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HTTP                                                             M. West
Internet-Draft                                                    Google
Intended status: Standards Track                       November 29, 2018
Expires: June 2, 2019


                         The 'Lang' Client Hint
                     draft-west-lang-client-hint-00

Abstract

   This document defines a Client Hint that aims to allow developers to
   opt-in to the ability to perform content negotiation based on the set
   of natural languages preferred by the user agent.  This new mechanism
   is intended to improve upon the privacy properties of the "Accept-
   Language" header, and eventually to supplant it entirely.

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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   This Internet-Draft will expire on June 2, 2019.

Copyright Notice

   Copyright (c) 2018 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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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The 'Lang' Client Hint  . . . . . . . . . . . . . . . . . . .   3
     2.1.  The 'Sec-CH-Lang' Header Field  . . . . . . . . . . . . .   3
     2.2.  Integration with Fetch  . . . . . . . . . . . . . . . . .   4
   3.  Security and Privacy Considerations . . . . . . . . . . . . .   4
     3.1.  Secure Transport  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Access Restrictions . . . . . . . . . . . . . . . . . . .   5
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   5
     4.1.  The 'Accept-Language' Header  . . . . . . . . . . . . . .   5
     4.2.  The 'Sec-CH-' prefix  . . . . . . . . . . . . . . . . . .   6
     4.3.  What about weight?  . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  'Sec-CH-Lang' Header Field  . . . . . . . . . . . . . . .   6
     5.2.  'Accept-Language' Header Field  . . . . . . . . . . . . .   7
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .   8
     A.1.  draft-west-ua-client-hints-00 . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Today, user agents generally specify a set of preferred languages as
   part of each HTTP request by sending the "Accept-Languages" header
   along with each request (defined in Section 5.3.5 of [RFC7231]).
   This header aims to give servers the opportunity to serve users the
   best content available in a language they understand.  For example,
   my browser currently sends the following header:

     Accept-Language: en-US, en;q=0.9, de;q=0.8

   This tells the server something along the lines of "This user prefers
   American English, but will accept any English at all.  If no English-
   languge content is available, try German!".  If the server has
   English-language content, it might redirect the user agent to that
   preferred content.  If not, it might try German.

   In the best case, this kind of content negotiation sincerely improves
   the user experience, giving them legible content they enjoy reading.
   This comes with a cost, however, as language preferences are fairly
   unique, and end up exposing quite a bit of entropy to the web.

   This document proposes a mechanism that might allow user agents to
   reduce the passive fingerprinting surface exposed by the "Accept-



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   Language" header by replacing it with a new "Sec-CH-Lang" Client Hint
   ([I-D.ietf-httpbis-client-hints]) that servers can opt-into
   receiving.  Rather than broadcasting this information to everyone on
   the network, all the time, user agents can make reasonable decisions
   about how to respond to given sites' requests for language
   preferences.

1.1.  Example

   A user navigates to "https://example.com/" for the first time.  Their
   user agent sends no language preferences along with the HTTP request.
   The server, however, is interested in rendering content consistent
   with the users' preferences, and requests this data by sending an
   "Accept-CH" header (Section 2.2.1 of [I-D.ietf-httpbis-client-hints])
   along with the response:

     Accept-CH: Lang

   In response, the user agent includes language preferences in
   subsequent requests:

     Sec-CH-Lang: "en-US", "en", "de"

1.2.  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.

2.  The 'Lang' Client Hint

   The 'Lang' Client Hint exposes a user agent's language preferences to
   a server.  The definitions below assume that each user agent has
   defined a "preferred languages list", which contains an arbitrary
   number of strings adhering to the "language-range" grammar defined in
   Section 2.1 of [RFC4647], and which is sorted in descending order of
   user preference.  The example given above, for instance, might result
   in the list << "en-US", "en", "de" >>.

2.1.  The 'Sec-CH-Lang' Header Field

   The "Sec-CH-Lang" request header field gives a server information
   about a user agent's language preferences.  It is a Structured Header
   ([I-D.ietf-httpbis-header-structure]) whose value MUST be a list
   ([I-D.ietf-httpbis-header-structure], Section 3.2).  Each item in the




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   list MUST be a string ([I-D.ietf-httpbis-header-structure],
   Section 3.7).

   The header's ABNF is:

     Sec-CH-Arch = sh-list

   To generate a "Sec-CH-Lang" header value for a given request, user
   agents MUST:

   1.  If the request's client-hints set includes "Lang", then:

       1.  Let "value" be a Structured Header whose value is an empty
           list.

       2.  For each item in the user agent's "preferred languages list":

           1.  Append the item to "value".

       3.  Set a header in request's header list whose name is "Sec-CH-
           Lang", and whose value is "value".

2.2.  Integration with Fetch

   The Fetch specification should call into the following algorithm in
   place of the current Step 1.4 in its HTTP-network-or-cache fetch
   algorithm.

   To set the language metadata for a request ("r"), the user agent MUST
   execute the following steps:

   1.  If request's header list does not contain "Accept-Language", then
       the user agent MAY append a header whose name is "Accept-
       Language" and whose value corresponds to the requirements in
       Section 5.3.5 of [RFC7231] to "request"'s header list.

   2.  Set request's "Sec-CH-Lang" header, as described in Section 2.1.

3.  Security and Privacy Considerations

3.1.  Secure Transport

   Client Hints will not be delivered to non-secure endpoints (see the
   secure transport requirements in Section 2.2.1 of
   [I-D.ietf-httpbis-client-hints]).  This means that language
   preferences will not be leaked over plaintext channels, reducing the
   opportunity for network attackers to build a profile of a given
   agent's behavior over time.



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3.2.  Delegation

   Client Hints will be delegated from top-level pages via Feature
   Policy (once a few patches against Fetch and Client Hints and Feature
   Policy land.  This reduces the likelihood that language preferences
   will be delivered along with subresource requests, which reduces the
   potential for passive fingerprinting.

   o  Fetch integration of Accept-CH opt-in:
      https://github.com/whatwg/fetch/issues/773

   o  HTML integration of Accept-CH-Lifetime and the ACHL cache:
      https://github.com/whatwg/html/issues/3774

   o  Adding new CH features to the CH list in Fetch:
      https://github.com/whatwg/fetch/issues/725

   o  Other PRs for adding the Feature Policy 3rd party opt-in:
      https://github.com/whatwg/fetch/issues/811 and
      https://github.com/wicg/feature-policy/issues/220

3.3.  Access Restrictions

   Language preferences expose quite a bit of entropy to the web.  User
   agents ought to exercise judgement before granting access to this
   information, and MAY impose restrictions above and beyond the secure
   transport and delegation requirements noted above.  For instance,
   user agents could choose to deliver the "Sec-CH-Lang" header only on
   navigation, but not on subresource requests.  Likewise, they could
   offer users control over the values revealed to servers, or gate
   access on explicit user interaction via a permission prompt or via a
   settings interface.

4.  Implementation Considerations

4.1.  The 'Accept-Language' Header

   User agents SHOULD deprecate the "Accept-Language" header in favor of
   the Client Hints model described in this document.  This deprecation
   can take place in stages, perhaps by first limiting the scopes in
   which the header is sent (navigations not subresources, etc), but the
   goal should be to remove the header entirely in favor of this opt-in
   model.








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4.2.  The 'Sec-CH-' prefix

   Based on some discussion in https://github.com/w3ctag/design-reviews/
   issues/320, it seems reasonable to forbid access to these headers
   from JavaScript, and demarcate them as browser-controlled client
   hints so they can be documented and included in requests without
   triggering CORS preflights.  A "Sec-CH-" prefix seems like a viable
   approach, but this bit might shift as the broader Client Hints
   discussions above coalesce into something more solid that lands in
   specs.

4.3.  What about weight?

   The "Accept-Language" header includes an optional weight along with
   each listed language (e.g.  "en;q=0.3" is less preferred than
   "de;q=0.9").  A potential application of that is expressing equal
   preference for two or more languages, but the challenge of exposing
   such an option to users (compared to an ordered list) seems to make
   practical use unlikely.  Moreover, widely-used implementations of the
   "Accept-Language" header blindly assign weights in exactly this way
   (see Chromium's "HttpUtil::GenerateAcceptLanguageHeader", and
   Firefox's "rust_prepare_accept_languages").

   In this document, I'm boldly (foolishly?) asserting that "q"
   weighting can be removed without impact, in favor of assigning
   semantic meaning to the ordering of the items in the header list.

5.  IANA Considerations

   This document intends to define the "Sec-CH-Lang" HTTP request header
   field, and to register it in the permanent message header field
   registry ([RFC3864]).

   It also intends to deprecate the "Accept-Language" header field.

5.1.  'Sec-CH-Lang' Header Field

   Header field name:  Sec-CH-Lang

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  this specification (Section 2.1)





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5.2.  'Accept-Language' Header Field

   Header field name:  Accept-Language

   Applicable protocol:  http

   Status:  deprecated

   Author/Change controller:  IETF

   Specification document:  this specification (Section 4.1), and
      Section 5.3.5 of [RFC7231]

6.  Normative References

   [I-D.ietf-httpbis-client-hints]
              Grigorik, I., "HTTP Client Hints", draft-ietf-httpbis-
              client-hints-06 (work in progress), July 2018.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-08 (work in progress),
              October 2018.

   [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>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004, <https://www.rfc-
              editor.org/info/rfc3864>.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006,
              <https://www.rfc-editor.org/info/rfc4647>.

   [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>.

   [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>.





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Appendix A.  Changes

A.1.  draft-west-ua-client-hints-00

   o  This specification sprang, fully-formed, from the head of Zeus.

Author's Address

   Mike West
   Google

   Email: mkwst@google.com







































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