Subscribing to YANG datastore push updates

YANG was originally designed for the Netconf protocol , which originally put most emphasis on configuration. However, YANG is not restricted to configuration data. YANG datastores, i.e. datastores that contain data modeled according using YANG, can contain configuration as well as operational data. It is therefore reasonable to expect that data in YANG datastores will increasingly be used to support applications that are not focused on managing configurations but that are, for example, related to service assurance. Service assurance applications typically involve monitoring operational state of networks and devices; of particular interest are changes that this data undergoes over time. Likewise, there are applications in which data and objects from one datastore need to be made available both to applications in other systems and to remote datastores . This requires mechanisms that allow remote systems to become quickly aware of any updates to allow to validate and maintain cross-network integrity and consistency. Traditional approaches to remote network state visibility rely heavily on polling. With polling, data is periodically explicitly retrieved by a client from a server to stay up-to-date. There are various issues associated with polling-based management: It introduces additional load on network, devices, and applications. Each polling cycle requires a separate yet arguably redundant request that results in an interrupt, requires parsing, consumes bandwidth. 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. A more effective alternative is when an application can request to be automatically updated as necessary of current content of the datastore (such as a subtree, or data in a subtree that meets a certain filter condition), and in which the server that maintains the datastore subsequently pushes those updates. However, such a solution does not currently exist. The need to perform polling-based management is typically considered an important shortcoming of management applications that rely on MIBs polled using SNMP . However, without a provision to support a push-based alternative, there is no reason to believe that management applications that operate on YANG datastores using protocols such as NETCONF or Restconf 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 . In addition, configuration change notifications have been defined in . 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. Furthermore, while delivery of updates using notifications is a viable option, some applications desire the ability to stream updates using other transports. Accordingly, there is a need for a service that allows client applications to dynamically subscribe to updates of a YANG datastore and that allows the server to push those updates, possibly using one of several delivery mechanisms. Additionally, support for subscriptions configured directly on the server are also useful when dynamic signaling is undesirable or unsupported. The requirements for such a service are documented in . This document proposes a solution. The solution builds on top of the Netconf Event Model which defines a mechanism for the management of event subscriptions. At its core, the solution that is defined here introduces a new set of event streams including datastore push updates that clients can subscribe to, as well as extensions to the event subscription model that allow to manage policies that define what and when updates are trigged. To this end, the document specifies a YANG data model that augments the YANG data model (and RPCs) defined as part of the NETCONF Event Model. Specifically, the solution features the following capabilities: An extension to event subscription mechanisms allows clients to subscribe to event streams containing automatic datastore updates. The subscription allows clients to specify which data they are interested in, what types of updates (e.g. create, delete, modify), and to provide optional filters with criteria that data must meet for updates to be sent. Furthermore, subscriptions 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 for a server to push back on requested subscription parameters. Because not every server may support every requested update policy 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 push update subscription parameters. For example, some servers may have a lower limit to the period with which they can send updates, or they may not support on-change updates for every piece of data. A mechanism to communicate the updates themselves. For this, the proposal leverages and extends existing YANG/Netconf/Restconf mechanisms, defining special notifications that carry updates. In addition, optional subscription parameters allow to specify which transport should be used to stream updates, and to define QoS extensions that allow to address aspects such as how to prioritize between streams of updates.

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. Dynamic subscription: A subscription negotiated between subscriber and publisher vian establish, modify, and delete RPCs respectively control plane signaling messages that are part of an existing management association between and publisher. Subscriber and receiver are the same system. NACM: NETCONF Access Control Model NETCONF: Network Configuration Protocol Publisher: A server that sends push updates to a receiver according to the terms of a subscription. In general, the publisher is also the "owner" of the subscription. Push-update stream: A conceptual data stream of a datastore that streams the entire datastore contents continuously and perpetually. Receiver: The target of push updates of a subscription. In case of a dynamic subscription, receiver and subscriber are the same system. However, in the case of a configured subscription, the receiver may be a different system than the one that configured the subscription. RPC: Remote Procedure Call SNMP: Simple Network Management Protocol Configured subscription: A subscription installed as part of a configuration datastore via a configuration interface. Subscriber: A client that negotiates a subscription with a server ("publisher"). A client that establishes a configured subscription is also considered a subscriber, even if it is not necessarily the receiver of a subscription. Subscription: A contract between a client ("subscriber") and a server ("publisher"), stipulating which information the client wishes to receive from the server (and which information the server has to provide to the client) without the need for further solicitation. Subscription filter: A filter that contains evaluation criteria which are evaluated against YANG objects of a subscription. An update is only published if the object meets the specified filter criteria. Subscription policy: A policy that specifies under what circumstances to push an update, e.g. whether updates are to be provided periodically or only whenever changes occur. Update: A data item containing the current value of a data node. Update trigger: A trigger, as specified by a subscription policy, that causes an update to be sent, respectively a data record to be generated. An example of a trigger is a change trigger, invoked when the value of a data node changes or a data node is created or deleted, or a time trigger, invoked after the laps of a periodic time interval. URI: Uniform Resource Identifier YANG: A data definition language for NETCONF YANG-Push: The subscription and push mechanism for YANG datastores that is specified in this document.

This document specifies a solution for a push update subscription service, which supports the dynamic as well as static (via configuration) creation of subscriptions to information updates of operational or configuration YANG data which are subsequently pushed from the server to the client. Dynamic subscriptions are initiated by clients who want to receive push updates. Servers respond to requests for the creation of subscriptions positively or negatively. Negative responses MAY include information about why the subscription was not accepted, in order to facilitate converging on an acceptable set of subscription parameters. Similarly, configured subscriptions are configured as part of a device's configuration. Once a subscription has been established, datastore push updates are pushed from the server to the receiver until the subscription ends. Accordingly, the solution encompasses several components: The subscription model for configuration and management of the subscriptions. The ability to provide hints for acceptable subscription parameters, in cases where a subscription desired by a client cannot currently be served. The stream of 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.

YANG-Push subscriptions are defined using a data model that is itself defined in YANG. This model augments the event subscription model defined in and introduces several new parameters, for example parameters that allow subscribers to specify what to include in an update, what triggers an update, and how to deliver updates. The subscription model assumes the presence of one or more conceptual perpetual datastreams of continuous updates that can be subscribed to. There are several datastreams with predefined semantics, such as the stream of updates of all operational data or the stream of updates of all config data. In addition, it is possible to define custom streams with customizable semantics. The model includes the list of update datastreams that are supported by a system and available for subscription. The subscription model augments the NETCONF event subscription model with a set of parameters as follows: An encoding for push updates. By default, updates are encoded using XML, but JSON can be requested as an option and other encodings may be supported in the future. An optional start time for the subscription. If the specified start time is in the past, the subscription goes into effect immediately. The start time also serves as anchor time for periodic subscriptions, from which intervals at which to send updates are calculated (see also below). An optional stop time for the subscription. Once the stop time is reached, the subscription is automatically terminated. A subscription policy definition regarding the update trigger when to send new updates. The trigger can be periodic or based on change. For periodic subscriptions, the trigger is defined by a parameter that defines the interval with which updates are to be pushed. The start time of the subscription serves as anchor time, defining one specific point in time at which an update needs to be sent. Update intervals always fall on the points in time that are a multiple of a period after the start time. EDITOR'S NOTE: A possible option to discuss concerns the introduction of an additional parameter "changes-only" for periodic subscription. Including this flag would results in sending at the end of each period an update containing only changes since the last update (i.e. a change-update as in the case of an on-change subscription), not a full snapshot of the subscribed information. Such an option might be interesting in case of data that is largely static and bandwidth-constrained environments. For on-change subscriptions, the trigger occurs whenever a change in the subscribed information is detected. On-change subscriptions have more complex semantics that can be guided by additional parameters. Please refer also to . One parameter specifies the dampening period, i.e. the interval that must pass before a successive update for the same data node is sent. The first time a change is detected, the update is sent immediately. If a subsequent change is detected, another update for that object is only sent once the dampening period has passed, containing the value of the data node that is then valid. Note that the dampening period applies to each object, not the set of all objects that are part of the same subscription. This means that on the first change of an object, an update for that object is immediately sent, regardless of whether or not for another object of the same subscription a dampening period is already in effect. Another parameter allows to restrict the types of changes for which updates are sent (changes to object values, object creation or deletion events). It is conceivable to augment the data model with additional parameters in the future to specify even more refined policies, such as parameters that specify the magnitude of a change that must occur before an update is triggered. A third parameter specifies whether or not a complete update with all the subscribed data should be sent at the beginning of a subscription to facilitate synchronization and establish the frame of reference for subsequent updates. EDITOR'S NOTE: Several semantic variations are conceivable. In one variation, an on-change notification is sent immediately, but successive dampening updates are sent at fixed period intervals, grouping all changes of objects for which changes have occurred since the sending of their last update and the current dampening period. Optionally, a filter, or set of filters, describing the subset of data items in the stream's data records that are of interest to the subscriber. The server should only send to the subscriber the data items that match the filter(s), when present. The absence of a filter indicates that all data items from the stream are of interest to the subscriber and all data records must be sent in their entirety to the subscriber. The following types of filters are introduced: subtree filters, with the same semantics as defined in [RFC 6241],and XPath filters. In addition, as with any subscription, an RFC 5277 filter MAY be used, with the same semantics as defined in [RFC 5277]. Additional filter types can be added through augmentations. Filters can be specified "inline" as part of the subscription, or can be configured separately and referenced by a subscription, in order to facilitate reuse of complex filters. The subscription data model is specified as part of the YANG data model described later in this specification. Specifically, the subscription parameters are defined in the "subscription-info" and "update-policy" groupings. Receiver information is defined in the "receiver-info" grouping. Information about the source address is defined in the "push-source-info" grouping. It is conceivable that additional subscription parameters might be added in the future. This can be accomplished through augmentation of the subscription data model.

A subscription rejection can be caused by the inability of the server to provide a stream with the requested semantics. For example, a server may not be able to support "on-change" updates for operational data, or only support them for a limited set of data nodes. Likewise, a server may not be able to support a requested update frequency. YANG-Push supports a simple negotiation between clients and servers for subscription parameters. The negotiation is limited to a single pair of subscription request and response. For negative responses, the server SHOULD include in the returned error what subscription parameters would have been accepted for the request. The returned acceptable parameters constitute suggestions that, when followed, increase the likelihood of success for subsequent requests. However, they are no guarantee that subsequent requests for this client or others will in fact be accepted. In case a subscriber requests an encoding other than XML, and this encoding is not supported by the server, the server simply indicates in the response that the encoding is not supported. A subscription negotiation capability has been introduced as part of the NETCON Event Notifications model. However, the ability to negotiate subscriptions is of particular importance in conjunction with push updates, as server implementations may have limitations with regards to what updates can be generated and at what velocity.

On-change subscriptions allow clients to subscribe to updates whenever changes to objects occur. As such, on-change subscriptions are of particular interest for data that changes relatively infrequently, yet that require applications to be notified with minimal delay when changes do occur. On-change subscriptions tend to be more difficult to implement than periodic subscriptions. Specifically, on-change subscriptions may involve a notion of state to see if a change occurred between past and current state, or the ability to tap into changes as they occur in the underlying system. Accordingly, on-change subscriptions may not be supported by all implementations or for every object. When an on-change subscription is requested for a datastream with a given subtree filter, where not all objects support on-change update triggers, the subscription request MUST be rejected. As a result, on-change subscription requests will tend to be directed at very specific, targeted subtrees with only few objects. Any updates for an on-change subscription will include only objects for which a change was detected. To avoid flooding clients with repeated updates for fast-changing objects, or objects with oscillating values, an on-change subscription allows for the definition of a dampening period. Once an update for a given object is sent, no other updates for this particular object are sent until the end of the dampening period. Values sent at the end of the dampening period are the values current when that dampening period expires. In addition, updates include information about objects that were deleted and ones that were newly created. On-change subscriptions can be refined to let users subscribe only to certain types of changes, for example, only to object creations and deletions, but not to modifications of object values. Additional refinements are conceivable. For example, in order to avoid sending updates on objects whose values undergo only a negligible change, additional parameters might be added to an on-change subscription specifying a policy that states how large or "significant" a change has to be before an update is sent. A simple policy is a "delta-policy" that states, for integer-valued data nodes, the minimum difference between the current value and the value that was last reported that triggers an update. Also more sophisticated policies are conceivable, such as policies specified in percentage terms or policies that take into account the rate of change. While not specified as part of this draft, such policies can be accommodated by augmenting the subscription data model accordingly.

Subscribed data is encoded in either XML or JSON format. A server MUST support XML encoding and MAY support JSON encoding. It is conceivable that additional encodings may be supported as options in the future. This can be accomplished by augmenting the subscription data model with additional identity statements used to refer to requested encodings.

In a periodic subscription, the data included as part of an update corresponds to data that could have been simply retrieved using a get operation and is encoded in the same way. XML encoding rules for data nodes are defined in . JSON encoding rules are defined in . This encoding is valid JSON, but also has special encoding rules to identify module namespaces and provide consistent type processing of YANG data.

In an on-change subscription, updates need to allow to differentiate between data nodes that were newly created since the last update, data nodes that were deleted, and data nodes whose value changed. XML encoding rules correspond to how data would be encoded in input to Netconf edit-config operations as specified in section 7.2, adding "operation" attributes to elements in the data subtree. Specifically, the following values will be utilized: create: The data identified by the element has been added since the last update. delete: The data identified by the element has been deleted since the last update. merge: The data identified by the element has been changed since the last update. replace: The data identified by the element has been replaced with the update contents since the last update. The remove value will not be utilized. Contrary to edit-config operations, the data is sent from the server to the client, not from the client to the server, and will not be restricted to configuration data. JSON encoding rules are roughly analogous to how data would be encoded in input to a YANG-patch operation, as specified in section 2.2. However, no edit-ids will be needed. Specifically, changes will be grouped under respective "operation" containers for creations, deletions, and modifications.

Subscriptions can specify filters for subscribed data. The following filters are supported in addition to RFC 5277 filters that apply to any event subscription: subtree-filter: A subtree filter specifies a subtree that the subscription refers to. When specified, updates will only concern data nodes from this subtree. Syntax and semantics correspond to that specified for section 6. xpath-filter: An XPath filter specifies an XPath expression applied to the data in an update, assuming XML-encoded data. Only a single filter can be applied to a subscription at a time. It is conceivable for implementations to support other filters. For example, an on-change filter might specify that changes in values should be sent only when the magnitude of the change since previous updates exceeds a certain threshold. It is possible to augment the subscription data model with additional filter types.

Pushing data based on a subscription could be considered analogous to a response to a data retrieval request, e.g. a "get" request. However, contrary to such a request, multiple responses to the same request may get sent over a longer period of time. A more suitable mechanism to consider is therefore that of a notification. There are however some specifics that need to be considered. Contrary to other notifications that are associated with alarms and unexpected event occurrences, push updates are solicited, i.e. tied to a particular subscription which triggered the notification, and arguably only of interest to the subscriber, respectively the intended receiver of the subscription. A subscription therefore needs to be able to distinguish between streams that underlie push updates and streams of other notifications. By the same token, notifications associated with updates and subscriptions to updates need to be distinguished from other notifications, in that they enter a datastream of push updates, not a stream of other event notifications. A push update notification contains several parameters: 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 . Alternative transports are conceivable but outside the scope of this specification. The solution specified in this document uses notifications to define datastore updates. The contents of the notification includes a set of explicitly defined data nodes. For this purpose, two new generic notifications are introduced, "push-update" and "push-change-update". Those notifications define how mechanisms that carry YANG notifications (e.g. Netconf notifications and Restconf) can be used to carry data records with updates of datastore contents as specified by a subscription. It is possible also map notifications to other transports and encodings and use the same subscription model; however, the definition of such mappings is outside the scope of this document. Push-update notification defines updates for a periodic subscription, as well as for the initial update of an on-change subscription used to synchronize the receiver at the start of a new subscription. The update record contains a data snippet that contains an instantiated subtree with the subscribed contents. The content of the update record is equivalent to the contents that would be obtained had the same data been explicitly retrieved using e.g. a Netconf "get"-operation, with the same filters applied. The contents of the notification conceptually represents the union of all data nodes in the yang modules supported by the server. However, in a YANG data model, it is not practical to model the precise data contained in the updates as part of the notification. This is because the specific data nodes supported depend on the implementing system and may even vary dynamically. Therefore, to capture this data, a single parameter that can represent any datastore contents is used, not parameters that represent data nodes one at a time. Push-change-update notification defines updates for on-change subscriptions. The update record here contains a data snippet that indicates the changes that data nodes have undergone, i.e. that indicates which data nodes have been created, deleted, or had changes to their values. The format follows the same format that operations that apply changes to a data tree would apply, indicating the creates, deletes, and modifications of data nodes. The following is an example of push notification. It contains an update for subscription 1011, including a subtree with root foo that contains a leaf, bar:

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1011

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some_string

]]> The following is an example of an on-change notification. It contains an update for subscription 89, including a new value for a leaf called beta, which is a child of a top-level container called alpha:

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1500

]]> The equivalent update when requesting json encoding:

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{ "ietf-yang-patch:yang-patch": { "patch-id": [ null ], "edit": [ { "edit-id": "edit1", "operation": "merge", "target": "/alpha/beta", "value": { "beta": 1500 } } ] } }

]]> When the beta leaf is deleted, the server may send

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]]>

There are two ways in which subscriptions can be managed, as specified in the NETCONF Event Notifications model: RPC-based and configuration based. Any given subscription is either RPC-based or configuration-based. There is no mixing-and-matching of RPC and configuration operations. Specifically, a configured subscription cannot be modified or deleted using RPC. Likewise, a subscription established via RPC cannot be modified or deleted through configuration operations. The following sections describe how subscription management is applied to YANG Push subscriptions.

RPC-based subscription allows a subscriber to establish a subscription via an RPC call. The subscriber and the receiver are the same entity, i.e. a subscriber cannot subscribe or in other ways interfere with a subscription on another receiver's behalf. The lifecycle of the subscription is dependent on the lifecyle of the transport session over which the subscription was requested. For example, when a Netconf session over which a subscription was established is torn down, the subscription is automatically terminated (and needs to be re-initiated when a new session is established). Alternatively, a subscriber can also decide to delete a subscription via another RPC. When an establish-subscription request is successful, the subscription identifier of the freshly established subscription is returned. A subscription can be rejected for multiple reasons, including the lack of authorization to establish a subscription, the lack of read authorization on the requested data node, or the inability of the server to provide a stream with the requested semantics. In such cases, no subscription is established. Instead, the subscription-result with the failure reason is returned as part of the RPC response. In addition, a set of alternative subscription parameters MAY be returned that would likely have resulted in acceptance of the subscription request, which the subscriber may try for a future subscription attempt. It should be noted that a rejected subscription does not result in the generation of an rpc-reply with an rpc-error element, as neither the specification of YANG-push specific errors nor the specification of additional data parameters to be returned in an error case are supported as part of a YANG data model. For instance, for the following request:

push-update 500 encode-xml ]]> the server might return:

error-insufficient-resources

2000 ]]> A subscriber that establishes a subscription using RPC can modify or delete the subscription using other RPCs. When the session between subscriber and publisher is terminated, the subscription is implicitly deleted.

Configuration-based subscription allows a subscription to be established as part of a server's configuration. This allows to persist subscriptions. Persisted subscriptions allow for a number of additional options than RPC-based subscriptions. As part of a configured subscription, a receiver needs to be specified. It is thus possible to have a different system acting as subscriber (the client creating the subscription) and as receiver (the client receiving the updates). In addition, a configured subscription allows to specify which transport protocol should be used, as well as the sender source (for example, a particular interface or an address of a specific VRF) from which updates are to be pushed. Configuration-based subscriptions cannot be modified or deleted using RPCs. Instead, configured subscriptions are deleted as part of regular configuration operations. Servers SHOULD reject attempts to modify configurations of active subscriptions. This way, race conditions in which a receiver may not be aware of changed subscription policies are avoided.

A receiver of subscription data may only be sent updates for which they have proper authorization. Data that is being pushed therefore needs to be subjected to a filter that applies all corresponding rules applicable at the time of a specific pushed update, removing any non-authorized data as applicable. The authorization model for data in YANG datastores is described in the Netconf Access Control Model . However, some clarifications to that RFC are needed so that the desired access control behavior is applied to pushed updates. One of these clarifications is that a subscription may only be established if the Receiver has read access to the target data node.

| operation | -------------> | data node | | allowed? | datastore | access | +-------------+ or state | allowed? | data access +-------------+ ]]> Likewise if a receiver no longer has read access permission to a target data node, the subscription must be abnormally terminated (with loss of access permission as the reason provided). Another clarification to is that each of the individual nodes in a pushed update must also go through access control filtering. This includes new nodes added since the last push update, as well as existing nodes. For each of these read access must be verified. The methods of doing this efficiently are left to implementation.

| access | ---> | add data node | | allowed? | | to update message | +-------------+ +-------------------+ ]]> If there are read access control changes applied under the target node, no notifications indicating the fact that this has occurred need to be provided.

Other possible operations include the ability for a Subscriber to request the suspension/resumption of a Subscription with a Publisher. However, subscriber driven suspension is not viewed as essential at this time, as a simpler alternative is to remove a subscription and reestablish it when needed. It should be noted that this does not affect the ability of the Publisher to suspend a subscription. This can occur in cases the server is not able to serve the subscription for a certain period of time, and indicated by a corresponding notification.

Particularly in the case of on-change push updates, it is important that push updates do not get lost. YANG-Push uses a secure and reliable transport. Notifications are not getting reordered, and in addition contain a time stamp. For those reasons, for the transport of push-updates, we believe that additional reliability mechanisms at the application level, such as sequence numbers for push updates, are not required. At the same time, it is conceivable that under certain circumstances, a push server is not able to generate the update notifications that it had committed to when accepting a subcription. In those circumstances, the server needs to inform the receiver of the situation. For this purpose, notifications are defined that a push server can use to inform subscribers/ receivers when a subscription is (temporarily) suspended, when a suspended subscription is resumed, and when a a subscription is terminated. This way, receivers will be able to rely on a subscription, knowing that they will be informed of any situations in which updates might be missed.

Depending on the subscription, the volume of updates can become quite large. There is no inherent limitation to the amount of data that can be included in a notification. That said, it may not always be practical to send the entire update in a single chunk. Implementations MAY therefore choose, at their discretion, to "chunk" updates and break them out into several update notifications.

There are several conceptual data streams introduced in this specification: yang-push includes the entirety of YANG data, including both configuration and operational data. operational-push includes all operational (read-only) YANG data config-push includes all YANG configuration data. It is conceivable to introduce other data streams with more limited scope, for example: operdata-nocounts-push, a datastream containing all operational (read-only) data with the exception of counters other custom datastreams Those data streams make particular sense for use cases involving service assurance (not relying on operational data), and for use cases requiring on-change update triggers which make no sense to support in conjunction with fast-changing counters. While it is possible to specify subtree filters on yang-push to the same effect, having those data streams greatly simplifies articulating subscriptions in such scenarios.

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. When providing access control to every node in a pushed update, it is possible to make and update efficient access control filters for an update. These filters can be set upon subscription and applied against a stream of updates. These filters need only be updated when (a) there is a new node added/removed from the subscribed tree with different permissions than its parent, or (b) read access permissions have been changed on nodes under the target node for the subscriber.

A new draft has been chartered by the Netconf Working Group to replace the current Netconf Event Model defined in RFC 5277. Future revisions of this document will leverage RFC 5277bis as applicable. It is anticipated that portions of the data model and subscription management that are now defined in this this document and that are not applicable only to YANG-Push, but to more general event subscriptions, will move to RFC 5277bis.

The YANG data model for datastore push subscriptions is depicted in the following figure.

module: ietf-yang-push augment /notif-bis:establish-subscription/notif-bis:input: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:establish-subscription/notif-bis:input/ | notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:establish-subscription/notif-bis:output: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:establish-subscription/notif-bis:output/ | notif-bis:result/notif-bis:no-success/notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:modify-subscription/notif-bis:input: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:modify-subscription/notif-bis:input/ | notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:modify-subscription/notif-bis:output: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:modify-subscription/notif-bis:output/ | notif-bis:result/notif-bis:no-success/notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:subscription-started: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:subscription-started/notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:subscription-modified: +---- subscription-start-time? yang:date-and-time +---- subscription-stop-time? yang:date-and-time +---- (update-trigger)? | +--:(periodic) | | +---- period yang:timeticks | +--:(on-change) {on-change}? | +---- no-synch-on-start? empty | +---- dampening-period yang:timeticks | +---- excluded-change* change-type +---- dscp? inet:dscp | {notif-bis:configured-subscriptions}? +---- subscription-priority? uint8 +---- subscription-dependency? string augment /notif-bis:subscription-modified/notif-bis:filter-type: +--:(update-filter) +---- (update-filter)? +--:(subtree) | +---- subtree-filter +--:(xpath) +---- xpath-filter? yang:xpath1.0 augment /notif-bis:filters/notif-bis:filter/notif-bis:filter-type: +--:(update-filter) +--rw (update-filter)? +--:(subtree) | +--rw subtree-filter +--:(xpath) +--rw xpath-filter? yang:xpath1.0 augment /notif-bis:subscription-config/notif-bis:subscription: +--rw subscription-start-time? yang:date-and-time +--rw subscription-stop-time? yang:date-and-time +--rw (update-trigger)? | +--:(periodic) | | +--rw period yang:timeticks | +--:(on-change) {on-change}? | +--rw no-synch-on-start? empty | +--rw dampening-period yang:timeticks | +--rw excluded-change* change-type +--rw dscp? inet:dscp | {notif-bis:configured-subscriptions}? +--rw subscription-priority? uint8 +--rw subscription-dependency? string augment /notif-bis:subscription-config/notif-bis:subscription/ | notif-bis:filter-type: +--:(update-filter) +--rw (update-filter)? +--:(subtree) | +--rw subtree-filter +--:(xpath) +--rw xpath-filter? yang:xpath1.0 augment /notif-bis:subscriptions/notif-bis:subscription: +--ro subscription-start-time? yang:date-and-time +--ro subscription-stop-time? yang:date-and-time +--ro (update-trigger)? | +--:(periodic) | | +--ro period yang:timeticks | +--:(on-change) {on-change}? | +--ro no-synch-on-start? empty | +--ro dampening-period yang:timeticks | +--ro excluded-change* change-type +--ro dscp? inet:dscp | {notif-bis:configured-subscriptions}? +--ro subscription-priority? uint8 +--ro subscription-dependency? string augment /notif-bis:subscriptions/notif-bis:subscription/ | notif-bis:filter-type: +--:(update-filter) +--ro (update-filter)? +--:(subtree) | +--ro subtree-filter +--:(xpath) +--ro xpath-filter? yang:xpath1.0 notifications: +---n push-update | +--ro subscription-id notif-bis:subscription-id | +--ro time-of-update? yang:date-and-time | +--ro (encoding)? | +--:(encode-xml) | | +--ro datastore-contents-xml? datastore-contents-xml | +--:(encode-json) {notif-bis:json}? | +--ro datastore-contents-json? datastore-contents-json +---n push-change-update {on-change}? +--ro subscription-id notif-bis:subscription-id +--ro time-of-update? yang:date-and-time +--ro (encoding)? +--:(encode-xml) | +--ro datastore-changes-xml? datastore-changes-xml +--:(encode-json) {notif-bis:json}? +--ro datastore-changes-yang? datastore-changes-json The components of the model are described in the following subsections.

Container "update-streams" is used to indicate which data streams are provided by the system and can be subscribed to. For this purpose, it contains a leaf list of data nodes identifying the supported streams.

Container "filters" contains a list of configurable data filters, each specified in its own list element. This allows users to configure filters separately from an actual subscription, which can then be referenced from a subscription. This facilitates the reuse of filter definitions, which can be important in case of complex filter conditions. One of three types of filters can be specified as part of a filter list element. Subtree filters follow syntax and semantics of RFC 6241 and allow to specify which subtree(s) to subscribe to. In addition, XPath filters can be specified for more complex filter conditions. Finally, filters can be specified using syntax and semantics of RFC5277. It is conceivable to introduce other types of filters; in that case, the data model needs to be augmented accordingly.

As an optional feature, configured-subscriptions, allows for the configuration of subscriptions as opposed to RPC. Subscriptions configurations are represented by list subscription-config. Each subscription is represented through its own list element and includes the following components: "subscription-id" is an identifier used to refer to the subscription. "stream" refers to the stream being subscribed to. The subscription model assumes the presence of perpetual and continuous streams of updates. Various streams are defined: "push-update" covers the entire set of YANG data in the server. "operational-push" covers all operational data, while "config-push" covers all configuration data. Other streams could be introduced in augmentations to the model by introducing additional identities. "encoding" refers to the encoding requested for the data updates. By default, updates are encoded using XML. However, JSON can be requested as an option if the json-enconding feature is supported. Other encodings may be supported in the future. "subscription-start-time" specifies when the subscription is supposed to start. The start time also serves as anchor time for periodic subscriptions (see below). "subscription-stop-time" specifies a stop time for the subscription. Once the stop time is reached, the subscription is automatically terminated. However, even when terminated, the subscription entry remains part of the configuration unless explicity deleted from the configuration. It is possible to effectively "resume" a stopped subscription by reconfiguring the stop time. Filters for a subscription can be specified using a choice, allowing to either reference a filter that has been separately configured or entering its definition inline. A choice of subscription policies allows to define when to send new updates - periodic or on change. For periodic subscriptions, the trigger is defined by a "period", a parameter that defines the interval with which updates are to be pushed. The start time of the subscription serves as anchor time, defining one specific point in time at which an update needs to be sent. Update intervals always fall on the points in time that are a multiple of a period after the start time. For on-change subscriptions, the trigger occurs whenever a change in the subscribed information is detected. On-change subscriptions have more complex semantics that is guided by additional parameters. "dampening-period" specifies the interval that must pass before a successive update for the same data node is sent. The first time a change is detected, the update is sent immediately. If a subsequent change is detected, another update is only sent once the dampening period has passed, containing the value of the data node that is then valid. "excluded-change" allows to restrict the types of changes for which updates are sent (changes to object values, object creation or deletion events). "no-synch-on-start" is a flag that allows to specify whether or not a complete update with all the subscribed data should be sent at the beginning of a subscription; if the flag is omitted, a complete update is sent to facilitate synchronization. It is conceivable to augment the data model with additional parameters in the future to specify even more refined policies, such as parameters that specify the magnitude of a change that must occur before an update is triggered. This is followed with a list of receivers for the subscription, indicating for each receiver the transport that should be used for push updates (if options other than Netconf are supported). It should be noted that the receiver does not have to be the same system that configures the subscription. Finally, "push-source" can be used to specify the source of push updates, either a specific interface or server address. A subscription established through configuration cannot be deleted using an RPC. Likewise, subscriptions established through RPC cannot be deleted through configuration. The deletion of a subscription, whether through RPC or configuration, results in immediate termination of the subsciption.

Subscriptions can be subjected to management themselves. For example, it is possible that a server may no longer be able to serve a subscription that it had previously accepted. Perhaps it has run out of resources, or internal errors may have occurred. When this is the case, a server needs to be able to temporarily suspend the subscription, or even to terminate it. More generally, the server should provide a means by which the status of subscriptions can be monitored. Container "subscriptions" contains the state of all subscriptions that are currently active. This includes subscriptions that were established (and have not yet been deleted) using RPCs, as well as subscriptions that have been configured as part of configuration. Each subscription is represented as a list element "datastore-push-subscription". The associated information includes an identifier for the subscription, a subscription status, as well as the various subscription parameters that are in effect. The subscription status indicates whether the subscription is currently active and healthy, or if it is degraded in some form. Leaf "configured-subscription" indicates whether the subscription came into being via configuration or via RPC. Subscriptions that were established by RPC are removed from the list once they expire (reaching stop-time )or when they are terminated. Subscriptions that were established by configuration need to be deleted from the configuration by a configuration editing operation.

A server needs to indicate any changes in status of a subscription to the receiver through a notification. Specifically, subscribers need to be informed of the following: A subscription has been temporarily suspended (including the reason) A subscription (that had been suspended earlier) is once again operational A subscription has been terminated (including the reason) 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.

YANG-Push subscriptions are established, modified, and deleted using three RPCs.

The subscriber sends an establish-subscription RPC with the parameters in section 3.1. For instance

push-update 500 encode-xml ]]> The server must respond explicitly positively (i.e., subscription accepted) or negatively (i.e., subscription rejected) to the request. Positive responses include the subscription-id of the accepted subscription. In that case a server may respond:

52 ]]> A subscription can be rejected for multiple reasons, including the lack of authorization to establish a subscription, the lack of read authorization on the requested data node, or the inability of the server to provide a stream with the requested semantics. . When the requester is not authorized to read the requested data node, the returned <error-info> indicates an authorization error and the requested node. For instance, if the above request was unauthorized to read node "ex:foo" the server may return:

error-data-not-authorized ]]> If a request is rejected because the server is not able to serve it, the server SHOULD include in the returned error what subscription parameters would have been accepted for the request. However, they are no guarantee that subsequent requests for this client or others will in fact be accepted. For example, for the following request:

push-update

encode-xml ]]> A server that cannot serve on-change updates but periodic updates might return the following:

error-no-such-option

100 ]]>

The subscriber may send a modify-subscription PRC for a subscription previously established using RPC The subscriber may change any subscription parameters by including the new values in the modify-subscription RPC. Parameters not included in the rpc should remain unmodified. For illustration purposes we include an exchange example where a subscriber modifies the period of the subscription.

push-update

1011

250 encode-xml ]]> The server must respond explicitly positively (i.e., subscription accepted) or negatively (i.e., subscription rejected) to the request. Positive responses include the subscription-id of the accepted subscription. In that case a server may respond:

1011 ]]> If the subscription modification is rejected, the server must send a response like it does for an establish-subscription and maintain the subscription as it was before the modification request. A subscription may be modified multiple times. A configured subscription cannot be modified using modify-subscription RPC. Instead, the configuration needs to be edited as needed.

To stop receiving updates from a subscription and effectively delete a subscription that had previously been established using an establish-subscription RPC, a subscriber can send a delete-subscription RPC, which takes as only input the subscription-id. For example

1011

]]> Configured subscriptions cannot be deleted via RPC, but have to be removed from the configuration.

file "ietf-yang-push@2016-06-15.yang" module ietf-yang-push { namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push"; prefix yp; import ietf-inet-types { prefix inet; } import ietf-yang-types { prefix yang; } import ietf-event-notifications { prefix notif-bis; } import ietf-5277-netconf { prefix notif; } organization "IETF"; contact "WG Web: WG List: WG Chair: Mahesh Jethanandani WG Chair: Mehmet Ersue Editor: Alexander Clemm Editor: Eric Voit Editor: Alberto Gonzalez Prieto Editor: Ambika Prasad Tripathy Editor: Einar Nilsen-Nygaard "; description "This module contains conceptual YANG specifications for YANG push."; revision 2016-06-15 { description "First revision to incorporate RFC 5277-bis."; reference "YANG Datastore Push, draft-ietf-netconf-yang-push-03"; } feature on-change { description "This feature indicates that on-change updates are supported."; } /* * IDENTITIES */ /* Additional errors for subscription operations */ identity error-data-not-authorized { base notif-bis:error; description "No read authorization for a requested data node."; } /* Additional types of streams */ identity update-stream { base notif:stream; description "Base identity to represent a conceptual system-provided datastream of datastore updates with predefined semantics."; } identity yang-push { base update-stream; description "A conceptual datastream consisting of all datastore updates, including operational and configuration data."; } identity operational-push { base update-stream; description "A conceptual datastream consisting of updates of all operational data."; } identity config-push { base update-stream; description "A conceptual datastream consisting of updates of all configuration data."; } identity custom-stream { base update-stream; description "A conceptual datastream for datastore updates with custom updates as defined by a user."; } /* Additional transport option */ identity restconf { base notif-bis:transport; description "Restconf notifications as a transport"; } /* * TYPE DEFINITIONS */ typedef datastore-contents-xml { type string; description "This type is be used to represent datastore contents, i.e. a set of data nodes with their values, in XML. The syntax corresponds to the syntax of the data payload returned in a corresponding Netconf get operation with the same filter parameters applied."; reference "RFC 6241 section 7.7"; } typedef datastore-changes-xml { type string; description "This type is used to represent a set of changes in a datastore encoded in XML, indicating for datanodes whether they have been created, deleted, or updated. The syntax corresponds to the syntax used to when editing a datastore using the edit-config operation in Netconf."; reference "RFC 6241 section 7.2"; } typedef datastore-contents-json { type string; description "This type is be used to represent datastore contents, i.e. a set of data nodes with their values, in JSON. The syntax corresponds to the syntax of the data payload returned in a corresponding RESTCONF get operation with the same filter parameters applied."; reference "RESTCONF Protocol"; } typedef datastore-changes-json { type string; description "This type is used to represent a set of changes in a datastore encoded in JSON, indicating for datanodes whether they have been created, deleted, or updated. The syntax corresponds to the syntax used to patch a datastore using the yang-patch operation with Restconf."; reference "draft-ietf-netconf-yang-patch"; } typedef filter-id { type uint32; description "A type to identify filters which can be associated with a subscription."; } typedef subscription-result { type identityref { base notif-bis:subscription-result; } description "The result of a subscription operation"; } typedef subscription-term-reason { type identityref { base notif-bis:subscription-errors; } description "Reason for a server to terminate a subscription."; } typedef subscription-susp-reason { type identityref { base notif-bis:subscription-errors; } description "Reason for a server to suspend a subscription."; } typedef encoding { type identityref { base notif-bis:encodings; } description "Specifies a data encoding, e.g. for a data subscription."; } typedef change-type { type enumeration { enum "create" { description "A new data node was created"; } enum "delete" { description "A data node was deleted"; } enum "modify" { description "The value of a data node has changed"; } } description "Specifies different types of changes that may occur to a datastore."; } typedef transport-protocol { type identityref { base notif-bis:transport; } description "Specifies transport protocol used to send updates to a receiver."; } typedef push-source { type enumeration { enum "interface-originated" { description "Pushes will be sent from a specific interface on a Publisher"; } enum "address-originated" { description "Pushes will be sent from a specific address on a Publisher"; } } description "Specifies from where objects will be sourced when being pushed off a publisher."; } typedef update-stream { type identityref { base update-stream; } description "Specifies a system-provided datastream."; } grouping update-filter { description "This groupings defines filters for push updates for a datastore tree. The filters define which updates are of interest in a push update subscription. Mixing and matching of multiple filters does not occur at the level of this grouping. When a push-update subscription is created, the filter can be a regular subscription filter, or one of the additional filters that are defined in this grouping."; choice update-filter { description "Define filters regarding which data nodes to include in push updates"; case subtree { description "Subtree filter."; anyxml subtree-filter { description "Subtree-filter used to specify the data nodes targeted for subscription within a subtree, or subtrees, of a conceptual YANG datastore. It may include additional criteria, allowing users to receive only updates of a limited set of data nodes that match those filter criteria. This will be used to define what updates to include in a stream of update events, i.e. to specify for which data nodes update events should be generated and specific match expressions that objects need to meet. The syntax follows the subtree filter syntax specified in RFC 6241, section 6."; reference "RFC 6241 section 6"; } } case xpath { description "XPath filter"; leaf xpath-filter { type yang:xpath1.0; description "Xpath defining the data items of interest."; } } } } grouping update-policy { description "This grouping describes the conditions under which an update will be sent as part of an update stream."; choice update-trigger { description "Defines necessary conditions for sending an event to the subscriber."; case periodic { description "The agent is requested to notify periodically the current values of the datastore or the subset defined by the filter."; leaf period { type yang:timeticks; mandatory true; description "Duraton of time which should occur between periodic push updates. Where the anchor of a start-time is available, the push will include the objects and their values which exist at an exact multiple of timeticks aligning to this start-time anchor."; } } case on-change { if-feature "on-change"; description "The agent is requested to notify changes in values in the datastore or a subset of it defined by a filter."; leaf no-synch-on-start { type empty; description "This leaf acts as a flag that determines behavior at the start of the subscription. When present, synchronization of state at the beginning of the subscription is outside the scope of the subscription. Only updates about changes that are observed from the start time, i.e. only push-change-update notifications are sent. When absent (default behavior), in order to facilitate a receiver's synchronization, a full update is sent when the subscription starts using a push-update notification, just like in the case of a periodic subscription. After that, push-change-update notifications are sent."; } leaf dampening-period { type yang:timeticks; mandatory true; description "Minimum amount of time that needs to have passed since the last time an update was provided."; } leaf-list excluded-change { type change-type; description "Use to restrict which changes trigger an update. For example, if modify is excluded, only creation and deletion of objects is reported."; } } } } grouping push-subscription-info { description "This grouping describes information concerning a push subscription that is need in addition to information already included in notif-bis:subscription-info."; leaf subscription-start-time { type yang:date-and-time; description "Designates the time at which a subscription is supposed to start, or immediately, in case the start-time is in the past. For periodic subscription, the start time also serves as anchor time from which the time of the next update is computed. The next update will take place at the next period interval from the anchor time. For example, for an anchor time at the top of a minute and a period interval of a minute, the next update will be sent at the top of the next minute."; } leaf subscription-stop-time { type yang:date-and-time; description "Designates the time at which a subscription will end. When a subscription reaches its stop time, it will be automatically deleted. No final push is required unless there is exact alignment with the end of a periodic subscription period."; } } grouping subscription-qos { description "This grouping describes Quality of Service information concerning a subscription. This information is passed to lower layers for transport priortization and treatment"; leaf dscp { if-feature "notif-bis:configured-subscriptions"; type inet:dscp; default "0"; description "The push update's IP packet transport priority. This is made visible across network hops to receiver. The transport priority is shared for all receivers of a given subscription."; } leaf subscription-priority { type uint8; description "Relative priority for a subscription. Allows an underlying transport layer perform informed load balance allocations between various subscriptions"; } leaf subscription-dependency { type string; description "Provides the Subscription ID of a parent subscription without which this subscription should not exist. In other words, there is no reason to stream these objects if another subscription is missing."; } } augment "/notif-bis:establish-subscription/notif-bis:input" { description "Define additional subscription parameters that apply specifically to push updates"; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:establish-subscription/notif-bis:input/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } augment "/notif-bis:establish-subscription/notif-bis:output" { description "Allow to return additional subscription parameters that apply specifically to push updates."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:establish-subscription/notif-bis:output/"+ "notif-bis:result/notif-bis:no-success/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } augment "/notif-bis:modify-subscription/notif-bis:input" { description "Define additional subscription parameters that apply specifically to push updates."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:modify-subscription/notif-bis:input/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } augment "/notif-bis:modify-subscription/notif-bis:output" { description "Allow to retun additional subscription parameters that apply specifically to push updates."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:modify-subscription/notif-bis:output/"+ "notif-bis:result/notif-bis:no-success/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } notification push-update { description "This notification contains a periodic push update. This notification shall only be sent to receivers of a subscription; it does not constitute a general-purpose notification."; leaf subscription-id { type notif-bis:subscription-id; mandatory true; description "This references the subscription because of which the notification is sent."; } leaf time-of-update { type yang:date-and-time; description "This leaf contains the time of the update."; } choice encoding { description "Distinguish between the proper encoding that was specified for the subscription"; case encode-xml { description "XML encoding"; leaf datastore-contents-xml { type datastore-contents-xml; description "This contains data encoded in XML, per the subscription."; } } case encode-json { if-feature "notif-bis:json"; description "JSON encoding"; leaf datastore-contents-json { type datastore-contents-json; description "This leaf contains data encoded in JSON, per the subscription."; } } } } notification push-change-update { if-feature "on-change"; description "This notification contains an on-change push update. This notification shall only be sent to the receivers of a subscription; it does not constitute a general-purpose notification."; leaf subscription-id { type notif-bis:subscription-id; mandatory true; description "This references the subscription because of which the notification is sent."; } leaf time-of-update { type yang:date-and-time; description "This leaf contains the time of the update, i.e. the time at which the change was observed."; } choice encoding { description "Distinguish between the proper encoding that was specified for the subscription"; case encode-xml { description "XML encoding"; leaf datastore-changes-xml { type datastore-changes-xml; description "This contains datastore contents that has changed since the previous update, per the terms of the subscription. Changes are encoded analogous to the syntax of a corresponding Netconf edit-config operation."; } } case encode-json { if-feature "notif-bis:json"; description "JSON encoding"; leaf datastore-changes-yang { type datastore-changes-json; description "This contains datastore contents that has changed since the previous update, per the terms of the subscription. Changes are encoded analogous to the syntax of a corresponding RESTCONF yang-patch operation."; } } } } augment "/notif-bis:subscription-started" { description "This augmentation adds push subscription parameters to the notification that a subscription has started and data updates are beginning to be sent. This notification shall only be sent to receivers of a subscription; it does not constitute a general-purpose notification."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:subscription-started/notif-bis:filter-type" { description "This augmentation allows to include additional update filters options to be included as part of the notification that a subscription has started."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } augment "/notif-bis:subscription-modified" { description "This augmentation adds push subscription parameters to the notification that a subscription has been modified. This notification shall only be sent to receivers of a subscription; it does not constitute a general-purpose notification."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:subscription-modified/notif-bis:filter-type" { description "This augmentation allows to include additional update filters options to be included as part of the notification that a subscription has been modified."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } augment "/notif-bis:filters/notif-bis:filter/notif-bis:filter-type" { description "This container adds additional update filter options to the list of configurable filters that can be applied to subscriptions. This facilitates the reuse of complex filters once defined."; case update-filter { uses update-filter; } } augment "/notif-bis:subscription-config/notif-bis:subscription" { description "Contains the list of subscriptions that are configured, as opposed to established via RPC or other means."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:subscription-config/notif-bis:subscription/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { uses update-filter; } } augment "/notif-bis:subscriptions/notif-bis:subscription" { description "Contains the list of currently active subscriptions, i.e. subscriptions that are currently in effect, used for subscription management and monitoring purposes. This includes subscriptions that have been setup via RPC primitives, e.g. establish-subscription, delete-subscription, and modify-subscription, as well as subscriptions that have been established via configuration."; uses push-subscription-info; uses update-policy; uses subscription-qos; } augment "/notif-bis:subscriptions/notif-bis:subscription/notif-bis:filter-type" { description "Add push filters to selection of filter types."; case update-filter { description "Additional filter options for push subscription."; uses update-filter; } } }

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      Subscriptions could be used to attempt to overload servers of YANG
      datastores. For this reason, it is important that the server has the
      ability to decline a subscription request if it would deplete its
      resources. In addition, a server needs to be able to suspend an existing
      subscription when needed. When this occur, the subscription status is
      updated accordingly and the clients are notified. Likewise, requests for
      subscriptions need to be properly authorized.

      A subscription could be used to retrieve data in subtrees that a
      client has not authorized access to. Therefore it is important that data
      pushed based on subscriptions is authorized in the same way that regular
      data retrieval operations are. Data being pushed to a client needs
      therefore to be filtered accordingly, just like if the data were being
      retrieved on-demand. The Netconf Authorization Control Model
      applies.

      A subscription could be configured on another receiver's behalf, with
      the goal of flooding that receiver with updates. One or more publishers
      could be used to overwhelm a receiver which doesn't even support
      subscriptions. Clients which do not want pushed data need only terminate
      or refuse any transport sessions from the publisher. In addition, the
      Netconf Authorization Control Model SHOULD be used to control and
      restrict authorization of subscription configuration.
    

    
      
        
          
             Which stream types to introduce. Current list includes streams for all operational and for all 
             config data.  Consider adding stream for operational data minus counters.  
             Also: assess implications of opstate implications on required data streams.  
            In addition to identifying which items go to which streams, 
            identifying and calling out which items (such as counters) 
            should not be "on-change subscribable" may be useful.
            Consider introducing a Yang extension to define if an object: 
            is-a-counter and/or not-notifiable.
            What QoS parameters should be supported for subscriptions.  
             Note: QoS parameters are applicable to buffering as well as temporarily loss of transport connectivity. 
             Implications of ephemeral requirements from I2RS
             Filters: YANG 1.1 allows filters to be defined in multiple places. 
             How do they intersect each other in a deterministic way.
             On-change subscription: consider providing publisher with capability to 
             initiate a refresh of contents rather than send deltas. Current proposal 
             allows for a "synch-on-start" option; such an option might be useful 
             also e.g. on resumption of a subscription that had been suspended.
             Do we need an extension for NACM to support filter out datastore nodes 
             for which the receiver has no read access? 
             (And how does this differ from existing GET, which must do the same filtering?) 
             In 5277, such filtering is done at the notification level. 
             Yang-push includes notification-content filtering. 
             This may be very expensive in terms of processing. 
             Andy suggestion: only accept Yang-push subscriptions for subtrees 
             the user has rights for all the nodes in the subtree. 
             Changes to those rights trigger a subscription termination. 
             Should we codify this, or let vendors determine when per 
             subtree filtering might be applied?
           
         
      
       
      Still need to agree on draft text
        
           Multiple receivers per configured subscription is ok.
           Proper behavior for on-change, including detecting and indicating changes within a dampening period.
           Still some details to work through, e.g., 
           do we add a counter for the number of object changes during a dampening period?
           Negotiate vs. auto-adjust. Cases where negotiate is needed exists for domain synchronization.
           Error messages can be used to transport information back as hints. 
           Alternative is dedicated RPC responses. 
           In either case specific contents of these negotiation messages must still be defined.
           Message format for synchronization (i.e. synch-on-start) still needs to be defined.
       
     
     
     
       Some editorial updates needed to reflect these 
       
       Periodic interval goes to seconds from timeticks
       Balancing Augment vs. Parallel Model structures (maximize augment)
       Moving from separate start/stop to Anchor time for Periodic
       
     
    
    
        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, and Guangying Zheng.