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Network Working Group                                      M. Nottingham
Internet-Draft                                                 Rackspace
Intended status: Informational                              July 4, 2012
Expires: January 5, 2013


           Indicating Details of Problems to Machines in HTTP
                    draft-nottingham-http-problem-00

Abstract

   This specification proposes a "Problem Detail" as an extensible way
   to carry machine-readable details of HTTP errors in a response, to
   avoid the need to invent new response formats for non-human
   consumers.

Note to Readers

   This specification is tentative (as all Internet-Drafts are, but even
   more so).  It's not yet clear whether defining this format will
   improve things enough to balance out the harm it may cause, and so it
   may disappear at any time.  Using it before it becomes an RFC may
   subject you to ridicule; you have been warned.

   What's the problem?  In a nutshell, this sort of thing might
   encourage tighter coupling, making applications more brittle.  On the
   other hand, lots of people are doing this already, so having a common
   way to do it -- with lots of caveats about how it can be misused --
   might be better.  Stay tuned.

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 http://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 January 5, 2013.

Copyright Notice



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   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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  The Problem Details JSON Object . . . . . . . . . . . . . . . . 4
   4.  Expressing Problem Details with Link Headers  . . . . . . . . . 5
   5.  Defining New Problem Details  . . . . . . . . . . . . . . . . . 5
   6.  Generating Problem Details  . . . . . . . . . . . . . . . . . . 7
   7.  Consuming Problem Details . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 9
   11. Normative References  . . . . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9























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1.  Introduction

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

   For example, 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 it's presented to a human in front of
   a Web browser, a non-browser client will have difficulty
   understanding the response, because it doesn't understand the
   structure of the markup.

   This specification defines a "Problem Detail" as an extensible way to
   carry machine-readable details of errors in a response, to avoid the
   need to invent new response formats.

   Problem details have:
   o  A generic status, identified and solely conveyed by the HTTP
      status code <http://www.iana.org/assignments/http-status-codes/>.
      For example, "403 Forbidden".
   o  An problem type that refines the status code semantics, identified
      by an absolute URI [RFC3986] and paired with a summary title.
   o  Optionally, detail that is specific to an instance of the problem.

   The common data model for problem details as a JSON [RFC4627] object.
   That object can be serialised as an "application/json-problem" HTTP
   response, or it can be conveyed using a Link HTTP response header
   field [RFC5988].

   Future work may include defining a HTML Microformat
   <http://microformats.org/> that allows this information to be carried
   in an HTML document as well.

   Note that Problem Details are 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 details in that format.

   Rather, the aim of this specification is to define a common 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",



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   "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]
   document, identified with the "application/json-problem" media type.
   Its root object has the following properties:

   o  "describedby" (string) - A URL that identifies the type of
      problem.  When dereferenced, it SHOULD provide human-readable
      documentation (e.g., using HTML).
   o  "title" (string) - A short, human-readable summary of the problem.
      It SHOULD NOT change from instance to instance of the problem,
      except for purposes of localisation.
   o  "detail" (string) - Optionally, additional, human readable detail
      specific to this instance of the problem.

   Problem details, when serialised as JSON objects, MUST include the
   "describedby" and "title" properties.  The "detail" property is
   always OPTIONAL.

   Problem types MAY extend their instances with additional properties.
   For example:

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

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

   Here, the out-of-credit problem (identified by its URI) indicates the
   reason for the 403 in "title", gives instance-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 property names.





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4.  Expressing Problem Details with Link Headers

   Because resources often use formats other than JSON to convey human-
   readable problem details, it is also possible to serialise Problem
   Details for non-browser consumption alongside them using a Link
   response header field [RFC5988].  For example:

      HTTP/1.1 403 Forbidden
      Content-Type: text/html
      Content-Language: en
      Link: <http://example.com/probs/out-of-credit>; rel="describedby";
               title="You do not have enough credits."; balance=30;
               account="http://api.example.com/account/12345"

   Here, the same problem as above is conveyed using the header in
   previous examples, but without the optional detail.

   Problem details are serialised into Link headers using the
   "describedby" link relation type, with the URL for the problem type
   as the link target.  The title is conveyed using the "title"
   parameter, and the detail can be conveyed using the "detail"
   parameter.

   Extension parameters on the JSON Object can also be expressed as
   target parameters on the link header, as long as the extension either
   defines them as a string, or explains how to express them as one.

   The title, detail, and extension properties MUST use the encoding
   specified in [RFC5987] if it falls outside of the US-ASCII character
   set.

   For example, the following are all valid:

      Link: <http://example.com/p/1>; rel="describedby"; title="Hi"
      Link: <ftp://example.com/p/1>; rel="describedby"; title="Hi"
      Link: <http://example.com/p/1>; rel="describedby";
            title="Hi there"; detail="something happened."
      Link: <http://example.com/p/1>; rel="describedby"; other="foo";
            title="Hi there"; title*=UTF-8''Hi%20there; another=bar;
            detail*=UTF-8''something%20happened


5.  Defining New Problem Details

   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.




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   Problem details are not a debugging tool for the underlying
   implementation; rather, they are a way to expose greater detail about
   the HTTP interface of the application itself.

   For example, an "out of credit" problem is an appropriate explanation
   as to why a request was forbidden.  In contrast, a description of the
   internal conditions that led to a 500 Internal Server Error along
   with a stack trace are not an appropriate use of this mechanism,
   because it exposes implementation detail, rather than explaining the
   interface.

   At the other end of the spectrum, 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 unnecessary,
   since a 403 Forbidden status code on a PUT request is self-
   explanatory.

   Finally, a problem domain 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 problems using
   HTTP server-driven content negotiation (i.e., the client uses the
   Accept request header to indicate a preference for problems).

   If defining a new problem still seems wise, one can be created by:
   1.  Nominating a describedby URL (typically, with the "http" scheme),
   2.  Choosing a title that appropriately describes it (think short),
       and
   3.  Nominating a 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.

   Optionally, a problem definition MAY specify additional properties 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 an extension is defined, its name SHOULD conform to token
   [RFC2616], so that it can be serialised in header and other formats.




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   Likewise, problems defining extensions should either make their
   values strings, or explain how to map their values to strings that
   are safe to include in HTTP headers (noting the special semantics of
   a comma there).


6.  Generating Problem Details

   The URL used in the describedby property MUST NOT be changed in any
   way from that defined by its specification; doing so turns it into a
   different problem that most consumers will not recognise.

   The title string SHOULD be sent as specified by the problem
   definition, although it MAY be localised (e.g., using server-driven
   content negotiation, with the "Accept-Language" request header).
   Remember that such responses are required by HTTP to include a Vary
   header, e.g.:

      HTTP/1.1 403 Forbidden
      Content-Type: text/html
      Content-Language: se
      Vary: Accept-Language
      Link: <http://example.com/probs/out-of-credit>;
            rel="describedby";
            title*=UTF-8''Du%20%C3%A4r%20ute%20p%C3%A5%20pengar.

   Note the use of RFC5987 encoding here.  Problem details SHOULD be
   served with Content-Language headers, even if negotiation isn't used,
   to aid in localisation.

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

   Software that generates problems should note that only one problem
   can be sent at at time.


7.  Consuming Problem Details

   Consumers should use the describedby URL as the primary identifier
   for the problem; 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).

   Likewise, the detail property SHOULD NOT be parsed for information by
   clients; extensions are more suitable and less error-prone ways to
   obtain such information.



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

   Finally, clients consuming problem details SHOULD NOT automatically
   deference the describedby URL; it is not intended for machine
   interaction.


8.  Security Considerations

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

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


9.  IANA Considerations

   This specification defines a new Internet media type, "application/
   json-problem":

      Type name: application
      Subtype name: json-problem
      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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10.  Acknowledgements

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


11.  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.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, August 2010.

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


Author's Address

   Mark Nottingham
   Rackspace

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













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