Comparison of NMDA datastores

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 7 February 2022.¶

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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.¶

Table of Contents

1. Introduction

The revised Network Management Datastore Architecture (NMDA) [RFC8342] introduces a set of new datastores that each hold YANG-defined data [RFC7950] and represent a different "viewpoint" on the data that is maintained by a server. New YANG datastores that are introduced include <intended>, which contains validated configuration data that a client application intends to be in effect, and <operational>, which contains operational state data (such as statistics) as well as configuration data that is actually in effect.¶

NMDA introduces in effect a concept of "lifecycle" for management data, distinguishing between data that is part of a configuration that was supplied by a user, configuration data that has actually been successfully applied and that is part of the operational state, and overall operational state that includes applied configuration data as well as status and statistics.¶

As a result, data from the same management model can be reflected in multiple datastores. Clients need to specify the target datastore to be specific about which viewpoint of the data they want to access. For example, a client application can differentiate whether they are interested in the configuration supplied to a server and that is supposed to be in effect, or the configuration that has been applied and is actually in effect on the server.¶

Due to the fact that data can propagate from one datastore to another, it is possible for differences between datastores to occur. Some of this is entirely expected, as there may be a time lag between when a configuration is given to the device and reflected in <intended>, until when it actually takes effect and is reflected in <operational>. However, there may be cases when a configuration item that was to be applied may not actually take effect at all or needs an unusually long time to do so. This can be the case due to certain conditions not being met, certain parts of the configuration not propagating because they are considered inactive, resource dependencies not being resolved, or even implementation errors in corner conditions.¶

When configuration that is in effect is different from configuration that was applied, many issues can result. It becomes more difficult to operate the network properly due to limited visibility of actual operational status which makes it more difficult to analyze and understand what is going on in the network. Services may be negatively affected (for example, degrading or breaking a customer service) and network resources may be misallocated.¶

Applications can potentially analyze any differences between two datastores by retrieving the contents from both datastores and comparing them. However, in many cases this will be at the same time costly and extremely wasteful.¶

This document introduces a YANG data model which defines RPCs, intended to be used in conjunction with NETCONF [RFC6241] or RESTCONF [RFC8040], that allow a client to request a server to compare two NMDA datastores and report any differences.¶

4. Data Model Overview

The core of the solution is a new management operation, <compare>, that compares the data tree contents of two datastores. The operation checks whether there are any differences in values or in data nodes that are contained in either datastore, and returns any differences as output. The output is returned in the format specified in YANG-Patch [RFC8072].¶

The YANG data model defines the <compare> operation as a new RPC. The operation takes the following input parameters:¶

• source: The source identifies the datastore that will serve as the reference for the comparison, for example <intended>.¶
• target: The target identifies the datastore to compare against the source, for example <operational>.¶
• filter-spec: This is a choice between different filter constructs to identify the parts of the datastore to be retrieved. It acts as a node selector that specifies which data nodes are within the scope of the comparison and which nodes are outside the scope. This allows a comparison operation to be applied only to a specific part of the datastore that is of interest, such as a particular subtree. Note, the filter does not allow expressions that match against data node values, since this may incur implementation difficulties and is not required for normal use cases.¶
• all: When set, this parameter indicates that all differences should be included, including differences pertaining to schema nodes that exist in only one of the datastores. When this parameter is not included, a prefiltering step is automatically applied to exclude data from the comparison that does not pertain to both datastores: if the same schema node is not present in both datastores, then all instances of that schema node and all its descendants are excluded from the comparison. This allows client applications to focus on the differences that constitute true mismatches of instance data without needing to specify more complex filter constructs.¶
• report-origin: When set, this parameter indicates that origin metadata should be included as part of RPC output. When this parameter is omitted, origin metadata in comparisons that involve <operational> is by default omitted. Note that origin metadata only applies to <operational> it is therefore also omitted in comparisons that do not involve <operational> regardless of whether or not the parameter is set.¶

The operation provides the following output parameter:¶

• differences: This parameter contains the list of differences. Those differences are encoded per the YANG-Patch data model defined in RFC8072. When a datastore node in the source of the comparison is not present in the target of the comparison, this can be indicated either as a "delete" or as a "remove" in the patch as there is no differentiation between those operations for the purposes of the comparison. The YANG-Patch data model is augmented to indicate the value of source datastore nodes in addition to the patch itself that would need to be applied to the source to produce the target. When the target datastore is <operational> and the input parameter "report-origin" is set, "origin" metadata is included as part of the patch. Including origin metadata can help in some cases explain the cause of a difference, for example when a data node is part of <intended> but the origin of the same data node in <operational> is reported as "system".¶

The data model is defined in the ietf-nmda-compare YANG module. Its structure is shown in the following figure. The notation syntax follows [RFC8340].¶

module: ietf-nmda-compare
rpcs:
   +---x compare
      +---w input
      |  +---w source            identityref
      |  +---w target            identityref
      |  +---w all?              empty
      |  +---w report-origin?    empty
      |  +---w (filter-spec)?
      |     +--:(subtree-filter)
      |     |  +---w subtree-filter?
      |     +--:(xpath-filter)
      |        +---w xpath-filter?     yang:xpath1.0 {nc:xpath}?
      +--ro output
         +--ro (compare-response)?
            +--:(no-matches)
            |  +--ro no-matches?    empty
            +--:(differences)
               +--ro differences
                  +--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?
                        +--ro source-value?

Authors' Addresses