One document matched: draft-nottingham-safe-hint-00.txt
Network Working Group M. Nottingham
Internet-Draft October 21, 2013
Intended status: Informational
Expires: April 24, 2014
The "safe" HTTP Client Hint
draft-nottingham-safe-hint-00
Abstract
This specification defines a "safe" HTTP Client Hint, expressing a
user preference to avoid "objectionable" content.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 24, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. The "safe" HTTP Client Hint . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Many Web sites have a "safe" mode, to assist those who don't want to
be exposed to "objectionable" content, or who don't want their
children to be exposed to such content. YouTube [youtube], Yahoo!
Search [yahoo], Google Search [google], Bing Search [bing], and many
other services have such a setting.
However, a user that wishes to have this preference honoured would
need to go to each Web site in turn, navigate to the appropriate
page, (possibly creating an account along the way) to get a cookie
[RFC6265] set in the browser. They would need to do this for each
browser on every device they use. As has been widely noted, this is
difficult [age-privacy].
This can be onerous to nearly impossible to achieve effectively,
because there are too many permutations of sites, user agents and
devices.
If instead this preference is proactively advertised by the user
agent, things become much simpler. A user agent that supports this
(whether it be an individual browser, or through an Operating System
HTTP library) need only be configured once to assure that the
preference is advertised to all sites that understand and choose to
act upon it. It's no longer necessary to go to each site that has
potentially "unsafe" content and configure a "safe" mode.
Furthermore, a proxy (for example, at a school) can be used to ensure
that the preference is associated with all (unencrypted) requests
flowing through it, helping to assure that clients behind it are not
exposed to "objectionable" content.
This specification defines how to associate this preference with a
request, as a HTTP Client Hint [grigorik-http-client-hints].
Note that this approach does not define what "safe" is; rather, it is
interpreted within the scope of each Web site that chooses to act
upon this information (or not). As such, it does not require
agreement upon what "safe" is, nor does it require application of
policy in the user agent or an intermediary (which can be problematic
for many reasons).
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. The "safe" HTTP Client Hint
When present in a request, the "safe" HTTP Client Hint indicates that
the user prefers content which is not objectionable, according to the
server's definition of the concept.
For example a request that includes the "safe" hint:
GET /foo.html HTTP/1.1
Host: www.example.org
User-Agent: ExampleBrowser/1.0
CH: safe
When configured to do so, user agents SHOULD include the "safe" hint
in every request, to ensure that the preference is applied (where
possible) to all resources.
For example, a Web browser might have a "Request Safe Browsing"
preference option. additionally, other clients MAY insert it; e.g.,
an operating system might choose to insert the hint in requests based
upon system-wide preferences, or a proxy might do so based upon its
configuration.
Servers that utilise the "safe" hint SHOULD document that they do so,
along with the criteria that they use to denote objectionable
content. If a site has more fine-grained degrees of "safety", it
SHOULD select a reasonable default to use, and document that; it MAY
use additional mechanisms (e.g., cookies) to fine-tune.
A response corresponding to the request above might have headers that
look like this:
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: text/html
Server: ExampleServer/2.0
Vary: CH
Note that the Vary response header needs to be sent if responses
associated with the resource might change depending on the value of
the "CH" header; this is not only true for those responses that have
changed, but also the "default" unchanged responses.
3. Security Considerations
The "safe" client hint is not a secure mechanism; it can be inserted
or removed by intermediaries with access to the data stream. Its
presence reveals information about the user, which may be of small
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assistance in "fingerprinting" the user (1 bit of information, to be
precise).
Due to its nature, including it in requests does not assure that all
content will actually be safe; it is only when servers elect to
honour it that it might change content.
Even then, a malicious server might adapt content so that it is even
less "safe" (by some definition of the word). As such, this
mechanism on its own is not enough to assure that only "safe" content
is seen; users who wish to ensure that will need to combine its use
with other techniques (e.g., content filtering).
Furthermore, the server and user may have differing ideas regarding
the semantics of "safe." As such, the "safety" of the user's
experience when browsing from site to site might (and probably will)
change.
4. IANA Considerations
This specification registers the "safe" HTTP Client Hint
[grigorik-http-client-hints]:
o Hint Name: safe
o Hint Value: boolean
o Description: Indicates that the user (or one responsible for them)
prefers "safe" / "unobjectionable" content.
o Contact: mnot@mnot.net
o Specification: (this document)
o Notes:
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[grigorik-http-client-hints] Grigorik, I., "HTTP Client Hints",
2013.
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5.2. Informative References
[RFC6265] Barth, A., "HTTP State Management
Mechanism", RFC 6265, April 2011.
[age-privacy] Moses, A., "Privacy concern as apps
share data from kids left to their own
devices", 2012, <http://
www.theage.com.au/technology/
technology-news/
privacy-concern-as-apps-share-data-
from-kids-left-to-their-own-devices-
20121222-2bso6.html>.
[bing] Microsoft, "Bing Help: Block Explicit
Web Sites", 2013, <http://
onlinehelp.microsoft.com/en-AU/bing/
ff808441.aspx>.
[google] Google, "SafeSearch: turn on or off",
2013, <http://support.google.com/
websearch/bin/
answer.py?p=settings_safesearch&
answer=510>.
[yahoo] Yahoo! Inc., "Yahoo! Search
Preferences", 2013, <http://
search.yahoo.com/preferences/
preferences>.
[youtube] Google, "How to access and turn on
Safety Mode?", 2013, <http://
support.google.com/youtube/bin/
answer.py?answer=174084>.
Appendix A. Acknowledgements
Thanks to Alissa Cooper, Ilya Grigorik and Emma Llanso for their
comments.
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: http://www.mnot.net/
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