The Constrained RESTful Application Language (CoRAL)
draft-hartke-t2trg-coral-07
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| Author | Klaus Hartke | ||
| Last updated | 2019-02-06 (Latest revision 2018-10-22) | ||
| Replaced by | draft-ietf-core-coral | ||
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draft-hartke-t2trg-coral-07
Thing-to-Thing Research Group K. Hartke
Internet-Draft Ericsson
Intended status: Experimental February 6, 2019
Expires: August 10, 2019
The Constrained RESTful Application Language (CoRAL)
draft-hartke-t2trg-coral-07
Abstract
The Constrained RESTful Application Language (CoRAL) defines a data
model and interaction model as well as two specialized serialization
formats for the description of typed connections between resources on
the Web ("links"), possible operations on such resources ("forms"),
as well as simple resource metadata.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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 August 10, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Data and Interaction Model . . . . . . . . . . . . . . . . . 4
3.1. Browsing Context . . . . . . . . . . . . . . . . . . . . 4
3.2. Documents . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Forms . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Form Fields . . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Embedded Representations . . . . . . . . . . . . . . . . 7
3.7. Navigation . . . . . . . . . . . . . . . . . . . . . . . 7
3.8. History Traversal . . . . . . . . . . . . . . . . . . . . 9
4. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Data Structure . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Documents . . . . . . . . . . . . . . . . . . . . . . 10
4.1.2. Links . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.3. Forms . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.4. Embedded Representations . . . . . . . . . . . . . . 12
4.1.5. Directives . . . . . . . . . . . . . . . . . . . . . 12
4.2. Dictionaries . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Dictionary References . . . . . . . . . . . . . . . . 13
4.2.2. Media Type Parameter . . . . . . . . . . . . . . . . 13
5. Textual Format . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Lexical Structure . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Line Terminators . . . . . . . . . . . . . . . . . . 15
5.1.2. White Space . . . . . . . . . . . . . . . . . . . . . 15
5.1.3. Comments . . . . . . . . . . . . . . . . . . . . . . 15
5.1.4. Identifiers . . . . . . . . . . . . . . . . . . . . . 15
5.1.5. IRIs and IRI References . . . . . . . . . . . . . . . 16
5.1.6. Literals . . . . . . . . . . . . . . . . . . . . . . 16
5.1.7. Punctuators . . . . . . . . . . . . . . . . . . . . . 19
5.2. Syntactic Structure . . . . . . . . . . . . . . . . . . . 20
5.2.1. Documents . . . . . . . . . . . . . . . . . . . . . . 20
5.2.2. Links . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2.3. Forms . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2.4. Embedded Representations . . . . . . . . . . . . . . 22
5.2.5. Directives . . . . . . . . . . . . . . . . . . . . . 23
6. Usage Considerations . . . . . . . . . . . . . . . . . . . . 24
6.1. Specifying CoRAL-based Applications . . . . . . . . . . . 24
6.1.1. Application Interfaces . . . . . . . . . . . . . . . 24
6.1.2. Resource Names . . . . . . . . . . . . . . . . . . . 25
6.1.3. Implementation Limits . . . . . . . . . . . . . . . . 25
6.2. Minting New Relation Types . . . . . . . . . . . . . . . 26
6.3. Expressing Registered Link Relation Types . . . . . . . . 27
6.4. Expressing Link Target Attributes . . . . . . . . . . . . 27
6.5. Expressing Simple RDF Statements . . . . . . . . . . . . 28
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6.6. Embedding CoRAL in CBOR Structures . . . . . . . . . . . 29
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
8.1. Media Type "application/coral+cbor" . . . . . . . . . . . 30
8.2. Media Type "text/coral" . . . . . . . . . . . . . . . . . 32
8.3. CoAP Content Formats . . . . . . . . . . . . . . . . . . 33
8.4. CBOR Tag . . . . . . . . . . . . . . . . . . . . . . . . 33
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1. Normative References . . . . . . . . . . . . . . . . . . 34
9.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Core Vocabulary . . . . . . . . . . . . . . . . . . 38
A.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 38
A.2. Form Relation Types . . . . . . . . . . . . . . . . . . . 38
A.3. Form Field Names . . . . . . . . . . . . . . . . . . . . 39
Appendix B. Default Dictionary . . . . . . . . . . . . . . . . . 39
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
The Constrained RESTful Application Language (CoRAL) is a language
for the description of typed connections between resources on the Web
("links"), possible operations on such resources ("forms"), as well
as simple resource metadata.
CoRAL is intended for driving automated software agents that navigate
a Web application based on a standardized vocabulary of link and form
relation types. It is designed to be used in conjunction with a Web
transfer protocol such as the Hypertext Transfer Protocol (HTTP)
[RFC7230] or the Constrained Application Protocol (CoAP) [RFC7252].
This document defines the CoRAL data and interaction model, as well
as two specialized CoRAL serialization formats.
The CoRAL data and interaction model is a superset of the Web Linking
model of RFC 8288 [RFC8288]. The data model consists of two primary
elements: "links" that describe the relationship between two
resources and the type of that relationship, and "forms" that
describe a possible operation on a resource and the type of that
operation. Additionally, the data model can describe simple resource
metadata in a way similar to the Resource Description Framework (RDF)
[W3C.REC-rdf11-concepts-20140225]. In contrast to RDF, the focus of
CoRAL however is on the interaction with resources, not just the
relationships between them. The interaction model derives from HTML
5 [W3C.REC-html52-20171214] and specifies how an automated software
agent can navigate between resources by following links and perform
operations on resources by submitting forms.
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The primary CoRAL serialization format is a compact, binary encoding
of links and forms in Concise Binary Object Representation (CBOR)
[RFC7049]. It is intended for environments with constraints on
power, memory, and processing resources [RFC7228] and shares many
similarities with the message format of the Constrained Application
Protocol (CoAP) [RFC7252]: For example, it uses numeric identifiers
instead of verbose strings for link and form relation types, and pre-
parses Uniform Resource Identifiers (URIs) [RFC3986] into (what CoAP
considers to be) their components, which simplifies URI processing
for constrained nodes a lot. As a result, link serializations in
CoRAL are often much more compact than equivalent serializations in
CoRE Link Format [RFC6690].
The secondary CoRAL serialization format is a lightweight, textual
encoding of links and forms that is intended to be easy to read and
write for humans. The format is loosely inspired by the syntax of
Turtle [W3C.REC-turtle-20140225] and is mainly intended for giving
examples.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Terms defined in this document appear in _cursive_ where they are
introduced.
2. Examples
[[NOTE TO READERS: Examples and test vectors will be provided on a
companion website.]]
3. 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.
3.1. Browsing Context
Borrowing from HTML 5 [W3C.REC-html52-20171214], each such agent
maintains a _browsing context_ in which the representations of Web
resources are processed. (In HTML 5, the browsing context typically
corresponds to a tab or window in a Web browser.)
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At any time, one representation in each browsing context is
designated the _active_ representation.
3.2. Documents
A resource representation in one of the CoRAL serialization formats
is called a CoRAL _document_. The Internationalized Resource
Identifier (IRI) [RFC3987] that was used to retrieve such a document
is called the document's _retrieval context_.
A CoRAL document consists of a list of zero or more links, forms, and
embedded resource representations, collectively called _elements_.
CoRAL serialization formats may define additional types of elements
for efficiency or convenience, such as a base for relative IRI
references [RFC3987].
3.3. Links
A _link_ describes a relationship between two resources on the Web
[RFC8288]. As defined in RFC 8288, it consists of a _link context_,
a _link relation type_, and a _link target_. In CoRAL, a link can
additionally have a nested list of zero or more elements, which take
the place of link target attributes.
A link can be viewed as a statement of the form "{link context} has a
{link relation type} resource at {link target}" where the link target
may be further described by nested elements.
The link relation type identifies the semantics of a link. In HTML 5
and RFC 8288, link relation types are typically denoted by an IANA-
registered name, such as "stylesheet" or "type". In CoRAL, they are
denoted by an IRI such as <http://www.iana.org/assignments/relation/
stylesheet> or <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>.
This allows for the creation of new link relation types without the
risk of collisions when from different organizations or domains of
knowledge. An IRI also can lead to documentation, schema, and other
information about the link relation type. These IRIs are primarily
used as identity tokens, though, and are compared using Simple String
Comparison (Section 5.1 of RFC 3987).
The link context and the link target are both either by an IRI or
(similarly to RDF) a literal. If the IRI scheme indicates a Web
transfer protocol such as HTTP or CoAP, then an agent can dereference
the IRI and navigate the browsing context to the referenced resource;
this is called _following the link_. A literal directly identifies a
value. This can be a Boolean value, an integer, a floating-point
number, a date/time value, a byte string, or a text string.
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A link can occur as a top-level element in a document or as a nested
element within a link. When a link occurs as a top-level element,
the link context implicitly is the document's retrieval context.
When a link occurs nested within a link, the link context of the
inner link is the link target of the outer link.
There are no restrictions on the cardinality of links; there can be
multiple links to and from a particular target, and multiple links of
the same or different types between a given link context and target.
However, the nested data structure constrains the description of a
resource graph to a tree: Links between linked resources can only be
described by further nesting links.
3.4. Forms
A _form_ provides instructions to an agent for performing an
operation on a Web resource. It consists of a _form context_, a
_form relation type_, a _request method_, and a _submission target_.
Additionally, a form may be accompanied by a list of _form fields_.
A form can be viewed as an instruction of the form "To perform a
{form relation type} operation on {form context}, make a {request
method} request to {submission target}" where the payload of the
request may be further described by any form fields.
The form relation type identifies the semantics of the operation.
Form relation types are denoted like link relation types by an IRI.
The form context is the resource on which an operation is ultimately
performed. To perform the operation, an agent needs to construct a
request with the specified request method and submission target as
the request IRI. The submission target typically is the same
resource as the form context, but may be a different resource.
Constructing and sending the request is called _submitting the form_.
If a form is accompanied by a list of form fields, as described in
the following section, then the agent also needs to construct a
payload that matches the specifications of the form fields and
include that in the request.
A form can occur as a top-level element in a document or as a nested
element within a link. When a form occurs as a top-level element,
the form context implicitly is the document's retrieval context.
When a form occurs nested within a link, the form context is the link
target of the enclosing link.
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3.5. Form Fields
Form fields provide further instructions to agents for constructing a
request payload.
A form field can directly identify one or more data items that need
to include in the request payload or can reference another resource
(such as a schema) that describes the structure of the payload. Form
fields may also provide other kinds of information, such as
acceptable media types for the payload.
A form field is a pair of a _form field name_ and a _form field
value_. The form field name identifies the semantics of the form
field. Form field names are denoted like link and form relation
types by an IRI. The form field value can either be an IRI, a
Boolean value, an integer, a floating-point number, a date/time
value, a byte string, or a text string.
3.6. Embedded Representations
When a document contains links to many resources and an agent needs a
representation of each link target, it may be inefficient to retrieve
each of these representations individually. To alleviate this,
documents can directly embed representations of resources.
An _embedded representation_ consists of a sequence of bytes, labeled
with _representation metadata_.
An embedded representation may be a full, partial, or inconsistent
version of the representation served from the IRI of the resource.
An embedded representation can occur as a top-level element in a
document or as a nested element within a link. When it occurs as a
top-level element, it provides an alternate representation of the
document's retrieval context. When it occurs nested within a link,
it provides a representation of link target of the enclosing link.
3.7. Navigation
An agent begins interacting with an application by performing a GET
request on an _entry point IRI_. The entry point IRI is the only IRI
an agent is expected to know before interacting with an application.
From there, the agent is expected to make all requests by following
links and submitting forms provided by the server in responses. The
entry point IRI can be obtained by manual configuration or through
some discovery process.
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If dereferencing the entry point IRI yields a CoRAL document (or any
other representation that implements the CoRAL data and interaction
model), then the agent makes this document the active representation
in the browsing context and proceeds as follows:
1. The first step for the agent is to decide what to do next, i.e.,
which type of link to follow or form to submit, based on the link
and form relation types it understands.
2. The agent then finds the link(s) or form(s) with the respective
relation type in the active representation. This may yield one
or more candidates, from which the agent will have to select the
most appropriate one. The set of candidates may be empty, for
example, when a transition is not supported or not allowed.
3. The agent selects one of the candidates based on the metadata
associated with each of these. Metadata includes the content
type of the target resource representation, the IRI scheme, the
request method, and other information that is provided as nested
elements in a link or form fields in a form.
If the selected candidate contains an embedded representation,
the agent MAY skip the following steps and immediately proceed
with step 8.
4. The agent obtains the _request IRI_ from the link target or
submission target. Fragment identifiers are not part of the
request IRI and MUST be separated from the rest of the IRI prior
to a dereference.
5. The agent constructs a new request with the request IRI. If the
agent is following a link, then the request method MUST be GET.
If the agent is submitting a form, then the request method MUST
be the one specified in the form. The request IRI may need to be
converted to a URI (Section 3.1 of RFC 3987) for protocols that
do not support IRIs.
The agent SHOULD set HTTP header fields and CoAP request options
according to metadata associated with the link or form (e.g., set
the HTTP Accept header field or the CoAP Accept option when the
media type of the target resource is provided). In the case of a
form with one or more form fields, the agent also MUST include a
request payload that matches the specifications of the form
fields.
6. The agent sends the request and receives the response.
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7. If a fragment identifier was separated from the request IRI, the
agent dereferences the fragment identifier within the received
representation.
8. The agent _updates the browsing context_ by making the (embedded
or received) representation the active representation.
9. Finally, the agent processes the representation according to the
semantics of the content type. If the representation is a CoRAL
document (or any other representation that implements the CoRAL
data and interaction model), this means the agent has the choice
of what to do next again -- and the cycle repeats.
3.8. History Traversal
A browsing context MAY entail a _session history_ that lists the
resource representations that the agent has processed, is processing,
or will process.
An entry in the session history consists of a resource representation
and the request IRI that was used to retrieve the representation.
New entries are added to the session history as the agent navigates
from resource to resource.
An agent can navigate a browsing context by _traversing the session
history_ in addition to following links and submitting forms. For
example, if an agent received a representation that doesn't contain
any further links or forms, it can revert the active representation
back to one it has visited earlier.
Traversing the history should take advantage of caches to avoid new
requests. An agent MAY reissue a safe request (e.g., a GET request)
if it doesn't have a fresh representation in its cache. An agent
MUST NOT reissue an unsafe request (e.g., a PUT or POST request).
4. Binary Format
This section defines the encoding of documents in the CoRAL binary
format.
A document in the binary format is a data item in Concise Binary
Object Representation (CBOR) [RFC7049]. The structure of this data
item is presented in the Concise Data Definition Language (CDDL)
[I-D.ietf-cbor-cddl]. The media type is "application/coral+cbor".
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4.1. Data Structure
The data structure of a document in the binary format is made up of
four kinds of elements: links, forms, embedded representations, and
(as an extension to the CoRAL data model) base directives. Base
directives provide a way to encode IRIs with a common base more
efficiently.
Elements are processed in the order they appear in the document.
Document processors need to maintain an _environment_ while iterating
an array of elements. The environment consists of two variables: the
_current context_ and the _current base_. Both the current context
and the current base are initially set to the document's retrieval
context.
4.1.1. Documents
The body of a document in the binary format is encoded as an array of
zero or more links, forms, embedded representations, and directives.
body = [*(link / form / representation / directive)]
4.1.2. Links
A link is encoded as an array that consists of the unsigned integer
2, followed by the link relation type and the link target, optionally
followed by a link body that contains nested elements.
link = [link: 2, relation, link-target, ?body]
The link relation type is encoded as a text string that conforms to
the syntax of an IRI [RFC3987].
relation = text
The link target is denoted by an IRI reference or represented by a
literal value. An IRI reference MUST be resolved against the current
base. The encoding of and resolution process for IRI references in
the binary format is described in RFC XXXX [I-D.hartke-t2trg-ciri].
The link target may be null, which indicates that the link target is
an unidentified resource.
link-target = ciri / literal
ciri = <Defined in Section X of RFC XXXX>
literal = bool / int / float / time / bytes / text / null
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The array of elements in the link body, if any, MUST be processed in
a fresh environment. Both the current context and the current base
in the new environment are initially set to the link target of the
enclosing link.
4.1.3. Forms
A form is encoded as an array that consists of the unsigned integer
3, followed by the form relation type, the request method, and the
submission target, optionally followed by a list of form fields.
form = [form: 3, relation, method, submission-target, ?form-
fields]
The form relation type is defined in the same way as a link relation
type (Section 4.1.2).
The method MUST refer to one of the request methods defined by the
Web transfer protocol identified by the scheme of the submission
target. It is encoded either as a text string or an unsigned
integer.
method = text / uint
For HTTP [RFC7230], the method MUST be encoded as 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. For
CoAP [RFC7252], the method MUST be encoded as an unsigned integer
(e.g., the unsigned integer 2 for the POST method); the set of
possible values is maintained in the IANA CoAP Method Codes Registry.
The submission target is denoted by an IRI reference. This IRI
reference MUST be resolved against the current base.
submission-target = ciri
4.1.3.1. Form Fields
A list of form fields is encoded as an array of zero or more name-
value pairs.
form-fields = [*(form-field-name, form-field-value)]
The list, if any, MUST be processed in a fresh environment. Both the
current context and the current base in the new environment are
initially set to the submission target of the enclosing form.
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A form field name is defined in the same way as a link relation type
(Section 4.1.2).
form-field-name = text
A form field value can be an IRI reference, a Boolean value, an
integer, a floating-point number, a date/time value, a byte string, a
text string, or null. An IRI reference MUST be resolved against the
current base.
form-field-value = ciri / literal
4.1.3.2. Short Forms
[[NOTE TO READERS: This section used to describe special elements for
compressing certain forms that were assumed to occur frequently. The
topic of encoding frequently occurring elements more efficiently will
be revisited when more real-world examples are available.]]
4.1.4. Embedded Representations
An embedded representation is encoded as an array that consists of
the unsigned integer 0, followed by the HTTP content type or CoAP
content format of the representation and a byte string containing the
representation data.
representation = [representation: 0, text / uint, bytes]
For HTTP, the content type MUST be specified as a text string in the
format defined in Section 3.1.1.1 of RFC 7231 [RFC7231]; the set of
possible values is maintained in the IANA Media Types Registry. For
CoAP, the content format MUST be specified as an unsigned integer;
the set of possible values is maintained in the IANA CoAP Content-
Formats Registry.
4.1.5. Directives
Directives provide the ability to manipulate the environment when
processing a list of elements. There is one type of directives
available: the Base directive.
directive = base-directive
4.1.5.1. Base Directives
A Base directive is encoded as an array that consists of the negative
integer -1, followed by a base.
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base-directive = [base: -1, base]
The base is denoted by an IRI reference. This IRI reference MUST be
resolved against the current context (not the current base).
base = ciri
The directive is processed by resolving the IRI reference against the
current context and assigning the result to the current base.
4.2. Dictionaries
The binary format can reference values from a dictionary to reduce
representation size and processing cost. Dictionary references can
used in place of link relation types, link targets, form relation
types, submission targets, form field names, and form field values.
4.2.1. Dictionary References
A dictionary reference is encoded as an unsigned integer. Where a
dictionary reference cannot be expressed unambiguously, the unsigned
integer is tagged with CBOR tag TBD6.
relation /= uint
link-target /= #6.TBD6(uint)
submission-target /= #6.TBD6(uint)
form-field-name /= uint
form-field-value /= #6.TBD6(uint)
4.2.2. Media Type Parameter
The "application/coral+cbor" media type is defined to have a
"dictionary" parameter that specifies the dictionary in use. The
dictionary is identified by a URI [RFC3986]. For example, a CoRAL
document that uses the dictionary identified by the URI
<http://example.com/dictionary> can use the following content type:
application/coral+cbor; dictionary="http://example.com/dictionary"
The URI serves only as an identifier; it does not necessarily have to
be dereferencable (or even use a dereferencable URI scheme). It is
permissible, though, to use a dereferencable URI and to serve a
representation that provides information about the dictionary in a
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human- or machine-readable way. (The format of such a representation
is outside the scope of this document.)
For simplicity, a CoRAL document can reference values only from one
dictionary; the value of the "dictionary" parameter MUST be a single
URI. If the "dictionary" parameter is absent, the default dictionary
specified in Appendix B of this document is assumed.
Once a dictionary has made an assignment, the assignment MUST NOT be
changed or removed. A dictionary, however, may contain additional
information about an assignment, which may change over time.
In CoAP [RFC7252], media types (including specific values for media
type parameters) are encoded as an unsigned integer called "content
format". For use with CoAP, each new CoRAL dictionary MUST register
a new content format in the IANA CoAP Content-Formats Registry.
5. Textual Format
This section defines the syntax of documents in the CoRAL textual
format using two grammars: The lexical grammar defines how Unicode
characters are combined to form line terminators, white space,
comments, and tokens. The syntactic grammar defines how tokens are
combined to form documents. Both grammars are presented in Augmented
Backus-Naur Form (ABNF) [RFC5234].
A document in the textual format is a Unicode string in a Unicode
encoding form [UNICODE]. The media type for such documents is "text/
coral". The "charset" parameter is not used; charset information is
transported inside the document in the form of an OPTIONAL Byte Order
Mark (BOM). The use of the UTF-8 encoding scheme [RFC3629], without
a BOM, is RECOMMENDED.
5.1. Lexical Structure
The lexical structure of a document in the textual format is made up
of four basic elements: line terminators, white space, comments, and
tokens. Of these, only tokens are significant in the syntactic
grammar. There are five kinds of tokens: identifiers, IRIs, IRI
references, literals, and punctuators.
token = identifier / iri / iriref / literal / punctuator
When several lexical grammar rules match a sequence of characters in
a document, the longest match takes priority.
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5.1.1. 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.)
5.1.2. 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.
5.1.3. 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.
5.1.4. Identifiers
An identifier token is a user-defined symbolic name. The rules for
identifiers correspond to those recommended by the Unicode Standard
Annex #31 [UNICODE-UAX31] using the following profile:
identifier = START *CONTINUE *(MEDIAL 1*CONTINUE)
START = <Any character with the XID_Start property>
CONTINUE = <Any character with the XID_Continue property>
MEDIAL = "-" / "." / "~" / %x58A / %xF0B
MEDIAL =/ %x2010 / %x2027 / %x30A0 / %x30FB
All identifiers MUST be converted into Unicode Normalization Form C
(NFC), as defined by the Unicode Standard Annex #15 [UNICODE-UAX15].
Comparison of identifiers is based on NFC and is case-sensitive
(unless otherwise noted).
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5.1.5. IRIs and IRI References
IRIs and IRI references are Unicode strings that conform to the
syntax defined in RFC 3987 [RFC3987]. An IRI reference can be either
an IRI or a relative reference. Both IRIs and IRI references are
enclosed in angle brackets ("<" and ">").
iri = "<" IRI ">"
iriref = "<" IRI-reference ">"
IRI = <Defined in Section 2.2 of RFC 3987>
IRI-reference = <Defined in Section 2.2 of RFC 3987>
5.1.6. Literals
A literal is a textual representation of a value. There are seven
types of literals: Boolean, integer, floating-point, date/time, byte
string, text string, and null.
literal = boolean / integer / float / datetime / bytes / text
literal =/ null
5.1.6.1. Boolean Literals
The case-insensitive tokens "true" and "false" denote the Boolean
values true and false, respectively.
boolean = "true" / "false"
5.1.6.2. Integer Literals
Integer literals denote an integer value of unspecified precision.
By default, integer literals are expressed in decimal, but they can
also be specified in an alternate base using a prefix: Binary
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)
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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
5.1.6.3. Floating-point Literals
Floating-point literals denote a floating-point number of unspecified
precision.
Floating-point literals consist of a sequence of decimal digits
followed by a fraction, an exponent, or both. The fraction consists
of a decimal point (".") followed by a sequence of decimal digits.
The exponent consists of the letter "e" in upper- or lowercase,
followed by an optional sign and a sequence of decimal digits that
indicate a power of 10 by which the value preceding the "e" is
multiplied.
Negative floating-point values are expressed by prepending a minus
sign ("-").
float = ["+" / "-"] 1*DIGIT [fraction] [exponent]
fraction = "." 1*DIGIT
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"
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5.1.6.4. Date/Time Literals
Date/time literals denote an instant in time.
A date/time literal consists of a sequence of characters in Internet
date/time format [RFC3339], enclosed in dollar signs.
datetime = DOLLAR date-time DOLLAR
date-time = <Defined in Section 5.6 of RFC 3339>
DOLLAR = %x24
5.1.6.5. Byte String Literals
Byte string literals denote an ordered sequence of bytes.
A byte string literal consists of a prefix and zero or more bytes
encoded in Base16, Base32, or Base64 [RFC4648] and 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
SQUOTE = %x27
5.1.6.6. Text String Literals
Text string literals denote a Unicode string.
A text string literal consists of zero or more Unicode characters
enclosed in double quotes. It can include simple escape sequences
(such as \t for the tab character) as well as hexadecimal and Unicode
escape sequences.
text = DQUOTE *(char / %x5C escape) DQUOTE
char = <Any character except %x22, %x5C, and line terminators>
escape = simple-escape / hexadecimal-escape / unicode-escape
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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
5.1.6.7. Null Literal
The case-insensitive tokens "null" and "_" denote the intentional
absence of any value.
null = "null" / "_"
5.1.7. Punctuators
Punctuator tokens are used for grouping and separating.
punctuator = "#" / ":" / "*" / "[" / "]" / "{" / "}" / "=" / "->"
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5.2. Syntactic Structure
The syntactic structure of a document in the textual format is made
up of four kinds of elements: links, forms, embedded representations,
and (as an extension to the CoRAL data model) directives. Directives
provide a way to make documents easier to read and write by setting a
base for relative IRI references and introducing shorthands for IRIs.
Elements are processed in the order they appear in the document.
Document processors need to maintain an _environment_ while iterating
a list of elements. The environment consists of three variables: the
_current context_, the _current base_, and the _current mapping from
identifiers to IRIs_. Both the current context and the current base
are initially set to the document's retrieval context. The current
mapping from identifiers to IRIs is initially empty.
5.2.1. Documents
The body of a document in the textual format consists of zero or more
links, forms, embedded representations, and directives.
body = *(link / form / representation / directive)
5.2.2. Links
A link consists of the link relation type, followed by the link
target, optionally followed by a link body enclosed in curly brackets
("{" and "}").
link = relation link-target ["{" body "}"]
The link relation type is denoted by either an IRI, a simple name, or
a qualified name.
relation = iri / simple-name / qualified-name
A simple name consists of an identifier. It is resolved to an IRI by
looking up the empty string in the current mapping from identifiers
to IRIs and appending the specified identifier to the result. It is
an error if the empty string is not present in the current mapping.
simple-name = identifier
A qualified name consists of two identifiers separated by a colon
(":"). It is resolved to an IRI by looking up the identifier on the
left hand side in the current mapping from identifiers to IRIs and
appending the identifier on the right hand side to the result. It is
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an error if the identifier on the left hand side is not present in
the current mapping.
qualified-name = identifier ":" identifier
The link target is denoted by an IRI reference or represented by a
value literal. An IRI reference MUST be resolved against the current
base. If the link target is null, the link target is an unidentified
resource.
link-target = iriref / literal
The list of elements in the link body, if any, MUST be processed in a
fresh environment. Both the current context and current base in this
environment are initially set to the link target of the enclosing
link. The mapping from identifiers to IRIs is initially set to a
copy of the mapping from identifiers to IRIs in the current
environment.
5.2.3. Forms
A form consists of the form relation type, followed by a "->" token,
the request method, and the submission target, optionally followed by
a list of form fields enclosed in square brackets ("[" and "]").
form = relation "->" method submission-target ["[" form-fields
"]"]
The form relation type is defined in the same way as a link relation
type (Section 5.2.2).
The method identifier MUST refer to one of the request methods
defined by the Web transfer protocol identified by the scheme of the
submission target. Method identifiers are case-insensitive and
constrained to Unicode characters in the Basic Latin block.
method = identifier
For HTTP [RFC7230], the set of possible method identifiers is
maintained in the IANA HTTP Method Registry. For CoAP [RFC7252], the
set of possible method identifiers is maintained in the IANA CoAP
Method Codes Registry.
The submission target is denoted by an IRI reference. This IRI
reference MUST be resolved against the current base.
submission-target = iriref
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5.2.3.1. Form Fields
A list of form fields consists of zero or more name-value pairs.
form-fields = *(form-field-name form-field-value)
The list, if any, MUST be processed in a fresh environment. Both the
current context and the current base in this environment are
initially set to the submission target of the enclosing form. The
mapping from identifiers to IRIs is initially set to a copy of the
mapping from identifiers to IRIs in the current environment.
The form field name is defined in the same way as a link relation
type (Section 5.2.2).
form-field-name = iri / simple-name / qualified-name
The form field value can be an IRI reference, Boolean literal,
integer literal, floating-point literal, byte string literal, text
string literal, or null. An IRI reference MUST be resolved against
the current base.
form-field-value = iriref / literal
5.2.4. Embedded Representations
An embedded representation consists of a "*" token, followed by the
representation data, optionally followed by representation metadata
enclosed in square brackets ("[" and "]").
representation = "*" bytes ["[" representation-metadata "]"]
Representation metadata consists of zero or more name-value pairs.
representation-metadata = *(metadata-name metadata-value)
This document specifies only one kind of metadata item, labeled with
the name "type": the HTTP content type or CoAP content format of the
representation.
metadata-name = "type"
metadata-value = text / integer
For HTTP, the content type MUST be specified as a text string in the
format defined in Section 3.1.1.1 of RFC 7231 [RFC7231]; the set of
possible values is maintained in the IANA Media Types Registry. For
CoAP, the content format MUST be specified as an integer; the set of
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possible values is maintained in the IANA CoAP Content-Formats
Registry.
A metadata item with the name "type" MUST NOT occur more than once.
If absent, its value defaults to content type "application/octet-
stream" or content format 42.
5.2.5. Directives
Directives provide the ability to manipulate the environment when
processing a list of elements. All directives start with a number
sign ("#") followed by a directive identifier. Directive identifiers
are case-insensitive and constrained to Unicode characters in the
Basic Latin block.
The following two types of directives are available: the Base
directive and the Using directive.
directive = base-directive / using-directive
5.2.5.1. Base Directives
A Base directive consists of a number sign ("#"), followed by the
case-insensitive identifier "base", followed by a base.
base-directive = "#" "base" base
The base is denoted by an IRI reference. The IRI reference MUST be
resolved against the current context (not the current base).
base = iriref
The directive is processed by resolving the IRI reference against the
current context and assigning the result to the current base.
5.2.5.2. Using Directives
A Using directive consists of a number sign ("#"), followed by the
case-insensitive identifier "using", optionally followed by an
identifier and an equals sign ("="), finally followed by an IRI. If
the identifier is not specified, it is assumed to be the empty
string.
using-directive = "#" "using" [identifier "="] iri
The directive is processed by adding the specified identifier and IRI
to the current mapping from identifiers to IRIs. It is an error if
the identifier is already present in the mapping.
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6. Usage Considerations
This section discusses some considerations in creating CoRAL-based
applications and managing link and form relation types.
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:
o Resources are identified by IRIs or represented by value literals.
o Interactions are based on the hypermedia interaction model of the
Web and the methods provided by the Web transfer protocol. The
semantics of possible interactions are identified by link and form
relation types.
o Representations are CoRAL documents encoded in the binary format
defined in Section 4 or the textual format defined in Section 5.
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:
o IRI schemes that identify the Web transfer protocol(s) used in the
application.
o Internet media types that identify the representation format(s)
used in the application, including the media type(s) of the CoRAL
serialization format(s).
o Link relation types that identify the semantics of links.
o Form relation types that identify the semantics of forms.
Additionally, for each form relation type, the permissible request
method(s).
o Form field names that identify the semantics of form fields.
Additionally, for each form field name, the permissible form field
value(s) and/or type(s).
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6.1.2. Resource Names
Resource names -- i.e., URIs [RFC3986] and IRIs [RFC3987] -- 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 and IRIs often include structured application data in the path
and query components, such as paths in a filesystem or keys in a
database. It is a common practice in many HTTP-based application
programming interfaces (APIs) to make this part of the application
specification, i.e., to prescribe fixed URI templates that are hard-
coded in implementations. There are a number of problems with this
practice [RFC7320], though.
In CoRAL-based applications, resource names are therefore not part of
the application specification -- they are an implementation detail.
The specification of a CoRAL-based application MUST NOT mandate any
particular form of resource name structure. BCP 190 [RFC7320]
describes the problematic practice of fixed URI structures in more
detail and provides some acceptable alternatives.
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
wish to limit these numbers and specify implementation limits that an
application implementation must at least support to be interoperable.
Applications may also mandate the following and other restrictions:
o use of only either the binary format or the text format;
o use of only either HTTP or CoAP as supported Web transfer
protocol;
o use of only dictionary references in the binary format for certain
link relation types, link targets, form relation types, submission
targets, form field names, and form field values;
o use of only either content type strings or content format IDs;
o use of IRI references only up to a specific string length;
o use of CBOR in a canonical format (see Section 3.9 of RFC 7049).
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6.2. Minting New Relation Types
New link relation types, form relation types, and form field names
can be minted by defining an IRI [RFC3987] that uniquely identifies
the item. Although the IRI can point to a resource that contains a
definition of the semantics of the relation type, 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 thare normalized according to Section 5.3 of RFC 3987.
Non-normalized forms that are best avoided include:
o Uppercase characters in scheme names and domain names
o Percent-encoding of characters where it is not required by the IRI
syntax
o Explicitly stated HTTP default port (e.g., <http://example.com/>
is preferable over <http://example.com:80/>)
o Completely empty path in HTTP IRIs (e.g., <http://example.com/> is
preferable over <http://example.com>)
o Dot segments ("/./" or "/../") in the path component of an IRI
o Lowercase hexadecimal letters within percent-encoding triplets
(e.g., "%3F" is preferable over "%3f")
o Punycode-encoding of Internationalized Domain Names in IRIs
o IRIs that are not in Unicode Normalization Form C [UNICODE-UAX15]
IRIs that identify link relation types, form relation types, and form
field names 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 4.2. These
impose a much smaller overhead but instead need to be assigned by an
authority to avoid collisions.
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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>
Note that registered link relation types are required to be
lowercased as per Section 3.3 of RFC 8288 [RFC8288].
(The convention of appending the link relation type to the prefix
"http://www.iana.org/assignments/relation/" to form an IRI is adopted
from Atom [RFC4287]. See also Appendix A.2 of RFC 8288 [RFC8288].)
6.4. Expressing Link Target Attributes
[[NOTE TO READERS: This section describes a mechanism to convert any
link target attributes to CoRAL in a way that allows a conversion
back without loss of information (round-trip conversion). It is
likely that this will be replaced by a specific set of unique link
relation types that match the known RFC 6690 attributes semantically
but do not round-trip in the presence of unknown attributes.]]
Link target attributes defined for use with CoRE Link Format
[RFC6690] (such as "hreflang", "media", "title", "title*", "type",
"ct", "rt", "if", "sz", and "obs") can be expressed in CoRAL by
nesting links under the respective link. The link relation type of
each such nested link is the lowercased attribute name appended to
the IRI <http://TBD2/>.
If the expressed link target attribute has a value, the target of the
nested link MUST be a text string; otherwise, the target MUST be the
Boolean value "true".
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#using iana = <http://www.iana.org/assignments/relation/>
#using attr = <http://TBD2/>
iana:item </patches/1> {
attr:type "application/json-patch+json"
attr:ct "51"
attr:sz "247"
attr:obs true
}
<=>
</patches/1>; rel=item; type="application/json-patch+json";
ct=51; sz=247; obs
Language information in attributes as per RFC 8187 [RFC8187], such as
in "title*" attributes, is expressed by nesting an additional link of
type <http://TBD2/hreflang> under the link representing the
attribute. The target of the nested link MUST be a text string
containing a language tag [RFC5646]. The attribute name is expressed
without the "*" character.
#using iana = <http://www.iana.org/assignments/relation/>
#using attr = <http://TBD2/>
iana:terms-of-service </tos> {
attr:title "Nutzungsbedingungen" { attr:hreflang "de" }
attr:title "Terms of use" { attr:hreflang "en" }
}
<=>
</tos>; rel=terms-of-service;
title*=UTF-8'de'Nutzungsbedingungen;
title*=UTF-8'en'Terms%20of%20use
Link target attributes that actually do not describe the link target
but the link itself (such as "rel", "anchor", and "rev") are excluded
from this provision and MUST NOT occur in a CoRAL document.
6.5. Expressing Simple RDF Statements
An RDF statement [W3C.REC-rdf11-concepts-20140225] says that some
relationship, indicated by a predicate, holds between two resources.
RDF predicates can therefore be good source for vocabulary to provide
resource metadata. For example, a CoRAL document could use the FOAF
vocabulary [FOAF] to describe the person or software that made it:
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#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>
}
6.6. Embedding CoRAL in CBOR Structures
Data items in the CoRAL binary format (Section 4) may be embedded in
other CBOR [RFC7049] data structures. Specifications using CDDL
[I-D.ietf-cbor-cddl] SHOULD reference the following CDDL definitions
for this purpose:
CoRAL-Document = body
CoRAL-Link = link
CoRAL-Form = form
7. Security Considerations
Parsers of CoRAL documents must operate on input that is assumed to
be untrusted. This means that parsers MUST fail gracefully in the
face of malicious inputs. Additionally, parsers MUST be prepared to
deal with resource exhaustion (e.g., resulting from the allocation of
big data items) or exhaustion of the call stack (stack overflow).
See Section 8 of RFC 7049 [RFC7049] for security considerations
relating to CBOR.
Implementers of the CoRAL textual format need to consider the
security aspects of handling Unicode input. See the Unicode Standard
Annex #36 [UNICODE-UAX36] for security considerations relating to
visual spoofing and misuse of character encodings. See Section 10 of
RFC 3629 [RFC3629] for security considerations relating to UTF-8.
CoRAL makes extensive use of IRIs and URIs. See Section 8 of RFC
3987 [RFC3987] for security considerations relating to IRIs. See
Section 7 of RFC 3986 [RFC3986] for security considerations relating
to URIs.
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
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compromised if an entity providing a given string is connected to the
wrong account or online resource based on different interpretations
of the string. See RFC 6943 [RFC6943] for security considerations
relating to identifiers in IRIs and other places.
CoRAL is intended to be used in conjunction with a Web transfer
protocol like HTTP or CoAP. See Section 9 of RFC 7230 [RFC7230],
Section 9 of RFC 7231 [RFC7231], etc., for security considerations
relating to HTTP. See Section 11 of RFC 7252 [RFC7252] for security
considerations relating to CoAP.
CoRAL does not define any specific mechanisms for protecting the
confidentiality and integrity of CoRAL documents. It relies on
application layer or transport layer mechanisms for this, such as
Transport Layer Security (TLS) [RFC8446].
CoRAL documents and the structure of a web of resources revealed from
automatically following links can disclose personal information and
other sensitive information. Implementations need to prevent the
unintentional disclosure of such information. See Section of 9 of
RFC 7231 [RFC7231] for additional considerations.
Applications using CoRAL ought to consider the attack vectors opened
by automatically following, trusting, or otherwise using links and
forms in CoRAL documents. Notably, a server that is authoritative
for the CoRAL representation of a resource may not necessarily be
authoritative for nested elements in the document. See Section 5 of
RFC 8288 [RFC8288] for related considerations.
Unless an application mitigates this risk by specifying more specific
rules, any link or form in a document where the link or form context
and the document's retrieval context don't share the same Web origin
[RFC6454] MUST be discarded ("same-origin policy").
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:
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N/A
Optional parameters:
dictionary - See Section 4.2 of [I-D.hartke-t2trg-coral].
Encoding considerations:
binary - See Section 4 of [I-D.hartke-t2trg-coral].
Security considerations:
See Section 7 of [I-D.hartke-t2trg-coral].
Interoperability considerations:
N/A
Published specification:
[I-D.hartke-t2trg-coral]
Applications that use this media type:
See Section 1 of [I-D.hartke-t2trg-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.hartke-t2trg-coral].
Intended usage:
COMMON
Restrictions on usage:
N/A
Author:
See the Author's Address section of [I-D.hartke-t2trg-coral].
Change controller:
IESG
Provisional registration?
No
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8.2. Media Type "text/coral"
This document registers the media type "text/coral" according to the
procedures of BCP 13 [RFC6838] and guidelines in RFC 6657 [RFC6657].
Type name:
text
Subtype name:
coral
Required parameters:
N/A
Optional parameters:
N/A
Encoding considerations:
binary - See Section 5 of [I-D.hartke-t2trg-coral].
Security considerations:
See Section 7 of [I-D.hartke-t2trg-coral].
Interoperability considerations:
N/A
Published specification:
[I-D.hartke-t2trg-coral]
Applications that use this media type:
See Section 1 of [I-D.hartke-t2trg-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.hartke-t2trg-coral].
Intended usage:
COMMON
Restrictions on usage:
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N/A
Author:
See the Author's Address section of [I-D.hartke-t2trg-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].
o Content Type: application/coral+cbor
Content Coding: identity
ID: TBD3
Reference: [I-D.hartke-t2trg-coral]
o Content Type: text/coral
Content Coding: identity
ID: TBD4
Reference: [I-D.hartke-t2trg-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 7049 [RFC7049].
o Tag: TBD6
Data Item: unsigned integer
Semantics: Dictionary reference
Reference: [I-D.hartke-t2trg-coral]
[[NOTE TO RFC EDITOR: Please replace all occurrences of "TBD6" in
this document with the code point assigned by IANA.]]
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9. References
9.1. Normative References
[I-D.hartke-t2trg-ciri]
Hartke, K., "Constrained Internationalized Resource
Identifiers", draft-hartke-t2trg-ciri-01 (work in
progress), February 2019.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR and JSON data structures", draft-ietf-cbor-
cddl-06 (work in progress), November 2018.
[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>.
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[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>.
[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>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[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.
[UNICODE-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms",
<http://unicode.org/reports/tr15/>.
[UNICODE-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax",
<http://unicode.org/reports/tr31/>.
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[UNICODE-UAX36]
The Unicode Consortium, "Unicode Standard Annex #36:
Unicode Security Considerations",
<http://unicode.org/reports/tr36/>.
9.2. Informative References
[FOAF] Brickley, D. and L. Miller, "FOAF Vocabulary Specification
0.99", January 2014,
<http://xmlns.com/foaf/spec/20140114.html>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[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>.
[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>.
[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>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
[RFC8187] Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", RFC 8187,
DOI 10.17487/RFC8187, September 2017,
<https://www.rfc-editor.org/info/rfc8187>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[W3C.REC-html52-20171214]
Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
S. Moon, "HTML 5.2", World Wide Web Consortium
Recommendation REC-html52-20171214, December 2017,
<https://www.w3.org/TR/2017/REC-html52-20171214>.
[W3C.REC-rdf-schema-20140225]
Brickley, D. and R. Guha, "RDF Schema 1.1", World Wide Web
Consortium Recommendation REC-rdf-schema-20140225,
February 2014,
<http://www.w3.org/TR/2014/REC-rdf-schema-20140225>.
[W3C.REC-rdf11-concepts-20140225]
Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
Concepts and Abstract Syntax", World Wide Web Consortium
Recommendation REC-rdf11-concepts-20140225, February 2014,
<http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225>.
[W3C.REC-turtle-20140225]
Prud'hommeaux, E. and G. Carothers, "RDF 1.1 Turtle",
World Wide Web Consortium Recommendation REC-turtle-
20140225, February 2014,
<http://www.w3.org/TR/2014/REC-turtle-20140225>.
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[W3C.REC-webarch-20041215]
Jacobs, I. and N. Walsh, "Architecture of the World Wide
Web, Volume One", World Wide Web Consortium
Recommendation REC-webarch-20041215, December 2004,
<http://www.w3.org/TR/2004/REC-webarch-20041215>.
Appendix A. Core Vocabulary
This section defines the core vocabulary for CoRAL: a set of link
relation types, form relation types, and form field names.
[[NOTE TO RFC EDITOR: Please replace all occurrences of "urn:TBD1" in
this document with an IETF-controlled IRI, such as "urn:ietf:..." or
"http://...ietf.org/...".]]
A.1. 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://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].
A.2. Form Relation Types
<urn:TBD1#create>
Indicates that the form's context is a collection and that a new
item can be created in that collection by submitting a suitable
representation. This form relation type is typically used with
the POST method [RFC7231] [RFC7252].
<urn:TBD1#update>
Indicates that the form's context can be updated by submitting a
suitable representation. This form relation type is typically
used with the PUT method [RFC7231] [RFC7252], PATCH method
[RFC5789] [RFC8132], or iPATCH method [RFC8132].
<urn:TBD1#delete>
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Indicates that the form's context can be deleted. This form
relation type is typically used with the DELETE method [RFC7231]
[RFC7252].
<urn:TBD1#search>
Indicates that the form's context can be searched by submitting a
search query. This form relation type is typically used with the
POST method [RFC7231] [RFC7252] or FETCH method [RFC8132].
A.3. Form Field Names
<urn:TBD1#accept>
Specifies an acceptable HTTP content type or CoAP content format
for the request payload. There may be multiple form fields with
this name. If a form does not include a form field with this
name, the server accepts any or no request payload, depending on
the form relation type.
For HTTP, the content type MUST be specified as a text string in
the format defined in Section 3.1.1.1 of RFC 7231 [RFC7231]; the
set of possible values is maintained in the IANA Media Types
Registry. For CoAP, the content format MUST be specified as an
unsigned integer; the set of possible values is maintained in the
IANA CoAP Content-Formats Registry.
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.
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 = <urn:TBD1#create>
4 = <urn:TBD1#update>
5 = <urn:TBD1#delete>
6 = <urn:TBD1#search>
7 = <urn:TBD1#accept>
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Acknowledgements
Thanks to Christian Amsuess for helpful comments and discussions that
have shaped the document.
Author's Address
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
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