Packed CBOR: Table set up by reference

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Table of Contents

1. Introduction

[ See abstract. ]¶

2. Setting up the tables by reference

CBOR tag TBD114 is defined with semantics similar to tags TBD113 and TBD1113 from [I-D.ietf-cbor-packed] in that it sets up tables around a rump.¶

cddl Packed-By-Reference = #6.114([count, source, rump]) rump = any source = CRI / URI count = (count-shared-and-argument // count-shared, count-argument // skip-shared, count-shared, skip-argument, count-argument) count-shared-and-argument = int count-shared = int count-argument = int skip-shared = int skip-argument = int ¶

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. 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 [RFC3986] or equivalently as a CRI (as defined in [I-D.ietf-core-href]. Later iterations of this document may introduce additional options. When the source identifier is dereferencable, all considerations from [I-D.bormann-t2trg-deref-id] 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. If a large number of items at the beginning of the source tables would not be used, the four-argument form of count 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 avoid the necessity of the four-argument form.¶

There is no short-hand for importing all the items, as the use of such a form would rule out any extensibility in source tables.¶

114([7, "tag:example.com,2023:books"
    [{"store": {
       "book": [
         {simple(1): "reference", simple(2): "Nigel Rees",
          simple(3): "Sayings of the Century", simple(0): simple(5)},
         {simple(1): simple(4), simple(2): "Evelyn Waugh",
          simple(3): "Sword of Honour", simple(0): 12.99},
         {simple(1): simple(4), simple(2): "Herman Melville",
          simple(3): "Moby Dick", simple(6): "0-553-21311-3",
          simple(0): simple(5)},
         {simple(1): simple(4), simple(2): "J. R. R. Tolkien",
          simple(3): "The Lord of the Rings",
          simple(6): "0-395-19395-8", simple(0): 22.99}],
       "bicycle": {"color": "red", simple(0): 19.95}}}]])

3. Nested table setups

Assembling tables from one or more source references can easily become verbose. 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). Those tables may be set up in a way that grows over time, so that more identifiers can be added later. The inner stage is set up using tag TBD114, and the source given is a packed reference.¶

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"¶

114([5, 6(100) / outer item 100 /, 114([2, 6(106) / outer item 101 /, [
  6(11) / outer item 0, "This class has students with the following names" /,
  6(0) / Franz, item 0 of german-names /,
  6(2) / George, item 0 of english-names /,
  6(1) / Fritz, item 1 of german-names /,
  6(7) / Jack, item 5 of english-names /
  ]])])

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.¶

4. Security Considerations

[ TBD ]¶

5. IANA Considerations

[ TBD: Request tag. ]¶

Acknowledgments

[ TBD ]¶

Author's Address