YANG Datastore SubscriptionHuaweiludwig@clemm.orgCisco Systemsevoit@cisco.comVMwareagonzalezpri@vmware.comCisco Systemsambtripa@cisco.comCisco Systemseinarnn@cisco.comYumaWorksandy@yumaworks.comEricssonbalazs.lengyel@ericsson.com
Operations & Management
NETCONFDraftVia the mechanism described in this document, subscriber applications may request a continuous, 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.Via the mechanism described in this document, subscriber applications may request a continuous, customized stream of updates from a YANG datastore. Providing such visibility into updates enables new capabilities based on the remote mirroring and monitoring of configuration and operational state.Traditional approaches to providing visibility into managed entities from remote have been built on polling. With polling, data is periodically requested and retrieved by a client from a server to stay up-to-date. However, there are issues associated with polling-based management:
Polling incurs significant latency. This latency prohibits many application types.Polling cycles may be missed, requests may be delayed or get lost, often when the network is under stress and the need for the data is the greatest.Polling requests may undergo slight fluctuations, resulting in intervals of different lengths. The resulting data is difficult to calibrate and compare.For applications that monitor for changes, many remote polling cycles place unwanted and ultimately wasteful load on the network, devices, and applications, particularly when changes occur only infrequently.A more effective alternative to polling is for an application to receive automatic and continuous updates from a targeted subset of a datastore. Accordingly, there is a need for a service that allows applications to subscribe to updates from a datastore and that enables the server (also referred to as publisher) to push and in effect stream those updates. The requirements for such a service have been documented in . This document defines a corresponding solution that is built on top of "Custom Subscription to Event Streams" . Supplementing that work are YANG data model augmentations, extended RPCs, and new datastore specific update notifications. Transport options for will work seamlessly with this solution. This document uses the terminology defined in , , , and . In addition, the following terms are introduced:
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, which 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, deleted). Also included in the update record may be other metadata, such as a subscription id of the subscription as part of 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.This document specifies a solution that provides a subscription service for updates from a datastore.
This solution supports dynamic as well as configured subscriptions to updates of datastore nodes.
Subscriptions specify when notification messages (also referred to as "push updates") should be sent and what data to include in update records.
Datastore node updates are subsequently pushed from the publisher to the receiver per the terms of the subscription.YANG-push subscriptions are defined using a YANG data model.
This model enhances the subscription model defined in
with capabilities that allow subscribers to subscribe to datastore node updates,
specifically to specify the update triggers defining when to generate update records as well as what to include in an update record.
Key enhancements include:Specification of selection filters which identify targeted YANG datastore nodes and/or datastore subtrees for which updates are to be pushed.Specification of update policies contain conditions which trigger the generation and pushing of new update records. There are two types of subscriptions, distinguished by how updates are triggered: periodic and on-change.
For periodic subscriptions, the update trigger is specified by two parameters that define when updates are to be pushed. These parameters are the period interval with which to report updates, and an "anchor time", i.e. a reference point in time that can be used to calculate at which points in time periodic updates need to be assembled and sent.For on-change subscriptions, an update trigger occurs whenever a change in the subscribed information is detected. Included are additional parameters that include:
Dampening period: In an on-change subscription, detected object changes should be sent as quickly as possible. However it may be undesirable to send a rapid series of object changes. Such behavior has the potential to exhaust resources in the publisher or receiver. In order to protect against that, a dampening period MAY be used to specify the interval which has to pass before successive update records for the same subscription are generated for a receiver. The dampening period collectively applies to the set of all datastore nodes selected by a single subscription. This means that when there is a change to one or more subscribed objects, an update record containing those objects is created immediately (when no dampening period is in effect) or at the end of a dampening period (when a dampening period is in fact in effect). If multiple changes to a single object occur during a dampening period, only the value that is in effect at the time when the update record is created is included. The dampening period goes into effect every time an update record completes assembly. Change type: This parameter can be used to reduce the types of datastore changes for which updates are sent (e.g., you might only send an update when an object is created or deleted, but not when an object value changes).No sync on start: defines whether or not a complete push-update of all subscribed data will be sent at the beginning of a subscription. Such early synchronization establishes the frame of reference for subsequent updates.An encoding (using anydata) for the contents of periodic and on-change push updates.A dynamic subscription request SHOULD be declined if a publisher's assessment is that it may be unable to provide update records meeting the terms of an "establish-subscription" or "modify-subscription" RPC request. In this case, a subscriber may quickly follow up with a new RPC request using different parameters. Random guessing of different parameters by a subscriber is to be discouraged. Therefore, in order to minimize the number of subscription iterations between subscriber and publisher,
a dynamic subscription supports a simple negotiation between subscribers and publishers for subscription parameters. This negotiation is in the form of supplemental information which should be inserted within error responses to a failed RPC request. This returned error response information, when considered, should increase the likelihood of success for subsequent RPC requests. Such hints include suggested periodic time intervals, acceptable dampening periods, and size estimates for the number or objects which would be returned from a proposed selection filter. However, there are no guarantees that subsequent requests which consider these hints will be accepted.
On-change subscriptions allow receivers to receive updates whenever changes to targeted objects occur. As such, on-change subscriptions are particularly effective for data that changes infrequently, yet for which applications need to be quickly notified whenever a change does occur with minimal delay.
On-change subscriptions tend to be more difficult to implement than periodic subscriptions.
Accordingly, on-change subscriptions may not be supported by all implementations or for every object.
Whether or not to accept or reject on-change subscription requests when the scope of the subscription contains objects for which on-change is not supported is up to the publisher implementation. A publisher MAY accept an on-change subscription even when the scope of the subscription contains objects for which on-change is not supported. In that case, updates are sent only for those objects within the scope that do support on-change updates, whereas other objects are excluded from update records, even if their values change. In order for a subscriber to determine whether objects support on-change subscriptions, objects are marked accordingly on a publisher. Accordingly, when subscribing, it is the responsibility of the subscriber to ensure it is aware of which objects support on-change and which do not. For more on how objects are so marked, see .
Alternatively, a publisher MAY decide to simply reject an on-change subscription in case the scope of the subscription contains objects for which on-change is not supported. In case of a configured subscription, the publisher MAY suspend the subscription.
To avoid flooding receivers with repeated updates for subscriptions containing fast-changing objects, or objects with oscillating values, an on-change subscription allows for the definition of a dampening period. Once an update record for a given object is generated, no other updates for this particular subscription will be created until the end of the dampening period. Values sent at the end of the dampening period are the values that are current at the end of the dampening period of all changed objects. Changed objects include those which were deleted or newly created during that dampening period. If an object has returned to its original value (or even has been created and then deleted) during the dampening-period, that value (and not the interim change) will still be sent. This will indicate churn is occurring on that object.
On-change subscriptions can be refined to let users subscribe only to certain types of changes. For example, a subscriber might only want object creations and deletions, but not modifications of object values. Putting it all together, following is the conceptual process for creating an update record as part of an on-change subscription:
Just before a change, or at the start of a dampening period, evaluate any filtering and any access control rules to ensure 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 YANG patch record for going from A to B.If there were any changes made between A and B which canceled each other out, insert into the YANG patch record the last change made, even if the new value is no different from the original value (since changes that were made in the interim were canceled out). In case the changes involve creating a new datastore node, then deleting it, the YANG patch record will indicate deletion of the datastore node. Similarly, in case the changes involve deleting a new datastore node, then recreating it, the YANG patch record will indicate creation of the datastore node. If the resulting patch record is non-empty, send it to the receiver.Note: In cases where a subscriber wants to have separate dampening periods for different objects, the subscriber has the option to create multiple subscriptions with different selection filters.
A subscription to updates from a datastore is intended to obviate the need for polling. However, in order to do so, it is critical that subscribers can rely on the subscription and have confidence that they will indeed receive the subscribed updates without having to worry about updates being silently dropped. In other words, a subscription constitutes a promise on the side of the publisher to provide the receivers with updates per the terms of the subscription.
Now, there are many reasons why a publisher may at some point no longer be able to fulfill the terms of the subscription, even if the subscription had been entered into with good faith. For example, the volume of 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 that is defined in this document mandates that a publisher notifies receivers immediately and reliably whenever it encounters a situation in which it is unable to keep the terms of the subscription, and provides the publisher with the option to suspend the subscription in such a case. 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 .
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.
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.In an on-change subscription, update records need to indicate not only values of changed datastore nodes but also the types of changes that occurred since the last update. Therefore, encoding rules for data in on-change updates will generally follow YANG-patch operation as specified in . The YANG-patch will describe what needs to be applied to the earlier state reported by the preceding update, to result in the now-current state. Note that contrary to , objects encapsulated are not restricted to only configuration objects.
A publisher indicates the type of change to a datastore node using
the different YANG patch operations: the "create" operation is used for
newly created objects (except entries in a user-ordered list), the
"delete" operation is used for deleted objects (including in
user-ordered lists), the "replace" operation is used when only the
object value changes, the "insert" operation is used when a new entry is
inserted in a list, and the "move" operation is used when an existing
entry in a user-ordered list is moved.
However, a patch must be able to do more than just describe the delta from the previous state to the current state. As per , it must also be able to identify whether transient changes have occurred on an object during a dampening period. To support this, it is valid to encode a YANG patch operation so that its application would result in no change between the previous and current state. This indicates that some churn has occurred on the object. An example of this would be a patch that indicates a "create" operation for a datastore node where the receiver believes one already exists, or a "replace" operation which replaces a previous value with the same value. Note that this means that the "create" and "delete" errors described in section 2.5 are not errors, and are valid operations with YANG-Push.
A subscription must specify both the selection filters and the datastore against which these selection filters will be applied. This information is used to choose and subsequently push data from the publisher's datastore to the receivers.Only a single selection filter can be applied to a subscription at a time. An RPC request proposing a new selection filter replaces any existing filter. The following selection filter types are included in the yang-push data model, and may be applied against a datastore:subtree: A subtree selection filter identifies one or more datastore subtrees. When specified, update records will only come from the datastore nodes of selected datastore subtree(s). The syntax and semantics correspond to that specified for section 6.xpath: An "xpath" selection filter is an XPath expression that returns a node set. When specified, updates will only come from the selected datastore nodes.These filters are intended to be used as selectors that define which objects are within the scope of a subscription. A publisher MUST support at least one type of selection filter.XPath itself provides powerful filtering constructs and care must be used in filter definition. Consider an XPath filter which only passes a datastore node when an interface is up. It is up to the receiver to understand implications of the presence or absence of objects in each update. When the set of selection filtering criteria is applied for a periodic subscription, then they are applied whenever a periodic
update record is constructed, and only datastore nodes that pass
the filter and to which a receiver has access are provided to
that receiver. If the same filtering criteria is applied to an on-change subscription, only the subset of those datastore nodes supporting on-change is provided. A datastore node which doesn't support on-change is never sent as part of an on-change subscription's "push-update" or "push-change-update" (see ).Contrary to traditional data retrieval requests, datastore subscription enables an unbounded series of update records to be streamed over time. Two generic YANG notifications for update records have been defined for this: "push-update" and "push-change-update". A "push-update" notification defines a complete, filtered update of the datastore per the terms of a subscription. This type of YANG notification is used for continuous updates of periodic subscriptions. A "push-update" notification can also be used for the on-change subscriptions in two cases. First, it MUST be used as the initial "push-update" if there is a need to synchronize the receiver at the start of a new subscription. It also MAY be sent if the publisher later chooses to resync an on-change subscription. The "push-update" update record contains an instantiated datastore subtree with all of the subscribed contents. The content of the update record is equivalent to the contents that would be obtained had the same data been explicitly retrieved using a datastore retrieval operation using the same transport with the same filters applied.A "push-change-update" notification is the most common type of update for on-change subscriptions. The update record in this case contains the set of changes that datastore nodes have undergone since the last notification message. In other words, this indicates which datastore nodes have been created, deleted, or have had changes to their values. In cases where multiple changes have occurred over the course of a dampening period and the object has not been deleted, the object's most current value is reported. (In other words, for each object, only one change is reported, not its entire history. Doing so would defeat the purpose of the dampening period.)"Push-update" and "push-change-update" are encoded and placed within notification messages, and ultimately queued for egress over the specified transport. The following is an example of a notification message for a subscription tracking the operational status of a single Ethernet interface (per ). This notification message is encoded XML over NETCONF as per .The following is an example of an on-change notification message for the same subscription.Of note in the above example is the 'patch-id' with a value of '0'. Per , the 'patch-id' is an arbitrary string. With YANG Push, the publisher SHOULD put into the 'patch-id' a counter starting at '0' which increments with every 'push-change-update' generated for a subscription. If used as a counter, this counter MUST be reset to '0' anytime a resynchronization occurs (i.e., with the sending of a 'push-update'). Also if used as a counter, the counter MUST be reset to '0' after passing a maximum value of '4294967295' (i.e. maximum value that can be represented using uint32 data type). Such a mechanism allows easy identification of lost or out-of-sequence update records.The RPCs defined within have been enhanced to support datastore subscription negotiation. Also, new error codes have been added that are able to indicate why a datastore subscription attempt has failed, along with new yang-data that MAY be used to include details on input parameters that might result in a successful subsequent RPC invocation.The establishment or modification of a datastore subscription can be rejected for multiple reasons. This includes a too large subtree request, or the inability of the publisher to push update records as frequently as requested. In such cases, no subscription is established. Instead, the subscription-result with the failure reason is returned as part of the RPC response. As part of this response, a set of alternative subscription parameters MAY be returned that would likely have resulted in acceptance of the subscription request. The subscriber may consider these as part of future subscription attempts.
In the case of a rejected request for an establishment of a datastore subscription, if there are hints, the hints SHOULD be transported within a yang-data "establish-subscription-datastore-error-info" container inserted into the RPC error response, in lieu of the "establish-subscription-stream-error-info" that is inserted in case of a stream subscription.
Below is a tree diagram for "establish-subscription-datastore-error-info".
All tree diagrams used in this document follow the notation
defined in
Similarly, in the case of a rejected request for modification of a datastore subscription, if there are hints, the hints SHOULD be transported within a yang-data "modify-subscription-datastore-error-info" container inserted into the RPC error response, in lieu of the "modify-subscription-stream-error-info" that is inserted in case of a stream subscription.
Below is a tree diagram for "modify-subscription-datastore-error-info".
A receiver of subscription data MUST only be sent updates for which it has proper authorization. A publisher MUST ensure that no non-authorized data is included in push updates. To do so, it needs to apply all corresponding checks applicable at the time of a specific pushed update and if necessary silently remove any non-authorized data from datastore subtrees. This enables YANG data pushed based on subscriptions to be authorized equivalently to a regular data retrieval (get) operation.Each "push-update" and "push-change-update" MUST have access control applied, as is depicted in the following diagram. This includes validating that read access is permitted for any new objects selected since the last notification message was sent to a particular receiver. To accomplish this, implementations SHOULD support the conceptual authorization model of , specifically section 3.2.4.A publisher MUST allow for the possibility that a subscription's selection filter references non-existent data or data that a receiver is not allowed to access. Such support permits a receiver the ability to monitor the entire lifecyle of some datastore tree without needing to explicitly enumerate every individual datastore node.
If, after access control has been applied, there are no objects remaining in an update record, then (in case of a periodic subscription) only a single empty "push-update" notification MUST be sent. Empty "push-change-update" messages (in case of an on-change subscription) MUST NOT be sent. This is required to ensure that clients cannot surreptitiously monitor objects that they do not have access to via carefully crafted selection filters. 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 which selects non-existent data or data that a receiver is not allowed to access. As reason, the error identity "unchanging-selection" SHOULD be returned. In addition, a publisher MAY choose to terminate a dynamic subscription or suspend a configured receiver when the authorization privileges of a receiver change, or the access controls for subscribed objects change. In that case, the publisher SHOULD include the error identity "unchanging-selection" as reason when sending the "subscription-terminated" notification. Such a capability enables the publisher to avoid having to support continuous and total filtering of a subscription's content for every update record. It also reduces the possibility of leakage of access-controlled objects.If read access into previously accessible nodes has been lost due to a receiver permissions change, this SHOULD be reported as a patch "delete" operation for on-change subscriptions. If not capable of handling such receiver permission changes with such a "delete", publisher implementations MUST force dynamic subscription re-establishment or configured subscription re-initialization so that appropriate filtering is installed.In some cases, a publisher supporting on-change notifications may not be able to push on-change updates for some object types. Reasons for this might be that the value of the datastore node changes frequently (e.g., 's in-octets counter), that small object changes are frequent and meaningless (e.g., a temperature gauge changing 0.1 degrees), or that the implementation is not capable of on-change notification for a particular object.In those cases, it will be important for client applications to have a way to identify for which objects on-change notifications are supported and for which ones they are not supported. Otherwise client applications will have no way of knowing whether they can indeed rely on their on-change subscription to provide them with the change updates that they are interested in. In other words, if implementations do not provide a solution and do not support comprehensive on-change notifiability, clients of those implementations will have no way of knowing what their on-change subscription actually covers.Implementations are therefore strongly advised to provide a solution to this problem. It is expected that such a solution will be standardized at some point in the future. In the meantime and until this occurs, implementations SHOULD provide their own solution.Particularly in the case of on-change updates, it is important that these updates do not get lost. In case the loss of an update is unavoidable, it is critical that the receiver is notified accordingly.Update records for a single subscription MUST NOT be resequenced prior to transport.It is conceivable that under certain circumstances, a publisher will recognize that it is unable to include within an update record the full set of objects desired per the terms of a subscription. In this case, the publisher MUST act as follows.
The publisher MUST set the "incomplete-update" flag on any update record which is known to be missing information.The publisher MAY choose to suspend the subscription as per . If the publisher does not create an update record at all, it MUST suspend the subscription.
When resuming an on-change subscription, the publisher SHOULD generate a complete patch from the previous update record. If this is not possible and the "sync-on-start" option is 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 these are possible, then an "incomplete-update" flag MUST be included on the next "push-change-update".Note: It is perfectly acceptable to have a series of "push-change-update" notifications (and even "push update" notifications) serially queued at the transport layer awaiting transmission. It is not required for the publisher to merge pending update records sent at the the same time.It is far preferable to decline a subscription request than to accept such a request when it cannot be met.Whether or not a subscription can be supported will be determined by a combination of several factors such as the subscription update trigger (on-change or periodic), the period in which to report changes (one second periods will consume more resources than one hour periods), the amount of data in the datastore subtree that is being subscribed to, and the number and combination of other subscriptions that are concurrently being serviced.The YANG data model for datastore push subscriptions is depicted in the following figure. The tree diagram follows the notiation defined in . New schema objects defined here (i.e., beyond those from ) are identified with "yp". For the reader's convenience, in order to compact the tree representation, some nodes that are defined in ietf-subscribed-notifications and that are not essential to the understanding of the data model defined here have been removed. This is indicated by "..." in the diagram where applicable.Selected components of the model are summarized below.Both configured and dynamic subscriptions are represented within the list "subscription". New parameters extending the basic subscription data model in include:
The targeted datastore from which the selection is being made.
The potential datastores include those from . A platform may also choose to support a custom datastore.A selection filter identifying yang nodes of interest within a datastore. Filter contents are specified via a reference to an existing filter, or via an in-line definition for only that subscription. Referenced filters allows an implementation to avoid evaluating filter acceptability during a dynamic subscription request. The case statement differentiates the options.For periodic subscriptions, triggered updates will occur at the boundaries of a specified time interval. These boundaries can be calculated from the periodic parameters:
a "period" which defines the duration between push updates. an "anchor-time"; update intervals fall on the points in time that are a multiple of a "period" from an "anchor-time". If "anchor-time" is not provided, then the "anchor-time" MUST be set with the creation time of the initial update record.For on-change subscriptions, assuming any dampening period has completed, triggering occurs whenever a change in the subscribed information is detected. On-change subscriptions have more complex semantics that is guided by its own set of parameters:
a "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. an "excluded-change" parameter which allows restriction of the types of changes for which updates should be sent (e.g., only add to an update record on object creation). a "sync-on-start" specifies whether a complete update with all the subscribed data is to be sent at the beginning of a subscription.Subscription state notifications and mechanism are reused from . Notifications "subscription-started" and "subscription-modified" have been augmented to include the datastore specific objects.Along with the subscribed content, there are other objects which might be part of a "push-update" or "push-change-update" notification.An "id" (that identifies the subscription) MUST be transported along with the subscribed contents.
This allows a receiver to differentiate which subscription resulted in a particular update record. A "time-of-update" which represents the time an update record snapshot was generated. A receiver MAY assume that at this point in time a publisher's objects have the values that were pushed.An "incomplete-update" leaf. This leaf indicates that not all changes which have occurred since the last update are actually included with this update. In other words, the publisher has failed to fulfill its full subscription obligations. (For example a datastore was unable to provide the full set of datastore nodes to a publisher process.) To facilitate re-synchronization of on-change subscriptions, a publisher MAY subsequently send a "push-update" containing a full selection snapshot of subscribed data.YANG-Push subscriptions are established, modified, and deleted using RPCs augmented from .The subscriber sends an establish-subscription RPC with the parameters in section 3.1. An example might look like:A positive response includes the "id" of the accepted subscription. In that case a publisher may respond:A subscription can be rejected for multiple reasons, including the lack of authorization to establish a subscription, no capacity to serve the subscription at the publisher, or the inability of the publisher to select datastore content at the requested cadence.If a request is rejected because the publisher is not able to serve it, the publisher SHOULD include in the returned error hints which help a subscriber understand subscription parameters might have been accepted for the request. These hints would be included within the yang-data structure "establish-subscription-error-datastore". However even with these hints, there are no guarantee that subsequent requests will in fact be accepted.The specific parameters to be returned as part of the RPC error response depend on
the specific transport that is used to manage the subscription. For example, in the case of NETCONF
, when a subscription
request is rejected, the NETCONF RPC reply
would be expected to include an "rpc-error" element with the following elements:
"error-type" of "application". "error-tag" of "operation-failed". Optionally, an "error-severity" of "error". Optionally, "error-info" containing XML-encoded data with hints for parameter settings
that might result in future RPC success per yang-data definition "establish-subscription-error-datastore".
For example, for the following request:a publisher that cannot serve on-change updates but periodic updates might return the following:The subscriber MAY invoke the "modify-subscription" RPC for a subscription it previously established. The subscriber will include newly desired values in the "modify-subscription" RPC. Parameters not included MUST remain unmodified. Below is an example where a subscriber attempts to modify the period and datastore XPath filter of a subscription.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 within the yang-data structure "modify-subscription-error-datastore". A subscription MAY be modified multiple times.The specific parameters to be returned in as part of the RPC error response depend on
the specific transport that is used to manage the subscription. In the case of NETCONF
, when a subscription
request is rejected, the NETCONF RPC reply
MUST include an "rpc-error" element with the following elements:
"error-type" of "application". "error-tag" of "operation-failed". Optionally, an "error-severity" of "error" (this MAY but does not have to be included). "error-path" pointing to the object or parameter that caused the rejection. Optionally, "error-info" containing XML-encoded data with hints for parameter settings
that might result in future RPC success per yang-data definition "modify-subscription-error-datastore".
A configured subscription cannot be modified using "modify-subscription" RPC. Instead, the configuration needs to be edited as needed.To stop receiving updates from a subscription and effectively delete a subscription that had previously been established using an "establish-subscription" RPC, a subscriber can send a "delete-subscription" RPC, which takes as only input the subscription's "id". This RPC is unmodified from .This RPC is supported only for on-change subscriptions previously established using an "establish-subscription" RPC. For example: On receipt, a publisher must either accept the request and quickly follow with a "push-update", or send an appropriate error within an rpc error response. Within an error response, the publisher MAY include supplemental information about the reasons within the yang-data structure "resync-subscription-error".To make subscription requests, the subscriber needs to know the YANG
datastore schemas used by the publisher, which are available via the YANG
Library module, ietf-yang-library.yang from . The receiver
is expected to know the YANG library information before starting a
subscription.The set of modules, revisions, features, and deviations can change at run-time (if supported by the publisher implementation). For this purpose, the YANG library provides a simple "yang-library-change" notification that informs the subscriber that the library has changed. In this case, a subscription may need to be updated to take the updates into account. The receiver may also need to be informed of module changes in order to process updates regarding datastore nodes from changed modules correctly.
This document registers the following namespace URI in the "IETF XML
Registry" :
URI: urn:ietf:params:xml:ns:yang:ietf-yang-push
Registrant Contact: The IESG.
XML: N/A; the requested URI is an XML namespace.
This document registers the following YANG module in the "YANG
Module Names" registry :
Name: ietf-yang-push
Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-push
Prefix: yp
Reference: draft-ietf-netconf-yang-push-19.txt (RFC form)
The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS .
The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.
There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability. (It should be noted that the YANG module augments the YANG module
from .
All security considerations that are listed there are relevant also for datastore subscriptions.
In the following, we focus on the data nodes that are newly introduced here.)
Subtree "selection-filter" under container "filters": This subtree allows to specify which objects or subtrees to include in a datastore subscription. An attacker could attempt to modify the filter. For example, the filter might be modified to result in very few objects being filtered in order to attempt to overwhelm the receiver. Alternatively, the filter might be modified to result in certain objects to be excluded from updates, in order to have certain changes go unnoticed. Subtree "datastore" in choice "target" in list "subscription": Analogous to "selection filter", an attacker might attempt to modify the objects being filtered in order to overwhelm a receiver with a larger volume of object updates than expected, or to have certain changes go unnoticed. Choice "update-trigger" in list "subscription": By modifying the update trigger, an attacker might alter the updates that are being sent in order to confuse a receiver, to withhold certain updates to be sent to the receiver, and/or to overwhelm a receiver. For example, an attacker might modify the period with which updates are reported for a periodic subscription, or it might modify the dampening period for an on-change subscription, resulting in greater delay of successive updates (potentially affecting responsiveness of applications that depend on the updates) or in a high volume of updates (to exhaust receiver resources). RPC "resync-subscription": This RPC allows a subscriber of an on-change subscription to request a full push of objects in the subscription's scope. This can result in a large volume of data. An attacker could attempt to use this RPC to exhaust resources on the server to generate the data, and attempt to overwhelm a receiver with the resulting data volume. 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.Custom Subscription to Event StreamsNETCONF Support for Event NotificationsRejection of an RPC for any reason is indicated by via RPC error response from the publisher. Valid RPC errors returned include both existing transport layer RPC error codes, such as those seen with NETCONF in , as well as subscription specific errors such as those defined within the YANG model. As a result, how subscription errors are encoded within an RPC error response is transport dependent. References to specific identities within the either the subscribed-notifications YANG model or the yang-push YANG model may be returned as part of the error responses resulting from failed attempts at datastore subscription. Following are valid errors per RPC (note: throughout this section the prefix 'sn' indicates an item imported from the subscribed-notifications.yang model):There is one final set of transport independent RPC error elements included in the YANG model. These are the following four yang-data structures for failed datastore subscriptions:A subscription may be unexpectedly terminated or suspended independent of any RPC or configuration operation. In such cases, indications of such a failure MUST be provided. To accomplish this, the following types of error identities may be returned within the corresponding subscription state change notification:(To be removed by RFC editor prior to publication)v18 - v19
Minor updates per WGLC comments. v17 - v18
Minor updates per WGLC comments. v16 - v17
Minor updates to YANG module, incorporating comments from Tom Petch. Updated references. v15 - v16
Updated security considerations. Updated references. Addressed comments from last call review, specifically comments received from Martin Bjorklund.v14 - v15
Minor text fixes. Includes a fix to on-change update calculation to cover churn when an object changes to and from a value during a dampening period.v13 - v14
Minor text fixes.v12 - v13
Hint negotiation models now show error examples.yang-data structures for rpc errors.v11 - v12
Included Martin's review clarifications.QoS moved to subscribed-notificationstime-of-update removed as it is redundant with RFC5277's eventTime, and other times from notification-messages.Error model moved to match existing implementationsOn-change notifiable removed, how to do this is implementation specific.NMDA model supported. Non NMDA version at https://github.com/netconf-wg/yang-push/v10 - v11
Promise model reference added.Error added for no-such-datastoreInherited changes from subscribed notifications (such as optional feature definitions).scrubbed the examples for proper encodingsv09 - v10
Returned to the explicit filter subtyping of v00-v05identityref to ds:datastore made explicitReturned ability to modify a selection filter via RPC.v08 - v09
Minor tweaks cleaning up text, removing appendicies, and making reference to revised-datastores.Subscription-id (now:id) optional in push updates, except when encoded in RFC5277, Section 4 one-way notification.Finished adding the text descibing the resync subscription RPC.Removed relationships to other drafts and future technology appendicies as this work is being explored elsewhere.Deferred the multi-line card issue to new draftsSimplified the NACM interactions.v07 - v08
Updated YANG models with minor tweaks to accommodate changes of ietf-subscribed-notifications.v06 - v07
Clarifying text tweaks.Clarification that filters act as selectors for subscribed datastore nodes; support for value filters not included but possible as a future extensionFilters don't have to be matched to existing YANG objectsv05 - v06
Security considerations updated.Base YANG model in [subscribe] updated as part of move to identities, YANG augmentations in this doc matched upTerms refined and text updates throughoutAppendix talking about relationship to other drafts added.Datastore replaces streamDefinitions of filters improvedv04 to v05
Referenced based subscription document changed to Subscribed Notifications from 5277bis.Getting operational data from filtersExtension notifiable-on-change addedNew appendix on potential futures. Moved text into there from several drafts.Subscription configuration section now just includes changed parameters from Subscribed NotificationsSubscription monitoring moved into Subscribed NotificationsNew error and hint mechanisms included in text and in the yang model.Updated examples based on the error definitionsGroupings updated for consistencyText updates throughoutv03 to v04
Updates-not-sent flag addedNot notifiable extension addedDampening period is for whole subscription, not single objectsMoved start/stop into rfc5277bisClient and Server changed to subscriber, publisher, and receiverAnchor time for periodicMessage format for synchronization (i.e. sync-on-start)Material moved into 5277bisQoS parameters supported, by not allowed to be modified by RPCText updates throughout