Network Working Group                                      M. Nottingham
Internet-Draft                                            March 16, 2014
Intended status: Informational
Expires: September 17, 2014

                       The "safe" HTTP Preference


   This specification defines a "safe" preference for HTTP, 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 September 17, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . . . 3
   2.  The "safe" Preference . . . . . . . . . . . . . . . . . . . . . 4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 6
   Appendix B.  Using "safe" on Your Web Site  . . . . . . . . . . . . 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 Preference [I-D.snell-http-prefer].

   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" Preference

   When present in a request, the "safe" preference 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" preference:

   GET /foo.html HTTP/1.1
   User-Agent: ExampleBrowser/1.0
   Prefer: safe

   When configured to do so, user agents SHOULD include the "safe"
   preference 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"
   option. additionally, other clients MAY insert it; e.g., an operating
   system might choose to insert the preference in requests based upon
   system-wide configuration, or a proxy might do so based upon its

   Servers that utilise the "safe" preference 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: Prefer

   Note that the Vary response header needs to be sent if responses
   associated with the resource might change depending on the value of
   the "Prefer" header; this is not only true for those responses that
   have changed, but also the "default" unchanged responses.

   NOTE: currently, the safe preference doesn't have a payload, but one
   could be used to indicate a "level" of safety desired; e.g.,
   "safe=hi" or "safe=lo".  Feedback appreciated.

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3.  Security Considerations

   The "safe" preference 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
   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" preference

   o  Preference: safe

   o  Value: (no value)

   o  Description: Indicates that the user (or one responsible for them)
      prefers "safe" or "unobjectionable" content.

   o  Reference: (this document)

   o  Notes:

5.  References

5.1.  Normative References

   [I-D.snell-http-prefer]  Snell, J., "Prefer Header for HTTP",
                            draft-snell-http-prefer-18 (work in
                            progress), January 2013.

   [RFC2119]                Bradner, S., "Key words for use in RFCs to

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                            Indicate Requirement Levels", BCP 14,
                            RFC 2119, March 1997.

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, <

   [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, <

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

Appendix A.  Acknowledgements

   Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso and Jeff Hughes
   for their comments.

Appendix B.  Using "safe" on Your Web Site

   Web sites that allow configuration of a "safe" mode can add support
   for the "safe" preference incrementally; since it will not be
   supported by all clients immediately, it is necessary to still have a
   "manual" safety configuration option.

   When honouring the safe preference, it is important that it not be
   possible to disable it through the Web UI, since "safe" may be
   inserted by an intermediary (e.g., at a school) or configured and

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   locked down by an administrator (e.g., a parent).

   The safe preference is designed to make as much of the Web a "safe"
   experience as possible; it is not intended to be configured site-by-
   site.  Therefore, if the user expresses a wish to disable "safe"
   mode, the site should remind them that the safe preference is being
   sent, and ask them to consult their administrator.

Author's Address

   Mark Nottingham


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