Guidelines for Authors and Reviewers of YANG Data Model Documents
RFC 6087

Internet Engineering Task Force (IETF)                        A. Bierman
Request for Comments: 6087                                       Brocade
Category: Informational                                     January 2011
ISSN: 2070-1721

   Guidelines for Authors and Reviewers of YANG Data Model Documents

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) implementations that utilize YANG
   data model modules.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6087.

Copyright Notice

   Copyright (c) 2011 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  3
     2.2.  NETCONF Terms  . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  YANG Terms . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.4.  Terms  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  General Documentation Guidelines . . . . . . . . . . . . . . .  5
     3.1.  Module Copyright . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Narrative Sections . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Definitions Section  . . . . . . . . . . . . . . . . . . .  6
     3.4.  Security Considerations Section  . . . . . . . . . . . . .  6
     3.5.  IANA Considerations Section  . . . . . . . . . . . . . . .  7
       3.5.1.  Documents that Create a New Namespace  . . . . . . . .  7
       3.5.2.  Documents that Extend an Existing Namespace  . . . . .  8
     3.6.  Reference Sections . . . . . . . . . . . . . . . . . . . .  8
   4.  YANG Usage Guidelines  . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Module Naming Conventions  . . . . . . . . . . . . . . . .  8
     4.2.  Identifiers  . . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Defaults . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.4.  Conditional Statements . . . . . . . . . . . . . . . . . . 10
     4.5.  XPath Usage  . . . . . . . . . . . . . . . . . . . . . . . 10
     4.6.  Lifecycle Management . . . . . . . . . . . . . . . . . . . 11
     4.7.  Module Header, Meta, and Revision Statements . . . . . . . 12
     4.8.  Namespace Assignments  . . . . . . . . . . . . . . . . . . 13
     4.9.  Top-Level Data Definitions . . . . . . . . . . . . . . . . 14
     4.10. Data Types . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.11. Reusable Type Definitions  . . . . . . . . . . . . . . . . 15
     4.12. Data Definitions . . . . . . . . . . . . . . . . . . . . . 15
     4.13. Operation Definitions  . . . . . . . . . . . . . . . . . . 17
     4.14. Notification Definitions . . . . . . . . . . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     6.1.  Security Considerations Section Template . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Module Review Checklist . . . . . . . . . . . . . . . 22
   Appendix B.  YANG Module Template  . . . . . . . . . . . . . . . . 24

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1.  Introduction

   The standardization of network configuration interfaces for use with
   the Network Configuration Protocol (NETCONF) [RFC4741] 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 YANG [RFC6020] data models.  YANG is used to
   define the data structures, protocol operations, and notification
   content used within a NETCONF server.  A server that supports a
   particular YANG module will support client NETCONF 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 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 [RFC4741].
   These guidelines are intended to be used by authors and reviewers to
   improve the readability and interoperability of published YANG data
   models.

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].

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   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 [RFC4741] and are not redefined
   here:

   o  capabilities

   o  client

   o  operation

   o  server

2.3.  YANG Terms

   The following terms are defined in [RFC6020] and are not redefined
   here:

   o  data node

   o  module

   o  namespace

   o  submodule

   o  version

   o  YANG

   o  YIN

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

2.4.  Terms

   The following terms are used throughout this document:

   published:  A stable release of a module or submodule, usually
      contained in an RFC.

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   unpublished:  An unstable release of a module or submodule, usually
      contained in an Internet-Draft.

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 [RFC2223] and updated in
   [RFC5741] 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

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/

   Each 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
   component.

   The '<CODE BEGINS>' tag SHOULD be followed by a string identifying
   the file name specified in Section 5.2 of [RFC6020].  The following
   example is for the '2010-01-18' revision of the 'ietf-foo' module:

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   <CODE BEGINS> file "ietf-foo@2010-01-18.yang"
   module ietf-foo {
       // ...
      revision 2010-01-18 {
         description "Latest revision";
         reference "RFC XXXX";
      }
      // ...
   }
   <CODE ENDS>

3.2.  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 YANG [RFC6020] or
   YANG Types [RFC6021] 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.

3.3.  Definitions Section

   This section contains the module(s) defined by the specification.
   These modules MUST be written using the YANG syntax defined in
   [RFC6020].  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.

   See Section 4 for guidelines on YANG usage.

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

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   Section 6.1 contains the security considerations template dated
   2010-06-16.  Authors MUST check the webpage at the URL listed above
   in case there is a more recent version available.

   In particular:

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

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

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

3.5.  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.

3.5.1.  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 YANG
   [RFC6020] specification includes the procedure for this purpose in
   its IANA Considerations section.

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3.5.2.  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.

3.6.  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
   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.

4.  YANG Usage Guidelines

   In general, modules in IETF Standards Track specifications MUST
   comply with all syntactic and semantic requirements of YANG
   [RFC6020].  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

   Modules contained in Standards Track documents SHOULD be named
   according to the guidelines in the IANA Considerations section of
   [RFC6020].

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   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.

   Once a module name is published, it MUST NOT be reused, even if the
   RFC containing the module is reclassified to 'Historic' status.

4.2.  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 12
   of [RFC6020].

4.3.  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         |
                     +---------------+---------------+

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4.4.  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 protocol capability, then a YANG 'feature' statement SHOULD
   be defined to indicate that the NETCONF 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).

4.5.  XPath Usage

   This section describes guidelines for using the XML Path Language
   [W3C.REC-xpath-19991116] (XPath) within YANG modules.

   The 'attribute' and 'namespace' axes are not supported in YANG, and
   MAY be empty in a NETCONF server implementation.

   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 'preceding', and 'following' axes SHOULD NOT be used.  These
   constructs rely on XML document order within a NETCONF 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).

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   The 'preceding-sibling' and 'following-sibling' axes SHOULD NOT used.
   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'.

   Data nodes that use the 'int64' and 'uint64' built-in type SHOULD NOT
   be used within numeric 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.

4.6.  Lifecycle Management

   The status statement MUST be present if its value is 'deprecated' or
   'obsolete'.

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

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

   The revision-date substatement within the imports 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.

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4.7.  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 Standards Track status, then the
   organization SHOULD be the IETF working group chartered to write the
   document.

   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.  In addition, the Area Director and other contact
   information MAY be present.

   The description statement MUST be present.  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,
   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.

   Each new revision MUST include a revision date that is higher than
   any other revision date in the module.  The revision date does not
   need to be updated if the module contents do not change in the new
   document revision.

   It is acceptable to reuse the same revision statement within
   unpublished versions (i.e., Internet-Drafts), but the revision date
   MUST be updated to a higher value each time the Internet-Draft is re-
   posted.

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4.8.  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:

   The following example URNs would be valid temporary 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 [RFC6020].

   The following examples of non-Standards-Track modules are only
   suggestions.  There are no guidelines for this type of URN in this
   document:

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

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

4.9.  Top-Level Data Definitions

   There SHOULD only be one top-level data node defined in each YANG
   module, if any data nodes are defined at all.

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

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

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

4.10.  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.

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

   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.

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

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

   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.

4.11.  Reusable Type Definitions

   If an appropriate derived type exists in any standard module, such as
   [RFC6021], then it SHOULD be used instead of defining a new derived
   type.

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

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

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

   The description statement MUST be present.

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

4.12.  Data Definitions

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

   o  anyxml

   o  augment

   o  choice

   o  container

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   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 '<b>' and '</b>', and MAY be used
   in such cases.  However, this construct SHOULD NOT be used if other
   YANG data node types can be used instead to represent the desired
   syntax and semantics.

   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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4.13.  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.

4.14.  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.

5.  IANA Considerations

   This document registers one URI in the IETF XML registry [RFC3688].
   The following registration has been made:

   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.

   Per this document, the following assignment has been made in the YANG
   Module Names Registry for the YANG module template in Appendix B.

       +---------------+-------------------------------------------+
       | Field         | Value                                     |
       +---------------+-------------------------------------------+
       | Name          | ietf-template                             |
       |               |                                           |
       | Namespace     | urn:ietf:params:xml:ns:yang:ietf-template |
       |               |                                           |
       | Prefix        | temp                                      |
       |               |                                           |
       | Reference     | RFC 6087                                  |
       +---------------+-------------------------------------------+

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6.  Security Considerations

   This document defines documentation guidelines for NETCONF 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://www.ops.ietf.org/netconf/yang-security-considerations.txt

   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.

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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 [RFC4741].  The lowest NETCONF layer is
   the secure transport layer and the mandatory-to-implement secure
   transport is SSH [RFC4742].

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

   There are a number of data nodes defined in this YANG module
   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
   Guidelines for MIB Documents [RFC4181], by C. M. Heard.

   The working group thanks Martin Bjorklund and Juergen Schoenwaelder
   for their extensive reviews and contributions to this document.

8.  References

8.1.  Normative References

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

   [RFC2223]  Postel, J. and J. Reynolds, "Instructions to RFC Authors",
              RFC 2223, October 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.

   [RFC4741]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
              December 2006.

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

   [RFC5741]  Daigle, L., Kolkman, O., and IAB, "RFC Streams, Headers,
              and Boilerplates", RFC 5741, December 2009.

   [W3C.REC-xpath-19991116]
              DeRose, S. and J. Clark, "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>.

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

   [RFC6021]  Schoenwaelder, J., "Common YANG Data Types", RFC 6021,
              October 2010.

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8.2.  Informative References

   [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.

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

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Appendix A.  Module Review Checklist

   This section is adapted from RFC 4181.

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

   1.  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.

   2.  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.

   3.  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/

   4.  Security Considerations section -- verify that the draft uses the
       latest approved template from the OPS area website (http://
       www.ops.ietf.org/netconf/yang-security-considerations.txt) and
       that the guidelines therein have been followed.

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

       XML Namespace Registry:  Register the YANG module namespace.

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

   6.  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

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       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).

   7.  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/

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

   9.  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 B.  YANG Module Template

<CODE BEGINS> file "ietf-template@2010-05-18.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>

        WG Chair: your-WG-chair
                  <mailto:your-WG-chair@example.com>

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

    // replace the first sentence in this description statement.
    // replace the copyright notice with the most recent
    // version, if it has been updated since the publication
    // of this document
    description
     "This module defines a template for other YANG modules.

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

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

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

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

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

    reference "RFC XXXX";

    // RFC Ed.: remove this note
    // Note: extracted from RFC 6087

    // replace '2010-05-18' with the module publication date
    // The format is (year-month-day)
    revision "2010-05-18" {
      description
        "Initial version";
    }

    // 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
   Brocade

   EMail: andy.bierman@brocade.com

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