Thing-to-Thing Research Group K. Hartke
Internet-Draft Universitaet Bremen TZI
Intended status: Experimental October 30, 2017
Expires: May 3, 2018
The Constrained RESTful Application Language (CoRAL)
draft-hartke-t2trg-coral-04
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 3, 2018.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Web Linking . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Links, Forms, and Metadata . . . . . . . . . . . . . . . 5
3. Data and Interaction Model . . . . . . . . . . . . . . . . . 6
3.1. Browsing Context . . . . . . . . . . . . . . . . . . . . 7
3.2. Documents . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Forms . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Form Data . . . . . . . . . . . . . . . . . . . . . . . . 9
3.6. Navigation . . . . . . . . . . . . . . . . . . . . . . . 9
3.7. History Traversal . . . . . . . . . . . . . . . . . . . . 11
4. Binary Format . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Data Structure . . . . . . . . . . . . . . . . . . . . . 11
4.1.1. Documents . . . . . . . . . . . . . . . . . . . . . . 11
4.1.2. Links . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.3. Forms . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.4. Directives . . . . . . . . . . . . . . . . . . . . . 14
5. Textual Format . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Lexical Structure . . . . . . . . . . . . . . . . . . . . 15
5.1.1. Line Terminators . . . . . . . . . . . . . . . . . . 15
5.1.2. White Space . . . . . . . . . . . . . . . . . . . . . 15
5.1.3. Comments . . . . . . . . . . . . . . . . . . . . . . 15
5.1.4. Identifiers . . . . . . . . . . . . . . . . . . . . . 16
5.1.5. 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. Directives . . . . . . . . . . . . . . . . . . . . . 22
6. Usage Considerations . . . . . . . . . . . . . . . . . . . . 23
6.1. Specifying CoRAL-based Applications . . . . . . . . . . . 23
6.1.1. Naming Resources . . . . . . . . . . . . . . . . . . 24
6.1.2. Implementation Limits . . . . . . . . . . . . . . . . 24
6.2. Minting New Relation Types . . . . . . . . . . . . . . . 25
6.3. Registering Relation Types . . . . . . . . . . . . . . . 26
6.4. Expressing Link Target Attributes . . . . . . . . . . . . 27
6.5. Embedding CoRAL in CBOR Structures . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8.1. Media Type "application/coral+cbor" . . . . . . . . . . . 29
8.2. Media Type "text/coral" . . . . . . . . . . . . . . . . . 30
8.3. CoAP Content Formats . . . . . . . . . . . . . . . . . . 31
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Core Vocabulary . . . . . . . . . . . . . . . . . . 35
A.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 36
A.2. Form Relation Types . . . . . . . . . . . . . . . . . . . 36
A.3. Form Field Names . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Default Profile . . . . . . . . . . . . . . . . . . 37
Appendix C. CBOR-encoded IRI References . . . . . . . . . . . . 37
C.1. Data Structure . . . . . . . . . . . . . . . . . . . . . 38
C.2. Options . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.3. Properties . . . . . . . . . . . . . . . . . . . . . . . 40
C.4. Reference Resolution . . . . . . . . . . . . . . . . . . 41
C.5. IRI Recomposition . . . . . . . . . . . . . . . . . . . . 42
C.6. CoAP Encoding . . . . . . . . . . . . . . . . . . . . . . 45
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 47
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 described in RFC 8288 [RFC8288]. The CoRAL data model consists
of two elements: _links_ that describe the relationships between
pairs of resources and the type of those relationships, and _forms_
that describe possible operations on resources and the type of those
operations. In addition, the model can describe simple resource
metadata in a way similar to the Resource Description Framework (RDF)
[W3C.REC-rdf11-concepts-20140225]. However, in contrast to RDF, the
focus of CoRAL is on the interaction with resources, not just on the
relationships between them. The CoRAL interaction model derives from
HTML 5 [W3C.REC-html51-20161101] 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]: It uses numeric identifiers instead of
verbose strings for link and form relation types, and pre-parses URIs
into (what CoAP considers to be) their components, which simplifies
URI processing greatly. As a result, link serializations are often
much more compact than equivalent serializations in CoRE Link Format
[RFC6690], including its CBOR variant [I-D.ietf-core-links-json].
Additionally, CoRAL supports the serialization of forms, which CoRE
Link Format does not support.
The second CoRAL serialization format is a lightweight, textual
encoding of links and forms that is intended to be easy to read and
write by humans. The format is loosely inspired by the syntax of
Turtle [W3C.REC-turtle-20140225] and is used for giving examples
throughout the document.
1.1. Requirements Notation
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.
2. Examples
2.1. Web Linking
At its core, CoRAL is just yet another serialization format for Web
links. For example, if an HTTP client sends the following request:
GET /TheBook/chapter3 HTTP/1.1
Host: example.com
and receives the following response:
HTTP/1.1 200 OK
Content-Type: text/coral
#using
next <./chapter4>
icon
license
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then the representation contains the following three links:
o one link of type "http://www.iana.org/assignments/relation/next"
from to
,
o one link of type "http://www.iana.org/assignments/relation/icon"
from to , and
o one link of type "http://www.iana.org/assignments/relation/
license" from to
.
This representation is equivalent to the following Link header field
[RFC8288]:
Link: <./chapter4>; rel="next",
; rel="icon",
; rel="license"
and the following HTML 5 [W3C.REC-html51-20161101] link elements:
2.2. Links, Forms, and Metadata
In its entirety, CoRAL is an expressive language for describing Web
links between resources, possible operations on these resources, and
simple resource metadata. For example, if an HTTP client sends the
following request:
GET /tasks HTTP/1.1
Host: example.com
and receives the following response:
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HTTP/1.1 200 OK
Content-Type: text/coral
#using
#using coral =
task {
description "Pick up the kids"
}
task {
description "Return the books to the library"
coral:delete -> DELETE
}
coral:create -> POST [coral:accept "example/task"]
then the representation contains the following six elements:
o one link of type "http://example.org/vocabulary#task" from
to ,
o one link of type "http://example.org/vocabulary#description" from
to "Pick up the kids",
o one link of type "http://example.org/vocabulary#task" from
to ,
o one link of type "http://example.org/vocabulary#description" from
to "Return the books to the library",
o one form of type "urn:ietf:rfc:XXXX#delete" that can be used to
delete by making a DELETE request to
, and
o one form of type "urn:ietf:rfc:XXXX#create" that can be used to
create a new item in by making a POST
request to with an "example/task"
payload.
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.
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3.1. Browsing Context
Borrowing from HTML 5 [W3C.REC-html51-20161101], 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.)
A browsing context has a _session history_ that lists the resource
representations that the agent has processed, is processing, or will
process. At any time, one representation in each browsing context is
designated the _active_ representation.
A session history consists of a flat list of session history entries.
Each _session history entry_ consists of a resource representation
and the Internationalized Resource Identifier (IRI) [RFC3987] that
was used to retrieve the representation. An entry can additionally
have other information associated with it. New entries are added to
the session history as the agent navigates from resource to resource.
3.2. Documents
A resource representation in one of the CoRAL serialization formats
is called a CoRAL _document_. The IRI 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 IRI for relative IRI references.
3.3. Links
A _link_ describes a relationship between two resources on the Web
[RFC8288]. It consists of a _link context_, a _link relation type_,
and a _link target_. A link can additionally have a nested list of
zero or more elements, which take the place of link target attributes
in CoRAL.
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 links and forms.
The link relation type identifies the semantics of a link. In HTML 5
and the RFC 8288 Link header field, a link relation type is typically
denoted by a registered name, such as "stylesheet" or "icon". In
CoRAL, a link relation type is denoted by an IRI or an unsigned
integer. IRIs allow the creation of new, unique relation types in a
decentralized fashion but can incur a high overhead in terms of
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message size. Small, unsigned integers on the other hand minimize
the overhead of link relation types in constrained environments, but
require the assignment of values by a registry to avoid collisions.
The link context and the link target are both resources on the Web.
Resources are denoted in CoRAL either by an IRI reference [RFC3987]
or (similar to RDF) by 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, which can be Boolean, an integer, a floating-point number, a
byte string, or a text string.
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 is implicitly 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 CoRAL data model constrains the description of a web of
resources 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 IRI_.
Additionally, a form can be accompanied by _form data_.
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 IRI_" where the payload of the request
may be further described by form data.
The form relation type identifies the semantics of the operation.
Like link relation types, the form relation type is denoted by an IRI
or an unsigned integer.
The form context is the resource on which an operation is ultimately
performed. To perform the operation, an agent must construct a
request with the specified request method and submission IRI. The
submission IRI typically refers to the form context, but MAY refer to
another resource. Constructing and sending the request is called
_submitting the form_.
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If a form is accompanied by form data (Section 3.5), the agent MUST
also construct a payload that matches the specifications of the form
data, and include it in the same request when 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 is implicitly the document's retrieval context.
When a form occurs nested within a link, the form context is the link
target of the enclosing link.
3.5. Form Data
Form data provides instructions for agents to construct a request
payload. It consists of a list of zero or more _form fields_. Each
form field consists of a _form field name_ and a _form field value_.
Form fields can either directly identify data items that need to be
included in the request payload or reference an external resource
(such as a schema) that describes the data. They can also provide
other information, such as acceptable data formats.
The form field name identifies the semantics of the form field. Like
link and form relation types, a form field name is denoted by an IRI
or an unsigned integer.
The form field value can be an IRI, a Boolean value, an integer, a
floating-point number, a byte string, or a text string.
3.6. 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.
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 proceeds as follows:
1. The first step for the agent is to decide what to do next, i.e.,
which type of link to follow or form to submit, based on the link
relation types and form relation types it understands.
2. The agent finds the link(s) or form(s) with the given relation
type in the active representation. This may yield one or more
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candidates, from which the agent must select the most appropriate
one in the next step. The set of candidates MAY be empty, for
example, if a transition is not supported or not allowed.
3. The agent selects one of the candidates based on the metadata
associated with the link(s) or form(s). Metadata typically
includes the media type of the target resource representation,
the IRI scheme, the request method, and other information that is
provided as nested elements in a link and form data in a form.
4. The agent resolves the IRI reference in the link or form as
specified in Section 5 of RFC 3986 [RFC3986] to obtain the
_request IRI_. Fragment identifiers are not part of the request
IRI and MUST be separated from the rest of the IRI prior to a
dereference. The request IRI may need to be converted to a URI
(Section 3.1 of RFC 3987 [RFC3987]) for protocols that do not
support IRIs.
5. The agent constructs a new request with the request IRI. If the
agent follows a link, the request method MUST be GET. If the
agent submits a form, the request method MUST be the one
specified in the form. The agent SHOULD set HTTP header fields
and CoAP request options according to provided metadata (e.g.,
set the HTTP Accept header field or the CoAP Accept option when
the media type of the target resource is provided). In case of a
form with form data, the agent MUST include a request payload
that matches the specifications of the form data.
6. The agent sends the request and retrieves the response.
7. If a fragment identifier was separated from the request IRI, the
agent dereferences the fragment identifier within the retrieved
representation.
8. The agent _updates the session history_: It removes all the
entries in the browsing context's session history after the
current entry. Then it appends a new entry at the end of the
history representing the new resource.
9. Finally, if response contains a CoRAL document or any other
representation that implements the CoRAL data and interaction
model, the agent can again decide what to do next and the cycle
repeats.
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3.7. History Traversal
An agent can also navigate a browsing context by traversing the
browsing context's session history. An agent can _traverse the
session history_ by updating the active representation to the that
entry.
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".
4.1. Data Structure
The data structure of a document in the binary format is made up of
three kinds of elements: links and forms, as defined by the CoRAL
data model, and base IRI directives. Base IRI directives provide a
way to encode IRI references with a common base more efficiently.
Elements are processed in the order they appear in the document.
Document processors need to maintain an _environment_ while iterating
an array of elements. The environment consists of three variables: a
_current context IRI_, a _current base IRI_, and a _current relation
type_. The current context IRI and current base IRI are initially
both set to the document's retrieval context. The current relation
type is initially set to the unsigned integer zero.
4.1.1. Documents
The body of a document in the binary format is encoded as an array of
zero or more links, forms, and directives.
body = [*(link / form / 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, target, ?body]
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The link relation type is encoded either as a text string containing
an absolute IRI reference or as an (unsigned or negative) integer
representing the difference to the current relation type. A link is
processed by updating the current relation type to the result of
adding the specified integer (or zero in the case of a text string)
to the current relation type.
relation = text / int
The link target is denoted by an IRI reference or represented by a
literal value. The IRI reference MAY be relative or absolute and
MUST be resolved against the current base IRI. The encoding of IRI
references in the binary format is described in Appendix C. The link
target MAY be null, which indicates that the link target is an
unidentified resource.
target = iri / literal / null
literal = bool / int / float / bytes / text
The array of elements in the link body (if any) MUST be processed in
a fresh environment. The current context IRI and current base IRI in
the new environment are initially both set to the link target of the
enclosing link. The current relation type in the new environment is
initially set to the current relation type.
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 submission method, and a
submission IRI reference, optionally followed by form data.
form = [form: 3, relation, method, iri, ?form-data]
The form relation type is encoded and processed 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 IRI.
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
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(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 IRI reference MAY be relative or absolute and MUST be
resolved against the current base IRI. The encoding of IRI
references in the binary format is described in Appendix C.
4.1.3.1. Form Data
Form data is encoded as an array of zero or more name-value pairs.
form-data = [*(form-field-name, form-field-value)]
Form data (if any) MUST be processed in a fresh environment. The
current context IRI and current base IRI in the new environment are
initially both set to the submission IRI of the enclosing form. The
current relation type in the new environment is initially set to the
current relation type.
A form field name is encoded and processed in the same way as a link
relation type (Section 4.1.2).
form-field-name = text / uint
A form field value can be an IRI reference, a Boolean value, an
integer, a floating-point number, a byte string, a text string, or
null. An IRI reference MAY be relative or absolute and MUST be
resolved against the current base IRI. The encoding of IRI
references in the binary format is described in Appendix C.
form-field-value = iri / bool / int / float / bytes / text / null
4.1.3.2. Short Forms
Forms in certain shapes can be encoded in a more efficient manner
using short forms. The following short forms are available:
form =/ [form.create: 4, ?accept: uint .size 2]
form =/ [form.update: 5, ?accept: uint .size 2]
form =/ [form.delete: 6]
If the scheme of the submission IRI indicates HTTP, the short forms
expand as follows:
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[4] -> [3, "urn:ietf:rfc:XXXX#create", "POST", []]
[4, x] -> [3, "urn:ietf:rfc:XXXX#create", "POST", [],
["urn:ietf:rfc:XXXX#accept", x]]
[5] -> [3, "urn:ietf:rfc:XXXX#update", "PUT", []]
[5, x] -> [3, "urn:ietf:rfc:XXXX#update", "PUT", [],
["urn:ietf:rfc:XXXX#accept", x]]
[6] -> [3, "urn:ietf:rfc:XXXX#delete", "DELETE", []]
If the scheme of the submission IRI indicates CoAP, the short forms
expand as follows:
[4] -> [3, "urn:ietf:rfc:XXXX#create", 2, []]
[4, x] -> [3, "urn:ietf:rfc:XXXX#create", 2, [],
["urn:ietf:rfc:XXXX#accept", x]]
[5] -> [3, "urn:ietf:rfc:XXXX#update", 3, []]
[5, x] -> [3, "urn:ietf:rfc:XXXX#update", 3, [],
["urn:ietf:rfc:XXXX#accept", x]]
[6] -> [3, "urn:ietf:rfc:XXXX#delete", 4, []]
The form relation types and form field names in the above expansions
are defined in Appendix A.
4.1.4. Directives
Directives provide the ability to manipulate the environment when
processing a list of elements. There is one directive available: the
Base URI directive.
directive = base-directive
4.1.4.1. Base URI Directives
A Base IRI directive is encoded as an array that consists of the
unsigned integer 1, followed by an IRI reference.
base-directive = [base: 1, iri]
The IRI reference MAY be relative or absolute and MUST be resolved
against the current context IRI. The encoding of IRI references in
the binary format is described in Appendix C.
The directive is processed by resolving the IRI reference against the
current context IRI and assigning the result to the current base IRI.
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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 the 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 four kinds of tokens: identifiers, IRI
references, literals, and punctuators.
When several lexical grammar rules match a sequence of characters in
a document, the longest match takes priority.
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.
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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 tokens 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 =
continue =
medial = "-" / "." / "~" / %xB7 / %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).
5.1.5. IRI References
An IRI reference is a Unicode string that conforms to the syntax
defined in RFC 3987 [RFC3987]. An IRI reference can be absolute or
relative and can contain a fragment identifier. IRI references are
enclosed in angle brackets ("<" and ">").
iri = "<" IRI-reference ">"
IRI-reference =
5.1.6. Literals
A literal is a textual representation of a value. There are six
types of literals: Boolean, integer, floating-point, byte string,
text string, and 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"
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5.1.6.2. Integer Literals
Integer literals denote integer values 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
5.1.6.3. Floating-point Literals
Floating-point literals denote floating-point numbers 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 ("-").
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floating-point = ["+" / "-"] 1*DIGIT [fraction] [exponent]
fraction = "." 1*DIGIT
exponent = (%x45 / %x65) ["+" / "-"] 1*DIGIT
Floating-point literals can additionally denote the special "Not-
a-Number" (NaN) value, positive infinity, and 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".
floating-point =/ "NaN"
floating-point =/ ["+" / "-"] "Infinity"
5.1.6.4. Byte String Literals
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 SQUOTE
base32 = %x62.33.32 SQUOTE SQUOTE
base64 = %x62.36.34 SQUOTE SQUOTE
SQUOTE = %x27
5.1.6.5. Text String Literals
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 =
escape = simple-escape / hexadecimal-escape / unicode-escape
simple-escape = %x30 / %x62 / %x74 / %x6E / %x76
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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
5.1.6.6. 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 three kinds of elements: links and forms, as defined by the
CoRAL data model, and directives. Directives provide a way to make
documents easier to read and write by defining a base IRI 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: a
_current context IRI_, a _current base IRI_, and a _current mapping
from identifiers to IRIs_. The current context IRI and current base
IRI are initially both 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, and directives.
body = *(link / form / 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 target ["{" body "}"]
The link relation type is denoted either by an absolute IRI
reference, a simple name, a qualified name, or an unsigned integer.
relation = iri / simple-name / qualified-name / integer
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 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 mapping.
qualified-name = identifier ":" identifier
The link target is denoted by an IRI reference or represented by a
value literal. The IRI reference MAY be relative or absolute and
MUST be resolved against the current base IRI. If the link target is
null, the link target is an unidentified resource.
target = iri / literal / null
literal = boolean / integer / floating-point / bytes / text
The list of elements in the link body (if any) MUST be processed in a
fresh environment. The current context IRI and current base IRI in
this environment are initially both 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,
a method identifier, and a submission IRI reference, optionally
followed by form data enclosed in square brackets ("[" and "]").
form = relation "->" method iri ["[" form-data "]"]
The form relation type is denoted in the same way as a link relation
type (Section 5.2.2).
The method identifier refers to one of the request methods defined by
the Web transfer protocol identified by the scheme of the submission
IRI. 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 IRI reference MAY be relative or absolute and MUST be
resolved against the current base IRI.
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5.2.3.1. Form Data
Form data consists of zero or more name-value pairs.
form-data = *(form-field-name form-field-value)
Form data MUST be processed in a fresh environment. The current
context IRI and current base IRI in this environment are initially
both set to the submission IRI 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 denoted in the same way as a link relation
type (Section 5.2.2).
form-field-name = iri / simple-name / qualified-name / integer
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 MAY be relative or
absolute and MUST be resolved against the current base IRI.
form-field-value = iri / boolean / integer
form-field-value =/ floating-point / bytes / text / null
5.2.4. 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 directives are available: Base IRI directives and Using
directives.
directive = base-directive / using-directive
5.2.4.1. Base IRI Directives
A Base IRI directive consists of a number sign ("#"), followed by the
case-insensitive identifier "base", followed by an IRI reference.
base-directive = "#" "base" iri
The IRI reference MAY be relative or absolute and MUST be resolved
against the current context IRI.
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The directive is processed by resolving the IRI reference against the
current context IRI and assigning the result to the current base IRI.
5.2.4.2. 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 absolute
IRI reference. If the identifier is not specified, it is assumed to
be the empty string.
using-directive = "#" "using" [identifier "="] iri
The IRI reference MUST be absolute.
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.
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
can be used.
Specifications for CoRAL-based applications need to list the specific
components used in the application and their identifiers. This
SHOULD include at least the following items:
o IRI schemes that identify the Web transfer protocol(s) used in the
application.
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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) or type(s).
6.1.1. Naming Resources
Resource names -- i.e., URIs [RFC3986] and IRIs [RFC3987] -- are a
cornerstone of Web-based applications. They enable 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 applications to
make this part of the application specification, i.e., they prescribe
fixed URI templates that are hard-coded in implementations. However,
there are a number of problems with this practice [RFC7320].
In CoRAL-based applications, resource names are 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.
6.1.2. 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 that run in constrained environments) MAY
wish to limit these numbers and specify implementation limits that an
application implementation MUST at least support to be interoperable.
Implementation limits MAY also include the following as well as other
items:
o use of only either the binary format or the text format;
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o use of only either HTTP or CoAP as the Web transfer protocol;
o use of only either IRIs or unsigned integers to denote link
relation types, form relation types, and form field names;
o use of only either short forms or long forms in the binary format;
o use of IRI references only up to a specific length;
o use of CBOR in a canonical format (Section 3.9 of RFC 7049
[RFC7049]).
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.
Link relation types registered in the IANA Link Relations Registry,
such as "collection" [RFC6573] or "icon" [W3C.REC-html51-20161101],
can be used in CoRAL by appending the registered name to the IRI
:
#using iana =
iana:collection
iana:icon
A good source for link relation types for resource metadata are RDF
predicates [W3C.REC-rdf11-concepts-20140225]. An RDF statement says
that some relationship, indicated by a predicate, holds between two
resources. RDF predicates and link relation types can therefore
often be used interchangeably. For example, a CoRAL document could
describe its creator by using the FOAF vocabulary [FOAF]:
#using iana =
#using foaf =
foaf:maker _ {
iana:type
foaf:familyName "Hartke"
foaf:givenName "Klaus"
foaf:mbox
}
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6.3. Registering Relation Types
IRIs that identify link relation types, form relation types, and form
field names do not need to be registered. The inclusion of DNS 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
representations. This can be a problem in constrained environments
[RFC7228]. Therefore, CoRAL alternatively allows the use of unsigned
integers to identify link relation types, form relation types, and
form field names. These impose a much smaller overhead but instead
need to be assigned by a registry to avoid collisions.
This document does not create a registry for such integers. Instead,
the media types for CoRAL documents in the binary and textual format
are defined to have a "profile" parameter [RFC6906] that determines
the registry in use. The registry is identified by a URI [RFC3986].
For example, a CoRAL document that uses the registry identified by
the URI can use the following media
type:
application/coral+cbor; profile="http://example.com/registry"
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 serve a
representation that provides information about the registry in a
human- or machine-readable way. (The format of such a representation
is outside the scope of this document.)
For simplicity, a CoRAL document can use unsigned integers from at
most one registry. The "profile" parameter of the CoRAL media types
MUST contain a single URI, not a white space separated list of URIs
as recommended in RFC 6906 [RFC6906]. If the "profile" parameter is
absent, the default profile specified in Appendix B is assumed.
A CoRAL registry SHOULD map each unsigned integer to a full IRI that
identifies a link relation type, form relation type, or form field
name. The namespaces for these three kinds of identifiers are
disjoint, i.e., the same integer MAY be assigned to a link relation
type, form relation type, and form field name without ambiguity.
Once an integer has been assigned, the assignment MUST NOT be changed
or removed. A registry MAY provide additional information about an
assignment (for example, whether a link relation type is deprecated).
In CoAP [RFC7252], media types (including specific values for their
parameters) are encoded as a small, unsigned integer called the
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content format. For use with CoAP, each CoRAL registry needs to
register a new content format in the IANA CoAP Content-Formats
Registry. Each such registered content format MUST specify a CoRAL
media type with a "profile" parameter that contains the registry URI.
6.4. Expressing Link Target Attributes
Link target attributes defined for use with CoRE Link Format
[RFC6690] (such as "type", "hreflang", "media", "ct", "rt", "if", and
"sz") can be expressed in CoRAL by nesting links under the respective
link and specifying the attribute name appended to the IRI
as the link relation type. The target of such nested
links MUST be a text string literal:
#using iana =
#using attr =
iana:item {
attr:type "application/json-patch+json"
attr:ct "51"
}
[[NOTE TO RFC EDITOR: Please replace all occurrences of "http://TBD/"
with a RFC-Editor-controlled IRI.]]
Link target attributes that do not actually describe the link target
but the link itself (such as "rel", "anchor", "rev", "title", and
"title*") are excluded from this provision and MUST NOT occur in a
CoRAL document.
6.5. 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-Body = body
CoRAL-Link = link
CoRAL-Form = form
CoRAL-IRI = iri
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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 stack depth (stack overflow).
See Section 8 of RFC 7049 [RFC7049] for security considerations
relating to parsing 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
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 locations.
CoRAL is intended to be used in conjunction with a Web transfer
protocol such as HTTP or CoAP. See Section 9 of RFC 7320 [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) [RFC5246].
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.
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Applications using CoRAL ought to consider the attack vectors opened
by automatically following, trusting, or otherwise using links and
forms in CoRAL documents. In particular, a server that is
authoritative for the CoRAL representation of a resource may not
necessarily be the authoritative source for nested links and forms.
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:
profile - See Section 6.3 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): N/A
Macintosh file type code(s): N/A
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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
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:
profile - See Section 6.3 of [I-D.hartke-t2trg-coral].
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:
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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): TEXT
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
8.3. CoAP Content Formats
This document registers CoAP content formats for the media types
"application/coral+cbor" and "text/coral" according to the procedures
of RFC 7252 [RFC7252].
o Media Type: application/coral+cbor
Content Coding: identity
ID: TBD (maybe 63)
Reference: [I-D.hartke-t2trg-coral]
o Media Type: text/coral
Content Coding: identity
ID: TBD (maybe 10063)
Reference: [I-D.hartke-t2trg-coral]
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9. References
9.1. Normative References
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-00
(work in progress), July 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, .
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, .
[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,
.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, .
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, .
[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,
.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
.
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[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, .
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, .
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017, .
[UNICODE] The Unicode Consortium, "The Unicode Standard",
.
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",
.
[UNICODE-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax",
.
[UNICODE-UAX36]
The Unicode Consortium, "Unicode Standard Annex #36:
Unicode Security Considerations",
.
9.2. Informative References
[FOAF] Brickley, D. and L. Miller, "FOAF Vocabulary Specification
0.99", January 2014,
.
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[I-D.ietf-core-links-json]
Li, K., Rahman, A., and C. Bormann, "Representing
Constrained RESTful Environments (CoRE) Link Format in
JSON and CBOR", draft-ietf-core-links-json-09 (work in
progress), July 2017.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, .
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010, .
[RFC6573] Amundsen, M., "The Item and Collection Link Relations",
RFC 6573, DOI 10.17487/RFC6573, April 2012,
.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
.
[RFC6903] Snell, J., "Additional Link Relation Types", RFC 6903,
DOI 10.17487/RFC6903, March 2013, .
[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
DOI 10.17487/RFC6906, March 2013, .
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, .
[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,
.
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[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, .
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, .
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
.
[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,
.
[W3C.REC-html51-20161101]
Faulkner, S., Eicholz, A., Leithead, T., and A. Danilo,
"HTML 5.1", World Wide Web Consortium Recommendation REC-
html51-20161101, November 2016,
.
[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,
.
[W3C.REC-turtle-20140225]
Prud'hommeaux, E. and G. Carothers, "RDF 1.1 Turtle",
World Wide Web Consortium Recommendation REC-turtle-
20140225, February 2014,
.
[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,
.
Appendix A. Core Vocabulary
This section defines the core vocabulary for CoRAL. It is
RECOMMENDED that all CoRAL registries assign an unsigned integer to
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each of these link relation types, form relation types, and form
field names.
[[NOTE TO RFC EDITOR: Please replace all occurrences of
"urn:ietf:rfc:XXXX#" with a RFC-Editor-controlled IRI.]]
A.1. Link Relation Types
Indicates that the link's context is an instance of the type
specified as the link's target; see Section 6 of RFC 6903
[RFC6903].
This link relation type serves in CoRAL the same purpose as the
RDF predicate identified by the IRI .
Indicates that the link's context is a collection and that the
link's target is a member of that collection; see Section 2.1 of
RFC 6573 [RFC6573].
Indicates that the link's target is a collection and that the
link's context is a member of that collection; see Section 2.2 of
RFC 6573 [RFC6573].
A.2. Form Relation Types
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].
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] or PATCH method
[RFC5789] [RFC8132].
Indicates that the form's context can be deleted. This form
relation type is typically used with the DELETE method [RFC7231]
[RFC7252].
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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
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 Profile
This section defines a default registry that is assumed when a CoRAL
media type without a "profile" parameter is used.
Link Relation Types
0 =
1 =
2 =
Form Relation Types
0 =
1 =
2 =
3 =
Form Fields
0 =
Appendix C. CBOR-encoded IRI References
URI references [RFC3986] and, secondarily, IRI references [RFC3987]
and are the most common usage of resource identifiers in hypertext
representation formats such as HTML 5 [W3C.REC-html51-20161101] and
the CoRE Link Format [RFC6690]. They encode the components of a
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resource identifier either as an absolute URI/IRI or as a relative
reference that is resolved against a base URI/IRI.
URI and IRI references are sequences of characters chosen from
limited subsets of the repertoires of US-ASCII characters and Unicode
characters, respectively. The individual components of a URI or IRI
are delimited by several reserved characters, which necessitates the
use of percent-encoding for reserved characters in a non-delimiting
function. The resolution of references involves parsing URI/IRI
references into their components, combining the components with those
of the base URI/IRI, merging paths, removing dot segments, and
recomposing the result into a URI/IRI reference string.
Altogether, proper processing of URIs is quite complex. This can be
a problem in particular in constrained environments [RFC7228] with
severe code size limitations. As a result, many implementations in
these environments choose to implement only an ad-hoc, informally-
specified, bug-ridden, non-interoperable subset of half of RFC 3986.
This section specifies CBOR-encoded IRI References, a serialization
format for IRI references that encodes the IRI components as CBOR
data items rather than text. Assuming that a CBOR implementation is
already present, typical operations on CBOR-encoded IRI references
such as parsing, reference resolution, and comparison can be
implemented much more easily than with the text-based format. A full
implementation that covers all corner cases of the specification can
be implemented in a relatively small amount of code.
CBOR-encoded IRI References are not capable of expressing all IRI
references permitted by the syntax of RFC 3987 [RFC3987]. The
supported subset covers all CoAP URIs [RFC7252] and most HTTP URIs
[RFC7230].
C.1. Data Structure
The encoding is very similar to the encoding of the request URI in
CoAP messages [RFC7252]. The components of an IRI reference are
encoded as a sequence of _options_. Each option consists of an
_option number_ identifying the type of option (scheme, host name,
etc.) and the _option value_.
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iri = [?(scheme: 1, text),
?(host.name: 2, text //
host.ip: 3, bytes .size 4 / bytes .size 16),
?(port: 4, uint .size 2),
?(path.type: 5, path-type),
*(path: 6, text),
*(query: 7, text),
?(fragment: 8, text)]
path-type = &(absolute-path: 0,
append-path: 1,
relative-path: 2,
append-relation: 3)
C.2. Options
The following options are defined:
scheme
Specifies the IRI scheme. IRI schemes have the same syntax as URI
schemes. The option value therefore MUST match the "scheme" rule
defined in Section 3.1 of RFC 3986.
host.name
Specifies the host of the IRI authority as a registered name.
host.ip
Specifies the host of the IRI authority as an IPv4 address
(4 bytes) or an IPv6 address (16 bytes).
port
Specifies the port number. The option value MUST be an unsigned
integer in the range 0 to 65535 (inclusive).
path.type
Specifies the type of the IRI path for reference resolution.
Possible values are 0 (absolute-path), 1 (append-path), 2
(relative-path), and 3 (append-relation).
path
Specifies one segment of the IRI path. This option can occur more
than once.
query
Specifies one argument of the IRI query. This option can occur
more than once.
fragment
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Specifies the fragment identifier.
The value of a "host.name", "path", "query", and "fragment" option
can be any Unicode string. No percent-encoding is performed.
C.3. Properties
A sequence of options is considered _well-formed_ if:
o the sequence of options is empty or starts with a "scheme",
"host.name", "host.ip", "port", "path.type", "path", "query", or
"fragment" option;
o a "scheme" option is followed by either a "host.name" or a
"host.ip" option;
o a "host.name" option is followed by a "port" option;
o a "host.ip" option is followed by a "port" option;
o a "port" option is followed by a "path", "query", or "fragment"
option or is at the end of the sequence;
o a "path.type" option is followed by a "path", "query", or
"fragment" option or is at the end of the sequence;
o a "path" option is followed by a "path", "query", or "fragment"
option or is at the end of the sequence;
o a "query" option is followed by a "query" or "fragment" option or
is at the end of the sequence; and
o a "fragment" option is at the end of the sequence.
A well-formed sequence of options is considered _absolute_ if the
sequence of options starts with a "scheme" option. A well-formed
sequence of options is considered _relative_ if the sequence of
options is empty or starts with an option other than the "scheme"
option.
An absolute sequence of options is considered _normalized_ if the
result of resolving the sequence of options against any base IRI
reference is equal to the input. (It doesn't matter what it is
resolved against, since it is already absolute.)
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C.4. Reference Resolution
This section defines how to resolve a CBOR-encoded IRI reference that
might be relative to a given base IRI.
Applications MUST resolve a well-formed sequence of options `href`
against an absolute sequence of options `base` by using an algorithm
that is functionally equivalent to the following Python 3.5 code.
def resolve(base, href, relation=None):
if not is_absolute(base) or not is_well_formed(href):
return None
result = []
type = PathType.RELATIVE_PATH
(option, value) = href[0]
if option == Option.HOST_IP:
option = Option.HOST_NAME
elif option == Option.PATH_TYPE:
href = href[1:]
type = value
option = Option.PATH
if option != Option.PATH or type == PathType.ABSOLUTE_PATH:
_copy_until(base, result, option)
else:
_copy_until(base, result, Option.QUERY)
if type == PathType.APPEND_RELATION:
_append_and_normalize(result, Option.PATH,
format(relation, "x"))
return result
if type == PathType.RELATIVE_PATH:
_remove_last_path_segment(result)
_copy_until(href, result, Option.END)
_append_and_normalize(href, Option.END, None)
return result
def _copy_until(input, output, end):
for (option, value) in input:
if option >= end:
break
_append_and_normalize(output, option, value)
def _append_and_normalize(output, option, value):
if option == Option.PATH:
if value == ".":
return
if value == "..":
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_remove_last_path_segment(output)
return
elif option > Option.PATH:
if len(output) >= 2 and \
output[-1] == (Option.PATH, "") and \
(output[-2][0] < Option.PATH_TYPE or \
output[-2] == (Option.PATH_TYPE, PathType.ABSOLUTE_PATH)):
_remove_last_path_segment(output)
if option >= Option.END:
return
output.append((option, value))
def _remove_last_path_segment(output):
if len(output) >= 1 and output[-1][0] == Option.PATH:
del output[-1]
C.5. IRI Recomposition
This section defines how to recompose an IRI from a sequence of
options that encodes an absolute IRI reference.
Applications MUST recompose an IRI from a sequence of options by
using an algorithm that is functionally equivalent to the following
Python 3.5 code.
To reduce variability, the hexadecimal notation when percent-encoding
octets SHOULD use uppercase letters. The text representation of IPv6
addresses SHOULD follow the recommendations in Section 4 of RFC 5952
[RFC5952].
def recompose(href):
if not is_absolute(href):
return None
result = ""
no_path = True
first_query = True
for (option, value) in href:
if option == Option.SCHEME:
result += value + ":"
elif option == Option.HOST_NAME:
result += "//" + _encode_ireg_name(value)
elif option == Option.HOST_IP:
result += "//" + _encode_ip_address(value)
elif option == Option.PORT:
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result += ":" + str(value)
elif option == Option.PATH:
result += "/" + _encode_path_segment(value)
no_path = False
elif option == Option.QUERY:
if no_path:
result += "/"
no_path = False
result += "?" if first_query else "&"
result += _encode_query_argument(value)
first_query = False
elif option == Option.FRAGMENT:
if no_path:
result += "/"
no_path = False
result += "#" + _encode_fragment(value)
if no_path:
result += "/"
no_path = False
return result
def _encode_ireg_name(s):
return "".join(c if _is_ireg_name_char(c) else
_encode_pct(c) for c in s)
def _encode_ip_address(b):
if len(b) == 4:
return ".".join(str(c) for c in b)
elif len(b) == 16:
return "[" + ... + "]" # see RFC 5952
def _encode_path_segment(s):
return "".join(c if _is_isegment_char(c) else
_encode_pct(c) for c in s)
def _encode_query_argument(s):
return "".join(c if _is_iquery_char(c) and c != "&" else
_encode_pct(c) for c in s)
def _encode_fragment(s):
return "".join(c if _is_ifragment_char(c) else
_encode_pct(c) for c in s)
def _encode_pct(s):
return "".join(
"%{0:0>2X}".format(c) for c in s.encode("utf-8"))
def _is_ireg_name_char(c):
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return _is_iunreserved(c) or _is_sub_delim(c)
def _is_isegment_char(c):
return _is_ipchar(c)
def _is_iquery_char(c):
return _is_ipchar(c) or _is_iprivate(c) or c == "/" or c == "?"
def _is_ifragment_char(c):
return _is_ipchar(c) or c == "/" or c == "?"
def _is_ipchar(c):
return _is_iunreserved(c) or _is_sub_delim(c) or \
c == ":" or c == "@"
def _is_iunreserved(c):
return _is_alpha(c) or _is_digit(c) or \
c == "-" or c == "." or c == "_" or c == "~" or \
_is_ucschar(c)
def _is_alpha(c):
return c >= "A" and c <= "Z" or c >= "a" and c <= "z"
def _is_digit(c):
return c >= "0" and c <= "9"
def _is_sub_delim(c):
return c == "!" or c == "$" or c == "&" or c == "'" or \
c == "(" or c == ")" or c == "*" or c == "+" or \
c == "," or c == ";" or c == "="
def _is_ucschar(c):
return c >= "\U000000A0" and c <= "\U0000D7FF" or \
c >= "\U0000F900" and c <= "\U0000FDCF" or \
c >= "\U0000FDF0" and c <= "\U0000FFEF" or \
c >= "\U00010000" and c <= "\U0001FFFD" or \
c >= "\U00020000" and c <= "\U0002FFFD" or \
c >= "\U00030000" and c <= "\U0003FFFD" or \
c >= "\U00040000" and c <= "\U0004FFFD" or \
c >= "\U00050000" and c <= "\U0005FFFD" or \
c >= "\U00060000" and c <= "\U0006FFFD" or \
c >= "\U00070000" and c <= "\U0007FFFD" or \
c >= "\U00080000" and c <= "\U0008FFFD" or \
c >= "\U00090000" and c <= "\U0009FFFD" or \
c >= "\U000A0000" and c <= "\U000AFFFD" or \
c >= "\U000B0000" and c <= "\U000BFFFD" or \
c >= "\U000C0000" and c <= "\U000CFFFD" or \
c >= "\U000D0000" and c <= "\U000DFFFD" or \
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c >= "\U000E1000" and c <= "\U000EFFFD"
def _is_iprivate(c):
return c >= "\U0000E000" and c <= "\U0000F8FF" or \
c >= "\U000F0000" and c <= "\U000FFFFD" or \
c >= "\U00100000" and c <= "\U0010FFFD"
C.6. CoAP Encoding
This section defines how to construct CoAP options from an absolute,
normalized, CBOR-encoded IRI Reference.
Applications MUST construct CoAP options by recomposing the sequence
of options to an IRI (Appendix C.5 of this document), mapping the IRI
to an URI (Section 3.1 of RFC 3987), and decomposing the URI into
CoAP options (Section 6.4 of RFC 7252).
The following illustrative Python 3.5 code is roughly equivalent to
this.
def coap(href, to_proxy=False):
if not is_absolute(href):
return None
result = b""
previous = 0
for (option, value) in href:
if option == Option.SCHEME:
pass
elif option == Option.HOST_NAME:
opt = 3 # Uri-Host
val = value.encode("utf-8")
result += _encode_coap_option(opt - previous, val)
previous = opt
elif option == Option.HOST_IP:
opt = 3 # Uri-Host
if len(value) == 4:
val = ".".join(str(c) for c in b).encode("utf-8")
elif len(value) == 16:
val = b"[" + ... + b"]" # see RFC 5952
result += _encode_coap_option(opt - previous, val)
previous = opt
elif option == Option.PORT:
opt = 7 # Uri-Port
val = value.to_bytes((value.bit_length() + 7) // 8, "big")
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result += _encode_coap_option(opt - previous, val)
previous = opt
elif option == Option.PATH:
opt = 11 # Uri-Path
val = value.encode("utf-8")
result += _encode_coap_option(opt - previous, val)
previous = opt
elif option == Option.QUERY:
opt = 15 # Uri-Query
val = value.encode("utf-8")
result += _encode_coap_option(opt - previous, val)
previous = opt
elif option == Option.FRAGMENT:
pass
if to_proxy:
(option, value) = href[0]
opt = 39 # Proxy-Scheme
val = value.encode("utf-8")
result += _encode_coap_option(opt - previous, val)
previous = opt
return result
def _encode_coap_option(delta, value):
length = len(value)
delta_nibble = _encode_coap_option_nibble(delta)
length_nibble = _encode_coap_option_nibble(length)
result = bytes([delta_nibble << 4 | length_nibble])
if delta_nibble == 13:
delta -= 13
result += bytes([delta])
elif delta_nibble == 14:
delta -= 256 + 13
result += bytes([delta >> 8, delta & 255])
if length_nibble == 13:
length -= 13
result += bytes([length])
elif length_nibble == 14:
length -= 256 + 13
result += bytes([length >> 8, length & 255])
result += value
return result
def _encode_coap_option_nibble(n):
if n < 13:
return n
elif n < 256 + 13:
return 13
elif n < 65536 + 256 + 13:
Hartke Expires May 3, 2018 [Page 46]
Internet-Draft Constrained RESTful Application Language October 2017
return 14
Author's Address
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
Hartke Expires May 3, 2018 [Page 47]