Original text:
This memo defines application/multipart-core, an application- independent media-type that can be used to combine representations of <del> zero or more different media types into a single message, such as a CoAP RFC7252 request or response body, with minimal framing overhead, each along with a CoAP Content-Format identifier. </del>  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.  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 <del> different formats to be played out in that sequence. </del>  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 <del> may not be relevant to the application.  This specification allows to indicate that an optional part is not present by substituting a null value for the representation of the part. </del>  A third situation that is common only ever has a single <del> representation in the sequence, which is one of a set of formats possible.  This kind of union of formats may also make the presence of the actual representation optional, the omission of which leads to a zero-length array. </del>  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 <del> application/multipart-core. </del>  1.1. respective content format. The collection is encoded as a CBOR RFC7049 array with an even number <del> 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. </del>  For example, a collection containing two representations, one with content format ID 42 and one with content format ID 0, looks like [42, h'0123456789abcdef', 0, h'3031323334'] For illustration, the structure of an application/multipart-core <del> representation can be described by the CDDL I-D.ietf-cbor-cddl specification in mct-cddl: </del>  This format is intended as a strict specification: An implementation MUST stop processing the representation if there is a CBOR well- 3. <del> 3.1. </del>  <del> When a client registers to observe a resource RFC7641 for which no </del>  representation is available yet, the server may send one or more 2.05 (Content) notifications before sending the first actual 2.05 <del> (Content) or 2.03 (Valid) notification.  The possible resulting sequence of notifications is shown in Figure 1. </del>  The specification of the Observe option requires that all notifications carry the same Content-Format.  The application/ For example, a single text/plain object (content-format 0) of value "Hello World" (11 characters) would be serialized as <del> 0x82 0x00 0x4b H e l l o 0x20 w o r l d </del>  In effect, the serialization for a single object is done by prefixing the object with information that there is one object (here: 0x82), (0x84 for two representations, 0x86 for three, ...) and the sequences of content-format and embedded representations follow.  5. 5.1.

Reviewer comments:
Address Benjamin Kaduk's outstanding DISCUSS.
No: with that, the sender sends along all formats and the recipient chooses based on its capabilities or preferences. Appropriate for single-shot mail, not at all appropriate for a request/response situation (at least not for the response part). URL Grüße, Carsten
Ah sorry, hadn't read properly (it's sender, not receiver).
LGTM

Revised text:
This memo defines application/multipart-core, an application- independent media-type that can be used to combine representations of  <ins> 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. </ins> 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. <ins> 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 resource URI and any further options (header fields), but three usage scenarios are envisioned: </ins> 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  <ins> various formats to be played out in that sequence, or search results returned in order of relevance. </ins> 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  <ins> 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. </ins> A third situation that is common only ever has a single  <ins> 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 array. </ins> 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  <ins> 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). </ins> 1.1. respective content format. The collection is encoded as a CBOR RFC7049 array with an even number  <ins> 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. </ins> For example, a collection containing two representations, one with content format ID 42 and one with content format ID 0, looks like [42, h'0123456789abcdef', 0, h'3031323334'] For illustration, the structure of an application/multipart-core  <ins> representation can be described by the CDDL RFC8610 specification in mct-cddl: </ins> This format is intended as a strict specification: An implementation MUST stop processing the representation if there is a CBOR well- 3.  <ins> This section illustrates one less obvious example for using application/multipart-core: combining it with observing a resource RFC7641 to handle pending results. </ins>  <ins> When a client registers to observe a resource for which no </ins> representation is available yet, the server may send one or more 2.05 (Content) notifications before sending the first actual 2.05  <ins> (Content) or 2.03 (Valid) notification.  A diagram depicting possible resulting sequences of notifications, identified by their respective response code, is shown in fig-sequence. </ins> The specification of the Observe option requires that all notifications carry the same Content-Format.  The application/ For example, a single text/plain object (content-format 0) of value "Hello World" (11 characters) would be serialized as  <ins> 0x82 0x00 0x4b H e l l o 0x20 W o r l d </ins> In effect, the serialization for a single object is done by prefixing the object with information that there is one object (here: 0x82), (0x84 for two representations, 0x86 for three, ...) and the sequences of content-format and embedded representations follow. <ins> For instance, the example from encoding 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). </ins> 5. 5.1.