Modeling JSON Text with YANG
draft-lhotka-yang-json-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
|
|
|---|---|---|---|
| Author | Ladislav Lhotka | ||
| Last updated | 2012-04-02 | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-lhotka-yang-json-00
NETMOD L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track April 02, 2012
Expires: October 4, 2012
Modeling JSON Text with YANG
draft-lhotka-yang-json-00
Abstract
This document defines rules for mapping data models expressed in YANG
to 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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 4, 2012.
Copyright Notice
Copyright (c) 2012 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.
Lhotka Expires October 4, 2012 [Page 1]
Internet-Draft Modeling JSON with YANG April 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Notation . . . . . . . . . . . . . . . . . . . 4
3. Specification of the Translation Procedure . . . . . . . . . . 5
3.1. Names and Namespaces . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . 7
3.3.8. The "identityref" Type . . . . . . . . . . . . . . . . 7
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
Lhotka Expires October 4, 2012 [Page 2]
Internet-Draft Modeling JSON with YANG April 2012
1. Introduction
The aim of this document is define rules for mapping data models
expressed in the YANG data modeling language [RFC6020] to JavaScript
Object Notation (JSON) text [RFC4627]. The result can be potentially
applied in two different ways:
1. JSON can be used in the context of the NETCONF protocol [RFC6241]
and with existing data models expressed in YANG.
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.
The former use case is not currently possible without further work
because all NETCONF and YANG standards so far assume that XML [XML]
is used for encoding both protocol messages and configuration data.
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
heavily 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. Translation of YANG namespaces is supported.
2. The information about datatypes of leaf nodes is used for
translating the leaf values to the most appropriate JSON
datatype.
3. Translation of XML attributes, mixed content, comments and
processing instructions is not supported.
Properties 1 and 2 mean that the translation is driven by a YANG data
model which must therefore be known in advance.
Lhotka Expires October 4, 2012 [Page 3]
Internet-Draft Modeling JSON with YANG April 2012
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
Lhotka Expires October 4, 2012 [Page 4]
Internet-Draft Modeling JSON with YANG April 2012
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 following subsections contain mainly rules for translating XML
documents to JSON text. Rules for the inverse translation are stated
only where needed, otherwise they can be easily inferred.
XML documents may be translated to JSON text only if they are valid
instances of a YANG data model, which must therefore be known
beforehand. There is one extra restriction beyond the standard YANG
rules: XML documents with mixed content - which is marginal in YANG
and allowed only in anyxml nodes - cannot be translated by this
procedure.
The semantics of several YANG statements, such as "rpc",
"notification", "config", "if-feature" or "default", are more or less
specific to NETCONF and may or may not be meaningful in a non-NETCONF
context. If such a statement appears in a data model that is used
for the translation procedure, then all YANG rules concerning this
statement MUST be observed.
For example, an application that uses YANG for validating JSON text
may decide to use the "default" statement for defining default values
of JSON's primitive datatypes (string, number or boolean). If it
does so, the default values SHALL also be taken into account for the
validation of semantic constraints such as those defined by "must"
statements.
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.
Most of the time, the namespace of JSON names is implicit. The
namespace MUST be explicitly specified whenever the namespace of a
name differs from that of its parent object, or whenever no parent
object exists. The namespace MUST NOT be explicitly specified
elsewhere.
Lhotka Expires October 4, 2012 [Page 5]
Internet-Draft Modeling JSON with YANG April 2012
In YANG terms, the namespace SHALL be specified only in the names of
(i) top-level objects and (ii) objects that augment the data tree of
another module (see Section 7.15 in [RFC6020]).
Where the namespace has to be specified in JSON text, it SHALL be
done in the following form:
<module name>:<local name>
Figure 1: Encoding a namespace identifier with a local name.
The translation procedure MUST correctly map YANG namespace URIs to
YANG module names and vice versa.
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 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).
* 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 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.
Lhotka Expires October 4, 2012 [Page 6]
Internet-Draft Modeling JSON with YANG April 2012
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.
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
Lhotka Expires October 4, 2012 [Page 7]
Internet-Draft Modeling JSON with YANG April 2012
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:
Lhotka Expires October 4, 2012 [Page 8]
Internet-Draft Modeling JSON with YANG April 2012
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:
Lhotka Expires October 4, 2012 [Page 9]
Internet-Draft Modeling JSON with YANG April 2012
{
"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 Martin Bjorklund and Phil Shafer for their
helpful comments and suggestions.
Lhotka Expires October 4, 2012 [Page 10]
Internet-Draft Modeling JSON with YANG April 2012
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>.
Lhotka Expires October 4, 2012 [Page 11]
Internet-Draft Modeling JSON with YANG April 2012
Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
Lhotka Expires October 4, 2012 [Page 12]