Internet Engineering Task Force (IETF) L. Colitti
Request for Comments: 7934 V. Cerf
BCP: 204 Google
Category: Best Current Practice S. Cheshire
ISSN: 2070-1721 D. Schinazi
July 2016 Host Address Availability Recommendations
This document recommends that networks provide general-purpose end
hosts with multiple global IPv6 addresses when they attach, and it
describes the benefits of and the options for doing so.
Status of This Memo
This memo documents an Internet Best Current Practice.
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). Further information on
BCPs is available in 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
Copyright (c) 2016 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Requirements Language . . . . . . . . . . . . . . . . . . 32. Common IPv6 Deployment Model . . . . . . . . . . . . . . . . 33. Benefits of Providing Multiple Addresses . . . . . . . . . . 34. Problems with Restricting the Number of Addresses per Host . 45. Overcoming Limits Using Network Address Translation . . . . . 56. Options for Providing More Than One Address . . . . . . . . . 67. Number of Addresses Required . . . . . . . . . . . . . . . . 88. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 89. Operational Considerations . . . . . . . . . . . . . . . . . 99.1. Host Tracking . . . . . . . . . . . . . . . . . . . . . . 99.2. Address Space Management . . . . . . . . . . . . . . . . 109.3. Addressing Link-Layer Scalability Issues via IP Routing . 1010. Security Considerations . . . . . . . . . . . . . . . . . . . 1111. References . . . . . . . . . . . . . . . . . . . . . . . . . 1111.1. Normative References . . . . . . . . . . . . . . . . . . 1111.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 151. Introduction
In most aspects, the IPv6 protocol is very similar to IPv4. This
similarity can create a tendency to think of IPv6 as 128-bit IPv4,
and thus lead network designers and operators to apply identical
configurations and operational practices to both. This is generally
a good thing because it eases the transition to IPv6 and the
operation of dual-stack networks. However, in some design and
operational areas, it can lead to carrying over IPv4 practices that
are limiting or not appropriate in IPv6 due to differences between
One such area is IP addressing, particularly IP addressing of hosts.
This is substantially different because unlike IPv4 addresses, IPv6
addresses are not a scarce resource. In IPv6, a single link provides
over four billion times more address space than the whole IPv4
Internet [RFC7421]. Thus, unlike IPv4, IPv6 networks are not forced
by address scarcity concerns to provide only one address per host.
Furthermore, providing multiple addresses has many benefits,
including application functionality and simplicity, privacy, and
flexibility to accommodate future applications. Another significant
benefit is the ability to provide Internet access without the use of
Network Address Translation (NAT). Providing only one IPv6 address
per host negates these benefits.
This document details the benefits of providing multiple addresses
per host, and the problems with not doing so. It recommends that
networks provide general-purpose end hosts with multiple global
addresses when they attach and lists current options for doing so.
It does not specify any changes to protocols or host behavior.
1.1. 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
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
2. Common IPv6 Deployment Model
IPv6 is designed to support multiple addresses, including multiple
global addresses, per interface (see Section 2.1 of [RFC4291] and
Section 5.9.4 of [RFC6434]). Today, many general-purpose IPv6 hosts
are configured with three or more addresses per interface: a link-
local address, a stable address (e.g., using 64-bit Extended Unique
Identifiers (EUI-64) or Opaque Interface Identifiers [RFC7217]), one
or more privacy addresses [RFC4941], and possibly one or more
temporary or non-temporary addresses obtained using the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) [RFC3315].
In most general-purpose IPv6 networks, hosts have the ability to
configure additional IPv6 addresses from the link prefix(es) without
explicit requests to the network. Such networks include all 3GPP
networks ([RFC6459], Section 5.2), in addition to Ethernet and Wi-Fi
networks using Stateless Address Autoconfiguration (SLAAC) [RFC4862].
3. Benefits of Providing Multiple Addresses
Today, there are many host functions that require more than one IP
address to be available to the host, including:
o Privacy addressing to prevent tracking by off-network hosts
o Multiple processors inside the same device. For example, in many
mobile devices, both the application processor and the baseband
processor need to communicate with the network, particularly for
technologies like I-WLAN [TS.24327] where the two processors share
the Wi-Fi network connection.
o Extending the network (e.g., "tethering").
o Running virtual machines on hosts.
o Translation-based transition technologies such as 464XLAT (a
combination of stateful and stateless translation) [RFC6877] that
translate between IPv4 and IPv6. Some of these technologies
require the availability of a dedicated IPv6 address in order to
determine whether inbound packets are translated or native
([RFC6877], Section 6.3).
o Identifier-locator addressing (ILA) [ILA].
o Future applications (e.g., per-application IPv6 addresses [TARP]).
Two examples of how the availability of multiple addresses per host
has already allowed substantial deployment of new applications
without explicit requests to the network are:
o 464XLAT. 464XLAT is usually deployed within a particular network;
in this model, the operator can ensure that the network is
appropriately configured to provide the customer-side translator
(CLAT) with the additional IPv6 address it needs to implement
464XLAT. However, there are deployments where the provider-side
translator (PLAT) (i.e., NAT64) is provided as a service by a
different network, without the knowledge or cooperation of the
residential ISP (e.g., the IPv6v4 Exchange Service [IPv6v4]).
This type of deployment is only possible because those residential
ISPs provide multiple IP addresses to their users, and thus those
users can freely obtain the extra IPv6 address required to run
o /64 sharing [RFC7278]. When the topology supports it, this is a
way to provide IPv6 tethering without needing to wait for network
operators to deploy DHCPv6 Prefix Delegation (PD), which is only
available in 3GPP release 10 or above ([RFC6459], Section 5.3).
4. Problems with Restricting the Number of Addresses per Host
Providing a restricted number of addresses per host implies that
functions that require multiple addresses either will be unavailable
(e.g., if the network provides only one IPv6 address per host, or if
the host has reached the limit of the number of addresses available)
or will only be available after an explicit request to the network is
granted. Requiring explicit requests to the network has the
o Increased latency, because a provisioning operation, and possibly
human intervention with an update to the Service Level Agreement
(SLA), must complete before the functionality is available.
o Uncertainty, because it is not known if a particular application
function will be available until the provisioning operation
succeeds or fails.
o Complexity, because implementations need to deal with failures and
somehow present them to the user. Failures may manifest as
timeouts, which may be slow and frustrating to users.
o Increased load on the network's provisioning servers.
Some operators may desire that their networks be configured to limit
the number of IPv6 addresses per host. Reasons might include
hardware limitations (e.g., Ternary Content-Addressable Memory (TCAM)
size or size constraints of the Neighbor Cache table), business
models (e.g., a desire to charge the network's users on a per-device
basis), or operational consistency with IPv4 (e.g., an IP address
management system that only supports one address per host). However,
hardware limitations are expected to ease over time, and an attempt
to generate additional revenue by charging per device may prove
counterproductive if customers respond (as they did with IPv4) by
using NAT, which results in no additional revenue, but leads to more
operational problems and higher support costs.
5. Overcoming Limits Using Network Address Translation
When the network limits the number of addresses available to a host,
this can mostly be overcome by end hosts by using NAT, and indeed in
IPv4 the scarcity of addresses is often mitigated by using NAT on the
host. Thus, the limits could be overcome in IPv6 as well by
implementing NAT66 on the host.
Unfortunately, NAT has well-known drawbacks. For example, it causes
application complexity due to the need to implement NAT traversal.
It hinders development of new applications. On mobile devices, it
reduces battery life due to the necessity of frequent keepalives,
particularly for UDP. Applications using UDP that need to work on
most of the Internet are forced to send keepalives at least every 30
seconds [KA]. For example, the QUIC protocol uses a 15-second
keepalive [QUIC]. Other drawbacks of NAT are well-known and
documented [RFC2993]. While IPv4 NAT is inevitable due to the
limited amount of IPv4 space available, that argument does not apply
to IPv6. Guidance from the Internet Architecture Board (IAB) is that
deployment of IPv6 NAT is not desirable [RFC5902].
The desire to overcome the problems listed in Section 4 without
disabling any features has resulted in developers implementing IPv6
NAT. There are fully stateful address+port NAT66 implementations in
client operating systems today: for example, Linux has supported
NAT66 since 2012 [L66]. At least one popular software hypervisor
also implemented NAT66 to work around these issues [V66]. Wide
deployment of networks that provide a restricted number of addresses
will cause proliferation of NAT66 implementations.
This is not a desirable outcome. It is not desirable for users
because they may experience application brittleness. It is likely
not desirable for network operators either, as they may suffer higher
support costs, and even when the decision to provide only one IPv6
address per device is dictated by the network's business model, there
may be little in the way of incremental revenue, because devices can
share their IPv6 address with other devices. Finally, it is not
desirable for operating system manufacturers and application
developers, who will have to build more complexity, lengthening
development time and/or reducing the time spent on other features.
Indeed, it could be argued that the main reason for deploying IPv6,
instead of continuing to scale the Internet using only IPv4 and
large-scale NAT44, is because doing so can provide all the hosts on
the planet with end-to-end connectivity that is constrained not by
accidental technical limitations, but only by intentional security
6. Options for Providing More Than One Address
Multiple IPv6 addresses can be provided in the following ways:
o Using Stateless Address Autoconfiguration (SLAAC) [RFC4862].
SLAAC allows hosts to create global IPv6 addresses on demand by
simply forming new addresses from the global prefix(es) assigned
to the link. Typically, SLAAC is used on shared links, but it is
also possible to use SLAAC while providing a dedicated /64 prefix
to each host. This is the case, for example, if the host is
connected via a point-to-point link such as in 3GPP networks, on a
network where each host has its own dedicated VLAN, or on a
wireless network where every Media Access Control (MAC) address is
placed in its own broadcast domain.
o Using stateful DHCPv6 address assignment [RFC3315]. Most DHCPv6
clients only ask for one non-temporary address, but the protocol
allows requesting multiple temporary and even multiple non-
temporary addresses, and the server could choose to provide
multiple addresses. It is also technically possible for a client
to request additional addresses using a different DHCP Unique
Identifier (DUID), though the DHCPv6 specification implies that
this is not expected behavior ([RFC3315], Section 9). The DHCPv6
server will decide whether to grant or reject the request based on
information about the client, including its DUID, MAC address, and
more. The maximum number of IPv6 addresses that can be provided
in a single DHCPv6 packet, given a typical MTU of 1500 bytes or
smaller, is approximately 30.
o DHCPv6 Prefix Delegation (PD) [RFC3633]. DHCPv6 PD allows the
client to request and be delegated a prefix, from which it can
autonomously form other addresses. If the prefix is shorter than
/64, it can be divided into multiple subnets that can be further
delegated to downstream clients. If the prefix is a /64, it can
be extended via L2 bridging, Neighbor Discovery (ND) proxying
[RFC4389], or /64 sharing [RFC7278], but it cannot be further
subdivided, as a prefix longer than /64 is outside the current
IPv6 specifications [RFC7421]. While the DHCPv6 Prefix Delegation
specification [RFC3633] assumes that the DHCPv6 client is a
router, DHCPv6 PD itself does not require that the client forward
IPv6 packets not addressed to itself, and thus does not require
that the client be an IPv6 router as defined in the IPv6
specification [RFC2460]. Also, in many cases (such as tethering,
or hosting virtual machines), hosts are already forwarding IPv6
packets and thus operating as IPv6 routers as defined in the IPv6
| | SLAAC | DHCPv6 | DHCPv6 | DHCPv4 |
| | | IA_NA / | PD | |
| | | IA_TA | | |
| Can extend network | No+ | No | Yes | Yes |
| | | | | (NAT44) |
| Can number "unlimited" | Yes* | Yes* | No | No |
| endpoints | | | | |
| Uses stateful, request- | No | Yes | Yes | Yes |
| based assignment | | | | |
| Is immune to Layer 3 on- | No | Yes | Yes | Yes |
| link resource exhaustion | | | | |
| attacks | | | | |
[*] Subject to network limitations, e.g., ND cache entry size limits.
[+] Except on certain networks, e.g., /64 sharing [RFC7278].
Table 1: Comparison of Multiple Address Assignment Options
7. Number of Addresses Required
If we itemize the use cases from Section 3, we can estimate the
number of addresses currently used in normal operations. In typical
implementations, privacy addresses use up to 7 addresses -- one per
day ([RFC4941], Section 3.5). Current mobile devices sharing an
uplink connection may typically support 8 downstream client devices,
with each one requiring one or more addresses. A client might choose
to run several virtual machines. Current implementations of 464XLAT
require the use of a separate address. Some devices require another
address for their baseband chip. Even a host performing just a few
of these functions simultaneously might need on the order of 20
addresses at the same time. Future applications designed to use an
address per application or even per resource will require many more.
These will not function on networks that enforce a hard limit on the
number of addresses provided to hosts. Thus, in general it is not
possible to estimate in advance how many addresses are required.
In order to avoid the problems described above and preserve the
Internet's ability to support new applications that use more than one
IPv6 address, it is RECOMMENDED that IPv6 network deployments provide
multiple IPv6 addresses from each prefix to general-purpose hosts.
To support future use cases, it is NOT RECOMMENDED to impose a hard
limit on the size of the address pool assigned to a host.
Particularly, it is NOT RECOMMENDED to limit a host to only one IPv6
address per prefix.
Due to the drawbacks imposed by requiring explicit requests for
address space (see Section 4), it is RECOMMENDED that the network
give the host the ability to use new addresses without requiring
explicit requests. This can be achieved either by allowing the host
to form new addresses autonomously (e.g., via SLAAC) or by providing
the host with a dedicated /64 prefix. The prefix MAY be provided
using DHCPv6 PD, SLAAC with per-device VLANs, or any other means.
Using stateful address assignment (DHCPv6 IA_NA or IA_TA) to provide
multiple addresses when the host connects (e.g., the approximately 30
addresses that can fit into a single packet) would accommodate
current clients, but it sets a limit on the number of addresses
available to hosts when they attach and therefore limits the
development of future applications.
9. Operational Considerations
9.1. Host Tracking
Some network operators -- often operators of networks that provide
services to third parties such as university campus networks -- are
required to track which IP addresses are assigned to which hosts on
their network. Maintaining persistent logs that map user IP
addresses and timestamps to hardware identifiers such as MAC
addresses may be used to attribute liability for copyright
infringement or other illegal activity.
It is worth noting that this requirement can be met without using
DHCPv6 address assignment. For example, it is possible to maintain
these mappings by monitoring the IPv6 neighbor table: routers
typically allow periodic dumps of the Neighbor Cache via the Simple
Network Management Protocol (SNMP) or other means, and many can be
configured to log every change to the Neighbor Cache. Using SLAAC
with a dedicated /64 prefix for each host simplifies tracking, as it
does not require logging every address formed by the host, but only
the prefix assigned to the host when it attaches to the network.
Similarly, providing address space using DHCPv6 PD has the same
tracking properties as DHCPv6 address assignment, but allows the
network to provide unrestricted address space.
Many large enterprise networks are fully dual stack and implement
address monitoring without using or supporting DHCPv6. The authors
are directly aware of several networks that operate in this way,
including the Universities of Loughborough, Minnesota, Reading,
Southampton, and Wisconsin, and Imperial College London, in addition
to the enterprise networks of the authors' employers.
It should also be noted that using DHCPv6 address assignment does not
ensure that the network can reliably track the IPv6 addresses used by
hosts. On any shared network without Layer 2 (L2) edge port
security, hosts are able to choose their own addresses regardless of
what address provisioning methodology the network operator believes
is in use. The only way to restrict the addresses used by hosts is
to use L2 security mechanisms that enforce that particular IPv6
addresses are used by particular link-layer addresses (for example,
Source Address Validation Improvement (SAVI) [RFC7039]). If those
mechanisms are available, it is possible to use them to provide
tracking; this form of tracking is more secure and reliable than
server logs because it operates independently of how addresses are
allocated. Finally, tracking address information via DHCPv6 server
logs is likely to become decreasingly viable due to ongoing efforts
to improve the privacy of DHCPv6 and MAC address randomization
9.2. Address Space Management
In IPv4, all but the world's largest networks can be addressed using
private space [RFC1918], with each host receiving one IPv4 address.
Many networks can be numbered in 192.168.0.0/16, which has roughly 65
thousand addresses. In IPv6, that is equivalent to a /48, with each
host receiving a /64 prefix. Under current Regional Internet
Registry (RIR) policies, a /48 is easy to obtain for an enterprise
network. Networks that need a bigger block of private space use
10.0.0.0/8, which has roughly 16 million addresses. In IPv6, that is
equivalent to a /40, with each host receiving a /64 prefix.
Enterprises of such size can easily obtain a /40 under current RIR
In the above cases, aggregation and routing can be equivalent to
IPv4: if a network aggregates per-host IPv4 addresses into prefixes
of length /32 - n, it can aggregate per-host /64 prefixes into the
same number of prefixes of length /64 - n.
Currently, residential users typically receive one IPv4 address and a
/48, /56, or /60 IPv6 prefix. While such networks do not provide
enough space to assign a /64 per host, such networks almost
universally use SLAAC, and thus do not pose any particular limit to
the number of addresses hosts can use.
Unlike IPv4 where addresses came at a premium, in all of these
networks there is enough IPv6 address space to supply clients with
multiple IPv6 addresses.
9.3. Addressing Link-Layer Scalability Issues via IP Routing
The number of IPv6 addresses on a link has a direct impact on
networking infrastructure nodes (routers, switches) and other nodes
on the link. Setting aside exhaustion attacks via L2 address
spoofing, every (L2, IP) address pair impacts networking hardware
requirements in terms of memory, Multicast Listener Discovery (MLD)
snooping, solicited node multicast groups, etc. Many of these costs
are incurred by neighboring hosts.
Hosts on such networks that create unreasonable numbers of addresses
risk impairing network connectivity for themselves and other hosts on
the network, and in extreme cases (e.g., hundreds or thousands of
addresses) may even find their network access restricted by denial-
of-service protection mechanisms.
We expect these scaling limitations to change over time as hardware
and applications evolve. However, switching to a dedicated /64
prefix per host can resolve these scaling limitations. If the prefix
is provided via DHCPv6 PD, or if the prefix can be used by only one
link-layer address (e.g., if the link layer uniquely identifies or
authenticates hosts based on MAC addresses), then there will be only
one routing entry and one ND cache entry per host on the network.
Furthermore, if the host is aware that the prefix is dedicated (e.g.,
if it was provided via DHCPv6 PD and not SLAAC), it is possible for
the host to assign IPv6 addresses from this prefix to an internal
virtual interface such as a loopback interface. This obviates the
need to perform Neighbor Discovery and Duplicate Address Detection on
the network interface for these addresses, reducing network traffic.
Thus, assigning a dedicated /64 prefix per host is operationally
prudent. Clearly, however, it requires more IPv6 address space than
using shared links, so the benefits provided must be weighed with the
operational overhead of address space management.
10. Security Considerations
As mentioned in Section 9.3, on shared networks using SLAAC, it is
possible for hosts to attempt to exhaust network resources and
possibly deny service to other hosts by creating unreasonable numbers
(e.g., hundreds or thousands) of addresses. Networks that provide
access to untrusted hosts can mitigate this threat by providing a
dedicated /64 prefix per host. It is also possible to mitigate the
threat by limiting the number of ND cache entries that can be created
for a particular host, but care must be taken to ensure that the
network does not prevent the legitimate use of multiple IP addresses
by non-malicious hosts.
Security issues related to host tracking are discussed in
11.1. 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,
11.2. Informative References
[ILA] Herbert, T., "Identifier-locator addressing for network
virtualization", Work in Progress, draft-herbert-nvo3-
ila-02, March 2016.
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