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rfc:rfc8407

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.

Bierman Best Current Practice [Page 1] RFC 8407 Guidelines for YANG Documents October 2018

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

Bierman Best Current Practice [Page 2] RFC 8407 Guidelines for YANG Documents October 2018

     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

Bierman Best Current Practice [Page 3] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 4] RFC 8407 Guidelines for YANG Documents October 2018

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

Bierman Best Current Practice [Page 5] RFC 8407 Guidelines for YANG Documents October 2018

 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".

Bierman Best Current Practice [Page 6] RFC 8407 Guidelines for YANG Documents October 2018

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

Bierman Best Current Practice [Page 7] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 8] RFC 8407 Guidelines for YANG Documents October 2018

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".

Bierman Best Current Practice [Page 9] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 10] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 11] RFC 8407 Guidelines for YANG Documents October 2018

 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.
  1. - if you have any writable data nodes (those are all the
  2. - "config true" nodes, and remember, that is the default)
  3. - 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)

Bierman Best Current Practice [Page 12] RFC 8407 Guidelines for YANG Documents October 2018

 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>
  1. - for all YANG modules you must evaluate whether any readable data
  2. - nodes (those are all the "config false" nodes, but also all other
  3. - nodes, because they can also be read via operations like get or
  4. - get-config) are sensitive or vulnerable (for instance, if they
  5. - might reveal customer information or violate personal privacy
  6. - laws such as those of the European Union if exposed to
  7. - 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>
  1. - if your YANG module has defined any RPC operations
  2. - 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.

Bierman Best Current Practice [Page 13] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 14] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 15] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 16] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 17] RFC 8407 Guidelines for YANG Documents October 2018

 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

Bierman Best Current Practice [Page 18] RFC 8407 Guidelines for YANG Documents October 2018

 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).

Bierman Best Current Practice [Page 19] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 20] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 21] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 22] RFC 8407 Guidelines for YANG Documents October 2018

 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
    }

Bierman Best Current Practice [Page 23] RFC 8407 Guidelines for YANG Documents October 2018

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;
      }

Bierman Best Current Practice [Page 24] RFC 8407 Guidelines for YANG Documents October 2018

      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.

Bierman Best Current Practice [Page 25] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 26] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 27] RFC 8407 Guidelines for YANG Documents October 2018

 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>

Bierman Best Current Practice [Page 28] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 29] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 30] RFC 8407 Guidelines for YANG Documents October 2018

     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";
     }

Bierman Best Current Practice [Page 32] RFC 8407 Guidelines for YANG Documents October 2018

     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.

Bierman Best Current Practice [Page 33] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 34] RFC 8407 Guidelines for YANG Documents October 2018

 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

Bierman Best Current Practice [Page 36] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 37] RFC 8407 Guidelines for YANG Documents October 2018

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

Bierman Best Current Practice [Page 38] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 39] RFC 8407 Guidelines for YANG Documents October 2018

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";
    }

Bierman Best Current Practice [Page 40] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 42] RFC 8407 Guidelines for YANG Documents October 2018

   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.

Bierman Best Current Practice [Page 43] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 44] RFC 8407 Guidelines for YANG Documents October 2018

 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].

Bierman Best Current Practice [Page 45] RFC 8407 Guidelines for YANG Documents October 2018

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 {

Bierman Best Current Practice [Page 46] RFC 8407 Guidelines for YANG Documents October 2018

         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.

Bierman Best Current Practice [Page 47] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 48] RFC 8407 Guidelines for YANG Documents October 2018

 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)

Bierman Best Current Practice [Page 49] RFC 8407 Guidelines for YANG Documents October 2018

 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
     }
  }

Bierman Best Current Practice [Page 50] RFC 8407 Guidelines for YANG Documents October 2018

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
     }
  }

Bierman Best Current Practice [Page 51] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 52] RFC 8407 Guidelines for YANG Documents October 2018

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.

Bierman Best Current Practice [Page 53] RFC 8407 Guidelines for YANG Documents October 2018

 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

Bierman Best Current Practice [Page 54] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 55] RFC 8407 Guidelines for YANG Documents October 2018

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>.

Bierman Best Current Practice [Page 56] RFC 8407 Guidelines for YANG Documents October 2018

 [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>.

Bierman Best Current Practice [Page 57] RFC 8407 Guidelines for YANG Documents October 2018

 [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>.

Bierman Best Current Practice [Page 58] RFC 8407 Guidelines for YANG Documents October 2018

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].

Bierman Best Current Practice [Page 59] RFC 8407 Guidelines for YANG Documents October 2018

 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.

Bierman Best Current Practice [Page 60] RFC 8407 Guidelines for YANG Documents October 2018

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

Bierman Best Current Practice [Page 61] RFC 8407 Guidelines for YANG Documents October 2018

      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.

Bierman Best Current Practice [Page 62] RFC 8407 Guidelines for YANG Documents October 2018

Author's Address

 Andy Bierman
 YumaWorks
 Email: andy@yumaworks.com

Bierman Best Current Practice [Page 63]

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