CoRE T. Fossati
Internet-Draft Nokia
Intended status: Standards Track K. Hartke
Expires: January 3, 2019 Ericsson
C. Bormann
Universitaet Bremen TZI
July 02, 2018
Multipart Content-Format for CoAP
draft-ietf-core-multipart-ct-01
Abstract
This memo defines application/multipart-core, an application-
independent media-type that can be used to combine representations of
several different media types into a single CoAP message-body with
minimal framing overhead, each along with a CoAP Content-Format
identifier.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Multipart Content-Format Encoding . . . . . . . . . . . . . . 3
3. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Observing Resources . . . . . . . . . . . . . . . . . . . 3
3.2. Implementation hints . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4.1. Registration of media type application/multipart-core . . 5
4.2. Registration of a Content-Format identifier for
application/multipart-core . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This memo defines application/multipart-core, an application-
independent media-type that can be used to combine representations of
several different media types into a single CoAP [RFC7252] message-
body with minimal framing overhead, each along with a CoAP Content-
Format identifier.
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.
Applications using the application/multipart-core Content-Format
define the internal structure of the application/multipart-core
representation.
For example, one way to structure the sub-types specific to an
application/multipart-core container is to always include them at the
same fixed position. This specification allows to indicate that an
optional part is not present by substituting a null value for the
representation of the part.
Optionally, an application might use the general format defined here,
but also register a new media type and an associated Content-Format
identifier -- typically one in the range 10000-64999 -- instead of
using application/multipart-core.
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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. The second, fourth, sixth, etc. element is a
byte string containing a representation, or the value "null" if an
optional part is indicated as not given. The first, third, fifth,
etc. element is an unsigned integer specifying the content format ID
of the representation following it. (Future extensions might want to
include additional alternative ways of specifying the media type of a
representation in such a position.)
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 [I-D.ietf-cbor-cddl]
specification in Figure 1:
multipart-core = [* multipart-part]
multipart-part = (type: uint .size 2, part: bytes / null)
Figure 1: CDDL for application/multipart-core
3. Usage Examples
3.1. Observing Resources
When a client registers to observe a resource [RFC7641] 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. The possible resulting
sequence of notifications is shown in Figure 1.
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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.
3.2. 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..66535 |
+----------------+------------+
Table 1: Serialization of content-format
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+-----------------------------+-------------------+
| Serialization | Length |
+-----------------------------+-------------------+
| 0x40..0x57 | 0..23 |
| 0x58 0xnn | 24..255 |
| 0x59 0xnn 0xnn | 256..66535 |
| 0x5a 0xnn 0xnn 0xnn 0xnn | 66536..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 about its content-format (here: 0x82
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.
4. IANA Considerations
4.1. 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
Encoding considerations: binary
Security considerations: See the Security Considerations Section of
RFCthis
Interoperability considerations: N/A
Published specification: RFCthis
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Applications that use this media type: Applications that need to
combine representations of potentially several media types into
one, e.g., EST-CoAP [I-D.ietf-ace-coap-est]
Fragment identifier considerations: N/A
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
4.2. 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 IETF Review space
(specifically, 256..999):
+----------------------------+----------+------+-----------+
| Media Type | Encoding | ID | Reference |
+----------------------------+----------+------+-----------+
| application/multipart-core | -- | TBD1 | RFCthis |
+----------------------------+----------+------+-----------+
5. Security Considerations
The security considerations of [RFC7049] apply. In particular,
resource exhaustion attacks may employ large values for the byte
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string size fields, or deeply nested structures of recursively
embedded application/multipart-core representations.
6. References
6.1. Normative References
[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>.
[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>.
6.2. Informative References
[I-D.ietf-ace-coap-est]
Stok, P., Kampanakis, P., Kumar, S., Richardson, M.,
Furuhed, M., and S. Raza, "EST over secure CoAP (EST-
coaps)", draft-ietf-ace-coap-est-03 (work in progress),
June 2018.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-02
(work in progress), February 2018.
[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>.
Acknowledgements
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.
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Authors' Addresses
Thomas Fossati
Nokia
Email: thomas.fossati@nokia.com
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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