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Network Working Group                                        I. Grigorik
Internet-Draft                                          December 6, 2013
Intended status: Informational
Expires: June 9, 2014


                           HTTP Client Hints
                  draft-grigorik-http-client-hints-01

Abstract

   An 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.

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 June 9, 2014.

Copyright Notice

   Copyright (c) 2013 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



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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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . . . 3
   2.  Client Hint Request Header Fields . . . . . . . . . . . . . . . 3
     2.1.  Hint Values . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.2.  Sending Client Hints  . . . . . . . . . . . . . . . . . . . 4
     2.3.  Server Processing of Client Hints . . . . . . . . . . . . . 5
       2.3.1.  Advertising Support for Client Hints  . . . . . . . . . 5
       2.3.2.  Interaction with Caches . . . . . . . . . . . . . . . . 5
   3.  The CH-DPR Client Hint  . . . . . . . . . . . . . . . . . . . . 6
   4.  The CH-RW Client Hint . . . . . . . . . . . . . . . . . . . . . 6
     4.1.  Confirming Selected DPR . . . . . . . . . . . . . . . . . . 6
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     5.1.  Relationship to the User-Agent Request Header . . . . . . . 7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9



























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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
   [I-D.ietf-httpbis-p2-semantics] 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.

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
   [I-D.ietf-httpbis-p1-messaging], Appendix B.  It includes by
   reference the OWS, field-name and quoted-string rules from that
   document, and the parameter rule from
   [I-D.ietf-httpbis-p2-semantics].


2.  Client Hint Request Header Fields

   A Client Hint request header field is a HTTP header field that is



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   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.

2.1.  Hint Values

   Client-Hint field-values consist of either a token or a comma-
   delimited list of parameters.

     client-hint-value = token | 1#parameter

   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.

2.2.  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.



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   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.

2.3.  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 "CH-DPR" request header field.

2.3.1.  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 CH-DPR and CH-RW
   headers to subsequent requests.

2.3.2.  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: CH-DPR;r=[1.5:]

   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.

     Key: CH-RW;r=[320:640]

   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



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   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: CH-DPR

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

     Vary: CH-DPR, CH-RW

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


3.  The CH-DPR Client Hint

   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.

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


4.  The CH-RW Client Hint

   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.

     CH-RW = 1*DIGIT

4.1.  Confirming Selected DPR

   The "DPR" header field indicates the ratio between physical pixels
   and density independent pixels of the selected response.

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

   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.




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   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.


5.  Examples

   For example, given the following request header:

     CH-DPR: 2.0
     CH-RW: 160

   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:

     DPR: 1.0

   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.

5.1.  Relationship to the User-Agent Request Header

   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.


6.  IANA Considerations

   This document defines the "Accept-CH", "CH-DPR", "CH-RW", and "DPR"
   HTTP request fields, and registers them in the Permanent Message
   Headers registry.






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


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.

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-25 (work in progress),
              November 2013.

   [I-D.ietf-httpbis-p2-semantics]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol



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              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-25 (work in progress),
              November 2013.

   [I-D.nottingham-http-browser-hints]
              Nottingham, M., "HTTP Origin and Hop Hints",
              draft-nottingham-http-browser-hints-05 (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.


Author's Address

   Ilya Grigorik

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

























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