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</style><link charset="utf-8" type="text/css" rel="stylesheet" href="http://www.w3.org/StyleSheets/TR/W3C-WD"></head>
<body style="display: inherit;"><div class="head"><p><a href="http://www.w3.org/"><img src="http://www.w3.org/Icons/w3c_home" alt="W3C" height="48" width="72"></a></p><h1 class="title" id="title">Tracking Preference Expression (DNT)</h1><h2 id="w3c-working-draft-14-november-2011">W3C Working Draft 14 November 2011</h2><dl><dt>This version:</dt><dd><a href="http://www.w3.org/TR/2011/WD-tracking-dnt-20111114/">http://www.w3.org/TR/2011/WD-tracking-dnt-20111114/</a></dd><dt>Latest published version:</dt><dd><a href="http://www.w3.org/TR/tracking-dnt/">http://www.w3.org/TR/tracking-dnt/</a></dd><dt>Latest editor's draft:</dt><dd><a href="http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html">http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html</a></dd><dt>Editor:</dt><dd><a href="http://roy.gbiv.com/">Roy T. Fielding</a>, <a href="http://www.adobe.com/">Adobe</a></dd>
</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2011 <a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.</p><hr></div>
<div class="introductory section" id="abstract"><h2>Abstract</h2>
This specification defines the technical mechanisms for expressing a
cross-site tracking preference via the <a class="internalDFN" href="#dfn-dnt-1">DNT</a> request header field in
HTTP, via an HTML DOM property readable by embedded scripts, and via
properties accessible to various user agent plug-in or extension APIs.
It also defines mechanisms for sites to signal whether and how they
honor this preference, both in the form of a machine-readable policy
at a well-known location for first-party sites and a <q>Tracking</q>
response header field for third-party resources that engage in
cross-site tracking, and a mechanism for allowing the user to approve
site-specific exceptions to DNT as desired.
</div><div id="sotd" class="introductory section"><h2>Status of This Document</h2><p><em>This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the <a href="http://www.w3.org/TR/">W3C technical reports index</a> at http://www.w3.org/TR/.</em></p>
<p>This draft does not represent a final working group consensus, though an attempt has been made to highlight areas where issues have been identified and present multiple alternatives if they have been discussed.</p>
<p>This document was published by the <a href="http://www.w3.org/2011/tracking-protection/">Tracking Protection Working Group</a> as a First Public Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to <a href="mailto:public-tracking@w3.org">public-tracking@w3.org</a> (<a href="mailto:public-tracking-request@w3.org?subject=subscribe">subscribe</a>, <a href="http://lists.w3.org/Archives/Public/public-tracking/">archives</a>). All feedback is welcome.</p><p>Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.</p><p>This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>. W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/49311/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.</p></div><div class="section" id="toc"><h2 class="introductory">Table of Contents</h2><ul class="toc"><li class="tocline"><a class="tocxref" href="#introduction"><span class="secno">1. </span>Introduction</a></li><li class="tocline"><a class="tocxref" href="#notational"><span class="secno">2. </span>Notational Conventions</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#requirements"><span class="secno">2.1 </span>Requirements</a></li><li class="tocline"><a class="tocxref" href="#notation"><span class="secno">2.2 </span>Formal Syntax</a></li><li class="tocline"><a class="tocxref" href="#terminology"><span class="secno">2.3 </span>Terminology</a></li></ul></li><li class="tocline"><a class="tocxref" href="#determining"><span class="secno">3. </span>Determining User Preference</a></li><li class="tocline"><a class="tocxref" href="#expressing"><span class="secno">4. </span>Expressing a Tracking Preference</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#dnt-header-field"><span class="secno">4.1 </span>DNT request header field</a></li><li class="tocline"><a class="tocxref" href="#js-dom"><span class="secno">4.2 </span>HTML DOM Interfaces</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#attributes"><span class="secno">4.2.1 </span>Attributes</a></li><li class="tocline"><a class="tocxref" href="#js-implements"><span class="secno">4.2.2 </span>Implements</a></li></ul></li><li class="tocline"><a class="tocxref" href="#plug-ins"><span class="secno">4.3 </span>Plug-In APIs</a></li></ul></li><li class="tocline"><a class="tocxref" href="#responding"><span class="secno">5. </span>Communicating a Tracking Status</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#response-goals"><span class="secno">5.1 </span>Goals</a></li><li class="tocline"><a class="tocxref" href="#response-criteria"><span class="secno">5.2 </span>Criteria</a></li><li class="tocline"><a class="tocxref" href="#response-options"><span class="secno">5.3 </span>Options</a></li><li class="tocline"><a class="tocxref" href="#response-policy"><span class="secno">5.4 </span>Machine-readable Tracking Policy</a></li><li class="tocline"><a class="tocxref" href="#response-header-field"><span class="secno">5.5 </span>Tracking response header field</a></li><li class="tocline"><a class="tocxref" href="#response-error"><span class="secno">5.6 </span>Status code for Tracking Required</a></li><li class="tocline"><a class="tocxref" href="#exceptions"><span class="secno">5.7 </span>Site-specific Exceptions</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#permanent-exceptions"><span class="secno">5.7.1 </span>Use case: Site-specific exceptions should persist</a></li></ul></li></ul></li><li class="tocline"><a class="tocxref" href="#acknowledgements"><span class="secno">A. </span>Acknowledgements</a></li><li class="tocline"><a class="tocxref" href="#resolved"><span class="secno">B. </span>Closed Issues</a></li><li class="tocline"><a class="tocxref" href="#postponed"><span class="secno">C. </span>Postponed Issues</a></li><li class="tocline"><a class="tocxref" href="#references"><span class="secno">D. </span>References</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#normative-references"><span class="secno">D.1 </span>Normative references</a></li><li class="tocline"><a class="tocxref" href="#informative-references"><span class="secno">D.2 </span>Informative references</a></li></ul></li></ul></div>
<div class="section" id="introduction">
<!--OddPage--><h2><span class="secno">1. </span>Introduction</h2>
<p>
The World Wide Web (WWW, or Web) consists of millions of sites
interconnected through the use of hypertext. Hypertext provides a
simple, page-oriented view of a wide variety of information that
can be traversed by selecting links, manipulating controls, and
supplying data via forms and search dialogs. A Web page is usually
composed of many different information sources beyond the initial
resource request, including embedded references to stylesheets,
inline images, javascript, and other elements that might be
automatically requested as part of the rendering or behavioral
processing defined for that page.
</p>
<p>
Each of the hypertext actions and each of the embedded resource
references might refer to any site on the Web, leading to a seamless
interaction with the user even though the pages might be composed of
information requested from many different and possibly independent
Web sites. From the user's perspective, they are simply visiting and
interacting with a single brand — the <dfn id="dfn-first-party">first-party</dfn> Web
property — and all of the technical details and protocol mechanisms
that are used to compose a page representing that brand are hidden
behind the scenes.
</p>
<p>
It has become common for Web site owners to collect data regarding
the usage of their sites for a variety of purposes, including what
led the user to visit their site (referrals), how effective the user
experience is within the site (web analytics), and the nature of who
is using their site (audience segmentation). In some cases, the data
collected is used to dynamically adapt the content (personalization)
or the advertising presented to the user (targeted advertising).
Data collection can occur both at the first-party site and via
third-party analytics providers through the insertion of tracking
elements on each page.
</p>
<p>
Advertising revenue is the single largest source of funding on the
Web. Since advertisers desire an audience that is receptive to
whatever they happen to be advertising, a significant premium is
assigned to sites that can demonstrate a favorable target audience,
and even more so for sites that are able to identify their audience
dynamically and adjust the advertising displayed to be specific to
the interests of that user. In an attempt to better understand or
predict those interests, some advertising mechanisms follow a user's
actions over time, collect data on the observed behavior, and use
that data for targeting future advertisements: a practice commonly
referred to as online behavioral advertising (OBA).
</p>
<p>
Like analytics data collection, Web sites often contract with
third-party advertising networks for the tasks of selecting,
delivering, and measuring the advertising shown on their sites, while
advertisers often contract with third-party verification companies
to provide independent accounting of ad impressions and fraud
detection.
</p>
<p>
There are numerous techniques for integrating advertising
networks into a website, though most involve some form of embedded
resource request to a site controlled by the advertising network.
Since the advertising networks are supplying ads for multiple sites,
they are capable of monitoring how often a given ad is displayed
to that same user agent across their entire network (frequency capping).
Naturally, advertisers consider frequency capping to be a desirable
feature, and thus it is common for advertisers to contractually limit
advertising campaigns to a maximum impression count per user. As a
result, advertising networks track users from site to site
even when OBA is not in use.
</p>
<p>
In many cases, Web users welcome the use of data collection for
personalization and targeted advertising, since it can allow a site
to tailor the user experience to their specific desires, reduce ads
that are irrelevant or repetitive, and avoid the imposition of more
direct revenue in the form of subscription-only services.
In other cases, personalization and targeting can be perceived as
<q>creepy</q>, intrusive, and sometimes simply incorrect.
In particular, targeting and personalization can evoke strong
negative feelings when data collected at a trusted site is used,
without the user's consent, for targeting ads on some other site
with which they have no personal trust relationship. When cross-site
tracking or cross-site sharing of data collection does not match the
user's expectations regarding privacy, the result can be a very
angry customer.
</p>
<p>
None of the participants in this Web of customization and targeted
advertising want to offend the user. For advertisers, it is
counterproductive. For Web site owners, it drives away their
audience and income. For advertising networks, it leads to blocking
and lost advertisers. Therefore, we need a mechanism for the user
to express their own preference regarding cross-site tracking that is
both simple to configure and efficient when implemented.
Likewise, since some Web sites may be dependent on the revenue
obtained from targeted advertising and unwilling (or unable) to
permit use of their content without cross-site data collection,
we need a mechanism for sites to alert the user to those requirements
and allow the user to configure an exception to DNT for specific sites.
</p>
<p>
This specification defines the HTTP request header field <a class="internalDFN" href="#dfn-dnt-1">DNT</a> for
expressing a tracking preference on the Web, a well-known location
(URI) for providing a machine-readable site-wide policy regarding DNT
compliance, and the HTTP response header field <a>Tracking</a> for
third-party resources engaged in dynamic tracking behavior to
communicate their compliance or non-compliance with the user's
expressed preference.
</p>
<p>
A companion document, <q><a href="http://www.w3.org/TR/tracking-compliance/">Tracking
Compliance and Scope</a></q>, more precisely defines the terminology
of tracking preferences, the scope of its applicability, and the
requirements on compliant first-party and third-party participants
when an indication of tracking preference is received.
</p>
</div>
<div class="section" id="notational">
<!--OddPage--><h2><span class="secno">2. </span>Notational Conventions</h2>
<div class="section" id="requirements">
<h3><span class="secno">2.1 </span>Requirements</h3>
<p>The key words <em title="must" class="rfc2119">must</em>,
<em title="must not" class="rfc2119">must not</em>,
<em title="required" class="rfc2119">required</em>,
<em title="should" class="rfc2119">should</em>,
<em title="should not" class="rfc2119">should not</em>,
<em title="recommended" class="rfc2119">recommended</em>,
<em title="may" class="rfc2119">may</em>, and
<em title="optional" class="rfc2119">optional</em> in this
specification are to be interpreted as described in
[<cite><a href="#bib-RFC2119" rel="biblioentry" class="bibref">RFC2119</a></cite>].</p>
</div>
<div class="section" id="notation">
<h3><span class="secno">2.2 </span>Formal Syntax</h3>
<p>This specification uses Augmented Backus-Naur Form [<cite><a href="#bib-ABNF" rel="biblioentry" class="bibref">ABNF</a></cite>]
to define network protocol syntax and WebIDL [<cite><a href="#bib-WEBIDL" rel="biblioentry" class="bibref">WEBIDL</a></cite>] for
defining scripting APIs.</p>
</div>
<div class="section" id="terminology">
<h3><span class="secno">2.3 </span>Terminology</h3>
<p>
This specification uses the term <dfn id="dfn-user-agent">user agent</dfn> to refer to
any of the various client programs capable of initiating HTTP
requests, including browsers, spiders (web-based robots),
command-line tools, native applications, and mobile apps [<cite><a href="#bib-HTTP11" rel="biblioentry" class="bibref">HTTP11</a></cite>].
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/13">ISSUE-13 </a>: What are the requirements for DNT on apps/native software in addition to browsers?<br>
<strong>[PENDING REVIEW]</strong>
The above paragraph aims at resolving this issue.
</p>
</div>
</div>
<div class="section" id="determining">
<!--OddPage--><h2><span class="secno">3. </span>Determining User Preference</h2>
<p>
The goal of this protocol is to allow a user to express their
personal preference regarding cross-site tracking to each server and
web application that they communicate with via HTTP, thereby allowing
each server to either adjust their behavior to meet the user's
expectations or reach a separate agreement with the user to satisfy
both parties. Key to that notion of expression is that it <em title="must" class="rfc2119">must</em>
reflect the user's preference, not the preference of some
institutional or
network-imposed mechanism outside the user's control.
</p>
<p>
The remainder of this specification defines the protocol in terms
of whether DNT is <dfn id="dfn-enabled">enabled</dfn> or <dfn id="dfn-not-enabled">not enabled</dfn>.
We do not specify how that preference is configured:
the user agent is responsible for determining the user experience
by which this preference is set.
</p>
<p>
For example, a user might configure their own user agent to
tell servers <q>do not track me cross-site</q>, install a plug-in
or extension that is specifically designed to add that expression,
or make a choice for privacy that then implicitly includes a
tracking preference (e.g., <q>Privacy settings: high</q>). For each
of these cases, we say that DNT is <a class="internalDFN" href="#dfn-enabled">enabled</a>.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/4">ISSUE-4</a>: What is the default for DNT in client configuration (opt-in or opt-out)?<br>
<strong>[PENDING REVIEW]</strong>
Proposed text above.
</p>
</div>
<div class="section" id="expressing">
<!--OddPage--><h2><span class="secno">4. </span>Expressing a Tracking Preference</h2>
<p>
When a user has configured a tracking preference, that preference
needs to be expressed to all mechanisms that might perform or
initiate tracking by third parties, including sites that the user agent
communicates with via HTTP, scripts that can extend behavior on
pages, and plug-ins or extensions that might be installed and
activated for various media types.
</p>
<div class="section" id="dnt-header-field">
<h3><span class="secno">4.1 </span>DNT request header field</h3>
<p>
The <dfn id="dfn-dnt">DNT</dfn> header field is hereby defined as the means for
expressing a user's tracking preference via HTTP [<cite><a href="#bib-HTTP11" rel="biblioentry" class="bibref">HTTP11</a></cite>].
A user agent <em title="must" class="rfc2119">must</em> send the <dfn id="dfn-dnt-1">DNT</dfn> header field on all HTTP
requests if (and only if) DNT is <a class="internalDFN" href="#dfn-enabled">enabled</a>. A user
agent <em title="must not" class="rfc2119">must not</em> send the <a class="internalDFN" href="#dfn-dnt-1">DNT</a> header field if DNT is
<a class="internalDFN" href="#dfn-not-enabled">not enabled</a>.
</p>
<pre class="abnf"><dfn id="dfn-dnt-field-name">DNT-field-name</dfn> = "DNT" ; case-insensitive
<dfn id="dfn-dnt-field-value">DNT-field-value</dfn> = ( "0" / "1" ) *DNT-extension ; case-sensitive
<dfn id="dfn-dnt-extension">DNT-extension</dfn> = %x21-2B / %x2D-7E ; visible ASCII except ","
</pre>
<p>
The DNT field-value sent by a user agent <em title="must" class="rfc2119">must</em> begin with the
character "1" (%x31) if DNT is <a class="internalDFN" href="#dfn-enabled">enabled</a> and there is not,
to the user agent's knowledge, a specific exception for the origin
server targeted by this request.
If DNT is <a class="internalDFN" href="#dfn-enabled">enabled</a> and there is a specific exception for
the target origin server via some mechanism understood by the
user agent, then the DNT field-value sent by a user agent <em title="must" class="rfc2119">must</em> begin
with the character "0" (%x30).
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/78">ISSUE-78</a>: What is the difference between absence of DNT header and DNT = 0?<br>
<strong>[PENDING REVIEW]</strong>
Proposed text above defines that a "0" may only be sent when DNT is
enabled and some mechanism known to the user agent has specifically
made an exception for this origin server. Note that we have not
defined such a mechanism (and probably won't do so). If DNT is
disabled or not implemented, no DNT header field is sent.
</p>
<pre class="example">GET /something/here HTTP/1.1
Host: example.com
DNT: 1</pre>
<p>
An HTTP intermediary <em title="must not" class="rfc2119">must not</em> add, delete, or modify the DNT header
field in requests forwarded through that intermediary unless that
intermediary has been specifically installed or configured to do so
by the user making the requests. For example, an Internet Service
Provider <em title="must not" class="rfc2119">must not</em> inject <q>DNT: 1</q> on behalf of all of their
users who have not selected a choice.
</p>
<p>
The remainder of the DNT field-value after the initial character is
reserved for future extensions. User agents that do not implement
such extensions <em title="must not" class="rfc2119">must not</em> send DNT-extension characters in the DNT
field-value. Servers that do not implement such extensions <em title="should" class="rfc2119">should</em>
ignore anything beyond the first character.
</p>
<p>
DNT extensions are to be interpreted as modifiers to the
main preference expressed by the first digit, such that the main
preference will be obeyed if the recipient does not understand the
extension. Hence, a DNT-field-value of "1xyz" can be thought of
as <q>DNT is enabled, but if you understand the refinements defined
by x, y, or z, then adjust my preferences according to those
refinements.</q> Extensions can only transmitted if DNT is
<a class="internalDFN" href="#dfn-enabled">enabled</a>. The extension syntax excludes the comma (",")
character in order to to differentiate valid field values from an
invalid occurrence of multiple DNT header fields that have been
combined as a single comma-separated list by a generic HTTP parser.
</p>
<p class="note">
Designers of future extensions should note that, if enabled,
DNT is sent on every request and is thus in the critical
path for a server attempting to read and act on every request.
Use as few extension characters as possible.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/82">ISSUE-82</a>: Should the DNT header be extensible with additional parameters?<br>
<strong>[PENDING REVIEW]</strong>
The above paragraphs allow for an extension string. At this point,
no extensions have been defined.
</p>
</div>
<div class="section" id="js-dom">
<h3><span class="secno">4.2 </span>HTML DOM Interfaces</h3>
<p>
The <a class="idlType" href="#idl-def-NavigatorDoNotTrack"><code>NavigatorDoNotTrack</code></a> interface provides a means for
the user's cross-site tracking preference to be expressed to
web applications running within a page rendered by the user agent.
</p>
<pre class="idl"><span class="idlInterface" id="idl-def-NavigatorDoNotTrack">[<span class="extAttr">NoInterfaceObject</span>]
interface <span class="idlInterfaceID">NavigatorDoNotTrack</span> {
<span class="idlAttribute"> readonly attribute <span class="idlAttrType"><a>DOMString</a></span> <span class="idlAttrName"><a href="#widl-NavigatorDoNotTrack-doNotTrack">doNotTrack</a></span>;</span>
};</span>
</pre><div class="section" id="attributes"><h4><span class="secno">4.2.1 </span>Attributes</h4><dl class="attributes"><dt id="widl-NavigatorDoNotTrack-doNotTrack"><code>doNotTrack</code> of type <span class="idlAttrType"><a>DOMString</a></span>, readonly</dt><dd>
When DNT is <a class="internalDFN" href="#dfn-enabled">enabled</a>, the doNotTrack attribute <em title="must" class="rfc2119">must</em> have a
string value that is the same as the <a class="internalDFN" href="#dfn-dnt-field-value">DNT-field-value</a>
defined in <a href="#dnt-header-field" class="sectionRef">section 4.1 DNT request header field</a>.
If DNT is <a class="internalDFN" href="#dfn-not-enabled">not enabled</a>, the value is <code>null</code>.
<div><em>No exceptions.</em></div></dd></dl></div>
<div class="section" id="js-implements">
<h4><span class="secno">4.2.2 </span>Implements</h4>
<pre class="idl"><span class="idlImplements"><a>Navigator</a> implements <a class="idlType" href="#idl-def-NavigatorDoNotTrack"><code>NavigatorDoNotTrack</code></a>;</span></pre><div class="idlImplementsDesc">
Objects implementing the <code>Navigator</code> interface
[<cite><a href="#bib-NAVIGATOR" rel="biblioentry" class="bibref">NAVIGATOR</a></cite>] (e.g., the <code>window.navigator</code> object)
<em title="must" class="rfc2119">must</em> also implement the <code>NavigatorDoNotTrack</code>
interface.
An instance of <code>NavigatorDoNotTrack</code> is obtained
by using binding-specific casting methods on an instance of
<code>Navigator</code>.
</div>
</div>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/84">ISSUE-84</a>: Do we need a JavaScript API / DOM property for client-side js access to Do Not Track status?<br>
<strong>[PENDING REVIEW]</strong>
We believe that we need such an API. This section proposes one.
</p>
</div>
<div class="section" id="plug-ins">
<h3><span class="secno">4.3 </span>Plug-In APIs</h3>
<p>
User agents often include user-installable component parts,
commonly known as <dfn id="dfn-plug-ins">plug-ins</dfn> or
<dfn id="dfn-browser-extensions">browser extensions</dfn>, that are capable of making their own
network requests. From the user's perspective, these components
are considered part of the user agent and thus ought to respect the
user's configuration of a tracking preference. However, plug-ins
do not normally have read access to the browser configuration.
Therefore, we will define here various mechanisms for communicating
the DNT preference via common plug-in APIs.
</p>
</div>
</div>
<div class="section" id="responding">
<!--OddPage--><h2><span class="secno">5. </span>Communicating a Tracking Status</h2>
The companion document, <q><a href="http://www.w3.org/TR/tracking-compliance/">Tracking
Compliance and Scope</a></q>, defines how service providers are
expected to comply when they receive an expression of the user's
tracking preference via any of the mechanisms described in
<a href="#expressing" class="sectionRef">section 4. Expressing a Tracking Preference</a>.
<p></p>
<p>
If no DNT preference is received, it may indicate either that
the user has chosen to allow cross-site tracking or that their
user agent does not support this protocol for expressing DNT
(e.g., user agents deployed prior to this protocol's existence).
In the absence of regulatory, legal, or other requirements, servers
are free to interpret the lack of a DNT header as they find most
appropriate for the given user, particularly when considered in
light of the user's privacy expectations and cultural circumstances.
</p>
<p>
This section defines how a server communicates its compliance with
tracking preferences, including whether it will honor the user's
preference, require some form of site-specific exception, or indicate
that it already has the user's permission via some other agreement
(e.g., a subscription or account agreement). Optionally, links can be
provided to human-readable information regarding the site's tracking
policies or where to go to opt-in, opt-out, or edit their personal
information.
</p>
<div class="section" id="response-goals">
<h3><span class="secno">5.1 </span>Goals</h3>
<p>
The following goals have been identified as reasons for having a
response from the server:
</p>
<ol>
<li>Auditing compliance by servers</li>
<li>Measuring deployment of DNT</li>
<li>Transparency
<ul>
<li>allow user awareness of DNT status per-site/element</li>
<li>indicate what elements on page have ack'd/honored DNT</li>
</ul>
</li>
<li>Guidance for site-specific exceptions</li>
</ol>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/98">ISSUE-98</a>: Consider applicable laws and regulations, such as Article 5(3) of the EU ePrivacy Directive</p>
</div>
<div class="section" id="response-criteria">
<h3><span class="secno">5.2 </span>Criteria</h3>
<p>
The following criteria have been identified as constraints on the
response design:
</p>
<ol>
<li>Minimize impact on cacheable responses</li>
<li>Simplicity</li>
<li>Scalability</li>
<li>Express fine-grained track/no-track for pieces of a site</li>
</ol>
</div>
<div class="section" id="response-options">
<h3><span class="secno">5.3 </span>Options</h3>
<p>
There have been many suggestions, but not much consensus, on how
servers ought to respond when DNT is enabled. The various
suggestions can be roughly categorized as follows:
</p>
<ul>
<li>No response.</li>
<li>A well-known location for machine-readable site-wide policy.</li>
<li>A static link header field for machine-readable policy.</li>
<li>A static header field indicating that DNT is honored.</li>
<li>A dynamic header field indicating that tracking is enabled or
disabled for this user (and why).</li>
</ul>
<p>
and also some combinations of the above. For example, we might
define that compliant servers provide a machine-readable site-wide
policy that indicates how they honor DNT, what sites are considered
the same brand, and links to resources for providing site-specific
exceptions to DNT or editing collected tracking data. We could
then limit use of a tracking response header field to only those
dynamic responses for third-party resources that engage in tracking.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/81">ISSUE-81</a>: Do we need a response at all from server?<br>
<strong>[PENDING REVIEW]</strong>
Yes: The users expect to be able to see whether a DNT header is
accepted, rejected, or sent into the void.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/79">ISSUE-79</a>: Should a server respond if a user sent DNT:0?</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/51">ISSUE-51</a>: Should 1st party have any response to DNT signal</p>
</div>
<div class="section" id="response-policy">
<h3><span class="secno">5.4 </span>Machine-readable Tracking Policy</h3>
<p>This can be defined as either a well-known location, as
defined by RFC5785, or as a Link header field sent in response
to any request (regardless of DNT).</p>
<ul>
<li>e.g., "http://example.com/.well-known/tracking"</li>
<li>machine-readable (JSON or similar)</li>
<li>include booleans for compliance</li>
<li>optionally include list of domains for same-brand scope</li>
<li>optionally include link to user-readable policy document</li>
<li>optionally include link to opt-in/out form</li>
<li>optionally include link to edit data collected</li>
<li>accessible before making use of site's services</li>
<li>allows third-party sites to indicate their own policies</li>
</ul>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/47">ISSUE-47</a>: Should the response from the server point to a URI of a policy (or an existing protocol) rather than a single bit in the protocol?</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/80">ISSUE-80</a>: Instead of responding with a Link: header URI, does it make sense to use a well-known location for this policy?</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/87">ISSUE-87</a>: Should there be an option for the server to respond with "I don't know what my policy is"</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/61">ISSUE-61</a>: A site could publish a list of the other domains that are associated with them</p>
</div>
<div class="section" id="response-header-field">
<h3><span class="secno">5.5 </span>Tracking response header field</h3>
<ul>
<li>sent on all responses?</li>
<li>sent only on dynamic/tracking responses?</li>
<li>different on dynamic vs static responses?
E.g, static headers for elements that never track (like <q>i am neutral</q>) and dynamic headers when <q>I am a tracking element and I accept your choice to not be tracked</q></li>
<li>does it indicate when a site believes it has an exemption from DNT,
such that the user can react appropriately if it isn't true. ...
The header could say <q>I see that you say DNT, but i am
tracking you for the following reasons.</q>
</li><li>it is sometimes contextual whether you are tracking or not.
</li></ul>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/76">ISSUE-76</a>: Should a server echo the DNT header to confirm receipt?</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/48">ISSUE-48</a>: Response from the server could both acknowledge receipt of a value and (separately) whether the server will honor it</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/90">ISSUE-90</a>: Interaction of DNT with caching and intermediaries</p>
</div>
<div class="section" id="response-error">
<h3><span class="secno">5.6 </span>Status code for Tracking Required</h3>
<p>An HTTP error response status code might be useful for indicating
that the site refuses service unless the user either logs into a
subscription account or agrees to an exception to DNT for this
site and its contracted third-party sites.
</p></div>
<div class="section" id="exceptions">
<h3><span class="secno">5.7 </span>Site-specific Exceptions</h3>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/43">ISSUE-43</a>: Sites should be able to let the user know their options when they arrive with Do Not Track</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/27">ISSUE-27</a>: How should the "opt back in" mechanism be designed?</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/46">ISSUE-46</a>: Enable users to do more granular blocking based on whether the site responds honoring Do Not Track</p>
<div class="section" id="permanent-exceptions">
<h4><span class="secno">5.7.1 </span>Use case: Site-specific exceptions should persist</h4>
<p>It would annoy users of DNT if they are presented with an
exception dialog each time they visit a site.</p>
<ol>
<li>User turns on DNT and visits Example.com</li>
<li>Example.com does not receive a signal it's on the
exception list</li>
<li>Example.com requests exception to DNT from user to access
content for free</li>
<li>User grants exception to Example.com (and perhaps
listed parties)</li>
<li>User views content</li>
<li>User returns to Example.com a week later</li>
<li>DNT signal is still turned on but Example.com is sent an
exception flag (or else doesn't send a DNT signal at all)
</li><li>In either case, it'll be important that Example.com know
to not trigger the exception request for this
user/web browser/device</li>
</ol>
</div>
</div>
</div>
<div id="acknowledgements" class="appendix section">
<!--OddPage--><h2><span class="secno">A. </span>Acknowledgements</h2>
<p>
This specification consists of input from many discussions within
and around the W3C Tracking Protection Working Group, along with
written contributions from
Roy&nbsp;T.&nbsp;Fielding (Adobe),
Tom&nbsp;Lowenthal (Mozilla),
Aleecia&nbsp;M.&nbsp;McDonald (Mozilla),
Matthias&nbsp;Schunter (IBM),
and Shane&nbsp;Wiley (Yahoo!).
</p>
<p>
The DNT header field is based on the original <em>Do Not Track</em>
submission by Jonathan&nbsp;Mayer (Stanford), Arvind&nbsp;Narayanan
(Stanford), and Sid&nbsp;Stamm (Mozilla).
The DOM API for <code>NavigatorDoNotTrack</code> is based on the
<em>Web Tracking Protection</em> submission by Andy&nbsp;Zeigler,
Adrian&nbsp;Bateman, and Eliot&nbsp;Graff (Microsoft).
Many thanks to Robin&nbsp;Berjon for ReSpec.js.
</p>
</div>
<div class="section" id="resolved">
<!--OddPage--><h2><span class="secno">B. </span>Closed Issues</h2>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/2">ISSUE-2</a>: What is the meaning of DNT (Do Not Track) header?<br>
<strong>[CLOSED]</strong>
"Does the presence of a DNT header field on requests always indicate
an explicit choice". <br>
The answer we agreed upon is "yes".
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/50">ISSUE-50</a>: Are DNT headers sent to first parties? <b>Yes</b></p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/70">ISSUE-70</a>: Does a past HTTP request with DNT set affect future HTTP requests? <b>No</b></p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/40">ISSUE-40</a>: Enable Do Not Track just for a session, rather than being stored<br>
<strong>[CLOSED]</strong>
Resolved in DNT Call 2011-10-26: The user agents are free to send
different DNT values for different sessions. We agreed that this is
a user-interface issue and out of scope on its own.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/68">ISSUE-68</a>: Should there be functionality for syncing preferences about tracking across different browsers?<br>
<strong>[CLOSED]</strong>
Resolved in DNT Call 2011-10-26: The user agents may or may not sync.
However, this is out of scope for this spec.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/42">ISSUE-42</a>: Feedback to the user from the browser when Do Not Track is turned on</p>
</div>
<div class="section" id="postponed">
<!--OddPage--><h2><span class="secno">C. </span>Postponed Issues</h2>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/44">ISSUE-44</a>: Ability to measure/detect who is honoring Do Not Track at a technical level<br>
<strong>[POSTPONED]</strong>
The info at the well-known URI declares whether a server promises to
follow DNT. Whether it actually does (or just pretends to do so) is
hard to determine and should be addressed later.
</p>
<p class="issue"><a href="http://www.w3.org/2011/tracking-protection/track/issues/64">ISSUE-64</a>: How does site preference management work with DNT<br>
<strong>[POSTPONED]</strong>
To what extent cookies can be used for preference management (such as
storing a language preference) will be resolved later.
</p>
</div>
<div class="appendix section" id="references"><!--OddPage--><h2><span class="secno">D. </span>References</h2><div class="section" id="normative-references"><h3><span class="secno">D.1 </span>Normative references</h3><dl class="bibliography"><dt id="bib-ABNF">[ABNF]</dt><dd>D. Crocker and P. Overell. <a href="http://www.ietf.org/rfc/rfc5234.txt"><cite>Augmented BNF for Syntax Specifications: ABNF.</cite></a> January 2008. Internet RFC 5234. URL: <a href="http://www.ietf.org/rfc/rfc5234.txt">http://www.ietf.org/rfc/rfc5234.txt</a>
</dd><dt id="bib-HTTP11">[HTTP11]</dt><dd>R. Fielding; et al. <a href="http://www.ietf.org/rfc/rfc2616.txt"><cite>Hypertext Transfer Protocol - HTTP/1.1.</cite></a> June 1999. Internet RFC 2616. URL: <a href="http://www.ietf.org/rfc/rfc2616.txt">http://www.ietf.org/rfc/rfc2616.txt</a>
</dd><dt id="bib-NAVIGATOR">[NAVIGATOR]</dt><dd>Ian Hickson, David Hyatt. <a href="http://dev.w3.org/html5/spec/timers.html#navigator"><cite>Navigator interface in HTML5.</cite></a> 15 April 2011. Editors' draft. (Work in progress.) URL: <a href="http://dev.w3.org/html5/spec/timers.html#navigator">http://dev.w3.org/html5/spec/timers.html#navigator</a>
</dd><dt id="bib-RFC2119">[RFC2119]</dt><dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key words for use in RFCs to Indicate Requirement Levels.</cite></a> March 1997. Internet RFC 2119. URL: <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
</dd><dt id="bib-WEBIDL">[WEBIDL]</dt><dd>Cameron McCormack. <a href="http://www.w3.org/TR/2008/WD-WebIDL-20081219"><cite>Web IDL.</cite></a> 19 December 2008. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/2008/WD-WebIDL-20081219">http://www.w3.org/TR/2008/WD-WebIDL-20081219</a>
</dd></dl></div><div class="section" id="informative-references"><h3><span class="secno">D.2 </span>Informative references</h3><p>No informative references.</p></div></div></body></html>