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NETCONF                                                         A. Clemm
Internet-Draft                                                Huawei USA
Intended status: Standards Track                                 E. Voit
Expires: November 16, 2019                                 Cisco Systems
                                                            May 15, 2019


                    Subscription to YANG Datastores
                    draft-ietf-netconf-yang-push-24

Abstract

   This document describes a mechanism that allows subscriber
   applications to request a continuous and customized stream of updates
   from a YANG datastore.  Providing such visibility into updates
   enables new capabilities based on the remote mirroring and monitoring
   of configuration and operational state.

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 November 16, 2019.

Copyright Notice

   Copyright (c) 2019 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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   3
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Subscription Model  . . . . . . . . . . . . . . . . . . .   5
     3.2.  Negotiation of Subscription Policies  . . . . . . . . . .   6
     3.3.  On-Change Considerations  . . . . . . . . . . . . . . . .   7
     3.4.  Reliability Considerations  . . . . . . . . . . . . . . .   8
     3.5.  Data Encodings  . . . . . . . . . . . . . . . . . . . . .   9
     3.6.  Defining the Selection with a Datastore . . . . . . . . .  10
     3.7.  Streaming Updates . . . . . . . . . . . . . . . . . . . .  11
     3.8.  Subscription Management . . . . . . . . . . . . . . . . .  13
     3.9.  Receiver Authorization  . . . . . . . . . . . . . . . . .  15
     3.10. On-Change Notifiable Datastore Nodes  . . . . . . . . . .  16
     3.11. Other Considerations  . . . . . . . . . . . . . . . . . .  17
   4.  A YANG Data Model for Management of Datastore Push
       Subscriptions . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.2.  Subscription Configuration  . . . . . . . . . . . . . . .  24
     4.3.  YANG Notifications  . . . . . . . . . . . . . . . . . . .  25
     4.4.  YANG RPCs . . . . . . . . . . . . . . . . . . . . . . . .  26
   5.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  31
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  50
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  50
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     10.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Appendix A.  Appendix A: Subscription Errors  . . . . . . . . . .  52
     A.1.  RPC Failures  . . . . . . . . . . . . . . . . . . . . . .  52
     A.2.  Notifications of Failure  . . . . . . . . . . . . . . . .  53



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   Appendix B.  Changes Between Revisions  . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  58

1.  Introduction

   Traditional approaches to provide visibility into managed entities
   from a remote system have been built on polling.  With polling, data
   is periodically requested and retrieved by a client from a server to
   stay up-to-date.  However, there are issues associated with polling-
   based management:

   o  Polling incurs significant latency.  This latency prohibits many
      types of application.

   o  Polling cycles may be missed and requests may be delayed or get
      lost, often when the network is under stress and the need for the
      data is the greatest.

   o  Polling requests may undergo slight fluctuations, resulting in
      intervals of different lengths.  The resulting data is difficult
      to calibrate and compare.

   o  For applications that monitor for changes, many remote polling
      cycles place unwanted and ultimately wasteful load on the network,
      devices, and applications, particularly when changes occur only
      infrequently.

   A more effective alternative to polling is for an application to
   receive automatic and continuous updates from a targeted subset of a
   datastore.  Accordingly, there is a need for a service that allows
   applications to subscribe to updates from a datastore and that
   enables the server (also referred to as publisher) to push and in
   effect stream those updates.  The requirements for such a service
   have been documented in [RFC7923].

   This document defines a corresponding solution that is built on top
   of "Custom Subscription to Event Streams"
   [I-D.draft-ietf-netconf-subscribed-notifications].  Supplementing
   that work are YANG data model augmentations, extended RPCs, and new
   datastore specific update notifications.  Transport options for
   [I-D.draft-ietf-netconf-subscribed-notifications] will work
   seamlessly with this solution.

2.  Definitions and Acronyms

   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



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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the terminology defined in [RFC7950], [RFC8341],
   [RFC8342], and [I-D.draft-ietf-netconf-subscribed-notifications].  In
   addition, the following terms are introduced:

   o  Datastore node: A node in the instantiated YANG data tree
      associated with a datastore.  In this document, datastore nodes
      are often also simply referred to as "objects"

   o  Datastore node update: A data item containing the current value of
      a datastore node at the time the datastore node update was
      created, as well as the path to the datastore node.

   o  Datastore subscription: A subscription to a stream of datastore
      node updates.

   o  Datastore subtree: A datastore node and all its descendant
      datastore nodes

   o  On-change subscription: A datastore subscription with updates that
      are triggered when changes in subscribed datastore nodes are
      detected.

   o  Periodic subscription: A datastore subscription with updates that
      are triggered periodically according to some time interval.

   o  Selection filter: Evaluation and/or selection criteria, which may
      be applied against a targeted set of objects.

   o  Update record: A representation of one or more datastore node
      updates.  In addition, an update record may contain which type of
      update led to the datastore node update (e.g., whether the
      datastore node was added, changed, deleted).  Also included in the
      update record may be other metadata, such as a subscription id of
      the subscription as part of which the update record was generated.
      In this document, update records are often also simply referred to
      as "updates".

   o  Update trigger: A mechanism that determines when an update record
      needs to be generated.

   o  YANG-Push: The subscription and push mechanism for datastore
      updates that is specified in this document.






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3.  Solution Overview

   This document specifies a solution that provides a subscription
   service for updates from a datastore.  This solution supports dynamic
   as well as configured subscriptions to updates of datastore nodes.
   Subscriptions specify when notification messages (also referred to as
   "push updates") should be sent and what data to include in update
   records.  Datastore node updates are subsequently pushed from the
   publisher to the receiver per the terms of the subscription.

3.1.  Subscription Model

   YANG-push subscriptions are defined using a YANG data model.  This
   model enhances the subscription model defined in
   [I-D.draft-ietf-netconf-subscribed-notifications] with capabilities
   that allow subscribers to subscribe to datastore node updates,
   specifically to specify the update triggers defining when to generate
   update records as well as what to include in an update record.  Key
   enhancements include:

   o  Specification of selection filters which identify targeted YANG
      datastore nodes and/or datastore subtrees for which updates are to
      be pushed.

   o  Specification of update policies contain conditions which trigger
      the generation and pushing of new update records.  There are two
      types of subscriptions, distinguished by how updates are
      triggered: periodic and on-change.

      *  For periodic subscriptions, the update trigger is specified by
         two parameters that define when updates are to be pushed.
         These parameters are the period interval with which to report
         updates, and an "anchor time", i.e. a reference point in time
         that can be used to calculate at which points in time periodic
         updates need to be assembled and sent.

      *  For on-change subscriptions, an update trigger occurs whenever
         a change in the subscribed information is detected.  Included
         are additional parameters that include:

         +  Dampening period: In an on-change subscription, detected
            object changes should be sent as quickly as possible.
            However it may be undesirable to send a rapid series of
            object changes.  Such behavior has the potential to exhaust
            resources in the publisher or receiver.  In order to protect
            against that, a dampening period MAY be used to specify the
            interval which has to pass before successive update records
            for the same subscription are generated for a receiver.  The



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            dampening period collectively applies to the set of all
            datastore nodes selected by a single subscription.  This
            means that when there is a change to one or more subscribed
            objects, an update record containing those objects is
            created immediately (when no dampening period is in effect)
            or at the end of a dampening period (when a dampening period
            is in fact in effect).  If multiple changes to a single
            object occur during a dampening period, only the value that
            is in effect at the time when the update record is created
            is included.  The dampening period goes into effect every
            time an update record completes assembly.

         +  Change type: This parameter can be used to reduce the types
            of datastore changes for which updates are sent (e.g., you
            might only send an update when an object is created or
            deleted, but not when an object value changes).

         +  Sync on start: defines whether or not a complete push-update
            of all subscribed data will be sent at the beginning of a
            subscription.  Such early synchronization establishes the
            frame of reference for subsequent updates.

   o  An encoding (using anydata) for the contents of periodic and on-
      change push updates.

3.2.  Negotiation of Subscription Policies

   A dynamic subscription request SHOULD be declined if a publisher's
   assessment is that it may be unable to provide update records meeting
   the terms of an "establish-subscription" or "modify-subscription" RPC
   request.  In this case, a subscriber may quickly follow up with a new
   RPC request using different parameters.

   Random guessing of different parameters by a subscriber is to be
   discouraged.  Therefore, in order to minimize the number of
   subscription iterations between subscriber and publisher, a dynamic
   subscription supports a simple negotiation between subscribers and
   publishers for subscription parameters.  This negotiation is in the
   form of supplemental information which should be inserted within
   error responses to a failed RPC request.  This returned error
   response information, when considered, should increase the likelihood
   of success for subsequent RPC requests.  Such hints include suggested
   periodic time intervals, acceptable dampening periods, and size
   estimates for the number or objects which would be returned from a
   proposed selection filter.  However, there are no guarantees that
   subsequent requests which consider these hints will be accepted.





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3.3.  On-Change Considerations

   On-change subscriptions allow receivers to receive updates whenever
   changes to targeted objects occur.  As such, on-change subscriptions
   are particularly effective for data that changes infrequently, yet
   for which applications need to be quickly notified whenever a change
   does occur with minimal delay.

   On-change subscriptions tend to be more difficult to implement than
   periodic subscriptions.  Accordingly, on-change subscriptions may not
   be supported by all implementations or for every object.

   Whether or not to accept or reject on-change subscription requests
   when the scope of the subscription contains objects for which on-
   change is not supported is up to the publisher implementation.  A
   publisher MAY accept an on-change subscription even when the scope of
   the subscription contains objects for which on-change is not
   supported.  In that case, updates are sent only for those objects
   within the scope that do support on-change updates, whereas other
   objects are excluded from update records, even if their values
   change.  In order for a subscriber to determine whether objects
   support on-change subscriptions, objects are marked accordingly on a
   publisher.  Accordingly, when subscribing, it is the responsibility
   of the subscriber to ensure it is aware of which objects support on-
   change and which do not.  For more on how objects are so marked, see
   Section 3.10.

   Alternatively, a publisher MAY decide to simply reject an on-change
   subscription in case the scope of the subscription contains objects
   for which on-change is not supported.  In case of a configured
   subscription, the publisher MAY suspend the subscription.

   To avoid flooding receivers with repeated updates for subscriptions
   containing fast-changing objects, or objects with oscillating values,
   an on-change subscription allows for the definition of a dampening
   period.  Once an update record for a given object is generated, no
   other updates for this particular subscription will be created until
   the end of the dampening period.  Values sent at the end of the
   dampening period are the values that are current at the end of the
   dampening period of all changed objects.  Changed objects include
   those which were deleted or newly created during that dampening
   period.  If an object has returned to its original value (or even has
   been created and then deleted) during the dampening-period, that
   value (and not the interim change) will still be sent.  This will
   indicate churn is occurring on that object.

   On-change subscriptions can be refined to let users subscribe only to
   certain types of changes.  For example, a subscriber might only want



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   object creations and deletions, but not modifications of object
   values.

   Putting it all together, following is the conceptual process for
   creating an update record as part of an on-change subscription:

   1.  Just before a change, or at the start of a dampening period,
       evaluate any filtering and any access control rules to ensure
       receiver is authorized to view all subscribed datastore nodes
       (filtering out any nodes for which this is not the case).  The
       result is a set "A" of datastore nodes and subtrees.

   2.  Just after a change, or at the end of a dampening period,
       evaluate any filtering and any (possibly new) access control
       rules.  The result is a set "B" of datastore nodes and subtrees.

   3.  Construct an update record, which takes the form of YANG patch
       record [RFC8072] for going from A to B.

   4.  If there were any changes made between A and B which canceled
       each other out, insert into the YANG patch record the last change
       made, even if the new value is no different from the original
       value (since changes that were made in the interim were canceled
       out).  In case the changes involve creating a new datastore node,
       then deleting it, the YANG patch record will indicate deletion of
       the datastore node.  Similarly, in case the changes involve
       deleting a new datastore node, then recreating it, the YANG patch
       record will indicate creation of the datastore node.

   5.  If the resulting patch record is non-empty, send it to the
       receiver.

   Note: In cases where a subscriber wants to have separate dampening
   periods for different objects, the subscriber has the option to
   create multiple subscriptions with different selection filters.

3.4.  Reliability Considerations

   A subscription to updates from a datastore is intended to obviate the
   need for polling.  However, in order to do so, it is critical that
   subscribers can rely on the subscription and have confidence that
   they will indeed receive the subscribed updates without having to
   worry about updates being silently dropped.  In other words, a
   subscription constitutes a promise on the side of the publisher to
   provide the receivers with updates per the terms of the subscription.

   Now, there are many reasons why a publisher may at some point no
   longer be able to fulfill the terms of the subscription, even if the



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   subscription had been entered into with good faith.  For example, the
   volume of datastore nodes may be larger than anticipated, the
   interval may prove too short to send full updates in rapid
   succession, or an internal problem may prevent objects from being
   collected.  For this reason, the solution that is defined in this
   document mandates that a publisher notifies receivers immediately and
   reliably whenever it encounters a situation in which it is unable to
   keep the terms of the subscription, and provides the publisher with
   the option to suspend the subscription in such a case.  This includes
   indicating the fact that an update is incomplete as part of a push-
   update or push-change-update notification, as well as emitting a
   subscription-suspended notification as applicable.  This is described
   further in Section 3.11.1.

   A publisher SHOULD reject a request for a subscription if it is
   unlikely that the publisher will be able to fulfill the terms of that
   subscription request.  In such cases, it is preferable to have a
   subscriber request a less resource intensive subscription than to
   deal with frequently degraded behavior.

   The solution builds on
   [I-D.draft-ietf-netconf-subscribed-notifications].  As defined there,
   any loss of underlying transport connection will be detected and
   result in subscription termination (in case of dynamic subscriptions)
   or suspension (in case of configured subscriptions), ensuring that
   situations will not occur in which the loss of update notifications
   would go unnoticed.

3.5.  Data Encodings

3.5.1.  Periodic Subscriptions

   In a periodic subscription, the data included as part of an update
   record corresponds to data that could have been read using a
   retrieval operation.

3.5.2.  On-Change Subscriptions

   In an on-change subscription, update records need to indicate not
   only values of changed datastore nodes but also the types of changes
   that occurred since the last update.  Therefore, encoding rules for
   data in on-change updates will generally follow YANG-patch operation
   as specified in [RFC8072].  The YANG-patch will describe what needs
   to be applied to the earlier state reported by the preceding update,
   to result in the now-current state.  Note that contrary to [RFC8072],
   objects encapsulated are not restricted to only configuration
   objects.




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   A publisher indicates the type of change to a datastore node using
   the different YANG patch operations: the "create" operation is used
   for newly created objects (except entries in a user-ordered list),
   the "delete" operation is used for deleted objects (including in
   user-ordered lists), the "replace" operation is used when only the
   object value changes, the "insert" operation is used when a new entry
   is inserted in a list, and the "move" operation is used when an
   existing entry in a user-ordered list is moved.

   However, a patch must be able to do more than just describe the delta
   from the previous state to the current state.  As per Section 3.3, it
   must also be able to identify whether transient changes have occurred
   on an object during a dampening period.  To support this, it is valid
   to encode a YANG patch operation so that its application would result
   in no change between the previous and current state.  This indicates
   that some churn has occurred on the object.  An example of this would
   be a patch that indicates a "create" operation for a datastore node
   where the receiver believes one already exists, or a "replace"
   operation which replaces a previous value with the same value.  Note
   that this means that the "create" and "delete" errors described in
   [RFC8072] section 2.5 are not errors, and are valid operations with
   YANG-Push.

3.6.  Defining the Selection with a Datastore

   A subscription must specify both the selection filters and the
   datastore against which these selection filters will be applied.
   This information is used to choose and subsequently push data from
   the publisher's datastore to the receivers.

   Only a single selection filter can be applied to a subscription at a
   time.  An RPC request proposing a new selection filter replaces any
   existing filter.  The following selection filter types are included
   in the YANG-push data model, and may be applied against a datastore:

   o  subtree: A subtree selection filter identifies one or more
      datastore subtrees.  When specified, update records will only come
      from the datastore nodes of selected datastore subtree(s).  The
      syntax and semantics correspond to that specified for [RFC6241]
      section 6.

   o  xpath: An "xpath" selection filter is an XPath expression that
      returns a node set.  (XPath is a query language for selecting
      nodes in an XML document.)  When specified, updates will only come
      from the selected datastore nodes.






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   These filters are intended to be used as selectors that define which
   objects are within the scope of a subscription.  A publisher MUST
   support at least one type of selection filter.

   XPath itself provides powerful filtering constructs and care must be
   used in filter definition.  Consider an XPath filter which only
   passes a datastore node when an interface is up.  It is up to the
   receiver to understand implications of the presence or absence of
   objects in each update.

   When the set of selection filtering criteria is applied for a
   periodic subscription, then they are applied whenever a periodic
   update record is constructed, and only datastore nodes that pass the
   filter and to which a receiver has access are provided to that
   receiver.  If the same filtering criteria is applied to an on-change
   subscription, only the subset of those datastore nodes supporting on-
   change is provided.  A datastore node which doesn't support on-change
   is never sent as part of an on-change subscription's "push-update" or
   "push-change-update" (see Section 3.7).

3.7.  Streaming Updates

   Contrary to traditional data retrieval requests, datastore
   subscription enables an unbounded series of update records to be
   streamed over time.  Two generic YANG notifications for update
   records have been defined for this: "push-update" and "push-change-
   update".

   A "push-update" notification defines a complete, filtered update of
   the datastore per the terms of a subscription.  This type of YANG
   notification is used for continuous updates of periodic
   subscriptions.  A "push-update" notification can also be used for the
   on-change subscriptions in two cases.  First, it MUST be used as the
   initial "push-update" if there is a need to synchronize the receiver
   at the start of a new subscription.  It also MAY be sent if the
   publisher later chooses to resync an on-change subscription.  The
   "push-update" update record contains an instantiated datastore
   subtree with all of the subscribed contents.  The content of the
   update record is equivalent to the contents that would be obtained
   had the same data been explicitly retrieved using a datastore
   retrieval operation using the same transport with the same filters
   applied.

   A "push-change-update" notification is the most common type of update
   for on-change subscriptions.  The update record in this case contains
   the set of changes that datastore nodes have undergone since the last
   notification message.  In other words, this indicates which datastore
   nodes have been created, deleted, or have had changes to their



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   values.  In cases where multiple changes have occurred over the
   course of a dampening period and the object has not been deleted, the
   object's most current value is reported.  (In other words, for each
   object, only one change is reported, not its entire history.  Doing
   so would defeat the purpose of the dampening period.)

   "Push-update" and "push-change-update" are encoded and placed within
   notification messages, and ultimately queued for egress over the
   specified transport.

   The following is an example of a notification message for a
   subscription tracking the operational status of a single Ethernet
   interface (per [RFC8343]).  This notification message is encoded XML
   over NETCONF as per
   [I-D.draft-ietf-netconf-netconf-event-notifications].

  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
   <eventTime>2017-10-25T08:00:11.22Z</eventTime>
   <push-update xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <id>1011</id>
     <datastore-contents>
        <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
         <interface>
           <name>eth0</name>
           <oper-status>up</oper-status>
         </interface>
       </interfaces>
     </datastore-contents>
   </push-update>
  </notification>

                          Figure 1: Push example

   The following is an example of an on-change notification message for
   the same subscription.
















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  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
   <eventTime>2017-10-25T08:22:33.44Z</eventTime>
   <push-change-update
        xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <id>89</id>
     <datastore-changes>
       <yang-patch>
         <patch-id>0</patch-id>
         <edit>
           <edit-id>edit1</edit-id>
           <operation>replace</operation>
           <target>/ietf-interfaces:interfaces</target>
           <value>
             <interfaces
                  xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
               <interface>
                 <name>eth0</name>
                 <oper-status>down</oper-status>
               </interface>
             </interfaces>
           </value>
         </edit>
       </yang-patch>
     </datastore-changes>
   </push-change-update>
  </notification>

                   Figure 2: Push example for on change

   Of note in the above example is the 'patch-id' with a value of '0'.
   Per [RFC8072], the 'patch-id' is an arbitrary string.  With YANG
   Push, the publisher SHOULD put into the 'patch-id' a counter starting
   at '0' which increments with every 'push-change-update' generated for
   a subscription.  If used as a counter, this counter MUST be reset to
   '0' anytime a resynchronization occurs (i.e., with the sending of a
   'push-update').  Also if used as a counter, the counter MUST be reset
   to '0' after passing a maximum value of '4294967295' (i.e. maximum
   value that can be represented using uint32 data type).  Such a
   mechanism allows easy identification of lost or out-of-sequence
   update records.

3.8.  Subscription Management

   The RPCs defined within
   [I-D.draft-ietf-netconf-subscribed-notifications] have been enhanced
   to support datastore subscription negotiation.  Also, new error codes
   have been added that are able to indicate why a datastore
   subscription attempt has failed, along with new YANG-data that MAY be



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   used to include details on input parameters that might result in a
   successful subsequent RPC invocation.

   The establishment or modification of a datastore subscription can be
   rejected for multiple reasons.  This includes a too large subtree
   request, or the inability of the publisher to push update records as
   frequently as requested.  In such cases, no subscription is
   established.  Instead, the subscription-result with the failure
   reason is returned as part of the RPC response.  As part of this
   response, a set of alternative subscription parameters MAY be
   returned that would likely have resulted in acceptance of the
   subscription request.  The subscriber may consider these as part of
   future subscription attempts.

   In the case of a rejected request for an establishment of a datastore
   subscription, if there are hints, the hints SHOULD be transported
   within a YANG-data "establish-subscription-datastore-error-info"
   container inserted into the RPC error response, in lieu of the
   "establish-subscription-stream-error-info" that is inserted in case
   of a stream subscription.

   Below is a tree diagram for "establish-subscription-datastore-error-
   info".  All tree diagrams used in this document follow the notation
   defined in [RFC8340]

          YANG-data establish-subscription-datastore-error-info
             +--ro establish-subscription-datastore-error-info
                +--ro reason?                identityref
                +--ro period-hint?           centiseconds
                +--ro filter-failure-hint?   string
                +--ro object-count-estimate? uint32
                +--ro object-count-limit?    uint32
                +--ro kilobytes-estimate?    uint32
                +--ro kilobytes-limit?       uint32

    Figure 3: Tree diagram for establish-subscription-datastore-error-
                                   info

   Similarly, in the case of a rejected request for modification of a
   datastore subscription, if there are hints, the hints SHOULD be
   transported within a YANG-data "modify-subscription-datastore-error-
   info" container inserted into the RPC error response, in lieu of the
   "modify-subscription-stream-error-info" that is inserted in case of a
   stream subscription.

   Below is a tree diagram for "modify-subscription-datastore-error-
   info".




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          YANG-data modify-subscription-datastore-error-info
             +--ro modify-subscription-datasore-error-info
                +--ro reason?                identityref
                +--ro period-hint?           centiseconds
                +--ro filter-failure-hint?   string
                +--ro object-count-estimate? uint32
                +--ro object-count-limit?    uint32
                +--ro kilobytes-estimate?    uint32
                +--ro kilobytes-limit?       uint32

    Figure 4: Tree diagram for modify-subscription-datastore-error-info

3.9.  Receiver Authorization

   A receiver of subscription data MUST only be sent updates for which
   it has proper authorization.  A publisher MUST ensure that no non-
   authorized data is included in push updates.  To do so, it needs to
   apply all corresponding checks applicable at the time of a specific
   pushed update and if necessary silently remove any non-authorized
   data from datastore subtrees.  This enables YANG data pushed based on
   subscriptions to be authorized equivalently to a regular data
   retrieval (get) operation.

   Each "push-update" and "push-change-update" MUST have access control
   applied, as is depicted in the following diagram.  This includes
   validating that read access is permitted for any new objects selected
   since the last notification message was sent to a particular
   receiver.  To accomplish this, implementations SHOULD support the
   conceptual authorization model of [RFC8341], specifically section
   3.2.4.

                         +-----------------+      +--------------------+
     push-update or -->  | datastore node  |  yes | add datastore node |
    push-change-update   | access allowed? | ---> | to update record   |
                         +-----------------+      +--------------------+

        Figure 5: Updated [RFC8341] access control for push updates

   A publisher MUST allow for the possibility that a subscription's
   selection filter references non-existent data or data that a receiver
   is not allowed to access.  Such support permits a receiver the
   ability to monitor the entire lifecyle of some datastore tree without
   needing to explicitly enumerate every individual datastore node.  If,
   after access control has been applied, there are no objects remaining
   in an update record, then (in case of a periodic subscription) only a
   single empty "push-update" notification MUST be sent.  Empty "push-
   change-update" messages (in case of an on-change subscription) MUST
   NOT be sent.  This is required to ensure that clients cannot



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   surreptitiously monitor objects that they do not have access to via
   carefully crafted selection filters.  By the same token, changes to
   objects that are filtered MUST NOT affect any dampening intervals.

   A publisher MAY choose to reject an establish-subscription request
   which selects non-existent data or data that a receiver is not
   allowed to access.  As reason, the error identity "unchanging-
   selection" SHOULD be returned.  In addition, a publisher MAY choose
   to terminate a dynamic subscription or suspend a configured receiver
   when the authorization privileges of a receiver change, or the access
   controls for subscribed objects change.  In that case, the publisher
   SHOULD include the error identity "unchanging-selection" as reason
   when sending the "subscription-terminated" respectively
   "subscription-suspended" notification.  Such a capability enables the
   publisher to avoid having to support continuous and total filtering
   of a subscription's content for every update record.  It also reduces
   the possibility of leakage of access-controlled objects.

   If read access into previously accessible nodes has been lost due to
   a receiver permissions change, this SHOULD be reported as a patch
   "delete" operation for on-change subscriptions.  If not capable of
   handling such receiver permission changes with such a "delete",
   publisher implementations MUST force dynamic subscription re-
   establishment or configured subscription re-initialization so that
   appropriate filtering is installed.

3.10.  On-Change Notifiable Datastore Nodes

   In some cases, a publisher supporting on-change notifications may not
   be able to push on-change updates for some object types.  Reasons for
   this might be that the value of the datastore node changes frequently
   (e.g., [RFC8343]'s in-octets counter), that small object changes are
   frequent and meaningless (e.g., a temperature gauge changing 0.1
   degrees), or that the implementation is not capable of on-change
   notification for a particular object.

   In those cases, it will be important for client applications to have
   a way to identify for which objects on-change notifications are
   supported and for which ones they are not supported.  Otherwise
   client applications will have no way of knowing whether they can
   indeed rely on their on-change subscription to provide them with the
   change updates that they are interested in.  In other words, if
   implementations do not provide a solution and do not support
   comprehensive on-change notifiability, clients of those
   implementations will have no way of knowing what their on-change
   subscription actually covers.





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   Implementations are therefore strongly advised to provide a solution
   to this problem.  For example, a solution might involve making
   discoverable to clients which objects are on-change notifiable,
   specified using another YANG data model.  It is expected that such a
   solution will be standardized at some point in the future.  In the
   meantime and until this occurs, implementations SHOULD provide their
   own solution.

3.11.  Other Considerations

3.11.1.  Robustness and reliability

   Particularly in the case of on-change updates, it is important that
   these updates do not get lost.  In case the loss of an update is
   unavoidable, it is critical that the receiver is notified
   accordingly.

   Update records for a single subscription MUST NOT be resequenced
   prior to transport.

   It is conceivable that under certain circumstances, a publisher will
   recognize that it is unable to include within an update record the
   full set of objects desired per the terms of a subscription.  In this
   case, the publisher MUST act as follows.

   o  The publisher MUST set the "incomplete-update" flag on any update
      record which is known to be missing information.

   o  The publisher MAY choose to suspend the subscription as per
      [I-D.draft-ietf-netconf-subscribed-notifications].  If the
      publisher does not create an update record at all, it MUST suspend
      the subscription.

   o  When resuming an on-change subscription, the publisher SHOULD
      generate a complete patch from the previous update record.  If
      this is not possible and the "sync-on-start" option is true for
      the subscription, then the full datastore contents MAY be sent via
      a "push-update" instead (effectively replacing the previous
      contents).  If neither of these are possible, then an "incomplete-
      update" flag MUST be included on the next "push-change-update".

   Note: It is perfectly acceptable to have a series of "push-change-
   update" notifications (and even "push update" notifications) serially
   queued at the transport layer awaiting transmission.  It is not
   required for the publisher to merge pending update records sent at
   the same time.





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   On the receiver side, what action to take when a record with an
   incomplete-update flag is received depends on the application.  It
   could simply choose to wait and do nothing.  It could choose to
   resynch, actively retrieving all subscribed information.  It could
   also choose to tear down the subscription and start a new one,
   perhaps with a lesser scope that contains less objects.

3.11.2.  Publisher capacity

   It is far preferable to decline a subscription request than to accept
   such a request when it cannot be met.

   Whether or not a subscription can be supported will be determined by
   a combination of several factors such as the subscription update
   trigger (on-change or periodic), the period in which to report
   changes (one second periods will consume more resources than one hour
   periods), the amount of data in the datastore subtree that is being
   subscribed to, and the number and combination of other subscriptions
   that are concurrently being serviced.

4.  A YANG Data Model for Management of Datastore Push Subscriptions

4.1.  Overview

   The YANG data model for datastore push subscriptions is depicted in
   the following figures.  The tree diagram that is used follows the
   notation defined in [RFC8340].  New schema objects defined here
   (i.e., beyond those from
   [I-D.draft-ietf-netconf-subscribed-notifications]) are identified
   with "yp".  For the reader's convenience, in order to compact the
   tree representation, some nodes that are defined in ietf-subscribed-
   notifications and that are not essential to the understanding of the
   data model defined here have been removed.  This is indicated by
   "..." in the diagram where applicable.

   Because the tree diagram is quite large, its depiction is broken up
   into several figures.  The first figure depicts the augmentations
   that are introduced in module ietf-yang-push to subscription
   configuration specified in module ietf-subscribed-notifications.












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   module: ietf-subscribed-notifications
       ...
       +--rw filters
       |  ...
       |  +--rw yp:selection-filter* [filter-id]
       |     +--rw yp:filter-id                   string
       |     +--rw (yp:filter-spec)?
       |        +--:(yp:datastore-subtree-filter)
       |        |  +--rw yp:datastore-subtree-filter?   <anydata>
       |        |          {sn:subtree}?
       |        +--:(yp:datastore-xpath-filter)
       |           +--rw yp:datastore-xpath-filter?     yang:xpath1.0
       |                   {sn:xpath}?
       +--rw subscriptions
          +--rw subscription* [id]
             |  ...
             +--rw (target)
             |  +--:(stream)
             |  |   ...
             |  +--:(yp:datastore)
             |     +--rw yp:datastore                     identityref
             |     +--rw (yp:selection-filter)?
             |        +--:(yp:by-reference)
             |        |  +--rw yp:selection-filter-ref
             |        |          selection-filter-ref
             |        +--:(yp:within-subscription)
             |           +--rw (yp:filter-spec)?
             |              +--:(yp:datastore-subtree-filter)
             |              |  +--rw yp:datastore-subtree-filter?
             |              |          <anydata> {sn:subtree}?
             |              +--:(yp:datastore-xpath-filter)
             |                 +--rw yp:datastore-xpath-filter?
             |                         yang:xpath1.0 {sn:xpath}?
             | ...
             +--rw (yp:update-trigger)
                +--:(yp:periodic)
                |  +--rw yp:periodic!
                |     +--rw yp:period         centiseconds
                |     +--rw yp:anchor-time?   yang:date-and-time
                +--:(yp:on-change) {on-change}?
                   +--rw yp:on-change!
                      +--rw yp:dampening-period?   centiseconds
                      +--rw yp:sync-on-start?      boolean
                      +--rw yp:excluded-change*    change-type


           Figure 6: Model structure: subscription configuration




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   The next figure depicts the augmentations of module ietf-yang-push
   made to RPCs specified in module ietf-subscribed-notifications.
   Specifically, these augmentations concern the establish-subscription
   and modify-subscription RPCs, which are augmented with parameters
   that are needed to specify datastore push subscriptions.


     rpcs:
       +---x establish-subscription
       |  +---w input
       |  |  ...
       |  |  +---w (target)
       |  |  |  +--:(stream)
       |  |  |  |  ...
       |  |  |  +--:(yp:datastore)
       |  |  |     +---w yp:datastore                   identityref
       |  |  |     +---w (yp:selection-filter)?
       |  |  |        +--:(yp:by-reference)
       |  |  |        |  +---w yp:selection-filter-ref
       |  |  |        |          selection-filter-ref
       |  |  |        +--:(yp:within-subscription)
       |  |  |           +---w (yp:filter-spec)?
       |  |  |              +--:(yp:datastore-subtree-filter)
       |  |  |              |  +---w yp:datastore-subtree-filter?
       |  |  |              |          <anydata> {sn:subtree}?
       |  |  |              +--:(yp:datastore-xpath-filter)
       |  |  |                 +---w yp:datastore-xpath-filter?
       |  |  |                         yang:xpath1.0 {sn:xpath}?
       |  |  | ...
       |  |  +---w (yp:update-trigger)
       |  |     +--:(yp:periodic)
       |  |     |  +---w yp:periodic!
       |  |     |     +---w yp:period         centiseconds
       |  |     |     +---w yp:anchor-time?   yang:date-and-time
       |  |     +--:(yp:on-change) {on-change}?
       |  |        +---w yp:on-change!
       |  |           +---w yp:dampening-period?   centiseconds
       |  |           +---w yp:sync-on-start?      boolean
       |  |           +---w yp:excluded-change*    change-type
       |  +--ro output
       |     +--ro id                            subscription-id
       |     +--ro replay-start-time-revision?   yang:date-and-time
       |             {replay}?
       +---x modify-subscription
       |  +---w input
       |     ...
       |     +---w (target)
       |     |  ...



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       |     |  +--:(yp:datastore)
       |     |     +---w yp:datastore                   identityref
       |     |     +---w (yp:selection-filter)?
       |     |        +--:(yp:by-reference)
       |     |        |  +---w yp:selection-filter-ref
       |     |        |          selection-filter-ref
       |     |        +--:(yp:within-subscription)
       |     |           +---w (yp:filter-spec)?
       |     |              +--:(yp:datastore-subtree-filter)
       |     |              |  +---w yp:datastore-subtree-filter?
       |     |              |          <anydata> {sn:subtree}?
       |     |              +--:(yp:datastore-xpath-filter)
       |     |                 +---w yp:datastore-xpath-filter?
       |     |                         yang:xpath1.0 {sn:xpath}?
       |     | ...
       |     +---w (yp:update-trigger)
       |        +--:(yp:periodic)
       |        |  +---w yp:periodic!
       |        |     +---w yp:period         centiseconds
       |        |     +---w yp:anchor-time?   yang:date-and-time
       |        +--:(yp:on-change) {on-change}?
       |           +---w yp:on-change!
       |              +---w yp:dampening-period?   centiseconds
       +---x delete-subscription
       |  ...
       +---x kill-subscription
          ...

     YANG-data (for placement into rpc error responses)
       ...

                      Figure 7: Model structure: RPCs

   The next figure depicts augmentations of module ietf-yang-push to the
   notifications that are specified in module ietf-subscribed-
   notifications.  The augmentations allow the inclusion of subscription
   configuration parameters that are specific to datastore push
   subscriptions as part of subscription-started and subscription-
   modified notifications.

     notifications:
       +---n replay-completed {replay}?
       |  ...
       +---n subscription-completed
       |  ...
       +---n subscription-started {configured}?
       |  |  ...
       |  +--ro (target)



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       |  |  ...
       |  |  +--:(yp:datastore)
       |  |     +--ro yp:datastore                   identityref
       |  |     +--ro (yp:selection-filter)?
       |  |        +--:(yp:by-reference)
       |  |        |  +--ro yp:selection-filter-ref
       |  |        |          selection-filter-ref
       |  |        +--:(yp:within-subscription)
       |  |           +--ro (yp:filter-spec)?
       |  |              +--:(yp:datastore-subtree-filter)
       |  |              |  +--ro yp:datastore-subtree-filter?
       |  |              |          <anydata> {sn:subtree}?
       |  |              +--:(yp:datastore-xpath-filter)
       |  |                 +--ro yp:datastore-xpath-filter?
       |  |                         yang:xpath1.0 {sn:xpath}?
       |  ...
       |  +--ro (yp:update-trigger)
       |     +--:(yp:periodic)
       |     |  +--ro yp:periodic!
       |     |     +--ro yp:period         centiseconds
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?   centiseconds
       |           +--ro yp:sync-on-start?      boolean
       |           +--ro yp:excluded-change*    change-type
       +---n subscription-resumed
       |  ...
       +---n subscription-modified
       |  ...
       |  +--ro (target)
       |  |  |  ...
       |  |  +--:(yp:datastore)
       |  |     +--ro yp:datastore                   identityref
       |  |     +--ro (yp:selection-filter)?
       |  |        +--:(yp:by-reference)
       |  |        |  +--ro yp:selection-filter-ref
       |  |        |          selection-filter-ref
       |  |        +--:(yp:within-subscription)
       |  |           +--ro (yp:filter-spec)?
       |  |              +--:(yp:datastore-subtree-filter)
       |  |              |  +--ro yp:datastore-subtree-filter?
       |  |              |          <anydata> {sn:subtree}?
       |  |              +--:(yp:datastore-xpath-filter)
       |  |                 +--ro yp:datastore-xpath-filter?
       |  |                         yang:xpath1.0 {sn:xpath}?
       |  ...
       |  +--ro (yp:update-trigger)?



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       |     +--:(yp:periodic)
       |     |  +--ro yp:periodic!
       |     |     +--ro yp:period         centiseconds
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?    centiseconds
       |           +--ro yp:sync-on-start?       boolean
       |           +--ro yp:excluded-change*     change-type
       +---n subscription-terminated
       |  ...
       +---n subscription-suspended
          ...


                 Figure 8: Model structure: Notifications

   The final figure in this section depicts the parts of module ietf-
   yang-push that are not simply augmentations to another module, but
   that are newly introduced.


   module: ietf-yang-push

     rpcs:
       +---x resync-subscription {on-change}?
          +---w input
             +---w id    sn:subscription-id

     YANG-data: (for placement into rpc error responses)
       +-- resync-subscription-error
       |  +--ro reason?                   identityref
       |  +--ro period-hint?              centiseconds
       |  +--ro filter-failure-hint?      string
       |  +--ro object-count-estimate?    uint32
       |  +--ro object-count-limit?       uint32
       |  +--ro kilobytes-estimate?       uint32
       |  +--ro kilobytes-limit?          uint32
       +-- establish-subscription-error-datastore
       |  +--ro reason?                   identityref
       |  +--ro period-hint?              centiseconds
       |  +--ro filter-failure-hint?      string
       |  +--ro object-count-estimate?    uint32
       |  +--ro object-count-limit?       uint32
       |  +--ro kilobytes-estimate?       uint32
       |  +--ro kilobytes-limit?          uint32
       +-- modify-subscription-error-datastore
          +--ro reason?                   identityref



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          +--ro period-hint?              centiseconds
          +--ro filter-failure-hint?      string
          +--ro object-count-estimate?    uint32
          +--ro object-count-limit?       uint32
          +--ro kilobytes-estimate?       uint32
          +--ro kilobytes-limit?          uint32

        notifications:
          +---n push-update
          |  +--ro id?                   sn:subscription-id
          |  +--ro datastore-contents?   <anydata>
          |  +--ro incomplete-update?    empty
          +---n push-change-update {on-change}?
             +--ro id?                  sn:subscription-id
             +--ro datastore-changes
             |  +--ro yang-patch
             |     +--ro patch-id    string
             |     +--ro comment?    string
             |     +--ro edit* [edit-id]
             |        +--ro edit-id      string
             |        +--ro operation    enumeration
             |        +--ro target       target-resource-offset
             |        +--ro point?       target-resource-offset
             |        +--ro where?       enumeration
             |        +--ro value?       <anydata>
             +--ro incomplete-update?   empty


           Figure 9: Model structure (non-augmentation portions

   Selected components of the model are summarized below.

4.2.  Subscription Configuration

   Both configured and dynamic subscriptions are represented within the
   list "subscription".  New parameters extending the basic subscription
   data model in [I-D.draft-ietf-netconf-subscribed-notifications]
   include:

   o  The targeted datastore from which the selection is being made.
      The potential datastores include those from [RFC8341].  A platform
      may also choose to support a custom datastore.

   o  A selection filter identifying YANG nodes of interest within a
      datastore.  Filter contents are specified via a reference to an
      existing filter, or via an in-line definition for only that
      subscription.  Referenced filters allows an implementation to
      avoid evaluating filter acceptability during a dynamic



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      subscription request.  The case statement differentiates the
      options.

   o  For periodic subscriptions, triggered updates will occur at the
      boundaries of a specified time interval.  These boundaries can be
      calculated from the periodic parameters:

      *  a "period" which defines the duration between push updates.

      *  an "anchor-time"; update intervals fall on the points in time
         that are a multiple of a "period" from an "anchor-time".  If
         "anchor-time" is not provided, then the "anchor-time" MUST be
         set with the creation time of the initial update record.

   o  For on-change subscriptions, assuming any dampening period has
      completed, triggering occurs whenever a change in the subscribed
      information is detected.  On-change subscriptions have more
      complex semantics that are guided by their own set of parameters:

      *  a "dampening-period" specifies the interval that must pass
         before a successive update for the subscription is sent.  If no
         dampening period is in effect, the update is sent immediately.
         If a subsequent change is detected, another update is only sent
         once the dampening period has passed for this subscription.

      *  an "excluded-change" parameter which allows restriction of the
         types of changes for which updates should be sent (e.g., only
         add to an update record on object creation).

      *  a "sync-on-start" specifies whether a complete update with all
         the subscribed data is to be sent at the beginning of a
         subscription.

4.3.  YANG Notifications

4.3.1.  State Change Notifications

   Subscription state notifications and mechanism are reused from
   [I-D.draft-ietf-netconf-subscribed-notifications].  Notifications
   "subscription-started" and "subscription-modified" have been
   augmented to include the datastore specific objects.

4.3.2.  Notifications for Subscribed Content

   Along with the subscribed content, there are other objects which
   might be part of a "push-update" or "push-change-update"
   notification.




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   An "id" (that identifies the subscription) MUST be transported along
   with the subscribed contents.  This allows a receiver to
   differentiate which subscription resulted in a particular update
   record.

   A "time-of-update" which represents the time an update record
   snapshot was generated.  A receiver MAY assume that at this point in
   time a publisher's objects had the values that were pushed.

   An "incomplete-update" leaf.  This leaf indicates that not all
   changes which have occurred since the last update are actually
   included with this update.  In other words, the publisher has failed
   to fulfill its full subscription obligations.  (For example a
   datastore was unable to provide the full set of datastore nodes to a
   publisher process.)  To facilitate re-synchronization of on-change
   subscriptions, a publisher MAY subsequently send a "push-update"
   containing a full selection snapshot of subscribed data.

4.4.  YANG RPCs

   YANG-Push subscriptions are established, modified, and deleted using
   RPCs augmented from
   [I-D.draft-ietf-netconf-subscribed-notifications].

4.4.1.  Establish-subscription RPC

   The subscriber sends an establish-subscription RPC with the
   parameters in section 3.1.  An example might look like:























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 <netconf:rpc message-id="101"
     xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
   <establish-subscription
       xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
       xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <yp:datastore
          xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
       ds:operational
     </yp:datastore>
     <yp:datastore-xpath-filter
         xmlns:ex="http://example.com/sample-data/1.0">
       /ex:foo
     </yp:datastore-xpath-filter>
     <yp:periodic>
       <yp:period>500</yp:period>
     </yp:periodic>
   </establish-subscription>
 </netconf:rpc>


                   Figure 10: Establish-subscription RPC

   A positive response includes the "id" of the accepted subscription.
   In that case a publisher may respond:

 <rpc-reply message-id="101"
    xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <id
      xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
       52
    </id>
 </rpc-reply>

          Figure 11: Establish-subscription positive RPC response

   A subscription can be rejected for multiple reasons, including the
   lack of authorization to establish a subscription, no capacity to
   serve the subscription at the publisher, or the inability of the
   publisher to select datastore content at the requested cadence.

   If a request is rejected because the publisher is not able to serve
   it, the publisher SHOULD include in the returned error hints which
   help a subscriber understand subscription parameters might have been
   accepted for the request.  These hints would be included within the
   YANG-data structure "establish-subscription-error-datastore".
   However even with these hints, there are no guarantee that subsequent
   requests will in fact be accepted.




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   The specific parameters to be returned as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  For NETCONF, those parameters are defined in
   [I-D.draft-ietf-netconf-netconf-event-notifications].  For example,
   for the following NETCONF request:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <establish-subscription
           xmlns=
             "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
           xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
         <yp:datastore
             xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
           ds:operational
         </yp:datastore>
         <yp:datastore-xpath-filter
             xmlns:ex="http://example.com/sample-data/1.0">
           /ex:foo
         </yp:datastore-xpath-filter>
         <yp:on-change>
           <yp:dampening-period>100</yp:dampening-period>
         </yp:on-change>
       </establish-subscription>
     </rpc>

            Figure 12: Establish-subscription request example 2

   a publisher that cannot serve on-change updates but that can serve
   periodic updates might return the following NETCONF response:

 <rpc-reply message-id="101"
   xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
   xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
   <rpc-error>
     <error-type>application</error-type>
     <error-tag>operation-failed</error-tag>
     <error-severity>error</error-severity>
     <error-path>/yp:periodic/yp:period</error-path>
     <error-info>
       <yp:establish-subscription-error-datastore>
         <yp:reason>yp:on-change-unsupported</yp:reason>
       </yp:establish-subscription-error-datastore>
     </error-info>
   </rpc-error>
 </rpc-reply>

        Figure 13: Establish-subscription error response example 2



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4.4.2.  Modify-subscription RPC

   The subscriber MAY invoke the "modify-subscription" RPC for a
   subscription it previously established.  The subscriber will include
   newly desired values in the "modify-subscription" RPC.  Parameters
   not included MUST remain unmodified.  Below is an example where a
   subscriber attempts to modify the period and datastore XPath filter
   of a subscription using NETCONF.

     <rpc message-id="102"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <modify-subscription
           xmlns=
             "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
           xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
         <id>1011</id>
         <yp:datastore
             xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
           ds:operational
         </yp:datastore>
         <yp:datastore-xpath-filter
             xmlns:ex="http://example.com/sample-data/1.0">
           /ex:bar
         </yp:datastore-xpath-filter>
         <yp:periodic>
           <yp:period>250</yp:period>
         </yp:periodic>
        </modify-subscription>
     </rpc>

                  Figure 14: Modify subscription request

   The publisher MUST respond to the subscription modification request.
   If the request is rejected, the existing subscription is left
   unchanged, and the publisher MUST send an RPC error response.  This
   response might have hints encapsulated within the YANG-data structure
   "modify-subscription-error-datastore".  A subscription MAY be
   modified multiple times.

   The specific parameters to be returned as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  For NETCONF, those parameters are specified in
   [I-D.draft-ietf-netconf-netconf-event-notifications].

   A configured subscription cannot be modified using "modify-
   subscription" RPC.  Instead, the configuration needs to be edited as
   needed.




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4.4.3.  Delete-subscription RPC

   To stop receiving updates from a subscription and effectively delete
   a subscription that had previously been established using an
   "establish-subscription" RPC, a subscriber can send a "delete-
   subscription" RPC, which takes as only input the subscription's "id".
   This RPC is unmodified from
   [I-D.draft-ietf-netconf-subscribed-notifications].

4.4.4.  Resync-subscription RPC

   This RPC is supported only for on-change subscriptions previously
   established using an "establish-subscription" RPC.  For example:

      <rpc message-id="103"
           xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
        <resync-subscription
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push"
          <id>1011</id>
        </resync-subscription>
       </netconf:rpc>

                      Figure 15: Resync subscription

   On receipt, a publisher must either accept the request and quickly
   follow with a "push-update", or send an appropriate error within an
   rpc error response.  Within an error response, the publisher MAY
   include supplemental information about the reasons within the YANG-
   data structure "resync-subscription-error".

4.4.5.  YANG Module Synchronization

   To make subscription requests, the subscriber needs to know the YANG
   datastore schemas used by the publisher, which are available via the
   YANG Library module, ietf-yang-library.yang from [RFC8525].  The
   receiver is expected to know the YANG library information before
   starting a subscription.

   The set of modules, revisions, features, and deviations can change at
   run-time (if supported by the publisher implementation).  For this
   purpose, the YANG library provides a simple "yang-library-change"
   notification that informs the subscriber that the library has
   changed.  In this case, a subscription may need to be updated to take
   the updates into account.  The receiver may also need to be informed
   of module changes in order to process updates regarding datastore
   nodes from changed modules correctly.





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5.  YANG Module

   This YANG module imports typedefs from [RFC6991], identities from
   [RFC8342], the YANG-data extension from [RFC8040], and the yang-patch
   grouping from [RFC8072].  In addition, it imports and augments many
   definitions from [I-D.draft-ietf-netconf-subscribed-notifications].

  <CODE BEGINS> file "ietf-yang-push@2019-05-15.yang"
  module ietf-yang-push {
    yang-version 1.1;
    namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push";
    prefix yp;

    import ietf-yang-types {
      prefix yang;
      reference
        "RFC 6991: Common YANG Data Types";
    }
    import ietf-subscribed-notifications {
      prefix sn;
      reference
        "draft-ietf-netconf-subscribed-notifications:
         Customized Subscriptions to a Publisher's Event Streams
         NOTE TO RFC Editor: Please replace above reference to
         draft-ietf-netconf-subscribed-notifications with RFC number
         when published (i.e. RFC xxxx).";
    }
    import ietf-datastores {
      prefix ds;
      reference
        "RFC 8342: Network Management Datastore Architecture (NMDA)";
    }
    import ietf-restconf {
      prefix rc;
      reference
        "RFC 8040: RESTCONF Protocol";
    }
    import ietf-yang-patch {
      prefix ypatch;
      reference
        "RFC 8072: YANG Patch Media Type";
    }

    organization
      "IETF NETCONF Working Group";
    contact
      "WG Web:   <http://tools.ietf.org/wg/netconf/>




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       WG List:  <mailto:netconf@ietf.org>

       Editor:   Alexander Clemm
                 <mailto:ludwig@clemm.org>
       Editor:   Eric Voit
                 <mailto:evoit@cisco.com>
       Editor:   Alberto Gonzalez Prieto
                 <mailto:agonzalezpri@vmware.com>
       Editor:   Ambika Prasad Tripathy
                 <mailto:ambtripa@cisco.com>
       Editor:   Einar Nilsen-Nygaard
                 <mailto:einarnn@cisco.com>
       Editor:   Andy Bierman
                 <mailto:andy@yumaworks.com>
       Editor:   Balazs Lengyel
                 <mailto:balazs.lengyel@ericsson.com>";
    description
      "This module contains YANG specifications for YANG push.

       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) 2019 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;
       see the RFC itself for full legal notices.";

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

    revision 2019-05-15 {
      description
        "Initial revision.
         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



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         (ietf-yang-push@2019-05-15.yang) to the date of RFC
         publication.
         (3) Please replace the following reference to
         draft-ietf-netconf-yang-push-24 with RFC number when
         published (i.e. RFC xxxx).";
      reference
        "draft-ietf-netconf-yang-push-24";
    }

    /*
     * FEATURES
     */

    feature on-change {
      description
        "This feature indicates that on-change triggered subscriptions
         are supported.";
    }

    /*
     * IDENTITIES
     */

    /* Error type identities for datastore subscription  */

    identity resync-subscription-error {
      description
        "Problem found while attempting to fulfill an
         'resync-subscription' RPC request.";
    }

    identity cant-exclude {
      base sn:establish-subscription-error;
      description
        "Unable to remove the set of 'excluded-changes'.  This means
         the publisher is unable to restrict 'push-change-update's to
         just the change types requested for this subscription.";
    }

    identity datastore-not-subscribable {
      base sn:establish-subscription-error;
      base sn:subscription-terminated-reason;
      description
        "This is not a subscribable datastore.";
    }

    identity no-such-subscription-resync {
      base resync-subscription-error;



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      description
        "Referenced subscription doesn't exist. This may be as a result
         of a non-existent subscription ID, an ID which belongs to
         another subscriber, or an ID for configured subscription.";
    }

    identity on-change-unsupported {
      base sn:establish-subscription-error;
      description
        "On-change is not supported for any objects which are
         selectable by this filter.";
    }

    identity on-change-sync-unsupported {
      base sn:establish-subscription-error;
      description
        "Neither sync on start nor resynchronization are supported for
         this subscription.  This error will be used for two
         reasons.  First if an 'establish-subscription' RPC includes
         'sync-on-start', yet the publisher can't support sending a
         'push-update' for this subscription for reasons other than
         'on-change-unsupported' or 'sync-too-big'.  And second, if the
         'resync-subscription' RPC is invoked either for an existing
         periodic subscription, or for an on-change subscription which
         can't support resynchronization.";
    }

    identity period-unsupported {
      base sn:establish-subscription-error;
      base sn:modify-subscription-error;
      base sn:subscription-suspended-reason;
      description
        "Requested time period or dampening-period is too short.  This
         can be for both periodic and on-change subscriptions (with or
         without dampening.) Hints suggesting alternative periods may
         be returned as supplemental information.";
    }

    identity update-too-big {
      base sn:establish-subscription-error;
      base sn:modify-subscription-error;
      base sn:subscription-suspended-reason;
      description
        "Periodic or on-change push update datatrees exceed a maximum
         size limit.  Hints on estimated size of what was too big may
         be returned as supplemental information.";
    }




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    identity sync-too-big {
      base sn:establish-subscription-error;
      base sn:modify-subscription-error;
      base resync-subscription-error;
      base sn:subscription-suspended-reason;
      description
        "Sync-on-start or resynchronization datatree exceeds a maximum
         size limit.  Hints on estimated size of what was too big may
         be returned as supplemental information.";
    }

    identity unchanging-selection {
      base sn:establish-subscription-error;
      base sn:modify-subscription-error;
      base sn:subscription-terminated-reason;
      description
        "Selection filter is unlikely to ever select datatree nodes.
         This means that based on the subscriber's current access
         rights, the publisher recognizes that the selection filter is
         unlikely to ever select datatree nodes which change.  Examples
         for this might be that node or subtree doesn't exist, read
         access is not permitted for a receiver, or static objects that
         only change at reboot have been chosen.";
    }

    /*
     * TYPE DEFINITIONS
     */

    typedef change-type {
      type enumeration {
        enum create {
          description
            "A change that refers to the creation of a new datastore
             node.";
        }
        enum delete {
          description
            "A change that refers to the deletion of a datastore
             node.";
        }
        enum insert {
          description
            "A change that refers to the insertion of a new
             user-ordered datastore node.";
        }
        enum move {
          description



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            "A change that refers to a reordering of the target
             datastore node.";
        }
        enum replace {
          description
            "A change that refers to a replacement of the target
             datastore node's value.";
        }
      }
      description
        "Specifies different types of datastore changes.


         This type is based on the edit operations defined for YANG
         Patch, with the difference that it is valid for a receiver to
         process an update record which performs a create operation on
         a datastore node the receiver believes exists, or to process a
         delete on a datastore node the receiver believes is missing.";
      reference
        "RFC 8072: YANG Patch Media Type, section 2.5";
    }

    typedef selection-filter-ref {
      type leafref {
        path "/sn:filters/yp:selection-filter/yp:filter-id";
      }
      description
        "This type is used to reference a selection filter.";
    }

    typedef centiseconds {
      type uint32;
      description
        "A period of time, measured in units of 0.01 seconds.";
      }

    /*
     * GROUP DEFINITIONS
     */

    grouping datastore-criteria {
      description
        "A grouping to define criteria for which selected objects
         from  a targeted datastore should be included in push
         updates.";
      leaf datastore {
        type identityref {
          base ds:datastore;



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        }
        mandatory true;
        description
          "Datastore from which to retrieve data.";
      }
      uses selection-filter-objects;
    }

    grouping selection-filter-types {
      description
        "This grouping defines the types of selectors for objects
         from a datastore.";
      choice filter-spec {
        description
          "The content filter specification for this request.";
        anydata datastore-subtree-filter {
          if-feature "sn:subtree";
          description
            "This parameter identifies the portions of the
             target datastore to retrieve.";
          reference
            "RFC 6241: Network Configuration Protocol, Section 6.";
        }
        leaf datastore-xpath-filter {
          if-feature "sn:xpath";
          type yang:xpath1.0;
          description
            "This parameter contains an XPath expression identifying
            the portions of the target datastore to retrieve.

            If the expression returns a node-set, all nodes in the
            node-set are selected by the filter.  Otherwise, if the
            expression does not return a node-set, the filter
            doesn't select any nodes.

            The expression is evaluated in the following XPath
            context:

              o   The set of namespace declarations is the set of prefix
                  and namespace pairs for all YANG modules implemented
                  by the server, where the prefix is the YANG module
                  name and the namespace is as defined by the
                  'namespace' statement in the YANG module.

                  If the leaf is encoded in XML, all namespace
                  declarations in scope on the 'stream-xpath-filter'
                  leaf element are added to the set of namespace
                  declarations.  If a prefix found in the XML is



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                  already present in the set of namespace declarations,
                  the namespace in the XML is used.

              o  The set of variable bindings is empty.

              o  The function library is the core function library, and
                 the XPath functions defined in section 10 in RFC 7950.

              o  The context node is the root node of the target
                 datastore.";
        }
      }
    }

    grouping selection-filter-objects {
      description
        "This grouping defines a selector for objects from a
         datastore.";
      choice selection-filter {
        description
          "The source of the selection filter applied to the
           subscription.  This will come either referenced from a global
           list, or be provided within the subscription itself.";
        case by-reference {
          description
            "Incorporate a filter that has been configured
             separately.";
          leaf selection-filter-ref {
            type selection-filter-ref;
            mandatory true;
            description
              "References an existing selection filter which is to be
               applied to the subscription.";
          }
        }
        case within-subscription {
          description
            "Local definition allows a filter to have the same
             lifecycle as the subscription.";
          uses selection-filter-types;
        }
      }
    }

    grouping update-policy-modifiable {
      description
        "This grouping describes the datastore specific subscription
         conditions that can be changed during the lifetime of the



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         subscription.";
      choice update-trigger {
        description
          "Defines necessary conditions for sending an event record to
           the subscriber.";
        case periodic {
          container periodic {
            presence "indicates a periodic subscription";
            description
              "The publisher is requested to notify periodically the
               current values of the datastore as defined by the
               selection filter.";
            leaf period {
              type centiseconds;
              mandatory true;
              description
                "Duration of time which should occur between periodic
                 push updates, in one hundredths of a second.";
            }
            leaf anchor-time {
              type yang:date-and-time;
              description
                "Designates a timestamp before or after which a series
                 of periodic push updates are determined. The next
                 update will take place at a whole multiple interval
                 from the anchor time.  For example, for an anchor time
                 is set for the top of a particular minute and a period
                 interval of a minute, updates will be sent at the top
                 of every minute this subscription is active.";
            }
          }
        }
        case on-change {
          if-feature "on-change";
          container on-change {
            presence "indicates an on-change subscription";
            description
              "The publisher is requested to notify changes in values
               in the datastore subset as defined by a selection
               filter.";
            leaf dampening-period {
              type centiseconds;
              default "0";
              description
                "Specifies the minimum interval between the assembly of
                 successive update records for a single receiver of a
                 subscription.  Whenever subscribed objects change, and
                 a dampening period interval (which may be zero) has



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                 elapsed since the previous update record creation for
                 a receiver, then any subscribed objects and properties
                 which have changed since the previous update record
                 will have their current values marshalled and placed
                 into a new update record.";
            }
          }
        }
      }
    }

    grouping update-policy {
      description
        "This grouping describes the datastore-specific subscription
         conditions of a subscription.";
      uses update-policy-modifiable {
        augment "update-trigger/on-change/on-change" {
          description
            "Includes objects not modifiable once subscription is
             established.";
          leaf sync-on-start {
            type boolean;
            default "true";
            description
              "When this object is set to false, it restricts an
               on-change subscription from sending push-update
               notifications.  When false, pushing a full selection per
               the terms of the selection filter MUST NOT be done for
               this subscription.  Only updates about changes,
               i.e. only push-change-update notifications are sent.
               When true (default behavior), in order to facilitate a
               receiver's synchronization, a full update is sent when
               the subscription starts using a push-update
               notification.  After that, push-change-update
               notifications are exclusively sent unless the publisher
               chooses to resync the subscription via a new push-update
               notification.";
          }
          leaf-list excluded-change {
            type change-type;
            description
              "Use to restrict which changes trigger an update.  For
               example, if modify is excluded, only creation and
               deletion of objects is reported.";
          }
        }
      }
    }



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    grouping hints {
      description
        "Parameters associated with some error for a subscription
         made upon a datastore.";
      leaf period-hint {
        type centiseconds;
        description
          "Returned when the requested time period is too short. This
           hint can assert a viable period for either a periodic push
           cadence or an on-change dampening interval.";
      }
      leaf filter-failure-hint {
        type string;
        description
          "Information describing where and/or why a provided filter
           was unsupportable for a subscription.";
      }
      leaf object-count-estimate {
        type uint32;
        description
          "If there are too many objects which could potentially be
           returned by the selection filter, this identifies the
           estimate of the number of objects which the filter would
           potentially pass.";
      }
      leaf object-count-limit {
        type uint32;
        description
          "If there are too many objects which could be returned by
           the selection filter, this identifies the upper limit of
           the publisher's ability to service for this subscription.";
      }
      leaf kilobytes-estimate {
        type uint32;
        description
          "If the returned information could be beyond the capacity
           of the publisher, this would identify the data size which
           could result from this selection filter.";
      }
      leaf kilobytes-limit {
        type uint32;
        description
          "If the returned information would be beyond the capacity
           of the publisher, this identifies the upper limit of the
           publisher's ability to service for this subscription.";
      }
    }




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    /*
     * RPCs
     */

    rpc resync-subscription {
      if-feature "on-change";
      description
        "This RPC allows a subscriber of an active on-change
         subscription to request a full push of objects.

         A successful invocation results in a push-update of all
         datastore nodes that the subscriber is permitted to access.
         This RPC can only be invoked on the same session on which the
         subscription is currently active.  In case of an error, a
         resync-subscription-error is sent as part of an error
         response.";
      input {
        leaf id {
          type sn:subscription-id;
          mandatory true;
          description
            "Identifier of the subscription that is to be resynced.";
        }
      }
    }

    rc:yang-data resync-subscription-error {
      container resync-subscription-error {
        description
          "If a 'resync-subscription' RPC fails, the subscription is
           not resynced and the RPC error response MUST indicate the
           reason for this failure.  This YANG-data MAY be inserted as
           structured data within a subscription's RPC error response
           to indicate the failure reason.";
        leaf reason {
          type identityref {
            base resync-subscription-error;
          }
          mandatory true;
          description
            "Indicates the reason why the publisher has declined a
             request for subscription resynchronization.";
        }
        uses hints;
      }
    }

    augment "/sn:establish-subscription/sn:input" {



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      description
        "This augmentation adds additional subscription parameters
         that apply specifically to datastore updates to RPC input.";
      uses update-policy;
    }

    augment "/sn:establish-subscription/sn:input/sn:target" {
      description
        "This augmentation adds the datastore as a valid target
         for the subscription to RPC input.";
      case datastore {
        description
          "Information specifying the parameters of an request for a
           datastore subscription.";
        uses datastore-criteria;
      }
    }

    rc:yang-data establish-subscription-datastore-error-info {
      container establish-subscription-datastore-error-info {
        description
          "If any 'establish-subscription' RPC parameters are
           unsupportable against the datastore, a subscription is not
           created and the RPC error response MUST indicate the reason
           why the subscription failed to be created.  This YANG-data
           MAY be inserted as structured data within a subscription's
           RPC error response to indicate the failure reason.  This
           YANG-data MUST be inserted if hints are to be provided back
           to the subscriber.";
        leaf reason {
          type identityref {
            base sn:establish-subscription-error;
          }
          description
            "Indicates the reason why the subscription has failed to
             be created to a targeted datastore.";
        }
        uses hints;
      }
    }

    augment "/sn:modify-subscription/sn:input" {
      description
        "This augmentation adds additional subscription parameters
         specific to datastore updates.";
      uses update-policy-modifiable;
    }




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    augment "/sn:modify-subscription/sn:input/sn:target" {
      description
        "This augmentation adds the datastore as a valid target
         for the subscription to RPC input.";
      case datastore {
        description
          "Information specifying the parameters of an request for a
           datastore subscription.";
        uses datastore-criteria;
      }
    }

    rc:yang-data modify-subscription-datastore-error-info {
      container modify-subscription-datastore-error-info {
        description
          "This YANG-data MAY be provided as part of a subscription's
           RPC error response when there is a failure of a
           'modify-subscription' RPC which has been made against a
           datastore.  This YANG-data MUST be used if hints are to be
           provides back to the subscriber.";
        leaf reason {
          type identityref {
            base sn:modify-subscription-error;
          }
          description
            "Indicates the reason why the subscription has failed to
             be modified.";
        }
        uses hints;
      }
    }

    /*
     * NOTIFICATIONS
     */

    notification push-update {
      description
        "This notification contains a push update, containing data
         subscribed to via a subscription.  This notification is sent
         for periodic updates, for a periodic subscription.  It can
         also be used for synchronization updates of an on-change
         subscription.  This notification shall only be sent to
         receivers of a subscription.  It does not constitute a
         general-purpose notification that would be subscribable as
         part of the NETCONF event stream by any receiver.";
      leaf id {
        type sn:subscription-id;



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        description
          "This references the subscription which drove the
           notification to be sent.";
      }
      anydata datastore-contents {
        description
          "This contains the updated data.  It constitutes a snapshot
           at the time-of-update of the set of data that has been
           subscribed to.  The snapshot corresponds to the same
           snapshot that would be returned in a corresponding get
           operation with the same selection filter parameters
           applied.";
      }
      leaf incomplete-update {
        type empty;
        description
          "This is a flag which indicates that not all datastore
           nodes subscribed to are included with this update.  In
           other words, the publisher has failed to fulfill its full
           subscription obligations, and despite its best efforts is
           providing an incomplete set of objects.";
      }
    }

    notification push-change-update {
      if-feature "on-change";
      description
        "This notification contains an on-change push update. This
         notification shall only be sent to the receivers of a
         subscription.  It does not constitute ageneral-purpose
         notification that would be subscribable as part of the
         NETCONF event stream by any receiver.";
      leaf id {
        type sn:subscription-id;
        description
          "This references the subscription which drove the
           notification to be sent.";
      }
      container datastore-changes {
        description
          "This contains the set of datastore changes of the target
           datastore starting at the time of the previous update, per
           the terms of the subscription.";
        uses ypatch:yang-patch;
      }
      leaf incomplete-update {
        type empty;
        description



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          "The presence of this object indicates not all changes which
           have occurred since the last update are included with this
           update.  In other words, the publisher has failed to
           fulfill its full subscription obligations, for example in
           cases where it was not able to keep up with a change
           burst.";
      }
    }

    augment "/sn:subscription-started" {
      description
        "This augmentation adds datastore-specific objects to
         the notification that a subscription has started.";
      uses update-policy;
    }

    augment "/sn:subscription-started/sn:target" {
      description
        "This augmentation allows the datastore to be included as
         part of the notification that a subscription has started.";
      case datastore {
        uses datastore-criteria {
          refine "selection-filter/within-subscription" {
            description
              "Specifies the selection filter and where it originated
               from.  If the 'selection-filter-ref' is populated, the
               filter within the subscription came from the 'filters'
               container.  Otherwise it is populated in-line as part of
               the subscription itself.";
          }
        }
      }
    }

    augment "/sn:subscription-modified" {
      description
        "This augmentation adds datastore-specific objects to
         the notification that a subscription has been modified.";
      uses update-policy;
    }

    augment "/sn:subscription-modified/sn:target" {
      description
        "This augmentation allows the datastore to be included as
         part of the notification that a subscription has been
         modified.";
      case datastore {
        uses datastore-criteria {



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          refine "selection-filter/within-subscription" {
            description
              "Specifies the selection filter and where it originated
               from.  If the 'selection-filter-ref' is populated, the
               filter within the subscription came from the 'filters'
               container.  Otherwise it is populated in-line as part of
               the subscription itself.";
          }
        }
      }
    }

    /*
     * DATA NODES
     */

    augment "/sn:filters" {
      description
        "This augmentation allows the datastore to be included as part
         of the selection filtering criteria for a subscription.";
      list selection-filter {
        key "filter-id";
        description
          "A list of pre-configured filters that can be applied
           to datastore subscriptions.";
        leaf filter-id {
          type string;
          description
            "An identifier to differentiate between selection
             filters.";
        }
        uses selection-filter-types;
      }
    }

    augment "/sn:subscriptions/sn:subscription" {
      when 'yp:datastore';
      description
        "This augmentation adds many datastore specific objects to a
         subscription.";
      uses update-policy;
    }

    augment "/sn:subscriptions/sn:subscription/sn:target" {
      description
        "This augmentation allows the datastore to be included as
         part of the selection filtering criteria for a subscription.";
      case datastore {



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        uses datastore-criteria;
      }
    }
  }

  <CODE ENDS>


6.  IANA Considerations

   This document registers the following namespace URI in the "IETF XML
   Registry" [RFC3688]:

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

   This document registers the following YANG module in the "YANG Module
   Names" registry [RFC6020]:

   Name: ietf-yang-push
   Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-push
   Prefix: yp
   Reference: draft-ietf-netconf-yang-push-21.txt (RFC form)

7.  Security Considerations

   The YANG module specified in this document defines 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.

   There are a number of data nodes defined in this YANG module that are
   writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability.  (It should be noted that the
   YANG module augments the YANG module from



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   [I-D.draft-ietf-netconf-subscribed-notifications].  All security
   considerations that are listed there are relevant also for datastore
   subscriptions.  In the following, we focus on the data nodes that are
   newly introduced here.)

   o  Subtree "selection-filter" under container "filters": This subtree
      allows to specify which objects or subtrees to include in a
      datastore subscription.  An attacker could attempt to modify the
      filter.  For example, the filter might be modified to result in
      very few objects being filtered in order to attempt to overwhelm
      the receiver.  Alternatively, the filter might be modified to
      result in certain objects to be excluded from updates, in order to
      have certain changes go unnoticed.

   o  Subtree "datastore" in choice "target" in list "subscription":
      Analogous to "selection filter", an attacker might attempt to
      modify the objects being filtered in order to overwhelm a receiver
      with a larger volume of object updates than expected, or to have
      certain changes go unnoticed.

   o  Choice "update-trigger" in list "subscription": By modifying the
      update trigger, an attacker might alter the updates that are being
      sent in order to confuse a receiver, to withhold certain updates
      to be sent to the receiver, and/or to overwhelm a receiver.  For
      example, an attacker might modify the period with which updates
      are reported for a periodic subscription, or it might modify the
      dampening period for an on-change subscription, resulting in
      greater delay of successive updates (potentially affecting
      responsiveness of applications that depend on the updates) or in a
      high volume of updates (to exhaust receiver resources).

   o  RPC "resync-subscription": This RPC allows a subscriber of an on-
      change subscription to request a full push of objects in the
      subscription's scope.  This can result in a large volume of data.
      An attacker could attempt to use this RPC to exhaust resources on
      the server to generate the data, and attempt to overwhelm a
      receiver with the resulting data volume.

   NACM provides one means to mitigate these threats on the publisher
   side.  In order to address those threats as a subscriber, a
   subscriber could monitor the subscription configuration for any
   unexpected changes.  For this, it can subscribe to updates to the
   YANG datastore nodes that represent his datastore subscriptions.  As
   this data volume is small, a paranoid subscriber could even revert to
   occasional polling to guard against a compromised subscription
   against subscription configuration updates itself.





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8.  Acknowledgments

   For their valuable comments, discussions, and feedback, we wish to
   acknowledge Tim Jenkins, Martin Bjorklund, Kent Watsen, Susan Hares,
   Yang Geng, Peipei Guo, Michael Scharf, Guangying Zheng, Tom Petch,
   Henk Birkholz, Reshad Rahman, Qin Wu, Rohit Ranade, and Rob Wilton.

9.  Contributors

      Alberto Gonzalez Prieto
      Microsoft
      albgonz@microsoft.com

      Ambika Prasad Tripathy
      Cisco Systems
      ambtripa@cisco.com

      Einar Nilsen-Nygaard
      Cisco Systems
      einarnn@cisco.com

      Andy Bierman
      YumaWorks
      andy@yumaworks.com

      Balazs Lengyel
      Ericsson
      balazs.lengyel@ericsson.com

10.  References

10.1.  Normative References

   [I-D.draft-ietf-netconf-subscribed-notifications]
              Voit, E., Clemm, A., Gonzalez Prieto, A., Tripathy, A.,
              and E. Nilsen-Nygaard, "Subscription to YANG Event
              Notifications", draft-ietf-netconf-subscribed-
              notifications-24 (work in progress), April 2019.

   [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>.




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   [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>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

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

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

   [RFC8072]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
              Media Type", RFC 8072, DOI 10.17487/RFC8072, February
              2017, <https://www.rfc-editor.org/info/rfc8072>.

   [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>.

10.2.  Informative References

   [I-D.draft-ietf-netconf-netconf-event-notifications]
              Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Dynamic subscription to YANG Events
              and DAtastores over NETCONF", April 2019.





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   [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>.

   [RFC7923]  Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
              for Subscription to YANG Datastores", RFC 7923,
              DOI 10.17487/RFC7923, June 2016,
              <https://www.rfc-editor.org/info/rfc7923>.

   [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>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [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>.

Appendix A.  Appendix A: Subscription Errors

A.1.  RPC Failures

   Rejection of an RPC for any reason is indicated by via RPC error
   response from the publisher.  Valid RPC errors returned include both
   existing transport layer RPC error codes, such as those seen with
   NETCONF in [RFC6241], as well as subscription specific errors such as
   those defined within the YANG model.  As a result, how subscription
   errors are encoded within an RPC error response is transport
   dependent.

   References to specific identities in the ietf-subscribed-
   notifications YANG model or the ietf-yang-push YANG model may be
   returned as part of the error responses resulting from failed
   attempts at datastore subscription.  For errors defined as part of
   ietf-subscribed-notifications, please refer to
   [I-D.draft-ietf-netconf-subscribed-notifications].  The errors
   introduced in this document, grouped per RPC, are as follows:






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       establish-subscription         modify-subscription
      ----------------------         -------------------
       cant-exclude                   period-unsupported
       datastore-not-subscribable     update-too-big
       on-change-unsupported          sync-too-big
       on-change-sync-unsupported     unchanging-selection
       period-unsupported
       update-too-big                resync-subscription
       sync-too-big                  --------------------
       unchanging-selection           no-such-subscription-resync
                                      sync-too-big


   There is one final set of transport independent RPC error elements
   included in the YANG model.  These are the following four YANG-data
   structures for failed datastore subscriptions:

   1. YANG-data establish-subscription-error-datastore
      This MUST be returned if information identifying the reason for an
      RPC error has not been placed elsewhere within the transport
      portion of a failed "establish-subscription" RPC response. This
      MUST be sent if hints are included.

   2. YANG-data modify-subscription-error-datastore
      This MUST be returned if information identifying the reason for an
      RPC error has not been placed elsewhere within the transport
      portion of a failed "modifiy-subscription" RPC response. This
      MUST be sent if hints are included.

   3. YANG-data sn:delete-subscription-error
      This MUST be returned if information identifying the reason for an
      RPC error has not been placed elsewhere within the transport
      portion of a failed "delete-subscription" or "kill-subscription"
      RPC response.

   4. YANG-data resync-subscription-error
      This MUST be returned if information identifying the reason for an
      RPC error has not been placed elsewhere within the transport
      portion of a failed "resync-subscription" RPC response.

A.2.  Notifications of Failure

   A subscription may be unexpectedly terminated or suspended
   independent of any RPC or configuration operation.  In such cases,
   indications of such a failure MUST be provided.  To accomplish this,
   a number of errors can be returned as part of the corresponding
   subscription state change notification.  For this purpose, the
   following error identities have been introduced in this document, in



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   addition to those that were already defined in
   [I-D.draft-ietf-netconf-subscribed-notifications]:


   subscription-terminated        subscription-suspended
   -----------------------        ----------------------
    datastore-not-subscribable     period-unsupported
    unchanging-selection           update-too-big
                                   synchronization-size


Appendix B.  Changes Between Revisions

   (To be removed by RFC editor prior to publication)

   v23 - v24

   o  Minor updates to address IESG review comments.  Moving five of the
      coauthors to contributors as requested.

   v22 - v23

   o  Minor updates to address IESG review comments.

   v21 - v22

   o  Minor updates per Martin Bjorklund's YANG doctor review.

   v20 - v21

   o  Minor updates, simplifying RPC input conditions.

   v19 - v20

   o  Minor updates per WGLC comments.

   v18 - v19

   o  Minor updates per WGLC comments.

   v17 - v18

   o  Minor updates per WGLC comments.

   v16 - v17

   o  Minor updates to YANG module, incorporating comments from Tom
      Petch.



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   o  Updated references.

   v15 - v16

   o  Updated security considerations.

   o  Updated references.

   o  Addressed comments from last call review, specifically comments
      received from Martin Bjorklund.

   v14 - v15

   o  Minor text fixes.  Includes a fix to on-change update calculation
      to cover churn when an object changes to and from a value during a
      dampening period.

   v13 - v14

   o  Minor text fixes.

   v12 - v13

   o  Hint negotiation models now show error examples.

   o  yang-data structures for rpc errors.

   v11 - v12

   o  Included Martin's review clarifications.

   o  QoS moved to subscribed-notifications

   o  time-of-update removed as it is redundant with RFC5277's
      eventTime, and other times from notification-messages.

   o  Error model moved to match existing implementations

   o  On-change notifiable removed, how to do this is implementation
      specific.

   o  NMDA model supported.  Non NMDA version at https://github.com/
      netconf-wg/yang-push/

   v10 - v11

   o  Promise model reference added.




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   o  Error added for no-such-datastore

   o  Inherited changes from subscribed notifications (such as optional
      feature definitions).

   o  scrubbed the examples for proper encodings

   v09 - v10

   o  Returned to the explicit filter subtyping of v00-v05

   o  identityref to ds:datastore made explicit

   o  Returned ability to modify a selection filter via RPC.

   v08 - v09

   o  Minor tweaks cleaning up text, removing appendicies, and making
      reference to revised-datastores.

   o  Subscription-id (now:id) optional in push updates, except when
      encoded in RFC5277, Section 4 one-way notification.

   o  Finished adding the text descibing the resync subscription RPC.

   o  Removed relationships to other drafts and future technology
      appendicies as this work is being explored elsewhere.

   o  Deferred the multi-line card issue to new drafts

   o  Simplified the NACM interactions.

   v07 - v08

   o  Updated YANG models with minor tweaks to accommodate changes of
      ietf-subscribed-notifications.

   v06 - v07

   o  Clarifying text tweaks.

   o  Clarification that filters act as selectors for subscribed
      datastore nodes; support for value filters not included but
      possible as a future extension

   o  Filters don't have to be matched to existing YANG objects

   v05 - v06



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   o  Security considerations updated.

   o  Base YANG model in [subscribe] updated as part of move to
      identities, YANG augmentations in this doc matched up

   o  Terms refined and text updates throughout

   o  Appendix talking about relationship to other drafts added.

   o  Datastore replaces stream

   o  Definitions of filters improved

   v04 to v05

   o  Referenced based subscription document changed to Subscribed
      Notifications from 5277bis.

   o  Getting operational data from filters

   o  Extension notifiable-on-change added

   o  New appendix on potential futures.  Moved text into there from
      several drafts.

   o  Subscription configuration section now just includes changed
      parameters from Subscribed Notifications

   o  Subscription monitoring moved into Subscribed Notifications

   o  New error and hint mechanisms included in text and in the YANG
      model.

   o  Updated examples based on the error definitions

   o  Groupings updated for consistency

   o  Text updates throughout

   v03 to v04

   o  Updates-not-sent flag added

   o  Not notifiable extension added

   o  Dampening period is for whole subscription, not single objects

   o  Moved start/stop into rfc5277bis



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   o  Client and Server changed to subscriber, publisher, and receiver

   o  Anchor time for periodic

   o  Message format for synchronization (i.e. sync-on-start)

   o  Material moved into 5277bis

   o  QoS parameters supported, by not allowed to be modified by RPC

   o  Text updates throughout

Authors' Addresses

   Alexander Clemm
   Huawei USA

   Email: ludwig@clemm.org


   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com



























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