| NETMOD | L. Lhotka |
| Internet-Draft | CZ.NIC |
| Intended status: Standards Track | April 22, 2014 |
| Expires: October 24, 2014 |
JSON Encoding of Data Modeled with YANG
draft-ietf-netmod-yang-json-00
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.
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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:
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:
<foo>123</foo>
"foo": 123
"foo": ["123"]
Item [it.dmd] above also means that, depending on the data model, the same XML element can be translated to different JSON objects. For example,
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]:
The following terms are defined in [XMLNS]:
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:
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.
| 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 |
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.
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 ambiguous, i.e., whenever the data model permits a sibling data node with the same local name. Otherwise, the namespace identifier is OPTIONAL.
module foomod {
namespace "http://example.com/foomod";
prefix "fm";
container foo {
leaf bar {
type boolean;
}
}
}
For example, consider the following YANG module:
{
"foo": {
"bar": true
}
}
If the data model consists only of this module, then the following is a valid JSON document:
module barmod {
namespace "http://example.com/barmod";
prefix "bm";
import foomod {
prefix fm;
}
augment "/fm:foo" {
leaf bar {
type uint8;
}
}
}
Now, assume the container "foo" is augmented from another module:
{
"foo": {
"foomod:bar": true,
"barmod:bar": 123
}
}
In the data model combining "foomod" and "barmod", we have two sibling data 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:
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.
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).
leaf foo {
type uint8;
}
<foo>123</foo>
"foo": 123
Example: For the leaf node definition
An XML element that is modeled as YANG container is translated to a name/object pair.
container bar {
leaf foo {
type uint8;
}
}
<bar>
<foo>123</foo>
</bar>
"bar": {
"foo": 123
}
Example: For the container definition
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).
leaf-list foo {
type uint8;
}
<foo>123</foo>
<foo>0</foo
"foo": [123, 0]
Example: For the leaf-list definition
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.
list bar {
key foo;
leaf foo {
type uint8;
}
leaf baz {
type string;
}
}
<bar>
<foo>123</foo>
<baz>zig</baz>
</bar>
<bar>
<foo>0</foo>
<baz>zag</baz>
</bar>
"bar": [
{
"foo": 123,
"baz": "zig"
},
{
"foo": 0,
"baz": "zag"
}
]
Example: For the list definition
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.
anyxml bar;
<bar>
<p xmlns="http://www.w3.org/1999/xhtml">
This is <em>very</em> cool.
</p>
</bar>
{
"bar": {
"p": "This is *very* cool."
}
}
Example: For the anyxml definition
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.
A "string" value is mapped to an identical JSON string, subject to JSON encoding rules.
A "boolean" value is mapped to the corresponding JSON value 'true' or 'false'.
An "enumeration" value is mapped in the same way as a string except that the permitted values are defined by "enum" statements in YANG.
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.
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.
A "leafref" value is mapped according to the same rules as the type of the leaf being referred to.
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.
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'.
leaf foo {
type empty;
}
<foo/>
"foo": [null]
Example: For the leaf definition
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.
leaf-list bar {
type union {
type uint16;
type string;
}
}
<bar>6378</bar>
<bar>14.5</bar>
<bar>infinity</bar>
"bar": [6378, "14.5", "infinity"]
For example, consider the following YANG definition:
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.
/ex:system/ex:user[ex:name='fred']
/example:system/example:user[example:name='fred']
/system/user[name='fred']
For example, assume "ex" is the prefix defined for the "example" module. Then the XML-encoded instance identifier
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:
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 "@".
<cask foo="a" bar="b">
...
</cask>
"cask": {
"@": {
"foo": "a",
"bar": "b"
}
...
}
<seq foo="a">
<name>one</name>
</seq>
<seq bar="b">
<name>two</name>
</seq>
"seq": [
{
"@": {
"foo": "a"
},
"name": "one"
},
{
"@": {
"bar": "b"
},
"name": "two"
}
]
Examples:
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.
<flag foo="a" bar="b">true</foo>
"flag": true,
"@flag": {
"foo": "a",
"bar": "b"
}
For example, the element
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.
<folio>6</folio>
<folio foo="a">3</folio>
<folio bar="b">7</folio>
<folio>8</folio>
"folio": [6, 3, 7, 8],
"@folio": [nil, {"foo": "a"}, {"bar": "b"}]
For example, a leaf-list instance with four entries
The encoding of attributes as specified above has the following two limitations:
TBD - register application/yang.data+json media type?
TBD.
The author wishes to thank Andy Bierman, Martin Bjorklund and Phil Shafer for their helpful comments and suggestions.
| [RFC7159] | Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC6020] | Bjorklund, M., "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. |
| [XMLNS] | Bray, T., Hollander, D., Layman, A., Tobin, R. and H. S. Thompson, "Namespaces in XML 1.0 (Third Edition)", World Wide Web Consortium Recommendation REC-xml-names-20091208, December 2009. |
| [IF-CFG] | Bjorklund, M., "A YANG Data Model for Interface Management", Internet-Draft draft-ietf-netmod-interfaces-cfg-16, January 2014. |
| [JSON-META] | Sakimura, N., "JSON Metadata", Internet-Draft draft-sakimura-json-metadata-01, November 2013. |
| [RESTCONF] | Bierman, A., Bjorklund, M., Watsen, K. and R. Fernando, "RESTCONF Protocol", Internet-Draft draft-ietf-netconf-restconf-00, March 2014. |
| [XPath] | Clark, J., "XML Path Language (XPath) Version 1.0", World Wide Web Consortium Recommendation REC-xpath-19991116, November 1999. |
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": "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": {
"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"
}
}
]
}
}