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NETCONF                                                         A. Clemm
Internet-Draft                                                    Huawei
Intended status: Standards Track                                 E. Voit
Expires: January 2, 2019                                   Cisco Systems
                                                      A. Gonzalez Prieto
                                                                  VMware
                                                             A. Tripathy
                                                       E. Nilsen-Nygaard
                                                           Cisco Systems
                                                              A. Bierman
                                                               YumaWorks
                                                              B. Lengyel
                                                                Ericsson
                                                            July 1, 2018


                      YANG Datastore Subscription
                    draft-ietf-netconf-yang-push-17

Abstract

   Via the mechanism described in this document, subscriber applications
   may request a continuous, customized stream of updates from a YANG
   datastore.  Providing such visibility into changes made upon YANG
   configuration and operational objects enables new capabilities based
   on the remote mirroring 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 January 2, 2019.








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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

   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  . . . . . . . . . . . . . . . . . .   4
   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  . . . . . . . . . . . . . . . . .  14
     3.10. On-change Notifiable YANG objects . . . . . . . . . . . .  15
     3.11. Other Considerations  . . . . . . . . . . . . . . . . . .  16
   4.  A YANG data model for management of datastore push
       subscriptions . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  Subscription configuration  . . . . . . . . . . . . . . .  22
     4.3.  YANG Notifications  . . . . . . . . . . . . . . . . . . .  23



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     4.4.  YANG RPCs . . . . . . . . . . . . . . . . . . . . . . . .  24
   5.  YANG module . . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  46
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  47
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  48
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  49
   Appendix A.  Appendix A: Subscription Errors  . . . . . . . . . .  50
     A.1.  RPC Failures  . . . . . . . . . . . . . . . . . . . . . .  50
     A.2.  Notifications of Failure  . . . . . . . . . . . . . . . .  51
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

1.  Introduction

   Traditional approaches to remote visibility 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
      application types.

   o  Polling cycles may be missed, 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 ultimately fruitless load on the network, devices,
      and applications.

   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 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



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   datastore specific update notifications.  Transport options for
   [I-D.draft-ietf-netconf-subscribed-notifications] will work
   seamlessly with this solution.

2.  Definitions and Acronyms

   This document uses the terminology defined in [RFC7950], [RFC8341],
   and [RFC8342].  In addition, the following terms are introduced:

   o  Datastore node: An instance of management information in a
      datastore.  Also known as "object".

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

   o  Datastore subscription: A subscription to updates regarding
      contents of a datastore.

   o  Datastore subtree: An instantiated datastore node and the
      datastore nodes that are hierarchically contained within it.

   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
      identifier of the subscription as part of which the update record
      was generated.

   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 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 should be sent and
   what data to include in update records.  YANG objects 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 data model that is itself
   defined in YANG.  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 subtrees within a datastore 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 which can be used to calculate at
         which point in time 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 such as:

         +  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
            of resources in the publisher or receiver.  In order to
            protect against that, a dampening period MAY be used to
            specify the interval which must pass before successive
            update records for the same subscription are generated for a
            receiver.  The dampening period collectively applies to the



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            set of all datastore nodes selected by a single subscription
            and sent to a single receiver.  This means that when there
            is a change to one or more subscribed objects, an update
            record containing those objects is created either
            immediately when no dampening period is in effect, or at the
            end of a dampening period.  If multiple changes to a single
            object occur during a dampening period, only the value that
            is in effect at the time 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 when an object is created or deleted, but
            not when an object value changes).

         +  No Synch 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 at different parameters by a subscriber is to be
   discouraged.  Therefore, in order to minimize the number of
   subscription iterations between subscriber and publisher, dynamic
   subscription supports a simple negotiation between subscribers and
   publishers for subscription parameters.  This negotiation is in the
   form of supplemental information which may 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 subscribers 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, whether or not their values
   actually 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 subscription MAY be suspended.

   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 current values of all changed objects which
   are current at the time the dampening period expires.  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, the last 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 push-change-update notification:

   1.  Just before a change, or at the start of a dampening period,
       evaluate any filtering and any access control rules.  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 a YANG patch record 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 for any object which otherwise wouldn't have appeared.

   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, multiple subscriptions with different
   objects in a selection filter can be created.

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
   subscription had been entered into with good faith.  For example, the
   volume of data objects 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.



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   A publisher SHOULD reject a request for a subscription if it is
   unlikely that the publisher will be able 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.

3.5.  Data Encodings

3.5.1.  Periodic Subscriptions

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

3.5.2.  On-Change Subscriptions

   In an on-change subscription, updates 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 configuration objects
   only.

   A publisher will indicate a change to the effect that a value of a
   datstore node has been updated by indicating a "replace" operation
   (applied to the datastore node) in the patch.  When a new datastore
   node was created (other than an element in a list), a publisher will
   indicate a "create" operation in the patch.  When a datastore node
   was deleted (other than an element in a list), the publisher
   indicates this by a "delete".  When a new list element was created or
   removed, the publisher indicates it by an "insert" or "remove",
   respectively.

   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 if 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 does a "create" operation for a datastore node where
   the receiver believes one already exists, or a "merge" 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.



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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 MUST remove
   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.  When specified, updates will only come from
      the selected datastore nodes.

   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.  As an example, consider an XPath filter
   with a boolean result; such a result will not provide an easily
   interpretable subset of a datastore.  Beyond the boolean example, it
   is quite possible to define an XPath filter where results are easy
   for an application to misinterpret.  Consider an XPath filter which
   only passes a datastore object 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, all selected datastore nodes 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".







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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 will 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 resynch 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
   values.  In cases where multiple changes have occurred 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.)

   These new "push-update" or "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
   port (per [RFC8343]).  This notification message is encoded XML over
   NETCONF as per [I-D.draft-ietf-netconf-netconf-event-notifications].









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<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">
   <subscription-id>1011</subscription-id>
   <datastore-contents>
     <interfaces-state xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
       <interface>
         <name>eth0</name>
         <oper-status>up</oper-status>
       </interface>
     </interfaces-state>
   </datastore-contents>
 </push-update>
</notification>

                          Figure 1: Push example

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

<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">
   <subscription-id>89</subscription-id>
   <datastore-changes>
     <yang-patch xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-patch">
       <patch-id>1</patch-id>
       <edit>
         <edit-id>edit1</edit-id>
         <operation>merge</operation>
         <target>/ietf-interfaces:interfaces-state</target>
         <value>
           <interfaces-state xmlns="http://foo.com/ietf-interfaces">
             <interface>
               <name>eth0</name>
               <oper-status>down</oper-status>
             </interface>
           </interfaces-state>
         </value>
       </edit>
     </yang-patch>
   </datastore-changes>
 </push-change-update>
</notification>

                   Figure 2: Push example for on change





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   Of note in the above example is the 'patch-id' with a value of '1'.
   Per [RFC8072], the 'patch-id' is an arbitrary string.  With YANG
   Push, the publisher SHOULD put into the 'patch-id' a counter starting
   at '1' which increments with every 'push-change-update' generated for
   a subscription.  If used as a counter, this counter MUST be reset to
   '1' 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 '1' the 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
   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, the hints MUST 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]









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          yang-data establish-subscription-datastore-error-info
             +--ro establish-subscription-datasore-error-info
                +--ro reason?                identityref
                +--ro period-hint?           yang:timeticks
                +--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, the hints MUST 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".

          yang-data modify-subscription-datastore-error-info
             +--ro modify-subscription-datasore-error-info
                +--ro reason?                identityref
                +--ro period-hint?           yang:timeticks
                +--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
   they have 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.

   A publisher MUST allow for the possibility that a subscription's
   selection filter references non-existent or access-protected data.
   Such support permits a receiver the ability to monitor the entire



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   lifecyle of some datastore tree.  In this case, all "push-update"
   notifications must be sent empty, and no "push-change-update"
   notifications will be sent until some data becomes visible for a
   receiver.

   A publisher MAY choose reject an establish-subscription request which
   selects non-existent or access-protected data.  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.  Such a
   capability enables the publisher to avoid having to support a
   continuous, and total filtering of an entire subscription's content.

   In these cases above, the error identity "unchanging-selection"
   SHOULD be returned.  This reduces the possibility of leakage of
   access controlled objects.

   Each "push-update" and "push-change-update" MUST have access control
   applied.  This includes validating that read access is permitted for
   any new objects selected since the last notification message was sent
   to a particular each receiver.  To accomplish this, implementations
   SHOULD support the conceptual authorization model of [RFC8342],
   specifically section 3.2.4.

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

      Figure 5: Updated [rfc6536bis] access control for push updates

   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 YANG objects

   In some cases, a publisher supporting on-change notifications may not
   be able to push updates for some object types on-change.  Reasons for
   this might be that the value of the datastore node changes frequently
   (e.g., [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.



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   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.

   Implementations are therefore strongly advised to provide a solution
   to this problem.  It is expected that such a solution will be
   standardized at some point in the future.  In the meantime and until
   this occurs, implementations will be expected to 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.  Or 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 take one or more of the following actions.

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

   o  It MAY choose to suspend a subscription as per
      [I-D.draft-ietf-netconf-subscribed-notifications].

   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 "no-synch-on-start" option is not
      present 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".



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   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 to merge pending update messages.  I.e., the dampening
   period applies to update record creation, not transmission.

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 figure.  The tree diagram follows the notiation 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.

   module: ietf-subscribed-notifications
       ...
       +--rw filters
       |  ...
       |  +--rw yp:selection-filter* [identifier]
       |     +--rw yp:identifier                  sn:filter-id
       |     +--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* [identifier]



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             |  ...
             +--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         yang:timeticks
                |     +--rw yp:anchor-time?   yang:date-and-time
                +--:(yp:on-change) {on-change}?
                   +--rw yp:on-change!
                      +--rw yp:dampening-period?    yang:timeticks
                      +--rw yp:no-synch-on-start?   empty
                      +--rw yp:excluded-change*     change-type

     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}?



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       |  |  |              +--:(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         yang:timeticks
       |  |     |     +---w yp:anchor-time?   yang:date-and-time
       |  |     +--:(yp:on-change) {on-change}?
       |  |        +---w yp:on-change!
       |  |           +---w yp:dampening-period?    yang:timeticks
       |  |           +---w yp:no-synch-on-start?   empty
       |  |           +---w yp:excluded-change*     change-type
       |  +--ro output
       |     +--ro identifier    subscription-id
       +---x modify-subscription
       |  +---w input
       |     ...
       |     +---w (target)
       |     |  ...
       |     |  +--:(yp:datastore)
       |     |     +---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         yang:timeticks
       |        |     +---w yp:anchor-time?   yang:date-and-time
       |        +--:(yp:on-change) {on-change}?
       |           +---w yp:on-change!
       |              +---w yp:dampening-period?   yang:timeticks
       +---x delete-subscription
       |  ...
       +---x kill-subscription
          ...




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

     notifications:
       +---n replay-completed {replay}?
       |  ...
       +---n subscription-completed
       |  ...
       +---n subscription-started {configured}?
       |  |  ...
       |  +--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)?
       |     +--:(yp:periodic)
       |     |  +--ro yp:periodic!
       |     |     +--ro yp:period         yang:timeticks
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?    yang:timeticks
       |           +--ro yp:no-synch-on-start?   empty
       |           +--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



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       |  |        |          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         yang:timeticks
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?    yang:timeticks
       |           +--ro yp:no-synch-on-start?   empty
       |           +--ro yp:excluded-change*     change-type
       +---n subscription-terminated
       |  ...
       +---n subscription-suspended
          ...

   module: ietf-yang-push

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

     yang-data: (for placement into rpc error responses)
       +-- resynch-subscription-error
       |  +--ro reason?                   identityref
       |  +--ro period-hint?              timeticks
       |  +--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?              timeticks
       |  +--ro filter-failure-hint?      string
       |  +--ro object-count-estimate?    uint32
       |  +--ro object-count-limit?       uint32
       |  +--ro kilobytes-estimate?       uint32



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       |  +--ro kilobytes-limit?          uint32
       +-- modify-subscription-error-datastore
          +--ro reason?                   identityref
          +--ro period-hint?              timeticks
          +--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 subscription-id?      sn:subscription-id
       |  +--ro incomplete-update?     empty
       |  +--ro datastore-contents?   <anydata>
       +---n push-change-update {on-change}?
          +--ro subscription-id?     sn:subscription-id
          +--ro incomplete-update?    empty
          +--ro datastore-changes?   <anydata>


                         Figure 6: Model structure

   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
      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:




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

      *  an "anchor-time"; update intervals always 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 is guided by its 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" flag 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 "no-synch-on-start" flag which 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.

   A "subscription-id" MUST be transported along with the subscribed
   contents.  An [RFC5277]  Section 4 one-way notification MAY be used
   for encoding updates.  Where it is, the relevant "subscription-id"
   MUST be encoded as the first element within each "push-update" or
   "push-change-update".  This allows a receiver to differentiate which
   subscription resulted in a particular push.




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

   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 providing 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:

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

                   Figure 7: Establish-subscription RPC

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




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

          Figure 8: 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.

   The specific parameters to be returned in as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  In the case of NETCONF
   [I-D.draft-ietf-netconf-netconf-event-notifications], when a
   subscription request is rejected, the NETCONF RPC reply MUST include
   an "rpc-error" element with the following elements:

   o  "error-type" of "application".

   o  "error-tag" of "operation-failed".

   o  Optionally, an "error-severity" of "error" (this MAY but does not
      have to be included).

   o  "error-app-tag" with the value being a string that corresponds to
      an identity with a base of "establish-subscription-error".

   o  Optionally, "error-info" containing XML-encoded data with hints
      for parameter settings that might result in future RPC success per
      yang-data definition "establish-subscription-error-datastore".




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   For example, for the following request:

   <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 netconf:type="xpath"
       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>
   </netconf:rpc>

            Figure 9: Establish-subscription request example 2

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


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

        Figure 10: 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" of a subscription.

 <netconf:rpc message-id="102"
    xmlns:netconf="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">
     <identifier>1011</identifier>
     <yp:datastore
     xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
       ds:operational
     </yp:datastore>
     <yp:datastore-xpath-filter
     netconf:type="xpath" 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>
 </netconf:rpc>

                  Figure 11: Modify subscription request

   The publisher MUST respond explicitly positively or negatively to the
   request.  If the subscription modification is rejected, the
   subscription is maintained as it was before the modification request.
   In addition, the publisher MUST send an rpc error response.  This rpc
   error response may contain hints encapsulated within the yang-data
   structure "modify-subscription-error-datastore".  A subscription MAY
   be modified multiple times.

   The specific parameters to be returned in as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  In the case of NETCONF
   [I-D.draft-ietf-netconf-netconf-event-notifications], when a
   subscription request is rejected, the NETCONF RPC reply MUST include
   an "rpc-error" element with the following elements:

   o  "error-type" of "application".

   o  "error-tag" of "operation-failed".



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   o  Optionally, an "error-severity" of "error" (this MAY but does not
      have to be included).

   o  "error-app-tag" with the value being a string that corresponds to
      an identity with a base of "modify-subscription-error".

   o  "error-path" pointing to the object or parameter that caused the
      rejection.

   o  Optionally, "error-info" containing XML-encoded data with hints
      for parameter settings that might result in future RPC success per
      yang-data definition "modify-subscription-error-datastore".

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

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
   "identifier".  This RPC is unmodified from
   [I-D.draft-ietf-netconf-subscribed-notifications].

4.4.4.  Resynch-subscription RPC

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

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

                           Resynch 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 "resynch-subscription-error".



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4.4.5.  YANG Module Synchronization

   To make subscription requests, the subscriber needs to know the YANG
   module library available on the publisher.  The YANG 1.0 module
   library information is sent by a NETCONF server in the NETCONF
   "hello" message.  For YANG 1.1 modules and all modules used with the
   RESTCONF [RFC8040] protocol, this information is provided by the YANG
   Library module (ietf-yang-library.yang from [RFC7895].  This YANG
   library information is important for the receiver to reproduce the
   set of object definitions used within the publisher.

   The YANG library includes a module list with the name, revision,
   enabled features, and applied deviations for each YANG module
   implemented by the publisher.  The receiver is expected to know the
   YANG library information before starting a subscription.  The
   "/modules-state/module-set-id" leaf in the "ietf-yang-library" module
   can be used to cache the YANG library information.

   The set of modules, revisions, features, and deviations can change at
   run-time (if supported by the publisher implementation).  In this
   case, the receiver needs to be informed of module changes before
   datastore nodes from changed modules can be processed correctly.  The
   YANG library provides a simple "yang-library-change" notification
   that informs the subscriber that the library has changed.  The
   receiver then needs to re-read the entire YANG library data for the
   replicated publisher in order to detect the specific YANG library
   changes.  The "ietf-netconf-notifications" module defined in
   [RFC6470] contains a "netconf-capability-change" notification that
   can identify specific module changes.  For example, the module URI
   capability of a newly loaded module will be listed in the "added-
   capability" leaf-list, and the module URI capability of an removed
   module will be listed in the "deleted-capability" leaf-list.

5.  YANG module

<CODE BEGINS> file "ietf-yang-push@2018-07-01.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;



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        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";
  }

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




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        Copyright (c) 2018 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
        draft-ietf-netconf-yang-push-17; see the RFC itself for full
        legal notices.

        NOTE TO RFC EDITOR: Please replace above reference to
        draft-ietf-netconf-yang-push-17 with RFC number when published
        (i.e. RFC xxxx).";

  revision 2018-07-01 {
    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
          (ietf-yang-push@2018-07-01.yang) to the date of RFC publication.
          (3) Please replace the following reference to
          draft-ietf-netconf-yang-push-17 with RFC number when published
          (i.e. RFC xxxx).";
    reference
      "draft-ietf-netconf-yang-push-17";
  }

 /*
  * FEATURES
  */

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

 /*
  * IDENTITIES
  */

  /* Error type identities for datastore subscription  */




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  identity resynch-subscription-error {
     description
      "Problem found while attempting to fulfill an
      'resynch-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-resynch {
    base resynch-subscription-error;
    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-synch-unsupported {
    base sn:establish-subscription-error;
    description
      "Neither synch on start nor resynchronization are supported for
      this subscription.  This error will be used for two reasons.
      First if an 'establish-subscription' RPC doesn't include
      'no-synch-on-start', yet the publisher can't support sending a
      'push-update' for this subscription for reasons other than
      'on-change-unsupported' or 'synchronization-size'.  And second,
      if the 'resynch-subscription' RPC is invoked either for an
      existing periodic subscription, or for an on-change subscription
      which can't support resynchronization.";



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  }

  identity period-unsupported {
    base sn:establish-subscription-error;
    base sn:modify-subscription-error;
    base sn:subscription-suspended-reason;
    description
      "Requested time 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 result-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.";
  }

  identity synchronization-size {
    base sn:establish-subscription-error;
    base sn:modify-subscription-error;
    base resynch-subscription-error;
    base sn:subscription-suspended-reason;
    description
      "Synch-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.";
  }




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  /*
   * TYPE DEFINITIONS
   */

  typedef change-type {
    type enumeration {
      enum "create" {
        description
          "A change that refers to the creation of a new data node.";
      }
      enum "delete" {
        description
          "A change that refers to the deletion of a data node.";
      }
      enum "insert" {
        description
          "A change that refers to the insertion of a new
          user-ordered data node.";
      }
      enum "merge" {
        description
          "A change that refers to a merging of a new value with a
          target data node.";
      }
      enum "move" {
        description
          "A change that refers to a reordering of the target data
          node";
      }
      enum "replace" {
        description
          "A change that refers to a replacement of the target data
          node's value.";
      }
      enum "remove" {
        description
          "A change that refers to the removal of a data node.";
      }
    }
    description
      "Specifies different types of datastore changes.";
    reference
      "RFC 8072 section 2.5, with a delta 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.";
  }



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  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.";
  }

  /*
   * 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;
       }
       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.";
      }
      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



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          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 are those in scope on
              the 'datastore-xpath-filter' leaf element.

           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;

      }
    }



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  }

  grouping update-policy-modifiable {
    description
      "This grouping describes the datastore specific subscription
       conditions that can be changed during the lifetime of the
       subscription.";
    choice update-trigger {
      description
        "Defines necessary conditions for sending an event record to
         the subscriber.";
      case periodic {
        container periodic {
          presence "indicates an 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 yang:timeticks;
            mandatory true;
            description
              "Duration of time which should occur between periodic
               push updates.";
          }
          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 yang:timeticks;
            default 0;



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            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 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 no-synch-on-start {
          type empty;
          description
            "The presence of this object restricts an on-change
             subscription from sending push-update notifications.  When
             present, 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 absent (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 resynch 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 yang:timeticks;
      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



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         publisher's ability to service for this subscription.";
    }
  }


  /*
   * RPCs
   */

   rpc resynch-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 objects that the subscriber is permitted to access.
       This RPC can only be invoked on the same session on which the
       subscription was established (using an establish-subscription
       RPC).  In case of an error, a resynch-subscription-error is
       sent as part of an error response.";
    input {
      leaf identifier {
        type sn:subscription-id;
        mandatory true;
        description
          "Identifier of the subscription that is to be resynched.";
      }
    }
  }

  rc:yang-data resynch-subscription-error {
    container resynch-subscription-error {
      description
        "If a 'resynch-subscription' RPC fails, the subscription is not
        resynched 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 resynch-subscription-error;
        }
        mandatory true;
        description
          "Indicates the reason why the publisher has declined a request
          for subscription resynchronization.";
      }
      uses hints;



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    }
  }

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



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    uses update-policy-modifiable;
  }

  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 selection-filter-objects;
    }
  }

  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



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       event stream by any receiver.";
    leaf subscription-id {
      type sn:subscription-id;
      description
        "This references the subscription which drove the notification
         to be sent.";
    }
    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.";
    }
    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.";
    }
  }

  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 a general-purpose
       notification.";
    leaf subscription-id {
      type sn:subscription-id;
      description
        "This references the subscription which drove the notification
         to be sent.";
    }
    leaf incomplete-update {
      type empty;
      description
        "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.";



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    }
    anydata 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.
         The datastore changes are encoded per RFC 8027
         (YANG Patch).";
    }
  }

  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 {



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       uses datastore-criteria {
          refine "selection-filter/within-subscription" {
          description
            "Specifies where the selection filter, and where it came
            from within the subscription and then populated within this
            notification. 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" {
    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 {
       uses datastore-criteria;



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    }
  }
}

<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-17.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
   [RFC5246].

   The NETCONF access control model [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:





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      First, it should be noted that the YANG module augments the YANG
      module from [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.

      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.

      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.

      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).

      RPC "resynch-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.

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, and Tom
   Petch.






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9.  References

9.1.  Normative References

   [I-D.draft-ietf-netconf-subscribed-notifications]
              Voit, E., Clemm, A., Gonzalez Prieto, A., Tripathy, A.,
              and E. Nilsen-Nygaard, "Custom Subscription to Event
              Streams", draft-ietf-netconf-subscribed-notifications-13
              (work in progress), June 2018.

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

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

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

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

   [RFC7895]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
              <https://www.rfc-editor.org/info/rfc7895>.

   [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>.

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




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   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

9.2.  Informative References

   [bergstra2014]
              Bergstra, J. and M. Burgess, "Promise Theory, Createspace
              Independent Publishers, ISBN-13 9781495437779", 2014.

   [I-D.draft-ietf-netconf-netconf-event-notifications]
              Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "NETCONF Support for Event
              Notifications", May 2018.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
              <https://www.rfc-editor.org/info/rfc5277>.

   [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>.





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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 within the either the subscribed-
   notifications YANG model or the yang-push YANG model may be returned
   as part of the error responses resulting from failed attempts at
   datastore subscription.  Following are valid errors per RPC (note:
   throughout this section the prefix 'sn' indicates an item imported
   from the subscribed-notifications.yang model):


   establish-subscription         modify-subscription
   ----------------------         -------------------
    cant-exclude                   sn:filter-unsupported
    datastore-not-subscribable     sn:insufficient-resources
    sn:dscp-unavailable            sn:no-such-subscription
    sn:filter-unsupported          period-unsupported
    sn:insufficient-resources      result-too-big
    on-change-unsupported          synchronization-size
    on-change-synch-unsupported    unchanging-selection
    period-unsupported
    result-too-big                resynch-subscription
    synchronization-size          --------------------
    unchanging-selection           no-such-subscription-resynch
                                   synchronization-size

   delete-subscription            kill-subscription
   ----------------------         -----------------
    sn:no-such-subscription        sn:no-such-subscription

   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:









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   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 resynch-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 "resynch-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,
   the following types of error identities may be returned within the
   corresponding subscription state change notification:


   subscription-terminated        subscription-suspended
   -----------------------        ----------------------
    datastore-not-subscribable     sn:insufficient-resources
    sn:filter-unavailable          period-unsupported
    sn:no-such-subscription        result-too-big
    sn:suspension-timeout          synchronization-size
    unchanging-selection

Appendix B.  Changes between revisions

   (To be removed by RFC editor prior to publication)

   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 optional in push updates, except when encoded in
      RFC5277, Section 4 one-way notification.

   o  Finished adding the text descibing the resynch 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. synch-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

   Email: ludwig@clemm.org


   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com


   Alberto Gonzalez Prieto
   VMware

   Email: agonzalezpri@vmware.com


   Ambika Prasad Tripathy
   Cisco Systems

   Email: ambtripa@cisco.com


   Einar Nilsen-Nygaard
   Cisco Systems

   Email: einarnn@cisco.com


   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com



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   Balazs Lengyel
   Ericsson

   Email: balazs.lengyel@ericsson.com















































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