Packed CBOR

1.1. Terminology

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.¶

Packed reference:

A shared item reference or an argument reference.¶

Shared item reference:

A reference to a shared item as defined in Section 2.2.¶

Argument reference:

A reference that combines a shared argument with a rump item as defined in Section 2.3.¶

Affix:

Prefix or suffix, used as an argument in an argument reference employing the default function "concatenation".¶

Function reference:

An argument reference that uses a tag for argument, rump, or both, causing the application of a function to reconstruct the data item.¶

Packing tables:

The pair of a shared item table and an argument table.¶

Current set:

The packing tables in effect at the data item under consideration.¶

Expansion:

The result of applying a packed reference in the context of given Packing tables.¶

The definitions of [STD94] apply. Specifically: The term "byte" is used in its now customary sense as a synonym for "octet"; "byte strings" are CBOR data items carrying a sequence of zero or more (binary) bytes, while "text strings" are CBOR data items carrying a sequence of zero or more Unicode code points (more precisely: Unicode scalar values), encoded in UTF-8 [STD63].¶

Where arithmetic is explained, this document uses the notation familiar from the programming language C, except that ".." denotes a range that includes both ends given, in the HTML and PDF versions, subtraction and negation are rendered as a hyphen ("-", as are various dashes), and superscript notation denotes exponentiation. For example, 2 to the power of 64 is notated: 2⁶⁴. In the plain-text version of this specification, superscript notation is not available and therefore is rendered by a surrogate notation. That notation is not optimized for this RFC; it is unfortunately ambiguous with C's exclusive-or and requires circumspection from the reader of the plain-text version.¶

2.1. Packing Tables

At any point within a data item making use of Packed CBOR, there is a Current Set of packing tables that applies.¶

There are two packing tables in a Current Set:¶

• Shared item table¶
• Argument table¶

Without any table setup, these two tables are empty arrays. Table setup can cause these arrays to be non-empty, where the elements are (potentially themselves packed) data items. Each of the tables is indexed by an unsigned integer (starting from 0). Such an index may be derived from information in tags and their content as well as from CBOR simple values.¶

Table setup mechanisms (see Section 3) may include all information needed for table setup within the packed CBOR data item, or they may refer external information. This information may be immutable, or may be intended to potentially grow over time. This raises the question of how a reference to a new item should be handled when the unpacker uses an older version of the external information.¶

If, during unpacking, an index is used that references an item that is unpopulated in (e.g., outside the size of) the table in use, this MAY be treated as an error by the unpacker and abort the unpacking. Alternatively, the unpacker MAY provide the special value 1112(undefined) (the simple value >undefined< as per Section 5.7 of [STD94], enclosed in the tag 1112) to the application and leave the error handling to the application. An unpacker SHOULD document which of these two alternatives has been chosen. CBOR based protocols that include the use of packed CBOR MAY require that unpacking errors are tolerated in some positions.¶

2.2. Referencing Shared Items

Shared items are stored in the shared item table of the Current Set.¶

The shared data items are referenced by using the reference data items in Table 1. When reconstructing the original data item, such a reference is replaced by the referenced data item, which is then recursively unpacked.¶

As examples in CBOR diagnostic notation (Section 8 of [STD94]), the first 22 elements of the shared item table are referenced by simple(0), simple(1), ... simple(15), 6(0), 6(-1), 6(1), 6(-2), 6(2), 6(-3). (The alternation between unsigned and negative integers for even/odd table index values — "zigzag encoding" — makes systematic use of shorter integer encodings first.)¶

Taking into account the encoding of these referring data items, there are 16 one-byte references, 48 two-byte references, 512 three-byte references, 131072 four-byte references, etc. As CBOR integers can grow to very large (or very negative) values, there is no practical limit to how many shared items might be used in a Packed CBOR item.¶

Note that the semantics of Tag 6 depend on its tag content: An integer turns the tag into a shared item reference, whereas a string or container (map or array) turns it into a straight (prefix) reference (see Table 2). Note also that the tag content of Tag 6 may itself be packed, so it may need to be unpacked to make this determination.¶

2.3. Referencing Argument Items

The argument table serves as a common table that can be used for argument references, i.e., for concatenation as well as references involving a function tag.¶

When referencing an argument, a distinction is made between straight and inverted references; if no function tag is involved, a straight reference combines a prefix out of the argument table with the rump data item, and an inverted reference combines a rump data item with a suffix out of the argument table.¶

Argument data items are referenced by using the reference data items in Table 2 and Table 3.¶

The tag number of the reference indicates a table index (an unsigned integer) leading to the "argument"; the tag content of the reference is the "rump item".¶

When reconstructing the original data item, such a reference is replaced by a data item constructed from the argument data item found in the table (argument, which might need to be recursively unpacked first) and the rump data item (rump, again possibly recursively unpacked).¶

Separate from the tag used as a reference, a tag ("function tag") may be involved to supply a function to be used in resolving the reference. It is crucial not to confuse reference tag and, if present, function tag.¶

A straight reference uses the argument as the provisional left-hand side and the rump data item as the right-hand side. An inverted reference uses the rump data item as the provisional left-hand side and the argument as the right-hand side.¶

In both cases, the provisional left-hand side is examined. If it is a tag ("function tag"), it is "unwrapped": The function tag's tag number is established as the function to be applied, and the tag content is kept as the unwrapped left-hand side. If the provisional left-hand side is not a tag, it is kept as the unwrapped left-hand side, and the function to be applied is concatenation, as defined below. The right-hand side is taken as is as the unwrapped right-hand side.¶

If a function tag was given, the reference is replaced by the result of applying the unpacking function to be computed to the left and right-hand sides. The unpacking function is defined by the definition of the tag number supplied. If that definition does not define an unpacking function, the result of the unpacking is not valid.¶

If no function tag was given, the reference is replaced by the left-hand side "concatenated" with the right-hand side, where concatenation is defined as in Section 2.4.¶

As a contrived (but short) example, if the argument table is ["foobar", h'666f6f62', "fo"], each of the following straight (prefix) references will unpack to "foobart": 6("t"), 225("art"), 226("obart") (the byte string h'666f6f62' == 'foob' is concatenated into a text string, and the last example is not an optimization).¶

Note that table index 0 of the argument table can be referenced both with tag 6 and tag 224, however tag 6 with an integer content is used for shared item references (see Table 1), so to combine index 0 with an integer rump, tag 224 needs to be used.¶

Taking into account the encoding and ignoring the less optimal tag 224, there is one single-byte straight (prefix) reference, 31 (2⁵-2⁰) two-byte references, 4064 (2¹²-2⁵) three-byte references, and 26843160 (2²⁸-2¹²) five-byte references for straight references. 268435455 (2²⁸) is an artificial limit, but should be high enough that there, again, is no practical limit to how many prefix items might be used in a Packed CBOR item. The numbers for inverted (suffix) references are one quarter of those, except that there is no single-byte reference and 8 two-byte references.¶

2.4. Concatenation

The concatenation function is defined as follows:¶

• If both left-hand side and right-hand side are arrays, the result of the concatenation is an array with all elements of the left-hand-side array followed by the elements of the right-hand side array.¶
• If both left-hand side and right-hand side are maps, the result of the concatenation is a map that is initialized with a copy of the left-hand-side map, and then filled in with the members of the right-hand side map, replacing any existing members that have the same key.¶

• If both left-hand side and right-hand side are one of the string types (not necessarily the same), the bytes of the left-hand side are concatenated with the bytes of the right-hand side. Byte strings concatenated with text strings need to contain valid UTF-8 data. The result of the concatenation gets the type of the unwrapped rump data item; this way a single argument table entry can be used to build both byte and text strings, depending on what type of rump is being used.¶
• If one side is one of the string types, and the other side is an array, the result of the concatenation is equivalent to the application of the "join" function (Section 4.1) to the string as the left-hand side and the array as the right-hand side. The original right-hand side of the concatenation determines the string type of the result.¶
• Other type combinations of left-hand side and right-hand side are not valid.¶

2.5. Discussion

This specification uses up a large number of Simple Values and Tags, in particular one of the rare one-byte tags and two thirds of the one-byte simple values. Since the objective is compression, this is warranted only based on a consensus that this specific format could be useful for a wide area of applications, while maintaining reasonable simplicity in particular at the side of the consumer.¶

A maliciously crafted Packed CBOR data item might contain a reference loop. A consumer/decompressor MUST protect against that.¶

3.1. Basic Packed CBOR

Two tags are predefined by this specification for packing table setup. They are defined in CDDL [RFC8610] as in Figure 1:¶

Basic-Packed-CBOR = #6.113([[*shared-and-argument-item], rump])
Split-Basic-Packed-CBOR =
                    #6.1113([[*shared-item], [*argument-item], rump])
rump = any
shared-and-argument-item = any
argument-item = any
shared-item = any

(This assumes the allocation of tag numbers 113 ('q') and 1113 for these tags.)¶

The array given as the first element of the content of tag 113 ("Basic-Packed-CBOR") is prepended to both the tables for shared items and arguments that apply to the entire tag (by default empty tables). The arrays given as the first and second element of the content of the tag 1113 ("Split-Basic-Packed-CBOR") are prepended to the tables for shared items and arguments that apply to the entire tag (by default empty tables). As discussed in the introduction to this section, references in the supplied new arrays use the new number space (where inherited items are shifted by the new items given), while the inherited items themselves use the inherited number space (so their semantics do not change by the mere action of inheritance).¶

The original CBOR data item can be reconstructed by recursively replacing shared and argument references encountered in the rump by their expansions.¶

4.1. Join Function Tags

Tag 106 ('j') defines the "join" unpacking function, based on the concatenation function (Section 2.4).¶

The join function expects an item that can be concatenated as its left-hand side, and an array of such items as its right-hand side. Joining works by sequentially applying the concatenation function to the elements of the right-hand-side array, interspersing the left-hand side as the "joiner".¶

An example in functional notation: join(", ", ["a", "b", "c"]) returns "a, b, c".¶

For a right-hand side of one or more elements, the first element determines the type of the result when text strings and byte strings are mixed in the argument. For a right-hand side of one element, the joiner is not used, and that element returned. For a right-hand side of zero elements, a neutral element is generated based on the type of the joiner (empty text/byte string for a text/byte string, empty array for an array, empty map for a map).¶

For an example, we assume this unpacked data item:¶

["https://packed.example/foo.html",
 "coap:://packed.example/bar.cbor",
 "mailto:support@packed.example"]

A packed form of this using straight references could be:¶

113([[106("packed.example")],
  [6(["https://", "/foo.html"]),
   6(["coap://", "/bar.cbor"]),
   6(["mailto:support@", ""])]
])

Tag 105 ('i') defines the "ijoin" unpacking function, which is exactly like that of tag 106, except that the left-hand side and right-hand side are interchanged ('i').¶

A packed form of the first example using inverted references and the ijoin tag could be:¶

113([["packed.example"],
  [216(105(["https://", "/foo.html"]),
   216(105(["coap://", "/bar.cbor"]),
   216("mailto:support@")]
])

A packed form of an array with many URIs that reference SenML items from the same place could be:¶

113([[105(["coaps://[2001::db8::1]/s/", ".senml"])],
  [6("temp-freezer"),
   6("temp-fridge"),
   6("temp-ambient")]
])

5.1. Tag Equivalence

A tag definition MAY declare Tag Equivalence to some existing structure for the tag, under some conditions defined by the new tag definition. This, in effect, extends all existing tag definitions that accept the named structure to accept the newly defined tag under the conditions given for the Tag Equivalence.¶

A number of limitations apply to Tag Equivalence, which therefore should be applied deliberately and sparingly:¶

• Tag Equivalence is a new concept, which may not be implemented by an existing generic decoder. A generic decoder not implementing tag equivalence might raise tag validity errors where Tag Equivalence says there should be none.¶
• A CBOR protocol MAY specify the use of Tag Equivalence, effectively limiting its full use to those generic encoders that implement it. Existing CBOR protocols that do not address Tag Equivalence implicitly have a new variant that allows Tag Equivalence (e.g., to support Packed CBOR with an existing protocol). A CBOR protocol that does address Tag Equivalence MAY be explicit about what kinds of Tag Equivalence it supports (e.g., only the reference tags employed by Packed CBOR and certain table setup tags).¶
• There is currently no way to express Tag Equivalence in CDDL. For Packed CBOR, CDDL would typically be used to describe the unpacked CBOR represented by it; further restricting the Packed CBOR is likely to lead to interoperability problems. (Note that, by definition, there is no need to describe Tag Equivalence on the receptacle side; only for the tag that declares Tag Equivalence.)¶
• The registry "CBOR Tags" [IANA.cbor-tags] currently does not have a way to record any equivalence claimed for a tag. A convention would be to alert to Tag Equivalence in the "Semantics (short form)" field of the registry.Needs to be done for the tag registrations here.¶

5.2. Tag Equivalence of Packed CBOR Tags

The reference tags in this specification declare their equivalence to the unpacked shared items or function results they represent.¶

The table setup tags 113 and 1113 declare its equivalence to the unpacked CBOR data item represented by it.¶

6.1. CBOR Tags Registry

In the registry "CBOR Tags" [IANA.cbor-tags], IANA is requested to allocate the tags defined in Table 4.¶

6.2. CBOR Simple Values Registry

In the registry "CBOR Simple Values" [IANA.cbor-simple-values], IANA is requested to allocate the simple values defined in Table 5.¶