YANG Modules Describing Capabilities for Systems and Datastore Update Notifications
draft-ietf-netconf-notification-capabilities-21

NETCONF                                                       B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                                A. Clemm
Expires: 18 April 2022                                         Futurewei
                                                               B. Claise
                                                                  Huawei
                                                         15 October 2021

 YANG Modules describing Capabilities for Systems and Datastore Update
                             Notifications
            draft-ietf-netconf-notification-capabilities-21

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 at
   implementation time, by making use of 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 18 April 2022.

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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 . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  19
     7.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  19
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Instance data example #1 . . . . . . . . . . . . . .  22
   Appendix B.  Instance data example #2 . . . . . . . . . . . . . .  24
   Appendix C.  Changes between revisions  . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   Servers and/or 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 API contract between the server and client.  Examples include
   maximum size of data that can be stored or transferred, information

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   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 the complete system, individual YANG datastores
   [RFC8342], 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:

   *  vendor independent

   *  machine-readable

   *  available in an identical format both at implementation time and
      at run time.

   Implementation-time information is needed by Network Management
   System (NMS) implementers.  An NMS implementation that supports
   notifications needs the information about a system's capability so it
   can send "on-change" notifications.  If the information is not
   documented in a way that is readily 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 operators need to plan their
   management practices and NMS implementation before they decide to buy
   the specific network node type.  Moreover, the decision to buy the
   node type sometimes depends on these management possibilities.

   Run-time capability information is needed:

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

   *  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 than 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 at run time.

   *  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].  When provided,
      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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   *  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 in the YANG-Push
   functionality.  As defined in [RFC8641] a publisher may allow
   subscribers to subscribe to updates from a datastore and will
   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:

   *  Supported (reporting) periods for "periodic" subscriptions

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

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

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

   *  Supported dampening periods for "on-change" subscriptions

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

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   Publishers might have some capabilities (or limitations) to document.
   For example, how many update notifications and how many datastore
   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 for.
   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.  The ietf-system-capabilies is used by other modules to
   augment in specific capability information.  Every set of such
   capabilities MUST 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.

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   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.

   Note: The solution is usable for both NMDA and non-NMDA systems.  For
   non-NMDA servers "config false" data is considered as if it were 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@2021-10-12.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";

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     }

     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 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.

            The same grouping MUST be used to define hierarchical capabilities
        supported both at the system level and datastore/data node 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. When stating a specific capability, the
        relative path for any specific capability must be the same
        under the system-capabilities container and under the
        per-node-capabilities list.

        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

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        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
        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 2021-10-12 {
       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@2021-10-12.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";
     }

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     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 {
         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, which represents a set of
              configuration datastores, must not be used.";
         }
         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.

              Note that the longest patch matching can be achieved
              by ordering more specific matches before less
              specific ones.";
           choice node-selection {
             description
               "A method to select some or all nodes within a datastore.";
             leaf node-selector {

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               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>

   <CODE ENDS>

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.

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   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.

   <CODE BEGINS> file "ietf-notification-capabilities@2021-10-12.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;

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       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
        that the publisher can support. (Note that for a specific
        subscription, the publisher MAY 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

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        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 2021-10-12 {
       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.

     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 {

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           type bits {
             bit config-changes {
               description
                 "The publisher is capable of sending
                  notifications for 'config true' 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 for all data nodes,
              'config false' data nodes, 'config true' data nodes, or
              no 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.";
           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

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              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 is
                likely to 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";
           }
         }
         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.";

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           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 for the selected data nodes.";
           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" {

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       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>

   <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] or as YANG instance data.
   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.

   This document outlines a framework for conveying system capability
   information that is inherently flexible and extensible.  While the
   full set of use cases is not known now, they may range as wide as
   conveying the minimum update period for periodic subscription updates
   and what protocols might be used for such notifications.  Knowledge
   of this type of value might, for example, allow an attacker to gain
   insight into how long unauthorized configuration changes might be

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   active prior to detection, and what communications channels might be
   disrupted to extend the period of non-detection.  Documents adding
   additional capabilities via augmenting this module are encouraged to
   document the security considerations of the new YANG nodes, according
   to the guidance in BCP 216.

   All protocol-accessible data nodes in augmented modules 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.  The data in file format also inherits all the security
   considerations of [I-D.ietf-netmod-yang-instance-file-format] which
   has additional considerations about read protections; and
   distinguishes between data at rest and in motion.

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 [RFC6020].  Following the format in [RFC6020], the following
   registrations are requested:

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

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      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
   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", Work in Progress, Internet-Draft, draft-ietf-
              netmod-yang-instance-file-format-19, 16 September 2021,
              <https://www.ietf.org/archive/id/draft-ietf-netmod-yang-
              instance-file-format-19.txt>.

   [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>.

   [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>.

   [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>.

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   [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>.

9.2.  Informative References

   [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>.

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Appendix A.  Instance data example #1

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

   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 RFC 8792) ===========

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=\
    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>acme-router-notification-capabilities</name>
  <content-schema>
    <module>ietf-system-capabilities@2021-10-12</module>
    <module>ietf-notification-capabilities@2021-10-12</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 except for two important counters,
    where a small dampening period is mandated.
  </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\

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      </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\
          </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>

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      </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.

========== NOTE: '\' line wrapping per RFC 8792) ===========

<?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@2021-10-12</module>
    <module>ietf-notification-capabilities@2021-10-12</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>

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      <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>
           <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)

   v20 - v21

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   *  IESG review

   v19 - v20

   *  IESG review

   v18 - v19

   *  IESG review

   v17 - v18

   *  IESG review

   v16 - v17

   *  AD review comments addressed

   v15 - v16

   *  Two editorial comments from document shepherd

   v14 - v15

   *  Address the last comments from document shepherd

   v13 - v14

   *  Updated according to sheperds review

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

   *  Added notes to the RFC editor

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

   *  Many editorial changes

   *  Replace YANG module prefix

   v12 - v13

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

   *  Introduced artwork folding in the examples

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

   *  Various editing changes

   v11 - v12

   *  Updated max-nodes-per-update description

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

   v10 - v11

   *  Updated examples

   *  Updated typedef notification-support

   v09 - v10

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

   *  Changed notification-support to bits with shorter names

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

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

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

   v08 - v09

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

   v07 - v08

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

   *  Renamed the top level container to system-capabilities

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

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

   v06 - v07

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

   v05 - v06

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

   *  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

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

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

   *  Small corrections, spelling, rewording

   *  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

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

   v02 - v03

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

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

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

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   v01 - v02

   *  Added instance data examples

   *  On-change capability can be defined per datastore

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

   *  Unified units used

   v00 - v01

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

Authors' Addresses

   Balazs Lengyel
   Ericsson
   1117 Budapest
   Magyar Tudosok korutja 11
   Hungary

   Email: balazs.lengyel@ericsson.com

   Alexander Clemm
   Futurewei
   2330 Central Expressway
   Santa Clara, CA 95050,
   United States of America

   Email: ludwig@clemm.org

   Benoit Claise
   Huawei

   Email: benoit.claise@huawei.com

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