Internet DRAFT - draft-lhotka-netmod-yang-json
draft-lhotka-netmod-yang-json
NETMOD L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track September 23, 2013
Expires: March 27, 2014
Modeling JSON Text with YANG
draft-lhotka-netmod-yang-json-02
Abstract
This document defines rules for presenting configuration and
operational state data defined using YANG as JSON text. It does so
by specifying a procedure for translating the subset of YANG-
compatible XML documents to JSON text, and vice versa.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 27, 2014.
Copyright Notice
Copyright (c) 2013 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Notation . . . . . . . . . . . . . . . . . . . 5
3. Specification of the Translation Procedure . . . . . . . . . . 6
3.1. Names and Namespaces . . . . . . . . . . . . . . . . . . . 7
3.2. Mapping XML Elements to JSON Objects . . . . . . . . . . . 9
3.2.1. The "leaf" Data Node . . . . . . . . . . . . . . . . . 9
3.2.2. The "container" Data Node . . . . . . . . . . . . . . 9
3.2.3. The "leaf-list" Data Node . . . . . . . . . . . . . . 10
3.2.4. The "list" Data Node . . . . . . . . . . . . . . . . . 10
3.2.5. The "anyxml" Data Node . . . . . . . . . . . . . . . . 11
3.3. Mapping YANG Datatypes to JSON Values . . . . . . . . . . 12
3.3.1. Numeric Datatypes . . . . . . . . . . . . . . . . . . 12
3.3.2. The "string" Type . . . . . . . . . . . . . . . . . . 12
3.3.3. The "boolean" Type . . . . . . . . . . . . . . . . . . 12
3.3.4. The "enumeration" Type . . . . . . . . . . . . . . . . 12
3.3.5. The "bits" Type . . . . . . . . . . . . . . . . . . . 12
3.3.6. The "binary" Type . . . . . . . . . . . . . . . . . . 12
3.3.7. The "leafref" Type . . . . . . . . . . . . . . . . . . 13
3.3.8. The "identityref" Type . . . . . . . . . . . . . . . . 13
3.3.9. The "empty" Type . . . . . . . . . . . . . . . . . . . 13
3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . . 13
3.3.11. The "instance-identifier" Type . . . . . . . . . . . . 14
3.4. IANA Considerations . . . . . . . . . . . . . . . . . . . 14
3.5. Security Considerations . . . . . . . . . . . . . . . . . 14
3.6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 14
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Normative References . . . . . . . . . . . . . . . . . . . 15
4.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. A Complete Example . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
The aim of this document is define rules for presenting configuration
and operational state data defined in the YANG data modeling
language [RFC6020] as JavaScript Object Notation (JSON) text [JSON].
The result can be potentially applied in two different ways:
1. JSON may be used instead of the standard XML [XML] encoding in
the context of the NETCONF protocol [RFC6241] and/or with
existing data models expressed in YANG. An example application
is the RESTCONF Protocol [RESTCONF].
2. Other documents that choose JSON to represent structured data can
use YANG for defining the data model, i.e., both syntactic and
semantic constraints that the data have to satisfy.
JSON mapping rules could be specified in a similar way as the XML
mapping rules in [RFC6020]. This would however require solving
several problems. To begin with, YANG uses XPath [XPath] quite
extensively, but XPath is not defined for JSON and such a definition
would be far from straightforward.
In order to avoid these technical difficulties, this document employs
an alternative approach: it defines a relatively simple procedure
which allows for translating the subset of XML that can be modeled
using YANG to JSON, and vice versa. Consequently, validation of a
JSON text against a data model can done by translating the JSON text
to XML, which is then validated according to the rules stated in
[RFC6020].
The translation procedure is adapted to YANG specifics and
requirements, namely:
1. The translation is driven by a concrete YANG data model and uses
information about data types to achieve better results than
generic XML-JSON translation procedures.
2. Various document types are supported, namely configuration data,
configuration + state data, RPC input and output parameters, and
notifications.
3. XML namespaces specified in the data model are mapped to
namespaces of JSON objects. However, explicit namespace
identifiers are rarely needed in JSON text.
4. Translation of XML attributes, mixed content, comments and
processing instructions is outside the scope of this document.
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Item 1 above also means that, depending on the data model, the same
XML element can be translated to different JSON objects. For
example,
<foo>123</foo>
is translated to
"foo": 123
if the "foo" node is defined as a leaf with the "uint8" datatype, or
to
"foo": ["123"]
if the "foo" node is defined as a leaf-list with the "string"
datatype, and the <foo> element has no siblings of the same name.
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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 [RFC6020]:
o anyxml
o augment
o container
o data node
o data tree
o datatype
o feature
o identity
o instance identifier
o leaf
o leaf-list
o list
o module
o submodule
The following terms are defined in [XMLNS]:
o local name
o prefixed name
o qualified name
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3. Specification of the Translation Procedure
The translation procedure defines a 1-1 correspondence between the
subset of YANG-compatible XML documents and JSON text. This means
that the translation can be applied in both directions and is always
invertible.
The translation procedure is applicable only to data hierarchies that
are modelled by a YANG data model. An input XML document MAY contain
enclosing elements representing NETCONF "Operations" and "Messages"
layers. However, these enclosing elements are ignored by the
translation procedure and do not appear in the resulting JSON
document.
Any YANG-compatible XML document can be translated, except documents
with mixed content. This is only a minor limitation since mixed
content is marginal in YANG - it is allowed only in "anyxml" nodes.
The following sections specify rules mainly for translating XML
documents to JSON text. Rules for the inverse translation are stated
only where necessary, otherwise they can be easily inferred.
REQUIRED parameters of the translation procedure are:
o YANG data model consisting of a set of YANG modules,
o type of the input document,
o optional features (defined via the "feature" statement) that are
considered active.
The permissible types of input documents are listed in Table 1
together with the corresponding part of the data model that is used
for the translation.
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+------------------------------+--------------------------------+
| Document Type | Data Model Section |
+------------------------------+--------------------------------+
| configuration and state data | main data tree |
| | |
| configuration | main data tree ("config true") |
| | |
| RPC input parameters | "input" nodes under "rpc" |
| | |
| RPC output parameters | "output" nodes under "rpc" |
| | |
| notification | "notification" nodes |
+------------------------------+--------------------------------+
Table 1: YANG Document Types
A particular application MAY decide to support only a subset of
document types from Table 1. For instance, RESTCONF Protocol
[RESTCONF] does not use notifications.
XML documents can be translated to JSON text only if they are valid
instances of the YANG data model and selected document type, also
taking into account the active features, if there are any.
The resulting JSON document is always a single object ([JSON],
Sec. 4) whose members are translated from the original XML document
using the rules specified in the following sections.
3.1. Names and Namespaces
The local part of a JSON name is always identical to the local name
of the corresponding XML element.
Each JSON name lives in a namespace which is uniquely identified by
the name of the YANG module where the corresponding data node is
defined. If the data node is defined in a submodule, then the
namespace identifier is the name of the main module to which the
submodule belongs. The translation procedure MUST correctly map YANG
namespace URIs to YANG module names and vice versa.
The namespace SHALL be expressed in JSON text by prefixing the local
name in the following way:
<module name>:<local name>
Figure 1: Encoding a namespace identifier with a local name.
The namespace identifier MUST be used for local names that are
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ambiguous, i.e., whenever the data model permits a sibling node with
the same local name. Otherwise, the namespace identifier is
OPTIONAL.
For example, consider the following YANG module:
module foomod {
namespace "http://example.com/foomod";
prefix "fm";
container foo {
leaf bar {
type boolean;
}
}
}
If the data model consists only of this module, then the following is
a valid JSON document:
{
"foo": {
"bar": true
}
}
Now, assume the container "foo" is augmented from another module:
module barmod {
namespace "http://example.com/barmod";
prefix "bm";
import foomod {
prefix fm;
}
augment "/fm:foo" {
leaf bar {
type uint8;
}
}
}
In the data model combining "foomod" and "barmod", we have two
sibling nodes with the same local name, namely "bar". In this case,
a valid JSON document has to specify an explicit namespace identifier
(module name) for both leaves:
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{
"foo": {
"foomod:bar": true,
"barmod:bar": 123
}
}
3.2. Mapping XML Elements to JSON Objects
XML elements that are modelled as YANG data nodes are translated to a
name/value pair where the name is formed from the name of the XML
element using the rules in Section 3.1. The value depends on the
type of the data node as specified in the following sections.
3.2.1. The "leaf" Data Node
An XML element that is modeled as YANG leaf is translated to a name/
value pair and the type of the value is derived from the YANG
datatype of the leaf (see Section 3.3 for the datatype mapping
rules).
Example: For the leaf node definition
leaf foo {
type uint8;
}
the XML element
<foo>123</foo>
corresponds to the JSON name/value pair
"foo": 123
3.2.2. The "container" Data Node
An XML element that is modeled as YANG container is translated to a
name/object pair.
Example: For the container node definition
container bar {
leaf foo {
type uint8;
}
}
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the XML element
<bar>
<foo>123</foo>
</bar>
corresponds to the JSON name/value pair
"bar": {
"foo": 123
}
3.2.3. The "leaf-list" Data Node
A sequence of one or more sibling XML elements with the same
qualified name that is modeled as YANG leaf-list is translated to a
name/array pair, and the array elements are primitive values whose
type depends on the datatype of the leaf-list (see Section 3.3).
Example: For the leaf-list node definition
leaf-list foo {
type uint8;
}
the XML elements
<foo>123</foo>
<foo>0</foo
corresponds to the JSON name/value pair
"foo": [123, 0]
3.2.4. The "list" Data Node
A sequence of one or more sibling XML elements with the same
qualified name that is modeled as YANG list is translated to a name/
array pair, and the array elements are JSON objects.
Unlike the XML encoding, where the list keys are required to come
before any other siblings, and in the order specified by the data
model, the order of members within a JSON list entry is arbitrary,
because JSON objects are fundamentally unordered collections of
members.
Example: For the list node definition
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list bar {
key foo;
leaf foo {
type uint8;
}
leaf baz {
type string;
}
}
the XML elements
<bar>
<foo>123</foo>
<baz>zig</baz>
</bar>
<bar>
<foo>0</foo>
<baz>zag</baz>
</bar>
corresponds to the JSON name/value pair
"bar": [
{
"foo": 123,
"baz": "zig"
},
{
"foo": 0,
"baz": "zag"
}
]
3.2.5. The "anyxml" Data Node
An XML element that is modeled as a YANG anyxml node is translated to
a name/object pair. The content of such an element is not modelled
by YANG, and there may not be a straightforward mapping to JSON text
(e.g., if it is a mixed XML content). Therefore, translation of
anyxml contents is necessarily application-specific and outside the
scope of this document.
Example: For the anyxml node definition
anyxml bar;
the XML element
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<bar>
<p xmlns="http://www.w3.org/1999/xhtml">
This is <em>very</em> cool.
</p>
</bar>
may be translated to the following JSON name/value pair:
{
"bar": {
"p": "This is *very* cool."
}
}
3.3. Mapping YANG Datatypes to JSON Values
3.3.1. Numeric Datatypes
A value of one of the YANG numeric datatypes ("int8", "int16",
"int32", "int64", "uint8", "uint16", "uint32", "uint64" and
"decimal64") is mapped to a JSON number using the same lexical
representation.
3.3.2. The "string" Type
A "string" value is mapped to an identical JSON string, subject to
JSON encoding rules.
3.3.3. The "boolean" Type
A "boolean" value is mapped to the corresponding JSON value 'true' or
'false'.
3.3.4. The "enumeration" Type
An "enumeration" value is mapped in the same way as a string except
that the permitted values are defined by "enum" statements in YANG.
3.3.5. The "bits" Type
A "bits" value is mapped to a string identical to the lexical
representation of this value in XML, i.e., space-separated names
representing the individual bit values that are set.
3.3.6. The "binary" Type
A "binary" value is mapped to a JSON string identical to the lexical
representation of this value in XML, i.e., base64-encoded binary
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data.
3.3.7. The "leafref" Type
A "leafref" value is mapped according to the same rules as the type
of the leaf being referred to.
3.3.8. The "identityref" Type
An "identityref" value is mapped to a string representing the
qualified name of the identity. Its namespace MAY be expressed as
shown in Figure 1. If the namespace part is not present, the
namespace of the name of the JSON object containing the value is
assumed.
3.3.9. The "empty" Type
An "empty" value is mapped to '[null]', i.e., an array with the
'null' value being its only element.
This representation was chosen instead of using simply 'null' in
order to facilitate the use of empty leafs in common programming
languages. When used in a boolean context, the '[null]' value,
unlike 'null', evaluates to 'true'.
Example: For the leaf node definition
leaf foo {
type empty;
}
the XML element
<foo/>
corresponds to the JSON name/value pair
"foo": [null]
3.3.10. The "union" Type
YANG "union" type represents a choice among multiple alternative
types. The actual type of the XML value MUST be determined using the
procedure specified in Sec. 9.12 of [RFC6020] and the mapping rules
for that type are used.
For example, consider the following YANG definition:
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leaf-list bar {
type union {
type uint16;
type string;
}
}
The sequence of three XML elements
<bar>6378</bar>
<bar>14.5</bar>
<bar>infinity</bar>
will then be translated to this name/array pair:
"bar": [6378, "14.5", "infinity"]
3.3.11. The "instance-identifier" Type
An "instance-identifier" value is a string representing a simplified
XPath specification. It is mapped to an analogical JSON string in
which all occurrences of XML namespace prefixes are either removed or
replaced with the corresponding module name according to the rules of
Section 3.1.
When translating such a value from JSON to XML, all components of the
instance-identifier MUST be given appropriate XML namespace prefixes.
It is RECOMMENDED that these prefixes be those defined via the
"prefix" statement in the corresponding YANG modules.
3.4. IANA Considerations
TBD.
3.5. Security Considerations
TBD.
3.6. Acknowledgments
The author wishes to thank Andy Bierman, Martin Bjorklund and Phil
Shafer for their helpful comments and suggestions.
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4. References
4.1. Normative References
[JSON] Bray, T., Ed., "The JSON Data Interchange Format",
draft-ietf-json-rfc4627bis-03 (work in progress),
September 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
Network Configuration Protocol (NETCONF)", RFC 6020,
September 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "NETCONF Configuration Protocol", RFC 6241,
June 2011.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., 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/2006/REC-xml-20060816>.
[XMLNS] Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
Thompson, "Namespaces in XML 1.0 (Third Edition)", World
Wide Web Consortium Recommendation REC-xml-names-20091208,
December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
4.2. Informative References
[IF-CFG] Bjorklund, M., "A YANG Data Model for Interface
Management", draft-ietf-netmod-interfaces-cfg-12 (work in
progress), July 2013.
[RESTCONF]
Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
"RESTCONF Protocol", draft-bierman-netconf-restconf-01
(work in progress), September 2013.
[XPath] Clark, J., "XML Path Language (XPath) Version 1.0", World
Wide Web Consortium Recommendation REC-xpath-19991116,
November 1999,
<http://www.w3.org/TR/1999/REC-xpath-19991116>.
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Appendix A. A Complete Example
The JSON document shown below was translated from a reply to the
NETCONF <get> request that can be found in Appendix D of [IF-CFG].
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 [IF-CFG]). The "if-mib" feature defined in the "ietf-
interfaces" module is considered to be active.
{
"interfaces": {
"interface": [
{
"name": "eth0",
"type": "ethernetCsmacd",
"enabled": false
},
{
"name": "eth1",
"type": "ethernetCsmacd",
"enabled": true,
"vlan-tagging": true
},
{
"name": "eth1.10",
"type": "l2vlan",
"enabled": true,
"base-interface": "eth1",
"vlan-id": 10
},
{
"name": "lo1",
"type": "softwareLoopback",
"enabled": true
}
]
},
"interfaces-state": {
"interface": [
{
"name": "eth0",
"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"
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}
},
{
"name": "eth1",
"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": "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": "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": "softwareLoopback",
"admin-status": "up",
"oper-status": "up",
"if-index": 1,
"statistics": {
"discontinuity-time": "2013-04-01T03:00:00+00:00"
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}
}
]
}
}
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Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
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