💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc7807.txt captured on 2024-05-10 at 13:48:36.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
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.
Nottingham & Wilde Standards Track [Page 1]
RFC 7807 Problem Details March 2016
Table of Contents
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
Nottingham & Wilde Standards Track [Page 2]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 3]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 4]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 5]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 6]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 7]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 8]
RFC 7807 Problem Details March 2016
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])
Nottingham & Wilde Standards Track [Page 9]
RFC 7807 Problem Details March 2016
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])
Nottingham & Wilde Standards Track [Page 10]
RFC 7807 Problem Details March 2016
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>.
Nottingham & Wilde Standards Track [Page 11]
RFC 7807 Problem Details March 2016
[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>.
Nottingham & Wilde Standards Track [Page 12]
RFC 7807 Problem Details March 2016
[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>.
Nottingham & Wilde Standards Track [Page 13]
RFC 7807 Problem Details March 2016
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:
Nottingham & Wilde Standards Track [Page 14]
RFC 7807 Problem Details March 2016
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.
Nottingham & Wilde Standards Track [Page 15]
RFC 7807 Problem Details March 2016
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/
Nottingham & Wilde Standards Track [Page 16]