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Versions: 00 01 02 draft-ietf-netmod-yang-json

NETMOD                                                         L. Lhotka
Internet-Draft                                                    CZ.NIC
Intended status: Standards Track                           April 2, 2013
Expires: October 4, 2013


                      Modeling JSON Text with YANG
                    draft-lhotka-netmod-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.

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
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   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, 2013.

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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   (http://trustee.ietf.org/license-info) in effect on the date of
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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 . . . . . . . . . . . . . . . . . . . . . . . . .  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 . . . . . . . . . . .  7
     3.3.  Mapping YANG Datatypes to JSON Values  . . . . . . . . . .  8
       3.3.1.  Numeric Types  . . . . . . . . . . . . . . . . . . . .  8
       3.3.2.  The "string" Type  . . . . . . . . . . . . . . . . . .  8
       3.3.3.  The "boolean" Type . . . . . . . . . . . . . . . . . .  8
       3.3.4.  The "enumeration" Type . . . . . . . . . . . . . . . .  8
       3.3.5.  The "bits" Type  . . . . . . . . . . . . . . . . . . .  8
       3.3.6.  The "binary" Type  . . . . . . . . . . . . . . . . . .  8
       3.3.7.  The "leafref" Type . . . . . . . . . . . . . . . . . .  8
       3.3.8.  The "identityref" Type . . . . . . . . . . . . . . . .  8
       3.3.9.  The "empty" Type . . . . . . . . . . . . . . . . . . .  9
       3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . .  9
       3.3.11. The "instance-identifier" Type . . . . . . . . . . . .  9
     3.4.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 12
     3.6.  Security Considerations  . . . . . . . . . . . . . . . . . 12
     3.7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 12
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     4.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
























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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 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 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 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.

   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.

   An implementation of the translation procedure MAY translate "anyxml"
   nodes and their contents from XML to JSON or vice versa, but the
   specific details of this translation are outside the scope of this
   document.  Note that the contents of "anyxml" nodes are not relevant
   for validity in terms of a YANG data model.

   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.


















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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 use only a subset of document
   types from Table 1.  For instance, YANG-API Protocol [YANG-API] 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.

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.




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   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  An XML element that is modeled as YANG leaf 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).

   o  An XML element that is modeled as YANG container is translated to
      a JSON object.

   o  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).

   o  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, which requires the list keys to come first and
      in the order specified by the data model, the order of members
      within a list entry is arbitrary, because JSON objects are
      fundamentally unordered collections of members.

   Note that the same XML element may be translated in different ways,
   depending on the definition of the corresponding data node in YANG.
   For example,

       <foo>42</foo>

   is translated to

       "foo": 42

   if the "foo" node is defined as a leaf with the "uint8" datatype, or
   to

       "foo": ["42"]



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   if the "foo" node is defined as a leaf-list with the "string"
   datatype.

3.3.  Mapping YANG Datatypes to JSON Values

3.3.1.  Numeric Types

   A value of one of the YANG numeric types ("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
   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



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   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.

   For example, consider the following YANG definition:

       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



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   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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   {
     "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.

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-01 (work in progress),
              November 2012.










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Author's Address

   Ladislav Lhotka
   CZ.NIC

   Email: lhotka@nic.cz













































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