Subscription to YANG Notifications for Datastore Updates
draft-ietf-netconf-yang-push-25

Network Working Group                                           A. Clemm
Internet-Draft                                        A. Gonzalez Prieto
Intended status: Standards Track                                 E. Voit
Expires: September 22, 2016                                  A. Tripathy
                                                       E. Nilsen-Nygaard
                                                           Cisco Systems
                                                          March 21, 2016


               Subscribing to YANG datastore push updates
                  draft-ietf-netconf-yang-push-02.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

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   This Internet-Draft will expire on September 22, 2016.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   5
   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 . . . . . . . . . . . . . . .  12
     3.5.  Subscription Filters  . . . . . . . . . . . . . . . . . .  13
     3.6.  Subscription State Model at the Publisher . . . . . . . .  13
     3.7.  Push Data Stream and Transport Mapping  . . . . . . . . .  14
     3.8.  Subscription management . . . . . . . . . . . . . . . . .  19
       3.8.1.  Subscription management by RPC  . . . . . . . . . . .  19
       3.8.2.  Subscription management by configuration  . . . . . .  21
     3.9.  Other considerations  . . . . . . . . . . . . . . . . . .  21
       3.9.1.  Authorization . . . . . . . . . . . . . . . . . . . .  21
       3.9.2.  Additional subscription primitives  . . . . . . . . .  22
       3.9.3.  Robustness and reliability considerations . . . . . .  23
       3.9.4.  Update size and fragmentation considerations  . . . .  23
       3.9.5.  Push data streams . . . . . . . . . . . . . . . . . .  23
       3.9.6.  Implementation considerations . . . . . . . . . . . .  24
       3.9.7.  Alignment with RFC 5277bis  . . . . . . . . . . . . .  25
   4.  A YANG data model for management of datastore push
       subscriptions . . . . . . . . . . . . . . . . . . . . . . . .  25
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  25
     4.2.  Update streams  . . . . . . . . . . . . . . . . . . . . .  31
     4.3.  Filters . . . . . . . . . . . . . . . . . . . . . . . . .  31
     4.4.  Subscription configuration  . . . . . . . . . . . . . . .  32
     4.5.  Subscription monitoring . . . . . . . . . . . . . . . . .  34



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     4.6.  Notifications . . . . . . . . . . . . . . . . . . . . . .  34
     4.7.  RPCs  . . . . . . . . . . . . . . . . . . . . . . . . . .  35
       4.7.1.  Establish-subscription RPC  . . . . . . . . . . . . .  35
       4.7.2.  Modify-subscription RPC . . . . . . . . . . . . . . .  37
       4.7.3.  Delete-subscription RPC . . . . . . . . . . . . . . .  39
   5.  YANG module . . . . . . . . . . . . . . . . . . . . . . . . .  39
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  62
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  63
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  63
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  64

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.

   o  It lacks robustness.  Polling cycles may be missed, requests may
      be delayed or get lost, often particularly in cases when the




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      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.

   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.  Additionally,
   support for static subscriptions configured directly on the server
   are also useful when 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
   that features the following capabilities:

   o  A mechanism that allows clients to dynamically subscribe to
      automatic datastore updates, and the means to manage those



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      subscription.  The subscription allows clients to specify which
      data they are interested in, and to provide optional filters with
      criteria that data must meet for updates to be sent.  Furthermore,
      subscription 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  An alternative mechanism that allows for the static configuration
      of subscriptions to automatic data store updates as part of a
      device configuration.  In addition to the aspects that are
      configurable for a dynamic subscription (filter criteria, update
      policy), static configuration of subscriptions allows for the
      definition of additional aspects such as intended receivers and
      alternative transport options.

   o  The ability for a server to push back on requested subscription
      parameters.  Because not every server may support every requested
      interval 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
      subscription parameters.

   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.

   This document specifies a YANG data model to manage subscriptions to
   data in YANG datastores, and to configure associated filters and data
   streams.  It defines extensions to RPCs defined in [RFC5277] that
   allow to extend notification subscriptions to subscriptions for
   datastore updates.  It also defines a notification that can be used
   to carry data updates and thus serve as push mechanism.

   Editor's note: A new draft [I-D.gonzalez-netconf-5277bis] has just
   been released to address many of the current Netconf Event Model's
   limitations, as chartered by the Netconf Working Group.  This draft
   will be leveraged in future revisions of this document.

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.



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   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 static subscription, the receiver may be a
   different system than the one that configured the subscription.

   RPC: Remote Procedure Call

   SNMP: Simple Network Management Protocol

   Static 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 static subscription 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.



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   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 subscription service, which
   supports the dynamic as well as static 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 include
   information about why the subscription was not accepted, in order to
   facilitate converging on an acceptable set of subscription
   parameters.  Similarly, static subscriptions are configured as part
   of a device's configuration.  Once a subscription has been
   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, with a set of associated services.

   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.




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   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 is based on the subscriptions defined in
   [RFC5277], which are also reused in Restconf.  The model is extended
   with several parameters, including a subscription type and a
   subscription ID.

   The subscription model assumes the presence of a conceptual perpetual
   datastream "push-update" of continuous datastore updates that can be
   subscribed to, although other datastreams may be supported as well.
   A subscription refers to a datastream and specifies filters that are
   to be applied to, it for example, to provide only those subsets of
   the information that match a filter criteria.  In addition, a
   subscription specifies a set of subscription parameters that define
   the trigger when data records should be sent, for example at periodic
   intervals or whenever underlying data items change.

   The complete set of subscription parameters for both dynamic and
   static subscriptions is as follows:

   o  The stream being subscribed to.  The subscription model assumes
      the presence of perpetual and continuous streams of updates.  The
      stream "push-update" is always available and covers the entire set
      of YANG data in the server, but a system may provide other streams
      to choose from.

   o  The datastore to target.  By default, the datastore will always be
      "running".  However, it is conceivable that implementations want
      to also support subscriptions to updates to other datastores.

   o  An encoding for the data 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).

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




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   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.

      *  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 is needed to specify 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 is only sent once the
            dampening period has passed, containing the value of the
            data node that is then valid.

         +  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
            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.

   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.  Three types of filters are
      supported: subtree filters, with the same semantics as defined in
      [RFC 6241], XPath filters, and RFC 5277 filter, 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



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      referenced by a subscription, in order to facilitate reuse of
      complex filters.

   In addition, for the configuration of static subscriptions, the
   following parameters are supported:

   o  One or more receiver IP addresses (and corresponding ports)
      intended as the destination for push updates for each
      subscription.  In addition the transport protocol for each
      destination may be defined.

   o  Optional parameters to identify an egress interface or IP address
      / VRF where a subscription updates should be pushed from the
      publisher.

   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.
   Likewise, a server may not be able to support a requested updated
   frequency, or a requested encoding.

   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.




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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
   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




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   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.

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.




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   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:

   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.

   o  RFC5277 filter: A filter that allows for matching of update
      notification records per RFC 5277.

   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.  Subscription State Model at the Publisher

   Below is the state machine for the publisher.  It is important to
   note that a subscription doesn't exist at the publisher until it is
   accepted and made active.  The mere request by a subscriber to
   establish a subscription is insufficient for that asserted
   subscription to be externally visible via this state machine.










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                          .-------.
                       | start |
                       '-------'
                           |
                        establish
                           |
                           |   .----------modify-------------.lex
                           v   v                              '
                     .-----------.                      .-----------.
          .--------. |           |------>suspend------->|           |
        modify      '|  active   |                      | suspended |
          '--------->|           |<----resume----<------|           |
                     '-----------'                      '-----------'
                           |                                  |
                        delete                             delete
                           |                                  |
                           v                                  |
                       .-------.                              |
                       |  end  |<-----------------------------'
                       '-------'

                Figure 1: Subscription states at publisher

   Of interest in this state machine are the following:

   o  Successful <establish-subscription> or <modify-subscription>
      requests put the subscription into an active state.

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

   o  A <delete-subscription> request will delete the entire
      subscription.

3.7.  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,



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   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.

   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.





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   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:

        <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 2: 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:









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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>1500</beta>
               </alpha>
             </datastore-changes-xml>
           </push-change-update>
        </notification>

                   Figure 3: 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 4: 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 5: 2nd push example for on change update

3.8.  Subscription management

   There are two ways in which subscriptions can be managed: 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.

   Note: In order to distinguish subscribing to a stream of YANG-Push
   updates from subscribing to a regular event stream and avoid
   confusion with the <creat-subscription> RPC defined in RFC 5277, the
   term "establish-subscription" is used instead of "create-
   subscription".

3.8.1.  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 6: 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 7: 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.

3.8.2.  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.

3.9.  Other considerations

3.9.1.  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 8: 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 9: 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.

3.9.2.  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.

3.9.3.  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.

3.9.4.  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.

3.9.5.  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.

3.9.6.  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.

3.9.7.  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
   +--ro update-streams
   |  +--ro update-stream*   update-stream
   +--rw filters
   |  +--rw filter* [filter-id]
   |     +--rw filter-id         filter-id
   |     +--rw (filter-type)?
   |        +--:(subtree)
   |        |  +--rw subtree-filter
   |        +--:(xpath)
   |        |  +--rw xpath-filter?     yang:xpath1.0
   |        +--:(rfc5277)
   |           +--rw filter
   +--rw subscription-config {configured-subscriptions}?
   |  +--rw yang-push-subscription* [subscription-id]
   |     +--rw subscription-id            subscription-id
   |     +--rw stream?                    update-stream
   |     +--rw encoding?                  encoding
   |     +--rw subscription-start-time?   yang:date-and-time
   |     +--rw subscription-stop-time?    yang:date-and-time
   |     +--rw (filterspec)?
   |     |  +--:(inline)
   |     |  |  +--rw (filter-type)?



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   |     |  |     +--:(subtree)
   |     |  |     |  +--rw subtree-filter
   |     |  |     +--:(xpath)
   |     |  |     |  +--rw xpath-filter?              yang:xpath1.0
   |     |  |     +--:(rfc5277)
   |     |  |        +--rw filter
   |     |  +--:(by-reference)
   |     |     +--rw filter-ref?                filter-ref
   |     +--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 receiver* [address]
   |     |  +--rw address     inet:host
   |     |  +--rw port?       inet:port-number
   |     |  +--rw protocol?   transport-protocol
   |     +--rw (push-source)?
   |     |  +--:(interface-originated)
   |     |  |  +--rw source-interface?          if:interface-ref
   |     |  +--:(address-originated)
   |     |     +--rw source-vrf?                uint32
   |     |     +--rw source-address             inet:ip-address-no-zone
   |     +--rw dscp?               inet:dscp {configured-subscriptions}?
   |     +--rw subscription-priority?     uint8
   |     +--rw subscription-dependency?   string
   +--ro subscriptions
      +--ro yang-push-subscription* [subscription-id]
         +--ro subscription-id            subscription-id
         +--ro configured-subscription?empty {configured-subscriptions}?
         +--ro subscription-status?       identityref
         +--ro stream?                    update-stream
         +--ro encoding?                  encoding
         +--ro subscription-start-time?   yang:date-and-time
         +--ro subscription-stop-time?    yang:date-and-time
         +--ro (filterspec)?
         |  +--:(inline)
         |  |  +--ro (filter-type)?
         |  |     +--:(subtree)
         |  |     |  +--ro subtree-filter
         |  |     +--:(xpath)
         |  |     |  +--ro xpath-filter?              yang:xpath1.0
         |  |     +--:(rfc5277)
         |  |        +--ro filter
         |  +--:(by-reference)
         |     +--ro filter-ref?                filter-ref



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         +--ro (update-trigger)?
         |  +--:(periodic)
         |  |  +--ro period                     yang:timeticks
         |  +--:(on-change) {on-change}?
         |     +--ro no-synch-on-start?         empty
         |     +--ro dampening-period           yang:timeticks
         |     +--ro excluded-change*           change-type
         +--ro receiver* [address]
         |  +--ro address     inet:host
         |  +--ro port?       inet:port-number
         |  +--ro protocol?   transport-protocol
         +--ro (push-source)?
         |  +--:(interface-originated)
         |  |  +--ro source-interface?          if:interface-ref
         |  +--:(address-originated)
         |     +--ro source-vrf?                uint32
         |     +--ro source-address             inet:ip-address-no-zone
         +--ro dscp?               inet:dscp {configured-subscriptions}?
         +--ro subscription-priority?     uint8
         +--ro subscription-dependency?   string
rpcs:
   +---x establish-subscription
   |  +---w input
   |  |  +---w stream?                    update-stream
   |  |  +---w encoding?                  encoding
   |  |  +---w subscription-start-time?   yang:date-and-time
   |  |  +---w subscription-stop-time?    yang:date-and-time
   |  |  +---w (filterspec)?
   |  |  |  +--:(inline)
   |  |  |  |  +---w (filter-type)?
   |  |  |  |     +--:(subtree)
   |  |  |  |     |  +---w subtree-filter
   |  |  |  |     +--:(xpath)
   |  |  |  |     |  +---w xpath-filter?              yang:xpath1.0
   |  |  |  |     +--:(rfc5277)
   |  |  |  |        +---w filter
   |  |  |  +--:(by-reference)
   |  |  |     +---w filter-ref?                filter-ref
   |  |  +---w (update-trigger)?
   |  |  |  +--:(periodic)
   |  |  |  |  +---w period                     yang:timeticks
   |  |  |  +--:(on-change) {on-change}?
   |  |  |     +---w no-synch-on-start?         empty
   |  |  |     +---w dampening-period           yang:timeticks
   |  |  |     +---w excluded-change*           change-type
   |  |  +---w dscp?               inet:dscp {configured-subscriptions}?
   |  |  +---w subscription-priority?     uint8
   |  |  +---w subscription-dependency?   string



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   |  +--ro output
   |     +--ro subscription-result        subscription-result
   |     +--ro (result)?
   |        +--:(success)
   |        |  +--ro subscription-id            subscription-id
   |        +--:(no-success)
   |           +--ro stream?                    update-stream
   |           +--ro encoding?                  encoding
   |           +--ro subscription-start-time?   yang:date-and-time
   |           +--ro subscription-stop-time?    yang:date-and-time
   |           +--ro (filterspec)?
   |           |  +--:(inline)
   |           |  |  +--ro (filter-type)?
   |           |  |     +--:(subtree)
   |           |  |     |  +--ro subtree-filter
   |           |  |     +--:(xpath)
   |           |  |     |  +--ro xpath-filter?            yang:xpath1.0
   |           |  |     +--:(rfc5277)
   |           |  |        +--ro filter
   |           |  +--:(by-reference)
   |           |     +--ro filter-ref?                filter-ref
   |           +--ro (update-trigger)?
   |           |  +--:(periodic)
   |           |  |  +--ro period                     yang:timeticks
   |           |  +--:(on-change) {on-change}?
   |           |     +--ro no-synch-on-start?         empty
   |           |     +--ro dampening-period           yang:timeticks
   |           |     +--ro excluded-change*           change-type
   |           +--ro dscp?         inet:dscp {configured-subscriptions}?
   |           +--ro subscription-priority?     uint8
   |           +--ro subscription-dependency?   string
   +---x modify-subscription
   |  +---w input
   |  |  +---w subscription-id?           subscription-id
   |  |  +---w stream?                    update-stream
   |  |  +---w encoding?                  encoding
   |  |  +---w subscription-start-time?   yang:date-and-time
   |  |  +---w subscription-stop-time?    yang:date-and-time
   |  |  +---w (filterspec)?
   |  |  |  +--:(inline)
   |  |  |  |  +---w (filter-type)?
   |  |  |  |     +--:(subtree)
   |  |  |  |     |  +---w subtree-filter
   |  |  |  |     +--:(xpath)
   |  |  |  |     |  +---w xpath-filter?              yang:xpath1.0
   |  |  |  |     +--:(rfc5277)
   |  |  |  |        +---w filter
   |  |  |  +--:(by-reference)



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   |  |  |     +---w filter-ref?                filter-ref
   |  |  +---w (update-trigger)?
   |  |  |  +--:(periodic)
   |  |  |  |  +---w period                     yang:timeticks
   |  |  |  +--:(on-change) {on-change}?
   |  |  |     +---w no-synch-on-start?         empty
   |  |  |     +---w dampening-period           yang:timeticks
   |  |  |     +---w excluded-change*           change-type
   |  |  +---w dscp?               inet:dscp {configured-subscriptions}?
   |  |  +---w subscription-priority?     uint8
   |  |  +---w subscription-dependency?   string
   |  +--ro output
   |     +--ro subscription-result        subscription-result
   |     +--ro stream?                    update-stream
   |     +--ro encoding?                  encoding
   |     +--ro subscription-start-time?   yang:date-and-time
   |     +--ro subscription-stop-time?    yang:date-and-time
   |     +--ro (filterspec)?
   |     |  +--:(inline)
   |     |  |  +--ro (filter-type)?
   |     |  |     +--:(subtree)
   |     |  |     |  +--ro subtree-filter
   |     |  |     +--:(xpath)
   |     |  |     |  +--ro xpath-filter?              yang:xpath1.0
   |     |  |     +--:(rfc5277)
   |     |  |        +--ro filter
   |     |  +--:(by-reference)
   |     |     +--ro filter-ref?                filter-ref
   |     +--ro (update-trigger)?
   |     |  +--:(periodic)
   |     |  |  +--ro period                     yang:timeticks
   |     |  +--:(on-change) {on-change}?
   |     |     +--ro no-synch-on-start?         empty
   |     |     +--ro dampening-period           yang:timeticks
   |     |     +--ro excluded-change*           change-type
   |     +--ro dscp?               inet:dscp {configured-subscriptions}?
   |     +--ro subscription-priority?     uint8
   |     +--ro subscription-dependency?   string
   +---x delete-subscription
      +---w input
         +---w subscription-id?   subscription-id
notifications:
   +---n push-update
   |  +--ro subscription-id            subscription-id
   |  +--ro time-of-update?            yang:date-and-time
   |  +--ro (encoding)?
   |     +--:(encode-xml)
   |     |  +--ro datastore-contents-xml?    datastore-contents-xml



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   |     +--:(encode-json) {json}?
   |        +--ro datastore-contents-json?   datastore-contents-json
   +---n push-change-update {on-change}?
   |  +--ro subscription-id           subscription-id
   |  +--ro time-of-update?           yang:date-and-time
   |  +--ro (encoding)?
   |     +--:(encode-xml)
   |     |  +--ro datastore-changes-xml?    datastore-changes-xml
   |     +--:(encode-json) {json}?
   |        +--ro datastore-changes-yang?   datastore-changes-json
   +---n subscription-started
   |  +--ro subscription-id            subscription-id
   |  +--ro stream?                    update-stream
   |  +--ro encoding?                  encoding
   |  +--ro subscription-start-time?   yang:date-and-time
   |  +--ro subscription-stop-time?    yang:date-and-time
   |  +--ro (filterspec)?
   |  |  +--:(inline)
   |  |  |  +--ro (filter-type)?
   |  |  |     +--:(subtree)
   |  |  |     |  +--ro subtree-filter
   |  |  |     +--:(xpath)
   |  |  |     |  +--ro xpath-filter?              yang:xpath1.0
   |  |  |     +--:(rfc5277)
   |  |  |        +--ro filter
   |  |  +--:(by-reference)
   |  |     +--ro filter-ref?                filter-ref
   |  +--ro (update-trigger)?
   |  |  +--:(periodic)
   |  |  |  +--ro period                     yang:timeticks
   |  |  +--:(on-change) {on-change}?
   |  |     +--ro no-synch-on-start?         empty
   |  |     +--ro dampening-period           yang:timeticks
   |  |     +--ro excluded-change*           change-type
   |  +--ro dscp?                 inet:dscp {configured-subscriptions}?
   |  +--ro subscription-priority?     uint8
   |  +--ro subscription-dependency?   string
   +---n subscription-suspended
   |  +--ro subscription-id    subscription-id
   |  +--ro reason?            subscription-susp-reason
   +---n subscription-resumed
   |  +--ro subscription-id    subscription-id
   +---n subscription-modified
   |  +--ro subscription-id            subscription-id
   |  +--ro stream?                    update-stream
   |  +--ro encoding?                  encoding
   |  +--ro subscription-start-time?   yang:date-and-time
   |  +--ro subscription-stop-time?    yang:date-and-time



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   |  +--ro (filterspec)?
   |  |  +--:(inline)
   |  |  |  +--ro (filter-type)?
   |  |  |     +--:(subtree)
   |  |  |     |  +--ro subtree-filter
   |  |  |     +--:(xpath)
   |  |  |     |  +--ro xpath-filter?              yang:xpath1.0
   |  |  |     +--:(rfc5277)
   |  |  |        +--ro filter
   |  |  +--:(by-reference)
   |  |     +--ro filter-ref?                filter-ref
   |  +--ro (update-trigger)?
   |  |  +--:(periodic)
   |  |  |  +--ro period                     yang:timeticks
   |  |  +--:(on-change) {on-change}?
   |  |     +--ro no-synch-on-start?         empty
   |  |     +--ro dampening-period           yang:timeticks
   |  |     +--ro excluded-change*           change-type
   |  +--ro dscp?                  inet:dscp {configured-subscriptions}?
   |  +--ro subscription-priority?     uint8
   |  +--ro subscription-dependency?   string
   +---n subscription-terminated
      +--ro subscription-id    subscription-id
      +--ro reason?            subscription-term-reason


                        Figure 10: 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.

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.





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   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
   static configuration of subscriptions, i.e. for subscriptions that
   are established via configuration 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).

   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.




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   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
      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.



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   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:

   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)





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   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 11: 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 12: 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 13: 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 14: 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 15: 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 16: 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 17: 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 18: 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@2016-03-21.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-interfaces {
    prefix if;
  }

  organization "IETF";
  contact



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    "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 2016-03-21 {
    description
      "Changes to grouping structure, RPC definitions, filter
       definitions, origin interface and receiver definitions.";
    reference "YANG Datastore Push, draft-ietf-netconf-yang-push-02";
  }

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

  feature json {
    description
      "This feature indicates that JSON encoding of push updates
       is supported.";
  }

  feature configured-subscriptions {



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

  identity subscription-result {
    description
      "Base identity for RPC responses to requests surrounding
       management (e.g. creation, modification) of
       subscriptions.";
  }

  identity ok {
    base subscription-result;
    description
      "OK - RPC was successful and was performed as requested.";
  }

  identity error {
    base subscription-result;
    description
      "RPC was not successful.
       Base identity for error return codes.";
  }

  identity error-no-such-subscription {
    base error;
    description
      "A subscription with the requested subscription ID
       does not exist.";
  }

  identity error-no-such-option {
    base error;
    description
      "A requested parameter setting is not supported.";
  }

  identity error-insufficient-resources {
    base error;
    description
      "The server has insufficient resources to support the
       subscription as requested.";
  }

  identity error-data-not-authorized {
    base error;
    description



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

  identity error-configured-subscription {
    base error;
    description
      "Cannot apply RPC to a configured subscription, i.e.
       to a subscription that was not established via RPC.";
  }

  identity error-other {
    base error;
    description
      "An unspecified error has occurred (catch all).";
  }

  identity subscription-stream-status {
    description
      "Base identity for the status of subscriptions and
       datastreams.";
  }

  identity active {
    base subscription-stream-status;
    description
      "Status is active and healthy.";
  }

  identity inactive {
    base subscription-stream-status;
    description
      "Status is inactive, for example outside the
       interval between start time and stop time.";
  }

  identity suspended {
    base subscription-stream-status;
    description
      "The status is suspended, meaning that the push server
       is currently unable to provide the negotiated updates
       for the subscription.";
  }

  identity subscription-errors {
    description
      "Base identity for subscription error status.
       This identity is not to be confused with error return
       codes for RPCs";



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  }

  identity internal-error {
    base subscription-errors;
    description
      "Subscription failures caused by server internal error.";
  }

  identity no-resources {
    base subscription-errors;
    description
      "Lack of resources, e.g. CPU, memory, bandwidth";
  }

  identity subscription-deleted {
    base subscription-errors;
    description
      "The subscription was terminated because the subscription
       was deleted.";
  }

  identity other {
    base subscription-errors;
    description
      "Fallback reason - any other reason";
  }

  identity event-stream {
    description
      "Base identity to represent a generic stream of event
       notifications.";
  }

  identity update-stream {
    base event-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 {



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    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 category of customizable datastream for datastore
       updates with contents that have defined by a user.";
  }

  identity netconf-stream {
    base event-stream;
    description
      "Default notification stream";
  }

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

  identity encode-xml {
    base encodings;
    description
      "Encode data using XML";
  }

  identity encode-json {
    base encodings;
    description
      "Encode data using JSON";
  }

  identity transport {
    description
   "An identity that represents a transport protocol for event updates";
  }




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  identity netconf {
    base transport;
    description
      "Netconf notifications as a transport";
  }

  identity restconf {
    base transport;
    description
      "Restconf notifications as a transport";
  }

  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



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

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

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

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

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

  typedef encoding {
    type identityref {
      base encodings;



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    }
    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";
      }
      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 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



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      "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.";
  }

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

  grouping datatree-filter {
    description
      "This grouping defines filters for a datastore tree.";
    choice filter-type {
      description
        "A filter needs to be a single filter of a given type.
         Mixing and matching of multiple filters does not occur
         at the level of this grouping.";
      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";
        }
      }



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      case xpath {
        description
          "XPath filter";
        leaf xpath-filter {
          type yang:xpath1.0;
          description
            "Xpath defining the data items of interest.";
        }
      }
      case rfc5277 {
        anyxml filter {
          description
            "Subtree filter per RFC 5277";
        }
      }
    }
  }

  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 {
      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.";



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        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
             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 subscription-info {
    description
      "This grouping describes basic information concerning a
       subscription.";
    leaf stream {
      type update-stream;
      description
        "The stream being subscribed to.";
    }
    leaf encoding {
      type encoding;
      default "encode-xml";



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      description
        "The type of encoding for the subscribed data.
         Default is XML";
    }
    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.";
    }
    choice filterspec {
      description
        "The filter to be applied to the stream as part of the
         subscription.  The filter defines which updates of the
         data stream are of interest to a subscriber.
         The filter can be specified in-line
         or configured separately and referenced here.
         If no filter is specified, the entire datatree
         is of interest.";
      case inline {
        description
          "Filter is defined as part of the subscription.";
        uses datatree-filter;
      }
      case by-reference {
        description
          "Incorporate a filter that has been configured
           separately.";
        leaf filter-ref {
          type filter-ref;
          description
            "References filter which is associated with the



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             subscription.";
        }
      }
    }
  }

  grouping push-source-info {
    description
      "Defines the sender source from which push updates
       for a configured subscription are pushed.";
    choice push-source {
      description
        "Identifies the egress interface on the Publisher from
         which pushed updates will or are being sent.";
      case interface-originated {
        description
          "When the push source is out of an interface on the
           Publisher established via static configuration.";
        leaf source-interface {
          type if:interface-ref;
          description
            "References the interface for pushed updates.";
        }
      }
      case address-originated {
        description
          "When the push source is out of an IP address on the
           Publisher established via static configuration.";
        leaf source-vrf {
          type uint32 {
            range "16..1048574";
          }
          description
            "Label of the vrf.";
        }
        leaf source-address {
          type inet:ip-address-no-zone;
          mandatory true;
          description
            "The source address for the pushed objects.";
        }
      }
    }
  }

  grouping receiver-info {
    description
      "Defines where and how to deliver push updates for a



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       configured subscription.  This includes
       specifying the receiver, as well as defining
       any network and transport aspects when pushing of
       updates occurs outside of Netconf or Restconf.";
    list receiver {
      key "address";
      description
        "A single host or multipoint address intended as a target
         for the pushed updates for a subscription.";
      leaf address {
        type inet:host;
        description
          "Specifies the address for the traffic to reach a
           remote host. One of the following must be
           specified: an ipv4 address, an ipv6 address,
           or a host name.";
      }
      leaf port {
        type inet:port-number;
        description
          "This leaf specifies the port number to use for messages
           destined for a receiver.";
      }
      leaf protocol {
        type transport-protocol;
        default "netconf";
        description
          "This leaf specifies the transport protocol used
           to deliver messages destined for the receiver.";
      }
    }
  }

 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 {
      if-feature 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.";
    }



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    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.";
    }
  }

  rpc establish-subscription {
    description
      "This RPC allows a subscriber to establish a subscription
       on its own behalf.  If successful, the subscription
       remains in effect for the duration of the subscriber's
       association with the publisher, or until the subscription
       is terminated by virtue of a delete-subscription request.
       In case an error (as indicated by subscription-result)
       is returned, the subscription is
       not establishd.  In that case, the RPC output
       MAY include suggested parameter settings
       that would have a high likelihood of succeeding in a
       subsequent establish-subscription request.";
    input {
      uses subscription-info;
      uses update-policy;
      uses subscription-qos;
    }
    output {
      leaf subscription-result {
        type subscription-result;
        mandatory true;
        description
          "Indicates whether subscription is operational,
           or if a problem was encountered.";
      }
      choice result {
        description
          "Depending on the subscription result, different
           data is returned.";
        case success {



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          description
            "This case is used when the subscription request
             was successful and a subscription was established as
             a result";
          leaf subscription-id {
            type subscription-id;
            mandatory true;
            description
              "Identifier used for this subscription.";
          }
        }
        case no-success {
          description
            "This case applies when a subscription request
             was not successful and no subscription was
             established as a result.  In this case,
             information MAY be returned that indicates
             suggested parameter settings that would have a
             high likelihood of succeeding in a subsequent
             establish-subscription request.";
          uses subscription-info;
          uses update-policy;
          uses subscription-qos;
        }
      }
    }
  }
  rpc modify-subscription {
    description
      "This RPC allows a subscriber to modify a subscription
       that was previously established using establish-subscription.
       If successful, the subscription
       remains in effect for the duration of the subscriber's
       association with the publisher, or until the subscription
       is terminated by virtue of a delete-subscription request.
       In case an error is returned (as indicated by
       subscription-result), the subscription is
       not modified and the original subscription parameters
       remain in effect.  In that case, the rpc error response
       MAY include suggested parameter settings
       that would have a high likelihood of succeeding in a
       subsequent modify-subscription request.";
    input {
      leaf subscription-id {
        type subscription-id;
        description
          "Identifier to use for this subscription.";
        }



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        uses subscription-info;
        uses update-policy;
        uses subscription-qos;
    }
    output {
      leaf subscription-result {
        type subscription-result;
        mandatory true;
        description
          "Indicates whether subscription was modified
           or if a problem was encountered.
           In case the subscription-result has a value
           other than OK, the original subscription was not
           changed.";
      }
      uses subscription-info;
      uses update-policy;
      uses subscription-qos;
    }
  }
  rpc delete-subscription {
    description
      "This RPC allows a subscriber to delete a subscription that
       was previously established using establish-subscription.";
    input {
      leaf subscription-id {
        type subscription-id;
        description
          "Identifier of the subscription that is to be deleted.
           Only subscriptions that were established using
           establish-subscription can be deleted via this RPC.";
      }
    }
  }
  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 subscription-id;
      mandatory true;
      description
        "This references the subscription because of which the
         notification is sent.";
    }
    leaf time-of-update {



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      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 "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
      "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 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



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        "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 "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.";
        }
      }
    }
  }
  notification subscription-started {
    description
      "This notification indicates 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.";
    leaf subscription-id {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";



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    }
    uses subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  notification subscription-suspended {
    description
      "This notification indicates that a suspension of the
       subscription by the server has occurred.  No further
       datastore updates will be sent until subscription
       resumes.
       This notification shall only be sent to receivers
       of a subscription; it does not constitute a general-purpose
       notification.";
    leaf subscription-id {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }
    leaf reason {
      type subscription-susp-reason;
      description
        "Provides a reason for why the subscription was
         suspended.";
    }
  }
  notification subscription-resumed {
    description
      "This notification indicates that a subscription that had
       previously been suspended has resumed. Datastore updates
       will once again be sent.";
    leaf subscription-id {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }
  }
  notification subscription-modified {
    description
      "This notification indicates that a subscription has
       been modified.  Datastore updates sent from this point
       on will conform to the modified terms of the
       subscription.";
    leaf subscription-id {
      type subscription-id;
      mandatory true;



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      description
        "This references the affected subscription.";
    }
    uses subscription-info;
    uses update-policy;
    uses subscription-qos;
  }
  notification subscription-terminated {
    description
      "This notification indicates that a subscription has been
       terminated.";
    leaf subscription-id {
      type subscription-id;
      mandatory true;
      description
        "This references the affected subscription.";
    }
    leaf reason {
      type subscription-term-reason;
      description
        "Provides a reason for why the subscription was
         terminated.";
    }
  }
  container update-streams {
    config false;
    description
      "This container contains a leaf list of built-in
       streams that are provided by the system.";
    leaf-list update-stream {
      type update-stream;
      description
        "Identifies a built-in stream that is supported by the
         system.  Streams are associated with their own identities,
         each of which carries a special semantics.";
    }
  }
  container filters {
    description
      "This container contains a list of configurable filters
       that can be applied to subscriptions.  This facilitates
       the reuse of complex filters once defined.";
    list filter {
      key "filter-id";
      description
        "A list of configurable filters that can be applied to
         subscriptions.";
      leaf filter-id {



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        type filter-id;
        description
          "An identifier to differentiate between filters.";
      }
      uses datatree-filter;
    }
  }
  container subscription-config {
    if-feature "configured-subscriptions";
    description
      "Contains the list of subscriptions that are configured,
       as opposed to established via RPC or other means.";
    list yang-push-subscription {
      key "subscription-id";
      description
        "Content of a yang-push subscription.";
      leaf subscription-id {
        type subscription-id;
        description
          "Identifier to use for this subscription.";
      }
      uses subscription-info;
      uses update-policy;
      uses receiver-info;
      uses push-source-info;
      uses subscription-qos;
    }
  }
  container subscriptions {
    config false;
    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.";
    list yang-push-subscription {
      key "subscription-id";
      config false;
      description
        "Content of a yang-push subscription.
         Subscriptions can be established using a control channel
         or RPC, or be established through configuration.";
      leaf subscription-id {
        type subscription-id;
        description



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          "Identifier of this subscription.";
      }
      leaf configured-subscription {
        if-feature "configured-subscriptions";
        type empty;
        description
          "The presence of this leaf indicates that the
           subscription originated from configuration, not through
           a control channel or RPC.";
      }
      leaf subscription-status {
        type identityref {
          base subscription-stream-status;
        }
        description
          "The status of the subscription.";
      }
      uses subscription-info;
      uses update-policy;
      uses receiver-info;
      uses push-source-info;
      uses subscription-qos;
    }
  }
}

<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



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   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.  References

7.1.  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>.

7.2.  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-03 (work in progress), April 2015.

   [I-D.gonzalez-netconf-5277bis]
              Gonzalez Prieto, A., Clemm, A., Voit, E., Tripathy, A.,
              and E. Nilsen-Nygaard, "Mounting YANG-defined information
              from remote datastores", draft-clemm-netmod-mount-03 (work
              in progress), March 2016.



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   [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-05 (work in progress), February 2016.

   [I-D.ietf-netconf-restconf]
              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", I-D draft-ietf-netconf-restconf-10, March 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-07 (work in progress), January
              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


   Eric Voit
   Cisco Systems

   EMail: evoit@cisco.com








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   Ambika Prasad Tripathy
   Cisco Systems

   EMail: ambtripa@cisco.com


   Einar Nilsen-Nygaard
   Cisco Systems

   EMail: einarnn@cisco.com









































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