CoRE Interfaces
draft-shelby-core-interfaces-02

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 concept of Function Set to guide the creation of RESTful profiles.

Status of this Memo

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This Internet-Draft will expire on September 06, 2012.

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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Function Set
• 3.1. Defining a Function Set
• 3.1.1. Path template
• 3.1.2. Resource Type
• 3.1.3. Interface Description
• 3.1.4. Data type
• 3.2. Discovery
• 3.3. Versioning
• 4. Interface Descriptions
• 4.1. Link List
• 4.2. Batch
• 4.3. Linked Batch
• 4.4. Sensor
• 4.5. Parameter
• 4.6. Read-only Parameter
• 4.7. Actuator
• 4.8. Resource Observation
• 4.9. Future Interfaces
• 4.10. WADL Description
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgments
• 8. Changelog
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Profile example
• Authors' Addresses

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 [I-D.ietf-core-link-format] and can be used by CoAP [I-D.ietf-core-coap] 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 document 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 the Function Set, which is a group of REST resources providing a service in a distributed system. 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 [I-D.ietf-core-link-format]. This specification makes use of the following additional terminology:

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.

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.

3. Function Set

This section defines how a specification can organize REST resources to create a new profile. A profile is structured into groups of resource types called Function Sets. 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 MAY have a subset of mandatory inputs, outputs and parameters to provide minimum interoperability. It MAY also be extended with manufacturer/user-specific resources. Other specifications defines the list of function sets available within a given profile. A device is composed of one or more Function Set instances. A profile specification MAY specify device profiles with mandatory function sets.

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.

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 MAY 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 MUST 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 MAY be exported to DNS-SD [I-D.cheshire-dnsext-dns-sd] for example.

The Resource Type parameter defines the value that MUST 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 MAY 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 4 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.

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 4.1 offers this functionality so a root resource SHOULD support this interface or a derived interface like CoRE Batch (See Section 4.2).

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 4 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 [I-D.ietf-core-link-format]. 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.shelby-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. Interface Descriptions

This section defines REST interfaces for Link List, Batch, Sensor, Parameter and Actuator 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.

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 Appendix A. These links are used in the subsequent examples below.

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: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",
    

4.1. 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 /s containing Sensor sub-resources /s/light, /s/temp and /s/humidity.

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"

    

4.2. 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-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" }],
}

    

4.3. 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 [I-D.ietf-core-link-format]. 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.

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

    

4.4. 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.

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" }],
}

    

4.5. 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 

    

4.6. 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

    

4.7. 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 request 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

    

4.8. Resource Observation

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 request how often a client is interested in receiving notifications and how much a resource 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.

Minimum Period:

When present, the minimum period indicates the minimum time in seconds the server SHOULD wait between sending notifications. In the absence of this parameter, the minimum period is up to the server.

Maximum Period:

When present, the maximum period indicated the maximum time in seconds the server SHOULD wait between sending notifications (regardless if it has changed). In the absence of this parameter, the maximum period is up to the server. The maximum period MUST be great 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.

The following example shows an Observation request using these query parameters. Here the value of Observe indicates the number of seconds since the observation was made to show the time.

Req: GET Observe /s/temp?pmin=10&pmax=60&st=1
Res: 2.05 Content Observe:0 (text/plain)
23.2

Res: 2.05 Content Observe:60 (text/plain)
23.0

Res: 2.05 Content Observe:80 (text/plain)
22.2

Res: 2.05 Content Observe:140 (text/plain)
21.8

    

4.9. 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.

4.10. 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 resource type"/>
    <method href="#read"/>
    <method href="#observe"/>
  </resource_type>

  <resource_type id="p">
    <doc title="Parameter resource type"/>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#update"/>
  </resource_type>

  <resource_type id="rp">
    <doc title="Read-only Parameter resource type"/>
    <method href="#read"/>
    <method href="#observe"/>
  </resource_type> 

  <resource_type id="a">
    <doc title="Actuator resource type"/>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#update"/>
    <method href="#toggle"/>
  </resource_type>

  <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="#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="#update"/>
    <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>

  <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="pmin" style="query" type="xsd:integer"/>
      <param name="pmax" style="query" type="xsd:integer"/>
      <param name="st" style="query" type="xsd:decimal"/>
    </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="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>

  

5. 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.

6. IANA Considerations

To be determined if a registry of interface descriptions should be created for CoRE, allowing other interface descriptions to be registered by other specifications (and if this document is the place to create such a registry).

7. 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.

8. Changelog

Changes from -01 to -02

• Defined a Function Set and its guidelines.
• Added the Link List interface.
• Added the Linked Batch interface.
• Improved the WADL interface definition.
• Added a simple profile example.

9. References

9.1. Normative References

9.2. Informative References

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.

Authors' Addresses