| Internet-Draft | CoRE Problem Details | May 2022 |
| Fossati & Bormann | Expires 26 November 2022 | [Page] |
This document defines a concise "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 for constrained environments. The format is inspired by, but intended to be more concise than, the Problem Details for HTTP APIs defined in RFC 7807.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-core-problem-details/.¶
Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (mailto: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.¶
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 26 November 2022.¶
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.¶
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 shapes of these data items 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 the vocabulary defined here). This pattern of communication is illustrated in Figure 1.¶
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.¶
The terminology from [RFC7252] and [STD94] applies. Readers are also expected to be familiar with the terminology from [RFC7807].¶
In this document, the structure of data is specified in CDDL [RFC8610] [RFC9165].¶
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.¶
A Concise Problem Details data item is a CBOR data item with the following structure:¶
problem-details = non-empty<{
? &(title: -1) => oltext
? &(detail: -2) => oltext
? &(instance: -3) => ~uri
? &(response-code: -4) => uint .size 1
? &(base-uri: -5) => ~uri
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
A number of problem detail entries, the Standard Problem Detail entries, are predefined (more predefined details can be registered, see Section 3.1).¶
Note that, unlike [RFC7807], Concise Problem Details data items have no explicit "problem type". Instead, the category (or, one could say, Gestalt) of the problem can be understood from the shape of the problem details offered. We talk of a "problem shape" for short.¶
A short, human-readable summary of the problem shape. It SHOULD NOT change from occurrence to occurrence of the same problem shape.¶
A human-readable explanation specific to this occurrence of the problem.¶
A URI reference that identifies the specific occurrence of the problem. It may or may not yield further information if dereferenced.¶
The CoAP response code (Sections 5.9 and 12.1.2 of [RFC7252]) generated by the origin server for this occurrence of the problem.¶
The base URI (Section 5.1 of [STD66]) that should be used to resolve relative URI references embedded in this Concise Problem Details data item.¶
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 (problem shape) 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].¶
The "response-code" member, if present, is only advisory; it conveys
the CoAP response code used for the convenience of the consumer.
Generators MUST use the same response code here as in the actual CoAP
response; the latter is needed to assure that generic CoAP software that
does not understand the problem-details format still behaves
correctly.
Consumers can use the response-code member to determine what the
original response code used by the generator was, in cases where it
has been changed (e.g., by an intermediary or cache), and when message
bodies persist without CoAP information (e.g., in an events log or analytics
database).
Generic CoAP software will still use the CoAP response code.
To support the use case of message body persistence without support by
the problem-details generator, the entity that persists the Concise
Problem Details data item can copy over the CoAP response code
that it received on the CoAP level.
Note that the "response-code" value is a numeric representation of the
actual code (see Section 3 of [RFC7252]), so it does not take the usual
presentation form that resembles an
HTTP status code -- 4.04 Not found is represented by the number 132.¶
The "base-uri" member is usually not present in the initial request-response communication as it can be inferred as per Section 5.1.3 of [STD66]. An entity that stores a Concise Problem Details data item or otherwise makes it available for consumers without this context might add in a base-uri member to allow those consumers to perform resolution of any relative URI references embedded in the data item.¶
This specification defines a generic problem details 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.¶
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).¶
Standard Problem Detail keys are negative integers, so they can never 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 data item MUST ignore any Standard Problem Detail entries that they do not recognize; this allows problem details to evolve.¶
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 (under control of 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 extensions 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, it 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.¶
{
/ 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"
}
}
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.¶
{
/ 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"
}
}
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 data item 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. The assumption is that this is done in a backward and forward compatible way.¶
Sometimes, Custom Problem Detail entries can evolve in a way where forward compatibility by "ignore unknown" would not be appropriate: e.g., when needing to add a "must-understand" member, which can only be ignored at the peril of misunderstanding the Concise Problem Details data item ("false interoperability"). In this case, a new Custom Problem Detail key can simply be registered for this case, keeping the old key backward and forward-compatible.¶
The security and privacy considerations outlined in Section 5 of [RFC7807] apply in full.¶
RFC Editor: please replace RFC XXXX with this RFC number and remove this note.¶
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:¶
a negative integer to be used as the value of the key¶
a name that could be used in implementations for the key¶
type of the data associated with the key in CDDL notation¶
a brief description¶
a reference document¶
Initial entries in this sub-registry are as follows:¶
| Key value | Name | CDDL Type | Brief description | Reference |
|---|---|---|---|---|
| -1 | title | text or tag 38 | short, human-readable summary of the problem shape | RFC XXXX |
| -2 | detail | text or tag 38 | human-readable explanation specific to this occurrence of the problem | RFC XXXX |
| -3 | instance | ~uri | URI reference identifying specific occurrence of the problem | RFC XXXX |
| -4 | response-code | uint .size 1 | CoAP response code | RFC XXXX |
| -5 | base-uri | ~uri | Base URI | RFC XXXX |
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:¶
an unsigned integer to be used as the value of the key¶
a name that could be used in implementations for the key¶
a brief description¶
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:¶
| Key value | Name | Brief description | Reference |
|---|---|---|---|
| 7807 | tunnel-7807 | Carry RFC 7807 problem details in a Concise Problem Details data item | RFC XXXX |
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-details+cbor | RFC XXXX, Section 5.3 |
application¶
concise-problem-details+cbor¶
none¶
none¶
binary (CBOR data item)¶
none¶
Section 5.3 of RFC XXXX¶
Clients and servers in the Internet of Things¶
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".)¶
CoRE WG mailing list (core@ietf.org), or IETF Applications and Real-Time Area (art@ietf.org)¶
COMMON¶
none¶
IETF¶
no¶
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-details+cbor | - | TBD1 | RFC XXXX |
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.¶
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.¶
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.¶
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 language-tagged string in CBOR has the tag 38 and consists of an array with a length of 2 or 3.¶
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; it can be explicitly given as null to express
the same while overriding any context that might be considered
applying to this element.
Note that the proper processing of Language and Direction Metadata is
an active area of investigation; the reader is advised to consult
ongoing standardization activities such as [STRING-META] when
processing the information represented in this tag.¶
In CDDL:¶
tag38 = #6.38([tag38-ltag, text, ?tag38-direction])
tag38-ltag = text .abnf ("Language-Tag" .det RFC5646)
tag38-direction = &(ltr: false, rtl: true, auto: null)
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)
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].¶
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)
¶
On certain occasions, it will be necessary to carry ("tunnel") [RFC7807] problem details in a Concise Problem Details data 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 data 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 Details entry with key 7807. Move the values for "type" and "status", if present, to the equivalent keys 0 and 1 of the Custom Problem Details 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. An HTTP Problem Type can then be registered that extracts top-level items from the Concise Problem Details data item in a similar way to the conversion described above, and which carries the rest of the Concise Problem Details data 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.¶
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, Marco Tiloca, Ari Keränen and Michael Richardson for review and comments on this document.¶
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.¶
Peter defined CBOR tag 38, basis of Appendix A.¶