NETMOD                                                        Q. Ma, Ed.
Internet-Draft                                                     Q. Wu
Updates: 8342, 6241, 8526, 8040 (if approved)                     Huawei
Intended status: Standards Track                                 C. Feng
Expires: 2 December 2024                                     31 May 2024


                      System-defined Configuration
                   draft-ietf-netmod-system-config-06

Abstract

   This document defines how a management client and server handle YANG-
   modeled configuration data that is instantiated by the server itself.
   The system-defined configuration can be referenced (e.g., leafref) by
   configuration explicitly created by a client.

   The Network Management Datastore Architecture (NMDA) defined in RFC
   8342 is updated with a read-only conventional configuration datastore
   called "system" to expose system-defined configuration.

   This document updates RFC 8342, RFC 6241, RFC 8526 and RFC 8040.

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
   working documents as Internet-Drafts.  The list of current Internet-
   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 2 December 2024.

Copyright Notice

   Copyright (c) 2024 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.



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   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 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 . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.3.  Updates to RFC 8342 . . . . . . . . . . . . . . . . . . .   5
     1.4.  Updates to RFC 6241 and RFC 8526  . . . . . . . . . . . .   6
     1.5.  Updates to RFC 8040 . . . . . . . . . . . . . . . . . . .   6
   2.  Kinds of System Configuration . . . . . . . . . . . . . . . .   6
     2.1.  Immediately-Present . . . . . . . . . . . . . . . . . . .   6
     2.2.  Conditionally-Present . . . . . . . . . . . . . . . . . .   6
   3.  The System Configuration Datastore (<system>) . . . . . . . .   7
   4.  Static Characteristics of <system>  . . . . . . . . . . . . .   7
     4.1.  Read-only to Clients  . . . . . . . . . . . . . . . . . .   7
     4.2.  May Change via Software Upgrades or Resource Changes  . .   7
     4.3.  No Impact to <operational>  . . . . . . . . . . . . . . .   8
   5.  Dynamic Behaviors . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Conceptual Model of Datastores  . . . . . . . . . . . . .   8
       5.1.1.  Origin Metadata Annotation  . . . . . . . . . . . . .  10
     5.2.  Explicit Declaration of System Configuration  . . . . . .  10
     5.3.  Servers Auto-configuring Referenced System Configuration
           ("resolve-system" parameter)  . . . . . . . . . . . . . .  10
       5.3.1.  NETCONF Support for "resolve-system" Parameter  . . .  11
       5.3.2.  RESTCONF Support for "resolve-system" Parameter . . .  12
         5.3.2.1.  Query Parameter . . . . . . . . . . . . . . . . .  12
     5.4.  Modifying (Overriding) System Configuration . . . . . . .  12
     5.5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.5.1.  Server Configuring of <running> Automatically . . . .  13
       5.5.2.  Declaring a System-defined Node in <running>
               Explicitly  . . . . . . . . . . . . . . . . . . . . .  18
       5.5.3.  Modifying a System-instantiated Leaf's Value  . . . .  19
       5.5.4.  Configuring Descendant Nodes of a System-defined
               Node  . . . . . . . . . . . . . . . . . . . . . . . .  20
   6.  Default Interactions  . . . . . . . . . . . . . . . . . . . .  21
   7.  Relation to Other Datastores  . . . . . . . . . . . . . . . .  21
     7.1.  The "factory-default" Datastore . . . . . . . . . . . . .  21
     7.2.  The "candidate" Datastore . . . . . . . . . . . . . . . .  21
   8.  The "ietf-system-datastore" Module  . . . . . . . . . . . . .  22
     8.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  22
     8.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  23
     8.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  24
   9.  The "ietf-netconf-resolve-system" Module  . . . . . . . . . .  26



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     9.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  26
     9.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  26
     9.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  26
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  The "IETF XML" Registry  . . . . . . . . . . . . . . . .  29
     10.2.  The "YANG Module Names" Registry . . . . . . . . . . . .  29
     10.3.  NETCONF Capability URN Registry  . . . . . . . . . . . .  29
     10.4.  RESTCONF Capability URN Registry . . . . . . . . . . . .  29
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  30
     11.1.  Regarding the "ietf-system-datastore" YANG Module  . . .  30
     11.2.  Regarding the "ietf-netconf-resolve-system" YANG
            Module . . . . . . . . . . . . . . . . . . . . . . . . .  30
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Key Use Cases  . . . . . . . . . . . . . . . . . . .  33
     A.1.  Device Powers On  . . . . . . . . . . . . . . . . . . . .  33
     A.2.  Client Commits Configuration  . . . . . . . . . . . . . .  34
     A.3.  Operator Installs Card into a Chassis . . . . . . . . . .  35
   Appendix B.  Changes between Revisions  . . . . . . . . . . . . .  36
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  38
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

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 to enable a server to better structure and
   expose the system configuration.  NETCONF/RESTCONF clients can
   benefit from a standard mechanism to retrieve what system
   configuration is available on a server.

   Some servers allow clients to reference a system-defined node which
   is not present in the datastore.  The absence of the system
   configuration in the datastore can render the datastore invalid from
   the perspective of a client or offline tools (e.g., missing leafref
   targets).  This document describes several approaches to bring the
   datastore to a valid state and satisfy referential integrity
   constraints.








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   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, 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 Network Management Datastore Architecture
   (NMDA) defined in [RFC8342] with a read-only conventional
   configuration datastore called "system" to expose system-defined
   configuration.

   As an alternative to clients explicitly copying referenced system-
   defined configuration so that the datastore is valid, a "resolve-
   system" parameter is defined to allow the server to copy referenced
   system nodes automatically.  This solution enables clients to
   reference nodes defined in <system>, override system-provided values,
   and configure descendant nodes of system-defined configuration.

   If a system-defined node is referenced, it refers to one of the
   following cases throughout this document:

   *  It is present in a leafref "path" statement and referred as the
      leafref value

   *  It is used as an "instance-identifier" type value

   *  It is present in an XPath expression of "when" or "must"
      constraints

   *  It is defined to satisfy the "mandatory" constraints

   *  It is defined to exactly satisfy the "min-element" constraints

   Conformance to this document requires the NMDA servers to implement
   the "ietf-system-datastore" YANG module (Section 8).

1.1.  Terminology

   This document assumes that the reader is familiar with the contents
   of [RFC6241], [RFC7950], [RFC8342], [RFC8407], and [RFC8525] and uses
   terminologies from those documents.

   The following terms are defined in this document:

   System configuration:  Configuration that is provided by the system





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      itself.  System configuration is present in the system
      configuration datastore (regardless of whether it is applied or
      referenced).  It is a different and separate concept from factory
      default configuration defined in [RFC8808] (which represents a
      preset initial configuration that is used to initialize the
      configuration of a server).


   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.


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 RFC 8342 to define a configuration datastore
   called "system" to hold system configuration (Section 3), it also
   redefines the term "conventional configuration datastore" from
   [RFC8342] to add "system" to the list of conventional configuration
   datastores.

   Configuration in <running> is merged with <system> to create the
   contents of <intended> after the configuration transformations to
   <running> (e.g., template expansion, removal of inactive
   configuration defined in [RFC8342]) have been performed
   (Section 5.1).

   The definition of "intended" origin metadata annotation identity is
   also updated (Section 5.1.1).



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1.4.  Updates to RFC 6241 and RFC 8526

   This document updates RFC 6241 to augment the NETCONF <edit-config>,
   <copy-config>, <validate>, and <commit> operations with an additional
   input parameter named "resolve-system", as specified in Section 5.3.

   This document also updates RFC 8526 to augment the NETCONF <edit-
   data> operation with the "resolve-system" parameter, as specified in
   Section 5.3.

1.5.  Updates to RFC 8040

   This document extends sections 4.8 and 9.1.1 in [RFC8040] to add a
   new query parameter "resolve-system" and corresponding query
   parameter capability URI (Section 5.3.2).

2.  Kinds of System Configuration

   This document defines two types of system configuration:
   configuration that is generated in <system> immediately when the
   device boots and configuration that is generated in <system> only
   when specific conditions being met on a device, they are described in
   Section 2.1 and Section 2.2, respectively.

2.1.  Immediately-Present

   Immediately-present refers to system configuration which is generated
   in <system> when the device is powered on, irrespective of physical
   resource present or not, a special functionality enabled or not.  An
   example of immediately-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 associated configuration; when the physical resource is not
   present (an interface card is removed), the system configuration will
   be automatically cleared.  Another example is when a special
   functionality is enabled, e.g., when a license or feature is enabled,
   specific configuration may be created by the system.








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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 of the
      tree, 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 manage operations via NETCONF and
      RESTCONF protocols.

   *  Origin: This document does not define any new origin identity.
      The "system" origin Metadata Annotation [RFC7952] is used to
      indicate the origin of a data item in system (Section 5.1.1).

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

   *  Defining YANG module: "ietf-system-datastore" (Section 8).

   The system configuration datastore doesn't persist across reboots.

4.  Static Characteristics of <system>

4.1.  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
   override the value of a system-initialized node (see Section 5.4).

4.2.  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) [RFC6470][RFC8639][RFC8641].





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   Generally speaking, any update of <system> should not cause the
   automatic update of <running> to not surprise clients with unexpected
   changes.  In particular, the behavior of system data migration during
   software upgrade is outside the scope of this document.  That said,
   here are some examples of how a server might handle this scenario
   ensuring <running> remains accurate:

   *  Servers migrate system configuration update in <running>.

   *  Servers reject the operation to change system configuration (e.g.,
      software upgrade fails) and needs the client to update the
      configuration in <running> as a prerequisite.  Servers are
      recommended to include some hints in error responses to help
      clients understand how <running> should be updated.

4.3.  No Impact to <operational>

   This work intends to have no impact to <operational> and does not
   define any new origin identity beyond Section 5.3.4 of [RFC8342].
   The existence of <system> enables a subset of those system generated
   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.

5.  Dynamic Behaviors

5.1.  Conceptual Model of Datastores

   Clients MAY reference nodes defined in <system>, override system-
   provided values, and configure descendant nodes of system-defined
   configuration in <running>.

   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>.  If <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 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.



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                +-------------+                 +-----------+
                | <candidate> |                 | <startup> |
                |  (ct, rw)   |<---+      +---->| (ct, rw)  |
                +-------------+    |      |     +-----------+
                       |           |      |           |
 +-----------+         |        +-----------+         |
 | <system>  |         +------->| <running> |<--------+
 | (ct, ro)  |                  | (ct, rw)  |
 +-----------+                  +-----------+
      |                              | // configuration transformations,
      |                              | // e.g., removal of nodes marked
      |           // merge           | // 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> is non-deletable to clients, even though a
   client may delete a copied system node from <running>.  If system
   initializes a value for a particular leaf which is overridden by the
   client with a different value in <running>, the client may delete it
   in <running>, in which case system-initialized value defined in
   <system> may still be in use and appear in <operational>.






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5.1.1.  Origin Metadata Annotation

   This document does not define any new origin identity when <system>
   interacts with <intended> and flows into <operational>.

   The "intended" identity of origin value defined in [RFC8342] to
   represent the origin of configuration provided by <intended>, this
   document updates its definition as origin source of configuration
   explicitly provided by <running>, 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.

   Configuration copied from <system> into <running> has its origin
   value reported as "intended" when it flows into <operational>.

5.2.  Explicit Declaration of System Configuration

   It is possible for a client to explicitly declare system
   configuration nodes with the same values as in <system>, by
   configuring a node (list/leaf-list entry, leaf, etc.) in the target
   datastore (e.g., <candidate> and <running>) that matches the same
   node and value in <system>.

   The explicit declaration of system-defined nodes can be useful, for
   example, when the client does not support the "resolve-system"
   parameter but needs the datastore to be referentially complete.  The
   client can explicitly copy the list entries (with at least the keys)
   that are referenced elsewhere in the datastore.  The client does not
   necessarily need to declare all the contents of the list entry (i.e.
   the descendant nodes), only the parts that are required to make the
   datastore appear valid.

5.3.  Servers Auto-configuring Referenced System Configuration
      ("resolve-system" parameter)

   This document defines a new parameter "resolve-system" to the input
   for some of the NETCONF and RESTCONF operations.  Clients that are
   aware of the "resolve-system" parameter MAY use this parameter to
   avoid the requirement to provide a referentially complete
   configuration.

   The "resolve-system" parameter is optional and has no value.  If it
   is present, and the server supports this capability, the server MUST
   copy referenced system nodes into the target datastore (e.g.,
   <candidate> and <running>) where it is missing from without the
   client doing the copy/paste explicitly, to resolve any references not
   resolved by the client.  If any child node of the copied



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   configuration has its value being set by the system that is not the
   schema default value, it MUST also be copied into the target
   datastore (Section 6).  The copy operation MUST NOT overwrite any
   explicit configuration in <running>.  The server copies the
   referenced system-defined nodes only when triggered by the "resolve-
   system" parameter.  Legacy clients don't see any changes in the
   server behaviors.

   The server may automatically configure the list entries (with at
   least the keys) that are referenced elsewhere by the clients.
   Similarly, not all the contents of the list entry (i.e., the
   descendant nodes) are necessarily copied by the server - only the
   parts that are required to make configuration valid.

   There is no distinction between the configuration automatically
   configured by the server and the one explicitly declared by the
   client, e.g., a read back of the datastore (e.g., NETCONF <get>/<get-
   config>/<get-data> operation, or RESTCONF GET method) returns
   automatically configured nodes.

   Note that even though an auto-configured node is allowed to be
   deleted from the target datastore by the client, the system may
   automatically recreate the deleted node to make configuration valid,
   when a "resolve-system" parameter is carried.  It is also possible
   that the operation request (e.g., <edit-config>) may not succeed due
   to incomplete referential integrity.

   Support for the "resolve-system" parameter is OPTIONAL.  Servers not
   supporting NMDA [RFC8342] MAY also implement this parameter without
   implementing the system configuration datastore, which would only
   eliminate the ability to retrieve the system configuration via
   protocol operations.  If a server implements <system>, referenced
   system configuration is copied from <system> into the target
   datastore when the "resolve-system" parameter is used; otherwise it
   is an implementation decision where to copy referenced system
   configuration.

5.3.1.  NETCONF Support for "resolve-system" Parameter

   This document defines a NETCONF protocol capability to indicate
   support for this parameter.  NETCONF server that supports "resolve-
   system" parameter MUST advertise the following capability identifier:

   urn:ietf:params:netconf:capability:resolve-system:1.0







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5.3.2.  RESTCONF Support for "resolve-system" Parameter

5.3.2.1.  Query Parameter

   The "resolve-system" parameter may be present in the request URI of
   some RESTCONF operations as shown in Figure 2.  This parameter is
   only allowed with no values carried.  If this parameter has any
   unexpected value, then a "400 Bad Request" status-line is returned.

  +----------------+---------+-----------------------------------------+
  | Name           | Methods | Description                             |
  +----------------+---------+-----------------------------------------+
  |resolve-system  | POST,   | resolve any references not resolved by  |
  |                | PUT     | the client and copy referenced          |
  |                | PATCH   | system configuration into <running>     |
  |                |         | automatically. This parameter can be    |
  |                |         | given in any order.                     |
  +----------------+---------+-----------------------------------------+

           Figure 2: RESTCONF "resolve-system" Query Parameter

5.3.2.1.1.  Query Parameter URI

   To enable a RESTCONF client to discover if the "resolve-system" query
   parameter is supported by the server, the following capability URI is
   defined, which is advertised by the server if supported, using the
   "ietf-restconf-monitoring" module defined in [RFC8040]:

   urn:ietf:params:restconf:capability:resolve-system:1.0

5.4.  Modifying (Overriding) System Configuration

   In some cases, a server may allow some parts of system configuration
   to be modified.  Modification of system configuration is achieved by
   the client writing configuration to <running> that overrides the
   system configuration.  Configurations defined in <running> take
   precedence over system configuration nodes in <system> if the server
   allows the nodes to be modified.

   For instance, descendant nodes in a system-defined list entry may be
   modifiable or not, even if some system configuration has been copied
   into <running> earlier.  If a system node is non-modifiable, then
   writing a different value for that node MUST return an error during a
   <edit-config>, <validate> or <commit> operation, depending on the
   target datastore.  The immutability of system configuration is
   defined in [I-D.ietf-netmod-immutable-flag].





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5.5.  Examples

   This section presents some sample data models and corresponding
   contents of various datastores with different dynamical behaviors
   above.  The XML snippets are used only for examples.

5.5.1.  Server Configuring of <running> Automatically

   In this subsection, the following fictional module is used:

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

     import ietf-inet-types {
       prefix "inet";
     }
     container applications {
       list application {
         key "name";
         leaf name {
           type string;
         }
         leaf protocol {
           type enumeration {
             enum tcp;
             enum udp;
           }
         }
         leaf destination-port {
           type inet:port-number;
         }
       }
     }
   }

   The server may predefine some applications as a convenience for
   clients.  The system-instantiated application entries may be present
   in <system> as follows:











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   <applications xmlns="urn:example:application">
     <application>
       <name>ftp</name>
       <protocol>tcp</protocol>
       <destination-port>21</destination-port>
     </application>
     <application>
       <name>tftp</name>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
     </application>
     <application>
       <name>smtp</name>
       <protocol>tcp</protocol>
       <destination-port>25</destination-port>
     </application>
   </applications>

   The client may also define its customized applications.  Suppose the
   configuration of applications is present in <running> as follows:

   <applications xmlns="urn:example:application">
     <application>
       <name>my-app-1</name>
       <protocol>tcp</protocol>
       <destination-port>2345</destination-port>
     </application>
     <application>
       <name>my-app-2</name>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
     </application>
   </applications>

   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.

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

     import example-application {
       prefix "app";
     }

     import ietf-inet-types {



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       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 "/app:applications/app:application/app:name";
               }
             }
           }
         }
         leaf packet-action {
           type enumeration {
             enum forward;
             enum drop;
             enum redirect;
           }
         }
       }
     }
   }

   If a client configures an ACL rule referencing system provided
   applications which are not present in <running>, the client may issue
   an <edit-config> operation with the parameter "resolve-system" to the
   NETCONF server as follows:








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   <rpc message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <edit-config>
       <target>
         <running/>
       </target>
       <config>
         <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-app-1</application>
             </matches>
             <packet-action>forward</packet-action>
           </acl-rule>
         </acl>
       </config>
       <resolve-system/>
     </edit-config>
   </rpc>


   The following gives the configuration of applications in <running>
   which is returned in the response to a follow-up retrieval operation:





















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   <applications xmlns="urn:example:application">
     <application>
       <name>my-app-1</name>
       <protocol>tcp</protocol>
       <destination-port>2345</destination-port>
     </application>
     <application>
       <name>my-app-2</name>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
     </application>
     <application>
       <name>ftp</name>
     </application>
     <application>
       <name>tftp</name>
     </application>
   </applications>


   And the configuration of applications is present in <operational> as
   follows:

   <applications xmlns="urn:example:application"
                 xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
                 or:origin="or:intended">
     <application>
       <name>my-app-1</name>
       <protocol>tcp</protocol>
       <destination-port>2345</destination-port>
     </application>
     <application>
       <name>my-app-2</name>
       <protocol>udp</protocol>
       <destination-port>69</destination-port>
     </application>
     <application>
       <name>ftp</name>
       <protocol or:origin="or:system">tcp</protocol>
       <destination-port or:origin="or:system">21</destination-port>
     </application>
     <application>
       <name>tftp</name>
       <protocol or:origin="or:system">udp</protocol>
       <destination-port or:origin="or:system">69</destination-port>
     </application>
   </applications>




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   Since the configuration of application "smtp" is not referenced by
   the client, some implementation may consider it not to be "in use",
   hence it is omitted from <operational> but only present in <system>.

5.5.2.  Declaring a System-defined Node in <running> Explicitly

   It's also possible for a client to explicitly declare the system-
   defined configurations that are referenced instead of using the
   "resolve-system" parameter.  For instance, in the above example, the
   client MAY also explicitly configure the following system defined
   applications "ftp" and "tftp" only with the list key "name":

   <applications xmlns="urn:example:application">
     <application>
       <name>ftp</name>
     </application>
     <application>
       <name>tftp</name>
     </application>
   </applications>

   And the configuration of ACL rules referencing application "ftp" and
   "tftp" are 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-app-1</application>
       </matches>
       <packet-action>forward</packet-action>
     </acl-rule>
   </acl>

   Once the data is written to <running>, it makes no difference whether
   it is explicitly declared by the client or automatically copied by
   the server.  The configuration for applications in <running> and
   <operational> would be identical to the ones in Section 5.5.1.







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5.5.3.  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 "exif";

     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 a loopback interface (named "lo0") with a
   MTU value "65536", a default IPv4 address of "127.0.0.1", and a
   default IPv6 address of "::1".  The configuration of "lo0" interface
   is 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 and adds the following
   configuration into <running>:

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

   Then the configuration of interfaces is present in <operational> 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>65535</mtu>
       <ip-address or:origin="or:system">127.0.0.1</ip-address>
       <ip-address or:origin="or:system">::1</ip-address>
     </interface>
   </interfaces>

5.5.4.  Configuring Descendant Nodes of a System-defined Node

   In the above example, image the client further configures 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>65535</mtu>
       <ip-address or:origin="or:system">127.0.0.1</ip-address>
       <ip-address or:origin="or:system">::1</ip-address>
     </interface>
   </interfaces>

6.  Default Interactions

   <system> should not contain the configuration using the schema
   default value, either specified in the "default" statement or
   described in the "description" statement.

   Any value provided by the system that is not the schema default value
   MUST be contained in <system>.  If system provides a value that is
   not the schema default value, and the node is not explicitly set by
   the client, it MUST be copied into the target datastore when its
   closest ancestor node needs to be copied to satisfy referential
   integrity constraints.

7.  Relation to Other Datastores

7.1.  The "factory-default" Datastore

   Any deletable system-provided configuration that is populated as part
   of <running> by the system at boot up, without being part of the
   contents of a <startup> datastore, must be defined in <factory-
   default> [RFC8808], which is used to initialize <running> when the
   device is first-time powered on or reset to its factory default
   condition.

   The <factory-reset> RPC operation can reset <system> to its factory
   default contents.

7.2.  The "candidate" Datastore

   If the device supports the :candidate or :private-candidate
   [I-D.ietf-netconf-privcand] capability, a client may edit the
   candidate or private-candidate datastore without expecting it to be
   valid until a <commit> or <validate> operation takes place.  The
   client may use the "resolve-system" parameter in one of the following
   situations:




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   *  The client makes an edit (e.g., NETCONF <edit-config>/<edit-data>,
      or RESTCONF edit operation) to the candidate/private-candidate
      datastore.  This is possible, though may not be required.

   *  The client issues a <validate> operation.

   *  The client issues a <commit> operation.

   In particular, [I-D.ietf-netconf-privcand] defines the concept of
   conflict, the server's copy referenced system nodes triggered by
   "resolve-system" parameter might conflict with the contents of
   <running>, the conflict resolution is no different than the
   resolution of conflict caused by configuration explicitly provided by
   the client.

8.  The "ietf-system-datastore" Module

8.1.  Data Model Overview

   This YANG module defines a new YANG identity named "system" that uses
   the "ds:datastore" identity defined in [RFC8342].  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].





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8.2.  Example Usage

   This section gives an example of data retrieval from <system>.  The
   fictional YANG module used following are from Appendix C.2 of
   [RFC8342].

   container bgp {
     leaf local-as {
       type uint32;
     }
     leaf peer-as {
       type uint32;
     }
     list peer {
       key name;
       leaf name {
         type inet:ip-address;
       }
       leaf local-as {
         type uint32;
         description
           "... Defaults to ../local-as.";
       }
       leaf peer-as {
         type uint32;
         description
           "... Defaults to ../peer-as.";
       }
       leaf local-port {
         type inet:port;
       }
       leaf remote-port {
         type inet:port;
         default 179;
       }
       leaf state {
         config false;
         type enumeration {
           enum init;
           enum established;
           enum closing;
         }
       }
     }
   }

   All the messages are presented in a protocol-independent manner.
   JSON is used to not imply a preferred encoding in this document.



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   Suppose the following BGP peer configuration is added to <running>:

   {
       "bgp": {
           "local-as": "64501",
           "peer-as": "64502",
           "peer": {
               "name": "2001:db8::2:3",
               "local-as": "64501",
               "peer-as": "64502"
           }
       }
   }

   The local port and remote port are used when the BGP peer connection
   is established.  Since both are not supplied explicitly in <running>
   and <intended>, the default value for "bgp/peer/remote-port" is used,
   and there is no default statement for "bgp/peer/local-port", the
   system will select a value for it.  So the contents of <system> are
   shown as follows:

   {
       "bgp": {
           "peer": {
               "name": "2001:db8::2:3",
               "local-port": "60794"
           }
       }
   }

8.3.  YANG Module

   <CODE BEGINS> file "ietf-system-datastore@2024-05-31.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 NETDOD (Network Modeling) Working Group";
     contact



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       "WG Web:   https://datatracker.ietf.org/wg/netmod/
        WG List:  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:datastore identity defined in [RFC8342].

        Copyright (c) 2024 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.

        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 2024-05-31 {
       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.";
     }
   }



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   <CODE ENDS>

9.  The "ietf-netconf-resolve-system" Module

   This YANG module is optional to implement.

9.1.  Data Model Overview

   The following tree diagram [RFC8340] illustrates the "ietf-netconf-
   resolve-system" module:

   module: ietf-netconf-resolve-system
     augment /nc:edit-config/nc:input:
       +---w resolve-system?   empty
     augment /nc:copy-config/nc:input:
       +---w resolve-system?   empty
     augment /nc:validate/nc:input:
       +---w resolve-system?   empty
     augment /nc:commit/nc:input:
       +---w resolve-system?   empty
     augment /ncds:edit-data/ncds:input:
       +---w resolve-system?   empty

9.2.  Example Usage

   Please refer to Section 5.5.1 for example usage of the "resolve-
   system" parameter.

9.3.  YANG Module

   This module imports modules "ietf-netconf" and "ietf-netconf-nmda",
   defined in [RFC6241] and [RFC8526], respectively.

   <CODE BEGINS> file "ietf-netconf-resolve-system@2024-05-31.yang"

   module ietf-netconf-resolve-system {
     yang-version 1.1;
     namespace
       "urn:ietf:params:xml:ns:yang:ietf-netconf-resolve-system";
     prefix ncrs;

     import ietf-netconf {
       prefix nc;
       reference
         "RFC 6241: Network Configuration Protocol (NETCONF)";
     }
     import ietf-netconf-nmda {
       prefix ncds;



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       reference
         "RFC 8526: NETCONF Extensions to Support the Network
          Management Datastore Architecture";
     }

     organization
       "IETF NETMOD (Network Modeling) Working Group";
     contact
       "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 an extension to the NETCONF protocol
        that allows the NETCONF client to control whether the server
        is allowed to copy referenced system configuration
        automatically without the client doing so explicitly.

         Copyright (c) 2024 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.

         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 2024-05-31 {
       description
         "Initial version.";
       reference



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         "RFC XXXX: System-defined Configuration";
     }

     grouping resolve-system-grouping {
       description
         "Define the resolve-system parameter grouping.";
       leaf resolve-system {
         type empty;
         description
           "When present, the server MUST copy referenced system
            configuration into the target configuration datastore
            where it is missing from without the client doing the
            copy/paste explicitly.";
       }
     }

     augment "/nc:edit-config/nc:input" {
       description
         "Adds the 'resolve-system' parameter to the input of the
          NETCONF <edit-config> operation.";
       uses resolve-system-grouping;
     }

     augment "/nc:copy-config/nc:input" {
       description
         "Adds the 'resolve-system' parameter to the input of the
          NETCONF <copy-config> operation.";
       uses resolve-system-grouping;
     }
     augment "/nc:validate/nc:input" {
       description
         "Adds the 'resolve-system' parameter to the input of the
          NETCONF <validate> operation.";
       uses resolve-system-grouping;
     }
     augment "/nc:commit/nc:input" {
       description
         "Adds the 'resolve-system' parameter to the input of the
          NETCONF <commit> operation.";
       uses resolve-system-grouping;
     }
     augment "/ncds:edit-data/ncds:input" {
       description
         "Adds the 'resolve-system' parameter to the input of the
          NETCONF <edit-data> operation.";
       uses resolve-system-grouping;
     }
   }



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   <CODE ENDS>

10.  IANA Considerations

10.1.  The "IETF XML" Registry

   This document registers two XML namespace URNs 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.

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

10.2.  The "YANG Module Names" Registry

   This document registers two module names in the 'YANG Module Names'
   registry, defined in [RFC6020].

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


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

10.3.  NETCONF Capability URN Registry

   This document registers the following capability identifier URN in
   the 'Network Configuration Protocol (NETCONF) Capability URNs'
   registry:

   urn:ietf:params:netconf:capability:resolve-system:1.0

10.4.  RESTCONF Capability URN Registry

   This document registers a capability in the 'RESTCONF Capability
   URNs' registry [RFC8040]:




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 Index            Capability Identifier
 -----------------------------------------------------------------------
 :resolve-system  urn:ietf:params:restconf:capability:resolve-system:1.0

11.  Security Considerations

11.1.  Regarding the "ietf-system-datastore" YANG Module

   The YANG module defined in this document extends the base operations
   for NETCONF [RFC6241] and RESTCONF [RFC8040].  The lowest NETCONF
   layer is the secure transport layer, and the mandatory-to-implement
   secure transport is Secure Shell (SSH) [RFC6242].  The lowest
   RESTCONF layer is HTTPS, and the mandatory-to-implement secure
   transport is TLS [RFC8446].

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

11.2.  Regarding the "ietf-netconf-resolve-system" YANG Module

   The YANG module defined in this document extends the base operations
   for NETCONF [RFC6241] and [RFC8526].  The lowest NETCONF layer is the
   secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [RFC8446].

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

   The security considerations for the base NETCONF protocol operations
   (see Section 9 of [RFC6241] apply to the new extended RPC operations
   defined in this document.  There is not any beyond the potential
   performance impacts of implementing the "resolve-system" parameter,
   which may mean employing some form of rate limiting or adapting the
   rate threshold might be a good idea to avoid DoS attacks.

12.  References

12.1.  Normative References







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   [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>.

   [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>.

   [RFC6470]  Bierman, A., "Network Configuration Protocol (NETCONF)
              Base Notifications", RFC 6470, DOI 10.17487/RFC6470,
              February 2012, <https://www.rfc-editor.org/info/rfc6470>.

   [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>.

   [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>.

   [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>.

   [RFC8526]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "NETCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8526,
              DOI 10.17487/RFC8526, March 2019,
              <https://www.rfc-editor.org/info/rfc8526>.

   [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>.

12.2.  Informative References



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   [I-D.ietf-netconf-privcand]
              Cumming, J. and R. Wills, "NETCONF Private Candidates",
              Work in Progress, Internet-Draft, draft-ietf-netconf-
              privcand-03, 30 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              privcand-03>.

   [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-00, 18
              March 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-netmod-immutable-flag-00>.

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

   [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>.

   [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>.

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

   [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>.

   [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>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.




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   [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>.

   [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>.

Appendix A.  Key Use Cases

   Following provides three use cases related to how <system> interacts
   with other datastores (e.g., <running>, <intended>, and
   <operational>).  The simple interface data model defined in
   Appendix C.3 of [RFC8342] is used.  For each use case, corresponding
   sample configuration in <running>, <system>, <intended> and
   <operational> are shown.  The XML snippets are used only for
   illustration purposes.

A.1.  Device Powers On

   When the device is powered on, suppose the system provides a 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>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
   </interfaces>

   In this case, the configuration of loopback interface is only present
   in <system>, the exact configuration also shows up in <intended>.
   The contents of <intended> appear as follows:

   <interfaces>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
   </interfaces>




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   And <operational> will show the system-provided loopback interface:

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

A.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:

   <interfaces>
     <interface>
       <name>et-0/0/0</name>
       <description>Test interface</description>
     </interface>
   </interfaces>

   And the contents of <system> keep unchanged since the interface is
   not physically present:

   <interfaces>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
   </interfaces>

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














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   <interfaces>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <description>Test interface</description>
     </interface>
   </interfaces>

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

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

A.3.  Operator Installs Card into a Chassis

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

   <interfaces>
     <interface>
       <name>et-0/0/0</name>
       <description>Test interface</description>
     </interface>
   </interfaces>

   And <system> might appear as follows:












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   <interfaces>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <mtu>1500</mtu>
     </interface>
   </interfaces>

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

   <interfaces>
     <interface>
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <description>Test interface</description>
       <mtu>1500</mtu>
     </interface>
   </interfaces>

   And the contents of <operational> appear as follows:

   <interfaces xmlns:or="urn:ietf:params:xml:ns:yang:ietf-origin"
               or:origin="or:intended">
     <interface or:origin="or:system">
       <name>lo0</name>
       <ip-address>127.0.0.1</ip-address>
       <ip-address>::1</ip-address>
     </interface>
     <interface>
       <name>et-0/0/0</name>
       <description>Test interface</description>
       <mtu or:origin="or:system">1500</mtu>
     </interface>
   </interfaces>

Appendix B.  Changes between Revisions

   v05 - v06




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   *  remove inactive-until-referenced system config

   *  add a new section (sec.6) to clarify the interplay bewteen system
      config and defaults

   *  add a new section (sec.7) to clarify relation to other datastores,
      which includes <factory-default> and <candidate>/<priv-candidate>

   *  leave the merge behavior of <system> and <running> unspecified

   *  augment <validate> and <commit> PRC operation to support "resolve-
      system" parameter

   *  editorial updates

   v04 - v05

   *  Explicitly state that system configuration copied from <system>
      into <running> have its origin value being reported as "intended"
      and update the examples accordingly to reflect it

   *  Update the definition of "intended" origin identity in 8342 to
      allow a subset of configuration in <intended> to use "system" as
      origin value

   *  State server behaviors of migrating updated system data into
      <running> is beyond the scope of this document, and give a couple
      of implementation examples

   *  Remove the related statement which mandates referenced system
      configuration must be copied into <running>

   *  Refine usage examples (e.g., fix validation errors, remove
      redundancy)

   v03 - v04

   *  Add some implementation consideration for "resolve-system"
      parameter

   *  Define a NETCONF capability identifier for "resolve-system"
      parameter so that the client can discover if it is supported by
      the server.

   *  state servers may upgrade copied system configuration in <running>
      as well during device upgrade or licensing change.

   v02 - v03



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   *  remove the merge mechanism related comments, as discussed in
      https://github.com/netconf-wg/netconf-next/issues/19

   *  Editorial changes

   v01 - v02

   *  Define referenced system configuration

   *  better clarify "resolve-system" parameter

   *  update Figure 2 in NMDA RFC

   *  Editorial changes

   v00 - v01

   *  Clarify why client's explicit copy is not preferred but cannot be
      avoided if resolve-system parameter is not defined

   *  Clarify active system configuration

   *  Update the timing when the server's auto copy should be enforced
      if a resolve-system parameter is used

   *  Editorial changes

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, Alexander Clemm, and Timothy Carey.

Contributors
















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

   Jan Lindblad
   Cisco Systems
   Email: jlindbla@cisco.com

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

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

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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