CBOR Encoding of Data Modeled with YANG

The specification of the YANG 1.1 data modelling language defines only 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. A new set of encoding rules has been defined to allow the use of the same data models in environments based on the JavaScript Object Notation (JSON) Data Interchange Format . This is accomplished in the JSON Encoding of Data Modeled with YANG specification . The aim of this document is to define a set of encoding rules for the Concise Binary Object Representation (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 .

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in . The following terms are defined in : action anydata anyxml data node data tree module notification RPC schema node schema tree submodule The following terms are defined in : member name name of an identity The following term is defined in : YANG hash This specification also makes use of the following terminology: child: A schema node defined within a collection such as a container, a list, a case, a notification, an RPC input, an RPC output, an action input, an action output. delta : Difference between the SID assigned to the current schema node and the SID assigned to the parent. parent: The collection in which a schema node is defined. structured identifier or SID: Unsigned integer used to identify different YANG items.

Within this document, CBOR binary contents are represented using an equivalent textual form called CBOR diagnostic notation as defined in section 6. This notation is used strictly for documentation purposes and is never used in the data serialization. CBOR content CBOR type Diagnostic notation Example CBOR encoding Unsigned integer 0 Decimal digits 123 or +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 The following extensions to the CBOR diagnostic notation are supported: Comments can be added to the end of each line. Any characters after a Pound sign (‘#’) outside of a string, up to the end of the line, are treated as a comment. Deltas are represented as positive numbers (e.g. +123).

This document defines CBOR encoding rules for YANG schema trees and their subtrees. Basic schema nodes such as leaf, leaf-list, list, anydata and anyxml can be encoded standalone. In this case, only the value of this schema node is encoded in CBOR. Identification of this value need to be provided by some external means when needed. A collection such as container, list instance, notification, RPC input, RPC output, action input and 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 three type of keys; SID as defined in , member names as defined in and YANG hash as defined by . Each of these key type is encoded using a specific CBOR type which allows their interpretation during the deserialization process. The end user of this mapping specification can mandate the use of a specific key type or a specific subset of key types. In order to minimize the size of the encoded data, the proposed mapping avoid any unnecessary meta-information beyond those natively supported by CBOR. For instance, CBOR tags are used sorely in the case of the union datatype to distinguish explicitly the use of different YANG datatypes encoded using the same CBOR major type. It is expected that entities generating and decoding CBOR contents have enough knowledge about the information processed in order to perform the expected task without the need of such extra meta-information. The CoAP Content-Format Option, or an HTTP Content-Type header field, conveys that the data is YANG-encoded CBOR in the first place.

Schema node instances 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 .

Leafs MUST be encoded based on the encoding rules specified in .

Collections such as containers, list instances, notifications, RPC inputs, RPC outputs, action inputs and action outputs MUST be encoded using a CBOR map data item (major type 5). A map is comprised of pairs of data items, with each data item consisting of a key and a value. Each key within the CBOR map is set to a data node identifier, each value is set to the value of this data node instance. This specification supports three type of keys; SID as defined in , member names as defined in and YANG hash as defined by . SIDs as keys Keys implemented using SIDs MUST be encoded using a CBOR unsigned integer (major type 0) or CBOR signed integer (major type 1), depending on the actual value. Keys are set to the delta of the associated SID, delta values are computed as follows: The delta value is equal to the SID of the current schema node minus the SID of the parent schema node. When no parent exists in the context of use of this container, the delta is set to the SID of the current schema node (a parent with SID equal to zero is assumed). Delta values may result in a negative number, clients and servers MUST support negative deltas. Member names as keys Keys implemented using member names MUST be encoded using a CBOR text string data item (major type 3). A namespace-qualified member name MUST be used for all members of a top-level collection, and then also whenever the namespaces of the schema node and its parent are different. In all other cases, the simple form of the member name MUST be used. Member names and namespaces are defined in section 4. YANG hashes as keys Keys implemented using YANG hashes MUST be encoded using a CBOR byte string data item (major type 2). Values MUST be encoded using the appropriate rules defined in and . Definition example :

SIDs example This example is encoded using the SIDs defined in Appendix C. CBOR diagnostic notation:

CBOR encoding:

Member names example CBOR diagnostic notation:

CBOR encoding:

YANG Hashes example CBOR diagnostic notation:

CBOR encoding:

A leaf-list MUST be encoded using a CBOR array data item (major type 4). Each entry of this array MUST be encoded using the rules defined by the YANG type specified. Definition example :

CBOR diagnostic notation: [ “ietf.org”, “ieee.org” ] CBOR encoding: 82 68 696574662e6f7267 68 696565652e6f7267

A list MUST be encoded using a CBOR array data item (major type 4). Each list instance within this CBOR array is encoded using a CBOR map data item (major type 5) based on the same rules as a YANG container as defined in . Definition example :

SIDs example SIDs used in this example are defined in Appendix C. It is important to note that the protocol or method using this mapping may carry a parent SID or may have the knowledge of this parent SID based on its context. In these cases, delta encoding can be performed based on this parent SID which minimizes the size of the encoded data. CBOR diagnostic notation:

CBOR encoding:

Member names example CBOR diagnostic notation:

CBOR encoding:

YANG hashes example CBOR diagnostic notation:

CBOR encoding:

An anydata serves as a container for an arbitrary set of schema nodes that otherwise appear as normal YANG-modeled data. An anydata 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: Keys MUST be set to valid SIDs, member names or YANG hashes. This rule applies to the key of the anydata node and the key of any inner schema node. The CBOR array MUST contain either unique scalar values (as a leaf-list, see ), or maps (as a list, see ). Values MUST follow the encoding rules of one of the datatypes listed in .

An anyxml instance is encoded as a CBOR key/value pair. The key of the anyxml schema node MUST be a valid SID, member name or YANG hash but the value is unrestricted, i.e., the value can be any CBOR encoded content.

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

CBOR diagnostic notation: 1280 CBOR encoding: 19 0500

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

CBOR diagnostic notation: -300 CBOR encoding: 39 012b

f type decimal64 MUST be encoded using either CBOR unsigned integer (major type 0) or CBOR signed integer (major type 1), depending on the actual value. Leafs oThe position of the decimal point is defined by the fraction-digits YANG statement and is not available in the CBOR encoding. Definition example :

CBOR diagnostic notation: 257 (Represents decimal value 2.57) CBOR encoding: 19 0101

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

CBOR diagnostic notation: “eth0” CBOR encoding: 64 65746830

Leafs of type boolean MUST be encoded using a CBOR true (major type 7, additional information 21) or false data item (major type 7, additional information 20). Definition example :

CBOR diagnostic notation: true CBOR encoding: f5

Leafs of type enumeration MUST be encoded using a CBOR unsigned integer (major type 0) or CBOR signed 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. Definition example :

CBOR diagnostic notation: 3 (Represents enumeration value ‘testing’) CBOR encoding: 03

Leafs of type bits MUST be encoded using a CBOR byte string data item (major type 2). Bits position are either explicitly assigned using the YANG statement ‘position’ or automatically assigned based on the algorithm defined in section 9.7.4.2. Bits position 0 to 7 are assigned to the first byte within the byte string, bits 8 to 15 to the second byte, and subsequent bytes are assigned similarly. Within each byte, bits are assigned from least to most significant. Definition example :

CBOR diagnostic notation: h’05’ (Represents bits disable-nagle and 10-Mb-only set) CBOR encoding: 41 05

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

CBOR diagnostic notation: h’1f1ce6a3f42660d888d92a4d8030476e’ CBOR encoding: 50 1f1ce6a3f42660d888d92a4d8030476e

Leafs of type leafref MUST be encoded using the rules of the schema node referenced by the ‘path’ YANG statement. Definition example :

CBOR diagnostic notation: “eth1” CBOR encoding: 64 65746831

This specification supports two approaches for encoding identityref, a SID as defined in or a name as defined in section 6.8. SIDs as identityref SIDs are globally unique and can be used as identityref. This approach is both compact and simple to implement. When SIDs are used, identityref MUST be encoded using a CBOR unsigned integer data item (major type 0) or CBOR signed integer (major type 1), depending on the actual value. The value represents the delta between the SID of the identity assigned to the leaf and the SID of the base identity defined for this leaf. Name as identityref Alternatively, an identityref may be encoded using a name as defined in section 6.8. When names are used, identityref MUST be encoded using a CBOR text string data item (major type 3). If the identity is defined in another module than the leaf node containing the identityref value, the namespace-qualified form MUST be used. Otherwise, both the simple and namespace-qualified forms are permitted. Definition example :

SIDs as identityref This example represents the encoding of identity “radius-pap” (SID 1705) assuming the base identity “radius-authentication-type” (SID 1703). CBOR diagnostic notation: 2 CBOR encoding: 02 Name as identityref CBOR diagnostic notation: “ietf-system:radius-pap” CBOR encoding: 76 696574662d73797374656d3a7261646975732d706170

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

CBOR diagnostic notation: null CBOR encoding: f6

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 prefixed by CBOR tag to avoid confusion between different YANG datatypes encoded using the same CBOR major type. bits decimal64 enumeration identityref instance-identifier leafref (Only when the datatype of the leaf referenced using the ‘path’ YANG statement require a CBOR tag) See for more information about these CBOR tags. Definition example :

CBOR diagnostic notation: “2001:db8:a0b:12f0::1” CBOR encoding: 74 323030313a6462383a6130623a313266303a3a31

This specification supports three approaches for encoding an instance-identifier, one based on SIDs as defined in , one based on names as defined in section 6.13 and one based on YANG hashes as defined in . SIDs as instance-identifier SIDs uniquely identify a data node. For a single instance data node, the SID is sufficient to identify this instance. For a multi-instance data node, a SID is combined with the list key(s) to identify each instance of this data node within the YANG list(s). Single instance data nodes MUST be encoded using a CBOR unsigned integer data item (major type 0) and set to the targeted data node SID. Multi-instances data nodes 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 data node SID. The following entries MUST contain the value of each key required to identify the instance of the targeted data node. These keys MUST be ordered as defined in the ‘key’ YANG statement, starting from top level list, and follow by each of the subordinate list(s). Names as instance-identifier The use of names as instance-identifier is defined in section 6.11. The resulting xpath MUST be encoded using a CBOR text string data item (major type 3). YANG hashes as instance-identifier When YANG hashes are used, xpath can be compressed based on the method defined by sections 4.1.4.1 and 4.1.4.2. The resulting hash MUST be encoded using a CBOR byte string data item (major type 2). Definition example :

First example based on SID In this example, a field of type instance-identifier identifies the data node “/system/contact” (SID 1728).

CBOR encoding:

First example based on name Same example as above based on names.

CBOR encoding:

First example based on YANG hash Same example assuming data node “/system/contact” is associated to YANG hash 0x09b06d17 or “JsG0X” in base64. CBOR diagnostic notation:

CBOR encoding:

Definition example :

Second example based on SID In this example, a field of type instance-identifier identify the data node “/system/authentication/user/authorized-key/key-data” (SID 1721) for the user name “bob” and the authorized-key name “admin”. CBOR diagnostic notation:

CBOR encoding:

Second example based on name Same example as above based on names. CBOR diagnostic notation:

CBOR encoding:

Second example based on YANG hash Same example assuming data node “/ietf-system:system/authentication/user/authorized-key/key-data” is associated to YANG hash 0x0d6e7afb or “Nbnr7” in base64. CBOR diagnostic notation:

CBOR encoding:

Third example based on SID This third example identify an instance within the list “/system/authentication/user” (SID 1717) corresponding to the user name “jack”. CBOR diagnostic notation:

CBOR encoding:

Third example based on name Same example as above based on names. CBOR diagnostic notation:

CBOR encoding:

Third example based on YANG hash Same example assuming data node “/ietf-system:system/authentication/user” is associated to YANG hash 0x2677c6c1 or “md8bB” in base64. CBOR diagnostic notation:

CBOR encoding:

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 of 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.

This specification requires the assignment of CBOR tags for the following YANG datatypes. These tags are added to the Tags Registry as defined in section 7.2 of . Tag Data Item Semantics Reference 40 bits YANG bits datatype RFC XXXX 41 decimal64 YANG decimal64 datatype RFC XXXX 42 enumeration YANG enumeration datatype RFC XXXX 43 identityref YANG identityref datatype RFC XXXX 44 instance-identifier YANG instance-identifier datatype RFC XXXX // RFC Ed.: update Tag values using allocated tags if needed and remove this note // RFC Ed.: replace XXXX with RFC number and remove this note

This document has been largely inspired by the extensive works done by Andy Bierman and Peter van der Stok 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 Ladislav Lhotka and Juergen Schoenwaelder.