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Versions: (draft-grigorik-http-client-hints) 00 01 02 03 04 05 06 07 08 09 10 11 12 14

HTTP Working Group                                           I. Grigorik
Internet-Draft                                                  Y. Weiss
Intended status: Experimental                                     Google
Expires: November 19, 2020                                  May 18, 2020


                           HTTP Client Hints
                   draft-ietf-httpbis-client-hints-14

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, user agents 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."




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   This Internet-Draft will expire on November 19, 2020.

Copyright Notice

   Copyright (c) 2020 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
     3.2.  Interaction with Caches . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     4.1.  Information Exposure  . . . . . . . . . . . . . . . . . .   7
     4.2.  Deployment and Security Risks . . . . . . . . . . . . . .   8
     4.3.  Abuse Detection . . . . . . . . . . . . . . . . . . . . .   9
   5.  Cost of Sending Hints . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Accept-CH . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  Since -01 . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.3.  Since -02 . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.4.  Since -03 . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.5.  Since -04 . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.6.  Since -05 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.7.  Since -06 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.8.  Since -07 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.9.  Since -08 . . . . . . . . . . . . . . . . . . . . . . . .  12



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     A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.13. Since -12 . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.14. Since -13 . . . . . . . . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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 user agent preferences.  Historically,
   applications that wanted to allow the server to optimize content
   delivery and user experience based on such capabilities had to rely
   on passive identification (e.g., by matching the User-Agent header
   field (Section 5.5.3 of [RFC7231]) against an established database of
   user agent signatures), use HTTP cookies [RFC6265] and URL
   parameters, or use some combination of these and similar mechanisms
   to enable ad hoc content negotiation.

   Such techniques are expensive to set up and maintain, and are not
   portable across both applications and servers.  They also make it
   hard for both user agent and server to understand which data are
   required and is in use during the negotiation:

   o  User agent detection cannot reliably identify all static
      variables, cannot infer dynamic user agent 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, traditional proactive content negotiation techniques often
   mean that user agents send these request headers prolifically.  This
   causes performance concerns (because it creates "bloat" in requests),




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   as well as privacy issues; passively providing such information
   allows servers to silently fingerprint the user.

   This document defines Client Hints, a framework that enables servers
   to opt-in to specific proactive content negotiation features,
   adapting their content accordingly, as well as guidelines for content
   negotiation mechanisms that use the framework.  This document also
   defines a new response header, Accept-CH, that allows an origin
   server to explicitly ask that user agents send these headers in
   requests.

   Client Hints mitigate performance concerns by assuring that user
   agents will only send the request headers when they're actually going
   to be used, and 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.

   The document does not define specific usages of Client Hints.  Such
   usages need to be defined in their respective specifications.

   One example of such usage is the User Agent Client Hints [UA-CH].

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].

2.  Client Hint Request Header Fields

   A Client Hint request header field is a HTTP header field that is
   used by HTTP user agents to indicate data that can be used by the
   server to select an appropriate response.  Each one conveys user
   agent preferences that the server can use to adapt and optimize the
   response.

2.1.  Sending Client Hints

   User agents choose what Client Hints to send in a request based on
   their default settings, user configuration, and server preferences
   expressed in "Accept-CH".  The user agent and server can use an opt-
   in mechanism outlined below to negotiate which header fields need to
   be sent to allow for efficient content adaption, and optionally use



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   additional mechanisms to negotiate delegation policies that control
   access of third parties to those same header fields.  Without such an
   opt-in, user agents SHOULD NOT send high-entropy hints, but MAY send
   low-entropy ones [CLIENT-HINTS-INFRASTRUCTURE].

   Implementers need to be aware of the passive fingerprinting
   implications when implementing support for Client Hints, and follow
   the considerations outlined in the Security Considerations
   (Section 4) 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.

   Servers MUST ignore hints they do not understand nor support.  There
   is no mechanism for servers to indicate user agents that hints were
   ignored.

   Furthermore, the server can generate additional response header
   fields (as specified by the hint or hints in use) that convey related
   values to aid client processing.

3.  Advertising Server Support

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

3.1.  The Accept-CH Response Header Field

   The Accept-CH response header field indicates server support for the
   hints indicated in its value.  Servers wishing to receive user agent
   information through Client Hints SHOULD add Accept-CH response header
   to their responses as early as possible.

   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 3.3.4 of [I-D.ietf-httpbis-header-structure]).  Its ABNF is:

     Accept-CH = sh-list

   For example:



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

   When a user agent receives an HTTP response containing "Accept-CH",
   that indicates that the origin opts-in to receive the indicated
   request header fields for subsequent same-origin requests.  The opt-
   in MUST be ignored if delivered over non-secure transport (using a
   scheme different from HTTPS).  It SHOULD be persisted and bound to
   the origin to enable delivery of Client Hints on subsequent requests
   to the server's origin, for the duration of the user's session (as
   defined by the user agent).  An opt-in overrides previous persisted
   opt-in values and SHOULD be persisted in its stead.

   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 will have to persist an Accept-
   CH preference bound to "https://example.com".  It will then use it
   for navigations to e.g. "https://example.com/foobar.html", but not to
   e.g. "https://foobar.example.com/".  It will similarly use the
   preference for any same-origin resource requests (e.g. to
   "https://example.com/image.jpg") initiated by the page constructed
   from the navigation's response, but not to cross-origin resource
   requests (e.g. "https://thirdparty.com/resource.js").  This
   preference will not extend to resource requests initiated to
   "https://example.com" from other origins (e.g. from navigations to
   "https://other-example.com/").

3.2.  Interaction with Caches

   When selecting a 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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4.  Security Considerations

4.1.  Information Exposure

   Request header fields used in features relying on this document
   expose information about the user's environment to enable proactive
   content negotiation.  Such information might reveal new information
   about the user and implementers ought to consider the following
   considerations, recommendations, and best practices.

   The underlying assumption is that exposing information about the user
   as a request header is equivalent (from a security perspective) to
   exposing this information by other means.  (For example, if the
   request's origin can access that information using JavsScript APIs,
   and transmit it to its servers).

   Because Client Hints is an explicit opt-in mechanism, that means that
   servers that want access to information about the user's environment
   need to actively ask for it, enabling clients and privacy researchers
   to keep track of which origins collect that data, and potentially act
   upon it.  The header-based opt-in means that we can remove passive
   fingerprinting vectors, such as the User-Agent string (enabling
   active access to that information through User-Agent Client Hints
   [4]), or otherwise expose information already available through
   script (e.g. the Save-Data Client Hint [5]), without increasing the
   passive fingerprinting surface.

   Therefore, features relying on this document to define Client Hint
   headers MUST NOT provide new information that is otherwise not
   available to the application via other means, such as existing
   request headers, HTML, CSS, or JavaScript.

   Such features need to take into account the following aspects of the
   information exposed:

   o  Entropy - Exposing highly granular data can be used to help
      identify users across multiple requests to different origins.
      Reducing the set of header 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.
   o  Sensitivity - The feature SHOULD NOT expose user-sensitive
      information.  To that end, information available to the
      application, but gated behind specific user actions (e.g. a
      permission prompt or user activation) SHOULD NOT be exposed as a
      Client Hint.




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   o  Change over time - The feature SHOULD NOT expose user information
      that changes over time, unless the state change itself is also
      exposed (e.g. through JavaScript callbacks).

   Different features will be positioned in different points in the
   space between low-entropy, non-sensitive and static information (e.g.
   user agent information), and high-entropy, sensitive and dynamic
   information (e.g. geolocation).  User agents need to consider the
   value provided by a particular feature vs these considerations, and
   MAY have different policies regarding that tradeoff on a per-feature
   basis.

   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 considering providing user choice mechanisms that
      allow users to balance privacy concerns against bandwidth
      limitations need to also consider that explaining to users the
      privacy implications involved, such as the risks of passive
      fingerprinting, is challenging and likely impractical.
   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, cookies, or similar, are cleared.

4.2.  Deployment and Security Risks

   Deployment of new request headers requires several considerations:

   o  Potential conflicts due to existing use of header field name
   o  Properties of the data communicated in header field value

   Authors of new Client Hints are advised to carefully consider whether
   they need to be able to be added by client-side content (e.g.,
   scripts), or whether they need to be exclusively set by the user
   agent.  In the latter case, the Sec- prefix on the header field name
   has the effect of preventing scripts and other application content
   from setting them in user agents.  Using the "Sec-" prefix signals to




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   servers that the user agent - and not application content - generated
   the values.  See [FETCH] for more information.

   By convention, request headers that are Client Hints are encouraged
   to use a CH- prefix, to make them easier to identify as using this
   framework; for example, CH-Foo or, with a "Sec-" prefix, Sec-CH-Foo.
   Doing so makes them easier to identify programmatically (e.g., for
   stripping unrecognised hints from requests by privacy filters).

4.3.  Abuse Detection

   A user agent that tracks access to active fingerprinting information
   SHOULD consider emission of Client Hints headers similarly to the way
   it would consider access to the equivalent API.

   Research into abuse of Client Hints might look at how HTTP responses
   that contain Client Hints differ from those with different values,
   and from those without.  This might be used to reveal which Client
   Hints are in use, allowing researchers to further analyze that use.

5.  Cost of Sending Hints

   While HTTP header compression schemes reduce the cost of adding HTTP
   header fields, sending Client Hints to the server incurs an increase
   in request byte size.  Servers SHOULD take that into account when
   opting in to receive Client Hints, and SHOULD NOT opt-in to receive
   hints unless they are to be used for content adaptation purposes.

   Due to request byte size increase, features relying on this document
   to define Client Hints MAY consider restricting sending those hints
   to certain request destinations [FETCH], where they are more likely
   to be useful.

6.  IANA Considerations

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

6.1.  Accept-CH

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





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7.  References

7.1.  Normative References

   [FETCH]    van Kesteren, A., "Fetch", n.d.,
              <https://fetch.spec.whatwg.org/>.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", draft-ietf-httpbis-header-structure-18 (work in
              progress), April 2020.

   [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>.

   [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>.

7.2.  Informative References

   [CLIENT-HINTS-INFRASTRUCTURE]
              Weiss, Y., "Client Hints Infrastructure", n.d.,
              <https://wicg.github.io/client-hints-infrastructure/>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [UA-CH]    West, M. and Y. Weiss, "User Agent Client Hints", n.d.,
              <https://wicg.github.io/ua-client-hints/>.



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7.3.  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

   [4] https://wicg.github.io/ua-client-hints/#http-ua-hints

   [5] https://wicg.github.io/savedata/#save-data-request-header-field

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.
   o  Changed document status to experimental.

A.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

A.4.  Since -03

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

A.5.  Since -04

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






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A.6.  Since -05

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

A.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

A.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

A.9.  Since -08

   o  PR 985: Describe the bytesize cost of hints.
   o  PR 776: Add Sec- and CH- prefix considerations.
   o  PR 1001: Clear CH persistence when cookies are cleared.

A.10.  Since -09

   o  PR 1064: Fix merge issues with "cost of sending hints".

A.11.  Since -10

   o  PR 1072: LC feedback from Julian Reschke.
   o  PR 1080: Improve list style.
   o  PR 1082: Remove section mentioning Variants.
   o  PR 1097: Editorial feedback from mnot.
   o  PR 1131: Remove unused references.
   o  PR 1132: Remove nested list.

A.12.  Since -11

   o  PR 1134: Re-insert back section.

A.13.  Since -12

   o  PR 1160: AD review.






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A.14.  Since -13

   o  PR 1171: Genart review.

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/


























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