Packed CBOR: Table set up by reference

Setting up the tables by reference CBOR tag TBD213 is defined with semantics similar to tags TBD113 and TBD1113 from in that it sets up tables around a rump. ([count, source, rump]) rump = any source = CRI / ~uri count = (count-shared-and-argument // ; similar to tag 113 count-shared, count-argument ) ; similar to tag 1113 count-shared-and-argument = uint count-shared = uint count-argument = uint tbd213 = 213 ; preliminary value, see IANA considerations ]]> The items inserted by the tables are not given explicitly, but picked out of tables known by their identifier given as source. Such a source needs to represent two lists of CBOR items, one for each kind of tables (one for shared item, one for arguments). The tag prepends some number of items out of those source lists to the tables that are used to decompress the rump. The identifier is given as a URI string (as defined in ) or equivalently as a CRI (as defined in ). Later iterations of this document may introduce additional options.If the stand-in concept of is generalized, the source item may become the raw list of tables, possibly disallowing the CRI and URI variants. Given that tags 113 and 1113 are capable of expressing cases where the source tables are present, tag TBD213 should then be used by using a dereferencing stand-in in the source position. When the source identifier is dereferenceable, all considerations from apply. (Simplifying: No dereferencing at runtime -- the recipient either knows it already or treats it as unknown). If the same number of items is prepended to both tables, their count is given as a single number; otherwise, the numbers are given separately. Encoders SHOULD use the most compact form of count, and SHOULD pick the lowest count(s) sufficient to encode the items contained in the rump. When those conflict, they may prioritize either. If the source supports evolution of sources (see ), disregarding that recommendation may pose an interoperability hazard.

Count vs. content of source The count encoded for the number of table entries given in a document will often mismatch with the number of entries the receiver of a document knows to be present in the given source.

Not all known entries are used If the encoded count is less than the number of known entries, this merely expresses that the originator of the document did not use the higher numbers. When a document's tables are populated from multiple sources, encoding the smallest possible count is useful because the table indices used throughout the document stay small and can thus be encoded concisely.

Unknown entries are used – evolution of sources If the encoded count is larger than the number of known entries, this indicates that the document may contain references that the receiver does not know. This can happen when a source has been evolved compatibly to contain more entries, compared to when the receiver learned of the source definition. Source entries beyond the receiver's knowledge stay unpopulated in the receiver's tables, but still shift existing entries to higher indices. Some CBOR protocols come with elements that support isolation of processing errors. For example, a CRI that uses unknown extensions is regarded as "unprocessable" (). It cannot be resolved, is unequal to any other CRI (unless they are identical), but does not inhibit the processing of its surrounding document. In such protocols, references to unpopulated table entries can be tolerated as described in . Care has to be taken around processing tag TBD1112: If that tag is produced in the course of unpacking, comparisons for identity are not reliable. Similarly, if the unpacking mechanism provides access to the serialized form of the unprocessable entity, identity comparisons are only reliable if the items being compared have the same table setup applied. Protocols may also pre-populated entries with values that are reserved in the protocol and specified to be ignored at reception. Later, when the entries are specified, concrete values take their places. This has roughly the same effect, but is harder to describe. (This paragraph may be removed later unless it is found to be particularly useful). Protocols that do not support error isolation need a way to negotiate the understood set of sources and table entries.

Evolution beyond adding items The content of tables may be altered in more ways than just adding entries that were previously unpopulated. Such changes are NOT RECOMMENDED, because while they can be done in a compatible way, providing criteria for this are out of scope of this document. If a later version of this document uses stand-in values more actively, this section will need to be revisited: In that case, the tables may be part of the outer source, and then those would grow internally.

Setup with skipped indices If a large number of items at the beginning of the source tables would not be used, there is an additional four-argument form of count that defines a number of items in the source tables that are skipped before selecting items into the table. This allows keeping the indices low and therefore compact. Source tables should be designed in such a way that commonly used items are at the start to minimize the necessity for the four-argument form.

Example Suppose the URI "tag:example.com,2023:byref" defines the items ["price", "category", "author", "title", "fiction", 8.95, "isbn"] in both tables. Then the example in figure 3 of can be written as: Assuming that the underlying CBOR protocol defines that unknown keys on goods may be ignored, an older receiver that only knows the first 5 entries of the source tables could still process the document, but would be missing all ISBNs and the price of one item.

Nested table setups Documents that use tables from multiple sources can easily spend many bytes on listing source identifiers. A pattern that reduces the verbosity while staying unambiguous are nested table setups, where the outer tables are extended to contain additional identifiers. In this pattern, tables are set up in two stages: The outer stage contains the CRIs or URIs that may later be used as source values. (It may also contain other items). The inner stage is set up using tag TBD213, and the source given is a packed reference. All table inputs can be evolved orthogonally as described in . If an unspecified entry is used as a source, the whole source content is considered unspecified.

Example of nested table setup In this example, the initial table set up is provided by the media type, and contains these items:

0: "This class has students with the following names"
100: "tag:example.com,2023:english-names.txt"
101: "tag:example.com,2023:german-names.txt"

Note that a constrained implementation of a decoder may not even have the fully expanded form of the URIs or CRIs available; it may only be capable of using these table entries in the source position and then find the shipped source lists.