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Encoding of Data Modeled with YANG in the Concise Binary Object Representation (CBOR)
RFC 9254

Document Type RFC - Proposed Standard (July 2022)
Authors Michel Veillette , Ivaylo Petrov , Alexander Pelov , Carsten Bormann , Michael Richardson
Last updated 2022-07-18
Replaces draft-veillette-core-yang-cbor-mapping
Stream Internet Engineering Task Force (IETF)
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OPSDIR Last Call Review Incomplete, due 2021-03-17
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Stream WG state Submitted to IESG for Publication
Document shepherd Marco Tiloca
Shepherd write-up Show Last changed 2021-12-20
IESG IESG state RFC 9254 (Proposed Standard)
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Responsible AD Francesca Palombini
Send notices to Carsten Bormann <cabo@tzi.org>, marco.tiloca@ri.se
IANA IANA review state Version Changed - Review Needed
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IANA expert review state Expert Reviews OK
RFC 9254


Internet Engineering Task Force (IETF)                 M. Veillette, Ed.
Request for Comments: 9254                       Trilliant Networks Inc.
Category: Standards Track                                 I. Petrov, Ed.
ISSN: 2070-1721                                  Google Switzerland GmbH
                                                                A. Pelov
                                                                  Acklio
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                           M. Richardson
                                                Sandelman Software Works
                                                               July 2022

    Encoding of Data Modeled with YANG in the Concise Binary Object
                         Representation (CBOR)

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.

   This document defines encoding rules for YANG in the Concise Binary
   Object Representation (CBOR) (RFC 8949).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9254.

Copyright Notice

   Copyright (c) 2022 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.  Terminology and Notation
   3.  Properties of the CBOR Encoding
     3.1.  CBOR Diagnostic Notation
     3.2.  YANG Schema Item iDentifier
     3.3.  Name
   4.  Encoding of Representation Nodes
     4.1.  The 'leaf'
       4.1.1.  Using SIDs in Keys
       4.1.2.  Using Names in Keys
     4.2.  The 'container' and Other Nodes from the Data Tree
       4.2.1.  Using SIDs in Keys
       4.2.2.  Using Names in Keys
     4.3.  The 'leaf-list'
       4.3.1.  Using SIDs in Keys
       4.3.2.  Using Names in Keys
     4.4.  The 'list' and the 'list' Entries
       4.4.1.  Using SIDs in Keys
       4.4.2.  Using Names in Keys
     4.5.  The 'anydata'
       4.5.1.  Using SIDs in Keys
       4.5.2.  Using Names in Keys
     4.6.  The 'anyxml'
       4.6.1.  Using SIDs in Keys
       4.6.2.  Using Names in Keys
   5.  Encoding of the 'yang-data' Extension
     5.1.  Using SIDs in Keys
     5.2.  Using Names in Keys
   6.  Representing YANG Data Types in CBOR
     6.1.  The Unsigned Integer Types
     6.2.  The Integer Types
     6.3.  The 'decimal64' Type
     6.4.  The 'string' Type
     6.5.  The 'boolean' Type
     6.6.  The 'enumeration' Type
     6.7.  The 'bits' Type
     6.8.  The 'binary' Type
     6.9.  The 'leafref' Type
     6.10. The 'identityref' Type
       6.10.1.  SIDs as 'identityref'
       6.10.2.  Name as 'identityref'
     6.11. The 'empty' Type
     6.12. The 'union' Type
     6.13. The 'instance-identifier' Type
       6.13.1.  SIDs as 'instance-identifier'
       6.13.2.  Names as 'instance-identifier'
   7.  Content-Types
   8.  Security Considerations
   9.  IANA Considerations
     9.1.  Media Types Registry
     9.2.  CoAP Content-Formats Registry
     9.3.  CBOR Tags Registry
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The specification of the YANG 1.1 data modeling language [RFC7950]
   defines an XML encoding for data instances, i.e., contents of
   configuration datastores, state data, RPC inputs and outputs, action
   inputs and outputs, and event notifications.

   An additional set of encoding rules has been defined in [RFC7951]
   based on "The JavaScript Object Notation (JSON) Data Interchange
   Format" [RFC8259].

   The aim of this document is to define a set of encoding rules for the
   Concise Binary Object Representation (CBOR) [RFC8949], collectively
   called "YANG-CBOR".  The resulting encoding is more compact compared
   to XML and JSON and more suitable for constrained nodes and/or
   constrained networks, as defined by [RFC7228].

2.  Terminology and Notation

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   SID values (and the SID deltas computed from them) shown in the
   examples are example values; these examples do not allocate the SIDs
   shown for specific items in the modules.

   The following terms are defined in [RFC7950]:

   *  action

   *  anydata

   *  anyxml

   *  data node

   *  data tree

   *  datastore

   *  feature

   *  identity

   *  module

   *  notification

   *  RPC

   *  schema node

   *  submodule

   The following term is defined in [RFC8040]:

   *  yang-data extension

   The following term is defined in [RFC8791]:

   *  YANG data structure

   This specification also makes use of the following terminology:

   YANG Schema Item iDentifier (or "YANG SID" or simply "SID"):
      63-bit unsigned integer used to identify different YANG items.

   delta:
      Difference between the current YANG SID and a reference YANG SID.
      A reference YANG SID is defined for each context for which deltas
      are used.

   absolute SID:
      A YANG SID that is not encoded as a delta.  This is usually called
      out explicitly only in positions where normally a delta would be
      found.

   representation tree:
      A YANG data tree, possibly enclosed by a representation of a
      schema node, such as a YANG data structure, a notification, an
      RPC, or an action.

   representation node:
      A node in a representation tree, i.e., a data tree node, or a
      representation of a schema node, such as a YANG data structure, a
      notification, an RPC, or an action.

   item:
      A schema node, an identity, a module, or a feature defined using
      the YANG modeling language.

   list entry:
      The data associated with a single entry of a list (see Section 7.8
      of [RFC7950]).

   container-like instance:
      An instance of a container, a YANG data structure, notification
      contents, RPC input, RPC output, action input, or action output
      (Section 4.2); a list entry in a list (Section 4.4); or an anydata
      node (Section 4.5).

   parent (of a representation node):
      The schema node of the closest enclosing representation node in
      which a given representation node is defined.

3.  Properties of the CBOR Encoding

   This document defines CBOR encoding rules for YANG data trees and
   their subtrees.

   A YANG data tree can be enclosed by a representation of a schema
   node, such as a YANG data structure, a notification, an RPC, or an
   action; this is called a representation tree.  The data tree nodes
   and the enclosing schema node representation, if any, are
   collectively called the representation nodes.

   A representation node, such as a container, list entry, YANG data
   structure, notification, RPC input, RPC output, action input, action
   output, or anydata node, is serialized using a CBOR map in which each
   schema node defined within is encoded using a key and a value.  This
   specification supports two types of CBOR keys: YANG Schema Item
   iDentifier (YANG SID), as defined in Section 3.2, and names, as
   defined in Section 3.3.  Each of these key types is encoded using a
   specific CBOR type that allows their interpretation during the
   deserialization process.  Protocols or mechanisms implementing this
   specification can mandate the use of a specific key type or allow the
   generator to choose freely per key.

   In order to minimize the size of the encoded data, the mapping avoids
   any unnecessary meta-information beyond that directly provided by the
   CBOR basic generic data model (Section 2 of [RFC8949]).  For
   instance, CBOR tags are used solely in the case of an absolute SID,
   anyxml data nodes, or the union datatype to explicitly distinguish
   the use of different YANG datatypes encoded using the same CBOR major
   type.

   Unless specified otherwise by the protocol or mechanism implementing
   this specification, the indefinite length encoding, as defined in
   Section 3.2 of [RFC8949], SHALL be supported by the CBOR decoders
   employed with YANG-CBOR.  (This enables an implementation to begin
   emitting an array or map before the number of entries in that
   structure is known, possibly also avoiding excessive locking or race
   conditions.  On the other hand, it deprives the receiver of the
   encoded data from advance announcement about some size information,
   so a generator should choose indefinite length encoding only when
   these benefits do accrue.)

   Data nodes implemented using a CBOR array, map, byte string, or text
   string can be instantiated but empty.  In this case, they are encoded
   with a length of zero.

   When representation nodes are serialized using the rules defined by
   this specification as part of an application payload, the payload
   SHOULD include information that would allow each node to be
   identified in a stateless way, for instance, the SID number
   associated with the node, the SID delta from another SID in the
   application payload, the namespace-qualified name, or the instance-
   identifier.

   Examples in Section 4 include a root CBOR map with a single entry
   having a key set to either a namespace-qualified name or a SID.  This
   root CBOR map is provided only as a typical usage example and is not
   part of the present encoding rules.  Only the value within this CBOR
   map is compulsory.

3.1.  CBOR Diagnostic Notation

   Within this document, CBOR binary contents are represented using an
   equivalent textual form called CBOR diagnostic notation, as defined
   in Section 8 of [RFC8949].  This notation is used strictly for
   documentation purposes and is never used in the data serialization.
   Table 1 below provides a summary of this notation.

      +==========+======+====================+===========+==========+
      | CBOR     | CBOR | Diagnostic         | Example   | CBOR     |
      | Content  | Type | Notation           |           | Encoding |
      +==========+======+====================+===========+==========+
      | Unsigned | 0    | Decimal digits     | 123       | 18 7B    |
      | integer  |      |                    |           |          |
      +----------+------+--------------------+-----------+----------+
      | Negative | 1    | Decimal digits     | -123      | 38 7A    |
      | integer  |      | prefixed by a      |           |          |
      |          |      | minus sign         |           |          |
      +----------+------+--------------------+-----------+----------+
      | Byte     | 2    | Hexadecimal value  | h'F15C'   | 42 F15C  |
      | string   |      | enclosed between   |           |          |
      |          |      | single quotes and  |           |          |
      |          |      | prefixed by an 'h' |           |          |
      +----------+------+--------------------+-----------+----------+
      | Text     | 3    | String of Unicode  | "txt"     | 63       |
      | string   |      | characters         |           | 747874   |
      |          |      | enclosed between   |           |          |
      |          |      | double quotes      |           |          |
      +----------+------+--------------------+-----------+----------+
      | Array    | 4    | Comma-separated    | [ 1, 2 ]  | 82 01 02 |
      |          |      | list of values     |           |          |
      |          |      | within square      |           |          |
      |          |      | brackets           |           |          |
      +----------+------+--------------------+-----------+----------+
      | Map      | 5    | Comma-separated    | { 1: 123, | A2       |
      |          |      | list of key :      | 2: 456 }  | 01187B   |
      |          |      | value pairs within |           | 021901C8 |
      |          |      | curly braces       |           |          |
      +----------+------+--------------------+-----------+----------+
      | Boolean  | 7/20 | false              | false     | F4       |
      +----------+------+--------------------+-----------+----------+
      |          | 7/21 | true               | true      | F5       |
      +----------+------+--------------------+-----------+----------+
      | Null     | 7/22 | null               | null      | F6       |
      +----------+------+--------------------+-----------+----------+
      | Not      | 7/23 | undefined          | undefined | F7       |
      | assigned |      |                    |           |          |
      +----------+------+--------------------+-----------+----------+

                 Table 1: CBOR Diagnostic Notation Summary

   Note: CBOR binary contents shown in this specification are annotated
   with comments.  These comments are delimited by slashes ("/"), as
   defined in Appendix G.6 of [RFC8610].

3.2.  YANG Schema Item iDentifier

   Some of the items defined in YANG [RFC7950] require the use of a
   unique identifier.  In both the Network Configuration Protocol
   (NETCONF) [RFC6241] and RESTCONF [RFC8040], these identifiers are
   implemented using text strings.  To allow the implementation of data
   models defined in YANG in constrained devices and constrained
   networks, a more compact method to identify YANG items is required.
   This compact identifier, called "YANG Schema Item iDentifier", is an
   unsigned integer limited to 63 bits of range (i.e.,
   0..9223372036854775807 or 0..0x7fffffffffffffff).  The following
   items are identified using YANG SIDs (often shortened to SIDs):

   *  identities

   *  data nodes

   *  RPCs and associated input(s) and output(s)

   *  actions and associated input(s) and output(s)

   *  YANG data structures

   *  notifications and associated information

   *  YANG modules and features

      |  Note that any structuring of modules into submodules is
      |  transparent to YANG-CBOR: SIDs are not allocated for the names
      |  of submodules, and any items within a submodule are effectively
      |  allocated SIDs as part of processing the module that includes
      |  them.

   To minimize their size, SIDs used as keys in CBOR maps are encoded
   using deltas, i.e., signed (negative or unsigned) integers that are
   added to the reference SID applying to the map.  The reference SID of
   an outermost map is zero, unless a different reference SID is
   unambiguously conferred from the environment in which the outermost
   map is used.  The reference SID of a map that is most directly
   embedded in a map entry with a name-based key is zero.  For all other
   maps, the reference SID is the SID computed for the map entry it is
   most directly embedded in.  (The embedding may be indirect if an
   array intervenes, e.g., in a YANG list.)  Where absolute SIDs are
   desired in map key positions (where a bare integer implies a delta),
   they need to be identified as absolute SID values by using CBOR tag
   number 47 (as defined in Section 4.2.1).

   Thus, conversion from SIDs to deltas and back to SIDs is a stateless
   process solely based on the data serialized or deserialized combined
   with, potentially, an outermost reference SID unambiguously conferred
   by the environment.

   Mechanisms and processes used to assign SIDs to YANG items and to
   guarantee their uniqueness are outside the scope of the present
   specification.  If SIDs are to be used, the present specification is
   used in conjunction with a specification defining this management.  A
   related document, i.e., [CORE-SID], is intended to serve as the
   definitive way to assign SID values for YANG modules managed by the
   IETF and recommends itself for YANG modules managed by non-IETF
   entities, as well.  The present specification has been designed to
   allow different methods of assignment to be used within separate
   domains.

   To provide implementations with a way to internally indicate the
   absence of a SID, the SID value 0 is reserved and will not be
   allocated; it is not used in interchange.

3.3.  Name

   This specification also supports the encoding of YANG item
   identifiers as text strings, similar to those used by the JSON
   encoding of data modeled with YANG [RFC7951].  This approach can be
   used to avoid the management overhead associated with SID allocation.
   The main drawback is the significant increase in size of the encoded
   data.

   YANG item identifiers implemented using names MUST be in one of the
   following forms:

   *  simple -- the identifier of the YANG item (i.e., schema node or
      identity).

   *  namespace-qualified -- the identifier of the YANG item is prefixed
      with the name of the module in which this item is defined,
      separated by the colon character (":").

   The name of a module determines the namespace of all YANG items
   defined in that module.  If an item is defined in a submodule, then
   the namespace-qualified name uses the name of the main module to
   which the submodule belongs.

   ABNF syntax [RFC5234] of a name is shown in Figure 1, where the
   production for "identifier" is defined in Section 14 of [RFC7950].

   name = [identifier ":"] identifier

     Figure 1: ABNF Production for a Simple or Namespace-Qualified Name

   A namespace-qualified name MUST be used for all members of a top-
   level CBOR map and then also whenever the namespaces of the
   representation node and its parent node are different.  In all other
   cases, the simple form of the name MUST be used.

   Definition example:

   module example-foomod {
     container top {
       leaf foo {
         type uint8;
       }
     }
   }

   module example-barmod {
     import example-foomod {
       prefix "foomod";
     }
     augment "/foomod:top" {
       leaf bar {
         type boolean;
       }
     }
   }

   A valid CBOR encoding of the 'top' container is as follows.

   CBOR diagnostic notation:

   {
     "example-foomod:top": {
       "foo": 54,
       "example-barmod:bar": true
     }
   }

   Both the 'top' container and the 'bar' leaf defined in a different
   YANG module as its parent container are encoded as namespace-
   qualified names.  The 'foo' leaf defined in the same YANG module as
   its parent container is encoded as a simple name.

4.  Encoding of Representation Nodes

   Representation nodes defined using the YANG modeling language are
   encoded using CBOR [RFC8949], based on the rules defined in this
   section.  We assume that the reader is already familiar with both
   YANG [RFC7950] and CBOR [RFC8949].

4.1.  The 'leaf'

   A 'leaf' MUST be encoded accordingly to its datatype using one of the
   encoding rules specified in Section 6.

   The following examples show the encoding of a 'hostname' leaf using a
   SID or a name.

   Definition example adapted from [RFC6991] and [RFC7317]:

   typedef domain-name {
     type string {
       pattern
         '((([a-zA-Z0-9_]([a-zA-Z0-9\-_]){0,61})?[a-zA-Z0-9]\.)*'
       + '([a-zA-Z0-9_]([a-zA-Z0-9\-_]){0,61})?[a-zA-Z0-9]\.?)'
       + '|\.';
       length "1..253";
     }
   }

   leaf hostname {
     type inet:domain-name;
   }

4.1.1.  Using SIDs in Keys

   As with all examples below, the delta in the outermost map assumes a
   reference YANG SID (current schema node) of 0.

   CBOR diagnostic notation:

   {
     1752 : "myhost.example.com"     / hostname (SID 1752) /
   }

   CBOR encoding:

   A1                                         # map(1)
      19 06D8                                 # unsigned(1752)
      72                                      # text(18)
         6D79686F73742E6578616D706C652E636F6D # "myhost.example.com"

4.1.2.  Using Names in Keys

   CBOR diagnostic notation:

   {
     "ietf-system:hostname" : "myhost.example.com"
   }

   CBOR encoding:

   A1                                         # map(1)
      74                                      # text(20)
         696574662D73797374656D3A686F73746E616D65
      72                                      # text(18)
         6D79686F73742E6578616D706C652E636F6D

4.2.  The 'container' and Other Nodes from the Data Tree

   Instances of containers, YANG data structures, notification contents,
   RPC inputs, RPC outputs, action inputs, and action outputs MUST be
   encoded using a CBOR map data item (major type 5).  The same encoding
   is also used for the list entries in a list (Section 4.4) and for
   anydata nodes (Section 4.5).  Collectively, we speak of these
   instances as "container-like instances".

   A map consists of pairs of data items, with each pair consisting of a
   key and a value.  Each key within the CBOR map is set to a schema
   node identifier, and each value is set to the value of this
   representation node according to the instance datatype.

   This specification supports two types of CBOR map keys: SID, as
   defined in Section 3.2, and names, as defined in Section 3.3.

   The following examples show the encoding of a 'system-state'
   container representation instance using SIDs or names.

   Definition example adapted from [RFC6991] and [RFC7317]:

   typedef date-and-time {
     type string {
       pattern '\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?'
             + '(Z|[\+\-]\d{2}:\d{2})';
     }
   }

   container system-state {

     container clock {
       leaf current-datetime {
         type date-and-time;
       }

       leaf boot-datetime {
         type date-and-time;
       }
     }
   }

4.2.1.  Using SIDs in Keys

   In the context of containers and other nodes from the data tree, CBOR
   map keys within inner CBOR maps can be encoded using deltas (bare
   integers) or absolute SIDs (tagged with tag number 47).

   Delta values are computed as follows:

   *  In the case of a 'container', deltas are equal to the SID of the
      current representation node minus the SID of the parent
      'container'.

   *  In the case of a 'list', deltas are equal to the SID of the
      current representation node minus the SID of the parent 'list'.

   *  In the case of an 'RPC input' or 'RPC output', deltas are equal to
      the SID of the current representation node minus the SID of the
      'RPC'.

   *  In the case of an 'action input' or 'action output', deltas are
      equal to the SID of the current representation node minus the SID
      of the 'action'.

   *  In the case of a 'notification content', deltas are equal to the
      SID of the current representation node minus the SID of the
      'notification'.

   CBOR diagnostic notation:

   {
     1720 : {                              / system-state (SID 1720) /
       1 : {                               / clock  (SID 1721) /
         2 : "2015-10-02T14:47:24Z-05:00", / current-datetime(SID 1723)/
         1 : "2015-09-15T09:12:58Z-05:00"  / boot-datetime (SID 1722) /
       }
     }
   }

   CBOR encoding:

   A1                                      # map(1)
      19 06B8                              # unsigned(1720)
      A1                                   # map(1)
         01                                # unsigned(1)
         A2                                # map(2)
            02                             # unsigned(2)
            78 1A                          # text(26)
               323031352D31302D30325431343A34373A32345A2D30353A3030
            01                             # unsigned(1)
            78 1A                          # text(26)
               323031352D30392D31355430393A31323A35385A2D30353A3030

                   Figure 2: System State Clock Encoding

4.2.2.  Using Names in Keys

   CBOR map keys implemented using names MUST be encoded using a CBOR
   text string data item (major type 3).  A namespace-qualified name
   MUST be used each time the namespace of a representation node and its
   parent differ.  In all other cases, the simple form of the name MUST
   be used.  Names and namespaces are defined in Section 4 of [RFC7951].

   The following example shows the encoding of a 'system' container
   representation node instance using names.

   CBOR diagnostic notation:

   {
     "ietf-system:system-state" : {
       "clock" : {
         "current-datetime" : "2015-10-02T14:47:24Z-05:00",
         "boot-datetime" : "2015-09-15T09:12:58Z-05:00"
       }
     }
   }

   CBOR encoding:

   A1                                      # map(1)
      78 18                                # text(24)
         696574662D73797374656D3A73797374656D2D7374617465
      A1                                   # map(1)
         65                                # text(5)
            636C6F636B                     # "clock"
         A2                                # map(2)
            70                             # text(16)
               63757272656E742D6461746574696D65
            78 1A                          # text(26)
               323031352D31302D30325431343A34373A32345A2D30353A3030
            6D                             # text(13)
               626F6F742D6461746574696D65
            78 1A                          # text(26)
               323031352D30392D31355430393A31323A35385A2D30353A3030

4.3.  The 'leaf-list'

   A leaf-list MUST be encoded using a CBOR array data item (major type
   4).  Each entry of this array MUST be encoded accordingly to its
   datatype using one of the encoding rules specified in Section 6.

   The following example shows the encoding of the 'search' leaf-list
   representation node instance containing two entries: "ietf.org" and
   "ieee.org".

   Definition example adapted from [RFC6991] and [RFC7317]:

   typedef domain-name {
     type string {
       pattern
         '((([a-zA-Z0-9_]([a-zA-Z0-9\-_]){0,61})?[a-zA-Z0-9]\.)*'
       + '([a-zA-Z0-9_]([a-zA-Z0-9\-_]){0,61})?[a-zA-Z0-9]\.?)'
       + '|\.';
       length "1..253";
     }
   }

   leaf-list search {
     type domain-name;
     ordered-by user;
   }

4.3.1.  Using SIDs in Keys

   CBOR diagnostic notation:

   {
     1746 : [ "ietf.org", "ieee.org" ]     / search (SID 1746) /
   }

   CBOR encoding:

   A1                        # map(1)
      19 06D2                # unsigned(1746)
      82                     # array(2)
         68                  # text(8)
            696574662E6F7267 # "ietf.org"
         68                  # text(8)
            696565652E6F7267 # "ieee.org"

4.3.2.  Using Names in Keys

   CBOR diagnostic notation:

   {
     "ietf-system:search" : [ "ietf.org", "ieee.org" ]
   }

   CBOR encoding:

   A1                                         # map(1)
      72                                      # text(18)
         696574662D73797374656D3A736561726368 # "ietf-system:search"
      82                                      # array(2)
         68                                   # text(8)
            696574662E6F7267                  # "ietf.org"
         68                                   # text(8)
            696565652E6F7267                  # "ieee.org"

4.4.  The 'list' and the 'list' Entries

   A list or a subset of a list MUST be encoded using a CBOR array data
   item (major type 4).  Each list entry within this CBOR array is
   encoded using a CBOR map data item (major type 5) based on the
   encoding rules of a container-like instance, as defined in
   Section 4.2.

   It is important to note that this encoding rule also applies to a
   'list' representation node instance that has a single entry.

   The following examples show the encoding of a 'server' list using
   SIDs or names.

   Definition example adapted from [RFC7317]:

   list server {
     key name;

     leaf name {
       type string;
     }
     choice transport {
       case udp {
         container udp {
           leaf address {
             type host;
             mandatory true;
           }
           leaf port {
             type port-number;
           }
         }
       }
     }
     leaf association-type {
       type enumeration {
         enum server;
         enum peer;
         enum pool;
       }
       default server;
     }
     leaf iburst {
       type boolean;
       default false;
     }
     leaf prefer {
       type boolean;
       default false;
     }
   }

4.4.1.  Using SIDs in Keys

   The encoding rules of each 'list' entry are defined in Section 4.2.1.

   CBOR diagnostic notation:

   {
     1756 : [                      / server (SID 1756) /
       {
         3 : "NRC TIC server",     / name (SID 1759) /
         5 : {                     / udp (SID 1761) /
           1 : "tic.nrc.ca",       / address (SID 1762) /
           2 : 123                 / port (SID 1763) /
         },
         1 : 0,                    / association-type (SID 1757) /
         2 : false,                / iburst (SID 1758) /
         4 : true                  / prefer (SID 1760) /
       },
       {
         3 : "NRC TAC server",     / name (SID 1759) /
         5 : {                     / udp (SID 1761) /
           1 : "tac.nrc.ca"        / address (SID 1762) /
         }
       }
     ]
   }

   CBOR encoding:

   A1                                      # map(1)
      19 06DC                              # unsigned(1756)
      82                                   # array(2)
         A5                                # map(5)
            03                             # unsigned(3)
            6E                             # text(14)
               4E52432054494320736572766572 # "NRC TIC server"
            05                             # unsigned(5)
            A2                             # map(2)
               01                          # unsigned(1)
               6A                          # text(10)
                  7469632E6E72632E6361     # "tic.nrc.ca"
               02                          # unsigned(2)
               18 7B                       # unsigned(123)
            01                             # unsigned(1)
            00                             # unsigned(0)
            02                             # unsigned(2)
            F4                             # primitive(20)
            04                             # unsigned(4)
            F5                             # primitive(21)
         A2                                # map(2)
            03                             # unsigned(3)
            6E                             # text(14)
               4E52432054414320736572766572 # "NRC TAC server"
            05                             # unsigned(5)
            A1                             # map(1)
               01                          # unsigned(1)
               6A                          # text(10)
                  7461632E6E72632E6361     # "tac.nrc.ca"

4.4.2.  Using Names in Keys

   The encoding rules of each 'list' entry are defined in Section 4.2.2.

   CBOR diagnostic notation:

   {
     "ietf-system:server" : [
       {
         "name" : "NRC TIC server",
         "udp" : {
           "address" : "tic.nrc.ca",
           "port" : 123
         },
         "association-type" : 0,
         "iburst" : false,
         "prefer" : true
       },
       {
         "name" : "NRC TAC server",
         "udp" : {
           "address" : "tac.nrc.ca"
         }
       }
     ]
   }

   CBOR encoding:

   A1                                      # map(1)
      72                                   # text(18)
         696574662D73797374656D3A736572766572
      82                                   # array(2)
         A5                                # map(5)
            64                             # text(4)
               6E616D65                    # "name"
            6E                             # text(14)
               4E52432054494320736572766572
            63                             # text(3)
               756470                      # "udp"
            A2                             # map(2)
               67                          # text(7)
                  61646472657373           # "address"
               6A                          # text(10)
                  7469632E6E72632E6361     # "tic.nrc.ca"
               64                          # text(4)
                  706F7274                 # "port"
               18 7B                       # unsigned(123)
            70                             # text(16)
               6173736F63696174696F6E2D74797065
            00                             # unsigned(0)
            66                             # text(6)
               696275727374                # "iburst"
            F4                             # primitive(20)
            66                             # text(6)
               707265666572                # "prefer"
            F5                             # primitive(21)
         A2                                # map(2)
            64                             # text(4)
               6E616D65                    # "name"
            6E                             # text(14)
               4E52432054414320736572766572
            63                             # text(3)
               756470                      # "udp"
            A1                             # map(1)
               67                          # text(7)
                  61646472657373           # "address"
               6A                          # text(10)
                  7461632E6E72632E6361     # "tac.nrc.ca"

4.5.  The 'anydata'

   An anydata node serves as a container for an arbitrary set of
   representation nodes that otherwise appear as normal YANG-modeled
   data.  An anydata representation node instance is encoded using the
   same rules as a container, i.e., using a CBOR map data item (major
   type 5) based on the encoding rules of a container-like instance, as
   defined in Section 4.2.

   The following example shows a possible use of an anydata node.  In
   this example, an anydata node is used to define a representation node
   containing a notification event; this representation node can be part
   of a YANG list to create an event logger.

   Definition example:

   module event-log {
     ...
     anydata last-event;                // SID 60123
   }

   This example also assumes the assistance of the following
   notification.

   module example-port {
     ...

     notification example-port-fault {  // SID 60200
       leaf port-name {                 // SID 60201
         type string;
       }
       leaf port-fault {                // SID 60202
         type string;
       }
     }
   }

4.5.1.  Using SIDs in Keys

   CBOR diagnostic notation:

   {
     60123 : {                   / last-event (SID 60123) /
       77 : {                    / example-port-fault (SID 60200) /
         1 : "0/4/21",           / port-name (SID 60201) /
         2 : "Open pin 2"        / port-fault (SID 60202) /
       }
     }
   }

   CBOR encoding:

   A1                               # map(1)
      19 EADB                       # unsigned(60123)
      A1                            # map(1)
         18 4D                      # unsigned(77)
         A2                         # map(2)
            01                      # unsigned(1)
            66                      # text(6)
               302F342F3231         # "0/4/21"
            02                      # unsigned(2)
            6A                      # text(10)
               4F70656E2070696E2032 # "Open pin 2"

   In some implementations, it might be simpler to use the absolute SID
   encoding (tag number 47) for the anydata root element.  CBOR
   diagnostic notation:

   {
     60123 : {                   / last-event (SID 60123) /
       47(60200) : {             / event-port-fault (SID 60200) /
         1 : "0/4/21",           / port-name (SID 60201) /
         2 : "Open pin 2"        / port-fault (SID 60202) /
       }
     }
   }

4.5.2.  Using Names in Keys

   CBOR diagnostic notation:

   {
     "event-log:last-event" : {
       "example-port:example-port-fault" : {
         "port-name" : "0/4/21",
         "port-fault" : "Open pin 2"
       }
     }
   }

   CBOR encoding:

   A1                                      # map(1)
      74                                   # text(20)
         6576656E742D6C6F673A6C6173742D6576656E74
      A1                                   # map(1)
         78 1F                             # text(31)
            6578616D706C652D706F72743A
            6578616D706C652D706F72742D6661756C74
         A2                                # map(2)
            69                             # text(9)
               706F72742D6E616D65          # "port-name"
            66                             # text(6)
               302F342F3231                # "0/4/21"
            6A                             # text(10)
               706F72742D6661756C74        # "port-fault"
            6A                             # text(10)
               4F70656E2070696E2032        # "Open pin 2"

4.6.  The 'anyxml'

   An anyxml representation node is used to serialize an arbitrary CBOR
   content, i.e., its value can be any CBOR binary object.  (The "xml"
   in the name is a misnomer that only applied to YANG-XML [RFC7950].)
   An anyxml value MAY contain CBOR data items tagged with one of the
   tags listed in Section 9.3.  The tags listed in Section 9.3 SHALL be
   supported.

   The following example shows a valid CBOR-encoded anyxml
   representation node instance consisting of a CBOR array containing
   the CBOR simple values 'true', 'null', and 'true'.

   Definition example adapted from [RFC7951]:

   module bar-module {
     ...
     anyxml bar;      // SID 60000
   }

4.6.1.  Using SIDs in Keys

   CBOR diagnostic notation:

   {
     60000 : [true, null, true]   / bar (SID 60000) /
   }

   CBOR encoding:

   A1         # map(1)
      19 EA60 # unsigned(60000)
      83      # array(3)
         F5   # primitive(21)
         F6   # primitive(22)
         F5   # primitive(21)

4.6.2.  Using Names in Keys

   CBOR diagnostic notation:

   {
     "bar-module:bar" : [true, null, true]   / bar (SID 60000) /
   }

   CBOR encoding:

   A1                                 # map(1)
      6E                              # text(14)
         6261722D6D6F64756C653A626172 # "bar-module:bar"
      83                              # array(3)
         F5                           # primitive(21)
         F6                           # primitive(22)
         F5                           # primitive(21)

5.  Encoding of the 'yang-data' Extension

   The yang-data extension [RFC8040] is used to define data structures
   in YANG that are not intended to be implemented as part of a
   datastore.

   The yang-data extension will specify a container that MUST be encoded
   using the encoding rules of nodes of data trees, as defined in
   Section 4.2.

   Just like YANG containers, the yang-data extension can be encoded
   using either SIDs or names.

   Definition example adapted from Appendix A of [CORE-COMI]:

   module ietf-coreconf {
     ...

     import ietf-restconf {
       prefix rc;
     }

     rc:yang-data yang-errors {
       container error {
         leaf error-tag {
           type identityref {
             base error-tag;
           }
         }
         leaf error-app-tag {
           type identityref {
             base error-app-tag;
           }
         }
         leaf error-data-node {
           type instance-identifier;
         }
         leaf error-message {
           type string;
         }
       }
     }
   }

5.1.  Using SIDs in Keys

   The yang-data extensions encoded using SIDs are carried in a CBOR map
   containing a single item pair.  The key of this item is set to the
   SID assigned to the yang-data extension container; the value is set
   to the CBOR encoding of this container, as defined in Section 4.2.

   This example shows a serialization example of the yang-errors yang-
   data extension, as defined in [CORE-COMI], using SIDs, as defined in
   Section 3.2.

   CBOR diagnostic notation:

   {
     1024 : {                      / error  (SID 1024) /
       4 : 1011,                   / error-tag (SID 1028) /
                                   / = invalid-value (SID 1011) /
       1 : 1018,                   / error-app-tag (SID 1025) /
                                   / = not-in-range (SID 1018) /
       2 : 1740,                   / error-data-node (SID 1026) /
                                   / = timezone-utc-offset (SID 1740) /
       3 : "Maximum exceeded"      / error-message (SID 1027) /
     }
   }

   CBOR encoding:

   A1                                      # map(1)
      19 0400                              # unsigned(1024)
      A4                                   # map(4)
         04                                # unsigned(4)
         19 03F3                           # unsigned(1011)
         01                                # unsigned(1)
         19 03FA                           # unsigned(1018)
         02                                # unsigned(2)
         19 06CC                           # unsigned(1740)
         03                                # unsigned(3)
         70                                # text(16)
            4D6178696D756D206578636565646564 # "Maximum exceeded"

5.2.  Using Names in Keys

   The yang-data extensions encoded using names are carried in a CBOR
   map containing a single item pair.  The key of this item is set to
   the namespace-qualified name of the yang-data extension container;
   the value is set to the CBOR encoding of this container, as defined
   in Section 4.2.

   This example shows a serialization example of the yang-errors yang-
   data extension, as defined in [CORE-COMI], using names, as defined
   Section 3.3.

   CBOR diagnostic notation:

   {
     "ietf-coreconf:error" : {
       "error-tag" : "invalid-value",
       "error-app-tag" : "not-in-range",
       "error-data-node" : "timezone-utc-offset",
       "error-message" : "Maximum exceeded"
     }
   }

   CBOR encoding:

   A1                                           # map(1)
      73                                        # text(19)
         696574662D636F7265636F6E663A6572726F72 # "ietf-coreconf:error"
      A4                                        # map(4)
         69                                     # text(9)
            6572726F722D746167                  # "error-tag"
         6D                                     # text(13)
            696E76616C69642D76616C7565          # "invalid-value"
         6D                                     # text(13)
            6572726F722D6170702D746167          # "error-app-tag"
         6C                                     # text(12)
            6E6F742D696E2D72616E6765            # "not-in-range"
         6F                                     # text(15)
            6572726F722D646174612D6E6F6465      # "error-data-node"
         73                                     # text(19)
            74696D657A6F6E652D7574632D6F6666736574
                                                # "timezone-utc-offset"
         6D                                     # text(13)
            6572726F722D6D657373616765          # "error-message"
         70                                     # text(16)
            4D6178696D756D206578636565646564    # "Maximum exceeded"

6.  Representing YANG Data Types in CBOR

   The CBOR encoding of an instance of a leaf or leaf-list
   representation node depends on the built-in type of that
   representation node.  The following subsection defines the CBOR
   encoding of each built-in type supported by YANG, as listed in
   Section 4.2.4 of [RFC7950].  Each subsection shows an example value
   assigned to a representation node instance of the discussed built-in
   type.

6.1.  The Unsigned Integer Types

   Leafs of type uint8, uint16, uint32, and uint64 MUST be encoded using
   a CBOR unsigned integer data item (major type 0).

   The following example shows the encoding of an 'mtu' leaf
   representation node instance set to 1280 bytes.

   Definition example adapted from [RFC8344]:

   leaf mtu {
     type uint16 {
       range "68..max";
     }
   }

   CBOR diagnostic notation: 1280

   CBOR encoding: 19 0500

6.2.  The Integer Types

   Leafs of type int8, int16, int32, and int64 MUST be encoded using
   either a CBOR unsigned integer (major type 0) or a CBOR negative
   integer (major type 1), depending on the actual value.

   The following example shows the encoding of a 'timezone-utc-offset'
   leaf representation node instance set to -300 minutes.

   Definition example adapted from [RFC7317]:

   leaf timezone-utc-offset {
     type int16 {
       range "-1500 .. 1500";
     }
   }

   CBOR diagnostic notation: -300

   CBOR encoding: 39 012B

6.3.  The 'decimal64' Type

   Leafs of type decimal64 MUST be encoded using a decimal fraction, as
   defined in Section 3.4.4 of [RFC8949].

   The following example shows the encoding of a 'my-decimal' leaf
   representation node instance set to 2.57.

   Definition example adapted from [RFC7317]:

   leaf my-decimal {
     type decimal64 {
       fraction-digits 2;
       range "1 .. 3.14 | 10 | 20..max";
     }
   }

   CBOR diagnostic notation: 4([-2, 257])

   CBOR encoding: C4 82 21 19 0101

6.4.  The 'string' Type

   Leafs of type string MUST be encoded using a CBOR text string data
   item (major type 3).

   The following example shows the encoding of a 'name' leaf
   representation node instance set to "eth0".

   Definition example adapted from [RFC8343]:

   leaf name {
     type string;
   }

   CBOR diagnostic notation: "eth0"

   CBOR encoding: 64 65746830

6.5.  The 'boolean' Type

   Leafs of type boolean MUST be encoded using a CBOR simple value
   'true' (major type 7, additional information 21) or 'false' (major
   type 7, additional information 20).

   The following example shows the encoding of an 'enabled' leaf
   representation node instance set to 'true'.

   Definition example adapted from [RFC7317]:

   leaf enabled {
     type boolean;
   }

   CBOR diagnostic notation: true

   CBOR encoding: F5

6.6.  The 'enumeration' Type

   Leafs of type enumeration MUST be encoded using a CBOR unsigned
   integer (major type 0) or CBOR negative integer (major type 1),
   depending on the actual value, or exceptionally as a tagged text
   string (see below).  Enumeration values are either explicitly
   assigned using the YANG statement 'value' or automatically assigned
   based on the algorithm defined in Section 9.6.4.2 of [RFC7950].

   The following example shows the encoding of an 'oper-status' leaf
   representation node instance set to 'testing'.

   Definition example adapted from [RFC7317]:

   leaf oper-status {
     type enumeration {
       enum up { value 1; }
       enum down { value 2; }
       enum testing { value 3; }
       enum unknown { value 4; }
       enum dormant { value 5; }
       enum not-present { value 6; }
       enum lower-layer-down { value 7; }
     }
   }

   CBOR diagnostic notation: 3

   CBOR encoding: 03

   Values of 'enumeration' types defined in a 'union' type MUST be
   encoded using a CBOR text string data item (major type 3) and MUST
   contain one of the names assigned by 'enum' statements in YANG (see
   also Section 6.12).  The encoding MUST be enclosed by the enumeration
   CBOR tag, as specified in Section 9.3.

   Definition example adapted from [RFC7950]:

   type union {
     type int32;
     type enumeration {
       enum unbounded;
     }
   }

   CBOR diagnostic notation: 44("unbounded")

   CBOR encoding: D8 2C 69 756E626F756E646564

6.7.  The 'bits' Type

   Keeping in mind that bit positions are either explicitly assigned
   using the YANG statement 'position' or automatically assigned based
   on the algorithm defined in Section 9.7.4.2 of [RFC7950], each
   element of type bits could be seen as a set of bit positions (or
   offsets from position 0) that have a value of either 1, which
   represents the bit being set, or 0, which represents that the bit is
   not set.

   Leafs of type bits MUST be encoded either using a CBOR array (major
   type 4) or byte string (major type 2) or exceptionally as a tagged
   text string (see below).  In case CBOR array representation is used,
   each element is either (1) a positive integer (major type 0 with
   value 0 being disallowed) that can be used to calculate the offset of
   the next byte string or (2) a byte string (major type 2) that carries
   the information regarding whether certain bits are set or not.  The
   initial offset value is 0, and each unsigned integer modifies the
   offset value of the next byte string by the integer value multiplied
   by 8.  For example, if the bit offset is 0 and there is an integer
   with value 5, the first byte of the byte string that follows will
   represent bit positions 40 to 47, with both ends included.  If the
   byte string has a second byte, it will carry information about bits
   48 to 55, and so on.  Within each byte, bits are assigned from least
   to most significant.  After the byte string, the offset is modified
   by the number of bytes in the byte string multiplied by 8.  Bytes
   with no bits set (zero bytes) at the end of the byte string are never
   generated.  If they occur at the end of the array, the zero bytes are
   simply omitted; if they occur at the end of a byte string preceding
   an integer, the zero bytes are removed and the integer is adjusted
   upwards by the number of zero bytes that were removed.  An example
   follows.

   The following example shows the encoding of an 'alarm-state' leaf
   representation node instance with the 'critical' (position 2),
   'warning' (position 8), and 'indeterminate' (position 128) flags set.

   typedef alarm-state {
     type bits {
       bit unknown;
       bit under-repair;
       bit critical;
       bit major;
       bit minor;
       bit warning {
         position 8;
       }
       bit indeterminate {
         position 128;
       }
     }
   }

   leaf alarm-state {
     type alarm-state;
   }

   CBOR diagnostic notation: [h'0401', 14, h'01']

   CBOR encoding: 83 42 0401 0E 41 01

   In a number of cases, the array would only need to have one element
   -- a byte string with a few bytes inside.  For this case, it is
   REQUIRED to omit the array element and have only the byte array that
   would have been inside.  To illustrate this, let us consider the same
   example YANG definition but this time encoding only 'under-repair'
   and 'critical' flags.  The result would be

   CBOR diagnostic notation: h'06'

   CBOR encoding: 41 06

   Elements in the array MUST be either byte strings that do not end in
   a zero byte or positive unsigned integers, where byte strings and
   integers MUST alternate, i.e., adjacent byte strings or adjacent
   integers are an error.  An array with a single byte string MUST
   instead be encoded as just that byte string.  An array with a single
   positive integer is an error.  Note that a recipient can handle
   trailing zero bytes in the byte strings using the normal rules
   without any issue, so an implementation MAY silently accept them.

   Values of 'bits' types defined in a 'union' type MUST be encoded
   using a CBOR text string data item (major type 3) and MUST contain a
   space-separated sequence of names of 'bits' that are set (see also
   Section 6.12).  The encoding MUST be enclosed by the bits CBOR tag,
   as specified in Section 9.3.

   The following example shows the encoding of an 'alarm-state' leaf
   representation node instance defined using a union type with the
   'under-repair' and 'critical' flags set.

   Definition example:

   leaf alarm-state-2 {
     type union {
       type alarm-state;
       type bits {
         bit extra-flag;
       }
     }
   }

   CBOR diagnostic notation: 43("under-repair critical")

   CBOR encoding: D8 2B 75 756E6465722D72657061697220637269746963616C

6.8.  The 'binary' Type

   Leafs of type binary MUST be encoded using a CBOR byte string data
   item (major type 2).

   The following example shows the encoding of an 'aes128-key' leaf
   representation node instance set to
   0x1f1ce6a3f42660d888d92a4d8030476e.

   Definition example:

   leaf aes128-key {
     type binary {
       length 16;
     }
   }

   CBOR diagnostic notation: h'1F1CE6A3F42660D888D92A4D8030476E'

   CBOR encoding: 50 1F1CE6A3F42660D888D92A4D8030476E

6.9.  The 'leafref' Type

   Leafs of type leafref MUST be encoded using the rules of the
   representation node referenced by the 'path' YANG statement.

   The following example shows the encoding of an 'interface-state-ref'
   leaf representation node instance set to "eth1".

   Definition example adapted from [RFC8343]:

   typedef interface-state-ref {
     type leafref {
       path "/interfaces-state/interface/name";
     }
   }

   container interfaces-state {
     list interface {
       key "name";
       leaf name {
         type string;
       }
       leaf-list higher-layer-if {
         type interface-state-ref;
       }
     }
   }

   CBOR diagnostic notation: "eth1"

   CBOR encoding: 64 65746831

6.10.  The 'identityref' Type

   This specification supports two approaches for encoding identityref:
   as a YANG Schema Item iDentifier, as defined in Section 3.2, or as a
   name, as defined in Section 6.8 of [RFC7951].  See Section 6.12 for
   an exceptional case when this representation needs to be tagged.

6.10.1.  SIDs as 'identityref'

   When representation nodes of type identityref are implemented using
   SIDs, they MUST be encoded using a CBOR unsigned integer data item
   (major type 0).  (Note that, as they are not used in the position of
   CBOR map keys, no delta mechanism is employed for SIDs used for
   identityref.)

   The following example shows the encoding of a 'type' leaf
   representation node instance set to the value 'iana-if-
   type:ethernetCsmacd' (SID 1880).

   Definition example adapted from [RFC7317]:

   identity interface-type {
   }

   identity iana-interface-type {
     base interface-type;
   }

   identity ethernetCsmacd {
     base iana-interface-type;
   }

   leaf type {
     type identityref {
       base interface-type;
     }
   }

   CBOR diagnostic notation: 1880

   CBOR encoding: 19 0758

6.10.2.  Name as 'identityref'

   Alternatively, an identityref MAY be encoded using a name, as defined
   in Section 3.3.  When names are used, identityref MUST be encoded
   using a CBOR text string data item (major type 3).  If the identity
   is defined in a different module than the leaf node containing the
   identityref data node, the namespace-qualified form MUST be used.
   Otherwise, both the simple and namespace-qualified forms are
   permitted.  Names and namespaces are defined in Section 3.3.

   The following example shows the encoding of the identity 'iana-if-
   type:ethernetCsmacd' using its namespace-qualified name.  This
   example is described in Section 6.10.1.

   CBOR diagnostic notation: "iana-if-type:ethernetCsmacd"

   CBOR encoding: 78 1B
   69616E612D69662D747970653A65746865726E657443736D616364

6.11.  The 'empty' Type

   Leafs of type empty MUST be encoded using the CBOR null value (major
   type 7, additional information 22).

   The following example shows the encoding of an 'is-router' leaf
   representation node instance when present.

   Definition example adapted from [RFC8344]:

   leaf is-router {
     type empty;
   }

   CBOR diagnostic notation: null

   CBOR encoding: F6

6.12.  The 'union' Type

   Leafs of type union MUST be encoded using the rules associated with
   one of the types listed.  When used in a union, the following YANG
   datatypes are enclosed by a CBOR tag to avoid confusion between
   different YANG datatypes encoded using the same CBOR major type.

   *  bits

   *  enumeration

   *  identityref

   *  instance-identifier

   See Section 9.3 for the assigned value of these CBOR tags.

   As mentioned in Sections 6.6 and in 6.7, 'enumeration' and 'bits' are
   encoded as a CBOR text string data item (major type 3) when defined
   within a 'union' type.  (This adds considerable complexity but is
   necessary because of an idiosyncrasy of the YANG data model for
   unions; the work-around allows compatibility to be maintained with
   the encoding of overlapping unions in XML and JSON.  See also
   Section 9.12 of [RFC7950].)

   The following example shows the encoding of an 'ip-address' leaf
   representation node instance when set to "2001:db8:a0b:12f0::1".

   Definition example adapted from [RFC6991]:

   typedef ipv4-address {
     type string {
       pattern
         '(([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])\.){3}'
       +  '([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])'
       + '(%[\p{N}\p{L}]+)?';
     }
   }

   typedef ipv6-address {
     type string {
       pattern '((:|[0-9a-fA-F]{0,4}):)([0-9a-fA-F]{0,4}:){0,5}'
             + '((([0-9a-fA-F]{0,4}:)?(:|[0-9a-fA-F]{0,4}))|'
             + '(((25[0-5]|2[0-4][0-9]|[01]?[0-9]?[0-9])\.){3}'
             + '(25[0-5]|2[0-4][0-9]|[01]?[0-9]?[0-9])))'
             + '(%[\p{N}\p{L}]+)?';
       pattern '(([^:]+:){6}(([^:]+:[^:]+)|(.*\..*)))|'
             + '((([^:]+:)*[^:]+)?::(([^:]+:)*[^:]+)?)'
             + '(%.+)?';
     }
   }

   typedef ip-address {
     type union {
       type ipv4-address;
       type ipv6-address;
     }
   }

   leaf address {
     type ip-address;
   }

   CBOR diagnostic notation: "2001:db8:a0b:12f0::1"

   CBOR encoding: 74 323030313A6462383A6130623A313266303A3A31

6.13.  The 'instance-identifier' Type

   This specification supports two approaches for encoding an instance-
   identifier: one based on YANG Schema Item iDentifier, as defined in
   Section 3.2, and one based on names, as defined in Section 3.3.  See
   Section 6.12 for an exceptional case when this representation needs
   to be tagged.

6.13.1.  SIDs as 'instance-identifier'

   SIDs uniquely identify a schema node.  In the case of a single
   instance schema node, i.e., a schema node defined at the root of a
   YANG module or submodule or schema nodes defined within a container,
   the SID is sufficient to identify this instance (representation
   node).  (Note that no delta mechanism is employed for SIDs used for
   identityref, see Section 6.10.1.)

   In the case of a representation node that is an entry of a YANG list,
   a SID is combined with the list key(s) to identify each instance
   within the YANG list(s).

   Instance-identifiers of single instance schema nodes MUST be encoded
   using a CBOR unsigned integer data item (major type 0) and set to the
   targeted schema node SID.

   Instance-identifiers of representation node entries of a YANG list
   MUST be encoded using a CBOR array data item (major type 4)
   containing the following entries:

   *  The first entry MUST be encoded as a CBOR unsigned integer data
      item (major type 0) and set to the targeted schema node SID.

   *  The following entries MUST contain the value of each key required
      to identify the instance of the targeted schema node.  These keys
      MUST be ordered as defined in the 'key' YANG statement, starting
      from the top-level list, and followed by each subordinate list(s).

   Examples within this section assume the definition of a schema node
   of type 'instance-identifier':

   Definition example adapted from [RFC7950]:

   container system {
     ...
     leaf reporting-entity {
       type instance-identifier;
     }

   *First example:*

   The following example shows the encoding of the 'reporting-entity'
   value referencing data node instance "/system/contact" (SID 1741).

   Definition example adapted from [RFC7317]:

   container system {

     leaf contact {
       type string;
     }

     leaf hostname {
       type inet:domain-name;
     }
   }

   CBOR diagnostic notation: 1741

   CBOR encoding: 19 06CD

   *Second example:*

   This example aims to show how a representation node entry of a YANG
   list is identified.  It uses a somewhat arbitrarily modified YANG
   module version from [RFC7317] by adding country to the leafs and keys
   of authorized-key.

   The following example shows the encoding of the 'reporting-entity'
   value referencing list instance "/system/authentication/user/
   authorized-key/key-data" (which is assumed to have SID 1734) for
   username "bob" and authorized-key with name "admin" and country
   "france".

   list user {
     key name;

     leaf name {
       type string;
     }

     leaf password {
       type ianach:crypt-hash;
     }

     list authorized-key {
       key "name country";

       leaf country {
         type string;
       }

       leaf name {
         type string;
       }

       leaf algorithm {
         type string;
       }

       leaf key-data {
         type binary;
       }
     }
   }

   CBOR diagnostic notation: [1734, "bob", "admin", "france"]

   CBOR encoding:

   84                 # array(4)
      19 06C6         # unsigned(1734)
      63              # text(3)
         626F62       # "bob"
      65              # text(5)
         61646D696E   # "admin"
      66              # text(6)
         6672616E6365 # "france"

   *Third example:*

   The following example shows the encoding of the 'reporting-entity'
   value referencing the list instance "/system/authentication/user"
   (SID 1730), corresponding to username "jack".

   CBOR diagnostic notation: [1730, "jack"]

   CBOR encoding:

   82             # array(2)
      19 06C2     # unsigned(1730)
      64          # text(4)
         6A61636B # "jack"

6.13.2.  Names as 'instance-identifier'

   An 'instance-identifier' value is encoded as a text string that is
   analogous to the lexical representation in XML encoding; see
   Section 9.13.2 of [RFC7950].  However, the encoding of namespaces in
   instance-identifier values follows the rules stated in Section 3.3,
   namely:

   *  The leftmost (top-level) data node name is always in the
      namespace-qualified form.

   *  Any subsequent data node name is in the namespace-qualified form
      if the node is defined in a module other than its parent node;
      otherwise, the simple form is used.  This rule also holds for node
      names appearing in predicates.

   For example,

   /ietf-interfaces:interfaces/interface[name='eth0']/ietf-ip:ipv4/ip

   is a valid instance-identifier value because the data nodes
   "interfaces", "interface", and "name" are defined in the module
   "ietf-interfaces", whereas "ipv4" and "ip" are defined in "ietf-ip".

   The resulting XML Path Language (XPath) MUST be encoded using a CBOR
   text string data item (major type 3).

   *First example:*

   This example is described in Section 6.13.1.

   CBOR diagnostic notation: "/ietf-system:system/contact"

   CBOR encoding:

   78 1B 2F696574662D73797374656D3A73797374656D2F636F6E74616374

   *Second example:*

   This example is described in Section 6.13.1.

   CBOR diagnostic notation (the line break is inserted for exposition
   only):

   "/ietf-system:system/authentication/user[name='bob']/
   authorized-key[name='admin'][country='france']/key-data"

   CBOR encoding:

   78 6B
      2F696574662D73797374656D3A73797374656D2F61757468656E74696361
      74696F6E2F757365725B6E616D653D27626F62275D2F617574686F72697A
      65642D6B65795B6E616D653D2761646D696E275D5B636F756E7472793D27
      6672616E6365275D2F6B65792D64617461

   *Third example:*

   This example is described in Section 6.13.1.

   CBOR diagnostic notation:

   "/ietf-system:system/authentication/user[name='jack']"

   CBOR encoding:

   78 34                                   # text(52)
      2F696574662D73797374656D3A73797374656D2F61757468656E74696361
      74696F6E2F757365725B6E616D653D276A61636B275D

7.  Content-Types

   This specification defines the media type application/yang-data+cbor,
   which can be used without parameters or with the id parameter set to
   either name or sid.

   This media type represents a YANG-CBOR document containing a
   representation tree.  If the media type parameter id is present,
   depending on its value, each representation node is identified by its
   associated namespace-qualified name, as defined in Section 3.3
   (id=name), or by its associated YANG SID (represented, e.g., in CBOR
   map keys as a SID delta or via tag number 47), as defined in
   Section 3.2 (id=sid), respectively.  If no id parameter is given,
   both forms may be present.

   The format of an application/yang-data+cbor representation is that of
   a CBOR map, mapping names, and/or SIDs (as defined above) into
   instance values (using the rules defined in Section 4).

   It is not foreseen at this point that the valid set of values for the
   id parameter will extend beyond name, sid, or being unset; if that
   does happen, any new value is foreseen to be of the form
   [a-z][a-z0-9]*(-[a-z0-9]+)*.

   In summary, this document defines three content-types, which are
   intended for use by different classes of applications:

   *  application/yang-data+cbor; id=sid -- for use by applications that
      need to be frugal with encoding space and text string processing
      (e.g., applications running on constrained nodes [RFC7228] or
      applications with particular performance requirements);

   *  application/yang-data+cbor; id=name -- for use by applications
      that do not want to engage in SID management and that have ample
      resources to manage text-string-based item identifiers (e.g.,
      applications that directly want to substitute application/
      yang.data+json with a more efficient representation without any
      other changes); and

   *  application/yang-data+cbor -- for use by more complex applications
      that can benefit from the increased efficiency of SID identifiers
      but also need to integrate databases of YANG modules before SID
      mappings are defined for them.

   All three content-types are based on the same representation
   mechanisms, parts of which are simply not used in the first and
   second cases.

   How the use of one of these content-types is selected in a transfer
   protocol is outside the scope of this specification.  The last
   paragraph of Section 5.2 of [RFC8040] discusses how to indicate and
   request the usage of specific content-types in RESTCONF.  Similar
   mechanisms are available in the Constrained Application Protocol
   (CoAP) [RFC7252] using the Content-Format and Accept Options;
   [CORE-COMI] demonstrates specifics on how Content-Format may be used
   to indicate the id=sid case.

8.  Security Considerations

   The security considerations of [RFC8949] and [RFC7950] apply.

   This document defines an alternative encoding for data modeled in the
   YANG data modeling language.  As such, this encoding does not
   contribute any new security issues in addition to those identified
   for the specific protocol or context for which it is used.

   To minimize security risks, software on the receiving side SHOULD
   reject all messages that do not comply to the rules of this document
   and reply with an appropriate error message to the sender.

   For instance, when the id parameter to the media type is used, it is
   important to properly reject identifiers of the other type to avoid
   scenarios where different implementations interpret a given content
   in different ways.

   When SIDs are in use, the interpretation of encoded data not only
   relies on having the right YANG modules but also on having the right
   SID mapping information.  Management and evolution of that mapping
   information therefore requires the same care as the management and
   evolution of the YANG modules themselves.  The procedures in
   [CORE-SID] are being defined with this in mind.

9.  IANA Considerations

9.1.  Media Types Registry

   IANA has added the following media type to the "Media Types" registry
   [IANA.media-types].

   +================+============================+===========+
   | Name           | Template                   | Reference |
   +================+============================+===========+
   | yang-data+cbor | application/yang-data+cbor | RFC 9254  |
   +----------------+----------------------------+-----------+

                  Table 2: Media Types Registry

   Type name:  application

   Subtype name:  yang-data+cbor

   Required parameters:  N/A

   Optional parameters:  id (see Section 7 of RFC 9254)

   Encoding considerations:  binary (CBOR)

   Security considerations:  see Section 8 of RFC 9254

   Interoperability considerations:  N/A

   Published specification:  RFC 9254

   Applications that use this media type:  applications that need a
      concise and efficient representation of YANG-modeled data

   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers specified for "application/yang-data+cbor" is
      as specified for "application/cbor".  (At publication of this
      document, there is no fragment identification syntax defined for
      "application/cbor".)

   Additional information:

   Magic number(s):  N/A

   File extension(s):  N/A

   Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  CORE WG
      mailing list (core@ietf.org) or IETF Applications and Real-Time
      Area (art@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  CoRE WG

   Change controller:  IETF

9.2.  CoAP Content-Formats Registry

   IANA has added the following Content-Formats to the "CoAP
   Content-Formats" subregistry, within the "Constrained RESTful
   Environments (CoRE) Parameters" registry [IANA.core-parameters].  The
   registration procedure is "Expert Review" for the 0-255 range and
   "IETF Review" for the 256-9999 range.

   +=====================================+==========+=====+===========+
   | Media Type                          | Encoding | ID  | Reference |
   +=====================================+==========+=====+===========+
   | application/yang-data+cbor          | -        | 340 | RFC 9254  |
   +-------------------------------------+----------+-----+-----------+
   | application/yang-data+cbor; id=name | -        | 341 | RFC 9254  |
   +-------------------------------------+----------+-----+-----------+
   | application/yang-data+cbor; id=sid  | -        | 140 | RFC 9254  |
   +-------------------------------------+----------+-----+-----------+

                  Table 3: CoAP Content-Format Registry

9.3.  CBOR Tags Registry

   IANA has allocated the following CBOR tag numbers in the "CBOR Tags"
   registry [IANA.cbor-tags] defined in Section 9.2 of [RFC8949].

    +=====+==================+============================+===========+
    | Tag | Data Item        | Semantics                  | Reference |
    +=====+==================+============================+===========+
    | 43  | text string      | YANG bits datatype; see    | RFC 9254  |
    |     |                  | Section 6.7.               |           |
    +-----+------------------+----------------------------+-----------+
    | 44  | text string      | YANG enumeration datatype; | RFC 9254  |
    |     |                  | see Section 6.6.           |           |
    +-----+------------------+----------------------------+-----------+
    | 45  | unsigned integer | YANG identityref datatype; | RFC 9254  |
    |     | or text string   | see Section 6.10.          |           |
    +-----+------------------+----------------------------+-----------+
    | 46  | unsigned integer | YANG instance-identifier   | RFC 9254  |
    |     | or text string   | datatype; see              |           |
    |     | or array         | Section 6.13.              |           |
    +-----+------------------+----------------------------+-----------+
    | 47  | unsigned integer | YANG Schema Item           | RFC 9254  |
    |     |                  | iDentifier (SID); see      |           |
    |     |                  | Section 3.2.               |           |
    +-----+------------------+----------------------------+-----------+

                        Table 4: CBOR Tags Registry

10.  References

10.1.  Normative References

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <https://www.iana.org/assignments/cbor-tags>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters/>.

   [IANA.media-types]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types/>.

   [RFC2119]  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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <https://www.rfc-editor.org/info/rfc7951>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8174]  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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8791]  Bierman, A., Björklund, M., and K. Watsen, "YANG Data
              Structure Extensions", RFC 8791, DOI 10.17487/RFC8791,
              June 2020, <https://www.rfc-editor.org/info/rfc8791>.

   [RFC8949]  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>.

10.2.  Informative References

   [CORE-COMI]
              Veillette, M., Ed., van der Stok, P., Ed., Pelov, A.,
              Bierman, A., and I. Petrov, Ed., "CoAP Management
              Interface (CORECONF)", Work in Progress, Internet-Draft,
              draft-ietf-core-comi-11, 17 January 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              comi-11>.

   [CORE-SID] Veillette, M., Ed., Pelov, A., Ed., Petrov, I., Ed.,
              Bormann, C., and M. Richardson, "YANG Schema Item
              iDentifier (YANG SID)", Work in Progress, Internet-Draft,
              draft-ietf-core-sid-18, 18 November 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              sid-18>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <https://www.rfc-editor.org/info/rfc7317>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [RFC8344]  Bjorklund, M., "A YANG Data Model for IP Management",
              RFC 8344, DOI 10.17487/RFC8344, March 2018,
              <https://www.rfc-editor.org/info/rfc8344>.

Acknowledgments

   This document has been largely inspired by the extensive work done by
   Andy Bierman and Peter van der Stok on [CORE-COMI].  [RFC7951] has
   also been a critical input to this work.  The authors would like to
   thank the authors and contributors of these two documents.

   The authors would also like to acknowledge the review, feedback, and
   comments from Ladislav Lhotka and Jürgen Schönwälder and from the
   Document Shepherd Marco Tiloca.  Extensive comments helped us further
   improve the document in the IESG review process; the authors would
   like to call out specifically the feedback and guidance by the
   responsible AD Francesca Palombini and the significant improvements
   suggested by IESG members Benjamin Kaduk and Rob Wilton.

Authors' Addresses

   Michel Veillette (editor)
   Trilliant Networks Inc.
   610 Rue du Luxembourg
   Granby Quebec J2J 2V2
   Canada
   Email: michel.veillette@trilliantinc.com

   Ivaylo Petrov (editor)
   Google Switzerland GmbH
   Brandschenkestrasse 110
   CH-8002 Zurich
   Switzerland
   Email: ivaylopetrov@google.com

   Alexander Pelov
   Acklio
   1137A avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France
   Email: a@ackl.io

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

   Michael Richardson
   Sandelman Software Works
   Canada
   Email: mcr+ietf@sandelman.ca