Problem Details for HTTP APIs
draft-nottingham-http-problem-02

Network Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Informational                         December 10, 2012
Expires: June 13, 2013

                     Problem Details for HTTP APIs
                    draft-nottingham-http-problem-02

Abstract

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

Note to Readers

   This draft should be discussed on the apps-discuss mailing list [1].

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 June 13, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Problem Details JSON Object  . . . . . . . . . . . . . . .  3
     3.1.  Required Members . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Optional Members . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Extension Members  . . . . . . . . . . . . . . . . . . . .  5
   4.  Defining New Problem Types . . . . . . . . . . . . . . . . . .  5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  HTTP Problems and XML . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   While HTTP [RFC2616] defines the status code as the primary indicator
   of generic response semantics, it is sometimes not fine-grained
   enough to convey helpful information about an error, particularly to
   non-human consumers of so-called "HTTP APIs".

   Consider a 403 Forbidden response that indicates that the client's
   account doesn't have enough credit.  While this can be adequately
   expressed in HTML if presented to a human in front of a Web browser,
   a non-browser client would have difficulty understanding the
   response, because it doesn't understand the structure of the markup.

   This specification defines conventions for carrying machine-readable
   details of errors in a response ("problem details"), to avoid the
   need to invent new, application-specific response formats.

   Conceptually, problem details are associated with a generic type
   (e.g., "out of credit").  Optionally, the specific occurrence of a
   problem can also be identified (e.g., "when Bob ran out of credit
   last Tuesday at 5:32 pm").  Both use URIs to assure global
   uniqueness, and provide the opportunity to fetch further information.

   Problem details are specified as a JSON [RFC4627] object; when
   occurring in a message body, they use the "application/
   api-problem+json" media type.  Appendix A defines how to translate
   problem details to an XML format, for those APIs that need it.

   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 format.

   Instead, the aim of this specification is to define a common error
   format for those applications that need one, so that they aren't
   required to define their own.

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 format for problem details is a JSON [RFC4627]

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   document, identified with the "application/api-problem+json" media
   type, whose root MUST be an object.

   For example:

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

   {
    "describedBy": "http://example.com/probs/out-of-credit",
    "title": "You do not have enough credit.",
    "detail": "Your current balance is 30, but that costs 50.",
    "supportId": "http://example.net/account/12345/msgs/abc",
    "balance": 30,
    "account": "http://example.net/account/12345"
   }

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

   Note that "describedBy" is case-sensitive in the JSON object, as are
   all other member names.

3.1.  Required Members

   The root object MUST have the following members:

   o  "describedBy" (string) - An absolute URI [RFC3986] that identifies
      the problem type.  When dereferenced, it SHOULD provide human-
      readable documentation for the problem type (e.g., using HTML).
   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 localisation.

   Consumers MUST use the describedBy 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 URL, and don't
   have the ability to discover them (e.g., offline log analysis).
   Consumers SHOULD NOT automatically dereference the describedBy URL.

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3.2.  Optional Members

   Furthermore, the root object MAY have the following members:

   o  "httpStatus" (number) - The HTTP status code set by the origin
      server for this occurrence of the problem.
   o  "detail" (string) - An human readable explanation specific to this
      occurance of the problem.
   o  "supportId" (string) - An absolute URI that identifies the
      specific occurrence of the problem.  It may or may not yield
      further information if dereferenced.

   The httpStatus 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.

   The detail member, if present, SHOULD focus on helping the client
   correct the problem, rather than giving debugging information.

   Consumers SHOULD NOT be parse the detail member for information;
   extensions are more suitable and less error-prone ways to obtain such
   information.

3.3.  Extension Members

   Finally, problem type definitions MAY extend the root object with
   additional members.

   Clients consuming problem details MUST ignore unrecognised
   extensions; this allows problem types to evolve and include
   additional information in the future.

4.  Defining New Problem Types

   Before defining a new type of problem detail, 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.  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.

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   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 on a PUT request is self-explanatory.

   Finally, an application may 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).

   New problem type definitions MUST document:
   1.  A describedBy URL (typically, with the "http" scheme),
   2.  A title that appropriately describes it (think short), and
   3.  The HTTP status code for it to be used with.

   Problem types MAY specify the use of the Retry-After response header
   in appropriate circumstances.

   A problem's describedBy URL SHOULD resolve to HTML documentation that
   explains how to resolve the problem.

   A problem type definition MAY specify additional members on the
   Problem Details JSON 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 conform to
   token [RFC2616], so that it can be serialised in formats other than
   JSON, and SHOULD be three characters or longer.

   Likewise, problem types defining extensions SHOULD either make their
   values strings, or explain how to map their values to strings, so
   that it's possible to include them in other formats.

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 serialised - the details given must also be
   scrutinised.

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   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 "httpStatus" 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.  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, 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:

      Type name: application
      Subtype name: api-problem+json
      Required parameters: None
      Optional parameters: None; unrecognised parameters
                           should be ignored
      Encoding considerations: Same as [RFC4627]
      Security considerations: see [this document]
      Interoperability considerations: None.
      Published specification: [this document]
      Applications that use this media type: HTTP
      Additional information:
        Magic number(s): n/a
        File extension(s): n/a
        Macintosh file type code(s): n/a
      Person & 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

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      Type name: application
      Subtype name: api-problem+xml
      Required parameters: None
      Optional parameters: None; unrecognised parameters
                           should be ignored
      Encoding considerations: Same as [RFC3023]
      Security considerations: see [this document]
      Interoperability considerations: None.
      Published specification: [this document]
      Applications that use this media type: HTTP
      Additional information:
        Magic number(s): n/a
        File extension(s): n/a
        Macintosh file type code(s): n/a
      Person & 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.  Acknowledgements

   The author would like to thank Jan Algermissen, Mike Amundsen, Subbu
   Allamaraju, Roy Fielding, Sam Johnston, Mike McCall, Julian Reschke,
   James Snell, and Erik Wilde for early review of this specification
   (even if some disagree with parts of it).

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

8.2.  Informative References

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

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

   [W3C.REC-xmlschema-0-20041028]
              Walmsley, P. and D. Fallside, "XML Schema Part 0: Primer
              Second Edition", World Wide Web Consortium
              Recommendation REC-xmlschema-0-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-0-20041028>.

URIs

   [1]  <https://www.ietf.org/mailman/listinfo/apps-discuss>

Appendix A.  HTTP Problems and XML

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

   The OPTIONAL XML Schema [W3C.REC-xmlschema-0-20041028] for the XML
   format is:

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   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     <xs:element name="problem">
       <xs:complexType>
         <xs:sequence maxOccurs="unbounded" minOccurs="0">
           <xs:element name="describedBy" type="xs:anyURI"/>
           <xs:element name="title" type="xs:string"/>
           <xs:element name="detail" type="xs:string" minOccurs="0"/>
           <xs:element name="httpStatus" type="xs:integer"
                       minOccurs="0"/>
           <xs:element name="supportId" type="xs:anyURI" minOccurs="0"/>
           <xs:any namespace="##any"
                   minOccurs="0" maxOccurs="unbounded"/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>
   </xs:schema>

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

   Extension arrays and objects can be serialised 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, an alternate version
   of the example above:

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

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

   would appear in XML as:

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

   <problem>
     <describedBy>http://example.com/probs/out-of-credit</describedBy>
     <title>You do not have enough credit.</title>
     <detail>Your current balance is 30, but that costs 50.</detail>
     <supportId>http://example.net/account/12345/msgs/abc</supportId>
     <more>
       <balance>30</balance>
       <accounts>
         <i>http://example.net/account/12345</i>
         <i>http://example.net/account/67890</i>
       </accounts>
     </more>
   </problem>

Author's Address

   Mark Nottingham
   Akamai

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

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