ARM

thomas.fossati@arm.com

Ericsson

Torshamnsgatan 23 Stockholm 16483 Sweden klaus.hartke@ericsson.com

Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

ART CoRE CoAP Multipart Content-Format 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 .

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Table of Contents

• .  Introduction
  • .  Requirements Language
• .  Multipart Content-Format Encoding
• .  Usage Example: Observing Resources
• .  Implementation Hints
• .  IANA Considerations
  • .  Registration of Media Type application/multipart-core
  • .  Registration of a Content-Format Identifier for application/multipart-core
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

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 ) 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 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) 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 (); 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 (), 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).

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 when, and only when, they appear in all capitals, as shown here.

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 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) specification in :

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

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.)

Usage Example: Observing Resources This section illustrates a less obvious example for using application/multipart-core: combining it with observing a resource 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 .

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

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 and carrying a single representation specified as the target Content-Format in the case in the middle of the figure.

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 . The second element is the object as a byte string, which is represented as a length as described in 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 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 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).

IANA Considerations

Registration of Media Type application/multipart-core IANA has registered the following media type :

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)

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

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: Addition to "CoAP Content-Formats" Registry

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

Security Considerations The security considerations of 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.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Enrollment over Secure Transport (EST) is used as a certificate provisioning protocol over HTTPS. Low-resource devices often use the lightweight Constrained Application Protocol (CoAP) for message exchanges. This document defines how to transport EST payloads over secure CoAP (EST-coaps), which allows constrained devices to use existing EST functionality for provisioning certificates. Work in Progress This second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK] This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON.

Acknowledgements Most of the text in this document is from earlier contributions by two of the authors, and . This earlier work was reorganized in this document based on the requirements in and discussions with , , and .

Authors' Addresses ARM

thomas.fossati@arm.com

Ericsson

Torshamnsgatan 23 Stockholm 16483 Sweden klaus.hartke@ericsson.com

Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org