HTTP Client Hints
draft-ietf-httpbis-client-hints-02

HTTP Working Group                                           I. Grigorik
Internet-Draft                                                    Google
Intended status: Experimental                            October 4, 2016
Expires: April 7, 2017

                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-02

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

   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
   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 April 7, 2017.

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Copyright Notice

   Copyright (c) 2016 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Client Hint Request Header Fields . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . . .   7
   5.  The Viewport-Width Client Hint  . . . . . . . . . . . . . . .   7
   6.  The Downlink Client Hint  . . . . . . . . . . . . . . . . . .   7
   7.  The Save-Data Client Hint . . . . . . . . . . . . . . . . . .   8
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Accept-CH  . . . . . . . . . . . . . . . . . . . . . . .  10
     10.2.  Content-DPR  . . . . . . . . . . . . . . . . . . . . . .  10
     10.3.  Downlink . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.4.  DPR  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.5.  Save-Data  . . . . . . . . . . . . . . . . . . . . . . .  10
     10.6.  Viewport-Width . . . . . . . . . . . . . . . . . . . . .  11
     10.7.  Width  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  Since -01 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.3.  Since -02 . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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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; 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 (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]
   and the OWS and field-name rules from [RFC7230].

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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.

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

   The client and server, or an intermediate proxy, 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 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
   (Section 7.1.4 of [RFC7231]), and optionally Key
   ([I-D.ietf-httpbis-key]), to indicate which hints can affect the
   selected response and whether the selected response is appropriate
   for a later request.

   Further, depending on the used hint, the server 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 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 ([W3C.REC-html5-20141028]).

     Accept-CH = #field-name

   For example:

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

   When a client receives Accept-CH, or if it is capable of processing
   the HTML response and finds an equivalent HTML meta element, it
   SHOULD append the Client-Hint header fields that match the advertised
   field-values to the header list of all subsequent requests.  For
   example, based on Accept-CH example above, a user agent could append
   DPR, Width, Viewport-Width, and Downlink header fields to all
   subresource requests initiated by the page constructed from the
   response.  Alternatively, a client can treat advertised support as a
   persistent origin preference and append same header fields on all
   future requests initiated to and by the resources associated with
   that origin.

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 needs to emit 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.

     Vary: DPR

   Above example indicates that the cache key needs to include the DPR
   header field.

     Vary: DPR, Width, Downlink

   Above example indicates that the cache key needs to include the DPR,
   Width, and Downlink 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 can return the following set of
   instructions:

     Key: DPR;partition=1.5:2.5:4.0

   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.

     Key: Width;div=320

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   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.

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

   Above example indicates that the cache key 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 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 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

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   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 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.

5.  The Viewport-Width Client Hint

   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.

6.  The Downlink Client Hint

   The "Downlink" request header field is a number 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 ([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.

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7.  The Save-Data Client Hint

   The "Save-Data" request header field consists of one or more tokens
   that indicate client's preference for reduced data usage, due to high
   transfer costs, slow connection speeds, or other reasons.

     Save-Data = sd-token *( OWS ";" OWS [sd-token] )
     sd-token = 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 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
   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.

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

     Downlink: 0.384

   The server knows that the client's maximum downlink speed is
   0.384Mbps (GPRS EDGE), and it 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 new
   information about the user's environment beyond what is already
   available to, and can be communicated by, the application at runtime
   via JavaScript and CSS.  For example, the application can obtain
   viewport width, image display width, and device pixel ratio via
   JavaScript, or through the use of CSS media queries and unique
   resource URLs even if JavaScript is disabled.  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 can make
   information about the user's environment available to origins that
   otherwise did not have access to this data, which may or may not be
   the desired outcome - e.g. this may enable an image optimization
   service to deliver a tailored asset, and it may reveal same
   information about the user to other origins that may not have had
   access to it before.  Similarly, sending highly granular data, such
   as image and viewport width may help identify users across multiple
   requests.  Restricting such field values to an enumerated range,
   where the user agent advertises a threshold value that is close but
   is not an exact representation of the current value, might reduce
   such fingerprinting risks.

   The implementers can provide mechanisms and policies to control how
   and when such hints are advertised: require origin opt-in and
   restrict delivery to same origin subrequests; limit delivery to
   requests that already carry indentifying information (e.g. cookies);
   modify delivery policy when in an "incognito" or a similar privacy
   mode; enable user configuration and opt in, and so on.

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.

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10.1.  Accept-CH

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

10.2.  Content-DPR

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

10.3.  Downlink

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

10.4.  DPR

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

10.5.  Save-Data

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

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10.6.  Viewport-Width

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

10.7.  Width

   o  Header field name: Width
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): 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>.

   [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>.

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   [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, T., 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

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

   [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

   o  Issue 200: Moved Key reference to informative.
   o  Issue 215: Extended passive fingerprinting and mitigation
      considerations.

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   o  Changed document status to experimental.

A.3.  Since -02

   None yet.

Author's Address

   Ilya Grigorik
   Google

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

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