HTTP defines proactive content negotiation to allow servers to select the appropriate response for a given request, based upon the user agent's characteristics, as expressed in request headers. In practice, user agents are often unwilling to send those request headers, because it is not clear whether they will be used, and sending them impacts both performance and privacy. This document defines an Accept-CH response header that servers can use to advertise their use of request headers for proactive content negotiation, along with a set of guidelines for the creation of such headers, colloquially known as "Client Hints."

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. 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). Not all documents approved by the IESG are candidates for any level of Internet Standard; see 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/rfc8942.

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There are thousands of different devices accessing the web, each with different device capabilities and preference information. These device capabilities include hardware and software characteristics, as well as dynamic user and user agent preferences. Historically, applications that wanted the server to optimize content delivery and user experience based on such capabilities had to rely on passive identification (e.g., by matching the User-Agent header field (Section 5.5.3 of RFC 7231) against an established database of user agent signatures), use HTTP cookies [RFC 6265] and URL parameters, or use some combination of these and similar mechanisms to enable ad hoc content negotiation. Such techniques are expensive to set up and maintain and are not portable across both applications and servers. They also make it hard for both user agent and server to understand which data are required and are in use during the negotiation:

• User agent detection cannot reliably identify all static variables, cannot infer dynamic user agent preferences, requires an external device database, is not cache friendly, and is reliant on a passive fingerprinting surface.
• Cookie-based approaches are not portable across applications and servers, impose additional client-side latency by requiring JavaScript execution, and are not cache friendly.
• URL parameters, similar to cookie-based approaches, suffer from lack of portability and are hard to deploy due to a requirement to encode content negotiation data inside of the URL of each resource.

Proactive content negotiation (Section 3.4.1 of RFC 7231) offers an alternative approach; user agents use specified, well-defined request headers to advertise their capabilities and characteristics, so that servers can select (or formulate) an appropriate response based on those request headers (or on other, implicit characteristics). However, traditional proactive content negotiation techniques often mean that user agents send these request headers prolifically. This causes performance concerns (because it creates "bloat" in requests), as well as privacy issues; passively providing such information allows servers to silently fingerprint the user. This document defines Client Hints, a framework that enables servers to opt-in to specific proactive content negotiation features, adapting their content accordingly, as well as guidelines for content negotiation mechanisms that use the framework. This document also defines a new response header, Accept-CH, that allows an origin server to explicitly ask that user agents send these headers in requests. Client Hints mitigate performance concerns by assuring that user agents will only send the request headers when they're actually going to be used, and they mitigate privacy concerns of passive fingerprinting by requiring explicit opt-in and disclosure of required headers by the server through the use of the Accept-CH response header, turning passive fingerprinting vectors into active ones. The document does not define specific usages of Client Hints. Such usages need to be defined in their respective specifications. One example of such usage is the User-Agent Client Hints [UA-CH].

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here. This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC 5234].

A Client Hints request header field is an HTTP header field that is used by HTTP user agents to indicate data that can be used by the server to select an appropriate response. Each one conveys user-agent preferences that the server can use to adapt and optimize the response.

User agents choose what Client Hints to send in a request based on their default settings, user configuration, and server preferences expressed in Accept-CH. The user agent and server can use an opt-in mechanism outlined below to negotiate which header fields need to be sent to allow for efficient content adaption, and they can optionally use additional mechanisms (e.g., as outlined in [CLIENT-HINTS-INFRASTRUCTURE]) to negotiate delegation policies that control access of third parties to those same header fields. User agents SHOULD require an opt-in to send any hints that are not considered low-entropy. See the low-entropy hint table at [CLIENT-HINTS-INFRASTRUCTURE] for examples of hints that expose low amounts of entropy. Implementers need to be aware of the fingerprinting implications when implementing support for Client Hints and follow the considerations outlined in the Security Considerations section of this document (see Section 4).

When presented with a request that contains one or more Client Hints header fields, servers can optimize the response based upon the information in them. When doing so, and if the resource is cacheable, the server MUST also generate a Vary response header field (Section 7.1.4 of RFC 7231) to indicate which hints can affect the selected response and whether the selected response is appropriate for a later request. Servers MUST ignore hints they do not understand nor support. There is no mechanism for servers to indicate to user agents that hints were ignored. Furthermore, the server can generate additional response header fields (as specified by the hint or hints in use) that convey related values to aid client processing.

Servers can advertise support for Client Hints using the mechanism described below.

The Accept-CH response header field indicates server support for the hints indicated in its value. Servers wishing to receive user agent information through Client Hints SHOULD add the Accept-CH response header to their responses as early as possible. Accept-CH is a Structured Header [RFC 8941]. Its value MUST be an sf-list (Section 3.1 of RFC 8941) whose members are Tokens (Section 3.3.4 of RFC 8941). Its ABNF is: For example: When a user agent receives an HTTP response containing Accept-CH, it indicates that the origin opts-in to receive the indicated request header fields for subsequent same-origin requests. The opt-in MUST be ignored if delivered over non-secure transport (using a scheme different from HTTPS). It SHOULD be persisted and bound to the origin to enable delivery of Client Hints on subsequent requests to the server's origin, for the duration of the user's session (as defined by the user agent). An opt-in overrides previous persisted opt-in values and SHOULD be persisted in its stead. Based on the Accept-CH example above, which is received in response to a user agent navigating to "https://site.example", and delivered over a secure transport, persisted Accept-CH preferences will be bound to "https://site.example". It will then use it for navigations to for example, "https://site.example/foobar.html", but not to, for example, "https://foobar.site.example/". It will similarly use the preference for any same-origin resource requests (e.g., to "https://site.example/image.jpg") initiated by the page constructed from the navigation's response, but not to cross-origin resource requests (e.g., "https://thirdparty.example/resource.js"). This preference will not extend to resource requests initiated to "https://site.example" from other origins (e.g., from navigations to "https://other.example/").

When selecting a response based on one or more Client Hints, and if the resource is cacheable, the server needs to generate a Vary response header field [RFC 7234] to indicate which hints can affect the selected response and whether the selected response is appropriate for a later request. The above example indicates that the cache key needs to include the Sec-CH-Example header field. The above example indicates that the cache key needs to include the Sec-CH-Example and Sec-CH-Example-2 header fields.

Request header fields used in features relying on this document expose information about the user's environment to enable privacy-preserving proactive content negotiation and avoid exposing passive fingerprinting vectors. However, implementers need to bear in mind that in the worst case, uncontrolled and unmonitored active fingerprinting is not better than passive fingerprinting. In order to provide user privacy benefits, user agents need to apply further policies that prevent abuse of the information exposed by features using Client Hints. The information exposed by features might reveal new information about the user, and implementers ought to consider the following considerations, recommendations, and best practices. The underlying assumption is that exposing information about the user as a request header is equivalent (from a security perspective) to exposing this information by other means. (For example, if the request's origin can access that information using JavaScript APIs and transmit it to its servers.) Because Client Hints is an explicit opt-in mechanism, it means that servers wanting access to information about the user's environment need to actively ask for it, enabling clients and privacy researchers to keep track of which origins collect that data, and potentially act upon it. The header-based opt-in means that removal of passive fingerprinting vectors is possible. As an example, the user agent can reduce the information exposed by the User-Agent string, while enabling active access to that information through User-Agent Client Hints [UA-CH]. Otherwise, the user agent can expose information already available through script (e.g., the Save-Data Client Hints <https://wicg.github.io/savedata/#save-data-request-header-field>), without increasing the passive fingerprinting surface. User agents supporting Client Hints features which send certain information to opted-in servers SHOULD avoid sending the equivalent information passively. Therefore, features relying on this document to define Client Hint headers MUST NOT provide new information that is otherwise not made available to the application by the user agent, such as existing request headers, HTML, CSS, or JavaScript. Such features need to take into account the following aspects of the exposed information:

Entropy:

Exposing highly granular data can be used to help identify users across multiple requests to different origins. Reducing the set of header field values that can be expressed, or restricting them to an enumerated range where the advertised value is close to but is not an exact representation of the current value, can improve privacy and reduce risk of linkability by ensuring that the same value is sent by multiple users.

Sensitivity:

The feature SHOULD NOT expose user-sensitive information. To that end, information available to the application, but gated behind specific user actions (e.g., a permission prompt or user activation), SHOULD NOT be exposed as a Client Hint.

Change over time:

The feature SHOULD NOT expose user information that changes over time, unless the state change itself is also exposed (e.g., through JavaScript callbacks).

Different features will be positioned in different points in the space between low-entropy, non-sensitive, and static information (e.g., user agent information) and high-entropy, sensitive, and dynamic information (e.g., geolocation). User agents need to consider the value provided by a particular feature vs. these considerations and may wish to have different policies regarding that tradeoff on a per-feature or other fine-grained basis. Implementers ought to consider both user- and server-controlled mechanisms and policies to control which Client Hints header fields are advertised:

• Implementers SHOULD restrict delivery of some or all Client Hints header fields to the opt-in origin only, unless the opt-in origin has explicitly delegated permission to another origin to request Client Hints header fields.
• Implementers that consider providing user-choice mechanisms that allow users to balance privacy concerns against bandwidth limitations need to also consider that explaining the privacy implications involved to users, such as the risks of passive fingerprinting, may be challenging or even impractical.
• Implementations specific to certain use cases or threat models MAY avoid transmitting some or all of the Client Hints header fields. For example, avoid transmission of header fields that can carry higher risks of linkability.

User agents MUST clear persisted opt-in preferences when any one of site data, browsing cache, cookies, or similar are cleared.

Deployment of new request headers requires several considerations:

• Potential conflicts due to existing use of a header field name
• Properties of the data communicated in a header field value

Authors of new Client Hints are advised to carefully consider whether they need to be able to be added by client-side content (e.g., scripts) or whether the Client Hints need to be exclusively set by the user agent. In the latter case, the Sec- prefix on the header field name has the effect of preventing scripts and other application content from setting them in user agents. Using the "Sec-" prefix signals to servers that the user agent -- and not application content -- generated the values. See [FETCH] for more information. By convention, request headers that are Client Hints are encouraged to use a CH- prefix, to make them easier to identify as using this framework; for example, CH-Foo or, with a "Sec-" prefix, Sec-CH-Foo. Doing so makes them easier to identify programmatically (e.g., for stripping unrecognized hints from requests by privacy filters). A Client Hints request header negotiated using the Accept-CH opt-in mechanism MUST have a field name that matches sf-token (Section 3.3.4 of RFC 8941).

A user agent that tracks access to active fingerprinting information SHOULD consider emission of Client Hints headers similar to the way it would consider access to the equivalent API. Research into abuse of Client Hints might look at how HTTP responses to requests that contain Client Hints differ from those with different values and from those without values. This might be used to reveal which Client Hints are in use, allowing researchers to further analyze that use.

Sending Client Hints to the server incurs an increase in request byte size. Some of this increase can be mitigated by HTTP header compression schemes, but each new hint sent will still lead to some increased bandwidth usage. Servers SHOULD take that into account when opting in to receive Client Hints and SHOULD NOT opt-in to receive hints unless they are to be used for content adaptation purposes. Due to request byte size increase, features relying on this document to define Client Hints MAY consider restricting sending those hints to certain request destinations [FETCH], where they are more likely to be useful.

Features relying on this document are expected to register added request header fields in the "Permanent Message Header Field Names" registry [RFC 3864]. This document defines the "Accept-CH" HTTP response header field; IANA has registered it in the same registry.

Header field name:

Accept-CH

Applicable protocol:

HTTP

Status:

experimental

Author/Change controller:

IETF

Specification document(s):

Section 3.1 of this RFC

Related information:

for Client Hints

[RFC2119]

S. Bradner, "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>.

[RFC3864]

G. Klyne, M. Nottingham, and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.

[RFC5234]

D. Crocker, and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC7231]

R. Fielding, and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC7234]

R. Fielding, M. Nottingham, and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.

[RFC8174]

B. Leiba, "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>.

[RFC8941]

M Nottingham, and P-H. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.

[CLIENT-HINTS-INFRASTRUCTURE]

Google, Y. Weiss, "Client Hints Infrastructure", July 2020,
<https://wicg.github.io/client-hints-infrastructure/>.

[FETCH]

WHATWG, "Fetch - Living Standard",
<https://fetch.spec.whatwg.org/>.

[RFC6265]

A. Barth, "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[UA-CH]

Google, M. West, and Y. Weiss, "User-Agent Client Hints", August 2020,
<https://wicg.github.io/ua-client-hints/>.

Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Ben Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Hardie, Jonas Sicking, Martin Thomson, and numerous other members of the IETF HTTP Working Group for invaluable help and feedback.

Ilya Grigorik

Yoav Weiss