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

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.¶

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):¶

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:¶

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

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.¶

4.1. 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;
}

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

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

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

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;
    }
  }
}

{
  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) /
    }
  }
}

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

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

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;
}

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

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

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

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;
  }
}

{
  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) /
      }
    }
  ]
}

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"

{
  "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"
      }
    }
  ]
}

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;
    }
  }
}

{
  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) /
    }
  }
}

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"

{
  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) /
    }
  }
}

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

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
}

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

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

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

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

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"

6.1. 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.¶

identity interface-type {
}

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

identity ethernetCsmacd {
  base iana-interface-type;
}

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

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.¶

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.¶

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

container system {

  leaf contact {
    type string;
  }

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

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;
    }
  }
}

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

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

78 1B 2F696574662D73797374656D3A73797374656D2F636F6E74616374

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

78 6B
   2F696574662D73797374656D3A73797374656D2F61757468656E74696361
   74696F6E2F757365725B6E616D653D27626F62275D2F617574686F72697A
   65642D6B65795B6E616D653D2761646D696E275D5B636F756E7472793D27
   6672616E6365275D2F6B65792D64617461

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

78 34                                   # text(52)
   2F696574662D73797374656D3A73797374656D2F61757468656E74696361
   74696F6E2F757365725B6E616D653D276A61636B275D

9.1. Media Types Registry

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

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.¶

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].¶