Internet DRAFT - draft-ietf-core-coral

draft-ietf-core-coral







Thing-to-Thing Research Group                                  K. Hartke
Internet-Draft                                                  Ericsson
Intended status: Standards Track                         August 29, 2019
Expires: March 1, 2020


          The Constrained RESTful Application Language (CoRAL)
                        draft-ietf-core-coral-00

Abstract

   The Constrained RESTful Application Language (CoRAL) defines a data
   model and interaction model as well as two specialized serialization
   formats for the description of typed connections between resources on
   the Web ("links"), possible operations on such resources ("forms"),
   as well as simple resource metadata.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 March 1, 2020.

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of
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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.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Data and Interaction Model  . . . . . . . . . . . . . . . . .   4
     2.1.  Browsing Context  . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Documents . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Forms . . . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.4.1.  Form Fields . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Embedded Representations  . . . . . . . . . . . . . . . .   7
     2.6.  Navigation  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.7.  History Traversal . . . . . . . . . . . . . . . . . . . .   9
   3.  Binary Format . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Data Structure  . . . . . . . . . . . . . . . . . . . . .  10
       3.1.1.  Documents . . . . . . . . . . . . . . . . . . . . . .  10
       3.1.2.  Links . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.1.3.  Forms . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.1.4.  Embedded Representations  . . . . . . . . . . . . . .  12
       3.1.5.  Directives  . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  Dictionaries  . . . . . . . . . . . . . . . . . . . . . .  13
       3.2.1.  Dictionary References . . . . . . . . . . . . . . . .  13
       3.2.2.  Media Type Parameter  . . . . . . . . . . . . . . . .  14
   4.  Textual Format  . . . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Lexical Structure . . . . . . . . . . . . . . . . . . . .  15
       4.1.1.  Line Terminators  . . . . . . . . . . . . . . . . . .  15
       4.1.2.  White Space . . . . . . . . . . . . . . . . . . . . .  15
       4.1.3.  Comments  . . . . . . . . . . . . . . . . . . . . . .  15
       4.1.4.  Identifiers . . . . . . . . . . . . . . . . . . . . .  15
       4.1.5.  IRIs and IRI References . . . . . . . . . . . . . . .  16
       4.1.6.  Literals  . . . . . . . . . . . . . . . . . . . . . .  16
       4.1.7.  Punctuators . . . . . . . . . . . . . . . . . . . . .  20
     4.2.  Syntactic Structure . . . . . . . . . . . . . . . . . . .  20
       4.2.1.  Documents . . . . . . . . . . . . . . . . . . . . . .  20
       4.2.2.  Links . . . . . . . . . . . . . . . . . . . . . . . .  20
       4.2.3.  Forms . . . . . . . . . . . . . . . . . . . . . . . .  21
       4.2.4.  Embedded Representations  . . . . . . . . . . . . . .  22
       4.2.5.  Directives  . . . . . . . . . . . . . . . . . . . . .  23
   5.  Usage Considerations  . . . . . . . . . . . . . . . . . . . .  24
     5.1.  Specifying CoRAL-based Applications . . . . . . . . . . .  24
       5.1.1.  Application Interfaces  . . . . . . . . . . . . . . .  24
       5.1.2.  Resource Identifiers  . . . . . . . . . . . . . . . .  25
       5.1.3.  Implementation Limits . . . . . . . . . . . . . . . .  25
     5.2.  Minting Vocabulary  . . . . . . . . . . . . . . . . . . .  26
     5.3.  Expressing Registered Link Relation Types . . . . . . . .  27
     5.4.  Expressing Simple RDF Statements  . . . . . . . . . . . .  27
     5.5.  Expressing Natural Language Texts . . . . . . . . . . . .  27
     5.6.  Embedding CoRAL in CBOR Data  . . . . . . . . . . . . . .  28



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     5.7.  Submitting CoRAL Documents  . . . . . . . . . . . . . . .  28
       5.7.1.  PUT Requests  . . . . . . . . . . . . . . . . . . . .  28
       5.7.2.  POST Requests . . . . . . . . . . . . . . . . . . . .  29
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Media Type "application/coral+cbor" . . . . . . . . . . .  31
     7.2.  Media Type "text/coral" . . . . . . . . . . . . . . . . .  32
     7.3.  CoAP Content Formats  . . . . . . . . . . . . . . . . . .  33
     7.4.  CBOR Tag  . . . . . . . . . . . . . . . . . . . . . . . .  34
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  34
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  36
   Appendix A.  Core Vocabulary  . . . . . . . . . . . . . . . . . .  38
     A.1.  Base  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.2.  Collections . . . . . . . . . . . . . . . . . . . . . . .  39
     A.3.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  40
     A.4.  CoAP  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Appendix B.  Default Dictionary . . . . . . . . . . . . . . . . .  42
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  42
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   The Constrained RESTful Application Language (CoRAL) is a language
   for the description of typed connections between resources on the Web
   ("links"), possible operations on such resources ("forms"), as well
   as simple resource metadata.

   CoRAL is intended for driving automated software agents that navigate
   a Web application based on a standardized vocabulary of link relation
   types and operation types.  It is designed to be used in conjunction
   with a Web transfer protocol such as the Hypertext Transfer Protocol
   (HTTP) [RFC7230] or the Constrained Application Protocol (CoAP)
   [RFC7252].

   This document defines the CoRAL data and interaction model, as well
   as two specialized CoRAL serialization formats.

   The CoRAL data and interaction model is a superset of the Web Linking
   model of RFC 8288 [RFC8288].  The data model consists of two primary
   elements: "links" that describe the relationship between two
   resources and the type of that relationship, and "forms" that
   describe a possible operation on a resource and the type of that
   operation.  Additionally, the data model can describe simple resource
   metadata in a way similar to the Resource Description Framework (RDF)
   [W3C.REC-rdf11-concepts-20140225].  In contrast to RDF, the focus of
   CoRAL however is on the interaction with resources, not just the
   relationships between them.  The interaction model derives from HTML



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   5 [W3C.REC-html52-20171214] and specifies how an automated software
   agent can navigate between resources by following links and perform
   operations on resources by submitting forms.

   The primary CoRAL serialization format is a compact, binary encoding
   of links and forms in Concise Binary Object Representation (CBOR)
   [RFC7049].  It is intended for environments with constraints on
   power, memory, and processing resources [RFC7228] and shares many
   similarities with the message format of the Constrained Application
   Protocol (CoAP) [RFC7252]: For example, it uses numeric identifiers
   instead of verbose strings for link relation types and operation
   types, and pre-parses Uniform Resource Identifiers (URIs) [RFC3986]
   into (what CoAP considers to be) their components, which simplifies
   URI processing for constrained nodes a lot.  As a result, link
   serializations in CoRAL are often much more compact than equivalent
   serializations in CoRE Link Format [RFC6690].

   The secondary CoRAL serialization format is a lightweight, textual
   encoding of links and forms that is intended to be easy to read and
   write for humans.  The format is loosely inspired by the syntax of
   Turtle [W3C.REC-turtle-20140225] and is mainly intended for giving
   examples.

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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Terms defined in this document appear in _cursive_ where they are
   introduced.

2.  Data and Interaction Model

   The Constrained RESTful Application Language (CoRAL) is designed for
   building Web-based applications [W3C.REC-webarch-20041215] in which
   automated software agents navigate between resources by following
   links and perform operations on resources by submitting forms.

2.1.  Browsing Context

   Borrowing from HTML 5 [W3C.REC-html52-20171214], each such agent
   maintains a _browsing context_ in which the representations of Web
   resources are processed.  (In HTML 5, the browsing context typically
   corresponds to a tab or window in a Web browser.)




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   At any time, one representation in each browsing context is
   designated the _active_ representation.

2.2.  Documents

   A resource representation in one of the CoRAL serialization formats
   is called a CoRAL _document_. The URI that was used to retrieve such
   a document is called the document's _retrieval context_.

   A CoRAL document consists of a list of zero or more links, forms, and
   embedded resource representations, collectively called _elements_.
   CoRAL serialization formats may define additional types of elements
   for efficiency or convenience, such as a base for relative URI
   references [RFC3986].

2.3.  Links

   A _link_ describes a relationship between two resources on the Web
   [RFC8288].  As defined in RFC 8288, it consists of a _link context_,
   a _link relation type_, and a _link target_. In CoRAL, a link can
   additionally have a nested list of zero or more elements, which take
   the place of link target attributes.

   A link can be viewed as a statement of the form "{link context} has a
   {link relation type} resource at {link target}" where the link target
   may be further described by nested elements.

   The link relation type identifies the semantics of a link.  In HTML 5
   and RFC 8288, link relation types are typically denoted by an IANA-
   registered name, such as "stylesheet" or "type".  In CoRAL, they are
   denoted by an Internationalized Resource Identifier (IRI) [RFC3987]
   such as <http://www.iana.org/assignments/relation/stylesheet> or
   <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>.  This allows for
   the creation of new link relation types without the risk of
   collisions when from different organizations or domains of knowledge.
   An IRI also can lead to documentation, schema, and other information
   about the link relation type.  These IRIs are only used as identity
   tokens, though, and are compared using Simple String Comparison
   (Section 5.1 of RFC 3987).

   The link context and the link target are both denoted by either a URI
   reference or a literal (similarly to RDF).  If the URI scheme
   indicates a Web transfer protocol such as HTTP or CoAP, then an agent
   can dereference the URI and navigate the browsing context to the
   referenced resource; this is called _following the link_.  A literal
   directly identifies a value: a Boolean value, an integer, a floating-
   point number, a date/time value, a byte string, or a text string.




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   A link can occur as a top-level element in a document or as a nested
   element within a link.  When a link occurs as a top-level element,
   the link context implicitly is the document's retrieval context.
   When a link occurs nested within a link, the link context of the
   inner link is the link target of the outer link.

   There are no restrictions on the cardinality of links; there can be
   multiple links to and from a particular target, and multiple links of
   the same or different types between a given link context and target.
   However, the nested data structure constrains the description of a
   resource graph to a tree: Links between linked resources can only be
   described by further nesting links.

2.4.  Forms

   A _form_ provides instructions to an agent for performing an
   operation on a Web resource.  It consists of a _form context_, an
   _operation type_, a _request method_, and a _submission target_.
   Additionally, a form may be accompanied by a list of _form fields_.

   A form can be viewed as an instruction of the form "To perform an
   {operation type} operation on {form context}, make a {request method}
   request to {submission target}" where the request may be further
   described by form fields.

   The operation type identifies the semantics of the operation.
   Operation types are denoted like link relation types by an IRI.

   The form context is the resource on which an operation is ultimately
   performed.  To perform the operation, an agent needs to construct a
   request with the specified method and the specified submission target
   as the request URI.  Usually, the submission target is the same
   resource as the form context, but it may be a different resource.
   Constructing and sending the request is called _submitting the form_.

   Form fields, specified in the next section, can be used to provide
   more detailed instructions to the agent for constructing the request.
   For example, form fields can instruct the agent to include a payload
   or certain headers in the request that must match the specifications
   of the form fields.

   A form can occur as a top-level element in a document or as a nested
   element within a link.  When a form occurs as a top-level element,
   the form context implicitly is the document's retrieval context.
   When a form occurs nested within a link, the form context is the link
   target of the enclosing link.





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2.4.1.  Form Fields

   Form fields provide further instructions to agents for constructing a
   request.

   For example, a form field could identify one or more data items that
   need to be included in the request payload or reference another
   resource (such as a schema) that describes the structure of the
   payload.  A form field could also provide other kinds of information,
   such as acceptable media types for the payload or expected request
   headers.  Form fields may be specific to the protocol used for
   submitting the form.

   A form field is the pair of a _form field type_ and a _form field
   value_.

   The form field type identifies the semantics of the form field.  Form
   field types are denoted like link relation types and operation types
   by an IRI.

   The form field value can be either a URI reference, a Boolean value,
   an integer, a floating-point number, a date/time value, a byte
   string, or a text string.

2.5.  Embedded Representations

   When a document contains links to many resources and an agent needs a
   representation of each link target, it may be inefficient to retrieve
   each of these representations individually.  To alleviate this,
   documents can directly embed representations of resources.

   An _embedded representation_ consists of a sequence of bytes, labeled
   with _representation metadata_.

   An embedded representation may be a full, partial, or inconsistent
   version of the representation served from the URI of the resource.

   An embedded representation can occur as a top-level element in a
   document or as a nested element within a link.  When it occurs as a
   top-level element, it provides an alternate representation of the
   document's retrieval context.  When it occurs nested within a link,
   it provides a representation of link target of the enclosing link.

2.6.  Navigation

   An agent begins interacting with an application by performing a GET
   request on an _entry point URI_. The entry point URI is the only URI
   an agent is expected to know before interacting with an application.



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   From there, the agent is expected to make all requests by following
   links and submitting forms provided by the server in responses.  The
   entry point URI can be obtained by manual configuration or through
   some discovery process.

   If dereferencing the entry point URI yields a CoRAL document (or any
   other representation that implements the CoRAL data and interaction
   model), then the agent makes this document the active representation
   in the browsing context and proceeds as follows:

   1.  The first step for the agent is to decide what to do next, i.e.,
       which type of link to follow or form to submit, based on the link
       relation types and operation types it understands.

   2.  The agent then finds the link(s) or form(s) with the respective
       type in the active representation.  This may yield one or more
       candidates, from which the agent will have to select the most
       appropriate one.  The set of candidates may be empty, for
       example, when a transition is not supported or not allowed.

   3.  The agent selects one of the candidates based on the metadata
       associated with each of these.  Metadata includes the content
       type of the target resource representation, the URI scheme, the
       request method, and other information that is provided as nested
       elements in a link or form fields in a form.

       If the selected candidate contains an embedded representation,
       the agent MAY skip the following steps and immediately proceed
       with step 8.

   4.  The agent obtains the _request URI_ from the link target or
       submission target.  Fragment identifiers are not part of the
       request URI and MUST be separated from the rest of the URI prior
       to a dereference.

   5.  The agent constructs a new request with the request URI.  If the
       agent is following a link, then the request method MUST be GET.
       If the agent is submitting a form, then the request method MUST
       be the one specified by the form.  An IRI may need to be
       converted to a URI (Section 3.1 of RFC 3987) for protocols that
       do not support IRIs.

       The agent should set HTTP header fields and CoAP request options
       according to metadata associated with the link or form (e.g., set
       the HTTP Accept header field or the CoAP Accept option when the
       media type of the target resource is provided).  Depending on the
       operation type of a form, the agent may also need to include a




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       request payload that matches the specifications of one or more
       form fields.

   6.  The agent sends the request and receives the response.

   7.  If a fragment identifier was separated from the request URI, the
       agent dereferences the fragment identifier within the received
       representation.

   8.  The agent _updates the browsing context_ by making the (embedded
       or received) representation the active representation.

   9.  Finally, the agent processes the representation according to the
       semantics of the content type.  If the representation is a CoRAL
       document (or any other representation that implements the CoRAL
       data and interaction model), this means the agent has the choice
       of what to do next again -- and the cycle repeats.

2.7.  History Traversal

   A browsing context MAY entail a _session history_ that lists the
   resource representations that the agent has processed, is processing,
   or will process.

   An entry in the session history consists of a resource representation
   and the request URI that was used to retrieve the representation.
   New entries are added to the session history as the agent navigates
   from resource to resource.

   An agent can navigate a browsing context by _traversing the session
   history_ in addition to following links and submitting forms.  For
   example, if an agent received a representation that doesn't contain
   any further links or forms, it can revert the active representation
   back to one it has visited earlier.

   Traversing the history should take advantage of caches to avoid new
   requests.  An agent MAY reissue a safe request (e.g., a GET request)
   when it doesn't have a fresh representation in its cache.  An agent
   MUST NOT reissue an unsafe request (e.g., a PUT or POST request)
   unless it intends to perform that operation again.

3.  Binary Format

   This section defines the encoding of documents in the CoRAL binary
   format.

   A document in the binary format is a data item in Concise Binary
   Object Representation (CBOR) [RFC7049].  The structure of this data



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   item is presented in the Concise Data Definition Language (CDDL)
   [RFC8610].  The media type is "application/coral+cbor".

   The following restrictions are placed on CBOR encoders: Byte strings
   and text strings MUST be encoded with definite length.  Integers and
   floating-point values MUST be encoded as such (e.g., a floating-point
   value of 0.0 must not be encoded as the integer 0).

3.1.  Data Structure

   The data structure of a document in the binary format is made up of
   four kinds of elements: links, forms, embedded representations, and
   (as an extension to the CoRAL data model) base directives.  Base
   directives provide a way to encode URI references with a common base
   more efficiently.

   Elements are processed in the order they appear in the document.
   Document processors need to maintain an _environment_ while iterating
   an array of elements.  The environment consists of two variables: the
   _current context_ and the _current base_. Both the current context
   and the current base are initially set to the document's retrieval
   context.

3.1.1.  Documents

   The body of a document in the binary format is encoded as an array of
   zero or more links, forms, embedded representations, and directives.

      document = body

      body = [*(link / form / representation / directive)]

3.1.2.  Links

   A link is encoded as an array that consists of the unsigned integer
   2, followed by the link relation type and the link target, optionally
   followed by a link body that contains nested elements.

      link = [2, relation-type, link-target, ?body]

   The link relation type is encoded as a text string that conforms to
   the syntax of an IRI [RFC3987].

      relation-type = text

   The link target is denoted by a Constrained Resource Identifier
   (CoRI) reference [I-D.ietf-core-href] or represented by a literal
   value.  A CoRI reference MUST be resolved against the current base.



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   The link target may be null, which indicates that the link target is
   an unidentified resource.

      link-target = CoRI / literal

      CoRI = <Defined in Section X of RFC XXXX>

      literal = bool / int / float / time / bytes / text / null

   The array of elements in the link body, if any, MUST be processed in
   a fresh environment.  Both the current context and the current base
   in the new environment are initially set to the link target of the
   enclosing link.

3.1.3.  Forms

   A form is encoded as an array that consists of the unsigned integer
   3, followed by the operation type and the submission target,
   optionally followed by a list of form fields.

      form = [3, operation-type, submission-target, ?form-fields]

   The operation type is defined in the same way as a link relation type
   (Section 3.1.2).

      operation-type = text

   The request method is either implied by the operation type or encoded
   as a form field.  If there are both, the form field takes precedence
   over the operation type.  Either way, the method MUST be defined for
   the Web transfer protocol identified by the scheme of the submission
   target.

   The submission target is denoted by a CoRI reference.  This CoRI
   reference MUST be resolved against the current base.

      submission-target = CoRI

3.1.3.1.  Form Fields

   A list of form fields is encoded as an array of zero or more type-
   value pairs.

      form-fields = [*(form-field-type, form-field-value)]

   The list, if any, MUST be processed in a fresh environment.  Both the
   current context and the current base in the new environment are
   initially set to the submission target of the enclosing form.



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   A form field type is defined in the same way as a link relation type
   (Section 3.1.2).

      form-field-type = text

   A form field value can be a CoRI reference, a Boolean value, an
   integer, a floating-point number, a date/time value, a byte string, a
   text string, or null.  A CoRI reference MUST be resolved against the
   current base.

      form-field-value = CoRI / literal

3.1.4.  Embedded Representations

   An embedded representation is encoded as an array that consists of
   the unsigned integer 0, followed by a byte string containing the
   representation data, optionally followed by representation metadata.

      representation = [0, bytes, ?representation-metadata]

   Representation metadata is encoded as an array of zero or more name-
   value pairs.

      representation-metadata = [*(metadata-name, metadata-value)]

   The metadata, if any, MUST be processed in a fresh environment.  All
   variables in the new environment are initially set to a copy of the
   variables in the current environment.

   The metadata name is defined in the same way as a link relation type
   (Section 3.1.2).

      metadata-name = text

   A metadata value can be a CoRI reference, a Boolean value, an
   integer, a floating-point number, a date/time value, a byte string, a
   text string, or null.  A CoRI reference MUST be resolved against the
   current base.

      metadata-value = CoRI / literal

3.1.5.  Directives

   Directives provide the ability to manipulate the environment when
   processing a list of elements.  There is one type of directives
   available: the Base directive.

      directive = base-directive



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3.1.5.1.  Base Directives

   A Base directive is encoded as an array that consists of the unsigned
   integer 1, followed by a base.

      base-directive = [1, base]

   The base is denoted by a CoRI reference.  This CoRI reference MUST be
   resolved against the current context (not the current base).

      base = CoRI

   The directive is processed by resolving the CoRI reference against
   the current context and assigning the result to the current base.

3.2.  Dictionaries

   The binary format can reference values from a dictionary to reduce
   representation size and processing cost.  Dictionary references can
   be used in place of link relation types, link targets, operation
   types, submission targets, form field types, form field values,
   representation metadata names, and representation metadata values.

3.2.1.  Dictionary References

   A dictionary reference is encoded as an unsigned integer.  Where a
   dictionary reference cannot be expressed unambiguously, the unsigned
   integer is tagged with CBOR tag TBD6.

      relation-type /= uint

      link-target /= #6.TBD6(uint)

      operation-type /= uint

      submission-target /= #6.TBD6(uint)

      form-field-type /= uint

      form-field-value /= #6.TBD6(uint)

      metadata-name /= uint

      metadata-value /= #6.TBD6(uint)







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3.2.2.  Media Type Parameter

   The "application/coral+cbor" media type is defined to have a
   "dictionary" parameter that specifies the dictionary in use.  The
   dictionary is identified by a URI [RFC3986].  For example, a CoRAL
   document that uses the dictionary identified by the URI
   <http://example.com/dictionary> can use the following content type:

      application/coral+cbor;dictionary="http://example.com/dictionary"

   The URI serves only as an identifier; it does not necessarily have to
   be dereferencable (or even use a dereferencable URI scheme).  It is
   permissible, though, to use a dereferencable URI and to serve a
   representation that provides information about the dictionary in a
   human- or machine-readable way.  (The format of such a representation
   is outside the scope of this document.)

   For simplicity, a CoRAL document can reference values only from one
   dictionary; the value of the "dictionary" parameter MUST be a single
   URI.  If the "dictionary" parameter is absent, the default dictionary
   specified in Appendix B of this document is assumed.

   Once a dictionary has made an assignment, the assignment MUST NOT be
   changed or removed.  A dictionary, however, may contain additional
   information about an assignment, which may change over time.

   In CoAP [RFC7252], media types (including specific values for media
   type parameters) are encoded as an unsigned integer called "content
   format".  For use with CoAP, each new CoRAL dictionary MUST register
   a new content format in the IANA CoAP Content-Formats Registry.

4.  Textual Format

   This section defines the syntax of documents in the CoRAL textual
   format using two grammars: The lexical grammar defines how Unicode
   characters are combined to form line terminators, white space,
   comments, and tokens.  The syntactic grammar defines how tokens are
   combined to form documents.  Both grammars are presented in Augmented
   Backus-Naur Form (ABNF) [RFC5234].

   A document in the textual format is a Unicode string in a Unicode
   encoding form [UNICODE].  The media type for such documents is "text/
   coral".  The "charset" parameter is not used; charset information is
   transported inside the document in the form of an OPTIONAL Byte Order
   Mark (BOM).  The use of the UTF-8 encoding scheme [RFC3629], without
   a BOM, is RECOMMENDED.





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4.1.  Lexical Structure

   The lexical structure of a document in the textual format is made up
   of four basic elements: line terminators, white space, comments, and
   tokens.  Of these, only tokens are significant in the syntactic
   grammar.  There are five kinds of tokens: identifiers, IRIs, IRI
   references, literals, and punctuators.

      token = identifier / iri / iriref / literal / punctuator

   When several lexical grammar rules match a sequence of characters in
   a document, the longest match takes priority.

4.1.1.  Line Terminators

   Line terminators divide text into lines.  A line terminator is any
   Unicode character with Line_Break class BK, CR, LF, or NL.  However,
   any CR character that immediately precedes a LF character is ignored.
   (This affects only the numbering of lines in error messages.)

4.1.2.  White Space

   White space is a sequence of one or more white space characters.  A
   white space character is any Unicode character with the White_Space
   property.

4.1.3.  Comments

   Comments are sequences of characters that are ignored when parsing
   text into tokens.  Single-line comments begin with the characters
   "//" and extend to the end of the line.  Delimited comments begin
   with the characters "/*" and end with the characters "*/".  Delimited
   comments can occupy a portion of a line, a single line, or multiple
   lines.

   Comments do not nest.  The character sequences "/*" and "*/" have no
   special meaning within a single-line comment; the character sequences
   "//" and "/*" have no special meaning within a delimited comment.

4.1.4.  Identifiers

   An identifier token is a user-defined symbolic name.  The rules for
   identifiers correspond to those recommended by the Unicode Standard
   Annex #31 [UNICODE-UAX31] using the following profile:

      identifier = START *CONTINUE *(MEDIAL 1*CONTINUE)

      START = <Any character with the XID_Start property>



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      CONTINUE = <Any character with the XID_Continue property>

      MEDIAL = "-" / "." / "~" / %x58A / %xF0B

      MEDIAL =/ %x2010 / %x2027 / %x30A0 / %x30FB

   All identifiers MUST be converted into Unicode Normalization Form C
   (NFC), as defined by the Unicode Standard Annex #15 [UNICODE-UAX15].
   Comparison of identifiers is based on NFC and is case-sensitive
   (unless otherwise noted).

4.1.5.  IRIs and IRI References

   IRIs and IRI references are Unicode strings that conform to the
   syntax defined in RFC 3987 [RFC3987].  An IRI reference can be either
   an IRI or a relative reference.  Both IRIs and IRI references are
   enclosed in angle brackets ("<" and ">").

      iri = "<" IRI ">"

      iriref = "<" IRI-reference ">"

      IRI = <Defined in Section 2.2 of RFC 3987>

      IRI-reference = <Defined in Section 2.2 of RFC 3987>

4.1.6.  Literals

   A literal is a textual representation of a value.  There are seven
   types of literals: Boolean, integer, floating-point, date/time, byte
   string, text string, and null.

      literal = boolean / integer / float / datetime / bytes / text

      literal =/ null

4.1.6.1.  Boolean Literals

   The case-insensitive tokens "true" and "false" denote the Boolean
   values true and false, respectively.

      boolean = "true" / "false"

4.1.6.2.  Integer Literals

   Integer literals denote an integer value of unspecified precision.
   By default, integer literals are expressed in decimal, but they can
   also be specified in an alternate base using a prefix: Binary



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   literals begin with "0b", octal literals begin with "0o", and
   hexadecimal literals begin with "0x".

   Decimal literals contain the digits "0" through "9".  Binary literals
   contain "0" and "1", octal literals contain "0" through "7", and
   hexadecimal literals contain "0" through "9" as well as "A" through
   "F" in upper- or lowercase.

   Negative integers are expressed by prepending a minus sign ("-").

      integer = ["+" / "-"] (decimal / binary / octal / hexadecimal)

      decimal = 1*DIGIT

      binary = %x30 (%x42 / %x62) 1*BINDIG

      octal = %x30 (%x4F / %x6F) 1*OCTDIG

      hexadecimal = %x30 (%x58 / %x78) 1*HEXDIG

      DIGIT = %x30-39

      BINDIG = %x30-31

      OCTDIG = %x30-37

      HEXDIG = %x30-39 / %x41-46 / %x61-66

4.1.6.3.  Floating-point Literals

   Floating-point literals denote a floating-point number of unspecified
   precision.

   Floating-point literals consist of a sequence of decimal digits
   followed by a fraction, an exponent, or both.  The fraction consists
   of a decimal point (".") followed by a sequence of decimal digits.
   The exponent consists of the letter "e" in upper- or lowercase,
   followed by an optional sign and a sequence of decimal digits that
   indicate a power of 10 by which the value preceding the "e" is
   multiplied.

   Negative floating-point values are expressed by prepending a minus
   sign ("-").

      float = ["+" / "-"] 1*DIGIT [fraction] [exponent]

      fraction = "." 1*DIGIT




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      exponent = (%x45 / %x65) ["+" / "-"] 1*DIGIT

   A floating-point literal can additionally denote either the special
   "Not-a-Number" (NaN) value, positive infinity, or negative infinity.
   The NaN value is produced by the case-insensitive token "NaN".  The
   two infinite values are produced by the case-insensitive tokens
   "+Infinity" (or simply "Infinity") and "-Infinity".

      float =/ "NaN"

      float =/ ["+" / "-"] "Infinity"

4.1.6.4.  Date/Time Literals

   Date/time literals denote an instant in time.

   A date/time literal consists of the prefix "dt" and a sequence of
   Unicode characters in Internet Date/Time Format [RFC3339], enclosed
   in single quotes.

      datetime = %x64.74 SQUOTE date-time SQUOTE

      date-time = <Defined in Section 5.6 of RFC 3339>

      SQUOTE = %x27

4.1.6.5.  Byte String Literals

   Byte string literals denote an ordered sequence of bytes.

   A byte string literal consists of a prefix and zero or more bytes
   encoded in Base16, Base32, or Base64 [RFC4648], enclosed in single
   quotes.  Byte string literals encoded in Base16 begin with "h" or
   "b16", byte string literals encoded in Base32 begin with "b32", and
   byte string literals encoded in Base64 begin with "b64".

      bytes = base16 / base32 / base64

      base16 = (%x68 / %x62.31.36) SQUOTE <Base16 encoded data> SQUOTE

      base32 = %x62.33.32 SQUOTE <Base32 encoded data> SQUOTE

      base64 = %x62.36.34 SQUOTE <Base64 encoded data> SQUOTE








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4.1.6.6.  Text String Literals

   Text string literals denote a Unicode string.

   A text string literal consists of zero or more Unicode characters
   enclosed in double quotes.  It can include simple escape sequences
   (such as \t for the tab character) as well as hexadecimal and Unicode
   escape sequences.

      text = DQUOTE *(char / %x5C escape) DQUOTE

      char = <Any character except %x22, %x5C, and line terminators>

      escape = simple-escape / hexadecimal-escape / unicode-escape

      simple-escape = %x30 / %x62 / %x74 / %x6E / %x76

      simple-escape =/ %x66 / %x72 / %x22 / %x27 / %x5C

      hexadecimal-escape = (%x78 / %x58) 2HEXDIG

      unicode-escape = %x75 4HEXDIG / %x55 8HEXDIG

      DQUOTE = %x22

   An escape sequence denotes a single Unicode code point.  For
   hexadecimal and Unicode escape sequences, the code point is expressed
   by the hexadecimal number following the "\x", "\X", "\u", or "\U"
   prefix.  Simple escape sequences indicate the code points listed in
   Table 1.

          +-----------------+------------+----------------------+
          | Escape Sequence | Code Point | Character Name       |
          +-----------------+------------+----------------------+
          |        \0       |   U+0000   | Null                 |
          |        \b       |   U+0008   | Backspace            |
          |        \t       |   U+0009   | Character Tabulation |
          |        \n       |   U+000A   | Line Feed            |
          |        \v       |   U+000B   | Line Tabulation      |
          |        \f       |   U+000C   | Form Feed            |
          |        \r       |   U+000D   | Carriage Return      |
          |        \"       |   U+0022   | Quotation Mark       |
          |        \'       |   U+0027   | Apostrophe           |
          |        \\       |   U+005C   | Reverse Solidus      |
          +-----------------+------------+----------------------+

                     Table 1: Simple Escape Sequences




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4.1.6.7.  Null Literal

   The case-insensitive tokens "null" and "_" denote the intentional
   absence of any value.

      null = "null" / "_"

4.1.7.  Punctuators

   Punctuator tokens are used for grouping and separating.

      punctuator = "#" / ":" / "*" / "[" / "]" / "{" / "}" / "=" / "->"

4.2.  Syntactic Structure

   The syntactic structure of a document in the textual format is made
   up of four kinds of elements: links, forms, embedded representations,
   and (as an extension to the CoRAL data model) directives.  Directives
   provide a way to make documents easier to read and write by setting a
   base for relative IRI references and introducing shorthands for IRIs.

   Elements are processed in the order they appear in the document.
   Document processors need to maintain an _environment_ while iterating
   a list of elements.  The environment consists of three variables: the
   _current context_, the _current base_, and the _current mapping from
   identifiers to IRIs_. Both the current context and the current base
   are initially set to the document's retrieval context.  The current
   mapping from identifiers to IRIs is initially empty.

4.2.1.  Documents

   The body of a document in the textual format consists of zero or more
   links, forms, embedded representations, and directives.

      document = body

      body = *(link / form / representation / directive)

4.2.2.  Links

   A link consists of the link relation type, followed by the link
   target, optionally followed by a link body enclosed in curly brackets
   ("{" and "}").

      link = relation-type link-target ["{" body "}"]

   The link relation type is denoted by either an IRI, a simple name, or
   a qualified name.



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      relation-type = iri / simple-name / qualified-name

   A simple name consists of an identifier.  It is resolved to an IRI by
   looking up the empty string in the current mapping from identifiers
   to IRIs and appending the specified identifier to the result.  It is
   an error if the empty string is not present in the current mapping.

      simple-name = identifier

   A qualified name consists of two identifiers separated by a colon
   (":").  It is resolved to an IRI by looking up the identifier on the
   left hand side in the current mapping from identifiers to IRIs and
   appending the identifier on the right hand side to the result.  It is
   an error if the identifier on the left hand side is not present in
   the current mapping.

      qualified-name = identifier ":" identifier

   The link target is denoted by an IRI reference or represented by a
   value literal.  An IRI reference MUST be resolved against the current
   base.  If the link target is null, the link target is an unidentified
   resource.

      link-target = iriref / literal

   The list of elements in the link body, if any, MUST be processed in a
   fresh environment.  Both the current context and current base in this
   environment are initially set to the link target of the enclosing
   link.  The mapping from identifiers to IRIs is initially set to a
   copy of the mapping from identifiers to IRIs in the current
   environment.

4.2.3.  Forms

   A form consists of the operation type, followed by a "->" token and
   the submission target, optionally followed by a list of form fields
   enclosed in square brackets ("[" and "]").

      form = operation-type "->" submission-target ["[" form-fields "]"]

   The operation type is defined in the same way as a link relation type
   (Section 4.2.2).

      operation-type = iri / simple-name / qualified-name

   The request method is either implied by the operation type or encoded
   as a form field.  If there are both, the form field takes precedence
   over the operation type.  Either way, the method MUST be defined for



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   the Web transfer protocol identified by the scheme of the submission
   target.

   The submission target is denoted by an IRI reference.  This IRI
   reference MUST be resolved against the current base.

      submission-target = iriref

4.2.3.1.  Form Fields

   A list of form fields consists of zero or more type-value pairs.

      form-fields = *(form-field-type form-field-value)

   The list, if any, MUST be processed in a fresh environment.  Both the
   current context and the current base in this environment are
   initially set to the submission target of the enclosing form.  The
   mapping from identifiers to IRIs is initially set to a copy of the
   mapping from identifiers to IRIs in the current environment.

   The form field type is defined in the same way as a link relation
   type (Section 4.2.2).

      form-field-type = iri / simple-name / qualified-name

   The form field value can be an IRI reference, Boolean literal,
   integer literal, floating-point literal, byte string literal, text
   string literal, or null.  An IRI reference MUST be resolved against
   the current base.

      form-field-value = iriref / literal

4.2.4.  Embedded Representations

   An embedded representation consists of a "*" token, followed by the
   representation data, optionally followed by representation metadata
   enclosed in square brackets ("[" and "]").

      representation = "*" bytes ["[" representation-metadata "]"]

   Representation metadata consists of zero or more name-value pairs.

      representation-metadata = *(metadata-name metadata-value)

   The metadata, if any, MUST be processed in a fresh environment.  All
   variables in the new environment are initially set to a copy of the
   variables in the current environment.




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   The metadata name is defined in the same way as a link relation type
   (Section 4.2.2).

      metadata-name = iri / simple-name / qualified-name

   The metadata value can be an IRI reference, Boolean literal, integer
   literal, floating-point literal, byte string literal, text string
   literal, or null.  An IRI reference MUST be resolved against the
   current base.

      metadata-value = iriref / literal

4.2.5.  Directives

   Directives provide the ability to manipulate the environment when
   processing a list of elements.  All directives start with a number
   sign ("#") followed by a directive identifier.  Directive identifiers
   are case-insensitive and constrained to Unicode characters in the
   Basic Latin block.

   The following two types of directives are available: the Base
   directive and the Using directive.

      directive = base-directive / using-directive

4.2.5.1.  Base Directives

   A Base directive consists of a number sign ("#"), followed by the
   case-insensitive identifier "base", followed by a base.

      base-directive = "#" "base" base

   The base is denoted by an IRI reference.  The IRI reference MUST be
   resolved against the current context (not the current base).

      base = iriref

   The directive is processed by resolving the IRI reference against the
   current context and assigning the result to the current base.

4.2.5.2.  Using Directives

   A Using directive consists of a number sign ("#"), followed by the
   case-insensitive identifier "using", optionally followed by an
   identifier and an equals sign ("="), finally followed by an IRI.  If
   the identifier is not specified, it is assumed to be the empty
   string.




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      using-directive = "#" "using" [identifier "="] iri

   The directive is processed by adding the specified identifier and IRI
   to the current mapping from identifiers to IRIs.  It is an error if
   the identifier is already present in the mapping.

5.  Usage Considerations

   This section discusses some considerations in creating CoRAL-based
   applications and vocabularies.

5.1.  Specifying CoRAL-based Applications

   CoRAL-based applications naturally implement the Web architecture
   [W3C.REC-webarch-20041215] and thus are centered around orthogonal
   specifications for identification, interaction, and representation:

   o  Resources are identified by IRIs or represented by value literals.

   o  Interactions are based on the hypermedia interaction model of the
      Web and the methods provided by the Web transfer protocol.  The
      semantics of possible interactions are identified by link relation
      types and operation types.

   o  Representations are CoRAL documents encoded in the binary format
      defined in Section 3 or the textual format defined in Section 4.
      Depending on the application, additional representation formats
      may be used.

5.1.1.  Application Interfaces

   Specifications for CoRAL-based applications need to list the specific
   components used in the application interface and their identifiers.
   This should include the following items:

   o  URI schemes that identify the Web transfer protocol(s) used in the
      application.

   o  Internet media types that identify the representation format(s)
      used in the application, including the media type(s) of the CoRAL
      serialization format(s).

   o  Link relation types that identify the semantics of links.

   o  Operation types that identify the semantics of forms.
      Additionally, for each operation type, the permissible request
      method(s).




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   o  Form field types that identify the semantics of form fields.
      Additionally, for each form field type, the permissible form field
      values.

   o  Metadata names that identify the semantics of representation
      metadata.  Additionally, for each metadata name, the permissible
      metadata values.

5.1.2.  Resource Identifiers

   URIs [RFC3986] are a cornerstone of Web-based applications.  They
   enable the uniform identification of resources and are used every
   time a client interacts with a server or a resource representation
   needs to refer to another resource.

   URIs often include structured application data in the path and query
   components, such as paths in a filesystem or keys in a database.  It
   is a common practice in many HTTP-based application programming
   interfaces (APIs) to make this part of the application specification,
   i.e., to prescribe fixed URI templates that are hard-coded in
   implementations.  There are a number of problems with this practice
   [RFC7320], though.

   In CoRAL-based applications, resource names are therefore not part of
   the application specification -- they are an implementation detail.
   The specification of a CoRAL-based application MUST NOT mandate any
   particular form of resource name structure.  BCP 190 [RFC7320]
   describes the problematic practice of fixed URI structures in more
   detail and provides some acceptable alternatives.

5.1.3.  Implementation Limits

   This document places no restrictions on the number of elements in a
   CoRAL document or the depth of nested elements.  Applications using
   CoRAL (in particular those running in constrained environments) may
   wish to limit these numbers and specify implementation limits that an
   application implementation must at least support to be interoperable.

   Applications may also mandate the following and other restrictions:

   o  use of only either the binary format or the text format;

   o  use of only either HTTP or CoAP as supported Web transfer
      protocol;

   o  use of only dictionary references in the binary format for certain
      vocabulary;




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   o  use of only either content type strings or content format IDs;

   o  use of CoRI references only up to a specific length;

   o  use of CBOR in a canonical format (see Section 3.9 of RFC 7049).

5.2.  Minting Vocabulary

   New link relation types, operation types, form field types, and
   metadata names can be minted by defining an IRI [RFC3987] that
   uniquely identifies the item.  Although the IRI can point to a
   resource that contains a definition of the semantics, clients SHOULD
   NOT automatically access that resource to avoid overburdening its
   server.  The IRI SHOULD be under the control of the person or party
   defining it, or be delegated to them.

   To avoid interoperability problems, it is RECOMMENDED that only IRIs
   are minted that are normalized according to Section 5.3 of RFC 3987.
   Non-normalized forms that are best avoided include:

   o  Uppercase characters in scheme names and domain names

   o  Percent-encoding of characters where it is not required by the IRI
      syntax

   o  Explicitly stated HTTP default port (e.g., <http://example.com/>
      is preferable over <http://example.com:80/>)

   o  Completely empty path in HTTP IRIs (e.g., <http://example.com/> is
      preferable over <http://example.com>)

   o  Dot segments ("/./" or "/../") in the path component of an IRI

   o  Lowercase hexadecimal letters within percent-encoding triplets
      (e.g., "%3F" is preferable over "%3f")

   o  Punycode-encoding of Internationalized Domain Names in IRIs

   o  IRIs that are not in Unicode Normalization Form C [UNICODE-UAX15]

   IRIs that identify vocabulary do not need to be registered.  The
   inclusion of domain names in IRIs allows for the decentralized
   creation of new IRIs without the risk of collisions.

   However, IRIs can be relatively verbose and impose a high overhead on
   a representation.  This can be a problem in constrained environments
   [RFC7228].  Therefore, CoRAL alternatively allows the use of unsigned
   integers to reference CBOR data items from a dictionary, as specified



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   in Section 3.2.  These impose a much smaller overhead but instead
   need to be assigned by an authority to avoid collisions.

5.3.  Expressing Registered Link Relation Types

   Link relation types registered in the IANA Link Relations Registry,
   such as "collection" [RFC6573] or "icon" [W3C.REC-html52-20171214],
   can be used in CoRAL by appending the registered name to the IRI
   <http://www.iana.org/assignments/relation/>:

      #using iana = <http://www.iana.org/assignments/relation/>

      iana:collection </items>
      iana:icon       </favicon.png>

   Note that registered link relation types are required to be
   lowercased, as per Section 3.3 of RFC 8288 [RFC8288].

   (The convention of appending the link relation types to the prefix
   "http://www.iana.org/assignments/relation/" to form IRIs is adopted
   from Atom [RFC4287]; see also Appendix A.2 of RFC 8288 [RFC8288].)

5.4.  Expressing Simple RDF Statements

   An RDF statement [W3C.REC-rdf11-concepts-20140225] says that some
   relationship, indicated by a predicate, holds between two resources.
   RDF predicates can therefore be good source for vocabulary to provide
   resource metadata.  For example, a CoRAL document could use the FOAF
   vocabulary [FOAF] to describe the person or software that made it:

      #using rdf = <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
      #using foaf = <http://xmlns.com/foaf/0.1/>

      foaf:maker null {
         rdf:type        <http://xmlns.com/foaf/0.1/Person>
         foaf:familyName "Hartke"
         foaf:givenName  "Klaus"
         foaf:mbox       <mailto:klaus.hartke@ericsson.com>
      }

5.5.  Expressing Natural Language Texts

   Text strings that are the target of a link can be associated with a
   language tag [RFC5646] and a base text direction (i.e., right-to-left
   or left-to-right) by nesting links of type <http://coreapps.org/
   base#language> and <http://coreapps.org/base#direction> under that
   link, respectively:




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      #using <http://coreapps.org/base#>
      #using iana = <http://www.iana.org/assignments/relation/>

      iana:terms-of-service </tos> {
         title "Nutzungsbedingungen" {
            language "de"
            direction "ltr"
         }
         title "Terms of use" {
            language "en-US"
            direction "ltr"
         }
      }

   The link relation types <http://coreapps.org/base#language> and
   <http://coreapps.org/base#direction> are defined in Appendix A.

5.6.  Embedding CoRAL in CBOR Data

   Data items in the CoRAL binary format (Section 3) may be embedded in
   other CBOR data [RFC7049] data.  Specifications using CDDL [RFC8610]
   SHOULD reference the following CDDL definitions for this purpose:

      CoRAL-Document = document

      CoRAL-Link = link

      CoRAL-Form = form

   For each embedded document, link, and form, the retrieval context,
   link context, and form context needs to be specified, respectively.

5.7.  Submitting CoRAL Documents

   By default, a CoRAL document is a representation that captures the
   current state of a resource.  The meaning of a CoRAL document changes
   when it is submitted in a request.  Depending on the request method,
   the CoRAL document can capture the intended state of a resource (PUT)
   or be subject to application-specific processing (POST).

5.7.1.  PUT Requests

   A PUT request with a CoRAL document enclosed in the request payload
   requests that the state of the target resource be created or replaced
   with the state described by the CoRAL document.  A successful PUT of
   a CoRAL document generally means that a subsequent GET on that same
   target resource would result in an equivalent document being sent in
   a success response.



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   An origin server SHOULD verify that a submitted CoRAL document is
   consistent with any constraints the server has for the target
   resource.  When a document is inconsistent with the target resource,
   the origin server SHOULD either make it consistent (e.g., by removing
   inconsistent elements) or respond with an appropriate error message
   containing sufficient information to explain why the document is
   unsuitable.

   The retrieval context of a CoRAL document in a PUT is the request URI
   of the request.

5.7.2.  POST Requests

   A POST request with a CoRAL document enclosed in the request payload
   requests that the target resource process the CoRAL document
   according to the resource's own specific semantics.

   The retrieval context of a CoRAL document in a POST is the request
   URI of the request.

6.  Security Considerations

   Parsers of CoRAL documents must operate on input that is assumed to
   be untrusted.  This means that parsers MUST fail gracefully in the
   face of malicious inputs (e.g., inputs not adhering to the data
   structure).  Additionally, parsers MUST be prepared to deal with
   resource exhaustion (e.g., resulting from the allocation of big data
   items) or exhaustion of the call stack (stack overflow).

   CoRAL documents intentionally do not feature the equivalent of XML
   entity references as to preclude the whole class of exponential XML
   entity expansion ("billion laughs") [CAPEC-197] and improper XML
   external entity [CAPEC-201] attacks.

   Implementers of the CoRAL binary format need to consider the security
   aspects of processing CBOR with the restrictions described in
   Section 3.  Notably, different number representations for the same
   numeric value are not equivalent in the CoRAL binary format.  See
   Section 8 of RFC 7049 [RFC7049] for security considerations relating
   to CBOR.

   Implementers of the CoRAL textual format need to consider the
   security aspects of handling Unicode input.  See the Unicode Standard
   Annex #36 [UNICODE-UAX36] for security considerations relating to
   visual spoofing and misuse of character encodings.  See Section 10 of
   RFC 3629 [RFC3629] for security considerations relating to UTF-8.





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   CoRAL makes extensive use of resource identifiers.  See Section 7 of
   RFC 3986 [RFC3986] for security considerations relating to URIs.  See
   Section 8 of RFC 3987 [RFC3987] for security considerations relating
   to IRIs.  See Section X of RFC XXXX [I-D.ietf-core-href] for security
   considerations relating to CoRIs.

   The security of applications using CoRAL can depend on the proper
   preparation and comparison of internationalized strings.  For
   example, such strings can be used to make authentication and
   authorization decisions, and the security of an application could be
   compromised if an entity providing a given string is connected to the
   wrong account or online resource based on different interpretations
   of the string.  See RFC 6943 [RFC6943] for security considerations
   relating to identifiers in IRIs and other places.

   CoRAL is intended to be used in conjunction with a Web transfer
   protocol like HTTP or CoAP.  See Section 9 of RFC 7230 [RFC7230],
   Section 9 of RFC 7231 [RFC7231], etc., for security considerations
   relating to HTTP.  See Section 11 of RFC 7252 [RFC7252] for security
   considerations relating to CoAP.

   CoRAL does not define any specific mechanisms for protecting the
   confidentiality and integrity of CoRAL documents.  It relies on
   application layer or transport layer mechanisms for this, such as
   Transport Layer Security (TLS) [RFC8446].

   CoRAL documents and the structure of a web of resources revealed from
   automatically following links can disclose personal information and
   other sensitive information.  Implementations need to prevent the
   unintentional disclosure of such information.  See Section of 9 of
   RFC 7231 [RFC7231] for additional considerations.

   Applications using CoRAL ought to consider the attack vectors opened
   by automatically following, trusting, or otherwise using links and
   forms in CoRAL documents.  Notably, a server that is authoritative
   for the CoRAL representation of a resource may not necessarily be
   authoritative for nested elements in the document.  See Section 5 of
   RFC 8288 [RFC8288] for related considerations.

   Unless an application mitigates this risk by specifying more specific
   rules, any link or form in a document where the link or form context
   and the document's retrieval context don't share the same Web origin
   [RFC6454] MUST be discarded ("same-origin policy").








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7.  IANA Considerations

7.1.  Media Type "application/coral+cbor"

   This document registers the media type "application/coral+cbor"
   according to the procedures of BCP 13 [RFC6838].

   Type name:
      application

   Subtype name:
      coral+cbor

   Required parameters:
      N/A

   Optional parameters:
      dictionary - See Section 3.2 of [I-D.ietf-core-coral].

   Encoding considerations:
      binary - See Section 3 of [I-D.ietf-core-coral].

   Security considerations:
      See Section 6 of [I-D.ietf-core-coral].

   Interoperability considerations:
      N/A

   Published specification:
      [I-D.ietf-core-coral]

   Applications that use this media type:
      See Section 1 of [I-D.ietf-core-coral].

   Fragment identifier considerations:
      As specified for "application/cbor".

   Additional information:
      Deprecated alias names for this type: N/A
      Magic number(s): N/A
      File extension(s): .coral.cbor
      Macintosh file type code(s): N/A

   Person & email address to contact for further information:
      See the Author's Address section of [I-D.ietf-core-coral].

   Intended usage:
      COMMON



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   Restrictions on usage:
      N/A

   Author:
      See the Author's Address section of [I-D.ietf-core-coral].

   Change controller:
      IESG

   Provisional registration?
      No

7.2.  Media Type "text/coral"

   This document registers the media type "text/coral" according to the
   procedures of BCP 13 [RFC6838] and guidelines in RFC 6657 [RFC6657].

   Type name:
      text

   Subtype name:
      coral

   Required parameters:
      N/A

   Optional parameters:
      N/A

   Encoding considerations:
      binary - See Section 4 of [I-D.ietf-core-coral].

   Security considerations:
      See Section 6 of [I-D.ietf-core-coral].

   Interoperability considerations:
      N/A

   Published specification:
      [I-D.ietf-core-coral]

   Applications that use this media type:
      See Section 1 of [I-D.ietf-core-coral].

   Fragment identifier considerations:
      N/A

   Additional information:



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      Deprecated alias names for this type: N/A
      Magic number(s): N/A
      File extension(s): .coral
      Macintosh file type code(s): N/A

   Person & email address to contact for further information:
      See the Author's Address section of [I-D.ietf-core-coral].

   Intended usage:
      COMMON

   Restrictions on usage:
      N/A

   Author:
      See the Author's Address section of [I-D.ietf-core-coral].

   Change controller:
      IESG

   Provisional registration?
      No

7.3.  CoAP Content Formats

   This document registers CoAP content formats for the content types
   "application/coral+cbor" and "text/coral" according to the procedures
   of RFC 7252 [RFC7252].

   o  Content Type: application/coral+cbor
      Content Coding: identity
      ID: TBD3
      Reference: [I-D.ietf-core-coral]

   o  Content Type: text/coral
      Content Coding: identity
      ID: TBD4
      Reference: [I-D.ietf-core-coral]

   [[NOTE TO RFC EDITOR: Please replace all occurrences of "TBD3" and
   "TBD4" in this document with the code points assigned by IANA.]]

   [[NOTE TO IMPLEMENTERS: Experimental implementations can use content
   format ID 65087 for "application/coral+cbor" and content format ID
   65343 for "text/coral" until IANA has assigned code points.]]






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7.4.  CBOR Tag

   This document registers a CBOR tag for dictionary references
   according to the procedures of RFC 7049 [RFC7049].

   o  Tag: TBD6
      Data Item: unsigned integer
      Semantics: Dictionary reference
      Reference: [I-D.ietf-core-coral]

   [[NOTE TO RFC EDITOR: Please replace all occurrences of "TBD6" in
   this document with the code point assigned by IANA.]]

8.  References

8.1.  Normative References

   [I-D.ietf-core-href]
              Hartke, K., "Constrained Resource Identifiers", draft-
              ietf-core-href-00 (work in progress), August 2019.

   [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>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [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,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.





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   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6657]  Melnikov, A. and J. Reschke, "Update to MIME regarding
              "charset" Parameter Handling in Textual Media Types",
              RFC 6657, DOI 10.17487/RFC6657, July 2012,
              <https://www.rfc-editor.org/info/rfc6657>.

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

              Note that this reference is to the latest version of
              Unicode, rather than to a specific release.  It is not
              expected that future changes in the Unicode specification
              will have any impact on this document.







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   [UNICODE-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms",
              <http://unicode.org/reports/tr15/>.

   [UNICODE-UAX31]
              The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax",
              <http://unicode.org/reports/tr31/>.

   [UNICODE-UAX36]
              The Unicode Consortium, "Unicode Standard Annex #36:
              Unicode Security Considerations",
              <http://unicode.org/reports/tr36/>.

8.2.  Informative References

   [CAPEC-197]
              MITRE, "CAPEC-197: XML Entity Expansion", July 2018,
              <https://capec.mitre.org/data/definitions/197.html>.

   [CAPEC-201]
              MITRE, "CAPEC-201: XML Entity Linking", July 2018,
              <https://capec.mitre.org/data/definitions/201.html>.

   [FOAF]     Brickley, D. and L. Miller, "FOAF Vocabulary Specification
              0.99", January 2014,
              <http://xmlns.com/foaf/spec/20140114.html>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <https://www.rfc-editor.org/info/rfc4287>.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/info/rfc5789>.

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

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

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <https://www.rfc-editor.org/info/rfc6943>.



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   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.

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

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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>.

   [W3C.REC-rdf-schema-20140225]
              Brickley, D. and R. Guha, "RDF Schema 1.1", World Wide Web
              Consortium Recommendation REC-rdf-schema-20140225,
              February 2014,
              <http://www.w3.org/TR/2014/REC-rdf-schema-20140225>.



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   [W3C.REC-rdf11-concepts-20140225]
              Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
              Concepts and Abstract Syntax", World Wide Web Consortium
              Recommendation REC-rdf11-concepts-20140225, February 2014,
              <http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225>.

   [W3C.REC-turtle-20140225]
              Prud&#039;hommeaux, E. and G. Carothers, "RDF 1.1 Turtle",
              World Wide Web Consortium Recommendation REC-turtle-
              20140225, February 2014,
              <http://www.w3.org/TR/2014/REC-turtle-20140225>.

   [W3C.REC-webarch-20041215]
              Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", World Wide Web Consortium
              Recommendation REC-webarch-20041215, December 2004,
              <http://www.w3.org/TR/2004/REC-webarch-20041215>.

Appendix A.  Core Vocabulary

   This section defines the core vocabulary for CoRAL: a set of link
   relation types, operation types, form field types, and metadata
   names.

A.1.  Base

   Link Relation Types:

   <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
      Indicates that the link's context is an instance of the class
      specified as the link's target, as defined by RDF Schema
      [W3C.REC-rdf-schema-20140225].

   <http://coreapps.org/base#title>
      Indicates that the link target is a human-readable label (e.g., a
      menu entry).

      The link target MUST be a text string.  The text string SHOULD be
      annotated with a language and text direction using nested links of
      type <http://coreapps.org/base#language> and <http://coreapps.org/
      base#direction>, respectively.

   <http://coreapps.org/base#language>
      Indicates that the link target is a language tag [RFC5646] that
      specifies the language of the link context.

      The link target MUST be a text string in the format specified in
      Section 2.1 of RFC 5646 [RFC5646].



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   <http://coreapps.org/base#direction>
      Indicates that the link target is a base text direction (right-to-
      left or left-to-right) that specifies the text directionality of
      the link context.

      The link target MUST be either the text string "rtl" or the text
      string "ltr".

   Operation Types:

   <http://coreapps.org/base#update>
      Indicates that the state of the form's context can be replaced
      with the state described by a representation submitted to the
      server.

      This operation type defaults to the PUT method [RFC7231] [RFC7252]
      for both HTTP and CoAP.  Typical overrides by a form field include
      the PATCH method [RFC5789] [RFC8132] for HTTP and CoAP and the
      iPATCH method [RFC8132] for CoAP.

   <http://coreapps.org/base#search>
      Indicates that the form's context can be searched by submitting a
      search query.

      This operation type defaults to the POST method [RFC7231] for HTTP
      and the FETCH method [RFC8132] for CoAP.  Typical overrides by a
      form field include the POST method [RFC7252] for CoAP.

A.2.  Collections

   Link Relation Types:

   <http://www.iana.org/assignments/relation/item>
      Indicates that the link's context is a collection and that the
      link's target is a member of that collection, as defined in
      Section 2.1 of RFC 6573 [RFC6573].

   <http://www.iana.org/assignments/relation/collection>
      Indicates that the link's target is a collection and that the
      link's context is a member of that collection, as defined in
      Section 2.2 of RFC 6573 [RFC6573].

   Operation Types:

   <http://coreapps.org/collections#create>
      Indicates that the form's context is a collection and that a new
      item can be created in that collection with the state defined by a
      representation submitted to the server.



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      This operation type defaults to the POST method [RFC7231]
      [RFC7252] for both HTTP and CoAP.

   <http://coreapps.org/collections#delete>
      Indicates that the form's context is a member of a collection and
      that the form's context can be removed from that collection.

      This operation type defaults to the DELETE method [RFC7231]
      [RFC7252] for both HTTP and CoAP.

A.3.  HTTP

   Form Field Types:

   <http://coreapps.org/http#method>
      Specifies the HTTP method for the request.

      The form field value MUST be a text string in the format defined
      in Section 4.1 of RFC 7231 [RFC7231].  The set of possible values
      is maintained in the IANA HTTP Method Registry.

      A form field of this type MUST NOT occur more than once in a form.
      If absent, it defaults to the request method implied by the form's
      operation type.

   <http://coreapps.org/http#accept>
      Specifies an acceptable HTTP content type for the request payload.
      There may be multiple form fields of this type.  If a form does
      not include a form field of this type, the server accepts any or
      no request payload, depending on the operation type.

      The form field value MUST be a text string in the format defined
      in Section 3.1.1.1 of RFC 7231 [RFC7231].  The possible set of
      media types and their parameters are maintained in the IANA Media
      Types Registry.

   Representation Metadata:

   <http://coreapps.org/http#type>
      Specifies the HTTP content type of the representation.

      The metadata value MUST be specified as a text string in the
      format defined in Section 3.1.1.1 of RFC 7231 [RFC7231].  The
      possible set of media types and their parameters are maintained in
      the IANA Media Types Registry.






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      Metadata of this type MUST NOT occur more than once for a
      representation.  If absent, its value defaults to content type
      "application/octet-stream".

A.4.  CoAP

   Form Field Types:

   <http://coreapps.org/coap#method>
      Specifies the CoAP method for the request.

      The form field value MUST be an integer identifying one of the
      CoAP request methods maintained in the IANA CoAP Method Codes
      Registry (e.g., the integer 2 for the POST method).

      A form field of this type MUST NOT occur more than once in a form.
      If absent, it defaults to the request method implied by the form's
      operation type.

   <http://coreapps.org/coap#accept>
      Specifies an acceptable CoAP content format for the request
      payload.  There may be multiple form fields of this type.  If a
      form does not include a form field of this type, the server
      accepts any or no request payload, depending on the operation
      type.

      The form field value MUST be an integer identifying one of the
      content formats maintained in the IANA CoAP Content-Formats
      Registry.

   Representation Metadata:

   <http://coreapps.org/coap#type>
      Specifies the CoAP content format of the representation.

      The metadata value MUST be an integer identifying one of the
      content formats maintained in the IANA CoAP Content-Formats
      Registry.

      Metadata of this type MUST NOT occur more than once for a
      representation.  If absent, it defaults to content format 42
      (i.e., content type "application/octet-stream" without a content
      coding).








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Appendix B.  Default Dictionary

   This section defines a default dictionary that is assumed when the
   "application/coral+cbor" media type is used without a "dictionary"
   parameter.

      +-----+-------------------------------------------------------+
      | Key | Value                                                 |
      +-----+-------------------------------------------------------+
      |   0 | <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>     |
      |   1 | <http://www.iana.org/assignments/relation/item>       |
      |   2 | <http://www.iana.org/assignments/relation/collection> |
      |   3 | <http://coreapps.org/collections#create>              |
      |   4 | <http://coreapps.org/base#update>                     |
      |   5 | <http://coreapps.org/collections#delete>              |
      |   6 | <http://coreapps.org/base#search>                     |
      |   7 | <http://coreapps.org/coap#accept>                     |
      |   8 | <http://coreapps.org/coap#type>                       |
      |   9 | <http://coreapps.org/base#language>                   |
      |  10 | <http://coreapps.org/coap#method>                     |
      |  11 | <http://coreapps.org/base#direction>                  |
      |  12 | "ltr"                                                 |
      |  13 | "rtl"                                                 |
      +-----+-------------------------------------------------------+

                        Table 2: Default Dictionary

Acknowledgements

   Thanks to Christian Amsuess, Carsten Bormann, Jaime Jimenez,
   Sebastian Kaebisch, Ari Keranen, Michael Koster, Matthias Kovatsch,
   Jim Schaad, and Niklas Widell 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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