Mapping YANG to Document Schema Definition Languages and Validating NETCONF Content
draft-ietf-netmod-dsdl-map-05

Abstract

This draft specifies the mapping rules for translating YANG data models into Document Schema Definition Languages (DSDL), a coordinated set of XML schema languages standardized as ISO 19757. The following DSDL schema languages are used by the mapping: RELAX NG, Schematron and DSRL. The mapping takes one or more YANG modules and produces a set of DSDL schemas for a selected target document type - datastore content, NETCONF PDU etc. Procedures for schema-based validation of such documents are also discussed.

Status of this Memo

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Copyright Notice

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Table of Contents

1.  Introduction
2.  Terminology and Notation
    2.1.  Glossary of New Terms
3.  Objectives and Motivation
4.  DSDL Schema Languages
    4.1.  RELAX NG
    4.2.  Schematron
    4.3.  Document Semantics Renaming Language (DSRL)
5.  Additional Annotations
    5.1.  Dublin Core Metadata Elements
    5.2.  RELAX NG DTD Compatibility Annotations
    5.3.  NETMOD-Specific Annotations
6.  Overview of the Mapping
7.  NETCONF Content Validation
8.  Design Considerations
    8.1.  Conceptual Data Tree
    8.2.  Modularity
    8.3.  Granularity
    8.4.  Handling of XML Namespaces
    8.5.  Features and Deviations
9.  Mapping YANG Data Models to the Conceptual Tree Schema
    9.1.  Occurrence Rules for Data Nodes
        9.1.1.  Optional and Mandatory Nodes
        9.1.2.  Implicit Nodes
    9.2.  Mapping YANG Groupings and Typedefs
        9.2.1.  YANG Refinements and Augments
        9.2.2.  Type Derivation Chains
    9.3.  Translation of XPath Expressions
    9.4.  YANG Language Extensions
10.  Mapping Conceptual Tree Schema to DSDL
    10.1.  Generating RELAX NG Schemas for Various Document Types
        10.1.1.  Reply to <get> or <get-config>
        10.1.2.  Remote Procedure Calls
        10.1.3.  Notifications
    10.2.  Mapping Semantic Constraints to Schematron
        10.2.1.  Validation Phases
    10.3.  Constraints on Mandatory Choice
    10.4.  Mapping Default Values to DSRL
11.  Mapping YANG Statements to Conceptual Tree Schema
    11.1.  The anyxml Statement
    11.2.  The argument Statement
    11.3.  The augment Statement
    11.4.  The base Statement
    11.5.  The belongs-to Statement
    11.6.  The bit Statement
    11.7.  The case Statement
    11.8.  The choice Statement
    11.9.  The config Statement
    11.10.  The contact Statement
    11.11.  The container Statement
    11.12.  The default Statement
    11.13.  The description Statement
    11.14.  The deviation Statement
    11.15.  The enum Statement
    11.16.  The error-app-tag Statement
    11.17.  The error-message Statement
    11.18.  The extension Statement
    11.19.  The feature Statement
    11.20.  The grouping Statement
    11.21.  The identity Statement
    11.22.  The if-feature Statement
    11.23.  The import Statement
    11.24.  The include Statement
    11.25.  The input Statement
    11.26.  The key Statement
    11.27.  The leaf Statement
    11.28.  The leaf-list Statement
    11.29.  The length Statement
    11.30.  The list Statement
    11.31.  The mandatory Statement
    11.32.  The max-elements Statement
    11.33.  The min-elements Statement
    11.34.  The module Statement
    11.35.  The must Statement
    11.36.  The namespace Statement
    11.37.  The notification Statement
    11.38.  The ordered-by Statement
    11.39.  The organization Statement
    11.40.  The output Statement
    11.41.  The path Statement
    11.42.  The pattern Statement
    11.43.  The position Statement
    11.44.  The prefix Statement
    11.45.  The presence Statement
    11.46.  The range Statement
    11.47.  The reference Statement
    11.48.  The require-instance Statement
    11.49.  The revision Statement
    11.50.  The rpc Statement
    11.51.  The status Statement
    11.52.  The submodule Statement
    11.53.  The type Statement
        11.53.1.  The empty Type
        11.53.2.  The boolean and binary Types
        11.53.3.  The bits Type
        11.53.4.  The enumeration and union Types
        11.53.5.  The identityref Type
        11.53.6.  The instance-identifier Type
        11.53.7.  The leafref Type
        11.53.8.  The numeric Types
        11.53.9.  The string Type
        11.53.10.  Derived Types
    11.54.  The typedef Statement
    11.55.  The unique Statement
    11.56.  The units Statement
    11.57.  The uses Statement
    11.58.  The value Statement
    11.59.  The when Statement
    11.60.  The yang-version Statement
    11.61.  The yin-element Statement
12.  Mapping NETMOD-specific annotations to DSDL Schema Languages
    12.1.  The @nma:config Annotation
    12.2.  The @nma:default Annotation
    12.3.  The @nma:implicit Annotation
    12.4.  The <nma:error-app-tag> Annotation
    12.5.  The <nma:error-message> Annotation
    12.6.  The <nma:instance-identifier> Annotation
    12.7.  The @nma:key Annotation
    12.8.  The @nma:leafref Annotation
    12.9.  The @nma:min-elements Annotation
    12.10.  The @nma:max-elements Annotation
    12.11.  The <nma:must> Annotation
    12.12.  The <nma:ordered-by> Annotation
    12.13.  The <nma:status> Annotation
    12.14.  The @nma:unique Annotation
    12.15.  The @nma:when Annotation
13.  IANA Considerations
14.  Security Considerations
15.  Acknowledgments
16.  References
    16.1.  Normative References
    16.2.  Informative References
Appendix A.  Schemas for NETMOD-Specific Annotations
    A.1.  NVDL Schema
    A.2.  Annotation Attributes for define Pattern
    A.3.  Annotation Elements for element Pattern
    A.4.  Annotation Attributes for element Pattern
    A.5.  Annotation attributes for group Pattern
    A.6.  Annotation attributes for choice and ref Patterns
    A.7.  Annotation attributes for element Pattern in the List Context
    A.8.  Annotation attributes for value Pattern
    A.9.  Named Patterns for All NETMOD-Specific Annotations
Appendix B.  Schema-Independent Library
Appendix C.  Mapping DHCP Data Model - A Complete Example
    C.1.  Input YANG Module
    C.2.  Conceptual Tree Schema
        C.2.1.  XML Syntax
        C.2.2.  Compact Syntax
    C.3.  Final DSDL Schemas
        C.3.1.  RELAX NG Schema for <get> Reply - XML Syntax
        C.3.2.  RELAX NG Schema for <get> Reply - Compact Syntax
    C.4.  Schematron Schema for <get> Reply
    C.5.  DSRL Schema for <get> Reply
Appendix D.  Change Log
    D.1.  Changes Between Versions -04 and -05
    D.2.  Changes Between Versions -03 and -04
    D.3.  Changes Between Versions -02 and -03
    D.4.  Changes Between Versions -01 and -02
    D.5.  Changes Between Versions -00 and -01
§  Authors' Addresses

1.  Introduction

The NETCONF Working Group has completed a base protocol used for configuration management [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.). This base specification defines protocol bindings and an XML container syntax for configuration and management operations, but does not include a modeling language or accompanying rules for how to model configuration and status information (in XML syntax) carried by NETCONF. The IETF Operations Area has a long tradition of defining data for SNMP Management Information Bases (MIBs) [RFC1157] (Case, J., Fedor, M., Schoffstall, M., and J. Davin, “Simple Network Management Protocol (SNMP),” May 1990.) using the SMI language [RFC2578] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” April 1999.) to model its data. While this specific modeling approach has a number of well-understood problems, most of the data modeling features provided by SMI are still considered extremely important. Simply modeling the valid syntax without the additional semantic relationships has caused significant interoperability problems in the past.

The NETCONF community concluded that a data modeling framework is needed to support ongoing development of IETF and vendor-defined management information modules. The NETMOD Working Group was chartered to address this problem by defining a new human-friendly modeling language based on SMIng [RFC3216] (Elliott, C., Harrington, D., Jason, J., Schoenwaelder, J., Strauss, F., and W. Weiss, “SMIng Objectives,” December 2001.) and called YANG [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.).

Since NETCONF uses XML for encoding its protocol data units (PDU), it is natural to express the constraints on NETCONF content using standard XML schema languages. For this purpose, the NETMOD WG selected the Document Schema Definition Languages (DSDL) that is being standardized as ISO/IEC 19757 [DSDL] (ISO/IEC, “Document Schema Definition Languages (DSDL) - Part 1: Overview,” 11 2004.). The DSDL framework comprises a set of XML schema languages that address grammar rules, semantic constraints and other data modeling aspects but also, and more importantly, do it in a coordinated and consistent way. While it is true that some DSDL parts have not been standardized yet and are still work in progress, the three parts that the YANG-to-DSDL mapping relies upon - RELAX NG, Schematron and DSRL - already have the status of an ISO/IEC International Standard and are supported in a number of software tools.

This document contains specification of a mapping that translates YANG data models to XML schemas utilizing a subset of the DSDL schema languages. The mapping procedure is divided into two steps: In the first step, the structure of the data tree, RPC signatures and notifications is expressed as a single RELAX NG grammar with simple annotations representing additional data model information (metadata, documentation, semantic constraints, default values etc.). The second step then generates a coordinated set of DSDL schemas that can validate specific XML documents such as client requests, server responses or notifications, perhaps also taking into account additional context such as active capabilities.

2.  Terminology and Notation

This document relies on many terms that are defined in the specifications of the NETCONF protocol [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.) and YANG data modeling language [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.). Also, the common terminology of XML and DSDL schema languages is used as defined in the respective standards [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), [DSDL] (ISO/IEC, “Document Schema Definition Languages (DSDL) - Part 1: Overview,” 11 2004.), [NVDL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 4: Namespace-Based Validation Dispatching Language (NVDL),” 6 2006.), [RNG] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. Second Edition.,” 12 2008.), [Schematron] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 3: Rule-Based Validation - Schematron,” 6 2006.) and [DSRL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 8: Document Semantics Renaming Language - DSRL,” 12 2008.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

In the text, we use the following typographic conventions:

• YANG statement keywords are delimited by single quotes.
• XML element names are delimited by "<" and ">" characters.
• Names of XML attributes are prefixed by the "@" character.
• Other literal values are delimited by double quotes.

XML elements names are always written with explicit namespace prefixes corresponding to the following XML vocabularies:

"a"

DTD compatibility annotations [RNG‑DTD] (Clark, J., Ed. and M. Murata, Ed., “RELAX NG DTD Compatibility,” December 2001.);

"dc"

Dublin Core metadata elements [RFC5013] (Kunze, J., “The Dublin Core Metadata Element Set,” August 2007.);

"dsrl"

Document Semantics Renaming Language [DSRL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 8: Document Semantics Renaming Language - DSRL,” 12 2008.);

"en"

NETCONF event notifications [RFC5277] (Chisholm, S. and H. Trevino, “NETCONF Event Notifications,” July 2008.);

"nc"

NETCONF protocol [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.);

"nma"

NETMOD-specific schema annotations (see Section 5.3 (NETMOD-Specific Annotations));

"nmf"

NETMOD-specific XPath extension functions (see Section 12.6 (The <nma:instance-identifier> Annotation));

"nmt"

Conceptual tree (see Section 8.1 (Conceptual Data Tree));

"rng"

RELAX NG [RNG] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. Second Edition.,” 12 2008.);

"sch"

ISO Schematron [Schematron] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 3: Rule-Based Validation - Schematron,” 6 2006.);

"xsd"

W3C XML Schema [XSD] (Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, “XML Schema Part 1: Structures Second Edition,” October 2004.).

The following table shows the mapping of these prefixes to namespace URIs.

2.1.  Glossary of New Terms

• ancestor datatype: Any datytype a given datatype is (transitively) derived from.
• ancestor built-in datatype: The built-in datatype that is at the start of the type derivation chain for a given datatype.
• conceptual data tree: A virtual XML document integrating all components of a YANG data model, i.e., configuration datastore, RPC methods (both requests and replies) and notifications. The conceptual tree is normally not instantiated, it only serves as a conceptual target for its schema. See Section 8.1 (Conceptual Data Tree) for details.
• implicit node: A node that, if missing, may be added to the information set of an XML document (configuration, RPC input or output, notification) without changing the meaning of that XML document.

3.  Objectives and Motivation

The main objective of this work is to complement YANG as a data modeling language by validation capabilities of DSDL schema languages, namely RELAX NG, Schematron and DSRL. This document describes the correspondence between grammatical, semantic and data type constraints expressed in YANG and equivalent DSDL constructs. The ultimate goal is to be able to capture all substantial information contained in YANG modules and express it in DSDL schemas. While the mapping from YANG to DSDL described in this document is in principle invertible, the inverse mapping from DSDL to YANG is not in its scope.

XML-based information models and XML-encoded data appear in several different forms in various phases of YANG data modeling and NETCONF workflow - configuration datastore contents, RPC requests and replies, and notifications. Moreover, RPC methods are characterized by an inherent diversity resulting from selective availability of capabilities and features. YANG modules can also define new RPC methods. The mapping should be able to accommodate this variability and generate schemas that are specifically tailored to a particular situation and thus considerably more effective for validation than generic all-encompassing schemas.

In order to cope with this variability, we assume that the schemas can be generated on demand for a particular purpose from the available collection of YANG modules and their lifetime will be relatively short. In other words, we don't envision that any collection of DSDL schemas will be created and maintained over an extended period of time in parallel to YANG modules.

The generated schemas are primarily intended as input to the existing XML schema validators and other off-the-shelf tools. However, the schemas may also be perused by developers and users as a formal representation of constraints on a particular XML-encoded data object. Consequently, our secondary goal is to keep the schemas as readable as possible. To this end, the complexity of the mapping is distributed into two steps:

1. The first step maps one or more YANG modules to a single RELAX NG schema of the so-called "conceptual tree", which contains grammatical constraints for the main data tree as well as RPCs and notifications. In order to record additional constraints that appear in the YANG modules but cannot be expressed in RELAX NG, the schema is augmented by simple annotations. The output of the first step can thus be considered a reasonably readable equivalent of the input YANG modules.
2. In the second step, the annotated RELAX NG schema from step 1 is transformed further to a coordinated set of fully conformant DSDL schemas containing constraints for a particular data object and a specific situation. The DSDL schemas are intended mainly for machine validation using off-the-shelf tools.

4.  DSDL Schema Languages

Document Schema Definition Languages (DSDL) is a framework of schema languages that is being developed as an international standard ISO/IEC 19757. Unlike other approaches to XML document validation, most notably W3C XML Schema (XSD) [XSD] (Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, “XML Schema Part 1: Structures Second Edition,” October 2004.), the DSDL framework adheres to the principle of "small languages": Each of the DSDL constituents is a stand-alone schema language with a relatively narrow purpose and focus. Together, these schema languages may be used in a coordinated way to accomplish various validation tasks.

The mapping described in this document uses three of the DSDL schema languages, namely RELAX NG, Schematron and DSRL.

4.1.  RELAX NG

RELAX NG (pronounced "relaxing") is an XML schema language for grammar-based validation and Part 2 of the ISO/IEC DSDL family of standards [RNG] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. Second Edition.,” 12 2008.). Like the W3C XML Schema language [XSD] (Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, “XML Schema Part 1: Structures Second Edition,” October 2004.), it is able to describe constraints on the structure and content of XML documents. However, unlike the DTD [XML] (Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.) and XSD schema languages, RELAX NG intentionally avoids any infoset augmentation such as defining default values. In the DSDL architecture, the particular task of defining and applying default values is delegated to another schema language, DSRL (see Section 4.3 (Document Semantics Renaming Language (DSRL))).

As its base datatype library, RELAX NG uses the W3C XML Schema Datatype Library [XSD‑D] (Biron, P. and A. Malhotra, “XML Schema Part 2: Datatypes Second Edition,” October 2004.), but unlike XSD, other datatype libraries may be used along with it or even replace it if necessary.

RELAX NG is very liberal in accepting annotations from other namespaces. With few exceptions, such annotations may be placed anywhere in the schema and need no encapsulating elements such as <xsd:annotation> in XSD.

RELAX NG schemas can be represented in two equivalent syntaxes: XML and compact. The compact syntax is described in Annex C of the RELAX NG specification [RNG‑CS] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. AMENDMENT 1: Compact Syntax,” 1 2006.), which was added to the standard in 2006 (Amendment 1). Automatic bidirectional conversions between the two syntaxes can be accomplished using several tools, for example Trang.

For its terseness and readability, the compact syntax is often the preferred form for publishing RELAX NG schemas whereas validators and other software tools usually work with the XML syntax. However, the compact syntax has two drawbacks:

• External annotations make the compact syntax schema considerably less readable. While in the XML syntax the annotating elements and attributes are represented in a simple and uniform way (XML elements and attributes from foreign namespaces), the compact syntax uses four different syntactic constructs: documentation, grammar, initial and following annotations. Therefore, the impact of annotations on readability is often much stronger for the compact syntax than for the XML syntax.
• In a computer program, it is more difficult to generate the compact syntax than the XML syntax. While a number of software libraries exist that make it easy to create an XML tree in memory and serialize it, no such aid is available for compact syntax.

For these reasons, the mapping specification in this document uses exclusively the XML syntax. Where appropriate, though, the schemas resulting from the translation may be presented in the equivalent compact syntax.

RELAX NG elements are qualified with the namespace URI "http://relaxng.org/ns/structure/1.0". The namespace of the W3C Schema Datatype Library is "http://www.w3.org/2001/XMLSchema-datatypes".

4.2.  Schematron

Schematron is Part 3 of DSDL that reached the status of a full ISO/IEC standard in 2006 [Schematron] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 3: Rule-Based Validation - Schematron,” 6 2006.). In contrast to the traditional schema languages such as DTD, XSD or RELAX NG, which are based on the concept of a formal grammar, Schematron utilizes a rule-based approach. Its rules may specify arbitrary conditions involving data from different parts of an XML document. Each rule consists of three essential components:

• context - an XPath expression that defines the set of locations where the rule is to be applied;
• assert or report condition - another XPath expression that is evaluated relative to the location matched by the context expression;
• human-readable message that is displayed when the assert condition is false or report condition is true.

The difference between the assert and report condition is that the former is positive in that it states a condition that a valid document has to satisfy, whereas the latter specifies an error condition.

Schematron draws most of its expressive power from XPath [XPath] (Clark, J. and S. DeRose, “XML Path Language (XPath) Version 1.0,” November 1999.) and XSLT [XSLT] (Clark, J., “XSL Transformations (XSLT) Version 1.0,” November 1999.). ISO Schematron allows for dynamic query language binding so that the following XML query languages can be used: STX, XSLT 1.0, XSLT 1.1, EXSLT, XSLT 2.0, XPath 1.0, XPath 2.0 and XQuery 1.0 (this list may be extended in the future).

The human-readable error messages are another feature that distinguishes Schematron from other common schema languages. The messages may even contain XPath expressions that are evaluated in the actual context and thus refer to existing XML document nodes and their contents.

The rules defined by a Schematron schema may be divided into several subsets known as phases. Validations may then be set up to include only selected phases. In the context of NETCONF data validation, this is useful for relaxing constraints that may not always apply. For example, the reference integrity may not be enforced for a candidate configuration.

Schematron elements are qualified with namespace URI "http://purl.oclc.org/dsdl/schematron".

4.3.  Document Semantics Renaming Language (DSRL)

DSRL (pronounced "disrule") is Part 8 of DSDL that reached the status of a full ISO/IEC standard in 2008 [DSRL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 8: Document Semantics Renaming Language - DSRL,” 12 2008.). Unlike RELAX NG and Schematron, it is specifically designed to modify XML information set of the validated document. While DSRL is primarily intended for renaming XML elements and attributes, it can also define default values for XML attributes and default content for XML elements so that elements or attributes with these default values are inserted if they are missing (or present and empty) in the validated documents. The latter feature is used by the YANG-to-DSDL mapping for representing YANG default contents consisting of leaf nodes with default values and their ancestor non-presence containers.

DSRL elements are qualified with namespace URI "http://purl.oclc.org/dsdl/dsrl".

5.  Additional Annotations

Besides the DSDL schema languages, the mapping also uses three sets of annotations that are added as foreign-namespace attributes and elements to RELAX NG schemas. Two of the annotation sets - Dublin Core elements and DTD compatibility annotations - are standard vocabularies for representing metadata and documentation, respectively. Although these data model items are not used for formal validation, they quite often carry important information for data model implementers. Therefore, they SHOULD be included in the conceptual tree schema and MAY also appear in the final validation schemas.

The third set are NETMOD-specific annotations conveying YANG semantic constraints and other information that cannot be expressed directly in RELAX NG. These annotations are only used in the first step of the mapping, i.e., in the conceptual tree schema. In the second mapping step, these annotations are converted to Schematron and DSRL rules.

5.1.  Dublin Core Metadata Elements

Dublin Core is a system of metadata elements that was originally created for describing metadata of World Wide Web resources in order to facilitate their automated lookup. Later it was accepted as a standard for describing metadata of arbitrary resources. This specification uses the definition from [RFC5013] (Kunze, J., “The Dublin Core Metadata Element Set,” August 2007.).

Dublin Core elements are qualified with namespace URI "http://purl.org/dc/terms".

5.2.  RELAX NG DTD Compatibility Annotations

DTD compatibility annotations are part of the RELAX NG DTD Compatibility specification [RNG‑DTD] (Clark, J., Ed. and M. Murata, Ed., “RELAX NG DTD Compatibility,” December 2001.). YANG-to-DSDL mapping uses only the <a:documentation> annotation for representing YANG 'description' and 'reference' texts.

Note that there is no intention to make the resulting schemas DTD-compatible, the main reason for using these annotations is technical: they are well supported and adequately interpreted by several RELAX NG tools.

DTD compatibility annotations are qualified with namespace URI "http://relaxng.org/ns/compatibility/annotations/1.0".

5.3.  NETMOD-Specific Annotations

NETMOD-specific annotations are XML elements and attributes qualified with the namespace URI "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1" that appear in various locations of the conceptual tree schema. YANG statements are mapped to these annotations in a straightforward way. In most cases, the annotation attributes and elements have the same name as the corresponding YANG statement.

Table 2 (NETMOD-specific annotations) lists alphabetically the names of NETMOD-specific annotation attributes (prefixed with "@") and elements (in angle brackets) along with a reference to the section where their use is described. Appendix A (Schemas for NETMOD-Specific Annotations) contains the formal specification of this annotation vocabulary.

Notes:

1. Appears only as subelement of <nma:must>.
2. Has an optional attribute @require-instance.
3. Has a mandatory attribute @assert and two optional subelements <nma:error-app-tag> and <nma:error-message>.

6.  Overview of the Mapping

This section gives an overview of the YANG-to-DSDL mapping, its inputs and outputs. Figure 1 (Structure of the mapping) presents an overall structure of the mapping:

                 +----------------+
                 | YANG module(s) |
                 +----------------+
                         |
                         |T
                         |
       +-------------------------------------+
       | RELAX NG schema for conceptual tree |
       +-------------------------------------+
            /       |           |       \
           /        |           |        \        +-------+
        Tg/       Tr|           |Tn       \       |library|
         /          |           |          \      +-------+
   +---------+   +-----+    +-------+    +------+
   |get reply|   | rpc |    | notif |    | .... |
   +---------+   +-----+    +-------+    +------+

The mapping procedure is divided into two steps:

1. Transformation T in the first step maps one or more YANG modules to a single RELAX NG schema for the conceptual tree (see Section 8.1 (Conceptual Data Tree)). Constraints that cannot be expressed directly in RELAX NG (list key definitions, 'must' statements etc.) and various documentation texts are recorded in the schema as foreign-namespace annotations.
2. In the second step, the conceptual tree schema is transformed in multiple ways to a coordinated set of DSDL schemas that can be used for validating a particular data object in a specific context. Figure 1 (Structure of the mapping) shows just three simple possibilities as examples. In the process, appropriate parts of the conceptual tree schema are extracted and specific annotations transformed to equivalent, but usually more complex, Schematron patterns, DSRL element maps etc.
3. As indicated in Figure 1 (Structure of the mapping), the second step of the mapping also uses a schema-independent library that contains common schema objects such as RELAX NG named pattern definitions.

An implementation of the mapping algorithm is supposed to accept one or more valid YANG modules as its input. It is important to be able to process multiple YANG modules together since multiple modules may be negotiated for a NETCONF session and the contents of the configuration datastore is then obtained as the union of data trees specified by the individual modules, which may also lead to multiple roots. In addition, the input modules may be further coupled by the 'augment' statement in which one module augments the data tree of another module.

It is also assumed that the algorithm has access, perhaps on demand, to all YANG modules that the input modules import (perhaps transitively).

The output of the first mapping step is an annotated RELAX NG schema for the conceptual tree - a virtual XML document containing, in its different subtrees, the entire datastore, all RPC requests and replies, and notifications as defined by the input YANG modules. By "virtual" we mean that such an XML document need not correspond to the actual layout of the configuration database in a NETCONF server, and will certainly never appear on the wire as the content of a NETCONF PDU. Hence, the RELAX NG schema for the conceptual tree is not intended for any direct validations but rather as a representation of a particular data model expressed in annotated RELAX NG and the common starting point for subsequent transformations that may produce practical validation schemas. The conceptual tree is further described in Section 8.1 (Conceptual Data Tree).

Other information contained in input YANG modules, such as semantic constraints or default values, are recorded in the conceptual tree schema as annotations - XML attributes or elements qualified with namespace URI "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1". Metadata describing the YANG modules are mapped to annotations utilizing Dublin Core elements (Section 5.1 (Dublin Core Metadata Elements)). Finally, documentation strings are mapped to <a:documentation> elements belonging to the DTD compatibility vocabulary (Section 5.2 (RELAX NG DTD Compatibility Annotations)).

The output from the second step is a coordinated set of three DSDL schemas corresponding to a specific data object and context:

• RELAX NG schema describing the grammatical and datatype constraints;
• Schematron schema expressing other constraints such as uniqueness of list keys or user-specified semantic rules;
• DSRL schema containing the specification of default contents.

7.  NETCONF Content Validation

This section describes how the schemas generated by the YANG-to-DSDL mapping are supposed to be applied for validating XML instance documents such as the content of a datastore or various NETCONF PDUs.

The validation proceeds in the following steps, which are also illustrated in Figure 2 (Outline of the validation procedure):

1. The XML instance document can be immediately checked for grammatical and data type validity using the RELAX NG schema.
2. Second, default values for leaf nodes have to be applied and their ancestor containers added where necessary. It is important to add the implicit nodes before the next validation step because YANG specification [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.) states that the data tree against which XPath expressions are evaluated already has all defaults filled-in. Note that this step modifies the information set of the validated XML document.
3. The semantic constraints are checked using the Schematron schema.

      +----------+                        +----------+
      |          |                        |   XML    |
      |   XML    |                        | document |
      | document |-----------o----------->|   with   |
      |          |           ^            | defaults |
      |          |           |            |          |
      +----------+           |            +----------+
           ^                 | filling-in       ^
           | grammar,        | defaults         | semantic
           | datatypes       |                  | constraints
           |                 |                  |
      +----------+       +--------+       +------------+
      | RELAX NG |       |  DSRL  |       | Schematron |
      |  schema  |       | schema |       |   schema   |
      +----------+       +--------+       +------------+

8.  Design Considerations

YANG modules could in principle be mapped to DSDL schemas in a number of ways. The mapping procedure described in this document uses several specific design decisions that are discussed in the following subsections.

8.1.  Conceptual Data Tree

DSDL schemas generated from YANG modules using the procedure described in this document are intended to be used for validating XML-encoded NETCONF data in various forms: every YANG-based model represents the contents of a datastore but also implies an array of schemas for all types of NETCONF PDUs. For a reasonably strict validation of a given data object, the schemas have to be quite specific. To begin with, effective validation of NETCONF PDU content requires separation of client and server schemas. While the decision about proper structuring of all PDU-validating schemas is beyond the scope of this document, the mapping procedure is designed to accommodate any foreseeable validation needs.

An essential part of the YANG-to-DSDL mapping procedure is nonetheless common to all validation needs: the grammar and datatype specifications for the datastore, RPCs and notifications expressed by one or more YANG modules have to be translated to RELAX NG. In order to be able to separate this common step, we introduce the notion of conceptual data tree: it is a virtual XML tree that contains full datastore, RPC requests with corresponding replies and notifications. The schema for the conceptual tree - a single RELAX NG schema with annotations - therefore has a quite similar logic as the input YANG module(s), the only major difference being the RELAX NG schema language.

For example, the conceptual data tree for a YANG module defining nodes in the namespace "http://example.com/ns/example" may be schematically represented as follows:

<nmt:netmod-tree
    xmlns:nmt="urn:ietf:params:xml:ns:netmod:conceptual-tree:1"
    xmlns:ex="http://example.com/ns/example">
  <nmt:top>
    ... configuration and status data ...
  </nmt:top>
  <nmt:rpc-methods>
    <nmt:rpc-method>
      <nmt:input>
        <ex:myrpc ...>
          ...
        </myrpc>
      </nmt:input>
      <nmt:output>
        ...
      </nmt:output>
    </nmt:rpc-method>
    ...
  </nmt:rpc-methods>
  <nmt:notifications>
    <nmt:notification>
      <ex:mynotif>
        ...
      </mynotif>
    </nmt:notification>
    ...
  </nmt:notifications>
</nmt:netmod>

The namespace URI "urn:ietf:params:xml:ns:netmod:conceptual-tree:1" identifies a simple vocabulary consisting of a few elements that encapsulate and separate the various parts of the conceptual data tree.

The conceptual tree schema is not intended for direct validation but rather serves as a well-defined starting point for subsequent transformations that generate validation schemas. In general, these transformations should be relatively simple - they will typically extract one or more subtrees from the conceptual tree schema, modify them as necessary and add encapsulating elements such as those from the NETCONF RPC layer.

Additional information contained in the source YANG module(s), such as semantic constraints and default values, is represented in the form of interim NETMOD-specific annotations that are included as foreign-namespace elements or attributes in the RELAX NG schema for the conceptual tree. In the second phase of the mapping, these annotations are typically translated to equivalent Schematron and DSRL rules.

As a useful side effect, by introducing the conceptual data tree we are also able to resolve the difficulties arising from the fact that a single datastore may consist of multiple parallel data hierarchies defined in one or more YANG modules, which cannot be mapped to a valid XML document. In the conceptual data tree, such multiple hierarchies appear under the single <nmt:top> element.

8.2.  Modularity

Both YANG and RELAX NG offer means for modularity, i.e., for splitting the contents of a full schema into separate modules and combining or reusing them in various ways. However, the approaches taken by YANG and RELAX NG differ. Modularity in RELAX NG is suitable for ad hoc combinations of a small number of schemas whereas YANG assumes a large set of modules similar to SNMP MIBs. The following differences are important:

• In YANG, whenever module A imports module B, it gets access to the definitions (groupings and typedefs) appearing at the top level of module B. However, no part of data tree from module B is imported along with it. In contrast, the <rng:include> pattern in RELAX NG imports both definitions of named patterns and the entire schema tree from the included schema.
• The names of imported YANG groupings and typedefs are qualified with the namespace of the imported module. On the other hand, the names of data nodes contained inside the imported groupings, when used within the importing module, become part of the importing module's namespace. In RELAX NG, the names of patterns are unqualified and so named patterns defined in both the importing and imported module share the same flat namespace. The contents of RELAX NG named patterns may either keep the namespace of the schema where they are defined or inherit the namespace of the importing module, analogically to YANG. However, in order to achieve the latter behavior, the imported module must be prepared in a special way as a library module that cannot be used as a stand-alone schema.

So the conclusion is that the modularity mechanisms of YANG and RELAX NG, albeit similar, are not directly compatible. Therefore, the corresponding design decision for the mapping algorithm is to collect all information in a single schema for the conceptual tree, even if it comes from multiple YANG modules or submodules. In other words, translations of imported groupings and typedefs are installed in the translation of importing module - but only if they are really used there.

NOTE: The 'include' statement that is used in YANG for including submodules has in fact the same semantics as the <rng:include> pattern. However, since we don't map the modular structure for YANG modules, it seems to have little sense to do it for submodules. Consequently, the contents of submodules appear directly in the conceptual tree schema, too.

8.3.  Granularity

RELAX NG supports different styles of schema structuring: One extreme, often called "Russian Doll", specifies the structure of an XML instance document in a single hierarchy. The other extreme, the flat style, uses a similar approach as the Data Type Definition (DTD) schema language - every XML element corresponds to a named pattern definition. In practice, some compromise between the two extremes is usually chosen.

YANG supports both styles in principle, too, but in most cases the modules are organized in a way that's closer to the "Russian Doll" style, which provides a better insight into the structure of the configuration data. Groupings are usually defined only for contents that are prepared for reuse in multiple places via the 'uses' statement. In contrast, RELAX NG schemas tend to be much flatter, because finer granularity is also needed in RELAX NG for extensibility of the schemas - it is only possible to replace or modify schema fragments that are factored out as named patterns. For YANG this is not an issue since its 'augment' and 'refine' statements can delve, by using path expressions, into arbitrary depths of existing structures.

In general, it not feasible to map YANG extension mechanisms to those of RELAX NG. For this reason, the mapping essentially keeps the granularity of the original YANG data model: definitions of named patterns in the resulting RELAX NG schema usually have direct counterparts in YANG groupings and definitions of derived types.

8.4.  Handling of XML Namespaces

Most modern XML schema languages including RELAX NG, Schematron and DSRL support schemas for so-called compound XML documents, which contain elements from multiple namespaces. This is useful for our purpose since YANG-to-DSDL mapping allows for multiple input YANG modules that naturally leads to compound document schemas.

RELAX NG offers two alternatives for defining the target namespaces in the schema:

1. First possibility is the traditional XML way via the @xmlns:xxx attribute.
2. One of the target namespace URIs may be declared using the @ns attribute.

In both cases, these attributes are typically attached to the <rng:grammar> element but may also appear elsewhere.

The design decision for the mapping is to use exclusively the alternative 1, since in this case all YANG modules are represented symmetrically, which makes further processing of the conceptual tree schema considerably easier. Moreover, this way the namespace prefixes declared in the input modules are recorded in the resulting schema - the prefix for the namespace declared using @ns would be lost.

DSRL schemas can declare any number of target namespaces via the standard XML attributes xmlns:xxx.

In contrast, Schematron requires all target namespaces to be defined in the <sch:ns> subelements of the root <sch:schema> element.

8.5.  Features and Deviations

YANG provides statements that allow for marking parts of the schema as conditional ('feature' and 'if-feature' statements) or declaring deviations from a data model ('deviation' statement). These statements are not handled by the YANG-to-DSDL mapping. Instead, it is assumed that all features and deviations are specified beforehand and the corresponding changes in the input YANG modules are already applied.

9.  Mapping YANG Data Models to the Conceptual Tree Schema

This section explains the main principles underlying the first step of the mapping. Its result is an annotated RELAX NG schema of the conceptual tree, which is described in Section 8.1 (Conceptual Data Tree).

As a special case, if the input YANG modules contain no data nodes (this is typical, e.g., for datatype library modules), an implementation MAY entirely remove the schema of the (empty) conceptual tree - the <rng:start> element with all its contents. The output RELAX NG schema will then contain only named pattern definitions translated from YANG groupings and typedefs.

Detailed specification of the mapping of individual YANG statements is contained in Section 11 (Mapping YANG Statements to Conceptual Tree Schema).

9.1.  Occurrence Rules for Data Nodes

In DSDL schema languages, occurrence constraints for a node are always localized together with that node. In a RELAX NG schema, for example, <rng:optional> pattern appears as the parent element of the pattern defining a leaf or non-leaf element. Similarly, DSRL specifies default content separately for every single node, be it a leaf or non-leaf element.

For leaf nodes in YANG modules, the occurrence constraints are also easily inferred from the substatements of 'leaf'. In contrast, for a YANG container it is often necessary to examine its entire subtree in order to determine the container's occurrence constraints.

Therefore, one of the goals of the first mapping step is to infer the occurrence constraints for all containers and mark accordingly the corresponding <rng:element> patterns in the conceptual tree schema so that any transformation procedure in the second mapping step can simply use this information and need not examine the subtree again.

First, it has to be decided whether a given container element must always be present in a valid configuration. If so, such a container element is called mandatory, otherwise it is called optional. This constraint is closely related to the notion of mandatory nodes in Section 3.1 in [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.). The only difference is that we also consider list keys to be mandatory.

The other occurrence constraint has to do with the semantics of the 'default' statement and the possibility of removing empty non-presence containers. As a result, the information set of a valid configuration may be modified by adding or removing certain leaf or container elements without changing the meaning of the configuration. In this document, such elements are called implicit.

Note that both occurrence constraints apply to containers at the top level of the data tree, and then also to other containers under the additional condition that their parent node exists in the instance document. For example, consider the following YANG fragment:

    container outer {
        presence 'Presence of "outer" means something.';
        container c1 {
            leaf foo {
                type uint8;
                default 1;
            }
        }
        container c2 {
            leaf-list bar {
                type uint8;
                min-elements 0;
            }
        }
        container c3 {
            leaf baz {
                type uint8;
                mandatory true;
            }
        }
    }

Here, container "outer" has the 'presence' substatement, which means that it is optional and not implicit. If "outer" is not present in a configuration, its child containers are not present as well. However, if "outer" does exist, it makes sense to ask which of its child containers are optional and which are implicit. In this case, "c1" is optional and implicit, "c2" is optional but not implicit and "c3" is mandatory (and therefore not implicit).

The following subsections give precise rules for determining whether a container is optional or mandatory and whether it is implicit. In order to simplify the recursive definition of these occurrence characteristics, it is useful to define them also for other types of data nodes, i.e., leaf, list, leaf-list and anyxml.

9.1.1.  Optional and Mandatory Nodes

The decision whether a given node is mandatory or optional is governed by the following rules:

• Leaf, anyxml and choice nodes are mandatory if they contain the substatement "mandatory true;". For a choice node this means that at least one node from exactly one case branch must exist.
• In addition, leaf nodes are mandatory if they are declared as list keys.
• List or leaf-list nodes are mandatory if they contain 'min-elements' substatement with argument value greater than zero.
• A container node is mandatory if its definition does not contain the 'presence' substatement and at least one of its child nodes is mandatory.

A node is optional if and only if it is not mandatory.

In RELAX NG, definitions of nodes that are optional must be explicitly wrapped in the <rng:optional> element. The mapping MUST use the above rules to determine whether a YANG node is optional and if so, insert the <rng:optional> element in the conceptual tree schema.

However, alternatives in <rng:choice> are never defined as optional in the conceptual tree schema. Therefore, 'anyxml', 'container', 'leaf', 'list' and 'leaf-list' statements appearing as children of 'choice' (so-called shorthand cases) are always mapped to mandatory RELAX NG patterns. If a choice in YANG is not mandatory, <rng:optional> is used to wrap the entire <rng:choice> pattern.

9.1.2.  Implicit Nodes

The following rules are used to determine whether a given node is implicit:

• List, leaf-list and anyxml nodes are never implicit.
• A leaf node is implicit if and only if it has a specified default value (either directly or via its datatype).
• A container node is implicit if and only if it does not have the 'presence' substatement, none of its children is mandatory and at least one child is implicit.

In the conceptual tree schema, all implicit containers, as well as leafs that obtain their default value from a typedef and don't have the @nma:default attribute, MUST be marked with @nma:implicit attribute having the value "true".

Note that Section 7.9.3 in [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.) specifies other rules that must be taken into account when deciding whether a given container or leaf appearing inside a case of a choice is ultimately implicit or not. Specifically, a leaf or container under a case can be implicit only if the case appears in the argument of the choice's 'default' statement. However, this is not sufficient by itself but also depends on the particular instance XML document, namely on the presence or absence of nodes from other (non-default) cases. The details are explained in Section 10.4 (Mapping Default Values to DSRL).

9.2.  Mapping YANG Groupings and Typedefs

YANG groupings and typedefs are generally mapped to RELAX NG named patterns. There are, however, several caveats that the mapping has to take into account.

First of all, YANG typedefs and groupings may appear at all levels of the module hierarchy and are subject to lexical scoping, see Section 5.5 in [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.). Second, top-level symbols from external modules are imported as qualified names represented using the external module namespace prefix and the name of the symbol. In contrast, named patterns in RELAX NG (both local and imported via the <rng:include> pattern) share the same namespace and within a grammar they are always global - their definitions may only appear at the top level as children of the <rng:grammar> element. Consequently, whenever YANG groupings and typedefs are mapped to RELAX NG named pattern definitions, their names MUST be disambiguated in order to avoid naming conflicts. The mapping uses the following procedure for mangling the names of groupings and type definitions:

• Names of groupings and typedefs appearing at the top level of the YANG module hierarchy are prefixed with the module name and two underscore characters ("__").
• Names of other groupings and typedefs, i.e., those that do not appear at the top level of a YANG module, are prefixed with the module name, double underscore, and then the names of all ancestor data nodes separated by double underscore.
• Since the names of groupings and typedefs in YANG have different namespaces, an additional underline character is added to the beginning of the mangled names of all groupings.

EXAMPLE. Consider the following YANG module which imports the standard module "ietf-inet-types" [Ytypes] (Schoenwaelder, J., Ed., “Common YANG Data Types,” February 2010.):

module example1 {
    namespace "http://example.com/ns/example1";
    prefix ex1;
    import "ietf-inet-types" {
        prefix "inet";
    }
    typedef vowels {
        type string {
            pattern "[aeiouy]*";
        }
    }
    grouping "grp1" {
        leaf "void" {
            type "empty";
        }
    }
    container "cont" {
        grouping "grp2" {
            leaf "address" {
                type "inet:ip-address";
            }
        }
        leaf foo {
            type vowels;
        }
        uses "grp1";
        uses "grp2";
    }
}

The resulting RELAX NG schema will then contain the following named pattern definitions (long regular expression patterns for IPv4 and IPv6 addresses are not shown):

<rng:define name="example1__vowels">
  <rng:data type="string">
    <rng:param name="pattern">[aeiouy]*</param>
  </rng:data>
</rng:define>

<rng:define name="_example1__grp1">
  <rng:optional>
    <rng:element name="ex1:void">
      <rng:empty/>
    </rng:element>
  </rng:optional>
</rng:define>

<rng:define name="_example1__cont__grp2">
  <rng:optional>
    <rng:element name="ex1:address">
      <rng:ref name="ietf-inet-types__ip-address"/>
    </rng:element>
  </rng:optional>
</rng:define>

<rng:define name="ietf-inet-types__ip-address">
  <rng:choice>
    <rng:ref name="ietf-inet-types__ipv4-address"/>
    <rng:ref name="ietf-inet-types__ipv6-address"/>
  </rng:choice>
</rng:define>

<rng:define name="ietf-inet-types__ipv4-address">
  <rng:data type="string">
    <rng:param name="pattern">... regex pattern ...</param>
  </rng:data>
</rng:define>

<rng:define name="ietf-inet-types__ipv6-address">
  <rng:data type="string">
    <rng:param name="pattern">... regex pattern ...</param>
  </rng:data>
</rng:define>

Note that type definitions from the "ietf-inet-types" module are mapped directly to the conceptual tree schema.

9.2.1.  YANG Refinements and Augments

YANG groupings represent a similar concept as named pattern definitions in RELAX NG and both languages also offer mechanisms for their subsequent modification. However, in RELAX NG the definitions themselves are modified whereas YANG allows for modifying expansions of groupings. Specifically, YANG provides two statements for this purpose that may appear as substatements of 'uses':

• 'refine' statement allows for changing parameters of a schema node inside the grouping referenced by the parent 'uses' statement;
• 'augment' statement can be used for adding new schema nodes to the grouping content.

Both 'refine' and 'augment' statements are quite powerful in that they can address, using XPath-like expressions as their arguments, schema nodes that are arbitrarily deep inside the grouping content. In contrast, modifications of named pattern definitions in RELAX NG are applied exclusively at the topmost level of the named pattern content. In order to achieve a modifiability of named patterns comparable to YANG, the RELAX NG schema would have to be extremely flat (cf. Section 8.3 (Granularity)) and very difficult to read.

Since the goal of the mapping described in this document is to generate ad hoc DSDL schemas, we decided to avoid these complications and instead expand the grouping and refine and/or augment it "in place". In other words, every 'uses' statement which has 'refine' and/or 'augment' substatements is replaced by the content of the corresponding grouping, the changes specified in the 'refine' and 'augment' statements are applied and the resulting YANG schema fragment is mapped as if the 'uses'/'grouping' indirection wasn't there.

If there are further 'uses' statements inside the grouping content, they may require expansion, too: it is necessary if the contained 'uses'/'grouping' pair lies on the "modification path" specified in the argument of a 'refine' or 'augment' statement.

EXAMPLE. Consider the following YANG module:

module example2 {
    namespace "http://example.com/ns/example2";
    prefix ex2;
    grouping leaves {
        uses fr;
        uses es;
    }
    grouping fr {
        leaf feuille {
            type string;
        }
    }
    grouping es {
        leaf hoja {
            type string;
        }
    }
    uses leaves;
}

The resulting conceptual tree schema contains three named pattern definitions corresponding to the three groupings, namely

<rng:define name="_example2__leaves">
  <rng:ref name="_example2__fr"/>
  <rng:ref name="_example2__es"/>
</rng:define>

<rng:define name="_example2__fr">
  <rng:optional>
    <rng:element name="ex2:feuille">
      <rng:data type="string"/>
    </rng:element>
  </rng:optional>
</rng:define>

<rng:define name="_example2__es">
  <rng:optional>
    <rng:element name="ex2:hoja">
      <rng:data type="string"/>
    </rng:element>
  </rng:optional>
</rng:define>

and the configuration data part of the conceptual tree schema is a single named pattern reference:

<rng:ref name="_example2__leaves"/>

Now assume that the "uses leaves" statement contains a 'refine' substatement, for example:

uses leaves {
    refine "hoja" {
        default "alamo";
    }
}

The resulting conceptual tree schema now contains just one named pattern definition - "_example2__fr". The other two groupings "leaves" and "es" have to be expanded because they both lie on the "modification path", i.e., contain the leaf "hoja" that is being refined. The configuration data part of the conceptual tree schema now looks like this:

<rng:ref name="_example2__fr"/>
<rng:optional>
  <rng:element name="ex2:hoja" nma:default="alamo">
    <rng:data type="string"/>
  </rng:element>
</rng:optional>

9.2.2.  Type Derivation Chains

RELAX NG has no equivalent of the type derivation mechanism in YANG that allows to modify a built-in type (perhaps in multiple steps) by adding new restrictions. When a derived YANG type is used without restrictions - as a substatement of either 'leaf' or another 'typedef' - the 'type' statement is mapped simply to the <rng:ref> element, i.e., a named pattern reference, and the type definition is mapped to a RELAX NG named pattern definition. However, if restrictions are specified as substatements of the 'type' statement, the type definition MUST be expanded at that point so that only the ancestor built-in type appears in the output schema, restricted with facets that correspond to the combination of all restrictions found along the type derivation chain and also in the 'type' statement.

EXAMPLE. Consider this YANG module:

module example3 {
    namespace "http://example.com/ns/example3";
    prefix ex3;
    typedef dozen {
        type uint8 {
            range 1..12;
        }
    }
    leaf month {
        type dozen;
    }

The 'type' statement in "leaf month" is mapped simply to the reference <rng:ref name="example3__dozen"/> and the corresponding named pattern is defined as follows:

<rng:define name="example3__dozen">
  <rng:data type="unsignedByte">
    <rng:param name="minInclusive">1</param>
    <rng:param name="maxInclusive">12</param>
  </rng:data>
</rng:define>

Assume now that the definition of leaf "month" is changed to

leaf month {
    type dozen {
        range 7..max;
    }
}

The output RELAX NG schema then won't contain any named pattern definition and leaf "month" will be mapped directly to

<rng:element name="ex3:month">
  <rng:data type="unsignedByte">
    <rng:param name="minInclusive">7</param>
    <rng:param name="maxInclusive">12</param>
  </rng:data>
</rng:element>

The mapping of type derivation chains may be further complicated by the presence of the 'default' statement in type definitions. In the simple case, when a type definition containing the 'default' statement is used without restrictions, the 'default' statement is mapped to the @nma:default attribute attached to the <rng:define> element.

However, if that type definition has to be expanded, the @nma:default annotation arising from the expanded type or ancestor types in the type derivation chain MUST be attached to the element where the expansion occurs. If there are multiple 'default' statements in consecutive steps of the type derivation, only the 'default' statement that is closest to the expanded type is used.

EXAMPLE. Consider this variation of the last example:

module example3bis {
    namespace "http://example.com/ns/example3bis";
    prefix ex3bis;
    typedef dozen {
        type uint8 {
            range 1..12;
        }
        default 7;
    }
    leaf month {
        type dozen;
    }

The 'typedef' statement in this module is mapped to the following named pattern definition:

<rng:define name="example3bis__dozen" @nma:default="7">
  <rng:data type="unsignedByte">
    <rng:param name="minInclusive">1</param>
    <rng:param name="maxInclusive">12</param>
  </rng:data>
</rng:define>

If the "dozen" type is restricted when used in the leaf "month" definition as in the previous example, the "dozen" type has to be expanded and @nma:default becomes an attribute of the <ex3bis:month> element definition:

<rng:element name="ex3bis:month" @nma:default="7">
  <rng:data type="unsignedByte">
    <rng:param name="minInclusive">7</param>
    <rng:param name="maxInclusive">12</param>
  </rng:data>
</rng:element>

However, if the definition of leaf "month" itself contained the 'default' substatement, the default specified for the "dozen" type will be ignored.

9.3.  Translation of XPath Expressions

YANG uses full XPath 1.0 syntax [XPath] (Clark, J. and S. DeRose, “XML Path Language (XPath) Version 1.0,” November 1999.) for the arguments of 'must', 'when' and 'path' statements. As the names of data nodes defined in a YANG module always belong to the namespace of that YANG module, YANG adopted a simplification similar to the concept of default namespace in XPath 2.0: node names needn't carry a namespace prefix inside the module where they are defined and the local module's namespace is assumed.

If an XPath expression is carried over to a NETMOD-specific annotation in the conceptual tree schema, it MUST be translated into a fully conformant XPath 1.0 expression that also reflects the hierarchy of the conceptual data tree:

1. Each unprefixed node name MUST be prepended with the local module's namespace prefix declared by the 'prefix' statement.
2. Absolute XPath expressions, i.e., those starting with a slash, MUST be prepended with appropriate path in the conceptual tree, according to the YANG specification of context for XPath expressions, see [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.), Sections 7.5.3 and 7.19.5.

Translation rule 2 means for example that absolute XPath expressions appearing in the main configuration data tree always start with "nmt:netmod-tree/nmt:top/", those appearing in a notification always start with "nmt:netmod-tree/nmt:notifications/nmt:notification/", etc.

EXAMPLE. YANG XPath expression "/dhcp/max-lease-time" appearing in the main configuration data will be translated to "nmt:netmod-tree/nmt:top/dhcp:dhcp/dhcp:max-lease-time".

The key identifiers and "descendant schema node identifiers" (see the ABNF production for and "descendant-schema-nodeid" in Section 12 of [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.)) are not XPath expressions but SHOULD be translated by adding local module prefixes as well.

9.4.  YANG Language Extensions

YANG allows for extending its own language in-line by adding new statements with keywords from special namespaces. Such extensions first have to be declared using the 'extension' statement and then can be used as the standard YANG statements, from which they are distinguished by a namespace prefix qualifying the extension keyword. RELAX NG has a similar extension mechanism - XML elements and attributes with names from foreign namespaces may be inserted at almost any place of a RELAX NG schema.

YANG language extensions may or may not have a meaning in the context of DSDL schemas. Therefore, an implementation MAY ignore any or all of the extensions. However, an extension that is not ignored MUST be mapped to XML element(s) and/or attribute(s) that exactly match the YIN form of the extension, see Section 11.1 in [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.).

EXAMPLE. Consider the following extension defined by the "acme" module:

extension documentation-flag {
    argument number;
}

This extension can then be used in the same or another module, for instance like this:

leaf folio {
    acme:documentation-flag 42;
    type string;
}

If this extension is honored by the mapping, it will be mapped to

<rng:element name="acme:folio">
   <acme:documentation-flag number="42"/>
   <rng:data type="string"/>
</rng:element>

Note that the 'extension' statement itself is not mapped in any way.

10.  Mapping Conceptual Tree Schema to DSDL

As explained in Section 6 (Overview of the Mapping), the second step of the YANG-to-DSDL mapping takes the conceptual tree schema and transforms it to various DSDL schemas prepared for validating instance XML documents. As an input parameter, this step takes in the simplest case just a specification of the NETCONF XML document type (or combination of multiple types) that is to be validated. These document type can be, for example, the content of a datastore, reply to <nc:get> or <nc:get-config>, other RPC requests or replies and notifications.

In general, the second mapping step has to accomplish the following three tasks:

1. Extract the part(s) of the conceptual tree schema that are appropriate for the requested document type. For example, if a <get> reply is to be validated, the subtree under <nmt:top> must be selected.
2. The schema must be adapted to the specific encapsulating XML elements mandated by the RPC layer. These are, for example, <nc:rpc> and <nc:data> elements in the case of a <get> reply or <en:notification> for a notification.
3. Finally, NETMOD-specific annotations that are relevant for the schema language of the generated schema must be mapped to the corresponding rules.

These three tasks are together much simpler than the first mapping step and can be effectively implemented using XSL transformations [XSLT] (Clark, J., “XSL Transformations (XSLT) Version 1.0,” November 1999.).

The following subsections describe the details of the second mapping step for the individual DSDL schema languages. Section 12 (Mapping NETMOD-specific annotations to DSDL Schema Languages) then contains a detailed specification for the mapping of all NETMOD-specific annotations.

10.1.  Generating RELAX NG Schemas for Various Document Types

With one minor exception, obtaining a validating RELAX NG schema from the conceptual tree schema really means only taking appropriate parts of the conceptual tree schema and assembling them in a new RELAX NG grammar, perhaps after removing all unwanted annotations. Depending on the XML document type that is the target for validation (<get>/<get-config> reply, RPC or notification) a corresponding top-level part of the grammar MUST be added as described in the following subsections.

Schemas for multiple alternative target document types can also be easily generated by enclosing the definitions for requested type in <rng:choice> element.

In order to avoid copying common named pattern definitions to every single output RELAX NG file, these schema-independent definitions SHOULD be collected in a library file which is then included by the validating RELAX NG schemas. Appendix B (Schema-Independent Library) has the listing of this library file.

The minor exception mentioned above is the annotation @nma:config, which must be observed if the target document type is <get-config> reply. In this case, each element definition that has this attribute with the value "false" MUST be removed from the schema together with its descendants. See Section 12.1 (The @nma:config Annotation) for more details.

10.1.1.  Reply to <get> or <get-config>

For a reply to <get> or <get-config>, the mapping must take the part of the conceptual tree schema under the definition of <nmt:top> and insert it in the following grammar:

<rng:grammar ... namespaces etc. ...>
  <rng:include href="relaxng-lib.rng"/>
  <rng:start>
    <rng:element name="nc:rpc-reply">
      <rng:ref name="message-id-attribute"/>
      <rng:element name="nc:data">
        ... patterns defining contents of "nmt:top" subtree ...
      </rng:element>
    </rng:element>
  </rng:start>
  ... named pattern definitions ...
</rng:grammar>

The definition for the named pattern "message-id-attribute" is found in the library file "relaxng-lib.rng" which is included on the second line (see Appendix B (Schema-Independent Library)).

Definitions of other named patterns MUST be copied literally from the conceptual tree schema to the resulting grammar. However, an implementation MAY choose to copy only those definitions that are really used in the particular output grammar.

10.1.2.  Remote Procedure Calls

For an RPC method named "myrpc" and defined in a YANG module with prefix "yam", the corresponding schema subtree is identified by the definition of <nmt:rpc-method> element whose <nmt:input> subelement has <yam:myrpc> as the only child.

The mapping must also take into account whether the target document type is an RPC request or reply. For "yam:myrpc" request, the resulting grammar looks as follows:

<rng:grammar ... namespaces etc. ...>
  <rng:include href="relaxng-lib.rng"/>
  <rng:start>
    <rng:element name="nc:rpc">
      <rng:ref name="message-id-attribute"/>
      <rng:element name="yam:myrpc">
        ... patterns defining contents of the subtree ...
        ... "nmt:rpc-method/nmt:input/yam:myrpc" ...
      </rng:element>
    </rng:element>
  </rng:start>
  ... named pattern definitions ...
</rng:grammar>

For "yam:myrpc" reply, the output grammar is

<rng:grammar ... namespaces etc. ...>
  <rng:include href="relaxng-lib.rng"/>
  <rng:start>
    <rng:element name="nc:rpc-reply">
      <rng:ref name="message-id-attribute"/>
      ... patterns defining contents of the corresponding ...
      ... "nmt:rpc-method/nmt:output" subtree ...
    </rng:element>
  </rng:start>
  ... named pattern definitions ...
</rng:grammar>

In both cases, exact copies of named pattern definitions from the conceptual tree schema MUST be inserted, but an implementation MAY choose to include only those used for the given RPC.

10.1.3.  Notifications

For a notification named "mynotif" and defined in a YANG module with prefix "yam", the corresponding schema subtree is identified by the definition of <nmt:notification> element that has the single child <yam:mynotif>.

The resulting grammar looks as follows:

<rng:grammar ... namespaces etc. ...>
  <rng:include href="relaxng-lib.rng"/>
  <rng:start>
    <rng:element name="en:notification">
      <rng:ref name="eventTime-element"/>
      <rng:element name="yam:mynotif">
        <!-- patterns defining contents of
             "nmt:rpc-notification/yam:mynotif" subtree -->
      </rng:element>
    </rng:element>
  </rng:start>
  <!-- named pattern definitions -->
</rng:grammar>

The definition of the named pattern "eventTime-element" is found in the "relaxng-lib.rng" library file, see Appendix B (Schema-Independent Library).

Again, exact copies of named pattern definitions from the conceptual tree schema MUST be inserted, but an implementation MAY choose to include only those used for the given notification.

10.2.  Mapping Semantic Constraints to Schematron

Schematron schemas tend to be much flatter and more uniform compared to RELAX NG. They have exactly four levels of XML hierarchy: <sch:schema>, <sch:pattern>, <sch:rule> and <sch:assert> or <sch:report>.

In a Schematron schema generated by the second mapping step, the basic unit of organization is a rule represented by the <sch:rule> element. Every rule corresponds to exactly one element definition pattern in the conceptual tree schema:

<sch:rule context="XELEM">
  ...
</sch:rule>

The value of the mandatory @context attribute of <sch:rule> (denoted as XELEM) MUST be set to the absolute path of the context element in the data tree. The <sch:rule> element contains the mappings of one or more of the following NETMOD-specific annotations, if they are attached to the context element: <nma:instance-identifier>, @nma:key, @nma:leafref, @nma:min-elements, @nma:max-elements, <nma:must>, @nma:unique and <nma:when>.

In the opposite direction, however, not every element definition pattern in the conceptual tree schema has a corresponding rule in the Schematron schema: definitions of elements the carry none of the above annotations are omitted.

Schematron rules may be further grouped into patterns represented by the <sch:pattern> element. The mapping uses patterns only for discriminating between subsets of rules that belong to different validation phases, see Section 10.2.1 (Validation Phases). Therefore, the <sch:schema> always has exactly two <sch:pattern> children: one named "standard" contains rules for all annotations except <nma:instance-identifier> and @nma:leafref, and another named "ref-integrity" containing rules for these two remaining annotations, i.e., referential integrity checks.

Element definitions in the conceptual tree schema that appear inside a named pattern definition (i.e., have <rng:define> among its ancestors) are subject to a different treatment. This is because their path in the data tree is not fixed - the named pattern may be referred to in multiple places. The mapping uses Schematron abstract rules to handle this case: An element definition inside a named pattern is mapped to an abstract rule and every use of the named pattern then extends (uses) this abstract rule in the concrete context.

EXAMPLE. Consider this element pattern annotated with <nma:must>:

<rng:element name="dhcp:default-lease-time">
  <rng:data type="unsignedInt"/>
  <nma:must assert=". &lt;= ../dhcp:max-lease-time">
    <nma:error-message>
      The default-lease-time must be less than max-lease-time
    </nma:error-message>
  </nma:must>
</rng:element>

If this element pattern appears outside any named pattern and as a child of <dhcp:dhcp> (as it does in the DHCP schema, see Appendix C.2 (Conceptual Tree Schema)), it is mapped to the following Schematron rule:

<sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                  dhcp:default-lease-time">
  <sch:assert test=". &lt;= ../dhcp:max-lease-time">
    The default-lease-time must be less than max-lease-time
  </sch:assert>
</sch:rule>

Now assume the element definition is inside a named pattern definition, say

<rng:define name="_dhcp__default-lease-time">
  <rng:element name="dhcp:default-lease-time">
    ... same content ...
  </rng:element>
</rng:define>

In this case it is mapped to an abstract rule, for instance

<sch:rule id="id31415926" abstract="true">
  <sch:assert test=". &lt;= ../dhcp:max-lease-time">
    The default-lease-time must be less than max-lease-time
  </sch:assert>
</sch:rule>

Any use of the named pattern definition via <rng:ref name="_dhcp__default-lease-time"/> then results in a new rule extending the abstract one, for example

<sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                  dhcp:default-lease-time">
  <sch:extends rule="id31415926"/>
</sch:rule>

Care must be taken that the value of the @context attribute in general consists of two parts in this case: its beginning is determined by the location of the <rng:ref> element in the main schema tree and the rest of the path comes from the relative position of the annotated element definition inside the named pattern. The situation becomes even more complex when the mapping has to deal with chained definitions of named patterns (<rng:ref> inside <rng:define>). The @context value then must be recursively glued together from multiple parts.

The mapping from the conceptual tree schema to Schematron proceeds in the following steps:

1. First, the active subtree(s) of the conceptual tree schema must be selected depending on the requested target document type. This procedure is identical to the RELAX NG case, including the handling of @nma:config if the target document type is <get-config> reply.
2. Namespaces of all input YANG modules, together with the namespaces of base NETCONF ("nc" prefix) or notifications ("en" prefix) MUST be declared using the <sch:ns> element, for example

      <sch:ns uri="http://example.com/ns/dhcp" prefix="dhcp"/>

3. Validation phases are defined (see Section 10.2.1 (Validation Phases)) and their constituting patterns "standard" and "ref-integrity" created.
4. For either validation phase, the input conceptual tree schema is scanned and element definitions with annotations relevant for the given phase are selected and a <sch:rule> is created for each of them. As explained above, the rule is abstract if the element definition appears inside a named pattern.
5. All annotations attached to the given element definition are then mapped using the mapping rules specified in Section 12 (Mapping NETMOD-specific annotations to DSDL Schema Languages). The resulting <sch:assert> or <sch:report> elements are the installed as children of the <sch:rule> element.

10.2.1.  Validation Phases

In certain situations it is useful to validate XML instance documents without enforcing the referential integrity constraints represented by the @nma:leafref and <nma:instance-identifier> annotations. For example, a candidate configuration referring to configuration parameters or state data of certain hardware will not pass full validation before the hardware is installed. To handle this, the Schematron mapping introduces two validation phases:

• Validation phase "full", which is the default, checks all semantic constraints.
• Validation phase "noref" is the same as "full" except it doesn't check referential integrity constraints.

A parameter identifying the validation phase to use has to be passed to the Schematron processor or otherwise "full" phase is assumed by default. How this is exactly done depends on the concrete Schematron processor and is outside the scope of this document.

The validation phases are defined in Schematron by listing the patterns that are to be applied for each phase. Therefore, the mapping puts the rules for referential integrity checking to a special <sch:pattern> with @id attribute set to "ref-integrity". The rules mapped from the remaining semantic constraints are put to another <sch:pattern> with @id attributes set to "standard".

With validation phases, the resulting Schematron schema has the following overall structure:

<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron">
  <sch:ns uri="..." prefix="..."/>
  ... more NS declarations ...
  <sch:phase id="full">
    <sch:active pattern="standard"/>
    <sch:active pattern="ref-integrity"/>
  </sch:phase>
  <sch:phase id="noref">
    <sch:active pattern="standard"/>
  </sch:phase>
  <sch:pattern id="standard">
    ... all rules except ref. integrity checks ...
  </sch:pattern>
  <sch:pattern id="ref-integrity">
    ... rules for ref. integrity checks ...
  </sch:pattern>
</sch:schema>

10.3.  Constraints on Mandatory Choice

In order to fully represent the semantics of YANG 'choice' statement with "mandatory true;" substatement, the RELAX NG grammar has to be combined with a special Schematron rule.

EXAMPLE. Consider the following module:

module example4 {
    namespace "http://example.com/ns/example4";
    prefix ex4;
    choice foobar {
        mandatory true;
        case foo {
            leaf foo1 {
                type uint8;
            }
            leaf foo2 {
                type uint8;
            }
        }
        leaf bar {
            type uint8;
        }
    }
}

In this module, all three leaf nodes in both case branches are optional but because of the "mandatory true;" statement, at least one of them must be present in a valid configuration. The 'choice' statement from this module is mapped to the following fragment of the conceptual tree schema:

<rng:choice>
  <rng:interleave>
    <rng:optional>
      <rng:element name="ex4:foo1">
        <rng:data type="unsignedByte"/>
      </rng:element>
    </rng:optional>
    <rng:optional>
      <rng:element name="ex4:foo2">
        <rng:data type="unsignedByte"/>
      </rng:element>
    </rng:optional>
  </rng:interleave>
  <rng:element name="ex4:bar">
    <rng:data type="unsignedByte"/>
  </rng:element>
</rng:choice>

In the second case branch, the "ex4:bar" element is defined as mandatory so that this element must be present in a valid configuration if this branch is selected. However, the two elements in the first branch "foo" cannot be both declared as mandatory since each of them alone suffices for a valid configuration. As a result, the above RELAX NG fragment would successfully validate configurations where none of the three leaf elements is present.

Therefore, mandatory choices, which can be recognized in the conceptual tree schema as <rng:choice> elements that do not have <optional> as their parent, have to be handled in a special way: For each mandatory choice where at least one of the cases contains more than one node, a rule MUST be present in the "standard" pattern of the Schematron schema enforcing the presence of at least one element from any of the cases. (RELAX NG schema guarantees that elements from different cases cannot be mixed together, that all mandatory nodes are present etc.).

For the example module above, the Schematron rule will be as follows:

<sch:rule context="/nc:rpc-reply/nc:data">
  <sch:assert test="ex4:foo1 or ex4:foo2 or ex4:bar">
    Node(s) from at least one case of choice "foobar" must exist.
  </sch:assert>
</sch:rule>

10.4.  Mapping Default Values to DSRL

DSRL is the only component of DSDL that is allowed to change the information set of the validated XML document. While DSRL also has other functions, YANG-to-DSDL mapping uses it only for specifying and applying default content. For XML instance documents based on YANG data models, insertion of default content may potentially take place for all implicit nodes defined by the rules in Section 9.1.2 (Implicit Nodes).

In DSRL, the default content of an element is specified using the <dsrl:default-content> element, which is a child of <dsrl:element-map>. Two sibling elements of <dsrl:default-content> determine the context for application of the default content, see [DSRL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 8: Document Semantics Renaming Language - DSRL,” 12 2008.):

• <dsrl:parent> element contains an XSLT pattern specifying the parent element; the default content is applied only if the parent element exists in the instance document.
• <dsrl:name> contains the XML name of the element which, if missing or empty, is inserted together with the content of <dsrl:default-content>.

The <dsrl:parent> element is optional in a general DSRL schema but for the purpose of the YANG-to-DSDL mapping this element MUST be always present in order to guarantee proper application of default content.

DSRL mapping only deals with element patterns defining implicit nodes (see Section 9.1.2 (Implicit Nodes)). In the conceptual tree schema, such element patterns are distinguished by having NETMOD-specific annotation attributes @nma:default or @nma:implicit, i.e., either

<rng:element name="ELEM" nma:default="DEFVALUE">
  ...
</rng:element>

or

<rng:element name="ELEM" nma:implicit="true">
  ...
</rng:element>

The former case applies to leaf nodes having the 'default' substatement, but also to leaf nodes that obtain their default value from a typedef, if this typedef is expanded according to the rules in Section 9.2.2 (Type Derivation Chains) so that the @nma:default annotation is attached directly to the leaf's element pattern.

The latter case is used for all implicit containers (see Section 9.1 (Occurrence Rules for Data Nodes)) and for leafs that obtain the default value from a typedef and don't have the @nma:default annotation.

In the simplest case, both element patterns are mapped to the following DSRL element map:

<dsrl:element-map>
  <dsrl:parent>XPARENT</dsrl:parent>
  <dsrl:name>ELEM</dsrl:name>
  <dsrl:default-content>DEFCONT</dsrl:default-content>
  </dsrl:element-map>

where XPARENT is the absolute XPath of ELEM's parent element in the data tree and DEFCONT is constructed as follows:

• If the implicit node ELEM is a leaf and has the @nma:default attribute, DEFCONT is set to the value of this attribute (denoted above as DEFVALUE).
• If the implicit node ELEM is a leaf and has the @nma:implicit attribute with the value "true", the default value has to be determined from the @nma:default attribute of the definition of ELEM's type (perhaps recursively) and used in place of DEFCONT in the above DSRL element map. See also Section 9.2.2 (Type Derivation Chains).
• Otherwise, the implicit node ELEM is a container and DEFCONT is constructed as an XML fragment containing all descendant elements of ELEM that have either @nma:implicit or @nma:default attribute.

The <rng:element>, <rng:group> or <rng:interleave>patterns appearing directly under <rng:choice> MUST NOT have either @nma:default or @nma:implicit annotation unless they belong to the default case of a YANG choice (see Section 11.12 (The default Statement)).

In addition, when mapping the default case of a choice, it has to be guaranteed that the default content is not applied if any node from any non-default case is present. This is accomplished by setting <dsrl:parent> in a special way:

<dsrl:parent>XPARENT[not (ELEM1|ELEM2|...|ELEMn)]</dsrl:parent>

where ELEM1, ELEM2, ... ELEMn are the names of all top-level nodes from all non-default cases. The rest of the element map is exactly as before.

EXAMPLE. Consider the following YANG module:

module example5 {
  namespace "http://example.com/ns/example5";
  prefix ex5;
  container outer {
    leaf leaf1 {
      type uint8;
      default 1;
    }
    choice one-or-two {
      default "one";
      container one {
        leaf leaf2 {
          type uint8;
          default 2;
        }
      }
      leaf leaf3 {
        type uint8;
        default 3;
      }
    }
  }
}

The DSRL schema generated for the "get-reply" target document type will be:

<dsrl:maps xmlns:dsrl="http://purl.oclc.org/dsdl/dsrl"
           xmlns:ex5="http://example.com/ns/example5"
           xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data</dsrl:parent>
    <dsrl:name>ex5:outer</dsrl:name>
    <dsrl:default-content>
      <ex5:leaf1>1</ex5:leaf1>
      <ex5:one><ex5:leaf2>2</ex5:leaf2></ex5:one>
    </dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>
      /nc:rpc-reply/nc:data/ex5:outer[not(ex5:leaf3)]
    </dsrl:parent>
    <dsrl:name>ex5:one</dsrl:name>
    <dsrl:default-content>
      <ex5:leaf2>2</ex5:leaf2>
    </dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/ex5:outer</dsrl:parent>
    <dsrl:name>ex5:leaf1</dsrl:name>
    <dsrl:default-content>1</dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/ex5:outer</dsrl:parent>
    <dsrl:name>ex5:one</dsrl:name>
    <dsrl:default-content>
      <ex5:leaf2>2</ex5:leaf2>
    </dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/ex5:outer/ex5:one</dsrl:parent>
    <dsrl:name>ex5:leaf2</dsrl:name>
    <dsrl:default-content>2</dsrl:default-content>
  </dsrl:element-map>
</dsrl:maps>

Note that the default value for "leaf3" defined in the YANG module is ignored because "leaf3" represents a non-default alternative of a choice and as such never becomes an implicit element.

11.  Mapping YANG Statements to Conceptual Tree Schema

Each subsection in this section is devoted to one YANG statement and provides the specification how the statement is mapped to the annotated RELAX NG schema of the conceptual tree. This is the first step of the mapping procedure as explained in Section 6 (Overview of the Mapping). The subsections are sorted alphabetically by the statement keyword.

Each YANG statement is mapped to an XML fragment, typically a single element or attribute but it may also be a larger structure. The mapping procedure is inherently recursive, which means that after finishing a statement the mapping continues with its substatements, if there are any, and a certain element of the resulting fragment becomes the parent of other fragments resulting from the mapping of substatements. Any changes to this default recursive procedure are explicitly specified.

YANG XML encoding rules translate to the following rules for ordering multiple subelements:

1. Within the <nmt:rpc-methods> subtree (i.e., for RPC input and output parameters) the order of subelements is fixed and their definitions in the conceptual tree schema MUST follow the order specified in the source YANG module.
2. When mapping the 'list' statement, all keys MUST come before any other subelements and in the same order as they are declared in the 'key' statement. The order of the remaining (non-key) subelements is not specified, so their definitions in the conceptual tree schema MUST be enclosed in the <rng:interleave> element.
3. Otherwise, all definitions of subelements in the conceptual tree schema MUST be enclosed in the <rng:interleave> element.

We use the following conventions:

• The argument of the statement being mapped is denoted by ARGUMENT.
• The element in the RELAX NG schema that becomes the parent of the resulting XML fragment is denoted by PARENT.

11.1.  The anyxml Statement

This statement is mapped to <rng:element> element and ARGUMENT with prepended local namespace prefix becomes the value of its @name attribute. The content of <rng:element> is

<rng:ref name="__anyxml__"/>

Substatements of the 'anyxml' statement, if any, MAY be mapped to additional children of the RELAX NG element definition.

If the 'anyxml' statement occurs in any of the input YANG modules, the following pattern definition MUST be added exactly once to the RELAX NG schema as a child of the <rng:grammar> element (cf. [Vli04] (van der Vlist, E., “RELAX NG,” 2004.), p. 172):

<rng:define name="__anyxml__">
  <rng:zeroOrMore>
    <rng:choice>
      <rng:attribute>
        <rng:anyName/>
      </rng:attribute>
      <rng:element>
        <rng:anyName/>
        <rng:ref name="__anyxml__"/>
      </rng:element>
      <rng:text/>
    </rng:choice>
  </rng:zeroOrMore>
</rng:define>

EXAMPLE: YANG statement in a module with namespace prefix "yam"

anyxml data {
    description "Any XML content allowed here.";
}

is mapped to the following fragment:

<rng:element name="yam:data">
    <a:documentation>Any XML content allowed here</a:documentation>
    <rng:ref name="__anyxml__"/>
</rng:element>

An anyxml node is optional if there is no "mandatory true;" substatement. The <rng:element> element then MUST be wrapped in <rng:optional>, except when the 'anyxml' statement is a child of the 'choice' statement and thus forms a shorthand case for that choice (see Section 9.1.1 (Optional and Mandatory Nodes) for details).

11.2.  The argument Statement

This statement is not mapped to the output schema, but see the rules for handling extensions in Section 9.4 (YANG Language Extensions).

11.3.  The augment Statement

As a substatement of 'uses', this statement is handled as a part of 'uses' mapping, see Section 11.57 (The uses Statement).

At the top level of a module or submodule, the 'augment' statement is used for augmenting the schema tree of another YANG module. If the latter module is not processed within the same mapping session, the top-level 'augment' statement MUST be ignored. Otherwise, the contents of the statement are added to the foreign module with the namespace of the module where the 'augment' statement appears.

11.4.  The base Statement

This statement is ignored as a substatement of 'identity' and handled within the 'identityref' type if it appears as a substatement of that type definition, see Section 11.53.5 (The identityref Type).

11.5.  The belongs-to Statement

This statement is not used since processing of submodules is always initiated from the main module, see Section 11.24 (The include Statement).

11.6.  The bit Statement

This statement is handled within the "bits" type, see Section 11.53.3 (The bits Type).

11.7.  The case Statement

This statement is mapped to <rng:group> or <rng:interleave> element, depending on whether the statement belongs to an RPC definition or not. If the argument of a sibling 'default' statement equals to ARGUMENT, @nma:implicit attribute with the value "true" MUST be added to that <rng:group> or <rng:interleave> element. The @nma:implicit attribute MUST NOT be used for nodes at the top-level of a non-default case (see Section 7.9.3 in [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.)).

11.8.  The choice Statement

This statement is mapped to <rng:choice> element.

Unless 'choice' has the 'mandatory' substatement with the value "true", the <rng:choice> element MUST be wrapped in <rng:optional>. The 'choice' statement with "mandatory true;" may require additional handling, see Section 10.3 (Constraints on Mandatory Choice).

The alternatives in <rng:choice> - mapped from either the 'case' statement or a shorthand case - MUST NOT be defined as optional.

11.9.  The config Statement

This statement is mapped to @nma:config attribute and ARGUMENT becomes its value.

11.10.  The contact Statement

This statement SHOULD NOT be used by the mapping since the output RELAX NG schema may result from multiple YANG modules created by different authors. The schema contains references to all input modules in the Dublin Core elements <dc:source>, see Section 11.34 (The module Statement). The original YANG modules are the authoritative sources of the authorship information.

11.11.  The container Statement

Using the rules specified in Section 9.1.1 (Optional and Mandatory Nodes), the mapping algorithm MUST determine whether the statement defines an optional container, and if so, insert the <rng:optional> element and make it the new PARENT.

The container defined by this statement is then mapped to the <rng:element> element, which becomes a child of PARENT and uses ARGUMENT with prepended local namespace prefix as the value of its @name attribute.

Finally, using the rules specified in Section 9.1.2 (Implicit Nodes), the mapping algorithm MUST determine whether the container is implicit, and if so, add the attribute @nma:implicit with the value "true" to the <rng:element> element.

11.12.  The default Statement

If this statement is a substatement of 'leaf', it is mapped to the @nma:default attribute of PARENT and ARGUMENT becomes its value.

As a substatement of 'typedef', the 'default' statement is also mapped to the @nma:default attribute with the value of ARGUMENT. The placement of this attribute depends on whether or not the type definition has to be expanded when it is used:

• If the type definition is not expanded, @nma:default becomes an attribute of the <rng:define> pattern resulting from the parent 'typedef' mapping.
• Otherwise, @nma:default becomes an attribute of the ancestor RELAX NG pattern inside which the expansion takes place.

Details and an example are given in Section 9.2.2 (Type Derivation Chains).

Finally, as a substatement of 'choice', the 'default' statement identifies the default case and is handled within the 'case' statement, see Section 11.7 (The case Statement). If the default case uses the shorthand notation where the 'case' statement is omitted, the @nma:implicit attribute with the value "true" is either attached to the node representing the default case in the shorthand notation or, alternatively, an extra <rng:group> element MAY be inserted and the @nma:implicit attribute attached to it. In the latter case, the net result is the same as if the 'case' statement wasn't omitted for the default case.

EXAMPLE. The following 'choice' statement in a module with namespace prefix "yam"

choice leaves {
    default feuille;
    leaf feuille { type empty; }
    leaf hoja { type empty; }
}

is either mapped directly to

<rng:choice>
  <rng:element name="yam:feuille" nma:implicit="true">
    <rng:empty/>
  </rng:element>
  <rng:element name="yam:hoja">
    <rng:empty/>
  </rng:element/>
</rng:choice>

or the default case may be wrapped in an extra <rng:group>:

<rng:choice>
  <rng:group nma:implicit="true">
    <rng:element name="yam:feuille">
      <rng:empty/>
    </rng:element>
  </rng:group>
  <rng:element name="yam:hoja">
    <rng:empty/>
  </rng:element/>
</rng:choice>

11.13.  The description Statement

This statement MAY be ignored. Otherwise, it is mapped to the DTD compatibility element <a:documentation> and ARGUMENT becomes its text.

In order to get properly formatted in the RELAX NG compact syntax, this element SHOULD be inserted as the first child of PARENT.

11.14.  The deviation Statement

This statement is ignored, see Section 8.5 (Features and Deviations).

11.15.  The enum Statement

This statement is mapped to <rng:value> element and ARGUMENT becomes its text. All substatements except 'status' are ignored because the <rng:value> element cannot contain annotation elements, see [RNG] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. Second Edition.,” 12 2008.), section 6.

11.16.  The error-app-tag Statement

This statement is ignored unless it is a substatement of 'must'. In the latter case it is mapped to the <nma:error-app-tag> element. See also Section 11.35 (The must Statement).

11.17.  The error-message Statement

This statement is ignored unless it is a substatement of 'must'. In the latter case it is mapped to the <nma:error-message> element. See also Section 11.35 (The must Statement).

11.18.  The extension Statement

This statement is ignored. However, extensions to the YANG language MAY be mapped as described in Section 9.4 (YANG Language Extensions).

11.19.  The feature Statement

This statement is ignored, see Section 8.5 (Features and Deviations).

11.20.  The grouping Statement

This statement is mapped to a RELAX NG named pattern definition <rng:define>, but only if the grouping defined by this statement is used without refinements and augments in at least one of the input modules. In this case, the named pattern definition becomes a child of the <rng:grammar> element and its name is ARGUMENT mangled according to the rules specified in Section 9.2 (Mapping YANG Groupings and Typedefs).

Whenever a grouping is used with additional refinements and/or augments, the grouping is expanded so that the refinements and augments may be applied in place to the prescribed schema nodes. See Section 9.2.1 (YANG Refinements and Augments) for further details and an example.

An implementation MAY offer the option of recording all 'grouping' statements as named patterns in the output RELAX NG schema even if they are not referenced. This is useful for mapping YANG "library" modules containing only 'typedef' and/or 'grouping' statements.

11.21.  The identity Statement

This statement is not specifically mapped. However, if the identity defined by this statement is used as the base for an "identityref" type in any of the input modules, or is derived from this base, ARGUMENT will appear as the text of one of the <rng:value> elements in the mapping of that "identityref" type. See Section 11.53.5 (The identityref Type) for more details and an example.

11.22.  The if-feature Statement

This statement is ignored, see Section 8.5 (Features and Deviations).

11.23.  The import Statement

This statement is not specifically mapped. The module whose name is in ARGUMENT has to be parsed so that the importing module be able to use its top-level groupings and typedefs and also augment the data tree of the imported module.

If the 'import' statement has the 'revision' substatement, the corresponding revision of the imported module MUST be used. The mechanism for finding a given module revision is outside the scope of this document.

11.24.  The include Statement

This statement is not specifically mapped. The submodule whose name is in ARGUMENT has to be parsed and its contents mapped exactly as if the submodule text was a subset of the main module text.

If the 'include' statement has the 'revision' substatement, the corresponding revision of the submodule MUST be used. The mechanism for finding a given submodule revision is outside the scope of this document.

11.25.  The input Statement

This statement is handled within 'rpc' statement, see Section 11.50 (The rpc Statement).

11.26.  The key Statement

This statement is mapped to @nma:key attribute. ARGUMENT MUST be translated so that every key is prefixed with the namespace prefix of the local module. The result of this translation then becomes the value of the @nma:key attribute.

11.27.  The leaf Statement

This statement is mapped to the <rng:element> element and ARGUMENT with prepended local namespace prefix becomes the value of its @name attribute.

A leaf is optional if there is no "mandatory true;" substatement and if the leaf is not declared among the keys of an enclosing list. The <rng:element> element then MUST be wrapped in <rng:optional>, except when the 'leaf' statement is a child of the 'choice' statement and thus forms a shorthand case for that choice (see Section 9.1.1 (Optional and Mandatory Nodes) for details).

11.28.  The leaf-list Statement

This statement is mapped to a block enclosed by either <rng:zeroOrMore> or <rng:oneOrMore> element depending on whether the argument of 'min-elements' substatement is "0" or positive, respectively (it is zero by default). This <rng:zeroOrMore> or <rng:oneOrMore> element becomes the PARENT.

<rng:element> is then added as a child element of PARENT and ARGUMENT with prepended local namespace prefix becomes the value of its @name attribute. If the 'leaf-list' statement has the 'min-elements' substatement and its argument is greater than one, additional attribute @nma:min-elements is attached to <rng:element> and the argument of 'min-elements' becomes the value of this attribute. Similarly, if there is the 'max-elements' substatement and its argument value is not "unbounded", attribute @nma:max-elements is attached to this element and the argument of 'max-elements' becomes the value of this attribute.

EXAMPLE. YANG leaf-list in a module with namespace prefix "yam"

leaf-list foliage {
    min-elements 3;
    max-elements 6378;
    ordered-by user;
    type string;
}

is mapped to the following RELAX NG fragment:

<rng:oneOrMore>
  <rng:element name="yam:foliage" nma:ordered-by="user"
               nma:min-elements="3" nma:max-elements="6378">
    <rng:data type="string"/>
  </rng:element>
</rng:oneOrMore>

11.29.  The length Statement

This statement is handled within the "string" type, see Section 11.53.9 (The string Type).

11.30.  The list Statement

This statement is mapped exactly as the 'leaf-list' statement, see Section 11.28 (The leaf-list Statement).

When mapping the substatements of 'list', the order of children of the list element MUST be specified so that list keys, if there are any, always appear in the same order as they are defined in the input YANG module and before other children, see [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.), Section 7.8.5. In particular, if any list key is defined in a grouping but the list node itself is not a part of the same grouping, and the position of the 'uses' statement would violate the above ordering requirement, the grouping MUST be expanded, i.e., the 'uses' statement replaced by the grouping contents.

For example, consider the following YANG fragment of a module with prefix "yam":

grouping keygrp {
    leaf clef {
        type uint8;
    }
}

list foo {
    key clef;
    leaf bar {
        type string;
    }
    leaf baz {
        type string;
    }
    uses keygrp;
}

is mapped to the following RELAX NG fragment:

<rng:zeroOrMore>
  <rng:element name="yam:foo" nma:key="yam:clef">
    <rng:element name="yam:clef">
      <rng:data type="unsignedByte"/>
    </rng:element>
    <rng:interleave>
      <rng:element name="yam:bar">
        <rng:data type="string"/>
      </rng:element>
      <rng:element name="yam:baz">
        <rng:data type="string"/>
      </rng:element>
    </rng:interleave>
  </rng:element>
</rng:zeroOrMore>

Note that the "keygrp" grouping is expanded and the "yam:clef" definition moved before the <rng:interleave> pattern.

11.31.  The mandatory Statement

This statement may appear as a substatement of 'leaf', 'choice' or 'anyxml' statement. If ARGUMENT is "true", the parent data node is mapped as mandatory, see Section 9.1.1 (Optional and Mandatory Nodes).

11.32.  The max-elements Statement

This statement is handled within 'leaf-list' or 'list' statements, see Section 11.28 (The leaf-list Statement).

11.33.  The min-elements Statement

This statement is handled within 'leaf-list' or 'list' statements, see Section 11.28 (The leaf-list Statement).

11.34.  The module Statement

This statement is not specifically mapped except that a <dc:source> element SHOULD be created as a child of <rng:grammar> and contain ARGUMENT as a reference to the input YANG module. See also Section 11.49 (The revision Statement).

With respect to the conceptual tree schema, substatements of 'module' MUST be mapped so that

• top level data elements be defined as descendants of the <nmt:top> element;
• elements mapped from 'rpc' statements be defined as descendants of the <nmt:rpc-methods> element;
• elements mapped from 'notification' statements be defined as descendants of the <nmt:notifications> element.

11.35.  The must Statement

This statement is mapped to the <nma:must> element. It has one mandatory attribute @assert (with no namespace), which contains ARGUMENT transformed into a valid XPath expression (see Section 9.3 (Translation of XPath Expressions)). The <nma:must> element may get other subelements resulting from mapping 'error-app-tag' and 'error-message' substatements. Other substatements of 'must', i.e., 'description' and 'reference', are ignored.

EXAMPLE. YANG statement

must 'current() <= ../max-lease-time' {
    error-message
        "The default-lease-time must be less than max-lease-time";
}

is mapped to

<nma:must assert="current()&lt;=../dhcp:max-lease-time">
  <nma:error-message>
    The default-lease-time must be less than max-lease-time
  </nma:error-message>
</nma:must>

11.36.  The namespace Statement

This statement is mapped to @xmlns:xxx attribute of the <rng:grammar> element where "xxx" is the namespace prefix specified by the sibling 'prefix' statement. ARGUMENT becomes the value of this attribute.

11.37.  The notification Statement

This statement is mapped to the following subtree in the RELAX NG schema ("yam" is the prefix of the local YANG module):

<rng:element name="nmt:notification">
  <rng:element name="yam:ARGUMENT">
    ...
  </rng:element>
</rng:element>

Substatements of 'notification' are mapped under <rng:element name="yam:ARGUMENT">.

The <rng:element name="nmt:rpc-notification"> element is a child of <rng:element name="nmt:notifications">.

11.38.  The ordered-by Statement

This statement is mapped to @nma:ordered-by attribute and ARGUMENT becomes the value of this attribute. See Section 11.28 (The leaf-list Statement) for an example.

11.39.  The organization Statement

This statement is not used by the mapping since the output conceptual tree schema may result from multiple YANG modules authored by different parties. The schema contains references to all input modules in the Dublin Core elements <dc:source>, see Section 11.34 (The module Statement). The original modules are the authoritative sources of the authorship information.

11.40.  The output Statement

This statement is handled within 'rpc' statement, see Section 11.50 (The rpc Statement).

11.41.  The path Statement

This statement is handled within "leafref" type, see Section 11.53.7 (The leafref Type).

11.42.  The pattern Statement

This statement is handled within "string" type, see Section 11.53.9 (The string Type).

11.43.  The position Statement

This statement is ignored.

11.44.  The prefix Statement

This statement is handled within the sibling 'namespace' statement, see Section 11.36 (The namespace Statement), or within the parent 'import' statement, see Section 11.23 (The import Statement). As a substatement of 'belongs-to' (in submodules), the 'prefix' statement is ignored.

11.45.  The presence Statement

This statement is mapped to the annotation attribute @nma:presence with the value "true". In addition, it influences the mapping of 'container' (Section 11.11 (The container Statement)): the parent container definition MUST be wrapped in <rng:optional>, regardless of its content. See also Section 9.1.1 (Optional and Mandatory Nodes).

11.46.  The range Statement

This statement is handled within numeric types, see Section 11.53.8 (The numeric Types).

11.47.  The reference Statement

This statement MAY be ignored. Otherwise, it is mapped to <a:documentation> element and its text is set to ARGUMENT prefixed with "See: ".

11.48.  The require-instance Statement

This statement is handled within "instance-identifier" type (Section 11.53.6 (The instance-identifier Type)).

11.49.  The revision Statement

The mapping uses only the most recent instance of the 'revision' statement, i.e., one with the latest date in ARGUMENT, which specifies the current revision of the input YANG module [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.). This date SHOULD be recorded, together with the name of the YANG module, in the corresponding Dublin Core element <dc:source> (see Section 11.34 (The module Statement)), for example in this form:

<dc:source>YANG module 'foo', revision 2010-03-02</dc:source>

The 'description' substatement of 'revision' is ignored.

11.50.  The rpc Statement

This statement is mapped to the following subtree in the RELAX NG schema ("yam" is the prefix of the local YANG module):

<rng:element name="nmt:rpc-method">
  <rng:element name="nmt:input">
    <rng:element name="yam:ARGUMENT">
      ... mapped content of 'input' ...
    </rng:element>
  </rng:element>
  <rng:element name="nmt:output">
    ... mapped content of 'output' ...
  </rng:element>
</rng:element>

As indicated in the schema fragment, contents of the 'input' substatement (if any) are mapped under <rng:element name="yam:ARGUMENT">. Similarly, contents of the 'output' substatement are mapped under <rng:element name="nmt:output">. If there is no 'output' substatement, the <rng:element name="nmt:output"> MUST NOT be present.

The <rng:element name="nmt:rpc-method"> element is a child of <rng:element name="nmt:rpc-methods">.

11.51.  The status Statement

This statement MAY be ignored. Otherwise, it is mapped to @nma:status attribute and ARGUMENT becomes its value.

11.52.  The submodule Statement

This statement is not specifically mapped. Its substatements are mapped as if they appeared directly in the module the submodule belongs to.

11.53.  The type Statement

Most YANG built-in datatypes have an equivalent in the XSD datatype library [XSD‑D] (Biron, P. and A. Malhotra, “XML Schema Part 2: Datatypes Second Edition,” October 2004.) as shown in Table 3 (YANG built-in datatypes with equivalents in the W3C XML Schema Type Library).

Two important datatypes of the XSD datatype library - "dateTime" and "anyURI" - are not built-in in YANG but defined as derived types in the standard modules [Ytypes] (Schoenwaelder, J., Ed., “Common YANG Data Types,” February 2010.): "date-and-time" in the "ietf-yang-types" module and "uri" in the "ietf-inet-types" module. However, the formal restrictions in the YANG type definitions are rather weak. Therefore, implementations of the YANG-to-DSDL mapping SHOULD detect these derived types in source YANG modules and map them to "dateType" and "anyURI", respectively.

Details about the mapping of individual YANG built-in types are given in the following subsections.

11.53.1.  The empty Type

This type is mapped to <rng:empty/>.

11.53.2.  The boolean and binary Types

These two built-in types do not allow any restrictions and are mapped simply by inserting <rng:data> element whose @type attribute is set to ARGUMENT mapped according to Table 3 (YANG built-in datatypes with equivalents in the W3C XML Schema Type Library) above.

11.53.3.  The bits Type

This type is mapped to <rng:list> and for each 'bit' substatement the following XML fragment is inserted as a child of <rng:list>:

<rng:optional>
  <rng:value>bit_name</rng:value>
</rng:optional>

where bit_name is the name of the bit as found in the argument of the corresponding 'bit' statement.

11.53.4.  The enumeration and union Types

These types are mapped to <rng:choice> element.

11.53.5.  The identityref Type

This type is mapped to <rng:choice> element with one or more <rng:value> subelements. Each of the <rng:value> subelements MUST have the @type attribute and its value set to "QName". One <rng:value> subelement for the base identity MUST always be present - it contains the argument of the 'base' substatement as its text. In addition, one <rng:value> substatement MUST be added for each identity declared locally or in an imported module that is derived from this base identity.

All namespace prefixes that are used for identities from imported modules MUST be appropriately defined.

EXAMPLE (taken from Section 7.16.3 of [YANG] (Bjorklund, M., Ed., “YANG - A data modeling language for NETCONF,” Febuary 2010.)). Consider the following two YANG modules:

module crypto-base {
    namespace "http://example.com/crypto-base";
    prefix "crypto";

    identity crypto-alg {
    description
        "Base identity from which all crypto algorithms
         are derived.";
    }
}

module des {
    namespace "http://example.com/des";
    prefix "des";

    import "crypto-base" {
        prefix "crypto";
    }

    identity des {
        base "crypto:crypto-alg";
        description "DES crypto algorithm";
    }

    identity des3 {
        base "crypto:crypto-alg";
        description "Triple DES crypto algorithm";
    }
}

If these two modules are imported to another module with namespace prefix "yam", leaf definition

leaf crypto {
    type identityref {
        base "crypto:crypto-alg";
    }
}

is mapped to

<rng:element name="yam:crypto">
  <rng:choice>
    <rng:value type="QName">crypto:crypto-alg</value>
    <rng:value type="QName">des:des</value>
    <rng:value type="QName">des:des3</value>
 </rng:choice>
</rng:element>

The "crypto" and "des" prefixes will typically be defined via attributes of the <rng:grammar> element.

11.53.6.  The instance-identifier Type

This type is mapped to <rng:data> element with @type attribute set to "string". In addition, empty <nma:instance-identifier> element MUST be inserted as a child of PARENT.

The argument of the 'require-instance' substatement, if it exists, becomes the value of the @require-instance attribute of <nma:instance-identifier>.

11.53.7.  The leafref Type

This type is mapped exactly as the type of the leaf given in the argument of 'path' substatement. In addition, @nma:leafref attribute MUST be added to PARENT. The argument of the 'path' substatement, translated according to Section 9.3 (Translation of XPath Expressions), is set as the value of this attribute.

11.53.8.  The numeric Types

YANG built-in numeric types are "int8", "int16", "int32", "int64", "uint8", "uint16", "uint32", "uint64" and "decimal64". They are mapped to <rng:data> element with @type attribute set to ARGUMENT translated according to Table 3 (YANG built-in datatypes with equivalents in the W3C XML Schema Type Library) above.

An exception is the "decimal64" type, which is mapped to the "decimal" type of the XSD datatype library. Its precision and number of fractional digits are controlled with the following facets, which MUST always be present:

• "totalDigits" facet set to the value of 19.
• "fractionDigits" facet set to the argument of the 'fraction-digits' substatement.

The fixed value of "totalDigits" corresponds to the maximum of 19 decimal digits for 64-bit integers.

For example, the statement

type decimal64 {
    fraction-digits 2;
}

is mapped to the following RELAX NG datatype:

<rng:data type="decimal">
  <rng:param name="totalDigits">19</rng:param>
  <rng:param name="fractionDigits">2</rng:param>
</rng:data>

All numeric types support the 'range' restriction, which is handled as follows:

If the range expression has just a single range, then

• if the range consists of a single number, the <rng:value> pattern is inserted and its content set to this number.
• Otherwise the range consists of both lower and upper bound and the following pair of datatype facets are inserted:

        <rng:param name="minInclusive">...</rng:param>
  

  and

         <rng:param name="maxInclusive">...</rng:param>
  

  Their contents are the lower and upper bound, respectively. If the lower bound is "min", the "minInclusive" facet is omitted and if the upper bound is "max", the "maxInclusive" facet is omitted.

If the range expression has multiple parts separated by "|", then the parent <rng:data> element must be repeated once for every range part and all such <rng:data> elements are wrapped in <rng:choice> element. Each <rng:data> element contains the <rng:value> pattern or the "minInclusive" and "maxInclusive" facets for one part of the range expression as described in the previous paragraph. For the "decimal64" type, the "totalDigits" and "fractionDigits" must be repeated inside each of the <rng:data> elements.

For example,

type int32 {
    range "-6378..0|42|100..max";
}

is mapped to the following RELAX NG fragment:

<rng:choice>
  <rng:data type="int">
    <rng:param name="minInclusive">-6378</rng:param>
    <rng:param name="maxInclusive">0</rng:param>
  </rng:data>
  <rng:data type="int">
    <rng:value>42</rng:value>
  </rng:data>
  <rng:data type="int">
    <rng:param name="minInclusive">100</rng:param>
  </rng:data>
</rng:choice>

11.53.9.  The string Type

This type is mapped to <rng:data> element with the @type attribute set to "string".

The 'length' restriction is handled analogically to the 'range' restriction for the numeric types (Section 11.53.8 (The numeric Types)):

If the length expression has just a single range, then

• if the length range consists of a single number L, the following datatype facet is inserted:

        <rng:param name="length">L</rng:param>.
  

• Otherwise the length range consists of both lower and upper bound and the following pair of datatype facets are inserted:

        <rng:param name="minLength">...</rng:param>
  

  and

        <rng:param name="maxLength">...</rng:param>
  

  Their contents are the lower and upper bound of the length range, respectively. If the lower bound is "min", the "minLength" facet is omitted and if the upper bound is "max", the "maxLength" facet is omitted.

If the length expression has of multiple parts separated by "|", then the parent <rng:data> element must be repeated once for every range part and all such <rng:data> elements are wrapped in <rng:choice> element. Each <rng:data> element contains the "length" or "minLength" and "maxLength" facets for one part of the length expression as described in the previous paragraph.

Every 'pattern' restriction of the "string" datatype is mapped to the "pattern" facet

<rng:param name="pattern">...</rng:param>

with content equal to the argument of the 'pattern' statement. All such "pattern" facets must be repeated inside each copy of the <rng:data> element, i.e., once for each length range.

For example,

type string {
    length "1|3..8";
    pattern "[A-Z][a-z]*";
}

is mapped to the following RELAX NG fragment:

<rng:choice>
  <rng:data type="string">
    <rng:param name="length">1</rng:param>
    <rng:param name="pattern">[A-Z][a-z]*</rng:param>
  </rng:data>
  <rng:data type="string">
    <rng:param name="minLength">3</rng:param>
    <rng:param name="maxLength">8</rng:param>
    <rng:param name="pattern">[A-Z][a-z]*</rng:param>
  </rng:data>
</rng:choice>

11.53.10.  Derived Types

If the 'type' statement refers to a derived type, it is mapped in one of the following ways depending on whether it contains any restrictions as its substatements:

1. Without restrictions, the 'type' statement is mapped simply to the <rng:ref> element, i.e., a reference to a named pattern. If the RELAX NG definition of this named pattern has not been added to the output schema yet, the corresponding type definition MUST be found and its mapping installed as a subelement of <rng:grammar>, see Section 11.54 (The typedef Statement). Even if a given derived type is used more than once in the input YANG modules, the mapping of the corresponding 'typedef' MUST be installed only once.
2. If any restrictions are present, the ancestor built-in type for the given derived type must be determined and the mapping of this base type MUST be used. Restrictions appearing at all stages of the type derivation chain MUST be taken into account and their conjunction added to the <rng:data> element which defines the basic type.

See Section 9.2.2 (Type Derivation Chains) for more details and an example.

11.54.  The typedef Statement

This statement is mapped to a RELAX NG named pattern definition <rng:define>, but only if the type defined by this statement is used without restrictions in at least one of the input modules. In this case, the named pattern definition becomes a child of the <rng:grammar> element and its name is ARGUMENT mangled according to the rules specified in Section 9.2 (Mapping YANG Groupings and Typedefs).

Whenever a derived type is used with additional restrictions, the ancestor built-in type for the derived type is used instead with restrictions (facets) that are a combination of all restrictions specified along the type derivation chain. See Section 11.53.10 (Derived Types) for further details and an example.

An implementation MAY offer the option of recording all 'typedef' statements as named patterns in the output RELAX NG schema even if they are not referenced. This is useful for mapping YANG "library" modules containing only 'typedef' and/or 'grouping' statements.

11.55.  The unique Statement

This statement is mapped to @nma:unique attribute. ARGUMENT is translated so that every node identifier in each of its components is prefixed with the namespace prefix of the local module, unless the prefix is already present. The result of this translation then becomes the value of the @nma:unique attribute.

For example, assuming that the local module prefix is "ex",

unique "foo ex:bar/baz"

is mapped to the following attribute/value pair:

nma:unique="ex:foo ex:bar/ex:baz"

11.56.  The units Statement

This statement is mapped to @nma:units attribute and ARGUMENT becomes its value. Implementations MAY ignore this statement.

11.57.  The uses Statement

If this statement has neither 'refine' nor 'augment' substatements, it is mapped to <rng:ref> element, i.e., a reference to a named pattern, and the value of its @name attribute is set to ARGUMENT mangled according to Section 9.2 (Mapping YANG Groupings and Typedefs). If the RELAX NG definition of the referenced named pattern has not been added to the output schema yet, the corresponding grouping MUST be found and its mapping installed as a subelement of <rng:grammar>, see Section 11.20 (The grouping Statement).

Otherwise, if the 'uses' statement has any 'refine' or 'augment' substatements, the corresponding grouping must be looked up and its contents is inserted under PARENT. See Section 9.2.1 (YANG Refinements and Augments) for further details and an example.

11.58.  The value Statement

This statement is ignored.

11.59.  The when Statement

This statement is mapped to @nma:when attribute and ARGUMENT, translated according to Section 9.3 (Translation of XPath Expressions), becomes it value.

11.60.  The yang-version Statement

This statement is not mapped to the output schema. However, an implementation SHOULD check that it is compatible with the YANG version declared by the statement (currently version 1).

11.61.  The yin-element Statement

This statement is not mapped to the output schema, but see the rules for extension handling in Section 9.4 (YANG Language Extensions).

12.  Mapping NETMOD-specific annotations to DSDL Schema Languages

This section contains mapping specification for individual NETMOD-specific annotations. In each case, the result of the mapping must be inserted into an appropriate context of the target DSDL schema as described in Section 10 (Mapping Conceptual Tree Schema to DSDL). The context is determined by the element definition in the conceptual tree schema to which the annotation is attached. In the rest of this section, we will denote CONTELEM the name of this context element properly qualified with its namespace prefix. Unless otherwise stated, Schematron asserts are descendants of the "standard" pattern and therefore active in both validation phases.

12.1.  The @nma:config Annotation

If this annotation is present with the value "false", the following rules MUST be observed for DSDL schemas of <nc:get-config> reply:

• When generating RELAX NG, the contents of the CONTELEM definition MUST be changed to <rng:notAllowed>.
• When generating Schematron or DSRL, the CONTELEM definition and all its descendants in the conceptual tree schema MUST be ignored.

12.2.  The @nma:default Annotation

This annotation is used for generating the DSRL schema as described in Section 10.4 (Mapping Default Values to DSRL).

12.3.  The @nma:implicit Annotation

This annotation is used for generating the DSRL schema as described in Section 10.4 (Mapping Default Values to DSRL).

12.4.  The <nma:error-app-tag> Annotation

This annotation currently has no mapping defined.

12.5.  The <nma:error-message> Annotation

This annotation is handled within <nma:must>, see Section 12.11 (The <nma:must> Annotation).

12.6.  The <nma:instance-identifier> Annotation

If this annotation element has the @require-instance attribute with the value "false", it is ignored. Otherwise it is mapped to the following Schematron assert:

<sch:assert test="nmf:evaluate(.)">
  The element pointed to by "CONTELEM" must exist.
</sch:assert>

The nmf:evaluate() function is an XSLT extension function (see Extension Functions in [XSLT] (Clark, J., “XSL Transformations (XSLT) Version 1.0,” November 1999.)) that evaluates an XPath expression at runtime. Such an extension function is provided by some XSLT processors, for example Saxon.

12.7.  The @nma:key Annotation

Assume this annotation attribute has the value "k_1 k_2 ... k_n", i.e., specifies n children of CONTELEM as list keys. The annotation is then mapped to the following Schematron report:

<sch:report test="CONDITION">
  Duplicate key of list "CONTELEM"
</sch:report>

where CONDITION has this form:

preceding-sibling::CONTELEM[C_1 and C_2 and ... and C_n]

Each sub-expression C_i, for i=1,2,...,n, specifies the condition for violation of uniqueness of key k_i, namely

k_i=current()/k_i

12.8.  The @nma:leafref Annotation

This annotation is mapped to the following assert:

<sch:assert test="PATH=.">
  Leafref "CONTELEM" must have the same value as "PATH"
</sch:assert>

where PATH is the value of @nma:leafref. The assert is a descendant of the "ref-integrity" pattern, which means that it will be used only for the "full" validation phase.

12.9.  The @nma:min-elements Annotation

This annotation is mapped to the following Schematron assert:

<sch:assert test="count(../CONTELEM)&gt;=MIN">
  List "CONTELEM" - item count must be at least MIN
</sch:assert>

where MIN is the value of @nma:min-elements.

12.10.  The @nma:max-elements Annotation

This annotation is mapped to the following Schematron assert:

<sch:assert test="count(../CONTELEM)&lt;=MAX">
  List "CONTELEM" - item count must be at most MAX
</sch:assert>

where MAX is the value of @nma:min-elements.

12.11.  The <nma:must> Annotation

This annotation is mapped to the following Schematron assert:

<sch:assert test="EXPRESSION">
  MESSAGE
</sch:assert>

where EXPRESSION is the value of the mandatory @assert attribute of <nma:must>. If the <nma:error-message> subelement exists, MESSAGE is set to its content, otherwise it is set to the default message "Condition EXPRESSION must be true".

12.12.  The <nma:ordered-by> Annotation

This annotation currently has no mapping defined.

12.13.  The <nma:status> Annotation

This annotation currently has no mapping defined.

12.14.  The @nma:unique Annotation

The mapping of this annotation is almost identical as for @nma:key, see Section 12.7 (The @nma:key Annotation), with two small differences:

• The value of @nma:unique is a list of descendant schema node identifiers rather than simple leaf names. However, the XPath expressions specified in Section 12.7 (The @nma:key Annotation) work without any modifications if the descendant schema node identifiers are substituted for k_1, k_2, ..., k_n.
• The message appearing as the text of <sch:report> is different: "Violated uniqueness for list CONTELEM".

12.15.  The @nma:when Annotation

This annotation is mapped to the following Schematron assert:

<sch:assert test="EXPRESSION">
  Node "CONTELEM" is only valid when "EXPRESSION" is true.
</sch:assert>

where EXPRESSION is the value of @nma:when.

13.  IANA Considerations

This document registers three namespace URIs in the IETF XML registry [RFC3688] (Mealling, M., “The IETF XML Registry,” January 2004.):

URI: urn:ietf:params:xml:ns:netmod:dsdl-annotations:1

URI: urn:ietf:params:xml:ns:netmod:conceptual-tree:1

URI: urn:ietf:params:xml:ns:netmod:xpath-extensions:1

14.  Security Considerations

This document defines a procedure that maps data models expressed in the YANG language to a coordinated set of DSDL schemas. The procedure itself has no security impact on the Internet.

DSDL schemas obtained by the mapping procedure may be used for validating the content of NETCONF protocol data units or entire datastores and thus provide additional validity checks above those performed by NETCONF server and client implementations supporting YANG data models. The strictness of this validation is directly derived from the source YANG modules that the validated XML data adhere to.

15.  Acknowledgments

The authors wish to thank the following individuals who provided helpful suggestions and/or comments on various versions of this document: Andy Bierman, Martin Bjorklund, Jirka Kosek and Phil Shafer.

16.  References

16.1. Normative References

16.2. Informative References

Appendix A.  Schemas for NETMOD-Specific Annotations

This appendix utilizes Namespace-Based Validation Dispatching Language (NVDL, Part 4 of DSDL) [NVDL] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 4: Namespace-Based Validation Dispatching Language (NVDL),” 6 2006.) to define NETMOD-specific annotations as extensions of the RELAX NG language. The NVDL schema may be used for validating conceptual tree schemas as compound XML documents consisting of RELAX NG sections and embedded sections with NETMOD-specific annotations. The NVDL schema dispatches the validation as follows:

1. RELAX NG sections identified by the namespace URI "http://relaxng.org/ns/structure/1.0" is validated with the standard RELAX NG schema for RELAX NG, see [RNG] (ISO/IEC, “Information Technology - Document Schema Definition Languages (DSDL) - Part 2: Regular-Grammar-Based Validation - RELAX NG. Second Edition.,” 12 2008.), Annex A.
2. NETMOD-specific annotation sections identified by the namespace URI "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1" are validated by several context-dependent RELAX NG schemas given below.
3. Other sections such as Dublin Core metadata or <a:documentation> annotation are attached to and validated together with the parent RELAX NG section.

A.1.  NVDL Schema

<CODE BEGINS> file "nmannot.nvdl"

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE nvdl:rules [<!ENTITY nmannot-uri
  "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1">
]>
<rules xmlns="http://purl.oclc.org/dsdl/nvdl/ns/structure/1.0"
       startMode="rng">
  <mode name="rng">
    <namespace ns="http://relaxng.org/ns/structure/1.0">
      <validate schema="http://relaxng.org/relaxng.rng"
                useMode="other">
        <context path="define" useMode="define"/>
        <context path="oneOrMore/element" useMode="list"/>
        <context path="element" useMode="element"/>
        <context path="choice/group|choice/interleave"
                 useMode="group-interleave"/>
        <context path="choice|ref" useMode="choice-ref"/>
        <context path="value" useMode="value"/>
      </validate>
    </namespace>
  </mode>
  <mode name="define">
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="define.rng"/>
    </namespace>
  </mode>
  <mode name="element">
    <namespace ns="&nmannot-uri;" match="elements">
      <validate schema="element-el.rng"/>
    </namespace>
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="element-att.rng"/>
    </namespace>
  </mode>
  <mode name="group-interleave">
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="group-interleave.rng"/>
    </namespace>
  </mode>
  <mode name="choice-ref">
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="choice-ref.rng"/>
    </namespace>
  </mode>
  <mode name="list">
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="list.rng"/>
    </namespace>
  </mode>
  <mode name="value">
    <namespace ns="&nmannot-uri;" match="attributes">
      <validate schema="value.rng"/>
    </namespace>
  </mode>
  <mode name="other">
    <namespace ns="&nmannot-uri;" match="elements attributes">
      <reject/>
    </namespace>
    <anyNamespace match="elements attributes">
      <attach/>
    </anyNamespace>
  </mode>
</rules>

<CODE ENDS>

A.2.  Annotation Attributes for define Pattern

<CODE BEGINS> file "define.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <group>
      <ref name="default-attribute"/>
      <ref name="status-attribute"/>
      <ref name="units-attribute"/>
    </group>
  </start>
</grammar>

<CODE ENDS>

A.3.  Annotation Elements for element Pattern

<CODE BEGINS> file "element-el.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <choice>
      <ref name="must-element"/>
      <ref name="instance-identifier-element"/>
    </choice>
  </start>
</grammar>

<CODE ENDS>

A.4.  Annotation Attributes for element Pattern

<CODE BEGINS> file "element-att.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <group>
      <ref name="config-attribute"/>
      <ref name="default-attribute"/>
      <ref name="implicit-attribute"/>
      <ref name="leafref-attribute"/>
      <ref name="presence-attribute"/>
      <ref name="status-attribute"/>
      <ref name="units-attribute"/>
      <ref name="when-attribute"/>
    </group>
  </start>
</grammar>

<CODE ENDS>

A.5.  Annotation attributes for group Pattern

<CODE BEGINS> file "group-interleave.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <group>
      <ref name="implicit-attribute"/>
      <ref name="status-attribute"/>
      <ref name="when-attribute"/>
    </group>
  </start>
</grammar>

<CODE ENDS>

A.6.  Annotation attributes for choice and ref Patterns

<CODE BEGINS> file "choice-ref.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <group>
      <ref name="status-attribute"/>
      <ref name="when-attribute"/>
    </group>
  </start>
</grammar>

<CODE ENDS>

A.7.  Annotation attributes for element Pattern in the List Context

<CODE BEGINS> file "list.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <group>
      <ref name="key-attribute"/>
      <ref name="min-elements-attribute"/>
      <ref name="max-elements-attribute"/>
      <ref name="ordered-by-attribute"/>
      <ref name="unique-attribute"/>
    </group>
  </start>
</grammar>

<CODE ENDS>

A.8.  Annotation attributes for value Pattern

<CODE BEGINS> file "value.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <include href="nmannotdefs.rng"/>
  <start>
    <ref name="status-attribute"/>
  </start>
</grammar>

<CODE ENDS>

A.9.  Named Patterns for All NETMOD-Specific Annotations

<CODE BEGINS> file "nmannotdefs.rng"

<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
         xmlns:nma="urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"
         datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">

<define name="config-attribute">
  <optional>
    <attribute name="nma:config">
      <data type="boolean"/>
    </attribute>
  </optional>
</define>

<define name="default-attribute">
  <optional>
    <attribute name="nma:default"/>
  </optional>
</define>

<define name="implicit-attribute">
  <optional>
    <attribute name="nma:implicit">
      <data type="boolean"/>
    </attribute>
  </optional>
</define>

<define name="error-app-tag-element">
  <optional>
    <element name="nma:error-app-tag">
      <text/>
    </element>
  </optional>
</define>

<define name="error-message-element">
  <optional>
    <element name="nma:error-message">
      <text/>
    </element>
  </optional>
</define>

<define name="instance-identifier-element">
  <element name="nma:instance-identifier">
    <optional>
      <attribute name="nma:require-instance">
        <data type="boolean"/>
      </attribute>
    </optional>
  </element>
</define>

<define name="key-attribute">
  <optional>
    <attribute name="nma:key">
      <list>
        <data type="QName"/>
      </list>
    </attribute>
  </optional>
</define>

<define name="leafref-attribute">
  <optional>
    <attribute name="nma:leafref">
      <data type="string"/>
    </attribute>
  </optional>
</define>

<define name="min-elements-attribute">
  <optional>
    <attribute name="nma:min-elements">
      <data type="nonNegativeInteger"/>
    </attribute>
  </optional>
</define>

<define name="max-elements-attribute">
  <optional>
    <attribute name="nma:max-elements">
      <data type="nonNegativeInteger"/>
    </attribute>
  </optional>
</define>

<define name="must-element">
    <element name="nma:must">
      <attribute name="assert">
        <data type="string"/>
      </attribute>
      <interleave>
        <ref name="error-app-tag-element"/>
        <ref name="error-message-element"/>
      </interleave>
    </element>
</define>

<define name="ordered-by-attribute">
  <optional>
    <attribute name="nma:ordered-by">
      <choice>
        <value>user</value>
        <value>system</value>
      </choice>
    </attribute>
  </optional>
</define>

<define name="presence-attribute">
  <optional>
    <attribute name="nma:presence">
      <value>true</value>
    </attribute>
  </optional>
</define>

<define name="status-attribute">
  <optional>
    <attribute name="nma:status">
      <choice>
        <value>current</value>
        <value>deprecated</value>
        <value>obsolete</value>
      </choice>
    </attribute>
  </optional>
</define>

<define name="unique-attribute">
  <optional>
    <attribute name="nma:unique">
      <list>
        <data type="token"/>
      </list>
    </attribute>
  </optional>
</define>

<define name="units-attribute">
  <optional>
    <attribute name="nma:units">
      <data type="string"/>
    </attribute>
  </optional>
</define>

<define name="when-attribute">
  <optional>
    <attribute name="nma:when">
      <data type="string"/>
    </attribute>
  </optional>
</define>

</grammar>

<CODE ENDS>

Appendix B.  Schema-Independent Library

In order to avoid copying the common named pattern definitions to every RELAX NG schema generated in the second mapping step, the definitions are collected in a library file - schema-independent library - which is included by the validating schemas under the file name "relaxng-lib.rng" (XML syntax) and "relaxng-lib.rnc" (compact syntax). The included definitions cover patterns for common elements from base NETCONF [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.) and event notifications [RFC5277] (Chisholm, S. and H. Trevino, “NETCONF Event Notifications,” July 2008.).

<CODE BEGINS> file "relaxng-lib.rng"

<?xml version="1.0" encoding="UTF-8"?>

<grammar xmlns="http://relaxng.org/ns/structure/1.0"
         xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
         xmlns:en="urn:ietf:params:xml:ns:netconf:notification:1.0"
         datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">

  <define name="message-id-attribute">
    <attribute name="message-id">
      <data type="string">
        <param name="maxLength">4095</param>
      </data>
    </attribute>
  </define>

  <define name="ok-element">
    <element name="nc:ok">
      <empty/>
    </element>
  </define>

  <define name="eventTime-element">
    <element name="en:eventTime">
      <data type="dateTime"/>
    </element>
  </define>
</grammar>

<CODE ENDS>

Appendix C.  Mapping DHCP Data Model - A Complete Example

This appendix demonstrates both steps of the YANG-to-DSDL mapping applied to the "canonical" DHCP tutorial data model. The single input YANG module is shown in Appendix C.1 (Input YANG Module) and the output schemas in the following two subsections.

The conceptual tree schema was obtained by the "rng" plugin of the pyang tool and the validating DSDL schemas by XSLT stylesheets that are also part of pyang distribution. RELAX NG schemas are shown in both XML and compact syntax. The latter was obtained from the former by using the Trang tool

Due to the limit of 72 characters per line, few long strings required manual editing, in particular the regular expression patterns for IP addresses etc. in the RELAX NG schemas. In the compact syntax we broke the patterns to appropriate segments and joined them with the concatenation operator "~". In the XML syntax, though, the long patterns had to be replaced by the placeholder string "... regex pattern ...". Also, line breaks were added to several documentation strings and Schematron messages. Other than that, the results of the automatic translations were not changed.

C.1.  Input YANG Module

module dhcp {
  namespace "http://example.com/ns/dhcp";
  prefix dhcp;

  import ietf-yang-types { prefix yang; }
  import ietf-inet-types { prefix inet; }

  organization
    "yang-central.org";
  description
    "Partial data model for DHCP, based on the config of
     the ISC DHCP reference implementation.";

  container dhcp {
    description
      "configuration and operational parameters for a DHCP server.";

    leaf max-lease-time {
      type uint32;
      units seconds;
      default 7200;
    }

    leaf default-lease-time {
      type uint32;
      units seconds;
      must '. <= ../max-lease-time' {
        error-message
          "The default-lease-time must be less than max-lease-time";
      }
      default 600;
    }

    uses subnet-list;

    container shared-networks {
      list shared-network {
        key name;

        leaf name {
          type string;
        }
        uses subnet-list;
      }
    }

    container status {
      config false;
      list leases {
        key address;

        leaf address {
          type inet:ip-address;
        }
        leaf starts {
          type yang:date-and-time;
        }
        leaf ends {
          type yang:date-and-time;
        }
        container hardware {
          leaf type {
            type enumeration {
              enum "ethernet";
              enum "token-ring";
              enum "fddi";
            }
          }
          leaf address {
            type yang:phys-address;
          }
        }
      }
    }
  }

  grouping subnet-list {
    description "A reusable list of subnets";
    list subnet {
      key net;
      leaf net {
        type inet:ip-prefix;
      }
      container range {
        presence "enables dynamic address assignment";
        leaf dynamic-bootp {
          type empty;
          description
            "Allows BOOTP clients to get addresses in this range";
        }
        leaf low {
          type inet:ip-address;
          mandatory true;
        }
        leaf high {
          type inet:ip-address;
          mandatory true;
        }
      }

      container dhcp-options {
        description "Options in the DHCP protocol";
        leaf-list router {
          type inet:host;
          ordered-by user;
          reference "RFC 2132, sec. 3.8";
        }
        leaf domain-name {
          type inet:domain-name;
          reference "RFC 2132, sec. 3.17";
        }
      }

      leaf max-lease-time {
        type uint32;
        units seconds;
        default 7200;
      }
    }
  }
}

C.2.  Conceptual Tree Schema

C.2.1.  XML Syntax

<?xml version="1.0" encoding="UTF-8"?>
<grammar
    xmlns="http://relaxng.org/ns/structure/1.0"
    xmlns:a="http://relaxng.org/ns/compatibility/annotations/1.0"
    xmlns:dc="http://purl.org/dc/terms"
    xmlns:dhcp="http://example.com/ns/dhcp"
    xmlns:nma="urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"
    xmlns:nmt="urn:ietf:params:xml:ns:netmod:conceptual-tree:1"
    datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
 <dc:creator>Pyang 0.9.3, CTS plugin</dc:creator>
 <dc:source>YANG module 'dhcp'</dc:source>
 <start>
  <element name="nmt:netmod-tree">
   <element name="nmt:top">
    <interleave>
     <optional>
      <element name="dhcp:dhcp" nma:implicit="true">
       <interleave>
        <a:documentation>
         configuration and operational parameters for a DHCP server
        </a:documentation>
        <optional>
         <element name="dhcp:max-lease-time"
                  nma:default="7200" nma:units="seconds">
          <data type="unsignedInt"/>
         </element>
        </optional>
        <optional>
         <element name="dhcp:default-lease-time"
                  nma:default="600" nma:units="seconds">
          <data type="unsignedInt"/>
          <nma:must assert=". &lt;= ../dhcp:max-lease-time">
           <nma:error-message>
            The default-lease-time must be less than max-lease-time.
           </nma:error-message>
          </nma:must>
         </element>
        </optional>
        <ref name="_dhcp__subnet-list"/>
        <optional>
         <element name="dhcp:shared-networks">
          <interleave>
           <zeroOrMore>
            <element name="dhcp:shared-network"
                     nma:key="dhcp:name">
             <element name="dhcp:name">
              <data type="string"/>
             </element>
             <interleave>
              <ref name="_dhcp__subnet-list"/>
             </interleave>
            </element>
           </zeroOrMore>
          </interleave>
         </element>
        </optional>
        <optional>
         <element name="dhcp:status" nma:config="false">
          <interleave>
           <zeroOrMore>
            <element name="dhcp:leases"
                     nma:key="dhcp:address">
             <element name="dhcp:address">
              <ref name="ietf-inet-types__ip-address"/>
             </element>
             <interleave>
              <optional>
               <element name="dhcp:starts">
                <ref name="ietf-yang-types__date-and-time"/>
               </element>
              </optional>
              <optional>
               <element name="dhcp:ends">
                <ref name="ietf-yang-types__date-and-time"/>
               </element>
              </optional>
              <optional>
               <element name="dhcp:hardware">
                <interleave>
                 <optional>
                  <element name="dhcp:type">
                   <choice>
                    <value>ethernet</value>
                    <value>token-ring</value>
                    <value>fddi</value>
                   </choice>
                  </element>
                 </optional>
                 <optional>
                  <element name="dhcp:address">
                   <ref name="ietf-yang-types__phys-address"/>
                  </element>
                 </optional>
                </interleave>
               </element>
              </optional>
             </interleave>
            </element>
           </zeroOrMore>
          </interleave>
         </element>
        </optional>
       </interleave>
      </element>
     </optional>
    </interleave>
   </element>
   <element name="nmt:rpc-methods">
    <empty/>
   </element>
   <element name="nmt:notifications">
    <empty/>
   </element>
  </element>
 </start>
 <define name="_dhcp__subnet-list">
  <interleave>
   <a:documentation>A reusable list of subnets</a:documentation>
   <zeroOrMore>
    <element name="dhcp:subnet" nma:key="dhcp:net">
     <element name="dhcp:net">
      <ref name="ietf-inet-types__ip-prefix"/>
     </element>
     <interleave>
      <optional>
       <element name="dhcp:range">
        <interleave>
         <optional>
          <element name="dhcp:dynamic-bootp">
           <a:documentation>
            Allows BOOTP clients to get addresses in this range
           </a:documentation>
           <empty/>
          </element>
         </optional>
         <element name="dhcp:low">
          <ref name="ietf-inet-types__ip-address"/>
         </element>
         <element name="dhcp:high">
          <ref name="ietf-inet-types__ip-address"/>
         </element>
        </interleave>
       </element>
      </optional>
      <optional>
       <element name="dhcp:dhcp-options">
        <interleave>
         <a:documentation>
          Options in the DHCP protocol
         </a:documentation>
         <zeroOrMore>
          <element name="dhcp:router" nma:ordered-by="user">
           <a:documentation>
            See: RFC 2132, sec. 3.8
           </a:documentation>
           <ref name="ietf-inet-types__host"/>
          </element>
         </zeroOrMore>
         <optional>
          <element name="dhcp:domain-name">
           <a:documentation>
            See: RFC 2132, sec. 3.17
           </a:documentation>
           <ref name="ietf-inet-types__domain-name"/>
          </element>
         </optional>
        </interleave>
       </element>
      </optional>
      <optional>
       <element name="dhcp:max-lease-time"
                nma:default="7200" nma:units="seconds">
        <data type="unsignedInt"/>
       </element>
      </optional>
     </interleave>
    </element>
   </zeroOrMore>
  </interleave>
 </define>
 <define name="ietf-inet-types__ip-prefix">
  <choice>
   <ref name="ietf-inet-types__ipv4-prefix"/>
   <ref name="ietf-inet-types__ipv6-prefix"/>
  </choice>
 </define>
 <define name="ietf-inet-types__ipv4-prefix">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ipv6-prefix">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ip-address">
  <choice>
   <ref name="ietf-inet-types__ipv4-address"/>
   <ref name="ietf-inet-types__ipv6-address"/>
  </choice>
 </define>
 <define name="ietf-inet-types__ipv4-address">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ipv6-address">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__host">
  <choice>
   <ref name="ietf-inet-types__ip-address"/>
   <ref name="ietf-inet-types__domain-name"/>
  </choice>
 </define>
 <define name="ietf-inet-types__domain-name">
  <data type="string">
   <param name="minLength">1</param>
   <param name="maxLength">253</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-yang-types__date-and-time">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-yang-types__phys-address">
  <data type="string">
   <param name="pattern">
    ([0-9a0-fA-F]{2}(:[0-9a0-fA-F]{2})*)?
   </param>
  </data>
 </define>
</grammar>

C.2.2.  Compact Syntax

namespace a = "http://relaxng.org/ns/compatibility/annotations/1.0"
namespace dc = "http://purl.org/dc/terms"
namespace dhcp = "http://example.com/ns/dhcp"
namespace nma = "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"
namespace nmt = "urn:ietf:params:xml:ns:netmod:conceptual-tree:1"

dc:creator [ "Pyang 0.9.3, CTS plugin" ]
dc:source [ "YANG module 'dhcp'" ]
start =
 element nmt:netmod-tree {
  element nmt:top {
   [ nma:implicit = "true" ]
   element dhcp:dhcp {

    ##
    ##   configuration and operational parameters for a DHCP server
    ##
    ([ nma:default = "7200" nma:units = "seconds" ]
     element dhcp:max-lease-time { xsd:unsignedInt }?
     & [ nma:default = "600" nma:units = "seconds" ]
      element dhcp:default-lease-time {
       xsd:unsignedInt
       >> nma:must [
         assert = ". <= ../dhcp:max-lease-time"
         nma:error-message [
          "\x{a}" ~
          "The default-lease-time must be less than max-lease-time." ~
          "\x{a}"
         ]
        ]
      }?
     & _dhcp__subnet-list
     & element dhcp:shared-networks {
       [ nma:key = "dhcp:name" ]
       element dhcp:shared-network {
        element dhcp:name { xsd:string },
        (_dhcp__subnet-list)
       }*
      }?
     & [ nma:config = "false" ]
      element dhcp:status {
       [ nma:key = "dhcp:address" ]
       element dhcp:leases {
        element dhcp:address { ietf-inet-types__ip-address },
        (element dhcp:starts { ietf-yang-types__date-and-time }?
        & element dhcp:ends { ietf-yang-types__date-and-time }?
        & element dhcp:hardware {
          element dhcp:type {
           "ethernet" | "token-ring" | "fddi"
          }?
          & element dhcp:address {
            ietf-yang-types__phys-address
           }?
         }?)
       }*
      }?)
   }?
  },
  element nmt:rpc-methods { empty },
  element nmt:notifications { empty }
 }
_dhcp__subnet-list =

 ## A reusable list of subnets
 ([ nma:key = "dhcp:net" ]
  element dhcp:subnet {
   element dhcp:net { ietf-inet-types__ip-prefix },
   (element dhcp:range {

    ##
    ##      Allows BOOTP clients to get addresses in this range
    ##
    element dhcp:dynamic-bootp { empty }?
    & element dhcp:low { ietf-inet-types__ip-address }
    & element dhcp:high { ietf-inet-types__ip-address }
   }?
   & element dhcp:dhcp-options {

     ##
     ##   Options in the DHCP protocol
     ##
     (
      ##
      ##            See: RFC 2132, sec. 3.8
      ##
      [ nma:ordered-by = "user" ]
      element dhcp:router { ietf-inet-types__host }*
      &
       ##
       ##           See: RFC 2132, sec. 3.17
       ##
       element dhcp:domain-name { ietf-inet-types__domain-name }?)
    }?
   & [ nma:default = "7200" nma:units = "seconds" ]
    element dhcp:max-lease-time { xsd:unsignedInt }?)
  }*)
ietf-inet-types__ip-prefix =
 ietf-inet-types__ipv4-prefix | ietf-inet-types__ipv6-prefix
ietf-inet-types__ipv4-prefix =
 xsd:string { pattern = "... regex pattern ..." }
ietf-inet-types__ipv6-prefix =
 xsd:string {
  pattern = "... regex pattern ..."
  pattern = "... regex pattern ..."
 }
ietf-inet-types__ip-address =
 ietf-inet-types__ipv4-address | ietf-inet-types__ipv6-address
ietf-inet-types__ipv4-address =
 xsd:string { pattern = "... regex pattern ..." }
ietf-inet-types__ipv6-address =
 xsd:string {
  pattern = "... regex pattern ..."
  pattern = "... regex pattern ..."
 }
ietf-inet-types__host =
 ietf-inet-types__ip-address | ietf-inet-types__domain-name
ietf-inet-types__domain-name =
 xsd:string {
  minLength = "1"
  maxLength = "253"
  pattern = "... regex pattern ..."
 }
ietf-yang-types__date-and-time =
 xsd:string { pattern = "... regex pattern ..." }
ietf-yang-types__phys-address =
 xsd:string {
  pattern = "([0-9a0-fA-F]{2}(:[0-9a0-fA-F]{2})*)?"
 }

C.3.  Final DSDL Schemas

This appendix contains DSDL schemas that were obtained from the conceptual tree schema in Appendix C.2 (Conceptual Tree Schema) by XSL transformations. These schemas can be directly used for validating a reply to unfiltered <get> with the contents corresponding to the DHCP data model.

The RELAX NG schema (again shown in both XML and compact syntax) includes the schema independent library from Appendix B (Schema-Independent Library).

C.3.1.  RELAX NG Schema for <get> Reply - XML Syntax

<?xml version="1.0" encoding="utf-8"?>
<grammar
    xmlns="http://relaxng.org/ns/structure/1.0"
    xmlns:a="http://relaxng.org/ns/compatibility/annotations/1.0"
    xmlns:dc="http://purl.org/dc/terms"
    xmlns:dhcp="http://example.com/ns/dhcp"
    xmlns:nma="urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"
    xmlns:nmt="urn:ietf:params:xml:ns:netmod:conceptual-tree:1"
    datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes"
    ns="urn:ietf:params:xml:ns:netconf:base:1.0">
 <include href="relaxng-lib.rng"/>
 <start>
  <element name="rpc-reply">
   <ref name="message-id-attribute"/>
   <element name="data">
    <interleave>
     <optional>
      <element name="dhcp:dhcp">
       <interleave>
        <optional>
         <element name="dhcp:max-lease-time">
          <data type="unsignedInt"/>
         </element>
        </optional>
        <optional>
         <element name="dhcp:default-lease-time">
          <data type="unsignedInt"/>
         </element>
        </optional>
        <ref name="_dhcp__subnet-list"/>
        <optional>
         <element name="dhcp:shared-networks">
          <interleave>
           <zeroOrMore>
            <element name="dhcp:shared-network">
             <element name="dhcp:name">
              <data type="string"/>
             </element>
             <interleave>
              <ref name="_dhcp__subnet-list"/>
             </interleave>
            </element>
           </zeroOrMore>
          </interleave>
         </element>
        </optional>
        <optional>
         <element name="dhcp:status">
          <interleave>
           <zeroOrMore>
            <element name="dhcp:leases">
             <element name="dhcp:address">
              <ref name="ietf-inet-types__ip-address"/>
             </element>
             <interleave>
              <optional>
               <element name="dhcp:starts">
                <ref name="ietf-yang-types__date-and-time"/>
               </element>
              </optional>
              <optional>
               <element name="dhcp:ends">
                <ref name="ietf-yang-types__date-and-time"/>
               </element>
              </optional>
              <optional>
               <element name="dhcp:hardware">
                <interleave>
                 <optional>
                  <element name="dhcp:type">
                   <choice>
                    <value>ethernet</value>
                    <value>token-ring</value>
                    <value>fddi</value>
                   </choice>
                  </element>
                 </optional>
                 <optional>
                  <element name="dhcp:address">
                   <ref name="ietf-yang-types__phys-address"/>
                  </element>
                 </optional>
                </interleave>
               </element>
              </optional>
             </interleave>
            </element>
           </zeroOrMore>
          </interleave>
         </element>
        </optional>
       </interleave>
      </element>
     </optional>
    </interleave>
   </element>
  </element>
 </start>
 <define name="_dhcp__subnet-list">
  <interleave>
   <zeroOrMore>
    <element name="dhcp:subnet">
     <element name="dhcp:net">
      <ref name="ietf-inet-types__ip-prefix"/>
     </element>
     <interleave>
      <optional>
       <element name="dhcp:range">
        <interleave>
         <optional>
          <element name="dhcp:dynamic-bootp">
           <empty/>
          </element>
         </optional>
         <element name="dhcp:low">
          <ref name="ietf-inet-types__ip-address"/>
         </element>
         <element name="dhcp:high">
          <ref name="ietf-inet-types__ip-address"/>
         </element>
        </interleave>
       </element>
      </optional>
      <optional>
       <element name="dhcp:dhcp-options">
        <interleave>
         <zeroOrMore>
          <element name="dhcp:router">
           <ref name="ietf-inet-types__host"/>
          </element>
         </zeroOrMore>
         <optional>
          <element name="dhcp:domain-name">
           <ref name="ietf-inet-types__domain-name"/>
          </element>
         </optional>
        </interleave>
       </element>
      </optional>
      <optional>
       <element name="dhcp:max-lease-time">
        <data type="unsignedInt"/>
       </element>
      </optional>
     </interleave>
    </element>
   </zeroOrMore>
  </interleave>
 </define>
 <define name="ietf-inet-types__ip-prefix">
  <choice>
   <ref name="ietf-inet-types__ipv4-prefix"/>
   <ref name="ietf-inet-types__ipv6-prefix"/>
  </choice>
 </define>
 <define name="ietf-inet-types__ipv4-prefix">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ipv6-prefix">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ip-address">
  <choice>
   <ref name="ietf-inet-types__ipv4-address"/>
   <ref name="ietf-inet-types__ipv6-address"/>
  </choice>
 </define>
 <define name="ietf-inet-types__ipv4-address">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__ipv6-address">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-inet-types__host">
  <choice>
   <ref name="ietf-inet-types__ip-address"/>
   <ref name="ietf-inet-types__domain-name"/>
  </choice>
 </define>
 <define name="ietf-inet-types__domain-name">
  <data type="string">
   <param name="minLength">1</param>
   <param name="maxLength">253</param>
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-yang-types__date-and-time">
  <data type="string">
   <param name="pattern">... regex pattern ...</param>
  </data>
 </define>
 <define name="ietf-yang-types__phys-address">
  <data type="string">
   <param name="pattern">
    ([0-9a0-fA-F]{2}(:[0-9a0-fA-F]{2})*)?
   </param>
  </data>
 </define>
</grammar>

C.3.2.  RELAX NG Schema for <get> Reply - Compact Syntax

default namespace = "urn:ietf:params:xml:ns:netconf:base:1.0"
namespace a = "http://relaxng.org/ns/compatibility/annotations/1.0"
namespace dc = "http://purl.org/dc/terms"
namespace dhcp = "http://example.com/ns/dhcp"
namespace nma = "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"
namespace nmt = "urn:ietf:params:xml:ns:netmod:conceptual-tree:1"

include "relaxng-lib.rnc"
start =
  element rpc-reply {
    message-id-attribute,
    element data {
      element dhcp:dhcp {
        element dhcp:max-lease-time { xsd:unsignedInt }?
        & element dhcp:default-lease-time { xsd:unsignedInt }?
        & _dhcp__subnet-list
        & element dhcp:shared-networks {
            element dhcp:shared-network {
              element dhcp:name { xsd:string },
              (_dhcp__subnet-list)
            }*
          }?
        & element dhcp:status {
            element dhcp:leases {
              element dhcp:address { ietf-inet-types__ip-address },
              (element dhcp:starts { ietf-yang-types__date-and-time }?
               & element dhcp:ends { ietf-yang-types__date-and-time }?
               & element dhcp:hardware {
                   element dhcp:type {
                     "ethernet" | "token-ring" | "fddi"
                   }?
                   & element dhcp:address {
                       ietf-yang-types__phys-address
                     }?
                 }?)
            }*
          }?
      }?
    }
  }
_dhcp__subnet-list =
  element dhcp:subnet {
    element dhcp:net { ietf-inet-types__ip-prefix },
    (element dhcp:range {
       element dhcp:dynamic-bootp { empty }?
       & element dhcp:low { ietf-inet-types__ip-address }
       & element dhcp:high { ietf-inet-types__ip-address }
     }?
     & element dhcp:dhcp-options {
         element dhcp:router { ietf-inet-types__host }*
         & element dhcp:domain-name { ietf-inet-types__domain-name }?
       }?
     & element dhcp:max-lease-time { xsd:unsignedInt }?)
  }*
ietf-inet-types__ip-prefix =
  ietf-inet-types__ipv4-prefix | ietf-inet-types__ipv6-prefix
ietf-inet-types__ipv4-prefix =
  xsd:string { pattern = "... regex pattern ..." }
ietf-inet-types__ipv6-prefix =
  xsd:string {
    pattern = "... regex pattern ..."
    pattern = "... regex pattern ..."
  }
ietf-inet-types__ip-address =
  ietf-inet-types__ipv4-address | ietf-inet-types__ipv6-address
ietf-inet-types__ipv4-address =
  xsd:string { pattern = "... regex pattern ..." }
ietf-inet-types__ipv6-address =
  xsd:string {
    pattern = "... regex pattern ..."
    pattern = "... regex pattern ..."
  }
ietf-inet-types__host =
  ietf-inet-types__ip-address | ietf-inet-types__domain-name
ietf-inet-types__domain-name =
  xsd:string {
    minLength = "1"
    maxLength = "253"
    pattern = "... regex pattern ..."
  }
ietf-yang-types__date-and-time =
  xsd:string { pattern = "... regex pattern ..." }
ietf-yang-types__phys-address =
  xsd:string {
    pattern =
      "\x{a}" ~
      "    ([0-9a0-fA-F]{2}(:[0-9a0-fA-F]{2})*)?\x{a}" ~
      "   "
  }

C.4.  Schematron Schema for <get> Reply

<?xml version="1.0" encoding="utf-8"?>
<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron">
  <sch:ns uri="http://example.com/ns/dhcp" prefix="dhcp"/>
  <sch:ns uri="urn:ietf:params:xml:ns:netconf:base:1.0"
          prefix="nc"/>
  <sch:phase id="full">
    <sch:active pattern="standard"/>
    <sch:active pattern="ref-integrity"/>
  </sch:phase>
  <sch:phase id="noref">
    <sch:active pattern="standard"/>
  </sch:phase>
  <sch:pattern id="standard">
    <sch:rule id="std-id2247270" abstract="true">
      <sch:report test="preceding-sibling::dhcp:subnet
                        [dhcp:net=current()/dhcp:net]">
      Duplicate key of list dhcp:subnet
      </sch:report>
    </sch:rule>
    <sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                       dhcp:default-lease-time">
      <sch:assert test=". &lt;= ../dhcp:max-lease-time">
        The default-lease-time must be less than max-lease-time.
      </sch:assert>
    </sch:rule>
    <sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/dhcp:subnet">
      <sch:extends rule="std-id2247270"/>
    </sch:rule>
    <sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                       dhcp:shared-networks/dhcp:shared-network">
      <sch:report test="preceding-sibling::dhcp:shared-network
                        [dhcp:name=current()/dhcp:name]">
        Duplicate key of list dhcp:shared-network
      </sch:report>
    </sch:rule>
    <sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                       dhcp:shared-networks/dhcp:shared-network/
                       dhcp:subnet">
      <sch:extends rule="std-id2247270"/>
    </sch:rule>
    <sch:rule context="/nc:rpc-reply/nc:data/dhcp:dhcp/
                       dhcp:status/dhcp:leases">
      <sch:report test="preceding-sibling::dhcp:leases
                        [dhcp:address=current()/dhcp:address]">
        Duplicate key of list dhcp:leases
      </sch:report>
    </sch:rule>
  </sch:pattern>
  <sch:pattern id="ref-integrity"/>
</sch:schema>

C.5.  DSRL Schema for <get> Reply

<?xml version="1.0" encoding="utf-8"?>
<dsrl:maps xmlns:dsrl="http://purl.oclc.org/dsdl/dsrl"
           xmlns:dhcp="http://example.com/ns/dhcp"
           xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/</dsrl:parent>
    <dsrl:name>dhcp:dhcp</dsrl:name>
    <dsrl:default-content>
      <dhcp:max-lease-time>7200</dhcp:max-lease-time>
      <dhcp:default-lease-time>600</dhcp:default-lease-time>
    </dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/dhcp:dhcp</dsrl:parent>
    <dsrl:name>dhcp:max-lease-time</dsrl:name>
    <dsrl:default-content>7200</dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>/nc:rpc-reply/nc:data/dhcp:dhcp</dsrl:parent>
    <dsrl:name>dhcp:default-lease-time</dsrl:name>
    <dsrl:default-content>600</dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>
      /nc:rpc-reply/nc:data/dhcp:dhcp/dhcp:subnet
    </dsrl:parent>
    <dsrl:name>dhcp:max-lease-time</dsrl:name>
    <dsrl:default-content>7200</dsrl:default-content>
  </dsrl:element-map>
  <dsrl:element-map>
    <dsrl:parent>
      /nc:rpc-reply/nc:data/dhcp:dhcp/dhcp:shared-networks/
      dhcp:shared-network/dhcp:subnet
    </dsrl:parent>
    <dsrl:name>dhcp:max-lease-time</dsrl:name>
    <dsrl:default-content>7200</dsrl:default-content>
  </dsrl:element-map>
</dsrl:maps>

Appendix D.  Change Log

D.1.  Changes Between Versions -04 and -05

• Leafs that take their default value from a typedef and are not annotated with @nma:default must have @nma:implicit="true".
• Changed code markers CODE BEGINS/ENDS to the form agreed by the WG.
• Derived types "date-and-time" and "uri" SHOULD be mapped to XSD "dateTime" and "anyURI" types, respectively.
• Clarified the notion of implicit nodes under under 'case' in Section 9.1.2 (Implicit Nodes).
• Moved draft-ietf-netmod-yang-types-06 to normative references.
• An extra <rng:group> is no more required for the default case of a choice in the shorthand notation.

D.2.  Changes Between Versions -03 and -04

• Implemented ordering rules for list children - keys must go first and appear in the same order as in the input YANG module.
• The 'case' statement is now mapped to either <rng:group> (inside RPCs) or <rng:interleave> (otherwise).
• A nma:default annotation coming from a datatype which the mapping expands is attached to the <rng:element> pattern where the expansion occurs. Added an example.
• Documentation statements ('description', 'reference', 'status') MAY be ignored.
• Single-valued numeric or length range parts are mapped to <rng:value> pattern or "length" facet.
• Example for "string" datatype was added.
• Appendix A (Schemas for NETMOD-Specific Annotations) now uses NVDL for defining NETMOD-specific annotations.
• Added CODE BEGINS/ENDS markers.
• Separated normative and informative references.
• Added URL for XPath extensions namespace.
• Added Section 2 (Terminology and Notation) (Terminology and Notation).
• Added Section 14 (Security Considerations) (Security Considerations).
• Added Section 15 (Acknowledgments) (Acknowledgments).
• Removed compact syntax schema from Appendix B (Schema-Independent Library).
• Editorial changes: symbolic citation labels.

D.3.  Changes Between Versions -02 and -03

• Changed @nma:default-case to @nma:implicit.
• Changed nma:leafref annotation from element to attribute.
• Added skeleton rule to Section 10.2 (Mapping Semantic Constraints to Schematron).
• Reworked Section 10.4 (Mapping Default Values to DSRL), added skeleton element maps,corrected the example.
• Added Section 8.5 (Features and Deviations) on 'feature' and 'deviation'.
• New Section 9.1 (Occurrence Rules for Data Nodes) integrating discussion of both optional/mandatory (was in -02) and implicit nodes (new).
• Reflected that key argument and schema node identifiers are no more XPath (should be in yang-07).
• Element patterns for implicit containers now must have @nma:implicit attribute.
• Removed "float32" and "float64" types and added mapping of "decimal64" with example.
• Removed mapping of 'require-instance' for "leafref" type.
• Updated RELAX NG schema for NETMOD-specific annotations.
• Updated the DHCP example.

D.4.  Changes Between Versions -01 and -02

• Moved Section 7 (NETCONF Content Validation) "NETCONF Content Validation" after Section 6 (Overview of the Mapping).
• New text about mapping defaults to DSRL, especially in Section 7 (NETCONF Content Validation) and Section 10.4 (Mapping Default Values to DSRL).
• Finished the DHCP example by adding the DSRL schema to Appendix C (Mapping DHCP Data Model - A Complete Example).
• New @nma:presence annotation was added - it is needed for proper handling of default content.
• Section 10.3 (Constraints on Mandatory Choice) "Constraints on Mandatory Choice" was added because these constraints require a combination of RELAX NG and Schematron.
• Fixed the schema for NETMOD-specific annotations by adding explicit prefix to all defined elements and attributes. Previously, the attributes had no namespace.
• Handling of 'feature', 'if-feature' and 'deviation' added.
• Handling of nma:instance-identifier via XSLT extension function.

D.5.  Changes Between Versions -00 and -01

• Attributes @nma:min-elements and @nma:max-elements are attached to <rng:element> (list entry) and not to <rng:zeroOrMore> or <rng:oneOrMore>.
• Keys and all node identifiers in 'key' and 'unique' statements are prefixed.
• Fixed the mapping of 'rpc' and 'notification'.
• Removed previous sec. 7.5 "RPC Signatures and Notifications" - the same information is now contained in Section 11.50 (The rpc Statement) and Section 11.37 (The notification Statement).
• Added initial "_" to mangled names of groupings.
• Mandated the use of @xmlns:xxx as the only method for declaring the target namespace.
• Added section "Handling of XML Namespaces" to explain the previous item.
• Completed DHCP example in Appendix C (Mapping DHCP Data Model - A Complete Example).
• Almost all text about the second mapping step is new.

Authors' Addresses