Stand-in Tags for YANG-CBOR
draft-bormann-cbor-yang-standin-03
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Carsten Bormann , Maria Matějka | ||
| Last updated | 2026-03-02 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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draft-bormann-cbor-yang-standin-03
CBOR (Concise Binary Object Representation) Maint. and Ext. C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Standards Track M. Matejka
Expires: 3 September 2026 CZ.NIC
2 March 2026
Stand-in Tags for YANG-CBOR
draft-bormann-cbor-yang-standin-03
Abstract
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
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-cbor-yang-standin/.
Discussion of this document takes place on the CBOR (Concise Binary
Object Representation) Maintenance and Extensions 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://github.com/cabo/yang-standin.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 3 September 2026.
Copyright Notice
Copyright (c) 2026 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Stand-In Tags . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. ietf-yang-types: Tag 1 (Date/Time) and Tag 100 (Date) . . 5
3.2. ietf-yang-types: Tags 37 (UUID), CPA113 (hex-string), and
CPA114 (dotted-quad) . . . . . . . . . . . . . . . . . . 6
3.3. ietf-inet-types: Tags 54 and 52 (IP addresses and
prefixes) . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Union handling . . . . . . . . . . . . . . . . . . . . . 12
4. Using Stand-In Tags . . . . . . . . . . . . . . . . . . . . . 13
4.1. The Standin File . . . . . . . . . . . . . . . . . . . . 13
4.2. Defining Stand-In Usage in Schema . . . . . . . . . . . . 15
4.3. Original stand-ins . . . . . . . . . . . . . . . . . . . 16
4.4. Legacy Round Trip . . . . . . . . . . . . . . . . . . . . 16
5. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. New CBOR Tags . . . . . . . . . . . . . . . . . . . . . . 17
7.2. stand-in tags? . . . . . . . . . . . . . . . . . . . . . 18
7.3. media-type parameters . . . . . . . . . . . . . . . . . . 18
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8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 20
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
(see abstract)
2. Conventions and Definitions
The terminology of [RFC9254] applies.
The present document focuses on a "basic" processing model where the
original source of YANG-modeled data serves as the encoder, and where
the decoder is part of the ultimate destination of those data.
// TODO: More elaborate processing models are possible and desirable;
several of the terms defined below are defined here only so they can
be used to discuss processing models of this kind.
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.
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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 a
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
[BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all
capitals, as shown here.
3. Stand-In Tags
As a complement to the representations defined in [RFC9254], the
present document defines efficient representations of certain types
of YANG-modeled data, based on both existing and newly registered
"stand-in" tags that are designed to represent the underlying data.
Whenever a stand-in tag is defined for a specific type, the
representation as specified in this document may be used. Otherwise,
the encoding is as specified in [RFC9254].
This section defines several standard stand-in tags for general use,
in many cases based on the type definitions of [RFC9911].
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(
// TODO: The detailed conditions under which certain transformations
can be applied to YANG-CBOR data representations after these have
been generated are TBD; several processing models are conceivable for
this. These processing models are out of scope of the present
document; work has started in
[I-D.marenamat-netmod-core-yang-transcoding]. One such condition
might be: Where information starts out in a legacy representation,
these tags are only used when an Unambiguous Round Trip can be
achieved.)
3.1. ietf-yang-types: Tag 1 (Date/Time) and Tag 100 (Date)
Section 3 of [RFC9911] defines the following types in ietf-yang-
types:
+===============+===========+===============+==========+
| YANG type | base type | specification | stand-in |
+===============+===========+===============+==========+
| date-and-time | string | [RFC6021] | tag 1 |
+---------------+-----------+---------------+----------+
| date | string | [RFC9911] | (none) |
+---------------+-----------+---------------+----------+
| date-no-zone | string | [RFC9911] | tag 100 |
+---------------+-----------+---------------+----------+
Table 1: Legacy date and date/time representation in
ietf-yang-types
Tag 1 (Section 3.4.2 of RFC 8949 [STD94]) can unambiguously stand in
for all date-and-time values that:
* do not specify a time zone (note that [RFC9911] recommends using
"Z", but allows using 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. (
// TODO: explore the use of tag 1001 for some of these cases.)
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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 [RFC8943] can unambiguously stand in for all date-no-zone
values.
3.2. ietf-yang-types: Tags 37 (UUID), CPA113 (hex-string), and CPA114
(dotted-quad)
Section 3 of [RFC9911] defines the following types in ietf-yang-
types:
+==============+===========+===============+============+
| YANG type | base type | specification | stand-in |
+==============+===========+===============+============+
| uuid | string | [RFC9911] | tag 37 |
+--------------+-----------+---------------+------------+
| hex-string | string | [RFC9911] | tag CPA113 |
+--------------+-----------+---------------+------------+
| mac-address | string | [RFC6021] | tag CPA113 |
+--------------+-----------+---------------+------------+
| phys-address | string | [RFC6021] | tag CPA113 |
+--------------+-----------+---------------+------------+
Table 2: Legacy UUID and colon-separated hexadecimal
representations in ietf- yang-types
These types are hexadecimal representations of byte strings, adorned
in various ways.
uuid stands for a 16-byte byte string (Section 4 of [RFC9562]),
represented in hexadecimal with ASCII minus/hyphen characters added
in specific positions. Tag 37 (see also Section 7 of
[I-D.bormann-cbor-notable-tags]) can be used as a binary stand-in for
this adorned hexadecimal representation. According to the
description of uuid in Section 3 of [RFC9911], "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 (
// TODO: align with processing model).
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 Section 7.1 to be an additional "Expected Later
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Encoding" tag (similar to tag 23, see Section 3.4.5.2 of RFC 8949
[STD94]), 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:
def tag_cpa113_to_legacy(s)
s.bytes.map{|x| "%02x" % x}.join(":")
end
Note: Section 2.4 of [RFC9542] 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 Table 2.
Note: Related stand-in semantics have not been defined so far for tag
21 or 22 that were defined alongside tag 23: 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.
+=============+===========+===============+============+
| YANG type | base type | specification | stand-in |
+=============+===========+===============+============+
| dotted-quad | string | [RFC6991] | tag CPA114 |
+-------------+-----------+---------------+------------+
Table 3: Legacy dotted-quad representation in ietf-
yang-types
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dotted-quad stands for an unsigned 32-bit integer (Section 3 of
[RFC9911]), represented as a text string in decimal values for 4 byte
values in network byte order, separated by ASCII dot characters. Tag
CPA114 can be used as a binary stand-in for this adorned bytewise
decimal representation.
IANA is requested to assign CPA114 as the CBOR tag to indicate a
value that is represented by a dotted-quad in YANG. The tag content
is an unsigned integer of a size not greater than 4 bytes
(0..0xFFFFFFFF, uint32). It is expected that when the data is being
displayed e.g. to human operator, the data will be shown as a string
of 4 decimal numbers giving the number as four bytes in network byte
order, separated by ASCII dot characters.
// 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.
// TODO: Better formulation of the dotted-quad expected later
encoding.
PROPOSED: RFC 9911 is clear that dotted-quad stands for an unsigned
integer that fits into 32 bits; we should not replace that with a
byte string.
RESOLVED: Should we fix the 4-byte length or make it more generic
with arbitrary length byte strings? (For usage as IPv4 similar
identifier -- see ietf-ospf.yang -- the 4-byte length is sufficient
and brings less complexity)
3.3. ietf-inet-types: Tags 54 and 52 (IP addresses and prefixes)
Section 4 of [RFC9911] defines in ietf-inet-types:
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+=========================+==============+===============+========+
| YANG type | base type | specification |stand-in|
+=========================+==============+===============+========+
| ip-address | union | [RFC6021] |(see |
| | | |union) |
+-------------------------+--------------+---------------+--------+
| ipv6-address | string | [RFC6021] |tag 54 |
+-------------------------+--------------+---------------+--------+
| ipv4-address | string | [RFC6021] |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 | [RFC9911] |(see |
| | | |union) |
+-------------------------+--------------+---------------+--------+
| ipv6-address-link-local | ipv6-address | [RFC9911] |tag 54 |
+-------------------------+--------------+---------------+--------+
| ipv4-address-link-local | ipv4-address | [RFC9911] |tag 52 |
+-------------------------+--------------+---------------+--------+
| ip-prefix | union | [RFC6021] |(see |
| | | |union) |
+-------------------------+--------------+---------------+--------+
| ipv6-prefix | string | [RFC6021] |tag 54 |
+-------------------------+--------------+---------------+--------+
| ipv4-prefix | string | [RFC6021] |tag 52 |
+-------------------------+--------------+---------------+--------+
| ip-address-and-prefix | union | [RFC9911] |(see |
| | | |union) |
+-------------------------+--------------+---------------+--------+
| ipv6-address-and-prefix | string | [RFC9911] |tag 54 |
+-------------------------+--------------+---------------+--------+
| ipv4-address-and-prefix | string | [RFC9911] |tag 52 |
+-------------------------+--------------+---------------+--------+
Table 4: Legacy representations in ietf-yang-types
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(
// TODO: align this and next paragraph with processing model.) An
intermediate encoder MAY normalize IPv6 addresses and prefixes that
do not comply with [RFC5952] 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'); only the first one complies
with [RFC5952]. 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 [RFC9164]:
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):
D8 36 # tag(54)
50 # bytes(16)
20010DB81234DEEDBEEFCAFEFACEFEED
CBOR encoding of legacy representation (40 bytes):
78 26 # text(38)
323030313A6462383A313233343A646565643A
626565663A636166653A666163653A66656564
# "2001:db8:1234:deed:beef:cafe:face:feed"
Stand-in representation of IPv6 prefix 2001:db8:1234::/48 is 54([48,
h'20010db81234']).
CBOR encoding of stand-in (12 bytes):
D8 36 # tag(54)
82 # array(2)
18 30 # unsigned(48)
46 # bytes(6)
20010DB81234 # " \u0001\r\xB8\u00124"
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CBOR encoding of legacy representation (19 bytes):
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):
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):
78 26 # text(38)
666538303A3A303230323A303266663A666666
663A666530333A303330332F36342565746830
# "fe80::0202:02ff:ffff:fe03:0303/64%eth0"
Stand-in representation of IPv4 address 192.0.2.1 is 52(h'c0000201').
CBOR encoding of stand-in (7 bytes):
D8 34 # tag(52)
68 # bytes(4)
C0000201
CBOR encoding of legacy representation (10 bytes):
69 # text(9)
3139322e302e322e31 # "192.0.2.1"
Stand-in representation of IPv4 prefix 192.0.2.0/24 is 52([24,
h'c0000200']).
CBOR encoding of stand-in (10 bytes):
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D8 34 # tag(52)
82 # array(2)
18 18 # unsigned(24)
48 # bytes(4)
C0000200
CBOR encoding of legacy representation (13 bytes):
6C # text(12)
3139322e302e322e302f3234 # "192.0.2.0/24"
Stand-in representation of IPv4 address combined with prefix
192.0.2.1/24 is 52([h'c0000201', 24]).
CBOR encoding of stand-in (10 bytes):
D8 34 # tag(52)
82 # array(2)
48 # bytes(4)
C0000201
18 18 # unsigned(24)
CBOR encoding of legacy representation (13 bytes):
6C # text(12)
3139322e302e322e312f3234 # "192.0.2.1/24"
TO DO: Check how the unions in [RFC9911] interact with this. E.g.,
the union ip-address needs to be parsed to decide between tag 54 and
tag 52.
3.4. Union handling
When the schema specifies a union data type for a node, there are
additional requirements on the encoder and decoder. The encoder MUST
be fully aware of data semantics and use the appropriate data type
and encoding. The decoder MUST use the data type information for
further processing, in a similar way as specified in Section 6.10 of
[RFC7951].
(
// TODO: Align the rest of the section with processing models.) 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.
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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.
4. Using Stand-In Tags
4.1. The Standin File
Encoder and decoder need some agreement over whether, where and how
stand-in tags are used. This agreement is embodied in a "standin
file", which has approximately the data content of Table 5
(representation TBD):
+=========================================+===========+=============+
| YANG typename | Tag | Description |
| | number(s) | (*) |
+=========================================+===========+=============+
| ietf-yang-types:date-and-time | 1 | Section 3.1 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:date-no-zone | 100 | Section 3.1 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:uuid | 37 | Section 3.2 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:hex-string | CPA113 | Section 3.2 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:mac-address | CPA113 | Section 3.2 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:phys-address | CPA113 | Section 3.2 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-yang-types:dotted-quad | CPA114 | Section 3.2 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ip-address | 54, 52 | Section 3.3 |
| | (see | of RFCthis |
| | union) | |
+-----------------------------------------+-----------+-------------+
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| ietf-inet-types:ipv6-address | 54 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv4-address | 52 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ip-address-no-zone | 54, 52 | Section 3.3 |
| | (see | of RFCthis |
| | union) | |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv6-address-no-zone | 54 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv4-address-no-zone | 52 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ip-address-link-local | 54, 52 | Section 3.3 |
| | (see | of RFCthis |
| | union) | |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv6-address-link-local | 54 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv4-address-link-local | 52 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ip-prefix | 54, 52 | Section 3.3 |
| | (see | of RFCthis |
| | union) | |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv6-prefix | 54 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv4-prefix | 52 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ip-address-and-prefix | 54, 52 | Section 3.3 |
| | (see | of RFCthis |
| | union) | |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv6-address-and-prefix | 54 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
| ietf-inet-types:ipv4-address-and-prefix | 52 | Section 3.3 |
| | | of RFCthis |
+-----------------------------------------+-----------+-------------+
Table 5: Information Content of Standin File
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(*) insert identityrefs from YANG module here
The YANG typename is a name composed of a module name and the name of
a type defined there, separated by a colon; names of built-in types
do not contain a module name or a colon. The tag number is the
number of the CBOR Tag used for representing certain instances of
this type. The description is a YANG identity (identified by a SID
for an identityref (Section 6.10.1 of [RFC9254]), or, for
experimental use, a name for an identityref (Section 6.10.2 of
[RFC9254])) defined in a YANG model; the description of the identity
either points to a section of the defining document that defines its
usage or contains this information outright.
(Note that there is currently no way to assign a SID to a typename;
this could be added to the definition of a ".sid" file in [RFC9595].)
Standin Files SHOULD generally be defined in a document such as the
present one; they SHOULD NOT be liberally made up for specific
applications (unless there are overriding concerns motivating
choosing ultimate efficiency over interoperability).
4.2. 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 of the
YANG-modeled data it describes 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.:
typedef efficient-date-and-time {
type date-and-time {
pattern '.*Z'
}
description
"The efficient-date-and-time type is a profile of the
date-and-time type that is intended to always enable using a
stand-in tag as per ((this document)), e.g., by not expressing
a time-zone-offset.
Not all restrictions that make this possible are expressed in
the above YANG string pattern.";
}
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(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 as specified in
Section 3 of [RFC9911], not -00:00 as was previously required by
Section 3 of [RFC6991] but not allowed by ISO 8601; see [RFC9557] for
references to versions of ISO 8601 and additional discussion of
this.)
// Note that this paragraph does not reference ISO 8601 because that
// is complicated and best done by consulting [RFC9557].
4.3. 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.
4.4. 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.
5. Negotiation
(
// TODO: Align with processing models.)
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
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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
* ?
6. Security Considerations
TODO Security
7. IANA Considerations
7.1. New CBOR Tags
In the registry "CBOR Tags" [IANA.cbor-tags], IANA is requested to
assign the tag in Table 6.
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+=============+==========+============================+=============+
| Tag | Data | Semantics | Reference |
| | Item | | |
+=============+==========+============================+=============+
| CPA113 | byte | Expected Later Encoding: | draft- |
| | string | colon-separated | bormann- |
| | | hexadecimal representation | yang- |
| | | of a byte string | standin, |
| | | | Section 3.2 |
+-------------+----------+----------------------------+-------------+
| CPA114 | unsigned | Expected Later Encoding: | |
| | integer | dot-separated decimal | |
| | | representation of network- | |
| | | byte-order ordered bytes | |
| | | of the integer. | |
+-------------+----------+----------------------------+-------------+
| draft- | | | |
| bormann- | | | |
| yang- | | | |
| standin, | | | |
| Section | | | |
| 3.2 | | | |
+-------------+----------+----------------------------+-------------+
Table 6: New CBOR Tag Defined by this Specification
7.2. stand-in tags?
ISSUE: Do we want to have a separate registry for stand-in files?
7.3. 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 parameters id=name/
id=sid to indicate another encoding decision.
8. References
8.1. Normative References
[BCP14] Best Current Practice 14,
<https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:
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Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/rfc/rfc5952>.
[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, DOI 10.17487/RFC7951, August 2016,
<https://www.rfc-editor.org/rfc/rfc7951>.
[RFC8943] Jones, M., Nadalin, A., and J. Richter, "Concise Binary
Object Representation (CBOR) Tags for Date", RFC 8943,
DOI 10.17487/RFC8943, November 2020,
<https://www.rfc-editor.org/rfc/rfc8943>.
[RFC9164] Richardson, M. and C. Bormann, "Concise Binary Object
Representation (CBOR) Tags for IPv4 and IPv6 Addresses and
Prefixes", RFC 9164, DOI 10.17487/RFC9164, December 2021,
<https://www.rfc-editor.org/rfc/rfc9164>.
[RFC9254] Veillette, M., Ed., Petrov, I., Ed., Pelov, A., Bormann,
C., and M. Richardson, "Encoding of Data Modeled with YANG
in the Concise Binary Object Representation (CBOR)",
RFC 9254, DOI 10.17487/RFC9254, July 2022,
<https://www.rfc-editor.org/rfc/rfc9254>.
[RFC9562] Davis, K., Peabody, B., and P. Leach, "Universally Unique
IDentifiers (UUIDs)", RFC 9562, DOI 10.17487/RFC9562, May
2024, <https://www.rfc-editor.org/rfc/rfc9562>.
[RFC9595] Veillette, M., Ed., Pelov, A., Ed., Petrov, I., Ed.,
Bormann, C., and M. Richardson, "YANG Schema Item
iDentifier (YANG SID)", RFC 9595, DOI 10.17487/RFC9595,
July 2024, <https://www.rfc-editor.org/rfc/rfc9595>.
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[RFC9911] Schönwälder, J., Ed., "Common YANG Data Types", RFC 9911,
DOI 10.17487/RFC9911, December 2025,
<https://www.rfc-editor.org/rfc/rfc9911>.
[STD94] Internet Standard 94,
<https://www.rfc-editor.org/info/std94>.
At the time of writing, this STD comprises the following:
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
8.2. Informative References
[I-D.bormann-cbor-notable-tags]
Bormann, C., "Notable CBOR Tags", Work in Progress,
Internet-Draft, draft-bormann-cbor-notable-tags-16, 25
February 2026, <https://datatracker.ietf.org/doc/html/
draft-bormann-cbor-notable-tags-16>.
[I-D.marenamat-netmod-core-yang-transcoding]
Matějka, M., "Transcoding Data Modeled with YANG", Work in
Progress, Internet-Draft, draft-marenamat-netmod-core-
yang-transcoding-00, 2 March 2026,
<https://datatracker.ietf.org/doc/html/draft-marenamat-
netmod-core-yang-transcoding-00>.
[RFC6021] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6021, DOI 10.17487/RFC6021, October 2010,
<https://www.rfc-editor.org/rfc/rfc6021>. This
specification is obsoleted by [RFC6991], which is
obsoleted by [RFC9911].
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/rfc/rfc6991>. This
specification is obsoleted by [RFC9911].
[RFC9542] Eastlake 3rd, D., Abley, J., and Y. Li, "IANA
Considerations and IETF Protocol and Documentation Usage
for IEEE 802 Parameters", BCP 141, RFC 9542,
DOI 10.17487/RFC9542, April 2024,
<https://www.rfc-editor.org/rfc/rfc9542>.
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[RFC9557] Sharma, U. and C. Bormann, "Date and Time on the Internet:
Timestamps with Additional Information", RFC 9557,
DOI 10.17487/RFC9557, April 2024,
<https://www.rfc-editor.org/rfc/rfc9557>.
List of Tables
Table 1: Legacy date and date/time representation in
ietf-yang-types
Table 2: Legacy UUID and colon-separated hexadecimal
representations in ietf-yang-types
Table 3: Legacy dotted-quad representation in ietf-yang-types
Table 4: Legacy representations in ietf-yang-types
Table 5: Information Content of Standin File (*) insert
identityrefs from YANG module here
Table 6: New CBOR Tag Defined by this Specification
Acknowledgments
TODO acknowledge.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Maria Matejka
CZ.NIC
Milesovska 1136/5
13000 Praha
Czechia
Email: maria.matejka@nic.cz, mq@jmq.cz
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