Guidelines for Authors and Reviewers of Documents Containing YANG Data Models
RFC 8407

Document Type RFC - Best Current Practice (October 2018; Errata)
Obsoletes RFC 6087
Also known as BCP 216
Last updated 2019-04-16
Replaces draft-bierman-netmod-rfc6087bis
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Internet Engineering Task Force (IETF)                        A. Bierman
Request for Comments: 8407                                     YumaWorks
BCP: 216                                                    October 2018
Obsoletes: 6087
Category: Best Current Practice
ISSN: 2070-1721

           Guidelines for Authors and Reviewers of Documents
                      Containing YANG Data Models

Abstract

   This memo provides guidelines for authors and reviewers of
   specifications containing YANG modules.  Recommendations and
   procedures are defined, which are intended to increase
   interoperability and usability of Network Configuration Protocol
   (NETCONF) and RESTCONF protocol implementations that utilize YANG
   modules.  This document obsoletes RFC 6087.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8407.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Changes since RFC 6087  . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  NETCONF Terms . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  YANG Terms  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  NMDA Terms  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Requirements Notation . . . . . . . . . . . . . . . . . .   8
   3.  General Documentation Guidelines  . . . . . . . . . . . . . .   8
     3.1.  Module Copyright  . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Code Components . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Example Modules . . . . . . . . . . . . . . . . . . .   9
     3.3.  Terminology Section . . . . . . . . . . . . . . . . . . .  10
     3.4.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Narrative Sections  . . . . . . . . . . . . . . . . . . .  10
     3.6.  Definitions Section . . . . . . . . . . . . . . . . . . .  11
     3.7.  Security Considerations Section . . . . . . . . . . . . .  11
       3.7.1.  Security Considerations Section Template  . . . . . .  12
     3.8.  IANA Considerations Section . . . . . . . . . . . . . . .  13
       3.8.1.  Documents That Create a New Namespace . . . . . . . .  14
       3.8.2.  Documents That Extend an Existing Namespace . . . . .  14
     3.9.  References Sections . . . . . . . . . . . . . . . . . . .  14
     3.10. Validation Tools  . . . . . . . . . . . . . . . . . . . .  14
     3.11. Module Extraction Tools . . . . . . . . . . . . . . . . .  15
     3.12. Module Usage Examples . . . . . . . . . . . . . . . . . .  15
   4.  YANG Usage Guidelines . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Module Naming Conventions . . . . . . . . . . . . . . . .  16
     4.2.  Prefixes  . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  18
       4.3.1.  Identifier Naming Conventions . . . . . . . . . . . .  18
     4.4.  Defaults  . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.5.  Conditional Statements  . . . . . . . . . . . . . . . . .  19
     4.6.  XPath Usage . . . . . . . . . . . . . . . . . . . . . . .  20
       4.6.1.  XPath Evaluation Contexts . . . . . . . . . . . . . .  20
       4.6.2.  Function Library  . . . . . . . . . . . . . . . . . .  21
       4.6.3.  Axes  . . . . . . . . . . . . . . . . . . . . . . . .  22
       4.6.4.  Types . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.6.5.  Wildcards . . . . . . . . . . . . . . . . . . . . . .  24
       4.6.6.  Boolean Expressions . . . . . . . . . . . . . . . . .  24
     4.7.  YANG Definition Lifecycle Management  . . . . . . . . . .  25
     4.8.  Module Header, Meta, and Revision Statements  . . . . . .  26
     4.9.  Namespace Assignments . . . . . . . . . . . . . . . . . .  28
     4.10. Top-Level Data Definitions  . . . . . . . . . . . . . . .  29
     4.11. Data Types  . . . . . . . . . . . . . . . . . . . . . . .  30
       4.11.1.  Fixed-Value Extensibility  . . . . . . . . . . . . .  30
       4.11.2.  Patterns and Ranges  . . . . . . . . . . . . . . . .  31
       4.11.3.  Enumerations and Bits  . . . . . . . . . . . . . . .  32

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       4.11.4.  Union Types  . . . . . . . . . . . . . . . . . . . .  33
       4.11.5.  Empty and Boolean  . . . . . . . . . . . . . . . . .  34
     4.12. Reusable Type Definitions . . . . . . . . . . . . . . . .  35
     4.13. Reusable Groupings  . . . . . . . . . . . . . . . . . . .  35
     4.14. Data Definitions  . . . . . . . . . . . . . . . . . . . .  36
       4.14.1.  Non-Presence Containers  . . . . . . . . . . . . . .  38
       4.14.2.  Top-Level Data Nodes . . . . . . . . . . . . . . . .  38
     4.15. Operation Definitions . . . . . . . . . . . . . . . . . .  39
     4.16. Notification Definitions  . . . . . . . . . . . . . . . .  39
     4.17. Feature Definitions . . . . . . . . . . . . . . . . . . .  40
     4.18. YANG Data Node Constraints  . . . . . . . . . . . . . . .  41
       4.18.1.  Controlling Quantity . . . . . . . . . . . . . . . .  41
       4.18.2.  "must" versus "when" . . . . . . . . . . . . . . . .  41
     4.19. "augment" Statements  . . . . . . . . . . . . . . . . . .  41
       4.19.1.  Conditional Augment Statements . . . . . . . . . . .  41
       4.19.2.  Conditionally Mandatory Data Definition Statements .  42
     4.20. Deviation Statements  . . . . . . . . . . . . . . . . . .  43
     4.21. Extension Statements  . . . . . . . . . . . . . . . . . .  44
     4.22. Data Correlation  . . . . . . . . . . . . . . . . . . . .  45
       4.22.1.  Use of "leafref" for Key Correlation . . . . . . . .  46
     4.23. Operational State . . . . . . . . . . . . . . . . . . . .  47
       4.23.1.  Combining Operational State and Configuration Data .  47
       4.23.2.  Representing Operational Values of Configuration
                Data . . . . . . . . . . . . . . . . . . . . . . . .  47
       4.23.3.  NMDA Transition Guidelines . . . . . . . . . . . . .  48
     4.24. Performance Considerations  . . . . . . . . . . . . . . .  52
     4.25. Open Systems Considerations . . . . . . . . . . . . . . .  52
     4.26. Guidelines for Constructs Specific to YANG 1.1  . . . . .  53
       4.26.1.  Importing Multiple Revisions . . . . . . . . . . . .  53
       4.26.2.  Using Feature Logic  . . . . . . . . . . . . . . . .  53
       4.26.3.  "anyxml" versus "anydata"  . . . . . . . . . . . . .  53
       4.26.4.  "action" versus "rpc"  . . . . . . . . . . . . . . .  53
     4.27. Updating YANG Modules (Published versus Unpublished)  . .  54
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  55
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  55
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  56
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  56
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  57
   Appendix A.  Module Review Checklist  . . . . . . . . . . . . . .  59
   Appendix B.  YANG Module Template . . . . . . . . . . . . . . . .  61
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  62
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  63

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1.  Introduction

   The standardization of network configuration interfaces for use with
   network configuration management protocols, such as the Network
   Configuration Protocol [RFC6241] and the RESTCONF protocol [RFC8040],
   requires a modular set of data models that can be reused and extended
   over time.

   This document defines a set of usage guidelines for documents
   containing YANG 1.1 [RFC7950] and YANG 1.0 [RFC6020] data models.
   YANG is used to define the data structures, protocol operations, and
   notification content used within a NETCONF and/or RESTCONF server.  A
   NETCONF or RESTCONF server that supports a particular YANG module
   will support client NETCONF and/or RESTCONF operation requests, as
   indicated by the specific content defined in the YANG module.

   Many YANG constructs are defined as optional to use, such as the
   "description" statement.  However, in order to make YANG modules more
   useful, it is desirable to define a set of usage guidelines that
   entails a higher level of compliance than the minimum level defined
   in the YANG specification [RFC7950].

   In addition, YANG allows constructs such as infinite length
   identifiers and string values, or top-level mandatory nodes, that a
   compliant server is not required to support.  Only constructs that
   all servers are required to support can be used in IETF YANG modules.

   This document defines usage guidelines related to the NETCONF
   operations layer and NETCONF content layer, as defined in [RFC6241],
   and the RESTCONF methods and RESTCONF resources, as defined in
   [RFC8040].

   These guidelines are intended to be used by authors and reviewers to
   improve the readability and interoperability of published YANG data
   models.

   Note that this document is not a YANG tutorial, and the reader is
   expected to know the YANG data modeling language before implementing
   the guidance in this document.

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1.1.  Changes since RFC 6087

   The following changes have been made to the guidelines published in
   [RFC6087]:

   o  Updated NETCONF reference from RFC 4741 to RFC 6241

   o  Updated NETCONF over the Secure Shell (SSH) citation from RFC 4742
      to RFC 6242

   o  Updated YANG Types reference from RFC 6021 to RFC 6991

   o  Updated obsolete URLs for IETF resources

   o  Changed top-level data node guideline

   o  Clarified XML Path Language (XPath) usage for a literal value
      representing a YANG identity

   o  Clarified XPath usage for a when-stmt

   o  Clarified XPath usage for "preceding-sibling" and
      "following-sibling" axes

   o  Added terminology guidelines

   o  Added mention of RFC 8174, which updates RFC 2119 by clarifying
      the use of capitalized key words

   o  Added YANG tree diagram guidelines

   o  Updated XPath guidelines for type conversions and function library
      usage

   o  Updated "Data Types" section

   o  Updated "Notification Definitions" section

   o  Clarified conditional key leaf nodes

   o  Clarified usage of "uint64" and "int64" data types

   o  Added text on YANG feature usage

   o  Added "Identifier Naming Conventions" section

   o  Clarified use of mandatory nodes with conditional augmentations

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   o  Clarified namespace and domain conventions for example modules

   o  Clarified conventions for identifying code components

   o  Added YANG 1.1 guidelines

   o  Added "YANG Data Node Constraints" section

   o  Added mention of the RESTCONF protocol

   o  Added guidelines for datastores revised by the Network Management
      Datastore Architecture (NMDA)

2.  Terminology

   The following terms are used throughout this document:

   o  published: A stable release of a module or submodule.  For
      example, the "Request for Comments" described in Section 2.1 of
      [RFC2026] is considered a stable publication.

   o  unpublished: An unstable release of a module or submodule.  For
      example the "Internet-Draft" described in Section 2.2 of [RFC2026]
      is considered an unstable publication that is a work in progress,
      subject to change at any time.

   o  YANG fragment: A set of YANG statements that are not intended to
      represent a complete YANG module or submodule.  These statements
      are not intended for actual use, except to provide an example of
      YANG statement usage.  The invalid syntax "..." is sometimes used
      to indicate that additional YANG statements would be present in a
      real YANG module.

   o  YANG tree diagram: A diagram representing the contents of a YANG
      module, as defined in [RFC8340].  It is also called a "tree
      diagram".

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2.1.  NETCONF Terms

   The following terms are defined in [RFC6241] and are not redefined
   here:

   o  capabilities

   o  client

   o  operation

   o  server

2.2.  YANG Terms

   The following terms are defined in [RFC7950] and are not redefined
   here:

   o  data node

   o  module

   o  namespace

   o  submodule

   o  version

   o  YANG

   o  YIN

   Note that the term 'module' may be used as a generic term for a YANG
   module or submodule.  When describing properties that are specific to
   submodules, the term 'submodule' is used instead.

2.3.  NMDA Terms

   The following terms are defined in [RFC8342] and are not redefined
   here:

   o  configuration

   o  conventional configuration datastore

   o  datastore

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   o  operational state

   o  operational state datastore

2.4.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  General Documentation Guidelines

   YANG modules under review are likely to be contained in Internet-
   Drafts (I-Ds).  All guidelines for I-D authors [ID-Guidelines] MUST
   be followed.  The guidelines for RFCs should be followed and are
   defined in the following: [RFC7322] (and any future RFCs that
   obsolete it), [RFC-STYLE], and [RFC7841].

   The following sections MUST be present in an I-D containing a module:

   o  Narrative sections

   o  Definition sections

   o  Security Considerations section

   o  IANA Considerations section

   o  References section

   There are three usage scenarios for YANG that can appear in an I-D or
   RFC:

   o  normative module or submodule

   o  example module or submodule

   o  example YANG fragment not part of any module or submodule

   The guidelines in this document refer mainly to a normative module or
   submodule but may be applicable to example modules and YANG fragments
   as well.

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3.1.  Module Copyright

   The module "description" statement MUST contain a reference to the
   latest approved IETF Trust Copyright statement, which is available
   online at:

       <https://trustee.ietf.org/license-info/>

3.2.  Code Components

   Each normative YANG module or submodule contained within an I-D or
   RFC is considered to be a code component.  The strings "<CODE
   BEGINS>" and "<CODE ENDS>" MUST be used to identify each code
   component.

   The "<CODE BEGINS>" tag SHOULD be followed by a string identifying
   the file name specified in Section 5.2 of [RFC7950].  The name string
   form that includes the revision date SHOULD be used.  The revision
   date MUST match the date used in the most recent revision of the
   module.

   The following example is for the "2016-03-20" revision of the
   "ietf-foo" module:

   <CODE BEGINS> file "ietf-foo@2016-03-20.yang"

       module ietf-foo {
         namespace "urn:ietf:params:xml:ns:yang:ietf-foo";
         prefix "foo";
         organization "...";
         contact "...";
         description "...";
         revision 2016-03-20 {
           description "Latest revision";
           reference "RFC XXXX: Foo Protocol";
         }
         // ... more statements
       }

   <CODE ENDS>

3.2.1.  Example Modules

   Example modules are not code components.  The <CODE BEGINS>
   convention MUST NOT be used for example modules.

   An example module SHOULD be named using the term "example", followed
   by a hyphen, followed by a descriptive name, e.g., "example-toaster".

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   See Section 4.9 regarding the namespace guidelines for example
   modules.

3.3.  Terminology Section

   A terminology section MUST be present if any terms are defined in the
   document or if any terms are imported from other documents.

3.4.  Tree Diagrams

   YANG tree diagrams provide a concise representation of a YANG module
   and SHOULD be included to help readers understand YANG module
   structure.  Guidelines on tree diagrams can be found in Section 3 of
   [RFC8340].

   If YANG tree diagrams are used, then an informative reference to the
   YANG tree diagrams specification MUST be included in the document.
   Refer to Section 2.2 of [RFC8349] for an example of such a reference.

3.5.  Narrative Sections

   The narrative part MUST include an overview section that describes
   the scope and field of application of the module(s) defined by the
   specification and that specifies the relationship (if any) of these
   modules to other standards, particularly to standards containing
   other YANG modules.  The narrative part SHOULD include one or more
   sections to briefly describe the structure of the modules defined in
   the specification.

   If the module or modules defined by the specification imports
   definitions from other modules (except for those defined in [RFC7950]
   or [RFC6991]) or are always implemented in conjunction with other
   modules, then those facts MUST be noted in the overview section; any
   special interpretations of definitions in other modules MUST be noted
   as well.  Refer to Section 2.3 of [RFC8349] for an example of this
   overview section.

   If the document contains a YANG module(s) that is compliant with NMDA
   [RFC8342], then the Introduction section should mention this fact.

   Example:

     The YANG data model in this document conforms to the Network
     Management Datastore Architecture defined in
     RFC 8342.

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   Consistent indentation SHOULD be used for all examples, including
   YANG fragments and protocol message instance data.  If line wrapping
   is done for formatting purposes, then this SHOULD be noted, as shown
   in the following example:

      [note: '\' line wrapping for formatting only]

      <myleaf xmlns="tag:example.com,2017:example-two">\
        this is a long value so the line needs to wrap to stay\
        within 72 characters\
      </myleaf>

3.6.  Definitions Section

   This section contains the module(s) defined by the specification.
   These modules SHOULD be written using the YANG 1.1 [RFC7950] syntax.
   YANG 1.0 [RFC6020] syntax MAY be used if no YANG 1.1 constructs or
   semantics are needed in the module.  If any of the imported YANG
   modules are written using YANG 1.1, then the module MUST be written
   using YANG 1.1.

   A YIN syntax version of the module MAY also be present in the
   document.  There MAY also be other types of modules present in the
   document, such as Structure of Management Information Version 2
   (SMIv2), which are not affected by these guidelines.

   Note that if the module itself is considered normative and not an
   example module or example YANG fragment, then all YANG statements
   within a YANG module are considered normative.  The use of keywords
   defined in [RFC2119] and [RFC8174] apply to YANG "description"
   statements in normative modules exactly as they would in any other
   normative section.

   Example YANG modules and example YANG fragments MUST NOT contain any
   normative text, including any all-uppercase reserved words from
   [RFC2119] and [RFC8174].

   Consistent indentation and formatting SHOULD be used in all YANG
   statements within a module.

   See Section 4 for guidelines on YANG usage.

3.7.  Security Considerations Section

   Each specification that defines one or more modules MUST contain a
   section that discusses security considerations relevant to those
   modules.

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   This section MUST be patterned after the latest approved template
   (available at <https://trac.ietf.org/trac/ops/wiki/yang-security-
   guidelines>).  Section 3.7.1 contains the security considerations
   template dated 2013-05-08 and last updated on 2018-07-02.  Authors
   MUST check the web page at the URL listed above in case there is a
   more recent version available.

   In particular:

   o  Writable data nodes that could be especially disruptive if abused
      MUST be explicitly listed by name, and the associated security
      risks MUST be explained.

   o  Readable data nodes that contain especially sensitive information
      or that raise significant privacy concerns MUST be explicitly
      listed by name, and the reasons for the sensitivity/privacy
      concerns MUST be explained.

   o  Operations (i.e., YANG "rpc" statements) that are potentially
      harmful to system behavior or that raise significant privacy
      concerns MUST be explicitly listed by name, and the reasons for
      the sensitivity/privacy concerns MUST be explained.

3.7.1.  Security Considerations Section Template

   X.  Security Considerations

   The YANG module specified in this document defines a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [RFC8446].

   The NETCONF access control model [RFC8341] provides the means to
   restrict access for particular NETCONF or RESTCONF users to a
   preconfigured subset of all available NETCONF or RESTCONF protocol
   operations and content.

    -- if you have any writable data nodes (those are all the
    -- "config true" nodes, and remember, that is the default)
    -- describe their specific sensitivity or vulnerability.

   There are a number of data nodes defined in this YANG module that are
   writable/creatable/deletable (i.e., "config true", which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)

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   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

   <list subtrees and data nodes and state why they are sensitive>

    -- for all YANG modules you must evaluate whether any readable data
    -- nodes (those are all the "config false" nodes, but also all other
    -- nodes, because they can also be read via operations like get or
    -- get-config) are sensitive or vulnerable (for instance, if they
    -- might reveal customer information or violate personal privacy
    -- laws such as those of the European Union if exposed to
    -- unauthorized parties)

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

   <list subtrees and data nodes and state why they are sensitive>

    -- if your YANG module has defined any RPC operations
    -- describe their specific sensitivity or vulnerability.

   Some of the RPC operations in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control access to these operations.  These are the
   operations and their sensitivity/vulnerability:

   <list RPC operations and state why they are sensitive>

3.8.  IANA Considerations Section

   In order to comply with IESG policy as set forth in
   <https://www.ietf.org/id-info/checklist.html>, every I-D that is
   submitted to the IESG for publication MUST contain an IANA
   Considerations section.  The requirements for this section vary
   depending on what actions are required of the IANA.  If there are no
   IANA considerations applicable to the document, then the IANA
   Considerations section will state that "This document has no IANA
   actions".  Refer to the guidelines in [RFC8126] for more details.

   Each normative YANG module MUST be registered in both the "IETF XML
   Registry" [RFC3688] [IANA-XML] and the "YANG Module Names" registry
   [RFC6020] [IANA-MOD-NAMES].  This applies to new modules and updated
   modules.  An example of an update registration for the
   "ietf-template" module can be found in Section 5.

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3.8.1.  Documents That Create a New Namespace

   If an I-D defines a new namespace that is to be administered by the
   IANA, then the document MUST include an IANA Considerations section
   that specifies how the namespace is to be administered.

   Specifically, if any YANG module namespace statement value contained
   in the document is not already registered with IANA, then a new entry
   in the "ns" subregistry within the "IETF XML Registry" MUST be
   requested from the IANA.

3.8.2.  Documents That Extend an Existing Namespace

   It is possible to extend an existing namespace using a YANG submodule
   that belongs to an existing module already administered by IANA.  In
   this case, the document containing the main module MUST be updated to
   use the latest revision of the submodule.

3.9.  References Sections

   For every import or include statement that appears in a module
   contained in the specification that identifies a module in a separate
   document, a corresponding normative reference to that document MUST
   appear in the Normative References section.  The reference MUST
   correspond to the specific module version actually used within the
   specification.

   For every normative reference statement that appears in a module
   contained in the specification that identifies a separate document, a
   corresponding normative reference to that document SHOULD appear in
   the Normative References section.  The reference SHOULD correspond to
   the specific document version actually used within the specification.
   If the reference statement identifies an informative reference that
   identifies a separate document, a corresponding informative reference
   to that document MAY appear in the Informative References section.

3.10.  Validation Tools

   All modules need to be validated before submission in an I-D.  The
   'pyang' YANG compiler is freely available from GitHub:

     <https://github.com/mbj4668/pyang>

   If the 'pyang' compiler is used to validate a normative module, then
   the "--ietf" command-line option MUST be used to identify any IETF
   guideline issues.

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   If the 'pyang' compiler is used to validate an example module, then
   the "--ietf" command-line option MAY be used to identify any IETF
   guideline issues.

   The "yanglint" program is also freely available from GitHub.

      <https://github.com/CESNET/libyang>

   This tool can be used to validate XPath statements within YANG
   modules.

3.11.  Module Extraction Tools

   A version of 'rfcstrip' that will extract YANG modules from an I-D or
   RFC is available.  The 'rfcstrip' tool that supports YANG module
   extraction is freely available at:

     <https://github.com/mbj4668/rfcstrip>

   This tool can be used to verify that the "<CODE BEGINS>" and "<CODE
   ENDS>" tags are used correctly and that the normative YANG modules
   can be extracted correctly.

   The "xym" tool is freely available on GitHub and can be used to
   extract YANG modules from a document.

      <https://github.com/xym-tool/xym>

3.12.  Module Usage Examples

   Each specification that defines one or more modules SHOULD contain
   usage examples, either throughout the document or in an appendix.
   This includes example instance document snippets in an appropriate
   encoding (e.g., XML and/or JSON) to demonstrate the intended usage of
   the YANG module(s).  Example modules MUST be validated.  Refer to
   Section 3.10 for tools that validate YANG modules.  If IP addresses
   are used, then a mix of either IPv4 and IPv6 addresses or IPv6
   addresses exclusively SHOULD be used in the examples.

4.  YANG Usage Guidelines

   Modules in IETF Standards Track specifications MUST comply with all
   syntactic and semantic requirements of YANG 1.1 [RFC7950].  See the
   exception for YANG 1.0 in Section 3.6.  The guidelines in this
   section are intended to supplement the YANG specification [RFC7950],
   which is intended to define a minimum set of conformance
   requirements.

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   In order to promote interoperability and establish a set of practices
   based on previous experience, the following sections establish usage
   guidelines for specific YANG constructs.

   Only guidelines that clarify or restrict the minimum conformance
   requirements are included here.

4.1.  Module Naming Conventions

   Normative modules contained in Standards Track documents MUST be
   named according to the guidelines in the IANA Considerations section
   of [RFC7950].

   A distinctive word or abbreviation (e.g., protocol name or working
   group abbreviation) SHOULD be used in the module name.  If new
   definitions are being defined to extend one or more existing modules,
   then the same word or abbreviation should be reused, instead of
   creating a new one.

   All published module names MUST be unique.  For a YANG module
   published in an RFC, this uniqueness is guaranteed by IANA.  For
   unpublished modules, the authors need to check that no other work in
   progress is using the same module name.

   Example modules are non-normative and SHOULD be named with the prefix
   "example-".

   It is suggested that a stable prefix be selected that represents the
   entire organization.  All normative YANG modules published by the
   IETF MUST begin with the prefix "ietf-".  Another standards
   organization, such as the IEEE, might use the prefix "ieee-" for all
   YANG modules.

   Once a module name is published, it MUST NOT be reused, even if the
   RFC containing the module is reclassified to "Historic" status.  A
   module name cannot be changed in YANG, and this would be treated as a
   new module, not a name change.

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4.2.  Prefixes

   All YANG definitions are scoped by the module containing the
   definition being referenced.  This allows definitions from multiple
   modules to be used, even if the names are not unique.  In the example
   below, the identifier "foo" is used in all three modules:

       module example-foo {
         namespace "tag:example.com,2017:example-foo";
         prefix f;

         container foo;
       }

       module example-bar {
         namespace "tag:example.com,2017:example-bar";
         prefix b;

         typedef foo { type uint32; }
       }

       module example-one {
         namespace "tag:example.com,2017:example-one";
         prefix one;
         import example-foo { prefix f; }
         import example-bar { prefix b; }

         augment "/f:foo" {
            leaf foo { type b:foo; }
         }
       }

   YANG defines the following rules for prefix usage:

   o  Prefixes are never used for built-in data types and YANG keywords.

   o  A prefix MUST be used for any external statement (i.e., a
      statement defined with the YANG "extension" statement).

   o  The proper module prefix MUST be used for all identifiers imported
      from other modules.

   o  The proper module prefix MUST be used for all identifiers included
      from a submodule.

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   The following guidelines apply to prefix usage of the current (local)
   module:

   o  The local module prefix SHOULD be used instead of no prefix in all
      path expressions.

   o  The local module prefix MUST be used instead of no prefix in all
      "default" statements for an "identityref" or "instance-identifier"
      data type.

   o  The local module prefix MAY be used for references to typedefs,
      groupings, extensions, features, and identities defined in the
      module.

   Prefix values SHOULD be short but are also likely to be unique.
   Prefix values SHOULD NOT conflict with known modules that have been
   previously published.

4.3.  Identifiers

   Identifiers for all YANG identifiers in published modules MUST be
   between 1 and 64 characters in length.  These include any construct
   specified as an "identifier-arg-str" token in the ABNF in Section 14
   of [RFC7950].

4.3.1.  Identifier Naming Conventions

   Identifiers SHOULD follow a consistent naming pattern throughout the
   module.  Only lowercase letters, numbers, and dashes SHOULD be used
   in identifier names.  Uppercase characters, the period character, and
   the underscore character MAY be used if the identifier represents a
   well-known value that uses these characters.  YANG does not permit
   any other characters in YANG identifiers.

   Identifiers SHOULD include complete words and/or well-known acronyms
   or abbreviations.  Child nodes within a container or list SHOULD NOT
   replicate the parent identifier.  YANG identifiers are hierarchical
   and are only meant to be unique within the set of sibling nodes
   defined in the same module namespace.

   It is permissible to use common identifiers such as "name" or "id" in
   data definition statements, especially if these data nodes share a
   common data type.

   Identifiers SHOULD NOT carry any special semantics that identify data
   modeling properties.  Only YANG statements and YANG extension
   statements are designed to convey machine-readable data modeling
   properties.  For example, naming an object "config" or "state" does

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   not change whether it is configuration data or state data.  Only
   defined YANG statements or YANG extension statements can be used to
   assign semantics in a machine-readable format in YANG.

4.4.  Defaults

   In general, it is suggested that substatements containing very common
   default values SHOULD NOT be present.  The following substatements
   are commonly used with the default value, which would make the module
   difficult to read if used everywhere they are allowed.

                     +--------------+---------------+
                     | Statement    | Default Value |
                     +--------------+---------------+
                     | config       | true          |
                     | mandatory    | false         |
                     | max-elements | unbounded     |
                     | min-elements | 0             |
                     | ordered-by   | system        |
                     | status       | current       |
                     | yin-element  | false         |
                     +--------------+---------------+

                            Statement Defaults

4.5.  Conditional Statements

   A module may be conceptually partitioned in several ways, using the
   "if-feature" and/or "when" statements.

   Data model designers need to carefully consider all modularity
   aspects, including the use of YANG conditional statements.

   If a data definition is optional, depending on server support for a
   NETCONF or RESTCONF protocol capability, then a YANG "feature"
   statement SHOULD be defined.  The defined "feature" statement SHOULD
   then be used in the conditional "if-feature" statement referencing
   the optional data definition.

   If any notification data, or any data definition, for a non-
   configuration data node is not mandatory, then the server may or may
   not be required to return an instance of this data node.  If any
   conditional requirements exist for returning the data node in a
   notification payload or retrieval request, they MUST be documented
   somewhere.  For example, a "when" or "if-feature" statement could
   apply to the data node, or the conditional requirements could be
   explained in a "description" statement within the data node or one of
   its ancestors (if any).

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   If any "if-feature" statements apply to a list node, then the same
   "if-feature" statements MUST apply to any key leaf nodes for the
   list.  There MUST NOT be any "if-feature" statements applied to any
   key leafs that do not also apply to the parent list node.

   There SHOULD NOT be any "when" statements applied to a key leaf node.
   It is possible that a "when" statement for an ancestor node of a key
   leaf will have the exact node-set result as the key leaf.  In such a
   case, the "when" statement for the key leaf is redundant and SHOULD
   be avoided.

4.6.  XPath Usage

   This section describes guidelines for using the XML Path Language
   (XPath) [W3C.REC-xpath] within YANG modules.

4.6.1.  XPath Evaluation Contexts

   YANG defines five separate contexts for evaluation of XPath
   statements:

   1.  The "running" datastore: collection of all YANG configuration
       data nodes.  The document root is the conceptual container (e.g.,
       "config" in the "edit-config" operation), which is the parent of
       all top-level data definition statements with a "config"
       statement value of "true".

   2.  State data + the "running" datastore: collection of all YANG data
       nodes.  The document root is the conceptual container, parent of
       all top-level data definition statements.

   3.  Notification: an event notification document.  The document root
       is the notification element.

   4.  RPC Input: The document root is the conceptual "input" node,
       which is the parent of all RPC input parameter definitions.

   5.  RPC Output: The document root is the conceptual "output" node,
       which is the parent of all RPC output parameter definitions.

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   Note that these XPath contexts cannot be mixed.  For example, a
   "when" statement in a notification context cannot reference
   configuration data.

       notification foo {
         leaf mtu {
           // NOT okay because when-stmt context is this notification
           when "/if:interfaces/if:interface[name='eth0']";
           type leafref {
             // Okay because path-stmt has a different context
             path "/if:interfaces/if:interface/if:mtu";
           }
         }
       }

   It is especially important to consider the XPath evaluation context
   for XPath expressions defined in groupings.  An XPath expression
   defined in a grouping may not be portable, meaning it cannot be used
   in multiple contexts and produce proper results.

   If the XPath expressions defined in a grouping are intended for a
   particular context, then this context SHOULD be identified in the
   "description" statement for the grouping.

4.6.2.  Function Library

   The "position" and "last" functions SHOULD NOT be used.  This applies
   to implicit use of the "position" function as well (e.g.,
   '//chapter[42]').  A server is only required to maintain the relative
   XML document order of all instances of a particular user-ordered list
   or leaf-list.  The "position" and "last" functions MAY be used if
   they are evaluated in a context where the context node is a user-
   ordered "list" or "leaf-list".

   The "id" function SHOULD NOT be used.  The "ID" attribute is not
   present in YANG documents, so this function has no meaning.  The YANG
   compiler SHOULD return an empty string for this function.

   The "namespace-uri" and "name" functions SHOULD NOT be used.
   Expanded names in XPath are different than YANG.  A specific
   canonical representation of a YANG-expanded name does not exist.

   The "lang" function SHOULD NOT be used.  This function does not apply
   to YANG because there is no "lang" attribute set with the document.
   The YANG compiler SHOULD return 'false' for this function.

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   The "local-name", "namespace-uri", "name", "string", and "number"
   functions SHOULD NOT be used if the argument is a node-set.  If so,
   the function result will be determined by the document order of the
   node-set.  Since this order can be different on each server, the
   function results can also be different.  Any function call that
   implicitly converts a node-set to a string will also have this issue.

   The "local-name" function SHOULD NOT be used to reference local names
   outside of the YANG module that defines the must or when expression
   containing the "local-name" function.  Example of a "local-name"
   function that should not be used:

      /*[local-name()='foo']

   The "derived-from-or-self" function SHOULD be used instead of an
   equality expression for identityref values.  This allows the
   identities to be conceptually augmented.

   Example:

      // do not use
      when "md-name-format = 'name-format-null'";

      // this is preferred
      when "derived-from-or-self(md-name-format, 'name-format-null')";

4.6.3.  Axes

   The "attribute" and "namespace" axes are not supported in YANG and
   MAY be empty in a NETCONF or RESTCONF server implementation.

   The "preceding" and "following" axes SHOULD NOT be used.  These
   constructs rely on XML document order within a NETCONF or RESTCONF
   server configuration database, which may not be supported
   consistently or produce reliable results across implementations.
   Predicate expressions based on static node properties (e.g., element
   name or value, and "ancestor" or "descendant" axes) SHOULD be used
   instead.  The "preceding" and "following" axes MAY be used if
   document order is not relevant to the outcome of the expression
   (e.g., check for global uniqueness of a parameter value).

   The "preceding-sibling" and "following-sibling" axes SHOULD NOT be
   used; however, they MAY be used if document order is not relevant to
   the outcome of the expression.

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   A server is only required to maintain the relative XML document order
   of all instances of a particular user-ordered list or leaf-list.  The
   "preceding-sibling" and "following-sibling" axes MAY be used if they
   are evaluated in a context where the context node is a user-ordered
   "list" or "leaf-list".

4.6.4.  Types

   Data nodes that use the "int64" and "uint64" built-in type SHOULD NOT
   be used within numeric or boolean expressions.  There are boundary
   conditions in which the translation from the YANG 64-bit type to an
   XPath number can cause incorrect results.  Specifically, an XPath
   "double" precision floating-point number cannot represent very large
   positive or negative 64-bit numbers because it only provides a total
   precision of 53 bits.  The "int64" and "uint64" data types MAY be
   used in numeric expressions if the value can be represented with no
   more than 53 bits of precision.

   Data modelers need to be careful not to confuse the YANG value space
   and the XPath value space.  The data types are not the same in both,
   and conversion between YANG and XPath data types SHOULD be considered
   carefully.

   Explicit XPath data type conversions MAY be used (e.g., "string",
   "boolean", or "number" functions), instead of implicit XPath data
   type conversions.

   XPath expressions that contain a literal value representing a YANG
   identity SHOULD always include the declared prefix of the module
   where the identity is defined.

   XPath expressions for "when" statements SHOULD NOT reference the
   context node or any descendant nodes of the context node.  They MAY
   reference descendant nodes if the "when" statement is contained
   within an "augment" statement, and the referenced nodes are not
   defined within the "augment" statement.

   Example:

      augment "/rt:active-route/rt:input/rt:destination-address" {
         when "rt:address-family='v4ur:ipv4-unicast'" {
           description
             "This augment is valid only for IPv4 unicast.";
         }
         // nodes defined here within the augment-stmt
         // cannot be referenced in the when-stmt
      }

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4.6.5.  Wildcards

   It is possible to construct XPath expressions that will evaluate
   differently when combined with several modules within a server
   implementation rather than when evaluated within the single module.
   This is due to augmenting nodes from other modules.

   Wildcard expansion is done within a server against all the nodes from
   all namespaces, so it is possible for a "must" or "when" expression
   that uses the '*' operator to always evaluate to false if processed
   within a single YANG module.  In such cases, the "description"
   statement SHOULD clarify that augmenting objects are expected to
   match the wildcard expansion.

      when /foo/services/*/active {
        description
          "No services directly defined in this module.
           Matches objects that have augmented the services container.";
      }

4.6.6.  Boolean Expressions

   The YANG "must" and "when" statements use an XPath boolean expression
   to define the test condition for the statement.  It is important to
   specify these expressions in a way that will not cause inadvertent
   changes in the result if the objects referenced in the expression are
   updated in future revisions of the module.

   For example, the leaf "foo2" must exist if the leaf "foo1" is equal
   to "one" or "three":

        leaf foo1 {
          type enumeration {
             enum one;
             enum two;
             enum three;
          }
        }

        leaf foo2 {
          // INCORRECT
          must "/f:foo1 != 'two'";
          type string;
        }

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        leaf foo2 {
          // CORRECT
          must "/f:foo1 = 'one' or /f:foo1 = 'three'";
          type string;
        }

   In the next revision of the module, leaf "foo1" is extended with a
   new enum named "four":

        leaf foo1 {
          type enumeration {
             enum one;
             enum two;
             enum three;
             enum four;
          }
        }

   Now the first XPath expression will allow the enum "four" to be
   accepted in addition to the "one" and "three" enum values.

4.7.  YANG Definition Lifecycle Management

   The YANG status statement MUST be present within a definition if its
   value is "deprecated" or "obsolete".  The status SHOULD NOT be
   changed from "current" directly to "obsolete".  An object SHOULD be
   available for at least one year with a "deprecated" status before it
   is changed to "obsolete".

   The module or submodule name MUST NOT be changed, once the document
   containing the module or submodule is published.

   The module namespace URI value MUST NOT be changed, once the document
   containing the module is published.

   The revision date substatement within the import statement SHOULD be
   present if any groupings are used from the external module.

   The revision date substatement within the include statement SHOULD be
   present if any groupings are used from the external submodule.

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   If an import statement is for a module from a stable source (e.g., an
   RFC for an IETF module), then a reference-stmt SHOULD be present
   within an import statement.

        import ietf-yang-types {
           prefix yang;
           reference "RFC 6991: Common YANG Data Types";
        }

   If submodules are used, then the document containing the main module
   MUST be updated so that the main module revision date is equal to or
   more recent than the revision date of any submodule that is (directly
   or indirectly) included by the main module.

   Definitions for future use SHOULD NOT be specified in a module.  Do
   not specify placeholder objects like the "reserved" example below:

       leaf reserved {
         type string;
         description
           "This object has no purpose at this time, but a future
            revision of this module might define a purpose
            for this object.";
         }
       }

4.8.  Module Header, Meta, and Revision Statements

   For published modules, the namespace MUST be a globally unique URI,
   as defined in [RFC3986].  This value is usually assigned by the IANA.

   The "organization" statement MUST be present.  If the module is
   contained in a document intended for IETF Standards Track status,
   then the organization SHOULD be the IETF working group (WG) chartered
   to write the document.  For other standards organizations, a similar
   approach is also suggested.

   The "contact" statement MUST be present.  If the module is contained
   in a document intended for Standards Track status, then the WG web
   and mailing information SHOULD be present, and the main document
   author or editor contact information SHOULD be present.  If
   additional authors or editors exist, their contact information MAY be
   present.  There is no need to include the contact information for WG
   Chairs.

   The "description" statement MUST be present.  For modules published
   within IETF documents, the appropriate IETF Trust Copyright text MUST
   be present, as described in Section 3.1.

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   If the module relies on information contained in other documents,
   which are not the same documents implied by the import statements
   present in the module, then these documents MUST be identified in the
   reference statement.

   A "revision" statement MUST be present for each published version of
   the module.  The "revision" statement MUST have a "reference"
   substatement.  It MUST identify the published document that contains
   the module.  Modules are often extracted from their original
   documents, and it is useful for developers and operators to know how
   to find the original source document in a consistent manner.  The
   "revision" statement MAY have a "description" substatement.

   The following example shows the revision statement for a published
   YANG module:

      revision "2012-02-22" {
        description
          "Initial version";
        reference
          "RFC 8341: Network Configuration
                     Access Control Model";
      }

   For an unpublished module, a complete history of each unpublished
   module revision is not required.  That is, within a sequence of draft
   versions, only the most recent revision need be recorded in the
   module.  Do not remove or reuse a revision statement for a published
   module.  A new revision date is not required unless the module
   contents have changed.  If the module contents have changed, then the
   revision date of that new module version MUST be updated to a date
   later than that of the previous version.

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   The following example shows the two revision statements for an
   unpublished update to a published YANG module:

      revision "2017-12-11" {
        description
          "Added support for YANG 1.1 actions and notifications tied to
           data nodes.  Clarify how NACM extensions can be used by other
           data models.";
        reference
          "RFC 8407: Network Configuration Protocol (NETCONF)
                     Access Control Model";
      }

      revision "2012-02-22" {
        description
          "Initial version";
        reference
          "RFC 8341: Network Configuration
                     Access Control Model";
      }

4.9.  Namespace Assignments

   It is RECOMMENDED that only valid YANG modules be included in
   documents, whether or not the modules are published yet.  This
   allows:

   o  the module to compile correctly instead of generating disruptive
      fatal errors.

   o  early implementors to use the modules without picking a random
      value for the XML namespace.

   o  early interoperability testing since independent implementations
      will use the same XML namespace value.

   Until a URI is assigned by the IANA, a proposed namespace URI MUST be
   provided for the namespace statement in a YANG module.  A value
   SHOULD be selected that is not likely to collide with other YANG
   namespaces.  Standard module names, prefixes, and URI strings already
   listed in the "YANG Module Names" registry MUST NOT be used.

   A standard namespace statement value SHOULD have the following form:

       <URN prefix string>:<module-name>

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   The following URN prefix string SHOULD be used for published and
   unpublished YANG modules:

       urn:ietf:params:xml:ns:yang:

   The following example URNs would be valid namespace statement values
   for Standards Track modules:

       urn:ietf:params:xml:ns:yang:ietf-netconf-partial-lock

       urn:ietf:params:xml:ns:yang:ietf-netconf-state

       urn:ietf:params:xml:ns:yang:ietf-netconf

   Note that a different URN prefix string SHOULD be used for modules
   that are not Standards Track.  The string SHOULD be selected
   according to the guidelines in [RFC7950].

   The following URIs exemplify what might be used by modules that are
   not Standards Track.  Note that the domain "example.com" SHOULD be
   used by example modules in IETF I-Ds.  These URIs are not intended to
   be dereferenced.  They are used for module namespace identification
   only.

   Example URIs using URLs per [RFC3986]:

       https://example.com/ns/example-interfaces

       https://example.com/ns/example-system

   Example URIs using tags per [RFC4151]:

       tag:example.com,2017:example-interfaces

       tag:example.com,2017:example-system

4.10.  Top-Level Data Definitions

   The top-level data organization SHOULD be considered carefully, in
   advance.  Data model designers need to consider how the functionality
   for a given protocol or protocol family will grow over time.

   The separation of configuration data and operational state SHOULD be
   considered carefully.  It is sometimes useful to define separate top-
   level containers for configuration and non-configuration data.  For
   some existing top-level data nodes, configuration data was not in
   scope, so only one container representing operational state was
   created.  Refer to NMDA [RFC8342] for details.

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   The number of top-level data nodes within a module SHOULD be
   minimized.  It is often useful to retrieve related information within
   a single subtree.  If data is too distributed, it becomes difficult
   to retrieve all at once.

   The names and data organization SHOULD reflect persistent
   information, such as the name of a protocol.  The name of the working
   group SHOULD NOT be used because this may change over time.

   A mandatory database data definition is defined as a node that a
   client must provide for the database to be valid.  The server is not
   required to provide a value.

   Top-level database data definitions MUST NOT be mandatory.  If a
   mandatory node appears at the top level, it will immediately cause
   the database to be invalid.  This can occur when the server boots or
   when a module is loaded dynamically at runtime.

4.11.  Data Types

   Selection of an appropriate data type (i.e., built-in type, existing
   derived type, or new derived type) is very subjective; therefore, few
   requirements can be specified on that subject.

   Data model designers SHOULD use the most appropriate built-in data
   type for the particular application.

   The signed numeric data types (i.e., "int8", "int16", "int32", and
   "int64") SHOULD NOT be used unless negative values are allowed for
   the desired semantics.

4.11.1.  Fixed-Value Extensibility

   If the set of values is fixed and the data type contents are
   controlled by a single naming authority, then an enumeration data
   type SHOULD be used.

       leaf foo {
         type enumeration {
           enum one;
           enum two;
         }
       }

   If extensibility of enumerated values is required, then the
   "identityref" data type SHOULD be used instead of an enumeration or
   other built-in type.

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       identity foo-type {
         description "Base for the extensible type";
       }

       identity one {
         base f:foo-type;
       }
       identity two {
         base f:foo-type;
       }

       leaf foo {
         type identityref {
           base f:foo-type;
         }
       }

   Note that any module can declare an identity with base "foo-type"
   that is valid for the "foo" leaf.  Identityref values are considered
   to be qualified names.

4.11.2.  Patterns and Ranges

   For string data types, if a machine-readable pattern can be defined
   for the desired semantics, then one or more pattern statements SHOULD
   be present.  A single-quoted string SHOULD be used to specify the
   pattern, since a double-quoted string can modify the content.  If the
   patterns used in a type definition have known limitations such as
   false negative or false positive matches, then these limitations
   SHOULD be documented within the typedef or data definition.

   The following typedef from [RFC6991] demonstrates the proper use of
   the "pattern" statement:

       typedef ipv4-address-no-zone {
         type inet:ipv4-address {
           pattern '[0-9\.]*';
         }
         ...
       }

   For string data types, if the length of the string is required to be
   bounded in all implementations, then a length statement MUST be
   present.

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   The following typedef from [RFC6991] demonstrates the proper use of
   the "length" statement:

       typedef yang-identifier {
         type string {
           length "1..max";
           pattern '[a-zA-Z_][a-zA-Z0-9\-_.]*';
           pattern '.|..|[^xX].*|.[^mM].*|..[^lL].*';
         }
         ...
       }

   For numeric data types, if the values allowed by the intended
   semantics are different than those allowed by the unbounded intrinsic
   data type (e.g., "int32"), then a range statement SHOULD be present.

   The following typedef from [RFC6991] demonstrates the proper use of
   the "range" statement:

       typedef dscp {
         type uint8 {
            range "0..63";
         }
         ...
       }

4.11.3.  Enumerations and Bits

   For "enumeration" or "bits" data types, the semantics for each "enum"
   or "bit" SHOULD be documented.  A separate "description" statement
   (within each "enum" or "bit" statement) SHOULD be present.

       leaf foo {
         // INCORRECT
         type enumeration {
           enum one;
           enum two;
         }
         description
           "The foo enum...
            one: The first enum
            two: The second enum";
       }

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       leaf foo {
         // CORRECT
         type enumeration {
           enum one {
             description "The first enum";
           }
           enum two {
             description "The second enum";
           }
         }
         description
           "The foo enum...  ";
       }

4.11.4.  Union Types

   The YANG "union" type is evaluated by testing a value against each
   member type in the union.  The first type definition that accepts a
   value as valid is the member type used.  In general, member types
   SHOULD be ordered from most restrictive to least restrictive types.

   In the following example, the "enumeration" type will never be
   matched because the preceding "string" type will match everything.

   Incorrect:

      type union {
        type string;
        type enumeration {
          enum up;
          enum down;
        }
      }

   Correct:

      type union {
        type enumeration {
          enum up;
          enum down;
        }
        type string;
      }

   It is possible for different member types to match, depending on the
   input encoding format.  In XML, all values are passed as string
   nodes; but in JSON, there are different value types for numbers,
   booleans, and strings.

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   In the following example, a JSON numeric value will always be matched
   by the "int32" type, but in XML the string value representing a
   number will be matched by the "string" type.  The second version will
   match the "int32" member type no matter how the input is encoded.

   Incorrect:

      type union {
        type string;
        type int32;
      }

   Correct:

      type union {
        type int32;
        type string;
      }

4.11.5.  Empty and Boolean

   YANG provides an "empty" data type, which has one value (i.e.,
   present).  The default is "not present", which is not actually a
   value.  When used within a list key, only one value can (and must)
   exist for this key leaf.  The type "empty" SHOULD NOT be used for a
   key leaf since it is pointless.

   There is really no difference between a leaf of type "empty" and a
   leaf-list of type "empty".  Both are limited to one instance.  The
   type "empty" SHOULD NOT be used for a leaf-list.

   The advantage of using type "empty" instead of type "boolean" is that
   the default (not present) does not take up any bytes in a
   representation.  The disadvantage is that the client may not be sure
   if an empty leaf is missing because it was filtered somehow or not
   implemented.  The client may not have a complete and accurate schema
   for the data returned by the server and may not be aware of the
   missing leaf.

   The YANG "boolean" data type provides two values ("true" and
   "false").  When used within a list key, two entries can exist for
   this key leaf.  Default values are ignored for key leafs, but a
   default statement is often used for plain boolean leafs.  The
   advantage of the "boolean" type is that the leaf or leaf-list has a
   clear representation for both values.  The default value is usually
   not returned unless explicitly requested by the client, so no bytes
   are used in a typical representation.

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   In general, the "boolean" data type SHOULD be used instead of the
   "empty" data type, as shown in the example below:

   Incorrect:

      leaf flag1 {
        type empty;
      }

   Correct:

      leaf flag2 {
        type boolean;
        default false;
      }

4.12.  Reusable Type Definitions

   If an appropriate derived type exists in any standard module, such as
   [RFC6991], then it SHOULD be used instead of defining a new derived
   type.

   If an appropriate units identifier can be associated with the desired
   semantics, then a units statement SHOULD be present.

   If an appropriate default value can be associated with the desired
   semantics, then a default statement SHOULD be present.

   If a significant number of derived types are defined, and it is
   anticipated that these data types will be reused by multiple modules,
   then these derived types SHOULD be contained in a separate module or
   submodule, to allow easier reuse without unnecessary coupling.

   The "description" statement MUST be present.

   If the type definition semantics are defined in an external document
   (other than another YANG module indicated by an import statement),
   then the reference statement MUST be present.

4.13.  Reusable Groupings

   A reusable grouping is a YANG grouping that can be imported by
   another module and is intended for use by other modules.  This is not
   the same as a grouping that is used within the module in which it is
   defined, but it happens to be exportable to another module because it
   is defined at the top level of the YANG module.

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   The following guidelines apply to reusable groupings, in order to
   make them as robust as possible:

   o  Clearly identify the purpose of the grouping in the "description"
      statement.

   o  There are five different XPath contexts in YANG (rpc/input, rpc/
      output, notification, "config true" data nodes, and all data
      nodes).  Clearly identify which XPath contexts are applicable or
      excluded for the grouping.

   o  Do not reference data outside the grouping in any "path", "must",
      or "when" statements.

   o  Do not include a "default" substatement on a leaf or choice unless
      the value applies on all possible contexts.

   o  Do not include a "config" substatement on a data node unless the
      value applies on all possible contexts.

   o  Clearly identify any external dependencies in the grouping
      "description" statement, such as nodes referenced by an absolute
      path from a "path", "must", or "when" statement.

4.14.  Data Definitions

   The "description" statement MUST be present in the following YANG
   statements:

   o  anyxml

   o  augment

   o  choice

   o  container

   o  extension

   o  feature

   o  grouping

   o  identity

   o  leaf

   o  leaf-list

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   o  list

   o  notification

   o  rpc

   o  typedef

   If the data definition semantics are defined in an external document,
   (other than another YANG module indicated by an import statement),
   then a reference statement MUST be present.

   The "anyxml" construct may be useful to represent an HTML banner
   containing markup elements, such as "<b>" and "</b>", and MAY be used
   in such cases.  However, this construct SHOULD NOT be used if other
   YANG data node types can be used instead to represent the desired
   syntax and semantics.

   It has been found that the "anyxml" statement is not implemented
   consistently across all servers.  It is possible that mixed-mode XML
   will not be supported or that configuration anyxml nodes will not
   supported.

   If there are referential integrity constraints associated with the
   desired semantics that can be represented with XPath, then one or
   more "must" statements SHOULD be present.

   For list and leaf-list data definitions, if the number of possible
   instances is required to be bounded for all implementations, then the
   max-elements statements SHOULD be present.

   If any "must" or "when" statements are used within the data
   definition, then the data definition "description" statement SHOULD
   describe the purpose of each one.

   The "choice" statement is allowed to be directly present within a
   "case" statement in YANG 1.1.  This needs to be considered carefully.
   Consider simply including the nested "choice" as additional "case"
   statements within the parent "choice" statement.  Note that the
   "mandatory" and "default" statements within a nested "choice"
   statement only apply if the "case" containing the nested "choice"
   statement is first selected.

   If a list defines any key leafs, then these leafs SHOULD be defined
   in order, as the first child nodes within the list.  The key leafs
   MAY be in a different order in some cases, e.g., they are defined in
   a grouping, and not inline in the list statement.

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4.14.1.  Non-Presence Containers

   A non-presence container is used to organize data into specific
   subtrees.  It is not intended to have semantics within the data model
   beyond this purpose, although YANG allows it (e.g., a "must"
   statement within the non-presence container).

   Example using container wrappers:

       container top {
          container foos {
             list foo { ... }
          }
          container bars {
             list bar { ... }
          }
       }

   Example without container wrappers:

       container top {
          list foo { ... }
          list bar { ... }
       }

   Use of non-presence containers to organize data is a subjective
   matter similar to use of subdirectories in a file system.  Although
   these containers do not have any semantics, they can impact protocol
   operations for the descendant data nodes within a non-presence
   container, so use of these containers SHOULD be considered carefully.

   The NETCONF and RESTCONF protocols do not currently support the
   ability to delete all list (or leaf-list) entries at once.  This
   deficiency is sometimes avoided by use of a parent container (i.e.,
   deleting the container also removes all child entries).

4.14.2.  Top-Level Data Nodes

   Use of top-level objects needs to be considered carefully:

   o  top-level siblings are not ordered

   o  top-level siblings are not static and depend on the modules that
      are loaded

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   o  for subtree filtering, retrieval of a top-level leaf-list will be
      treated as a content-match node for all top-level-siblings

   o  a top-level list with many instances may impact performance

4.15.  Operation Definitions

   If the operation semantics are defined in an external document (other
   than another YANG module indicated by an import statement), then a
   reference statement MUST be present.

   If the operation impacts system behavior in some way, it SHOULD be
   mentioned in the "description" statement.

   If the operation is potentially harmful to system behavior in some
   way, it MUST be mentioned in the Security Considerations section of
   the document.

4.16.  Notification Definitions

   The "description" statement MUST be present.

   If the notification semantics are defined in an external document
   (other than another YANG module indicated by an import statement),
   then a reference statement MUST be present.

   If the notification refers to a specific resource instance, then this
   instance SHOULD be identified in the notification data.  This is
   usually done by including "leafref" leaf nodes with the key leaf
   values for the resource instance.  For example:

     notification interface-up {
       description "Sent when an interface is activated.";
       leaf name {
         type leafref {
           path "/if:interfaces/if:interface/if:name";
         }
       }
     }

   Note that there are no formal YANG statements to identify any data
   node resources associated with a notification.  The "description"
   statement for the notification SHOULD specify if and how the
   notification identifies any data node resources associated with the
   specific event.

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4.17.  Feature Definitions

   The YANG "feature" statement is used to define a label for a set of
   optional functionality within a module.  The "if-feature" statement
   is used in the YANG statements associated with a feature.  The
   description-stmt within a feature-stmt MUST specify any interactions
   with other features.

   The set of YANG features defined in a module should be considered
   carefully.  Very fine granular features increase interoperability
   complexity and should be avoided.  A likely misuse of the feature
   mechanism is the tagging of individual leafs (e.g., counters) with
   separate features.

   If there is a large set of objects associated with a YANG feature,
   then consider moving those objects to a separate module, instead of
   using a YANG feature.  Note that the set of features within a module
   is easily discovered by the reader, but the set of related modules
   within the entire YANG library is not as easy to identity.  Module
   names with a common prefix can help readers identity the set of
   related modules, but this assumes the reader will have discovered and
   installed all the relevant modules.

   Another consideration for deciding whether to create a new module or
   add a YANG feature is the stability of the module in question.  It
   may be desirable to have a stable base module that is not changed
   frequently.  If new functionality is placed in a separate module,
   then the base module does not need to be republished.  If it is
   designed as a YANG feature, then the module will need to be
   republished.

   If one feature requires implementation of another feature, then an
   "if-feature" statement SHOULD be used in the dependent "feature"
   statement.

   For example, feature2 requires implementation of feature1:

      feature feature1 {
        description "Some protocol feature";
      }

      feature feature2 {
        if-feature "feature1";
        description "Another protocol feature";
      }

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4.18.  YANG Data Node Constraints

4.18.1.  Controlling Quantity

   The "min-elements" and "max-elements" statements can be used to
   control how many list or leaf-list instances are required for a
   particular data node.  YANG constraint statements SHOULD be used to
   identify conditions that apply to all implementations of the data
   model.  If platform-specific limitations (e.g., the "max-elements"
   supported for a particular list) are relevant to operations, then a
   data model definition statement (e.g., "max-ports" leaf) SHOULD be
   used to identify the limit.

4.18.2.  "must" versus "when"

   "must" and "when" YANG statements are used to provide cross-object
   referential tests.  They have very different behavior.  The "when"
   statement causes data node instances to be silently deleted as soon
   as the condition becomes false.  A false "when" expression is not
   considered to be an error.

   The "when" statement SHOULD be used together with "augment" or "uses"
   statements to achieve conditional model composition.  The condition
   SHOULD be based on static properties of the augmented entry (e.g.,
   list key leafs).

   The "must" statement causes a datastore validation error if the
   condition is false.  This statement SHOULD be used for enforcing
   parameter value restrictions that involve more than one data node
   (e.g., end-time parameter must be after the start-time parameter).

4.19.  "augment" Statements

   The YANG "augment" statement is used to define a set of data
   definition statements that will be added as child nodes of a target
   data node.  The module namespace for these data nodes will be the
   augmenting module, not the augmented module.

   A top-level "augment" statement SHOULD NOT be used if the target data
   node is in the same module or submodule as the evaluated "augment"
   statement.  The data definition statements SHOULD be added inline
   instead.

4.19.1.  Conditional Augment Statements

   The "augment" statement is often used together with the "when"
   statement and/or "if-feature" statement to make the augmentation
   conditional on some portion of the data model.

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   The following example from [RFC7223] shows how a conditional
   container called "ethernet" is added to the "interface" list only for
   entries of the type "ethernetCsmacd".

        augment "/if:interfaces/if:interface" {
            when "if:type = 'ianaift:ethernetCsmacd'";

            container ethernet {
                leaf duplex {
                    ...
                }
            }
        }

4.19.2.  Conditionally Mandatory Data Definition Statements

   YANG has very specific rules about how configuration data can be
   updated in new releases of a module.  These rules allow an "old
   client" to continue interoperating with a "new server".

   If data nodes are added to an existing entry, the old client MUST NOT
   be required to provide any mandatory parameters that were not in the
   original module definition.

   It is possible to add conditional "augment" statements such that the
   old client would not know about the new condition and would not
   specify the new condition.  The conditional "augment" statement can
   contain mandatory objects only if the condition is false, unless
   explicitly requested by the client.

   Only a conditional "augment" statement that uses the "when" statement
   form of a condition can be used in this manner.  The YANG features
   enabled on the server cannot be controlled by the client in any way,
   so it is not safe to add mandatory augmenting data nodes based on the
   "if-feature" statement.

   The XPath "when" statement condition MUST NOT reference data outside
   of the target data node because the client does not have any control
   over this external data.

   In the following dummy example, it is okay to augment the "interface"
   entry with "mandatory-leaf" because the augmentation depends on
   support for "some-new-iftype".  The old client does not know about
   this type, so it would never select this type; therefore, it would
   not add a mandatory data node.

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     module example-module {

       yang-version 1.1;
       namespace "tag:example.com,2017:example-module";
       prefix mymod;

       import iana-if-type { prefix iana; }
       import ietf-interfaces { prefix if; }

       identity some-new-iftype {
          base iana:iana-interface-type;
       }

       augment "/if:interfaces/if:interface" {
          when "if:type = 'mymod:some-new-iftype'";

          leaf mandatory-leaf {
             type string;
             mandatory true;
          }
       }
     }

   Note that this practice is safe only for creating data resources.  It
   is not safe for replacing or modifying resources if the client does
   not know about the new condition.  The YANG data model MUST be
   packaged in a way that requires the client to be aware of the
   mandatory data nodes if it is aware of the condition for this data.
   In the example above, the "some-new-iftype" identity is defined in
   the same module as the "mandatory-leaf" data definition statement.

   This practice is not safe for identities defined in a common module
   such as "iana-if-type" because the client is not required to know
   about "my-module" just because it knows about the "iana-if-type"
   module.

4.20.  Deviation Statements

   Per RFC 7950, Section 7.20.3, the YANG "deviation" statement is not
   allowed to appear in IETF YANG modules, but it can be useful for
   documenting server capabilities.  Deviation statements are not
   reusable and typically not shared across all platforms.

   There are several reasons that deviations might be needed in an
   implementation, e.g., an object cannot be supported on all platforms,
   or feature delivery is done in multiple development phases.
   Deviation statements can also be used to add annotations to a module,
   which does not affect the conformance requirements for the module.

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   It is suggested that deviation statements be defined in separate
   modules from regular YANG definitions.  This allows the deviations to
   be platform specific and/or temporary.

   The order that deviation statements are evaluated can affect the
   result.  Therefore, multiple deviation statements in the same module,
   for the same target object, SHOULD NOT be used.

   The "max-elements" statement is intended to describe an architectural
   limit to the number of list entries.  It is not intended to describe
   platform limitations.  It is better to use a "deviation" statement
   for the platforms that have a hard resource limit.

   Example documenting platform resource limits:

     Wrong: (max-elements in the list itself)

        container backups {
          list backup {
             ...
             max-elements  10;
             ...
          }
        }

     Correct: (max-elements in a deviation)

        deviation /bk:backups/bk:backup {
          deviate add {
             max-elements  10;
          }
        }

4.21.  Extension Statements

   The YANG "extension" statement is used to specify external
   definitions.  This appears in the YANG syntax as an
   "unknown-statement".  Usage of extension statements in a published
   module needs to be considered carefully.

   The following guidelines apply to the usage of YANG extensions:

   o  The semantics of the extension MUST NOT contradict any YANG
      statements.  Extensions can add semantics not covered by the
      normal YANG statements.

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   o  The module containing the extension statement MUST clearly
      identify the conformance requirements for the extension.  It
      should be clear whether all implementations of the YANG module
      containing the extension need to also implement the extension.  If
      not, identify what conditions apply that would require
      implementation of the extension.

   o  The extension MUST clearly identify where it can be used within
      other YANG statements.

   o  The extension MUST clearly identify if YANG statements or other
      extensions are allowed or required within the extension as
      substatements.

4.22.  Data Correlation

   Data can be correlated in various ways, using common data types,
   common data naming, and common data organization.  There are several
   ways to extend the functionality of a module, based on the degree of
   coupling between the old and new functionality:

   o  inline: update the module with new protocol-accessible objects.
      The naming and data organization of the original objects is used.
      The new objects are in the original module namespace.

   o  augment: create a new module with new protocol-accessible objects
      that augment the original data structure.  The naming and data
      organization of the original objects is used.  The new objects are
      in the new module namespace.

   o  mirror: create new objects in a new module or the original module,
      except use a new naming scheme and data location.  The naming can
      be coupled in different ways.  Tight coupling is achieved with a
      "leafref" data type, with the "require-instance" substatement set
      to "true".  This method SHOULD be used.

   If the new data instances are not limited to the values in use in the
   original data structure, then the "require-instance" substatement
   MUST be set to "false".  Loose coupling is achieved by using key
   leafs with the same data type as the original data structure.  This
   has the same semantics as setting the "require-instance" substatement
   to "false".

   The relationship between configuration and operational state has been
   clarified in NMDA [RFC8342].

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4.22.1.  Use of "leafref" for Key Correlation

   Sometimes it is not practical to augment a data structure.  For
   example, the correlated data could have different keys or contain
   mandatory nodes.

   The following example shows the use of the "leafref" data type for
   data correlation purposes:

   Not preferred:

      list foo {
         key name;
         leaf name {
           type string;
         }
         ...
      }

      list foo-addon {
         key name;
         config false;
         leaf name {
           type string;
         }
         ...
      }

   Preferred:

      list foo {
         key name;
         leaf name {
           type string;
         }
         ...
      }

      list foo-addon {
         key name;
         config false;
         leaf name {
           type leafref {
             path "/foo/name";
             require-instance false;
           }
         }
         leaf addon {

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           type string;
           mandatory true;
         }
      }

4.23.  Operational State

   The modeling of operational state with YANG has been refined over
   time.  At first, only data that has a "config" statement value of
   "false" was considered to be operational state.  This data was not
   considered to be part of any datastore, which made the YANG XPath
   definition much more complicated.

   Operational state is now modeled using YANG according to the new NMDA
   [RFC8342] and conceptually contained in the operational state
   datastore, which also includes the operational values of
   configuration data.  There is no longer any need to duplicate data
   structures to provide separate configuration and operational state
   sections.

   This section describes some data modeling issues related to
   operational state and guidelines for transitioning YANG data model
   design to be NMDA compatible.

4.23.1.  Combining Operational State and Configuration Data

   If possible, operational state SHOULD be combined with its associated
   configuration data.  This prevents duplication of key leafs and
   ancestor nodes.  It also prevents race conditions for retrieval of
   dynamic entries and allows configuration and operational state to be
   retrieved together with minimal message overhead.

      container foo {
        ...
        // contains "config true" and "config false" nodes that have
        // no corresponding "config true" object (e.g., counters)
      }

4.23.2.  Representing Operational Values of Configuration Data

   If possible, the same data type SHOULD be used to represent the
   configured value and the operational value, for a given leaf or leaf-
   list object.

   Sometimes the configured value set is different than the operational
   value set for that object, for example, the "admin-status" and
   "oper-status" leafs in [RFC8343].  In this case, a separate object
   MAY be used to represent the configured and operational values.

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   Sometimes the list keys are not identical for configuration data and
   the corresponding operational state.  In this case, separate lists
   MAY be used to represent the configured and operational values.

   If it is not possible to combine configuration and operational state,
   then the keys used to represent list entries SHOULD be the same type.
   The "leafref" data type SHOULD be used in operational state for key
   leafs that have corresponding configuration instances.  The
   "require-instance" statement MAY be set to "false" (in YANG 1.1
   modules only) to indicate instances are allowed in the operational
   state that do not exist in the associated configuration data.

   The need to replicate objects or define different operational state
   objects depends on the data model.  It is not possible to define one
   approach that will be optimal for all data models.

   Designers SHOULD describe and justify any NMDA exceptions in detail,
   such as the use of separate subtrees and/or separate leafs.  The
   "description" statements for both the configuration and the
   operational state SHOULD be used for this purpose.

4.23.3.  NMDA Transition Guidelines

   YANG modules SHOULD be designed with the assumption that they will be
   used on servers supporting the operational state datastore.  With
   this in mind, YANG modules SHOULD define "config false" nodes
   wherever they make sense to the data model.  "Config false" nodes
   SHOULD NOT be defined to provide the operational value for
   configuration nodes, except when the value space of a configured and
   operational value may differ, in which case a distinct "config false"
   node SHOULD be defined to hold the operational value for the
   configured node.

   The following guidelines are meant to help modelers develop YANG
   modules that will maximize the utility of the model with both current
   and new implementations.

   New modules and modules that are not concerned with the operational
   state of configuration information SHOULD immediately be structured
   to be NMDA compatible, as described in Section 4.23.1.  This
   transition MAY be deferred if the module does not contain any
   configuration datastore objects.

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   The remaining are options that MAY be followed during the time that
   NMDA mechanisms are being defined.

   (a)  Modules that require immediate support for the NMDA features
        SHOULD be structured for NMDA.  A temporary non-NMDA version of
        this type of module MAY exist, as either an existing model or a
        model created by hand or with suitable tools that mirror the
        current modeling strategies.  Both the NMDA and the non-NMDA
        modules SHOULD be published in the same document, with NMDA
        modules in the document main body and the non-NMDA modules in a
        non-normative appendix.  The use of the non-NMDA module will
        allow temporary bridging of the time period until NMDA
        implementations are available.

   (b)  For published models, the model should be republished with an
        NMDA-compatible structure, deprecating non-NMDA constructs.  For
        example, the "ietf-interfaces" model in [RFC7223] has been
        restructured as an NMDA-compatible model in [RFC8343].  The
        "/interfaces-state" hierarchy has been marked "status
        deprecated".  Models that mark their "/foo-state" hierarchy with
        "status deprecated" will allow NMDA-capable implementations to
        avoid the cost of duplicating the state nodes, while enabling
        non-NMDA-capable implementations to utilize them for access to
        the operational values.

   (c)  For models that augment models that have not been structured
        with the NMDA, the modeler will have to consider the structure
        of the base model and the guidelines listed above.  Where
        possible, such models should move to new revisions of the base
        model that are NMDA compatible.  When that is not possible,
        augmenting "state" containers SHOULD be avoided, with the
        expectation that the base model will be re-released with the
        state containers marked as deprecated.  It is RECOMMENDED to
        augment only the "/foo" hierarchy of the base model.  Where this
        recommendation cannot be followed, then any new "state" elements
        SHOULD be included in their own module.

4.23.3.1.  Temporary Non-NMDA Modules

   A temporary non-NMDA module allows a non-NMDA-aware client to access
   operational state from an NMDA-compliant server.  It contains the
   top-level "config false" data nodes that would have been defined in a
   legacy YANG module (before NMDA).

   A server that needs to support both NMDA and non-NMDA clients can
   advertise both the new NMDA module and the temporary non-NMDA module.
   A non-NMDA client can use separate "foo" and "foo-state" subtrees,
   except the "foo-state" subtree is located in a different (temporary)

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   module.  The NMDA module can be used by a non-NMDA client to access
   the conventional configuration datastores and the deprecated <get>
   operation to access nested "config false" data nodes.

   To create the temporary non-NMDA model from an NMDA model, the
   following steps can be taken:

   o  Change the module name by appending "-state" to the original
      module name

   o  Change the namespace by appending "-state" to the original
      namespace value

   o  Change the prefix by appending "-s" to the original prefix value

   o  Add an import to the original module (e.g., for typedef
      definitions)

   o  Retain or create only the top-level nodes that have a "config"
      statement value "false".  These subtrees represent "config false"
      data nodes that were combined into the configuration subtree;
      therefore, they are not available to non-NMDA aware clients.  Set
      the "status" statement to "deprecated" for each new node.

   o  The module description SHOULD clearly identify the module as a
      temporary non-NMDA module

4.23.3.2.  Example: Create a New NMDA Module

   Create an NMDA-compliant module, using combined configuration and
   state subtrees, whenever possible.

     module example-foo {
       namespace "urn:example.com:params:xml:ns:yang:example-foo";
       prefix "foo";

       container foo {
         // configuration data child nodes
         // operational value in operational state datastore only
         // may contain "config false" nodes as needed
       }
    }

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4.23.3.3.  Example: Convert an Old Non-NMDA Module

   Do not remove non-compliant objects from existing modules.  Instead,
   change the status to "deprecated".  At some point, usually after 1
   year, the status MAY be changed to "obsolete".

   Old Module:

     module example-foo {
       namespace "urn:example.com:params:xml:ns:yang:example-foo";
       prefix "foo";

       container foo {
         // configuration data child nodes
       }

       container foo-state {
         config false;
         // operational state child nodes
       }
    }

   Converted NMDA Module:

     module example-foo {
       namespace "urn:example.com:params:xml:ns:yang:example-foo";
       prefix "foo";

       container foo {
         // configuration data child nodes
         // operational value in operational state datastore only
         // may contain "config false" nodes as needed
         // will contain any data nodes from old foo-state
       }

       // keep original foo-state but change status to deprecated
       container foo-state {
         config false;
         status deprecated;
         // operational state child nodes
       }
    }

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4.23.3.4.  Example: Create a Temporary NMDA Module

   Create a new module that contains the top-level operational state
   data nodes that would have been available before they were combined
   with configuration data nodes (to be NMDA compliant).

     module example-foo-state {
       namespace "urn:example.com:params:xml:ns:yang:example-foo-state";
       prefix "foo-s";

       // import new or converted module; not used in this example
       import example-foo { prefix foo; }

       container foo-state {
         config false;
         status deprecated;
         // operational state child nodes
       }
    }

4.24.  Performance Considerations

   It is generally likely that certain YANG statements require more
   runtime resources than other statements.  Although there are no
   performance requirements for YANG validation, the following
   information MAY be considered when designing YANG data models:

   o  Lists are generally more expensive than containers

   o  "when" statement evaluation is generally more expensive than
      "if-feature" or "choice" statements

   o  "must" statements are generally more expensive than "min-entries",
      "max-entries", "mandatory", or "unique" statements

   o  "identityref" leafs are generally more expensive than
      "enumeration" leafs

   o  "leafref" and "instance-identifier" types with "require-instance"
      set to true are generally more expensive than if
      "require-instance" is set to false

4.25.  Open Systems Considerations

   Only the modules imported by a particular module can be assumed to be
   present in an implementation.  An open system MAY include any
   combination of YANG modules.

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4.26.  Guidelines for Constructs Specific to YANG 1.1

   The set of guidelines for YANG 1.1 will grow as operational
   experience is gained with the new language features.  This section
   contains an initial set of guidelines for new YANG 1.1 language
   features.

4.26.1.  Importing Multiple Revisions

   Standard modules SHOULD NOT import multiple revisions of the same
   module into a module.  This MAY be done if independent definitions
   (e.g., enumeration typedefs) from specific revisions are needed in
   the importing module.

4.26.2.  Using Feature Logic

   The YANG 1.1 feature logic is much more expressive than YANG 1.0.  A
   "description" statement SHOULD describe the "if-feature" logic in
   text, to help readers understand the module.

   YANG features SHOULD be used instead of the "when" statement, if
   possible.  Features are advertised by the server, and objects
   conditional by the "if-feature" statement are conceptually grouped
   together.  There is no such commonality supported for "when"
   statements.

   Features generally require less server implementation complexity and
   runtime resources than objects that use "when" statements.  Features
   are generally static (i.e., set when a module is loaded and not
   changed at runtime).  However, every client edit might cause a "when"
   statement result to change.

4.26.3.  "anyxml" versus "anydata"

   The "anyxml" statement MUST NOT be used to represent a conceptual
   subtree of YANG data nodes.  The "anydata" statement MUST be used for
   this purpose.

4.26.4.  "action" versus "rpc"

   The use of "action" statements or "rpc" statements is a subjective
   design decision.  RPC operations are not associated with any
   particular data node.  Actions are associated with a specific data
   node definition.  An "action" statement SHOULD be used if the
   protocol operation is specific to a subset of all data nodes instead
   of all possible data nodes.

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   The same action name MAY be used in different definitions within
   different data node.  For example, a "reset" action defined with a
   data node definition for an interface might have different parameters
   than for a power supply or a VLAN.  The same action name SHOULD be
   used to represent similar semantics.

   The NETCONF Access Control Model (NACM) [RFC8341] does not support
   parameter-based access control for RPC operations.  The user is given
   permission (or not) to invoke the RPC operation with any parameters.
   For example, if each client is only allowed to reset their own
   interface, then NACM cannot be used.

   For example, NACM cannot enforce access control based on the value of
   the "interface" parameter, only the "reset" operation itself:

      rpc reset {
        input {
          leaf interface {
            type if:interface-ref;
            mandatory true;
            description "The interface to reset.";
          }
        }
      }

   However, NACM can enforce access control for individual interface
   instances, using a "reset" action.  If the user does not have read
   access to the specific "interface" instance, then it cannot invoke
   the "reset" action for that interface instance:

      container interfaces {
        list interface {
          ...
          action reset { }
        }
      }

4.27.  Updating YANG Modules (Published versus Unpublished)

   YANG modules can change over time.  Typically, new data model
   definitions are needed to support new features.  YANG update rules
   defined in Section 11 of [RFC7950] MUST be followed for published
   modules.  They MAY be followed for unpublished modules.

   The YANG update rules only apply to published module revisions.  Each
   organization will have their own way to identify published work that
   is considered to be stable and unpublished work that is considered to

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   be unstable.  For example, in the IETF, the RFC document is used for
   published work, and the I-D is used for unpublished work.

5.  IANA Considerations

   The following registration in the "ns" subregistry of the "IETF XML
   Registry" [RFC3688] was detailed in [RFC6087] and has been updated by
   IANA to reference this document.

       URI: urn:ietf:params:xml:ns:yang:ietf-template

       Registrant Contact: The IESG.

       XML: N/A, the requested URI is an XML namespace.

   The following assignment was detailed in [RFC6087] and has been
   updated by IANA in the "YANG Module Names" registry.  This document
   has also been added as a reference for the "YANG Module Names"
   registry itself as it contains the template necessary for
   registration in Appendix B.

         +-----------+-------------------------------------------+
         | Field     | Value                                     |
         +-----------+-------------------------------------------+
         | Name      | ietf-template                             |
         | Namespace | urn:ietf:params:xml:ns:yang:ietf-template |
         | Prefix    | temp                                      |
         | Reference | RFC 8407                                  |
         +-----------+-------------------------------------------+

                         YANG Registry Assignment

6.  Security Considerations

   This document defines documentation guidelines for NETCONF or
   RESTCONF content defined with the YANG data modeling language;
   therefore, it does not introduce any new or increased security risks
   into the management system.

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7.  References

7.1.  Normative References

   [ID-Guidelines]
              Housley, R., "Guidelines to Authors of Internet-Drafts",
              December 2010,
              <https://www.ietf.org/standards/ids/guidelines/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5378]  Bradner, S., Ed. and J. Contreras, Ed., "Rights
              Contributors Provide to the IETF Trust", BCP 78, RFC 5378,
              DOI 10.17487/RFC5378, November 2008,
              <https://www.rfc-editor.org/info/rfc5378>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

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   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [W3C.REC-xpath]
              Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", W3C Recommendation REC-xpath-19991116,
              November 1999,
              <http://www.w3.org/TR/1999/REC-xpath-19991116>.

7.2.  Informative References

   [IANA-MOD-NAMES]
              IANA, "YANG Module Names",
              <https://www.iana.org/assignments/yang-parameters/>.

   [IANA-XML] IANA, "IETF XML Registry",
              <https://www.iana.org/assignments/xml-registry/>.

   [RFC-STYLE]
              RFC Editor, "Style Guide",
              <http://www.rfc-editor.org/styleguide/>.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,
              <https://www.rfc-editor.org/info/rfc4151>.

   [RFC4181]  Heard, C., Ed., "Guidelines for Authors and Reviewers of
              MIB Documents", BCP 111, RFC 4181, DOI 10.17487/RFC4181,
              September 2005, <https://www.rfc-editor.org/info/rfc4181>.

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   [RFC6087]  Bierman, A., "Guidelines for Authors and Reviewers of YANG
              Data Model Documents", RFC 6087, DOI 10.17487/RFC6087,
              January 2011, <https://www.rfc-editor.org/info/rfc6087>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <https://www.rfc-editor.org/info/rfc7223>.

   [RFC7322]  Flanagan, H. and S. Ginoza, "RFC Style Guide", RFC 7322,
              DOI 10.17487/RFC7322, September 2014,
              <https://www.rfc-editor.org/info/rfc7322>.

   [RFC7841]  Halpern, J., Ed., Daigle, L., Ed., and O. Kolkman, Ed.,
              "RFC Streams, Headers, and Boilerplates", RFC 7841,
              DOI 10.17487/RFC7841, May 2016,
              <https://www.rfc-editor.org/info/rfc7841>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [RFC8349]  Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
              Routing Management (NMDA Version)", RFC 8349,
              DOI 10.17487/RFC8349, March 2018,
              <https://www.rfc-editor.org/info/rfc8349>.

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Appendix A.  Module Review Checklist

   This section is adapted from RFC 4181.

   The purpose of a YANG module review is to review the YANG module for
   both technical correctness and adherence to IETF documentation
   requirements.  The following checklist may be helpful when reviewing
   an I-D:

   o  I-D Boilerplate -- verify that the document contains the required
      I-D boilerplate (see <https://www.ietf.org/id-info/
      guidelines.html>), including the appropriate statement to permit
      publication as an RFC, and that the I-D boilerplate does not
      contain references or section numbers.

   o  Abstract -- verify that the abstract does not contain references,
      that it does not have a section number, and that its content
      follows the guidelines in <https://www.ietf.org/id-info/
      guidelines.html>.

   o  Copyright Notice -- verify that the document has the appropriate
      text regarding the rights that document contributors provide to
      the IETF Trust [RFC5378].  Verify that it contains the full IETF
      Trust copyright notice at the beginning of the document.  The IETF
      Trust Legal Provisions (TLP) can be found at:

      <https://trustee.ietf.org/license-info/>

   o  Security Considerations section -- verify that the document uses
      the latest approved template from the Operations and Management
      (OPS) area website (see <https://trac.ietf.org/area/ops/trac/wiki/
      yang-security-guidelines>) and that the guidelines therein have
      been followed.

   o  IANA Considerations section -- this section must always be
      present.  For each module within the document, ensure that the
      IANA Considerations section contains entries for the following
      IANA registries:

         XML Namespace Registry: Register the YANG module namespace.

         YANG Module Registry: Register the YANG module name, prefix,
         namespace, and RFC number, according to the rules specified in
         [RFC6020].

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   o  References -- verify that the references are properly divided
      between normative and informative references, that RFCs 2119 and
      8174 are included as normative references if the terminology
      defined therein is used in the document, that all references
      required by the boilerplate are present, that all YANG modules
      containing imported items are cited as normative references, and
      that all citations point to the most current RFCs, unless there is
      a valid reason to do otherwise (for example, it is okay to include
      an informative reference to a previous version of a specification
      to help explain a feature included for backward compatibility).
      Be sure citations for all imported modules are present somewhere
      in the document text (outside the YANG module).  If a YANG module
      contains reference or "description" statements that refer to an
      I-D, then the I-D is included as an informative reference.

   o  License -- verify that the document contains the Simplified BSD
      License in each YANG module or submodule.  Some guidelines related
      to this requirement are described in Section 3.1.  Make sure that
      the correct year is used in all copyright dates.  Use the approved
      text from the latest TLP document, which can be found at:

      <https://trustee.ietf.org/license-info/>

   o  Other Issues -- check for any issues mentioned in
      <https://www.ietf.org/id-info/checklist.html> that are not covered
      elsewhere.

   o  Technical Content -- review the actual technical content for
      compliance with the guidelines in this document.  The use of a
      YANG module compiler is recommended when checking for syntax
      errors.  A list of freely available tools and other information,
      including formatting advice, can be found at:

      <https://trac.ietf.org/trac/netconf/wiki>
       and
      <https://trac.ietf.org/trac/netmod/wiki>

      Checking for correct syntax, however, is only part of the job.
      It is just as important to actually read the YANG module document
      from the point of view of a potential implementor.  It is
      particularly important to check that "description" statements are
      sufficiently clear and unambiguous to allow interoperable
      implementations to be created.

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Appendix B.  YANG Module Template

   <CODE BEGINS> file "ietf-template@2016-03-20.yang"

   module ietf-template {
     yang-version 1.1;

     // replace this string with a unique namespace URN value

     namespace "urn:ietf:params:xml:ns:yang:ietf-template";

     // replace this string, and try to pick a unique prefix

     prefix temp;

     // import statements here: e.g.,
     // import ietf-yang-types { prefix yang; }
     // import ietf-inet-types { prefix inet; }
     // identify the IETF working group if applicable

     organization
       "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

     // update this contact statement with your info

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/your-wg-name/>
        WG List:  <mailto:your-wg-name@ietf.org>

        Editor:   your-name
                  <mailto:your-email@example.com>";

     // replace the first sentence in this description statement.
     // replace the copyright notice with the most recent
     // version, if it has been updated since the publication
     // of this document

     description
       "This module defines a template for other YANG modules.

        Copyright (c) <insert year> IETF Trust and the persons
        identified as authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Simplified BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions

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        Relating to IETF Documents
        (http://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see
        the RFC itself for full legal notices.";

     // RFC Ed.: replace XXXX with actual RFC number and remove
     // this note

     // replace '2016-03-20' with the module publication date
     // the format is (year-month-day)

     revision 2016-03-20 {
       description
         "what changed in this revision";
       reference "RFC XXXX: <Replace With Document Title>";
     }

     // extension statements
     // feature statements
     // identity statements
     // typedef statements
     // grouping statements
     // data definition statements
     // augment statements
     // rpc statements
     // notification statements
     // DO NOT put deviation statements in a published module
   }

   <CODE ENDS>

Acknowledgments

   The structure and contents of this document are adapted from
   "Guidelines for Authors and Reviewers of MIB Documents" [RFC4181], by
   C. M. Heard.

   The working group thanks Martin Bjorklund, Juergen Schoenwaelder,
   Ladislav Lhotka, Jernej Tuljak, Lou Berger, Robert Wilton, Kent
   Watsen, and William Lupton for their extensive reviews and
   contributions to this document.

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Author's Address

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com

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