IPv6 Operations                                               L. Colitti
Internet-Draft                                                   V. Cerf
Intended status: Best Current Practice                            Google
Expires: June 12, 2016                                       S. Cheshire
                                                             D. Schinazi
                                                              Apple Inc.
                                                       December 10, 2015

               Host address availability recommendations


   This document recommends that networks provide general-purpose end
   hosts with multiple global IPv6 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.

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   This Internet-Draft will expire on June 12, 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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   include Simplified BSD License text as described in Section 4.e of

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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 restricted 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  . . . . . . . . . . . . . . . . .   8
     9.1.  Stateful addressing and host tracking . . . . . . . . . .   8
     9.2.  Address space management  . . . . . . . . . . . . . . . .   9
     9.3.  Addressing link layer scalability issues via IP routing .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 addressing, 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",
   "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 ([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 Opaque Interface
   Identifiers [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
   ([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

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

   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
      [RFC6877] that provide IPv4 over IPv6.  Some of these require the
      availability of a dedicated IPv6 address in order to determine
      whether inbound packets are translated or native ([RFC6877]
      section 6.3).

   o  ILA ("Identifier-locator addressing") [I-D.herbert-nvo3-ila].

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

   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,
      and in this model the operator can ensure that the network is
      appropriately configured to provide the CLAT with the additional
      IPv6 address it needs to implement 464XLAT.  However, there are
      deployments where the 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
      <http://www.jpix.ad.jp/en/service/ipv6v4.html>).  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 PD, which is only available in 3GPP
      release 10 ([RFC6459] section 5.3).

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

   Assigning a restricted 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.

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   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 to configure their networks to limit the
   number of IPv6 addresses per host.  Reasons might include hardware
   limitations (e.g., TCAM or neighbor cache table size constraints),
   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

   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 <http://www.ietf.org/proceedings/88/slides/slides-88-tsvarea-
   10.pdf>.  For example, the QUIC protocol uses a 15-second keepalive
   [I-D.tsvwg-quic-protocol].  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 IAB is that deployment of IPv6 NAT is not
   desirable [RFC5902].

   The desire to overcome the problems listed in Section 4 without
   disabling any features has resulted in developers implementing IPv6
   NAT.  There are fully-stateful address+port NAT66 implementations in
   client operating systems today: for example, Linux has supported
   NAT66 since late 2012 <http://kernelnewbies.org/Linux_3.7#head-
   103e14959eeb974bbd4e862df8afe7c118ba2beb>.  A popular software
   hypervisor also recently implemented NAT66 to work around these
   issues <https://communities.vmware.com/docs/DOC-29954>.  Wide

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   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 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 constrained not by
   accidental technical limitations, but only by intentional security

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 technically possible for a
      client to request additional addresses using a different 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 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.  If the prefix is shorter than
      /64, it can be divided into multiple subnets which can be further
      delegated to downstream clients.  If the prefix is a /64, it can
      be extended via L2 bridging, 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].

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   |                          | SLAAC |    DHCPv6   | DHCPv6 |  DHCPv4 |
   |                          |       |   IA_NA /   |   PD   |         |
   |                          |       |    IA_TA    |        |         |
   | Extend network           |  Yes  |      No     |  Yes   |   Yes   |
   |                          |       |             |        | (NAT44) |
   | "Unlimited" endpoints    |  Yes* |     Yes*    |   No   |    No   |
   | Stateful, request-based  |   No  |     Yes     |  Yes   |   Yes   |
   | 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 assignment options

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 ([RFC4941] section 3.5).  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 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

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 (e.g., if it requires DHCPv6 to connect), it is
   RECOMMENDED that the network assign a /64 prefix to every host (e.g.,
   via DHCPv6 PD).  Using DHCPv6 IA_NA or IA_TA to request a sufficient
   number of addresses (e.g. 32) would accommodate current clients but

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   sets a limit on the number of addresses available to hosts when they
   attach and would limit the development of future applications.
   Assigning prefixes longer than a /64 will limit the flexibility of
   the host to further assign addresses to any internal functions,
   virtual machines, or downstream clients that require address space -
   for example, by not allowing the use of SLAAC.

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 - 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 avoid liability for copyright infringement
   or other illegal activity.

   It is worth noting that this requirement can be met without using
   stateful addressing mechanisms such as DHCPv6.  For example, it is
   possible to maintain these mappings by scraping IPv6 neighbor tables,
   as routers typically allow periodic dumps of the neighbor cache via
   SNMP or other means, and many can be configured to log every change
   to the neighbor cache.

   It is also worth noting that without L2 edge port security, hosts are
   still able to choose their own addresses - DHCPv6 does not offer any
   enforcement of what addresses a host is allowed to use.  Such
   guarantees 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.  This form of tracking is much more secure and reliable
   than DHCP server logs 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

   Thus, host tracking does not necessarily require the use of stateful
   address assignment mechanisms such as DHCPv6.  Indeed, many large
   enterprise networks, including the enterprise networks of the
   authors' employers, are fully dual-stack but do not currently use or
   support DHCPv6.  The authors are directly aware of several networks
   that operate in this way, including Universities of Loughborough,
   Minnesota, Reading, Southampton, Wisconsin and Imperial College

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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 which has roughly 64k
   addresses.  In IPv6, that is equivalent to a /48, with each of 64k
   hosts receiving a /64 prefix.  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 has roughly 16 million addresses.  In IPv6, that is equivalent
   to a /40, with each host receiving /64 prefix.  Enterprises of such
   size can easily obtain a /40 under current RIR policies.

   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 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 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 DHCPv6 PD model with one /64 prefix per host
   resolves these scaling limitations, with only one routing entry and
   one ND cache entry per device on the network.

   Also, a DHCPv6 PD model with a dedicated /64 per host makes it
   possible for the host not to assign global IPv6 addresses directly to
   its physical network interface, but instead to assign them to an
   internal interface such as a loopback interface.  This obviates the

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   need to perform Neighbour Discovery and Duplicate Address Detection
   for anything other than the link-local address on its physical
   network interface, reducing network traffic.

10.  Acknowledgements

   The authors thank Tore Anderson, Brian Carpenter, David Farmer,
   Wesley George, Erik Kline, Shucheng (Will) Liu, Dieter Siegmund, Mark
   Smith, Sander Steffann and James Woodyatt for their input and

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,
              DOI 10.17487/RFC2119, March 1997,

13.2.  Informative References

              Herbert, T., "Identifier-locator addressing for network
              virtualization", draft-herbert-nvo3-ila-01 (work in
              progress), October 2015.

              Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              profile for DHCP clients", draft-ietf-dhc-anonymity-
              profile-04 (work in progress), October 2015.

              Iyengar, 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.

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   [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., 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,

   [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>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC5902]  Thaler, D., Zhang, L., and G. Lebovitz, "IAB Thoughts on
              IPv6 Network Address Translation", RFC 5902,
              DOI 10.17487/RFC5902, 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>.

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   [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,

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, 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,
              DOI 10.17487/RFC7217, 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,
              DOI 10.17487/RFC7278, June 2014,

   [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.

Authors' Addresses

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

   Email: lorenzo@google.com

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   Vint Cerf
   1875 Explorer St
   10th Floor
   Reston, VA  20190

   Email: vint@google.com

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

   Email: cheshire@apple.com

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

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