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RFC 8641

Subscription to YANG Notifications for Datastore Updates

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Top   ToC   RFC8641 - Page 1
Internet Engineering Task Force (IETF)                          A. Clemm
Request for Comments: 8641                                     Futurewei
Category: Standards Track                                        E. Voit
ISSN: 2070-1721                                            Cisco Systems
                                                          September 2019


        Subscription to YANG Notifications for Datastore Updates

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8641.
Top   ToC   RFC8641 - Page 2
Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   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.
Top   ToC   RFC8641 - Page 3
Table of Contents

   1. Introduction ....................................................4
   2. Definitions .....................................................4
   3. Solution Overview ...............................................6
      3.1. Subscription Model .........................................6
      3.2. Negotiation of Subscription Policies .......................7
      3.3. On-Change Considerations ...................................8
      3.4. Reliability Considerations .................................9
      3.5. Data Encodings ............................................10
      3.6. Defining the Selection with a Datastore ...................11
      3.7. Streaming Updates .........................................12
      3.8. Subscription Management ...................................15
      3.9. Receiver Authorization ....................................16
      3.10. On-Change Notifiable Datastore Nodes .....................18
      3.11. Other Considerations .....................................18
   4. A YANG Data Model for Management of Datastore Push
      Subscriptions ..................................................20
      4.1. Overview ..................................................20
      4.2. Subscription Configuration ................................27
      4.3. YANG Notifications ........................................28
      4.4. YANG RPCs .................................................29
   5. YANG Module for YANG-Push ......................................34
   6. IANA Considerations ............................................51
   7. Security Considerations ........................................51
   8. References .....................................................53
      8.1. Normative References ......................................53
      8.2. Informative References ....................................55
   Appendix A. Subscription Errors ...................................56
     A.1. RPC Failures ...............................................56
     A.2. Failure Notifications ......................................57
   Acknowledgments ...................................................58
   Contributors ......................................................58
   Authors' Addresses ................................................58
Top   ToC   RFC8641 - Page 4
1.  Introduction

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

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

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

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

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

   A more effective alternative to polling is for an application to
   receive automatic and continuous updates from a targeted subset of a
   datastore.  Accordingly, there is a need for a service that
   (1) allows applications to subscribe to updates from a datastore and
   (2) enables the server (also referred to as 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 [RFC8639].  Supplementing that work are YANG data model
   augmentations, extended RPCs, and new datastore-specific update
   notifications.  Transport options provided in [RFC8639] will work
   seamlessly with this solution.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.
Top   ToC   RFC8641 - Page 5
   This document uses the terminology defined in [RFC7950], [RFC8341],
   [RFC8342], and [RFC8639].  In addition, this document defines the
   following terms:

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

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

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

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

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

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

   o  Selection filter: Evaluation and/or selection criteria that 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, or deleted).  Also included in
      the update record may be other metadata, such as a subscription ID
      of the subscription for which the update record was generated.  In
      this document, update records are often also simply referred to as
      "updates".

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

   o  YANG-Push: The subscription and push mechanism for datastore
      updates that is specified in this document.
Top   ToC   RFC8641 - Page 6
3.  Solution Overview

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

3.1.  Subscription Model

   YANG-Push subscriptions are defined using a YANG data model.  This
   model enhances the subscription model defined in [RFC8639] 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  The specification of selection filters that identify targeted YANG
      datastore nodes and/or datastore subtrees for which updates are to
      be pushed.

   o  The specification of update policies that contain conditions that
      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 (1) the period interval with which to
         report updates and (2) an "anchor-time", i.e., a reference
         point in time that can be used to calculate at which points in
         time periodic updates need to be assembled and sent.

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

         +  "dampening-period": In an on-change subscription, detected
            object changes should be sent as quickly as possible.
            However, it may be undesirable to send a rapid series of
            object changes.  Such behavior has the potential to exhaust
            resources in the publisher or receiver.  In order to protect
            against this type of scenario, a dampening period MAY be
            used to specify the interval that has to pass before
            successive update records for the same subscription are
            generated for a receiver.  The dampening period collectively
Top   ToC   RFC8641 - Page 7
            applies to the set of all datastore nodes selected by a
            single subscription.  This means that when there is a change
            to one or more subscribed objects, an update record
            containing those objects is created immediately (when no
            dampening period is in effect) or at the end of a dampening
            period (when a dampening period is in fact in effect).  If
            multiple changes to a single object occur during a dampening
            period, only the value that is in effect at the time when
            the update record is created is included.  The dampening
            period goes into effect every time the assembly of an update
            record is completed.

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

         +  "sync-on-start": This parameter defines whether or not a
            complete "push-update" (Section 3.7) 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
   determines that it may be unable to provide update records meeting
   the terms of an "establish-subscription" or "modify-subscription" RPC
   request.  In this case, a subscriber may quickly follow up with a new
   RPC request using different parameters.

   Random guessing of different parameters by a subscriber is to be
   discouraged.  Therefore, in order to minimize the number of
   subscription iterations between subscriber and publisher, a dynamic
   subscription supports a simple negotiation between subscribers and
   publishers for subscription parameters.  This negotiation is in the
   form of supplemental information that should be inserted into 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 of objects that would be returned from a
   proposed selection filter.  However, there are no guarantees that
   subsequent requests that consider these hints will be accepted.
Top   ToC   RFC8641 - Page 8
3.3.  On-Change Considerations

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

   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 of the subscription that do support on-change
   updates, whereas other objects are excluded from update records, even
   if their values change.  In order for a subscriber to determine
   whether objects support on-change subscriptions, objects are marked
   accordingly on a publisher.  Accordingly, when subscribing, it is the
   responsibility of the subscriber to ensure that 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 if the scope of the subscription contains objects for
   which on-change is not supported.  In the case of a configured
   subscription, the publisher MAY suspend the subscription.

   To avoid flooding receivers with repeated updates for subscriptions
   containing fast-changing objects or objects with oscillating values,
   an on-change subscription allows for the definition of a dampening
   period.  Once an update record for a given object is generated, no
   other updates for this particular subscription will be created until
   the end of the dampening period.  Values sent at the end of the
   dampening period are the values that are current at the end of the
   dampening period of all changed objects.  Changed objects include
   those objects that were deleted or newly created during that
   dampening period.  If an object has returned to its original value
   (or even has been created and then deleted) during the dampening
   period, that value (and not the interim change) will still be sent.
   This will indicate that churn is occurring on that object.
Top   ToC   RFC8641 - Page 9
   On-change subscriptions can be refined to let users subscribe only to
   certain types of changes.  For example, a subscriber might only want
   object creations and deletions, but not modifications of object
   values.

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

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

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

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

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

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

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

3.4.  Reliability Considerations

   A subscription to updates from a datastore is intended to obviate the
   need for polling.  However, in order to do so, it is critical that
   subscribers can rely on the subscription and have confidence that
   they will indeed receive the subscribed updates without having to
   worry about updates being silently dropped.  In other words, a
   subscription constitutes a promise on the side of the publisher to
   provide the receivers with updates per the terms of the subscription.
Top   ToC   RFC8641 - Page 10
   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 initiated in good faith.  For example, the
   volume of datastore nodes may be larger than anticipated, the
   interval may prove too short to send full updates in rapid
   succession, or an internal problem may prevent objects from being
   collected.  For this reason, the solution defined in this document
   (1) mandates that a publisher notify receivers immediately and
   reliably whenever it encounters a situation in which it is unable to
   keep the terms of the subscription and (2) provides the publisher
   with the option to suspend the subscription in such a case.  This
   includes indicating the fact that an update is incomplete as part of
   a "push-update" or "push-change-update" notification, as well as
   emitting a "subscription-suspended" notification as applicable.  This
   is described further in Section 3.11.1.

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

   The solution builds on [RFC8639].  As defined therein, any loss of an
   underlying transport connection will be detected and result in
   subscription termination (in the case of dynamic subscriptions) or
   suspension (in the case of configured subscriptions), ensuring that
   situations where the loss of update notifications would go unnoticed
   will not occur.

3.5.  Data Encodings

3.5.1.  Periodic Subscriptions

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

3.5.2.  On-Change Subscriptions

   In an on-change subscription, update records need to indicate not
   only values of changed datastore nodes but also the types of changes
   that occurred since the last update.  Therefore, encoding rules for
   data in on-change updates will generally follow YANG Patch operations
   as specified in [RFC8072].  The YANG Patch operations will describe
   what needs to be applied to the earlier state reported by the
   preceding update in order to result in the now-current state.  Note
   that objects referred to in an update are not limited to
Top   ToC   RFC8641 - Page 11
   configuration data but can include any objects (including operational
   data), whereas [RFC8072] patches apply only to configuration data in
   configuration datastores.

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

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

3.6.  Defining the Selection with a Datastore

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

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

   o  subtree: A subtree selection filter identifies one or more
      datastore subtrees.  When specified, update records will only come
      from the datastore nodes of selected datastore subtree(s).  The
      syntax and semantics correspond to those specified in [RFC6241],
      Section 6.
Top   ToC   RFC8641 - Page 12
   o  xpath: An "xpath" selection filter is an XPath expression that
      returns a node set.  (XPath is a query language for selecting
      nodes in an XML document; see [XPATH] for details.)  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.  Consider an XPath filter that only passes
   a datastore node when an interface is up.  It is up to the receiver
   to understand the implications of the presence or absence of objects
   in each update.

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

3.7.  Streaming Updates

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

   A "push-update" notification defines a complete, filtered update of
   the datastore per the terms of a subscription.  This type of YANG
   notification is used for continuous updates of periodic
   subscriptions.  A "push-update" notification can also be used for the
   on-change subscriptions in two cases.  First, it MUST be used as the
   initial "push-update" if there is a need to synchronize the receiver
   at the start of a new subscription.  Second, it MAY be sent if the
   publisher later chooses to resync an on-change subscription.  The
   "push-update" update record contains an instantiated datastore
   subtree with all of the subscribed contents.  The content of the
   update record is equivalent to the contents that would be obtained
   had the same data been explicitly retrieved using a datastore
   retrieval operation using the same transport with the same filters
   applied.
Top   ToC   RFC8641 - Page 13
   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, have been deleted, or have had changes to
   their values.  In cases where multiple changes have occurred over the
   course of a dampening period and the object has not been deleted, the
   object's most current value is reported.  (In other words, for each
   object, only one change is reported, not its entire history.  Doing
   so would defeat the purpose of the dampening period.)

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

   Figure 1 provides an example of a notification message for a
   subscription tracking the operational status of a single Ethernet
   interface (per [RFC8343]).  This notification message is encoded XML
   [W3C.REC-xml-20081126] over the Network Configuration Protocol
   (NETCONF) as per [RFC8640].

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

                          Figure 1: Push Example
Top   ToC   RFC8641 - Page 14
   Figure 2 provides 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">
     <id>89</id>
     <datastore-changes>
       <yang-patch>
         <patch-id>0</patch-id>
         <edit>
           <edit-id>edit1</edit-id>
           <operation>replace</operation>
           <target>/ietf-interfaces:interfaces</target>
           <value>
             <interfaces
                  xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
               <interface>
                 <name>eth0</name>
                 <oper-status>down</oper-status>
               </interface>
             </interfaces>
           </value>
         </edit>
       </yang-patch>
     </datastore-changes>
   </push-change-update>
  </notification>

       Figure 2: Push Example for an On-Change Notification Message

   Of note in the above example is the "patch-id" with a value of "0".
   Per [RFC8072], the "patch-id" is an arbitrary string.  With
   YANG-Push, the publisher SHOULD put into the "patch-id" a counter
   starting at "0" that increments with every "push-change-update"
   generated for a subscription.  If used as a counter, this counter
   MUST be reset to "0" any time a resynchronization occurs (i.e., with
   the sending of a "push-update").  Also, if used as a counter, the
   counter MUST be reset to "0" after passing a maximum value of
   "4294967295" (i.e., the maximum value that can be represented using
   the uint32 data type).  Such a mechanism allows easy identification
   of lost or out-of-sequence update records.
Top   ToC   RFC8641 - Page 15
3.8.  Subscription Management

   The RPCs defined in [RFC8639] 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, including a subtree request that is
   too large or the inability of the publisher to push update records as
   frequently as requested.  In such cases, no subscription is
   established.  Instead, a subscription result that includes the reason
   for the failure 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 including such
   parameters in future subscription attempts.

   In the case of a rejected request for establishment of a datastore
   subscription, if there are hints, the hints SHOULD be transported in
   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 the case of a
   stream subscription.

   Figure 3 shows a tree diagram for "establish-subscription-datastore-
   error-info".  All tree diagrams used in this document follow the
   notation defined in [RFC8340].

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

   Figure 3: "establish-subscription-datastore-error-info" Tree Diagram
Top   ToC   RFC8641 - Page 16
   Similarly, in the case of a rejected request for modification of a
   datastore subscription, if there are hints, the hints SHOULD be
   transported in 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 the case
   of a stream subscription.

   Figure 4 shows a tree diagram for "modify-subscription-datastore-
   error-info".

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

     Figure 4: "modify-subscription-datastore-error-info" Tree Diagram

3.9.  Receiver Authorization

   A receiver of subscription data MUST only be sent updates for which
   it has proper authorization.  A publisher MUST ensure that no
   unauthorized 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 unauthorized
   data from datastore subtrees.  This enables YANG data that is pushed
   based on subscriptions to be authorized in a way that is equivalent
   to a regular data retrieval ("get") operation.

   Each "push-update" and "push-change-update" MUST have access control
   applied, as depicted in Figure 5.  This includes validating that read
   access is permitted for any new objects selected since the last
   notification message was sent to a particular receiver.  A publisher
   MUST silently omit data nodes from the results that the client is not
   authorized to see.  To accomplish this, implementations SHOULD apply
   the conceptual authorization model of [RFC8341], specifically
   Section 3.2.4, extended to apply analogously to data nodes included
   in notifications, not just <rpc-reply> messages sent in response to
   <get> and <get-config> requests.
Top   ToC   RFC8641 - Page 17
                         +-----------------+      +--------------------+
     push-update or -->  | datastore node  |  yes | add datastore node |
    push-change-update   | access allowed? | ---> | to update record   |
                         +-----------------+      +--------------------+

                 Figure 5: Access Control for Push Updates

   A publisher MUST allow for the possibility that a subscription's
   selection filter references nonexistent data or data that a receiver
   is not allowed to access.  Such support permits a receiver the
   ability to monitor the entire lifecycle of some datastore tree
   without needing to explicitly enumerate every individual datastore
   node.  If, after access control has been applied, there are no
   objects remaining in an update record, then the effect varies given
   if the subscription is a periodic or on-change subscription.  For a
   periodic subscription, an empty "push-update" notification MUST be
   sent, so that clients do not get confused into thinking that an
   update was lost.  For an on-change subscription, a "push-update"
   notification MUST NOT be sent, so that clients remain unaware of
   changes made to nodes they don't have read-access for.  By the same
   token, changes to objects that are filtered MUST NOT affect any
   dampening intervals.

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

   If read access into previously accessible nodes has been lost due to
   a receiver permissions change, this SHOULD be reported as a patch
   "delete" operation for on-change subscriptions.  If not capable of
   handling such receiver permission changes with such a "delete",
   publisher implementations MUST force dynamic subscription
   re-establishment or configured subscription reinitialization so that
   appropriate filtering is installed.
Top   ToC   RFC8641 - Page 18
3.10.  On-Change Notifiable Datastore Nodes

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

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

   Implementations are therefore strongly advised to provide a solution
   to this problem.  One solution might involve making discoverable to
   clients which objects are on-change notifiable, specified using
   another YANG data model.  Such a solution is specified in
   [Yang-Push-Notif-Cap].  Until this solution is standardized,
   implementations SHOULD provide their own solution.

3.11.  Other Considerations

3.11.1.  Robustness and Reliability

   It is important that updates as discussed in this document, and
   on-change updates in particular, do not get lost.  If the loss of an
   update is unavoidable, it is critical that the receiver be notified
   accordingly.

   Update records for a single subscription MUST NOT be resequenced
   prior to transport.
Top   ToC   RFC8641 - Page 19
   It is conceivable that, under certain circumstances, a publisher will
   recognize that it is unable to include in an update record the full
   set of objects desired per the terms of a subscription.  In this
   case, the publisher MUST act as follows.

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

   o  The publisher MAY choose to suspend the subscription as per
      [RFC8639].  If the publisher does not create an update record at
      all, it MUST suspend the subscription.

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

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

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

3.11.2.  Publisher Capacity

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

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


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