The Constrained RESTful Application Language (CoRAL)
draft-ietf-core-coral-02
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| Last updated | 2020-01-08 | ||
| Replaces | draft-hartke-t2trg-coral | ||
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draft-ietf-core-coral-02
CoRE Working Group K. Hartke
Internet-Draft Ericsson
Intended status: Standards Track 8 January 2020
Expires: 11 July 2020
The Constrained RESTful Application Language (CoRAL)
draft-ietf-core-coral-02
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-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 July 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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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
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
6.7. Embedding CoRAL in CBOR Representations
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 RFC 8288
[RFC8288] and primarily consists 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.
Additionally, the data model can describe simple resource metadata in
similarly to statements in the Resource Description Framework (RDF)
[W3C.REC-rdf11-concepts-20140225]. In contrast to RDF, the focus of
CoRAL, however, is not on the description of a resource graph, but on
the discovery of possible future application states of a software
agent.
The interaction model derives from HTML [W3C.REC-html52-20171214] and
specifies how an automated software agent can change the application
state, i.e., navigate between resources by following links and
perform operations on resources by 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)
[I-D.ietf-cbor-7049bis]. The format 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 considerably simplifies URI processing for
constrained nodes that already implement CoAP. 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
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.
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_.
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 a base for relative URI references.
2.3. Links
A _link_ describes a relationship between two resources on the Web
[RFC8288]. As in RFC 8288, a link in CoRAL has a _link context_, a
_link relation type_, and a _link target_. However, in CoRAL, links
do 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 (which is done in RFC 8288 with target attributes)
and the chaining of links (which can be done in RFC 8288 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.
The link relation type identifies the semantics of a link. While in
HTML and RFC 8288 these are typically denoted by an IANA-registered
name, such as "stylesheet" or "type", link relation types in CoRAL
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. 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 RFC 3987.
The link context and the link target can be both either a URI, a
literal value, or an anonymous resource. If the scheme of a URI
indicates a Web transfer protocol such as HTTP or CoAP, an agent can
dereference the URI and navigate the browsing context to the link
target; this is called _following the link_. A literal value can be
either a Boolean value, an integer, a floating-point number, a date/
time instant, a byte string, or a text string. An anonymous resource
is a resource which is not identified by a URI and is not a literal.
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 same resource as 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. 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.
The operation type identifies the semantics of the operation.
Operation types are denoted (like link relation types) by an IRI.
Both the form context and the submission target are denoted by a URI.
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_.
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 same
resource as 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, form fields can instruct the agent to include a certain
payload or certain header fields in the request. Form fields might
describe a payload by identifying acceptable media types, referencing
a schema, or listing a number of data items that need to be included.
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.
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, a Boolean value, an
integer, a floating-point number, a date/time instant, a byte string,
a text string, or null. A null denotes the intentional absence of
any form field value.
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
that the agent is expected to know before interacting with the
application. From then on, the agent is expected to make all
requests by following links and submitting forms that are provided by
servers in responses. The entry point URI can 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), 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 type of form to submit, based on
the link relation types and operation types it knows.
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 a transition is not supported or not allowed.
3. The agent selects one of the candidates based on the order of
appearance in the document and the resource metadata associated
with them in the form of nested elements and form fields.
Examples for resource metadata include the indication of a
content type for the target resource representation, the URI
scheme of a link target, or the request method of a form.
4. The agent obtains the _request URI_ from the link target or
submission target. Link targets and submission targets may be
denoted by relative URI references, which need to be resolved 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 indicated 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 the nested elements of a link or form fields of a
form (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.
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 content type. If the representation is a CoRAL
document (or any other representation that implements the CoRAL
data and interaction model), the agent has the choice of what to
do next again -- and the cycle repeats.
2.7. History Traversal
Each browsing context has a _session history_ that lists the resource
representations that the agent has processed, is processing, or will
process. The maximum length of the session history is up for the
agent to decide and may be zero.
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.
In addition to following links and submitting forms, an agent can
decide tp navigate a browsing context by _traversing the session
history_. For example, when an agent receives a representation that
does not contain any further links or forms, it can set 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 does not have a fresh representation in its cache. An agent
MUST NOT reissue an unsafe request (e.g., a PUT or a 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 encoded in Concise Binary Object
Representation (CBOR) [I-D.ietf-cbor-7049bis]. The encoding MUST
satisfy the Core Deterministic Encoding Requirements specified in
Section XX of RFC XXXX [I-D.ietf-cbor-7049bis].
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 RFC 8610 [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_. Both the current context
and the current base are 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
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 (CoRI)
reference [I-D.ietf-core-href]. The CoRI reference MUST be resolved
against the current context (not the current base).
baseURI = CoRI
CoRI = <Defined in Section XX of RFC XXXX>
The directive is processed by resolving the CoRI 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 CoRI references.
IRI = CoRI
A CoRI reference is processed by resolving it to an IRI as specified
in Section XX of RFC XXXX [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 encoded as a text string that conforms to
the syntax of an IRI [RFC3987].
relation-type = text
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. Both the current context and current base in this
environment are initially set to the link target of the enclosing
link.
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 encoded as a text string that conforms to the
syntax of an IRI [RFC3987].
operation-type = text
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.
Both the current context and the current base in this environment are
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 encoded as a text string that conforms to the
syntax of an IRI [RFC3987].
form-field-type = text
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. Both the current context and current base in this
environment are initially set to the form field value of the
enclosing form field.
3.2. Dictionary Compression
A document in the binary format can reference values from an external
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,
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 is
otherwise not allowed. For example, a dictionary reference that is
used in place of a link relation type is not allowed to refer to a
Boolean value.
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 [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
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 [RFC7252], media types (including specific values for their
parameters) 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
with IANA in the 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 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
token =/ 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 rules for
identifiers correspond to those recommended by the 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 = "-" / "." / "~" / %x58A / %xF0B
MEDIAL =/ %x2010 / %x2027 / %x30A0 / %x30FB
All identifiers MUST be converted into Unicode Normalization Form C
(NFC), which is defined in Unicode Standard Annex #15 [UAX15].
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 RFC 3987 [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"
float =/ ["+" / "-"] "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
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
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_. 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.
4.2.2. 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
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 from a mapping from identifiers to IRIs using names. There
are three kinds of names: simple names, qualified names, and
predefined names.
IRI = IRIref / simple-name / qualified-name / predefined-name
Both 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
RFC 3987 [RFC3987] using the current base.
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 2. 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 2: 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. 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.
4.2.5. 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.
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.
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. Both the current context and current base in this
environment are initially set to the form field value of the
enclosing form field. 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 can 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 [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 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
[I-D.nottingham-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.
BCP 190 [I-D.nottingham-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 give specific implementation limits that an
application implementation must at least support 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 CoRI 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 [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:
* 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 [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
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 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>
The convention of appending the relation type name 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].
Note that registered relation type names are required to be lowercase
ASCII letters (Section 3.3 of RFC 8288).
6.4. Expressing Simple RDF Statements
An RDF statement [W3C.REC-rdf11-concepts-20140225] says that some
relationship, indicated by a predicate, holds between two resources.
Existing RDF vocabularies can therefore be 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 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:
#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.
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 <http://coreapps.org/base#>
#using http = <http://coreapps.org/http#>
#using iana = <http://www.iana.org/assignments/relation/>
iana:icon </favicon.gif> {
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.
6.7. Embedding CoRAL in CBOR Representations
Data items in the CoRAL binary format (Section 3) may be embedded in
other CBOR structures [I-D.ietf-cbor-7049bis]. Specifications using
CDDL [RFC8610] can 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 specification for the
embedding CBOR structure needs to specify the document retrieval
context, the link context, and the form context, respectively.
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 XX of RFC XXXX
[I-D.ietf-cbor-7049bis] 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 RFC
3629 [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
RFC 3986 [RFC3986] for security considerations relating to URIs. See
Section 8 of RFC 3987 [RFC3987] for security considerations relating
to IRIs. See Section XX 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 strings.
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
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 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. See Section 5 of RFC 8288 [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 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 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 BCP 13 [RFC6838] and guidelines of 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 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 RFC 7252 [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 can 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 RFC XXXX [I-D.ietf-cbor-7049bis].
* 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-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", Work in Progress, Internet-Draft,
draft-ietf-cbor-7049bis-12, 18 December 2019,
<https://tools.ietf.org/html/draft-ietf-cbor-7049bis-12>.
[I-D.ietf-core-href]
Hartke, K., "Constrained Resource Identifiers", Work in
Progress, Internet-Draft, draft-ietf-core-href-02, 8
January 2020,
<https://tools.ietf.org/html/draft-ietf-core-href-02>.
[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>.
[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>.
[UAX15] The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms",
<http://unicode.org/reports/tr15/>.
[UAX31] The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax",
<http://unicode.org/reports/tr31/>.
[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.
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>.
[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>.
[I-D.nottingham-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>.
[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>.
[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>.
[UTR36] The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations",
<http://unicode.org/reports/tr36/>.
[UTS39] The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms",
<http://unicode.org/reports/tr39/>.
[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'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 RFC 5646 [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 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 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.
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.
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 RFC 7231 [RFC7231]. The possible set of media
types and their parameters are maintained in the IANA Media Types
Registry.
A link of this type MUST NOT occur more than once for the link
context. If absent, its value defaults to the 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.
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 one of the content
formats maintained in the IANA CoAP Content-Formats Registry.
A link of this type MUST NOT occur more than once for the link
context. If absent, it defaults to content format 42 (i.e., the
content type "application/octet-stream" without a content coding).
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 3: Default Dictionary
Appendix C. Change Log
This section is to be removed before publishing as an RFC.
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.
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