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Network Working Group                                        I. Grigorik
Internet-Draft                                                    Google
Intended status: Informational                         September 4, 2014
Expires: March 8, 2015


                           HTTP Client Hints
                  draft-grigorik-http-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 allows clients to indicate what formats they
   prefer, Client Hints allow clients to indicate a list of device and
   agent specific preferences.

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 March 8, 2015.

Copyright Notice

   Copyright (c) 2014 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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   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 . . . . . . . . . . . . . . . . . . . . . . 5
   4.  The RW Client Hint  . . . . . . . . . . . . . . . . . . . . . . 6
     4.1.  Confirming Selected DPR . . . . . . . . . . . . . . . . . . 6
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8



























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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) 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 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].



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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.

   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 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 "DPR" request header field.

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, RW

   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 and RW
   headers to all subsequent requests.





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2.2.2.  Interaction with Caches

   Client Hints MAY be combined with Key ([I-D.fielding-http-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;r=[1.5:]

   Above examples indicates that the cache key should be based on the
   DPR header, and the resource should be cached and made available for
   any client whose device pixel ratio is 1.5, or higher.

     Key: RW;r=[320:640]

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

     Vary: DPR

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

     Vary: DPR, RW

   Above example indicates that the cache key should be based on the DPR
   and RW headers.


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.






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4.  The RW Client Hint

   The "RW" header field is a number that, in requests, indicates the
   Resource Width (RW) in CSS px, which is either the display width of
   the requested resource (e.g. display width of an image), or the
   layout viewport width if the resource does not have a display width
   (e.g. a non-image asset).

     RW = 1*DIGIT

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

4.1.  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 (RW) 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.


5.  Example

   For example, given the following request headers:

     DPR: 2.0
     RW: 160

   The server knows that the device pixel ratio is 2.0, and that the
   intended display width of requested resource is 160 CSS px.



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   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.


6.  IANA Considerations

   This document defines the "Accept-CH", "DPR", and "RW" 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: Ilya Grigorik, ilya@igvita.com
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: RW
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: Ilya Grigorik, ilya@igvita.com
   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: Ilya Grigorik, ilya@igvita.com
   o  Specification document(s): [this document]
   o  Related information: for Client Hints
   o  Header field name: Accept-CH
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: Ilya Grigorik, ilya@igvita.com
   o  Specification document(s): [this document]
   o  Related information: for Client Hints





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7.  Security Considerations

   The 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.


8.  Normative References

   [I-D.fielding-http-key]
              Fielding, R. and M. Nottingham, "The Key HTTP Response
              Header Field", draft-fielding-http-key-02 (work in
              progress), February 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230,
              June 2014.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.


Author's Address

   Ilya Grigorik
   Google

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












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