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Austria christian@amsuess.com

cbor cbor packed uri cri 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.

Introduction [ See abstract. ]

Setting up the tables by reference CBOR tag TBD114 is defined with semantics similar to tags TBD113 and TBD1113 from 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 or equivalently as a CRI (as defined in . Later iterations of this document may introduce additional options. When the source identifier is dereferencable, 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. 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.

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:

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"

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.

Security Considerations [ TBD ]

IANA Considerations [ TBD: Request tag. ]

Normative References Universität Bremen TZI The Concise Binary Object Representation (CBOR, RFC 8949 == STD 94) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. CBOR does not provide any forms of data compression. CBOR data items, in particular when generated from legacy data models, often allow considerable gains in compactness when applying data compression. While traditional data compression techniques such as DEFLATE (RFC 1951) can work well for CBOR encoded data items, their disadvantage is that the receiver needs to decompress the compressed form to make use of the data. This specification describes Packed CBOR, a simple transformation of a CBOR data item into another CBOR data item that is almost as easy to consume as the original CBOR data item. A separate decompression step is therefore often not required at the receiver. // The present version (-09) provides two table setup tags (common, // split setup) and discusses behavior in case of references to // unpopulated table entries during unpacking. Universität Bremen TZI In a protocol or an application environment, it is often important to be able to create unambiguous identifiers for some meaning (concept or some entity). Due to the simplicity of creating URIs, these have become popular for this purpose. Beyond the purpose of identifiers to be uniquely associated with a meaning, some of these URIs are in principle _dereferenceable_, so something can be placed that can be retrieved when encountering such a URI. The present -00 version is a stub to draw some attention to the opportunity that this pattern would benefit from a common description, documenting its benefits and pitfalls, and some mitigations for the latter. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] Universität Bremen TZI Fraunhofer SIT The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that represents the URI components in Concise Binary Object Representation (CBOR) instead of a sequence of characters. This simplifies parsing, comparison and reference resolution in environments with severe limitations on processing power, code size, and memory size. // (This "cref" paragraph will be removed by the RFC editor:) The // present revision -13 of this draft picks up some additional // discussion points and is intended as input to the CoRE WG meeting // at IETF 117.

Acknowledgments [ TBD ]