CBOR Encoding of Data Modeled with YANG
draft-ietf-core-yang-cbor-16

Document Type Active Internet-Draft (core WG)
Authors Michel Veillette  , Ivaylo Petrov  , Alexander Pelov  , Carsten Bormann 
Last updated 2021-07-20 (latest revision 2021-06-24)
Replaces draft-veillette-core-yang-cbor-mapping
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Carsten Bormann  for 92 days
Francesca Palombini  for 105 days
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Michel Veillette  for 92 days
Ivaylo Petrov  for 92 days
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Internet Engineering Task Force                        M. Veillette, Ed.
Internet-Draft                                   Trilliant Networks Inc.
Intended status: Standards Track                          I. Petrov, Ed.
Expires: 27 December 2021                        Google Switzerland GmbH
                                                                A. Pelov
                                                                  Acklio
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                            25 June 2021

                CBOR Encoding of Data Modeled with YANG
                      draft-ietf-core-yang-cbor-16

Abstract

   This document defines encoding rules for serializing configuration
   data, state data, RPC input and RPC output, action input, action
   output, notifications and the yang-data extension defined within YANG
   modules using the Concise Binary Object Representation (CBOR, RFC
   8949).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 December 2021.

Copyright Notice

   Copyright (c) 2021 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

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Properties of the CBOR Encoding . . . . . . . . . . . . . . .   5
     3.1.  CBOR diagnostic notation  . . . . . . . . . . . . . . . .   6
     3.2.  YANG Schema Item iDentifier . . . . . . . . . . . . . . .   8
     3.3.  Name  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Encoding of YANG Schema Node Instances  . . . . . . . . . . .  10
     4.1.  The 'leaf'  . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  10
       4.1.2.  Using names in keys . . . . . . . . . . . . . . . . .  11
     4.2.  The 'container' and other nodes from the data tree  . . .  11
       4.2.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  12
       4.2.2.  Using names in keys . . . . . . . . . . . . . . . . .  13
     4.3.  The 'leaf-list' . . . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  14
       4.3.2.  Using names in keys . . . . . . . . . . . . . . . . .  15
     4.4.  The 'list' and 'list' entries . . . . . . . . . . . . . .  15
       4.4.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  16
       4.4.2.  Using names in keys . . . . . . . . . . . . . . . . .  18
     4.5.  The 'anydata' . . . . . . . . . . . . . . . . . . . . . .  20
       4.5.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  21
       4.5.2.  Using names in keys . . . . . . . . . . . . . . . . .  22
     4.6.  The 'anyxml'  . . . . . . . . . . . . . . . . . . . . . .  23
       4.6.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . .  23
       4.6.2.  Using names in keys . . . . . . . . . . . . . . . . .  24
   5.  Encoding of 'yang-data' extension . . . . . . . . . . . . . .  24
     5.1.  Using SIDs in keys  . . . . . . . . . . . . . . . . . . .  25
     5.2.  Using names in keys . . . . . . . . . . . . . . . . . . .  26
   6.  Representing YANG Data Types in CBOR  . . . . . . . . . . . .  27
     6.1.  The unsigned integer Types  . . . . . . . . . . . . . . .  27
     6.2.  The integer Types . . . . . . . . . . . . . . . . . . . .  28
     6.3.  The 'decimal64' Type  . . . . . . . . . . . . . . . . . .  28
     6.4.  The 'string' Type . . . . . . . . . . . . . . . . . . . .  29
     6.5.  The 'boolean' Type  . . . . . . . . . . . . . . . . . . .  29
     6.6.  The 'enumeration' Type  . . . . . . . . . . . . . . . . .  30
     6.7.  The 'bits' Type . . . . . . . . . . . . . . . . . . . . .  31
     6.8.  The 'binary' Type . . . . . . . . . . . . . . . . . . . .  33
     6.9.  The 'leafref' Type  . . . . . . . . . . . . . . . . . . .  33
     6.10. The 'identityref' Type  . . . . . . . . . . . . . . . . .  34
       6.10.1.  SIDs as identityref  . . . . . . . . . . . . . . . .  34
       6.10.2.  Name as identityref  . . . . . . . . . . . . . . . .  35

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     6.11. The 'empty' Type  . . . . . . . . . . . . . . . . . . . .  35
     6.12. The 'union' Type  . . . . . . . . . . . . . . . . . . . .  36
     6.13. The 'instance-identifier' Type  . . . . . . . . . . . . .  37
       6.13.1.  SIDs as instance-identifier  . . . . . . . . . . . .  37
       6.13.2.  Names as instance-identifier . . . . . . . . . . . .  40
   7.  Content-Types . . . . . . . . . . . . . . . . . . . . . . . .  42
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  42
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
     9.1.  Media-Types Registry  . . . . . . . . . . . . . . . . . .  42
     9.2.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  43
     9.3.  CBOR Tags Registry  . . . . . . . . . . . . . . . . . . .  44
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  44
     10.2.  Informative References . . . . . . . . . . . . . . . . .  45
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  46
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

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].  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.

   The following terms are defined in [RFC7950]:

   *  action

   *  anydata

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   *  anyxml

   *  data node

   *  data tree

   *  datastore

   *  feature

   *  identity

   *  module

   *  notification

   *  RPC

   *  schema node

   *  schema tree

   *  submodule

   The following terms are defined in [RFC8040]:

   *  yang-data extension

   This specification also makes use of the following terminology:

   *  child: A schema node defined as a child node of a container, a
      list, a case, a notification, an RPC input, an RPC output, an
      action input, or an action output.

   *  YANG Schema Item iDentifier (YANG SID or simply SID): 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.

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

   *  list entry: the data associated with a single element of a list.

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   *  parent: The container, list, case, notification, RPC input, RPC
      output, action input or action output node in which a schema node
      is defined.

3.  Properties of the CBOR Encoding

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

   An instance of a schema node such as container, list, notification,
   RPC input, RPC output, action input, or action output is serialized
   using a CBOR map in which each child schema node 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 which allows their interpretation during
   the deserialization process.  Protocols or mechanisms implementing
   this specification can mandate the use of a specific key type.

   In order to minimize the size of the encoded data, the proposed
   mapping avoids any unnecessary meta-information beyond those natively
   supported by CBOR.  For instance, CBOR tags are used solely in the
   case of a SID not encoded as delta, anyxml schema nodes, or the union
   datatype, to distinguish explicitly 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 lengths encoding as defined in
   Section 3.2 of [RFC8949] SHALL be supported by CBOR decoders.

   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 schema nodes are serialized using the rules defined by this
   specification as part of an application payload, the payload SHOULD
   include information that would allow a stateless way to identify each
   node, such as the SID number associated with the node, 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.

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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.

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      +==========+======+====================+===========+==========+
      | 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 [RFC8610] Appendix G.6.

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3.2.  YANG Schema Item iDentifier

   Some of the items defined in YANG [RFC7950] require the use of a
   unique identifier.  In both Network Configuration Protocol (NETCONF)
   [RFC6241] and RESTCONF [RFC8040], these identifiers are implemented
   using 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.  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)

   *  notifications and associated information

   *  YANG modules, submodules, and features

   To minimize their size, SIDs used as keys in inner CBOR maps are
   typically encoded using deltas.  Conversion from SIDs to deltas and
   back to SIDs are stateless processes solely based on the data
   serialized or deserialized.  These SIDs may also be encoded as
   absolute number when enclosed by CBOR tag 47.

   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.
   One example for such a specification is [I-D.ietf-core-sid].

3.3.  Name

   This specification also supports the encoding of YANG item
   identifiers as 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:

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   *  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 data node
   and its parent node are different.  In all other cases, the simple
   form of the name SHOULD 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:

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   {
     "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 simple name.

4.  Encoding of YANG Schema Node Instances

   Schema node instances 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 shows the encoding of a 'hostname' leaf using
   a SID or a name.

   Definition example from [RFC7317]:

   typedef domain-name {
     type string {
       length "1..253";
       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]\.?
           )|\.';
     }
   }

   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:

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   {
     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, lists, notification contents, RPC inputs,
   RPC outputs, action inputs, and action outputs schema nodes MUST be
   encoded using a CBOR map data item (major type 5).  A map is
   comprised 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, each value is set to the value of this schema node
   instance according to the instance datatype.

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

   The following examples shows the encoding of a 'system-state'
   container schema node instance using SIDs or names.

   Definition example from [RFC7317]:

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   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 or SIDs.
   In the case of deltas, they MUST be encoded using a CBOR unsigned
   integer (major type 0) or CBOR negative integer (major type 1),
   depending on the actual delta value.  In the case of SID, they are
   encoded using the SID value enclosed by CBOR tag 47 as defined in
   Section 9.3.

   Delta values are computed as follows:

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

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

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

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   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 schema 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
   schema 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"
       }
     }
   }

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   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
   schema node instance containing two entries, "ietf.org" and
   "ieee.org".

   Definition example [RFC7317]:

   typedef domain-name {
     type string {
       length "1..253";
       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]\.?
                )|\.';
     }
   }

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

4.3.1.  Using SIDs in keys

   CBOR diagnostic notation:

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   {
     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 '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 collection as defined in Section 4.2.

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

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

   Definition example from [RFC7317]:

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   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.
   Deltas of list members are equal to the SID of the current schema
   node minus the SID of the 'list'.

   CBOR diagnostic notation:

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   {
     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:

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   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:

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   {
     "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:

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   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 serves as a container for an arbitrary set of schema nodes
   that otherwise appear as normal YANG-modeled data.  An anydata schema
   node instance is encoded using the same rules as a container, i.e.,
   CBOR map.  The requirement that anydata content can be modeled by
   YANG implies the following:

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   *  CBOR map keys of any inner schema nodes MUST be set to valid
      deltas or names.

   *  The CBOR array MUST contain either unique scalar values (as a
      leaf-list, see Section 4.3), or maps (as a list, see Section 4.4).

   *  CBOR map values MUST follow the encoding rules of one of the
      datatypes listed in Section 4.

   The following example shows a possible use of an anydata.  In this
   example, an anydata is used to define a schema node containing a
   notification event; this schema 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:

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   {
     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)
            18 4E                   # unsigned(78)
            66                      # text(6)
               302F342F3231         # "0/4/21"
            18 4F                   # unsigned(79)
            6A                      # text(10)
               4F70656E2070696E2032 # "Open pin 2"

   In some implementations, it might be simpler to use the absolute SID
   tag encoding for the anydata root element.  The resulting encoding is
   as follows:

   {
     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"
       }
     }
   }

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   CBOR encoding:

   A1                                      # map(1)
      74                                   # text(20)
         6576656E742D6C6F673A6C6173742D6576656E74
      A1                                   # map(1)
         78 20                             # text(32)
            6578616D706C652D706F72743A206578616D7
            06C652D706F72742D6661756C74
         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 schema node is used to serialize an arbitrary CBOR content,
   i.e., its value can be any CBOR binary object. 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 schema node
   instance consisting of a CBOR array containing the CBOR simple values
   'true', 'null' and 'true'.

   Definition example 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:

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   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 '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 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 from [I-D.ietf-core-comi] Appendix A:

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   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 [I-D.ietf-core-comi] using SIDs as
   defined in Section 3.2.

   CBOR diagnostic notation:

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   {
     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

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 3.3.

   This example shows a serialization example of the yang-errors yang-
   data extension as defined in [I-D.ietf-core-comi] using names as
   defined Section 3.3.

   CBOR diagnostic notation:

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   {
     "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

6.  Representing YANG Data Types in CBOR

   The CBOR encoding of an instance of a leaf or leaf-list schema node
   depends on the built-in type of that schema node.  The following sub-
   section 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 schema 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).

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

   Definition example 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 CBOR unsigned integer (major type 0) or CBOR negative integer
   (major type 1), depending on the actual value.

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

   Definition example 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
   schema node instance set to 2.57.

   Definition example from [RFC7317]:

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   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 schema node
   instance set to "eth0".

   Definition example 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 schema
   node instance set to 'true'.

   Definition example from [RFC7317]:

   leaf enabled {
     type boolean;
   }

   CBOR diagnostic notation: true

   CBOR encoding: F5

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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.  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
   schema node instance set to 'testing'.

   Definition example 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.  The
   encoding MUST be enclosed by the enumeration CBOR tag as specified in
   Section 9.3.

   Definition example from [RFC7950]:

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

   CBOR diagnostic notation: 44("unbounded")

   CBOR encoding: D8 2C 69 756E626F756E646564

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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 or byte
   string (major type 2).  In case CBOR array representation is used,
   each element is either 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 a byte string (major type 2) that carries the
   information 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 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 at the
   end of the byte string are removed.  An example follows.

   The following example shows the encoding of an 'alarm-state' leaf
   schema node instance with the 'critical' (position 3), 'warning'
   (position 8) and 'indeterminate' (position 128) flags set.

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   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 small number of bytes inside.  For this case, it
   is expected 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 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.

   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.  The
   encoding MUST be enclosed by the bits CBOR tag as specified in
   Section 9.3.

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   The following example shows the encoding of an 'alarm-state' leaf
   schema 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
   schema 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 schema
   node referenced by the 'path' YANG statement.

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

   Definition example from [RFC8343]:

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   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].

6.10.1.  SIDs as identityref

   When schema nodes of type identityref are implemented using SIDs,
   they MUST be encoded using a CBOR unsigned integer data item (major
   type 0).  (Note that no delta mechanism is employed for SIDs used for
   identityref.)

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

   Definition example from [RFC7317]:

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   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 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
   schema node instance when present.

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   Definition example 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 Section 6.6 and in Section 6.7, 'enumeration' and
   'bits' are encoded as a CBOR text string data item (major type 3)
   when defined within a 'union' type.

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

   Definition example from [RFC7317]:

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   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.

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.

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   In the case of a schema node member of a YANG list, a SID is combined
   with the list key(s) to identify each instance within the YANG
   list(s).

   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.

   Schema node members 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 of the subordinate
      list(s).

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

   Definition example 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 from [RFC7317]:

   container system {

     leaf contact {
       type string;
     }

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

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   CBOR diagnostic notation: 1741

   CBOR encoding: 19 06CD

   *Second example:*

   The following example shows the encoding of the 'reporting-entity'
   value referencing list instance "/system/authentication/user/
   authorized-key/key-data" (SID 1734) for user name "bob" and
   authorized-key "admin".

   Definition example from [RFC7317]:

   list user {
     key name country;

     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:

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   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 user name "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 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, and
      the simple form is used otherwise.  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".

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   The resulting 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 1c 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']/key-data"

   CBOR encoding:

   78 59
      2F696574662D73797374656D3A73797374656D2F61757468656E74696361
      74696F6E2F757365725B6E616D653D27626F62275D2F617574686F72697A
      65642D6B65795B6E616D653D2761646D696E275D2F6B65792D64617461

   *Third example:*

   This example is described in Section 6.13.1.

   CBOR diagnostic notation:

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

   CBOR encoding:

   78 33
      2F696574662D73797374656D3A73797374656D2F61757468656E74696361
      74696F6E2F757365725B6E616D653D27626F62275D

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7.  Content-Types

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

   This media-type represents a CBOR YANG document containing one or
   multiple data node values.  Depending on the presence and value of
   the media-type parameter "id", each data node is identified by its
   associated namespace qualified name as defined in Section 3.3
   ("id=name"), by its associated YANG SID (represented as a SID delta
   or via tag 47) as defined in Section 3.2 ("id=sid"), or either of
   these (no "id" parameter given).

   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]+)*".

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.

9.  IANA Considerations

9.1.  Media-Types Registry

   This document adds the following Media-Type to the "Media Types"
   registry.

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

                             Table 2

   // RFC Ed.: please replace RFC XXXX with this RFC number and remove
   this note.

   Type name:  application
   Subtype name:  yang-data+cbor
   Required parameters:  none
   Optional parameters:  id (see Section 7 of RFC XXXX)
   Encoding considerations:  binary (CBOR)
   Security considerations:  see Section 8 of RFC XXXX
   Published specification:  RFC XXXX
   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:  none
   Author/Change controller:  IETF

9.2.  CoAP Content-Formats Registry

   This document adds the following Content-Format to the "CoAP Content-
   Formats", within the "Constrained RESTful Environments (CoRE)
   Parameters" registry, where TBD3 comes from the "Expert Review" 0-255
   range and TBD1 and TBD2 come from the "IETF Review" 256-9999 range.

   +============================+================+======+===========+
   | Content Type               | Content Coding | ID   | Reference |
   +============================+================+======+===========+
   | application/yang-data+cbor | -              | TBD1 | RFC XXXX  |
   +----------------------------+----------------+------+-----------+
   | application/yang-          | -              | TBD2 | RFC XXXX  |
   | data+cbor; id=name         |                |      |           |
   +----------------------------+----------------+------+-----------+
   | application/yang-          | -              | TBD3 | RFC XXXX  |
   | data+cbor; id=sid          |                |      |           |
   +----------------------------+----------------+------+-----------+

                                Table 3

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   // RFC Ed.: please replace TBDx with assigned IDs, remove the
   requested ranges, and remove this note.
   // RFC Ed.: please replace RFC XXXX with this RFC number and remove
   this note.

9.3.  CBOR Tags Registry

   This specification requires the assignment of CBOR tags for the
   following YANG datatypes.  These tags are added to the CBOR Tags
   Registry as defined in Section 9.2 of [RFC8949].

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

                                 Table 4

   // RFC Ed.: please replace RFC XXXX with RFC number and remove this
   note

10.  References

10.1.  Normative References

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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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

   [I-D.ietf-core-comi]
              Veillette, M., Stok, P. V. D., Pelov, A., Bierman, A., and
              I. Petrov, "CoAP Management Interface (CORECONF)", Work in
              Progress, Internet-Draft, draft-ietf-core-comi-11, 17
              January 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-comi-11.txt>.

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   [I-D.ietf-core-sid]
              Veillette, M., Pelov, A., and I. Petrov, "YANG Schema Item
              iDentifier (YANG SID)", Work in Progress, Internet-Draft,
              draft-ietf-core-sid-15, 17 January 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-sid-
              15.txt>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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 works done
   by Andy Bierman and Peter van der Stok on [I-D.ietf-core-comi].
   [RFC7951] has also been a critical input to this work.  The authors
   would like to thank the authors and contributors to these two drafts.

   The authors would also like to acknowledge the review, feedback, and
   comments from Ladislav Lhotka and Jürgen Schönwälder.

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Internet-Draft   CBOR Encoding of Data Modeled with YANG       June 2021

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
   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

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