Subscribing to YANG datastore push updates
draft-clemm-netconf-yang-push-00.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 the client per a subscription policy, without requiring additional client requests.

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Table of Contents

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

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 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 up-to-date.

There are various issues associated with polling-based management:

• 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 bandwidth.
• 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.
• 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 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 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: [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.

• 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.
• 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 parameters.
• 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

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 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

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:

• The subscription model for configuration and management of the subscriptions, with a set of associated services.
• The ability to negotiate subscription parameters, in cases where a subscription desired by a client cannot currently be served.
• 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 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:

• 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.
• 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.)
• An identifier for the subscription.
• An optional start time. Used to trigger replays starting at the provided time. Its semantics are those in [RFC 5277].
• An optional stop time. Used to limit temporarily the events of interest. Its semantics are those in [RFC 5277].
• 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.

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.

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: [RFC5277]. Alternative transports are conceivable but outside the scope of this specification.

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

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.

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

 
   <netconf:rpc message-id="101"
            xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
       <create-subscription
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <stream>push-update</stream>
           <subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
              datastore-push:1.0">
                     my-sub
          </subscription-id>
          <filter netconf:type="xpath"
                     xmlns:ex="http://example.com/foo/1.0"
                     select="/ex:foo"/>
          <period xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">
                    500
          </period>
       </create-subscription>
   </netconf:rpc>
             

                 
<rpc-reply message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <rpc-error>
           <error-type>application</error-type>
           <error-tag>operation-not-supported</error-tag>
           <error-severity>error</error-severity>
           <error-info>
               <supported-subscription xmlns="urn:ietf:params:xml:ns:
                   netconf:datastore-push:1.0">
                   <period>3000</period>
               </supported-subscription>
           </error-info>
       </rpc-error>
</rpc-reply>
             

                 

   <netconf:rpc message-id="102"
            xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
       <delete-subscription
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
          <subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
            datastore-push:1.0">
                     my-sub
          </subscription-id>
     </delete-subscription>
   </netconf:rpc>
             

                 
   <netconf:rpc message-id="103"
            xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
       <create-subscription
              xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
           <stream>push-update</stream>
           <subscription-id 
              xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">
                     my-sub
           </subscription-id>
           <filter netconf:type="xpath"
                     xmlns:ex="http://example.com/foo/1.0"
                     select="/ex:foo"/>
           <period xmlns="urn:ietf:params:xml:ns:netconf:
              datastore-push:1.0">
                    3000
       </period>
       </create-subscription>
   </netconf:rpc>
       

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)?
            +--:(periodic)
            |  +--ro period?                yang:timeticks
            +--:(on-change)
               +--ro dampening-period       yang:timeticks
               +--ro (change-policy)?
                  +--:(delta-policy)
                     +--ro delta?                 uint32          
		    

<CODE BEGINS>
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;

  import ietf-yang-types {
    prefix yang;
  }

  organization "IETF";
  contact
    "Editor:   Alexander Clemm
               <mailto:alex@cisco.com>
     
     Editor:   Alberto Gonzalez Prieto
               <mailto:albertgo@cisco.com>
     
     Editor:   Eric Voit
               <mailto:evoit@cisco.com>";
  description
    "This module contains conceptual YANG specifications
     for datastore push.";

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

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

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

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

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

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

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

  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.";
  }

  grouping subscription-stream-policy {
    description
      "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 {
      description
        "Datastore subtree of interest.";
      type subtree-filter;
    }
    leaf xpath-filter {
      type yang:xpath1.0;
      description
        "Xpath defining the data items of interest.";
    }
    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;
          description
            "Elapsed time between notifications.";
        }
      }
      case on-change {
        description
          "The agent is requested to notify changes in 
           values in the datastore or a subset of it defined 
           by a filter.";
        leaf 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.";
        }
        choice change-policy {
          description
            "Policy describing necessary conditions for 
             sending an event  to the subscriber.";
          case delta-policy {
            leaf delta {
              type uint32;
              description
                "For integer, minimum difference 
                 between current and last reports 
                 values that can trigger an update.";
            }
          }
        }
      }
    }
  }

  grouping subscription-info {
    description
      "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;
      description
        "The name of the stream subscribed to.";
    }
    leaf start-time {
      type yang:date-and-time;
      description
        "Starting time for replays.";
      reference "RFC 5277, Section 2.1.1";
    }
    leaf stop-time {
      type yang:date-and-time;
      description
        "Time limit for events of interest.";
      reference "RFC 5277, Section 2.1.1";
    }
  }

  notification push-update {
    description
      "This notification contains an update from a datastore";
    leaf subscription-id {
      type subscription-identifier;
      mandatory true;
      description
        "This references the subscription because of which the 
         notification is sent.";
    }
    leaf datastore-contents {
      type datastore-contents;
      description
        "This contains datastore contents 
         per the subscription.";
    }
  }
  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.";
    leaf subscription-id {
      type subscription-identifier;
      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-identifier;
      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-identifier;
      mandatory true;
      description
        "This references the affected subscription.";
    }
    uses subscription-info;
    uses subscription-stream-policy;
  }
  notification subscription-terminated {
    description
      "This notification indicates that a subscription has been 
       terminated.";
    leaf subscription-id {
      type subscription-identifier;
      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 streams {
    if-feature custom-streams;
    list stream {
      key "stream-name";
      description
        "A user-definable stream.";
      leaf stream-name {
        type string;
        mandatory true;
        description
          "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 {
      description
        "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;
        description
          "Identifier to use for this subscription.";
      }
      leaf subscription-status {
        type identityref {
          base subscription-stream-status;
        }
        description
          "The status of the subscription.";
      }
      uses subscription-info;
      uses subscription-stream-policy;
    }
  }
}

<CODE ENDS>