YANG Modules describing Capabilities for Systems and Datastore Update Notifications
draft-ietf-netconf-notification-capabilities-16
Document Type Active Internet-Draft (netconf WG)
Authors Balázs Lengyel  , Alexander Clemm  , Benoît Claise 
Last updated 2021-04-16
Replaces draft-lengyel-netconf-notification-capabilities
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NETCONF                                                       B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                                A. Clemm
Expires: October 18, 2021                                      Futurewei
                                                               B. Claise
                                                                  Huawei
                                                          April 16, 2021

 YANG Modules describing Capabilities for Systems and Datastore Update
                             Notifications
            draft-ietf-netconf-notification-capabilities-16

Abstract

   This document defines two YANG modules,"ietf-system-capabilities" and
   "ietf-notification-capabilities".

   The module "ietf-system-capabilities" provides a placeholder
   structure that can be used to discover YANG related system
   capabilities for servers.  The module can be used to report
   capability information from the server at run-time or implementation-
   time, per the YANG Instance Data File Format.

   The module "ietf-notification-capabilities" augments "ietf-system-
   capabilities" to specify capabilities related to Subscription to YANG
   Notifications for Datastore Updates.

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 October 18, 2021.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Providing System Capability Information . . . . . . . . . . .   4
   3.  Providing YANG-Push Notification Capabilities Information . .   5
   4.  System Capabilities Model . . . . . . . . . . . . . . . . . .   6
     4.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Notification Capabilities Model . . . . . . . . . . . . . . .  11
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  18
     7.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Instance data example #1 . . . . . . . . . . . . . .  20
   Appendix B.  Instance data example #2 . . . . . . . . . . . . . .  22
   Appendix C.  Changes between revisions  . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Servers and/or a publishers often have capabilities, values
   describing operational behavior, that need to be conveyed to clients,
   which is enabled by the YANG modules described in this document.

   There is a need to publish this capability information as it is part
   of the contract between the server and client.  Examples include

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   maximum size of data that can be stored or transferred, information
   about counters (whether a node supports "on-change" telemetry), etc.
   Such capabilities are often dependent on a vendor's implementation or
   the available resources at deployment.  Many such capabilities are
   specific to either the complete system, individual YANG datastores
   [RFC8342] or specific parts of the YANG schema, or even individual
   data nodes.  It is a goal of this document to provide a common way of
   representing such capabilities in a format that is:

   o  vendor independent

   o  machine readable

   o  available in identical format both at implementation-time and run-
      time

   Implementation-time information is needed by Network Management
   System (NMS) implementers.  A NMS implementation that wants to
   support notifications, needs the information about a system's
   capability to send "on-change" notifications.  If the information is
   not documented in a way available to the NMS designer, but only as
   instance data from the network node once it is deployed, the NMS
   implementation will be delayed, because it has to wait for the
   network node to be ready.  In addition, the assumption that all NMS
   implementers will have a correctly configured network node available
   to retrieve data from is an expensive proposition and may not always
   hold.  (An NMS may need to be able to handle many dozens of network
   node types.)  Often a fully functional NMS is a requirement for
   introducing a new network node type into a network, so delaying NMS
   readiness effectively also delays the time at which a new network
   node type can be introduced into the network.

   Implementation-time information is needed by system integrators.
   When introducing a network node type into their network, operators
   often need to integrate the node type into their own management
   system.  The NMS may have management functions that depend on "on-
   change" notifications.  The network operator needs to plan his
   management practices and NMS implementation before he even decides to
   buy the specific network node type.  Moreover the decision to buy the
   node type sometimes depends on these management possibilities.

   Run-time information is needed:

   o  for any "purely model driven" application, e.g., a NETCONF-
      browser.  Such applications depend on reading models and
      capabilities at run-time to support all the publisher's available
      functionality.

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   o  in case the capability might change during run-time e.g., due to
      licensing, HW constraints etc.

   o  to check that capability information provided earlier, at
      implementation-time is what the publisher has implemented.

1.1.  Terminology

   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.

   The terms "YANG-Push", "on-change subscription" and "periodic
   subscription" are used as defined in [RFC8641].

   The terms "subscriber", "publisher" and "receiver" are used as
   defined in [RFC8639].

   The term "server" is used as defined in [RFC8342].

   The terms "YANG instance data file format", "instance data", and
   "instance data set" are used as defined in
   [I-D.ietf-netmod-yang-instance-file-format].

   In addition, this document defines the following terms:

   "Implementation-time information": Information about the server's
   behavior that is made available during the implementation of the
   server, available from a source other then a running server.

   "Run-time information": Information about the server's behavior that
   is available from the running server via management protocols such as
   NETCONF [RFC6241] or RESTCONF [RFC8040].

2.  Providing System Capability Information

   Capability information is represented by instance-data based on one
   or more "capability defining YANG modules".  This allows a user to
   discover capabilities both at implementation-time and run-time.

   o  For the implementation-time use-case: Capabilities SHOULD be
      provided by the implementer as YANG instance data files complying
      to [I-D.ietf-netmod-yang-instance-file-format].  The file MUST be
      available already at implementation-time retrievable in a way that
      does not depend on a live network node.  E.g., download from
      product website.

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   o  For the run-time use-case: Capabilities SHOULD be available via
      NETCONF [RFC6241] or RESTCONF [RFC8040] from the live server
      (implementing the publisher) during run-time.  Implementations
      that support changing these capabilities at run-time SHOULD
      support "on-change" notifications about the system-capabilities
      container.

   The module "ietf-system-capabilities" provides a placeholder
   structure to be used to specify any YANG related system capability.

   The module "ietf-notification-capabilities" is defined to allow a
   publisher to specify capabilities related to "Subscription to YANG
   Notifications for Datastore Updates" [RFC8641], also known as YANG-
   Push, augmenting "ietf-system-capabilities".

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

3.  Providing YANG-Push Notification Capabilities Information

   A specific case is the need to specify capabilities is the YANG-Push
   functionality.  As defined in [RFC8641] a publisher may allow
   subscribers to subscribe to updates from a datastore and subsequently
   push such update notifications to the receiver.  Notifications may be
   sent periodically or "on-change" (more or less immediately after each
   change).

   A publisher supporting YANG-Push has a number of capabilities defined
   in [RFC8641] that are often determined during the implementation of
   the publisher.  These include:

   o  Supported (reporting) periods for "periodic" subscriptions

   o  Maximum number of objects that can be sent in an update

   o  The set of datastores or data nodes for which "periodic"
      notification is supported

   Additional capabilities if the optional "on-change" feature is
   supported include:

   o  Supported dampening periods for "on-change" subscriptions

   o  The set of datastores or data nodes for which "on-change"
      notification is supported

   Publishers might have some capabilities (or limitations) to document.
   For example, how many update notifications and how many datastore

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   node updates they can send out in a certain time-period.  Other
   publishers might not support "periodic" subscriptions to all
   datastores.  In some cases, a publisher supporting "on-change"
   notifications will not be able to push updates for some object types
   "on-change".  Reasons for this might be that the value of the
   datastore node changes frequently (e.g., in-octets counter), that
   small object changes are frequent and irrelevant to the receiver
   (e.g., a temperature gauge changing 0.1 degrees within a
   predetermined and acceptable range), or that the implementation is
   not capable of on-change notification for a particular object.  In
   all those cases, it will be important for subscriber applications to
   have a way to identify which objects "on-change" notifications are
   supported and for which ones not.

   Faced with the reality that support for "on-change" notification does
   not mean that such notifications will be sent for any specific data
   node, subscriber/management applications can not rely on the "on-
   change" functionality unless the subscriber has some means to
   identify which objects "on-change" notifications are supported.  YANG
   models are meant to be used as an interface contract.  Without
   identification of the data nodes actually supporting "on-change",
   this contract would be incomplete.

   Clients of a server (and subscribers to a publisher, as subscribers
   are also clients) need a method to gather capability information.

4.  System Capabilities Model

   The module "ietf-system-capabilities" is defined to provide a
   structure that can be used to discover (as read-only operational
   state) any YANG related system capability.

   This module itself does not contain any capabilities; it provides
   augmentation points for capabilities to be defined in subsequent YANG
   modules.  It SHOULD be used by other modules to augment-in specific
   capability information.  Every set of such capabilities SHOULD be
   wrapped in a container under the augment statement to cleanly
   separate different groups of capabilities.  These "wrapper
   containers" SHALL be augmented in at /sysc:system-capabilities and
   /sysc:system-capabilities/sysc:datastore-capabilities/sysc:per-node-
   capabilities.

   Capability values can be specified on system level, datastore level
   (by selecting all nodes in the datastore) or for specific data nodes
   of a specific datastore (and their contained sub-trees).  Capability
   values specified for a specific datastore or node-set override values
   specified on the system level.

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   Note: The solution is usable for both NMDA and non-NMDA systems.  For
   non-NMDA servers "config false" data is considered as if it was part
   of the running datastore.

4.1.  Tree Diagram

   The following tree diagram [RFC8340] provides an overview of the data
   model.

 module: ietf-system-capabilities
   +--ro system-capabilities
      +--ro datastore-capabilities* [datastore]
         +--ro datastore         -> /yanglib:yang-library/datastore/name
         +--ro per-node-capabilities* []
            +--ro (node-selection)?
               +--:(node-selector)
                  +--ro node-selector?   nacm:node-instance-identifier

4.2.  YANG Module

   This YANG module imports typedefs from [RFC8341] and a reference path
   from [RFC8525].

<CODE BEGINS> file "ietf-system-capabilities@2021-04-02.yang"

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

  import ietf-netconf-acm {
    prefix nacm;
    reference
      "RFC 8341: Network Configuration Access Control Model";
  }
  import ietf-yang-library {
    prefix yanglib;
    description
      "This module requires ietf-yang-library to be implemented.
       Revision 2019-01-04 or a revision derived from it
       is REQUIRED.";
    reference
      "RFC8525: YANG Library";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group";
  contact

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

     Editor:   Balazs Lengyel
               <mailto:balazs.lengyel@ericsson.com>";
  description
    "This module specifies a structure to specify system
     capabilities for a server or a publisher. System capabilities
     may include capabilities of a NETCONF or RESTCONF server or a
     notification publisher.

     This module does not contain any specific capabilities, it only
     provides a structure where containers containing the actual
     capabilities are augmented in.

     Capability values can be specified on system level,
     datastore level (by selecting all nodes in the datastore) or
     for specific data nodes of a specific datastore (and their
     contained sub-trees).
     Capability values specified for a specific datastore or
     node-set override values specified on the system/publisher level.

     To find a capability value for a specific data node in a
     specific datastore the user SHALL:

     1) search for a datastore-capabilities list entry for
     the specific datastore.

     2) If the datastore entry is found within that entry, process all
     per-node-capabilities entries in the order they appear in the list.
     The first entry that specifies the specific capability and has a
     node-selector selecting the specific data node defines the
     capability value.

     3) If the capability value is not found above and the specific
     capability is specified under the system-capabilities container
     (outside the datastore-capabilities list), this value shall be
     used.

     4) If no values are found in the previous steps, the
     system/publisher is not capable of providing a value. Possible
     reasons are: it is unknown, the capability is changing for some
     reason, there is no specified limit, etc. In this case the
     system's behavior is unspecified.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document

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     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.

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

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

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

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

  revision 2021-04-02 {
    description
      "Initial version
       NOTE TO RFC EDITOR:
       (1)Please replace the above revision date to
         the date of RFC publication when published.
       (2) Please replace the date in the file name
         (ietf-system-capabilities@2021-04-02.yang)
         to the date of RFC publication.
       (3) Please replace the following reference
         with RFC number when published
         (i.e. RFC xxxx).";
    reference
      "RFC XXXX: YANG Modules describing Capabilities for Systems
       and Datastore Update Notifications";
  }

  container system-capabilities {
    config false;
    description
      "System capabilities.
       Capability values specified here at the system level
       are valid for all datastores and are used when the
       capability is not specified on the datastore level
       or for specific data nodes.";
    // augmentation point for system level capabilities
    list datastore-capabilities {

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      key "datastore";
      description
        "Capabilities values per datastore.

         For non-NMDA servers/publishers 'config false' data is
         considered as if it was part of the running datastore.";
      leaf datastore {
        type leafref {
          path
            "/yanglib:yang-library/yanglib:datastore/yanglib:name";
        }
        description
          "The datastore for which capabilities are defined.
           Only one specific datastore can be specified
           e.g., ds:conventional is not allowed.";
      }
      list per-node-capabilities {
        description
          "Each list entry specifies capabilities for the selected
           data nodes. The same capabilities apply for the data nodes
           in the subtree below the selected nodes.

           The system SHALL order the entries according to their
           precedence. The order of the entries MUST NOT change unless
           the underlying capabilities also change.";
        choice node-selection {
          description
            "A method to select all or some nodes within a datastore.";
          leaf node-selector {
            type nacm:node-instance-identifier;
            description
              "Selects the data nodes for which capabilities are
               specified. The special value '/' denotes all data nodes
               in the datastore, consistent with the path leaf node on
               page 41 [RFC8341].";
            reference
              "RFC 8341: Network Configuration Access Control Model";
          }
        }
        // augmentation point for datastore or data node level
        // capabilities
      }
    }
  }
}

   <CODE ENDS>

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5.  Notification Capabilities Model

   The YANG module "ietf-notification-capabilities" provides YANG-Push
   related capability information.

5.1.  Tree Diagram

   The following tree diagram [RFC8340] provides an overview of the data
   model.

   module: ietf-notification-capabilities
     augment /sysc:system-capabilities:
       +--ro subscription-capabilities
          +--ro max-nodes-per-update?               uint32
          +--ro periodic-notifications-supported?   notification-support
          +--ro (update-period)?
          |  +--:(minimum-update-period)
          |  |  +--ro minimum-update-period?        uint32
          |  +--:(supported-update-period)
          |     +--ro supported-update-period*      uint32
          +--ro on-change-supported?                notification-support
          |       {yp:on-change}?
          +--ro minimum-dampening-period?           uint32
          |       {yp:on-change}?
          +--ro supported-excluded-change-type*     union
                  {yp:on-change}?
     augment /sysc:system-capabilities/sysc:datastore-capabilities
               /sysc:per-node-capabilities:
       +--ro subscription-capabilities
          +--ro max-nodes-per-update?               uint32
          +--ro periodic-notifications-supported?   notification-support
          +--ro (update-period)?
          |  +--:(minimum-update-period)
          |  |  +--ro minimum-update-period?        uint32
          |  +--:(supported-update-period)
          |     +--ro supported-update-period*      uint32
          +--ro on-change-supported?                notification-support
          |       {yp:on-change}?
          +--ro minimum-dampening-period?           uint32
          |       {yp:on-change}?
          +--ro supported-excluded-change-type*     union
                  {yp:on-change}?

5.2.  YANG Module

   This YANG module imports a feature and typedefs from [RFC8641] and
   also imports the "ietf-system-capabilities" specified in this
   document.

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 <CODE BEGINS> file "ietf-notification-capabilities@2021-04-02.yang"

 module ietf-notification-capabilities {
   yang-version 1.1;
   namespace
     "urn:ietf:params:xml:ns:yang:ietf-notification-capabilities";
   prefix notc;

   import ietf-yang-push {
     prefix yp;
     description
       "This module requires ietf-yang-push to be implemented.";
     reference
       "RFC 8641: Subscription to YANG Notifications for Datastore
         Updates";
   }
   import ietf-system-capabilities {
     prefix sysc;
     description
       "This module requires ietf-system-capabilities to be
        implemented.";
     reference
       "RFC XXXX: YANG Modules describing Capabilities for Systems
        and Datastore Update Notifications";
   }

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

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

      Editor:   Balazs Lengyel
                <mailto:balazs.lengyel@ericsson.com>";
   description
     "This module specifies YANG-Push [RFC 8641] related publisher
      capabilities.

      The module contains:

      - specification of which data nodes support 'on-change' or
      'periodic' notifications.

      - capabilities related to the throughput of notification data
      the publisher can support. (Note that for a specific

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      subscription the publisher MAY still allow only longer periods
      or smaller updates depending on e.g., actual load conditions.)

      Capability values can be specified on system/publisher level,
      datastore level or for specific data nodes of a specific
      datastore (and their contained sub-trees), as defined in the
      ietf-system-capabilities module.

      If different data nodes covered by a single subscription
      have different values for a specific capability, then using
      values that are only acceptable for some of these data nodes,
      but not for others, may result in the rejection of the
      subscription.

      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.

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

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

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

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

   revision 2021-04-02 {
     description
       "Initial version";
     reference
       "RFC XXXX: YANG Modules describing Capabilities for Systems
        and Datastore Update Notifications";
   }

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

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   grouping subscription-capabilities {
     description
       "Capabilities related to YANG-Push subscriptions
        and notifications";
     container subscription-capabilities {
       description
         "Capabilities related to YANG-Push subscriptions
          and notifications";
       typedef notification-support {
         type bits {
           bit config-changes {
             description
               "The publisher is capable of sending
                notifications for 'config false' nodes for the
                relevant scope and subscription type.";
           }
           bit state-changes {
             description
               "The publisher is capable of sending
                notifications for 'config false' nodes for the relevant
                scope and subscription type.";
           }
         }
         description
           "Type for defining whether 'on-change' or
            'periodic' notifications are supported 'config false'
            data nodes, 'config true' date nodes, no data nodes,
            or all data nodes.

            If the bit config-changes or state-changes is set
            for a datastore, or a set of nodes that does not contain
            nodes with the indicated config value, this has no
            effect, as if no support was declared. E.g. indicating
            support for state-changes for a candidate datastore has
            no effect.";
       }

       leaf max-nodes-per-update {
         type uint32 {
           range "1..max";
         }
         description
           "Maximum number of data nodes that can be sent
            in an update. The publisher MAY support more data nodes,
            but SHOULD support at least this number.

            May be used to avoid the 'update-too-big' error
            during subscription.";

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         reference
           "RFC 8641: Subscription to YANG Notifications for Datastore
            Updates, the 'update-too-big' error/identity";
       }
       leaf periodic-notifications-supported {
         type notification-support;
         description
           "Specifies whether the publisher is capable of
            sending 'periodic' notifications for the selected
            data nodes including any subtrees that may exist
            below them.";
         reference
           "RFC 8641: Subscription to YANG Notifications for Datastore
            Updates, 'periodic' subscription concept";
       }
       choice update-period {
         description
           "Supported update period value or values for
            'periodic' subscriptions.";
         leaf minimum-update-period {
           type uint32;
           units "centiseconds";
           description
             "Indicates the minimal update period that is
              supported for a 'periodic' subscription.

              A subscription request to the selected data
              nodes with a smaller period than what this leaf
              specifies will result in a 'period-unsupported' error.";
           reference
             "RFC 8641: Subscription to YANG Notifications for Datastore
              Updates, the period leaf in the ietf-yang-push YANG
              module";
         }
         leaf-list supported-update-period {
           type uint32;
           units "centiseconds";
           description
             "Supported update period values for a 'periodic'
              subscription.

              A subscription request to the selected data nodes with a
              period not included in the leaf-list will result in a
              'period-unsupported' error.";
           reference
             "RFC 8641: Subscription to YANG Notifications for Datastore
              Updates, the period leaf in the ietf-yang-push YANG
              module";

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         }
       }
       leaf on-change-supported {
         if-feature "yp:on-change";
         type notification-support;
         description
           "Specifies whether the publisher is capable of
            sending 'on-change' notifications for the selected
            data nodes and the subtree below them.";
         reference
           "RFC 8641: Subscription to YANG Notifications for Datastore
            Updates, on-change concept";
       }
       leaf minimum-dampening-period {
         if-feature "yp:on-change";
         type uint32;
         units "centiseconds";
         description
           "The minimum dampening-period supported for 'on-change'
            subscriptions for the selected data nodes.

            If this value is present and greater than zero,
            that implies dampening is mandatory.";
         reference
           "RFC 8641: Subscription to YANG Notifications for
            Datastore Updates, the dampening-period leaf in the
            ietf-yang-push YANG module";
       }
       leaf-list supported-excluded-change-type {
         if-feature "yp:on-change";
         type union {
           type enumeration {
             enum none {
               value -2;
               description
                 "None of the change types can be excluded.";
             }
             enum all {
               value -1;
               description
                 "Any combination of change types can be excluded.";
             }
           }
           type yp:change-type;
         }
         description
           "The change types that can be excluded in
            YANG-Push subscriptions.";

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         reference
           "RFC 8641: Subscription to YANG Notifications for Datastore
            Updates, the change-type typedef in the ietf-yang-push
            YANG module";
       }
     }
   }

   augment "/sysc:system-capabilities" {
     description
       "Add system level capabilities";
     uses subscription-capabilities {
       refine
         "subscription-capabilities/supported-excluded-change-type" {
         default "none";
       }
     }
   }

   augment "/sysc:system-capabilities/sysc:datastore-capabilities"
         + "/sysc:per-node-capabilities" {
     description
       "Add datastore and node level capabilities";
     uses subscription-capabilities {
       refine
         "subscription-capabilities/supported-excluded-change-type" {
         default "none";
       }
     }
   }
 }

   <CODE ENDS>

6.  Security Considerations

   The YANG modules specified in this document define a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or 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 or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

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   All protocol-accessible data nodes are read-only and cannot be
   modified.  The data in these modules is not security sensitive.
   Access control may be configured, to avoid exposing the read-only
   data.

   When that data is in file format, data should be protected against
   modification or unauthorized access using normal file handling
   mechanisms.

7.  IANA Considerations

7.1.  The IETF XML Registry

   This document registers two URIs in the IETF XML registry [RFC3688].
   Following the format in [RFC3688], the following registrations are
   requested:

      URI: urn:ietf:params:xml:ns:yang:ietf-system-capabilities
      Registrant Contact: The NETCONF WG of the IETF.
      XML: N/A, the requested URI is an XML namespace.

      URI: urn:ietf:params:xml:ns:yang:ietf-notification-capabilities
      Registrant Contact: The NETCONF WG of the IETF.
      XML: N/A, the requested URI is an XML namespace.

7.2.  The YANG Module Names Registry

   This document registers two YANG modules in the YANG Module Names
   registry.  Following the format in [RFC7950], the the following
   registrations are requested:

      name:       ietf-system-capabilities
      namespace:  urn:ietf:params:xml:ns:yang:ietf-system-capabilities
      prefix:     sysc
      reference:  RFC XXXX

      name:       ietf-notification-capabilities
      namespace:
           urn:ietf:params:xml:ns:yang:ietf-notification-capabilities
      prefix:     notc
      reference:  RFC XXXX

8.  Acknowledgments

   For their valuable comments, discussions, and feedback, we wish to
   acknowledge Andy Bierman, Juergen Schoenwaelder, Rob Wilton, Kent
   Watsen, Eric Voit, Joe Clarke, Martin Bjorklund, Ladislav Lhotka, Qin

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   Wu, Mahesh Jethanandani, Ran Tao, Reshad Rahman and other members of
   the Netmod WG.

9.  References

9.1.  Normative References

   [I-D.ietf-netmod-yang-instance-file-format]
              Lengyel, B. and B. Claise, "YANG Instance Data File
              Format", draft-ietf-netmod-yang-instance-file-format-12
              (work in progress), April 2020.

   [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>.

   [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>.

   [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>.

   [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>.

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9.2.  Informative References

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

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

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

   [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>.

   [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>.

   [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>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

Appendix A.  Instance data example #1

   The following instance data example describes the notification
   capabilities of a hypothetical "acme-router".  The router implements
   the running, and operational datastores.  Every change can be
   reported "on-change" from the running datastore, but only "config
   false" nodes and some "config false" data from the operational
   datastore.  Interface statistics are not reported "on-change" only
   two important counters.  Datastore subscription capabilities are not
   reported "on-change" as they never change on the acme-router during
   run-time.

========== NOTE: '\' line wrapping per BCP YYY (RFC YYYY) ===========

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=\

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    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>acme-router-notification-capabilities</name>
  <content-schema>
    <module>ietf-system-capabilities@2021-04-02</module>
    <module>ietf-notification-capabilities@2021-04-02</module>
  </content-schema>
  <description>Defines the notification capabilities of an acme-router.
    The router only has running, and operational datastores.
    Every change can be reported on-change from the running
    datastore, but only "config false" nodes and some "config
    false" data from the operational datastore. Statistics are
    not reported on-change only two important counters, for these
    a smaller dampening period is possible.
  </description>
  <content-data>
    <system-capabilities \
     xmlns="urn:ietf:params:xml:ns:yang:ietf-system-capabilities" \
     xmlns:notc=\
       "urn:ietf:params:xml:ns:yang:ietf-notification-capabilities" \
     xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
     <notc:subscription-capabilities>
      <notc:minimum-update-period>500</notc:minimum-update-period>
      <notc:max-nodes-per-update>2000</notc:max-nodes-per-update>
      <notc:minimum-dampening-period>100</notc:minimum-dampening-period>
      <notc:periodic-notifications-supported>\
         config-changes state-changes\
      </notc:periodic-notifications-supported>
      <notc:on-change-supported>\
         config-changes state-changes\
      </notc:on-change-supported>
      <notc:supported-excluded-change-type>\
         all\
      </notc:supported-excluded-change-type>
     </notc:subscription-capabilities>
      <datastore-capabilities>
        <datastore>ds:operational</datastore>
        <per-node-capabilities>
          <node-selector>\
              /if:interfaces/if:interface[if:name='lo']\
          </node-selector>
          <notc:subscription-capabilities>
            <notc:on-change-supported/>
            <notc:periodic-notifications-supported/>
          </notc:subscription-capabilities>
        </per-node-capabilities>
        <per-node-capabilities>
          <node-selector>\
              /if:interfaces/if:interface/if:statistics/if:in-octets\

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          </node-selector>
          <notc:subscription-capabilities>
            <notc:minimum-dampening-period>10
              </notc:minimum-dampening-period>
            <notc:on-change-supported>\
              state-changes\
            </notc:on-change-supported>
          </notc:subscription-capabilities>
        </per-node-capabilities>
        <per-node-capabilities>
          <node-selector>\
              /if:interfaces/if:interface/if:statistics/if:out-octets\
          </node-selector>
          <notc:subscription-capabilities>
            <notc:minimum-dampening-period>10
              </notc:minimum-dampening-period>
            <notc:on-change-supported>\
              state-changes\
            </notc:on-change-supported>
          </notc:subscription-capabilities>
        </per-node-capabilities>
        <per-node-capabilities>
          <node-selector>\
              /if:interfaces/if:interface/if:statistics\
          </node-selector>
          <notc:subscription-capabilities>
            <notc:on-change-supported/>
          </notc:subscription-capabilities>
        </per-node-capabilities>
      </datastore-capabilities>
    </system-capabilities>
  </content-data>
</instance-data-set>

   Figure 1: Notification Capabilities with data node specific settings

Appendix B.  Instance data example #2

   The following examples use artwork folding [RFC8792] for better
   formatting.

   The following instance data example describes the notification
   capabilities of a hypothetical "acme-switch".  The switch implements
   the running, candidate and operational datastores.  Every change can
   be reported "on-change" from the running datastore, nothing from the
   candidate datastore and all "config false" data from the operational
   datastore.  "periodic" subscriptions are supported for running and
   operational, but not for candidate datastores.

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========== NOTE: '\' line wrapping per BCP YYY (RFC YYYY) ===========

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=\
    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>acme-switch-notification-capabilities</name>
  <content-schema>
    <module>ietf-system-capabilities@2021-04-02</module>
    <module>ietf-notification-capabilities@2021-04-02</module>
  </content-schema>
  <description>Notification capabilities of acme-switch.
    Acme-switch implements the running, candidate and operational
    datastores. Every change can be reported on-change from the
    running datastore, nothing from the candidate datastore and
    all "config false" data from the operational datastore. Periodic
    subscriptions are supported for running and operational, but not
    for candidate datastore.
  </description>
  <content-data>
    <system-capabilities \
     xmlns="urn:ietf:params:xml:ns:yang:ietf-system-capabilities" \
     xmlns:notc=\
       "urn:ietf:params:xml:ns:yang:ietf-notification-capabilities" \
     xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
     <notc:subscription-capabilities>
      <notc:minimum-update-period>500</notc:minimum-update-period>
      <notc:max-nodes-per-update>2000</notc:max-nodes-per-update>
      <notc:minimum-dampening-period>100</notc:minimum-dampening-period>
      <notc:periodic-notifications-supported>\
        config-changes state-changes\
      </notc:periodic-notifications-supported>
     </notc:subscription-capabilities>
     <datastore-capabilities>
       <datastore>ds:operational</datastore>
       <per-node-capabilities>
         <node-selector>/</node-selector>
         <notc:subscription-capabilities>
           <notc:on-change-supported>\
             state-changes\
           </notc:on-change-supported>
         </notc:subscription-capabilities>
       </per-node-capabilities>
     </datastore-capabilities>
     <datastore-capabilities>
       <datastore>ds:candidate</datastore>
       <per-node-capabilities>
         <node-selector>/</node-selector>
         <notc:subscription-capabilities>

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           <notc:on-change-supported/>
           <notc:periodic-notifications-supported/>
         </notc:subscription-capabilities>
       </per-node-capabilities>
     </datastore-capabilities>
     <datastore-capabilities>
      <datastore>ds:running</datastore>
       <per-node-capabilities>
         <node-selector>/</node-selector>
         <notc:subscription-capabilities>
           <notc:on-change-supported>\
             config-changes\
           </notc:on-change-supported>
         </notc:subscription-capabilities>
       </per-node-capabilities>
     </datastore-capabilities>
    </system-capabilities>
  </content-data>
</instance-data-set>

     Figure 2: Notification Capabilities with datastore level settings

Appendix C.  Changes between revisions

   Note to RFC Editor (To be removed by RFC Editor)

   v15 - v16

   o  Two editorial comments from document shepherd

   v14 - v15

   o  Address the last comments from document shepherd

   v13 - v14

   o  Updated according to sheperds review

   o  Added to import, which imported modules need to be implemented.

   o  Added notes to the RFC editor

   o  Re-arrange the sections, for a better reading flow

   o  Many editorial changes

   o  Replace YANG module prefix

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   v12 - v13

   o  Rearranged order of notification capability leafs into 3 groups:
      generic, specific to periodic subscriptions, specific to on-
      change.

   o  Introduced artwork folding in the examples

   o  Updated to follow draft-ietf-netmod-yang-instance-file-format-10

   o  Various editing changes

   v11 - v12

   o  Updated max-nodes-per-update description

   o  Reformatted YANG models with pyang -f yang --keep-comments --yang-
      line-length 69

   v10 - v11

   o  Updated examples

   o  Updated typedef notification-support

   v09 - v10

   o  Removed description text from imports about the need for
      implementing the imported module.

   o  Changed notification-support to bits with shorter names

   o  Assigned enum values to supported-excluded-change-type

   o  Made node-selector a choice to allow for future alternative
      selection methods.

   o  Changed precedence of per-node-capabilities entries.  Precedence
      is now according to the position of entries in the list.

   v08 - v09

   o  Split the YANG module into two: ietf-system-capabilities and ietf-
      notification-capabilities.  Restructured/updated the draft
      accordingly.

   v07 - v08

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   o  Prepared the YANG model to include other non-YANG-Push related
      capabilities.

   o  Renamed the top level container to system-capabilities

   o  Added a container subscription-capabilities to the grouping
      subscription-capabilities to contain all subscription related
      capabilities

   o  Updated examples according to draft-ietf-netmod-yang-instance-
      file-format-06.

   v06 - v07

   o  Updated examples according to draft-ietf-netmod-yang-instance-
      file-format-05.

   v05 - v06

   o  Providing the capability data is only a "SHOULD" recommendation.
      Some reviewers wanted MUST some wanted much less.

   o  The YANG module import statements now indicate the imported
      modules that must be implemented not just available as import as
      requested by the YangDoctors review.

   v04 - v05

   o  Added new capabilities periodic-notifications-supported and
      supported-excluded-change-type.

   o  Restructured YANG module to make the node-selector's usage similar
      to how NACM uses it: "/" means the whole datastore.

   o  Small corrections, spelling, rewording

   o  Replaced the term server with the term publisher except in cases
      where we speak about datastores and functionality based on get,
      getconfig operations.  In this latter case it is really the server
      functionality that is discussed

   v03 - v04

   o  Clarified recommended support for on-change notifications about
      the datastore-subscription-capabilities.

   v02 - v03

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   o  Allow throughput related capabilities to be defined on top,
      datastore or data node level.  Described that specific capability
      values always override generic ones.

   o  Indicate that non-NMDA servers can also use this model.

   o  Updated according to draft-ietf-netmod-yang-instance-file-
      format-04

   v01 - v02

   o  Added instance data examples

   o  On-change capability can be defined per datastore

   o  Added "if-feature yp:on-change" where relevant

   o  Unified units used

   v00 - v01

   o  Add more capabilities: minimum period, supported period max-number
      of objects, min dampening period, dampening supported

Authors' Addresses

   Balazs Lengyel
   Ericsson
   Magyar Tudosok korutja 11
   1117 Budapest
   Hungary

   Email: balazs.lengyel@ericsson.com

   Alexander Clemm
   Futurewei
   2330 Central Expressway
   Santa Clara, CA 95050
   USA

   Email: ludwig@clemm.org

   Benoit Claise
   Huawei

   Email: benoit.claise@huawei.com

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