Internet Engineering Task Force (IETF) T. Fossati
Request for Comments: 9876 Linaro
Updates: 7252 E. Dijk
Category: Standards Track IoTconsultancy.nl
ISSN: 2070-1721 September 2025
Updates to the IANA Registration Procedures for Constrained Application
Protocol (CoAP) Content-Formats
Abstract
This document updates RFC 7252 by modifying the registration
procedures for the "CoAP Content-Formats" IANA registry, within the
"Constrained RESTful Environments (CoRE) Parameters" registry group.
This document also introduces a new column, "Media Type", to the
registry. Furthermore, this document reserves Content-Format
identifiers 64998 and 64999 for use in documentation.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9876.
Copyright Notice
Copyright (c) 2025 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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Table of Contents
1. Introduction
2. Conventions and Definitions
3. Security Considerations
4. IANA Considerations
4.1. CoAP Content-Formats Registry
4.1.1. Temporary Content-Format Registrations
4.1.2. Addition of the Media Type Column to the Registry
4.1.3. Expert Review Procedure
4.1.4. Preferred Format for the Content Type Field
4.1.5. Examples of Invalid Registration Requests
4.2. New Note and Reference Additions
4.3. Reserving Content-Format Identifiers 64998 and 64999 for
Documentation
5. References
5.1. Normative References
5.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Section 12.3 of [RFC7252] describes the registration procedures for
the "CoAP Content-Formats" IANA registry within the "Constrained
RESTful Environments (CoRE) Parameters" registry group
[IANA.core-params]. (Note that the columns of this registry have
been revised according to [Err4954].) In particular, it defines the
rules for obtaining Constrained Application Protocol (CoAP) Content-
Format identifiers from the "IETF Review with Expert Review or IESG
Approval with Expert Review" range of the registry (256-9999) as well
as from the "First Come First Served" (FCFS) range of the registry
(10000-64999). For the FCFS range, these rules do not involve the
designated expert (DE) and are managed solely by IANA personnel to
finalize the registration.
Unfortunately, the rules do not explicitly require checking that the
combination of Content-Type (i.e., Media Type with optional
parameters) and Content Coding associated with the requested CoAP
Content-Format is semantically valid. This task is generally non-
trivial, requires knowledge from multiple documents and technologies,
and should not be solely demanded from the registrar. This lack of
guidance may engender confusion in both the registering party and the
registrar, and it has already led to erroneous registrations.
This document updates [RFC7252] by modifying the registration
procedures for the "CoAP Content-Formats" registry to mitigate the
risk of unintentional or malicious errors. These updates amend the
different ranges of the registry, introduce a review procedure to be
performed for most ranges of the registry, and allow the registration
of temporary Content-Format identifiers. This document also
introduces a new column, "Media Type", to the registry. Furthermore,
this document reserves Content-Format identifiers 64998 and 64999 for
use in documentation.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the terms "Media Type", "Content Coding",
"Content-Type", and "Content Format" as defined in Section 2 of
[RFC9193]. In this document, those terms are fully capitalized.
3. Security Considerations
This document hardens updates the registration procedures of CoAP Content-
Formats in ways that to reduce the chances of malicious manipulation of the
associated registry.
Otherwise, it does not change the Security Considerations of
[RFC7252].
4. IANA Considerations
This document updates the IANA procedures defined in [RFC7252] for
registering CoAP Content-Formats as described in Section 4.1. It
also adds a new note concerning temporary registrations (Section 4.2)
and reserves Content-Format IDs 64998 and 64999 for documentation
(Section 4.3).
4.1. CoAP Content-Formats Registry
This section and its subsections replace Section 12.3 of [RFC7252].
Internet Media Types are identified by a string, such as
"application/xml" [RFC2046]. In order to minimize the overhead of
using Media Types to indicate the format of payloads, [RFC7252] has
defined a registry for a subset of Internet Media Types to be used in
CoAP and assigned each, in combination with a Content Coding, a
numeric identifier. The name of the registry is "CoAP Content-
Formats", within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
Each entry in the registry must include the Content Type, the Content
Coding (if any), the Media Type registered with IANA, the numeric
identifier in the range 0-65535 to be used for that Media Type in
CoAP, the Content Coding associated with this
identifier, and a reference to a document describing what a payload with
that Media Type means semantically.
CoAP does not include a separate way to convey Content Coding
information with a request or response; for that reason, the Content
Coding (if any) is also specified for each identifier. If multiple
Content Codings will be used with a Media Type, then a separate
Content-Format identifier for each is to be registered. Similarly,
other parameters related to an Internet Media Type can be defined for
a CoAP Content-Format entry.
The registration procedures for CoAP Content-Formats are described in
Table 1.
+=============+===============+=====================================+
| Range | Registration | Note |
| | Procedures | |
+=============+===============+=====================================+
| 0-255 | Expert Review | Review procedure described in |
| | | RFC 9876, Section 4.1.3. |
+-------------+---------------+-------------------------------------+
| 256-9999 | IETF Review | Review procedure described in |
| | with Expert | RFC 9876, Section 4.1.3 |
| | Review or | |
| | IESG Approval | |
| | with Expert | |
| | Review | |
+-------------+---------------+-------------------------------------+
| 10000-19999 | Expert Review | Review procedure described in |
| | | RFC 9876, Section 4.1.3. |
+-------------+---------------+-------------------------------------+
| 20000-32999 | First Come | FCFS is allowed if the |
| | First Served | registration has no parameters, |
| | | the registration has an empty |
| | | Content Coding, the Media Type is |
| | | not yet used in this registry, |
| | | and the Media Type is registered |
| | | (or approved for registration) in |
| | | the "Media Types" registry |
| | | [IANA.media-types]. |
+-------------+---------------+-------------------------------------+
| 33000-64997 | Expert Review | Review procedure described in |
| | | RFC 9876, Section 4.1.3. |
+-------------+---------------+-------------------------------------+
| 64998-64999 | Reserved for | |
| | Documentation | |
+-------------+---------------+-------------------------------------+
| 65000-65535 | Experimental | No operational use |
| | Use | |
+-------------+---------------+-------------------------------------+
Table 1: Registration Procedures for CoAP Content-Formats
Because the namespace of single-byte identifiers is so small, the
IANA policy for additions in the range 0-255 inclusive to the
registry is "Expert Review" as described in Section 4.5 of RFC 8126
[BCP26]. For the handling of temporary allocations within the 0-255
range, see also Section 4.1.1, Paragraph 6.
The 256-9999 range has registration procedures requiring "IETF Review
with Expert Review" or "IESG Approval with Expert Review". In
particular:
* All assignments according to "IETF Review with Expert Review" are
made on an "IETF Review" basis per Section 4.8 of RFC 8126 [BCP26]
with "Expert Review" additionally required per Section 4.5 of RFC
8126 [BCP26].
The procedure for early IANA allocation of Standards Track code
points defined in [RFC7120] also applies. When such a procedure
is used, IANA will ask the designated expert(s) to approve the
early allocation before registration. In addition, working group
chairs are encouraged to consult the expert(s) early during the
process outlined in Section 3.1 of [RFC7120].
* All assignments according to "IESG Approval with Expert Review"
are made on an "IESG Approval" basis per Section 4.10 of RFC 8126
[BCP26] with "Expert Review" additionally required per Section 4.5
of RFC 8126 [BCP26].
The registration policy for the 10000-19999 and 33000-64997 ranges is
"Expert Review", following the procedure described in Section 4.1.3.
The registration policy for the 20000-32999 range is FCFS. A
registration request for this range must consist solely of a
registered Media Type name in the "Content Type" field, without any
parameter names or "Content Coding", and the Media Type must not have
been used in this registry yet. If the criteria do not apply, a
registration for a different range (which requires "Expert Review")
can be requested.
The identifiers between 65000 and 65535 inclusive are reserved for
experiments. They are not meant for vendor-specific use of any kind
and MUST NOT be used in operational deployments.
In machine-to-machine (M2M) applications, it is not expected that
generic Internet Media Types such as text/plain, application/xml, or
application/octet-stream are useful for real applications in the long
term. It is recommended that M2M applications making use of CoAP
request new Internet Media Types from IANA indicating semantic
information about how to create or parse a payload. For example, a
Smart Energy application payload carried as Concise Binary Object
Representation (CBOR) might request a more specific type like
application/se+cbor.
4.1.1. Temporary Content-Format Registrations
This section clarifies that the "CoAP Content-Formats" registry
allows temporary registrations within the 0-64998 0-64997 range.
A temporary registration may be created, for example, by an IANA
early allocation action [RFC7120]. If the referenced Media Type is
provisional (that is, included in the "Provisional Standard Media
Type Registry" [IANA.prov-media-types]), then a created registration
is always temporary.
A temporary registration is marked as such by IANA in the
corresponding registry entry. Once the required registration
procedure (defined in Table 1) for the temporary ID has successfully
completed, and the referenced Media Type is included in the "Media
Types" registry [IANA.media-types], IANA must remove any indication
about the temporary nature of the registration so that the entry
becomes permanent.
If a temporary registration does not successfully complete the
registration procedure, IANA must remove the entry and set the
Content-Format ID value back to "Unassigned". This may happen, for
example, when an Internet-Draft requesting a Content-Format ID is
abandoned. If a temporary registration (in any range) refers to a
provisional Media Type that is abandoned, IANA must remove the entry
and set the Content-Format ID value back to "Unassigned".
Note that in the 10000-64998 10000-64997 range, the abandonment of a document
requesting a Content-Format ID does not cause an entry to be removed.
That is because the required registration procedure for this range
does not require completion of any standards process, nor does it
require a registering document.
Temporary registrations within the 0-255 range are exempt from the
formal renewal process outlined in [RFC7120]. Specifically, IANA
will not monitor the removal of registrations in this range.
Instead, the designated experts direct IANA to carry out this task.
4.1.2. Addition of the Media Type Column to the Registry
To assist users of the "CoAP Content-Formats" registry in finding
detailed information about the Media Type associated with each CoAP
Content-Format, and to ensure that a Media Type exists before a new
entry can be registered, IANA has added the new column "Media Type"
to the registry. This new column is placed directly to the right of the
existing "Content Type" column.
The "Media Type" field for each entry lists the (base) Media Type
name and provides a hyperlink to registration information for that
Media Type as recorded by IANA. If the Media Type is provisional,
the hyperlink points to the "Provisional Standard Media Type
Registry" [IANA.prov-media-types]. If a provisional Media Type
becomes a permanent Media Type, IANA must update the "Media Type"
field in the associated registry entries to ensure the hyperlink
directs to the registration information for that Media Type.
In a registration request, the requester does not need to fill out
the "Media Type" field separately, as the necessary information is
already provided in the "Content Type" field of the request.
4.1.3. Expert Review Procedure
The DE designated expert is instructed to perform the "Expert Review",
as described by the following checklist:
1. The combination of Content-Type and Content Coding for which the
registration is requested must not be already present in the
"CoAP Content-Formats" registry.
2. The Media Type associated with the requested Content-Format must
be either registered in the "Media Types" registry
[IANA.media-types] or approved for registration. Alternatively,
it may be listed in the "Provisional Standard Media Type
Registry" [IANA.prov-media-types]. The use of provisional
standard Media Types is only permitted for Content-Format
identifiers within the ranges of 0-255 and 256-9999.
3. The optional parameter names must have been defined in
association with the Media Type, and any parameter values
associated with such parameter names must be as permitted.
4. The Content Type must be in the preferred format defined in
Section 4.1.4.
5. If a Content Coding is specified, it must exist (or must have
been approved for registration) in the "HTTP Content Coding
Registry" within the "Hypertext Transfer Protocol (HTTP)
Parameters" registry group [IANA.http-params].
For the 0-255 range, in addition to the checks described above, the
DE
designated expert is instructed to also evaluate the requested code
point concerning the limited availability of the 1-byte code point
space. For the ranges 256-9999, 10000-19999, and 33000-64997, a
similar criterion may also apply where combinations of Media Type
parameters and Content Coding choices consume considerable code point
space.
4.1.4. Preferred Format for the Content Type Field
This section defines the preferred string format for including a
requested Content Type in the "CoAP Content-Formats" registry.
During the review process, the designated expert(s) or IANA may
rewrite a requested Content Type into this preferred string format
before approval.
The preferred string format is as defined in Section 8.3.1 of
[RFC9110] and follows these rules:
1. For any case-insensitive elements, lowercase characters are used.
2. Parameter values are only quoted if the value is such that it
requires use of a quoted-string per Section 5.6.6 of [RFC9110].
Otherwise, a parameter value is included unquoted.
3. A single semicolon character without any adjacent whitespace
characters is used as the separator between the Media Type and
parameters.
4.1.5. Examples of Invalid Registration Requests
This section provides examples of registration requests for the "CoAP
Content-Formats" registry that are invalid but would be approved
under the procedure defined in Section 12.3 of [RFC7252]. The
checklist defined in Section 4.1.3 should prevent any of these
attempts from succeeding. These examples serve as a representative,
but not exhaustive, sample to train the DE's designated expert's eye on
invalid registration attempts.
All the example registration requests use two CoAP Content-Format
identifiers: 64998 and 64999.
For each of the following example registration requests, one can
create a similar instance where the requested registration is for a
CoAP Content-Format identifier within the "IETF Review with Expert
Review or IESG Approval with Expert Review" range. Likewise, such
registrations must not be allowed to succeed.
4.1.5.1. The Media Type is Unknown
The registrant requests an FCFS Content-Format ID for an unknown
Media Type:
+==========================+================+=======+
| Content Type | Content Coding | ID |
+==========================+================+=======+
| application/unknown+cbor | - | 64999 |
+--------------------------+----------------+-------+
Table 2: Attempt at Registering Content-Format
for an Unknown Media Type
4.1.5.2. The Media Type Parameter is Unknown
The registrant requests an FCFS Content-Format ID for an existing
Media Type with an unknown parameter:
+======================================+================+=======+
| Content Type | Content Coding | ID |
+======================================+================+=======+
| application/cose;unknown-parameter=1 | - | 64999 |
+--------------------------------------+----------------+-------+
Table 3: Attempt at Registering Content-Format for a Media
Type with an Unknown Parameter
4.1.5.3. The Media Type Parameter Value is Invalid
The registrant requests an FCFS Content-Format ID for an existing
Media Type with an invalid parameter value:
+====================================+================+=======+
| Content Type | Content Coding | ID |
+====================================+================+=======+
| application/cose;cose-type=invalid | - | 64999 |
+------------------------------------+----------------+-------+
Table 4: Attempt at Registering Content-Format for a Media
Type with an Invalid Parameter Value
4.1.5.4. The Content Coding is Unknown
The registrant requests an FCFS Content-Format ID for an existing
Media Type with an unknown Content Coding:
+========================+================+=======+
| Content Type | Content Coding | ID |
+========================+================+=======+
| application/senml+cbor | inflate | 64999 |
+------------------------+----------------+-------+
Table 5: Attempt at Registering Content-Format
with Unknown Content Coding
4.1.5.5. Duplicate Entry with Default Media Type Parameters
The registrant requests an FCFS Content-Format ID for a Media Type
that includes a parameter set to its default value, while a
(hypothetical) Content-Format ID 64998 is already registered for this
Media Type without that parameter. As a result, this could lead to
the creation of two separate Content-Format IDs for the same
"logical" entry.
+==================================+================+=======+
| Content Type | Content Coding | ID |
+==================================+================+=======+
| application/my | - | 64998 |
+----------------------------------+----------------+-------+
| application/my;parameter=default | - | 64999 |
+----------------------------------+----------------+-------+
Table 6: Attempt at Registering an Equivalent Logical
Entry with a Different Content-Format ID (1)
4.1.5.6. Duplicate Entry with Default Content Coding
The registrant requests an FCFS Content-Format ID for the "identity"
Content Coding, which is the default coding. If accepted, this
request would duplicate an entry with (hypothetical) Content-Format
ID 64998 where the "Content Coding" field is left empty.
+================+================+=======+
| Content Type | Content Coding | ID |
+================+================+=======+
| application/my | - | 64998 |
+----------------+----------------+-------+
| application/my | identity | 64999 |
+----------------+----------------+-------+
Table 7: Attempt at Registering an
Equivalent Logical Entry with a
Different Content-Format ID (2)
4.1.5.7. Duplicate Entry with Equivalent Parameter
The registrant requests an FCFS Content-Format ID for a Media Type
that includes a parameter. The value of this parameter appears
distinct from that of a (hypothetical) previously registered Content-
Format ID 64998 that also includes this parameter. However, the
semantics of the parameter value are identical to the existing
registration.
In this example, the eat_profile parameter value (which can be any
URI) is set as a Uniform Resource Name (URN) [RFC8141]. Since the
Namespace Identifier (see example in this example) for URNs is
defined as case insensitive, the two registrations are semantically
identical.
+=====================================+================+=======+
| Content Type | Content Coding | ID |
+=====================================+================+=======+
| application/ | - | 64998 |
| eat+cwt;eat_profile="urn:example:1" | | |
+-------------------------------------+----------------+-------+
| application/ | - | 64999 |
| eat+cwt;eat_profile="urn:EXAMPLE:1" | | |
+-------------------------------------+----------------+-------+
Table 8: Attempt at Registering an Equivalent Logical Entry
with a Different Content-Format ID (3)
4.2. New Note and Reference Additions
IANA has added the following note to the registry:
| Note: As per RFC 9876, temporary registrations within the 0-255
| range are approved by designated experts. These registrations are
| not subject to the formal renewal process in [RFC7120].
IANA has also listed this document as an additional reference for the
registry.
4.3. Reserving Content-Format Identifiers 64998 and 64999 for
Documentation
IANA has reserved Content-Format identifiers 64998 and 64999 for use
in documentation.
5. References
5.1. Normative References
[BCP26] Best Current Practice 26,
<https://www.rfc-editor.org/info/bcp26>.
At the time of writing, this BCP comprises the following:
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>.
[IANA.core-params]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<https://www.iana.org/assignments/core-parameters>.
[IANA.http-params]
IANA, "Hypertext Transfer Protocol (HTTP) Parameters",
<https://www.iana.org/assignments/http-parameters>.
[IANA.media-types]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[IANA.prov-media-types]
IANA, "Provisional Standard Media Type Registry",
<https://www.iana.org/assignments/provisional-standard-
media-types>.
[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>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9193] Keränen, A. and C. Bormann, "Sensor Measurement Lists
(SenML) Fields for Indicating Data Value Content-Format",
RFC 9193, DOI 10.17487/RFC9193, June 2022,
<https://www.rfc-editor.org/info/rfc9193>.
5.2. Informative References
[Err4954] RFC Errata, Erratum ID 4954, RFC 7252,
<https://www.rfc-editor.org/errata/eid4954>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
Acknowledgments
Thank you Amanda Baber, Carsten Bormann, Christer Holmberg, Éric
Vyncke, Francesca Palombini, Ketan Talaulikar, Marco Tiloca, Mohamed
Boucadair, Paul Wouters, Renzo Navas, and Rich Salz for your reviews,
comments, suggestions, and fixes.
Authors' Addresses
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org
Esko Dijk
IoTconsultancy.nl
Email: esko.dijk@iotconsultancy.nl