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Applications and Real-Time Concise Binary Object Representation Maintenance and Extensions 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.

Decoder:

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

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 BCP 14 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 representations in ietf-yang-types

YANG type	base type	specification	stand-in
date-and-time	string		tag 1
date-with-zone-offset	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-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-pretty D8 36 # tag(54) 50 # bytes(16) 20010DB81234DEEDBEEFCAFEFACEFEED CBOR encoding of legacy representation (40 bytes): cbor-pretty 78 26 # text(38) 323030313A6462383A313233343A646565643A626565663A636166653A666163653A66656564 Stand-in representation of IPv6 prefix 2001:db8:1234::/48 is 54([48, h'20010db81234']). CBOR encoding of stand-in (12 bytes): cbor-pretty D8 36 # tag(54) 82 # array(2) 18 30 # unsigned(48) 46 # bytes(6) 20010DB81234 # " \u0001\r\xB8\u00124" CBOR encoding of legacy representation (19 bytes): cbor-pretty 72 # text(18) 323030313A6462383A313233343A3A2F3438 # "2001:db8:1234::/48" 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-pretty D8 36 # tag(54) 83 # array(3) 50 # bytes(16) FE8000000000020202FFFFFFFE030303 18 40 # unsigned(64) 44 # bytes(4) 65746830 # "eth0" CBOR encoding of legacy representation (40 bytes): cbor-pretty 78 26 # text(38) 666538303A3A303230323A303266663A666666663A666530333A303330332F36342565746830 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 TO DO: formally define the YANG extension

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. This places a requirement on the producer (which needs to have the capability to produce YANG-CBOR where those stand-in tags are used, in place of legacy representations). A producer MUST NOT employ legacy representations where stand-in tags are required by the consumer. 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

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 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.

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. Jacobs University This document introduces a collection of common data types to be used with the YANG data modeling language. This document 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] 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.

Acknowledgments TODO acknowledge.