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CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                   M. Vial
Expires: April 21, 2016                               Schneider-Electric
                                                               M. Koster
                                                                     ARM
                                                        October 19, 2015


  Reusable Interface Definitions for Constrained RESTful Environments
                     draft-ietf-core-interfaces-04

Abstract

   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.

   Dynamic linking of state updates between resources, either on an
   endpoint or between endpoints, is defined with the concept of Link
   Bindings.  We also define conditional observation attributes that
   work with Link Bindings or with simple CoAP Observe [RFC7641].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.







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Copyright Notice

   Copyright (c) 2015 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.  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 . . . . . . . . . . . . .   7
     4.4.  Links and Items in Collections  . . . . . . . . . . . . .   7
     4.5.  Queries on Collections  . . . . . . . . . . . . . . . . .   9
     4.6.  Observing Collections . . . . . . . . . . . . . . . . . .   9
     4.7.  Hypermedia Controls on Collections  . . . . . . . . . . .   9
     4.8.  Collection Types  . . . . . . . . . . . . . . . . . . . .  10
     4.9.  The collection+senml+json Content-Format  . . . . . . . .  10
   5.  Link Bindings and Observe Attributes  . . . . . . . . . . . .  11
     5.1.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Binding methods . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Binding table . . . . . . . . . . . . . . . . . . . . . .  14
     5.4.  Resource Observation Attributes . . . . . . . . . . . . .  14
   6.  Interface Descriptions  . . . . . . . . . . . . . . . . . . .  15
     6.1.  Link List . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.2.  Batch . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.3.  Linked Batch  . . . . . . . . . . . . . . . . . . . . . .  18
     6.4.  Hypermedia Collection . . . . . . . . . . . . . . . . . .  19
     6.5.  Sensor  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.6.  Parameter . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.7.  Read-only Parameter . . . . . . . . . . . . . . . . . . .  21
     6.8.  Actuator  . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.9.  Binding . . . . . . . . . . . . . . . . . . . . . . . . .  22
     6.10. Future Interfaces . . . . . . . . . . . . . . . . . . . .  23
     6.11. WADL Description  . . . . . . . . . . . . . . . . . . . .  23
   7.  Function Sets and Profiles  . . . . . . . . . . . . . . . . .  29



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     7.1.  Defining a Function Set . . . . . . . . . . . . . . . . .  29
       7.1.1.  Path template . . . . . . . . . . . . . . . . . . . .  30
       7.1.2.  Resource Type . . . . . . . . . . . . . . . . . . . .  30
       7.1.3.  Interface Description . . . . . . . . . . . . . . . .  30
       7.1.4.  Data type . . . . . . . . . . . . . . . . . . . . . .  31
     7.2.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.3.  Versioning  . . . . . . . . . . . . . . . . . . . . . . .  31
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     12.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Appendix A.  Profile example  . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

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
   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
   document.

   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.

   This document introduces the concept of a Link Binding, which defines
   a new link relation type to create a dynamic link between resources
   over which to exchange state updates.  Specifically, a Link Binding
   is a link for binding the state of 2 resources together such that
   updates to one are sent over the link to the other.  CoRE Link Format
   representations are used to configure, inspect, and maintain Link
   Bindings.  This document additionally defines a set of conditional
   Observe Attributes for use with Link Bindings and with the standalone
   CoRE Observe [RFC7641] method.

   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.  Interface types
   may also be provided with hypermedia controls and affordances to
   drive client interaction using the principles of HATEOAS.  In this
   case, the Interface Types serve as constructor templates for resource
   organization and hypermedia annotation.

2.  Terminology

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

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

   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
      subresources.




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   Link Binding:  A unidirectional logical link between a source
      resource and a destination resource, over which state information
      is synchronized.

   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
      specification.

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

   Service Discovery:  The process making it possible for a web client
      to automatically detect devices and Function Sets offered by these
      devices on a CoRE network.

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, Linked Batch, and Hypermedia Collection.

   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.





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   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].

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





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   Items may be dynamically created in a collection along with their
   hyperlinks.  This provides an "item factory" pattern which can serve
   as a resource creation mechanism for dynamic resources.  This pattern
   is also useful for creating temporary resources for the
   implementation of dynamic phenomena like commands, actions, and
   events using REST design patterns.  Item creation uses the collection
   Content-Format which allows specification of links and item state in
   a single representation.

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
   operation.

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

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

   Items:  application/senml+json, text/plain

   Collection:  application/collection+senml+json

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:





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   </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 [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.

   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 a 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.




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   The collection resource SHOULD represented using the
   collection+senml+json Content-Format.  The Hypermedia Collection type
   is the only collection type which supports this representation.
   Reading a collection using this content-format returns a
   representation of the links and the items in the collection.
   Performing a POST operation using this Content-Format MAY create one
   or more new item(s) and their corresponding links in the collection.
   Performing a PUT operation on this resource replaces the entire set
   of links and items with the payload.  This Content-Format is
   described in section Section 4.9.  Implementations MAY provide an
   alternate method using POST in a Content-Format used by the items in
   the collection which creates a default link-value and system-assigned
   resource name.  Such implementations MAY create sub-resources of the
   collection resource.

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 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.  Hypermedia Controls on Collections

   Additional Hypermedia controls may be defined to enable clients to
   automatically consume the collection resources.  Typically, the
   developer may map application level semantics onto collection
   operations.  For example, invoking an Action on an actuator may be
   defined as creating an Action item resource in a collection of
   Actions associated with the actuator, each item in the collection
   representing a past, current, or future action to be processed by the
   actuator.  Removing the item could cancel any pending or curent long-
   running action, and removing a completed action could free up
   resources for new actions to be invoked.  A Hypermedia control for
   this pattern might provide a semantic name for the action, for



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   example "Change Brightness", and might direct the client to supply a
   SenML representation of parameters for the action as well as provide
   instructions on what method (POST) to use and how to construct the
   URI (the collection URI in this case) if required.  An example of
   this hypermedia control is shown below.

4.8.  Collection Types

   There are four 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 | core.lb  | link-format, senml               |
   |          Hypermedia | core.hc  | link-format, senml,              |
   |          Collection |          | collection+senml                 |
   |             Binding | core.bnd | link-format                      |
   +---------------------+----------+----------------------------------+

   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
   Description.  The Hypermedia Collection SHOULD expose hypermedia
   controls to applications to indicate which methods and functions are
   supported.

4.9.  The collection+senml+json Content-Format

   The collection+senml+json Content-Format is used to represent all of
   the attributes and resources of a collection in a single format.
   This is accomplished by extending the SenmL format by adding a links
   element "l".  The links element is formatted as an array of links in
   the application/link-format+json Content-Format with the tag "l"
   which follows the structure of the "e" element.  An example of this
   format is given below.














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   {
     "bn":"/ep/sen/"
     "e":[
       { "n": "light", "v": 123, "u": "lx" },
       { "n": "temp", "v": 27.2, "u": "degC" },
       { "n": "humidity", "v": 80, "u": "%RH" }],
     "l":[
       { "href":"/ep/sen/", "rel":"self", "if": "core.hc", "rt": "ms" },
       { "href":"light", "rt":"core.s" },
       { "href":"temp", "rt":"core.s" },
       { "href":"humidity", "rt":"core.s" }]
   }


5.  Link Bindings and Observe Attributes

   In a M2M RESTful environment, endpoints may directly exchange the
   content of their resources to operate the distributed system.  For
   example, a light switch may supply on-off control information that
   may be sent directly to a light resource for on-off control.
   Beforehand, a configuration phase is necessary to determine how the
   resources of the different endpoints are related to each other.  This
   can be done either automatically using discovery mechanisms or by
   means of human intervention and a so-called commissioning tool.  In
   this document the abstract relationship between two resources is
   called a link Binding.  The configuration phase necessitates the
   exchange of binding information so a format recognized by all CoRE
   endpoints is essential.  This document defines a format based on the
   CoRE Link-Format to represent binding information along with the
   rules to define a binding method which is a specialized relationship
   between two resources.  The purpose of a binding is to synchronize
   the content between a source resource and a destination resource.
   The destination resource MAY be a group resource if the authority
   component of the destination URI contains a group address (either a
   multicast address or a name that resolves to a multicast address).
   Since a binding is unidirectional, the binding entry defining a
   relationship is present only on one endpoint.  The binding entry may
   be located either on the source or the destination endpoint depending
   on the binding method.  The following table gives a summary of the
   binding methods described in more detail in Section 5.2.

          +---------+------------+-------------+---------------+
          | Name    | Identifier | Location    | Method        |
          +---------+------------+-------------+---------------+
          | Polling | poll       | Destination | GET           |
          | Observe | obs        | Destination | GET + Observe |
          | Push    | push       | Source      | PUT           |
          +---------+------------+-------------+---------------+



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5.1.  Format

   Since Binding involves the creation of a link between two resources,
   Web Linking and the CoRE Link-Format are a natural way to represent
   binding information.  This involves the creation of a new relation
   type, purposely named "boundto".  In a Web link with this relation
   type, the target URI contains the location of the source resource and
   the context URI points to the destination resource.  The Web link
   attributes allow a fine-grained control of the type of
   synchronization exchange along with the conditions that trigger an
   update.  This specification defines the attributes below:

           +--------------------+-----------+------------------+
           | Attribute          | Parameter | Value            |
           +--------------------+-----------+------------------+
           | Binding method     | bind      | xsd:string       |
           | Minimum Period (s) | pmin      | xsd:integer (>0) |
           | Maximum Period (s) | pmax      | xsd:integer (>0) |
           | Change Step        | st        | xsd:decimal (>0) |
           | Greater Than       | gt        | xsd:decimal      |
           | Less Than          | lt        | xsd:decimal      |
           +--------------------+-----------+------------------+

   Bind Method:  This is the identifier of a binding method which
      defines the rules to synchronize the destination resource.  This
      attribute is mandatory.

   Minimum Period:  When present, the minimum period indicates the
      minimum time to wait (in seconds) before sending a new
      synchronization message (even if it has changed).  In the absence
      of this parameter, the minimum period is up to the notifier.

   Maximum Period:  When present, the maximum period indicates the
      maximum time in seconds between two consecutive state
      synchronization messages (regardless if it has changed).  In the
      absence of this parameter, the maximum period is up to the
      notifier.  The maximum period MUST be greater than the minimum
      period parameter (if present).

   Change Step:  When present, the change step indicates how much the
      value of a resource SHOULD change before sending a new
      notification (compared to the value of the last notification).
      This parameter has lower priority than the period parameters, thus
      even if the change step has been fulfilled, the time since the
      last notification SHOULD be between pmin and pmax.

   Greater Than:  When present, Greater Than indicates the upper limit
      value the resource value SHOULD cross before sending a new



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      notification.  This parameter has lower priority than the period
      parameters, thus even if the Greater Than limit has been crossed,
      the time since the last notification SHOULD be between pmin and
      pmax.

   Less Than:  When present, Less Than indicates the lower limit value
      the resource value SHOULD cross before sending a new notification.
      This parameter has lower priority than the period parameters, thus
      even if the Less Than limit has been crossed, the time since the
      last notification SHOULD be between pmin and pmax.

5.2.  Binding methods

   A binding method defines the rules to generate the web-transfer
   exchanges that will effectively send content from the source resource
   to the destination resource.  The description of a binding method
   must define the following aspects:

   Identifier:  This is value of the "bind" attribute used to identify
      the method.

   Location:  This information indicates whether the binding entry is
      stored on the source or on the destination endpoint.

   REST Method:  This is the REST method used in the Request/Response
      exchanges.

   Conditions:  A binding method definition must state how the condition
      attributes of the abstract binding definition are actually used in
      this specialized binding.

   This specification supports 3 binding methods described below.

   Polling:  The Polling method consists of sending periodic GET
      requests from the destination endpoint to the source resource and
      copying the content to the destination resource.  The binding
      entry for this method MUST be stored on the destination endpoint.
      The destination endpoint MUST ensure that the polling frequency
      does not exceed the limits defined by the pmin and pmax attributes
      of the binding entry.  The copying process MAY filter out content
      from the GET requests using value-based conditions (e.g Change
      Step, Less Than, Greater Than).

   Observe:  The Observe method creates an observation relationship
      between the destination endpoint and the source resource.  On each
      notification the content from the source resource is copied to the
      destination resource.  The creation of the observation
      relationship requires the CoAP Observation mechanism [RFC7641]



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      hence this method is only permitted when the resources are made
      available over CoAP.  The binding entry for this method MUST be
      stored on the destination endpoint.  The binding conditions are
      mapped as query string parameters (see Section 5.4).

   Push:  When the Push method is assigned to a binding, the source
      endpoint sends PUT requests to the destination resource when the
      binding condition attributes are satisfied for the source
      resource.  The source endpoint MUST only send a notification
      request if the binding conditions are met.  The binding entry for
      this method MUST be stored on the source endpoint.

5.3.  Binding table

   The binding table is a special resource that gives access to the
   bindings on a endpoint.  A binding table resource MUST support the
   Binding interface defined in Section 6.9.  A profile SHOULD allow
   only one resource table per endpoint.

5.4.  Resource Observation Attributes

   When resource interfaces following this specification are made
   available over CoAP, the CoAP Observation mechanism [RFC7641] MAY be
   used to observe any changes in a resource, and receive asynchronous
   notifications as a result.  In addition, a set of query string
   parameters are defined here to allow a client to control how often a
   client is interested in receiving notifications and how much a
   resource value should change for the new representation to be
   interesting.  These query parameters are described in the following
   table.  A resource using an interface description defined in this
   specification and marked as Observable in its link description SHOULD
   support these observation parameters.  The Change Step parameter can
   only be supported on resources with an atomic numeric value.

   These query parameters MUST be treated as resources that are read
   using GET and updated using PUT, and MUST NOT be included in the
   Observe request.  Multiple parameters MAY be updated at the same time
   by including the values in the query string of a PUT.  Before being
   updated, these parameters have no default value.












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         +----------------+------------------+------------------+
         | Resource       | Parameter        | Data Format      |
         +----------------+------------------+------------------+
         | Minimum Period | /{resource}?pmin | xsd:integer (>0) |
         | Maximum Period | /{resource}?pmax | xsd:integer (>0) |
         | Change Step    | /{resource}?st   | xsd:decimal (>0) |
         | Less Than      | /{resource}?lt   | xsd:decimal      |
         | Greater Than   | /{resource}?gt   | xsd:decimal      |
         +----------------+------------------+------------------+

   Minimum Period:  When present, the minimum period indicates the
      minimum time to wait (in seconds) before sending a new
      synchronization message (even if it has changed).  In the absence
      of this parameter, the minimum period is up to the notifier.

   Maximum Period:  When present, the maximum period indicates the
      maximum time in seconds between two consecutive state
      synchronization messages (regardless if it has changed).  In the
      absence of this parameter, the maximum period is up to the
      notifier.  The maximum period MUST be greater than the minimum
      period parameter (if present).

   Change Step:  When present, the change step indicates how much the
      value of a resource SHOULD change before sending a new
      notification (compared to the value of the last notification).
      This parameter has lower priority than the period parameters, thus
      even if the change step has been fulfilled, the time since the
      last notification SHOULD be between pmin and pmax.

   Greater Than:  When present, Greater Than indicates the upper limit
      value the resource value SHOULD cross before sending a new
      notification.  This parameter has lower priority than the period
      parameters, thus even if the Greater Than limit has been crossed,
      the time since the last notification SHOULD be between pmin and
      pmax.

   Less Than:  When present, Less Than indicates the lower limit value
      the resource value SHOULD cross before sending a new notification.
      This parameter has lower priority than the period parameters, thus
      even if the Less Than limit has been crossed, the time since the
      last notification SHOULD be between pmin and pmax.

6.  Interface Descriptions

   This section defines REST interfaces for Link List, Batch, Sensor,
   Parameter, Actuator and Binding table resources.  Variants such as
   Linked Batch or Read-Only Parameter are also presented.  Each type is
   described along with its Interface Description attribute value and



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   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 | 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         |
   |         Binding | core.bnd | GET, POST,      | link-format        |
   |                 |          | DELETE          |                    |
   +-----------------+----------+-----------------+--------------------+

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











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   Req: GET /.well-known/core
   Res: 2.05 Content (application/link-format)
   </s/>;rt="simple.sen";if="core.b",
   </s/lt>;rt="simple.sen.lt";if="core.s",
   </s/tmp>;rt="simple.sen.tmp";if="core.s";obs,
   </s/hum>;rt="simple.sen.hum";if="core.s",
   </a/>;rt="simple.act";if="core.b",
   </a/1/led>;rt="simple.act.led";if="core.a",
   </a/2/led>;rt="simple.act.led";if="core.a",
   </d/>;rt="simple.dev";if="core.ll",
   </l/>;if="core.lb",

6.1.  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
   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"


6.2.  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)
   {"e":[
       { "n": "light", "v": 123, "u": "lx" },
       { "n": "temp", "v": 27.2, "u": "degC" },
       { "n": "humidity", "v": 80, "u": "%RH" }],
   }


6.3.  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


6.4.  Hypermedia Collection

   The Hypermedia Collection interface MAY provide a full set of the
   methods and link relation types described in section Section 4 of
   this document.

   The following example interacts with a Hypermedia Collection at
   /act1/actions/ by creating a new resource with Parameter sub-
   resources newVal, tTime.  The example depicts an actuation operation
   with a new actuator value of 86.3% and a transition time of 10
   seconds.  The returned location of the created resource is then read,
   and a response is returned which includes the remaining time for the
   operation to complete "rTime".  Then, the operation is cancelled by
   sending a DELETE operation to the location of the created resource
   that represents the running action.





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   Req: POST /Act1/Actions/
   Content-Format: application/collection+senml_json
   Pl: [{"n":newVal", "v":86.3}, {"n":tTime", "v":10}]
   Res: 2.01 Created
   Location: Action1234

   Req: GET /Act1/Actions/Action1234
   Accepts: application/senml+json
   Res: 2.05 Content
   Pl: [{"n":newVal", "v":86.3},
        {"n":tTime", "v":10},
        {"n":"rTime", "v":"8.87"}]

   Req: DELETE /Act1/Actions/Action1234
   Res: 2.02 Deleted

   Req: GET /Act1/Actions/Action1234
   Res: 4.04 Not Found


6.5.  Sensor

   The Sensor interface allows the value of a sensor resource to be read
   (GET).  The Media type of the resource can be either plain text or
   SenML.  Plain text MAY be used for a single measurement that does not
   require meta-data.  For a measurement with meta-data such as a unit
   or time stamp, SenML SHOULD be used.  A resource with this interface
   MAY use SenML to return multiple measurements in the same
   representation, for example a list of recent measurements.

   The following are examples of Sensor interface requests in both text/
   plain and application/senml+json.


   Req: GET /s/humidity (Accept: text/plain)
   Res: 2.05 Content (text/plain)
   80

   Req: GET /s/humidity (Accept: application/senml+json)
   Res: 2.05 Content (application/senml+json)
   {"e":[
       { "n": "humidity", "v": 80, "u": "%RH" }],
   }








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6.6.  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


6.7.  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


6.8.  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.




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   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


6.9.  Binding

   The Binding interface is used to manipulate a binding table.  A
   request with a POST method and a content format of application/link-
   format simply appends new bindings to the table.  All links in the
   payload MUST have a relation type "boundTo".  A GET request simply
   returns the current state of a binding table whereas a DELETE request
   empties the table.

   The following example shows requests for adding, retrieving and
   deleting bindings in a binding table.


   Req: POST /bnd/ (Content-Format: application/link-format)
   <coap://sensor.example.com/s/light>;
     rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"
   Res: 2.04 Changed

   Req: GET /bnd/
   Res: 2.05 Content (application/link-format)
   <coap://sensor.example.com/s/light>;
     rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"

   Req: DELETE /bnd/
   Res: 2.04 Changed






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6.10.  Future Interfaces

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

6.11.  WADL Description

   This section defines the formal Web Application Description Langauge
   (WADL) definition of these CoRE interface descriptions.


<?xml version="1.0" standalone="yes"?>

<application xmlns="http://research.sun.com/wadl/2006/10"
             xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             xmlns:senml="urn:ietf:params:xml:ns:senml">

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

  <doc title="CoRE Interfaces"/>

  <resource_type id="s">
    <doc title="Sensor interface type"/>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
  </resource_type>

  <resource_type id="p">
    <doc title="Parameter interfacee type"/>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
    <method href="#update"/>
  </resource_type>

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



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    <method href="#setattr"/>
  </resource_type>

  <resource_type id="a">
    <doc title="Actuator interface type"/>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
    <method href="#update"/>
    <method href="#apply"/>
  </resource_type>

  <resource_type id="ll">
    <doc title="Link List interface type"/></doc>
    <method href="#listLinks"/>
  </resource_type>

  <resource_type id="b">
    <doc title="Batch of sub-resources interface type">The methods read,
     observe, update and apply are applied to each sub-
     resource of the requested resource that supports it. Mixed
     sub-resource types can be supported.</doc>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
    <method href="#update"/>
    <method href="#apply"/>
    <method href="#listLinks"/>
  </resource_type>

  <resource_type id="lb">
    <doc title="Linked Batch interface type">. The methods read,
     obervableRead, update and apply are applied to each linked
     resource of the requested resource that supports it. Mixed
     linked resource types can be supported.</doc>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
    <method href="#update"/>
    <method href="#apply"/>
    <method href="#listLinks"/>
    <method href="#appendLinks"/>



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    <method href="#clearLinks"/>
  </resource_type>

  <resource_type id="hc">
    <doc title="Hypermedia Collection interface type">.</doc>
    <method href="#read"/>
    <method href="#observe"/>
    <method href="#observe-cancel"/>
    <method href="#getattr"/>
    <method href="#setattr"/>
    <method href="#update"/>
    <method href="#apply"/>
    <method href="#listLinks"/>
    <method href="#appendLinks"/>
    <method href="#clearLinks"/>
    <method href="#updateLinks"/>
    <method href="#readCollection"/>
    <method href="#addItem"/>
  </resource_type>

  <resource_type id="bnd">
    <doc title="Binding table resource type">A modifiable list of
    links. Each link MUST have the relation type "boundTo".</doc>
    <method href="#listLinks"/>
    <method href="#appendLinks"/>
    <method href="#clearLinks"/>
  </resource_type>

  <method id="read" name="GET">
    <doc>Retrieve the value of a sensor, an actuator or a parameter.
     Both HTTP and CoAP support this method.</doc>
    <request>
    </request>
    <response status="200">
      <representation mediaType="text/plain"/>
      <representation mediaType="application/senml+exi"/>
      <representation mediaType="application/senml+xml"/>
      <representation mediaType="application/senml+json"/>
    </response>
    <response status="2.05">
      <representation mediaType="text/plain"/>

  <representation mediaType="application/senml+exi"/>
      <representation mediaType="application/senml+xml"/>
      <representation mediaType="application/senml+json"/>
    </response>
  </method>




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  <method id="observe" name="GET">
    <doc>Observe the value of a sensor, an actuator or a parameter.
     Only CoAP supports this method since it requires the CoRE
     Observe mechanism.</doc>
    <request>
      <param name="observe" style="header" type="xsd:integer">
        <option value = 0/>
      </param>
    </request>
    <response status="2.05">
      <representation mediaType="text/plain"/>
      <representation mediaType="application/senml+exi"/>
      <representation mediaType="application/senml+xml"/>
      <representation mediaType="application/senml+json"/>
    </response>
  </method>

  <method id="observe-cancel" name="GET">
    <doc>Cancel observation in progress.
     Only CoAP supports this method since it requires the CoRE
     Observe mechanism.</doc>
    <request>
      <param name="observe" style="header" type="xsd:integer">
        <option value = 1/>
      </param>
    </request>
    <response status="2.05">
      <representation mediaType="text/plain"/>
      <representation mediaType="application/senml+exi"/>
      <representation mediaType="application/senml+xml"/>
      <representation mediaType="application/senml+json"/>
    </response>
  </method>

  <method id="update" name="PUT">
    <doc>Control the actuator or update a parameter with a new value
    or command. Both HTTP and CoAP support this method.</doc>
    <request>
      <representation mediaType="text/plain"/>
      <representation mediaType="application/senml+exi"/>
      <representation mediaType="application/senml+xml"/>
      <representation mediaType="application/senml+json"/>
    </request>
    <response status="200"/>
    <response status="2.04"/>
  </method>

  <method id="getattr" name="GET">



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    <doc>Retrieve the observe attributes associated with a resource.
        Both HTTP and CoAP support this method.</doc>
    <request>
      <doc>This request MUST contain an Accept option with
      application/link-format when the resource supports
      other GET methods.</doc>
      <representation mediaType="application/link-format"/>
    </request>
    <response status="200">
      <representation mediaType="application/link-format"/>
    </response>
    <response status="2.05">
      <representation mediaType="application/link-format"/>
    </response>
  </method>

  <method id="setattr" name="PUT">
    <doc>Set the values of some or all of the observe attributes
    associated with a resource.
    Both HTTP and CoAP support this method.</doc>
    <request>
      <param name="pmin" style="query" type="xsd:integer"/>
      <param name="pmax" style="query" type="xsd:integer"/>
      <param name="lt" style="query" type="xsd:decimal"/>
      <param name="gt" style="query" type="xsd:decimal"/>
      <param name="st" style="query" type="xsd:decimal"/>
    </request>
    <response status="200">
    </response>
    <response status="2.04">
    </response>
  </method>

  <method id="apply" name="POST">
    <doc>Apply the value, if supplied, to resources. Both HTTP and CoAP
    support this method.</doc>
    <request>
      <doc>The apply function may contain a payload to be applied.</doc>
    </request>
    <response status="200"/>
    <response status="2.04"/>
  </method>

  <method id="listLinks" name="GET">
    <doc>Retrieve the list of Web links associated to a resource.
     Both HTTP and CoAP support this method.</doc>
    <request>
    <doc>This request MUST contain an Accept option with



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    application/link-format when the resource supports
    other GET methods.</doc>
    </request>
    <response status="200">
      <representation mediaType="application/link-format"/>
    </response>
    <response status="2.05">
      <representation mediaType="application/link-format"/>
    </response>
  </method>

  <method id="appendLinks" name="POST">
    <doc>Append new Web links to a resource which is a collection
    of links. Both HTTP and CoAP support this method.</doc>
    <request>
      <representation mediaType="application/link-format"/>
    </request>
    <response status="200"/>
    <response status="2.04"/>
  </method>

  <method id="clearLinks" name="DELETE">
    <doc>Clear all Web Links in a resource which is a collection
    of links. Both HTTP and CoAP support this method.</doc>
    <request>
    </request>
    <response status="200"/>
    <response status="2.02"/>
  </method>

  <method id="updateLinks" name="PATCH">
    <doc>Update all Web Links in a resource which is a collection
    of links. Both HTTP and CoAP support this method.</doc>
    <doc>This request MUST contain a Content-Format option with
    application/merge-patch+json.</doc>
    <request>
    </request>
    <response status="200"/>
    <response status="2.04"/>
  </method>

    <method id="addItem" name="POST">
    <doc>Add zero or more items to the collection with their links. Both HTTP and CoAP support this method.</doc>
    <doc>This request MAY contain a Content-Format option with
    application/collection+senml+json.</doc>
    <doc>This request MAY contain a Content-Format option with
    application/senml+json.</doc>
    <request>



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    </request>
    <response status="200"/>
    <response status="2.01"/>
  </method>

    <method id="readCollection" name="GET">
    <doc>REturn a representation of both links and items in the collection. Both HTTP and CoAP support this method.</doc>
    <doc>This request MUST contain an Accepts option with
    application/collection+senml+json.</doc>
   <request>
    </request>
    <response status="200"/>
    <response status="2.05"/>
  </method>

</application>


7.  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-
   specific resources.  A device is composed of one or more Function Set
   instances.

   An example of function sets can be found from the CoRE Resource
   Directory specification that defines REST interfaces for
   registration, group and lookup [I-D.ietf-core-resource-directory].
   The OMA Lightweight M2M standard [REF] also defines a function set
   structure called an Objects that use integer path, instance and
   resource URI segments.  OMA Objects can be defined and then
   registered with an OMA maintained registry [REF].  This section is
   simply meant as a guideline for the definition of other such REST
   interfaces, either custom or part of other specifications.

7.1.  Defining a Function Set

   In a Function Set, types of resources are defined.  Each type
   includes a human readable name, a path template, a Resource Type for
   discovery, the Interface Definition and the data type and allowed
   values.  A Function Set definition may also include a field
   indicating if a sub-resource is mandatory or optional.





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7.1.1.  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.

7.1.2.  Resource Type

   Each root resource of a Function Set is assigned a Resource Type
   parameter, therefore making it possible to discover it.  Each sub-
   resource of a Function Set is also assigned a Resource Type
   parameter.  This Resource Type is used for resource discovery and is
   usually necessary to discover optional resources supported on a
   specific device.  The Resource Type of a Function Set may also be
   used for service discovery and can be exported to DNS-SD [RFC6763]
   for example.

   The Resource Type parameter defines the value that should be included
   in the rt= field of the CoRE Link Format when describing a link to
   this resource.  The value SHOULD be in the form "namespace.type" for
   root resources and "namespace.type.subtype" for sub-resources.  This
   naming convention facilitates resource type filtering with the
   /.well-known/core resource.  However a profile could allow mixing in
   foreign namespace references within a Function Set to import external
   references from other object models (e.g.  SenML and UCUM).

7.1.3.  Interface Description

   The Interface Description parameter defines the REST interface for
   that type of resource.  Several base interfaces are defined in
   Section 6 of this document.  For a given profile, the Interface
   Description may be inferred from the Resource Type.  In that case the
   Interface Description MAY be elided from link descriptions of
   resource types defined in the profile, but should be included for
   custom extensions to the profile.



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   The root resource of a Function Set should provide a list of links to
   its sub-resources in order to offer gradual reveal of resources.  The
   CoRE Link List interface defined in Section 6.1 offers this
   functionality so a root resource should support this interface or a
   derived interface like CoRE Batch (See Section 6.2).

7.1.4.  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 6 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.

7.2.  Discovery

   A device conforming to a profile SHOULD make its resources
   discoverable by providing links to the resources on the path /.well-
   known/core as defined in [RFC6690].  All resources hosted on a device
   SHOULD be discoverable either with a direct link in /.well-known/core
   or by following successive links starting from /.well-known/core.

   The root path of a Function Set instance SHOULD be directly
   referenced in /.well-known/core in order to offer discovery at the
   first discovery stage.  A device with more than 10 individual
   resources SHOULD only expose Function Set instances in /.well-known/
   core to limit the size of this resource.

   In addition, a device MAY register its resources to a Resource
   Directory using the registration interface defined in
   [I-D.ietf-core-resource-directory] if such a directory is available.

7.3.  Versioning

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

8.  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.



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9.  IANA Considerations

   The interface description types defined require registration.

   The new link relations type "boundto" and "grp" require registration.

10.  Acknowledgments

   Acknowledgement is given to colleagues from the SENSEI project who
   were critical in the initial development of the well-known REST
   interface concept, to members of the IPSO Alliance where further
   requirements for interface types have been discussed, and to Szymon
   Sasin, Cedric Chauvenet, Daniel Gavelle and Carsten Bormann who have
   provided useful discussion and input to the concepts in this
   document.

11.  Changelog

   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.




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   o  Removed query parameters from Observe definition.

   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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12.  References

12.1.  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>.

12.2.  Informative References

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-04
              (work in progress), July 2015.

   [I-D.jennings-core-senml]
              Jennings, C., Shelby, Z., Arkko, J., and A. Keranen,
              "Media Types for Sensor Markup Language (SENML)", draft-
              jennings-core-senml-01 (work in progress), July 2015.

   [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>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <http://www.rfc-editor.org/info/rfc6763>.

   [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>.





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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.

Appendix A.  Profile example

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

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

                           List of Function Sets

       +-------+----------+----------------+---------+------------+
       |  Type | Path     | RT             | IF      | Data Type  |
       +-------+----------+----------------+---------+------------+
       |  Name | /d/name  | simple.dev.n   | core.p  | xsd:string |
       | Model | /d/model | simple.dev.mdl | core.rp | xsd:string |
       +-------+----------+----------------+---------+------------+

                      Device Description Function Set

   +-------------+-------------+----------------+--------+-------------+
   |        Type | Path        | RT             | IF     | Data Type   |
   +-------------+-------------+----------------+--------+-------------+
   |       Light | /s/light    | simple.sen.lt  | core.s | xsd:decimal |
   |             |             |                |        | (lux)       |
   |    Humidity | /s/humidity | simple.sen.hum | core.s | xsd:decimal |
   |             |             |                |        | (%RH)       |
   | Temperature | /s/temp     | simple.sen.tmp | core.s | xsd:decimal |
   |             |             |                |        | (degC)      |
   +-------------+-------------+----------------+--------+-------------+

                           Sensors Function Set










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

                          Actuators Function Set

Authors' Addresses

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   FINLAND

   Phone: +1-408-203-9434
   Email: zach.shelby@arm.com


   Matthieu Vial
   Schneider-Electric
   Grenoble
   FRANCE

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


   Michael Koster
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

   Email: michael.koster@arm.com















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