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Versions: 00 01 02 03 04 05 06 07 08 09 10 11            Standards Track
Network Working Group                                      M. Nottingham
Intended status: Standards Track                         October 3, 2014
Expires: April 6, 2015

                       The "safe" HTTP Preference


   This specification defines a "safe" preference for HTTP requests,
   expressing a desire 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 6, 2015.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The "safe" Preference . . . . . . . . . . . . . . . . . . . .   3
   3.  Implementation Status . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Setting "safe" from Web Browsers . . . . . . . . . .   7
   Appendix C.  Supporting "safe" on Web Sites . . . . . . . . . . .   7

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"

   However, that goal is often difficult to achieve, because of the 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, for each browser on every device used.

   If this desire 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.

   Furthermore, a proxy (for example, at a school) can associate the
   preference 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 declare this desire in requests as
   a HTTP Preference [RFC7240].

   Note that this specification does not precisely define what "safe"
   is; rather, it is interpreted within the scope of each Web site that
   chooses to act upon this information (or not).

   That said, the intent of "safe" is to allow end users (or those
   acting on their behalf) to express a desire to avoid content that is
   considered "objectionable" within the cultural context of that site;
   usually (but not always) content that is unsuitable for minors.  The
   "safe" preference ought not be used for other purposes.

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   It is also important to note that the "safe" preference is not a
   reliable indicator that the end user is a child; other users might
   have a desire for unobjectionable content, and some children might
   browse without the preference being set.

   Simply put, it is a statement by (or on behalf of) the end user to
   the effect "If your site has a 'safe' setting, this user is hereby
   opting into that, according to your definition of the term."

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
   content which is not objectionable is preferred, 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
   available to all requested resources.

   See Appendix B for advice specific to Web browsers wishing to support

   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 [RFC6265]) to

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   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 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 [RFC7234] Section 4.1 for more information the interaction
   between Vary and Web caching.

   See Appendix C for additional advice specific to Web servers wishing
   to use "safe".

3.  Implementation Status

   _Note to RFC Editor: Please remove this section before publication._

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft.  The description of implementations in this section
   is intended to assist the IETF in its decision processes in
   progressing drafts to RFCs.  Please note that the listing of any
   individual implementation here does not imply endorsement by the
   IETF.  Furthermore, no effort has been spent to verify the
   information presented here that was supplied by IETF contributors.
   This is not intended as, and must not be construed to be, a catalog
   of available implementations or their features.  Readers are advised
   to note that other implementations may exist.

   o  Microsoft Internet Explorer - see http://support.microsoft.com/

   o  Microsoft Bing

   o  Mozilla Firefox - see https://bugzilla.mozilla.org/

   o  Cisco - see http://blogs.cisco.com/security/filtering-explicit-

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   o  YouTube - based upon testing the live site (not formally

4.  Security Considerations

   The "safe" preference is not a secure mechanism; it can be inserted
   or removed by intermediaries with access to the request stream.  Its
   presence reveals limited information about the user, which may be of
   small assistance in "fingerprinting" the user.

   By its nature, including "safe" in requests does not assure that all
   content will actually be safe; it is only when servers elect to honor
   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; those 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)

5.  IANA Considerations

   This specification registers the "safe" preference [RFC7240]:

   o  Preference: safe

   o  Value: (no value)

   o  Description: Indicates that "safe" / "unobjectionable" content is

   o  Reference: (this document)

   o  Notes:

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC7240]  Snell, J., "Prefer Header for HTTP", RFC 7240, June 2014.

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6.2.  Informative References

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

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June

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Appendix A.  Acknowledgements

   Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso, Jeff Hughes,
   Lorrie Cranor, Doug Turner and Dave Crocker 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.  Supporting "safe" on Web Sites

   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 have another way to configure

   When honoring 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).  If per-site
   configuration is present and the safe preference is received, the
   safer interpretation is always used.

   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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