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 Description 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.
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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 too 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:
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 Thing Description Directory. The following mechanism is provided by the Thing or the Thing Description Directory so that Consumers can discover the Thing Description or a URL that point to the Thing Description.
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 Thing Description Directory, this MUST be the Directory Description of the Thing Description Directory.
A Thing or Thing
Description Directory may use the Well-Known Uniform
Resource Identifier [RFC8615]
to advertise its presence. The Thing or Thing
Description Directory registers its own Thing or
Directory Description into the following path:
When a request is made at the above Well-Known URI, 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 Thing Description Directory 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 Thing
Description Directory MUST be
_wot. And the Service
Name to indicate the Thing
Description Directory MUST be
The Service Names
_directory._sub._wot are tentative. The
following Service Names are used in the existing
_wot-servient. To use a Service Name,
registration to "Underscored and Globally Scoped DNS Node
Names" Registry [RFC8552]
In addition, the following
information MUST be included in the
TXT record that is pointed to by the Service
Directory. If omitted, the type is assumed to be
The following key/value pairs are used in the
retrieve: Absolute path name of the API to get
an array of Thing Description IDs from the directory
register: Absolute path name of the API to
register a Directory Description with the Thing
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 Thing Description Directory using DNS-SD and mDNS.
A Thing or Thing Description Directory may advertise its presence using the Constrained RESTful Environment (CoRE) Link Format [RFC6690]. And, a Thing or Thing Description Directory may use the CoRE Resource Directory [CoRE-RD] to register a link to the Thing or Directory Description.
The resource type
rt) of the Link that targets the Thing
Description of the Thing MUST be
wot.thing. The resource
type of the Link that targets the Directory Description of
Description Directory MUST be
The resource types
wot.directory are tentative. See also § 8. IANA Considerations.
A Thing or Thing Description Directory may advertise its presence using the Decentralized Identifier (DID) [DID-CORE].
The DID Document obtained by resolving the DID of a Thing or Thing Description Directory MUST contains a Service Endpoint which point to Thing Description of the Thing or Directory Description of the Thing Description Directory.
To do: Description of supported explorations, and requirements for new exploration mechanisms.
DirectoryDescriptionfrom the discovery context or URI
Example 3 which describes the API of the Thing Description Directory is an example of this TD type.
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)
Content-Type header, and the TD in body.
The server MAY provide alternative
representations through server-driven content
negotiation, that is by honoring 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
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.
As shown in Figure 4, the Thing Description Directory can contain zero or more TDs. For every TD, the directory may additionally maintain the registration information for bookkeeping and search purposes. 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 ontology of a TD maintained by the directory is illustrated in Figure 5.
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,
type can be omitted which
defaults to "about:blank", and
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, and listing (CRUDL) interfaces. The operations are described below:
The API MUST allow registration of a TD
object passed as request body. The request SHOULD contain
header for JSON serialization of TD. The TD
object must be validated in accordance with
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
createTD action in Example 3 and elaborated
PUTrequest 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).
POSTrequest. 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].
Registration of TDs using non-idempotent HTTP
POST method enables creation of anonymous TDs
id attribute). The
producer can distinguish between the created TDs
using the unique-system generated IDs given in
the response Location header.
A side-effect of this is that clients will be able to register duplicate TDs accidentally or on purpose.
Need to clarify:
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,
Content-Type header, and the requested TD in
body. The retrieve operation is specified as
retrieveTD property in Example 3.
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:
PUTrequest to the location corresponding to the existing TD. The request SHOULD contain
application/td+jsonContent-Type header for JSON serialization of TD. The TD object must be validated in accordance with § 220.127.116.11.6 Validation. Upon success, the server MUST respond with 204 (No Content) status. This operation is specified as
updateTDproperty 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
PATCHrequest 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+jsonContent-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
nullbut 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 § 18.104.22.168.6 Validation. Upon success, the server MUST respond with a 204 (No Content) status. This operation is specified as
updatePartialTDproperty in Example 3.
idnot found (for
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
deleteTD property in Example 3.
The listing endpoint provides a way to query the collection of TD objects from the directory. The Search API may be used to retrieve Partial TDs or TD fragments; see § 22.214.171.124 Search.
The list of TDs MUST be retrieved from the directory
using an HTTP
A successful response
MUST have 200 (OK) status,
Content-Type header, and an array of TDs in the
Serializing and returning the full list of TDs may be burdensome to servers. As such, servers should serialize incrementally and utilize protocol-specific mechanisms to respond in chunks. HTTP/1.1 servers SHOULD perform chunked Transfer-Encoding [RFC7230] to respond the data incrementally. Most HTTP/1.1 clients automatically process the data received with chunked transfer encoding. Memory-constrained applications which require the full list should consider processing the received data incrementally. Chunked transfer encoding is not supported in HTTP/2. HTTP/2 servers SHOULD respond the data incrementally using HTTP Frames [RFC7540].
The paginated list operation is specified as
retrieveTDs property in Example 3.
The syntactic validation of TD objects before storage is RECOMMENDED to prevent common erroneous submissions. The server MAY use Thing Description JSON Schema to validate standard TD vocabulary, or a more comprehensive JSON Schema to also validate extensions.
If the server fails to
validate the TD object, it MUST inform the client with
necessary details to identify and resolve the
errors. The validation error
MUST be described as
Problem Details [RFC7807]
with an extension field called
validationErrors, set to an array of
description fields. This is
necessary to represent the error in a
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 query and payload specification are subject to change for simplification and consistency with other API features (e.g. Anonymous TD, partial TD format [RFC7396]).
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
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
registration event in
typequery parameters. For example, in response to query
?type=created_td&type=deleted_td, the server must only deliver events of types
deleted_td. At the absence of any
typequery parameter, the server must deliver all types of events.
td_idquery parameters. For example, the query
?type=updated_td&td_id=urn:example:1234must result in
updated_tdevents for the TD identified with
sparqlquery parameters. If the server does not support a given search query parameter, it MUST reject the request with 501 (Not Implemented) status.
include_changesquery parameter is set to
true, the create event data object MAY include the created TD as the value of
include_changesquery 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
include_changesquery 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
To discuss further: Federated queries to other TDDs, Spatial and network-limited queries, Links
GETrequest. The request MUST contain a valid JSONPath [JSONPATH] as searching parameter. A successful response MUST have 200 (OK) status, contain
application/jsonContent-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
searchJSONPathproperty in Example 3.
List of errors:
GETrequest. The request MUST contain a valid XPath [xpath-31] as search parameter. A successful response MUST have 200 (OK) status, contain
application/jsonContent-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
searchXPathproperty in Example 3.
List of errors:
GETrequests. The support for SPARQL search using HTTP
POSTmethod is OPTIONAL.
UPDATEqueries 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
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
searchSPARQLproperty 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.
IANA will be asked to allocate the following values into
the Resource Type (
rt=) Link Target Attribute
Values sub-registry of the Constrained Restful Environments
(CoRE) Parameters registry defined in [RFC6690].
||Thing Description of a Thing||[wot-discovery], § 5.4 CoRE Link Format and CoRE Resource Directory|
||Directory Description of a Thing Description Directory||[wot-discovery], § 5.4 CoRE Link Format and CoRE Resource Directory|
DirectoryDescriptionidentifying TDs of directories and a type
LinkDescriptionto identify a TD that is just a link to another TD (useful for referring to remote directories in order to support federation).
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.