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Versions: (draft-hartke-t2trg-coral) 00 01 02 03

CoRE Working Group                                             K. Hartke
Internet-Draft                                                  Ericsson
Intended status: Standards Track                            9 March 2020
Expires: 10 September 2020


          The Constrained RESTful Application Language (CoRAL)
                        draft-ietf-core-coral-03

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"),
   and simple resource metadata.

Note to Readers

   This note is to be removed before publishing as an RFC.

   The issues list for this Internet-Draft can be found at
   <https://github.com/core-wg/coral/labels/coral>.

   Companion material for this Internet-Draft can be found at
   <https://github.com/core-wg/coral>.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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   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
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   This Internet-Draft will expire on 10 September 2020.

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Table of Contents

   1.  Introduction
     1.1.  Data and Interaction Model
     1.2.  Serialization Formats
     1.3.  Notational Conventions
   2.  Data and Interaction Model
     2.1.  Browsing Context
     2.2.  Documents
     2.3.  Links
     2.4.  Forms
     2.5.  Form Fields
     2.6.  Navigation
     2.7.  History Traversal
   3.  Binary Format
     3.1.  Data Structure
       3.1.1.  Documents
       3.1.2.  Directives
       3.1.3.  IRIs
       3.1.4.  Links
       3.1.5.  Forms
       3.1.6.  Form Fields
     3.2.  Dictionary Compression
       3.2.1.  Dictionary References
       3.2.2.  Media Type Parameter
     3.3.  Export Interface
   4.  Textual Format
     4.1.  Lexical Structure
       4.1.1.  Line Terminators
       4.1.2.  White Space
       4.1.3.  Comments
       4.1.4.  Identifiers
       4.1.5.  Literals
       4.1.6.  Punctuators
     4.2.  Syntactic Structure
       4.2.1.  Documents
       4.2.2.  Directives
       4.2.3.  IRIs
       4.2.4.  Links
       4.2.5.  Forms
       4.2.6.  Form Fields
   5.  Document Semantics
     5.1.  Submitting Documents
       5.1.1.  PUT Requests
       5.1.2.  POST Requests
     5.2.  Returning Documents
       5.2.1.  Success Responses
       5.2.2.  Redirection Responses
       5.2.3.  Error Responses
   6.  Usage Considerations
     6.1.  Specifying CoRAL-based Applications
       6.1.1.  Application Interfaces
       6.1.2.  Resource Identifiers
       6.1.3.  Implementation Limits
     6.2.  Minting Vocabulary
     6.3.  Expressing Registered Link Relation Types
     6.4.  Expressing Simple RDF Statements
     6.5.  Expressing Natural Language Texts
     6.6.  Embedding Representations in CoRAL
   7.  Security Considerations
   8.  IANA Considerations
     8.1.  Media Type "application/coral+cbor"
     8.2.  Media Type "text/coral"
     8.3.  CoAP Content Formats
     8.4.  CBOR Tag
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Core Vocabulary
     A.1.  Base
     A.2.  Collections
     A.3.  HTTP
     A.4.  CoAP
   Appendix B.  Default Dictionary
   Appendix C.  Change Log
   Acknowledgements
   Author's Address

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"), and
   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 model and interaction model as
   well as two specialized CoRAL serialization formats.

1.1.  Data and Interaction Model

   The data model derives from the Web Linking model of [RFC8288] and
   consists primarily of two 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.

   The data model can additionally make simple statements about
   resources in a way similar to the Resource Description Framework
   (RDF) [W3C.REC-rdf11-concepts-20140225].  In contrast to RDF,
   however, the focus of CoRAL is not on the description of a graph of
   resources, but on the discovery of possible future application
   states.

   The interaction model derives from the processing model of HTML
   [W3C.REC-html52-20171214] and specifies how an automated software
   agent can change the application state by navigating between
   resources following links and performing operations on resources
   submitting forms.

1.2.  Serialization Formats

   The primary serialization format is a compact, binary encoding of
   links and forms in Concise Binary Object Representation (CBOR)
   [RFC7049bis].  This format is intended for environments with
   constraints on power, memory, and processing resources [RFC7228] and
   shares many similarities with the message format of CoAP: In place of
   verbose strings, small numeric identifiers are used to encode link
   relation types and operation types.  Uniform Resource Identifiers
   (URIs) [RFC3986] are pre-parsed into (what CoAP considers to be)
   their components, which considerably simplifies URI processing for
   constrained nodes that already have a CoAP implementation.  As a
   result, link serializations in CoRAL are often much more compact and
   easier to process than equivalent serializations in CoRE Link Format
   [RFC6690].

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

1.3.  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 (rendered in plain text as the new term surrounded by
   underscores).

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, the browsing context typically
   corresponds to a tab or window in a Web browser.)

   At any time, one representation in a 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_.  This URI is
   also considered the base URI for relative URI references in the
   document.

   A CoRAL document consists of a list of zero or more links and forms,
   collectively called _elements_.  CoRAL serialization formats may
   define additional types of elements for efficiency or convenience,
   such as an embedded base URI that takes precedence over the
   document's base URI.

2.3.  Links

   A _link_ describes a relationship between two resources on the Web.
   As in [RFC8288], a link in CoRAL has a _link context_, a _link
   relation type_, and a _link target_.  However, a link in CoRAL does
   not have target attributes.  Instead, a link may have a list of zero
   or more nested elements.  These enable both the description of
   resource metadata and the chaining of links, which is done in
   [RFC8288] by setting the anchor of one link to the target of another.

      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.

   A link relation type identifies the semantics of a link.  In HTML and
   in [RFC8288], link relation types are typically denoted by an IANA-
   registered name, such as "stylesheet" or "type".  In CoRAL, all link
   relation types are in contrast 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 decentralized creation of new link relation types without the
   risk of collisions when from different organizations or domains of
   knowledge.  IRIs can also lead to documentation, schema, and other
   information about a link relation type.  In CoRAL documents, these
   IRIs are only used as identity tokens, though, and are compared with
   Simple String Comparison as specified in Section 5.3.1 of [RFC3987].

   Link contexts and link targets can both be either a URI, a literal
   value, or an anonymous resource.  If the link target is a URI and the
   URI scheme indicates a Web transfer protocol like HTTP or CoAP, an
   agent can dereference the URI and navigate the browsing context to
   its target resource; this is called _following the link_.  Literal
   values are distinct and distinguishable from URIs and directly
   identify data by means of a literal representation.  A literal value
   can be either a Boolean value, an integer number, a floating-point
   number, a date/time instant, a byte string, or a text string.  An
   anonymous resource is a resource that is neither identified by a URI
   nor a literal representation.

   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
   nested link is the link target of the enclosing 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 nesting nature of the data model constrains the
   description of resource relations to a tree: Relations 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 resource on the Web.  A form has a _form context_, an
   _operation type_, a _request method_, and a _submission target_.
   Additionally, a form may be accompanied by a list of zero or more
   _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.

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

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

   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.

2.5.  Form Fields

   Form fields can be used to provide more detailed instructions to
   agents for constructing the request when submitting a form.  For
   example, a form field could instruct an agent to include a certain
   payload or header field in the request.  A payload could, for
   instance, be described by form fields providing acceptable media
   types, a reference to schema information, or a number of individual
   data items that the agents needs to supply.  Form fields can be
   specific to the Web transfer protocol that is used for submitting the
   form.

   A form field is a pair of a _form field type_ and a _form field
   value_.  Additionally, a form field may have a list of zero or more
   nested elements that further describe the form field value.

   A 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.

   Form field values can be either a URI, a Boolean value, an integer
   number, a floating-point number, a date/time instant, a byte string,
   a text string, or null.  A null indicates the intentional absence of
   any form field value.

2.6.  Navigation

   An agent begins the interaction with an application by performing a
   GET request on an _entry point URI_. The entry point URI is the only
   URI that the agent is expected to know beforehand.  From then on, the
   agent is expected to make all requests by following links and
   submitting forms that are provided in the responses resulting from
   the requests.  The entry point URI could be obtained through some
   discovery process or manual configuration.

   If dereferencing the entry point URI yields a CoRAL document (or any
   other representation that implements the CoRAL data and interaction
   model), 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.

       An agent may follow a link without understanding the link
       relation type, e.g., for the sake of pre-fetching or building a
       search index.  However, an agent MUST NOT submit a form without
       understanding the operation type.

   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 can be empty, for
       example, when an application state transition is not supported or
       not allowed.

   3.  The agent selects one of the candidates based on the metadata
       associated them (in the form of form fields and nested elements)
       and their order of appearance in the document.  Examples for
       relevant metadata could include the indication of a media type
       for the target resource representation, the URI scheme of a
       target resource, or the request method of an operation.

   4.  The agent obtains the _request URI_ from the link target or
       submission target.  Link targets and submission targets can be
       denoted by relative URI references, which need to be resolved
       against a base URI to obtain the request URI.  Fragment
       identifiers are not part of the request URI and MUST be separated
       from the rest of the URI prior to the next step.

   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 supplied by the form.  An IRI may need to be converted
       to a URI (see Section 3.1 of [RFC3987]) for protocols that do not
       support IRIs.

       The agent SHOULD set HTTP header fields and CoAP request options
       according to the metadata (e.g., set the HTTP Accept header field
       or the CoAP Accept option when a media type for the target
       resource is provided).  Depending on the operation type of a
       form, the agent may also have to include a request payload that
       matches the specifications of some 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 selects the fragment indicated by the fragment identifier
       within the received representation according to the semantics of
       its media type.

   8.  The agent updates the browsing context by making the (selected
       fragment of the) received representation the active
       representation.

   9.  Finally, the agent processes the representation according to the
       semantics of its media type.  If the representation is a CoRAL
       document (or any other representation that implements the CoRAL
       data and interaction model), the agent again has the choice of
       what to do next.  Go to step 1.

2.7.  History Traversal

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

   A session history consists of session history entries.  The number of
   session history entries may be limited and dependent on the agent.
   An agent with severe constraints on memory size might only have
   enough memory for the most recent entry.

   An entry in the session history consists of a resource representation
   and the representation's retrieval context.  New entries are added to
   the session history as the agent navigates from resource to resource,
   discarding entries that are no longer used.

   An agent can decide to navigate a browsing context (in addition to
   following links and submitting forms) by _traversing the session
   history_.  For example, when an agent receives a response with a
   representation that does not contain any further links or forms, it
   can navigate back to a resource representation it has visited earlier
   and make that the active representation.

   Traversing the history SHOULD take advantage of caches to avoid new
   requests.  An agent may reissue a safe request (e.g., a GET) when it
   does not have a fresh representation in its cache.  An agent MUST NOT
   reissue an unsafe request (e.g., a PUT or POST) unless it actually
   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 encoded in Concise Binary Object
   Representation (CBOR) [RFC7049bis].  The encoding MUST satisfy the
   Core Deterministic Encoding Requirements specified in Section 4.2.1
   of [RFC7049bis].

   The CBOR structure of a document is presented in the Concise Data
   Definition Language (CDDL) [RFC8610].  All CDDL rules not defined in
   this document are defined in Appendix D of [RFC8610].

   The media type of documents in the binary format is "application/
   coral+cbor".

3.1.  Data Structure

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

3.1.1.  Documents

   A document in the binary format is encoded as a CBOR array that
   contains zero or more elements.  An element is either a link, a form,
   or a directive.

      document = [*element]

      element = link / form / directive

   The 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_.  The current context and
   the current base are both initially set to the document's retrieval
   context.

3.1.2.  Directives

   Directives provide the ability to manipulate the environment while
   processing elements.

   There is a single type of directives available: the Base directive.

      directive = base-directive

   It is an error if a document processor encounters any other type of
   directive.

3.1.2.1.  Base Directives

   A Base directive is encoded as a CBOR array that contains the
   unsigned integer 1 and a base URI.

      base-directive = [1, baseURI]

   The base URI is denoted by a Constrained Resource Identifier (CRI)
   reference [I-D.ietf-core-href].  The CRI reference MUST be resolved
   against the current context (not the current base).

      baseURI = CRI-Reference

      CRI-Reference = <Defined in Section XX of RFC XXXX>

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

3.1.3.  IRIs

   IRIs in links and forms are encoded as CRI references.

      IRI = CRI-Reference

   A CRI reference is processed by resolving it to an IRI as specified
   in Section 5.2 of [I-D.ietf-core-href] using the current base.

3.1.4.  Links

   A link is encoded as a CBOR array that contains the unsigned integer
   2, the link relation type, the link target, and, optionally, an array
   of zero or more nested elements.

      link = [2, relation-type, link-target, ?[*element]]

   The link relation type is an IRI.

      relation-type = IRI

   The link target is either an IRI, a literal value, or null.

      link-target = IRI / literal / null

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

   The nested elements, if any, MUST be processed in a fresh
   environment.  The current context is set to the link target of the
   enclosing link.  The current base is initially set to the link
   target, if the link target is an IRI; otherwise, it is set to the
   current base of the current environment.

3.1.5.  Forms

   A form is encoded as a CBOR array that contains the unsigned integer
   3, the operation type, the submission target, and, optionally, an
   array of zero or more form fields.

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

   The operation type is an IRI.

      operation-type = IRI

   The submission target is an IRI.

      submission-target = IRI

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

   The form fields, if any, MUST be processed in a fresh environment.
   The current context is set to an unspecified URI that represents the
   enclosing form.  The current base is initially set to the submission
   target of the enclosing form.

3.1.6.  Form Fields

   A form field is encoded as a CBOR sequence that consists of a form
   field type, a form field value, and, optionally, an array of zero or
   more nested elements.

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

   The form field type is an IRI.

      form-field-type = IRI

   The form field value is either an IRI, a literal value, or null.

      form-field-value = IRI / literal / null

   The nested elements, if any, MUST be processed in a fresh
   environment.  The current context is set to the form field value of
   the enclosing form field.  The current base is initially set to the
   form field value, if the form field value is an IRI; otherwise, it is
   set to the current base of the current environment.

3.2.  Dictionary Compression

   A document in the binary format MAY reference values from an external
   dictionary.  This helps 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, and form field 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, as follows:

      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)

   A dictionary reference MUST NOT refer to a dictionary value that
   would otherwise not be syntactically allowed in that position.  For
   example, a dictionary reference in the position of a link relation
   type cannot refer to a Boolean value; it can only refer to an IRI.

3.2.2.  Media Type Parameter

   The "application/coral+cbor" media type for documents in the binary
   format is defined to have a "dictionary" parameter that specifies the
   dictionary in use.  The dictionary is identified by a URI.  For
   example, a CoRAL document that uses the dictionary identified by the
   URI <http://example.com/dictionary> would have 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
   machine- or human-readable way.  (The representation format and
   security considerations of such a representation are 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.

   The "dictionary" parameter is OPTIONAL.  If it 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, media types (including specific values for their parameters,
   plus an optional content coding) are encoded as an unsigned integer
   called the "content format" of a representation.  For use with CoAP,
   each new CoRAL dictionary therefore needs to have a new content
   format registered in the CoAP Content Formats Registry
   [CORE-PARAMETERS].

3.3.  Export Interface

   The definition of documents, links, and forms in the CoRAL binary
   format can be reused in other CBOR-based protocols.  Specifications
   using CDDL should reference the following rules for this purpose:

      CoRAL-Document = document
      CoRAL-Link = link
      CoRAL-Form = form

   For each embedded document, link, and form, the CBOR-based protocol
   needs to specify the document retrieval context, link context, and
   form context, respectively.

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 of textual media types [RFC6657] is
   not used; instead, 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.

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 three kinds of tokens: identifier tokens, literal
   tokens, and punctuator tokens.

      token = identifier / IRIref / boolean / integer / float
            / datetime / bytes / text / null / 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 syntax for
   identifiers corresponds to the Default Identifier Syntax in Unicode
   Standard Annex #31 [UAX31] with the following profile:

      identifier = START *CONTINUE *(MEDIAL 1*CONTINUE)
      START = <Any character with the XID_Start property>
      CONTINUE = <Any character with the XID_Continue property>
      MEDIAL = <Any character from Table XX>

                +-----------------------------------------+
                | Code Point                              |
                +=========================================+
                | U+002D HYPHEN-MINUS                     |
                +-----------------------------------------+
                | U+002E FULL STOP                        |
                +-----------------------------------------+
                | U+007E TILDE                            |
                +-----------------------------------------+
                | U+058A ARMENIAN HYPHEN                  |
                +-----------------------------------------+
                | U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG |
                +-----------------------------------------+
                | U+2010 HYPHEN                           |
                +-----------------------------------------+
                | U+2027 HYPHENATION POINT                |
                +-----------------------------------------+
                | U+30A0 KATAKANA-HIRAGANA DOUBLE HYPHEN  |
                +-----------------------------------------+
                | U+30FB KATAKANA MIDDLE DOT              |
                +-----------------------------------------+

                         Table 1: Medial Characters

   All identifiers MUST be converted into Unicode Normalization Form C
   (NFC) [Unicode].  Comparison of identifiers is based on NFC and case-
   sensitive (unless otherwise noted).

4.1.5.  Literals

   A literal token is a textual representation of a value.

4.1.5.1.  IRI Reference Literals

   IRI reference tokens denote references to resources on the Web.

   An IRI reference literal consists of a Unicode string that conforms
   to the syntax defined in [RFC3987].  An IRI reference is either an
   IRI or a relative reference.  IRI references are enclosed in angle
   brackets ("<" and ">").

      IRIref = "<" IRI-reference ">"

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

4.1.5.2.  Boolean Literals

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

      boolean = "true" / "false"

4.1.5.3.  Integer Literals

   Integer literal tokens 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 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

4.1.5.4.  Floating-point Literals

   Floating-point literal tokens 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
      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" / ["+" / "-"] "Infinity"

4.1.5.5.  Date/Time Literals

   Date/time literal tokens 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

4.1.5.6.  Byte String Literals

   Byte string literal tokens 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

4.1.5.7.  Text String Literals

   Text string literal tokens 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 DQUOTE, %x5C, and line terminators>
      escape = simple-escape / hexadecimal-escape / unicode-escape
      simple-escape = %x30 / %x62 / %x74 / %x6E / %x76
                    / %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 2.

            +-----------------+------------------------------+
            | Escape Sequence | Code Point                   |
            +=================+==============================+
            |        \0       | U+0000 NULL                  |
            +-----------------+------------------------------+
            |        \b       | U+0008 BACKSPACE             |
            +-----------------+------------------------------+
            |        \t       | U+0009 HORIZONTAL TABULATION |
            +-----------------+------------------------------+
            |        \n       | U+000A LINE FEED             |
            +-----------------+------------------------------+
            |        \v       | U+000B VERTICAL TABULATION   |
            +-----------------+------------------------------+
            |        \f       | U+000C FORM FEED             |
            +-----------------+------------------------------+
            |        \r       | U+000D CARRIAGE RETURN       |
            +-----------------+------------------------------+
            |        \"       | U+0022 QUOTATION MARK        |
            +-----------------+------------------------------+
            |        \'       | U+0027 APOSTROPHE            |
            +-----------------+------------------------------+
            |        \\       | U+005C REVERSE SOLIDUS       |
            +-----------------+------------------------------+

                     Table 2: Simple Escape Sequences

4.1.5.8.  Null Literal

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

      null = "null" / "_"

4.1.6.  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 three kinds of elements: links, forms, 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.

4.2.1.  Documents

   A document in the textual format consists of a sequence of zero or
   more elements.  An element is either a link, a form, or a directive.

      document = *element

      element = link / form / directive

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

4.2.2.  Directives

   Directives provide the ability to manipulate the environment while
   processing elements.

   All directives start with a number sign ("#") followed by an
   identifier.  The identifier is case-insensitive and restricted 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

   It is an error if a document processor encounters any other type of
   directive.

4.2.2.1.  Base Directives

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

      base-directive = "#" "base" baseIRI

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

      baseIRI = IRIref

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

4.2.2.2.  Using Directives

   A Using directive consists of a number sign ("#"), followed by the
   case-insensitive token "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.

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

   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 or if the IRI is not
   an IRI but a relative reference.

4.2.3.  IRIs

   IRIs in links and forms can be either denoted by an IRI reference or
   looked up in a mapping from identifiers to IRIs.  Lookups can be done
   in three ways: using a simple name, a qualified name, or a predefined
   name.

      IRI = IRIref / simple-name / qualified-name / predefined-name

   All IRI references and names are processed by resolving them to an
   IRI, as described in the following sub-sections.

4.2.3.1.  IRI References

   An IRI reference is resolved to an IRI as specified in Section 6.5 of
   [RFC3987] using the current base as the base URI.

4.2.3.2.  Simple Names

   A simple name consists of an identifier.

      simple-name = identifier

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

4.2.3.3.  Qualified Names

   A qualified name consists of two identifiers separated by a colon
   (":").

      qualified-name = identifier ":" identifier

   A qualified name is resolved to an IRI by looking up the identifier
   given on the left hand side in the current mapping from identifiers
   to IRIs.  The identifier given on the right hand side is appended to
   the result.  It is an error if the identifier on the left hand side
   is not present in the mapping.

4.2.3.4.  Predefined Names

   A predefined name consists of a commercial at sign ("@") followed by
   an identifier.  The identifier is case-insensitive and restricted to
   Unicode characters in the Basic Latin block.

      predefined-name = "@" identifier

   A predefined name is resolved to an IRI by looking up the identifier
   in Table 3.  It is an error if the identifier is not present in the
   table.

           +------------+--------------------------------------+
           | Identifier | IRI                                  |
           +============+======================================+
           | direction  | <http://coreapps.org/base#direction> |
           +------------+--------------------------------------+
           | language   | <http://coreapps.org/base#language>  |
           +------------+--------------------------------------+

                         Table 3: Predefined Names

4.2.4.  Links

   A link consists of the link relation type, followed by the link
   target, optionally followed by a sequence of zero or more nested
   elements enclosed in curly brackets ("{" and "}").

      link = relation-type link-target ["{" *element "}"]

   The link relation type is an IRI.

      relation-type = IRI

   The link target is either an IRI, a literal value, or null.

      link-target = IRI / literal / null

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

   The nested elements, if any, MUST be processed in a fresh
   environment.  The current context is set to the link target of the
   enclosing link.  The current base is initially set to the link
   target, if the link target is an IRI; otherwise, it is set to the
   current base of the current environment.  The mapping from
   identifiers to IRIs is initially set to a copy of the mapping from
   identifiers to IRIs in the current environment.

4.2.5.  Forms

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

      form = operation-type "->" submission-target ["[" *form-field "]"]

   The operation type is an IRI.

      operation-type = IRI

   The submission target is an IRI.

      submission-target = IRI

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

   The form fields, if any, MUST be processed in a fresh environment.
   The current context is set to an unspecified URI that represents the
   enclosing form.  The current base is 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.

4.2.6.  Form Fields

   A form field consists of a form field type, followed by a form field
   value, optionally followed by a sequence of zero or more nested
   elements enclosed in curly brackets ("{" and "}").

      form-field = form-field-type form-field-value ["{" *element "}"]

   The form field type is an IRI.

      form-field-type = IRI

   The form field value is either an IRI, a literal value, or null.

      form-field-value = IRI / literal / null

   The nested elements, if any, MUST be processed in a fresh
   environment.  The current context is set to the form field value of
   the enclosing form field.  The current base is initially set to the
   form field value, if the form field value is an IRI; otherwise, it is
   set to the current base of the current environment.  The mapping from
   identifiers to IRIs is initially set to a copy of the mapping from
   identifiers to IRIs in the current environment.

5.  Document Semantics

5.1.  Submitting 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).

5.1.1.  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.

   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 and the base URI of a CoRAL document in a PUT
   are the request URI of the request.

5.1.2.  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 defined by the
   target resource's processing semantics; it may be an unspecified URI.
   The base URI of the document is the request URI of the request.

5.2.  Returning Documents

   In a response, the meaning of a CoRAL document changes depending on
   the request method and the response status code.  For example, a
   CoRAL document in a successful response to a GET represents the
   current state of the target resource, whereas a CoRAL document in a
   successful response to a POST might represent either the processing
   result or the new resource state.  A CoRAL document in an error
   response represents the error condition, usually describing the error
   state and what next steps are suggested for resolving it.

5.2.1.  Success Responses

   Success responses have a response status code that indicates that the
   client's request was successfully received, understood, and accepted
   (2xx in HTTP, 2.xx in CoAP).  When the representation in a success
   response does not describe the state of the target resource, it
   describes result of processing the request.  For example, when a
   request has been fulfilled and has resulted in one or more new
   resources being created, a CoRAL document in the response can link to
   and describe the resource(s) created.

   The retrieval context and the base URI of a CoRAL document
   representing the current state of a resource are the request URI of
   the request.

   The retrieval context of a CoRAL document representing a processing
   result is an unspecified URI that refers to the processing result
   itself.  The base URI of the document is the request URI of the
   request.

5.2.2.  Redirection Responses

   Redirection responses have a response status code that indicates that
   further action needs to be taken by the agent (3xx in HTTP).  A
   redirection response, for example, might indicate that the target
   resource is available at a different URI or the server offers a
   choice of multiple matching resources, each with its own specific
   URI.

   In the latter case, the representation in the response might contain
   a list of resource metadata and URI references (i.e., links) from
   which the agent can choose the most preferred one.

   The retrieval context of a CoRAL document representing such multiple
   choices in a redirection response is an unspecified URI that refers
   to the redirection itself.  The base URI of the document is the
   request URI of the request.

5.2.3.  Error Responses

   Error response have a response status code that indicates that either
   the request cannot be fulfilled or the server failed to fulfill an
   apparently valid request (4xx or 5xx in HTTP, 4.xx or 5.xx in CoAP).
   A representation in an error response describes the error condition.

   The retrieval context of a CoRAL document representing such an error
   condition is an unspecified URI that refers to the error condition
   itself.  The base URI of the document is the request URI of the
   request.

6.  Usage Considerations

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

6.1.  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:

   *  Resources are identified by IRIs or represented by literal values.

   *  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.

   *  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.

6.1.1.  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:

   *  The Web transfer protocols supported.

   *  The representation formats used, identified by their Internet
      media types, including the CoRAL serialization formats.

   *  The link relation types used.

   *  The operation types used.  Additionally, for each operation type,
      the permissible request methods.

   *  The form field types used.  Additionally, for each form field
      type, the permissible form field values.

6.1.2.  Resource Identifiers

   URIs 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 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.
   However, there are a number of problems with this practice
   [RFC7320bis].

   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.

   [RFC7320bis] describes the problematic practice of fixed URI
   structures in more detail and provides some acceptable alternatives.

6.1.3.  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
   limit these numbers and define specific implementation limits that an
   implementation must support at least to be interoperable.

   Applications may also mandate the following and other restrictions:

   *  Use of only either the binary format or the text format.

   *  Use of only either HTTP or CoAP as the supported Web transfer
      protocol.

   *  Use of only dictionary references in the binary format for certain
      vocabulary.

   *  Use of URI references and CRI references only up to a specific
      length.

6.2.  Minting Vocabulary

   New link relation types, operation types, and form field types can be
   minted by defining an IRI that uniquely identifies the item.
   Although the IRI may 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 [RFC3987].
   Non-normalized forms that are best avoided include:

   *  Uppercase characters in scheme names and domain names

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

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

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

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

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

   *  Punycode-encoding of Internationalized Domain Names in IRIs

   *  IRIs that are not in Unicode Normalization Form C

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

6.3.  Expressing Registered Link Relation Types

   Link relation types registered in the Link Relations Registry
   [LINK-RELATIONS], 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>

   The convention of appending the relation type name to the prefix
   <http://www.iana.org/assignments/relation/> to form IRIs is adopted
   from the Atom Syndication Format [RFC4287]; see also Appendix A.2 of
   [RFC8288].

   Note that registered relation type names are required to be lowercase
   ASCII letters (see Section 3.3 of [RFC8288]).

6.4.  Expressing Simple RDF Statements

   In RDF [W3C.REC-rdf11-concepts-20140225], a statement says that some
   relationship, indicated by a predicate, holds between two resources.
   Existing RDF vocabularies can therefore be a good source for link
   relation types that describe 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>
      }

6.5.  Expressing Natural Language Texts

   Text strings can be associated with a Language Tag [RFC5646] and a
   base text direction (right-to-left or left-to-right) by nesting links
   of types <http://coreapps.org/base#language> and
   <http://coreapps.org/base#direction>, respectively:

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

      iana:terms-of-service </tos> {
         base:title "Nutzungsbedingungen" {
            @language  "de"
            @direction "ltr"
         }
         base: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.

6.6.  Embedding Representations in CoRAL

   When a document links to many Web resources and an agent needs a
   representation of each of them, it can be inefficient to retrieve
   each representations individually.  To minimize round-trips,
   documents can embed representations of resources.

   A representation can be embedded in a document by including a link of
   type <http://coreapps.org/base#representation>:

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

      iana:icon </favicon.gif> {
         base:representation
            b64'R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAIAOw==' {
            http:type "image/gif"
         }
      }

   An embedded representation SHOULD have a nested link of type
   <http://coreapps.org/http#type> or <http://coreapps.org/coap#type>
   that indicates the content type of the representation.

   The link relation types <http://coreapps.org/base#representation>,
   <http://coreapps.org/http#type>, and <http://coreapps.org/coap#type>
   are defined in Appendix A.

7.  Security Considerations

   CoRAL document processors need to be fully prepared for all types of
   hostile input that may be designed to corrupt, overrun, or achieve
   control of the agent processing the document.  For example, hostile
   input may be constructed to overrun buffers, allocate very big data
   structures, or exhaust the stack depth by setting up deeply nested
   elements.  Processors need to have appropriate resource management to
   mitigate these attacks.

   CoRAL serialization formats intentionally do not feature the
   equivalent of XML entity references so as to preclude the entire
   class of attacks relating to them, such as exponential XML entity
   expansion ("billion laughs") [CAPEC-197] and malicious XML entity
   linking [CAPEC-201].

   Implementers of the CoRAL binary format need to consider the security
   aspects of decoding CBOR.  See Section 10 of [RFC7049bis] for
   security considerations relating to CBOR.  In particular, different
   number encodings for the same numeric value are not equivalent in
   CoRAL (e.g., a floating-point value of 0.0 is not the same as the
   integer 0).

   Implementers of the CoRAL textual format need to consider the
   security aspects of handling Unicode input.  See Unicode Technical
   Report #36 [UTR36] for security considerations relating to visual
   spoofing and misuse of character encodings.  See Section 10 of
   [RFC3629] for security considerations relating to UTF-8.  See Unicode
   Technical Standard #39 [UTS39] for security mechanisms that can be
   used to detect possible security problems relating to Unicode.

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

   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 [RFC6943] for security considerations relating to
   identifiers in IRIs and other strings.

   CoRAL is intended to be used in conjunction with a Web transfer
   protocol like HTTP or CoAP.  See Section 9 of [RFC7230], Section 9 of
   [RFC7231], etc., for security considerations relating to HTTP.  See
   Section 11 of [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
   security mechanisms on the application layer or transport layer 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 9 of
   [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.  See Section 5 of [RFC8288] for related
   considerations.

   In particular, when a CoRAL document is the representation of a
   resource, the server that is authoritative for that resource may not
   necessarily be authoritative for nested elements in the document.  In
   this case, unless an application defines specific rules, any link or
   form where the link/form context and the document's retrieval context
   do not share the same Web Origin [RFC6454] should be discarded
   ("same-origin policy").

8.  IANA Considerations

8.1.  Media Type "application/coral+cbor"

   This document registers the media type "application/coral+cbor"
   according to the procedures of [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 7 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

   Restrictions on usage:
      N/A

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

   Change controller:
      IESG

   Provisional registration?
      No

8.2.  Media Type "text/coral"

   This document registers the media type "text/coral" according to the
   procedures of [RFC6838] and guidelines of [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 7 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:
      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

8.3.  CoAP Content Formats

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

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

   *  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 may use content
   format ID 65087 for "application/coral+cbor" and content format ID
   65343 for "text/coral" until IANA has assigned code points.]]

8.4.  CBOR Tag

   This document registers a CBOR tag for dictionary references
   according to the procedures of [RFC7049bis].

   *  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.]]

9.  References

9.1.  Normative References

   [I-D.ietf-core-href]
              Hartke, K., "Constrained Resource Identifiers", Work in
              Progress, Internet-Draft, draft-ietf-core-href-03, 9 March
              2020,
              <https://tools.ietf.org/html/draft-ietf-core-href-03>.

   [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>.

   [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>.

   [RFC7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", Work in Progress, Internet-Draft,
              draft-ietf-cbor-7049bis-13, 8 March 2020,
              <https://tools.ietf.org/html/draft-ietf-cbor-7049bis-13>.

   [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, Version
              12.1.0", ISBN 978-1-936213-25-2, May 2019,
              <http://www.unicode.org/versions/Unicode12.1.0/>.

9.2.  Informative References

   [CAPEC-197]
              MITRE, "CAPEC-197: XML Entity Expansion", 30 September
              2019, <https://capec.mitre.org/data/definitions/197.html>.

   [CAPEC-201]
              MITRE, "CAPEC-201: XML Entity Linking", 30 September 2019,
              <https://capec.mitre.org/data/definitions/201.html>.

   [CORE-PARAMETERS]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <http://www.iana.org/assignments/core-parameters>.

   [FOAF]     Brickley, D. and L. Miller, "FOAF Vocabulary Specification
              0.99", 14 January 2014,
              <http://xmlns.com/foaf/spec/20140114.html>.

   [HAL]      Kelly, M., "JSON Hypertext Application Language", Work in
              Progress, Internet-Draft, draft-kelly-json-hal-08, 12 May
              2016,
              <https://tools.ietf.org/html/draft-kelly-json-hal-08>.

   [HTTP-METHODS]
              IANA, "Hypertext Transfer Protocol (HTTP) Method
              Registry", <http://www.iana.org/assignments/http-methods>.

   [LINK-RELATIONS]
              IANA, "Link Relations",
              <http://www.iana.org/assignments/link-relations>.

   [MEDIA-TYPES]
              IANA, "Media Types",
              <http://www.iana.org/assignments/media-types>.

   [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>.

   [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>.

   [RFC7320bis]
              Nottingham, M., "URI Design and Ownership", Work in
              Progress, Internet-Draft, draft-nottingham-rfc7320bis-03,
              5 January 2020, <https://tools.ietf.org/html/draft-
              nottingham-rfc7320bis-03>.

   [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>.

   [UAX31]    The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax", Revision 31,
              February 2019,
              <http://www.unicode.org/reports/tr31/tr31-31.html>.

   [UTR36]    The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", Revision 15, September
              2014, <http://www.unicode.org/reports/tr36/tr36-15.html>.

   [UTS39]    The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", Revision 20, May 2019,
              <http://www.unicode.org/reports/tr39/tr39-20.html>.

   [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, 14 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, 25
              February 2014,
              <http://www.w3.org/TR/2014/REC-rdf-schema-20140225>.

   [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, 25 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, 25 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, 15 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, and form field types.

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 [RFC5646].

   <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".

   <http://coreapps.org/base#representation>
      Indicates that the link target is a representation of the link
      context.

      The link target MUST be a byte string.

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

      A link with this link relation type 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.

   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 [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 [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.

      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 [RFC7231].  The possible set of values is
      maintained in the HTTP Methods Registry [HTTP-METHODS].

      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 [RFC7231].  The possible set of media types
      and their parameters is maintained in the Media Types Registry
      [MEDIA-TYPES].

   Link Relation Types:

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

      The link target MUST be a text string in the format defined in
      Section 3.1.1.1 of [RFC7231].  The possible set of media types and
      their parameters is maintained in the Media Types Registry
      [MEDIA-TYPES].

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 a CoAP method
      (e.g., the integer 2 for the POST method).  The possible set of
      values is maintained in the CoAP Method Codes Registry
      [CORE-PARAMETERS].

      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 a CoAP content
      format.  The possible set of values is maintained in the CoAP
      Content Formats Registry [CORE-PARAMETERS].

   Link Relation Types:

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

      The link target MUST be an integer identifying a CoAP content
      format (e.g., the integer 42 for the content type "application/
      octet-stream" without a content coding).  The possible set of
      values is maintained in the CoAP Content Formats Registry
      [CORE-PARAMETERS].

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"                                                 |
      +-----+-------------------------------------------------------+
      |  14 | <http://coreapps.org/base#representation>             |
      +-----+-------------------------------------------------------+

                        Table 4: Default Dictionary

Appendix C.  Change Log

   This section is to be removed before publishing as an RFC.

   Changes from -02 to -03:

   *  Changed the binary format to express relation types, operation
      types and form field types using [I-D.ietf-core-href] (#2).

   *  Clarified the current context and current base for nested elements
      and form fields (#53).

   *  Minor editorial improvements (#27).

   Changes from -01 to -02:

   *  Added nested elements to form fields.

   *  Replaced the special construct for embedded representations with
      links.

   *  Changed the textual format to allow simple/qualified names
      wherever IRI references are allowed.

   *  Introduced predefined names in the textual format (#39).

   *  Minor editorial improvements and bug fixes (#16 #28 #31 #37 #39).

   Changes from -00 to -01:

   *  Added a section on the semantics of CoRAL documents in responses.

   *  Minor editorial improvements.

Acknowledgements

   CoRAL is heavily inspired by Mike Kelly's JSON Hypertext Application
   Language [HAL].

   The recommendations for minting IRIs have been adopted from RDF 1.1
   Concepts and Abstract Syntax [W3C.REC-rdf11-concepts-20140225] to
   ease the interoperability between RDF predicates and link relation
   types.

   Thanks to Christian Amsuess, Carsten Bormann, Thomas Fossati, Jaime
   Jimenez, Jim Schaad, Sebastian Kaebisch, Ari Keranen, Michael Koster,
   Matthias Kovatsch and Niklas Widell for helpful comments and
   discussions that have shaped the document.

Author's Address

   Klaus Hartke
   Ericsson
   Torshamnsgatan 23
   SE-16483 Stockholm
   Sweden

   Email: klaus.hartke@ericsson.com


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