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.
Status of This Document
This section describes the status of this document at
the time of its publication. Other documents may supersede this
document. A list of current W3C publications and the
latest revision of this technical report can be found in the
W3C technical reports
index at https://www.w3.org/TR/.
Status Update (June 2020): The link to the commit history was fixed in-place on June 23 2020 after the renaming of a branch.
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as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation
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Claim(s) must disclose the information in accordance with
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6 of the W3C
Patent Policy.
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
Thing (much like the index.html of a Web
site). A TD instance has four main components: textual metadata
about the Thing itself, a set of Interaction Affordances that
indicate how the Thing can be used, schemas for the
data exchanged with the Thing for
machine-understandability, and, finally, Web links to
express any formal or informal relation to other Things or documents on the Web.
The Interaction Model of W3C WoT defines three types
of Interaction Affordances: Properties
(PropertyAffordance
class) can be used for sensing and controlling parameters, such
as getting the current value or setting an operation state.
Actions (ActionAffordance class)
model invocation of physical (and hence time-consuming)
processes, but can also be used to abstract RPC-like calls of
existing platforms. Events (EventAffordance 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.
In general, the TD provides metadata for different Protocol Bindings identified by URI schemes
[RFC3986]
(e.g., http, coap, etc.
[IANA-URI-SCHEMES]),
content types based on media types [RFC2046]
(e.g., application/json,
application/xml, application/cbor,
application/exi, 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.
Example 1
shows a TD instance and illustrates the Interaction Model with Properties, Actions, and
Events by describing a lamp Thing with the title
MyLampThing.
From this TD example, we know there exists one Property affordance with the title
status. 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
https://mylamp.example.com/status (announced
within the forms structure by the
href 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.
In a similar manner, an Action
affordance is specified to toggle the switch status using
the POST method on the
https://mylamp.example.com/toggle resource, where
POST is again a default assumption for invoking Actions.
The Event affordance 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
HTTP with its long polling subprotocol on
https://mylamp.example.com/oh.
This example also specifies the basic security
scheme, requiring a username and password for access. Note that
a security scheme is first given a name in
securityDefinitions and then activated by
specifying that name in a security 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
nosec security scheme to indicate that no access
control mechanisms are used. Additional examples will be
provided later.
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
forms field. Example 2 extends
the TD sample from Example 1 by introducing a second defintion
in the @context to declare the prefix
saref as referring to SAREF, the Smart Appliance
Reference Ontology [SMARTM2M].
This IoT ontology includes terms interpreted as semantic labels
that can be set as values of the @type field,
giving the semantics of Things and their
Interaction Affordances. In the
example below, the Thing is labelled with
saref:LightSwitch, the statusProperty is labelled with
saref:OnOffState and the toggleAction with saref:ToggleCommand.
The declaration mechanism inside some @context
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 § D. JSON-LD Context Usage and the
documentation under the namespace IRIs, e.g., https://www.w3.org/2019/wot/td).
2.
Conformance
As well as sections marked as non-normative, all authoring
guidelines, diagrams, examples, and notes in this specification
are non-normative. Everything else in this specification is
normative.
The key words MAY, MUST, MUST NOT,
RECOMMENDED, SHOULD, and SHOULD NOT
in this document are to be interpreted as described in BCP 14 [RFC2119]
[RFC8174]
when, and only when, they appear in all capitals, as shown
here.
The fundamental WoT terminology such as Thing,
Consumer, Thing Description
(TD),
Interaction
Model, Interaction Affordance,
Property, Action,
Event, Protocol
Binding, Servient,
WoT
Interface, WoT Runtime, etc. is defined in
Section
3 of the WoT Architecture specification [WOT-ARCHITECTURE].
In addition, this specification introduces the following
definitions:
TD Context
Extension
A mechanism to extend Thing
Descriptions with additional Vocabulary
Terms. It is the basis for semantic annotations and
extensions to core mechanisms such as Protocol Bindings,
Security Schemes, and Data Schemas.
A system that can serialize some internal representation of
a Thing Description in a given
format and/or deserialize it from that format. A TD Processor must detect semantically
inconsistent Thing Descriptions,
that is, Thing Descriptions
that cannot satisfy constraints on the Instance Relation of the
Thing class. For that purpose, a TD Processor may compute canonical forms of
Thing Descriptions in which all
possible Default Values are assigned. A
TD Processor is typically a
sub-system of a WoT Runtime.
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).
TD Serialization or TD Document
Textual or binary representation of Thing Descriptions that can be
stored and exchanged between Servients. A
TD Serialization follows a given
representation format, identified by a media type when
exchanged over the network. The default representation
format for Thing Descriptions
is JSON-based as defined by this specification.
Vocabulary
A collection of Vocabulary Terms,
identified by a namespace IRI.
Term and Vocabulary Term
A character string. When a Term is part of a
Vocabulary, i.e., prefixed by a namespace IRI, it
is called a Vocabulary Term. For the sake of
readability, Vocabulary Terms present in this
document are always written in a compact form and not as
full IRIs.
In the present specification, Vocabulary Terms
are always presented in their compact form. Their expanded form
can be accessed under the namespace IRI of the Vocabulary they belong to. These namespaces follow
the structure of § 5.3 Class
Definitions. Each Vocabulary used in the TD Information Model has its own namespace IRI,
as follows:
Vocabulary
Namespace IRI
Core
https://www.w3.org/2019/wot/td#
Data Schema
https://www.w3.org/2019/wot/json-schema#
Security
https://www.w3.org/2019/wot/security#
Hypermedia Controls
https://www.w3.org/2019/wot/hypermedia#
The Vocabularies are independent from each other. They
may be reused and extended in other W3C specifications. Every
breaking change in the design of a Vocabulary will require
the assignment of a new year-based namespace URI. Note that to
maintain the general coherence of the TD Information
Model, the associated JSON-LD context file is versioned
such that every version has its own URI (v1,
v1.1, v2, ...) to also identify
non-breaking changes, in particular the addition of new
Terms.
Because a Vocabulary 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 § 9.1 Context Fetching
Privacy Risk).
The UML diagram shown next gives an overview of the
TD Information Model. It represents all classes as
tables and the associations that exist between classes,
starting from the class Thing, as directed arrows. For the
sake of readability, the diagram was split in four parts, one
for each of the four base Vocabularies.
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 TD Information
Model accordingly.
All definitions in this section refer to sets,
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
Object is a data structure recursively
defined as follows:
a set of name-value pairs where the name is a
Term and the value is another Object, is also an Object.
Though this definition does not prevent Objects to include multiple name-value pairs with
the same name, they are generally not considered in this
specification. An Object whose elements only have
numbers as names is called an Array. Similarly,
an Object whose elements only have Terms (that do
not belong to any Vocabulary) as names is called a
Map. All names appearing in some name-value
pair in a Map are assumed to be
unique within the scope of the Map.
On the basis of these two functions, an
Instance
Relation can be defined for a pair composed of an
Object and a Class. This relation
is defined as constraints to be satisfied. That is, an
Object is an instance of a Class if the two
following constraints are both satisfied:
According to the definition above, an Object would be an instance of every Simple Type, regardless of its structure. Instead,
another definition for the Instance
Relation is introduced for Simple Types: an
Object is an instance of a Simple Type if it is a Term with a given
lexical form (e.g., true, false for
the boolean type, 1,
2, 3, ... for the
unsignedInt type, etc.).
Moreover, additional Classes, called
Parameterized Classes, can
be derived from the generic Map and Array structures. An Object is a Map
of some Class, that is, an instance of the
Map type parameterized with some Class, if it is a Map such that the
value in all the name-value pairs it contains is an instance
of this Class. The same applies to Arrays.
Finally, a Class is a Subclass of
some other Class if every instance of the
former is also an instance of the latter.
By convention, Simple Types are
denoted by names starting with lowercase. The TD Information Model references the following
Simple Types defined in XML
Schema [XMLSCHEMA11-2-20120405]:
string, anyURI,
dateTime, integer,
unsignedInt, double, and
boolean. Their definition (i.e., the
specification of their lexical form) is outside of the scope
of the TD Information Model.
The formalization introduced here does not consider the
possible relation between Objects as abstract
data structures and physical world objects such as Things. However, care was given to the possibility
of re-interpreting all Vocabulary Terms
involved in the TD Information Model as RDF
resources, so as to integrate them in a larger model of the
physical world (an ontology). For details about semantic
processing, please refer to § D. JSON-LD
Context Usage and the documentation under the namespace
IRIs, e.g., https://www.w3.org/2019/wot/td.
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.
Define the base URI that is used for all
relative URI references throughout a TD document.
In TD instances, all relative URIs are resolved
relative to the base URI using the algorithm
defined in [RFC3986].
base does not affect the URIs used in
@context and the IRIs used within
Linked Data [LINKED-DATA]
graphs that are relevant when semantic processing
is applied to TD instances.
Set of form hypermedia controls that describe how
an operation can be performed. Forms are
serializations of Protocol Bindings. In this
version of TD, all operations that can be
described at the Thing level are concerning how
to interact with the Thing's Properties collectively
at once.
The @context
name-value pair MUST contain the anyURI
https://www.w3.org/2019/wot/td/v1 either
directly when of type anyURI or as first
element when of type Array.When @context
is an Array, the anyURI
https://www.w3.org/2019/wot/td/v1MAY be followed by elements of
type anyURI or type Map in any
order, while it is RECOMMENDED to include only one
Map with all the name-value pairs in the
@contextArray.Maps contained in an @contextArrayMAY
contain name-value pairs, where the value is a namespace
identifier of type anyURI and the name a
Term or prefix denoting that namespace.One Map
contained in an @contextArraySHOULD contain a name-value pair that
defines the default language for the Thing Description,
where the name is the Term@language and the value is a well-formed
language tag as defined by [BCP47] (e.g.,
en, de-AT, gsw-CH,
zh-Hans, zh-Hant-HK,
sl-nedis).
The computation of the base direction of all
human-readable text strings is defined by the following
set of rules:
If no
language tag is given, the base direction SHOULD be inferred
through first-strong heuristics or detection algorithms
such as the CLDR Likely Subtags [LDML].
Outside
of MultiLanguageMaps, the base
direction MAY be
inferred from the language tag of the default
language.
Inside
of MultiLanguageMaps, the base
direction of each value of the name-value pairs
MAY be inferred
from the language tag given in the corresponding
name.
In cases
where a language can be written in more than one script
with different base directions, the corresponding
language tag given in @language or
MultiLanguageMapsMUST include a script
subtag, so that an appropriate base direction can be
inferred. An example is Azeri, which is written
LTR when Latin script is used (specified using
az-Latn) and RTL when Arabic script is
used (specified using az-Arab).
TD Processors should be aware of
certain special cases when processing bidirectional text.
They should take care to use bidi isolation when
presenting strings to users, particularly when embedding
in surrounding text (e.g., for Web user interface). Mixed
direction text can occur in any language, even when the
language is properly identified.
TD producers should attempt to provide mixed direction
strings in a way that can be displayed successfully by a
naive user agent. For example, if an RTL string begins
with an LTR run (such as a number or a brand or trade
name in Latin script), including an RLM character at the
start of the string or wrapping opposite direction runs
in bidi controls can assist in proper display.
Strings on the Web: Language and Direction
Metadata [string-meta]
provides some guidance and illustrates a number of
pitfalls when using bidirectional text.
In addition to the
explicitly provided Interaction
Affordances in the properties,
actions, and eventsArrays, a Thing can also
provide meta-interactions, which are indicated by
Form instances in its optional
formsArray.
When
the formsArray of a
Thing instance contains Form
instances, the string values assigned to the name
op, either directly or within an Array, MUST be one of the following operation
types: readallproperties,
writeallproperties,
readmultipleproperties, or
writemultipleproperties. (See
an example for an
usage of form in a Thing instance.)
The data schema for each of these meta-interactions is
constructed by combining the data schemas of each
PropertyAffordance instance in a single
ObjectSchema instance, where the
propertiesMap of the
ObjectSchema instance contains each data
schema of the PropertyAffordances identified
by the name of the corresponding
PropertyAffordances instance.
If not specified otherwise (e.g., through a TD Context Extension), the request data of the
readmultipleproperties operation is an
Array that contains the intended
PropertyAffordances instance names, which is
serialized to the content type specified by the
Form instance.
5.3.1.2
InteractionAffordance
Metadata of a Thing that shows 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.
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 pushing the new state after
a change.
A hint that indicates whether Servients hosting
the Thing and Intermediaries should provide a
Protocol Binding that supports the
observeproperty operation for this
Property.
Property instances are also instances of
the class DataSchema. Therefore, it can contain the
type, unit,
readOnly and writeOnly
members, among others.
PropertyAffordance is a Subclass of the
InteractionAffordanceClass and
the DataSchemaClass.
When a Form instance is within a
PropertyAffordance instance, the value
assigned to opMUST be one of readproperty,
writeproperty, observeproperty,
unobserveproperty or an Array
containing a combination of these terms.
5.3.1.4
ActionAffordance
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).
Signals if the Action is safe (=true) or not.
Used to signal if there is no internal state (cf.
resource state) is changed when invoking an Action.
In that case responses can be cached as
example.
Indicates whether the Action is idempotent
(=true) or not. Informs whether the Action can be
called repeatedly with the same result, if present,
based on the same input.
ActionAffordance is a Subclass of the
InteractionAffordanceClass.
When a Form instance is within an
ActionAffordance instance, the value
assigned to op MUST
be invokeaction.
5.3.1.5
EventAffordance
An Interaction Affordance that describes an event
source, which asynchronously pushes event data to
Consumers (e.g., overheating
alerts).
EventAffordance is a Subclass of the
InteractionAffordanceClass.
When
a Form instance is within an EventAffordance
instance, the value assigned to opMUST be either
subscribeevent,
unsubscribeevent, or both terms within an
Array.
5.3.1.6 VersionInfo
Metadata of a Thing that provides version information
about the TD document. If required, additional version
information such as firmware and hardware version (term
definitions outside of the TD namespace) can be extended
via the TD Context Extension
mechanism.
It is recommended that the values within instances of
the VersionInfoClass follow the
semantic versioning pattern, where a sequence of three
numbers separated by a dot indicates the major version,
minor version, and patch version, respectively. See
[SEMVER] for
details.
5.3.1.7 MultiLanguage
A Map providing a set of
human-readable texts in different languages identified by
language tags described in [BCP47]. See
§ 6.3.2
Human-Readable Metadata for example usages of this
container in a Thing Description instance.
Each name of the
MultiLanguageMapMUST be a language tag as
defined in [BCP47].Each value of the
MultiLanguageMapMUST be of type
string.
5.3.2 Data Schema Vocabulary
Definitions
The data schema vocabulary definition is reflecting a
very common subset of the terms defined by JSON Schema
[JSON-SCHEMA].
It is noted that data schema definitions within Thing
Description instances are not limited to this defined
subset and may use additional terms found in JSON Schema
using a TD Context Extension for the
additional terms as described in § 7. TD Context
Extensions, otherwise these terms are semantically
ignored by TD Processors (for details about
semantic processing, please refer to § D. JSON-LD Context Usage and the
documentation under the namespace IRIs, e.g., https://www.w3.org/2019/wot/td).
A data schema is an abstract notation for data contained
in data formats. In a TD, concrete data formats are
specified in Forms (see § 5.3.4.2
Form) using content types. When the value
of a content type in an instance of the Form is
application/json, the data schema can be
processed directly by JSON Schema processors. Otherwise,
Web of Things (WoT) Binding Templates [WOT-BINDING-TEMPLATES]
defines data schema's available mappings to other content
types such as XML [xml].
If the content type in an instance of the Form is not
application/json and if no mapping is defined
for the content type, specifying a data schema does not
make sense for the content type.
5.3.2.1 DataSchema
Metadata that describes the data format used. It can
be used for validation.
The format string values are known from a
fixed set of values and their corresponding format rules
defined in [JSON-SCHEMA]
(Section 7.3 Defined Formats in particular). Servients MAY use the
format value to perform additional
validation accordingly.When a value that is
not found in the known set of values is assigned to
format, such a validation SHOULD succeed.
5.3.2.2 ArraySchema
Metadata describing data of type Array. This
Subclass is indicated by the
value array assigned to type in
DataSchema instances.
Metadata describing data of type string.
This Subclass is indicated by the
value string assigned to type
in DataSchema instances.
5.3.2.8 NullSchema
Metadata describing data of type null.
This Subclass is indicated by the
value null assigned to type in
DataSchema instances. This Subclass
describes only one acceptable value, namely
null. It can be used as part of a
oneOf declaration, where it is used to
indicate, that the data can also be
null.
5.3.3 Security Vocabulary
Definitions
This specification provides a selection of
well-established security mechanisms that are directly
built into protocols eligible as Protocol Bindings for W3C WoT or are widely in
use with those protocols. The current set of HTTP security
schemes is partly based on
OpenAPI 3.0.1 (see also [OPENAPI]).
However while the HTTP security schemes, Vocabulary, and syntax given in this
specification share many similarities with OpenAPI, they
are not compatible.
A security configuration corresponding to identified
by the Vocabulary Termnosec (i.e., "scheme":
"nosec"), indicating there is no authentication or
other mechanism required to access the resource.
5.3.3.3
BasicSecurityScheme
Basic Authentication [RFC7617]
security configuration identified by the Vocabulary Termbasic (i.e.,
"scheme": "basic"), using an unencrypted
username and password. This scheme should be used with
some other security mechanism providing confidentiality,
for example, TLS.
Digest Access Authentication [RFC7616]
security configuration identified by the Vocabulary Termdigest (i.e.,
"scheme": "digest"). This scheme is similar
to basic authentication but with added features to avoid
man-in-the-middle attacks.
API key authentication security configuration
identified by the Vocabulary Termapikey (i.e., "scheme":
"apikey"). This is for the case where the access
token is opaque and is not using a standard token
format.
Bearer Token [RFC6750]
security configuration identified by the Vocabulary Termbearer (i.e.,
"scheme": "bearer") for situations where
bearer tokens are used independently of OAuth2. If the
oauth2 scheme is specified it is not
generally necessary to specify this scheme as well as it
is implied. For format, the value
jwt indicates conformance with
[RFC7519],
jws indicates conformance with
[RFC7797],
cwt indicates conformance with
[RFC8392], and
jwe indicates conformance with
[RFC7516], with
values for alg interpreted consistently with
those standards. Other formats and
algorithms for bearer tokens MAY be specified in vocabulary
extensions.
OAuth2 authentication security configuration for
systems conformant with [RFC6749]
and [RFC8252],
identified by the Vocabulary Termoauth2 (i.e., "scheme":
"oauth2"). For the code
flow both authorization and
tokenMUST be included. If no
scopes are defined in the
SecurityScheme then they are considered to
be empty.
Set of authorization scope identifiers provided
as an array. These are provided in tokens returned
by an authorization server and associated with
forms in order to identify what resources a client
may access and how. The values associated with a
form should be chosen from those defined in an
OAuth2SecurityScheme active on that
form.
The present model provides a representation for (typed)
Web links and Web forms exposed by a Thing. The
Link class definition is reflecting a very
common subset of the terms defined in Web Linking
[RFC8288]. The
defined terms can be used, e.g., to describe the relation
to another Thing such as a Lamp Thing
is controlled by a Switch Thing. The
Form class corresponds to a newly introduced
form of hypermedia control to manipulate the state of
Things (and other Web resources).
5.3.4.1
Link
A link can be viewed as a statement of the form
"link context has a relation
type resource at link target",
where the optional target attributes may
further describe the resource.
A form can be viewed as a statement of "To perform an
operation type operation on form
context, make a request method
request to submission target" where the
optional form fields may further describe
the required request. In Thing Descriptions, the
form context is the surrounding Object,
such as Properties, Actions, and Events or the Thing
itself for meta-interactions.
Indicates the semantic intention of performing
the operation(s) described by the form. For
example, the Property interaction allows get and
set operations. The protocol binding may contain a
form for the get operation and a different form for
the set operation. The op attribute indicates which
form is for which and allows the client to select
the correct form for the operation required. op can
be assigned one or more interaction verb(s) each
representing a semantic intention of an
operation.
string or Array of string (one of
readproperty,
writeproperty,
observeproperty,
unobserveproperty,
invokeaction,
subscribeevent,
unsubscribeevent,
readallproperties,
writeallproperties,
readmultipleproperties, or
writemultipleproperties)
href
Target IRI of a link or submission target of a
form.
Content coding values indicate an encoding
transformation that has been or can be applied to a
representation. Content codings are primarily used
to allow a representation to be compressed or
otherwise usefully transformed without losing the
identity of its underlying media type and without
loss of information. Examples of content coding
include "gzip", "deflate", etc. .
Indicates the exact mechanism by which an
interaction will be accomplished for a given
protocol when there are multiple options. For
example, for HTTP and Events, it indicates which of
several available mechanisms should be used for
asynchronous notifications such as long polling
(longpoll), WebSub [websub]
(websub), Server-Sent Events
(sse) [html] (also known as
EventSource). Please note that there is no
restriction on the subprotocol selection and other
mechanisms can also be announced by this
subprotocol term.
Set of authorization scope identifiers provided
as an array. These are provided in tokens returned
by an authorization server and associated with
forms in order to identify what resources a client
may access and how. The values associated with a
form should be chosen from those defined in an
OAuth2SecurityScheme active on that
form.
This optional term can be used if, e.g., the
output communication metadata differ from input
metadata (e.g., output contentType differ from the
input contentType). The response name contains
metadata that is only valid for the response
messages.
The list of possible operation types of a form is
fixed. As of this version of the specification, it only
includes the well-known types necessary to implement the
WoT interaction model described in [WOT-ARCHITECTURE].
Future versions of the standard may extend this list but
operations types
SHOULD NOT be
arbitrarily set by servients.
The optional response name-value pair can
be used to provide metadata for the expected response
message. With the core vocabulary, it only includes
content type information, but TD Context Extensions could
be applied. If no
response name-value pair is provided, it
MUST be assumed
that the content type of the response is equal to the
content type assigned to the Form instance. Note
that contentType within an
ExpectedResponseClass does not
have a Default Value. For instance, if
the value of the content type of the form is
application/xml the assumed value of the
content type of the response will be also
application/xml.
In some use cases, input and output data might be
represented in a different form, for instance an Action
that accepts JSON, but returns an image. In such a case,
the optional response name-value pair can
describe the content type of the expected response.
If the content type of
the expected response differs from the content type of
the form, the Form instance MUST include a name-value
pair with the name response. For
instance, an ActionAffordance could only
accept application/json for its input data,
while it will respond with an image/jpeg
content type for its output data. In that case the
content types differ and the response
name-value pair has to be used to provide response
content type (image/jpeg) information to the
Consumer.
5.3.4.3
ExpectedResponse
Communication metadata describing the expected
response message.
Array of string
with the elements readproperty and
writeproperty
If defined within an instance of
PropertyAffordance
Form
op
invokeaction
If defined within an instance of
ActionAffordance
Form
op
subscribeevent
If defined within an instance of
EventAffordance
BasicSecurityScheme
in
header
DigestSecurityScheme
in
header
BearerSecurityScheme
in
header
APIKeySecurityScheme
in
query
DigestSecurityScheme
qop
auth
BearerSecurityScheme
alg
ES256
BearerSecurityScheme
format
jwt
6. TD Representation Format
WoT Thing Descriptions represent Things and are modeled and
structured based on § 5. TD Information
Model. This section defines a JSON-based representation
format for Things, a serialization of instances
of the ClassThing defined by the TD
Information Model.
The JSON serialization of the TD Information
Model is aligned with the syntax of JSON-LD 1.1
[json-ld11] in
order to streamline semantic evaluation. Hence, the TD
representation format can be processed either as raw JSON or
with a JSON-LD 1.1 processor (for details about semantic
processing, please refer to § D. JSON-LD Context
Usage and the documentation under the namespace IRIs, e.g.,
https://www.w3.org/2019/wot/td).
In order to support interoperable internationalization,
TDs
MUST be serialized
according to the requirements defined in Section 8.1 of RFC8259
[RFC8259]
for open ecosystems. In summary, this requires the
following:
Implementations MUST NOT add a byte order
mark (U+FEFF) to the beginning of a TD document.
TD ProcessorsMAY ignore the presence
of a byte order mark rather than treating it as an
error.
6.1 Mapping to JSON Types
The TD Information Model is constructed,
so that there is an easy mapping between model Objects and JSON types. Every Class instances
maps to a JSON object, where each name-value pair of the
Class instance is a member of the JSON object.
Every Simple Type mentioned in § 5.3 Class Definitions (i.e.,
string, anyURI,
dateTime, integer,
unsignedInt, double, and
boolean) maps to a primitive JSON type (string,
number, boolean), as per the rules listed below. These rules
apply to values in name-value pairs:
Values that are of type
string or anyURIMUST be serialized as JSON
strings.
Values that are of type
dateTimeMUST be serialized as JSON strings following
the "date-time" format specified by [RFC3339].
Examples would include 2019-05-24T13:12:45Z
and 2015-07-11T09:32:26+08:00. Values that are of type
dateTimeSHOULD use the literal Z
representing the UTC time zone instead of an
offset.
Values that are of type
integer or unsignedIntMUST be serialized as JSON
numbers without a fraction or exponent part.
Values that are of type
doubleMUST be serialized as JSON number.
Values that are of type
booleanMUST be serialized as JSON boolean.
Every complex type of the TD Information Model
(i.e., Arrays, Maps, and Class instances) maps to a structured JSON type
(array and object), as per the rules listed below:
A
value of type ArrayMUST be serialized as JSON array, with each
value of the name-value pairs as element of the JSON array
ordered by the numeric name of the pair.
A
value of type MapMUST be serialized as JSON object, with each
name-value pair as member of the JSON object.
The following example shows the TD instance from Example 1 with a
checkbox to also include the members with Default Values (=checkbox checked). These members
can be omitted (=checkbox unchecked) to simplify the TD
serialization. Note that a TD Processor
interprets these omitted members identically as if they were
explicitly present with a given Default
Value.
Please note that, depending on the Protocol Binding used, additional protocol-specific
Vocabulary Terms may apply. They may also have
associated Default Values, and hence can also
be omitted as explained in this subsection. Further
information can be found in § 8.3 Protocol
Bindings.
6.3 Information Model Serialization
6.3.1 Thing Root Object
A Thing Description is a data structure rooted at an
Object of type Thing. In turn, a JSON
serialization of the Thing Description is a JSON object,
which is the root of a syntax tree constructed from the
TD Information Model.
The root
element of a TD SerializationMUST be a JSON object that includes a
member with the name @context and a value of
type string or array that equals or respectively contains
https://www.w3.org/2019/wot/td/v1.
In general, this URI is used to identify the TD
representation format version defined by this
specification. For JSON-LD processing [json-ld11], this URI
specifies the Thing Description context file. An
@context of type array indicates TD Context Extensions (see § 7. TD Context
Extensions for details).
All name-value pairs of an instance
of Thing, where the name is a Vocabulary Term in the Signature of
Thing, MUST be serialized as JSON members of the root
object.
A TD snippet for a serialized root object including all
mandatory and optional members is given below:
All
values assigned to version,
securityDefinitions, properties,
actions, and events in an
instance of the ClassThingMUST be serialized as
JSON objects.
All
values assigned to links, and
forms in an instance of the ClassThingMUST be serialized as JSON arrays
containing JSON objects as defined in § 6.3.8
links and § 6.3.9
forms, respectively.
The value assigned to
security in an instance of ClassThingMUST be serialized as
JSON string or as JSON array whose elements are JSON
strings.
6.3.2 Human-Readable Metadata
JSON members named title and
description are used within a TD document to
provide human-readable metadata. They can be used as
comments for developers inspecting a TD document or as
display texts for user interface.
As defined in § 5.3.1.1
Thing, the base text direction used to
display human-readable metadata can either be estimated
using heuristics such as the first-strong rule or inferred
from language information. In TD documents the default
language is defined by a value assigned to
@language in the @context, and
this, along with a script subtag if necessary, can be used
to determine a base text direction. However,
when interpreting human-readable text, each human-readable
string value MUST be
processed independently. In other words, a TD Processor cannot carry forward changes in
direction from one string to another, or infer direction
for one string from another one elsewhere in the TD.
Note
Strings on the Web [STRING-META]
suggests both strong-first and language-based inferencing
as means to determine the base text direction. Given that
the Thing Description format is based on JSON-LD 1.1
[json-ld11], which currently
lacks explicit direction metadata, these approaches are
currently considered appropriate at the time of this
publication. However, if JSON-LD 1.1 adopts support for
explicit base direction metadata as recommended by
[STRING-META],
the Thing Description format should be updated to take
advantage of that feature.
A TD snippet using title and
description is shown below. The default
language is set to en through the definition
of the @language member within a JSON object
in the @context array.
The JSON members named titles and
descriptions are used within the TD document
to provide human-readable metadata in multiple languages
within a single TD document. All name-value
pairs of a MultiLanguageMapMUST be serialized as
members of a JSON object, where the name is a well-formed
language tag as defined by [BCP47] and the
value is a human-readable string in the language indicated
by the tag. See § 5.3.1.7
MultiLanguage for details. All
MultiLanguage object within a TD document
SHOULD contain the
same set of language members.
A TD snippet using titles and
descriptions at different levels is given
below:
TD instances may also combine the use of
title and description with
titles and descriptions.
When title and
titles or description and
descriptions are present within the same JSON
object, the values of title and
descriptionMAY be seen as the default text.When title and
titles or description and
descriptions are present in a TD document,
each title and description member
SHOULD have a
corresponding titles and
descriptions member, respectively. The
language of the default text is indicated by the default
language, which is usually set by the creator of the Thing
Description instance.
Another possibility to set the default language is
through a language negotiation mechanism, such as the
Accept-Language header field of HTTP.
In cases where
the default language has been negotiated, an
@language member MUST be present to indicate the result of the
negotiation and the corresponding default language of the
returned content.When
the default language has been negotiated successfully, TD
documents SHOULD
include the appropriate matching values for the members
title and description in
preference to MultiLanguage objects in
titles and descriptions
members.Note
however that Things MAY choose to not support such
dynamically-generated TDs nor to support language
negotiation (e.g., because of resource
constraints).
6.3.3
version
All
name-value pairs of an instance of
VersionInfo, where the name is a Vocabulary Term included in the Signature of VersionInfo, MUST be serialized as JSON
members with the Vocabulary Term as
name.
A TD snippet of a version information object is given
below:
The version member is intended as container
for additional application- and/or device-specific version
information based on TD Context
Extensions. See § 7.1 Semantic
Annotations for details.
6.3.4
securityDefinitions and
security
In a Thing instance, the value assigned to
securityDefinitions is a Map of
instances of SecurityScheme. All name-value pairs
of a Map of SecurityScheme instances
MUST be
serialized as members of the JSON object that results from
serializing the Map; the name of a pair MUST be serialized as a
JSON string and the value of the pair, an instance of
SecurityScheme, MUST be serialized as a JSON object.
All name-value pairs of an instance
of one of the Subclasses of
SecurityScheme, where the name is a Vocabulary Term included in the Signature of that Subclass or in the
Signature of
SecurityScheme, MUST be serialized as members of the JSON
object that results from serializing the
SecuritySchemeSubclass's instance,
with the Vocabulary Term as
name.
The following TD snippet shows a simple security
configuration specifying basic username/password
authentication in the header. The value given for
in is actually the Default Value (header) and could be
omitted. A named security configuration must be given in
the securityDefinitions map. That definition
must be activated by including its JSON name in the
security member, which can be of type string
when only one definition is activated.
Here is a more complex example: a TD snippet showing
digest authentication on a proxy combined with bearer token
authentication on the Thing. In the
digest scheme, the Default Value of in (i.e.,
header) is omitted, but still applies. Note
that the corresponding private security configuration such
as username/password and tokens must be configured in the
Consumer to interact successfully. When
activating multiple security definitions, the
security member becomes an array.
Security configuration in the TD is mandatory.
At least one security definition
MUST be activated
through the security array at the Thing level
(i.e., in the TD root object). This configuration
can be seen as the default security mechanism required to
interact with the Thing. Security
definitions MAY also
be activated at the form level by including a
security member in form objects, which
overrides (i.e., completely replace) all definitions
activated at the Thing level.
The nosec security scheme is provided for
the case that no security is needed. The minimal security
configuration for a Thing is activation
of the nosec security scheme at the Thing
level, as shown in the following example:
To give a more complex example, suppose we have a
Thing where all Interaction Affordances require
basic authentication except for one, for which no
authentication is required. For the status
Property and the toggle Action,
basic authentication is required and defined
at the Thing level. For the overheating Event,
however, no authentication is required, and hence the
security configuration is overridden at the form level.
Security configurations can also can be specified for
different forms within the same Interaction Affordance. This may
be required for devices that support multiple protocols,
for example HTTP and CoAP [RFC7252],
which support different security mechanisms. This is also
useful when alternative authentication mechanisms are
allowed. Here is a TD snippet demonstrating three possible
ways to activate a Property affordance: via HTTPS with
basic authentication, with digest authentication, with
bearer token authentication. In other words, the use of
different security configurations within multiple forms
provides a way to combine security mechanisms in an "OR"
fashion. In contrast, putting multiple security
configurations in the same security member
combines them in an "AND" fashion, since in that case they
would all need to be satisfied to allow activation of the
Interaction Affordance. Note that
activating one (default) configuration at the Thing level
is still mandatory.
As another more complex example, OAuth2 makes use of
scopes. These are identifiers that may appear in tokens and
must match with corresponding identifiers in a resource to
allow access to that resource (or Interaction Affordance in the case
of W3C WoT).
For example, in the following, the status
Property can be read by Consumers using
bearer tokens containing the scope limited,
but the configure Action can only be invoked
with a token containing the special scope.
Scopes are not identical to roles, but are often associated
with them; for example, perhaps only those in an
administrative role are authorized to perform "special"
interactions. Tokens can have more than one scope. In this
example, an administrator would probably be issued tokens
with both the limited and special
scopes, while ordinary users would only be issued tokens
with the limited scope.
The value assigned to properties in a
Thing instance is a Map of instances of
PropertyAffordance. All name-value pairs
of a Map of PropertyAffordance instances
MUST be
serialized as members of the JSON object that results from
serializing the Map; the name of a pair MUST be serialized as a
JSON string and the value of the pair, an instance of
PropertyAffordance, MUST be serialized as a JSON object.
All name-value pairs of an instance of
PropertyAffordance, where the name is a
Vocabulary Term included in (one
of) the Signatures of
PropertyAffordance,
InteractionAffordance, or
DataSchema, MUST be serialized as members of the JSON
object that results from serializing the
PropertyAffordance instance, with the Vocabulary Term as name. See § 6.3.10 Data
Schemas for details on serializing DataSchema
instances.
The value assigned to
forms in an instance of
PropertyAffordanceMUST be serialized as a JSON array containing
one or more JSON object serializations as defined in
§ 6.3.9
forms.
A snippet for two Property affordances
is given below:
6.3.6
actions
In a Thing instance, the value assigned to
actions is a Map of instances of
ActionAffordance. All name-value pairs of
a Map of ActionAffordance instances
MUST be
serialized as members of the JSON object that results from
serializing the Map; the name of a pair MUST be serialized as a
JSON string and the value of the pair, an instance of
ActionAffordance, MUST be serialized as a JSON object.
All
name-value pairs of an instance of
ActionAffordance, where the name is a Vocabulary Term included in (one of) the Signatures of ActionAffordance or
InteractionAffordance, MUST be serialized as members of the JSON
object that results from serializing the
ActionAffordance instance, with the Vocabulary Term as name.
The values assigned to
input and output in an instance
of ActionAffordanceMUST be serialized as JSON objects. They
rely on the ClassDataSchema, whose serialization
is defined in § 6.3.10 Data
Schemas.
The value assigned to forms
in an instance of ActionAffordanceMUST be serialized as a JSON
array containing one or more JSON object serializations as
defined in § 6.3.9
forms.
A TD snippet of an Action affordance is given below:
6.3.7
events
In a Thing instance, the value assigned to
events is a map of instances of
EventAffordance. All name-value pairs of
a Map of EventAffordance instances
MUST be
serialized as members of the JSON object that results from
serializing the Map; the name of a pair MUST be serialized as a
JSON string and the value of the pair, an instance of
EventAffordance, MUST be serialized as a JSON object.
All
name-value pairs of an instance of
EventAffordance, where the name is a Vocabulary Term included in (one of) the Signatures of EventAffordance or
InteractionAffordance, MUST be serialized as members of the JSON
object that results from serializing the
EventAffordance instance, with the Vocabulary Term as name.
The values assigned to
subscription, data, and
cancellation in an instance of
EventAffordanceMUST be serialized as JSON objects. They
rely on the ClassDataSchema, whose serialization
is defined in § 6.3.10 Data
Schemas.
The
value assigned to forms in an instance of
EventAffordanceMUST be serialized as a JSON array containing
one or more JSON object serializations as defined in
§ 6.3.9
forms.
A TD snippet of an Event object is given below:
Event affordances have been defined in a flexible
manner, in order to adopt existing (e.g., WebSub
[websub]) or
customer-oriented event mechanisms (e.g., Webhooks). For
this reason, subscription and
cancellation can be defined according to the
desired mechanism. Please find further details in
[WOT-BINDING-TEMPLATES].
Example § A.3 Webhook Event
Example illustrates how Events can use
subscription and cancellation to
describe Webhooks.
6.3.8
links
All
name-value pairs of an instance of Link, where
the name is a Vocabulary Term included in the
Signature of Link,
MUST be serialized as
members of the JSON object that results from serializing
the Link instance, with the Vocabulary Term as name.
A TD snippet of a link object in the links
array is given below:
6.3.9
forms
All
name-value pairs of an instance of Form, where
the name is a Vocabulary Term included in the
Signature of Form,
MUST be serialized as
members of the JSON object that results from serializing
the Form instance, with the Vocabulary Term as name.
A TD snippet of a form object in the forms
array is given below:
href may also carry a URI that contains
dynamic variables such as p and d in
http://192.168.1.25/left?p=2&d=1. In that case the URI
can be defined as template as defined in [RFC6570]:
http://192.168.1.25/left{?p,d}.
In such a case, the URI Template
variables MUST be
collected in the JSON-object based
uriVariables member with the associated
(unique) variable names as JSON names.
The serialization of each
value in the map assigned to uriVariables in
an instance of FormMUST rely on the ClassDataSchema,
whose serialization is defined in § 6.3.10 Data
Schemas.
A TD snippet using a URI Template and
uriVariables is given below:
The contentType member is used to assign a
media type [RFC2046]
including media type parameters as attribute-value pairs
separated by a ; character. Example:
In some use cases, the form metadata of the Interaction Affordance not only
describes the request, but also provides metadata for the
expected response. For instance, an Action
takePhoto defines an input schema
to submit parameter settings of a camera (aperture
priority, timer, etc.) using JSON for the request payload
(i.e., "contentType": "application/json"). The
output of this action is the photo taken, which is
available in JPEG format, for example. In such cases, the
response member is used to indicate the
representation format of the response payload (e.g.,
"contentType": "image/jpeg"). Here no
output schema is required, as the content type
fully specifies the representation format.
If present, the value assigned to
response in an instance of FormMUST be a JSON
object.If present, the response object
MUST contain a
contentType member as defined in the Class definition of ExpectedResponse.
A form snippet with the
response member is shown below based on the
takePhoto Action described above:
When forms is present at the top level, it
can be used to describe meta interactions offered by a
Thing. For example, the operation types
"readallproperties" and "writeallproperties" are for meta
interactions with a Thing by which
Consumers can read and write all properties at
once. In the example below, a forms member is
included in the TD root object and the Consumer can use the submission target
https://mylamp.example.com/allproperties both
to read or write all Properties (i.e., on,
brightness, and timer) of the
Thing in a single protocol transaction.
In the case of operation type
writeallproperties, it is expected that the
Consumer provides all writable (non
readOnly) properties and the (new) assigned
values (e.g., within payload). Similarly, for the
writemultipleproperties operation type, it is
expected that the Consumer provides
writable (non readOnly) properties. On the
Thing side, Thing is expected to
return readable (non writeOnly) properties in
the case of readmultipleproperties and
readallproperties operation types.
6.3.10 Data Schemas
The data schemas of the WoT Thing Description defined
through the DataSchemaClass are
based on a subset of the JSON Schema terms [JSON-SCHEMA].
Thus, serializations of the TD data schemas can be fed
directly into JSON Schema validator implementations to
validate the data exchanged with Things.
Data schema serialization applies to
PropertyAffordance instances, the values
assigned to input and output in
ActionAffordance instances, the values
assigned to subscription, data,
and cancellation in
EventAffordance instances, and the value
assigned to uriVariables in instances of
Subclasses of InteractionAffordance
(when a form object
uses a URI Template).
All
name-value pairs of an instance of one of the Subclasses of DataSchema, where the
name is a Vocabulary Term included in the
Signature of that Subclass or in the Signature of
DataSchema, MUST be serialized as members of the JSON
object that results from serializing the
DataSchemaSubclass's instance,
with the Vocabulary Term as
name.
The value assigned to
properties in an instance of
ObjectSchemaMUST be serialized as a JSON object.
The values assigned to
enum, required, and
oneOf in an instance of
DataSchemaMUST be serialized as a JSON array.
The value assigned to
items in an instance of
ArraySchemaMUST be serialized as a JSON object or a JSON
array containing JSON objects.
A TD snippet data schema members is given below. Note
that the surrounding object may be a data schema object
(e.g., for input and output) or a
Property object, which would contain additional
members.
The terms readOnly and
writeOnly can be used signal which data items
are exchanged in read interactions (i.e., when reading a
Property) and which in write interactions (i.e., when
writing a Property). This can be used as workaround when
Properties of an unconventional Thing exhibit
different data for reading and writing, which can be the
case when augmenting an existing device or service with a
Thing Description.
A TD snippet with the usage of readOnly and
writeOnly is given below:
When the status Property is read, the
status data is returned using a latestStatus
member in the payload. To update the status
Property, the new value must be provided through a
newStatusValue member in the payload.
As an additional feature, a Thing Description instance
allows the usage of a unit member within data
schemas. This can be used to associate a unit of measure to
a data item. Its string value can be selected freely.
However, it is recommended to select units defined in
well-known Vocabularies. See § 7. TD Context
Extensions for an example.
6.4
Identification
The JSON-based serialization of Thing Descriptions is
identified by the media type application/td+json
or the CoAP Content-Format ID 432 (see § 10. IANA Considerations).
7.
TD Context Extensions
This section is non-normative.
In addition to the standard Vocabulary definitions
in § 5. TD Information
Model, the WoT Thing Description offers the possibility to
add context knowledge from additional namespaces. This
mechanism can be used to enrich the Thing Description instances
with additional (e.g., domain-specific) semantics. It can also
be used to import additional Protocol Bindings
or new security schemes in the future.
For such TD Context Extensions, the Thing
Descriptions use the @context mechanism known from
JSON-LD [json-ld11].
When using TD Context Extensions, the value of
@context of the ClassThing
is an Array with additional elements of type
anyURI identifying JSON-LD context files or
Map containing namespace IRIs as defined in § 5.3.1.1 Thing.
TD Context Extensions allow for
additional Vocabulary Terms to a Thing
Description instance. If the included namespaces are based on
Class definitions such as those provided by the RDF
Schema or OWL, they can be used to annotate any Class instance of a Thing Description semantically
by associating the instance to a such an external Class definition. This is done by assigning a
Class name to the @type name-value
pair or including Class name in its Array value for multiple associations/annotations.
Following the serialization rules in § 6.1 Mapping to JSON Types,
@type is either serialized as JSON string or as
JSON array. @type is the JSON-LD keyword
[json-ld11]
used to set the type of a node.
TD Context Extensions also allow the
inclusion of additional name-value pairs and well-defined
values within any Class instance of a Thing
Description. These pairs and values are defined through the
included Vocabulary Terms and are serialized
as additional members in the corresponding JSON objects or
values of existing members, respectively. Examples are
additional version metadata for the Thing or units
of measure for data items.
As an example, the TD snippet given below extends the
version information container by adding version numbers for
the hardware and firmware of the Thing, and uses values
from external Vocabularies for the Thing and for the data schema unit: SAREF, also used in Example 2, and OM,
the Ontology of Units of Measure [RIJGERSBERG]. These
Vocabularies are used as examples—others may exist,
in particular in the home automation domain.
In many cases, TD Context Extensions
may be used to annotate pieces of a data schema, to be able
to semantically process the state information of the physical
world object, which is represented by the data exchanged
during an interaction (e.g., in the payload of a response).
For example, a semantic description of this state information
in RDF can be embedded in the TD Document and
pieces of a data schema can be individually annotated as
referring to specific parts of that RDF-modeled state of the
physical world object.
The TD snippet below uses SAREF to describe the state of a
lamp. The external Vocabulary Termssn:forProperty, taken from SSN, the Semantic
Sensor Network Ontology [VOCAB-SSN], is
being used to link the data schema of the statusProperty with the actual on/off state of the
physical world object.
In Example 2, the state of the Thing is given by the status
affordance itself and possible state changes are given by the
toggle affordance. In other words, the state of
the physical world object directly provides the Interaction Affordances of the
Thing. This design is satisfactory for simple
cases. In more elaborate cases, however, several affordances
may be available for the same physical state. In the example
above, the fullStatusProperty
provides an alternative, more verbose representation for the
state of the lamp.
The following TD example uses a fictional CoAP Protocol Binding, as no such Protocol Binding is available at the time of
writing this specification. This TD Context
Extension assumes that there is a CoAP in RDF
vocabulary similar to HTTP Vocabulary in RDF
1.0 [HTTP-in-RDF10] that
is accessible via an example namespace
http://www.example.org/coap-binding#. The
supplemented cov:methodName member instructs the
Consumer which CoAP method has to be applied (e.g.,
GET for the CoAP Method Code 0.01,
POST for the CoAP Method Code 0.02, or
iPATCH for CoAP Method Code 0.07).
7.3 Adding Security Schemes
Finally, new security schemes that are not included in
§ 5.3.3 Security
Vocabulary Definitions can be imported using the TD Context Extension mechanism. This example uses a
fictional ACE security scheme based on [ACE]
that is, for this example, defined by the namespace at
http://www.example.org/ace-security#. Note that
such additional security schemes must be Subclasses of the ClassSecurityScheme.
The following assertions relate to the behavior of
components of a WoT system, as opposed to the representation or
information model of the TD. However, note that TDs are
descriptive, and may in particular be used to describe
pre-existing network interfaces. In these cases, assertions
cannot be made that constrain the behavior of such pre-existing
interfaces. Instead, the assertions must be interpreted as
constraints on the TD to accurately represent such
interfaces.
8.1 Security Configurations
To enable secure interoperation, security configurations
must accurately reflect the requirements of the Thing:
If a Thing requires a
specific access mechanism for an interaction, that
mechanism MUST be
specified in the security configuration of the Thing
Description.
If a Thing does not
require a specific access mechanism for an interaction,
that mechanism MUST
NOT be specified in the security configuration of the
Thing Description.
Some security protocols may ask for authentication
information dynamically, including required encoding or
encryption schemes. One consequence of the above is that if a
protocol asks for a form of security credentials or an
encoding or encryption scheme not declared in the Thing
Description then the Thing Description is to be considered
invalid.
8.2 Data
Schemas
The data schemas provided in the TD should accurately
represent the data payloads returned and accepted by the
described Thing in the interactions specified
in the TD. In general, Consumers should
follow the data schemas strictly, not generating anything not
given in the WoT Thing Description, but should accept
additional data from the Thing not given
explicitly in the WoT Thing Description. In general, Things are described by WoT Thing
Descriptions, but Consumers are constrained
to follow WoT Thing Descriptions when interacting with
Things.
A Thing acting as a
Consumer when interacting with another target
Thing described in a WoT Thing Description
MUST generate data
organized according to the data schemas given in the
corresponding interactions.
A WoT Thing Description MUST accurately describe the
data returned and accepted by each interaction.
A ThingMAY return additional data from
an interaction even when such data is not described in the
data schemas given in its WoT Thing Description.
This applies to ObjectSchema and
ArraySchema (when items is an
Array of DataSchema) where there can be
additional properties or items in the data returned. This
behaves as if "additionalProperties":true or
"additionalItems":true as defined in
[JSON-SCHEMA].
A Thing acting
as a Consumer when interacting with
another ThingMUST accept without error any additional data
not described in the data schemas given in the Thing
Description of the target Thing. This
applies to ObjectSchema and
ArraySchema (when items is an
Array of DataSchema) where there can be
additional properties or items in the data returned. This
behaves as if "additionalProperties":true or
"additionalItems":true as defined in
[JSON-SCHEMA].
A Thing acting
as a Consumer when interacting with
another ThingMUST NOT generate data not described in the
data schemas given in the Thing Description of that
Thing.
A Thing acting as a
Consumer when interacting with another ThingMUST
generate URIs according to the URI Templates, base URIs,
and form href parameters given in the Thing Description of
the target Thing.
URI Templates, base URIs, and href
members in a WoT Thing Description MUST accurately describe the WoT Interface of the Thing.
Every form in a WoT Thing Description must have a
submission target, given by the href member. The
URI scheme of this submission target indicates what Protocol Binding the Thing implements
[WOT-ARCHITECTURE].
For instance, if the target starts with http or
https, a Consumer can then
infer the Thing implements the Protocol Binding based on HTTP and it should expect
HTTP-specific terms in the form instance (see next section,
§ 8.3.1 Protocol
Binding based on HTTP).
Every form in a WoT Thing
Description MUST
follow the requirements of the Protocol Binding indicated by the URI scheme of
its href member.
Every form in a WoT Thing
Description MUST
accurately describe requests (including request headers, if
present) accepted by the Thing in an
interaction.
8.3.1 Protocol Binding based on HTTP
Per default the Thing Description supports the Protocol Binding based on HTTP by including the
HTTP RDF vocabulary definitions from HTTP Vocabulary in
RDF 1.0 [HTTP-in-RDF10].
This vocabulary can be directly used within TD instances by
the usage of the prefix htv, which points to
http://www.w3.org/2011/http#. Further details
of Protocol Binding based on HTTP can
be found in [WOT-BINDING-TEMPLATES].
To interact with a Thing that
implements the Protocol Binding
based on HTTP, a Consumer needs to
know what HTTP method to use when submitting a form. In the
general case, a Thing Description can explicitly include a
term indicating the method, i.e.,
htv:methodName. For the sake of conciseness,
the Protocol Binding based on HTTP
defines Default Values for the operation
types listed below, which also aims at convergence of the
methods expected by Things (e.g., GET to
read, PUT to write). When no method is indicated in a
form representing an Protocol Binding
based on HTTP, a Default ValueMUST be assumed as
shown in the following table.
Please refer to [WOT-BINDING-TEMPLATES]
for information on how to describe IoT platforms and
ecosystems.
9. Security and Privacy Considerations
This section is non-normative.
In general the security measures taken to protect a WoT
system will depend on the threats and attackers that system may
face and the value of the assets needs to protect. In addition
privacy risks will depend on the association of Things with
identifiable people and both the direct information and the
inferred information available from such an association. A
detailed discussion of security and privacy considerations for
the Web of Things, including a threat model that can be adapted
to various circumstances, is presented in the informative
document [WOT-SECURITY-GUIDELINES].
This section discusses only security and privacy risks and
possible mitigations directly relevant to the WoT Thing
Description.
A WoT Thing Description can describe both secure and
insecure network interfaces. When a Thing Description is
retro-fitted to an existing network interface, no change in the
security status of the network interface is to be expected.
The use of a WoT Thing Description introduces the security
and privacy risks given in the following sections. After each
risk, we suggest some possible mitigations.
9.1 Context Fetching Privacy Risk
Fetching the vocabulary files given in the
@context member of any JSON-LD [json-ld11] document can be a privacy
risk. In the case of the WoT, an attacker can observe the
network traffic produced by such fetches and can use the
metadata of the fetch, such as the destination IP address, to
infer information about the device especially if
domain-specific vocabularies are used. This is a risk even if
the connection is encrypted, and is related to DNS privacy
leaks.
Mitigation:
Avoid actual fetching of vocabulary files. Vocabulary
files should be cached whenever possible. Ideally they
would be made immutable, built into the interpreting
device, and not fetched at all, with the URI in the
@context member serving only as an identifier
of the (known) vocabulary. This requires the use of strict
version control, as updates should use a new URI to ensure
that existing URIs can refer to immutable data. Use
well-known standard vocabulary files whenever possible to
improve the chances that the context file will be available
locally to systems interpreting the metadata in a Thing
Description.
9.2 Immutable Identifiers Privacy Risk
A Thing Description containing an identifier
(id) may describe a Thing that is associated
with an identifiable person. Such identifiers pose various
risks including tracking. However, if the identifier is also
immutable, then the tracking risk is amplified, since a
device may be sold or given to another person and the known
ID used to track that person.
Mitigation:
All identifiers should be mutable, and there should be a
mechanism to update the id of a Thing. Specifically, the id of a
Thing should not be fixed in hardware. This
does, however, conflict with the Linked Data ideal that
identifiers are fixed URIs. In many circumstances it will
be acceptable to only allow updates to identifiers if a
Thing is reinitialized. In this case as a
software entity the old Thing ceases to
exist and a new Thing is created. This can be
sufficient to break a tracking chain when, for example, a
device is sold to a new owner. Alternatively, if more
frequent changes are desired during the operational phase
of a device, a mechanism can be put into place to notify
only authorized users of the change in identifier when a
change is made. Note however that some classes of
devices, e.g., medical devices, may require immutable IDs
by law in some jurisdictions. In this case extra
attention should be paid to secure access to files, such
as Thing Descriptions, containing such immutable
identifiers. It may also be desirable to not share the
"true" immutable identifier in such a case in the TD
whenever possible.
9.3 Fingerprinting Privacy Risk
As noted above, the id member in a TD can
pose a privacy risk. However, even if the id is
updated as described to mitigate its tracking risk, it may
still be possible to associate a TD with a particular
physical device, and from there to an identifiable person,
through fingerprinting.
Even if a specific device instance cannot be identified
through fingerprinting, it may be possible to infer the type
of a device from the information in the TD, such as the set
of interactions, and use this type to infer private
information about an identifiable person, such as a medical
condition.
Mitigation:
Only authorized users should be provided access to the
Thing Description for a Thing, and only
the amount of information needed for the level of
authorization and the use case should be provided. If the
TD is only distributed to authorized users through secure
and confidential channels, for example through a
directory service that requires authentication, then
external unauthorized parties will not have access to the
TD to fingerprint it. To further mitigate this risk,
information not necessary for a particular use case of a
TD should be omitted whenever possible. For example, for
an ad-hoc connection to a device where the Consumer does
not store state about the Thing, the id can
be omitted. If the Consumer does not need certain
interactions for its use case, they can be omitted. If
the Consumer is not authorized to use certain
interactions, they can likewise be omitted. If the
Consumer does not have any capability to display
human-readable information such as titles or
descriptions, they can be omitted or replaced with
zero-length strings.
9.4 Globally Unique Identifier
Privacy Risk
Globally unique identifiers pose a privacy risk if a
centralized authority is needed to create and distribute
them, since then a third party has knowledge of the
identifiers.
Mitigation:
The id field in TDs are intentionally not
required to be globally unique. There are several
cryptographic mechanisms available to generate suitable IDs
in a distributed fashion that do not require a central
registry. These mechanisms typically have a very low
probability of generating duplicate identifiers, and this
needs to be taken into account in the system design; for
example, by detecting duplicates and regenerating IDs when
necessary. The scope of IDs also does not need to be
global: it is acceptable to use identifiers that only
distinguish Things in a certain context, such as within a
home or factory.
9.5 TD Interception and Tampering
Security Risk
Intercepting and tampering with TDs can be used to launch
man-in-the-middle attacks, for example by rewriting URLs in
TDs to redirect accesses to a malicious intermediary that can
capture or manipulate data.
Mitigation:
Obtain Thing Descriptions only through mutually
authenticated channels. This ensures that the Consumer and the server are both sure of the
identity of the other party to the communication. This is
also necessary in order to deliver TDs only to authorized
users.
9.6 Context Interception and
Tampering Security Risk
Intercepting and tampering with context files can be used
to facilitate attacks by modifying the interpretation of
vocabulary.
Mitigation:
Ideally context files would only be obtained through
authenticated channels but it is notable (and unfortunate)
that many contexts are indicated using HTTP URLs, which are
vulnerable to interception and modification if
dereferenced. However, if context files are immutable and
cached, and dereferencing is avoided whenever possible,
then this risk can be reduced.
9.7 Inferencing of Personally
Identifiable Information Privacy Risk
In many locales, in order to protect the privacy of users,
there are legal requirements for the handling of personally
identifiable information, that is, information that can be
associated with a particular person. Such information can of
course be generated by IoT devices directly. However, the
existence and metadata of IoT devices (the kind of data
stored in a Thing Description) can also contain or be used to
infer personally identifiable information. This information
can be as simple as the fact that a certain person owns a
certain type of device, which can lead to additional
inferences about that person.
Mitigation:
Treat a Thing Description associated with a personal
device as if it contained personally identifiable
information. As an example application of this principle,
consider how to obtain user consent. Consent for usage of
personally identifiable data generated by a Thing is often obtained when a Thing is paired with system consuming the data,
which is frequently also when the Thing Description is
registered with a local directory or the system consuming
the Thing Description in order to access the device. In
this case, consent for using data from a Thing can be combined with consent for
accessing the Thing Description of the Thing. As a second example, if we consider a TD
to contain personally identifiable information, then it
should not be retained indefinitely or used for purposes
other than those for which consent was given.
Since WoT Thing Description is
intended to be a pure data exchange format for Thing metadata, the serialization SHOULD NOT be passed
through a code execution mechanism such as JavaScript's
eval() function to be parsed. An
(invalid) document may contain code that, when executed,
could lead to unexpected side effects compromising the
security of a system.
WoT Thing Descriptions can be evaluated with a JSON-LD
1.1 processor, which typically follows links to remote
contexts (i.e., TD context extensions, see
W3C WoT
Thing Description, section 7) automatically,
resulting in the transfer of files without the explicit
request of the Consumer for each
one. If remote contexts are served by third parties, it
may allow them to gather usage patterns or similar
information leading to privacy concerns. While
implementations on resource-constrained devices are
expected to perform raw JSON processing (as opposed to
JSON-LD processing), implementations in general
SHOULD statically
cache vetted versions of their supported context
extensions and not to follow links to remote
contexts. Supported context extensions can be
managed through a secure software update mechanism
instead.
Context Extensions (see
W3C WoT
Thing Description, section 7) that are loaded from
the Web over non-secure connections, such as HTTP, run
the risk of being altered by an attacker such that they
may modify the TD Information
Model in a way that could compromise security.
For this reason, Consumer again SHOULD vet and cache remote contexts before
allowing the system to use it.
Given that JSON-LD processing usually includes the
substitution of long IRIs [RFC3987]
with short terms, WoT Thing Descriptions may expand
considerably when processed using a JSON-LD 1.1 processor
and, in the worst case, the resulting data might consume
all of the recipient's resources. ConsumersSHOULD treat any TD metadata with due
skepticism.
Comment: An MQTT client frequently publishes the
illuminance data (number is serialized in text format) to
the topic /illuminance by the MQTT broker
running behind the address 192.168.1.187:1883.
Example 34:
MyIlluminanceSensor with MQTT Protocol Binding
Context Extensions: use HTTP Protocol Binding supplements
(htv prefix already included in TD context)
Offered affordances: 1 Event
Security: none
Protocol Binding: HTTP
Comment: WebhookThing provides an Event
affordance temperature which periodically
pushes the latest temperature value to the Consumer using a Webhook mechanism, where the
Thing sends POST requests to a callback URI
provided by the Consumer. To describe this, the
subscription member defines a write-only
parameter callbackURL, which must be
submitted through the subscribeevent form.
The read-only parameter subscriptionID is
returned by the subscription. The WebhookThing
will then periodically POST to this callback URI with a
payload defined by data. To unsubscribe, the
Consumer has to submit the
unsubscribeevent form, which makes use of a
URI Template. The uriVariables member
informs the Consumer to include the
subscriptionID string. This can be further
automated by using a TD Context
Extension to include proper semantic annotations.
Alternatively, one can imagine unsubscribing using the
cancellation member similarly to
subscription and combine this with a
unsubscribeevent form that describes a POST
request with payload to unsubscribe.
Example 35:
Temperature Event with subscription and
cancellation
{
"@context": "https://www.w3.org/2019/wot/td/v1",
"id": "urn:dev:ops:32473-Thing-1234",
"title": "WebhookThing",
"description": "Webhook-based Event with subscription and unsubscribe form.",
"securityDefinitions": {"nosec_sc": {"scheme": "nosec"}},
"security": ["nosec_sc"],
"events": {
"temperature": {
"description": "Provides periodic temperature value updates.",
"subscription": {
"type": "object",
"properties": {
"callbackURL": {
"type": "string",
"format": "uri",
"description": "Callback URL provided by subscriber for Webhook notifications.",
"writeOnly": true
},
"subscriptionID": {
"type": "string",
"description": "Unique subscription ID for cancellation provided by WebhookThing.",
"readOnly": true
}
}
},
"data": {
"type": "number",
"description": "Latest temperature value that is sent to the callback URL."
},
"cancellation": {
"type": "object",
"properties": {
"subscriptionID": {
"type": "integer",
"description": "Required subscription ID to cancel subscription.",
"writeOnly": true
}
}
},
"uriVariables": {
"subscriptionID": { "type": "string" }
},
"forms": [
{
"op": "subscribeevent",
"href": "http://192.168.0.124:8080/events/temp/subscribe",
"contentType": "application/json",
"htv:methodName": "POST"
},
{
"op": "unsubscribeevent",
"href": "http://192.168.0.124:8080/events/temp/{subscriptionID}",
"htv:methodName": "DELETE"
}
]
}
}
}
B. JSON Schema for TD Instance
Validation
This section is non-normative.
Below is a JSON Schema [JSON-SCHEMA]
document for syntactically validating Thing Description
instances serialized in JSON based format.
Note
The Thing Description defined by this document
allows for adding external vocabularies by using
@context mechanism known from JSON-LD
[json-ld11],
and the terms in those external vocabularies can be used in
addition to the terms defined in § 5. TD Information
Model. For this reason, the below JSON schema is
intentionally non-strict in that regard. You can replace the
value of additionalProperties schema property
true with false in different
scopes/levels in order to perform a stricter validation in
case no external vocabularies are used.
Note
Please note that some JSON Schema validation
tools do not support the iri string format.
A Thing Description Template is a description for a
class of Things. It describes the properties,
actions, events and common metadata that are shared for an
entire group of Things, to enable the common handling
of thousands of devices by a cloud server, which is not
practical on a per-Thing basis. The Thing Description
Template uses the same core vocabulary and information model
from § 5. TD Information
Model.
The Thing Description Template is a logical description of
the interface and possible interaction with devices
(properties, actions and events), however it does not contain
device-specific information, such as a serial number, GPS
location, security information or concrete protocol
endpoints.
Since a Thing Description Template does not contain a
Protocol Binding to specific endpoints
and does not define a specific security mechanism, the
forms and securityDefinitions and
security keys must not be present.
The same Thing Description Template can be implemented by
Things from multiple vendors, a Thing can
implement multiple Thing Description Templates, define
additional metadata (vendor, location, security) and define
bindings to concrete protocols. To avoid conflicts between
properties, actions and events from different Thing Description
Templates that are combined into a common Thing, all these
identifiers must be unique within a Thing.
A common Thing Description Template for a class of devices
enables writing applications across vendors and creates a more
attractive market for application developers. A concrete Thing
Description can implement multiple Thing Description Templates
and thus can aggregate function blocks into a combined
device.
The business models of cloud vendors are typically built on
managing thousands of identical devices. All devices with the
same Thing Description Template can be managed in the same way
by cloud applications. It is easy to create multiple simulated
devices, if the interface and the instance are treated
separately.
Since a Thing Description Template is a subset of the Thing
Description in which some optional and mandatory Vocabulary Terms do not exist, however, it can be
serialized in the same way and in the same formats as a Thing
Description. Note that Thing Template instances cannot be
validated in the same way as Thing Description instances due to
some missing mandatory terms.
C.1 Thing Description Template
Examples
This section shows a
Thing Description Template for a lamp and a Thing Description
Template for a buzzer.
C.1.1 Thing Description Template:
Lamp
Example
36:
MyLampThingTemplate serialized in JSON
{
"@context": ["https://www.w3.org/2019/wot/td/v1"],
"@type" : "ThingTemplate",
"title": "Lamp Thing Description Template",
"description" : "Lamp Thing Description Template",
"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"}
}
}
}
C.1.2 Thing Description Template:
Buzzer
Example
37:
MyBuzzerThingTemplate serialized in JSON
{
"@context": ["https://www.w3.org/2019/wot/td/v1"],
"@type" : "ThingTemplate",
"title": "Buzzer Thing Description Template",
"description" : "Thing Description Template of a buzzer that makes noise for 10 seconds",
"actions": {
"buzz": {
"description" : "buzz for 10 seconds"
}
}
}
D.
JSON-LD Context Usage
This section is non-normative.
The present specification introduces the TD
Information Model as a set of constraints over different
Vocabularies, i.e. sets of Vocabulary Terms.
This section briefly explains how a machine-readable definition
of these constraints can be integrated into client
applications, by making use of the mandatory
@context of a TD document.
Accessing the TD Information Model from a TD
document is done in two steps. First, clients must retrieve a
mapping from JSON strings to IRIs. This mapping is defined as a
JSON-LD context, as explained later. Second, clients can access
the constraints defined on these IRIs by dereferencing them.
Constraints are defined as logical axioms in the RDF format,
readily interpretable by client programs.
All Vocabulary Terms referenced in
§ 5. TD Information
Model are serialized as (compact) JSON strings in a TD
document. However, each of these terms is unambiguously
identified by a full IRI, as per the first Linked Data
principle [LINKED-DATA]. The
mappings from JSON keys to IRIs is what the
@context value of a TD points to. For instance,
the file at
This JSON file follows the JSON-LD 1.1 syntax
[JSON-LD11].
Numerous JSON-LD libraries can automatically process the
@context of a TD and expand all the JSON strings
it includes.
Once every Vocabulary Term of a TD is expanded to
a IRI, the second step consists in dereferencing this IRI to
get fragments of the TD Information Model
that refer to that Vocabulary Term. For
instance, dereferencing the IRI
results in an RDF document stating that the term
ObjectSchema is a Class and more
precisely, a sub-class of DataSchema. Such logical
axioms are represented in RDF using formalisms of various
complexity: here, sub-class relations are expressed as RDF
Schema axioms [RDF-SCHEMA]. Moreover, these axioms
may be serialized in various formats. Here, they are serialized
in the Turtle format [TURTLE]:
<https://www.w3.org/2019/wot/json-schema#ObjectSchema>
a rdfs:Class .
<https://www.w3.org/2019/wot/json-schema#ObjectSchema>
rdfs:subClassOf <https://www.w3.org/2019/wot/json-schema#DataSchema> .
By default, if a user agent does not perform any content
negotiation, a human-readable HTML documentation is returned
instead of the RDF document. To negotiate content, clients must
include the HTTP header Accept: text/turtle in
their request.
In section § 8.1 Security
Configurations, clarified the interaction between the
dynamic authentication information asked by some security
protocols and the security configurations information
declared in the Thing Description.
At-risk features CertSecurityScheme,
PublicSecurityScheme,
PoPSecurityScheme as well as
implicit, password and
client flows in OAuth2SecurityScheme
were removed.
In section § 6.3.9
forms, clarified the expectations of
Consumers and Things for operation types
writemultipleproperties,
readmultipleproperties and
readallproperties.
In section § 8.3.1 Protocol
Binding based on HTTP, the contexts in which default
values GET or PUT are used for
vocabulary term htv:methodName were clarified.
The editors would like to thank Michael Koster, Michael
Lagally, Kazuyuki Ashimura, Ege Korkan, Daniel Peintner, Toru
Kawaguchi, María Poveda, Dave Raggett, Kunihiko Toumura,
Takeshi Yamada, Ben Francis, Manu Sporny, Klaus Hartke, Addison
Phillips, Jose M. Cantera, Tomoaki Mizushima, Soumya Kanti
Datta and Benjamin Klotz for providing contributions, guidance
and expertise.
Also, many thanks to the W3C staff and all other
current and former active Participants of the W3C Web of Things Interest
Group (WoT IG) and Working Group (WoT WG) for their support,
technical input and suggestions that led to improvements to
this document.
Finally, special thanks to Joerg Heuer for leading the WoT
IG for 2 years from its inception and guiding the group to come
up with the concept of WoT building blocks including the Thing
Description.
