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rfc:bcp:bcp204

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

                                                            Apple Inc.
                                                             July 2016
             Host Address Availability Recommendations

Abstract

 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
 http://www.rfc-editor.org/info/rfc7934.

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Colitti, et al. Best Current Practice [Page 1] RFC 7934 Host Address Availability Recommendations July 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
 2.  Common IPv6 Deployment Model  . . . . . . . . . . . . . . . .   3
 3.  Benefits of Providing Multiple Addresses  . . . . . . . . . .   3
 4.  Problems with Restricting the Number of Addresses per Host  .   4
 5.  Overcoming Limits Using Network Address Translation . . . . .   5
 6.  Options for Providing More Than One Address . . . . . . . . .   6
 7.  Number of Addresses Required  . . . . . . . . . . . . . . . .   8
 8.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   8
 9.  Operational Considerations  . . . . . . . . . . . . . . . . .   9
   9.1.  Host Tracking . . . . . . . . . . . . . . . . . . . . . .   9
   9.2.  Address Space Management  . . . . . . . . . . . . . . . .  10
   9.3.  Addressing Link-Layer Scalability Issues via IP Routing .  10
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
   11.2.  Informative References . . . . . . . . . . . . . . . . .  11
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1. 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
 the protocols.
 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.

Colitti, et al. Best Current Practice [Page 2] RFC 7934 Host Address Availability Recommendations July 2016

 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
    [RFC4941].
 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.

Colitti, et al. Best Current Practice [Page 3] RFC 7934 Host Address Availability Recommendations July 2016

 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
    464XLAT.
 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
 following drawbacks:
 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.

Colitti, et al. Best Current Practice [Page 4] RFC 7934 Host Address Availability Recommendations July 2016

 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

Colitti, et al. Best Current Practice [Page 5] RFC 7934 Host Address Availability Recommendations July 2016

 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
 policies.

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

Colitti, et al. Best Current Practice [Page 6] RFC 7934 Host Address Availability Recommendations July 2016

    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
    specification [RFC2460].
 +--------------------------+-------+-------------+--------+---------+
 |                          | 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

Colitti, et al. Best Current Practice [Page 7] RFC 7934 Host Address Availability Recommendations July 2016

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.

8. Recommendations

 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.

Colitti, et al. Best Current Practice [Page 8] RFC 7934 Host Address Availability Recommendations July 2016

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
 [RFC7844].

Colitti, et al. Best Current Practice [Page 9] RFC 7934 Host Address Availability Recommendations July 2016

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
 policies.
 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

Colitti, et al. Best Current Practice [Page 10] RFC 7934 Host Address Availability Recommendations July 2016

 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
 Section 9.1.

11. References

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,
            <http://www.rfc-editor.org/info/rfc2119>.

11.2. Informative References

 [ILA]      Herbert, T., "Identifier-locator addressing for network
            virtualization", Work in Progress, draft-herbert-nvo3-
            ila-02, March 2016.

Colitti, et al. Best Current Practice [Page 11] RFC 7934 Host Address Availability Recommendations July 2016

 [IPv6v4]   Japan Internet Exchange, "IPv6v4 Exchange Service", April
            2013, <http://www.jpix.ad.jp/en/service/ipv6v4.html>.
 [KA]       Roskind, J., "Quick UDP Internet Connections", November
            2013, <http://www.ietf.org/proceedings/88/slides/
            slides-88-tsvarea-10.pdf>.
 [L66]      McHardy, P., "netfilter: ipv6: add IPv6 NAT support",
            Linux commit 58a317f1061c894d2344c0b6a18ab4a64b69b815,
            August 2012, <https://git.kernel.org/cgit/linux/kernel/
            git/torvalds/linux.git/commit/
            ?id=58a317f1061c894d2344c0b6a18ab4a64b69b815>.
 [QUIC]     Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
            A UDP-Based Secure and Reliable Transport for HTTP/2",
            Work in Progress, draft-tsvwg-quic-protocol-02, January
            2016.
 [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
            and E. Lear, "Address Allocation for Private Internets",
            BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
            <http://www.rfc-editor.org/info/rfc1918>.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
            December 1998, <http://www.rfc-editor.org/info/rfc2460>.
 [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
            DOI 10.17487/RFC2993, November 2000,
            <http://www.rfc-editor.org/info/rfc2993>.
 [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
            C., and M. Carney, "Dynamic Host Configuration Protocol
            for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
            2003, <http://www.rfc-editor.org/info/rfc3315>.
 [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
            Host Configuration Protocol (DHCP) version 6", RFC 3633,
            DOI 10.17487/RFC3633, December 2003,
            <http://www.rfc-editor.org/info/rfc3633>.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, DOI 10.17487/RFC4291, February
            2006, <http://www.rfc-editor.org/info/rfc4291>.
 [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
            Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
            2006, <http://www.rfc-editor.org/info/rfc4389>.

Colitti, et al. Best Current Practice [Page 12] RFC 7934 Host Address Availability Recommendations July 2016

 [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
            Address Autoconfiguration", RFC 4862,
            DOI 10.17487/RFC4862, September 2007,
            <http://www.rfc-editor.org/info/rfc4862>.
 [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
            Extensions for Stateless Address Autoconfiguration in
            IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
            <http://www.rfc-editor.org/info/rfc4941>.
 [RFC5902]  Thaler, D., Zhang, L., and G. Lebovitz, "IAB Thoughts on
            IPv6 Network Address Translation", RFC 5902,
            DOI 10.17487/RFC5902, July 2010,
            <http://www.rfc-editor.org/info/rfc5902>.
 [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
            Requirements", RFC 6434, DOI 10.17487/RFC6434, December
            2011, <http://www.rfc-editor.org/info/rfc6434>.
 [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,
            <http://www.rfc-editor.org/info/rfc6459>.
 [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
            Combination of Stateful and Stateless Translation",
            RFC 6877, DOI 10.17487/RFC6877, April 2013,
            <http://www.rfc-editor.org/info/rfc6877>.
 [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,
            <http://www.rfc-editor.org/info/rfc7039>.
 [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
            Interface Identifiers with IPv6 Stateless Address
            Autoconfiguration (SLAAC)", RFC 7217,
            DOI 10.17487/RFC7217, April 2014,
            <http://www.rfc-editor.org/info/rfc7217>.
 [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,
            <http://www.rfc-editor.org/info/rfc7278>.

Colitti, et al. Best Current Practice [Page 13] RFC 7934 Host Address Availability Recommendations July 2016

 [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,
            <http://www.rfc-editor.org/info/rfc7421>.
 [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
            Profiles for DHCP Clients", RFC 7844,
            DOI 10.17487/RFC7844, May 2016,
            <http://www.rfc-editor.org/info/rfc7844>.
 [TARP]     Gleitz, PM. and SB. Bellovin, "Transient Addressing for
            Related Processes: Improved Firewalling by Using IPv6 and
            Multiple Addresses per Host", In Proceedings of the
            Eleventh Usenix Security Symposium, August 2001,
            <https://www.usenix.org/legacy/events/sec01/gleitz.html>.
 [TS.24327] 3GPP, "Mobility between 3GPP Wireless Local Area Network
            (WLAN) interworking (I-WLAN) and 3GPP systems; General
            Packet Radio System (GPRS) and 3GPP I-WLAN aspects; Stage
            3", 3GPP TS 24.327, June 2011,
            <http://www.3gpp.org/DynaReport/24327.htm>.
 [V66]      Oracle, "What's New in VirtualBox 4.3?", October 2013,
            <https://blogs.oracle.com/fatbloke/entry/
            what_s_new_in_virtualbox>.

Acknowledgements

 The authors thank Tore Anderson, Brian Carpenter, David Farmer,
 Wesley George, Geoff Huston, Erik Kline, Victor Kuarsingh, Shucheng
 (Will) Liu, Shin Miyakawa, Dieter Siegmund, Mark Smith, Sander
 Steffann, Fred Templin, and James Woodyatt for their input and
 contributions.

Colitti, et al. Best Current Practice [Page 14] RFC 7934 Host Address Availability Recommendations July 2016

Authors' Addresses

 Lorenzo Colitti
 Google
 Roppongi 6-10-1
 Minato, Tokyo  106-6126
 Japan
 Email: lorenzo@google.com
 Vint Cerf
 Google
 1875 Explorer Street
 10th Floor
 Reston, VA  20190
 United States of America
 Email: vint@google.com
 Stuart Cheshire
 Apple Inc.
 1 Infinite Loop
 Cupertino, CA  95014
 United States of America
 Email: cheshire@apple.com
 David Schinazi
 Apple Inc.
 1 Infinite Loop
 Cupertino, CA  95014
 United States of America
 Email: dschinazi@apple.com

Colitti, et al. Best Current Practice [Page 15]

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