YANG JSON Encoding: JSON Encoding of Data Modeled with YANG
Rules for writing YANG configuration and state data as JSON text.
Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, August 2016, <https://www.rfc-editor.org/info/rfc7951>.
@misc{rfc7951,
series = {Request for Comments},
number = 7951,
howpublished = {RFC 7951},
publisher = {RFC Editor},
doi = {10.17487/RFC7951},
url = {https://www.rfc-editor.org/info/rfc7951},
author = {L. Lhotka},
title = {{JSON Encoding of Data Modeled with YANG}},
pagetotal = 20,
year = 2016,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) L. Lhotka
Request for Comments: 7951 CZ.NIC
Category: Standards Track August 2016
ISSN: 2070-1721
JSON Encoding of Data Modeled with YANG
Abstract
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7951.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Properties of the JSON Encoding . . . . . . . . . . . . . . . 4
4. Names and Namespaces . . . . . . . . . . . . . . . . . . . . 5
5. Encoding of YANG Data Node Instances . . . . . . . . . . . . 7
5.1. The "leaf" Data Node . . . . . . . . . . . . . . . . . . 7
5.2. The "container" Data Node . . . . . . . . . . . . . . . . 8
5.3. The "leaf-list" Data Node . . . . . . . . . . . . . . . . 8
5.4. The "list" Data Node . . . . . . . . . . . . . . . . . . 9
5.5. The "anydata" Data Node . . . . . . . . . . . . . . . . . 9
5.6. The "anyxml" Data Node . . . . . . . . . . . . . . . . . 10
5.7. Metadata Objects . . . . . . . . . . . . . . . . . . . . 11
6. Representing YANG Data Types in JSON Values . . . . . . . . . 11
6.1. Numeric Types . . . . . . . . . . . . . . . . . . . . . . 11
6.2. The "string" Type . . . . . . . . . . . . . . . . . . . . 11
6.3. The "boolean" Type . . . . . . . . . . . . . . . . . . . 11
6.4. The "enumeration" Type . . . . . . . . . . . . . . . . . 12
6.5. The "bits" Type . . . . . . . . . . . . . . . . . . . . . 12
6.6. The "binary" Type . . . . . . . . . . . . . . . . . . . . 12
6.7. The "leafref" Type . . . . . . . . . . . . . . . . . . . 12
6.8. The "identityref" Type . . . . . . . . . . . . . . . . . 12
6.9. The "empty" Type . . . . . . . . . . . . . . . . . . . . 13
6.10. The "union" Type . . . . . . . . . . . . . . . . . . . . 14
6.11. The "instance-identifier" Type . . . . . . . . . . . . . 15
7. I-JSON Compliance . . . . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. A Complete Example . . . . . . . . . . . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The Network Configuration Protocol (NETCONF) [RFC6241] uses XML [XML]
for encoding data in its Content Layer. Other management protocols
might want to use other encodings while still benefiting from using
YANG [RFC7950] as the data modeling language.
For example, the RESTCONF protocol [RESTCONF] supports two encodings:
XML (media type "application/yang.data+xml") and JavaScript Object
Notation (JSON) (media type "application/yang.data+json").
The specification of the YANG 1.1 data modeling language [RFC7950]
defines only XML encoding of data trees, i.e., configuration data,
state data, input/output parameters of Remote Procedure Call (RPC)
operations or actions, and notifications. The aim of this document
is to define rules for encoding the same data as JSON text [RFC7159].
2. Terminology and Notation
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 [RFC2119].
The following terms are defined in [RFC7950]:
o action
o anydata
o anyxml
o augment
o container
o data node
o data tree
o identity
o instance identifier
o leaf
o leaf-list
o list
o module
o RPC operation
o submodule
The following terms are defined in [RFC6241]:
o configuration data
o notification
o state data
3. Properties of the JSON Encoding
This document defines JSON encoding for YANG data trees and their
subtrees. It is always assumed that the top-level structure in JSON-
encoded data is an object.
Instances of YANG data nodes (leafs, containers, leaf-lists, lists,
anydata nodes, and anyxml nodes) are encoded as members of a JSON
object, i.e., name/value pairs. Section 4 defines how the name part
is formed, and the following sections deal with the value part. The
encoding rules are identical for all types of data trees, i.e.,
configuration data, state data, parameters of RPC operations,
actions, and notifications.
With the exception of "anydata" encoding (Section 5.5), all rules in
this document are also applicable to YANG 1.0 [RFC6020].
Unlike XML element content, JSON values carry partial type
information (number, string, boolean). The JSON encoding is defined
so that this information is never in conflict with the data type of
the corresponding YANG leaf or leaf-list.
With the exception of anyxml and schema-less anydata nodes, it is
possible to map a JSON-encoded data tree to XML encoding as defined
in [RFC7950], and vice versa. However, such conversions require the
YANG data model to be available.
In order to achieve maximum interoperability while allowing
implementations to use a variety of existing JSON parsers, the JSON
encoding rules follow, as much as possible, the constraints of the
I-JSON (Internet JSON) restricted profile [RFC7493]. Section 7
discusses I-JSON conformance in more detail.
4. Names and Namespaces
A JSON object member name MUST be in one of the following forms:
o simple - identical to the identifier of the corresponding YANG
data node.
o namespace-qualified - the data node identifier is prefixed with
the name of the module in which the data node is defined,
separated from the data node identifier by the colon character
(":").
The name of a module determines the namespace of all data node names
defined in that module. If a data node is defined in a submodule,
then the namespace-qualified member name uses the name of the main
module to which the submodule belongs.
ABNF syntax [RFC5234] of a member name is shown in Figure 1, where
the production for "identifier" is defined in Section 14 of
[RFC7950].
member-name = [identifier ":"] identifier
Figure 1: ABNF Production for a JSON Member Name
A namespace-qualified member name MUST be used for all members of a
top-level JSON object and then also whenever the namespaces of the
data node and its parent node are different. In all other cases, the
simple form of the member name MUST be used.
For example, consider the following YANG module:
module example-foomod {
namespace "http://example.com/foomod";
prefix "foomod";
container top {
leaf foo {
type uint8;
}
}
}
If the data model consists only of this module, then the following is
valid JSON-encoded configuration data:
{
"example-foomod:top": {
"foo": 54
}
}
Note that the member of the top-level object uses the namespace-
qualified name but the "foo" leaf doesn't because it is defined in
the same module as its parent container "top".
Now, assume that the container "top" is augmented from another
module, "example-barmod":
module example-barmod {
namespace "http://example.com/barmod";
prefix "barmod";
import example-foomod {
prefix "foomod";
}
augment "/foomod:top" {
leaf bar {
type boolean;
}
}
}
Valid JSON-encoded configuration data containing both leafs may then
look like this:
{
"example-foomod:top": {
"foo": 54,
"example-barmod:bar": true
}
}
The name of the "bar" leaf is prefixed with the namespace identifier
because its parent is defined in a different module.
Explicit namespace identifiers are sometimes needed when encoding
values of the "identityref" and "instance-identifier" types. The
same form of namespace-qualified name as defined above is then used.
See Sections 6.8 and 6.11 for details.
5. Encoding of YANG Data Node Instances
Every data node instance is encoded as a name/value pair where the
name is formed from the data node identifier using the rules of
Section 4. The value depends on the category of the data node, as
explained in the following subsections.
Character encoding MUST be UTF-8.
5.1. The "leaf" Data Node
A leaf instance is encoded as a name/value pair where the value can
be a string, number, literal "true" or "false", or the special array
"[null]", depending on the type of the leaf (see Section 6 for the
type encoding rules).
Example: For the leaf node definition
leaf foo {
type uint8;
}
the following is a valid JSON-encoded instance:
"foo": 123
5.2. The "container" Data Node
A container instance is encoded as a name/object pair. The
container's child data nodes are encoded as members of the object.
Example: For the container definition
container bar {
leaf foo {
type uint8;
}
}
the following is a valid JSON-encoded instance:
"bar": {
"foo": 123
}
5.3. The "leaf-list" Data Node
A leaf-list is encoded as a name/array pair, and the array elements
are values of some scalar type, which can be a string, number,
literal "true" or "false", or the special array "[null]", depending
on the type of the leaf-list (see Section 6 for the type encoding
rules).
The ordering of array elements follows the same rules as the ordering
of XML elements representing leaf-list entries in the XML encoding.
Specifically, the "ordered-by" properties (Section 7.7.7 in
[RFC7950]) MUST be observed.
Example: For the leaf-list definition
leaf-list foo {
type uint8;
}
the following is a valid JSON-encoded instance:
"foo": [123, 0]
5.4. The "list" Data Node
A list instance is encoded as a name/array pair, and the array
elements are JSON objects.
The ordering of array elements follows the same rules as the ordering
of XML elements representing list entries in the XML encoding.
Specifically, the "ordered-by" properties (Section 7.7.7 in
[RFC7950]) MUST be observed.
Unlike the XML encoding, where list keys are required to precede any
other siblings within a list entry and appear in the order specified
by the data model, the order of members in a JSON-encoded list entry
is arbitrary because JSON objects are fundamentally unordered
collections of members.
Example: For the list definition
list bar {
key foo;
leaf foo {
type uint8;
}
leaf baz {
type string;
}
}
the following is a valid JSON-encoded instance:
"bar": [
{
"foo": 123,
"baz": "zig"
},
{
"baz": "zag",
"foo": 0
}
]
5.5. The "anydata" Data Node
The anydata data node serves as a container for an arbitrary set of
nodes that otherwise appear as normal YANG-modeled data. A data
model for anydata content may or may not be known at runtime. In the
latter case, converting JSON-encoded instances to the XML encoding
defined in [RFC7950] may be impossible.
An anydata instance is encoded in the same way as a container, i.e.,
as a name/object pair. The requirement that anydata content can be
modeled by YANG implies the following rules for the JSON text inside
the object:
o It is valid I-JSON [RFC7493].
o All object member names satisfy the ABNF production in Figure 1.
o Any JSON array contains either only unique scalar values (as a
leaf-list; see Section 5.3) or only objects (as a list; see
Section 5.4).
o The "null" value is only allowed in the single-element array
"[null]" corresponding to the encoding of the "empty" type; see
Section 6.9.
Example: For the anydata definition
anydata data;
the following is a valid JSON-encoded instance:
"data": {
"ietf-notification:notification": {
"eventTime": "2014-07-29T13:43:01Z",
"example-event:event": {
"event-class": "fault",
"reporting-entity": {
"card": "Ethernet0"
},
"severity": "major"
}
}
}
5.6. The "anyxml" Data Node
An anyxml instance is encoded as a JSON name/value pair. The value
MUST satisfy I-JSON constraints.
Example: For the anyxml definition
anyxml bar;
the following is a valid JSON-encoded instance:
"bar": [true, null, true]
5.7. Metadata Objects
Apart from instances of YANG data nodes, a JSON document MAY contain
special object members whose name starts with the "@" character
(COMMERCIAL AT). Such members are used for special purposes, such as
encoding metadata [RFC7952]. The exact syntax and semantics of such
members are outside the scope of this document.
6. Representing YANG Data Types in JSON Values
The type of the JSON value in an instance of the leaf or leaf-list
data node depends on the type of that data node, as specified in the
following subsections.
6.1. Numeric Types
A value of the "int8", "int16", "int32", "uint8", "uint16", or
"uint32" type is represented as a JSON number.
A value of the "int64", "uint64", or "decimal64" type is represented
as a JSON string whose content is the lexical representation of the
corresponding YANG type as specified in Sections 9.2.1 and 9.3.1 of
[RFC7950].
For example, if the type of the leaf "foo" in Section 5.1 was
"uint64" instead of "uint8", the instance would have to be encoded as
"foo": "123"
The special handling of 64-bit numbers follows from the I-JSON
recommendation to encode numbers exceeding the IEEE 754-2008
double-precision range [IEEE754-2008] as strings; see Section 2.2 in
[RFC7493].
6.2. The "string" Type
A "string" value is represented as a JSON string, subject to JSON
string encoding rules.
6.3. The "boolean" Type
A "boolean" value is represented as the corresponding JSON literal
name "true" or "false".
6.4. The "enumeration" Type
An "enumeration" value is represented as a JSON string -- one of the
names assigned by "enum" statements in YANG.
The representation is identical to the lexical representation of the
"enumeration" type in XML; see Section 9.6 in [RFC7950].
6.5. The "bits" Type
A "bits" value is represented as a JSON string -- a space-separated
sequence of names of bits that are set. The permitted bit names are
assigned by "bit" statements in YANG.
The representation is identical to the lexical representation of the
"bits" type; see Section 9.7 in [RFC7950].
6.6. The "binary" Type
A "binary" value is represented as a JSON string -- base64 encoding
of arbitrary binary data.
The representation is identical to the lexical representation of the
"binary" type in XML; see Section 9.8 in [RFC7950].
6.7. The "leafref" Type
A "leafref" value is represented using the same rules as the type of
the leaf to which the leafref value refers.
6.8. The "identityref" Type
An "identityref" value is represented as a string -- the name of an
identity. If the identity is defined in a module other than the leaf
node containing the identityref value, the namespace-qualified form
(Section 4) MUST be used. Otherwise, both the simple and namespace-
qualified forms are permitted.
For example, consider the following schematic module:
module example-mod {
...
import ietf-interfaces {
prefix if;
}
...
leaf type {
type identityref {
base "if:interface-type";
}
}
}
A valid instance of the "type" leaf is then encoded as follows:
"type": "iana-if-type:ethernetCsmacd"
The namespace identifier "iana-if-type" must be present in this case
because the "ethernetCsmacd" identity is not defined in the same
module as the "type" leaf.
6.9. The "empty" Type
An "empty" value is represented as "[null]", i.e., an array with the
"null" literal being its only element. For the purposes of this
document, "[null]" is considered an atomic scalar value.
This encoding of the "empty" type was chosen instead of using simply
"null" in order to facilitate the use of empty leafs in common
programming languages where the "null" value of a member is treated
as if the member is not present.
Example: For the leaf definition
leaf foo {
type empty;
}
a valid instance is
"foo": [null]
6.10. The "union" Type
A value of the "union" type is encoded as the value of any of the
member types.
When validating a value of the "union" type, the type information
conveyed by the JSON encoding MUST also be taken into account. JSON
syntax thus provides additional means for resolving the member type
of the union that are not available in XML encoding.
For example, consider the following YANG definition:
leaf bar {
type union {
type uint16;
type string;
}
}
In RESTCONF [RESTCONF], it is possible to set the value of "bar" in
the following way when using the "application/yang.data+xml"
media type:
<bar>13.5</bar>
because the value may be interpreted as a string, i.e., the
second member type of the union. When using the
"application/yang.data+json" media type, however, this is an error:
"bar": 13.5
In this case, the JSON encoding indicates that the value is supposed
to be a number rather than a string, and it is not a valid "uint16"
value.
Conversely, the value of
"bar": "1"
is to be interpreted as a string.
6.11. The "instance-identifier" Type
An "instance-identifier" value is encoded as a string that is
analogical to the lexical representation in XML encoding; see
Section 9.13.2 in [RFC7950]. However, the encoding of namespaces in
instance-identifier values follows the rules stated in Section 4,
namely:
o The leftmost (top-level) data node name is always in the
namespace-qualified form.
o 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".
7. I-JSON Compliance
I-JSON [RFC7493] is a restricted profile of JSON that guarantees
maximum interoperability for protocols that use JSON in their
messages, no matter what JSON encoders/decoders are used in protocol
implementations. The encoding defined in this document therefore
observes the I-JSON requirements and recommendations as closely as
possible.
In particular, the following properties are guaranteed:
o Character encoding is UTF-8.
o Member names within the same JSON object are always unique.
o The order of JSON object members is never relied upon.
o Numbers of any type supported by YANG can be exchanged reliably.
See Section 6.1 for details.
The JSON encoding defined in this document deviates from I-JSON only
in the representation of the "binary" type. In order to remain
compatible with XML encoding, the base64 encoding scheme is used
(Section 6.6), whilst I-JSON recommends base64url instead.
8. Security Considerations
This document defines an alternative encoding for data modeled in the
YANG data modeling language. As such, it doesn't contribute any new
security issues beyond those discussed in Section 17 of [RFC7950].
This document defines no mechanisms for signing and encrypting data
modeled with YANG. Under normal circumstances, data security and
integrity are guaranteed by the management protocol in use, such as
NETCONF [RFC6241] or RESTCONF [RESTCONF]. If this is not the case,
external mechanisms, such as Public-Key Cryptography Standards (PKCS)
#7 [RFC2315] or JSON Object Signing and Encryption (JOSE) [RFC7515]
[RFC7516], need to be considered.
JSON processing is rather different from XML, and JSON parsers may
thus suffer from different types of vulnerabilities than their XML
counterparts. To minimize these new security risks, software on the
receiving side SHOULD reject all messages that do not comply with the
rules of this document and reply with an appropriate error message to
the sender.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<http://www.rfc-editor.org/info/rfc7493>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<http://www.rfc-editor.org/info/rfc7950>.
9.2. Informative References
[IEEE754-2008]
IEEE, "IEEE Standard for Floating-Point Arithmetic",
IEEE 754-2008, DOI 10.1109/IEEESTD.2008.4610935, 2008,
<http://standards.ieee.org/findstds/
standard/754-2008.html>.
[RESTCONF] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", Work in Progress,
draft-ietf-netconf-restconf-16, August 2016.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
<http://www.rfc-editor.org/info/rfc2315>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<http://www.rfc-editor.org/info/rfc6020>.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
<http://www.rfc-editor.org/info/rfc7223>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<http://www.rfc-editor.org/info/rfc7516>.
[RFC7952] Lhotka, L., "Defining and Using Metadata with YANG",
RFC 7952, DOI 10.17487/RFC7952, August 2016,
<http://www.rfc-editor.org/info/rfc7952>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation
REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
Appendix A. A Complete Example
The JSON document shown below represents the same data as the reply
to the NETCONF <get> request appearing in Appendix D of [RFC7223].
The data model is a combination of two YANG modules:
"ietf-interfaces" and "ex-vlan" (the latter is an example module from
Appendix C of [RFC7223]). The "if-mib" feature defined in the
"ietf-interfaces" module is supported.
{
"ietf-interfaces:interfaces": {
"interface": [
{
"name": "eth0",
"type": "iana-if-type:ethernetCsmacd",
"enabled": false
},
{
"name": "eth1",
"type": "iana-if-type:ethernetCsmacd",
"enabled": true,
"ex-vlan:vlan-tagging": true
},
{
"name": "eth1.10",
"type": "iana-if-type:l2vlan",
"enabled": true,
"ex-vlan:base-interface": "eth1",
"ex-vlan:vlan-id": 10
},
{
"name": "lo1",
"type": "iana-if-type:softwareLoopback",
"enabled": true
}
]
},
"ietf-interfaces:interfaces-state": {
"interface": [
{
"name": "eth0",
"type": "iana-if-type:ethernetCsmacd",
"admin-status": "down",
"oper-status": "down",
"if-index": 2,
"phys-address": "00:01:02:03:04:05",
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
}
},
{
"name": "eth1",
"type": "iana-if-type:ethernetCsmacd",
"admin-status": "up",
"oper-status": "up",
"if-index": 7,
"phys-address": "00:01:02:03:04:06",
"higher-layer-if": [
"eth1.10"
],
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
}
},
{
"name": "eth1.10",
"type": "iana-if-type:l2vlan",
"admin-status": "up",
"oper-status": "up",
"if-index": 9,
"lower-layer-if": [
"eth1"
],
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
}
},
{
"name": "eth2",
"type": "iana-if-type:ethernetCsmacd",
"admin-status": "down",
"oper-status": "down",
"if-index": 8,
"phys-address": "00:01:02:03:04:07",
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
}
},
{
"name": "lo1",
"type": "iana-if-type:softwareLoopback",
"admin-status": "up",
"oper-status": "up",
"if-index": 1,
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
}
}
]
}
}
Acknowledgements
The author wishes to thank Andy Bierman, Martin Bjorklund, Dean
Bogdanovic, Balazs Lengyel, Juergen Schoenwaelder, and Phil Shafer
for their helpful comments and suggestions.
Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
ISO Plain Language
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§1Why a JSON Encoding for YANG Data
The Network Configuration Protocol (NETCONF) uses XML to encode data in its content layer. Other management protocols may want to use other encodings while still benefiting from YANG as the data modeling language. The RESTCONF protocol, for example, supports two encodings: XML with the media type application/yang.data+xml, and JavaScript Object Notation (JSON) with the media type application/yang.data+json.
The YANG 1.1 data modeling language defines only an XML encoding of data trees, meaning configuration data, state data, the input and output parameters of Remote Procedure Call operations or actions, and notifications. This document defines the rules for encoding the same data as JSON text.
The rules apply to every kind of data tree, and with the exception of the anydata encoding they also apply to YANG 1.0. The top-level structure in JSON-encoded data is always an object, and instances of YANG data nodes are encoded as members of that object, that is, as name and value pairs.
Unlike XML element content, JSON values carry partial type information, because a JSON value is a number, a string or a boolean. The encoding is defined so that this information is never in conflict with the data type of the corresponding YANG leaf or leaf-list. Except for anyxml and schema-less anydata nodes, a JSON-encoded data tree can be mapped to the XML encoding and back, although such conversions need the YANG data model to be available.
YANG describes the shape of network data, and this document describes how to write that data as JSON. One JSON object holds everything, and each part of the model becomes a member of it.
§4Names and Namespaces
A JSON object member name MUST be in one of two forms. The simple form is identical to the identifier of the corresponding YANG data node. The namespace-qualified form prefixes the data node identifier with the name of the module in which the node is defined, separated by a colon. The ABNF production is member-name = [identifier ":"] identifier. The name of a module determines the namespace of all the data node names defined in it, and a node defined in a submodule uses the name of the main module it belongs to.
The namespace-qualified form MUST be used for all members of a top-level JSON object, and also whenever the namespace of the data node differs from that of its parent. In all other cases the simple form MUST be used. So in a model with a single module, the top-level container carries the module prefix while the leaves inside it do not. If the container is augmented from a second module, the augmented leaf does carry a prefix, because it comes from a different module than its parent.
Explicit namespace identifiers are sometimes needed when encoding values of the identityref and instance-identifier types, where the same namespace-qualified form is used. For an instance-identifier, the leftmost, top-level data node name is always namespace-qualified, and any subsequent name is qualified when the node is defined in a module other than its parent. The same rule holds for names inside predicates.
A name gets a module prefix when it comes from a different module than its parent, and the top level is always prefixed. Inside one module, plain names are enough.
{
"example-foomod:top": {
"foo": 54,
"example-barmod:bar": true
}
}§5How Each Kind of Node Is Encoded
A leaf is a name and value pair, where the value is a string, a number, the literal true or false, or the special array [null], depending on the leaf's type. A container is a name and object pair, with the container's child data nodes as members of that object. A leaf-list is a name and array pair, where the array holds scalar values of that type. A list is a name and array pair, where the array elements are JSON objects. Character encoding MUST be UTF-8.
The ordering of array elements follows the same rules as the XML encoding, so the ordered-by properties of YANG MUST be observed. The order of members within a list entry, however, is arbitrary, because JSON objects are unordered collections, unlike XML list entries where the keys come first in a specified order.
An anydata node holds an arbitrary set of nodes that otherwise appear as normal modeled data, and it is encoded like a container, as a name and object pair. A data model for the content may or may not be known at runtime, and without one, converting the instance to XML may be impossible. Because the content must still be modelable by YANG, the JSON text inside must be valid I-JSON, all member names must satisfy the member name production, any array must hold either only unique scalar values or only objects, and the null value is only allowed in the single-element array [null] that encodes the empty type.
An anyxml instance is a name and value pair whose value must satisfy I-JSON. Apart from instances of data nodes, a JSON document MAY contain special object members whose name starts with the at sign, used for purposes such as encoding metadata; the syntax and meaning of those members are outside the scope of this document.
Each kind of node has its own shape: a leaf is a value, a container is an object, a leaf-list is an array of values, and a list is an array of objects. The order of entries in an array matters, but the order of the fields inside an entry does not.
- leafA name and a value: string, number, true or false, or [null].
- containerA name and an object, with the child nodes as members.
- leaf-listA name and an array of scalar values.
- listA name and an array of objects.
- anydataA name and an object holding arbitrary modeled data.
- anyxmlA name and any JSON value that satisfies I-JSON.
§6How YANG Types Become JSON Values
The type of the JSON value depends on the type of the data node. The integer types int8, int16, int32, uint8, uint16 and uint32 are represented as a JSON number. The int64, uint64 and decimal64 types are represented as a JSON string holding the lexical representation of the YANG type. This special handling of 64-bit numbers follows the I-JSON recommendation to encode numbers outside the IEEE 754 double-precision range as strings.
The string type is a JSON string, and the boolean type is the literal true or false. The enumeration type is a JSON string, one of the names assigned by enum statements. The bits type is a JSON string holding a space-separated sequence of the names of the bits that are set. The binary type is a JSON string holding the base64 encoding of arbitrary binary data. A leafref value uses the same rules as the type of the leaf it refers to.
An identityref value is a string holding the name of an identity. If the identity is defined in a module other than the leaf holding the identityref, the namespace-qualified form MUST be used; otherwise either form is allowed. The empty type is represented as [null], an array whose only element is the null literal, which counts as an atomic scalar value for this purpose. It was written that way rather than as a plain null so that empty leaves work in programming languages where a null member value is treated as if the member were absent.
A union value is encoded as the value of any of its member types, and the type information carried by the JSON syntax must also be taken into account when validating it. So for a union of uint16 and string, the JSON value 13.5 is an error, because JSON says it is a number and it is not a valid uint16, while the value "1" is a string. An instance-identifier value is a string analogous to its lexical representation in XML, with the namespace rules described earlier.
Each YANG type has one fixed JSON shape. Wide numbers and decimals become strings so that no precision is lost, and the empty type becomes a one-element array holding null.
| YANG type | JSON value |
|---|---|
| int8, int16, int32, uint8, uint16, uint32 | Number |
| int64, uint64, decimal64 | String with the lexical representation |
| string | String |
| boolean | The literal true or false |
| enumeration | String, one of the enum names |
| bits | String, space-separated bit names |
| binary | String, base64 encoded |
| leafref | The same rules as the leaf it refers to |
| identityref | String, the name of the identity |
| empty | [null] |
| union | The value of any of the member types |
| instance-identifier | String, like the XML lexical form |
§7Following the I-JSON Profile
I-JSON is a restricted profile of JSON that guarantees maximum interoperability for protocols that use JSON in their messages, whatever JSON encoders and decoders the implementations happen to use. The encoding defined here follows the constraints of that profile as closely as possible, so that a wide range of existing JSON parsers can be used.
Four properties are guaranteed. The character encoding is UTF-8. Member names within the same JSON object are always unique. The order of JSON object members is never relied upon. And numbers of any type supported by YANG can be exchanged reliably.
The encoding deviates from I-JSON in exactly one place, the representation of the binary type. To stay compatible with the XML encoding, base64 is used, while I-JSON recommends base64url instead.
The rules follow a stricter JSON profile so that any parser can read the data. The only departure is that binary data uses base64 rather than the URL-safe variant.
§8Security Considerations
This document defines an alternative encoding for data modeled in YANG, so it adds no security issues beyond those discussed in Section 17 of the YANG 1.1 specification.
It defines no mechanism for signing or encrypting YANG-modeled data. Under normal circumstances, data security and integrity are guaranteed by the management protocol in use, such as NETCONF or RESTCONF. Where that is not the case, external mechanisms such as PKCS number 7 or the JSON Object Signing and Encryption specifications need to be considered.
JSON processing differs from XML processing, so JSON parsers can suffer from different kinds of vulnerability than XML parsers. To minimize these new risks, software on the receiving side SHOULD reject all messages that do not comply with the rules of this document and reply with an appropriate error message to the sender.
The document adds no new security rules of its own. It does warn that JSON parsers have their own failure modes, so a receiver should refuse anything that does not follow these rules.