Subscribing to Event NotificationsSympotechalex@sympotech.comCisco Systemsalbertgo@cisco.comCisco Systemsevoit@cisco.comCisco Systemseinarnn@cisco.comCisco Systemsambtripa@cisco.comCienaschishol@ciena.comCisco Systemshtrevino@cisco.com
Operations & Management
NETCONFDraftThis document defines capabilities and operations for subscribing to content and providing asynchronous notification message delivery on that content. 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 when using in conjunction with draft-ietf-netconf-netconf-event-notifications are intended to replace RFC 5277.This document defines mechanisms that provide an asynchronous message notification delivery service in a protocol-agnostic manner.
This document defines capabilities and operations for providing asynchronous message notification delivery
for notifications including those 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
(defined in )
and Restconf
(defined in ).
The capabilities
and operations defined in this document are intended to replace RFC 5277,
along with their mapping onto NETCONF transport.
The motivation for this work is to enable the sending of transport agnostic asynchronous notification messages driven by a YANG Subscription that are consistent with the data model (content) and security model. Predating this work was used within a NETCONF implementation. [RFC5277] which defined a limited defines a notification mechanism for for NETCONF. However, there are various [RFC5277] has limitations:, many of which have been exposed in [RFC7923].The scope of the work aims at meeting the operational needs of network subscriptions:Ability to dynamically or statically subscribe to event notifications available on a publisher.Ability to negotiate acceptable dynamic subscription parameters.Ability to filter the subset of notifications to be pushed with stream-specific semantics. Ability for the notification payload to be interpreted independently of the transport protocol. (In other words, the encoded notification fully describes itself.) Mechanism to communicate the notifications.Ability to replay locally logged notifications.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.Configured subscription: A subscription installed via a configuration interface which persists across reboots.Dynamic subscription: A subscription agreed between subscriber and publisher via create, establish, modify, and delete RPC control plane signaling messages.Event: An occurrence of something 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 set of information intended for a Receiver indicating that one or more Event(s) have occurred. Details of the Event(s) may be included within the Notification.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.NACM: NETCONF Access Control Model.OAM: Operations, Administration, Maintenance.Publisher: An entity responsible for streaming Event Notifications per the terms of a SubscriptionsReceiver: A target to which a publisher pushes event notifications. For dynamic subscriptions, the receiver and subscriber will often be the same entity.RPC: Remote Procedure Call.Stream (also referred to as "event stream"): A continuous ordered set of events grouped under an explicit criteria.Subscriber: An entity able to request and negotiate a contract for the receipt of event notifications from a publisher.Subscription: A contract with a publisher, stipulating which information receiver(s) wishes to have pushed from the publisher without the need for further solicitation.This document describes mechanisms for subscribing and receiving event notifications from an event server publisher. This document builds on top of the capabilities defined in , extending them, and generalizing them to be protocol-agnostic.Some enhancements over RFC 5277 include the ability to have multiple subscriptions on a single transport session, to terminate a single subscriptions without terminating the transport session, and to modify existing subscriptions. These enhancements do not affect existing RFC 5277 subscribers that do not support these particular subscription requirements.The solution supports subscribing to event notifications using two mechanisms:Dynamic subscriptions, where a subscriber initiates a subscription negotiation with a publisher via RPC.
A subscriber initiates a negotiation by issuing a subscription request. If the publisher wants to serve this request, it will accept it, and then start pushing event notifications as negotiated.
If the publisher does not wish to serve it as requested, it may respond with subscription parameters which it would have accepted.Configured subscriptions, which is an optional mechanism that enables managing subscriptions via a configuration interface so that a publisher can send event notifications to configured receiver(s).Some key characteristics of configured and dynamic subscriptions include:The lifetime of a dynamic subscription is limited by the lifetime of the subscriber session used to establish it. Typically loss of the transport session tears down any dependent dynamic subscriptions.The lifetime of a configured subscription is driven by configuration being present on the running configuration.
This implies configured subscriptions persist across reboots, and persists even when transport is unavailable.
This also means configured subscriptions do not support negotiation.Subscriptions can be modified or terminated at any point of their lifetime. configured subscriptions can be modified by any configuration client with write rights on the configuration of the subscription.Note that there is no mixing-and-matching of dynamic and configured subscriptions. Specifically, a configured subscription cannot be modified or deleted using RPC. Similarly, a subscription created via RPC cannot be modified through configuration operations.The publisher may decide to terminate a dynamic subscription at any time. Similarly, it 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 publisher running out of resources to serve the subscription, or by internal errors on the publisher. An event stream is a set of events available for subscription from a publisher. 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. That said, some event streams will be standardized whereas others may be vendor specific. One standardized event stream is the "NETCONF" notification event stream. The NETCONF event stream contains all NETCONF XML event notifications supported by the publisher, except for those belonging only to streams that
explicitly indicate that they must be excluded from the NETCONF stream, such as notifications that serve OAM and signaling purposes.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.A publisher maintains a list of available event streams as operational data. This list contains both standardized and vendor-specific event streams. A client can retrieve this list like any other YANG-defined data, for example using the <get> operation when using NETCONF. a publisher implementation SHOULD support the ability to perform filtering of notification records per RFC 5277. (TODO: since 5277 is to be obsoleted, we should describe the filter here.)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.Of interest in this state machine are the following: Successful <establish-subscription> or
<modify-subscription> requests put the subscription into an
active state.Failed <modify-subscription> requests will leave the
subscription in its previous state, with no visible change to any
streaming updates.A <delete-subscription> request will delete the entire
subscription.The YANG data model for event notifications is depicted in this section. The data model is structured as follows:
"Streams" contains a list of event streams that are supported by the publisher and that can be subscribed to."Filters" contains a configurable list of filters that can be applied
to a subscription. This allows users to reference an
existing filter definition as an alternative to defining a filter inline
for each subscription. "Subscription-config" contains the configuration of configured subscriptions. The parameters of each configured subscription are a superset of the parameters of a dynamic subscription and use the same groupings. In addition, the configured subscriptions must also specify intended receivers and may specify the push source from which to send the stream of notification messages."Subscriptions" contains a list of all subscriptions on a publisher, both
configured and dynamic. It can be used to retrieve information about the
subscriptions which an
publisher is serving.
The data model also contains a number of notifications that allow a publisher to signal information about a subscription. Finally, the data model contains
a number of RPC definitions that are used to manage dynamic subscriptions.
Dynamic subscriptions are managed via RPC.This operation includes and extends the "create-subscription" operation defined in RFC 5277.
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.The input parameters of the operation are those of create subscription plus:
filter-ref: filters that have been previously (and separately) configured can be referenced by a subscription.
This mechanism enables the reuse of filters.encoding: by default, updates are encoded using XML. Other encodings may be supported, such as JSON. If the publisher cannot satisfy the request, it sends a negative <subscription-result> element.If the subscriber has no authorization to establish the subscription,
the <subscription-result> indicates an authorization error.
If the request is rejected because the publisher is not able to serve it,
the publisher 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 subscriber or others will be accepted.
For instance, consider a subscription from ,
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. This operation permits modifying the terms of a dynamic subscription previously established. Subscriptions created
by configuration cannot be modified. Dynamic subscriptions can be modified one or multiple times. If the publisher accepts the request, it immediately starts sending events based on the new terms, completely ignoring the previous ones. If the publisher 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 subset of the establish subscription request parameters which are allowed to be dynamically modified.Dynamic subscriptions established via RPC can only be modified (or deleted) via RPC using the same transport session used to establish that subscription.Configured subscriptions cannot be modified (or deleted) using RPCs. Instead, configured subscriptions are modified (or deleted) as part of regular configuration operations. Publishers MUST reject any attempts to modify (or delete) configured subscriptions via RPC.This operation permits canceling a subscription previously established. If the publisher accepts the request, it immediately stops sending events for the subscription. If the publisher rejects the request, all subscriptions remain as prior to the request. That is, the request has no impact whatsoever.Subscriptions created via RPC can only be deleted via RPC using the same transport session used for subscription establishment. Configured subscriptions cannot be deleted using RPCs. Instead, configured subscriptions are deleted as part of regular configuration operations. Publishers 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 publisher can satisfy the request, it sends an OK element. If the publisher cannot satisfy the request, it sends an error-rpc element.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 permissions for 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:
One or more receiver IP addresses (and corresponding ports)
intended as the destination for push updates for each
subscription. In addition, the transport protocol for each
destination may be defined.Optional parameters to identify an egress interface or IP
address / VRF where a subscription updates should be pushed from
the publisher.Configured subscriptions are 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 subscriber 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 publisher sends to its 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 such as a specific egress interface.For example at subscription establishment, a client may send:if the request is accepted, the publisher would reply:if the request is not accepted because the publisher cannot serve it, the publisher may reply:Configured subscriptions can be modified using configuration operations against the top-level subtree subscription-config.Immediately after a subscription is successfully modified, the publisher 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, subscription-started to a specific receiver) or removed (i.e., subscription-terminated to a specific receiver.)Subscriptions can be deleted using configuration operations against the top-level subtree subscription-config. For example, in RESTCONF:Immediately after a subscription is successfully deleted, the publisher sends to all receivers a control-plane notification stating the subscription has been terminated (subscription-terminated).Once a subscription has been set up, the publisher streams (asynchronously) the event notifications per the terms of the subscription. We refer to these as data plane notifications. For dynamic subscriptions set up via RPC operations, event notifications are sent over the 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 . Implementations must support time zones.The event notifications must also include the subscription-id if the establish-subscription was used in its establishment, or if it was configured via an operational interface. The event notification also contains notification-specific tagged content, if any.The following is an example of an event notification from :The equivalent using json encoding would beIn addition to data plane notifications, a publisher may send control plane notifications to indicate to receivers that an event related to the subscription management has occurred.
Control plane notifications are unlike other notifications in that they are not general-purpose
notifications. They cannot be filtered out,
and they are delivered only to the receiver of a subscription. The definition of control plane notifications is distinct from other notifications
by making use of a YANG extension tagging them as control plane notification.
Control plane notifications include indications that a replay of notifications has been
completed, that a subscription is done sending notifications because an end time has been reached,
and that a subscription has started, been modified, been terminated, or been suspended.
They are described in the following subsections.
This notification is originally defined in . It is sent to indicate that all of the replay notifications have been sent. This notification must not be sent for any other reason. In the case of a subscription without a stop time or a stop time which has not been reached, after the <replayComplete> notification has been sent, it can be expected that any notifications generated since the start of the subscription creation will be sent, followed by notifications in sequence as they arise naturally within the system.This notification is originally defined in . It is sent to indicate that a subscription, which includes a stop time, has finished passing events.This notification indicates that a configured subscription has started and data updates are beginning to be sent. This notification includes the parameters of the subscription, except for the receiver(s) addressing information and push-source information. Note that for RPC-based subscriptions, no such notifications are sent.This notification indicates that a configured subscription has been modified successfully. This notification includes the parameters of the subscription, except for the receiver(s) addressing information and push-source information. Note that for RPC-based subscriptions, no such notifications are sent.This notification indicates that a subscription has been terminated by the publisher. The notification includes the reason for the termination. The publisher may decide to terminate a subscription when it is running out of resources for serving it, an internal error occurs, etc. Publisher-driven terminations are notified to all receivers. The management plane can also terminate configured subscriptions using configuration operations.Subscribers can terminate via RPC subscriptions established via RPC. In such cases, no subscription-terminated notifications are sent. This notification indicates that a publisher has suspended a subscription. The notification includes the reason for the suspension. A possible reason is the lack of resources to serve it. No further data plane notifications will be sent until the subscription resumes. Suspensions are notified to the subscriber (in the case of dynamic subscriptions) and all receivers (in the case of configured subscriptions). This notification indicates that a previously suspended dubscription has been resumed. Data plane notifications generated in the future will be sent after the subscription terms. Resumptions are notified to the subscriber (in the case of dynamic subscriptions) and all receivers (in the case of configured subscriptions).Capabilities are advertised in messages sent by each peer during session establishment . Publishers supporting the features in this document must advertise both capabilities "urn:ietf:params:netconf:capability:notification:1.0" and "urn:ietf:params:netconf:capability:notification:1.1".An example of a hello message by a publisher during session establishment would be:Subscribers that only support recognize capability "urn:ietf:params:netconf:capability:notification:1.0" and ignore capability "urn:ietf:params:netconf:capability:notification:1.1". This allows them interacting with the publisher as per [RFC5277]. Subscribers that support the features in this document recognize both capabilities. This allows them interacting with the publisher as per this document.Note that to support backwards compatibility, the yang models in this document include two types of naming conventions. That used in , e.g., replayComplete; and that commonly used in yang models, e.g., subscription-started.The <notification> elements are never sent before the transport layer, including capabilities exchange, has been established and the manager has been securely established.A secure transport is highly recommended and the publisher must ensure that the user has sufficient authorization to perform the function they are requesting against the specific subset of content involved. When a <get> is received that refers to the content defined in this memo, clients should only be able to view the content for which they have sufficient privileges. <create-subscription> and <establish-subscription> operations can be considered like deferred <get>, and the content that different users can access may vary. This different access is reflected in the <notificationt> to which different users are able to subscribe. The contents of notifications, as well as the names of event streams, may contain sensitive information and care should be taken to ensure that they are viewed only by authorized users. The publisher MUST NOT include any content in a notification that the user is not authorized to view.If a malicious or buggy subscriber sends a number of <create-subscription> requests, then these subscriptions accumulate and may use up system resources. In such a situation, subscriptions can be terminated by terminating the suspect underlying NETCONF sessions using the <kill-session> operation. If the subscriber uses <establish-subscription>, the publisher can also suspend or terminate subscriptions with per-subscription granularity.A subscription could be configured on another receiver's behalf, with the goal of flooding that receiver with updates. One or more publishers could be used to overwhelm a receiver, which doesn't even support subscriptions. Subscribers that do not want pushed data need only terminate or refuse any transport sessions from the publisher. In addition, the NETCONF Authorization Control Model SHOULD be used to control and restrict authorization of subscription configuration. This control models permits specifying per-user permissions to receive specific event notification types. The permissions are specified as a set of access control rules.Note that streams can define additional authorization requirements. For instance, in , each of the elements in its data plane notifications must also go through access control.For their valuable comments, discussions, and feedback, we wish to acknowledge Andy Bierman, Yang Geng, Peipei Guo, Susan Hares, Tim Jenkins, Balazs Lengyel, Kent Watsen, Michael Scharf, and Guangying Zheng.
The YANG 1.1 Data Modeling LanguageRESTCONF ProtocolSubscribing to YANG datastore push updatesNETCONF support for event notificationsRestconf and HTTP transport for event notifications(To be removed by RFC editor prior to publication)EN1 - Definition of basic set of Stream types. What streams are provided and what do they contain (includes default 5277 stream).EN2 - Clarify interplay between filter definitions and different streams.
Includes information in subtrees of event payloads.EN3 - Mechanisms for diagnostics, e.g. deal with dropped updates, monitoring when they occur, etcEN4 - How to allow for seamless integration with non-standard encodings and transports (like GPB/GRPC). Specify requirements encoding and transport must meet, provide examples. EN7 - Detecting loss of a sequential update notification, and mechanisms to resend. Implications to transports must be thought through.EN6 - Stream discovery. Allow to discover additional stream properties. EN12 - Test-only option for a subscription is desired. But it still needs to be defined.EN14 - Ensure that Configured Subscriptions are fully defined in YANG model.EN5 - This draft obsoletes 5277, as opposed to being in parallel with it EN8 - No mandatory transportEN15 - Term for Dynamic and Static Subscriptions (move to "Configured")EN9 - Multiple receivers per Configured Subscription is ok.EN13 - RFC6241 Subtree-filter definition in 5277bis cannot apply to elements of an event. Must explicitly define how 6241 doesn't apply filtering within a 5277bis event.EN10 - Replay support will be provided for selected stream types (modify vs. delete)EN11 - Required layering security requirements/considerations will be added into the YANG model for Configured Subscriptions. It will be up to the transport to meet these requirements.(To be removed by RFC editor prior to publication)v00 - v01YANG Model changes. New groupings for subscription info to allow restriction of what is changable via RPC. Removed notifications for adding and removing receivers of configured subscriptions.Expanded/renamed defintions from event server to publisher, and client to subscriber as applicable. Updated the definitions to include and expand on RFC 5277.Removal of redundant with other draftsMany other clean-ups of wording and terminology