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Multipart Content-Format for the Constrained Application Protocol (CoAP)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8710.
Authors Thomas Fossati , Klaus Hartke , Carsten Bormann
Last updated 2020-09-10 (Latest revision 2019-08-21)
Replaces draft-bormann-core-maybe, draft-fossati-core-multipart-ct
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Working Group Repo
Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Jaime Jimenez
Shepherd write-up Show Last changed 2019-03-20
IESG IESG state Became RFC 8710 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alexey Melnikov
Send notices to Jaime Jimenez <>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
CoRE                                                          T. Fossati
Internet-Draft                                                       ARM
Intended status: Standards Track                               K. Hartke
Expires: February 22, 2020                                      Ericsson
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         August 21, 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 message, such as a
   CoAP request or response body, with minimal framing overhead, each
   along with a CoAP Content-Format identifier.

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

   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 February 22, 2020.

Copyright Notice

   Copyright (c) 2019 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
   ( 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

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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Multipart Content-Format Encoding . . . . . . . . . . . . . .   4
   3.  Usage Example: Observing Resources  . . . . . . . . . . . . .   4
   4.  Implementation Hints  . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Registration of media type application/multipart-core . .   6
     5.2.  Registration of a Content-Format identifier for
           application/multipart-core  . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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, into a single representation, with minimal framing
   overhead.  This combined representation may then be carried in a
   single message, such as a CoAP [RFC7252] request or response body.

   This simple and efficient binary framing mechanism can be employed to
   create application specific request and response 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

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   resource URI and any further options (header fields), but three usage
   scenarios are envisioned:

   The individual representations in an application/multipart-core 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

   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, 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",
   "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.

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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:

      [42, h'0123456789abcdef', 0, h'3031323334']

   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.

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         __________       __________       __________
        |          |     |          |     |          |
   ---->|   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  | Value      |
                      | 0x00..0x17     | 0..23      |
                      | 0x18 0xnn      | 24..255    |
                      | 0x19 0xnn 0xnn | 256..65535 |

                 Table 1: Serialization of content-format

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            | 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..      |

                  Table 2: Serialization of object length

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

      0x82 0x00 0x4b H e l l o 0x20 W o r l d

   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:

      0x84 (*) 0x182A 0x48 0x0123456789ABCDEF (+) 0x00 0x45 0x3031323334

   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:  application

   Subtype name:  multipart-core

   Required parameters:  N/A

   Optional parameters:  N/A

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   Encoding considerations:  binary

   Security considerations:  See the Security Considerations Section of

   Interoperability considerations:  N/A

   Published specification:  RFCthis

   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

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  CoRE WG

   Change controller:  IESG

   Provisional registration? (standards tree only):  no

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

   IANA is requested to register the following Content-Format to the
   "CoAP Content-Formats" subregistry, within the "Constrained RESTful

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   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

              Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
              "EST over secure CoAP (EST-coaps)", draft-ietf-ace-coap-
              est-12 (work in progress), June 2019.

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   [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


   Klaus Hartke
   Torshamnsgatan 23
   Stockholm  SE-16483


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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921

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