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For this RFC, original HTML is available from the RFC-Editor: RFC8908

PROPOSED STANDARD

Internet Engineering Task Force (IETF)                     T. Pauly, Ed.
Request for Comments: 8908                                    Apple Inc.
Category: Standards Track                                D. Thakore, Ed.
ISSN: 2070-1721                                                CableLabs
                                                          September 2020


                           Captive Portal API

Abstract

   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

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8908.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

Table of Contents

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

1.  Introduction

   This document describes a HyperText Transfer Protocol (HTTP)
   Application Programming 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 [CAPPORT-ARCH].  Specifically, the API provides:

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

   *  A URI of a user-facing web portal that can be used to get out of
      captivity.

   *  Authenticated and encrypted connections, using TLS for connections
      to both the API and user-facing web portal.

2.  Terminology

   This document leverages the terminology and components described in
   [CAPPORT-ARCH] 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.

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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
       captivity 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
   [RFC8910].

4.  API Connection Details

   The API server endpoint MUST be accessed over HTTP using an https URI
   [RFC2818] and SHOULD use the default https port.  For example, if the
   Captive Portal API server is hosted at "example.org", the URI of the
   API could be "https://example.org/captive-portal/api".

   The client SHOULD NOT assume that the URI of the API server for a
   given network will stay the same and SHOULD rely on the discovery or
   provisioning process each time it joins the network.

   As described in Section 3 of [CAPPORT-ARCH], 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 set
   of IP addresses, the system needs to ensure that the same IP
   addresses are 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 SHOULD be distinct per client.  Thus, depending on how
   the Captive Portal system is configured, the URI will 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 "https://example.org/captive-portal/api/X54PD39JV" as
   its API URI.

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; second, it provides the client of the API an
   opportunity to authenticate the server that is hosting the API.  This
   authentication allows the client to ensure that the entity providing
   the Captive Portal API has a valid certificate for the hostname
   provisioned by the network using the mechanisms defined in [RFC8910],
   by validating that a DNS-ID [RFC6125] on the certificate is equal to
   the provisioned hostname.

   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, especially for clients that do
   not support OCSP stapling, a captive network SHOULD permit
   connections to OCSP responders or Certificate Revocation Lists (CRLs)
   that are referenced by certificates provided by the API server.  For
   more discussion on certificate revocation checks, see Section 6.5 of
   BCP 195 [RFC7525].  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 mechanisms
   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
   if the network redirects a cleartext webpage to a web portal.

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:

    +=========+=========+============================================+
    | Key     | Type    | Description                                |
    +=========+=========+============================================+
    | 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 will still be allowed    |
    |         |         | enough access for it to perform server     |
    |         |         | authentication (Section 4.1).              |
    +---------+---------+--------------------------------------------+

                                 Table 1

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

    +====================+=========+==================================+
    | Key                | Type    | Description                      |
    +====================+=========+==================================+
    | user-portal-url    | string  | Provides the URL of a web portal |
    |                    |         | that MUST be accessed over TLS   |
    |                    |         | with which a user can interact.  |
    +--------------------+---------+----------------------------------+
    | venue-info-url     | string  | Provides the URL of a webpage or |
    |                    |         | site that SHOULD be accessed     |
    |                    |         | over TLS 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  | number  | An integer that 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., |
    |                    |         | captive=true) or when the        |
    |                    |         | session is not time-limited.     |
    +--------------------+---------+----------------------------------+
    | bytes-remaining    | number  | An integer that indicates the    |
    |                    |         | number of bytes remaining, after |
    |                    |         | which the client will be 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, including IP         |
    |                    |         | headers.  Captive Portal systems |
    |                    |         | might not count traffic to       |
    |                    |         | whitelisted servers, such as the |
    |                    |         | API server, but clients cannot   |
    |                    |         | rely on such 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.,           |
    |                    |         | captive=true) or when the        |
    |                    |         | session is not byte-limited.     |
    +--------------------+---------+----------------------------------+

                                  Table 2

   The valid JSON keys can be extended by adding entries to the Captive
   Portal API Keys Registry (Section 8.2).  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.

   Captive Portal JSON content can contain per-client data that is not
   appropriate to store in an intermediary cache.  Captive Portal API
   servers SHOULD set the Cache-Control header field in any responses to
   "private" or a more restrictive value, such as "no-store" [RFC7234].

   Client behavior for issuing requests for updated JSON content is
   implementation specific and can be based on user interaction or the
   indications of seconds and bytes remaining in a given session.  If at
   any point the client does not receive valid JSON content from the API
   server, either due to an error or due to receiving no response, the
   client SHOULD continue to apply the most recent valid content it had
   received or, if no content had been received previously, proceed to
   interact with the captive network as if the API capabilities were not
   present.

6.  Example Interaction

   Upon discovering the URI of the API server, a client connected to a
   captive network will query the API server to retrieve information
   about its captive state and conditions to escape captivity.  In this
   example, the client discovered the URI "https://example.org/captive-
   portal/api/X54PD39JV" using one of the mechanisms defined in
   [RFC8910].

   To request the Captive Portal JSON content, a client sends an HTTP
   GET request:

   GET /captive-portal/api/X54PD39JV HTTP/1.1
   Host: example.org
   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": "https://example.org/portal.html"
   }

   Upon receiving this information, the client will use it to direct the
   user to 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 external network access, the client SHOULD query the
   API server again to verify that it is no longer captive.

   When the client requests the Captive Portal JSON content after
   gaining external network access, the server responds with updated
   JSON content:

   HTTP/1.1 200 OK
   Cache-Control: private
   Date: Mon, 02 Mar 2020 05:08:13 GMT
   Content-Type: application/captive+json

   {
      "captive": false,
      "user-portal-url": "https://example.org/portal.html",
      "venue-info-url": "https://flight.example.com/entertainment",
      "seconds-remaining": 326,
      "can-extend-session": true
   }

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
   encrypted.

   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 the hostname
   that was provisioned by the network using [RFC8910].  Note that this
   validation only confirms that the API server matches what the
   network's provisioning mechanism (such as DHCP or IPv6 Router
   Advertisements) provided; it is not validating the security of those
   provisioning mechanisms or the user's trust relationship to the
   network.

7.1.  Privacy Considerations

   Information passed between a client and the user-facing web portal
   may include a user's personal information, such as a full name and
   credit card details.  Therefore, it is important that both the user-
   facing web portal and the API server that points a client to the web
   portal are only accessed over encrypted connections.

   It is important to note that although communication to the user-
   facing web portal requires use of TLS, the authentication only
   validates that the web portal server matches the name in the URI
   provided by the API server.  Since this is not a name that a user
   typed in, the hostname of the website that would be presented to the
   user may include "confusable characters", which can mislead the user.
   See Section 12.5 of [RFC8264] for a discussion of confusable
   characters.

8.  IANA Considerations

   IANA has registered the "application/captive+json" media type
   (Section 8.1) and created 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".

   Type name:  application

   Subtype name:  captive+json

   Required parameters:  N/A

   Optional parameters:  N/A

   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:  RFC 8908

   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.

   Fragment identifier considerations:  N/A

   Additional Information:  N/A

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

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  CAPPORT IETF WG

   Change controller:  IETF

8.2.  Captive Portal API Keys Registry

   IANA has created a new registry called "Captive Portal API Keys",
   which reserves 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].

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

   Reference:  A reference to a specification that defines the key and
      explains its usage.

   New assignments for the "Captive Portal API Keys" registry will be
   administered by IANA using the Specification Required policy
   [RFC8126].  The designated expert is expected to validate the
   existence of documentation describing new keys in a permanent,
   publicly available specification, such as an Internet-Draft or RFC.
   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 nongeneric use cases, particularly
   ones that are not specified as part of an IETF document, are
   encouraged to use a domain-specific prefix.

9.  References

9.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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,
              <https://www.rfc-editor.org/info/rfc6960>.

   [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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

9.2.  Informative References

   [CAPPORT-ARCH]
              Larose, K., Dolson, D., and H. Liu, "CAPPORT
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-capport-architecture-08, 11 May 2020,
              <https://tools.ietf.org/html/draft-ietf-capport-
              architecture-08>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/info/rfc8264>.

   [RFC8910]  Kumari, W. and E. Kline, "Captive-Portal Identification in
              DHCP and Router Advertisement (RA)", RFC 8910,
              DOI 10.17487/RFC8910, September 2020,
              <https://www.rfc-editor.org/info/rfc8910>.

Acknowledgments

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

Authors' Addresses

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

   Email: tpauly@apple.com


   Darshak Thakore (editor)
   CableLabs
   858 Coal Creek Circle
   Louisville, CO 80027
   United States of America

   Email: d.thakore@cablelabs.com


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