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Table of Contents

1. Introduction

The Concise Binary Object Representation (CBOR) [RFC7049] can be used for serialization of data in the JSON [RFC8259] data model or in its own, somewhat expanded, data model. When serializing a sequence of such values, it is sometimes convenient to have a format where these sequences can simply be concatenated to obtain a serialization of the concatenated sequence of values or to encode a sequence of values that might grow at the end by just appending further CBOR data items.¶

This document describes the concept and format of "CBOR Sequences", which are composed of zero or more encoded CBOR data items. CBOR Sequences can be consumed (and produced) incrementally without requiring a streaming CBOR parser that is able to deliver substructures of a data item incrementally (or a streaming encoder able to encode from substructures incrementally).¶

This document defines and registers the "application/cbor-seq" media type in the "Media Types" registry along with a Constrained Application Protocol (CoAP) Content-Format identifier. Media type structured syntax suffixes [RFC6838] were introduced as a way for a media type to signal that it is based on another media type as its foundation. CBOR [RFC7049] defines the "+cbor" structured syntax suffix. This document defines and registers the "+cbor-seq" structured syntax suffix in the "Structured Syntax Suffix Registry".¶

2. CBOR Sequence Format

Formally, a CBOR Sequence is a sequence of bytes that is recursively defined as either of the following:¶

• an empty (zero-length) sequence of bytes¶
• the sequence of bytes making up an encoded CBOR data item [RFC7049] followed by a CBOR Sequence.¶

In short, concatenating zero or more encoded CBOR data items generates a CBOR Sequence. (Consequently, concatenating zero or more CBOR Sequences also results in a CBOR Sequence.)¶

There is no end-of-sequence indicator. (If one is desired, CBOR encoding an array of the CBOR data model values being encoded, employing either a definite or an indefinite length encoding, as a single CBOR data item may actually be the more appropriate representation.)¶

CBOR Sequences, unlike JSON Text Sequences [RFC7464], do not use a marker between items. This is possible because CBOR-encoded data items are self delimiting and the end can always be calculated. (Note that, while the early object/array-only form of JSON was self delimiting as well, this stopped being the case when simple values such as single numbers were made valid JSON documents.)¶

Decoding a CBOR Sequence works as follows:¶

• If the CBOR Sequence is an empty sequence of bytes, the result is an empty sequence of CBOR data model values.¶
• Otherwise, one must decode a single CBOR data item from the bytes of the CBOR sequence and insert the resulting CBOR data model value at the start of the result of repeating this decoding process recursively with the remaining bytes. (A streaming decoder would therefore simply deliver zero or more CBOR data model values, each as soon as the bytes making it up are available.)¶

This means that if any data item in the sequence is not well formed, it is not possible to reliably decode the rest of the sequence. (An implementation may be able to recover from some errors in a sequence of bytes that is almost, but not entirely, a well-formed encoded CBOR data item. Handling malformed data is outside the scope of this specification.)¶

This also means that the CBOR Sequence format can reliably detect truncation of the bytes making up the last CBOR data item in the sequence, but it cannot detect entirely missing CBOR data items at the end. A CBOR Sequence decoder that is used for consuming streaming CBOR Sequence data may simply pause for more data (e.g., by suspending and later resuming decoding) in case a truncated final item is being received.¶

3. The "+cbor-seq" Structured Syntax Suffix

The use case for the "+cbor-seq" structured syntax suffix is analogous to that for "+cbor": it SHOULD be used by a media type when parsing the bytes of the media type object, as a CBOR Sequence leads to a meaningful result that is at least sometimes not just a single CBOR data item. (Without the qualification at the end, this sentence is trivially true for any +cbor media type, which of course should continue to use the "+cbor" structured syntax suffix.)¶

Applications encountering a "+cbor-seq" media type can then either simply use generic processing if all they need is a generic view of the CBOR Sequence or use generic CBOR Sequence tools for initial parsing and then implement their own specific processing on top of that generic parsing tool.¶

4. Practical Considerations

my-embedded-cbor-seq = bytes .cborseq my-array
my-array = [* my-element]
my-element = my-foo / my-bar

; This defines an array, the elements of which are to be used
; in a CBOR sequence:
my-sequence = [* my-element]
my-element = my-foo / my-bar

[1, 2, 3]

1, 2, 3

; This defines an array of CBOR byte strings, the elements of which
; are to be used in a CBOR sequence:
my-sequence = [* my-element]
my-element = bytes .cbor my-element-structure
my-element-structure = my-foo / my-bar

5. Security Considerations

The security considerations of CBOR [RFC7049] apply. This format provides no cryptographic integrity protection of any kind but can be combined with security specifications such as CBOR Object Signing and Encryption (COSE) [RFC8152] to do so. (COSE protections can be applied to an entire CBOR sequence or to each of the elements of the sequence independently; in the latter case, additional effort may be required if there is a need to protect the relationship of the elements in the sequence.)¶

As usual, decoders must operate on input that is assumed to be untrusted. This means that decoders MUST fail gracefully in the face of malicious inputs.¶

6. IANA Considerations

Acknowledgements

This document has mostly been generated from [RFC7464] by Nico Williams and [RFC8091] by Erik Wilde, which do a similar but slightly more complicated exercise for JSON [RFC8259]. Laurence Lundblade raised an issue on the CBOR mailing list that pointed out the need for this document. Jim Schaad and John Mattsson provided helpful comments.¶

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