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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std"
     docName="draft-ietf-capport-rfc7710bis-10" number="8910"
     ipr="trust200902" consensus="true"
     obsoletes="7710" updates="3679">
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  <?rfc strict="yes"?>

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     tocInclude="true" symRefs="true" sortRefs="true" version="3">
  <front>
    <!--c WK: Set long title. -->

    <title abbrev="DHCP Captive-Portal">Captive-Portal Identification in DHCP
    / RA</title>
    and Router Advertisements (RAs)</title>
    <seriesInfo name="RFC" value="8910"/>
    <author fullname="Warren Kumari" initials="W." surname="Kumari">
      <organization>Google</organization>
      <address>
        <postal>
          <street>1600 Amphitheatre Parkway</street>
          <city>Mountain View, CA</city>
          <code>94043</code>

          <country>US</country>
          <country>United States of America</country>
        </postal>
        <email>warren@kumari.net</email>
      </address>
    </author>
    <author fullname="Erik Kline" initials="E." surname="Kline">
      <organization>Loon</organization>
      <address>
        <postal>
          <street>1600 Amphitheatre Parkway</street>
          <city>Mountain View, CA</city>
          <code>94043</code>

          <country>US</country>
          <country>United States of America</country>
        </postal>
        <phone/>
        <email>ek@loon.com</email>
      </address>
    </author>
    <date month="July" month="September" year="2020"/>

<keyword>Captive Portal</keyword>
<keyword>Walled Garden</keyword>
<keyword>Coffee-shop</keyword>
<keyword>Hotel</keyword>

    <abstract>
      <t>In many environments offering short-term or temporary Internet access
      (such as coffee shops), it is common to start new connections in a
      captive portal mode. This highly restricts what the user can do
      until the user has satisfied the captive portal conditions.</t>
      <t>This document describes a DHCPv4 and DHCPv6 option and a Router Advertisement
      (RA) option to inform clients that they are behind some sort of
      captive portal enforcement device, and that they will need to satify satisfy the
      Captive Portal conditions to get
      Internet access. It is not a full solution to address all of the issues
      that clients may have with captive portals; it is designed to be one
      component of a standardized approach for hosts to interact with such
      portals. While this document defines how the network operator may convey
      the captive portal API endpoint to hosts, the specific methods of
      satisfying and interacting with the captive portal are out of
      scope of this document.</t>
      <t>This document replaces <xref target="RFC7710"/>.  <xref target="RFC7710"/> RFC 7710, which used DHCP code point 160.
      Due to a conflict, this document specifies 114.  Consequently, this
      document also updates <xref target="RFC3679"/>.</t> RFC 3679.</t>
    </abstract>

  </front>
  <middle>
    <section title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>In many environments, users need to connect to a captive portal
      device and agree to an Acceptable Use Policy (AUP) and / or and/or provide
      billing information before they can access the Internet. Regardless of
      how that mechanism operates, this document provides functionality
      to allow the client to know when it is
      behind a captive portal and how to contact it.</t>
      <t>In order to present users with the payment or AUP pages, presently a captive
      portal enforcement device presently has to intercept the user's connections and
      redirect the user to a captive portal server, using methods that are
      very similar to man-in-the-middle (MITM) attacks. As increasing focus is
      placed on security, and end nodes adopt a more secure stance, these
      interception techniques will become less effective and/or more
      intrusive.</t>
      <t>This document describes a DHCPv4 <xref target="RFC2131"/> target="RFC2131" format="default"/> and DHCPv6
      <xref target="RFC8415"/> target="RFC8415" format="default"/> option (Captive-Portal) and an IPv6
      Router Advertisement (RA) <xref target="RFC4861"/> target="RFC4861" format="default"/> option that informs
      clients that they are behind a captive portal enforcement device and
      the API endpoint that the host can contact for more information.</t>
      <t>This document replaces RFC 7710 <xref target="RFC7710"/>.</t> target="RFC7710"
      format="default"/>, which used DHCP code point 160. Due to a conflict,
      this document specifies 114.  Consequently, this document also updates
      <xref target="RFC3679"/>.</t>
      <section title="Requirements Notation">
        <t>The numbered="true" toc="default">
        <name>Requirements Notation</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> here.
        </t>
      </section>
    </section>

    <section anchor="option" title="The numbered="true" toc="default">
      <name>The Captive-Portal Option"> Option</name>
      <t>The Captive-Portal DHCP / RA DHCP/RA Option informs the client that it may be
      behind a captive portal and provides the URI to access an API as defined
      by [draft-ietf-capport-api]. <xref target="RFC8908"/>. This is primarily intended to improve the
      user experience by showing the user the captive portal information faster and more
      reliably. Note that, for the foreseeable future, captive portals will
      still need to implement interception techniques to serve legacy
      clients, and clients will need to perform probing to detect captive
      portals";
      portals; nonetheless, the mechanism provided by this document provides
      a more reliable and performant way to do so, and is therefore the preferred
      mechanism for captive portal detection.</t>
      <t>Clients that support the Captive Portal DHCP option SHOULD <bcp14>SHOULD</bcp14> include the
      option in the Parameter Request List in DHCPREQUEST messages. DHCP servers
      MAY
      <bcp14>MAY</bcp14> send the Captive Portal option without any explicit request.</t>

      <t>In order to support multiple "classes" of clients (e.g. (e.g., IPv4 only,
      IPv6 only with DHCPv6 (<xref target="RFC8415"/>), target="RFC8415" format="default"/>), and
      IPv6 only with RA) RA), the captive network can provision the client with the
      URI via multiple methods (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The
      captive portal operator SHOULD <bcp14>SHOULD</bcp14> ensure that the URIs
      provisioned by each method are identical to reduce the chance of
      operational problems.  As the maximum length of the URI that can be
      carried in IPv4 DHCP is 255 bytes, URIs longer than this SHOULD NOT <bcp14>SHOULD
      NOT</bcp14> be provisioned by any of the IPv6 options described in this
      document.  In IPv6-only environments environments, this restriction can be
      relaxed.</t>
      <t>In all variants of this option, the URI MUST <bcp14>MUST</bcp14> be that of the captive
      portal API endpoint [draft-ietf-capport-api]. (<xref target="RFC8908"/>).
      </t>

      <t>A captive portal MAY <bcp14>MAY</bcp14> do content negotiation (<xref target="RFC7231"/>
      section 3.4)
      target="RFC7231" sectionFormat="of" section="3.4"/>) and attempt to
      redirect clients querying without an explicit indication of support for
      the captive portal API content type
      (i.e. (i.e., without
      application/capport+json listed explicitly anywhere within an Accept
      header vis. field as described in <xref target="RFC7231"/> section 5.3). target="RFC7231" sectionFormat="of"
      section="5.3"/>). In so doing, the captive portal SHOULD <bcp14>SHOULD</bcp14>
      redirect the client to the value associated with the "user-portal-url"
      API key. When performing such content negotiation (<xref target="RFC7231"/> Section 3.4),
      target="RFC7231" sectionFormat="of" section="3.4"/>), implementors of
      captive portals need to keep in mind that such responses might be
      cached, and therefore SHOULD <bcp14>SHOULD</bcp14> include an appropriate Vary
      header field (<xref target="RFC7231"/> Section 7.1.4) target="RFC7231" sectionFormat="of"
      section="7.1.4"/>) or set the Cache-Control header field in any
      responses to "private", "private" or a more restrictive value such as "no-store" <xref target="RFC7234"/> Section 5.2.2.3).
      (<xref target="RFC7234" sectionFormat="of" section="5.2.2.3"/>).
      </t>

      <t>The URI SHOULD NOT <bcp14>SHOULD NOT</bcp14> contain an IP address literal. Exceptions to this
      might include networks with only one operational IP address family where
      DNS is either not available or not fully functional until the captive
      portal has been satisfied. Use of iPAddress IP Address certificates (<xref target="RFC3779"/>) target="RFC3779" format="default"/>)
      adds considerations that are out of scope for this document.</t>
      <t>Networks with no captive portals may explicitly indicate this
      condition by using this option with the IANA-assigned URI for this
      purpose. Clients observing the URI value
      "urn:ietf:params:capport:unrestricted" may forego time-consuming forms of
      captive portal detection.</t>
      <section anchor="dhcpv4opt" title="IPv4 numbered="true" toc="default">
        <name>IPv4 DHCP Option"> Option</name>
        <t>The format of the IPv4 Captive-Portal DHCP option is shown
        below.<figure>
            <artwork><![CDATA[
        below.</t>

<figure title="Captive-Portal DHCPv4 Option Format">
        <artwork name="" type="" align="left" 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          | Len           | URI (variable length) ...     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                   ...URI continued...                         .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
          </figure></t>

        <t><list style="symbols">
            <t>Code: The
</figure>

<ul empty="true">

<li>
<dl>

<dt>Code:
</dt>
<dd>The Captive-Portal DHCPv4 Option (114) (one octet)</t>

            <t>Len: The octet).
</dd>

<dt>Len:
</dt>
<dd>The length (one octet), in octets, of the URI.</t>

            <t>URI: The URI.
</dd>

<dt>URI:
</dt>
<dd>The URI for the captive portal API endpoint to which the
            user should connect (encoded following the rules in <xref
            target="RFC3986"/>).</t>
          </list>See <xref target="RFC2132"/>, Section 2
	    target="RFC3986" format="default"/>).
</dd>

</dl>
</li>
</ul>
        <t>See <xref target="RFC2132" sectionFormat="of" section="2"/> for more on the format
          of IPv4 DHCP options.</t>
        <t>Note that the URI parameter is not null terminated.</t>
      </section>
      <section anchor="dhcpv6opt" title="IPv6 numbered="true" toc="default">
        <name>IPv6 DHCP Option"> Option</name>
        <t>The format of the IPv6 Captive-Portal DHCP option is shown below.
        <figure>
            <artwork><![CDATA[
        </t>
<figure title="Captive-Portal DHCPv6 Option Format">
        <artwork name="" type="" align="left" 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          option-code          |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                      URI (variable length)                    .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
          </figure></t>

        <t><list style="symbols">
            <t>option-code: The
</figure>

<ul empty="true">
<li>
<dl>

<dt>option-code:
</dt>
<dd>The Captive-Portal DHCPv6Option DHCPv6 Option (103) (two
            octets)</t>

            <t>option-len: The octets).
</dd>

<dt>option-len:
</dt>
<dd>The unsigned 16-bit length, in octets, of the URI.
            </t>

            <t>URI: The
</dd>

<dt>URI:
</dt>
<dd>The URI for the captive portal API endpoint to which the user should
connect (encoded following the rules in <xref
            target="RFC3986"/>).</t>
          </list>See <xref target="RFC7227"/>, Section 5.7 target="RFC3986" format="default"/>).
</dd>

</dl>

</li>
</ul>
        <t>See <xref target="RFC7227" sectionFormat="of" section="5.7"/> for
        more examples of DHCP Options with URIs. See <xref target="RFC8415"/>, Section 21.1 target="RFC8415"
        sectionFormat="of" section="21.1"/> for more on the format of IPv6
        DHCP options.</t>
        <t>Note that the URI parameter is not null terminated.</t>
        <t>As the maximum length of the URI that can be carried in IPv4 DHCP is
        255 bytes, URIs longer than this SHOULD NOT <bcp14>SHOULD NOT</bcp14> be provisioned via
        IPv6 DHCP options.</t>
      </section>
      <section anchor="v6ndopt" title="The numbered="true" toc="default">
        <name>The Captive-Portal IPv6 RA Option"> Option</name>
        <t>This section describes the Captive-Portal Router Advertisement
        option.</t>

        <t><figure>
            <artwork><![CDATA[
<figure title="Captive-Portal RA Option Format">
        <artwork name="" type="" align="left" 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |              URI              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 2: Captive-Portal RA Option Format]]></artwork>
          </figure></t>

        <t><list style="hanging">
            <t hangText="Type">37</t>

            <t hangText="Length">8-bit
]]></artwork>
</figure>

<ul empty="true">
<li>
<dl newline="false" spacing="normal">
          <dt>Type:</dt>
          <dd>37</dd>
          <dt>Length:</dt>
          <dd>8-bit unsigned integer. The length of the
            option (including the Type and Length fields) in units of 8
            bytes.</t>

            <t hangText="URI">The
            bytes.</dd>
          <dt>URI:</dt>
          <dd>The URI for the captive portal API endpoint to
            which the user should connect. This MUST <bcp14>MUST</bcp14> be padded with NUL
            (0x00) to make the total option length (including the Type and
            Length fields) a multiple of 8 bytes.</t>
          </list></t> bytes.</dd>
        </dl>
</li>
</ul>

        <t>Note that the URI parameter is not guaranteed to be null terminated.</t>
        <t>As the maximum length of the URI that can be carried in IPv4 DHCP is
        255 bytes, URIs longer than this SHOULD NOT <bcp14>SHOULD NOT</bcp14> be provisioned via
        IPv6 RA options.</t>
      </section>
    </section>
    <section anchor="precedence" title="Precedence numbered="true" toc="default">
      <name>Precedence of API URIs"> URIs</name>
      <t>A device may learn about Captive Portal API URIs through more than
      one of (or indeed all of) the above options. Implementations can select
      their own precedence order (e.g., prefer one of the IPv6 options before
      the DHCPv4 option, or vice versa, et cetera).</t>
      <t>If the URIs learned via more than one option described in <xref
      target="option"/> target="option" format="default"/> are not all identical, this condition should be logged
      for the device owner or administrator; it is a network configuration error
      if the learned URIs are not all identical.</t>
    </section>
    <section anchor="iana" title="IANA Considerations">
      <t>This document requests one numbered="true" toc="default">
      <name>IANA Considerations</name>

      <t>IANA has registered a new IETF URN protocol parameter (<xref
      target="RFC3553"/>) entry. This document
      target="RFC3553" format="default"/>). IANA has also requests a reallocation
      of reallocated two
      DHCPv4 option codes (see <xref target="exp106"/> target="exp106" format="default"/> for
      background) and updated the references for background).</t>

      <t>Thanks IANA!</t> previously registered DHCPv6
      and IPv6 ND options.</t>
      <section anchor="iana-urn"
               title="Captive numbered="true" toc="default">
        <name>Captive Portal Unrestricted Identifier">
       <t>This document registers Identifier</name>
        <t>IANA has registered a new entry under in the IETF "IETF URN Sub-namespace
	for Registered Protocol Parameter Identifiers Identifiers" registry defined in
        <xref target="RFC3553"/>:
       <list style="hanging">
         <t hangText="Registered target="RFC3553" format="default"/>:
        </t>
        <dl newline="false" spacing="compact">
          <dt>Registered Parameter Identifier:">capport:unrestricted</t>
         <t hangText="Reference:">RFC TBD (this document)</t>
         <t hangText="IANA Identifier:</dt>
          <dd>capport:unrestricted</dd>
          <dt>Reference:</dt>
          <dd>RFC 8910</dd>
          <dt>IANA Registry Reference:"><xref target="RFC3553"/></t>
      </list></t> Reference:</dt>
          <dd>RFC 8910</dd>
        </dl>
        <t>Only one value is defined (see URN above). No hierarchy is defined and
      therefore
        and, therefore, no sub-namespace registrations are possible.</t>
      </section>
      <section anchor="ietf_dhcpv4_option_code_change"
               title="BOOTP numbered="true" toc="default">
        <name>BOOTP Vendor Extensions and DHCP Options Code Change">
        <t>
      [ RFC Ed: Please remove before publication:
      RFC7710 uses DHCP Code 160 -- unfortunately, it was discovered that this option code is already widely used by Polycom (see appendix).
      Option 114 (URL) is currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter Siegmund, dieter@apple.com - Reason to recover: Never published in an RFC)
      Tommy Pauly (Apple) and Dieter Siegmund confirm that this codepoint hasn't been used, and Apple is willing to relinquish it for use in CAPPORT.
      Please see thread: https://mailarchive.ietf.org/arch/msg/captive-portals/TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ]
        </t>

        <t>The IANA is requested to update Change</name>

        <t>IANA has updated the "BOOTP Vendor Extensions and DHCP
        Options" registry
          (https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml) (&lt;<eref
        target="https://www.iana.org/assignments/bootp-dhcp-parameters"/>&gt;)
	as follows.</t>

        <t><figure><artwork><![CDATA[   Tag: 114
   Name: DHCP Captive-Portal
   Data Length: N
   Meaning: DHCP Captive-Portal
   Reference: [THIS-RFC]]]></artwork></figure></t>

        <t><figure><artwork><![CDATA[   Tag: 160
   Name: Unassigned
   Data Length:
   Meaning: Previously

<dl spacing="compact">
<dt>Tag:</dt>
<dd>114</dd>

<dt>Name:</dt>
<dd>DHCP Captive-Portal</dd>

<dt>Data Length:</dt>
<dd>N</dd>

<dt>Meaning:</dt>
<dd>DHCP Captive-Portal</dd>

<dt>Reference:</dt>
<dd>RFC 8910</dd>
</dl>

<dl spacing="compact">
<dt>Tag:</dt>
<dd>160</dd>

<dt>Name:</dt>
<dd>Unassigned</dd>

<dt>Data Length:</dt>
<dd></dd>

<dt>Meaning:</dt>
<dd>Previously assigned by RFC7710; <xref target="RFC7710"/>; known to also be used by Polycom.
   Reference: [THIS-RFC][RFC7710]]]></artwork></figure></t> Polycom.</dd>

<dt>Reference:</dt>
<dd><xref target="RFC7710"/> RFC 8910</dd>
</dl>

      </section>

      <section anchor="iana_update_dhcv6_and_icmpv6"
               title="Update numbered="true" toc="default">
        <name>Update DHCPv6 and IPv6 ND Options Registries">
        <t>This document requests that Registries</name>

        <t>IANA has updated the DHCPv6 (103 - DHCP Captive-Portal) and IPv6 ND
	(37 - DHCP Captive-Portal) options previously
        registered in <xref target="RFC7710"/> be updated target="RFC7710" format="default"/> to reference this
        document.</t>
      </section>
    </section>
    <section anchor="security" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>By removing or reducing the need for captive portals to perform
      MITM hijacking, this mechanism improves security by
      making the portal and its actions visible, rather than hidden, and
      reduces the likelihood that users will disable useful security
      safeguards like DNSSEC validation, VPNs, etc etc. in order to interact with
      the captive portal.  In addition, because the system knows that it is
      behind a captive portal, it can know not to send cookies, credentials,
      etc.  By handing out a URI
      which that is protected with TLS, the captive
      portal operator can attempt to reassure the user that the captive portal
      is not malicious.</t>
      <t>Clients processing these options SHOULD <bcp14>SHOULD</bcp14> validate that the option's
      contents conform to the validation requirements for URIs, including
      those described in
      <xref target="RFC3986"/>.</t> target="RFC3986" format="default"/>.</t>
      <t>Each of the options described in this document is presented to a
      node using the same protocols used to provision other information
      critical to the node's successful configuration on a network. The
      security considerations applicable to each of these provisioning
      mechanisms also apply when the node is attempting to learn the
      information conveyed in these options. In the absence of security
      measures like RA Guard RA-Guard (<xref target="RFC6105"/>, target="RFC6105" format="default"/>, <xref target="RFC7113"/>)
      target="RFC7113" format="default"/>) or DHCP Shield DHCPv6-Shield <xref target="RFC7610"/>,
      target="RFC7610" format="default"/>, an attacker could inject, modify,
      or block DHCP messages or RAs.</t>

      <t>An attacker with the ability to inject DHCP messages or RAs
      could include an option from this document to force users to contact
      an address of his the attacker's choosing. As an An attacker with this capability could
      simply list themselves as the default gateway (and so intercept all the
      victim's traffic); this does not provide them with significantly more
      capabilities, but because this document removes the need for
      interception, the attacker may have an easier time performing the
      attack.</t>
      <t>However, as the operating systems and application(s) that make use of
      this information know that they are connecting to a captive portal
      device (as opposed to intercepted connections where the OS/application
      may not know that they are connecting to a captive portal or hostile device)
      device), they can render the page in a sandboxed environment and take
      other precautions, precautions such as clearly labeling the page as untrusted. The
      means of sandboxing and a user interface presenting this information is
      not covered in this document - document; by its nature nature, it is implementation
      specific and best left to the application and user interface
      designers.</t>
      <t>Devices and systems that automatically connect to an open network
      could potentially be tracked using the techniques described in this
      document (forcing the user to continually re-satisfy resatisfy the Captive Portal
      conditions,
      conditions or exposing their browser fingerprint). However,
      similar tracking can already be performed with the presently common
      captive portal mechanisms, so this technique does not give the attackers
      more capabilities.</t>
      <t>Captive portals are increasingly hijacking TLS connections to force
      browsers to talk to the portal. Providing the portal's URI via a DHCP or
      RA option is a cleaner technique, and reduces user expectations of being
      hijacked -
      hijacked; this may improve security by making users more reluctant to
      accept TLS hijacking, which can be performed from beyond the network
      associated with the captive portal.</t>
    </section>

    <section title="Acknowledgements">
      <t>This document is a -bis of RFC7710. Thanks to all of the original
      authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve Sheng),
      and original contributors.</t>

      <t>Also thanks to the CAPPORT WG for all of the discussion and
      improvements including contributions and review from Joe Clarke,
      Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens Schimpe,
      Martin Thomson, Michael Richardson,
      Remi Nguyen Van, Subash Tirupachur Comerica, Bernie Volz,
      and Tommy Pauly.</t>
    </section>

  </middle>
  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.2131'?>

      <?rfc include='reference.RFC.2132'?>

      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.3553'?>

      <?rfc include='reference.RFC.3986'?>

      <?rfc include='reference.RFC.4861'?>

      <?rfc include='reference.RFC.7227'?>

      <?rfc include='reference.RFC.7231'?>

      <?rfc include='reference.RFC.7234'?>

      <?rfc include='reference.RFC.8174'?>

      <?rfc include='reference.RFC.8415'?>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <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.2132.xml"/>
        <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.3553.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3986.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.7227.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7231.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7234.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8415.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3679.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3779.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6105.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7113.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7610.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7710.xml"/>

   <reference anchor="RFC8908" target="https://www.rfc-editor.org/info/rfc8908">
     <front>
       <title>Captive Portal API</title>
       <author initials="T" surname="Pauly" fullname="Tommy Pauly" role="editor">
         <organization />
       </author>
       <author initials="D" surname="Thakore" fullname="Darshak Thakore" role="editor">
         <organization />
       </author>
       <date month="September" year="2020" />
     </front>
     <seriesInfo name="RFC" value="8908" />
     <seriesInfo name="DOI" value="10.17487/RFC8908"/>
   </reference>

      </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.3679'?>
      <?rfc include='reference.RFC.3779'?>
      <?rfc include='reference.RFC.6105'?>
      <?rfc include='reference.RFC.7113'?>
      <?rfc include='reference.RFC.7610'?>
      <?rfc include='reference.RFC.7710'?>
    </references>

    <section title="Changes / Author Notes.">
      <t>[RFC Editor: Please remove this section before publication ]</t>

      <t>From initial to -00.</t>

      <t><list style="symbols">
          <t>Import of RFC7710.</t>
        </list></t>

      <t>From -00 to -01.</t>

      <t><list style="symbols">
          <t>Remove link-relation text.</t>
          <t>Clarify option should be in DHCPREQUEST parameter list.</t>
          <t>Uppercase some SHOULDs.</t>
        </list></t>
    </section>

    <section anchor="diff7710" title="Changes numbered="true" toc="default">
      <name>Changes from RFC 7710"> 7710</name>
      <t>This document incorporates the following changes from <xref
      target="RFC7710"/>.</t>

      <t><list style="numbers">
          <t>Clarify target="RFC7710" format="default"/>.</t>
      <ol spacing="normal" type="1">
        <li>Clarified that IP string literals are NOT RECOMMENDED.</t>

          <t>Clarify <bcp14>NOT RECOMMENDED</bcp14>.</li>
        <li>Clarified that the option URI MUST <bcp14>MUST</bcp14> be that of the captive portal
          API endpoint.</t>

          <t>Clarify endpoint.</li>
        <li>Clarified that captive portals MAY <bcp14>MAY</bcp14> do content negotiation.</t>

          <t>Added negotiation.</li>
        <li>Added text about Captive Portal API URI precedence in the event
          of a network configuration error.</t>

          <t>Added error.</li>
        <li>Added urn:ietf:params:capport:unrestricted URN.</t>

          <t>Notes URN.</li>
        <li>Noted that the DHCPv4 Option Code changed from 160 to 114.</t>
        </list></t> 114.</li>
      </ol>
    </section>
    <section anchor="exp106"
             title="Observations From numbered="true" toc="default">
      <name>Observations from IETF 106 Network Experiment"> Experiment</name>

      <t>During IETF 106 in Singapore Singapore, an <eref
        target="https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments"
          >experiment</eref>
      target="https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments">experiment</eref>
      enabling clients compatible with the Captive Portal API compatible clients to discover a
      venue-info-url (see <eref
      target="https://tickets.meeting.ietf.org/wiki/CAPPORT"> experiment
      description</eref> for more detail) revealed that some Polycom devices
      on the same network made use of DHCPv4 option code 160 for <eref
        target="https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-Standardization-160-vs-66/td-p/72577"
        >other
      target="https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-Standardization-160-vs-66/td-p/72577">other
      purposes</eref>.</t>
      <t>The presence of DHCPv4 Option code 160 holding a value indicating the
      Captive Portal API URL caused these devices to not function as desired.
      For this reason, this document requests IANA deprecate has deprecated option code 160 and
      reallocate
      allocated a different value to be used for the Captive Portal API URL.</t>
    </section>

    <section numbered="false" toc="default">
      <name>Acknowledgements</name>

      <t>This document is a -bis of RFC 7710. Thanks to all of the original
      authors (<contact fullname="Warren Kumari"/>, <contact fullname="Olafur
      Gudmundsson"/>, <contact fullname="Paul Ebersman"/>, and <contact fullname="Steve Sheng"/>)
      and original contributors.</t>
      <t>Also thanks to the CAPPORT WG for all of the discussion and
      improvements, including contributions and review from <contact fullname="Joe Clarke"/>,
      <contact fullname="Lorenzo Colitti"/>, <contact fullname="Dave
      Dolson"/>, <contact fullname="Hans Kuhn"/>, <contact fullname="Kyle
      Larose"/>, <contact fullname="Clemens Schimpe"/>,
      <contact fullname="Martin Thomson"/>, <contact fullname="Michael Richardson"/>,
      <contact fullname="Remi Nguyen Van"/>, <contact fullname="Subash
      Tirupachur Comerica"/>, <contact fullname="Bernie Volz"/>,
      and <contact fullname="Tommy Pauly"/>.</t>
    </section>
  </back>
</rfc>