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CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                   M. Vial
Expires: May 13, 2015                                 Schneider-Electric
                                                        November 9, 2014


                            CoRE Interfaces
                     draft-ietf-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 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
   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 May 13, 2015.

Copyright Notice

   Copyright (c) 2014 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  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 . . . . . . . . . . . . . . . . . . . .  9
     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  . . . . . . . . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Profile example . . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24





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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) |
           +--------------------+-----------+------------------+







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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.

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.

   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.







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   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).

   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 upon
      change of 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.


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



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   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.


   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",







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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-
   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.





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   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      |
           +--------------+----------------+------------------+
           | Change Step  | /{resource}?st | xsd:decimal (>0) |
           | Less Than    | /{resource}?lt | xsd:decimal      |
           | Greater Than | /{resource}?gt | xsd:decimal      |
           +--------------+----------------+------------------+








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   Change Step:  When set, 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.

   Less Than:  When set, the value of the resource MUST be less than
      this parameter in order to send a new notification.  This
      parameter has lower priority than the period parameters.

   Greater Than:  When set, the value of the resource MUST be greater
      than this parameter in order to send a new notification.  This
      parameter has lower priority than the period parameters.

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"?>

   <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>




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     <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"/>



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       <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>
       </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">



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       <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>



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     <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>



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 -01 to -02





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   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

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [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-15 (work in progress),
              October 2014.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Bormann, C., and S. Krco, "CoRE Resource
              Directory", draft-ietf-core-resource-directory-01 (work in
              progress), December 2013.

   [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

       +-------+----------+----------------+---------+------------+
       |  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




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   +-------------+-------------+----------------+--------+-------------+
   |        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


   Matthieu Vial
   Schneider-Electric
   Grenoble,
   FRANCE

   Phone: +33 (0)47657 6522
   Email: matthieu.vial@schneider-electric.com










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