Web of Things (WoT) Architecture

W3C Candidate Recommendation

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Matthias Kovatsch (Huawei)
Ryuichi Matsukura (Fujitsu Ltd.)
Michael Lagally (Oracle Corp.)
Toru Kawaguchi (Panasonic Corp.)
Kunihiko Toumura (Hitachi, Ltd.)
Kazuo Kajimoto (Former Editor, when at Panasonic)
GitHub w3c/wot-architecture
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The W3C Web of Things (WoT) was created to enable interoperability across IoT platforms and application domains.

WoT provides mechanisms to formally describe IoT interfaces to allow IoT devices and services to communicate with each other, independent of their underlying implementation, and across multiple networking protocols. In addition WoT offers a standardized way to define and program IoT behavior.

This WoT Architecture specification describes the abstract architecture for the W3C Web of Things. It is derived from a set of requirements that were derived from use cases for multiple application domains. The architecture can be mapped onto a variety of concrete deployment scenarios, several example patterns of which are given.

This specification is focused on the scope of W3C WoT standardization, which is broken down into so-called building blocks. It introduces the four initial WoT building blocks, which are defined and described in detail in separate specifications, and explains their interworking:

The WoT Thing Description is the central building block, as it allows to describe the metadata and network-facing interfaces of Things.

The informational WoT Binding Templates provide guidelines on how to define so-called Protocol Bindings for the description of these network-facing interfaces and provides examples for a number of existing IoT ecosystems and standards.

The optional WoT Scripting API enables the implementation of the application logic of a Thing using a common JavaScript API similar to the Web browser APIs. This simplifies IoT application development and enables portability across vendors and devices.

The WoT Security and Privacy Considerations represent a cross-cutting building block, which should be applied to any system implementing W3C WoT. It focuses on the secure implementation and configuration of Things.

This specification also covers non-normative architectural aspects and conditions for the deployment of WoT systems. These guidelines are described in the context of deployment scenarios.

Overall, the goal is to preserve and complement existing IoT standards and solutions. In general, W3C WoT is designed to describe what exists rather than to prescribe what to implement.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

Editor's note: The W3C WoT WG is asking for feedback

Please contribute to this draft using the GitHub Issue feature of the WoT Architecture repository. For feedback on security and privacy considerations, please use the WoT Security and Privacy Considerations repository to file issues, as they are cross-cutting over all our specifications.

This document describes an abstract architecture design. However, there is a preliminary implementation report which describes concrete implementations based on the associated WoT Thing Description specification. 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 preliminary implementation report.

This document was published by the Web of Things Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation.

GitHub Issues are preferred for discussion of this specification. Alternatively, you can send comments to our mailing list. Please send them to public-wot-wg@w3.org (archives).

W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 13 June 2019.

Please see the Working Group's Preliminary Implementation Report for the associated WoT Thing Description specification.

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 March 2019 W3C Process Document.

1. Introduction

The goals of the Web of Things (WoT) are to improve the interoperability and usability of the Internet of Things (IoT). Through a collaboration involving many stakeholders over the past years, several building blocks have been identified that address these challenges. The first set of WoT building blocks is now defined:

This specification serves as an umbrella for W3C WoT specifications and defines the basics such as terminology and the underlying abstract architecture of the W3C Web of Things. In particular, the purpose of this specification is to provide:

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Terminology

This section is normative.

This specification uses the following terms as defined here. The WoT prefix is used to avoid ambiguity for terms that are (re)defined specifically for Web of Things concepts.

An Interaction Affordance that allows to invoke a function of the Thing, which manipulates state (e.g., toggling a lamp on or off) or triggers a process on the Thing (e.g., dim a lamp over time).
Binding Templates
A re-usable collection of blueprints for the communication with different IoT platforms. The blueprints provide information to map Interaction Affordances to platform-specific messages through WoT Thing Description as well as implementation notes for the required protocol stacks or dedicated communication drivers.
to consume a Thing
To parse and process a TD document and from it create a Consumed Thing software abstraction as interface for the application in the local runtime environment.
Consumed Thing
A software abstraction that represents a remote Thing used by the local application. The abstraction might be created by a native WoT Runtime, or instantiated as an object through the WoT Scripting API.
An entity that can process WoT Thing Descriptions (including its JSON-based representation format) and interact with Things (i.e., consume Things).
Digital Twin
A digital twin is a virtual representation of a device or a group of devices that resides on a cloud or edge node. It can be used to represent real-world devices which may not be continuously online, or to run simulations of new applications and services, before they get deployed to the real devices.
Domain-specific vocabulary
Linked Data vocabulary that can be used in the WoT Thing Description, but is not defined by W3C WoT.
Edge device
A device that provides an entry point into enterprise or service provider core networks. Examples include gateways, routers, switches, multiplexers, and a variety of other access devices.
An Interaction Affordance that describes an event source, which asynchronously pushes event data to Consumers (e.g., overheating alerts).
to expose a Thing
To create an Exposed Thing software abstraction in the local runtime environment to manage the state of a Thing and interface with the behavior implementation.
Exposed Thing
A software abstraction that represents a locally hosted Thing that can be accessed over the network by remote Consumers. The abstraction might be created by a native WoT Runtime, or instantiated as an object through the WoT Scripting API.
Hypermedia Control
A serialization of a Protocol Binding in hypermedia, that is, either a Web link [RFC8288] for navigation or a Web form for performing other operations. Forms can be seen as request templates provided by the Thing to be completed and sent by the Consumer.
Interaction Affordance
Metadata of a Thing that shows and describes the possible choices to Consumers, thereby suggesting how Consumers may interact with the Thing. There are many types of potential affordances, but W3C WoT defines three types of Interaction Affordances: Properties, Actions, and Events. A fourth Interaction Affordance is navigation, which is already available on the Web through linking.
Interaction Model
An intermediate abstraction that formalizes and narrows the mapping from application intent to concrete protocol operations. In W3C WoT, the defined set of Interaction Affordances constitutes the Interaction Model.
An entity between Consumers and Things that can proxy, augment, or compose Things and republish a WoT Thing Description that points to the WoT Interface on the Intermediary instead of the original Thing. For Consumers, an Intermediary may be indistinguishable from a Thing, following the Layered System constraint of REST.
IoT platform
A specific IoT ecosystem such as OCF, oneM2M, or Mozilla Project Things with its own specifications for application-facing APIs, data model, and protocols or protocol configurations.
Personally Identifiable Information (PII)
Information that can be associated with a unique individual.
The system should maintain the confidentiality of Personally Identifiable Information.
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.
Protocol Binding
The mapping from an Interaction Affordance to concrete messages of a specific protocol, thereby informing Consumers how to activate the Interaction Affordance. W3C WoT serializes Protocol Bindings as hypermedia controls.
The system should preserve its integrity and functionality even when subject to attack.
A software stack that implements the WoT building blocks. A Servient can host and expose Things and/or host Consumers that consume Things. Servients can support multiple Protocol Bindings to enable interaction with different IoT platforms.
An extension mechanism to a transfer protocol that must be known to interact successfully. An example is long polling for HTTP.
Short for WoT Thing Description.
TD Vocabulary
A controlled Linked Data vocabulary by W3C WoT to tag the metadata of Things in the WoT Thing Description including communication metadata of WoT Binding Templates.
Thing or Web 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.
Thing Directory
A directory service for TDs that provides a Web interface to register TDs (similar to [CoRE-RD]) and look them up (e.g., using SPARQL queries or the CoRE RD lookup interface [CoRE-RD]).
Transfer Protocol
The underlying, standardized application layer protocol without application-specific requirements or constraints on options or subprotocol mechanisms. Examples are HTTP, CoAP, or MQTT.
Virtual Thing
An instance of a Thing that represents a Thing that is located on another system component.
WoT Interface
The network-facing interface of a Thing that is described by a WoT Thing Description.
WoT Runtime
A runtime system that maintains an execution environment for applications, and is able to expose and/or consume Things, to process WoT Thing Descriptions, to maintain private security metadata, and to interface with Protocol Binding implementations. A WoT Runtime may have a custom API or use the optional WoT Scripting API.
WoT Scripting API
The application-facing programming interface provided by a Servient in order to ease the implementation of behavior or applications running in a WoT Runtime. It is comparable to the Web browser APIs. The WoT Scripting API is an optional building block for W3C WoT.
WoT Servient
Synonym for Servient.
WoT Thing Description or Thing Description
Structured data describing a Thing. A WoT Thing Description comprises general metadata, domain-specific metadata, Interaction Affordances (which include the supported Protocol Bindings), and links to related Things. The WoT Thing Description format is the central building block of W3C WoT.

4. Use Cases

This section is non-normative.

This section presents the application domains and use cases targeted by the W3C WoT and which are used to derive the abstract architecture discussed in §  7. WoT Building Blocks.

The Web of Things architecture does not put any limitations on use cases and application domains. Various application domains have been considered to collect common patterns that have to be satisfied by the abstract architecture.

The following sections are not exhaustive. Rather they serve as illustrations, where connected things can provide additional benefit or enable new scenarios.

4.1 Application Domains

4.1.1 Consumer

In the consumer space there are multiple assets that benefit from being connected. Lights and air conditioners can be turned off based on room occupancy. Window blinds can be closed automatically based on weather conditions and presence. Energy and other resource consumption can be optimized based on usage patterns and predictions.

The consumer use cases in this section includes Smart Home use case.

Figure 1 shows an example of a Smart Home. In this case, gateways are connected to edge devices such as sensors, cameras and home appliances through corresponding local communication protocols such as KNX, ECHONET, ZigBee, DECT ULE and Wi-SUN. Multiple gateways can exist in one home, while each gateway can support multiple local protocols.

Gateways can be connected to the cloud through the internet, while some appliances can be connected to the cloud directly. Services running in the cloud collect data from edge devices and analyze the data, then provide value to users through the edge devices and other UX devices.

smart home use case
Figure 1 Smart Home

Smart home provides consumer benefits such as remote access and control, voice control and home automation. Smart home also enables device manufacturers to monitor and maintain devices remotely. Smart home can realize value added services such as energy management and security surveillance.

4.1.2 Industrial

The industrial use cases in this section are applicable to different industry verticals.
Due to the nature of overlaps in the application scenarios, different verticals have similar use cases. Example: Smart Factory

Figure 2 shows an example of a Smart Factory. In this case, field-level, cell and line controllers automate different factory equipment based on industrial communication protocols such as PROFINET, Modbus, OPC UA TSN, EtherCAT, or CAN. An industrial edge device collects selected data from various controllers and makes it available to a cloud backend service, e.g., for remote monitoring via a dashboard or analyzes it for preventive maintenance.

smart factory use case
Figure 2 Smart Factory

Smart factories require advanced monitoring of the connected manufacturing equipment as well of the manufactured products. They benefit from predictions of machine failures and early discovery of anomalies to prevent costly downtime and maintenance efforts.

Additionally, monitoring of connected manufacturing equipment and the environment at the production facility for the presence of poisonous gases, excessive noise or heat increases the safety of the workers and reduces the risks of incidents or accidents.

Real-time monitoring and KPI calculations of production equipment helps to detect productivity problems and optimize the supply chain.

4.1.3 Transportation & Logistics

Monitoring of vehicles, fuel costs, maintenance needs and assignments helps to optimize the full utilization of the vehicle fleet.

Shipments can be tracked to be en-route to ensure consistent quality and condition of the transported goods. This is especially useful to assert the integrity of the cold-chain from warehouses to refrigerated trucks to delivery.

Centralized monitoring and management of stock in warehouses and yards can prevent out of stock and excessive stock situations.

4.1.4 Utilities

Automated reading of residential and C&I (Commercial and Industrial) meters, and billing offers continuous insights into resource consumption and potential bottlenecks.

Monitoring the condition and output of distributed renewable energy generation equipment enables optimization of distributed energy resources.

Monitoring and remote-controlling of distribution equipment helps to automate the distribution process.

Continuous monitoring of generation and distribution infrastructure is improving safety of utilities crew in the field.

4.1.5 Oil and Gas

Offshore platform monitoring, leakage detection and prediction of pipelines as well as monitoring and controlling the levels in tanks and reservoirs helps to improve the industrial safety for the workforce as well as for the environment.

Automated calculation of a distributed stock through various storage tanks and delivery pipes/trucks allows for improved planning and resource optimization.

4.1.6 Insurance

Proactive Asset Monitoring of high value assets such as connected structures, fleet vehicles, etc. mitigates the risk of severe damage and high costs due to predictions and early detection of incidents.

Usage based insurance can be offered with usage tracking and customized insurance policies.

Predictive weather monitoring and re-routing fleet vehicles to covered garages can limit loss due to hail damage, tree damage.

4.1.7 Engineering and Construction

Monitoring for industrial safety reduces the risks of security hazards. Monitoring of assets at construction site can prevent damage and loss.

4.1.8 Agriculture

Soil condition monitoring and creating optimal plans for watering, fertilizing as well as monitoring the produce conditions optimize the quality and output of agricultural produce.

4.1.9 Healthcare

Data collection and analytics of clinical trial data helps to gain insights into new areas.

Remote patient monitoring mitigates the risk of undetected critical situations for elderly people and patients after hospitalization.

4.1.10 Environment Monitoring

Environment monitoring typically relies on a lot of distributed sensors that send their measurement data to common gateways, edge devices and cloud services.

Monitoring of air pollution, water pollution and other environmental risk factors such as fine dust, ozone, volatile organic compound, radioactivity, temperature, humidity to detect critical environment conditions can prevent unrecoverable health or environment damages.

4.1.11 Smart Cities

Monitoring of Bridges, Dams, Levees, Canals for material condition, deterioration, vibrations discovers maintenance repair work and prevents significant damage. Monitoring of highways and providing appropriate signage ensures optimized traffic flow.

Smart Parking is optimizing and tracking the usage and availability of parking spaces and automates billing/reservations.

Smart control of street lights based on presence detection, weather predictions, etc. reduces cost.

Garbage containers can be monitored to optimize the waste management and the trash collection route.

4.1.12 Smart Buildings

Monitoring the energy usage throughout the building helps to optimize resource consumption and reduce waste.

Monitoring the equipment in the buildings such as HVAC, Elevators, etc. and fixing problems early improves the satisfaction of occupants.

4.1.13 Connected Car

Monitoring of operation status, prediction of service needs optimizes maintenance needs and costs. Driver safety is enhanced with notifications of an early warning system for critical road and traffic conditions. Connected Car Example

Figure 3 shows an example of a Connected Car. In this case, a gateway connects to car components through CAN and to the car navigation system through a proprietary interface. Services running in the cloud collect data pushed from car components and analyze the data from multiple cars to determine traffic patterns. The gateway can also consume cloud services, in this case, to get traffic data and show it to the driver through the car navigation system.

connected car use case
Figure 3 Connected Car

Monitoring of operation status, prediction of service needs optimizes maintenance needs and costs. Driver safety is enhanced with notifications of an early warning system for critical road and traffic conditions.

4.2 Common Patterns

This section introduces common use case patterns that illustrate how devices/things interact with controllers, other devices, agents and servers. In this section, we use the term client role as an initiator of a transport protocol, and the term server role as a passive component of a transport protocol. This does not imply prescribing a specific role on any system component. A device can be in a client and server role simultaneously.

One example of this dual role is a sensor, that registers itself with a cloud service and regularly sends sensor readings to the cloud. In the response messages the cloud can adjust the transmission rate of the sensor's messages or select specific sensor attributes, that are to be transmitted in future messages. Since the sensor registers itself with the cloud and initiates connections, it is in the 'client' role. However, since it also reacts to requests, that are transmitted in response messages, it also fulfills a 'server' role.

The following sections illustrate the roles, tasks, and use case patterns with increasing complexity. They are not exhaustive and are presented to motivate for the WoT architecture and building blocks that are defined in later sections of this specification.

4.2.1 Device Controllers

The first use case is a local device controlled by a user-operated remote controller as depicted in Figure 4 . A remote controller can access an electronic appliance through the local home network directly. In this case, the remote controller can be implemented by a browser or native application.

In this pattern, at least one device like the electronic appliance has a server role that can accept a request from the other devices and responds to them, and sometimes initiates a mechanical action. The other device like the remote controller has a client role that can send a message with a request, like to read a sensor value or to turn on the device. Moreover, to emit a current state or event notification of a device, the device may have a client role that can send a message to another device, which has server roles.

smart home device use case
Figure 4 Device Control

4.2.2 Thing-to-Thing

Figure 5 shows an example of a direct Thing-to-Thing interaction. The scenario is as follows: a sensor detects a change of the room condition, for example the temperature exceeding a threshold, and issues a control message like "turn on" to the electronic appliance. The sensor unit can issue some trigger messages to other devices.

In this case, when two devices that have server roles are connected, at least one device must have also a client role that issues a message to the other to actuate or notify.

smart home t2t use case
Figure 5 Control Agent

4.2.3 Remote Access

This use case contains a mobile remote controller (e.g., on a smartphone) as shown in Figure 6 . The remote controller can switch between different network connections and protocols, e.g., between a cellular network and a home network, which is using protocols such as Wi-Fi and Bluetooth. When the controller is in the home network it is a trusted device and no additional security or access control is required. When it is outside of the trusted network, additional access control and security mechanisms must be applied to ensure a trusted relationship. Note that in this scenario the network connectivity may change due to switching between different network access points or cellular base stations.

In this pattern, the remote controller and the electronic appliance have a client and a server role as in the related scenario in Figure 4 .

smart home multi use case
Figure 6 Multiple Network Interfaces

4.2.4 Smart Home Gateways

Figure 7 shows a use case using a Smart Home gateway. The smart home gateway is placed between a home network and the Internet. The gateway manages electronic appliances inside the house and can receive commands from a remote controller over the Internet, e.g., from a smartphone as in the previous use case. It is also is a virtual representation of a device. The smart home gateway typically offers proxy and firewall functionality.

In this pattern, the home gateway has both a client and a server role. When the remote controller actuates the electronic appliance, it can connect to the electronic appliance in the client role and to the remote controller with the server role. When the electronic appliance emits a message to the remote controller, the gateway act as server roles for the electric appliance, and it act as client roles for the remote controller.

smart home gateway use case
Figure 7 Smart Home Gateway

4.2.5 Edge Devices

An Edge Device or Edge Gateway is similar to a Smart Home gateway. We use the term to indicate additional tasks that are carried out by the edge gateway. Whereas the home gateway in Figure 8 primarily just bridges between the public and the trusted network, the edge device has local compute capabilities and typically bridges between different protocols. Edge devices are typically used in industrial solutions, where they can provide preprocessing, filtering and aggregation of data provided by connected devices and sensors.

edge device use case
Figure 8 Edge device

4.2.6 Digital Twins

A digital twin is a virtual representation, i.e. a model of a device or a group of devices that resides on a cloud server or edge device. It can be used to represent real-world devices which may not be continuously online, or to run simulations of new applications and services, before they get deployed to the real devices.

digital twin use case
Figure 9 Digital Twin

Digital twins can model a single device, or they can aggregate multiple devices in a virtual representation of the combined devices.

digital twin multiple devices use case
Figure 10 Digital Twin for Multiple Devices

Digital twins can be realized in different ways, depending on whether a device is already connected to the cloud, or whether it is connected to a gateway, which itself is connected to the cloud. Cloud-ready Devices

Figure 11 shows an example where electronic appliances are connected directly to the cloud. The cloud mirrors the appliances and, acting as a digital twin, can receive commands from remote controllers (e.g., a smartphone). Authorized controllers can be located anywhere, as the digital twin is globally reachable.

smart home cloud use case 1
Figure 11 Appliance twin for a Cloud-ready Devices Legacy Devices

Figure 12 shows an example where legacy electronic appliances cannot directly connect to the cloud. Here, a gateway is needed to relay the connection. The gateway works as:

  • integrator of a variety of legacy communication protocols both in the physical and logical view
  • firewall toward the Internet
  • privacy filter which substitutes real image and/or speech, and logs data locally
  • local agent in case the network connection is interrupted
  • emergency services running locally when fire alarms and similar events occur

The cloud mirrors the gateway with all connected appliances and acts as a digital twin that manages them in the cloud in conjunction with the gateway. Furthermore, the cloud can receive commands from remote controllers (e.g., a smartphone), which can be located anywhere.

smart home cloud use case 2
Figure 12 A Digital Twin for a Legacy Device

4.2.7 Multi-Cloud

Typical IoT deployments consist of multiple (thousands) of devices. Without a standardized mechanism, the management of firmware updates for specific clouds require a lot of effort and hinders wider scale IoT adoption.

The primary benefit of a standardized mechanism for describing devices and device types is the capability of deploying devices to different cloud environments without the need of doing customization at device software / firmware level, i.e., installing cloud specific code to a device. This implies that the solution is flexible enough to describe devices in a way that allows on-boarding and using devices in multiple IoT cloud environments.

This drives adoption of Web of Things devices, since it enables easy usage of new devices in an existing deployment, as well as migration of existing devices from one cloud to the other.

4.2.8 Cross-domain Collaboration

Figure 13 show an example of a cross-domain collaboration. In this case, each system involves other systems in other domains, such as Smart Factory with Smart City, Smart City with Smart Home. This type of system is called "Symbiotic" ecosystem, as shown in [IEC-FOTF]. There are two collaboration models: direct collaboration and indirect collaboration. In the direct collaboration model, systems exchange information directly with each other in a peer-to-peer manner. In the indirect collaboration, systems exchange information via some collaboration platform. In order to maintain and continue this collaboration, each system provides the metadata of their capabilities and interfaces and adapts itself to others.

cross domain direct use case cross domain indirect use case
Figure 13 Cross-domain collaboration

4.3 Summary

The previous section described various architecture patterns. In these patterns, some functional entities such as the devices including the legacy devices, controllers, gateways and cloud servers are located at physical locations such as inside building, outside buildings, and data centers. Figure 14 is an overview that shows the combinations and communication paths of these entities.

In a transport protocol layer, each entity arbitrarily selects a suitable role for communications. For example, a device may act as a server when the device provides a service to indefinite number of applications. On the other hand, if a device has limited or intermittent network connectivity, they may act as a client and actively send message to an application when network is available. Regardless of this, in application layer, an application sees that a device provides abstract interfaces to interact and the application can interact with the device using their abstract interfaces.

use case summary
Figure 14 Use Case Overview

5. Requirements

This section is normative.

5.1 Functional Requirements

This section defines the properties required in an abstract Web of Things (WoT) architecture.

5.1.1 Common Principles
  • WoT architecture should enable mutual interworking of different eco-systems using web technology.
  • WoT architecture should be based on the web architecture using RESTful APIs.
  • WoT architecture should allow to use multiple payload formats which are commonly used in the web.
  • WoT architecture must enable different device architectures and must not force a client or server implementation of system components.
  • Flexibility

    There are a wide variety of physical device configurations for WoT implementations. The WoT abstract architecture should be able to be mapped to and cover all of the variations.

  • Compatibility

    There are already many existing IoT solutions and ongoing IoT standardization activities in many business fields. The WoT should provide a bridge between these existing and developing IoT solutions and Web technology based on WoT concepts. The WoT should be upwards compatible with existing IoT solutions and current standards.

  • Scalability

    WoT must be able to scale for IoT solutions that incorporate thousands to millions of devices. These devices may offer the same capabilities even though they are created by different manufacturers.

  • Interoperability

    WoT must provide interoperability across device and cloud manufacturers. It must be possible to take a WoT enabled device and connect it with a cloud service from different manufacturers out of the box.

5.1.2 Thing functionalities
  • WoT architecture should allow things to have functionalities such as
    • reading thing's status information
    • updating thing's status information which might cause actuation
    • subscribing to, receiving and unsubscribing to notifications of changes of the thing's status information.
    • invoking functions with input and output parameters which would cause certain actuation or calculation.
    • subscribing to, receiving and unsubscribing to event notifications that are more general than just reports of state transitions.

5.1.3 Search and discovery
  • WoT architecture should allow clients to know thing's attributes, functionalities and their access points, prior to access to the thing itself.
  • WoT architecture should allow clients to search things by its attributes and functionalities.
  • WoT architecture should allow semantic search of things providing required functionalities based on a unified vocabulary, regardless of naming of the functionalities.

5.1.4 Description mechanism
  • WoT architecture should support a common description mechanism which enables describing things and their functions.
  • Such descriptions should be not only human-readable, but also machine-readable.
  • Such descriptions should allow semantic annotation of its structure and described contents.
  • Such description should be able to be exchanged using multiple formats which are commonly used in the web.

5.1.5 Description of attributes
  • WoT architecture should allow describing thing's attributes such as
    • name
    • explanation
    • version of spec, format and description itself
    • links to other related things and metadata information
  • Such descriptions should support internationalization.

5.1.6 Description of functionalities

5.1.7 Network
  • WoT architecture should support multiple web protocols which are commonly used.
  • Such protocols include
    1. protocols commonly used in the internet and
    2. protocols commonly used in the local area network
  • WoT architecture should allow using multiple web protocols to access to the same functionality.
  • WoT architecture should allow using a combination of multiple protocols to the functionalities of the same thing (e.g., HTTP and WebSocket).

5.1.8 Deployment
  • WoT architecture should support a wide variety of thing capabilities such as edge devices with resource restrictions and virtual things on the cloud, based on the same model.
  • WoT architecture should support multiple levels of thing hierarchy with intermediate entities such as gateways and proxies.
  • WoT architecture should support accessing things in the local network from the outside of the local network (the internet or another local network), considering network address translation.

5.1.9 Application
  • WoT architecture should allow describing applications for a wide variety of things such as edge device, gateway, cloud and UI/UX device, using web standard technology based on the same model.

5.1.10 Legacy adoption
  • WoT architecture should allow mapping of legacy IP and non-IP protocols to web protocols, supporting various topologies, where such legacy protocols are terminated and translated.
  • WoT architecture should allow transparent use of existing IP protocols without translation, which follow RESTful architecture.
  • WoT architecture must not enforce client or server roles on devices and services. An IoT device can be either a client or a server, or both, depending on the system architecture; the same is true of edge and cloud services.

5.2 Technical Requirements

§  4.2 Common Patterns defines the Web of Things abstract architecture by showing various use cases and enumerating patterns for combining architectural components. This section describes technical requirements derived from the abstract architecture.

5.2.1 Components consisting Web of Things and the Web of Things architecture

The use cases help to identify basic components such as devices and applications, that access and control those devices, proxies (i.e., gateways and edge devices) that are located between devices. An additional component useful in some use cases is the directory, which assists with discovery.

Those components are connected to the internet or field networks in offices, factories or other facilities. Note that all components involved may be connected to a single network in some cases, however, in general components can be deployed across multiple networks.

5.2.2 Devices

Access to devices is made using a description of their functions and interfaces. This description is called Thing Description (TD). A Thing Description includes a general metadata about the device, information models representing functions, transport protocol description for operating on information models, and security information.

General metadata contains device identifiers (URI), device information such as serial number, production date, location and other human readable information.

Information models defines device attributes, and represent device’s internal settings, control functionality and notification functionality. Devices that have the same functionality have the same information model regardless of the transport protocols used.

Because many systems based on Web of Things architecture are crossing system Domains, vocabularies and meta data (e.g., ontologies) used in information models should be commonly understood by involved parties. In addition to REST transports, PubSub transports are also supported.

Security information includes descriptions about authentication, authorization and secure communications. Devices are required to put TDs either inside them or at locations external to the devices, and to make TDs accessible so that other components can find and access them.

5.2.3 Applications

Applications need to be able to generate and use network and program interfaces based on metadata (descriptions).

Applications have to be able to obtain these descriptions through the network, therefore, need to be able to conduct search operations and acquire the necessary descriptions over the network.

5.2.4 Digital Twins

Digital Twins need to generate program interfaces internally based on metadata (descriptions), and to represent virtual devices by using those program interfaces. A twin has to produce a description for the virtual device and make it externally available.

Identifiers of virtual devices need to be newly assigned, therefore, are different from the original devices. This makes sure that virtual devices and the original devices are clearly recognized as separate entities. Transport and security mechanisms and settings of the virtual devices can be different from original devices if necessary. Virtual devices are required to have descriptions provided either directly by the twin or to have them available at external locations. In either case it is required to make the descriptions available so that other components can find and use the devices associated with them.

5.2.5 Discovery

For TDs of devices and virtual devices to be accessible from devices, applications and twins, there needs to be a common way to share TDs. Directories can serve this requirement by providing functionalities to allow devices and twins themselves automatically or the users to manually register the descriptions.

Descriptions of the devices and virtual devices need to be searchable by external entities. Directories have to be able to process search operations with search keys such as keywords from the general description in the device description or information models.

5.2.6 Security

Security information related to devices and virtual devices needs to be described in device descriptions. This includes information for authentication/authorization and payload encryption.

WoT architecture should support multiple security mechanism commonly used in the web, such as Basic, Digest, Bearer and OAuth2.0.

5.2.7 Accessibility

The Web of Things primarily targets machine-to-machine communication. The humans involved are usually developers that integrate Things into applications. End-users will be faced with the front-ends of the applications or the physical user interfaces provided by devices themselves. Both are out of scope of the W3C WoT specifications. Given the focus on IoT instead of users, accessibility is not a direct requirement, and hence is not addressed within this specification.

There is, however, an interesting aspect on accessibility: Fulfilling the requirements above enables machines to understand the network-facing API of devices. This can be utilized by accessibility tools to provide user interfaces of different modality, thereby removing barriers to using physical devices and IoT-related applications.

6. WoT Architecture

This section is normative.

To address the use cases in Section 4 and fulfill the requirements in Section 5, the Web of Things (WoT) builds on top of the concept of Web Things – usually simply called Things – that can be used by so-called Consumers. This section provides the background and normative assertions to define the overall W3C Web of Things architecture. As the Web of Things addresses stakeholders from different domains, certain aspects of Web technology are explained in more detail, in particular the concept of hypermedia.

6.1 Overview

A Thing is the abstraction of a physical or virtual entity (e.g., a device or a room) and is described by standardized metadata. In W3C WoT, the description metadata MUST be a WoT Thing Description (TD) [wot-thing-description]. Consumers MUST be able to parse and process the TD representation format, which is based on JSON [RFC8259]. The format can be processed either through classic JSON libraries or a JSON-LD processor, as the underlying information model is graph-based and its serialization compatible with JSON-LD 1.1 [json-ld-syntax]. A TD is instance-specific (i.e., describes an individual Thing, not types of Things) and is the default external, textual (Web) representation of a Thing. There MAY be other representations of a Thing such as an HTML-based user interface, simply an image of the physical entity, or even non-Web representations in closed systems.

To be a Thing, however, at least one TD representation MUST be available. The WoT Thing Description is a standardized, machine-understandable representation format that allows Consumers to discover and interpret the capabilities of a Thing (through semantic annotations) and to adapt to different implementations (e.g., different protocols or data structures) when interacting with a Thing, thereby enabling interoperability across different IoT platforms, i.e., different ecosystems and standards.

consumer thing
Figure 15 Consumer-Thing interaction

A Thing can also be the abstraction of a virtual entity. A virtual entity is the composition of one or more Things (e.g., a room consisting of several sensors and actuators). One option for the composition is to provide a single, consolidated WoT Thing Description that contains the superset of capabilities for the virtual entity. In cases where the composition is rather complex, its TD may link to hierarchical sub-Things within the composition. The main TD acts as entry point and only contain general metadata and potentially overarching capabilities. This allows grouping of certain aspects of more complex Things.

Linking does not only apply to hierarchical Things, but relations between Things and other resources in general. Link relation types express how Things relate, for instance, a switch controlling a light or a room monitored by a motion sensor. Other resources related to a Thing can be manuals, catalogs for spare parts, CAD files, a graphical UI, or any other document on the Web. Overall, Web linking among Things makes the Web of Things navigable, for both humans and machines. This can be further facilitated by providing Thing directories that manage a catalog of available Things, usually by caching their TD representation. In summary, WoT Thing Descriptions MAY link to other Things and other resources on the Web to form a Web of Things.

linked things
Figure 16 Linked Things

Things must be hosted on networked system components with a software stack to realize interaction through a network-facing interface, the WoT Interface of a Thing. One example of this is an HTTP server running on an embedded device with sensors and actuators interfacing the physical entity behind the Thing abstraction. However, W3C WoT does not mandate where Things are hosted; it can be on the IoT device directly, an Edge device such as a gateway, or the cloud.

A typical deployment challenge is a scenario, where local networks are not reachable from the Internet, usually because of IPv4 Network Address Translation (NAT) or firewall devices. To remedy this situation, W3C WoT allows for Intermediaries between Things and Consumers.

Intermediaries can act as proxies for Things, where the Intermediary has a WoT Thing Description similar to the original Thing, but which points to the WoT Interface provided by the Intermediary. Intermediaries may also augment existing Things with additional capabilities or compose a new Thing out of multiple available Things, thereby forming a virtual entity. To Consumers, Intermediaries look like Things, as they possess WoT Thing Descriptions and provide a WoT Interface, and hence might be indistinguishable from Things in a layered system architecture like the Web [REST]. An identifier in the WoT Thing Description MUST allow for the correlation of multiple TDs representing the same original Thing or ultimately unique physical entity.

Figure 17 Intermediary

Another remedy for restricted local networks is binding the WoT Interface to a protocol that establishes the connection from the Thing within the local network to a publicly reachable Consumer.

Things MAY be bundled together with a Consumer to enable Thing-to-Thing interaction. Usually, the Consumer behavior is embedded in the software component, which is also implementing the behavior of the Thing. The configuration of the Consumer behavior MAY be exposed through the Thing.

The concepts of W3C WoT are applicable to all levels relevant for IoT applications: the device level, edge level, and cloud level. This fosters common interfaces and APIs across the different levels and enables various integration patterns such as Thing-to-Thing, Thing-to-Gateway, Thing-to-Cloud, Gateway-to-Cloud, and even cloud federation, i.e., interconnecting cloud computing environments of two or more service providers, for IoT applications. Figure 18 gives an overview how the WoT concepts introduced above can be applied and combined to address the use cases summarized in §  4.3 Summary.

architecture overview
Figure 18 Abstract Architecture of W3C WoT

6.2 Affordances

A central aspect in W3C WoT is the provision of machine-understandable metadata (i.e., WoT Thing Descriptions). Ideally, such metadata is self-descriptive, so that Consumers are able to identify what capabilities a Thing provides and how to use the provided capabilities. A key to this self-descriptiveness lies in the concept of affordances.

The term affordance originates in ecological psychology, but was adopted in the field of Human-Computer Interaction [HCI] based on the definition by Donald Norman: "'Affordance' refers to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used." [NORMAN]

An example for this is a door with a handle. The door handle is an affordance, which suggests that the door can be opened. For humans, a door handle usually also suggests how the door can be opened; an American knob suggests twisting, a European lever handle suggests pressing down.

The hypermedia principle, which is one of the core foundations of the REST architectural style [REST], demands that any piece of information available on the Web be linked to other pieces of information so that the consumer of the information gets explicit knowledge about how to navigate the Web and control Web applications. Here, the simultaneous presentation of information and control (provided in the form of hyperlinks) is a mechanism that affords Web clients the means to drive Web applications. In this context, an affordance is the description of a hyperlink (e.g., via a link relation type and link target attributes) suggesting Web clients how to navigate and possibly how to act on the linked resource. Hence, links provide navigation affordances.

Drawn from this hypermedia principle, the Web of Things defines Interaction Affordances as metadata of a Thing that shows and describes the possible choices to Consumers, thereby suggesting how Consumers may interact with the Thing. A general Interaction Affordance is navigation, which is activated by following a link, thereby enabling Consumers to browse the Web of Things. §  6.4 Interaction Model defines three more types of Interaction Affordances for W3C WoT: Properties, Actions, and Events.

Overall, this W3C WoT definition is aligned with HCI and interaction designers, who create physical Things, as well as the REST and microservice community, who is working on Web services in general.

6.3 Web Thing

A Web Thing has four architectural aspects of interest: its behavior, its Interaction Affordances, its security configuration, and its Protocol Bindings, as depicted in Figure 19 . The behavior aspect of a Thing includes both the autonomous behavior and the handlers for the Interaction Affordances. The Interaction Affordances provide a model of how Consumers can interact with the Thing through abstract operations, but without reference to a specific network protocol or data encoding. The protocol binding adds the additional detail needed to map each Interaction Affordance to concrete messages of a certain protocol. In general, different concrete protocols may be used to support different subsets of Interaction Affordances, even within a single Thing. The security configuration aspect of a Thing represents the mechanisms used to control access to the Interaction Affordances and the management of related public and private metadata.

web thing
Figure 19 Architectural Aspects of a Thing

6.4 Interaction Model

Originally, a Web resource usually represented a document on the World Wide Web that can simply be fetched by a Web client. With the introduction of Web services, resources became more generic interaction entities that can implement any kind of behavior. This very high level of abstraction makes it hard to provide a loose coupling between applications and resources due to the manifold interaction possibilities. As a result, at the time of writing typical API descriptions consist of a static mapping from an application intent to a resource address, method, request payload structure, response payload structure, and expected errors. This imposes a tight coupling between Web client and Web service.

The Interaction Model of W3C WoT introduces an intermediate abstraction that formalizes the mapping from application intent to concrete protocol operations and also narrows the possibilities how Interaction Affordances can be modeled.

In addition to navigation affordances (i.e., Web links), Things MAY offer three other types of Interaction Affordances defined by this specification: Properties, Actions, and Events. While this narrow waist allows to decouple Consumers and Things, these four types of Interaction Affordances are still able to model virtually all interaction possibilities found in IoT devices and services.

6.4.1 Properties

A Property is an Interaction Affordance that exposes the state of the Thing. The state exposed by a Property MUST be retrievable (readable). Optionally, the state exposed by a Property MAY be updated (writeable). Things MAY choose to make Properties observable by pushing the new state after a change (cf. Observing Resources [RFC7641]). Write-only state should be updated through an Action.

If the data is not fully specified by the Protocol Binding used (e.g., through a Media Type), Properties MAY contain one data schema for the exposed state.

Examples of Properties are sensor values (read-only), stateful actuators (read-write), configuration parameters (read-write), Thing status (read-only or read-write), or computation results (read-only).

6.4.2 Actions

An Action is an Interaction Affordance that allows to invoke a function of the Thing. An Action MAY manipulate state that is not directly exposed (cf. Properties), manipulate multiple Properties at a time, or manipulate Properties based on internal logic (e.g., toggle). Invoking an Action MAY also trigger a process on the Thing that manipulates state (including physical state through actuators) over time.

If the data is not fully specified by the Protocol Binding used (e.g., through a Media Type), Actions MAY contain data schemas for optional input parameters and output results.

Examples of Actions are changing multiple Properties simultaneously, changing Properties over time such as fading the brightness of a light (dimming) or with a process that shall not be disclosed such as a proprietary control loop algorithm, or invoking a long-lasting process such as printing a document.

6.4.3 Events

An Event Interaction Affordance describes an event source that pushes data asynchronously from the Thing to the Consumer. Here not state, but state transitions (i.e., events) are communicated. Events MAY be triggered through conditions that are not exposed as Properties.

If the data is not fully specified by the Protocol Binding used (e.g., through a Media Type), Events MAY contain data schemas for the event data and possible subscription control messages (e.g., to subscribe with a Webhook callback URI).

Examples of Events are discrete events such as an alarm or samples of a time series that are pushed regularly.

6.5 Hypermedia Controls

On the Web, an affordance is the simultaneous presentation of information and controls, such that the information becomes the affordance through which the user obtains choices. For humans, the information is usually text or images describing or decorating a hyperlink. The control is a Web link, which includes at least the URI of the target resource, which can be dereferenced by the Web browser (i.e., the link can be followed). But also machines can follow links in a meaningful way, when the Web link is further described by a relation type and a set of target attributes. A hypermedia control is the machine-understandable description of how to activate an affordance. Hypermedia controls usually originate from a Web server and are discovered in-band while a Web client is interacting with the server. This way, Web servers can drive clients through Web applications dynamically, by taking their current state and other factors such as authorization into account. This is opposed to out-of-band interface descriptions that need to be preinstalled or hardcoded into clients (e.g., RPC, WS-* Web services, HTTP services with fixed URI-method-response definitions).

W3C WoT makes use of two kinds of hypermedia controls: Web links [RFC8288], the well-established control to navigate the Web, and Web forms as a more powerful control to enable any kind of operation. Links are already used in other IoT standards and IoT platforms such as CoRE Link Format [RFC6690], OMA LWM2M [LWM2M], and OCF [OCF]. Form is a new concept that besides W3C WoT is also introduced by the Constrained RESTful Application Language (CoRAL) [CoRAL] defined by the IETF.

6.5.2 Forms

Forms enable Consumers (or Web clients in the broader sense) to perform operations that go beyond dereferencing a URI (e.g., to manipulate the state of a Thing). Consumers do so by filling out and submitting the form to its submission target. This usually requires more detailed information about the contents of the (request) message than a link can provide (e.g., method, header fields, or other protocol options). Forms can be seen as a request template, where the provider pre-filled parts of the information according to its own interface and state, and left parts blank to be filled by the Consumers (or Web client in general).

W3C WoT defines forms as new hypermedia control. Note that the definition in CoRAL is virtually identical, and hence compatible [CoRAL]. A form is comprised of:

  • a form context,
  • an operation type,
  • a submission target,
  • a request method, and
  • optionally form fields.

A form can be viewed as a statement of "To perform a operation type operation on form context , issue a request method request to submission target " where the optional form fields may further describe the required request.

Form contexts and submission targets MUST both be Internationalized Resource Identifiers (IRIs) [RFC3987]. However, in the common case, they will also be URIs [RFC3986], because many protocols (such as HTTP) do not support IRIs.

Form context and submission target MAY point to the same resource or different resources, where the submission target resource implements the operation for the context.

The operation type identifies the semantics of the operation. Operation types are denoted similar to link relation types:

  • Well-known operation types MUST follow the ABNF LOALPHA *( LOALPHA / DIGIT / "." / "-" ). Well-known operation types MUST be compared using a case-insensitive comparison. The well-known operation types for the Web of Things defined by this specification are given in Table 1.
  • The set of predefined operation types MAY be augmented by Extension operation types chosen by applications. Extension operation types MUST be URIs [RFC3986] that uniquely identify the type. Extension operation types MUST be compared as strings using a case-insensitive comparison. Nevertheless, all-lowercase URIs SHOULD be used for extension operation types.

The request method MUST identify one method of the standard set of the protocol identified by the submission target URI scheme.

Form fields are optional and MAY further specify the expected request message for the given operation. Note that this is not limited to the payload, but may affect also protocol headers. Form fields MAY depend on the protocol used for the submission target as specified in the URI scheme. Examples are HTTP header fields, CoAP options, the protocol-independent Media Type including parameters (i.e., full content type) for the request payload, or information about the expected response.

Table 1 Well-known Operation Types for the Web of Things
Operation Type Description
readproperty Identifies the read operation on Property Affordances to retrieve the corresponding data.
writeproperty Identifies the write operation on Property Affordances to update the corresponding data.
observeproperty Identifies the observe operation on Property Affordances to be notified with the new data when the Property was updated.
unobserveproperty Identifies the unobserve operation on Property Affordances to stop the corresponding notifications.
invokeaction Identifies the invoke operation on Action Affordances to perform the corresponding action.
subscribeevent Identifies the subscribe operation on Event Affordances to be notified by the Thing when the event occurs.
unsubscribeevent Identifies the unsubscribe operation on Event Affordances to stop the corresponding notifications.
readallproperties Identifies the readallproperties operation on Things to retrieve the data of all Properties in a single interaction.
writeallproperties Identifies the writeallproperties operation on Things to update the data of all writable Properties in a single interaction.
readmultipleproperties Identifies the readmultipleproperties operation on Things to retrieve the data of selected Properties in a single interaction.
writemultipleproperties Identifies the writemultipleproperties operation on Things to update the data of selected writable Properties in a single interaction.
Editor's note

As of this specification, the well-known operation types are a fixed set that results from the WoT Interaction Model. Other specifications may define further well-known operation types that are valid for their respective document format or form serialization. Later versions of this specification or another specification may set up an IANA registry in the future to enable extension and a more generic Web form model that may be applied beyond WoT specifications.

6.6 Protocol Bindings

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]. It informs the Consumer how to activate the Interaction Affordance through a network-facing interface. The Protocol Bindings follow the Uniform Interface constraint of REST [REST] to support interoperability. Thus, not all communication protocols are eligible to implement Protocol Bindings for W3C WoT; the requirements are given in the assertions below.

In the door example given in §  6.2 Affordances, the Protocol Binding corresponds to the door handle at the level of knob vs lever, which suggests how the door can be opened.

6.6.1 Hypermedia-driven

Interaction Affordances MUST include one or more Protocol Bindings. Protocol Bindings MUST be serialized as hypermedia controls (see §  6.5 Hypermedia Controls) to be self-descriptive on how to activate the Interaction Affordance. The hypermedia controls MUST originate from the authority managing the Thing that is providing the corresponding Interaction Affordance. The authority can be the Thing itself, producing the TD document at runtime (based on its current state and including network parameters such as its IP address) or serving it from memory with only the current network parameters inserted. The authority can also be an external entity that has full and up-to-date knowledge of the Thing including its network parameters and internal structure (e.g., software stack). This enables a loose coupling between Things and Consumers, allowing for an independent lifecycle and evolution. The hypermedia controls MAY be cached outside the Thing and used for offline processing if caching metadata is available to determine the freshness.

6.6.2 URIs

Eligible protocols for W3C WoT MUST have an associated URI scheme that is registered with IANA [RFC4395]. Hypermedia controls rely on URIs to identify link and submission targets. Thereby, the URI scheme (the first component up to ":") identifies the communication protocol to be used for Interaction Affordances with the Thing. W3C WoT refers to these protocols as transfer protocols.

6.6.3 Standard Set of Methods

Eligible protocols for W3C WoT MUST be based on a standard set of methods that are known a priori. The standard set of methods makes messages self-descriptive to enable intermediate processing of Interaction Affordances, for instance by proxies or to translate between Protocol Bindings [REST]. Furthermore, it allows Consumers to have re-usable protocol stacks of common transfer protocols such as HTTP, CoAP, or MQTT, avoiding Thing-specific code or plugins for Consumers.

6.6.4 Media Types

All data (a.k.a. content) exchanged when activating Interaction Affordances MUST be identified by a Media Type [RFC6838] in the Protocol Binding. Media Types are labels to identify representation formats, for instance application/json for JSON [RFC8259] or application/cbor for CBOR [RFC7049]. They are managed by IANA.

Some Media Types might need additional parameters to fully specify the representation format used. Examples are text/plain; charset=utf-8 or application/ld+json; profile="http://www.w3.org/ns/json-ld#compacted". This needs to be taken into account in particular when describing data to be sent to Things. There might also be standardized transformations on the data such as content coding [RFC7231]. Protocol Bindings MAY have additional information that specifies representation formats in more detail than the Media Type alone.

Note that many Media Types only identify a generic serialization format that does not provide further semantics for its elements (e.g., XML, JSON, CBOR). Thus, the corresponding Interaction Affordances SHOULD declare a data schema to provide more detailed syntactic metadata for the data exchanged.

6.7 WoT System Components and their Interconnectivity

Section §  6.1 Overview described the WoT architecture in terms of the abstract WoT architecture components such as Things, Consumers and Intermediaries. When those abstract WoT architecture components are implemented as a software stack to take a specific role in the WoT architecture, such software stacks are called Servients. Systems that are based on the WoT architecture involve Servients, which are communicating with each other to achieve the goals of a system.

This section uses system configuration diagrams to illustrate how Servients work together to build systems based on the WoT architecture.

A Thing can be implemented by a Servient. In a Thing, a Servient software stack contains a representation of a Thing called Exposed Thing, and makes its WoT Interface available to Consumers of the Thing. This Exposed Thing may be used by other software components on the Servient (e.g., applications) to implement the behavior of the thing.

servient as a thing
Figure 20 Servient as a Thing

On the other hand, Consumers are always implemented by Servients, as they must be able to process the Thing Description (TD) format and must have a protocol stack that can be configured through Protocol Binding information contained in the TDs.

In a Consumer, a Servient software stack provides a representation of a Thing called Consumed Thing, and makes it available to those applications running on the Servient that need to process TDs to interact with Things.

servient as a consumer
Figure 21 Servient as a Consumer

A Consumed Thing instance in the Servient software stack serves to separate the protocol level complexity from applications. It is communicating with Exposed Things on behalf of the application.

Similarly, an Intermediary is yet another WoT architecture component implemented by a Servient. An Intermediary is located between a Thing and its Consumers, performing the roles of both a Consumer (to the Thing) and a Thing (to the Consumers). In an Intermediary, a Servient software stack contains the representations of both a Consumer (Consumed Thing) and a Thing (Exposed Thing).

servient as an intermediary
Figure 22 Servient as an Intermediary

6.7.1 Direct Communication

Figure 23 shows direct communication between a Thing, which is exposing Interaction Affordances through Thing Descriptions, and a Consumer that uses the Thing by means of the Interaction Affordances. Direct communication applies when both Servients use the same network protocol(s) and are accessible to each other.

hight-level architecture of consumer and thing
Figure 23 High-level architecture of Consumer and Thing

An Exposed Thing is the software representation of a Thing abstraction, serving a WoT Interface of the Interaction Affordances provided by the Thing.

A Consumed Thing is the software representation of a remote Thing being consumed by a Consumer, serving as the interface to the remote Thing for the applications. A Consumer can generate a Consumed Thing instance by parsing and processing a TD document. Interactions between a Consumer and a Thing are performed by the Consumed Thing and the Exposed Thing exchanging messages over a direct network connection between them.

6.7.2 Indirect Communication

In Figure 24 , a Consumer and a Thing connect to each other via an Intermediary. An Intermediary is required if the Servients use different protocols or if they are on different networks that require authentication and provide access control (e.g. firewalls).

high-level architecture with intermediary
Figure 24 High-level architecture with Intermediary

An Intermediary combines Exposed Thing and Consumed Thing functionality. The functionality of Intermediaries includes relaying messages for the Interaction Affordances between a Consumer and a Thing, optionally caching the Thing's data for faster response, and transforming communication when the functionality of the Thing is extended by the Intermediary. In an Intermediary, a Consumed Thing creates a proxy object of the Exposed Thing of a Thing, and a Consumer can access the proxy object (i.e., the Exposed Thing of the Intermediary) through its own Consumed Thing.

Consumer and Intermediary can communicate in a different protocol than Intermediary and Thing For example, an Intermediary can provide a bridge between a Thing that uses CoAP and the application of a Consumer that uses HTTP.

Even when there are multiple different protocols used between Intermediary and Things, Consumer can indirectly communicate with those Things using a single protocol through the Intermediary. The same is true for the authentication. The Consumed Thing of a Consumer only needs to authenticate with the Exposed Things of the Intermediary using a single security mechanism, while the Intermediary might need multiple security mechanism to authenticate with different Things.

Usually, an Intermediary generates the Thing Description for its proxy object based on the Thing Description of the originating Thing. Depending on the requirements of the use cases, the TD for the proxy object may either use the same identifier as the TD of the original Thing, or it gets assigned a new identifier. The If necessary, a TD generated by an Intermediary MAY contain interfaces for other communication protocols.

7. WoT Building Blocks

This section is normative.

The Web of Things (WoT) building blocks allow the implementation of systems that conform with the abstract WoT Architecture. The specifics of these building blocks are defined in separate specification; this section provides an overview and a summary.

The WoT building blocks support each of the architectural aspects of a Thing discussed in §  6.3 Web Thing and depicted in Figure 19 . The individual building blocks are shown in the context of an abstract Thing in Figure 25 . This is an abstract view and does not represent any particular implementation; instead it illustrates the relationship between the building blocks and the main architectural aspects of a Thing. In this figure the WoT building blocks are highlighted with black outlines. Security, a cross-cutting concern, is separated into public and protected private components. The WoT Scripting API is optional and the Binding Templates are informative.

wot building blocks
Figure 25 Relationship of WoT Building Blocks to the Architectural Aspects of a Thing.

In the following sections we will provide additional information on each WoT building block: the WoT Thing Description, the WoT Binding Templates, and the WoT Scripting API. Security, although it is a cross-cutting concern, can be considered a fourth building block.

7.1 WoT Thing Description

The WoT Thing Description (TD) specification [wot-thing-description] defines an information model based on a semantic vocabulary and a serialized representation based on JSON. TDs provide rich metadata for Things in a way that is both human-readable and machine-understandable. Both the information model and the representation format of TDs are aligned with Linked Data [LINKED-DATA], so that besides raw JSON processing, implementations may choose to make use of JSON-LD [JSON-LD11] and graph databases to enable powerful semantic processing of the metadata.

A Thing Description (TD) describes Thing instances with general metadata such as name, ID, descriptions, and also can provide relation metadata through links to related Things or other documents. TDs also contain Interaction Affordance metadata based on the interaction model defined in §  6.4 Interaction Model; public security configuration metadata; and communications metadata defining protocol bindings. The TD can be seen as the index.html for Things, as it provides the entry point to learn about the provided services and related resources, both of which are described using hypermedia controls.

Ideally, the TD is created and/or hosted by the Thing itself and retrieved upon discovery. Yet it can also be hosted externally when a Thing has resource restrictions (e.g., limited memory space, limited power) or when an existing device is retrofitted to become part of the Web of Things. A common pattern to improve discovery (e.g., for constrained devices) and to facilitate device management is to register TDs with a directory. It is recommended that Consumers use a TD caching mechanism combined with a notification mechanism, which will inform them when it is required to fetch a new version of the TD, in case the Thing is updated.

For semantic interoperability, TDs may make use of a domain-specific vocabulary, for which explicit extension points are provided. However, development of any particular domain-specific vocabulary is currently out-of-scope of the W3C WoT standardization activity.

Three examples of potentially useful external IoT vocabularies are SAREF [SAREF], iot.schema.org [iot-schema-org], and the W3C Semantic Sensor Network ontology [vocab-ssn]. Use of such external vocabularies in TDs is optional. In the future additional domain-specific vocabularies may be developed and used with TDs.

Overall, the WoT Thing Description building block fosters interoperability in two ways: First, TDs enable machine-to-machine communication in the Web of Things. Second, TDs can serve as a common, uniform format for developers to document and retrieve all the details necessary to create applications that can access IoT devices and make use of their data.

7.2 WoT Binding Templates

This section is non-normative.

The IoT uses a variety of protocols for accessing devices, since no single protocol is appropriate in all contexts. Thus, a central challenge for the Web of Things is to enable interactions with the plethora of different IoT platforms (e.g., OCF, oneM2M, OMA LWM2M, OPC UA) and devices that do not follow any particular standard, but provide an eligible interface over a suitable network protocol. WoT is tackling this variety through Protocol Bindings, which must meet a number of constraints (see §  6.6 Protocol Bindings).

The non-normative WoT Binding Templates specification [wot-binding-templates] provides a collection of communication metadata blueprints that give guidance on how to interact with different IoT platforms. When describing a particular IoT device or service, the Binding Template for the corresponding IoT Platform can be used to look up the communication metadata that must be provided in the Thing Description to support that platform.

binding templates
Figure 26 From Binding Templates to Protocol Bindings

Figure 26 shows how Binding Templates are applied. A WoT Binding Template is created only once for each IoT Platform and can then be reused in all TDs for devices of that platform. The Consumer that is processing a TD must implement the required Protocol Binding by including a corresponding protocol stack and by configuring the stack (or its messages) according to the information given in the TD.

The communication metadata of Protocol Bindings spans five dimensions:

7.3 WoT Scripting API

This section is non-normative.

The WoT Scripting API is an optional "convenience" building block of W3C WoT that eases IoT application development by providing an ECMAScript-based API [ECMAScript] similar to the Web browser APIs. By integrating a scripting runtime system into the WoT Runtime, the WoT Scripting API enables using portable application scripts that define the behavior of Things, Consumers, and Intermediaries.

Traditionally, IoT device logic is implemented in firmware, which results in productivity constraints similar to that of embedded development, including a relatively complex update process. The WoT Scripting API in contrast supports enables implementing device logic by reusable scripts executed in a runtime system for IoT applications not dissimilar to that of a Web browser, and aims to improve productivity and reduce integration costs. Furthermore, standardized APIs enable portability for application modules, for instance, to move compute-intense logic from a device up to a local gateway, or to move time-critical logic from the cloud down to a gateway or edge node.

The non-normative WoT Scripting API specification [wot-scripting-api] defines the structure and algorithms of the programming interface that allows scripts to discover, fetch, consume, produce, and expose WoT Thing Descriptions. The runtime system of the WoT Scripting API instantiates local objects that act as an interface to other Things and their Interaction Affordances (Properties, Actions, and Events). It also allows scripts to expose Things, that is, to define and implement Interaction Affordances and publish a Thing Description.

7.4 WoT Security and Privacy Guidelines

This section is non-normative.

Security is a cross-cutting concern and should be considered in all aspects of system design. In the WoT architecture, security is supported by certain explicit features, such as support for public security metadata in TDs and by separation of concerns in the design of the WoT Scripting API. The specification for each building block also includes a discussion of particular security and privacy considerations of that building block. Another non-normative specification, the WoT Security and Privacy Considerations [wot-security], provides additional cross-cutting security and privacy guidance.

8. Servient Implementation

This section is non-normative.

As defined in §  6.7 WoT System Components and their Interconnectivity, a Servient is a software stack that implements the WoT building blocks presented in the previous section. Servients can host and expose Things and/or consume Things (i.e., host Consumers). Depending on the Protocol Binding, Servients can perform in both server and client role, hence the portmanteau naming.

The previous section describes how the WoT building blocks conceptually relate to each other and how they correspond to the abstract WoT Architecture (see §  6. WoT Architecture). When implementing these concepts, a more detailed view is necessary that takes certain technical aspects into account. This section describes the detailed architecture of a Servient implementation.

Figure 27 shows a Servient implementation that is using the (optional) WoT Scripting API building block. Here, the WoT Runtime is also a Scripting Runtime system that, in addition to managing the WoT-specific aspects, also interprets and executes the application scripts. Servients that support the WoT Scripting API usually run on powerful devices, edge nodes, or in the cloud. The WoT Architecture does not limit the application-facing API of the WoT Runtime to JavaScript/ECMAScript. Also other runtime systems can be used to implement a Servient.

Section §  8.8.1 Native WoT API presents an alternative Servient implementation without the WoT Scripting API building block. The WoT Runtime may use any programming language for its application-facing API. Usually, it is the native language of the Servient software stack, for instance C/C++ for embedded Servients or Java for cloud-based Servients. It may also be an alternative scripting language such as Lua to combine the benefits of application scripts with low resource consumption.

architecture implementation
Figure 27 Implementation of a Servient using the WoT Scripting API

The role and functionality of each module shown in Figure 27 is explained in the following sections.

8.1 Behavior Implementation

The behavior defines the overall application logic of a Thing, which has several aspects:

It includes autonomous behavior of Things (e.g., sampling of sensors or control loops for actuators), the handlers for Interaction Affordances (i.e., the concrete actions taken when an affordance is activated), Consumer behavior (e.g., controlling a Thing or realizing mashups), and Intermediary behavior (e.g., simply proxying a Thing or composing virtual entities). The behavior implementation within a Servient defines which Things, Consumers, and Intermediaries are hosted on this component.

Figure 27 depicts Servients that are implementing the optional WoT Scripting API building block, where portable application scripts written in JavaScript [ECMAScript] define the behavior. They are executed by a scripting runtime system that is part of the WoT Runtime (when providing the WoT Scripting API or any other script-based API). They are portable, as they are written against the common WoT Scripting API definitions, and hence can be executed by any Servient featuring this building block. This makes it possible to shift application logic between system components, for instance moving a Consumer from the cloud to an edge node to meet networking requirements, or to move an Intermediary to the cloud to fulfill growing resource demands. Portable applications enable to 'install' additional behavior after the deployment of a Servient.

In principle, any programming language and API can be used in order to define the behavior of a Thing, as long as the Interaction Affordances are presented externally through a WoT Interface. The adaption between application-facing API and the protocol stack is handled by the WoT Runtime. See §  8.8.1 Native WoT API for behavior implementation without the WoT Scripting API building block.

8.2 WoT Runtime

Technically, the Thing abstraction and its Interaction Model is implemented in a runtime system. This WoT Runtime maintains the execution environment for the behavior implementation and is able to expose and/or consume Things, and hence must be able to fetch, process, serialize, and serve WoT Thing Descriptions.

Every WoT Runtime has an application-facing interface (i.e., an API) for the behavior implementation. The optional WoT Scripting API building block shown in Figure 27 defines such an application-facing interface that follows the Thing abstraction and enables the deployment of behavior implementations during runtime through application scripts. See §  8.8.1 Native WoT API for alternative APIs, which can also only be available during compile time. In general, application logic should be executed in isolated execution environments to prevent unauthorized access to the management aspects of the WoT Runtime, in particular the private security configuration. In multi-tenant Servients, additional execution environment isolation is required for the different tenants.

A WoT Runtime needs to provide certain operations to manage the lifecycle of Things, or more precisely their software abstractions and descriptions. A lifecycle management (LCM) system may encapsulate those lifecycle operations within a Servient and use internal interfaces to realize the lifecycle management. The details of such operations vary among different implementations. The WoT Scripting API includes LCM functionality, and hence represents one possible implementation of such a system.

The WoT Runtime must interface with the protocol stack implementation of the Servient, as it decouples the behavior implementation from the details of the Protocol Bindings. The WoT Runtime usually also interfaces with the underlying system, for instance, to access local hardware such as attached sensors and actuators or to access system services such as storage. Both interfaces are implementation-specific, yet the WoT Runtime must provide the necessary adaption to the implemented Thing abstraction.

8.3 WoT Scripting API

The WoT Scripting API building block defines an ECMAScript API that closely follows the WoT Thing Description specification [wot-thing-description]. It defines the interface between behavior implementations and a scripting-based WoT Runtime. Other, simpler APIs may be implemented on top of it, similar to, for instance, jQuery for the Web browser APIs.

See [wot-scripting-api] for more details.

8.4 Exposed Thing and Consumed Thing Abstractions

The WoT Runtime instantiates software representations of Things based on their TDs. These software representations provide the interface towards the behavior implementation.

The Exposed Thing abstraction represents a Thing hosted locally and accessible from the outside through the protocol stack implementation of the Servient. The behavior implementation can fully control Exposed Things by defining their metadata and Interaction Affordances, and providing their autonomous behavior.

The Consumed Thing abstraction represents a remotely hosted Thing for Consumers that needs to be accessed using a communication protocol. Consumed Things are proxy objects or stubs. The behavior implementation is restricted to reading their metadata and activating their Interaction Affordances as described in the corresponding TD. Consumed Things can also represent system features such as local hardware or devices behind proprietary or legacy communication protocols. In this case, the WoT Runtime must provide the necessary adaptation between system API and Consumed Thing. Furthermore, it must provide corresponding TDs and make them available to the behavior implementation, for instance, by extending whatever discovery mechanism is provided by the WoT Runtime through the application-facing API (e.g., the discover() method defined in the WoT Scripting API [wot-scripting-api]).

When using the WoT Scripting API, Exposed Thing and Consumed Thing are JavaScript objects, which can be created, operated on, and destroyed by application scripts. However, access may be restricted through a security configuration, for instance, in multi-tenant Servients.

8.5 Private Security Configuration

Private security metadata is also conceptually managed by the WoT Runtime but is intentionally not made directly accessible to the application. In fact, in the most secure hardware implementations, such private security data is stored in a separate, isolated memory (e.g., on a secure element or TPM) and only an abstract set of operations (possibly even implemented by an isolated processor and software stack) is provided that limit the attack surface and prevent external disclosure of this data. Private security data is used transparently by the Protocol Binding to authorize and protect the integrity and confidentiality of interactions.

8.6 Protocol Stack Implementation

The protocol stack of a Servient implements the WoT Interface of the Exposed Things and is used by Consumers to access the WoT Interface of remote Things (via Consumed Things). It produces the concrete protocol messages to interact over the network. Servients may implement multiple protocols, and hence support multiple Protocol Bindings to enable interaction with different IoT Platforms.

In many cases, where standard protocols are used, generic protocol stacks can be used to produce the platform-specific messages (e.g., one for HTTP(S) dialects, one for CoAP(S) dialects, and one for MQTT solutions, etc.). In this case, the communication metadata from the Thing Description is used to select and configure the right stack (e.g., HTTP with the right header fields or CoAP with the right options). Parsers and serializers for the expected payload representation format (JSON, CBOR, XML, etc.) as identified by the Internet Media Type can also be shared across these generic protocol stacks.

See [wot-binding-templates] for details.

8.7 System API

An implementation of a WoT Runtime may provide local hardware or system services to behavior implementations through the Thing abstraction, as if they were accessible over a communication protocol. In this case, the WoT Runtime should enable the behavior implementation to instantiate Consumed Things that internally interface with the system instead of the protocol stack. This can be done by listing such system Things, which are only available in the local WoT Runtime, in the results of the discovery mechanism provided by the application-facing WoT Runtime API.

A device may also be physically external to a Servient, but connected via a proprietary protocol or a protocol not eligible as WoT Interface (see §  6.6 Protocol Bindings). In this case, the WoT Runtime may access legacy devices with such protocols (e.g., ECHONET Lite, BACnet, X10, I2C, SPI, etc.) through proprietary APIs, but may again choose to expose them to the behavior implementation via a Thing abstraction. A Servient can then act as gateway to the legacy devices. This should only be done if the legacy device cannot be described directly using a WoT Thing Description.

The behavior implementation may also access local hardware or system services (e.g., storage) through a proprietary API or other means. This is, however, out of scope of the W3C WoT standardization, as it hinders portability.

8.8 Alternative Servient and WoT Implementations

The WoT Scripting API building block is optional. Alternative Servient implementations are possible, where the WoT Runtime offers an alternative API for the application logic, which may be written in any programming language.

Furthermore, devices or services unaware of W3C WoT can still be consumed, when it is possible to provide a well-formed WoT Thing Description for them. In this case, the TD describes a WoT Interface of a Thing that has a black-box implementation.

8.8.1 Native WoT API

There are various reasons why a developer may choose to implement a Servient without using the WoT Scripting API. This may be due to insufficient memory or computing resources, so the developer cannot use the required software stack or a fully-featured scripting engine. Alternatively, to support their use case (for example, a proprietary communications protocol) the developer may have to use specific functions or libraries only available through a particular programming environment or language.

In this case, a WoT Runtime can still be used, but with an equivalent abstraction and functionality exposed using an alternative application-facing interface instead of the WoT Scripting API Except for the latter, all block descriptions in §  8. Servient Implementation are also valid for Figure 28 .

implementation native
Figure 28 Implementation of a Servient Using a Native WoT API

8.8.2 Existing Device Described with a Thing Description

It is also possible to integrate existing IoT devices or services into the W3C Web of Things and to use them as Things by creating a Thing Description for these devices or services. Such a TD can either be created manually or via a tool or service. For example, a TD could be generated by a service that provides automatic translation of metadata provided by another, ecosystem-dependent machine-readable format. This can only be done, however, if the target device is using protocols that can be described using a Protocol Binding. The requirements for this are given in §  6.6 Protocol Bindings Much of the previous discussion also implies that a Thing provides its own Thing Description. While this is a useful pattern it is not mandatory. In particular, it may not be possible to modify existing devices to provide their own Thing Description directly. In this case the Thing Description will have to be provided separately using a service such as a directory or some other external and separate distribution mechanism.

implementation existing
Figure 29 Integration of Existing IoT Devices into W3C WoT

9. WoT Deployments

This section is non-normative.

This section discusses how the Web of Things works as a whole, when devices and services that implement Things and Consumers are connected together in various topologies and deployment scenarios.

Before discussing specific topologies, we will first review the roles that Things and Consumers can play in a WoT network and the relationships they have with the Exposed Thing and Consumed Thing software abstractions. Exposed Thing and Consumed Thing are internally available to the behavior implementations of Servients in the roles of Things and Consumers, respectively.

9.1 Thing and Consumer Roles

A Servient in the role of a Thing creates an Exposed Thing based on a Thing Description (TD). TDs are published and made available to other Servients that are in the roles of Consumers or Intermediaries. TDs may be published in various different ways: the TD might be registered with a management system such as the Thing Directory service, or a Thing may provide the requesters with a TD upon receiving a request for a TD. It is even possible to statically associate a TD with Thing in certain application scenarios.

A Servient in the role of a Consumer obtains the TD of a Thing using a discovery mechanism and creates a Consumed Thing based on the obtained TD. The concrete discovery mechanism depends on the individual deployment scenario: It could be provided by a management system such as a Thing Directory, a discovery protocol, through static assignment, etc. Internal system functions of a device, such as interacting with attached sensors and actuators, can also optionally be represented as Consumed Thing abstractions.

The functions supported by the Consumed Thing are provided to the Consumer's behavior implementation through a programming language interface. In the WoT Scripting API, Consumed Things are represented by objects. The behavior implementation (that is, the application logic) running in a Thing can engage through Interaction Affordances with Consumers by using the programming language interface provided by the Exposed Thing.

A Thing does not necessarily represent a physical device. Things can also represent a collection of devices, or virtual services running in a gateway or in the cloud. Likewise, a Consumer may represent an application or service running on a gateway or cloud. Consumers can also be implemented on edge devices. In Intermediaries, a single Servient performs both the roles of a Thing and a Consumer simultaneously which are sharing a single WoT Runtime.

9.2 Topology of WoT Systems and Deployment Scenarios

Various topologies and deployment scenarios of WoT systems are discussed in this section. These are only example patterns and other interconnection topologies are also possible. The topologies described here are derived from the Web of Things use cases (§  4. Use Cases) as well as the technical requirements extracted from them (§  5. Requirements).

9.2.1 Consumer and Thing on the same Network

In the simplest interconnection topology, illustrated by Figure 30 , the Consumer and Thing are on the same network and can communicate directly with each other without any intermediaries. One use case where this topology arises is when the Consumer is an orchestration service or some other IoT application running on a gateway and the Thing is a device interfacing to a sensor or an actuator. However, the client/server relationship could easily be reversed; the client could be a device in the Consumer role accessing a service running as a Thing on a gateway or in the cloud.

consumer and thing on the same network
Figure 30 Consumer and Thing on the Same Network

If the Thing is in the cloud and the Consumer is on a local network (see Figure 1 for an example in a Smart Home use case) the actual network topology may be more complex, for example requiring NAT traversal and disallowing certain forms of discovery. In such cases one of the more complex topologies discussed later may be more appropriate.

9.2.2 Consumer and Thing connected via Intermediaries

An Intermediary plays both Thing and Consumer roles on the network and supports both the Exposed Thing and Consumed Thing software abstractions within its WoT Runtime. Intermediaries can be used for proxying between devices and networks or for Digital Twins. Intermediary acting as a Proxy

One simple application of an Intermediary is a proxy for a Thing. When the Intermediary acts as a proxy, it has interfaces with two separate networks or protocols. This may involve the implementation of additional security mechanisms such as providing TLS endpoints. Generally proxies do not modify the set of interactions, so the TD exposed by the Intermediary will have the same interactions as the consumed TD, however the connection metadata is modified.

To implement this proxy pattern, the Intermediary obtains a TD of a Thing and creates a Consumed Thing. It creates a proxy object of the Thing as a software implementation that has the same Interaction Affordances. It then creates a TD for the proxy object with a new identifier and possibly with new communications metadata (protocol bindings) and/or new public security configuration metadata. Finally, an Exposed Thing is created based on this TD, and the Intermediary notifies other Consumers or Intermediaries of the TD via an appropriate publication mechanism.

consumer and thing connect via a intermediary acting as a proxy
Figure 31 Consumer and Thing Connect via a Intermediary Acting as a Proxy Intermediary acting as a Digital Twin

More complex Intermediaries may be known as Digital Twins. A Digital Twin may or may not modify the protocols or translate between networks, but they provide additional services, such as state caching, deferred updates, or even predictive simulation of the behavior of the target device. For example, if an IoT device has limited power, it may choose to wake up relatively infrequently, synchronize with a digital twin, and then immediately go to sleep again. In this case, typically the Digital Twins runs on a less power-constrained device (such as in the cloud or on a gateway) and is able to respond to interactions on the constrained device's behalf. Requests for the current state of properties may also be satisfied by the Digital Twins using cached state. Requests that arrive when the target IoT device is sleeping may be queued and sent to it when it wakes up. To implement this pattern, the Intermediary, i.e., the digital twin needs to know when the device is awake. The device implementation as a Thing may need to include a notification mechanism for that. This could be implemented using a separate Consumer/Thing pair, or by using Event interactions for this purpose.

9.2.3 Devices in a Local Network Controlled from a Cloud Service

In Smart Home use cases, devices (sensors and home appliances) connected to a home network are often monitored and, in some cases, also controlled by cloud services. There is usually a NAT device between the home network to which the devices are connected and the cloud. The NAT device translates IP addresses as well as often providing firewall services, which block connections selectively. The local devices and cloud services can only communicate with each other if the communication can successfully traverse the gateway.

A typical structure, adopted in ITU-T Recommendation Y.4409/Y.2070 [Y.4409-Y.2070] , is shown in Figure 32 . In this structure there is both a local and a remote Intermediary. The local Intermediary aggregates the Interaction Affordances from multiple Thing into a (set of) Exposed Things, which can all be mapped onto a common protocol (for example, HTTP, with all interactions mapped to a single URL namespace with a common base server and using a single port). This provides the remote Intermediary with a simple way to access all the Things behind the NAT device, assuming the local Intermediary has used a converged protocol that can traverse the NAT device and has some way to expose this service to the Internet (STUN, TURN, DyDNS, etc). In addition, the local Intermediary can function as a Thing proxy, so even when the connected Things each use a different protocol (HTTP, MQTT, CoAP, etc.) and/or a different set of ecosystem conventions, the Exposed Thing can converge them into a single protocol so that Consumers do not need to be aware of the various protocols the Things use.

In Figure 32 , there are two clients connected to the remote Intermediary, which has aggregated the services that reside behind the NAT border and may provide additional protocol translation or security services. In particular, the local Intermediary may be on a network with limited capacity and making that service directly available to all users may not be feasible. In this case access to the local Intermediary is only provided to the remote Intermediary. The remote Intermediary then implements a more general access control mechanism and may also perform caching or throttling to protect the consumer from excess traffic. Those consumers also will use a single protocol suitable for the open Internet (e.g., HTTPS) to communicate with the Intermediary, which makes the development of clients much simpler.

In this topology there is NAT and firewall functionality between the consumers and things, but the local and remote Intermediaries work together to tunnel all communications through the firewall, so the consumers and things need to know nothing about the firewall. The paired Intermediaries also protect the home devices by providing access control and traffic management.

cloud applications
Figure 32 Cloud Applications Implemented as Consumers Connected to Local Devices implemented as Things via Paired Intermediaries

In more difficult cases the NAT and firewall traversal may not work exactly as shown. In particular, an ISP may not support publicly accessible addresses, or STUN/TURN and/or DyDNS may not be supported or available. In this case the Intermediaries may alternative reverse the client/server roles between them to set up an initial connection (with the local Intermediary first connecting to the remote Intermediary in the cloud), then the pair of Intermediaries may establish a tunnel (using for example, a Secure WebSocket, which uses TLS to protect the connection). The tunnel can then be used to encode all communications between the Intermediaries using a custom protocol. In this case the initial connection can still be made over HTTPS using standard ports, and from the local Intermediary to the remote Intermediary identically to a normal browser/web server interaction. This should be able to traverse most home firewalls, and since the connection is outgoing, network address translation will not cause any problems. However, even though a custom tunneling protocol is needed, the remote Intermediary can still translate this custom protocol back into standard external protocols. The connected Consumers and Things do not need to know about it. It is also possible to extend this example to use cases where both Things and Consumers can connect on either side of the NAT boundary. This however also requires a bidirectional tunnel to be established between the two Intermediaries.

9.2.4 Discovery Using a Thing Directory

Once local devices (and possibly services) can be monitored or controlled by services on cloud, a variety of additional services can be built on top. For example, a cloud application could change a device's operating condition based on an analysis of collected data.

However when the remote Intermediary is a part of a cloud platform servicing client applications, the clients need to be able to find device information by, for example, accessing a directory of connected devices. For simplicity in the figure below we have assumed all local devices are implemented as Things and all cloud applications as Consumers. To make the metadata of local devices implemented as Things available to the cloud applications, their metadata can be registered with a Thing Directory service. This metadata is specifically the TDs of the local devices modified to reflect the security configuration and communication metadata provided by the remote Intermediary. A client application then can obtain the metadata it needs to communicate with local devices to achieve its functionality by querying the Thing Directory.

cloud directory
Figure 33 Cloud Service with Thing Directory

In more complex situations, not shown in the figure, there may also be cloud services that act as Things. These can also register themselves with the Thing Directory. Since a Thing Directory is a Web service, it should be visible to the local devices through the NAT or firewall device and its interface can even be provided with its own TD. Local devices acting as Consumers can then discover the Things in the cloud via the Thing Directory and connect to the Things directly or via the local Intermediary if, for instance, protocol translation is needed.

9.2.5 Service-to-Service Connections Across Multiple Domains

Multiple cloud eco-systems each based on different IoT platforms can work together to make a larger, system-of-systems eco-system. Building on the previously discussed structure of a cloud application eco-system, the figure below shows two eco-systems connected to each other to make a system-of-systems. Consider the case in which a client in one eco-system (i.e., Consumer A below) needs to use a server in another eco-system (i.e., Thing B below). There is more than one mechanism to achieve this cross eco-systems application-device integration. Below, two mechanisms are explained, each using a figure, to show how this can be achieved. Connection Through Thing Directory Synchronization

In Figure 34 , two Thing Directories synchronize information, which makes it possible for Consumer A to obtain the information of Thing B through Thing Directory A. As described in previous sections, remote Intermediary B maintains a shadow implementation of Thing B. By obtaining the TD of this shadow device, Consumer A is able to use Thing B through the remote Intermediary B.

service sync directory
Figure 34 Multiple Cloud Connections Through Thing Directory Synchronization Connection Through Proxy Synchronization

In Figure 35 , two remote Intermediaries synchronize device information. When a shadow of Thing B is created in remote Intermediary B, the shadow’s TD is simultaneously synchronized into remote Intermediary A. Remote Intermediary A in turn creates its own shadow of Thing B, and registers the TD with Thing Directory A. With this mechanism, synchronization between Thing Directories is not necessary.

service sync proxy
Figure 35 Multiple Cloud Connections Through Intermediary Synchronization

10. Security and Privacy Considerations

This section is non-normative.

Security is a cross-cutting issue that needs to be taken into account in all WoT building blocks and WoT implementations. This chapter summarizes some general issues and guidelines to help preserve the security and privacy of WoT implementations. For a more detailed and complete analysis of security and privacy issues, see the WoT Security and Privacy Considerations specification [wot-security].

Overall, the goal of the WoT is to describe the existing access mechanisms and properties of IoT devices and services, including security. In general, W3C WoT is designed to describe what exists rather than to prescribe what to implement.

However, the WoT architecture should enable the use of best practices in security and privacy. The WoT security architecture must support the goals and mechanisms of the IoT protocols and systems it connects to. These systems vary in their security requirements and risk tolerance, so security mechanisms will also vary based on these factors.

Security and privacy are especially important in the IoT domain since IoT devices need to operate autonomously and in many cases have access to both personal data and/or can be in control of safety-critical systems. Compared to personal systems, IoT devices are subject to different and in some cases higher risks than IT systems. It is also important to protect IoT systems so that they can not be used to launch attacks on other computer systems.

In general, security and privacy cannot be guaranteed. It is not possible for the WoT to turn an insecure system into a secure one. However, the WoT architecture needs to do no harm: it should support security and privacy at least as well as the systems it connects to.

10.1 WoT Thing Description Risks

The metadata contained in a WoT Thing Description (TD) is potentially sensitive. As a best practice, TDs should be used together with integrity protection mechanisms and access control policies, and should be provided only to authorized users.

Please refer to the Security and Privacy Consideration section of the WoT Thing Description specification for additional details and discussion.

10.1.1 Thing Description Private Security Data Risk

TDs are designed to carry only public security data. Producers of TDs must ensure that no private security information is included in TDs. There should be a strict separation of public and private security data. A TD should contain only public security information, letting Consumers know what they need to do to access as system if and only if they have authorization. Authorization in turn should be based on separately managed private information.

The built-in TD security schemes defined in the TD specification do not support the encoding of private security data. However, there is a risk that other fields such as human-readable descriptions might be used (incorrectly...) to encode this information, or new security schemes might be defined and deployed via the extension mechanism that encode such information.

Creators of TDs and extensions meant to be used in TDs must ensure that no private security data is ever stored in TDs.

10.1.2 Thing Description Personally Identifiable Information Risk

Thing descriptions can potentially contain Personally Identifiable Information of various types. Even if it is not explicit, the presence of semantic information in a TD and its association with a person can be used to infer information about that person. For example, the association of uniquely identifiable TDs exposed by mobile devices whose location can be determined can be a tracking risk.

Generally, personally identifiable information in a TD should be limited as much as possible. In some cases, however, it cannot be avoided. The potential presence of PII in a TD means that TD should be treated like other forms of PII. They should be stored and transmitted in a secure fashion, should only be cached for limited times, should be deleted upon request, should only be used for the purpose for which they were provided with user consent, and they should otherwise satisfy local requirements (including any legal requirements) for the use of PII.

Storage of PII in TDs should be minimized as much as possible. Even without explicit PII in TDs, a tracking and identification privacy risk may exist. To minimize this risk, TDs should generally be treated as if they contained PII and subject to the same management policies as other PII. They should only be provided to authorized Consumers.

10.1.3 Thing Description Communication Metadata Risk

The WoT Binding Templates must correctly support the security mechanisms employed by the underlying IoT Platform for that platform to be considered eligible for use with WoT. Due to the automation of network interactions necessary to deploy IoT at scale, operators need to ensure that Things are exposed and consumed in a way that is compliant with their security policies.

Whenever possible, TD creators should use the vetted communication metadata provided in the WoT Binding Templates. When generating TDs for an IoT ecosystem not covered by the WoT Binding Templates, ensure that all the security requirements of the IoT Platform are satisfied.

10.2 WoT Scripting API Security and Privacy Risks

The WoT Runtime implementation and the WoT Scripting API should have mechanisms to prevent malicious access to the system and isolate scripts in multi-tenant Servients . More specifically the WoT Runtime implementation when used with the WoT Scripting API should take into account the following security and privacy risks and implement the recommended mitigations.

10.2.1 Cross-Script Security and Privacy Risk

In basic WoT setups, all scripts running inside the WoT Runtime are considered trusted, distributed by the manufacturer, and therefore there is no strong need to perform strict isolation between each running script instance. However, depending on device capabilities, deployment use case scenarios, and risk level it might be desirable to do so. For example, if one script handles sensitive privacy-related PII data and is well-audited, it might be desirable to separate it from the rest of the script instances to minimize the risk of data exposure in case some other script inside the same system gets compromised during runtime. Another example is mutual co-existence of different tenants on a single WoT device. In this case each WoT runtime instance will be hosting a different tenant, and isolation between them is required.

The WoT Runtime should perform isolation of script instances and their data in cases when scripts handle privacy-related or other critical security data. Similarly, the WoT Runtime implementation should perform isolation of WoT Runtime instances and their data if a WoT device has more than one tenant. Such isolation can be performed within the WoT Runtime using platform security mechanisms available on the device. For more information see Sections "WoT Servient Single-Tenant" and "WoT Servient Multi-Tenant" of the WoT Security and Privacy Considerations specification [wot-security].

10.2.2 Physical Device Direct Access Security and Privacy Risk

In case a script is compromised or malfunctions the underlying physical device (and potentially surrounded environment) can be damaged if a script can use directly exposed native device interfaces. If such interfaces lack safety checks on their inputs, they might bring the underlying physical device (or environment) to an unsafe state.

The WoT Runtime should avoid directly exposing the native device interfaces to the script developers. Instead a WoT Runtime implementation should provide a hardware abstraction layer for accessing the native device interfaces. Such hardware abstraction layer should refuse to execute commands that might put the device (or environment) to an unsafe state. Additionally, in order to reduce the damage to a physical WoT device in cases a script gets compromised, it is important to minimize the number of interfaces that are exposed or accessible to a particular script based on its functionality.

10.3 WoT Runtime Security and Privacy Risks

10.3.1 Provisioning and Update Security Risk

If the WoT Runtime implementation supports post-manufacturing provisioning or updates of itself, scripts, or any related data (including security credentials), it can be a major attack vector. An attacker can try to modify any above described element during the update or provisioning process or simply provision attacker's code and data directly.

Post-manufacturing provisioning or update of scripts, the WoT Runtime itself or any related data should be done in a secure fashion. A set of recommendations for secure update and post-manufacturing provisioning can be found in the WoT Security and Privacy Considerations specification [wot-security].

10.3.2 Security Credentials Storage Security and Privacy Risk

Typically the WoT Runtime needs to store the security credentials that are provisioned to a WoT device to operate in a network. If an attacker can compromise the confidentiality or integrity of these credentials, then it can obtain access to assets, impersonate other WoT Things, devices, or services, or launch Denial-Of-Service (DoS) attacks.

The WoT Runtime should securely store any provisioned security credentials, guaranteeing their integrity and confidentiality. In case there are more than one tenant on a single WoT-enabled device, a WoT Runtime implementation should guarantee isolation of each tenant's provisioned security credentials. Additionally, in order to minimize a risk that provisioned security credentials get compromised, the WoT Runtime implementation should not expose any API for scripts to query the provisioned security credentials. Such credentials (or event better, abstract operations that use them but do not expose them) should only be accessible to the Protocol Binding implementation that uses them.

A. Recent Specification Changes

Changes from First Public Working Draft

Earlier Publications

B. Acknowledgments

Special thanks to Michael McCool, Takuki Kamiya, Kazuyuki Ashimura, Sebastian Käbisch, Zoltan Kis, Elena Reshetova, Ari Keränen, Kazuaki Nimura, Klaus Hartke, and Philippe Le Hegaret for their contributions to this document.

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.

The WoT WG also would like to appreciate the pioneering efforts regarding the concept of "Web of Things" that started as an academic initiative in the form of publications such as [wot-pioneers-1] [wot-pioneers-2] [wot-pioneers-3] [wot-pioneers-4] and, starting in 2010, a yearly International Workshop on the Web of Things.

C. References

C.1 Normative references
Link Relations. IANA. URL: https://www.iana.org/assignments/link-relations/
MQTT Version 3.1.1. OASIS Standard. 2014.
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
Uniform Resource Identifier (URI): Generic Syntax. T. Berners-Lee; R. Fielding; L. Masinter. IETF. January 2005. Internet Standard. URL: https://tools.ietf.org/html/rfc3986
Internationalized Resource Identifiers (IRIs). M. Duerst; M. Suignard. IETF. January 2005. Proposed Standard. URL: https://tools.ietf.org/html/rfc3987
Guidelines and Registration Procedures for New URI Schemes. T. Hansen; T. Hardie; L. Masinter. IETF. February 2006. Best Current Practice. URL: https://tools.ietf.org/html/rfc4395
Augmented BNF for Syntax Specifications: ABNF. D. Crocker, Ed.; P. Overell. IETF. January 2008. Internet Standard. URL: https://tools.ietf.org/html/rfc5234
Media Type Specifications and Registration Procedures. N. Freed; J. Klensin; T. Hansen. IETF. January 2013. Best Current Practice. URL: https://tools.ietf.org/html/rfc6838
Concise Binary Object Representation (CBOR). C. Bormann; P. Hoffman. IETF. October 2013. Proposed Standard. URL: https://tools.ietf.org/html/rfc7049
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content. R. Fielding, Ed.; J. Reschke, Ed.. IETF. June 2014. Proposed Standard. URL: https://tools.ietf.org/html/rfc7231
The Constrained Application Protocol (CoAP). Z. Shelby; K. Hartke; C. Bormann. IETF. June 2014. Proposed Standard. URL: https://tools.ietf.org/html/rfc7252
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://tools.ietf.org/html/rfc8174
The JavaScript Object Notation (JSON) Data Interchange Format. T. Bray, Ed.. IETF. December 2017. Internet Standard. URL: https://tools.ietf.org/html/rfc8259
Web Linking. M. Nottingham. IETF. October 2017. Proposed Standard. URL: https://tools.ietf.org/html/rfc8288
Web of Things (WoT) Thing Description. Sebastian Käbisch; Takuki Kamiya. W3C. 21 October 2018. W3C Working Draft. URL: https://www.w3.org/TR/wot-thing-description/

C.2 Informative references
The Constrained RESTful Application Language (CoRAL). Klaus Hartke. IETF. March 2019. Internet-Draft. URL: https://tools.ietf.org/html/draft-hartke-t2trg-coral
CoRE Resource Directory. IETF. 11 March 2019. Internet-Draft. URL: https://tools.ietf.org/html/draft-ietf-core-resource-directory-20
ECMAScript Language Specification. Ecma International. URL: https://tc39.github.io/ecma262/
Server-Sent Events. Ian Hickson. W3C. 3 February 2015. W3C Recommendation. URL: https://www.w3.org/TR/eventsource/
The Encyclopedia of Human-Computer Interaction, 2nd Ed. Interaction Design Foundation. 2013.
Factory of the future. IEC. October 2015. URL: https://www.iec.ch/whitepaper/pdf/iecWP-futurefactory-LR-en.pdf
iot.schema.org. URL: https://iot.schema.org/
JSON-LD 1.0. Manu Sporny; Gregg Kellogg; Markus Lanthaler. W3C. 16 January 2014. W3C Recommendation. URL: https://www.w3.org/TR/json-ld/
JSON-LD 1.1. Gregg Kellogg; Pierre-Antoine Champin. W3C. 10 May 2019. W3C Working Draft. URL: https://www.w3.org/TR/json-ld11/
Linked Data Design Issues. Tim Berners-Lee. W3C. 27 July 2006. W3C-Internal Document. URL: https://www.w3.org/DesignIssues/LinkedData.html
Lightweight Machine to Machine Technical Specification: Core. OMA SpecWorks. August 2018. Approved Version: 1.1. URL: http://openmobilealliance.org/release/LightweightM2M/V1_1-20180710-A/OMA-TS-LightweightM2M_Core-V1_1-20180710-A.pdf
The Psychology of Everyday Things. Basic Books. 1988.
OCF Core Specification. Open Connectivity Foundation. April 2019. Version 2.0.2. URL: https://openconnectivity.org/developer/specifications
REST: Architectural Styles and the Design of Network-based Software Architectures. University of California, Irvine. 2000. PhD thesis.
Security Architecture for the Internet Protocol. S. Kent; K. Seo. IETF. December 2005. Proposed Standard. URL: https://tools.ietf.org/html/rfc4301
Known Issues and Best Practices for the Use of Long Polling and Streaming in Bidirectional HTTP. S. Loreto; P. Saint-Andre; S. Salsano; G. Wilkins. IETF. April 2011. Informational. URL: https://tools.ietf.org/html/rfc6202
Datagram Transport Layer Security Version 1.2. E. Rescorla; N. Modadugu. IETF. January 2012. Proposed Standard. URL: https://tools.ietf.org/html/rfc6347
Constrained RESTful Environments (CoRE) Link Format. Z. Shelby. IETF. August 2012. Proposed Standard. URL: https://tools.ietf.org/html/rfc6690
The OAuth 2.0 Authorization Framework. D. Hardt, Ed.. IETF. October 2012. Proposed Standard. URL: https://tools.ietf.org/html/rfc6749
Observing Resources in the Constrained Application Protocol (CoAP). K. Hartke. IETF. September 2015. Proposed Standard. URL: https://tools.ietf.org/html/rfc7641
Use Cases for Authentication and Authorization in Constrained Environments. L. Seitz, Ed.; S. Gerdes, Ed.; G. Selander; M. Mani; S. Kumar. IETF. January 2016. Informational. URL: https://tools.ietf.org/html/rfc7744
The Transport Layer Security (TLS) Protocol Version 1.3. E. Rescorla. IETF. August 2018. Proposed Standard. URL: https://tools.ietf.org/html/rfc8446
Smart Appliances REFerence (SAREF) ontology. ETSI. November 2015. URL: https://sites.google.com/site/smartappliancesproject/ontologies/reference-ontology
Semantic Sensor Network Ontology. Armin Haller; Krzysztof Janowicz; Simon Cox; Danh Le Phuoc; Kerry Taylor; Maxime Lefrançois. W3C. 19 October 2017. W3C Recommendation. URL: https://www.w3.org/TR/vocab-ssn/
Web of Things (WoT) Protocol Binding Templates. Michael Koster. W3C. 5 April 2018. W3C Note. URL: https://www.w3.org/TR/wot-binding-templates/
Mobile Service Interaction with the Web of Things. E. Rukzio, M. Paolucci; M. Wagner, H. Berndt; J. Hamard; A. Schmidt. Proceedings of 13th International Conference on Telecommunications (ICT 2006), Funchal, Madeira island, Portugal. May 2006.
Putting Things to REST. Erik Wilde. UCB iSchool Report 2007-015, UC Berkeley, Berkeley, CA, USA. November 2007.
Poster Abstract: Dyser – Towards a Real-Time Search Engine for the Web of Things. Benedikt Ostermaier; B. Maryam Elahi; Kay Römer; Michael Fahrmair; Wolfgang Kellerer. Proceedings of ACM SenSys 2008, Raleigh, NC, USA. November 2008.
A Resource Oriented Architecture for the Web of Things. Dominique Guinard; Vlad Trifa; Erik Wilde. Proceedings of Internet of Things 2010 International Conference (IoT 2010). Tokyo, Japan. November 2010.
Web of Things (WoT) Scripting API. Zoltan Kis; Kazuaki Nimura; Daniel Peintner; Johannes Hund. W3C. 29 November 2018. W3C Working Draft. URL: https://www.w3.org/TR/wot-scripting-api/
Web of Things (WoT) Security and Privacy Considerations. Elena Reshetova; Michael McCool. W3C. 3 December 2018. W3C Note. URL: https://www.w3.org/TR/wot-security/
ITU-T Rec. Y.4409/Y.2070 (01/2015) Requirements and architecture of the home energy management system and home network services . ITU-T. January 2015. Recommendation.