Thing-to-Thing Research Group K. Hartke
Internet-Draft Universitaet Bremen TZI
Intended status: Experimental October 17, 2016
Expires: April 20, 2017
The Constrained RESTful Application Language (CoRAL)
draft-hartke-t2trg-coral-01
Abstract
The Constrained RESTful Application Language (CoRAL) is a compact,
binary representation format for building RESTful, hypermedia-driven
applications that run in constrained environments.
Status of This Memo
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This Internet-Draft will expire on April 20, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Options . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Deletable . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Format . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Href.* . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Method . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.6. Relation . . . . . . . . . . . . . . . . . . . . . . . . 10
4.7. Title . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.8. Updatable . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 10
5.1. Establish a Base URI . . . . . . . . . . . . . . . . . . 10
5.2. Transform References . . . . . . . . . . . . . . . . . . 11
5.3. Remove Dot Segments . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. CoRAL Option Number Registry . . . . . . . . . . . . . . 12
7.2. Media Type . . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Structured Syntax Suffix . . . . . . . . . . . . . . . . 15
7.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 18
A.2. CoRE Lighting . . . . . . . . . . . . . . . . . . . . . . 23
A.3. CoRE Link Format . . . . . . . . . . . . . . . . . . . . 24
A.4. CoRE Interfaces . . . . . . . . . . . . . . . . . . . . . 25
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Constrained RESTful Environments (CoRE) realize the Web architecture
[W3C.REC-webarch-20041215] in a suitable form for constrained nodes
and networks [RFC7228].
In the Web, hypertext documents contain links and forms that allow a
user to navigate between resources and submit information to a server
for processing. By annotating these elements with machine-readable
link relation types [RFC5988] and form relation types, it is possible
to extend this interaction model to machine-to-machine communication.
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This document describes the Constrained RESTful Application Language
(CoRAL), a compact serialization format for Web links and forms that
is based on the Concise Binary Object Representation (CBOR) [RFC7049]
and that aligns closely with the Constrained Application Protocol
(CoAP) [RFC7252].
1.1. Terminology
Readers are expected to be familiar with the terms and concepts
described in [RFC5988] and [I-D.hartke-core-apps].
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
[RFC2119].
2. Model
CoRAL is designed for building hypermedia-driven Web applications in
which software agents navigate between resources by following links
and interact with resources by submitting forms.
Each agent maintains a "browsing context", an environment in which
resource representations are processed. (In the traditional Web, the
browsing context corresponds to a tab or window in a Web browser.) A
browsing context has a session history, which lists the resources
that that browsing context has visited, is visiting, or will visit.
At any time, one resource in a browsing context is designated the
"current" resource. Following a link or submitting a form causes the
browsing context to navigate to a new resource.
A link indicates a relationship between two resources, the link
context and the link target, and affords the navigation between these
two. The semantics of the relationship are identified by a link
relation type, which in CoRAL can be IANA-registered or application-
specific. To minimize round-trips, a link in CoRAL can optionally
embed a (complete or partial) representation of the link target.
Furthermore, a link target can be an anonymous resource in CoRAL; in
this case, the link turns into a "literal" which consists only of a
link relation type and a representation.
A form similarly indicates a relationship between two resources, the
form context and the form target, and affords the interaction with
the context through the submission of the form to the target. In
many cases, the target of a form is the same resource as the context,
but this is not required. The semantics of a form are identified by
a form relation type, which in CoRAL again can be IANA-registered or
application-specific. The submission of a form typically requires
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the agent to construct a payload that is included with the request.
For this purpose, a form indicates the acceptable content formats for
the payload and can optionally embed a detailed description of the
expected data, for example, in the form of a list of form fields.
(The syntax for such a description is outside this document's scope.)
The CoRAL interaction model is as follows:
1. The first step for an 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 current resource. This may yield one or more
candidates from which the agent must select the most appropriate
one. The set of candidates may be empty if the transition is not
allowed, for example, when the agent is unauthorized. The format
of links and forms in CoRAL is specified in Section 3.
3. The agent selects one of the candidates based on the metadata
associated with the link or form. Metadata can include the
content format of the target resource representation, the URI
scheme, the request method and other attributes that describe the
target. Metadata is encoded in CoRAL as CoAP-style options,
which are specified in Section 4.
4. The agent resolves the URI reference in the link or form to its
absolute form in order to obtain the "request URI". CoRAL
encodes URI references like CoAP as a sequence of options, which
significantly simplifies the implementation of URI processors.
The process of reference resolution is specified in Section 5.
5. The agent constructs a new request with the request URI. If the
agent follows a link, the request method is GET; if the agent
submits a form, the request method is indicated by an option.
The agent should set request parameters according to the link/
form attributes (e.g., set the CoAP Accept option when the
content format of the target resource is indicated). In the case
of a form, the agent also needs to construct a request payload
that matches the specifications of the form.
6. Finally, the agent sends the request and retrieves the response.
The agent processes the enclosed representation, updates the
browsing context to the new resource, and can decide again what
to do next.
An agent can furthermore navigate a browsing context by traversing
the browsing context's session history. The session history consists
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of a flat list of session history entries. Each session history
entry consists of a URI and may have other information associated
with it. Session history entries are added to the session history as
the agent navigates from resource to resource. An agent can traverse
the session history to any entry by updating the browsing context to
the resource for that entry.
3. Format
CoRAL can be used as a standalone representation format or embedded
in representations in other formats. As a standalone format, CoRAL
representations have the media type "application/coral" or a media
type derived from CoRAL with the structured syntax suffix "+coral".
CoRAL is typically embedded in CBOR-based media types, but other
media types can embed CoRAL as well. The CoRAL format is in all
cases the same.
The top-level structure of CoRAL is a CoRAL document. A CoRAL
document consists of a sequence of links, forms, literals and bases,
which are collectively called elements. Elements consist of a number
indicating the element type, a "href type" that indicates how CoRAL-
encoded URI references are to be interpreted in reference resolution,
a sequence of zero or more options and, optionally, a body.
Link, form and literal elements come in two flavors: a "fat" format
that includes all the items listed above, and a "tiny" format. The
tiny formats provide a concise way to express elements that match
certain patterns. Base elements are always in the "fat" format.
They encode a base URI for reference resolution and apply to all
subsequent elements until the next base element is encountered.
In the Web, link relation types are identified by strings, such as
"stylesheet", "terms-of-service" or "item". In order to minimize the
overhead of using these relation types in constrained environments,
[I-D.hartke-core-apps] extends the IANA Link Relation Types registry
with a numeric identifier for each type. CoRAL uses these (non-
negative) numeric identifiers instead of the textual names. The same
optimization is applied to form relation types, CoAP content formats
and CoAP request methods.
Applications can use negative numbers to indicate application-
specific link relation types, form relation types and content
formats, which do not need to be IANA-registered. The mappings from
numbers to textual names need to be provided by the respective media
type definition (i.e., a media type with the "+coral" suffix or a
media type embedding CoRAL; the "application/coral" media type itself
does not define any application-specific values).
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CoRAL defines a number of options that can be included in link, form,
literal and base elements. Options are used to encode the relation
type, the target resource URI and target attributes. They are
serialized as a sequence of unwrapped pairs where each pair consists
of a CoRAL option number and an option value. The pairs MUST be
sorted such that the option numbers are in ascending order.
Using the notation of [I-D.greevenbosch-appsawg-cbor-cddl], the CoRAL
data format can be expressed as follows:
document = [*element]
element = tiny-link / tiny-literal / tiny-form
/ fat-link / fat-literal / fat-form
/ base
tiny-link = [1, href-type, relation]
tiny-literal = [2, href-type, relation, format, body]
tiny-form = [3, href-type, relation, accept]
base = [4, href-type, options]
fat-link = [5, href-type, options, ?body]
fat-literal = [6, href-type, options, body]
fat-form = [7, href-type, options, ?body]
href-type = &(append-relation: 0,
absolute-path: 1,
append-path: 2,
relative-path: 3)
relation = int
format = int
accept = int
options = [*(option-number, option-value)]
option-number = uint
option-value = uint / int / text / bytes
body = bytes
The tiny formats expand as follows:
[1, H, R] -> [5, H, [1, R]]
[2, H, R, F, B] -> [6, H, [1, R, 4, F], B]
[3, H, R, A] -> [7, H, [1, R, 3, A]]
4. Options
Table 1 summarizes the CoRAL options defined in this document.
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+-----+---+----------------+--------+--------+-------------+
| No. | R | Name | Format | Length | Default |
+-----+---+----------------+--------+--------+-------------+
| 1 | x | Relation | int | | (none) |
| 2 | | Method | uint | | 2 (POST) |
| 3 | x | Accept | int | | (none) |
| 4 | x | Format | int | | (none) |
| 5 | | Href.Scheme | text | 1-255 | (none) |
| 6 | | Href.Host.Name | text | 1-255 | (none) |
| 7 | | Href.Host.IPv4 | bytes | 4 | (none) |
| 8 | | Href.Host.IPv6 | bytes | 16 | (none) |
| 9 | | Href.Port | uint | | (see below) |
| 10 | x | Href.Path | text | 0-255 | (none) |
| 11 | x | Href.Query | text | 0-255 | (none) |
| 12 | | Href.Fragment | text | 0-255 | (none) |
| 13 | | Title | text | 0-255 | (none) |
| 14 | | Updatable | bool | | false |
| 15 | | Deletable | bool | | false |
+-----+---+----------------+--------+--------+-------------+
Table 1: Options
The option properties are defined as follows:
Number: An option is identified by an option number.
Repeatable (R): An option that is repeatable MAY be included one or
more times in an element. An option that is not repeatable MUST
NOT be included more than once. If an agent encounters an option
with more occurrences than the option is defined for, each extra
occurrence MUST be ignored.
Format: Option values are defined to have a certain format, which is
the CBOR encoding of the specified type.
Length: Option values with types "text" and "bytes" are defined to
have a specific length, often in the form of an upper and lower
bound. The length of an option value MUST NOT be outside the
defined range. If an agent encounters an option with a length
outside the defined range, that option MUST be ignored.
Default Value: Options can be defined to have a default value. If
the value of an option is intended to be this default value, the
option SHOULD NOT be included in the element. If the option is
not present, the default value MUST be assumed.
The individual options are explained in the following subsections.
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4.1. Accept
The Accept Option indicates the acceptable content formats for the
representation included in a form submission.
The option value of an Accept Option is either one of the content
format IDs registered in the CoAP Content-Formats registry (>= 0) or
one of the application-specific content format IDs defined by the
media type (< 0).
If a form does not include an Accept Option, the service accepts any
content format.
4.2. Deletable
The Deletable Option, when present in a link, defines a form that can
be used to delete the link target. The context and target of that
form are the link target, the submission method is DELETE and no
representation must be submitted.
4.3. Format
The Format Option, when present in a link or a form, provides a hint
indicating what the content format of the payload of the CoAP
response should be when following the link or submitting the form.
Note that this is only a hint; it does not override the Content-
Format Option included in the CoAP response. If the Format Option
occurs more than once, an agent can set the Accept Option in its CoAP
request to request a particular content format.
The Format Option is REQUIRED if a link embeds a representation in
the link body. The Format Option is also REQUIRED in a literal. In
both cases the first occurrence of the option indicates the content
format of the embedded representation.
The option value of a Format Option is either one of the content
format IDs registered in the CoAP Content-Formats registry (>= 0) or
one of the application-specific content format IDs defined by the
media type (< 0).
4.4. Href.*
The Href.Scheme, Href.Host.Name, Href.Host.IPv4, Href.IPv6,
Href.Port, Href.Path, Href.Query and Href.Fragment Options are used
to specify the target resource URI of a link or form. They hold the
following values:
o the Href.Scheme Option specifies the URI scheme name,
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o the Href.Host.Name Option specifies the host as a registered name
[RFC3986],
o the Href.Host.IPv4 Option specifies the host as a 32-bit IPv4
address,
o the Href.Host.IPv6 Option specifies the host as a 128-bit IPv6
address,
o the Href.Port Option specifies the port number,
o each Href.Path Option specifies one segment of the path,
o each Href.Query Option specifies one argument of the query, and
o the Href.Fragment Option specifies the fragment identifier.
The Href.Host.Name, Href.Host.IPv4 and Href.Host.IPv6 options are
mutually exclusive.
The default value of the Href.Port Option is the default port for the
URI scheme.
Table 2 lists the permitted Href.* options by Href Type. A 'yes'
indicates that an option of this type MAY be present; a 'no'
indicates that an option of this type MUST NOT be present. The
resolution of a sequence of Href.* options against a base URI is
specified in Section 5.
+----------------+------------+------------+-----------+------------+
| | absolute- | relative- | append- | append- |
| | path | path | path | relation |
+----------------+------------+------------+-----------+------------+
| Href.Scheme | yes | no | no | no |
| Href.Host.Name | yes | no | no | no |
| Href.Host.IPv4 | yes | no | no | no |
| Href.Host.IPv6 | yes | no | no | no |
| Href.Port | yes | no | no | no |
| Href.Path | yes | yes | yes | no |
| Href.Query | yes | yes | yes | no |
| Href.Fragment | yes | yes | yes | no |
+----------------+------------+------------+-----------+------------+
Table 2: Permitted Href.* Options by Href Type
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4.5. Method
The Method Option, when present in a form, indicates the CoAP method
to use for form submission. The option value is one of the CoAP
method codes registered in the CoAP Method Codes registry. The
option value defaults to the POST method when the Method Option is
not present in a form.
4.6. Relation
The Relation Option indicates the link relation type of a link or
literal and the form relation type of a form. At least one Relation
Option is REQUIRED in each link, literal and form element.
The option value of a Relation Option is either one of the relation
type IDs registered in the Link Relation Types registry (>= 0) or one
of the application-specific relation type IDs defined by the media
type (< 0).
4.7. Title
The Title Option, when present, is used to label the target of a link
such that it can be used as a human-readable identifier (e.g., a menu
entry).
4.8. Updatable
The Updatable Option, when present in a link, defines a form that can
be used to update the link target. The context and target of that
form are the link target, the submission method is PUT and the
content format of the submitted representation must be one of the
formats indicated by the Format Option in the link.
5. Reference Resolution
This section defines the process of resolving a URI reference within
a link or form to an absolute URI suitable for inclusion in a CoAP
request.
5.1. Establish a Base URI
URI references can be relative and thus are only usable when a base
URI is known. This means that a base URI must be established before
the use of all URI references that might be relative.
The base URI of a reference in a link or form is established as
specified in Section 5.1 of [RFC3986]. CoRAL supports a "Base URI
Embedded in Content" in the form of base elements. A base element
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applies to all subsequent elements in a document until the next base
element is encountered. The URI reference in a base element itself
is resolved relative to the base URI of next lower precedence.
5.2. Transform References
The following pseudocode describes an algorithm for transforming a
URI reference R into its target URI T using the base URI B, the href
type H, and the link or form relation type S. The URI reference and
base URI are assumed to be pre-parsed into a sequence of Href.*
options; the result is returned as a sequence of Href.* options as
well.
if (R starts with Href.Scheme) then
T = R
elif (R starts with Href.Host.*) then
T = [ (k, v) | (k, v) <- B, k == Href.Scheme ] ++
[ (k, v) | (k, v) <- R, k > Href.Scheme ]
elif (R starts with Href.Port) then
T = [ (k, v) | (k, v) <- B, k < Href.Port ] ++
[ (k, v) | (k, v) <- R, k >= Href.Port ]
elif (H is append-relation) then
T = [ (k, v) | (k, v) <- B, k <= Href.Path ] ++
[ (Href.Path, (hex S)) ]
elif (H is append-path) then
T = [ (k, v) | (k, v) <- B, k <= Href.Path ] ++
[ (k, v) | (k, v) <- R, k >= Href.Path ]
elif (H is relative-path) then
T = [ (k, v) | (k, v) <- B, k < Href.Path ] ++
(init [ (k, v) | (k, v) <- B, k == Href.Path ]) ++
[ (k, v) | (k, v) <- R, k >= Href.Path ]
elif (H is absolute-path) then
T = [ (k, v) | (k, v) <- B, k < Href.Path ] ++
[ (k, v) | (k, v) <- R, k >= Href.Path ]
endif
The "init" function returns all the elements of the input list except
the last one. For example, (init [1, 2, 3]) returns [1, 2] and (init
[]) returns [].
The "hex" function returns a hexadecimal representation of the input
number. For example, (hex -421) returns "-1A5" and (hex 0) returns
"0".
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5.3. Remove Dot Segments
After transforming a the URI reference into its target URI, the
special path segments "." and ".." need to be removed. Although
there are many ways to accomplish this removal process, we describe a
simple method using two string buffers.
1. The input buffer is initialized with the sequence of path
segments and the output buffer is initialized to the empty
sequence.
2. While the input buffer is not empty, loop as follows:
* If the input buffer begins with a "." path segment, then
remove this segment from the input buffer; otherwise,
* if the input buffer begins with a ".." path segment, then
remove this segment from the input buffer and and remove the
last segment (if any) from the output buffer; otherwise,
* move the first path segment in the input buffer to the end of
the output buffer.
3. Finally, the sequence of path segments in the target URI is
replaced by the sequence of path segments in the output buffer.
6. Security Considerations
TODO.
7. IANA Considerations
7.1. CoRAL Option Number Registry
This document establishes the CoRAL Option Number registry for the
option numbers used in CoRAL. The registry is located within the
CoRE Parameters registry.
7.1.1. Registering New Option Numbers
Option numbers are registered on the advice of a Designated Expert
(appointed by the IESG or their delegate), with a Specification
Required (using terminology from [RFC5226]).
Registration requests consist of the completed registration template
below, typically published in an RFC. However, to allow for the
allocation of values prior to publication, the Designated Expert may
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approve registration once they are satisfied that a specification
will be published.
The registration template is:
o Option Number:
o Option Name:
o Reference:
7.1.2. Initial Registry Contents
The CoRAL Option Number registry's initial contents are:
o Option Number: 1
Option Name: Relation
Reference: [RFCXXXX]
o Option Number: 2
Option Name: Method
Reference: [RFCXXXX]
o Option Number: 3
Option Name: Accept
Reference: [RFCXXXX]
o Option Number: 4
Option Name: Format
Reference: [RFCXXXX]
o Option Number: 5
Option Name: Href.Scheme
Reference: [RFCXXXX]
o Option Number: 6
Option Name: Href.Host.Name
Reference: [RFCXXXX]
o Option Number: 7
Option Name: Href.Host.IPv4
Reference: [RFCXXXX]
o Option Number: 8
Option Name: Href.Host.IPv6
Reference: [RFCXXXX]
o Option Number: 9
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Option Name: Href.Port
Reference: [RFCXXXX]
o Option Number: 10
Option Name: Href.Path
Reference: [RFCXXXX]
o Option Number: 11
Option Name: Href.Query
Reference: [RFCXXXX]
o Option Number: 12
Option Name: Href.Fragment
Reference: [RFCXXXX]
o Option Number: 13
Option Name: Title
Reference: [RFCXXXX]
o Option Number: 14
Option Name: Updatable
Reference: [RFCXXXX]
o Option Number: 15
Option Name: Deletable
Reference: [RFCXXXX]
7.2. Media Type
This document registers the media type "application/coral" in the
"Media Types" registry.
Type name:
application
Subtype name:
coral
Required parameters:
N/A
Optional parameters:
N/A
Encoding considerations:
CoRAL is a binary encoding.
Security considerations:
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See Section 6 of [RFCXXXX].
Interoperability considerations:
There are no known interoperability issues.
Published specification:
[RFCXXXX]
Applications that use this media type:
Hypermedia-driven Web applications that run in constrained nodes
and networks.
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 "Author's Address" section of [RFCXXXX].
Intended usage:
COMMON
Restrictions on usage:
N/A
Author:
See "Author's Address" section of [RFCXXXX].
Change controller:
IESG
7.3. Structured Syntax Suffix
This document registers the suffix "+coral" in the "Structured Syntax
Suffix" registry.
Name:
Constrained RESTful Application Language (CoRAL)
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+suffix:
+coral
References:
[RFCXXXX]
Encoding considerations:
CoRAL is a binary format.
Interoperability considerations:
There are no known interoperability issues.
Fragment identifier considerations:
The syntax and semantics of fragment identifiers specified for
+coral are as specified for "application/coral". (At publication
of this document, there is no fragment identification syntax
defined for "application/coral".)
Security considerations:
See Section 6 of [RFCXXXX].
Contact:
See "Author's Address" section of [RFCXXXX].
Author/Change controller:
IESG
7.4. CoAP Content-Format
This document registers a content format for the "application/coral"
media type in the "CoAP Content-Formats" registry.
o Media Type: application/coral
Encoding: -
ID: 70
Reference: [RFCXXXX]
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
8.2. Informative References
[FOAF] Brickley, D. and L. Miller, "FOAF Vocabulary Specification
0.99", January 2014,
<http://xmlns.com/foaf/spec/20140114.html>.
[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C. and H. Birkholz, "CBOR data definition language
(CDDL): a notational convention to express CBOR data
structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
in progress), September 2016.
[I-D.hartke-core-apps]
Hartke, K., "CoRE Application Descriptions", draft-hartke-
core-apps-04 (work in progress), October 2016.
[I-D.hartke-core-lighting]
Hartke, K., "CoRE Lighting", draft-hartke-core-lighting-00
(work in progress), September 2015.
[I-D.ietf-core-interfaces]
Shelby, Z., Vial, M., Koster, M., and C. Groves, "Reusable
Interface Definitions for Constrained RESTful
Environments", draft-ietf-core-interfaces-05 (work in
progress), July 2016.
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[I-D.ietf-core-links-json]
Li, K., Rahman, A., and C. Bormann, "Representing CoRE
Formats in JSON and CBOR", draft-ietf-core-links-json-06
(work in progress), July 2016.
[I-D.kelly-json-hal]
Kelly, M., "JSON Hypertext Application Language", draft-
kelly-json-hal-08 (work in progress), May 2016.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://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,
<http://www.rfc-editor.org/info/rfc7228>.
[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>.
[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. Examples
A.1. Overview
A.1.1. Links
At its core, CoRAL is yet another serialization format for Web links.
For example, the following Web link (in RFC 5988 syntax):
<coap://example.com/info/tos>;rel=terms-of-service;type=text/plain
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can be serialized in CoRAL as the following CBOR data item (in CBOR
extended diagnostic format; forward slashes indicate comments):
[ [ 5, / fat-link /
1, / absolute-path /
[ 1, 70, / relation = terms-of-service /
4, 0, / format = text\plain /
5, "coap", / href.scheme = "coap" /
6, "example.com", / href.host.name = "example.com" /
10, "info", / href.path = "info" /
10, "tos" ] ] ] / href.path = "tos" /
Multiple links are serialized as items of the top-level array:
[ [ 5, / fat-link /
3, / relative-path /
[ 1, 26, / relation = first /
4, 0, / format = text\plain /
10, "page1" ] ], / href.path = "page1" /
[ 5, / fat-link /
3, / relative-path /
[ 1, 55, / relation = previous /
4, 0, / format = text\plain /
10, "page6" ] ], / href.path = "page6" /
[ 5, / fat-link /
3, / relative-path /
[ 1, 41, / relation = next /
4, 0, / format = text\plain /
10, "page8" ] ], / href.path = "page8" /
[ 5, / fat-link /
3, / relative-path /
[ 1, 34, / relation = last /
4, 0, / format = text\plain /
10, "page42" ] ] ] / href.path = "page42" /
The Format Option, when present, is a hint indicating what the CoAP
content format of the result of dereferencing the link should be. If
more than one format is available, the Format Option can be repeated:
[ [ 5, / fat-link /
3, / relative-path /
[ 1, 33, / relation = item /
4, 47, / format = application\exi /
4, 50, / format = application\json /
4, 60, / format = application\cbor /
10, "item1" ] ] ] / href.path = "item1" /
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A.1.2. Embedding
If a representation links to many resources, it may be inefficient to
retrieve a representation of each link target individually. For this
reason, CoRAL supports the embedding of a representation of the link
target in the link itself:
[ [ 5, / fat-link /
3, / relative-path /
[ 1, 33, / relation = item /
4, 50, / format = application\json /
10, "item1" ], / href.path = "item1" /
h'7b20227461736b223a20
2252657475726e207468
6520626f6f6b7320746f
20746865206c69627261
7279222c202261737369
676e6565223a2022416c
69636522207d' ] ]
where the byte string in this example encodes the following JSON
object:
{
"task": "Return the books to the library",
"assignee": "Alice"
}
By embedding representations, it is possible to use CoRAL as a (very
basic) substitute for RDF [W3C.REC-rdf11-concepts-20140225]. For
example, the RDF graph (in Turtle [W3C.REC-turtle-20140225] syntax)
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
<> foaf:name "John Doe" ;
foaf:age 32 ;
foaf:homepage <coap://www.doe.example/> .
can be serialized in CoRAL as follows:
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[ [ 2, / tiny-literal /
2, / append-path /
-100, / relation = name /
0, / format = text\plain /
h'4a6f686e20446f65' ], / "John Doe" /
[ 2, / tiny-literal /
2, / append-path /
-101, / relation = name /
7, / format = uint8 /
h'20' ], / 32 /
[ 5, / fat-link /
1, / absolute-path /
[ 1, -102, / relation = homepage /
5, "coap", / href.scheme = "coap" /
6, "www.doe.example" / href.host.name = "www.doe.example" /
] ] ]
A.1.3. Namespaces
The link relation type in a serialized link may be from the "global"
or the "local" namespace. The global namespace is indicated by an
unsigned number and is made up of the link relation types registered
with IANA. The local namespace is indicated by a negative number and
is defined by the media type of the CoRAL representation.
By default, CoRAL representations have the "application/coral" media
type where the local namespace is empty. However, it is possible to
create new media types based on CoRAL and to register these with the
"+coral" suffix. In this case, the media type specification can fill
the local namespace with application-specific link relation types.
For example, a media type "application/example.shop+coral" could
define the following set of local link relation types:
+-----+----------------------------------+
| ID | Meaning |
+-----+----------------------------------+
| -80 | http://example.com/rels/order |
| -81 | http://example.com/rels/basket |
| -82 | http://example.com/rels/customer |
+-----+----------------------------------+
Similarly, a media type "application/example.foaf+coral" could define
the following mapping from link relation type IDs to the FOAF RDF
model [FOAF]:
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+------+------------------------------------+
| ID | Meaning |
+------+------------------------------------+
| -100 | http://xmlns.com/foaf/0.1/name |
| -101 | http://xmlns.com/foaf/0.1/age |
| -102 | http://xmlns.com/foaf/0.1/homepage |
+------+------------------------------------+
A.1.4. Forms
In addition to Web links, CoRAL supports forms. An agent can use a
form to perform an operation on the form context, such as updating a
resource or creating a new item in a collection.
Similar to link relation types, the semantics of a form are indicated
by the form relation type. The Href.* Options encode the URI of the
form target to which the agent should submit the form. A form
additionally encodes the submission method (POST, PUT, PATCH, DELETE)
and the description of a representation that the service expects as
part of form submission:
[ [ 7, / fat-form /
3, / relative-path /
[ 1, 1, / relation = create-item /
2, 2, / method = POST /
3, 60, / accept = application\cbor /
10, "items" ] ] ] / href.path = "items" /
The Accept Option specifies the content format of the expected
representation. A content format can use the body of a form to
describe the expected representation in more detail, for example, by
specifying a set of form fields that the agent needs to fill out:
[ [ 7, / fat-form /
3, / relative-path /
[ 1, 1, / relation = create-item /
2, 2, / method = POST /
3, -65535, / accept = example\form /
10, "items" ], / href.path = "items" /
h'6e616d652c206167652c / "name, age, homepage" /
20686f6d6570616765' ] ]
A.1.5. Editing
The target resource of a link may be editable. In this case, the
representation of such a resource typically contains one or more
forms that allow an agent to edit the resource. However, it may be
inefficient to include these forms every time a representation of the
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link target is retrieved and more efficient to include them in
representations that link to the resource. CoRAL supports this with
two options.
Setting the Updatable Option in a link to true defines a form that
can be used to update the target resource. The context and target of
that form are both the target of the link, the submission method is
PUT and the content format of the submitted representation must be
one of the formats indicated by the Format Option in the link. For
example, given the following CoRAL representation, an agent can
change the recipient by making a PUT request to <./to> with the new
value in "text/plain" format:
[ [ 5, / fat-link /
3, / relative-path /
[ 1, -120, / relation = sender /
4, 0, / format = text\plain /
10, "from", / href.path = "from" /
14, true ], / updatable = true /
h'4a756c696574' ], / "Juliet" /
[ 5, / fat-link /
3, / relative-path /
[ 1, -121, / relation = recipient /
4, 0, / format = text\plain /
10, "to", / href.path = "to" /
14, true ], / updatable = true /
h'526f6d656f' ], / "Romeo" /
[ 5, / fat-link /
3, / relative-path /
[ 1, -122, / relation = message /
4, 0, / format = text\plain /
10, "message", / href.path = "message" /
14, true ], / updatable = true /
h'4172742074686f75206e / "Art thou not Romeo, /
6f7420526f6d656f2c20 / and a Montague?" /
616e642061204d6f6e74
616775653f' ] ]
Setting the Deletable Flag in a link to true likewise defines a form
that can be used to delete the target resource.
A.2. CoRE Lighting
CoRE Lighting [I-D.hartke-core-lighting] defines a benchmark scenario
for the exploration of hypermedia-oriented design in constrained,
RESTful environments. The bulletin board example in Section 5.2.1 of
[I-D.hartke-core-lighting] can be encoded in CoRAL as follows:
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[[7, 3, [1, 1, 3, 65200, 10, "bulletins"]],
[5, 1, [1, 33, 4, 65200, 6, "light-bulb.example"], <<1>>],
[5, 1, [1, 33, 4, 65200, 6, "remote-control.example"], <<2>>]]
where <<1>> is a byte string that encodes the following CoRAL
structure:
[[5, 3, [1, -100, 4, 65202, 10, "config"]],
[2, 3, -101, 0, "Light 2"],
[2, 3, -102, 0, "Illuminates the couch."],
[2, 3, -103, 0, "Living Room"]]
and <<2>> is a byte string that encodes the following CoRAL
structure:
[[5, 3, [1, 1, 4, 65203, 10, "state"]],
[2, 3, -101, 0, "LRC 1"],
[2, 3, -102, 0, "Controls Light 2."],
[2, 3, -103, 0, "Living Room"]]
Table 3 shows a comparison of sizes of the example encoded in CoRAL
and JSON.
+--------+-----------+
| Format | Size |
+--------+-----------+
| JSON | 515 bytes |
| CoRAL | 245 bytes |
+--------+-----------+
Table 3: Size Comparison
A.3. CoRE Link Format
The example in this section is based on an example on page 14 of
[RFC6690]:
</sensors>;ct=40;title="Sensor Index",
</sensors/temp>;rt="temperature-c";if="sensor",
</sensors/light>;rt="light-lux";if="sensor",
<http://www.example.com/sensors/t123>;anchor="/sensors/temp"
;rel="describedby",
</t>;anchor="/sensors/temp";rel="alternate"
The example can be encoded in CoRAL as follows:
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[[4, 1, [10, "sensors"]],
[5, 2, [4, 40, 13, "Sensor Index"]],
[5, 2, [10, "temp", 16, "temperature-c", 17, "sensor"]],
[5, 2, [10, "light", 16, "light-lux", 17, "sensor"]],
[5, 1, [1, 18, 5, "http", 6, "www.example.com",
10, "sensors", 10, "t123", 18, "/sensors/temp"]],
[5, 1, [1, 2, 10, "t", 18, "/sensors/temp"]]]
Table 4 shows a comparison of sizes of the example encoded in CoRAL
and a number of Link Format variants.
+--------------------+-----------+
| Format | Size |
+--------------------+-----------+
| Link Format | 251 bytes |
| Link Format (JSON) | 320 bytes |
| Link Format (CBOR) | 203 bytes |
| CoRAL | 181 bytes |
+--------------------+-----------+
Table 4: Size Comparison
A.4. CoRE Interfaces
The example in this section is based on an example on page 10 of
[I-D.ietf-core-interfaces]:
</s/>;rt="simple.sen";if="core.b",
</s/lt>;rt="simple.sen.lt";if="core.s",
</s/tmp>;rt="simple.sen.tmp";if="core.s";obs,
</s/hum>;rt="simple.sen.hum";if="core.s",
</a/>;rt="simple.act";if="core.b",
</a/1/led>;rt="simple.act.led";if="core.a",
</a/2/led>;rt="simple.act.led";if="core.a",
</d/>;rt="simple.dev";if="core.ll",
</l/>;if="core.lb"
The example can be encoded in CoRAL as follows:
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[[5, 1, [10, "d", 10, "", 16, "simple.dev", 17, "core.ll"]],
[5, 1, [10, "l", 10, "", 17, "core.lb"]],
[4, 1, [17, "core.b"]],
[5, 2, [10, "s", 10, "", 16, "simple.sen"]],
[5, 2, [10, "a", 10, "", 16, "simple.act"]],
[4, 1, [10, "s", 17, "core.s"]],
[5, 2, [10, "lt", 16, "simple.sen.lt"]],
[5, 2, [10, "tmp", 16, "simple.sen.tmp", 19, true]],
[5, 2, [10, "hum", 16, "simple.sen.hum"]],
[4, 1, [10, "a", 16, "simple.act.led", 17, "core.a"]],
[5, 2, [10, "1", 10, "led"]],
[5, 2, [10, "2", 10, "led"]]]
Table 5 shows a comparison of sizes of the example encoded in CoRAL
and a number of Link Format variants.
+--------------------+-----------+
| Format | Size |
+--------------------+-----------+
| Link Format | 332 bytes |
| Link Format (JSON) | 456 bytes |
| Link Format (CBOR) | 264 bytes |
| CoRAL | 248 bytes |
+--------------------+-----------+
Table 5: Size Comparison
Acknowledgements
This specification is heavily inspired by the JSON Hypertext
Application Language (HAL) [I-D.kelly-json-hal]; the author of and
contributors to that specification are acknowledged for their great
work.
Yassin Nasir Hassan suggested placing the hypermedia controls for
modifying a link target in the link context rather than in the
representation of the link target.
Thanks to Carsten Bormann, Jaime Jimenez and Matthias Kovatsch for
helpful comments and discussions that have shaped the document.
Author's Address
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Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
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