| Network Working Group | A. Clemm |
| Internet-Draft | Y. Qu |
| Intended status: Standards Track | Futurewei Technologies, Inc. |
| Expires: September 20, 2018 | J. Tantsura |
| Nuage Networks | |
| March 19, 2018 |
Comparison of NMDA datastores
draft-clemm-netconf-nmda-diff-03
This document defines an RPC operation to compare management datastores that comply with the NMDA architecture.
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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 at least conceptually 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, allowing to clearly distinguish 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 both 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. This way, an application can differentiate whether they are (for example) interested in the configuration that has been applied and is actually in effect, or in the configuration that was supplied by a client and that is supposed to be in effect.
Due to the fact that data can propagate from one datastore to another, it is possibly 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, 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 status which makes it more difficult to analyze and understand what is going on in the network. Services may be negatively affected (for example, breaking a service instance resulting in service is not properly delivered to a customer) and network resources 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.
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.
At the core of the solution is a new management operation, <compare>, that allows to compare two datastores for the same data. The operation checks whether there are any differences in values or in objects 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:
As part of the differences, it will be useful to include "origin" metadata where applicable, specifically when the target datastore is <operational>. This can help explain the cause of a difference, for example when a data item is part of <intended> but the origin in <operational> is reported as "system". How to best report "origin" metadata is an item for further study, specifically whether it should be automatically returned per default or whether its reporting should be controlled using another RPC parameter.
module: ietf-nmda-compare
rpcs:
+---x compare
+---w input
| +---w source identityref
| +---w target identityref
| +---w (filter-spec)?
| | +--:(subtree-filter)
| | | +---w subtree-filter? <anydata>
| | +--:(xpath-filter)
| | +---w xpath-filter? yang:xpath1.0 {nc:xpath}?
+--ro output
+--ro differences
Structure of ietf-nmda-compare
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].
<CODE BEGINS> file "ietf-nmda-compare@2018-03-19.yang"
module ietf-nmda-compare {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-nmda-compare";
prefix cp;
import ietf-yang-types {
prefix yang;
}
import ietf-datastores {
prefix ds;
}
import ietf-yang-patch {
prefix ypatch;
}
import ietf-netconf {
prefix nc;
}
organization "IETF";
contact
"WG Web: <http://tools.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>
Author: Alexander Clemm
<mailto:ludwig@clemm.org>
Author: Yingzhen Qu
<mailto:yingzhen.qu@huawei.com>
Author: Jeff Tantsura
<mailto:jefftant.ietf@gmail.com>";
description
"The YANG data model defines a new operation, <compare>, that
can be used to compare NMDA datastores.";
revision 2018-03-19 {
description
"Initial revision";
reference
"RFC XXXX: Comparison of NMDA datastores";
}
/* RPC */
rpc compare {
description
"NMDA compare operation.";
input {
leaf source {
type identityref {
base ds:datastore;
}
mandatory true;
description
"The source datastore to be compared.";
}
leaf target {
type identityref {
base ds:datastore;
}
mandatory true;
description
"The target datastore to be compared.";
}
choice filter-spec {
description
"Identifies the portions of the datastores to be
compared.";
anydata subtree-filter {
description
"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;
description
"This parameter contains an XPath expression
identifying the portions of the target
datastore to retrieve.";
}
}
}
output {
container differences {
uses ypatch:yang-patch;
description
"The list of differences, encoded per RFC8072.";
}
}
}
}
<CODE ENDS>
The following example compares the difference between <operational> and <intended> for object "explicit-router-id", as defined in data module [I-D.draft-ietf-ospf-yang].
RPC request:
<rpc message-id=“101”
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0”>
<compare>
<source>operational</source>
<target>intended</target>
<filter-spec>
<xpath-filter>/ospf/instance/explicit-router-id</xpath-filter>
</filter-spec>
</compare>
</rpc>
RPC reply, when a difference is detected:
<rpc-reply
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
message-id="101">
<differences>
<yang-patch>
<patch-id>ospf router-id</patch-id>
<comment>diff between running and startup</comment>
<edit>
<edit-id>1</edit-id>
<operation>replace</operation>
<target>/ospf/instance/explicit-router-id</target>
<value>
<explicit-router-id>1.1.1.1<explicit-router-id>
</value>
</edit>
</yang-patch>
</differences>
</rpc-reply>
RPC reply when no difference is detected:
<rpc-reply
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
message-id="101">
<differences/>
</rpc-reply>
It is conceivable to extend the compare operation with a number of possible additional features in the future.
For one, 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.
A second additional feature could apply a prefiltering step to exclude data from the comparison that can occur in only one datastore but not the other. This could be controlled by adding an additional flag as input to the RPC. For example, if one datastore has only configuration data, data nodes for which config is false could be excluded from the comparison. Of course, those data nodes still constitute a difference, and not every user may be aware of the underlying distinction. As an alternative, a user could also explicitly exclude such nodes from the comparison through use a corresponding filter construct.
This document registers one URI in the IETF XML registry [RFC3688]. Following the format in [RFC3688], the following registration is requested:
This document registers a YANG module in the YANG Module Names registry [RFC6020]. Following the format in [RFC6020], the following registration is requested:
Comparing discrepancies between datastores 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 NACM in order to require proper authorization for requests to be made. Second, server implementations can limit the number of requests that they serve in any one time interval, potentially rejecting requests made at a higher frequency than the implementation can reasonably sustain.
We thank Rob Wilton, Andy Bierman, Martin Bjorklund, and Mahesh Jethanandani for valuable feedback and suggestions on an earlier revision of this document.