Internet-Draft Prefix per client using DHCPv6 PD February 2024
Colitti, et al. Expires 29 August 2024 [Page]
Workgroup:
v6ops Working Group
Internet-Draft:
draft-ietf-v6ops-dhcp-pd-per-device
Published:
Intended Status:
Informational
Expires:
Authors:
L. Colitti
Google, LLC
J. Linkova, Ed.
Google
X. Ma, Ed.
Google

Using DHCPv6-PD to Allocate Unique IPv6 Prefix per Client in Large Broadcast Networks

Abstract

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

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 29 August 2024.

1. Introduction

Often, large 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 clients obtain prefixes from the network. This provides two important advantages.

First, it offers better scalability. Unlike IPv4, IPv6 allows (and often requires) hosts to have multiple addresses (see Section 16 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/hashes (Neighbor Cache on first-hop routers, Neighbor Discovery Proxy caches on L2 devices etc). On VXLAN [RFC7348] networks each address might be represented as a route so 8 addresses instead of 1 requires the devices to support 8 times more routes, etc. If an infrastructure device resources are exhausted, the device might drop some IPv6 addresses from the corresponding tables, while the address owner might be still using the address to send traffic. This leads to traffic blackholing and 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 arbitrary number of addresses.

Second, it 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 PD, 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/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 behaviour of the network. Host behaviour is not defined in this specification.

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 which connects to a network and acquires addresses. The node may wish to obtain addresses for its own use, or 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].

ND: Neighbor Discovery, [RFC4861].

PIO: Prefix Information Option, [RFC4862].

SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].

DHCPv6-PD: DHCPv6 ([RFC8415]) mechanism to delegate IPv6 prefixes to clients.

4. Design Principles

Instead of all clients on a given link forming addresses from the same shared prefix assigned to that link:

  • A device acts as DHCP-PD client and requests a prefix via DHCPv6-PD by sending an IA_PD request.
  • The first-hop router acts as a DHCPv6 relay and sends the request to the DHCPv6-PD servers. In smaller networks it's entirely possible for the first-hop router to act as a DHCPv6-PD server and assign the prefix from a larger pool allocated for the given link or the whole site.
  • The allocated prefix is installed into the first-hop router routing table as a route pointing to the client's link-local address. That route can be redistributed into a dynamic routing protocol (if the network is running one). For the router and all other infrastructure devices that prefix is considered off-link, so traffic to that prefix does not trigger any ND packets, other than the minimum ND required to sustain 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 [RFC7084] requirement WAA-7. 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 this specification.

DHCPv6 Servers Pool 2001:db8:ddd0::/48 for clients on 2001:db8:c001::/64 link IPv6 Network DHCPv6 DHCPv6 Relay-Forward Relay-Forward DHCPv6 Route for 2001:db8:ddd0::/48 DHCPv6 Relay-Reply Relay-Reply First-hop router/DHCPv6 relay First-hop Router/DHCPv6 relay Route 2001:db8:ddd0:1::/64 -> fe80::aaaa Route 2001:db8:ddd0:1::/64 -> fe80::aaaa Route 2001:db8:ddd0:2::/64 -> fe80::cccc Route 2001:db8:ddd0:2::/64 -> fe80::cccc Shared IPv6 link 2001:db8:c001::/64 legacy client B DHCPv6 DHCPv6 no DHCPv6-PD support DHCPv6 DHCPv6 Request Reply link-local address fe80::bbbb Request Reply global address 2001:db8:c001::bbbb Client C link-local address fe80::cccc delegated prefix 2001:db8:ddd0:2::/64 Client A link-local address: fe80::aaaa delegated prefix: 2001:db8:ddd0:1::/64 Router Advertisement PIO 2001:db8:ddd0:2::/64 virtual system virtual system 2001:db8:ddd0:1::f00 2001:db8:ddd0:1::2345 2001:db8:ddd0:1::cafe 2001:db8:ddd0:1::abc Tethered device1 Tethered device2 2001:db8:ddd0:2::5555 2001:db8:ddd0:2::6666
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 (DHCPv6 prefix delegation) and allocating 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 3-5 addresses per client might introduce scalability issues.
  • Networks with 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 lower number of clients, SLAAC is a better and simpler 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, and 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.

Other deployment models, such as prefix pools shared over multiple links or routers, are possible, but 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.

When using a dedicated prefix pool for each link, 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.

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) to 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 delegated, some prefixes 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 explicitly allows it using the Server Unicast option ([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 [I-D.ietf-dhc-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.

For security reasons, some networks block onlink 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 onlink, 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 [RFC8168] recommendations 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/T2 timers). In particular the following factors shall be taken into account:

  • the expected maximum number of clients;
  • average duration of a client connection;
  • 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 SSIDs);
  • expected level of recurring clients (for example, a corporate authenticated WiFI network might be using longer timers than an open public Wi-Fi).

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, 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 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 using [RFC7084] (see requirement L-2), and a prefix length of /64 is required to provide global connectivity for stub networks as per [I-D.ietf-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.

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 onlink 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 SSID 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 (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 behaviour 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 behaviour: 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, quite often different links have different security policies applied (for example, corporate internal network vs guest network), hence clients on different links need to use different prefixes.

10. Antispoofing and SAVI Interaction

Enabling the unicast Reverse Path Forwarding (uRPF) 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 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]) for protecting link-local addresses and creating SAVI bindings for DHCPv6-PD assigned prefixes would prevent spoofing.

Some infrastructure devices do not implement SAVI as defined in [RFC7039] but perform other forms of address tracking and snooping for security or performance improvement purposes (e.g. ND proxy). This is very common behaviour for wireless devices (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 Neighbour Advertisements. In DHCPv6-PD environments, however, the attacker can only learn about other clients's 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 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 ones), 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 would request an unique prefix suitable for SLAAC, the home network might run out of prefixes, if the prefix allocated to the CPE by its ISP is too small (e.g. if an ISP allocates /60, it would only allow 16 clients to request /64). 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 only use the prefix delegated via DHCPv6-PD. Starting both SLAAC using the PIO prefix and DHCPv6-PD and deprecating that 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 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 to use the prefix in PIO or not, if the network provides the PIO with A flag set.

To allow the network to signal DHCPv6-PD support, [I-D.collink-6man-pio-pflag] defines a new PIO flag, indicating that DHCPv6-PD is the preferred method of obtaining prefixes. It should be noted that the deployment model described in this document does not depend on [I-D.collink-6man-pio-pflag] and can be implemented without deploying that PIO flag. For example, devices acting as [RFC7084] compatible routers would be able to receive prefixes via DHCPv-PD.

12. Benefits

The proposed solution provides the following benefits:

  • The network devices' 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.
  • 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 PIO with '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].
  • DHCP-PD logs and first-hop routers routing tables provide complete information on IPv6 to MAC mapping, which can be used for forensics and troubleshooting. Such information is much less dynamic than ND cache and therefore it's much easier for an operator to collect and process it.
  • A dedicated prefix per client allows the network administrator to create per-device security policies (ACLs) even if the client is using temporary addresses. This mitigates one of the issues described in [I-D.ietf-opsec-ipv6-addressing].
  • The cost of having multiple addresses is offloaded to the clients. Hosts are free to create and use as many addresses as they need.
  • Fate sharing: all global addresses used by a given client are routed as a single prefix. Either all of them work or not, which makes the failures easier to diagnose and mitigate.
  • Lower level of multicast traffic: less Neighbor Discovery ([RFC4861]) multicast packets, as there are only clients link-local addresses the routers need to resolve. Also, no DAD traffic except for clients' link-local addresses.
  • Ability to extend the network transparently. If the client uses SLAAC, delegating a prefix allows the client to provide connectivity to other hosts, like as it is possible in IPv4 with NAT.

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.

Networks that use the proposed mechanism instead of SLAAC or in addition to SLAAC, SHOULD minimally:

  • Ensure that when a client requests a prefix, the prefix is randomly assigned and not allocated deterministically.
  • Use short prefix lifetimes, to ensure that when a client disconnects and reconnects it gets a different prefix.

To provide privacy roughly equivalent to SLAAC with temporary addresses ([RFC8981]), the network SHOULD allow the client to have two prefixes 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. The prefix's lifetimes SHOULD 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 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 on average consume just under 1.05 prefixes.

14. IANA Considerations

This memo includes no request to IANA.

15. Security Considerations

A malicious (or just misbehaving) client might attempt to exhaust the DHCP-PD pool by sending a large number of requests with differing DUIDs. This is not a new issue, as the same attack might be implemented in DHCPv4 or DHCPv6 for IA_NA requests. 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.

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. Appendix: 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 4. Other common scenarios where the number of addresses per host's interface might increase significantly, include but are not limited to:

  • Devices running containers/namespaces: each container/namespace would have muliple 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 ULA [RFC4193]and non-ULA prefixes together or network 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's been observed that networks often do limit the number of on-link addresses per device, likely in an attempt to protect the network resources and prevent DoS attacks.

The most common scenario of network-imposed limitations is Neighbor Discovery (ND) proxy. Many enterprise-scale wireless solutions implement ND proxy to reduce 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 a single MAC such a table can contain. When the limit is exceeded the behaviour is implementation-dependent. Some vendors just fail to install N+1 address to the table. Other 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 implict signal, with traffic being silently dropped.

17. References

17.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4193]
Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, , <https://www.rfc-editor.org/info/rfc4193>.
[RFC7084]
Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, DOI 10.17487/RFC7084, , <https://www.rfc-editor.org/info/rfc7084>.
[RFC5460]
Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, DOI 10.17487/RFC5460, , <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, , <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, , <https://www.rfc-editor.org/info/rfc6877>.
[RFC8168]
Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint Issues", RFC 8168, DOI 10.17487/RFC8168, , <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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8987>.

17.2. Informative References

[RFC4861]
Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc7513>.
[RFC7653]
Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6 Active Leasequery", RFC 7653, DOI 10.17487/RFC7653, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8200>.
[RFC8504]
Chown, T., Loughney, J., and T. Winters, "IPv6 Node Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504, , <https://www.rfc-editor.org/info/rfc8504>.
Colitti, L., Linkova, J., Ma, X., and D. Lamparter, "Signalling DHCPv6 Prefix Delegation Availability to Hosts", Work in Progress, Internet-Draft, draft-collink-6man-pio-pflag-03, , <https://datatracker.ietf.org/doc/html/draft-collink-6man-pio-pflag-03>.
[I-D.ietf-dhc-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-04, , <https://datatracker.ietf.org/doc/html/draft-ietf-dhc-rfc8415bis-04>.
[I-D.ietf-opsec-ipv6-addressing]
Gont, F. and G. Gont, "Implications of IPv6 Addressing on Security Operations", Work in Progress, Internet-Draft, draft-ietf-opsec-ipv6-addressing-00, , <https://datatracker.ietf.org/doc/html/draft-ietf-opsec-ipv6-addressing-00>.
[I-D.ietf-snac-simple]
Lemon, T. and J. Hui, "Automatically Connecting Stub Networks to Unmanaged Infrastructure", Work in Progress, Internet-Draft, draft-ietf-snac-simple-03, , <https://datatracker.ietf.org/doc/html/draft-ietf-snac-simple-03>.

Acknowledgements

Thanks to Harald Alvestrand, Nick Buraglio, Brian Carpenter, 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, Eduard Vasilenko, Timothy Winters, Chongfeng Xie for the discussions, their input and all contribution.

Authors' Addresses

Lorenzo Colitti
Google, LLC
Shibuya 3-21-3,
Japan
Jen Linkova (editor)
Google
1 Darling Island Rd
Pyrmont NSW 2009
Australia
Xiao Ma (editor)
Google
Shibuya 3-21-3,
Japan