System-defined Configuration
draft-ietf-netmod-system-config-19

NETMOD                                                        Q. Ma, Ed.
Internet-Draft                                                     Q. Wu
Updates: 8342 (if approved)                                       Huawei
Intended status: Standards Track                                 C. Feng
Expires: 25 July 2026                                    21 January 2026

                      System-defined Configuration
                   draft-ietf-netmod-system-config-19

Abstract

   The Network Management Datastore Architecture (NMDA) in RFC 8342
   defines several configuration datastores holding configuration.  The
   contents of these configuration datastores are controlled by clients.
   This document introduces the concept of system configuration
   datastore holding configuration controlled by the system on which a
   server is running.  The system configuration can be referenced (e.g.,
   leafref) by configuration explicitly created by clients.

   This document updates RFC 8342.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 July 2026.

Copyright Notice

   Copyright (c) 2026 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

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.3.  Updates to RFC 8342 . . . . . . . . . . . . . . . . . . .   5
   2.  Kinds of System Configuration . . . . . . . . . . . . . . . .   5
     2.1.  Always-Present  . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Conditionally-Present . . . . . . . . . . . . . . . . . .   6
   3.  The System Configuration Datastore (<system>) . . . . . . . .   6
   4.  Conceptual Model of Datastores  . . . . . . . . . . . . . . .   7
   5.  Static Characteristics  . . . . . . . . . . . . . . . . . . .   9
     5.1.  Read-only to Clients  . . . . . . . . . . . . . . . . . .   9
     5.2.  No Changes to <operational> . . . . . . . . . . . . . . .   9
   6.  Dynamic Behaviors . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  May Change via Software Upgrades or Resource Changes  . .   9
     6.2.  Referencing System Configuration  . . . . . . . . . . . .  10
     6.3.  Overriding System Configuration . . . . . . . . . . . . .  10
     6.4.  Configuring Descendant nodes of System Configuration  . .  10
   7.  The "ietf-system-datastore" Module  . . . . . . . . . . . . .  10
     7.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  11
     7.2.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  The "IETF XML" Registry . . . . . . . . . . . . . . . . .  13
     8.2.  The "YANG Module Names" Registry  . . . . . . . . . . . .  13
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
     10.1.  Considerations for the "ietf-system-datastore" YANG
            Module . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.2.  Considerations for System Configuration  . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Example of Dynamic Behaviors . . . . . . . . . . . .  17
     A.1.  Referencing System-defined Nodes  . . . . . . . . . . . .  17
     A.2.  Modifying a System-instantiated Leaf's Value  . . . . . .  23
     A.3.  Configuring Descendant Nodes of a System-defined Node . .  24
   Appendix B.  Key Use Cases  . . . . . . . . . . . . . . . . . . .  25
     B.1.  Device Powers On  . . . . . . . . . . . . . . . . . . . .  27
     B.2.  Client Commits Configuration  . . . . . . . . . . . . . .  27
     B.3.  Operator Installs Card into a Chassis . . . . . . . . . .  29
     B.4.  Client further Commits Configuration  . . . . . . . . . .  30

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   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  32
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   The Network Management Datastore Architecture (NMDA) [RFC8342]
   defines system configuration as the configuration that is supplied by
   the device itself and appears in <operational> when it is in use
   (Figure 2 in [RFC8342]).

   However, there is a desire from operators to enable a server to
   better expose the system configuration, regardless of whether it is
   in use.  For example, some implementations define the system
   configuration which must be referenced to be active.  NETCONF/
   RESTCONF clients can benefit from a standard mechanism to retrieve
   what system configuration is available on a server.

   Some servers allow the descendant nodes of system-defined
   configuration to be configured or modified.  For example, the system
   configuration may contain an almost empty physical interface, whose
   existence in the system configuration is tied to the presence of
   particular hardware, while the client needs to be able to add,
   modify, or remove a number of descendant nodes.  Some descendant
   nodes may not be modifiable (e.g., the interface "type" set by the
   system).

   This document updates the NMDA defined in [RFC8342] with a read-only
   conventional configuration datastore called "system" to expose
   system-defined configuration.  The solution enables configuration
   explicitly created by the clients to reference nodes defined in
   <system>, override system-provided values, and configure descendant
   nodes of system-defined configuration.

   The solution defined in this document requires the use of NMDA for
   both clients and servers.  Conformance to this document requires NMDA
   servers implement the "ietf-system-datastore" YANG module
   (Section 7).

1.1.  Terminology

   This document assumes that the reader is familiar with the contents
   of [RFC6241], [RFC7950], [RFC8342], and [RFC8525] and uses
   terminologies from those documents.  The terms "device" and "server"
   are used interchangeably in this document.

   The following terms are defined in this document:

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   system configuration:  RFC 8342 defines it as "Configuration that is
      supplied by the device itself."  The definition herein refines
      that definition of system configuration to represent configuration
      present in the system configuration datastore (regardless of
      whether it is applied or referenced).  It may also be referred to
      as "system-defined configuration" or "system-provided
      configuration" throughout this document.  The system configuration
      discussed in this document cannot be deletable; configuration
      provided by the server that is deletable is outside the scope of
      this document.

   system configuration datastore:  A configuration datastore holding
      configuration provided by the system itself.  This datastore is
      referred to as "<system>".

   This document redefines the term "conventional configuration
   datastore" in Section 3 of [RFC8342] to add "system" to the list of
   conventional configuration datastores:

   conventional configuration datastore:  One of the following set of
      configuration datastores: <running>, <startup>, <candidate>,
      <system>, and <intended>.  These datastores share a common
      datastore schema, and protocol operations allow copying data
      between these datastores.  The term "conventional" is chosen as a
      generic umbrella term for these datastores.  Note while protocol
      operations allow copying data between conventional datastores, the
      read-only nature of datastores such as <system> and <intended>
      restricts clients from copying data into them.

   system node:  An instance in the data tree that is provided by the
      system itself.  System node may also be called "system-defined
      node" or "system-provided node" throughout this document.

   referenced node:  A referenced node is one of:

      *  Targets of leafref values defined via the "path" statement.

      *  Targets of "instance-identifier" type values.

      *  Nodes present in an XPath expression of "when" constraints.

      *  Nodes present in an XPath expression of "must" constraints.

      *  Nodes defined to satisfy the "mandatory true" constraints.

      *  Nodes defined to satisfy the "min-elements" constraints.

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1.2.  Requirements Language

   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.

1.3.  Updates to RFC 8342

   This document updates [RFC8342] to define a configuration datastore
   called "system" that holds system configuration (Section 3).  The
   document also redefines the term "conventional configuration
   datastore" from RFC8342 to include "system" in the list.

   To ensure the validity of <intended> when clients interact with
   system configuration (e.g., configuration provided by clients
   references system configuration) and allow the existence of system-
   defined templates and inactive system configuration, configuration in
   <running> is merged with <system> to create the contents of
   <intended> after the configuration transformations (e.g., template
   expansion, removal of inactive configuration defined in [RFC8342])
   have been performed, as described in Section 4.  Specifications in
   [RFC8342] related to the processing of system configuration are
   updated by the mechanism defined in this document.

   Additionally, this document also updates the definition of "intended"
   origin metadata annotation identity defined in Section 5.3.4 of
   [RFC8342].  The "intended" identity of origin value defined in
   [RFC8342] represents the origin of configuration provided by
   <intended>.  This document updates that definition as the origin
   source of configuration explicitly provided by clients, and allows a
   subset of configuration in <intended> that flows from <system> yet is
   not configured or overridden explicitly in <running> to use "system"
   as its origin value.  As per Section 5.3.4 of [RFC8342], all
   configuration with origin value being reported as "intended" MUST
   originate from <running>, which includes any configuration in
   <system> that has been copied into <running>.  Configuration that is
   in <system> and not also present in <running> MUST be reported as
   origin "system" in <operational>.

2.  Kinds of System Configuration

   This document defines two types of system configuration.
   Configuration that is always-present and configuration that is
   conditionally-present.  These types of system configuration are
   described in Section 2.1 and Section 2.2, respectively.

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2.1.  Always-Present

   Always-present refers to system configuration which is generated in
   <system> when the device is powered on, irrespective of whether
   physical resources are present or whether a special functionality is
   enabled.  An example of always-present system configuration is an
   always-existing loopback interface.

2.2.  Conditionally-Present

   Conditionally-present refers to system configuration which is
   generated in <system> based on specific conditions being met in a
   system.  For example, if a physical resource is present (e.g., an
   interface card is inserted), the system automatically detects it and
   loads the associated configuration; when the physical resource is not
   present (an interface card is removed), the system configuration will
   automatically be removed from <system>.  Another example is when a
   special functionality (e.g., a license or feature) is enabled,
   specific configuration may be created by the system.

3.  The System Configuration Datastore (<system>)

   Following guidelines for defining datastores in the Appendix A of
   [RFC8342], this document introduces a new datastore resource named
   "system" that represents the system configuration.  NMDA servers
   compliant with this document MUST implement a system configuration
   datastore, and they SHOULD also implement <intended>.

   *  Name: "system".

   *  YANG modules: all.

   *  YANG nodes: all "config true" data nodes up to the root node,
      generated by the system.

   *  Management operations: The datastore can be read using network
      management protocols such as NETCONF and RESTCONF, but its
      contents cannot be changed by management operations via NETCONF
      and RESTCONF protocols.

   *  Origin: This document does not define any new origin identity.
      The "system" identity of origin metadata annotation [RFC7952] is
      used to indicate the origin of a data item provided in <system>.

   *  Protocols: YANG-driven management protocols, such as NETCONF and
      RESTCONF.

   *  Defining YANG module: "ietf-system-datastore" (Section 7).

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   The system configuration datastore does not persist across reboots.

4.  Conceptual Model of Datastores

   Clients may provide configuration nodes that reference nodes defined
   in <system>, override system-provided values, and configure
   descendant nodes of system-defined configuration in <running>, as
   detailed in Section 6.

   To ensure the validity of <intended>, configuration in <running> is
   merged with <system> to become <intended>, in which process,
   configuration appearing in <running> takes precedence over the same
   node in <system>.  Since it is unspecified how to merge configuration
   before transformations, if <system> or <running> includes
   configuration that requires further transformation (e.g., template
   expansion, removal of inactive configuration defined in [RFC8342])
   before it can be applied, configuration transformations MUST be
   performed independently on each datastore before <running> is merged
   with <system>.

   Whenever configuration in <system> changes, the server MUST also
   immediately update and validate <intended>.

   As a result, Figure 2 in Section 5 of [RFC8342] is updated with the
   below conceptual model of datastores which incorporates the system
   configuration datastore.  For completeness, this model also include
   <factory-default> introduced in [RFC8808].

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                              +-----------------+
                              |<factory-default>|
                        +-----|    (ct, ro)     |-----+
                        |     +-----------------+     |
   //"factory-reset" RPC|             |               |
                        v             |               v
                +-------------+       |         +-----------+
                | <candidate> |       |         | <startup> |
                |  (ct, rw)   |<---+  |   +---->| (ct, rw)  |
                +-------------+    |  |   |     +-----------+
                       |           |  v   |           |
 +-----------+         |        +-----------+         |
 | <system>  |         +------->| <running> |<--------+
 | (ct, ro)  |                  | (ct, rw)  |
 +-----------+                  +-----------+
      |                              |
      |                              |
      |                              | // configuration transformations,
      +--------------+---------------+ // e.g., removal of nodes marked
                     |                 // as "inactive", expansion of
                     |                 // templates
                     v
               +------------+
               | <intended> |  // subject to validation
               | (ct, ro)   |
               +------------+
                      |       // changes applied, subject to
                      |       // local factors, e.g., missing
                      |       // resources, delays
  dynamic             |
  configuration       |   +-------- learned configuration
  datastores -----+   |   +-------- default configuration
                  |   |   |
                  v   v   v
              +---------------+
              | <operational> | <-- system state
              | (ct + cf, ro) |
              +---------------+

 ct = config true; cf = config false
 rw = read-write; ro = read-only
 boxes denote named datastores

              Figure 1: Architectural Model of Datastores

   Configuration in <system> cannot be deleted by clients (e.g., a list
   entry can never be removed from <system> through protocol
   operations), even though a node defined in <system> may be overridden

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   in <running>.  If the system initializes a value for a particular
   leaf which is overridden by the client with a different value in
   <running> (Section 6.3), and if the node in <running> is removed at a
   later time, the system-initialized value defined in <system> appears
   in <intended> and may come into use eventually if applied
   successfully.

   Configuration may disappear from <system> due to, e.g., resources no
   longer available.  In such cases, configuration for missing resources
   can still remain in <running> and <intended>, but it will not be
   applied and appear in <operational>.  This is further clarified in
   Section 5.3.2 of [RFC8342].

5.  Static Characteristics

5.1.  Read-only to Clients

   The system datastore is read-only (i.e., edits towards <system>
   directly MUST be denied), though the client may be allowed to provide
   configuration that overrides the value of a system-initialized node
   (see Section 6.3).

5.2.  No Changes to <operational>

   This work does not change the definition of <operational>, nor does
   it impact the contents of <operational>, as specified in [RFC8342].
   It clarifies origin reporting, i.e., the origin of nodes sourced from
   <system> is reported as "system" unless explicitly configured or
   overridden in <running>.  <system> enables system-defined nodes to be
   defined like configuration, i.e., made visible to clients in order
   for being referenced or configurable prior to present in
   <operational>.  "config false" nodes are out of scope, hence existing
   "config false" nodes are not impacted by this work.

6.  Dynamic Behaviors

6.1.  May Change via Software Upgrades or Resource Changes

   The contents of <system> MAY change dynamically under various
   conditions, such as license change, software upgrade, and system-
   controlled resources change (see Section 2.2).  The updates of system
   configuration may be obtained through YANG notifications (e.g., on-
   change notification) [RFC8639][RFC8641].

   If system configuration changes (e.g., during a software upgrade),
   <running> SHOULD remain a valid configuration data tree.  Any
   mechanisms to achieve this are outside the scope of this document.

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6.2.  Referencing System Configuration

   Clients may create configuration data in <running> that references
   nodes in <system>.  Some implementations may define system nodes
   solely as a convenience for clients to reference.  It is also
   possible for the clients to define their customized nodes for
   reference.

   Appendix A.1 provides an example of a client referencing system-
   defined nodes.

6.3.  Overriding System Configuration

   Although <system> is read-only, in some cases, a server may allow
   some parts of system configuration (e.g., a leaf's value) to be
   overridden, (note the distinction between <system> and system
   configuration).  Overriding of system configuration is achieved by
   the client writing configuration data in <running> that overrides the
   values of matched configuration nodes at the corresponding level in
   <system>.  Configurations defined in <running> take precedence over
   system configuration nodes in <system> if the server allows the nodes
   to be overridden (some implementations may have immutable system
   configuration which is identified by the server using an immutable
   metadata annotation, see [I-D.ietf-netmod-immutable-flag] for
   details), regardless of whether a system-instantiated value changes
   subsequently.

   Appendix A.2 provides an example of a client overriding a system-
   instantiated leaf's value.

6.4.  Configuring Descendant nodes of System Configuration

   A server may also allow a client to add nodes to a list entry in
   <system> by writing those additional nodes in <running>.  Those
   additional data nodes may not exist in <system> (i.e., an addition
   rather than an override).

   Appendix A.3 provides an example of a client configuring descendant
   nodes of a system-defined node.

7.  The "ietf-system-datastore" Module

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7.1.  Data Model Overview

   This YANG module defines a new YANG identity named "system" that uses
   the "ds:conventional" identity defined in [RFC8342] as its base.  A
   client can discover the system configuration datastore support on the
   server by reading the YANG library information from the operational
   state datastore.

   The system datastore is defined as a conventional configuration
   datastore and shares a common datastore schema with other
   conventional datastores.

   The following diagram illustrates the relationship amongst the
   "identity" statements defined in the "ietf-system-datastore" and
   "ietf-datastores" YANG modules:

   Identities:
       +--- datastore
       |  +--- conventional
       |  |  +--- running
       |  |  +--- candidate
       |  |  +--- startup
       |  |  +--- system
       |  |  +--- intended
       |  +--- dynamic
       |  +--- operational

   The diagram above uses syntax that is similar to but not defined in
   [RFC8340].

7.2.  YANG Module

   <CODE BEGINS> file "ietf-system-datastore@2026-01-22.yang"

   module ietf-system-datastore {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-system-datastore";
     prefix sysds;

     import ietf-datastores {
       prefix ds;
       reference
         "RFC 8342: Network Management Datastore Architecture(NMDA)";
     }

     organization
       "IETF NETMOD (Network Modeling) Working Group";
     contact

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       "WG Web:   <https://datatracker.ietf.org/wg/netmod/>
        WG List:  <mailto:netmod@ietf.org>

        Author: Qiufang Ma
                <mailto:maqiufang1@huawei.com>
        Author: Qin Wu
                <mailto:bill.wu@huawei.com>
        Author: Chong Feng
                <mailto:fengchongllly@gmail.com>";
     description
       "This module defines a new YANG identity that uses the
        ds:conventional identity defined in [RFC8342].

        Copyright (c) 2026 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 Revised
        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 XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
        itself for full legal notices.";

     revision 2026-01-22 {
       description
         "Initial version.";
       reference
         "RFC XXXX: System-defined Configuration";
     }

     identity system {
       base ds:conventional;
       description
         "This read-only datastore contains the configuration
          provided by the system itself.";
     }
   }

   <CODE ENDS>

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7.3.  Example Usage

   The following example shows how the configuration in <system> could
   be retrieved in a NETCONF <get-data> RPC operation.  The example uses
   the "example-application" fictional data model defined in
   Appendix A.1.

   <rpc message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <get-data xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-nmda"
        xmlns:sysds="urn:ietf:params:xml:ns:yang:ietf-system-datastore">
       <datastore>sysds:system</datastore>
       <subtree-filter>
         <applications xmlns="urn:example:application"/>
       </subtree-filter>
     </get-data>
   </rpc>

   When using the RESTCONF protocol, the system configuration datastore
   can be accessed via the resource: {+restconf}/ds/ietf-system-
   datastore:system.  The following example uses an HTTP GET method to
   request "applications" configuration:

   GET /restconf/ds/ietf-system-datastore:system/\
      example-application:applications HTTP/1.1
   Host: example.com
   Accept: application/yang-data+xml

8.  IANA Considerations

8.1.  The "IETF XML" Registry

   IANA is requested to register the following XML namespace URI in the
   'IETF XML registry', following the format defined in [RFC3688].

      URI: urn:ietf:params:xml:ns:yang:ietf-system-datastore
      Registrant Contact: The IESG.
      XML: N/A, the requested URIs are XML namespaces.

8.2.  The "YANG Module Names" Registry

   IANA is requested to register the following YANG module in the 'YANG
   Module Names' registry, defined in [RFC6020].

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         name: ietf-system-datastore
         prefix: sysds
         namespace: urn:ietf:params:xml:ns:yang:ietf-system-datastore
         maintained by IANA? N
         RFC: XXXX // RFC Ed.: replace XXXX and remove this comment

9.  Operational Considerations

   System configuration exists regardless of whether the server
   implements <system> or not.  The introduction of <system> provides a
   standardized way to expose system configuration within NMDA.

   NMDA clients that are not aware of <system> will continue to operate
   correctly.  They will interact only with datastores such as
   <running>, <candidate>, <intended>, and <operational> as before.  The
   presence of <system> does not change the fundamental behavior for
   such legacy clients.  Operators should be aware that to fully
   leverage the capabilities defined in this document, client
   applications need to be updated to recognize and interact with
   <system>.

10.  Security Considerations

10.1.  Considerations for the "ietf-system-datastore" YANG Module

   This section is modeled after the template described in Section 3.7
   of [I-D.ietf-netmod-rfc8407bis].

   The "ietf-system-datastore" YANG module defines a data model that is
   designed to be accessed via YANG-based management protocols, such as
   NETCONF [RFC6241] and RESTCONF [RFC8040].  These protocols have to
   use a secure transport layer (e.g., SSH [RFC4252], TLS
   [I-D.ietf-tls-rfc8446bis], and QUIC [RFC9000]) and have to use mutual
   authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   The YANG module only defines an identity that uses the
   "ds:conventional" identity as its base.  The module by itself does
   not expose any data nodes that are writable, data nodes that contain
   read-only state, or RPCs.  As such, there are no additional security
   issues related to the YANG module that need to be considered.

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10.2.  Considerations for System Configuration

   The system datastore, while read-only to clients, may contain
   sensitive information such as hardware identifiers, security
   policies, and critical system resources.  Read access to sensitive
   system nodes and subtrees within the datastore MUST be controlled to
   prevent unauthorized disclosure.  Implementations are strongly
   advised to log all access attempts to sensitive system configuration
   for audit purposes.

   Furthermore, while <system> cannot be modified directly, system
   configuration may be overridden as a merging result (Section 6.3).
   An attacker may configure a leaf that shadows a sensitive node in
   <system>.  Misconfiguration in <running> could lead to unintended
   system behavior including security policy bypass and availability
   risks.  Unauthorized modification to sensitive contents MUST be
   prevented to avoid those negative effects on the network.

11.  References

11.1.  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>.

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, DOI 10.17487/RFC7952, August 2016,
              <https://www.rfc-editor.org/info/rfc7952>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [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>.

11.2.  Informative References

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   [I-D.ietf-netmod-immutable-flag]
              Ma, Q., Wu, Q., Lengyel, B., and H. Li, "YANG Metadata
              Annotation for Immutable Flag", Work in Progress,
              Internet-Draft, draft-ietf-netmod-immutable-flag-07, 12
              January 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-netmod-immutable-flag-07>.

   [I-D.ietf-netmod-rfc8407bis]
              Bierman, A., Boucadair, M., and Q. Wu, "Guidelines for
              Authors and Reviewers of Documents Containing YANG Data
              Models", Work in Progress, Internet-Draft, draft-ietf-
              netmod-rfc8407bis-28, 5 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              rfc8407bis-28>.

   [I-D.ietf-tls-rfc8446bis]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8446bis-14, 13 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8446bis-14>.

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

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [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>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [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>.

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   [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>.

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.

   [RFC8639]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Subscription to YANG Notifications",
              RFC 8639, DOI 10.17487/RFC8639, September 2019,
              <https://www.rfc-editor.org/info/rfc8639>.

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

   [RFC8808]  Wu, Q., Lengyel, B., and Y. Niu, "A YANG Data Model for
              Factory Default Settings", RFC 8808, DOI 10.17487/RFC8808,
              August 2020, <https://www.rfc-editor.org/info/rfc8808>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

Appendix A.  Example of Dynamic Behaviors

   This section presents some sample data models and corresponding
   contents of various datastores with different dynamic behaviors
   described in Section 6.  The XML snippets are used only for
   illustration purposes.  Note this section does not show the contents
   of <intended> as they are related to the configuration in
   <operational> assuming the intended configuration is applied
   successfully.  Also note that if the "origin" metadata annotation for
   configuration is unspecified in snippets, it is inherited from its
   parent node.

A.1.  Referencing System-defined Nodes

   In this subsection, the following fictional module is used:

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   module example-application {
     yang-version 1.1;
     namespace "urn:example:application";
     prefix "ex-app";

     import ietf-inet-types {
       prefix "inet";
     }
     container applications {
       list application {
         key "name";
         leaf name {
           type string;
         }
         leaf app-id {
           type string;
         }
         leaf protocol {
           type enumeration {
             enum tcp;
             enum udp;
           }
           mandatory true;
         }
         leaf destination-port {
           default "0";
           type inet:port-number;
         }
         leaf description {
           type string;
         }
         container security-protection {
           presence "Indicates that security protection is enabled.";
           leaf risk-level {
             type enumeration {
               enum high;
               enum low;
             }
           }
           //additional leafs for security-specific configuration...
         }
       }
     }
   }

   A fictional ACL YANG module is used as follows, which defines a
   leafref for the leaf-list "application" data node to refer to an
   existing application name.

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   module example-acl {
     yang-version 1.1;
     namespace "urn:example:acl";
     prefix "ex-acl";

     import example-application {
       prefix "ex-app";
     }

     import ietf-inet-types {
       prefix "inet";
     }

     container acl {
       list acl-rule {
         key "name";
         leaf name {
           type string;
         }
         container matches {
           choice l3 {
             container ipv4 {
               leaf src-address {
                 type inet:ipv4-prefix;
               }
               leaf dst-address {
                 type inet:ipv4-prefix;
               }
             }
           }
           choice applications {
             leaf-list application {
               type leafref {
                 path "/ex-app:applications/ex-app:application"
                    + "/ex-app:name";
               }
             }
           }
         }
         leaf packet-action {
           type enumeration {
             enum forward;
             enum drop;
             enum redirect;
           }
         }
       }
     }

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   }

   The server may predefine some applications as a convenience for
   clients, these applications are immediately-present system
   configuration.  When the device is powered on, the system-
   instantiated application entries may be present in <system> as
   follows:

   <applications xmlns="urn:example:application">
     <application>
       <name>ftp</name>
       <app-id>001</app-id>
       <protocol>tcp</protocol>
       <destination-port>21</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
     <application>
       <name>tftp</name>
       <app-id>002</app-id>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
     <application>
       <name>smtp</name>
       <app-id>003</app-id>
       <protocol>tcp</protocol>
       <destination-port>25</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
   </applications>

   The client may also define customized applications.  Those
   applications may be present in <running> as follows:

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   <applications xmlns="urn:example:application">
     <application>
       <name>my-smtp</name>
       <app-id>101</app-id>
       <protocol>tcp</protocol>
       <destination-port>2345</destination-port>
       <description>customized smtp application</description>
       <security-protection>
         <risk-level>high</risk-level>
       </security-protection>
     </application>
     <application>
       <name>my-foo</name>
       <app-id>102</app-id>
       <protocol>udp</protocol>
       <destination-port>1024</destination-port>
       <description>customized application</description>
     </application>
   </applications>

   If a client configures an ACL rule referencing some system-provided
   or customized applications, the configuration of ACL rule may be
   shown as follows:

   <acl xmlns="urn:example:acl">
     <acl-rule>
       <name>allow-access-to-ftp-tftp</name>
       <matches>
         <ipv4>
           <src-address>198.51.100.0/24</src-address>
           <dst-address>192.0.2.0/24</dst-address>
         </ipv4>
         <application>ftp</application>
         <application>tftp</application>
         <application>my-smtp</application>
       </matches>
       <packet-action>forward</packet-action>
     </acl-rule>
   </acl>

   As different entries of application configuration in <system> and
   <running> are merged to create <intended>, and there are no merging
   conflicts in the contents between <system> and <running>,
   <operational> might contain the configuration of applications with
   the values of origin reflecting the source of entries as follows:

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   <applications xmlns="urn:example:application"
                 xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
                 or:origin="or:intended">
     <application>
       <name>my-smtp</name>
       <app-id>101</app-id>
       <protocol>tcp</protocol>
       <destination-port>2345</destination-port>
       <description>customized smtp application</description>
       <security-protection>
         <risk-level>high</risk-level>
       </security-protection>
     </application>
     <application>
       <name>my-foo</name>
       <app-id>102</app-id>
       <protocol>udp</protocol>
       <destination-port>1024</destination-port>
       <description>customized application</description>
     </application>
     <application or:origin="or:system">
       <name>ftp</name>
       <app-id>001</app-id>
       <protocol>tcp</protocol>
       <destination-port>21</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
     <application or:origin="or:system">
       <name>tftp</name>
       <app-id>002</app-id>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
     <application or:origin="or:system">
       <name>smtp</name>
       <app-id>003</app-id>
       <protocol>tcp</protocol>
       <destination-port>25</destination-port>
       <security-protection>
         <risk-level>low</risk-level>
       </security-protection>
     </application>
   </applications>

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A.2.  Modifying a System-instantiated Leaf's Value

   This subsection uses the following fictional interface YANG module:

   module example-interface {
     yang-version 1.1;
     namespace "urn:example:interface";
     prefix "ex-if";

     import ietf-inet-types {
       prefix "inet";
     }

     container interfaces {
       list interface {
         key name;
         leaf name {
           type string;
         }
         leaf description {
           type string;
         }
         leaf mtu {
           type uint32;
         }
         leaf-list ip-address {
           type inet:ip-address;
         }
       }
     }
   }

   Suppose the system provides an always-present loopback interface
   (named "lo0") with an MTU value "65536", a default IPv4 address of
   "127.0.0.1", and a default IPv6 address of "::1".  The configuration
   of "lo0" interface may be present in <system> as follows:

   <interfaces xmlns="urn:example:interface">
     <interface>
       <name>lo0</name>
       <mtu>65536</mtu>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
   </interfaces>

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   A client modifies the value of MTU to 9216 by adding the following
   configuration into <running>:

   <interfaces xmlns="urn:example:interface">
     <interface>
       <name>lo0</name>
       <mtu>9216</mtu>
     </interface>
   </interfaces>

   Since the MTU value provided by the client takes precedence over the
   system-provided value, and the "origin" value of configuration
   provided by the client is set to "intended", the configuration of
   interfaces that is present in <operational> may be as follows:

   <interfaces xmlns="urn:example:interface"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface>
       <name>lo0</name>
       <mtu>9216</mtu>
       <ip-address or:origin="or:system">127.0.0.1</ip-address>
       <ip-address or:origin="or:system">::1</ip-address>
     </interface>
   </interfaces>

A.3.  Configuring Descendant Nodes of a System-defined Node

   Based on the example in Appendix A.2, imagine the client further adds
   the description node of a "lo0" interface in <running> as follows:

   <interfaces xmlns="urn:example:interface">
     <interface>
       <name>lo0</name>
       <description>loopback</description>
     </interface>
   </interfaces>

   The configuration of interface "lo0" is present in <operational> as
   follows:

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   <interfaces xmlns="urn:example:interface"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface>
       <name>lo0</name>
       <description>loopback</description>
       <mtu>9216</mtu>
       <ip-address or:origin="or:system">127.0.0.1</ip-address>
       <ip-address or:origin="or:system">::1</ip-address>
     </interface>
   </interfaces>

Appendix B.  Key Use Cases

   This section updates the "Interface Example" supplied in Appendix C.3
   of [RFC8342].

   This section provides several use cases related to how <system>
   interacts with other datastores (e.g., <candidate>, <running>,
   <intended>, and <operational>).  The following fictional interface
   data model is used:

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   module example-interface-management {
     yang-version 1.1;
     namespace "urn:example:interfacemgmt";
     prefix "ex-ifm";

     import ietf-inet-types {
       prefix "inet";
     }

     container interfaces {
       list interface {
         key "name";
         leaf name {
           type string;
         }
         leaf type {
           type enumeration {
             enum ethernet;
             enum atm;
             enum loopback;
           }
         }
         leaf enabled {
           type boolean;
           default "true";
         }
         leaf-list ip-address {
           type inet:ip-address;
         }
         leaf speed {
           when "../type = 'ethernet'";
           type enumeration {
             enum 10Mb;
             enum 100Mb;
           }
         }
         leaf description {
           type string;
         }
       }
     }
   }

   For each use case, corresponding sample configuration in <running>,
   <system>, <intended> and <operational> are shown.  The XML snippets
   are used only for illustration purposes.

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B.1.  Device Powers On

   When the device is powered on, suppose the system provides an always-
   present loopback interface (named "lo0") which is not explicitly
   configured in <running>.  Thus, no configuration for interfaces
   appears in <running>;

   And the contents of <system> are:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

   In this case, the configuration of loopback interface is only present
   in <system>, the configuration of interface in <intended> would be
   identical to the one in <system> shown above.

   And <operational> will show the system-provided loopback interface,
   note that <operational> also includes the default value specified in
   the YANG module:

   <interfaces xmlns="urn:example:interfacemgmt"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:system">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <enabled or:origin="or:default">true</enabled>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

B.2.  Client Commits Configuration

   If a client creates an interface "et-0/0/0" but the interface does
   not physically exist at this point, what is in <running> appears as
   follows:

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   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>et-0/0/0</name>
       <ip-address>192.168.10.10</ip-address>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

   And the contents of <system> remain unchanged, only containing the
   "lo0" loopback interface, since the interface "et-0/0/0" is not
   physically present:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

   The contents of <intended> represent the merged data of <system> and
   <running>:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <ip-address>192.168.10.10</ip-address>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

   Since the interface named "et-0/0/0" does not exist, the associated
   configuration is not present in <operational>, which appears as
   follows:

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   <interfaces xmlns="urn:example:interfacemgmt"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface or:origin="or:system">
       <name>lo0</name>
       <type>loopback</type>
       <enabled or:origin="or:default">true</enabled>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

B.3.  Operator Installs Card into a Chassis

   When the interface is installed by the operator, the system will
   detect it and generate the associated conditionally-present interface
   configuration in <system>.  The contents of <running> keep unchanged:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>et-0/0/0</name>
       <ip-address>192.168.10.10</ip-address>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

   And <system> might appear as follows:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type>ethernet</type>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

   Then <intended> contains the merged configuration of <system> and
   <running>:

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   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type>ethernet</type>
       <ip-address>192.168.10.10</ip-address>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

   And the contents of <operational> appear as follows:

   <interfaces xmlns="urn:example:interfacemgmt"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface or:origin="or:system">
       <name>lo0</name>
       <type>loopback</type>
       <enabled or:origin="or:default">true</enabled>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type or:origin="or:system">ethernet</type>
       <enabled or:origin="or:default">true</enabled>
       <ip-address>192.168.10.10</ip-address>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

B.4.  Client further Commits Configuration

   If the client further sets the speed of interface "et-0/0/0" in
   <running>:

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   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>et-0/0/0</name>
       <speed>10Mb</speed>
     </interface>
   </interfaces>

   The contents of <system> keep unchanged:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type>ethernet</type>
       <description>system-defined interface</description>
     </interface>
   </interfaces>

   And the contents of <intended> which represents a merged results of
   <running> and <system> are as follows:

   <interfaces xmlns="urn:example:interfacemgmt">
     <interface>
       <name>lo0</name>
       <type>loopback</type>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type>ethernet</type>
       <ip-address>192.168.10.10</ip-address>
       <speed>10Mb</speed>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

   And <operational> would appear as follows:

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   <interfaces xmlns="urn:example:interfacemgmt"
               xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface or:origin="or:system">
       <name>lo0</name>
       <type>loopback</type>
       <enabled>true</enabled>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
       <description>system-defined interface</description>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <type or:origin="or:system">ethernet</type>
       <enabled or:origin="or:default">true</enabled>
       <ip-address>192.168.10.10</ip-address>
       <speed>10Mb</speed>
       <description>pre-provisioned interface</description>
     </interface>
   </interfaces>

Acknowledgements

   The authors would like to thank for following for discussions and
   providing input to this document: Balazs Lengyel, Robert Wilton,
   Juergen Schoenwaelder, Andy Bierman, Martin Bjorklund, Mohamed
   Boucadair, Michal Vaško, Alexander Clemm, and Timothy Carey.

Contributors

   Kent Watsen
   Watsen Networks
   Email: kent+ietf@watsen.net

   Jan Lindblad
   Cisco Systems
   Email: jlindbla@cisco.com

   Jason Sterne
   Nokia
   Email: jason.sterne@nokia.com

   Chongfeng Xie
   China Telecom
   Beijing
   China
   Email: xiechf@chinatelecom.cn

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Authors' Addresses

   Qiufang Ma (editor)
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing
   Jiangsu, 210012
   China
   Email: maqiufang1@huawei.com

   Qin Wu
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing
   Jiangsu, 210012
   China
   Email: bill.wu@huawei.com

   Chong Feng
   Email: fengchongllly@gmail.com

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