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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Web of Things (WoT) Thing Description</title>
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, href: "https://github.com/w3c/wot-security/blob/master/wot-security-best-practices.md"
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, href: "https://w3c.github.io/wot-binding-templates/"
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, date: "12 January 2018"
}
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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.
A Thing Description describes the metadata and interfaces of Things,
where a Thing 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 Things in a machine-understandable way.
A Thing Description instance can be hosted by the Thing itself or hosted
externally when a Thing has resource restrictions (e.g., limited memory space)
or when a Web of Things-compatible legacy device is retrofitted
with a Thing Description.
</p>
</section>
<section id="sotd">
<p>
Implementers need to be aware that this specification is considered unstable.
Vendors interested in implementing this specification before it eventually
reaches the Candidate Recommendation phase should subscribe to the
<a href="https://github.com/w3c/wot-thing-description">repository</a>
and take part in the discussions.
</p>
<!--
<p>This document is governed by the <a id=
"w3c_process_revision" href=
"https://www.w3.org/2019/Process-20190301/">1 March 2019
<abbr title="World Wide Web Consortium">W3C</abbr> Process
Document</a>.</p>
-->
<p>
<span class="at-risk">Text and table entries highlighted with a yellow background
indicates a feature associated with an at-risk assertion for which
insufficient implementation experience exists.</span>
When an entire section is at-risk the words
"<span class="at-risk">This section is at risk.</span>"
will be placed at the start of the section and highlighted with a yellow background.</span>
</p>
</section>
<section id="introduction" class="informative">
<h1>Introduction</h1>
<p>
The Thing Description (TD) model is a central building block in the W3C Web of
Things (WoT) and can be considered as the entry point of a Thing
(much like the <i>index.html</i> of a Web site). A TD instance has four main
components: textual metadata about the <a href="#thing">Thing</a> itself, a collection of
<a href="#interactionaffordance">affordances</a> to interact with the Thing on the
basis of WoT's high-level interaction paradigm (involving the classes
<a href="#propertyaffordance"><code>PropertyAffordance</code></a>,
<a href="#actionaffordance"><code>ActionAffordance</code></a> and
<a href="#eventaffordance"><code>EventAffordance</code></a>),
<a href="#sec-data-schema-vocabulary-definition">schemas</a> for the information
exchanged with the Thing for machine-understandability,
and, finally, <a href="#sec-web-linking-vocabulary-definition">Web links</a> to
express any formal or informal relation to other Things.
</p>
<p>
Property affordances (<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.
Action affordances (<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.
Event affordances (<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.
In general, the TD provides metadata for different communication bindings
identified by URI schemes [[iana-uri-schemes]] (e.g., <code>http</code>, <code>coap</code>, etc.),
content types based on media types (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 keys refers to terms of
the TD core 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 context extension and RDF processing.
</p>
<p>
<a href="#simple-thing-description-sample">Example 1</a> shows a TD instance and
illustrates the TD model's <a href="#propertyaffordance"><code>PropertyAffordance</code></a>,
<a href="#actionaffordance"><code>ActionAffordance</code></a> and
<a href="#eventaffordance"><code>EventAffordance</code></a>
classes by describing a lamp Thing with the name <i>MyLampThing</i>.
</p>
<aside class="example" id="simple-thing-description-sample" title="Thing Description Sample">
<pre>{
"@context": "https://www.w3.org/2019/td/v1",
"id": "urn:dev:ops:32473-WoTLamp-1234",
"name": "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 name <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> name), 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 and explained in [[WoT-Binding-Templates]].
</p>
<p>
In a similar manner, an <a href="#actionaffordance">action affordance</a> is specified to toggle the
switch status using the POST method applied to 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 Thing.
Here, a subscription to be notified upon a possible overheating event
of the lamp can be obtained by using the HTTP with its long polling
sub-protocol at <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 a
<code>securityDefinition</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 Thing 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 defintion in the <code>@context</code> to declare the
prefix <code>saref</code> as referring to the SAREF vocabulary namespace [[smartM2M]].
The SAREF vocabulary includes terms to describe lighting devices and other home automation
devices that one can embed in a TD as semantic labels as values for the <code>@type</code>
property. In the present example, the Thing is labelled with <code>saref:LightingDevice</code>,
the <code>status</code> property affordance is labelled with <code>saref:OnOffState</code>
and the <code>toggle</code> action affordance with <code>saref:ToggleCommand</code>.
</p>
<aside class="example" id="thing-description-full-serialization" title="Thing Description with context extension">
<pre>
{
"@context": [
"https://www.w3.org/2019/td/v1",
{
"saref": "https://w3id.org/saref#"
}
],
"id": "urn:dev:ops:32473-WoTLamp-1234",
"name": "MyLampThing",
"@type": "saref:LightingDevice",
"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": {
"@type": "saref:NotifyCommand",
"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
this specification [[json-ld11]]. The TD instance can be also processed as an RDF
document (details are given in <a href="#note-jsonld11-processing"></a>).
</p>
</section>
<section id="terminology">
<h2>Terminology</h2>
<p>The generic WoT terminology is defined in [[WOT-ARCHITECTURE]]:
<dfn data-lt="Things">Thing</dfn>,
<dfn data-lt="Thing Descriptions">Thing Description</dfn>
(in short <dfn>TD</dfn>),
<dfn>Web of Things</dfn> (in short <b><i>WoT</i></b>),
<dfn>WoT Interface</dfn> etc.
</p>
<p>
In addition, this document introduces the following definitions:
</p>
<dl>
<dt>
<dfn id="dfn-inf-model">TD Information Model</dfn>
</dt>
<dd>
Set of constraints and equivalence relations applying to pre-defined vocabularies
(i.e., a core vocabulary with extension like JSON Schema, Web Linking and WoT Security),
thus defining the semantics of these vocabularies. Constraints and
equivalence relations are typically expressed in terms of class signature,
property value assignment and property type on arbitrary object-oriented data
structures.
</dd>
<dt>
<dfn id="dfn-td-processor">TD Processor</dfn>
</dt>
<dd>
A system that can serialize some internal representation of a TD
in a given format and deserialize it from that format. If two TDs have different
serializations but are semantically equivalent, a TD processor must be able to
construct a canonical (i.e., identical) representation when deserializing them.
For instance, a TD in which default values were omitted is equivalent to a TD that
would include default values. Moreover, a TD processor must also detect semantically
inconsistent TDs, for which no representation should exist. A TD processor is
typically a sub-system of a WoT runtime.
</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 TD that can be stored and exchanged between
servients. A TD serialization follows a given format, provided as a content type
when exchanging it. The reference serialization format for TDs is JSON.
</dd>
<dt>
<dfn id="dfn-vocab">Vocabulary</dfn>
</dt>
<dd>
A collection of vocabulary terms, identified by a namespace IRI.
</dd>
<dt>
<dfn id="dfn-vocab-term">Vocabulary Term</dfn>
</dt>
<dd>
A class name (like <code>Thing</code>), a property name (like
<code>forms</code>) or a property value (like <code>readproperty</code>).
</dd>
</dl>
</section>
<section>
<h1>Namespaces</h1>
<p>
The TD information model presented in this document has its foundations in
RDF, with a mapping to JSON. This mapping is provided as a JSON-LD context
file, available at the following URI:
</p>
<p>
<code>https://www.w3.org/2019/td/v1</code>
</p>
<section>
<h2>Reference Namespaces</h2>
<p>
All terms defined in the context are grouped by vocabulary, mirroring the
structure of the present document (see
<a href="#thing-description-json-ld-context"></a> for more details on the
context). Each vocabulary is assigned a year-based namespace URI, as follows:
</p>
<table class="def">
<thead>
<tr>
<th>Vocabulary</th>
<th>Namespace URI</th>
</tr>
</thead>
<tbody>
<tr>
<td>Core</td>
<td><code>https://www.w3.org/2019/td#</code></td>
</tr>
<tr>
<td>Data Schema</td>
<td><code>https://www.w3.org/2019/json-schema#</code></td>
</tr>
<tr>
<td>Security</td>
<td><code>https://www.w3.org/2019/wot-security#</code></td>
</tr>
<tr>
<td>Web Linking</td>
<td><code>https://www.w3.org/2019/web-linking#</code></td>
</tr>
</tbody>
</table>
<p>
Vocabularies are independent from each other. They may be reused and
extended in other W3C recommendations. For every breaking change in the
design of a vocabulary, it will be assigned a new year-based namespace URI.
Note that to maintain the general coherence of the TD information model,
the JSON-LD context itself is versioned, such that every version has its own URI
(<code>v1</code>, <code>v1.1</code>, <code>v2</code>, ...).
</p>
<p>
Because a vocabulary under some namespace URI 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 URI is to
optimize payload sizes of messages exchanged between constrained devices. It
also avoids any privacy leakage resulting from devices accessing publicly
available vocabularies private networks (see also
<a href="#sec-security-consideration-context"></a>).
</p>
</section>
<section class="informative">
<h2>Alternative Namespaces</h2>
<p>
For convenience, generic namespace URIs redirect to the latest year-based
URI for each vocabulary. They are defined as follows:
</p>
<table class="def">
<thead>
<tr>
<th>Vocabulary</th>
<th>Namespace URI</th>
</tr>
</thead>
<tbody>
<tr>
<td>Core</td>
<td><code>https://www.w3.org/ns/td#</code></td>
</tr>
<tr>
<td>Data Schema</td>
<td><code>https://www.w3.org/ns/json-schema#</code></td>
</tr>
<tr>
<td>Security</td>
<td><code>https://www.w3.org/ns/wot-security#</code></td>
</tr>
<tr>
<td>Web Linking</td>
<td><code>https://www.w3.org/ns/web-linking#</code></td>
</tr>
</tbody>
</table>
<p>
Embedding generic namespace URIs in applications is however not encouraged,
since newer versions of the vocabularies may break functionalities. Their
usage should be reserved for applications with powerful infrastructures, like
analytics on large production plants or buildings based on knowledge graphs
that may include devices referencing different versions of these vocabularies.
</p>
</section>
</section>
<section id="conformance">
<p>
A Thing Description instance complies with this specification if it follows
the normative statements in Section
<a href="#sec-vocabulary-definition"></a>
and Section
<a href="#sec-td-serialization"></a>
regarding Thing Description serialization.
</p>
<p>
A JSON Schema [[?JSON-SCHEMA-VALIDATION]] is provided in Appendix
<a href="#json-schema-4-validation"></a>
to validate Thing Description instances based on JSON serialization.
</p>
<p class="ednote">
In the future some information about RDF validation will be provided.
</p>
</section>
<section id="sec-vocabulary-definition" class="normative">
<h1>Information Model</h1>
<!-- h1>Vocabulary</h1 -->
<p>This section introduces the TD information model.
The TD information model serves as the conceptual basis
for the serialization and processing of Thing Description described
in later sections in this document.<p>
<section>
<h2>Overview</h2>
<p>
The TD information model is built upon the following, independent vocabularies:
<ul>
<li>
the <em>core</em> TD vocabulary, which reflects the
<a href="https://www.w3.org/TR/wot-architecture/#sec-interaction-model">WoT interaction model</a>
including property, action and event affordances [[!WOT-ARCHITECTURE]]
</li>
<li>
the <em>Data Schema</em> vocabulary, including (a subset of) the terms defined in
JSON Schema [[?JSON-SCHEMA-VALIDATION]]
</li>
<li>
the <em>WoT Security</em> vocabulary, defining security mechanisms
and configuration requirements to implement them
</li>
<li>
the <em>Web Linking</em> vocabulary, encoding the main principles of RESTful
communication using Web links and forms
</li>
</ul>
</p>
<p>
Each of these vocabularies is essentially a set of terms that can
be used to build data structures, interpreted as objects in the
traditional object-oriented sense. Formally, an object is either denoted
by a term or a set of property-value pairs such that properties are terms
and values denote other objects. Terms may also denote classes (or sets)
of objects.
</p>
<p>
In this context, the TD information model is to be understood as a
set of constraints and equivalence relations between such data structures
that specifically denote WoT Things, i.e. physical world objects.
The constraints of the TD information model are threefolds:
<ul>
<li>
<em>class signature</em>: for all objects denoted by a set of
property-value pairs in the same class, there is a unique set of
terms such that all property names for an object are in this set
(the signature of the class).
</li>
<li>
<em>property value assignment</em>: for all objects denoted by
a set of property-value pairs in the same class, there is a set
of terms such that every term in this set is the property of
some property-value pair for each object (mandatory terms).
</li>
<li>
<em>property type</em>: for all objects denoted by a set of
property-value pairs in the same class, if two pairs for two
distinct objects have the same property, their respective values
must be in the same class (same type).
</li>
</ul>
In turn, the equivalence relations of the TD information model express
that two distinct data structures denote the same object. In practice,
equivalences are defined between data structures that omit certain
property-value pairs, assuming default values, and data structures that
explicitly include these values.
</p>
<p>
These abstract constraints can be intuitively represented as a UML
class diagram, in which classes are shown as tables, each row
giving a property name and a simple type, separated by a colon
(<code>:</code>). A simple type is a class with no signature.
Classes are connected with arrows if there is a property type constraint
between them (associations) or if one denotes a subset of the other
(inheritance).
</p>
<!-- TODO mention cardinalities -->
<p>
The diagram shown next represents all constraints on WoT Things included
in the TD information model. For the sake of readability, the diagram was
split in four parts, one for each of the four base vocabularies.
</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="ednote">
The following diagrams are automatically generated from the ontology files. The layout will be improved in one of the next TD updates.
</p>
<figure id="td-core-model">
<img src="visualization/td.svg" alt="TD core model figure"/>
<figcaption>TD core model (<a href="visualization/td.svg">Click SVG file</a>)</figcaption>
</figure>
<figure id="td-data-schema-model">
<img src="visualization/json-schema.svg" alt="TD data schema model figure"/>
<figcaption>JSON schema model (<a href="visualization/json-schema.svg">Click SVG file</a>)</figcaption>
</figure>
<figure id="td-security-model">
<img src="visualization/wot-security.svg" alt="TD security model figure"/>
<figcaption>WoT security model (<a href="visualization/wot-security.svg">Click SVG file</a>)</figcaption>
</figure>
<figure id="web-linking-model">
<img src="visualization/web-linking.svg" alt="Web linking model figure"/>
<figcaption>Web linking model (<a href="visualization/web-linking.svg">Click SVG file</a>)</figcaption>
</figure>
<p>
Details on the TD information model are given next as tables. First, equivalence relations
are provided in the form of default values for properties for some class. Second, constraints
on the vocabularies mentioned above are provided as class definitions where each table
gives the signature of a class and each row of a table gives some value assignment and
some type for each property, along with a description.
</p>
</section>
<section id="sec-default-values">
<h3>Default Values for Vocabulary Terms</h3>
<p>
<span class="rfc2119-assertion" id="td-vocabulary-defaults">
All equivalence relations expressed in Tables in Section <a href="#sec-default-values"></a>
MUST be followed by a TD processor.
</span>
</p>
<p>
The follow table gives default value assignments when a TD does not
assign certain property to any value.
</p>
<table class="def">
<thead>
<tr>
<th><a>Vocabulary term</a></th>
<th>Default value</th>
<th>Context</th>
</tr>
</thead>
<tbody>
<tr class="rfc2119-default-assertion" id="td-default-contentType">
<td>
<code>contentType</code>
</td>
<td><code>application/json</code></td>
<td></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-safe">
<td>
<code>safe</code>
</td>
<td><code>false</code></td>
<td></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-idempotent">
<td>
<code>idempotent</code>
</td>
<td><code>false</code></td>
<td></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-op-properties">
<td>
<code>op</code>
</td>
<td>Array of strings with values <code>readproperty</code> followed by
<code>writeproperty</code></td>
<td><code>PropertyAffordance</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-op-actions">
<td>
<code>op</code>
</td>
<td><code>invokeaction</code></td>
<td><code>ActionAffordance</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-op-events">
<td>
<code>op</code>
</td>
<td><code>subscribeevent</code></td>
<td><code>EventAffordance</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-in-1">
<td>
<code>in</code>
</td>
<td><code>header</code></td>
<td><code>BasicSecurityScheme</code></br>
<code>DigestSecurityScheme</code></br>
<code>BearerSecurityScheme</code></br>
<code>PoPSecurityScheme</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-in-2">
<td>
<code>in</code>
</td>
<td><code>query</code></td>
<td><code>APIKeySecurityScheme</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-qop">
<td>
<code>qop</code>
</td>
<td><code>auth</code></td>
<td><code>DigestSecurityScheme</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-alg">
<td>
<code>alg</code>
</td>
<td><code>ES256</code></td>
<td><code>BearerSecurityScheme</code></br>
<code>PoPSecurityScheme</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-format">
<td>
<code>format</code>
</td>
<td><code>jwt</code></td>
<td><code>BearerSecurityScheme</code></br>
<code>PoPSecurityScheme</code></td>
</tr>
<tr class="rfc2119-default-assertion" id="td-default-flow">
<td>
<code>flow</code>
</td>
<td><code>implicit</code></td>