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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902"
     updates="2563" obsoletes="" category="std"
     docName="draft-ietf-dhc-v6only-08">

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     docName="draft-ietf-dhc-v6only-08" number="8925" submissionType="IETF"
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  <front>
    <!-- The abbreviated title is used in the page header - it is only necessary if the
         full title is longer than 39 characters -->

    <title>IPv6-Only-Preferred
    <title>IPv6-Only Preferred Option for DHCPv4</title>

    <!-- add 'role="editor"' below for the editors if appropriate -->
    <seriesInfo name="RFC" value="8925"/>
    <author fullname="Lorenzo Colitti" initials="L." surname="Colitti">
      <organization>Google</organization>
      <address>
        <postal>
          <street>Shibuya 3-21-3</street>
          <city>Shibuya</city>
          <region>Tokyo</region>
          <region>Shibuya, Tokyo</region>
          <code>150-0002</code>
          <country>JP</country>
          <country>Japan</country>
        </postal>

        <phone></phone>
        <email>lorenzo@google.com</email>
      </address>
    </author>
    <author fullname="Jen Linkova" initials="J." surname="Linkova">
      <organization>Google</organization>
      <address>
        <postal>
          <street>1 Darling Island Rd</street>
          <city>Pyrmont</city>
          <region>NSW</region>
          <code>2009</code>
          <country>AU</country>
          <country>Australia</country>
        </postal>

        <phone></phone>
        <email>furry@google.com</email>
      </address>
    </author>
    <author fullname="Michael C. Richardson" initials="M." surname="Richardson">
      <organization abbrev="Sandelman">Sandelman Software Works</organization>
      <address>
        <email>mcr+ietf@sandelman.ca</email>

        <uri>http://www.sandelman.ca/</uri>
        <uri>https://www.sandelman.ca/</uri>
      </address>
    </author>
    <author fullname="Tomek Mrugalski" initials="T." surname="Mrugalski">
      <organization abbrev="ISC">Internet Systems Consortium, Inc.</organization>
      <address>
        <postal>
          <street>950 Charter Street</street>
          <city>Redwood City</city>
          <region>CA</region>
          <code>94063</code>
          <country>USA</country>
          <street>PO Box 360</street>
          <city>Newmarket</city>
          <region>NH</region>
          <code>03857</code>
          <country>United States of America</country>
        </postal>
        <email>tomasz.mrugalski@gmail.com</email>
      </address>
    </author>

    <date/>

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    <area>Internet</area>

    <workgroup>Dynamic Host Configuration</workgroup>

    <!-- WG name at the upperleft corner of the doc,
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    <date month="October" year="2020"/>

    <abstract>
      <t>
          This document specifies a DHCPv4 option to indicate that a host supports an IPv6-only mode and is willing to forgo obtaining an IPv4 address if the network provides IPv6 connectivity.
          It also updates RFC2563 RFC 2563 to specify the DHCPv4 server behavior when the server receives a DHCPDISCOVER not containing the Auto-Configure option but containing the new option defined in this document.
      </t>
    </abstract>
  </front>
  <middle>
    <section title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>
          One of the biggest challenges of deploying IPv6-only LANs is that
          such networks might contain a 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 <xref target="RFC6877"/> target="RFC6877"
          format="default"/> deployed), others might still have IPv4 dependencies and need IPv4 addresses to operate properly.
          To incrementally rollout roll out IPv6-only, network operators might need to provide IPv4 on demand demand, whereby a host receives an IPv4 address if it needs it, while IPv6-only capable IPv6-only-capable hosts (such as modern mobile devices) are not allocated IPv4 addresses.
Traditionally
Traditionally, that goal is achieved by placing IPv6-only capable IPv6-only-capable devices into in
a dedicated IPv6-only network segment or WiFi SSID, Wi-Fi Service Set Identifier (SSID),
while dual-stack devices reside in another network with IPv4 and DHCPv4
enabled.
However However, such an approach has a number of drawbacks, including including, but not
limited to:
		    <list style="symbols">
			    <t> to, the following:
      </t>
      <ul spacing="normal">
        <li>
          Doubling the number of network segments leads to operational
          complexity and impact on performance impact, -- for instance instance, due to high
          memory utilization caused by an increased number of ACL Access Control
          List (ACL) entries.
			    </t>
			    <t>
          </li>
        <li>
          Placing a host into in the correct network segment is problematic.
          For example, in the case of 802.11 Wi-Fi Wi-Fi, the user might select the wrong SSID.
          In the case of wired 802.1x authentication authentication, the authentication
          server might not have all the information required to make the
          correct decision and choose between an IPv6-only VLAN and a dual-stack VLAN.
			    </t>
		    </list>
	    </t>
          </li>
      </ul>
      <t>
          It would be beneficial for IPv6 deployment if operators could implement IPv6-mostly (or IPv4-on-demand) segments where IPv6-only hosts co-exist coexist with legacy dual-stack devices.
          The trivial solution of disabling the IPv4 stack on IPv6-only capable IPv6-only-capable hosts is not feasible feasible, as those clients must be able to operate on IPv4-only networks as well.
          While IPv6-only capable 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, such an approach might be practically undesirable. undesirable in practice.
          One important reason is that when a host connects to a network, it does not know if whether the network is IPv4-only, dual-stack dual-stack, or IPv6-only.
          To ensure that the connectivity over whatever protocol is present becomes available as soon as possible 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 that are visible to users, as it would be
          disruptive for applications which that have already started using the assigned IPv4 address already.
			    Instead address.
          Instead, it would be useful to have a mechanism which that 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, NAT64 <xref target="RFC6146"/>) target="RFC6146" format="default"/>) is available.
		    The proposed
          This document provides such a solution is to introduce via a new DHCPv4 option which that
          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 IPv4 on demand, such clients would
not be supplied with IPv4 addresses, while IPv4 addresses would be provided on IPv4-only or dual-stack segments without NAT64 services IPv4 addresses will be provided. services.
      </t>
      <t>
          <xref target="RFC2563"/> introduces target="RFC2563" format="default"/> introduced the Auto-Configure DHCPv4 option and describes DHCPv4 servers server behavior if no address is chosen for a host. This document updates <xref target="RFC2563"/> target="RFC2563" format="default"/> to modify the server behavior if the DHCPOFFER contains the IPv6-only IPv6-Only Preferred option.
      </t>
      <section title="Requirements Language"> numbered="true" toc="default">
        <name>Requirements Language</name>
       <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
                        "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
       "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
       "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
       "<bcp14>SHOULD NOT</bcp14>",
       "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
       "<bcp14>MAY</bcp14>", and
                        "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document
       are to be interpreted as described in
                        BCP 14 BCP&nbsp;14
       <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
       when, they appear in all capitals, as shown here.</t>
      </section>
      <section title="Terminology">
	      <t>
			      Dual-stack numbered="true" toc="default">
        <name>Terminology</name>
        <dl newline="false" spacing="normal">
        <dt>Dual-stack network or device: a device:</dt>
        <dd>A network or device which that has both versions of the Internet Protocol (IPv4 and IPv6) enabled and operational.
	      </t>
	      <t>
		      IPv6-only capable host: a operational.</dd>
        <dt>IPv6-only-capable host:</dt><dd>A host which that does not require an IPv4 address and can operate on IPv6-only networks.
            More precisely, IPv6-only capability is specific to a given
            interface of the host: if some application applications on a host require IPv4
            and the 464XLAT CLAT (customer-side translator) <xref target="RFC6877"/> target="RFC6877"
            format="default"/> is only enabled on one interface, the host is
            IPv6-only capable if connected to a NAT64 network via that
            interface. This document implies that IPv6-only capable IPv6-only-capable hosts
            reach IPv4-only destinations via a NAT64 service provided by the
            network. <xref target="v6onlydef" /> format="default"/> discusses
            hypothetical scenarios of for other transition technologies being used.
	      </t>
	      <t>
		      IPv4-requiring host: a used.</dd>
        <dt>IPv4-requiring host:</dt><dd>A host which that is not IPv6-only capable and can not cannot operate in an IPv6-only network providing NAT64 service.
	      </t>
	      <t>
		      IPv4-on-demand: a service.</dd>
        <dt>IPv4 on demand:</dt><dd>A deployment scenario where 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 opt in to receiving receive IPv4 addresses.
	      </t>
	      <t>
		      IPv6-mostly network: a
</dd>
        <dt>IPv6-mostly network:</dt><dd>A network which that provides NAT64
        (possibly with DNS64) service as well as IPv4 connectivity and allows
        the coexistence of IPv6-only, dual-stack dual-stack, and IPv4-only hosts on the same segment.
            Such a deployment scenario allows operators to incrementally turn off IPv4 on end hosts, while still providing IPv4 to devices which that require IPv4 to operate.
		      But, IPv6-only capable
            But IPv6-only-capable devices need not be assigned IPv4 addresses.
	      </t>
	      <t>
			      IPv6-only mode: a addresses.</dd>
        <dt>IPv6-only mode:</dt><dd>A mode of operation when where a host acts as an IPv6-only capable IPv6-only-capable host and does not have IPv4 addresses assigned (except that IPv4 link-local addresses <xref target="RFC3927"/> target="RFC3927" format="default"/> may have been configured).
	      </t>
	      <t>
		      IPv6-only network: a configured).</dd>
        <dt>IPv6-only network:</dt><dd>A network which that does not provide routing functionality for IPv4 packets.
            Such networks may or may not allow intra-LAN IPv4 connectivity.
            An IPv6-only network usually provides access to IPv4-only resources via NAT64 <xref target="RFC6146"/>.
	      </t>
	      <t>
		      NAT64: Network target="RFC6146" format="default"/>.</dd>
        <dt>NAT64:</dt><dd>Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers <xref target="RFC6146"/>.
	      </t>
	      <t>
		      RA:    Router Advertisement, a target="RFC6146" format="default"/>.</dd>
        <dt>Router Advertisement (RA):</dt><dd>A message used by IPv6 routers to advertise their presence presence, together
            with various link and Internet parameters <xref target="RFC4861"/>.
	      </t>
	      <t>
		      DNS64: a target="RFC4861" format="default"/>.</dd>
        <dt>DNS64:</dt><dd>A mechanism for synthesizing AAAA records from A records <xref target="RFC6147"/>.
	      </t>
	      <t>
		      Network target="RFC6147" format="default"/>.</dd>
        <dt>Network attachment event: A Link Up event:</dt><dd>A link up event, as described by
        <xref target="RFC4957" /> which format="default"/>, that results in a host detecting an available network.
	      </t>
	      <t>
			       Disabling network.</dd>
        <dt>Disabling the IPv4 stack on the host interface: the host interface:</dt><dd>
        <t>Host behavior when the host:
		       <list style="symbols">
			       <t>
					       does host</t>
        <ul>
          <li>does not send any IPv4 packets from that interface,
			       </t>
			       <t>
					       drops interface,</li>
          <li>drops all IPv4 packets received on that interface and

			       </t>
			       <t>
					       does interface, and</li>
          <li>does not forward any IPv4 packets to that interface.
			       </t>
		       </list>
	       </t> interface.</li>
        </ul>
      </dd>
    </dl>
      </section>
    </section>
    <section title="Reasons numbered="true" toc="default">
      <name>Reasons to Signal IPv6-Only Support in DHCPv4 Packets"> Packets</name>
      <t>
          For networks which that contain a mix of both IPv6-only capable IPv6-only-capable hosts and
          IPv4-requiring hosts, hosts and which that utilize DHCPv4 for configuring the IPv4 network stack on hosts, it seems natural to leverage the same protocol to signal that IPv4 is discretional on a given segment.
          An ability to remotely disable IPv4 on a host can be seen as a new denial-of-service attack vector.
          The proposed approach provided in this document limits the attack surface to DHCPv4-related
          attacks without introducing new vulnerable elements.
      </t>
      <t>
          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 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 defined in this document does not introduce any
          additional delays to the process of configuring an IP stack on
          hosts.
          If the network does not support IPv6-only/IPv4-on-demand mode, an IPv6-only capable
          IPv6-only-capable host would configure an IPv4 address as quickly as on
          any other host.
      </t>
      <t>
          Being a client/server protocol, DHCPv4 allows IPv4 to be selectively disabled on a per-host basis on a given network segment.
			    Coexistence
          The coexistence of IPv6-only, dual-stack 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.
      </t>
    </section>
    <section title="IPv6-Only numbered="true" toc="default">
      <name>IPv6-Only Preferred Option"> Option</name>
      <section anchor="Format" title="Option format"> numbered="true" toc="default">
        <name>Option Format</name>
        <figure align="center" anchor="fig_Option"
                             title="IPv6-Only anchor="fig_Option">
          <name>IPv6-Only Preferred Option Format"> Format</name>
          <artwork align="center"><![CDATA[ align="center" name="" type="" alt=""><![CDATA[
  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) (cont.)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                      ]]></artwork>
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
        </figure>
        <t>Fields:</t>
                      <texttable style="none">
                              <ttcol></ttcol>
                              <ttcol></ttcol>

			      <c>Code: </c>   <c> 8-bit
        <dl newline="false" spacing="normal">
              <dt>Code:</dt>

              <dd>8-bit identifier of the IPv6-Only Preferred option code as
              assigned by IANA: TBD. 108.
            The client includes the Code in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST messages as described in <xref target="v4client"/>.</c>

                              <c>Length:</c>   <c> 8-bit target="v4client" format="default"/>.</dd>
              <dt>Length:</dt>
              <dd>8-bit unsigned integer.  The length of the option option, excluding the Code and Length Fields.  The server MUST <bcp14>MUST</bcp14> set the length field to 4. The client MUST <bcp14>MUST</bcp14> ignore the IPv6-Only Preferred option if the length field value is not 4.</c>
			      <c>Value:</c>   <c> 32-bit 4.</dd>
              <dt>Value:</dt>
              <dd><t>32-bit unsigned integer.
            The number of seconds for which the client should disable DHCPv4 for (V6ONLY_WAIT configuration variable).
            If the server pool is explicitly configured with a V6ONLY_WAIT timer timer, the server MUST <bcp14>MUST</bcp14> set the field to that configured value. Otherwise Otherwise, the server MUST <bcp14>MUST</bcp14> set it to zero.
            The client MUST <bcp14>MUST</bcp14> process that field as described in <xref target="v4client"/>.
					      The target="v4client" format="default"/>.</t>
            <t>The client never sets this field field, as it never sends the full option but includes the option code in the Parameter Request List as described in <xref target="v4client"/>.
			      </c>
                              <c></c><c></c>
       </texttable> target="v4client" format="default"/>.</t></dd>
        </dl>
      </section>
      <section anchor="v4client" title="DHCPv4 numbered="true" toc="default">
        <name>DHCPv4 Client Behavior"> Behavior</name>
        <t>
        A DHCPv4 client SHOULD <bcp14>SHOULD</bcp14> allow a device
        administrator to configure IPv6-only preferred mode capability 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 capable, the
        DHCPv4 client MUST NOT <bcp14>MUST NOT</bcp14> consider the host to be an IPv6-only capable
        IPv6-only-capable host for the purpose of sending/receiving DHCPv4 packets over any other interfaces.
        </t>
        <t>
             The DHCPv4 client on an IPv4-requiring host MUST NOT <bcp14>MUST
	     NOT</bcp14> include the IPv6-only IPv6-Only Preferred option code in the Parameter Request List of any DHCPv4 packets and MUST <bcp14>MUST</bcp14> ignore that option in packets received from DHCPv4 servers.
        </t>
        <t>
             DHCPv4 clients running on IPv6-only capable IPv6-only-capable hosts SHOULD <bcp14>SHOULD</bcp14> include the IPv6-only 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 enabled interface per-enabled-interface basis.
        </t>
        <t>
             If the client did not include the IPv6-only IPv6-Only Preferred option code
             in the Parameter Request List option in the DHCPDISCOVER or
             DHCPREQUEST message message, it MUST <bcp14>MUST</bcp14> ignore the IPv6-only IPv6-Only
             Preferred option in any messages received from the server.
        </t>
        <t>
             If the client includes the IPv6-only IPv6-Only Preferred option code in the Parameter Request List and the DHCPOFFER message from the server contains a valid IPv6-only IPv6-Only Preferred option, the client SHOULD NOT <bcp14>SHOULD NOT</bcp14> request the IPv4 address provided in the DHCPOFFER.
             If the IPv6-only IPv6-Only Preferred option returned by the server contains
             a value greater than or equal to MIN_V6ONLY_WAIT, the client SHOULD <bcp14>SHOULD</bcp14> set the V6ONLY_WAIT timer to that value.
             Otherwise, the client SHOULD <bcp14>SHOULD</bcp14> set the V6ONLY_WAIT timer to MIN_V6ONLY_WAIT.
             The client SHOULD <bcp14>SHOULD</bcp14> stop the DHCPv4 configuration process for V6ONLY_WAIT seconds or until a network attachment event, whichever happens first.
             The host MAY <bcp14>MAY</bcp14> disable the IPv4 stack completely on the affected interface for V6ONLY_WAIT seconds or until the network attachment event, whichever happens first.
        </t>
        <t>
             The IPv6-only IPv6-Only Preferred option SHOULD <bcp14>SHOULD</bcp14> be included in the Parameter Request List option in DHCPREQUEST messages (after receiving a DHCPOFFER without this option, for a an INIT-REBOOT, or when renewing or rebinding a leased address).
             If the DHCPv4 server responds with a DHCPACK that includes the IPv6-only
             IPv6-Only Preferred option, the client behaviour client's behavior depends on the client's state.
             If the client is in the INIT-REBOOT state state, it SHOULD
             <bcp14>SHOULD</bcp14> stop the DHCPv4 configuration process or
             disable the IPv4 stack completely for V6ONLY_WAIT seconds or
             until the network attachment event, whichever happens first.
             It also MAY <bcp14>MAY</bcp14> send a DHCPRELEASE message.
             If the client is in any other state state, it SHOULD <bcp14>SHOULD</bcp14> continue to use the assigned IPv4 address until further DHCPv4 reconfiguration events.
        </t>
        <t>
             If the client includes the IPv6-only IPv6-Only Preferred option code in the
             Parameter Request List and the server responds with a DHCPOFFER message without a valid IPv6-only IPv6-Only Preferred option, the client MUST <bcp14>MUST</bcp14> proceed as normal with a DHCPREQUEST.
        </t>
        <t>
             If the client waits for multiple DHCPOFFER responses and selects one of them, it MUST <bcp14>MUST</bcp14> follow the processing for the IPv6-only IPv6-Only Preferred option based on the selected response.
             A client MAY <bcp14>MAY</bcp14> use the presence of the IPv6-only IPv6-Only Preferred option as a selection criteria. criterion.
        </t>
        <t>
             When an IPv6-only capable IPv6-only-capable client receives the IPv6-Only Preferred
             option from the server, the client MAY configurean <bcp14>MAY</bcp14> configure an IPv4 link-local address <xref target="RFC3927"/>. target="RFC3927" format="default"/>.
             In that case IPv6-only capable case, IPv6-only-capable devices might still be able to communicate over IPv4 to other devices on the link.
             The Auto-Configure Option option <xref target="RFC2563"/> target="RFC2563"
             format="default"/> can be used to control the autoconfiguration
             of IPv4 link-local addresses autoconfiguration. addresses.
             <xref target="autoconf"/> target="autoconf" format="default"/> discusses the
             interaction between the IPv6-only IPv6-Only Preferred option and the Auto-Configure options. option.
        </t>
      </section>
      <section anchor="v4srv" title="DHCPv4 numbered="true" toc="default">
        <name>DHCPv4 Server Behavior"> Behavior</name>
        <t>
             The DHCPv4 server SHOULD <bcp14>SHOULD</bcp14> be able to configure all or individual pools to include the IPv6-only preferred IPv6-Only Preferred option in DHCPv4 responses if the client included the option code in the Parameter Request List option. List.
             The DHCPv4 server MAY <bcp14>MAY</bcp14> have a configuration option to specify the V6ONLY_WAIT timer for all or individual IPv6-mostly pools.
        </t>
        <t>
             The server MUST NOT <bcp14>MUST NOT</bcp14> include the IPv6-only IPv6-Only
	     Preferred  option in the DHCPOFFER or DHCPACK message if the YIADDR field in the message does not belong to a
	     selected pool is not configured as IPv6-mostly.
             The server MUST NOT <bcp14>MUST NOT</bcp14> include the IPv6-only 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.
        </t>
        <t>
             If the IPv6-only IPv6-Only Preferred option is present in the Parameter Request List received from the client and the corresponding DHCPv4 pool is explicitly configured as belonging to an IPv6-mostly network segment, the server MUST <bcp14>MUST</bcp14> include the IPv6-only IPv6-Only Preferred option when responding with the DHCPOFFER or DHCPACK message.
             If the server responds with the IPv6-only IPv6-Only Preferred option and the V6ONLY_WAIT timer is configured for the pool, the server MUST <bcp14>MUST</bcp14> copy the configured value to the IPv6-only IPv6-Only Preferred option value field.
		       Otherwise
             Otherwise, it MUST <bcp14>MUST</bcp14> set the field to zero.

             The server SHOULD NOT <bcp14>SHOULD NOT</bcp14> assign an address from the pool.
		       Instead
             Instead, it SHOULD <bcp14>SHOULD</bcp14> return 0.0.0.0 as the offered address.
             Alternatively, if offering 0.0.0.0 is not feasible, feasible -- for example
             example, due to some limitations of the server or the network infrastructure,
             infrastructure -- the server MAY <bcp14>MAY</bcp14> include in the
             DHCPOFFER an available IPv4 address from the pool into the DHCPOFFER pool, as per recommendations in <xref target="RFC2131"/>. target="RFC2131" format="default"/>.
             In this case, the offered address MUST <bcp14>MUST</bcp14> be a valid address that is not committed to any other client.
             Because the client is not expected ever expected to request this address, the server SHOULD NOT <bcp14>SHOULD NOT</bcp14> reserve the address and SHOULD NOT <bcp14>SHOULD NOT</bcp14> verify its uniqueness.
             If the client then issues a DHCPREQUEST for the address, the
             server MUST <bcp14>MUST</bcp14> process it per <xref target="RFC2131"/>, target="RFC2131"
             format="default"/>, including replying with a DHCPACK for the
             address if in the meantime it has not been committed to another client.
             client in the meantime.
        </t>
        <t>
             If a client includes both a Rapid-Commit Rapid Commit option <xref target="RFC4039"/>
             target="RFC4039" format="default"/> and an IPv6-Only Preferred
             option in the DHCPDISCOVER message message, the server SHOULD NOT <bcp14>SHOULD
             NOT</bcp14> honor the Rapid-Commit Rapid Commit option if the response to the client would contain the IPv6-only IPv6-Only Preferred option to the client. option.
             It SHOULD <bcp14>SHOULD</bcp14> instead respond with a DHCPOFFER as indicated above.
        </t>
        <t>
             If the server receives a DHCPREQUEST containing the IPv6-only IPv6-Only Preferred option for the address from a pool configured as IPv6-mostly, the server MUST <bcp14>MUST</bcp14> process it per <xref target="RFC2131"/>. target="RFC2131" format="default"/>.
        </t>
        <section anchor="autoconf" title="Interaction numbered="true" toc="default">
          <name>Interaction with RFC2563"> RFC 2563</name>
          <t>
<xref target="RFC2563"/> target="RFC2563" format="default"/> defines an Auto-Configure DHCPv4
option to disable IPv4 link-local address configuration for IPv4
clients. Clients can support both, neither or just one of both the IPv6-Only Preferred option and the
Auto-Configure options. option, just one of the options, or neither option.
If a client sends both the IPv6-Only Preferred option and the Auto-Configure options option, the network administrator can prevent the host from configuring an IPv4 link-local address on an IPv6-mostly network.
To achieve this this, the server needs to send a DHCPOFFER which that contains a 'yiaddr'
of 0x00000000, 0.0.0.0, and the Auto-Configure flag saying set to "DoNotAutoConfigure".
</t>
          <t>
However
However, special care should be taken in a situation when where a server supports
both options and receives just an IPv6-Only Preferred option from a client.
Section 2.3 of
<xref target="RFC2563"/> target="RFC2563" sectionFormat="of" section="2.3"/>
 states that if no address is chosen for the host (which would be the case for IPv6-only capable
 IPv6-only-capable clients on an IPv6-mostly network) then: network), then
"If the DHCPDISCOVER does not contain the Auto-Configure option, it is not answered."
Such behavior would be undesirable for clients supporting the IPv6-Only
Preferred option without supporting the Auto-Configure option option, as they would
not receive any response from the server and would keep asking, requesting a response instead of disabling DHCPv4 for V6ONLY_WAIT seconds.
Therefore
Therefore, the following update is made to Section 2.3 of <xref target="RFC2563"/>" target="RFC2563" sectionFormat="of" section="2.3"/>.
</t>
          <t>
OLD TEXT:
</t>
<t>
---
</t>
<t>
However,
     <blockquote><t>However, if no address is chosen for the host, a few additional steps MUST <bcp14>MUST</bcp14> be taken.
</t> taken.</t>
          <t>
If the DHCPDISCOVER does not contain the Auto-Configure option, it is not answered.
</t>
<t>
---
</t>
</blockquote>
          <t>
NEW TEXT:
</t>
<t>
---
</t>
<t>
However,

     <blockquote><t>However, if no address is chosen for the host, a few additional steps MUST <bcp14>MUST</bcp14> be taken.
</t>
          <t>
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.
</t>
<t>
---
</t>
</blockquote>
        </section>
      </section>
      <section anchor="vars" title="Constants numbered="true" toc="default">
        <name>Constants and Configuration Variables">
	       <texttable style="none">
                              <ttcol></ttcol>
			      <ttcol></ttcol>
			      <c>V6ONLY_WAIT</c> <c>The Variables</name>
        <dl newline="false" spacing="normal">
              <dt>V6ONLY_WAIT:</dt>
              <dd>The time for which the client SHOULD <bcp14>SHOULD</bcp14> stop the DHCPv4 configuration process. The value MUST NOT <bcp14>MUST NOT</bcp14> be less than MIN_V6ONLY_WAIT seconds. Default: 1800 seconds</c>
			      <c>MIN_V6ONLY_WAIT</c> <c>The seconds</dd>
              <dt>MIN_V6ONLY_WAIT:</dt>
              <dd>The lower boundary for V6ONLY_WAIT. Value: 300 seconds</c>
			      <c></c><c></c>
			       </texttable> seconds</dd>
        </dl>
      </section>
    </section>
    <section anchor="v6onlydef" title="IPv6-Only numbered="true" toc="default">
      <name>IPv6-Only Transition Technologies Considerations"> Technology Considerations</name>
      <t>
Until IPv6 adoption in the Internet reaches 100%, communication between an
IPv6-only host and an IPv4-only destination requires some form of a transition mechanism deployed in the network.
At the time of writing, the only such mechanism that is widely supported by end hosts is NAT64 <xref target="RFC6146"/> target="RFC6146" format="default"/> (either with or without 464XLAT).
Therefore
Therefore, the IPv6-only 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 DHCPv4 server to return the IPv6-only IPv6-Only Preferred option unless the network provides NAT64 service.
</t>
      <t>
Hypothetically, it is possible for multiple transition technologies to
coexist. In such scenario a 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
                However, it seems unlikely that any new transition technology would arise and be widely adopted in any the foreseeable future.
		Therefore
                Therefore, adding support for non-existing technologies seems
                to be suboptimal and the proposed mechanism suboptimal, so this document implies that NAT64 is used to facilitate
connectivity between IPv6 and IPv4.
                In the unlikely event that a new transition mechanism becomes widely deployed, the applicability of the IPv6-Only-Preferred IPv6-Only Preferred option to that mechanism will depend on the nature of the new mechanism.
                If the new mechanism is designed in such a way that it's fully transparent for hosts that support NAT64 and the IPv6-Only-Preferred IPv6-Only Preferred option, then the option can continue to be used with the new mechanism.
                If the new mechanism is not compatible with NAT64, NAT64 and implementation on the host side is required to support it, then a new DHCPv4 option needs to be defined.
</t>
      <t>
It should be also be noted that declaring a host (technically, a host interface) IPv6-only capable is a policy decision. For example,
 <list style="symbols">
<t>
      </t>
      <ul spacing="normal">
        <li>
An operating system OS vendor may make such a decision and configure their DHCPv4
clients to send the IPv6-Only Preferred option by default if the OS has
a 464XLAT CLAT <xref target="RFC6877"/> target="RFC6877" format="default"/> enabled.
</t>
<t>
</li>
        <li>
An enterprise network administrator may provision the corporate hosts as
IPv6-only capable if all applications that users are supposed to run have been
tested in an IPv6-only environment (or if a 464XLAT CLAT is enabled on the devices).
</t>
<t>
IoT
</li>
        <li>
Internet of Things (IoT) devices may be shipped in IPv6-only capable IPv6-only-capable mode if they are designed to
connect to an IPv6-enabled cloud destination only.
</t>
</list>

</t>
</li>
      </ul>
    </section>
    <section anchor="IANA" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>

      <t>The IANA is requested to assign has assigned a new DHCPv4 Option option code for the IPv6-Only Preferred option
      from the BOOTP "BOOTP Vendor Extensions and DHCPv4 Options 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.
      </t>

<texttable anchor="option_table">
    <ttcol align="left">Tag</ttcol>
    <ttcol align="left">Name</ttcol>
    <ttcol align="left">Data Length</ttcol>
    <ttcol align="left">Meaning</ttcol>
    <ttcol align="left">Reference</ttcol>
    <c>TBD (proposed value: 108)</c>
    <c>IPv6-only Preferred option</c>
    <c>4</c>
    <c>Number
      <eref target="https://www.iana.org/assignments/bootp-dhcp-parameters/" brackets="angle"/>.
      </t>

      <dl newline="false" spacing="compact">
        <dt>Tag:</dt>
            <dd>108</dd>
        <dt>Name:</dt>
            <dd>IPv6-Only Preferred</dd>
        <dt>Data Length:</dt>
            <dd>4</dd>
        <dt>Meaning:</dt>
            <dd>Number of seconds to disable that DHCPv4 for</c>
    <c>draft-ietf-dhc-v6only</c>
</texttable> should be disabled</dd>
        <dt>Reference:</dt>
            <dd>RFC 8925</dd>
      </dl>
    </section>
    <section anchor="Security" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>
          An attacker might send a spoofed DHCPOFFER containing IPv6-only an IPv6-Only Preferred option with the value field set to a large number, such as 0xffffffff, effectively disabling DHCPv4 on clients supporting the option.
          If the network is IPv4-only IPv4-only, such clients would lose connectivity, while on a dual-stack network without NAT64 service service, only connectivity to IPv4-only destinations would be affected.
			    The recovery
          Recovery from such an attack would require triggering a network attachment event.
			    However

However, it should be noted that if the network does not provide
protection from a rogue DHCPv4 server server, the similar attack vector can
be executed by offering an invalid address and setting the Lease Time
option <xref target="RFC2132"/> value field to 0xffffffff.
          The latter attack would affect all hosts, hosts -- not just hosts that support the IPv6-only IPv6-Only Preferred option.
			    Therefore
          Therefore, the security measures against rogue DHCPv4 servers would
          be sufficient to prevent the attacks specific to IPv6-only the IPv6-Only Preferred option.
			    Additionally
          Additionally, such attacks can only be executed if the victim
          prefers the rogue DHCPOFFER over the legitimate ones.
			    Therefore offers.
          Therefore, for the attack to be successful successful, the attacker needs to
          know the selection criteria used by the client and to be able to make
          its rogue offer more preferable. preferable to other offers.
      </t>
      <t>
          It should be noted that disabling IPv4 on a host upon receiving the IPv6-only IPv6-Only Preferred option from the DHCPv4 server protects the host from IPv4-related attacks and therefore could be considered a security feature feature, as it reduces the attack surface.
      </t>
    </section>
  </middle>
  <back>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2131.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2563.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3927.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4039.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>
      <references>
        <name>Informative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2132.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4957.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6146.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6147.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6877.xml"/>
      </references>
    </references>
    <section anchor="Acknowledgements" title="Acknowledgements"> numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>
      Thanks to the following people (in alphabetical order) for their review
      and feedback: Mohamed Boucadair, Martin Duke, Russ Housley, Sheng Jiang, Benjamin Kaduk, Murray Kucherawy, Ted Lemon, Roy Marples, Bjorn Mork, Alvaro Retana, Peng Shuping, Pascal Thubert, Bernie Volz, Eric Vyncke, Robert Wilton.
			    Authors <contact fullname="Mohamed Boucadair"/>, <contact
      fullname="Martin Duke"/>, <contact fullname="Russ Housley"/>, <contact
      fullname="Sheng Jiang"/>, <contact fullname="Benjamin Kaduk"/>, <contact
      fullname="Murray Kucherawy"/>, <contact fullname="Ted Lemon"/>, <contact
      fullname="Roy Marples"/>, <contact fullname="Bjorn Mork"/>, <contact
      fullname="Alvaro Retana"/>, <contact fullname="Peng Shuping"/>, <contact
      fullname="Pascal Thubert"/>, <contact fullname="Bernie Volz"/>, <contact
      fullname="Éric Vyncke"/>, and <contact fullname="Robert Wilton"/>.
      The authors would like to thank Bob Hinden and Brian Carpenter <contact fullname="Bob Hinden"/> and <contact fullname="Brian Carpenter"/> for the initial idea of signaling IPv6-only capability to hosts.
      Special thanks to Erik Kline, Mark Townsley and Maciej Zenczykowski <contact fullname="Erik Kline"/>, <contact fullname="Mark Townsley"/>, and <contact fullname="Maciej Zenczykowski"/> for the discussion which that led to the idea of signalling signaling IPv6-only capability over DHCPv4.
      </t>
    </section>
  </middle>

  <!--  *****BACK MATTER ***** -->

  <back>
    <references title="Normative References">
      &RFC2119;
      &RFC2131;
      &RFC2563;
      &RFC3927;
      &RFC4039;
      &RFC8174;
    </references>

    <references title="Informative References">
      <!-- &RFC6052; -->
      &RFC4861;
      &RFC4957;
      &RFC6146;
      &RFC6147;
      &RFC6877;
    </references>
  </back>
</rfc>