Captive Portal Interaction                                 T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                         D. Thakore, Ed.
Expires: 2 October 2020                                        CableLabs
                                                           31 March 2020

                           Captive Portal API


   This document describes an HTTP API that allows clients to interact
   with a Captive Portal system.  With this API, clients can discover
   how to get out of captivity and fetch state about their Captive
   Portal sessions.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 2 October 2020.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  API Connection Details  . . . . . . . . . . . . . . . . . . .   3
     4.1.  Server Authentication . . . . . . . . . . . . . . . . . .   4
   5.  API State Structure . . . . . . . . . . . . . . . . . . . . .   5
   6.  Example Interaction . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     7.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Captive Portal API JSON Media Type Registration . . . . .   7
     8.2.  Captive Portal API Keys Registry  . . . . . . . . . . . .   8
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document describes a HyperText Transfer Protocol (HTTP)
   Application Program Interface (API) that allows clients to interact
   with a Captive Portal system.  The API defined in this document has
   been designed to meet the requirements in the Captive Portal
   Architecture [I-D.ietf-capport-architecture].  Specifically, the API

   *  The state of captivity (whether or not the client has access to
      the Internet)

   *  A URI that a client browser can present to a user to get out of

   *  An encrypted connection (using TLS for connections to both the API
      and user portal)

2.  Terminology

   This document leverages the terminology and components described in
   [I-D.ietf-capport-architecture] and additionally defines the
   following terms:

   *  Captive Portal Client: The client that interacts with the Captive
      Portal API is typically some application running on the User
      Equipment that is connected to the Captive Network.  This is also
      referred to as the "client" in this document.

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   *  Captive Portal API Server: The server exposing the API's defined
      in this document to the client.  This is also referred to as the
      "API server" in this document.

3.  Workflow

   The Captive Portal Architecture defines several categories of
   interaction between clients and Captive Portal systems:

   1.  Provisioning, in which a client discovers that a network has a
       captive portal, and learns the URI of the API server.

   2.  API Server interaction, in which a client queries the state of
       the captive portal and retrieves the necessary information to get
       out of captivity.

   3.  Enforcement, in which the enforcement device in the network
       blocks disallowed traffic.

   This document defines the mechanisms used in the second category.  It
   is assumed that the location of the Captive Portal API server has
   been discovered by the client as part of Provisioning.  A set of
   mechanisms for discovering the API Server endpoint is defined in

4.  API Connection Details

   The API server endpoint MUST be accessed using HTTP over TLS (HTTPS)
   and SHOULD be served on port 443 [RFC2818].  The client SHOULD NOT
   assume that the URI for a given network attachment will stay the
   same, and SHOULD rely on the discovery or provisioning process each
   time it joins the network.

   For example, if the Captive Portal API server is hosted at
   "", the URI of the API could be "

   As described in Section 3 of [I-D.ietf-capport-architecture], the
   identity of the client needs to be visible to the Captive Portal API
   server in order for the server to correctly reply with the client's
   portal state.  If the identifier used by the Captive Portal system is
   the client's IP address, the system needs to ensure that the same IP
   address is visible to both the API server and the enforcement device.

   If the API server needs information about the client identity that is
   not otherwise visible to it, the URI provided to the client during
   provisioning can be distinct per client.  Thus, depending on how the

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   Captive Portal system is configured, the URI might be unique for each
   client host and between sessions for the same client host.

   For example, a Captive Portal system that uses per-client session
   URIs could use "" as its

4.1.  Server Authentication

   The purpose of accessing the Captive Portal API over an HTTPS
   connection is twofold: first, the encrypted connection protects the
   integrity and confidentiality of the API exchange from other parties
   on the local network; and second, it provides the client of the API
   an opportunity to authenticate the server that is hosting the API.
   This authentication is aimed at allowing a user to be reasonably
   confident that the entity providing the Captive Portal API has a
   valid certificate for the hostname in the URI (such as
   "").  The hostname of the API SHOULD be displayed to the
   user in order to indicate the entity which is providing the API

   Clients performing revocation checking will need some means of
   accessing revocation information for certificates presented by the
   API server.  Online Certificate Status Protocol [RFC6960] (OCSP)
   stapling, using the TLS Certificate Status Request extension
   [RFC6066] SHOULD be used.  OCSP stapling allows a client to perform
   revocation checks without initiating new connections.  To allow for
   other forms of revocation checking, a captive network could permit
   connections to OCSP responders or Certificate Revocation Lists (CRLs)
   that are referenced by certificates provided by the API server.  In
   addition to connections to OCSP responders and CRLs, a captive
   network SHOULD also permit connections to Network Time Protocol (NTP)
   [RFC5905] servers or other time-sync mechnisms to allow clients to
   accurately validate certificates.

   Certificates with missing intermediate certificates that rely on
   clients validating the certificate chain using the URI specified in
   the Authority Information Access (AIA) extension [RFC5280] SHOULD NOT
   be used by the Captive Portal API server.  If the certificates do
   require the use of AIA, the captive network MUST allow client access
   to the host specified in the URI.

   If the client is unable to validate the certificate presented by the
   API server, it MUST NOT proceed with any of the behavior for API
   interaction described in this document.  The client will proceed to
   interact with the captive network as if the API capabilities were not
   present.  It may still be possible for the user to access the network
   by being redirected to a web portal.

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5.  API State Structure

   The Captive Portal API data structures are specified in JavaScript
   Object Notation (JSON) [RFC8259].  Requests and responses for the
   Captive Portal API use the "application/captive+json" media type.
   Clients SHOULD include this media type as an Accept header in their
   GET requests, and servers MUST mark this media type as their Content-
   Type header in responses.

   The following key MUST be included in the top-level of the JSON
   structure returned by the API server:

   *  "captive" (boolean): indicates whether the client is in a state of
      captivity, i.e it has not satisfied the conditions to access the
      external network.  If the client is captive (i.e. captive=true),
      it can still be allowed enough access for it to perform server
      authentication Section 4.1.

   The following keys can be optionally included in the top-level of the
   JSON structure returned by the API server:

   *  "user-portal-url" (string): provides the URL of a web portal with
      which a user can interact.

   *  "venue-info-url" (string): provides the URL of a webpage or site
      on which the operator of the network has information that it
      wishes to share with the user (e.g., store info, maps, flight
      status, or entertainment).

   *  "can-extend-session" (boolean): indicates that the URL specified
      as "user-portal-url" allows the user to extend a session once the
      client is no longer in a state of captivity.  This provides a hint
      that a client system can suggest accessing the portal URL to the
      user when the session is near its limit in terms of time or bytes.

   *  "seconds-remaining" (integer): indicates the number of seconds
      remaining, after which the client will be placed into a captive
      state.  The API server SHOULD include this value if the client is
      not captive (i.e. captive=false) and the client session is time-
      limited, and SHOULD omit this value for captive clients (i.e.

   *  "bytes-remaining" (integer): indicates the number of bytes
      remaining, after which the client will be in placed into a captive
      state.  The byte count represents the sum of the total number of
      IP packet (layer 3) bytes sent and received by the client.
      Captive portal systems might not count traffic to whitelisted
      servers, such as the API server, but clients cannot rely on such

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      behavior.  The API server SHOULD include this value if the client
      is not captive (i.e. captive=false) and the client session is
      byte-limited, and SHOULD omit this value for captive clients (i.e.

   The valid JSON keys can be extended by adding entries to the Captive
   Portal API Keys Registry Section 8.  If a client receives a key that
   it does not recognize, it MUST ignore the key and any associated
   values.  All keys other than the ones defined in this document as
   "required" will be considered optional.

6.  Example Interaction

   A client connected to a captive network upon discovering the URI of
   the API server will query the API server to retrieve information
   about its captive state and conditions to escape captivity.  To
   request the Captive Portal JSON content, a client sends an HTTP GET

   GET /captive-portal/api/X54PD
   Accept: application/captive+json

   The server then responds with the JSON content for that client:

   HTTP/1.1 200 OK
   Cache-Control: private
   Date: Mon, 02 Mar 2020 05:07:35 GMT
   Content-Type: application/captive+json

      "captive": true,
      "user-portal-url": "",
      "venue-info-url": "",
      "seconds-remaining": 326,
      "can-extend-session": true

   Upon receiving this information the client will provide this
   information to the user so that they may navigate the web portal (as
   specified by the user-portal-url value) to enable access to the
   external network.  Once the user satisfies the requirements for
   extenal network access, the client SHOULD query the API server again
   to verify that it is no longer captive.

   Captive Portal JSON content can contain per-client data that is not
   appropriate to store in an intermediary cache.  Captive Portal API

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   servers SHOULD set the Cache-Control header in any responses to
   "private", or a more restrictive value [RFC7234].

7.  Security Considerations

   One of the goals of this protocol is to improve the security of the
   communication between client hosts and Captive Portal systems.
   Client traffic is protected from passive listeners on the local
   network by requiring TLS-encrypted connections between the client and
   the Captive Portal API server, as described in Section 4.  All
   communication between the clients and the API server MUST be

   In addition to encrypting communications between clients and Captive
   Portal systems, this protocol requires a basic level of
   authentication from the API server, as described in Section 4.1.
   Specifically, the API server MUST present a valid certificate on
   which the client can perform revocation checks.  This allows the
   client to ensure that the API server has authority for a hostname
   that can be presented to a user.

   It is important to note that while the server authentication checks
   can validate a specific hostname, it is certainly possible for the
   API server to present a valid certificate for a hostname that uses
   non-standard characters or is otherwise designed to trick the user
   into believing that its hostname is some other, more trustworthy,
   name.  This is a danger of any scenario in which a hostname is not
   typed in by a user.

7.1.  Privacy Considerations

   Information passed between a client and a Captive Portal system may
   include a user's personal information, such as a full name and credit
   card details.  Therefore, it is important that Captive Portal API
   Servers do not allow access to the Captive Portal API over
   unencrypted sessions.

8.  IANA Considerations

   IANA is requested to create a registration for an "application/
   captive+json" media type (Section 8.1) and a registry for fields in
   that format (Section 8.2).

8.1.  Captive Portal API JSON Media Type Registration

   This document registers the media type for Captive Portal API JSON
   text, "application/captive+json".

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   Type name:  application

   Subtype name:  captive+json

   Required parameters:  None

   Optional parameters:  None

   Encoding considerations:  Encoding considerations are identical to
      those specified for the "application/json" media type.

   Security considerations:  See Section 7

   Interoperability considerations:  This document specifies format of
      conforming messages and the interpretation thereof.

   Published specification:  This document

   Applications that use this media type:  This media type is intended
      to be used by servers presenting the Captive Portal API, and
      clients connecting to such captive networks.

   Additional information:  None

   Person & email address to contact for further information:  See
      Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  None


   Change controller:  IETF

8.2.  Captive Portal API Keys Registry

   IANA is asked to create and maintain a new registry called "Captive
   Portal API Keys", which will reserve JSON keys for use in Captive
   Portal API data structures.  The initial contents of this registry
   are provided in Section 5.

   Each entry in the registry contains the following fields:

   Key:  The JSON key being registered, in string format.

   Type:  The type of the JSON value to be stored, as one of the value
      types defined in [RFC8259].

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   Description:  A brief description explaining the meaning of the
      value, how it might be used, and/or how it should be interpreted
      by clients.

   New assignments for Captive Portal API Keys Registry will be
   administered by IANA through Expert Review [RFC8126].  The Designated
   Expert is expected to validate the existence of documentation
   describing new keys in a permanent publicly available specification.
   The expert is expected to validate that new keys have a clear meaning
   and do not create unnecessary confusion or overlap with existing
   keys.  Keys that are specific to non-generic use cases, particularly
   ones that are not specified as part of an IETF document, are
   encouraged to use a domain-specific prefix.

9.  Acknowledgments

   This work in this document was started by Mark Donnelly and Margaret
   Cullen.  Thanks to everyone in the CAPPORT Working Group who has
   given input.

10.  References

10.1.  Normative References

              Kumari, W. and E. Kline, "Captive-Portal Identification in
              DHCP / RA", Work in Progress, Internet-Draft, draft-ietf-
              capport-rfc7710bis-03, 30 March 2020,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,

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              DOI 10.17487/RFC6066, January 2011,

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

10.2.  Informative References

              Larose, K., Dolson, D., and H. Liu, "CAPPORT
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-capport-architecture-06, 15 February 2020,

Authors' Addresses

   Tommy Pauly (editor)
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America


   Darshak Thakore (editor)
   858 Coal Creek Circle
   Louisville, CO 80027,

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   United States of America


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