Internet-Draft Comparison of NMDA datastores August 2021
Clemm, et al. Expires 7 February 2022 [Page]
Network Working Group
Intended Status:
Standards Track
A. Clemm
Y. Qu
J. Tantsura
A. Bierman

Comparison of NMDA datastores


This document defines an RPC operation to compare management datastores that comply with the NMDA architecture.

Status of This Memo

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This Internet-Draft will expire on 7 February 2022.

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.

2. Key Words

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.

3. Definitions and Acronyms

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:

The operation provides the following output parameter:

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
   +---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)?
            |  +--ro no-matches?    empty
               +--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?
Figure 1: Structure of ietf-nmda-compare

5. YANG Data Model

<CODE BEGINS> file "ietf-nmda-compare@2021-08-06.yang"

module ietf-nmda-compare {

  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-nmda-compare";

  prefix cmp;

  import ietf-yang-types {
    prefix yang;
    reference "RFC 6991: Common YANG Data Types";
  import ietf-datastores {
    prefix ds;
    reference "RFC 8342: Network Management Datastore
               Architecture (NMDA)";
  import ietf-yang-patch {
    prefix ypatch;
    reference "RFC 8072: YANG Patch Media Type";
  import ietf-netconf {
    prefix nc;
    reference "RFC6241: Network Configuration Protocol (NETCONF)";

  organization "IETF";
    "WG Web:   <>
     WG List:  <>

     Author: Alexander Clemm

     Author: Yingzhen Qu

     Author: Jeff Tantsura

     Author: Andy Bierman

    "The YANG data model defines a new operation, <compare>, that
     can be used to compare NMDA datastores.

     Copyright (c) 2021 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

     This version of this YANG module is part of
     draft-ietf-netmod-nmda-diff-12; see the RFC itself for full
     legal notices.

     NOTE TO RFC EDITOR: Please replace above reference to
     draft-ietf-netmod-nmda-diff-12 with RFC number when published
     (i.e. RFC xxxx).";

  revision 2021-08-06 {
      "Initial revision.
       (1)Please replace the above revision date to
       the date of RFC publication when published.
       (2) Please replace the date in the file name
       (ietf-nmda-compare@2021-08-06.yang) to the date of RFC
       (3) Please replace the following reference to
       draft-ietf-netmod-nmda-diff-12 with RFC number when published
       (i.e. RFC xxxx).";
      "draft-ietf-netmod-nmda-diff-12: Comparison of NMDA

  /* RPC */
  rpc compare {
      "NMDA datastore compare operation.";
    input {
      leaf source {
        type identityref {
          base ds:datastore;
        mandatory true;
          "The source datastore to be compared.";
      leaf target {
        type identityref {
          base ds:datastore;
        mandatory true;
          "The target datastore to be compared.";
      leaf all {
        type empty;
          "When this leaf is provided, all data nodes are compared,
           whether their schema node pertains to both datastores or
           not. When this leaf is omitted, a prefiltering step is
           automatically applied that excludes data nodes from the
           comparison that can occur in only one datastore but not
           the other.  Specifically, if one of the datastores
           (source or target) contains only configuration data and
           the other datastore is <operational>, data nodes for
           which config is false are excluded from the comparison.";
      leaf report-origin {
        type empty;
          "When this leaf is provided, origin metadata is
           included as part of RPC output. When this leaf is
           omitted, origin metadata in comparisons that involve
           <operational> is by default omitted.";
      choice filter-spec {
          "Identifies the portions of the datastores to be
        anydata subtree-filter {
            "This parameter identifies the portions of the
             target datastore to retrieve.";
          reference "RFC 6241, Section 6.";
        leaf xpath-filter {
          if-feature nc:xpath;
          type yang:xpath1.0;
            "This parameter contains an XPath expression
             identifying the portions of the target
             datastore to retrieve.";
          reference "RFC 6991: Common YANG Data Types";
    output {
      choice compare-response {
          "Comparison results.";
        leaf no-matches {
          type empty;
            "This leaf indicates that the filter did not match
             anything and nothing was compared.";
        container differences {
            "The list of differences, encoded per RFC8072 with an
             augmentation to include source values where applicable.
             When a datastore node in the source is not present in
             the target, this can be indicated either as a 'delete'
             or as a 'remove' as there is no difference between
             them for the purposes of the comparison.";
          uses ypatch:yang-patch {
            augment "yang-patch/edit" {
                "Provide the value of the source of the patch,
                 respectively of the comparison, in addition to
                 the target value, where applicable.";
              anydata source-value {
                when "../operation = 'delete'"
                  + "or ../operation = 'merge'"
                  + "or ../operation = 'move'"
                  + "or ../operation = 'replace'"
                  + "or ../operation = 'remove'";
                  "The anydata 'value' is only used for 'delete',
                   'move', 'merge', 'replace', and 'remove'
              reference "RFC 8072: YANG Patch Media Type";


6. Example

The following example compares the difference between <operational> and <intended> for a subtree under "interfaces". The subtree contains a subset of objects that are defined in a YANG data model for the management of interfaces defined in [RFC8343]. For the purposes of understanding the subsequent example, the following excerpt of the data model whose instantiation is the basis of the comparison is provided:

container interfaces {
    "Interface parameters.";
  list interface {
    key "name";
    leaf name {
      type string;
        "The name of the interface".
    leaf description {
      type string;
        "A textual description of the interface.";
    leaf enabled {
      type boolean;
      default "true";
        "This leaf contains the configured, desired state of the

The contents of <intended> and <operational> datastores:

<interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
    <description>ip interface</description>

  <interface or:origin="or:learned">

<operational> does not contain an instance for leaf "description" that is contained in <intended>. Another leaf, "enabled", has different values in the two datastores, being "true" in <operational> and "false" in <intended>. A third leaf, "name", is the same in both cases. The origin of the leaf instances in <operational> is "learned", which may help explain the discrepancies.

RPC request to compare <operational> (source of the comparison) with <intended>(target of the comparison):

<rpc message-id="101"
  <compare xmlns="urn:ietf:params:xml:ns:yang:ietf-nmda-compare"

RPC reply, when a difference is detected:

      <patch-id>interface status</patch-id>
        diff between operational (source) and intended (target)
           <if:enabled or:origin="or:learned">true</if:enabled>
          <if:description>ip interface<description>

The same request in RESTCONF (using JSON format):

POST /restconf/operations/ietf-nmda-compare:compare HTTP/1.1
Content-Type: application/yang-data+json
Accept: application/yang-data+json

{ "ietf-nmda-compare:input" : {
   "source" : "ietf-datastores:operational",
   "target" : "ietf-datastores:intended",
   "report-origin" : null,
   "xpath-filter" : "/ietf-interfaces:interfaces"

The same response in RESTCONF (using JSON format):

HTTP/1.1 200 OK
Date: Thu, 24 Jan 2019 20:56:30 GMT
Server: example-server
Content-Type: application/yang-data+json

{ "ietf-nmda-compare:output" : {
    "differences" : {
      "ietf-yang-patch:yang-patch" : {
        "patch-id" : "interface status",
        "comment" : "diff between intended (source) and operational",
        "edit" : [
            "edit-id" : "1",
            "operation" : "replace",
            "target" : "/ietf-interfaces:interface=eth0/enabled",
            "value" : {
               "ietf-interfaces:interface/enabled" : "false"
            "source-value" : {
               "ietf-interfaces:interface/enabled" : "true",
               "@ietf-interfaces:interface/enabled" : {
                 "ietf-origin:origin" : "ietf-origin:learned"
            "edit-id" : "2",
            "operation" : "create",
            "target" : "/ietf-interfaces:interface=eth0/description",
            "value" : {
               "ietf-interface:interface/description" : "ip interface"

7. Performance Considerations

The compare operation can be computationally expensive. While responsible client applications are expected to use the operation responsibly and sparingly only when warranted, implementations need to be aware of the fact that excessive invocation of this operation will burden system resources and need to ensure that system performance will not be adversely impacted. One possibility for an implementation to mitigate against this is to limit the number of requests that are served to a client, or to any number of clients, in any one time interval, by rejecting requests made at a higher frequency than the implementation can reasonably sustain.

While useful, tools such as YANG Data Models that allow for the monitoring of server resources, system performance, and statistics about RPCs and RPC rates are outside the scope of this document. When defined, any such model should be general in nature and not limited to the RPC operation defined in this document.

8. IANA Considerations

8.1. Updates to the IETF XML Registry

This document registers one URI in the IETF XML registry [RFC3688]. Following the format in [RFC3688], the following registration is requested:

8.2. Updates to the YANG Module Names Registry

This document registers a YANG module in the YANG Module Names registry [RFC6020]. Following the format in [RFC6020], the following registration is requested:

9. Security Considerations

The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC8446].

The NETCONF access control model [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.

NACM specifies access for the server in its entirety and the same access rules apply to all datastores. Any subtrees to which a requestor does not have read access are silently skipped and not included in the comparison.

The RPC operation defined in this YANG module, "compare", may be considered sensitive or vulnerable in some network environments. It is thus important to control access to this operation. This is the sensitivity/vulnerability of RPC operation "compare":

Comparing datastores for differences requires a certain amount of processing resources at the server. An attacker could attempt to attack a server by making a high volume of comparison requests. Server implementations can guard against such scenarios in several ways. For one, they can implement the NETCONF access control model in order to require proper authorization for requests to be made. Second, server implementations can limit the number of requests that they serve to a client in any one time interval, rejecting requests made at a higher frequency than the implementation can reasonably sustain.

10. Acknowledgments

We thank Rob Wilton, Martin Bjorklund, Mahesh Jethanandani, Lou Berger, Kent Watsen, Phil Shafer, Ladislav Lhotka, Tim Carey, and Reshad Rahman for valuable feedback and suggestions.

11. References

11.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <>.
Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, , <>.
Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, , <>.
Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, , <>.
Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, , <>.
Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <>.
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <>.
Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch Media Type", RFC 8072, DOI 10.17487/RFC8072, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, , <>.
Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, , <>.
Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K., and R. Wilton, "Network Management Datastore Architecture (NMDA)", RFC 8342, DOI 10.17487/RFC8342, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.

11.2. Informative References

Bjorklund, M., "A YANG Data Model for Interface Management", RFC 8343, DOI 10.17487/RFC8343, , <>.

Appendix A. Possible Future Extensions

It is conceivable to extend the compare operation with a number of possible additional features in the future.

Specifically, it is possible to define an extension with an optional feature for dampening. This will allow clients to specify a minimum time period for which a difference must persist for it to be reported. This will enable clients to distinguish between differences that are only fleeting from ones that are not and that may represent a real operational issue and inconsistency within the device.

For this purpose, an additional input parameter can be added to specify the dampening period. Only differences that pertain for at least the dampening time are reported. A value of 0 or omission of the parameter indicates no dampening. Reporting of differences MAY correspondingly be delayed by the dampening period from the time the request is received.

To implement this feature, a server implementation might run a comparison when the RPC is first invoked and temporarily store the result. Subsequently, it could wait until after the end of the dampening period to check whether the same differences are still observed. The differences that still persist are then returned.

Authors' Addresses

Alexander Clemm
2330 Central Expressway
Santa Clara,, CA 95050
United States of America
Yingzhen Qu
2330 Central Expressway
Santa Clara,, CA 95050
United States of America
Jeff Tantsura
Andy Bierman