HTTP Client Hints

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, clients 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.”¶

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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 client preferences. Applications that want to allow the server to optimize content delivery and user experience based on such capabilities have, historically, had to rely on passive identification (e.g., by matching User-Agent (Section 5.5.3 of [RFC7231]) header field against an established database of client signatures), used HTTP cookies and URL parameters, or use some combination of these and similar mechanisms to enable ad hoc content negotiation.¶

• User agent detection cannot reliably identify all static variables, cannot infer dynamic client preferences, requires 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 [RFC7231]) 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.¶

However, proactive content negotiation requires clients to 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 agent.¶

This document defines a new response header, Accept-CH, that allows an origin server to explicitly ask that clients send these headers in requests. It also defines guidelines for content negotiation mechanisms that use it, colloquially referred to as Client Hints.¶

Client Hints mitigate the performance concerns by assuring that clients will only send the request headers when they’re actually going to be used, and the 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.¶

This document defines the Client Hints infrastructure, a framework that enables servers to opt-in to specific proactive content negotiation features, which will enable them to adapt their content accordingly. However, it does not define any specific features that will use that infrastructure. Those features will be defined in their respective specifications.¶

A Client Hint request header field is a HTTP header field that is used by HTTP clients to indicate configuration data that can be used by the server to select an appropriate response. Each one conveys client preferences that the server can use to adapt and optimize the response.¶

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

The request header fields defined in this document, and those that extend it, expose information about the user’s environment to enable proactive content negotiation. Such information may reveal new information about the user and implementers ought to consider the following considerations, recommendations, and best practices.¶

Transmitted Client Hints header fields SHOULD NOT provide new information that is otherwise not available to the application via other means, such as using HTML, CSS, or JavaScript. Further, sending highly granular data, such as image and viewport width may help identify users across multiple requests. Reducing the set of field values that can be expressed, or restricting them to an enumerated range where the advertised value is close 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. However, such precautions can still be insufficient for some types of data, especially data that can change over time.¶

• 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 MAY provide user choice mechanisms so that users may balance privacy concerns with bandwidth limitations. However, implementers should also be aware that explaining the privacy implications of passive fingerprinting to users may be challenging.
• Implementations specific to certain use cases or threat models MAY avoid transmitting some or all of Client Hints header fields. For example, avoid transmission of header fields that can carry higher risks of linkability.

Implementers SHOULD support Client Hints opt-in mechanisms and MUST clear persisted opt-in preferences when any one of site data, browsing history, browsing cache, or similar, are cleared.¶

This document defines the “Accept-CH” HTTP response field, and registers it in the Permanent Message Header Fields registry.¶

Client Hints may be combined with Variants response header field [VARIANTS] to enable fine-grained control of the cache key for improved cache efficiency. Features that define Client Hints will need to specify the related variants algorithms as described in Section 6 of [VARIANTS].¶

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.¶