Thing-to-Thing Data Hub
draft-hartke-t2trg-data-hub-06

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Thing-to-Thing Research Group                                  K. Hartke
Internet-Draft                                                  Ericsson
Intended status: Experimental                                May 9, 2020
Expires: November 10, 2020

                        Thing-to-Thing Data Hub
                     draft-hartke-t2trg-data-hub-06

Abstract

   A "Thing-to-Thing Data Hub" is a RESTful, hypermedia-driven Web
   application that can be used in Thing-to-Thing communications to
   share data items such as thing descriptions, configurations, resource
   descriptions, or firmware updates at a central location.

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
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   This Internet-Draft will expire on November 10, 2020.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Data Hubs . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Data Items  . . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Data Item Representation  . . . . . . . . . . . . . .   4
     2.2.  Data Collections  . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Data Collection Representation  . . . . . . . . . . .   5
       2.2.2.  Filter Query Representation . . . . . . . . . . . . .   5
     2.3.  Data Hub Discovery  . . . . . . . . . . . . . . . . . . .   5
     2.4.  Interactions  . . . . . . . . . . . . . . . . . . . . . .   6
       2.4.1.  Getting All Data Items  . . . . . . . . . . . . . . .   6
       2.4.2.  Getting Data Items by Metadata  . . . . . . . . . . .   6
       2.4.3.  Creating a Data Item  . . . . . . . . . . . . . . . .   6
       2.4.4.  Reading a Data Item . . . . . . . . . . . . . . . . .   7
       2.4.5.  Observing a Data Item . . . . . . . . . . . . . . . .   7
       2.4.6.  Updating a Data Item  . . . . . . . . . . . . . . . .   7
       2.4.7.  Deleting a Data Item  . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In Thing-to-Thing communication, there is often a need to share data
   items of common interest through a central location.  For example,
   the Resource Directory [I-D.ietf-core-resource-directory] aggregates
   descriptions of Web resources held on constrained nodes, which
   enables other nodes to easily discover these resources; a Thing
   Directory [W3C.CR-wot-architecture-20190516] stores metadata of IoT
   devices, allowing clients to discover interaction affordances and
   supported protocol bindings of Things; a Firmware Server
   [I-D.ietf-suit-architecture] stores firmware images and manifests,
   making this data available to deployed devices, commissioning tools,
   and other services.

   As more and more Thing-to-Thing applications are implemented, it
   becomes increasingly important being able to not only share resource
   descriptions and firmware updates but also many other kinds of data,
   such as default configurations for new devices, service locations, or
   certificate revocation lists.  Resource directories and firmware
   servers are not a good fit for these kinds of data, as they're
   specialized to their use cases and generally not accepting any other

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   kinds of data.  The creation of new, specialized applications for
   every type of data is not practical in the long term.

   This document defines a simple "data hub" application, a RESTful Web
   application with a machine-understandable hypermedia API.  A "data
   hub" generalizes the concept of a central repository for different
   applications and is suitable for constrained environments [RFC7228].
   Specifically, it enables clients to share data items in any format
   and provides means for creating, reading, observing, updating,
   deleting, and finding data items at a data hub server.

   Data hubs are primarily intended to be accessible over the
   Constrained Application Protocol (CoAP) [RFC7252].

   Features:

   o  General

      The data hub generalizes the concept of a directory or repository
      to data items of any Internet media type.  This means that
      applications using the data hub aren't stuck forever with the same
      media types or limited to just resource descriptions or firmware
      updates.

   o  Searchable

      Clients can retrieve a subset of data items from a data hub based
      on item metadata.

   o  Observable

      Data items published to a data hub are exposed as resources.  As
      such, they can be observed for changes [RFC7641] over CoAP.  This
      allows clients to stay informed of information that other clients
      update over time.  As a result, the data hub functions similar to
      a Publish-Subscribe Broker [I-D.ietf-core-coap-pubsub].

   o  Evolvable

      The key differentiator of the data hub compared to Resource
      Directory [I-D.ietf-core-resource-directory] and CoAP Publish-
      Subscribe Broker [I-D.ietf-core-coap-pubsub] lies in the
      evolvability of the application -- the ability to respond
      effectively to the need for changes without negatively impacting
      existing and new clients.

      Data hubs enable fine-grained evolvability by driving all
      interactions by machine-understandable hypermedia elements.

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      Features can be added, changed or removed in a safe, backwards-
      compatible way simply by updating the data hub representation to
      expose appropriate links and forms.

1.1.  Notational Conventions

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

2.  Data Hubs

   The "Thing-to-Thing Data Hub" application consists of two types of
   resources: a "data collection" and a number of "data items" that have
   been shared (Figure 1).

                             ___
                      Data  /   \
                Collection  \___/
                                \
                                 \____________________
                                  \___    \___        \___
                            Data  /   \   /   \  ...  /   \
                           Items  \___/   \___/       \___/

                     Figure 1: Resources of a Data Hub

2.1.  Data Items

   A data item is a resource that is a member of the data collection
   resource.

2.1.1.  Data Item Representation

   The representation of a data item can be of any media type.  However,
   a data hub can restrict the media types it accepts for publication
   (see Section 2.2.1).

2.2.  Data Collections

   A data collection is a collection resource that contains data item
   resources.

   Design Note:  In this version of this document, a data hub has only a
      depth of one level; i.e., all data item resources are organized
      directly under the top-level data collection resource.  This could
      be extended to multiple levels in a future version.

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2.2.1.  Data Collection Representation

   The representation of a data collection is a CoRAL document
   [I-D.ietf-core-coral] containing the following elements:

      A form of type <http://coreapps.org/collections#create> containing
      the following fields:

         For each content format accepted for publication, a form field
         of type <http://coreapps.org/coap#accept> indicating that
         content format.  The absence of any form fields of this type
         indicates that any content format is accepted.

      A form of type <http://coreapps.org/base#search>.

      For each data item in the data collection, a link of type
      <http://www.iana.org/assignments/relation/item> [RFC6573]
      targeting the data item and containing the following elements:

         A form of type <http://coreapps.org/base#update>.

         A form of type <http://coreapps.org/collections#delete>.

         Optionally, a (complete or partial) embedded representation of
         the data item.

   The document MAY additionally contain other links and forms not
   described in this document.  For example, a document could contain a
   link with the <http://www.iana.org/assignments/relation/alternate>
   link relation type [W3C.REC-html52-20171214] that references an
   alternate representation of a data item.

   Any of the links and forms MUST be omitted if following or submitting
   it can never lead to a successful outcome, for example, because the
   client is not authorized or the server does not support the feature.

2.2.2.  Filter Query Representation

   TODO.

2.3.  Data Hub Discovery

   In this version of this document, clients are assumed to be pre-
   configured with the URI of a data collection at a data hub.

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2.4.  Interactions

2.4.1.  Getting All Data Items

   A client can list all data items in a data collection by making a GET
   request to the data collection URI (e.g., after discovering the data
   hub as described in Section 2.3).

   On success, the server returns a 2.05 (Content) response with a
   representation of the collection.  As specified in Section 2.2.1
   above, this representation includes links to (and, optionally,
   representations of) the data items in the data collection as well as
   forms for creating, updating, deleting, and finding data items.

2.4.2.  Getting Data Items by Metadata

   If the representation of a data collection contains a form of type
   <http://coreapps.org/base#search>, the client can filter the data
   collection by submitting this form with a search query (see
   Section 2.2.2).

   Implementations of this version of this document MUST use the method
   implied by the <http://coreapps.org/base#search> operation type,
   i.e., the FETCH method [RFC8132].  Any form indicating a different
   method MUST be ignored.

   On success, the server returns a 2.05 (Content) response with a
   representation of a list of data items in the collection (see
   Section 2.2.1) that match the query.

2.4.3.  Creating a Data Item

   If the representation of a data collection contains a form of type
   <http://coreapps.org/collections#create>, the client can create a new
   data item in the data collection by submitting this form with a
   representation in one of the acceptable media types.  The acceptable
   media types are indicated by form fields of type
   <http://coreapps.org/coap#accept>.

   Implementations of this version of this document MUST use the method
   implied by the <http://coreapps.org/collections#create> operation
   type, i.e., the POST method [RFC7252].  Any form indicating a
   different method MUST be ignored.

   On success, the server returns a 2.01 (Created) response.  The
   location of the created data item is conveyed using the Location-Path
   and Location-Query options [RFC7252].

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2.4.4.  Reading a Data Item

   A client can read a data item by following a link with the
   <http://www.iana.org/assignments/relation/item> link relation type in
   the representation of the data collection.

2.4.5.  Observing a Data Item

   A client can observe a data item by following a link with the
   <http://www.iana.org/assignments/relation/item> link relation type in
   the representation of the data collection and observing the target
   resource as specified in RFC 7641 [RFC7641].

2.4.6.  Updating a Data Item

   If the representation of a data collection includes a form of type
   <http://coreapps.org/base#update> nested within the link to a data
   item, a client can update the data item by submitting this form with
   a representation of the updated data item.

   Implementations of this version of this document MUST use the method
   implied by the <http://coreapps.org/base#update> operation type,
   i.e., the PUT method [RFC7252].  Any form indicating a different
   method MUST be ignored.

   On success, the server returns a 2.04 (Changed) response.

2.4.7.  Deleting a Data Item

   If the representation of a data collection includes a form of type
   <http://coreapps.org/collections#delete> nested within the link to a
   data item, the client can delete the data item by submitting this
   form.

   Implementations of this version of this document MUST use the method
   implied by the <http://coreapps.org/collections#delete> operation
   type, i.e., the DELETE method [RFC7252].  Any form indicating a
   different method MUST be ignored.

   On success, the server returns a 2.02 (Deleted) response.

3.  Security Considerations

   The data hub application relies on a Web transfer protocol like CoAP
   to exchange representations in a CoRAL serialization format.  See
   Section 11 of RFC 7252 [RFC7252] and Section 7 of RFC 7641 [RFC7641]
   for security considerations relating to CoAP.  See Section XX of RFC

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   XXXX [I-D.ietf-core-coral] for security considerations relating to
   CoRAL.

   The data hub application does not define any specific mechanisms for
   protecting the confidentiality and integrity of messages exchanged
   between a data hub and a client.  It is recommended that
   implementations employ application layer or transport layer
   mechanisms for interactions with a data hub.

   The data hub application does not define any specific mechanisms for
   protecting the confidentiality and integrity of representations of
   data items shared through a data hub.  For scenarios where end-to-end
   security matters, such as for firmware updates
   [I-D.ietf-suit-information-model], implementations should employ an
   object security mechanism.

4.  IANA Considerations

   This document has no IANA actions.

5.  References

5.1.  Normative References

   [I-D.ietf-core-coral]
              Hartke, K., "The Constrained RESTful Application Language
              (CoRAL)", draft-ietf-core-coral-03 (work in progress),
              March 2020.

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

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

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

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

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   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

5.2.  Informative References

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-09 (work in
              progress), September 2019.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-24 (work in progress), March 2020.

   [I-D.ietf-suit-architecture]
              Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
              Firmware Update Architecture for Internet of Things",
              draft-ietf-suit-architecture-08 (work in progress),
              November 2019.

   [I-D.ietf-suit-information-model]
              Moran, B., Tschofenig, H., and H. Birkholz, "An
              Information Model for Firmware Updates in IoT Devices",
              draft-ietf-suit-information-model-05 (work in progress),
              January 2020.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [W3C.CR-wot-architecture-20190516]
              Kovatsch, M., Matsukura, R., Lagally, M., Kawaguchi, T.,
              Toumura, K., and K. Kajimoto, "Web of Things (WoT)
              Architecture", World Wide Web Consortium Candidate
              Recommendation CR-wot-architecture-20190516, May 2019,
              <https://www.w3.org/TR/2019/CR-wot-architecture-20190516>.

   [W3C.REC-html52-20171214]
              Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
              S. Moon, "HTML 5.2", World Wide Web Consortium
              Recommendation REC-html52-20171214, December 2017,
              <https://www.w3.org/TR/2017/REC-html52-20171214>.

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Acknowledgements

   Thanks to Christian Amsuess and Jaime Jimenez for helpful comments
   and discussions that have shaped the document.

Author's Address

   Klaus Hartke
   Ericsson
   Torshamnsgatan 23
   Stockholm  SE-16483
   Sweden

   Email: klaus.hartke@ericsson.com

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