Network Working Group                                      M. Nottingham
Internet-Draft                                          October 21, 2013
Intended status: Informational
Expires: April 24, 2014

                      The "safe" HTTP Client Hint


   This specification defines a "safe" HTTP Client Hint, expressing a
   user preference to avoid "objectionable" content.

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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

   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",
   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
   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

   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

   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)

4.  IANA Considerations

   This specification registers the "safe" HTTP Client Hint

   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:

   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",

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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://

   [bing]                        Microsoft, "Bing Help: Block Explicit
                                 Web Sites", 2013, <http://

   [google]                      Google, "SafeSearch: turn on or off",
                                 2013, <

   [yahoo]                       Yahoo! Inc., "Yahoo! Search
                                 Preferences", 2013, <http://

   [youtube]                     Google, "How to access and turn on
                                 Safety Mode?", 2013, <http://

Appendix A.  Acknowledgements

   Thanks to Alissa Cooper, Ilya Grigorik and Emma Llanso for their

Author's Address

   Mark Nottingham


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