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<h1 class="title p-name" id="title">Social Web Protocols</h1>
<h2 id="w3c-working-group-note-25-december-2017"><abbr title="World Wide Web Consortium">W3C</abbr> Working Group Note <time class="dt-published" datetime="2017-12-25">25 December 2017</time></h2>
<dl>
<dt>This version:</dt>
<dd><a class="u-url" href="https://www.w3.org/TR/2017/NOTE-social-web-protocols-20171225/">https://www.w3.org/TR/2017/NOTE-social-web-protocols-20171225/</a></dd>
<dt>Latest published version:</dt>
<dd><a href="https://www.w3.org/TR/social-web-protocols/">https://www.w3.org/TR/social-web-protocols/</a></dd>
<dt>Latest editor's draft:</dt>
<dd><a href="https://w3c-social.github.io/social-web-protocols">https://w3c-social.github.io/social-web-protocols</a></dd>
<dt>Previous version:</dt>
<dd><a href="https://www.w3.org/TR/2017/WD-social-web-protocols-20170504/">https://www.w3.org/TR/2017/WD-social-web-protocols-20170504/</a></dd>
<dt>Editor:</dt>
<dd class="p-author h-card vcard" data-editor-id="69000"><a class="u-url url p-name fn" href="http://rhiaro.co.uk/">Amy Guy</a>, <a class="p-org org h-org h-card" href="http://inf.ed.ac.uk/">University of Edinburgh</a>, <span class="ed_mailto"><a class="u-email email" href="mailto:amy@rhiaro.co.uk">amy@rhiaro.co.uk</a></span></dd>
<dt>Repository:</dt>
<dd>
<a href="https://github.com/w3c-social/social-web-protocols">
Git repository
</a>
</dd>
<dd>
<a href="https://github.com/w3c-social/social-web-protocols/issues">
Issues
</a>
</dd>
<dd>
<a href="https://github.com/w3c-social/social-web-protocols/gh-pages">
Commits
</a>
</dd>
</dl>
<p class="copyright">
<a href="https://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> ©
2017
<a href="https://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup>
(<a href="https://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>,
<a href="https://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
<a href="https://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>).
<abbr title="World Wide Web Consortium">W3C</abbr> <a href="https://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a href="https://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and
<a rel="license" href="https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document">permissive document license</a>
rules apply.
</p>
<hr title="Separator for header">
</div>
<section id="abstract" class="introductory"><h2 id="abstract-0">Abstract</h2>
The Social Web Protocols are a collection of standards which enable various aspects of decentralised social interaction on the Web. This document describes the purposes of each, and how they fit together.
</section>
<section id="sotd" class="introductory"><h2 id="status-of-this-document">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 <abbr title="World Wide Web Consortium">W3C</abbr> publications and the latest revision of this technical report can be found in the <a href="https://www.w3.org/TR/"><abbr title="World Wide Web Consortium">W3C</abbr> technical reports index</a> at https://www.w3.org/TR/.</em>
</p>
<p>
This document was published by the <a href="https://www.w3.org/Social/WG">Social Web Working Group</a> as a Working Group Note.
Comments regarding this document are welcome. Please send them to
<a href="mailto:public-socialweb@w3.org">public-socialweb@w3.org</a>
(<a href="mailto:public-socialweb-request@w3.org?subject=subscribe">subscribe</a>,
<a href="https://lists.w3.org/Archives/Public/public-socialweb/">archives</a>).
</p>
<p>
Publication as a Working Group Note does not imply endorsement by the <abbr title="World Wide Web Consortium">W3C</abbr>
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="https://www.w3.org/Consortium/Patent-Policy/"><abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>.
<abbr title="World Wide Web Consortium">W3C</abbr> maintains a <a href="https://www.w3.org/2004/01/pp-impl/72531/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="https://www.w3.org/Consortium/Patent-Policy/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with
<a href="https://www.w3.org/Consortium/Patent-Policy/#sec-Disclosure">section
6 of the <abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>.
</p>
<p>This document is governed by the <a id="w3c_process_revision" href="https://www.w3.org/2017/Process-20170301/">1 March 2017 <abbr title="World Wide Web Consortium">W3C</abbr> Process Document</a>.
</p>
</section><nav id="toc"><h2 class="introductory" id="table-of-contents">Table of Contents</h2><ol class="toc"><li class="tocline"><a href="#introduction" class="tocxref"><span class="secno">1. </span>Introduction</a></li><li class="tocline"><a href="#overview" class="tocxref"><span class="secno">2. </span>Overview</a><ol class="toc"><li class="tocline"><a href="#contents" class="tocxref"><span class="secno">2.1 </span>Contents</a></li><li class="tocline"><a href="#definitions" class="tocxref"><span class="secno">2.2 </span>Definitions</a></li><li class="tocline"><a href="#swwg-specs" class="tocxref"><span class="secno">2.3 </span>Specifications</a><ol class="toc"><li class="tocline"><a href="#requirements" class="tocxref"><span class="secno">2.3.1 </span>Requirements</a></li><li class="tocline"><a href="#how-do-the-specs-relate-to-each-other" class="tocxref"><span class="secno">2.3.2 </span>How do these specifications relate to each other?</a></li></ol></li></ol></li><li class="tocline"><a href="#reading" class="tocxref"><span class="secno">3. </span>Reading</a><ol class="toc"><li class="tocline"><a href="#content-representation" class="tocxref"><span class="secno">3.1 </span>Content representation</a><ol class="toc"><li class="tocline"><a href="#content-other" class="tocxref"><span class="secno">3.1.1 </span>Other ways of representing content</a></li></ol></li><li class="tocline"><a href="#objects" class="tocxref"><span class="secno">3.2 </span>Objects</a></li><li class="tocline"><a href="#collections" class="tocxref"><span class="secno">3.3 </span>Collections</a><ol class="toc"><li class="tocline"><a href="#streams-special" class="tocxref"><span class="secno">3.3.1 </span>Special streams</a></li><li class="tocline"><a href="#inbox-interop" class="tocxref"><span class="secno">3.3.2 </span>Inboxes</a></li></ol></li></ol></li><li class="tocline"><a href="#publishing" class="tocxref"><span class="secno">4. </span>Publishing</a><ol class="toc"><li class="tocline"><a href="#creating" class="tocxref"><span class="secno">4.1 </span>Creating</a></li><li class="tocline"><a href="#updating" class="tocxref"><span class="secno">4.2 </span>Updating</a></li><li class="tocline"><a href="#deleting" class="tocxref"><span class="secno">4.3 </span>Deleting</a></li></ol></li><li class="tocline"><a href="#subscribing" class="tocxref"><span class="secno">5. </span>Subscribing</a><ol class="toc"><li class="tocline"><a href="#subscribing-with-follow" class="tocxref"><span class="secno">5.1 </span>Subscribing with <code>as:Follow</code></a></li><li class="tocline"><a href="#subscribing-hub" class="tocxref"><span class="secno">5.2 </span>Subscribing with a hub</a></li></ol></li><li class="tocline"><a href="#delivery" class="tocxref"><span class="secno">6. </span>Delivery</a><ol class="toc"><li class="tocline"><a href="#targeting-and-discovery" class="tocxref"><span class="secno">6.1 </span>Targeting and discovery</a></li><li class="tocline"><a href="#delivery-generic" class="tocxref"><span class="secno">6.2 </span>Generic notifications</a></li><li class="tocline"><a href="#delivery-activities" class="tocxref"><span class="secno">6.3 </span>Activity notifications</a></li><li class="tocline"><a href="#delivery-mentioning" class="tocxref"><span class="secno">6.4 </span>Mentioning</a><ol class="toc"><li class="tocline"><a href="#webmention-updates" class="tocxref"><span class="secno">6.4.1 </span>Updates and deletes</a></li></ol></li><li class="tocline"><a href="#delivery-interop" class="tocxref"><span class="secno">6.5 </span>Delivery interop</a><ol class="toc"><li class="tocline"><a href="#wm-as-as" class="tocxref"><span class="secno">6.5.1 </span>Webmention as AS2 / ActivityPub</a></li><li class="tocline"><a href="#ldn-to-wm" class="tocxref"><span class="secno">6.5.2 </span>LDN to Webmention</a></li><li class="tocline"><a href="#wm-to-ldn" class="tocxref"><span class="secno">6.5.3 </span>Webmention to LDN</a></li></ol></li></ol></li><li class="tocline"><a href="#profiles" class="tocxref"><span class="secno">7. </span>Profiles</a><ol class="toc"><li class="tocline"><a href="#relationships" class="tocxref"><span class="secno">7.1 </span>Relationships</a></li><li class="tocline"><a href="#auth" class="tocxref"><span class="secno">7.2 </span>Authentication, authorization, and access control</a></li></ol></li><li class="tocline"><a href="#related-specs" class="tocxref"><span class="secno">8. </span>Related specifcations</a></li><li class="tocline"><a href="#change-log" class="tocxref"><span class="secno">A. </span>Change Log</a><ol class="toc"><li class="tocline"><a href="#changes-since-2017-05-04" class="tocxref"><span class="secno">A.1 </span>Changes from 4 May 2017 WD to this version</a></li><li class="tocline"><a href="#changes-since-2016-11-16" class="tocxref"><span class="secno">A.2 </span>Changes from 16 November 2016 WD to this version</a></li><li class="tocline"><a href="#changes-since-2016-11-01" class="tocxref"><span class="secno">A.3 </span>Changes from 1 November 2016 WD to this version</a></li><li class="tocline"><a href="#changes-since-2016-10-12" class="tocxref"><span class="secno">A.4 </span>Changes from 12 October 2016 WD to this version</a></li><li class="tocline"><a href="#changes-since-2016-08-23" class="tocxref"><span class="secno">A.5 </span>Changes from 23 August to 12 October 2016</a></li><li class="tocline"><a href="#changes-since-2016-06-03" class="tocxref"><span class="secno">A.6 </span>Changes from 3 June to 23 August 2016</a></li></ol></li><li class="tocline"><a href="#references" class="tocxref"><span class="secno">B. </span>References</a><ol class="toc"><li class="tocline"><a href="#informative-references" class="tocxref"><span class="secno">B.1 </span>Informative references</a></li></ol></li></ol></nav>
<section id="introduction">
<!--OddPage--><h2 id="x1-introduction"><span class="secno">1. </span>Introduction</h2>
<p>The <abbr title="World Wide Web Consortium">W3C</abbr> Social Web Working Group ran from June 2014 to December 2017, published seven recommendations and some notes. This is one of the notes, and it describes the work at the time of the group closing.</p>
<p>This work is by no means done, and the various communities involved have by no means gone to sleep. The endeavour continues in the <a href="https://www.w3.org/wiki/SocialCG">Social Web Community Group</a>, in which you are warmly invited to particiate.</p>
</section>
<section id="overview">
<!--OddPage--><h2 id="x2-overview"><span class="secno">2. </span>Overview</h2>
<p>People and the content they create are the core components of the social web; they make up the social graph. This document describes a standard way in which people can:</p>
<ul>
<li>create, update and delete social content;</li>
<li>connect with other people by subscribing to their content;</li>
<li>interact with other peoples' content;</li>
<li>be notified when other people interact with their content.</li>
</ul>
<p>regardless of <em>what that content</em> is or <em>where it is stored</em>.</p>
<p>These components are core building blocks for interoperable decentralised social systems.</p>
<p>Each of these components can be implemented independently as needed, or all together in one system, as well as extended to meet domain-specific requirements. Users can store their social data across any number of compliant servers, and use compliant clients hosted elsewhere to interact with their own content and the content of others.</p>
<p>The <a href="https://www.w3.org/2013/socialweb/social-wg-charter">Social Web Working Group Charter</a> proposes the following deliverables, which have been met by the Group, and will be referred to throughout this document:</p>
<dl>
<dt>Social Data Syntax</dt>
<dd>A JSON-based syntax to allow the transfer of social information, such as status updates, across differing social systems.</dd>
<dt>Social API</dt>
<dd>A document that defines a specification for a client-side API that lets developers embed and format third party information such as social status updates.</dd>
<dt>Federation Protocol</dt>
<dd>A Web protocol to allow the federation of activity-based status updates and other data (such as profile information) between heterogeneous Web-based social systems. Federation should include multiple servers sharing updates within a client-server architecture, and allow decentralized social systems to be built.</dd>
</dl>
<section id="contents">
<h3 id="x2-1-contents"><span class="secno">2.1 </span>Contents</h3>
<p>This is an overview of the specifications of the Social Web Working Group.</p>
<p>Due to the diversity of viable technical approaches proposed within the Working Group, several specifications reached recommendation which provide overlapping functionality through differing mechanisms. This document exists to provide informative guidance on how and when to implement the various specifications, as well as some hints towards routes to bridging between similar specifications.</p>
</section>
<section id="definitions">
<h3 id="x2-2-definitions"><span class="secno">2.2 </span>Definitions</h3>
<p>This document is divided into five sections, pertaining to the components above, and derived from the <a href="https://www.w3.org/wiki/Socialwg/Social_API/Requirements">Social API Requirements</a>. The various Social Web Working Group specifications implement different combinations of these requirements (see <a href="#requirements" class="sec-ref"><span class="secno">2.3.1</span> <span class="sec-title">Requirements</span></a>) so for clarity the requirement are defined here.</p>
<dl>
<dt><a href="#reading">reading</a></dt>
<dd>covers specifications which describe the syntax and vocabulary to be used for social content. Useful for applications which expose feeds, as well as those which consume them.</dd>
<dt>publishing</dt>
<dd>covers Social API specifications which tell clients how to instruct servers to <a href="#publishing">create</a>, <a href="#updating">update</a> or <a href="#deleting">delete</a> content.</dd>
<dt><a href="#subscribing">subscribing</a></dt>
<dd>covers Federation Protocol specifications which describe how to ask for notifications about content, and subsequently how those notifications should be delivered.</dd>
<dt><a href="#delivery">delivery</a></dt>
<dd>covers Federation Protocol specifications which describe how to deliver notifications which have not been solicited through subscription.</dd>
<dt><a href="#profiles">profiles</a></dt>
<dd>covers specifications which describe profiles.</dd>
</dl>
</section>
<section id="swwg-specs">
<h3 id="x2-3-specifications"><span class="secno">2.3 </span>Specifications</h3>
<p>These are the specifications produced by the Social Web Working Group. New implementation reports and feedback are always welcome (details for where to submit these are at the top of each document).</p>
<dl>
<dt>[<cite><a class="bibref" href="#bib-Activitypub">Activitypub</a></cite>]</dt>
<dd>JSON(-LD)-based APIs for client-to-server interactions (ie. publishing) and server-to-server interactions (ie. federation).</dd>
<dt>ActivityStreams 2.0 [<cite><a class="bibref" href="#bib-activitystreams-core">activitystreams-core</a></cite>] and [<cite><a class="bibref" href="#bib-activitystreams-vocabulary">activitystreams-vocabulary</a></cite>]</dt>
<dd>The syntax and vocabulary for <a href="#reading">representing</a> social activities, actors, objects, and collections in JSON(-LD).</dd>
<dt>Linked Data Notifications ([<cite><a class="bibref" href="#bib-LDN">LDN</a></cite>])</dt>
<dd>A JSON-LD-based protocol for <a href="#delivery">delivery</a>.</dd>
<dt>[<cite><a class="bibref" href="#bib-Micropub">Micropub</a></cite>]</dt>
<dd>A form-encoding and JSON-based API for client-to-server interactions (ie. publishing).</dd>
<dt>[<cite><a class="bibref" href="#bib-Webmention">Webmention</a></cite>]</dt>
<dd>A form-encoding-based protocol for <a href="#delivery">delivery</a>.</dd>
<dt>[<cite><a class="bibref" href="#bib-WebSub">WebSub</a></cite>]</dt>
<dd>A protocol for <a href="#subscribing">subscription</a> to any resource and <a href="#delivery">delivery</a> of updates about it.</dd>
</dl>
<p>Specifications which are not Social Web Working Group recommendations, but which are nonetheless relevent to the charter deliverables, are described in <a href="#related-specs" class="sec-ref"><span class="secno">8.</span> <span class="sec-title">Related specifcations</span></a>.</p>
<section id="requirements">
<h4 id="x2-3-1-requirements"><span class="secno">2.3.1 </span>Requirements</h4>
<p>This table shows the high level requirements according to the <a href="https://www.w3.org/2013/socialweb/social-wg-charter.html">Social Web Working Group charter</a> and the <a href="https://www.w3.org/wiki/Socialwg/Social_API/Requirements">Social API Requirements</a>, and how the specifications of the Working Group overlap with respect to each.</p>
<p>This table lists the requirements met by the core functionality of each specification. Some specifications may additionally touch on or make suggestions about other requirements, which can be useful when deciding how to combine specifications, and is highlighted as appropriate throughout this document.</p>
<table class="th90">
<tbody><tr>
<td></td>
<td colspan="3">Data syntax</td>
<td colspan="4">Social API</td>
<td colspan="2">Federation Protocol</td>
<td></td>
</tr>
<tr>
<th></th>
<th><a href="#content-representation">Vocabulary</a></th>
<th><a href="#content-representation">Syntax</a></th>
<th><a href="#profiles">Profiles</a></th>
<th><a href="#reading">Read</a></th>
<th><a href="#publishing">Create</a></th>
<th><a href="#updating">Update</a></th>
<th><a href="#deleting">Delete</a></th>
<th><a href="#subscribing">Subscription</a></th>
<th><a href="#delivery">Delivery</a></th>
</tr>
<tr>
<td style="text-align:left;">ActivityPub</td>
<td></td>
<td></td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td style="text-align:left;">ActivityStreams 2.0</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr>
<td style="text-align:left;">Linked Data Notifications</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>X</td>
</tr>
<tr>
<td style="text-align:left;">Micropub</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>X</td>
<td>X</td>
<td>X</td>
<td></td>
<td></td>
</tr>
<tr>
<td style="text-align:left;">Webmention</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>X</td>
</tr>
<tr>
<td style="text-align:left;">WebSub<br></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>X</td>
<td></td>
</tr>
</tbody></table>
</section>
<section id="how-do-the-specs-relate-to-each-other">
<h4 id="x2-3-2-how-do-these-specifications-relate-to-each-other"><span class="secno">2.3.2 </span>How do these specifications relate to each other?</h4>
<p>As a quick reference, some key relationships between specifications are as follows:</p>
<ul>
<li><strong>ActivityPub and ActivityStreams 2.0</strong>: ActivityPub uses the AS2 syntax and vocabulary for the payload of all requests.</li>
<li><strong>ActivityPub and Linked Data Notifications</strong>: ActivityPub specialises LDN as the mechanism for delivery of notifications by requiring that payloads are AS2. <em>Inbox</em> endpoint discovery is the same. LDN receivers can understand requests from ActivityPub federated servers, but ActivityPub servers can't necessarily understand requests from generic LDN senders.</li>
<li><strong>ActivityStreams 2.0 and Linked Data Notifications</strong>: LDN <em class="rfc2119" title="MAY">MAY</em> use the AS2 syntax and vocabulary for the payload of notification requests.</li>
<li><strong>Webmention and Linked Data Notifications</strong>: Overlapping functionality that needs to be bridged due to different content types of requests. An LDN request <em class="rfc2119" title="MAY">MAY</em> contain the equivalent data as a Webmention request, but not necessarily vice versa.</li>
<li><strong>ActivityPub and Micropub</strong>: Overlapping functionality that needs to be bridged due to different vocabularies and possibly different content types of requests. Micropub specifies client-to-server interactions for content creation; ActivityPub specifies this, <em>plus</em> side-effects and server-to-server interactions.</li>
<li><strong>Micropub and Webmention</strong>: Are complementary but independent. Content can be created with Micropub, then Webmentions can be sent to any URLs referenced by the content.</li>
<li><strong>Micropub and Linked Data Notifications</strong>: Are complementary but independent. Content can be created with Micropub, then LDN discovery can be commenced on any relevant resources referenced by the content.</li>
<li><strong>Micropub and WebSub</strong>: Are complementary but independent. Content can be created with Micropub, then a WebSub <em>hub</em> can handle delivery to subscribers.</li>
<li><strong>ActivityPub and WebSub</strong>: Overlapping functionality using different content types and verification mechanisms.</li>
</ul>
</section>
</section>
</section>
<section id="reading">
<!--OddPage--><h2 id="x3-reading"><span class="secno">3. </span>Reading</h2>
<p>If you are a content publisher, this section is about how you should publish your content. If you are a content consumer, this is what you should expect to consume.</p>
<section id="content-representation">
<h3 id="x3-1-content-representation"><span class="secno">3.1 </span>Content representation</h3>
<p><strong>ActivityStreams 2.0</strong> is the recommended syntax ([<cite><a class="bibref" href="#bib-activitystreams-core">activitystreams-core</a></cite>]) and vocabulary ([<cite><a class="bibref" href="#bib-activitystreams-vocabulary">activitystreams-vocabulary</a></cite>]) for social data. ActivityStreams 2.0 represents content and interactions as <em>Objects</em> and <em>Activities</em>. The ActivityStreams vocabulary defines a finite set of common Object and Activity types and properties, as well as an extension mechanism for applications which want to expand on or specialise these.</p>
<p>ActivityStreams 2.0 content is served with the Content-Type <code>application/activity+json</code> or for JSON-LD extended implementations, <code>application/ld+json; profile="https://www.w3.org/ns/activitystreams"</code>. Consumers should recognise both of these Content-Types as ActivityStreams 2.0; <span title="after much wringing of hands and gnashing of teeth, the WG decided that">they should be treated as equivalent</span>.</p>
<p>In order to claim ActivityStreams 2.0 conformance, a data publisher must use ActivityStreams 2.0 vocabulary terms where available, and <em>only</em> use other vocabularies <em>in addition</em>, or where no appropriate ActivityStreams 2.0 term exists.</p>
<div class="note"><div role="heading" class="note-title marker" id="h-note" aria-level="4"><span>Note</span><span style="text-transform: none">: ActivityStreams 1.0</span></div><p class="">ActivityStreams 2.0 builds upon [<cite><a class="bibref" href="#bib-AS1">AS1</a></cite>] and is not fully backwards compatible; <a href="http://www.w3.org/TR/activitystreams-core#activitystreams-1.0">the relationship between AS1 and AS2 is documented in the AS2 spec</a>. If you have implemented [<cite><a class="bibref" href="#bib-AS1">AS1</a></cite>], you should transition to ActivityStreams 2.0.</p></div>
<p>To make content available as ActivityStreams 2.0 JSON, one could do so directly when requested with an appropriate <code>Accept</code> header (eg. <code>application/activity+json</code> or <code>application/ld+json</code>), or indirectly via a <code>rel="alternate" type="application/activity+json"</code> link . This link could be to a different domain, for third-party services which dynamically generate ActivityStreams 2.0 JSON on behalf of a publisher.</p>
<div class="note"><div role="heading" class="note-title marker" id="h-note-0" aria-level="4"><span>Note</span><span style="text-transform: none">: JSON-LD</span></div><div class="">
<p>When sent as JSON-LD, ActivityStreams 2.0 must be <a href="https://json-ld.org/spec/latest/json-ld-api/#compaction">compacted</a>, and must not use the <code>@graph</code> keyword. This is because an ActivityStreams 2.0 document requires a single <code>Object</code> as the root. To serve multiple resources, you can instead nest them.</p>
<p>The <a href="https://www.w3.org/ns/activitystreams#">ActivityStreams 2.0 JSON-LD context</a> (https://www.w3.org/ns/activitystreams#) aliases <code>@id</code> and <code>@type</code> to <code>id</code> and <code>type</code> respectively.</p>
</div></div>
<p><strong>[<cite><a class="bibref" href="#bib-Activitypub">Activitypub</a></cite>]</strong> uses ActivityStreams 2.0 for all data, and also <em>extends</em> ActivityStreams 2.0 with additional terms. Thus, ActivityPub requires responses to have the Content-Type <code>application/ld+json; profile="https://www.w3.org/ns/activitystreams"</code>. Publishers are however expected to also respect requests with the <code>application/activity+json</code> Accept header.</p>
<section id="content-other">
<h4 id="x3-1-1-other-ways-of-representing-content"><span class="secno">3.1.1 </span>Other ways of representing content</h4>
<p>If you don't like ActivityStreams 2.0, there are still some specifications you can use for particular tasks.</p>
<p><strong>[<cite><a class="bibref" href="#bib-LDN">LDN</a></cite>]</strong> notification contents can use any vocabulary, so long as the data is available as JSON-LD. Thus notifications <em class="rfc2119" title="MAY">MAY</em> use ActivityStreams 2.0, but don't have to.</p>
<p><strong>[<cite><a class="bibref" href="#bib-Micropub">Micropub</a></cite>]</strong> clients which expect to read data (this would usually be clients for <em><a href="#updating">updating</a></em>) are expecting it as JSON in the parsed microformats2 syntax ([<cite><a class="bibref" href="#bib-microformats2-parsing">microformats2-parsing</a></cite>]).</p>
<p><strong>[<cite><a class="bibref" href="#bib-WebSub">WebSub</a></cite>]</strong> is agnostic as to the Content-Type used by publishers; hubs are expected to deliver the new content to subscribers as-is produced by the publisher, at the publisher's <code>topic</code> URL.</p>
</section>
</section>
<section id="objects">
<h3 id="x3-2-objects"><span class="secno">3.2 </span>Objects</h3>
<p>All ActivityStreams 2.0 objects have URLs in the <code>id</code> property, which return the properties of an object. The response may vary depending on the requester's right to access the content.</p>
</section>
<section id="collections">
<h3 id="x3-3-collections"><span class="secno">3.3 </span>Collections</h3>
<p>Each ActivityStreams 2.0 collection has a URL in the <code>id</code> property which returns the contents of the stream (according to the requester's right to access). This could be paged - ActivityStreams 2.0 provides <a href="https://www.w3.org/TR/activitystreams-core/#paging">a paging mechanism</a>. It may include additional metadata about the stream (such as title, description).</p>
<p>Each <a href="#objects">object</a> in a collection contains at least its URL (<code>id</code>), as well as optionally other properties of the object.</p>
<p>Collections may represent changing streams of objects, or fixed sets. One <a href="#profiles">profile</a> may be associated with one or more streams of content. Streams may be generated automatically or manually, and might be segregated by post type, topic, audience, or any arbitrary criteria decided by the curator of the stream. A profile may include links to multiple streams, which a consumer could follow to read or subscribe to.</p>
<section id="streams-special">
<h4 id="x3-3-1-special-streams"><span class="secno">3.3.1 </span>Special streams</h4>
<p><strong>[<cite><a class="bibref" href="#bib-Activitypub">Activitypub</a></cite>]</strong> specifies two streams that <em class="rfc2119" title="MUST">MUST</em> be accessible from a profile via the following properties:</p>
<ul>
<li><code>inbox</code>: A reference to an ActivityStreams 2.0 collection comprising all the objects received by the actor.</li>
<li><code>outbox</code>: An ActivityStreams 2.0 collection comprising all the objects produced by the actor.</li>
</ul>
<p>ActivityPub also specifies further properties for accessing additional streams; <code>following</code> and <code>followers</code> are expected, and the rest are optional:</p>
<ul>
<li><code>following</code>: An ActivityStreams 2.0 collection of the actors that this actor is following.</li>
<li><code>followers</code>: An ActivityStreams 2.0 collection of the actors that follow this actor