NETCONF A. Clemm
Internet-Draft Huawei
Intended status: Standards Track E. Voit
Expires: January 2, 2019 Cisco Systems
A. Gonzalez Prieto
VMware
A. Tripathy
E. Nilsen-Nygaard
Cisco Systems
A. Bierman
YumaWorks
B. Lengyel
Ericsson
July 1, 2018

YANG Datastore Subscription
draft-ietf-netconf-yang-push-17

Abstract

Via the mechanism described in this document, subscriber applications may request a continuous, customized stream of updates from a YANG datastore. Providing such visibility into changes made upon YANG configuration and operational objects enables new capabilities based on the remote mirroring of configuration and operational state.

Status of This Memo

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

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This Internet-Draft will expire on January 2, 2019.

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Table of Contents

1. Introduction

Traditional approaches to remote visibility 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:

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 allows applications to subscribe to updates from a datastore and that enables 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 "Custom Subscription to Event Streams" [I-D.draft-ietf-netconf-subscribed-notifications]. Supplementing that work are YANG data model augmentations, extended RPCs, and new datastore specific update notifications. Transport options for [I-D.draft-ietf-netconf-subscribed-notifications] will work seamlessly with this solution.

2. Definitions and Acronyms

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

3. Solution Overview

This document specifies a solution that provides 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 should be sent and what data to include in update records. YANG objects 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 data model that is itself defined in YANG. This model enhances the subscription model defined in [I-D.draft-ietf-netconf-subscribed-notifications] 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:

3.2. Negotiation of Subscription Policies

A dynamic subscription request SHOULD be declined if a publisher's assessment is 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 at different parameters by a subscriber is to be discouraged. Therefore, in order to minimize the number of subscription iterations between subscriber and publisher, dynamic subscription supports a simple negotiation between subscribers and publishers for subscription parameters. This negotiation is in the form of supplemental information which may be inserted within 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 or objects which would be returned from a proposed selection filter. However, there are no guarantees that subsequent requests which consider these hints will be accepted.

3.3. On-Change Considerations

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

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 that do support on-change updates whereas other objects are excluded from update records, whether or not their values actually 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 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 in case the scope of the subscription contains objects for which on-change is not supported. In case of a configured subscription, the subscription MAY be suspended.

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 current values of all changed objects which are current at the time the dampening period expires. Changed objects include those which 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, the last change will still be sent. This will indicate 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, following is the conceptual process for creating an push-change-update notification:

  1. Just before a change, or at the start of a dampening period, evaluate any filtering and any access control rules. 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 a YANG patch record for going from A to B.
  4. If there were any changes made between A and B which canceled each other out, insert into the YANG patch record the last change made for any object which otherwise wouldn't have appeared.
  5. If the resulting 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, multiple subscriptions with different objects in a selection filter can be created.

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 entered into with good faith. For example, the volume of data objects 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 that is defined in this document mandates that a publisher notifies receivers immediately and reliably whenever it encounters a situation in which it is unable to keep the terms of the subscription, and provides the publisher with the option to suspend the subscription in such a case.

A publisher SHOULD reject a request for a subscription if it is unlikely that the publisher will be able 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.

3.5. Data Encodings

3.5.1. Periodic Subscriptions

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

3.5.2. On-Change Subscriptions

In an on-change subscription, updates 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 operation as specified in [RFC8072]. The YANG-patch will describe what needs to be applied to the earlier state reported by the preceding update, to result in the now-current state. Note that contrary to [RFC8072], objects encapsulated are not restricted to configuration objects only.

A publisher will indicate a change to the effect that a value of a datstore node has been updated by indicating a "replace" operation (applied to the datastore node) in the patch. When a new datastore node was created (other than an element in a list), a publisher will indicate a "create" operation in the patch. When a datastore node was deleted (other than an element in a list), the publisher indicates this by a "delete". When a new list element was created or removed, the publisher indicates it by an "insert" or "remove", respectively.

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 if 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 and current state. This indicates that some churn has occurred on the object. An example of this would be a patch that does a "create" operation for a datastore node where the receiver believes one already exists, or a "merge" operation which replaces a previous value with the same value. Note that this means that the "create" and "delete" errors described in [RFC8072] section 2.5 are not errors, and are valid operations with 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 MUST remove any existing filter. The following selection filter types are included in the yang-push data model, and may be applied against a datastore:

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. As an example, consider an XPath filter with a boolean result; such a result will not provide an easily interpretable subset of a datastore. Beyond the boolean example, it is quite possible to define an XPath filter where results are easy for an application to misinterpret. Consider an XPath filter which only passes a datastore object when an interface is up. It is up to the receiver to understand implications of the presence or absence of objects in each update.

When the set of selection filtering criteria is applied for a periodic subscription, all selected datastore nodes to which a receiver has access are provided to that receiver. If the same filtering criteria is applied to an on-change subscription, only the subset of those datastore nodes supporting on-change is provided. A datastore node which doesn't support on-change is never sent as part of an on-change subscription's "push-update" or "push-change-update".

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: "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 will be used as the initial "push-update" if there is a need to synchronize the receiver at the start of a new subscription. It also MAY be sent if the publisher later chooses to resynch 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, deleted, or have had changes to their values. In cases where multiple changes have occurred 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.)

These new "push-update" or "push-change-update" are encoded and placed within notification messages, and ultimately queued for egress over the specified transport.

The following is an example of a notification message for a subscription tracking the operational status of a single Ethernet port (per [RFC8343]). This notification message is encoded XML over NETCONF as per [I-D.draft-ietf-netconf-netconf-event-notifications].

 
<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">
   <subscription-id>1011</subscription-id>
   <datastore-contents>
     <interfaces-state xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
       <interface>
         <name>eth0</name>
         <oper-status>up</oper-status>
       </interface> 
     </interfaces-state> 
   </datastore-contents>
 </push-update>
</notification>
          

Figure 1: Push example

The following is 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">
   <subscription-id>89</subscription-id>
   <datastore-changes>
     <yang-patch xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-patch">
       <patch-id>1</patch-id>
       <edit>
         <edit-id>edit1</edit-id>
         <operation>merge</operation>
         <target>/ietf-interfaces:interfaces-state</target>
         <value>
           <interfaces-state xmlns="http://foo.com/ietf-interfaces">
             <interface>
               <name>eth0</name>
               <oper-status>down</oper-status>
             </interface> 
           </interfaces-state>
         </value>
       </edit>
     </yang-patch>
   </datastore-changes>
 </push-change-update>
</notification>
          

Figure 2: Push example for on change

Of note in the above example is the 'patch-id' with a value of '1'. Per [RFC8072], the 'patch-id' is an arbitrary string. With YANG Push, the publisher SHOULD put into the 'patch-id' a counter starting at '1' which increments with every 'push-change-update' generated for a subscription. If used as a counter, this counter MUST be reset to '1' anytime a resynchronization occurs (i.e., with the sending of a 'push-update'). Also if used as a counter, the counter MUST be reset to '1' the after passing a maximum value of '4294967295' (i.e. maximum value that can be represented using uint32 data type). Such a mechanism allows easy identification of lost or out-of-sequence update records.

3.8. Subscription Management

The RPCs defined within [I-D.draft-ietf-netconf-subscribed-notifications] 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. This includes a too large subtree request, or the inability of the publisher to push update records as frequently as requested. In such cases, no subscription is established. Instead, the subscription-result with the failure reason 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 these as part of future subscription attempts.

In the case of a rejected request for an establishment of a datastore subscription, the hints MUST be transported within 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 case of a stream subscription.

Below is 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-datasore-error-info
             +--ro reason?                identityref
             +--ro period-hint?           yang:timeticks
             +--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: Tree diagram for establish-subscription-datastore-error-info

Similarly, in the case of a rejected request for modification of a datastore subscription, the hints MUST be transported within 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 case of a stream subscription.

Below is a tree diagram for "modify-subscription-datastore-error-info".

 
       yang-data modify-subscription-datastore-error-info
          +--ro modify-subscription-datasore-error-info
             +--ro reason?                identityref
             +--ro period-hint?           yang:timeticks
             +--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: Tree diagram for modify-subscription-datastore-error-info

3.9. Receiver Authorization

A receiver of subscription data MUST only be sent updates for which they have proper authorization. A publisher MUST ensure that no non-authorized 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 non-authorized data from datastore subtrees. This enables YANG data pushed based on subscriptions to be authorized equivalently to a regular data retrieval (get) operation.

A publisher MUST allow for the possibility that a subscription's selection filter references non-existent or access-protected data. Such support permits a receiver the ability to monitor the entire lifecyle of some datastore tree. In this case, all "push-update" notifications must be sent empty, and no "push-change-update" notifications will be sent until some data becomes visible for a receiver.

A publisher MAY choose reject an establish-subscription request which selects non-existent or access-protected data. 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. Such a capability enables the publisher to avoid having to support a continuous, and total filtering of an entire subscription's content.

In these cases above, the error identity "unchanging-selection" SHOULD be returned. This reduces the possibility of leakage of access controlled objects.

Each "push-update" and "push-change-update" MUST have access control applied. This includes validating that read access is permitted for any new objects selected since the last notification message was sent to a particular each receiver. To accomplish this, implementations SHOULD support the conceptual authorization model of [RFC8342], specifically section 3.2.4.

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

Figure 5: Updated [rfc6536bis] access control for push updates

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 re-initialization so that appropriate filtering is installed.

3.10. On-change Notifiable YANG objects

In some cases, a publisher supporting on-change notifications may not be able to push updates for some object types on-change. Reasons for this might be that the value of the datastore node changes frequently (e.g., [RFC8343]'s in-octets counter), that small object changes are frequent and meaningless (e.g., a temperature gauge changing 0.1 degrees), or that 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. It is expected that such a solution will be standardized at some point in the future. In the meantime and until this occurs, implementations will be expected to provide their own solution.

3.11. Other Considerations

3.11.1. Robustness and reliability

Particularly in the case of on-change updates, it is important that these updates do not get lost. Or in case the loss of an update is unavoidable, it is critical that the receiver is 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 within an update record the full set of objects desired per the terms of a subscription. In this case, the publisher MUST take one or more of the following actions.

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 to merge pending update messages. I.e., the dampening period applies to update record creation, not transmission.

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 the following figure. The tree diagram follows the notiation defined in [RFC8340]. New schema objects defined here (i.e., beyond those from [I-D.draft-ietf-netconf-subscribed-notifications]) are identified with "yp". For the reader's convenience, in order to compact the tree representation, some nodes that are defined in ietf-subscribed-notifications and that are not essential to the understanding of the data model defined here have been removed. This is indicated by "..." in the diagram where applicable.

module: ietf-subscribed-notifications
    ...
    +--rw filters
    |  ...
    |  +--rw yp:selection-filter* [identifier]
    |     +--rw yp:identifier                  sn:filter-id
    |     +--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* [identifier]
          |  ...
          +--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         yang:timeticks
             |     +--rw yp:anchor-time?   yang:date-and-time
             +--:(yp:on-change) {on-change}?
                +--rw yp:on-change!
                   +--rw yp:dampening-period?    yang:timeticks
                   +--rw yp:no-synch-on-start?   empty
                   +--rw yp:excluded-change*     change-type

  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         yang:timeticks
    |  |     |     +---w yp:anchor-time?   yang:date-and-time
    |  |     +--:(yp:on-change) {on-change}?
    |  |        +---w yp:on-change!
    |  |           +---w yp:dampening-period?    yang:timeticks
    |  |           +---w yp:no-synch-on-start?   empty
    |  |           +---w yp:excluded-change*     change-type
    |  +--ro output
    |     +--ro identifier    subscription-id
    +---x modify-subscription
    |  +---w input
    |     ...
    |     +---w (target)
    |     |  ...
    |     |  +--:(yp:datastore)
    |     |     +---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         yang:timeticks
    |        |     +---w yp:anchor-time?   yang:date-and-time
    |        +--:(yp:on-change) {on-change}?
    |           +---w yp:on-change!
    |              +---w yp:dampening-period?   yang:timeticks
    +---x delete-subscription
    |  ...
    +---x kill-subscription
       ...

  yang-data (for placement into rpc error responses)
    ...
             
  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         yang:timeticks
    |     |     +--ro yp:anchor-time?   yang:date-and-time
    |     +--:(yp:on-change) {on-change}?
    |        +--ro yp:on-change!
    |           +--ro yp:dampening-period?    yang:timeticks
    |           +--ro yp:no-synch-on-start?   empty
    |           +--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         yang:timeticks
    |     |     +--ro yp:anchor-time?   yang:date-and-time
    |     +--:(yp:on-change) {on-change}?
    |        +--ro yp:on-change!
    |           +--ro yp:dampening-period?    yang:timeticks
    |           +--ro yp:no-synch-on-start?   empty
    |           +--ro yp:excluded-change*     change-type
    +---n subscription-terminated
    |  ...
    +---n subscription-suspended
       ...

module: ietf-yang-push

  rpcs:
    +---x resynch-subscription {on-change}?
       +---w input
          +---w identifier    sn:subscription-id

  yang-data: (for placement into rpc error responses)
    +-- resynch-subscription-error
    |  +--ro reason?                   identityref
    |  +--ro period-hint?              timeticks
    |  +--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?              timeticks
    |  +--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?              timeticks
       +--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 subscription-id?      sn:subscription-id
    |  +--ro incomplete-update?     empty
    |  +--ro datastore-contents?   <anydata>
    +---n push-change-update {on-change}?
       +--ro subscription-id?     sn:subscription-id
       +--ro incomplete-update?    empty
       +--ro datastore-changes?   <anydata>

        

Figure 6: Model structure

Selected components of the model are summarized below.

4.2. Subscription configuration

Both configured and dynamic subscriptions are represented within the list "subscription". New parameters extending the basic subscription data model in [I-D.draft-ietf-netconf-subscribed-notifications] include:

4.3. YANG Notifications

4.3.1. State Change Notifications

Subscription state notifications and mechanism are reused from [I-D.draft-ietf-netconf-subscribed-notifications]. 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 which might be part of a "push-update" or "push-change-update" notification.

A "subscription-id" MUST be transported along with the subscribed contents. An [RFC5277] Section 4 one-way notification MAY be used for encoding updates. Where it is, the relevant "subscription-id" MUST be encoded as the first element within each "push-update" or "push-change-update". This allows a receiver to differentiate which subscription resulted in a particular push.

A "time-of-update" which represents the time an update record snapshot was generated. A receiver MAY assume that a publisher's objects have these pushed values at this point in time.

An "incomplete-update" leaf. This leaf indicates that not all changes which 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 providing the full set of datastore nodes to a publisher process.) To facilitate re-synchronization 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 [I-D.draft-ietf-netconf-subscribed-notifications].

4.4.1. Establish-subscription RPC

The subscriber sends an establish-subscription RPC with the parameters 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>
      <yp:source xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
        ds:operational
      </yp:source>
      <xpath-filter 
          xmlns:ex="http://example.com/sample-data/1.0"
          select="/ex:foo"/>
    </yp:datastore>
    <yp:periodic>
      <yp:period>500</yp:period>
    </yp:periodic>
 </establish-subscription>
</netconf:rpc>          
    

Figure 7: Establish-subscription RPC

A positive response includes the "identifier" of the accepted subscription. In that case a publisher MAY respond:

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

Figure 8: 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 which help a subscriber understand subscription parameters might have been accepted for the request. These hints would be included within the yang-data structure "establish-subscription-error-datastore". However even with these hints, there are no guarantee that subsequent requests will in fact be accepted.

The specific parameters to be returned in as part of the RPC error response depend on the specific transport that is used to manage the subscription. In the case of NETCONF [I-D.draft-ietf-netconf-netconf-event-notifications], when a subscription request is rejected, the NETCONF RPC reply MUST include an "rpc-error" element with the following elements:

For example, for the following request:

<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 netconf:type="xpath" 
    xmlns:ex="http://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>
</netconf:rpc>
          

Figure 9: Establish-subscription request example 2

a publisher that cannot serve on-change updates but periodic updates might return the following:

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

Figure 10: 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. Below is an example where a subscriber attempts to modify the "period" of a subscription.

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

Figure 11: Modify subscription request

The publisher MUST respond explicitly positively or negatively to the request. If the subscription modification is rejected, the subscription is maintained as it was before the modification request. In addition, the publisher MUST send an rpc error response. This rpc error response may contain hints encapsulated within the yang-data structure "modify-subscription-error-datastore". A subscription MAY be modified multiple times.

The specific parameters to be returned in as part of the RPC error response depend on the specific transport that is used to manage the subscription. In the case of NETCONF [I-D.draft-ietf-netconf-netconf-event-notifications], when a subscription request is rejected, the NETCONF RPC reply MUST include an "rpc-error" element with the following elements:

A configured subscription cannot be modified using "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 only input the subscription's "identifier". This RPC is unmodified from [I-D.draft-ietf-netconf-subscribed-notifications].

4.4.4. Resynch-subscription RPC

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

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

Resynch subscription

On receipt, a publisher must either accept the request and quickly follow with a "push-update", or send an appropriate error within an rpc error response. Within an error response, the publisher may include supplemental information about the reasons within the yang-data structure "resynch-subscription-error".

4.4.5. YANG Module Synchronization

To make subscription requests, the subscriber needs to know the YANG module library available on the publisher. The YANG 1.0 module library information is sent by a NETCONF server in the NETCONF "hello" message. For YANG 1.1 modules and all modules used with the RESTCONF [RFC8040] protocol, this information is provided by the YANG Library module (ietf-yang-library.yang from [RFC7895]. This YANG library information is important for the receiver to reproduce the set of object definitions used within the publisher.

The YANG library includes a module list with the name, revision, enabled features, and applied deviations for each YANG module implemented by the publisher. The receiver is expected to know the YANG library information before starting a subscription. The "/modules-state/module-set-id" leaf in the "ietf-yang-library" module can be used to cache the YANG library information.

The set of modules, revisions, features, and deviations can change at run-time (if supported by the publisher implementation). In this case, the receiver needs to be informed of module changes before datastore nodes from changed modules can be processed correctly. The YANG library provides a simple "yang-library-change" notification that informs the subscriber that the library has changed. The receiver then needs to re-read the entire YANG library data for the replicated publisher in order to detect the specific YANG library changes. The "ietf-netconf-notifications" module defined in [RFC6470] contains a "netconf-capability-change" notification that can identify specific module changes. For example, the module URI capability of a newly loaded module will be listed in the "added-capability" leaf-list, and the module URI capability of an removed module will be listed in the "deleted-capability" leaf-list.

5. YANG module

<CODE BEGINS> file "ietf-yang-push@2018-07-01.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
	  "draft-ietf-netconf-subscribed-notifications: 
	   Customized Subscriptions to a Publisher's Event Streams
	   
	   NOTE TO RFC Editor: Please replace above reference to 
	   draft-ietf-netconf-subscribed-notifications with RFC number
	   when published (i.e. RFC xxxx).";
  }
  import ietf-datastores {
    prefix ds;
	reference
	  "RFC 8342: Network Management Datastore Architecture (NMDA)";
  }
  import ietf-restconf   { 
    prefix rc;   
	reference
      "RFC 8040: RESTCONF Protocol";	
  }  
  
  organization "IETF";
  contact
    "WG Web:   <http://tools.ietf.org/wg/netconf/>
     WG List:  <mailto:netconf@ietf.org>

     Editor:   Alexander Clemm
               <mailto:ludwig@clemm.org>
     
     Editor:   Eric Voit
               <mailto:evoit@cisco.com>

     Editor:   Alberto Gonzalez Prieto
               <mailto:agonzalezpri@vmware.com>
               
     Editor:   Ambika Prasad Tripathy
               <mailto:ambtripa@cisco.com>
               
     Editor:   Einar Nilsen-Nygaard
               <mailto:einarnn@cisco.com>
               
     Editor:   Andy Bierman
               <mailto:andy@yumaworks.com>
     
     Editor:   Balazs Lengyel
               <mailto:balazs.lengyel@ericsson.com>";
               
  description
    "This module contains YANG specifications for YANG push.
	
	Copyright (c) 2018 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 
	draft-ietf-netconf-yang-push-17; see the RFC itself for full 
	legal notices.
	
	NOTE TO RFC EDITOR: Please replace above reference to 
	draft-ietf-netconf-yang-push-17 with RFC number when published 
	(i.e. RFC xxxx).";

  revision 2018-07-01 {
    description
      "Initial revision.
	  NOTE TO RFC EDITOR: 
	  (1)Please replace the above revision date to 
	  the date of RFC publication when published.  
	  (2) Please replace the date in the file name  
	  (ietf-yang-push@2018-07-01.yang) to the date of RFC publication. 
	  (3) Please replace the following reference to 
	  draft-ietf-netconf-yang-push-17 with RFC number when published 
	  (i.e. RFC xxxx).";
    reference 
      "draft-ietf-netconf-yang-push-17";
  }

 /*
  * FEATURES
  */
    
  feature on-change {
    description
      "This feature indicates that on-change triggered subscriptions 
       are supported.";
  }

 /*
  * IDENTITIES
  */
  
  /* Error type identities for datastore subscription  */
  
  identity resynch-subscription-error {
     description
      "Problem found while attempting to fulfill an 
      'resynch-subscription' RPC request. "; 
  }      
  
  identity cant-exclude {
    base sn:establish-subscription-error;
    description 
      "Unable to remove the set of 'excluded-changes'.  This means the
      publisher is unable to restrict 'push-change-update's 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-resynch {
    base resynch-subscription-error;
    description
     "Referenced subscription doesn't exist. This may be as a result of 
      a non-existent subscription ID, an ID which belongs to another 
      subscriber, or an ID for configured subscription.";
  }
  
  identity on-change-unsupported {
    base sn:establish-subscription-error;
    description 
      "On-change is not supported for any objects which are selectable 
      by this filter.";
  }
  
  identity on-change-synch-unsupported {
    base sn:establish-subscription-error;
    description 
      "Neither synch on start nor resynchronization are supported for 
      this subscription.  This error will be used for two reasons. 
      First if an 'establish-subscription' RPC doesn't include 
      'no-synch-on-start', yet the publisher can't support sending a 
      'push-update' for this subscription for reasons other than 
      'on-change-unsupported' or 'synchronization-size'.  And second, 
      if the 'resynch-subscription' RPC is invoked either for an 
      existing periodic subscription, or for an on-change subscription 
      which can't support resynchronization.";
  }

  identity period-unsupported {
    base sn:establish-subscription-error;
    base sn:modify-subscription-error;
    base sn:subscription-suspended-reason;
    description 
      "Requested time 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 result-too-big {
    base sn:establish-subscription-error;
    base sn:modify-subscription-error;
    base sn:subscription-suspended-reason;
    description 
      "Periodic or on-change push update datatrees exceed a maximum 
      size limit.  Hints on estimated size of what was too big may 
      be returned as supplemental information.";
  }
  
  identity synchronization-size {
    base sn:establish-subscription-error;
    base sn:modify-subscription-error;
    base resynch-subscription-error;
    base sn:subscription-suspended-reason;
    description 
      "Synch-on-start or resynchronization datatree exceeds a maximum 
      size limit.  Hints on 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 
      "Selection filter is unlikely to ever select datatree nodes.  This
      means that based on the subscriber's current access rights, the 
      publisher recognizes that the selection filter is unlikely to ever 
      select datatree nodes which change. Examples for this might be 
      that 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 data node.";
      }
      enum "delete" {
        description
          "A change that refers to the deletion of a data node."; 
      }
      enum "insert" {
        description
          "A change that refers to the insertion of a new 
          user-ordered data node.";
      }
      enum "merge" {
        description
          "A change that refers to a merging of a new value with a
          target data node.";
      }
      enum "move" {
        description
          "A change that refers to a reordering of the target data 
          node";
      }
      enum "replace" {
        description
          "A change that refers to a replacement of the target data
          node's value."; 
      }
      enum "remove" {
        description
          "A change that refers to the removal of a data node.";
      }
    }
    description
      "Specifies different types of datastore changes.";
    reference 
      "RFC 8072 section 2.5, with a delta that it is valid for a 
      receiver to process an update record which 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.";
  }

  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.";
  }

  /*
   * 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.";
      }
      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:

           o  The set of namespace declarations are those in scope on
              the 'datastore-xpath-filter' leaf element.
              
           o  The set of variable bindings is empty.
           
           o  The function library is 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.";
      }
    }   
  }
  
  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 come either referenced from a global list, or be 
        provided within the subscription itself.";
      case by-reference {
        description
          "Incorporate a filter that has been configured separately.";
        leaf selection-filter-ref {
          type selection-filter-ref;
          mandatory true;
          description
            "References an existing selection filter which is to be 
             applied to the subscription.";
        }
      }
      case within-subscription {
        description
          "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 an periodic subscription";
          description
            "The publisher is requested to notify periodically the  
            current values of the datastore as defined by the selection 
            filter.";
          leaf period {
            type yang:timeticks;
            mandatory true;
            description
              "Duration of time which should occur between periodic 
               push updates.";
          }
          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 whole multiple interval from the  
               anchor time.  For example, for an anchor time 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  
               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 changes in values in
             the datastore subset as defined by a selection filter.";
          leaf dampening-period {
            type yang:timeticks;
            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, 
              then any subscribed objects and properties which have 
              changed since the previous update record will have their 
              current values marshalled and placed into 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 not modifiable once subscription is 
           established.";
        leaf no-synch-on-start {
          type empty;
          description
            "The presence of this object restricts an on-change 
             subscription from sending push-update notifications.  When 
             present, 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 absent (default behavior), 
             in order to facilitate a receiver's synchronization, a full
             update is sent when the subscription starts using a 
             push-update notification.  After that, push-change-update 
             notifications are exclusively sent unless the publisher 
             chooses to resynch the subscription via a new push-update
             notification.";
        }
        leaf-list excluded-change {
          type change-type;
          description
            "Use to restrict which changes trigger an update.
             For example, if modify is excluded, only creation and
             deletion of objects is reported.";
        }
      }
    }
  }

  grouping hints {
    description 
      "Parameters associated with some error for a subscription made 
      upon a datastore."; 
    leaf period-hint { 
      type yang:timeticks;
      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 which could potentially be 
         returned by the selection filter, this identifies the estimate 
         of the number of objects which the filter would potentially 
         pass.";
    }
    leaf object-count-limit { 
      type uint32;
      description
        "If there are too many objects which could be returned by the 
         selection filter, this identifies the upper limit of the 
         publisher's ability to service for this subscription.";
    }
    leaf kilobytes-estimate { 
      type uint32;
      description
        "If the returned information could be beyond the capacity of  
         the publisher, this would identify the data size which 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 for this subscription.";
    }
  }  
  
  
  /*
   * RPCs
   */
 
   rpc resynch-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 objects that the subscriber is permitted to access.  
       This RPC can only be invoked on the same session on which the 
       subscription was established (using an establish-subscription 
       RPC).  In case of an error, a resynch-subscription-error is 
       sent as part of an error response.";
    input {
      leaf identifier {
        type sn:subscription-id;
        mandatory true;
        description
          "Identifier of the subscription that is to be resynched.";
      }
    }
  }

  rc:yang-data resynch-subscription-error {
    container resynch-subscription-error {
      description
        "If a 'resynch-subscription' RPC fails, the subscription is not 
        resynched and the RPC error response MUST indicate the reason 
        for this failure. This yang-data MAY be inserted as structured 
        data within a subscription's RPC error response to indicate the 
        failure reason.";
      leaf reason {
        type identityref {
          base resynch-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 an 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 within a subscription's RPC error 
        response to indicate the failure reason.  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 an request for a 
         datastore subscription.";                       
      uses selection-filter-objects;
    }
  }

  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 which has been made against a 
        datastore.  This yang-data MUST be used if hints are to be
        provides 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, containing data 
       subscribed to via a subscription. This notification is sent for 
       periodic updates, for a periodic subscription.  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 subscription-id {
      type sn:subscription-id;
      description
        "This references the subscription which drove the notification
         to be sent.";
    }
    leaf incomplete-update {
      type empty;
      description
        "This is a flag which 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.";
    }
    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.";
    }        
  }
  
  notification push-change-update {
    if-feature "on-change";
    description
      "This notification contains an on-change push update. This 
       notification shall only be sent to the receivers of a 
       subscription; it does not constitute a general-purpose
       notification.";
    leaf subscription-id {
      type sn:subscription-id;
      description
        "This references the subscription which drove the notification
         to be sent.";
    }
    leaf incomplete-update {
      type empty;
      description
        "The presence of this object indicates not all changes which
         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 change burst.";
    }
    anydata 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.
         The datastore changes are encoded per RFC 8027 
         (YANG Patch).";
    }
  }

  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 within 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 where the selection filter, and where it came 
            from within the subscription and then populated within this 
            notification. If the 'selection-filter-ref' is populated, 
            the filter within 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 pre-configured 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" {
    description
      "This augmentation adds many datastore specific objects to a 
       subscription.";
    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: draft-ietf-netconf-yang-push-17.txt (RFC form)

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 [RFC5246].

The NETCONF access control model [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:

8. 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, and Tom Petch.

9. References

9.1. Normative References

[I-D.draft-ietf-netconf-subscribed-notifications] Voit, E., Clemm, A., Gonzalez Prieto, A., Tripathy, A. and E. Nilsen-Nygaard, "Custom Subscription to Event Streams", Internet-Draft draft-ietf-netconf-subscribed-notifications-13, June 2018.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010.
[RFC6470] Bierman, A., "Network Configuration Protocol (NETCONF) Base Notifications", RFC 6470, DOI 10.17487/RFC6470, February 2012.
[RFC6991] Schoenwaelder, J., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013.
[RFC7895] Bierman, A., Bjorklund, M. and K. Watsen, "YANG Module Library", RFC 7895, DOI 10.17487/RFC7895, June 2016.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016.
[RFC8040] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017.
[RFC8072] Bierman, A., Bjorklund, M. and K. Watsen, "YANG Patch Media Type", RFC 8072, DOI 10.17487/RFC8072, February 2017.
[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, March 2018.
[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.

9.2. Informative References

[bergstra2014] Bergstra, J. and M. Burgess, "Promise Theory, Createspace Independent Publishers, ISBN-13 9781495437779", 2014.
[I-D.draft-ietf-netconf-netconf-event-notifications] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard, E. and A. Tripathy, "NETCONF Support for Event Notifications", May 2018.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011.
[RFC7923] Voit, E., Clemm, A. and A. Gonzalez Prieto, "Requirements for Subscription to YANG Datastores", RFC 7923, DOI 10.17487/RFC7923, June 2016.
[RFC8340] Bjorklund, M. and L. Berger, "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018.
[RFC8343] Bjorklund, M., "A YANG Data Model for Interface Management", RFC 8343, DOI 10.17487/RFC8343, March 2018.

Appendix A. Appendix A: Subscription Errors

A.1. RPC Failures

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

References to specific identities within the either the subscribed-notifications YANG model or the yang-push YANG model may be returned as part of the error responses resulting from failed attempts at datastore subscription. Following are valid errors per RPC (note: throughout this section the prefix 'sn' indicates an item imported from the subscribed-notifications.yang model):

    

establish-subscription         modify-subscription 
----------------------         -------------------  
 cant-exclude                   sn:filter-unsupported
 datastore-not-subscribable     sn:insufficient-resources
 sn:dscp-unavailable            sn:no-such-subscription          
 sn:filter-unsupported          period-unsupported    
 sn:insufficient-resources      result-too-big
 on-change-unsupported          synchronization-size
 on-change-synch-unsupported    unchanging-selection
 period-unsupported
 result-too-big                resynch-subscription
 synchronization-size          --------------------
 unchanging-selection           no-such-subscription-resynch
                                synchronization-size
 
delete-subscription            kill-subscription
----------------------         -----------------
 sn:no-such-subscription        sn:no-such-subscription
              

There is one final set of transport independent RPC error elements included in the YANG model. These are the following 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 within 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 within the transport 
      portion of a failed "modifiy-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 within the transport 
      portion of a failed "delete-subscription" or "kill-subscription" 
      RPC response. 

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

A.2. Notifications of Failure

A subscription may be unexpectedly terminated or suspended independent of any RPC or configuration operation. In such cases, indications of such a failure MUST be provided. To accomplish this, the following types of error identities may be returned within the corresponding subscription state change notification:

            
 
subscription-terminated        subscription-suspended
-----------------------        ---------------------- 
 datastore-not-subscribable     sn:insufficient-resources
 sn:filter-unavailable          period-unsupported
 sn:no-such-subscription        result-too-big
 sn:suspension-timeout          synchronization-size
 unchanging-selection           
              

Appendix B. Changes between revisions

(To be removed by RFC editor prior to publication)

v16 - v17

v15 - v16

v14 - v15

v13 - v14

v12 - v13

v11 - v12

v10 - v11

v09 - v10

v08 - v09

v07 - v08

v06 - v07

v05 - v06

v04 to v05

v03 to v04

Authors' Addresses

Alexander Clemm Huawei EMail: ludwig@clemm.org
Eric Voit Cisco Systems EMail: evoit@cisco.com
Alberto Gonzalez Prieto VMware EMail: agonzalezpri@vmware.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