Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 7807                                        Akamai
Category: Standards Track                                       E. Wilde
ISSN: 2070-1721                                               March 2016

Problem Details for HTTP APIs

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7807.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The Problem Details JSON Object . . . . . . . . . . . . . . .   3
     3.1.  Members of a Problem Details Object . . . . . . . . . . .   5
     3.2.  Extension Members . . . . . . . . . . . . . . . . . . . .   6
   4.  Defining New Problem Types  . . . . . . . . . . . . . . . . .   6
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Predefined Problem Types  . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  application/problem+json  . . . . . . . . . . . . . . . .   9
     6.2.  application/problem+xml . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  HTTP Problems and XML  . . . . . . . . . . . . . . .  14
   Appendix B.  Using Problem Details with Other Formats . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1. Introduction

   HTTP [RFC7230] 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 [W3C.REC-html5-20141028] response body, non-human consumers of
   so-called "HTTP APIs" are usually not.

   This specification defines simple JSON [RFC7159] and XML
   [W3C.REC-xml-20081126] 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.

   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 [RFC7159] object; when
   formatted as a JSON document, it uses the "application/problem+json"
   media type.  Appendix A 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.

2. Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3. The Problem Details JSON Object

   The canonical model for problem details is a JSON [RFC7159] 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:

   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) [RFC4918] code could be used to encapsulate multiple status
   messages.

3.1. Members of a Problem Details Object

   A problem details object can have the following members:

   o  "type" (string) - A URI reference [RFC3986] that identifies the
      problem type.  This specification encourages that, when
      dereferenced, it provide human-readable documentation for the
      problem type (e.g., using HTML [W3C.REC-html5-20141028]).  When
      this member is not present, its value is assumed to be
      "about:blank".

   o  "title" (string) - 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).

   o  "status" (number) - The HTTP status code ([RFC7231], Section 6)
      generated by the origin server for this occurrence of the problem.

   o  "detail" (string) - A human-readable explanation specific to this
      occurrence of the problem.

   o  "instance" (string) - A URI reference that identifies the specific
      occurrence of the problem.  It may or may not yield further
      information if dereferenced.

   Consumers MUST use the "type" string as the primary identifier for
   the problem type; 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).
   Consumers SHOULD NOT automatically dereference the type URI.

   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.

   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.

   Note that both "type" and "instance" accept relative URIs; this means
   that they must be resolved relative to the document's base URI, as
   per [RFC3986], Section 5.

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.

   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
   [RFC7231], Section 5.3.2).

   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 ([RFC7231], Section 7.1.3) in appropriate
   circumstances.

   A problem's type URI SHOULD resolve to HTML [W3C.REC-html5-20141028]
   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 [RFC5988] 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.

   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. Predefined Problem Types

   This specification reserves the use of one URI as a problem type:

   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.

   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

This specification defines two new Internet media types [RFC6838].

6.1. application/problem+json

   Type name:  application

   Subtype name:  problem+json

   Required parameters:  None

   Optional parameters:  None; unrecognized parameters should be ignored

   Encoding considerations:  Same as [RFC7159]

   Security considerations:  see Section 5 of this document

   Interoperability considerations:  None

   Published specification:  RFC 7807 (this document)

   Applications that use this media type:  HTTP

   Fragment identifier considerations:  Same as for application/json
      ([RFC7159])

   Additional information:

      Deprecated alias names for this type:  n/a

      Magic number(s):  n/a

      File extension(s):  n/a

      Macintosh file type code(s):  n/a

   Person and email address to contact for further information:
      Mark Nottingham <mnot@mnot.net>

   Intended usage:  COMMON

   Restrictions on usage:  None.

   Author:  Mark Nottingham <mnot@mnot.net>

   Change controller:  IESG

6.2. application/problem+xml

   Type name:  application

   Subtype name:  problem+xml

   Required parameters:  None

   Optional parameters:  None; unrecognized parameters should be ignored

   Encoding considerations:  Same as [RFC7303]

   Security considerations:  see Section 5 of this document

   Interoperability considerations:  None

   Published specification:  RFC 7807 (this document)

   Applications that use this media type:  HTTP

   Fragment identifier considerations:  Same as for application/xml (as
      specified by Section 5 of [RFC7303])

   Additional information:

      Deprecated alias names for this type:  n/a

      Magic number(s):  n/a

      File extension(s):  n/a

      Macintosh file type code(s):  n/a

   Person and email address to contact for further information:
      Mark Nottingham <mnot@mnot.net>

   Intended usage:  COMMON

   Restrictions on usage:  None.

   Author:  Mark Nottingham <mnot@mnot.net>

   Change controller:  IESG

7. References

7.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc3986>.

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

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", W3C Recommendation REC-xml-20081126, November
              2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.

7.2. Informative References

   [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.

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

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

   [RFC6694]  Moonesamy, S., Ed., "The "about" URI Scheme", RFC 6694,
              DOI 10.17487/RFC6694, August 2012,
              <http://www.rfc-editor.org/info/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,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <http://www.rfc-editor.org/info/rfc7303>.

   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", W3C
              Recommendation REC-html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

   [W3C.REC-rdfa-core-20130822]
              Adida, B., Birbeck, M., McCarron, S., and I. Herman, "RDFa
              Core 1.1 - Second Edition", W3C Recommendation
              REC-rdfa-core-20130822, August 2013,
              <http://www.w3.org/TR/2013/REC-rdfa-core-20130822>.

   [W3C.REC-xml-stylesheet-20101028]
              Clark, J., Pieters, S., and H. Thompson, "Associating
              Style Sheets with XML documents 1.0 (Second Edition)", W3C
              Recommendation REC-xml-stylesheet-20101028, October 2010,
              <http://www.w3.org/TR/2010/REC-xml-stylesheet-20101028>.

Appendix A. HTTP Problems and XML

   Some HTTP-based APIs use XML [W3C.REC-xml-20081126] as their primary
   format convention.  Such APIs can express problem details using the
   format defined in this appendix.

   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:

   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 [W3C.REC-xml-stylesheet-20101028].  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 B. 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 [W3C.REC-html5-20141028] 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.

   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 [W3C.REC-rdfa-core-20130822].

   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
   Akamai

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

   Erik Wilde

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