Guidelines for Authors and Reviewers of YANG Data Model Documents
draft-ietf-netmod-rfc6087bis-13

Versions: (draft-bierman-netmod-rfc6087bis)   00           Informational
          01 02 03 04 05 06 07 08 09 10 11 12 13                        
Network Working Group                                         A. Bierman
Internet-Draft                                                 YumaWorks
Obsoletes: 6087 (if approved)                              June 18, 2017
Intended status: Informational
Expires: December 20, 2017


   Guidelines for Authors and Reviewers of YANG Data Model Documents
                    draft-ietf-netmod-rfc6087bis-13

Abstract

   This memo provides guidelines for authors and reviewers of Standards
   Track specifications containing YANG data model modules.  Applicable
   portions may be used as a basis for reviews of other YANG data model
   documents.  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 data model modules.  This document
   obsoletes RFC 6087.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 20, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  6
     2.2.  NETCONF Terms  . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  YANG Terms . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Terms  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.4.1.  YANG Tree Diagrams . . . . . . . . . . . . . . . . . .  7
   3.  General Documentation Guidelines . . . . . . . . . . . . . . .  8
     3.1.  Module Copyright . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Code Components  . . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Example Modules  . . . . . . . . . . . . . . . . . . .  9
     3.3.  Terminology Section  . . . . . . . . . . . . . . . . . . .  9
     3.4.  Tree Diagrams  . . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  Narrative Sections . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Definitions Section  . . . . . . . . . . . . . . . . . . . 11
     3.7.  Security Considerations Section  . . . . . . . . . . . . . 11
     3.8.  IANA Considerations Section  . . . . . . . . . . . . . . . 12
       3.8.1.  Documents that Create a New Namespace  . . . . . . . . 12
       3.8.2.  Documents that Extend an Existing Namespace  . . . . . 12
     3.9.  Reference Sections . . . . . . . . . . . . . . . . . . . . 12
     3.10. Validation Tools . . . . . . . . . . . . . . . . . . . . . 13
     3.11. Module Extraction Tools  . . . . . . . . . . . . . . . . . 13
     3.12. Module Usage Examples  . . . . . . . . . . . . . . . . . . 13
   4.  YANG Usage Guidelines  . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Module Naming Conventions  . . . . . . . . . . . . . . . . 14
     4.2.  Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.3.  Identifiers  . . . . . . . . . . . . . . . . . . . . . . . 16
       4.3.1.  Identifier Naming Conventions  . . . . . . . . . . . . 16
     4.4.  Defaults . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.5.  Conditional Statements . . . . . . . . . . . . . . . . . . 17
     4.6.  XPath Usage  . . . . . . . . . . . . . . . . . . . . . . . 18
       4.6.1.  XPath Evaluation Contexts  . . . . . . . . . . . . . . 18
       4.6.2.  Function Library . . . . . . . . . . . . . . . . . . . 19
       4.6.3.  Axes . . . . . . . . . . . . . . . . . . . . . . . . . 20
       4.6.4.  Types  . . . . . . . . . . . . . . . . . . . . . . . . 20
       4.6.5.  Wildcards  . . . . . . . . . . . . . . . . . . . . . . 21
       4.6.6.  Boolean Expressions  . . . . . . . . . . . . . . . . . 21
     4.7.  Lifecycle Management . . . . . . . . . . . . . . . . . . . 22
     4.8.  Module Header, Meta, and Revision Statements . . . . . . . 23
     4.9.  Namespace Assignments  . . . . . . . . . . . . . . . . . . 24



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     4.10. Top-Level Data Definitions . . . . . . . . . . . . . . . . 25
     4.11. Data Types . . . . . . . . . . . . . . . . . . . . . . . . 26
       4.11.1. Fixed Value Extensibility  . . . . . . . . . . . . . . 26
       4.11.2. Patterns and Ranges  . . . . . . . . . . . . . . . . . 27
       4.11.3. Enumerations and Bits  . . . . . . . . . . . . . . . . 28
       4.11.4. Union Types  . . . . . . . . . . . . . . . . . . . . . 29
       4.11.5. Empty and Boolean  . . . . . . . . . . . . . . . . . . 30
     4.12. Reusable Type Definitions  . . . . . . . . . . . . . . . . 31
     4.13. Reusable Groupings . . . . . . . . . . . . . . . . . . . . 32
     4.14. Data Definitions . . . . . . . . . . . . . . . . . . . . . 32
       4.14.1. Non-Presence Containers  . . . . . . . . . . . . . . . 34
       4.14.2. Top-Level Data Nodes . . . . . . . . . . . . . . . . . 34
     4.15. Operation Definitions  . . . . . . . . . . . . . . . . . . 35
     4.16. Notification Definitions . . . . . . . . . . . . . . . . . 35
     4.17. Feature Definitions  . . . . . . . . . . . . . . . . . . . 36
     4.18. YANG Data Node Constraints . . . . . . . . . . . . . . . . 36
       4.18.1. Controlling Quantity . . . . . . . . . . . . . . . . . 36
       4.18.2. must vs. when  . . . . . . . . . . . . . . . . . . . . 37
     4.19. Augment Statements . . . . . . . . . . . . . . . . . . . . 37
       4.19.1. Conditional Augment Statements . . . . . . . . . . . . 37
       4.19.2. Conditionally Mandatory Data Definition Statements . . 38
     4.20. Deviation Statements . . . . . . . . . . . . . . . . . . . 39
     4.21. Extension Statements . . . . . . . . . . . . . . . . . . . 40
     4.22. Data Correlation . . . . . . . . . . . . . . . . . . . . . 41
       4.22.1. Use of Leafref for Key Correlation . . . . . . . . . . 42
     4.23. Operational Data . . . . . . . . . . . . . . . . . . . . . 43
     4.24. Performance Considerations . . . . . . . . . . . . . . . . 43
     4.25. Open Systems Considerations  . . . . . . . . . . . . . . . 44
     4.26. YANG 1.1 Guidelines  . . . . . . . . . . . . . . . . . . . 44
       4.26.1. Importing Multiple Revisions . . . . . . . . . . . . . 44
       4.26.2. Using Feature Logic  . . . . . . . . . . . . . . . . . 44
       4.26.3. anyxml vs. anydata . . . . . . . . . . . . . . . . . . 45
       4.26.4. action vs. rpc . . . . . . . . . . . . . . . . . . . . 45
     4.27. Updating YANG Modules (Published vs. Unpublished)  . . . . 46
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 47
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 48
     6.1.  Security Considerations Section Template . . . . . . . . . 48
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 50
   8.  Changes Since RFC 6087 . . . . . . . . . . . . . . . . . . . . 51
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 53
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 53
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 56
     A.1.  v12 to v13 . . . . . . . . . . . . . . . . . . . . . . . . 56
     A.2.  v11 to v12 . . . . . . . . . . . . . . . . . . . . . . . . 56
     A.3.  v10 to v11 . . . . . . . . . . . . . . . . . . . . . . . . 56
     A.4.  v09 to v10 . . . . . . . . . . . . . . . . . . . . . . . . 56
     A.5.  v08 to v09 . . . . . . . . . . . . . . . . . . . . . . . . 56



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     A.6.  v07 to v08 . . . . . . . . . . . . . . . . . . . . . . . . 57
     A.7.  v06 to v07 . . . . . . . . . . . . . . . . . . . . . . . . 57
     A.8.  v05 to v06 . . . . . . . . . . . . . . . . . . . . . . . . 57
     A.9.  v04 to v05 . . . . . . . . . . . . . . . . . . . . . . . . 58
     A.10. v03 ot v04 . . . . . . . . . . . . . . . . . . . . . . . . 58
     A.11. v02 to v03 . . . . . . . . . . . . . . . . . . . . . . . . 58
     A.12. v01 to v02 . . . . . . . . . . . . . . . . . . . . . . . . 59
     A.13. v00 to v01 . . . . . . . . . . . . . . . . . . . . . . . . 59
   Appendix B.  Module Review Checklist . . . . . . . . . . . . . . . 60
   Appendix C.  YANG Module Template  . . . . . . . . . . . . . . . . 62
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 64








































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1.  Introduction

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

   This document defines a set of usage guidelines for Standards Track
   documents containing [RFC7950] data models.  YANG is used to define
   the data structures, protocol operations, and notification content
   used within a NETCONF and/or RESTCONF server.  A 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.

   This document is similar to the Structure of Management Information
   version 2 (SMIv2) usage guidelines specification [RFC4181] in intent
   and structure.  However, since that document was written a decade
   after SMIv2 modules had been in use, it was published as a 'Best
   Current Practice' (BCP).  This document is not a BCP, but rather an
   informational reference, intended to promote consistency in documents
   containing YANG modules.

   Many YANG constructs are defined as optional to use, such as the
   description statement.  However, in order to maximize
   interoperability of NETCONF and RESTCONF implementations utilizing
   YANG data models, it is desirable to define a set of usage guidelines
   that may require a higher level of compliance than the minimum level
   defined in the YANG specification.

   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 using this
   document.




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2.  Terminology

2.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   RFC 2119 language is used here to express the views of the NETMOD
   working group regarding content for YANG modules.  YANG modules
   complying with this document will treat the RFC 2119 terminology as
   if it were describing best current practices.

2.2.  NETCONF Terms

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

   o  capabilities

   o  client

   o  operation

   o  server

2.3.  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.



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2.4.  Terms

   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.

2.4.1.  YANG Tree Diagrams

   A simplified graphical representation of the data model is used in
   this document.  The meaning of the symbols in these diagrams is
   defined in [I-D.ietf-netmod-yang-tree-diagrams].


























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3.  General Documentation Guidelines

   YANG data model modules under review are likely to be contained in
   Internet-Drafts.  All guidelines for Internet-Draft authors MUST be
   followed.  The RFC Editor provides guidelines for authors of RFCs,
   which are first published as Internet-Drafts.  These guidelines
   should be followed and are defined in [RFC7322] and updated in
   [RFC7841] and "RFC Document Style" [RFC-STYLE].

   The following sections MUST be present in an Internet-Draft
   containing a module:

   o  Narrative sections

   o  Definitions section

   o  Security Considerations section

   o  IANA Considerations section

   o  References section

   There are three usage scenarios for YANG that can appear in an
   Internet-Draft 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 complete
   module or submodule, but may be applicable to example modules and
   YANG fragments as well.

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:

       http://trustee.ietf.org/license-info/

3.2.  Code Components

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



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   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 following
   example is for the '2010-01-18' 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";
         }
         // ... 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".

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.

   If YANG tree diagrams are used, then a sub-section explaining the
   YANG tree diagram syntax MUST be present, containing the following
   text:

     A simplified graphical representation of the data model is used in
     this document.  The meaning of the symbols in these diagrams is
     defined in [I-D.ietf-netmod-yang-tree-diagrams].







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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.  Tree diagrams MAY be split into sections to correspond to
   document structure.

   The following example shows a simple YANG tree diagram:

      +--rw top-level-config-container
      |  +--rw config-list* [key-name]
      |     +--rw key-name                    string
      |     +--rw optional-parm?              string
      |     +--rw mandatory-parm              identityref
      |     +--ro read-only-leaf              string
      +--ro top-level-nonconfig-container
         +--ro nonconfig-list* [name]
            +--ro name               string
            +--ro type               string

   The 'pyang' compiler can be used to produce the tree diagram, using
   the '-f tree' command line parameter.

   If the YANG module is comprised of groupings only, then the tree
   diagram SHOULD contain the groupings.  The 'pyang' compiler can be
   used to produce a tree diagram with groupings using the '-f tree
   --tree-print-groupings" command line parameters.

   If the YANG module contains notifications, then the tree diagram
   SHOULD contain the notifications.  If the YANG module contains RPC
   statements, then the tree diagram SHOULD contain the RPC statements.

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(s) defined by the specification imports definitions
   from other modules (except for those defined in the [RFC7950] or
   [RFC6991] documents), or are always implemented in conjunction with
   other modules, then those facts MUST be noted in the overview
   section, as MUST be noted any special interpretations of definitions
   in other modules.



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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.

   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 SMIv2, which are not affected by these guidelines.

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

   Example YANG modules MUST NOT contain any normative text, including
   any reserved words from [RFC2119].

   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.

   This section MUST be patterned after the latest approved template
   (available at http://trac.tools.ietf.org/area/ops/trac/wiki/
   yang-security-guidelines).  Section 6.1 contains the security
   considerations template dated 2013-05-08.  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



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      concerns MUST be explicitly listed by name and the reasons for the
      sensitivity/privacy concerns MUST be explained.

3.8.  IANA Considerations Section

   In order to comply with IESG policy as set forth in
   http://www.ietf.org/id-info/checklist.html, every Internet-Draft 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 stating that there are no actions is removed
   by the RFC Editor before publication.  Refer to the guidelines in
   [RFC5226] for more details.

   Each normative YANG module MUST be registered in the XML namespace
   Registry [RFC3688], and the YANG Module Names Registry [RFC6020].
   This applies to new modules and updated modules.  Examples of these
   registrations for the "ietf-template" module can be found in
   Section 5.

3.8.1.  Documents that Create a New Namespace

   If an Internet-Draft 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 YANG
   Namespace registry entry MUST be requested from the IANA.  The
   [RFC7950] specification includes the procedure for this purpose in
   its IANA Considerations section.

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.  Reference Sections

   For every import or include statement that appears in a module
   contained in the specification, which 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



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   used within the specification.

   For every normative reference statement that appears in a module
   contained in the specification, which 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, which
   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 Internet
   Draft.  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.

   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.

3.11.  Module Extraction Tools

   A version of 'rfcstrip' is available which will extract YANG modules
   from an Internet Draft or RFC.  The 'rfcstrip' tool which supports
   YANG module extraction is freely available:

     http://www.yang-central.org/twiki/pub/Main/YangTools/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.

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).






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4.  YANG Usage Guidelines

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

   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 acronym (e.g., protocol name or working group
   acronym) 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 acronym 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 representing 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
   a new module, not a name change.







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4.2.  Prefixes

   All YANG definitions are scoped by the module containing the
   definition being referenced.  This allows definitions from multiple
   modules to be used, even if the names are not unique.  In the example
   below, the identifier "foo" is used in all 3 modules:

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

         container foo;
       }

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

         typedef foo { type uint32; }
       }

       module example-one {
         namespace "http://example.com/ns/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 allowed 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.

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



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   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 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 13
   of [RFC7950].

4.3.1.  Identifier Naming Conventions

   Identifiers SHOULD follow a consistent naming pattern throughout the
   module.  Only lower-case letters, numbers, and dashes SHOULD be used
   in identifier names.  Upper-case characters and the underscore
   character MAY be used if the identifier represents a well-known value
   that uses these characters.

   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 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
   modelling properties.  Only YANG statements and YANG extension
   statements are designed to convey machine readable data modelling
   properties.  For example, naming an object "config" or "state" does
   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.




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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 to indicate that the NETCONF or RESTCONF
   capability is supported within the data model.

   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).

   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 leaf that do not also apply to the parent list node.



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   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
   [W3C.REC-xpath-19991116] (XPath) within YANG modules.

4.6.1.  XPath Evaluation Contexts

   YANG defines 5 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.

   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 OK because when-stmt context is this notification
           when "/if:interfaces/if:interface[name='eth0']";
           type leafref {
             // OK because path-stmt has a different context
             path "/if:interfaces/if:interface/if:mtu";
           }
         }
       }



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   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.

   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 defining the must or when expression
   containing the 'local-name' function.  Example of a local-name
   function that should not be used:

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







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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, '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 used,
   however they MAY be used if document order is not relevant to the
   outcome of the expression.

   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



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   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
      }

4.6.5.  Wildcards

   It is possible to construct XPath expressions that will evaluate
   differently when combined with several modules within a server
   implementation, then 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 will 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



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   to "one" or "three":

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

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

        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.  Lifecycle Management

   The status statement MUST be present 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 '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



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   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.

   If submodules are used, then the document containing the main module
   MUST be updated so that the main module revision date is equal 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 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 working
   group web and mailing information MUST 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.

   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.

   If the module relies on information contained in other documents,



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   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.

   It is not required to keep the full revision history of draft
   versions (e.g., modules contained within Internet-Drafts).  That is,
   within a sequence of draft versions, only the most recent revision
   need be recorded in the module.  However, whenever a new (i.e.
   changed) version is made available (e.g., via a new version of an
   Internet-Draft), the revision date of that new version MUST be
   updated to a date later than that of the previous version.

4.9.  Namespace Assignments

   It is RECOMMENDED that only valid YANG modules be included in
   documents, whether or not they 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 Registry MUST NOT be used.

   A standard namespace statement value SHOULD have the following form:

       <URN prefix string>:<module-name>

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

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



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   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 non-
   Standards-Track modules.  The string SHOULD be selected according to
   the guidelines in [RFC7950].

   The following URIs exemplify what might be used by non Standards
   Track modules.  Note that the domain "example.com" SHOULD be used by
   example modules in IETF drafts.

   Example URIs using URLs per RFC 3986 [RFC3986]:

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

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

   Example URIs using tags per RFC 4151 [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 data 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 data was
   created.

   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, is becomes difficult
   to retrieve all at once.

   The names and data organization SHOULD reflect persistent



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   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, and therefore
   few requirements can be specified on that subject.

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

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

4.11.1.  Fixed Value Extensibility

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

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

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










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

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

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

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

4.11.2.  Patterns and Ranges

   For string data types, if a machine-readable pattern can be defined
   for the desired semantics, then one or more pattern statements SHOULD
   be present.  A single quoted string SHOULD be used to specify the
   pattern, since a double-quoted string can modify the content.

   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.

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







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       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.























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

       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:




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      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.

   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



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   implemented.  The client may not have a complete and accurate schema
   for the data returned by the server, and 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.

   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.




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   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 it is
   defined, but happens to be exportable to another module because it is
   defined at the top-level of the YANG module.

   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 5 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" sub-statement on a leaf or choice
      unless the value applies on all possible contexts.

   o  Do not include a "config" sub-statement 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 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





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   o  container

   o  extension

   o  feature

   o  grouping

   o  identity

   o  leaf

   o  leaf-list

   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 '&lt;b&gt;' and '&lt;/b&gt;', 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 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.



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   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.

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., "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 sub-directories in a file system.  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

   -top-level siblings are not ordered -top-level siblings not are not
   static, and depends on the modules that are loaded





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

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

4.15.  Operation Definitions

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

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

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

4.16.  Notification Definitions

   The description statement MUST be present.

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

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

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

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






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4.17.  Feature Definitions

   The YANG "feature" statement is used to define a label for a set of
   optional functionality within a module.  The "if-feature" statement
   is used in the YANG statements associated with a feature.

   The set of YANG features available in a module should be considered
   carefully.  The description-stmt within a feature-stmt MUST specify
   any interactions with other 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";
      }

4.18.  YANG Data Node Constraints

4.18.1.  Controlling Quantity

   The "min-elements" and "max-elements" statements can be use to
   control how many list or leaf-list instances are required for a



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   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 vs. when

   The "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 the "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.

   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".





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        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 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 target data node because the client does not have any control over
   this external data.

   In the following dummy example, it is OK 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, and therefore not be
   adding a mandatory data node.










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     module example-module {
       namespace "http://example.com/ns/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 {
             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

   The YANG "deviation" statement cannot 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.

   It is suggested that deviation statements be defined in separate



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   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.

   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



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      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 sub-
      statements.

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 new a 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" sub-statement 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" sub-statement
   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" sub-
   statement to "false".

   It is sometimes useful to separate configuration and operational
   data, so that they do not not even share the exact same naming
   characteristics.  The correlation between configuration the
   operational data that is affected by changes in configuration is a
   complex problem.  There may not be a simple 1:1 relationship between
   a configuration data node and an operational data node.  Further work
   is needed in YANG to clarify this relationship.  Protocol work may
   also be needed to allow a client to retrieve this type of information



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   from a server.  At this time the best practice is to clearly document
   any relationship to other data structures in the "description"
   statement.

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:


















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      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 {
           type string;
           mandatory true;
         }
      }

4.23.  Operational Data

   In YANG, any data that has a "config" statement value of "false"
   could be considered operational data.  The relationship between
   configuration (i.e., "config" statement has a value of "true") and
   operational data can be complex.

   The original set of datastores defined in NETCONF (i.e., candidate,
   running, and startup) are not sufficient to fully manage a device
   with multiple sources of configuration data.  In additional, a
   separate datastore is needed to store operational state and other
   data such as statistics.  Refer to
   [I-D.ietf-netmod-revised-datastores] for details on this new "revised
   datastore" architecture.  Guidelines for usage of the new datastores
   (including the operational datastore) is defined in
   [I-D.dsdt-nmda-guidelines].

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:





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   o  Lists are generally more expensive than containers

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

   o  "must" statement is 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

   A YANG module MUST NOT be designed such that the set of modules found
   on a server implementation can be predetermined in advance.  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.

4.26.  YANG 1.1 Guidelines

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

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 the authors can
   demonstrate that the "avoided" definitions from the most recent of
   the multiple revisions are somehow broken or harmful to
   interoperability.

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 if-feature are conceptually grouped together.  There
   is no such commonality supported for "when" statements.




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

4.26.3.  anyxml vs. 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 vs. 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.

   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) [RFC6536] does not support
   parameter 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 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 access control for individual



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   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 vs. 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 which
   is considered to be stable, and unpublished work which is considered
   to be unstable.  For example, in the IETF, the RFC document is used
   for published work, and the Internet-Draft is used for unpublished
   work.



























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5.  IANA Considerations

     -- RFC Ed: These registries need to be updated to reference this
        RFC instead of RFC 6087 for the ietf-template module, and
        remove this note.

   This document registers one URI in the IETF XML registry [RFC3688].

   The following registration has been made in [RFC6087] and updated by
   this document.

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

       Registrant Contact: The NETMOD WG of the IETF.

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

   The following assignment has been made in [RFC6087] and updated by
   this document in the YANG Module Names Registry, or the YANG module
   template in Appendix C.

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

                         YANG Registry Assignment




















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6.  Security Considerations

   This document defines documentation guidelines for NETCONF or
   RESTCONF content defined with the YANG data modeling language.  The
   guidelines for how to write a Security Considerations section for a
   YANG module are defined in the online document

   http://trac.tools.ietf.org/area/ops/trac/wiki/
   yang-security-guidelines

   This document does not introduce any new or increased security risks
   into the management system.

   The following section contains the security considerations template
   dated 2010-06-16.  Be sure to check the webpage at the URL listed
   above in case there is a more recent version available.

   Each specification that defines one or more YANG modules MUST contain
   a section that discusses security considerations relevant to those
   modules.  This section MUST be patterned after the latest approved
   template (available at

    http://www.ops.ietf.org/netconf/yang-security-considerations.txt).

   In particular, writable data nodes that could be especially
   disruptive if abused MUST be explicitly listed by name and the
   associated security risks MUST be spelled out.

   Similarly, 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.

   Further, if new RPC operations have been defined, then the security
   considerations of each new RPC operation MUST be explained.

6.1.  Security Considerations Section Template

   X. Security Considerations

   The YANG module defined in this memo is designed to be accessed via
   the NETCONF protocol [RFC6241].  The lowest NETCONF layer is the
   secure transport layer and the mandatory-to-implement secure
   transport is SSH [RFC6242].

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



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   There are a number of data nodes defined in this YANG module which
   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)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

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


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

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

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


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

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

    <list RPC operations and state why they are sensitive>














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7.  Acknowledgments

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

   The working group thanks Martin Bjorklund, Juergen Schoenwaelder,
   Ladislav Lhotka, and Jernej Tuljak for their extensive reviews and
   contributions to this document.











































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8.  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 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 XPath usage for a literal value representing a YANG
      identity

   o  Clarified XPath usage for a when-stmt

   o  Clarified XPath usage for 'proceeding-sibling' and
      'following-sibling' axes

   o  Added terminology guidelines

   o  Added YANG tree diagram definition and guideline

   o  Updated XPath guidelines for type conversions and function library
      usage.

   o  Updated data types section

   o  Updated notifications section

   o  Clarified conditional key leaf nodes

   o  Clarify usage of 'uint64' and 'int64' data types

   o  Added text on YANG feature usage

   o  Added Identifier Naming Conventions

   o  Clarified use of mandatory nodes with conditional augmentations

   o  Clarified namespace and domain conventions for example modules

   o  Clarified conventions for identifying code components




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   o  Added YANG 1.1 guidelines

   o  Added Data Model Constraints section

   o  Added mention of RESTCONF protocol

   o  Added mention of Revised Datastores and associated guidelines












































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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5378]  Bradner, S. and J. Contreras, "Rights Contributors Provide
              to the IETF Trust", BCP 78, RFC 5378, November 2008.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6991]  Schoenwaelder, J., "Common YANG Data Types", RFC 6991,
              July 2013.

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

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

9.2.  Informative References

   [I-D.dsdt-nmda-guidelines]
              Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Guidelines for YANG Module Authors
              (NMDA)", draft-dsdt-nmda-guidelines-01 (work in progress),
              May 2017.

   [I-D.ietf-netmod-revised-datastores]
              Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore
              Architecture", draft-ietf-netmod-revised-datastores-02
              (work in progress), May 2017.




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   [I-D.ietf-netmod-yang-tree-diagrams]
              Bjorklund, M. and L. Berger, "YANG Tree Diagrams",
              draft-ietf-netmod-yang-tree-diagrams-00 (work in
              progress), June 2017.

   [RFC-STYLE]
              Braden, R., Ginoza, S., and A. Hagens, "RFC Document
              Style", September 2009,
              <http://www.rfc-editor.org/rfc-style-guide/rfc-style>.

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

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

   [RFC4181]  Heard, C., "Guidelines for Authors and Reviewers of MIB
              Documents", BCP 111, RFC 4181, September 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6087]  Bierman, A., "Guidelines for Authors and Reviewers of YANG
              Data Model Documents", RFC 6087, January 2011.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, June 2011.

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

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              March 2012.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, May 2014.

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

   [RFC7841]  Halpern, J., Ed., Daigle, L., Ed., and O. Kolkman, Ed.,



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              "RFC Streams, Headers, and Boilerplates", RFC 7841,
              DOI 10.17487/RFC7841, May 2016,
              <http://www.rfc-editor.org/info/rfc7841>.

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












































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Appendix A.  Change Log

       -- RFC Ed.: remove this section before publication.

A.1.  v12 to v13

   o  Clarify that the revision-date SHOULD be used in a CODE BEGINS
      YANG file extraction macro.

   o  Clarify the IANA requirements section wrt/ XML namespace and YANG
      module name registries.

   o  Clarify YANG Usage section wrt/ XML and/or JSON encoding format.

   o  Update Operation Data section to consider revised datastores.

   o  Add reference to YANG Tree Diagrams and update 2 sections that use
      this reference.

   o  Add reference to Revised Datastores and guidelines drafts

A.2.  v11 to v12

   o  fix incorrect location of new Module Usage Examples section

A.3.  v10 to v11

   o  updated YANG tree diagram syntax to align with pyang 1.7.1

   o  added general guideline to include module usage examples

A.4.  v09 to v10

   o  clarified <CODE BEGINS> is only for normative modules

   o  clarified example module namespace URI conventions

   o  clarified pyang usage for normative and example modules

   o  updated YANG tree diagrams section with text from RFC 8022

A.5.  v08 to v09

   o  fixed references

   o  added mention of RESTCONF to abstract and intro





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   o  created separate section for code components

   o  fixed document status

A.6.  v07 to v08

   o  changed CODE BEGINS guideline for example modules

   o  updated tree diagram guidelines

   o  clarified published and unpublished terms

   o  added section on Empty and Boolean data types

   o  clarified how to update the revision statement

   o  updated operational state guidelines

   o  added 'YANG fragment' to terminology section

A.7.  v06 to v07

   o  update contact statement guideline

   o  update example modules guidelines

   o  add guidelines on top-level data nodes

   o  add guideline on use of NP containers

   o  added guidelines on union types

   o  add guideline on deviations

   o  added section on open systems considerations

   o  added guideline about definitions reserved for future use

A.8.  v05 to v06

   o  Changed example 'my-module' to 'example-module'

   o  Added section Updating YANG Modules (Published vs. Unpublished)

   o  Added Example Modules section

   o  Added "<EXAMPLE BEGINS>" convention for full example modules




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   o  Added section on using action vs. rpc

   o  Changed term "operational state" to "operational data"

   o  Added section on YANG Data Node Constraints

   o  Added guidelines on using must vs. when statements

   o  Made ietf-foo module validate for I-D submission

A.9.  v04 to v05

   o  Clarified that YANG 1.1 SHOULD be used but YANG 1.0 MAY be used if
      no YANG 1.1 features needed

   o  Changed SHOULD follow YANG naming conventions to MUST follow (for
      standards track documents only)

   o  Clarified module naming conventions for normative modules, example
      modules, and modules from other SDOs.

   o  Added prefix value selection guidelines

   o  Added new section on guidelines for reusable groupings

   o  Made header guidelines less IETF-specific

   o  Added new section on guidelines for extension statements

   o  Added guidelines for nested "choice" statement within a "case"
      statement

A.10.  v03 ot v04

   o  Added sections for deviation statements and performance
      considerations

   o  Added YANG 1.1 section

   o  Updated YANG reference from 1.0 to 1.1

A.11.  v02 to v03

   o  Updated draft based on github data tracker issues added by Benoit
      Clause (Issues 12 - 18)






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A.12.  v01 to v02

   o  Updated draft based on mailing list comments.

A.13.  v00 to v01

   All issues from the issue tracker have been addressed.

     https://github.com/netmod-wg/rfc6087bis/issues

   o  Issue 1: Tree Diagrams: Added Section 2.4.1 so RFCs with YANG
      modules can use an Informative reference to this RFC for tree
      diagrams.  Updated guidelines to reference this RFC when tree
      diagrams are used

   o  Issue 2: XPath function restrictions: Added paragraphs in XPath
      usage section for 'id', 'namespace-uri', 'name', and 'lang'
      functions

   o  Issue 3: XPath function document order issues: Added paragraph in
      XPath usage section about node-set ordering for 'local-name',
      'namespace-uri', 'name', 'string' and 'number' functions.  Also
      any function that implicitly converts a node-set to a string.

   o  Issue 4: XPath preceding-sibling and following-sibling: Checked
      and text in XPath usage section already has proposed text from
      Lada.

   o  Issue 5: XPath 'when-stmt' reference to descendant nodes: Added
      exception and example in XPath Usage section for augmented nodes.

   o  Issue 6: XPath numeric conversions: Changed 'numeric expressions'
      to 'numeric and boolean expressions'

   o  Issue 7: XPath module containment: Added sub-section on XPath
      wildcards

   o  Issue 8: status-stmt usage: Added text to Lifecycle Management
      section about transitioning from active to deprecated and then to
      obsolete.

   o  Issue 9: resource identification in notifications: Add text to
      Notifications section about identifying resources and using the
      leafref data type.

   o  Issue 10: single quoted strings: Added text to Data Types section
      about using a single-quoted string for patterns.




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Appendix B.  Module Review Checklist

   This section is adapted from RFC 4181.

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

   o  I-D Boilerplate -- verify that the draft contains the required
      Internet-Draft boilerplate (see
      http://www.ietf.org/id-info/guidelines.html), including the
      appropriate statement to permit publication as an RFC, and that
      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
      http://www.ietf.org/id-info/guidelines.html.

   o  Copyright Notice -- verify that the draft has the appropriate text
      regarding the rights that document contributers 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:


      http://trustee.ietf.org/license-info/

   o  Security Considerations section -- verify that the draft uses the
      latest approved template from the OPS area website (http://
      trac.tools.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 [RFC7950].






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   o  References -- verify that the references are properly divided
      between normative and informative references, that RFC 2119 is
      included as a normative reference 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 OK 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).

   o  License -- verify that the draft 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 Trust Legal Provisions (TLP) document, which
      can be found at:


      http://trustee.ietf.org/license-info/

   o  Other Issues -- check for any issues mentioned in
      http://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
      can be found at:


      http://trac.tools.ietf.org/wg/netconf/trac/wiki

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









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Appendix C.  YANG Module Template

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

      module ietf-template {

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

          This version of this YANG module is part of RFC XXXX; see



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          the RFC itself for full legal notices.";

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

        reference "RFC XXXX";


        // RFC Ed.: remove this note
        // Note: extracted from RFC XXXX


        // 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 "document containing this module";
        }

        // 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>









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Author's Address

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com













































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