NETMOD L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track April 21, 2014
Expires: October 23, 2014
JSON Encoding of Data Modeled with YANG
draft-ietf-netmod-yang-json-00
Abstract
This document defines rules for representing configuration and 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. A JSON encoding of XML attributes is
also defined so as to allow for including metadata in JSON documents.
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 October 23, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 4
3. Specification of the Translation Procedure . . . . . . . . . 5
3.1. Names and Namespaces . . . . . . . . . . . . . . . . . . 6
3.2. Mapping XML Elements to JSON Objects . . . . . . . . . . 8
3.2.1. The "leaf" Data Node . . . . . . . . . . . . . . . . 8
3.2.2. The "container" Data Node . . . . . . . . . . . . . . 8
3.2.3. The "leaf-list" Data Node . . . . . . . . . . . . . . 9
3.2.4. The "list" Data Node . . . . . . . . . . . . . . . . 9
3.2.5. The "anyxml" Data Node . . . . . . . . . . . . . . . 10
3.3. Mapping YANG Datatypes to JSON Values . . . . . . . . . . 11
3.3.1. Numeric Datatypes . . . . . . . . . . . . . . . . . . 11
3.3.2. The "string" Type . . . . . . . . . . . . . . . . . . 11
3.3.3. The "boolean" Type . . . . . . . . . . . . . . . . . 11
3.3.4. The "enumeration" Type . . . . . . . . . . . . . . . 11
3.3.5. The "bits" Type . . . . . . . . . . . . . . . . . . . 12
3.3.6. The "binary" Type . . . . . . . . . . . . . . . . . . 12
3.3.7. The "leafref" Type . . . . . . . . . . . . . . . . . 12
3.3.8. The "identityref" Type . . . . . . . . . . . . . . . 12
3.3.9. The "empty" Type . . . . . . . . . . . . . . . . . . 12
3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . 13
3.3.11. The "instance-identifier" Type . . . . . . . . . . . 13
4. Encoding Metadata in JSON . . . . . . . . . . . . . . . . . . 14
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. A Complete Example . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The aim of this document is define rules for representing
configuration and state data defined using the YANG data modeling
language [RFC6020] as JavaScript Object Notation (JSON)
text [RFC7159]. 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].
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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. Section 4 defines JSON encoding of XML attributes. Although XML
attributes cannot be modeled with YANG, they are often used for
attaching metadata to elements, and a standard JSON encoding is
therefore needed.
5. Translation of XML mixed content, comments and processing
instructions is outside the scope of this document.
Item 1 above also means that, depending on the data model, the same
XML element can be translated to different JSON objects. For
example,
123
is translated to
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"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 element has no siblings of the same name.
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]:
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o local name
o prefixed name
o qualified name
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 it 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 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 data
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" data nodes under "rpc" |
| | |
| RPC output parameters | "output" data nodes under "rpc" |
| | |
| notification | "notification" data nodes |
+------------------------------+---------------------------------+
Table 1: YANG Document Types
When translating XML to JSON, the type of the input document can
often be determined form the encapsulating elements belonging to the
"Operations" or "Messages" layer as defined by the NETCONF protocol
(see Sec. 1.2 in [RFC6241]).
A particular application MAY decide to support only a subset of
document types from Table 1.
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 ([RFC7159],
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:
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:
Figure 1: Encoding a namespace identifier with a local name.
The namespace identifier MUST be used for local names that are
ambiguous, i.e., whenever the data model permits a sibling data 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 data nodes with the same local name, namely "bar". In this
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case, a valid JSON document has to specify an explicit namespace
identifier (module name) for both leaves:
{
"foo": {
"foomod:bar": true,
"barmod:bar": 123
}
}
3.2. Mapping XML Elements to JSON Objects
An XML element that is modelled as a YANG data node is 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
123
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 definition
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container bar {
leaf foo {
type uint8;
}
}
the XML element
123
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 definition
leaf-list foo {
type uint8;
}
the XML elements
123
0
123
zig
0
zag
correspond 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 data 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.
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Example: For the anyxml definition
anyxml bar;
the XML element
This is very cool.
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.
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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
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 encoding 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 definition
leaf foo {
type empty;
}
the XML element
corresponds to the JSON name/value pair
"foo": [null]
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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:
leaf-list bar {
type union {
type uint16;
type string;
}
}
The sequence of three XML elements
6378
14.5
infinity
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.
For example, assume "ex" is the prefix defined for the "example"
module. Then the XML-encoded instance identifier
/ex:system/ex:user[ex:name='fred']
corresponds to the following JSON-encoded instance identifier:
/example:system/example:user[example:name='fred']
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or simply
/system/user[name='fred']
if the local names of the data nodes "system", "user" and "name" are
unambiguous.
4. Encoding Metadata in JSON
By design, YANG does not allow for modeling XML attributes. However,
attributes are often used in XML instance documents for attaching
various types of metadata information to elements. It is therefore
desirable to have a standard way for representing attributes in JSON
documents as well.
The metadata encoding defined in the rest of this section satisfies
the following two important requirements:
1. There has to be a way for adding metadata to instances of all
types of YANG data nodes, i.e., leafs, containers, list and leaf-
list entries, and anyxml nodes.
2. The encoding of YANG data node instances as defined in the
previous sections must not change.
Existing proposals for metadata encoding in JSON, such as
[JSON-META], are oriented on rather specific uses of metadata, and
fall short with respect to the first requirement.
All attributes assigned to an XML element are mapped in JSON to
members (name/value pairs) of a single object, henceforth denoted as
the metadata object. The placement of this object depends on the
type of the element from YANG viewpoint, as specified in the
following paragraphs.
For an XML element that is translated to a JSON object (i.e., a
container, anyxml node and list entry), the metadata object is added
as a new member of that object with the name "@".
Examples:
o If "cask" is a container or anyxml node, the XML instance with
attributes
...
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is mapped to the following JSON object:
"cask": {
"@": {
"foo": "a",
"bar": "b"
}
...
}
o If "seq" is a list, then the pair of XML elements
one
two
is mapped to the following JSON array:
"seq": [
{
"@": {
"foo": "a"
},
"name": "one"
},
{
"@": {
"bar": "b"
},
"name": "two"
}
]
In order to assign attributes to a leaf instance, a sibling name/
value pair is added, where the name is the symbol "@" concatenated
with the identifier of the leaf.
For example, the element
true
is mapped to the following two name/value pairs:
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"flag": true,
"@flag": {
"foo": "a",
"bar": "b"
}
Finally, for a leaf-list instance, which is represented as a JSON
array with primitive values, attributes may be assigned to one or
more entries by adding a sibling name/value pair, where the name is
the symbol "@" concatenated with the identifier of the leaf-list, and
the value is a JSON array whose i-th element is the metadata object
with attributes assigned to the i-th entry of the leaf-list, or nil
if the i-th entry has no attributes.
Trailing nil values in the array, i.e., those following the last non-
nil metadata object, MAY be omitted.
For example, a leaf-list instance with four entries
6
3
7
8
is mapped to the following two name/value pairs:
"folio": [6, 3, 7, 8],
"@folio": [nil, {"foo": "a"}, {"bar": "b"}]
The encoding of attributes as specified above has the following two
limitations:
o Mapping of namespaces of XML attributes is undefined.
o Attribute values can only be strings, other data types are not
supported.
5. IANA Considerations
TBD - register application/yang.data+json media type?
6. Security Considerations
TBD.
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7. Acknowledgments
The author wishes to thank Andy Bierman, Martin Bjorklund and Phil
Shafer for their helpful comments and suggestions.
8. References
8.1. Normative References
[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.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[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,
.
[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,
.
8.2. Informative References
[IF-CFG] Bjorklund, M., "A YANG Data Model for Interface
Management", draft-ietf-netmod-interfaces-cfg-16 (work in
progress), January 2014.
[JSON-META]
Sakimura, N., "JSON Metadata", draft-sakimura-json-
metadata-01 (work in progress), November 2013.
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[RESTCONF]
Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
"RESTCONF Protocol", draft-ietf-netconf-restconf-00 (work
in progress), March 2014.
[XPath] Clark, J., "XML Path Language (XPath) Version 1.0", World
Wide Web Consortium Recommendation REC-xpath-19991116,
November 1999,
.
Appendix A. A Complete Example
The JSON document shown below was translated from a reply to the
NETCONF 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": "iana-if-type:ethernetCsmacd",
"enabled": false
},
{
"name": "eth1",
"type": "iana-if-type:ethernetCsmacd",
"enabled": true,
"vlan-tagging": true
},
{
"name": "eth1.10",
"type": "iana-if-type:l2vlan",
"enabled": true,
"base-interface": "eth1",
"vlan-id": 10
},
{
"name": "lo1",
"type": "iana-if-type:softwareLoopback",
"enabled": true
}
]
},
"interfaces-state": {
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"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"
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}
},
{
"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"
}
}
]
}
}
Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
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