Network Working Group                                           A. Clemm
Internet-Draft                                        A. Gonzalez Prieto
Intended status: Standards Track                                 E. Voit
Expires: September 10, 2015                                Cisco Systems
                                                           March 9, 2015

               Subscribing to YANG datastore push updates


   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
   the client per a subscription policy, without requiring additional
   client requests.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   5
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Subscription Model  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Negotiation of Subscription Policies  . . . . . . . . . .   8
     3.3.  Custom Datastreams  . . . . . . . . . . . . . . . . . . .   8
     3.4.  Push Data Stream and Transport Mapping  . . . . . . . . .   9
     3.5.  Subscription operations . . . . . . . . . . . . . . . . .  10
     3.6.  A YANG data model for management of datastore push
           subscriptions . . . . . . . . . . . . . . . . . . . . . .  13
     3.7.  Other considerations  . . . . . . . . . . . . . . . . . .  15
       3.7.1.  Authorization . . . . . . . . . . . . . . . . . . . .  15
       3.7.2.  Additional subscription primitives  . . . . . . . . .  15
       3.7.3.  Robustness and reliability considerations . . . . . .  16
       3.7.4.  Implementation considerations . . . . . . . . . . . .  16
   4.  YANG module . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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 contain operational data.
   It is therefore reasonable to expect that data in YANG datastores
   will increasingly be used to support applications that are not

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   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 [peermount-req][peermount].  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 rely heavily on polling, in which data is
   periodically explicitly retrieved by a client from a server to stay

   There are various issues associated with polling-based management:

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

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

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

   More effective is an alternative in which an application can request
   to be automatically updated 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 [RFC6241] or RESTconf [restconf] will be

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   any more effective, as they would follow the same request/response

   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 subscribe to updates of a YANG datastore and that
   allows the server to push those updates.  The requirements for such a
   service are documented in [pub-sub-reqs].  This document proposes a
   solution that addresses those requirements.  The solution features
   the following:

   o  A mechanism that allows clients to subscribe to automatic
      datastore updates, and the means to manage those 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.  Furthemore, 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  The ability to negotiate 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

   o  A mechanism is 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.

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2.  Definitions and Acronyms

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

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

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

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

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

   NACM: NETCONF Access Control Model

   NETCONF: Network Configuration Protocol

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

   RPC: Remote Procedure Call

   SNMP: Simple Network Management Protocol

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

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

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

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

   Update trigger: A trigger, as specified by a subscription policy,
   that causes an update to be sent, respectively a data record to be
   generated.  An example of a trigger is a change trigger, invoked when
   the value of a data node changes or a data node is created or

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   deleted, or a time trigger, invoked after the laps of a periodic time

   URI: Uniform Resource Identifier

   YANG: A data definition language for NETCONF

3.  Solution Overview

   This document specifies a solution that allows clients to subscribe
   to information updates in a YANG datastore, which are subsequently
   pushed from the server to the client.

   Subscriptions are initiated by clients.  Servers respond to a
   subscription request explicitly positively or negatively.  Negative
   responses include information about why the subscription was not
   accepted, in order to facilitate converging on an accepable set of
   subscription parameters.  Once a subscription has been established,
   datastore push updates are pushed from the server to the subscribing
   client 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 negotiate subscription parameters, in cases where a
      subscription desired by a client cannot currently be served.

   o  The stream of datastore push updates.

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

3.1.  Subscription Model

   Yang-push subscriptions are defined using a data model.  This model
   is based on the subscriptions defined in [RFC-5277], which is also
   reused in RESTconf.  The model is extended with several parameters,
   including a subscription type and a subscription ID.

   A subscription refers to a datastream.  The subscription model
   assumes the presence of a conceptual perpetual datastream "push-
   update" of continuous datastore updates of infinite time resolution.
   A subscription refers to this 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

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   subscription specifies a subscription policy that defines 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 is as follows:

   o  The name of the stream to subscribe to.  The subscription model
      always assumes the presence of a perpetual and continuous stream
      of updates.  The stream is called "push-update".  However, as
      mentioned, it is possible to subscribe to other datastreams, such
      as custom datastreams which are separately configured.

   o  Optional filter(s), 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.  Two filtering mechanisms are provided: subtree
      filtering and Xpath filtering, with the semantics described in
      [RFC 5277 Section 3.6].  (Additional filter types can be added
      through extensions.)

   o  An identifier for the subscription.

   o  An optional start time.  Used to trigger replays starting at the
      provided time.  Its semantics are those in [RFC 5277].

   o  An optional stop time.  Used to limit temporarily the events of
      interest.  Its semantics are those in [RFC 5277].

   o  For subscriptions to "push-update", a subscription policy
      definition regarding the update trigger to send new updates.  The
      trigger can be periodic or based on change.  For periodic
      subscriptions, the trigger is defines by the interval with which
      to push updates.  For on-change subscriptions, the trigger is
      defined using the dampening interval with which to push repeated
      changes, an indicator for the magnitude of changes, etc.

   It is conceivable that additional subscription parameters might be
   added in the future.  For example, a parameter might be introduced
   that allows to specify which encoding method for updates to use.  The
   list could accordingly be extended and augmented in the future.

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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.  Providing "on-
   change" updates for operational data can be computationally expensive
   and an agent may decide not to support them or supporting them for a
   small number of subscribers or for a limited set of data nodes.

   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 are no guarantee for subsequent requests for
   this client or others.

3.3.  Custom Datastreams

   Optionally, it is possible to introduce other datastreams (beyond the
   datastore-push datastream) with custom semantics.  Some datastreams
   can be custom configured.  The support of this is tied to a separate
   feature.  The configuration of a custom datastream specifies the
   trigger conditions under which new data records for the stream are
   generated, and which updates the corresponding data records contain.
   For example, the configuration of a datastream can specify which
   subsets of data nodes in a datastore the datastream should contain,
   which filter criteria the updates need to meet, and under what
   conditions to create updates - for example, periodically or whenever
   a data item changes.

   A subscription that refers to a custom datastream can specify a set
   of filters, like for the "push-update" datastream.  However, the
   policy as to when updates are triggered (periodically or on change)
   needs to be the same as the policy of the datastream and cannot be
   modified.  It is not possible, for example, to define a custom
   datastream which creates on-change updates, yet subscribe to that
   datastream with periodic updates.

   While conceptually similar, the choice between subscribing to
   datastream "push-update" or configuring and subscribing to a custom
   datastream can be thought of as analogous to the choice between
   operating a nozzle that is connected to a hose, or controlling the
   faucet (custom datastream).  Operating the nozzle is for most uses
   simpler; however, the option to operate the faucet instead can
   provide additional flexibility in some scenarios.

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3.4.  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 is therefore that of a notification.
   Contrary to notifications associated with alarms and unexpected event
   occurrences, push updates are solicited, i.e. tied tied to a
   particular subscription which triggered the notification.  (An
   alternative conceptual model would consider a subscription an "opt-
   in" filter on a continuous stream of updates.)

   The 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,

   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
   communicate datastore updates.  The contents of the notification
   includes a set of explicitly defined data nodes.  For this purpose, a
   new generic notification is introduced, "push-update" notification.
   This notification is used to carry a data record with updates of
   datastore contents as specified by a subscription.

   The update record consists of a data snippet that contains an
   instantiated datastore subtree with the subscribed contents.  Data
   nodes that do not match filter criteria are removed.  Likewise, in
   the case of a subscription with "on-change" subscription policy, data
   nodes that have not undergone change are omitted.  The contents 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, excluding
   the following statements: "mandatory", "must", "min-elements", "max-
   elements", "when", and "default".  However, in a YANG data model, it
   is not practical to model the precise data contained in the updates
   as part of the notification, because the specific data nodes
   supported depends 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.

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

  <?xml version="1.0" encoding="UTF-8"?>
  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
     <subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
     <datastore-contents xmlns="urn:ietf:params:xml:ns:netconf:

                          Figure 1: Push example

3.5.  Subscription operations

   There are several operations associated with subscriptions.  At the
   most basic level, clients need to be able to create subscriptions, as
   well as delete subscriptions when they are no longer needed.

   RFC 5277 specifies an operation to create subscriptions for event
   streams, <create-subscription>.  This operation is leveraged and
   extended to create datastore-push subscriptions.  Specifically, an
   additional parameter is added to allow for the specification of
   trigger policy.

   To support datastore push, a server MUST support the interleave
   capability specified in [RFC5277].  This is required to allow for

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   modification of what data is being subscribed to without needing to
   establish a separate Netconf session.

   The example below illustrates a subscription for a periodic push of
   all data under a container called foo.

   <netconf:rpc message-id="101"
           <subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
          <filter netconf:type="xpath"
          <period xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">

                      Figure 2: Subscription example

   The example below illustrates a subscription response, where an agent
   does not support frequent periodic updates, and suggests a different
   sampling rate to the client.

<rpc-reply message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
               <supported-subscription xmlns="urn:ietf:params:xml:ns:

                Figure 3: Subscription negotiation example

   RFC 5277 does not specify operations to delete subscriptions.
   Instead, it assumes that an event subscription is associated with its

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   own Netconf session.  When the session is torn down, the subscription
   is implicitly deleted.  Likewise, there is no operation to modify a
   subscription.  Modifying a subscription requires tearing down a
   Netconf session, starting a new one, and creating a new subscription.
   Furthermore, each session only supports a single subscription.
   Establishing multiple subscriptions requires multiple concurrent
   Netconf sessions.

   To facilitate datastore-push subscriptions, an additional RPC is
   introduced, <delete-subscription>.

   The <delete-subscription> operation takes as parameter a subscription
   ID.  As a result of the operation, the subscription is removed and no
   more data records will be sent.

     <netconf:rpc message-id="102"
            <subscription-id xmlns="urn:ietf:params:xml:ns:netconf:

                      Figure 4: Subscription deletion

   Finally, a separate operation to modify a subscription is introduced,
   <modify-subscription>.  This operation takes the same parameters as
   <create-subscription>, but refers to an existing subscription.  Of
   course, a subscription could also be deleted and another be created.
   However, modify operation avoids issues regarding the synchronization
   of creation and deletion operations, such as potential loss or
   duplication of updates.  Also, a modify operation allows to simply
   extend an existing subscription beyond the initial subscription end

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   <netconf:rpc message-id="103"
           <filter netconf:type="xpath"
           <period xmlns="urn:ietf:params:xml:ns:netconf:

                       Figure 5: Modify subscription

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

   Subscriptions as well as datastreams 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.  When custom datastreams are
   supported, those datastreams need to be configured and monitored as

   For this purpose, a YANG data model is introduced, which is depicted
   in the following figure.

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   module: ietf-datastore-push
      +--rw streams {custom-streams}?
      |  +--rw stream* [stream-name]
      |     +--rw stream-name         string
      |     +--ro stream-status?      identityref
      |     +--rw subtree-filter?     subtree-filter
      |     +--rw xpath-filter?       yang:xpath1.0
      |     +--rw (update-trigger)?
      |        +--:(periodic)
      |        |  +--rw period?             yang:timeticks
      |        +--:(on-change)
      |           +--rw dampening-period    yang:timeticks
      |           +--rw (change-policy)?
      |              +--:(delta-policy)
      |                 +--rw delta?              uint32
      +--rw subscriptions
         +--ro datastore-push-subscription* [subscription-id]
            +--ro subscription-id        subscription-identifier
            +--ro subscription-status?   identityref
            +--ro stream?                string
            +--ro start-time?            yang:date-and-time
            +--ro stop-time?             yang:date-and-time
            +--ro subtree-filter?        subtree-filter
            +--ro xpath-filter?          yang:xpath1.0
            +--ro (update-trigger)?
               |  +--ro period?                yang:timeticks
                  +--ro dampening-period       yang:timeticks
                  +--ro (change-policy)?
                        +--ro delta?                 uint32

                         Figure 6: Model structure

   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 paramters.  The subscription status indicates whether
   the subscription is currently active and healthy, or if it is
   degraded in some form.  Subscriptions are automatically removed from
   the list once they expire or are terminated.  Because subscriptions
   are managed using their own set of operation primitives, they are

   An optional feature, custom-streams, is introduced to allow for the
   configuration of custom datastreams.  Custom datastreams are
   represented through a separate list, consisting of information used

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   to configure those datastreams.  This information consititutes mostly
   configuration information, with the exception of parameters used to
   indicate the status and health of the datastream.

   In addition, a server needs to indicate any changes in status to the
   subscriber through a notification.  Specifically, subscribers need to
   be informed of the following:

   o  A subscription has been temporarily suspended(including the

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

   o  A subscription has been abnormally terminated (including the

   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.

3.7.  Other considerations

3.7.1.  Authorization

   A client may only receive updates to data that the client has proper
   authorization for.  Normal authorization rules apply.  Data that is
   being pushed therefore needs to be subjected to a filter that applies
   the corresponding rules, removing any non-authorized data as

   The authorization model for data in YANG datastores is described in
   the Netconf Access Control Model [RFC6536].

3.7.2.  Additional subscription primitives

   Other possible operations include the ability to suspend and resume
   subscriptions.  However, those operations are not viewed as
   essential, as it is always possible to alternatively simply remove a
   subscription and recreate it when needed.

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3.7.3.  Robustness and reliability considerations

   Particularly in the case of on-change push updates, it is important
   that push updates do not get lost.  However, datastore-push uses a
   secure and reliable transport.  Notifations are not getting
   reordered, and in addition contain a time stamp.  For those reasons,
   we believe that additional reliability mechanisms at the application
   level, such as sequence numbers for push updates, are not required.

3.7.4.  Implementation considerations

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

   Even periodic retrieval 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.

4.  YANG module

   file "ietf-datastore-push@2014-03-09.yang"

   module ietf-datastore-push {
     namespace "urn:XXXX:params:xml:ns:yang:ietf-datastore-push";
     prefix datastore-push;

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     import ietf-yang-types {
       prefix yang;

     organization "IETF";
       "Editor:   Alexander Clemm

        Editor:   Alberto Gonzalez Prieto

        Editor:   Eric Voit
       "This module contains conceptual YANG specifications
        for datastore push.";

     revision 2014-03-09 {
         "Initial revision.";
       reference "Datastore push.";

     feature custom-streams {
         "This feature allows users to configure datastore update
          streams in addition to the stream provided by default,

     identity subscription-stream-status {
         "Base identity for the status of subscriptions and

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

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

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     identity in-error {
       base subscription-stream-status;
         "The status is in error or degraded, meaning that
          stream and/or subscription are currently unable to provide
          the negotiated updates.";

     identity subscription-errors {
         "Base identity for subscription errors.";

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

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

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

     typedef datastore-contents {
       type string;
         "This type is be used to represent datastore contents,
          including a filtered datastore subtree per a set of
          subscription parameters. ";

     typedef subtree-filter {
       type string;
         "This type is used to define a subtree filter.
          Its precise syntax is TBD.";

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     typedef subscription-identifier {
       type string {
         length "1 .. max";
         "A client-provided identifier for the subscription.";

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

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

     grouping subscription-stream-policy {
         "This grouping contains the parameters which describe
          the policy which data is pushed as part of a
          subscription or a data stream.";
       leaf subtree-filter {
           "Datastore subtree of interest.";
         type subtree-filter;
       leaf xpath-filter {
         type yang:xpath1.0;
           "Xpath defining the data items of interest.";
       choice update-trigger {
           "Defines necessary conditions for sending an event  to
            the subscriber.";
         case periodic {
             "The agent is requested to notify periodically the
              current values of the datastore or the subset
              defined by the filter.";

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           leaf period {
             type yang:timeticks;
               "Elapsed time between notifications.";
         case on-change {
             "The agent is requested to notify changes in
              values in the datastore or a subset of it defined
              by a filter.";
           leaf dampening-period {
             type yang:timeticks;
             mandatory true;
               "Minimum amount of time that needs to have
                passed since the last time an update was
           choice change-policy {
               "Policy describing necessary conditions for
                sending an event  to the subscriber.";
             case delta-policy {
               leaf delta {
                 type uint32;
                   "For integer, minimum difference
                    between current and last reports
                    values that can trigger an update.";

     grouping subscription-info {
         "This grouping describes basic information concerning a
          subscription, without the subscription policy which is
          defined separately to be shareable with the definition
          of a datastream.";
       leaf stream {
         type string;
           "The name of the stream subscribed to.";

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       leaf start-time {
         type yang:date-and-time;
           "Starting time for replays.";
         reference "RFC 5277, Section 2.1.1";
       leaf stop-time {
         type yang:date-and-time;
           "Time limit for events of interest.";
         reference "RFC 5277, Section 2.1.1";

     notification push-update {
         "This notification contains an update from a datastore";
       leaf subscription-id {
         type subscription-identifier;
         mandatory true;
           "This references the subscription because of which the
            notification is sent.";
       leaf datastore-contents {
         type datastore-contents;
           "This contains datastore contents
            per the subscription.";
     notification subscription-suspended {
         "This notification indicates that a suspension of the
          subscription by the server has occurred.  No further
          datastore updates will be sent until subscription
       leaf subscription-id {
         type subscription-identifier;
         mandatory true;
           "This references the affected subscription.";
       leaf reason {
         type subscription-susp-reason;
           "Provides a reason for why the subscription was

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     notification subscription-resumed {
         "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-identifier;
         mandatory true;
           "This references the affected subscription.";
     notification subscription-modified {
         "This notification indicates that a subscription has
          been modified.  Datastore updates sent from this point
          on will conform to the modified terms of the
       leaf subscription-id {
         type subscription-identifier;
         mandatory true;
           "This references the affected subscription.";
       uses subscription-info;
       uses subscription-stream-policy;
     notification subscription-terminated {
         "This notification indicates that a subscription has been
       leaf subscription-id {
         type subscription-identifier;
         mandatory true;
           "This references the affected subscription.";
       leaf reason {
         type subscription-term-reason;
           "Provides a reason for why the subscription was
     container streams {
       if-feature custom-streams;

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       list stream {
         key "stream-name";
           "A user-definable stream.";
         leaf stream-name {
           type string;
           mandatory true;
             "The name assigned to the stream.";
         leaf stream-status {
           type identityref {
             base subscription-stream-status;
           config false;
         uses subscription-stream-policy;
     container subscriptions {
       list datastore-push-subscription {
           "Content of a yang-push subscription.
            Subscriptions are created using a dedicated RPC, hence
            they do not constitute configuration information.";
         config false;
         key "subscription-id";
         leaf subscription-id {
           type subscription-identifier;
             "Identifier to use for this subscription.";
         leaf subscription-status {
           type identityref {
             base subscription-stream-status;
             "The status of the subscription.";
         uses subscription-info;
         uses subscription-stream-policy;


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5.  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.

6.  References

6.1.  Normative References

   [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol (SNMP)", STD 15, RFC
              1157, May 1990.

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

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

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

   [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, March

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6.2.  Informative References

              Clemm, A., Medved, J., and E. Voit, "Mounting YANG-defined
              information from remote datastores", draft-clemm-netmod-
              mount-02 (work in progress), October 2014.

              Voit, E., Clemm, A., Bansal, S., Tripathy, A., and P.
              Yellai, "Requirements for Peer Mounting of YANG subtrees
              from Remote Datastores", draft-voit-netmod-peer-mount-
              requirements-00 (work in progress), September 2014.

              Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
              for Subscription to YANG Datastores", draft-ietf-i2rs-pub-
              sub-requirements-00 (work in progress), March 2015.

              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", I-D draft-ietf-netconf-restconf-03, October

Authors' Addresses

   Alexander Clemm
   Cisco Systems


   Alberto Gonzalez Prieto
   Cisco Systems


   Eric Voit
   Cisco Systems


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