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<!DOCTYPE html>
<html lang='en-US'>
<head>
<!-- link rel="stylesheet" href="testing/atrisk.css" -->
<meta charset='utf-8'>
<title>Web of Things (WoT) Architecture 1.1</title>
<script class="remove" src="https://www.w3.org/Tools/respec/respec-w3c"></script>
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editors: [
{
name: "Michael Lagally",
w3cid: "47166",
company: "Oracle Corp.",
companyURL: "https://www.oracle.com/"
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name: "Ryuichi Matsukura",
w3cid: "64284",
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company : "Intel Corp.",
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href: "https://github.com/w3c/wot-architecture/graphs/contributors"
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data: [{
value: "We are on GitHub",
href: "https://github.com/w3c/wot-architecture/"
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href: "https://github.com/w3c/wot-architecture/issues"
}]
}],
localBiblio: {
"CoRE-RD": {
href: "https://datatracker.ietf.org/doc/html/draft-ietf-core-resource-directory-21",
title: "CoRE Resource Directory",
authors: ["M. Koster", "C. Bormann", "P. van der Stok", "C. Amsuess"],
status: "Internet-Draft",
publisher: "IETF",
date: "13 June 2019"
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"IEC-FOTF": {
href: "https://www.iec.ch/basecamp/factory-future",
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publisher: "IEC",
date: "October 2015"
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"IOT-SCHEMA-ORG": {
href: "https://www.w3.org/community/iotschema/",
title: "Schema Extensions for IoT Community Group"
},
"REST": {
href: "https://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf",
title: "REST: Architectural Styles and the Design of Network-based Software Architectures",
authors: ["Roy Thomas Fielding"],
status: "PhD thesis",
publisher: "University of California, Irvine",
date: "2000"
},
"SAREF": {
href: "https://sites.google.com/site/smartappliancesproject/ontologies/reference-ontology",
title: "Smart Appliances REFerence (SAREF) ontology",
publisher: "ETSI",
date: "November 2015"
},
"SOLID": {
href: "https://solidproject.org/TR/",
title: "Solid Technical Reports",
publisher: "W3C Solid CG",
date: "April 2021"
},
"HCI": {
href: "https://www.interaction-design.org/literature/book/the-encyclopedia-of-human-computer-interaction-2nd-ed",
title: "The Encyclopedia of Human-Computer Interaction, 2nd Ed",
publisher: "Interaction Design Foundation",
date: "2013"
},
"NORMAN": {
title: "The Psychology of Everyday Things",
authors: ["Donald A. Norman"],
publisher: "Basic Books",
date: "1988"
},
"MQTT": {
href: "https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html",
title: "MQTT Version 3.1.1 Plus Errata 01",
authors: ["Andrew Banks", "Rahul Gupta"],
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date: "December 2015"
},
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href: "https://openconnectivity.org/developer/specifications/",
title: "OCF Core Specification",
publisher: "Open Connectivity Foundation",
status: "Version 2.0.2",
date: "April 2019"
},
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href: "https://www.w3.org/TR/wot-usecases/",
title: "Web of Things (WoT) Use Cases and Requirements",
publisher: "W3C",
authors: ["Michael Lagally", "Michael McCool", "Ryuichi Matsukura", "Tomoaki Mizushima"],
status: "Editor's Draft",
date: "Oct 2020"
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title: "Lightweight Machine to Machine Technical Specification: Core",
publisher: "OMA SpecWorks",
status: "Approved Version: 1.1",
date: "August 2018"
},
"CoRAL": {
href: "https://datatracker.ietf.org/doc/html/draft-ietf-core-coral/",
title: "The Constrained RESTful Application Language (CoRAL)",
authors: ["Christian Amsüss","Thomas Fossati"],
publisher: "IETF",
status: "Internet-Draft",
date: "March 2022"
},
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href: "https://pdfs.semanticscholar.org/3ee3/a2e8ce93fbf9ba14ad54e12adaeb1f3ca392.pdf",
title: "Mobile Service Interaction with the Web of Things",
authors: ["E. Rukzio, M. Paolucci", "M. Wagner, H. Berndt", "J. Hamard", "A. Schmidt"],
publisher: "Proceedings of 13th International Conference on Telecommunications (ICT 2006), Funchal, Madeira island, Portugal",
date: "May 2006"
},
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href: "https://escholarship.org/uc/item/1786t1dm",
title: "Putting Things to REST",
authors: ["Erik Wilde"],
publisher: "UCB iSchool Report 2007-015, UC Berkeley, Berkeley, CA, USA",
date: "November 2007"
},
"WOT-PIONEERS-3": {
href: "https://www.vs.inf.ethz.ch/publ/papers/ostermai-poster-2008.pdf",
title: "Poster Abstract: Dyser – Towards a Real-Time Search Engine for the Web of Things",
authors: ["Benedikt Ostermaier",
"B. Maryam Elahi",
"Kay Römer",
"Michael Fahrmair",
"Wolfgang Kellerer"],
publisher: "Proceedings of ACM SenSys 2008, Raleigh, NC, USA",
date: "November 2008"
},
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href: "https://ieeexplore.ieee.org/abstract/document/5678452",
title: "A Resource Oriented Architecture for the Web of Things",
authors: ["Dominique Guinard", "Vlad Trifa", "Erik Wilde"],
publisher: "Proceedings of Internet of Things 2010 International Conference (IoT 2010). Tokyo, Japan",
date: "November 2010"
},
"Y.4409-Y.2070": {
href: "https://www.itu.int/rec/T-REC-Y.2070-201501-I",
title: "ITU-T Rec. Y.4409/Y.2070 (01/2015) Requirements and architecture of the home energy management system and home network services ",
publisher: "ITU-T",
status: "Recommendation",
date: "January 2015"
},
"ISO-IEC-2382": {
href: "https://www.iso.org/obp/ui/#iso:std:iso-iec:2382:ed-1:v2:en",
title: "Information technology — Vocabulary",
publisher: "ISO",
date: "2015"
},
"ISO-IEC-27000": {
href: "https://www.iso.org/obp/ui/#iso:std:iso-iec:27000:ed-5:v1:en",
title: "Information technology — Security techniques — Information security management systems — Overview and vocabulary",
publisher: "ISO",
date: "2018"
},
"ISO-IEC-29100": {
href: "https://www.iso.org/obp/ui/#iso:std:iso-iec:29100:ed-1:v1:en",
title: "Information technology — Security techniques — Privacy framework",
publisher: "ISO",
date: "2011"
}
, "WOT-THING-DESCRIPTION": {
title: "Web of Things (WoT) Thing Description 1.1"
, href: "https://www.w3.org/TR/wot-thing-description11/"
, authors: [
"Sebastian Kaebisch"
, "Takuki Kamiya"
, "Michael McCool"
, "Victor Charpenay"
]
, publisher: "W3C"
, date: "November 2020"
}
, "WOT-DISCOVERY": {
title: "Web of Things (WoT) Discovery"
, href: "https://www.w3.org/TR/wot-discovery/"
, authors: [
"Andrea Cimmino"
, "Michael McCool"
, "Farshid Tavakolizadeh"
, "Kunihiko Toumura"
]
, publisher: "W3C"
, date: "November 2020"
}
, "WOT-SECURITY": {
title: "Web of Things (WoT) Security and Privacy Guidelines"
, href: "https://www.w3.org/TR/wot-security/"
//, href: "https://w3c.github.io/wot-security/"
, authors: [
, "Michael McCool"
, "Elena Reshetova"
]
, publisher: "W3C"
, date: "March 2019"
}
}
};
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</head>
<body>
<section id="abstract">
<p>The W3C Web of Things (WoT) enables
interoperability across IoT platforms and application
domains.
The goal of the WoT is to preserve and complement existing
IoT standards and solutions. The W3C WoT architecture is
designed to describe what exists, and only prescribes new mechanisms
when necessary.
</p>
<p>This <em>WoT Architecture</em> specification describes the abstract
architecture for the W3C Web of Things.
This abstract architecture is based on
requirements that were derived from use cases for
multiple application domains.
Several modular building blocks were identified whose detailed
specifications are given in other documents.
This document describes how these building blocks are related and work together.
The WoT abstract architecture defines a basic conceptual
framework that can be mapped onto a variety of concrete deployment scenarios,
several examples of which are given.
However, the abstract architecture described in this specification does not
itself define concrete mechanisms or prescribe any concrete implementation.
</p>
</section>
<section id="sotd">
<p>
This document describes an abstract architecture.
However, there is an
<a href="https://w3c.github.io/wot-architecture/testing/report11.html">Implementation Report</a>
that describes a set of concrete implementations
following the W3C Web of Things architecture.
It also references the other implementation
reports for the various WoT building blocks.
</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 Architecture 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>
-->
<p class="at-risk">At-risk assertions are marked with yellow highlighting.</span></p>
</section>
<section id="introduction" class="informative">
<h1>Introduction</h1>
<p>The goals of the <em>Web of Things</em> (WoT) are to improve the interoperability
and usability of the Internet of Things (IoT). Through a collaboration
involving many stakeholders over many years, several building
blocks have been identified that help address these challenges.
</p>
<p>A set of over 30 WoT <em>use cases</em> were contributed by stakeholders from multiple industries for various
application domains.
These have been collected and were published in the <em>WoT Use Cases and Requirements</em>
<a href="https://www.w3.org/TR/wot-usecases/">https://www.w3.org/TR/wot-usecases/</a> document.
</p>
<p>
The collection of use cases is classified into two categories:
</p>
<ul>
<li>Horizontal use cases that address multiple domains</li>
<li>Domain specific (vertical) use cases for a single application domain</li>
</ul>
<p>These use cases and requirements drive the creation and further evolution
of the W3C WoT specification family.</p>
<p>The WoT architecture specification is focused on the scope of W3C WoT standardization,
which can be broken down into these building blocks as well as the abstract
architecture that defines how they are related.
<p>
The architecture document serves multiple purposes:
</p>
<p>
The building blocks are defined and described in detail in separate specifications.
In addition to defining the abstract architecture and its terminology and
conceptual framework,
this specification also serves as an introduction to the WoT building blocks,
and explains their interworking:</p>
<ul>
<li>The <em>Web of Things (WoT) Thing Description</em> [[?WOT-THING-DESCRIPTION]]
normatively provides a machine-readable
data format for describing the metadata and network-facing interfaces of Things.
It is based upon the fundamental concepts introduced in this document, such as
interaction affordances.
</li>
<li>The <em>Web of Things (WoT) Binding Templates</em> [[?WOT-BINDING-TEMPLATES]]
provides informational guidelines on how to define network-facing interfaces in Things for
particular protocols and IoT ecosystems, which we call Protocol Bindings.
The document also provides examples for a number of existing IoT
ecosystems and standards.
</li>
<li>The <em>Web of Things (WoT) Discovery</em> [[?WOT-DISCOVERY]]
specification defines a distribution mechanism for WoT metadata
(<a>Thing Descriptions</a>).
The WoT Discovery process uses existing mechanisms for first
contact, but provides for access control before serving detailed metadata.
It includes support for directories and self-description.
</li>
<li>The <em>Web of Things (WoT) Scripting API</em> [[?WOT-SCRIPTING-API]],
which is optional, enables the implementation of
the application logic of a Thing using a common JavaScript API
similar to the Web browser APIs. This simplifies IoT application
development and enables portability across vendors and devices.
</li>
<li>The <em>Web of Things (WoT) Security and Privacy Guidelines</em> [[?WOT-SECURITY]]
represent a cross-cutting building block.
This informational document provides guidelines for the secure implementation and
configuration of Things,
and discusses issues which should be considered in any systems implementing W3C WoT.
However, it should be emphasized that
security and privacy can only be fully evaluated in the context
of a complete set of concrete mechanisms for a specific implementation,
which goes beyond the scope of the WoT abstract architecture.
This is especially
true when the WoT architecture is used descriptively for pre-existing systems,
since the W3C WoT cannot constrain the behavior of such systems, it can only
describe them.
In this document we also discuss privacy and security risks and their mitigation
at a high level in sections <a href="#sec-security-considerations"></a>
and <a href="#sec-privacy-considerations"></a>.
</li>
</ul>
<p>This specification also covers non-normative architectural aspects
and conditions for the deployment of WoT systems.
These guidelines are described in the context of example deployment scenarios,
although this specification does not require specific concrete
implementations.
</p>
<p>This specification serves as an umbrella for W3C WoT
specifications and defines the basics such as terminology
and the underlying abstract architecture of the W3C Web of
Things. In summary, the purpose of this specification is to
provide:</p>
<ul>
<li>a set of application domains in <a href="#sec-application-domains"></a>
that were considered to identify use cases for the W3C WoT Architecture,
</li>
<li>a set of common deployment patterns in
<a href="#sec-common-deployment-patterns"></a>,
</li>
<li>a definition of the abstract architecture in
<a href="#sec-wot-architecture"></a>,
</li>
<li>an overview of a set of WoT building blocks
and their interplay in <a href="#sec-building-blocks"></a>,
</li>
<li>an informative guideline on how to map the abstract architecture to
concrete implementations in
<a href="#sec-servient-implementation"></a>,
</li>
<li>informative examples of deployment scenarios in
<a href="#sec-deployment-scenario"></a>,
</li>
<li>and a set of high level
security and privacy considerations to be aware of when
implementing a system based on the W3C WoT architecture in
<a href="#sec-security-considerations"></a> and
<a href="#sec-privacy-considerations"></a>, respectively.
</li>
</ul>
<p>Additional requirements, use cases, conceptual features and new building blocks
are collected in future versions of the <em>WoT Use Cases and Requirement </em>
<a href="https://www.w3.org/TR/wot-usecases/">https://www.w3.org/TR/wot-usecases/</a> document.
</p>
</section>
<section id="conformance"></section>
<section id="terminology" class="informative">
<h1>Terminology</h1>
<p>This specification uses the following terms as defined here.
The WoT prefix is used to avoid ambiguity for terms that are
(re)defined specifically for Web of Things concepts.</p>
<p>
In case of a conflict of a definition with terminology used in another
WoT document, the definition of the WoT Architecture takes precedence.
</p>
<dl>
<dt>
<dfn>Action</dfn>
</dt>
<dd>An Interaction Affordance that allows to invoke a
function of the Thing, which manipulates state
(e.g., toggling a lamp on or off)
or triggers a process on the Thing (e.g., dim a lamp over time).</dd>
<dt>
<dfn data-lt="WoT Anonymous Thing Description" class="lint-ignore">Anonymous TD</dfn>
</dt>
<dd>A Thing Description without a user-defined identifier (`id` attribute).</dd>
<dt>
<dfn>Connected Device</dfn>
</dt>
<dd>
A synonym for <a>Device</a>.</dd>
<dt>
<dfn data-lt="WoT Binding Templates">Binding
Templates</dfn>
</dt>
<dd>
A re-usable collection of blueprints that enable a Thing Description
to be used with a specific protocol, data payload format or an IoT
platform that combine both of them in specific ways. This is done through
additional descriptive vocabularies, Thing Models and examples that aim
to guide the implementers of Things and Consumers alike. </dd>
<dt>
<dfn>Consumed Thing</dfn>
</dt>
<dd>A software abstraction that represents a remote
Thing used by the local application. The abstraction might be
created by a native WoT Runtime, or instantiated
as an object through the WoT Scripting API.</dd>
<dt>
<dfn>Content Type</dfn>
</dt>
<dd>Identifier for the format of the message body.
Also known as media type and MIME type [[RFC2046]]. </dd>
<dt>
<dfn class="lint-ignore">Consuming a Thing</dfn>
</dt>
<dd>To parse and process a TD document and from it create a Consumed
Thing software abstraction as interface for the application in the local
runtime environment.</dd>
<dt>
<dfn>Consumer</dfn>
</dt>
<dd>An entity that can process WoT Thing Descriptions
(including its JSON-based representation format)
and interact with Things (i.e., consume Things).</dd>
<dt>
<dfn>Data Schema</dfn>
</dt>
<dd>A data schema describes the information model and the related payload structure
and corresponding data items that are passed between <a>Things</a>
and <a>Consumers</a> during interactions.</dd>
<dt>
<dfn>Device</dfn>
</dt>
<dd>A Device is a physical entity that has a network interface.
Devices can be described by a <a>Thing Description</a> and are a kind of <a>Thing</a>.
A synonym for <a>Connected Device</a>.
Compare with <a>Service</a>.</dd>
<dt>
<dfn>Digital Twin</dfn>
</dt>
<dd>A digital twin is type of <a>Virtual Thing</a>
that resides on a cloud or edge node.
Digital Twins may be used to represent and provide
a network interface for
real-world devices which may not be continuously online
(see also <a>Shadows</a>),
may be able to run simulations of new applications and services
before they get deployed to the real devices,
may be able to maintain a history of past state or behaviour,
and may be able to predict future state or behaviour.
Digital Twins typically have more functionality than
simple <a>Shadows</a>.
</dd>
<dt>
<dfn class="lint-ignore">Directory</dfn>
</dt>
<dd>A <a>Service</a> that maintains a set of data or metadata describing other <a>Services</a>
or <a>Things</a>. An example would be a <a>WoT Thing Description Directory</a>.</dd>
<dt><dfn data-lt="WoT Discovery">Discovery</dfn>
<dd>Mechanisms defined by WoT for distributing and accessing
<a>WoT Thing Descriptions</a> on the network,
either locally or remotely.</dd>
<dt>
<dfn data-lt="WoT Discoverer" class="lint-ignore">Discoverer</dfn>
</dt>
<dd>An entity which acts as a client of a WoT Discovery process to discover
and fetch a <a>Thing Description</a>,
e.g. by being introduced to and searching a
<a>Thing Description Directory</a> exploration service or by
fetching a <a>Thing Description</a> directly from
the well-known endpoint on a Thing.
</dd>
<dt>
<dfn>Domain-specific Vocabulary</dfn>
</dt>
<dd>Linked Data vocabulary that can be used in the WoT
Thing Description, but is not defined by W3C WoT.</dd>
<dt>
<dfn>Edge Device</dfn>
</dt>
<dd>A device that provides an entry point into
enterprise or service provider core networks. Examples
include hubs, gateways, routers, switches, multiplexers, and a
variety of other access devices.</dd>
<dt>
<dfn data-lt="WoT Enriched Thing Description" class="lint-ignore">Enriched TD</dfn>
</dt>
<dd>A Thing Description embedded with additional attributes
for bookkeeping and discovery.</dd>
<dt>
<dfn>Event</dfn>
</dt>
<dd>An Interaction Affordance that describes an event source,
which asynchronously pushes event data to Consumers
(e.g., overheating alerts).</dd>
<dt>
<dfn data-lt="WoT Exploration">Exploration</dfn>
</dt>
<dd>A discovery mechanism that provides access to detailed metadata in the
form of one or more Thing Descriptions. Exploration mechanisms are in
general protected by security mechanism and are accessible only to authorized users.
</dd>
<dt>
<dfn>Exposed Thing</dfn>
</dt>
<dd>A software abstraction that represents a locally hosted Thing
that can be accessed over the network by remote Consumers.
The abstraction might be created by a native WoT Runtime,
or instantiated as an object through the WoT Scripting API.</dd>
<dt>
<dfn class="lint-ignore">Exposing a Thing</dfn>
</dt>
<dd>To create an Exposed Thing software abstraction in the
local runtime environment to manage the state of a Thing
and interface with the behavior implementation.</dd>
<dt>
<dfn>Hypermedia Control</dfn>
</dt>
<dd>A serialization of a Protocol Binding in hypermedia, that is,
either a Web link [[RFC8288]] for navigation or a Web form for
performing other operations. Forms can be seen as request templates
provided by the Thing to be completed and sent by the Consumer.</dd>
<dt>
<dfn>Interaction Affordance</dfn>
</dt>
<dd>
Metadata of a Thing that shows and describes the possible choices to Consumers,
thereby suggesting how Consumers may interact with the Thing.
There are many types of potential affordances, but
W3C WoT defines three types of Interaction Affordances:
Properties, Actions, and Events.
A fourth Interaction Affordance is navigation, which is already available on the Web through linking.</dd>
<dt>
<dfn>Interaction Model</dfn>
</dt>
<dd>An intermediate abstraction that formalizes and narrows the
mapping from application intent to concrete protocol operations.
In W3C WoT, the defined set of Interaction Affordances constitutes the Interaction Model.</dd>
<dt>
<dfn>Intermediary</dfn>
</dt>
<dd>An entity between Consumers and Things that can proxy, augment, or compose Things
and republish a WoT Thing Description that points to the WoT Interface on the Intermediary instead of the
original Thing.
For Consumers, an Intermediary may be indistinguishable from a Thing, following the Layered System constraint of
REST.</dd>
<dt>
<dfn data-lt="WoT Introduction">Introduction</dfn>
</dt>
<dd>A "first contact" discovery mechanism, whose result is a URL that
references an exploration mechanism. Introduction mechanisms themselves
should not directly provide metadata, and in general are designed to be
open.
</dd>
<dt>
<dfn>IoT Platform</dfn>
</dt>
<dd>A specific IoT ecosystem such as OCF, oneM2M, or
Mozilla Project Things with its own specifications for
application-facing APIs, data model, and protocols or
protocol configurations.</dd>
<dt>
<dfn class="lint-ignore">Metadata</dfn>
</dt>
<dd>Data that provides a description of an entity's abstract characteristics.
For example, a <a>Thing Description</a> is Metadata for a <a>Thing</a>.</dd>
<dt>
<dfn data-lt="Personally Identifiable Information">Personally Identifiable Information (PII)</dfn>
</dt>
<dd>
Any information that can be used to identify the natural person to whom such information relates,
or is or might be directly or indirectly linked to a natural person.
We use the same definition as [[ISO-IEC-29100]].
</dd>
<dt>
<dfn class="lint-ignore">Orchestration</dfn>
</dt>
<dd>
The automation of the behavior of a collection of things.
Orchestration is combining individual things with rules or services
into a new service or virtual <a>Thing</a>.
</dd>
<dt>
<dfn>Partial TD</dfn>
</dt>
<dd>
A <a>Partial TD</a> is an object that follows the same hierarchical structure of the <a>TD</a> information model,
but it is not required to contain all the mandatory elements.
<p>
Note: An example for the usage of a <a>Partial TD</a> is in <a>WoT Scripting API</a>,
where it is used as input for the creation of <a>Exposed Things</a>.
</p></dd>
<dt>
<dfn class="lint-ignore">Privacy</dfn>
</dt>
<dd>Freedom from intrusion into the private life or affairs of an individual when that intrusion results from
undue or illegal gathering and use of data about that individual.
We use the same definition as [[ISO-IEC-2382]].
See also <a>Personally Identifiable Information</a> and <a>Security</a>,
as well as other related definitions in [[ISO-IEC-29100]].
</dd>
<dt>
<dfn>Private Security Data</dfn>
</dt>
<dd>
Private Security Data is that component of a Thing's <a>Security Configuration</a> that is
kept secret and is not shared with other devices or users. An example would be private keys in a PKI
system. Ideally such data is stored in a separate memory inaccessible to the application
and is only used via abstract operations, such as signing, that do not reveal the secret
information even to the application using it.</dd>
<dt>
<dfn>Producer</dfn>
</dt>
<dd>An entity that can create WoT Thing Descriptions
for a specific Thing.</dd>
<dt>
<dfn>Profile</dfn>
</dt>
<dd>A technical specification which provides a set of assertions such that any <a>Consumer</a>
which conforms with the those assertions is out-of-the-box interoperable with any <a>Thing</a>
which also conforms with those assertions.
</dd>
<dt>
<dfn>Property</dfn>
</dt>
<dd>An Interaction Affordance that exposes state of the Thing.
This state can then be retrieved (read) and optionally updated (write).
Things can also choose to make Properties observable by notifying <a>Consumers</a>
about a state change.</dd>
<dt>
<dfn data-lt="WoT Protocol Binding">Protocol Binding</dfn>
</dt>
<dd>The mapping from an Interaction Affordance to concrete messages of a specific protocol,
thereby informing <a>Consumers</a> how to activate the <a>Interaction Affordance</a>.
W3C WoT serializes Protocol Bindings as <a>hypermedia controls</a>.</dd>
<dt>
<dfn>Public Security Metadata</dfn>
</dt>
<dd>
Public Security Metadata is that component of a Thing's <a>Security Configuration</a> which
describes the security mechanisms and access rights necessary to access a Thing.
It does not include any secret information or concrete data (including public keys), and does
not by itself, provide access to the Thing. Instead, it describes the mechanisms by which access
may be obtained by authorized users, including how they must authenticate themselves.
</dd>
<dt>
<dfn data-lt="WoT Registrant" class="lint-ignore">Registrant</dfn>
</dt>
<dd>An entity which registers a <a>Thing Description</a>
with a <a>Thing Description Directory</a>.
This entity may or may not be the <a>Thing</a> that
the registered <a>Thing Description</a> describes.
</dd>
<dt>
<dfn>Security</dfn>
</dt>
<dd>Preservation of the confidentiality, integrity and availability of information.
Properties such as authenticity, accountability, non-repudiation, and reliability may also be involved.
This definition is adapted from the definition of <i>Information Security</i> in [[ISO-IEC-27000]], which
also includes additional definitions of each of the more specific properties mentioned.
Please refer to this document for other related definitions.
We additionally note that it is desirable that these properties be maintained both in normal operation
and when the system is subject to attack.
</dd>
<dt>
<dfn>Security Configuration</dfn>
</dt>
<dd>The combination of Public Security Metadata, Private Security Data, and any other configuration
information (such as public keys) necessary to operationally configure the security mechanisms of a Thing.</dd>
<dt>
<dfn>Service</dfn>
</dt>
<dd>A Service is a software entity that has a network interface.
Services can be described by a <a>Thing Description</a> and are a kind of <a>Thing</a>.
See also <a>Virtual Thing</a>.
Compare with <a>Device</a>.</dd>
<dt>
<dfn>Servient</dfn>
</dt>
<dd>A software stack that implements the WoT building
blocks. A Servient can host and expose Things and/or host Consumers that consume Things.
Servients can support multiple Protocol Bindings to enable
interaction with different IoT platforms.</dd>
<dt>
<dfn>Shadow</dfn>
</dt>
<dd>A Shadow is a <a>Virtual Thing</a> that
maintains a copy of the state and mediates interactions with another
<a>Thing</a>.
A Shadow aims to achieve eventual consistency with the state
of the Thing it represents.
If a Shadow has more functionality than simply mirroring state
it may be better to refer to it as a <a>Digital Twin</a>.
</dd>
<dt>
<dfn>Subprotocol</dfn>
</dt>
<dd>An extension mechanism to a transport protocol that
must be known to interact successfully.
An example is long polling for HTTP.</dd>
<dt>
<dfn class="lint-ignore">System</dfn>
</dt>
<dd>An entity consisting of multiple interacting components.</dd>
<dt>
<dfn>TD</dfn>
</dt>
<dd>Short for <a>WoT Thing Description</a>.</dd>
<dt>
<dfn>TDD</dfn>
</dt>
<dd>Short for <a>WoT Thing Description Directory</a>.</dd>
<dt>
<dfn class="lint-ignore">TD Vocabulary</dfn>
</dt>
<dd>A Linked Data vocabulary controlled by W3C WoT to
tag the metadata of Things in the WoT Thing Description
including communication metadata of WoT Binding
Templates.</dd>
<dt>
<dfn class="lint-ignore">TD Context Extension</dfn>
</dt>
<dd>A mechanism to extend <a>Thing Descriptions</a> with additional <a>Vocabulary
Terms</a> using <code>@context</code> as specified in JSON-LD[[?JSON-LD11]].
It is the basis for semantic annotations and extensions to core
mechanisms such as Protocol Bindings, Security Schemes, and Data Schemas.</dd>
<dt>
<dfn class="lint-ignore">TD Server</dfn>
</dt>
<dd>Short for <a>Thing Description Server</a>.</dd>
<dt>
<dfn>Thing</dfn> or <dfn>Web Thing</dfn>
</dt>
<dd>An abstraction of a physical or a virtual entity
whose metadata and interfaces are described by a WoT
Thing Description, whereas a virtual entity is the
composition of one or more Things.</dd>
<dt>
<dfn data-lt="WoT Thing Description Directory">Thing Description Directory</dfn>
</dt>
<dd>A directory service for TDs that provides a Web
interface to register TDs and look them up
(e.g., using JSONPath or SPARQL queries).
A recommended API and feature set is defined in [[WOT-DISCOVERY]], and is
used as an optional part of the <a>WoT Discovery</a> process.
</dd>
<dt>
<dfn>TD Fragment</dfn>
</dt>
<dd>
A <a>TD Fragment</a> is a substructure of the data model of a TD.
It is a valid object structure that can be validated syntactically against a part of the TD information model
defined in chapter 5 of the <a>Thing Description</a>specification, however the fragment may omit some context that allows full validation.
</dd>
<dt>
<dfn data-lt="WoT Thing Description Server">Thing Description Server</dfn>
</dt>
<dd>A Thing Description Server is a web resource, addressed by a URL, that can provide
a Thing Description when accessed.
Its requirements are defined in [[WOT-DISCOVERY]], and is
used as an optional part of the <a>WoT Discovery</a> process.
</dd>
<dt>
<dfn data-lt="WoT Thing Model">Thing Model</dfn>
</dt>
<dd>A <a>Thing Model</a> is a description for a class of Things that have the same
capabilities. It describes the <a>Properties</a>, <a>Actions</a>, and <a>Events</a> and common metadata that are
shared for an entire group of <a>Things</a>. Compared to a Thing Description, a Thing Model does not contain enough
information to identify or interact with a Thing instance.</dd>
<dt>
<dfn>Transport Protocol</dfn>
</dt>
<dd>The underlying, standardized application layer
protocol without application-specific requirements or
constraints on options or subprotocol mechanisms.
Examples are HTTP, CoAP, or MQTT.
</dd>
<dt>
<dfn>Trusted Environment</dfn>
</dt>
<dd>Set of devices that assume each other's claims of
identity are authentic without proof and allow relatively unrestricted
access to one another over a common protected network.
</dd>
<dt>
<dfn>Virtual Thing</dfn>
</dt>
<dd>A <a>Service</a> that
represents,
augments the functionality of,
provides an improved interface to,
or
stands in place of
one or more other <a>Things</a>.
A Virtual Thing will often act as an <a>Intermediary</a>.
Examples include <a>Shadows</a> and <a>Digital Twins</a>.
</dd>
<dt>
<dfn>Vocabulary</dfn>
</dt>
<dd>
A collection of <a>Vocabulary Terms</a>, identified by a namespace IRI.
</dd>
<dt>
<dfn>Term</dfn>
and
<dfn>Vocabulary Term</dfn>
</dt>
<dd>
A character string. When a <a>Term</a> is part of a <a>Vocabulary</a>, i.e., prefixed by
a namespace IRI[[RFC3987]], it is called a <a>Vocabulary Term</a>. For the sake of readability,
<a>Vocabulary Terms</a> present in this document are always written in a compact
form and not as full IRIs.
</dd>
<dt>
<dfn>WoT Interface</dfn>
</dt>
<dd>The network-facing interface of a Thing
that is described by a WoT Thing Description.</dd>
<dt>
<dfn>WoT Profile</dfn>
</dt>
<dd>Synonym for <a>Profile</a></dd>
<dt>
<dfn>WoT Runtime</dfn>
</dt>
<dd>A runtime system that maintains an execution
environment for applications, and is able to expose and/or
consume Things, to process WoT Thing Descriptions, to maintain Security
Configurations, and to interface with Protocol Binding implementations.
A WoT Runtime may have a custom API or use the optional WoT Scripting API.</dd>
<dt>
<dfn>WoT Scripting API</dfn>
</dt>
<dd>The application-facing programming interface
provided by a Servient in order to ease
the implementation of behavior or applications running in a WoT
Runtime. It is comparable to the Web browser APIs.
The WoT Scripting API is an optional building block for W3C WoT.</dd>
<dt>
<dfn class="lint-ignore">WoT Servient</dfn>
</dt>
<dd>Synonym for Servient.</dd>
<dt>
<dfn>WoT Thing Description</dfn> or <dfn>Thing Description</dfn>
</dt>
<dd>Structured data describing a Thing. A WoT Thing Description comprises
general metadata, domain-specific metadata, Interaction Affordances
(which include the supported Protocol Bindings), and links to related Things.
The WoT Thing Description format is the central building block of W3C WoT.</dd>
</dl>
<!-- Device categories -->
<section id="device-categories">
<h2>Device Categories</h2>
<p>
In a deployment of WoT conforming to the <a href="https://www.w3.org/TR/wot-architecture/#architecture-abstract">WoT Abstract Architecture</a>
we see a variety of different device types.
They range (sorted in the order of footprint and capabilities)
from small embedded <em>node</em> devices to <em>gateways</em> or <em>hubs</em>
to powerful <em>edge</em> devices and <em>cloud</em> servers.
Interoperability between these devices implies that a core set of features and functionalities
is available on <em>all</em> of them.
</p>
<p>
The following device categories describe the footprint and characteristics of
typical representatives of these classes.
This is used to identify the possible features and use cases for these device classes.
</p>
<p>
These categories are aligned with the classes defined by the IETF [[RFC7228]] for constrained devices,
however the classes have been extended to larger devices and
bounds on typical sizes of RAM and Flash/ROM are provided.
Memory and storage size are both easier to quantify and more limiting than performance,
so that is the basis of this categorization.
This is not a strict categorization.
Categories may overlap and not all memory may be available for user applications.
</p>
<table class="def">
<tr>
<th>Category</th>
<th>data size (RAM)</th>
<th>code size (Flash, ROM, ...)</th>
<th>Typical Representative</th>
<th>Remarks, typical application scenarios</th>
</tr>
<tr>
<td>Class-0, C0</td>
<td><< 10 KiB</td>
<td><< 100 KiB</td>
<td>small footprint microcontrollers</td>
<td>Sensor nodes without secure communication</td>
</tr>
<tr>
<td>Class-1, C1</td>
<td>~ 10 KiB</td>
<td>~ 100 KiB</td>
<td>microcontrollers</td> <!-- could just call this a "node" or "basic node" -->
<td>Sensors that typically supports secure communication protocols such as TLS, DTLS</td>
</tr><tr>
<td>Class-2, C2</td>
<td>~ 64 KiB</td>