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Versions: 00 01 02                                                      
Network Working Group                                           A. Cedik
Internet-Draft                                            shipcloud GmbH
Intended status: Standards Track                                E. Wilde
Expires: October 1, 2020                                           Axway
                                                          March 30, 2020

             Communicating Warning Information in HTTP APIs


   This document defines a new HTTP header field Content-Warning and a
   standard response format for representing warning information in HTTP

Note to Readers

   This draft should be discussed on the rfc-interest mailing list

   Online access to all versions and files is available on GitHub

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 https://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 October 1, 2020.

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

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   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Content-Warning Header Field  . . . . . . . . . . . . . . . .   3
   4.  The "embedded-warning" Content-Warning Type . . . . . . . . .   4
   5.  JSON Warning Format . . . . . . . . . . . . . . . . . . . . .   4
   6.  Example with HTTP Header Field and Embedded Warning . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  HTTP Header Field Content-Warning . . . . . . . . . . . .   6
     8.2.  Content-Warning Type Registry . . . . . . . . . . . . . .   6
       8.2.1.  Registration Procedure  . . . . . . . . . . . . . . .   6
       8.2.2.  Initial Registry Content  . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.

   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

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   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",
   document are to be interpreted as described in [RFC2119].

3.  Content-Warning Header Field

   The Content-Warning header field 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".

      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 8.2.  Clients SHOULD ignore Content-Warning types that
      they do not know.

      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").

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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 header field looks like this:

   Content-Warning: "embedded-warning"; 1590190500

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 an 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
   can be returned to the API 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.

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   Entries in these individual objects follow the pattern described in

   When warnings are present the Content-Warning header 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 Header 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 API client was successful.

   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

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7.  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.

8.  IANA Considerations

8.1.  HTTP Header Field Content-Warning

   This specification registers the following entry in the Permanent
   Message Header Field Names registry established by [RFC3864]:

   o  Header 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:

8.2.  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.

8.2.1.  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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8.2.2.  Initial Registry Content

   The registry has been populated with the registered values shown

   | Content-Warning Type   | Reference                        |
   | embedded-warning       | this RFC, Section 4              |

9.  References

9.1.  Normative References

              Nottingham, M., "Binary Structured HTTP Headers", draft-
              nottingham-binary-structured-headers-02 (work in
              progress), March 2020.

              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-14 (work in progress),
              October 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [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,

   [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,

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   [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,

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,

   [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,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

9.2.  Informative References

              Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", World Wide Web Consortium
              Recommendation REC-webarch-20041215, December 2004,

Appendix A.  Acknowledgements

   Thanks for comments and suggestions provided by ...

Authors' Addresses

   Andre Cedik
   shipcloud GmbH

   Email: andre.cedik@googlemail.com

   Erik Wilde

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/

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