Problem Details for HTTP APIs
draft-nottingham-http-problem-03

Network Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Informational                                  E. Wilde
Expires: August 29, 2013                                             EMC
                                                       February 25, 2013

                     Problem Details for HTTP APIs
                    draft-nottingham-http-problem-03

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
   invent new error 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
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   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The Problem Details JSON Object  . . . . . . . . . . . . . . .  4
     3.1.  Required Members . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Optional Members . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Extension Members  . . . . . . . . . . . . . . . . . . . .  5
   4.  Defining New Problem Types . . . . . . . . . . . . . . . . . .  6
     4.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  HTTP Problems and XML . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   HTTP [RFC2616] status codes are sometimes not sufficient to convey
   helpful information about an error.  While humans behind Web browsers
   can be informed about the nature of the problem with an HTML response
   body, non-human consumers of so-called "HTTP APIs" are usually not.

   This specification defines simple JSON [RFC4627] 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).

   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.

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

   Additionally, problems 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 may be useful for support or forensic
   purposes.  See below for a full list.

   The data model for problem details is a JSON [RFC4627] object; when
   formatted as a JSON document, it uses the "application/
   api-problem+json" media type.  Appendix A defines how to express them
   in an equivalent XML format, which uses the "application/
   api-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

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   accommodate describing the relevant details in that application's
   format.

   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 re-define the
   semantics of existing HTTP status codes.

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 [RFC4627] object.

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

   For example, a HTTP response carrying JSON problem details:

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

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

   Here, the out-of-credit problem (identified by its problemType URI)
   indicates the reason for the 403 in "title", gives a reference for
   the specific problem occurrence with "problemInstance", 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 "problemType" is case-sensitive in the JSON object, as are
   all other member names.

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3.1.  Required Members

   A problem details object MUST have the following members:

   o  "problemType" (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 problemType 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 don't
   have the ability to discover them (e.g., offline log analysis).
   Consumers SHOULD NOT automatically dereference the problemType URI.

3.2.  Optional Members

   Furthermore, a problem details 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
      occurrence of the problem.
   o  "problemInstance" (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 problem details
   object with additional members.

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

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.  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 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 problemType URI (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 problemType URI SHOULD resolve to HTML documentation that
   explains how to resolve the problem.

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   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] and SHOULD consist of characters
   from ALPHA, DIGIT and "_" (so that it can be serialised in formats
   other than JSON), and SHOULD be three characters or longer.

4.1.  Example

   For example, if your 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 convention.

   To do so, you might look for an already-defined problemType URI that
   suits your purposes.  If one is available, you can reuse that URI by
   documenting use of both this specification and that specific URI
   value in your API.

   If one isn't available, you'd reference this specification, mint and
   document a new problemType 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.  This information could appear within the rest of your
   API's documentation.

   In summary: a problemInstance URI will always identify a specific
   occurrence of a problem.  On the other hand, problemType 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.

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 [RFC6838]:

      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 authors would like to thank Jan Algermissen, Mike Amundsen, Subbu
   Allamaraju, Roy Fielding, Sam Johnston, Mike McCall, Julian Reschke,
   and James Snell 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.

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

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

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

8.2.  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.

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

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

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013.

   [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>.

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 problem details using the
   format defined in this appendix.

   The OPTIONAL RELAX NG schema [ISO-19757-2] for the XML format is:

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

   start |= problem
   problem =
     element problem {
       (element problemType { xsd:anyURI }
        & element title { xsd:string }
        & element detail { xsd:string }?
        & element httpStatus { xsd:positiveInteger }?
        & element problemInstance { xsd:anyURI }?),
       anyNsElement
     }
   anyNsElement =
     (element ns:* { anyNsElement | text }
      | attribute * { text })*

   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 would appear in XML as:

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

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

   Note that this format uses an XML Namespace.  This is primarily to
   allow embedding it into other formats; it does not imply that it can
   be extended with content from 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 serialised

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   using that namespace.

Authors' Addresses

   Mark Nottingham
   Akamai

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

   Erik Wilde
   EMC

   Email: erik.wilde@emc.com

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