NETMOD L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track June 05, 2012
Expires: December 7, 2012
Modeling JSON Text with YANG
draft-lhotka-yang-json-01
Abstract
This document defines rules for mapping data models expressed in YANG
to configuration and operational state data encoded 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . 6
3.3. Mapping YANG Datatypes to JSON Values . . . . . . . . . . 7
3.3.1. Numeric Types . . . . . . . . . . . . . . . . . . . . 7
3.3.2. The "string" Type . . . . . . . . . . . . . . . . . . 7
3.3.3. The "boolean" Type . . . . . . . . . . . . . . . . . . 7
3.3.4. The "enumeration" Type . . . . . . . . . . . . . . . . 7
3.3.5. The "bits" Type . . . . . . . . . . . . . . . . . . . 7
3.3.6. The "binary" Type . . . . . . . . . . . . . . . . . . 7
3.3.7. The "leafref" Type . . . . . . . . . . . . . . . . . . 8
3.3.8. The "identityref" Type . . . . . . . . . . . . . . . . 8
3.3.9. The "empty" Type . . . . . . . . . . . . . . . . . . . 8
3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . . 8
3.3.11. The "instance-identifier" Type . . . . . . . . . . . . 8
3.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. IANA Considerations . . . . . . . . . . . . . . . . . . . 10
3.6. Security Considerations . . . . . . . . . . . . . . . . . 10
3.7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 10
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Normative References . . . . . . . . . . . . . . . . . . . 11
4.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The aim of this document is define rules for mapping data models
expressed in the YANG data modeling language [RFC6020] to
configuration and operational state data encoded as JavaScript Object
Notation (JSON) text [RFC4627]. 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 YANG-API Protocol [YANG-API].
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 to translate 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 in JSON text are rarely needed.
4. Translation of XML attributes, mixed content, comments and
processing instructions is not supported.
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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 model
o data node
o data tree
o datatype
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.
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 subsections 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,
o type of the input XML document,
o optional features (defined via the "feature" statement) that are
considered active.
The permissible types of XML 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" nodes under "rpc" |
| | |
| RPC output parameters | "output" nodes under "rpc" |
| | |
| notification | "notification" nodes |
+------------------------------+--------------------------------+
Table 1: YANG Document Types
A particular application may decide to use only a subset of document
types from Table 1. For instance, YANG-API Protocol [YANG-API] does
not use notifications.
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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.
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
ambiguous, i.e., whenever the data model permits a sibling node with
the same local name. Otherwise, the namespace identifier is
OPTIONAL.
When mapping namespaces from JSON text to XML, the resulting XML
document may use default namespace declarations (via the "xmlns"
attribute), prefix-based namespace declarations (via attributes
beginning with "xmlns:"), or any combination thereof following the
rules stated in [XMLNS]. If prefixed names are used, their prefix
SHOULD be the one defined by the "prefix" statement in the YANG
module where each data node is defined.
3.2. Mapping XML Elements to JSON Objects
XML elements are translated to JSON objects in a straightforward way:
o XML elements that have no siblings of the same qualified name
correspond either to a name/value pair or to a JSON object:
* An XML element which is modeled as a leaf in YANG is translated
to a name/value pair and the JSON datatype of the value is
derived from the YANG datatype of the leaf (see Section 3.3 for
the datatype mapping rules).
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* An XML element which is modeled as a container in YANG is
translated to an object.
o A sequence of sibling XML elements with the same qualified name
(modeled as a list or leaf-list in YANG) corresponds to a JSON
array. If the sequence is modeled as a leaf-list in YANG, then
the array elements are primitive values (strings, numbers or
booleans) whose type depends on the datatype of the leaf-list (see
Section 3.3). If the sequence is modeled as a list in YANG, then
the array elements are JSON objects.
3.3. Mapping YANG Datatypes to JSON Values
3.3.1. Numeric Types
YANG numeric types ("int8", "int16", "int32", "int64", "uint8",
"uint16", "uint32", "uint64" and "decimal64") are mapped to JSON
numbers whose decimal representation is the YANG canonical form of
the number. Hexadecimal values MUST be converted to decimal.
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., a space-separated list of
bit values.
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.
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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, subject to the same constraints as the
XML value.
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'.
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.
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. Example
Consider a simple data model defined by the following YANG module:
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module ex-json {
namespace "http://example.com/ex-json";
prefix ej;
import ietf-inet-types {
prefix inet;
}
container top {
list address {
key "seqno";
leaf seqno {
type uint8;
}
leaf ip {
type inet:ip-address;
mandatory true;
}
}
container phases {
typedef angle {
type decimal64 {
fraction-digits 2;
}
units "radians";
}
leaf max-phase {
default "6.28";
type angle;
}
leaf-list phase {
type angle;
must ". <= ../max-phase";
min-elements 1;
}
}
}
}
Figure 2: Example YANG module.
By using the translation procedure defined in this document, we can
conclude that the following JSON text is valid according to the data
model:
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{
"ex-json:top": {
"address": [
{
"seqno": 1,
"ip": "192.0.2.1"
},
{
"seqno": 2,
"ip": "2001:db8:0:1::1"
}
],
"phases": {
"phase": [0.79, 1.04, 3.14]
}
}
}
Figure 3: Example JSON text.
Note that the semantic constraint specified by the "must" statement
in Figure 2 is satisfied by all elements of the "phase" array because
the default value of 6.28 is used for the absent "max-phase" leaf.
3.5. IANA Considerations
TBD.
3.6. Security Considerations
TBD.
3.7. 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[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
[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>.
[YANG-API]
Bierman, A. and M. Bjorklund, "YANG-API Protocol",
draft-bierman-netconf-yang-api-00 (work in progress),
May 2012.
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Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
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