CoRE Z. Shelby
Internet-Draft ARM
Intended status: Informational M. Vial
Expires: January 7, 2016 Schneider-Electric
M. Koster
ARM
July 6, 2015
CoRE Interfaces
draft-ietf-core-interfaces-03
Abstract
This document defines well-known REST interface descriptions for
Batch, Sensor, Parameter and Actuator types for use in contrained web
servers using the CoRE Link Format. A short reference is provided
for each type that can be efficiently included in the interface
description attribute of the CoRE Link Format. These descriptions
are intended to be for general use in resource designs or for
inclusion in more specific interface profiles. In addition, this
document defines the concepts of Function Set and Binding. The
former is the basis element to create RESTful profiles and the latter
helps the configuration of links between resources located on one or
more endpoints.
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
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on January 7, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Function Set . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Defining a Function Set . . . . . . . . . . . . . . . . . 4
3.1.1. Path template . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Resource Type . . . . . . . . . . . . . . . . . . . . 5
3.1.3. Interface Description . . . . . . . . . . . . . . . . 5
3.1.4. Data type . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Versioning . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Binding methods . . . . . . . . . . . . . . . . . . . . . 8
4.3. Binding table . . . . . . . . . . . . . . . . . . . . . . 9
5. Interface Descriptions . . . . . . . . . . . . . . . . . . . . 10
5.1. Link List . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Batch . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Linked Batch . . . . . . . . . . . . . . . . . . . . . . . 12
5.4. Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.5. Parameter . . . . . . . . . . . . . . . . . . . . . . . . 14
5.6. Read-only Parameter . . . . . . . . . . . . . . . . . . . 14
5.7. Actuator . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.8. Binding . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.9. Resource Observation Attributes . . . . . . . . . . . . . 16
5.10. Future Interfaces . . . . . . . . . . . . . . . . . . . . 17
5.11. WADL Description . . . . . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
9. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Profile example . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
The Constrained RESTful Environments (CoRE) working group aims at
realizing the REST architecture in a suitable form for the most
constrained nodes (e.g. 8-bit microcontrollers with limited RAM and
ROM) and networks (e.g. 6LoWPAN). CoRE is aimed at machine-to-
machine (M2M) applications such as smart energy and building
automation.
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 and static interfaces result in fragility. The
discovery of resources provided by an HTTP Web Server is typically
called Web Linking [RFC5988]. The use of Web Linking for the
description and discovery of resources hosted by constrained web
servers is specified by the CoRE Link Format [RFC6690] and can be
used by CoAP [RFC7252] or HTTP servers. The CoRE Link Format defines
an attribute that can be used to describe the REST interface of a
resource, and may include a link to a description document. This
memo describes how other specifications can combine resources with a
well-known interface to create new CoRE RESTful profiles. A CoRE
profile is based on the concept of Function Set, which is a group of
REST resources providing a service in a distributed system. In
addition, the notion of Binding is introduced in order to create a
synchronization link between two resources. This document also
defines well-known interface descriptions for Batch, Sensor,
Parameter and Actuator types to compose new Function Sets or for
standalone use in a constrained web server. A short reference is
provided for each type that can be efficiently included in the
interface description (if=) attribute of the CoRE Link Format.
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:
Function Set: A group of well-known REST resources that provides a
particular service.
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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.
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.
Binding: A unidirectional logical link between a source resource and
a destination resource.
3. Function Set
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.
3.1. 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.
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3.1.1. 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.
3.1.2. 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).
3.1.3. Interface Description
The Interface Description parameter defines the REST interface for
that type of resource. Several base interfaces are defined in
Section 5 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.
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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 5.1 offers this
functionality so a root resource should support this interface or a
derived interface like CoRE Batch (See Section 5.2).
3.1.4. 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 5 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 types for the CoRE
interfaces or define new interfaces with new content types.
3.2. 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.
3.3. Versioning
A profile should track Function Set changes to avoid incompatibility
issues. Evolutions in a Function Set SHOULD be backward compatible.
4. Bindings
In a M2M RESTful environment, endpoints exchange the content of their
resources to operate the distributed system. 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 Binding.
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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 4.2.
+---------+------------+-------------+---------------+
| Name | Identifier | Location | Method |
+---------+------------+-------------+---------------+
| Polling | poll | Destination | GET |
| Observe | obs | Destination | GET + Observe |
| Push | push | Source | PUT |
+---------+------------+-------------+---------------+
4.1. Format
Since Binding lies in 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 |
+--------------------+-----------+------------------+
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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 syncronization
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.
4.2. 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.
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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 relies on the Publish/Subscribe pattern
thus an observation relationship is created 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
[I-D.ietf-core-observe] 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 5.9).
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.
4.3. 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 5.8. A profile SHOULD allow
only one resource table per endpoint.
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5. 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
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 |
+-------------------+----------+------------------------------------+
| Link List | core.ll | GET |
| Batch | core.b | GET, PUT, POST (where applicable) |
| Linked Batch | core.lb | GET, PUT, POST, DELETE (where |
| | | applicable) |
| Sensor | core.s | GET |
| Parameter | core.p | GET, PUT |
| Read-only | core.rp | GET |
| Parameter | | |
| Actuator | core.a | GET, PUT, POST |
| Binding | core.bnd | GET, POST, 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.
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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",
5.1. 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 type, 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 type. 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"
5.2. 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), set
(PUT) or toggle (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-
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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" }],
}
5.3. 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 type of application/link-format simply appends
new resources to the collection. The links in the payload MUST
reference a resource on the web server with an absolute path. A
DELETE request empties the current collection of links. 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.
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Req: POST /l (Content-type: 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-type: 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: 2.04 Changed
5.4. 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.
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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" }],
}
5.5. 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 set (PUT). Plain text or SenML Media types MAY
be returned from this type of interface.
The following example shows request for reading and setting a
parameter.
Req: GET /d/name
Res: 2.05 Content (text/plain)
node5
Req: PUT /d/name (text/plain)
outdoor
Res: 2.04 Changed
5.6. Read-only Parameter
The Read-only Parameter interface allows configuration parameters to
be read (GET) but not set. 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
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5.7. 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 a new actuator value set (PUT). In
addition, this interface defines the use of POST (with no body) to
toggle an actuator 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 set.
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
5.8. Binding
The Binding interface is used to manipulate a binding table. A
request with a POST method and a content type 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.
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Req: POST /bnd (Content-type: 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
5.9. Resource Observation Attributes
When resource interfaces following this specification are made
available over CoAP, the CoAP Observation mechanism
[I-D.ietf-core-observe] 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 set using PUT, and MUST NOT be included in the Observe
request. Multiple parameters MAY be set at the same time by
including the values in the query string of a PUT. Before being set,
these parameters have no default value.
+----------------+------------------+------------------+
| 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 |
+----------------+------------------+------------------+
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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 syncronization
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.
5.10. Future Interfaces
It is expected that further interface descriptions will be defined in
this and other specifications. Potential interfaces to be considered
for this specifications include:
Collection: This resource would be a container that allows sub-
resources to be added or removed.
5.11. WADL Description
This section defines the formal Web Application Description Langauge
(WADL) definition of these CoRE interface descriptions.
<?xml version="1.0" standalone="yes"?>
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<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 resource 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 resource 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 resource type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
</resource_type>
<resource_type id="a">
<doc title="Actuator resource type"/>
<method href="#read"/>
<method href="#observe"/>
<method href="#observe-cancel"/>
<method href="#getattr"/>
<method href="#setattr"/>
<method href="#update"/>
<method href="#toggle"/>
</resource_type>
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<resource_type id="ll">
<doc title="Link List type"/></doc>
<method href="#listLinks"/>
</resource_type>
<resource_type id="b">
<doc title="Batch of sub-resources type">The methods read,
observe, update and toggle 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="#toggle"/>
<method href="#listLinks"/>
</resource_type>
<resource_type id="lb">
<doc title="Linked Batch resource type">. The methods read,
obervableRead, update and toggle 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="#listLinks"/>
<method href="#appendLinks"/>
<method href="#clearLinks"/>
</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>
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</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>
<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>
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</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">
<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>
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<method id="toggle" name="POST">
<doc>Toggle the values of actuator resources. Both HTTP and CoAP
support this method.</doc>
<request>
<doc>The toggle function is only applicable if the request
is empty.</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
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="2.04"/>
</method>
</application>
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6. 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.
7. IANA Considerations
The interface description types defined require registration.
The new link relation type "boundto" requires registration.
8. 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.
9. Changelog
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.
o Removed query parameters from Observe definition.
o Added observe-cancel definition to WADL and to observable
interfaces.
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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.
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
10.2. Informative References
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP",
draft-ietf-core-observe-16 (work in progress),
December 2014.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-03
(work in progress), June 2015.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
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
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+-------+----------+----------------+---------+------------+
| 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
+------+------------+----------------+--------+-------------+
| 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
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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
FINLAND
Phone:
Email: michael.koster@arm.com
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