Packed CBOR: Table set up by reference
draft-amsuess-cbor-packed-by-reference-00

cbor                                                           C. Amsüss
Internet-Draft                                              11 July 2023
Intended status: Standards Track                                        
Expires: 12 January 2024

                 Packed CBOR: Table set up by reference
               draft-amsuess-cbor-packed-by-reference-00

Abstract

   Packed CBOR is a compression mechanism for Concise Binary Object
   Representation (CBOR) that can be used without a decompression step.
   This document introduces a means for setting up its tables by means
   of dereferencable identifiers, and introduces a pattern of using it
   without sending long identifiers.

Status of This Memo

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   This Internet-Draft will expire on 12 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Setting up the tables by reference  . . . . . . . . . . . . .   2
     2.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Nested table setups . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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).

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   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.

2.1.  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 [I-D.ietf-cbor-packed]
   can be written as:

   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:

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   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. ]

6.  Normative References

   [I-D.bormann-t2trg-deref-id]
              Bormann, C. and C. Amsüss, "The "dereferenceable
              identifier" pattern", Work in Progress, Internet-Draft,
              draft-bormann-t2trg-deref-id-01, 9 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-bormann-
              t2trg-deref-id-01>.

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   [I-D.ietf-cbor-packed]
              Bormann, C., "Packed CBOR", Work in Progress, Internet-
              Draft, draft-ietf-cbor-packed-09, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              packed-09>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-13, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-13>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

Acknowledgments

   [ TBD ]

Author's Address

   Christian Amsüss
   Austria
   Email: christian@amsuess.com

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