Re: [netmod] Issue with path statements in RPC

[tom petch <[e-mail]>]

From: netmod <[e-mail]> on behalf of Italo Busi <[e-mail]>
Sent: 04 August 2020 08:49

We have found some issues with RPC XPaths when developing the YANG code for https://tools.ietf.org/html/draft-ietf-teas-yang-path-computation

As discussed during the TEAS WG session in IETF 108, this issue has been raised on pyang github: https://github.com/mbj4668/pyang/issues/662

It was also suggested to check with Netmod WG what should be the correct behavior according to RFC7950.

<tp>
Italo

While waiting for those with greater understanding than I to come up with the answer about whether or not 'input' should or should not be present in the XPath, I had another look at RFC7950 and decided that I could argue it either way; it comes down to schema tree versus data tree, both of which figure in the definition of leafref,  but this is just my speculation,

Tom Petch.

The following code is accepted by pyang 1.75 but not by pyang 2.1:

    type leafref {
      path "/te:tunnels-path-compute/te:input/"
         + "te:path-compute-info/"
        + "te-pc:tunnel-attributes/te-pc:tunnel-name";
    }

The following code instead is accepted by pyang 2.1 but not by pyang 1.7.5:

    type leafref {
      path "/te:tunnels-path-compute/"
         + "te:path-compute-info/"
        + "te-pc:tunnel-attributes/te-pc:tunnel-name";
    }

Moreover the following when statement, which is quite useful to constraint which information is provided by the RPC output based on some attributes in the RPC input, is accepted by pyang 1.7.5 but not accepted by pyang 2.1;

  augment "/te:tunnels-actions/te:output" {
    description
      "Augment Tunnels Action RPC input with path delete result";

    container path-computed-delete-result {
      when "derived-from-or-self(../../te:input/te:action-info/"
         + "te:action, 'tunnel-action-path-compute-delete')";
      description "Path Delete RPC output";
      leaf-list path-compute-transaction-id {
        type string;
        description
          "The list of the transaction-id values of the
           transient states that have been successfully deleted";
      }
    }   // container path-computed-delete-result
  }   // path-delete rpc output


Could you please help us to understand what is the correct behavior?

In case the correct behavior is the one of pyang 2.1, our understanding is that it would not be possible to provide in YANG when constraints for the data nodes in the RPC output depending on specific values of the data nodes used in the RPC input. Therefore these constraints should be specified as behavior and implemented in the back-end.

Is our understanding correct?

Thanks for your help

Sergio and Italo (on behalf of co-authors/contributors)

Italo Busi
Principal Optical Transport Network Research Engineer

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