Dynamic Host Configuration                                    L. Colitti
Internet-Draft                                                J. Linkova
Intended status: Standards Track                                  Google
Expires: June 6, 11, 2020                                     M. Richardson
                                                            T. Mrugalski
                                                        December 4, 9, 2019

                  IPv6-Only-Preferred Option for DHCP


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

Status of This Memo

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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  . . . . . . . . . . . . . . . . .   4   5
     3.1.  Option format . . . . . . . . . . . . . . . . . . . . . .   4   5
     3.2.  DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . .   5
     3.3.  DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . .   6   7
     3.4.  Configuration Variables . . . . . . . . . . . . . . . . .   7   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9  10

1.  Introduction

   One of the biggest challenges of deploying IPv6-only LANs is that
   such networks might contain rather heterogeneous collection of end hosts.
   Some of them 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).  At the same
   time some devices might still have IPv4 dependencies and need IPv4
   connectivity to operate properly.  To incrementaly incrementally rollout
   IPv6-only, network operators need to provide IPv4-as-a-service when
   whereby a host receives an IPv4 address if it needs it, while
   IPv6-only capable devices (such as modern mobile devices) are not
   allocated IPv4 addresses.  Deploying separate LAN segments for
   IPv6-only and for dual-stack hosts (such as two WiFi SSIDs or two
   VLANs) is undesirable for a number of reasons, including but not
   limited to:

   o  Doubling the number of network segments which leads to operational
      complexity and performance impact, for instance due to TCAM
      utilization increase from 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
      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.

   Therefore it

   It would be beneficial for IPv6 deployment if operators could
   implement IPv4-mostly IPv6-mostly (or IPv4-as-a-Service) 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 provdes 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 immidiately. 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 IPv4 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 IPv4 if
   the network provides IPv6-only connectivty connectivity (as NAT64 and DNS64).  If
   the particular network segment provides IPv4-as-a-service such
   clients would not be supplied with IPv4 addresses, while on IPv4-only
   or dual-stack segments without NAT64 services IPv4 addresses will be

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 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 IPv4 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 CLAT [RFC6877] is only
   enabled on one interface, the host is IPv6-only capable if connected
   to a NAT64 network via that interface.

   IPv4-as-a-Service: 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; addresses.

   IPv6-mostly network: a network which provides NAT64 (possibly with
   DNS64) service as well as IPv4 connectivity.  Such deployment
   scenario allows operators to incrementaly 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

   IPv6-Only network: a network which does not provide routing
   functionality for IPv4 packets.  Such network networks may or may not allow
   intra-LAN IPv4 connectivity.  IPv6-Only network usually provide 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

   DNS64: a mechanism for synthesizing AAAA records from A records

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

   For networks which contains contain both IPv6-capable and IPv4-requiring
   devices and utilizes utilize DHCP for configuring IPv4 network stack on hosts,
   it seems only natural to leverage the same protocol to signal that
   IPv4 is discretional on the 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 signalling signaling is that IPv4 will be
   disabled only if both the client and the server indicate IPv6-only
   capability.  It allows IPv6-only capable clients hosts to turn off IPv6 IPv4 only
   upon receiving an explicit signal from the network and operate in
   dual-stack or IPv4-only mode otherwise.

   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
     |     Type      |   Length      |           Value               |
     |         Value (contd)         |

                Figure 1: IPv6-Only Preferred Option Format


   Type   8-bit identifier of the IPv6-Only Preferred option type as
          assigned by IANA: TBD
   Length  8-bit unsigned integer.  The length of the option (excluding excluding
          the Type and Length Fields.  The server MUST set the length
          field to 0. 4. The receiver MUST ignore the IPv6-Only Preferred
          option if the the length field value is not 0. 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 conifgured with 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 300 seconds.

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
   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.

   Clients not capable of operating in an IPv6-only NAT64 environment
   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.

   IPv6-only capable clients SHOULD include the IPv6-only Preferred
   option 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 in the
   DHCPDISCOVER or DHCPREQUEST message it MUST ignore the 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 the client SHOULD set the V6ONLY_WAIT timer to that value.  If
   the server returns zero value 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 attachement attachment event happens.  The host MAY disable IPv4 stack
   completely for V6ONLY_WAIT seconds or until the network disconnection
   event heppens. happens.

   The client SHOULD include the IPv6-only Preferred 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 heppens. happens.  Alternatively the client MAY continue
   to use the assigned IPv4 address until further DHCP reconfiguration

   If the client includes the IPv6-only Preferred option in the
   Parameter Request List and the server responds with DHCPOFFER message
   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 server the client
   sends the DHCPREQUEST to did not include processing for the IPv6-only Preferred
   option in the DHCPOFFER, based on the selected response.  A client MUST NOT stop MAY use the DHCP
   configuration process or disable IPv4 stack even if other servers
   include presence
   of the IPv6-only Preferred option in their responses. as a selection criteria.

   When an IPv6-only capable client receives the IPv6-Only Preferred
   option from the server, The 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.

3.3.  DHCPv4 Server Behaviour

   The DHCP server SHOULD have a configuration option to mark configure the
   given DHCP pool as belonging to with an IPv6-only preferred option.  The DHCP server
   MAY have a configuration option to specify V6ONLY_WAIT timer for all
   or individual IPv6-mostly network segment. 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 respond the IPv6-only Preferred option when
   responding with the DHPCOFFER DHCPOFFER or DHCPACK message.  If the pool server
   responds with the IPv6-only Preferred option and the V6ONLY_WAIT
   timer is explicitly configured with a dedicated IPv4
   address to be returned for the pool, the server MUST copy the configured
   value to the IPv6-only capable clients Preferred option value field.  Otherwise it
   MUST set the field to zero.  The server MUST
   specify that SHOULD include an available
   IPv4 address from the pool into the DHCPOFFER as per recommendations
   in [RFC2131] but SHOULD NOT reserve the client's network address and MUST SHOULD NOT verify
   its uniqueness.  Otherwise the server SHOULD follow the
   recommendations in [RFC2131].  The client is not expected to use that IPv4 address
   so if the client responds with the DHCPREQUEST message for that
   address the server SHOULD respond with DHCPNAK.

   As an optional optimization an IPv6-mostly pool MAY be configured
   with a dedicated IPv4 address to be returned to IPv6-only capable
   clients.  In that case the server SHOULD specify that address as the
   client's network address and MUST NOT verify its uniqueness.

   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 woul would
   contain the IPv6-only Preferred option to the client.  It SHOULD
   instead respond with a DHCPOFFER so that the IP address does not need
   to be reserved for the client until the lease expires.

3.4.  Configuration Variables

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

4.  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                | Type  |
                  | IPv6-only Preferred option | (TBD) |

                                  Table 1

5.  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.

6.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   review and feedback: Mohamed Bboucadair, Bjorn Mork, Bernie Volz. 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.

7.  References
7.1.  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,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              DOI 10.17487/RFC3927, May 2005,

   [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,

   [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,

   [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>.

7.2.  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,

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,

Authors' Addresses

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

   Email: lorenzo@google.com

   Jen Linkova
   1 Darling Island Rd
   Pyrmont, NSW  2009

   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

   Email: tomasz.mrugalski@gmail.com