Internet DRAFT - draft-cedik-http-warning
draft-cedik-http-warning
Network Working Group A. Cedik
Internet-Draft shipcloud GmbH
Intended status: Standards Track E. Wilde
Expires: March 28, 2021 Axway
September 24, 2020
Communicating Warning Information in HTTP APIs
draft-cedik-http-warning-02
Abstract
This document defines a new HTTP field Content-Warning and a standard
response format for representing warning information in HTTP APIs.
Note to Readers
This draft should be discussed on the rfc-interest mailing list
(<https://lists.w3.org/Archives/Public/ietf-http-wg/>).
Online access to all versions and files is available on GitHub
(<https://github.com/dret/I-D/tree/master/http-warning>).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 28, 2021.
Copyright Notice
Copyright (c) 2020 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
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Content-Warning Field . . . . . . . . . . . . . . . . . . . . 3
3.1. HTTP request methods . . . . . . . . . . . . . . . . . . 4
4. The "embedded-warning" Content-Warning Type . . . . . . . . . 4
4.1. Allowed HTTP request methods for embedded-warning . . . . 4
5. JSON Warning Format . . . . . . . . . . . . . . . . . . . . . 5
6. Example with HTTP Field and Embedded Warning . . . . . . . . 5
7. Cache Considerations . . . . . . . . . . . . . . . . . . . . 6
7.1. Caching the "embedded-warning" Content-Warning type . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8.1. Absence of a response body . . . . . . . . . . . . . . . 7
8.2. Absence of warnings in the response body . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9.1. HTTP Field Content-Warning . . . . . . . . . . . . . . . 8
9.2. Content-Warning Type Registry . . . . . . . . . . . . . . 8
9.2.1. Registration Procedure . . . . . . . . . . . . . . . 8
9.2.2. Initial Registry Content . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Many current APIs are based on HTTP [RFC7230] as their application
protocol. Their response handling model is based on the assumption
that requests either are successful or they fail. In both cases
(success and failure) an HTTP status code [RFC7231] is returned to
convey either fact.
But response status is not always strictly either success or failure.
For example, there are cases where an underlying system returns a
response with data that cannot be defined as a clear error. API
providers who are integrating such a service might want to return a
success response nonetheless, but returning a HTTP status code of
e.g. 200 OK without any additional information is not the only
possible approach in this case.
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As defined in the principles of Web architecture
[W3C.REC-webarch-20041215], agents that "recover from errors by
making a choice without the user's consent are not acting on the
user's behalf". Therefore APIs should be able to communicate what
has happened to their consumers, which then allows clients or users
to make more informed decisions. Note that this specification
specifically targets warnings and not errors, meaning that while it
may be useful for clients to understand the warning condition and act
on it, they also may choose to ignore it and treat the response as a
successful one.
This document defines a warning code and a standard response
structure for communicating and representing warning information in
HTTP APIs. The goal is to allow HTTP providers to have a
standardized way of communicating to their consumers that while the
response can be considered to represent success, there is warning
information available that they might want to take into account.
As a general guideline, warning information should be considered to
be any information that can be safely ignored (treating the response
as if it did not communicate or embed any warning information), but
that might help clients and users to make better decisions.
2. Notational Conventions
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. Content-Warning Field
The Content-Warning field can be found in the header or trailer
section (see Section 4.6 of [I-D.ietf-httpbis-semantics]) of http
responses and allows to represent different kinds of warning
information via HTTP. It is defined as a Structured Header List
[I-D.ietf-httpbis-header-structure]. Its ABNF is:
Content-Warning = sh-list
Each member of the list MUST have exactly the two parameters "type"
and "date".
o The "type" parameter represents the warning that is being
signaled. Its value is defined as a sh-token and SHOULD be a type
that is registered in the Content-Warning type registry
Section 9.2. Clients SHOULD ignore Content-Warning types that
they do not know.
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o The "date" parameter defines the last occurrence of this warning
as a structured headers date as defined in
[I-D.ietf-binary-structured-headers] (e.g. "1581410465").
Intermediaries of a response are not allowed to modify existing
Content-Warning fields, but can add additional entries if warnings
are produced while they are handling a response.
3.1. HTTP request methods
The Content-Warning Field is not tied to any specific HTTP request
method, although specific values MAY only be used with a single or a
subset of methods. The information as to which HTTP request methods
are support for a single Content-Warning Type MUST be defined in the
definition of the Content-Warning Type.
4. The "embedded-warning" Content-Warning Type
This document introduces the Content-Warning Type "embedded-warning".
As mentioned in the introduction (Section 1), HTTP requests can be
successful or they can fail. They can also result in a state where
the original intent was satisfied, but a side effect happened that
should be conveyed back to the client.
To make it easier for clients to handle such an event, the Content-
Warning type "embedded-warning" MAY be returned. In this case, the
client MAY either treat the response according to its HTTP status
code, or in addition the client MAY use the embedded warning
information to understand the nature of the warning.
The "embedded-warning" type does not prescribe the way in which
warnings are represented. The assumption is that the response will
have embedded information that allows the client to learn about the
nature of the warning. The following section describes a JSON
structure that MAY be used to represent the warning. HTTP services
are free to use this or other formats to represent the warning
information they are embedding.
An exemplary Content-Warning field looks like this:
Content-Warning: "embedded-warning"; 1590190500
4.1. Allowed HTTP request methods for embedded-warning
The embedded-warning Content-Warning Type infers, that there is more
information in the responses body. Therefore all HTTP request
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methods that MAY have content in their body MAY also return embedded
warnings.
HTTP request methods that do not return a body in their response
SHOULD NOT return the embedded-warning Content-Warning Type.
The HTTP request method HEAD is an exception since it is allowed to
return headers that are meant for being returned when sending a GET
request. Therefore it MAY return the embedded-warning Content-
Warning Type, although the body will be empty.
5. JSON Warning Format
The JSON warning format uses the JSON format described in [RFC8259].
It is intended to be used as a building block in the response schemas
of JSON-based APIs.
In many current designs of JSON-based HTTP APIs, services represent
response data as members of the returned JSON object. In order to
make it easier for consumers to identify information about warnings,
a top-level member is defined that contains all warning information
in a representation. A "warnings" member MUST encapsulate the
warnings that will be returned to the client.
When a condition occurs that can not be defined as a "hard error"
(i.e., that allows clients to continue treating the resulting
response as a success), additional information about this condition
SHOULD be returned to the client. The "warnings" member MUST be an
array that is structured with one object for each and every warning
message that is returned to the client.
Entries in these individual objects follow the pattern described in
[RFC7807].
When warnings are present the Content-Warning field (as defined in
Section 3) SHOULD be set to indicate that warnings have be returned.
This way a client will not have to parse the response body to find
out whether a warnings member is present.
6. Example with HTTP Field and Embedded Warning
Since warnings do not have an effect on the returned HTTP status
code, the response status code SHOULD be in the 2xx range, indicating
that the intent of the client was successful.
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POST /example HTTP/1.1
Host: example.com
Accept: application/json
HTTP/1.1 200 OK
Content-Type: application/json
Content-Warning: "embedded-warning"; 1590190500
{
"request_id": "2326b087-d64e-43bd-a557-42171155084f",
"warnings": [
{
"detail": "Street name was too long. It has been shortened...",
"instance": "https://example.com/shipments/3a186c51/msgs/c94d",
"status": "200",
"title": "Street name too long. It has been shortened.",
"type": "https://example.com/errors/shortened_entry"
},
{
"detail": "City for this zipcode unknown. Code for shipment..",
"instance": "https://example.com/shipments/3a186c51/msgs/5927",
"status": "200",
"title": "City for zipcode unknown.",
"type": "https://example.com/errors/city_unknown"
}
],
"id": "3a186c51d4281acb",
"carrier_tracking_no": "84168117830018",
"tracking_url": "http://example.com/3a186c51d",
"label_url": "http://example.com/shipping_label_3a186c51d.pdf",
"price": 3.4
}
This example shows that the original intent was successful. If the
original request was in fact not successful, a different status code
SHOULD be returned. Embedded warnings are not tied to a specific
http status code. Therefore they can be combined with every status
code.
7. Cache Considerations
The Content-Warning field itself does not encourage a specific
handling when it comes to caching responses. It is up to the
Content-Warning type to specify if caching can be used or not.
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7.1. Caching the "embedded-warning" Content-Warning type
The reasons for returning the Content-Warning Type "embedded-warning"
can be manifold. A system could e.g. return warnings due to
circumstances in the backend that can either still exist on
subsequent requests or that have been solved in the meantime.
Intermediaries can fall into the same category. When a warning
occurs, it can add warnings to the response making it possible to
debug what happened at the intermediary. The reason for said warning
can persist or may disappear on subsequent requests.
Therefore caching embedded-warnings SHOULD NOT be done. As one can't
predict if the reason for returning embedded-warnings is still
persistent.
8. Security Considerations
API providers need to exercise care when reporting warnings.
Malicious actors could use this information for orchestrating
attacks. Social engineering can also be a factor when warning
information is returned by the API.
Clients processing warning information SHOULD make sure the right
type of content was transmitted by checking the content-type header
as well as the content-warning field. Content in the bodys warnings
object SHOULD be processed accordingly. If no content-warning field
was provided, clients are advised to ignore the content provided in
the bodys warnings object.
8.1. Absence of a response body
As described in Section 4.1 the embedded-warning Content-Warning type
is expecting a body to be returned in the http response unless the
HEAD method has been used for the request.
Therefore API clients SHOULD only parse a responses' body when the
Content-Warning type is "embedded-warning". When the body is absent,
a client SHOULD stop processing the response and return an adequate
error message.
If an intermediary discovers a missing response body it MAY adjust
the response to return a http status code of 500 - internal server
error (see Section 6.6.1 of [RFC7231]).
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8.2. Absence of warnings in the response body
When the response body does not contain warnings a client MAY use
appropriate ways to inform the api provider about the fact. An error
message MAY be
If an intermediary discovers missing warnings in the response body it
MAY adjust the response to return warnings containing this
information.
9. IANA Considerations
9.1. HTTP Field Content-Warning
This specification registers the following entry in the Permanent
Message Field Names registry established by [RFC3864]:
o Field name: Content-Warning
o Applicable protocol: HTTP
o Status: standard
o Author/Change Controller: IETF
o Specification document(s): [this document]
o Related information:
9.2. Content-Warning Type Registry
The "Content-Warning Type Registry" defines the namespace for new
Content-Warning types. This specification establishes a new registry
according to the guidelines given in [RFC8126]. This new registry
should not be included in an existing group of registries.
9.2.1. Registration Procedure
A registration MUST include the following fields:
o Content-Warning Type: Name of the Content-Warning Type
o Reference: Pointer to a specification text
The registration policy for this registry is "Specification Required"
as defined by [RFC8126], Section 4.6. They MUST follow the "sh-
token" syntax defined by [I-D.ietf-httpbis-header-structure].
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9.2.2. Initial Registry Content
The registry has been populated with the registered values shown
below:
+------------------------+----------------------------------+
| Content-Warning Type | Reference |
+------------------------+----------------------------------+
| embedded-warning | this RFC, Section 4 |
+------------------------+----------------------------------+
10. References
10.1. Normative References
[I-D.ietf-binary-structured-headers]
Nottingham, M., "Binary Structured HTTP Headers", draft-
nottingham-binary-structured-headers-02 (work in
progress), March 2020.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-14 (work in progress),
October 2019.
[I-D.ietf-httpbis-semantics]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", draft-ietf-httpbis-semantics-07
(work in progress), March 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[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,
<https://www.rfc-editor.org/info/rfc7230>.
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[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,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/info/rfc7807>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
10.2. Informative References
[W3C.REC-webarch-20041215]
Jacobs, I. and N. Walsh, "Architecture of the World Wide
Web, Volume One", World Wide Web Consortium
Recommendation REC-webarch-20041215, December 2004,
<http://www.w3.org/TR/2004/REC-webarch-20041215>.
Appendix A. Acknowledgements
Thanks for comments and suggestions provided by Roy Fielding, Mark
Nottingham, and Roberto Polli.
Authors' Addresses
Andre Cedik
shipcloud GmbH
Email: andre.cedik@googlemail.com
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Erik Wilde
Axway
Email: erik.wilde@dret.net
URI: http://dret.net/netdret/
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