CoRE Application Descriptions
draft-hartke-core-apps-02

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Thing-to-Thing Research Group                                  K. Hartke
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                           August 18, 2015
Expires: February 19, 2016

                     CoRE Application Descriptions
                       draft-hartke-core-apps-02

Abstract

   The interfaces of RESTful, hypertext-driven applications consist of
   reusable, self-descriptive components such as Internet media types
   and link relation types.  This document defines a template that
   application designers can use to describe their application's
   interface in a structured way so that other parties can develop
   interoperable clients and servers or reuse the components in their
   own applications.

Note to Readers

   This Internet-Draft should be discussed on the Thing-to-Thing
   Research Group (T2TRG) mailing list <t2trg@irtf.org>.

Status of This Memo

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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

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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Application Descriptions  . . . . . . . . . . . . . . . . . .   3
     2.1.  Communication Protocols . . . . . . . . . . . . . . . . .   3
     2.2.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Internet Media Types  . . . . . . . . . . . . . . . . . .   4
     2.4.  Representation Formats  . . . . . . . . . . . . . . . . .   5
     2.5.  Link Relation Types . . . . . . . . . . . . . . . . . . .   6
     2.6.  Form Relation Types . . . . . . . . . . . . . . . . . . .   7
     2.7.  Well-Known Locations  . . . . . . . . . . . . . . . . . .   7
     2.8.  URI Structures  . . . . . . . . . . . . . . . . . . . . .   7
   3.  Template  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Form Relation Type Registry . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is designed to
   enable applications implementing the REST architectural style [REST]
   in Constrained-Node Networks [RFC7228].

   As CoAP applications are implemented and deployed, it becomes
   increasingly important to be able to describe them in some structured
   way in order to promote interoperability and reuse.  Previous efforts
   like WADL [WADL] however focus on code generation from machine-
   readable service descriptions and are not truly RESTful [DWNWADL].

   REST application interfaces (APIs) are by definition hypertext-driven
   [RESTAPI].  This means that the interface is a description of the
   common vocabulary between the client and the server, centered around
   Internet media types and link relation types, rather than a static
   list of resources and the operations supported on them.

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   RESTful applications are often easy to understand, but require some
   design effort.  This is because application designers do not only
   have to take current requirements into consideration, but also
   anticipate changes that may be required in the future.  The reward is
   long-term stability and evolvability ("design for decades").  REST is
   intended for long-lived network-based applications that span multiple
   organizations [RESTAPI].

   This document defines a template for describing the interface of
   RESTful, hypertext-driven applications in Constrained RESTful
   Environments (CoRE).

2.  Application Descriptions

   A CoRE Application Description is a named set of reusable, self-
   descriptive components.  It is comprised of:

   o  URI schemes that identify communication protocols,

   o  Internet media types that identify representation formats,

   o  link relation types that identify link semantics,

   o  form relation types that identify form semantics, and

   o  optionally, well-known locations.

   Together, these components provide the specific, in-band instructions
   for interfacing with a given service.

2.1.  Communication Protocols

   The foundation of a hypertext-driven REST API are the communication
   protocol(s) spoken between a client and a server.  Although HTTP/1.1
   [RFC7230] is by far the most common communication protocol for REST
   APIs, a REST API should typically not be dependent on any specific
   communication protocol.

2.2.  URI Schemes

   The use of a particular protocol is guided by URI schemes [RFC7595]
   that describe the syntax and semantics of URI references found in
   links and forms (Section 2.4).

   A URI scheme refers to a family of protocols, typically distinguished
   by a version number.  For example, the "http" URI scheme refers to
   the two members of the HTTP family of protocols: HTTP/1.1 [RFC7230],
   and HTTP/2 [RFC7540].  The specific HTTP version is negotiated

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   between the client and the server through version indicators in the
   protocol or the TLS application-layer protocol negotiation (ALPN)
   extension [RFC7301].

      IANA maintains a list of registered URI schemes at
      <http://www.iana.org/assignments/uri-schemes>.

2.3.  Internet Media Types

   One of the most important aspect of hypertext-driven communications
   is the concept of media types [RFC6838].  Media types are used to
   label representations so that it is known how the representation
   should be interpreted, and how it is encoded.  The core of an
   application description should be one or more hypertext media types.

   A media type identifies a versioned series of representation formats
   (Section 2.4): a media type does not identify a particular version of
   a representation format; rather, the media type identifies the
   family, and includes provisions for version indicator(s) embedded in
   the representations themselves to determine more precisely the nature
   of how the data is to be interpreted.  A new media type is only
   needed to designate a completely incompatible format [MIMEWEB].

   Media types consist of a top-level type and a subtype, structured
   into trees.  Optionally, media types can have parameters.  For
   example, the media type "text/plain; charset=utf-8" is a subtype for
   generic text under the "text" top-level type in the standards tree,
   and has a parameter "charset" set to "utf-8".

   Media types can be further refined by structured type name suffixes
   (e.g., "+xml" appended to the base subtype name; see Section 4.2.8 of
   RFC 6838), or by subtype information embedded in the representations
   themselves (e.g., "xmlns" declarations in XML documents [XMLNS]).
   Structured type name suffixes should be preferred, because embedded
   subtype information cannot be negotiated (e.g., using the CoAP Accept
   option).

   A media type must be determined from in-band information (e.g., from
   the CoAP Content-Format option).  Clients must not assume a structure
   from the application context or other out-of-band information.

      IANA maintains a list of registered Internet media types at
      <http://www.iana.org/assignments/media-types>.

      IANA maintains a list of registered structured suffixes at
      <http://www.iana.org/assignments/media-type-structured-suffix>.

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      IANA maintains a list of registered CoAP content formats at
      <http://www.iana.org/assignments/core-parameters>.

2.4.  Representation Formats

   In RESTful applications, clients and servers exchange representations
   that capture the current or intended state of a resource and that are
   labeled with a media type.  A representation is a sequence of bytes
   whose structure and semantics are specified by a representation
   format, a set of rules for encoding information.

   Representation formats should generally allow clients with different
   goals, so they can do different things with the same data.  The
   specification of a representation format "describes a problem space,
   not a prescribed relationship between client and server.  Client and
   server must share an understanding of the representations they're
   passing back and forth, but they don't need to have the same idea of
   what the problem is that needs to be solved." [WEBAPIS]

   Representation formats and their specifications evolve over time.  It
   is part of the responsibility of the designer of a new version of a
   format to try to insure both forward and backward compatibility: new
   documents should work reasonably (with some fallback) with old
   processors, and old documents should work reasonably with new
   processors [MIMEWEB].

   Representation formats enable hypertext-driven applications when they
   support the expression of hypermedia controls: links (Section 2.4.1)
   and/or forms (Section 2.4.2).

2.4.1.  Links

   A link is the primary means for a client to change application state,
   i.e., to navigate from one resource to another.  A link is a typed
   connection between two resources [RFC5988], and is comprised of:

   o  a context (usually the current resource),

   o  a link relation type that identifies the semantics of the link
      (Section 2.5),

   o  a target resource URI, and

   o  optionally, attributes that describe the link target.

   A link can be viewed as a statement of the form "{context IRI} has a
   {relation type} resource at {target URI}, which has {target
   attributes}" [RFC5988].  For example, <http://example.com/> might

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   have a "terms-of-service" resource at <http://example.com/tos>, which
   has the media type "text/html".

   There are two special kind of links:

   o  An embedding link is a link with the additional hint that it, when
      processed, should be substituted with a representation of the
      referenced resource.  Thus, traversing an embedding link adds to
      the application state, rather than replacing it.

   o  A templated link is a link where the client constructs the target
      resource URI from provided in-band instructions.  The specific
      rules for such instructions are described by the representation
      format.  URI Templates [RFC6570] provide a generic way to
      construct URIs through variable expansion.

2.4.2.  Forms

   A form is the primary means for a client to change resource state.
   It is comprised of:

   o  a context (usually the current resource),

   o  a form relation type that identifies the semantics of the form
      (Section 2.6),

   o  a target resource URI,

   o  a submission method (PUT, POST, PATCH, or DELETE), and

   o  and a description of a representation that the service accepts as
      part of form submission.  This description can be a set of form
      fields, or simply a list of acceptable media types.

   A form can be viewed as an instruction of the form "To {relation
   type} {context}, make a {method} request to {target URI}".  For
   example, to "create-item" in <http://example.com/collection/>, a
   client might make a POST request to <http://example.com/collection/>.

   Note:  A form with a submission method of GET is, strictly speaking,
      a templated link, since it provides a way to construct a URI and
      does not change resource state.

2.5.  Link Relation Types

   A link relation type identifies the semantics of a link [RFC5988].
   For example, a link with the relation type "copyright" indicates that

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   the resource identified by the target URI is a statement of the
   copyright terms applying to the current context.

   Relation types are not to be confused with media types [RFC6838];
   they do not identify the format of the representation that results
   when the link is dereferenced.  Rather, they only describe how the
   current context is related to another resource.

      IANA maintains a list of registered link relation types at
      <http://www.iana.org/assignments/link-relations>.

2.6.  Form Relation Types

   A form relation type identifies the semantics of a form.  For
   example, a form with the relation type "create-item" indicates that a
   new item can be created within the current context by making a
   request to the resource identified by the target URI.

      IANA maintains a list of registered link relation types at
      <TBD>.

2.7.  Well-Known Locations

   Some applications may require the discovery of information about a
   host ("site-wide metadata").  For example, [RFC6415] defines a
   metadata document format for describing hosts; similarly, [RFC6690]
   defines a link format for the discovery of resources hosted by a
   server.

   Applications that need to define a resource for site-wide metadata
   can register new "well-known locations".  [RFC5785] defines a path
   prefix in "http" and "https" URIs for this purpose, "/.well-known/";
   [RFC7252] extends this concept to "coap" and "coaps" URIs.

      IANA maintains a list of registered well-known URIs at
      <http://www.iana.org/assignments/well-known-uris>.

2.8.  URI Structures

   Application descriptions must not constrain URI structures in ways
   that aren't explicitly allowed by [RFC3986].  In particular,
   mandating particular forms of URI substructure is inappropriate.
   [RFC7320] describes this problematic practice and provides some
   acceptable alternatives for use in application descriptions.

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3.  Template

   Application name:

   URI schemes:

   Media types:

   Link relations:

   Form relations:

   Well-known locations:

   Interoperability considerations:

   Security considerations:

   Contact:

   Author/Change controller:

4.  Security Considerations

   The security considerations of [RFC3986], [RFC5785], [RFC5988],
   [RFC6570], [RFC6838], [RFC7320], and [RFC7595] are inherited.

   All components of an application description are expected to contain
   clear security considerations.  Application descriptions should
   further contain security considerations that need to be taken into
   account for the security of the overall application.

5.  IANA Considerations

5.1.  Form Relation Type Registry

   This specification establishes the Form Relation Type registry.

5.1.1.  Registering New Form Relation Types

   Form relation types are registered in the same way as link relation
   types [RFC5988], i.e., they are registered on the advice of a
   Designated Expert, with a Specification Required.

   The requirements for registered relation types are adopted from
   [RFC5988], Section 4.1.

   The registration template is:

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   o  Relation Name:

   o  Description:

   o  Reference:

   o  Notes: [optional]

5.1.2.  Initial Registry Contents

   The Form Relation Type registry's initial contents are:

   o  Relation Name: create-item
      Description: Refers to a resource that can be used to create a
      resource in a collection of resources.
      Reference: [RFCXXXX]

   o  Relation Name: delete
      Description: Refers to a resource that can be used to delete a
      resource in a collection of resources.
      Reference: [RFCXXXX]

   o  Relation Name: update
      Description: Refers to a resource that can be used to update the
      state of the form's context.
      Reference: [RFCXXXX]

6.  Acknowledgements

   Thanks to Jan Algermissen, Mike Amundsen, Olaf Bergmann, Carsten
   Bormann, Stefanie Gerdes, Mike Kelly, Michael Koster, Matthias
   Kovatsch, Julian Reschke, Teemu Savolainen, Bilhanan Silverajan, and
   Erik Wilde for helpful comments and discussions that have shaped the
   document.

   Some of the text in this document has been borrowed from [RESTAPI],
   [RFC5988], [RFC7320], and [MIMEWEB].  All errors are my own.

   This work was funded in part by Nokia.

7.  References

7.1.  Normative References

   [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>.

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   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

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

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <http://www.rfc-editor.org/info/rfc6570>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,
              <http://www.rfc-editor.org/info/rfc7320>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <http://www.rfc-editor.org/info/rfc7595>.

7.2.  Informative References

   [DWNWADL]  Gregorio, J., "Do we need WADL?", June 2007,
              <http://bitworking.org/news/193/Do-we-need-WADL>.

   [MIMEWEB]  Masinter, L., "MIME and the Web", draft-masinter-mime-web-
              info-02 (work in progress), January 2011.

   [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>.

   [RESTAPI]  Fielding, R., "REST APIs must be hypertext-driven",
              October 2008, <http://roy.gbiv.com/untangled/2008/
              rest-apis-must-be-hypertext-driven>.

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   [RFC6415]  Hammer-Lahav, E., Ed. and B. Cook, "Web Host Metadata",
              RFC 6415, DOI 10.17487/RFC6415, October 2011,
              <http://www.rfc-editor.org/info/rfc6415>.

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

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

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

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

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

   [WADL]     Hadley, M., "Web Application Description Language", World
              Wide Web Consortium Member Submission SUBM-wadl-20090831,
              August 2009,
              <http://www.w3.org/Submission/2009/SUBM-wadl-20090831>.

   [WEBAPIS]  Richardson, L. and M. Amundsen, "RESTful Web APIs",
               O'Reilly, September 2013.

   [XMLNS]    Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", World
              Wide Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

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Author's Address

   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   EMail: hartke@tzi.org

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