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Applications and Real-Time CBOR (Concise Binary Object Representation) Maint. and Ext. efficient YANG YANG (RFC 7950) is a data modeling language used to model configuration data, state data, parameters and results of Remote Procedure Call (RPC) operations or actions, and notifications. YANG-CBOR (RFC 9254) defines encoding rules for YANG in the Concise Binary Object Representation (CBOR) (RFC 8949). While the overall structure of YANG-CBOR is encoded in an efficient, binary format, YANG itself has its roots in XML and therefore traditionally encodes some information such as date/times and IP addresses/prefixes in a verbose text form. This document defines how to use existing CBOR tags for this kind of information in YANG-CBOR as a "stand-in" for the text-based information that would be found in the original form of YANG-CBOR. About This Document Status information for this document may be found at . Discussion of this document takes place on the CBOR (Concise Binary Object Representation) Maintenance and Extensions Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction (see abstract)

Conventions and Definitions The terminology of applies.

Legacy representation:

The (often text-based) representation for a YANG data item as used in YANG-XML, YANG-JSON, and (unchanged) YANG-CBOR.

Stand-in tag:

A CBOR tag that can supply the information that is equivalent to a legacy representation in a more efficient format (e.g., using binary data).

Encoder:

The party which generates (sends) CBOR data described by YANG.

Intermediate Encoder:

An encoder which isn't the original author of the data, converting it from legacy representation.

Aggressive Intermediate Encoder:

An intermediate encoder that might choose to discard some information of a legacy representation in order to be able to use a stand-in tag. Such a choice may be based on knowledge of the Decoder's handling of such information (e.g, to accommodate intolerant decoders), or it may be a general characteristic of the service provided by the intermediate encoder (e.g., in order to serve as a legacy-eschewing encoder).

Legacy-Eschewing Encoder:

An encoder that does not generate legacy representations in places where a stand-in tag might instead be used. An intermediate encoder may need to be aggressive to achieve this.

Decoder:

The party which receives and parses CBOR data described by YANG.

Intolerant Decoder:

A decoder that does not accept legacy representations in places where a stand-in tag might instead be used. Such a decoder is designed to interoperate only with an legacy-eschewing encoder.

Intermediate Decoder:

A decoder which isn't the final recipient of the data, converting it to legacy representation.

Data Transfer:

A series of actions, generally beginning by data origination, encoding, continuing by optional intermediate transcoding, sending and receiving, and finally decoding and consuming.

Round Trip:

Part of a data transfer between an encoder generating CBOR data with stand-in tags and a decoder parsing the data.

Legacy Round Trip:

A Round Trip where the encoder is an intermediate encoder or the decoder is an intermediate decoder and any of these converts from or to the legacy representation.

Unambiguous Round Trip:

A Legacy Round Trip that provides exactly the same legacy representation (not just semantically equivalent). The stand-in tag is also said to "unambiguously stand in" for the legacy representation.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in () () when, and only when, they appear in all capitals, as shown here.

Stand-In Tags This document defines two sets of stand-in tags. Where information starts out in a legacy representation, these tags are only used when an Unambiguous Round Trip can be achieved.

ietf-yang-types: Tag 1 (Date/Time) and Tag 100 (Date) defines the following types in ietf-yang-types: Legacy date and date/time representations in ietf-yang-types

YANG type	base type	specification	stand-in
date-and-time	string		tag 1
date	string		(none)
date-no-zone	string		tag 100

Tag 1 (Section of RFC 8949 ) can unambiguously stand in for all date-and-time values that:

• do not specify a time zone (note that uses the legacy "-00:00" format for time-zone-free date-times)
• are not an inserted leap second (23:59:60 or 23:59:61)
• do not have trailing zeroes in the fractional part of the seconds.
• do not have fractional parts of the seconds with a precision that cannot be represented in floating-point tag content in a tag 1.

All other date-and-time values stay in legacy representation. Tag 1 uses an integer tag content for all date-and-time values without fractional seconds and a floating-point tag content for values that have fractional seconds given. Tag 100 can unambiguously stand in for all date-no-zone values.

ietf-yang-types: Tags 37 (UUID) and CPA113 (hex-string) defines the following types in ietf-yang-types: Legacy UUID and colon-separated hexadecimal representations in ietf-yang-types

YANG type	base type	specification	stand-in
uuid	string		tag 37
hex-string	string		tag CPA113
mac-address	string		tag CPA113
phys-address	string		tag CPA113

These types are hexadecimal representations of byte strings, adorned in various ways. uuid stands for a 16-byte byte string (), represented in hexadecimal with ASCII minus/hyphen characters added in specific positions. Tag 37 (see also Section 7 of ) can be used as a binary stand-in for this adorned hexadecimal representation. According to the description of uuid in , "the canonical representation uses lowercase characters". For consistency with this specification, an intermediate decoder of a tag 37 stand-in MUST use lowercase characters in the uuid hex string generated. hex-string, and the similar, but more specific types mac-address and phys-address, stand for byte strings in various lengths (exactly 6 bytes for mac-address, variable-length for the others), represented in hexadecimal with ASCII colon characters added between the representations of each of the bytes. This specification defines tag number CPA113 to be an additional "Expected Later Encoding" tag (similar to tag 23, see Section of RFC 8949 ), except that the expected encoding of CPA113 includes colons and uses lowercase hex digits. The following example implementation of the transformation in a decoder shows the use of lowercase hex characters (%02x as opposed to %02X) and the insertion of colon characters between the hex-represented bytes: Note: defines tag number 48 for MAC addresses. This could be used in place of tag CPA113, but only for MAC addresses, not for other byte strings of a similar form. This specification therefore requests IANA to assign a new CBOR tag that can be used as a stand-in for all instances of colon-separated text strings of hexadecimally represented bytes, as shown in . Note Related tags have not been defined so far for tag 21 or 22 defined alongside tag 23, as YANG has a base type "binary" that is encoded in base64 classic in YANG-XML and YANG-JSON, but already encoded in a binary byte string in YANG-CBOR; use cases that might actually use base type "string" for base64-encoded data in YANG have not been considered. However, tag 21 or 22 could be used as stand-in tags if that is useful for some specific YANG model not considered here. RFC-Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. For each usage of the term "CPA", please remove the prefix "CPA" from the indicated value and replace the residue with the value assigned by IANA; perform an analogous substitution for all other occurrences of the prefix "CPA" in the document. Finally, please remove this note.

ietf-inet-types: Tags 54 and 52 (IP addresses and prefixes) defines in ietf-inet-types: Legacy representations in ietf-yang-types

YANG type	base type	specification	stand-in
ip-address	union		(see union)
ipv6-address	string		tag 54
ipv4-address	string		tag 52
ip-address-no-zone	union	RFC 6991	(see union)
ipv6-address-no-zone	ipv6-address	RFC 6991	tag 54
ipv4-address-no-zone	ipv4-address	RFC 6991	tag 52
ip-address-link-local	union		(see union)
ipv6-address-link-local	ipv6-address		tag 54
ipv4-address-link-local	ipv4-address		tag 52
ip-prefix	union		(see union)
ipv6-prefix	string		tag 54
ipv4-prefix	string		tag 52
ip-address-and-prefix	union		(see union)
ipv6-address-and-prefix	string		tag 54
ipv4-address-and-prefix	string		tag 52

An intermediate encoder MAY normalize IPv6 addresses and prefixes that do not comply with but can be converted into the stand-in representation. For example, IPv6 address written as 2001:db8:: is the same as 2001:0db8::0:0 and both would be converted to 54(h'20010db8000000000000000000000000'), anyway only the first one complies with . The encoder MAY refuse to convert the latter one. If the schema specifies ip-prefix, an intermediate encoder MAY normalize prefixes with non-zero bits after the prefix end. For example, if the legacy representation of ipv6-prefix is 2001:db8:1::/40, the encoder may either refuse it as malformed or convert it to 2001:db8::/40 and represent as 54([40, h'20010db8']). The encoder implementation should be clear about which normalizations are employed and how. Adapted examples from : Stand-in representation of IPv6 address 2001:db8:1234:deed:beef:cafe:face:feed is 54(h'20010db81234deedbeefcafefacefeed'). CBOR encoding of stand-in (19 bytes): CBOR encoding of legacy representation (40 bytes): Stand-in representation of IPv6 prefix 2001:db8:1234::/48 is 54([48, h'20010db81234']). CBOR encoding of stand-in (12 bytes): CBOR encoding of legacy representation (19 bytes): Stand-in representation of IPv6 link-local address fe80::0202:02ff:ffff:fe03:0303/64%eth0 is 54([h'fe8000000000020202fffffffe030303', 64, 'eth0']). CBOR encoding of stand-in (27 bytes): CBOR encoding of legacy representation (40 bytes): TO DO: adapt more examples from TO DO: Check how the unions in and interact with this. E.g., the union ip-address needs to be parsed to decide between tag 54 and tag 52.

Union handling When the schema specifies a union data type for a node, there are additional requirements on the encoder and decoder. An encoder which is fully aware of data semantics MUST use the appropriate data type, even though it isn't formally specified by the schema. If an intermediate encoder doesn't fully understand the data semantics, it needs to find out which type the data actually is to choose the right stand-in. If more types are possible, it MAY choose any of these which allow for an Unambiguous Round Trip, otherwise it SHOULD keep the legacy representation. If a decoder receives data for a union-typed node, it MUST accept any data type of the union, even though it may violate additional constraints outside the schema.

Using Stand-In Tags

Defining Stand-In Usage in Schema Requiring modifications to a YANG model in order to use it with stand-in tags would pose significant deployment hurdles to using stand-in tags. A YANG model may want to restrict the information content in such a way that stand-in tags can always be used, e.g., by using date-no-zone in place of date where that is applicable, or by excluding features of a YANG data type that cannot be represented in a stand-in-tag. ISSUE: Should this document define such restricted types, e.g.: (This particular example is additionally problematic since the usual way to indicate the absence of time zone information in ISO 8601 date-times is using Z as the time zone indicated, not -00:00 as is required by but not allowed by ISO 8601; see for additional discussion of this.) Note that this paragraph does not reference ISO 8601 because that is complicated and best done by consulting .

Original stand-ins The simplest situation is when no intermediate encoders and decoders are involved in the data transfer, therefore the round trip is not legacy. In this case, no conversions are involved and data is validated using the schema extension from the previous section.

Legacy Round Trip Producing a stand-in MUST be triggered by schema usage. Intermediate encoders MUST NOT encode stand-ins when no schema is available. It's generally not recommended to do a legacy round trip where both the encoder and decoder are converting from and to the legacy representation.

Negotiation Introducing stand-in tags in YANG-CBOR requires some form of consent between the producer and the consumer of YANG-CBOR information:

• A producer that creates YANG-CBOR containing stand-in tags needs to know whether the consumer supports stand-in tags, and, possibly, which specific stand-in tags it supports. We speak about the capability of a consumer to consume stand-in tags. A producer MUST NOT employ stand-in tags unless it knows about the capabilities of the consumer. A consumer SHOULD indicate its capabilities for consuming stand-in tags.
• A consumer may not want to implement certain legacy text-based representations where more efficient (and easy to implement) stand-in tags are available, i.e., it may use an intolerant decoder. This places a requirement on the producer to use a legacy-eschewing encoder (which therefore needs to have the capability to produce YANG-CBOR where those stand-in tags are used, in place of legacy representations). Where the consumer employs an intolerant decoder, stand-in tags are required by the consumer: for interoperating with a producer's encoder, this MUST be legacy-eschewing, i.e. it MUST NOT employ legacy representations. A consumer that has requirements for only receiving stand-in tags in place of legacy representations, MUST indicate this to the producer.

ISSUE: Where do we put those two aspects of negotiation?

• NETCONF negotiation
• yang-library
• media-type parameters
• ?

Security Considerations TODO Security

IANA Considerations

New CBOR Tags In the registry "" , IANA is requested to assign the tag in . New CBOR Tag Defined by this Specification

Tag	Data Item	Semantics	Reference
CPA113	byte string	Expected Later Encoding: colon-separated hexadecimal representation of a byte string	draft-bormann-yang-standin,

stand-in tags? ISSUE: Do we want to have a separate registry for stand-in tags? They already are CBOR tags and thus in the registry, but might get lost in the bulk of that (and are only identified as YANG-CBOR stand-in Tags in the specification).

media-type parameters ISSUE: Should the use of stand-in tags be mentioned in the various YANG-CBOR-based media types (as a media type parameter)? Compare how application/yang-data+cbor can use id=name/id=sid to indicate another encoding decision.

References Normative References YANG (RFC 7950) is a data modeling language used to model configuration data, state data, parameters and results of Remote Procedure Call (RPC) operations or actions, and notifications. This document defines encoding rules for YANG in the Concise Binary Object Representation (CBOR) (RFC 8949). This specification defines two Concise Binary Object Representation (CBOR) tags for use with IPv6 and IPv4 addresses and prefixes. Constructor University This document defines a collection of common data types to be used with the YANG data modeling language. This version of the document adds several new type definitions and obsoletes RFC 6991. As IPv6 deployment increases, there will be a dramatic increase in the need to use IPv6 addresses in text. While the IPv6 address architecture in Section 2.2 of RFC 4291 describes a flexible model for text representation of an IPv6 address, this flexibility has been causing problems for operators, system engineers, and users. This document defines a canonical textual representation format. It does not define a format for internal storage, such as within an application or database. It is expected that the canonical format will be followed by humans and systems when representing IPv6 addresses as text, but all implementations must accept and be able to handle any legitimate RFC 4291 format. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR), as specified in RFC 7049, 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. In CBOR, one point of extensibility is the definition of CBOR tags. RFC 7049 defines two tags for time: CBOR tag 0 (date/time string as per RFC 3339) and tag 1 (POSIX "seconds since the epoch"). Since then, additional requirements have become known. This specification defines a CBOR tag for a date text string (as per RFC 3339) for applications needing a textual date representation within the Gregorian calendar without a time. It also defines a CBOR tag for days since the date 1970-01-01 in the Gregorian calendar for applications needing a numeric date representation without a time. This specification is the reference document for IANA registration of the CBOR tags defined. The Concise Binary Object Representation (CBOR) 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. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document introduces a collection of common data types to be used with the YANG data modeling language. [STANDARDS-TRACK] This specification defines UUIDs (Universally Unique IDentifiers) -- also known as GUIDs (Globally Unique IDentifiers) -- and a Uniform Resource Name namespace for UUIDs. A UUID is 128 bits long and is intended to guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System (NCS), later in the Open Software Foundation's (OSF's) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the OSF DCE specification with the kind permission of the OSF (now known as "The Open Group"). Information from earlier versions of the OSF DCE specification have been incorporated into this document. This document obsoletes RFC 4122. IANA In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document defines an extension to the timestamp format defined in RFC 3339 for representing additional information, including a time zone. It updates RFC 3339 in the specific interpretation of the local offset Z, which is no longer understood to "imply that UTC is the preferred reference point for the specified time". Some IETF protocols make use of Ethernet frame formats and IEEE 802 parameters. This document discusses several aspects of such parameters and their use in IETF protocols, specifies IANA considerations for assignment of points under the IANA Organizationally Unique Identifier (OUI), and provides some values for use in documentation. This document obsoletes RFC 7042. Universität Bremen TZI The Concise Binary Object Representation (CBOR, RFC 8949) 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. In CBOR, one point of extensibility is the definition of CBOR tags. RFC 8949's original edition, RFC 7049, defined a basic set of 16 tags as well as a registry that can be used to contribute additional tag definitions [IANA.cbor-tags]. Since RFC 7049 was published, at the time of writing some 190 definitions of tags and ranges of tags have been added to that registry. The present document provides a roadmap to a large subset of these tag definitions. Where applicable, it points to an IETF standards or standard development document that specifies the tag. Where no such document exists, the intention is to collect specification information from the sources of the registrations. After some more development, the present document is intended to be useful as a reference document for the IANA registrations of the CBOR tags the definitions of which have been collected.

Acknowledgments TODO acknowledge.