Internet DRAFT - draft-west-lang-client-hint
draft-west-lang-client-hint
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 2, 2019.
Copyright Notice
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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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