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Versions: (draft-clemm-netconf-yang-push) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19

Network Working Group                                           A. Clemm
Internet-Draft                                        A. Gonzalez Prieto
Intended status: Standards Track                                 E. Voit
Expires: December 17, 2016                                   A. Tripathy
                                                       E. Nilsen-Nygaard
                                                           Cisco Systems
                                                           June 15, 2016


               Subscribing to YANG datastore push updates
                  draft-ietf-netconf-yang-push-03.txt

Abstract

   This document defines a subscription and push mechanism for YANG
   datastores.  This mechanism allows client applications to request
   updates from a YANG datastore, which are then pushed by the server to
   a receiver per a subscription policy, without requiring additional
   client 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 December 17, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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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
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   6
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Subscription Model  . . . . . . . . . . . . . . . . . . .   8
     3.2.  Negotiation of Subscription Policies  . . . . . . . . . .  10
     3.3.  On-Change Considerations  . . . . . . . . . . . . . . . .  11
     3.4.  Data Encodings  . . . . . . . . . . . . . . . . . . . . .  12
       3.4.1.  Periodic Subscriptions  . . . . . . . . . . . . . . .  12
       3.4.2.  On-Change Subscriptions . . . . . . . . . . . . . . .  13
     3.5.  Subscription Filters  . . . . . . . . . . . . . . . . . .  13
     3.6.  Push Data Stream and Transport Mapping  . . . . . . . . .  14
     3.7.  Subscription management . . . . . . . . . . . . . . . . .  18
       3.7.1.  Subscription management by RPC  . . . . . . . . . . .  18
       3.7.2.  Subscription management by configuration  . . . . . .  20
     3.8.  Other considerations  . . . . . . . . . . . . . . . . . .  20
       3.8.1.  Authorization . . . . . . . . . . . . . . . . . . . .  20
       3.8.2.  Additional subscription primitives  . . . . . . . . .  21
       3.8.3.  Robustness and reliability considerations . . . . . .  22
       3.8.4.  Update size and fragmentation considerations  . . . .  22
       3.8.5.  Push data streams . . . . . . . . . . . . . . . . . .  22
       3.8.6.  Implementation considerations . . . . . . . . . . . .  23
       3.8.7.  Alignment with RFC 5277bis  . . . . . . . . . . . . .  24
   4.  A YANG data model for management of datastore push
       subscriptions . . . . . . . . . . . . . . . . . . . . . . . .  24
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  24
     4.2.  Update streams  . . . . . . . . . . . . . . . . . . . . .  28
     4.3.  Filters . . . . . . . . . . . . . . . . . . . . . . . . .  29
     4.4.  Subscription configuration  . . . . . . . . . . . . . . .  29
     4.5.  Subscription monitoring . . . . . . . . . . . . . . . . .  31
     4.6.  Notifications . . . . . . . . . . . . . . . . . . . . . .  31



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     4.7.  RPCs  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
       4.7.1.  Establish-subscription RPC  . . . . . . . . . . . . .  32
       4.7.2.  Modify-subscription RPC . . . . . . . . . . . . . . .  34
       4.7.3.  Delete-subscription RPC . . . . . . . . . . . . . . .  36
   5.  YANG module . . . . . . . . . . . . . . . . . . . . . . . . .  36
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  51
   7.  Issues that are currently being worked and resolved . . . . .  51
     7.1.  Unresolved issues under discussion  . . . . . . . . . . .  51
     7.2.  Agreement in principal  . . . . . . . . . . . . . . . . .  52
     7.3.  Closed Issues . . . . . . . . . . . . . . . . . . . . . .  52
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  53
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  53
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Introduction

   YANG [RFC6020] was originally designed for the Netconf protocol
   [RFC6241], which originally put most emphasis on configuration.
   However, YANG is not restricted to configuration data.  YANG
   datastores, i.e. datastores that contain data modeled according using
   YANG, can contain configuration as well as operational data.  It is
   therefore reasonable to expect that data in YANG datastores will
   increasingly be used to support applications that are not focused on
   managing configurations but that are, for example, related to service
   assurance.

   Service assurance applications typically involve monitoring
   operational state of networks and devices; of particular interest are
   changes that this data undergoes over time.  Likewise, there are
   applications in which data and objects from one datastore need to be
   made available both to applications in other systems and to remote
   datastores [I-D.voit-netmod-yang-mount-requirements]
   [I-D.clemm-netmod-mount].  This requires mechanisms that allow remote
   systems to become quickly aware of any updates to allow to validate
   and maintain cross-network integrity and consistency.

   Traditional approaches to remote network state visibility rely
   heavily on polling.  With polling, data is periodically explicitly
   retrieved by a client from a server to stay up-to-date.

   There are various 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.



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   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 is when an application can request to be
   automatically updated as necessary of current content of the
   datastore (such as a subtree, or data in a subtree that meets a
   certain filter condition), and in which the server that maintains the
   datastore subsequently pushes those updates.  However, such a
   solution does not currently exist.

   The need to perform polling-based management is typically considered
   an important shortcoming of management 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 using
   protocols such as NETCONF or Restconf [I-D.ietf-netconf-restconf]
   will be any more effective, as they would follow the same request/
   response pattern.

   While YANG allows the definition of notifications, such notifications
   are generally intended to indicate the occurrence of certain well-
   specified event conditions, such as a the onset of an alarm condition
   or the occurrence of an error.  A capability to subscribe to and
   deliver 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 client
   applications to dynamically subscribe to updates of a YANG datastore
   and that allows the server to push those updates, possibly using one
   of several delivery mechanisms.  Additionally, support for
   subscriptions configured directly on the server are also useful when




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   dynamic signaling is undesirable or unsupported.  The requirements
   for such a service are documented in [I-D.i2rs-pub-sub-requirements].

   This document proposes a solution.  The solution builds on top of the
   Netconf Event Model [I-D.gonzalez-netconf-5277bis] which defines a
   mechanism for the management of event subscriptions.  At its core,
   the solution that is defined here introduces a new set of event
   streams including datastore push updates that clients can subscribe
   to, as well as extensions to the event subscription model that allow
   to manage policies that define what and when updates are trigged.  To
   this end, the document specifies a YANG data model that augments the
   YANG data model (and RPCs) defined as part of the NETCONF Event
   Model.

   Specifically, the solution features the following capabilities:

   o  An extension to event subscription mechanisms allows clients to
      subscribe to event streams containing automatic 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 optional filters with criteria that data
      must meet for updates to be sent.  Furthermore, subscriptions can
      specify a policy that directs when updates are provided.  For
      example, a client may request to be updated periodically in
      certain intervals, or whenever data changes occur.

   o  The ability for a server to push back on requested subscription
      parameters.  Because not every server may support every requested
      update policy for every piece of data, it is necessary for a
      server to be able to indicate whether or not it is capable of
      supporting a requested subscription, and possibly allow to
      negotiate push update subscription parameters.  For example, some
      servers may have a lower limit to the period with which they can
      send updates, or they may not support on-change updates for every
      piece of data.

   o  A mechanism to communicate the updates themselves.  For this, the
      proposal leverages and extends existing YANG/Netconf/Restconf
      mechanisms, defining special notifications that carry updates.  In
      addition, optional subscription parameters allow to specify which
      transport should be used to stream updates, and to define QoS
      extensions that allow to address aspects such as how to prioritize
      between streams of updates.








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2.  Definitions and Acronyms

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

   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.

   Dynamic subscription: A subscription negotiated between subscriber
   and publisher vian establish, modify, and delete RPCs respectively
   control plane signaling messages that are part of an existing
   management association between and publisher.  Subscriber and
   receiver are the same system.

   NACM: NETCONF Access Control Model

   NETCONF: Network Configuration Protocol

   Publisher: A server that sends push updates to a receiver according
   to the terms of a subscription.  In general, the publisher is also
   the "owner" of the subscription.

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

   Receiver: The target of push updates of a subscription.  In case of a
   dynamic subscription, receiver and subscriber are the same system.
   However, in the case of a configured subscription, the receiver may
   be a different system than the one that configured the subscription.

   RPC: Remote Procedure Call

   SNMP: Simple Network Management Protocol

   Configured subscription: A subscription installed as part of a
   configuration datastore via a configuration interface.

   Subscriber: A client that negotiates a subscription with a server
   ("publisher").  A client that establishes a configured subscription



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   is also considered a subscriber, even if it is not necessarily the
   receiver of a subscription.

   Subscription: A contract between a client ("subscriber") and a server
   ("publisher"), stipulating which information the client wishes to
   receive from the server (and which information the server has to
   provide to the client) without the need for further solicitation.

   Subscription 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.

   Subscription policy: A policy that specifies under what circumstances
   to push an update, e.g. whether updates are to be provided
   periodically or only whenever changes occur.

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

   Update trigger: A trigger, as specified by a subscription policy,
   that causes an update to be sent, respectively a data record to be
   generated.  An example of a trigger is 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.

   URI: Uniform Resource Identifier

   YANG: A data definition language for NETCONF

   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, which supports the dynamic as well as static (via
   configuration) creation of subscriptions to information updates of
   operational or configuration YANG data which are subsequently pushed
   from the server to the client.

   Dynamic subscriptions are initiated by clients who want to receive
   push updates.  Servers respond to requests for the creation of
   subscriptions positively or negatively.  Negative responses MAY
   include information about why the subscription was not accepted, in
   order to facilitate converging on an acceptable set of subscription
   parameters.  Similarly, configured subscriptions are configured as
   part of a device's configuration.  Once a subscription has been




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   established, datastore push updates are pushed from the server to the
   receiver until the subscription ends.

   Accordingly, the solution encompasses several components:

   o  The subscription model for configuration and management of the
      subscriptions.

   o  The ability to provide hints for acceptable subscription
      parameters, in cases where a subscription desired by a client
      cannot currently be served.

   o  The stream of push updates.

   In addition, there are a number of additional considerations, such as
   the tie-in of the mechanisms with security mechanisms.  Each of those
   aspects will be discussed in the following subsections.

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.gonzalez-netconf-5277bis] and introduces several new
   parameters, for example parameters that allow subscribers to specify
   what to include in an update, what triggers an update, and how to
   deliver updates.

   The subscription model assumes the presence of one or more conceptual
   perpetual datastreams of continuous updates that can be subscribed
   to.  There are several datastreams with predefined semantics, such as
   the stream of updates of all operational data or the stream of
   updates of all config data.  In addition, it is possible to define
   custom streams with customizable semantics.  The model includes the
   list of update datastreams that are supported by a system and
   available for subscription.

   The subscription model augments the NETCONF event subscription model
   with a set of parameters as follows:

   o  An encoding for push updates.  By default, updates are encoded
      using XML, but JSON can be requested as an option and other
      encodings may be supported in the future.

   o  An optional start time for the subscription.  If the specified
      start time is in the past, the subscription goes into effect
      immediately.  The start time also serves as anchor time for
      periodic subscriptions, from which intervals at which to send
      updates are calculated (see also below).



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   o  An optional stop time for the subscription.  Once the stop time is
      reached, the subscription is automatically terminated.

   o  A subscription policy definition regarding the update trigger when
      to send new updates.  The trigger can be periodic or based on
      change.

      *  For periodic subscriptions, the trigger is defined by a
         parameter that defines the interval with which updates are to
         be pushed.  The start time of the subscription serves as anchor
         time, defining one specific point in time at which an update
         needs to be sent.  Update intervals always fall on the points
         in time that are a multiple of a period after the start time.

      *  EDITOR'S NOTE: A possible option to discuss concerns 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 information.  Such an option might be interesting in
         case of data that is largely static and bandwidth-constrained
         environments.

      *  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 specifies the dampening period, i.e. the
            interval that must pass before a successive update for the
            same data node is sent.  The first time a change is
            detected, the update is sent immediately.  If a subsequent
            change is detected, another update for that object is only
            sent once the dampening period has passed, containing the
            value of the data node that is then valid.  Note that the
            dampening period applies to each object, not the set of all
            objects that are part of the same subscription.  This means
            that on the first change of an object, an update for that
            object is immediately sent, regardless of whether or not for
            another object of the same subscription a dampening period
            is already in effect.

         +  Another parameter allows to restrict the types of changes
            for which updates are sent (changes to object values, object
            creation or deletion events).  It is conceivable to augment
            the data model with additional parameters in the future to
            specify even more refined policies, such as parameters that



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            specify the magnitude of a change that must occur before an
            update is triggered.

         +  A third parameter specifies whether or not a complete update
            with all the subscribed data should be sent at the beginning
            of a subscription to facilitate synchronization and
            establish the frame of reference for subsequent updates.

         +  EDITOR'S NOTE: Several semantic variations are conceivable.
            In one variation, an on-change notification is sent
            immediately, but successive dampening updates are sent at
            fixed period intervals, grouping all changes of objects for
            which changes have occurred since the sending of their last
            update and the current dampening period.

   o  Optionally, a filter, or set of filters, describing the subset of
      data items in the stream's data records that are of interest to
      the subscriber.  The server should only send to the subscriber the
      data items that match the filter(s), when present.  The absence of
      a filter indicates that all data items from the stream are of
      interest to the subscriber and all data records must be sent in
      their entirety to the subscriber.  The following types of filters
      are introduced: subtree filters, with the same semantics as
      defined in [RFC 6241],and XPath filters.  In addition, as with any
      subscription, an RFC 5277 filter MAY be used, with the same
      semantics as defined in [RFC 5277].  Additional filter types can
      be added through augmentations.  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.

   The subscription data model is specified as part of the YANG data
   model described later in this specification.  Specifically, the
   subscription parameters are defined in the "subscription-info" and
   "update-policy" groupings.  Receiver information is defined in the
   "receiver-info" grouping.  Information about the source address is
   defined in the "push-source-info" grouping.  It is conceivable that
   additional subscription parameters might be added in the future.
   This can be accomplished through augmentation of the subscription
   data model.

3.2.  Negotiation of Subscription Policies

   A subscription rejection can be caused by the inability of the server
   to provide a stream with the requested semantics.  For example, a
   server may not be able to support "on-change" updates for operational
   data, or only support them for a limited set of data nodes.




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   Likewise, a server may not be able to support a requested update
   frequency.

   YANG-Push supports a simple negotiation between clients and servers
   for subscription parameters.  The negotiation is limited to a single
   pair of subscription request and response.  For negative responses,
   the server SHOULD include in the returned error what subscription
   parameters would have been accepted for the request.  The returned
   acceptable parameters constitute suggestions that, when followed,
   increase the likelihood of success for subsequent requests.  However,
   they are no guarantee that subsequent requests for this client or
   others will in fact be accepted.

   In case a subscriber requests an encoding other than XML, and this
   encoding is not supported by the server, the server simply indicates
   in the response that the encoding is not supported.

   A subscription negotiation capability has been introduced as part of
   the NETCON Event Notifications model.  However, the ability to
   negotiate subscriptions is of particular importance in conjunction
   with push updates, as server implementations may have limitations
   with regards to what updates can be generated and at what velocity.

3.3.  On-Change Considerations

   On-change subscriptions allow clients 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, the subscription request MUST be rejected.  As a result,
   on-change subscription requests will tend to be directed at very
   specific, targeted subtrees with only few objects.

   Any updates for an on-change subscription will include only objects
   for which a change was detected.  To avoid flooding clients with
   repeated updates for fast-changing objects, or objects with
   oscillating values, an on-change subscription allows for the



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   definition of a dampening period.  Once an update for a given object
   is sent, no other updates for this particular object are sent until
   the end of the dampening period.  Values sent at the end of the
   dampening period are the values current when that dampening period
   expires.  In addition, updates include information about objects that
   were deleted and ones that were newly created.

   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.

   Additional refinements are conceivable.  For example, in order to
   avoid sending updates on objects whose values undergo only a
   negligible change, additional parameters might be added to an on-
   change subscription specifying a policy that states how large or
   "significant" a change has to be before an update is sent.  A simple
   policy is a "delta-policy" that states, for integer-valued data
   nodes, the minimum difference between the current value and the value
   that was last reported that triggers an update.  Also more
   sophisticated policies are conceivable, such as policies specified in
   percentage terms or policies that take into account the rate of
   change.  While not specified as part of this draft, such policies can
   be accommodated by augmenting the subscription data model
   accordingly.

3.4.  Data Encodings

   Subscribed data is encoded in either XML or JSON format.  A server
   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 [RFC6020].  JSON encoding rules are defined
   in [I-D.ietf-netmod-yang-json].  This encoding is valid JSON, but
   also has special encoding rules to identify module namespaces and
   provide consistent type processing of YANG data.







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3.4.2.  On-Change Subscriptions

   In an on-change subscription, updates need to allow to differentiate
   between data nodes that were newly created since the last update,
   data nodes that were deleted, and data nodes whose value changed.

   XML encoding rules correspond to how data would be encoded in input
   to Netconf edit-config operations as specified in [RFC6241] section
   7.2, adding "operation" attributes to elements in the data subtree.
   Specifically, the following values will be utilized:

   o  create: The data identified by the element has been added since
      the last update.

   o  delete: The data identified by the element has been deleted since
      the last update.

   o  merge: The data identified by the element has been changed since
      the last update.

   o  replace: The data identified by the element has been replaced with
      the update contents since the last update.

   The remove value will not be utilized.

   Contrary to edit-config operations, the data is sent from the server
   to the client, not from the client to the server, and will not be
   restricted to configuration data.

   JSON encoding rules are roughly analogous to how data would be
   encoded in input to a YANG-patch operation, as specified in
   [I-D.ietf-netconf-yang-patch] section 2.2.  However, no edit-ids will
   be needed.  Specifically, changes will be grouped under respective
   "operation" containers for creations, deletions, and modifications.

3.5.  Subscription Filters

   Subscriptions can specify filters for subscribed data.  The following
   filters are supported in addition to RFC 5277 filters that apply to
   any event subscription:

   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.



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   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.

   A more suitable mechanism to consider is therefore 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, push updates are solicited,
   i.e. tied to a particular subscription which triggered the
   notification, and arguably only of interest to the subscriber,
   respectively the intended receiver of the subscription.  A
   subscription therefore needs to be able to distinguish between
   streams that underlie push updates and streams of other
   notifications.  By the same token, notifications associated with
   updates and subscriptions to updates need to be distinguished from
   other notifications, in that they enter a datastream of push updates,
   not a stream of other event notifications.

   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  A data node that contains a representation of the datastore
      subtree containing the updates.  The subtree is filtered per
      access control rules to contain only data that the subscriber is
      authorized to see.  Also, depending on the subscription type,
      i.e., specifically for on-change subscriptions, the subtree
      contains only the data nodes that contain actual changes.  (This
      can be simply a node of type string or, for XML-based encoding,
      anyxml.)

   Notifications are sent using <notification> elements as defined in
   [RFC5277].  Alternative transports are conceivable but outside the
   scope of this specification.





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   The solution specified in this document uses notifications to define
   datastore updates.  The contents of the notification includes a set
   of explicitly defined data nodes.  For this purpose, two new generic
   notifications are introduced, "push-update" and "push-change-update".
   Those notifications define how mechanisms that carry YANG
   notifications (e.g.  Netconf notifications and Restconf) can be used
   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.

   Push-update notification defines updates for a periodic subscription,
   as well as for the initial update of an on-change subscription used
   to synchronize the receiver at the start of a new subscription.  The
   update record contains a data snippet that contains an instantiated
   subtree with the subscribed contents.  The content of the update
   record is equivalent to the contents that would be obtained had the
   same data been explicitly retrieved using e.g. a Netconf "get"-
   operation, with the same filters applied.

   The contents of the notification conceptually represents the union of
   all data nodes in the yang modules supported by the server.  However,
   in a YANG data model, it is not practical to model the precise data
   contained in the updates as part of the notification.  This is
   because the specific data nodes supported depend on the implementing
   system and may even vary dynamically.  Therefore, to capture this
   data, a single parameter that can represent any datastore contents is
   used, not parameters that represent data nodes one at a time.

   Push-change-update notification defines updates for on-change
   subscriptions.  The update record here contains a data snippet that
   indicates the changes that data nodes have undergone, i.e. that
   indicates which data nodes have been created, deleted, or had changes
   to their values.  The format follows the same format that operations
   that apply changes to a data tree would apply, indicating the
   creates, deletes, and modifications of data nodes.

   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>2015-03-09T19:14:56Z</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>2015-03-09T19:14:56Z</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>2015-03-09T19:14:56Z</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 server 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>2015-03-09T19:14:56Z</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

   There are two ways in which subscriptions can be managed, as
   specified in the NETCONF Event Notifications model: RPC-based and
   configuration based.  Any given subscription is either RPC-based or
   configuration-based.  There is no mixing-and-matching of RPC and
   configuration operations.  Specifically, a configured subscription
   cannot be modified or deleted using RPC.  Likewise, a subscription
   established via RPC cannot be modified or deleted through
   configuration operations.

   The following sections describe how subscription management is
   applied to YANG Push subscriptions.

3.7.1.  Subscription management by RPC

   RPC-based subscription allows a subscriber to establish a
   subscription via an RPC call.  The subscriber and the receiver are
   the same entity, i.e. a subscriber cannot subscribe or in other ways
   interfere with a subscription on another receiver's behalf.  The
   lifecycle of the subscription is dependent on the lifecyle of the
   transport session over which the subscription was requested.  For
   example, when a Netconf session over which a subscription was
   established is torn down, the subscription is automatically
   terminated (and needs to be re-initiated when a new session is
   established).  Alternatively, a subscriber can also decide to delete
   a subscription via another RPC.






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   When an establish-subscription request is successful, the
   subscription identifier of the freshly established subscription is
   returned.

   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
   server 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:

   <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 server might return:












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   <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

   A subscriber that establishes a subscription using RPC can modify or
   delete the subscription using other RPCs.  When the session between
   subscriber and publisher is terminated, the subscription is
   implicitly deleted.

3.7.2.  Subscription management by configuration

   Configuration-based subscription allows a subscription to be
   established as part of a server's configuration.  This allows to
   persist subscriptions.  Persisted subscriptions allow for a number of
   additional options than RPC-based subscriptions.  As part of a
   configured subscription, a receiver needs to be specified.  It is
   thus possible to have a different system acting as subscriber (the
   client creating the subscription) and as receiver (the client
   receiving the updates).  In addition, a configured subscription
   allows to specify which transport protocol should be used, as well as
   the sender source (for example, a particular interface or an address
   of a specific VRF) from which updates are to be pushed.

   Configuration-based subscriptions cannot be modified or deleted using
   RPCs.  Instead, configured subscriptions are deleted as part of
   regular configuration operations.  Servers SHOULD reject attempts to
   modify configurations of active subscriptions.  This way, race
   conditions in which a receiver may not be aware of changed
   subscription policies are avoided.

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, removing
   any non-authorized data as applicable.





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   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 the target data node.

                    +-------------+                 +-------------+
       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
   target data node, 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 push update,
   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 target
   node, no notifications indicating the fact that this has occurred
   need to be provided.

3.8.2.  Additional subscription primitives

   Other possible operations include the ability for a Subscriber to
   request the suspension/resumption of a Subscription with a Publisher.
   However, subscriber driven suspension is not viewed as essential at
   this time, as a simpler alternative is to remove a subscription and
   reestablish it when needed.



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   It should be noted that this does not affect the ability of the
   Publisher to suspend a subscription.  This can occur in cases the
   server is not able to serve the subscription for a certain period of
   time, and indicated by a corresponding notification.

3.8.3.  Robustness and reliability considerations

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

   YANG-Push uses a secure and reliable transport.  Notifications are
   not getting reordered, and in addition contain a time stamp.  For
   those reasons, for the transport of push-updates, we believe that
   additional reliability mechanisms at the application level, such as
   sequence numbers for push updates, are not required.

   At the same time, it is conceivable that under certain circumstances,
   a push server is not able to generate the update notifications that
   it had committed to when accepting a subcription.  In those
   circumstances, the server needs to inform the receiver of the
   situation.  For this purpose, notifications are defined that a push
   server can use to inform subscribers/ receivers when a subscription
   is (temporarily) suspended, when a suspended subscription is resumed,
   and when a a subscription is terminated.  This way, receivers will be
   able to rely on a subscription, knowing that they will be informed of
   any situations in which updates might be missed.

3.8.4.  Update size and fragmentation considerations

   Depending on the subscription, the volume of updates can become quite
   large.  There is no inherent limitation to the amount of data that
   can be included in a notification.  That said, it may not always be
   practical to send the entire update in a single chunk.
   Implementations MAY therefore choose, at their discretion, to "chunk"
   updates and break them out into several update notifications.

3.8.5.  Push data streams

   There are several conceptual data streams introduced in this
   specification:

   o  yang-push includes the entirety of YANG data, including both
      configuration and operational data.

   o  operational-push includes all operational (read-only) YANG data

   o  config-push includes all YANG configuration data.




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   It is conceivable to introduce other data streams with more limited
   scope, for example:

   o  operdata-nocounts-push, a datastream containing all operational
      (read-only) data with the exception of counters

   o  other custom datastreams

   Those data streams make particular sense for use cases involving
   service assurance (not relying on operational data), and for use
   cases requiring on-change update triggers which make no sense to
   support in conjunction with fast-changing counters.  While it is
   possible to specify subtree filters on yang-push to the same effect,
   having those data streams greatly simplifies articulating
   subscriptions in such scenarios.

3.8.6.  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 of operational state alone, to be able to
   push it, can consume considerable system resources.  Configuration
   data may in many cases be persisted in an actual database or a
   configuration file, where retrieval of the database content or the
   file itself is reasonably straightforward and computationally
   inexpensive.  However, retrieval of operational data may, depending
   on the implementation, require invocation of APIs, possibly on an
   object-by-object basis, possibly involving additional internal
   interrupts, etc.

   For those reasons, if is important for an implementation to
   understand what subscriptions it can or cannot support.  It is far
   preferrable to decline a subscription request, than to accept it only
   to result in subsequent failure later.

   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.





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   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.

3.8.7.  Alignment with RFC 5277bis

   A new draft has been chartered by the Netconf Working Group to
   replace the current Netconf Event Model defined in RFC 5277.  Future
   revisions of this document will leverage RFC 5277bis as applicable.
   It is anticipated that portions of the data model and subscription
   management that are now defined in this this document and that are
   not applicable only to YANG-Push, but to more general event
   subscriptions, will move to RFC 5277bis.

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.

   module: ietf-yang-push
   augment /notif-bis:establish-subscription/notif-bis:input:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?
      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks
      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:establish-subscription/notif-bis:input/
      |       notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0



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   augment /notif-bis:establish-subscription/notif-bis:output:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?
      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks
      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:establish-subscription/notif-bis:output/
      |     notif-bis:result/notif-bis:no-success/notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0
   augment /notif-bis:modify-subscription/notif-bis:input:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?
      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks
      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:modify-subscription/notif-bis:input/
      |     notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0
   augment /notif-bis:modify-subscription/notif-bis:output:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?



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      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks
      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:modify-subscription/notif-bis:output/
      |     notif-bis:result/notif-bis:no-success/notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0
   augment /notif-bis:subscription-started:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?
      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks
      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:subscription-started/notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0
   augment /notif-bis:subscription-modified:
      +---- subscription-start-time?   yang:date-and-time
      +---- subscription-stop-time?    yang:date-and-time
      +---- (update-trigger)?
      |  +--:(periodic)
      |  |  +---- period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +---- no-synch-on-start?         empty
      |     +---- dampening-period           yang:timeticks



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      |     +---- excluded-change*           change-type
      +---- dscp?                      inet:dscp
      |       {notif-bis:configured-subscriptions}?
      +---- subscription-priority?     uint8
      +---- subscription-dependency?   string
   augment /notif-bis:subscription-modified/notif-bis:filter-type:
      +--:(update-filter)
         +---- (update-filter)?
            +--:(subtree)
            |  +---- subtree-filter
            +--:(xpath)
               +---- xpath-filter?     yang:xpath1.0
   augment /notif-bis:filters/notif-bis:filter/notif-bis:filter-type:
      +--:(update-filter)
         +--rw (update-filter)?
            +--:(subtree)
            |  +--rw subtree-filter
            +--:(xpath)
               +--rw xpath-filter?     yang:xpath1.0
   augment /notif-bis:subscription-config/notif-bis:subscription:
      +--rw subscription-start-time?   yang:date-and-time
      +--rw subscription-stop-time?    yang:date-and-time
      +--rw (update-trigger)?
      |  +--:(periodic)
      |  |  +--rw period                     yang:timeticks
      |  +--:(on-change) {on-change}?
      |     +--rw no-synch-on-start?         empty
      |     +--rw dampening-period           yang:timeticks
      |     +--rw excluded-change*           change-type
      +--rw dscp?                      inet:dscp
      |    {notif-bis:configured-subscriptions}?
      +--rw subscription-priority?     uint8
      +--rw subscription-dependency?   string
   augment /notif-bis:subscription-config/notif-bis:subscription/
      |      notif-bis:filter-type:
      +--:(update-filter)
         +--rw (update-filter)?
            +--:(subtree)
            |  +--rw subtree-filter
            +--:(xpath)
               +--rw xpath-filter?     yang:xpath1.0
   augment /notif-bis:subscriptions/notif-bis:subscription:
      +--ro subscription-start-time?   yang:date-and-time
      +--ro subscription-stop-time?    yang:date-and-time
      +--ro (update-trigger)?
      |  +--:(periodic)
      |  |  +--ro period                     yang:timeticks
      |  +--:(on-change) {on-change}?



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      |     +--ro no-synch-on-start?         empty
      |     +--ro dampening-period           yang:timeticks
      |     +--ro excluded-change*           change-type
      +--ro dscp?                      inet:dscp
      |                          {notif-bis:configured-subscriptions}?
      +--ro subscription-priority?     uint8
      +--ro subscription-dependency?   string
   augment /notif-bis:subscriptions/notif-bis:subscription/
      |      notif-bis:filter-type:
      +--:(update-filter)
         +--ro (update-filter)?
            +--:(subtree)
            |  +--ro subtree-filter
            +--:(xpath)
               +--ro xpath-filter?     yang:xpath1.0
   notifications:
      +---n push-update
      |  +--ro subscription-id            notif-bis:subscription-id
      |  +--ro time-of-update?            yang:date-and-time
      |  +--ro (encoding)?
      |     +--:(encode-xml)
      |     |  +--ro datastore-contents-xml?    datastore-contents-xml
      |     +--:(encode-json) {notif-bis:json}?
      |        +--ro datastore-contents-json?   datastore-contents-json
      +---n push-change-update {on-change}?
         +--ro subscription-id           notif-bis:subscription-id
         +--ro time-of-update?           yang:date-and-time
         +--ro (encoding)?
            +--:(encode-xml)
            |  +--ro datastore-changes-xml?    datastore-changes-xml
            +--:(encode-json) {notif-bis:json}?
               +--ro datastore-changes-yang?   datastore-changes-json

                         Figure 9: Model structure

   The components of the model are described in the following
   subsections.

4.2.  Update streams

   Container "update-streams" is used to indicate which data streams are
   provided by the system and can be subscribed to.  For this purpose,
   it contains a leaf list of data nodes identifying the supported
   streams.







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4.3.  Filters

   Container "filters" contains a list of configurable data filters,
   each specified in its own list element.  This allows users to
   configure filters separately from an actual subscription, which can
   then be referenced from a subscription.  This facilitates the reuse
   of filter definitions, which can be important in case of complex
   filter conditions.

   One of three types of filters can be specified as part of a filter
   list element.  Subtree filters follow syntax and semantics of RFC
   6241 and allow to specify which subtree(s) to subscribe to.  In
   addition, XPath filters can be specified for more complex filter
   conditions.  Finally, filters can be specified using syntax and
   semantics of RFC5277.

   It is conceivable to introduce other types of filters; in that case,
   the data model needs to be augmented accordingly.

4.4.  Subscription configuration

   As an optional feature, configured-subscriptions, allows for the
   configuration of subscriptions as opposed to RPC.  Subscriptions
   configurations are represented by list subscription-config.  Each
   subscription is represented through its own list element and includes
   the following components:

   o  "subscription-id" is an identifier used to refer to the
      subscription.

   o  "stream" refers to the stream being subscribed to.  The
      subscription model assumes the presence of perpetual and
      continuous streams of updates.  Various streams are defined:
      "push-update" covers the entire set of YANG data in the server.
      "operational-push" covers all operational data, while "config-
      push" covers all configuration data.  Other streams could be
      introduced in augmentations to the model by introducing additional
      identities.

   o  "encoding" refers to the encoding requested for the data updates.
      By default, updates are encoded using XML.  However, JSON can be
      requested as an option if the json-enconding feature is supported.
      Other encodings may be supported in the future.

   o  "subscription-start-time" specifies when the subscription is
      supposed to start.  The start time also serves as anchor time for
      periodic subscriptions (see below).




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   o  "subscription-stop-time" specifies a stop time for the
      subscription.  Once the stop time is reached, the subscription is
      automatically terminated.  However, even when terminated, the
      subscription entry remains part of the configuration unless
      explicity deleted from the configuration.  It is possible to
      effectively "resume" a stopped subscription by reconfiguring the
      stop time.

   o  Filters for a subscription can be specified using a choice,
      allowing to either reference a filter that has been separately
      configured or entering its definition inline.

   o  A choice of subscription policies allows to define when to send
      new updates - periodic or on change.

      *  For periodic subscriptions, the trigger is defined by a
         "period", a parameter that defines the interval with which
         updates are to be pushed.  The start time of the subscription
         serves as anchor time, defining one specific point in time at
         which an update needs to be sent.  Update intervals always fall
         on the points in time that are a multiple of a period after the
         start time.

      *  For on-change subscriptions, the trigger occurs whenever a
         change in the subscribed information is detected.  On-change
         subscriptions have more complex semantics that is guided by
         additional parameters. "dampening-period" specifies the
         interval that must pass before a successive update for the same
         data node 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, containing the value of the data node that is then
         valid. "excluded-change" allows to restrict the types of
         changes for which updates are sent (changes to object values,
         object creation or deletion events).  "no-synch-on-start" is a
         flag that allows to specify whether or not a complete update
         with all the subscribed data should be sent at the beginning of
         a subscription; if the flag is omitted, a complete update is
         sent to facilitate synchronization.  It is conceivable to
         augment the data model with additional parameters in the future
         to specify even more refined policies, such as parameters that
         specify the magnitude of a change that must occur before an
         update is triggered.

   o  This is followed with a list of receivers for the subscription,
      indicating for each receiver the transport that should be used for
      push updates (if options other than Netconf are supported).  It




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      should be noted that the receiver does not have to be the same
      system that configures the subscription.

   o  Finally, "push-source" can be used to specify the source of push
      updates, either a specific interface or server address.

   A subscription established through configuration cannot be deleted
   using an RPC.  Likewise, subscriptions established through RPC cannot
   be deleted through configuration.

   The deletion of a subscription, whether through RPC or configuration,
   results in immediate termination of the subsciption.

4.5.  Subscription monitoring

   Subscriptions can be subjected to management themselves.  For
   example, it is possible that a server may no longer be able to serve
   a subscription that it had previously accepted.  Perhaps it has run
   out of resources, or internal errors may have occurred.  When this is
   the case, a server needs to be able to temporarily suspend the
   subscription, or even to terminate it.  More generally, the server
   should provide a means by which the status of subscriptions can be
   monitored.

   Container "subscriptions" contains the state of all subscriptions
   that are currently active.  This includes subscriptions that were
   established (and have not yet been deleted) using RPCs, as well as
   subscriptions that have been configured as part of configuration.

   Each subscription is represented as a list element "datastore-push-
   subscription".  The associated information includes an identifier for
   the subscription, a subscription status, as well as the various
   subscription parameters that are in effect.  The subscription status
   indicates whether the subscription is currently active and healthy,
   or if it is degraded in some form.  Leaf "configured-subscription"
   indicates whether the subscription came into being via configuration
   or via RPC.

   Subscriptions that were established by RPC are removed from the list
   once they expire (reaching stop-time )or when they are terminated.
   Subscriptions that were established by configuration need to be
   deleted from the configuration by a configuration editing operation.

4.6.  Notifications

   A server needs to indicate any changes in status of a subscription to
   the receiver through a notification.  Specifically, subscribers need
   to be informed of the following:



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   o  A subscription has been temporarily suspended (including the
      reason)

   o  A subscription (that had been suspended earlier) is once again
      operational

   o  A subscription has been terminated (including the reason)

   o  A subscription has been modified (including the current set of
      subscription parameters in effect)

   Finally, a server might provide additional information about
   subscriptions, such as statistics about the number of data updates
   that were sent.  However, such information is currently outside the
   scope of this specification.

4.7.  RPCs

   YANG-Push subscriptions are established, modified, and deleted using
   three RPCs.

4.7.1.  Establish-subscription RPC

   The subscriber sends an establish-subscription RPC with the
   parameters in section 3.1.  For instance


   <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 10: Establish-subscription RPC

   The server 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 server 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 11: 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
   server to provide a stream with the requested semantics. .

   When the requester is not authorized to read the requested data node,
   the returned <error-info> indicates an authorization error and the
   requested node.  For instance, if the above request was unauthorized
   to read node "ex:foo" the server 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">
          error-data-not-authorized
       </subscription-result>
   </rpc-reply>

         Figure 12: Establish-subscription access denied response

   If a request is rejected because the server is not able to serve it,
   the server SHOULD include in the returned error what subscription
   parameters would have been accepted for the request.  However, they
   are no guarantee that subsequent requests for this client 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 13: Establish-subscription request example 2

   A server 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">
          error-no-such-option
       </subscription-result>
       <period>100</period>
   </rpc-reply>

        Figure 14: Establish-subscription error response example 2

4.7.2.  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>
         <filter netconf:type="xpath"
              xmlns:ex="http://example.com/sample-data/1.0"
              select="/ex:foo"/>
         <period>250</period>
         <encoding>encode-xml</encoding>
      </modify-subscription>
   </netconf:rpc>

                  Figure 15: Modify subscription request

   The server 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 server 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>
      <subscription-id
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
         1011
      </subscription-id>
   </rpc-reply>

                  Figure 16: Modify subscription response

   If the subscription modification is rejected, the server 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.






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4.7.3.  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

   <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">
      <ok/>
   </rpc-reply>


                      Figure 17: Delete subscription

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

5.  YANG module

<CODE BEGINS> file "ietf-yang-push@2016-06-15.yang"
module ietf-yang-push {
  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-event-notifications {
    prefix notif-bis;
  }
  import ietf-5277-netconf {
    prefix notif;
  }



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  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:alex@cisco.com>

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

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

     Editor:   Ambika Prasad Tripathy
               <mailto:ambtripa@cisco.com>

     Editor:   Einar Nilsen-Nygaard
               <mailto:einarnn@cisco.com>";
  description
    "This module contains conceptual YANG specifications
     for YANG push.";

  revision 2016-06-15 {
    description
      "First revision to incorporate RFC 5277-bis.";
    reference "YANG Datastore Push, draft-ietf-netconf-yang-push-03";
  }

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

 /*
  * IDENTITIES
  */

  /* Additional errors for subscription operations */
  identity error-data-not-authorized {
    base notif-bis:error;



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    description
      "No read authorization for a requested data node.";
  }

  /* Additional types of streams */
  identity update-stream {
    base notif:stream;
    description
      "Base identity to represent a conceptual system-provided
       datastream of datastore updates with predefined semantics.";
  }

  identity yang-push {
    base update-stream;
    description
      "A conceptual datastream consisting of all datastore
       updates, including operational and configuration data.";
  }

  identity operational-push {
    base update-stream;
    description
      "A conceptual datastream consisting of updates of all
       operational data.";
  }

  identity config-push {
    base update-stream;
    description
      "A conceptual datastream consisting of updates of all
       configuration data.";
  }

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

  /* Additional transport option */
  identity restconf {
    base notif-bis:transport;
    description
      "Restconf notifications as a transport";
  }

  /*



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   * TYPE DEFINITIONS
   */

  typedef datastore-contents-xml {
    type string;
    description
      "This type is be used to represent datastore contents,
       i.e. a set of data nodes with their values, in XML.
       The syntax corresponds to the syntax of the data payload
       returned in a corresponding Netconf get operation with the
       same filter parameters applied.";
    reference "RFC 6241 section 7.7";
  }

  typedef datastore-changes-xml {
    type string;
    description
      "This type is used to represent a set of changes in a
       datastore encoded in XML, indicating for datanodes whether
       they have been created, deleted, or updated.  The syntax
       corresponds to the syntax used to when editing a
       datastore using the edit-config operation in Netconf.";
    reference "RFC 6241 section 7.2";
  }

  typedef datastore-contents-json {
    type string;
    description
      "This type is be used to represent datastore contents,
       i.e. a set of data nodes with their values, in JSON.
       The syntax corresponds to the syntax of the data
       payload returned in a corresponding RESTCONF get
       operation with the same filter parameters applied.";
    reference "RESTCONF Protocol";
  }

  typedef datastore-changes-json {
    type string;
    description
      "This type is used to represent a set of changes in a
       datastore encoded in JSON, indicating for datanodes whether
       they have been created, deleted, or updated.  The syntax
       corresponds to the syntax used to patch a datastore
       using the yang-patch operation with Restconf.";
    reference "draft-ietf-netconf-yang-patch";
  }

  typedef filter-id {



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    type uint32;
    description
      "A type to identify filters which can be associated with a
       subscription.";
  }

  typedef subscription-result {
    type identityref {
      base notif-bis:subscription-result;
    }
    description
      "The result of a subscription operation";
  }

  typedef subscription-term-reason {
    type identityref {
      base notif-bis:subscription-errors;
    }
    description
      "Reason for a server to terminate a subscription.";
  }

  typedef subscription-susp-reason {
    type identityref {
      base notif-bis:subscription-errors;
    }
    description
      "Reason for a server to suspend a subscription.";
  }

  typedef encoding {
    type identityref {
      base notif-bis:encodings;
    }
    description
      "Specifies a data encoding, e.g. for a data subscription.";
  }

  typedef change-type {
    type enumeration {
      enum "create" {
        description
          "A new data node was created";
      }
      enum "delete" {
        description
          "A data node was deleted";
      }



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      enum "modify" {
        description
          "The value of a data node has changed";
      }
    }
    description
      "Specifies different types of changes that may occur
       to a datastore.";
  }

  typedef transport-protocol {
    type identityref {
      base notif-bis:transport;
    }
    description
      "Specifies transport protocol used to send updates to a
       receiver.";
  }

  typedef push-source {
    type enumeration {
      enum "interface-originated" {
        description
          "Pushes will be sent from a specific interface on a
           Publisher";
      }
      enum "address-originated" {
        description
          "Pushes will be sent from a specific address on a
           Publisher";
      }
    }
    description
      "Specifies from where objects will be sourced when being pushed
       off a publisher.";
  }

  typedef update-stream {
    type identityref {
      base update-stream;
    }
    description
      "Specifies a system-provided datastream.";
  }

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



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       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.
             It may include additional criteria,
             allowing users to receive only updates of a limited
             set of data nodes that match those filter criteria.
             This will be used to define what
             updates to include in a stream of update events, i.e.
             to specify for which data nodes update events should be
             generated and specific match expressions that objects
             need to meet. The syntax follows the subtree filter
             syntax specified in RFC 6241, section 6.";
          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 {
    description
      "This grouping describes the conditions under which an
       update will be sent as part of an update stream.";
    choice update-trigger {



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      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 or the subset
           defined by the filter.";
        leaf period {
          type yang:timeticks;
          mandatory true;
          description
            "Duraton 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.";
        }
      }
      case on-change {
        if-feature "on-change";
        description
          "The agent is requested to notify changes in
           values in the datastore or a subset of it defined
           by a filter.";
        leaf no-synch-on-start {
          type empty;
          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 are sent.";
        }
        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



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             provided.";
        }
        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 push-subscription-info {
    description
      "This grouping describes information concerning a
       push subscription that is need in addition to information
       already included in notif-bis:subscription-info.";
    leaf subscription-start-time {
      type yang:date-and-time;
      description
        "Designates the time at which a subscription is supposed
         to start, or immediately, in case the start-time is in
         the past. For periodic subscription, the start time also
         serves as anchor time from which the time of the next
         update is 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.";
    }
    leaf subscription-stop-time {
      type yang:date-and-time;
      description
        "Designates the time at which a subscription will end.
         When a subscription reaches its stop time, it will be
         automatically deleted. No final push is required unless there
         is exact alignment with the end of a periodic subscription
         period.";
    }
  }

  grouping subscription-qos {
    description
      "This grouping describes Quality of Service information
       concerning a subscription.  This information is passed to lower
       layers for transport priortization and treatment";
    leaf dscp {



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      if-feature "notif-bis:configured-subscriptions";
      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 subscription-priority {
      type uint8;
      description
        "Relative priority for a subscription.   Allows an underlying
         transport layer perform informed load balance allocations
         between various subscriptions";
    }
    leaf subscription-dependency {
      type string;
      description
        "Provides the Subscription ID of a parent subscription
         without which this subscription should not exist. In
         other words, there is no reason to stream these objects
         if another subscription is missing.";
    }
  }

  augment "/notif-bis:establish-subscription/notif-bis:input" {
    description
      "Define additional subscription parameters that apply
       specifically to push updates";
    uses push-subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  augment "/notif-bis:establish-subscription/notif-bis:input/notif-bis: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 "/notif-bis:establish-subscription/notif-bis:output" {
    description
      "Allow to return additional subscription parameters that apply
       specifically to push updates.";
    uses push-subscription-info;



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



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  notification push-update {
    description
      "This notification contains a periodic push update.
       This notification shall only be sent to receivers
       of a subscription; it does not constitute a general-purpose
       notification.";
    leaf subscription-id {
      type notif-bis: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.";
    }
    choice encoding {
      description
        "Distinguish between the proper encoding that was specified
         for the subscription";
      case encode-xml {
        description
          "XML encoding";
        leaf datastore-contents-xml {
          type datastore-contents-xml;
          description
            "This contains data encoded in XML,
             per the subscription.";
        }
      }
      case encode-json {
        if-feature "notif-bis:json";
        description
          "JSON encoding";
        leaf datastore-contents-json {
          type datastore-contents-json;
          description
            "This leaf contains data encoded in JSON,
             per the subscription.";
        }
      }
    }
  }
  notification push-change-update {
    if-feature "on-change";
    description



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      "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 notif-bis: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.";
    }
    choice encoding {
      description
        "Distinguish between the proper encoding that was specified
         for the subscription";
      case encode-xml {
        description
          "XML encoding";
        leaf datastore-changes-xml {
          type datastore-changes-xml;
          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 Netconf edit-config
             operation.";
        }
      }
      case encode-json {
        if-feature "notif-bis:json";
        description
          "JSON encoding";
        leaf datastore-changes-yang {
          type datastore-changes-json;
          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 RESTCONF yang-patch
             operation.";
        }
      }



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    }
  }
  augment "/notif-bis: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 push-subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  augment "/notif-bis:subscription-started/notif-bis: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 "/notif-bis: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 push-subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  augment "/notif-bis:subscription-modified/notif-bis: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;
    }
  }



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  augment "/notif-bis:filters/notif-bis:filter/notif-bis: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.";
    case update-filter {
      uses update-filter;
    }
  }
  augment "/notif-bis:subscription-config/notif-bis:subscription" {
    description
      "Contains the list of subscriptions that are configured,
       as opposed to established via RPC or other means.";
    uses push-subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  augment "/notif-bis:subscription-config/notif-bis:subscription/notif-bis:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {
      uses update-filter;
    }
  }
  augment "/notif-bis:subscriptions/notif-bis: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 push-subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  augment "/notif-bis:subscriptions/notif-bis:subscription/notif-bis:filter-type" {
    description
      "Add push filters to selection of filter types.";
    case update-filter {
      description
        "Additional filter options for push subscription.";
      uses update-filter;
    }
  }
}



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<CODE ENDS>

6.  Security Considerations

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

   A subscription could be used to retrieve data in subtrees that a
   client has not 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 client
   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.  Clients 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.

7.  Issues that are currently being worked and resolved

7.1.  Unresolved issues under discussion

   o  Which stream types to introduce.  Current list includes streams
      for all operational and for all config data.  Consider adding
      stream for operational data minus counters.  Also: assess
      implications of opstate implications on required data streams.

   o  In addition to identifying which items go to which streams,
      identifying and calling out which items (such as counters) should
      not be "on-change subscribable" may be useful.  Consider
      introducing a Yang extension to define if an object: is-a-counter
      and/or not-notifiable.

   o  What QoS parameters should be supported for subscriptions.  Note:
      QoS parameters are applicable to buffering as well as temporarily
      loss of transport connectivity.

   o  Implications of ephemeral requirements from I2RS



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   o  Filters: YANG 1.1 allows filters to be defined in multiple places.
      How do they intersect each other in a deterministic way.

   o  On-change subscription: consider providing publisher with
      capability to initiate a refresh of contents rather than send
      deltas.  Current proposal allows for a "synch-on-start" option;
      such an option might be useful also e.g. on resumption of a
      subscription that had been suspended.

   o  Do we need an extension for NACM to support filter out datastore
      nodes for which the receiver has no read access?  (And how does
      this differ from existing GET, which must do the same filtering?)
      In 5277, such filtering is done at the notification level.  Yang-
      push includes notification-content filtering.  This may be very
      expensive in terms of processing.  Andy suggestion: only accept
      Yang-push subscriptions for subtrees the user has rights for all
      the nodes in the subtree.  Changes to those rights trigger a
      subscription termination.  Should we codify this, or let vendors
      determine when per subtree filtering might be applied?

7.2.  Agreement in principal

   Still need to agree on draft text

   o  Multiple receivers per configured subscription is ok.

   o  Proper behavior for on-change, including detecting and indicating
      changes within a dampening period.

   o  Still some details to work through, e.g., do we add a counter for
      the number of object changes during a dampening period?

   o  Negotiate vs. auto-adjust.  Cases where negotiate is needed exists
      for domain synchronization.  Error messages can be used to
      transport information back as hints.  Alternative is dedicated RPC
      responses.  In either case specific contents of these negotiation
      messages must still be defined.

   o  Message format for synchronization (i.e. synch-on-start) still
      needs to be defined.

7.3.  Closed Issues

   Some editorial updates needed to reflect these

   o  Periodic interval goes to seconds from timeticks

   o  Balancing Augment vs. Parallel Model structures (maximize augment)



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   o  Moving from separate start/stop to Anchor time for Periodic

8.  Acknowledgments

   For their valuable comments, discussions, and feedback, we wish to
   acknowledge Andy Bierman, Yang Geng, Peipei Guo, Susan Hares, Tim
   Jenkins, Balazs Lengyel, Kent Watsen, and Guangying Zheng.

9.  References

9.1.  Normative References

   [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol (SNMP)", RFC 1157,
              DOI 10.17487/RFC1157, May 1990,
              <http://www.rfc-editor.org/info/rfc1157>.

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

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

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

   [RFC6470]  Bierman, A., "Network Configuration Protocol (NETCONF)
              Base Notifications", RFC 5277, February 2012.

   [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>.

9.2.  Informative References

   [I-D.clemm-netmod-mount]
              Clemm, A., Medved, J., and E. Voit, "Mounting YANG-defined
              information from remote datastores", draft-clemm-netmod-
              mount-04 (work in progress), March 2016.







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   [I-D.gonzalez-netconf-5277bis]
              Gonzalez Prieto, A., Clemm, A., Voit, E., Tripathy, A.,
              Nilsen-Nygaard, E., Chisholm, S., and H. Trevino,
              "Subscribing to YANG-Defined Event Notifications", draft-
              clemm-netmod-mount-03 (work in progress), June 2016.

   [I-D.i2rs-pub-sub-requirements]
              Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
              for Subscription to YANG Datastores", draft-ietf-i2rs-pub-
              sub-requirements-09 (work in progress), May 2016.

   [I-D.ietf-netconf-restconf]
              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", I-D draft-ietf-netconf-restconf-13, April 2016.

   [I-D.ietf-netconf-yang-patch]
              Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
              Media Type", draft-ietf-netconf-yang-patch-08 (work in
              progress), March 2016.

   [I-D.ietf-netmod-yang-json]
              Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              draft-ietf-netmod-yang-json-10 (work in progress), March
              2016.

   [I-D.voit-netmod-yang-mount-requirements]
              Voit, E., Clemm, A., and S. Mertens, "Requirements for
              Peer Mounting of YANG subtrees from Remote Datastores",
              draft-voit-netmod-yang-mount-requirements-00 (work in
              progress), March 2016.

Authors' Addresses

   Alexander Clemm
   Cisco Systems

   EMail: alex@cisco.com


   Alberto Gonzalez Prieto
   Cisco Systems

   EMail: albertgo@cisco.com








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   Eric Voit
   Cisco Systems

   EMail: evoit@cisco.com


   Ambika Prasad Tripathy
   Cisco Systems

   EMail: ambtripa@cisco.com


   Einar Nilsen-Nygaard
   Cisco Systems

   EMail: einarnn@cisco.com



































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