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Versions: (draft-shelby-core-interfaces) 00 01 02 03 04 05 06 07 08 09

CoRE Working Group                                             Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                   Z. Vial
Expires: September 14, 2017                           Schneider-Electric
                                                               M. Koster
                                                             SmartThings
                                                               C. Groves
                                                                  Huawei
                                                          March 13, 2017


  Reusable Interface Definitions for Constrained RESTful Environments
                     draft-ietf-core-interfaces-09

Abstract

   This document defines a set of Constrained RESTful Environments
   (CoRE) Link Format Interface Descriptions [RFC6690] applicable for
   use in constrained environments.  These include the: Actuator,
   Paramter, Read-only parameter, Sensor, Batch, Linked Batch and Link
   List interfaces.

   The Batch, Linked Batch and Link List interfaces make use of resource
   collections.  This document further describes how collections relate
   to interfaces.

   Many applications require a set of interface descriptions in order
   provide the required functionality.  This document defines the
   concept of function sets to specify this set of interfaces and
   resources.

   _Editor's note: The git repository for the draft is found at
   https://github.com/core-wg/interfaces_

   _Editor's note: Two open issues are proposals for: Removing the
   binding interface in favour of the link list interface.  Changing
   "rel" type from one attribute to two to indicate src and
   destination._

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/.



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   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 September 14, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Collections . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Introduction to Collections . . . . . . . . . . . . . . .   5
     3.2.  Use Cases for Collections . . . . . . . . . . . . . . . .   6
     3.3.  Content-Formats for Collections . . . . . . . . . . . . .   6
     3.4.  Links and Items in Collections  . . . . . . . . . . . . .   7
     3.5.  Queries on Collections  . . . . . . . . . . . . . . . . .   8
     3.6.  Observing Collections . . . . . . . . . . . . . . . . . .   8
     3.7.  Collection Types  . . . . . . . . . . . . . . . . . . . .   8
   4.  Interface Descriptions  . . . . . . . . . . . . . . . . . . .   9
     4.1.  Link List . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Batch . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Linked Batch  . . . . . . . . . . . . . . . . . . . . . .  12
     4.4.  Sensor  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.5.  Parameter . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  Read-only Parameter . . . . . . . . . . . . . . . . . . .  14
     4.7.  Actuator  . . . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Link List . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Batch . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Linked Batch  . . . . . . . . . . . . . . . . . . . . . .  16
     6.4.  Sensor  . . . . . . . . . . . . . . . . . . . . . . . . .  16



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     6.5.  Parameter . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.6.  Read-only parameter . . . . . . . . . . . . . . . . . . .  17
     6.7.  Actuator  . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Current Usage of Interfaces and Function Sets  . . .  22
     A.1.  Constrained RESTful Environments (CoRE) Link Format
           (IETF)  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.2.  CoRE Resource Directory  (IETF) . . . . . . . . . . . . .  23
     A.3.  Open Connectivity Foundation (OCF)  . . . . . . . . . . .  23
     A.4.  oneM2M  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.5.  OMA LWM2M . . . . . . . . . . . . . . . . . . . . . . . .  24
   Appendix B.  Resource Profile example . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   IETF Standards for machine to machine communication in constrained
   environments describe a REST protocol and a set of related
   information standards that may be used to represent machine data and
   machine metadata in REST interfaces.  CoRE Link-format is a standard
   for doing Web Linking [RFC5988] in constrained environments.  SenML
   [I-D.ietf-core-senml] is a simple data model and representation
   format for composite and complex structured resources.  CoRE Link-
   Format and SenML can be used by CoAP [RFC7252] or HTTP servers.

   The discovery of resources offered by a constrained server is very
   important in machine-to-machine applications where there are no
   humans in the loop.  Machine application clients must be able to
   adapt to different resource organizations without advance knowledge
   of the specific data structures hosted by each connected thing.  The
   use of Web Linking for the description and discovery of resources
   hosted by constrained web servers is specified by CoRE Link Format
   [RFC6690].  CoRE Link Format additionally defines a link attribute
   for interface description ("if") that can be used to describe the
   REST interface of a resource, and may include a link to a description
   document.

   This document defines a set of Link Format interface descriptions for
   some common design patterns that enable the server side composition
   and organization, and client side discovery and consumption, of
   machine resources using Web Linking.  A client discovering the "if"
   link attribute will be able to consume resources based on its
   knowledge of the expected interface types.  In this sense the




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   Interface Type acts in a similar way as a Content-Format, but as a
   selector for a high level functional abstraction.

   An interface description describes a resource in terms of it's
   associated content formats, data types, URI templates, REST methods,
   parameters, and responses.  Basic interface descriptions are defined
   for sensors, actuators, and properties.

   A set of collection types is defined for organizing resources for
   discovery, and for various forms of bulk interaction with resource
   sets using typed embedding links.

   This document first defines the concept of collection interface
   descriptions.  It then defines a number of generic interface
   descriptions that may be used in contrained environments.  Several of
   these interface descriptions utilise collections.

   Whilst this document assumes the use of CoAP [RFC7252], the REST
   interfaces described can also be realized using HTTP [RFC7230].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   This document requires readers to be familiar with all the terms and
   concepts that are discussed in [RFC5988] and [RFC6690].  This
   document makes use of the following additional terminology:

   Gradual Reveal:  A REST design where resources are discovered
      progressively using Web Linking.

   Interface Description:  The Interface Description describes the
      generic REST interface to interact with a resource or a set of
      resources.  Its use is described via the Interface Description
      'if' attribute which is an opaque string used to provide a name or
      URI indicating a specific interface definition used to interact
      with the target resource.  One can think of this as describing
      verbs usable on a resource.

   Resource Discovery:  The process allowing a web client to identify
      resources being hosted on a web server.

   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.



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3.  Collections

3.1.  Introduction to Collections

   A Collection is a resource which represents one or more related
   resources.  [RFC6573] describes the "item" and "collection" Link
   Relation. "item" link relation identifies a member of collection.
   "collection" indicates the collection that an item is a member of.
   For example: A collection might be a resource representing catalog of
   products, an item is a resource related to an individual product.

   Section 1.2.2/[RFC6690] also describes resource collections.

   This document uses the concept of "collection" and applies it to
   interface descriptions.  A collection interface description consists
   of a set of links and a set of items pointed to by the links which
   may be sub-resources of the collection resource.  The collection
   interface descriptions described in this document are Link List,
   Batch and Linked Batch.

   The links in a collection are represented in CoRE Link-Format
   Content-Formats including JSON and CBOR variants, and the items in
   the collection may be represented by senml, including JSON and CBOR
   variants.  In general, a collection may support items of any
   available Content-Format.

   A particular resource item may be a member of more than one
   collection at a time by being linked to, but may only be a
   subresource of one collection.

   Some collections may have pre-configured items and links, and some
   collections may support dynamic creation and removal of items and
   links.  Likewise, modification of items in some collections may be
   permitted, and not in others.

   Collections may support link embedding, which is analogous to an
   image tag (link) causing the image to display inline in a browser
   window.  Resources pointed to by embedded links in collections may be
   interacted with using bulk operations on the collection resource.
   For example, performing a GET on a collection resource may return a
   single representation containing all of the linked resources.

   Links in collections may be selected for processing by a particular
   request by using Query Filtering as described in CoRE Link-Format
   [RFC6690].






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3.2.  Use Cases for Collections

   Collections may be used to provide gradual reveal of resources on an
   endpoint.  There may be a small set of links at the .well-known/core
   location, which may in turn point to other collections of resources
   that represent device information, device configuration, device
   management, and various functional clusters of resources on the
   device.

   A collection may provide resource encapsulation, where link embedding
   may be used to provide a single resource with which a client may
   interact to obtain a set of related resource values.  For example, a
   collection for manufacturer parameters may consist of manufacturer
   name, date of manufacture, location of manufacture, and serial number
   resources which can be read as a single senml data object.

   A collection may be used to group a set of like resources for bulk
   state update or actuation.  For example, the brightness control
   resources of a number of luminaries may be grouped by linking to them
   in a collection.  The collection type may support receiving a single
   update form a client and sending that update to each resource item in
   the collection.

   Items may be sub-resources of the collection resource.  This enables
   updates to multiple items in the collection to be processed together
   within the context of the collection resource.

3.3.  Content-Formats for Collections

   The collection interfaces by default use CoRE Link-Format for the
   link representations and SenML or text/plain for representations of
   items.  The examples given are for collections that expose resources
   and links in these formats.  In addition, a new "collection" Content-
   Format is defined based on the SenML framework which represents both
   links and items in the collection.

   The choice of whether to return a representation of the links or of
   the items or of the collection format is determined by the Accept
   header option in the request.  Likewise, the choice of updating link
   metadata or item data or the collection resource itself is determined
   by the Content-Format option in the header of the update request
   operation.

   The default Content-Formats for collection types described in this
   document are:

   Links:  application/link-format, application/link-format+json




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   Items:  application/senml+json, text/plain

3.4.  Links and Items in Collections

   Links use CoRE Link-Format representation by default and may point to
   any resource reachable from the context of the collection.  This
   includes absolute links and links that point to other network
   locations if the context of the collection allows.  Links to sub-
   resources in the collection MUST have a path-element starting with
   the resource name, as per [RFC3986].  Links to resources in the
   global context MUST start with a root path identifier [RFC5988].
   Links to other collections are formed per [RFC3986].

   Examples of links:

   </sen/>;if="core.lb"':  Link to the /sen/ collection describing it as
      a core.lb type collection (Linked Batch)

   </sen/>;rel="grp"':  Link to the /sen/ collection indicating that
      /sen/ is a member of a group in the collection in which the link
      appears.

   <"/sen/temp">;rt="temperature"':  An absolute link to the resource at
      the path /sen/temp

   <temp>;rt="temperature":  Link to the temp subresource of the
      collection in which this link appears."

   <temp>;anchor="/sen/":  A link to the temp subresource of the
      collection /sen/ which is assumed not to be a subresource of the
      collection in which the link appears ,but is expected to be
      identified in the collection by resource name.

   Links in the collection MAY be Read, Updated, Added, or Removed using
   the CoRE Link-Format or JSON Merge-Patch Content-Formats on the
   collection resource.  Reading links uses the GET method and returns
   an array or list containing the link-values of all selected links.
   Links may be added to the collection using POST or PATCH methods.
   Updates to links MUST use the PATCH method and MAY use query
   filtering to select links for updating.  The PATCH method on links
   MUST use the JSON Merge-Patch Content-Format (application/merge-
   patch+json) specified in [RFC7396].

   Items in the collection SHOULD be represented using the SenML
   (application/senml+json) or plain text (text/plain) Content-Formats,
   depending on whether the representation is of a single data point or
   multiple data points.  Items MAY be represented using any supported
   Content-Format.



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   Link Embedding enables the bulk processing of items in the collection
   using a single operation targeting the collection resource.  A subset
   of resources in the collection may be selected for operation using
   Query Filtering.  Bulk Read operations using GET return a SenML
   representation of all selected resources.  Bulk item Update
   operations using PUT or POST apply the payload document to all
   selected resource items in the collection, using either a Batch or
   Group update policy.  A Batch update is performed by applying the
   resource values in the payload document to all resources in the
   collection that match any resource name in the payload document.
   Group updates are performed by applying the payload document to each
   item in the collection.  Group updates are indicated by the link
   relation type rel="grp" in the link.

3.5.  Queries on Collections

   Collections MAY support query filtering as defined in CoRE Link-
   Format [RFC6690].  Operations targeting either the links or the items
   MAY select a subset of links and items in the collection by using
   query filtering.  The Content-Format specified in the request header
   selects whether links or items are targeted by the operation.

3.6.  Observing Collections

   Resource Observation via [I-D.ietf-core-dynlink] using CoAP [RFC7252]
   MAY be supported on items in a collection.  A subset of the
   conditional observe parameters MAY be specified to apply.  In most
   cases pmin and pmax are useful.  Resource observation on a
   collection's items resource returns the collection representation.
   Observation Responses, or notifications, SHOULD provide the
   collection representations in SenML Content-Format.  Notifications
   MAY include multiple observations of the colection resource, with
   SenML time stamps indicating the observation times.

3.7.  Collection Types

   There are three collection types defined in this document:














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            +-----------------+---------+--------------------+
            | Collection Type | if=     | Content-Format     |
            +-----------------+---------+--------------------+
            | Link List       | core.ll | link-format        |
            |                 |         |                    |
            | Batch           | core.b  | link-format, senml |
            |                 |         |                    |
            | Linked Batch    | core.lb | link-format, senml |
            +-----------------+---------+--------------------+

                     Table 1: Collection Type Summary

   The interface description defined in this document describes the
   methods and functions that may be applied to the collections.

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 document.  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.















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     +--------------+---------+-----------------+--------------------+
     | Interface    | if=     | Methods         | Content-Formats    |
     +--------------+---------+-----------------+--------------------+
     | Link List    | core.ll | GET             | link-format        |
     |              |         |                 |                    |
     | Batch        | core.b  | GET, PUT, POST  | link-format, senml |
     |              |         |                 |                    |
     | Linked Batch | core.lb | GET, PUT, POST, | link-format, senml |
     |              |         |                 |                    |
     |              |         | DELETE          |                    |
     |              |         |                 |                    |
     | Sensor       | core.s  | GET             | link-format,       |
     |              |         |                 |                    |
     |              |         |                 | text/plain         |
     |              |         |                 |                    |
     | Parameter    | core.p  | GET, PUT        | link-format,       |
     |              |         |                 |                    |
     |              |         |                 | text/plain         |
     |              |         |                 |                    |
     | Read-only    | core.rp | GET             | link-format,       |
     |              |         |                 |                    |
     | Parameter    |         |                 | text/plain         |
     |              |         |                 |                    |
     | Actuator     | core.a  | GET, PUT, POST  | link-format,       |
     |              |         |                 |                    |
     |              |         |                 | text/plain         |
     +--------------+---------+-----------------+--------------------+

                  Table 2: Interface Description Summary

   The following is an example of links in the CoRE Link Format using
   these interface descriptions.  The resource hierarchy is based on a
   simple resource profile defined in Appendix B.  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/light>;rt="simple.sen.lt";if="core.s",
   </s/temp>;rt="simple.sen.tmp";if="core.s";obs,
   </s/humidity>;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",

                    Figure 1: Binding Interface Example



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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 format.  However if the
   resource does not support any other form of content-format the Accept
   option MAY be elided.

   Note: The use of an Accept option with application/link-format is
   recommended even though it is not strictly needed for the link list
   interface because this interface is extended by the batch and linked
   batch interfaces where different content-formats are possible.

   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"

4.2.  Batch

   The Batch interface is used to manipulate a collection of sub-
   resources at the same time.  The Batch interface description supports
   the same methods as its sub-resources, and can be used to read (GET),
   update (PUT) or apply (POST) the values of those sub-resource with a
   single resource representation.  The sub-resources of a Batch MAY be
   heterogeneous, a method used on the Batch only applies to sub-
   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.  For
   example: a GET with an Accept:application/link-format on a resource
   utilizing the batch interface will return the sub-resource links.



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   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 [RFC6690].  A request with a POST
   method and a content format of application/link-format simply appends
   new resource links to the collection.  The links in the payload MUST
   reference a resource on the web server with an absolute path.  A
   DELETE request removes the entire collection.  All other requests
   available for a basic Batch are still valid for a Linked Batch.

   The following example interacts with a Linked Batch /l/ and creates a
   collection containing /s/light, /s/temp and /s/humidity in 2 steps.
























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   Req: POST /l/ (Content-Format: application/link-format)
   </s/light>,</s/temp>
   Res: 2.04 Changed

   Req: GET /l/
   Res: 2.05 Content (application/senml+json)
   {"e":[
      { "n": "/s/light", "v": 123, "u": "lx" },
      { "n": "/s/temp", "v": 27.2, "u": "degC" },
   }

   Req: POST /l/ (Content-Format: application/link-format)
   </s/humidity>
   Res: 2.04 Changed

   Req: GET /l/ (Accept: application/link-format)
   Res: 2.05 Content (application/link-format)
   </s/light>,</s/temp>,</s/humidity>

   Req: GET /l/
   Res: 2.05 Content (application/senml+json)
   {"e":[
      { "n": "/s/light", "v": 123, "u": "lx" },
      { "n": "/s/temp", "v": 27.2, "u": "degC" },
      { "n": "/s/humidity", "v": 80, "u": "%RH" }],
   }

   Req: DELETE /l/
   Res: 2.02 Deleted

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.









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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" }],
   }

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 update (PUT).  Plain text or SenML Media types
   MAY be returned from this type of interface.

   The following example shows request for reading and updating a
   parameter.

   Req: GET /d/name
   Res: 2.05 Content (text/plain)
   node5

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

4.6.  Read-only Parameter

   The Read-only Parameter interface allows configuration parameters to
   be read (GET) but not updated.  Plain text or SenML Media types MAY
   be returned from this type of interface.

   The following example shows request for reading such a parameter.

   Req: GET /d/model
   Res: 2.05 Content (text/plain)
   SuperNode200

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 the actuator value can be updated
   (PUT).  In addition, this interface allows the use of POST to change



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   the state of an actuator, for example to toggle between its possible
   values.  Plain text or SenML Media types MAY be returned from this
   type of interface.  A resource with this interface MAY use SenML to
   include multiple measurements in the same representation, for example
   a list of recent actuator values or a list of values to updated.

   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.  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

   This document registers the following CoRE Interface Description
   (if=) Link Target Attribute Values.

6.1.  Link List

   Attribute Value:  core.ll

   Description:  The Link List interface is used to retrieve a list of
      resources on a web server.





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   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.2.  Batch

   Attribute Value:  core.b

   Description: The Batch interface is used to manipulate a collection
   of sub-resources at the same time.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.3.  Linked Batch

   Attribute Value:  core.lb

   Description:  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.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.4.  Sensor

   Attribute Value:  core.s

   Description:  The Sensor interface allows the value of a sensor
      resource to be read.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.5.  Parameter

   Attribute Value: core.p





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   Description:  The Parameter interface allows configurable parameters
      and other information to be modeled as a resource.  The value of
      the parameter can be read or update.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.6.  Read-only parameter

   Attribute Value:  core.rp

   Description:  The Read-only Parameter interface allows configuration
      parameters to be read but not updated.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

6.7.  Actuator

   Attribute Value:  core.a

   Description:  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 or the actuator value can be updated.  In
      addition, this interface allows the use of POST to change the
      state of an actuator, for example to toggle between its possible
      values.

   Reference:  This document.  Note to RFC Editor - please insert the
      appropriate RFC reference.

   Notes:  None

7.  Acknowledgements

   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 descriptions 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.



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8.  Changelog

   Changes from -08 to 09:

   o  Section 3.6: Modified to indicate that the entire collection
      resource is returned.

   o  General: Added editor's note with open issues.

   Changes from -07 to 08:

   o  Section 3.3: Modified Accepts to Accept header option.

   o  Addressed the editor's note in Section 4.1 to clarify the use of
      the Accept option.

   Changes from -06 to 07:

   o  Corrected Figure 1 sub-resource names e.g. tmp to temp and hum to
      humidity.

   o  Addressed the editor's note in Section 4.2.

   o  Removed section on function sets and profiles as agreed to at the
      IETF#97.

   Changes from -05 to -06:

   o  Updated the abstract.

   o  Section 1: Updated introduction.

   o  Section 2: Alphabetised the order

   o  Section 2: Removed the collections definition in favour of the
      complete definition in the collections section.

   o  Removed section 3 on interfaces in favour of an updated definition
      in section 1.3.

   o  General: Changed interface type to interface description as that
      is the term defined in RFC6690.

   o  Removed section on future interfaces.

   o  Section 8: Updated IANA considerations.





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   o  Added Appendix A to discuss current state of the art wrt to
      collections, function sets etc.

   Changes from -04 to -05:

   o  Removed Link Bindings and Observe attributes.  This functionality
      is now contained in I-D.ietf-core-dynlink.

   o  Hypermedia collections have been removed.  This is covered in a
      new T2TRG draft.

   o  The WADL description has been removed.

   o  Fixed minor typos.

   o  Updated references.

   Changes from -03 to -04:

   o  Fixed tickets #385 and #386.

   o  Changed abstract and into to better describe content.

   o  Focus on Interface and not function set/profiles in intro.

   o  Changed references from draft-core-observe to RFC7641.

   o  Moved Function sets and Profiles to section after Interfaces.

   o  Moved Observe Attributes to the Link Binding section.

   o  Add a Collection section to describe the collection types.

   o  Add the Hypermedia Collection Interface Description.

   Changes from -02 to -03:

   o  Added lt and gt to binding format section.

   o  Added pmin and pmax observe parameters to Observation Attributes.

   o  Changed the definition of lt and gt to limit crossing.

   o  Added definitions for getattr and setattr to WADL.

   o  Added getattr and setattr to observable interfaces.

   o  Removed query parameters from Observe definition.



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   o  Added observe-cancel definition to WADL and to observable
      interfaces.

   Changes from -01 to -02:

   o  Updated the date and version, fixed references.

   o  "Removed pmin and pmax observe parameters "[Ticket #336]"."

   Changes from -00 to WG Document -01

   o  Improvements to the Function Set section.

   Changes from -05 to WG Document -00

   o  Updated the date and version.

   Changes from -04 to -05

   o  Made the Observation control parameters to be treated as resources
      rather than Observe query parameters.  Added Less Than and Greater
      Than parameters.

   Changes from -03 to -04

   o  Draft refresh

   Changes from -02 to -03

   o  Added Bindings

   o  Updated all rt= and if= for the new Link Format IANA rules

   Changes from -01 to -02

   o  Defined a Function Set and its guidelines.

   o  Added the Link List interface.

   o  Added the Linked Batch interface.

   o  Improved the WADL interface definition.

   o  Added a simple profile example.







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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

9.2.  Informative References

   [I-D.ietf-core-dynlink]
              Shelby, Z., Vial, M., Koster, M., and C. Groves, "Dynamic
              Resource Linking for Constrained RESTful Environments",
              draft-ietf-core-dynlink-02 (work in progress), February
              2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-10
              (work in progress), March 2017.

   [I-D.ietf-core-senml]
              Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
              Bormann, "Media Types for Sensor Measurement Lists
              (SenML)", draft-ietf-core-senml-05 (work in progress),
              March 2017.

   [OIC-Core]
              "OIC Resource Type Specification v1.1.0", 2016,
              <https://openconnectivity.org/resources/specifications>.

   [OIC-SmartHome]
              "OIC Smart Home Device Specification v1.1.0", 2016,
              <https://openconnectivity.org/resources/specifications>.








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   [OMA-TS-LWM2M]
              "Lightweight Machine to Machine Technical Specification",
              2016, <http://technical.openmobilealliance.org/Technical/
              technical-information/release-program/current-releases/
              oma-lightweightm2m-v1-0>.

   [oneM2MTS0008]
              "TS 0008 v1.3.2 CoAP Protocol Binding", 2016,
              <http://www.onem2m.org/technical/published-documents>.

   [oneM2MTS0023]
              "TS 0023 v2.0.0 Home Appliances Information Model and
              Mapping", 2016,
              <http://www.onem2m.org/technical/published-documents>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC6573]  Amundsen, M., "The Item and Collection Link Relations",
              RFC 6573, DOI 10.17487/RFC6573, April 2012,
              <http://www.rfc-editor.org/info/rfc6573>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <http://www.rfc-editor.org/info/rfc7396>.

Appendix A.  Current Usage of Interfaces and Function Sets

   Editor's note: This appendix will be removed.  It is only included
   for information.

   This appendix analyses the current landscape with regards the
   definition and use of collections, interfaces and function sets/
   profiles.  This should be considered when considering the scope of
   this document.




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   In summary it can be seen that there is a lack of consistancy of the
   definition and usage of interface description and function sets.

A.1.  Constrained RESTful Environments (CoRE) Link Format (IETF)

   [RFC6690] assumes that different deployments or application domains
   will define the appropriate REST Interface Descriptions along with
   Resource Types to make discovery meaningful.  It highlights that
   collections are often used for these interfaces.

   Whilst 3.2/[RFC6690] defines a new Interface Description 'if'
   attribute the procedures around it are about the naming of the
   interface not what information should be included in the
   documentation about the interface.

   Function sets are not discussed.

A.2.  CoRE Resource Directory (IETF)

   [I-D.ietf-core-resource-directory] uses the concepts of collections,
   interfaces and function sets.

   If defines a number of interfaces: discovery, registration,
   registration update, registration removal, read endpoint links,
   update endpoint links, registration request interface, removal
   request interface and lookup interface.  However it does not assign
   an inteface description identifier (if=) to these interfaces.

   It does define a resource directory function set which specifies
   relevant content formats and interfaces to be used between a resource
   directory and endpoints.  However it does not follow the format
   proposed by this document.

A.3.  Open Connectivity Foundation (OCF)

   The OIC Core Specification [OIC-Core] most closely aligns with the
   work in this specification.  It makes use of interface descriptions
   as per [RFC6690] and has registered several interface identifiers
   (https://www.iana.org/assignments/core-parameters/core-
   parameters.xhtml#if-link-target-att-value).  These interface
   descriptors are similar to those defined in this specification.  From
   a high level perspective:









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   links list:   OCF (oic.if.ll) -> IETF (core.ll)
                 Note: it's called "link list" in the IETF.
   linked batch: OCF (oic.if.b) -> IETF (core.lb)
   read-only:    OCF (oic.if.r) -> IETF (core.rp)
   read-write:   OCF (oic.if.rw) -> IETF (core.p)
   actuator:     OCF (oic.if.a) -> IETF (core.a)
   sensor:       OCF (oic.if.s) -> IETF (core.s)
   batch:        No OCF equivalent -> IETF (core.b)

   Some of the OCF interfaces make use of collections.

   The OIC Core specification does not use the concept of function sets.
   It does however discuss the concept of profiles.  The OCF defines two
   sets of documents.  The core specification documents such as
   [OIC-Core] and vertical profile specification documents which provide
   specific information for specific applications.  The OIC Smart Home
   Device Specification [OIC-SmartHome] is one such specification.  It
   provides information on the resource model, discovery and data types.

A.4.  oneM2M

   OneM2M describes a technology independent functional architecture
   [oneM2MTS0023].  In this archictecture the reference points between
   functional entities are called "interfaces".  This usage does not
   match the [RFC6690] concept of interfaces.  A more direct comparison
   is that of 10.2/[oneM2MTS0023] that defines basic procedures and
   resource type-specific procedures utilising REST type create,
   retrieve, update, delete, notify actions.

   [oneM2MTS0023] does not refer to resource collections however does
   define "Group Management Procedures" in 10.2.7/[oneM2MTS0023].  It
   does allow bulk management of member resources.

   [oneM2MTS0023] does not use the term "function set". [oneM2MTS0008]
   describes the binding with the CoAP protocol.  In some respects this
   document provides a profile of the CoAP protocol in terms of the
   protocol elements that need to be supported.  However it does not
   define any interface descriptions nor collections.

A.5.  OMA LWM2M

   [OMA-TS-LWM2M] utilises the concept of interfaces.  It defines the
   following interfaces: Bootstrap, Client Registration, Device
   Management and Service Enablement and Information Reporting.  It
   defines that these have a particular direction (Uplink/Downlink) and
   indicates the operations that may be applied to the interface (i.e.
   Request Bootstrap, Write, Delete, Register, Update, De-Register,
   Create, Read, Write, Delete, Execute, Write Attributes, Discover,



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   Observe, Cancel Observation, Notify).  It then further defines which
   objects may occur over the interface.  In 6/[OMA-TS-LWM2M] resource
   model, identifier and data formats are described.

   Whilst it does not formally describe the use of "collections" the use
   of a multiple resource TLV allows a hierarchy of resource/sub-
   resource.

   It does not identify the interfaces through an Interface Description
   (if=) attribute.

   It does not use the term function set.  Informally the specification
   could be considered as a function set.

   Note: It refers to draft-ietf-core-interfaces-00.  It also makes use
   of the binding/observation attributes from draft-ietf-dynlink-00 but
   does not refer to that document.

Appendix B.  Resource Profile example

   The following is a short definition of simple device resource
   profile.  This simplistic profile is for use in the examples of this
   document.

         +--------------------+-----------+------------+---------+
         | Functions          | Root Path | RT         | IF      |
         +--------------------+-----------+------------+---------+
         | Device Description | /d        | simple.dev | core.ll |
         |                    |           |            |         |
         | Sensors            | /s        | simple.sen | core.b  |
         |                    |           |            |         |
         | Actuators          | /a        | simple.act | core.b  |
         +--------------------+-----------+------------+---------+

                   Table 3: Functional list of resources

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

                   Table 4: Device Description Resources






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

                         Table 5: Sensor Resources

       +------+------------+----------------+--------+-------------+
       | Type | Path       | RT             | IF     | Data Type   |
       +------+------------+----------------+--------+-------------+
       | LED  | /a/{#}/led | simple.act.led | core.a | xsd:boolean |
       +------+------------+----------------+--------+-------------+

                        Table 6: Actuator Resources

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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   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

   Email: michael.koster@smartthings.com


   Christian Groves
   Huawei
   Australia

   Email: cngroves.std@gmail.com





































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