NETCONF A. Gonzalez Prieto
Internet-Draft A. Clemm
Intended status: Standards Track E. Voit
Expires: December 17, 2016 E. Nilsen-Nygaard
A. Tripathy
Cisco Systems
S. Chisholm
Ciena
H. Trevino
Cisco Systems
June 15, 2016

Subscribing to YANG-Defined Event Notifications
draft-gonzalez-netconf-5277bis-02

Abstract

This document defines capabilities and operations for providing asynchronous message notification delivery for notifications defined using YANG. Notification delivery can occur over a variety of protocols used commonly in conjunction with YANG, such as NETCONF and Restconf. The capabilities and operations defined in this document along with their mapping onto NETCONF transport (to be specified in a separate document, but still included in the current document version) are intended to obsolete RFC 5277.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 17, 2016.

Copyright Notice

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

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

This document defines mechanisms that provide an asynchronous message notification delivery service for the NETCONF protocol . This is an optional capability built on top of the base NETCONF definition. This document defines capabilities and operations for providing asynchronous message notification delivery for notifications defined using YANG, including capabilities and operations necessary to establish, monitor, and support subscriptions to notification delivery.

Notification delivery can occur over a variety of protocols used commonly in conjunction with YANG, such as NETCONF [RFC6241] and Restconf [I-D.ietf-netconf-restconf]. The capabilities and operations defined in this document along with their mapping onto NETCONF transport (to be specified in a separate document, but still included in the current document version) are intended to obsolete RFC 5277.

Editor's note: The current version of this document specifies both capabilities and operations for providing asynchronous notification delivery, as well as mapping of those capabilities and operations onto NETCONF. The transport mapping to NETCONF will be moved into a separate document and will be removed in future revisions of this document.

1.1. Motivation

The motivation for this work is to enable the sending of asynchronous notification messages that are consistent with the data model (content) and security model used within a NETCONF implementation.

[RFC5277] defines a notification mechanism for NETCONF. However, there are various limitations:

The scope of the work aims at meeting the following operational needs:

1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Event: Something that happens that may be of interest. (e.g., a configuration change, a fault, a change in status, crossing a threshold, or an external input to the system.)

Event notification: A message sent by a server to a receiver indicating that an event (of interest to the subscriber) has occurred. Events can trigger notifications if an interested party has subscribed to the stream(s) it belongs to.

Stream (also referred to as "event stream"): A continuous flow of event, status, state, or other information.

Subscriber: An entity able to request and negotiate a contract for the receipt of event notifications from a NETCONF server.

Receiver: A target to which a NETCONF server pushes event notifications. In many deployments, the receiver and subscriber will be the same entity.

Subscription: A contract between a subscriber and a NETCONF server, stipulating which information the receiver wishes to have pushed from the server without the need for further solicitation.

Filter: Evaluation criteria, which may be applied against a targeted set of objects/events in a subscription. Information traverses the filter only if specified filter criteria are met.

Dynamic subscription: A subscription agreed between subscriber and NETCONF server via create, establish, modify, and delete RPC control plane signaling messages.

Configured subscription: A subscription installed via a configuration interface.

Operation: In this document, this term refers to NETCONF protocol operations [RFC6241] defined in support of NETCONF notifications.

NACM: NETCONF Access Control Model.

RPC: Remote Procedure Call.

1.3. Solution Overview

This document describes mechanisms for subscribing and receiving event notifications from a NETCONF server. This document enhances the capabilities of RFC 5277 while maintaining backwards capability with existing implementations. It is intended that a final version of this document might obsolete RFC 5277.

The enhancements over [RFC5277] include the ability to terminate subscriptions without terminating the client session, to modify existing subscriptions, and to have multiple subscriptions on a NETCONF session.

These enhancements do not affect [RFC5277] clients that do not support these particular subscription requirements.

The solution supports subscribing to event notifications using two mechanisms.

  1. Dynamic subscriptions, where a NETCONF client initiates a subscription negotiation with a NETCONF server. Here a client initiates a negotiation by issuing a subscription request. If the agent wants to serve this request, it will accept it, and then start pushing event notifications as negotiated. If the agent does not wish to serve it as requested, it may respond with subscription parameters, which it would have accepted.
  2. Configured subscriptions, which is an optional mechanism that enables managing subscriptions via a configuration interface so that a NETCONF agent sends event notifications to given receiver(s).

Some key characteristics of configured and dynamic subscriptions include:

Note that there is no mixing-and-matching of RPC and configuration operations. Specifically, a configured subscription cannot be modified or deleted using RPC. Similarly, a subscription created via RPC cannot be modified through configuration operations.

The NETCONF agent may decide to terminate a dynamic subscription at any time. Similarly the NETCONF agent may decide to temporarily suspend the sending of event notifications for either configured or dynamic subscriptions. Such termination or suspension may be driven by the agent running out of resources to serve the subscription, or by internal errors on the server.

2. Solution

2.1. Event Streams

An event stream is a set of events available for subscription from a server. It is out of the scope of this document to identify a) how streams are defined, b) how events are defined/generated, and c) how events are assigned to streams.

The following is a high-level description of the flow of a notification. Note that it does not mandate and/or preclude an implementation. As events are raised, they are assigned to streams. An event may be assigned to multiple streams. The event is distributed to subscribers and receivers based on the current subscriptions and access control. Access control is needed because if any receiver of that subscription does not have permission to receive an event, then it never makes it into a notification, and processing of the event is completed for that subscription.

2.2. Event Stream Discovery

A server maintains a list of available event streams as operational data. A client can retrieve this list like any other YANG-defined data, for example using the <get> operation when using NETCONF.

2.3. Default Event Stream

A NETCONF server implementation supporting the notification capability MUST support the "NETCONF" notification event stream. This stream contains all NETCONF XML event notifications supported by the NETCONF server, except for those belonging only to streams that explicitly indicate that they must be excluded from the NETCONF stream. The exact string "NETCONF" is used during the advertisement of stream support during the <get> operation on <streams> and during the <create-subscription> and <establish-subscription> operations.

2.4. Filters

A NETCONF Server implementation SHOULD support the ability to perform filtering of notification records per RFC 5277.

2.5. Subscription State Model at the Publisher

Below is the state machine of a subscription 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.

 
                       .-------.
                    | 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:

2.6. Data Model Trees for Event Notifications

The YANG data models for event notifications are depicted in the following sections.

2.6.1. Data Model Tree for RFC5277 (netconf namespace)

module: ietf-5277-netconf
rpcs:
   +---x create-subscription    
      +--ro input     
         +--ro stream?      string
         +--ro (filter-type)?
         |  +--:(rfc5277)
         |     +--ro filter       
         +--ro startTime?   yang:date-and-time
         +--ro stopTime?    yang:date-and-time

2.6.2. Data Model Tree for RFC5277 (netmod namespace)

module: ietf-5277-netmod
   +--rw netconf
      +--rw streams
         +--rw stream* [name]
            +--rw name                     string
            +--rw description              string
            +--rw replaySupport            boolean
            +--rw replayLogCreationTime    yang:date-and-time
            +--rw replayLogAgedTime        yang:date-and-time
notifications:
   +---n replayComplete          
   +---n notificationComplete

2.6.3. Data Model for RFC5277-bis Extensions

                
module: ietf-event-notifications
   +--ro streams
   |  +--ro stream*   notif:stream
   +--rw filters
   |  +--rw filter* [filter-id]
   |     +--rw filter-id    filter-id
   |     +--rw (filter-type)?
   |        +--:(rfc5277)
   |           +--rw filter
   +--rw subscription-config {configured-subscriptions}?
   |  +--rw subscription* [subscription-id]
   |     +--rw subscription-id     subscription-id
   |     +--rw stream?             stream
   |     +--rw (filter-type)?
   |     |  +--:(rfc5277)
   |     |  |  +--rw filter
   |     |  +--:(by-reference)
   |     |     +--rw filter-ref?         filter-ref
   |     +--rw startTime?          yang:date-and-time
   |     +--rw stopTime?           yang:date-and-time
   |     +--rw encoding?           encoding
   |     +--rw receivers
   |     |  +--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
   +--ro subscriptions
      +--ro subscription* [subscription-id]
         +--ro subscription-id            subscription-id
         +--ro configured-subscription?   empty {configured-subscriptions}?
         +--ro subscription-status?       subscription-status
         +--ro stream?                    stream
         +--ro (filter-type)?
         |  +--:(rfc5277)
         |  |  +--ro filter
         |  +--:(by-reference)
         |     +--ro filter-ref?                filter-ref
         +--ro startTime?                 yang:date-and-time
         +--ro stopTime?                  yang:date-and-time
         +--ro encoding?                  encoding
         +--ro receivers
         |  +--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
augment /netmod-notif:replayComplete:
   +---- subscription-id?   subscription-id
augment /netmod-notif:notificationComplete:
   +---- subscription-id?   subscription-id
augment /netmod-notif:netconf/netmod-notif:streams:
   +--rw exclude-from-NETCONF-stream?   empty
rpcs:
   +---x establish-subscription
   |  +---w input
   |  |  +---w stream?       stream
   |  |  +---w (filter-type)?
   |  |  |  +--:(rfc5277)
   |  |  |  |  +---w filter
   |  |  |  +--:(by-reference)
   |  |  |     +---w filter-ref?   filter-ref
   |  |  +---w startTime?    yang:date-and-time
   |  |  +---w stopTime?     yang:date-and-time
   |  |  +---w encoding?     encoding
   |  +--ro output
   |     +--ro subscription-result    subscription-result
   |     +--ro (result)?
   |        +--:(success)
   |        |  +--ro subscription-id        subscription-id
   |        +--:(no-success)
   |           +--ro stream?                stream
   |           +--ro (filter-type)?
   |           |  +--:(rfc5277)
   |           |  |  +--ro filter
   |           |  +--:(by-reference)
   |           |     +--ro filter-ref?            filter-ref
   |           +--ro startTime?             yang:date-and-time
   |           +--ro stopTime?              yang:date-and-time
   |           +--ro encoding?              encoding
   +---x modify-subscription
   |  +---w input
   |  |  +---w subscription-id?   subscription-id
   |  |  +---w stream?            stream
   |  |  +---w (filter-type)?
   |  |  |  +--:(rfc5277)
   |  |  |  |  +---w filter
   |  |  |  +--:(by-reference)
   |  |  |     +---w filter-ref?        filter-ref
   |  |  +---w startTime?         yang:date-and-time
   |  |  +---w stopTime?          yang:date-and-time
   |  |  +---w encoding?          encoding
   |  +--ro output
   |     +--ro subscription-result    subscription-result
   |     +--ro (result)?
   |        +--:(success)
   |        |  +--ro subscription-id        subscription-id
   |        +--:(no-success)
   |           +--ro stream?                stream
   |           +--ro (filter-type)?
   |           |  +--:(rfc5277)
   |           |  |  +--ro filter
   |           |  +--:(by-reference)
   |           |     +--ro filter-ref?            filter-ref
   |           +--ro startTime?             yang:date-and-time
   |           +--ro stopTime?              yang:date-and-time
   |           +--ro encoding?              encoding
   +---x delete-subscription
      +---w input
      |  +---w subscription-id    subscription-id
      +--ro output
         +--ro subscription-result    subscription-result
notifications:
   +---n subscription-started
   |  +--ro subscription-id    subscription-id
   |  +--ro stream?            stream
   |  +--ro (filter-type)?
   |  |  +--:(rfc5277)
   |  |  |  +--ro filter
   |  |  +--:(by-reference)
   |  |     +--ro filter-ref?        filter-ref
   |  +--ro startTime?         yang:date-and-time
   |  +--ro stopTime?          yang:date-and-time
   |  +--ro encoding?          encoding
   +---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?            stream
   |  +--ro (filter-type)?
   |  |  +--:(rfc5277)
   |  |  |  +--ro filter
   |  |  +--:(by-reference)
   |  |     +--ro filter-ref?        filter-ref
   |  +--ro startTime?         yang:date-and-time
   |  +--ro stopTime?          yang:date-and-time
   |  +--ro encoding?          encoding
   +---n subscription-terminated
   |  +--ro subscription-id    subscription-id
   |  +--ro reason?            subscription-term-reason
   +---n added-to-subscription
   |  +--ro subscription-id    subscription-id
   |  +--ro stream?            stream
   |  +--ro (filter-type)?
   |  |  +--:(rfc5277)
   |  |  |  +--ro filter
   |  |  +--:(by-reference)
   |  |     +--ro filter-ref?        filter-ref
   |  +--ro startTime?         yang:date-and-time
   |  +--ro stopTime?          yang:date-and-time
   |  +--ro encoding?          encoding
   +---n removed-from-subscription
      +--ro subscription-id    subscription-id

2.7. Creating a Subscription

Editor's note: The following section needs updating. NETCONF mapping will move to a separate document.

This operation is fully defined in [RFC5277]. It allows a subscriber to request the creation of a dynamic subscription. If successful, the subscription remains in effect for the duration of the NETCONF session.

This operation is included in the document for supporting backwards compatibility with [RFC5277] clients. New clients are expected not to use this operation, but establish subscriptions as defined in Section 2.8

2.7.1. Parameters

The input parameters of the operation are:

If the server can satisfy the request, it sends a positive acknowledgement.

If the request cannot be completed for any reason, an error is returned along with an error reason. Subscription requests can fail for several reasons, including if a filter with invalid syntax is provided or if the name of a non-existent stream is provided. Other errors include:

2.8. Establishing a Subscription

Editor's note: The following section needs updating. NETCONF mapping will move to a separate document.

This operation is an evolution of the create subscription operation. It allows a subscriber to request the creation of a subscription both via RPC and configuration operations. When invoking the RPC, establish-subscription permits negotiating the subscription terms, changing them dynamically and enabling multiple subscriptions overs a single NETCONF session (if interleaving [RFC6241] is supported), and canceling subscriptions without terminating the NETCONF session.

The input parameters of the operation are those of create subscription plus:

If the NETCONF server cannot satisfy the request, the server sends a negative <subscription-result> element.

If the client has no authorization to establish the subscription, the <subscription-result> indicates an authorization error. If the 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 when it was processed. However, they is no guarantee that subsequent requests with those parameters for this client or others will be accepted. For instance, consider a subscription from [I-D.ietf-netconf-yang-push], which augments the establish-subscription with some additional parameters, including "period".

Subscription requests will fail if a filter with invalid syntax is provided or if the name of a non-existent stream is provided.

3. Modifying a Subscription

Editor's note: The following section needs updating. NETCONF mapping will move to a separate document.

This operation permits modifying the terms of a subscription previously established. Subscriptions created by configuration cannot be modified. Dynamic subscriptions can be modified one or multiple times. If the server accepts the request, it immediately starts sending events based on the new terms, completely ignoring the previous ones. If the server rejects the request, the subscription remains as prior to the request. That is, the request has no impact whatsoever. The contents of negative responses to modify-subscription requests are the same as in establish subscription requests.

Dynamic subscriptions established via RPC can only be modified (or deleted) via RPC using the same session used to establish it. Configured subscriptions cannot be modified (or deleted) using RPCs. Instead, configured subscriptions are modified (or deleted) as part of regular configuration operations. Servers MUST reject any attempts to modify (or delete) configured subscriptions via RPC.

The parameters to modify-subscription are those of establish-subscription plus a mandatory subscription-id.

If the NETCONF server can satisfy the request, the server sends a positive subscription-result. This response is like that to an establish-subscription request without the subscription-id, which would be redundant.

If the NETCONF server cannot satisfy the request, the server sends a negative subscription-result. Its contents and semantics are identical to those to an establish-subscription request.

4. Deleting a Subscription

Editor's note: The following section needs updating. NETCONF mapping will move to a separate document.

This operation permits canceling a subscription previously established. Created subscriptions cannot be explicitly deleted. If the server accepts the request, it immediately stops sending events for the subscription. If the server rejects the request, all subscriptions remain as prior to the request. That is, the request has no impact whatsoever. A request may be rejected because the provided subscription identifier is incorrect.

Subscriptions created via RPC can only be deleted via RPC using the same session used for establishment. Configured subscriptions cannot be deleted using RPCs. Instead, configured subscriptions are deleted as part of regular configuration operations. Servers MUST reject any RPC attempt to delete configured subscriptions.

The only parameter to delete-subscription is the identifier of the subscription to delete.

If the NETCONF server can satisfy the request, the server sends an OK element.

If the NETCONF server cannot satisfy the request, the server sends an error-rpc element.

5. Configured Subscriptions

A configured subscription is a subscription installed via a configuration interface.

Configured subscriptions persist across reboots, and persist even when transport is unavailable. This also means configured subscriptions do not support negotiation.

Configured subscriptions can be modified by any configuration client with write rights on the configuration of the subscription. Subscriptions can be modified or terminated at any point of their lifetime.

Supporting configured subscriptions is optional and advertised using the "configured-subscriptions" feature.

In addition to subscription parameters that apply to dynamic subscriptions, the following additional parameters apply to configured subscriptions:

5.1. Creating a Configured Subscription

Configured subscriptions cannot be created via configuration operations. New clients should use the mechanisms described in Section 5.2 for establishing configured subscriptions.

5.2. Establishing a Configured Subscription

Subscriptions can be established using configuration operations against the top-level subtree subscription-config. There are two key differences between RPC and configuration operations for subscription establishment. Firstly, configuration operations do not support negotiation while RPCs do. Secondly, while RPCs mandate that the client establishing the subscription is the only receiver of the notifications, configuration operations permit specifying receivers independent of any tracked subscriber. Immediately after a subscription is successfully established, the server sends to the receivers a control-plane notification stating the subscription has been established (subscription-started).

Because there is no explicit association with an existing transport session, configured configuration operations require additional parameters to indicate the receivers of the notifications and possibly the source of the notifications (i.e., a specific interface or server address).

For example at subscription establishment, a NETCONF client may send:

<rpc message-id="101" 
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" 
       xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
    <edit-config>
        <target>
            <running/>
        </target>
        <subscription-config 
            xmlns="urn:ietf:params:xml:ns:netconf:notification:1.1">
            <subscription>
                <subscription-id>
                    1922
                </subscription-id>
                <stream>
                    foo
                </stream>
                <receiver>
                    <address>
                        1.2.3.4
                    </address>
                    <port>
                        1234
                    </port>
                </receiver>
            </subscription>
        </subscription-config>
    </edit-config>
</rpc>

Figure 2: Establish configured subscription

if the request is accepted, the server would reply:

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

Figure 3: Response to a successful configured subscription establishment

if the request is not accepted because the server cannot serve it, the server may reply:

<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <rpc-error>
        <error-type>application</error-type>
        <error-tag>resource-denied</error-tag>
        <error-severity>error</error-severity>
        <error-message xml:lang="en">
            Temporarily the server cannot serve this 
            subscription due to the current workload.
        </error-message>       
    </rpc-error>
</rpc-reply>

Figure 4: Response to a failed configured subscription establishment

5.3. Modifying a Configured Subscription

Configured subscriptions can be modified using configuration operations against the top-level subtree subscription-config.

Immediately after a subscription is successfully modified, the server sends to the existing receivers a control-plane notification stating the subscription has been modified (i.e., subscription-modified).

If the modification involved adding and/or removing receivers, those modified receivers are sent control-plane notifications, indicating they have been added (i.e, added-to-subscription, with the same contents as a modified-subscription) or removed (i.e., removed-from-subscription)

5.4. Deleting a Configured Subscription

Subscriptions can be deleted using configuration operations against the top-level subtree subscription-config. For example, in NETCONF:

<rpc message-id="101" 
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <edit-config>
        <target>
            <running/>
        </target>
        <subscription-config 
        xmlns:xc="urn:ietf:params:xml:ns:netconf:notification:1.1">
            <subscription xc:operation="delete">
                <subscription-id>
                    1922
                </subscription-id >
           </subscription>
        </subscription-config>
    </edit-config>
</rpc>

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

Figure 5: Deleting a configured subscription

Immediately after a subscription is successfully deleted, the server sends to the receivers a control-plane notification stating the subscription has been terminated (subscription-terminated).

6. Event (Data Plane) Notifications

Once a subscription has been set up, the NETCONF server sends (asynchronously) the event notifications from the subscribed stream. We refer to these as data plane notifications. For dynamic subscriptions set up via RPC operations, event notifications are sent over the NETCONF session used to create or establish the subscription. For configured subscriptions, event notifications are sent over the specified connections.

An event notification is sent to the receiver(s) when an event of interest (i.e., meeting the specified filtering criteria) has occurred. An event notification is a complete and well-formed XML document. Note that <notification> is not a Remote Procedure Call (RPC) method but rather the top-level element identifying the one-way message as a notification. Note that event notifications never trigger responses.

The event notification always includes an <eventTime> element. It is the time the event was generated by the event source. This parameter is of type dateTime and compliant to [RFC3339]. Implementations must support time zones.

The event notification also contains notification-specific tagged content, if any. With the exception of <eventTime>, the content of the notification is beyond the scope of this document.

For the encodings other than XML, notifications include an additional XML element so that the notification is a well-formed XML. The element is <notification-contents-{encoding}>, E.g., <notification-contents-json>. That element contains the notification contents in the desired encoding

The following is an example of an event notification from [RFC6020]: