CoRE Z. Shelby
Internet-Draft ARM
Intended status: Informational M. Vial
Expires: April 21, 2016 Schneider-Electric
M. Koster
ARM
October 19, 2015
Reusable Interface Definitions for Constrained RESTful Environments
draft-ietf-core-interfaces-04
Abstract
This document defines a set of reusable REST resource design patterns
suitable for use in constrained environments, based on IETF CoRE
standards for information representation and information exchange.
Interface types for Sensors, Actuators, Parameters, and resource
Collections are defined using the "if" link attribute defined by CoRE
Link Format [RFC6690]. Clients may use the "if" attribute to
determine how to consume resources.
Dynamic linking of state updates between resources, either on an
endpoint or between endpoints, is defined with the concept of Link
Bindings. We also define conditional observation attributes that
work with Link Bindings or with simple CoAP Observe [RFC7641].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 21, 2016.
Shelby, et al. Expires April 21, 2016 [Page 1]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Interface Types . . . . . . . . . . . . . . . . . . . . . . . 5
4. Collections . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Introduction to Collections . . . . . . . . . . . . . . . 5
4.2. Use Cases for Collections . . . . . . . . . . . . . . . . 6
4.3. Content-Formats for Collections . . . . . . . . . . . . . 7
4.4. Links and Items in Collections . . . . . . . . . . . . . 7
4.5. Queries on Collections . . . . . . . . . . . . . . . . . 9
4.6. Observing Collections . . . . . . . . . . . . . . . . . . 9
4.7. Hypermedia Controls on Collections . . . . . . . . . . . 9
4.8. Collection Types . . . . . . . . . . . . . . . . . . . . 10
4.9. The collection+senml+json Content-Format . . . . . . . . 10
5. Link Bindings and Observe Attributes . . . . . . . . . . . . 11
5.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Binding methods . . . . . . . . . . . . . . . . . . . . . 13
5.3. Binding table . . . . . . . . . . . . . . . . . . . . . . 14
5.4. Resource Observation Attributes . . . . . . . . . . . . . 14
6. Interface Descriptions . . . . . . . . . . . . . . . . . . . 15
6.1. Link List . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Batch . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3. Linked Batch . . . . . . . . . . . . . . . . . . . . . . 18
6.4. Hypermedia Collection . . . . . . . . . . . . . . . . . . 19
6.5. Sensor . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.6. Parameter . . . . . . . . . . . . . . . . . . . . . . . . 21
6.7. Read-only Parameter . . . . . . . . . . . . . . . . . . . 21
6.8. Actuator . . . . . . . . . . . . . . . . . . . . . . . . 21
6.9. Binding . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.10. Future Interfaces . . . . . . . . . . . . . . . . . . . . 23
6.11. WADL Description . . . . . . . . . . . . . . . . . . . . 23
7. Function Sets and Profiles . . . . . . . . . . . . . . . . . 29
Shelby, et al. Expires April 21, 2016 [Page 2]
Internet-DrafReusable Interface Definitions for Constrained October 2015
7.1. Defining a Function Set . . . . . . . . . . . . . . . . . 29
7.1.1. Path template . . . . . . . . . . . . . . . . . . . . 30
7.1.2. Resource Type . . . . . . . . . . . . . . . . . . . . 30
7.1.3. Interface Description . . . . . . . . . . . . . . . . 30
7.1.4. Data type . . . . . . . . . . . . . . . . . . . . . . 31
7.2. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 31
7.3. Versioning . . . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Profile example . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
IETF Standards for machine to machine communication in constrained
environments describe a REST protocol and a set of related
information standards that may be used to represent machine data and
machine metadata in REST interfaces.. CoRE Link-format is a standard
for doing Web Linking [RFC5988] in constrained environments. SenML
is a simple data model and representation format for composite and
complex structured resources. CoRE Link-Format and SenML can be used
by CoAP [RFC7252] or HTTP servers.
The discovery of resources offered by a constrained server is very
important in machine-to-machine applications where there are no
humans in the loop. Machine application clients must be able to
adapt to different resource organizations without advance knowledge
of the specific data structures hosted by each connected thing. The
use of Web Linking for the description and discovery of resources
hosted by constrained web servers is specified by CoRE Link Format
[RFC6690]. CoRE Link Format additionally defines a link attribute
for Interface Type ("if") that can be used to describe the REST
interface of a resource, and may include a link to a description
document.
This document defines a set of Link Format compatible Interface Types
for some common design patterns that enable the server side
composition and organization, and client side discovery and
consumption, of machine resources using Web Linking. An Interface
Type may describe a resource in terms of it's associated content
formats, data types, URI templates, REST methods, parameters, and
responses. Basic interface types are defined for sensors, actuators,
and properties. A set of collection types is defined for organizing
Shelby, et al. Expires April 21, 2016 [Page 3]
Internet-DrafReusable Interface Definitions for Constrained October 2015
resources for discovery, and for various forms of bulk interaction
with resource sets using typed embedding links.
This document introduces the concept of a Link Binding, which defines
a new link relation type to create a dynamic link between resources
over which to exchange state updates. Specifically, a Link Binding
is a link for binding the state of 2 resources together such that
updates to one are sent over the link to the other. CoRE Link Format
representations are used to configure, inspect, and maintain Link
Bindings. This document additionally defines a set of conditional
Observe Attributes for use with Link Bindings and with the standalone
CoRE Observe [RFC7641] method.
Interface Types may be used in the composition of Function Sets and
Profiles. Function Sets and Profiles are described and an example is
given of a sensor and actuator device profile using Function Sets
composed from the Interface Types described in this document.
This document describes a set of Interface Types which are referenced
by the "if" link attribute and used to implement reusable design
patterns and functional abstractions. A client discovering the "if"
link attribute will be able to consume resources based on its
knowledge of the expected interface types. In this sense the
Interface Type acts in a similar way as a Content-Format, but as a
selector for a high level functional abstraction. Interface types
may also be provided with hypermedia controls and affordances to
drive client interaction using the principles of HATEOAS. In this
case, the Interface Types serve as constructor templates for resource
organization and hypermedia annotation.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC5988] and [RFC6690]. This
specification makes use of the following additional terminology:
Interface Type: A resource attribute which describes the interface
exposed by the resource in terms of content formats, REST methods,
parameters, and other related characteristics.
Collection: A resource which contains set of related resources,
referenced by a list of links and optionally consisting of
subresources.
Shelby, et al. Expires April 21, 2016 [Page 4]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Link Binding: A unidirectional logical link between a source
resource and a destination resource, over which state information
is synchronized.
Resource Discovery: The process allowing a web client to identify
resources being hosted on a web server.
Gradual Reveal: A REST design where resources are discovered
progressively using Web Linking.
Function Set: A group of well-known REST resources that provides a
particular service.
Profile: A group of well-known Function Sets defined by a
specification.
Device: An IP smart object running a web server that hosts a group
of Function Set instances from a profile.
Service Discovery: The process making it possible for a web client
to automatically detect devices and Function Sets offered by these
devices on a CoRE network.
3. Interface Types
An Interface Type definition may describe a resource in terms of it's
associated content formats, data types, URI templates, REST methods,
parameters, and responses.
4. Collections
4.1. Introduction to Collections
A Collection is a resource which represents one or more related
resources. Within this document, a collection refers to a collection
with characteristics defined in this document. A Collection
Interface Type consists of a set of links and a set of items pointed
to by the links which may be sub-resources of the collection
resource. The collection types described in this document are Link
List, Batch, Linked Batch, and Hypermedia Collection.
The links in a collection are represented in CoRE Link-Format
Content-Formats including JSON and CBOR variants, and the items in
the collection may be represented by senml, including JSON and CBOR
variants. In general, a collection may support items of any
available Content-Format.
Shelby, et al. Expires April 21, 2016 [Page 5]
Internet-DrafReusable Interface Definitions for Constrained October 2015
A particular resource item may be a member of more than one
collection at a time by being linked to, but may only be a
subresource of one collection.
Some collections may have pre-configured items and links, and some
collections may support dynamic creation and removal of items and
links. Likewise, modification of items in some collections may be
permitted, and not in others.
Collections may support link embedding, which is analogous to an
image tag (link) causing the image to display inline in a browser
window. Resources pointed to by embedded links in collections may be
interacted with using bulk operations on the collection resource.
For example, performing a GET on a collection resource may return a
single representation containing all of the linked resources.
Links in collections may be selected for processing by a particular
request by using Query Filtering as described in CoRE Link-Format
[RFC6690].
4.2. Use Cases for Collections
Collections may be used to provide gradual reveal of resources on an
endpoint. There may be a small set of links at the .well-known/core
location, which may in turn point to other collections of resources
that represent device information, device configuration, device
management, and various functional clusters of resources on the
device.
A collection may provide resource encapsulation, where link embedding
may be used to provide a single resource with which a client may
interact to obtain a set of related resource values. For example, a
collection for manufacturer parameters may consist of manufacturer
name, date of manufacture, location of manufacture, and serial number
resources which can be read as a single senml data object.
A collection may be used to group a set of like resources for bulk
state update or actuation. For example, the brightness control
resources of a number of luminaries may be grouped by linking to them
in a collection. The collection type may support receiving a single
update form a client and sending that update to each resource item in
the collection.
Items may be sub-resources of the collection resource. This enables
updates to to multiple items in the collection to be processed
together within the context of the collection resource.
Shelby, et al. Expires April 21, 2016 [Page 6]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Items may be dynamically created in a collection along with their
hyperlinks. This provides an "item factory" pattern which can serve
as a resource creation mechanism for dynamic resources. This pattern
is also useful for creating temporary resources for the
implementation of dynamic phenomena like commands, actions, and
events using REST design patterns. Item creation uses the collection
Content-Format which allows specification of links and item state in
a single representation.
4.3. Content-Formats for Collections
The collection interfaces by default use CoRE Link-Format for the
link representations and SenML or text/plain for representations of
items. The examples given are for collections that expose resources
and links in these formats. In addition, a new "collection" Content-
Format is defined based on the SenML framework which represents both
links and items in the collection.
The choice of whether to return a representation of the links or of
the items or of the collection format is determined by the accepts
header option in the request. Likewise, the choice of updating link
metadata or item data or the collection resource itself is determined
by the Content-Format option in the header of the update request
operation.
The default Content-Formats for collection types described in this
document are:
Links: application/link-format, application/link-format+json
Items: application/senml+json, text/plain
Collection: application/collection+senml+json
4.4. Links and Items in Collections
Links use CoRE Link-Format representation by default and may point to
any resource reachable from the context of the collection. This
includes absolute links and links that point to other network
locations if the context of the collection allows. Links to sub-
resources in the collection MUST have a path-element starting with
the resource name, as per RFC3986 [RFC3986]. Links to resources in
the global context MUST start with a root path identifier
[RFC5988].Links to other collections are formed per RFC3986.
Examples of links:
Shelby, et al. Expires April 21, 2016 [Page 7]
Internet-DrafReusable Interface Definitions for Constrained October 2015
</sen/>;if="core.lb" : Link to the /sen/ collection describing it as
a core.lb type collection (Linked Batch)
</sen/>;rel="grp" : Link to the /sen/ collection indicating that
/sen/ is a member of a group in the collection in which the link
appears.
<"/sen/temp">;rt="temperature" : An absolute link to the resource at
the path /sen/temp
<temp>;rt="temperature" : Link to the temp subresource of the
collection in which this link appears.
<temp>;anchor="/sen/" : A link to the temp subresource of the
collection /sen/ which is assumed not to be a subresource of the
collection in which the link appears ,but is expected to be
identified in the collection by resource name.
Links in the collection MAY be Read, Updated, Added, or Removed using
the CoRE Link-Format or JSON Merge-Patch Content-Formats on the
collection resource. Reading links uses the GET method and returns
an array or list containing the link-values of all selected links.
Links may be added to the collection using POST or PATCH methods.
Updates to links MUST use the PATCH method and MAY use query
filtering to select links for updating. The PATCH method on links
MUST use the JSON Merge-Patch Content-Format (application/merge-
patch+json) specified in RFC7396 [RFC7396] .
Items in the collection SHOULD be represented using the SenML
(application/senml+json) or plain text (text/plain) Content-Formats,
depending on whether the representation is of a single data point or
multiple data points. Items MAY be represented using any supported
Content-Format.
Link Embedding enables the bulk processing of items in the collection
using a single operation targeting the collection resource. A subset
of resources in the collection may be selected for operation using
Query Filtering. Bulk Read operations using GET return a SenML
representation of all selected resources. Bulk item Update
operations using PUT or POST apply the payload document to all
selected resource items in the collection, using a either a Batch or
Group update policy. A Batch update is performed by applying the
resource values in the payload document to all resources in the
collection that match any resource name in the payload document.
Group updates are performed by applying the payload document to each
item in the collection. Group updates are indicated by the link
relation type rel="grp" in the link.
Shelby, et al. Expires April 21, 2016 [Page 8]
Internet-DrafReusable Interface Definitions for Constrained October 2015
The collection resource SHOULD represented using the
collection+senml+json Content-Format. The Hypermedia Collection type
is the only collection type which supports this representation.
Reading a collection using this content-format returns a
representation of the links and the items in the collection.
Performing a POST operation using this Content-Format MAY create one
or more new item(s) and their corresponding links in the collection.
Performing a PUT operation on this resource replaces the entire set
of links and items with the payload. This Content-Format is
described in section Section 4.9. Implementations MAY provide an
alternate method using POST in a Content-Format used by the items in
the collection which creates a default link-value and system-assigned
resource name. Such implementations MAY create sub-resources of the
collection resource.
4.5. Queries on Collections
Collections MAY support query filtering as defined in CoRE Link-
Format [RFC6690]. Operations targeting either the links or the items
MAY select a subset of links and items in the collection by using
query filtering. The Content-Format specified in the request header
selects whether links or items are targeted by the operation.
4.6. Observing Collections
Resource Observation using CoAP [RFC7252] MAY be supported on items
in a collection. A subset of the conditional observe parameters MAY
be specified to apply. In most cases pmin and pmax are useful.
Resource observation on a collection's items resource MAY report any
changes of resource state in any item in the collection. Observation
Responses, or notifications, SHOULD provide representations of the
resources that have changed in SenML Content-Format. Notifications
MAY include multiple observations of a particular resource, with
SenML time stamps indicating the observation times.
4.7. Hypermedia Controls on Collections
Additional Hypermedia controls may be defined to enable clients to
automatically consume the collection resources. Typically, the
developer may map application level semantics onto collection
operations. For example, invoking an Action on an actuator may be
defined as creating an Action item resource in a collection of
Actions associated with the actuator, each item in the collection
representing a past, current, or future action to be processed by the
actuator. Removing the item could cancel any pending or curent long-
running action, and removing a completed action could free up
resources for new actions to be invoked. A Hypermedia control for
this pattern might provide a semantic name for the action, for
Shelby, et al. Expires April 21, 2016 [Page 9]
Internet-DrafReusable Interface Definitions for Constrained October 2015
example "Change Brightness", and might direct the client to supply a
SenML representation of parameters for the action as well as provide
instructions on what method (POST) to use and how to construct the
URI (the collection URI in this case) if required. An example of
this hypermedia control is shown below.
4.8. Collection Types
There are four collection types defined in this document:
+---------------------+----------+----------------------------------+
| Collection Type | if= | Content-Formats |
+---------------------+----------+----------------------------------+
| Link List | core.ll | link-format |
| Batch | core.b | link-format, senml |
| Linked Batch | core.lb | link-format, senml |
| Hypermedia | core.hc | link-format, senml, |
| Collection | | collection+senml |
| Binding | core.bnd | link-format |
+---------------------+----------+----------------------------------+
Each collection type MAY support a subset of the methods and
functions described above. For the first three collection types, the
methods and functions are defined in the corresponding Interface
Description. The Hypermedia Collection SHOULD expose hypermedia
controls to applications to indicate which methods and functions are
supported.
4.9. The collection+senml+json Content-Format
The collection+senml+json Content-Format is used to represent all of
the attributes and resources of a collection in a single format.
This is accomplished by extending the SenmL format by adding a links
element "l". The links element is formatted as an array of links in
the application/link-format+json Content-Format with the tag "l"
which follows the structure of the "e" element. An example of this
format is given below.
Shelby, et al. Expires April 21, 2016 [Page 10]
Internet-DrafReusable Interface Definitions for Constrained October 2015
{
"bn":"/ep/sen/"
"e":[
{ "n": "light", "v": 123, "u": "lx" },
{ "n": "temp", "v": 27.2, "u": "degC" },
{ "n": "humidity", "v": 80, "u": "%RH" }],
"l":[
{ "href":"/ep/sen/", "rel":"self", "if": "core.hc", "rt": "ms" },
{ "href":"light", "rt":"core.s" },
{ "href":"temp", "rt":"core.s" },
{ "href":"humidity", "rt":"core.s" }]
}
5. Link Bindings and Observe Attributes
In a M2M RESTful environment, endpoints may directly exchange the
content of their resources to operate the distributed system. For
example, a light switch may supply on-off control information that
may be sent directly to a light resource for on-off control.
Beforehand, a configuration phase is necessary to determine how the
resources of the different endpoints are related to each other. This
can be done either automatically using discovery mechanisms or by
means of human intervention and a so-called commissioning tool. In
this document the abstract relationship between two resources is
called a link Binding. The configuration phase necessitates the
exchange of binding information so a format recognized by all CoRE
endpoints is essential. This document defines a format based on the
CoRE Link-Format to represent binding information along with the
rules to define a binding method which is a specialized relationship
between two resources. The purpose of a binding is to synchronize
the content between a source resource and a destination resource.
The destination resource MAY be a group resource if the authority
component of the destination URI contains a group address (either a
multicast address or a name that resolves to a multicast address).
Since a binding is unidirectional, the binding entry defining a
relationship is present only on one endpoint. The binding entry may
be located either on the source or the destination endpoint depending
on the binding method. The following table gives a summary of the
binding methods described in more detail in Section 5.2.
+---------+------------+-------------+---------------+
| Name | Identifier | Location | Method |
+---------+------------+-------------+---------------+
| Polling | poll | Destination | GET |
| Observe | obs | Destination | GET + Observe |
| Push | push | Source | PUT |
+---------+------------+-------------+---------------+
Shelby, et al. Expires April 21, 2016 [Page 11]
Internet-DrafReusable Interface Definitions for Constrained October 2015
5.1. Format
Since Binding involves the creation of a link between two resources,
Web Linking and the CoRE Link-Format are a natural way to represent
binding information. This involves the creation of a new relation
type, purposely named "boundto". In a Web link with this relation
type, the target URI contains the location of the source resource and
the context URI points to the destination resource. The Web link
attributes allow a fine-grained control of the type of
synchronization exchange along with the conditions that trigger an
update. This specification defines the attributes below:
+--------------------+-----------+------------------+
| Attribute | Parameter | Value |
+--------------------+-----------+------------------+
| Binding method | bind | xsd:string |
| Minimum Period (s) | pmin | xsd:integer (>0) |
| Maximum Period (s) | pmax | xsd:integer (>0) |
| Change Step | st | xsd:decimal (>0) |
| Greater Than | gt | xsd:decimal |
| Less Than | lt | xsd:decimal |
+--------------------+-----------+------------------+
Bind Method: This is the identifier of a binding method which
defines the rules to synchronize the destination resource. This
attribute is mandatory.
Minimum Period: When present, the minimum period indicates the
minimum time to wait (in seconds) before sending a new
synchronization message (even if it has changed). In the absence
of this parameter, the minimum period is up to the notifier.
Maximum Period: When present, the maximum period indicates the
maximum time in seconds between two consecutive state
synchronization messages (regardless if it has changed). In the
absence of this parameter, the maximum period is up to the
notifier. The maximum period MUST be greater than the minimum
period parameter (if present).
Change Step: When present, the change step indicates how much the
value of a resource SHOULD change before sending a new
notification (compared to the value of the last notification).
This parameter has lower priority than the period parameters, thus
even if the change step has been fulfilled, the time since the
last notification SHOULD be between pmin and pmax.
Greater Than: When present, Greater Than indicates the upper limit
value the resource value SHOULD cross before sending a new
Shelby, et al. Expires April 21, 2016 [Page 12]
Internet-DrafReusable Interface Definitions for Constrained October 2015
notification. This parameter has lower priority than the period
parameters, thus even if the Greater Than limit has been crossed,
the time since the last notification SHOULD be between pmin and
pmax.
Less Than: When present, Less Than indicates the lower limit value
the resource value SHOULD cross before sending a new notification.
This parameter has lower priority than the period parameters, thus
even if the Less Than limit has been crossed, the time since the
last notification SHOULD be between pmin and pmax.
5.2. Binding methods
A binding method defines the rules to generate the web-transfer
exchanges that will effectively send content from the source resource
to the destination resource. The description of a binding method
must define the following aspects:
Identifier: This is value of the "bind" attribute used to identify
the method.
Location: This information indicates whether the binding entry is
stored on the source or on the destination endpoint.
REST Method: This is the REST method used in the Request/Response
exchanges.
Conditions: A binding method definition must state how the condition
attributes of the abstract binding definition are actually used in
this specialized binding.
This specification supports 3 binding methods described below.
Polling: The Polling method consists of sending periodic GET
requests from the destination endpoint to the source resource and
copying the content to the destination resource. The binding
entry for this method MUST be stored on the destination endpoint.
The destination endpoint MUST ensure that the polling frequency
does not exceed the limits defined by the pmin and pmax attributes
of the binding entry. The copying process MAY filter out content
from the GET requests using value-based conditions (e.g Change
Step, Less Than, Greater Than).
Observe: The Observe method creates an observation relationship
between the destination endpoint and the source resource. On each
notification the content from the source resource is copied to the
destination resource. The creation of the observation
relationship requires the CoAP Observation mechanism [RFC7641]
Shelby, et al. Expires April 21, 2016 [Page 13]
Internet-DrafReusable Interface Definitions for Constrained October 2015
hence this method is only permitted when the resources are made
available over CoAP. The binding entry for this method MUST be
stored on the destination endpoint. The binding conditions are
mapped as query string parameters (see Section 5.4).
Push: When the Push method is assigned to a binding, the source
endpoint sends PUT requests to the destination resource when the
binding condition attributes are satisfied for the source
resource. The source endpoint MUST only send a notification
request if the binding conditions are met. The binding entry for
this method MUST be stored on the source endpoint.
5.3. Binding table
The binding table is a special resource that gives access to the
bindings on a endpoint. A binding table resource MUST support the
Binding interface defined in Section 6.9. A profile SHOULD allow
only one resource table per endpoint.
5.4. Resource Observation Attributes
When resource interfaces following this specification are made
available over CoAP, the CoAP Observation mechanism [RFC7641] MAY be
used to observe any changes in a resource, and receive asynchronous
notifications as a result. In addition, a set of query string
parameters are defined here to allow a client to control how often a
client is interested in receiving notifications and how much a
resource value should change for the new representation to be
interesting. These query parameters are described in the following
table. A resource using an interface description defined in this
specification and marked as Observable in its link description SHOULD
support these observation parameters. The Change Step parameter can
only be supported on resources with an atomic numeric value.
These query parameters MUST be treated as resources that are read
using GET and updated using PUT, and MUST NOT be included in the
Observe request. Multiple parameters MAY be updated at the same time
by including the values in the query string of a PUT. Before being
updated, these parameters have no default value.
Shelby, et al. Expires April 21, 2016 [Page 14]
Internet-DrafReusable Interface Definitions for Constrained October 2015
+----------------+------------------+------------------+
| Resource | Parameter | Data Format |
+----------------+------------------+------------------+
| Minimum Period | /{resource}?pmin | xsd:integer (>0) |
| Maximum Period | /{resource}?pmax | xsd:integer (>0) |
| Change Step | /{resource}?st | xsd:decimal (>0) |
| Less Than | /{resource}?lt | xsd:decimal |
| Greater Than | /{resource}?gt | xsd:decimal |
+----------------+------------------+------------------+
Minimum Period: When present, the minimum period indicates the
minimum time to wait (in seconds) before sending a new
synchronization message (even if it has changed). In the absence
of this parameter, the minimum period is up to the notifier.
Maximum Period: When present, the maximum period indicates the
maximum time in seconds between two consecutive state
synchronization messages (regardless if it has changed). In the
absence of this parameter, the maximum period is up to the
notifier. The maximum period MUST be greater than the minimum
period parameter (if present).
Change Step: When present, the change step indicates how much the
value of a resource SHOULD change before sending a new
notification (compared to the value of the last notification).
This parameter has lower priority than the period parameters, thus
even if the change step has been fulfilled, the time since the
last notification SHOULD be between pmin and pmax.
Greater Than: When present, Greater Than indicates the upper limit
value the resource value SHOULD cross before sending a new
notification. This parameter has lower priority than the period
parameters, thus even if the Greater Than limit has been crossed,
the time since the last notification SHOULD be between pmin and
pmax.
Less Than: When present, Less Than indicates the lower limit value
the resource value SHOULD cross before sending a new notification.
This parameter has lower priority than the period parameters, thus
even if the Less Than limit has been crossed, the time since the
last notification SHOULD be between pmin and pmax.
6. Interface Descriptions
This section defines REST interfaces for Link List, Batch, Sensor,
Parameter, Actuator and Binding table resources. Variants such as
Linked Batch or Read-Only Parameter are also presented. Each type is
described along with its Interface Description attribute value and
Shelby, et al. Expires April 21, 2016 [Page 15]
Internet-DrafReusable Interface Definitions for Constrained October 2015
valid methods. These are defined for each interface in the table
below. These interfaces can support plain text and/or SenML Media
types.
The if= column defines the Interface Description (if=) attribute
value to be used in the CoRE Link Format for a resource conforming to
that interface. When this value appears in the if= attribute of a
link, the resource MUST support the corresponding REST interface
described in this section. The resource MAY support additional
functionality, which is out of scope for this specification.
Although these interface descriptions are intended to be used with
the CoRE Link Format, they are applicable for use in any REST
interface definition.
The Methods column defines the methods supported by that interface,
which are described in more detail below.
+-----------------+----------+-----------------+--------------------+
| Interface | if= | Methods | Content-Formats |
+-----------------+----------+-----------------+--------------------+
| Link List | core.ll | GET | link-format |
| Batch | core.b | GET, PUT, POST | link-format, senml |
| Linked Batch | core.lb | GET, PUT, POST, | link-format, senml |
| | | DELETE | |
| Sensor | core.s | GET | link-format, |
| | | | text/plain |
| Parameter | core.p | GET, PUT | link-format, |
| | | | text/plain |
| Read-only | core.rp | GET | link-format, |
| Parameter | | | text/plain |
| Actuator | core.a | GET, PUT, POST | link-format, |
| | | | text/plain |
| Binding | core.bnd | GET, POST, | link-format |
| | | DELETE | |
+-----------------+----------+-----------------+--------------------+
The following is an example of links in the CoRE Link Format using
these interface descriptions. The resource hierarchy is based on a
simple profile defined in Appendix A. These links are used in the
subsequent examples below.
Shelby, et al. Expires April 21, 2016 [Page 16]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Req: GET /.well-known/core
Res: 2.05 Content (application/link-format)
</s/>;rt="simple.sen";if="core.b",
</s/lt>;rt="simple.sen.lt";if="core.s",
</s/tmp>;rt="simple.sen.tmp";if="core.s";obs,
</s/hum>;rt="simple.sen.hum";if="core.s",
</a/>;rt="simple.act";if="core.b",
</a/1/led>;rt="simple.act.led";if="core.a",
</a/2/led>;rt="simple.act.led";if="core.a",
</d/>;rt="simple.dev";if="core.ll",
</l/>;if="core.lb",
6.1. Link List
The Link List interface is used to retrieve (GET) a list of resources
on a web server. The GET request SHOULD contain an Accept option
with the application/link-format content format; however if the
resource does not support any other form of GET methods the Accept
option MAY be elided. The Accept option SHOULD only include the
application/link-format content format. The request returns a list
of URI references with absolute paths to the resources as defined in
CoRE Link Format. This interface is typically used with a parent
resource to enumerate sub-resources but may be used to reference any
resource on a web server.
Link List is the base interface to provide gradual reveal of
resources on a CoRE web server, hence the root resource of a Function
Set SHOULD implement this interface or an extension of this
interface.
The following example interacts with a Link List /d containing
Parameter sub-resources /d/name, /d/model.
Req: GET /d/ (Accept:application/link-format)
Res: 2.05 Content (application/link-format)
</d/name>;rt="simple.dev.n";if="core.p",
</d/model>;rt="simple.dev.mdl";if="core.rp"
6.2. Batch
The Batch interface is used to manipulate a collection of sub-
resources at the same time. The Batch interface type supports the
same methods as its sub-resources, and can be used to read (GET),
update (PUT) or apply (POST) the values of those sub-resource with a
single resource representation. The sub-resources of a Batch MAY be
heterogeneous, a method used on the Batch only applies to sub-
Shelby, et al. Expires April 21, 2016 [Page 17]
Internet-DrafReusable Interface Definitions for Constrained October 2015
resources that support it. For example Sensor interfaces do not
support PUT, and thus a PUT request to a Sensor member of that Batch
would be ignored. A batch requires the use of SenML Media types in
order to support multiple sub-resources.
In addition, The Batch interface is an extension of the Link List
interface and in consequence MUST support the same methods.
The following example interacts with a Batch /s/ with Sensor sub-
resources /s/light, /s/temp and /s/humidity.
Req: GET /s/
Res: 2.05 Content (application/senml+json)
{"e":[
{ "n": "light", "v": 123, "u": "lx" },
{ "n": "temp", "v": 27.2, "u": "degC" },
{ "n": "humidity", "v": 80, "u": "%RH" }],
}
6.3. Linked Batch
The Linked Batch interface is an extension of the Batch interface.
Contrary to the basic Batch which is a collection statically defined
by the web server, a Linked Batch is dynamically controlled by a web
client. A Linked Batch resource has no sub-resources. Instead the
resources forming the batch are referenced using Web Linking
[RFC5988] and the CoRE Link Format [RFC6690]. A request with a POST
method and a content format of application/link-format simply appends
new resource links to the collection. The links in the payload MUST
reference a resource on the web server with an absolute path. A
DELETE request removes the entire collection. All other requests
available for a basic Batch are still valid for a Linked Batch.
The following example interacts with a Linked Batch /l/ and creates a
collection containing /s/light, /s/temp and /s/humidity in 2 steps.
Shelby, et al. Expires April 21, 2016 [Page 18]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Req: POST /l/ (Content-Format: application/link-format)
</s/light>,</s/temp>
Res: 2.04 Changed
Req: GET /l/
Res: 2.05 Content (application/senml+json)
{"e":[
{ "n": "/s/light", "v": 123, "u": "lx" },
{ "n": "/s/temp", "v": 27.2, "u": "degC" },
}
Req: POST /l/ (Content-Format: application/link-format)
</s/humidity>
Res: 2.04 Changed
Req: GET /l/ (Accept: application/link-format)
Res: 2.05 Content (application/link-format)
</s/light>,</s/temp>,</s/humidity>
Req: GET /l/
Res: 2.05 Content (application/senml+json)
{"e":[
{ "n": "/s/light", "v": 123, "u": "lx" },
{ "n": "/s/temp", "v": 27.2, "u": "degC" },
{ "n": "/s/humidity", "v": 80, "u": "%RH" }],
}
Req: DELETE /l/
Res: 2.02 Deleted
6.4. Hypermedia Collection
The Hypermedia Collection interface MAY provide a full set of the
methods and link relation types described in section Section 4 of
this document.
The following example interacts with a Hypermedia Collection at
/act1/actions/ by creating a new resource with Parameter sub-
resources newVal, tTime. The example depicts an actuation operation
with a new actuator value of 86.3% and a transition time of 10
seconds. The returned location of the created resource is then read,
and a response is returned which includes the remaining time for the
operation to complete "rTime". Then, the operation is cancelled by
sending a DELETE operation to the location of the created resource
that represents the running action.
Shelby, et al. Expires April 21, 2016 [Page 19]
Internet-DrafReusable Interface Definitions for Constrained October 2015
Req: POST /Act1/Actions/
Content-Format: application/collection+senml_json
Pl: [{"n":newVal", "v":86.3}, {"n":tTime", "v":10}]
Res: 2.01 Created
Location: Action1234
Req: GET /Act1/Actions/Action1234
Accepts: application/senml+json
Res: 2.05 Content
Pl: [{"n":newVal", "v":86.3},
{"n":tTime", "v":10},
{"n":"rTime", "v":"8.87"}]
Req: DELETE /Act1/Actions/Action1234
Res: 2.02 Deleted
Req: GET /Act1/Actions/Action1234
Res: 4.04 Not Found
6.5. Sensor
The Sensor interface allows the value of a sensor resource to be read
(GET). The Media type of the resource can be either plain text or
SenML. Plain text MAY be used for a single measurement that does not
require meta-data. For a measurement with meta-data such as a unit
or time stamp, SenML SHOULD be used. A resource with this interface
MAY use SenML to return multiple measurements in the same
representation, for example a list of recent measurements.
The following are examples of Sensor interface requests in both text/
plain and application/senml+json.
Req: GET /s/humidity (Accept: text/plain)
Res: 2.05 Content (text/plain)
80
Req: GET /s/humidity (Accept: application/senml+json)
Res: 2.05 Content (application/senml+json)
{"e":[
{ "n": "humidity", "v": 80, "u": "%RH" }],
}
Shelby, et al. Expires April 21, 2016 [Page 20]
Internet-DrafReusable Interface Definitions for Constrained October 2015
6.6. Parameter
The Parameter interface allows configurable parameters and other
information to be modeled as a resource. The value of the parameter
can be read (GET) or update (PUT). Plain text or SenML Media types
MAY be returned from this type of interface.
The following example shows request for reading and updating a
parameter.
Req: GET /d/name
Res: 2.05 Content (text/plain)
node5
Req: PUT /d/name (text/plain)
outdoor
Res: 2.04 Changed
6.7. Read-only Parameter
The Read-only Parameter interface allows configuration parameters to
be read (GET) but not updated. Plain text or SenML Media types MAY
be returned from this type of interface.
The following example shows request for reading such a parameter.
Req: GET /d/model
Res: 2.05 Content (text/plain)
SuperNode200
6.8. Actuator
The Actuator interface is used by resources that model different
kinds of actuators (changing its value has an effect on its
environment). Examples of actuators include for example LEDs,
relays, motor controllers and light dimmers. The current value of
the actuator can be read (GET) or the actuator value can be updated
(PUT). In addition, this interface allows the use of POST to change
the state of an actuator, for example to toggle between its possible
values. Plain text or SenML Media types MAY be returned from this
type of interface. A resource with this interface MAY use SenML to
include multiple measurements in the same representation, for example
a list of recent actuator values or a list of values to updated.
Shelby, et al. Expires April 21, 2016 [Page 21]
Internet-DrafReusable Interface Definitions for Constrained October 2015
The following example shows requests for reading, setting and
toggling an actuator (turning on a led).
Req: GET /a/1/led
Res: 2.05 Content (text/plain)
0
Req: PUT /a/1/led (text/plain)
1
Res: 2.04 Changed
Req: POST /a/1/led (text/plain)
Res: 2.04 Changed
Req: GET /a/1/led
Res: 2.05 Content (text/plain)
0
6.9. Binding
The Binding interface is used to manipulate a binding table. A
request with a POST method and a content format of application/link-
format simply appends new bindings to the table. All links in the
payload MUST have a relation type "boundTo". A GET request simply
returns the current state of a binding table whereas a DELETE request
empties the table.
The following example shows requests for adding, retrieving and
deleting bindings in a binding table.
Req: POST /bnd/ (Content-Format: application/link-format)
<coap://sensor.example.com/s/light>;
rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"
Res: 2.04 Changed
Req: GET /bnd/
Res: 2.05 Content (application/link-format)
<coap://sensor.example.com/s/light>;
rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"
Req: DELETE /bnd/
Res: 2.04 Changed
Shelby, et al. Expires April 21, 2016 [Page 22]
Internet-DrafReusable Interface Definitions for Constrained October 2015
6.10. Future Interfaces
It is expected that further interface descriptions will be defined in
this and other specifications.
6.11. WADL Description
This section defines the formal Web Application Description Langauge
(WADL) definition of these CoRE interface descriptions.
<?xml version="1.0" standalone="yes"?>
<application xmlns="http://research.sun.com/wadl/2006/10"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:senml="urn:ietf:params:xml:ns:senml">
<grammars>
<include href="http://tools.ietf.org/html/draft-jennings-senml"/>
</grammars>
<doc title="CoRE Interfaces"/>
<resource_type id="s">
<doc title="Sensor interface type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
</resource_type>
<resource_type id="p">
<doc title="Parameter interfacee type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
</resource_type>
<resource_type id="rp">
<doc title="Read-only Parameter interface type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
Shelby, et al. Expires April 21, 2016 [Page 23]
Internet-DrafReusable Interface Definitions for Constrained October 2015
<method href="#setattr"/>
</resource_type>
<resource_type id="a">
<doc title="Actuator interface type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
<method href="#apply"/>
</resource_type>
<resource_type id="ll">
<doc title="Link List interface type"/></doc>
<method href="#listLinks"/>
</resource_type>
<resource_type id="b">
<doc title="Batch of sub-resources interface type">The methods read,
observe, update and apply are applied to each sub-
resource of the requested resource that supports it. Mixed
sub-resource types can be supported.</doc>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
<method href="#apply"/>
<method href="#listLinks"/>
</resource_type>
<resource_type id="lb">
<doc title="Linked Batch interface type">. The methods read,
obervableRead, update and apply are applied to each linked
resource of the requested resource that supports it. Mixed
linked resource types can be supported.</doc>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
<method href="#apply"/>
<method href="#listLinks"/>
<method href="#appendLinks"/>
Shelby, et al. Expires April 21, 2016 [Page 24]
Internet-DrafReusable Interface Definitions for Constrained October 2015
<method href="#clearLinks"/>
</resource_type>
<resource_type id="hc">
<doc title="Hypermedia Collection interface type">.</doc>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
<method href="#apply"/>
<method href="#listLinks"/>
<method href="#appendLinks"/>
<method href="#clearLinks"/>
<method href="#updateLinks"/>
<method href="#readCollection"/>
<method href="#addItem"/>
</resource_type>
<resource_type id="bnd">
<doc title="Binding table resource type">A modifiable list of
links. Each link MUST have the relation type "boundTo".</doc>
<method href="#listLinks"/>
<method href="#appendLinks"/>
<method href="#clearLinks"/>
</resource_type>
<method id="read" name="GET">
<doc>Retrieve the value of a sensor, an actuator or a parameter.
Both HTTP and CoAP support this method.</doc>
<request>
</request>
<response status="200">
<representation mediaType="text/plain"/>
<representation mediaType="application/senml+exi"/>
<representation mediaType="application/senml+xml"/>
<representation mediaType="application/senml+json"/>
</response>
<response status="2.05">
<representation mediaType="text/plain"/>
<representation mediaType="application/senml+exi"/>
<representation mediaType="application/senml+xml"/>
<representation mediaType="application/senml+json"/>
</response>
</method>
Shelby, et al. Expires April 21, 2016 [Page 25]
Internet-DrafReusable Interface Definitions for Constrained October 2015
<method id="observe" name="GET">
<doc>Observe the value of a sensor, an actuator or a parameter.
Only CoAP supports this method since it requires the CoRE
Observe mechanism.</doc>
<request>
<param name="observe" style="header" type="xsd:integer">
<option value = 0/>
</param>
</request>
<response status="2.05">
<representation mediaType="text/plain"/>
<representation mediaType="application/senml+exi"/>
<representation mediaType="application/senml+xml"/>
<representation mediaType="application/senml+json"/>
</response>
</method>
<method id="observe-cancel" name="GET">
<doc>Cancel observation in progress.
Only CoAP supports this method since it requires the CoRE
Observe mechanism.</doc>
<request>
<param name="observe" style="header" type="xsd:integer">
<option value = 1/>
</param>
</request>
<response status="2.05">
<representation mediaType="text/plain"/>
<representation mediaType="application/senml+exi"/>
<representation mediaType="application/senml+xml"/>
<representation mediaType="application/senml+json"/>
</response>
</method>
<method id="update" name="PUT">
<doc>Control the actuator or update a parameter with a new value
or command. Both HTTP and CoAP support this method.</doc>
<request>
<representation mediaType="text/plain"/>
<representation mediaType="application/senml+exi"/>
<representation mediaType="application/senml+xml"/>
<representation mediaType="application/senml+json"/>
</request>
<response status="200"/>
<response status="2.04"/>
</method>
<method id="getattr" name="GET">
Shelby, et al. Expires April 21, 2016 [Page 26]
Internet-DrafReusable Interface Definitions for Constrained October 2015
<doc>Retrieve the observe attributes associated with a resource.
Both HTTP and CoAP support this method.</doc>
<request>
<doc>This request MUST contain an Accept option with
application/link-format when the resource supports
other GET methods.</doc>
<representation mediaType="application/link-format"/>
</request>
<response status="200">
<representation mediaType="application/link-format"/>
</response>
<response status="2.05">
<representation mediaType="application/link-format"/>
</response>
</method>
<method id="setattr" name="PUT">
<doc>Set the values of some or all of the observe attributes
associated with a resource.
Both HTTP and CoAP support this method.</doc>
<request>
<param name="pmin" style="query" type="xsd:integer"/>
<param name="pmax" style="query" type="xsd:integer"/>
<param name="lt" style="query" type="xsd:decimal"/>
<param name="gt" style="query" type="xsd:decimal"/>
<param name="st" style="query" type="xsd:decimal"/>
</request>
<response status="200">
</response>
<response status="2.04">
</response>
</method>
<method id="apply" name="POST">
<doc>Apply the value, if supplied, to resources. Both HTTP and CoAP
support this method.</doc>
<request>
<doc>The apply function may contain a payload to be applied.</doc>
</request>
<response status="200"/>
<response status="2.04"/>
</method>
<method id="listLinks" name="GET">
<doc>Retrieve the list of Web links associated to a resource.
Both HTTP and CoAP support this method.</doc>
<request>
<doc>This request MUST contain an Accept option with
Shelby, et al. Expires April 21, 2016 [Page 27]
Internet-DrafReusable Interface Definitions for Constrained October 2015
application/link-format when the resource supports
other GET methods.</doc>
</request>
<response status="200">
<representation mediaType="application/link-format"/>
</response>
<response status="2.05">
<representation mediaType="application/link-format"/>
</response>
</method>
<method id="appendLinks" name="POST">
<doc>Append new Web links to a resource which is a collection
of links. Both HTTP and CoAP support this method.</doc>
<request>
<representation mediaType="application/link-format"/>
</request>
<response status="200"/>
<response status="2.04"/>
</method>
<method id="clearLinks" name="DELETE">
<doc>Clear all Web Links in a resource which is a collection
of links. Both HTTP and CoAP support this method.</doc>
<request>
</request>
<response status="200"/>
<response status="2.02"/>
</method>
<method id="updateLinks" name="PATCH">
<doc>Update all Web Links in a resource which is a collection
of links. Both HTTP and CoAP support this method.</doc>
<doc>This request MUST contain a Content-Format option with
application/merge-patch+json.</doc>
<request>
</request>
<response status="200"/>
<response status="2.04"/>
</method>
<method id="addItem" name="POST">
<doc>Add zero or more items to the collection with their links. Both HTTP and CoAP support this method.</doc>
<doc>This request MAY contain a Content-Format option with
application/collection+senml+json.</doc>
<doc>This request MAY contain a Content-Format option with
application/senml+json.</doc>
<request>
Shelby, et al. Expires April 21, 2016 [Page 28]
Internet-DrafReusable Interface Definitions for Constrained October 2015
</request>
<response status="200"/>
<response status="2.01"/>
</method>
<method id="readCollection" name="GET">
<doc>REturn a representation of both links and items in the collection. Both HTTP and CoAP support this method.</doc>
<doc>This request MUST contain an Accepts option with
application/collection+senml+json.</doc>
<request>
</request>
<response status="200"/>
<response status="2.05"/>
</method>
</application>
7. Function Sets and Profiles
This section defines how a set of REST resources can be created
called a function set. A Function Set is similar to a function block
in the sense that it consists of input, output and parameter
resources and contains internal logic. A Function Set can have a
subset of mandatory inputs, outputs and parameters to provide minimum
interoperability. It can also be extended with manufacturer/user-
specific resources. A device is composed of one or more Function Set
instances.
An example of function sets can be found from the CoRE Resource
Directory specification that defines REST interfaces for
registration, group and lookup [I-D.ietf-core-resource-directory].
The OMA Lightweight M2M standard [REF] also defines a function set
structure called an Objects that use integer path, instance and
resource URI segments. OMA Objects can be defined and then
registered with an OMA maintained registry [REF]. This section is
simply meant as a guideline for the definition of other such REST
interfaces, either custom or part of other specifications.
7.1. Defining a Function Set
In a Function Set, types of resources are defined. Each type
includes a human readable name, a path template, a Resource Type for
discovery, the Interface Definition and the data type and allowed
values. A Function Set definition may also include a field
indicating if a sub-resource is mandatory or optional.
Shelby, et al. Expires April 21, 2016 [Page 29]
Internet-DrafReusable Interface Definitions for Constrained October 2015
7.1.1. Path template
A Function Set is a container resource under which its sub-resources
are organized. The profile defines the path to each resource of a
Function Set in a path template. The template can contain either
relative paths or absolute paths depending on the profile needs. An
absolute Function Set should be located at its recommended root path
on a web server, however it can be located under an alternative path
if necessary (for example multi-purpose devices, gateways etc.). A
relative Function Set can be instantiated as many times as needed on
a web server with an arbitrary root path. However some Function Sets
(e.g. device description) only make sense as singletons.
The path template includes a possible index {#} parameter, and
possible fixed path segments. The index {#} allows for multiple
instances of this type of resource, and can be any string. The root
path and the indexes are the only variable elements in a path
template. All other path segments should be fixed.
7.1.2. Resource Type
Each root resource of a Function Set is assigned a Resource Type
parameter, therefore making it possible to discover it. Each sub-
resource of a Function Set is also assigned a Resource Type
parameter. This Resource Type is used for resource discovery and is
usually necessary to discover optional resources supported on a
specific device. The Resource Type of a Function Set may also be
used for service discovery and can be exported to DNS-SD [RFC6763]
for example.
The Resource Type parameter defines the value that should be included
in the rt= field of the CoRE Link Format when describing a link to
this resource. The value SHOULD be in the form "namespace.type" for
root resources and "namespace.type.subtype" for sub-resources. This
naming convention facilitates resource type filtering with the
/.well-known/core resource. However a profile could allow mixing in
foreign namespace references within a Function Set to import external
references from other object models (e.g. SenML and UCUM).
7.1.3. Interface Description
The Interface Description parameter defines the REST interface for
that type of resource. Several base interfaces are defined in
Section 6 of this document. For a given profile, the Interface
Description may be inferred from the Resource Type. In that case the
Interface Description MAY be elided from link descriptions of
resource types defined in the profile, but should be included for
custom extensions to the profile.
Shelby, et al. Expires April 21, 2016 [Page 30]
Internet-DrafReusable Interface Definitions for Constrained October 2015
The root resource of a Function Set should provide a list of links to
its sub-resources in order to offer gradual reveal of resources. The
CoRE Link List interface defined in Section 6.1 offers this
functionality so a root resource should support this interface or a
derived interface like CoRE Batch (See Section 6.2).
7.1.4. Data type
The Data Type field defines the type of value (and possible range)
that is returned in response to a GET for that resource or accepted
with a PUT. The interfaces defined in Section 6 make use of plain
text and SenML Media types for the actual format of this data. A
profile may restrict the list of supported content formats for the
CoRE interfaces or define new interfaces with new content types.
7.2. Discovery
A device conforming to a profile SHOULD make its resources
discoverable by providing links to the resources on the path /.well-
known/core as defined in [RFC6690]. All resources hosted on a device
SHOULD be discoverable either with a direct link in /.well-known/core
or by following successive links starting from /.well-known/core.
The root path of a Function Set instance SHOULD be directly
referenced in /.well-known/core in order to offer discovery at the
first discovery stage. A device with more than 10 individual
resources SHOULD only expose Function Set instances in /.well-known/
core to limit the size of this resource.
In addition, a device MAY register its resources to a Resource
Directory using the registration interface defined in
[I-D.ietf-core-resource-directory] if such a directory is available.
7.3. Versioning
A profile should track Function Set changes to avoid incompatibility
issues. Evolutions in a Function Set SHOULD be backward compatible.
8. Security Considerations
An implementation of a client needs to be prepared to deal with
responses to a request that differ from what is specified in this
document. A server implementing what the client thinks is a resource
with one of these interface descriptions could return malformed
representations and response codes either by accident or maliciously.
A server sending maliciously malformed responses could attempt to
take advantage of a poorly implemented client for example to crash
the node or perform denial of service.
Shelby, et al. Expires April 21, 2016 [Page 31]
Internet-DrafReusable Interface Definitions for Constrained October 2015
9. IANA Considerations
The interface description types defined require registration.
The new link relations type "boundto" and "grp" require registration.
10. Acknowledgments
Acknowledgement is given to colleagues from the SENSEI project who
were critical in the initial development of the well-known REST
interface concept, to members of the IPSO Alliance where further
requirements for interface types have been discussed, and to Szymon
Sasin, Cedric Chauvenet, Daniel Gavelle and Carsten Bormann who have
provided useful discussion and input to the concepts in this
document.
11. Changelog
Changes from -03 to -04
o Fixed tickets #385 and #386
o Changed abstract and into to better describe content
o Focus on Interface and not function set/profiles in intro
o Changed references from draft-core-observe to RFC7641
o Moved Function sets and Profiles to section after Interfaces
o Moved Observe Attributes to the Link Binding section
o Add a Collection section to describe the collection types
o Add the Hypermedia Collection Interface Description
Changes from -02 to -03
o Added lt and gt to binding format section.
o Added pmin and pmax observe parameters to Observation Attributes
o Changed the definition of lt and gt to limit crossing.
o Added definitions for getattr and setattr to WADL.
o Added getattr and setattr to observable interfaces.
Shelby, et al. Expires April 21, 2016 [Page 32]
Internet-DrafReusable Interface Definitions for Constrained October 2015
o Removed query parameters from Observe definition.
o Added observe-cancel definition to WADL and to observable
interfaces.
Changes from -01 to -02
o Updated the date and version, fixed references.
o Removed pmin and pmax observe parameters [Ticket #336]
Changes from -00 to WG Document -01
o Improvements to the Function Set section.
Changes from -05 to WG Document -00
o Updated the date and version.
Changes from -04 to -05
o Made the Observation control parameters to be treated as resources
rather than Observe query parameters. Added Less Than and Greater
Than parameters.
Changes from -03 to -04
o Draft refresh
Changes from -02 to -03
o Added Bindings
o Updated all rt= and if= for the new Link Format IANA rules
Changes from -01 to -02
o Defined a Function Set and its guidelines.
o Added the Link List interface.
o Added the Linked Batch interface.
o Improved the WADL interface definition.
o Added a simple profile example.
Shelby, et al. Expires April 21, 2016 [Page 33]
Internet-DrafReusable Interface Definitions for Constrained October 2015
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
12.2. Informative References
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-04
(work in progress), July 2015.
[I-D.jennings-core-senml]
Jennings, C., Shelby, Z., Arkko, J., and A. Keranen,
"Media Types for Sensor Markup Language (SENML)", draft-
jennings-core-senml-01 (work in progress), July 2015.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<http://www.rfc-editor.org/info/rfc7396>.
Shelby, et al. Expires April 21, 2016 [Page 34]
Internet-DrafReusable Interface Definitions for Constrained October 2015
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
Appendix A. Profile example
The following is a short definition of simple profile. This
simplistic profile is for use in the examples of this document.
+--------------------+-----------+------------+---------+
| Function Set | Root Path | RT | IF |
+--------------------+-----------+------------+---------+
| Device Description | /d | simple.dev | core.ll |
| Sensors | /s | simple.sen | core.b |
| Actuators | /a | simple.act | core.b |
+--------------------+-----------+------------+---------+
List of Function Sets
+-------+----------+----------------+---------+------------+
| Type | Path | RT | IF | Data Type |
+-------+----------+----------------+---------+------------+
| Name | /d/name | simple.dev.n | core.p | xsd:string |
| Model | /d/model | simple.dev.mdl | core.rp | xsd:string |
+-------+----------+----------------+---------+------------+
Device Description Function Set
+-------------+-------------+----------------+--------+-------------+
| Type | Path | RT | IF | Data Type |
+-------------+-------------+----------------+--------+-------------+
| Light | /s/light | simple.sen.lt | core.s | xsd:decimal |
| | | | | (lux) |
| Humidity | /s/humidity | simple.sen.hum | core.s | xsd:decimal |
| | | | | (%RH) |
| Temperature | /s/temp | simple.sen.tmp | core.s | xsd:decimal |
| | | | | (degC) |
+-------------+-------------+----------------+--------+-------------+
Sensors Function Set
Shelby, et al. Expires April 21, 2016 [Page 35]
Internet-DrafReusable Interface Definitions for Constrained October 2015
+------+------------+----------------+--------+-------------+
| Type | Path | RT | IF | Data Type |
+------+------------+----------------+--------+-------------+
| LED | /a/{#}/led | simple.act.led | core.a | xsd:boolean |
+------+------------+----------------+--------+-------------+
Actuators Function Set
Authors' Addresses
Zach Shelby
ARM
150 Rose Orchard
San Jose 95134
FINLAND
Phone: +1-408-203-9434
Email: zach.shelby@arm.com
Matthieu Vial
Schneider-Electric
Grenoble
FRANCE
Phone: +33 (0)47657 6522
Email: matthieu.vial@schneider-electric.com
Michael Koster
ARM
150 Rose Orchard
San Jose 95134
USA
Email: michael.koster@arm.com
Shelby, et al. Expires April 21, 2016 [Page 36]
