    Internet Engineering Task Force (IETF)                           E. Voit
    Request for Comments: 8639                                 Cisco Systems
    Category: Standards Track                                       A. Clemm
    ISSN: 2070-1721                                                Futurewei
                                                          A. Gonzalez Prieto
                                                                   Microsoft
                                                           E. Nilsen-Nygaard
                                                                 A. Tripathy
                                                               Cisco Systems
                                                              September 2019

                       Subscription to YANG Notifications

    Abstract

       This document defines a YANG data model and associated mechanisms
       enabling subscriber-specific subscriptions to a publisher's event
       streams.  Applying these elements allows a subscriber to request and
       receive a continuous, customized feed of publisher-generated
       information.

    Status of This Memo

       This is an Internet Standards Track document.

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

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8639.

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    Copyright Notice

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

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

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

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    RFC 8639                Subscribed Notifications          September 2019

    Table of Contents

       1. Introduction ....................................................3
          1.1. Motivation .................................................4
          1.2. Terminology ................................................4
          1.3. Solution Overview ..........................................6
          1.4. Relationship to RFC 5277 ...................................7
       2. Solution ........................................................8
          2.1. Event Streams ..............................................8
          2.2. Event Stream Filters .......................................9
          2.3. QoS ........................................................9
          2.4. Dynamic Subscriptions .....................................10
          2.5. Configured Subscriptions ..................................19
          2.6. Event Record Delivery .....................................27
          2.7. Subscription State Change Notifications ...................28
          2.8. Subscription Monitoring ...................................33
          2.9. Support for the "ietf-subscribed-notifications"
               YANG Module ...............................................34
       3. YANG Data Model Tree Diagrams ..................................34
          3.1. The "streams" Container ...................................34
          3.2. The "filters" Container ...................................35
          3.3. The "subscriptions" Container .............................35
       4. Event Notification Subscription YANG Module ....................37
       5. IANA Considerations ............................................66
       6. Implementation Considerations ..................................66
       7. Transport Requirements .........................................67
       8. Security Considerations ........................................68
       9. References .....................................................72
          9.1. Normative References ......................................72
          9.2. Informative References ....................................74
       Appendix A. Example Configured Transport Augmentation .............75
       Acknowledgments ...................................................77
       Authors' Addresses ................................................77

    1.  Introduction

       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.

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    RFC 8639                Subscribed Notifications          September 2019

       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.

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

    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:

       o  multiple subscriptions on a single transport session

       o  support for dynamic and configured subscriptions

       o  modification of an existing subscription in progress

       o  per-subscription operational counters

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

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

       o  independence from transport

    1.2.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

       o  Client: Defined in [RFC8342].

       o  Configuration: Defined in [RFC8342].

       o  Configuration datastore: Defined in [RFC8342].

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

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

       o  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.

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

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

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

       o  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.

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

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

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

       o  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.

       o  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].

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    1.3.  Solution Overview

       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.

       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:

       o  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.

       o  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.

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       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).

       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.

    1.4.  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:

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

       o  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].

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

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

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

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

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

       o  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.

    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.

    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.

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       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.

    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.

    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:

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

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

       o  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.

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       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.

       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.

    2.4.  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].

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    2.4.1.  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

       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".

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       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.

    2.4.2.  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.

       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.

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       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

       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.

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       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

    2.4.2.1.  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.

       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".

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       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.

    2.4.3.  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).

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

           +---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.

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       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.

           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

    2.4.4.  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.

    2.4.5.  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.

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

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

                  Figure 7: "kill-subscription" RPC Tree Diagram

    2.4.6.  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.

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       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:

       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.

    2.5.  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:

       o  persistence across publisher reboots,

       o  persistence even when transport is unavailable, and

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

       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

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       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:

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

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

       o  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.

    2.5.1.  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

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       "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
       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).

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       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
       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

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       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".

    2.5.2.  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

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       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.

    2.5.3.  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
       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.)

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    2.5.4.  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").

    2.5.5.  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.

    2.5.6.  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.

       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

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       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.

    2.5.7.  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.

       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.

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    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

       [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.

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    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.

    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
       referenced filter under the "within-subscription" subtree.

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

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       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

    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.

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       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:

       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.

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       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.

    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

       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.

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    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

    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.

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

            Figure 15: "subscription-resumed" Notification Tree Diagram

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    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

    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

    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.

       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

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       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.

    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.

    3.  YANG Data Model Tree Diagrams

       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.

    3.1.  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.

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       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

    3.2.  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.

         +--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

    3.3.  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.

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       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
               |       {dscp}?
               +--rw weighting?                                 uint8 {qos}?
               +--rw dependency?
               |       subscription-id {qos}?
               +--rw transport?                                 transport
               |       {configured}?
               +--rw encoding?                                  encoding
               +--rw purpose?                                   string
               |       {configured}?

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               +--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

    4.  Event Notification Subscription YANG Module

       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].

    <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";

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      }
      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
          "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',

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         '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
          "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.";
      }

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      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
           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";
      }

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      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
           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 {

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        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
           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;

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        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.";
      }

      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

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          "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.";
        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";
      }

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      /* 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.";
      }

      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

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          "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.

               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

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               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
                     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.";

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        }
        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
             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 {

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              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;
              }
            }
          }
        }
        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 {

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              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.";
            }
          }
        }
        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

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             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
          "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.";

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          }
          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
            "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.";
          }
        }
      }

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      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.";
          }
          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

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               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.
               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.";
          }
        }

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      }

      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.";
          }
        }
      }

      /*
       * 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.";

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        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
            "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.";

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        }
      }

      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:
               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'.

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                 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 {
          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 {

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          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
            "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;

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            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.";
          }
        }
      }
      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.";

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        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
                 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;

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                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
                  "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.";
              }
            }
          }

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          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
                   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.";
              }

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              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 {
                  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'.";
                  }

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                  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";
                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>

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    5.  IANA Considerations

       IANA has registered one URI in the "ns" subregistry of the "IETF XML
       Registry" [RFC3688] maintained at <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>.  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

    6.  Implementation Considerations

       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

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       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.

    7.  Transport Requirements

       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.

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       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].

    8.  Security Considerations

       The YANG module specified in this document defines a schema for data
       that is designed to be accessed via network management protocols such
       as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
       is the secure transport layer, and the mandatory-to-implement secure
       transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
       is HTTPS, and the mandatory-to-implement secure transport is TLS
       [RFC5246].

       The 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.

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       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.

       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"

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

       o  "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:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       o  "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"

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

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

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       Container: "/subscriptions"

       o  "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.

       o  "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

       o  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"

       o  No special considerations.

       RPC: "establish-subscription"

       o  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"

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

       RPC: "modify-subscription"

       o  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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    9.  References

    9.1.  Normative References

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

       [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
                  "Definition of the Differentiated Services Field (DS
                  Field) in the IPv4 and IPv6 Headers", RFC 2474,
                  DOI 10.17487/RFC2474, December 1998,
                  <https://www.rfc-editor.org/info/rfc2474>.

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

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

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

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

       [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
                  and A. Bierman, Ed., "Network Configuration Protocol
                  (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
                  <https://www.rfc-editor.org/info/rfc6241>.

       [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
                  Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
                  <https://www.rfc-editor.org/info/rfc6242>.

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

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

       [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
                  RFC 7951, DOI 10.17487/RFC7951, August 2016,
                  <https://www.rfc-editor.org/info/rfc7951>.

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

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

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

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

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

       [RFC8529]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
                  Liu, "YANG Data Model for Network Instances", RFC 8529,
                  DOI 10.17487/RFC8529, March 2019,
                  <https://www.rfc-editor.org/info/rfc8529>.

       [W3C.REC-xml-20081126]
                  Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
                  F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
                  Edition)", World Wide Web Consortium Recommendation
                  REC-xml-20081126, November 2008,
                  <https://www.w3.org/TR/2008/REC-xml-20081126>.

       [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
                  Version 1.0", November 1999,
                  <https://www.w3.org/TR/1999/REC-xpath-19991116>.

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    9.2.  Informative References

       [RESTCONF-Notif]
                  Voit, E., Rahman, R., Nilsen-Nygaard, E., Clemm, A.,
                  and A. Bierman, "Dynamic subscription to YANG Events
                  and Datastores over RESTCONF", Work in Progress,
                  draft-ietf-netconf-restconf-notif-15, June 2019.

       [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
                  Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
                  October 2013, <https://www.rfc-editor.org/info/rfc7049>.

       [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
                  Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
                  DOI 10.17487/RFC7540, May 2015,
                  <https://www.rfc-editor.org/info/rfc7540>.

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

       [RFC8071]  Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
                  RFC 8071, DOI 10.17487/RFC8071, February 2017,
                  <https://www.rfc-editor.org/info/rfc8071>.

       [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
                  Interchange Format", STD 90, RFC 8259,
                  DOI 10.17487/RFC8259, December 2017,
                  <https://www.rfc-editor.org/info/rfc8259>.

       [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
                  BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
                  <https://www.rfc-editor.org/info/rfc8340>.

       [RFC8640]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
                  E., and A. Tripathy, "Dynamic Subscription to YANG Events
                  and Datastores over NETCONF", RFC 8640,
                  DOI 10.17487/RFC8640, September 2019,
                  <https://www.rfc-editor.org/info/rfc8640>.

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

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    Appendix A.  Example Configured Transport Augmentation

       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.";
         }

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         augment
           "/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.

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    Acknowledgments

       For their valuable comments, discussions, and feedback, we wish to
       acknowledge Andy Bierman, Tim Jenkins, Martin Bjorklund, Kent Watsen,
       Balazs Lengyel, Robert Wilton, Sharon Chisholm, Hector Trevino, Susan
       Hares, Michael Scharf, and Guangying Zheng.

    Authors' Addresses

       Eric Voit
       Cisco Systems

       Email: evoit@cisco.com

       Alexander Clemm
       Futurewei

       Email: ludwig@clemm.org

       Alberto Gonzalez Prieto
       Microsoft

       Email: alberto.gonzalez@microsoft.com

       Einar Nilsen-Nygaard
       Cisco Systems

       Email: einarnn@cisco.com

       Ambika Prasad Tripathy
       Cisco Systems

       Email: ambtripa@cisco.com

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