).

At the time of writing, the only prefix size that will allow devices
to use SLAAC is 64 bits. Also, as noted in [RFC7421], using an
interface identifier (IID) shorter than 64 bits and a subnet prefix
longer than 64 bits is outside the current IPv6 specifications.
Choosing longer prefixes would require the client and any connected
system to use other address assignment mechanisms. This would limit
the applicability of the proposed solution, as other mechanisms are
not currently supported by many hosts.

For the same reasons, a prefix length of /64 or shorter is required
to extend the network as described in [RFC7084] (see requirement
L-2), and a prefix length of /64 is required to provide global
connectivity for stub networks as per [SNAC-SIMPLE].

Assigning a prefix of sufficient size to support SLAAC is possible on
large networks. In general, any network that numbers clients from an
IPv4 prefix of length X (e.g., X=/18, X=/24) would require an IPv6
prefix of length X+32 (e.g., X=/40, X=/56) to provide a /64 prefix to
every device. As an example, Section 9.2 of [RFC7934] suggests that
even a very large network that assigns every single one of the 16
million IPv4 addresses in 10.0.0.0/8 would only need an IPv6 /40. A
/40 prefix is a small amount of address space: there are 32 times
more /40s in the current IPv6 unicast range 2000::/3 than there are
IPv4 addresses. Existing sites that currently use a /48 prefix
cannot support more than 64k clients in this model without
renumbering, though many networks of such size have Local Internet
Registry (LIR) status and can justify bigger address blocks.

Note that assigning a prefix of sufficient size to support SLAAC does
not require that subtended nodes use SLAAC; they can use other
address assignment mechanisms as well.

9. Client Mobility

As per Section 18.2.12 of [RFC8415], when the client moves to a new
link, it MUST initiate a Rebind/Reply message exchange. Therefore,
when the client moves between network attachment points, it would
refresh its delegated prefix the same way it refreshes addresses
assigned (via SLAAC or DHCPv6 IA_NA) from a shared on-link prefix:

* When a client moves from between different attachment points on
the same link (e.g., roams between two APs while connected to the
same wireless network or moves between two switchports belonging
to the same VLAN), the delegated prefix does not change, and the
first-hop routers have a route for the prefix with the nexthop set
to the client link-local address on that link. As per requirement
S-2 in Section 4.3 of [RFC8987], the DHCPv6-relays (the first-hop
routers) MUST retain the route for the delegating prefix until the
route is released or removed due to expiring DHCP timers.
Therefore, if the client reconnects to the same link, the prefix
doesn't change.

* When a client moves to a different link, the DHCPv6 server
provides the client with a new prefix, so the behavior is
consistent with SLAAC or DHCPv6-assigned addresses, which are also
different on the new link.

In theory, DHCPv6 servers can delegate the same prefix to the same
client even if the client changes the attachment points. However,
while allowing the client to keep the same prefix while roaming
between links might provide some benefits for the client, it is not
feasible without changing DHCPv6 relay behavior: after the client
moves to a new link, the DHCPv6 relays would retain the route
pointing to the client's link-local address on the old link for the
duration of DHCPv6 timers (see requirement S-2, Section 4.3 of
[RFC8987]). As a result, the first-hop routers would have two routes
for the same prefix pointing to different links, causing connectivity
issues for the client.

It should be noted that addressing clients from a shared on-link
prefix also does not allow clients to keep addresses while roaming
between links, so the proposed solution is not different in that
regard. In addition to that, different links often have different
security policies applied (for example, corporate internal networks
versus guest networks), hence clients on different links need to use
different prefixes.

10. Antispoofing and SAVI Interaction

Enabling unicast Reverse Path Forwarding (uRPF) [RFC3704] on the
first-hop router interfaces towards clients provides the first layer
of defense against spoofing. A spoofed packet sent by a malicious
client would be dropped by the router unless the spoofed address
belongs to a prefix delegated to another client on the same
interface. Therefore the malicious client can only spoof addresses
already delegated to another client on the same link or another
client's link-local address.

Source Address Validation Improvement (SAVI) [RFC7039] provides more
reliable protection against address spoofing. Administrators
deploying the proposed solution on SAVI-enabled infrastructure SHOULD
ensure that SAVI perimeter devices support DHCPv6-PD snooping to
create the correct binding for the delegated prefixes (see
[RFC7513]). Using FCFS SAVI [RFC6620] to protect link-local
addresses and create SAVI bindings for DHCPv6-PD assigned prefixes
would prevent spoofing.

Some infrastructure devices do not implement SAVI as defined in
[RFC7039]; instead, they perform other forms of address tracking and
snooping for security or performance improvement purposes (e.g., ND
proxy). This is very common behavior for wireless devices (such as
access points and controllers). Administrators SHOULD ensure that
such devices are able to snoop DHCPv6-PD packets so the traffic from
the delegated prefixes is not dropped.

It should be noted that using DHCPv6-PD makes it harder for an
attacker to select the spoofed source address. When all clients are
using the same shared link to form addresses, the attacker might
learn addresses used by other clients by listening to multicast
Neighbor Solicitations and Neighbor Advertisements. In DHCPv6-PD
environments, however, the attacker can only learn about other
clients' global addresses by listening to multicast DHCPv6 messages,
which are not transmitted so often, and may not be received by the
client at all because they are sent to multicast groups that are
specific to DHCPv6 servers and relays.

11. Migration Strategies and Co-existence with SLAAC Using Prefixes
from the PIO

It would be beneficial for the network to explicitly indicate its
support of DHCPv6-PD for connected clients.

* In small networks (e.g., home networks), where the number of
clients is not too high, the number of available prefixes becomes
a limiting factor. If every phone or laptop in a home network
were to request a unique prefix suitable for SLAAC, the home
network might run out of prefixes, if the prefix allocated to the
Customer Premises Equipment (CPE) by its ISP is too long. For
example, if an ISP delegates a /60, the CPE would only be able to
delegate fifteen /64 prefixes to clients. So while the enterprise
network administrator might want all phones in the network to
request a prefix, it would be highly undesirable for the same
phone to request a prefix when connecting to a home network.

* When the network supports both a unique prefix per client and a
PIO with A=1 as address assignment methods, it's highly desirable
for the client NOT to use the PIO prefix to form global addresses
and instead only use the prefix delegated via DHCPv6-PD. Starting
both SLAAC using the PIO prefix and DHCPv6-PD, and then
deprecating the SLAAC addresses after receiving a delegated prefix
would be very disruptive for applications. If the client
continues to use addresses formed from the PIO prefix, it would
not only undermine the benefits of the proposed solution (see
Section 12), but it would also introduce complexity and
unpredictability in the source address selection. Therefore, the
client needs to know what address assignment method to use and
whether or not to use the prefix in the PIO, if the network
provides the PIO with the 'A' flag set.

The deployment model described in this document does not require the
network to signal support of DHCPv6-PD: for example, devices acting
as compatible routers [RFC7084] will be able to receive prefixes via
DHCPv6-PD even without such signaling. Also, some clients may decide
to start DHCPv6-PD and acquire prefixes if they detect that the
network does not provide addresses via SLAAC. To fully achieve the
benefits described in this section, [PIO-PFLAG] defines a new PIO
flag to signal that DHCPv6-PD is the preferred method of obtaining
prefixes.

12. Benefits

The proposed solution provides the following benefits:

* Network device resources (e.g., memory) need to scale to the
number of devices, not the number of IPv6 addresses. The first-
hop routers have a single route per device pointing to the
device's link-local address. This can potentially enable hardware
cost savings; for example, if hardware such as wireless LAN
controllers is limited to supporting only a specific number of
client addresses, or in VXLAN deployments where each client
address consumes one routing table entry.

* The cost of having multiple addresses is offloaded to the clients.
Hosts are free to create and use as many addresses as they need
without imposing any additional costs onto the network.

* If all clients connected to the given link support this mode of
operation and can generate addresses from the delegated prefixes,
there is no reason to advertise a common prefix assigned to that
link in the PIO with the 'A' flag set. Therefore, it is possible
to remove the global shared prefix from that link and the router
interface completely, so no global addresses are on-link for the
link. This would lead to reducing the attack surface for Neighbor
Discovery attacks described in [RFC6583].

* DHCPv6-PD logs and routing tables obtained from first-hop routers
provide complete information on IPv6 to MAC mapping, which can be
used for forensics and troubleshooting. Such information is much
less dynamic than the ND cache; therefore, it's much easier for an
operator to collect and process it.

* A dedicated prefix per client allows the network administrator to
create security policies per device (such as ACLs) even if the
client is using temporary addresses. This mitigates one of the
issues described in [IPv6-ADDRESS].

* Fate sharing: all global addresses used by a given client are
routed as a single prefix. Either all of them work or none of
them work, which makes failures easier to diagnose and mitigate.

* Lower level of multicast traffic: less Neighbor Discovery
[RFC4861] multicast packets, as the routers need to resolve only
the clients' link-local addresses. Also, there is no Duplicate
Address Detection (DAD) traffic except for the clients' link-local
addresses.

* Ability to extend the network transparently. If the network
delegates to the client a prefix of sufficient size to support
SLAAC, the client can provide connectivity to other hosts, as is
possible in IPv4 with NAT (e.g., by acting as an IPv6 Customer
Edge (CE) router as described in [RFC7084]).

13. Privacy Considerations

If an eavesdropper or information collector is aware that a given
client is using the proposed mechanism, then they may be able to
track the client based on its prefix. The privacy implications of
this are equivalent to the privacy implications of networks using
stateful DHCPv6 address assignment: in both cases, the IPv6 addresses
are determined by the server, either because the server assigns a
full 128-bit address in a shared prefix, or because the server
determines what prefix is delegated to the client. Administrators
deploying the proposed mechanism can use similar methods to mitigate
the impact as the ones used today in networks that use stateful
DHCPv6 address assignment.

Except for networks (such as datacenter networks) where hosts do not
need temporary addresses [RFC8981], the network SHOULD:

* Ensure that when a client requests a prefix, the prefix is
randomly assigned and not allocated deterministically.

* Use short prefix lifetimes (e.g., hours) to ensure that when a
client disconnects and reconnects it gets a different prefix.

* Allow the client to have more than one prefix at the same time.
This allows the client to rotate prefixes using a mechanism
similar to temporary addresses, but that operates on prefixes
instead of on individual addresses. In this case, the prefix's
lifetime MUST be short enough to allow the client to use a
reasonable rotation interval without using too much address space.
For example, if every 24 hours the client asks for a new prefix
and stops renewing the old prefix, and the Valid Lifetime of
delegated prefixes is one hour, then the client will consume two
prefixes for one hour out of 24 hours, and thus will consume just
under 1.05 prefixes on average.

14. IANA Considerations

This document has no IANA actions.

15. Security Considerations

A malicious (or just misbehaving) client might attempt to exhaust the
DHCPv6-PD pool by sending a large number of requests with differing
DHCP Unique Identifiers (DUIDs). To prevent a misbehaving client
from denying service to other clients, the DHCPv6 server or relay
MUST support limiting the number of prefixes delegated to a given
client at any given time.

Networks can protect against malicious clients by authenticating
devices using tokens that cannot be spoofed (e.g., 802.1x
authentication) and limiting the number of link-local addresses or
MAC addresses that each client is allowed to use. Note that this is
not a new issue, as the same attack might be implemented using DHCPv4
or DHCPv6 IA_NA requests; in particular, while it is unlikely for
clients to be able to exhaust an IA_NA address pool, clients using
IA_NA can exhaust other resources such as DHCPv6 and routing
infrastructure resources such as server RAM, ND cache entries,
Ternary Content-Addressable Memory (TCAM) entries, SAVI entries, etc.

A malicious client might request a prefix and then release it very
quickly, causing routing convergence events on the relays. The
impact of this attack can be reduced if the network rate-limits the
amount of broadcast and multicast messages from the client.

Delegating the same prefix for the same client introduces privacy
concerns. The proposed mitigation is discussed in Section 13.

Spoofing scenarios and prevention mechanisms are discussed in
Section 10.

16. References

16.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.

[RFC5460] Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
DOI 10.17487/RFC5460, February 2009,
<https://www.rfc-editor.org/info/rfc5460>.

[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, DOI 10.17487/RFC6620, May 2012,
<https://www.rfc-editor.org/info/rfc6620>.

[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.

[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.

[RFC8168] Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint
Issues", RFC 8168, DOI 10.17487/RFC8168, May 2017,
<https://www.rfc-editor.org/info/rfc8168>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

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

[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>.

[RFC8987] Farrer, I., Kottapalli, N., Hunek, M., and R. Patterson,
"DHCPv6 Prefix Delegating Relay Requirements", RFC 8987,
DOI 10.17487/RFC8987, February 2021,
<https://www.rfc-editor.org/info/rfc8987>.

16.2. Informative References

[ADDR-NOTIFICATION]
Kumari, W., Krishnan, S., Asati, R., Colitti, L., Linkova,
J., and S. Jiang, "Registering Self-generated IPv6
Addresses using DHCPv6", Work in Progress, Internet-Draft,
draft-ietf-dhc-addr-notification-13, 16 May 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dhc-
addr-notification-13>.

[IPv6-ADDRESS]
Gont, F. and G. Gont, "Implications of IPv6 Addressing on
Security Operations", Work in Progress, Internet-Draft,
draft-ietf-opsec-ipv6-addressing-00, 2 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-opsec-
ipv6-addressing-00>.

[PIO-PFLAG]
Colitti, L., Linkova, J., Ma, X., and D. Lamparter,
"Signaling DHCPv6 Prefix per Client Availability to
Hosts", Work in Progress, Internet-Draft, draft-ietf-6man-
pio-pflag-11, 4 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
pio-pflag-11>.

[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.

[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>.

[RFC6459] Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
Partnership Project (3GPP) Evolved Packet System (EPS)",
RFC 6459, DOI 10.17487/RFC6459, January 2012,
<https://www.rfc-editor.org/info/rfc6459>.

[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583,
DOI 10.17487/RFC6583, March 2012,
<https://www.rfc-editor.org/info/rfc6583>.

[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, DOI 10.17487/RFC7039, October 2013,
<https://www.rfc-editor.org/info/rfc7039>.

[RFC7278] Byrne, C., Drown, D., and A. Vizdal, "Extending an IPv6
/64 Prefix from a Third Generation Partnership Project
(3GPP) Mobile Interface to a LAN Link", RFC 7278,
DOI 10.17487/RFC7278, June 2014,
<https://www.rfc-editor.org/info/rfc7278>.

[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<https://www.rfc-editor.org/info/rfc7348>.

[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<https://www.rfc-editor.org/info/rfc7421>.

[RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
Validation Improvement (SAVI) Solution for DHCP",
RFC 7513, DOI 10.17487/RFC7513, May 2015,
<https://www.rfc-editor.org/info/rfc7513>.

[RFC7653] Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6
Active Leasequery", RFC 7653, DOI 10.17487/RFC7653,
October 2015, <https://www.rfc-editor.org/info/rfc7653>.

[RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
"Host Address Availability Recommendations", BCP 204,
RFC 7934, DOI 10.17487/RFC7934, July 2016,
<https://www.rfc-editor.org/info/rfc7934>.

[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>.

[RFC8415bis]
Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T.
Winters, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", Work in Progress, Internet-Draft, draft-ietf-
dhc-rfc8415bis-05, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dhc-
rfc8415bis-05>.

[RFC8501] Howard, L., "Reverse DNS in IPv6 for Internet Service
Providers", RFC 8501, DOI 10.17487/RFC8501, November 2018,
<https://www.rfc-editor.org/info/rfc8501>.

[RFC8504] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>.

[SNAC-SIMPLE]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-05, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-snac-
simple-05>.

Appendix A. Multiple Addresses Considerations

While a typical IPv4 host normally has only one IPv4 address per
interface, an IPv6 device almost always has multiple addresses
assigned to its interface. At the very least, a host can be expected
to have one link-local address, one temporary address, and, in most
cases, one stable global address. On a network providing NAT64
service, an IPv6-only host running the 464XLAT customer-side
translator (CLAT) [RFC6877] would use a dedicated 464XLAT address,
configured via SLAAC (see Section 6.3 of [RFC6877]), which brings the
total number of addresses to four. Other common scenarios where the
number of addresses per host interface might increase significantly
include but are not limited to:

* Devices running containers or namespaces: each container or
namespace would have multiple addresses as described above. As a
result, a device running just a few containers in a bridge mode
can easily have 20 or more IPv6 addresses on the given link.

* Networks assigning multiple prefixes to a given link: multihomed
networks, networks using Unique Local IPv6 Unicast Addresses (ULA,
[RFC4193]) and non-ULA prefixes together, or networks performing a
graceful renumbering from one prefix to another.

[RFC7934] discusses this aspect and explicitly states that IPv6
deployments SHOULD NOT limit the number of IPv6 addresses a host can
have. However, it has been observed that networks often do limit the
number of on-link addresses per device, likely in an attempt to
protect network resources and prevent DoS attacks.

The most common scenario of network-imposed limitations is ND proxy.
Many enterprise-scale wireless solutions implement ND proxy to reduce
the amount of broadcast and multicast downstream (AP to clients)
traffic and provide SAVI functions. To perform ND proxy, a device
usually maintains a table containing IPv6 and MAC addresses of
connected clients. At least some implementations have hardcoded
limits on how many IPv6 addresses per single MAC such a table can
contain. When the limit is exceeded, the behavior is implementation
dependent. Some vendors just fail to install an N+1 address to the
table. Others delete the oldest entry for this MAC and replace it
with the new address. In any case, the affected addresses lose
network connectivity without receiving any implicit signal, with
traffic being silently dropped.

Acknowledgements

Thanks to Harald Alvestrand, Nick Buraglio, Brian Carpenter, Tim
Chown, Roman Danyliw, Gert Doering, David Farmer, Fernando Gont, Joel
Halpern, Nick Hilliard, Bob Hinden, Martin Hunek, Erik Kline, Warren
Kumari, David Lamparter, Andrew McGregor, Tomek Mrugalski, Alexandre
Petrescu, Jurgen Schonwalder, Pascal Thubert, Ole Troan, Eric Vyncke,
Eduard Vasilenko, Timothy Winters, Chongfeng Xie, and Peter Yee for
the discussions, their input, and all contributions.

Authors' Addresses

Lorenzo Colitti
Google, LLC
Shibuya 3-21-3,
Japan
Email: [email protected]

Jen Linkova (editor)
Google
1 Darling Island Rd
Pyrmont New South Wales 2009
Australia
Email: [email protected], [email protected]

Xiao Ma (editor)
Google
Shibuya 3-21-3,
Japan
Email: [email protected]