YANG Module Tags
RFC 8819



Internet Engineering Task Force (IETF)                          C. Hopps
Request for Comments: 8819                                     L. Berger
Updates: 8407                                    LabN Consulting, L.L.C.
Category: Standards Track                                  D. Bogdanovic
ISSN: 2070-1721                                           Volta Networks
                                                            January 2021

                            YANG Module Tags

Abstract

   This document provides for the association of tags with YANG modules.
   The expectation is for such tags to be used to help classify and
   organize modules.  A method for defining, reading, and writing
   modules tags is provided.  Tags may be registered and assigned during
   module definition, assigned by implementations, or dynamically
   defined and set by users.  This document also provides guidance to
   future model writers; as such, this document updates RFC 8407.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8819.

Copyright Notice

   Copyright (c) 2021 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
   (https://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
     1.1.  Some Possible Use Cases for YANG Module Tags
     1.2.  Conventions Used in This Document
   2.  Tag Values
     2.1.  IETF Tags
     2.2.  Vendor Tags
     2.3.  User Tags
     2.4.  Reserved Tags
   3.  Tag Management
     3.1.  Module Definition Tagging
     3.2.  Implementation Tagging
     3.3.  User Tagging
   4.  Tags Module Structure
     4.1.  Tags Module Tree
     4.2.  YANG Module
   5.  Other Classifications
   6.  Guidelines to Model Writers
     6.1.  Define Standard Tags
   7.  IANA Considerations
     7.1.  YANG Module Tag Prefixes Registry
     7.2.  IETF YANG Module Tags Registry
     7.3.  Updates to the IETF XML Registry
     7.4.  Updates to the YANG Module Names Registry
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Examples
   Appendix B.  Non-NMDA State Module
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The use of tags for classification and organization is fairly
   ubiquitous not only within IETF protocols but in the internet itself
   (e.g., "#hashtags").  One benefit of using tags for organization over
   a rigid structure is that it is more flexible and can more easily
   adapt over time as technologies evolve.  Tags can be usefully
   registered, but they can also serve as a non-registered mechanism
   available for users to define themselves.  This document provides a
   mechanism to define tags and associate them with YANG modules in a
   flexible manner.  In particular, tags may be registered as well as
   assigned during module definition, assigned by implementations, or
   dynamically defined and set by users.

   This document defines a YANG module [RFC7950] that provides a list of
   module entries to allow for adding or removing tags as well as
   viewing the set of tags associated with a module.

   This document defines an extension statement to indicate tags that
   SHOULD be added by the module implementation automatically (i.e.,
   outside of configuration).

   This document also defines an IANA registry for tag prefixes as well
   as a set of globally assigned tags.

   Section 6 provides guidelines for authors of YANG data models.

   This document updates [RFC8407].

   The YANG data model in this document conforms to the Network
   Management Datastore Architecture (NMDA) defined in [RFC8342].

1.1.  Some Possible Use Cases for YANG Module Tags

   During this document's development, there were requests for example
   uses of module tags.  The following are a few example use cases for
   tags.  This list is certainly not exhaustive.

   One example use of tags would be to help filter different discrete
   categories of YANG modules supported by a device.  For example, if
   modules are suitably tagged, then an XPath query can be used to list
   all of the vendor modules supported by a device.

   Tags can also be used to help coordination when multiple, semi-
   independent clients are interacting with the same devices.  For
   example, one management client could mark that some modules should
   not be used because they have not been verified to behave correctly,
   so that other management clients avoid querying the data associated
   with those modules.

   Tag classification is useful for users searching module repositories
   (e.g., YANG catalog).  A query restricted to the 'ietf:routing'
   module tag could be used to return only the IETF YANG modules
   associated with routing.  Without tags, a user would need to know the
   name of all the IETF routing protocol YANG modules.

   Future management protocol extensions could allow for filtering
   queries of configuration or operational state on a server based on
   tags (for example, return all operational state related to system
   management).

1.2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Tag Values

   All tags SHOULD begin with a prefix indicating who owns their
   definition.  An IANA registry (Section 7.1) is used to support
   registering tag prefixes.  Currently, three prefixes are defined.  No
   further structure is imposed by this document on the value following
   the registered prefix, and the value can contain any YANG type
   'string' characters except carriage returns, newlines, and tabs.

   Again, except for the conflict-avoiding prefix, this document is
   purposefully not specifying any structure on (i.e., restricting) the
   tag values.  The intent is to avoid arbitrarily restricting the
   values that designers, implementers, and users can use.  As a result
   of this choice, designers, implementers, and users are free to add or
   not add any structure they may require to their own tag values.

2.1.  IETF Tags

   An IETF tag is a tag that has the prefix "ietf:".  All IETF tags are
   registered with IANA in a registry defined later in this document
   (Section 7.2).

2.2.  Vendor Tags

   A vendor tag is a tag that has the prefix "vendor:".  These tags are
   defined by the vendor that implements the module and are not
   registered; however, it is RECOMMENDED that the vendor include extra
   identification in the tag to avoid collisions, such as using the
   enterprise or organization name following the "vendor:" prefix (e.g.,
   vendor:example.com:vendor-defined-classifier).

2.3.  User Tags

   A user tag is any tag that has the prefix "user:".  These tags are
   defined by the user/administrator and are not meant to be registered.
   Users are not required to use the "user:" prefix; however, doing so
   is RECOMMENDED as it helps avoid collisions.

2.4.  Reserved Tags

   Any tag not starting with the prefix "ietf:", "vendor:", or "user:"
   is reserved for future use.  These tag values are not invalid but
   simply reserved in the context of specifications (e.g., RFCs).

3.  Tag Management

   Tags can become associated with a module in a number of ways.  Tags
   may be defined and associated at module design time, at
   implementation time, or via user administrative control.  As the main
   consumer of tags are users, users may also remove any tag, no matter
   how the tag became associated with a module.

3.1.  Module Definition Tagging

   A module definition MAY indicate a set of tags to be added by the
   module implementer.  These design-time tags are indicated using the
   module-tag extension statement.

   If the module is defined in an IETF Standards Track document, the
   tags MUST be IETF tags (Section 2.1).  Thus, new modules can drive
   the addition of new IETF tags to the IANA registry defined in
   Section 7.2, and the IANA registry can serve as a check against
   duplication.

3.2.  Implementation Tagging

   An implementation MAY include additional tags associated with a
   module.  These tags SHOULD be IETF tags (i.e., registered) or vendor-
   specific tags.

3.3.  User Tagging

   Tags of any kind, with or without a prefix, can be assigned and
   removed by the user using normal configuration mechanisms.  In order
   to remove a tag from the operational datastore, the user adds a
   matching "masked-tag" entry for a given module.

4.  Tags Module Structure

4.1.  Tags Module Tree

   The tree associated with the "ietf-module-tags" module follows.  The
   meaning of the symbols can be found in [RFC8340].

       module: ietf-module-tags
         +--rw module-tags
            +--rw module* [name]
               +--rw name          yang:yang-identifier
               +--rw tag*          tag
               +--rw masked-tag*   tag

                  Figure 1: YANG Module Tags Tree Diagram

4.2.  YANG Module

   <CODE BEGINS> file "ietf-module-tags@2021-01-04.yang"
   module ietf-module-tags {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-module-tags";
     prefix tags;

     import ietf-yang-types {
       prefix yang;
     }

     organization
       "IETF NetMod Working Group (NetMod)";
     contact
       "WG Web:  <https://datatracker.ietf.org/wg/netmod/>
        WG List: <mailto:netmod@ietf.org>

        Author: Christian Hopps
                <mailto:chopps@chopps.org>

        Author: Lou Berger
                <mailto:lberger@labn.net>

        Author: Dean Bogdanovic
                <mailto:ivandean@gmail.com>";

     description
       "This module describes a mechanism associating tags with YANG
        modules.  Tags may be IANA assigned or privately defined.

        Copyright (c) 2021 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Simplified BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC 8819
        (https://www.rfc-editor.org/info/rfc8819); see the RFC itself
        for full legal notices.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.";

     revision 2021-01-04 {
       description
         "Initial revision.";
       reference
         "RFC 8819: YANG Module Tags";
     }

     typedef tag {
       type string {
         length "1..max";
         pattern '[\S ]+';
       }
       description
         "A tag is a type of 'string' value that does not include
          carriage return, newline, or tab characters.  It SHOULD begin
          with a registered prefix; however, tags without a registered
          prefix SHOULD NOT be treated as invalid.";
     }

     extension module-tag {
       argument tag;
       description
         "The argument 'tag' is of type 'tag'.  This extension statement
          is used by module authors to indicate the tags that SHOULD be
          added automatically by the system.  As such, the origin of the
          value for the predefined tags should be set to 'system'
          [RFC8342].";
     }

     container module-tags {
       description
         "Contains the list of modules and their associated tags.";
       list module {
         key "name";
         description
           "A list of modules and their associated tags.";
         leaf name {
           type yang:yang-identifier;
           mandatory true;
           description
             "The YANG module name.";
         }
         leaf-list tag {
           type tag;
           description
             "Tags associated with the module.  See the IANA 'YANG
              Module Tag Prefixes' registry for reserved prefixes and
              the IANA 'IETF YANG Module Tags' registry for IETF tags.

              The 'operational' state [RFC8342] view of this list is
              constructed using the following steps:

              1) System tags (i.e., tags of 'system' origin) are added.
              2) User-configured tags (i.e., tags of 'intended' origin)
              are added.
              3) Any tag that is equal to a masked-tag is removed.";
         }
         leaf-list masked-tag {
           type tag;
           description
             "The list of tags that should not be associated with this
              module.  The user can remove (mask) tags from the
              operational state datastore [RFC8342] by adding them to
              this list.  It is not an error to add tags to this list
              that are not associated with the module, but they have no
              operational effect.";
         }
       }
     }
   }
   <CODE ENDS>

                        Figure 2: Module Tags Module

5.  Other Classifications

   It is worth noting that a different YANG module classification
   document exists [RFC8199].  That document only classifies modules in
   a logical manner and does not define tagging or any other mechanisms.
   It divides YANG modules into two categories (service or element) and
   then into one of three origins: standard, vendor, or user.  It does
   provide a good way to discuss and identify modules in general.  This
   document defines IETF tags to support the classification style
   described in [RFC8199].

6.  Guidelines to Model Writers

   This section updates [RFC8407].

6.1.  Define Standard Tags

   A module MAY indicate, using module-tag extension statements, a set
   of tags that are to be automatically associated with it (i.e., not
   added through configuration).

   module example-module {
     namespace "https://example.com/yang/example";
     prefix "ex";
     //...
     import module-tags { prefix tags; }

     tags:module-tag "ietf:some-new-tag";
     tags:module-tag "ietf:some-other-tag";
     // ...
   }

   The module writer can use existing standard tags or use new tags
   defined in the model definition, as appropriate.  For IETF
   standardized modules, new tags MUST be assigned in the IANA registry
   defined below, see Section 7.2.

7.  IANA Considerations

7.1.  YANG Module Tag Prefixes Registry

   IANA has created the "YANG Module Tag Prefixes" subregistry in the
   "YANG Module Tags" registry.

   This registry allocates tag prefixes.  All YANG module tags SHOULD
   begin with one of the prefixes in this registry.

   Prefix entries in this registry should be short strings consisting of
   lowercase ASCII alpha-numeric characters and a final ":" character.

   The allocation policy for this registry is Specification Required
   [RFC8126].  The Reference and Assignee values should be sufficient to
   identify and contact the organization that has been allocated the
   prefix.

   The initial values for this registry are as follows.

        +=========+========================+===========+==========+
        | Prefix  | Description            | Reference | Assignee |
        +=========+========================+===========+==========+
        | ietf:   | IETF tags allocated in | RFC 8819  | IETF     |
        |         | the IANA "IETF YANG    |           |          |
        |         | Module Tags" registry. |           |          |
        +---------+------------------------+-----------+----------+
        | vendor: | Non-registered tags    | RFC 8819  | IETF     |
        |         | allocated by the       |           |          |
        |         | module implementer.    |           |          |
        +---------+------------------------+-----------+----------+
        | user:   | Non-registered tags    | RFC 8819  | IETF     |
        |         | allocated by and for   |           |          |
        |         | the user.              |           |          |
        +---------+------------------------+-----------+----------+

                                  Table 1

   Other standards development organizations (SDOs) wishing to allocate
   their own set of tags should allocate a prefix from this registry.

7.2.  IETF YANG Module Tags Registry

   IANA has created the "IETF YANG Module Tags" subregistry within the
   "YANG Module Tags" registry . This registry appears below the "YANG
   Module Tag Prefixes" registry.

   This registry allocates tags that have the registered prefix "ietf:".
   New values should be well considered and not achievable through a
   combination of already existing IETF tags.  IANA assigned tags must
   conform to Net-Unicode as defined in [RFC5198], and they shall not
   need normalization.

   The allocation policy for this registry is IETF Review [RFC8126].

   The initial values for this registry are as follows.

    +============================+=======================+===========+
    | Tag                        | Description           | Reference |
    +============================+=======================+===========+
    | ietf:network-element-class | Network element as    | [RFC8199] |
    |                            | defined in [RFC8199]. |           |
    +----------------------------+-----------------------+-----------+
    | ietf:network-service-class | Network service as    | [RFC8199] |
    |                            | defined in [RFC8199]. |           |
    +----------------------------+-----------------------+-----------+
    | ietf:sdo-defined-class     | Module is defined by  | [RFC8199] |
    |                            | a standards           |           |
    |                            | organization.         |           |
    +----------------------------+-----------------------+-----------+
    | ietf:vendor-defined-class  | Module is defined by  | [RFC8199] |
    |                            | a vendor.             |           |
    +----------------------------+-----------------------+-----------+
    | ietf:user-defined-class    | Module is defined by  | [RFC8199] |
    |                            | the user.             |           |
    +----------------------------+-----------------------+-----------+
    | ietf:hardware              | Relates to hardware   | RFC 8819  |
    |                            | (e.g., inventory).    |           |
    +----------------------------+-----------------------+-----------+
    | ietf:software              | Relates to software   | RFC 8819  |
    |                            | (e.g., installed OS). |           |
    +----------------------------+-----------------------+-----------+
    | ietf:protocol              | Represents a protocol | RFC 8819  |
    |                            | (often combined with  |           |
    |                            | another tag to        |           |
    |                            | refine).              |           |
    +----------------------------+-----------------------+-----------+
    | ietf:qos                   | Relates to quality of | RFC 8819  |
    |                            | service.              |           |
    +----------------------------+-----------------------+-----------+
    | ietf:network-service-app   | Relates to a network  | RFC 8819  |
    |                            | service application   |           |
    |                            | (e.g., an NTP server, |           |
    |                            | DNS server, DHCP      |           |
    |                            | server, etc.).        |           |
    +----------------------------+-----------------------+-----------+
    | ietf:system-management     | Relates to system     | RFC 8819  |
    |                            | management (e.g., a   |           |
    |                            | system management     |           |
    |                            | protocol such as      |           |
    |                            | syslog, TACAC+, SNMP, |           |
    |                            | NETCONF, etc.).       |           |
    +----------------------------+-----------------------+-----------+
    | ietf:oam                   | Relates to            | RFC 8819  |
    |                            | Operations,           |           |
    |                            | Administration, and   |           |
    |                            | Maintenance (e.g.,    |           |
    |                            | BFD).                 |           |
    +----------------------------+-----------------------+-----------+
    | ietf:routing               | Relates to routing.   | RFC 8819  |
    +----------------------------+-----------------------+-----------+
    | ietf:security              | Related to security.  | RFC 8819  |
    +----------------------------+-----------------------+-----------+
    | ietf:signaling             | Relates to control-   | RFC 8819  |
    |                            | plane signaling.      |           |
    +----------------------------+-----------------------+-----------+
    | ietf:link-management       | Relates to link       | RFC 8819  |
    |                            | management.           |           |
    +----------------------------+-----------------------+-----------+

                                 Table 2

7.3.  Updates to the IETF XML Registry

   This document registers a URI in the "IETF XML Registry" [RFC3688].
   Following the format in [RFC3688], the following registrations have
   been made:

   URI:  urn:ietf:params:xml:ns:yang:ietf-module-tags
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

   URI:  urn:ietf:params:xml:ns:yang:ietf-module-tags-state
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

7.4.  Updates to the YANG Module Names Registry

   This document registers two YANG modules in the "YANG Module Names"
   registry [RFC6020].  Following the format in [RFC6020], the following
   registrations have been made:

   name:  ietf-module-tags
   namespace:  urn:ietf:params:xml:ns:yang:ietf-module-tags
   prefix:  tags
   reference:  RFC 8819

   name:  ietf-module-tags-state
   namespace:  urn:ietf:params:xml:ns:yang:ietf-module-tags-state
   prefix:  tags-s
   reference:  RFC 8819

8.  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 Secure Shell (SSH) [RFC6242].

   This document adds the ability to associate tag metadata with YANG
   modules.  This document does not define any actions based on these
   associations, and none are yet defined; therefore, it does not by
   itself introduce any new security considerations directly.

   Users of the tag metadata may define various actions to be taken
   based on the tag metadata.  These actions and their definitions are
   outside the scope of this document.  Users will need to consider the
   security implications of any actions they choose to define, including
   the potential for a tag to get 'masked' by another user.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8199]  Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
              Classification", RFC 8199, DOI 10.17487/RFC8199, July
              2017, <https://www.rfc-editor.org/info/rfc8199>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,
              <https://www.rfc-editor.org/info/rfc8407>.

9.2.  Informative References

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <https://www.rfc-editor.org/info/rfc5198>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

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

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

Appendix A.  Examples

   The following is a fictional NETCONF example result from a query of
   the module tags list.  For the sake of brevity, only a few module
   results are shown.

   <ns0:data xmlns:ns0="urn:ietf:params:xml:ns:netconf:base:1.0">
     <t:module-tags
      xmlns:t="urn:ietf:params:xml:ns:yang:ietf-module-tags">
       <t:module>
         <t:name>ietf-bfd</t:name>
         <t:tag>ietf:network-element-class</t:tag>
         <t:tag>ietf:oam</t:tag>
         <t:tag>ietf:protocol</t:tag>
         <t:tag>ietf:sdo-defined-class</t:tag>
       </t:module>
       <t:module>
         <t:name>ietf-isis</t:name>
         <t:tag>ietf:network-element-class</t:tag>
         <t:tag>ietf:protocol</t:tag>
         <t:tag>ietf:sdo-defined-class</t:tag>
         <t:tag>ietf:routing</t:tag>
       </t:module>
       <t:module>
         <t:name>ietf-ssh-server</t:name>
         <t:tag>ietf:network-element-class</t:tag>
         <t:tag>ietf:protocol</t:tag>
         <t:tag>ietf:sdo-defined-class</t:tag>
         <t:tag>ietf:system-management</t:tag>
       </t:module>
     </t:module-tags>
   </ns0:data>

                   Figure 3: Example NETCONF Query Output

Appendix B.  Non-NMDA State Module

   As per [RFC8407], the following is a non-NMDA module to support
   viewing the operational state for non-NMDA compliant servers.

   <CODE BEGINS> file "ietf-module-tags-state@2021-01-04.yang"
   module ietf-module-tags-state {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-module-tags-state";
     prefix tags-s;

     import ietf-yang-types {
       prefix yang;
     }
     import ietf-module-tags {
       prefix tags;
     }

     organization
       "IETF NetMod Working Group (NetMod)";
     contact
       "WG Web:  <https://datatracker.ietf.org/wg/netmod/>
        WG List: <mailto:netmod@ietf.org>

        Author: Christian Hopps
                <mailto:chopps@chopps.org>

        Author: Lou Berger
                <mailto:lberger@labn.net>

        Author: Dean Bogdanovic
                <mailto:ivandean@gmail.com>";

     description
       "This module describes a mechanism associating tags with YANG
        modules.  Tags may be IANA assigned or privately defined.

        This is a temporary non-NMDA module that is for use by
        implementations that don't yet support NMDA.

        Copyright (c) 2021 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Simplified BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC 8819
        (https://www.rfc-editor.org/info/rfc8819); see the RFC itself
        for full legal notices.";

     revision 2021-01-04 {
       description
         "Initial revision.";
       reference
         "RFC 8819: YANG Module Tags";
     }

     container module-tags-state {
       config false;
       status deprecated;
       description
         "Contains the list of modules and their associated tags.";
       list module {
         key "name";
         status deprecated;
         description
           "A list of modules and their associated tags.";
         leaf name {
           type yang:yang-identifier;
           mandatory true;
           status deprecated;
           description
             "The YANG module name.";
         }
         leaf-list tag {
           type tags:tag;
           status deprecated;
           description
             "Tags associated with the module.  See the IANA 'YANG
              Module Tag Prefixes' registry for reserved prefixes and
              the IANA 'IETF YANG Module Tags' registry for IETF tags.

              The contents of this list is constructed using the
              following steps:

              1) System tags (i.e., tags of added by the system) are
              added.
              2) User-configured tags (i.e., tags added by
              configuration) are added.
              3) Any tag that is equal to a masked-tag present in the
              corresponding ietf-module-tags:module-tags:module-tag leaf
              list for this module is removed.";
         }
       }
     }
   }
   <CODE ENDS>

                Figure 4: Non-NMDA Module Tags State Module

Acknowledgements

   Special thanks to Robert Wilton for his help improving the
   introduction and providing the example use cases, as well as
   generating the non-NMDA module.

Authors' Addresses

   Christian Hopps
   LabN Consulting, L.L.C.

   Email: chopps@chopps.org

   Lou Berger
   LabN Consulting, L.L.C.

   Email: lberger@labn.net

   Dean Bogdanovic
   Volta Networks

   Email: ivandean@gmail.com