Subscription to YANG Notifications for Datastore Updates

Abstract

This document describes a mechanism that allows subscriber applications to request a continuous and customized stream of updates from a YANG datastore. Providing such visibility into updates enables new capabilities based on the remote mirroring and monitoring of configuration and operational state.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8641.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................4
   2. Definitions .....................................................4
   3. Solution Overview ...............................................6
      3.1. Subscription Model .........................................6
      3.2. Negotiation of Subscription Policies .......................7
      3.3. On-Change Considerations ...................................8
      3.4. Reliability Considerations .................................9
      3.5. Data Encodings ............................................10
      3.6. Defining the Selection with a Datastore ...................11
      3.7. Streaming Updates .........................................12
      3.8. Subscription Management ...................................15
      3.9. Receiver Authorization ....................................16
      3.10. On-Change Notifiable Datastore Nodes .....................18
      3.11. Other Considerations .....................................18
   4. A YANG Data Model for Management of Datastore Push
      Subscriptions ..................................................20
      4.1. Overview ..................................................20
      4.2. Subscription Configuration ................................27
      4.3. YANG Notifications ........................................28
      4.4. YANG RPCs .................................................29
   5. YANG Module for YANG-Push ......................................34
   6. IANA Considerations ............................................51
   7. Security Considerations ........................................51
   8. References .....................................................53
      8.1. Normative References ......................................53
      8.2. Informative References ....................................55
   Appendix A. Subscription Errors ...................................56
     A.1. RPC Failures ...............................................56
     A.2. Failure Notifications ......................................57
   Acknowledgments ...................................................58
   Contributors ......................................................58
   Authors' Addresses ................................................58

1. Introduction

Traditional approaches for providing visibility into managed entities from a remote system have been built on polling. With polling, data is periodically requested and retrieved by a client from a server to stay up to date. However, there are issues associated with polling- based management:

• Polling incurs significant latency. This latency prohibits many types of applications.

• Polling cycles may be missed, and requests may be delayed or get lost -- often when the network is under stress and the need for the data is the greatest.

• Polling requests may undergo slight fluctuations, resulting in intervals of different lengths. The resulting data is difficult to calibrate and compare.

• For applications that monitor for changes, many remote polling cycles place unwanted and ultimately wasteful load on the network, devices, and applications, particularly when changes occur only infrequently.

A more effective alternative to polling is for an application to receive automatic and continuous updates from a targeted subset of a datastore. Accordingly, there is a need for a service that (1) allows applications to subscribe to updates from a datastore and (2) enables the server (also referred to as the "publisher") to push and, in effect, stream those updates. The requirements for such a service have been documented in [RFC7923].

This document defines a corresponding solution that is built on top of [RFC8639]. Supplementing that work are YANG data model augmentations, extended RPCs, and new datastore-specific update notifications. Transport options provided in [RFC8639] will work seamlessly with this solution.

2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

   This document uses the terminology defined in [RFC7950], [RFC8341],
   [RFC8342], and [RFC8639].  In addition, this document defines the
   following terms:

• Datastore node: A node in the instantiated YANG data tree associated with a datastore. In this document, datastore nodes are often also simply referred to as "objects".

• Datastore node update: A data item containing the current value of a datastore node at the time the datastore node update was created, as well as the path to the datastore node.

• Datastore subscription: A subscription to a stream of datastore node updates.

• Datastore subtree: A datastore node and all its descendant datastore nodes.

• On-change subscription: A datastore subscription with updates that are triggered when changes in subscribed datastore nodes are detected.

• Periodic subscription: A datastore subscription with updates that are triggered periodically according to some time interval.

• Selection filter: Evaluation and/or selection criteria that may be applied against a targeted set of objects.

• Update record: A representation of one or more datastore node updates. In addition, an update record may contain which type of update led to the datastore node update (e.g., whether the datastore node was added, changed, or deleted). Also included in the update record may be other metadata, such as a subscription ID of the subscription for which the update record was generated. In this document, update records are often also simply referred to as "updates".

• Update trigger: A mechanism that determines when an update record needs to be generated.

• YANG-Push: The subscription and push mechanism for datastore updates that is specified in this document.

3. Solution Overview

This document specifies a solution that provides a subscription service for updates from a datastore. This solution supports dynamic as well as configured subscriptions to updates of datastore nodes. Subscriptions specify when notification messages (also referred to as "push updates") should be sent and what data to include in update records. Datastore node updates are subsequently pushed from the publisher to the receiver per the terms of the subscription.

3.1. Subscription Model

YANG-Push subscriptions are defined using a YANG data model. This model enhances the subscription model defined in [RFC8639] with capabilities that allow subscribers to subscribe to datastore node updates -- specifically, to specify the update triggers defining when to generate update records as well as what to include in an update record. Key enhancements include:

• The specification of selection filters that identify targeted YANG datastore nodes and/or datastore subtrees for which updates are to be pushed.

• The specification of update policies that contain conditions that trigger the generation and pushing of new update records. There are two types of subscriptions, distinguished by how updates are triggered: periodic and on-change.

• For periodic subscriptions, the update trigger is specified by two parameters that define when updates are to be pushed. These parameters are (1) the period interval with which to report updates and (2) an "anchor-time", i.e., a reference point in time that can be used to calculate at which points in time periodic updates need to be assembled and sent.

• For on-change subscriptions, an update trigger occurs whenever a change in the subscribed information is detected. The following additional parameters are included:

         +  "dampening-period": In an on-change subscription, detected
         
            object changes should be sent as quickly as possible.
            However, it may be undesirable to send a rapid series of
            object changes.  Such behavior has the potential to exhaust
            resources in the publisher or receiver.  In order to protect
            against this type of scenario, a dampening period MAY be
            used to specify the interval that has to pass before
            successive update records for the same subscription are
            generated for a receiver.  The dampening period collectively

applies to the set of all datastore nodes selected by a single subscription. This means that when there is a change to one or more subscribed objects, an update record containing those objects is created immediately (when no dampening period is in effect) or at the end of a dampening period (when a dampening period is in fact in effect). If multiple changes to a single object occur during a dampening period, only the value that is in effect at the time when the update record is created is included. The dampening period goes into effect every time the assembly of an update record is completed.

• "change-type": This parameter can be used to reduce the types of datastore changes for which updates are sent (e.g., you might only send an update when an object is created or deleted, but not when an object value changes).

• "sync-on-start": This parameter defines whether or not a complete "push-update" (Section 3.7) of all subscribed data will be sent at the beginning of a subscription. Such early synchronization establishes the frame of reference for subsequent updates.

• An encoding (using anydata) for the contents of periodic and on-change push updates.

3.2. Negotiation of Subscription Policies

A dynamic subscription request SHOULD be declined if a publisher determines that it may be unable to provide update records meeting the terms of an "establish-subscription" or "modify-subscription" RPC request. In this case, a subscriber may quickly follow up with a new RPC request using different parameters.

Random guessing of different parameters by a subscriber is to be discouraged. Therefore, in order to minimize the number of subscription iterations between subscriber and publisher, a dynamic subscription supports a simple negotiation between subscribers and publishers for subscription parameters. This negotiation is in the form of supplemental information that should be inserted into error responses to a failed RPC request. This returned error response information, when considered, should increase the likelihood of success for subsequent RPC requests. Such hints include suggested periodic time intervals, acceptable dampening periods, and size estimates for the number of objects that would be returned from a proposed selection filter. However, there are no guarantees that subsequent requests that consider these hints will be accepted.

3.3. On-Change Considerations

On-change subscriptions allow receivers to receive updates whenever changes to targeted objects occur. As such, on-change subscriptions are particularly effective for data that changes infrequently but for which applications need to be quickly notified, with minimal delay, whenever a change does occur.

On-change subscriptions tend to be more difficult to implement than periodic subscriptions. Accordingly, on-change subscriptions may not be supported by all implementations or for every object.

Whether or not to accept or reject on-change subscription requests when the scope of the subscription contains objects for which on-change is not supported is up to the publisher implementation. A publisher MAY accept an on-change subscription even when the scope of the subscription contains objects for which on-change is not supported. In that case, updates are sent only for those objects within the scope of the subscription that do support on-change updates, whereas other objects are excluded from update records, even if their values change. In order for a subscriber to determine whether objects support on-change subscriptions, objects are marked accordingly on a publisher. Accordingly, when subscribing, it is the responsibility of the subscriber to ensure that it is aware of which objects support on-change and which do not. For more on how objects are so marked, see Section 3.10.

Alternatively, a publisher MAY decide to simply reject an on-change subscription if the scope of the subscription contains objects for which on-change is not supported. In the case of a configured subscription, the publisher MAY suspend the subscription.

To avoid flooding receivers with repeated updates for subscriptions containing fast-changing objects or objects with oscillating values, an on-change subscription allows for the definition of a dampening period. Once an update record for a given object is generated, no other updates for this particular subscription will be created until the end of the dampening period. Values sent at the end of the dampening period are the values that are current at the end of the dampening period of all changed objects. Changed objects include those objects that were deleted or newly created during that dampening period. If an object has returned to its original value (or even has been created and then deleted) during the dampening period, that value (and not the interim change) will still be sent. This will indicate that churn is occurring on that object.

On-change subscriptions can be refined to let users subscribe only to certain types of changes. For example, a subscriber might only want object creations and deletions, but not modifications of object values.

Putting it all together, the conceptual process for creating an update record as part of an on-change subscription is as follows:

1. Just before a change, or at the start of a dampening period, evaluate any filtering and any access control rules to ensure that a receiver is authorized to view all subscribed datastore nodes (filtering out any nodes for which this is not the case). The result is a set "A" of datastore nodes and subtrees.

2. Just after a change, or at the end of a dampening period, evaluate any filtering and any (possibly new) access control rules. The result is a set "B" of datastore nodes and subtrees.

3. Construct an update record, which takes the form of a YANG Patch record [RFC8072] for going from A to B.

4. If there were any changes made between A and B that canceled each other out, insert into the YANG Patch record the last change made, even if the new value is no different from the original value (since changes that were made in the interim were canceled out). If the changes involve creating a new datastore node and then deleting it, the YANG Patch record will indicate the deletion of the datastore node. Similarly, if the changes involve deleting a new datastore node and then recreating it, the YANG Patch record will indicate the creation of the datastore node.

5. If the resulting YANG Patch record is non-empty, send it to the receiver.

Note: In cases where a subscriber wants to have separate dampening periods for different objects, the subscriber has the option to create multiple subscriptions with different selection filters.

3.4. Reliability Considerations

A subscription to updates from a datastore is intended to obviate the need for polling. However, in order to do so, it is critical that subscribers can rely on the subscription and have confidence that they will indeed receive the subscribed updates without having to worry about updates being silently dropped. In other words, a subscription constitutes a promise on the side of the publisher to provide the receivers with updates per the terms of the subscription.

Now, there are many reasons why a publisher may at some point no longer be able to fulfill the terms of the subscription, even if the subscription had been initiated in good faith. For example, the volume of datastore nodes may be larger than anticipated, the interval may prove too short to send full updates in rapid succession, or an internal problem may prevent objects from being collected. For this reason, the solution defined in this document (1) mandates that a publisher notify receivers immediately and reliably whenever it encounters a situation in which it is unable to keep the terms of the subscription and (2) provides the publisher with the option to suspend the subscription in such a case. This includes indicating the fact that an update is incomplete as part of a "push-update" or "push-change-update" notification, as well as emitting a "subscription-suspended" notification as applicable. This is described further in Section 3.11.1.

A publisher SHOULD reject a request for a subscription if it is unlikely that the publisher will be able to fulfill the terms of that subscription request. In such cases, it is preferable to have a subscriber request a less resource-intensive subscription than to deal with frequently degraded behavior.

The solution builds on [RFC8639]. As defined therein, any loss of an underlying transport connection will be detected and result in subscription termination (in the case of dynamic subscriptions) or suspension (in the case of configured subscriptions), ensuring that situations where the loss of update notifications would go unnoticed will not occur.

3.5. Data Encodings

3.5.1. Periodic Subscriptions

In a periodic subscription, the data included as part of an update record corresponds to data that could have been read using a retrieval operation.

3.5.2. On-Change Subscriptions

In an on-change subscription, update records need to indicate not only values of changed datastore nodes but also the types of changes that occurred since the last update. Therefore, encoding rules for data in on-change updates will generally follow YANG Patch operations as specified in [RFC8072]. The YANG Patch operations will describe what needs to be applied to the earlier state reported by the preceding update in order to result in the now-current state. Note that objects referred to in an update are not limited to configuration data but can include any objects (including operational data), whereas [RFC8072] patches apply only to configuration data in configuration datastores.

A publisher indicates the type of change to a datastore node using the different YANG Patch operations: the "create" operation is used for newly created objects (except entries in a user-ordered list), the "delete" operation is used for deleted objects (including in user-ordered lists), the "replace" operation is used when only the object value changes, the "insert" operation is used when a new entry is inserted in a list, and the "move" operation is used when an existing entry in a user-ordered list is moved.

However, a patch must be able to do more than just describe the delta from the previous state to the current state. As per Section 3.3, it must also be able to identify whether transient changes have occurred on an object during a dampening period. To support this, it is valid to encode a YANG Patch operation so that its application would result in no change between the previous state and the current state. This indicates that some churn has occurred on the object. An example of this would be a patch that indicates a "create" operation for a datastore node where the receiver believes one already exists or a "replace" operation that replaces a previous value with the same value. Note that this means that the "create" and "delete" errors as described in [RFC8072], Section 2.5 are not errors in the case of YANG-Push (i.e., they are considered valid operations for YANG-Push).

3.6. Defining the Selection with a Datastore

A subscription must specify both the selection filters and the datastore against which these selection filters will be applied. This information is used to choose and subsequently push data from the publisher's datastore to the receivers.

Only a single selection filter can be applied to a subscription at a time. An RPC request proposing a new selection filter replaces any existing filter. The following selection filter types are included in the YANG-Push data model and may be applied against a datastore:

• subtree: A subtree selection filter identifies one or more datastore subtrees. When specified, update records will only come from the datastore nodes of selected datastore subtree(s). The syntax and semantics correspond to those specified in [RFC6241], Section 6.

• xpath: An "xpath" selection filter is an XPath expression that returns a node set. (XPath is a query language for selecting nodes in an XML document; see [XPATH] for details.) When specified, updates will only come from the selected datastore nodes.

These filters are intended to be used as selectors that define which objects are within the scope of a subscription. A publisher MUST support at least one type of selection filter.

XPath itself provides powerful filtering constructs, and care must be used in filter definition. Consider an XPath filter that only passes a datastore node when an interface is up. It is up to the receiver to understand the implications of the presence or absence of objects in each update.

When the set of selection-filtering criteria is applied for a periodic subscription, these criteria are applied whenever a periodic update record is constructed, and only datastore nodes that pass the filter and to which a receiver has access are provided to that receiver. If the same filtering criteria are applied to an on-change subscription, only the subset of those datastore nodes supporting on-change is provided. A datastore node that doesn't support on-change is never sent as part of an on-change subscription's "push-update" or "push-change-update" (Section 3.7).

3.7. Streaming Updates

Contrary to traditional data retrieval requests, datastore subscription enables an unbounded series of update records to be streamed over time. Two generic YANG notifications for update records have been defined for this scenario: "push-update" and "push-change-update".

A "push-update" notification defines a complete, filtered update of the datastore per the terms of a subscription. This type of YANG notification is used for continuous updates of periodic subscriptions. A "push-update" notification can also be used for the on-change subscriptions in two cases. First, it MUST be used as the initial "push-update" if there is a need to synchronize the receiver at the start of a new subscription. Second, it MAY be sent if the publisher later chooses to resync an on-change subscription. The "push-update" update record contains an instantiated datastore subtree with all of 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 a datastore retrieval operation using the same transport with the same filters applied.

A "push-change-update" notification is the most common type of update for on-change subscriptions. The update record in this case contains the set of changes that datastore nodes have undergone since the last notification message. In other words, this indicates which datastore nodes have been created, have been deleted, or have had changes to their values. In cases where multiple changes have occurred over the course of a dampening period and the object has not been deleted, the object's most current value is reported. (In other words, for each object, only one change is reported, not its entire history. Doing so would defeat the purpose of the dampening period.)

"push-update" and "push-change-update" are encoded and placed in notification messages and are ultimately queued for egress over the specified transport.

Figure 1 provides an example of a notification message for a subscription tracking the operational status of a single Ethernet interface (per [RFC8343]). This notification message is encoded XML [W3C.REC-xml-20081126] over the Network Configuration Protocol (NETCONF) as per [RFC8640].

  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
   <eventTime>2017-10-25T08:00:11.22Z</eventTime>
   <push-update xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <id>1011</id>
     <datastore-contents>
        <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
         <interface>
           <name>eth0</name>
           <oper-status>up</oper-status>
         </interface>
       </interfaces>
     </datastore-contents>
   </push-update>
  </notification>

Figure 1: Push Example

Figure 2 provides an example of an on-change notification message for the same subscription.

  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
   <eventTime>2017-10-25T08:22:33.44Z</eventTime>
   <push-change-update
        xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <id>89</id>
     <datastore-changes>
       <yang-patch>
         <patch-id>0</patch-id>
         <edit>
           <edit-id>edit1</edit-id>
           <operation>replace</operation>
           <target>/ietf-interfaces:interfaces</target>
           <value>
             <interfaces
                  xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
               <interface>
                 <name>eth0</name>
                 <oper-status>down</oper-status>
               </interface>
             </interfaces>
           </value>
         </edit>
       </yang-patch>
     </datastore-changes>
   </push-change-update>
  </notification>

Figure 2: Push Example for an On-Change Notification Message

Of note in the above example is the "patch-id" with a value of "0". Per [RFC8072], the "patch-id" is an arbitrary string. With YANG-Push, the publisher SHOULD put into the "patch-id" a counter starting at "0" that increments with every "push-change-update" generated for a subscription. If used as a counter, this counter MUST be reset to "0" any time a resynchronization occurs (i.e., with the sending of a "push-update"). Also, if used as a counter, the counter MUST be reset to "0" after passing a maximum value of "4294967295" (i.e., the maximum value that can be represented using the uint32 data type). Such a mechanism allows easy identification of lost or out-of-sequence update records.

3.8. Subscription Management

The RPCs defined in [RFC8639] have been enhanced to support datastore subscription negotiation. Also, new error codes have been added that are able to indicate why a datastore subscription attempt has failed, along with new yang-data that MAY be used to include details on input parameters that might result in a successful subsequent RPC invocation.

The establishment or modification of a datastore subscription can be rejected for multiple reasons, including a subtree request that is too large or the inability of the publisher to push update records as frequently as requested. In such cases, no subscription is established. Instead, a subscription result that includes the reason for the failure is returned as part of the RPC response. As part of this response, a set of alternative subscription parameters MAY be returned that would likely have resulted in acceptance of the subscription request. The subscriber may consider including such parameters in future subscription attempts.

In the case of a rejected request for establishment of a datastore subscription, if there are hints, the hints SHOULD be transported in a yang-data "establish-subscription-datastore-error-info" container inserted into the RPC error response, in lieu of the "establish- subscription-stream-error-info" that is inserted in the case of a stream subscription.

Figure 3 shows a tree diagram for "establish-subscription-datastore- error-info". All tree diagrams used in this document follow the notation defined in [RFC8340].

          yang-data establish-subscription-datastore-error-info
            +--ro establish-subscription-datastore-error-info
               +--ro reason?                identityref
               +--ro period-hint?           centiseconds
               +--ro filter-failure-hint?   string
               +--ro object-count-estimate? uint32
               +--ro object-count-limit?    uint32
               +--ro kilobytes-estimate?    uint32
               +--ro kilobytes-limit?       uint32

Figure 3: "establish-subscription-datastore-error-info" Tree Diagram

Similarly, in the case of a rejected request for modification of a datastore subscription, if there are hints, the hints SHOULD be transported in a yang-data "modify-subscription-datastore-error-info" container inserted into the RPC error response, in lieu of the "modify-subscription-stream-error-info" that is inserted in the case of a stream subscription.

Figure 4 shows a tree diagram for "modify-subscription-datastore- error-info".

          yang-data modify-subscription-datastore-error-info
            +--ro modify-subscription-datastore-error-info
               +--ro reason?                identityref
               +--ro period-hint?           centiseconds
               +--ro filter-failure-hint?   string
               +--ro object-count-estimate? uint32
               +--ro object-count-limit?    uint32
               +--ro kilobytes-estimate?    uint32
               +--ro kilobytes-limit?       uint32

Figure 4: "modify-subscription-datastore-error-info" Tree Diagram

3.9. Receiver Authorization

A receiver of subscription data MUST only be sent updates for which it has proper authorization. A publisher MUST ensure that no unauthorized data is included in push updates. To do so, it needs to apply all corresponding checks applicable at the time of a specific pushed update and, if necessary, silently remove any unauthorized data from datastore subtrees. This enables YANG data that is pushed based on subscriptions to be authorized in a way that is equivalent to a regular data retrieval ("get") operation.

Each "push-update" and "push-change-update" MUST have access control applied, as depicted in Figure 5. This includes validating that read access is permitted for any new objects selected since the last notification message was sent to a particular receiver. A publisher MUST silently omit data nodes from the results that the client is not authorized to see. To accomplish this, implementations SHOULD apply the conceptual authorization model of [RFC8341], specifically Section 3.2.4, extended to apply analogously to data nodes included in notifications, not just <rpc-reply> messages sent in response to <get> and <get-config> requests.

                         +-----------------+      +--------------------+
     push-update or -->  | datastore node  |  yes | add datastore node |
    push-change-update   | access allowed? | ---> | to update record   |
                         +-----------------+      +--------------------+

Figure 5: Access Control for Push Updates

A publisher MUST allow for the possibility that a subscription's selection filter references nonexistent data or data that a receiver is not allowed to access. Such support permits a receiver the ability to monitor the entire lifecycle of some datastore tree without needing to explicitly enumerate every individual datastore node. If, after access control has been applied, there are no objects remaining in an update record, then the effect varies given if the subscription is a periodic or on-change subscription. For a periodic subscription, an empty "push-update" notification MUST be sent, so that clients do not get confused into thinking that an update was lost. For an on-change subscription, a "push-update" notification MUST NOT be sent, so that clients remain unaware of changes made to nodes they don't have read-access for. By the same token, changes to objects that are filtered MUST NOT affect any dampening intervals.

A publisher MAY choose to reject an "establish-subscription" request that selects nonexistent data or data that a receiver is not allowed to access. The error identity "unchanging-selection" SHOULD be returned as the reason for the rejection. In addition, a publisher MAY choose to terminate a dynamic subscription or suspend a configured receiver when the authorization privileges of a receiver change or the access controls for subscribed objects change. In that case, the publisher SHOULD include the error identity "unchanging- selection" as the reason when sending the "subscription-terminated" or "subscription-suspended" notification, respectively. Such a capability enables the publisher to avoid having to support continuous and total filtering of a subscription's content for every update record. It also reduces the possibility of leakage of access-controlled objects.

If read access into previously accessible nodes has been lost due to a receiver permissions change, this SHOULD be reported as a patch "delete" operation for on-change subscriptions. If not capable of handling such receiver permission changes with such a "delete", publisher implementations MUST force dynamic subscription re-establishment or configured subscription reinitialization so that appropriate filtering is installed.

3.10. On-Change Notifiable Datastore Nodes

In some cases, a publisher supporting on-change notifications may not be able to push on-change updates for some object types. Reasons for this might be that the value of the datastore node changes frequently (e.g., the in-octets counter as defined in [RFC8343]), small object changes are frequent and meaningless (e.g., a temperature gauge changing 0.1 degrees), or the implementation is not capable of on-change notification for a particular object.

In those cases, it will be important for client applications to have a way to identify for which objects on-change notifications are supported and for which ones they are not supported. Otherwise, client applications will have no way of knowing whether they can indeed rely on their on-change subscription to provide them with the change updates that they are interested in. In other words, if implementations do not provide a solution and do not support comprehensive on-change notifiability, clients of those implementations will have no way of knowing what their on-change subscription actually covers.

Implementations are therefore strongly advised to provide a solution to this problem. One solution might involve making discoverable to clients which objects are on-change notifiable, specified using another YANG data model. Such a solution is specified in [Yang-Push-Notif-Cap]. Until this solution is standardized, implementations SHOULD provide their own solution.

3.11. Other Considerations

3.11.1. Robustness and Reliability

It is important that updates as discussed in this document, and on-change updates in particular, do not get lost. If the loss of an update is unavoidable, it is critical that the receiver be notified accordingly.

Update records for a single subscription MUST NOT be resequenced prior to transport.

It is conceivable that, under certain circumstances, a publisher will recognize that it is unable to include in an update record the full set of objects desired per the terms of a subscription. In this case, the publisher MUST act as follows.

• The publisher MUST set the "incomplete-update" flag on any update record that is known to be missing information.

• The publisher MAY choose to suspend the subscription as per [RFC8639]. If the publisher does not create an update record at all, it MUST suspend the subscription.

• When resuming an on-change subscription, the publisher SHOULD generate a complete patch from the previous update record. If this is not possible and the "sync-on-start" option is set to "true" for the subscription, then the full datastore contents MAY be sent via a "push-update" instead (effectively replacing the previous contents). If neither scenario above is possible, then an "incomplete-update" flag MUST be included on the next "push-change-update".

Note: It is perfectly acceptable to have a series of "push-change- update" notifications (and even "push-update" notifications) serially queued at the transport layer awaiting transmission. It is not required for the publisher to merge pending update records sent at the same time.

On the receiver side, what action to take when a record with an "incomplete-update" flag is received depends on the application. It could simply choose to wait and do nothing. It could choose to resync, actively retrieving all subscribed information. It could also choose to tear down the subscription and start a new one, perhaps with a smaller scope that contains fewer objects.

3.11.2. Publisher Capacity

It is far preferable to decline a subscription request than to accept such a request when it cannot be met.

Whether or not a subscription can be supported will be determined by a combination of several factors, such as the subscription update trigger (on-change or periodic), the period in which to report changes (one-second periods will consume more resources than one-hour periods), the amount of data in the datastore subtree that is being subscribed to, and the number and combination of other subscriptions that are concurrently being serviced.

4. A YANG Data Model for Management of Datastore Push Subscriptions

4.1. Overview

The YANG data model for datastore push subscriptions is depicted in Figures 6 through 9. The tree diagram that is used follows the notation defined in [RFC8340]. New schema objects defined here (i.e., beyond those from [RFC8639]) are identified with "yp". For the reader's convenience, in order to compact the tree representation, some nodes that are defined in the ietf-subscribed- notifications YANG module [RFC8639] and therefore are not essential to the understanding of the data model defined here have been removed. This is indicated by "..." in the diagram where applicable.

Because the tree diagram is quite large, its depiction is broken up into four figures. Figure 6 depicts the augmentations that are introduced in YANG module ietf-yang-push to the subscription configuration specified in YANG module ietf-subscribed-notifications.

module: ietf-subscribed-notifications

     ...
     +--rw filters
     |  ...
     |  +--rw yp:selection-filter* [filter-id]
     |     +--rw yp:filter-id                   string
     |     +--rw (yp:filter-spec)?
     |        +--:(yp:datastore-subtree-filter)
     |        |  +--rw yp:datastore-subtree-filter?   <anydata>
     |        |          {sn:subtree}?
     |        +--:(yp:datastore-xpath-filter)
     |           +--rw yp:datastore-xpath-filter?     yang:xpath1.0
     |                   {sn:xpath}?
     +--rw subscriptions
     
        +--rw subscription* [id]
           |  ...
           +--rw (target)
           |  +--:(stream)
           |  |   ...
           |  +--:(yp:datastore)
           |     +--rw yp:datastore                     identityref
           |     +--rw (yp:selection-filter)?
           |        +--:(yp:by-reference)
           |        |  +--rw yp:selection-filter-ref
           |        |          selection-filter-ref
           |        +--:(yp:within-subscription)
           |           +--rw (yp:filter-spec)?
           |              +--:(yp:datastore-subtree-filter)
           |              |  +--rw yp:datastore-subtree-filter?
           |              |          <anydata> {sn:subtree}?
           |              +--:(yp:datastore-xpath-filter)
           |                 +--rw yp:datastore-xpath-filter?
           |                         yang:xpath1.0 {sn:xpath}?
           | ...
           +--rw (yp:update-trigger)
              +--:(yp:periodic)
              |  +--rw yp:periodic!
              |     +--rw yp:period         centiseconds
              |     +--rw yp:anchor-time?   yang:date-and-time
              +--:(yp:on-change) {on-change}?
                 +--rw yp:on-change!
                    +--rw yp:dampening-period?   centiseconds
                    +--rw yp:sync-on-start?      boolean
                    +--rw yp:excluded-change*    change-type

Figure 6: Data Model Structure: Subscription Configuration

Figure 7 depicts the augmentations of YANG module ietf-yang-push made to RPCs specified in YANG module ietf-subscribed-notifications [RFC8639]. Specifically, these augmentations concern the "establish- subscription" and "modify-subscription" RPCs, which are augmented with parameters that are needed to specify datastore push subscriptions.

rpcs:

       +---x establish-subscription
       |  +---w input
       |  |  ...
       |  |  +---w (target)
       |  |  |  +--:(stream)
       |  |  |  |  ...
       |  |  |  +--:(yp:datastore)
       |  |  |     +---w yp:datastore                   identityref
       |  |  |     +---w (yp:selection-filter)?
       |  |  |        +--:(yp:by-reference)
       |  |  |        |  +---w yp:selection-filter-ref
       |  |  |        |          selection-filter-ref
       |  |  |        +--:(yp:within-subscription)
       |  |  |           +---w (yp:filter-spec)?
       |  |  |              +--:(yp:datastore-subtree-filter)
       |  |  |              |  +---w yp:datastore-subtree-filter?
       |  |  |              |          <anydata> {sn:subtree}?
       |  |  |              +--:(yp:datastore-xpath-filter)
       |  |  |                 +---w yp:datastore-xpath-filter?
       |  |  |                         yang:xpath1.0 {sn:xpath}?
       |  |  | ...
       |  |  +---w (yp:update-trigger)
       |  |     +--:(yp:periodic)
       |  |     |  +---w yp:periodic!
       |  |     |     +---w yp:period         centiseconds
       |  |     |     +---w yp:anchor-time?   yang:date-and-time
       |  |     +--:(yp:on-change) {on-change}?
       |  |        +---w yp:on-change!
       |  |           +---w yp:dampening-period?   centiseconds
       |  |           +---w yp:sync-on-start?      boolean
       |  |           +---w yp:excluded-change*    change-type
       |  +--ro output
       |     +--ro id                            subscription-id
       |     +--ro replay-start-time-revision?   yang:date-and-time
       |             {replay}?
       
       +---x modify-subscription
       |  +---w input
       |     ...
       |     +---w (target)
       |     |  ...
       |     |  +--:(yp:datastore)
       |     |     +---w yp:datastore                   identityref
       |     |     +---w (yp:selection-filter)?
       |     |        +--:(yp:by-reference)
       |     |        |  +---w yp:selection-filter-ref
       |     |        |          selection-filter-ref
       |     |        +--:(yp:within-subscription)
       |     |           +---w (yp:filter-spec)?
       |     |              +--:(yp:datastore-subtree-filter)
       |     |              |  +---w yp:datastore-subtree-filter?
       |     |              |          <anydata> {sn:subtree}?
       |     |              +--:(yp:datastore-xpath-filter)
       |     |                 +---w yp:datastore-xpath-filter?
       |     |                         yang:xpath1.0 {sn:xpath}?
       |     | ...
       |     +---w (yp:update-trigger)
       |        +--:(yp:periodic)
       |        |  +---w yp:periodic!
       |        |     +---w yp:period         centiseconds
       |        |     +---w yp:anchor-time?   yang:date-and-time
       |        +--:(yp:on-change) {on-change}?
       |           +---w yp:on-change!
       |              +---w yp:dampening-period?   centiseconds
       +---x delete-subscription
       |  ...
       +---x kill-subscription
          ...

yang-data (for placement into RPC error responses)

...

Figure 7: Data Model Structure: RPCs

Figure 8 depicts augmentations of YANG module ietf-yang-push to the notifications that are specified in YANG module ietf-subscribed- notifications. The augmentations allow the inclusion of subscription configuration parameters that are specific to datastore push subscriptions as part of "subscription-started" and "subscription- modified" notifications.

notifications:

       +---n replay-completed {replay}?
       |  ...
       +---n subscription-completed
       |  ...
       +---n subscription-started {configured}?
       |  |  ...
       |  +--ro (target)
       |  |  ...
       |  |  +--:(yp:datastore)
       |  |     +--ro yp:datastore                   identityref
       |  |     +--ro (yp:selection-filter)?
       |  |        +--:(yp:by-reference)
       |  |        |  +--ro yp:selection-filter-ref
       |  |        |          selection-filter-ref
       |  |        +--:(yp:within-subscription)
       |  |           +--ro (yp:filter-spec)?
       |  |              +--:(yp:datastore-subtree-filter)
       |  |              |  +--ro yp:datastore-subtree-filter?
       |  |              |          <anydata> {sn:subtree}?
       |  |              +--:(yp:datastore-xpath-filter)
       |  |                 +--ro yp:datastore-xpath-filter?
       |  |                         yang:xpath1.0 {sn:xpath}?
       |  ...
       |  +--ro (yp:update-trigger)
       |     +--:(yp:periodic)
       |     |  +--ro yp:periodic!
       |     |     +--ro yp:period         centiseconds
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?   centiseconds
       |           +--ro yp:sync-on-start?      boolean
       |           +--ro yp:excluded-change*    change-type
       +---n subscription-resumed
       |  ...
       
       +---n subscription-modified
       |  ...
       |  +--ro (target)
       |  |  |  ...
       |  |  +--:(yp:datastore)
       |  |     +--ro yp:datastore                   identityref
       |  |     +--ro (yp:selection-filter)?
       |  |        +--:(yp:by-reference)
       |  |        |  +--ro yp:selection-filter-ref
       |  |        |          selection-filter-ref
       |  |        +--:(yp:within-subscription)
       |  |           +--ro (yp:filter-spec)?
       |  |              +--:(yp:datastore-subtree-filter)
       |  |              |  +--ro yp:datastore-subtree-filter?
       |  |              |          <anydata> {sn:subtree}?
       |  |              +--:(yp:datastore-xpath-filter)
       |  |                 +--ro yp:datastore-xpath-filter?
       |  |                         yang:xpath1.0 {sn:xpath}?
       |  ...
       |  +--ro (yp:update-trigger)?
       |     +--:(yp:periodic)
       |     |  +--ro yp:periodic!
       |     |     +--ro yp:period         centiseconds
       |     |     +--ro yp:anchor-time?   yang:date-and-time
       |     +--:(yp:on-change) {on-change}?
       |        +--ro yp:on-change!
       |           +--ro yp:dampening-period?    centiseconds
       |           +--ro yp:sync-on-start?       boolean
       |           +--ro yp:excluded-change*     change-type
       +---n subscription-terminated
       |  ...
       +---n subscription-suspended
          ...

Figure 8: Data Model Structure: Notifications

Finally, Figure 9 depicts the parts of YANG module ietf-yang-push that are newly introduced in this document (i.e., that are not simply augmentations of another YANG module).

module:

           ietf-yang-push

rpcs:

       +---x resync-subscription {on-change}?
          +---w input
             +---w id    sn:subscription-id

yang-data (for placement into RPC error responses):

       +-- resync-subscription-error
       |  +--ro reason?                   identityref
       |  +--ro period-hint?              centiseconds
       |  +--ro filter-failure-hint?      string
       |  +--ro object-count-estimate?    uint32
       |  +--ro object-count-limit?       uint32
       |  +--ro kilobytes-estimate?       uint32
       |  +--ro kilobytes-limit?          uint32
       +-- establish-subscription-error-datastore
       |  +--ro reason?                   identityref
       |  +--ro period-hint?              centiseconds
       |  +--ro filter-failure-hint?      string
       |  +--ro object-count-estimate?    uint32
       |  +--ro object-count-limit?       uint32
       |  +--ro kilobytes-estimate?       uint32
       |  +--ro kilobytes-limit?          uint32
       +-- modify-subscription-error-datastore
          +--ro reason?                   identityref
          +--ro period-hint?              centiseconds
          +--ro filter-failure-hint?      string
          +--ro object-count-estimate?    uint32
          +--ro object-count-limit?       uint32
          +--ro kilobytes-estimate?       uint32
          +--ro kilobytes-limit?          uint32

notifications:

          +---n push-update
          |  +--ro id?                   sn:subscription-id
          |  +--ro datastore-contents?   <anydata>
          |  +--ro incomplete-update?    empty
          +---n push-change-update {on-change}?
             +--ro id?                   sn:subscription-id
             +--ro datastore-changes
             |  +--ro yang-patch
             |     +--ro patch-id    string
             |     +--ro comment?    string
             |     +--ro edit* [edit-id]
             |        +--ro edit-id      string
             |        +--ro operation    enumeration
             |        +--ro target       target-resource-offset
             |        +--ro point?       target-resource-offset
             |        +--ro where?       enumeration
             |        +--ro value?       <anydata>
             +--ro incomplete-update?    empty

Figure 9: Data Model Structure: Non-augmentation Portions

Selected components of the data model are summarized below.

4.2. Subscription Configuration

Both configured and dynamic subscriptions are represented in the list "subscription". New parameters extending the basic subscription data model in [RFC8639] include:

• The targeted datastore from which the selection is being made. The potential datastores include those from [RFC8342]. A platform may also choose to support a custom datastore.

• A selection filter identifying YANG nodes of interest in a datastore. Filter contents are specified via a reference to an existing filter or via an in-line definition for only that subscription. Referenced filters allow an implementation to avoid evaluating filter acceptability during a dynamic subscription request. The "case" statement differentiates the options.

• For periodic subscriptions, triggered updates will occur at the boundaries of a specified time interval. These boundaries can be calculated from the periodic parameters:

• a "period" that defines the duration between push updates.

• an "anchor-time"; update intervals fall on the points in time that are a multiple of a "period" from an "anchor-time". If an "anchor-time" is not provided, then the "anchor-time" MUST be set with the creation time of the initial update record.

• For on-change subscriptions, assuming that any dampening period has completed, triggering occurs whenever a change in the subscribed information is detected. On-change subscriptions have more-complex semantics that are guided by their own set of parameters:

• a "dampening-period" that specifies the interval that must pass before a successive update for the subscription is sent. If no dampening period is in effect, the update is sent immediately. If a subsequent change is detected, another update is only sent once the dampening period has passed for this subscription.

• an "excluded-change" that allows the restriction of the types of changes for which updates should be sent (e.g., only add to an update record on object creation).

• a "sync-on-start" that specifies whether a complete update with all the subscribed data is to be sent at the beginning of a subscription.

4.3. YANG Notifications

4.3.1. State Change Notifications

Subscription state notifications and mechanisms are reused from [RFC8639]. Notifications "subscription-started" and "subscription- modified" have been augmented to include the datastore-specific objects.

4.3.2. Notifications for Subscribed Content

Along with the subscribed content, there are other objects that might be part of a "push-update" or "push-change-update" notification.

• An "id" (that identifies the subscription). This object MUST be transported along with the subscribed contents. It allows a receiver to determine which subscription resulted in a particular update record.

• An "incomplete-update" leaf. This leaf indicates that not all changes that have occurred since the last update are actually included with this update. In other words, the publisher has failed to fulfill its full subscription obligations. (For example, a datastore was unable to provide the full set of datastore nodes to a publisher process.) To facilitate the resynchronization of on-change subscriptions, a publisher MAY subsequently send a "push-update" containing a full selection snapshot of subscribed data.

4.4. YANG RPCs

YANG-Push subscriptions are established, modified, and deleted using RPCs augmented from [RFC8639].

4.4.1. "establish-subscription" RPC

The subscriber sends an "establish-subscription" RPC with the parameters listed in Section 3.1. An example might look like:

 <netconf:rpc message-id="101"
     xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
   <establish-subscription
       xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
       xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
     <yp:datastore
          xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
       ds:operational
     </yp:datastore>
     <yp:datastore-xpath-filter
         xmlns:ex="https://example.com/sample-data/1.0">
       /ex:foo
     </yp:datastore-xpath-filter>
     <yp:periodic>
       <yp:period>500</yp:period>
     </yp:periodic>
   </establish-subscription>
 </netconf:rpc>

Figure 10: "establish-subscription" RPC

A positive response includes the "id" of the accepted subscription. In that case, a publisher may respond as follows:

 <rpc-reply message-id="101"
    xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <id
      xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
       52
    </id>
 </rpc-reply>

Figure 11: "establish-subscription" Positive RPC Response

A subscription can be rejected for multiple reasons, including the lack of authorization to establish a subscription, no capacity to serve the subscription at the publisher, or the inability of the publisher to select datastore content at the requested cadence.

If a request is rejected because the publisher is not able to serve it, the publisher SHOULD include in the returned error hints that help a subscriber understand what subscription parameters might have been accepted for the request. These hints would be included in the yang-data structure "establish-subscription-error-datastore". However, even with these hints, there are no guarantees that subsequent requests will in fact be accepted.

The specific parameters to be returned as part of the RPC error response depend on the specific transport that is used to manage the subscription. For NETCONF, those parameters are defined in [RFC8640]. For example, for the following NETCONF request:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <establish-subscription
           xmlns=
             "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
           xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
         <yp:datastore
             xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
           ds:operational
         </yp:datastore>
         <yp:datastore-xpath-filter
             xmlns:ex="https://example.com/sample-data/1.0">
           /ex:foo
         </yp:datastore-xpath-filter>
         <yp:on-change>
           <yp:dampening-period>100</yp:dampening-period>
         </yp:on-change>
       </establish-subscription>
     </rpc>

Figure 12: "establish-subscription" Request: Example 2

A publisher that cannot serve on-change updates but can serve periodic updates might return the following NETCONF response:

 <rpc-reply message-id="101"
   xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
   xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
   <rpc-error>
     <error-type>application</error-type>
     <error-tag>operation-failed</error-tag>
     <error-severity>error</error-severity>
     <error-path>/yp:periodic/yp:period</error-path>
     <error-info>
       <yp:establish-subscription-error-datastore>
         <yp:reason>yp:on-change-unsupported</yp:reason>
       </yp:establish-subscription-error-datastore>
     </error-info>
   </rpc-error>
 </rpc-reply>

Figure 13: "establish-subscription" Error Response: Example 2

4.4.2. "modify-subscription" RPC

The subscriber MAY invoke the "modify-subscription" RPC for a subscription it previously established. The subscriber will include newly desired values in the "modify-subscription" RPC. Parameters not included MUST remain unmodified. Figure 14 provides an example where a subscriber attempts to modify the period and datastore XPath filter of a subscription using NETCONF.

     <rpc message-id="102"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <modify-subscription
           xmlns=
             "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
           xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
         <id>1011</id>
         <yp:datastore
             xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
           ds:operational
         </yp:datastore>
         <yp:datastore-xpath-filter
             xmlns:ex="https://example.com/sample-data/1.0">
           /ex:bar
         </yp:datastore-xpath-filter>
         <yp:periodic>
           <yp:period>250</yp:period>
         </yp:periodic>
        </modify-subscription>
     </rpc>

Figure 14: "modify-subscription" Request

The publisher MUST respond to the subscription modification request. If the request is rejected, the existing subscription is left unchanged, and the publisher MUST send an RPC error response. This response might have hints encapsulated in the yang-data structure "modify-subscription-error-datastore". A subscription MAY be modified multiple times.

The specific parameters to be returned as part of the RPC error response depend on the specific transport that is used to manage the subscription. For NETCONF, those parameters are specified in [RFC8640].

A configured subscription cannot be modified using a "modify-subscription" RPC. Instead, the configuration needs to be edited as needed.

4.4.3. "delete-subscription" RPC

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 its only input the subscription's "id". This RPC is unmodified from [RFC8639].

4.4.4. "resync-subscription" RPC

This RPC is supported only for on-change subscriptions previously established using an "establish-subscription" RPC. For example:

      <rpc message-id="103"
           xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
        <resync-subscription
            xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
          <id>1011</id>
        </resync-subscription>
      </rpc>

Figure 15: "resync-subscription"

On receipt, a publisher must either (1) accept the request and quickly follow with a "push-update" or (2) send an appropriate error in an RPC error response. In its error response, the publisher MAY include, in the yang-data structure "resync-subscription-error", supplemental information about the reasons for the error.

4.4.5. YANG Module Synchronization

To make subscription requests, the subscriber needs to know the YANG datastore schemas used by the publisher. These schemas are available in the YANG library module ietf-yang-library.yang as defined in [RFC8525]. The receiver is expected to know the YANG library information before starting a subscription.

The set of modules, revisions, features, and deviations can change at runtime (if supported by the publisher implementation). For this purpose, the YANG library provides a simple "yang-library-change" notification that informs the subscriber that the library has changed. In this case, a subscription may need to be updated to take the updates into account. The receiver may also need to be informed of module changes in order to process updates regarding datastore nodes from changed modules correctly.

5. YANG Module for YANG-Push

   This YANG module imports typedefs from [RFC6991], identities from
   [RFC8342], the "yang-data" extension from [RFC8040], and the
   "yang-patch" grouping from [RFC8072].  In addition, it imports and
   augments many definitions from [RFC8639].  It also references
   [RFC6241], [XPATH] ("XML Path Language (XPath) Version 1.0"), and
   [RFC7950].
   
   <CODE BEGINS> file "ietf-yang-push@2019-09-09.yang"
   module ietf-yang-push {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push";
     prefix yp;
   
     import ietf-yang-types {
       prefix yang;
       reference
         "RFC 6991: Common YANG Data Types";
     }
     import ietf-subscribed-notifications {
       prefix sn;
       reference
         "RFC 8639: Subscription to YANG Notifications";
     }
     import ietf-datastores {
       prefix ds;
       reference
         "RFC 8342: Network Management Datastore Architecture (NMDA)";
     }
     import ietf-restconf {
       prefix rc;
       reference
         "RFC 8040: RESTCONF Protocol";
     }
     import ietf-yang-patch {
       prefix ypatch;
       reference
         "RFC 8072: YANG Patch Media Type";
     }
   
     organization
       "IETF NETCONF (Network Configuration) Working Group";
     contact
       "WG Web:  <https:/datatracker.ietf.org/wg/netconf/>
        WG List: <mailto:netconf@ietf.org>
   
        Author:  Alexander Clemm
                 <mailto:ludwig@clemm.org>
   
        Author:  Eric Voit
                 <mailto:evoit@cisco.com>";

description

"This module contains YANG specifications for YANG-Push.

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here.

Copyright © 2019 IETF Trust and the persons identified as authors of the code. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info).

This version of this YANG module is part of RFC 8641; see the RFC itself for full legal notices.";

     revision 2019-09-09 {
       description
         "Initial revision.";
       reference
         "RFC 8641: Subscriptions to YANG Datastores";
     }
     
     /*
      * FEATURES
      */
     
     feature on-change {
       description
         "This feature indicates that on-change triggered subscriptions
          are supported.";
     }
     
     /*
      * IDENTITIES
      */
     
     /* Error type identities for datastore subscription */
     
     identity resync-subscription-error {
       description
         "Problem found while attempting to fulfill a
          'resync-subscription' RPC request.";
     }
     
     identity cant-exclude {
       base sn:establish-subscription-error;
       description
         "Unable to remove the set of 'excluded-change' parameters.
          This means that the publisher is unable to restrict
          'push-change-update' notifications to just the change types
          requested for this subscription.";
     }
     
     identity datastore-not-subscribable {
       base sn:establish-subscription-error;
       base sn:subscription-terminated-reason;
       description
         "This is not a subscribable datastore.";
     }
     
     identity no-such-subscription-resync {
       base resync-subscription-error;
       description
         "The referenced subscription doesn't exist.  This may be as a
          result of a nonexistent subscription ID, an ID that belongs to
          another subscriber, or an ID for a configured subscription.";
     }
     
     identity on-change-unsupported {
       base sn:establish-subscription-error;
       description
         "On-change is not supported for any objects that are
          selectable by this filter.";
     }
     
     identity on-change-sync-unsupported {
       base sn:establish-subscription-error;
       description
         "Neither 'sync-on-start' nor resynchronization is supported for
          this subscription.  This error will be used for two reasons:
          (1) if an 'establish-subscription' RPC includes
          'sync-on-start' but the publisher can't support sending a
          'push-update' for this subscription for reasons other than
          'on-change-unsupported' or 'sync-too-big'
     
          (2) if the 'resync-subscription' RPC is invoked for either an
          existing periodic subscription or an on-change subscription
          that can't support resynchronization.";
     }
     
     identity period-unsupported {
       base sn:establish-subscription-error;
       base sn:modify-subscription-error;
       base sn:subscription-suspended-reason;
       description
         "The requested time period or 'dampening-period' is too short.
          This can be for both periodic and on-change subscriptions
          (with or without dampening).  Hints suggesting alternative
          periods may be returned as supplemental information.";
     }
     
     identity update-too-big {
       base sn:establish-subscription-error;
       base sn:modify-subscription-error;
       base sn:subscription-suspended-reason;
       description
         "Periodic or on-change push update data trees exceed a maximum
          size limit.  Hints on the estimated size of what was too big
          may be returned as supplemental information.";
     }
     
     identity sync-too-big {
       base sn:establish-subscription-error;
       base sn:modify-subscription-error;
       base resync-subscription-error;
       base sn:subscription-suspended-reason;
       description
         "The 'sync-on-start' or resynchronization data tree exceeds a
          maximum size limit.  Hints on the estimated size of what was
          too big may be returned as supplemental information.";
     }

identity unchanging-selection {

       base sn:establish-subscription-error;
       base sn:modify-subscription-error;
       base sn:subscription-terminated-reason;
       description
         "The selection filter is unlikely to ever select data tree
          nodes.  This means that based on the subscriber's current
          access rights, the publisher recognizes that the selection
          filter is unlikely to ever select data tree nodes that change.
          Examples for this might be that the node or subtree doesn't
          exist, read access is not permitted for a receiver, or static
     
          objects that only change at reboot have been chosen.";
     }
     
     /*
      * TYPE DEFINITIONS
      */

typedef change-type {

       type enumeration {
         enum create {
           description
             "A change that refers to the creation of a new
              datastore node.";
         }
         enum delete {
           description
             "A change that refers to the deletion of a
              datastore node.";
         }
         enum insert {
           description
             "A change that refers to the insertion of a new
              user-ordered datastore node.";
         }
         enum move {
           description
             "A change that refers to a reordering of the target
              datastore node.";
         }
         enum replace {
           description
             "A change that refers to a replacement of the target
              datastore node's value.";
         }
       }
       description
         "Specifies different types of datastore changes.
     
          This type is based on the edit operations defined for
          YANG Patch, with the difference that it is valid for a
          receiver to process an update record that performs a
          'create' operation on a datastore node the receiver believes
          exists or to process a delete on a datastore node the
          receiver believes is missing.";
       reference
         "RFC 8072: YANG Patch Media Type, Section 2.5";
     }
     typedef selection-filter-ref {
       type leafref {
         path "/sn:filters/yp:selection-filter/yp:filter-id";
       }
       description
         "This type is used to reference a selection filter.";
     }
     
     typedef centiseconds {
       type uint32;
       description
         "A period of time, measured in units of 0.01 seconds.";
     }
     
     /*
      * GROUP DEFINITIONS
      */
     
     grouping datastore-criteria {
       description
         "A grouping to define criteria for which selected objects from
          a targeted datastore should be included in push updates.";
       leaf datastore {
         type identityref {
           base ds:datastore;
         }
         mandatory true;
         description
           "Datastore from which to retrieve data.";
       }
       uses selection-filter-objects;
     }

grouping selection-filter-types {

       description
         "This grouping defines the types of selectors for objects
          from a datastore.";
       choice filter-spec {
         description
           "The content filter specification for this request.";
         anydata datastore-subtree-filter {
           if-feature "sn:subtree";
           description
             "This parameter identifies the portions of the
              target datastore to retrieve.";
           reference
             "RFC 6241: Network Configuration Protocol (NETCONF),
                        Section 6";
         }
         leaf datastore-xpath-filter {
           if-feature "sn:xpath";
           type yang:xpath1.0;
           description
             "This parameter contains an XPath expression identifying
              the portions of the target datastore to retrieve.

If the expression returns a node set, all nodes in the node set are selected by the filter. Otherwise, if the expression does not return a node set, the filter doesn't select any nodes.

The expression is evaluated in the following XPath context:

• The set of namespace declarations is the set of prefix and namespace pairs for all YANG modules implemented by the server, where the prefix is the YANG module name and the namespace is as defined by the 'namespace' statement in the YANG module.

If the leaf is encoded in XML, all namespace declarations in scope on the 'stream-xpath-filter' leaf element are added to the set of namespace declarations. If a prefix found in the XML is already present in the set of namespace declarations, the namespace in the XML is used.

o The set of variable bindings is empty.

• The function library is comprised of the core function library and the XPath functions defined in Section 10 in RFC 7950.

              o  The context node is the root node of the target
                 datastore.";
           reference
             "XML Path Language (XPath) Version 1.0
              (https://www.w3.org/TR/1999/REC-xpath-19991116)
              RFC 7950: The YANG 1.1 Data Modeling Language,
                        Section 10";
         }
       }
     }

grouping selection-filter-objects {

       description
     
         "This grouping defines a selector for objects from a
          datastore.";
       choice selection-filter {
         description
           "The source of the selection filter applied to the
            subscription.  This will either (1) come referenced from a
            global list or (2) be provided in the subscription itself.";
         case by-reference {
           description
             "Incorporates a filter that has been configured
              separately.";
           leaf selection-filter-ref {
             type selection-filter-ref;
             mandatory true;
             description
               "References an existing selection filter that is to be
                applied to the subscription.";
           }
         }
         case within-subscription {
           description
             "A local definition allows a filter to have the same
              lifecycle as the subscription.";
           uses selection-filter-types;
         }
       }
     }

grouping update-policy-modifiable {

       description
         "This grouping describes the datastore-specific subscription
          conditions that can be changed during the lifetime of the
          subscription.";
       choice update-trigger {
         description
           "Defines necessary conditions for sending an event record to
            the subscriber.";
         case periodic {
           container periodic {
             presence "indicates a periodic subscription";
             description
               "The publisher is requested to periodically notify the
                receiver regarding the current values of the datastore
                as defined by the selection filter.";
             leaf period {
               type centiseconds;
               mandatory true;
               description
                 "Duration of time that should occur between periodic
                  push updates, in units of 0.01 seconds.";
             }
             leaf anchor-time {
               type yang:date-and-time;
               description
                 "Designates a timestamp before or after which a series
                  of periodic push updates are determined.  The next
                  update will take place at a point in time that is a
                  multiple of a period from the 'anchor-time'.
                  For example, for an 'anchor-time' that is set for the
                  top of a particular minute and a period interval of a
                  minute, updates will be sent at the top of every
                  minute that this subscription is active.";
             }
           }
         }
         case on-change {
           if-feature "on-change";
           container on-change {
             presence "indicates an on-change subscription";
             description
               "The publisher is requested to notify the receiver
                regarding changes in values in the datastore subset as
                defined by a selection filter.";
             leaf dampening-period {
               type centiseconds;
               default "0";
               description
                 "Specifies the minimum interval between the assembly of
                  successive update records for a single receiver of a
                  subscription.  Whenever subscribed objects change and
                  a dampening-period interval (which may be zero) has
                  elapsed since the previous update record creation for
                  a receiver, any subscribed objects and properties
                  that have changed since the previous update record
                  will have their current values marshalled and placed
                  in a new update record.";
             }
           }
         }
       }
     }

grouping update-policy {

description

"This grouping describes the datastore-specific subscription

conditions of a subscription.";

       uses update-policy-modifiable {
         augment "update-trigger/on-change/on-change" {
           description
             "Includes objects that are not modifiable once a
              subscription is established.";
           leaf sync-on-start {
             type boolean;
             default "true";
             description
               "When this object is set to 'false', (1) it restricts an
                on-change subscription from sending 'push-update'
                notifications and (2) pushing a full selection per the
                terms of the selection filter MUST NOT be done for
                this subscription.  Only updates about changes
                (i.e., only 'push-change-update' notifications)
                are sent.  When set to 'true' (the default behavior),
                in order to facilitate a receiver's synchronization,
                a full update is sent, via a 'push-update' notification,
                when the subscription starts.  After that,
                'push-change-update' notifications are exclusively sent,
                unless the publisher chooses to resync the subscription
                via a new 'push-update' notification.";
           }
           leaf-list excluded-change {
             type change-type;
             description
               "Used to restrict which changes trigger an update.  For
                example, if a 'replace' operation is excluded, only the
                creation and deletion of objects are reported.";
           }
         }
       }
     }

grouping hints {

       description
         "Parameters associated with an error for a subscription
          made upon a datastore.";
       leaf period-hint {
         type centiseconds;
         description
           "Returned when the requested time period is too short.  This
            hint can assert a viable period for either a periodic push
            cadence or an on-change dampening interval.";
       }
       leaf filter-failure-hint {
         type string;
         description
           "Information describing where and/or why a provided filter
            was unsupportable for a subscription.";
       }
       leaf object-count-estimate {
         type uint32;
         description
           "If there are too many objects that could potentially be
            returned by the selection filter, this identifies the
            estimate of the number of objects that the filter would
            potentially pass.";
       }
       leaf object-count-limit {
         type uint32;
         description
           "If there are too many objects that could be returned by
            the selection filter, this identifies the upper limit of
            the publisher's ability to service this subscription.";
       }
       leaf kilobytes-estimate {
         type uint32;
         description
           "If the returned information could be beyond the capacity
            of the publisher, this would identify the estimated
            data size that could result from this selection filter.";
       }
       leaf kilobytes-limit {
         type uint32;
         description
           "If the returned information would be beyond the capacity
            of the publisher, this identifies the upper limit of the
            publisher's ability to service this subscription.";
       }
     }
     
     /*
      * RPCs
      */

rpc resync-subscription {

if-feature "on-change";
description

"This RPC allows a subscriber of an active on-change

subscription to request a full push of objects.

A successful invocation results in a 'push-update' of all datastore nodes that the subscriber is permitted to access. This RPC can only be invoked on the same session on which the subscription is currently active. In the case of an error, a

          'resync-subscription-error' is sent as part of an error
          response.";
       input {
         leaf id {
           type sn:subscription-id;
           mandatory true;
           description
             "Identifier of the subscription that is to be resynced.";
         }
       }
     }
     
     rc:yang-data resync-subscription-error {
       container resync-subscription-error {
         description
           "If a 'resync-subscription' RPC fails, the subscription is
            not resynced and the RPC error response MUST indicate the
            reason for this failure.  This yang-data MAY be inserted as
            structured data in a subscription's RPC error response
            to indicate the reason for the failure.";
         leaf reason {
           type identityref {
             base resync-subscription-error;
           }
           mandatory true;
           description
             "Indicates the reason why the publisher has declined a
              request for subscription resynchronization.";
         }
         uses hints;
       }
     }
     
     augment "/sn:establish-subscription/sn:input" {
       description
         "This augmentation adds additional subscription parameters
          that apply specifically to datastore updates to RPC input.";
       uses update-policy;
     }

augment "/sn:establish-subscription/sn:input/sn:target" {

       description
         "This augmentation adds the datastore as a valid target
          for the subscription to RPC input.";
       case datastore {
         description
           "Information specifying the parameters of a request for a
            datastore subscription.";
         uses datastore-criteria;
       }
     }
     
     rc:yang-data establish-subscription-datastore-error-info {
       container establish-subscription-datastore-error-info {
         description
           "If any 'establish-subscription' RPC parameters are
            unsupportable against the datastore, a subscription is not
            created and the RPC error response MUST indicate the reason
            why the subscription failed to be created.  This yang-data
            MAY be inserted as structured data in a subscription's
            RPC error response to indicate the reason for the failure.
            This yang-data MUST be inserted if hints are to be provided
            back to the subscriber.";
         leaf reason {
           type identityref {
             base sn:establish-subscription-error;
           }
           description
             "Indicates the reason why the subscription has failed to
              be created to a targeted datastore.";
         }
         uses hints;
       }
     }
     
     augment "/sn:modify-subscription/sn:input" {
       description
         "This augmentation adds additional subscription parameters
          specific to datastore updates.";
       uses update-policy-modifiable;
     }
     
     augment "/sn:modify-subscription/sn:input/sn:target" {
       description
         "This augmentation adds the datastore as a valid target
          for the subscription to RPC input.";
       case datastore {
         description
           "Information specifying the parameters of a request for a
            datastore subscription.";
         uses datastore-criteria;
       }
     }
     
     rc:yang-data modify-subscription-datastore-error-info {
       container modify-subscription-datastore-error-info {
     
         description
           "This yang-data MAY be provided as part of a subscription's
            RPC error response when there is a failure of a
            'modify-subscription' RPC that has been made against a
            datastore.  This yang-data MUST be used if hints are to be
            provided back to the subscriber.";
         leaf reason {
           type identityref {
             base sn:modify-subscription-error;
           }
           description
             "Indicates the reason why the subscription has failed to
              be modified.";
         }
         uses hints;
       }
     }
     
     /*
      * NOTIFICATIONS
      */

notification push-update {

       description
         "This notification contains a push update that in turn contains
          data subscribed to via a subscription.  In the case of a
          periodic subscription, this notification is sent for periodic
          updates.  It can also be used for synchronization updates of
          an on-change subscription.  This notification shall only be
          sent to receivers of a subscription.  It does not constitute
          a general-purpose notification that would be subscribable as
          part of the NETCONF event stream by any receiver.";
       leaf id {
         type sn:subscription-id;
         description
           "This references the subscription that drove the
            notification to be sent.";
       }
       anydata datastore-contents {
         description
           "This contains the updated data.  It constitutes a snapshot
            at the time of update of the set of data that has been
            subscribed to.  The snapshot corresponds to the same
            snapshot that would be returned in a corresponding 'get'
            operation with the same selection filter parameters
            applied.";
       }
       leaf incomplete-update {
     
         type empty;
         description
           "This is a flag that indicates that not all datastore
            nodes subscribed to are included with this update.  In
            other words, the publisher has failed to fulfill its full
            subscription obligations and, despite its best efforts, is
            providing an incomplete set of objects.";
       }
     }
     
     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 that would be subscribable as part of the
          NETCONF event stream by any receiver.";
       leaf id {
         type sn:subscription-id;
         description
           "This references the subscription that drove the
            notification to be sent.";
       }
       container datastore-changes {
         description
           "This contains the set of datastore changes of the target
            datastore, starting at the time of the previous update, per
            the terms of the subscription.";
         uses ypatch:yang-patch;
       }
       leaf incomplete-update {
         type empty;
         description
           "The presence of this object indicates that not all changes
            that have occurred since the last update are included with
            this update.  In other words, the publisher has failed to
            fulfill its full subscription obligations -- for example,
            in cases where it was not able to keep up with a burst of
            changes.";
       }
     }

augment "/sn:subscription-started" {

       description
         "This augmentation adds datastore-specific objects to
          the notification that a subscription has started.";
       uses update-policy;
     }
     
     augment "/sn:subscription-started/sn:target" {
       description
         "This augmentation allows the datastore to be included as
          part of the notification that a subscription has started.";
       case datastore {
         uses datastore-criteria {
           refine "selection-filter/within-subscription" {
             description
               "Specifies the selection filter and where it originated
                from.  If the 'selection-filter-ref' is populated, the
                filter in the subscription came from the 'filters'
                container.  Otherwise, it is populated in-line as part
                of the subscription itself.";
           }
         }
       }
     }
     
     augment "/sn:subscription-modified" {
       description
         "This augmentation adds datastore-specific objects to
          the notification that a subscription has been modified.";
       uses update-policy;
     }
     
     augment "/sn:subscription-modified/sn:target" {
       description
         "This augmentation allows the datastore to be included as
          part of the notification that a subscription has been
          modified.";
       case datastore {
         uses datastore-criteria {
           refine "selection-filter/within-subscription" {
             description
               "Specifies the selection filter and where it originated
                from.  If the 'selection-filter-ref' is populated, the
                filter in the subscription came from the 'filters'
                container.  Otherwise, it is populated in-line as part
                of the subscription itself.";
           }
         }
       }
     }
     
     /*
      * DATA NODES
      */
     
     augment "/sn:filters" {
       description
         "This augmentation allows the datastore to be included as part
          of the selection-filtering criteria for a subscription.";
       list selection-filter {
         key "filter-id";
         description
           "A list of preconfigured filters that can be applied
            to datastore subscriptions.";
         leaf filter-id {
           type string;
           description
             "An identifier to differentiate between selection
              filters.";
         }
         uses selection-filter-types;
       }
     }
     
     augment "/sn:subscriptions/sn:subscription" {
       when 'yp:datastore';
       description
         "This augmentation adds objects to a subscription that are
          specific to a datastore subscription, i.e., a subscription to
          a stream of datastore node updates.";
       uses update-policy;
     }
   
     augment "/sn:subscriptions/sn:subscription/sn:target" {
       description
         "This augmentation allows the datastore to be included as
          part of the selection-filtering criteria for a subscription.";
       case datastore {
         uses datastore-criteria;
       }
     }
   }
   
   <CODE ENDS>

6. IANA Considerations

This document registers the following namespace URI in the "IETF XML Registry" [RFC3688]:

   URI: urn:ietf:params:xml:ns:yang:ietf-yang-push

Registrant Contact: The IESG.

XML:

N/A; the requested URI is an XML namespace.

This document registers the following YANG module in the "YANG Module Names" registry [RFC6020]:

Name:

         ietf-yang-push
   
   Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-push

Prefix:

           yp

Reference:

              RFC 8641

7. Security Considerations

The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC8446].

The Network Configuration Access Control Model (NACM) [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.

There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability. (It should be noted that the YANG module defined in this document augments the YANG module defined in [RFC8639]. All security considerations that are listed in

[RFC8639] are also relevant for datastore subscriptions. In the following list, we focus on the new data nodes that are introduced in this document.)

• Subtree "selection-filter" under container "filters": This subtree allows a subscriber to specify which objects or subtrees to include in a datastore subscription. An attacker could attempt to modify the filter. For example, the filter might be modified to result in very few objects being filtered in order to attempt to overwhelm the receiver. Alternatively, the filter might be modified to result in certain objects being excluded from updates, in which case certain changes would go unnoticed.

• Subtree "datastore" in choice "target" in list "subscription": Analogous to "selection filter", an attacker might attempt to modify the objects being filtered in order to overwhelm a receiver with a larger volume of object updates than expected or cause certain changes to go unnoticed.

• Choice "update-trigger" in list "subscription": By modifying the update trigger, an attacker might alter the updates that are being sent in order to confuse a receiver, withhold certain updates to be sent to the receiver, and/or overwhelm a receiver. For example, an attacker might modify the period with which updates are reported for a periodic subscription, or it might modify the dampening period for an on-change subscription, resulting in a greater delay for successive updates (potentially affecting the responsiveness of applications that depend on the updates) or in a high volume of updates (to exhaust receiver resources).

The NACM provides one means to mitigate these threats on the publisher side. In order to address those threats as a subscriber, the subscriber could monitor the subscription configuration for any unexpected changes and subscribe to updates to the YANG datastore nodes that represent its datastore subscriptions. As this volume of data is small, a paranoid subscriber could even revert to occasional polling to guard against a compromised subscription against subscription configuration updates itself.

Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability:

• Subtree "selection-filter" under container "filters": If access control is not properly configured, can expose system internals to those who should not have access to this information.

• Subtree "datastore" in choice "target" in list "subscription": If access control is not properly configured, can expose system internals to those who should not have access to this information.

• Choice "update-trigger" in list "subscription": If access control is not properly configured, can expose system internals to those who should not have access to this information.

Some of the RPC operations in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control access to these operations. These are the operations and their sensitivity/vulnerability:

• RPC "resync-subscription": This RPC allows a subscriber of an on-change subscription to request a full push of objects in the subscription's scope. This can result in a large volume of data. An attacker could attempt to use this RPC to exhaust resources on the server to generate the data and could then attempt to overwhelm a receiver with the resulting large volume of data.

8. References

8.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.
   
   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.
   
   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.
   
   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.
   
   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.
   
   [RFC8072]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
              Media Type", RFC 8072, DOI 10.17487/RFC8072,
              February 2017, <https://www.rfc-editor.org/info/rfc8072>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.
   
   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.
   
   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.
   
   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.
   
   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.
   
   [RFC8639]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Subscription to YANG Notifications",
              RFC 8639, DOI 10.17487/RFC8639, September 2019,
              <https://www.rfc-editor.org/info/rfc8639>.

[W3C.REC-xml-20081126]

              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation
              REC-xml-20081126, November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126>.
   
   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", November 1999,
              <https://www.w3.org/TR/1999/REC-xpath-19991116>.

8.2. Informative References

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.
   
   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.
   
   [RFC7923]  Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
              for Subscription to YANG Datastores", RFC 7923,
              DOI 10.17487/RFC7923, June 2016,
              <https://www.rfc-editor.org/info/rfc7923>.
   
   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.
   
   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.
   
   [RFC8640]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Dynamic Subscription to YANG Events
              and Datastores over NETCONF", RFC 8640,
              DOI 10.17487/RFC8640, September 2019,
              <https://www.rfc-editor.org/info/rfc8640>.

[Yang-Push-Notif-Cap]

              Lengyel, B., Clemm, A., and B. Claise, "Yang-Push
              Notification Capabilities", Work in Progress,
              draft-ietf-netconf-notification-capabilities-04,
              September 2019.

Appendix A. Subscription Errors

A.1. RPC Failures

Rejection of an RPC for any reason is indicated via an RPC error response from the publisher. Valid RPC errors returned include both (1) existing transport-layer RPC error codes, such as those seen with NETCONF in [RFC6241] and (2) subscription-specific errors, such as those defined in the YANG data model. As a result, how subscription errors are encoded in an RPC error response is transport dependent.

References to specific identities in the ietf-subscribed- notifications YANG module [RFC8639] or the ietf-yang-push YANG module may be returned as part of the error responses resulting from failed attempts at datastore subscription. For errors defined as part of the ietf-subscribed-notifications YANG module, please refer to [RFC8639]. The errors defined in this document, grouped per RPC, are as follows:

      establish-subscription          modify-subscription
      ---------------------------     ---------------------
      
       cant-exclude                    period-unsupported
       datastore-not-subscribable      update-too-big
       on-change-unsupported           sync-too-big
       on-change-sync-unsupported      unchanging-selection
       period-unsupported
       update-too-big                 resync-subscription
       sync-too-big                   ----------------------------
       unchanging-selection            no-such-subscription-resync
                                       sync-too-big

There is one final set of transport-independent RPC error elements included in the YANG data model. These are the four yang-data structures for failed datastore subscriptions:

   1.  yang-data "establish-subscription-error-datastore": This MUST be
       returned if information identifying the reason for an RPC error
       has not been placed elsewhere in the transport portion of a
       failed "establish-subscription" RPC response.  This MUST be sent
       if hints are included.
   
   2.  yang-data "modify-subscription-error-datastore": This MUST be
       returned if information identifying the reason for an RPC error
       has not been placed elsewhere in the transport portion of a
       failed "modify-subscription" RPC response.  This MUST be sent if
       hints are included.
   
   3.  yang-data "sn:delete-subscription-error": This MUST be returned
       if information identifying the reason for an RPC error has not
       been placed elsewhere in the transport portion of a failed
       "delete-subscription" or "kill-subscription" RPC response.

4. yang-data "resync-subscription-error": This MUST be returned if information identifying the reason for an RPC error has not been placed elsewhere in the transport portion of a failed "resync-subscription" RPC response.

A.2. Failure Notifications

A subscription may be unexpectedly terminated or suspended independently of any RPC or configuration operation. In such cases, indications of such a failure MUST be provided. To accomplish this, a number of errors can be returned as part of the corresponding subscription state change notification. For this purpose, the following error identities are introduced in this document, in addition to those that were already defined in [RFC8639]:

   subscription-terminated        subscription-suspended
   ---------------------------    ----------------------
   
    datastore-not-subscribable     period-unsupported
    unchanging-selection           update-too-big
                                   synchronization-size

Acknowledgments

For their valuable comments, discussions, and feedback, we wish to acknowledge Tim Jenkins, Martin Bjorklund, Kent Watsen, Susan Hares, Yang Geng, Peipei Guo, Michael Scharf, Guangying Zheng, Tom Petch, Henk Birkholz, Reshad Rahman, Qin Wu, Rohit Ranade, and Rob Wilton.

Contributors

The following individuals made substantial contributions to this document and should be considered coauthors. Their contributions include information contained in the YANG module provided in Section 5 of this document.

   Alberto Gonzalez Prieto
   Microsoft
   Email: alberto.gonzalez@microsoft.com
   
   Ambika Prasad Tripathy
   Cisco Systems
   Email: ambtripa@cisco.com
   
   Einar Nilsen-Nygaard
   Cisco Systems
   Email: einarnn@cisco.com
   
   Andy Bierman
   YumaWorks
   Email: andy@yumaworks.com
   
   Balazs Lengyel
   Ericsson
   Email: balazs.lengyel@ericsson.com

Authors' Addresses

Alexander Clemm
Futurewei

Email:

          ludwig@clemm.org

Eric Voit
Cisco Systems

Email:

          evoit@cisco.com