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NETCONF                                                         A. Clemm
Internet-Draft                                                    Huawei
Intended status: Standards Track                                 E. Voit
Expires: September 1, 2017                            A. Gonzalez Prieto
                                                             A. Tripathy
                                                       E. Nilsen-Nygaard
                                                           Cisco Systems
                                                              A. Bierman
                                                               YumaWorks
                                                              B. Lengyel
                                                                Ericsson
                                                       February 28, 2017


               Subscribing to YANG datastore push updates
                    draft-ietf-netconf-yang-push-05

Abstract

   This document defines a subscription and push mechanism for YANG
   datastores.  This mechanism allows subscriber applications to request
   updates from a YANG datastore, which are then pushed by the publisher
   to a receiver per a subscription policy, without requiring additional
   subscriber requests.

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 http://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 September 1, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   5
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Subscription Model  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Negotiation of Subscription Policies  . . . . . . . . . .   7
     3.3.  On-Change Considerations  . . . . . . . . . . . . . . . .   8
     3.4.  Data Encodings  . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  YANG object filters . . . . . . . . . . . . . . . . . . .  10
     3.6.  Push Data Stream and Transport Mapping  . . . . . . . . .  10
     3.7.  Subscription management . . . . . . . . . . . . . . . . .  14
     3.8.  Other considerations  . . . . . . . . . . . . . . . . . .  15
   4.  A YANG data model for management of datastore push
       subscriptions . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.2.  Filters . . . . . . . . . . . . . . . . . . . . . . . . .  25
     4.3.  Subscription configuration  . . . . . . . . . . . . . . .  26
     4.4.  Notifications . . . . . . . . . . . . . . . . . . . . . .  27
     4.5.  RPCs  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   5.  YANG module . . . . . . . . . . . . . . . . . . . . . . . . .  34
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  47
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  47
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  47
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  47
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  48



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   Appendix A.  Technologies to be considered for future iterations   49
     A.1.  Proxy YANG Subscription when the Subscriber and Receiver
           are different . . . . . . . . . . . . . . . . . . . . . .  49
     A.2.  OpState and Filters . . . . . . . . . . . . . . . . . . .  49
     A.3.  Splitting push updates  . . . . . . . . . . . . . . . . .  50
     A.4.  Potential Subscription Parameters . . . . . . . . . . . .  50
   Appendix B.  Issues that are currently being worked and resolved   51
   Appendix C.  Changes between revisions  . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  52

1.  Introduction

   YANG [RFC7950] was originally designed for the Netconf protocol
   [RFC6241] which focused on configuration data.  However, YANG can be
   used to model both configuration and operational data.  It is
   therefore reasonable to expect YANG datastores will increasingly be
   used to support applications that care about about both.

   For example, service assurance applications will need to be aware of
   any remote updates to configuration and operational objects.  Rapid
   awareness of object changes will enable such things as validating and
   maintaining cross-network integrity and consistency, or monitoring
   state and key performance indicators of remote devices.

   Traditional approaches to remote visibility have been built on
   polling.  With polling, data is periodically explicitly retrieved by
   a client from a server to stay up-to-date.  However, there are issues
   associated with polling-based management:

   o  It introduces additional load on network, devices, and
      applications.  Each polling cycle requires a separate yet arguably
      redundant request that results in an interrupt, requires parsing,
      consumes bandwidth.

   o  It lacks robustness.  Polling cycles may be missed, requests may
      be delayed or get lost, often particularly in cases when the
      network is under stress and hence exactly when the need for the
      data is the greatest.

   o  Data may be difficult to calibrate and compare.  Polling requests
      may undergo slight fluctuations, resulting in intervals of
      different lengths which makes data hard to compare.  Likewise,
      pollers may have difficulty issuing requests that reach all
      devices at the same time, resulting in offset polling intervals
      which again make data hard to compare.

   A more effective alternative to polling is when an application can
   request to be automatically updated on current relevant content of a



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   datastore.  If such a request is accepted, interesting updates will
   subsequently be pushed from that datastore.

   Dependence on polling-based management is typically considered an
   important shortcoming of applications that rely on MIBs polled using
   SNMP [RFC1157].  However, without a provision to support a push-based
   alternative, there is no reason to believe that management
   applications that operate on YANG datastores will be any more
   effective, as they would follow the same request/response pattern.

   While YANG allows the definition of push notifications, such
   notifications generally indicate the occurrence of certain well-
   specified event conditions, such as the onset of an alarm condition
   or the occurrence of an error.  A capability to subscribe to and
   deliver such pre-defined event notifications has been defined in
   [RFC5277].  In addition, configuration change notifications have been
   defined in [RFC6470].  These change notifications pertain only to
   configuration information, not to operational state, and convey the
   root of the subtree to which changes were applied along with the
   edits, but not the modified data nodes and their values.
   Furthermore, while delivery of updates using notifications is a
   viable option, some applications desire the ability to stream updates
   using other transports.

   Accordingly, there is a need for a service that allows applications
   to dynamically subscribe to updates of a YANG datastore and that
   allows the publisher to push those updates, possibly using one of
   several delivery mechanisms.  Additionally, support for subscriptions
   configured directly on the publisher are also useful when dynamic
   signaling is undesirable or unsupported.  The requirements for such a
   service are documented in [RFC7923].

   This document proposes a solution.  The solution builds on top of the
   NETCONF WG's Subscribed Notifications draft [I-D:netconf-sub-notif].
   At its core, the solution defined here suppliments that work by
   introducing datastore push update mechanisms, and providing
   corresponding extensions to the event subscription model.  The
   document also includes YANG data model augmentations which extend the
   model and RPCs defined within [I-D:netconf-sub-notif].

   Key capabilities worth highlighting include:

   o  Additions to event subscription mechanisms which allow clients to
      subscribe to datastore updates.  The subscription allows clients
      to specify which data they are interested in, what types of
      updates (e.g., create, delete, modify), and to provide filter
      criteria that data must meet for updates to be sent.  Furthermore,
      subscriptions can specify a policy that directs when updates are



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      provided.  For example, a client may request to be updated
      periodically in certain intervals, or whenever data changes occur.

   o  Format and contents of the YANG push updates themselves.

   o  The ability for a publisher to push back on requested subscription
      parameters.  Because not every publisher may support every
      requested update policy for every piece of data, it is necessary
      for a publisher to be able to indicate whether or not it is
      capable of supporting a requested subscription, and possibly allow
      to hints at subscription parameters which might have succeeded.

   o  Subscription parameters which allow the specification of QoS
      extensions to address prioritization between independent streams
      of updates.

2.  Definitions and Acronyms

   Many of the terms in this document are defined in
   [I-D:netconf-sub-notif].  Please see that document for these
   definitions.

   Data node: An instance of management information in a YANG datastore.

   Data node update: A data item containing the current value/property
   of a Data node at the time the data node update was created.

   Data record: A record containing a set of one or more data node
   instances and their associated values.

   Datastore: A conceptual store of instantiated management information,
   with individual data items represented by data nodes which are
   arranged in hierarchical manner.

   Datastream: A continuous stream of data records, each including a set
   of updates, i.e. data node instances and their associated values.

   Data subtree: An instantiated data node and the data nodes that are
   hierarchically contained within it.

   Push-update stream: A conceptual data stream of a datastore that
   streams the entire datastore contents continuously and perpetually.

   Update: A data item containing the current value of a data node.

   Update notification: An Event Notification including those data node
   update(s) to be pushed in order to meet the obligations of a single




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   Subscription.  All included data node updates must reflect the state
   of a Datastore at a snapshot in time.

   Update record: A representation of a data node update as a data
   record.  An update record can be included as part of an update
   stream.  It can also be logged for retrieval.  In general, an update
   record will include the value/property of a data node.  It may also
   include information about the type of data node update, i.e. whether
   the data node was modified/updated, or newly created, or deleted.

   Update trigger: A mechanism, as specified by a Subscription Policy,
   that determines when a data node update is to be communicated. (e.g.,
   a change trigger, invoked when the value of a data node changes or a
   data node is created or deleted, or a time trigger, invoked after the
   laps of a periodic time interval.)

   YANG object filter: A filter that contains evaluation criteria which
   are evaluated against YANG objects of a subscription.  An update is
   only published if the object meets the specified filter criteria.

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

3.  Solution Overview

   This document specifies a solution for a push update subscription
   service.  This solution supports the dynamic as well as configured
   subscriptions to information updates from YANG datastores.  A
   subscription might target exposed operational and/or configuration
   YANG objects on a device.  YANG objects are subsequently pushed from
   the publisher to the receiver per the terms of the subscription.

3.1.  Subscription Model

   YANG-push subscriptions are defined using a data model that is itself
   defined in YANG.  This model augments the event subscription model
   defined in [I-D:netconf-sub-notif] and introduces new capabilities
   that allow subscribers to specify what to include in an update
   notification and what triggers such an update notification.

   o  Enhancements to filters.  Specifically the filter must at least
      identify at least one targeted yang data node/subtree.  The filter
      may also define additional yang nodes/subtrees to include or
      exclude.  The publisher must only send to the receiver those data
      node updates that can traverse applied filter.  Filters can be
      specified "inline" as part of the subscription, or can be
      configured separately and referenced by a subscription in order to
      facilitate reuse of complex filters.



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   o  A subscription policy definition regarding the update trigger when
      to send new update notifications.

      *  For periodic subscriptions, the trigger is defined by two
         parameters that defines the interval with which updates are to
         be pushed.  These parameters are the period/interval of
         reporting duration, and an anchor time which can be used to
         calculate at which times updates needs to be assembled and
         sent.

      *  For on-change subscriptions, the trigger occurs whenever a
         change in the subscribed information is detected.  On-change
         subscriptions have more complex semantics that can be guided by
         additional parameters.  Please refer also to Section 3.3.

         +  One parameter specifiing the dampening period.  This period
            is the interval which must pass before a successive update
            notification for the same Subscription is sent.  Note that
            the dampening period applies to the set of all data nodes
            within a single subscription.  This means that on the first
            change of an object, an update notification containing that
            object is sent either immediately or at the end of a
            dampening period already in effect.

         +  Another parameter allowing the restriction of the types of
            changes for which updates are sent (changes to object
            values, object creation or deletion events).

         +  A third parameter specifing whether or not a complete push-
            update with all the subscribed data should be sent at the
            beginning of a subscription.  Such a push provides the
            receiver the current state, and establish the frame of
            reference for subsequent updates.

   o  Anydata encoding for the contents of periodic and on-change push
      updates.

   The subscription data model is described via augmentations to
   [I-D:netconf-sub-notif] later in this specification.  It is
   conceivable that additional subscription parameters might be
   interesting.  Augmentations to the subscription data model may be
   used for this.

3.2.  Negotiation of Subscription Policies

   A dynamic subscription request SHOULD be declined based on
   publisher's assessment that it may be unable to provide a filtered
   update notifications that would meet the terms of the request.  But a



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   subscriber may quickly follow up with a new subscription request
   using different parameters.

   Random guessing at different parameters should be discouraged.
   Therefore to minimize the number of subscription iterations between
   subscriber and publisher, dynamic subscriptions must support a simple
   negotiation between subscribers and publishers for subscription
   parameters.  This negotiation is limited to either an establish or
   modify subscription request, followed by no-success response.  The
   no-success message, where available SHOULD include in the returned
   error information parameters.  The returned parameters provide
   information that, when followed, increase the likelihood of success
   for subsequent requests.  However, there are no guarantee that
   subsequent requests for this subscriber will in fact be accepted.

   Negotiable parameters which may be returned from a publisher beyond
   those from [I-D:netconf-sub-notif] include: hints at acceptable time
   intervals, size estimates forn the number or objects which would be
   returned from a filter, and the names of targeted objects not found
   in the publisher's YANG tree.

3.3.  On-Change Considerations

   On-change subscriptions allow subscribers to subscribe to updates
   whenever changes to objects occur.  As such, on-change subscriptions
   are of particular interest for data that changes relatively
   infrequently, yet that require applications to be notified with
   minimal delay when changes do occur.

   On-change subscriptions tend to be more difficult to implement than
   periodic subscriptions.  Specifically, on-change subscriptions may
   involve a notion of state to see if a change occurred between past
   and current state, or the ability to tap into changes as they occur
   in the underlying system.  Accordingly, on-change subscriptions may
   not be supported by all implementations or for every object.

   When an on-change subscription is requested for a datastream with a
   given subtree filter, where not all objects support on-change update
   triggers, only the objects supporting on-change will be provided.
   For more on how objects are so marked, see Section 3.8.5

   Any updates for an on-change subscription will include only supported
   objects for which a change was detected and which met the filtering
   criteria.  To avoid flooding receivers with repeated updates for
   fast-changing objects, or objects with oscillating values, an on-
   change subscription allows for the definition of a dampening period.
   Once an update for a given object is sent, no other updates for this
   particular subscription are sent until the end of the dampening



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   period.  Values sent at the end of the dampening period are the
   current values of all changed objects which are current at the time
   the dampening period expires.  Changed objects includes those which
   were deleted or newly created during that dampening period.

   On-change subscriptions can be refined to let users subscribe only to
   certain types of changes, for example, only to object creations and
   deletions, but not to modifications of object values.

3.4.  Data Encodings

   Subscribed data is encoded in either XML or JSON format.  A publisher
   MUST support XML encoding and MAY support JSON encoding.

   It is conceivable that additional encodings may be supported as
   options in the future.  This can be accomplished by augmenting the
   subscription data model with additional identity statements used to
   refer to requested encodings.

3.4.1.  Periodic Subscriptions

   In a periodic subscription, the data included as part of an update
   corresponds to data that could have been simply retrieved using a get
   operation and is encoded in the same way.  XML encoding rules for
   data nodes are defined in [RFC7950].  JSON encoding rules are defined
   in [RFC7951].  This encoding is valid JSON, but also has special
   encoding rules to identify module namespaces and provide consistent
   type processing of YANG data.

3.4.2.  On-Change Subscriptions

   In an on-change subscription, updates need to indicate not only
   values of changed data nodes but also the types of changes that
   occurred since the last update, such as whether data nodes were newly
   created since the last update or whether they were merely modified,
   as well as which data nodes were deleted.

   Encoding rules for data in on-change updates correspond to how data
   would be encoded in a YANG-patch operation as specified in [RFC8072].
   The "YANG-patch" would in this case be applied to the earlier state
   reported by the preceding update, to result in the now-current state
   of YANG data.  Of course, contrary to a YANG-patch operation, the
   data is sent from the publisher to the receiver and is not restricted
   to configuration data.







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3.5.  YANG object filters

   Subscriptions can specify filters for subscribed data.  The following
   filters are supported:

   o  subtree-filter: A subtree filter specifies a subtree that the
      subscription refers to.  When specified, updates will only concern
      data nodes from this subtree.  Syntax and semantics correspond to
      that specified for [RFC6241] section 6.

   o  xpath-filter: An XPath filter specifies an XPath expression
      applied to the data in an update, assuming XML-encoded data.

   Only a single filter can be applied to a subscription at a time.

   It is conceivable for implementations to support other filters.  For
   example, an on-change filter might specify that changes in values
   should be sent only when the magnitude of the change since previous
   updates exceeds a certain threshold.  It is possible to augment the
   subscription data model with additional filter types.

3.6.  Push Data Stream and Transport Mapping

   Pushing data based on a subscription could be considered analogous to
   a response to a data retrieval request, e.g., a "get" request.
   However, contrary to such a request, multiple responses to the same
   request may get sent over a longer period of time.

   An applicable mechanism is that of a notification.  There are however
   some specifics that need to be considered.  Contrary to other
   notifications that are associated with alarms and unexpected event
   occurrences, update notifications are solicited, i.e. tied to a
   particular subscription which triggered the notification.

   A push update notification contains several parameters:

   o  A subscription correlator, referencing the name of the
      subscription on whose behalf the notification is sent.

   o  Data nodes containing a representation of the datastore subtree(s)
      containing the updates.  In all cases, the subtree(s) are filtered
      per access control rules to contain only data that the subscriber
      is authorized to see.  For on-change subscriptions, the subtree
      may only contain the data nodes which have changed since the start
      of the previous dampening interval.

   This document introduces two generic notifications: "push-update" and
   "push-change-update".  Those notifications may be encapsulated on a



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   transport (e.g., NETCONF or HTTP) to carry data records with updates
   of datastore contents as specified by a subscription.  It is possible
   also map notifications to other transports and encodings and use the
   same subscription model; however, the definition of such mappings is
   outside the scope of this document.

   A push-update notification defines a complete update of the datastore
   per the terms of a subscription.  This type of notification is used
   for continuous updates of periodic subscriptions.  A push-update
   notification can also used be for the on-change subscriptions in two
   cases.  First it will be used as the initial push-update if there is
   a need to synchronize the receiver at the start of a new
   subscription.  It also may be sent if the publisher later chooses to
   resynch a previously synched on-change subscription.  The push-update
   record contains a data snippet that contains an instantiated subtree
   with the subscribed contents.  The content of the update notification
   is equivalent to the contents that would be obtained had the same
   data been explicitly retrieved using e.g., a Netconf "get"-operation,
   with the same filters applied.

   The contents of the push-update notification conceptually represents
   the union of all data nodes in the yang modules supported by the
   publisher.  However, in a YANG data model, it is not practical to
   model the precise data contained in the updates as part of the
   notification.  To capture this data, a single parameter that can
   encapsulate the full set of subscribed datastore contents is used,
   not parameters that represent data nodes one at a time.

   A push-change-update notification is the most common type of update
   for on-change subscriptions.  The update record in this case contains
   a data snippet that indicates the full set of changes that data nodes
   have undergone since the last notification of YANG objects.  In other
   words, this indicates which data nodes have been created, deleted, or
   have had changes to their values.  The format of the data snippet
   follows YANG-patch [RFC8072], i.e. the same format that would be used
   with a YANG-patch operation to apply changes to a data tree,
   indicating the creates, deletes, and modifications of data nodes.
   Please note that as the update can include a mix of configuration and
   operational data

   The following is an example of push notification.  It contains an
   update for subscription 1011, including a subtree with root foo that
   contains a leaf, bar:








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        <notification
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <eventTime>2015-03-09T19:14:56Z</eventTime>
           <push-update
               xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
             <subscription-id>1011</subscription-id>
             <time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
             <datastore-contents-xml>
                <foo>
                   <bar>some_string</bar>
                </foo>
             </datastore-contents-xml>
           </push-update>
        </notification>

                          Figure 1: Push example

   The following is an example of an on-change notification.  It
   contains an update for subscription 89, including a new value for a
   leaf called beta, which is a child of a top-level container called
   alpha:

        <notification
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <eventTime>2015-03-09T19:14:56Z</eventTime>
           <push-change-update xmlns=
               "urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
             <subscription-id>89</subscription-id>
             <time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
             <datastore-changes-xml>
               <alpha xmlns="http://example.com/sample-data/1.0" >
                 <beta>1500</beta>
               </alpha>
             </datastore-changes-xml>
           </push-change-update>
        </notification>

                   Figure 2: Push example for on change

   The equivalent update when requesting json encoding:











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        <notification
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <eventTime>2015-03-09T19:14:56Z</eventTime>
           <push-change-update xmlns=
               "urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
             <subscription-id>89</subscription-id>
             <time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
             <datastore-changes-json>
               {
                "ietf-yang-patch:yang-patch": {
                "patch-id": [
                  null
                ],
                "edit": [
                  {
                      "edit-id": "edit1",
                      "operation": "merge",
                      "target": "/alpha/beta",
                      "value": {
                          "beta": 1500
                      }
                  }
                ]
               }
             }
             </datastore-changes-json>
           </push-change-update>
        </notification>

              Figure 3: Push example for on change with JSON

   When the beta leaf is deleted, the publisher may send



















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        <notification
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <eventTime>2015-03-09T19:14:56Z</eventTime>
           <push-change-update xmlns=
               "urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
             <subscription-id>89</subscription-id>
             <time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
             <datastore-changes-xml>
               <alpha xmlns="http://example.com/sample-data/1.0" >
                 <beta urn:ietf:params:xml:ns:netconf:base:1.0:
                     operation="delete"/>
               </alpha>
             </datastore-changes-xml>
           </push-change-update>
        </notification>

              Figure 4: 2nd push example for on change update

3.7.  Subscription management

   A [I-D:netconf-sub-notif] subscription needs enhancment to support
   YANG Push subscription negotiation.  Specifically, these enhancements
   are needed to signal to the subscriber why an attempt has failed.

   A subscription can be rejected for multiple reasons, including the
   lack of authorization to establish a subscription, the lack of read
   authorization on the requested data node, or the inability of the
   publisher to provide a stream with the requested semantics.  In such
   cases, no subscription is established.  Instead, the subscription-
   result with the failure reason is returned as part of the RPC
   response.  In addition, a set of alternative subscription parameters
   MAY be returned that would likely have resulted in acceptance of the
   subscription request, which the subscriber may try for a future
   subscription attempt.

   It should be noted that a rejected subscription does not result in
   the generation of an rpc-reply with an rpc-error element, as neither
   the specification of YANG-push specific errors nor the specification
   of additional data parameters to be returned in an error case are
   supported as part of a YANG data model.

   For instance, for the following request:









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   <netconf:rpc message-id="101"
      xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
      <establish-subscription
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         <stream>push-update</stream>
         <filter netconf:type="xpath"
               xmlns:ex="http://example.com/sample-data/1.0"
               select="/ex:foo"/>
         <period>500</period>
         <encoding>encode-xml</encoding>
      </establish-subscription>
   </netconf:rpc>

                 Figure 5: Establish-Subscription example

   the publisher might return:

   <rpc-reply message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
      <subscription-result
        xmlns="http://urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
        error-insufficient-resources
      </subscription-result>
      <period>2000</period>
   </rpc-reply>

                     Figure 6: Error response example

3.8.  Other considerations

3.8.1.  Authorization

   A receiver of subscription data may only be sent updates for which
   they have proper authorization.  Data that is being pushed therefore
   needs to be subjected to a filter that applies all corresponding
   rules applicable at the time of a specific pushed update, silently
   removing any non-authorized data from subtrees.

   The authorization model for data in YANG datastores is described in
   the Netconf Access Control Model [RFC6536].  However, some
   clarifications to that RFC are needed so that the desired access
   control behavior is applied to pushed updates.

   One of these clarifications is that a subscription may only be
   established if the receiver has read access to every data node
   specifically named within the subscription filter.





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                    +-------------+                 +-------------+
       subscription |  protocol   |                 |   target    |
       request -->  |  operation  | ------------->  |  data node  |
                    |  allowed?   |   datastore     |   access    |
                    +-------------+   or state      |  allowed?   |
                                      data access   +-------------+


                 Figure 7: Access control for subscription

   Likewise if a receiver no longer has read access permission to a data
   node named/targeted within a filter, the subscription must be
   abnormally terminated (with loss of access permission as the reason
   provided).

   Another clarification to [RFC6536] is that each of the individual
   nodes in a pushed update must also go through access control
   filtering.  This includes new nodes added since the last update
   notification, as well as existing nodes.  For each of these read
   access must be verified.  The methods of doing this efficiently are
   left to implementation.

                      +-------------+      +-------------------+
       subscription   |  data node  |  yes |                   |
       update  -->    |   access    | ---> | add data node     |
                      |  allowed?   |      | to update message |
                      +-------------+      +-------------------+

                 Figure 8: Access control for push updates

   If there are read access control changes applied under the data node
   named/targeted within a filter, no notifications indicating the fact
   that this has occurred should be provided.

3.8.2.  Robustness and reliability considerations

   Particularly in the case of on-change push updates, it is important
   that push updates do not get lost.

   Update notifications will typically traverse a secure and reliable
   transport.  Notifications will not be reordered, and will also
   contain a time stamp.  Despite these protections for on-change, it is
   possible that complete update notifications get lost.  For this
   reason, update sequence numbers for push-change-updates may be
   included in a subscription so that an application can determine if an
   update has been lost.





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   At the same time, it is conceivable that under certain circumstances,
   a publisher will recognize that it is unable to include within an
   update notification the full set of objects desired per the terms of
   a subscription.  In this case, the publisher must take one or more of
   the following actions.

   o  A publisher must set the updates-not-sent flag on any update
      notification which is known to be missing information.

   o  It may choose to suspend and resume a subscription as per
      [I-D:netconf-sub-notif].

   o  When resuming an on-change subscription, the publisher should
      generate a complete patch from the previous update notification.
      If this is not possible and the synch-on-start option is
      configured, then the full datastore contents may be sent instead
      (effectively replacing the previous contents).  If neither of
      these are possible, then an updates-not-sent flag must be included
      on the next push-change-update.

3.8.3.  Update size and fragmentation considerations

   Depending on the subscription, the volume of updates can become quite
   large.  Additionally, based on the platform, it is possible that
   push-updates for a single subscription are best sent independently
   from different line-cards.  Therefore, it may not always be practical
   to send the entire update in a single chunk.  Implementations of
   push-update MAY therefore choose, at their discretion, to "chunk"
   updates and break them out into several push-update notifications.
   In this case the updates-not-sent flag will indicate that no single
   push-update is complete.  Push-change-updates may also be chunked as
   long as none of the changed objects in the separate pushes are state-
   entangled.

3.8.4.  Implementation considerations

   Implementation specifics are outside the scope of this specification.
   That said,it should be noted that monitoring of operational state
   changes inside a system can be associated with significant
   implementation challenges.

   Even periodic retrieval and push of operational counters may consume
   considerable system resources.  In addition the on-change push of
   small amounts of configuration data may, depending on the
   implementation, require invocation of APIs, possibly on an object-by-
   object basis, possibly involving additional internal interrupts, etc.





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   For those reasons, it is important for an implementation to
   understand what subscriptions it can or cannot support.  It is far
   preferable to decline a subscription request then to accept such a
   request when it cannot be met.

   Whether or not a subscription can be supported will in general be
   determined by a combination of several factors, including the
   subscription policy (on-change or periodic, with on-change in general
   being the more challenging of the two), the period in which to report
   changes (1 second periods will consume more resources than 1 hour
   periods), the amount of data in the subtree that is being subscribed
   to, and the number and combination of other subscriptions that are
   concurrently being serviced.

   When providing access control to every node in a pushed update, it is
   possible to make and update efficient access control filters for an
   update.  These filters can be set upon subscription and applied
   against a stream of updates.  These filters need only be updated when
   (a) there is a new node added/removed from the subscribed tree with
   different permissions than its parent, or (b) read access permissions
   have been changed on nodes under the target node for the subscriber.

3.8.5.  Identifying on-change notifiable YANG objects

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

   Support for on-change notification is usually specific to the
   individual YANG model and/or implementation so it is possible to
   define in design time.  System integrators need this information
   (without reading any data from a live node).

   The default assumption is that no data nodes support on-change
   notification.  Schema nodes and subtrees that support on-change
   notifications MUST be marked as such with the YANG extension
   notifiable-on-change.

   If the model designer wants to add the notifiable-on-change statement
   to an existing module, but wants to avoid modifying the text of the
   existing module, the notifiable-on-change statement may be added
   using deviation statements.





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    extension notifiable-on-change {
      Indicates whether changes to the data node are reportable in
      on-change subscriptions.

      The statement MUST only be a substatement of the leaf, leaf-list,
      container, list, anyxml, anydata  statements. Zero or One
      notifiable-on-change statement is allowed per parent statement. NO
      substatements are allowed.

      The argument is a boolean value indicating whether on-change
      notifications are supported. If notifiable-on-change is not
      specified, the default is the same as the parent data node's
      value. For top level data nodes the default value is false.";

      argument value;
    }

                      Figure 9: Notifiable Extension

   When an on-change subscription is established data-nodes marked with
   notifiable-on-change false; will be automatically filtered out.  This
   also means that authorization checks need be performed on them.

    deviation /sys:system/sys:system-time {
      deviate add {
         yp:notifiable-on-change false;
      }
    }

                       Figure 10: Deviation Example

4.  A YANG data model for management of datastore push subscriptions

4.1.  Overview

   The YANG data model for datastore push subscriptions is depicted in
   the following figure.  Following Yang tree convention in the
   depiction, brackets enclose list keys, "rw" means configuration, "ro"
   operational state data, "?" designates optional nodes, "*" designates
   nodes that can have multiple instances.  Parantheses with a name in
   the middle enclose choice and case nodes.  A "+" at the end of a line
   indicates that the line is to be concatenated with the subsequent
   line.  New YANG tree notation is the i] which indicates that the node
   in that line has been brought in / imported from another model, and
   an (a) which indicates this is the specific imported node augmented.
   In the figure below, all have been imported from 5277bis.  The model
   consists mostly of augmentations to RPCs and notifications defined in




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   the data model for subscriptions for event notifications of
   [I-D:netconf-sub-notif].

  (Note: the yp indicates augmentations from yang push above and
  beyond the event-notifications model)
  module: ietf-subscribed-notifications
      +--ro streams
      |  +--ro stream*   stream
      +--rw filters
      |  +--rw filter* [identifier]
      |     +--rw identifier           filter-id
      |     +--rw (filter-type)?
      |        +--:(by-reference)
      |        |  +--rw filter-ref?          filter-ref
      |        +--:(event-filter)
      |        |  +--rw filter?
      |        +--:(yp:update-filter)
      |           +--rw (yp:update-filter)?
      |              +--:(yp:subtree)
      |              |  +--rw yp:subtree-filter?
      |              +--:(yp:xpath)
      |                 +--rw yp:xpath-filter?     yang:xpath1.0
      +--rw subscription-config {configured-subscriptions}?
      |  +--rw subscription* [identifier]
      |     +--rw identifier              subscription-id
      |     +--rw stream?                 stream
      |     +--rw encoding?               encoding
      |     +--rw stop-time?              yang:date-and-time
      |     +--rw (filter-type)?
      |     |  +--:(by-reference)
      |     |  |  +--rw filter-ref?             filter-ref
      |     |  +--:(event-filter)
      |     |  |  +--rw filter?
      |     |  +--:(yp:update-filter)
      |     |     +--rw (yp:update-filter)?
      |     |        +--:(yp:subtree)
      |     |        |  +--rw yp:subtree-filter?
      |     |        +--:(yp:xpath)
      |     |           +--rw yp:xpath-filter?        yang:xpath1.0
      |     +--rw receivers
      |     |  +--rw receiver* [address]
      |     |     +--rw address     inet:host
      |     |     +--rw port        inet:port-number
      |     |     +--rw protocol?   transport-protocol
      |     +--rw (notification-origin)?
      |     |  +--:(interface-originated)
      |     |  |  +--rw source-interface?       if:interface-ref
      |     |  +--:(address-originated)



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      |     |     +--rw source-vrf?             string
      |     |     +--rw source-address?         inet:ip-address-no-zone
      |     +--rw (yp:update-trigger)?
      |     |  +--:(yp:periodic)
      |     |  |  +--rw yp:period               yang:timeticks
      |     |  |  +--rw yp:anchor-time?         yang:date-and-time
      |     |  +--:(yp:on-change) {on-change}?
      |     |     +--rw yp:dampening-period     yang:timeticks
      |     |     +--rw yp:no-synch-on-start?   empty
      |     |     +--rw yp:excluded-change*     change-type
      |     +--rw yp:dscp?                inet:dscp
      |     +--rw yp:weighting?           uint8
      |     +--rw yp:dependency?          sn:subscription-id
      +--ro subscriptions
         +--ro subscription* [identifier]
            +--ro identifier                 subscription-id
            +--ro configured-subscription?
            |                       empty {configured-subscriptions}?
            +--ro stream?                    stream
            +--ro encoding?                  encoding
            +--ro replay-start-time?         yang:date-and-time
            +--ro stop-time?                 yang:date-and-time
            +--ro (filter-type)?
            |  +--:(by-reference)
            |  |  +--ro filter-ref?                filter-ref
            |  +--:(event-filter)
            |  |  +--ro filter?
            |  +--:(yp:update-filter)
            |     +--ro (yp:update-filter)?
            |        +--:(yp:subtree)
            |        |  +--ro yp:subtree-filter?
            |        +--:(yp:xpath)
            |           +--ro yp:xpath-filter?           yang:xpath1.0
            +--ro (notification-origin)?
            |  +--:(interface-originated)
            |  |  +--ro source-interface?          if:interface-ref
            |  +--:(address-originated)
            |     +--ro source-vrf?              string
            |     +--ro source-address?          inet:ip-address-no-zone
            +--ro receivers
            |  +--ro receiver* [address]
            |     +--ro address                   inet:host
            |     +--ro port                      inet:port-number
            |     +--ro protocol?                 transport-protocol
            |     +--ro pushed-notifications?     yang:counter64
            |     +--ro excluded-notifications?   yang:counter64
            +--ro subscription-status?       subscription-status
            +--ro (yp:update-trigger)?



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            |  +--:(yp:periodic)
            |  |  +--ro yp:period                  yang:timeticks
            |  |  +--ro yp:anchor-time?            yang:date-and-time
            |  +--:(yp:on-change) {on-change}?
            |     +--ro yp:dampening-period        yang:timeticks
            |     +--ro yp:no-synch-on-start?      empty
            |     +--ro yp:excluded-change*        change-type
            +--ro yp:dscp?                   inet:dscp
            +--ro yp:weighting?              uint8
            +--ro yp:dependency?             sn:subscription-id

    rpcs:
      +---x establish-subscription
      |  +---w input
      |  |  +---w stream?                 stream
      |  |  +---w encoding?               encoding
      |  |  +---w replay-start-time?      yang:date-and-time
      |  |  +---w stop-time?              yang:date-and-time
      |  |  +---w (filter-type)?
      |  |  |  +--:(by-reference)
      |  |  |  |  +---w filter-ref?             filter-ref
      |  |  |  +--:(event-filter)
      |  |  |  |  +---w filter?
      |  |  |  +--:(yp:update-filter)
      |  |  |     +---w (yp:update-filter)?
      |  |  |        +--:(yp:subtree)
      |  |  |        |  +---w yp:subtree-filter?
      |  |  |        +--:(yp:xpath)
      |  |  |           +---w yp:xpath-filter?        yang:xpath1.0
      |  |  +---w (yp:update-trigger)?
      |  |  |  +--:(yp:periodic)
      |  |  |  |  +---w yp:period               yang:timeticks
      |  |  |  |  +---w yp:anchor-time?         yang:date-and-time
      |  |  |  +--:(yp:on-change) {on-change}?
      |  |  |     +---w yp:dampening-period     yang:timeticks
      |  |  |     +---w yp:no-synch-on-start?   empty
      |  |  |     +---w yp:excluded-change*     change-type
      |  |  +---w yp:dscp?                inet:dscp
      |  |  +---w yp:weighting?           uint8
      |  |  +---w yp:dependency?          sn:subscription-id
      |  +--ro output
      |     +--ro subscription-result         subscription-result
      |     +--ro (result)?
      |        +--:(no-success)
      |        |  +--ro filter-failure?             string
      |        |  +--ro replay-start-time-hint?     yang:date-and-time
      |        |  +--ro yp:period-hint?             yang:timeticks
      |        |  +--ro yp:error-path?              string



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      |        |  +--ro yp:object-count-estimate?   uint32
      |        |  +--ro yp:object-count-limit?      uint32
      |        |  +--ro yp:kilobytes-estimate?      uint32
      |        |  +--ro yp:kilobytes-limit?         uint32
      |        +--:(success)
      |           +--ro identifier                  subscription-id
      +---x modify-subscription
      |  +---w input
      |  |  +---w identifier?            subscription-id
      |  |  +---w stop-time?             yang:date-and-time
      |  |  +---w (filter-type)?
      |  |  |  +--:(by-reference)
      |  |  |  |  +---w filter-ref?            filter-ref
      |  |  |  +--:(event-filter)
      |  |  |  |  +---w filter?
      |  |  |  +--:(yp:update-filter)
      |  |  |     +---w (yp:update-filter)?
      |  |  |        +--:(yp:subtree)
      |  |  |        |  +---w yp:subtree-filter?
      |  |  |        +--:(yp:xpath)
      |  |  |           +---w yp:xpath-filter?       yang:xpath1.0
      |  |  +---w (yp:update-trigger)?
      |  |     +--:(yp:periodic)
      |  |     |  +---w yp:period              yang:timeticks
      |  |     |  +---w yp:anchor-time?        yang:date-and-time
      |  |     +--:(yp:on-change) {on-change}?
      |  |        +---w yp:dampening-period    yang:timeticks
      |  +--ro output
      |     +--ro subscription-result         subscription-result
      |     +--ro (result)?
      |        +--:(no-success)
      |           +--ro filter-failure?             string
      |           +--ro yp:period-hint?             yang:timeticks
      |           +--ro yp:error-path?              string
      |           +--ro yp:object-count-estimate?   uint32
      |           +--ro yp:object-count-limit?      uint32
      |           +--ro yp:kilobytes-estimate?      uint32
      |           +--ro yp:kilobytes-limit?         uint32
      +---x delete-subscription
      |  +---w input
      |  |  +---w identifier    subscription-id
      |  +--ro output
      |     +--ro subscription-result    subscription-result
      +---x kill-subscription
         +---w input
         |  +---w identifier    subscription-id
         +--ro output
            +--ro subscription-result    subscription-result



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    notifications:
      +---n replay-complete
      |  +--ro identifier    subscription-id
      +---n notification-complete
      |  +--ro identifier    subscription-id
      +---n subscription-started
      |  +--ro identifier              subscription-id
      |  +--ro stream?                 stream
      |  +--ro encoding?               encoding
      |  +--ro replay-start-time?      yang:date-and-time
      |  +--ro stop-time?              yang:date-and-time
      |  +--ro (filter-type)?
      |  |  +--:(by-reference)
      |  |  |  +--ro filter-ref?             filter-ref
      |  |  +--:(event-filter)
      |  |  |  +--ro filter?
      |  |  +--:(yp:update-filter)
      |  |     +--ro (yp:update-filter)?
      |  |        +--:(yp:subtree)
      |  |        |  +--ro yp:subtree-filter?
      |  |        +--:(yp:xpath)
      |  |           +--ro yp:xpath-filter?        yang:xpath1.0
      |  +--ro (yp:update-trigger)?
      |  |  +--:(yp:periodic)
      |  |  |  +--ro yp:period               yang:timeticks
      |  |  |  +--ro yp:anchor-time?         yang:date-and-time
      |  |  +--:(yp:on-change) {on-change}?
      |  |     +--ro yp:dampening-period     yang:timeticks
      |  |     +--ro yp:no-synch-on-start?   empty
      |  |     +--ro yp:excluded-change*     change-type
      |  +--ro yp:dscp?                inet:dscp
      |  +--ro yp:weighting?           uint8
      |  +--ro yp:dependency?          sn:subscription-id
      +---n subscription-resumed
      |  +--ro identifier    subscription-id
      +---n subscription-modified
      |  +--ro identifier              subscription-id
      |  +--ro stream?                 stream
      |  +--ro encoding?               encoding
      |  +--ro replay-start-time?      yang:date-and-time
      |  +--ro stop-time?              yang:date-and-time
      |  +--ro (filter-type)?
      |  |  +--:(by-reference)
      |  |  |  +--ro filter-ref?             filter-ref
      |  |  +--:(event-filter)
      |  |  |  +--ro filter?
      |  |  +--:(yp:update-filter)
      |  |     +--ro (yp:update-filter)?



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      |  |        +--:(yp:subtree)
      |  |        |  +--ro yp:subtree-filter?
      |  |        +--:(yp:xpath)
      |  |           +--ro yp:xpath-filter?        yang:xpath1.0
      |  +--ro (yp:update-trigger)?
      |  |  +--:(yp:periodic)
      |  |  |  +--ro yp:period               yang:timeticks
      |  |  |  +--ro yp:anchor-time?         yang:date-and-time
      |  |  +--:(yp:on-change) {on-change}?
      |  |     +--ro yp:dampening-period     yang:timeticks
      |  |     +--ro yp:no-synch-on-start?   empty
      |  |     +--ro yp:excluded-change*     change-type
      |  +--ro yp:dscp?                inet:dscp
      |  +--ro yp:weighting?           uint8
      |  +--ro yp:dependency?          sn:subscription-id
      +---n subscription-terminated
      |  +--ro identifier        subscription-id
      |  +--ro error-id          subscription-errors
      |  +--ro filter-failure?   string
      +---n subscription-suspended
         +--ro identifier        subscription-id
         +--ro error-id          subscription-errors
         +--ro filter-failure?   string
  module: ietf-yang-push

    notifications:
      +---n push-update
      |  +--ro subscription-id       sn:subscription-id
      |  +--ro time-of-update?       yang:date-and-time
      |  +--ro updates-not-sent?     empty
      |  +--ro datastore-contents?
      +---n push-change-update {on-change}?
         +--ro subscription-id      sn:subscription-id
         +--ro time-of-update?      yang:date-and-time
         +--ro updates-not-sent?    empty
         +--ro datastore-changes?

                        Figure 11: Model structure

   Selected components of the model are summarized in the following
   subsections.

4.2.  Filters

   Filters can be supported via a reference to an entry in the filter
   container, or via direct embedding within a subscription itself.
   When a reference is used, it becomes possible to configure filters
   independently of the lifecycle of a subscription.  This facilitates



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   the reuse of filter definitions, which can be important in case of
   complex filter conditions.  Referenced filters can also allow an
   implementation to avoid evaluating filter acceptability during a
   dynamic subscription request.

   Whether referenced or in-line, filters used for yang-push must be of
   case update-filters, and must follow the syntax and semantics of RFC
   6241.  It is not expected that implementations will support
   comprehensive XPATH syntax and boundless complexity.  It will be up
   to implementations to describe what is viable, but the goal is to
   provide equivalent capabilities to what is available with a GET.
   Yang-push implementations must reject dynamic subscriptions or
   suspend configured subscriptions if they include filters which are
   unsupportable on a platform.

   It is conceivable that other types of filters will be introduced for
   yang-push in the future.  To support such filter types, additional
   filter cases can augment the data model.

4.3.  Subscription configuration

   Both configured and dynamic subscriptions are represented within the
   list subscription-config.  Each subscription has own list elements.
   New and enhanced parameters extending the basic subscription data
   model in [I-D:netconf-sub-notif] include:

   o  An update filter identifying yang nodes of interest.  Filter
      contents are specified via a reference to an existing filter, or
      via an in-line definition for only that subscription.  The case
      statement differentates the options.

   o  For periodic subscriptions, triggered updates will occur at the
      boundaries of a specified time interval.  The periodic parameters
      which define this interval include:

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

      *  an "anchor-time".  Update intervals always fall on the points
         in time that are a multiple of a period after the anchor time.
         If anchor time is not provided, then the anchor time must be
         set for when the initial push update can be sent.

   o  When used in conjunction with period, the boundaries of periodic
      update periods may be calculated.

   o  For on-change subscriptions, assuming the dampening period has
      completed, triggered occurs whenever a change in the subscribed




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      information is detected.  On-change subscriptions have more
      complex semantics that is guided by its own set of parameters:

      *  a "dampening-period" specifies the interval that must pass
         before a successive update for the subscription is sent.  The
         first time a change is detected, the update is sent
         immediately.  If a subsequent change is detected, another
         update is only sent once the dampening period has passed for
         this subscription has passed.

      *  an "excluded-change" flag which allows restriction of the types
         of changes for which updates should be sent (changes to object
         values, object creation or deletion events).

      *  a "no-synch-on-start" flag which specifies whether a complete
         update with all the subscribed data should be sent at the
         beginning of a subscription.

   o  Optional qos parameters to indicate the treatment of a
      susbcription relative to other traffic between publisher and
      receiver.  These include:

      *  A "dscp" QoS marking which should be stamped on packets to show
         network QoS treatment.

      *  A "weighting" so that bandwidth proportional to this weighting
         can be allocated to this subscription relative to others for
         that receiver.

      *  a "dependency" upon another subscription.  No push should be
         sent until all updates for the referenced subscription have
         been queued and sent.

   o  A subscription's weighting should work identically to stream
      dependency weighting as described within RFC 7540, section 5.3.2.

   o  A subscription's dependency should work identically to stream
      dependency as described within RFC 7540, sections 5.3.1, 5.3.3,
      and 5.3.4.  If a dependency is attempted via an RPC, but the
      referenced subscription does not exist, the dependency will be
      removed.

4.4.  Notifications








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4.4.1.  Monitoring and OAM Notifications

   OAM notifications and mechanism are with one exception reused from
   [I-D:netconf-sub-notif].

   The one excpetion is the excluded-notifications object is not
   applicable for yang-push.  This is because discarded notifications
   for datastore does not have meaning in this context, and should
   always be zero.

4.4.2.  Update Notifications

   The data model introduces two YANG notifications for the actual
   updates themselves.

   Notification "push-update" is used to send a complete snapshot of the
   data that has been subscribed to, with all YANG object filters
   applied.  The notification is used for periodic subscription updates
   in a periodic subscription.

   The notification can also be used in an on-change subscription for
   the purposes of allowing a receiver to "synch" on a complete set of
   susbcribed datastore contents.  This will be done the start of an on-
   change subscription, unless no-synch-on-start is specified for that
   subscription.  In addition, this notification MAY be used during the
   subscription.  This might be a useful thing to do if change updates
   were not sent as expected (as indicated by the "updates-not-sent"
   flag, or an identification of loss in pushed updates), or for general
   resynchronization of a datastore extract at longer period intervals
   (such as once per day) to mitigate the possibility of any
   application-dependent synchronization drift.  A mandatory requirement
   defining when to sending a push-update notification in conjunction
   with on-change subscription is not asserted in this specification
   beyond synch-on-start.  However an on-change receiver must be able to
   handle an unsolicited push-update as a state synchonization reset.

   The format and syntax of the contained update notification data
   corresponds to the format and syntax of data that would be returned
   in a corresponding get operation with the same filter parameters
   applied.

   Notification "push-change-update" is used to send data updates for
   changes that have occurred in the subscribed data.  This notification
   is used only in conjunction with on-change subscriptions.

   The data updates are encoded analogous to the syntax of a
   corresponding yang-patch operation.  It corresponds to the data that
   would be contained in a yang-patch operation applied to the YANG



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   datastore at the previous update, to result in the current state (and
   applying it also to operational data).

   If the application detects a discontinuity in the updates it is
   pushing, the notification can include a flag "updates-not-sent".
   This is a flag which indicates that not all changes which have
   occurred since the last update are actually included with this
   update.  In other words, the publisher has failed to fulfill its full
   subscription obligations, for example in cases where it was not able
   to keep up with a change burst.  To facilitate synchronization, a
   publisher MAY subsequently send a push-update containing a full
   snapshot of subscribed data.

4.5.  RPCs

   YANG-Push subscriptions are established, modified, and deleted using
   RPCs augmented from [I-D:netconf-sub-notif].

4.5.1.  Establish-subscription RPC

   The subscriber sends an establish-subscription RPC with the
   parameters in section 3.1.  An example might look like:

   <netconf:rpc message-id="101"
      xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
      <establish-subscription
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         <filter netconf:type="xpath"
               xmlns:ex="http://example.com/sample-data/1.0"
               select="/ex:foo"/>
         <period>500</period>
         <encoding>encode-xml</encoding>
      </establish-subscription>
   </netconf:rpc>

                   Figure 12: Establish-subscription RPC

   The publisher must respond explicitly positively (i.e., subscription
   accepted) or negatively (i.e., subscription rejected) to the request.
   Positive responses include the subscription-id of the accepted
   subscription.  In that case a publisher may respond:










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

          Figure 13: Establish-subscription positive RPC response

   A subscription can be rejected for multiple reasons, including the
   lack of authorization to establish a subscription, the lack of read
   authorization on the requested data node, or the inability of the
   publisher to provide a stream with the requested semantics.

   When the requester is not authorized to read the requested data node,
   the returned information indicates the node is unavailable.  For
   instance, if the above request was unauthorized to read node "ex:foo"
   the publisher may return:


   <rpc-reply message-id="101"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <subscription-result
             xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
          subtree-unavailable
       </subscription-result>
       <filter-failure
             xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
          /ex:foo
       </filter-failure>
   </rpc-reply>

         Figure 14: Establish-subscription access denied response

   If a request is rejected because the publisher is not able to serve
   it, the publisher SHOULD include in the returned error what
   subscription parameters would have been accepted for the request.
   However, there are no guarantee that subsequent requests for this
   subscriber or others will in fact be accepted.

   For example, for the following request:





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   <netconf:rpc message-id="101"
      xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
      <establish-subscription
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         <stream>push-update</stream>
         <filter netconf:type="xpath"
               xmlns:ex="http://example.com/sample-data/1.0"
               select="/ex:foo"/>
         <dampening-period>10</dampening-period>
         <encoding>encode-xml</encoding>
      </establish-subscription>
   </netconf:rpc>

            Figure 15: Establish-subscription request example 2

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

   <rpc-reply message-id="101"
         xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <subscription-result
             xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
          period-unsupported
       </subscription-result>
       <period-hint>100</period-hint>
   </rpc-reply>

        Figure 16: Establish-subscription error response example 2

4.5.2.  Modify-subscription RPC

   The subscriber may send a modify-subscription PRC for a subscription
   previously established using RPC The subscriber may change any
   subscription parameters by including the new values in the modify-
   subscription RPC.  Parameters not included in the rpc should remain
   unmodified.  For illustration purposes we include an exchange example
   where a subscriber modifies the period of the subscription.














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   <netconf:rpc message-id="102"
      xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
      <modify-subscription
           xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         <stream>push-update</stream>
         <subscription-id>
            1011
         </subscription-id>
         <period>250</period>
      </modify-subscription>
   </netconf:rpc>

                  Figure 17: Modify subscription request

   The publisher must respond explicitly positively (i.e., subscription
   accepted) or negatively (i.e., subscription rejected) to the request.
   Positive responses include the subscription-id of the accepted
   subscription.  In that case a publisher may respond:

   <rpc-reply message-id="102"
      xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
      <subscription-result
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         ok
      </subscription-result>
   </rpc-reply>

                  Figure 18: Modify subscription response

   If the subscription modification is rejected, the publisher must send
   a response like it does for an establish-subscription and maintain
   the subscription as it was before the modification request.  A
   subscription may be modified multiple times.

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

4.5.3.  Delete-subscription RPC

   To stop receiving updates from a subscription and effectively delete
   a subscription that had previously been established using an
   establish-subscription RPC, a subscriber can send a delete-
   subscription RPC, which takes as only input the subscription-id.  For
   example:






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

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

                      Figure 19: Delete subscription

   Configured subscriptions cannot be deleted via RPC, but have to be
   removed from the configuration.

4.5.4.  YANG Module Synchronization

   In order to fully support datastore replication, the receiver needs
   to know the YANG module library that is in use by server that is
   being replicated.  The YANG 1.0 module library information is sent by
   a NETCONF server in the NETCONF 'hello' message.  For YANG 1.1
   modules and all modules used with the RESTCONF [RFC8040] protocol,
   this information is provided by the YANG Library module (ietf-yang-
   library.yang from [RFC7895].  The YANG library information is
   important for the receiver to reproduce the set of object definitions
   used by the replicated datastore.

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

   The set of modules, revisions, features, and deviations can change at
   run-time (if supported by the server implementation).  In this case,
   the receiver needs to be informed of module changes before data nodes
   from changed modules can be processed correctly.  The YANG library
   provides a simple "yang-library-change" notification that informs the
   client that the library has changed somehow.  The receiver then needs



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   to re-read the entire YANG library data for the replicated server in
   order to detect the specific YANG library changes.  The "ietf-
   netconf-notifications" module defined in [RFC6470] contains a
   "netconf-capability-change" notification that can identify specific
   module changes.  For example, the module URI capability of a newly
   loaded module will be listed in the "added-capability" leaf-list, and
   the module URI capability of an removed module will be listed in the
   "deleted-capability" leaf-list.

5.  YANG module

<CODE BEGINS> file "ietf-yang-push@2017-02-08.yang"
module ietf-yang-push {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push";
  prefix yp;

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

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

     WG Chair: Mahesh Jethanandani
               <mailto:mjethanandani@gmail.com>

     WG Chair: Mehmet Ersue
               <mailto:mehmet.ersue@nokia.com>

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

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

     Editor:   Alberto Gonzalez Prieto
               <mailto:albertgo@cisco.com>

     Editor:   Ambika Prasad Tripathy



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               <mailto:ambtripa@cisco.com>

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

     Editor:   Andy Bierman
               <mailto:andy@yumaworks.com>

     Editor:   Balazs Lengyel
               <mailto:balazs.lengyel@ericsson.com>";

  description
    "This module contains conceptual YANG specifications
     for YANG push.";

  revision 2017-02-08 {
    description
      "Updates to simplify modify-subscription, add anchor-time";
    reference
      "YANG Datastore Push, draft-ietf-netconf-yang-push-05";
  }

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

 /*
  * IDENTITIES
  */

  /* Additional errors for subscription operations */
  identity period-unsupported {
    base sn:error;
    description
      "Requested time period is too short. This can be for both
       periodic and on-change dampening.";
  }

  identity qos-unsupported {
    base sn:error;
    description
      "Subscription QoS parameters not supported on this platform.";
  }

  identity dscp-unavailable {
    base sn:error;



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    description
      "Requested DSCP marking not allocatable.";
  }

  identity on-change-unsupported {
    base sn:error;
    description
      "On-change not supported.";
  }

  identity synch-on-start-unsupported {
    base sn:error;
    description
      "On-change synch-on-start not supported.";
  }

  identity synch-on-start-datatree-size {
    base sn:error;
    description
      "Synch-on-start would push a datatree which exceeds size limit.";
  }

  identity reference-mismatch {
    base sn:error;
    description
      "Mismatch in filter key and referenced yang subtree.";
  }

  identity subtree-unavailable {
    base sn:error;
    description
      "Referenced yang subtree doesn't exist, or is a node where read
       access is not permitted.";
  }

  identity datatree-size {
    base sn:error;
    description
      "Resulting push updates would exceed size limit.";
  }

  /* Additional types of streams */
  identity yang-push {
    base sn:stream;
    description
      "A conceptual datastream consisting of all datastore updates,
       including operational and configuration data.";
  }



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  identity custom-stream {
    base sn:stream;
    description
      "A conceptual datastream for datastore updates with custom
       updates as defined by a user.";
  }

  /* Additional transport option */
  identity http2 {
    base sn:transport;
    description
      "HTTP2 notifications as a transport";
  }

  /*
   * TYPE DEFINITIONS
   */

  typedef filter-id {
    type uint32;
    description
      "A type to identify filters which can be associated with a
       subscription.";
  }

  typedef change-type {
    type enumeration {
      enum "create" {
        description
          "A new data node was created";
      }
      enum "delete" {
        description
          "A data node was deleted";
      }
      enum "modify" {
        description
          "The value of a data node has changed";
      }
    }
    description
      "Specifies different types of changes that may occur to a
       datastore.";
  }

  grouping update-filter {
    description
      "This groupings defines filters for push updates for a



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       datastore tree.  The filters define which updates are of
       interest in a push update subscription.  Mixing and matching
       of multiple filters does not occur at the level of this
       grouping. When a push-update subscription is created, the
       filter can be a regular subscription filter, or one of the
       additional filters that are defined in this grouping.";
    choice update-filter {
      description
        "Define filters regarding which data nodes to include
         in push updates";
      case subtree {
        description
          "Subtree filter.";
        anyxml subtree-filter {
          description
            "Subtree-filter used to specify the data nodes targeted
             for subscription within a subtree, or subtrees, of a
             conceptual YANG datastore.  Objects matching the filter
             criteria will traverse the filter. The syntax follows
             the subtree filter syntax specified in RFC 6241.";
          reference "RFC 6241 section 6";
        }
      }
      case xpath {
        description
          "XPath filter";
        leaf xpath-filter {
          type yang:xpath1.0;
          description
            "Xpath defining the data items of interest.";
        }
      }
    }
  }

  grouping update-policy-modifiable {
    description
      "This grouping describes the datastore specific subscription
       conditions that can be changed during the lifetime of the
       subscription.";
    choice update-trigger {
      description
        "Defines necessary conditions for sending an event to
         the subscriber.";
      case periodic {
        description
          "The agent is requested to notify periodically the current
           values of the datastore as defined by the filter.";



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        leaf period {
          type yang:timeticks;
          mandatory true;
          description
            "Duration of time which should occur between periodic
             push updates.  Where the anchor of a start-time is
             available, the push will include the objects and their
             values which exist at an exact multiple of timeticks
             aligning to this start-time anchor.";
        }
        leaf anchor-time {
          type yang:date-and-time;
          description
            "Designates a timestamp from which the series of periodic
             push updates are computed. The next update will take place
             at the next period interval from the anchor time.  For
             example, for an anchor time at the top of a minute and a
             period interval of a minute, the next update will be sent
             at the top of the next minute.";
        }
      }
      case on-change {
        if-feature "on-change";
        description
          "The agent is requested to notify changes in values in the
           datastore subset as defined by a filter.";
        leaf dampening-period {
          type yang:timeticks;
          mandatory true;
          description
            "Minimum amount of time that needs to have passed since the
             last time an update was provided for the subscription.";
        }
      }
    }
  }

  grouping update-policy {
    description
      "This grouping describes the datastore specific subscription
       conditions of a subscription.";
    uses update-policy-modifiable {
      augment "update-trigger/on-change" {
        description
          "Includes objects not modifiable once subscription is
           established.";
        leaf no-synch-on-start {
          type empty;



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          description
            "This leaf acts as a flag that determines behavior at the
             start of the subscription.  When present, synchronization
             of state at the beginning of the subscription is outside
             the scope of the subscription. Only updates about changes
             that are observed from the start time, i.e. only push-
             change-update notifications are sent. When absent (default
             behavior), in order to facilitate a receiver's
             synchronization, a full update is sent when the
             subscription starts using a push-update notification, just
             like in the case of a periodic subscription.  After that,
             push-change-update notifications only are sent unless the
             Publisher chooses to resynch the subscription again.";
        }
        leaf-list excluded-change {
          type change-type;
          description
            "Use to restrict which changes trigger an update.
             For example, if modify is excluded, only creation and
             deletion of objects is reported.";
        }
      }
    }
  }

  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 {
      type inet:dscp;
      default "0";
      description
        "The push update's IP packet transport priority. This is made
         visible across network hops to receiver. The transport
         priority is shared for all receivers of a given
         subscription.";
    }
    leaf weighting {
      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



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        "RFC-7540, section 5.3.2";
    }
    leaf dependency {
      type sn:subscription-id;
      description
        "Provides the Subscription ID of a parent subscription which has
         absolute priority 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 of the
         parent has something ready to push.";
      reference
        "RFC-7540, section 5.3.1";
    }
  }

  grouping update-error-hints {
    description
      "Allow return additional negotiation hints that apply
       specifically to push updates.";
    leaf period-hint {
      type yang:timeticks;
      description
        "Returned when the requested time period is too short. This hint
         can assert an viable period for both periodic push cadence and
         on-change dampening.";
    }
    leaf error-path {
      type string;
      description
        "Reference to a YANG path which is associated with the error
         being returned.";
    }
    leaf object-count-estimate {
      type uint32;
      description
        "If there are too many objects which could potentially be
         returned by the filter, this identifies the estimate of the
         number of objects which the filter would potentially pass.";
    }
    leaf object-count-limit {
      type uint32;
      description
        "If there are too many objects which could be returned by the
         filter, this identifies the upper limit of the publisher's
         ability to service for this subscription.";
    }
    leaf kilobytes-estimate {
      type uint32;



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      description
        "If the returned information could be beyond the capacity of the
         publisher, this would identify the data size which could result
         from this filter.";
    }
    leaf kilobytes-limit {
      type uint32;
      description
        "If the returned information would be beyond the capacity of the
         publisher, this identifies the upper limit of the publisher's
         ability to service for this subscription.";
    }
  }

  augment "/sn:establish-subscription/sn:input" {
    description
      "Define additional subscription parameters that apply
       specifically to push updates";
    uses update-policy;
    uses update-qos;
  }
  augment "/sn:establish-subscription/sn:input/"+
          "sn:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {
      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
  augment "/sn:establish-subscription/sn:output/"+
    "sn:result/sn:no-success" {
    description
      "Add push datastore error info and hints to RPC output.";
    uses update-error-hints;
  }
  augment "/sn:modify-subscription/sn:input" {
    description
      "Define additional subscription parameters that apply
       specifically to push updates.";
    uses update-policy-modifiable;
  }
  augment "/sn:modify-subscription/sn:input/"+
          "sn:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {



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      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
  augment "/sn:modify-subscription/sn:output/"+
    "sn:result/sn:no-success" {
    description
      "Add push datastore error info and hints to RPC output.";
    uses update-error-hints;
  }

  notification push-update {
    description
      "This notification contains a push update, containing data
       subscribed to via a subscription. This notification is sent for
       periodic updates, for a periodic subscription.  It can also be
       used for synchronization updates of an on-change subscription.
       This notification shall only be sent to receivers of a
       subscription; it does not constitute a general-purpose
       notification.";
    leaf subscription-id {
      type sn:subscription-id;
      mandatory true;
      description
        "This references the subscription because of which the
         notification is sent.";
    }
    leaf time-of-update {
      type yang:date-and-time;
      description
        "This leaf contains the time of the update.";
    }
    leaf updates-not-sent {
      type empty;
      description
        "This is a flag which indicates that not all data nodes
         subscribed to are included with this update. In other words,
         the publisher has failed to fulfill its full subscription
         obligations. This may lead to intermittent loss of
         synchronization of data at the client.  Synchronization at the
         client can occur when the next push-update is received.";
    }
    anydata datastore-contents {
      description
        "This contains the updated data.  It constitutes a snapshot
         at the time-of-update of the set of data that has been
         subscribed to.  The format and syntax of the data



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         corresponds to the format and syntax of data that would be
         returned in a corresponding get operation with the same
         filter parameters applied.";
    }
  }
  notification push-change-update {
    if-feature "on-change";
    description
      "This notification contains an on-change push update. This
       notification shall only be sent to the receivers of a
       subscription; it does not constitute a general-purpose
       notification.";
    leaf subscription-id {
      type sn:subscription-id;
      mandatory true;
      description
        "This references the subscription because of which the
         notification is sent.";
    }
    leaf time-of-update {
      type yang:date-and-time;
      description
        "This leaf contains the time of the update, i.e. the time at
         which the change was observed.";
    }
    leaf updates-not-sent {
      type empty;
      description
        "This is a flag which indicates that not all changes which
         have occurred since the last update are included with this
         update.  In other words, the publisher has failed to
         fulfill its full subscription obligations, for example in
         cases where it was not able to keep up with a change burst.
         To facilitate synchronization, a publisher MAY subsequently
         send a push-update containing a full snapshot of subscribed
         data. Such a push-update might also be triggered by a
         subscriber requesting an on-demand synchronization.";
    }
    anydata datastore-changes {
      description
        "This contains datastore contents that has changed since the
         previous update, per the terms of the subscription.  Changes
         are encoded analogous to the syntax of a corresponding yang-
         patch operation, i.e. a yang-patch operation applied to the
         YANG datastore implied by the previous update to result in the
         current state (and assuming yang-patch could also be applied to
         operational data).";
    }



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  }
  augment "/sn:subscription-started" {
    description
      "This augmentation adds push subscription parameters to the
       notification that a subscription has started and data updates are
       beginning to be sent. This notification shall only be sent to
       receivers of a subscription; it does not constitute a general-
       purpose notification.";
    uses update-policy;
    uses update-qos;
  }
  augment "/sn:subscription-started/sn:filter-type" {
    description
      "This augmentation allows to include additional update filters
       options to be included as part of the notification that a
       subscription has started.";
    case update-filter {
      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
  augment "/sn:subscription-modified" {
    description
      "This augmentation adds push subscription parameters to the
       notification that a subscription has been modified. This
       notification shall only be sent to receivers of a subscription;
       it does not constitute a general-purpose notification.";
    uses update-policy;
    uses update-qos;
  }
  augment "/sn:subscription-modified/sn:filter-type" {
    description
      "This augmentation allows to include additional update
       filters options to be included as part of the notification
       that a subscription has been modified.";
    case update-filter {
      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
  augment "/sn:filters/sn:filter/"+
          "sn:filter-type" {
    description
      "This container adds additional update filter options to the list
       of configurable filters that can be applied to subscriptions.
       This facilitates the reuse of complex filters once defined.";



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    case update-filter {
      uses update-filter;
    }
  }
  augment "/sn:subscription-config/sn:subscription" {
    description
      "Contains the list of subscriptions that are configured,
       as opposed to established via RPC or other means.";
    uses update-policy;
    uses update-qos;
  }
  augment "/sn:subscription-config/sn:subscription/"+
          "sn:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {
      uses update-filter;
    }
  }
  augment "/sn:subscriptions/sn:subscription" {
    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, e.g. establish-subscription, delete-subscription,
       and modify-subscription, as well as subscriptions that
       have been established via configuration.";
    uses update-policy;
    uses update-qos;
  }
  augment "/sn:subscriptions/sn:subscription/"+
          "sn:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {
      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
}

<CODE ENDS>







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6.  Security Considerations

   Subscriptions could be used to attempt to overload publishers of YANG
   datastores.  For this reason, it is important that the publisher has
   the ability to decline a subscription request if it would deplete its
   resources.  In addition, a publisher needs to be able to suspend an
   existing subscription when needed.  When this occur, the subscription
   status is updated accordingly and the receivers are notified.
   Likewise, requests for subscriptions need to be properly authorized.

   A subscription could be used to retrieve data in subtrees that a
   receiver has no authorized access to.  Therefore it is important that
   data pushed based on subscriptions is authorized in the same way that
   regular data retrieval operations are.  Data being pushed to a
   receiver needs therefore to be filtered accordingly, just like if the
   data were being retrieved on-demand.  The Netconf Authorization
   Control Model applies.

   A subscription could be configured on another receiver's behalf, with
   the goal of flooding that receiver with updates.  One or more
   publishers could be used to overwhelm a receiver which doesn't even
   support subscriptions.  Receivers which do not want pushed data need
   only terminate or refuse any transport sessions from the publisher.
   In addition, the Netconf Authorization Control Model SHOULD be used
   to control and restrict authorization of subscription configuration.

   For both configured and dynamic subscriptions it is essential to
   authenticate and authorize that receiver via some transport level
   mechanism before sending any push updates.

7.  Acknowledgments

   For their valuable comments, discussions, and feedback, we wish to
   acknowledge Tim Jenkins, Kent Watsen, Susan Hares, Yang Geng, Peipei
   Guo, Michael Scharf, Sharon Chisholm, and Guangying Zheng.

8.  References

8.1.  Normative References

   [I-D:netconf-sub-notif]
              Clemm, A., Gonzalez Prieto, A., Voit, E., Tripathy, A.,
              and E. Nilsen-Nygaard, "Subscribing to YANG-Defined Event
              Notifications", draft-ietf-netconf-subscribed-
              notifications-00 (work in progress), February 2017.






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

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              DOI 10.17487/RFC6536, March 2012,
              <http://www.rfc-editor.org/info/rfc6536>.

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

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

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

   [RFC8072]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
              Media Type", RFC 8072, DOI 10.17487/RFC8072, February
              2017, <http://www.rfc-editor.org/info/rfc8072>.

8.2.  Informative References

   [RFC1157]  Case, J., "A Simple Network Management Protocol (SNMP)",
              RFC 1157, May 1990.

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, July 2008.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)",
              RFC 6241, June 2011.

   [RFC7923]  Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
              for Subscription to YANG Datastores", RFC 7923, June 2016.

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








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Appendix A.  Technologies to be considered for future iterations

A.1.  Proxy YANG Subscription when the Subscriber and Receiver are
      different

   The properties of Dynamic and Configured Subscriptions can be
   combined to enable deployment models where the Subscriber and
   Receiver are different.  Such separation can be useful with some
   combination of:

   o  An operator does not want the subscription to be dependent on the
      maintenance of transport level keep-alives.  (Transport
      independence provides different scalability characteristics.)

   o  There is not a transport session binding, and a transient
      Subscription needs to survive in an environment where there is
      unreliable connectivity with the Receiver and/or Subscriber.

   o  An operator wants the Publisher to include highly restrictive
      capacity management and Subscription security mechanisms outside
      of domain of existing operational or programmatic interfaces.

   To build a Proxy Subscription, first the necessary information must
   be signaled as part of the <establish-subscription>.  Using this set
   of Subscriber provided information; the same process described within
   section 3 will be followed.

   After a successful establishment, if the Subscriber wishes to track
   the state of Receiver subscriptions, it may choose to place a
   separate on-change Subscription into the "Subscriptions" subtree of
   the YANG Datastore on the Publisher.

A.2.  OpState and Filters

   Currently there are ongoing discusssions to revise the concept of
   datastores, allowing for proper handling and distinction of intended
   versus applied configurations and extending the notion of a datastore
   to operational data.  When finalized, the new concept may open up the
   possibility for new types of subscription filters, for example,
   targeting specific datastores and targeting (potentially) differences
   in datatrees across different datastores.

   Likewise, it is conceivable that filters are defined that apply to
   metadata, such as data nodes for which metadata has been defined that
   meets a certain criteria.

   Defining any such subscription filters at this point would be highly
   speculative in nature.  However, it should be noted that



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   corresponding extensions may be defined in future specifications.
   Any such extensions will be straightforward to accommodate by
   introducing a model that defines new filter types, and augmenting the
   new filter type into the subscription model.

A.3.  Splitting push updates

   Push updates may become fairly large and extend across multiple
   subsystems in a YANG-Push Server.  As a result, it conceivable to not
   combine all updates into a single update message, but to split
   updates into multiple separate update messages.  Such splitting could
   occur along multiple criteria: limiting the number of data nodes
   contained in a single update, grouping updates by subtree, grouping
   updates by internal subsystems (e.g., by line card), or grouping them
   by other criteria.

   Splitting updates bears some resemblance to fragmenting packets.  In
   effect, it can be seen as fragmenting update messages at an
   application level.  However, from a transport perspective, splitting
   of update messages is not required as long as the transport does not
   impose a size limitation or provides its own fragmentation mechanism
   if needed.  We assume this to be the case for YANG-Push.  In the case
   of NETCONF, RESTCONF, HTTP/2, no limit on message size is imposed.
   In case of other transports, any message size limitations need to be
   handled by the corresponding transport mapping.

   There may be some scenarios in which splitting updates might still
   make sense.  For example, if updates are collected from multiple
   independent subsystems, those updates could be sent separately
   without need for combining.  However, if updates were to be split,
   other issues arise.  Examples include indicating the number of
   updates to the receiver, dinstinguishing a missed fragment from a
   missed update, and the ordering with which updates are received.
   Proper addressing those issues would result in considerable
   complexity, while resulting in only very limited gains.  In addition,
   if a subscription is found to result in updates that are too large, a
   publisher can always reject the request for a subscription while the
   subscriber is always free to break a subscription up into multiple
   subscriptions.

A.4.  Potential Subscription Parameters

   A possible is the introduction of an additional parameter "changes-
   only" for periodic subscription.  Including this flag would results
   in sending at the end of each period an update containing only
   changes since the last update (i.e. a change-update as in the case of
   an on-change subscription), not a full snapshot of the subscribed




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   information.  Such an option might be interesting in case of data
   that is largely static and bandwidth-constrained environments.

Appendix B.  Issues that are currently being worked and resolved

   (To be removed by RFC editor prior to publication)

   Issue #6: Data plane notifications and layered headers.  Specifically
   how do we want to enable standard header unificiation and bundle
   support vs. the data plane notifications currently defined.

Appendix C.  Changes between revisions

   (To be removed by RFC editor prior to publication)

   v04 to v05

   o  Referenced based subscription document changed to Subscribed
      Notifications from 5277bis.

   o  Getting operational data from filters

   o  Extension notifiable-on-change added

   o  New appendix on potential futures.  Moved text into there from
      several drafts.

   o  Subscription configuration section now just includes changed
      parameters from Subscribed Notifications

   o  Subscription monitoring moved into Subscribed Notifications

   o  New error and hint mechanisms included in text and in the yang
      model.

   o  Updated examples based on the error definitions

   o  Groupings updated for consistency

   o  Text updates throughout

   v03 to v04

   o  Updates-not-sent flag added

   o  Not notifiable extension added

   o  Dampening period is for whole subscription, not single objects



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   o  Moved start/stop into rfc5277bis

   o  Client and Server changed to subscriber, publisher, and receiver

   o  Anchor time for periodic

   o  Message format for synchronization (i.e. synch-on-start)

   o  Material moved into 5277bis

   o  QoS parameters supported, by not allowed to be modified by RPC

   o  Text updates throughout

Authors' Addresses

   Alexander Clemm
   Huawei

   Email: ludwig@clemm.org


   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com


   Alberto Gonzalez Prieto
   Cisco Systems

   Email: albertgo@cisco.com


   Ambika Prasad Tripathy
   Cisco Systems

   Email: ambtripa@cisco.com


   Einar Nilsen-Nygaard
   Cisco Systems

   Email: einarnn@cisco.com







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   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com


   Balazs Lengyel
   Ericsson

   Email: balazs.lengyel@ericsson.com









































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