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CAPPORT Architecture

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This is an older version of an Internet-Draft that was ultimately published as RFC 8952.
Authors Kyle Larose , David Dolson
Last updated 2017-09-29
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Internet Engineering Task Force                                K. Larose
Internet-Draft                                                 D. Dolson
Intended status: Informational                                  Sandvine
Expires: April 1, 2018                                September 28, 2017

                          CAPPORT Architecture


   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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 1, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Components  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  User Equipment  . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Provisioning Service  . . . . . . . . . . . . . . . . . .   5
       2.2.1.  DHCP or Router Advertisements . . . . . . . . . . . .   6
       2.2.2.  Provisioning Domains  . . . . . . . . . . . . . . . .   6
     2.3.  Captive Portal API Server . . . . . . . . . . . . . . . .   6
     2.4.  Captive Portal Enforcement  . . . . . . . . . . . . . . .   7
     2.5.  ICMP/ICMP6  . . . . . . . . . . . . . . . . . . . . . . .   8
     2.6.  Component Diagram . . . . . . . . . . . . . . . . . . . .   8
   3.  Solution Workflow . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Initial Connection  . . . . . . . . . . . . . . . . . . .  10
     3.2.  Conditions Expire . . . . . . . . . . . . . . . . . . . .  10
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     6.1.  Trusting the Network  . . . . . . . . . . . . . . . . . .  11
     6.2.  Authenticated APIs  . . . . . . . . . . . . . . . . . . .  12
     6.3.  Risk of Nuisance Captive Portal . . . . . . . . . . . . .  12
     6.4.  User Options  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   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.

   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.

   Problems with captive portal implementations have been described in
   [I-D.nottingham-capport-problem].  [If that document cannot be
   published, consider putting its best parts into an appendix of this

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   This document standardizes an architecture for implementing captive
   portals that provides tools for addressing most of those problems.
   We are guided by these principles:

   o  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.

   o  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.

   o  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.)

   o  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).

   o  The architecture MUST provide a path of incremental migration,
      acknowledging a huge variety of portals and end-user device
      implementations and software versions.

   o  The architecture SHOULD improve security by providing mechanisms
      for trust, allowing alerts from trusted network operators to be
      distinguished from attacks from untrusted agents.

   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.)

   The architecture uses the following mechanisms:

   o  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 [RFC7710].
      Other protocols (such as RADIUS), Provisioning Domains
      [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.

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   o  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.

   o  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).

   The architecture attempts to provide privacy, authentication, and
   safety mechanisms to the extent possible.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   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).

   Captive Portal Conditions: site-specific requirements that a user or
   device must satisfy in order to gain access to the wider network.

   Captive Portal Enforcement: The network equipment which enforces the
   traffic restriction and notifies the User Equipment it is in a
   captive state.

   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.

   Captive Portal Server: The web server providing a user interface for
   assisting the user in satisfying the conditions to escape captivity.

2.  Components

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2.1.  User Equipment

   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
   Section 2.4, until site-specific requirements have been met.

   At this time we consider only devices with web browsers, with web
   applications being the means of satisfying Captive Portal Conditions.

   o  An example interactive User Equipment is a smart phone.

   o  SHOULD support provisioning of the URI for the Captive Portal API
      (e.g., by DHCP)

   o  MAY distinguish Captive Portal API access per network interface,
      in the manner of Provisioning Domain Architecture [RFC7556].

   o  SHOULD have a mechanism for notifying the user of the Captive

   o  SHOULD have a web browser so that the user may navigate the
      Captive Portal user interface.

   o  SHOULD be able to receive and validate the Captive Portal ICMP
      message types, and to access the Captive Portal API in response.

   o  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".

   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.

2.2.  Provisioning Service

   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.

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2.2.1.  DHCP or Router Advertisements

   A standard for providing a portal URI using DHCP or Router
   Advertisements is described in [RFC7710].  The CAPPORT architecture
   expects this URI to indicate the API described in Section 2.3.

   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?]

2.2.2.  Provisioning Domains

   Although still a work in progress,
   [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 Section 2.3.

   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=<URI of portal>.  The key-value pair
   should provide a different result when access is not restricted.
   E.g., key="captivePortal" and value="".

   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.

   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
   Section 2.5).

2.3.  Captive Portal API Server

   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.

   The URI of this API will have been provisioned to the User Equipment.
   (Refer to Section 2.2).

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   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.

   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.

   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 Section 6.)

   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.

   This document does not specify the details of the API.

   The CAPPORT API SHOULD support TLS for privacy and server

2.4.  Captive Portal Enforcement

   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.

   The Captive Portal Enforcement component:

   o  Allows traffic through for allowed User Equipment.

   o  Blocks (discards) traffic and sends ICMP notifications for
      disallowed User Equipment.

   o  Permits disallowed User Equipment to access necessary APIs and web
      pages to fulfill requirements of exiting captivity.

   o  Updates allow/block rules per User Equipment in response to
      operations from the Captive Portal back-end.

   As an upgrade path, captive portals MAY continue to support methods
   that work today, such as modification of port-80 HTTP responses to

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   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.]

2.5.  ICMP/ICMP6

   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.).

   A mechanism to trigger captive portal work-flows in the User
   Equipment is proposed in [I-D.wkumari-capport-icmp-unreach].

   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

   The ICMP messages MUST NOT be sent to the Internet devices.  The
   indications are only sent to the User Equipment.

   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.

2.6.  Component Diagram

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   The following diagram shows the communication between each component.

   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

   In the diagram:

   o  During provisioning (e.g., DHCP), the User Equipment acquires the
      URI for the CAPPORT API.

   o  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.

   o  The User Equipment attempts to communicate to the external network
      through the Captive Portal enforcement device.

   o  The Captive Portal Enforcement device either allows the User
      Equipment's packets to the external network, or responds with an
      ICMP message.

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   o  The CAPPORT web portal server directs the Captive Portal
      Enforcement device to either allow or deny external network access
      for the User Equipment.

   Although the provisioning, API, and web portal functions are shown as
   discrete blocks, they could of course be combined into a single

3.  Solution Workflow

   This section aims to improve understanding by describing a possible
   workflow of solutions adhering to the architecture.

3.1.  Initial Connection

   This section describes a possible work-flow when User Equipment
   initially joins a Captive Network.

   1.  The User Equipment joins the Captive Network by acquiring a DHCP
       lease, RA, or similar, acquiring provisioning information.

   2.  The User Equipment learns the URI for the Captive Portal API from
       the provisioning information (e.g., [RFC7710]).

   3.  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.)

   4.  If necessary, the User navigates the web portal to gain access to
       the external network.

   5.  The Captive Portal API server indicates to the Captive Portal
       Enforcement device that the User Equipment is allowed to access
       the external network.

   6.  The User Equipment attempts a connection outside the captive

   7.  If the requirements have been satisfied, the access is permitted;
       otherwise the "Expired" behavior occurs.

   8.  The User Equipment accesses the network until conditions Expire.

3.2.  Conditions Expire

   This section describes a possible work-flow when conditions expire
   and the user visits the portal again (e.g., low quota, or time

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   1.  Pre-condition: the Captive Portal Enforcement has been configured
       to detect an expiry condition, which has now occurred.

   2.  The User Equipment sends a packet to the outside network.

   3.  The Captive Portal Enforcement detects that the packet is from an
       expired User Equipment.

   4.  The Captive Portal Enforcement sends an ICMP message to the User
       Equipment indicating that it needs to refresh its access.

   5.  The User Equipment verifies the ICMP message is plausible.

   6.  The User Equipment queries the Captive Portal API to refresh
       parameters and status of the Captive Network.

   7.  If necessary, the User once again navigates the web portal to
       gain access to the external network.

   8.  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.

   9.  The User Equipment accesses the external network.

4.  Acknowledgements

   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

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

6.1.  Trusting the Network

   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 [RFC7556] provides some discussion on authenticating an

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   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.

   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.

6.2.  Authenticated APIs

   The solution described here assumes that when the User Equipment
   needs to trust the API server, server authentication will be utilized
   using TLS mechanisms.

6.3.  Risk of Nuisance Captive Portal

   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

      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.

      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.

      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.

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      Even when redirected, the User Equipment securely authenticates
      with API servers.

6.4.  User Options

   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.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

   [RFC7710]  Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng,
              "Captive-Portal Identification Using DHCP or Router
              Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710,
              December 2015, <>.

7.2.  Informative References

              Pfister, P., Schinazi, D., Pauly, T., Vyncke, E., and B.
              Bruneau, "Discovering Provisioning Domain Names and Data",
              draft-bruneau-intarea-provisioning-domains-02 (work in
              progress), July 2017.

              Nottingham, M., "Captive Portals Problem Statement",
              draft-nottingham-capport-problem-01 (work in progress),
              April 2016.

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              Bird, D. and W. Kumari, "Captive Portal ICMP Messages",
              draft-wkumari-capport-icmp-unreach-02 (work in progress),
              April 2017.

Authors' Addresses

   Kyle Larose
   408 Albert Street
   Waterloo, ON  N2L 3V3

   Phone: +1 519 880 2400

   David Dolson
   408 Albert Street
   Waterloo, ON  N2L 3V3

   Phone: +1 519 880 2400

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