HTTP Client Hintsilya@igvita.comhttp://www.igvita.com/
General
Internet-DraftAn increasing diversity of Web-connected device form factors and software capabilities has created a need to deliver varying, or optimized content for each device.HTTP Client Hints can be used as input to proactive content negotiation; just as the Accept header allowed clients to indicate what formats they prefer, Client Hints allow clients to indicate a list of device and agent specific preferences.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 (UA) 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, UA sniffing has numerous limitations:UA detection cannot reliably identify all static variablesUA detection cannot infer any dynamic client preferencesUA detection requires an external device databaseUA detection is not cache friendlyA popular alternative strategy is to use HTTP cookies to communicate some information about the client. However, this approach is also not cache friendly, bound by same origin policy, and imposes additional client-side latency by requiring JavaScript execution to create and manage HTTP cookies.This document defines a set of new request header fields that allow the client to perform proactive content negotiation by indicating a list of device and agent specific preferences, through a mechanism similar to the Accept header which is used to indicate preferred response formats.The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this
document are to be interpreted as described in .This document uses the Augmented Backus-Naur Form (ABNF) notation of
with the list rule extension defined in
, Appendix B. It includes by reference the
OWS, field-name and quoted-string rules from that document, and the
parameter rule from .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 a list of client preferences that the server can use to adapt and optimize the response.Client Hint request headers share a common syntax. As a convention, those defined in this specification have names prefixed with “CH-“, but this is only a convenience.This document defines a selection of Client Hint request header fields, and can be referenced by other specifications wishing to use the same syntax and processing model.Client-Hint field-values consist of either a token or a comma-delimited list of parameters.When the value is a token, it can either be boolean or a numeric value. Where possible, this form SHOULD be used, so that the hints don’t consume too much space in requests.Boolean values are indicated by the presence of the hint field-name in the request headers. If the hint name is absent in the last message containing the client hint header field, it is considered false.Numeric values are indicated by the full field-value contents (single value), or by the digits after “=” of the hint name (parameter value), up to the first non-digit character. If the hint does not have an argument, its value is assumed to be 0.Note that HTTP allows headers with comma-separated values to be conveyed using multiple instances of the same header field; as a result, the hints are collected from all instances of the same header on the message in question before being considered complete. If the same hint is used more than once, then the last hint overrides all previous occurrences, and the final ordering of unique hints is not significant.Clients control which Client Hint headers and their respective header fields are communicated, based on their default settings, user configuration and/or preferences. The user may be given the choice to enable, disable, or override specific hints.The client and server, or an intermediate proxy, may use an opt-in mechanism to negotiate which fields should be reported to allow for efficient content adaption.Servers can modify the response sent based upon Client Hints. When doing so, it MUST confirm the selection for certain hints and indicate the value of selected resource via corresponding response header. For example, this specification defines “DPR” that corresponds to the “CH-DPR” request header field.Servers can advertise support for Client Hints using the Accept-CH header or an equivalent HTML meta element with http-equiv attribute.For example:When a client receives Accept-CH, it SHOULD append the Client Hint headers that match the advertised field-values. For example, based on Accept-CH example above, the client would append CH-DPR and CH-RW headers to subsequent requests.Client Hints MAY be combined with Key () to enable fine-grained control of the cache key for improved cache efficiency. For example, the server may return the following set of instructions:Above examples indicates that the cache key should be based on the CH-DPR header, and the resource should be cached and made available for any client whose device pixel ratio is 1.5, or higher.Above example indicates that the cache key should be based on the CH-RW header, and the resource should be cached and made available for any request whose display width falls between 320 and 640px.In absence of support for fine-grained control of the cache key via the Key header field, Vary response header can be used to indicate that served resource has been adapted based on specified Client Hint preferences.Above example indicates that the cache key should be based on the CH-DPR header.Above example indicates that the cache key should be based on the CH-DPR and CH-RW headers.The “CH-DPR” header field indicates the client’s current Device Pixel Ratio (DPR), the ratio between physical pixels and density independent pixels on the device.The “CH-RW” header field indicates the client’s current Resource Width (RW), the display width of the requested resource in density independent pixels on the device.The “DPR” header field indicates the ratio between physical pixels and density independent pixels of the selected response.DPR ratio affects the calculation of intrinsic size of the image 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 resource whenever CH-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 DPR value contradicts previous client-side DPR indication, the server returned value must take precedence.The server does not need to confirm resource width (RW) selection as this value can be derived from the resource itself once it is decoded by the client.For example, given the following request header:The server knows that the device pixel ratio is 2.0, and that the intended display width of requested resource is 160px, as measured by density independent pixels on the device.If the server uses above hints to perform resource selection, it must confirm its selection via the DPR response header to allow the client to calculate the appropriate intrinsic size of the image resource. The server does not need to confirm resource width, only the ratio between physical pixels and density independent pixels of the selected image resource:The DPR response header indicates to the client that the server has selected resource with DPR ratio of 1.0. The client may 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.Client Hints does not supersede or replace User-Agent. Existing device detection mechanisms can continue to use both mechanisms if necessary. By advertising its capabilities within a request header, Client Hints allows for cache friendly and proactive content negotiation.This document defines the “Accept-CH”, “CH-DPR”, “CH-RW”, and “DPR” HTTP request fields, and registers them in the Permanent Message Headers registry.Header field name: CH-DPRApplicable protocol: HTTPStatus: InformationalAuthor/Change controller: Ilya Grigorik, ilya@igvita.comSpecification document(s): [this document]Related information: for Client HintsHeader field name: CH-RWApplicable protocol: HTTPStatus: InformationalAuthor/Change controller: Ilya Grigorik, ilya@igvita.comSpecification document(s): [this document]Related information: for Client HintsHeader field name: DPRApplicable protocol: HTTPStatus: InformationalAuthor/Change controller: Ilya Grigorik, ilya@igvita.comSpecification document(s): [this document]Related information: for Client HintsHeader field name: Accept-CHApplicable protocol: HTTPStatus: InformationalAuthor/Change controller: Ilya Grigorik, ilya@igvita.comSpecification document(s): [this document]Related information: for Client HintsThe client controls which header fields are communicated and when. In cases such as incognito or anonymous profile browsing, the header can be omitted if necessary.Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keyword
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. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
Note that the force of these words is modified by the requirement
level of the document in which they are used.
Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication transform for IPsec). To ensure that such quantities have consistent values and interpretations across all implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA).</t><t> In order for IANA to manage a given namespace prudently, it needs guidelines describing the conditions under which new values can be assigned or when modifications to existing values can be made. If IANA is expected to play a role in the management of a namespace, IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a namespace and provides guidelines for authors on the specific text that must be included in documents that place demands on IANA.</t><t> This document obsoletes RFC 2434. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes general security concerns for implementations.Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentThe Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.HTTP Origin and Hop HintsOver time, HTTP clients -- especially Web browsers -- have adapted how they use the protocol based upon common server configurations and behaviours. While this is necessary in the common case, it can be detrimental for performance and interoperability. This document establishes a mechanism whereby both origin servers and intermediaries can make hints available to clients about their preferences and capabilities, without imposing undue overhead on their interactions or requiring support for them. This is intended to allow clients to safely optimise connections to servers.The Key HTTP Response Header FieldThe 'Key' header field for HTTP responses allows an origin server to describe the cache key for a negotiated response: a short algorithm that can be used upon later requests to determine if the same response is reusable. Key has the advantage of avoiding an additional round trip for validation whenever a new request differs slightly, but not significantly, from prior requests. Key also informs user agents of the request characteristics that might result in different content, which can be useful if the user agent is not sending Accept* fields in order to reduce the risk of fingerprinting.