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rfc:rfc6087

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.

Bierman Informational [Page 1] RFC 6087 Guidelines for YANG Documents January 2011

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

Bierman Informational [Page 2] RFC 6087 Guidelines for YANG Documents January 2011

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].

Bierman Informational [Page 3] RFC 6087 Guidelines for YANG Documents January 2011

 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.

Bierman Informational [Page 4] RFC 6087 Guidelines for YANG Documents January 2011

 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:

Bierman Informational [Page 5] RFC 6087 Guidelines for YANG Documents January 2011

 <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).

Bierman Informational [Page 6] RFC 6087 Guidelines for YANG Documents January 2011

 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.

Bierman Informational [Page 7] RFC 6087 Guidelines for YANG Documents January 2011

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].

Bierman Informational [Page 8] RFC 6087 Guidelines for YANG Documents January 2011

 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         |
                   +---------------+---------------+

Bierman Informational [Page 9] RFC 6087 Guidelines for YANG Documents January 2011

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).

Bierman Informational [Page 10] RFC 6087 Guidelines for YANG Documents January 2011

 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.

Bierman Informational [Page 11] RFC 6087 Guidelines for YANG Documents January 2011

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.

Bierman Informational [Page 12] RFC 6087 Guidelines for YANG Documents January 2011

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:

Bierman Informational [Page 13] RFC 6087 Guidelines for YANG Documents January 2011

    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.

Bierman Informational [Page 14] RFC 6087 Guidelines for YANG Documents January 2011

 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

Bierman Informational [Page 15] RFC 6087 Guidelines for YANG Documents January 2011

 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.

Bierman Informational [Page 16] RFC 6087 Guidelines for YANG Documents January 2011

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                                  |
     +---------------+-------------------------------------------+

Bierman Informational [Page 17] RFC 6087 Guidelines for YANG Documents January 2011

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.

Bierman Informational [Page 18] RFC 6087 Guidelines for YANG Documents January 2011

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].
  1. - if you have any writable data nodes (those are all the
  2. - "config true" nodes, and remember, that is the default)
  3. - describe their specific sensitivity or vulnerability.
 There are a number of data nodes defined in this YANG module
 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>
  1. - for all YANG modules you must evaluate whether any readable data
  2. - nodes (those are all the "config false" nodes, but also all other
  3. - nodes, because they can also be read via operations like get or
  4. - get-config) are sensitive or vulnerable (for instance, if they
  5. - might reveal customer information or violate personal privacy
  6. - laws such as those of the European Union if exposed to
  7. - unauthorized parties)
 Some of the readable data nodes in this YANG module may be
 considered sensitive or vulnerable in some network environments.
 It is thus important to control read access (e.g., via get,
 get-config, or notification) to these data nodes.  These are the
 subtrees and data nodes and their sensitivity/vulnerability:
  <list subtrees and data nodes and state why they are sensitive>
  1. - if your YANG module has defined any rpc operations
  2. - describe their specific sensitivity or vulnerability.
 Some of the RPC operations in this YANG module may be considered
 sensitive or vulnerable in some network environments.  It is thus
 important to control access to these operations.  These are the
 operations and their sensitivity/vulnerability:
  <list RPC operations and state why they are sensitive>

Bierman Informational [Page 19] RFC 6087 Guidelines for YANG Documents January 2011

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.

Bierman Informational [Page 20] RFC 6087 Guidelines for YANG Documents January 2011

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>.

Bierman Informational [Page 21] RFC 6087 Guidelines for YANG Documents January 2011

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

Bierman Informational [Page 22] RFC 6087 Guidelines for YANG Documents January 2011

     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

Bierman Informational [Page 24] RFC 6087 Guidelines for YANG Documents January 2011

    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>

Bierman Informational [Page 25] RFC 6087 Guidelines for YANG Documents January 2011

Author's Address

 Andy Bierman
 Brocade
 EMail: andy.bierman@brocade.com

Bierman Informational [Page 26]

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