IPv6 Operations                                               L. Colitti
Internet-Draft                                                   V. Cerf
Intended status: Best Current Practice                            Google
Expires: January 24, 2016                                    S. Cheshire
                                                             D. Schinazi
                                                              Apple Inc.
                                                           July 23, 2015

               Host address availability recommendations


   This document recommends that networks provide general-purpose end
   hosts with multiple global addresses when they attach, and describes
   the benefits of and the options for doing so.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 24, 2016.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Common IPv6 deployment model  . . . . . . . . . . . . . . . .   3
   3.  Benefits of multiple addresses  . . . . . . . . . . . . . . .   3
   4.  Problems with assigning a limited number of addresses per
       host  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Overcoming limits using Network Address Translation . . . . .   5
   6.  Options for obtaining more than one address . . . . . . . . .   6
   7.  Number of addresses required  . . . . . . . . . . . . . . . .   7
   8.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Operational considerations  . . . . . . . . . . . . . . . . .   7
     9.1.  Stateful addressing and host tracking . . . . . . . . . .   7
     9.2.  Address space management  . . . . . . . . . . . . . . . .   8
     9.3.  Addressing link layer scalability issues via IP routing .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   12. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     13.2.  Informative References . . . . . . . . . . . . . . . . .   9
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 areas it can lead
   to carrying over IPv4 practices that are not appropriate in IPv6 due
   to significant differences between the protocols.

   One such area is IP adressing, particularly IP addressing of hosts.
   This is substantially different because unlike IPv4 addresses, IPv6
   addresses are not a scarce resource.  In IPv6, each link has a
   virtually unlimited amount of address space [RFC7421].  Thus, unlike
   IPv4, IPv6 networks are not forced by address availability
   considerations to assign only one address per host.  On the other
   hand, assigning multiple addresses has many benefits including
   application functionality and simplicity, privacy, future
   applications, and the ability to deploy the Internet without the use

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   of NAT.  Assigning only one IPv6 address per host negates these

   This document describes the benefits of assigning 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.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Common IPv6 deployment model

   IPv6 is designed to support multiple addresses, including multiple
   global addresses, per interface ([RFC4291] section 2.1, [RFC6434]
   section 5.9.4).  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 EUI-64 or [RFC7217]), one or
   more privacy addresses [RFC4941], and possibly one or more temporary
   or non-temporary addresses assigned using DHCPv6 [RFC3315].

   In most general-purpose IPv6 networks, including all 3GPP networks
   (see [RFC6459] section 5.2) and Ethernet and Wi-Fi networks using
   SLAAC [RFC4862], IPv6 hosts have the ability to configure additional
   IPv6 addresses from the link prefix(es) without explicit requests to
   the network.

3.  Benefits of multiple addresses

   Today, there are many host functions that require more than one IP
   address to be available to the host:

   o  Privacy addressing to prevent tracking by off-network hosts (e.g.,

   o  Multiple processors inside the same device.  For example, in many
      mobile devices both the application processor and baseband
      processor need to communicate with the network, particularly for
      recent technologies like ePDG.

   o  Extending the network (e.g., tethering).

   o  Running virtual machines on hosts.

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   o  Translation-based transition technologies such as 464XLAT that
      provide IPv4 over IPv6.  Current implementations require the
      availability of a dedicated IPv6 address in order to determine
      whether inbound packets are translated or native.

   o  ILA ("Identifier-locator addressing"): https://tools.ietf.org/

   o  Future applications (e.g., per-application IPv6 addresses, such as
      described in [TARP]).

   Example 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 [RFC6877]. 464XLAT is usually deployed within a particular
      network operator's network, but there are deployment models where
      the PLAT is provided as a service by a different network (e.g.,

   o  /64 sharing [RFC7278].  This was a way to provide IPv6 tethering
      without needing to wait for network operators to deploy DHCPv6 PD,
      which is only available in 3GPP release 10.

4.  Problems with assigning a limited number of addresses per host

   Assigning a limited number of addresses per host implies that
   functions that require multiple addresses will either 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 that the functions will only be available after an explicit
   request to the network is granted.  The necessity of explicit
   requests has the following drawbacks:

   o  Increased latency, because a provisioning operation, and possibly
      human intervention with an update to the service level agreement,
      must complete before the functionality is available.

   o  Uncertainty, because it is not known in advance if a particular
      operation function will be available.

   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.

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   Some operators may desire to configure their networks to limit the
   number of IPv6 addresses per host.  Reasons might include hardware
   limitations (e.g., TCAM or neighbour cache table size constraints),
   operational consistency with IPv4 (e.g., an IP address management
   system that only supports one address per host), or business models
   (e.g., a desire to charge the network's users on a per-device basis).

5.  Overcoming limits using Network Address Translation

   These limits can mostly be overcome by end hosts by using NAT, and
   indeed in IPv4 most of these functions are provided 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 [2].  For example, the QUIC protocol uses a 15-second
   keepalive [I-D.tsvwg-quic-protocol].  Other drawbacks are described
   in [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 IAB is that deployment of IPv6 NAT is not desirable

   If networks that provide limited amount of addresses become widely
   deployed, then the desire to overcome the problems listed in
   Section 4 without disabling any features may result in operating
   system manufacturers implementing IPv6 NAT.

   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 assign 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 limited not by
   technical factors but only by security policies.

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6.  Options for obtaining more than one address

   Multiple IPv6 addresses can be obtained in the following ways:

   o  Using Stateless Address Autoconfiguration [RFC4862].  SLAAC allows
      hosts to create global IPv6 addresses on demand by simply forming
      new addresses from the global prefix assigned to the link.

   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 assign the
      client multiple addresses.  It is also possible for a client to
      request additional addresses using a different DUID.  The DHCPv6
      server will decide whether to grant or reject the request based on
      information about the client, including its DUID, MAC address, and
      so on.

   o  DHCPv6 prefix delegation [RFC3633].  DHCPv6 PD allows the client
      to request and be delegated a prefix, from which it can
      autonomously form other addresses.  The prefix can also be
      hierarchically delegated to downstream clients, or, if it is a
      /64, it be reshared with downstream clients via ND proxying
      [RFC4389] or /64 sharing [RFC7278].

   |                        |  SLAAC  |   DHCPv6   |  DHCPv6 |  DHCPv4 |
   |                        |         |  IA_NA /   |    PD   |         |
   |                        |         |   IA_TA    |         |         |
   | Autonomously form      |   Yes   |     No     |   Yes   |   Yes   |
   | addresses              |   (/64  |            |   (/64  | (NAT44) |
   |                        |  share) |            |  share) |         |
   | "Unlimited" endpoints  |   Yes*  |    Yes*    |    No   |    No   |
   | Stateful, request-     |    No   |    Yes     |   Yes   |   Yes   |
   | based                  |         |            |         |         |
   | Immune to layer 3 on-  |    No   |    Yes     |   Yes   |   Yes   |
   | link resource          |         |            |         |         |
   | exhaustion attacks     |         |            |         |         |

   [*] Subject to network limitations, e.g., ND cache entry size limits.

         Table 1: Comparison of multiple address assigment options

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7.  Number of addresses required

   If we itemize the use cases from section Section 3, we can estimate
   the number of addresses currently used in normal operations.  In
   typical implementations, privacy addresses use up to 8 addresses (one
   per day).  Current mobile devices may typically support 8 clients,
   with each one requiring one or more addresses.  A client might choose
   to run several virtual machines.  Current implementations of 464XLAT
   require use of a separate address.  Some devices require another
   address for their baseband chip.  Even a host performing only several
   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.

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
   when they connect to the network.  To support future use cases, it is
   RECOMMENDED to not impose a hard limit on the size of the address
   pool assigned to a host.  If the network requires explicit requests
   for address space, a /64 prefix is desirable.  Using DHCPv6 IA_NA or
   IA_TA to request a sufficient number of addresses (e.g. 32) would
   accomodate current clients but sets a limit on the number of
   addresses available to hosts when they attach and would limit the
   development of future applications.

9.  Operational considerations

9.1.  Stateful addressing and host tracking

   Some network operators - often operators of networks that provide
   services to third parties such as university campus networks - have
   made the argument that the only feasible IPv6 deployment mechanism is
   DHCPv6, due to the need to be able to track at all times IPv6
   addresses are assigned to which hosts.  (Example: [3] ).  One reason
   frequently cited for this is protection from liability for copyright
   infringement or other illegal activity by maintaining persistent logs
   that map user IP addresses and timestamps to hardware identifiers
   such as MAC addresses.

   It is worth noting that using DHCPv6 does not by itself ensure that
   hosts will actually use the addresses assigned to them by the network
   as opposed to using any other address on the prefix.  Such guarantees

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   can only be provided by link-layer security mechanisms that enforce
   that particular IPv6 addresses are used by particular link-layer
   addresses (for example: SAVI [RFC7039]).  If those mechanisms are
   available, it is possible to use them to provide tracking, instead.
   This form of tracking is much more reliable because it operates
   independently of how addresses are allocated.

   Additionally, attempts to track this sort of information via DHCPv6
   are likely to become decreasingly viable due to ongoing efforts to
   improve the privacy of DHCP: [I-D.ietf-dhc-anonymity-profile].

   Many large enterprise networks, including the enterprise networks of
   the authors, are fully dual-stack but do not currently use or support

9.2.  Address space management

   In IPv4, all but the world's largest networks can be addressed using
   private space [RFC1918], and with each host receiving one IPv4
   address.  Many networks can be numbered in which has
   roughly 64k addresses.  In IPv6, that is equivalent to assigning one
   /64 per host out of a /48.  Under current RIR policies, a /48 is easy
   to obtain for an enterprise network.

   Networks that need a bigger block of private space use,
   which is is roughly 16 million addresses.  In IPv6, that is
   equivalent to assigning a /64 per host out of a /40.  Enterprises of
   such size can easily obtain a /40 under current RIR policies.

   Currently, residential users receive one IPv4 address and a /48, /56
   or /60 IPv6 prefix.  While such networks do not have enough space to
   assign a /64 per device, today such networks almost universally use

   Unlike IPv4 where addresses came at a premium, in all 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 direct impact for
   networking infrastructure nodes (routers, switches) and other nodes
   on the link.  Setting aside exhaustion attacks via Layer 2 address
   spoofing, every (Layer 2, IP) address pair impacts networking
   hardware requirements in terms of memory, MLD snooping, solicited
   node multicast groups, etc.  Many of these same impacts can be felt
   by neighboring hosts.  Switching to a DHCPv6 PD model means there are

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   only forwarding decisions, with only one routing entry and one ND
   cache entry per device on the network.

10.  Acknowledgements

   The authors thank Dieter Siegmund, Mark Smith, Sander Steffann, James
   Woodyatt and Tore Anderson for their input and contributions.

11.  IANA Considerations

   This memo includes no request to IANA.

12.  Security Considerations

   None so far.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

13.2.  Informative References

              Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              profile for DHCP clients", draft-ietf-dhc-anonymity-
              profile-01 (work in progress), June 2015.

              Jana, J. and I. Swett, "QUIC: A UDP-Based Secure and
              Reliable Transport for HTTP/2", draft-tsvwg-quic-
              protocol-01 (work in progress), July 2015.

   [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,

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              DOI 10.17487/RFC2993, November 2000,

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

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   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [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>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC5902]  Thaler, D., Zhang, L., and G. Lebovitz, "IAB Thoughts on
              IPv6 Network Address Translation", RFC 5902, July 2010.

   [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., Soininen, J., Patil, B., Savolainen, T.,
              Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, January 2012.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation", RFC
              6877, April 2013.

   [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,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217, April 2014.

   [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, June

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   [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,

   [TARP]     Gleitz, PM. and SM. Bellovin, "Transient Addressing for
              Related Processes: Improved Firewalling by Using IPv6 and
              Multiple Addresses per Host", August 2001.

13.3.  URIs

   [1] http://www.jpix.ad.jp/en/service/ipv6v4.html

   [2] http://www.ietf.org/proceedings/88/slides/

   [3] https://code.google.com/p/android/issues/detail?id=32621#c60

Authors' Addresses

   Lorenzo Colitti
   Roppongi 6-10-1
   Minato, Tokyo  106-6126

   Email: lorenzo@google.com

   Vint Cerf
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: vint@google.com

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014

   Email: cheshire@apple.com

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   David Schinazi
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014

   Email: dschinazi@apple.com

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