HTTP Client Hints
draft-ietf-httpbis-client-hints-07

HTTP Working Group                                           I. Grigorik
Internet-Draft                                                    Google
Intended status: Experimental                             March 11, 2019
Expires: September 12, 2019

                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-07

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 two response headers, Accept-CH and Accept-CH-
   Lifetime, 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."

   It also defines an initial set of 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/.

   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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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   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 September 12, 2019.

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
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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
       2.2.1.  Advertising Support via Accept-CH Header Field  . . .   5
       2.2.2.  The Accept-CH-Lifetime Header Field . . . . . . . . .   5
       2.2.3.  Interaction with Caches . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Accept-CH . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Accept-CH-Lifetime  . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Interaction with Key Response Header Field . . . . .   9
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  10
     B.1.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.2.  Since -01 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.3.  Since -02 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.4.  Since -03 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.5.  Since -04 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.6.  Since -05 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.7.  Since -06 . . . . . . . . . . . . . . . . . . . . . . . .  10
     B.8.  Since -07 . . . . . . . . . . . . . . . . . . . . . . . .  11

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   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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
   (Section 5.5.3 of [RFC7231]) header field 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, user agent sniffing has numerous
   limitations:

   o  User agent detection cannot reliably identify all static variables
   o  User agent detection cannot infer any dynamic client preferences
   o  User agent detection requires an external device database
   o  User agent detection is not cache friendly

   A popular alternative strategy is to use HTTP cookies ([RFC6265]) to
   communicate some information about the user agent.  However, this
   approach is also not cache friendly, bound by same origin policy, and
   often imposes additional client-side latency by requiring JavaScript
   execution to create and manage HTTP cookies.

   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, for a period of time bounded by the Accept-CH-Lifetime
   response header.  It also defines guidelines for content negotiation
   mechanisms that use it, colloquially referred to as Client Hints.

   Client Hints mitigate the performance concerns by assuring that
   clients will only send the request headers when they're actually

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   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.

   This document does not supersede or replace the User-Agent header
   field.  Existing device detection mechanisms can continue to use both
   mechanisms if necessary.  By advertising user agent capabilities
   within a request header field, Client Hints allow 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", "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.

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   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.

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.

2.2.1.  Advertising Support via Accept-CH Header Field

   Servers can advertise support for Client Hints using the Accept-CH
   header field or an equivalent HTML meta element with http-equiv
   attribute ([HTML5]).

     Accept-CH = #field-name

   For example:

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

   When a client receives an HTTP response advertising support for
   Client Hints, it should process it as origin ([RFC6454]) opt-in to
   receive Client Hint header fields advertised in the field-value.  The
   opt-in MUST be delivered over a secure transport.

   For example, based on Accept-CH example above, a user agent could
   append the Sec-CH-Example and Sec-CH-Example-2 header fields to all
   same-origin resource requests initiated by the page constructed from
   the response.

2.2.2.  The Accept-CH-Lifetime Header Field

   Servers can ask the client to remember the set of Client Hints that
   the server supports for a specified period of time, to enable
   delivery of Client Hints on subsequent requests to the server's
   origin ([RFC6454]).

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     Accept-CH-Lifetime = #delta-seconds

   When a client receives an HTTP response that contains Accept-CH-
   Lifetime header field, the field-value indicates that the Accept-CH
   preference SHOULD be persisted and bound to the origin, and be
   considered stale after response's age ([RFC7234], section 4.2) is
   greater than the specified number of seconds.  The preference MUST be
   delivered over a secure transport, 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
     Accept-CH-Lifetime: 86400

   For example, based on the Accept-CH and Accept-CH-Lifetime 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" for up to 86400 seconds (1 day), 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.

   If Accept-CH-Lifetime occurs in a message more than once, the last
   value overrides all previous occurrences.

2.2.3.  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.

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3.  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.

   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.

4.  IANA Considerations

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

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4.1.  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 of this document
   o  Related information: for Client Hints

4.2.  Accept-CH-Lifetime

   o  Header field name: Accept-CH-Lifetime
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/Change controller: IETF
   o  Specification document(s): Section 2.2.2 of this document
   o  Related information: for Client Hints

5.  References

5.1.  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>.

   [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>.

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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,
              <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>.

5.2.  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>.

Appendix A.  Interaction with Key Response Header Field

   Client Hints may be combined with Key response header field ([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: Sec-CH-Example;partition=1.5:2.5:4.0

   Above example indicates that the cache key needs to include the value
   of the Sec-CH-Example header field with three segments: less than
   1.5, 1.5 to less than 2.5, and 4.0 or greater.

     Key: Width;Sec-CH-Example=320

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   Above example indicates that the cache key needs to include the value
   of the Sec-CH-Example header field and be partitioned into groups of
   320: 0-320, 320-640, and so on.

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

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

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B.8.  Since -07

   o  Removed specific features to be defined in other specifications

Acknowledgements

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

Author's Address

   Ilya Grigorik
   Google

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

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