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: for distributing a single WoT
Thing Description from a regular web service, including as a
special case self-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.
Status of This Document
This section describes the status of this document at
the time of its publication. 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/.
Future updates to this specification may incorporate
new features.
Publication as a Proposed Recommendation does not imply
endorsement by W3C and its Members.
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.
The W3C
Membership and other interested parties are invited to review
the document and send comments through 10 July 2023. Advisory
Committee Representatives should consult their WBS
questionnaires. Note that substantive technical comments
were expected during the Candidate Recommendation review period
that ended 07 June 2023.
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.
The Web of Things (WoT) defines an architecture that
supports the integration and use of web technologies with IoT
devices. The WoT Architecture [wot-architecture11]
document defines the basic concepts and patterns of usage
supported. However, the WoT Thing Description [wot-thing-description11]
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 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
[Discovery-Categorization-IoT],
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-architecture11]
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 distribution of single TDs from regular
web services, with a special case of this being
self-description. Finally, we discuss some security and privacy
considerations, including a set of potential risks and
mitigations.
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, OPTIONAL,
RECOMMENDED, SHOULD, and SHOULD NOT
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 non-normative.
The fundamental WoT terminology such as Thing, Thing Description (TD), Thing
Model (TM), Property, Action,
Event, Anonymous TD, Discoverer, Discovery, Exploration, Introduction, Thing Description Server
(TD Server), Thing Description
Directory (TDD), Partial TD, Enriched TD are defined in Section
3 of the WoT Architecture 1.1 specification
[wot-architecture11].
4.
Architecture
This section is non-normative.
Figure 1 shows an overview of the WoT
Discovery process. Discovery uses a two-phase architecture to
resolve the competing requirements to be both open and to
restrict access to metadata to authorized entities. In the
first phase, one or more of a set of relatively open
"Introduction" mechanisms may be used to generate a set of
candidate URLs. These URLs do not themselves contain metadata,
but are used in the second stage to reference "Exploration"
services that can actually provide metadata, after
authentication, in the form of Thing Descriptions.
Figure
1Discovery architecture overview. A set of open
Introduction mechanisms provides a set of URLs, which point
at Exploration services that only provide metadata (TDs)
after suitable authentication. Thing Links and Thing
Description Directories provide additional flexibility but
retrieving further results from these is at the discretion
of the application.
The intention is that Introduction mechanisms are relatively
open "first contact" mechanisms to provide a starting point for
the rest of the Discovery process. In this document we specify
details on several Introduction mechanisms, suitable for
different use cases, including both local and non-local
scenarios, but Introductions can in fact be provided by any
mechanism that can return a URL. Introductions, however, do not
include any security or privacy controls and so should not
provide metadata directly. Instead, the URLs provided by
Introduction mechanisms reference "Exploration" services.
Exploration services actually do provide metadata, but only
after suitable authentication and access controls have been
applied.
The Discovery process can produce a set of URLs as
output from its Introduction phase, even if only one
Introduction mechanism is used (some Introduction mechanisms
can themselves return multiple URLs). The final output after
the Exploration phase can also be a set of Thing
Descriptions.
Each URL provided by the Introduction phase always points at
an Exploration service endpoint that will return a single Thing
Description. In the simplest case this URL references an
ordinary resource provided by a web server which provides the
Thing Description of a Thing describing an IoT endpoint device.
As a special case of this, for self-describing Things an
Introduction URL might point directly at an endpoint provided
by a Thing serving its own Thing Description.
In general Thing Descriptions might be provided in various
ways and in particular may not be self-describing. For
example,
brownfield devices not designed with the WoT
specification in mind will not be capable of serving their
own Thing Descriptions;
battery-powered devices may not be online most of the
time; and
some devices may not be powerful enough to manage and
serve TDs directly.
The Thing Description for such Things should be provided by
separate services.
This document specifies two special cases that allow for
more flexibility:
A Thing Description referenced by an Introduction URL may
describe a Thing Description
Directory service. A Thing Description Directory service
(which is still a Thing) maintains a (possibly dynamic)
database of Thing Descriptions. Thing Description Directories
that maintain large numbers of Thing Descriptions may also
support queries that can be used to selectively retrieve
Thing Descriptions.
The second special case is a Thing Link. A Thing Link
is also a Thing Description, but rather than describing a
Thing directly, it holds a link to a Thing Description hosted
elsewhere. A Thing Description Directory can also store Thing
Links which can redirect to other Thing Description
Directories, allowing for a linked directory structure.
It is not mandatory for the Discovery process to
retrieve the contents of Thing Description Directories and
return them as part of the results, because in general this
might result in a huge set of results. Instead the application
should scan the results for Thing Description Directory TDs and
decide whether to retrieve TDs from them, possibly selectively.
Likewise, it is not required to follow Thing Links
automatically; instead the application may choose to follow
them selectively.
5.
Discoverer Process
In this section we will describe the WoT Discovery process
from the point of view of a client, and what it means to say
that a client supports WoT Discovery. We will use the term
Discoverer for an entity
that is a client of the WoT Discovery process. A Discoverer may
or may not be a full Consumer. A Discoverer does however need
to read and extract information from special TDs for
Directories and Thing Links and use specific affordances and
links provided in them. Conversely, a Consumer may not support
Discovery, although it is recommended [wot-architecture11].
The WoT Discovery process is designed so that nearly any
client that can fetch a single TD given a single URI can be
said to support WoT Discovery. Of course, Discoverers may
support more powerful Discovery mechanisms, but some of these
have additional requirements. Some Introduction mechanisms can
return multiple URLs, each of which can in turn be used to
fetch at least one TD. So even without a TDD, it is possible to
discover multiple TDs.
The following assertions describe the specific
responsibilities of a Discoverer:
A Discoverer MUST support at least one Introduction
mechanism. The simplest Introduction mechanism,
Direct, simply provides a single URL of a target TD. This
assertion results in different minimal requirements depending
on which Introduction mechanism is selected out of the
several available. For example, when Direct is used as the
sole Introduction mechanism, at a minimum a Discoverer must
be able to accept a single URL pointing at a TD. Note,
however, that this TD could still be describing a Directory.
Another option would be for a Discoverer to implement, as its
sole Introduction mechanism, an "automatic" Introduction
mechanism such as DNS-SD, possibly in combination with mDNS.
In this case the application code running in the Discoverer
would not have to supply any additional information to invoke
Discovery.
A Discoverer MUST support fetching a TD from at least one
URL provided as part of the Introduction process. In
other words, not only must a Discoverer accept URLs pointing
at TDs, it must be able to fetch that TD, for example by
using a GET for an HTTP URL. It is permissible to split the
implementation of a Discoverer into two components, one that
fetches/reads a TD and sets up a configuration during
installation to be used by a run-time component that cannot
itself read TDs. In this case, however, the latter component
is not by itself a Discoverer.
A Discoverer MAY support multiple invocations of the same
Introduction mechanism. If multiple invocations of a
single Introduction mechanism is supported, or if multiple
Introduction mechanisms are used, then the Discoverer needs
to manage a set of Introduction results. Some Introduction
mechanisms themselves return a set of results, e.g. CoRE-RD,
DID, or DNS-SD. Some mechanisms, such as scanning Bluetooth
beacons, may intrinsically result in separate multiple
"invocations". However, this Discoverer requirement is
optional to support an important special case: it is
permissible to have a simple Discoverer that only supports a
single invocation of a Direct Introduction, directly
specifying a single TD via a URL.
A Discoverer MUST be able to merge URLs resulting from
multiple Introduction mechanisms, multiple results from a
single Introduction mechanism, and multiple Introduction
invocations into a single set. Note that even if only
one Introduction mechanism is supported multiple results
might be produced. This assertion states that the overall
output of the Introduction phase is a single set of URLs. The
word "set" is used here in the mathematical sense: the
results are unordered and unique. Uniqueness means that if
multiple Introductions have the same result, they are merged
into a single result. If the Discoverer only supports one
Introduction mechanism and that mechanism can only produce
one URL, then the "merge" is trivial.
A Discoverer MUST be able to identify whether a TD fetched
from an Introduction URL has Thing Directory or
Thing Link
type. This implies that the Discoverer needs to be
able to check the @type field and make this
distinction. A Discoverer can decide whether or not to follow
links or fetch TDD contents. There are some use cases where a
Consumer may not expand certain URLs, for example links
pointing at external resources, or when a TDD contains many
TDs and fetching them all would exceed the Consumer's memory
processing capabilities.
A Discoverer MAY fetch additional TDs from any Exploration
mechanism described in its initial set of TDs (including, in
particular, Thing Description Directories) and add them into
the set of TD results. This only adds the results of
fetching TDs from an Exploration mechanism to the set of
results. These new results do not delete the original TD
describing the Exploration mechanism. A Discoverer
implementation can decide whether or not to fetch TDD
contents. Implementers should consider the fact that TDDs may
be large and fetching the contents of an entire TDD might be
prohibitively expensive.
A Discoverer MAY fetch source TDs from the targets of the
links in a Thing Link
described in its initial set of TDs and add them into the set
of TD results. This only adds the results of fetching
TDs from a Thing Link to the set of results. These new
results do not delete the original TD describing the Thing
Link. A Discoverer implementation can decide whether or not
to fetch the targets of Thing Links.
A Discoverer MAY fetch additional TDs iteratively from any
Thing Link
or Exploration mechanism described in its set of TDs and add
them into the set of TD results. This only adds the
results of fetching TDs from an Exploration mechanism to the
set of results. These new results do not delete the original
TD describing the Exploration mechanism.
A Discoverer MAY terminate fetching additional TDs at any
point or for any reason.
A Discoverer MAY fetch additional TDs by following links or
fetching additional TDs from Exploration mechanisms (e.g.
TDDs) in any order. The Discoverer can fetch
additional TDs in any order from any Exploration mechanism
and is not required to use all of them.
A
Discoverer MUST track which TDs
describing links or Exploration mechanisms have already been
fetched and avoid fetching duplicate results. This is
to avoid infinite loops. Note that Thing Description
Directories can easily refer to each other, for example, so
fetching the TDs from a TDD, then fetching all those TDs will
give another copy of the original TDD's TD. This second copy
should NOT trigger further fetches. Note that tracking cannot
depend only on the value of "id" fields in TDs since this
field is optional.
The above process supports a way to let Directories
reference other Directories without duplicating their TDs: a
Directory wanting to reference other Directories should include
a Thing Link
with a "describedby" relation to the TD of the other Directory
service. Then the above process would expand the Thing Link to
obtain the actual TD of the Directory, and then (optionally)
use the appropriate Directory affordance to access the contents
of the linked Directory. Note that such a Thing Link points at
the TD of the Directory, not at the Directory itself. These may
or may not be hosted in the same location.
Recursively fetching the contents of such linked
directories, especially without a specific query or filter,
could easily result in downloading a large amount of data. Such
recursive expansion should be limited to use cases that require
it, such as inventory, auditing, or indexing.
URLs for Directory services can also be used with the
federation capabilities of SPARQL queries, noted below, which
in most cases will be a more efficient way to collect specific
information from a set of distributed directory services.
However, SPARQL requires the URL of a SPARQL endpoint for such
federation, which can be found in the TDs of Directories
supporting SPARQL queries. This is not the same as the URL
pointing at the TD of a Directory.
6. Introduction Mechanisms
This chapter describes mechanisms for initial contact with
Things or Thing
Description Directories. Any of the following mechanisms
may be provided by the Thing or the Thing
Description Directory to Consumers. The result of an
introduction mechanism is always a URL (address) of an
exploration service which can be used to obtain detailed
metadata (TDs) after suitable authentication. It is also
possible for multiple introduction mechanisms to be used and
the results merged. No particular introduction mechanism is
mandatory, as long as the URL of at least one exploration
service is somehow obtained.
6.1
Direct
To obtain an URL of an exploration
service, any mechanism that results in a single URL
MAY be used. 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 7. Exploration Mechanisms. 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 Thing Description of the Thing
Description Directory.
6.2
Well-Known URIs
A Thing or Thing
Description DirectoryMAY use
the Well-Known Uniform Resource Identifier [RFC8615]
to advertise its presence.If a
Thing or Thing
Description Directory use the Well-Known Uniform Resource
Identifier [RFC8615]
to advertise its presence, it MUST
register its own Thing Description into the following path:
/.well-known/wot.
When a request is
made at the above Well-Known URI, the server MUST return a Thing Description as prescribed
in 7. Exploration Mechanisms.
6.3 DNS-Based Service Discovery
A
Thing or Thing
Description DirectoryMAY use
DNS-Based Service Discovery (DNS-SD)[RFC6763].
This can be also be used on the same local network in
combination with Multicast DNS (mDNS)[RFC6762].
The following table lists the service names and protocols
for advertising their presense. These are normative since
each has a valid use with existing exploration
mechanisms:
Service name
Thing or TDD
Protocol
_wot._tcp
Thing
HTTP over TCP, HTTP over TLS/TCP, CoAP over TCP, or
CoAP over TLS/TCP
_directory._sub._wot._tcp
TDD
HTTP over TCP, HTTP over TLS/TCP, CoAP over TCP, or
CoAP over TLS/TCP
_wot._udp
Thing
CoAP over UDP or CoAP over DTLS/UDP
The following additional service name has been defined for
future use. This definition is however informative since
there is currently no defined directory service using CoAP
over UDP:
Service name
Thing or TDD
Protocol
_directory._sub._wot._udp
TDD
CoAP over UDP or CoAP over DTLS/UDP
For TCP-based services, the following information
MUST be included in the
TXT record that is pointed to by the Service
Instance Name:
td
Absolute pathname of the Thing Description of the Thing
or Thing Description of the Thing
Description Directory.
type
Type of the Thing Description, i.e.
Thing or Directory. If omitted,
the type is assumed to be Thing.
scheme
Scheme part of URL. One of the following values can
be specified, with the standard registered URI
interpretations [RFC7595]:
http (HTTP over TCP), https
(HTTP over TLS/TCP), coap+tcp (CoAP over
TCP), or coaps+tcp (CoAP over TLS/TCP). If
omitted, the scheme is assumed to be
http.
For UDP-based services, the following information
MUST be included in the
TXT record that is pointed to by the Service
Instance Name:
td
Absolute pathname of the Thing Description of the Thing
or Thing Description of the Thing
Description Directory.
type
Type of the Thing Description, i.e.
Thing or Directory. If omitted,
the type is assumed to be Thing.
scheme
Scheme part of URL. One of the following values can
be specified, with the standard registered URI
interpretations [RFC7595]:
coap (CoAP over UDP) or coaps
(CoAP over DTLS/UDP). If omitted, the scheme is assumed
to be coap.
Figure
2Using mDNS for discovery of the Thing
Description of a ThingFigure 3Using mDNS
for discovery of the Thing Description of a Thing
Description Directory (TDD)
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 Thing Description of the
Thing
Description DirectoryMUST be
wot.directory.
6.5
DID Documents
A
Thing or Thing
Description Directory using a Decentralized Identifier
(DID) [DID-CORE]
MAY advertise the location of its
TD by including a DID Service
Endpoint of type WotThing or
WotDirectory, respectively, in the DID Document
that the TD's identifier resolves to.
In order to define Service
Endpoints for WoT Discovery, the DID Document obtained by
resolving the DID of a Thing or Thing
Description DirectoryMUST
include the URL
https://www.w3.org/2022/wot/discovery-did in its
@context [did-spec-registries].
If the DID Document
obtained by resolving the DID of a Thing or Thing
Description Directory contains a Service Endpoint of type
WotThing or WotDirectory,
respectively, then this Service Endpoint MUST refer to the TD describing that Thing (when using
the WotThing service name) or the TD describing that Thing
Description Directory (when using the
WotDirectory service name), respectively
[did-spec-registries].
7. Exploration Mechanisms
This section defines the supported exploration mechanisms
after providing some common background material.
7.1
Overview
Figure 4The
high-level class diagram of the exploration mechanisms,
depicting how Thing Description Servers and Thing
Description Directories provide TDs. A Self-describing Thing is a
special case of a Thing Description Server that is also a
Thing. A Thing Description Directory acts as a Thing
Description Server for each Thing Description it
contains.
Figure 4 depicts the high-level
information model for TD Servers (serving single
TDs, including those for self-description) and Thing
Description Directory services. A Thing
Description Directory may contain TDs and at the same time is also a
Thing, which means it has its own TD. A directory also hosts
web service endpoints for retrieving individual TDs for other
Things and each of these can be used as a TD Server. A Thing
may in general host its own TD in which case it is a
Self-Describing Thing. Self-description is not mandatory for
directories, but Self-Describing Thing Description
Directories are possible that are both Thing Description
Directories and Self-Describing Things.
Figure 5The
ontology of Thing Descriptions in the Discovery
context.
Figure 5 illustrates the Discovery
ontology as an extension of the Thing ontology.
The ontology includes a class for metadata that are
associated with TDs stored in a directory. This class is
called RegistrationInformation and described
as part of the directory specification in 7.3.1.1 Registration
Information.
The Discovery ontology also defines two new Thing
Description classes, described in the following sections,
that may be used to model special exploratory metadata:
ThingDirectory
and ThingLink.
7.1.1.1
ThingDirectory
A TD which
describes a Thing Description Directory instance
MUST use type
ThingDirectory from the discovery context or
URI
https://www.w3.org/2022/wot/discovery#ThingDirectory.
A TD which describes
a reference to another TD MUST
use type ThingLink from the discovery
context or URI
https://www.w3.org/2022/wot/discovery#ThingLink.A Thing Link
MUST define the referenced TD as
a Link with describedby link relation type,
application/td+json media type and
href set to the target URL.
A Thing Link can be used in various scenarios. For
example:
A self-describing Thing with limited computational
resources intends to describe itself: host a minimal
TD (Thing Link) locally
and references a larger one with the full details
hosted at a different URL, perhaps in a directory.
A self-describing Thing or proxy has a very large
or dynamic description: registers a small or static
TD (Thing Link) in a
directory which references the actual TD hosted at the edge.
7.1.2 Security Bootstrapping
The purpose of an exploration service is to serve TDs,
but only after suitable authentication, and only to
authorized parties. However, in some cases a Discoverer may
not know what security credentials are needed to access a
TD via an exploration service, particularly in ad-hoc
scenarios. Since upon first access to an exploration
service a Discoverer will not yet have access to the TD if
suitable authentication credentials are not provided, the
Discoverer can't depend on the security metadata held in
TDs to know what kind of authentication and authorization
information is needed. If the Discoverer has no prior
knowledge, it will have to depend on existing security
negotation support to bootstrap access, at least to the TD
itself.
We define the following for the HTTP protocol, for which
security negotiation processes already exist. However, most
of the HTTP negotiation processes assume there is a human
user in the loop, but this is also appropriate for WoT
Discovery, since this problem will typically occur when a
user is trying to access a public WoT service or perform
integration of a new device. In this case the purpose of
negotiation is to provide guidance on what credentials are
needed to access the system.
In cases when exploration services are being used to
automate system management it would be best to
pre-establish what credentials (and authentication
mechanisms) are needed to access the relevant exploration
services and security bootstrapping would not be required.
For this reason security bootstrapping is not a mandatory
feature, and can be omitted or disabled on devices that are
to be used with pre-established security mechanisms.
Security bootstrapping may also only be necessary on the
first access to a TD. Once a Discoverer has
determined what credentials and authentication mechanism
are required to access a particular exploration service,
they can retain this information and attempt to use them
for future accesses. Note however that depending on the
security scheme used, credentials themselves may expire and
may need to be re-established periodically.
Security bootstrapping
MAY be provided on any HTTP
endpoint that serves a TD. As mentioned above,
disabling or omitting security bootstrapping is permissible
if security mechanisms have been previously established.
For example, if an installation wants to use the OAuth2
client flow and provide potential clients with
an address of an authentication server to use in advance,
then security bootstrapping can be disabled, since the
alternative would be to include other (and potentially
weaker) forms of authentication.
In the HTTP protocol, the authentication and
authorization mechanisms to be used can generally be
negotiated by the HTTP server returning a "401
(Unauthorized)" response code in conjunction with a
WWW-Authenticate header that specifies the
information required. To gain access, the client then needs
to make another request with the necessary information.
There are several authentication schemes registered with
IANA. However, not all of these are in wide use, some
are experimental, and there is only partial overlap with
the schemes supported by TDs. Also, note that the
oauth scheme in the IANA registration refers
to OAuth1, which is deprecated, so it should not be used.
The relevant OAuth2 flow, the code flow,
instead of a 401 response begins with a redirection to an
authentication server, eventually resulting in credentials
(bearer tokens in the case of WoT) that can be used for
access.
Given these considerations, to enable security
bootstrapping on a wide variety of devices as well as on
browsers, the following constraints should be observed:
If security bootstrapping is
enabled on an exploration service, after initial
contact using the URL provided by an introduction
mechanism, the exploration service MUST reply with an HTTP response code
appropriate for one of the Basic, Bearer, Digest, or
OAuth2 security schemes if authentication information
has not been provided but access can be granted when it
is. Note that if the exploration service does
not want to provide access for some reason, or if
security bootstrapping is disabled, it can ignore the
request or reply with another code such as 404 or 403.
Also, if no authentication is required, then the system
can reply immediately with the requested TD as if
authentication information were provided. Bypassing
authentication however is only appropriate if the TD
served as a response does not contain and cannot be
used to infer Personally Identifiable Information; see
9. Privacy
Considerations.
If security bootstrapping is enabled on an
exploration service using one of the following
IANA-registered HTTP Authentication Schemes: Basic,
Bearer, or Digest, then (following the standards for
these authentication schemes) a 401 HTTP response at an
API endpoint intended to serve a TD must include a
WWW-Authenticate header and any other
headers describing the required authorizations. For
details of the requirements, the IANA registry should be
consulted for each of the above authentication schemes.
Likewise, following the OAuth2 specifications, if the
OAuth2 code flow is used during security
bootstrapping, the "302 (Found)" or "303 (See Other)"
response code must be used for redirection to the
authentication server, with access credentials eventually
being represented with bearer tokens. Note that the other
OAuth2 flow supported in WoT Thing Description 1.1,
client, expects the initial access to be to
the authentication server, not the final endpoint, and so
cannot be used via security bootstrapping. These
requirements also apply only to endpoints of exploration
services that might need to support security
bootstrapping, that is those that serve TDs, not to other
endpoints that might be provided by the same exploration
service. In particular, these requirements apply only to
URLs that can be referenced by introduction mechanisms,
not to (for example) event subscription endpoints.
There are relevant Security and Privacy Considerations
in [wot-architecture11]
and [wot-thing-description11]
regarding when authentication is required for access to TDs
and the use of secure transport. See also 9. Privacy Considerations. In summary,
secure transport (e.g. TLS) is required for public services
and strongly recommended even on private networks (even if
there is no authentication requirement, to protect the
confidentiality of queries), and serving requests without
authentication and authorization should only be considered
in limited circumstances when no Personally Identifiable
Information is present or can be inferred.
7.2 Thing Description Server
Any web service that can be referenced by a URL and return
a TD with appropriate authentication and access controls can
be used as an exploration mechanism. We will refer to this as
a Thing Description Server or TD Server. A TD Server does not
need to be a Thing. In particular, a TD can be hosted on an
ordinary web server and referenced by its URL.
A TD Server can be used to support self-description. For
self-description, a Thing hosts
its own TD and makes it available via
a web resource identified with a URL. Such a web resource
however is not included as an affordance in the TD itself.
This web resource may or may not be the same as the
well-known URL used as an Introduction mechanism defined in
6.2 Well-Known
URIs.
Use of secure transport is subject to assertions given in
the Security Considerations and Privacy Considerations
sections of the [wot-architecture11]
and [wot-thing-description11]
specifications, which define scenarios where secure transport
is recommended or mandatory and mutual authentication is
recommended.
A TD Server distributing a TD using the following
protocols is subject to the following constraints:
HTTP
An HTTP-based TD Server
providing a TDMUST serve that resource with a
GET method.A
successful response from an HTTP-based TD Server
providing a TDMUST have 200 (OK) status and
the TD in the body.A successful
response with JSON serialization MUST contain either
application/json or
application/td+json in the Content-Type
header. Here application/td+json is
preferred as it is more specific and implies
application/json. The default
serialization format for successful response bodies
MUST be JSON, with JSON-LD 1.1
[JSON-LD11] syntax. The
JSON-LD syntax allows semantic extensions and processing.
An HTTP-based
TD Server providing a TDMAY
provide alternative representations through server-driven
content negotiation, that is by honoring the request's
Accept and Accept-Encoding headers and responding with
the supported TD serialization and equivalent
Content-Type and Content-Encoding headers. In
addition, following the process normatively defined in
the WoT Thing Description 1.1 specification
[wot-thing-description11],
an HTTP-based TD Server providing a TD may provide modified TDs or
error responses using a different default language after
server-driven content negotiation, that is by honoring
the request's Accept-Language header.
An HTTP-based TD Server
providing a TDMUST respond to
HEAD requests by returning only the headers
equivalent to those returned by a GET
request to the same endpoint. This enables clients
to retrieve HTTP headers such as the Content-Length in
advance to know the size of the TD (in bytes) and decide on an
efficient query strategy.
In constrained environments, a single TD may be too large to process
for the server or clients. See 10.1 Incremental
Transfer for protocol-specific recommendations on
incremental transfer of the requested payload.
Error responses:
401 (Unauthorized): No authentication.
403 (Forbidden): Insufficient rights to the
resource.
CoAP
A CoAP-based TD Server
providing a TDMUST serve that resource with a
GET method.A
successful response from a CoAP-based TD Server providing
a TDMUST have a 2.05 (Content) status, contain
a Content-Format option with value 50
(application/json) or 432
(application/td+json), and the TD in the payload.
Content-Format 432 is preferred as it is more specific
and implies Content-Format 50. Note that the payload
might be split over multiple message exchanges using
block-wise transfer [RFC7959].
A CoAP-based
TD Server providing a TDMAY
provide alternative representations through server-driven
content negotiation, that is by honouring the request's
Accept option and responding with the supported TD serialization and equivalent
Content-Format option.
A CoAP-based TD Server providing a TDSHOULD respond to requests containing a
Size2 option by including the size estimate of the
TD in its next
response. This is relevant when obtaining a
TD using block-wise
transfer and enables clients to abort the retrieval if
the total payload size should be too large for them to
process.
In constrained environments, a single TD may be too large to process
for the server or clients. See 10.1 Incremental
Transfer for protocol-specific recommendations on
incremental transfer of the requested payload.
Error responses:
4.01 (Unauthorized): No authentication.
4.03 (Forbidden): Insufficient rights to the
resource.
7.3 Thing Description Directory
A Thing
Description Directory (TDD or Directory for short) is a
Thing that provides a service to
manage a set of TDs describing other Things.
The ontology of a TD in the Discovery context was
introduced in
Figure 5. The
RegistrationInformation class is associated
with TDs that
are stored in a directory. The following table lists the
registration information attributes for use within
TDs that embed or
reference the Discovery context. Note that only an
Enriched
TD embeds the registration information. An
Enriched
TDMUST contain in its
@context the URI
https://www.w3.org/2022/wot/discovery.
In this table, client refers to the producer or consumer
of a TD and
server refers to the Thing
Description Directory.
Whenever an absolute time is
expressed using
dateTime it MUST be
interpreted as date-time as specified in [RFC3339].
Specifically, the timezone offset is not optional.
Vocabulary term
Description
Client Assignment
Server Assignment
Type
created
Provides the absolute time when the TD instance
was created inside the directory.
This MAY be set by
the directory and returned to consumers.
Provides the absolute time when the TD instance
registration expires.
The producer MAY set
this to indicate the absolute expiry time during
the registration.
For servers that support expirable TDs: If
ttl (relative expiry) is present,
the server MUST ignore
client assignments to expires and
instead compute and set it internally.
Time-to-live: relative amount of time in seconds
from the registration time until when the TD
instance registration expires.
The producer MAY set
this to indicate the relative expiry time during
the registration.
For servers that support expirable TDs: The
server MUST use
ttl to calculate the
expires (absolute expiry) value.
optional
read-only
number
retrieved
The absolute time at which the TD was retrieved
from the server.
This is useful for clients that intend to
process other absolute timestamps but do not have
an internal clock or other means of acquiring the
current time.
Producers can set the expiry time to inform the
directory and other consumers about the validity of the
TD registrations. The expiry is also a useful indicator
to inform the consumers about expiry of dynamic TDs, e.g. when changes to
metadata such as geolocation or properties are expected
to be valid for a limited period. Consumers may rely on
the expiry time to know how long a retrieved TD will be valid and when they
need to request a more recent one. Consumers who retrieve
an expired TD may consider it as metadata of an inactive
client.
For the servers, the expiry time is useful for
implementing automatic removal of obsolete or accidental
registrations. Servers SHOULD periodically purge TDs that are
past their expiry times. Prescribing a global
mandate or upper limit for the expiry time is
application-specific and beyond the scope of this
specification. Servers may mandate or set a configurable
upper limit to expiry times and refuse incompliant
requests. The purging by servers is particularly
beneficial when interacting with clients (e.g. IoT
devices) that are unable to explicitly deregister their
TDs. This could be due
to protocol-specific limitations, failure, destruction,
or ungraceful decommissioning. Such clients should set a
reasonably short expiry time and periodically extend it
during the normal operation. The expiry can be extended
by updating the registration either fully or partially,
including an update that makes no changes to the TD; see
7.3.2.1.3 Update.
If a client ceases to operate, a directory with purging
capability will automatically remove its
registration.
7.3.1.3 Anonymous TD
Identifiers
The directory assigns local identifiers to Anonymous
TDs to enable management and retrieval of such TDs
from the directory. In situations where the
server exposes an Anonymous TD (e.g. retrieval, listing,
search), it MUST add the local
identifier as id of the TD to allow local
referencing.The local identifier
SHOULD be a UUID Version 4,
presented as a URN [RFC4122].
UUID Version 4 is a random or pseudo-random number which
does not carry unintended information about the host or
the resource.
7.3.2 Directory Service API
Directory services provide access to System
User Data and need to provide appropriate security and
privacy protections. The use of secure transport protocols
and access controls for authenticity and confidentiality in
implementations of the WoT Directory Service API are
governed by the Security Considerations and Privacy
Considerations given in [wot-architecture11].
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. All HTTP API error
responses described using Problem Details MUST be encoded using UTF-8.HTTP API error responses
MAY report details in different
languages using proactive negotiation, if the
Accept-Language header field has been set in
the HTTP request [RFC7231].
The APIs set the HTTP status codes as defined in Section
6 of [RFC7231].
The list of used error codes include (but are not limited
to) the following:
400 (Bad Request): Invalid client input in body,
query, or headers. This is accompanied by an appropriate
response message.
401 (Unauthorized): The request lacks valid
authentication credentials. As noted in 7.1.2 Security
Bootstrapping, this is the first step in
authentication negotiation, needed to bootstrap secure
access to TDs. Information on what credentials are
required will be included in a
WWW-Authenticate header.
403 (Forbidden): Insufficient rights to access the
resource.
404 (Not Found): TD or endpoint does not exist. This
is accompanied by an appropriate response message.
For
each HTTP endpoint that responds to the GET
method, the server MUST accept
HEAD requests and return only the
headers. This allows clients to retrieve headers
such as the Content-Length without receiving the body and
decide on a suitable strategy to query the information. For
example, a constrained client can request only the
necessary parts of an object (using an appropriate search
query) or retrieve a list of items in small subsets.
In constrained environments, a single TD may be too
large to process for the server or clients. This affects
both read (i.e. retrieving one or more TDs or TD fragments)
and write (i.e. submitting a TD or Partial TD) operations.
See 10.1 Incremental
Transfer for protocol-specific recommendations on
incremental transfer of the payloads.
To ensure the safe transmission of data over the URL, it
is expected that both client and server percent
encode/decode characters that conflict with delimiters in
the rest of the URL. These characters are defined as
unsafe in Section 2.2 of [RFC1738].
Unsafe characters can result in unexpected behavior if they
appear in the URL, for example, when the resource ID
included in the path is a URL by itself, or in the search
query string.
The directory APIs include mandatory, recommended, and
optional features. When a directory is unable
to answer a request because of unsupported recommended or
optional features, it SHOULD
inform the client about the absence of those features by
returning appropriate HTTP errors. The following
examples can be used as guideline for implementations:
If the missing feature is to customize functionality
of an existing API use 400 (Bad Request) or 501 (Not
Implemented).
If an API endpoint is not provided (e.g.
/search/sparql endpoint), use 404 (Not
Found).
If a method is not supported on an existing API
endpoint (e.g. PATCH for
/things endpoint), use 405 (Method Not
Allowed).
The process of modifying the default language of a TD
using translations already provided in a TD is normatively
described in the WoT Thing Description 1.1 specification
[wot-thing-description11].
Using this process, a Directory server may provide modified
TDs, or its own TD, using a different default language
after server-driven content negotiation, that is by
honouring the request's Accept-Language header.
7.3.2.1 Things API
The Things API is a RESTful HTTP API served at the
/things endpoint providing interfaces to
create, retrieve, update, delete, and list (CRUDL)
TDs. The design of this
API is in accordance with [RFC7231]
and [REST-IOT].
The HTTP API follows these general rules:
The API MUST provide the interface to list
TDs. The Search API allows filtering and
selection from this list; see 7.3.2.3
Search API.
The API MAY
provide the interfaces to create, read, update, and
delete (CRUD) individual TDs.
A directory that provides both read and write over
HTTP is considered a full HTTP directory.
Full HTTP directories SHOULD implement all of CRUDL (create,
read, update, delete, and list) interfaces. It
is practical to implement only the interfaces to read
and list if the directory serves a static collection
of TDs.
This is also useful for directories that intend to
expose only retrieval operations over HTTP, and
perform other operations via out-of-band mechanisms.
To expose read-only
access, the directory MUST
enforce access control on create, update, and delete
interfaces.
The default
serialization format for all request and success
response bodies MUST be JSON,
with JSON-LD 1.1 [JSON-LD11] syntax to support
extensions and semantic processing.
Directories
MAY accept alternative
representations based on request's indicated
Content-Type or Content-Encoding headers. This
is useful for applications that need to provide
representations other than raw JSON as input to a
directory.
Directories
MAY provide alternative
representations through server-driven content
negotiation, that is by honouring the request's Accept
and Accept-Encoding headers and responding with the
supported TD representation and equivalent Content-Type
and Content-Encoding headers. This is useful for
applications that need to retrieve representations
other than raw JSON from a directory, such as
Gzip-compressed JSON.
The CRUDL operations are described in the following
sections:
7.3.2.1.1 Creation
Creation refers to the registration of a new
TD inside the
directory.
The TD object is validated in accordance with
7.3.2.1.6
Validation. Note that a TD may or may not be
generated by the Thing it describes. For
brownfield devices in particular a separate Discoverer process
or service may be required that generates and registers
a TD for a Thing
on its behalf.
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 elaborated
below:
A TD that has an
idMUST be
submitted to the directory in the body of an HTTP
PUT request at
/things/{id} endpoint, where
id is the unique TD identifier,
present inside the TD object. An Anonymous
TD is handled differently; see below.
The request
SHOULD contain
application/td+json Content-Type
header for JSON serialization of TD. The TD
object is validated in accordance with 7.3.2.1.6
Validation. 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).
An Anonymous
TDMUST be submitted
to the directory in the body of an HTTP
POST request at /things
endpoint.The
request SHOULD contain
application/td+json Content-Type
header for JSON serialization of TD. The TD
object is validated in accordance with 7.3.2.1.6
Validation. The directory
MUST assign a local
identifier to any Anonymous
TD to enable local management and retrieval
from the directory. The scheme of the
system-generated ID is described in 7.3.1.3
Anonymous TD Identifiers. Upon
successful processing, the server MUST respond with 201 (Created)
status and a Location header containing the
system-generated URI of created TD resource.
The system-generated URI includes the
system-generated identifier of the TD and can be
used subsequently to query the TD from the
directory.
A server that supports expirable TDs will realize
such functionality as described in 7.3.1.2
Registration Expiry. In particular, if
ttl (relative expiry) is given during the
creation, such servers will calculate and store the
expires value.
7.3.2.1.2 Retrieval
The retrieval of an existing TD
MUST be done using an HTTP
GET request at /things/{id}
endpoint, where id is the unique TD
identifier.A successful response
MUST have 200 (OK) status and
the requested TD in the body.A successful
response with JSON serialization MUST contain either
application/json or
application/td+json in the Content-Type
header. Here application/td+json is
preferred as it is more specific and implies
application/json. Note that the default
serialization is JSON with JSON-LD syntax, and
alternative serializations can be negotiated; see
7.3.2.1 Things
API.
This is an Enriched
TD which includes the registration information such
as the creation and modification time of the TD within
the directory.
The example below shows a retrieved Anonymous
TD that is in Enriched
TD form and has local identifier
urn:uuid:48951ff3-4019-4e67-b217-dbbf011873dc.
The following is an example of a retrieved TD that
was registered with a relative expiry time of 3600
seconds (one hour). The server has calculated the
absolute expiry time as one hour after the modification
time.
For the sake of readability, the time values in this
example are set to exact numbers. In realistic
settings, time values may include fractions.
7.3.2.1.3 Update
The update operations are to replace or partially
modify an existing TD.
The update operations are described below:
A modified TD MUST replace an existing one when
submitted using an HTTP PUT request at
/things/{id} endpoint, where
id is the identifier of the existing
TD.The request
SHOULD contain
application/td+json Content-Type
header for JSON serialization of TD. The TD
object is validated in accordance with 7.3.2.1.6
Validation. Upon success, the server
MUST respond with 204 (No
Content) status.
A server that supports expirable TDs will
realize such functionality as described in 7.3.1.2
Registration Expiry. If ttl
(relative expiry) is set during the update
operation, the server will calculate and set the
expires (absolute expiry) value.
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.
An existing TD
MUST be partially modified
when the modified parts are submitted using an HTTP
PATCH request at
/things/{id} endpoint, where
id is the identifier of the existing
TD.The partial
update MUST be processed
using the JSON merge patch 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 does 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 is validated in
accordance with 7.3.2.1.6
Validation. Upon success, the
server MUST respond with a
204 (No Content) status.
A server that supports expirable TDs will
realize such functionality as described in 7.3.1.2
Registration Expiry. During the partial update
operation, if the resulting TD has ttl
(relative expiry), the server will calculate and
set a new expires (absolute expiry)
value.
A patch operation is particularly useful to
efficiently extend the expiry of a registration
that uses a ttl (relative expiry)
value. This is typically done by submitting an
empty merge patch document, i.e. an empty JSON
object. This effectively translates to performing a
partial update operation that updates nothing, but
triggers the recalculation of expires
(absolute expiry) value. This expiry functionality
only works if the server supports it as defined in
7.3.1.2
Registration Expiry.
The following example is a merge patch document
to update only the base and
registration expires fields of a
TD:
7.3.2.1.4 Deletion
A delete operation MUST be done using an HTTP
DELETE request at
/things/{id}, where id is the
identifier of the existing TD.A
successful response MUST have
204 (No Content) status. The retrieve operation
is specified as deleteThing property in
7.3.2.4 API Specification (Thing
Model).
7.3.2.1.5 Listing
The listing endpoint provides different ways to
query the collection of full TD objects from the
directory.
In many scenarios, retrieving parts instead of full
TD objects is preferred because only a subset of
elements are needed (e.g. id and
href of a property for all TDs) and to
save networking resources. The Search API allows
querying parts of TD objects; see 7.3.2.3 Search
API.
The directory MUST allow retrieval of existing TDs
using HTTP GET requests at the
/things endpoint.A
successful response MUST have
200 (OK) status and an array of TDs in the body.A successful
response with JSON serialization MUST contain either
application/json or
application/ld+json in the Content-Type
header. Here application/ld+json is
preferred as it is more specific and implies
application/json. Note that the default
serialization is JSON with JSON-LD syntax, and
alternative serializations can be negotiated; see
7.3.2.1 Things
API.
There may be scenarios in which clients need to
retrieve the collection in small subsets of TDs. While
the Search API (7.3.2.3 Search
API) does offer the ability to query a specific
range, it may not be optimal, nor developer-friendly.
The server MAY support pagination to return the
collection in small subsets. The pagination must
be based on the following rules:
When the
limit query parameter is set to a
positive integer, the server MAY respond with a subset of TDs
totalling to less than or equal to the requested
number.
When
there are more TDs after a returned subset of the
collection, the response MUST contain a next Link
header [RFC8288] with the URL of the
next subset.The
next link MUST
include all arguments needed to produce the same set
of data and its ordering, in particular the same
limit argument given on the initial
request as well as a zero-based offset
argument anchored at the beginning of the next
subset.The
link MUST be absolute or
relative to directory API's base URL.
Moreover, it may include additional arguments that
are necessary for ordering or session
management.
All
paged responses MUST contain
a canonical Link header [RFC8288] pointing to
the collection and include an etag
parameter to represent the current state of the
collection. The link may be absolute or
relative to directory API's base URL. The
etag value could be a revision number,
timestamp, or UUID Version 4, set whenever the TD
collection changes in a way that affects the ordering
of the TDs. The clients may rely on the
etag value to know whether the
collection remains consistent across paginated
retrieval of the collection. For example, creation or
deletion of TDs or update of TD fields used for
ordering may shift the calculated paging window.
The
collection MUST be sorted in
ascending order using Unicode code point order by the
unique identifier of TDs.
This above specification follows a subset of Linked
Data Paging [LDP-Paging]
to allow optional pagination of the JSON-LD array.
Additional parts of Linked Data Paging may be
implemented for examples to honour client's query
preference or to add other link relations for semantic
annotation and alternative navigation links.
The following example provides a walk-through of the
paginated retrieval of TDs:
As an
alternative to an array of TDs as the body of the
response, the server MAY send
a more verbose payload allowing server-side
information, such as pagination information, to be
included in addition to the actual data.
The alternative pagination format is inspired by
Hydra Advanced Concepts, more concretely the
Partial Collection View. Adapted to our purposes
and using the members field to accomodate the
array of TDs, it looks as follows for the listing
endpoint:
To tell the server which format to send, the
additional query parameter
?format=array|collection can be added to
the request. ?format=array is the default
parameter, does not have to be provided explicitly, and
yields to a server response of the pure array of TDs.
?format=collection should yield to a
server response with the format as described in
Example 9.
Additional forms of validation can be added to
support various use cases. For example, a use case may
require stateful validation of the input TDs to ensure
that the version value is initialized and
updated according to pre-defined rules.
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
objects with field and
description fields. This is
necessary to represent the error in a machine-readable
way. All validation error
responses described using Problem Details MUST be encoded using UTF-8. As
already stated in general for error reporting,
validation error responses may report details in
different languages using proactive negotiation, if the
Accept-Language header field has been set
in the HTTP request [RFC7231].
Example 10 is an example
error response with two validation errors.
7.3.2.2 Events API
The Notification API is to notify clients about the
changes to TDs
maintained within the directory. Directories
MAY implement the Notification
API.
The Notification API MUST follow the Server-Sent Events (SSE)
[HTML]
specifications to serve events to clients at
/events endpoint. 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. The server SHOULD provide an event ID as the
id field in each event and respond to
re-connecting clients by delivering all missed
events.
The rest of this section describes the implementation
details on top of the SSE protocol. Realizing the
notification functionality using other protocols such as
MQTT are possible and may be formalized in future
versions of this specification.
Event Types
The server MUST produce events attributed to the
lifecycle of the Thing Descriptions within the
directory using thing_created,
thing_updated, and
thing_deleted event types.
Event Filtering
The API enables server-side filtering of events to
reduce resource consumption by delivering only the
events required by clients. Client libraries may
offer additional filtering capabilities on the
client-side.
The server MUST support event filtering based on
the event type given by the client upon
subscription.
For example, given the URI Template
/events{/type}:
/events/thing_created instructs
the server to only deliver events of type
thing_created
/events instructs the server to
deliver all events
The clients need to subscribe separately to
receive a subset of the events (e.g. only
thing_created and
thing_deleted) from the server. When
using HTTP/2, multiple subscriptions on the same
domain (HTTP streams) get multiplexed on a single
connection.
Event Data
The event data MUST contain the JSON serialization of
the event object. The event data object is a
Partial
TD or the whole TD object depending on the
request:
The event data
object MUST at least
include the identifier of the TD created,
updated, or deleted at that event in Partial
TD form.
When
diff query parameter is set to
true and the event has
thing_created type, the server
MAY return the whole TD
object as event data.
When
diff query parameter is set to
true and the event has
thing_updated type, the server
MAY inform the client
about the updated parts following the JSON Merge
Patch [RFC7396]
format.A
thing_updated event data that is
based on JSON Merge Patch [RFC7396] MUST always include the identifier
of the TD regardless of whether it is
changed.
The following example shows the event
triggered on update of the TD from Example 12:
The
diff query parameter MUST be ignored for
thing_deleted events. In other
words, the server shall not include additional
properties in the payload of
thing_deleted events when
diff is set to true.
When a server which does not support the
diff query parameter is requested with
such query parameter, it should reject the request.
This is to inform clients about the lack of such
functionality at connection time and avoid runtime
exceptions caused by missing event data
attributes.
The Notification API is specified as three event
affordances in 7.3.2.4 API
Specification (Thing Model), namely:
thingCreated, thingUpdated, and
thingDeleted.
Note: SSE
Authorization Header
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.
7.3.2.3 Search API
This specification describes three search APIs:
syntactic search with JSONPath [JSONPATH],
syntactic search with XPath [xpath-31], and
semantic search with SPARQL [sparql11-overview].
The Directory MAY implement semantic search with
SPARQL. JSONPath and XPath are informative and
subject to change.
It is RECOMMENDED that directories implement a
search API to efficiently serve TDs based on client-specific
queries.
7.3.2.3.1 Syntactic search:
JSONPath
This section is non-normative.
Support
for JSONPath Search API is optional. If implemented,
the JSONPath API must allow searching TDs using an HTTP
GET request at
/search/jsonpath?query={query} endpoint,
where query is the JSONPath expression.
The request must contain a valid JSONPath
[JSONPATH]
as searching parameter. A successful response must have
200 (OK) status, contain application/json
in the Content-Type header, and a set of complete TDs
or a set of TD fragments in the body. The syntactic
search with JSONPath is specified as
searchJSONPath action in 7.3.2.4 API Specification (Thing
Model).
7.3.2.3.2 Syntactic search:
XPath
This section is non-normative.
Support
for XPath Search API is optional. If implemented, the
XPath query API must allow searching TDs using an HTTP
GET request at
/search/xpath?query={query} endpoint,
where query is the XPath expression. The
request must contain a valid XPath 3.1 [xpath-31] as
search parameter. A successful response must have 200
(OK) status, contain application/json in
the Content-Type header, and the query response in JSON
serialization in the body. The data schema for the
response is defined implicitly by the query and the
XPath specification. The syntactic search with XPath is
specified as searchXPath action in
7.3.2.4 API Specification (Thing
Model).
7.3.2.3.3 Semantic search:
SPARQL
Support for SPARQL Search API is optional.
If implemented, the SPARQL
search API MUST allow
searching TDs using the SPARQL 1.1 protocol
[sparql11-overview].The SPARQL API
MUST accept queries using HTTP
GET requests at
/search/sparql?query={query} endpoint,
where query is the SPARQL
expression.The support for SPARQL
search using HTTP POST method at
/search/sparql endpoint is OPTIONAL.A successful
request with a query SELECT or
ASKMUST return a
response 200 (OK) status, and contain
application/json by default in the
Content-Type header.A
successful request with a query CONSTRUCT
and DESCRIBEMUST
return a response 200 (OK) status, and contain
application/ld+json by default in the
Content-Type header.A
request with any query different from
SELECT, ASK,
CONSTRUCT or DESCRIBEMUST return a response 400
(Bad Request). The semantic search with SPARQL
is specified as searchSPARQL action in
7.3.2.4 API Specification (Thing
Model). A WoT Thing Description
Directory MAY implement
federation in its SPARQL query API.If implemented,
the SPARQL API MUST implement
the SPARQL 1.1 Federated Query standard
[sparql11-overview].
7.3.2.4 API Specification
(Thing Model)
A template for the API of Thing Description
Directories is given here as a Thing Model. The Thing
Model is normative (except where noted) but should not be
considered as the sole reference to implement or interact
with a Thing Description Directory. Please refer also to
the specifications given in 7.3.2 Directory
Service API.
The searchJSONPath and
searchXPath affordances given in this Thing
Model are not normative and are provided for information
only.
Note that a contentType is required by
the WoT TD 1.1 specification [wot-thing-description11]
for instances of the ExpectedResponse class
(i.e., the JSON object corresponding to the key
response in a form), even when the body of
the HTTP response is supposed to be empty. In this case,
application/x-empty can be used to ensure
that the TD generated from the TM below is valid.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
"https://www.w3.org/2022/wot/discovery"
],
"@type": [
"tm:ThingModel",
"ThingDirectory"
],
"title": "Thing Description Directory (TDD) Thing Model",
"version": {
"model": "1.0.0"
},
"base": "{{DIRECTORY_BASE_URL}}",
"tm:optional": [
"/actions/createThing",
"/actions/createAnonymousThing",
"/actions/retrieveThing",
"/actions/updateThing",
"/actions/partiallyUpdateThing",
"/actions/deleteThing",
"/actions/searchJSONPath",
"/actions/searchXPath",
"/actions/searchSPARQL",
"/events/thingCreated",
"/events/thingUpdated",
"/events/thingDeleted"
],
"properties": {
"things": {
"description": "Retrieve all Thing Descriptions",
"uriVariables": {
"offset": {
"title": "Number of TDs to skip before the page",
"type": "number",
"default": 0
},
"limit": {
"title": "Number of TDs in a page",
"type": "number"
},
"format": {
"title": "Payload format",
"type": "string",
"enum": [
"array",
"collection"
],
"default": "array"
}
},
"forms": [
{
"href": "/things{?offset,limit,format}",
"htv:methodName": "GET",
"response": {
"description": "Success response",
"htv:statusCodeValue": 200,
"contentType": "application/ld+json",
"htv:headers": [
{
"htv:fieldName": "Link"
}
]
},
"additionalResponses": [
{
"description": "Invalid query arguments",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
}
},
"actions": {
"createThing": {
"description": "Create a Thing Description",
"uriVariables": {
"id": {
"@type": "ThingID",
"title": "Thing Description ID",
"type": "string",
"format": "iri-reference"
}
},
"input": {
"description": "The schema is implied by the content type",
"type": "object"
},
"forms": [
{
"href": "/things/{id}",
"htv:methodName": "PUT",
"contentType": "application/td+json",
"response": {
"description": "Success response",
"htv:statusCodeValue": 201
},
"additionalResponses": [
{
"description": "Invalid serialization or TD",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
},
"createAnonymousThing": {
"description": "Create an anonymous Thing Description",
"input": {
"description": "The schema is implied by the content type",
"type": "object"
},
"forms": [
{
"href": "/things",
"htv:methodName": "POST",
"contentType": "application/td+json",
"response": {
"description": "Success response including the system-generated URI",
"htv:headers": [
{
"description": "System-generated URI",
"htv:fieldName": "Location"
}
],
"htv:statusCodeValue": 201
},
"additionalResponses": [
{
"description": "Invalid serialization or TD",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
},
"retrieveThing": {
"description": "Retrieve a Thing Description",
"uriVariables": {
"id": {
"@type": "ThingID",
"title": "Thing Description ID",
"type": "string",
"format": "iri-reference"
}
},
"output": {
"description": "The schema is implied by the content type",
"type": "object"
},
"safe": true,
"idempotent": true,
"forms": [
{
"href": "/things/{id}",
"htv:methodName": "GET",
"response": {
"description": "Success response",
"htv:statusCodeValue": 200,
"contentType": "application/td+json"
},
"additionalResponses": [
{
"description": "TD with the given id not found",
"contentType": "application/problem+json",
"htv:statusCodeValue": 404
}
]
}
]
},
"updateThing": {
"description": "Update a Thing Description",
"uriVariables": {
"id": {
"@type": "ThingID",
"title": "Thing Description ID",
"type": "string",
"format": "iri-reference"
}
},
"input": {
"description": "The schema is implied by the content type",
"type": "object"
},
"forms": [
{
"href": "/things/{id}",
"htv:methodName": "PUT",
"contentType": "application/td+json",
"response": {
"description": "Success response",
"htv:statusCodeValue": 204
},
"additionalResponses": [
{
"description": "Invalid serialization or TD",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
},
"partiallyUpdateThing": {
"description": "Partially update a Thing Description",
"uriVariables": {
"id": {
"@type": "ThingID",
"title": "Thing Description ID",
"type": "string",
"format": "iri-reference"
}
},
"input": {
"description": "The schema is implied by the content type",
"type": "object"
},
"forms": [
{
"href": "/things/{id}",
"htv:methodName": "PATCH",
"contentType": "application/merge-patch+json",
"response": {
"description": "Success response",
"htv:statusCodeValue": 204
},
"additionalResponses": [
{
"description": "Invalid serialization or TD",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
},
{
"description": "TD with the given id not found",
"contentType": "application/problem+json",
"htv:statusCodeValue": 404
}
]
}
]
},
"deleteThing": {
"description": "Delete a Thing Description",
"uriVariables": {
"id": {
"@type": "ThingID",
"title": "Thing Description ID",
"type": "string",
"format": "iri-reference"
}
},
"forms": [
{
"href": "/things/{id}",
"htv:methodName": "DELETE",
"response": {
"description": "Success response",
"htv:statusCodeValue": 204
},
"additionalResponses": [
{
"description": "TD with the given id not found",
"contentType": "application/problem+json",
"htv:statusCodeValue": 404
}
]
}
]
},
"searchJSONPath": {
"description": "JSONPath syntactic search. This affordance is not normative and is provided for information only.",
"uriVariables": {
"query": {
"title": "A valid JSONPath expression",
"type": "string"
}
},
"output": {
"description": "The schema depends on the given query",
"type": "object"
},
"safe": true,
"idempotent": true,
"forms": [
{
"href": "/search/jsonpath?query={query}",
"htv:methodName": "GET",
"response": {
"description": "Success response",
"contentType": "application/json",
"htv:statusCodeValue": 200
},
"additionalResponses": [
{
"description": "JSONPath expression not provided or contains syntax errors",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
},
"searchXPath": {
"description": "XPath syntactic search. This affordance is not normative and is provided for information only.",
"uriVariables": {
"query": {
"title": "A valid XPath expression",
"type": "string"
}
},
"output": {
"description": "The schema depends on the given query",
"type": "object"
},
"safe": true,
"idempotent": true,
"forms": [
{
"href": "/search/xpath?query={query}",
"htv:methodName": "GET",
"response": {
"description": "Success response",
"contentType": "application/json",
"htv:statusCodeValue": 200
},
"additionalResponses": [
{
"description": "XPath expression not provided or contains syntax errors",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
},
"searchSPARQL": {
"description": "SPARQL semantic search",
"uriVariables": {
"query": {
"title": "A valid SPARQL 1.1. query",
"type": "string"
}
},
"output": {
"description": "The schema depends on the given query",
"type": "object"
},
"safe": true,
"idempotent": true,
"forms": [
{
"href": "/search/sparql?query={query}",
"htv:methodName": "GET",
"response": {
"description": "Success response",
"contentType": "application/json",
"htv:statusCodeValue": 200
},
"additionalResponses": [
{
"description": "SPARQL query not provided or contains syntax errors",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
},
{
"href": "/search/sparql",
"htv:methodName": "POST",
"response": {
"description": "Success response",
"contentType": "application/json",
"htv:statusCodeValue": 200
},
"additionalResponses": [
{
"description": "SPARQL query not provided or contains syntax errors",
"contentType": "application/problem+json",
"htv:statusCodeValue": 400
}
]
}
]
}
},
"events": {
"thingCreated": {
"description": "Registration of Thing Descriptions inside the directory",
"uriVariables": {
"diff": {
"description": "Receive the full created TD as event data",
"type": "boolean"
}
},
"data": {
"title": "Partial/Full TD",
"type": "object"
},
"forms": [
{
"op": "subscribeevent",
"href": "/events/thing_created{?diff}",
"subprotocol": "sse",
"htv:headers": [
{
"description": "ID of the last event for reconnection",
"htv:fieldName": "Last-Event-ID"
}
],
"response": {
"contentType": "text/event-stream"
}
}
]
},
"thingUpdated": {
"description": "Updates to Thing Descriptions within the directory",
"uriVariables": {
"diff": {
"description": "Include TD changes inside event data",
"type": "boolean"
}
},
"data": {
"title": "Partial TD",
"type": "object",
"contentMediaType": "application/merge-patch+json"
},
"forms": [
{
"op": "subscribeevent",
"href": "/events/thing_updated{?diff}",
"subprotocol": "sse",
"htv:headers": [
{
"description": "ID of the last event for reconnection",
"htv:fieldName": "Last-Event-ID"
}
],
"response": {
"contentType": "text/event-stream"
}
}
]
},
"thingDeleted": {
"description": "Deletion of Thing Descriptions from the directory",
"data": {
"title": "Partial TD",
"type": "object"
},
"forms": [
{
"op": "subscribeevent",
"href": "/events/thing_deleted",
"subprotocol": "sse",
"htv:headers": [
{
"description": "ID of the last event for reconnection",
"htv:fieldName": "Last-Event-ID"
}
],
"response": {
"contentType": "text/event-stream"
}
}
]
}
}
}
8. Security Considerations
Security is a cross-cutting issue that needs to be
considered in all WoT building blocks and WoT implementations.
This chapter summarizes some general issues and guidelines to
help preserve the security of concrete WoT discovery
implementations. For a more detailed and complete analysis of
both security and privacy issues, see the WoT Security and
Privacy Guidelines specification [WOT-SECURITY].
WoT Thing and WoT TDDs are also web services and should be
implemented using best practices for web services. In addition
to the specific security considerations below, the security
risks and mitigations discussed in guides such as the OWASP Top
10 [OWASP-Top-10] should be evaluated,
and if applicable, addressed.
8.1 Denial of Service
Certain functions of the directory service, in particular
search queries, may require significant resources to execute
and this fact can be used to launch denial of service (DoS)
attacks against WoT Thing Description Directory services. In
such an attack a WoT Directory would be overloaded by
requests from the attacker and unable to service other
requests.
Mitigations:
A WoT Thing Description Directory implementation
should limit the number of queries per unit time from
the same requestor.
A WoT Thing Description Directory implementation
should limit the complexity of queries (for example,
the total length of the query expression or its
depth).
A WoT Thing Description Directory implementation
should use a watchdog timer to abort queries that take
more than a certain maximum
(implementation-configurable) amount of time.
8.2 Amplification and Distributed
Denial of Service
It may also be possible to use elements of WoT Discovery
mechanisms to launch distributed denial of service (DDoS)
attacks against other targets. In such an attack the WoT
Discovery service itself is not the target. Instead, an
aspect of the WoT Discovery service would be exploited to
generate amplified network traffic that overloads a third
party, the actual target. Such an attack has two
requirements: first, the ability to redirect traffic to a
third party, and second, an intermediary service that can be
exploited to amplify network traffic from the attacker.
Redirection of network traffic is possible in some protocols,
such as unsecured CoAP, by modifying source information in
headers. Amplification is possible by taking advantage of
three multiplicative factors: the ratio of request to
response payload sizes, use of "observe" in protocols like
CoAP (which can give multiple results for one request), and
use of multicast (which can allow multiple servers to respond
to one request). Services which do not support authentication
are ideal intermediaries for such an indirect attack.
Unfortunately, the Introduction mechanisms for WoT Discovery
are meant to provide open access mechanisms to initiate
discovery and might be exploited for this purpose.
Mitigations:
Open implementations of
Introduction mechanisms SHOULD
NOT support observe or similar extended result
subprotocols.
Open implementations of Introduction mechanisms
should not respond to multicast requests unless this is
absolutely required by the protocol. If support for
multicast is required, in the case of CoAP, the
observations made in [AMPLIFICATION-ATTACKS]
are relevant, since the number of servers that might
respond to a multicast request during discovery will
generally not be known in advance.
Limit the size of responses to the minimum. The
total size of responses to an Introduction on the
public internet (outside of a protected local network)
should be less than 3x the size of the total size of
request, and this should include any error responses.
This is consistent with DDOS mitigations in
[RFC9000]
(QUIC) and HTTP/3. Here "total size" includes any
headers required by the protocol itself, as well as
padding in the request to allow for a larger
response.
Introductions should rate-limit responses to any
particular request source.
Introduction mechanisms on a
segmented network behind a firewall (e.g. a LAN)
SHOULD NOT respond to requests
that are (apparently) from outside that
LAN.
Of particular concern are Introduction mechanisms
that can return multiple results, such as CoRE-RD and
DID. It may be necessary to use
authentication/authorization on such Introduction
mechanisms as well if the other mitigations above are not
sufficient. A recommended alternative is to move multiple
results from such Introductions into a WoT TDD, which can
then be protected by appropriate authentication and
authorization measures. Then the open Introduction
mechanism only has to return one result, the URL of the
TDD. Introduction mechanisms that are visible on the open
internet should be especially careful to implement the
above mitigations, and perhaps avoid Introduction
mechanisms that can return multiple URLs completely.
8.3 Self-Discovery on LANs
On a LAN, certificates and browsers may not be able to
property set up TLS for HTTPS because browsers expect
certificates pointing to a publicly-visible URL. Using HTTP
is common practice inside LANs but in combination with
self-description it means that WoT Things would be
essentially be making TDs visible to everyone with access to
the private LAN. Even if security mechanisms such as HTTP
passwords were used, these are not effective (they can be
easily discovered by a traffic analyser) without transport
security.
Mitigations:
PSK
(pre-shared keys) SHOULD be used
if possible on LANs, meaning one of the ciphersuites in
[RFC4279].
This does require that Things are assigned PSKs in a
common security domain, which is typically done by
following an onboarding process. Unfortunately, specific
onboarding processes are currently out of scope of the
WoT specifications.
An alternative is to depend on local network security
(i.e. WEP). This is not the best solution from a security
or privacy point of view but may be acceptable in some
contexts. Note however that all users with access to the
network would in turn have access to all TDs via
self-description. If Things cannot be individually
secured with transport security and authentication and
authorization, a separate network SHOULD be set up, i.e. with an alternative
SSID, and used only for IoT devices. Using a
segmented network reduces the need for distributing the
password to this network to those who need access to the
set of IoT devices connected to it.
Another alternative is to use a reverse proxy service
based in the cloud. Secure setup can be accomplished if
the IoT device has access to the cloud, since the proxy
server can have a public URL and the initial connection
can use HTTPS, then open a secure tunnel over a
websocket. The proxy can in turn re-expose a secure
endpoint, and possible add authentication. The
disadvantages of this approach including depending on an
external cloud service and the need to expose an external
access point (which is itself a security risk). The first
disadvantage can be addressed by hosting the proxy
service locally and exposing a public URL using e.g.
dynamic DNS if the local server is connected through an
ISP. If Things cannot be individually secured with
transport security and authentication and authorization,
then they may be made available for general access via a
proxy that can provide suitable access controls.
9. Privacy Considerations
Privacy is a cross-cutting issue that needs to be considered
in all WoT building blocks and WoT implementations. This
chapter summarizes some general issues and guidelines to help
preserve the privacy of concrete WoT discovery implementations.
For a more detailed and complete analysis of both 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 privacy of existing discovery schemes by
using a two-phase approach and allowing for the enforcement of
authorization before metadata release. However several 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. For
privacy we distinguish the following broad classes of use case
scenarios:
Institutional
Both the Things producing metadata and the Consumers of
that metadata are owned and controlled by an institution or
representatives of an institution. Example: Automation in a
factory where a control system is accessing the state of an
assembly line in order to evaluate quality.
Service
The Things producing metadata are owned and controlled by
an institution or representatives of an institution while the
consumers are individuals. Example: driver of an electric
vehicle accessing the TD for a charge station in order to
check status of a charge.
Personal
Both the Things producing metadata and the Consumers of
that metadata are owned and controlled by the same
individual. Example: A smart home control system for charging
an electric car from home-attached solar panels, both home
and car owned by the same person.
Personal Peer-to-Peer
The Things producing metadata and the Consumers of that
metadata are owned and controlled by different individuals.
Example: A smart home control system for charging a guest's
electric car from home-attached solar panels.
Institutional Peer-to-Peer
The Things producing metadata and the Consumers of that
metadata are owned and controlled by different institutions.
Example: A utility provides and manages power delivered to a
factory, and the factory provides an interface for the
utility to negotiate on-demand power usage reductions.
Client
The Things producing metadata are owned and controlled by
an individual while the consumers are an institution or
representatives of an institution. Example: A personal
electric vehicle exposes an interface to a public charging
station so that the charging station can evaluate the charge
status of the vehicle.
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.
In the following we make frequent reference to "tracking".
This term covers multiple privacy risks, including location
tracking and behavioral profiling. In general, the definition
of "profiling" given in Article 4 of the GDPR [GDPR-Defs]
is to be considered equivalent to "tracking" as used in this
document.
With these definitions and categories established, we will
now discuss some specific privacy risks and potential
mitigations.
9.1 Location Tracking and
Profiling
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:
The TD or Thing could report actual location
information so no inferencing by network range is
needed;
A directory could support actual search by
location;
Absence of a TD in a directory could reveal negative
location information. For example, if a TD times out and is
removed from a directory, that means the device has not
been present for a while, which could be used to infer that
a person is NOT in a particular location, e.g. not at
home.
Location tracking is not the only profiling risk. In
general, "profiling" includes any mechanism used to evaluate
information about a person, including economic status,
health, preferences, interests, reliability, and behavior.
Some of the metadata in a TD can be used to infer information
of this kind if the described Thing can be associated with a
person. Some of the mitigations below are also applicable to
this more general definition of profiling.
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.
Mitigations:
There are a few options to mitigate these risks:
To avoid location tracking and other forms of
profiling, a WoT Thing associated with a person may
disable registration with public directories.
Registration would still be possible with personal
directories, for example, a home gateway, but a user
could disable registration at other locations. This has
the disadvantage that functionality is lost: personal
devices cannot be discovered in public locations. This
could be addressed by having internet-accessible
private discovery services. For example, the user's
home gateway could provide an internet-accessible
service, but with access control limiting use to
authorized users.
To avoid location tracking and other forms of
profiling, a WoT Thing associated with a person should
use anonymous TDs when registering with a public
directories. In some cases, it may be possible to use
anonymous TDs and omit explicit IDs from TDs submitted
to a TDD. In this case the TDD will generate a local ID
valid only in that TDD. This does, however, complicate
update since the client needs to remember the local ID
assigned by the TDD. Anonymous TDs also do not prevent
tracking by other means, such as fingerprinting.
To avoid location tracking and other forms of
profiling, a WoT Thing associated with a person may
periodically generate new IDs. Using fixed IDs makes it
exceptionally easy to track devices. This problem also
occurs in DHCP with MAC address and there is a similar
partial mitigation: generate new random IDs
periodically. There are however, a few issues. First of
all, other identification information in the TD needs
to be hidden. For example, client IDs issued by CSPs
for API security should be omitted from TDs if they
cannot be easily changed. Second, if the device
generates a new ID, the user may still need to know the
current ID to find the device via discovery. This can
be accomplished however by generating new IDs using a
deterministic cryptographic generator that is a
function of the current time. However, note that
regenerating IDs alone does not make tracking
impossible since a TD might be fingerprinted. Also,
updating an ID might be observable to the owner of the
directory service, who could track and record the
updated ID. Even if the TD is deleted and reinserted
the association could be inferred. This is however
exactly parallel to the situation with DHCP and
rotation of MAC addresses. In general, however,
generating new IDs at least for each service or person
to which a TD is supplied makes it harder to connect
registration events at different locations and times.
It is also prudent to generate new identifiers upon
major changes in configuration, such as unregistering
from a local network or hub and registering with a new
one (which typically indicates a change in ownership).
There is a related issue with long-lived IP addresses
which might need to be updated periodically to mitigate
tracking. In the context of ipv6 [RFC8981]
discusses this. Finally, there is a problem with
devices that require immutable identifiers, e.g.
medical devices in such jurisdictions. This is
discussed in [wot-thing-description11],
but in summary the problem can be avoided if such
immutable identifiers are made available only as
protected properties, e.g. via affordances requiring
authentication, not in the TD, and the TD identifier
itself (if used) is independent of the immutable
identifier, and so can be made mutable.
To reduce the risk of
negative location inferencing, access to private
directories SHOULD be limited
by using access controls. If an attacker cannot
access the service, they cannot retrieve information to
infer location. Access rights provided to guests (e.g.
for Peer-to-Peer Personal scenarios) should be
appropriately time-limited. Use of long time-to-live
values may be appropriate in other cases. In addition,
TDs should be updated in a directory only when they
change. For example, the TD for a car may only be
updated when new car firmware is available providing
new services, and the time-to-live might be set at one
month (covering most absences).
When explicit location
information is available, whether stored in a TD or
available in a property, additional care SHOULD be taken to only share the TD
and/or access to the device with trusted partners,
including directories. If the TD must be shared
with a public directory, the location information may
be stripped.
9.2
Query Tracking
A directory service could potentially record and track
queries by an individual, identifying that individual by
their authenticated identity provided. Then the set of
queries associated with an individual could be used to
profile that individual, and specific queries may also reveal
personal information about an individual.
Mitigations:
When accessing a public directory, like any other
public web service, users and implementations should use
an anonymous identity provider. In particular, OAuth2 can
provide tokens which don't identify specific individuals,
they just assert access rights proven elsewhere.
10. Performance Considerations
This section is non-normative.
10.1 Incremental Transfer
TD objects are not constrained in
size. They may become expensive to process and transfer
individually or collectively. A single TD or a list of TDs
could be too large for a constrained device, serving its own
TD to consumers, submitting it to a directory, or consuming
other TDs. To meet such requirements, servers should support
incremental transfer of payloads using protocol-specific
mechanisms:
HTTP/1.1 servers should support chunked
Transfer-Encoding [RFC7230]
to receive and server data incrementally.
HTTP/2 servers should handle the data incrementally
using
HTTP Frames [RFC7540].
The HTTP/1.1 chunked transfer encoding is not supported
in HTTP/2.
CoAP servers must suppoert block-wise transfer
[RFC7959]
when relying on an unreliable transport protocol like UDP
and should support it when using a reliable transport
protocol like TCP or WebSockets [RFC8323].
Most HTTP servers and clients automatically process the
data that is transferred in chunks. Memory-constrained
clients should consider consuming the received data
incrementally, instead of trying to load a whole object in
memory for de-serialization.
11.
IANA Considerations
11.1 Well-Known URI
Registration
IANA will be asked to allocate the following value into
the Well-Known URI defined in [RFC8615].
URI suffix: wot
11.2 Service Name Registration
IANA will be asked to allocate the following value into
Service Name and Transport Protocol Port Number Registry
defined in [RFC6335].
Service Name: wot
Transport Protocol: TCP, UDP
Description: Service name to search for the Thing
Description of a Thing or a Thing Description Directory in
the W3C Web
of Things
Port Number: N/A
Assignment notes: We use _directory
subtype for a Thing Description Directory.
11.3 CoRE Resource Types
Registration
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].
The following JSON Schema specifies the extensions used
in Enriched
TDs. It can be used for validating TDs by a TDD as prescribed in 7.3.2.1.6 Validation.
Define sorting order to be based on Unicode code
points.
Clarify 3x rule for DDoS migitation to allow padding in
request, and require it only outside of protected local
networks.
Define content types to be used for CoAP servers,
paralleling TD servers.
Update DID service names and refine DID assertions and
add examples to clarify use for Introductions.
Update DNS-SD section to include explicit scheme in TXT
field.
Replace reference to draft CoRE-RD with final
specification published as RFC9176.
B.3 Changes from 2 June 2021 Working
Draft
Use wot for well-known URI service
name.
Refer to TD and Architecture specs for general
constraints on secure transport.
Define requirements for CoAP-based TD Servers.
Introduction of TD Server exploration, with
self-description as a special case.
Elaboration of class diagram to clarify that not all
Directories need to be self-describing.
Clarify and consolidate error codes and ontology.
Add assertions for UTF-8 supporting
internationalization.
Update overview figure showing introduction and
exploration mechanisms.
Add Discoverer section defining requirements for
discovery clients.
Use Thing Model instead of a Thing Description example
for Directory API.
Refactor Directory Service API (td->thing, split
anonymous create action, retrieve one and search as
actions, listing as things property, split events)
Add amplification DDOS security consideration.
Split Security and Privacy Considerations into separate
sections.
Define status of search mechanisms: JSON Path, XPath,
and SPARQL.
Pagination as array and pagination as collection.
Updates to events API.
TD expiry management.
HEAD method support.
B.4 Changes from 24 November 2020 First
Public Working Draft
Update name of directory service to "Thing Description
Directory" with acronym TDD, to avoid confusion with the
acronym TD used for Thing Descriptions.
Elaboration of complete API for registration of a Thing
Description with a Thing Description Directory, including
support for creation, update, retrieval, deletion, and
listing.
Add recommendation that TDs be validated before storage
in a directory. Note: this corresponds to work under way in
the TD spec to precisely define appropriate levels of
automatic validation for different contexts, including
directory registration.
Addition of Security and Privacy Considerations
regarding Denial of Service and Location Tracking.
Addition of IANA Considerations regarding CoRE Resource
Types.
Definition of Enriched TD to support embedded metadata
in TDs returned by a TDD.
Updates to description of exploration to avoid
unnecessary dependence on HTTP (in the future, other
protocols may be supported).
Use of "resource type" instead of "endpoint type" when
describing entry in CoRE RD.
Addition of ontology for TDDs that include the type
DirectoryDescription identifying TDs of
directories and a type LinkDescription to
identify a TD that is just a link to another TD (useful for
referring to remote directories in order to support
federation).
Add section for self-description exploration mechanism.
This supports the use case where a Thing hosts its own TD
and peer-to-peer Thing communication.
Addition of diagram showing the directory information
model.
Removal of reference to Issue 16, citing CRUD interface
requirements. Replaced with CRUDL (CRUD + Listing)
interface requirement.
MAY assertion added to
explicitly permit implementation of SPARQL federation, and
a requirement that if implemented, it needs to follow the
SPARQL 1.1 specification.
Updates to references: more recent versions of CoRE RD,
DID Core, Eventsource, JSON-LD, and JSON Path
specifications. Use of datatracker service (and URLs) to
reference IETF RFCs. Addition of normative references to
RFC7396 and RFC7540.
C.
Acknowledgments
Special thanks to Arne Broering, Soumya Kanti Datta, and
Christian Bonnet for their survey work on existing discovery
mechannisms in IoT. Special thanks to Philipp Blum, Victor
Charpeney, Ben Francis, Christian Glomb, Daniel Peintner,
Christine Perey, Jan Romann, and Elodie Thieblin for their
contributions to this document.
Many thanks to the W3C staff, especially
Kazuyuki Ashimura and Dominique Hazael-Massieux, 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.
