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Versions: (draft-link-dhc-v6only) 00

Dynamic Host Configuration                                    L. Colitti
Internet-Draft                                                J. Linkova
Updates: 2563 (if approved)                                       Google
Intended status: Standards Track                           M. Richardson
Expires: September 10, 2020                                    Sandelman
                                                            T. Mrugalski
                                                                     ISC
                                                           March 9, 2020


                  IPv6-Only-Preferred Option for DHCP
                        draft-ietf-dhc-v6only-00

Abstract

   This document specifies a DHCP option to indicate that a host
   supports an IPv6-only mode and willing to forgo obtaining an IPv4
   address if the network provides IPv6 connectivity.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 10, 2020.

Copyright Notice

   Copyright (c) 2020 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
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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
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . .   4
   3.  IPv6-Only Preferred Option  . . . . . . . . . . . . . . . . .   5
     3.1.  Option format . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . .   5
     3.3.  DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . .   7
       3.3.1.  Interoperability with RFC2563 . . . . . . . . . . . .   8
     3.4.  Constants and Configuration Variables . . . . . . . . . .   9
   4.  IPv6-Only Transition Technologies Considerations  . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   One of the biggest challenges of deploying IPv6-only LANs is that
   such networks might contain rather heterogeneous collection of hosts.
   While some hosts are capable of operating in IPv6-only mode (either
   because the OS and all applications are IPv6-only capable or because
   the host has some form of 464XLAT [RFC6877] deployed), others might
   still have IPv4 dependencies and need IPv4 addresses to operate
   properly.  To incrementally rollout IPv6-only, network operators
   might need to provide IPv4 on demand whereby a host receives an IPv4
   address if it needs it, while IPv6-only capable hosts (such as modern
   mobile devices) are not allocated IPv4 addresses.  Traditionally that
   goal is achieved by placing IPv6-only capable devices into a
   dedicated IPv6-only network segment or WiFi SSID, while dual-stack
   devices reside in another network with IPv4 and DHCP enabled.
   However such approach has a number of drawbacks, including but not
   limited to:

   o  Doubling the number of network segments leads to operational
      complexity and performance impact, for instance due to high memory
      utilization caused by an increased number of ACL entries.

   o  Placing a host into the correct network segment is problematic.
      For example, in the case of 802.11 Wi-Fi the user might select the



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      wrong SSID.  In the case of wired 802.1x authentication the
      authentication server might not have all the information required
      to make the correct decision and choose between an IPv6-only and a
      dual-stack VLAN.

   It would be beneficial for IPv6 deployment if operators could
   implement IPv6-mostly (or IPv4-on-demand) segments where IPv6-only
   hosts co-exist with legacy dual-stack devices.  The trivial solution
   of disabling IPv4 stack on IPv6-only capable hosts is not feasible as
   those clients must be able to operate on IPv4-only networks as well.
   While IPv6-only capable devices might use a heuristic approach to
   learning if the network provides IPv6-only functionality and stop
   using IPv4 if it does, it might be practically undesirable.  One
   important reason is that when a host connects to a network, it does
   not know if the network is IPv4-only, dual-stack or IPv6-only.  To
   ensure that the connectivity over whatever protocol is present
   becomes available as soon as possible the host usually starts
   configuring both IPv4 and IPv6 immediately.  If hosts were to delay
   requesting IPv4 until IPv6 reachability is confirmed, that would
   penalize IPv4-only and dual-stack networks, which does not seem
   practical.  Requesting IPv4 and then releasing it later, after IPv6
   reachability is confirmed, might cause user-visible errors as it
   would be disruptive for applications which have started using the
   assigned IPv4 address already.  Instead it would be useful to have a
   mechanism which would allow a host to indicate that its request for
   an IPv4 address is optional and a network to signal that IPv6-only
   functionality (such as NAT64) is available.  The proposed solution is
   to introduce a new DHCP option which a client uses to indicate that
   it does not need an IPv4 address if the network provides IPv6-only
   connectivity (as NAT64 and DNS64).  If the particular network segment
   provides IPv4-on-demand such clients would not be supplied with IPv4
   addresses, while on IPv4-only or dual-stack segments without NAT64
   services IPv4 addresses will be provided.

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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   IPv6-only capable host: a host which does not require an IPv4 address
   and can operate on IPv6-only networks.  Strictly speaking IPv6-only
   capability is specific to a given interface of the host: if some
   application on a host require IPv4 and 464XLAT CLAT [RFC6877] is only



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   enabled on one interface, the host is IPv6-only capable if connected
   to a NAT64 network via that interface.

   IPv4-on-demand: a deployment scenario when end hosts are expected to
   operate in IPv6-only mode by default and IPv4 addresses can be
   assigned to some hosts if those hosts explicitly opt-in to receiving
   IPv4 addresses.

   IPv6-mostly network: a network which provides NAT64 (possibly with
   DNS64) service as well as IPv4 connectivity and allows coexistence of
   IPv6-only, dual-stack and IPv4-only hosts on the same segment.  Such
   deployment scenario allows operators to incrementally turn off IPv4
   on end hosts, while still providing IPv4 to devices which require
   IPv4 to operate.  But, IPv6-only capable devices need not be assigned
   IPv4 addresses.

   IPv6-Only network: a network which does not provide routing
   functionality for IPv4 packets.  Such networks may or may not allow
   intra-LAN IPv4 connectivity.  IPv6-Only network usually provides
   access to IPv4-only resources via NAT64 [RFC6147].

   NAT64: Network Address and Protocol Translation from IPv6 Clients to
   IPv4 Servers [RFC6146].

   RA: Router Advertisement, a message used by IPv6 routers to advertise
   their presence together with various link and Internet parameters
   [RFC4861].

   DNS64: a mechanism for synthesizing AAAA records from A records
   [RFC6147].

2.  Reasons to Signal IPv6-Only Support in DHCPv4 Packets

   For networks which contain both IPv6-capable and IPv4-requiring
   devices and utilize DHCP for configuring the IPv4 network stack on
   hosts, it seems only natural to leverage the same protocol to signal
   that IPv4 is discretional on a given segment.  Such an approach
   limits the attack surface to DHCP-related attacks without introducing
   new vulnerable elements.

   Another benefit of using DHCPv4 for signaling is that IPv4 will be
   disabled only if both the client and the server indicate IPv6-only
   capability.  It allows IPv6-only capable hosts to turn off IPv4 only
   upon receiving an explicit signal from the network and operate in
   dual-stack or IPv4-only mode otherwise.  In addition, the proposed
   mechanism does not introduce any additional delays to the process of
   configuring IP stack on hosts.  If the network does not support IPv6-




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   only/IPv4-on-demand mode, an IPv6-only capable host would configure
   an IPv4 address as quickly as on any other host.

   Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the
   same LAN would not only allow network administrators to preserve
   scarce IPv4 addresses but would also drastically simplify incremental
   deployment of IPv6-only networks, positively impacting IPv6 adoption.

3.  IPv6-Only Preferred Option

3.1.  Option format



      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Length      |           Value               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Value (contd)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 1: IPv6-Only Preferred Option Format

   Fields:

   Code   8-bit identifier of the IPv6-Only Preferred option code as
          assigned by IANA: TBD
   Length  8-bit unsigned integer.  The length of the option excluding
          the Code and Length Fields.  The server MUST set the length
          field to 4. The receiver MUST ignore the IPv6-Only Preferred
          option if the length field value is not 4.
   Value                                          32-bit unsigned
          integer.                                       The number of
          seconds the client should disable DHCPv4 for (V6ONLY_WAIT
          configuration variable).
          If the server pool is explicitly configured with a V6ONLY_WAIT
          timer the server MUST set the field to that configured value.
          Otherwise the server MUST set it to zero.
          The client MUST ignore V6ONLY_WAIT timer received from the
          server if the value is less than MIN_V6ONLY_WAIT.

3.2.  DHCPv4 Client Behaviour

   A DHCP client SHOULD allow a device administrator to configure
   IPv6-only preferred mode either for a specific interface (to indicate
   that the device is IPv6-only capable if connected to a NAT64 network



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   via that interface) or for all interfaces.  If only a specific
   interface is configured as IPv6-only capable the DHCP client MUST NOT
   be considered as an IPv6-capable for the purpose of sending/receiving
   DHCP packets over any other interfaces.

   If a host is not capable of operating in an IPv6-only NAT64
   environment its DHCP client MUST NOT include the IPv6-only Preferred
   option in the Parameter Request List of any DHCP packets and MUST
   ignore that option in packets received from DHCP servers.

   DHCP clients running on IPv6-only capable hosts SHOULD include the
   IPv6-only Preferred option code in the Parameter Request List in
   DHCPDISCOVER and DHCPREQUEST messages for interfaces so enabled and
   follow the processing as described below on a per interface enabled
   basis.

   If the client did not include the IPv6-only Preferred option code in
   the Parameter Request List option in the DHCPDISCOVER or DHCPREQUEST
   message it MUST ignore the IPv6-only Preferred option in any messages
   received from the server.

   If the client includes the IPv6-only Preferred option in the
   Parameter Request List and the DHCPOFFER message from the server
   contains a valid IPv6-only Preferred option, the client MUST NOT
   configure the IPv4 address provided in the DHCPOFFER.  If the
   IPv6-only Preferred option returned by the server contains non-zero
   value greater or equal to MIN_V6ONLY_WAIT, the client SHOULD set the
   V6ONLY_WAIT timer to that value.  Otherwise, the client MUST use its
   own configuration for V6ONLY_WAIT timer.  The client SHOULD stop the
   DHCP configuration process for at least V6ONLY_WAIT seconds or until
   a network attachment event happens.  The host MAY disable the IPv4
   stack completely for V6ONLY_WAIT seconds or until the network
   disconnection event happens.

   The client SHOULD include the IPv6-only Preferred option in the
   Parameter Request List option in DHCPREQUEST messages (after
   receiving a DHCPOFFER without this option, for a INIT-REBOOT, or when
   renewing or rebinding a leased address).  If the DHCP server responds
   with a DHCPACK that includes the IPv6-only Preferred option, the
   client MAY send a DHCPRELEASE message and MAY either stop the DHCP
   configuration process or disable IPv4 stack completely for
   V6ONLY_WAIT seconds or until the network disconnection event happens.
   Alternatively the client MAY continue to use the assigned IPv4
   address until further DHCP reconfiguration events.

   If the client includes the IPv6-only Preferred option in the
   Parameter Request List and the server responds with DHCPOFFER message




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   without a valid IPv6-only Preferred option, the client MUST proceed
   as normal with a DHCPREQUEST.

   If the client waits for multiple DHCPOFFER responses and selects one
   of them, it MUST follow the processing for the IPv6-only Preferred
   option based on the selected response.  A client MAY use the presence
   of the IPv6-only Preferred option as a selection criteria.

   When an IPv6-only capable client receives the IPv6-Only Preferred
   option from the server, the client MAY configure IPv4 link-local
   address [RFC3927].  In that case IPv6-Only capable devices might
   still be able to communicate over IPv4 to other devices on the link.
   The Auto-Configure Option [RFC2563] can be used to control IPv4 link-
   local addresses autoconfiguration.  Section 3.3.1 discusses
   interoperability between the IPv6-only Preferred and the Auto-
   Configure options.

3.3.  DHCPv4 Server Behaviour

   The DHCP server SHOULD be able to configure certain pools to include
   the IPv6-only preferred option in DHCP responses if the client
   included the option code in the Parameter Request List option.  The
   DHCP server MAY have a configuration option to specify V6ONLY_WAIT
   timer for all or individual IPv6-mostly pools.

   The server MUST NOT include the IPv6-only Preferred option in the
   DHCPOFFER or DHCPACK message if the YIADDR field in the message does
   not belong to a pool configured as IPv6-mostly.  The server MUST NOT
   include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK
   message if the option was not present in the Parameter Request List
   sent by the client.

   If the IPv6-only Preferred option is present in the Parameter Request
   List received from the client and the corresponding DHCP pool is
   explicitly configured as belonging to an IPv6-mostly network segment,
   the server MUST include the IPv6-only Preferred option when
   responding with the DHCPOFFER or DHCPACK message.  If the server
   responds with the IPv6-only Preferred option and the V6ONLY_WAIT
   timer is configured for the pool, the server MUST copy the configured
   value to the IPv6-only Preferred option value field.  Otherwise it
   MUST set the field to zero.  The server SHOULD not assign an address
   for the pool.  Instead it SHOULD return 0.0.0.0 as the offered
   address.  Alternatively, the server MAY include an available IPv4
   address from the pool into the DHCPOFFER as per recommendations in
   [RFC2131] but SHOULD NOT reserve the address and SHOULD NOT verify
   its uniqueness.





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   If a client includes both a Rapid-Commit option [RFC4039] and
   IPv6-Only Preferred option in the DHCPDISCOVER message the server
   SHOULD NOT honor the Rapid-Commit option if the response would
   contain the IPv6-only Preferred option to the client.  It SHOULD
   instead respond with a DHCPOFFER as indicated above.

3.3.1.  Interoperability with RFC2563

   [RFC2563] defines an Auto-Configure DHCP option to disable IPv4 link-
   local address configuration for IPv4 clients.  Clients can support
   both, neither or just one of IPv6-Only Preferred and Auto-Configure
   options.  If a client sends both IPv6-Only Preferred and Auto-
   Configure options the network administrator can prevent the host from
   configuring an IPv4 link-local address on IPv6-mostly network.  To
   achieve this the server needs to send DHCPOFFER which contains a
   'yiaddr' of 0x00000000, and the Auto-Configure flag saying
   "DoNotAutoConfigure".

   However special care should be taken in a situation when a server
   supports both options and receives just IPv6-Only Preferred option
   from a client.  Section 2.3 of [RFC2563] states that if no address is
   chosen for the host (which would be the case for IPv6-only capable
   clients on IPv6-mostly network) then: "If the DHCPDISCOVER does not
   contain the Auto-Configure option, it is not answered."  Such
   behaviour would be undesirable for clients supporting the IPv6-Only
   Preferred option w/o supporting the Auto-Configure option as they
   would not receive any response from the server and would keep asking,
   instead of disabling DHCP for V6ONLY_WAIT second.  Therefore the
   following update is proposed to Section 2.3 of [RFC2563]"

   OLD TEXT:

   ---

   However, if no address is chosen for the host, a few additional steps
   MUST be taken.

   If the DHCPDISCOVER does not contain the Auto-Configure option, it is
   not answered.

   ---

   NEW TEXT:

   ---

   However, if no address is chosen for the host, a few additional steps
   MUST be taken.



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   If the DHCPDISCOVER does not contain the Auto-Configure option and
   the IPv6-Only Preferred option is not present, it is not answered.
   If the DHCPDISCOVER does not contain the Auto-Configure option but
   contains the IPv6-Only Preferred option, the processing rules for the
   IPv6-Only Preferred option apply.

   ---

3.4.  Constants and Configuration Variables

     V6ONLY_WAIT     The minimum time the client SHOULD stop the DHCP
                     configuration process for. MUST be no less than
                     MIN_V6ONLY_WAIT seconds. Default: 1800 seconds
     MIN_V6ONLY_WAIT The lower boundary for V6ONLY_WAIT. Value: 300
                     seconds

4.  IPv6-Only Transition Technologies Considerations

   Until IPv6 adoption in the Internet reaches 100%, communication
   between an IPv6-only host and IPv4-only destination requires some
   form of transition mechanism deployed in the network.  At the time of
   writing, the only such mechanism is NAT64 [RFC6146].  Therefore the
   IPv6-only Preferred option is only sent by hosts capable of operating
   on NAT64 networks.  In a typical deployment scenario, a network
   administrator would not configure the DHCP server to return the
   IPv6-only Preferred option unless the network provides NAT64 service.

   Hypothetically it is possible for multiple transition technologies to
   coexist.  In such scenario some form of negotiation would be required
   between a client and a server to ensure that the transition
   technology supported by the client is the one the network provides.
   However it seems unlikely that any new transition technology would
   arise and be widely adopted in any foreseeable future.  Therefore
   adding support for non-existing technologies seems to be suboptimal
   and the proposed mechanism implies that NAT64 is used to facilitate
   connectivity between IPv6 and IPv4.

   It should be also noted that declaring a host or (strictly speaking,
   a host interface) IPv6-only capable is a policy decision.  For
   example,

   o  An operating system vendor may make such decision and configure
      their DHCP clients to send the IPv6-Only Preferred option by
      default if the OS has 464XLAT CLAT [RFC6877] enabled.

   o  An enterprise network administrator may provision the corporate
      hosts as IPv6-only capable if all applications users are supposed




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      to run have been tested in IPv6-only environment (or if 464XLAT
      CLAT is enabled on the devices).

   o  IoT devices may be shipped in IPv6-only capable mode if they are
      designed to connect to IPv6-enabled cloud destination only.

5.  IANA Considerations

   The IANA is requested to assign a new DHCP Option code for the
   IPv6-Only Preferred option from the BOOTP Vendor Extensions and DHCP
   Options registry, located at https://www.iana.org/assignments/bootp-
   dhcp-parameters/bootp-dhcp-parameters.xhtml#options .  If possible,
   please assign option code 108.

                  +----------------------------+-------+
                  | Option Name                | Code  |
                  +----------------------------+-------+
                  | IPv6-only Preferred option | (TBD) |
                  +----------------------------+-------+

                                  Table 1

6.  Security Considerations

   The proposed mechanism is not introducing any new security
   implications.  While clients using the IPv6-only Preferred option are
   vulnerable to attacks related to a rogue DHCP server, enabling
   IPv6-only Preferred option does not provide an attacker with any
   additional mechanisms.

   It should be noted that disabling IPv4 on a host upon receiving the
   IPv6-only Preferred option from the DHCP server protects the host
   from IPv4-related attacks and therefore could be considered a
   security feature.

7.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   review and feedback: Mohamed Boucadair, Ted Lemon, Roy Marples, Bjorn
   Mork, Bernie Volz (AI: add more names here).  Authors would like to
   thank Bob Hinden and Brian Carpenter for the initial idea of
   signaling IPv6-only capability to hosts.

8.  References







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8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2563]  Troll, R., "DHCP Option to Disable Stateless Auto-
              Configuration in IPv4 Clients", RFC 2563,
              DOI 10.17487/RFC2563, May 1999,
              <https://www.rfc-editor.org/info/rfc2563>.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              DOI 10.17487/RFC3927, May 2005,
              <https://www.rfc-editor.org/info/rfc3927>.

   [RFC4039]  Park, S., Kim, P., and B. Volz, "Rapid Commit Option for
              the Dynamic Host Configuration Protocol version 4
              (DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005,
              <https://www.rfc-editor.org/info/rfc4039>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.




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   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

Authors' Addresses

   Lorenzo Colitti
   Google
   Shibuya 3-21-3
   Shibuya, Tokyo  150-0002
   JP

   Email: lorenzo@google.com


   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont, NSW  2009
   AU

   Email: furry@google.com


   Michael C. Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/


   Tomek Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   Email: tomasz.mrugalski@gmail.com












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