Apple Inc.

One Apple Park Way Cupertino CA 95014 United States of America tpauly@apple.com

CableLabs

858 Coal Creek Circle Louisville CO 80027 United States of America d.thakore@cablelabs.com

Captive Portal Interaction 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 .

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 () 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

• .  Introduction
• .  Terminology
• .  Workflow
• .  API Connection Details
  • .  Server Authentication
• .  API State Structure
• .  Example Interaction
• .  Security Considerations
  • .  Privacy Considerations
• .  IANA Considerations
  • .  Captive Portal API JSON Media Type Registration
  • .  Captive Portal API Keys Registry
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Authors' Addresses

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 . 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.

Terminology This document leverages the terminology and components described in 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 when, and only when, they appear in all capitals, as shown here.

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 .

API Connection Details The API server endpoint MUST be accessed over HTTP using an https URI 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 , 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.

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 , by validating that a DNS-ID 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 (OCSP) stapling, using the TLS Certificate Status Request extension , 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 BCP 195. In addition to connections to OCSP responders and CRLs, a captive network SHOULD also permit connections to Network Time Protocol (NTP) 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 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.

API State Structure The Captive Portal API data structures are specified in JavaScript Object Notation (JSON) . 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 ().

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.

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

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 . 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 }

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 . 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 . 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 . 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.

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 for a discussion of confusable characters.

IANA Considerations IANA has registered the "application/captive+json" media type () and created a registry for fields in that format ().

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

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

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 . 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 .

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 . 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.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] The Network Time Protocol (NTP) is widely used to synchronize computer clocks in the Internet. This document describes NTP version 4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3), described in RFC 1305, as well as previous versions of the protocol. NTPv4 includes a modified protocol header to accommodate the Internet Protocol version 6 address family. NTPv4 includes fundamental improvements in the mitigation and discipline algorithms that extend the potential accuracy to the tens of microseconds with modern workstations and fast LANs. It includes a dynamic server discovery scheme, so that in many cases, specific server configuration is not required. It corrects certain errors in the NTPv3 design and implementation and includes an optional extension mechanism. [STANDARDS-TRACK] This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK] This document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. Informative References This document describes a CAPPORT architecture. DHCP or Router Advertisements, an optional signaling protocol, and an HTTP API are used to provide the solution. The role of Provisioning Domains (PvDs) is described. Work in Progress Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases. Application protocols using Unicode code points in protocol strings need to properly handle such strings in order to enforce internationalization rules for strings placed in various protocol slots (such as addresses and identifiers) and to perform valid comparison operations (e.g., for purposes of authentication or authorization). This document defines a framework enabling application protocols to perform the preparation, enforcement, and comparison of internationalized strings ("PRECIS") in a way that depends on the properties of Unicode code points and thus is more agile with respect to versions of Unicode. As a result, this framework provides a more sustainable approach to the handling of internationalized strings than the previous framework, known as Stringprep (RFC 3454). This document obsoletes RFC 7564.

Acknowledgments This work was started by and . Thanks to everyone in the CAPPORT Working Group who has given input.

Authors' Addresses Apple Inc.

One Apple Park Way Cupertino CA 95014 United States of America tpauly@apple.com

CableLabs

858 Coal Creek Circle Louisville CO 80027 United States of America d.thakore@cablelabs.com