draft-ietf-capport-architecture.xml

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 <!-- ***** FRONT MATTER ***** -->

 <front>
   <!-- The abbreviated title is used in the page header - it is only necessary if the 
        full title is longer than 39 characters -->

   <title abbrev="CAPPORT Architecture">CAPPORT Architecture</title>

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    <author fullname="Kyle Larose" initials="K." surname="Larose">
      <organization>Sandvine</organization>
      <address>
        <postal>
          <street>408 Albert Street</street>
          <!-- Reorder these if your country does things differently -->
          <city>Waterloo</city>
          <region>ON</region>
          <code>N2L 3V3</code>
          <country>Canada</country>
        </postal>
        <phone>+1 519 880 2400</phone>
        <email>klarose@sandvine.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>

    <author fullname="David Dolson" initials="D." surname="Dolson">
      <organization>Sandvine</organization>
      <address>
        <postal>
          <street>408 Albert Street</street>
          <!-- Reorder these if your country does things differently -->
          <city>Waterloo</city>
          <region>ON</region>
          <code>N2L 3V3</code>
          <country>Canada</country>
        </postal>
        <phone>+1 519 880 2400</phone>
        <email>ddolson@sandvine.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>


    <date year="2017" />

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   <area>art</area>

   <workgroup>Internet Engineering Task Force</workgroup>

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   <abstract>
     <t>This document aims to document consensus on the CAPPORT architecture.
        DHCP or Router Advertisements, ICMP, and an HTTP API are used to
        provide the solution. The role of Provisioning Domains (PvDs) is
        described.
     </t>
   </abstract>
 </front>

 <middle>
   <section title="Introduction">
     <t>
       In this document, "Captive Portal" is used to describe a network to
       which a device may be voluntarily attached, such that network access is
       limited until some requirements have been fulfilled.  Typically a user
       is required to use a web browser to fulfil requirements imposed by the
       network operator, such as reading advertisements, accepting an
       acceptable-use policy, or providing some form of credentials.
     </t>
     <t>
       Implementations generally require a web server, some method to
       allow/block traffic, and some method to alert the user.  Common methods
       of alerting the user involve modifying HTTP or DNS traffic.
     </t>
     <t>
       Problems with captive portal implementations have been described in
       <xref target="I-D.nottingham-capport-problem"/>. [If that document
       cannot be published, consider putting its best parts into an appendix of
       this document.]
     </t>
     <t>
       This document standardizes an architecture for implementing captive portals
       that provides tools for addressing most of those problems. We are guided
       by these principles:
       <list style="symbols">
         <t>Solutions SHOULD NOT require the forging of responses from DNS or
            HTTP servers, or any other protocol.  In particular, solutions
            SHOULD NOT require man-in-the-middle proxy of TLS traffic.</t>
         <t>Solutions MUST operate at the layer of Internet Protocol (IP) or
            above, not being specific to any particular access technology such
            as Cable, WiFi or 3GPP.</t>
         <t>Solutions SHOULD allow a device to be alerted that it is in a
            captive network when attempting to use any application on the
            network.  (Versus requiring a user to visit a clear-text HTTP
            site in order to receive a notification.)</t>
         <t>The state of captivity SHOULD be explicitly available to devices
            (in contrast to modification of DNS or HTTP, which is only
            indirectly machine-detectable by the client--by comparing
            responses to well-known queries with expected responses).</t>
         <t>The architecture MUST provide a path of incremental migration,
            acknowledging a huge variety of portals and end-user device
            implementations and software versions.</t>
         <t>The architecture SHOULD improve security by providing mechanisms
            for trust, allowing alerts from trusted network operators to be
            distinguished from attacks from untrusted agents.</t>
       </list>
     </t>
     <t>
       A side-benefit of the architecture described in this document is that
       devices without user interfaces are able to identify parameters of
       captivity.  (However, this document does not yet describe a mechanism
       for such devices to escape captivity.)
     </t>
     <t>
       The architecture uses the following mechanisms:
       <list style="symbols">
         <t>Network provisioning protocols provide end-user devices with a
            URI for the API that end-user devices query for information about
            what is required to escape captivity. DHCP, DHCPv6, and
            Router-Advertisement options for this purpose are available in
            <xref target="RFC7710"/>. Other protocols (such as RADIUS),
            Provisioning Domains
            <xref target="I-D.bruneau-intarea-provisioning-domains"/>, or
            static configuration may also be used.
            A device MAY query this API at any time to determine whether the
            network is holding the device in a captive state.
         </t>
         <t>End-user devices are notified of captivity with ICMP/ICMP6 messages
            in response to traffic.
            This notification can work with any Internet protocol, not just
            clear-text HTTP. This notification does not carry the portal URI;
            rather it provides a notification to the User Equipment that it
            is in a captive state.
         </t>
         <t>
            Receipt of the ICMP/ICMP6 messages informs an end-user device that
            it is captive. 
            In response, the device SHOULD query the provisioned API to obtain
            information about the network state.
            The device MAY take immediate action to satisfy the portal
            (according to its configuration/policy).
         </t>
       </list>
     </t>
     <t>
       The architecture attempts to provide privacy, authentication, and safety mechanisms
       to the extent possible.
     </t>
     <section title="Requirements Language">
       <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in <xref
       target="RFC2119">RFC 2119</xref>.</t>
     </section>
     <section title="Terminology">
       <t>Captive Network: A network which limits communication of attached
          devices to restricted hosts until the user has satisfied
          Captive Portal Conditions, after which access is permitted to a wider
          set of hosts (typically the internet).</t>
       <t>Captive Portal Conditions: site-specific requirements that a user
          or device must satisfy in order to gain access to the wider network.</t>
       <t>Captive Portal Enforcement: The network equipment which enforces the
          traffic restriction and notifies the User Equipment it is in a captive
          state.</t>
       <t>Captive Portal User Equipment: Also known as User Equipment. A device
          which has voluntarily joined a network for purposes of communicating
          beyond the constraints of the captive network.</t>
       <t>Captive Portal Server: The web server providing a user interface for
          assisting the user in satisfying the conditions to escape
          captivity.</t>
     </section>

   </section>

   <section title="Components">

     <section anchor="section_client" title="User Equipment">
      <t>
       The User Equipment is the device that a user desires to be attached to
       a network with full access to all hosts on the network (e.g., to have
       Internet access). The User Equipment communication is typically
       restricted by the Captive Portal Enforcement, described in <xref target="section_capport_enforcement"/>,
       until site-specific requirements have been met.
      </t>
      <t>
        At this time we consider only devices with web browsers, with web
        applications being the means of satisfying Captive Portal Conditions.
      </t>
      <t>
      <list style="symbols">
        <t>An example interactive User Equipment is a smart phone.</t>
        <t>SHOULD support provisioning of the URI for the Captive Portal API (e.g., by DHCP)</t>
        <t>MAY distinguish Captive Portal API access per network interface, in the manner
           of Provisioning Domain Architecture <xref target="RFC7556"/>.</t>
        <t>SHOULD have a mechanism for notifying the user of the Captive Portal</t>
        <t>SHOULD have a web browser so that the user may navigate the Captive Portal
           user interface.</t>
        <t>SHOULD be able to receive and validate the Captive Portal ICMP message
           types, and to access the Captive Portal API in response.</t>
        <t>MAY restrict application access to networks not granting full
           network access. E.g., a device connected to a mobile network may
           be connecting to a WiFi network; the operating system MAY
           avoid updating the default route until network access restrictions
           have been lifted (excepting access to the Captive Portal server).
           This has been termed "make before break".</t>
      </list>
      </t>
      <t>
        None of the above requirements are mandatory because (a) we do not wish
        to say users or devices must seek access beyond the captive network,
        (b) the requirements may be fulfilled by manually visiting the captive
        portal web application, and (c) legacy devices must continue to be
        supported.
      </t>
     </section>

     <section anchor="section_provisioning" title="Provisioning Service">
       <t>
         Here we discuss candidate mechanisms for provisioning the User
         Equipment with the URI of the API to query captive portal state and
         navigate the portal.
       </t>
       <section anchor="section_dhcp" title="DHCP or Router Advertisements">
         <t>
           A standard for providing a portal URI using DHCP or Router
           Advertisements is described in <xref target="RFC7710"/>.  The
           CAPPORT architecture expects this URI to indicate the API described
           in <xref target="section_api"/>.
         </t>
         <t>
           Although it is not clear from RFC7710  what protocol should be
           executed at the specified URI, some readers might have assumed it to
           be an HTML page, and hence there might be User Equipment assuming a
           browser should open this URI.  For backwards compatibility, it is
           RECOMMENDED that the server check the "Accept" field when serving
           the URI, and serve HTML pages for "text/html" and serve the API for
           "application/json". [REVISIT: are these details appropriate?]
         </t>
       </section>
       <section anchor="section_pvd" title="Provisioning Domains">
         <t>
           Although still a work in progress,
           <xref target="I-D.bruneau-intarea-provisioning-domains"/>
           proposes a mechanism for User Equipment to be provided with PvD
           Bootstrap Information containing the URI for a JSON file containing
           key-value pairs to be downloaded over HTTPS.
           This JSON file would fill the role of the Captive Portal API
           described in <xref target="section_api"/>.
         </t>
         <t>
           One key-value pair can be used to indicate the network
           has restricted access, requiring captive portal navigation by a user.
           E.g., key="captivePortal" and value=&lt;URI of portal&gt;.
           The key-value pair should provide a different result when
           access is not restricted. E.g., key="captivePortal" and value="".
         </t>
         <t>
           This JSON file is extensible, allowing new key-value pairs
           to indicate such things as network access expiry time,
           URI for API access by IOT devices, etc.
         </t>
         <t>
           The PvD server MUST support multiple (repeated) queries from each
           User Equipment, always returning the current captive portal
           information.  The User Equipment is expected to make this query upon
           receiving (and validating) an ICMP Captive Portal message
           (see <xref target="section_icmp"/>).
         </t>
       </section>
     </section>

     <section anchor="section_api" title="Captive Portal API Server">
       <t>
         The purpose of a Captive Portal API is to permit a query of Captive
         Portal state without interrupting the user.  This API thereby removes
         the need for a device to perform clear-text "canary" HTTP queries to
         check for response tampering.
       </t>
       <t>
         The URI of this API will have been provisioned to the User Equipment.
         (Refer to <xref target="section_provisioning"/>).
       </t>
       <t>
         This architecture expects the User Equipment to query the API when the
         User Equipment attaches to the network and multiple times thereafter.
         Therefore the API MUST support multiple repeated queries from the same
         User Equipment, returning current state of captivity for the
         equipment.
       </t>
       <t>
         At minimum the API MUST provide: (1) the state of captivity and (2) a URI
         for a browser to present the portal application to the user.
         The API SHOULD provide evidence to the caller that it supports
         the present architecture.
       </t>
       <t>
         When user equipment receives (and validates) ICMP Captive Portal
         alerts, the user equipment SHOULD query the API to check the state.
         The User Equipment SHOULD rate-limit these API queries in the event of
         ICMP flooding by an attacker. (See <xref target="Security"/>.)
       </t>
       <t>
         The API MUST be extensible to support future use-cases by allowing
         extensible information elements. Suggestions include quota information,
         expiry time, method of providing credentials, security token for
         validating ICMP messages.
       </t>
       <t>
         This document does not specify the details of the API.
       </t>
        <t>
          The CAPPORT API SHOULD support TLS for privacy and server authentication.
        </t>
     </section>

     <section anchor="section_capport_enforcement" title="Captive Portal Enforcement">
      <t>
        The Captive Portal Enforcement component restricts network access to 
        User Equipment according to site-specific policy. Typically User Equipment
        is permitted access to a small number of services and
        is denied general network access until it has performed some action.
      </t>
      <t>
       The Captive Portal Enforcement component:
       <list style="symbols">
        <t>Allows traffic through for allowed User Equipment.</t>
        <t>Blocks (discards) traffic and sends ICMP notifications for disallowed User Equipment.</t>
        <t>Permits disallowed User Equipment to access necessary APIs and web pages to
           fulfill requirements of exiting captivity.</t>
        <t>Updates allow/block rules per User Equipment in response to operations
           from the Captive Portal back-end.</t>
       </list>
      </t>
      <t>
        As an upgrade path, captive portals MAY continue to support methods
        that work today, such as modification of port-80 HTTP responses to
        redirect users to the portal.  Various user-equipment vendors probe
        canary URLs to identify the state captivity [reference Mariko
        Kobayashi's survey]. While doing so, ICMP messages SHOULD also be sent,
        to activate work-flows in supporting devices.
        [TODO: give some thought to precise recommendations for backwards compatibility.]
      </t>
     </section>
     <section anchor="section_icmp" title="ICMP/ICMP6">
       <t>
         ICMP messages have been selected for indicating to a sender that
         packets could not be delivered for reason of a network policy (in
         particular, captive portal). ICMP is already used to indicate reasons
         that packets could not be delivered (network unreachable, port
         unreachable, packet too large, etc.).
       </t>
       <t>
         A mechanism to trigger captive portal work-flows in the User Equipment
         is proposed in <xref target="I-D.wkumari-capport-icmp-unreach"/>.
       </t>
       <t>
         The Captive Portal Enforcement function is REQUIRED to send such
         ICMP messages when disallowed User Equipment attempts to send
         to the network.
         These ICMP messages MUST be rate-limited to a configurable rate.
       </t>
       <t>
         The ICMP messages MUST NOT be sent to the Internet devices. The indications
         are only sent to the User Equipment.
       </t>
       <t>
         The User Equipment operating system is NOT REQUIRED to deliver the
         impact of the ICMP message to the application that triggered it.
         The User Equipment might be able to satisfy the Captive Portal
         requirements quickly enough that existing transport connections are
         not impacted.
       </t>
     </section>
     <section title="Component Diagram">
     <t>
       <figure>
           <preamble>
            The following diagram shows the communication between each component.
           </preamble>
           <artwork><![CDATA[
o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
. CAPTIVE NETWORK                                               .
.                                            +--------------+   .
. +------------+   Provision API URI         | Provisioning |   .
. |            |<---------------------------+|  Service     |   .
. |   User     |                             +--------------+   .
. | Equipment  |   Query CAPPORT status;     +-------------+    .
. |            |+--------------------------->| CAPPORT API |    .
. |            |                             |  Server     |    .
. |            |                             +------+------+    .
. |            |                                    |Status     .
. |            |   Portal user interface     +------+------+    .
. |            |+--------------------------> | CAPPORT     |    .
. +------------+                             | web portal  |    .
.       ^  |  Connection Attempt             +-------------+    .
.       |  |  to prohibited service.                   |        .
.       |  +------------------> +---------------+  Allow/Deny   . 
.       |                       |               |    Rules      . 
.       |   ICMP Unreachable    | Captive Portal|     |         .
.       +---------------------+ | Enforcement   | <---+         .
.                               +---------------+               .
.                                       |                       .
.                            To/from external network           .
.                                       |                       .
.                                       |                       .
o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
                                  EXTERNAL NETWORK

        Figure 1: Captive Portal Architecture Component Diagram
           ]]></artwork>
       </figure>
        In the diagram:
             <list style="symbols">
              <t>During provisioning (e.g., DHCP), the User Equipment acquires
                 the URI for the CAPPORT API.</t>
              <t>The User Equipment queries the API to learn of its state of
                 captivity. If captive, the User Equipment presents the portal
                 user interface to the user.</t>
              <t>The User Equipment attempts to communicate to the external
                 network through the Captive Portal enforcement device.</t>
              <t>The Captive Portal Enforcement device either allows the User
                 Equipment's packets to the external network, or responds with
                 an ICMP message.</t>
              <t>The CAPPORT web portal server directs the Captive Portal
                 Enforcement device to either allow or deny external network
                 access for the User Equipment.</t>
             </list>
    </t>
    <t>
      Although the provisioning, API, and web portal functions are shown as
      discrete blocks, they could of course be combined into a single element.
    </t>
    </section>
   </section>

   <section anchor="section_workflow" title="Solution Workflow">
    <t>
      This section aims to improve understanding by describing a possible
      workflow of solutions adhering to the architecture.
    </t>
    <section title="Initial Connection">
    <t>
      This section describes a possible work-flow when User Equipment initially
      joins a Captive Network.
    </t>
    <t> 
    <list style="numbers">
      <t>The User Equipment joins the Captive Network by acquiring a DHCP
         lease, RA, or similar, acquiring provisioning information.</t>
      <t>The User Equipment learns the URI for the Captive Portal API from the
         provisioning information (e.g., <xref target="RFC7710"/>).</t>
      <t>The User Equipment accesses the CAPPORT API to receive parameters
         of the Captive Network, including web-portal URI. (This step replaces
         the clear-text query to a canary URL.)</t>
      <t>If necessary, the User navigates the web portal to gain access to the
         external network.</t>
      <t>The Captive Portal API server indicates to the Captive Portal Enforcement
         device that the User Equipment is allowed to access the external network.</t>
      <t>The User Equipment attempts a connection outside the captive network</t>
      <t>If the requirements have been satisfied, the access is permitted;
         otherwise the "Expired" behavior occurs.</t>
      <t>The User Equipment accesses the network until conditions Expire.</t>
    </list>
    </t>
    </section>
    <section title="Conditions Expire">
    <t>
      This section describes a possible work-flow when conditions expire and
      the user visits the portal again (e.g., low quota, or time expiry).
     </t>
    <t>
     <list style="numbers">
      <t>Pre-condition: the Captive Portal Enforcement has been configured to
         detect an expiry condition, which has now occurred.</t>
      <t>The User Equipment sends a packet to the outside network.</t>
      <t>The Captive Portal Enforcement detects that the packet is from an
         expired User Equipment.</t>
      <t>The Captive Portal Enforcement sends an ICMP message to
         the User Equipment indicating that it needs to refresh its access.
         <xref target="I-D.wkumari-capport-icmp-unreach"/>.</t>
      <t>The User Equipment verifies the ICMP message is plausible.</t>
      <t>The User Equipment queries the Captive Portal API to refresh
         parameters and status of the Captive Network.</t>
      <t>If necessary, the User once again navigates the web portal to gain
         access to the external network.</t>
      <t>The Captive Portal API Server gives more quota (time, bytes, etc.) to
         the User Equipment by indicating to the Captive Portal Enforcement the new, extended quota.</t>
      <t>The User Equipment accesses the external network.</t>
     </list>
    </t>
    </section>
   </section>

   <section anchor="Acknowledgements" title="Acknowledgements">
     <t>The authors thank various individuals for their feedback on
        the mailing list and during the IETF98 hackathon: 
        David Bird,
        Eric Kline,
        Alexis La Goulette,
        Alex Roscoe,
        Darshak Thakore,
        and Vincent van Dam.
     </t>
   </section>


   <!-- Possibly a 'Contributors' section ... -->

   <section anchor="IANA" title="IANA Considerations">
     <t>This memo includes no request to IANA.</t>

   </section>

   <section anchor="Security" title="Security Considerations">
     <!--All drafts are required to have a security considerations section.  See RFC3552 -->
     <section title="Trusting the Network">
       <t>
         When joining a network, some trust is placed in the network operator.
         This is usually considered to be a decision by a user on the basis of
         the reputation of an organization. However, once a user makes such a
         decision, protocols can support authenticating a network is operated
         by who claims to be operating it.  The Provisioning Domain
         Architecture <xref target="RFC7556"/> provides some discussion on
         authenticating an operator.
       </t>
       <t>
         Given that a user chooses to visit a Captive Portal URI, the URI location
         SHOULD be securely provided to the user's device. E.g., the DHCPv6 AUTH
         option can sign this information.
       </t>
       <t>
         If a user decides to incorrectly trust an attacking network, they might
         be convinced to visit an attacking web page and unwittingly provide
         credentials to an attacker. Browsers can authenticate servers but
         cannot detect cleverly misspelled domains, for example.
       </t>
     </section>
     <section title="Authenticated APIs">
       <t>
         The solution described here assumes that when the User Equipment needs to
         trust the API server, server authentication will be utilized using TLS
         mechanisms.
       </t>
     </section>
     <section title="Risk of Nuisance Captive Portal">
       <t>
         It is possible for any user on the Internet to send ICMP packets in an attempt
         to cause the receiving equipment to go to the captive portal. This has been
         considered and addressed in the following ways:
         <list>
           <t>The ICMP packet does not carry the URL, making this method safer
              than HTTP 3xx-redirect methods currently in use.
              The User Equipment does not have to use clear-text HTTP to solicit
              the URL of the portal.
           </t>
           <t>Because the ICMP messages will carry embedded packets sent by the
              sender, the receiver of the ICMP message can validate that the
              transport header is plausibly one it sent (i.e., the
              transport-layer 5-tuple matches an open connection; there is no
              need to save every packet it sent).
              This validation requires an off-path attacker to guess the
              5-tuple in order to affect a flow.
              It is trivial for an on-path attacker to send a plausible ICMP
              packet.  (This is not a new ICMP attack.)
              The impact of getting a valid ICMP packet to the User Equipment
              is that it will visit the CAPPORT API to check the status.
              For this reason we recommend the User Equipment rate-limit these
              requests to the API.
           </t>
           <t>We considered that the ICMP packet could carry a short secret
              token that would be known to the User Equipment and Captive
              Portal Enforcement device but would not be available to an
              attacker, even to an on-path attacker.
              Although possible to guess by brute force, the impact would be at
              worst a nuisance visit to the API.  We suggest that a 32-bit
              token would be sufficient to deter nuisance attacks.
           </t>
           <t>Even when redirected, the User Equipment securely authenticates with API servers.
           </t>
         </list>
       </t>
     </section>
     <section title="User Options">
       <t>
         The ICMP messaging informs the User Equipment that it is being held
         captive.  There is no requirement that the User Equipment do something
         about this.
         Devices MAY permit users to disable automatic reaction to
         captive-portal indications for privacy reasons.
         However, there is the trade-off that the user doesn't get notified
         when network access is restricted.
         Hence, end-user devices MAY allow users to manually control captive
         portal interactions, possibly on the granularity of Provisioning
         Domains.
       </t>
     </section>
   </section>
 </middle>

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

 <back>
   <!-- References split into informative and normative -->

   <!-- There are 2 ways to insert reference entries from the citation libraries:
    1. define an ENTITY at the top, and use "ampersand character"RFC2629; here (as shown)
    2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml"?> here
       (for I-Ds: include="reference.I-D.narten-iana-considerations-rfc2434bis.xml")

    Both are cited textually in the same manner: by using xref elements.
    If you use the PI option, xml2rfc will, by default, try to find included files in the same
    directory as the including file. You can also define the XML_LIBRARY environment variable
    with a value containing a set of directories to search.  These can be either in the local
    filing system or remote ones accessed by http (http://domain/dir/... ).-->

   <references title="Normative References">
     <!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?-->
     &RFC2119;
     <?rfc include="reference.RFC.7710.xml"?>
     <?rfc include="reference.RFC.7556.xml"?>

   </references>
   <references title="Informative References">
     <?rfc include="reference.I-D.wkumari-capport-icmp-unreach.xml"?>
     <?rfc include="reference.I-D.nottingham-capport-problem.xml"?>
     <?rfc include="reference.I-D.bruneau-intarea-provisioning-domains.xml"?>
   </references>

<!--
   <section anchor="app-additional" title="Additional Stuff">
     <t>This becomes an Appendix.</t>
   </section>
-->
 </back>
</rfc>