RFC8710: Multipart Content-Format for the Constrained Application Protocol (CoAP)

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Internet Engineering Task Force (IETF)                        T. Fossati
Request for Comments: 8710                                           ARM
Category: Standards Track                                      K. Hartke
ISSN: 2070-1721                                                 Ericsson
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                           February 2020

Multipart Content-Format for the Constrained Application Protocol (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 (each with a Constrained
   Application Protocol (CoAP) Content-Format identifier) into a single
   representation, with minimal framing overhead.

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

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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Multipart Content-Format Encoding
   3.  Usage Example: Observing Resources
   4.  Implementation Hints
   5.  IANA Considerations
     5.1.  Registration of Media Type application/multipart-core
     5.2.  Registration of a Content-Format Identifier for
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Authors' Addresses

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 with a CoAP Content-Format
   identifier [RFC7252]) into a single representation, with minimal
   framing overhead.

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

   As the name of the media type suggests, application/multipart-core
   was inspired by the multipart media types initially defined in the
   original set of MIME specifications [RFC2046] and later.  However,
   while those needed to focus on the syntactic aspects of integrating
   multiple representations into one email, 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]); however, the semantics are relaxed to
   allow for both ordered and unordered collections of media types.

      Historical Note: Experience with multipart/mixed in email has
      shown that recipients that care about order of included body parts
      will process them in the order they are listed inside multipart/
      mixed, and recipients that don't care about the order will ignore
      it anyway.  The media type multipart/parallel that was intended
      for unordered collections didn't deploy.

   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:

   In one case, 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 another case, an application may be more interested in a bag of
   representations (which are distinguished by their Content-Format
   identifiers), 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 common situation only has a single representation in the
   sequence, and the sender 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, an application 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",
   "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:

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

   For illustration, the structure of an application/multipart-core
   representation can be described by the Concise Data Definition
   Language (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 unless
   some streaming processing has already happened.)

3.  Usage Example: Observing Resources

   This section illustrates a 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 that indicate the lack of an actual
   representation.  Later on, when one becomes available, the server
   will send 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., by 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

            | 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 follows:

   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),
   information about its Content-Format (here: 0x00), and information
   regarding 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, etc.), 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 (+)
   marks the start of the second representation (Content-Format 0, byte
   string length 5).

5.  IANA Considerations

5.1.  Registration of Media Type application/multipart-core

   IANA has registered the following media type [RFC6838]:

   Type name:  application

   Subtype name:  multipart-core

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See the Security Considerations section of
      RFC 8710.

   Interoperability considerations:  N/A

   Published specification:  RFC 8710

   Applications that use this media type:  Applications that need to
      combine representations of zero or more different media types into
      one, e.g., EST over secure CoAP (EST-CoAP) [EST-COAPS]

   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers specified for application/multipart-core are
      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 has registered the following Content-Format in the "CoAP
   Content-Formats" subregistry within the "Constrained RESTful
   Environments (CoRE) Parameters" registry:

        | Media Type                 | Encoding | ID | Reference |
        | application/multipart-core | -        | 62 | RFC 8710  |

           Table 3: Addition to "CoAP Content-Formats" Registry

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 employ 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, <https://www.rfc-editor.org/info/rfc7049>.

   [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, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

              Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
              "EST over secure CoAP (EST-coaps)", Work in Progress,
              Internet-Draft, draft-ietf-ace-coap-est-18, 6 January

   [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, <https://www.rfc-editor.org/info/rfc8610>.


   Most of the text in this document is from earlier contributions by
   two of the authors, Thomas Fossati and Klaus Hartke.  This earlier
   work was reorganized in this document based on the requirements in
   [EST-COAPS] and discussions with Michael Richardson, Panos Kampanis,
   and Peter van der Stok.

Authors' Addresses

   Thomas Fossati

   Email: thomas.fossati@arm.com

   Klaus Hartke
   Torshamnsgatan 23
   SE-16483 Stockholm

   Email: klaus.hartke@ericsson.com

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921
   Email: cabo@tzi.org