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NETMOD                                                         L. Lhotka
Internet-Draft                                                    CESNET
Intended status: Standards Track                                 R. Mahy
Expires: August 15, 2009                                     Plantronics
                                                             S. Chisholm
                                                                  Nortel
                                                       February 11, 2009


  Mapping YANG to Document Schema Definition Languages and Validating
                            NETCONF Content
                     draft-ietf-netmod-dsdl-map-00

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Abstract

   This draft describes the mapping rules for translating YANG data
   models into XML schemas using Document Schema Definition Languages
   (DSDL).


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Objectives and Motivation  . . . . . . . . . . . . . . . . . .  8
   3.  DSDL Schema Languages  . . . . . . . . . . . . . . . . . . . . 10
     3.1.   RELAX NG  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.   Schematron  . . . . . . . . . . . . . . . . . . . . . . . 11
     3.3.   Document Schema Renaming Language (DSRL)  . . . . . . . . 12
   4.  Additional Annotations . . . . . . . . . . . . . . . . . . . . 13
     4.1.   Dublin Core Metadata Elements . . . . . . . . . . . . . . 13
     4.2.   RELAX NG DTD Compatibility Annotations  . . . . . . . . . 13
     4.3.   NETMOD-specific Annotations . . . . . . . . . . . . . . . 14
   5.  Overview of the Mapping  . . . . . . . . . . . . . . . . . . . 16
   6.  Design Considerations  . . . . . . . . . . . . . . . . . . . . 19
     6.1.   Conceptual Data Tree  . . . . . . . . . . . . . . . . . . 19
     6.2.   Modularity  . . . . . . . . . . . . . . . . . . . . . . . 21
     6.3.   Granularity . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Mapping Data Model Structure to the Conceptual Tree Schema . . 23
     7.1.   Optional and Mandatory Content  . . . . . . . . . . . . . 23
     7.2.   Mapping YANG Groupings and Typedefs . . . . . . . . . . . 23
       7.2.1.    YANG Refinements and Augments  . . . . . . . . . . . 26
       7.2.2.    Type derivation chains . . . . . . . . . . . . . . . 29
     7.3.   Translation of XPath Expressions  . . . . . . . . . . . . 30
     7.4.   YANG Language Extensions  . . . . . . . . . . . . . . . . 31
     7.5.   RPC Signatures and Notifications  . . . . . . . . . . . . 32
   8.  Mapping Conceptual Tree Schema to DSDL . . . . . . . . . . . . 33
     8.1.   Mapping Semantic Constraints to Schematron  . . . . . . . 33
       8.1.1.    Library of Schema-independent Schematron Patterns  . 33
     8.2.   Mapping Default Values to DSRL  . . . . . . . . . . . . . 33
   9.  NETCONF Content Validation . . . . . . . . . . . . . . . . . . 34
     9.1.   Validation Phases . . . . . . . . . . . . . . . . . . . . 34
   10. Mapping YANG Statements to Annotated RELAX NG  . . . . . . . . 36
     10.1.  The anyxml Statement  . . . . . . . . . . . . . . . . . . 36
     10.2.  The argument Statement  . . . . . . . . . . . . . . . . . 37
     10.3.  The augment Statement . . . . . . . . . . . . . . . . . . 37
     10.4.  The base Statement  . . . . . . . . . . . . . . . . . . . 38
     10.5.  The belongs-to Statement  . . . . . . . . . . . . . . . . 38
     10.6.  The bit Statement . . . . . . . . . . . . . . . . . . . . 38
     10.7.  The case Statement  . . . . . . . . . . . . . . . . . . . 38
     10.8.  The choice Statement  . . . . . . . . . . . . . . . . . . 38
     10.9.  The config Statement  . . . . . . . . . . . . . . . . . . 38



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     10.10. The contact Statement . . . . . . . . . . . . . . . . . . 38
     10.11. The container Statement . . . . . . . . . . . . . . . . . 38
     10.12. The default Statement . . . . . . . . . . . . . . . . . . 39
     10.13. The description Statement . . . . . . . . . . . . . . . . 39
     10.14. The enum Statement  . . . . . . . . . . . . . . . . . . . 40
     10.15. The error-app-tag Statement . . . . . . . . . . . . . . . 40
     10.16. The error-message Statement . . . . . . . . . . . . . . . 40
     10.17. The extension Statement . . . . . . . . . . . . . . . . . 40
     10.18. The grouping Statement  . . . . . . . . . . . . . . . . . 40
     10.19. The identity Statement  . . . . . . . . . . . . . . . . . 41
     10.20. The import Statement  . . . . . . . . . . . . . . . . . . 41
     10.21. The include Statement . . . . . . . . . . . . . . . . . . 41
     10.22. The input Statement . . . . . . . . . . . . . . . . . . . 41
     10.23. The key Statement . . . . . . . . . . . . . . . . . . . . 41
     10.24. The leaf Statement  . . . . . . . . . . . . . . . . . . . 41
     10.25. The leaf-list Statement . . . . . . . . . . . . . . . . . 42
     10.26. The length Statement  . . . . . . . . . . . . . . . . . . 42
     10.27. The list Statement  . . . . . . . . . . . . . . . . . . . 42
     10.28. The mandatory Statement . . . . . . . . . . . . . . . . . 43
     10.29. The max-elements Statement  . . . . . . . . . . . . . . . 43
     10.30. The min-elements Statement  . . . . . . . . . . . . . . . 43
     10.31. The module Statement  . . . . . . . . . . . . . . . . . . 43
     10.32. The must Statement  . . . . . . . . . . . . . . . . . . . 43
     10.33. The namespace Statement . . . . . . . . . . . . . . . . . 44
     10.34. The notification Statement  . . . . . . . . . . . . . . . 45
     10.35. The ordered-by Statement  . . . . . . . . . . . . . . . . 45
     10.36. The organization Statement  . . . . . . . . . . . . . . . 45
     10.37. The output Statement  . . . . . . . . . . . . . . . . . . 46
     10.38. The path Statement  . . . . . . . . . . . . . . . . . . . 46
     10.39. The pattern Statement . . . . . . . . . . . . . . . . . . 46
     10.40. The position Statement  . . . . . . . . . . . . . . . . . 46
     10.41. The prefix Statement  . . . . . . . . . . . . . . . . . . 46
     10.42. The presence Statement  . . . . . . . . . . . . . . . . . 46
     10.43. The range Statement . . . . . . . . . . . . . . . . . . . 46
     10.44. The reference Statement . . . . . . . . . . . . . . . . . 46
     10.45. The require-instance Statement  . . . . . . . . . . . . . 46
     10.46. The revision Statement  . . . . . . . . . . . . . . . . . 47
     10.47. The rpc Statement . . . . . . . . . . . . . . . . . . . . 47
     10.48. The status Statement  . . . . . . . . . . . . . . . . . . 47
     10.49. The submodule Statement . . . . . . . . . . . . . . . . . 47
     10.50. The type Statement  . . . . . . . . . . . . . . . . . . . 47
       10.50.1.  The empty Type . . . . . . . . . . . . . . . . . . . 48
       10.50.2.  The boolean and binary Types . . . . . . . . . . . . 48
       10.50.3.  The bits Type  . . . . . . . . . . . . . . . . . . . 49
       10.50.4.  The enumeration and union Types  . . . . . . . . . . 49
       10.50.5.  The identityref Type . . . . . . . . . . . . . . . . 49
       10.50.6.  The instance-identifier Type . . . . . . . . . . . . 51
       10.50.7.  The leafref Type . . . . . . . . . . . . . . . . . . 51



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       10.50.8.  The numeric Types  . . . . . . . . . . . . . . . . . 51
       10.50.9.  The string Type  . . . . . . . . . . . . . . . . . . 52
       10.50.10. Derived Types  . . . . . . . . . . . . . . . . . . . 53
     10.51. The typedef Statement . . . . . . . . . . . . . . . . . . 53
     10.52. The unique Statement  . . . . . . . . . . . . . . . . . . 54
     10.53. The units Statement . . . . . . . . . . . . . . . . . . . 54
     10.54. The uses Statement  . . . . . . . . . . . . . . . . . . . 54
     10.55. The value Statement . . . . . . . . . . . . . . . . . . . 54
     10.56. The when Statement  . . . . . . . . . . . . . . . . . . . 54
     10.57. The yang-version Statement  . . . . . . . . . . . . . . . 54
     10.58. The yin-element Statement . . . . . . . . . . . . . . . . 54
   11. Mapping NETMOD-specific annotations to Schematron and DSRL . . 55
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 56
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
   Appendix A.  RELAX NG Schema for NETMOD-specific Annotations . . . 59
     A.1.   XML Syntax  . . . . . . . . . . . . . . . . . . . . . . . 59
     A.2.   Compact Syntax  . . . . . . . . . . . . . . . . . . . . . 62
   Appendix B.  Schematron Library  . . . . . . . . . . . . . . . . . 63
   Appendix C.  Translation of the DHCP Data Model  . . . . . . . . . 64
     C.1.   XML Syntax  . . . . . . . . . . . . . . . . . . . . . . . 64
     C.2.   Compact Syntax  . . . . . . . . . . . . . . . . . . . . . 68
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 72





























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1.  Introduction

   The NETCONF Working Group has completed a base protocol used for
   configuration management [1].  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) [2] using the SMI language [3] 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 rather than 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 [4] and called YANG [5].

   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 [6].  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 the 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.




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   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 [7].

   In the text, we also use the following typographic conventions:

   o  YANG statement keywords are delimited by single quotes.

   o  Literal values are delimited by double quotes.

   o  XML element names are delimited by "<" and ">" characters.

   o  Names of XML attributes are prefixed by the "@" character.

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

   a  DTD compatibility annotations [8]

   dc Dublin Core metadata elements

   nc NETCONF protocol [1]

   nma  NETMOD-specific schema annotations

   nmt  Conceptual tree

   dsrl  Document Schema Renaming Language[9]

   rng  RELAX NG[10]

   sch  ISO Schematron[11]

   xsd  W3C XML Schema[12]

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














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     +--------+-----------------------------------------------------+
     | Prefix | Namespace URI                                       |
     +--------+-----------------------------------------------------+
     | a      | http://relaxng.org/ns/compatibility/annotations/1.0 |
     |        |                                                     |
     | dc     | http://purl.org/dc/terms                            |
     |        |                                                     |
     | nc     | urn:ietf:params:xml:ns:netconf:base:1.0             |
     |        |                                                     |
     | nma    | urn:ietf:params:xml:ns:netmod:dsdl-annotations:1    |
     |        |                                                     |
     | nmt    | urn:ietf:params:xml:ns:netmod:conceptual-tree:1     |
     |        |                                                     |
     | dsrl   | http://purl.oclc.org/dsdl/dsrl                      |
     |        |                                                     |
     | rng    | http://relaxng.org/ns/structure/1.0                 |
     |        |                                                     |
     | sch    | http://purl.oclc.org/dsdl/schematron                |
     |        |                                                     |
     | xsd    | http://www.w3.org/2001/XMLSchema                    |
     +--------+-----------------------------------------------------+

          Table 1: Used namespace prefixes and corresponding URIs




























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2.  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, primarily RELAX NG and Schematron.  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-encoded data appear in several different forms in various phases
   of the 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 efficient than
   generic all-encompassing schemas.

   In order to cope with this variability, we assume that the schemas
   can be generated on demand 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 extended periods 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 resulting
       schema should thus be considered a reasonably readable equivalent
       of the input YANG modules.





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   2.  In the second step, the annotated RELAX NG schema from step 1 is
       transformed further to a coordinated set of 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.














































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3.  DSDL Schema Languages

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

3.1.  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 [10].  Like the W3C XML Schema language [12], it is able to
   describe constraints on the structure and contents of XML documents.
   However, unlike the DTD [13] 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 3.3).

   As its base datatype library, RELAX NG uses the W3C XML Schema
   Datatype Library [14], 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 element such as
   <xsd:annotation> in XSD.

   RELAX NG schema can be represented using two equivalent syntaxes: XML
   and compact.  The compact syntax is described in Annex C of the RELAX
   NG specification [15], which was added to the standard in 2006
   (Amendment 1).  Automatic bidirectional conversions between the two
   syntaxes can be accomplished using for example Trang [22].

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

   o  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
      on readability that results from adding annotations is often much
      stronger for the compact syntax than for the XML syntax.

   o  It is more difficult to programmatically generate compact syntax
      than XML syntax.  While a number of software libraries exist that



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      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 use
   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".

3.2.  Schematron

   Schematron is Part 3 of DSDL that reached the status of a full ISO/
   IEC standard in 2006 [11].  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 parts:

   o  Context - an XPath expression that defines the set of locations
      where the rule is to be applied,

   o  Assert or report condition - another XPath expression that is
      evaluated relative to the location matched by the context
      expression.

   o  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.  The mapping described in this document uses exclusively
   the positive (assert) form.

   Schematron draws most of its expressive power from XPath [16] and
   XSLT [17].  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 future).

   The human-readable error messages are another feature that
   distinguishes Schematron from other schema languages such as RELAX NG
   or XSD.  The messages may even contain XPath expressions that are



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   evaluated in the actual context and thus refer to existing XML
   document nodes and their content.

   ISO Schematron introduced the concept of _abstract patterns_ whose
   purpose is similar to functions in programming languages.  The
   mapping described in this document uses a library of abstract
   patterns for specifying generic constraints such as uniqueness of
   certain leaf values in list items.

   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".

3.3.  Document Schema Renaming Language (DSRL)

   DSRL (pronounced "disrule") is Part 8 of DSDL that reached the status
   of a full ISO/IEC standard in 2008 [9].  Unlike RELAX NG and
   Schematron, it is specifically designed to modify XML information set
   of the validated document.  The primary application for DSRL is
   renaming XML elements and attributes.  DSRL can also define default
   values for XML attributes and elements so that elements or attributes
   with these default values are inserted if they are missing in the
   validated documents.  The latter feature is used by the YANG-to-DSDL
   mapping for representing YANG defaults for leaf nodes.

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


















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4.  Additional Annotations

   In addition to the DSDL schema languages, the mapping uses three sets
   of annotations that are added as foreign-namespace elements and
   attributes 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.  While these data model items may not be used for
   formal validation, they quite often carry important information.
   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 semantic
   constraints and other information that cannot be expressed natively
   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.

4.1.  Dublin Core Metadata Elements

   Dublin Core [23] 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 found in [18].

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

4.2.  RELAX NG DTD Compatibility Annotations

   DTD compatibility annotations are part of the RELAX NG DTD
   Compatibility specification [8].  The 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:
   it is 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".







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4.3.  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 in the conceptual tree schema.  YANG statements are
   mapped to these annotations in a very straightforward way.  In
   particular, the annotation attributes and elements always have the
   same name as the corresponding YANG statement.

   Table 2 lists alphabetically the names of NETMOD-specific annotation
   elements (in angle brackets) and attributes (prefixed with "@") along
   with a reference to the section where their use is described.
   Appendix A then contains the RELAX NG schema of this annotation
   vocabulary.




































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              +---------------------------+---------+------+
              | annotation                | section | note |
              +---------------------------+---------+------+
              | @nma:config               | 10.9    |      |
              |                           |         |      |
              | @nma:default              | 10.12   |      |
              |                           |         |      |
              | @nma:default-case         | 10.7    |      |
              |                           |         |      |
              | <nma:error-app-tag>       | 10.15   | 1    |
              |                           |         |      |
              | <nma:error-message>       | 10.16   | 1    |
              |                           |         |      |
              | <nma:instance-identifier> | 10.50.6 | 2    |
              |                           |         |      |
              | @nma:key                  | 10.23   |      |
              |                           |         |      |
              | <nma:leafref>             | 10.50.7 | 2    |
              |                           |         |      |
              | @nma:min-elements         | 10.25   |      |
              |                           |         |      |
              | @nma:max-elements         | 10.25   |      |
              |                           |         |      |
              | <nma:must>                | 10.32   | 3    |
              |                           |         |      |
              | @nma:ordered-by           | 10.35   |      |
              |                           |         |      |
              | @nma:status               | 10.48   |      |
              |                           |         |      |
              | @nma:unique               | 10.48   |      |
              |                           |         |      |
              | @nma:units                | 10.48   |      |
              |                           |         |      |
              | @nma:when                 | 10.56   |      |
              +---------------------------+---------+------+

                   Table 2: NETMOD-specific annotations

   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>.





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5.  Overview of the Mapping

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

                    +----------------+
                    | YANG module(s) |
                    +----------------+
                            |
                            |T
                            |
            +---------------------------------+
            | DSDL schema for conceptual tree |
            +---------------------------------+
               /       |           |       \      +-------+
              /        |           |        \     |library|
           Td/       Ts|           |Tc       \    +-------+
            /          |           |          \
      +---------+   +------+    +------+    +------+
      |datastore|   |server|    |client|    | .... |
      +---------+   +------+    +------+    +------+

                    Figure 1: Structure of the mapping

   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 6.1).  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 simple
       annotations belonging to special namespaces.

   2.  In the second step, the conceptual tree schema may be 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 shows just three simplest 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:
       default-content> elements etc.

   3.  As indicated in Figure 1, the second step of the mapping also
       uses a schema-independent library that contains mainly Schematron
       abstract patterns corresponding to common YANG concepts such as
       the uniqueness constraint for list keys.




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   An implementation of the mapping algorithm accepts 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, perhaps with multiple root
   nodes.  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 module(s) imports (transitively).

   The output of the first mapping step is an annotated RELAX NG schema
   for the conceptual tree, which represents a virtual XML document
   containing, in its different subtrees, the entire datastore, all RPC
   requests and replies, and notifications 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 agent, 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 RELAX NG
   and the common starting point for subsequent transformations that
   will typically produce validation schemas.  The conceptual tree is
   further described in Section 6.1.

   Other information contained in input YANG modules, such as semantic
   constraints or default values, are recorded as annotations - XML
   elements or attributes 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 4.1).  Finally, documentation strings
   are mapped to the <a:documentation> element belonging to the DTD
   compatibility vocabulary (Section 4.2).

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

   o  RELAX NG schema describing the grammatical and datatype
      constraints;

   o  Schematron schema expressing other constraints such as uniqueness
      of list keys or user-specified semantic rules;

   o  DSRL schema containing a specification of default values.

   An implementation SHOULD allow for selecting a subset of schema



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   languages and annotation types to be used for output.  For example, a
   user might want to select pure RELAX NG without any annotations.

















































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6.  Design Considerations

   YANG modules could 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.

6.1.  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 (full datastore and several
   types of PDUs): every YANG-based model represents the contents of a
   full 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.  However, the decision about proper
   structuring of all PDU-validating schemas is beyond the scope of this
   document.  However, the mapping procedure described in this document
   is designed to accommodate any foreseeable validation needs.

   An essential part of the YANG-to-DSDL mapping procedure is
   nonetheless common to all validation approaches: the schemas 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.

   The conceptual data tree may be schematically represented as follows:
















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   <nmt:netmod-tree
       xmlns:nmt="urn:ietf:params:xml:ns:netmod:conceptual-tree:1">
     <nmt:main>
       ... configuration and status data ...
     </nmt:main>
     <nmt:rpc-methods>
       <nmt:rpc-method name="...">
         <nmt:input>
           ...
         </nmt:input>
         <nmt:output>
           ...
         </nmt:output>
       </nmt:rpc-method>
       ...
     </nmt:rpcs>
     <nmt:notifications>
       <nmt:notification name="...">
         ...
       </nmt:notification>
       ...
     </nmt:notifications>
   </nmt:netmod>

   The namespace URI "urn:ietf:params:xml:ns:netmod: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 various validation schemas.  Such
   transformations should be relatively simple, they will typically
   extract one or several 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 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, the interim annotations
   are extracted and 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 stemming from the fact that
   a single configuration repository 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



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   multiple hierarchies appear under the single <nmt:main> element.

6.2.  Modularity

   Both YANG and RELAX NG offer means for modularity, i.e., for
   splitting the contents into separate modules (schemas) 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:

   o  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 module B's data tree 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.

   o  The names of imported YANG groupings and typedefs are qualified
      with the namespace of the imported module.  On the other hand, the
      data nodes contained inside the imported groupings, when used
      within the importing module, become part of the importing
      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.




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6.3.  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 is introduced inside a new named
   pattern.  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.























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7.  Mapping Data Model Structure to the Conceptual Tree Schema

   This section explains the main principles underlying the first step
   of the mapping.  Details about mapping individual YANG statements are
   contained in Section 10.

7.1.  Optional and Mandatory Content

   In YANG, all leaf nodes are optional unless they contain substatement

   mandatory true;

   or unless they are declared as list keys.

   Lists or leaf-lists are optional unless they contain 'min-elements'
   substatement with argument value greater than zero.

   A slightly more complicated situation arises for YANG containers.
   First, containers with the 'presence' substatement are always
   optional since their presence or absence carries specific
   information.  On the other hand, non-presence containers may be
   omitted if they are empty.  For the purposes of the YANG-to-DSDL
   mapping, we declare a non-presence container as optional if and only
   if the following two conditions hold:

   1.  none of its descendant leaf nodes is mandatory or, if such a leaf
       exists, it is enclosed in an intervening container with presence;

   2.  none of its descendant list or leaf-list nodes has 'min-elements'
       substatement with argument value greater than zero or, if such a
       list or leaf-list exists, it is enclosed in an intervening
       container with presence.

   In RELAX NG, all elements that are optional must be explicitly
   wrapped in the <rng:optional> element.  The mapping algorithm thus
   uses the above rules to determine whether a YANG node is optional and
   if so, insert the <rng:optional> element in the RELAX NG schema.

7.2.  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 [5],
   Section 5.5.  Moreover, top-level symbols from external modules are
   imported as qualified names represented using the external module



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   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:

   o  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 ("__").

   o  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.

   For example, consider the following YANG module which imports the
   standard module "inet-types" [19]:






























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   module example1 {
       namespace "http://example.com/ns/example1";
       prefix "ex1";
       import "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):


















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   <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="t:void">
         <rng:empty/>
       </rng:element>
     </rng:optional>
   </rng:define>

   <rng:define name="example1__cont__grp2">
     <rng:optional>
       <rng:element name="t:address">
         <rng:ref name="inet-types__ip-address"/>
       </rng:element>
     </rng:optional>
   </rng:define>

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

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

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

7.2.1.  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



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   statements for this purpose that may appear as substatements of
   'uses':

   o  'refine' statement allows for changing parameters of a schema node
      inside the grouping referenced by the parent 'uses' statement;

   o  '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 a subset of XPath 1.0 expressions as
   arguments, schema nodes that are arbitrarily deep inside the grouping
   content.  In contrast, definitions of named patterns in RELAX NG
   operate 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 6.3) 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 virtually 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:

















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

   <rng:define name="example2__es">
     <rng:optional>
       <rng:element name="hoja">
         <rng:data type="string"/>
       </rng:element>
     </rng:optional>
   </rng:define>


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




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   <rng:ref name="example2__leaves"/>

   Now assume that the "leave" expansion is refined:

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

   The resulting conceptual tree schema now contains just one named
   pattern definition - "example__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 now
   looks like this:

   <rng:ref name="example2__fr"/>
     <rng:optional>
       <rng:element name="hoja" nma:default="alamo">
         <rng:data type="string"/>
       </rng:element>
     </rng:optional>

7.2.2.  Type derivation chains

   RELAX NG has no equivalent of the type derivation mechanism in YANG,
   where a base built-in type may be modified (in multiple steps) by
   adding new restrictions.  Therefore, when mapping YANG derived types
   with restrictions, the derived types MUST be "unwound" all the way
   back to the base built-in type.  At the same time, all restrictions
   found along the type derivation chain MUST be combined and their
   intersection used as facets restricting the corresponding type in
   RELAX NG.

   When a derived YANG type is used without restrictions, the 'type'
   statement is mapped simply to the <rng:ref> element, i.e., a named
   pattern reference.  However, if restrictions are specified as
   substatements of the 'type' statement, the type MUST be expanded at
   that point so that only the built-in type appears in the output
   schema, restricted with facets that again correspond to the
   combination of all restrictions found along the type derivation chain
   and also in the 'type' statement.

   EXAMPLE.  Consider this YANG module:






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

7.3.  Translation of XPath Expressions

   YANG uses full XPath 1.0 syntax [16] for the arguments of 'must' and
   'when' statements and a subset thereof in several other statements.
   However, since the names of data nodes defined by YANG modules are
   always namespace qualified, YANG adopted a simplification similar to
   the concept of _default namespace_ in XPath 2.0: node names needn't



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   carry a namespace prefix inside the module where they are defined, in
   which case the module's namespace is assumed.

   If an XPath expression is carried over to a NETMOD-specific
   annotation in the first mapping step, 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 [16], sections 7.5.3 and 7.19.5.

   Translation rule 2 means that absolute XPath expressions appearing in
   the main configuration data tree always start with "nmt:netmod-tree/
   nmt:main/", those appearing in "my-notif" notification always start
   with "nmt:netmod-tree/nmt:notifications/
   nmt:notification[@name='my-notif']/", etc.

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

   [Editor's note: We may want to introduce "$root" variable that always
   contains the appropriate partial path in conceptual tree.  The
   translated XPath in the example would then become "$root/dhcp:dhcp/
   dhcp:max-lease-time".]

7.4.  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 native statements, only with 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 every 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.




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


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

7.5.  RPC Signatures and Notifications

   In the conceptual tree schema, YANG definitions of RPC methods are
   mapped to the subtree under "/nmt:netmod-tree/nmt:rpc-methods".  Each
   RPC method corresponds to one subelement of <nmt:rpc-methods>, namely
   <nmt:rpc-method name="..."> where the value of the @name attribute is
   set to the name of the method.  In turn, the <nmt:rpc-method> element
   has two subelements, <nmt:input> and <nmt:output> (both are optional)
   that contain input and input parameters of the given RPC method,
   respectively.

   Analogically, the content of each notification is mapped inside the
   element <nmt:notification name="...">, which is a subelement of
   "/nmt:netmod-tree/nmt:notifications".












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8.  Mapping Conceptual Tree Schema to DSDL

   [Editor's note: This section is not finished yet.  We need to write a
   mapping specification for each of the NETMOD-specific annotations.]

8.1.  Mapping Semantic Constraints to Schematron

   TBD

8.1.1.  Library of Schema-independent Schematron Patterns

   This section outlines the schema-independent library of Schematron
   patterns.

   TBD

8.2.  Mapping Default Values to DSRL

   TBD
































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9.  NETCONF Content Validation

   [Editor's note: This section is not finished yet.  We have to figure
   out what are the NETCONF objects we want to validate, and also the
   validation contexts and modes.  The concept of validation phases
   outlined below is just one part of the problem.  However, these
   questions are not DSDL-specific and should be addressed by the WG.
   One issue is DSDL-specific though: we have to find a way for
   validating compound documents using both NETCONF and NETMOD schemas.
   NVDL is a good candidate.]

9.1.  Validation Phases

   Validation of a configuration datastore or NETCONF PDUs may occur in
   different logical phases, adding more tests with each phase.  We use
   three levels or phases for validating an instance document.  There is
   a level of validation which is appropriate even for loose XML
   document fragments which still maintain their hierarchy (_fragment
   phase_), another level of validation is appropriate for a cohesive
   XML document which may however not be able to validate relational
   integrity checks against some operational state (_standard phase_),
   and finally there is validation which cover everything including all
   relational integrity checks (_full validation phase_).  For example,
   in NETCONF an edit-config operation can cause the replacement a small
   fragment of XML.  A candidate configuration may be waiting for
   application but can't check the readiness of a piece of hardware that
   the configuration refers to.

   From the NETCONF perspective, these three phases can be considered to
   have the following scope:

   1.  the fragment phase: This can be run against individual NETCONF
       operations.

   2.  the standard phase: This can be run against the candidate
       configuration, but won't always pass.

   3.  the full validation phase: This can be run against a running
       configuration.

   During the Fragment phase validation it is verified that the content
   is well-formed and appropriate to the operation.

   During Standard phase validation (all rules except for leafref
   checking):

   o  Verify that mandatory nodes are present.




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   o  Check the uniqueness for unique and key annotations.

   o  Print a warning if any manual validation rules are present.

   During Full phase validation: add leafref checks.

   The mechanism how the phase specification is passed to the Schematron
   validator is outside the scope of this document.  For example, it can
   be accomplished via command line parameters.

   <sch:phase id="fragment">
     <sch:active pattern="lease-time"/>
     <sch:active pattern="nonconfig"/>
   </sch:phase>
   <sch:phase id="std">
     <sch:active pattern="lease-time"/>
     <sch:active pattern="nonconfig"/>
     <sch:active pattern="key"/>
   </sch:phase>
   <sch:phase id="full">
     <sch:active pattern="lease-time"/>
     <sch:active pattern="nonconfig"/>
     <sch:active pattern="key"/>
     <sch:active pattern="keyref"/>
   </sch:phase>


























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10.  Mapping YANG Statements to Annotated RELAX NG

   Each subsection in this section is devoted to one YANG statement and
   describes how the statement is mapped to the annotated RELAX NG
   schema of the conceptual tree.  This is the first step of the mapping
   procedure, see Section 5.  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 algorithm 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.  We use the following notation:

   o  The argument of the statement being mapped is denoted by ARGUMENT.

   o  The element in the RELAX NG schema that becomes the parent of the
      resulting XML fragment is denoted by PARENT.

10.1.  The anyxml Statement

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

   <rng:ref name="__anyxml__"/>

   Substatements of the 'anyxml' statement are 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. [20], p.
   172):















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   <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

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

   maps to the following fragment:

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

10.2.  The argument Statement

   This statement is not mapped to the output schema, but see the rules
   for extension handling in Section 7.4.

10.3.  The augment Statement

   As a substatement of 'uses', this statement is handled as a part of
   'uses' mapping, see Section 10.54.

   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.







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10.4.  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 10.50.5.

10.5.  The belongs-to Statement

   This statement is not used since processing of submodules is always
   initiated from the main module, see Section 10.21.

10.6.  The bit Statement

   Handled within the "bits" type, see Section 10.50.3.

10.7.  The case Statement

   This statement is mapped to <rng:group> element.  If the argument of
   a sibling 'default' statement equals to ARGUMENT, @nma:default-case
   attribute with the value of "true" is added to that <rng:group>
   element.

10.8.  The choice Statement

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

   Unless 'choice' has the 'mandatory' substatement with the value of
   "true", the <rng:choice> element MUST be wrapped in <rng:optional>.

10.9.  The config Statement

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

10.10.  The contact Statement

   This statement is not 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 10.31.  The original
   modules are the authoritative sources of the authorship information.

10.11.  The container Statement

   Using the procedure outlined in Section 7.1, 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.



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

10.12.  The default Statement

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

   As a substatement of 'choice', the 'default' statement identifies the
   default case and is handled within the 'case' statement, see
   Section 10.7.  If the default case uses the shorthand notation where
   the 'case' statement is omitted, an extra <rng:group> element MUST be
   inserted with @nma:default-case attribute set to "true".  The net
   result is then the same as if the 'case' statement wasn't omitted for
   the default case.

   EXAMPLE.  The following 'choice' statement

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

   is mapped to

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

10.13.  The description Statement

   This statement is ignored if it appears at the top level of each
   input YANG module.  The description can be found in the source module
   that is referred to by Dublin Core element <dc:source> and use
   ARGUMENT as its content.

   Otherwise, this statement is mapped to the DTD compatibility element
   <a:documentation> and ARGUMENT becomes its text.



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

10.14.  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 annotations, see [10], Section 6.

10.15.  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 10.32.

10.16.  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 10.32.

10.17.  The extension Statement

   This statement is ignored.  However, extensions to the YANG language
   MAY be mapped as described in Section 7.4.

10.18.  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 7.2.

   Whenever a grouping is used with additional refinements and/or
   augments, the grouping is expanded so that the refinements and
   augments may be applied directly to the prescribed schema nodes.  See
   Section 7.2.1 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.







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10.19.  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, ARGUMENT will appear as the text of
   one of the <rng:value> elements in the mapping of that "identityref"
   type.  See Section 10.50.5 for more details and an example.

10.20.  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.

10.21.  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.

10.22.  The input Statement

   This statement is mapped to <rng:element> and its @name attribute is
   set to "nmt:input".

10.23.  The key Statement

   This statement is mapped to @nma:key attribute and ARGUMENT becomes
   the value of this attribute.

10.24.  The leaf Statement

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

   The leaf is optional if there is no "mandatory true;" substatement
   and if the leaf is not declared among the keys of an enclosing list.



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   In this case, the <rng:element> element MUST be wrapped in <rng:
   optional>.

10.25.  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.

   Further, if the argument of 'min-elements' is greater than one,
   attribute @nma:min-elements is attached to PARENT and the argument of
   'min-elements' becomes the value of this attribute.

   If there is the 'max-elements' substatement, attribute @nma:max-
   elements is attached to PARENT and the argument of 'max-elements'
   becomes the value of this attribute.

   Then <rng:element> is added as a child element of PARENT and ARGUMENT
   becomes the value of its @name attribute.

   EXAMPLE.  YANG leaf-list

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

   is mapped to the following RELAX NG fragment:

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

10.26.  The length Statement

   Handled within the "string" type, see Section 10.50.9.

10.27.  The list Statement

   This statement is mapped exactly as the 'leaf-list' statement, see
   Section 10.25.



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10.28.  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 7.1.

10.29.  The max-elements Statement

   This statement is handled within 'leaf-list' or 'list' statements,
   see Section 10.25.

10.30.  The min-elements Statement

   This statement is handled within 'leaf-list' or 'list' statements,
   see Section 10.25.

10.31.  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 10.46.

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

   o  top level data elements be defined as children of the <nmt:top>
      element;

   o  elements mapped from 'rpc' statements be defined as children of
      the <nmt:rpc-methods> element;

   o  elements mapped from 'notification' statements be defined as
      children of the <nmt:notifications> element.

10.32.  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 7.3).
   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





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   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>

10.33.  The namespace Statement

   ARGUMENT of this statement - namespace URI of the input module - is
   mapped to an XML attribute of the <rng:grammar> element (the root of
   the RELAX NG schema) in the following way:

   1.  For one of the input modules, its namespace URI MAY become the
       value of the @ns attribute.

   2.  For the remaining input modules, the namespace URI becomes the
       value of the @xmlns:xxx attribute where "xxx" is a unique prefix,
       which SHOULD be set to the argument of the 'prefix' statement in
       the same module (see Section 10.41).

   In case 2, names of all data nodes appearing as values of the @name
   attribute of the <rng:element> elements in the RELAX NG schema MUST
   use the given prefix whereas in case 1 the values have no prefix.

   Namespace URI of all input modules MAY be mapped using the method in
   2.  The advantage of this approach is that the recommended prefixes
   are recorded in the output RELAX NG schema for all input modules.

   EXAMPLE: Assume we have the following two YANG modules as input:

   module foo-module {
       namespace "http://example.com/ns/foo";
       prefix foo;
       ...
   }

   and






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   module bar-module {
       namespace "http://example.com/ns/bar";
       prefix bar;
       ...
   }

   The <rng:grammar> element in the output schema may then be either

   <rng:grammar xmlns="http://relaxng.org/ns/structure/1.0"
             xmlns:foo="http://example.com/ns/foo"
             xmlns:bar="http://example.com/ns/bar"
             ...
             >
                 ...
             </rng:grammar>

   or

   <rng:grammar xmlns="http://relaxng.org/ns/structure/1.0"
             ns="http://example.com/ns/foo"
             xmlns:bar="http://example.com/ns/bar"
             ...
             >
                 ...
             </rng:grammar>

   The third possibility is analogical to the previous one, only with
   the roles of "foo" and "bar" exchanged.

10.34.  The notification Statement

   This element is mapped to <rng:element> and its @name attribute is
   set to "nmt:notification" element.  This element is defined as a
   child of the <nmt:notifications> element in the conceptual tree.

10.35.  The ordered-by Statement

   This statement is mapped to @nma:ordered-by attribute and ARGUMENT
   becomes the value of this attribute.  See Section 10.25 for an
   example.

10.36.  The organization Statement

   This statement is not used by the mapping since the output RELAX NG
   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 10.31.  The original
   modules are the authoritative sources of the authorship information.



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10.37.  The output Statement

   This statement is mapped to <rng:element> and its @name attribute is
   set to "nmt:output".

10.38.  The path Statement

   Handled within "leafref" type, see Section 10.50.7.

10.39.  The pattern Statement

   Handled within "string" type, see Section 10.50.9.

10.40.  The position Statement

   This statement is ignored.

10.41.  The prefix Statement

   ARGUMENT of this statement - the recommended namespace prefix for the
   input module - MAY be mapped to a namespace prefix declared using the
   @xmlns:xxx attribute of the <rng:grammar> element in the output
   schema, where "xxx" is replaced by ARGUMENT.  For multiple input
   modules, the mapping of prefixes depends on how their namespace URIs
   are used, see Section 10.33.

10.42.  The presence Statement

   This statement influences the mapping of 'container' (Section 10.11):
   it makes the parent container optional, regardless of its content.
   See also Section 7.1.

10.43.  The range Statement

   Handled within numeric types, see Section 10.50.8.

10.44.  The reference Statement

   This statement is ignored if it appears at the top level of a module
   or submodule.

   Otherwise, this statement is mapped to <a:documentation> element and
   its text is set to ARGUMENT prefixed with "See: ".

10.45.  The require-instance Statement

   Handled within the types "leafref" (Section 10.50.7) and "instance-
   identifier" (Section 10.50.6).



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10.46.  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 [5].  This
   date SHOULD be recorded, together with the name of the YANG module,
   in the corresponding Dublin Core element <dc:source> (see
   Section 10.31), for example in this form:

   <dc:source>YANG module 'foo', revision 2009-01-19</dc:source>

   The 'description' substatement of 'revision' is not used.

10.47.  The rpc Statement

   This element is mapped to <rng:element> and its @name attribute is
   set to "nmt:rpc-method" element.  This element is defined as a child
   of the <nmt:rpc-methods> element in the conceptual tree.

10.48.  The status Statement

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

10.49.  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.

10.50.  The type Statement

   Most YANG built-in types have an equivalent in the XSD datatype
   library [14] as shown in Table 3.

















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     +-----------+---------------+----------------------------------+
     | YANG type | XSD type      | Meaning                          |
     +-----------+---------------+----------------------------------+
     | int8      | byte          | 8-bit integer value              |
     |           |               |                                  |
     | int16     | short         | 16-bit integer value             |
     |           |               |                                  |
     | int32     | int           | 32-bit integer value             |
     |           |               |                                  |
     | int64     | long          | 64-bit integer value             |
     |           |               |                                  |
     | uint8     | unsignedByte  | 8-bit unsigned integer value     |
     |           |               |                                  |
     | uint16    | unsignedShort | 16-bit unsigned integer value    |
     |           |               |                                  |
     | uint32    | unsignedInt   | 32-bit unsigned integer value    |
     |           |               |                                  |
     | uint64    | unsignedLong  | 64-bit unsigned integer value    |
     |           |               |                                  |
     | float32   | float         | 32-bit IEEE floating-point value |
     |           |               |                                  |
     | float64   | double        | 64-bit IEEE floating-point value |
     |           |               |                                  |
     | string    | string        | character string                 |
     |           |               |                                  |
     | boolean   | boolean       | "true" or "false"                |
     |           |               |                                  |
     | binary    | base64Binary  | binary data in base64 encoding   |
     +-----------+---------------+----------------------------------+

     Table 3: Selected datatypes from the W3C XML Schema Type Library

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

10.50.1.  The empty Type

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

10.50.2.  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.







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10.50.3.  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.

10.50.4.  The enumeration and union Types

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

10.50.5.  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 a
   @type attribute and its value set to "QName".  One <rng:value>
   subelement with argument of the 'base' substatement as its text MUST
   always be present.  In addition, one <rng:value> substatement MUST be
   added for each identity declared locally or in an imported module
   that has the argument of the 'base' substatement as its base
   identity.

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

   EXAMPLE (taken from [5], Section 7.6.13).  Consider the following two
   YANG modules:



















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   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, leaf definition

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

   is mapped to

   <rng:element name="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>




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

10.50.6.  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 'require-instance' substatement, if it exists, is mapped to the
   @require-instance attribute of <nma:instance-identifier>.

10.50.7.  The leafref Type

   This type is mapped to <rng:data> element with @type attribute set to
   the type of the leaf given in the argument of 'path' substatement.
   In addition, <nma:leafref> element MUST be inserted as a child of
   PARENT.  The argument value of the 'path' substatement is set as the
   text of this element.

   The 'require-instance' substatement, if it exists, is mapped to the
   @require-instance attribute of <nma:leafref>.

10.50.8.  The numeric Types

   YANG built-in numeric types are "int8", "int16", "int32", "int64",
   "uint8", "uint16", "uint32", "uint64", "float32" and "float64".  They
   are mapped to <rng:data> element with @type attribute set to ARGUMENT
   mapped according to Table 3.

   All numeric types support the 'range' restriction, which is handled
   in the following way:

   o  If the range expression consists of a single range part, insert
      the pair of RELAX NG facets

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

      and

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

      Their contents are the lower and upper bound of the range part,
      respectively.  If the range part consists of a single number, both
      "minInclusive" and "maxInclusive" facets use this value as their
      content.  If the lower bound is "min", the "minInclusive" facet is
      omitted and if the upper bound is "max", the "maxInclusive" facet
      is omitted.



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   o  If the range expression has multiple parts separated by "|", then
      repeat the <rng:data> element once for every range part and wrap
      them all in <rng:choice> element.  Each <rng:data> element
      contains the "minInclusive" and "maxInclusive" facets for one part
      of the range expression as described in the previous item.

   For example, the 'typedef' statement

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

   appearing at the top level of the "example" module is mapped to the
   following RELAX NG fragment:

   <rng:define name="example__rt">
     <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:param name="minInclusive">42</rng:param>
         <rng:param name="maxInclusive">42</rng:param>
       </rng:data>
       <rng:data type="int">
         <rng:param name="minInclusive">100</rng:param>
       </rng:data>
     </rng:choice>
   </rng:define>

10.50.9.  The string Type

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

   For the 'pattern' restriction, insert <rng:param> element with @name
   attribute set to "pattern".  The argument of the 'pattern' statement
   (regular expression) becomes the content of this element.

   The 'length' restriction is handled in the same way as the 'range'
   restriction for the numeric types, with the additional twist that if
   the length expression has multiple parts, the "pattern" facet

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




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   if there is any, must be repeated inside each copy of the <rng:data>
   element, i.e., for each length part.

10.50.10.  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 'typedef' must be
       found and its mapping installed as a subelement of <rng:grammar>,
       see Section 10.51.  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 base type for the given
       derived type must be determined and the mapping of this base type
       is used.  Restrictions appearing at all stages of the derivation
       chain must be taken into account and their conjunction added to
       the <rng:data> element which defines the basic type.

   See Section 7.2.2 for more details and an example.

10.51.  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 7.2.

   Whenever a derived type is used with additional restrictions, the the
   base 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 10.50.10 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.






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10.52.  The unique Statement

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

10.53.  The units Statement

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

10.54.  The uses Statement

   If this statement has neither 'refine' nor 'augment' substatements,
   it is mapped to <rng:ref> element and the value of its @name
   attribute is set to ARGUMENT mangled according to Section 7.2

   If there are any 'refine' or 'augment' substatements, the
   corresponding grouping must be looked up and its contents is inserted
   as children of PARENT.  See Section 7.2.1 for further details and an
   example.

10.55.  The value Statement

   This statement is ignored.

10.56.  The when Statement

   This statement is mapped to @nma:when attribute and ARGUMENT becomes
   it value.

10.57.  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).

10.58.  The yin-element Statement

   This statement is not mapped to the output schema, but see the rules
   for extension handling in Section 7.4.











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11.  Mapping NETMOD-specific annotations to Schematron and DSRL

   TBD
















































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12.  IANA Considerations

   This document registers two namespace URIs in the IETF XML registry
   [21]:

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


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










































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13.  References

   [1]   Enns, R., "NETCONF Configuration Protocol", RFC 4741,
         December 2006.

   [2]   Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
         Network Management Protocol (SNMP)", STD 15, RFC 1157,
         May 1990.

   [3]   McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder,
         Ed., "Structure of Management Information Version 2 (SMIv2)",
         STD 58, RFC 2578, April 1999.

   [4]   Elliott, C., Harrington, D., Jason, J., Schoenwaelder, J.,
         Strauss, F., and W. Weiss, "SMIng Objectives", RFC 3216,
         December 2001.

   [5]   Bjorklund, M., Ed., "YANG - A data modeling language for
         NETCONF", draft-ietf-netmod-yang-03 (work in progress),
         January 2009.

   [6]   ISO/IEC, "Document Schema Definition Languages (DSDL) - Part 1:
         Overview", ISO/IEC 19757-1, 11 2004.

   [7]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [8]   Clark, J., Ed. and M. Murata, Ed., "RELAX NG DTD
         Compatibility", OASIS Committee Specification 3 December 2001,
         December 2001.

   [9]   ISO/IEC, "Information Technology - Document Schema Definition
         Languages (DSDL) - Part 8: Document Semantics Renaming Language
         - DSRL", ISO/IEC 19757-8:2008(E), 12 2008.

   [10]  ISO/IEC, "Information Technology - Document Schema Definition
         Languages (DSDL) - Part 2: Regular-Grammar-Based Validation -
         RELAX NG. Second Edition.", ISO/IEC 19757-2:2008(E), 12 2008.

   [11]  ISO/IEC, "Information Technology - Document Schema Definition
         Languages (DSDL) - Part 3: Rule-Based Validation - Schematron",
         ISO/IEC 19757-3:2006(E), 6 2006.

   [12]  Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, "XML
         Schema Part 1: Structures Second Edition", World Wide Web
         Consortium Recommendation REC-xmlschema-1-20041028,
         October 2004,
         <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.



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   [13]  Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F.
         Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
         Edition)", World Wide Web Consortium Recommendation REC-xml-
         20060816, August 2006,
         <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [14]  Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes Second
         Edition", World Wide Web Consortium Recommendation REC-
         xmlschema-2-20041028, October 2004,
         <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

   [15]  ISO/IEC, "Information Technology - Document Schema Definition
         Languages (DSDL) - Part 2: Regular-Grammar-Based Validation -
         RELAX NG. AMENDMENT 1: Compact Syntax", ISO/IEC 19757-2:2003/
         Amd. 1:2006(E), 1 2006.

   [16]  Clark, J. and S. DeRose, "XML Path Language (XPath) Version
         1.0", World Wide Web Consortium Recommendation REC-xpath-
         19991116, November 1999,
         <http://www.w3.org/TR/1999/REC-xpath-19991116>.

   [17]  Clark, J., "XSL Transformations (XSLT) Version 1.0", World Wide
         Web Consortium Recommendation REC-xslt-19991116, November 1999.

   [18]  Kunze, J., "The Dublin Core Metadata Element Set", RFC 5013,
         August 2007.

   [19]  Schoenwaelder, J., Ed., "Common YANG Data Types",
         draft-ietf-netmod-yang-types-01 (work in progress),
         November 2008.

   [20]  van der Vlist, E., "RELAX NG", O'Reilly , 2004.

   [21]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
         January 2004.

   [22]  <http://www.thaiopensource.com/relaxng/trang.html>

   [23]  <http://dublincore.org/>

   [24]  <http://www.yang-central.org/twiki/bin/view/Main/DhcpTutorial>

   [25]  <http://thaiopensource.com/relaxng/trang.html>








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Appendix A.  RELAX NG Schema for NETMOD-specific Annotations

   This appendix contains the RELAX NG schema for the NETMOD-specific
   annotations in both XML and compact syntax.

   [Editor's note: It is currently only a set of named pattern
   definitions as templates for the annotation elements and attributes.
   We should find a way how to connect this to the schema for RELAX NG,
   which these annotations extend.  One option may be NVDL or it can
   also be done as in the spec for DTD compatibility annotations.]

A.1.  XML Syntax

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

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

   <define name="default-attribute">
     <attribute name="default"/>
   </define>

   <define name="default-case-attribute">
     <attribute name="default-case">
       <data type="boolean"/>
     </attribute>
   </define>

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

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



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   </define>

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

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

   <define name="leafref-element">
     <element name="leafref">
       <optional>
         <attribute name="require-instance">
           <data type="boolean"/>
         </attribute>
       </optional>
       <data type="string"/>
     </element>
   </define>

   <define name="min-elements-attribute">
     <attribute name="min-elements">
       <data type="integer"/>
     </attribute>
   </define>

   <define name="max-elements-attribute">
     <attribute name="max-elements">
       <data type="integer"/>
     </attribute>
   </define>

   <define name="must-element">
     <element name="must">
       <attribute name="assert">
         <data type="string"/>
       </attribute>



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       <interleave>
         <ref name="err-app-tag-element"/>
         <ref name="err-message-element"/>
       </interleave>
     </element>
   </define>

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

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

   <define name="unique-attribute">
     <attribute name="unique">
       <list>
         <data type="string"/>
       </list>
     </attribute>
   </define>

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

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

   </grammar>




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A.2.  Compact Syntax

   default namespace =
       "urn:ietf:params:xml:ns:netmod:dsdl-annotations:1"

   config-attribute = attribute config { xsd:boolean }
   default-attribute = attribute default { text }
   default-case-attribute = attribute default-case { xsd:boolean }
   error-app-tag-element = element error-app-tag { text }?
   error-message-element = element error-message { text }?
   instance-identifier-element =
     element instance-identifier {
       attribute require-instance { xsd:boolean }?
     }
   key-attribute =
     attribute key {
       list { xsd:QName }
     }
   leafref-element =
     element leafref {
       attribute require-instance { xsd:boolean }?,
       xsd:string
     }
   min-elements-attribute = attribute min-elements { xsd:integer }
   max-elements-attribute = attribute max-elements { xsd:integer }
   must-element =
     element must {
       attribute assert { xsd:string },
       (err-app-tag-element & err-message-element)
     }
   ordered-by-attribute = attribute ordered-by { "user" | "system" }
   status-attribute =
     attribute status { "current" | "deprecated" | "obsolete" }
   unique-attribute =
     attribute unique {
       list { xsd:string }
     }
   units-attribute = attribute units { xsd:string }
   when-attribute = attribute when { xsd:string }












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Appendix B.  Schematron Library

   This appendix contains the schema-independent library of Schematron
   abstract patterns.

   [Editor's note: It is incomplete.]

   <!-- Uniqueness of list keys -->
   <sch:pattern abstract="true" id="key">
     <sch:rule context="$context">
       <sch:assert test="count($context[$key=current()/$key])=1">
         The key "<value-of select="$key"/>" needs to be unique
         within the list at: <value-of select="$context"/>.
       </sch:assert>
     </sch:rule>
   </sch:pattern>

   <!-- Check of listref target -->
   <sch:pattern abstract="true" id="keyref">
     <sch:rule context="$keyref-context">
       <sch:assert test="$key-context[$key=current()]">
         The contents of "<value-of select="$keyref-context"/>"
         must be a '</name>' with the key
         "<value-of select="$key"/>" in this context:
         <value-of select="$key-context"/>.
       </sch:assert>
     </sch:rule>
   </sch:pattern>























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Appendix C.  Translation of the DHCP Data Model

   This appendix demonstrates output of the YANG->DSDL mapping algorithm
   applied to the "canonical" DHCP tutorial [24] data model.

   Appendix C.1 shows the result of the mapping algorithm in the RELAX
   NG XML syntax and Appendix C.2 the same in the compact syntax, which
   was obtained using the Trang tool [25].

   The long regular expressions for IP addresses etc. that would exceed
   the limit of 72 characters per line were trimmed.  Other than that,
   the results of the automatic translations were not changed.

C.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:rng-annot:1"
      xmlns:nmt="urn:ietf:params:xml:ns:netmod:tree:1"
      datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
    <dc:creator>Pyang 0.9.3, RELAX NG 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">
                <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="current() &lt;= ../max-lease-time">



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                      <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">
                    <zeroOrMore>
                      <element name="dhcp:shared-network"
                               nma:key="name">
                        <element name="dhcp:name">
                          <data type="string"/>
                        </element>
                        <ref name="dhcp__subnet-list"/>
                      </element>
                    </zeroOrMore>
                  </element>
                </optional>
                <optional>
                  <element name="dhcp:status"
                           nma:config="false">
                    <zeroOrMore>
                      <element name="dhcp:leases"
                               nma:key="address">
                        <element name="dhcp:address">
                          <ref name="inet-types__ip-address"/>
                        </element>
                        <optional>
                          <element name="dhcp:starts">
                            <ref name="yang-types__date-and-time"/>
                          </element>
                        </optional>
                        <optional>
                          <element name="dhcp:ends">
                            <ref name="yang-types__date-and-time"/>
                          </element>
                        </optional>
                        <optional>
                          <element name="dhcp:hardware">
                            <optional>
                              <element name="dhcp:type">
                                <choice>
                                  <value>ethernet</value>
                                  <value>token-ring</value>
                                  <value>fddi</value>
                                </choice>
                              </element>



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                            </optional>
                            <optional>
                              <element name="dhcp:address">
                                <ref name="yang-types__phys-address"/>
                              </element>
                            </optional>
                          </element>
                        </optional>
                      </element>
                    </zeroOrMore>
                  </element>
                </optional>
              </element>
            </optional>
          </interleave>
        </element>
        <element name="nmt:rpc-methods">
          <empty/>
        </element>
        <element name="nmt:notifications">
          <empty/>
        </element>
      </element>
    </start>
    <define name="dhcp__subnet-list">
      <a:documentation>A reusable list of subnets</a:documentation>
      <zeroOrMore>
        <element name="dhcp:subnet" nma:key="net">
          <element name="dhcp:net">
            <ref name="inet-types__ip-prefix"/>
          </element>
          <optional>
            <element name="dhcp:range">
              <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="inet-types__ip-address"/>
              </element>
              <element name="dhcp:high">
                <ref name="inet-types__ip-address"/>
              </element>
            </element>
          </optional>



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          <optional>
            <element name="dhcp:dhcp-options">
              <a:documentation>Options in the DHCP
              protocol</a:documentation>
              <zeroOrMore>
                <element name="dhcp:router"
                         nma:ordered-by="user">
                  <ref name="inet-types__host"/>
                  <a:documentation>See: RFC 2132,
                  sec. 3.8</a:documentation>
                </element>
              </zeroOrMore>
              <optional>
                <element name="dhcp:domain-name">
                  <ref name="inet-types__domain-name"/>
                  <a:documentation>See: RFC 2132,
                  sec. 3.17</a:documentation>
                </element>
              </optional>
            </element>
          </optional>
          <optional>
            <element name="dhcp:max-lease-time"
                     nma:default="7200"
                     nma:units="seconds">
              <data type="unsignedInt"/>
            </element>
          </optional>
        </element>
      </zeroOrMore>
    </define>
    <define name="inet-types__ip-prefix">
      <choice>
        <ref name="inet-types__ipv4-prefix"/>
        <ref name="inet-types__ipv6-prefix"/>
      </choice>
    </define>
    <define name="inet-types__ipv4-prefix">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="inet-types__ipv6-prefix">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="inet-types__ip-address">



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      <choice>
        <ref name="inet-types__ipv4-address"/>
        <ref name="inet-types__ipv6-address"/>
      </choice>
    </define>
    <define name="inet-types__ipv4-address">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="inet-types__ipv6-address">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="inet-types__host">
      <choice>
        <ref name="inet-types__ip-address"/>
        <ref name="inet-types__domain-name"/>
      </choice>
    </define>
    <define name="inet-types__domain-name">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="yang-types__date-and-time">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
    <define name="yang-types__phys-address">
      <data type="string">
        <param name="pattern">... removed ...</param>
      </data>
    </define>
  </grammar>

C.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:rng-annot:1"
namespace nmt = "urn:ietf:params:xml:ns:netmod:tree:1"

dc:creator [ "Pyang 0.9.3, RELAX NG plugin" ]
dc:source [ "YANG module 'dhcp'" ]



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start =
  element nmt:netmod-tree {
    element nmt:top {

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

## A reusable list of subnets
dhcp__subnet-list =



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  [ nma:key = "net" ]
  element dhcp:subnet {
    element dhcp:net { inet-types__ip-prefix },
    element dhcp:range {

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

    ## Options in the DHCP protocol
    element dhcp:dhcp-options {
      [ nma:ordered-by = "user" ]
      element dhcp:router {
        inet-types__host
        >> a:documentation [ "See: RFC 2132, sec. 3.8" ]
      }*,
      element dhcp:domain-name {
        inet-types__domain-name
        >> a:documentation [ "See: RFC 2132, sec. 3.17" ]
      }?
    }?,
    [ nma:default = "7200" nma:units = "seconds" ]
    element dhcp:max-lease-time { xsd:unsignedInt }?
  }*
inet-types__ip-prefix =
  inet-types__ipv4-prefix | inet-types__ipv6-prefix
inet-types__ipv4-prefix =
  xsd:string {
    pattern = "... removed ..."
  }
inet-types__ipv6-prefix =
  xsd:string {
    pattern = "... removed ..."
  }
inet-types__ip-address =
  inet-types__ipv4-address | inet-types__ipv6-address
inet-types__ipv4-address =
  xsd:string {
    pattern = "... removed ..."
  }
inet-types__ipv6-address =
  xsd:string {
    pattern = "... removed ..."
  }
inet-types__host = inet-types__ip-address | inet-types__domain-name
inet-types__domain-name =



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  xsd:string {
    pattern = "... removed ..."
  }
yang-types__date-and-time =
  xsd:string {
    pattern = "... removed ..."
  }
yang-types__phys-address =
  xsd:string {
    pattern = "([0-9a0-fA-F]{2}(:[0-9a0-fA-F]{2})*)?"
  }








































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Authors' Addresses

   Ladislav Lhotka
   CESNET

   Email: lhotka@cesnet.cz


   Rohan Mahy
   Plantronics

   Email: rohan@ekabal.com


   Sharon Chisholm
   Nortel

   Email: schishol@nortel.com

































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