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BEST CURRENT PRACTICE
Errata Exist
Internet Engineering Task Force (IETF)                        L. ColittiRequest for Comments: 7934                                       V. CerfBCP: 204                                                          GoogleCategory: Best Current Practice                              S. CheshireISSN: 2070-1721                                              D. Schinazi                                                              Apple Inc.                                                               July 2016Host Address Availability RecommendationsAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  Common IPv6 Deployment Model  . . . . . . . . . . . . . . . .33.  Benefits of Providing Multiple Addresses  . . . . . . . . . .3   4.  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  . . . . . . . . . . . . . . . .10     9.3.  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   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 (seeSection 2.1 of [RFC4291] andSection 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 inSection 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 supportedColitti, 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, andColitti, 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 OptionsColitti, et al.           Best Current Practice                 [Page 7]

RFC 7934        Host Address Availability Recommendations      July 20167.  Number of Addresses Required   If we itemize the use cases fromSection 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 (seeSection 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 20169.  Operational Considerations9.1.  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 20169.2.  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 prefixColitti, 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 inSection 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 inSection 9.1.11.  References11.1.  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 2016Authors' 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.comColitti, et al.           Best Current Practice                [Page 15]

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