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NETCONF                                                          E. Voit
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                                A. Clemm
Expires: February 16, 2019                                        Huawei
                                                      A. Gonzalez Prieto
                                                               Microsoft
                                                       E. Nilsen-Nygaard
                                                             A. Tripathy
                                                           Cisco Systems
                                                         August 15, 2018


        Customized Subscriptions to a Publisher's Event Streams
             draft-ietf-netconf-subscribed-notifications-16

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 for
   and receive a continuous, custom feed of publisher generated
   information.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 16, 2019.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Solution Overview . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Relationship to RFC-5277  . . . . . . . . . . . . . . . .   6
   2.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Event Streams . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Event Stream Filters  . . . . . . . . . . . . . . . . . .   7
     2.3.  QoS . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.4.  Dynamic Subscriptions . . . . . . . . . . . . . . . . . .   9
     2.5.  Configured Subscriptions  . . . . . . . . . . . . . . . .  16
     2.6.  Event Record Delivery . . . . . . . . . . . . . . . . . .  24
     2.7.  Subscription State Notifications  . . . . . . . . . . . .  24
     2.8.  Subscription Monitoring . . . . . . . . . . . . . . . . .  30
     2.9.  Advertisement . . . . . . . . . . . . . . . . . . . . . .  31
   3.  YANG Data Model Trees . . . . . . . . . . . . . . . . . . . .  31
     3.1.  Event Streams Container . . . . . . . . . . . . . . . . .  31
     3.2.  Filters Container . . . . . . . . . . . . . . . . . . . .  32
     3.3.  Subscriptions Container . . . . . . . . . . . . . . . . .  32
   4.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .  34
   5.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .  61
     5.1.  IANA Considerations . . . . . . . . . . . . . . . . . . .  61
     5.2.  Implementation Considerations . . . . . . . . . . . . . .  61
     5.3.  Transport Requirements  . . . . . . . . . . . . . . . . .  62
     5.4.  Security Considerations . . . . . . . . . . . . . . . . .  62
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  66
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  66
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  66
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  68
   Appendix A.  Example Configured Transport Augmentation  . . . . .  69
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  71
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  75

1.  Introduction

   This document defines a YANG data model and associated mechanisms
   enabling subscriber-specific subscriptions to a publisher's event
   streams.  Effectively this enables a 'subscribe then publish'
   capability where the customized information needs and access



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   permissions of each target receiver are understood by the publisher
   before subscribed event records are marshaled and pushed.  The
   receiver then gets a continuous, custom feed of publisher generated
   information.

   While the functionality defined in this document is transport-
   agnostic, transports like NETCONF [RFC6241] or RESTCONF [RFC8040] can
   be used to configure or dynamically signal subscriptions, and there
   are bindings defined for subscribed event record delivery for NETCONF
   within [I-D.draft-ietf-netconf-netconf-event-notifications], and for
   HTTP2 or HTTP1.1 within [I-D.draft-ietf-netconf-restconf-notif].

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

1.1.  Motivation

   Various limitations in [RFC5277] are discussed in [RFC7923].
   Resolving these 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.

   Client: defined in [RFC8342].

   Configuration: defined in [RFC8342].



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   Configuration datastore: defined in [RFC8342].

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

   Dynamic subscription: A subscription created dynamically by a
   subscriber via a remote procedure call.

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

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

   Event record: A set of information detailing an event.

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

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

   Notification message: Information intended for a receiver indicating
   that one or more event(s) have occurred.

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

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

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

   Subscription: A contract with a publisher, stipulating which
   information 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 within a
   publisher.  This mechanism is 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 at
       subscription parameters that, had they been present, would 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 of a configured subscription.
       Support for configured subscriptions is optional, with its
       availability advertised via a YANG feature.

   Additional characteristics differentiating configured from dynamic
   subscriptions include:

   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 within the publisher's
      applied configuration.  Being tied to configuration operations
      implies configured subscriptions can be configured to persist
      across reboots, and implies 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 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 subscription established via RPC cannot be
   modified through configuration operations.  Also note that transport
   specific transport drafts based on this specification MUST detail the
   life cycles of both dynamic and configured subscriptions.

   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 within [RFC5277].  Especially when
   extending an existing [RFC5277] implementation, it is important to
   understand what has been reused and what has been replaced.  Key
   relationships between these two documents include:

   o  this document defines a transport independent capability,
      [RFC5277] is specific to NETCONF.

   o  the data model in this document is used instead of the data model
      in Section 3.4 of [RFC5277] for the new operations.

   o  the RPC operations in this draft replaces the operation "create-
      subscription" 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 new document concurrently.

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

2.  Solution

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



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2.1.  Event Streams

   An event stream is a named entity on a publisher which exposes a
   continuously updating set of event records.  Each event stream is
   available for subscription.  It is out of the scope of this document
   to identify a) how streams are defined (other than the NETCONF
   stream), b) how event records are defined/generated, and c) how event
   records are assigned to streams.

   There is only one reserved event stream name within this document:
   "NETCONF".  The "NETCONF" event stream contains all NETCONF XML event
   record information supported by the publisher, except for the
   subscription state notifications described in Section 2.7.  Among
   these included NETCONF XML event records are individual YANG 1.1
   notifications described in section 7.16 of [RFC7950].  Each of these
   YANG 1.1 notifications will be treated as a distinct event record.
   Beyond the "NETCONF" stream, implementations MAY define additional
   event streams.

   As 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 within an event stream for which the receiver has no
   read access.  As an example of how this might be accomplished, see
   [RFC8341] section 3.4.6.  Note that per Section 2.7 of this document,
   subscription state change notifications are never filtered out.

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

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

2.2.  Event Stream Filters

   This document defines an extensible filtering mechanism.  The filter
   itself is a boolean test which is placed on the content of an event
   record.  A 'false' filtering result causes the event message to be
   excluded from delivery to a receiver.  A filter never results in
   information being stripped from within an event record prior to that
   event record being encapsulated within a notification message.  The



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   two optional event stream filtering syntaxes supported are [XPATH]
   and subtree [RFC6241].

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

2.3.  QoS

   This document provide for several 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
      destined for that receiver.

   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 [RFC2474] DSCP
   marking being placed within the IP header of any resulting
   notification messages and subscription state change notifications.

   For the "weighting" parameter, when concurrently dequeuing
   notification messages from multiple subscriptions to a receiver, the
   publisher MUST allocate bandwidth to each subscription proportionally
   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 within an RPC
   references a subscription which 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.








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2.4.  Dynamic Subscriptions

   Dynamic subscriptions are managed via protocol operations (in the
   form of [RFC7950], Section 7.14 RPCs) made against targets located
   within 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 within [I-D.ietf-netconf-yang-push]'s YANG model.

2.4.1.  Dynamic Subscription State Model

   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 insufficient 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 for a dynamic subscription

   Of interest in this state machine are the following:

   o  Successful "establish-subscription" or "modify-subscription" RPCs
      put the subscription into the active state.





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

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

   o  A publisher may choose to suspend a subscription when there is
      insufficient CPU or bandwidth available to service the
      subscription.  This is notified to a subscriber with a
      "subscription-suspended" 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 an active state.

   o  Even without a "modify-subscription" request, a publisher may
      return a subscription to the active state should the resource
      constraints become sufficient again.  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 transport selected by the subscriber
   to reach the publisher MUST be able to support multiple "establish-
   subscription" requests made within the same transport session.

   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  Where the transport used by the RPC supports multiple encodings,
      an optional "encoding" for the event records pushed.  Note: 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 "start-time" for the subscription.  The "start-time"
      MUST be in the past and indicates that the subscription is
      requesting a replay of previously generated information from the



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      event stream.  For more on replay, see Section 2.4.2.1.  Where
      there is no "start-time", the subscription starts immediately.

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

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

       +---x establish-subscription
          +---w input
          |  +---w (target)
          |  |  +--:(stream)
          |  |     +---w (stream-filter)?
          |  |     |  +--:(by-reference)
          |  |     |  |  +---w stream-filter-ref
          |  |     |  |          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 identifier                    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 which if
   considered in a subsequent "establish-subscription" RPC, may result
   in a successful subscription establishment.  Any such hints MUST be




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   transported within a yang-data "establish-subscription-stream-error-
   info" container included within the RPC error response.

       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 RPC yang-data tree diagram

2.4.2.1.  Requesting a replay of event records

   Replay provides the ability to establish a subscription which is also
   capable of passing recently generated event records.  In other words,
   as the subscription initializes itself, it sends any previously
   generated content from within the target event stream which meets the
   filter and timeframe criteria.  The end of these historical event
   records is identified via a "replay-completed" 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 they are placed into the
   stream.

   Replay is an optional feature which 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 within the
   publisher, or when event record entries in the log are purged.

   The inclusion of a "replay-start-time" within 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".

   If the time the replay starts is later than the time marked within
   any event records retained within 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 stream.  An event
   stream that does support replay is not expected to have an unlimited
   supply of saved notifications available to accommodate any given



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   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 model 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.  If the publisher accepts the requested
   modifications, it acknowledges success to the subscriber, then
   immediately starts sending event records based on the new terms.

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

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

       +---x modify-subscription
          +---w input
             +---w identifier               subscription-id
             +---w (target)
             |  +--:(stream)
             |     +---w (stream-filter)?
             |        +--:(by-reference)
             |        |  +---w stream-filter-ref
             |        |          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.)



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

   Below is a tree diagram for "modify-subscription-RPC-yang-data".  All
   objects contained in this tree are described within the included YANG
   model within 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 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.  If the delete request
   matches a known subscription established on the same transport
   session, then it MUST be deleted; otherwise it MUST be rejected with
   no changes to the publisher.

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

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

              Figure 6: delete-subscription RPC tree diagram

   Dynamic subscriptions can only be deleted via this RPC using the same
   transport session previously used for subscription establishment.
   Configured subscriptions cannot be deleted using RPCs.




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2.4.5.  Killing a Dynamic Subscription

   The "kill-subscription" operation permits an operator to end a
   dynamic subscription which is not associated with the transport
   session used for the RPC.  A publisher MUST terminate any dynamic
   subscription identified by RPC request.

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

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

        +---x kill-subscription
          +---w input
             +---w identifier    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 RPC error processing described in this
   section.  In all cases, RPC error information returned will use
   existing transport layer RPC structures, such as those seen with
   NETCONF in [RFC6241] Appendix A, or with RESTCONF in [RFC8040]
   Section 7.1.  These structures MUST be able to encode subscription
   specific errors identified below and defined within this document's
   YANG model.

   As a result of this mixture, how subscription errors are encoded
   within an RPC error response is transport dependent.  Following are
   valid errors which can occur for each RPC:













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    establish-subscription         modify-subscription
    ----------------------         -------------------
    dscp-unavailable               filter-unsupported
    encoding-unsupported           insufficient-resources
    filter-unsupported             no-such-subscription
    history-unavailable
    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 above, see
   [I-D.draft-ietf-netconf-netconf-event-notifications], Figure 10.

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

   1.  "establish-subscription-stream-error-info": This MUST be returned
       if an RPC error reason has not been placed elsewhere within 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 if
       an RPC error reason has not been placed elsewhere within 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 if an RPC
       error reason has not been placed elsewhere within 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 of their
   lifetime.  Multiple configured subscriptions MUST be supportable over
   a single transport session.

   Configured subscriptions have several characteristics distinguishing
   them from dynamic subscriptions:



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   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, supporting configured subscriptions is optional and
   advertised using the "configured" feature.  On a receiver of a
   configured subscription, support for dynamic subscriptions is
   optional except where replaying missed event records is required.

   In addition to the subscription parameters available to dynamic
   subscriptions 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 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 VRF on which to reach
         receivers.  This VRF is a network instance as defined within
         [I-D.draft-ietf-rtgwg-ni-model].  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 describing these parameters is shown in Figure 20
   within Section 3.3.  All parameters are described within the YANG
   model in Section 4.






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2.5.1.  Configured Subscription State Model

   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 valid or invalid
   states.  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 state model 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 which fails a
   re-evaluation.  See (2) in the diagram.  Second, the publisher might
   determine that the subscription is no longer supportable.  This could
   be for reasons of an unexpected but sustained increase in an event
   stream's event records, degraded CPU capacity, a more complex
   referenced filter, or other higher priority subscriptions which have
   usurped resources.  See (3) in the diagram.  No matter the case, a



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   "subscription-terminated" notification is sent to any receivers in an
   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 active or suspended states.
   Finally, a subscription may be deleted by configuration (4).

   When a subscription is in the valid state, a publisher will attempt
   to connect with all receivers of a configured subscription and
   deliver notification messages.  Below is the state machine for each
   receiver of a configured subscription.  This receiver state machine
   is fully contained within 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)|timeout |             |
     |   |          |<-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 which include the word 'receiver' show the possible
   states for an individual receiver of a valid configured subscription.
   * indicates a state change notification

   Figure 9: Receiver state 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 "connecting".  If
   transport connectivity is not available to any receiver and there are
   any notification messages to deliver, a transport session is
   established (e.g., through [RFC8071]).  Individual receivers are
   moved to the active state when a "subscription-started" 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 if both transport connectivity can be



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   verified to the receiver, and event records are not being dropped due
   to a publisher buffer overflow.  The result is that a receiver will
   remain active on the publisher as long as events aren't being lost,
   or the receiver cannot be reached.  In addition, a configured
   subscription's receiver MUST be moved to connecting if transport
   connectivity cannot be achieved, or if the receiver is reset via the
   "reset" action (b), (c).  For more on reset, see Section 2.5.5.

   A configured subscription's receiver MUST be moved to the suspended
   state if there is transport connectivity between the publisher and
   receiver, but notification messages are failing to be delivered due
   to publisher buffer overflow, or notification messages are not able
   to be generated for that receiver due to insufficient CPU (d).  This
   is indicated to the receiver by the "subscription-suspended" state
   change notification.

   A configured subscription receiver MUST be returned to the active
   state from the suspended state when notification messages are able to
   be generated, bandwidth is sufficient to handle the notification
   messages, and a receiver has successfully been sent a "subscription-
   resumed" or "subscription-modified" state change notification (e).
   The choice as to which of these two 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" 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 within the 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 [I-D.ietf-netconf-yang-push]
   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



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   whether the notification messages need to come from a specific egress
   interface on the publisher.  Receiver specific transport connectivity
   parameters MUST be configured via transport specific augmentations to
   this specification.  See Section 2.5.7 for details."

   After a subscription is successfully established, the publisher
   immediately sends a "subscription-started" 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 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
   overflow, 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 of
   [I-D.draft-ietf-netconf-netconf-event-notifications].

   Note that is possible to configure replay on a configured
   subscription.  This capability is to allow a configured subscription
   to exist on a system so that event records generated during and
   following boot can be buffered and pushed as soon as the transport
   session is established.

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 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 is 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.



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   Also note that if multiple modifications have occurred during the
   suspension, only the latest one need be sent to the receiver.)

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 state change
   notification stating the subscription has ended (i.e., "subscription-
   terminated").

2.5.5.  Resetting a Configured Subscription Receiver

   It is possible that a configured subscription to a receiver needs to
   be reset.  This is accomplished via the "reset" action within the
   YANG model at "/subscriptions/subscription/receivers/receiver/reset".
   This re-initialization 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 events for the subscribed stream.  All buffered event
   which have been retained since the last publisher restart will be
   sent.

   Replay of events 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 possibility of resending of event
   records logged prior to publisher restart.  It also ensures the same
   records will be sent to each configured receiver, regardless of the
   speed of transport connectivity establishment to each receiver.
   Finally, establishing restart as the earliest potential time for
   event records to be included within notification messages, a well-
   understood timeframe for replay is defined.

   As a result, when any configured subscription receivers become
   active, buffered event records will be sent immediately after the
   "subscription-started" notification.  The leading event record sent



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   will be the first event record subsequent to the latest of three
   different times: the "replay-log-creation-time", "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 a 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" within 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 timegap
   where records generated on the publisher were not successully
   received.  And with this information, the receiver may choose to
   dynamically subscribe to retrieve any event records placed into the
   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 model defined within 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 model does
   include 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 least one YANG
   model which 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 be encoded in a
   <notification> message as defined within [RFC5277], Section 4.  And
   per [RFC5277]'s "eventTime" object definition, the "eventTime" is
   populated with the event occurrence time.

   The following example within [RFC7950] section 7.16.3 is an example
   of a compliant message:

      <notification
             xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
          <eventTime>2007-09-01T10:00:00Z</eventTime>
          <link-failure xmlns="http://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

   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"
   notifications has been sent, respectively.

2.7.  Subscription State Notifications

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




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   Subscription state notifications are unlike other notifications in
   that they are never included in any stream.  Instead, they are
   inserted (as defined in this section) within the sequence of
   notification messages sent to a particular receiver.  Subscription
   state notifications cannot be 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
   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
   notification.

   The complete set of subscription state 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 state
   change notification are all the parameters of the subscription,
   except for the receiver(s) transport connection information and
   origin information indicating where notification messages will egress
   the publisher.  Note that if a referenced filter from the "filters"
   container has been used within 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.

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


















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      +---n subscription-started {configured}?
         +--ro identifier
         |       subscription-id
         +--ro (target)
         |  +--:(stream)
         |     +--ro (stream-filter)?
         |     |  +--:(by-reference)
         |     |  |  +--ro stream-filter-ref
         |     |  |          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.

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




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       +---n subscription-modified
          +--ro identifier               subscription-id
          +--ro (target)
          |  +--:(stream)
          |     +--ro (stream-filter)?
          |     |  +--:(by-reference)
          |     |  |  +--ro stream-filter-ref        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.  Where a configured subscription has been modified during the
       suspension of a receiver, the notification will be delayed until
       the receiver's suspension is lifted.  In this situation, the
       notification indicates that the subscription has been both
       modified and resumed.

   2.  While this state change will most commonly be used with
       configured subscriptions, with dynamic subscriptions, there is
       also one time this notification will be sent.  A "subscription-
       modified" state change notification MUST be sent if the contents
       of the filter identified by the subscription's "stream-filter-
       ref" leaf has changed.







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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 event records to a receiver for the following
   reasons:

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

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

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

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

   Each of the reasons above correspond one-to-one with a "reason"
   identityref specified within the YANG model.

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

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


       Figure 13: subscription-terminated notification tree diagram

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

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




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   constraints stop a publisher from serving a valid subscription.  The
   two conditions where is this 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 within the "reason" object.  No further
   notification 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 within the included YANG model
   within Section 4.

       +---n subscription-suspended
          +--ro identifier    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 state
   change notification may now be sent.

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

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

         Figure 15: subscription-resumed notification tree diagram

2.7.6.  subscription-completed

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






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

       +---n subscription-completed
          +--ro identifier    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 message containing an event record with a timestamp
   later than (1) the "stop-time" or (2) the subscription's start time.

   If a subscription contains no "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 within the included YANG model
   within Section 4.

       +---n replay-completed
          +--ro identifier    subscription-id

           Figure 17: replay-completed notification tree diagram

2.8.  Subscription Monitoring

   In the operational datastore, the container "subscriptions" maintains
   the state of all dynamic subscriptions, as well as all configured
   subscriptions.  Using datastore retrieval operations, or subscribing
   to the "subscriptions" container [I-D.ietf-netconf-yang-push] allows
   the state of subscriptions and their connectivity to receivers to be
   monitored.

   Each subscription in the operational datastore is represented as a
   list element.  Included in this list are event counters for each
   receiver, the state of each receiver, as well as the subscription
   parameters currently in effect.  The appearance of the leaf
   "configured-subscription-state" indicates that a particular
   subscription came into being via configuration.  This leaf also
   indicates if current state of that subscription is valid, invalid,
   and concluded.




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

   Dynamic subscriptions are removed from the operational 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 within the operational datastore and
   as a result are not configurable.

2.9.  Advertisement

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

3.  YANG Data Model Trees

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

3.1.  Event 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.

     +--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: Stream Container tree diagram




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   Above is a tree diagram for the streams container.  All objects
   contained in this tree are described within the included YANG model
   within Section 4.

3.2.  Filters Container

   The "filters" container maintains a list of all subscription filters
   that persist outside the life-cycle of a single subscription.  This
   enables pre-defined filters which may be referenced by more than one
   subscription.

       +--rw filters
          +--rw stream-filter* [identifier]
             +--rw identifier               filter-id
             +--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: Filter Container tree diagram

   Above is a tree diagram for the filters container.  All objects
   contained in this tree are described within the included YANG model
   within Section 4.

3.3.  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 which a publisher is
   serving.

    +--rw subscriptions
       +--rw subscription* [identifier]
          +--rw identifier
          |       subscription-id
          +--rw (target)
          |  +--:(stream)
          |     +--rw (stream-filter)?
          |     |  +--:(by-reference)
          |     |  |  +--rw stream-filter-ref
          |     |  |          stream-filter-ref
          |     |  +--:(within-subscription)
          |     |     +--rw (filter-spec)?
          |     |        +--:(stream-subtree-filter)
          |     |        |  +--rw stream-subtree-filter?   <anydata>
          |     |        |          {subtree}?



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          |     |        +--:(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}?
          +--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 count-sent?       yang:zero-based-counter64
                +--ro count-excluded?   yang:zero-based-counter64
                +--ro state             enumeration
                +---x reset {configured}?
                   +--ro output
                      +--ro time    yang:date-and-time

                   Figure 20: Subscriptions tree diagram

   Above is a tree diagram for the subscriptions container.  All objects
   contained in this tree are described within the included YANG model
   within Section 4.




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4.  Data Model

   This module imports typedefs from [RFC6991], [RFC8343], and
   [RFC8040], and it references [I-D.draft-ietf-rtgwg-ni-model],
   [XPATH], [RFC6241], [RFC7540], [RFC7951] and [RFC7950].

   [ note to the RFC Editor - please replace XXXX within this YANG model
   with the number of this document, and XXXY with the number of
   [I-D.draft-ietf-rtgwg-ni-model] ]

   [ note to the RFC Editor - please replace the two dates within the
   YANG module with the date of publication ]

<CODE BEGINS> file "ietf-subscribed-notifications@2018-08-15.yang"
module ietf-subscribed-notifications {
  yang-version 1.1;
  namespace
    "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications";

  prefix sn;

  import ietf-inet-types {
    prefix inet;
    reference
      "RFC 6991: Common YANG Data Types";
  }
  import ietf-interfaces {
    prefix if;
    reference
      "RFC 8343: A YANG Data Model for Interface Management";
  }
  import ietf-network-instance {
    prefix ni;
    reference
      "draft-ietf-rtgwg-ni-model-12: YANG 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";



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  contact
    "WG Web:   <http:/tools.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
    "Contains a YANG specification for subscribing to event records
    and receiving matching content within notification messages.

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

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

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

  revision 2018-08-15 {
    description
      "Initial version";
    reference
    "RFC XXXX: Customized Subscriptions to a Publisher's Event Streams";
  }

  /*
   * FEATURES
   */

  feature configured {
    description



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      "This feature indicates that configuration of subscription is
      supported.";
  }

  feature dscp {
    description
      "This feature indicates a publisher supports the placement of
      suggested prioritization levels for network transport within
      notification messages.";
  }

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

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

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

  feature qos {
    description
      "This feature indicates 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 which 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 {



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    description
      "This feature indicates support for YANG subtree filtering.";
    reference "RFC 6241, Section 6.";
  }

  feature supports-vrf {
    description
      "This feature indicates a publisher supports VRF configuration
      for configured subscriptions.  VRF support for dynamic
      subscriptions does not require this feature.";
    reference "RFC XXXY, Section 6.";
  }

  feature xpath {
    description
      "This feature indicates support for xpath filtering.";
    reference "http://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 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 to 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
      "Problem found while attempting to fulfill either a
      'delete-subscription' RPC request or a 'kill-subscription'
      RPC request.";
  }

  identity establish-subscription-error {



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     description
      "Problem found while attempting to fulfill an
      'establish-subscription' RPC request.";
  }

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

  identity subscription-suspended-reason {
     description
      "Problem condition communicated to a receiver as part of a
      'subscription-terminated' notification.";
  }

  identity subscription-terminated-reason {
     description
      "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 mark notification messages with a
      prioritization information in a way which 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 which doesn't exist, or to which they do not
     have access permissions.";
  }

  identity filter-unsupported {
    base establish-subscription-error;



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

  identity history-unavailable {
    base establish-subscription-error;
    if-feature "replay";
    description
     "Replay request too far into the past. This means the publisher
      does store historic information for the requested stream, but
      not back to the requested timestamp.";
  }

  identity insufficient-resources {
    base establish-subscription-error;
    base modify-subscription-error;
    base subscription-suspended-reason;
    description
      "The publisher has insufficient 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 non-existent subscription ID, an ID which belongs to another
      subscriber, or an ID for 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;



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    description
     "Not a subscribable 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 previously suspended subscription. The publisher
      has eliminated the subscription as it exceeded a time limit for
      suspension.";
  }

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

  /* Identities for encodings */

  identity configurable-encoding {
    description
      "If a transport identity derives from this identity, it means
       that it supports configurable encodings.";
  }

  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



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      "RFC 7951 - JSON Encoding of Data Modeled with YANG";
  }

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

  /*
   * TYPEDEFs
   */

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

  typedef filter-id {
    type string;
    description
      "A type to identify filters which can be associated with a
       subscription.";
  }

  typedef stream-filter-ref {
    type leafref {
      path "/sn:filters/sn:stream-filter/sn:identifier";
    }
    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 stream.";
  }

  typedef subscription-id {
    type uint32;
    description



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      "A type for subscription identifiers.";
  }

  typedef transport {
    type identityref {
      base transport;
    }
    description
      "Specifies 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 within the
          event stream.  For example, if the notification message
          contains an instance of a notification defined in YANG, then
          the top-level element is the name of the YANG notification.

          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, 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



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           individual, delineated event records as contained within
           the event stream.  For example, if the notification message
           contains an instance of a notification defined in YANG,
           then the top-level element is the name of the YANG
           notification, and the root node has this top-level element
           as the only child.

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

           The expression is evaluated in the following XPath context:

             o  The set of namespace declarations are those in scope on
                the 'stream-xpath-filter' leaf element.

             o  The set of variable bindings is empty.

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

             o  The context node is the root node.";
        reference
          "http://www.w3.org/TR/1999/REC-xpath-19991116
           RFC 7950, Section 10.";

      }
    }
  }

  grouping update-qos {
    description
      "This grouping describes Quality of Service information
       concerning a subscription.  This information is passed to lower
       layers for transport prioritization and treatment";
    leaf dscp {
      if-feature "dscp";
      type inet:dscp;
      default "0";
      description
        "The desired network transport priority level. This is the
         priority set on notification messages encapsulating the results
         of the subscription.  This transport priority is shared for all
         receivers of a given subscription.";
    }
    leaf weighting {



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      if-feature "qos";
      type uint8 {
         range "0 .. 255";
      }
      description
        "Relative weighting for a subscription. Allows an underlying
         transport layer perform informed load balance allocations
         between various subscriptions";
      reference
        "RFC-7540, section 5.3.2";
    }
    leaf dependency {
      if-feature "qos";
      type subscription-id;
      description
        "Provides the 'subscription-id' of a parent subscription which
         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 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, section 5.3.1";
    }
  }

  grouping subscription-policy-modifiable {
    description
      "This grouping describes all objects which may be changed
      in a subscription.";
    choice target {
      mandatory true;
      description
        "Identifies the source of information against which a
        subscription is being applied, as well as specifics on the
        subset of information desired from that source.";
      case stream {
        choice stream-filter {
          description
            "An event stream filter can be applied to a subscription.
            That filter will come either referenced from a global list,
            or be provided within the subscription itself.";
          case by-reference {
            description



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              "Apply a filter that has been configured separately.";
            leaf stream-filter-ref {
              type stream-filter-ref;
              mandatory true;
              description
                "References an existing stream filter which is to
                be applied to an event stream for the subscription.";
            }
          }
          case within-subscription {
            description
              "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 which can be passed over the RPCs defined in this
       model.";
    uses subscription-policy-modifiable {
      augment target/stream {
        description
          "Adds additional objects which can be modified by RPC.";
        leaf stream {
          type stream-ref {
            require-instance false;
          }
          mandatory true;
          description
            "Indicates the event stream to be considered for
            this subscription.";
        }



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        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, with event records being selected needing to
            be at or after the start at the time specified.  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;
      mandatory true;
      description
        "This leaf specifies the transport used to deliver
        messages destined to all receivers of a subscription.";
    }
    leaf encoding {
      when 'not(../transport) or derived-from(../transport,
      "sn:configurable-encoding")';
      type encoding;
      description
        "The type of encoding for notification messages.   For a
        dynamic subscription, if not included as part of an establish-
        subscription RPC, the encoding will be populated with the
        encoding used by that RPC.  For a configured subscription, if
        not explicitly configured the encoding with be the default
        encoding for an underlying transport.";
    }
    leaf purpose {
      if-feature "configured";



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      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. In case an error occurs, or the
       publisher cannot meet the terms of a subscription, an RPC error
       is returned, 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 identifier {
        type subscription-id;
        mandatory true;
        description
          "Identifier used for this subscription.";
      }
      leaf replay-start-time-revision {
        if-feature "replay";
        type yang:date-and-time;
          description
            "If a replay has been requested, this represents the
            earliest time covered by the event buffer for the requested
            stream.  The value of this object is the
            'replay-log-aged-time' if it exists.  Otherwise it is the



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            '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 within a subscription's RPC error
        response to indicate the failure reason.  This yang-data MUST be
        inserted if hints are to be provided back to the subscriber.";
      leaf reason {
        type identityref {
          base establish-subscription-error;
        }
        description
          "Indicates the reason why the subscription has failed to
          be created to a targeted stream.";
        }
      leaf filter-failure-hint {
        type string;
          description
            "Information describing where and/or why a provided filter
             was unsupportable for a subscription.";
      }
    }
  }

  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 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



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       'modify-subscription' will return a suspended subscription to an
       'active' state.";
    input {
      leaf identifier {
        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 which has been made against a
        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
          which 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.";
      }
    }
  }

  rpc delete-subscription {
    description
      "This RPC allows a subscriber to delete a subscription that
       was previously created from 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 an
       'delete-subscription-error-info' structure.";



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    input {
      leaf identifier {
        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 {
    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 an
       'delete-subscription-error-info' structure.";
    input {
      leaf identifier {
        type subscription-id;
        mandatory true;
        description
          "Identifier of the subscription that is to be deleted. Only
           subscriptions that were created using
           'establish-subscription' can be deleted via this RPC.";
      }
    }
  }

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



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        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. It must not be sent for any other
       reason.";
    leaf identifier {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }
  }

  notification subscription-completed {
    sn:subscription-state-notification;
    if-feature "configured";
    description
      "This notification is sent to indicate that a subscription has
       finished passing event records, as the 'stop-time' has been
       reached.";
    leaf identifier {
      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 state change notification includes both



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       modified and non-modified aspects of a subscription.";
    leaf identifier {
      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-ref' is populated, the filter within 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 identifier {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }
  }

  notification subscription-started {
    sn:subscription-state-notification;
    if-feature "configured";
    description
      "This notification indicates that a subscription has started and
        notifications are beginning to be sent. This notification shall
       only be sent to receivers of a subscription; it does not
       constitute a general-purpose notification.";
    leaf identifier {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }



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    uses subscription-policy {
      refine "target/stream/replay-start-time" {
         description
           "Indicates the time that a replay using for the streaming of
           buffered event records.  This will be populated with the most
           recent of the following: 'replay-log-creation-time',
           '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-ref' is populated, the filter within the
          subscription came from the 'filters' container.  Otherwise it
          is populated in-line as part of the subscription.";
      }
      augment "target/stream" {
        description
          "This augmentation adds additional parameters specific to a
          subscription-started notification.";
        leaf replay-previous-event-time {
          when "../replay-start-time";
          if-feature "replay";
          type yang:date-and-time;
            description
            "If there is at least one event in the replay buffer prior
            to 'replay-start-time', this gives the time of the event
            generated immediately prior to the 'replay-start-time'.

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

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



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    leaf identifier {
      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 which resulted in the suspension.";
    }
  }

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


  /*
   * DATA NODES
   */

  container streams {
    config false;
    description
      "This container contains information on the built-in streams
      provided by the publisher.";
    list stream {



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      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;
        mandatory true;
        description
          "A description of the event stream, including such information
           as the type of event records that are available within this
           event stream.";
      }
      leaf replay-support {
        if-feature "replay";
        type empty;
        description
          "Indicates that event record replay is available on this
          stream.";
      }
      leaf replay-log-creation-time {
        when "../replay-support";
        if-feature "replay";
        type yang:date-and-time;
        mandatory true;
        description
          "The timestamp of the creation of the log used to support the
          replay function on this 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 {
        if-feature "replay";
        type yang:date-and-time;
        description
          "The timestamp associated with last event record which has
           been aged out of the log.  This timestamp identifies how far
           back into history this replay log extends, if it doesn't
           extend back to the 'replay-log-creation-time'.  This object
           MUST be present if replay is supported and any event records
           have been aged out of the log.";
      }



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    }
  }

  container filters {
    description
      "This container 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 "identifier";
      description
        "A list of pre-configured filters that can be applied to
        subscriptions.";
      leaf identifier {
        type filter-id;
        description
          "An identifier 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 setup via RPC primitives as well as subscriptions
       that have been established via configuration.";
    list subscription {
      key "identifier";
      description
        "The identity and specific parameters of a subscription.
         Subscriptions within this list can be created using a control
         channel or RPC, or be established through configuration.

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



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            subscription.  If an event stream has been removed,
            and no longer can be referenced by an active subscription,
            send a 'subscription-terminated' notification with
            'stream-unavailable' as the reason.  If a configured
            subscription refers to a non-existent stream, move that
            subscription to the 'invalid' state.";
        }
        augment "target/stream" {
          description
            "Enables objects to added to a configured stream
            subscription";
          leaf configured-replay {
            if-feature "configured";
            if-feature "replay";
            type empty;
            description
              "The presence of this leaf indicates that replay for the
              configured subscription should start at the earliest time
              in the event log, or at the publisher boot time, which
              ever 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 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 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";



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            }
            description
              "VRF from which notification messages should egress a
              publisher.";
          }
          leaf source-address {
            type inet:ip-address-no-zone;
            description
              "The source address for the notification messages.  If a
              source VRF exists, but this object doesn't, a publisher's
              default address for that VRF must be used.";
          }
        }
      }
      leaf configured-subscription-state {
        if-feature "configured";
        type enumeration {
          enum valid {
            value 1;
            description
              "Connection is active and healthy.";
          }
          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,
                but 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 system established state
          of the subscription.";
      }
      container receivers {
        description
          "Set of receivers in a subscription.";
        list receiver {
          key "name";
          min-elements 1;



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          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 augmentated to a specific receiver
            within this list.";
          leaf name {
            type string;
            description
              "Identifies a unique receiver for a subscription.";
          }
          leaf count-sent {
            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 subscription
              transitions to the valid state.";
          }
          leaf count-excluded {
            type yang:zero-based-counter64;
            config false;
            description
              "The number of event records explicitly removed either
              via 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 'count-sent' is initialized.";
          }
          leaf state {
            type enumeration {
              enum active {
                value 1;
                description
                  "Receiver is currently being sent any applicable
                  notification messages for the subscription.";
              }
              enum suspended {
                value 2;
                description
                  "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



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                  "A subscription has been configured, but a
                  'subscription-started' state change notification needs
                  to be successfully received before notification
                  messages are sent.

                  If the 'reset' action is invoked for a receiver of an
                  active configured subscription, the state must be
                  moved to 'connecting'.";
              }
              enum timeout {
                value 4;
                if-feature "configured";
                description
                  "A subscription has failed in sending a subscription
                  started state change to the receiver.
                  Additional attempts at 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 info it
              is possible to determine whether a subscriber is currently
              generating notification messages intended for that
              receiver.";
          }
          action reset {
            if-feature "configured";
            description
              "Allows the reset of this configured subscription receiver
              to the 'connecting' state. This enables the
              connection process to be re-initiated.";
            output {
              leaf time {
                type yang:date-and-time;
                mandatory true;
                description
                  "Time a publisher returned the receiver to a
                  'connecting' state.";
              }
            }
          }
        }
      }
    }
  }



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}
<CODE ENDS>

5.  Considerations

5.1.  IANA Considerations

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

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

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

   Name: ietf-subscribed-notifications
   Namespace: urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications
   Prefix: sn
   Reference: draft-ietf-netconf-ietf-subscribed-notifications-11.txt
   (RFC form)

5.2.  Implementation Considerations

   To support deployments including both configured and dynamic
   subscriptions, it is recommended to split subscription identifiers
   into static and dynamic halves.  That way it eliminates the
   possibility of collisions if the configured subscriptions attempt to
   set a subscription-id which might have already been dynamically
   allocated.  A best practice is to use lower half the "identifier"
   object's integer space when that "identifier" is assigned by an
   external entity (such as with a configured subscription).  This
   leaves the upper half of subscription identifiers 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 against the set of
   receivers using the subscription identifier as a handle for that set.
   But for streaming updates, state change notifications are local to a
   receiver.  In this specification it is the case that receivers get no
   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 identifier
   across the receivers to allow that correlation.



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   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 which failed to be delivered just prior to the reboot.
   Delivering these event records be accomplished by leveraging the
   "eventTime" from the last event record received prior to the receipt
   of a "subscription-started" state change notification.  With this
   "eventTime" and the "replay-start-time" from the "subscription-
   started" notification, an independent dynamic subscription can be
   established which retrieves any event records which may have been
   generated but not sent to the receiver.

5.3.  Transport Requirements

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

   For both configured and dynamic subscriptions the publisher MUST
   authenticate a receiver via some transport level mechanism before
   sending any event records for which they are 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 and the publisher MUST
   ensure that the receiver has sufficient authorization to perform the
   function they are requesting against the specific subset of content
   involved.

   A specific transport specification 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 notifications MUST be sent on the same transport
   session to ensure the properly ordered delivery.

   Additional transport requirements will be dictated by the choice of
   transport used with a subscription.  For an example of such
   requirements with NETCONF transport, see
   [I-D.draft-ietf-netconf-netconf-event-notifications].

5.4.  Security Considerations

   The YANG module specified in this document defines a schema for data
   that is designed to be accessed via network management transports
   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




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   RESTCONF layer is HTTPS, and the mandatory-to-implement secure
   transport is TLS [RFC5246].

   The NETCONF 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 operations
   and content.

   One subscription identifier can be used for two or more receivers of
   the same configured subscription.  But due to the possibility of
   different access control permissions per receiver, it cannot be
   assumed that each receiver is getting identical updates.

   With configured subscriptions, one or more publishers could be used
   to overwhelm a receiver.  Notification messages SHOULD NOT be sent to
   any receiver which 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, the receiver SHOULD retrieve any event
   records generated since the last event record was received.  This can
   be accomplish by establishing a separate dynamic replay subscription
   with the same filtering criteria with the publisher", assuming the
   publisher supports the "replay" feature.

   For dynamic subscriptions, implementations need to protect against
   malicious or buggy subscribers which may send a large number
   "establish-subscription" requests, thereby using 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, terminate the
   underlying transport session.

   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
   where there is a specific sensitivity/vulnerability:

   Container: "/filters"

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



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   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 less important updates.

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

   o  "identifier": can overwrite an existing subscription, perhaps one
      configured by another entity.

   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 into 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 containing
      no content permitted for the targeted receivers.

   o  "stream-filter-ref": could be set to a filter which is irrelevant
      to the event stream.

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





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

   o  "weighting": placing 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 have no access to this
      information.

   o  "replay-support": if access control is not properly configured,
      can expose logs to those who should have no access.

   Container: "/subscriptions"

   o  "count-excluded": leaf can provide information about filtered
      event records.  A network operator should have permissions to know
      about such filtering.

   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.



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   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 or otherwise 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 or otherwise reject the request.

6.  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.

7.  References

7.1.  Normative References

   [I-D.draft-ietf-rtgwg-ni-model]
              Berger, L., Hopps, C., and A. Lindem, "YANG Network
              Instances", draft-ietf-rtgwg-ni-model-12 (work in
              progress), March 2018.

   [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>.



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   [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>.

   [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>.




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

   [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>.

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

7.2.  Informative References

   [I-D.draft-ietf-netconf-netconf-event-notifications]
              Clemm, Alexander., Voit, Eric., Gonzalez Prieto, Alberto.,
              Nilsen-Nygaard, E., and A. Tripathy, "NETCONF support for
              event notifications", May 2018,
              <https://datatracker.ietf.org/doc/
              draft-ietf-netconf-netconf-event-notifications/>.

   [I-D.draft-ietf-netconf-restconf-notif]
              Voit, Eric., Clemm, Alexander., Tripathy, A., Nilsen-
              Nygaard, E., and Alberto. Gonzalez Prieto, "Restconf and
              HTTP transport for event notifications", May 2018,
              <https://datatracker.ietf.org/doc/
              draft-ietf-netconf-restconf-notif/>.

   [I-D.ietf-netconf-yang-push]
              Clemm, Alexander., Voit, Eric., Gonzalez Prieto, Alberto.,
              Tripathy, A., Nilsen-Nygaard, E., Bierman, A., and B.
              Lengyel, "YANG Datastore Subscription", May 2018,
              <https://datatracker.ietf.org/doc/
              draft-ietf-netconf-yang-push/>.

   [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>.






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   [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>.

   [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>.

Appendix A.  Example Configured Transport Augmentation

   This appendix provides a non-normative example of how the YANG model
   defined in Section 4 may be enhanced to incorporate the configuration
   parameters needed to support the transport connectivity process.  In
   this example, connectivity via an imaginary transport type of "foo"
   is explored.  For more on the overall need, see Section 2.5.7.

   The YANG model defined in this section 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.  Within this augmentation are the
   transport configuration parameters "address" and "port" which are
   necessary to make the connect to the receiver.



























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module ietf-foo-subscribed-notifications {
  yang-version 1.1;
  namespace
    "urn:ietf:params:xml:ns:yang:ietf-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 transport protocol for subscribed notifications.";
  }

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

           Figure 21: Example Transport Augmentation for NETCONF



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   This example YANG model 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 model must be defined.

   [ note to the RFC Editor - this YANG module is not encapsulated in
   CODE BEGINS and CODE ENDS as it is just an example, and not intended
   to be included in any supported IETF inventory of models.  ]

Appendix B.  Changes between revisions

   (To be removed by RFC editor prior to publication)

   v15 - v16

   o  Mandatory empty case "transport" removed.

   o  Appendix case turned from "netconf" to "foo".

   v14 - v15

   o  Text tweaks.

   o  Mandatory empty case "transport" added for transport parameters.
      This includes a new section and an appendix explaining it.

   v13 - v14

   o  Removed the 'address' leaf.

   o  Replay is now of type 'empty' for configured.

   v12 - v13

   o  Tweaks from Kent's comments

   o  Referenced in YANG model updated per Tom Petch's comments

   o  Added leaf replay-previous-event-time

   o  Renamed the event counters, downshifted the subscription states

   v11 - v12

   o  Tweaks from Kent's, Tim's, and Martin's comments

   o  Clarified dscp text, and made its own feature

   o  YANG model tweaks alphabetizing, features.



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   v10 - v11

   o  access control filtering of events in streams included to match
      RFC5277 behavior

   o  security considerations updated based on YANG template.

   o  dependency QoS made non-normative on HTTP2 QoS

   o  tree diagrams referenced for each figure using them

   o  reference numbers placed into state machine figures

   o  broke configured replay into its own section

   o  many tweaks updates based on LC and YANG doctor reviews

   o  trees and YANG model reconciled were deltas existed

   o  new feature for interface originated.

   o  dscp removed from the qos feature

   o  YANG model updated in a way which collapses groups only used once
      so that they are part of the 'subscriptions' container.

   o  alternative encodings only allowed for transports which support
      them.

   v09 - v10

   o  Typos and tweaks

   v08 - v09

   o  NMDA model supported.  Non NMDA version at https://github.com/
      netconf-wg/rfc5277bis/

   o  Error mechanism revamped to match to embedded implementations.

   o  Explicitly identified error codes relevant to each RPC/
      Notification

   v07 - v08

   o  Split YANG trees to separate document subsections.





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   o  Clarified configured state machine based on Balazs comments, and
      moved it into the configured subscription subsections.

   o  Normative reference to Network Instance model for VRF

   o  One transport for all receivers of configured subscriptions.

   o  QoS section moved in from yang-push

   v06 - v07

   o  Clarification on state machine for configured subscriptions.

   v05 - v06

   o  Made changes proposed by Martin, Kent, and others on the list.
      Most significant of these are stream returned to string (with the
      SYSLOG identity removed), intro section on 5277 relationship, an
      identity set moved to an enumeration, clean up of definitions/
      terminology, state machine proposed for configured subscriptions
      with a clean-up of subscription state options.

   o  JSON and XML become features.  Also Xpath and subtree filtering
      become features

   o  Terminology updates with event records, and refinement of filters
      to just event stream filters.

   o  Encoding refined in establish-subscription so it takes the RPC's
      encoding as the default.

   o  Namespaces in examples fixed.

   v04 - v05

   o  Returned to the explicit filter subtyping of v00

   o  stream object changed to 'name' from 'stream'

   o  Cleaned up examples

   o  Clarified that JSON support needs notification-messages draft.

   v03 - v04

   o  Moved back to the use of RFC5277 one-way notifications and
      encodings.




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   v03 - v04

   o  Replay updated

   v02 - v03

   o  RPCs and Notification support is identified by the Notification
      2.0 capability.

   o  Updates to filtering identities and text

   o  New error type for unsupportable volume of updates

   o  Text tweaks.

   v01 - v02

   o  Subscription status moved under receiver.

   v00 - v01

   o  Security considerations updated

   o  Intro rewrite, as well as scattered text changes

   o  Added Appendix A, to help match this to related drafts in progress

   o  Updated filtering definitions, and filter types in yang file, and
      moved to identities for filter types

   o  Added Syslog as an event stream

   o  HTTP2 moved in from YANG-Push as a transport option

   o  Replay made an optional feature for events.  Won't apply to
      datastores

   o  Enabled notification timestamp to have different formats.

   o  Two error codes added.

   v01 5277bis - v00 subscribed notifications

   o  Kill subscription RPC added.

   o  Renamed from 5277bis to Subscribed Notifications.

   o  Changed the notification capabilities version from 1.1 to 2.0.



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   o  Extracted create-subscription and other elements of RFC5277.

   o  Error conditions added, and made specific in return codes.

   o  Simplified yang model structure for removal of 'basic' grouping.

   o  Added a grouping for items which cannot be statically configured.

   o  Operational counters per receiver.

   o  Subscription-id and filter-id renamed to identifier

   o  Section for replay added.  Replay now cannot be configured.

   o  Control plane notification renamed to subscription state
      notification

   o  Source address: Source-vrf changed to string, default address
      option added

   o  In yang model: 'info' changed to 'policy'

   o  Scattered text clarifications

   v00 - v01 of 5277bis

   o  YANG Model changes.  New groupings for subscription info to allow
      restriction of what is changeable via RPC.  Removed notifications
      for adding and removing receivers of configured subscriptions.

   o  Expanded/renamed definitions from event server to publisher, and
      client to subscriber as applicable.  Updated the definitions to
      include and expand on RFC 5277.

   o  Removal of redundancy with other drafts

   o  Many other clean-ups of wording and terminology

Authors' Addresses

   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com







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   Alexander Clemm
   Huawei

   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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