Problem Details for HTTP APIs
draft-ietf-httpapi-rfc7807bis-01

HTTPAPI                                                    M. Nottingham
Internet-Draft                                                          
Obsoletes: 7807 (if approved)                                   E. Wilde
Intended status: Standards Track                                        
Expires: 16 April 2022                                          S. Dalal
                                                         13 October 2021

                     Problem Details for HTTP APIs
                    draft-ietf-httpapi-rfc7807bis-01

Abstract

   This document defines a "problem detail" as a way to carry machine-
   readable details of errors in a HTTP response to avoid the need to
   define new error response formats for HTTP APIs.

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-httpapi/rfc7807bis.

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 16 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The Problem Details JSON Object . . . . . . . . . . . . . . .   4
     3.1.  Members of a Problem Details Object . . . . . . . . . . .   5
       3.1.1.  "type"  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  "status"  . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.3.  "title" . . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.4.  "detail"  . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.5.  "instance"  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Extension Members . . . . . . . . . . . . . . . . . . . .   8
   4.  Defining New Problem Types  . . . . . . . . . . . . . . . . .   8
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Registered Problem Types  . . . . . . . . . . . . . . . .  10
       4.2.1.  about:blank . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  JSON Schema for HTTP Problems  . . . . . . . . . . .  14
   Appendix B.  HTTP Problems and XML  . . . . . . . . . . . . . . .  15
   Appendix C.  Using Problem Details with Other Formats . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   HTTP [HTTP] status codes are sometimes not sufficient to convey
   enough information about an error to be helpful.  While humans behind
   Web browsers can be informed about the nature of the problem with an
   HTML [HTML5] response body, non-human consumers of so-called "HTTP
   APIs" are usually not.

   This specification defines simple JSON [RFC8259] and XML [XML]
   document formats to suit this purpose.  They are designed to be
   reused by HTTP APIs, which can identify distinct "problem types"
   specific to their needs.

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   Thus, API clients can be informed of both the high-level error class
   (using the status code) and the finer-grained details of the problem
   (using one of these formats).

   For example, consider a response that indicates that the client's
   account doesn't have enough credit.  The 403 Forbidden status code
   might be deemed most appropriate to use, as it will inform HTTP-
   generic software (such as client libraries, caches, and proxies) of
   the general semantics of the response.

   However, that doesn't give the API client enough information about
   why the request was forbidden, the applicable account balance, or how
   to correct the problem.  If these details are included in the
   response body in a machine-readable format, the client can treat it
   appropriately; for example, triggering a transfer of more credit into
   the account.

   This specification does this by identifying a specific type of
   problem (e.g., "out of credit") with a URI [RFC3986]; HTTP APIs can
   do this by nominating new URIs under their control, or by reusing
   existing ones.

   Additionally, problem details can contain other information, such as
   a URI that identifies the specific occurrence of the problem
   (effectively giving an identifier to the concept "The time Joe didn't
   have enough credit last Thursday"), which can be useful for support
   or forensic purposes.

   The data model for problem details is a JSON [RFC8259] object; when
   formatted as a JSON document, it uses the "application/problem+json"
   media type.  Appendix B defines how to express them in an equivalent
   XML format, which uses the "application/problem+xml" media type.

   Note that problem details are (naturally) not the only way to convey
   the details of a problem in HTTP; if the response is still a
   representation of a resource, for example, it's often preferable to
   accommodate describing the relevant details in that application's
   format.  Likewise, in many situations, there is an appropriate HTTP
   status code that does not require extra detail to be conveyed.

   Instead, the aim of this specification is to define common error
   formats for those applications that need one, so that they aren't
   required to define their own, or worse, tempted to redefine the
   semantics of existing HTTP status codes.  Even if an application
   chooses not to use it to convey errors, reviewing its design can help
   guide the design decisions faced when conveying errors in an existing
   format.

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2.  Requirements

   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.

3.  The Problem Details JSON Object

   The canonical model for problem details is a JSON [RFC8259] object.

   When serialized as a JSON document, that format is identified with
   the "application/problem+json" media type.

   For example, an HTTP response carrying JSON problem details:

   HTTP/1.1 403 Forbidden
   Content-Type: application/problem+json
   Content-Language: en

   {
    "type": "https://example.com/probs/out-of-credit",
    "title": "You do not have enough credit.",
    "detail": "Your current balance is 30, but that costs 50.",
    "instance": "/account/12345/msgs/abc",
    "balance": 30,
    "accounts": ["/account/12345",
                 "/account/67890"]
   }

   Here, the out-of-credit problem (identified by its type URI)
   indicates the reason for the 403 in "title", gives a reference for
   the specific problem occurrence with "instance", gives occurrence-
   specific details in "detail", and adds two extensions; "balance"
   conveys the account's balance, and "accounts" gives links where the
   account can be topped up.

   The ability to convey problem-specific extensions allows more than
   one problem to be conveyed.  For example:

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   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json
   Content-Language: en

   {
   "type": "https://example.net/validation-error",
   "title": "Your request parameters didn't validate.",
   "invalid_params": [ {
                         "name": "age",
                         "reason": "must be a positive integer"
                       },
                       {
                         "name": "color",
                         "reason": "must be 'green', 'red' or 'blue'"}
                     ]
   }

   Note that this requires each of the subproblems to be similar enough
   to use the same HTTP status code.  If they do not, the 207 (Multi-
   Status) code [RFC4918] could be used to encapsulate multiple status
   messages.

3.1.  Members of a Problem Details Object

   Problem detail objects can have the following members.  If the type
   of a member's value does not match the specified type, the member
   MUST be ignored -- i.e., processing will continue as if the member
   had not been present.

3.1.1.  "type"

   The "type" member is a JSON string containing a URI reference
   [RFC3986] that identifies the problem type.  Consumers MUST use the
   "type" URI (after resolution, if necessary) as the primary identifier
   for the problem type.

   When this member is not present, its value is assumed to be
   "about:blank".

   If the type URI is a locator (e.g., those with a "http" or "https"
   scheme), dereferencing it SHOULD provide human-readable documentation
   for the problem type (e.g., using HTML [HTML5]).  However, consumers
   SHOULD NOT automatically dereference the type URI, unless they do so
   in the course of providing information to developers (e.g., when a
   debugging tool is in use).

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   When "type" contains a relative URI, it is resolved relative to the
   document's base URI, as per [RFC3986], Section 5.  However, using
   relative URIs can cause confusion, and they might not be handled
   correctly by all implementations.

   For example, if the two resources "https://api.example.org/foo/
   bar/123" and "https://api.example.org/widget/456" both respond with a
   "type" equal to the relative URI reference "example-problem", when
   resolved they will identify different resources
   ("https://api.example.org/foo/bar/example-problem" and
   "https://api.example.org/widget/example-problem" respectively).  As a
   result, it is RECOMMENDED that absolute URIs be used in "type" when
   possible, and that when relative URIs are used, they include the full
   path (e.g., "/types/123").

   The type URI can also be a non-resolvable URI.  For example, the tag
   URI scheme [RFC4151] can be used to uniquely identify problem types:

   tag:mnot@mnot.net,2021-09-17:OutOfLuck

   Non-resolvable URIs ought not be used when there is some future
   possibility that it might become desireable to do so.  For example,
   if the URI above were used in an API and later a tool was adopted
   that resolves type URIs to discover information about the error,
   taking advantage of that capability would require switching to a
   resolvable URI, thereby creating a new identity for the problem type
   and thus introducing a breaking change.

3.1.2.  "status"

   The "status" member is a JSON number indicating the HTTP status code
   ([HTTP], Section 15) generated by the origin server for this
   occurrence of the problem.

   The "status" member, if present, is only advisory; it conveys the
   HTTP status code used for the convenience of the consumer.
   Generators MUST use the same status code in the actual HTTP response,
   to assure that generic HTTP software that does not understand this
   format still behaves correctly.  See Section 5 for further caveats
   regarding its use.

   Consumers can use the status member to determine what the original
   status 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 HTTP information.  Generic HTTP software will still
   use the HTTP status code.

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3.1.3.  "title"

   The "title" member is a JSON string containing a short, human-
   readable summary of the problem type.

   It SHOULD NOT change from occurrence to occurrence of the problem,
   except for purposes of localization (e.g., using proactive content
   negotiation; see [HTTP], Section 12.1).

   The "title" string is advisory and included only for users who are
   not aware of the semantics of the URI and do not have the ability to
   discover them (e.g., offline log analysis).

3.1.4.  "detail"

   The "detail" member is a JSON string containing a human-readable
   explanation specific to this occurrence of the problem.

   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 are more suitable and less error-prone ways to obtain such
   information.

3.1.5.  "instance"

   The "instance" member is a JSON string containing a URI reference
   that identifies the specific occurrence of the problem.

   When the "instance" URI is dereferenceable, the problem details
   object can be fetched from it.  It might also return information
   about the problem occurrence in other formats through use of
   proactive content negotiation (see [HTTP], Section 12.5.1).

   When the "instance" URI is not dereferenceable, it serves as a unique
   identifier for the problem occurrence that may be of significance to
   the server, but is opaque to the client.

   When "instance" contains a relative URI, it is resolved relative to
   the document's base URI, as per [RFC3986], Section 5.  However, using
   relative URIs can cause confusion, and they might not be handled
   correctly by all implementations.

   For example, if the two resources "https://api.example.org/foo/
   bar/123" and "https://api.example.org/widget/456" both respond with
   an "instance" equal to the relative URI reference "example-instance",
   when resolved they will identify different resources

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   ("https://api.example.org/foo/bar/example-instance" and
   "https://api.example.org/widget/example-instance" respectively).  As
   a result, it is RECOMMENDED that absolute URIs be used in "instance"
   when possible, and that when relative URIs are used, they include the
   full path (e.g., "/instances/123").

3.2.  Extension Members

   Problem type definitions MAY extend the problem details object with
   additional members.

   For example, our "out of credit" problem above defines two such
   extensions -- "balance" and "accounts" to convey additional, problem-
   specific information.

   Clients consuming problem details MUST ignore any such extensions
   that they don't recognize; this allows problem types to evolve and
   include additional information in the future.

   Note that because extensions are effectively put into a namespace by
   the problem type, it is not possible to define new "standard" members
   without defining a new media type.

4.  Defining New Problem Types

   When an HTTP API needs to define a response that indicates an error
   condition, it might be appropriate to do so by defining a new problem
   type.

   Before doing so, it's important to understand what they are good for,
   and what's better left to other mechanisms.

   Problem details are not a debugging tool for the underlying
   implementation; rather, they are a way to expose greater detail about
   the HTTP interface itself.  Designers of new problem types need to
   carefully consider the Security Considerations (Section 5), in
   particular, the risk of exposing attack vectors by exposing
   implementation internals through error messages.

   Likewise, truly generic problems -- i.e., conditions that could
   potentially apply to any resource on the Web -- are usually better
   expressed as plain status codes.  For example, a "write access
   disallowed" problem is probably unnecessary, since a 403 Forbidden
   status code in response to a PUT request is self-explanatory.

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   Finally, an application might have a more appropriate way to carry an
   error in a format that it already defines.  Problem details are
   intended to avoid the necessity of establishing new "fault" or
   "error" document formats, not to replace existing domain-specific
   formats.

   That said, it is possible to add support for problem details to
   existing HTTP APIs using HTTP content negotiation (e.g., using the
   Accept request header to indicate a preference for this format; see
   [HTTP], Section 12.5.1).

   New problem type definitions MUST document:

   1.  a type URI (typically, with the "http" or "https" scheme),

   2.  a title that appropriately describes it (think short), and

   3.  the HTTP status code for it to be used with.

   Problem type definitions MAY specify the use of the Retry-After
   response header ([HTTP], Section 10.2.3) in appropriate
   circumstances.

   A problem's type URI SHOULD resolve to HTML [HTML5] documentation
   that explains how to resolve the problem.

   A problem type definition MAY specify additional members on the
   problem details object.  For example, an extension might use typed
   links [RFC8288] to another resource that can be used by machines to
   resolve the problem.

   If such additional members are defined, their names SHOULD start with
   a letter (ALPHA, as per [RFC5234], Appendix B.1) and SHOULD consist
   of characters from ALPHA, DIGIT ([RFC5234], Appendix B.1), and "_"
   (so that it can be serialized in formats other than JSON), and they
   SHOULD be three characters or longer.

4.1.  Example

   For example, if you are publishing an HTTP API to your online
   shopping cart, you might need to indicate that the user is out of
   credit (our example from above), and therefore cannot make the
   purchase.

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   If you already have an application-specific format that can
   accommodate this information, it's probably best to do that.
   However, if you don't, you might consider using one of the problem
   details formats -- JSON if your API is JSON-based, or XML if it uses
   that format.

   To do so, you might look for an already-defined type URI that suits
   your purposes.  If one is available, you can reuse that URI.

   If one isn't available, you could mint and document a new type URI
   (which ought to be under your control and stable over time), an
   appropriate title and the HTTP status code that it will be used with,
   along with what it means and how it should be handled.

   In summary: an instance URI will always identify a specific
   occurrence of a problem.  On the other hand, type URIs can be reused
   if an appropriate description of a problem type is already available
   someplace else, or they can be created for new problem types.

4.2.  Registered Problem Types

   This specification defines the HTTP Problem Type registry for common,
   widely-used problem type URIs, to promote reuse.

   Registration requests are reviewed and approved by a Designated
   Expert, as per [RFC8126], Section 4.5.  A specification document is
   appreciated, but not required.

   When evaluating requests the Expert(s) should consider community
   feedback, how well-defined the problem type is, and this
   specification's requirements.  Vendor-specific, application-specific,
   and deployment-specific values are not registrable.

   Registrations MAY use the prefix "https://iana.org/assignments/http-
   problem-types#", and are encouraged to do so when a stable, neutral
   URI is desirable.

   Registration requests should use the following template:

   *  Type URI: [a URI for the problem type]

   *  Title: [a short description of the problem type]

   *  Recommended HTTP status code: [what status code is most
      appropriate to use with the type]

   *  Reference: [to a specification defining the type]

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   See the registry at https://iana.org/assignments/http-problem-types
   (https://iana.org/assignments/http-problem-types) for details on
   where to send registration requests.

4.2.1.  about:blank

   This specification registers one Problem Type, "about:blank".

   *  Type URI: about:blank

   *  Title: See HTTP Status Code

   *  Recommended HTTP status code: N/A

   *  Reference: [this document]

   The "about:blank" URI [RFC6694], when used as a problem type,
   indicates that the problem has no additional semantics beyond that of
   the HTTP status code.

   When "about:blank" is used, the title SHOULD be the same as the
   recommended HTTP status phrase for that code (e.g., "Not Found" for
   404, and so on), although it MAY be localized to suit client
   preferences (expressed with the Accept-Language request header).

   Please note that according to how the "type" member is defined
   (Section 3.1), the "about:blank" URI is the default value for that
   member.  Consequently, any problem details object not carrying an
   explicit "type" member implicitly uses this URI.

5.  Security Considerations

   When defining a new problem type, the information included must be
   carefully vetted.  Likewise, when actually generating a problem --
   however it is serialized -- the details given must also be
   scrutinized.

   Risks include leaking information that can be exploited to compromise
   the system, access to the system, or the privacy of users of the
   system.

   Generators providing links to occurrence information are encouraged
   to avoid making implementation details such as a stack dump available
   through the HTTP interface, since this can expose sensitive details
   of the server implementation, its data, and so on.

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   The "status" member duplicates the information available in the HTTP
   status code itself, thereby bringing the possibility of disagreement
   between the two.  Their relative precedence is not clear, since a
   disagreement might indicate that (for example) an intermediary has
   modified the HTTP status code in transit (e.g., by a proxy or cache).

   As such, those defining problem types as well as generators and
   consumers of problems need to be aware that generic software (such as
   proxies, load balancers, firewalls, and virus scanners) are unlikely
   to know of or respect the status code conveyed in this member.

6.  IANA Considerations

   Please update the "application/problem+json" and "application/
   problem+xml" registrations in the Internet media types registry
   [RFC6838]. to refer to this document.

   Please create the HTTP Problem Types Registry, as specified in
   Section 4.2, and populate it with "about:blank" as per Section 4.2.1.

7.  References

7.1.  Normative References

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [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>.

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

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

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   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, 26 November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126>.

7.2.  Informative References

   [HTML5]    WHATWG, "HTML - Living Standard", n.d.,
              <https://html.spec.whatwg.org>.

   [I-D.draft-bhutton-json-schema-00]
              Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON
              Schema: A Media Type for Describing JSON Documents", Work
              in Progress, Internet-Draft, draft-bhutton-json-schema-00,
              8 December 2020, <https://datatracker.ietf.org/doc/html/
              draft-bhutton-json-schema-00>.

   [ISO-19757-2]
              International Organization for Standardization,
              "Information Technology -- Document Schema Definition
              Languages (DSDL) -- Part 2: Grammar-based Validation --
              RELAX NG", ISO/IEC 19757-2, 2003.

   [RDFA]     Adida, B., Birbeck, M., McCarron, S., and I. Herman, "RDFa
              Core 1.1 - Third Edition", World Wide Web Consortium
              Recommendation REC-rdfa-core-20150317, 17 March 2015,
              <https://www.w3.org/TR/2015/REC-rdfa-core-20150317>.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,
              <https://www.rfc-editor.org/rfc/rfc4151>.

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   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,
              <https://www.rfc-editor.org/rfc/rfc4918>.

   [RFC6694]  Moonesamy, S., Ed., "The "about" URI Scheme", RFC 6694,
              DOI 10.17487/RFC6694, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6694>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/rfc/rfc6838>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8288>.

   [XSLT]     Clark, J., Pieters, S., and H. Thompson, "Associating
              Style Sheets with XML documents 1.0 (Second Edition)",
              World Wide Web Consortium Recommendation REC-xml-
              stylesheet-20101028, 28 October 2010,
              <https://www.w3.org/TR/2010/REC-xml-stylesheet-20101028>.

Appendix A.  JSON Schema for HTTP Problems

   This section presents a non-normative JSON Schema
   [I-D.draft-bhutton-json-schema-00] for HTTP Problem Details.  If
   there is any disagreement between it and the text of the
   specification, the latter prevails.

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   # NOTE: '\' line wrapping per RFC 8792
   {
     "$schema": "https://json-schema.org/draft/2020-12/schema",
     "title": "A problem object RFC 7807bis",
     "type": "object",
     "properties": {
       "type": {
         "type": "string",
         "format": "uri-reference",
         "description": "A URI reference RFC3986 that identifies the \
   problem type."
       },
       "title": {
         "type": "string",
         "description": "A short, human-readable summary of the \
   problem type. It SHOULD NOT change from occurrence to occurrence \
   of the problem, except for purposes of localization (e.g., using \
   proactive content negotiation; see RFC7231, Section 3.4)"
       },
       "status": {
         "type": "integer",
         "description": "The HTTP status code (RFC7231, Section 6) \
   generated by the origin server for this occurrence of the problem.",
         "minimum": 100,
         "maximum": 599
       },
       "detail": {
         "type": "string",
         "description": "A human-readable explanation specific to \
   this occurrence of the problem."
       },
       "instance": {
         "type": "string",
         "format": "uri-reference",
         "description": "A URI reference that identifies the \
   specific occurrence of the problem. It may or may not yield \
   further information if dereferenced."
       }
     }
   }

Appendix B.  HTTP Problems and XML

   Some HTTP-based APIs use XML [XML] as their primary format
   convention.  Such APIs can express problem details using the format
   defined in this appendix.

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   The RELAX NG schema [ISO-19757-2] for the XML format is as follows.
   Keep in mind that this schema is only meant as documentation, and not
   as a normative schema that captures all constraints of the XML
   format.  Also, it would be possible to use other XML schema languages
   to define a similar set of constraints (depending on the features of
   the chosen schema language).

      default namespace ns = "urn:ietf:rfc:7807"

      start = problem

      problem =
        element problem {
          (  element  type            { xsd:anyURI }?
           & element  title           { xsd:string }?
           & element  detail          { xsd:string }?
           & element  status          { xsd:positiveInteger }?
           & element  instance        { xsd:anyURI }? ),
          anyNsElement
        }

      anyNsElement =
        (  element    ns:*  { anyNsElement | text }
         | attribute  *     { text })*

   The media type for this format is "application/problem+xml".

   Extension arrays and objects are serialized into the XML format by
   considering an element containing a child or children to represent an
   object, except for elements that contain only child element(s) named
   'i', which are considered arrays.  For example, the example above
   appears in XML as follows:

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   HTTP/1.1 403 Forbidden
   Content-Type: application/problem+xml
   Content-Language: en

   <?xml version="1.0" encoding="UTF-8"?>
   <problem xmlns="urn:ietf:rfc:7807">
     <type>https://example.com/probs/out-of-credit</type>
     <title>You do not have enough credit.</title>
     <detail>Your current balance is 30, but that costs 50.</detail>
     <instance>https://example.net/account/12345/msgs/abc</instance>
     <balance>30</balance>
     <accounts>
       <i>https://example.net/account/12345</i>
       <i>https://example.net/account/67890</i>
     </accounts>
   </problem>

   Note that this format uses an XML namespace.  This is primarily to
   allow embedding it into other XML-based formats; it does not imply
   that it can or should be extended with elements or attributes in
   other namespaces.  The RELAX NG schema explicitly only allows
   elements from the one namespace used in the XML format.  Any
   extension arrays and objects MUST be serialized into XML markup using
   only that namespace.

   When using the XML format, it is possible to embed an XML processing
   instruction in the XML that instructs clients to transform the XML,
   using the referenced XSLT code [XSLT].  If this code is transforming
   the XML into (X)HTML, then it is possible to serve the XML format,
   and yet have clients capable of performing the transformation display
   human-friendly (X)HTML that is rendered and displayed at the client.
   Note that when using this method, it is advisable to use XSLT 1.0 in
   order to maximize the number of clients capable of executing the XSLT
   code.

Appendix C.  Using Problem Details with Other Formats

   In some situations, it can be advantageous to embed problem details
   in formats other than those described here.  For example, an API that
   uses HTML [HTML5] might want to also use HTML for expressing its
   problem details.

   Problem details can be embedded in other formats either by
   encapsulating one of the existing serializations (JSON or XML) into
   that format or by translating the model of a problem detail (as
   specified in Section 3) into the format's conventions.

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   For example, in HTML, a problem could be embedded by encapsulating
   JSON in a script tag:

   <script type="application/problem+json">
     {
      "type": "https://example.com/probs/out-of-credit",
      "title": "You do not have enough credit.",
      "detail": "Your current balance is 30, but that costs 50.",
      "instance": "/account/12345/msgs/abc",
      "balance": 30,
      "accounts": ["/account/12345",
                   "/account/67890"]
     }
   </script>

   or by inventing a mapping into RDFa [RDFA].

   This specification does not make specific recommendations regarding
   embedding problem details in other formats; the appropriate way to
   embed them depends both upon the format in use and application of
   that format.

Acknowledgements

   The authors would like to thank Jan Algermissen, Subbu Allamaraju,
   Mike Amundsen, Roy Fielding, Eran Hammer, Sam Johnston, Mike McCall,
   Julian Reschke, and James Snell for review of this specification.

Authors' Addresses

   Mark Nottingham
   Prahran
   Australia

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

   Erik Wilde

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/

   Sanjay Dalal

   Email: sanjay.dalal@cal.berkeley.edu
   URI:   https://github.com/sdatspun2

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