NETCONF Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track E. Voit
Expires: January 17, 2019 Cisco Systems
July 16, 2018

Concise YANG Telemetry
draft-birkholz-yang-core-telemetry-01

Abstract

This document defines CoAP operations that implement the capabilities of YANG Datastore Subscriptions and YANG Customized Subscriptions for the CoAP Management Interface (CoMI). The ‘/s’ resource, as defined in CoMI, is extended analogously to include a set of sub-resources, each of them representing an observable resource identified by its subscription-id. Specific additions include but are not limited to new FETCH Body definitions and simplified subtree subscriptions to intermediate data nodes in YANG datastore modules using SID.

Status of This Memo

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

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This Internet-Draft will expire on January 17, 2019.

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Table of Contents

1. Introduction

The YANG management interface for constrained devices and networks, called CoAP Management Interface (CoMI), is defined in [I-D.ietf-core-comi] and covers the capabilities as defined by YANG 1.1 [RFC7950]. The most essential characteristics of CoMI are the use of:

This document defines additions to CoMI called Concise YANG Telemetry that:

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.

2. Terminology

Due to the utilization of CoAP, the interaction model of CoMI is quite similar to RESTCONF [RFC8040]. RESTCONF supports subscriptions to a YANG datastore via notification statements in YANG modules, which—when subscribed to via the base subscription YANG RPC defined in [RFC7950]—result in Series [I-D.bormann-t2trg-stp] of Server Sent Events [W3C.REC-eventsource-20150203]}. A corresponding Event Stream specification for NETCONF [RFC6241] Event Notifications is defined in section 3.2.3 of [RFC5277]. To simplify corresponding terminology (and especially consolidate the impedance mismatch of the terms Notifications and Events), this document defines the following new term:

YANG Telemetry:
A Series of YANG modeled data items, potentially composing Update Records, sent from a YANG datastore [RFC8342] to a YANG receiver [I-D.ietf-netconf-subscribed-notifications] either solicited or unsolicited in a fashion that can guarantee well-defined levels of Visibility with respect to Data Node Value changes.

Please note: while the focus of YANG is typically on management and operations, the scope of YANG Telemetry extends into the Security Area with respect to Security Events. Because of this, YANG Telemetry characteristics that address security requirements, such as Visibility and Resilient YANG Subscriptions, are addressed in this document.

The definition of YANG Telemetry is based on the following terminology:

Data Node Value:
Defined in CoMI as the value assigned to a data node instance. Data node values are serialized into the payload according to the rules defined in section 4 of [I-D.ietf-core-yang-cbor]. Changes in Data Node Values are conveyed via data node updates referred to as Update Records.
Update Record:
Defined in [I-D.ietf-netconf-yang-push], in the context of Concise YANG Telemetry, an Update Record composes a single YANG data item in a Series of YANG Notifications or YANG Notification Bundle Messages conveying the changes to a YANG datastore’s module’s Data Node Values.
Resilient Subscription:
A subscription [I-D.ietf-netconf-subscribed-notifications] used to create Concise YANG Telemetry that continues to function in a given scope of CoMI Clients, or discoverable homes (see CoAP Call Home, respectively), if the current receiver of Concise YANG Telemetry is rendered unavailable.
This subscription characteristic enables a CoMI datastore to detect loss of a specific CoMI Client that is the target of Concise YANG Telemetry, provided that the CoMI Client is part of a group of CoMI Clients that supports this kind of fail-over mechanism. This subscription characteristic can have a deteriorating effect on the level of assurance with respect to Visibility and can therefore result in missed Updated Records, which a “new home” has to be notified about.
Series:
Series Transfer Pattern are described in [I-D.bormann-t2trg-stp] as the conveyance of a sequence of associated data items over time, where a client is able to obtain the Series and to learn about new items.
YANG Event Stream Subscriptions or a YANG Datastore Subscription creates an specific Series Transfer Pattern composed of individual YANG Notifications or YANG Notification Bundle Messages that include related updated records.
Subscription Characteristics:
The set of attributes associated with an active YANG Subscription.
The set of attributes is defined by the data definition statements [RFC7950] defined by the augments in YANG Customized Subscriptions to a Publisher’s Event Streams [I-D.ietf-netconf-subscribed-notifications], YANG Datastore Subscription [I-D.ietf-netconf-yang-push] and corresponding CoAP Tokens for Concise YANG Telemetry.
Visibility:
A level of assurance that Update Records will be received by a YANG Client.
There might be reasons, such as resource exhaustion or dampening settings, that result in Update Records being lost in transit or not being emitted by the YANG datastore. Sequential Message-IDs or specific YANG Notifications that report, e.g., about past events of resource exhaustion will inform the YANG Client about the characteristics of the loss of Update Records.
YANG Receiver:
A subset of clients as defined in [RFC8342]. A YANG Receiver is the entity that can receive and process – in contrast to access, which is mandatory in order to create a subscription, but not mandatory in order to receive the corresponding telemetry stream – YANG-defined data originating from a server, over some network management protocol.
YANG Receiver is a representation agnostic term. Specific network protocols that operate on YANG Servers via YANG Clients use representations, such as XML, JSON, or CBOR.
YANG Data Item:
An instance of YANG a modules Data Node Values or the changes to Data Node Values, conveyed as data in motion, serialized in a specific representation, such as XML, JSON, or CBOR. A Data Node Update as defined in [I-D.ietf-netconf-yang-push] is a YANG data item.
YANG Datastore:
Defined in [RFC8342], a datastore is a conceptual place to store and access information. A datastore might be implemented, for example, using files, a database, flash memory locations, or combinations thereof.
YANG Notification:
Defined in [RFC8342], a Notification is a server-initiated message indicating that a certain event has been recognized by the server.
YANG Notification (Bundle) Message:
An encapsulation header for one or more YANG notifications as defined in [I-D.ietf-netconf-notification-messages]. The message header includes a specific set of well-known objects, which are of potential use to networking layers prior being interpreted by some receiving application layer process.
Examples of header object include, but are not limited to: timestamps, signatures, or evidence about the integrity of the agents creating messages or notifications.
YANG Subscription:
A Subscription in the context of YANG is defined in [I-D.ietf-netconf-subscribed-notifications] as a contract with a publisher, stipulating which information one or more receivers wish to have pushed from the publisher without the need for further solicitation.
In the context of Concise YANG Telemetry, this is a parent term that encompasses the concepts of YANG Datastore Subscriptions and YANG Customized Subscriptions.

2.1. CoAP Terminology

In addition to the illustration of the scope of YANG Telemetry above, this section highlights the most important terms that are vital to the functionality of Concise YANG Telemetry with respect to the Constrained Application Protocol:

CoAP Call Home:
A procedure similar to the one defined in [RFC8071] NETCONF Call Home and RESTCONF Call Home, in which a YANG datastore can trigger a YANG Client to initiate a YANG Subscription (typically by taking on the role of a CoAP client and indicating to a CoAP node that is including a YANG Client to initiate a YANG Subscription).
CoAP Client:
The entity that emits CoAP requests to a CoAP Node with server capabilities. In the context of Concise YANG Telemetry, a YANG client, for example. that is creating dynamic subscriptions is a CoAP Client.
CoAP Token:
A token used to match a CoAP response with a CoAP request [RFC7252]. It is intended for use as a client-local identifier to differentiate between concurrent requests (similar to a request ID).
A CoAP Token must be known, or at least deterministically inferable by a YANG Client in order to receive a Series of CoAP responses that compose Concise YANG Telemetry.

3. Summary of the Additions to CoMI

This documents defines the binding of YANG Datastore Subscriptions and YANG Customized Subscriptions to the CoAP Management Interface. In summary, these additions include:

3.1. Telemetry-Specific CoMI datastores

The CoMI architecture (and YANG in general) assumes that both YANG client and YANG datastores (server) retain or have access to knowledge about the same YANG specification (see Figure 1 in [I-D.ietf-core-comi]). This is not necessarily true for a YANG Telemetry capable CoMI server. Highly constrained nodes can emit Series of subscribed notifications without previous solicitation: this allows them to create well-formed YANG-modeled Telemetry from hard-coded building blocks of YANG-modeled data, which are in compliance to YANG modules. In consequence, while taking on the role of a YANG datastore, a YANG Subscription capable CoMI server SHOULD be capable to process YANG queries, but MAY not be due to the lack of corresponding functions or knowledge of the resulting structures received.

As these flavors of YANG datastores are not necessarily able to create CoMI responses based on client request, it is likely that highly constrained datastores initiate a Call-Home procedure (see [RFC8071]) acting as if a request was already received (see Configured Subscription above), soliciting a very specific request they can fulfill (dynamic subscription) or rendezvous via a discoverable YANG Zero Touch entity. In all these usage scenarios, the datastore intends to create device specific YANG Telemetry to be conveyed to corresponding YANG clients.

In essence, incorporating a complete YANG module on a CoMI datastore that is capable of YANG Subscriptions is not required to enable the initiation of Concise YANG Telemetry within a very specific scope. This kin of Telemetry-Specific CoMI datastore is therefore not a fully YANG 1.1 compliant datastore, but able to create valid YANG modeled YANG Data Items.

4. Subscription Content and State

Two generic YANG notification statements for Update Records are introduced by YANG Datastore Subscriptions [I-D.ietf-netconf-yang-push] augments to enable the following capabilities:

push-update:
A notification that includes a complete (and potentially filtered) update of data node values of YANG datastore nodes per the terms of a subscription.
push-change-update:
A notification that includes an incremental (and potentially filtered) update of data node values of YANG datastore nodes since the last (change-)update notification.

Every Update Record in a Series that is generated in the context of a subscription is emitted per the characteristics of the subscription state maintained by the CoMI datastore. Subscription state can be created on the CoMI datastore during manufacturing, onboarding, enrollment, deployment, or maintenance of the CoMI datastore. Most typically, subscription state is created by a YANG Client (e.g. a Network Management System) via a dynamic subscription.

4.1. Selection Filter

A vital part of the subscription state that defines the content of a YANG Telemetry stream is the filter expression associated with the subscription characteristics. A filter expression enables a CoMI datastore to emit only a subset of potential notification content; reducing the volume of data in motion, significantly.

Three types of Filter Expressions enable a CoMI datastore to emit filtered subsets of data node value updates:

Subtree Filter Expression:
A SID pointing to a specific data node in a YANG module (including notification statements) is used to create update records that include updates about the identified data node and its potential child nodes. Effectively, a single SID points to the root node of the subtree update records are created for.
XPATH Filter Expression:
A more detailed selection of SIDs and corresponding data node values that update records are created for. The corresponding representation of XPATH Filter Expressions for COMI is defined in [I-D.ietf-core-yang-cbor].
Conditional SID Selectors (experimental):
The SID concept introduced by CoMI and represented via YANG modeled date conveyed using CBOR allows for a simplified Filter Expression data model that retains most of the capabilities of an XPATH Filter Expression, while using a significantly simpler model definition.

5. Subscription Characteristics

Distinct YANG Telemetry streams are defined by the following three primary subscription characteristics:

  1. Subscription Trigger (dynamic / configured)
  2. Subscription Interval (periodic / on-change)
  3. Subscription Type (stream / datastore)

These characteristics define how subscription state is created and how the resulting Telemetry streams behave. In general, corresponding subscription state can be created by a YANG client via the “establish-subscription” RPC as defined in [I-D.ietf-netconf-subscribed-notifications].

5.1. Subscription Trigger

There are three options how to establish a YANG Telemetry stream via YANG Subscriptions:

  1. a YANG client starts to receive a Telemetry stream from a YANG datastore, without previous solicitation. To enable this, persistent subscription characteristics must have been configured on the YANG datastore earlier (e.g. during onboarding), but may be left unknown to the YANG Client. They then have to be acquired or inferred via procedures that are—at the time of this writing—out-of-scope of this document. In essence, corresponding CoAP Tokens are unknown to the YANG Client when the first YANG Data Item is received from the datastore.
  2. a YANG datastore initiates contact with a YANG client via a Rendezvous, Join, or Call Home procedure and triggers the creation of a Telemetry stream by the YANG Client. In this case, the subscription characteristics are provisioned by the YANG Client and come into effect after a YANG datastore successfully discovered the corresponding YANG Client that is its home. In essence, the corresponding CoAP Tokens are created by the CoAP Client and are conveyed to the YANG datastore via the traditional CoAP interaction model.
  3. a YANG client starts to receive a Telemetry stream from a YANG datastore, after being solicited to do so. In this case, persistent subscription characteristics are known by a YANG Client and a YANG datastore before deployment or acquired after their deployment. In essence, corresponding CoAP Tokens and Subscription State are already known by the YANG Client and the YANG datastore when the first YANG Data Item is received from the datastore.

5.1.1. CoMI Configured Subscriptions

CoAP defines a strict coupling of request and corresponding response messages via the CoAP Token. Every CoAP Request MUST include a CoAP Token that is generated by the CoAP Client. Analogously, every CoAP response that is associated with that request MUST include the corresponding CoAP Token in order for the CoAP Request not to be discarded.

In order to enable this type of YANG Subscription, one or more CoMI Clients have to retain or gain knowledge about the corresponding CoAP Tokens (via declarative guidance, a distribution mechanism, or by inferring them via a Call Home procedure), for which they are intended to receive Concise YANG Telemetry by. This implies the existence of a deployed solution that enables a secure and resilient distribution of corresponding CoAP Tokens in a group of CoMI Clients. The exact architecture of this solution is—at the time of this writing—out-of-scope of this document.

5.1.2. Dynamic Subscriptions

This subscription trigger requires knowledge about potential YANG datastores to subscribe to by a YANG Client. This subscription characteristics have to be pre-configured or discoverable by the YANG Client. A typical procedure to facilitate a dynamic subscription is the Call Home discovery mechanism.

5.2. Subscription Interval

The subscription interval is a specific Subscription Characteristic that defines the events that trigger emission of an Update Record in the context of a YANG Subscription. There are two types of subscription interval: periodic subscription and on-change subscription.

5.2.1. Periodic Subscription

A periodic subscription uses a timer in order to emit an Update Record in the context of a YANG Subscription. This type of Subscription Characteristic is intended to be used if Data Node Values change with a regular, predicable cadence. A typically example of Data Node Values that benefit from this type of Subscription Characteristic is used are PDU Counters.

5.2.2. On-Change Subscription

An on-change subscription uses the event of a Data Node Value change to emit an Update Record in the context of a YANG Subscription. This type of Subscription Characteristic is intended to be used if Data Node Values changes only occasionally, but conveyance of information about that change in a timely fashion is required. Typical examples are, deployment of a new IEEE 802.1AR LDevID, or the modification of an ACL by a logged in user.

5.2.2.1. On-Change Subscription Prerequisites

An on-change subscription capability MUST be explicitly annotated in a YANG module definition in order to prevent a meaningless or harmful association of Subscription Characteristics to a YANG Subscription. E.g. it is advisable not to allow for on-change subscriptions to data definition statements that provide Data Node Values representing rapidly changing counters.

The actual syntax and corresponding semantics of data definition statements that are intended to allow for associating an on-change Subscription Characteristic with a YANG Datastore Subscription to a YANG module is out-of-scope of [I-D.ietf-netconf-yang-push] or [I-D.ietf-netconf-subscribed-notifications] and—in consequence—also out-of-scope of this document. Mechanisms being proposed include [draft-lengyel-netconf-notification-capabilities-01].

5.2.2.2. Visibility

In usage scenarios that require a high level of assurance with respect to Visibility (most prominently security-related events) it is vital for a YANG Client to gain knowledge about a deterioration of Visibility of Update Records.

In order to request complete Visibility of every Data Node Value change via a corresponding Update Record, the dampening-period (see section 4.2. in [I-D.ietf-netconf-yang-push] MUST be set to 0. The use of YANG Notification Bundle Messages [I-D.ietf-netconf-notification-messages] can mitigate the deteriorating impact of a dampening-period higher than 0, but can still result in missed Update Records in an constrained-node environment.

If the conveyance of an on-change Update Record failed, lost records can be recognized as a sequence number of pushed updates is included with each sequential update record.

If the assambly of an Update Record is incomplete, datastore MUST include an “updates-not-sent” flag in the next Update Record.

There are usage scenarios, in which complete Visibility of every Change to Data Node Values, but only the information that there was a Data Node Values change occurred is appropriate. In these cases, a dampening-period higher than 0 should be used. A prominent example are “interface-flapping” events.

5.3. Subscription Type

The YANG Telemetry subscription trigger mechanisms illustrated above creates subscription state between a YANG client and a YANG datastore. As long as this subscription state between these two entities persists, a datastore can emit Series of YANG notifications to a YANG client, if appropriate conditions are met, e.g. the YANG client expects solicited event notifications coming from the datastore due to a dynamic subscription.

YANG Datastore Subscription [I-D.ietf-netconf-yang-push] and YANG Subscribed Notifications [I-D.ietf-netconf-subscribed-notifications] extend this mechanism by enabling subscriptions to changes of YANG module data node state in a YANG datastore resulting in two types of sources - or two different types of YANG Notification Series [I-D.bormann-t2trg-stp], respectively:

5.3.1. Stream Subscription

5.3.2. Datastore Subscription

6. Resilient Subscriptions

In usage scenarios with a group of more than one CoMI Client a CoMI datastore can potentially convey Concise YANG Telemetry to, a YANG Subscription can be maintained in a more resilient manner. Emitting a CoAP response in a confirmable message enables a CoMI datastore to detect that a corresponding CoMI Client became unavailable (due to missing confirmation messages). In order to create a Resilient Subscription, a detected loss of a CoMI Client MUST immediately re-trigger the CoAP Call Home procedure in order to discover an equivalent “new home” to send the corresponding Concise YANG Telemetry Stream to. The maximum interval between confirmable message as a part of the Concise YANG Telemetry stream is 24 hours. The interval can be chosen smaller and appropriate to the requirements of the usage scenario. Theoretically – but not necessarily advisable in a constrained-node environment – every CoAP response can be send in a confirmable message.

7. Subscription Management (better word?)

7.1. YANG RPCs

7.2. NETCONF Access Control Model [RFC6536bis]

8. Selection Filters

8.1. SID for subtree Selection Filter

8.2. CBOR-YANG for XPATH-like Selection Filter

9. Update Triggers for Periodic Subscriptions

9.1. Interval

9.2. Anchor Time

10. Update Triggers for On-Change Subscriptions

10.1. dampening period

10.2. change-type

10.3. no sync-on-start

11. YANG Telemetry Operations for COMI

Every subscription-id is created by the YANG datastore and is used in the corresponding subscription state to provide the root identifier, by which dedicated subscription characteristics are associated with an established subscription. In consequence, the basic interaction model of Concise YANG Telemetry is split into two operations that are initiated by the YANG client in sequence:

11.1. Extension of the CoMI Event Stream Resource

A standard CoMI datastore as defined in [I-D.ietf-core-comi] typically uses the datastore resource “/c” to provide the YANG datastore tree and the resource “/s” to provide the YANG notification stream. Sub-resources under “/c” are represented in the format of /c/sid.

Concise YANG Telemetry extends the scope of the “/s” resource. Sub-resources under “/s” are represented as /s/key, where key is a numeric string representation of the subscription identifier, e.g. “/s/65536/”. The key representation reduces the ambiguity with respect to sid, which uses an URI safe base64 representation.

11.2. Extension of the CoMI Datastore Resource

Each subscription identifier key is instantiated as a sub-resource of the “/c/subid” resource, a YANG tree instance of the subscription characteristics yang:ietf-subscribed-notifications/subscriptions (as defined in YANG Datastore Subscription [I-D.ietf-netconf-yang-push], which augments ietf-subscribed-notification defined in [I-D.ietf-netconf-subscribed-notifications]) is provided here for each active subscription.

11.3. Extension of the YANG Subscription Mechanism

YANG Customized Subscriptions provides augmented RPC for establishing, modifying, deleting, or killing a subscription. CoMI uses the same module as YANG Datastore Subscription and provides a corresponding interface to allow for a corresponding confirmable POST message to RPC resources (see [I-D.ietf-core-comi] Section 5.3.2.).

Concise YANG Telemetry also defines the capabilities to point confirmable FETCH messages – including the Observe option - to sub-resources provided by “/c”. If the body of the FETCH message includes a CBOR modeled [I-D.ietf-core-yang-cbor] subtree filter expression, a new subscription is created and a corresponding subscription id is returned. Additionally, a corresponding subscription sub-resource under “/s” is created.

As usual in CoMI, iPATCH requests can be used to perform a number of operations on the datastore in one request, such as deleting, creating, and updating subscriptions.

12. Upcoming Features and Stories

13. IANA considerations

This document includes no requests to IANA, but solutions drafts incubated via this document might.

14. Security Considerations

This document includes no security considerations, but solution drafts incubated via this document will.

15. Acknowledgements

Carsten Bormann, Klaus Hartke, Michel Veillette

16. Change Log

First version -00

17. Normative References

[I-D.bormann-t2trg-stp] Bormann, C. and K. Hartke, "The Series Transfer Pattern (STP)", Internet-Draft draft-bormann-t2trg-stp-01, July 2018.
[I-D.ietf-core-comi] Veillette, M., Stok, P., Pelov, A. and A. Bierman, "CoAP Management Interface", Internet-Draft draft-ietf-core-comi-03, June 2018.
[I-D.ietf-core-yang-cbor] Veillette, M., Pelov, A., Somaraju, A., Turner, R. and A. Minaburo, "CBOR Encoding of Data Modeled with YANG", Internet-Draft draft-ietf-core-yang-cbor-06, February 2018.
[I-D.ietf-netconf-notification-messages] Voit, E., Birkholz, H., Bierman, A., Clemm, A. and T. Jenkins, "Notification Message Headers and Bundles", Internet-Draft draft-ietf-netconf-notification-messages-03, February 2018.
[I-D.ietf-netconf-subscribed-notifications] Voit, E., Clemm, A., Prieto, A., Nilsen-Nygaard, E. and A. Tripathy, "Customized Subscriptions to a Publisher's Event Streams", Internet-Draft draft-ietf-netconf-subscribed-notifications-14, July 2018.
[I-D.ietf-netconf-yang-push] Clemm, A., Voit, E., Prieto, A., Tripathy, A., Nilsen-Nygaard, E., Bierman, A. and B. Lengyel, "YANG Datastore Subscription", Internet-Draft draft-ietf-netconf-yang-push-17, July 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016.
[RFC8040] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017.
[RFC8071] Watsen, K., "NETCONF Call Home and RESTCONF Call Home", RFC 8071, DOI 10.17487/RFC8071, February 2017.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[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.

Authors' Addresses

Henk Birkholz Fraunhofer SIT Rheinstrasse 75 Darmstadt, 64295 Germany EMail: henk.birkholz@sit.fraunhofer.de
Eric Voit Cisco Systems EMail: evoit@cisco.com