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<!doctype html>
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</head>
<body>
<section id="abstract">
<p>
This document describes a formal information model and a common representation for a Web of Things (WoT) Thing
Description Next Version. A Thing Description describes the metadata and interfaces of <a>Things</a>, where a
<a>Thing</a> is an abstraction of a physical or virtual entity that provides interactions to and participates in
the Web of Things. Thing Descriptions provide a set of interactions based on a small vocabulary that makes it
possible both to integrate diverse devices and to allow diverse applications to interoperate. Thing
Descriptions, by default, are encoded in a JSON format that also allows JSON-LD processing. The latter provides
a powerful foundation to represent knowledge about <a>Things</a> in a machine-understandable way. A Thing
Description instance can be hosted by the <a>Thing</a> itself or hosted externally when a <a>Thing</a> has
resource restrictions (e.g., limited memory space) or when a Web of Things-compatible legacy device is
retrofitted with a Thing Description. Furthermore, this document introduces the Thing Model, which allows
authors to describe only the model or class of an Internet of Things (IoT) entity. Thing Models can be seen as a
template for Thing Description instances, but with reduced constraints such as no or few requirements for
specific communication metadata.
</p>
<p>
This specification continues the work of [[WOT-THING-DESCRIPTION11]] with no guarantees on backwards
compatibility. In case of no backwards compatibility, there will be concrete guidelines for implementers to
migrate to the new version.
</p>
</section>
<section id="sotd">
<!-- AT-RISK
<p>
The following <span class="at-risk">at-risk features</span> (marked in yellow throughout this specification)
may be removed due to insufficient implementation experience
<a href="https://w3c.github.io/wot-thing-description/testing/report11.html">reported</a>
and/or comments received during the CR period:
</p>
<ul>
<li>
Whole sections related to the security schemes described in
<a href="#certsecurityscheme"></a>,
<a href="#publicsecurityscheme"></a>, and
<a href="#popsecurityscheme"></a>.
</li>
<li>
Vocabulary terms and assertions related to the
<code>implicit</code>, <code>password</code>, and <code>client</code> flows in <a href="#oauth2securityscheme"></a>.
</li>
<li>All default values related to the above in <a href="#sec-default-values"></a>.</li>
<li>A <a href="#td-writeall-consumer">behavioral assertion</a> for <code>writeallproperties</code> that allows rejection of incomplete writes.</li>
</ul>
-->
<!-- <p>
Future updates to this specification may incorporate
<a href="https://www.w3.org/2021/Process-20211102/#allow-new-features">new features</a>.
</p> -->
<!-- <p>
The Web of Things Working Group intends to submit this document for consideration as a
<abbr title="World Wide Web Consortium">W3C</abbr> Proposed Recommendation after at least the minimum CR review
period has passed. However, before PR transition is requested, any features or assertions currently marked as
at-risk that did not appear in the TD 1.0 specification and do not have at least two implementations at that
time will either be removed or converted into informative statements, as appropriate.
</p> -->
</section>
<section id="introduction" class="informative">
<h1>Introduction</h1>
<section id="introduction-td">
<h2>Thing Description</h2>
<p>
The WoT Thing Description (TD) is a central building block in the W3C Web of Things (WoT) and can be
considered as the entry point of a <a>Thing</a> (much like the <i>index.html</i> of a Web site). A TD instance
has five main components: textual metadata about the <a href="#thing">Thing</a> itself, a set of
<a href="#interactionaffordance">Interaction Affordances</a> that indicate how the <a>Thing</a> can be used,
<a href="#sec-data-schema-vocabulary-definition">schemas</a> for the data exchanged with the <a>Thing</a> for
machine-understandability, <a href="#sec-security-vocabulary-definition">Security Definitions</a> to provide
metadata about the security mechanisms that must be used for interactions, and, finally,
<a href="#sec-hypermedia-vocabulary-definition">Web links</a> to express any formal or informal relation to
other <a>Things</a> or documents on the Web.
</p>
<p>
The <a>Interaction Model</a> of W3C WoT defines three types of <a>Interaction Affordances</a>: Properties (<a
href="#propertyaffordance"
><code>PropertyAffordance</code></a
>
class) can be used for sensing and controlling parameters, such as getting the current value or setting an
operation state. Actions (<a href="#actionaffordance"><code>ActionAffordance</code></a> class) model
invocation of physical (and hence time-consuming) processes, but can also be used to abstract RPC-like calls
of existing platforms. Events (<a href="#eventaffordance"><code>EventAffordance</code></a> class) are used for
the push model of communication where notifications, discrete events, or streams of values are sent
asynchronously to the receiver. See [[wot-architecture11]] for details.
</p>
<p>
In general, the TD provides metadata for different <a>Protocol Bindings</a> identified by URI schemes
[[RFC3986]] (e.g., <code>http</code>, <code>coap</code>, etc. [[?IANA-URI-SCHEMES]]), content types based on
media types [[RFC2046]] (e.g., <code>application/json</code>, <code>application/xml</code>,
<code>application/cbor</code>, <code>application/exi</code>, etc. [[?IANA-MEDIA-TYPES]]), and security
mechanisms (for authentication, authorization, confidentiality, etc.). Serialization of TD instances is based
on JSON [[RFC8259]], where JSON names refer to terms of the TD vocabulary, as defined in this specification
document. In addition the JSON serialization of TDs follows the syntax of JSON-LD 1.1 [[?JSON-LD11]] to enable
extensions and rich semantic processing.
</p>
<p>
<a href="#simple-thing-description-sample">Example 1</a> shows a TD instance and illustrates the
<a>Interaction Model</a> with Properties, Actions, and Events by describing a lamp <a>Thing</a> with the title
<i>MyLampThing</i>.
</p>
<aside class="example" id="simple-thing-description-sample" title="Thing Description sample">
<pre>
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"id": "urn:uuid:0804d572-cce8-422a-bb7c-4412fcd56f06",
"title": "MyLampThing",
"securityDefinitions": {
"basic_sc": {"scheme": "basic", "in": "header"}
},
"security": "basic_sc",
"properties": {
"status": {
"type": "string",
"forms": [{"href": "https://mylamp.example.com/status"}]
}
},
"actions": {
"toggle": {
"forms": [{"href": "https://mylamp.example.com/toggle"}]
}
},
"events": {
"overheating": {
"data": {"type": "string"},
"forms": [{
"href": "https://mylamp.example.com/oh",
"subprotocol": "longpoll"
}]
}
}
}</pre
>
</aside>
<p>
From this TD example, we know there exists one <a href="#propertyaffordance">Property affordance</a> with the
title <i>status</i>. In addition, information is provided to indicate that this Property is accessible via
(the secure form of) the HTTP protocol with a GET method at the URI
<code>https://mylamp.example.com/status</code> (announced within the <code>forms</code> structure by the
<code>href</code> member), and will return a string-based status value. The use of the GET method is not
stated explicitly, but is one of the default assumptions defined by this document.
</p>
<p>
In a similar manner, an <a href="#actionaffordance">Action affordance</a> is specified to toggle the switch
status using the POST method on the <code>https://mylamp.example.com/toggle</code> resource, where POST is
again a default assumption for invoking Actions.
</p>
<p>
The <a href="#eventaffordance">Event affordance</a> enables a mechanism for asynchronous messages to be sent
by a <a>Thing</a>. Here, a subscription to be notified upon a possible overheating event of the lamp can be
obtained by using HTTP with its long polling subprotocol on <code>https://mylamp.example.com/oh</code>.
</p>
<p>
This example also specifies the <code>basic</code> security scheme, requiring a username and password for
access. Note that a security scheme is first given a name in <code>securityDefinitions</code> and then
activated by specifying that name in a <code>security</code> section. In combination with the use of the HTTP
protocol this example demonstrates the use of HTTP Basic Authentication. Specification of at least one
security scheme at the top level is mandatory, and gives the default access requirements for every resource.
However, security schemes can also be specified per-form, with configurations given at the form level
overriding configurations given at the <code>Thing</code> level, allowing for the specification of
fine-grained access control. It is also possible to use a special <code>nosec</code> security scheme to
indicate that no access control mechanisms are used. Additional examples will be provided later.
</p>
<p>
The Thing Description offers the possibility to add contextual definitions in some namespace. This mechanism
can be used to integrate additional semantics to the content of the Thing Description instance, provided that
formal knowledge, e.g., logic rules for a specific domain of application, can be found under the given
namespace. Contextual information can also help specify some configurations and behavior of the underlying
communication protocols declared in the <code>forms</code> field.
<a href="#thing-description-full-serialization">Example 2</a> extends the TD sample from Example 1 by
introducing a second definition in the <code>@context</code> to declare the prefix <code>saref</code> as
referring to <a href="https://ontology.tno.nl/saref.ttl">SAREF</a>, the Smart Appliance Reference Ontology
[[SMARTM2M]]. This IoT ontology includes terms interpreted as semantic labels that can be set as values of the
<code>@type</code> field, giving the semantics of <a>Things</a> and their <a>Interaction Affordances</a>. In
the example below, the <a>Thing</a> is labelled with <code>saref:LightSwitch</code>, the <code>status</code>
<a>Property</a> is labelled with <code>saref:OnOffState</code> and the <code>toggle</code> <a>Action</a> with
<code>saref:ToggleCommand</code>.
</p>
<aside
class="example"
id="thing-description-full-serialization"
title="Thing Description with TD Context Extension for semantic annotations"
>
<pre>
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{ "saref": "https://w3id.org/saref#" }
],
"id": "urn:uuid:300f4c4b-ca6b-484a-88cf-fd5224a9a61d",
"title": "MyLampThing",
"@type": "saref:LightSwitch",
"securityDefinitions": {
"basic_sc": {"scheme": "basic", "in": "header"}
},
"security": "basic_sc",
"properties": {
"status": {
"@type": "saref:OnOffState",
"type": "string",
"forms": [{
"href": "https://mylamp.example.com/status"
}]
}
},
"actions": {
"toggle": {
"@type": "saref:ToggleCommand",
"forms": [{
"href": "https://mylamp.example.com/toggle"
}]
}
},
"events": {
"overheating": {
"data": {"type": "string"},
"forms": [{
"href": "https://mylamp.example.com/oh"
}]
}
}
}</pre
>
</aside>
<p>
The declaration mechanism inside some
<code>@context</code> is specified by JSON-LD. A TD instance complies to version 1.1 of that specification
[[?json-ld11]]. Hence, a TD instance can be also processed as an RDF document (for details about semantic
processing, please refer to Appendix <a href="#json-ld-ctx-usage"></a> and the documentation under the
namespace IRIs, e.g., <a href="https://www.w3.org/2019/wot/td">https://www.w3.org/2019/wot/td</a>).
</p>
</section>
<section id="introduction-tm">
<h2>Thing Model</h2>
<p>
One of the main intentions of a <a>Thing Description</a> is to provide a <a>Consumer</a> with all the details
necessary to successfully interact with a <a>Thing</a>. In some IoT application scenarios, a fully detailed
<a>Thing Description</a>, e.g., with communication metadata is not necessary (e.g., IoT ecosystems may
implicitly handle communication separately), or may not be available because a new entity has not yet been
deployed (e.g., IP address is not yet known). Sometimes, also a kind of class definition is required that
forces capability definitions that should be available for all created instances (e.g., large-scale production
of new devices).
</p>
<p>
In order to address the above-mentioned scenarios or others, the <a>Thing Model</a> can be used that mainly
provides the data model definitions within <a>Things</a>' <a>Properties</a>, <a>Actions</a>, and/or
<a>Events</a> and can be potentially used as template for creating <a>Thing Description</a> instances. In the
following a sample <a>Thing Model</a> is presented that can be seen as a model for the
<a>Thing Description</a> instance in <a href="#simple-thing-description-sample"></a>.
</p>
<pre class="example" title="Thing Model sample" id="td-model-example-lamp">
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type": "tm:ThingModel",
"title": "Lamp Thing Model",
"properties": {
"status": {
"description": "current status of the lamp (on|off)",
"type": "string",
"readOnly": true
}
},
"actions": {
"toggle": {
"description": "Turn the lamp on or off"
}
},
"events": {
"overheating": {
"description": "Lamp reaches a critical temperature (overheating)",
"data": {"type": "string"}
}
}
}
</pre>
<p>
<a>Thing Model</a> definitions are identified by the <code>"@type": "tm:ThingModel"</code>. As the example
shows, it does not provide details about a single <a>Thing</a> instance due to the lack of communication and
security metadata. This specification presents a mechanism for deriving valid
<a>Thing Description</a> instances from such <a>Thing Model</a> definitions. In addition, other design
concepts are specified, including how to override, extend, and reuse existing <a>Thing Model</a> definitions.
</p>
</section>
</section>
<section id="conformance">
<p>
A Thing Description instance complies with this specification if it follows the normative statements in
<a href="#sec-vocabulary-definition"></a>
and
<a href="#sec-td-serialization"></a>
regarding Thing Description serialization.
</p>
<p>
A JSON Schema [[?JSON-SCHEMA]] to validate Thing Description instances is provided in Appendix
<a href="#json-schema-for-validation"></a>.
</p>
</section>
<section id="terminology" class="informative">
<h2>Terminology</h2>
<p>
The fundamental WoT terminology such as <dfn>Thing</dfn>, <dfn>Consumer</dfn>, <dfn>Producer</dfn>,
<dfn>Thing Description</dfn> (<dfn class="lint-ignore">TD</dfn>), <dfn>Partial TD</dfn>,
<dfn>Thing Model</dfn> (<dfn class="lint-ignore">TM</dfn>), <dfn>Interaction Model</dfn>,
<dfn>Interaction Affordance</dfn>, <dfn class="lint-ignore">IoT Platform</dfn>, <dfn>Property</dfn>,
<dfn>Action</dfn>, <dfn>Event</dfn>, <dfn class="lint-ignore">Data Schema</dfn>,
<dfn class="lint-ignore">Content Type</dfn>, <dfn>Protocol Binding</dfn>, <dfn>Servient</dfn>,
<dfn>Vocabulary</dfn>, <dfn>Term</dfn>, <dfn id="dfn-vocab-term">Vocabulary Term</dfn>,
<dfn>WoT Interface</dfn>, and
<dfn>WoT Runtime</dfn>
are defined in <a href="https://www.w3.org/TR/wot-architecture/#terminology">Section 3</a>
of the WoT Architecture specification [[wot-architecture11]].
</p>
<p>In addition, this specification introduces the following definitions:</p>
<dl>
<dt>
<dfn id="dfn-semantic-tag">Semantic Tag</dfn>, <dfn id="dfn-semantic-annotation">Semantic Annotation</dfn>
</dt>
<dd>
A JSON-LD mechanism that links definitions in a <a>Thing Descriptions</a> document to concepts in an (RDF)
ontology. This allows Thing Description authors to provide further context and express domain knowledge in a
standardized way. In a <a>Thing Descriptions</a> document, this can be achieved using
<code>@type</code> members, and through the use of string prefixes using a colon (<code>:</code>).
</dd>
<dt>
<dfn id="dfn-context-ext">TD Context Extension</dfn>
</dt>
<dd>
A mechanism to extend <a>Thing Descriptions</a> with additional <a>Vocabulary Terms</a>. It is the basis for
<a>semantic annotations</a> and extensions to core mechanisms such as Protocol Bindings, Security Schemes, and
Data Schemas.
</dd>
<dt>
<dfn id="dfn-inf-model">TD Information Model</dfn>
</dt>
<dd>
Set of <a>Class</a> definitions constructed from pre-defined <a>Vocabularies</a> on which constraints apply,
thus defining the semantics of these <a>Vocabularies</a>. Class definitions are typically expressed in terms
of a <a>Signature</a> (a set of <a>Vocabulary Terms</a>) and functions over that <a>Signature</a>. The
<a>TD Information Model</a> also includes <a>Default Values</a>, defined as a global function over
<a>Classes</a>.
</dd>
<dt>
<dfn id="dfn-td-processor">TD Processor</dfn>
</dt>
<dd>
A system that can serialize some internal representation of a <a>Thing Description</a> in a given format
and/or deserialize it from that format. A <a>TD Processor</a> can follow validation steps to detect
semantically inconsistent <a>Thing Descriptions</a>, that is, <a>Thing Descriptions</a> that cannot satisfy
constraints on the <a>Instance Relation</a> of the <code>Thing</code> class. For that purpose, a
<a>TD Processor</a> can compute fill in the forms of <a>Thing Descriptions</a> in which all possible
<a>Default Values</a> are assigned. A <a>TD Processor</a> is typically a sub-system of a <a>WoT Runtime</a>.
Implementations of a TD Processor can be a TD <a>producer</a> (able to serialize to TD Documents) or a TD
consumer (able to deserialize from TD Documents) or both.
</dd>
<dt>
<dfn id="dfn-td-serialization">TD Serialization</dfn>
or
<dfn id="dfn-td-document">TD Document</dfn>
</dt>
<dd>
Textual or binary representation of <a>Thing Descriptions</a> that can be stored and exchanged between
<a>Servients</a>. A <a>TD Serialization</a> follows a given representation format, identified by a media type
when exchanged over the network. The default representation format for <a>Thing Descriptions</a> is JSON-based
as defined by this specification.
</dd>
<dt>
<dfn id="dfn-td-level" class="lint-ignore">Levels of a TD</dfn> (including
<dfn id="dfn-td-thing-level">Thing Level</dfn>,
<dfn id="dfn-td-affordance-level" class="lint-ignore">Affordance Level</dfn>,
<dfn id="dfn-td-data-schema-level" class="lint-ignore">Data Schema Level</dfn>,
<dfn id="dfn-td-forms-level" class="lint-ignore">Forms Level</dfn>)
</dt>
<dd>
The scope of a TD or TM instance at the given hierarchy level. For example, the root of a TD, where terms such
as <code>@context</code> are defined is the Thing level, <code>forms</code> are defined within the Affordance
level, <code>type</code>, <code>maximum</code> are defined within the Data Schema level and
<code>href</code> is defined within the Forms level. Even if not defined, other levels can be used such as
Links level.
</dd>
<dt><dfn id="dfn-subspecification">Binding Template Subspecification</dfn>, <dfn>subspecification</dfn></dt>
<dd>
A binding template document that is published separately from this document that specifies a binding template
for a protocol, payload format or platform. All binding template subspecifications respect the rules set in
the <a href="binding-templates">Binding Templates</a> section.
</dd>
</dl>
<p>These definitions are further developed in <a href="#preliminary-definitions"></a>.</p>
</section>
<section class="normative">
<h1>Namespaces</h1>
<p>
The version of the <a>TD Information Model</a> defined in <a href="#sec-vocabulary-definition"></a> of this
specification is identified by the following IRI:
</p>
<p>
<code>https://www.w3.org/2022/wot/td/v1.1</code>
</p>
<p>
This IRI [[RFC3987]], which is also a URI [[!RFC3986]], can be dereferenced to obtain a
<a href="https://www.w3.org/2022/wot/td/v1.1">JSON-LD context file</a> [[?json-ld11]], allowing the compact
strings in <a>TD Documents</a> to be expanded to full IRI-based <a>Vocabulary Terms</a>. However, this
processing is only required when transforming JSON-based <a>TD Documents</a> to RDF, an optional feature of
<a>TD Processor</a>
implementations.
</p>
<p>
In the present specification, <a>Vocabulary Terms</a> are always presented in their compact form. Their expanded
form can be accessed under the namespace IRI of the <a>Vocabulary</a> they belong to. These namespaces follow
the structure of <a href="#class-definitions"></a>. Each <a>Vocabulary</a> used in the
<a>TD Information Model</a> has its own namespace IRI, as follows:
</p>
<table class="def numbered">
<caption>
Namespaces used in TDs
</caption>
<thead>
<tr>
<th>Vocabulary</th>
<th>Namespace IRI</th>
</tr>
</thead>
<tbody>
<tr>
<td>Core</td>
<td><code>https://www.w3.org/2019/wot/td#</code></td>
</tr>
<tr>
<td>Data Schema</td>
<td><code> https://www.w3.org/2019/wot/json-schema#</code></td>
</tr>
<tr>
<td>Security</td>
<td><code>https://www.w3.org/2019/wot/security#</code></td>
</tr>
<tr>
<td>Hypermedia Controls</td>
<td><code>https://www.w3.org/2019/wot/hypermedia#</code></td>
</tr>
</tbody>
</table>
<p>
All vocabularies that are additionally used for <a>Thing Model</a> definitions have the following namespace IRI:
</p>
<table class="def numbered">
<caption>
Namespaces used in TMs
</caption>
<thead>
<tr>
<th>Vocabulary</th>
<th>Namespace IRI</th>
</tr>
</thead>
<tbody>
<tr>
<td>Thing Model</td>
<td><code>https://www.w3.org/2022/wot/tm#</code></td>
</tr>
</tbody>
</table>
<p>
The <a>Vocabularies</a> are independent from each other. They may be reused and extended in other W3C
specifications. Every breaking change in the design of a <a>Vocabulary</a> will require the assignment of a new
year-based namespace URI. Note that to maintain the general coherence of the <a>TD Information Model</a>, the
associated JSON-LD context file is versioned such that every version has its own URI (<code>v1</code>,
<code>v1.1</code>, <code>v2</code>, ...) to also identify non-breaking changes, in particular the addition of
new <a>Terms</a>.
</p>
<p>
Because a <a>Vocabulary</a> under some namespace IRI can only undergo non-breaking changes, its content can be
safely cached or embedded in applications. One advantage of exposing relatively static content under a namespace
IRI is to optimize payload sizes of messages exchanged between constrained devices. It also avoids any privacy
leakage resulting from devices accessing publicly available vocabularies from private networks (see also
<a href="#sec-privacy-consideration"></a>).
</p>
</section>
<section id="sec-vocabulary-definition" class="normative">
<h1>TD Information Model</h1>
<p>
This section introduces the <a>TD Information Model</a>. The <a>TD Information Model</a> serves as the
conceptual basis for the processing of Thing Descriptions and their serialization, which is described separately
in <a href="#sec-td-serialization"></a>.
</p>
<p></p>
<section>
<h2>Overview</h2>
<p>The <a>TD Information Model</a> is built upon the following, independent <a>Vocabularies</a>:</p>
<ul>
<li>
the <em>core</em> TD <a>Vocabulary</a>, which reflects the <a>Interaction Model</a> with the
<a>Properties</a>, <a>Actions</a>, and <a>Events</a> <a>Interaction Affordances</a> [[wot-architecture11]]
</li>
<li>
the <em>Data Schema</em> <a>Vocabulary</a>, including (a subset of) the terms defined by JSON Schema
[[?JSON-SCHEMA]]
</li>
<li>
the <em>WoT Security</em> <a>Vocabulary</a>, identifying security mechanisms and requirements for their
configuration
</li>
<li>
the <em>Hypermedia Controls</em> <a>Vocabulary</a>, encoding the main principles of RESTful communication
using Web links and forms
</li>
</ul>
<p>
Each of these <a>Vocabularies</a> is essentially a set of <a>Terms</a> that can be used to build data
structures, interpreted as objects in the traditional object-oriented sense. Objects are instances of classes
and have properties. In the context of W3C WoT, they denote <a>Things</a> and their
<a>Interaction Affordances</a>. A formal definition of objects is given in
<a href="#preliminary-definitions"></a>. The main elements of the <a>TD Information Model</a> are then
presented in <a href="#class-definitions"></a>. Certain object properties may be omitted in a TD when
<a>Default Values</a> exist. A list of defaults is given in <a href="#sec-default-values"></a>.
</p>
<p>
The UML diagram shown next gives an overview of the <a>TD Information Model</a>. It represents all classes as
tables and the associations that exist between classes, starting from the class
<a href="#thing"><code>Thing</code></a
>, as directed arrows. For the sake of readability, the diagram was split in four parts, one for each of the
four base <a>Vocabularies</a>.
</p>
<!--<p><a href="http://visualdataweb.de/webvowl/#iri=https://rawgit.com/w3c/wot-thing-description/TD-JSON-LD-1.1/ontology/td.ttl">Click here for the visualization</a></p>-->
<figure id="td-core-model">
<img src="visualization/td.svg" alt="UML diagram of the TD information model for the TD core vocabulary" />
<figcaption>TD core vocabulary</figcaption>
</figure>
<hr />
<figure id="td-data-schema-model">
<img
src="visualization/jsonschema.svg"
alt="UML diagram of the TD information model for the Data schema vocabulary"
/>
<figcaption>Data schema vocabulary</figcaption>
</figure>
<hr />
<figure id="td-security-model">
<img
src="visualization/wotsec.svg"
alt="UML diagram of the TD information model for the WoT security vocabulary"
/>
<figcaption>WoT security vocabulary</figcaption>
</figure>
<hr />
<figure id="hypermedia-model">
<img
src="visualization/hctl.svg"
alt="UML diagram of the TD information model for the hypermedia controls vocabulary"
/>
<figcaption>Hypermedia controls vocabulary</figcaption>
</figure>
</section>
<section id="preliminary-definitions">
<h2>Preliminaries</h2>
<p>
To provide a model that can be easily processed by both, simple rules on a tree-based document (i.e., raw JSON
processing) and rich Semantic Web tooling (i.e., JSON-LD processing), this document defines the following
formal preliminaries to construct the <a>TD Information Model</a> accordingly.
</p>
<p>
All definitions in this section refer to <em>sets</em>, which intuitively are collections of elements that can
themselves be sets. All arbitrarily complex data structures can be defined in terms of sets. In particular, an
<dfn>Object</dfn> is a data structure recursively defined as follows:
</p>
<ul>
<li>a <a>Term</a>, which may or may not belong to a <a>Vocabulary</a>, is an <a>Object</a>.</li>
<li>
a set of name-value pairs where the name is a <a>Term</a> and the value is another <a>Object</a>, is also an
<a>Object</a>.
</li>
</ul>
<p>
Though this definition does not prevent <a>Objects</a> to include multiple name-value pairs with the same