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.
The following features are considered at risk of removal due to potentially insufficient implementation experience (reports).
proxy
in ,
qop
in
and scopes
term in .
Text or table entries highlighted with a yellow background indicate an at-risk feature for which insufficient implementation experience currently exists. In each such case the annotation "This feature is at risk" is attached. When an entire section is at risk the annotation "This section is at risk" is placed at the start of the section and highlighted with a yellow background. When an entire section is at risk individual items within that section are not highlighted or annotated.
The Working Group seeks implementation feedback, having set the requirement of at least two implementations of each feature as the exit criteria for the Candidate Recommendation phase. The group aims to obtain reports from one TD producer and one TD consumer for each feature if applicable. For details, including definitions of implementation, TD producer, and TD consumer, see the draft implementation report.
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 [[iana-uri-schemes]] (e.g., http
, coap
, etc.),
content types based on media types (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 a
securityDefinition
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 the SAREF vocabulary namespace [[smartM2M]].
The SAREF vocabulary includes terms to describe lighting devices and other home automation
devices that one can embed in a TD as semantic labels in the @type
property. In the present example, the Thing is labelled with saref:LightSwitch
,
the form of the status
property affordance is labelled with saref:GetCommand
,
the form of the toggle
action affordance with saref:ToggleCommand
and
the form the overheating
event affordance with saref:NotifyCommand
.
The declaration mechanism inside some
@context
is specified by JSON-LD. A TD instance complies to version 1.1 of
this specification [[?json-ld11]]. The TD instance can be also processed as an RDF
document (details are given in ).
A Thing Description instance complies with this specification if it follows the normative statements in and regarding Thing Description serialization.
A JSON Schema [[?JSON-SCHEMA-VALIDATION]] to validate Thing Description instances is provided in Appendix .
A set of SHACL shapes [[SHACL]] is provided under the TD namespace IRI as an alternative to validate Thing Description instances after transformation to RDF. See Appendix .
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:
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.
The version of the TD Information Model defined in of this specification is identified by the following IRI:
https://www.w3.org/2019/wot/td/v1
This IRI, which is also a URI, can be dereferenced to obtain a JSON-LD context file [[?json-ld11]], allowing the compact strings in TD Documents to be expanded to full IRI-based Vocabulary Terms. However, this processing is only required when transforming JSON-based TD Documents to RDF, an optional feature of TD Processor implementations. See for more details on this aspect.
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 . 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 ).
This section introduces the TD Information Model. The TD Information Model serves as the conceptual basis for the processing of Thing Descriptions and their serialization, which is described separately in .
The TD Information Model is built upon the following, independent Vocabularies:
Overview
Each of these Vocabularies is essentially a set of Terms that can be used to build data structures, interpreted as objects in the traditional object-oriented sense. Objects are instances of classes and have properties. In the context of W3C WoT, they denote Things and their Interaction Affordances. A formal definition of objects is given in . The main elements of the TD Information Model are then presented in . Certain object properties may be omitted in a TD when Default Values exist. A list of defaults is given in .
The UML diagram shown next gives an overview of the TD Information Model.
It represents all classes as tables and the assocations 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 processes 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:
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.
Moreover, Objects can be instances of some Class. A Class, which is denoted by a Vocabulary Term, is first defined by a set of Vocabulary Terms called a Signature. A Class whose Signature is empty is called a Simple Type.
The Signature of a Class allows to construct two functions that further
define Classes: an Assignment Function and a Type Function.
The Assignment Function of a Class takes a Vocabulary Term of the
Class's Signature as input and returns either true
or
false
as output. Intuitively, the Assignment Function indicates
whether an element of the Signature is mandatory or optional when instantiating
the Class. The Type Function of a Class also takes a Vocabulary
Term of the Class's Signature as input and returns another Class
as output. These functions are partial: their domain is limited to the
Signature of the Class being defined.
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:
true
, the Object includes a name-value pair with the
Vocabulary Term as name.
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.
Given all definitions above, the TD Information Model is to be understood
as a set of Class definitions,
which include a Class name (a Vocabulary Term),
a Signature (a set of Vocabulary Terms),
an Assignment Function,
and a Type Function.
These Class definitions are provided as tables in .
For each table, the values "mandatory" (respectively, "optional") in the assignment column
indicates that the Assignment Function returns true
(respectively, false
) for the corresponding Vocabulary Term.
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.
In addition, the TD Information Model defines a global function on
pairs of Vocabulary Terms. The function takes a Class name and
another Vocabulary Term as input and
returns an Object. If the returned Object is different from
null
, it represents the Default Value for some
assignment on the input Vocabulary Term in an instance of the input
Class. This function allows to relax the constraint
defined above on the Assignment Function:
an Object is an instance of a Class if it includes all mandatory
assignments or if Default Value exist for the missing
assignments. All Default Values are given in the table of
. In each table of
, the assignment column contains the value
"with default" if a Default Value is available for the corresponding
combination of Class and Vocabulary Term in 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). This aspect is dealt with in Appendix .
A TD Processor MUST be able to detect that a TD does not meet Class instantiation constraints on all Classes defined in , , , and .
Thing
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@context | JSON-LD keyword to define short-hand names called terms that are used throughout a TD document. | mandatory | anyURI or Array |
@type | JSON-LD keyword to label the object with semantic tags (or types). | optional | string or Array of string |
id | unique identifier of the Thing (URI, e.g. custom URN). | mandatory | anyURI |
title | Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | mandatory | string |
titles | Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). | optional | MultiLanguage |
description | Provides additional (human-readable) information based on a default language. | optional | string |
descriptions | Can be used to support (human-readable) information in different languages. | optional | MultiLanguage |
version | Provides version information. | optional | VersionInfo |
created | Provides information when the TD instance was created. | optional | dateTime |
modified | Provides information when the TD instance was last modified. | optional | dateTime |
support | Provides information about the TD maintainer as URI scheme (e.g., mailto [[RFC6068]], tel [[RFC3966]], https). | optional | anyURI |
base | 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 [[?RFC3987]] used within Linked Data [[?LINKED-DATA]] graphs that are relevant when semantic processing is applied to TD instances. | optional | anyURI |
properties | All Property-based interaction affordance of the Thing. | optional | Map of PropertyAffordance |
actions | All Action-based interaction affordance of the Thing. | optional | Map of ActionAffordance |
events | All Event-based interaction affordance of the Thing. | optional | Map of EventAffordance |
links | Provides Web links to arbitrary resources that relate to the specified Thing Description. | optional | Array of Link |
forms | 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 property affordances collectively at once. | optional | Array of Form |
security | Set of security definition names, chosen from those defined in securityDefinitions. These must all be satisfied for access to resources. | mandatory | string or Array of string |
securityDefinitions | Set of named security configurations (definitions only). Not actually applied unless names are used in a security section. | mandatory | Map of SecurityScheme |
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/v1
MAY 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 @context
Array.
Maps contained in an @context
Array MAY 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 @context
Array SHOULD 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:
MultiLanguage
Maps,
the base direction MAY be inferred from the language tag of the default language.
MultiLanguage
Maps,
the base direction of each value of the name-value pairs
MAY be inferred from the language tag given in the corresponding name.
@language
or MultiLanguage
Maps
MUST 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 events
Arrays,
a Thing can also provide meta-interactions,
which are indicated by Form
instances in its optional forms
Array.
When the forms
Array 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 opteration 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 properties
Map 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.
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type | JSON-LD keyword to label the object with semantic tags (or types). | optional | string or Array of string |
title | Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | optional | string |
titles | Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). | optional | MultiLanguage |
description | Provides additional (human-readable) information based on a default language. | optional | string |
descriptions | Can be used to support (human-readable) information in different languages. | optional | MultiLanguage |
forms | Set of form hypermedia controls that describe how an operation can be performed. Forms are serializations of Protocol Bindings. | mandatory | Array of Form |
uriVariables | Define URI template variables as collection based on DataSchema declarations. | optional | Map of DataSchema |
The class InteractionAffordance
has the following subclasses:
PropertyAffordance
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
observable | A hint that indicates whether Servients hosting the Thing and Intermediaries should provide a Protocol Binding that supports the observeproperty operation for this Property. | optional | boolean |
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 InteractionAffordance
Class and the DataSchema
Class.
When a Form instance is within a PropertyAffordance
Class,
the value assigned to op
MUST be one of readproperty
, writeproperty
, observeproperty
,
unobserveproperty
or an Array containing a combination of these terms.
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., dimm a lamp over time).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
input | Used to define the input data schema of the action. | optional | DataSchema |
output | Used to define the output data schema of the action. | optional | DataSchema |
safe | 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. | with default | boolean |
idempotent | 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. | with default | boolean |
ActionAffordance
is a Subclass of the InteractionAffordance
Class.
When a Form instance is within an instance of the ActionAffordance
Class,
the value assigned to op MUST be invokeaction
.
EventAffordance
An Interaction Affordance that describes an event source, which asynchronously pushes event data to Consumers (e.g., overheating alerts).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
subscription | Defines data that needs to be passed upon subscription, e.g., filters or message format for setting up Webhooks. | optional | DataSchema |
data | Defines the data schema of the Event instance messages pushed by the Thing. | optional | DataSchema |
cancellation | Defines any data that needs to be passed to cancel a subscription, e.g., a specific message to remove a Webhook. | optional | DataSchema |
EventAffordance
is a Subclass of the InteractionAffordance
Class.
When a Form instance is within an instance of the EventAffordance
Class,
the value assigned to op
MUST be either subscribeevent
, unsubscribeevent
, or both terms within an Array.
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
instance | Provides a version identicator of this TD instance. | mandatory | string |
It is recommended that the values within instances of the VersionInfo
Class 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.
MultiLanguage
A Map providing a set of human-readable texts in different languages identified by language tags described in [[BCP47]]. See for example usages of this container in a Thing Description instance.
Each name of the MultiLanguage
Map
MUST be a language tag as defined in [[!BCP47]].
Each value of the MultiLanguage
Map
MUST be of type string
.
The data schema definition is reflecting a very common subset of the terms defined by JSON Schema [[?JSON-SCHEMA-VALIDATION]]. 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. In that case, it is recommended to use a TD Context Extension for the additional terms as described in , otherwise these terms are semantically ignored by TD Processors (also see ).
DataSchema
Metadata that describes the data format used. It can be used for validation.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type | JSON-LD keyword to label the object with semantic tags (or types) | optional | string or Array of string |
title | Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | optional | string |
titles | Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). | optional | MultiLanguage |
description | Provides additional (human-readable) information based on a default language. | optional | string |
descriptions | Can be used to support (human-readable) information in different languages. | optional | MultiLanguage |
type | Assignment of JSON-based data types compatible with JSON Schema (one of boolean, integer, number, string, object, array, or null). | optional | string (one of object , array , string , number , integer , boolean , or null ) |
const | Provides a constant value. | optional | any type |
unit | Provides unit information that is used, e.g., in international science, engineering, and business. | optional | string |
oneOf | Used to ensure that the data is valid against one of the specified schemas in the array. | optional | Array of DataSchema |
enum | Restricted set of values provided as an array. | optional | Array of any type |
readOnly | Boolean value that is a hint to indicate whether a property interaction / value is read only (=true) or not (=false). | with default | boolean |
writeOnly | Boolean value that is a hint to indicate whether a property interaction / value is write only (=true) or not (=false). | with default | boolean |
format | Allows validation based on a format pattern such as "date-time", "email", "uri", etc. (Also see below.) | optional | string |
The class DataSchema
has the following subclasses:
The format
string values are known from a fixed set of values and their corresponding format rules
defined in [[?JSON-SCHEMA-VALIDATION]] (Section 7.3 Defined Formats in particluar).
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.
ArraySchema
Metadata describing data of type Array. This Subclass is indicated by the value array
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
items | Used to define the characteristics of an array. | optional | DataSchema or Array of DataSchema |
minItems | Defines the minimum number of items that have to be in the array. | optional | unsignedInt |
maxItems | Defines the maximum number of items that have to be in the array. | optional | unsignedInt |
BooleanSchema
Metadata describing data of type boolean
. This Subclass is indicated by the value boolean
assigned to type
in DataSchema
instances.
NumberSchema
Metadata describing data of type number
. This Subclass is indicated by the value number
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
minimum | Specifies a minimum numeric value. Only applicable for associated number or integer types. | optional | double |
maximum | Specifies a maximum numeric value. Only applicable for associated number or integer types. | optional | double |
IntegerSchema
Metadata describing data of type integer
. This Subclass is indicated by the value integer
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
minimum | Specifies a minimum numeric value. Only applicable for associated number or integer types. | optional | integer |
maximum | Specifies a maximum numeric value. Only applicable for associated number or integer types. | optional | integer |
ObjectSchema
Metadata describing data of type object
. This Subclass is indicated by the value object
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
properties | Data schema nested definitions. | optional | Map of DataSchema |
required | Defines which members of the object type are mandatory. | optional | Array of string |
StringSchema
Metadata describing data of type string
. This Subclass is indicated by the value string
assigned to type
in DataSchema
instances.
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
.
This specification provides a selection of well-established security mechanisms that are directly built into protocols eligable 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.
SecurityScheme
Metadata describing the configuration of a security mechanism. The value assigned to the name scheme
MUST be defined within a Vocabulary included in the Thing Description, either in the standard Vocabulary defined in or in a TD Context Extension.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type | JSON-LD keyword to label the object with semantic tags (or types). | optional | string or Array of string |
scheme | Identification of the security mechanism being configured. | mandatory | string (e.g. nosec , basic , cert , digest , bearer , pop , psk , public , oauth2 , or apikey ) |
description | Provides additional (human-readable) information based on a default language. | optional | string |
descriptions | Can be used to support (human-readable) information in different languages. | optional | MultiLanguage |
proxy | URI of the proxy server this security configuration provides access to. If not given, the corresponding security configuration is for the endpoint. This feature is at risk. | optional | anyURI |
The class SecurityScheme
has the following subclasses:
NoSecurityScheme
A security configuration corresponding to identified by the Vocabulary Term nosec
(i.e., "scheme": "nosec"
), indicating there is no authentication or other mechanism required to access the resource.
BasicSecurityScheme
Basic authentication security configuration identified by the Vocabulary Term basic
(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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
in | Specifies the location of security authentication information. | with default | string (one of header , query , body , or cookie ) |
name | Name for query, header, or cookie parameters. | optional | string |
DigestSecurityScheme
Digest authentication security configuration identified by the Vocabulary Term digest
(i.e., "scheme": "digest"
). This scheme is similar to basic authentication but with added features to avoid man-in-the-middle attacks.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
qop | Quality of protection. This feature is at risk. | with default | string (one of auth , or auth-int ) |
in | Specifies the location of security authentication information. | with default | string (one of header , query , body , or cookie ) |
name | Name for query, header, or cookie parameters. | optional | string |
APIKeySecurityScheme
This section is at risk.
API key authentication security configuration identified by the Vocabulary Term apikey
(i.e., "scheme": "apikey"
). This is for the case where the access token is opaque and is not using a standard token format.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
in | Specifies the location of security authentication information. | with default | string (one of header , query , body , or cookie ) |
name | Name for query, header, or cookie parameters. | optional | string |
BearerSecurityScheme
Bearer token authentication security configuration identified by the Vocabulary Term bearer
(i.e., "scheme": "bearer"
). This scheme is intended 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..
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
authorization | URI of the authorization server. | optional | anyURI |
alg | Encoding, encryption, or digest algorithm. | with default | string (e.g. MD5 , ES256 , or ES512-256 ) |
format | Specifies format of security authentication information. | with default | string (e.g. jwt , cwt , jwe , or jws ) |
in | Specifies the location of security authentication information. | with default | string (one of header , query , body , or cookie ) |
name | Name for query, header, or cookie parameters. | optional | string |
CertSecurityScheme
This section is at risk.
Certificate-based asymmetric key security configuration conformant with [[!X509V3]] identified by the Vocabulary Term cert
(i.e., "scheme": "cert"
).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
identity | Identifier providing information which can be used for selection or confirmation. | optional | string |
PSKSecurityScheme
This section is at risk.
Pre-shared key authentication security configuration identified by the Vocabulary Term psk
(i.e., "scheme": "psk"
).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
identity | Identifier providing information which can be used for selection or confirmation. | optional | string |
PublicSecurityScheme
This section is at risk.
Raw public key asymmetric key security configuration identified by the Vocabulary Term public
(i.e., "scheme": "public"
).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
identity | Identifier providing information which can be used for selection or confirmation. | optional | string |
PoPSecurityScheme
This section is at risk.
Proof-of-possession (PoP) token authentication security configuration identified by the Vocabulary Term pop
(i.e., "scheme": "pop"
). Here 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 PoP tokens MAY be specified in vocabulary extensions..
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
authorization | URI of the authorization server. | optional | anyURI |
alg | Encoding, encryption, or digest algorithm. | with default | string (e.g. MD5 , ES256 , or ES512-256 ) |
format | Specifies format of security authentication information. | with default | string (e.g. jwt , cwt , jwe , or jws ) |
in | Specifies the location of security authentication information. | with default | string (one of header , query , body , or cookie ) |
name | Name for query, header, or cookie parameters. | optional | string |
OAuth2SecurityScheme
This section is at risk.
OAuth2 authentication security configuration for systems conformant with [[!RFC6749]] and [[!RFC8252]], identified by the Vocabulary Term oauth2
(i.e., "scheme": "oauth2"
). For the implicit
flow authorization
MUST be included. For the password
and client
flows token
MUST be included. For the code
flow both authorization
and token
MUST be included. If no scopes
are defined in the SecurityScheme
then they are considered to be empty.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
authorization | URI of the authorization server. | optional | anyURI |
token | URI of the token server. | optional | anyURI |
refresh | URI of the refresh server. | optional | anyURI |
scopes | 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 feature is at risk. | optional | string or Array of string |
flow | Authorization flow. | with default | string (one of implicit , password , client , or code ) |
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).
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
href | target IRI of a link or submission target of a form. | mandatory | anyURI |
type | Target attribute providing a hint indicating what the media type [IANA-MEDIA-TYPES] of the result of dereferencing the link should be. | optional | string |
rel | A link relation type identifies the semantics of a link. | optional | string |
anchor | By default, the context, or anchor, of a link conveyed in the Link header field is the URL of the representation it is associated with, as defined in RFC7231, Section 3.1.4.1, and is serialized as a URI. | optional | anyURI |
Form
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.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
op | 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. | with default | 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. | mandatory | anyURI |
contentType | Assign a content type based on a media type [[IANA-MEDIA-TYPES]] (e.g., 'text/plain') and potential parameters (e.g., 'charset=utf-8') for the media type. | with default | string |
contentCoding | 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. . | optional | string |
subprotocol | 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, websub (also see https://www.w3.org/TR/websub/), or server sent events (also see https://www.w3.org/TR/eventsource/). Please note that there is no restriction on the sub-protocol selection and other mechanisms can also be announced by this subprotocol term. | optional | string (e.g. longpoll , websub , or sse ) |
security | Set of security definition names, chosen from those defined in securityDefinitions. These must all be satisfied for access to resources. | optional | string or Array of string |
scopes | 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 feature is at risk. | optional | string or Array of string |
response | This optional term can be used if, e.g., the output communication metadata differ from input metdata (e.g., output contentType differ from the input contentType). The response name contains metadata that is only valid for the reponse messages. | optional | ExpectedResponse |
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 ExpectedResponse
Class 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.
Possible values for the contentCoding
property can be found e.g. in
the
IANA HTTP content coding registry.
ExpectedResponse
Communication metadata describing the expected response message.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
contentType | Assign a content type based on a media type [[IANA-MEDIA-TYPES]] (e.g., 'text/plain') and potential parameters (e.g., 'charset=utf-8') for the media type. | mandatory | string |
When assignments in a TD are missing, a TD Processor MUST follow the Default Value assignments expressed in the table of .
The following table gives all Default Values defined in the TD Information Model.
Class | Vocabulary Term | Default Value | Comment |
---|---|---|---|
Form
|
contentType
|
application/json |
|
DataSchema |
readOnly
|
false |
|
DataSchema |
writeOnly
|
false |
|
ActionAffordance |
safe
|
false |
|
ActionAffordance |
idempotent
|
false |
|
Form |
op
|
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 |
|
PoPSecurityScheme
This feature is at risk. |
in
|
header |
|
APIKeySecurityScheme
This feature is at risk. |
in
|
query |
|
DigestSecurityScheme
This feature is at risk. |
qop
|
auth |
|
BearerSecurityScheme
|
alg
|
ES256 |
|
PoPSecurityScheme
This feature is at risk. |
alg
|
ES256 |
|
BearerSecurityScheme
|
format
|
jwt |
|
PoPSecurityScheme
This feature is at risk. |
format
|
jwt |
|
OAuth2SecurityScheme
This feature is at risk. |
flow
|
implicit |
WoT Thing Descriptions represent Things and are modeled and structured based on
.
This section defines a JSON-based representation format for Things,
a serialization of instances of the Class Thing
defined by the TD Information Model.
A TD Processor MUST be able to serialize Thing Descriptions into the JSON format [[!RFC8259]] and deserialize Thing Descriptions from that format, according to the rules noted in and .
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, as further detailed in .
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:
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
(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:
string
or
anyURI
MUST be serialized as JSON strings.
dateTime
MUST 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 dateTime
SHOULD use the literal Z
representing the UTC time zone instead
of an offset.
integer
or unsignedInt
MUST be serialized as JSON numbers without a fraction or exponent part.
double
MUST be serialized as JSON number.
boolean
MUST 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 Thing Description serialization may omit Vocabulary Term for which Default Values are defined, as listed in the table given in .
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 .
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 Serialization MUST 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 for details).
{ "@context": "https://www.w3.org/2019/wot/td/v1", ... }
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:
{ "@context": "https://www.w3.org/2019/wot/td/v1", "@type": "Thing", "id": "urn:dev:ops:32473-Thing-1234", "title": "MyThing", "titles": {...}, "description": "Human readable information.", "descriptions": {...}, "support": "mailto:support@example.com", "version" : {...}, "created" : "2018-11-14T19:10:23.824Z", "modified" : "2019-06-01T09:12:43.124Z", "securityDefinitions": {...}, "security": ..., "base": "https://servient.example.com/", "properties": {...}, "actions": {...}, "events": {...}, "links": [...], "forms": [...] }
All values assigned to
version
,
securityDefinitions
,
properties
,
actions
, and
events
in an instance of the Class Thing
MUST be serialized as JSON objects.
All values assigned to
links
, and
forms
in an instance of the Class Thing
MUST be serialized as JSON arrays containing JSON objects
as defined in
and , respectively.
The value assigned to
security
in an instance of Class Thing
MUST be serialized as JSON string or as JSON array whose elements are JSON strings.
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 , 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.
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.
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "@language" : "en" } ], "title": "MyThing", "description": "Human readable information.", ... "properties": { "on": { "title" : "On/Off", "type": "boolean", "forms": [...] }, "status": { "title" : "Status", "type": "object", ... "forms": [...] } }, ... }
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 MultiLanguage
Map
MUST 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 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:
{ "@context": "https://www.w3.org/2019/wot/td/v1", "title": "MyThing", "titles": { "en":"MyThing", "de": "MeinDing", "ja" : "私の物", "zh-Hans" : "我的东西", "zh-Hant" : "我的東西" }, "descriptions": { "en":"Human readable information.", "de": "Menschenlesbare Informationen.", "ja" : "人間が読むことができる情報", "zh-Hans" : "人们可阅读的信息", "zh-Hant" : "人們可閱讀的資訊" }, ... "properties": { "on": { "titles": { "en": "On/Off", "de": "An/Aus", "ja": "オンオフ", "zh-Hans": "开关", "zh-Hant": "開關" }, "type": "boolean", "forms": [...] }, "status": { "titles": { "en": "Status", "de": "Zustand", "ja": "状態", "zh-Hans": "状态", "zh-Hant": "狀態" }, "type": "object", ... "forms": [...] } }, ... }
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 description
MAY 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 the language of the default text is indicated by the default language,
which is usually set by the creator of the Thing Description instance.
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "@language" : "de" } ], "title": "MyThing", "titles": { "en":"MyThing", "de": "MeinDing", "ja" : "私の物", "zh-Hans" : "我的东西", "zh-Hant" : "我的東西" }, "description": "Menschenlesbare Informationen.", "descriptions": { "en":"Human readable information.", "de": "Menschenlesbare Informationen.", "ja" : "人間が読むことができる情報", "zh-Hans" : "人们可阅读的信息", "zh-Hant" : "人們可閱讀的資訊" }, ... "properties": { "on": { "title" : "An/Aus", "titles": { "en": "On/Off", "de": "An/Aus", "ja": "オンオフ", "zh-Hans": "开关", "zh-Hant": "開關" }, "type": "boolean", "forms": [...] }, "status": { "title" : "Zustand", "titles": { "en": "Status", "de": "Zustand", "ja": "状態", "zh-Hans": "状态", "zh-Hant": "狀態" }, "type": "object", ... "forms": [...] } }, ... }
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).
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:
{ ... "version": { "instance": "1.2.1" }, ... }
The version
member is intended as container for additional application- and/or device-specific version information based
on TD Context Extensions. See for details.
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 SecurityScheme
Subclass'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.
... "securityDefinitions": { "basic_sc": { "scheme": "basic", "in": "header" } }, "security": "basic_sc", ...
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.
... "securityDefinitions": { "proxy_sc": { "scheme": "digest", "proxy": "https://portal.example.com/" }, "bearer_sc": { "in":"header", "scheme": "bearer", "format": "jwt", "alg": "ES256", "authorization": "https://servient.example.com:8443/" } }, "security": ["proxy_sc", "bearer_sc"], ...
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:
{ "@context": "https://www.w3.org/2019/wot/td/v1", "id": "urn:dev:ops:32473-Thing-1234", "title": "MyThing", "description": "Human readable information.", "support": "https://servient.example.com/contact", "securityDefinitions": { "nosec_sc": { "scheme": "nosec" }}, "security": "nosec_sc", "properties": {...}, "actions": {...}, "events": {...}, "links": [...] }
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.
{ ... "securityDefinitions": { "basic_sc": {"scheme": "basic"}, "nosec_sc": {"scheme": "nosec"} }, "security": ["basic_sc"], ... "properties": { "status": { ... "forms": [{ "href": "https://mylamp.example.com/status" }] } }, "actions": { "toggle": { ... "forms": [{ "href": "https://mylamp.example.com/toggle" }] } }, "events": { "overheating": { ... "forms": [{ "href": "https://mylamp.example.com/oh", "security": ["nosec_sc"] }] } } }
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, via HTTPS
via digest authentication, or via CoAPS with a pre-shared key (PSK).
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.
{ ... "securityDefinitions": { "basic_sc": { "scheme": "basic" }, "digest_sc": { "scheme": "digest", "qop": "auth", "in": "header" }, "psk_sc": { "scheme": "psk" } }, "security": ["basic_sc"], ... "properties": { "status": { ... "forms": [{ "href": "https://mylamp.example.com/status" }, { "href": "https://mylamp.example.com/status", "security": ["digest_sc"] }, { "href": "coaps://mylamp.example.com:5684/status", "security": ["psk_sc"] }] } }, ... }
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.
{ ... "securityDefinitions": { "oauth2_sc": { "scheme": "oauth2", ... "flow": "implicit", "authorization": "https://example.com/authorization", "scopes": ["limited", "special"] } }, "security": ["oauth2_sc"], ... "properties": { "status": { ... "forms": [{ "href": "https://scopes.example.com/status", "scopes": ["limited"] }] } }, "action": { "configure": { ... "forms": [{ "href": "https://scopes.example.com/configure", "scopes": ["special"] }] } }, ... }
properties
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 for
details on serializing DataSchema
instances.
The value assigned to forms
in an instance of PropertyAffordance
MUST be serialized as a JSON array
containing one or more JSON object serializations as defined in .
A snippet for two Property affordances is given below:
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
ActionAffordance
MUST be serialized as JSON objects.
They rely on the the Class DataSchema
,
whose serialization is defined in .
The value assigned to forms
in an instance of ActionAffordance
MUST be serialized as a JSON array
containing one or more JSON object serializations as defined in .
A TD snippet of an Action affordance is given below:
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 EventAffordance
MUST be serialized as JSON objects.
They rely on the the Class DataSchema
,
whose serialization is defined in .
The value assigned to forms
in an instance of EventAffordance
MUST be serialized as a JSON array
containing one or more JSON object serializations as defined in .
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) 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 illustrates how Events can use subscription
and cancellation
to describe Webhooks.
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:
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.
If required, form objects MAY be supplemented with protocol-specific Vocabulary Terms identified with a prefix. See also .
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 Form
MUST
rely on the Class DataSchema
,
whose serialization is defined in .
A TD snippet using a URI Template and uriVariables
is given below:
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "eg": "http://www.example.org/iot" } ], ... "actions": { "LeftDown": { ... "uriVariables": { "p" : { "type": "integer", "minimum": 0, "maximum": 16, "@type": "eg:SomeKindOfAngle" }, "d" : { "type": "integer", "minimum": 0, "maximum": 1, "@type": "eg:Direction" } }, "forms": [{ "href" : "http://192.168.1.25/left{?p,d}", "htv:methodName": "GET" }] }, ... }, ... }
The contentType
member is used to assign a media types [[!IANA-MEDIA-TYPES]]
including media type parameters as attribute-value pairs separated by a ;
character.
Example:
... "contentType" : "text/plain; charset=utf-8", ...
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
Form
MUST 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:
{ ... "actions": { "takePhoto": { ... "forms": [{ "op": "invokeaction", "href": "http://camera.example.com/api/snapshot", "contentType": "application/json", "response": { "contentType": "image/jpeg" } }] } }, ... }
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.
{ ... "properties": { "on": { "type": "boolean", "forms": [...] }, "brightness": { "type": "number", "forms": [...] }, "timer": { "type": "integer", "forms": [...] } }, ... "forms": [{ "op": "readallproperties", "href": "https://mylamp.example.com/allproperties", "contentType": "application/json", "htv:methodName": "GET" }, { "op": "writeallproperties", "href": "https://mylamp.example.com/allproperties", "contentType": "application/json", "htv:methodName": "PUT" }] }
The data schemas of the WoT Thing Description defined through the
DataSchema
Class
are based on a subset of the JSON Schema terms [[?JSON-SCHEMA-VALIDATION]].
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 DataSchema
Subclass's
instance, with the Vocabulary Term as name.
The value assigned to properties
in an instance of
ObjectSchema
MUST be serialized as a JSON object.
The values assigned to enum
,
required
,
and oneOf
in an instance of DataSchema
MUST be serialized as a JSON array.
The value assigned to items
in an instance of
ArraySchema
MUST 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:
... "properties": { "status": { "description": "Read or write On/Off status.", "type": "object", "properties": { "latestStatus": { "type": "string", "enum": ["On", "Off"], "readOnly": true }, "newStatusValue": { "type": "string", "enum": ["On", "Off"], "writeOnly": true } }, forms: [...] } } ...
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 based on existing definitions
by integrating the corresponding namespace
(e.g., from Smart Appliances REFerence (SAREF) ontology or Ontology of units of Measure (OM)).
See for details.
The JSON-based serialization of Thing Descriptions is identified by
the media type application/td+json
or the
CoAP Content-Format ID T.B.D.
(see ).
CoAP-based WoT implementations can use the experimental Content-Format
65100
until the final Content-Format ID has been assigned.
In addition to the standard Vocabulary definitions in , 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 Class Thing
is an Array with additional elements of type anyURI
identifying JSON-LD context files
or Map containing namespace IRIs as defined in .
The serialization rules for complex types in
define the serialization of an extended @context
name-value pair.
A snippet with TD Context Extensions is given below:
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "iot": "http://example.org/iot", "cov": "http://www.example.org/coap-binding#" }, "https://schema.org/" ], ... }
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 ,
@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 addind version numbers for the hardware and firmware of the Thing, semantically annotates a Property, and uses a well-defined value for the data schema unit.
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "v": "http://www.example.org/versioningTerms#", "saref": "https://w3id.org/saref#", "om": "http://www.wurvoc.org/vocabularies/om-1.8/" } ], "@type": "Thing", "version": { "instance": "1.2.1", "v:firmware": "0.9.1", "v:hardware": "1.0" }, ... "properties": { "temperature": { "@type": "saref:Temperature", "description": "Temperature value of the weather station", "type": "number", "minimum": -32.5, "maximum": 55.2, "unit": "om:degree_Celsius", "forms": [...] }, ... }, ... }
In many cases, context extension may be used to annotate pieces of
a data schema, as in the TD snippet below. The example references
SSN/SOSA [[vocab-ssn]] and OM 1.8, an ontology for units of measure
[[Rijgersberg-et-al-2013]]. Note
that object assigned to temperature
is a data schema and not
the actual temperature property of the physical world object. More details
on modeling the physical world can be found in Appendix
.
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "sosa": "http://www.w3.org/ns/sosa/", "ssn": "http://www.w3.org/ns/ssn/", "om": "http://www.wurvoc.org/vocabularies/om-1.8/" } ], "@type": "Thing", "id": "urn:dev:ops:32473-WoTLamp-1234", "sosa:observes": { "@id": "urn:dev:ops:32473-WoTStation-1234/temperature", "@type": "om:Temperature" }, ... "properties": { "temperature": { "ssn:forProperty": "urn:dev:ops:32473-WoTStation-1234/temperature", "description": "Temperature measurement of the weather station", "type": "number", "minimum": -32.5, "maximum": 55.2, "unit": "om:degree_Celsius", "forms": [...] }, ... }, ... }
With the TD Context Extensions in a Thing Description,
the communication metadata can be supplemented or new Protocol Bindings added
through additional Vocabulary Terms serialized into JSON objects representing a Form
instance.
(see also ).
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 RDF vocabulary similar to [[HTTP-in-RDF10]]
that is accessable via the 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).
Finally, new security schemes that are not included in
can be imported using the TD Context Extension mechanism.
This example uses a fictional ACE security scheme
based on [[?draft-ietf-ace-oauth-authz]] 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
Class SecurityScheme
.
{ @context: [ "https://www.w3.org/2019/wot/td/v1", { "cov": "http://www.example.org/coap-binding#", "ace": "http://www.example.org/ace-security#" } ], ... "securityDefinitions": { "ace_sc": { "scheme": "ace:ACESecurityScheme", ... "ace:as": "coaps://as.example.com/token", "ace:audience": "coaps://rs.example.com", "ace:scopes": ["limited", "special"], "ace:cnonce": true } }, "security": ["ace_sc"], "properties": { "status": { ... "forms": [{ "op": "readproperty", "href": "coaps://rs.example.com/status", "contentType": "application/cbor", "cov:methodName": "GET", "ace:scopes": ["limited"] }] } }, "action": { "configure": { ... "forms": [{ "op": "invokeaction", "href": "coaps://rs.example.com/configure", "contentType": "application/cbor", "cov:methodName": "POST", "ace:scopes": ["special"] }] } }, ... }
Note that all security schemes defined in are already part of the TD context and need not to be included through a context extension.
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 behaviour of such pre-existing interfaces. Instead, the assertions must be interpreted as constraints on the TD to accurately represent such interfaces.
To enable secure interoperation, security configurations must accurately reflect the requirements of the Thing:
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 Decription. In general, Things are described by WoT Thing Descriptions, but Consumers are constrained to follow WoT Thing Descriptions when interacting with Things.
A Protocol Binding is the mapping from an Interaction Affordance to concrete messages of a
specific protocol such as HTTP [[!RFC7231]], CoAP [[!RFC7252]], or MQTT [[!MQTT]]. Protocol Bindings of
Interaction Affordances are serialized as forms
as defined in .
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
HTTP Protocol Binding and it should expect HTTP-specific terms in the
form instance (see next section, ).
href
member.
Per default the Thing Description supports the HTTP Protocol Binding and
includes the HTTP RDF vocabulary set definitions from [[HTTP-in-RDF10]] and can be
directly used within TD instances by the usage of the prefix htv
, which
points to http://www.w3.org/2011/http#
.
To interact with a Thing that implements the HTTP Protocol Binding, 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 HTTP Protocol Binding defines Default Values for each operation type,
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 HTTP
Protocol Binding, a Default Value MUST be assumed as shown in
the following table.
Vocabulary term | Default value | Context |
---|---|---|
htv:methodName
|
GET |
Form with operation type readproperty
|
htv:methodName
|
PUT |
Form with operation type writeproperty
|
htv:methodName
|
POST |
Form with operation type invokeaction
|
For example, the following Default Values should be assumed for forms in the introductory TD example:
The number of Protocol Bindings a Thing can implement is not restricted. Other Protocol Bindings (e.g., for CoAP, MQTT, or OPC UA) are intended to be standardized in separate documents such as a protocol Vocabulary similar to HTTP Vocabulary in RDF 1.0 [[HTTP-in-RDF10]] or specifications including Default Value definitions. Such protocols can be simply integrated into the TD by the usage of the context extension mechanism ().
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. 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-CONSIDERATIONS]]. 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.
Deferencing 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 deferences and can use the metadata
of the dereference, 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.
@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.
The fact that a Thing Description contains a unique identifier means that should it be associated with a person it can be used to track that person and therefore pose a risk to privacy.
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.
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 a person, through fingerprinting.
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.
Intercepting and tampering with context files can be used to facilitate attacks by modifying the interpretation of vocabulary.
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.
application/td+json
Media Type Registration
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 ) 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 ) 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 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. Consumers SHOULD treat any TD metadata with due skepticism.
Rules for processing both conforming and non-conforming content are defined in this specification.
IANA assigns compact CoAP Content-Format IDs for media types in the CoAP Content-Formats subregistry within the Constrained RESTful Environments (CoRE) Parameters registry [[RFC7252]]. The Content-Format ID for WoT Thing Description is (t.b.d.) in the 256-9999 range (IETF Review or IESG Approval).
Feature list of the Thing:
{ "@context": [ "https://www.w3.org/2019/wot/td/v1", { "cov": "http://www.example.org/coap-binding#" } ], "id": "urn:dev:ops:32473-WoTLamp-1234", "title": "MyLampThing", "description" : "MyLampThing uses JSON serialization", "securityDefinitions": {"psk_sc":{"scheme": "psk"}}, "security": ["psk_sc"], "properties": { "status": { "description" : "Shows the current status of the lamp", "type": "string", "forms": [{ "href": "coaps://mylamp.example.com/status", "cov:methodName" : "GET" }] } }, "actions": { "toggle": { "description" : "Turn on or off the lamp", "forms": [{ "href": "coaps://mylamp.example.com/toggle", "cov:methodName" : "POST" }] } }, "events": { "overheating": { "description" : "Lamp reaches a critical temperature (overheating)", "data": {"type": "string"}, "forms": [{ "href": "coaps://mylamp.example.com/oh", "cov:methodName" : "GET", "subprotocol" : "cov:observe" }] } } }
Feature list of the Thing:
{ "@context": "https://www.w3.org/2019/wot/td/v1", "title": "MyLightSensor", "id": "urn:dev:ops:32473-WoTLightSensor-1234", "securityDefinitions": {"nosec_sc": {"scheme": "nosec"}}, "security": ["nosec_sc"], "events": { "lightSensor": { "data":{"type": "integer"}, "forms": [ { "href": "mqtt://192.168.1.187:1883/lightSensor", "contentType" : "text/plain" } ] } } }
Feature list of the Thing:
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 context extension to include proper semantic annotations.
Alternatively, one can imagine unsubscribing using the concellation
member similarly to subscription
and combine this with a unsubscribeevent
form that describes a POST request with payload to unsubscribe.
{ "@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" } ] } } }
Below is a JSON Schema [[?JSON-SCHEMA-VALIDATION]] document for syntactically validating Thing Description instances serialized in JSON based format.
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 .
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.
Please note that some JSON Schema validation tools do not support the iri
string format.
The following JSON Schema for validating TD instances does not require the terms with Default Values to be present. Thus the terms with Default Values are optional. (see also )
{ "title": "WoT TD Schema - 08 May 2019", "description": "JSON Schema for validating TD instances against the TD model. TD instances can be with or without terms that have default values", "$schema ": "http://json-schema.org/draft-07/schema#", "definitions": { "thing-context-w3c-uri": { "type": "string", "enum": [ "https://www.w3.org/2019/wot/td/v1" ] }, "thing-context": { "oneOf": [{ "type": "array", "items": { "anyOf": [{ "$ref": "#/definitions/anyUri" }, { "type": "object" } ] }, "contains": { "$ref": "#/definitions/thing-context-w3c-uri" } }, { "$ref": "#/definitions/thing-context-w3c-uri" } ] }, "type_declaration": { "oneOf": [{ "type": "string" }, { "type": "array", "items": { "type": "string" } } ] }, "property_element": { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "title": { "$ref": "#/definitions/title" }, "titles": { "$ref": "#/definitions/titles" }, "uriVariables": { "type": "object", "additionalProperties": { "$ref": "#/definitions/dataSchema" } }, "forms": { "type": "array", "minItems": 1, "items": { "$ref": "#/definitions/form_element_property" } }, "observable": { "type": "boolean" }, "writeOnly": { "type": "boolean" }, "readOnly": { "type": "boolean" }, "oneOf": { "type": "array", "items": { "$ref": "#/definitions/dataSchema" } }, "unit": { "type": "string" }, "enum": { "type": "array", "minItems": 1, "uniqueItems": true }, "format": { "type": "string" }, "const": {}, "type": { "type": "string", "enum": [ "boolean", "integer", "number", "string", "object", "array", "null" ] }, "items": { "oneOf": [{ "$ref": "#/definitions/dataSchema" }, { "type": "array", "items": { "$ref": "#/definitions/dataSchema" } } ] }, "maxItems": { "type": "integer", "minimum": 0 }, "minItems": { "type": "integer", "minimum": 0 }, "minimum": { "type": "number" }, "maximum": { "type": "number" }, "properties": { "additionalProperties": { "$ref": "#/definitions/dataSchema" } }, "required": { "type": "array", "items": { "type": "string" } } }, "required": [ "forms" ], "additionalProperties": true }, "action_element": { "type": "object", "properties": { "description": { "type": "string" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "title": { "$ref": "#/definitions/title" }, "titles": { "$ref": "#/definitions/titles" }, "uriVariables": { "type": "object", "additionalProperties": { "$ref": "#/definitions/dataSchema" } }, "@type": { "$ref": "#/definitions/type_declaration" }, "forms": { "type": "array", "minItems": 1, "items": { "$ref": "#/definitions/form_element_action" } }, "input": { "$ref": "#/definitions/dataSchema" }, "output": { "$ref": "#/definitions/dataSchema" }, "safe": { "type": "boolean" }, "idempotent": { "type": "boolean" } }, "required": [ "forms" ], "additionalProperties": true }, "event_element": { "type": "object", "properties": { "description": { "type": "string" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "title": { "$ref": "#/definitions/title" }, "titles": { "$ref": "#/definitions/titles" }, "uriVariables": { "type": "object", "additionalProperties": { "$ref": "#/definitions/dataSchema" } }, "@type": { "$ref": "#/definitions/type_declaration" }, "forms": { "type": "array", "minItems": 1, "items": { "$ref": "#/definitions/form_element_event" } }, "subscription": { "$ref": "#/definitions/dataSchema" }, "data": { "$ref": "#/definitions/dataSchema" }, "cancellation": { "$ref": "#/definitions/dataSchema" }, "type": { "not": {} }, "enum": { "not": {} }, "const": { "not": {} } }, "required": [ "forms" ], "additionalProperties": true }, "form_element_property": { "type": "object", "properties": { "href": { "$ref": "#/definitions/anyUri" }, "op": { "oneOf": [{ "type": "string", "enum": [ "readproperty", "writeproperty", "observeproperty", "unobserveproperty" ] }, { "type": "array", "items": { "type": "string", "enum": [ "readproperty", "writeproperty", "observeproperty", "unobserveproperty" ] } } ] }, "contentType": { "type": "string" }, "security": { "type": "array", "items": { "type": "string" } }, "scopes": { "type": "array", "items": { "type": "string" } }, "subProtocol": { "type": "string", "enum": [ "longpoll", "websub", "sse" ] }, "response": { "type": "object", "properties": { "contentType": { "type": "string" } } } }, "required": [ "href" ], "additionalProperties": true }, "form_element_action": { "type": "object", "properties": { "href": { "$ref": "#/definitions/anyUri" }, "op": { "oneOf": [{ "type": "string", "enum": [ "invokeaction" ] }, { "type": "array", "items": { "type": "string", "enum": [ "invokeaction" ] } } ] }, "contentType": { "type": "string" }, "security": { "type": "array", "items": { "type": "string" } }, "scopes": { "type": "array", "items": { "type": "string" } }, "subProtocol": { "type": "string", "enum": [ "longpoll", "websub", "sse" ] }, "response": { "type": "object", "properties": { "contentType": { "type": "string" } } } }, "required": [ "href" ], "additionalProperties": true }, "form_element_event": { "type": "object", "properties": { "href": { "$ref": "#/definitions/anyUri" }, "op": { "oneOf": [{ "type": "string", "enum": [ "subscribeevent", "unsubscribeevent" ] }, { "type": "array", "items": { "type": "string", "enum": [ "subscribeevent", "unsubscribeevent" ] } } ] }, "contentType": { "type": "string" }, "security": { "type": "array", "items": { "type": "string" } }, "scopes": { "type": "array", "items": { "type": "string" } }, "subProtocol": { "type": "string", "enum": [ "longpoll", "websub", "sse" ] }, "response": { "type": "object", "properties": { "contentType": { "type": "string" } } } }, "required": [ "href" ], "additionalProperties": true }, "form_element_root": { "type": "object", "properties": { "href": { "$ref": "#/definitions/anyUri" }, "op": { "oneOf": [{ "type": "string", "enum": [ "readallproperties", "writeallproperties", "readmultipleproperties", "writemultipleproperties" ] }, { "type": "array", "items": { "type": "string", "enum": [ "readallproperties", "writeallproperties", "readmultipleproperties", "writemultipleproperties" ] } } ] }, "contentType": { "type": "string" }, "security": { "type": "array", "items": { "type": "string" } }, "scopes": { "type": "array", "items": { "type": "string" } }, "subProtocol": { "type": "string", "enum": [ "longpoll", "websub", "sse" ] }, "response": { "type": "object", "properties": { "contentType": { "type": "string" } } } }, "required": [ "href" ], "additionalProperties": true }, "description": { "type": "string" }, "title": { "type": "string" }, "descriptions": { "type": "object" }, "titles": { "type": "object" }, "dataSchema": { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "title": { "$ref": "#/definitions/title" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "titles": { "$ref": "#/definitions/titles" }, "writeOnly": { "type": "boolean" }, "readOnly": { "type": "boolean" }, "oneOf": { "type": "array", "items": { "$ref": "#/definitions/dataSchema" } }, "unit": { "type": "string" }, "enum": { "type": "array", "minItems": 1, "uniqueItems": true }, "format": { "type": "string" }, "const": {}, "type": { "type": "string", "enum": [ "boolean", "integer", "number", "string", "object", "array", "null" ] }, "items": { "oneOf": [{ "$ref": "#/definitions/dataSchema" }, { "type": "array", "items": { "$ref": "#/definitions/dataSchema" } } ] }, "maxItems": { "type": "integer", "minimum": 0 }, "minItems": { "type": "integer", "minimum": 0 }, "minimum": { "type": "number" }, "maximum": { "type": "number" }, "properties": { "additionalProperties": { "$ref": "#/definitions/dataSchema" } }, "required": { "type": "array", "items": { "type": "string" } } } }, "link_element": { "type": "object", "properties": { "anchor": { "$ref": "#/definitions/anyUri" }, "href": { "$ref": "#/definitions/anyUri" }, "rel": { "type": "string" }, "type": { "type": "string" } }, "required": [ "href" ], "additionalProperties": true }, "securityScheme": { "oneOf": [{ "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "nosec" ] } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "basic" ] }, "in": { "type": "string", "enum": [ "header", "query", "body", "cookie" ] }, "name": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "cert" ] }, "identity": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "digest" ] }, "qop": { "type": "string", "enum": [ "auth", "auth-int" ] }, "in": { "type": "string", "enum": [ "header", "query", "body", "cookie" ] }, "name": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "bearer" ] }, "authorization": { "$ref": "#/definitions/anyUri" }, "alg": { "type": "string", "enum": [ "MD5", "ES256", "ES512-256" ] }, "format": { "type": "string", "enum": [ "jwt", "jwe", "jws" ] }, "in": { "type": "string", "enum": [ "header", "query", "body", "cookie" ] }, "name": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "psk" ] }, "identity": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "public" ] }, "identity": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "oauth2" ] }, "authorization": { "$ref": "#/definitions/anyUri" }, "token": { "$ref": "#/definitions/anyUri" }, "refresh": { "$ref": "#/definitions/anyUri" }, "scopes": { "type": "array", "items": { "type": "string" } }, "flow": { "type": "string", "enum": [ "implicit", "password", "client", "code" ] } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "apikey" ] }, "in": { "type": "string", "enum": [ "header", "query", "body", "cookie" ] }, "name": { "type": "string" } }, "required": [ "scheme" ] }, { "type": "object", "properties": { "@type": { "$ref": "#/definitions/type_declaration" }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "proxy": { "$ref": "#/definitions/anyUri" }, "scheme": { "type": "string", "enum": [ "pop" ] }, "authorization": { "$ref": "#/definitions/anyUri" }, "format": { "type": "string", "enum": [ "jwt", "jwe", "jws" ] }, "alg": { "type": "string", "enum": [ "MD5", "ES256", "ES512-256" ] }, "in": { "type": "string", "enum": [ "header", "query", "body", "cookie" ] }, "name": { "type": "string" } }, "required": [ "scheme" ] } ] }, "anyUri": { "type": "string", "format": "iri-reference" } }, "type": "object", "properties": { "id": { "type": "string", "format": "uri" }, "title": { "$ref": "#/definitions/title" }, "titles": { "$ref": "#/definitions/titles" }, "properties": { "type": "object", "additionalProperties": { "$ref": "#/definitions/property_element" } }, "actions": { "type": "object", "additionalProperties": { "$ref": "#/definitions/action_element" } }, "events": { "type": "object", "additionalProperties": { "$ref": "#/definitions/event_element" } }, "description": { "$ref": "#/definitions/description" }, "descriptions": { "$ref": "#/definitions/descriptions" }, "version": { "type": "object", "properties": { "instance": { "type": "string" } }, "required": [ "instance" ] }, "links": { "type": "array", "items": { "$ref": "#/definitions/link_element" } }, "forms": { "type": "array", "minItems": 1, "items": { "$ref": "#/definitions/form_element_root" } }, "base": { "$ref": "#/definitions/anyUri" }, "securityDefinitions": { "type": "object", "minProperties": 1, "additionalProperties": { "$ref": "#/definitions/securityScheme" } }, "support": { "$ref": "#/definitions/anyUri" }, "created": { "type": "string" }, "modified": { "type": "string" }, "security": { "type": "array", "minItems": 1, "items": { "type": "string" } }, "@type": { "$ref": "#/definitions/type_declaration" }, "@context": { "$ref": "#/definitions/thing-context" } }, "required": [ "title", "id", "security", "securityDefinitions", "@context" ], "additionalProperties": true }
A Thing 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 Template uses the same core vocabulary and information model from section 5.
The Thing Template enables:
The Thing 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 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 Template can be implemented by Things from multiple vendors, a Thing can implement multiple Thing Templates, define additional metadata (vendor, location, security) and define bindings to concrete protocols. To avoid conflicts between properties, actions and events from different Thing Templates that are combined into a common Thing, all these identifiers must be unique within a Thing.
A common Thing 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 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 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 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.
{ "@context": ["https://www.w3.org/2019/wot/td/v1"], "@type" : "ThingTemplate", "title": "Lamp Thing Template", "description" : "Lamp Thing 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"} } } }
{ "@context": ["https://www.w3.org/2019/wot/td/v1"], "@type" : "ThingTemplate", "title": "Buzzer Thing Template", "description" : "Thing template of a buzzer that makes noise for 10 seconds", "actions": { "buzz": { "description" : "buzz for 10 seconds" } } }
To integrate a Thing Description instance with external knowledge and contextual information (like SAREF annotations), the use of JSON-LD 1.1 and its processing API [[json-ld11-api]], as well as RDF-based tools [[rdf11-concepts]] and libraries is highly recommended. This appendix introduces two non-normative elements of the Thing Description model based on RDF: the default JSON-LD context for TD and the TD ontology.
A TD document can be transformed into RDF by a standard JSON-LD processor
(to then be uploaded to an RDF store for further processing).
The following procedure loads the context of the TD, given by the
term @context
, and generates RDF triples:
When applying this procedure to the introductory TD example () with all Default Values, the following triples are obtained:
Most importantly, the JSON-LD context maps JSON strings to RDF IRIs (hence, the notion
of context: in two different contexts, the same string can have different
meanings). The mapping here is rather straightforward: every vocabulary of the TD
information model is defined its own namespace and every vocabulary term maps to some
name, appended to the corresponding namespace. Strings that are not part of any
vocabulary map to RDF literals (strings with an optional datatype). For instance,
title
maps to https://www.w3.org/2019/wot/td#title
and
MyLampThing
remains unchanged. Applying this mapping is called
context expansion.
Then, after expanding JSON strings to full IRIs, all map structures of the
TD document must be turned into RDF triples. In a simple case, a
triple is produced for every key/value pair in the map. For instance, the
very first triple of the example above was produced from "title": "myLampThing"
.
Other cases rely on specific JSON-LD features. Most of these features were introduced in the 1.1 version of the standard and thus require an appropriate JSON-LD 1.1 implementation. They are listed below:
@
but in certain cases, it is
more convenient to define aliases for these keywords, e.g. to more easily process JSON-LD
document in a specific programming environment. In the TD core vocabulary, id
is an alias for @id
, which must be an IRI used as the subject of triples when
producing RDF. For this reason, the first triples in the example have
urn:dev:ops:32473-WoTLamp-1234
as a subject instead of an arbitrary
blank node.
properties
is used both in the core vocabulary and in the data schema
vocabulary. Yet, a JSON string cannot map to two IRIs simultaneously in the same context
definition. The mapping from properties
to
https://www.w3.org/2019/wot/json-schema#properties
must therefore be scoped
to instances of DataSchema
only.
status
, toggle
and overheating
appear as keys in a map but they do not produce triples in which they become predicates. This special
structure is called an index map.
Without any indication in the context, the index key is lost when an index map is turned into RDF.
To make the transformation to RDF reversible, it is possible to indicate that index keys should
produce triples with a given predicate. Each vocabulary can include an RDF term for this purpose.
In the core vocabulary, the property https://www.w3.org/2019/wot/td#name
is used.
security
definitions. JSON-LD 1.1 includes another indexing mechanism for
that purpose. In the above example, securityDefinitions
does not produce any
triple.
The last two JSON-LD features (property-based data indexing and indexing without a predicate) are still experimental; they have not been published in any Editors' Draft yet.
As presented in the previous section, all Vocabulary Terms (including those denoting Classes) follow the RDF convention of belonging to a Vocabulary identified by a namespace IRI. As a result, the TD information model can be reformulated in the Web Ontology Language (OWL) [[owl2-overview]]. Class definitions become OWL axioms defined on the Vocabulary Terms of the TD model. The axioms that are particularly of interest in the present document are those of the TD ontology available under the TD namespace IRI, both as an RDF file and as a human-readable HTML documentation. Default Values cannot be directly translated to OWL, they are therefore ignored in the TD ontology.
The axioms included in the TD ontology may be used to validate a TD but its primary
purpose is to serve as an entry point for a deeper semantic description of WoT Things
that would make clients behave in a more autonomous way, as desired in WoT
[[wot-architecture]]. A semantic description is nothing more than a description of the
physical world, in which WoT systems may have tangible effects. OWL should be the
preferred language for this purpose. OWL is indeed a language to specify ontologies,
that is, conceptualizations of the physical world. To this end, an ontology does not cover
one possible model of the physical world by constructing abstract data structures. Instead,
it provides necessary constraints on all possible models of the world for these models to
be sound. This principle, at the basis of model theory, is what the semantics of RDF
(and OWL) is based on [[rdf11-mt]]. In the following diagram, because the only fact
we know about urn:dev:ops:32473-WoTLamp-1234
is that it is a light
switch with generic affordances, it could be any light switch, be it mechanical,
digital or even virtual.
As a consequence, the TD ontology is meant to be integrated into a larger ontology, as it would not suffice to describe physical world objects alone. An ontology is generally designed for a specific domain of application, like transportation or home automation. For the latter domain, the TD ontology can e.g. be integrated with SAREF, as illustrated in several examples in this document.
An intelligent WoT client can be assigned the task of turning a light off
in ontological terms, using SAREF. For instance, the task could be expressed as
a post-condition on the saref:OnOffState
of some saref:LightSwitch
.
Assuming a server exposes the TD of ,
the client has two possibilities to perform this task: it can either write
the status
property as off
or it can invoke the action
toggle
and see if the resulting status
is indeed
equal to off
.
Yet, in the TD example, the switch state is not directly exposed. Assuming
the client has no prior knowledge of the meaning of status
,
it must infer possible operations from the semantic annotations available in the TD
and its domain-specific ontology, that includes SAREF. The following ontological
axioms can be formulated for SAREF, in the OWL Manchester syntax
[[owl2-manchester-syntax]]:
They respectively state that a light switch must have an on/off state and that
a toggle command must act on some on/off state. They may seem intuitive
to humans but it is only possible for the client to know what to do from such
axioms. If we assume that a saref:GetCommand
always exposes the state of the Thing
, the client can then
automatically infer what form(s) to submit to change the switch state: a
writeproperty
on the saref:GetCommand
or an
invokeaction
on the saref:ToggleCommand
. Note that the
result of the operation cannot be known for certain, as the TD does not expose
its internal state. However, by selecting certain models of the physical world,
either by statistical means or by further axiomatization, clients can still
reach a certain level of autonomy with an ontological approach.
descriptions
and titles
Vocabulary Terms).@language
to declare a default language and specified how to infer text direction in human-readable metadata.InteractionPattern
Class to InteractionAffordance
.@context
and optional @type
Vocabulary Terms to the Thing Class.created
and modified
Vocabulary Terms to the Thing Class to provide information as to when the TD instance was created or last modified, resp.version
vocabulary term to the Thing Class to provide version information.readallproperties
, writeallproperties
, readmultipleproperties
, and writemultipleproperties
.@type
Vocabulary Term to the InteractionAffordance Class to make semantic annotations part of the Information Model.uriVariables
Vocabulary Terms to InteractionAffordance Class to describe URI Template variables.safe
and idempotent
Vocabulary Terms to the ActionAffordance Class to provide information about the safeness and idempotency of the Action.subscription
and cancellation
Vocabulary Terms to the EventAffordance Class to describe data that needs to be passed upon subscription and cancellation, resp.label
to title
in the InteractionAffordance Class to be consistent with JSON Schema.security
and scopes
Vocabulary Terms from the InteractionAffordance Class to simplify mechanism.writable
vocabulary term from the PropertyAffordance Class and made Property affordances both readable and writable by default; this can be overridden using the op
vocabulary term of the Form Class.mediaType
to type
in the Link Class to align with RFC 8288.response
vocabulary term to the Form Class to describe possible response messages.mediaType
to contentType
in the Form Class to reflect that request might need to set also media type parameters.rel
to op
in the Form Class to resolve perceived conflicts with link relations.unobserveproperty
to the enumerated values for the op
vocabulary term.@type
vocabulary term to the DataSchema Class to make semantic annotations part of the Information Model.unit
vocabulary term to the DataSchema Class to provide unit metadata.oneOf
vocabulary term to the DataSchema Class to allow for alternative schemas.format
vocabulary term to the DataSchema Class to allow for pattern validation.@type
vocabulary term to the SecurityScheme Class to make semantic annotations part of the Information Model.securityDefinitions
vocabulary term to the Thing Class to declare security definitions.security
vocabulary term in the Thing and Form Classes to be used to activate declared security definitions.securityDefinitions
and security
configuration is mandatory.Url
postfixes from security scheme Vocabulary Terms.Changes from Second Public Working Draft are described in the Third Public Working Draft
The editors would like to thank Matthias Kovatsch, 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 and Jose M. Cantera for providing contributions, guidance and expertise.
Also, many thanks to the W3C staff and all other 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.