CoRE Interfaces
draft-shelby-core-interfaces-01

CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                        January 31, 2012
Expires: August 3, 2012

                            CoRE Interfaces
                    draft-shelby-core-interfaces-01

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.

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 August 3, 2012.

Copyright Notice

   Copyright (c) 2012 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

Shelby                   Expires August 3, 2012                 [Page 1]
Internet-Draft               CoRE Interfaces                January 2012

   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Interface Descriptions . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Batch  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Sensor . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Parameter  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Read-only Parameter  . . . . . . . . . . . . . . . . . . .  6
     3.5.  Actuator . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.6.  Resource Observation . . . . . . . . . . . . . . . . . . .  7
     3.7.  Future Interfaces  . . . . . . . . . . . . . . . . . . . .  8
     3.8.  WADL Description . . . . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13

Shelby                   Expires August 3, 2012                 [Page 2]
Internet-Draft               CoRE Interfaces                January 2012

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 defines well-known interface descriptions for Batch, Sensor,
   Parameter, Read-only Parameter and Actuator types for use in
   contrained web servers.  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].

3.  Interface Descriptions

   This section defines REST interfaces for Batch, Sensor, Parameter and
   Actuator resources.  Each type is described along with its Interface
   Description attribute value, recommended path pattern 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

Shelby                   Expires August 3, 2012                 [Page 3]
Internet-Draft               CoRE Interfaces                January 2012

   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 Path column defines an example path pattern that a resource of
   this type might use for simple end-points.  These interfaces are
   expected to also be used in other path patterns.  In any case,
   resource paths SHOULD be discoverable as described in
   [I-D.ietf-core-link-format].

   The Methods column defines the methods supported by that interface,
   which are described in more detail below.

   +-----------------+---------+------------+--------------------------+
   |       Interface | if=     | Path       | Methods                  |
   |                 |         | Example    |                          |
   +-----------------+---------+------------+--------------------------+
   |           Batch | core#b  | /s,/p,/a   | GET, PUT, POST (where    |
   |                 |         |            | applicable)              |
   |          Sensor | core#s  | /s/{name}  | GET                      |
   |       Parameter | core#p  | /p/{name}  | GET, PUT                 |
   |       Read-only | core#rp | /p/{name}  | GET                      |
   |       Parameter |         |            |                          |
   |        Actuator | core#a  | /a/{name}  | GET, PUT, POST           |
   +-----------------+---------+------------+--------------------------+

   The following is an example of links in the CoRE Link Format using
   these interface descriptions.  These links are used in the subsequent
   examples below.

   Req: GET /.well-known/core
   Res: 2.05 Content (application/link-format)
   </s>;if="core#b",
   </s/light>;if="core#s",
   </s/temp>;if="core#s";obs,
   </s/humidity>;if="core#s",
   </p/name>;if="core#p",
   </p/model>;if="core#rp",
   </a>;if="core#b",
   </a/led1>;if="core#a",
   </a/led2>;if="core#a"

Shelby                   Expires August 3, 2012                 [Page 4]
Internet-Draft               CoRE Interfaces                January 2012

3.1.  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) or
   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.

   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" }],
   }

3.2.  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                   Expires August 3, 2012                 [Page 5]
Internet-Draft               CoRE Interfaces                January 2012

   }

3.3.  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 /p/name
   Res: 2.05 Content (text/plain)
   node5

   Req: PUT /p/name (text/plain)
   outdoor
   Res: 2.04 Changed

3.4.  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 /p/model
   Res: 2.05 Content (text/plain)
   SuperNode200

3.5.  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

Shelby                   Expires August 3, 2012                 [Page 6]
Internet-Draft               CoRE Interfaces                January 2012

   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/led1
   Res: 2.05 Content (text/plain)
   0

   Req: PUT /a/led1 (text/plain)
   1
   Res: 2.04 Changed

   Req: POST /a/led1 (text/plain)
   Res: 2.04 Changed

   Req: GET /a/led1
   Res: 2.05 Content (text/plain)
   0

3.6.  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.

           +--------------------+-----------+------------------+
           | Query              | Parameter | Value            |
           +--------------------+-----------+------------------+
           | Minimum Period (s) | pmin      | xsd:integer (>0) |
           | Maximum Period (s) | pmax      | xsd:integer (>0) |
           | Change Step        | st        | xsd:decimal (>0) |
           +--------------------+-----------+------------------+

Shelby                   Expires August 3, 2012                 [Page 7]
Internet-Draft               CoRE Interfaces                January 2012

   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

3.7.  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:

   Batch of Links:  This resource would contain a list of links,
      pointing to resources that will be manipulated by requesting a
      method on that resource.

Shelby                   Expires August 3, 2012                 [Page 8]
Internet-Draft               CoRE Interfaces                January 2012

   Collection:  This resource would be a container that allows sub-
      resources to be added or removed.

3.8.  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">

   <grammars>
   <include href="http://tools.ietf.org/html/draft-jennings-senml"/>
   </grammars>

   <doc title="CoRE Interfaces"/>

   <resource_type id="b">
    <doc title="Batch of sub-resources type. The method is applied
    to each sub-resource of the requested resource that
    supports it. Mixed sub-resource types can be supported."/>
    <method id="b_request" name="GET">
       <doc>Retrieve the representations of all resources under
       this collection in a single request.</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="200">
         <representation mediaType="application/senml+exi"/>
         <representation mediaType="application/senml+xml"/>
         <representation mediaType="application/senml+json"/>
       </response>
    </method>
   <method id="b_update" name="PUT">
       <doc>Update the representations of all resources under
       this collection in a single request that support
       PUT.</doc>
       <request>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </request>
       <response status="204"/>

Shelby                   Expires August 3, 2012                 [Page 9]
Internet-Draft               CoRE Interfaces                January 2012

    </method>
    <method id="b_toggle" name="POST">
       <doc>Toggle the values of actuator resources that support
       POST in the collection.</doc>
       <request>
       </request>
       <response status="204"/>
    </method>
   </resource_type>

   <resource_type id="s">
    <doc title="Sensor resource type"/>
    <method id="s_request" name="GET">
       <doc>Retrieve the value of the sensor</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="200">
          <representation mediaType="text/plain"/>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </response>
    </method>
   </resource_type>

   <resource_type id="p">
    <doc title="Parameter resource type"/>
    <method id="p_request" name="GET">
       <doc>Retrieve the value of the parameter</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="200">
          <representation mediaType="text/plain"/>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </response>
    </method>
    <method id="p_update" name="PUT">
       <doc>Update to the parameter value</doc>
       <request>
          <representation mediaType="text/plain"/>

Shelby                   Expires August 3, 2012                [Page 10]
Internet-Draft               CoRE Interfaces                January 2012

          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </request>
       <response status="204"/>
    </method>
   </resource_type>

   <resource_type id="rp">
    <doc title="Read-only Parameter resource type"/>
    <method id="p_request" name="GET">
       <doc>Retrieve the value of the parameter</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="200">
          <representation mediaType="text/plain"/>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </response>
    </method>
   </resource_type>

   <resource_type id="a">
    <doc title="Actuator resource type"/>
    <method id="a_request" name="GET">
       <doc>Retrieve the value of the actuator</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="200">
          <representation mediaType="text/plain"/>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>
          <representation mediaType="application/senml+json"/>
       </response>
    </method>
    <method id="a_actuate" name="PUT">
       <doc>Control the actuator with a new value or command</doc>
       <request>
          <representation mediaType="text/plain"/>
          <representation mediaType="application/senml+exi"/>
          <representation mediaType="application/senml+xml"/>

Shelby                   Expires August 3, 2012                [Page 11]
Internet-Draft               CoRE Interfaces                January 2012

          <representation mediaType="application/senml+json"/>
       </request>
       <response status="204"/>
    </method>
    <method id="a_toggle" name="POST">
       <doc>Toggle the value of the actuator</doc>
       <request>
       </request>
       <response status="204"/>
    </method>
   </resource_type>

   </application>

4.  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.

5.  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).

6.  Acknowledgments

   Acknowledgement is given to colleagues from the SENSEI project who
   were critical in the initial development of the well-known REST
   interface conept, 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.

   Special thanks to Matthieu Vial for helping develop ideas for the
   batch, link batch and collection resources and for providing useful

Shelby                   Expires August 3, 2012                [Page 12]
Internet-Draft               CoRE Interfaces                January 2012

   comments and the document.

7.  Changelog

8.  References

8.1.  Normative References

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-11 (work in progress),
              January 2012.

   [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.

8.2.  Informative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

   [I-D.ietf-core-observe]
              Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
              draft-ietf-core-observe-03 (work in progress),
              October 2011.

Author's Address

   Zach Shelby
   Sensinode
   Kidekuja 2
   Vuokatti  88600
   FINLAND

   Phone: +358407796297
   Email: zach@sensinode.com

Shelby                   Expires August 3, 2012                [Page 13]