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Versions: (draft-li-core-cbor-equivalents) 00 01 02 03 04 05 06 07 08 09

CoRE Working Group                                                 K. Li
Internet-Draft                                             Alibaba Group
Intended status: Standards Track                               A. Rahman
Expires: October 29, 2017                                   InterDigital
                                                         C. Bormann, Ed.
                                                 Universitaet Bremen TZI
                                                          April 27, 2017


Representing Constrained RESTful Environments (CoRE) Link Format in JSON
                                and CBOR
                     draft-ietf-core-links-json-08

Abstract

   JavaScript Object Notation, JSON (RFC7159) 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 (RFC5988) 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).  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 http://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 October 29, 2017.



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Copyright Notice

   Copyright (c) 2017 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
   (http://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 . . . . . . . . . . . . . . . . . . . . . . .   3
   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  . . . . . . . . .   7
     2.5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.5.1.  Link Format to JSON Example . . . . . . . . . . . . .   8
       2.5.2.  Link Format to CBOR Example . . . . . . . . . . . . .   9
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Media types . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  CoAP Content-Format Registration  . . . . . . . . . . . .  12
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Reference implementation . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Web Linking [RFC5988] 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].





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   The JavaScript Object Notation (JSON) [RFC7159] 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")

      *  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

      *  Do not cater for RFC 5988 complications caused by HTTP header
         character set issues [RFC2047]

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



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   [RFC2119] when they appear in ALL CAPS.  These words may also appear
   in this document in lower case as plain English words, absent their
   normative meanings.

   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], and [RFC8075]

   IoT: Internet of Things

   JSON: JavaScript Object Notation [RFC7159]

   The objective of the JSON and CBOR mappings defined in this document
   is to contain information of the formats specified in [RFC5988] 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 [RFC5988] 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.

   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;




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   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
   boolean, or an array of strings or booleans, 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 [RFC7159]).

   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 [RFC5988] though.  No attempt has been made
   to decode the possibly space-separated values for rt=, if=, and rel=
   into JSON arrays.)

   The URI-Reference is represented as a name/value pair with the name
   "href" and the URI-Reference as the value.  (Rationale: This usage 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]).

   As a non-normative summary, the resulting structure can be
   represented in CBOR Data Definition Language (CDDL)
   [I-D.greevenbosch-appsawg-cbor-cddl] as:








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   links = [* link]
   link = {
     href: tstr    ; resource URI
     * tstr => value
   }
   value = tstr   ; text value -- the normal case
         / true   ; no value given, just the name
         / [2* tstr/true ] ; repeats for two or more

                   Figure 1: CoRE Link Format Data Model

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:

          +----------+---------------+-------------------------+
          | 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.

   *Adam's Issue number 2: What should an implementation do that
   receives a link-format that doesn't heed the above MUST?  Is there an
   onus to check it?*



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   This leads to the following CDDL representation for the CBOR
   encoding:

   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,
   )
   value = tstr   ; text value -- the normal case
         / true   ; no value given, just the name
         / [2* tstr/true ] ; 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.  A
   work-in-progress revision of [RFC5988], [I-D.nottingham-rfc5988bis],
   clarifies that this is indeed intended to be the case.  However,
   existing 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.



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   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]



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   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")



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        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(12 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



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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 [RFC7159], 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



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   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].  All IDs are assigned from the
   "Expert Review" (0-255) range.  The assigned IDs are show in Table 2.

                 +------------------------------+-------+
                 | Media type                   | ID    |
                 +------------------------------+-------+
                 | application/link-format+cbor | TBD64 |
                 | application/link-format+json | TBD54 |
                 +------------------------------+-------+

                     Table 2: CoAP Content-Format IDs




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4.  Security Considerations

   The security considerations relevant to the data model of [RFC6690],
   as well as those of [RFC7049] and [RFC7159] 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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://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, <http://www.rfc-editor.org/info/rfc7049>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

5.2.  Informative References

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-greevenbosch-appsawg-
              cbor-cddl-10 (work in progress), March 2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-10
              (work in progress), March 2017.

   [I-D.nottingham-rfc5988bis]
              Nottingham, M., "Web Linking", draft-nottingham-
              rfc5988bis-05 (work in progress), April 2017.





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   [MNOT11]   Nottingham, M., "Linking in JSON", November 2011,
              <http://www.mnot.net/blog/2011/11/25/linking_in_json>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <http://www.rfc-editor.org/info/rfc2047>.

   [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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc8075>.

   [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.

   # <CODE BEGINS>
   require 'strscan'
   require 'json'
   require 'cbor-pretty'

   class String
     def as_utf8
       force_encoding(Encoding::UTF_8)
     end
   end



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   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,
     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 ';'



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                   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?
           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 do |k, v|
                if String === v
                  if MUSTBEQUOTED[k] || v !~ /\A#{PTOKENCHAR}+\z/
                      "#{k}=\"#{v.gsub(/[\\"]/) { |x| "\\#{x}"}}\""
                  else
                    "#{k}=#{v}"
                  end
                else
                  "#{k}"
                end
              end].join(';')
           end.join(",")
         end
       end
     end
   end



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   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>

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.




































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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






















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