Concise Problem Details For CoAP APIs
draft-ietf-core-problem-details-03

CoRE Working Group                                            T. Fossati
Internet-Draft                                                       arm
Intended status: Standards Track                              C. Bormann
Expires: 7 November 2022                          Universität Bremen TZI
                                                              6 May 2022

                 Concise Problem Details For CoAP APIs
                   draft-ietf-core-problem-details-03

Abstract

   This document defines a "problem detail" as a way to carry machine-
   readable details of errors in a REST response to avoid the need to
   define new error response formats for REST APIs.  The format is
   inspired by, but intended to be more concise than, the Problem
   Details for HTTP APIs defined in RFC 7807.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the CORE Working Group
   mailing list (core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/core-problem-details.

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 7 November 2022.

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Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Basic Problem Details . . . . . . . . . . . . . . . . . . . .   3
   3.  Extending Concise Problem Details . . . . . . . . . . . . . .   5
     3.1.  Standard Problem Detail Entries . . . . . . . . . . . . .   5
     3.2.  Custom Problem Detail Entries . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Standard Problem Detail Key registry  . . . . . . . . . .   9
     5.2.  Custom Problem Detail Key registry  . . . . . . . . . . .  10
     5.3.  Media Type  . . . . . . . . . . . . . . . . . . . . . . .  11
     5.4.  Content-Format  . . . . . . . . . . . . . . . . . . . . .  12
     5.5.  CBOR Tag 38 . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Language-Tagged Strings  . . . . . . . . . . . . . .  14
     A.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  14
     A.2.  Detailed Semantics  . . . . . . . . . . . . . . . . . . .  14
     A.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Appendix B.  Interworking with RFC 7807 . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   REST response status information such as CoAP response codes
   (Section 5.9 of [RFC7252]) is sometimes not sufficient to convey
   enough information about an error to be helpful.  This specification
   defines a simple and extensible framework to define CBOR [STD94] data
   items to suit this purpose.  It is designed to be reused by REST
   APIs, which can identify distinct "problem types" specific to their
   needs.  Thus, API clients can be informed of both the high-level
   error class (using the response code) and the finer-grained details
   of the problem (using this vocabulary), as shown in Figure 1.

                    .--------.          .--------.
                    |  CoAP  |          |  CoAP  |
                    | Client |          | Server |
                    '----+---'          '---+----'
                         |                  |
                         | Request          |
                         o----------------->|
                         |                  | (failure)
                         |<-----------------o
                         |   Error Response |
                         |      with a CBOR |
                         | data item giving |
                         |  Problem Details |
                         |                  |

                Figure 1: Problem Details: Example with CoAP

   The framework presented is largely inspired by the Problem Details
   for HTTP APIs defined in [RFC7807].  Appendix B discusses
   applications where interworking with [RFC7807] is required.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Basic Problem Details

   A Concise Problem Details data item is a CBOR data item with the
   following structure (notated in CDDL [RFC8610]):

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   problem-details = non-empty<{
     ? &(title: -1) => oltext
     ? &(detail: -2) => oltext
     ? &(instance: -3) => ~uri
     ? &(response-code: -4) => uint .size 1
     standard-problem-detail-entries
     custom-problem-detail-entries
   }>

   standard-problem-detail-entries = (
     * nint => any
   )

   custom-problem-detail-entries = (
     * (uint/~uri) => { + any => any }
   )

   non-empty<M> = (M) .and ({ + any => any })

   oltext = text / tag38 ; see Appendix A for tag38

                     Figure 2: Problem Detail Data Item

   A number of problem detail entries, the Standard Problem Detail
   entries, are predefined (more predefined details can be registered,
   see Section 3.1):

   The title (key -1):
      A short, human-readable summary of the problem type.  It SHOULD
      NOT change from occurrence to occurrence of the problem.

   The detail (key -2):
      A human-readable explanation specific to this occurrence of the
      problem.

   The instance (key -3):
      A URI reference that identifies the specific occurrence of the
      problem.  It may or may not yield further information if
      dereferenced.

   The response-code (key -4)
      The CoAP response code (Section 5.9 of [RFC7252]) generated by the
      origin server for this occurrence of the problem.

   Note that, unlike [RFC7807], Concise Problem Details data items have
   no explicit type.

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   Both "title" and "detail" can use either an unadorned CBOR text
   string (text) or a language-tagged text string (tag38); see
   Appendix A for the definition of the latter.

   The "title" string is advisory and included to give consumers a
   shorthand for the category of the error encountered.

   The "detail" member, if present, ought to focus on helping the client
   correct the problem, rather than giving debugging information.
   Consumers SHOULD NOT parse the "detail" member for information;
   extensions (see Section 3) are more suitable and less error-prone
   ways to obtain such information.

   Note that the "instance" URI reference may be relative; this means
   that it must be resolved relative to the representation's base URI,
   as per Section 5 of [STD66].

   Note that the "response code" value is a numeric representation of
   the actual code, so it does not take the usual form that resembles an
   HTTP status code -- 4.04 Not found is represented by the number 132.

3.  Extending Concise Problem Details

   This specification defines a generic problem type container with only
   a minimal set of attributes to make it usable.

   It is expected that applications will extend the base format by
   defining new attributes.

   These new attributes fall into two categories: generic and
   application specific.

   Generic attributes will be allocated in the standard-problem-detail-
   entries slot according to the registration procedure defined in
   Section 3.1.

   Application-specific attributes will be allocated in the custom-
   problem-detail-entries slot according to the procedure described in
   Section 3.2.

3.1.  Standard Problem Detail Entries

   Beyond the Standard Problem Detail keys defined in Figure 2,
   additional Standard Problem Detail keys can be registered for use in
   the standard-problem-detail-entries slot (see Section 5.1).

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   Standard Problem Detail keys are negative integers, so they never can
   conflict with Custom Problem Detail keys defined for a specific
   application domain (which are unsigned integers or URIs.)

   In summary, the keys for Standard Problem Detail entries are in a
   global namespace that is not specific to a particular application
   domain.

   Consumers of a Concise Problem Details instance MUST ignore any
   Standard Problem Detail entries that they do not recognize; this
   allows problem details to evolve.

3.2.  Custom Problem Detail Entries

   Applications may extend the Problem Details data item with additional
   entries to convey additional, application-specific information.

   Such new entries are allocated in the custom-problem-detail-entries
   slot, and carry a nested map specific to that application.  The map
   key can either be an (absolute!)  URI (controlled by the entity
   defining this extension), or an unsigned integer.  Only the latter
   needs to be registered (Section 5.2).

   Within the nested map any number of attributes can be given for a
   single extension.  The semantics of each custom attribute MUST be
   described in the documentation for the extension; for extension that
   are registered (i.e., are identified by an unsigned int) that
   documentation goes along with the registration.

   The unsigned integer form allows a more compact representation, in
   exchange, authors are expected to comply with the required
   registration and documentation process.  In comparison, the URI form
   is less space-efficient but requires no registration.  It is
   therefore useful for experimenting during the development cycle and
   for applications deployed in environments where producers and
   consumers of Concise Problem Details are more tightly integrated.
   (The URI form thus covers the potential need we might otherwise have
   for a "private use" range for the unsigned integers.)

   Note that the URI given for the extension is for identification
   purposes only and, even if dereferenceable in principle, MUST NOT be
   dereferenced in the normal course of handling problem details (i.e.,
   outside diagnostic/debugging procedures involving humans).

   An example of a custom extension using a URI as custom-problem-
   detail-entries key is shown in Figure 3.

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      {
        / title /         -1: "title of the error",
        / detail /        -2: "detailed information about the error",
        / instance /      -3: "coaps://pd.example/FA317434",
        / response-code / -4: 128, / 4.00 /

        "tag:3gpp.org,2022-03:TS29112": {
          / cause /  0: "machine readable error cause",
          / invalidParams / 1: [
            [
              / param / "first parameter name",
              / reason / "must be a positive integer"
            ],
            [
              / param / "second parameter name"
            ]
          ],
          / supportedFeatures / 2: "d34db33f"
        }
      }

                  Figure 3: Example Extension with URI key

   Obviously, an SDO like 3GPP can also easily register such a custom
   problem detail entry to receive a more efficient unsigned integer
   key; the same example but using a registered unsigned int as custom-
   problem-detail-entries key is shown in Figure 4.

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      {
        / title /         -1: "title of the error",
        / detail /        -2: "detailed information about the error",
        / instance /      -3: "coaps://pd.example/FA317434",
        / response-code / -4: 128, / 4.00 /

        /example value 4711 not actually registered like this:/
        4711: {
          / cause /  0: "machine readable error cause",
          / invalidParams / 1: [
            [
              / param / "first parameter name",
              / reason / "must be a positive integer"
            ],
            [
              / param / "second parameter name"
            ]
          ],
          / supportedFeatures / 2: "d34db33f"
        }
      }

       Figure 4: Example Extension with unsigned int (registered) key

   In summary, the keys for the maps used inside Custom Problem Detail
   entries are defined specifically to the identifier of that Custom
   Problem Detail entry, the documentation of which defines these
   internal entries, typically chosen to address a given application
   domain.  Consumers of a Concise Problem Details instance MUST ignore
   any Custom Problem Detail entries, or keys inside the Custom Problem
   Detail entries, that they do not recognize; this allows Custom
   Problem Detail entries to evolve and include additional information
   in the future.

4.  Security Considerations

   The security and privacy considerations outlined in Section 5 of
   [RFC7807] apply in full.

5.  IANA Considerations

   // RFC Editor: please replace RFC XXXX with this RFC number and
   // remove this note.

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5.1.  Standard Problem Detail Key registry

   This specification defines a new sub-registry for Standard Problem
   Detail Keys in the CoRE Parameters registry [IANA.core-parameters],
   with the policy "specification required" [RFC8126].

   Each entry in the registry must include:

   Key value:
      a negative integer to be used as the value of the key

   Name:
      a name that could be used in implementations for the key

   CDDL type:
      type of the data associated with the key in CDDL notation

   Brief description:
      a brief description

   reference:
      a reference document

   Initial entries in this sub-registry are as follows:

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   +=======+===============+=======+======================+===========+
   | Key   | Name          | CDDL  | Brief description    | Reference |
   | value |               | Type  |                      |           |
   +=======+===============+=======+======================+===========+
   | -1    | title         | text  | short, human-        | RFCXXXX   |
   |       |               |       | readable summary of  |           |
   |       |               |       | the problem type     |           |
   +-------+---------------+-------+----------------------+-----------+
   | -2    | detail        | text  | human-readable       | RFCXXXX   |
   |       |               |       | explanation specific |           |
   |       |               |       | to this occurrence   |           |
   |       |               |       | of the problem       |           |
   +-------+---------------+-------+----------------------+-----------+
   | -3    | instance      | ~uri  | URI reference        | RFCXXXX   |
   |       |               |       | identifying specific |           |
   |       |               |       | occurrence of the    |           |
   |       |               |       | problem              |           |
   +-------+---------------+-------+----------------------+-----------+
   | -4    | response-code | uint  | CoAP response code   | RFCXXXX   |
   |       |               | .size |                      |           |
   |       |               | 1     |                      |           |
   +-------+---------------+-------+----------------------+-----------+

     Table 1: Initial Entries in Standard Problem Detail Key registry

5.2.  Custom Problem Detail Key registry

   This specification defines a new sub-registry for Custom Problem
   Detail Keys in the CoRE Parameters registry [IANA.core-parameters],
   with the policy "first come first served" [RFC8126].

   Each entry in the registry must include:

   Key value:
      an unsigned integer to be used as the value of the key

   Name:
      a name that could be used in implementations for the key

   Brief description:
      a brief description

   Reference:
      a reference document that provides a description of the map,
      including a CDDL description, that describes all inside keys and
      values

   Initial entries in this sub-registry are as follows:

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      +=======+=============+===========================+===========+
      | Key   | Name        | Brief description         | Reference |
      | value |             |                           |           |
      +=======+=============+===========================+===========+
      | 7807  | tunnel-7807 | Carry RFC 7807 problem    | RFCXXXX   |
      |       |             | details in a Concise      |           |
      |       |             | Problem Details data item |           |
      +-------+-------------+---------------------------+-----------+

       Table 2: Initial Entries in Custom Problem Detail Key registry

5.3.  Media Type

   IANA is requested to add the following Media-Type to the "Media
   Types" registry [IANA.media-types].

   +============================+============================+=========+
   |Name                        |Template                    |Reference|
   +============================+============================+=========+
   |concise-problem-details+cbor|application/concise-problem-|RFCXXXX, |
   |                            |details+cbor                |Section  |
   |                            |                            |5.3      |
   +----------------------------+----------------------------+---------+

      Table 3: New Media Type application/concise-problem-details+cbor

   Type name:  application
   Subtype name:  concise-problem-details+cbor
   Required parameters:  none
   Optional parameters:  none
   Encoding considerations:  binary (CBOR data item)
   Security considerations:  Section 4 of RFC XXXX
   Interoperability considerations:  none
   Published specification:  Section 5.3 of RFC XXXX
   Applications that use this media type:  Clients and servers in the
      Internet of Things
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/cbor".  (At
      publication of RFC XXXX, there is no fragment identification
      syntax defined for "application/cbor".)
   Person & email address to contact for further information:  CoRE WG
      mailing list (core@ietf.org), or IETF Applications and Real-Time
      Area (art@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  none
   Author/Change controller:  IETF
   Provisional registration:  no

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5.4.  Content-Format

   IANA is requested to register a Content-Format number in the "CoAP
   Content-Formats" sub-registry, within the "Constrained RESTful
   Environments (CoRE) Parameters" Registry [IANA.core-parameters], as
   follows:

   +==============================+================+======+===========+
   | Content-Type                 | Content Coding | ID   | Reference |
   +==============================+================+======+===========+
   | application/concise-problem- | -              | TBD1 | RFC XXXX  |
   | details+cbor                 |                |      |           |
   +------------------------------+----------------+------+-----------+

                       Table 4: New Content-Format

   TBD1 is to be assigned from the space 256..999.

   In the registry as defined by Section 12.3 of [RFC7252] at the time
   of writing, the column "Content-Type" is called "Media type" and the
   column "Content Coding" is called "Encoding".
   // This paragraph to be removed by RFC editor.

5.5.  CBOR Tag 38

   In the registry "CBOR Tags" [IANA.cbor-tags], IANA has registered
   CBOR Tag 38.  IANA is requested to replace the reference for this
   registration with Appendix A, RFC XXXX.

6.  References

6.1.  Normative References

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <https://www.iana.org/assignments/cbor-tags>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters>.

   [IANA.media-types]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

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   [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/rfc/rfc2119>.

   [RFC4647]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
              Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
              2006, <https://www.rfc-editor.org/rfc/rfc4647>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/rfc/rfc5646>.

   [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/rfc/rfc7252>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7807>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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/rfc/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [STD66]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [STD94]    Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

6.2.  Informative References

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   [I-D.ietf-httpapi-rfc7807bis]
              Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
              for HTTP APIs", Work in Progress, Internet-Draft, draft-
              ietf-httpapi-rfc7807bis-02, 16 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-
              rfc7807bis-02>.

   [RDF]      Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
              Concepts and Abstract Syntax", W3C Recommendation, 25
              February 2014,
              <http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [Unicode-14.0.0]
              The Unicode Consortium, "The Unicode Standard, Version
              14.0.0", Mountain View: The Unicode Consortium,
              ISBN 978-1-936213-29-0, September 2021,
              <https://www.unicode.org/versions/Unicode14.0.0/>.  Note
              that while this document references a version that was
              recent at the time of writing, the statements made based
              on this version are expected to remain valid for future
              versions.

Appendix A.  Language-Tagged Strings

   This appendix serves as the archival documentation for CBOR Tag 38, a
   tag for serializing language-tagged text strings in CBOR.  The text
   of this appendix is adapted from the specification text supplied for
   its initial registration.  It has been extended to allow
   supplementing the language tag by a direction indication.

A.1.  Introduction

   In some cases it is useful to specify the natural language of a text
   string.  This specification defines a tag that does just that.  One
   technology that supports language-tagged strings is the Resource
   Description Framework (RDF) [RDF].

A.2.  Detailed Semantics

   A language-tagged string in CBOR has the tag 38 and consists of an
   array with a length of 2 or 3.

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   The first element is a well-formed language tag under Best Current
   Practice 47 ([RFC5646] and [RFC4647]), represented as a UTF-8 text
   string (major type 3).

   The second element is an arbitrary UTF-8 text string (major type 3).
   Both the language tag and the arbitrary string can optionally be
   annotated with CBOR tags; this is not shown in the CDDL below.

   The optional third element, if present, is a Boolean value that
   indicates a direction: false for "ltr" direction, true for "rtl"
   direction.  If the third element is absent, no indication is made
   about the direction.

   In CDDL:

   tag38 = #6.38([tag38-ltag, text, ?tag38-direction])
   tag38-ltag = text  .abnf ("Language-Tag" .det RFC5646)
   tag38-direction = &(ltr: false, rtl: true)

   RFC5646 = '
    Language-Tag  = langtag             ; normal language tags
                  / privateuse          ; private use tag
                  / legacy              ; legacy tags

    langtag       = language
                    ["-" script]
                    ["-" region]
                    *("-" variant)
                    *("-" extension)
                    ["-" privateuse]

    language      = 2*3ALPHA            ; shortest ISO 639 code
                    ["-" extlang]       ; sometimes followed by
                                        ; extended language subtags
                  / 4ALPHA              ; or reserved for future use
                  / 5*8ALPHA            ; or registered language subtag

    extlang       = 3ALPHA              ; selected ISO 639 codes
                    *2("-" 3ALPHA)      ; permanently reserved

    script        = 4ALPHA              ; ISO 15924 code

    region        = 2ALPHA              ; ISO 3166-1 code
                  / 3DIGIT              ; UN M.49 code

    variant       = 5*8alphanum         ; registered variants
                  / (DIGIT 3alphanum)

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    extension     = singleton 1*("-" (2*8alphanum))

                                        ; Single alphanumerics
                                        ; "x" reserved for private use
    singleton     = DIGIT               ; 0 - 9
                  / %x41-57             ; A - W
                  / %x59-5A             ; Y - Z
                  / %x61-77             ; a - w
                  / %x79-7A             ; y - z

    privateuse    = "x" 1*("-" (1*8alphanum))

    legacy        = irregular / regular ; different word in RFC

    irregular     = "en-GB-oed" / "i-ami" / "i-bnn" / "i-default" /
                  "i-enochian" / "i-hak" / "i-klingon" / "i-lux" /
                  "i-mingo" / "i-navajo" / "i-pwn" / "i-tao" / "i-tay" /
                  "i-tsu" / "sgn-BE-FR" / "sgn-BE-NL" / "sgn-CH-DE"

    regular       = "art-lojban" / "cel-gaulish" / "no-bok" / "no-nyn" /
                  "zh-guoyu" / "zh-hakka" / "zh-min" / "zh-min-nan" /
                  "zh-xiang"

    alphanum      = (ALPHA / DIGIT)     ; letters and numbers

    ALPHA         =  %x41-5A / %x61-7A  ; A-Z / a-z
    DIGIT         =  %x30-39            ; 0-9
   '

   NOTE: Language tags of any combination of case are allowed.  But
   section 2.1.1 of [RFC5646], part of Best Current Practice 47,
   recommends a case combination for language tags, that encoders that
   support tag 38 may wish to follow when generating language tags.

   Data items with tag 38 that do not meet the criteria above are
   invalid (see Section 5.3.2 of [STD94]).

   NOTE: The Unicode Standard [Unicode-14.0.0] includes a set of
   characters designed for tagging text (including language tagging), in
   the range U+E0000 to U+E007F.  Although many applications, including
   RDF, do not disallow these characters in text strings, the Unicode
   Consortium has deprecated these characters and recommends annotating
   language via a higher-level protocol instead.  See the section
   "Deprecated Tag Characters" in Section 23.9 of [Unicode-14.0.0].

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A.3.  Examples

   Examples in this section are given in CBOR diagnostic mode, and then
   as a pretty-printed hexadecimal representation of the encoded item.

   The following example shows how the English-language string "Hello"
   is represented.

   38(["en", "Hello"])

   D8 26               # tag(38)
      82               # array(2)
         62            # text(2)
            656E       # "en"
         65            # text(5)
            48656C6C6F # "Hello"

   The following example shows how the French-language string "Bonjour"
   is represented.

   38(["fr", "Bonjour"])

   D8 26                   # tag(38)
      82                   # array(2)
         62                # text(2)
            6672           # "fr"
         67                # text(7)
            426F6E6A6F7572 # "Bonjour"

   The following example shows how the Hebrew-language string "שלום"
   (HEBREW LETTER SHIN, HEBREW LETTER LAMED, HEBREW LETTER VAV, HEBREW
   LETTER FINAL MEM, U+05E9 U+05DC U+05D5 U+05DD) is represented.  Note
   the rtl direction expressed by setting the third element in the array
   to "true".

   38(["he", "שלום", true])

   D8 26                     # tag(38)
      83                     # array(3)
         62                  # text(2)
            6865             # "he"
         68                  # text(8)
            D7A9D79CD795D79D # "שלום"
         F5                  # primitive(21)

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Appendix B.  Interworking with RFC 7807

   On certain occasions, it will be necessary to carry ("tunnel")
   [RFC7807] problem details in a Concise Problem Details item.

   This appendix defines a Custom Problem Details entry for that
   purpose.  This is assigned Custom Problem Detail key 7807 in
   Section 5.2.  Its structure is:

   tunnel-7807 = {
     ? &(type: 0) => ~uri
     ? &(status: 1) => 0..999
     * text => any
   }

   To carry an [RFC7807] problem details JSON object in a Concise
   Problem Details item, first convert the JSON object to CBOR as per
   Section 6.2 of [STD94].  Create an empty Concise Problem Details data
   item.

   Move the values for "title", "detail", and "instance", if present,
   from the [RFC7807] problem details to the equivalent Standard Problem
   Detail entries.  Create a Custom Problem Detail entry with key 7807.
   Move the values for "type" and "status", if present, to the
   equivalent keys 0 and 1 of the Custom Problem Detail entry.  Move all
   remaining key/value pairs (additional members as per Section 3.2 of
   [RFC7807]) in the converted [RFC7807] problem details object to the
   Custom Problem Details map unchanged.

   The inverse direction, carrying Concise Problem Details in a Problem
   Details JSON object requires the additional support provided by
   [I-D.ietf-httpapi-rfc7807bis], which is planned to create the HTTP
   Problem Types Registry.  A Problem Type can then be registered that
   extracts top-level items from the Concise Problem Details item in a
   similar way to the conversion described above, and which carries the
   rest of the Concise Problem Details item in an additional member via
   base64url encoding without padding (Section 5 of [RFC4648]).  Details
   can be defined in a separate document when the work on
   [I-D.ietf-httpapi-rfc7807bis] is completed.

Acknowledgments

   Mark Nottingham and Erik Wilde, authors of RFC 7807.  Klaus Hartke
   and Jaime Jiménez, co-authors of an earlier generation of this
   specification.  Christian Amsüss and Marco Tiloca for review and
   comments on this document.

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   For Appendix A, John Cowan and Doug Ewell are also to be
   acknowledged.  The content of an earlier version of this appendix was
   also discussed in the "apps-discuss at ietf.org" and "ltru at
   ietf.org" mailing lists.

Contributors

   Peter Occil
   Email: poccil14 at gmail dot com
   URI:   http://peteroupc.github.io/CBOR/

   Peter defined CBOR tag 38, basis of Appendix A.

Authors' Addresses

   Thomas Fossati
   arm
   Email: thomas.fossati@arm.com

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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