An increasing diversity of Web-connected devices and software capabilities has created a need to deliver optimized content for each device.
This specification defines an extensible and configurable set of HTTP request header fields, colloquially known as Client Hints, to address this. They are intended to be used as input to proactive content negotiation; just as the Accept header field allows user agents to indicate what formats they prefer, Client Hints allow user agents to indicate device and agent specific preferences.
Discussion of this draft takes place on the HTTP working group mailing list (email@example.com), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
Working Group information can be found at http://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/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.
One way to infer some of these capabilities is through User-Agent (Section 5.5.3 of [RFC7231]) header field detection against an established database of client signatures. However, this technique requires acquiring such a database, integrating it into the serving path, and keeping it up to date. However, even once this infrastructure is deployed, user agent sniffing has numerous limitations:
This document defines a set of new request header fields that allow user agent to perform proactive content negotiation (Section 3.4.1 of [RFC7231]) by indicating device and agent specific preferences, through a mechanism similar to the Accept header field which is used to indicate preferred response formats.
Client Hints does not supersede or replace the User-Agent header field. Existing device detection mechanisms can continue to use both mechanisms if necessary. By advertising its capabilities within a request header field, Client Hints allows for cache friendly and proactive content negotiation.
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.
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.
Clients control which Client Hints are sent in requests, based on their default settings, user configuration, and server preferences. The client and server can use an opt-in mechanism outlined below to negotiate which fields should be sent to allow for efficient content adaption, and optinally use additional mechanisms to negotiate delegation policies that control access of third parties to same fields.
Implementers should be aware of the passive fingerprinting implications when implementing support for Client Hints, and follow the considerations outlined in “Security Considerations” section of this document.
When presented with a request that contains one or more client hint 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 [RFC7231]) to indicate which hints can affect the selected response and whether the selected response is appropriate for a later request.
Further, depending on the hint used, the server can generate additional response header fields to convey related values to aid client processing. For example, this specification defines the “Content-DPR” response header field that needs to be returned by the server when the “DPR” hint is used to select the response.
Servers can advertise support for Client Hints using the Accept-CH header field or an equivalent HTML meta element with http-equiv attribute ([HTML5]).
Accept-CH = #field-name
Accept-CH: DPR, Width, Viewport-Width
When a client receives an HTTP response advertising support for Client Hints, it should process it as origin ([RFC6454]) opt-in to receive Client Hint header fields advertised in the field-value. The opt-in MUST be delivered over a secure transport.
For example, based on Accept-CH example above, a user agent could append DPR, Width, and Viewport-Width header fields to all same-origin resource requests initiated by the page constructed from the response.
Servers can ask the client to remember the set of Client Hints that the server supports for a specified period of time, to enable delivery of Client Hints on subsequent requests to the server’s origin ([RFC6454]).
Accept-CH-Lifetime = #delta-seconds
When a client receives an HTTP response that contains Accept-CH-Lifetime header field, the field-value indicates that the Accept-CH preference SHOULD be persisted and bound to the origin, and be considered stale after response’s age ([RFC7234], section 4.2) is greater than the specified number of seconds. The preference MUST be delivered over a secure transport, and MUST NOT be persisted for an origin that isn’t HTTPS.
Accept-CH: DPR, Width Accept-CH: Viewport-Width Accept-CH-Lifetime: 86400
For example, based on the Accept-CH and Accept-CH-Lifetime example above, which is received from “https://bar.example.com” in response to a resource request initiated by “https://foo.example.com”, both delivered over a secure transport: a user agent SHOULD persist an Accept-CH preference bound to “https://foo.example.com”, for requests initiated to “https://bar.example.com” from “https://foo.example.com”, for up to 86400 seconds (1 day). This preference SHOULD NOT extend to requests initiated to “https://bar.example.com” from other origins.
If Accept-CH-Lifetime occurs in a message more than once, the last value overrides all previous occurrences.
When selecting an optimized response based on one or more Client Hints, and if the resource is cacheable, the server needs to generate a Vary response header field ([RFC7234]) to indicate which hints can affect the selected response and whether the selected response is appropriate for a later request.
Above example indicates that the cache key needs to include the DPR header field.
Vary: DPR, Width
Above example indicates that the cache key needs to include the DPR and Width header fields.
The “DPR” request header field is a number that indicates the client’s current Device Pixel Ratio (DPR), which is the ratio of physical pixels over CSS px (Section 5.2 of [CSSVAL]) of the layout viewport (Section 9.1.1 of [CSS2]) on the device.
DPR = 1*DIGIT [ "." 1*DIGIT ]
If DPR occurs in a message more than once, the last value overrides all previous occurrences.
The “Content-DPR” response header field is a number that indicates the ratio between physical pixels over CSS px of the selected image response.
Content-DPR = 1*DIGIT [ "." 1*DIGIT ]
DPR ratio affects the calculation of intrinsic size of image resources on the client - i.e. typically, the client automatically scales the natural size of the image by the DPR ratio to derive its display dimensions. As a result, the server MUST explicitly indicate the DPR of the selected image response whenever the DPR hint is used, and the client MUST use the DPR value returned by the server to perform its calculations. In case the server returned Content-DPR value contradicts previous client-side DPR indication, the server returned value MUST take precedence.
Note that DPR confirmation is only required for image responses, and the server does not need to confirm the resource width as this value can be derived from the resource itself once it is decoded by the client.
If Content-DPR occurs in a message more than once, the last value overrides all previous occurrences.
The “Width” request header field is a number that indicates the desired resource width in physical px (i.e. intrinsic size of an image). The provided physical px value is a number rounded to the smallest following integer (i.e. ceiling value).
Width = 1*DIGIT
If the desired resource width is not known at the time of the request or the resource does not have a display width, the Width header field can be omitted. If Width occurs in a message more than once, the last value overrides all previous occurrences.
The “Viewport-Width” request header field is a number that indicates the layout viewport width in CSS px. The provided CSS px value is a number rounded to the smallest following integer (i.e. ceiling value).
Viewport-Width = 1*DIGIT
If Viewport-Width occurs in a message more than once, the last value overrides all previous occurrences.
For example, given the following request header fields:
DPR: 2.0 Width: 320 Viewport-Width: 320
The server knows that the device pixel ratio is 2.0, that the intended display width of the requested resource is 160 CSS px (320 physical pixels at 2x resolution), and that the viewport width is 320 CSS px.
If the server uses above hints to perform resource selection for an image asset, it must confirm its selection via the Content-DPR response header to allow the client to calculate the appropriate intrinsic size of the image response. The server does not need to confirm resource width, only the ratio between physical pixels and CSS px of the selected image resource:
The Content-DPR response header field indicates to the client that the server has selected resource with DPR ratio of 1.0. The client can use this information to perform additional processing on the resource - for example, calculate the appropriate intrinsic size of the image resource such that it is displayed at the correct resolution.
The request header fields defined in this specification, 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.
Implementers ought to consider both user and server controlled mechanisms and policies to control which Client Hints header fields are advertised:
Implementers SHOULD support Client Hints opt-in mechanisms and MUST clear persisted opt-in preferences when site data, browsing history, browsing cache, or similar, are cleared.
This document defines the “Accept-CH”, “DPR”, “Viewport-Width”, and “Width” HTTP request fields, “Accept-CH”, “Accept-CH-Lifetime”, and “Content-DPR” HTTP response field, and registers them in the Permanent Message Header Fields registry.
Client Hints may be combined with Key response header field ([KEY]) to enable fine-grained control of the cache key for improved cache efficiency. For example, the server can return the following set of instructions:
Above example indicates that the cache key needs to include the value of the DPR header field with three segments: less than 1.5, 1.5 to less than 2.5, and 4.0 or greater.
Above example indicates that the cache key needs to include the value of the Width header field and be partitioned into groups of 320: 0-320, 320-640, and so on.
Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Yoav Weiss, Ben Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Hardie, Jonas Sicking, and numerous other members of the IETF HTTP Working Group for invaluable help and feedback.