Internet DRAFT - draft-ietf-core-links-json
draft-ietf-core-links-json
CoRE Working Group K. Li
Internet-Draft Alibaba Group
Intended status: Standards Track A. Rahman
Expires: August 30, 2018 InterDigital
C. Bormann, Ed.
Universitaet Bremen TZI
February 26, 2018
Representing Constrained RESTful Environments (CoRE) Link Format in JSON
and CBOR
draft-ietf-core-links-json-10
Abstract
JavaScript Object Notation, JSON (RFC 8259) is a text-based data
format which is popular for Web based data exchange. Concise Binary
Object Representation, CBOR (RFC7049) is a binary data format which
has been optimized for data exchange for the Internet of Things
(IoT). For many IoT scenarios, CBOR formats will be preferred since
it can help decrease transmission payload sizes as well as
implementation code sizes compared to other data formats.
Web Linking (RFC 8288) provides a way to represent links between Web
resources as well as the relations expressed by them and attributes
of such a link. In constrained networks, a collection of Web links
can be exchanged in the CoRE link format (RFC 6690). Outside of
constrained environments, it may be useful to represent these
collections of Web links in JSON, and similarly, inside constrained
environments, in CBOR. This specification defines a common format
for this.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 30, 2018.
Li, et al. Expires August 30, 2018 [Page 1]
Internet-Draft Links-in-JSON February 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Web Links in JSON and CBOR . . . . . . . . . . . . . . . . . 4
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Information Model . . . . . . . . . . . . . . . . . . . . 4
2.3. Additional Encoding Step for CBOR . . . . . . . . . . . . 6
2.4. Converting JSON or CBOR to Link-Format . . . . . . . . . 8
2.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.1. Link Format to JSON Example . . . . . . . . . . . . . 9
2.5.2. Link Format to CBOR Example . . . . . . . . . . . . . 10
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
3.1. Media types . . . . . . . . . . . . . . . . . . . . . . . 12
3.2. CoAP Content-Format Registration . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Normative References . . . . . . . . . . . . . . . . . . 14
5.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Reference implementation . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Web Linking [RFC8288] provides a way to represent links between Web
resources as well as the relations expressed by them and attributes
of such a link. In constrained networks, a collection of Web links
can be exchanged in the CoRE link format [RFC6690] to enable resource
discovery, for instance by using the CoAP protocol [RFC7252].
Li, et al. Expires August 30, 2018 [Page 2]
Internet-Draft Links-in-JSON February 2018
The JavaScript Object Notation (JSON) [RFC8259] is a lightweight,
text-based, language-independent data interchange format. JSON is
popular in the Web development environment as it is easy for humans
to read and write.
The Concise Binary Object Representation (CBOR) [RFC7049] is a binary
data format which requires extremely small code size, allows very
compact message representation, and provides extensibility without
the need for version negotiation. CBOR is especially well suited for
IoT environments because of these efficiencies.
When converting between a bespoke syntax such as that defined by
[RFC6690] and JSON or CBOR, many small decisions have to be made. If
left without guidance, it is likely that a number of slightly
incompatible dialects will emerge. This specification defines a
common format for representing CoRE Web Linking in JSON and CBOR.
Note that there is a separate question on how to represent Web links
pointing out of JSON documents, as discussed for example in [MNOT11].
While there are good reasons to stay as compatible as possible to
developments in this area, the present specification is solving a
different problem.
1.1. Objectives
This specification has been designed based on the following
objectives:
o Canonical mapping
* lossless conversion in both directions between any pair of
[RFC6690], JSON, and CBOR ("round-tripping"), unless prevented
by a limitation of [RFC6690]
* but not attempting to ensure that a sequence of conversions
from one of the formats through one or both of the others and
back to the original would result in a bit-wise identical
representation
o The simplest thing that could possibly work.
While the formats defined in this document are based on the above
objectives, they are general enough that they can be used for other
applications of links in the Web. The same basic formats can be used
for Web links that do not default to the "hosts" relation type (as is
defined in [RFC6690]) and that allow percent encoding and general IRI
syntax in what is an URI-Reference field in [RFC6690]. Also,
specific support has been added for internationalized link attributes
Li, et al. Expires August 30, 2018 [Page 3]
Internet-Draft Links-in-JSON February 2018
such as "title*", including their language tags (while staying
limited to UTF-8 as the character set).
1.2. Terminology
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.
The term "byte" is used in its now customary sense as a synonym for
"octet".
CoAP: Constrained Application Protocol [RFC7252]
CBOR: Concise Binary Object Representation [RFC7049]
CoRE: Constrained RESTful Environments, the field of work underlying
[RFC6690], [RFC7049], [RFC7252], [RFC7641], [RFC7959], [RFC8075], and
[RFC8323]
IoT: Internet of Things
JSON: JavaScript Object Notation [RFC8259]
The objective of the JSON and CBOR mappings defined in this document
is to contain information of the formats specified in [RFC8288] and
[RFC6690]. This specification therefore uses the names of the ABNF
productions used in those documents.
2. Web Links in JSON and CBOR
2.1. Background
Web Linking [RFC8288] provides a way to represent links between Web
resources as well as the relations expressed by them and attributes
of such a link. In constrained networks, a collection of Web links
can be exchanged in the CoRE link format [RFC6690] to enable resource
discovery, for instance by using the CoAP protocol [RFC7252] and in
conjunction with the CoRE resource directory
[I-D.ietf-core-resource-directory].
2.2. Information Model
This section discusses the information model underlying the CORE Link
Format payload.
Li, et al. Expires August 30, 2018 [Page 4]
Internet-Draft Links-in-JSON February 2018
An "application/link-format" document is a collection of Web links
("link-value"), each of which is a collection of attributes ("link-
param") applied to a "URI-Reference".
We straightforwardly map:
o the collection of Web links to a JSON or CBOR array of links;
o each link to a JSON object or CBOR map, mapping attribute names to
attribute values.
In the object representing a "link-value", each target attribute or
other parameter ("link-param") is represented by a JSON name/value
pair (member). The name is a string representation of the parameter
or attribute name (as in "parmname"). The value can be a string, a
language-tagged string, a boolean, or an array of these, as described
below.
If the attribute value ("ptoken" or "quoted-string") is present, and
a Link attribute with this name ("parmname") is present just once in
the "link-value", the value is a string representation of the
parameter or attribute value ("ptoken" or "quoted-string"). "quoted-
string" productions are parsed (i.e, the outer quotes removed and the
backslash constructions evaluated) as defined in [RFC6690] and its
referenced documents, before placing them in JSON strings (in the
representation of which they may gain back additional decorations
such as backslashes as defined in [RFC8259]).
Attribute values represented as per [RFC8187], e.g. for the "title*"
attribute, are converted in a language-tagged string; the attribute
name is then represented without the "*" character. A language-
tagged string is represented as a CBOR map (JSON object) that carries
the language tag as the key for a single member and the attribute
value in UTF-8 form as its value.
If no attribute value ("ptoken" or "quoted-string") is present, the
presence of the attribute name is indicated by using the Boolean
value "true" as the value.
If a Link attribute ("parmname") is present more than once in a
"link-value", its values are then represented as a JSON array of JSON
string values or "true"; this array becomes the value of the JSON
name/value pair where the attribute name is the JSON name.
Attributes occurring just once MUST NOT be represented as JSON arrays
but MUST be directly represented as JSON strings or "true". (Note
that [RFC6690] has cut down on the use of repeated parameter names;
they are still allowed by [RFC8288] though. No attempt has been made
to decode the possibly space-separated values for rt=, if=, and rel=
Li, et al. Expires August 30, 2018 [Page 5]
Internet-Draft Links-in-JSON February 2018
into JSON arrays.) Recipients MUST NOT accept documents that violate
this requirement.
The URI-Reference is represented as a name/value pair with the name
"href" and the URI-Reference as the value, with the latter converted
to an IRI-Reference as per Section 3.2 of [RFC3987] (Rationale: The
usage of "href" is consistent with the use of "href" as a query
parameter for link-format query filtering and with link-format
reserving the link parameter "href" specifically for this use
[RFC6690]. The usage of an IRI-Reference is consistent with the
mandate in [RFC6690] that percent-encoding be processed. Note that
the format is able to represent IRIs the URIs for which cannot be
represented in [RFC6690] as not all percent-encoded constructions are
amenable to the pre-processing required by [RFC6690].)
As a convenient reference, the resulting structure can be described
in CBOR Data Definition Language (CDDL) [I-D.ietf-cbor-cddl] as in
Figure 1 (informative).
links = [* link]
link = {
href: tstr ; resource URI
* tstr => value
}
value1 = tstr ; text value -- the normal case
/ { tstr => tstr } ; language tag and value
/ true ; no value given, just the name
value = value1
/ [2* value1 ] ; repeats for two or more
Figure 1: CoRE Link Format Data Model (JSON)
2.3. Additional Encoding Step for CBOR
The above specification for JSON might have been used as is for the
CBOR encoding as well. However, to further reduce message sizes, an
extra encoding step is performed: "href" and some commonly occurring
attribute names are encoded as small integers.
The substitution is defined in Table 1:
Li, et al. Expires August 30, 2018 [Page 6]
Internet-Draft Links-in-JSON February 2018
+----------+---------------+-------------------------+
| name | encoded value | origin |
+----------+---------------+-------------------------+
| href | 1 | [RFC6690], [RFCthis] |
| rel | 2 | [RFC5988] Section 5.3 |
| anchor | 3 | [RFC5988] Section 5.2 |
| rev | 4 | [RFC5988] Section 5.3 |
| hreflang | 5 | [RFC5988] Section 5.4 |
| media | 6 | [RFC5988] Section 5.4 |
| title | 7 | [RFC5988] Section 5.4 |
| type | 8 | [RFC5988] Section 5.4 |
| rt | 9 | [RFC6690] Section 3.1 |
| if | 10 | [RFC6690] Section 3.2 |
| sz | 11 | [RFC6690] Section 3.3 |
| ct | 12 | [RFC7252] Section 7.2.1 |
| obs | 13 | [RFC7641] Section 6 |
+----------+---------------+-------------------------+
Table 1: Integer Encoding of common attribute names
This list of substitutions is fixed by the present specification; no
future expansion of the list is foreseen. "href" as well as all
attribute names in this list MUST be represented by their integer
substitutions and MUST NOT use the attribute name in text form.
Recipients MUST NOT accept documents that violate this requirement.
As a convenient reference, the resulting structure can be described
in CBOR Data Definition Language (CDDL) [I-D.ietf-cbor-cddl] as in
Figure 2 (informative).
Li, et al. Expires August 30, 2018 [Page 7]
Internet-Draft Links-in-JSON February 2018
links = [* link]
link = {
href => tstr ; resource URI
* label => value
}
href = 1
label = tstr / &(
rel: 2, anchor: 3, rev: 4,
hreflang: 5, media: 6, title: 7,
type: 8, rt: 9, if: 10,
sz: 11, ct: 12, obs: 13,
)
value1 = tstr ; text value -- the normal case
/ { tstr => tstr } ; language tag and value
/ true ; no value given, just the name
value = value1
/ [2* value1 ] ; repeats for two or more
Figure 2: CoRE Link Format Data Model (CBOR)
2.4. Converting JSON or CBOR to Link-Format
When a JSON or CBOR representation needs to be converted back to
link-format, the above process is performed in inverse. Since link-
format allows serializing link parameter values both in unqouted form
("ptoken") or in quoted form ("quoted-string"), a decision has to be
made for each value. Where the syntax of "ptoken" does not allow the
value to be represented, the quoted form clearly needs to be used.
However, when both forms are possible, the decision is arbitrary.
The recently republished Web Linking specification, [RFC8288],
clarifies that this is indeed intended to be the case. However,
previous specifications of link attributes, including those in
[RFC5988] and [RFC6690], sometimes have made this decision in a
specific way by only including one or the other alternative in the
ABNF given for a link parameter. This requires a converter to know
about all these cases, including those that have not been defined yet
at the time of writing the converter. This problem becomes even
harder by the fact that there is no central registry of link-
attribute names.
Obviously, the conversion back to link-format needs to result in a
valid link-format document. The reference implementation in
Appendix A has addressed this problem with the following two rules:
o Where a "ptoken" representation is possible, that is used instead
of "quoted-string". This rule covers most of the special cases
listed above.
Li, et al. Expires August 30, 2018 [Page 8]
Internet-Draft Links-in-JSON February 2018
o As a special exception to the above rule, the four link attributes
"anchor", "title", "rt", and "if" are always expressed as "quoted-
string". This rule covers these specific four cases.
This set of rules is based on the hope that future definitions of
link attributes will no longer hardcode one or the other
serialization.
2.5. Examples
The examples in this section are based on an example on page 15 of
[RFC6690] (Figure 3).
</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"
Figure 3: Example from page 15 of [RFC6690]
2.5.1. Link Format to JSON Example
The link-format document in Figure 3 becomes (321 bytes, line breaks
shown are not part of the minimally-sized JSON document):
"[{"href":"/sensors","ct":"40","title":"Sensor
Index"},{"href":"/sensors/temp","rt":"temperature-
c","if":"sensor"},{"href":"/sensors/light","rt":"light-
lux","if":"sensor"},{"href":"http://www.example.com/sensors/
t123","anchor":"/sensors/
temp","rel":"describedby"},{"href":"/t","anchor":"/sensors/
temp","rel":"alternate"}] "
To demonstrate the handling of value-less and array-valued
attributes, we extend the link-format example by examples of these
(Figure 4; the "obs" attribute is defined in Section 6 of [RFC7641],
while the "foo" attribute is for exposition only):
</sensors>;ct=40;title="Sensor Index",
</sensors/temp>;rt="temperature-c";if="sensor";obs,
</sensors/light>;rt="light-lux";if="sensor",
<http://www.example.com/sensors/t123>;anchor="/sensors/temp"
;rel="describedby";foo="bar";foo=3;ct=4711,
</t>;anchor="/sensors/temp";rel="alternate"
Figure 4: Example derived from page 15 of [RFC6690]
Li, et al. Expires August 30, 2018 [Page 9]
Internet-Draft Links-in-JSON February 2018
The link-format document in Figure 4 becomes the JSON document in
Figure 5 (some spacing and indentation added):
[{"href":"/sensors","ct":"40","title":"Sensor Index"},
{"href":"/sensors/temp","rt":"temperature-c","if":"sensor",
"obs":true},
{"href":"/sensors/light","rt":"light-lux","if":"sensor"},
{"href":"http://www.example.com/sensors/t123",
"anchor":"/sensors/temp","rel":"describedby",
"foo":["bar","3"],"ct":"4711"},
{"href":"/t","anchor":"/sensors/temp","rel":"alternate"}]
Figure 5: Example derived from page 15 of [RFC6690]
Note that the conversion is unable to convert the string-valued "ct"
attribute to a number, which would be the natural type for a Content-
Format value; similarly, both "foo" values are treated as strings
independently of whether they are quoted or numeric in syntax.
2.5.2. Link Format to CBOR Example
This examples shows conversion from link format to CBOR format.
The link-format document in Figure 3 becomes (in CBOR diagnostic
format):
[{1: "/sensors", 12: "40", 7: "Sensor Index"},
{1: "/sensors/temp", 9: "temperature-c", 10: "sensor"},
{1: "/sensors/light", 9: "light-lux", 10: "sensor"},
{1: "http://www.example.com/sensors/t123", 3: "/sensors/temp",
2: "describedby"},
{1: "/t", 3: "/sensors/temp", 2: "alternate"}]
or, in hexadecimal (203 bytes):
85 # array(number of data items:5)
a3 # map(# data item pairs:3)
01 # unsigned integer(value:1,"href")
68 # text string(8 bytes)
2f73656e736f7273 # "/sensors"
0c # unsigned integer(value:12,"ct")
62 # text(2)
3430 # "40"
07 # unsigned integer(value:7,"title")
6c # text string(12 bytes)
53656e736f7220496e646578 # "Sensor Index"
a3 # map(# data item pairs:3)
01 # unsigned integer(value:1,"href")
Li, et al. Expires August 30, 2018 [Page 10]
Internet-Draft Links-in-JSON February 2018
6d # text string(13 bytes)
2f73656e736f72732f74
656d70 # "/sensors/temp"
09 # unsigned integer(value:9,"rt")
6d # text string(13 bytes)
74656d70657261747572
652d63 # "temperature-c"
0a # unsigned integer(value:10,"if")
66 # text string(6 bytes)
73656e736f72 # "sensor"
a3 # map(# data item pairs:3)
01 # unsigned integer(value:1,"href")
6e # text string(14 bytes)
2f73656e736f72732f6c
69676874 # "/sensors/light"
09 # unsigned integer(value:9,"rt")
69 # text string(9 bytes)
6c696768742d6c7578 # "light-lux"
0a # unsigned integer(value:10,"if")
66 # text string(6 bytes)
73656e736f72 # "sensor"
a3 # map(# data item pairs:3)
01 # unsigned integer(value:1,"href")
78 23 # text string(35 bytes)
687474703a2f2f777777
2e6578616d706c652e63
6f6d2f73656e736f7273
2f74313233 # "http://www.example.com/sensors/t123"
03 # unsigned integer(value:3,"anchor")
6d # text string(13 bytes)
2f73656e736f72732f74
656d70 # "/sensors/temp"
02 # unsigned integer(value:2,"rel")
6b # text string(11 bytes)
6465736372696265646279 # "describedby"
a3 # map(# data item pairs:3)
01 # unsigned integer(value:1,"href")
62 # text string(2 bytes)
2f74 # "/t"
03 # unsigned integer(value:3,"anchor")
6d # text string(13 bytes)
2f73656e736f72732f74
656d70 # "/sensors/temp"
02 # unsigned integer(value:2,"rel")
69 # text string(9 bytes)
616c7465726e617465 # "alternate"
Figure 6: Web Links Encoded in CBOR
Li, et al. Expires August 30, 2018 [Page 11]
Internet-Draft Links-in-JSON February 2018
3. IANA Considerations
3.1. Media types
This specification registers the following additional Internet Media
Types:
Type name: application
Subtype name: link-format+json
Required parameters: None
Optional parameters: None
Encoding considerations: Resources that use the "application/link-
format+json" media type are required to conform to the
"application/json" Media Type and are therefore subject to the
same encoding considerations specified in [RFC8259], Section 11.
Security considerations: See Section 4 of [RFCthis].
Published specification: [RFCthis].
Applications that use this media type: Applications that interchange
collections of Web links based on CoRE link format [RFC6690] in
JSON.
Additional information:
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): TEXT
Person & email address to contact for further information:
Carsten Bormann <cabo@tzi.org>
Intended usage: COMMON
Change controller: IESG
and
Type name: application
Subtype name: link-format+cbor
Li, et al. Expires August 30, 2018 [Page 12]
Internet-Draft Links-in-JSON February 2018
Required parameters: None
Optional parameters: None
Encoding considerations: Resources that use the "application/link-
format+cbor" media type are required to conform to the
"application/cbor" Media Type and are therefore subject to the
same encoding considerations specified in [RFC7049], Section 7.
Security considerations: See Section 4 of [RFCthis].
Published specification: [RFCthis].
Applications that use this media type: Applications that interchange
collections of Web links based on CoRE link format [RFC6690] in
CBOR.
Additional information:
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): CBOR
Person & email address to contact for further information:
Kepeng Li <kepeng.lkp@alibaba-inc.com>
Intended usage: COMMON
Change controller: IESG
3.2. CoAP Content-Format Registration
IANA is requested to assign CoAP Content-Format IDs for the above
media types in the "CoAP Content-Formats" sub-registry, within the
"CoRE Parameters" registry [RFC7252]. The ID for "application/link-
format+cbor" is assigned from the "Expert Review" (0-255) range,
while the ID for "application/link-format+json" is assigned from the
"IETF review" range. The assigned IDs are show in Table 2.
Li, et al. Expires August 30, 2018 [Page 13]
Internet-Draft Links-in-JSON February 2018
+------------------------------+--------+--------+-----------+
| Media type | Coding | ID | Reference |
+------------------------------+--------+--------+-----------+
| application/link-format+cbor | - | TBD64 | [RFCthis] |
| application/link-format+json | - | TBD504 | [RFCthis] |
+------------------------------+--------+--------+-----------+
Table 2: CoAP Content-Format IDs
4. Security Considerations
The security considerations relevant to the data model of [RFC6690],
as well as those of [RFC7049] and [RFC8259] apply.
5. References
5.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8187] Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", RFC 8187,
DOI 10.17487/RFC8187, September 2017,
<https://www.rfc-editor.org/info/rfc8187>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
Li, et al. Expires August 30, 2018 [Page 14]
Internet-Draft Links-in-JSON February 2018
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
5.2. Informative 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-01
(work in progress), January 2018.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-12 (work in progress), October 2017.
[MNOT11] Nottingham, M., "Linking in JSON", November 2011,
<http://www.mnot.net/blog/2011/11/25/linking_in_json>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
Li, et al. Expires August 30, 2018 [Page 15]
Internet-Draft Links-in-JSON February 2018
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RUBY] "Information technology -- Programming languages -- Ruby",
ISO/IEC 30170:2012, April 2012.
Appendix A. Reference implementation
A reference implementation of a converter from [RFC6690] link-format
to JSON and CBOR (and back to link-format) in the programming
language Ruby [RUBY] is reproduced below. (Note that this
implementation does not handle [RFC8187]-encoded attributes.) For
pretty-printing the binary CBOR, this uses the "cbor-diag" gem (Ruby
library), which may need to be installed by "gem install cbor-diag".
# <CODE BEGINS>
require 'strscan'
require 'json'
require 'cbor-pretty'
class String
def as_utf8
force_encoding(Encoding::UTF_8)
end
end
module CoRE
module Links
def self.map_to_true(a)
Hash[a.map{ |t| [t, true]}]
end
PTOKENCHAR = %r"[\[\]\w!#-+\--/:<-?^-`{-~@]"
QUOSTRCHAR = %r{(?:[^"\\]|\\.)} # to be used inside "
ATTRCHAR = %r"[\w!#$&+.^`|~-]"
MUSTBEQUOTED = map_to_true(%w{anchor title rt if})
ANCHORNAME = "href"
SCANATTR =
%r{(#{ATTRCHAR}+)(?:=(?:(#{PTOKENCHAR}+)|"(#{QUOSTRCHAR}*)"))?} # "
RAWMAPPINGS = <<-DATA
href: 1, rel: 2, anchor: 3,
rev: 4, hreflang: 5, media: 6,
title: 7, type: 8, rt: 9,
if: 10, sz: 11, ct: 12,
Li, et al. Expires August 30, 2018 [Page 16]
Internet-Draft Links-in-JSON February 2018
obs: 13,
DATA
MAPPINGS = Hash.new {|h, k| k}
RAWMAPPINGS.scan(/([-\w]+)\s*:\s*([-\w]+),/) do |n, v|
MAPPINGS[n] = Integer(v)
end
def self.parse(*args)
WLNK.parse(*args)
end
class WLNK
attr_accessor :resources
def initialize(r = []) # make sure the keys are strings
@resources = r.to_ary # make sure it's an Array
end
def self.parse(s, robust = true)
wl = WLNK.new
ss = StringScanner.new(s.as_utf8)
ss.skip(/\s+/) if robust
while ss.scan(%r{<([^>]+)>})
res = { ANCHORNAME => ss[1].as_utf8 }
ss.skip(/\s*/) if robust
while ss.skip(/;/)
ss.skip(/\s*/) if robust
unless ss.scan(SCANATTR)
raise ArgumentError, "must have attribute behind ';'
at: #{ss.peek(20).inspect} (byte #{ss.pos})"
end
key = ss[1].as_utf8
value = ss[2] ||
(ss[3] ? ss[3].gsub(/\\(.)/) { $1 } : true)
if res[key]
res[key] = Array(res[key]) << value
else
res[key] = value
end
ss.skip(/\s*/) if robust
end
wl.resources << res
break unless ss.skip(/,/)
ss.skip(/\s*/) if robust
end
ss.skip(/\s*/) if robust
raise ArgumentError, "link-format unparseable at:
#{ss.peek(20).inspect} (byte #{ss.pos})" unless ss.eos?
Li, et al. Expires August 30, 2018 [Page 17]
Internet-Draft Links-in-JSON February 2018
wl
end
def to_json
JSON.pretty_generate(@resources)
end
def to_cbor
CBOR.encode(@resources.map {|r|
Hash[r.map { |k, v| [MAPPINGS[k], v] }]})
end
def to_wlnk
resources.map do |res|
res = res.dup
u = res.delete(ANCHORNAME)
["<#{u}>", *res.map { |k, v| wlnk_item(k, v) }].join(';')
end.join(",")
end
private
def wlnk_item(k, v)
case v
when String
if MUSTBEQUOTED[k] || v !~ /\A#{PTOKENCHAR}+\z/
"#{k}=\"#{v.gsub(/[\\"]/) { |x| "\\#{x}"}}\""
else
"#{k}=#{v}"
end
when Array
v.map{ |v1| wlnk_item(k, v1) }.join(';')
when true
"#{k}"
else
fail "Don't know how to represent #{{k=>v}.inspect}"
end
end
end
end
end
lf = CoRE::Links.parse(ARGF.read)
puts lf.to_json # JSON
puts CBOR.pretty(lf.to_cbor) # CBOR "pretty" binary form
puts lf.to_wlnk # RFC 6690 link-format
# <CODE ENDS>
Li, et al. Expires August 30, 2018 [Page 18]
Internet-Draft Links-in-JSON February 2018
Acknowledgements
Special thanks to Bert Greevenbosch who was an author on the initial
version of a contributing document as well as the original author on
the CDDL notation.
Hannes Tschofenig made many helpful suggestions for improving this
document.
Li, et al. Expires August 30, 2018 [Page 19]
Internet-Draft Links-in-JSON February 2018
Authors' Addresses
Kepeng LI
Alibaba Group
Wenyixi Road, Yuhang District
Hangzhou, Zhejiang 311121
China
Email: kepeng.lkp@alibaba-inc.com
Akbar Rahman
InterDigital Communications, LLC
1000 Sherbrooke Street West
Montreal, Quebec H3A 3G4
Canada
Phone: +1-514-585-0761
Email: akbar.rahman@interdigital.com
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Li, et al. Expires August 30, 2018 [Page 20]
