HTTP Client Hints
draft-ietf-httpbis-client-hints-00

HTTP Working Group                                           I. Grigorik
Internet-Draft                                                    Google
Intended status: Standards Track                       November 24, 2015
Expires: May 27, 2016

                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-00

Abstract

   An increasing diversity of Web-connected devices and software
   capabilities has created a need to deliver optimized content for each
   device.

   This specification defines a 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 allows clients to indicate what formats they
   prefer, Client Hints allow clients to indicate a list of device and
   agent specific preferences.

Note to Readers

   The issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/client-hints .

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 27, 2016.

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Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Client Hint Request Header Fields . . . . . . . . . . . . . .   3
     2.1.  Sending Client Hints  . . . . . . . . . . . . . . . . . .   4
     2.2.  Server Processing of Client Hints . . . . . . . . . . . .   4
       2.2.1.  Advertising Support for Client Hints  . . . . . . . .   4
       2.2.2.  Interaction with Caches . . . . . . . . . . . . . . .   5
   3.  The DPR Client Hint . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Confirming Selected DPR . . . . . . . . . . . . . . . . .   6
   4.  The Width Client Hint . . . . . . . . . . . . . . . . . . . .   6
   5.  The Viewport-Width Client Hint  . . . . . . . . . . . . . . .   7
   6.  The Downlink Client Hint  . . . . . . . . . . . . . . . . . .   7
   7.  The Save-Data Hint  . . . . . . . . . . . . . . . . . . . . .   7
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   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.

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   However, even once this infrastructure is deployed, UA sniffing has
   numerous limitations:

   o  UA detection cannot reliably identify all static variables
   o  UA detection cannot infer any dynamic client preferences
   o  UA detection requires an external device database
   o  UA detection is not cache friendly

   A 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 [RFC7231] 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.

   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.

1.1.  Notational Conventions

   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 [RFC2119].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234] with the list rule extension defined in [RFC7230],
   Appendix B.  It includes by reference the DIGIT rule from [RFC5234];
   the OWS, field-name and quoted-string rules from [RFC7230]; and the
   parameter rule from [RFC7231].

2.  Client Hint Request Header Fields

   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.

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   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.

2.1.  Sending Client Hints

   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.

2.2.  Server Processing of Client Hints

   Servers MAY respond with an optimized response based on one or more
   received hints from the client.  When doing so, and if the resource
   is cacheable, the server MUST also emit a Vary response header field
   ([RFC7234]), and optionally Key ([I-D.ietf-httpbis-key]), to indicate
   which hints were used and whether the selected response is
   appropriate for a later request.

   Further, depending on the used hint, the server MAY also need to emit
   additional response header fields to confirm the property of the
   response, such that the client can adjust its processing.  For
   example, this specification defines "Content-DPR" response header
   field that MUST be returned by the server when the "DPR" hint is used
   to select the response.

2.2.1.  Advertising Support for Client Hints

   Servers can advertise support for Client Hints using the Accept-CH
   header or an equivalent HTML meta element with http-equiv attribute.

     Accept-CH = #token

   For example:

     Accept-CH: DPR, Width, Viewport-Width, Downlink

   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 DPR, Width,
   Viewport-Width, and Downlink headers to all subsequent requests.

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2.2.2.  Interaction with Caches

   When selecting an optimized response based on one or more Client
   Hints, and if the resource is cacheable, the server MUST also emit a
   Vary response header field ([RFC7234]) to indicate which hints were
   used and whether the selected response is appropriate for a later
   request.

     Vary: DPR

   Above example indicates that the cache key should be based on the DPR
   header.

     Vary: DPR, Width, Downlink

   Above example indicates that the cache key should be based on the
   DPR, Width, and Downlink headers.

   Client Hints MAY be combined with Key ([I-D.ietf-httpbis-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:

     Key: DPR;partition=1.5:2.5:4.0

   Above example indicates that the cache key should be based on the
   value of the DPR header with three segments: less than 1.5, 1.5 to
   less than 2.5, and 4.0 or greater.

     Key: Width;div=320

   Above example indicates that the cache key should be based on the
   value of the Width header and be partitioned into groups of 320:
   0-320, 320-640, and so on.

     Key: Downlink;partition=0.5:1.0:3.0:5.0:10

   Above example indicates that the cache key should be based on the
   (Mbps) value of the Downlink header with six segments: less than 0.5,
   0.5 to less than 1.0, 1.0 to less than 3.0, 3.0 to less than 5.0, 5.0
   to less than 10; 10 or higher.

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3.  The DPR Client Hint

   The "DPR" header field is a number that, in requests, indicates the
   client's current Device Pixel Ratio (DPR), which is the ratio of
   physical pixels over CSS px of the layout viewport on the device.

     DPR = 1*DIGIT [ "." 1*DIGIT ]

   If DPR occurs in a message more than once, the last value overrides
   all previous occurrences.

3.1.  Confirming Selected DPR

   The "Content-DPR" 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.

4.  The Width Client Hint

   The "Width" header field is a number that, in requests, indicates the
   resource width in physical px (i.e. intrinsic size of an image).  The
   provided physical px value is a number rounded to the largest
   smallest following integer (i.e. ceiling value).

     Width = 1*DIGIT

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   If the resource width is not known at the time of the request or the
   resource does not have a display width, the Width header field may be
   omitted.  If Width occurs in a message more than once, the last value
   overrides all previous occurrences.

5.  The Viewport-Width Client Hint

   The "Viewport-Width" header field is a number that, in requests,
   indicates the layout viewport width in CSS px.  The provided CSS px
   value is a number rounded to the largest 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.

6.  The Downlink Client Hint

   The "Downlink" header field is a number that, in requests, indicates
   the client's maximum downlink speed in megabits per second (Mbps), as
   defined by the "downlinkMax" attribute in the W3C Network Information
   API.

     Downlink = 1*DIGIT [ "." 1*DIGIT ]

   If Downlink occurs in a message more than once, the minimum value
   should be used to override other occurrences.

7.  The Save-Data Hint

   The "Save-Data" header field is a token that, in requests, indicates
   client's preference for reduced data usage, due to high transfer
   costs, slow connection speeds, or other reasons.

     Save-Data = "on"

   The token is a signal indicating explicit user opt-in into a reduced
   data usage mode on the client, and when communicated to origins
   allows them to deliver alternate content honoring such preference -
   e.g. smaller image and video resources, alternate markup, and so on.

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8.  Examples

   For example, given the following request headers:

     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 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:

     Content-DPR: 1.0

   The Content-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.

   Alternatively, the server could select an alternate resource based on
   the maximum downlink speed advertised in the request headers:

     Downlink: 0.384

   The server knows that the client's maximum downlink speed is
   0.384Mbps (GPRS EDGE), and it may use this information to select an
   optimized resource - for example, an alternate image asset,
   stylesheet, HTML document, media stream, and so on.

9.  Security Considerations

   Client Hints defined in this specification do not expose any new
   information about the user's environment beyond what is already
   available to, and may be communicated by, the application at runtime
   via JavaScript - e.g. viewport and image display width, device pixel
   ratio, and so on.

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   However, implementors should consider the privacy implications of
   various methods to enable delivery of Client Hints - see "Sending
   Client Hints" section.  For example, sending Client Hints on all
   requests may make information about the user's environment available
   to origins that otherwise did not have access to this data (e.g.
   origins hosting non-script resources), which may or not be the
   desired outcome.  The implementors may want to provide mechanisms to
   control such behavior via explicit opt-in, or other mechanisms.
   Similarly, the implementors should consider how and whether delivery
   of Client Hints is affected when the user is in "incognito" or
   similar privacy mode.

10.  IANA Considerations

   This document defines the "Accept-CH", "DPR", "Width", and "Downlink"
   HTTP request fields, "Content-DPR" HTTP response field, and registers
   them in the Permanent Message Header Fields registry.

   o  Header field name: DPR
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Width
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Viewport-Width
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Downlink
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Content-DPR
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints

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   o  Header field name: Accept-CH
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Save-Data
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document]
   o  Related information: for Client Hints

11.  Normative References

   [I-D.ietf-httpbis-key]
              Fielding, R. and m. mnot, "The Key HTTP Response Header
              Field", draft-ietf-httpbis-key-00 (work in progress),
              October 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

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

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

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Author's Address

   Ilya Grigorik
   Google

   Email: ilya@igvita.com
   URI:   https://www.igvita.com/

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