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Applications and Real-Time Area (art) Internet Engineering Task Force CBOR Based on the Concise Binary Object Representation (CBOR, RFC 8949), this document defines encoding rules for representing configuration data, state data, parameters and results of Remote Procedure Call (RPC) operations or actions, and notifications, defined using YANG (RFC 7950).

Introduction The specification of the YANG 1.1 data modeling language 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 based on the JavaScript Object Notation (JSON) Data Interchange Format . The aim of this document is to define a set of encoding rules for the Concise Binary Object Representation (CBOR) , collectively called YANG-CBOR. The resulting encoding is more compact compared to XML and JSON and more suitable for Constrained Nodes and/or Constrained Networks as defined by .

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 when, and only when, they appear in all capitals, as shown here. The following terms are defined in :

• action
• anydata
• anyxml
• data node
• data tree
• datastore
• feature
• identity
• module
• notification
• RPC
• schema node
• submodule

The following term is defined in :

• yang-data extension

The following term is defined in :

• YANG data structure

This specification also makes use of the following terminology:

• YANG Schema Item iDentifier (YANG SID or simply SID): 63-bit unsigned integer used to identify different YANG items.
• delta: Difference between the current YANG SID and a reference YANG SID. A reference YANG SID is defined for each context for which deltas are used.
• absolute SID: YANG SID not encoded as a delta. This is usually called out explicitly only in positions where normally a delta would be found.
• representation tree: a YANG data tree, possibly enclosed by a representation of a schema node such as a YANG data structure, a notification, an RPC, or an action.
• representation node: a node in a representation tree, i.e., a data tree node, or a representation of a schema node such as a YANG data structure, a notification, an RPC, or an action.
• item: A schema node, an identity, a module, or a feature defined using the YANG modeling language.
• list entry: the data associated with a single entry of a list (see ).
• parent (of a representation node): the schema node of the closest enclosing representation node in which a given representation node is defined.

Properties of the CBOR Encoding This document defines CBOR encoding rules for YANG data trees and their subtrees. A YANG data tree can be enclosed by a representation of a schema node such as a YANG data structure, a notification, an RPC, or an action; this is called a representation tree. The data tree nodes and the enclosing schema node representation, if any, are collectively called the representation nodes. A representation node such as container, list entry, YANG data structure, notification, RPC input, RPC output, action input, or action output is serialized using a CBOR map in which each schema node defined within is encoded using a key and a value. This specification supports two types of CBOR keys; YANG Schema Item iDentifier (YANG SID) as defined in and names as defined in . 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 or allow the generator to choose freely per key. In order to minimize the size of the encoded data, the mapping avoids any unnecessary meta-information beyond that directly provided by the CBOR basic generic data model (). For instance, CBOR tags are used solely in the case of an absolute SID, anyxml data 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 length encoding as defined in SHALL be supported by the CBOR decoders employed with YANG-CBOR. (This enables an implementation to begin emitting an array or map before the number of entries in that structure is known, possibly also avoiding excessive locking or race conditions. On the other hand, it deprives the receiver of the encoded data from advance announcement about some size information, so a generator should choose indefinite length encoding only when these benefits do accrue.) Data nodes implemented using a CBOR array, map, byte string, or text string can be instantiated but empty. In this case, they are encoded with a length of zero. When representation nodes are serialized using the rules defined by this specification as part of an application payload, the payload SHOULD include information that would allow 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 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.

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

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

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

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

• identities
• data nodes
• RPCs and associated input(s) and output(s)
• actions and associated input(s) and output(s)
• YANG data structures
• notifications and associated information
• YANG modules and features

Note that any structuring of modules into submodules is transparent to YANG-CBOR: SIDs are not allocated for the names of submodules, and any items within a submodule are effectively allocated SIDs as part of processing the module that includes them. To minimize their size, SIDs used as keys in CBOR maps are encoded using deltas, i.e., signed (negative or unsigned) integers that are added to the reference SID applying to the map. The reference SID of an outermost map is zero, unless a different reference SID is unambiguously conferred from the environment in which the outermost map is used. The reference SID of a map that is most directly embedded in a map entry with a name-based key is zero. For all other maps, the reference SID is the SID computed for the map entry it is most directly embedded in. (The embedding may be indirect if an array intervenes, e.g., in a YANG list.) Where absolute SIDs are desired in map key positions (where a bare integer implies a delta), they need to be identified as absolute SID values by using CBOR tag number 47 (as defined in ). 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, , 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.

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 . This approach can be used to avoid the management overhead associated with SID allocation. The main drawback is the significant increase in size of the encoded data. YANG item identifiers implemented using names MUST be in one of the following forms:

• simple -- the identifier of the YANG item (i.e., schema node or identity).
• namespace qualified -- the identifier of the YANG item is prefixed with the name of the module in which this item is defined, separated by the colon character (":").

The name of a module determines the namespace of all YANG items defined in that module. If an item is defined in a submodule, then the namespace qualified name uses the name of the main module to which the submodule belongs. ABNF syntax of a name is shown in , where the production for "identifier" is defined in .

ABNF Production for a simple or namespace qualified name

A namespace qualified name MUST be used for all members of a top-level CBOR map and then also whenever the namespaces of the representation node and its parent node are different. In all other cases, the simple form of the name MUST be used. Definition example: A valid CBOR encoding of the 'top' container is as follows. CBOR diagnostic notation: 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.

Encoding of Representation Nodes Representation nodes defined using the YANG modeling language are encoded using CBOR based on the rules defined in this section. We assume that the reader is already familiar with both YANG and CBOR .

The 'leaf' A 'leaf' MUST be encoded accordingly to its datatype using one of the encoding rules specified in . The following examples show the encoding of a 'hostname' leaf using a SID or a name. Definition example adapted from and :

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: CBOR encoding:

Using names in keys CBOR diagnostic notation: CBOR encoding:

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 (). 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, 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 and names as defined in . The following examples show the encoding of a 'system-state' container representation instance using SIDs or names. Definition example adapted from and :

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

• In the case of a 'container', deltas are equal to the SID of the current representation node minus the SID of the parent 'container'.
• In the case of a 'list', deltas are equal to the SID of the current representation node minus the SID of the parent 'list'.
• In the case of an 'RPC input' or 'RPC output', deltas are equal to the SID of the current representation node minus the SID of the 'RPC'.
• In the case of an 'action input' or 'action output', deltas are equal to the SID of the current representation node minus the SID of the 'action'.
• In the case of a 'notification content', deltas are equal to the SID of the current representation node minus the SID of the 'notification'.

CBOR diagnostic notation: CBOR encoding:

System state clock encoding

Using names in keys CBOR map keys implemented using names MUST be encoded using a CBOR text string data item (major type 3). A namespace-qualified name MUST be used each time the namespace of a representation node and its parent differ. In all other cases, the simple form of the name MUST be used. Names and namespaces are defined in . The following example shows the encoding of a 'system' container representation node instance using names. CBOR diagnostic notation: CBOR encoding:

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 . 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 and :

Using SIDs in keys CBOR diagnostic notation: CBOR encoding:

Using names in keys CBOR diagnostic notation: CBOR encoding:

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 . 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 simplified from :

Using SIDs in keys The encoding rules of each 'list' entry are defined in . CBOR diagnostic notation: CBOR encoding:

Using names in keys The encoding rules of each 'list' entry are defined in . CBOR diagnostic notation: CBOR encoding:

The 'anydata' An anydata 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., CBOR map. The requirement that anydata content can be modeled by YANG implies the following:

• CBOR map keys of any inner representation nodes MUST be set to valid deltas or names.
• CBOR arrays MUST contain either unique scalar values (as a leaf-list, see ), or maps (as a list, see ).
• CBOR map values MUST follow the encoding rules of one of the datatypes listed in .

The following example shows a possible use of an anydata. In this example, an anydata 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: This example also assumes the assistance of the following notification.

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

Using names in keys CBOR diagnostic notation: CBOR encoding:

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 .) An anyxml value MAY contain CBOR data items tagged with one of the tags listed in . The tags listed in 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 from :

Using SIDs in keys CBOR diagnostic notation: CBOR encoding:

Using names in keys CBOR diagnostic notation: CBOR encoding:

Encoding of 'yang-data' extension The yang-data extension is used to define data structures in YANG that are not intended to be implemented as part of a datastore. The yang-data extension will specify a container that MUST be encoded using the encoding rules of nodes of data trees as defined in . Just like YANG containers, the yang-data extension can be encoded using either SIDs or names. Definition example from Appendix A:

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 . This example shows a serialization example of the yang-errors yang-data extension as defined in using SIDs as defined in . CBOR diagnostic notation: CBOR encoding:

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 . This example shows a serialization example of the yang-errors yang-data extension as defined in using names as defined . CBOR diagnostic notation: CBOR encoding:

Representing YANG Data Types in CBOR The CBOR encoding of an instance of a leaf or leaf-list representation node depends on the built-in type of that representation node. The following sub-section defines the CBOR encoding of each built-in type supported by YANG as listed in . Each subsection shows an example value assigned to a representation node instance of the discussed built-in type.

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 from : CBOR diagnostic notation: 1280 CBOR encoding: 19 0500

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 representation node instance set to -300 minutes. Definition example from : CBOR diagnostic notation: -300 CBOR encoding: 39 012B

The 'decimal64' Type Leafs of type decimal64 MUST be encoded using a decimal fraction as defined in . The following example shows the encoding of a 'my-decimal' leaf representation node instance set to 2.57. Definition example from : CBOR diagnostic notation: 4([-2, 257]) CBOR encoding: C4 82 21 19 0101

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 from : CBOR diagnostic notation: "eth0" CBOR encoding: 64 65746830

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 from : CBOR diagnostic notation: true CBOR encoding: F5

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 . The following example shows the encoding of an 'oper-status' leaf representation node instance set to 'testing'. Definition example from : 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 ). The encoding MUST be enclosed by the enumeration CBOR tag as specified in . Definition example from : CBOR diagnostic notation: 44("unbounded") CBOR encoding: D8 2C 69 756E626F756E646564

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 , 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), or exceptionally as a tagged text string (see below). 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 (zero bytes) at the end of the byte string are never generated: If they would 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 adjusted upwards by the number of zero bytes removed. An example follows. The following example shows the encoding of an 'alarm-state' leaf representation node instance with the 'critical' (position 3), 'warning' (position 8) and 'indeterminate' (position 128) flags set. 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 ). The encoding MUST be enclosed by the bits CBOR tag as specified in . 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: CBOR diagnostic notation: 43("under-repair critical") CBOR encoding: D8 2B 75 756E6465722D72657061697220637269746963616C

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: CBOR diagnostic notation: h'1F1CE6A3F42660D888D92A4D8030476E' CBOR encoding: 50 1F1CE6A3F42660D888D92A4D8030476E

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 from : CBOR diagnostic notation: "eth1" CBOR encoding: 64 65746831

The 'identityref' Type This specification supports two approaches for encoding identityref: as a YANG Schema Item iDentifier as defined in , or as a name as defined in . See for an exceptional case when this representation needs to be tagged.

SIDs as identityref When representation nodes of type identityref are implemented using SIDs, they MUST be encoded using a CBOR unsigned integer data item (major type 0). (Note that, as they are not used in the position of CBOR map keys, no delta mechanism is employed for SIDs used for identityref.) The following example shows the encoding of a 'type' leaf representation node instance set to the value 'iana-if-type:ethernetCsmacd' (SID 1880). Definition example from : CBOR diagnostic notation: 1880 CBOR encoding: 19 0758

Name as identityref Alternatively, an identityref MAY be encoded using a name as defined in . 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 . The following example shows the encoding of the identity 'iana-if-type:ethernetCsmacd' using its namespace qualified name. This example is described in . CBOR diagnostic notation: "iana-if-type:ethernetCsmacd" CBOR encoding: 78 1b 69616E612D69662D747970653A65746865726E657443736D616364

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 from : CBOR diagnostic notation: null CBOR encoding: F6

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 for the assigned value of these CBOR tags. As mentioned in and in , '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 workaround allows compatibility to be maintained with the encoding of overlapping unions in XML and JSON. See also .) 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 ): CBOR diagnostic notation: "2001:db8:a0b:12f0::1" CBOR encoding: 74 323030313A6462383A6130623A313266303A3A31

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

SIDs as instance-identifier SIDs uniquely identify a schema node. In the case of a single instance schema node, i.e., a schema node defined at the root of a YANG module or submodule or schema nodes defined within a container, the SID is sufficient to identify this instance (representation node). (Note that no delta mechanism is employed for SIDs used for identityref, see .) In the case of a representation node that is an entry of a YANG list, a SID is combined with the list key(s) to identify each instance within the YANG list(s). Instance identifiers of single instance schema nodes MUST be encoded using a CBOR unsigned integer data item (major type 0) and set to the targeted schema node SID. Instance identifiers of representation node entries of a YANG list MUST be encoded using a CBOR array data item (major type 4) containing the following entries:

• The first entry MUST be encoded as a CBOR unsigned integer data item (major type 0) and set to the targeted schema node SID.
• The following entries MUST contain the value of each key required to identify the instance of the targeted schema node. These keys MUST be ordered as defined in the 'key' YANG statement, starting from the top level list, and followed by each of the subordinate list(s).

Examples within this section assume the definition of a schema node of type 'instance-identifier': Definition example from : First example: The following example shows the encoding of the 'reporting-entity' value referencing data node instance "/system/contact" (SID 1741). Definition example from : CBOR diagnostic notation: 1741 CBOR encoding: 19 06CD Second example: This example aims to show how a representation node entry of a YANG list is identified. It uses a somewhat arbitrarily modified YANG module version from by adding country to the leafs and keys of authorized-key. The following example shows the encoding of the 'reporting-entity' value referencing list instance "/system/authentication/user/authorized-key/key-data" (which is assumed to have SID 1734) for username "bob" and authorized-key with name "admin" and country "france". CBOR diagnostic notation: [1734, "bob", "admin", "france"] CBOR encoding: Third example: The following example shows the encoding of the 'reporting-entity' value referencing the list instance "/system/authentication/user" (SID 1730) corresponding to username "jack". CBOR diagnostic notation: [1730, "jack"] CBOR encoding:

Names as instance-identifier An "instance-identifier" value is encoded as a text string that is analogous to the lexical representation in XML encoding; see . However, the encoding of namespaces in instance-identifier values follows the rules stated in , 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". The resulting xpath MUST be encoded using a CBOR text string data item (major type 3). First example: This example is described in . CBOR diagnostic notation: "/ietf-system:system/contact" CBOR encoding: Second example: This example is described in . CBOR diagnostic notation (the line break is inserted for exposition only): CBOR encoding: Third example: This example is described in . CBOR diagnostic notation: CBOR encoding:

Content-Types This specification defines the media-type application/yang-data+cbor, which can be used without parameters or with the id parameter set to either name or sid. This media-type represents a YANG-CBOR document containing a representation tree. If the media-type parameter id is present, depending on its value, each representation node is identified by its associated namespace qualified name as defined in (id=name), or by its associated YANG SID (represented, e.g., in CBOR map keys as a SID delta or via tag number 47) as defined in (id=sid), respectively. If no id parameter is given, both forms may be present. The format of an application/yang-data+cbor representation is that of a CBOR map, mapping names and/or SIDs (as defined above) into instance values (using the rules defined in ). It is not foreseen at this point that the valid set of values for the id parameter will extend beyond name, sid, or being unset; if that does happen, any new value is foreseen to be of the form [a-z][a-z0-9]*(-[a-z0-9]+)*. In summary, this document defines three content-types, which are intended for use by different classes of applications:

• application/yang-data+cbor; id=sid -- for use by applications that need to be frugal with encoding space and text string processing (e.g., applications running on constrained nodes , or applications with particular performance requirements);
• application/yang-data+cbor; id=name -- for use by applications that do not want to engage in SID management, and that have ample resources to manage text-string based item identifiers (e.g., applications that directly want to substitute application/yang.data+json with a more efficient representation without any other changes);
• application/yang-data+cbor -- for use by more complex applications that can benefit from the increased efficiency of SID identifiers but also need to integrate databases of YANG modules before SID mappings are defined for them.

All three content-types are based on the same representation mechanisms, parts of which are simply not used in the first and second case. How the use of one of these content types is selected in a transfer protocol is outside the scope of this specification. The last paragraph of discusses how to indicate and request the usage of specific content-types in RESTCONF. Similar mechanisms are available in CoAP using the Content-Format and Accept Options; demonstrates specifics on how Content-Format may be used to indicate the id=sid case.

Security Considerations The security considerations of and apply. This document defines an alternative encoding for data modeled in the YANG data modeling language. As such, this encoding does not contribute any new security issues in addition to those identified for the specific protocol or context for which it is used. To minimize security risks, software on the receiving side SHOULD reject all messages that do not comply to the rules of this document and reply with an appropriate error message to the sender. For instance, when the 'id' parameter to the media type is used, it is important to properly reject identifiers of the other type, to avoid scenarios where different implementations interpret a given content in different ways. When SIDs are in use, the interpretation of encoded data not only relies on having the right YANG modules, but also on having the right SID mapping information. Management and evolution of that mapping information therefore requires the same care as the management and evolution of the YANG modules themselves. The procedures in are being defined with this in mind.

IANA Considerations

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

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

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

Type name:

application

Subtype name:

yang-data+cbor

Required parameters:

N/A

Optional parameters:

id (see of RFC XXXX)

Encoding considerations:

binary (CBOR)

Security considerations:

see 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

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-data+cbor; id=name	-	TBD2	RFC XXXX
application/yang-data+cbor; id=sid	-	TBD3	RFC XXXX

// 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.

CBOR Tags Registry In the registry "" , as per , IANA has allocated the CBOR tags in for the YANG datatypes listed. CBOR tags defined by this specification

Tag	Data Item	Semantics	Reference
43	text string	YANG bits datatype; see	RFC XXXX
44	text string	YANG enumeration datatype; see .	RFC XXXX
45	unsigned integer or text string	YANG identityref datatype; see .	RFC XXXX
46	unsigned integer or text string or array	YANG instance-identifier datatype; see .	RFC XXXX
47	unsigned integer	YANG Schema Item iDentifier (SID); see .	RFC XXXX

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

References Normative References YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). This document defines encoding rules for representing configuration data, state data, parameters of Remote Procedure Call (RPC) operations or actions, and notifications defined using YANG as JavaScript Object Notation (JSON) text. This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. This document describes YANG mechanisms for defining abstract data structures with YANG. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. IANA In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Trilliant Networks Inc. Acklio Google Switzerland GmbH Universität Bremen TZI Sandelman Software Works YANG Schema Item iDentifiers (YANG SID) are globally unique 63-bit unsigned integers used to identify YANG items, as a more compact method to identify YANG items that can be used for efficiency and in constrained environments (RFC 7228). This document defines the semantics, the registration, and assignment processes of YANG SIDs for IETF managed YANG modules. To enable the implementation of these processes, this document also defines a file format used to persist and publish assigned YANG SIDs. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Trilliant Networks Inc. consultant Acklio YumaWorks Acklio This document describes a network management interface for constrained devices and networks, called CoAP Management Interface (CORECONF). The Constrained Application Protocol (CoAP) is used to access datastore and data node resources specified in YANG, or SMIv2 converted to YANG. CORECONF uses the YANG to CBOR mapping and converts YANG identifier strings to numeric identifiers for payload size reduction. CORECONF extends the set of YANG based protocols, NETCONF and RESTCONF, with the capability to manage constrained devices and networks. The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in the standardization work for constrained-node networks. This document defines a YANG data model for the configuration and identification of some common system properties within a device containing a Network Configuration Protocol (NETCONF) server. This document also includes data node definitions for system identification, time-of-day management, user management, DNS resolver configuration, and some protocol operations for system management. This document defines a YANG data model for the management of network interfaces. It is expected that interface-type-specific data models augment the generic interfaces data model defined in this document. The data model includes definitions for configuration and system state (status information and counters for the collection of statistics). The YANG data model in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document obsoletes RFC 7223. This document defines a YANG data model for management of IP implementations. The data model includes configuration and system state. The YANG data model in this document conforms to the Network Management Datastore Architecture defined in RFC 8342. This document obsoletes RFC 7277.

Acknowledgments This document has been largely inspired by the extensive works done by and on . 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 and .