HTTP Working Group                                           I. Grigorik
Internet-Draft                                                    Google
Intended status: Standards Track                       November 24, 2015
Expires:                            May 27, 31, 2016
Expires: December 2, 2016

                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-00
                   draft-ietf-httpbis-client-hints-01

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 field 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

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), 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 .

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
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   This Internet-Draft will expire on May 27, December 2, 2016.

Copyright Notice

   Copyright (c) 2015 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Client Hint Request Header Fields . . . . . . . . . . . . . .   3   4
     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   7
   5.  The Viewport-Width Client Hint  . . . . . . . . . . . . . . .   7
   6.  The Downlink Client Hint  . . . . . . . . . . . . . . . . . .   7
   7.  The Save-Data Client Hint . . . . . . . . . . . . . . . . . .   7
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Accept-CH  . . . . . . . . . . . . . . . . . . . . . . .   9
     10.2.  Content-DPR  . . . . . . . . . . . . . . . . . . . . . .   9
     10.3.  Downlink . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.4.  DPR  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.5.  Save-Data  . . . . . . . . . . . . . . . . . . . . . . .  10
     10.6.  Viewport-Width . . . . . . . . . . . . . . . . . . . . .  10
     10.7.  Width  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . .   7
   8.  Examples . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Changes  . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . .  12
     A.1.  Since -00 . . . .   8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . .  12
     A.2.  Since -01 .   9
   11. Normative References . . . . . . . . . . . . . . . . . . . .  10 . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11  12

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)
   (UA; Section 5.5.3 of [RFC7231]) 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:

   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 ([RFC6265]) 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] (Section 3.4.1 of
   [RFC7231]) by indicating a list of 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.

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.

   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 can be given the choice
   to enable, disable, or override specific hints.

   The client and server, or an intermediate proxy, may can 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]),
   (Section 7.1.4 of [RFC7231]), 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 can 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 needs
   to 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 field or an equivalent HTML meta element with http-equiv attribute.
   attribute ([W3C.REC-html5-20141028]).

     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
   header fields 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 header fields to all subsequent
   requests.

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 needs to 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 needs to include the DPR
   header.
   header field.

     Vary: DPR, Width, Downlink

   Above example indicates that the cache key should be based on needs to include the DPR,
   Width, and Downlink headers. header fields.

   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 can 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 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.

     Key: Width;div=320

   Above example indicates that the cache key should be based on needs to include the value
   of the Width header field 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 needs to include the
   (Mbps) value of the Downlink header field 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.

3.  The DPR Client Hint

   The "DPR" request header field is a number that, in requests, that indicates the
   client's current Device Pixel Ratio (DPR), which is the ratio of
   physical pixels over CSS px (Section 5.2 of
   [W3C.CR-css-values-3-20150611]) 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.

3.1.  Confirming Selected DPR

   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 MUST explicitly indicate
   the DPR of the selected image response whenever the DPR hint is used,
   and the client must 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 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" request header field is a number that, in requests, 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
   largest 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 may
   can 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" request header field is a number that, in requests, that 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" request header field is a number that, in requests, that indicates the
   client's maximum downlink speed in megabits per second (Mbps), as
   defined by the "downlinkMax" attribute in the W3C Network Information
   API.
   API ([NETINFO]).

     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 Client Hint

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

     Save-Data : sd-token *( ";" [sd-token] )
     sd-token = "on"

   The token

   This document defines the "on" sd-token value, which is used as 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.  New token
   and extension token values can only be defined by revisions of this
   specification.

8.  Examples

   For example, given the following request headers: 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:

     Content-DPR: 1.0

   The Content-DPR response header field indicates to the client that
   the server has selected resource with DPR ratio of 1.0.  The client may
   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.

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

     Downlink: 0.384

   The server knows that the client's maximum downlink speed is
   0.384Mbps (GPRS EDGE), and it may can 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 can be communicated by, the application at runtime
   via JavaScript - e.g. viewport and image display width, device pixel
   ratio, and so on.

   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 can 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 might or not be the
   desired outcome.  The implementors may want to can 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.

10.1.  Accept-CH

   o  Header field name: DPR Accept-CH
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 2.2.1
   o  Related information: for Client Hints

10.2.  Content-DPR

   o  Header field name: Width Content-DPR
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 3.1 of this document
   o  Related information: for Client Hints

10.3.  Downlink

   o  Header field name: Viewport-Width Downlink
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 6 of this document
   o  Related information: for Client Hints

10.4.  DPR

   o  Header field name: Downlink DPR
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 3 of this document
   o  Related information: for Client Hints

10.5.  Save-Data

   o  Header field name: Content-DPR Save-Data
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 7 of this document
   o  Related information: for Client Hints

10.6.  Viewport-Width

   o  Header field name: Accept-CH Viewport-Width
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 5 of this document
   o  Related information: for Client Hints

10.7.  Width

   o  Header field name: Save-Data Width
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): [this document] Section 4 of this document
   o  Related information: for Client Hints

11.  References

11.1.  Normative References

   [CSS2]     Bos, B., Celic, T., Hickson, I., and H. Lie, "Cascading
              Style Sheets Level 2 Revision 1 (CSS 2.1) Specification",
              W3C Recommendation REC-CSS2-20110607, June 2011,
              <http://www.w3.org/TR/2011/REC-CSS2-20110607>.

   [I-D.ietf-httpbis-key]
              Fielding, R. and m. mnot, M. Nottingham, "The Key HTTP Response
              Header Field", draft-ietf-httpbis-key-00 draft-ietf-httpbis-key-01 (work in
              progress),
              October 2015. March 2016.

   [NETINFO]  Caceres, M., Moreno, F., and I. Grigorik, "Network
              Information API", December 2015, <https://w3c.github.io/
              netinfo/>.

   [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>.

   [W3C.CR-css-values-3-20150611]
              Atkins, T. and E. Etemad, "CSS Values and Units Module
              Level 3", World Wide Web Consortium CR CR-css-values-
              3-20150611, June 2015,
              <http://www.w3.org/TR/2015/CR-css-values-3-20150611>.

   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O&#039;Connor, E., and S. Pfeiffer, "HTML5",
              World Wide Web Consortium Recommendation REC-
              html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

11.2.  Informative References

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

Appendix A.  Changes

A.1.  Since -00

   o  Issue 168 (make Save-Data extensible) updated ABNF.
   o  Issue 163 (CH review feedback) editorial feedback from httpwg
      list.
   o  Issue 153 (NetInfo API citation) added normative reference.

A.2.  Since -01

   None yet.

Author's Address

   Ilya Grigorik
   Google

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