Guidelines for Authors and Reviewers of YANG Data Model Documents
draft-ietf-netmod-rfc6087bis-04

Internet-Draft        Guidelines for YANG Documents            July 2015

5.13.  Data Definitions

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

   o  anyxml

   o  augment

   o  choice

   o  container

   o  extension

   o  feature

   o  grouping

   o  identity

   o  leaf

   o  leaf-list

   o  list

   o  notification

   o  rpc

   o  typedef

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

   The 'anyxml' construct may be useful to represent an HTML banner
   containing markup elements, such as '<b>' and '</b>', and
   MAY be used in such cases.  However, this construct SHOULD NOT be
   used if other YANG data node types can be used instead to represent
   the desired syntax and semantics.

   It has been found that the 'anyxml' statement is not implemented
   consistently across all servers.  It is possible that mixed mode XML
   will not be supported, or configuration anyxml nodes will not
   supported.

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   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.

5.14.  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.

5.15.  Notification Definitions

   The description statement MUST be present.

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

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

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

   Note that there are no formal YANG statements to identify any data

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   node resources associated with a notification.  The description
   statement for the notification SHOULD specify if and how the
   notification identifies any data node resources associated with the
   specific event.

5.16.  Feature Definitions

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

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

   If there is a large set of objects associated with a YANG feature,
   then consider moving those objects to a separate module, instead of
   using a YANG feature.  Note that the set of features within a module
   is easily discovered by the reader, but the set of related modules
   within the entire YANG library is not as easy to identity.  Module
   names with a common prefix can help readers identity the set of
   related modules, but this assumes the reader will have discovered and
   installed all the relevant modules.

   Another consideration for deciding whether to create a new module or
   add a YANG feature is the stability of the module in question.  It
   may be desirable to have a stable base module that is not changed
   frequently.  If new functionality is placed in a separate module,
   then the base module does not need to be republished.  If it is
   designed as a YANG feature then the module will need to be
   republished.

   If one feature requires implementation of another feature, then an
   "if-feature" statement SHOULD be used in the dependent "feature"
   statement.

   For example, feature2 requires implementation of feature1:

      feature feature1 {
        description "Some protocol feature";
      }

      feature feature2 {
        if-feature "feature1";
        description "Another protocol feature";
      }

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5.17.  Augment Statements

   The YANG "augment" statement is used to define a set of data
   definition statements that will be added as child nodes of a target
   data node.  The module namespace for these data nodes will be the
   augmenting module, not the augmented module.

   A top-level "augment" statement SHOULD NOT be used if the target data
   node is in the same module or submodule as the evaluated "augment"
   statement.  The data definition statements SHOULD be added inline
   instead.

5.17.1.  Conditional Augment Statements

   The "augment" statement is often used together with the "when"
   statement and/or "if-feature" statement to make the augmentation
   conditional on some portion of the data model.

   The following example from [RFC7223] shows how a conditional
   container called "ethernet" is added to the "interface" list only for
   entries of the type "ethernetCsmacd".

        augment "/if:interfaces/if:interface" {
            when "if:type = 'ianaift:ethernetCsmacd'";

            container ethernet {
                leaf duplex {
                    ...
                }
            }
        }

5.17.2.  Conditionally Mandatory Data Definition Statements

   YANG has very specific rules about how configuration data can be
   updated in new releases of a module.  These rules allow an "old
   client" to continue interoperating with a "new server".

   If data nodes are added to an existing entry, the old client MUST NOT
   be required to provide any mandatory parameters that were not in the
   original module definition.

   It is possible to add conditional augment statements such that the
   old client would not know about the new condition, and would not
   specify the new condition.  The conditional augment statement can
   contain mandatory objects only if the condition is false unless
   explicitly requested by the client.

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   Only a conditional augment statement that uses the "when" statement
   form of condition can be used in this manner.  The YANG features
   enabled on the server cannot be controlled by the client in any way,
   so it is not safe to add mandatory augmenting data nodes based on the
   "if-feature" statement.

   The XPath "when" statement condition MUST NOT reference data outside
   of target data node because the client does not have any control over
   this external data.

   In the following dummy example, it is OK to augment the "interface"
   entry with "mandatory-leaf" because the augmentation depends on
   support for "some-new-iftype".  The old client does not know about
   this type so it would never select this type, and therefore not be
   adding a mandatory data node.

     module my-module {
       ...

       identity some-new-iftype {
          base iana:iana-interface-type;
       }

       augment "/if:interfaces/if:interface" {
          when "if:type = 'mymod:some-new-iftype'";

          leaf mandatory-leaf {
             mandatory true;
             ...
          }
       }
     }

   Note that this practice is safe only for creating data resources.  It
   is not safe for replacing or modifying resources if the client does
   not know about the new condition.  The YANG data model MUST be
   packaged in a way that requires the client to be aware of the
   mandatory data nodes if it is aware of the condition for this data.
   In the example above, the "some-new-iftype" identity is defined in
   the same module as the "mandatory-leaf" data definition statement.

   This practice is not safe for identities defined in a common module
   such as "iana-if-type" because the client is not required to know
   about "my-module" just because it knows about the "iana-if-type"
   module.

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5.18.  Deviation Statements

   The YANG "deviation" statement cannot appear in IETF YANG modules,
   but it can be useful for documenting server capabilities.  Deviation
   statements are not reusable and typically not shared across all
   platforms.

   There are several reasons that deviations might be needed in an
   implementation, e.g., an object cannot be supported on all platforms,
   or feature delivery is done in multiple development phases.

   It is suggested that deviation statements be defined in separate
   modules from regular YANG definitions.  This allows the deviations to
   be platform-specific and/or temporary.

   The "max-elements" statement is intended to describe an architectural
   limit to the number of list entries.  It is not intended to describe
   platform limitations.  It is better to use a "deviation" statement
   for the platforms that have a hard resource limit.

   Example documenting platform resource limits:

     Wrong: (max-elements in the list itself)

        container backups {
          list backup {
             ...
             max-elements  10;
             ...
          }
        }

     Correct: (max-elements in a deviation)

        deviation /bk:backups/bk:backup {
          deviate add {
             max-elements  10;
          }
        }

5.19.  Data Correlation

   Data can be correlated in various ways, using common data types,
   common data naming, and common data organization.  There are several
   ways to extend the functionality of a module, based on the degree of
   coupling between the old and new functionality:

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   o  inline: update the module with new protocol-accessible objects.
      The naming and data organization of the original objects is used.
      The new objects are in the original module namespace.

   o  augment: create a new module with new protocol-accessible objects
      that augment the original data structure.  The naming and data
      organization of the original objects is used.  The new objects are
      in the new module namespace.

   o  mirror: create new objects in a new module or the original module,
      except use new a naming scheme and data location.  The naming can
      be coupled in different ways.  Tight coupling is achieved with a
      "leafref" data type, with the "require-instance" sub-statement set
      to "true".  This method SHOULD be used.

   If the new data instances are not limited to the values in use in the
   original data structure, then the "require-instance" sub-statement
   MUST be set to "false".  Loose coupling is achieved by using key
   leafs with the same data type as the original data structure.  This
   has the same semantics as setting the "require-instance" sub-
   statement to "false".

   It is sometimes useful to separate configuration and operational
   state, so that they do not not even share the exact same naming
   characteristics.  The correlation between configuration the
   operational state data that is affected by changes in configuration
   is a complex problem.  There may not be a simple 1:1 relationship
   between a configuration data node and an operational data node.
   Further work is needed in YANG to clarify this relationship.
   Protocol work may also be needed to allow a client to retrieve this
   type of information from a server.  At this time the best practice is
   to clearly document any relationship to other data structures in the
   "description" statement.

5.20.  Operational State

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

   One challenge for client developers is determining if the configured
   value is being used, which requires the developer to know which
   operational state parameters are associated with the particular
   configuration object (or group of objects).

   The simplest interaction between configuration and operational state
   is "none".  For example, the arbitrary administrative name or

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   sequence number assigned to an access control rule.  The configured
   value is always the value that is being used by the system.

   However, some configuration parameters interact with routing and
   other signalling protocols, such that the operational value in use by
   the system may not be the same as the configured value.  Other
   parameters specify the desired state, but environmental and other
   factors can cause the actual state to be different.

   For example a "temperature" configuration setting only represents the
   desired temperature.  An operational state parameter is needed that
   reports the actual temperature in order to determine if the cooling
   system is operating correctly.  YANG has no mechanism other than the
   "description" statement to associate the desired temperature and the
   actual temperature.

   Careful consideration needs to be given to the location of
   operational state data.  It can either be located within the
   configuration subtree for which it applies, or it can be located
   outside the particular configuration subtree.  Placing operation
   state within the configuration subtree is appropriate if the
   operational values can only exist if the configuration exists.

   The "interfaces" and "interfaces-state" subtrees defined in [RFC7223]
   are an example of a complex relationship between configuration and
   operational state.  The operational values can include interface
   entries that have been discovered or initialized by the system.  An
   interface may be in use that has not been configured at all.
   Therefore, the operational state for an interface cannot be located
   within the configuration for that same interface.

   Sometimes the configured value represents some sort of procedure to
   be followed, in which the system will select an actual value, based
   on protocol negotiation.

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      leaf duplex-admin-mode {
        type enumeration {
          enum auto;
          enum half;
          enum full;
        }
      }

      leaf duplex-oper-mode {
        config false;
        type enumeration {
          enum half;
          enum full;
        }
      }

   For example a "duplex" mode configuration may be "auto" to auto-
   negotiate the actual value to be used.  The operational parameter
   will never contain the value "auto".  It will always contain the
   result of the auto-negotiation, such as "half" or "full".  This is
   just one way in which the configuration data model is not exactly the
   same as the operational data model.  Another is if the detailed
   properties of the data are different for configured vs. learned
   entries.

   If all the data model properties are aligned between configuration
   and operational data, then it can be useful to define the
   configuration parameters within a grouping, and then replicate that
   grouping within the operational state portion of the data model.

       grouping parms {
          // do not use config-stmt in any of the nodes
          // placed in this grouping
       }

       container foo {
         uses parms;  // these are all config=true by default
         state {
           config false;  // only exists if foo config exists
           uses parms;
         }
       }

   Note that this mechanism can also be used if the configuration and
   operational state data are in separate sub-trees:

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       container bar { // bar config can exist without bar-state
         config true;
         uses parms;
       }

       container bar-state {  // bar-state can exist without bar
         config false;
         uses parms;
       }

   The need to replicate objects or define different operational state
   objects depends on the data model.  It is not possible to define one
   approach that will be optimal for all data models.  Designers SHOULD
   describe the relationship in detail between configuration objects and
   any associated operational state objects.  The "description"
   statements for both the configuration and the operational state
   SHOULD be used for this purpose.

5.21.  Performance Considerations

   It is generally likely that certain YANG statements require more
   runtime resources than other statements.  Although there are no
   performance requirements for YANG validation, the following
   information MAY be considered when designing YANG data models:

   o  Lists are generally more expensive than containers

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

   o  "must" statement is generally more expensive than "min-entries",
      "max-entries", "mandatory", or "unique" statements

   o  "identityref" leafs are generally more expensive than
      "enumeration" leafs

   o  "leafref" and "instance-identifier" types with "requite-instance"
      set to true are generally more expensive than if
      "require-instance" is set to false

5.22.  YANG 1.1 Guidelines

   TODO: need more input on YANG 1.1 guidelines

5.22.1.  Importing Multiple Revisions

   Standard modules SHOULD NOT import multiple revisions of the same
   module into a module.  This MAY be done if the authors can

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

5.22.2.  Using Feature Logic

   The YANG 1.1 feature logic is much more expressive than YANG 1.0.  A
   "description" statement SHOULD describe the "if-feature" logic in
   text, to help readers understand the module.

   YANG features SHOULD be used instead of the "when" statement, if
   possible.  This reduces server implementation complexity and might
   reduce runtime resource requirements as well.

5.22.3.  anyxml vs. anydata

   The "anyxml" statement MUST NOT be used to represent a conceptual
   subtree of YANG data nodes.  The "anydata" statment MUST be used for
   this purpose.

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6.  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 C.

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

                         YANG Registry Assignment

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

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

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

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

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

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

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

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

7.1.  Security Considerations Section Template

   X. Security Considerations

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

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

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   There are a number of data nodes defined in this YANG module which
   are writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

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

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

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

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

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

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

    <list RPC operations and state why they are sensitive>

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8.  Acknowledgments

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

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

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9.  Changes Since RFC 6087

   The following changes have been made to the guidelines published in
   [RFC6087]:

   o  Updated NETCONF reference from RFC 4741 to RFC 6241

   o  Updated NETCONF over SSH citation from RFC 4742 to RFC 6242

   o  Updated YANG Types reference from RFC 6021 to RFC 6991

   o  Updated obsolete URLs for IETF resources

   o  Changed top-level data node guideline

   o  Clarified XPath usage for a literal value representing a YANG
      identity

   o  Clarified XPath usage for a when-stmt

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

   o  Added terminology guidelines

   o  Added YANG tree diagram definition and guideline

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

   o  Updated data types section

   o  Updated notifications section

   o  Clarified conditional key leaf nodes

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

   o  Added text on YANG feature usage

   o  Added Identifier Naming Conventions

   o  Clarified use of mandatory nodes with conditional augmentations

   o  Clarified namespace and domain conventions for example modules

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10.  References

10.1.  Normative References

   [I-D.ietf-netmod-rfc6020bis]
              Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)",
              draft-ietf-netmod-rfc6020bis-06 (work in progress),
              July 2015.

   [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.

   [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.

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

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

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

10.2.  Informative References

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

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

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

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

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

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Appendix A.  Change Log

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

A.1.  03 ot 04

   o  Added sections for deviation statements and performance
      considerations

   o  Added YANG 1.1 section

   o  Updated YANG reference from 1.0 to 1.1

A.2.  02 to 03

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

A.3.  01 to 02

   o  Updated draft based on mailing list comments.

A.4.  00 to 01

   All issues from the issue tracker have been addressed.

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

   o  Issue 1: Tree Diagrams: Added Section 3 so RFCs with YANG modules
      can use an Informative reference to this RFC for tree diagrams.
      Updated guidelines to reference this RFC when tree diagrams are
      used

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

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

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

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

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

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

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

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

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

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Appendix B.  Module Review Checklist

   This section is adapted from RFC 4181.

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

   o  I-D Boilerplate -- verify that the draft contains the required
      Internet-Draft boilerplate (see
      http://www.ietf.org/id-info/guidelines.html), including the
      appropriate statement to permit publication as an RFC, and that
      I-D boilerplate does not contain references or section numbers.

   o  Abstract -- verify that the abstract does not contain references,
      that it does not have a section number, and that its content
      follows the guidelines in
      http://www.ietf.org/id-info/guidelines.html.

   o  Copyright Notice -- verify that the draft has the appropriate text
      regarding the rights that document contributers provide to the
      IETF Trust [RFC5378].  Verify that it contains the full IETF Trust
      copyright notice at the beginning of the document.  The IETF Trust
      Legal Provisions (TLP) can be found at:

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

   o  Security Considerations section -- verify that the draft uses the
      latest approved template from the OPS area website (http://
      trac.tools.ietf.org/area/ops/trac/wiki/yang-security-guidelines)
      and that the guidelines therein have been followed.

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

      XML Namespace Registry: Register the YANG module namespace.

      YANG Module Registry: Register the YANG module name, prefix,
         namespace, and RFC number, according to the rules specified
         in [I-D.ietf-netmod-rfc6020bis].

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   o  References -- verify that the references are properly divided
      between normative and informative references, that RFC 2119 is
      included as a normative reference if the terminology defined
      therein is used in the document, that all references required by
      the boilerplate are present, that all YANG modules containing
      imported items are cited as normative references, and that all
      citations point to the most current RFCs unless there is a valid
      reason to do otherwise (for example, it is OK to include an
      informative reference to a previous version of a specification to
      help explain a feature included for backward compatibility).  Be
      sure citations for all imported modules are present somewhere in
      the document text (outside the YANG module).

   o  License -- verify that the draft contains the Simplified BSD
      License in each YANG module or submodule.  Some guidelines related
      to this requirement are described in Section 4.1.  Make sure that
      the correct year is used in all copyright dates.  Use the approved
      text from the latest Trust Legal Provisions (TLP) document, which
      can be found at:

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

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

   o  Technical Content -- review the actual technical content for
      compliance with the guidelines in this document.  The use of a
      YANG module compiler is recommended when checking for syntax
      errors.  A list of freely available tools and other information
      can be found at:

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

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

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Appendix C.  YANG Module Template

    <CODE BEGINS> file "ietf-template@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

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          set forth in Section 4.c of the IETF Trust's Legal Provisions
          Relating to IETF Documents
          (http://trustee.ietf.org/license-info).

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

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

        reference "RFC XXXX";

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

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

        // extension statements

        // feature statements

        // identity statements

        // typedef statements

        // grouping statements

        // data definition statements

        // augment statements

        // rpc statements

        // notification statements

        // DO NOT put deviation statements in a published module

      }

    <CODE ENDS>

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Author's Address

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com

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