Internet Engineering Task Force                               A. Bierman
Internet-Draft                                         InterWorking Labs
Intended status: Informational                          January 15,                            April 20, 2010
Expires: July 19, October 22, 2010

   Guidelines for Authors and Reviewers of YANG Data Model Documents
                    draft-ietf-netmod-yang-usage-03
                    draft-ietf-netmod-yang-usage-04

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 NETCONF
   implementations which utilize YANG data model modules.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts.
   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 19, October 22, 2010.

Copyright Notice

   Copyright (c) 2010 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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  5
     2.2.  NETCONF Terms  . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  YANG Terms . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Terms  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  General Documentation Guidelines . . . . . . . . . . . . . . .  7
     3.1.  YANG Data Model Boilerplate Section  Module Copyright . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Narrative Sections . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Definitions Section  . . . . . . . . . . . . . . . . . . .  8
     3.4.  Security Considerations Section  . . . . . . . . . . . . .  8
     3.5.  IANA Considerations Section  . . . . . . . . . . . . . . .  8
       3.5.1.  Documents that Create a New Name Space . . . . . . . .  8
       3.5.2.  Documents that Extend an Existing Name Space . . . . .  9
     3.6.  Reference Sections . . . . . . . . . . . . . . . . . . . .  9
     3.7.  Copyright Notices  . . . . . . . . . . . . . . . . . . . .  9
     3.8.  Intellectual Property Section  . . . . . . . . . . . . . .  9
   4.  YANG Usage Guidelines  . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Module Naming Conventions  . . . . . . . . . . . . . . . . 10
     4.2.  Identifiers  . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Defaults . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  Conditional Statements . . . . . . . . . . . . . . . . . . 11
     4.5.  Lifecycle Management . . . . . . . . . . . . . . . . . . . 12
     4.6.  Header Contents  . . . . . . . . . . . . . . . . . . . . . 12
     4.7.  Temporary Namespace Assignments  . . . . . . . . . . . . . 13
     4.8.  Top Level Database Objects . . . . . . . . . . . . . . . . 14
     4.9.  Data Types . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.10. Reusable Type Definitions  . . . . . . . . . . . . . . . . 15
     4.11. Object Definitions . . . . . . . . . . . . . . . . . . . . 16
     4.12. RPC Operation Definitions  . . . . . . . . . . . . . . . . . . . . . 17
     4.13. Notification Definitions . . . . . . . . . . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Module Review Checklist . . . . . . . . . . . . . . . 22
   Appendix B.  YANG Module Template  . . . . . . . . . . . . . . . . 24
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 27
     C.1.  Changes from 00 03 to 01 04  . . . . . . . . . . . . . . . . . . 27
     C.2.  Changes from 01 to 02 to 03  . . . . . . . . . . . . . . . . . . 27 28
     C.3.  Changes from 01 to 02  . . . . . . . . . . . . . . . . . . 28
     C.4.  Changes from 00 to 03 01  . . . . . . . . . . . . . . . . . . 27 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.  Introduction

   The standardization of network configuration interfaces for use with
   the NETCONF [RFC4741] protocol 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 [I-D.ietf-netmod-yang] data models.  It is
   similar to the MIB usage guidelines specification [RFC4181] in intent
   and structure.

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

    The NETCONF stack can be conceptually partitioned into four layers.

         Layer                Example
        +-------------+   +--------------------+ +-------------------+
    (4) |   Content   |   | Configuration data | | Notification data |
        +-------------+   +--------------------+ +-------------------+
               |                    |                   |
        +-------------+   +-----------------+           |
    (3) | Operations  |   |  <edit-config>  |           |
        +-------------+   +-----------------+           |
               |                    |                   |
        +-------------+   +--------------------+  +----------------+
    (2) |  Messages   |   | <rpc>, <rpc-reply> |  | <notification> |
        +-------------+   +--------------------+  +----------------+
               |                    |                   |
        +-------------+   +-----------------------------------------+
    (1) |   Secure    |   | SSH, TLS, BEEP/TLS, SOAP/HTTP/TLS, ...  |
        | Transports  |   |                                         |
        +-------------+   +-----------------------------------------+

                                 Figure 1

   This document defines usage guidelines related to the NETCONF
   operations layer (3), layer, and NETCONF content layer (4). layer, as defined in [RFC4741].

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 YANG module content.  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  application

   o  capabilities

   o  client

   o  operation

   o  RPC

   o  server

2.3.  YANG Terms

   The following terms are defined in [I-D.ietf-netmod-yang] and are not
   redefined here:

   o  data node

   o  module

   o  submodule

   o  namespace

   o  submodule

   o  version

2.4.  Terms

   The following terms are

   Note that the term 'module' may be used throughout this document:

   o  module: Generic as a generic term for a YANG data model
   module or submodule.  When describing properties which are specific
   to submodules, the term 'YANG submodule', or simply 'submodule' is used instead.

   o  Published Document:

2.4.  Terms

   The following terms are used throughout this document:

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

   o  Unpublished Document:

   unpublished:  An unstable release of a module, 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.  These guidelines are available online at:

   http://www.rfc-editor.org/rfc-editor/instructions2authors.txt

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

   o  YANG data model boilerplate section

   o  Narrative sections

   o  Definitions section

   o  Security Considerations section

   o  IANA Considerations section

   o  References section

3.1.  YANG Data Model Boilerplate Section

   This section  Module Copyright

   The module description statement MUST contain a verbatim copy of the latest approved
   Internet-Standard Management Framework boilerplate,
   IETF Trust Copyright statement, which is available on-line, in
   section 4 of the Trust Legal Provisions (TLP) document, at:

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

   Each YANG module or submodule contained within an Internet Draft or RPC
   RFC MUST be identified as a 'Code Component'.  The strings '<CODE
   BEGINS>' and '<CODE ENDS>' SHOULD be used to identify each Code
   Component.

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 module 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 import definitions from
   other modules (except for those defined in the YANG

   [I-D.ietf-netmod-yang] or YANG Types [I-D.ietf-netmod-yang-types]
   documents) or are always implemented in conjunction with other
   modules, then those facts MUST be noted in the overview section, as
   MUST any special interpretations of objects in other modules.

3.3.  Definitions Section

   This section contains the module(s) defined by the specification.
   These modules MUST be written in YANG [I-D.ietf-netmod-yang].

   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 [ed: URL TBD]). http://www.ops.ietf.org/yang-security.html).

   In particular, writable module objects that could be especially
   disruptive if abused MUST be explicitly listed by name and the
   associated security risks MUST be spelled out; similarly, readable
   module objects 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.

3.5.  IANA Considerations Section

   In order to comply with IESG policy as set forth in
   http://www.ietf.org/ID-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 what actions are required of the IANA.

3.5.1.  Documents that Create a New Name Space

   If an Internet-Draft defines a new name space that is to be
   administered by the IANA, then the document MUST include an IANA
   Considerations section, that specifies how the name space 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
   specification includes the procedure for this purpose in its IANA
   Considerations section.

3.5.2.  Documents that Extend an Existing Name Space

   If an Internet-Draft defines any extensions

   It is possible to extend an existing namespace using a YANG Namespace submodule
   which belongs to an existing module already administered by the IANA, then IANA.  In
   this case, the document MUST include an
   IANA Considerations section, specifies how containing the name space extension
   is to main module MUST be administered.

   Specifically, if any YANG submodule belongs-to value updated to
   use the latest revision of the submodule.

3.6.  Reference Sections

   For every import or include statement which appears in a module
   contained in the
   document is associated with specification, which identifies a module that contains a namespace
   statement value equal to a YANG Namespace already administered by the
   IANA, then the existing YANG Namespace must be updated to include the
   new submodule.

3.6.  Reference Sections

   For every import or include statement which appears in a module
   contained in the specification, which identifies a module in 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 reference statement which 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.

3.7.  Copyright Notices

   The proper copyright notices MUST be present in the module
   description statement.  Refer to the IETF Trust Legal Provision for
   the exact legal text that needs to be included.

3.8.  Intellectual Property Section

   The proper IPR statements MUST be present in the document, according
   to the most current Internet Draft boilerplate.  Refer to the IETF
   Trust Legal Provision for the exact legal text that needs to be
   included.

4.  YANG Usage Guidelines

   In general, modules in IETF standards-track specifications MUST
   comply with all syntactic and semantic requirements of YANG.
   [I-D.ietf-netmod-yang].  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 which clarify or restrict the minimum conformance
   requirements are included here.

4.1.  Module Naming Conventions

   Modules contained in standards track documents SHOULD be named with
   according to the prefix 'ietf-'.  Other types of modules MUST NOT use guidelines in the 'ietf-'
   prefix string. IANA considerations section of
   [I-D.ietf-netmod-yang].

   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.

   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 published modules, submodules, typedefs,
   groupings, data objects, rpcs, operations, and notifications MUST be
   between 1 and 64 characters in length.

4.3.  Defaults

   In general, it is suggested that sub-statements containing default
   values SHOULD NOT be present.  For example, 'status current;',
   'config true;', 'mandatory false;', and 'max-elements unbounded;' are
   common defaults which would make the module difficult to read if used
   everywhere they are allowed.

   Instead, it is suggested that common statements SHOULD only be used
   when being set to a value other than the default value.

4.4.  Conditional Statements

   A module may be conceptually partitioned in several ways, using the
   'if-feature' and/or 'when' statements.  In addition, NETCONF
   capabilities are designed to identify optional functionality.

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

   Objects SHOULD NOT directly reference NETCONF capabilities, in order
   to specify optional behavior.  Instead, a 'feature' statement SHOULD
   be defined to represent the instead of a NETCONF capability, and the 'if-feature'
   statement SHOULD be used within the optional object definition.

   If the condition associated with the desired semantics is not
   dependent on any particular instance value within the database, then
   an 'if-feature' statement SHOULD be used instead of a 'when'
   statement.

   All 'must' and 'when' statements MUST contain valid XPath.  If any
   name tests are present, they MUST contain valid module prefixes and
   data node names.  References to non-existent nodes are considered
   invalid in YANG, even though they are permitted in XPath.

   The 'attribute' and 'namespace' axis SHOULD NOT be used because the
   associated XML node types are not supported in YANG, and may not be
   supported consistently across NETCONF server implementations.

   The 'position' and 'last' functions SHOULD NOT MAY be used.  Also, used with caution, within
   a single server implementation.  These functions may be useful in
   some cases when processing user-ordered lists.  A server is only
   required to maintain the XML order of 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., name, value,
   ancestors, descendants) SHOULD be used instead.

   The 'preceding-sibling' and 'following-sibling' axes MAY be used,
   with caution.  A server is not required to maintain a persistent or
   deterministic XML document order, which will affect use of these
   axes.

   Implicit 'position' function calls within predicates SHOULD NOT be
   used. (e.g., //chapter[42]).

   Data nodes which use the 'int64' and 'uint64' built-in type SHOULD
   NOT MAY be
   used with caution, within relational 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.

   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.

4.5.  Lifecycle Management

   The status statement SHOULD NOT be present if its value is 'current'.
   It 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 SHOULD MUST NOT be changed, once the document
   containing the module is published.

   The revision-date sub-statement (within the imports statement) SHOULD
   be present if any groupings are used from the external module.

   The revision-date sub-statement (within the include statement) SHOULD
   be present if any groupings are used from the external sub-module.

   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 which is
   (directly or indirectly) included by the main module.

4.6.  Header Contents

   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 documented 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 documented document intended for standards-track status, then the working
   group WEB and mailing information MUST be present, and the document
   author contact information SHOULD be present.  In addition, the Area
   Director and other contact information MAY be present.

   The description statement MUST be present.  If the module is
   contained in an unpublished document, then the file name of this
   document SHOULD be identified in the description statement.  This  The appropriate IETF
   Trust Copyright text MUST be removed when the document is published. present, as described in Section 3.1.

   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 reference statement MUST be present.  It MUST identify the
   published document which contains the module.

   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 which contains
   the module.

   Each new revision MUST include a revision date which is higher than
   any other revision date in the module.

   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-
   published.

4.7.  Temporary Namespace Assignments

   It is desirable to include only valid YANG modules in documents,
   whether they are published yet or not.  This allows:

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

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

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

   Until a URI is assigned by the IANA, a temporary namespace URI MUST
   be provided for the namespace statement in a YANG module.  A value
   SHOULD be selected which is not likely to collide with other YANG
   namespaces.

   An unpublished module namespace statement value SHOULD include the
   field 'DRAFT-nn', where 'nn' is replaced by the current Internet
   Draft number.

   If the YANG module has been previously published, then the RPC being
   updated needs to be identified.  In this case, an unpublished module
   namespace statement value SHOULD include the field
   'DRAFT-XXXXBIS-nn', where 'XXXX' is replaced by the RFC number being
   updated, and 'nn' is replaced by the current Internet Draft number.

   A temporary standard namespace statement value SHOULD have the following form:

   <URN prefix string>:<module-name>:<draft-field> string>:<module-name>

   The suggested following URN prefix string that SHOULD be used is shown below.
   This value will be defined by the IANA. for published and
   unpublished YANG modules

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

   The following example URNs would be valid temporary namespace
   statement values:

      urn:ietf:params:xml:ns:yang:ietf-netconf-partial-lock:DRAFT-09

      urn:ietf:params:xml:ns:yang:ietf-netconf-state:DRAFT-07

      urn:ietf:params:xml:ns:yang:ietf-netconf:DRAFT-4741BIS-01 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 [I-D.ietf-netmod-yang].

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

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

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

4.8.  Top Level Database Objects

   There SHOULD only be one top-level data node defined in each YANG
   module.  However, there MAY be more than one if needed.

   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 object is defined as a node that a client must
   provide for the database to be valid.  The server will not provide a
   value under any conditions.

   Top-level database objects 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.

   Top level objects are declared in YANG as mandatory with the
   mandatory statement or the min-elements statement.  All nested non-
   presence containers are transparent, so a mandatory node nested
   within one or more non-presence containers causes the top-level
   container to be considered mandatory.

4.9.  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 not required to
   be unbounded
   bounded in all implementations, then a length statement SHOULD be
   present.

   For string data types, object semantics SHOULD NOT rely on
   preservation of leading and trailing whitespace characters.

   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.

4.10.  Reusable Type Definitions

   If an appropriate derived type exists in any standard module, such as
   [I-D.ietf-netmod-yang-types], 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,
   then the reference statement SHOULD be present.

4.11.  Object Definitions

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

   o  extension

   o  feature

   o  identity

   o  typedef

   o  grouping

   o  augment

   o  rpc

   o  notification

   The description statement MUST be present in the following data
   definition constructs:

   o  container

   o  leaf

   o  leaf-list

   o  list

   o  choice

   o  anyxml
   If the object semantics are defined in an external document, then a
   reference statement SHOULD be present.

   The 'anyxml' construct MUST NOT MAY be used with caution within configuration
   data.  This may be useful to represent an HTML banner for example.
   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 objects, if the number of possible instances
   is not required to be unbounded bounded for all implementations, then the
   max-elements statement max-
   elements statements SHOULD be present.

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

4.12.  RPC  Operation Definitions

   The description statement MUST be present. present in 'rpc' statements
   defining new operations.

   If the RPC method operation semantics are defined in an external document, then
   a reference statement SHOULD be present.

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

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

4.13.  Notification Definitions

   The description statement MUST be present.

   If the notification semantics are defined in an external document,
   then a reference statement SHOULD be present.

5.  IANA Considerations

   There are no actions requested of IANA at this time.

6.  Security Considerations

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

7.  Acknowledgments

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

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.

   [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.

   [I-D.ietf-netmod-yang]
              Bjorklund, M., "YANG - A data modeling language for
              NETCONF", draft-ietf-netmod-yang-10 draft-ietf-netmod-yang-12 (work in progress),
              January
              April 2010.

   [I-D.ietf-netmod-yang-types]
              Schoenwaelder, J., "Common YANG Data Types",
              draft-ietf-netmod-yang-types-05
              draft-ietf-netmod-yang-types-08 (work in progress),
              December 2009.
              April 2010.

8.2.  Informative References

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

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
   a draft document:

   1.  I-D Boilerplate -- verify that the draft contains the required
       Internet-Draft boilerplate (see
       http://www.ietf.org/ietf/1id-guidelines.txt), 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/ietf/1id-guidelines.txt.

   3.   YANG Module Boilerplate  IETF Trust Copyright -- verify that the draft contains the latest
       approved SNMP Network Management Framework TLP boilerplate
        from the OPS area web site
        (http://www.ops.ietf.org/mib-boilerplate.html). [ed: real URL
        TBD] as described in Section 3.1.

   4.  Security Considerations Section -- verify that the draft uses the
       latest approved template from the OPS area web site
        (http://www.ops.ietf.org/mib-security.html)
       (http://www.ops.ietf.org/yang-security.html) and that the
       guidelines therein have been followed.

   5.  IANA Considerations Section -- this section must always be
       present.  If the draft requires no action from the IANA, ensure
        that this is explicitly noted.  If  For each module within the draft requires URI values
        to be assigned, document, ensure that the
       IANA Considerations section contains entries for the information specified in [TBD] of these guidelines.
        If the draft contains the initial version of an IANA-maintained
        module, verify that the [TBD] invocation contains maintenance
        instructions that comply with the requirements in RFC 2434.  In
        the latter case, the following
       IANA Considerations section that will
        appear in registries:

       XML Namespace Registry:  Register the YANG module namespace.

       YANG Module Registry:  Register the YANG module name, prefix,
          namespace, and RFC MUST contain a pointer number, according to the actual IANA-
        maintained module. rules specified in
          [I-D.ietf-netmod-yang].

   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
       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).

   7.  Copyright Notices -- verify that the draft contains an
       abbreviated IETF Trust copyright notice in the description
       statement of each YANG module or sub-module, and that it contains
       the full IETF Trust copyright notice and disclaimer specified in Sections 5.4 and
        5.5 of RFC 3978 at the end of the document.
       Make sure that the correct year is used in all copyright dates.

   8.   IPR Notice -- if the draft does not contains a verbatim copy of
       Use the IPR notice specified in Section 5 of RFC 3979, recommend
        that approved text from the IPR notice latest Trust Legal Provisions
       (TLP) document, which can be included.

   9. found at:

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

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

   10.

   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; see [YANG tool URL TBD] for more information.  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.

Appendix B.  YANG Module Template

<CODE BEGINS> file "ietf-template.yang"

module ietf-template {

    // replace this string with a unique namespace URN value
    namespace
      "urn:ietf:params:xml:ns:yang:ietf-template:DRAFT-02";

    // 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) 2009 2010 IETF Trust and the persons identified as
      the document authors.  All rights reserved.

      Redistribution and use in source and binary forms, with or
      without modification, are is permitted provided that the
      following conditions are met:

      - Redistributions of source code must retain the above
        copyright notice, this list of conditions pursuant to, and subject
      to the
        following disclaimer.

      - Redistributions in binary form must reproduce the above
        copyright notice, this list of conditions and license terms contained in, the
        following disclaimer Simplified BSD License
      set forth in the documentation and/or other
        materials provided with the distribution.

      - Neither the name Section 4.c of Internet Society, IETF or IETF
        Trust, nor the names of specific contributors, may be
        used IETF Trust's Legal Provisions
      Relating to endorse or promote products derived from this
        software without specific prior written permission.

      THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
      CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED
      WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
      WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
      PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
      OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
      INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
      (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
      GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
      BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
      LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
      (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
      OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
      POSSIBILITY OF SUCH DAMAGE. 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 draft-ietf-netmod-yang-usage-02.txt draft-ietf-netmod-yang-usage-04.txt

    // replace YYYY-MM-DD with a real date (year-month-day)
    // here is an example revision date: 2009-08-12
    revision YYYY-MM-DD {
      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>
                                 Figure 2 1

Appendix C.  Change Log

C.1.  Changes from 00 03 to 01 04

   o  Added transport 'TLS' to  Removed figure 1. 1 to reduce duplication, just refer to 4741bis
      draft.

   o  Added note about RFC 2119 terminology.  Fixed bugs and typos found in WGLC reviews.

   o  Corrected URL for instructions  Removed some guidelines and referring to authors. YANG draft instead of
      duplicating YANG rules here.

   o  Updated  Changed security guidelines so they refer to the IETF Trust TLP
      instead of MIB-specific references.

   o  Change temporary namespace procedures section. guidelines so the DRAFT-XX and RFC-nnnn
      suffix strings are not used.

   o  Updated  Changed some MIB boilerplate so it refers to YANG boilerplate
      instead.

   o  Introduced dangling URL reference to online YANG security
      guidelines on module contact, reference,

      http://www.ops.ietf.org/yang-security.html

      Text from Bert Wijnen will be completed soon and organization
      statements. posted online,
      and then this URL will be finalized.

   o  Moved reference for identifying the source document inside the
      each revision statement.

   o  Removed guideline about valid XPath since YANG already requires
      valid XPath.

   o  Added note guideline that strings should not rely on use preservation of preceding-sibling
      leading and following-sibling axes
      in trailing whitespace characters.

   o  Relaxed some XPath expressions. and anyxml guidelines from SHOULD NOT or MUST
      NOT to MAY use with caution.

   o  Added section on temporary namespace statement values.  Updated the TLP text within the example module again.

   o  Added section on top level database objects.  Reversed order of change log so most recent entries are first.

C.2.  Changes from 02 to 03

   o  Updated figure 1 to align with 4741bis draft.

   o  Updated guidelines for import-by-revision and include-by-revision.

   o  Added ietf-template.yang appendix.

C.2. file name to code begins convention in ietf-template module.

C.3.  Changes from 01 to 02

   o  Updated figure 1 per mailing list comments.

   o  Updated suggested organization to include the working group name.

   o  Updated ietf-template.yang to use new organization statement
      value.

   o  Updated Code Component requirements as per new TLP.

   o  Updated ietf-template.yang to use new Code Component begin and end
      markers.

   o  Updated references to the TLP in a couple sections.

   o  Change manager/agent terminology to client/server.

C.3.

C.4.  Changes from 02 00 to 03 01

   o  Updated  Added transport 'TLS' to figure 1 1.

   o  Added note about RFC 2119 terminology.

   o  Corrected URL for instructions to align with 4741bis draft. authors.

   o  Updated namespace procedures section.

   o  Updated guidelines for import-by-revision on module contact, reference, and include-by-revision. organization
      statements.

   o  Added file name to code begins convention note on use of preceding-sibling and following-sibling axes
      in ietf-template module. XPath expressions.

   o  Added section on temporary namespace statement values.

   o  Added section on top level database objects.

   o  Added ietf-template.yang appendix.

Author's Address

   Andy Bierman
   InterWorking Labs

   Email: andyb@iwl.com