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The W3C Web of Things (WoT) is intended to enable interoperability across IoT platforms and application domains. One key mechanism for accomplishing this goal is the definition and use of metadata describing the interactions an IoT device or service makes available over the network at a suitable level of abstraction. The WoT Thing Description specification satisfies this objective.
However, in order to use a Thing its Thing Description first has to be obtained. The WoT Discovery process described in this document addresses this problem. WoT Discovery needs to support the distribution of WoT Thing Descriptions in a variety of use cases. This includes ad-hoc and engineered systems; during development and at runtime; and on both local and global networks. The process also needs to work with existing discovery mechanisms, be secure, protect private information, and be able to efficiently handle updates to WoT Thing Descriptions and the dynamic and diverse nature of the IoT ecosystem.
The WoT Discovery process is divided into two phases, Introduction, and Exploration. The Introduction phase leverages existing discovery mechanisms but does not directly expose metadata; they are simply used to discover Exploration services, which provide metadata but only after secure authentication and authorization. This document normatively defines two Exploration services, one for WoT Thing self-description with a single WoT Thing Description and a searchable WoT Thing Directory service for collections of Thing Descriptions. A variety of Introduction services are also described and where necessary normative definitions are given to support them.
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/.
This document was published by the Web of Things Working Group as an Editor's Draft.
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This document is governed by the 15 September 2020 W3C Process Document.
The Web of Things (WoT) defines an architecture that supports the integration and use of web technologies with IoT devices. The WoT Architecture [wot-architecture] document defines the basic concepts and patterns of usage supported. However, the WoT Thing Description [wot-thing-description] is a key specification for WoT Discovery since it is the purpose of WoT Discovery to make WoT Thing Descriptions available. Specifically, WoT Discovery has to allow authenticated and authorized entities (and only those entities) to find WoT Thing Descriptions satisfying a set of criteria, such as being near a certain location, or having certain semantics, or containing certain interactions. Conversely, in order to support security and privacy objectives, the WoT Discovery process must not leak information to unauthorized entities. This includes leaking information that a given entity is requesting certain information, not just the information distributed in the Thing Descriptions themselves.
There are already a number of discovery mechanisms defined, so we have to establish why we are proposing a new one. First, many existing discovery mechanisms have relatively weak security and privacy protections. One of our objectives is to establish a mechanism that not only uses best practices to protect metadata, but that can be upgraded to support future best practices as needed. Second, we are using discovery in a broad sense to include both local and non-local mechanisms. While a local mechanism might use a broadcast protocol, non-local mechanisms might go beyond the current network segment where broadcast is not scalable, and so a different approach, such as a search service, is needed. Our approach is to use existing mechanisms as needed to bootstrap into a more general and secure metadata distribution system. Third, the metadata we are distributing, the WoT Thing Description, is highly structured and includes rich data such as data schemas and semantic annotations. Existing discovery mechanisms based on a list of simple key-value pairs are not appropriate. At the same time, use of existing standards for semantic data query, such as SPARQL [SPARQL11-OVERVIEW], while potentially suitable for some advanced use cases, might require to much effort for many anticipated IoT applications. Therefore in order to address more basic applications, we also define some simpler query mechanisms.
After defining some basic terminology, we will summarize the basic use cases and requirements for WoT Discovery. These are a subset of the more detailed and exhaustive use cases and requirements presented in the WoT Use Cases [wot-usecases] and WoT Architecture [wot-architecture] documents. Then we will describe the basic architecture of the WoT Discovery process, which uses a two-phase Introduction/Exploration approach. The basic goal of this architecture is to be able to use existing discovery standards to bootstrap access to protected discovery services, but to distribute detailed metadata only to authorized users, and to also protect those making queries from eavesdroppers as much as possible. We then describe details of specific Introduction and Exploration mechanisms. In particular, we define in detail a normative API for a WoT Thing Description Directory (WoT TDD) service that provides a search mechanism for collections of WoT Thing Descriptions that can be dynamically registered by Things or entities acting on their behalf. The WoT Discovery mechanism however also supports self-description by individual Things and one issue we address is how to distinguish between these two approaches. Finally, we discuss some security and privacy considerations, including a set of potential risks and mitigations.
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, OPTIONAL, RECOMMENDED, 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.
This section is non-normative.
The fundamental WoT terminology such as Thing, Thing Description (TD), Property, Action, Event are defined in Section 3 of the WoT Architecture specification [WOT-ARCHITECTURE].
In addition, this specification introduces the following definitions:
id attribute).
This section is non-normative.
Figure 1 shows an overview of discovery process.
To do: an overview of the two-phase approach and its purpose, which is to support controlled and authenticated access to metadata by authorized users only.
This chapter describes a mechanism for discovering a Thing or a Directory Service. The following mechanism is provided by the Thing or the Directory Service so that Consumer can discover the Thing Description or a URL that point to the Thing Description.
The following sub-sections define assertions without
using the rfc2119-assertion class.
Any mechanism that results in a single URL. This includes Bluetooth beacons, QR codes, and written URLs to be typed by a user. A request on all such URLs MUST result in a TD as prescribed in § 6.1 Self-description. For self-describing Things, this can be the TD of the Thing itself. If the URL references a Directory, this MUST be the TD of the Directory service.
A Thing or Directory Service MAY use the Well-Known Uniform Resource Identifier [RFC8615]
to advertise its presence. The Thing or Directory Service registers its own Thing Description
into the following path:
/.well-known/wot-thing-description.
When a request is made at the above path, the server MUST return a Thing Description as prescribed in § 6.1 Self-description.
The service name in Well-Known URI (wot-thing-description) is tentative.
"Well-Known URIs" registry and contents of registration request is described in Section 3.1 of [RFC8615].
A Thing or Directory Service MAY use the DNS-Based Service Discovery (DNS-SD)[RFC6763]. This can be also be used to discover them on the same link by combining Multicast DNS (mDNS)[RFC6762].
In DNS-SD, format of the Service Instance Name is Instance.Service.Domain.
The Service part is a pair of labels following the conventions of [RFC2782].
The first label has an underscore followed by the Service Name,
and the second label describes the protocol.
The Service Name to indicate the Thing or Directory Service MUST be _wot.
And the Service Name to indicate the Directory Service MUST be _directory._sub._wot.
The Service Names _wot and _directory._sub._wot
are tentative. The following Service Names are used in the existing implementations:
_wot,
_device._sub._wot,
_directory._sub._wot,
_webthing.
To use a Service Name, registration to "Underscored and Globally Scoped DNS Node Names"
Registry [RFC8552] is required.
In addition, the following information MUST be included in the TXT
record that is pointed to by the Service Instance Name:
tdtypeThing or Directory.
If omitted, the type is assumed to be Thing.
The following key/value pairs are used in the existing implementations:
retrieve:
Absolute path name of the API to get an array of Thing Description IDs
from the Directory Service.
register:
Absolute path name of the API to register a Thing Description with the Directory Service.
path:
The URI of the thing description on the Web Thing's web server
td:
Prefix of Directory Service API
tls:
Value of 1 if the Web Thing supports connections via HTTPS.
Figure 2 and Figure 3 shows example sequences of discovery of Thing and Directory Service using DNS-SD and mDNS.
A Thing or Directory Service MAY advertise its presence using the Constrained RESTful Environment (CoRE) Link Format [RFC6690]. And, a Thing or Directory Service MAY use the CoRE Resource Directory [CoRE-RD] to register a link to the Thing Description.
The endpoint type(et) of the Link that targets the Thing Description of the Thing
MUST be wot.thing.
The endpoint type of the Link that targets the Thing Description of the Directory Service
MUST be wot.directory.
The endpoint types wot.thing and wot.directory are tentative.
A Thing or Directory Service MAY advertise its presence using the Decentralized Identifier (DID) [DID-CORE].
The DID Document obtained by resolving the DID of a Thing or Directory Service MUST contains a Service Endpoint which point to Thing Description of the Thing or Directory Service.
To do: Description of supported explorations, and requirements for new exploration mechanisms.
DirectoryDescription from the
discovery context or URI https://www.w3.org/2021/wot/discovery#DirectoryDescription.
Example 3 which describes the API of the Thing Description Directory is an example of this TD type.
LinkDescription from the
discovery context or URI https://www.w3.org/2021/wot/discovery#LinkDescription.
The Link Description MUST define the reference TD as a Link with
describedby link relation type, application/td+json media type
and href set to the target URL.
Example 2 is an example Link Description.
The context and type URIs are tentative and subject to change.
The self-description is an exploration mechanism in which a Thing hosts its own TD and exposes it at a URL or through others means. If exposed at a URL (e.g. over HTTP or CoAP), the URL may be advertised via one of the § 5. Introduction Mechanisms. The hosted TD may also be registered inside a Thing Description Directory as prescribed in § 6.2 Directory.
The self-description using the following protocols must be according to the given specification:
The HTTP-based self-description SHOULD be over HTTPS (HTTP Over TLS).
The HTTP server MUST serve the TD with a GET method.
A successful response MUST have 200 (OK) status, contain application/td+json
Content-Type header, and the TD in body.
The server MAY provide alternative representations through
server-driven content negotiation, that is by honouring the
request's Accept header and responding with the supported
TD serialization and equivalent Content-Type header.
The server SHOULD serve the requests after performing necessary
authentication and authorization.
Error responses:
To do: Describe mechanisms for TDs to be hosted in a searchable directory service.
To Do: Formal definition of information contained in a directory and its organization.
Figure 5 shows the information model of the
Thing Description Directory.
A directory can contain zero or more TDs.
For every TD, the directory may maintain the registration information for
bookkeeping and search purposes. The information may also include the
unique identifier of the TD within the directory.
This is useful for when a TD has been registered under an identifier
which differs from TD's internal id field.
When available, consumers may request the registration information
via the respective API.
A TD which embeds the registration information is called an Enriched TD.
The HTTP API responses must use appropriate status codes described in this section for success and error responses. The HTTP API MUST use the Problem Details [RFC7807] format to carry error details in HTTP client error (4xx) and server error (5xx) responses. This enables both machines and humans to know the high-level error class and fine-grained details.
The Problem Details error type field is a URI reference which
could used to map the occurred error to WoT-specific error class.
There are few open issues raising the lack of WoT-specific error types:
wot-discovery#44,
wot-thing-description#303,
wot-scripting-api#200.
For now, type can be omitted which defaults to "about:blank", and title
should be set to HTTP status text.
Below is a generic Thing Description for the Directory HTTP API with OAuth2 security. The Thing Description alone should not be considered as the full specification to implement or interact with a directory. Additional details for every interaction are described in human-readable form in the subsequent sections.
Need to confirm if equivalent OpenAPI spec can be easily created out of the TD in Example 3. If yes, a sentence may be added indicating this possibility.
The Registration API is a RESTful HTTP API in accordance with the recommendations defined in [RFC7231] and [REST-IOT]. The default serialization format for all request and response bodies MUST be JSON, with JSON-LD 1.1 [JSON-LD11] syntax to support extensions and semantic processing. Directories MAY accept additional representations based on request's indicated Content-Type or Content-Encoding, and provide additional representations through server-driven content negotiation.
The Registration API MUST provide create, retrieve, update, delete (CRUD) interfaces.
The API MUST allow registration of a TD object passed as request body.
The request SHOULD contain application/td+json Content-Type header for
JSON serialization of TD.
The TD object must be validated in accordance with § 6.2.2.1.5 Validation.
A TD which is identified with an id attribute MUST be handled
differently with one that has no identifier (Anonymous TD).
The create operations are specified as createTD action in
Example 3
and elaborated below:
PUT request at a
target location (HTTP path) containing the unique TD id.
Upon successful processing, the server MUST respond with
201 (Created) status.
Note: If the target location corresponds to an existing TD, the request shall instead proceed as an Update operation and respond the appropriate status code (see Update section).
POST request.
Upon successful processing, the server MUST respond with 201 (Created) status
and a Location header containing a system-generated identifier for the TD.
The identifier SHOULD be a Version 4 UUID URN [RFC4122].
Error responses:
The server should employ a mechanism to eliminate duplication of TDs submitted with
a POST request. The spec need to have recommendations on how to perform this.
A TD MUST be retrieved from the directory using an HTTP GET request,
including the identifier of the TD as part of the path.
A successful response MUST have 200 (OK) status, contain application/td+json
Content-Type header, and the requested TD in body.
The retrieve operation is specified as retrieveTD property in
Example 3.
Error responses:
id not found.
Need to specify and describe the query argument to toggle enrichment of TD with registration info.
The API MUST allow modifications to an existing TD as full replacement or partial updates. The update operations are described below:
PUT request
to the location corresponding to the existing TD.
The request SHOULD contain application/td+json Content-Type header for JSON
serialization of TD.
The TD object must be validated in accordance with § 6.2.2.1.5 Validation.
Upon success, the server MUST respond with 204 (No Content) status.
This operation is specified as updateTD property in
Example 3.
Note: If the target location does not correspond to an existing TD,
the request shall instead proceed as a Create operation and respond
the appropriate status code (see Create section). In other words, an HTTP PUT
request acts as a create or update operation.
PATCH request to the location corresponding to the existing TD.
The partial update MUST be processed using the JSON merge patch format
format described in [RFC7396].
The request MUST contain application/merge-patch+json Content-Type header for JSON
serialization of the merge patch document.
The input MUST be in Partial TD form and conform to the
original TD structure.
If the input contains members that appear in the original TD,
their values are replaced. If a member do not appear in the
original TD, that member is added. If the member is set to null
but appear in the original TD, that member is removed.
Members with object values are processed recursively.
After applying the modifications, the TD object must be validated in accordance with § 6.2.2.1.5 Validation.
Upon success, the server MUST respond with a 204 (No Content) status.
This operation is specified as updatePartialTD property in
Example 3.
Error responses:
id not found (for PATCH only).
A TD MUST be removed from the directory when an HTTP DELETE request
is submitted to the location corresponding to the existing TD.
A successful response MUST have 204 (No Content) status.
The retrieve operation is specified as deleteTD property in
Example 3.
Error responses:
id not found.
It would be nice to include an example validation result in the uniform error format (i.e. RFC7807 Problem Details).
How much validation does a directory need to do?
To do: Other administrative functions not having to do with CRUD of individual records, for example, security configuration. Also, administrator roles may expand the capabilities of administrators for management of records (for instance, the ability to delete a record they did not create).
The Notification API is to notify clients about the changes
to Thing Descriptions maintained within the directory.
The Notification API MUST follow the Server-Sent Events [EVENTSOURCE]
specifications to serve events to clients.
In particular, the server responds to successful requests with 200 (OK) status
and text/event-stream Content Type. Re-connecting clients may continue from
the last event by providing the last event ID as Last-Event-ID header value.
This API is specified as registration event in Example 3.
created_td, updated_td, deleted_td
keywords respectively.
type query parameters.
For example, in response to query ?type=created_td&type=deleted_td,
the server must only deliver events of types created_td and deleted_td.
At the absence of any type query parameter, the server must deliver all
types of events.
td_id query parameters.
For example, the query ?type=updated_td&td_id=urn:example:1234 must
result in updated_td events for the TD identified with urn:example:1234.
jsonpath, xpath,
or sparql query parameters.
If the server does not support a given search query parameter, it
MUST reject the request with 501 (Not Implemented) status.
td_id field.
include_changes query parameter is set to true, the create
event data object MAY include the created TD as the value of created_td
field.
include_changes query parameter is set to true, the update
event data object MAY include the updated parts of the TD in
Partial TD form as the value of td_updates field.
include_changes query parameter, it MUST
reject the request with 501 (Not Implemented) status.
Some early SSE implementations (including HTML5 EventSource) do not allow setting custom headers in the initial HTTP request. Authorization header is required in few OAuth2 flows and passing it as a query parameter is not advised. There are polyfills for browsers and modern libraries which allow setting Authorization header.
Sub-API to search a directory, e.g. issue a query.
There are different forms and levels of query possible, for example,
syntactic (JSONPath, XPath) vs. semantic (SPARQL),
and the more advanced query types may not be
supported by all directories.
So this API will have further subsections,
some of which will be optional.
Search also includes a sub-API for managing listing the contents (eg returned by
a query) including handling pagination, etc.
Note that one special form of query will be able to return everything.
Results may be subject to the requestor's authorization.
To discuss further:
Federated queries to other TDDs, Spatial and network-limited queries, Links
GET request.
The request MUST contain a valid JSONPath [JSONPATH] as searching parameter.
A successful response MUST have 200 (OK) status, contain application/json Content-Type header, and in the body a set of complete TDs or a set of TD fragments.
The syntactic search with JSONPath is specified as searchJSONPath property in
Example 3.
List of errors:
GET request.
The request MUST contain a valid XPath [xpath-31] as search parameter.
A successful response MUST have 200 (OK) status, contain application/json Content-Type header, and in the body a set of complete TDs or a set of TD fragments.
The syntactic search with XPath is specified as searchXPath property in
Example 3.
List of errors:
GET requests.
The support for SPARQL search using HTTP POST method is OPTIONAL.
UPDATE queries are out of the scope for the API.
A successful response MUST have 200 (OK) status, and depending on the type of query contain by default as Content-Type header application/ld+json for CONSTRUCT and DESCRIBE queries or application/json for SELECT or ASK. The response body MAY contain TD fragments or a set of TDs depending on the query.
The semantic search with SPARQL is specified as searchSPARQL property in
Example 3.
List of errors:
Minimum security and privacy requirements for confidentiality, authentication, access control, etc.
This section is non-normative.
Security and privacy are cross-cutting issues that need to be considered in all WoT building blocks and WoT implementations. This chapter summarizes some general issues and guidelines to help preserve the security and privacy of concrete WoT discovery implementations. For a more detailed and complete analysis of security and privacy issues, see the WoT Security and Privacy Guidelines specification [WOT-SECURITY].
The WoT discovery architecture is designed to avoid a dependence on the security and privacy of existing discovery schemes by using a two-phase approach and requiring authorization before metadata release. However several security and privacy risks still exist. These are listed below along with possible mitigations. The level of risk to privacy in particular depends on the use case and whether there is a risk that information related to a person might be distributed in a fashion inconsistent with the privacy desires of that person. We distinguish the following broad classes of use case scenarios:
All of these in fact carry privacy risks. Even in the case of factory automation, there is the chance that data about employee performance would be captured and would have to be managed appropriately.
With these categories established, we will now discuss some specific risks and potential mitigations.
Certain functions of the directory service, in particular search queries, may require significant resources to execute and this fact can be used to launch DDoS attacks against WoT Thing Description Directory services.
This is mostly of concern in the Service scenario, where the metadata requester is a private individual and the provider is an institution. In some cases this risk may appear in Peer-to-Peer scenarios as well.
A discovery service may potentially allow the approximate location of a person to be determined without their consent. This risk occurs in some specific circumstances which can be avoided or mitigated. It is also similar to the risk posed by other network services such as DHCP and DNS.
For this risk to occur, there first has to be an IoT device that can be reliably associated with a person's location, such as a necessary medical device or a vehicle. Note that the risk only applies to personal use cases, not institutional ones. Secondly, the device has to be configured to register automatically with the nearest directory service. In this case, the location of the device can be inferred from the network range of the directory service and the location of the person inferred from the location of the device.
There are a few variants of this:
Some of these risks are shared by similar services. For example, DCHP automatically responds to requests for IP addresses on a local network, and devices typically provide an identifier (a MAC address) as part of this process, and the DHCP server maintains a registry. In theory, someone with access to the DHCP server in, say, a cafe, could use this information to track someone's phone and infer their location.
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.