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.
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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 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, 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 present document uses the terminology defined in the WoT Architecture [wot-architecture] document, and also the additional terms defined here. The WoT prefix is used to avoid ambiguity for terms that are (re)defined specifically for Web of Things concepts.
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.
Any mechanism that results in a single URL. This includes Bluetooth beacons, QR codes, and written URLs to be typed by a user. A GET on all such URLs MUST result in a TD. 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
to advertise its presence. The Thing or Directory Service
registers its own Thing Description into the following path:
When the HTTP GET access is made to the above path, the
HTTP server MUST return a Thing
Description with the
content-type set to
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
And the Service Name to indicate the Directory Service
The Service Names
_directory._sub._wot are tentative. The
following Service Names are used in the existing
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
record that is pointed to by the Service Instance Name:
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 Thing Description with the Directory
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
The endpoint types
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.
To do: Describe mechanisms for devices to self-describe, hosting their own TDs.
To do: Describe mechanisms for TDs to be hosted in a searchable directory service.
A Directory can be distinguished from a Thing by the use
@type including the semantic term
To Do: Formal definition of information contained in a directory and its organization.
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
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:
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 2. 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 SHOULD be validated syntactically
Thing Description JSON Schema [WoT-Thing-Description].
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 2 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].
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
property in Example 2.
The API MUST allow modifications to existing
TDs as full replacement or partial updates.
The request SHOULD contain
application/td+json Content-Type header
for JSON serialization of TD. The update
operations are described below:
PUTrequest to the location corresponding to the existing TD. The TD object SHOULD be validated syntactically using the Thing Description JSON Schema [WoT-Thing-Description]. Upon success, the server MUST respond with 204 (No Content) status. This operation is specified as
updateTDproperty in Example 2.
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
acts as a create or update operation. An HTTP
PATCH may be used for an update-only
PATCHrequest to the location corresponding to the existing TD. The modified parts MUST be in Partial TD form and conform to the original TD structure. The input MAY include other existing parts of the TD or the whole TD object. When the whole TD object is provided as input, the operation acts as an update-only action. After applying the modifications, the TD object SHOULD be validated syntactically using the Thing Description JSON Schema [WoT-Thing-Description]. Upon success, the server MUST respond with a 204 (No Content) status. This operation is specified as
updatePartialTDproperty in Example 2.
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 as
deleteTD property in Example 2.
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
specifications to serve events to clients. In
particular, the server responds to successful requests
with 200 (OK) status and
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 Example
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 authorization.
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 2.
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 2.
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 2.
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].
To do, some discussion of general security and privacy concerns and mitigations. Note that the architecture above is designed to address many such points, for example the two-phase approach and "authorization before metadata release" principles, so this would be a summary and a recap.
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.