[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-grigorik-http-client-hints) 00 01 02 03 04 05 06 07 08

HTTP Working Group                                           I. Grigorik
Internet-Draft                                                  Y. Weiss
Intended status: Experimental                                     Google
Expires: May 21, 2020                                  November 18, 2019


                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-08

Abstract

   HTTP defines proactive content negotiation to allow servers to select
   the appropriate response for a given request, based upon the user
   agent's characteristics, as expressed in request headers.  In
   practice, clients are often unwilling to send those request headers,
   because it is not clear whether they will be used, and sending them
   impacts both performance and privacy.

   This document defines an Accept-CH response header that servers can
   use to advertise their use of request headers for proactive content
   negotiation, along with a set of guidelines for the creation of such
   headers, colloquially known as "Client Hints."

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/ [1].

   Working Group information can be found at http://httpwg.github.io/
   [2]; source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/client-hints [3].

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 https://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."




Grigorik & Weiss          Expires May 21, 2020                  [Page 1]


Internet-Draft              HTTP Client Hints              November 2019


   This Internet-Draft will expire on May 21, 2020.

Copyright Notice

   Copyright (c) 2019 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
   (https://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  . . . . . . . . . . . . . . . . .   4
   2.  Client Hint Request Header Fields . . . . . . . . . . . . . .   4
     2.1.  Sending Client Hints  . . . . . . . . . . . . . . . . . .   4
     2.2.  Server Processing of Client Hints . . . . . . . . . . . .   5
   3.  Advertising Server Support  . . . . . . . . . . . . . . . . .   5
     3.1.  The Accept-CH Response Header Field . . . . . . . . . . .   5
       3.1.1.  Interaction with Caches . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Accept-CH . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
     6.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Interaction with Variants Response Header Field  . .   9
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .   9
     B.1.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .   9
     B.2.  Since -01 . . . . . . . . . . . . . . . . . . . . . . . .   9
     B.3.  Since -02 . . . . . . . . . . . . . . . . . . . . . . . .   9
     B.4.  Since -03 . . . . . . . . . . . . . . . . . . . . . . . .   9
     B.5.  Since -04 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.6.  Since -05 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.7.  Since -06 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.8.  Since -07 . . . . . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10





Grigorik & Weiss          Expires May 21, 2020                  [Page 2]


Internet-Draft              HTTP Client Hints              November 2019


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.  Applications that want
   to allow the server to optimize content delivery and user experience
   based on such capabilities have, historically, had to rely on passive
   identification (e.g., by matching User-Agent (Section 5.5.3 of
   [RFC7231]) header field against an established database of client
   signatures), used HTTP cookies and URL parameters, or use some
   combination of these and similar mechanisms to enable ad hoc content
   negotiation.

   Such techniques are expensive to setup and maintain, are not portable
   across both applications and servers, and make it hard to reason for
   both client and server about which data is required and is in use
   during the negotiation:

   o  User agent detection cannot reliably identify all static
      variables, cannot infer dynamic client preferences, requires
      external device database, is not cache friendly, and is reliant on
      a passive fingerprinting surface.
   o  Cookie based approaches are not portable across applications and
      servers, impose additional client-side latency by requiring
      JavaScript execution, and are not cache friendly.
   o  URL parameters, similar to cookie based approaches, suffer from
      lack of portability, and are hard to deploy due to a requirement
      to encode content negotiation data inside of the URL of each
      resource.

   Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
   alternative approach; user agents use specified, well-defined request
   headers to advertise their capabilities and characteristics, so that
   servers can select (or formulate) an appropriate response.

   However, proactive content negotiation requires clients to send these
   request headers prolifically.  This causes performance concerns
   (because it creates "bloat" in requests), as well as privacy issues;
   passively providing such information allows servers to silently
   fingerprint the user agent.

   This document defines a new response header, Accept-CH, that allows
   an origin server to explicitly ask that clients send these headers in
   requests.  It also defines guidelines for content negotiation
   mechanisms that use it, colloquially referred to as Client Hints.





Grigorik & Weiss          Expires May 21, 2020                  [Page 3]


Internet-Draft              HTTP Client Hints              November 2019


   Client Hints mitigate the performance concerns by assuring that
   clients will only send the request headers when they're actually
   going to be used, and the privacy concerns of passive fingerprinting
   by requiring explicit opt-in and disclosure of required headers by
   the server through the use of the Accept-CH response header.

   This document defines the Client Hints infrastructure, a framework
   that enables servers to opt-in to specific proactive content
   negotiation features, which will enable them to adapt their content
   accordingly.  However, it does not define any specific features that
   will use that infrastructure.  Those features will be defined in
   their respective specifications.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   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].

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
   client preferences that the server can use to adapt and optimize the
   response.

2.1.  Sending Client Hints

   Clients control which Client Hints are sent in requests, based on
   their default settings, user configuration, and server preferences.
   The client and server can use an opt-in mechanism outlined below to
   negotiate which fields should be sent to allow for efficient content
   adaption, and optionally use additional mechanisms to negotiate
   delegation policies that control access of third parties to same
   fields.

   Implementers should be aware of the passive fingerprinting
   implications when implementing support for Client Hints, and follow
   the considerations outlined in "Security Considerations" section of
   this document.



Grigorik & Weiss          Expires May 21, 2020                  [Page 4]


Internet-Draft              HTTP Client Hints              November 2019


2.2.  Server Processing of Client Hints

   When presented with a request that contains one or more client hint
   header fields, servers can optimize the response based upon the
   information in them.  When doing so, and if the resource is
   cacheable, the server MUST also generate a Vary response header field
   (Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
   selected response and whether the selected response is appropriate
   for a later request.

   Further, depending on the hint used, the server can generate
   additional response header fields to convey related values to aid
   client processing.

3.  Advertising Server Support

   Servers can advertise support for Client Hints using the mechnisms
   described below.

3.1.  The Accept-CH Response Header Field

   The Accept-CH response header field or the equivalent HTML meta
   element with http-equiv attribute ([HTML5]) indicate server support
   for particular hints indicated in its value.

   Accept-CH is a Structured Header [I-D.ietf-httpbis-header-structure].
   Its value MUST be an sh-list (Section 3.1 of
   [I-D.ietf-httpbis-header-structure]) whose members are tokens
   (Section 3.7 of [I-D.ietf-httpbis-header-structure]).  Its ABNF is:

     Accept-CH = sh-list

   For example:

     Accept-CH: Sec-CH-Example, Sec-CH-Example-2

   When a client receives an HTTP response advertising support for
   provided list of Clients Hints, it SHOULD process it as origin
   ([RFC6454]) opt-in to receive Client Hint header fields advertised in
   the field-value, for subsequent same-origin requests.

   o  The opt-in MUST be delivered over a secure transport.
   o  The opt-in SHOULD be persisted and bound to the origin to enable
      delivery of Client Hints on subsequent requests to the server's
      origin, and MUST NOT be persisted for an origin that isn't HTTPS.

     Accept-CH: Sec-CH-Example, Sec-CH-Example-2
     Accept-CH: Sec-CH-Example-3



Grigorik & Weiss          Expires May 21, 2020                  [Page 5]


Internet-Draft              HTTP Client Hints              November 2019


   For example, based on the Accept-CH example above, which is received
   in response to a user agent navigating to "https://example.com", and
   delivered over a secure transport: a user agent SHOULD persist an
   Accept-CH preference bound to "https://example.com" and use it for
   user agent navigations to "https://example.com" and any same-origin
   resource requests initiated by the page constructed from the
   navigation's response.  This preference SHOULD NOT extend to resource
   requests initiated to "https://example.com" from other origins.

3.1.1.  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 generate
   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: Sec-CH-Example

   Above example indicates that the cache key needs to include the Sec-
   CH-Example header field.

     Vary: Sec-CH-Example, Sec-CH-Example-2

   Above example indicates that the cache key needs to include the Sec-
   CH-Example and Sec-CH-Example-2 header fields.

4.  Security Considerations

   The request header fields defined in this document, and those that
   extend it, expose information about the user's environment to enable
   proactive content negotiation.  Such information may reveal new
   information about the user and implementers ought to consider the
   following considerations, recommendations, and best practices.

   Transmitted Client Hints header fields SHOULD NOT provide new
   information that is otherwise not available to the application via
   other means, such as using HTML, CSS, or JavaScript.  Further,
   sending highly granular data, such as image and viewport width may
   help identify users across multiple requests.  Reducing the set of
   field values that can be expressed, or restricting them to an
   enumerated range where the advertised value is close but is not an
   exact representation of the current value, can improve privacy and
   reduce risk of linkability by ensuring that the same value is sent by
   multiple users.  However, such precautions can still be insufficient
   for some types of data, especially data that can change over time.





Grigorik & Weiss          Expires May 21, 2020                  [Page 6]


Internet-Draft              HTTP Client Hints              November 2019


   Implementers ought to consider both user and server controlled
   mechanisms and policies to control which Client Hints header fields
   are advertised:

   o  Implementers SHOULD restrict delivery of some or all Client Hints
      header fields to the opt-in origin only, unless the opt-in origin
      has explicitly delegated permission to another origin to request
      Client Hints header fields.
   o  Implementers MAY provide user choice mechanisms so that users may
      balance privacy concerns with bandwidth limitations.  However,
      implementers should also be aware that explaining the privacy
      implications of passive fingerprinting to users may be
      challenging.
   o  Implementations specific to certain use cases or threat models MAY
      avoid transmitting some or all of Client Hints header fields.  For
      example, avoid transmission of header fields that can carry higher
      risks of linkability.

   Implementers SHOULD support Client Hints opt-in mechanisms and MUST
   clear persisted opt-in preferences when any one of site data,
   browsing history, browsing cache, or similar, are cleared.

5.  IANA Considerations

   This document defines the "Accept-CH" HTTP response field, and
   registers it in the Permanent Message Header Fields registry.

5.1.  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 3.1 of this document
   o  Related information: for Client Hints

6.  References

6.1.  Normative References

   [HTML5]    Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O'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>.






Grigorik & Weiss          Expires May 21, 2020                  [Page 7]


Internet-Draft              HTTP Client Hints              November 2019


   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-14 (work in progress),
              October 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc6265>.




Grigorik & Weiss          Expires May 21, 2020                  [Page 8]


Internet-Draft              HTTP Client Hints              November 2019


   [VARIANTS]
              Nottingham, M., "HTTP Representation Variants", draft-
              ietf-httpbis-variants-05 (work in progress), March 2019.

6.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] http://httpwg.github.io/

   [3] https://github.com/httpwg/http-extensions/labels/client-hints

Appendix A.  Interaction with Variants Response Header Field

   Client Hints may be combined with Variants response header field
   [VARIANTS] to enable fine-grained control of the cache key for
   improved cache efficiency.  Features that define Client Hints will
   need to specify the related variants algorithms as described in
   Section 6 of [VARIANTS].

Appendix B.  Changes

B.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.

B.2.  Since -01

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

B.3.  Since -02

   o  Issue 239: Updated reference to CR-css-values-3
   o  Issue 240: Updated reference for Network Information API
   o  Issue 241: Consistency in IANA considerations
   o  Issue 250: Clarified Accept-CH

B.4.  Since -03

   o  Issue 284: Extended guidance for Accept-CH
   o  Issue 308: Editorial cleanup
   o  Issue 306: Define Accept-CH-Lifetime



Grigorik & Weiss          Expires May 21, 2020                  [Page 9]


Internet-Draft              HTTP Client Hints              November 2019


B.5.  Since -04

   o  Issue 361: Removed Downlink
   o  Issue 361: Moved Key to appendix, plus other editorial feedback

B.6.  Since -05

   o  Issue 372: Scoped CH opt-in and delivery to secure transports
   o  Issue 373: Bind CH opt-in to origin

B.7.  Since -06

   o  Issue 524: Save-Data is now defined by NetInfo spec, dropping
   o  PR 775: Removed specific features to be defined in other
      specifications

B.8.  Since -07

   o  Issue 761: Clarified that the defined headers are response
      headers.
   o  Issue 730: Replaced Key reference with Variants.
   o  Issue 700: Replaced ABNF with structured headers.
   o  PR 878: Removed Accept-CH-Lifetime based on feedback at IETF 105

Acknowledgements

   Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Ben
   Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Hardie,
   Jonas Sicking, Martin Thomson, and numerous other members of the IETF
   HTTP Working Group for invaluable help and feedback.

Authors' Addresses

   Ilya Grigorik
   Google

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


   Yoav Weiss
   Google

   Email: yoav@yoav.ws
   URI:   https://blog.yoav.ws/






Grigorik & Weiss          Expires May 21, 2020                 [Page 10]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/