Network Configuration                                          R. Wilton
Internet-Draft                                             Cisco Systems
Intended status: Informational                            9 January 2025
Expires: 13 July 2025


         YANG-Push Operational Data Observability Enhancements
                draft-wilton-netconf-yp-observability-01

Abstract

   *This version of the document is aimed to be a base reference point
   to compare against to see how YANG Push Lite compares to the two core
   RFCs [RFC8639] & [RFC8641] that it is based on.  The next draft
   revision would serve as a better starting point to see the proposed
   protocol & data model for YANG Push Lite.*

   YANG Push Lite is a simplified specification of YANG Push,
   specifically optimized for observability of operational data.

   This early draft proposes some enhancements to YANG-Push to optimize
   its behavior for operational data telemetry.  It also lists some
   additional issues that could potentially be discussed if there is
   further interest in this work, in particular whether we should be
   attempting extensions to YANG-Push (as this document is currently
   framed) or instead should attempt to define a new 'YANG-Push lite'.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://rgwilton.github.io/draft-yp-observability/draft-wilton-
   netconf-yp-observability.html.  Status information for this document
   may be found at https://datatracker.ietf.org/doc/draft-wilton-
   netconf-yp-observability/.

   Discussion of this document takes place on the Network Configuration
   Working Group mailing list (mailto:netconf@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/netconf/.
   Subscribe at https://www.ietf.org/mailman/listinfo/netconf/.

   Source for this draft and an issue tracker can be found at
   https://github.com/rgwilton/draft-yp-observability.







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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 13 July 2025.

Copyright Notice

   Copyright (c) 2025 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Document Status . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Acknowledgement to the authors of RFC 8639 and RFC 8641 . . .   4
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Introduction text from Subscribed Notifications (RFC
           8639) . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Introduction text from YANG Push (RFC 8641) . . . . . . .   6
   4.  Conventions and Definitions . . . . . . . . . . . . . . . . .   7
     4.1.  Definitions taken from Subscribed Notifications (RFC
           8639) . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Definitions taken from YANG Push (RFC 8641) . . . . . . .   8
   5.  Subscribed Notifications Solution Overview (all from RFC
           8639) . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Relationship to RFC 5277  . . . . . . . . . . . . . . . .  11
     5.2.  Solution  . . . . . . . . . . . . . . . . . . . . . . . .  11



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       5.2.1.  Event Streams . . . . . . . . . . . . . . . . . . . .  12
       5.2.2.  Event Stream Filters  . . . . . . . . . . . . . . . .  13
       5.2.3.  QoS . . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.4.  Configured Subscriptions  . . . . . . . . . . . . . .  14
       5.2.5.  Dynamic Subscriptions . . . . . . . . . . . . . . . .  22
       5.2.6.  Event Record Delivery . . . . . . . . . . . . . . . .  30
       5.2.7.  Subscription State Change Notifications . . . . . . .  31
       5.2.8.  Subscription Monitoring . . . . . . . . . . . . . . .  36
       5.2.9.  Support for the "ietf-subscribed-notifications" YANG
               Module  . . . . . . . . . . . . . . . . . . . . . . .  37
   6.  YANG Push Solution Overview (all from RFC 8641) . . . . . . .  37
     6.1.  Subscription Model  . . . . . . . . . . . . . . . . . . .  38
       6.1.1.  Negotiation of Subscription Policies  . . . . . . . .  39
       6.1.2.  On-Change Considerations  . . . . . . . . . . . . . .  40
       6.1.3.  Reliability Considerations  . . . . . . . . . . . . .  42
       6.1.4.  Data Encodings  . . . . . . . . . . . . . . . . . . .  42
       6.1.5.  Defining the Selection with a Datastore . . . . . . .  43
       6.1.6.  Streaming Updates . . . . . . . . . . . . . . . . . .  44
       6.1.7.  Subscription Management . . . . . . . . . . . . . . .  47
       6.1.8.  Receiver Authorization  . . . . . . . . . . . . . . .  48
       6.1.9.  On-Change Notifiable Datastore Nodes  . . . . . . . .  50
       6.1.10. Other Considerations  . . . . . . . . . . . . . . . .  50
     6.2.  Subscription State Change Notifications . . . . . . . . .  52
   7.  YANG Data Model Tree Diagrams (from RFC 8639) . . . . . . . .  52
     7.1.  The "streams" Container . . . . . . . . . . . . . . . . .  52
     7.2.  The "filters" Container . . . . . . . . . . . . . . . . .  52
     7.3.  The "subscriptions" Container . . . . . . . . . . . . . .  53
   8.  A YANG Data Model for Management of Datastore Push
           Subscriptions (from RFC 8641) . . . . . . . . . . . . . .  54
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  54
     8.2.  Subscription Configuration  . . . . . . . . . . . . . . .  61
     8.3.  YANG Notifications  . . . . . . . . . . . . . . . . . . .  62
       8.3.1.  State Change Notifications  . . . . . . . . . . . . .  62
       8.3.2.  Notifications for Subscribed Content  . . . . . . . .  62
     8.4.  YANG RPCs . . . . . . . . . . . . . . . . . . . . . . . .  63
       8.4.1.  "establish-subscription" RPC  . . . . . . . . . . . .  63
       8.4.2.  "modify-subscription" RPC . . . . . . . . . . . . . .  66
       8.4.3.  "delete-subscription" RPC . . . . . . . . . . . . . .  67
       8.4.4.  "resync-subscription" RPC . . . . . . . . . . . . . .  67
       8.4.5.  YANG Module Synchronization . . . . . . . . . . . . .  67
   9.  YANG Model  . . . . . . . . . . . . . . . . . . . . . . . . .  68
   10. Implementation Considerations (from RFC 8639) . . . . . . . . 113
   11. Transport Requirements (from RFC 8639)  . . . . . . . . . . . 114
   12. Security Considerations . . . . . . . . . . . . . . . . . . . 114
     12.1.  Security Considerations from RFC 8639  . . . . . . . . . 115
     12.2.  Security Considerations from RFC 8641  . . . . . . . . . 119
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 121
     13.1.  IANA Considerations from RFC 8639  . . . . . . . . . . . 121



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     13.2.  IANA Considerations from RFC 8641  . . . . . . . . . . . 121
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 122
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
     Normative References  . . . . . . . . . . . . . . . . . . . . . 122
     Informative References  . . . . . . . . . . . . . . . . . . . . 122
   Appendix A.  Example Configured Transport Augmentation (from RFC
           8639) . . . . . . . . . . . . . . . . . . . . . . . . . . 124
   Appendix B.  Subscription Errors (from RFC 8641)  . . . . . . . . 125
     B.1.  RPC Failures  . . . . . . . . . . . . . . . . . . . . . . 125
     B.2.  Failure Notifications . . . . . . . . . . . . . . . . . . 126
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 127

1.  Document Status

   *PLEASE READ - This version of the document is aimed to be a base
   reference point () to compare against to see how YANG Push Lite
   compares to the two core RFCs [RFC8639] & [RFC8641] that it is based
   on.  The next draft revision would serve as a better starting point
   to see the proposed protocol & data model for YANG Push Lite.*

   Based on the feedback received during the IETF 121 NETCONF session,
   this document has currently been written as a self-contained
   lightweight replacement for [RFC8639] and [RFC8641], defining a
   separate configuration data model.  It leverages
   [I-D.draft-netana-netconf-notif-envelope] as the envelope for the
   generated notifications with a new update message format.

2.  Acknowledgement to the authors of RFC 8639 and RFC 8641

   This document is intended to be a lightweight alternative for
   [RFC8639] and [RFC8641], but it intentionally reuses substantial part
   of the design and data model of those RFCs.

   For ease of reference, this document, at least initially, sources
   most of the text and basis for the YANG model directly from those
   RFCs, rather than creating a separate document that would contain
   numerous references.

   Hence, the authors of this draft would like to thank and acknowledge
   the very significant previous work by the authors and of those RFCs,
   i.e., Eric Voit, Alex Clemm, Alberto Gonzalez Prieto, Einar Nilsen-
   Nygaard, Ambika Prasad Tripathy, and also all those who contributed
   to the underlying work upon which this document is heavily based.








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3.  Introduction

   [I-D.ietf-nmop-yang-message-broker-integration] describes an
   architecture for how YANG-Push [RFC8641] can be integrated
   effectively with message brokers, e.g., [Kafka], that is part of a
   wider architecture for a *Network Anomaly Detection Framework*,
   specified in [I-D.ietf-nmop-network-anomaly-architecture].

   YANG-Push is a key part of these architectures, but through
   experience of implementing YANG-Push specifically for the use cases
   described in the above architecture documents, it became clear that
   there are aspects of YANG-Push that are not optimal for these use
   cases for neither producer or consumer, particular as they relate to
   operational data.

   For the consumer of the telemetry data, there is a requirement to
   associate a schema with the instance-data that will be provided by a
   subscription.  One approach is to fetch and build the entire schema
   for the device, e.g., by fetching YANG library, and then use the
   subscription XPath to select the relevant subtree of the schema that
   applies only to the subscription.  The problem with this approach is
   that if the schema ever changes, e.g., after a software update, then
   it is reasonably likely of some changes occurring with the global
   device schema even if there are no changes to the schema subtree
   under the subscription path.  Hence, it would be helpful to identify
   and version the schema associated with a particular subscription
   path, and also to encoded the instance data relatively to the
   subscription path rather than as an absolute path from the root of
   the operational datastore.

3.1.  Introduction text from Subscribed Notifications (RFC 8639)

   This document defines a YANG data model and associated mechanisms
   enabling subscriber-specific subscriptions to a publisher's event
   streams.  This effectively enables a "subscribe, then publish"
   capability where the customized information needs and access
   permissions of each target receiver are understood by the publisher
   before subscribed event records are marshaled and pushed.  The
   receiver then gets a continuous, customized feed of publisher-
   generated information.

   While the functionality defined in this document is transport
   agnostic, transports like the Network Configuration Protocol
   (NETCONF) [RFC6241] or RESTCONF [RFC8040] can be used to configure or
   dynamically signal subscriptions.  Bindings for subscribed event
   record delivery for NETCONF and RESTCONF are defined in [RFC8640] and
   [RESTCONF-Notif], respectively.




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   The YANG data model defined in this document conforms to the Network
   Management Datastore Architecture defined in [RFC8342].

3.1.1.  Motivation

   Various limitations to subscriptions as described in [RFC5277] were
   alleviated to some extent by the requirements provided in [RFC7923].
   Resolving any remaining issues is the primary motivation for this
   work.  Key capabilities supported by this document include:

   *  multiple subscriptions on a single transport session

   *  support for dynamic and configured subscriptions

   *  modification of an existing subscription in progress

   *  per-subscription operational counters

   *  negotiation of subscription parameters (through the use of hints
      returned as part of declined subscription requests)

   *  subscription state change notifications (e.g., publisher-driven
      suspension, parameter modification)

   *  independence from transport

3.2.  Introduction text from YANG Push (RFC 8641)

   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:

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

   *  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.

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







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   *  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.

4.  Conventions and 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.

4.1.  Definitions taken from Subscribed Notifications (RFC 8639)

   *  Client: Defined in [RFC8342].

   *  Configuration: Defined in [RFC8342].

   *  Configuration datastore: Defined in [RFC8342].

   *  Configured subscription: A subscription installed via
      configuration into a configuration datastore.

   *  Dynamic subscription: A subscription created dynamically by a
      subscriber via a Remote Procedure Call (RPC).

   *  Event: An occurrence of something that may be of interest.
      Examples include a configuration change, a fault, a change in
      status, crossing a threshold, or an external input to the system.

   *  Event occurrence time: A timestamp matching the time an
      originating process identified as when an event happened.




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   *  Event record: A set of information detailing an event.

   *  Event stream: A continuous, chronologically ordered set of events
      aggregated under some context.

   *  Event stream filter: Evaluation criteria that may be applied
      against event records in an event stream.  Event records pass the
      filter when specified criteria are met.

   *  Notification message: Information intended for a receiver
      indicating that one or more events have occurred.

   *  Publisher: An entity responsible for streaming notification
      messages per the terms of a subscription.

   *  Receiver: A target to which a publisher pushes subscribed event
      records.  For dynamic subscriptions, the receiver and subscriber
      are the same entity.

   *  Subscriber: A client able to request and negotiate a contract for
      the generation and push of event records from a publisher.  For
      dynamic subscriptions, the receiver and subscriber are the same
      entity.

   *  Subscription: A contract with a publisher, stipulating the
      information that one or more receivers wish to have pushed from
      the publisher without the need for further solicitation.

   All YANG tree diagrams used in this document follow the notation
   defined in [RFC8340].

4.2.  Definitions taken from YANG Push (RFC 8641)

   This document uses the terminology defined in [RFC7950], [RFC8341],
   [RFC8342], and [RFC8639].  In addition, this document defines the
   following terms:

   *  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".

   *  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.

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




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

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

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

   *  Selection filter: Evaluation and/or selection criteria that may be
      applied against a targeted set of objects.

   *  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".

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

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

5.  Subscribed Notifications Solution Overview (all from RFC 8639)

   This document describes a transport-agnostic mechanism for
   subscribing to and receiving content from an event stream in a
   publisher.  This mechanism operates through the use of a
   subscription.

   Two types of subscriptions are supported:

   1.  Dynamic subscriptions, where a subscriber initiates a
       subscription negotiation with a publisher via an RPC.  If the
       publisher is able to serve this request, it accepts it and then
       starts pushing notification messages back to the subscriber.  If
       the publisher is not able to serve it as requested, then an error
       response is returned.  This response MAY include hints for
       subscription parameters that, had they been present, may have
       enabled the dynamic subscription request to be accepted.






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   2.  Configured subscriptions, which allow the management of
       subscriptions via a configuration so that a publisher can send
       notification messages to a receiver.  Support for configured
       subscriptions is optional, with its availability advertised via a
       YANG feature.

   Additional characteristics differentiating configured from dynamic
   subscriptions include the following:

   *  The lifetime of a dynamic subscription is bound by the transport
      session used to establish it.  For connection-oriented stateful
      transports like NETCONF, the loss of the transport session will
      result in the immediate termination of any associated dynamic
      subscriptions.  For connectionless or stateless transports like
      HTTP, a lack of receipt acknowledgment of a sequential set of
      notification messages and/or keep-alives can be used to trigger a
      termination of a dynamic subscription.  Contrast this to the
      lifetime of a configured subscription.  This lifetime is driven by
      relevant configuration being present in the publisher's applied
      configuration.  Being tied to configuration operations implies
      that (1) configured subscriptions can be configured to persist
      across reboots and (2) a configured subscription can persist even
      when its publisher is fully disconnected from any network.

   *  Configured subscriptions can be modified by any configuration
      client with write permission on the configuration of the
      subscription.  Dynamic subscriptions can only be modified via an
      RPC request made by the original subscriber or by a change to
      configuration data referenced by the subscription.

   Note that there is no mixing and matching of dynamic and configured
   operations on a single subscription.  Specifically, a configured
   subscription cannot be modified or deleted using RPCs defined in this
   document.  Similarly, a dynamic subscription cannot be directly
   modified or deleted by configuration operations.  It is, however,
   possible to perform a configuration operation that indirectly impacts
   a dynamic subscription.  By changing the value of a preconfigured
   filter referenced by an existing dynamic subscription, the selected
   event records passed to a receiver might change.

   Also note that transport-specific specifications based on this
   specification MUST detail the lifecycle of dynamic subscriptions as
   well as the lifecycle of configured subscriptions (if supported).








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   A publisher MAY terminate a dynamic subscription at any time.
   Similarly, it MAY decide to temporarily suspend the sending of
   notification messages for any dynamic subscription, or for one or
   more receivers of a configured subscription.  Such termination or
   suspension is driven by internal considerations of the publisher.

5.1.  Relationship to RFC 5277

   This document is intended to provide a superset of the subscription
   capabilities initially defined in [RFC5277].  It is important to
   understand what has been reused and what has been replaced,
   especially when extending an existing implementation that is based on
   [RFC5277].  Key relationships between these two documents include the
   following:

   *  This document defines a transport-independent capability;
      [RFC5277] is specific to NETCONF.

   *  For the new operations, the data model defined in this document is
      used instead of the data model defined in Section 3.4 of
      [RFC5277].

   *  The RPC operations in this document replace the operation <create-
      subscription> as defined in [RFC5277], Section 4.

   *  The <notification> message of [RFC5277], Section 4 is used.

   *  The included contents of the "NETCONF" event stream are identical
      between this document and [RFC5277].

   *  A publisher MAY implement both the Notification Management Schema
      and RPCs defined in [RFC5277] and this document concurrently.

   *  Unlike [RFC5277], this document enables a single transport session
      to intermix notification messages and RPCs for different
      subscriptions.

   *  A subscription "stop-time" can be specified as part of a
      notification replay.  This supports a capability analogous to the
      <stopTime> parameter of [RFC5277].  However, in this
      specification, a "stop-time" parameter can also be applied without
      replay.

5.2.  Solution

   Per the overview provided in Section 1.3, this section details the
   overall context, state machines, and subsystems that may be assembled
   to allow the subscription of events from a publisher.



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5.2.1.  Event Streams

   An event stream is a named entity on a publisher; this entity exposes
   a continuously updating set of YANG-defined event records.  An event
   record is an instantiation of a "notification" YANG statement.  If
   the "notification" is defined as a child to a data node, the
   instantiation includes the hierarchy of nodes that identifies the
   data node in the datastore (see Section 7.16.2 of [RFC7950]).  Each
   event stream is available for subscription.  Identifying a) how event
   streams are defined (other than the NETCONF stream), b) how event
   records are defined/generated, and c) how event records are assigned
   to event streams is out of scope for this document.

   There is only one reserved event stream name in this document:
   "NETCONF".  The "NETCONF" event stream contains all NETCONF event
   record information supported by the publisher, except where an event
   record has explicitly been excluded from the stream.  Beyond the
   "NETCONF" stream, implementations MAY define additional event
   streams.

   As YANG-defined event records are created by a system, they may be
   assigned to one or more streams.  The event record is distributed to
   a subscription's receiver(s) where (1) a subscription includes the
   identified stream and (2) subscription filtering does not exclude the
   event record from that receiver.

   Access control permissions may be used to silently exclude event
   records from an event stream for which the receiver has no read
   access.  See [RFC8341], Section 3.4.6 for an example of how this
   might be accomplished.  Note that per Section 2.7 of this document,
   subscription state change notifications are never filtered out.

   If no access control permissions are in place for event records on an
   event stream, then a receiver MUST be allowed access to all the event
   records.  If subscriber permissions change during the lifecycle of a
   subscription and event stream access is no longer permitted, then the
   subscription MUST be terminated.

   Event records MUST NOT be delivered to a receiver in a different
   order than the order in which they were placed on an event stream.











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5.2.2.  Event Stream Filters

   This document defines an extensible filtering mechanism.  The filter
   itself is a boolean test that is placed on the content of an event
   record.  A "false" filtering result causes the event record to be
   excluded from delivery to a receiver.  A filter never results in
   information being stripped from an event record prior to that event
   record being encapsulated in a notification message.  The two
   optional event stream filtering syntaxes supported are [XPATH] and
   subtree [RFC6241].

   If no event stream filter is provided in a subscription, all event
   records on an event stream are to be sent.

5.2.3.  QoS

   This document provides for several Quality of Service (QoS)
   parameters.  These parameters indicate the treatment of a
   subscription relative to other traffic between publisher and
   receiver.  Included are:

   *  A "dscp" marking to differentiate prioritization of notification
      messages during network transit.

   *  A "weighting" so that bandwidth proportional to this weighting can
      be allocated to this subscription relative to other subscriptions.

   *  A "dependency" upon another subscription.

   If the publisher supports the "dscp" feature, then a subscription
   with a "dscp" leaf MUST result in a corresponding Differentiated
   Services Code Point (DSCP) marking [RFC2474] being placed in the IP
   header of any resulting notification messages and subscription state
   change notifications.  A publisher MUST respect the DSCP markings for
   subscription traffic egressing that publisher.

   Different DSCP code points require different transport connections.
   As a result, where TCP is used, a publisher that supports the "dscp"
   feature must ensure that a subscription's notification messages are
   returned in a single TCP transport session where all traffic shares
   the subscription's "dscp" leaf value.  If this cannot be guaranteed,
   any "establish-subscription" RPC request SHOULD be rejected with a
   "dscp-unavailable" error.








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   For the "weighting" parameter, when concurrently dequeuing
   notification messages from multiple subscriptions to a receiver, the
   publisher MUST allocate bandwidth to each subscription proportional
   to the weights assigned to those subscriptions.  "Weighting" is an
   optional capability of the publisher; support for it is identified
   via the "qos" feature.

   If a subscription has the "dependency" parameter set, then any
   buffered notification messages containing event records selected by
   the parent subscription MUST be dequeued prior to the notification
   messages of the dependent subscription.  If notification messages
   have dependencies on each other, the notification message queued the
   longest MUST go first.  If a "dependency" included in an RPC
   references a subscription that does not exist or is no longer
   accessible to that subscriber, that "dependency" MUST be silently
   removed.  "Dependency" is an optional capability of the publisher;
   support for it is identified via the "qos" feature.

   "Dependency" and "weighting" parameters will only be respected and
   enforced between subscriptions that share the same "dscp" leaf value.

   There are additional types of publisher capacity overload that this
   specification does not address, as they are out of scope.  For
   example, the prioritization of which subscriptions have precedence
   when the publisher CPU is overloaded is not discussed.  As a result,
   implementation choices will need to be made to address such
   considerations.

5.2.4.  Configured Subscriptions

   A configured subscription is a subscription installed via
   configuration.  Configured subscriptions may be modified by any
   configuration client with the proper permissions.  Subscriptions can
   be modified or terminated via configuration at any point during their
   lifetime.  Multiple configured subscriptions MUST be supportable over
   a single transport session.

   Configured subscriptions have several characteristics distinguishing
   them from dynamic subscriptions:

   *  persistence across publisher reboots,

   *  persistence even when transport is unavailable, and

   *  an ability to send notification messages to more than one
      receiver.  (Note that receivers are unaware of the existence of
      any other receivers.)




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   On the publisher, support for configured subscriptions is optional
   and advertised using the "configured" feature.  On a receiver of a
   configured subscription, support for dynamic subscriptions is
   optional.  However, if replaying missed event records is required for
   a configured subscription, support for dynamic subscription is highly
   recommended.  In this case, a separate dynamic subscription can be
   established to retransmit the missing event records.

   In addition to the subscription parameters available to dynamic
   subscriptions as described in Section 2.4.2, the following additional
   parameters are also available to configured subscriptions:

   *  A "transport", which identifies the transport protocol to use to
      connect with all subscription receivers.

   *  One or more receivers, each intended as the destination for event
      records.  Note that each individual receiver is identifiable by
      its "name".

   *  Optional parameters to identify where traffic should egress a
      publisher:

      -  A "source-interface", which identifies the egress interface to
         use from the publisher.  Publisher support for this parameter
         is optional and advertised using the "interface-designation"
         feature.

      -  A "source-address" address, which identifies the IP address to
         stamp on notification messages destined for the receiver.

      -  A "source-vrf", which identifies the Virtual Routing and
         Forwarding (VRF) instance on which to reach receivers.  This
         VRF is a network instance as defined in [RFC8529].  Publisher
         support for VRFs is optional and advertised using the
         "supports-vrf" feature.

      If none of the above parameters are set, notification messages
      MUST egress the publisher's default interface.

   A tree diagram that includes these parameters is provided in
   Figure 20 in Section 3.3.  These parameters are described in the YANG
   module in Section 4.









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5.2.4.1.  Configured Subscription State Machine

   Below is the state machine for a configured subscription on the
   publisher.  This state machine describes the three states ("valid",
   "invalid", and "concluded") as well as the transitions between these
   states.  Start and end states are depicted to reflect configured
   subscription creation and deletion events.  The creation or
   modification of a configured subscription initiates an evaluation by
   the publisher to determine if the subscription is in the "valid"
   state or the "invalid" state.  The publisher uses its own criteria in
   making this determination.  If in the "valid" state, the subscription
   becomes operational.  See (1) in the diagram below.

 .........
 : start :-.
 :.......: |
      create  .---modify-----.----------------------------------.
           |  |              |                                  |
           V  V          .-------.         .......         .---------.
  .----[evaluate]--no--->|invalid|-delete->: end :<-delete-|concluded|
  |                      '-------'         :.....:         '---------'
  |-[evaluate]--no-(2).      ^                ^                 ^
  |        ^          |      |                |                 |
 yes       |          '->unsupportable      delete           stop-time
  |      modify         (subscription-   (subscription-   (subscription-
  |        |             terminated*)     terminated*)      concluded*)
  |        |                 |                |                 |
 (1)       |                (3)              (4)               (5)
  |   .---------------------------------------------------------------.
  '-->|                         valid                                 |
      '---------------------------------------------------------------'


 Legend:
   Dotted boxes: subscription added or removed via configuration
   Dashed boxes: states for a subscription
   [evaluate]: decision point on whether the subscription
               is supportable
   (*): resulting subscription state change notification

     Figure 8: Publisher's State Machine for a Configured Subscription

   A subscription in the "valid" state may move to the "invalid" state
   in one of two ways.  First, it may be modified in a way that fails a
   re-evaluation.  See (2) in the diagram.  Second, the publisher might
   determine that the subscription is no longer supportable.  This could
   be because of an unexpected but sustained increase in an event
   stream's event records, degraded CPU capacity, a more complex



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   referenced filter, or other subscriptions that have usurped
   resources.  See (3) in the diagram.  No matter the case, a
   "subscription-terminated" notification is sent to any receivers in
   the "active" or "suspended" state.  A subscription in the "valid"
   state may also transition to the "concluded" state via (5) if a
   configured stop time has been reached.  In this case, a
   "subscription-concluded" notification is sent to any receivers in the
   "active" or "suspended" state.  Finally, a subscription may be
   deleted by configuration (4).

   When a subscription is in the "valid" state, a publisher will attempt
   to connect with all receivers of a configured subscription and
   deliver notification messages.  Below is the state machine for each
   receiver of a configured subscription.  This receiver state machine
   is fully contained in the state machine of the configured
   subscription and is only relevant when the configured subscription is
   in the "valid" state.

     .-----------------------------------------------------------------.
     |                         valid                                   |
     |   .----------.                           .------------.         |
     |   | receiver |---timeout---------------->|  receiver  |         |
     |   |connecting|<----------------reset--(c)|disconnected|         |
     |   |          |<-transport                '------------'         |
     |   '----------'  loss,reset------------------------------.       |
     |      (a)          |                                     |       |
     |  subscription-   (b)                                   (b)      |
     |  started*    .--------.                             .---------. |
     |       '----->|        |(d)-insufficient CPU,------->|         | |
     |              |receiver|    buffer overflow          |receiver | |
     | subscription-| active |                             |suspended| |
     |   modified*  |        |<----CPU, b/w sufficient,-(e)|         | |
     |        '---->'--------'     subscription-modified*  '---------' |
     '-----------------------------------------------------------------'

     Legend:
       Dashed boxes that include the word "receiver" show the possible
       states for an individual receiver of a valid configured
       subscription.

      * indicates a subscription state change notification

      Figure 9: Receiver State Machine for a Configured Subscription
                              on a Publisher

   When a configured subscription first moves to the "valid" state, the
   "state" leaf of each receiver is initialized to the "connecting"
   state.  If transport connectivity is not available to any receivers



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   and there are any notification messages to deliver, a transport
   session is established (e.g., per [RFC8071]).  Individual receivers
   are moved to the "active" state when a "subscription-started"
   subscription state change notification is successfully passed to that
   receiver (a).  Event records are only sent to active receivers.
   Receivers of a configured subscription remain active on the publisher
   if both (1) transport connectivity to the receiver is active and (2)
   event records are not being dropped due to a publisher's sending
   capacity being reached.  In addition, a configured subscription's
   receiver MUST be moved to the "connecting" state if the receiver is
   reset via the "reset" action (b), (c).  For more on the "reset"
   action, see Section 2.5.5.  If transport connectivity cannot be
   achieved while in the "connecting" state, the receiver MAY be moved
   to the "disconnected" state.

   A configured subscription's receiver MUST be moved to the "suspended"
   state if there is transport connectivity between the publisher and
   receiver but (1) delivery of notification messages is failing due to
   a publisher's buffer capacity being reached or (2) notification
   messages cannot be generated for that receiver due to insufficient
   CPU (d).  This is indicated to the receiver by the "subscription-
   suspended" subscription state change notification.

   A configured subscription's receiver MUST be returned to the "active"
   state from the "suspended" state when notification messages can be
   generated, bandwidth is sufficient to handle the notification
   messages, and a receiver has successfully been sent a "subscription-
   resumed" or "subscription-modified" subscription state change
   notification (e).  The choice as to which of these two subscription
   state change notifications is sent is determined by whether the
   subscription was modified during the period of suspension.

   Modification of a configured subscription is possible at any time.  A
   "subscription-modified" subscription state change notification will
   be sent to all active receivers, immediately followed by notification
   messages conforming to the new parameters.  Suspended receivers will
   also be informed of the modification.  However, this notification
   will await the end of the suspension for that receiver (e).

   The mechanisms described above are mirrored in the RPCs and
   notifications defined in this document.  It should be noted that
   these RPCs and notifications have been designed to be extensible and
   allow subscriptions into targets other than event streams.  For
   instance, the YANG module defined in Section 5 of [RFC8641] augments
   "/sn:modify-subscription/sn:input/sn:target".






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5.2.4.2.  Creating a Configured Subscription

   Configured subscriptions are established using configuration
   operations against the top-level "subscriptions" subtree.

   Because there is no explicit association with an existing transport
   session, configuration operations MUST include additional parameters
   beyond those of dynamic subscriptions.  These parameters identify
   each receiver, how to connect with that receiver, and possibly
   whether the notification messages need to come from a specific egress
   interface on the publisher.  Receiver-specific transport connectivity
   parameters MUST be configured via transport-specific augmentations to
   this specification.  See Section 2.5.7 for details.

   After a subscription is successfully established, the publisher
   immediately sends a "subscription-started" subscription state change
   notification to each receiver.  It is quite possible that upon
   configuration, reboot, or even steady-state operations, a transport
   session may not be currently available to the receiver.  In this
   case, when there is something to transport for an active
   subscription, transport-specific "call home" operations [RFC8071]
   will be used to establish the connection.  When transport
   connectivity is available, notification messages may then be pushed.

   With active configured subscriptions, it is allowable to buffer event
   records even after a "subscription-started" has been sent.  However,
   if events are lost (rather than just delayed) due to replay buffer
   capacity being reached, a new "subscription-started" must be sent.
   This new "subscription-started" indicates an event record
   discontinuity.

   To see an example of subscription creation using configuration
   operations over NETCONF, see Appendix A.

5.2.4.3.  Modifying a Configured Subscription

   Configured subscriptions can be modified using configuration
   operations against the top-level "subscriptions" subtree.

   If the modification involves adding receivers, added receivers are
   placed in the "connecting" state.  If a receiver is removed, the
   subscription state change notification "subscription-terminated" is
   sent to that receiver if that receiver is active or suspended.

   If the modification involves changing the policies for the
   subscription, the publisher sends to currently active receivers a
   "subscription-modified" notification.  For any suspended receivers, a
   "subscription-modified" notification will be delayed until the



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   receiver's subscription has been resumed.  (Note: In this case, the
   "subscription-modified" notification informs the receiver that the
   subscription has been resumed, so no additional "subscription-
   resumed" need be sent.  Also note that if multiple modifications have
   occurred during the suspension, only the "subscription-modified"
   notification describing the latest one need be sent to the receiver.)

5.2.4.4.  Deleting a Configured Subscription

   Subscriptions can be deleted through configuration against the top-
   level "subscriptions" subtree.

   Immediately after a subscription is successfully deleted, the
   publisher sends to all receivers of that subscription a subscription
   state change notification stating that the subscription has ended
   (i.e., "subscription-terminated").

5.2.4.5.  Resetting a Configured Subscription's Receiver

   It is possible that a configured subscription to a receiver needs to
   be reset.  This is accomplished via the "reset" action in the YANG
   module at "/subscriptions/subscription/receivers/receiver/reset".
   This action may be useful in cases where a publisher has timed out
   trying to reach a receiver.  When such a reset occurs, a transport
   session will be initiated if necessary, and a new "subscription-
   started" notification will be sent.  This action does not have any
   effect on transport connectivity if the needed connectivity already
   exists.

5.2.4.6.  Replay for a Configured Subscription

   It is possible to do replay on a configured subscription.  This is
   supported via the configuration of the "configured-replay" object on
   the subscription.  The setting of this object enables the streaming
   of the buffered event records for the subscribed event stream.  All
   buffered event records that have been retained since the last
   publisher restart will be sent to each configured receiver.

   Replay of event records created since restart is useful.  It allows
   event records generated before transport connectivity establishment
   to be passed to a receiver.  Setting the restart time as the earliest
   configured replay time precludes the possibility of resending event
   records that were logged prior to publisher restart.  It also ensures
   that the same records will be sent to each configured receiver,
   regardless of the speed of transport connectivity establishment to
   each receiver.  Finally, by establishing restart as the earliest
   potential time for event records to be included in notification
   messages, a well-understood timeframe for replay is defined.



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   As a result, when any configured subscription's receivers become
   active, buffered event records will be sent immediately after the
   "subscription-started" notification.  If the publisher knows the last
   event record sent to a receiver and the publisher has not rebooted,
   the next event record on the event stream that meets filtering
   criteria will be the leading event record sent.  Otherwise, the
   leading event record will be the first event record meeting filtering
   criteria subsequent to the latest of three different times: the
   "replay-log-creation-time", the "replay-log-aged-time", or the most
   recent publisher boot time.  The "replay-log-creation-time" and
   "replay-log-aged-time" are discussed in Section 2.4.2.1.  The most
   recent publisher boot time ensures that duplicate event records are
   not replayed from a previous time the publisher was booted.

   It is quite possible that a receiver might want to retrieve event
   records from an event stream prior to the latest boot.  If such
   records exist where there is a configured replay, the publisher MUST
   send the time of the event record immediately preceding the "replay-
   start-time" in the "replay-previous-event-time" leaf.  Through the
   existence of the "replay-previous-event-time", the receiver will know
   that earlier events prior to reboot exist.  In addition, if the
   subscriber was previously receiving event records with the same
   subscription "id", the receiver can determine if there was a time gap
   where records generated on the publisher were not successfully
   received.  And with this information, the receiver may choose to
   dynamically subscribe to retrieve any event records placed in the
   event stream before the most recent boot time.

   All other replay functionality remains the same as with dynamic
   subscriptions as described in Section 2.4.2.1.

5.2.4.7.  Transport Connectivity for a Configured Subscription

   This specification is transport independent.  However, supporting a
   configured subscription will often require the establishment of
   transport connectivity.  And the parameters used for this transport
   connectivity establishment are transport specific.  As a result, the
   YANG module defined in Section 4 is not able to directly define and
   expose these transport parameters.

   It is necessary for an implementation to support the connection
   establishment process.  To support this function, the YANG data model
   defined in this document includes a node where transport-specific
   parameters for a particular receiver may be augmented.  This node is
   "/subscriptions/subscription/receivers/receiver".  By augmenting
   transport parameters from this node, system developers are able to
   incorporate the YANG objects necessary to support the transport
   connectivity establishment process.



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   The result of this is the following requirement.  A publisher
   supporting the feature "configured" MUST also support at least one
   YANG data model that augments transport connectivity parameters on
   "/subscriptions/subscription/receivers/receiver".  For an example of
   such an augmentation, see Appendix A.

5.2.5.  Dynamic Subscriptions

   Dynamic subscriptions are managed via protocol operations (in the
   form of RPCs, per [RFC7950], Section 7.14) made against targets
   located in the publisher.  These RPCs have been designed extensibly
   so that they may be augmented for subscription targets beyond event
   streams.  For examples of such augmentations, see the RPC
   augmentations in the YANG data model provided in [RFC8641].

5.2.5.1.  Dynamic Subscription State Machine

   Below is the publisher's state machine for a dynamic subscription.
   Each state is shown in its own box.  It is important to note that
   such a subscription doesn't exist at the publisher until an
   "establish-subscription" RPC is accepted.  The mere request by a
   subscriber to establish a subscription is not sufficient for that
   subscription to be externally visible.  Start and end states are
   depicted to reflect subscription creation and deletion events.

                     .........
                     : start :
                     :.......:
                         |
                 establish-subscription
                         |
                         |   .-------modify-subscription--------.
                         v   v                                  |
                   .-----------.                          .-----------.
         .--------. | receiver  |--insufficient CPU, b/w-->| receiver  |
     modify-       '|  active   |                          | suspended |
     subscription   |           |<----CPU, b/w sufficient--|           |
         ---------->'-----------'                          '-----------'
                         |                                      |
               delete/kill-subscription                     delete/kill-
                         |                                 subscription
                         v                                      |
                     .........                                  |
                     :  end  :<---------------------------------'
                     :.......:

     Figure 1: Publisher's State Machine for a Dynamic Subscription




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   Of interest in this state machine are the following:

   o Successful "establish-subscription" or "modify-subscription" RPCs
   move the subscription to the "active" state.

   o Failed "modify-subscription" RPCs will leave the subscription in
   its previous state, with no visible change to any streaming updates.

   o A "delete-subscription" or "kill-subscription" RPC will end the
   subscription, as will reaching a "stop-time".

   o A publisher may choose to suspend a subscription when there is not
   sufficient CPU or bandwidth available to service the subscription.
   This is announced to the subscriber via the "subscription- suspended"
   subscription state change notification.

   o A suspended subscription may be modified by the subscriber (for
   example, in an attempt to use fewer resources).  Successful
   modification returns the subscription to the "active" state.

   o Even without a "modify-subscription" request, a publisher may
   return a subscription to the "active" state when sufficient resources
   are again available.  This is announced to the subscriber via the
   "subscription-resumed" subscription state change notification.

5.2.5.2.  Establishing a Dynamic Subscription

   The "establish-subscription" RPC allows a subscriber to request the
   creation of a subscription.

   The input parameters of the operation are:

   o A "stream" name, which identifies the targeted event stream against
   which the subscription is applied.

   o An event stream filter, which may reduce the set of event records
   pushed.

   o If the transport used by the RPC supports multiple encodings, an
   optional "encoding" for the event records pushed.  If no "encoding"
   is included, the encoding of the RPC MUST be used.

   o An optional "stop-time" for the subscription.  If no "stop-time" is
   present, notification messages will continue to be sent until the
   subscription is terminated.






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   o An optional "replay-start-time" for the subscription.  The "replay-
   start-time" MUST be in the past and indicates that the subscription
   is requesting a replay of previously generated information from the
   event stream.  For more on replay, see Section 2.4.2.1.  If there is
   no "replay-start-time", the subscription starts immediately.

   If the publisher can satisfy the "establish-subscription" request, it
   replies with an identifier for the subscription and then immediately
   starts streaming notification messages.

   Below is a tree diagram for "establish-subscription".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---x establish-subscription
         +---w input
         |  +---w (target)
         |  |  +--:(stream)
         |  |     +---w (stream-filter)?
         |  |     |  +--:(by-reference)
         |  |     |  |  +---w stream-filter-name
         |  |     |  |          stream-filter-ref
         |  |     |  +--:(within-subscription)
         |  |     |     +---w (filter-spec)?
         |  |     |        +--:(stream-subtree-filter)
         |  |     |        |  +---w stream-subtree-filter?   <anydata>
         |  |     |        |          {subtree}?
         |  |     |        +--:(stream-xpath-filter)
         |  |     |           +---w stream-xpath-filter?
         |  |     |                   yang:xpath1.0 {xpath}?
         |  |     +---w stream                               stream-ref
         |  |     +---w replay-start-time?
         |  |             yang:date-and-time {replay}?
         |  +---w stop-time?
         |  |       yang:date-and-time
         |  +---w dscp?                                      inet:dscp
         |  |       {dscp}?
         |  +---w weighting?                                 uint8
         |  |       {qos}?
         |  +---w dependency?
         |  |       subscription-id {qos}?
         |  +---w encoding?                                  encoding
         +--ro output
             +--ro id                            subscription-id
             +--ro replay-start-time-revision?   yang:date-and-time
                     {replay}?

           Figure 2: "establish-subscription" RPC Tree Diagram




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   A publisher MAY reject the "establish-subscription" RPC for many
   reasons, as described in Section 2.4.6.  The contents of the
   resulting RPC error response MAY include details on input parameters
   that, if considered in a subsequent "establish-subscription" RPC, may
   result in successful subscription establishment.  Any such hints MUST
   be transported in a yang-data "establish-subscription-stream-error-
   info" container included in the RPC error response.

   Below is a tree diagram for "establish-subscription-stream-error-
   info" RPC yang-data.  All objects contained in this tree are
   described in the YANG module in Section 4.

       yang-data establish-subscription-stream-error-info
         +--ro establish-subscription-stream-error-info
           +--ro reason?                   identityref
           +--ro filter-failure-hint?      string

           Figure 3: "establish-subscription-stream-error-info"
                       RPC yang-data Tree Diagram

5.2.5.2.1.  Requesting a Replay of Event Records

   Replay provides the ability to establish a subscription that is also
   capable of passing event records generated in the recent past.  In
   other words, as the subscription initializes itself, it sends any
   event records in the target event stream that meet the filter
   criteria that have an event time that is after the "replay-start-
   time" and also have an event time before the "stop-time" should this
   "stop-time" exist.  The end of these historical event records is
   identified via a "replay-completed" subscription state change
   notification.  Any event records generated since the subscription
   establishment may then follow.  For a particular subscription, all
   event records will be delivered in the order in which they are placed
   in the event stream.

   Replay is an optional feature that is dependent on an event stream
   supporting some form of logging.  This document puts no restrictions
   on the size or form of the log, where it resides in the publisher, or
   when event record entries in the log are purged.

   The inclusion of a "replay-start-time" in an "establish-subscription"
   RPC indicates a replay request.  If the "replay-start-time" contains
   a value that is earlier than what a publisher's retained history
   supports, then if the subscription is accepted, the actual
   publisher's revised start time MUST be set in the returned "replay-
   start-time-revision" object.





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   A "stop-time" parameter may be included in a replay subscription.
   For a replay subscription, the "stop-time" MAY be earlier than the
   current time but MUST be later than the "replay-start-time".

   If the given "replay-start-time" is later than the time marked in any
   event records retained in the replay buffer, then the publisher MUST
   send a "replay-completed" notification immediately after a successful
   "establish-subscription" RPC response.

   If an event stream supports replay, the "replay-support" leaf is
   present in the "/streams/stream" list entry for the event stream.  An
   event stream that does support replay is not expected to have an
   unlimited supply of saved notifications available to accommodate any
   given replay request.  To assess the timeframe available for replay,
   subscribers can read the leafs "replay-log-creation-time" and
   "replay-log-aged-time".  See Figure 18 for the YANG tree and
   Section 4 for the YANG module describing these elements.  The actual
   size of the replay log at any given time is a publisher-specific
   matter.  Control parameters for the replay log are outside the scope
   of this document.

5.2.5.3.  Modifying a Dynamic Subscription

   The "modify-subscription" operation permits changing the terms of an
   existing dynamic subscription.  Dynamic subscriptions can be modified
   any number of times.  Dynamic subscriptions can only be modified via
   this RPC using a transport session connecting to the subscriber.  If
   the publisher accepts the requested modifications, it acknowledges
   success to the subscriber, then immediately starts sending event
   records based on the new terms.

   Subscriptions created by configuration cannot be modified via this
   RPC.  However, configuration may be used to modify objects referenced
   by the subscription (such as a referenced filter).

   Below is a tree diagram for "modify-subscription".  All objects
   contained in this tree are described in the YANG module in Section 4.














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       +---x modify-subscription
         +---w input
             +---w id
             |       subscription-id
             +---w (target)
             |  +--:(stream)
             |     +---w (stream-filter)?
             |        +--:(by-reference)
             |        |  +---w stream-filter-name
             |        |          stream-filter-ref
             |        +--:(within-subscription)
             |           +---w (filter-spec)?
             |              +--:(stream-subtree-filter)
             |              |  +---w stream-subtree-filter?   <anydata>
             |              |          {subtree}?
             |              +--:(stream-xpath-filter)
             |                 +---w stream-xpath-filter?
             |                         yang:xpath1.0 {xpath}?
             +---w stop-time?
                     yang:date-and-time

             Figure 4: "modify-subscription" RPC Tree Diagram

   If the publisher accepts the requested modifications on a currently
   suspended subscription, the subscription will immediately be resumed
   (i.e., the modified subscription is returned to the "active" state).
   The publisher MAY immediately suspend this newly modified
   subscription through the "subscription-suspended" notification before
   any event records are sent.

   If the publisher rejects the RPC request, the subscription remains as
   it was prior to the request.  That is, the request has no impact
   whatsoever.  Rejection of the RPC for any reason is indicated via an
   RPC error as described in Section 2.4.6.  The contents of such a
   rejected RPC MAY include hints on inputs that (if considered) may
   result in a successfully modified subscription.  These hints MUST be
   transported in a yang-data "modify-subscription-stream-error-info"
   container inserted into the RPC error response.

   Below is a tree diagram for "modify-subscription-stream-error-info"
   RPC yang-data.  All objects contained in this tree are described in
   the YANG module in Section 4.









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       yang-data modify-subscription-stream-error-info
         +--ro modify-subscription-stream-error-info
           +--ro reason?                identityref
           +--ro filter-failure-hint?   string

             Figure 5: "modify-subscription-stream-error-info"
                       RPC yang-data Tree Diagram

5.2.5.4.  Deleting a Dynamic Subscription

   The "delete-subscription" operation permits canceling an existing
   subscription.  If the publisher accepts the request and the publisher
   has indicated success, the publisher MUST NOT send any more
   notification messages for this subscription.

   Below is a tree diagram for "delete-subscription".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---x delete-subscription
         +---w input
             +---w id     subscription-id

             Figure 6: "delete-subscription" RPC Tree Diagram

   Dynamic subscriptions can only be deleted via this RPC using a
   transport session connecting to the subscriber.  Configured
   subscriptions cannot be deleted using RPCs.

5.2.5.5.  Killing a Dynamic Subscription

   The "kill-subscription" operation permits an operator to end a
   dynamic subscription that is not associated with the transport
   session used for the RPC.  A publisher MUST terminate any dynamic
   subscription identified by the "id" parameter in the RPC request, if
   such a subscription exists.

   Configured subscriptions cannot be killed using this RPC.  Instead,
   configured subscriptions are deleted as part of regular configuration
   operations.  Publishers MUST reject any RPC attempt to kill a
   configured subscription.

   Below is a tree diagram for "kill-subscription".  All objects
   contained in this tree are described in the YANG module in Section 4.








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       +---x kill-subscription
         +---w input
             +---w id     subscription-id

             Figure 7: "kill-subscription" RPC Tree Diagram

5.2.5.6.  RPC Failures

   Whenever an RPC is unsuccessful, the publisher returns relevant
   information as part of the RPC error response.  Transport-level error
   processing MUST be done before the RPC error processing described in
   this section.  In all cases, RPC error information returned by the
   publisher will use existing transport-layer RPC structures, such as
   those seen with NETCONF (Appendix A of [RFC6241]) or RESTCONF
   (Section 7.1 of [RFC8040]).  These structures MUST be able to encode
   subscription-specific errors identified below and defined in this
   document's YANG data model.

   As a result of this variety, how subscription errors are encoded in
   an RPC error response is transport dependent.  Valid errors that can
   occur for each RPC are as follows:

     establish-subscription         modify-subscription
     ----------------------         ----------------------
     dscp-unavailable               filter-unsupported
     encoding-unsupported           insufficient-resources
     filter-unsupported             no-such-subscription
     insufficient-resources
     replay-unsupported

     delete-subscription            kill-subscription
     ----------------------         ----------------------
     no-such-subscription           no-such-subscription

   To see a NETCONF-based example of an error response from the list
   above, see the "no-such-subscription" error response illustrated in
   [RFC8640], Figure 10.

   There is one final set of transport-independent RPC error elements
   included in the YANG data model defined in this document: three yang-
   data structures that enable the publisher to provide to the receiver
   any error information that does not fit into existing transport-layer
   RPC structures.  These structures are:








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   1.  "establish-subscription-stream-error-info": This MUST be returned
       with the leaf "reason" populated if an RPC error reason has not
       been placed elsewhere in the transport portion of a failed
       "establish-subscription" RPC response.  This MUST be sent if
       hints on how to overcome the RPC error are included.

   2.  "modify-subscription-stream-error-info": This MUST be returned
       with the leaf "reason" populated if an RPC error reason has not
       been placed elsewhere in the transport portion of a failed
       "modify-subscription" RPC response.  This MUST be sent if hints
       on how to overcome the RPC error are included.

   3.  "delete-subscription-error-info": This MUST be returned with the
       leaf "reason" populated if an RPC error reason has not been
       placed elsewhere in the transport portion of a failed "delete-
       subscription" or "kill-subscription" RPC response.

5.2.6.  Event Record Delivery

   Whether dynamic or configured, once a subscription has been set up,
   the publisher streams event records via notification messages per the
   terms of the subscription.  For dynamic subscriptions, notification
   messages are sent over the session used to establish the
   subscription.  For configured subscriptions, notification messages
   are sent over the connections specified by the transport and each
   receiver of a configured subscription.

   A notification message is sent to a receiver when an event record is
   not blocked by either the specified filter criteria or receiver
   permissions.  This notification message MUST include an <eventTime>
   object, as shown in [RFC5277], Section 4.  This <eventTime> MUST be
   at the top level of a YANG structured event record.

   The following example of XML [W3C.REC-xml-20081126], adapted from
   Section 4.2.10 of [RFC7950], illustrates a compliant message:

     <notification
             xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
         <eventTime>2007-09-01T10:00:00Z</eventTime>
         <link-failure xmlns="https://acme.example.com/system">
             <if-name>so-1/2/3.0</if-name>
             <if-admin-status>up</if-admin-status>
             <if-oper-status>down</if-oper-status>
         </link-failure>
     </notification>

               Figure 10: Subscribed Notification Message




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   [RFC5277], Section 2.2.1 states that a notification message is to be
   sent to a subscriber that initiated a <create-subscription>.  With
   this document, this statement from [RFC5277] should be more broadly
   interpreted to mean that notification messages can also be sent to a
   subscriber that initiated an "establish-subscription" or to a
   configured receiver that has been sent a "subscription-started".

   When a dynamic subscription has been started or modified with
   "establish-subscription" or "modify-subscription", respectively,
   event records matching the newly applied filter criteria MUST NOT be
   sent until after the RPC reply has been sent.

   When a configured subscription has been started or modified, event
   records matching the newly applied filter criteria MUST NOT be sent
   until after the "subscription-started" or "subscription-modified"
   notification has been sent, respectively.

5.2.7.  Subscription State Change Notifications

   In addition to sending event records to receivers, a publisher MUST
   also send subscription state change notifications when events related
   to subscription management have occurred.

   Subscription state change notifications are unlike other
   notifications in that they are never included in any event stream.
   Instead, they are inserted (as defined in this section) into the
   sequence of notification messages sent to a particular receiver.
   Subscription state change notifications cannot be dropped or filtered
   out, they cannot be stored in replay buffers, and they are delivered
   only to impacted receivers of a subscription.  The identification of
   subscription state change notifications is easy to separate from
   other notification messages through the use of the YANG extension
   "subscription-state-notif".  This extension tags a notification as a
   subscription state change notification.

   The complete set of subscription state change notifications is
   described in the following subsections.

5.2.7.1.  "subscription-started"

   This notification indicates that a configured subscription has
   started, and event records may be sent.  Included in this
   subscription state change notification are all the parameters of the
   subscription, except for (1) transport connection information for one
   or more receivers and (2) origin information indicating where
   notification messages will egress the publisher.  Note that if a
   referenced filter from the "filters" container has been used in the
   subscription, the notification still provides the contents of that



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   referenced filter under the "within-subscription" subtree.

   Note that for dynamic subscriptions, no "subscription-started"
   notifications are ever sent.

   Below is a tree diagram for "subscription-started".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n subscription-started {configured}?
         +--ro id
         |       subscription-id
         +--ro (target)
         |  +--:(stream)
         |     +--ro (stream-filter)?
         |     |  +--:(by-reference)
         |     |  |  +--ro stream-filter-name
         |     |  |          stream-filter-ref
         |     |  +--:(within-subscription)
         |     |     +--ro (filter-spec)?
         |     |        +--:(stream-subtree-filter)
         |     |        |  +--ro stream-subtree-filter?   <anydata>
         |     |        |          {subtree}?
         |     |        +--:(stream-xpath-filter)
         |     |           +--ro stream-xpath-filter?     yang:xpath1.0
         |     |                   {xpath}?
         |     +--ro stream                               stream-ref
         |     +--ro replay-start-time?
         |     |       yang:date-and-time {replay}?
         |     +--ro replay-previous-event-time?
         |             yang:date-and-time {replay}?
         +--ro stop-time?
         |       yang:date-and-time
         +--ro dscp?                                      inet:dscp
         |       {dscp}?
         +--ro weighting?                                 uint8 {qos}?
         +--ro dependency?
         |       subscription-id {qos}?
         +--ro transport?                                 transport
         |       {configured}?
         +--ro encoding?                                  encoding
         +--ro purpose?                                   string
                 {configured}?

       Figure 11: "subscription-started" Notification Tree Diagram







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5.2.7.2.  "subscription-modified"

   This notification indicates that a subscription has been modified by
   configuration operations.  It is delivered directly after the last
   event records processed using the previous subscription parameters,
   and before any event records processed after the modification.

   Below is a tree diagram for "subscription-modified".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n subscription-modified
         +--ro id
         |       subscription-id
         +--ro (target)
         |  +--:(stream)
         |     +--ro (stream-filter)?
         |     |  +--:(by-reference)
         |     |  |  +--ro stream-filter-name
         |     |  |          stream-filter-ref
         |     |  +--:(within-subscription)
         |     |     +--ro (filter-spec)?
         |     |        +--:(stream-subtree-filter)
         |     |        |  +--ro stream-subtree-filter?   <anydata>
         |     |        |          {subtree}?
         |     |        +--:(stream-xpath-filter)
         |     |           +--ro stream-xpath-filter?     yang:xpath1.0
         |     |                   {xpath}?
         |     +--ro stream                               stream-ref
         |     +--ro replay-start-time?
         |             yang:date-and-time {replay}?
         +--ro stop-time?
         |       yang:date-and-time
         +--ro dscp?                                      inet:dscp
         |       {dscp}?
         +--ro weighting?                                 uint8 {qos}?
         +--ro dependency?
         |       subscription-id {qos}?
         +--ro transport?                                 transport
         |       {configured}?
         +--ro encoding?                                  encoding
         +--ro purpose?                                   string
                 {configured}?

       Figure 12: "subscription-modified" Notification Tree Diagram

   A publisher most often sends this notification directly after the
   modification of any configuration parameters impacting a configured
   subscription.  But it may also be sent at two other times:



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   1.  If a configured subscription has been modified during the
       suspension of a receiver, the notification will be delayed until
       the receiver's suspension is lifted.  In this situation, the
       notification indicates that the subscription has been both
       modified and resumed.

   2.  A "subscription-modified" subscription state change notification
       MUST be sent if the contents of the filter identified by the
       subscription's "stream-filter-ref" leaf have changed.  This state
       change notification is to be sent for a filter change impacting
       any active receivers of a configured or dynamic subscription.

5.2.7.3.  "subscription-terminated"

   This notification indicates that no further event records for this
   subscription should be expected from the publisher.  A publisher may
   terminate the sending of event records to a receiver for the
   following reasons:

   1.  Configuration that removes a configured subscription, or a "kill-
       subscription" RPC that ends a dynamic subscription.  These are
       identified via the reason "no-such-subscription".

   2.  A referenced filter is no longer accessible.  This reason is
       identified by the "filter-unavailable" identity.

   3.  The event stream referenced by a subscription is no longer
       accessible by the receiver.  This reason is identified by the
       "stream-unavailable" identity.

   4.  A suspended subscription has exceeded some timeout.  This reason
       is identified by the "suspension-timeout" identity.

   Each reason listed above derives from the "subscription-terminated-
   reason" base identity specified in the YANG data model in this
   document.

   Below is a tree diagram for "subscription-terminated".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n subscription-terminated
         +--ro id        subscription-id
         +--ro reason    identityref

     Figure 13: "subscription-terminated" Notification Tree Diagram






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   Note: This subscription state change notification MUST be sent to a
   dynamic subscription's receiver when the subscription ends
   unexpectedly.  This might happen when a "kill-subscription" RPC is
   successful or when some other event, not including reaching the
   subscription's "stop-time", results in a publisher choosing to end
   the subscription.

5.2.7.4.  "subscription-suspended"

   This notification indicates that a publisher has suspended the
   sending of event records to a receiver and also indicates the
   possible loss of events.  Suspension happens when capacity
   constraints stop a publisher from serving a valid subscription.  The
   two conditions where this is possible are:

   1.  "insufficient-resources", when a publisher is unable to produce
       the requested event stream of notification messages, and

   2.  "unsupportable-volume", when the bandwidth needed to get
       generated notification messages to a receiver exceeds a
       threshold.

   These conditions are encoded in the "reason" object.  No further
   notifications will be sent until the subscription resumes or is
   terminated.

   Below is a tree diagram for "subscription-suspended".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n subscription-suspended
         +--ro id        subscription-id
         +--ro reason    identityref

       Figure 14: "subscription-suspended" Notification Tree Diagram

5.2.7.5.  "subscription-resumed"

   This notification indicates that a previously suspended subscription
   has been resumed under the unmodified terms previously in place.
   Subscribed event records generated after the issuance of this
   subscription state change notification may now be sent.

   Below is a tree diagram for "subscription-resumed".  All objects
   contained in this tree are described in the YANG module in Section 4.







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       +---n subscription-resumed
         +--ro id    subscription-id

       Figure 15: "subscription-resumed" Notification Tree Diagram

5.2.7.6.  "subscription-completed"

   This notification indicates that a subscription that includes a
   "stop-time" has successfully finished passing event records upon
   reaching that time.

   Below is a tree diagram for "subscription-completed".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n subscription-completed {configured}?
         +--ro id    subscription-id

       Figure 16: "subscription-completed" Notification Tree Diagram

5.2.7.7.  "replay-completed"

   This notification indicates that all of the event records prior to
   the current time have been passed to a receiver.  It is sent before
   any notification messages containing an event record with a timestamp
   later than (1) the "stop-time" or (2) the subscription's start time.

   If a subscription does not contain a "stop-time" or has a "stop-time"
   that has not been reached, then after the "replay-completed"
   notification has been sent, additional event records will be sent in
   sequence as they arise naturally on the publisher.

   Below is a tree diagram for "replay-completed".  All objects
   contained in this tree are described in the YANG module in Section 4.

       +---n replay-completed {replay}?
         +--ro id    subscription-id

         Figure 17: "replay-completed" Notification Tree Diagram

5.2.8.  Subscription Monitoring

   In the operational state datastore, the "subscriptions" container
   maintains the state of all dynamic subscriptions as well as all
   configured subscriptions.  Using datastore retrieval operations
   [RFC8641] or subscribing to the "subscriptions" container
   (Section 3.3) allows the state of subscriptions and their
   connectivity to receivers to be monitored.




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   Each subscription in the operational state datastore is represented
   as a list element.  Included in this list are event counters for each
   receiver, the state of each receiver, and the subscription parameters
   currently in effect.  The appearance of the leaf "configured-
   subscription-state" indicates that a particular subscription came

   into being via configuration.  This leaf also indicates whether the
   current state of that subscription is "valid", "invalid", or
   "concluded".

   To understand the flow of event records in a subscription, there are
   two counters available for each receiver.  The first counter is
   "sent-event-records", which shows the number of events identified for
   sending to a receiver.  The second counter is "excluded-event-
   records", which shows the number of event records not sent to a
   receiver.  "excluded-event-records" shows the combined results of
   both access control and per-subscription filtering.  For configured
   subscriptions, counters are reset whenever the subscription's state
   is evaluated as "valid" (see (1) in Figure 8).

   Dynamic subscriptions are removed from the operational state
   datastore once they expire (reaching "stop-time") or when they are
   terminated.  While many subscription objects are shown as
   configurable, dynamic subscriptions are only included in the
   operational state datastore and as a result are not configurable.

5.2.9.  Support for the "ietf-subscribed-notifications" YANG Module

   Publishers supporting this document MUST indicate support of the YANG
   module "ietf-subscribed-notifications" in the YANG library of the
   publisher.  In addition, if supported, the optional features "encode-
   xml", "encode-json", "configured", "supports-vrf", "qos", "xpath",
   "subtree", "interface-designation", "dscp", and "replay" MUST be
   indicated.

6.  YANG Push Solution Overview (all from RFC 8641)

   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.







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6.1.  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:

   *  The specification of selection filters that identify targeted YANG
      datastore nodes and/or datastore subtrees for which updates are to
      be pushed.

   *  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:

      -  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:























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         o  "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
            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.

         o  "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).

         o  "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.

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

6.1.1.  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



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   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.

6.1.2.  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



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   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.

   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.






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6.1.3.  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 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.

6.1.4.  Data Encodings

6.1.4.1.  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.





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6.1.4.2.  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

   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).

6.1.5.  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.







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   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:

   *  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.

   *  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).

6.1.6.  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



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   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.

   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_.



















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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">
    <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

   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



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   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.

6.1.7.  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].









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           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

   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

6.1.8.  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



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   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.

                         +-----------------+      +--------------------+
     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.






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   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.

6.1.9.  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.

6.1.10.  Other Considerations

6.1.10.1.  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.




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   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:

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

   *  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.

   *  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.

6.1.10.2.  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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6.2.  Subscription State Change Notifications

7.  YANG Data Model Tree Diagrams (from RFC 8639)

   This section contains tree diagrams for nodes defined in Section 4.
   For tree diagrams of subscription state change notifications, see
   Section 2.7.  For the tree diagrams for the RPCs, see Section 2.4.

7.1.  The "streams" Container

   A publisher maintains a list of available event streams as
   operational data.  This list contains both standardized and vendor-
   specific event streams.  This enables subscribers to discover what
   streams a publisher supports.

   Below is a tree diagram for the "streams" container.  All objects
   contained in this tree are described in the YANG module in Section 4.

     +--ro streams
       +--ro stream* [name]
           +--ro name                        string
           +--ro description                 string
           +--ro replay-support?             empty {replay}?
           +--ro replay-log-creation-time    yang:date-and-time
           |       {replay}?
           +--ro replay-log-aged-time?       yang:date-and-time
                   {replay}?

               Figure 18: "streams" Container Tree Diagram

7.2.  The "filters" Container

   The "filters" container maintains a list of all subscription filters
   that persist outside the lifecycle of a single subscription.  This
   enables predefined filters that may be referenced by more than one
   subscription.

   Below is a tree diagram for the "filters" container.  All objects
   contained in this tree are described in the YANG module in Section 4.












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     +--rw filters
       +--rw stream-filter* [name]
           +--rw name                           string
           +--rw (filter-spec)?
             +--:(stream-subtree-filter)
             |  +--rw stream-subtree-filter?   <anydata> {subtree}?
             +--:(stream-xpath-filter)
                 +--rw stream-xpath-filter?     yang:xpath1.0 {xpath}?

               Figure 19: "filters" Container Tree Diagram

7.3.  The "subscriptions" Container

   The "subscriptions" container maintains a list of all subscriptions
   on a publisher, both configured and dynamic.  It can be used to
   retrieve information about the subscriptions that a publisher is
   serving.

   Below is a tree diagram for the "subscriptions" container.  All
   objects contained in this tree are described in the YANG module in
   Section 4.

     +--rw subscriptions
       +--rw subscription* [id]
           +--rw id
           |       subscription-id
           +--rw (target)
           |  +--:(stream)
           |     +--rw (stream-filter)?
           |     |  +--:(by-reference)
           |     |  |  +--rw stream-filter-name
           |     |  |          stream-filter-ref
           |     |  +--:(within-subscription)
           |     |     +--rw (filter-spec)?
           |     |        +--:(stream-subtree-filter)
           |     |        |  +--rw stream-subtree-filter?   <anydata>
           |     |        |          {subtree}?
           |     |        +--:(stream-xpath-filter)
           |     |           +--rw stream-xpath-filter?
           |     |                   yang:xpath1.0 {xpath}?
           |     +--rw stream                               stream-ref
           |     +--ro replay-start-time?
           |     |       yang:date-and-time {replay}?
           |     +--rw configured-replay?                   empty
           |             {configured,replay}?
           +--rw stop-time?
           |       yang:date-and-time
           +--rw dscp?                                      inet:dscp



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           |       {dscp}?
           +--rw weighting?                                 uint8 {qos}?
           +--rw dependency?
           |       subscription-id {qos}?
           +--rw transport?                                 transport
           |       {configured}?
           +--rw encoding?                                  encoding
           +--rw purpose?                                   string
           |       {configured}?
           +--rw (notification-message-origin)? {configured}?
           |  +--:(interface-originated)
           |  |  +--rw source-interface?
           |  |          if:interface-ref {interface-designation}?
           |  +--:(address-originated)
           |     +--rw source-vrf?
           |     |       -> /ni:network-instances/network-instance/name
           |     |       {supports-vrf}?
           |     +--rw source-address?
           |             inet:ip-address-no-zone
           +--ro configured-subscription-state?             enumeration
           |       {configured}?
           +--rw receivers
             +--rw receiver* [name]
                 +--rw name                      string
                 +--ro sent-event-records?
                 |       yang:zero-based-counter64
                 +--ro excluded-event-records?
                 |       yang:zero-based-counter64
                 +--ro state                     enumeration
                 +---x reset {configured}?
                   +--ro output
                       +--ro time    yang:date-and-time

             Figure 20: "subscriptions" Container Tree Diagram

8.  A YANG Data Model for Management of Datastore Push Subscriptions
    (from RFC 8641)

8.1.  Overview

   The YANG data model for datastore push subscriptions is depicted in
   Figures 6 through 9.  The tree diagram that is used follows the
   notation defined in [RFC8340].  New schema objects defined here
   (i.e., beyond those from [RFC8639]) are identified with "yp".  For
   the reader's convenience, in order to compact the tree
   representation, some nodes that are defined in the ietf-subscribed-
   notifications YANG module [RFC8639] and therefore are not essential
   to the understanding of the data model defined here have been



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   removed.  This is indicated by "..." in the diagram where applicable.

   Because the tree diagram is quite large, its depiction is broken up
   into four figures.  Figure 6 depicts the augmentations that are
   introduced in YANG module ietf-yang-push to the subscription
   configuration specified in YANG module ietf-subscribed-notifications.













































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

         Figure 6: Data Model Structure: Subscription Configuration





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   Figure 7 depicts the augmentations of YANG module ietf-yang-push made
   to RPCs specified in YANG module ietf-subscribed-notifications
   [RFC8639].  Specifically, these augmentations concern the "establish-
   subscription" and "modify-subscription" RPCs, which are augmented
   with parameters that are needed to specify datastore push
   subscriptions.

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



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

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

                   Figure 7: Data Model Structure: RPCs

   Figure 8 depicts augmentations of YANG module ietf-yang-push to the
   notifications that are specified in YANG module ietf-subscribed-
   notifications.  The augmentations allow the inclusion of subscription
   configuration parameters that are specific to datastore push
   subscriptions as part of "subscription-started" and "subscription-
   modified" notifications.

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



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



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

                  Figure 8: Data Model Structure: Notifications

   Finally, Figure 9 depicts the parts of YANG module ietf-yang-push
   that are newly introduced in this document (i.e., that are not simply
   augmentations of another YANG module).

   module: ietf-yang-push

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

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



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         +--ro object-count-estimate?    uint32
         +--ro object-count-limit?       uint32
         +--ro kilobytes-estimate?       uint32
         +--ro kilobytes-limit?          uint32
       notifications:
         +---n push-update
         |  +--ro id?                   sn:subscription-id
         |  +--ro datastore-contents?   <anydata>
         |  +--ro incomplete-update?    empty
         +---n push-change-update {on-change}?
             +--ro id?                   sn:subscription-id
             +--ro datastore-changes
             |  +--ro yang-patch
             |     +--ro patch-id    string
             |     +--ro comment?    string
             |     +--ro edit* [edit-id]
             |        +--ro edit-id      string
             |        +--ro operation    enumeration
             |        +--ro target       target-resource-offset
             |        +--ro point?       target-resource-offset
             |        +--ro where?       enumeration
             |        +--ro value?       <anydata>
             +--ro incomplete-update?    empty

         Figure 9: Data Model Structure: Non-augmentation Portions

   Selected components of the data model are summarized below.

8.2.  Subscription Configuration

   Both configured and dynamic subscriptions are represented in the list
   "subscription".  New parameters extending the basic subscription data
   model in [RFC8639] include:

   *  The targeted datastore from which the selection is being made.
      The potential datastores include those from [RFC8342].  A platform
      may also choose to support a custom datastore.

   *  A selection filter identifying YANG nodes of interest in 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 allow an implementation to avoid
      evaluating filter acceptability during a dynamic subscription
      request.  The "case" statement differentiates the options.

   *  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" that defines the duration between push updates.

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

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

      -  a "dampening-period" that 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" that allows the restriction of the types
         of changes for which updates should be sent (e.g., only add to
         an update record on object creation).

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

8.3.  YANG Notifications

8.3.1.  State Change Notifications

   Subscription state notifications and mechanisms are reused from
   [RFC8639].  Notifications "subscription-started" and "subscription-
   modified" have been augmented to include the datastore-specific
   objects.

8.3.2.  Notifications for Subscribed Content

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

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






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

8.4.  YANG RPCs

   YANG-Push subscriptions are established, modified, and deleted using
   RPCs augmented from [RFC8639].

8.4.1.  "establish-subscription" RPC

   The subscriber sends an "establish-subscription" RPC with the
   parameters listed 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
          xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
       ds:operational
     </yp:datastore>
     <yp:datastore-xpath-filter
         xmlns:ex="https://example.com/sample-data/1.0">
       /ex:foo
     </yp:datastore-xpath-filter>
     <yp:periodic>
       <yp:period>500</yp:period>
     </yp:periodic>
   </establish-subscription>
 </netconf:rpc>

                  Figure 10: "establish-subscription" RPC

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








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

         Figure 11: "establish-subscription" Positive RPC Response

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

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

   The specific parameters to be returned as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  For NETCONF, those parameters are defined in
   [RFC8640].  For example, for the following NETCONF request:
























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

         Figure 12: "establish-subscription" Request: Example 2

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

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

       Figure 13: "establish-subscription" Error Response: Example 2









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8.4.2.  "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.  Figure 14 provides an example
   where a subscriber attempts to modify the period and datastore XPath
   filter of a subscription using NETCONF.

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

                 Figure 14: "modify-subscription" Request

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

   The specific parameters to be returned as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  For NETCONF, those parameters are specified in
   [RFC8640].

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




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8.4.3.  "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 its only input the subscription's
   "id".  This RPC is unmodified from [RFC8639].

8.4.4.  "resync-subscription" RPC

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

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

                     Figure 15: "resync-subscription"

   On receipt, a publisher must either (1) accept the request and
   quickly follow with a "push-update" or (2) send an appropriate error
   in an RPC error response.  In its error response, the publisher MAY
   include, in the yang-data structure "resync-subscription-error",
   supplemental information about the reasons for the error.

8.4.5.  YANG Module Synchronization

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

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






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9.  YANG Model

   RFC 8639: This module imports typedefs from [RFC6991], [RFC8343],
   [RFC8341], [RFC8529], and [RFC8040].  It references [RFC6241],
   [XPATH] ("XML Path Language (XPath) Version 1.0"), [RFC7049],
   [RFC8259], [RFC7950], [RFC7951], and [RFC7540].

   RFC 8641: This YANG module imports typedefs from [RFC6991],
   identities from [RFC8342], the "yang-data" extension from [RFC8040],
   and the "yang-patch" grouping from [RFC8072].  In addition, it
   imports and augments many definitions from [RFC8639].  It also
   references [RFC6241], [XPATH] ("XML Path Language (XPath) Version
   1.0"), and [RFC7950].

   <CODE BEGINS> file "ietf-subscribed-notifications@2019-09-09.yang"
   module ietf-subscribed-notifications {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications";
     prefix sn;

     import ietf-inet-types {
       prefix inet;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-interfaces {
       prefix if;
       reference
         "RFC 8343: A YANG Data Model for Interface Management";
     }
     import ietf-netconf-acm {
       prefix nacm;
       reference
         "RFC 8341: Network Configuration Access Control Model";
     }
     import ietf-network-instance {
       prefix ni;
       reference
         "RFC 8529: YANG Data Model for Network Instances";
     }
     import ietf-restconf {
       prefix rc;
       reference
         "RFC 8040: RESTCONF Protocol";
     }
     import ietf-yang-types {
       prefix yang;
       reference



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         "RFC 6991: Common YANG Data Types";
     }

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

        Author:  Alexander Clemm
                 <mailto:ludwig@clemm.org>

        Author:  Eric Voit
                 <mailto:evoit@cisco.com>

        Author:  Alberto Gonzalez Prieto
                 <mailto:alberto.gonzalez@microsoft.com>

        Author:  Einar Nilsen-Nygaard
                 <mailto:einarnn@cisco.com>

        Author:  Ambika Prasad Tripathy
                 <mailto:ambtripa@cisco.com>";
     description
       "This module defines a YANG data model for subscribing to event
        records and receiving matching content in notification messages.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.

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

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

        This version of this YANG module is part of RFC 8639; see the
        RFC itself for full legal notices.";

     revision 2019-09-09 {
       description



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         "Initial version.";
       reference
         "RFC 8639: A YANG Data Model for Subscriptions to
                    Event Notifications";
     }

     /*
      * FEATURES
      */

     feature configured {
       description
         "This feature indicates that configuration of subscriptions is
          supported.";
     }

     feature dscp {
       description
         "This feature indicates that a publisher supports the ability
          to set the Differentiated Services Code Point (DSCP) value in
          outgoing packets.";
     }

     feature encode-json {
       description
         "This feature indicates that JSON encoding of notification
          messages is supported.";
     }

     feature encode-xml {
       description
         "This feature indicates that XML encoding of notification
          messages is supported.";
     }

     feature interface-designation {
       description
         "This feature indicates that a publisher supports sourcing all
          receiver interactions for a configured subscription from a
          single designated egress interface.";
     }

     feature qos {
       description
         "This feature indicates that a publisher supports absolute
          dependencies of one subscription's traffic over another
          as well as weighted bandwidth sharing between subscriptions.
          Both of these are Quality of Service (QoS) features that allow



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          differentiated treatment of notification messages between a
          publisher and a specific receiver.";
     }

     feature replay {
       description
         "This feature indicates that historical event record replay is
          supported.  With replay, it is possible for past event records
          to be streamed in chronological order.";
     }

     feature subtree {
       description
         "This feature indicates support for YANG subtree filtering.";
       reference
         "RFC 6241: Network Configuration Protocol (NETCONF),
                    Section 6";
     }

     feature supports-vrf {
       description
         "This feature indicates that a publisher supports VRF
          configuration for configured subscriptions.  VRF support for
          dynamic subscriptions does not require this feature.";
       reference
         "RFC 8529: YANG Data Model for Network Instances,
                    Section 6";
     }

     feature xpath {
       description
         "This feature indicates support for XPath filtering.";
       reference
         "XML Path Language (XPath) Version 1.0
          (https://www.w3.org/TR/1999/REC-xpath-19991116)";
     }

     /*
      * EXTENSIONS
      */

     extension subscription-state-notification {
       description
         "This statement applies only to notifications.  It indicates
          that the notification is a subscription state change
          notification.  Therefore, it does not participate in a regular
          event stream and does not need to be specifically subscribed
          to in order to be received.  This statement can only occur as



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          a substatement of the YANG 'notification' statement.  This
          statement is not for use outside of this YANG module.";
     }

     /*
      * IDENTITIES
      */
     /* Identities for RPC and notification errors */

     identity delete-subscription-error {
       description
         "Base identity for the problem found while attempting to
          fulfill either a 'delete-subscription' RPC request or a
          'kill-subscription' RPC request.";
     }

     identity establish-subscription-error {
       description
         "Base identity for the problem found while attempting to
          fulfill an 'establish-subscription' RPC request.";
     }

     identity modify-subscription-error {
       description
         "Base identity for the problem found while attempting to
          fulfill a 'modify-subscription' RPC request.";
     }

     identity subscription-suspended-reason {
       description
         "Base identity for the problem condition communicated to a
          receiver as part of a 'subscription-suspended'
          notification.";
     }

     identity subscription-terminated-reason {
       description
         "Base identity for the problem condition communicated to a
          receiver as part of a 'subscription-terminated'
          notification.";
     }

     identity dscp-unavailable {
       base establish-subscription-error;
       if-feature "dscp";
       description
         "The publisher is unable to mark notification messages with
          prioritization information in a way that will be respected



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          during network transit.";
     }

     identity encoding-unsupported {
       base establish-subscription-error;
       description
         "Unable to encode notification messages in the desired
          format.";
     }

     identity filter-unavailable {
       base subscription-terminated-reason;
       description
         "Referenced filter does not exist.  This means a receiver is
          referencing a filter that doesn't exist or to which it
          does not have access permissions.";
     }

     identity filter-unsupported {
       base establish-subscription-error;
       base modify-subscription-error;
       description
         "Cannot parse syntax in the filter.  This failure can be from
          a syntax error or a syntax too complex to be processed by the
          publisher.";
     }

     identity insufficient-resources {
       base establish-subscription-error;
       base modify-subscription-error;
       base subscription-suspended-reason;
       description
         "The publisher does not have sufficient resources to support
          the requested subscription.  An example might be that
          allocated CPU is too limited to generate the desired set of
          notification messages.";
     }

     identity no-such-subscription {
       base modify-subscription-error;
       base delete-subscription-error;
       base subscription-terminated-reason;
       description
         "Referenced subscription doesn't exist.  This may be as a
          result of a nonexistent subscription ID, an ID that belongs to
          another subscriber, or an ID for a configured subscription.";
     }




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     identity replay-unsupported {
       base establish-subscription-error;
       if-feature "replay";
       description
         "Replay cannot be performed for this subscription.  This means
          the publisher will not provide the requested historic
          information from the event stream via replay to this
          receiver.";
     }

     identity stream-unavailable {
       base subscription-terminated-reason;
       description
         "Not a subscribable event stream.  This means the referenced
          event stream is not available for subscription by the
          receiver.";
     }

     identity suspension-timeout {
       base subscription-terminated-reason;
       description
         "Termination of a previously suspended subscription.  The
          publisher has eliminated the subscription, as it exceeded a
          time limit for suspension.";
     }

     identity unsupportable-volume {
       base subscription-suspended-reason;
       description
         "The publisher does not have the network bandwidth needed to
          get the volume of generated information intended for a
          receiver.";
     }

     /* Identities for encodings */

     identity configurable-encoding {
       description
         "If a transport identity derives from this identity, it means
          that it supports configurable encodings.  An example of a
          configurable encoding might be a new identity such as
          'encode-cbor'.  Such an identity could use
          'configurable-encoding' as its base.  This would allow a
          dynamic subscription encoded in JSON (RFC 8259) to request
          that notification messages be encoded via the Concise Binary
          Object Representation (CBOR) (RFC 7049).  Further details for
          any specific configurable encoding would be explored in a
          transport document based on this specification.";



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       reference
         "RFC 8259: The JavaScript Object Notation (JSON) Data
                    Interchange Format
          RFC 7049: Concise Binary Object Representation (CBOR)";
     }

     identity encoding {
       description
         "Base identity to represent data encodings.";
     }

     identity encode-xml {
       base encoding;
       if-feature "encode-xml";
       description
         "Encode data using XML as described in RFC 7950.";
       reference
         "RFC 7950: The YANG 1.1 Data Modeling Language";
     }

     identity encode-json {
       base encoding;
       if-feature "encode-json";
       description
         "Encode data using JSON as described in RFC 7951.";
       reference
         "RFC 7951: JSON Encoding of Data Modeled with YANG";
     }

     /* Identities for transports */

     identity transport {
       description
         "An identity that represents the underlying mechanism for
          passing notification messages.";
     }

     /*
      * TYPEDEFs
      */

     typedef encoding {
       type identityref {
         base encoding;
       }
       description
         "Specifies a data encoding, e.g., for a data subscription.";
     }



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     typedef stream-filter-ref {
       type leafref {
         path "/sn:filters/sn:stream-filter/sn:name";
       }
       description
         "This type is used to reference an event stream filter.";
     }

     typedef stream-ref {
       type leafref {
         path "/sn:streams/sn:stream/sn:name";
       }
       description
         "This type is used to reference a system-provided
          event stream.";
     }

     typedef subscription-id {
       type uint32;
       description
         "A type for subscription identifiers.";
     }

     typedef transport {
       type identityref {
         base transport;
       }
       description
         "Specifies the transport used to send notification messages
          to a receiver.";
     }

     /*
      * GROUPINGS
      */

     grouping stream-filter-elements {
       description
         "This grouping defines the base for filters applied to event
          streams.";
       choice filter-spec {
         description
           "The content filter specification for this request.";
         anydata stream-subtree-filter {
           if-feature "subtree";
           description
             "Event stream evaluation criteria encoded in the syntax of
              a subtree filter as defined in RFC 6241, Section 6.



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              The subtree filter is applied to the representation of
              individual, delineated event records as contained in the
              event stream.

              If the subtree filter returns a non-empty node set, the
              filter matches the event record, and the event record is
              included in the notification message sent to the
              receivers.";
           reference
             "RFC 6241: Network Configuration Protocol (NETCONF),
                        Section 6";
         }
         leaf stream-xpath-filter {
           if-feature "xpath";
           type yang:xpath1.0;
           description
             "Event stream evaluation criteria encoded in the syntax of
              an XPath 1.0 expression.

              The XPath expression is evaluated on the representation of
              individual, delineated event records as contained in
              the event stream.

              The result of the XPath expression is converted to a
              boolean value using the standard XPath 1.0 rules.  If the
              boolean value is 'true', the filter matches the event
              record, and the event record is included in the
              notification message sent to the receivers.

              The expression is evaluated in the following XPath
              context:

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

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

                 o  The set of variable bindings is empty.

                 o  The function library is comprised of the core



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                    function library and the XPath functions defined in
                    Section 10 in RFC 7950.

                 o  The context node is the root node.";
           reference
             "XML Path Language (XPath) Version 1.0
              (https://www.w3.org/TR/1999/REC-xpath-19991116)
              RFC 7950: The YANG 1.1 Data Modeling Language,
                        Section 10";
         }
       }
     }

     grouping update-qos {
       description
         "This grouping describes QoS information concerning a
          subscription.  This information is passed to lower layers
          for transport prioritization and treatment.";
       leaf dscp {
         if-feature "dscp";
         type inet:dscp;
         default "0";
         description
           "The desired network transport priority level.  This is the
            priority set on notification messages encapsulating the
            results of the subscription.  This transport priority is
            shared for all receivers of a given subscription.";
       }
       leaf weighting {
         if-feature "qos";
         type uint8 {
           range "0 .. 255";
         }
         description
           "Relative weighting for a subscription.  Larger weights get
            more resources.  Allows an underlying transport layer to
            perform informed load-balance allocations between various
            subscriptions.";
         reference
           "RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2),
                      Section 5.3.2";
       }
       leaf dependency {
         if-feature "qos";
         type subscription-id;
         description
           "Provides the 'subscription-id' of a parent subscription.
            The parent subscription has absolute precedence should



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            that parent have push updates ready to egress the publisher.
            In other words, there should be no streaming of objects from
            the current subscription if the parent has something ready
            to push.

            If a dependency is asserted via configuration or via an RPC
            but the referenced 'subscription-id' does not exist, the
            dependency is silently discarded.  If a referenced
            subscription is deleted, this dependency is removed.";
         reference
           "RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2),
                      Section 5.3.1";
       }
     }

     grouping subscription-policy-modifiable {
       description
         "This grouping describes all objects that may be changed
          in a subscription.";
       choice target {
         mandatory true;
         description
           "Identifies the source of information against which a
            subscription is being applied as well as specifics on the
            subset of information desired from that source.";
         case stream {
           choice stream-filter {
             description
               "An event stream filter can be applied to a subscription.
                That filter will either come referenced from a global
                list or be provided in the subscription itself.";
             case by-reference {
               description
                 "Apply a filter that has been configured separately.";
               leaf stream-filter-name {
                 type stream-filter-ref;
                 mandatory true;
                 description
                   "References an existing event stream filter that is
                    to be applied to an event stream for the
                    subscription.";
               }
             }
             case within-subscription {
               description
                 "A local definition allows a filter to have the same
                  lifecycle as the subscription.";
               uses stream-filter-elements;



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             }
           }
         }
       }
       leaf stop-time {
         type yang:date-and-time;
         description
           "Identifies a time after which notification messages for a
            subscription should not be sent.  If 'stop-time' is not
            present, the notification messages will continue until the
            subscription is terminated.  If 'replay-start-time' exists,
            'stop-time' must be for a subsequent time.  If
            'replay-start-time' doesn't exist, 'stop-time', when
            established, must be for a future time.";
       }
     }

     grouping subscription-policy-dynamic {
       description
         "This grouping describes the only information concerning a
          subscription that can be passed over the RPCs defined in this
          data model.";
       uses subscription-policy-modifiable {
         augment "target/stream" {
           description
             "Adds additional objects that can be modified by an RPC.";
           leaf stream {
             type stream-ref {
               require-instance false;
             }
             mandatory true;
             description
               "Indicates the event stream to be considered for
                this subscription.";
           }
           leaf replay-start-time {
             if-feature "replay";
             type yang:date-and-time;
             config false;
             description
               "Used to trigger the 'replay' feature for a dynamic
                subscription, where event records that are selected
                need to be at or after the specified starting time.  If
                'replay-start-time' is not present, this is not a replay
                subscription and event record push should start
                immediately.  It is never valid to specify start times
                that are later than or equal to the current time.";
           }



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         }
       }
       uses update-qos;
     }

     grouping subscription-policy {
       description
         "This grouping describes the full set of policy information
          concerning both dynamic and configured subscriptions, with the
          exclusion of both receivers and networking information
          specific to the publisher, such as what interface should be
          used to transmit notification messages.";
       uses subscription-policy-dynamic;
       leaf transport {
         if-feature "configured";
         type transport;
         description
           "For a configured subscription, this leaf specifies the
            transport used to deliver messages destined for all
            receivers of that subscription.";
       }
       leaf encoding {
         when 'not(../transport) or derived-from(../transport,
         "sn:configurable-encoding")';
         type encoding;
         description
           "The type of encoding for notification messages.  For a
            dynamic subscription, if not included as part of an
            'establish-subscription' RPC, the encoding will be populated
            with the encoding used by that RPC.  For a configured
            subscription, if not explicitly configured, the encoding
            will be the default encoding for an underlying transport.";
       }
       leaf purpose {
         if-feature "configured";
         type string;
         description
           "Open text allowing a configuring entity to embed the
            originator or other specifics of this subscription.";
       }
     }

     /*
      * RPCs
      */

     rpc establish-subscription {
       description



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         "This RPC allows a subscriber to create (and possibly
          negotiate) a subscription on its own behalf.  If successful,
          the subscription remains in effect for the duration of the
          subscriber's association with the publisher or until the
          subscription is terminated.  If an error occurs or the
          publisher cannot meet the terms of a subscription, an RPC
          error is returned, and the subscription is not created.
          In that case, the RPC reply's 'error-info' MAY include
          suggested parameter settings that would have a higher
          likelihood of succeeding in a subsequent
          'establish-subscription' request.";
       input {
         uses subscription-policy-dynamic;
         leaf encoding {
           type encoding;
           description
             "The type of encoding for the subscribed data.  If not
              included as part of the RPC, the encoding MUST be set by
              the publisher to be the encoding used by this RPC.";
         }
       }
       output {
         leaf id {
           type subscription-id;
           mandatory true;
           description
             "Identifier used for this subscription.";
         }
         leaf replay-start-time-revision {
           if-feature "replay";
           type yang:date-and-time;
           description
             "If a replay has been requested, this object represents
              the earliest time covered by the event buffer for the
              requested event stream.  The value of this object is the
              'replay-log-aged-time' if it exists.  Otherwise, it is
              the 'replay-log-creation-time'.  All buffered event
              records after this time will be replayed to a receiver.
              This object will only be sent if the starting time has
              been revised to be later than the time requested by the
              subscriber.";
         }
       }
     }

     rc:yang-data establish-subscription-stream-error-info {
       container establish-subscription-stream-error-info {
         description



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           "If any 'establish-subscription' RPC parameters are
            unsupportable against the event stream, 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 in a
            subscription's RPC error response to indicate the reason for
            the failure.  This yang-data MUST be inserted if hints are
            to be provided back to the subscriber.";
         leaf reason {
           type identityref {
             base establish-subscription-error;
           }
           description
             "Indicates the reason why the subscription has failed to
              be created to a targeted event stream.";
         }
         leaf filter-failure-hint {
           type string;
           description
             "Information describing where and/or why a provided
              filter was unsupportable for a subscription.  The
              syntax and semantics of this hint are
              implementation specific.";
         }
       }
     }

     rpc modify-subscription {
       description
         "This RPC allows a subscriber to modify a dynamic
          subscription's parameters.  If successful, the changed
          subscription parameters remain in effect for the duration of
          the subscription, until the subscription is again modified, or
          until the subscription is terminated.  In the case of an error
          or an inability to meet the modified parameters, the
          subscription is not modified and the original subscription
          parameters remain in effect.  In that case, the RPC error MAY
          include 'error-info' suggested parameter hints that would have
          a high likelihood of succeeding in a subsequent
          'modify-subscription' request.  A successful
          'modify-subscription' will return a suspended subscription to
          the 'active' state.";
       input {
         leaf id {
           type subscription-id;
           mandatory true;
           description
             "Identifier to use for this subscription.";



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         }
         uses subscription-policy-modifiable;
       }
     }

     rc:yang-data modify-subscription-stream-error-info {
       container modify-subscription-stream-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 that has been made against an
            event stream.  This yang-data MUST be used if hints are to
            be provided back to the subscriber.";
         leaf reason {
           type identityref {
             base modify-subscription-error;
           }
           description
             "Information in a 'modify-subscription' RPC error response
              that indicates the reason why the subscription to an event
              stream has failed to be modified.";
         }
         leaf filter-failure-hint {
           type string;
           description
             "Information describing where and/or why a provided
              filter was unsupportable for a subscription.  The syntax
              and semantics of this hint are
              implementation specific.";
         }
       }
     }

     rpc delete-subscription {
       description
         "This RPC allows a subscriber to delete a subscription that
          was previously created by that same subscriber using the
          'establish-subscription' RPC.

          If an error occurs, the server replies with an 'rpc-error'
          where the 'error-info' field MAY contain a
          'delete-subscription-error-info' structure.";
       input {
         leaf id {
           type subscription-id;
           mandatory true;
           description
             "Identifier of the subscription that is to be deleted.



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              Only subscriptions that were created using
              'establish-subscription' from the same origin as this RPC
              can be deleted via this RPC.";
         }
       }
     }

     rpc kill-subscription {
       nacm:default-deny-all;
       description
         "This RPC allows an operator to delete a dynamic subscription
          without restrictions on the originating subscriber or
          underlying transport session.

          If an error occurs, the server replies with an 'rpc-error'
          where the 'error-info' field MAY contain a
          'delete-subscription-error-info' structure.";
       input {
         leaf id {
           type subscription-id;
           mandatory true;
           description
             "Identifier of the subscription that is to be deleted.
              Only subscriptions that were created using
              'establish-subscription' can be deleted via this RPC.";
         }
       }
     }

     rc:yang-data delete-subscription-error-info {
       container delete-subscription-error-info {
         description
           "If a 'delete-subscription' RPC or a 'kill-subscription' RPC
            fails, the subscription is not deleted and the RPC error
            response MUST indicate the reason for this failure.  This
            yang-data MAY be inserted as structured data in a
            subscription's RPC error response to indicate the reason
            for the failure.";
         leaf reason {
           type identityref {
             base delete-subscription-error;
           }
           mandatory true;
           description
             "Indicates the reason why the subscription has failed to be
              deleted.";
         }
       }



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     }

     /*
      * NOTIFICATIONS
      */

     notification replay-completed {
       sn:subscription-state-notification;
       if-feature "replay";
       description
         "This notification is sent to indicate that all of the replay
          notifications have been sent.";
       leaf id {
         type subscription-id;
         mandatory true;
         description
           "This references the affected subscription.";
       }
     }

     notification subscription-completed {
       sn:subscription-state-notification;
       if-feature "configured";
       description
         "This notification is sent to indicate that a subscription has
          finished passing event records, as the 'stop-time' has been
          reached.";
       leaf id {
         type subscription-id;
         mandatory true;
         description
           "This references the gracefully completed subscription.";
       }
     }

     notification subscription-modified {
       sn:subscription-state-notification;
       description
         "This notification indicates that a subscription has been
          modified.  Notification messages sent from this point on will
          conform to the modified terms of the subscription.  For
          completeness, this subscription state change notification
          includes both modified and unmodified aspects of a
          subscription.";
       leaf id {
         type subscription-id;
         mandatory true;
         description



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           "This references the affected subscription.";
       }
       uses subscription-policy {
         refine "target/stream/stream-filter/within-subscription" {
           description
             "Filter applied to the subscription.  If the
              'stream-filter-name' is populated, the filter in the
              subscription came from the 'filters' container.
              Otherwise, it is populated in-line as part of the
              subscription.";
         }
       }
     }

     notification subscription-resumed {
       sn:subscription-state-notification;
       description
         "This notification indicates that a subscription that had
          previously been suspended has resumed.  Notifications will
          once again be sent.  In addition, a 'subscription-resumed'
          indicates that no modification of parameters has occurred
          since the last time event records have been sent.";
       leaf id {
         type subscription-id;
         mandatory true;
         description
           "This references the affected subscription.";
       }
     }

     notification subscription-started {
       sn:subscription-state-notification;
       if-feature "configured";
       description
         "This notification indicates that a subscription has started
          and notifications will now be sent.";
       leaf id {
         type subscription-id;
         mandatory true;
         description
           "This references the affected subscription.";
       }
       uses subscription-policy {
         refine "target/stream/replay-start-time" {
           description
             "Indicates the time that a replay is using for the
              streaming of buffered event records.  This will be
              populated with the most recent of the following:



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              the event time of the previous event record sent to a
              receiver, the 'replay-log-creation-time', the
              'replay-log-aged-time', or the most recent publisher
              boot time.";
         }
         refine "target/stream/stream-filter/within-subscription" {
           description
             "Filter applied to the subscription.  If the
              'stream-filter-name' is populated, the filter in the
              subscription came from the 'filters' container.
              Otherwise, it is populated in-line as part of the
              subscription.";
         }
         augment "target/stream" {
           description
             "This augmentation adds additional parameters specific to a
              'subscription-started' notification.";
           leaf replay-previous-event-time {
             when '../replay-start-time';
             if-feature "replay";
             type yang:date-and-time;
             description
               "If there is at least one event in the replay buffer
                prior to 'replay-start-time', this gives the time of
                the event generated immediately prior to the
                'replay-start-time'.

                If a receiver previously received event records for
                this configured subscription, it can compare this time
                to the last event record previously received.  If the
                two are not the same (perhaps due to a reboot), then a
                dynamic replay can be initiated to acquire any missing
                event records.";
           }
         }
       }
     }

     notification subscription-suspended {
       sn:subscription-state-notification;
       description
         "This notification indicates that a suspension of the
          subscription by the publisher has occurred.  No further
          notifications will be sent until the subscription resumes.
          This notification shall only be sent to receivers of a
          subscription; it does not constitute a general-purpose
          notification.";
       leaf id {



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         type subscription-id;
         mandatory true;
         description
           "This references the affected subscription.";
       }
       leaf reason {
         type identityref {
           base subscription-suspended-reason;
         }
         mandatory true;
         description
           "Identifies the condition that resulted in the suspension.";
       }
     }

     notification subscription-terminated {
       sn:subscription-state-notification;
       description
         "This notification indicates that a subscription has been
          terminated.";
       leaf id {
         type subscription-id;
         mandatory true;
         description
           "This references the affected subscription.";
       }
       leaf reason {
         type identityref {
           base subscription-terminated-reason;
         }
         mandatory true;
         description
           "Identifies the condition that resulted in the termination.";
       }
     }

     /*
      * DATA NODES
      */

     container streams {
       config false;
       description
         "Contains information on the built-in event streams provided by
          the publisher.";
       list stream {
         key "name";
         description



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           "Identifies the built-in event streams that are supported by
            the publisher.";
         leaf name {
           type string;
           description
             "A handle for a system-provided event stream made up of a
              sequential set of event records, each of which is
              characterized by its own domain and semantics.";
         }
         leaf description {
           type string;
           description
             "A description of the event stream, including such
              information as the type of event records that are
              available in this event stream.";
         }
         leaf replay-support {
           if-feature "replay";
           type empty;
           description
             "Indicates that event record replay is available on this
              event stream.";
         }
         leaf replay-log-creation-time {
           when '../replay-support';
           if-feature "replay";
           type yang:date-and-time;
           mandatory true;
           description
             "The timestamp of the creation of the log used to support
              the replay function on this event stream.  This time
              might be earlier than the earliest available information
              contained in the log.  This object is updated if the log
              resets for some reason.";
         }
         leaf replay-log-aged-time {
           when '../replay-support';
           if-feature "replay";
           type yang:date-and-time;
           description
             "The timestamp associated with the last event record that
              has been aged out of the log.  This timestamp identifies
              how far back in history this replay log extends, if it
              doesn't extend back to the 'replay-log-creation-time'.
              This object MUST be present if replay is supported and any
              event records have been aged out of the log.";
         }
       }



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     }
     container filters {
       description
         "Contains a list of configurable filters that can be applied to
          subscriptions.  This facilitates the reuse of complex filters
          once defined.";
       list stream-filter {
         key "name";
         description
           "A list of preconfigured filters that can be applied to
            subscriptions.";
         leaf name {
           type string;
           description
             "A name to differentiate between filters.";
         }
         uses stream-filter-elements;
       }
     }
     container subscriptions {
       description
         "Contains the list of currently active subscriptions, i.e.,
          subscriptions that are currently in effect, used for
          subscription management and monitoring purposes.  This
          includes subscriptions that have been set up via
          RPC primitives as well as subscriptions that have been
          established via configuration.";
       list subscription {
         key "id";
         description
           "The identity and specific parameters of a subscription.
            Subscriptions in this list can be created using a control
            channel or RPC or can be established through configuration.

            If the 'kill-subscription' RPC or configuration operations
            are used to delete a subscription, a
            'subscription-terminated' message is sent to any active or
            suspended receivers.";
         leaf id {
           type subscription-id;
           description
             "Identifier of a subscription; unique in a given
              publisher.";
         }
         uses subscription-policy {
           refine "target/stream/stream" {
             description
               "Indicates the event stream to be considered for this



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                subscription.  If an event stream has been removed
                and can no longer be referenced by an active
                subscription, send a 'subscription-terminated'
                notification with 'stream-unavailable' as the reason.
                If a configured subscription refers to a nonexistent
                event stream, move that subscription to the
                'invalid' state.";
           }
           refine "transport" {
             description
               "For a configured subscription, this leaf specifies the
                transport used to deliver messages destined for all
                receivers of that subscription.  This object is
                mandatory for subscriptions in the configuration
                datastore.  This object (1) is not mandatory for dynamic
                subscriptions in the operational state datastore and
                (2) should not be present for other types of dynamic
                subscriptions.";
           }
           augment "target/stream" {
             description
               "Enables objects to be added to a configured stream
                subscription.";
             leaf configured-replay {
               if-feature "configured";
               if-feature "replay";
               type empty;
               description
                 "The presence of this leaf indicates that replay for
                  the configured subscription should start at the
                  earliest time in the event log or at the publisher
                  boot time, whichever is later.";
             }
           }
         }
         choice notification-message-origin {
           if-feature "configured";
           description
             "Identifies the egress interface on the publisher
              from which notification messages are to be sent.";
           case interface-originated {
             description
               "When notification messages are to egress a specific,
                designated interface on the publisher.";
             leaf source-interface {
               if-feature "interface-designation";
               type if:interface-ref;
               description



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                 "References the interface for notification messages.";
             }
           }
           case address-originated {
             description
               "When notification messages are to depart from a
                publisher using a specific originating address and/or
                routing context information.";
             leaf source-vrf {
               if-feature "supports-vrf";
               type leafref {
                 path "/ni:network-instances/ni:network-instance/ni:name";
               }
               description
                 "VRF from which notification messages should egress a
                  publisher.";
             }
             leaf source-address {
               type inet:ip-address-no-zone;
               description
                 "The source address for the notification messages.
                  If a source VRF exists but this object doesn't, a
                  publisher's default address for that VRF must
                  be used.";
             }
           }
         }
         leaf configured-subscription-state {
           if-feature "configured";
           type enumeration {
             enum valid {
               value 1;
               description
                 "The subscription is supportable with its current
                  parameters.";
             }
             enum invalid {
               value 2;
               description
                 "The subscription as a whole is unsupportable with its
                  current parameters.";
             }
             enum concluded {
               value 3;
               description
                 "A subscription is inactive, as it has hit a
                  stop time.  It no longer has receivers in the
                  'active' or 'suspended' state, but the subscription



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                  has not yet been removed from configuration.";
             }
           }
           config false;
           description
             "The presence of this leaf indicates that the subscription
              originated from configuration, not through a control
              channel or RPC.  The value indicates the state of the
              subscription as established by the publisher.";
         }
         container receivers {
           description
             "Set of receivers in a subscription.";
           list receiver {
             key "name";
             min-elements 1;
             description
               "A host intended as a recipient for the notification
                messages of a subscription.  For configured
                subscriptions, transport-specific network parameters
                (or a leafref to those parameters) may be augmented to a
                specific receiver in this list.";
             leaf name {
               type string;
               description
                 "Identifies a unique receiver for a subscription.";
             }
             leaf sent-event-records {
               type yang:zero-based-counter64;
               config false;
               description
                 "The number of event records sent to the receiver.  The
                  count is initialized when a dynamic subscription is
                  established or when a configured receiver
                  transitions to the 'valid' state.";
             }
             leaf excluded-event-records {
               type yang:zero-based-counter64;
               config false;
               description
                 "The number of event records explicitly removed via
                  either an event stream filter or an access control
                  filter so that they are not passed to a receiver.
                  This count is set to zero each time
                  'sent-event-records' is initialized.";
             }
             leaf state {
               type enumeration {



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                 enum active {
                   value 1;
                   description
                     "The receiver is currently being sent any
                      applicable notification messages for the
                      subscription.";
                 }
                 enum suspended {
                   value 2;
                   description
                     "The receiver state is 'suspended', so the
                      publisher is currently unable to provide
                      notification messages for the subscription.";
                 }
                 enum connecting {
                   value 3;
                   if-feature "configured";
                   description
                     "A subscription has been configured, but a
                      'subscription-started' subscription state change
                      notification needs to be successfully received
                      before notification messages are sent.

                      If the 'reset' action is invoked for a receiver of
                      an active configured subscription, the state
                      must be moved to 'connecting'.";
                 }
                 enum disconnected {
                   value 4;
                   if-feature "configured";
                   description
                     "A subscription has failed to send a
                      'subscription-started' state change to the
                      receiver.  Additional connection attempts are not
                      currently being made.";
                 }
               }
               config false;
               mandatory true;
               description
                 "Specifies the state of a subscription from the
                  perspective of a particular receiver.  With this
                  information, it is possible to determine whether a
                  publisher is currently generating notification
                  messages intended for that receiver.";
             }
             action reset {
               if-feature "configured";



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               description
                 "Allows the reset of this configured subscription's
                  receiver to the 'connecting' state.  This enables the
                  connection process to be reinitiated.";
               output {
                 leaf time {
                   type yang:date-and-time;
                   mandatory true;
                   description
                     "Time at which a publisher returned the receiver to
                      the 'connecting' state.";
                 }
               }
             }
           }
         }
       }
     }
   }
   <CODE ENDS>

            Figure 1: YANG module ietf-subscribed-notifications

   <CODE BEGINS> file "ietf-yang-push@2019-09-09.yang"
   module ietf-yang-push {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push";
     prefix yp;

     import ietf-yang-types {
       prefix yang;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-subscribed-notifications {
       prefix sn;
       reference
         "RFC 8639: Subscription to YANG Notifications";
     }
     import ietf-datastores {
       prefix ds;
       reference
         "RFC 8342: Network Management Datastore Architecture (NMDA)";
     }
     import ietf-restconf {
       prefix rc;
       reference
         "RFC 8040: RESTCONF Protocol";



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     }
     import ietf-yang-patch {
       prefix ypatch;
       reference
         "RFC 8072: YANG Patch Media Type";
     }

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

        Author:  Alexander Clemm
                 <mailto:ludwig@clemm.org>

        Author:  Eric Voit
                 <mailto:evoit@cisco.com>";

     description
       "This module contains YANG specifications for YANG-Push.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.

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

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

        This version of this YANG module is part of RFC 8641; see the
        RFC itself for full legal notices.";

     revision 2019-09-09 {
       description
         "Initial revision.";
       reference
         "RFC 8641: Subscriptions to YANG Datastores";
     }




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     /*
      * FEATURES
      */

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

     /*
      * IDENTITIES
      */

     /* Error type identities for datastore subscription */

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

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

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

     identity no-such-subscription-resync {
       base resync-subscription-error;
       description
         "The referenced subscription doesn't exist.  This may be as a
          result of a nonexistent subscription ID, an ID that belongs to
          another subscriber, or an ID for a configured subscription.";
     }

     identity on-change-unsupported {
       base sn:establish-subscription-error;



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       description
         "On-change is not supported for any objects that are
          selectable by this filter.";
     }

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

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

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

     identity sync-too-big {
       base sn:establish-subscription-error;
       base sn:modify-subscription-error;
       base resync-subscription-error;
       base sn:subscription-suspended-reason;
       description
         "The 'sync-on-start' or resynchronization data tree exceeds a
          maximum size limit.  Hints on the estimated size of what was



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          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
         "The selection filter is unlikely to ever select data tree
          nodes.  This means that based on the subscriber's current
          access rights, the publisher recognizes that the selection
          filter is unlikely to ever select data tree nodes that change.
          Examples for this might be that the node or subtree doesn't
          exist, read access is not permitted for a receiver, or static
          objects that only change at reboot have been chosen.";
     }

     /*
      * TYPE DEFINITIONS
      */

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



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       }
       description
         "Specifies different types of datastore changes.

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

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

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

     /*
      * GROUP DEFINITIONS
      */

     grouping datastore-criteria {
       description
         "A grouping to define criteria for which selected objects from
          a targeted datastore should be included in push updates.";
       leaf datastore {
         type identityref {
           base ds:datastore;
         }
         mandatory true;
         description
           "Datastore from which to retrieve data.";
       }
       uses selection-filter-objects;
     }

     grouping selection-filter-types {



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

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

              The expression is evaluated in the following XPath
              context:

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

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

              o  The set of variable bindings is empty.

              o  The function library is comprised of the core
                 function library and the XPath functions defined in
                 Section 10 in RFC 7950.



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              o  The context node is the root node of the target
                 datastore.";
           reference
             "XML Path Language (XPath) Version 1.0
              (https://www.w3.org/TR/1999/REC-xpath-19991116)
              RFC 7950: The YANG 1.1 Data Modeling Language,
                        Section 10";
         }
       }
     }

     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 either (1) come referenced from a
            global list or (2) be provided in the subscription itself.";
         case by-reference {
           description
             "Incorporates a filter that has been configured
              separately.";
           leaf selection-filter-ref {
             type selection-filter-ref;
             mandatory true;
             description
               "References an existing selection filter that is to be
                applied to the subscription.";
           }
         }
         case within-subscription {
           description
             "A local definition allows a filter to have the same
              lifecycle as the subscription.";
           uses selection-filter-types;
         }
       }
     }

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



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           "Defines necessary conditions for sending an event record to
            the subscriber.";
         case periodic {
           container periodic {
             presence "indicates a periodic subscription";
             description
               "The publisher is requested to periodically notify the
                receiver regarding the current values of the datastore
                as defined by the selection filter.";
             leaf period {
               type centiseconds;
               mandatory true;
               description
                 "Duration of time that should occur between periodic
                  push updates, in units of 0.01 seconds.";
             }
             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 point in time that is a
                  multiple of a period from the 'anchor-time'.
                  For example, for an 'anchor-time' that 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 that 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 the receiver
                regarding changes in values in the datastore subset as
                defined by a selection filter.";
             leaf dampening-period {
               type centiseconds;
               default "0";
               description
                 "Specifies the minimum interval between the assembly of
                  successive update records for a single receiver of a
                  subscription.  Whenever subscribed objects change and
                  a dampening-period interval (which may be zero) has
                  elapsed since the previous update record creation for
                  a receiver, any subscribed objects and properties



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                  that have changed since the previous update record
                  will have their current values marshalled and placed
                  in 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 that are not modifiable once a
              subscription is established.";
           leaf sync-on-start {
             type boolean;
             default "true";
             description
               "When this object is set to 'false', (1) it restricts an
                on-change subscription from sending 'push-update'
                notifications and (2) 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 set to 'true' (the default behavior),
                in order to facilitate a receiver's synchronization,
                a full update is sent, via a 'push-update' notification,
                when the subscription starts.  After that,
                'push-change-update' notifications are exclusively sent,
                unless the publisher chooses to resync the subscription
                via a new 'push-update' notification.";
           }
           leaf-list excluded-change {
             type change-type;
             description
               "Used to restrict which changes trigger an update.  For
                example, if a 'replace' operation is excluded, only the
                creation and deletion of objects are reported.";
           }
         }
       }
     }

     grouping hints {



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

     /*



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      * RPCs
      */

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

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

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

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



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         "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 a 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 in a subscription's
            RPC error response to indicate the reason for the failure.
            This yang-data MUST be inserted if hints are to be provided
            back to the subscriber.";
         leaf reason {
           type identityref {
             base sn:establish-subscription-error;
           }
           description
             "Indicates the reason why the subscription has failed to
              be created to a targeted datastore.";
         }
         uses hints;
       }
     }

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

     augment "/sn:modify-subscription/sn:input/sn:target" {



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

     rc:yang-data modify-subscription-datastore-error-info {
       container modify-subscription-datastore-error-info {
         description
           "This yang-data MAY be provided as part of a subscription's
            RPC error response when there is a failure of a
            'modify-subscription' RPC that has been made against a
            datastore.  This yang-data MUST be used if hints are to be
            provided 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 that in turn contains
          data subscribed to via a subscription.  In the case of a
          periodic subscription, this notification is sent for periodic
          updates.  It can also be used for synchronization updates of
          an on-change subscription.  This notification shall only be
          sent to receivers of a subscription.  It does not constitute
          a general-purpose notification that would be subscribable as
          part of the NETCONF event stream by any receiver.";
       leaf id {
         type sn:subscription-id;
         description



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

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



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            that 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 burst of
            changes.";
       }
     }

     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 in the subscription came from the 'filters'
                container.  Otherwise, it is populated in-line as part
                of the subscription itself.";
           }
         }
       }
     }

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

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



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             description
               "Specifies the selection filter and where it originated
                from.  If the 'selection-filter-ref' is populated, the
                filter in 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 preconfigured filters that can be applied
            to datastore subscriptions.";
         leaf filter-id {
           type string;
           description
             "An identifier to differentiate between selection
              filters.";
         }
         uses selection-filter-types;
       }
     }

     augment "/sn:subscriptions/sn:subscription" {
       when 'yp:datastore';
       description
         "This augmentation adds objects to a subscription that are
          specific to a datastore subscription, i.e., a subscription to
          a stream of datastore node updates.";
       uses update-policy;
     }

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



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         uses datastore-criteria;
       }
     }
   }
   <CODE ENDS>

                    Figure 2: YANG module ietf-yang-push

10.  Implementation Considerations (from RFC 8639)

   To support deployments that include both configured and dynamic
   subscriptions, it is recommended that the subscription "id" domain be
   split into static and dynamic halves.  This will eliminate the
   possibility of collisions if the configured subscriptions attempt to
   set a "subscription-id" that might have already been dynamically
   allocated.  A best practice is to use the lower half of the "id"
   object's integer space when that "id" is assigned by an external
   entity (such as with a configured subscription).  This leaves the
   upper half of the subscription integer space available to be
   dynamically assigned by the publisher.

   If a subscription is unable to marshal a series of filtered event
   records into transmittable notification messages, the receiver should
   be suspended with the reason "unsupportable-volume".

   For configured subscriptions, operations are performed against the
   set of receivers using the subscription "id" as a handle for that
   set.  But for streaming updates, subscription state change
   notifications are local to a receiver.  In the case of this
   specification, receivers do not get any information from the
   publisher about the existence of other receivers.  But if a network
   operator wants to let the receivers correlate results, it is useful
   to use the subscription "id" across the receivers to allow that
   correlation.  Note that due to the possibility of different access
   control permissions per receiver, each receiver may actually get a
   different set of event records.

   For configured replay subscriptions, the receiver is protected from
   duplicated events being pushed after a publisher is rebooted.
   However, it is possible that a receiver might want to acquire event
   records that failed to be delivered just prior to the reboot.
   Delivering these event records can be accomplished by leveraging the
   <eventTime> [RFC5277] from the last event record received prior to
   the receipt of a "subscription-started" subscription state change
   notification.  With this <eventTime> and the "replay-start-time" from
   the "subscription-started" notification, an independent dynamic
   subscription can be established that retrieves any event records that
   may have been generated but not sent to the receiver.



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11.  Transport Requirements (from RFC 8639)

   This section provides requirements for any subscribed notification
   transport supporting the solution presented in this document.

   The transport selected by the subscriber to reach the publisher MUST
   be able to support multiple "establish-subscription" requests made in
   the same transport session.

   For both configured and dynamic subscriptions, the publisher MUST
   authenticate a receiver via some transport-level mechanism before
   sending any event records that the receiver is authorized to see.  In
   addition, the receiver MUST authenticate the publisher at the
   transport level.  The result is mutual authentication between the
   two.

   A secure transport is highly recommended.  Beyond this, the publisher
   MUST ensure that the receiver has sufficient authorization to perform
   the function it is requesting against the specific subset of content
   involved.

   A specification for a transport built upon this document may or may
   not choose to require the use of the same logical channel for the
   RPCs and the event records.  However, the event records and the
   subscription state change notifications MUST be sent on the same
   transport session to ensure properly ordered delivery.

   A specification for a transport MUST identify any encodings that are
   supported.  If a configured subscription's transport allows different
   encodings, the specification MUST identify the default encoding.

   A subscriber that includes a "dscp" leaf in an "establish-
   subscription" request will need to understand and consider what the
   corresponding DSCP value represents in the domain of the publisher.

   Additional transport requirements will be dictated by the choice of
   transport used with a subscription.  For an example of such
   requirements, see [RFC8640].

12.  Security Considerations

   TODO.  New YANG models will be defined that need to document their
   security considerations, but otherwise the security considerations in
   YANG-Push should be sufficient.  Note, we should use the new security
   considerations template, which will allow this section to be
   considerable shorter.





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12.1.  Security Considerations from RFC 8639

   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 Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   With configured subscriptions, one or more publishers could be used
   to overwhelm a receiver.  To counter this, notification messages
   SHOULD NOT be sent to any receiver that does not support this
   specification.  Receivers that do not want notification messages need
   only terminate or refuse any transport sessions from the publisher.

   When a receiver of a configured subscription gets a new
   "subscription-started" message for a known subscription where it is
   already consuming events, it may indicate that an attacker has done
   something that has momentarily disrupted receiver connectivity.  To
   acquire events lost during this interval, the receiver SHOULD
   retrieve any event records generated since the last event record was
   received.  This can be accomplished by establishing a separate
   dynamic replay subscription with the same filtering criteria with the
   publisher, assuming that the publisher supports the "replay" feature.

   For dynamic subscriptions, implementations need to protect against
   malicious or buggy subscribers that may send a large number of
   "establish-subscription" requests and thereby use up system
   resources.  To cover this possibility, operators SHOULD monitor for
   such cases and, if discovered, take remedial action to limit the
   resources used, such as suspending or terminating a subset of the
   subscriptions or, if the underlying transport is session based,
   terminating the underlying transport session.

   The replay mechanisms described in Sections 2.4.2.1 and 2.5.6 provide
   access to historical event records.  By design, the access control
   model that protects these records could enable subscribers to view
   data to which they were not authorized at the time of collection.







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   Using DNS names for configured subscription's receiver "name" lookups
   can cause situations where the name resolves differently than
   expected on the publisher, so the recipient would be different than
   expected.

   An attacker that can cause the publisher to use an incorrect time can
   induce message replay by setting the time in the past and can
   introduce a risk of message loss by setting the time in the future.

   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:

   Container: "/filters"

   *  "stream-subtree-filter": Updating a filter could increase the
      computational complexity of all referencing subscriptions.

   *  "stream-xpath-filter": Updating a filter could increase the
      computational complexity of all referencing subscriptions.

   Container: "/subscriptions"

   The following considerations are only relevant for configuration
   operations made upon configured subscriptions:

   *  "configured-replay": Can be used to send a large number of event
      records to a receiver.

   *  "dependency": Can be used to force important traffic to be queued
      behind updates that are not as important.

   *  "dscp": If unvalidated, can result in the sending of traffic with
      a higher-priority marking than warranted.

   *  "id": Can overwrite an existing subscription, perhaps one
      configured by another entity.

   *  "name": Adding a new key entry can be used to attempt to send
      traffic to an unwilling receiver.

   *  "replay-start-time": Can be used to push very large logs, wasting
      resources.




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   *  "source-address": The configured address might not be able to
      reach a desired receiver.

   *  "source-interface": The configured interface might not be able to
      reach a desired receiver.

   *  "source-vrf": Can place a subscription in a virtual network where
      receivers are not entitled to view the subscribed content.

   *  "stop-time": Could be used to terminate content at an inopportune
      time.

   *  "stream": Could set a subscription to an event stream that does
      not contain content permitted for the targeted receivers.

   *  "stream-filter-name": Could be set to a filter that is not
      relevant to the event stream.

   *  "stream-subtree-filter": A complex filter can increase the
      computational resources for this subscription.

   *  "stream-xpath-filter": A complex filter can increase the
      computational resources for this subscription.

   *  "weighting": Allocating a large weight can overwhelm the dequeuing
      of other subscriptions.

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

   Container: "/streams"

   *  "name": If access control is not properly configured, can expose
      system internals to those who should not have access to this
      information.

   *  "replay-support": If access control is not properly configured,
      can expose logs to those who should not have access.

   Container: "/subscriptions"








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   *  "excluded-event-records": This leaf can provide information about
      filtered event records.  A network operator should have the proper
      permissions to know about such filtering.  However, exposing the
      count of excluded events to a receiver could leak information
      about the presence of access control filters that might be in
      place for that receiver.

   *  "subscription": Different operational teams might have a desire to
      set varying subsets of subscriptions.  Access control should be
      designed to permit read access to just the allowed set.

   Some of the RPC operations in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control access to these operations.  These are the
   operations and their sensitivity/vulnerability:

   RPC: all

   *  If a malicious or buggy subscriber sends an unexpectedly large
      number of RPCs, the result might be an excessive use of system
      resources on the publisher just to determine that these
      subscriptions should be declined.  In such a situation,
      subscription interactions MAY be terminated by terminating the
      transport session.

   RPC: "delete-subscription"

   *  No special considerations.

   RPC: "establish-subscription"

   *  Subscriptions could overload a publisher's resources.  For this
      reason, publishers MUST ensure that they have sufficient resources
      to fulfill this request; otherwise, they MUST reject the request.

   RPC: "kill-subscription"

   *  The "kill-subscription" RPC MUST be secured so that only
      connections with administrative rights are able to invoke this
      RPC.

   RPC: "modify-subscription"

   *  Subscriptions could overload a publisher's resources.  For this
      reason, publishers MUST ensure that they have sufficient resources
      to fulfill this request; otherwise, they MUST reject the request.





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12.2.  Security Considerations from RFC 8641

   The YANG module specified in this document defines a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [RFC8446].

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   There are a number of data nodes defined in this YANG module that are
   writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability.  (It should be noted that the
   YANG module defined in this document augments the YANG module defined
   in [RFC8639].  All security considerations that are listed in

   [RFC8639] are also relevant for datastore subscriptions.  In the
   following list, we focus on the new data nodes that are introduced in
   this document.)

   *  Subtree "selection-filter" under container "filters": This subtree
      allows a subscriber 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 being excluded from updates,
      in which case certain changes would 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 cause
      certain changes to 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, withhold certain updates to
      be sent to the receiver, and/or overwhelm a receiver.  For



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      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 a
      greater delay for successive updates (potentially affecting the
      responsiveness of applications that depend on the updates) or in a
      high volume of updates (to exhaust receiver resources).

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

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

   *  Subtree "selection-filter" under container "filters": If access
      control is not properly configured, can expose system internals to
      those who should not have access to this information.

   *  Subtree "datastore" in choice "target" in list "subscription": If
      access control is not properly configured, can expose system
      internals to those who should not have access to this information.

   *  Choice "update-trigger" in list "subscription": If access control
      is not properly configured, can expose system internals to those
      who should not have access to this information.

   Some of the RPC operations in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control access to these operations.  These are the
   operations and their sensitivity/vulnerability:

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






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13.  IANA Considerations

   TODO - This document will need to register new YANG models with IANA.

13.1.  IANA Considerations from RFC 8639

   IANA has registered one URI in the "ns" subregistry of the "IETF XML
   Registry" [RFC3688] maintained at https://www.iana.org/assignments/
   xml-registry (https://www.iana.org/assignments/xml-registry).  The
   following registration has been made per the format in [RFC3688]:

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

   IANA has registered one YANG module in the "YANG Module Names"
   registry [RFC6020] maintained at https://www.iana.org/assignments/
   yang-parameters (https://www.iana.org/assignments/yang-parameters).
   The following registration has been made per the format in [RFC6020]:

   Name: ietf-subscribed-notifications Namespace:
   urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications Prefix: sn
   Reference: RFC 8639

13.2.  IANA Considerations from RFC 8641

   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: RFC 8641






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Acknowledgments

   This inital draft is early work is based on discussions with various
   folk, particularly Thomas Graf, Holger Keller, Dan Voyer, Nils
   Warnke, and Alex Huang Feng; but also wider conversations that
   include: Benoit Claise, Pierre Francois, Paolo Lucente, Jean
   Quilbeuf, among others.

References

Normative References

   [I-D.draft-netana-netconf-notif-envelope]
              Feng, A. H., Francois, P., Graf, T., and B. Claise,
              "Extensible YANG model for YANG-Push Notifications", Work
              in Progress, Internet-Draft, draft-netana-netconf-notif-
              envelope-01, 3 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-netana-
              netconf-notif-envelope-01>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [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/rfc/rfc8340>.

   [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/rfc/rfc8341>.

Informative References

   [Consistency]
              Wikipedia, "Consistency (database systems)",
              <https://en.wikipedia.org/wiki/
              Consistency_(database_systems)>.







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   [EventualConsistency]
              Rouse, M., "Eventual Consistency",
              <https://www.techopedia.com/definition/29165/eventual-
              consistency>.

   [I-D.draft-ietf-netconf-http-client-server]
              Watsen, K., "YANG Groupings for HTTP Clients and HTTP
              Servers", Work in Progress, Internet-Draft, draft-ietf-
              netconf-http-client-server-23, 15 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
              http-client-server-23>.

   [I-D.ietf-nmop-network-anomaly-architecture]
              Graf, T., Du, W., and P. Francois, "An Architecture for a
              Network Anomaly Detection Framework", Work in Progress,
              Internet-Draft, draft-ietf-nmop-network-anomaly-
              architecture-01, 20 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-nmop-
              network-anomaly-architecture-01>.

   [I-D.ietf-nmop-yang-message-broker-integration]
              Graf, T. and A. Elhassany, "An Architecture for YANG-Push
              to Message Broker Integration", Work in Progress,
              Internet-Draft, draft-ietf-nmop-yang-message-broker-
              integration-05, 19 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-nmop-
              yang-message-broker-integration-05>.

   [Kafka]    Apache.org, "Apache Kafka", <https://kafka.apache.org/>.

   [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/rfc/rfc8342>.

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

   [RFC8639]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Subscription to YANG Notifications",
              RFC 8639, DOI 10.17487/RFC8639, September 2019,
              <https://www.rfc-editor.org/rfc/rfc8639>.

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/rfc/rfc8641>.




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Appendix A.  Example Configured Transport Augmentation (from RFC 8639)

   This appendix provides a non-normative example of how the YANG module
   defined in Section 4 may be enhanced to incorporate the configuration
   parameters needed to support the transport connectivity process.
   This example is not intended to be a complete transport model.  In
   this example, connectivity via an imaginary transport type of "foo"
   is explored.  For more on the overall objectives behind configuring
   transport connectivity for a configured subscription, see
   Section 2.5.7.

   The YANG module example defined in this appendix contains two main
   elements.  First is a transport identity "foo".  This transport
   identity allows a configuration agent to define "foo" as the selected
   type of transport for a subscription.  Second is a YANG case
   augmentation "foo", which is made to the
   "/subscriptions/subscription/receivers/receiver" node of Section 4.
   In this augmentation are the transport configuration parameters
   "address" and "port", which are necessary to make the connection to
   the receiver.

   module example-foo-subscribed-notifications {
     yang-version 1.1;
     namespace
       "urn:example:foo-subscribed-notifications";

     prefix fsn;

     import ietf-subscribed-notifications {
       prefix sn;
     }
     import ietf-inet-types {
       prefix inet;
     }

     description
       "Defines 'foo' as a supported type of configured transport for
       subscribed event notifications.";

     identity foo {
       base sn:transport;
       description
         "Transport type 'foo' is available for use as a configured
         subscription's transport protocol for subscribed
         notifications.";
     }

     augment



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       "/sn:subscriptions/sn:subscription/sn:receivers/sn:receiver" {
       when 'derived-from(../../../transport, "fsn:foo")';
       description
         "This augmentation makes transport parameters specific to 'foo'
         available for a receiver.";
       leaf address {
         type inet:host;
         mandatory true;
         description
           "Specifies the address to use for messages destined for a
           receiver.";
       }
       leaf port {
         type inet:port-number;
         mandatory true;
         description
           "Specifies the port number to use for messages destined for a
           receiver.";
       }
     }
   }

                 Figure 21: Example Transport Augmentation
                     for the Fictitious Protocol "foo"

   This example YANG module for transport "foo" will not be seen in a
   real-world deployment.  For a real-world deployment supporting an
   actual transport technology, a similar YANG module must be defined.

Appendix B.  Subscription Errors (from RFC 8641)

B.1.  RPC Failures

   Rejection of an RPC for any reason is indicated via an RPC error
   response from the publisher.  Valid RPC errors returned include both
   (1) existing transport-layer RPC error codes, such as those seen with
   NETCONF in [RFC6241] and (2) subscription-specific errors, such as
   those defined in the YANG data model.  As a result, how subscription
   errors are encoded in an RPC error response is transport dependent.

   References to specific identities in the ietf-subscribed-
   notifications YANG module [RFC8639] or the ietf-yang-push YANG module
   may be returned as part of the error responses resulting from failed
   attempts at datastore subscription.  For errors defined as part of
   the ietf-subscribed-notifications YANG module, please refer to
   [RFC8639].  The errors defined in this document, grouped per RPC, are
   as follows:




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

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

   1.  yang-data "establish-subscription-error-datastore": This MUST be
       returned if information identifying the reason for an RPC error
       has not been placed elsewhere in 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 in the transport portion of a
       failed "modify-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 in the transport portion of a failed
       "delete-subscription" or "kill-subscription" RPC response.

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

B.2.  Failure Notifications

   A subscription may be unexpectedly terminated or suspended
   independently of any RPC or configuration operation.  In such cases,
   indications of such a failure MUST be provided.  To accomplish this,
   a number of errors can be returned as part of the corresponding
   subscription state change notification.  For this purpose, the
   following error identities are introduced in this document, in
   addition to those that were already defined in [RFC8639]:




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

Author's Address

   Robert Wilton
   Cisco Systems
   Email: rwilton@cisco.com








































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