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</head>
<body>
<section id="abstract">
<p>
This document describes a formal model and a common representation
for a Web of Things (WoT) Thing Description 1.1.
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 to describe
only the model or class of an 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 describes a superset of the features defined in Thing Description 1.0 [[WOT-THING-DESCRIPTION]].
Unless otherwise specified, documents created with version 1.0 of this specification remain
compatible with Thing Description 1.1.
</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/report.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>
-->
</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-ARCHITECTURE]] 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:dev:ops:32473-WoTLamp-1234",
"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://w3id.org/saref">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:dev:ops:32473-WoTLamp-1234",
"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>Thing Description</dfn> (<dfn>TD</dfn>),
<dfn>Partial TD</dfn>,
<dfn>Thing Model</dfn> (<dfn>TM</dfn>),
<dfn>Interaction Model</dfn>,
<dfn>Interaction Affordance</dfn>,
<dfn>Property</dfn>,
<dfn>Action</dfn>,
<dfn>Event</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-ARCHITECTURE]].
</p>
<p>
In addition, this specification introduces the following definitions:
</p>
<dl>
<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 semantic annotations 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> must 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> may compute 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 may be a TD producer only (able to serialize to TD Documents)
or a TD consumer only (able to deserialize from TD Documents).
</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>
</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">
<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">
<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-security-consideration-context"></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>
<section>
<h2>Overview</h2>
<p>
The <a>TD Information Model</a> is built upon the following, independent <a>Vocabularies</a>:
<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-ARCHITECTURE]]
</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>
<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>-->
<p class="note">
The following figures are automatically generated based on the underlying ontology definitions.
</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 (<a href="visualization/td.svg">SVG file</a>)</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 (<a href="visualization/jsonschema.svg">SVG file</a>)</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 (<a href="visualization/wotsec.svg">SVG file</a>)</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 (<a href="visualization/hctl.svg">SVG file</a>)</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 name, they are generally not considered
in this specification. An <a>Object</a> whose elements only have numbers
as names is called an <dfn>Array</dfn>. Similarly, an <a>Object</a> whose
elements only have <a>Term</a>s (that do not belong to any <a>Vocabulary</a>)
as names is called a <dfn>Map</dfn>. All names appearing in some name-value pair
in a <a>Map</a> are assumed to be <em>unique</em> within the scope of the
<a>Map</a>.
</p>
<p>
Moreover, <a>Object</a>s can be instances of some <dfn>Class</dfn>.
A <a>Class</a>, which is denoted by a <a>Vocabulary Term</a>, is
first defined by a set of <a>Vocabulary Terms</a> called a <dfn>Signature</dfn>. A