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Network Working Group                                      M. Nottingham
Intended status: Informational                              May 30, 2014
Expires: December 1, 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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 1, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The "safe" Preference . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Setting "safe" from Web Browsers . . . . . . . . . .   6
   Appendix C.  Using "safe" on Your Web Site  . . . . . . . . . . .   6

1.  Introduction

   Many Web sites have a "safe" mode, to assist those who don't want to
   be exposed (or have their children exposed) to "objectionable"
   content.  YouTube [youtube], Yahoo!  Search [yahoo], Google Search
   [google], Bing Search [bing], and many other services have such a

   However, those who wish to have this preference honoured 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.

   This is onerous to achieve effectively, because there are so many
   permutations of sites, user agents and devices.

   If this preference is proactively advertised by the user agent,
   things become much simpler.  A user agent that supports doing so
   (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].

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   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].

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
   Host: www.example.org
   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"

   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

   Origin servers that utilize the "safe" preference SHOULD document
   that they do so, along with the criteria that they use to denote
   objectionable content.  If a server 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:

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   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 cacheable
   responses associated with the resource might change depending on the
   value of the "Prefer" header.  This is not only true for those
   responses that are "safe", but also the default "unsafe" response.

   See [I-D.ietf-httpbis-p6-cache] for more information.

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 "safe" in requests does not assure that
   all content will actually be safe; it is only when servers elect to
   honour it that content might be "safe".

   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.

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   o  Reference: (this document)

   o  Notes:

5.  References

5.1.  Normative References

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

5.2.  Informative References

              Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", draft-ietf-
              httpbis-p6-cache-26 (work in progress), February 2014.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

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

   [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,

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Appendix A.  Acknowledgements

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

Appendix B.  Setting "safe" from Web Browsers

   As discussed in Section 2, there are many possible ways for the
   "safe" preference to be generated.  One possibility is for a Web
   browser to allow its users to configure the preference to be sent.

   When doing so, it is important not to misrepresent the preference as
   binding to Web sites.  For example, an appropriate setting might be a
   checkbox with wording such as:

     [] Request "safe" content from Web sites

   ... along with further information available upon request (e.g., from
   a "help" system).

   Browsers might also allow the "safe" preference to be "locked" - that
   is, prevent modification without administrative access, or a

Appendix C.  Using "safe" on Your Web Site

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

   When honouring the safe preference, it is important that it not be
   possible to disable it through the Web interface, since "safe" may be
   inserted by an intermediary (e.g., at a school) or configured and
   locked down by an administrator (e.g., a parent).  When both the
   "safe" preference and per-site configuration are present, the
   preference takes precedence.

   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 (since "safe" might
   be set by an intermediary or locked-down Operating System

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   As explained in Section 2, responses that change based upon the
   presence of the "safe" preference need to either carry the "Vary:
   Prefer" response header field, or be uncacheable by shared caches
   (e.g., with a "Cache-Control: private" response header field).  This
   is to avoid an unsafe cached response being served to a client that
   prefers safe content (or vice versa).

Author's Address

   Mark Nottingham

   EMail: mnot@mnot.net
   URI:   http://www.mnot.net/

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