Packed CBOR: Table set up by reference
draft-amsuess-cbor-packed-by-reference-02
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Christian Amsüss | ||
| Last updated | 2024-03-04 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources |
Other Repository
|
||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-amsuess-cbor-packed-by-reference-02
cbor C. Amsüss
Internet-Draft 4 March 2024
Intended status: Standards Track
Expires: 5 September 2024
Packed CBOR: Table set up by reference
draft-amsuess-cbor-packed-by-reference-02
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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://chrysn.codeberg.page/packed-by-reference/draft-amsuess-cbor-
packed-by-reference.html. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-amsuess-cbor-
packed-by-reference/.
Discussion of this document takes place on the CBOR Working Group
mailing list (mailto:cbor@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at
https://www.ietf.org/mailman/listinfo/cbor/.
Source for this draft and an issue tracker can be found at
https://codeberg.org/chrysn/packed-by-reference.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Amsüss Expires 5 September 2024 [Page 1]
Internet-Draft Packed CBOR: Table set up by reference March 2024
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Setting up the tables by reference . . . . . . . . . . . . . 3
2.1. Count vs. content of source . . . . . . . . . . . . . . . 4
2.1.1. Not all known entries are used . . . . . . . . . . . 4
2.1.2. Unknown entries are used - evolutution of sources . . 4
2.2. Setup with skipped indices . . . . . . . . . . . . . . . 5
2.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Nested table setups . . . . . . . . . . . . . . . . . . . . . 6
3.1. Example of nested table setup . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. CBOR Tags Registry . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
[ See abstract. ]
Amsüss Expires 5 September 2024 [Page 2]
Internet-Draft Packed CBOR: Table set up by reference March 2024
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.
Packed-By-Reference = #6.<tbd114>([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
tbd114 = 114 ; 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 [RFC3986] or
equivalently as a CRI (as defined in [I-D.ietf-core-href]. Later
iterations of this document may introduce additional options.
// If the stand-in concept of [I-D.bormann-cbor-yang-standin] 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 TBD114 should then be used by using a
// dereferencing stand-in in the source position. 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.
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 priorize either. If the source
supports evolution of sources (see Section 2.1.2), disregarding that
recommendation may pose an interoperability hazard.
Amsüss Expires 5 September 2024 [Page 3]
Internet-Draft Packed CBOR: Table set up by reference March 2024
2.1. 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.
2.1.1. 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.
2.1.2. Unknown entries are used - evolutution 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 with elements that support isolation of
processing errors. For example, a CRI that uses unknown extensions
is regarded as "unprocessable" (Section 5.2.1 of
[I-D.ietf-core-href]). 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 Section 2.1 of [I-D.ietf-cbor-packed].
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).
Amsüss Expires 5 September 2024 [Page 4]
Internet-Draft Packed CBOR: Table set up by reference March 2024
If a protocol has isolated error processing only around some
elements, encoders need to take care to not use entries in unisolated
positions that may be unpopulated at the decoder. The protocol and
source authors need to provide appropriate guidance.
Protocols that do not support error isolation need a way to negotiate
the understood set of sources and table entries.
If an implementation does not support any elements with isolated
error processing at all, a receiver of a document may already stop
processing the document when encountering a setup by reference that
includes undefined elements.
2.1.2.1. 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 it 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.
2.2. 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.
count //= (
skip-shared, count-shared, skip-argument, count-argument
)
skip-shared = uint
skip-argument = uint
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.
Amsüss Expires 5 September 2024 [Page 5]
Internet-Draft Packed CBOR: Table set up by reference March 2024
2.3. 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}}}]])
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.
3. 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 TBD114, and the source given is a packed
reference.
All table inputs can be evolved orthogonally as described in
Section 2.1.2. If an unspecified entry is used as a source, the
whole source content is considered unspecified.
Amsüss Expires 5 September 2024 [Page 6]
Internet-Draft Packed CBOR: Table set up by reference March 2024
3.1. 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"
114([5, 6(42) / outer item 100 /, 114([2, 6(45) / outer item 101, currently item 105 /, [
simple(11) / outer item 0, currenlty item 11, "This class has students with the following names" /,
simple(0) / item 0 of german-names, "Franz" /,
simple(2) / item 0 of english-names, currently item 2, "George" /,
simple(1) / item 1 of german-names, "Fritz" /,
simple(7) / item 5 of english-names, currently item 7, "Jack" /
]])])
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
5.1. CBOR Tags Registry
In the registry "CBOR Tags", IANA is requested to allocate one tag:
* Tag: 114
* Data item: Array [count, source, rump]
* Semantics: "Packed CBOR: table setup"
* Reference: This document
6. References
6.1. Normative References
Amsüss Expires 5 September 2024 [Page 7]
Internet-Draft Packed CBOR: Table set up by reference March 2024
[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-03, 2 March 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-
t2trg-deref-id-03>.
[I-D.ietf-cbor-packed]
Bormann, C. and M. Gütschow, "Packed CBOR", Work in
Progress, Internet-Draft, draft-ietf-cbor-packed-12, 2
March 2024, <https://datatracker.ietf.org/doc/html/draft-
ietf-cbor-packed-12>.
[I-D.ietf-cbor-update-8610-grammar]
Bormann, C., "Updates to the CDDL grammar of RFC 8610",
Work in Progress, Internet-Draft, draft-ietf-cbor-update-
8610-grammar-04, 2 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
update-8610-grammar-04>.
[I-D.ietf-core-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-14, 9 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-14>.
[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>.
6.2. Informative References
[I-D.bormann-cbor-yang-standin]
Bormann, C. and M. Matějka, "Stand-in Tags for YANG-CBOR",
Work in Progress, Internet-Draft, draft-bormann-cbor-yang-
standin-00, 21 February 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
yang-standin-00>.
Appendix A. Change log
From -01 to -02:
* Add text on use of unpopulated items, and rationale to count in
general.
Amsüss Expires 5 September 2024 [Page 8]
Internet-Draft Packed CBOR: Table set up by reference March 2024
* Split 4-argument form into its own subsection
* Fix erroneous example
* Augment CDDL with comments and [I-D.ietf-cbor-update-8610-grammar]
* Add considerations for splitting between loading and importing
through stand-ins
* Write IANA considerations
* Editorial changes
Acknowledgments
[ TBD ]
Author's Address
Christian Amsüss
Austria
Email: christian@amsuess.com
Amsüss Expires 5 September 2024 [Page 9]