[netmod] Please clarify implementation about ‘when’

"Fengchong (frank)" <frank.fengchong@huawei.com> Fri, 06 September 2019 07:19 UTC

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From: "Fengchong (frank)" <frank.fengchong@huawei.com>
To: "netmod@ietf.org" <netmod@ietf.org>
CC: Yangshouchuan <yangshouchuan@huawei.com>, Yangang <yangang@huawei.com>, Qin Wu <bill.wu@huawei.com>
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Date: Fri, 6 Sep 2019 07:18:39 +0000
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Hi all,

In RFC7950 secton 8, several description about when:
In section 8.2<https://tools.ietf.org/html/rfc7950#section-8.2>.2>.  Configuration Data Modifications
   o  If a request modifies a configuration data node such that any
      node's "when" expression becomes false, then the node in the data
      tree with the "when" expression is deleted by the server.
In 8.3.1<https://tools.ietf.org/html/rfc7950#section-8.3.1>.1>.  Payload Parsing

   o  If data for a node tagged with "when" is present and the "when"

      condition evaluates to "false", the server MUST reply with an

      "unknown-element" <error-tag> in the <rpc-error>.

In 8.3.2<https://tools.ietf.org/html/rfc7950#section-8.3.2>.2>.  NETCONF <edit-config> Processing

Modification requests for nodes tagged with "when", and the "when"

      condition evaluates to "false".  In this case, the server MUST

      reply with an "unknown-element" <error-tag> in the <rpc-error>.

YANG module:
module foo {
   namespace “http://foo.com”;
   prefix “foo”;
Leaf a {…}
Leaf b {
  When “a = 10”;
}
}
Scene 1:
The first edit-config request:
<edit-config>
   <target>
      <candidate/>
   </target>
   <config>
      <a xmlns= “http://foo.com”>3</a>
   </config>
</edit-config>
This request will set a = 3.

The second request:
<edit-config>
   <target>
      <candidate/>
   </target>
   <config>
      <a xmlns= “http://foo.com”>10</a>
      <b xmlns= “http://foo.com”>5</b>
   </config>
</edit-config>

According 8.3.1, in rpc payload parsing phase, the a’s value in candidate datastore is 3,so leaf b’s when condition is evaluated to false, server will report ‘unknown-element’ error.
Is it expected by user?
Scene 2:
The first edit-config request:
<edit-config>
   <target>
      <candidate/>
   </target>
   <config>
      <a xmlns= “http://foo.com”>10</a>
   </config>
</edit-config>
This request will set a = 10.

The second request:
<edit-config>
   <target>
      <candidate/>
   </target>
   <config>
      <a xmlns= “http://foo.com”>3</a>
      <b xmlns= “http://foo.com”>5</b>
   </config>
</edit-config>
According 8.3.1, in rpc payload parsing phase, the a’s value in candidate datastore is 10, so leaf b’s when condition is evaluated to true, server will accept this request in payload parsing phase.

In edit-config request processing phase, if leaf a’s modification is processed firstly, the a’s value will be changed to 3, so the b’s when condition will be false, when server process b’s modification, b will be treated as unknown-element, the edit-config request will fail.
If leaf b’s modification is processed firstly, server will accept this modification ,because b’s when condition is true, and when server process a’s modification , this modification will be accepted, and b’s when condition will be evaluated to false, leaf b will be deleted automatically, the edit-config request will be OK.

How server should process this situation?