CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                   M. Vial
Expires: January 6, 2017                              Schneider-Electric
                                                               M. Koster
                                                               C. Groves
                                                            July 5, 2016

  Reusable Interface Definitions for Constrained RESTful Environments


   This document defines a set of reusable REST resource design patterns
   suitable for use in constrained environments, based on IETF CoRE
   standards for information representation and information exchange.

   Interface types for Sensors, Actuators, Parameters, and resource
   Collections are defined using the "if" link attribute defined by CoRE
   Link Format [RFC6690].  Clients may use the "if" attribute to
   determine how to consume resources.

   Editor's note: This version removes the observe notify functionality.
   Further work is needed on this draft to

   1  Focus the interfaces draft exclusively on interfaces and
       collections, and make it clear that this is not the IETF
       officialy endorsed way to use REST.  Build compatibility with OCF
       collections to the most reasonable extent, otherwise, try to fnid
       best practice guidance.

   2  Tone down the formality of function set definition and remove the
       perception that CoRE Interfaces defines REST function sets.
       Instead, find some descriptive language that accomplishes the
       same thing in RD, Pubsub, Interfaces, and other drafts that want
       to define REST API profiles for mapping functions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 January 6, 2017.

Copyright Notice

   Copyright (c) 2016 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
   ( 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.  Interface Types . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Collections . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Introduction to Collections . . . . . . . . . . . . . . .   5
     4.2.  Use Cases for Collections . . . . . . . . . . . . . . . .   6
     4.3.  Content-Formats for Collections . . . . . . . . . . . . .   6
     4.4.  Links and Items in Collections  . . . . . . . . . . . . .   7
     4.5.  Queries on Collections  . . . . . . . . . . . . . . . . .   8
     4.6.  Observing Collections . . . . . . . . . . . . . . . . . .   8
     4.7.  Collection Types  . . . . . . . . . . . . . . . . . . . .   8
   5.  Interface Descriptions  . . . . . . . . . . . . . . . . . . .   9
     5.1.  Link List . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Batch . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Linked Batch  . . . . . . . . . . . . . . . . . . . . . .  11
     5.4.  Sensor  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.5.  Parameter . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.6.  Read-only Parameter . . . . . . . . . . . . . . . . . . .  13
     5.7.  Actuator  . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.8.  Future Interfaces . . . . . . . . . . . . . . . . . . . .  14
   6.  Function Sets and Profiles  . . . . . . . . . . . . . . . . .  14

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     6.1.  Defining a Function Set . . . . . . . . . . . . . . . . .  15
       6.1.1.  Path template . . . . . . . . . . . . . . . . . . . .  15
       6.1.2.  Resource Type . . . . . . . . . . . . . . . . . . . .  15
       6.1.3.  Interface Description . . . . . . . . . . . . . . . .  16
       6.1.4.  Data type . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Versioning  . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Profile example  . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 Type ("if") that can be used to describe the REST
   interface of a resource, and may include a link to a description

   This document defines a set of Link Format compatible Interface Types
   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.  An Interface
   Type may describe a resource in terms of it's associated content
   formats, data types, URI templates, REST methods, parameters, and
   responses.  Basic interface types are defined for sensors, actuators,
   and properties.  A set of collection types is defined for organizing

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   resources for discovery, and for various forms of bulk interaction
   with resource sets using typed embedding links.

   Interface Types may be used in the composition of Function Sets and
   Profiles.  Function Sets and Profiles are described and an example is
   given of a sensor and actuator device profile using Function Sets
   composed from the Interface Types described in this document.

   This document describes a set of Interface Types which are referenced
   by the "if" link attribute and used to implement reusable design
   patterns and functional abstractions.  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
   Interface Type acts in a similar way as a Content-Format, but as a
   selector for a high level functional abstraction.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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:

   Interface Type:  A resource attribute which describes the interface
      exposed by the resource in terms of content formats, REST methods,
      parameters, and other related characteristics.

   Collection:  A resource which contains set of related resources,
      referenced by a list of links and optionally consisting of

   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.

   Function Set:  A group of well-known REST resources that provides a
      particular service.

   Profile:  A group of well-known Function Sets defined by a

   Device:  An IP smart object running a web server that hosts a group
      of Function Set instances from a profile.

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   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.

3.  Interface Types

   An Interface Type definition may describe a resource in terms of it's
   associated content formats, data types, URI templates, REST methods,
   parameters, and responses.

4.  Collections

4.1.  Introduction to Collections

   A Collection is a resource which represents one or more related
   resources.  Within this document, a collection refers to a collection
   with characteristics defined in this document.  A Collection
   Interface Type 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 types 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

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4.2.  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

   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 to multiple items in the collection to be processed
   together within the context of the collection resource.

4.3.  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 accepts
   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

   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

4.4.  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 [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=""  : Link to the /sen/ collection describing it as
      a 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

   <"/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 [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

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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.

4.5.  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.

4.6.  Observing Collections

   Resource Observation 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 MAY report any changes of resource state in any item in the
   collection.  Observation Responses, or notifications, SHOULD provide
   representations of the resources that have changed in SenML Content-
   Format.  Notifications MAY include multiple observations of a
   particular resource, with SenML time stamps indicating the
   observation times.

4.7.  Collection Types

   There are three collection types defined in this document:

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

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   Each collection type MAY support a subset of the methods and
   functions described above.  For the first three collection types, the
   methods and functions are defined in the corresponding Interface

5.  Interface Descriptions

   This section defines REST interfaces for Link List, Batch, Sensor,
   Parameter and Actuator resources.  Variants such as Linked Batch or
   Read-Only Parameter are also presented.  Each type is described along
   with its Interface Description attribute value and valid methods.
   These are defined for each interface in the table below.  These
   interfaces can support plain text and/or SenML Media types.

   The if= column defines the Interface Description (if=) attribute
   value to be used in the CoRE Link Format for a resource conforming to
   that interface.  When this value appears in the if= attribute of a
   link, the resource MUST support the corresponding REST interface
   described in this section.  The resource MAY support additional
   functionality, which is out of scope for this specification.
   Although these interface descriptions are intended to be used with
   the CoRE Link Format, they are applicable for use in any REST
   interface definition.

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

   |       Interface | if=     | Methods          | Content-Formats    |
   |       Link List | core.ll | GET              | link-format        |
   |           Batch | core.b  | GET, PUT, POST   | link-format, senml |
   |    Linked Batch | | 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         |

   The following is an example of links in the CoRE Link Format using
   these interface descriptions.  The resource hierarchy is based on a
   simple profile defined in Appendix A.  These links are used in the
   subsequent examples below.

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   Req: GET /.well-known/core
   Res: 2.05 Content (application/link-format)

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 format; 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 format.  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

   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)

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),
   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-

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   resources that support it.  For example Sensor interfaces do not
   support PUT, and thus a PUT request to a Sensor member of that Batch
   would be ignored.  A batch requires the use of SenML Media types in
   order to support multiple sub-resources.

   In addition, The Batch interface is an extension of the Link List
   interface and in consequence MUST support the same methods.

   The following example interacts with a Batch /s/ with Sensor sub-
   resources /s/light, /s/temp and /s/humidity.

   Req: GET /s/
   Res: 2.05 Content (application/senml+json)
       { "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 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)
   Res: 2.04 Changed

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

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

   Req: GET /l/ (Accept: application/link-format)
   Res: 2.05 Content (application/link-format)

   Req: GET /l/
   Res: 2.05 Content (application/senml+json)
      { "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

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)

   Req: GET /s/humidity (Accept: application/senml+json)
   Res: 2.05 Content (application/senml+json)
       { "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 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

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

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

5.6.  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)

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

   Req: PUT /a/1/led (text/plain)
   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)

5.8.  Future Interfaces

   It is expected that further interface descriptions will be defined in
   this and other specifications.

6.  Function Sets and Profiles

   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-

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   specific resources.  A device is composed of one or more Function Set

   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.

6.1.  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.

6.1.1.  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.

6.1.2.  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

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   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).

6.1.3.  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.

   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).

6.1.4.  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 formats for the
   CoRE interfaces or define new interfaces with new content types.

6.2.  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

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   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.

6.3.  Versioning

   A profile should track Function Set changes to avoid incompatibility
   issues.  Evolutions in a Function Set SHOULD be backward compatible.

7.  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.

8.  IANA Considerations

   The interface description types defined require registration.

   The new link relations type "grp" requires registration.

9.  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

10.  Changelog

   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.

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   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.

   o  Added observe-cancel definition to WADL and to observable

   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

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   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.

11.  References

11.1.  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,

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,

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   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

11.2.  Informative References

              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-07
              (work in progress), March 2016.

              Jennings, C., Shelby, Z., Arkko, J., and A.
              Ker&#258;[currency units]nen, "Media Types for Sensor
              Markup Language (SenML)", draft-ietf-core-senml-00 (work
              in progress), April 2016.

   [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,

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 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.

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         |       Function Set | Root Path | RT         | IF      |
         | Device Description | /d        | | 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  |   | core.p  | xsd:string |
       | Model | /d/model | | core.rp | xsd:string |

                      Device Description Function Set

   |        Type | Path        | RT             | IF     | Data Type   |
   |       Light | /s/light    |  | 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

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   Zach Shelby
   150 Rose Orchard
   San Jose  95134

   Phone: +1-408-203-9434

   Matthieu Vial

   Phone: +33 (0)47657 6522

   Michael Koster
   665 Clyde Avenue
   Mountain View  94043


   Christian Groves


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