Internet-Draft Concise Problem Details: Body Error Posi February 2023
Amsüss Expires 22 August 2023 [Page]
Intended Status:
C. Amsüss

Concise Problem Details: Body Error Position


This defines a single standard problem detail for use with the Concise Problem Details format: Request Body Error Position. Using this detail, the server can point at the position inside the client's request body that induced the error.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (, which is archived at

Source for this draft and an issue tracker can be found at

Status of This Memo

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This Internet-Draft will expire on 22 August 2023.

Table of Contents

1. Introduction

Concise Problem Details for CoAP APIs [RFC9290] describes how a server can provide details about an error processing a client request, and how to extend these error messages. This document uses that extension mechanism and adds the Request Body Error Position detail.

1.1. Terminology

The description of the problem detail uses the term "body" as defined in [RFC7959].

1.2. Document lifecycle

Registering a standard problem detail merely requires a specification, not an RFC (let alone of a particular track), and has been performed based on version -00 of this document.

Publication as an RFC has not been pursued in -00, nor is it at the time of writing. It will expire as an Inetnet Draft, but nonetheless be usable as the permanent reference for the IANA registration.

Should a need for further development or a more official publication arise, the document may be picked up again at a later time. For example, that might be done in the style of [I-D.bormann-cbor-notable-tags].

2. Request Body Error Position

The Request Body Error Position problem detail indicates that the error described by the Concise Problem Details response resulted from processing the request body. The numeric value indicates a byte position inside that body that corresponds to the error. The precise error position for invalid data may vary by implementation -- for example, if a numeric value inside a CBOR ([STD94]) item exceeds the expected range, it may indicate the number's initial byte (typically if the implementation doesn't even implement the indicated argument size) or the argument (if it implements it).

When the request's content format indicated a non-identity content coding, the offset points into the uncompressed body. Consequently, this error detail is not suitable for pointing out errors that occur during uncompressing.

The main envisioned use of this option is for the client to highlight or back-annotate (eg. to counteract minification, or to display it on some diagnostic notation) the erroneous item in the request body for a human author.

3. Usage example

The figures in this section illustrate a CoAP [RFC7252] message exchange using CBOR [STD94] bodies, and a hypothetical CoAP tool's output that utilizes this error detail.

Req: FETCH coap://
Content-Format: 60 (application/cbor)
Payload (diagnostic notation):
  {7: 74656, 44: 8472}

Res: 4.00 Bad Request
Content-Format: 257 (application/concise-problem-details+cbor)
Payload (diagnostic notation):
    / title /                        -1: "Unknown query key",
    / request-body-error-position / -25: 8
Figure 1: Messages exchanged between client and server
$ coap post coap:// cbor '{7: 74656, 44: 8472}'
Error: Bad Request: Unknown query key
{7: 74656, 44: 8472}
           ^^ The server indicated that the error occurred here.
Figure 2: Output of a hypothetical CoAP client that utilizes the Request Body Error Position detail

4. Security Considerations

Producing a Request Body Error Position detail gives the client some information about the internal workings of the server. If application designers intend to minimize the amount of information obtainable about the server, they need to weigh that goal against usability, and may prefer not to expose this (or any other) detail.

The Request Body Error Position detail can be used by malicious clients to explore the borders of acceptable content. This can be mitigated by limiting this (or other) details to suitably authorized users, or, where possible, only parsing data from trusted sources in the first place.

5. IANA considerations

A new entry has been assigned in the "Standard Problem Detail Keys" subregistry of the "Constrained RESTful Environments (CoRE) Parameters" registry.

Key value:

The value -25 is suggested



CDDL type:


Brief description:

Byte index inside the request body at which the error became apparent


This document

Change controller:

IETF CoRE working group

6. Acknowledgements

Michael Richardson provided good input for the Securitiy Considerations.

7. References

7.1. Normative References

Fossati, T. and C. Bormann, "Concise Problem Details for Constrained Application Protocol (CoAP) APIs", RFC 9290, DOI 10.17487/RFC9290, , <>.
Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, , <>.

7.2. Informative References

Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, .
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Bormann, C., "Notable CBOR Tags", Work in Progress, Internet-Draft, draft-bormann-cbor-notable-tags-07, , <>.

Appendix A. Change log

Since -00:

Author's Address

Christian Amsüss