RFC 8641

Subscription to YANG Notifications for Datastore Updates

📅 published September 2019, from ietf

RFC 8641 💖 Subscription to YANG Notifications for Datastore Updates

okay so bestie, this is RFC 8641, and the title is "Subscription to YANG Notifications for Datastore Updates" 🎀 The authors are A. Clemm at Futurewei and E. Voit at Cisco Systems ✨ It's a Standards Track doc from the Internet Engineering Task Force (IETF), it's ISSN 2070-1721, and it's dated September 2019 📅 The header doesn't list any Obsoletes or Updates fields, so it's not tagged as replacing or changing anything else 🙈 Its RFC number is 8641, obvi 💅

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 ⚖️

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

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

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 🦋

Okay, so traditional ways of seeing what's going on with managed devices from a remote system have been built on polling 🔁 With polling, a client periodically asks a server for data to stay up to date. But polling has some real issues:

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 way better move than polling is for an application to receive automatic and continuous updates from a targeted subset of a datastore ✨ So there's 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 that service are documented in [RFC7923].

This document defines the solution, and it's built on top of [RFC8639] 🧁 It adds YANG data model augmentations, extended RPCs, and new datastore-specific update notifications. The 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]. It also 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 🌈

Here's the big picture. This document specifies a solution that provides a subscription service for updates from a datastore 💜 It 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 then pushed from the publisher to the receiver per the terms of the subscription 💅✨

3.1. Subscription Model 💖

okay so YANG-Push subscriptions are defined using a YANG data model 🎀 This model builds on the subscription model from [RFC8639] and adds extra powers so subscribers can subscribe to datastore node updates. Specifically, they get to decide the update triggers (when to generate update records) AND what goes inside each update record 🦋 The key upgrades are:

Selection filters: these identify the targeted YANG datastore nodes and/or datastore subtrees you want updates pushed for 🌸
Update policies: these hold the conditions that trigger generating and pushing new update records. There are two types, sorted by how the updates get triggered: periodic and on-change ✨
- For periodic subscriptions, the trigger is set by two parameters: (1) the period interval for reporting updates, and (2) an "anchor-time", which is basically a reference point in time used to work out when periodic updates should get assembled and sent ⏰
- For on-change subscriptions, an update trigger happens whenever a change in the subscribed info is detected. These extra parameters come along for the ride:
  - dampening-period: okay this one is a lifesaver 🛟 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, because that behavior has the potential to exhaust resources in the publisher or receiver. To protect against that kind 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. So when there's 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 one is actually in effect). If multiple changes to a single object happen during a dampening period, only the value in effect when the update record is created gets included. The dampening period goes into effect every time the assembly of an update record is completed 💅
  - change-type: this can be used to reduce the types of datastore changes that trigger updates. For example, you might only send an update when an object is created or deleted, but not when its value changes 🙈
  - sync-on-start: this defines whether or not a complete "push-update" (Section 3.7) of all subscribed data gets sent at the beginning of a subscription. This early sync establishes the frame of reference for the updates that follow 🧚
An encoding (using anydata) for the contents of periodic and on-change push updates 🍓

3.2. Negotiation of Subscription Policies 🤝

so 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 can quickly follow up with a new RPC request using different parameters 🔁

random guessing of different parameters by a subscriber is to be discouraged 🙅‍♀️ Therefore, 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 takes the form of supplemental information that should be inserted into error responses to a failed RPC request. When considered, this returned error response information 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 which consider these hints will be accepted 😭

3.3. On-Change Considerations 🦋

on-change subscriptions let receivers get updates whenever changes to targeted objects occur 👀 As such, they 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, so they 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 that don't support on-change. In that case, updates are sent only for the objects within the scope that do support on-change updates, and the other objects are excluded from update records, even if their values change. For a subscriber to determine whether objects support on-change subscriptions, objects are marked accordingly on a publisher. So when subscribing, it is the responsibility of the subscriber to make sure it knows which objects support on-change and which do not. For more on how objects are marked, see Section 3.10 📌

Heads up: a publisher MAY also decide to simply reject an on-change subscription if the scope contains objects that don't support on-change. For a configured subscription, the publisher MAY suspend the subscription instead.

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 that point, for all changed objects. Changed objects include those 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 indicates that churn is occurring on that object 🌀

on-change subscriptions can be refined so 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 goes like this 💜

Just before a change, or at the start of a dampening period, evaluate any filtering and any access control rules to make sure 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.
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.
Construct an update record, which takes the form of a YANG Patch record [RFC8072] for going from A to B.
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 the changes made in the interim 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.
If the resulting YANG Patch record is non-empty, send it to the receiver 📤

Note: in cases where a subscriber wants 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 🙌 But for that to work, it is critical that subscribers can rely on the subscription and have confidence that they will actually receive the subscribed updates, without having to worry about updates being silently dropped. In other words, a subscription is a promise from 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 was started 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 hits a situation where it is unable to keep the terms of the subscription, and (2) gives the publisher 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 there, any loss of an underlying transport connection will be detected and will result in subscription termination (for dynamic subscriptions) or suspension (for configured subscriptions). This makes sure situations where the loss of update notifications would go unnoticed will not happen 🔒

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 ✨

Okay so for on-change subscriptions, update records need to say more than just the new values of the nodes that changed 👀. They also need to say what kind of change happened since the last update. That's why the encoding rules generally follow YANG Patch operations from [RFC8072] 💅. The YANG Patch operations describe what needs to be applied to the earlier state reported by the preceding update to get to the now-current state. Also, objects in an update aren't limited to configuration data 🌸. They can include any objects, operational data too, whereas [RFC8072] patches only apply to configuration data in configuration datastores.

A publisher tells the receiver what kind of change happened to a datastore node by 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.
The "move" operation is used when an existing entry in a user-ordered list is moved.

But wait, there's more 🦋. 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 make that work, it is valid to encode a YANG Patch operation so that applying it results in no change between the previous state and the current state. This signals that some churn has happened on the object 🌀. An example would be a patch with a "create" operation for a datastore node the receiver believes already exists, or a "replace" operation that swaps a previous value for the same value. Note that the "create" and "delete" errors described in [RFC8072], Section 2.5 are not errors in the case of YANG-Push. They're considered valid operations for YANG-Push, so no drama 😌.

3.6. Defining the Selection with a Datastore

A subscription must specify both the selection filters and the datastore those filters get applied against 🧚. This info is what's used to choose and then 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 YANG-Push data model includes the following selection filter types, which 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 the 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 meant to act as selectors that define which objects are within the scope of a subscription 🔍. XPath itself has powerful filtering constructs, so care must be taken when defining a filter 🫶. Consider an XPath filter that only passes a datastore node when an interface is up. It's up to the receiver to understand what the presence or absence of objects in each update means.

A publisher MUST support at least one type of selection filter 💜.

When the set of selection-filtering criteria is applied for a periodic subscription, those criteria are applied whenever a periodic update record is constructed. Only datastore nodes that pass the filter and that a receiver has access to are provided to that receiver ✨. If the same filtering criteria are applied to an on-change subscription, only the subset of those datastore nodes that support 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

Unlike traditional data retrieval requests, datastore subscriptions can stream an unbounded series of update records 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. It can also be used for on-change subscriptions in two cases. First, it MUST be used as the initial "push-update" if the receiver needs to be synchronized 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 what would be obtained had the same data been explicitly retrieved using a datastore retrieval operation over 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, it indicates which datastore nodes have been created, deleted, or had their values changed. When multiple changes have occurred over 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 they're ultimately queued for egress over the specified transport 📤.

Figure 1 shows 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 in 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 shows 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

Okay bestie, the RPCs from [RFC8639] got a whole glow-up 💅 and can now negotiate datastore subscriptions ✨. There are also brand-new error codes that explain why a datastore subscription attempt flopped 😭, plus new yang-data that MAY be used to include details on input parameters that might lead to a successful follow-up RPC invocation 🎀.

Rejections can happen for a bunch of reasons, like a subtree request that's way too big or the publisher not being able to push update records as often as you asked 🙈. When that happens, no subscription gets made. Instead, you get a subscription result that includes the reason for the failure in the RPC response. A set of alternative subscription parameters MAY be returned too, the ones that would likely have gotten a yes 🦋. The subscriber may consider including such parameters in future attempts 💜.

If a datastore subscription establishment gets rejected and there are hints, the hints SHOULD be transported in a yang-data container called establish-subscription-datastore-error-info. This gets inserted into the RPC error response, instead of the establish-subscription-stream-error-info you'd see for a stream subscription 🌸.

Figure 3 shows a tree diagram for establish-subscription-datastore-error-info. All tree diagrams 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

Same energy for modifications! If a datastore subscription modification gets rejected and there are hints, the hints SHOULD be transported in a yang-data container called modify-subscription-datastore-error-info, inserted into the RPC error response. It replaces the modify-subscription-stream-error-info you'd get with 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

Security time, and it's giving serious 🔐. 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 that, it needs to run all the relevant checks at the time of each specific pushed update and, if needed, quietly strip any unauthorized data out of the datastore subtrees 🤫. This means YANG data pushed through subscriptions gets authorized in a way that's equivalent to a regular data retrieval (get) operation 💖.

Each push-update and push-change-update MUST have access control applied, as shown in Figure 5 👑. That includes checking 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 pull this off, implementations SHOULD apply the conceptual authorization model of [RFC8341], specifically Section 3.2.4, extended so it works on data nodes included in notifications too, 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 isn't allowed to access 🦋. This lets a receiver watch the entire lifecycle of a datastore tree without having to list every single datastore node by hand ✨. If there are no objects left in an update record after access control has been applied, the outcome depends on whether the subscription is periodic or on-change. For a periodic subscription, an empty push-update notification MUST be sent, so clients don't get confused and think an update got lost 😭. For an on-change subscription, a push-update notification MUST NOT be sent, so clients stay unaware of changes 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. Also, 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. This is iconic for the publisher, because it doesn't have to keep doing continuous and total filtering of a subscription's content for every update record 💅. It also cuts down the chance of access-controlled objects leaking 🔐.

If read access to previously accessible nodes gets lost because a receiver's permissions changed, this SHOULD be reported as a patch delete operation for on-change subscriptions. If a publisher can't handle receiver permission changes with that kind of delete, publisher implementations MUST force dynamic subscription re-establishment or configured subscription reinitialization, so the right filtering gets installed 😭🔥.

3.10. On-Change Notifiable Datastore Nodes

Sometimes a publisher that supports on-change notifications just can't push on-change updates for certain object types 🙃. Reasons might be that the value of a datastore node changes super frequently (like the in-octets counter defined in [RFC8343]), that small object changes are frequent and meaningless (like a temperature gauge bouncing around by 0.1 degrees), or that the implementation simply can't do on-change notification for a particular object 🌡️.

In those cases, it's really important for client applications to be able to tell which objects support on-change notifications and which ones don't 👀. Otherwise, clients have no way of knowing whether they can actually rely on their on-change subscription to deliver the changes they care about 😩. In other words, if implementations don't provide a solution and don't support comprehensive on-change notifiability, clients have no way of knowing what their on-change subscription actually covers 💀.

So implementations are strongly advised to provide a solution to this problem 🫶. One option is making it discoverable to clients which objects are on-change notifiable, specified using another YANG data model. That kind of 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

Updates like the ones in this document, and on-change updates especially, must not get lost 🥺. If losing an update is unavoidable, it's critical that the receiver gets notified about it 🚨.

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

Sometimes a publisher realizes it can't fit the full set of objects the subscription asked for into an update record. In that 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 doesn't 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 that isn't 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 of those scenarios is possible, then an incomplete-update flag MUST be included on the next push-change-update 🔁.

Note: It's perfectly acceptable to have a series of push-change-update notifications (and even push-update notifications) serially queued at the transport layer waiting to be sent 📬. It's not required for the publisher to merge pending update records sent at the same time 🌸.

On the receiver side, what to do when a record with an incomplete-update flag shows up depends on the application 🦋. It could just wait and do nothing 😌. It could choose to resync, actively retrieving all subscribed information 🔄. It could also tear down the subscription and start a new one, maybe 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 one that can't be met 🙅‍♀️💅.

Whether a subscription can be supported gets decided by a mix of factors 🧮. These include the subscription update trigger (on-change or periodic), the period in which changes get reported (one-second periods use way more resources than one-hour periods ⏱️), the amount of data in the datastore subtree being subscribed to, and the number and combination of other subscriptions being serviced at the same time 🌪️.

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

4.1. Overview

The YANG data model for datastore push subscriptions is shown in Figures 6 through 9 🦋 The trees follow the notation from [RFC8340] ✨ Any brand-new schema objects (the ones that aren't already in [RFC8639]) are marked with "yp" 💅 These trees are huge, so some nodes from the ietf-subscribed-notifications YANG module [RFC8639] that aren't essential to understanding this model got trimmed to keep things compact 🙈 Wherever that happened, you'll see "..." in the diagram.

Because the full tree is so big, it's split into four figures 🎀 Figure 6 shows the augmentations that ietf-yang-push adds to the subscription configuration in 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 covers the augmentations ietf-yang-push makes to the RPCs defined in ietf-subscribed-notifications [RFC8639] 💜 The big ones are the "establish-subscription" and "modify-subscription" RPCs, which get extra parameters so you can actually 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 shows the augmentations of ietf-yang-push to the notifications defined in ietf-subscribed-notifications ✨ These let "subscription-started" and "subscription-modified" notifications carry the datastore push configuration parameters 💖

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 shows the parts of ietf-yang-push that are brand new in this document 🧚 (so, not just augmentations of some other 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

Okay so this is the notifications tree, and it's giving very neat, very organised 🎀 ✨ Here are the bits of the data model we're looking at, bestie 💅

4.2. Subscription Configuration 🦋

Both configured and dynamic subscriptions live in the list subscription. The new parameters that extend the basic subscription data model from [RFC8639] are:

The datastore 🍓 The targeted datastore that the selection is being made from. The possible datastores include the ones from [RFC8342], and a platform may also choose to support a custom datastore.
The selection filter 🔥 A filter that identifies the YANG nodes you care about in a datastore. The filter contents are either a reference to an existing filter or an in-line definition that only applies to that one subscription. Referenced filters let an implementation skip evaluating whether the filter is acceptable during a dynamic subscription request. The case statement is what tells the options apart.
Periodic subscriptions ⏰ For these, triggered updates happen at the boundaries of a specified time interval. You can work out those boundaries from the periodic parameters:
- period defines the duration between push updates.
- 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.
On-change subscriptions 👑 Assuming any dampening period has completed, triggering happens whenever a change in the subscribed information is detected. These have more-complex semantics, guided by their own set of parameters:
- dampening-period 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.
- excluded-change 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).
- sync-on-start 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]. The 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. Main character energy 💖

An id (that identifies the subscription). This object MUST be transported along with the subscribed contents. It lets a receiver figure out 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 that are 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 a 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 gives 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. 🌈

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. It 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 🔄

okay so, before the subscriber can even make a subscription request, it needs to know which YANG datastore schemas the publisher is using 🧠. Those live in the YANG library module ietf-yang-library.yang, as defined in [RFC8525]. The receiver is expected to know the YANG library info before it starts a subscription 💅.

The set of modules, revisions, features, and deviations can change at runtime (if the publisher implementation supports that) 🦋. So the YANG library has a simple "yang-library-change" notification that tells the subscriber the library has changed 🔔. When that happens, a subscription might need an update to take the changes into account 🫶. The receiver may also need to hear about module changes so it can process updates about datastore nodes from changed modules correctly ✨.

5. YANG Module for YANG-Push 🎀

this YANG module borrows typedefs from [RFC6991], identities from [RFC8342], the "yang-data" extension from [RFC8040], and the "yang-patch" grouping from [RFC8072] 🧁. It also imports and augments a whole bunch of stuff from [RFC8639] 💖. And it 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 (c) 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:

              o  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.

Okay bestie, we're still deep in the YANG module, and this first bit is all about how the XPath filter picks up its namespace declarations and what context it runs in 💅

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.

o 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; } } }


Next up is the grouping that handles the update policy knobs. This is where you pick *when* updates fly out to the subscriber: on a periodic timer, or whenever something actually changes ✨

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."; } } } } }


Now the full `update-policy` grouping. This one adds the stuff that can't be changed once the subscription is up, like `sync-on-start` and the `excluded-change` list. The `sync-on-start` bit is a big deal, so read it carefully 🫶

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."; } } } }


Next is the `hints` grouping, which is basically the extra info a publisher can send back when a subscription request goes sideways. Think of it as the "here's what would've worked" note 📝

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."; } }


Time for the RPCs section of the module, starting with `resync-subscription`. It lets a subscriber ask for a full push again, and the error reply has its own yang-data container ✨

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; } }


Last up in this part: the augmentations that bolt datastore-specific stuff onto `establish-subscription` and `modify-subscription`. Lots of little pieces fitting together, very satisfying 🧚

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; } } }


# 6. IANA Considerations ✨

<lavender>Okay, so IANA gets two fresh registrations from us, bestie 🎀</lavender>

First up, we register a namespace URI in the "IETF XML Registry" [RFC3688]. The registrant is The IESG 💅 The XML field is N/A, since the requested URI is just an XML namespace, so there's no XML file to point to 🦋

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

Registrant Contact: The IESG.

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


Then we register a YANG module in the "YANG Module Names" registry [RFC6020] 🌸 It's called ietf-yang-push, it uses that same namespace, its prefix is yp, and the reference points right back at RFC 8641 🫶

Name: ietf-yang-push

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

Prefix: yp

Reference: RFC 8641


<rainbow>Registries sorted, periodt ✨👑</rainbow>

# 7. Security Considerations 🔐✨

okay bestie, this part is a little serious, but it's also giving main character energy 💅. The YANG module in this document defines a schema for data that's meant to be reached through network management protocols like 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 its mandatory-to-implement secure transport is TLS [RFC8446].

The Network Configuration Access Control Model (NACM) [RFC8341] is how you restrict particular NETCONF or RESTCONF users to a preset subset of all the available protocol operations and content. It's the bouncer at the club 🧚

There are a bunch of data nodes in this YANG module that are writable, creatable, and deletable (that's `config true`, which is the default) 🍓. These nodes might be sensitive or vulnerable in some network environments, so writing to them without proper protection can mess with network operations. Here are the subtrees and data nodes, plus why they're sensitive. (Also, the YANG module in this document augments the YANG module from [RFC8639], so all the security considerations listed in [RFC8639] are relevant for datastore subscriptions too. In the list below, we focus on the new data nodes this document introduces 🦋.)

- Subtree `selection-filter` under container `filters`: This subtree lets a subscriber say which objects or subtrees to include in a datastore subscription. An attacker could try to modify the filter. For example, the filter could be changed so that very few objects get filtered in, which tries to overwhelm the receiver. Or the filter could be changed so that certain objects get excluded from updates, and then certain changes go unnoticed 👀.
- Subtree `datastore` in choice `target` in list `subscription`: Same vibe as the "selection filter". An attacker might try to modify the objects being filtered to overwhelm a receiver with a bigger volume of object updates than expected, or to make certain changes go unnoticed 🙈.
- Choice `update-trigger` in list `subscription`: If an attacker modifies the update trigger, they can change the updates being sent to confuse a receiver, withhold certain updates from being sent to the receiver, and/or overwhelm a receiver. For example, an attacker might change the period at which updates are reported for a periodic subscription, or change the dampening period for an on-change subscription. That could mean a bigger delay for successive updates (which can hurt the responsiveness of apps that depend on the updates) or a high volume of updates that exhausts receiver resources 🔥.

The NACM gives the publisher side one way to mitigate these threats. If you want to protect yourself as a subscriber, you can monitor the subscription configuration for unexpected changes and subscribe to updates to the YANG datastore nodes that represent your datastore subscriptions. Since that volume of data is small, a paranoid subscriber could even go back to occasional polling to guard against a compromised subscription. Yes, you can poll the subscription configuration updates yourself 😭🫶.

Some of the readable data nodes in this YANG module might be sensitive or vulnerable in some network environments, so it's important to control read access to them (for example, via `get`, `get-config`, or `notification`). Here's the rundown:

- Subtree `selection-filter` under container `filters`: If access control isn't properly configured, this can expose system internals to people who shouldn't have access to that information.
- Subtree `datastore` in choice `target` in list `subscription`: If access control isn't properly configured, this can expose system internals to people who shouldn't have access to that information.
- Choice `update-trigger` in list `subscription`: If access control isn't properly configured, this can expose system internals to people who shouldn't have access to that information.

Some of the RPC operations in this YANG module might be sensitive or vulnerable in some network environments too, so it's important to control access to them. Here are the operations and their sensitivity:

- RPC `resync-subscription`: This RPC lets a subscriber of an on-change subscription ask for a full push of the objects in the subscription's scope. That can mean a huge amount of data 📦. An attacker could try to use this RPC to exhaust the server's resources while it generates the data, and then try to overwhelm a receiver with the resulting large volume of data 💀.

# 8. References ✨

## 8.1. Normative References

Here are all the normative references, bestie. These are the ones you really need to know about 📚.

[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

These ones are the extra reading, bestie. Super useful, just not strictly required 🧠🌸.

[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

Okay so when an RPC gets rejected, for any reason, the publisher sends back an RPC error response 🙅‍♀️. Two kinds of errors can show up in there: (1) the regular transport-layer RPC error codes, like the ones you see with NETCONF in [RFC6241] 🔌, and (2) subscription-specific errors, like the ones defined in the YANG data model 💅. That means how subscription errors get encoded inside an RPC error response depends on the transport 🚚.

References to specific identities in the `ietf-subscribed-notifications` YANG module [RFC8639] or the `ietf-yang-push` YANG module may come back as part of the error responses when a datastore subscription attempt fails. For errors defined in the `ietf-subscribed-notifications` YANG module, go check out [RFC8639] 📖. The errors defined in this document, grouped per RPC, look like this 👀:

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's one last set of transport-independent RPC error elements in the YANG data model, and 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 can get unexpectedly terminated or suspended, and it can happen totally independently of any RPC or configuration operation 😱. When that happens, indications of such a failure MUST be provided 🚨. To make that happen, a bunch of errors can be returned as part of the matching subscription state change notification 💌. The following error identities are introduced in this document, on top of the ones 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 🫶 literally a whole squad of legends, no notes 👑.

# Contributors 💜

<pink>The following individuals made substantial contributions to this document and should be considered coauthors.</pink> 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

<rainbow>And that's the whole thing, bestie! 🌈 We officially made it through RFC 8641 💅✨</rainbow>

📦 where this page comes from

ietf: rfc8641 2019-09
💖 girlified by claude-haiku-5-5 on 2026-10-18: a language model's rewrite of the original, under its terms