CoRE T. Fossati
Internet-Draft ARM
Intended status: Standards Track K. Hartke
Expires: February 24, 2020 Ericsson
C. Bormann
Universität Bremen TZI
August 23, 2019

Multipart Content-Format for CoAP


This memo defines application/multipart-core, an application-independent media type that can be used to combine representations of zero or more different media types into a single representation, with minimal framing overhead, each along with a CoAP Content-Format identifier.

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Table of Contents

1. Introduction

This memo defines application/multipart-core, an application-independent media type that can be used to combine representations of zero or more different media types, each along with a CoAP Content-Format identifier [RFC7252], into a single representation, with minimal framing overhead. This combined representation may then be carried in a single message, for instance as the body or payload of a transfer protocol request or response.

This simple and efficient binary framing mechanism can be employed to create application specific message bodies which build on multiple already existing media types.

As the name of the media type suggests, it is inspired by the multipart media types that started to be defined with the original set of MIME specifications [RFC2046]. However, while those needed to focus on the syntactic aspects of integrating multiple representations into one e-mail, transfer protocols providing full data transparency such as CoAP as well as readily available encoding formats such as the Concise Binary Object Representation (CBOR) [RFC7049] shift the focus towards the intended use of the combined representations. In this respect, the basic intent of the application/multipart-core media type is like that of multipart/mixed (Section 5.1.3 of [RFC2046]). The detailed semantics of the representations are refined by the context established by the application in the accompanying request parameters, e.g., the resource URI and any further options (header fields), but three usage scenarios are envisioned:

The individual representations in an application/multipart-core message body occur in a sequence, which may be employed by an application where such a sequence is natural, e.g. for a number of audio snippets in various formats to be played out in that sequence, or search results returned in order of relevance.

In other cases, an application may be more interested in a bag of representations, which are distinguished by their Content-Format identifier, such as an audio snippet and a text representation accompanying it. In such a case, the sequence in which these occur may not be relevant to the application. This specification adds the option of substituting a null value for the representation of an optional part, which indicates that the part is not present.

A third situation that is common only ever has a single representation in the sequence, where the sender already selects just one of a set of formats possible for this situation. This kind of union “type” of formats may also make the presence of the actual representation optional, the omission of which leads to a zero-length array.

Where these rules are not sufficient for an application, it might still use the general format defined here, but register a new media type and an associated Content-Format identifier to associate the representation with these more specific semantics instead of using the application/multipart-core media type.

Also, future specifications might want to define rough equivalents for other multipart media types with specific semantics not covered by the present specification, such as multipart/alternative (Section 5.1.4 of [RFC2046]), where several alternative representations are provided in the message body, but only one of those is to be selected by the recipient for its use (this is less likely to be useful in a constrained environment that has facilities for pre-flight discovery).

1.1. Requirements Language

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.

2. Multipart Content-Format Encoding

A representation of media type application/multipart-core contains a collection of zero or more representations, each along with their respective content format.

The collection is encoded as a CBOR [RFC7049] array with an even number of elements. Counting from zero, the odd-numbered elements are a byte string containing a representation, or the value null if an optional part is indicated as not given. The (even-numbered) element preceding each of these is an unsigned integer specifying the content format ID of the representation following it.

For example, a collection containing two representations, one with content format ID 42 and one with content format ID 0, looks like this in CBOR diagnostic notation:

For illustration, the structure of an application/multipart-core representation can be described by the CDDL [RFC8610] specification in Figure 1:

multipart-core = [* multipart-part]
multipart-part = (type: uint .size 2, part: bytes / null)

Figure 1: CDDL for application/multipart-core

This format is intended as a strict specification: An implementation MUST stop processing the representation if there is a CBOR well-formedness error, a deviation from the structure defined above, or any residual data left after processing the CBOR data item. (This generally means the representation is not processed at all except if some streaming processing has already happened.)

3. Usage Example: Observing Resources

This section illustrates one less obvious example for using application/multipart-core: combining it with observing a resource [RFC7641] to handle pending results.

When a client registers to observe a resource for which no representation is available yet, the server may send one or more 2.05 (Content) notifications before sending the first actual 2.05 (Content) or 2.03 (Valid) notification. A diagram depicting possible resulting sequences of notifications, identified by their respective response code, is shown in Figure 2.

      __________       __________       __________
     |          |     |          |     |          |
---->|   2.05   |---->|  2.05 /  |---->|  4.xx /  |
     |  Pending |     |   2.03   |     |   5.xx   |
     |__________|     |__________|     |__________|
        ^   \ \          ^    \           ^
         \__/  \          \___/          /

Figure 2: Sequence of Notifications

The specification of the Observe option requires that all notifications carry the same Content-Format. The application/multipart-core media type can be used to provide that Content-Format: e.g., carrying an empty list of representations in the case marked as “Pending” in Figure 2, and carrying a single representation specified as the target content-format in the case in the middle of the figure.

4. Implementation Hints

This section describes the serialization for readers that may be new to CBOR. It does not contain any new information.

An application/multipart-core representation carrying no representations is represented by an empty CBOR array, which is serialized as a single byte with the value 0x80.

An application/multipart-core representation carrying a single representation is represented by a two-element CBOR array, which is serialized as 0x82 followed by the two elements. The first element is an unsigned integer for the Content-Format value, which is represented as described in Table 1. The second element is the object as a byte string, which is represented as a length as described in Table 2 followed by the bytes of the object.

Serialization of content-format
Serialization Value
0x00..0x17 0..23
0x18 0xnn 24..255
0x19 0xnn 0xnn 256..65535
Serialization of object length
Serialization Length
0x40..0x57 0..23
0x58 0xnn 24..255
0x59 0xnn 0xnn 256..65535
0x5a 0xnn 0xnn 0xnn 0xnn 65536..4294967295
0x5b 0xnn .. 0xnn (8 bytes) 4294967296..

For example, a single text/plain object (content-format 0) of value “Hello World” (11 characters) would be serialized as

In effect, the serialization for a single object is done by prefixing the object with information that there is one object (here: 0x82), about its content-format (here: 0x00) and its length (here: 0x4b).

For more than one representation included in an application/multipart-core representation, the head of the CBOR array is adjusted (0x84 for two representations, 0x86 for three, …) and the sequences of content-format and embedded representations follow.

For instance, the example from Section 2 would be serialized as:

where (*) marks the start of the information about the first representation (content-format 42, byte string length 8) and, (+), of the second representation (content-format 0, byte string length 5).

5. IANA Considerations

5.1. Registration of media type application/multipart-core

IANA is requested to register the following media type [RFC6838]:

Type name:
Subtype name:
Required parameters:
Optional parameters:
Encoding considerations:
Security considerations:
See the Security Considerations Section of RFCthis
Interoperability considerations:
Published specification:
Applications that use this media type:
Applications that need to combine representations of zero or more different media types into one, e.g., EST-CoAP [I-D.ietf-ace-coap-est]
Fragment identifier considerations:
The syntax and semantics of fragment identifiers specified for “application/multipart-core” is as specified for “application/cbor”. (At publication of this document, there is no fragment identification syntax defined for “application/cbor”.)
Additional information:
Deprecated alias names for this type:
Magic number(s):
File extension(s):
Macintosh file type code(s):

Person & email address to contact for further information:
Intended usage:
Restrictions on usage:
Change controller:
Provisional registration? (standards tree only):

5.2. Registration of a Content-Format identifier for application/multipart-core

IANA is requested to register the following Content-Format to the “CoAP Content-Formats” subregistry, within the “Constrained RESTful Environments (CoRE) Parameters” registry, from the Expert Review space (0..255):

Media Type Encoding ID Reference
application/multipart-core TBD1 RFCthis

6. Security Considerations

The security considerations of [RFC7049] apply. In particular, resource exhaustion attacks may employ large values for the byte string size fields, or deeply nested structures of recursively embedded application/multipart-core representations.

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.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

7.2. Informative References

[I-D.ietf-ace-coap-est] Stok, P., Kampanakis, P., Richardson, M. and S. Raza, "EST over secure CoAP (EST-coaps)", Internet-Draft draft-ietf-ace-coap-est-12, June 2019.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996.
[RFC6838] Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013.
[RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015.
[RFC8610] Birkholz, H., Vigano, C. and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019.


Most of the text in this draft is from earlier contributions by two of the authors, Thomas Fossati and Klaus Hartke. The re-mix in this document is based on the requirements in [I-D.ietf-ace-coap-est], based on discussions with Michael Richardson, Panos Kampanis and Peter van der Stok.

Authors' Addresses

Thomas Fossati ARM EMail:
Klaus Hartke Ericsson Torshamnsgatan 23 Stockholm, SE-16483 Sweden EMail:
Carsten Bormann Universität Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63921 EMail: