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Versions: 00 01

Thing-to-Thing Research Group                                  M. Koster
Internet-Draft                                               SmartThings
Intended status: Experimental                             March 13, 2017
Expires: September 14, 2017


                Media Types for Hypertext Sensor Markup
                       draft-koster-t2trg-hsml-01

Abstract

   The scale and scope of the worldwide web has been in part driven by
   the availability of HTML as a common serialization, data model, and
   interaction model for structured resources on the web.  By contrast,
   the general use of appropriate hypermedia techniques for machine
   interfaces has been limited by the lack of a common format for
   serialization and exchange of structured machine resources and
   sensor/actuator data which includes or embeds standardized hypermedia
   controls.  The IRTF Thing to Thing Research Group [T2TRG] has a
   charter to investigate the use of REST design style [REST]for machine
   interactions.  The W3C Web of Things Interest Group [W3C-WoT] are
   investigating abstract hypermedia controls and interaction models for
   machines.  Machine optimized content formats exist for web links
   [RFC5988] [RFC6690] and for data items [I-D.ietf-core-senml].

   Structured data which contains both links and items is known as the
   collection pattern.  This draft describes media types for
   representation of machine resources structured as collections.  A
   simple, reusable data model is described with a representation
   format, using a well known set of keywords to expose hypermedia
   controls, which inform clients how to perform state transfer
   operations on resources.  The underlying assumptions regarding
   transfer layer processing are specified in this document.  The HSML
   media type described in this document is compatible with SenML and
   CoRE Link-format by reusing the keyword identifiers and element
   structures from these content formats.  Representations of HSML
   document content may be obtained in CoRE Link-Format and SenML
   content formats.

Status of This Memo

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

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



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

   This Internet-Draft will expire on September 14, 2017.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   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.  Scope of this document  . . . . . . . . . . . . . . . . . . .   4
   2.  Overview and Use Case Requirements  . . . . . . . . . . . . .   4
   3.  Data Model and Interaction Model  . . . . . . . . . . . . . .   4
     3.1.  Informative Representation Examples . . . . . . . . . . .   5
     3.2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Collections . . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Link Embedding  . . . . . . . . . . . . . . . . . . . . .   5
       3.4.1.  Batch operations on multiple items in a collection  .   6
       3.4.2.  Collective operation on groups of linked resources  .   6
   4.  Abstract Transfer Model . . . . . . . . . . . . . . . . . . .   6
   5.  Collections . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Base element  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Link element  . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Item element  . . . . . . . . . . . . . . . . . . . . . .   8
       5.3.1.  Items embedded in the collection  . . . . . . . . . .   8
       5.3.2.  Items stored as collections . . . . . . . . . . . . .   9
   6.  Representation Formats  . . . . . . . . . . . . . . . . . . .  11
     6.1.  Example Serialization Formats . . . . . . . . . . . . . .  11
       6.1.1.  Collection Formats  . . . . . . . . . . . . . . . . .  12
       6.1.2.  Link Formats  . . . . . . . . . . . . . . . . . . . .  13
       6.1.3.  Item Formats  . . . . . . . . . . . . . . . . . . . .  13
   7.  URI and Parameter Processing  . . . . . . . . . . . . . . . .  15
     7.1.  URI Path Processing . . . . . . . . . . . . . . . . . . .  15
     7.2.  URI Parameter processing  . . . . . . . . . . . . . . . .  15



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       7.2.1.  Resource selection  . . . . . . . . . . . . . . . . .  16
   8.  Transfer Model Mapping to Collections . . . . . . . . . . . .  16
     8.1.  Target Resource is Collection, Format is Collection . . .  16
       8.1.1.  RETRIEVE  . . . . . . . . . . . . . . . . . . . . . .  16
       8.1.2.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  17
       8.1.3.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  18
       8.1.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  Target Resource is Collection, Format is Link . . . . . .  21
       8.2.1.  RETRIEVE  . . . . . . . . . . . . . . . . . . . . . .  21
       8.2.2.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  21
       8.2.3.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  22
       8.2.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  23
     8.3.  Target Resource is Collection, Format is Item . . . . . .  24
       8.3.1.  Link Embedding Items  . . . . . . . . . . . . . . . .  24
       8.3.2.  RETRIEVE  . . . . . . . . . . . . . . . . . . . . . .  24
       8.3.3.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  25
       8.3.4.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  26
       8.3.5.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  27
     8.4.  Target Resource is Item . . . . . . . . . . . . . . . . .  28
       8.4.1.  RETRIEVE  . . . . . . . . . . . . . . . . . . . . . .  28
       8.4.2.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  28
       8.4.3.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  29
       8.4.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  29
     8.5.  Groups  . . . . . . . . . . . . . . . . . . . . . . . . .  29
       8.5.1.  RETRIEVE  . . . . . . . . . . . . . . . . . . . . . .  30
       8.5.2.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  31
       8.5.3.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  32
       8.5.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  33
   9.  Link extensions . . . . . . . . . . . . . . . . . . . . . . .  34
     9.1.  Actions . . . . . . . . . . . . . . . . . . . . . . . . .  34
     9.2.  Link Bindings and Monitors  . . . . . . . . . . . . . . .  35
   10. Reserved Identifiers  . . . . . . . . . . . . . . . . . . . .  36
     10.1.  Default namespace  . . . . . . . . . . . . . . . . . . .  37
     10.2.  URI Processing Parameters  . . . . . . . . . . . . . . .  37
     10.3.  Link Keywords  . . . . . . . . . . . . . . . . . . . . .  37
       10.3.1.  Link Relation Types  . . . . . . . . . . . . . . . .  38
       10.3.2.  Link Attribute Types . . . . . . . . . . . . . . . .  38
     10.4.  Item Keywords  . . . . . . . . . . . . . . . . . . . . .  38
     10.5.  Link Parameters used in Actions  . . . . . . . . . . . .  39
     10.6.  Link Parameters used in Link Bindings and Monitors . . .  40
     10.7.  Conditional Observe Parameters used in Monitors  . . . .  41
     10.8.  Link Attribute Values  . . . . . . . . . . . . . . . . .  41
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
     11.1.  Media Types  . . . . . . . . . . . . . . . . . . . . . .  42
     11.2.  CoRE Parameters Content Formats  . . . . . . . . . . . .  43
     11.3.  Link Parameters  . . . . . . . . . . . . . . . . . . . .  43
     11.4.  Link Relation Types  . . . . . . . . . . . . . . . . . .  44
     11.5.  New CoRE Interface Types . . . . . . . . . . . . . . . .  44



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     11.6.  Transfer Layer Methods . . . . . . . . . . . . . . . . .  44
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  44
     12.1.  Object Signing . . . . . . . . . . . . . . . . . . . . .  44
     12.2.  Signed Embedded Time Stamps  . . . . . . . . . . . . . .  44
     12.3.  Signed Embedded Access Control . . . . . . . . . . . . .  45
     12.4.  Secure State Updates . . . . . . . . . . . . . . . . . .  45
     12.5.  Object Signing and Encryption  . . . . . . . . . . . . .  45
   13. Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  45
   14. Informative References  . . . . . . . . . . . . . . . . . . .  47
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  48

1.  Scope of this document

   This is a broadly scoped document which specifies representation
   formats, data models, interaction models, transfer mapping, URI
   processing, and design pattern extensions including actions and
   monitors.

   The features listed above and new features may be specified and
   extended as needed in other documents which refer to this document.

2.  Overview and Use Case Requirements

   Use case requirements include the following.

   A standardized way to expose self-describing resource representations
   using embedded hyperlinks and link annotations.

   A standardized way of organizing and interacting with resource
   instances using hypermedia controls such as links and forms.

   A standardized encapsulation of resources for modeling things,
   capabilities, groups, indices, and other common structures.

3.  Data Model and Interaction Model

   The HSML data model consists of collections containing links which
   point to items.  An instance of a collection is a resource and is
   identified by a URI.

   Links are standard web links as in [RFC5988] or [RFC6690].  Items are
   identified by links in collections.

   Links in a collection may point to items within the context of the
   collection or they may point to items external to the collection, on
   the same server or on other servers.





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   Items are data elements, either within the context the collection, or
   outide the context of the collection.  An instance of an item is a
   resource and is identified by a URI.

   An Item may only be in the context of one collection, but may be
   identified by any number of links in any number of collections.

   Items in the collection that use an HSML compatible data model, for
   example SenML, see [I-D.ietf-core-senml], may be embedded in the
   collection and transferred either along with the links or separately
   from links.

3.1.  Informative Representation Examples

   JSON formatted examples are used in this document to illustrate
   normative and informative concepts.  Representations in other formats
   may be derived from the JSON representations.  For example, compact
   binary mappings may be defined using available models.

3.2.  Links

   Links follow the specifications in [RFC5988] and [RFC6690] with
   extensions to implement actions and monitors as described in this
   document and any referencing extension documents.

   HSML Links may be stored in Resource Directories for discovery using
   CoRE Resource Directory [I-D.ietf-core-resource-directory].

3.3.  Collections

   Collections contain one or more links and extended links, and may
   contain data items referred to by the links.  A representation of a
   collection may contain both links and data items, plus any extended
   links such as action forms.

3.4.  Link Embedding

   Link embedding enables the transfer of one or more items in a
   collection using a single transfer operation.  This document
   describes two types of link embedding for items in the collection.
   Batch link embedding allows one or more resource instances (item) to
   each contribute part of an aggregate (collection) representation.
   Group link embedding allows a particular operation to be repeated for
   each member (item) of a group (collection).







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3.4.1.  Batch operations on multiple items in a collection

   A collection of items enables operations on more than one item at a
   time by exposing a structured a representation of multiple resources
   in the collection.

   Applications may select resources by using URI parameters, and
   transfer representations of multiple named resources using the HSML
   or SenML multi-item formats.

3.4.2.  Collective operation on groups of linked resources

   Resource links in the collection may specify group transfer
   semantics, where transfer operations are routed to each resource in
   the collection specified by a group link.  Group responses are
   aggregated using a multi-item format which identifies each item by
   URI.

4.  Abstract Transfer Model

   The HSML media type assumes a transfer model capable of interacting
   with representations using a simple CRUD model, allowing for basic
   life cycle operations on resources and collections.

   CREATE
      Create an instance of a resource as specified using the payload as
      a constructor.  Optionally return a reference to the created
      resource.  Typically uses POST in CoAP [RFC7252] or HTTP, may use
      PUBLISH in pubsub protocols.

   RETRIEVE
      Obtain a representation of the selected resource.  Typically uses
      GET in CoAP or HTTP, could use SUBSCRIBE with message retention in
      pubsub.

   UPDATE
      Replace or partially replace the representation of the selected
      resource.  Typically uses PUT or PATCH in CoAP and HTTP, could use
      PUBLISH in pubsub in the frequent case that CREATE and UPDATE are
      not needed on the same resource.

   DELETE
      Remove the representation of the selected resource.  Typically
      uses DELETE in CoAP or HTTP.  There is no natural mapping to
      pubsub if a remove operation is not provided.

   OBSERVE




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      Obtain a sequence of representations of the selected resource,
      indicating state updates which occur on the resource.  Typically
      uses CoAP Observe, HTTP EventSource, MQTT SUBSCRIBE.  OBSERVE is
      the transfer equivalent of performing a RETRIEVE on the resource
      immediately following each state change of the resource.

5.  Collections

   Collection representations in HSML include Base Elements, Link
   Elements, and Item Elements.

5.1.  Base element

   A base elements describes the context under which to interpret values
   embedded in subsequent items within the representation of a
   collection.

   The base identifier element (bi) may contain an absolute URI or an
   absolute path reference from which to base relative references found
   in the links.  It functions as a base URI embedded in content as per
   [RFC3986] Section 5.1.1

   URI reference follows the definition in [RFC3986] Section 5.

   The format of base elements are specified in [I-D.ietf-core-senml].
   Figure 1 is an example of a base element.

       {
         "bi": "/sensors/"
       }

                        Figure 1: Example Base URI

   Other base items from SenML are permissible, including base time (bt)
   and base value(bv).  If additional senml base values are present, the
   client MUST interpret the items in the collection in the context of
   the applicable base elements.  For example, if there is a "bv" or
   base value elment, all of the returned values from items in the
   collection MUST be added to the base value as per
   [I-D.ietf-core-senml].

5.2.  Link element

   A link element is a hyperlink based on the structure and syntax of
   [RFC6690] and [I-D.ietf-core-links-json].  An example link element is
   shown in Figure 2.





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       {
         "href": "temp",
         "rt": "some.sensor.temp"
       }

                      Figure 2: Example Link Element

5.3.  Item element

   An item element in a collection is a data element that is referenced
   by a link in the collection.

   Items in the collection are indicated by hyperlink references
   ("href") that serve as selection variables for matching to URI
   parameters and resource names ("n")in multi-resource
   representataions.  Reference resolution should use the rules defined
   in [RFC3986].

   Items may be embedded in the collection, they may be subresources of
   the collection, or they may be items in other collections referenced
   by links in the collection.  An example item element is shown in
   Figure 3

       {
         "n": "temp",
         "v": 27
       }

                      Figure 3: Example Item Element

5.3.1.  Items embedded in the collection

   Items may be stored as simple sets of key-value pairs in the context
   of the collection.  Links about these items may be obtained from the
   collection that contains them.
















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      [
        {
          "bi": "/sensors/"
        },
        {
          "href": "temp",
          "rel": "item"
        },
        {
          "href": "humid",
          "rel": "item"
        },
        {
          "n": "temp",
          "v": 27
        },
        {
          "n": "humid",
          "v": 50
        }
      ]

                 Figure 4: Items Embedded in a Collection

5.3.2.  Items stored as collections

   Alternatively, items themselves may be stored as single-item
   collections, pointed to by links in another collection.  Items stored
   as collections may contain an item with a zero length href and name,
   and a self link for the item as shown in the collection
   representation of the item in Figure 5.  Items stored in this way may
   be augmented by adding additional resources and link content to the
   collection.  Items stored as collections may offer link format and
   collection format representations.

















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      base collection:
      [
        {
          "bi": "/sensors/"
        },
        {
          "href": "temp/"
        },
        {
          "href": "humid/"
        }
      ]

      "temp" item:
      [
        {
          "bi": "/sensors/temp/"
        },
        {
          "href": "", //may be elided
          "rel": ["self","item"]
        },
        {
          "n": "", //may be elided
          "v": 27
        }
      ]

      "humid" item:
      [
        {
          "bi": "/sensors/humid/"
        },
        {
          "rel": ["self","item"]
        },
        {
          "v": 50
        }
      ]

                  Figure 5: Items as Separate Collections

   Items embedded in collections, and items linked and stored as
   separate collections, will all be returned using the item
   representation format as shown in Figure 6.  A client interacting
   with the items representation of the example collection at /sensors/




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   would not need to understand the difference between embedded items
   and linked items that exposed similar content.

       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                  Figure 6: Example Items Representation

6.  Representation Formats

   The HSML media type includes multiple content types and interface
   types [I-D.ietf-core-interfaces] to enable the client to select
   representations that optimize communication for the workflow.
   Representation formats include links and items together (collection
   formats), links alone (link formats), or items alone (item formats).

   Link formats are useful for discovery workflow, item formats are
   useful for interaction with resource state machines, and link+item
   formats are useful for constructing and modifying resource instances.

   In addition to HSML native formats, standard CoRE Link-Format
   [RFC6690] and SenML formats [I-D.ietf-core-senml] may be exposed.

6.1.  Example Serialization Formats

   Figure 7 shows an example document in hsml+json format.  This example
   contains a base element, three link elements, and two item elements.













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       RETRIEVE /sensors/ accept=application/hsml+json
       or
       RETRIEVE /sensors/ accept=application/hsml.collection+json
       or
       RETRIEVE /sensors/
         accept=application/hsml+json?if=hsml.collection
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp"
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                    Figure 7: Example Collection Format

   The HSML media type defines content formats and corresponding CoRE
   Interface Types that may select partial representations of the
   resource for interaction.

6.1.1.  Collection Formats

   Collection formats as shown in Figure 7 expose all of the elements of
   a resource, including items, links, and link extensions.








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6.1.2.  Link Formats

   Link content formats, when used in an "accept" option or "content-
   type" option in a transfer header, or when selected by the
   "if=hsml.link" URI parameter, will select the link elements in the
   collection for interaction as in Figure 8.

       RETRIEVE /sensors/ accept=application/hsml.link+json
       or
       RETRIEVE /sensors/ accept=application/hsml+json?if=hsml.link
       Response Payload:
       [
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp"
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         }
       ]

                       Figure 8: Example Lnk Format

   CoRE link-format content formats, for example application/link-
   format+json, select RFC6690 compliant links, and may not include
   representations of extended links (rel=action, rel=monitor).

6.1.3.  Item Formats

   Item content formats, when used in an "accept" option or "content-
   type" option in a transfer header, or when selected by the
   "if=hsml.item" URI parameter, will select the item elements in the
   collection for interaction as in Figure 9.













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       RETRIEVE /sensors/ accept=application/hsml.item+json
       or
       RETRIEVE /sensors/ accept=application/hsml+json?if=hsml.item
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                       Figure 9: Example Item Format

   URI Parameters for matching link attributes and relations may be used
   to select items when item representations are being specified using
   either content-format (accept) or interface parameters (if=).  For
   example:

       RETRIEVE /sensors/?if=hsml.item&rt=some.sensor.temp
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 27
         }
       ]

              Figure 10: Item Selection Using Link Parameter

   SenML content formats select data records and return SenML compliant
   resource names. "bn" may optionally be returned when compliant
   resource names "n" may be resolved through simple string
   concatenation as per [I-D.ietf-core-senml].








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7.  URI and Parameter Processing

   The HSML media type defines URI reference processing and URI Query
   processing but does not in general define fragment (#) references in
   URIs.

   If fragment references are provided in a particular transfer
   implementation, they should be used to select single items in
   collections in accordance with current practice.

7.1.  URI Path Processing

   The path part of the URI reference used to indicate HSML resources
   may be used as a reference to a collection or to an item in a
   collection.  Collection references should contain the trailing slash
   character "/" in accordance with [RFC3986].  Server implementations
   should return links to collections with the trailing "/", and should
   attempt to accept references to collections without the trailing "/"
   if such references can be used to construct unambiguous references.

   References to items in a collection should not contain the trailing
   "/" character.  Servers should return items in response to references
   that do not contain the trailing "/" character, and should attempt to
   accept references to items in collections with the trailing "/" if
   such references can be used to construct unambiguous references.

   URI references may be routed to collections in the order in which
   path segments appear in the reference, from left to right reading the
   path string, separated by "/" characters.

   URI references may alternatively be routed as opaque strings to
   resources.  In this case, the resolution of relative references to
   items in a collection should be possible by concatenating the
   relative reference to the context URI of the collection.  Note that
   this may enforce certain naming conventions such as the trailing
   slash in practice.

7.2.  URI Parameter processing

   URI Parameters, typically mapped as query parameters in HTTP and
   CoAP, are used for selecting resources, selecting partial
   representations, and otherwise modifying aspects of the expected or
   included representation.  In this way, they may be considered part of
   the URI, since they help identify a unique representation to be
   transferred.






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7.2.1.  Resource selection

   URI Parameters may be used to select resources in a collection for
   transfer.  This is done using the common parameter matching rules
   specified in [RFC6690].

   Resource selection is performed based on matching URI Parameters with
   Link Parameters of all links in the collection which are exposed by
   the indicated media type and interface type.  URI Parameters listed
   in Section 10.2 are excluded from the matching process.

   The target resource selection depends on the content-format specified
   in the request or the interface type specified in the URI parameters.

   The collection content-formats or interface types select all links
   and items in the collection, including link extensions.  URI
   parameters included in the request should be matched against link
   parameters for selecting links and associated items.

   The link content formats or interface types select all links in the
   collection.  URI parameters included in the request should be matched
   against link parameters for link selection.

   The item content formats or interface types select all items in the
   collection.  URI parameters included in the request should be matched
   against link parameters associated with items in the collection for
   item selection as shown in Figure 10.

8.  Transfer Model Mapping to Collections

8.1.  Target Resource is Collection, Format is Collection

   When the reference of a request targets a collection resource, using
   a collection format, the representation may contain both links and
   items.  It is implied that operations using this format will interact
   with both links and items.  The collection format is indicated by
   using a collection content type in the accept or content-type header,
   or by specifying a collection interface type e.g. if=hsml.collection.

8.1.1.  RETRIEVE

   Retrieve returns a representation of selected elements, consisting of
   a list of elements in the collection, including base element, links,
   and optionally representations of items, as shown in Figure 11.
   Elements may include link extenstions, for example action links and
   monitor links.





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       RETRIEVE /sensors/ accept=application/hsml.collection+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp"
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                      Figure 11: Retrieve Collection

8.1.2.  UPDATE

   Update replaces all selected elements in the collection with elements
   included in the payload.  Update operations may include replace (PUT)
   and partial update (PATCH) oprations where supported in the transfer
   protocol.  The server response should indicate that the resource was
   Changed.














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       UPDATE /sensors/?href=temp
         content-type=application/hsml.collection+json
       Payload:
       [
         {
           "rt": ["some.sensor.temp", "some.other.type"]
         }
       ]
       Response: Changed

       RETRIEVE /sensors/ accept=application/hsml.collection+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": ["some.sensor.temp", "some.other.type"]
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                   Figure 12: Update Item in Collection

8.1.3.  CREATE

   Create adds elements to a collection, including links and items,
   where the elements are specified by representations included in the
   payload.  Hints and directives about the created resource may be
   included in the payload as link parameters, for example a value for
   href, specifying the location of the created resource.  The server is
   expected to return the location of created resource instances to the



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   client in a header or other response parameter.  For example, the
   "Location" option in CoAP or "Location" header in HTTP should be used
   to identify the created resource.  The server response should
   indicate that a resource was Created.

       CREATE /sensors/ content-type=application/hsml.collection+json
       Payload:
       [
         {
           "href": "barometer",
           "rt": "some.sensor.mbar"
         },
         {
           "n": "barometer",
           "v": 993
         }
       ]
       Response: Created
       Location: "barometer"

       RETRIEVE /sensors/ accept=application/hsml.collection+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "barometer",
           "rt": "some.sensor.mbar"
         },
         {
           "href": "temp",
           "rt": ["some.sensor.temp", "some.other.type"]
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         },
         {
           "n": "barometer",
           "v": 993
         },
         {
           "n": "temp",



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           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                   Figure 13: Create Item in Collection

8.1.4.  DELETE

   Delete removes selected elements from the collection.  If no elements
   are selected, delete removes the entire collection.  The server
   response should indicate that the resource was Deleted.

       DELETE /sensors/?href=barometer
       Response: Deleted

       RETRIEVE /sensors/ accept=application/hsml.collection+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": ["some.sensor.temp", "some.other.type"]
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                   Figure 14: Delete Item in Collection



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8.2.  Target Resource is Collection, Format is Link

   When a collection is referenced and the link format is indicated,
   using a link content format in the header or specifying a link
   interface type, e.g. if=hsml.link, it is expected that the request
   will interact with the links in the collection.

8.2.1.  RETRIEVE

   Retrieve returns a list containing selected links, as shown in
   Figure 15.

       RETRIEVE /sensors/ accept=application/hsml.link+json
       Response Payload:
       [
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": ["some.sensor.temp", "some.other.type"]
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         }
       ]

       RETRIEVE /sensors/?rt=some.sensor.temp
       accept=application/hsml.link+json
       Response Payload:
       [
         {
           "href": "temp",
           "rt": ["some.sensor.temp", "some.other.type"]
         }
       ]


                         Figure 15: Retrieve Links

8.2.2.  UPDATE

   Update modifies selected links, replacing link elements with elements
   included in the payload.  Update operations may include replace (PUT)
   and partial update (PATCH) oprations where supported in the transfer




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   protocol.  The server response should indicate that the resource was
   Changed.

       UPDATE /sensors/?href=temp
       content-type=application/hsml.link+json
       Payload:
       [
         {
           "rt": "some.sensor.temp"
         }
       ]

       RETRIEVE /sensors/ accept=application/hsml.link+json
       Response Payload:
       [
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp",
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         }
       ]

                          Figure 16: Update Links

8.2.3.  CREATE

   Create adds links to the collection, where the links are included in
   the payload.  The server response should indicate that the resource
   was Changed.















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       CREATE /sensors/ content-type=application/hsml.link+json
       Payload:
       [
         {
           "href": "/sensor-group/"
         }
       ]
       Response: Changed

       RETRIEVE /sensors/ accept=application/hsml.link+json
       Response Payload:
       [
         {
           "href": "/sensor-group/"
         },
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp",
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         }
       ]


                          Figure 17: Create Links

8.2.4.  DELETE

   Delete removes selected links from the collection.  The server
   response should indicate that the resource was Changed.  If links
   point to items in the context of the collection, either remove the
   items as well as the links, or leave the collection as is and return
   a method error (Method Not Allowed).












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       DELETE /sensors/?href=sensor-group
       Response: Changed

       RETRIEVE /sensors/ accept=application/hsml.link+json
       Response Payload:
       [
         {
           "anchor": "/sensors/",
           "rel": ["self", "index"]
         },
         {
           "href": "temp",
           "rt": "some.sensor.temp",
         },
         {
           "href": "humid",
           "rt": "some.sensor.humid"
         }
       ]

                          Figure 18: Delete Links

8.3.  Target Resource is Collection, Format is Item

   Whan a collection is referenced and the item format is indicated,
   either by including an item content type in the request header or
   using an item interface type, e.g. if=hsml.item, it is expected that
   the request will interact with the items in a collection.

   Specifying item interaction with a collection invokes the link
   embedding operations.

8.3.1.  Link Embedding Items

   Collective operations on items in collections are invoked by using
   the URI of the collections, along with URI parameters, to select one
   or more items in the collection.

   Items which are compatible with the HSML item format may be returned
   with multiple items embedded in a single representation.

8.3.2.  RETRIEVE

   Retrieve returns a list containing a base element and a composite
   representation of the selected items as shown in Figure 19.






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       RETRIEVE /sensors/ accept=application/hsml.item+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 27
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

       RETRIEVE /sensors/?href=temp
       accept=application/hsml.item+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 27
         }
       ]

                         Figure 19: Retrieve Items

8.3.3.  UPDATE

   Update modifies selected items, replacing items in the collection
   with items included in the payload which match by name "n" value.
   Update operations may include replace (PUT) and partial update
   (PATCH) oprations where supported in the transfer protocol.  The
   server response should indicate that the resource was Changed.













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       UPDATE /sensors/ content-type=application/hsml.item+json
       Payload:
       [
         {
           "n": "temp",
           "v": 30
         }
       ]
       Response: Changed

       RETRIEVE /sensors/ accept=application/hsml.item+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 30
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                          Figure 20: Update Items

8.3.4.  CREATE

   Create adds new items to the collection along with system-constructed
   links.  Link content is determined by the resource type or traits
   defined by application semantics.  Server is expected to return the
   location of created resource instances to the client in a header or
   other response parameter.  For example, the "Location" option in CoAP
   or "Location" header in HTTP should be used to identify the created
   resource.  THe server response should indicate that a resource was
   Created.













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       CREATE /sensors/ content-type=application/hsml.item+json
       Payload:
       [
         {
           "n": "barometer",
           "v": 1002
         }
       ]
       Response: Created

       RETRIEVE /sensors/ accept=application/hsml.item+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 30
         },
         {
           "n": "barometer",
           "v": 1002
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                          Figure 21: Create Items

8.3.5.  DELETE

   Delete removes selected items and corresponding links from the
   collection.  The server response should indicate that the resource
   was Deleted.  If no items are selected, return a not found error.














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       DELETE /sensors/?href=barometer
       Response: Deleted

       RETRIEVE /sensors/ accept=application/hsml.item+json
       Response Payload:
       [
         {
           "bi": "/sensors/"
         },
         {
           "n": "temp",
           "v": 30
         },
         {
           "n": "humid",
           "v": 50
         }
       ]

                          Figure 22: Delete Items

8.4.  Target Resource is Item

   When the URI of a reference points to an item in a collection, it is
   expected that the request will interact with a single item.

8.4.1.  RETRIEVE

   Retrieve returns a representation of the item in the content type
   according to the accept option of the RETRIEVE request, or using a
   system defined content-format if there is no accept option provided.

       RETRIEVE /sensors/temp accept=text/plain
       Response Payload:
       30

                       Figure 23: Retrieve One Item

8.4.2.  UPDATE

   Update replaces the resource state with the state defined in the
   supplied representation according to the content-type or ct option.
   Update operations may include replace (PUT) and partial update
   (PATCH) oprations where supported in the transfer protocol.  The
   server response should indicate that the resource was Changed.






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       UPDATE /sensors/temp content-type=text/plain
       Payload:
       33

       RETRIEVE /sensors/temp accept=text/plain
       Response Payload:
       33

                        Figure 24: Update One Item

8.4.3.  CREATE

   Not Defined, application dependent.

8.4.4.  DELETE

   Delete removes any links to the item from the collection, and removes
   the item.  If the item is stored as a collection, delete removes the
   collection.  The server response should indicate that the resource
   was Deleted.

       DELETE /sensors/temp

       RETRIEVE /sensors/temp accept=text/plain
       Response: Not Found

                        Figure 25: Delete One Item

8.5.  Groups

   Group transfer operations are provided by collections that contain
   links with the "grp" relation value.

   Transfer operations which specify the collection URI as target and
   use the item content format are routed to the resolved URI of each
   link in the collection that contains the "grp" relation.

   URI Parameters used for resource selection and matching are sent to
   the target URIs of all links that contain the "grp" relation.

   Responses from the selected group resources are aggregated and by
   default returned as a single response.  The group response SHOULD be
   returned as an outer array where such representation is available,
   for example a JSON array which contains elements consisting of SenML
   responses.






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   Optionally, a chunked response may be specified, if provided by the
   transfer implementation, in which the response from each group member
   is returned individually within a sequence of responses.

   The return code should be based on successful responses from link
   targets.  An implementation of a group collection may choose to allow
   some rejected responses from link targets, depending on the
   composition of the link targets.  A group may not be required to be
   composed of link targets that always accept all requests; this is at
   the discretion of the resource designer.

   No mechanism is provided in this document to enable a client to
   inspect the separate return codes from each group link target
   resource.  Multiple transfer headers may be supplied in some
   representations, or mapped to metadata in others.

   The following examples assume the prior example from Figure 5 indexed
   by a group collection as in Figure 26.

       RETRIEVE /sensor-group/ accept=application/hsml.collection+json
       Response Payload:
       [
         {
           "bi": "/sensor-group/"
         },
         {
           "anchor": "/sensor-group/",
           "rel": ["self", "index"]
         },
         {
           "href": "/sensors/temp/",
           "rel": "grp"
         },
         {
           "href": "/sensors/humid/",
           "rel": "grp"
         }
       ]

                    Figure 26: Example Group Collection

8.5.1.  RETRIEVE

   Retrieve requests are routed to each link in the collection that
   contains the "grp" relation.  The response from each link target is
   returned as an element in an array representation.





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       RETRIEVE /sensor-group/ accept=application/hsml.item+json
       Response Payload:
       [
         [
           {
             "bi": "/sensors/temp/"
           },
           {
             "v": 33
           }
         ],
         [
           {
             "bi": "/sensors/humid/"
           },
           {
             "v": 41
           }
         ]
       ]

                         Figure 27: Group Retrieve

8.5.2.  UPDATE

   Update requests are routed to each link in the collection that
   contains the "grp" relation.  The target resource of each group link
   processes the request, including URI parameters and content format.
   The result code returned should indicate that the resource is Changed
   if any resource state may have been updated.





















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       UPDATE /sensor-group/ content-type=application/hsml.item+json
       Payload:
       [
         {
           "v": 0
         }
       ]
       Response: Changed

       RETRIEVE /sensor-group/ accept=application/hsml.item+json
       Response Payload:
       [
         [
           {
             "bi": "/sensors/temp/"
           },
           {
             "v": 0
           }
         ],
         [
           {
             "bi": "/sensors/humid/"
           },
           {
             "v": 0
           }
         ]
       ]

                          Figure 28: Group Update

8.5.3.  CREATE

   Create requests are routed to each link in the collection that
   contains the "grp" relation.  In the example shown in Figure 29, an
   additional named resource is being created within each (collection
   type) item to hold a location value for that item.  The result code
   should indicate that a resource was Created if any resource was
   created as a result of the create operation.











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       CREATE /sensor-group/ content-type=application/hsml.item+json
       Payload:
       [
         {
           "n": "location",
           "vs": "living room"
         }
       ]
       Response: Created

       RETRIEVE /sensor-group/ accept=application/hsml.item+json
       Response Payload:
       [
         [
           {
             "bi": "/sensors/temp/"
           },
           {
             "v": 0
           },
           {
             "n": "location",
             "vs": "living room"
           }

         ],
         [
           {
             "bi": "/sensors/humid/"
           },
           {
             "v": 0
           },
           {
             "n": "location",
             "vs": "living room"
           }
         ]
       ]

                          Figure 29: Group Create

8.5.4.  DELETE

   Delete requests are routed to each link in the collection that
   contains the "grp" relation.  In the example shown in Figure 30, the
   URI parameter ?href=location selects the resource at the relative URI
   reference "location" at each group link target for delete.  The



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   result code should indicate that a resource was Deleted if any
   resource was deleted as a result of the delete operation.

       DELETE /sensor-group/?href=location
       Response: Deleted

       RETRIEVE /sensor-group/ accept=application/hsml.item+json

       Response Payload:
       [
         [
           {
             "bi": "/sensors/temp/"
           },
           {
             "v": 0
           }
         ],
         [
           {
             "bi": "/sensors/humid/"
           },
           {
             "v": 0
           }
         ]
       ]


                          Figure 30: Group Delete

9.  Link extensions

9.1.  Actions

   Actions are hypermedia controls, indicated by a rel=action value in a
   link, used to construct transfer operations that change the state of
   resources.  The use roughly follows the use of forms in HTML
   [RFC1866], with semantics more consistent with links.  See
   Section 10.5 for more information.

   An example Action element is shown in Figure 31.









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       {
         "rel": "action",
         "type": "st.on",
         "href": "switchcommand",
         "method": "create",
         "accept": "text/plain",
         "schema": {"type": "string", "enum": ["on"]}
       }

       {
         "rel": "action",
         "type": "st.off",
         "href": "switchcommand",
         "method": "create",
         "accept": "text/plain",
         "schema": {"enum": ["off"]}
       }

                     Figure 31: Example Action Element

   These Action elements inform the client that to perform a type
   "st.on" or "st.off" action on the context resource, perform a CREATE
   method on the "switchcommand" URI relative to the context URI, using
   the text/plain content type, with a payload as defined by the
   "schema" parameter.  This example uses a free-form fragment of JSON-
   Schema language to differentiate, by action payloads, the "st.on" and
   "st.off" actions, which are mapped to the same URI and method.

9.2.  Link Bindings and Monitors

   Link Bindings and Monitors are hypermedia controls, indicated by a
   rel=boundto or rel=monitor value in a link, used to construct
   transfer operations that consume or expose state changes of
   resources.  A monitor invokes a state transfer operation from the
   link context to a target resource.  A Link Binding follows the
   semantics defined in [I-D.groves-core-dynlink], and invokes a state
   transfer in the opposite direction, that is from the link target to
   the link context.

   Monitors use the IANA registered link relation "monitor", defined in
   [RFC5989].  Link Bindings use the link relation type "boundto",
   defined in [I-D.groves-core-dynlink].

   Monitors have a set of accept parameters that indicate how the
   context resource is being observed, a set of filter parameters that
   indicate the conditions for generating a state change in the monitor,
   and a set of target parameters that indicate how state changes are to




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   be applied to the monitor resource.  See Section 10.6 for more
   information.

   An example Monitor element is shown in Figure 32.

       {
         "rel": "monitor",
         "href": "tank-level-events",
         "content-type": "application/senml+json",
         "transfer-method": "create",
         "pmin": 600,
         "pmax": 3600,
         "nbul": 20,
         "nbll": 80
       }

                    Figure 32: Example Monitor Element

   This Monitor element defines a monitor resource at the "tank-level-
   events" URI relative to the context URI, which OBSERVEs the context
   URI, and updates the "tank-level-events" resource using the CREATE
   method to add JSON items to the collection, according to the given
   conditional parameters no more frequently than once every 600
   seconds, at least once every 3600 seconds, when the reading is in the
   notification band, which has a lower limit of 80 and wraps around
   zero to an upper limit of 20.  This has the effect of defining a low
   level alert notification and high level alert notification.

10.  Reserved Identifiers

   This section defines the common reserved identifiers that are
   expected to be processed by implementations of HSML clients and
   servers.  There are many more relation types and link parameters
   defined and registered with IANA.  Implementations should not
   restrict processing to the keywords identified in this document; they
   should accept all IANA registered keywords as valid identifiers.

   Many of the keywords listed are defined in other RFCs and IETF
   documents.  This document does not redefine any existing keywords.
   Where a definition exists, the existing definition will be used.
   Where multiple conflicting definitions exist, this document will
   indicate the required definition.

   New definitions are summarized in Section 11.







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10.1.  Default namespace

   Identifiers in representations using the HSML media types are assumed
   to use the default namespace defined in Section 10 of this document.
   An identifier that does not containan explicit namespace identifier
   is assumed to be in the default namespace.

   For example, if the identifier "method" is encountered and it doesn't
   resolve to an IANA registered parameter (reg-parameter in [RFC5988])
   resolution should be attempted using the definition of "method" in
   this document.

10.2.  URI Processing Parameters

   The following URI Parameters are used to filter representations
   according to specific processing rules and should not be used to
   attempt to match link parameters.

   "if"  Interface type, used to select a partial representation of a
      collection

   "count"  Indicates the number of items to be returned from the
      collection

   "start"  Indicates the array index of the item in the collection to
      select as the first item to be returned

   "page"  Page number, in units of count

10.3.  Link Keywords

   The following keywords are reserved for use in an HSML serialization
   to indicate elements of a web link

   "anchor"
      Overrides the default resource context of the link

   "rel"
      Link relation type as defined in [RFC5988] and [RFC6690]

   "href"
      Target of a link reference.  This may be a relative path reference
      in the collection, e.g. "currentValue" or an absolute path
      reference on the server, "/sensors/temp/currentValue", or an
      absulute URI, for example "https://example.com/sensors/temp/
      currentValue"





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10.3.1.  Link Relation Types

   The following keywords are reserved for use in a HSML serialization
   to indicate types of link relations, and are used for values of
   "rel".

   "self"
      Refers to the collection that contains the link

   "item"
      The link points to an item in the collection, indicating
      eligibility for collective interaction using link embedding as
      described in Section 3.4 and Section 8.3.1.

   "grp"
      The item the link points to is available for collective
      interaction through the collection URI according to group
      semantics described in Section 8.5.

10.3.2.  Link Attribute Types

   The following keywords are reserved for use in a HSML serialization
   to indicate types of link attributes

   "rt"
      The resource type(s) of the item

   "u"
      Units of measure

   "ct"
      The CoAP content-format number(s) associated with the item

   "content-type"
      The media type string(s) associated with the item

   "obs"
      Presence of this attribute indicates that the associated resource
      is observable

10.4.  Item Keywords

   The following keywords are reserved for use in a HSML serialization
   to indicate elements within the serialization.  Some of these are
   defined in [I-D.ietf-core-senml].

   "bi"




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      The base URI of the collection, relative to the service location
      e.g. "/sensors/temp/" Thi sis a new definition for HSML

   "bt"
      The base time that corresponds to the encapsulated state of the
      collection

   "t"
      The time stamp that corresponds to the encapsulated state of the
      item in the collection, relative to the base time "bt"

   "n"
      The name or URI of the resource, relative to the base name or base
      URI

   "u"
      Units of measure

   "v"
      Number value

   "vb"
      Boolean value

   "vs"
      String value

10.5.  Link Parameters used in Actions

   "anchor"
      May override the default context of an action

   "rel"
      Indicates that this control is an action when rel contains the
      value "action"

   "href"
      URI for mapping or invoking the action specified in the action
      control.

   "type"
      Additional indicator of the action being exposed, can be used with
      "rel"

   "method"
      Transfer method to use on a particular action

   "accept"



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      The Content-Types or CoAP content-formats that are accepted on
      create and update methods

   "content-type"
      The media type string(s) that are exposed by retrieve and observe
      methods

   "ct"
      The CoAP content-format number(s) exposed

   "schema"
      Indicates the schema to use for constructing or interpreting
      transfer payloads, may be a literal value or a URI pointing to an
      instance of a schema

10.6.  Link Parameters used in Link Bindings and Monitors

   "anchor"
      May override the context URI of a link binding or monitor with any
      observable resource

   "rel"
      Indicates that this control is a monitor when rel contains the
      value "monitor" or a link binding when rel contains the value
      "boundto"

   "href"
      The URI of the resource used to monitor context URI, where
      transfer operations will be sent.

   "accept"
      The media type string or CoAP content-format to request from the
      observed resource

   "content-type"
      The media type string to use in the transfer operation

   "ct"
      the CoAP content-format number to use in the transfer operation

   "accept-method"
      (HSML extension) Transfer method to use in request from the
      observed resource, default is OBSERVE

   "transfer-method"
      (HSML extension) Transfer method to use for notifications, default
      is UPDATE




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   "accept-schema"
      (HSML extension) Schema to use in interpreting the observed
      resource payload, required if transfer-schema is used.

   "transfer-schema"
      (HSML extension) Schema to use in constructing the notification
      transfer payload, default is to transfer the accepted payload
      unmodified to the target resource.

10.7.  Conditional Observe Parameters used in Monitors

   "pmin"
      Minimum time between notifications from a monitor

   "pmax"
      Maximum time between notifications from a monitor

   "gth"
      Value to match or exceed to determine notification condition

   "lth"
      Value to match or be less than to determine notification condition

   "st"
      Value change +/- from last report to determine notification
      condition

   "eq"
      Value to match, or change from, to determine notification
      condition

   "bmn"
      Defines a lower limit, at or above which notification is enabled

   "bmx"
      Defines an upper limit, at or below which notification is enabled

   "iv"
      Starts the notification state machine with an initial value

10.8.  Link Attribute Values

   The following keywords are reserved for use in a HSML serialization
   to indicate values of link attributes

   "create"
      Transfer layer CREATE operation, value of "method" or "target-
      method"



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   "retrieve"
      Transfer layer RETRIEVE operation, value of "method" or "accept-
      method"

   "update"
      Transfer layer UPDATE operation, value of "method" or "target-
      method"

   "delete"
      Transfer layer DELETE operation, value of "method" or "target-
      method"

   "observe"
      Transfer layer OBSERVE operation, value of "method" or "accept-
      method"

11.  IANA Considerations

11.1.  Media Types

   Type

   o  application

   Subtypes

   o  hsml

   o  hsml.collection

   o  hsml.link

   o  hsml.item

   Media type strings

   o  application/hsml

   o  application/hsml.collection

   o  application/hsml.link

   o  application/hsml.item

   o  application/hsml+json

   o  application/hsml.collection+json




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   o  application/hsml.link+json

   o  application/hsml.item+json

11.2.  CoRE Parameters Content Formats

   (subject to Structured Syntax encoding rules TBD)

   o  22000 - application/hsml+json

   o  22001 - application/hsml.link+json

   o  22002 - application/hsml.item+json

11.3.  Link Parameters

   o  method

   o  schema

   o  content-type

   o  ct

   o  accept-method

   o  transfer-method

   o  accept-schema

   o  transfer-schema

   The following should be registered in the CoRE dynamic linking draft
   [I-D.groves-core-dynlink].

   o  pmin

   o  pmax

   o  bmn

   o  bmx

   o  iv

   o  lth

   o  gth



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   o  st

   o  eq

11.4.  Link Relation Types

   o  grp

11.5.  New CoRE Interface Types

   o  hsml.collection

   o  hsml.item

   o  hsml.link

11.6.  Transfer Layer Methods

   These definitions may use the default namespace and do not need to be
   registered with IANA

   o  create

   o  retrieve

   o  update

   o  delete

   o  observe

12.  Security Considerations

12.1.  Object Signing

   Collection representations are resource state encapsulations and may
   be transmitted and stored as signed objects in order to protect the
   integrity of data and metadata, including time and embedded access
   control information.

12.2.  Signed Embedded Time Stamps

   The collection may include time stamps (bt and t) that are signed
   with the object data and metadata.







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12.3.  Signed Embedded Access Control

   The collection representation may include embedded access control
   information, also signed with the metadata, that can instruct the
   server to enforce a particular access policy for transfer requests.

12.4.  Secure State Updates

   Representations submitted to a server to update the state of a
   resource (UPDATE, CREATE, DELETE) may also contain embedded signed
   assertions which may be used by the server to decide whether to apply
   or reject the update.

12.5.  Object Signing and Encryption

   Object signing and encryption SHOULD use the mechanisms specified in
   IETF documents for secure JSON Objects [RFC7516] and CBOR Objects
   [I-D.ietf-cose-msg] [I-D.selander-ace-object-security].

13.  Terminology

   Client
      Having a client role in a REST operation, transmitting a request
      and receiving one or more responses.

   Server
      Having a server role in a REST operation, the origin of data items
      or proxy for the origin.  A server is also an authority for a URI
      namespace [RFC3986].

   Resource
      Server endpoint for a REST operation, identified by a URI
      [RFC3986]

   Representation
      An encoded form of the state of a resource.  The encoding rules
      may be specified in a media type or content type.  Clients and
      servers exchange representations of resources in order to effect
      application state changes [REST].

   URI
      Uniform Resource Identifier, used to identify a resource in a link
      or as a reference [RFC3986]

   Reference
      An identifier used to select or identify a particular resource.
      References are constructed by clients to identify resources when




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      interacting with servers.  Servers match references in client
      requests against URIs of hosted resources.

   Media Type, also Content-Format, Content-Type
      A set of rules for encoding, transfer, and prosessing resource
      representations

   Hypermedia
      Design style which uses metadata in the form of hyperlinks to
      structure resources in relation to each other

   Collection
      A composite resource that contains links and optionally data items

   Link, also Hyperlink
      A metadata element as described in [RFC5988] and [RFC6690] that
      contains a pointer to and description of some data element.

   Item
      A data item pointed to by one or more links in one or more
      collections.

   Context
      The context of a link is the subject of the link or the enclosing
      scope.  In this document the collection is the default context for
      links in the collection.

   Target
      The target of a link is the resource being pointed to or
      described.  Links in a collection point to and describe items as
      link targets.

   Transfer Layer
      A set of predefined message types used to implement state transfer
      semantics, for example REST.

   Request
      A message sent from a client to a server identifying the resource,
      representation, and method to use for the interaction with the
      server.

   Response
      A message sent from a server to a client in response to a request,
      which communicates the state of the identified resource.

   Operation or Method





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      The state transition type requested by the client for the server
      to perform on the identified resource.  Indicated by the transfer
      layer method, for example, RETRIEVE, UPDATE, CREATE, DELETE.

   Pubsub
      A transfer layer semantic interface based on the publish-subscribe
      paradigm, allowing for asynchronous messages to be routed on
      demand.

14.  Informative References

   [I-D.groves-core-dynlink]
              Shelby, Z., Vial, M., Koster, M., and C. Groves, "Dynamic
              Resource Linking for Constrained RESTful Environments",
              draft-groves-core-dynlink-00 (work in progress), July
              2016.

   [I-D.ietf-core-interfaces]
              Shelby, Z., Vial, M., Koster, M., and C. Groves, "Reusable
              Interface Definitions for Constrained RESTful
              Environments", draft-ietf-core-interfaces-08 (work in
              progress), February 2017.

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing CoRE
              Formats in JSON and CBOR", draft-ietf-core-links-json-06
              (work in progress), July 2016.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-09
              (work in progress), October 2016.

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

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

   [I-D.selander-ace-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-selander-ace-
              object-security-06 (work in progress), October 2016.




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   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000,
              <http://www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [RFC1866]  Berners-Lee, T. and D. Connolly, "Hypertext Markup
              Language - 2.0", RFC 1866, DOI 10.17487/RFC1866, November
              1995, <http://www.rfc-editor.org/info/rfc1866>.

   [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>.

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

   [RFC5989]  Roach, A., "A SIP Event Package for Subscribing to Changes
              to an HTTP Resource", RFC 5989, DOI 10.17487/RFC5989,
              October 2010, <http://www.rfc-editor.org/info/rfc5989>.

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

   [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>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

   [T2TRG]    IRTF, ., "IRTF Thing to Thing Research Group", 2016,
              <https://datatracker.ietf.org/rg/t2trg/charter/>.

   [W3C-WoT]  WoT IG, ., "W3C Web of Things Interest Group", 2016,
              <https://www.w3.org/WoT/IG/>.

Author's Address








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   Michael Koster
   SmartThings
   1281 Lawrence Station Road
   Sunnyvale  94089
   USA

   Phone: +1-707-502-5136
   Email: michael.koster@smartthings.com











































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