Keywords: CoAP, Multipart Content-Format
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)
Abstract
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
https://www.rfc-editor.org/info/rfc8710.
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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
application/multipart-core
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
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",
"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:
[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
Content-Format
+-----------------------------+-------------------+
| 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
follows:
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
application/cbor.)
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:
iesg@ietf.org
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/
multipart-core
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<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>.
7.2. Informative References
[EST-COAPS]
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
2020,
<https://tools.ietf.org/html/draft-ietf-ace-coap-est-18>.
[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>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[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>.
Acknowledgements
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
ARM
Email: thomas.fossati@arm.com
Klaus Hartke
Ericsson
Torshamnsgatan 23
SE-16483 Stockholm
Sweden
Email: klaus.hartke@ericsson.com
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org