Re: [netmod] Issue with path statements in RPC

tom petch <ietfc@btconnect.com> Mon, 10 August 2020 09:39 UTC

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From: tom petch <ietfc@btconnect.com>
To: Martin Björklund <mbj+ietf@4668.se>, "Italo.Busi@huawei.com" <Italo.Busi@huawei.com>
CC: "teas@ietf.org" <teas@ietf.org>, "netmod@ietf.org" <netmod@ietf.org>
Thread-Topic: [netmod] Issue with path statements in RPC
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Date: Mon, 10 Aug 2020 09:38:48 +0000
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Subject: Re: [netmod] Issue with path statements in RPC
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From: netmod <netmod-bounces@ietf.org> on behalf of Martin Björklund <mbj+ietf@4668.se>
Sent: 10 August 2020 10:24

Italo Busi <Italo.Busi@huawei.com> wrote:
> 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.
>
> 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";
>     }

The leafref path is an XPath expression, which operates on the data
tree (instance data).  The "input" node is not present in the instance
data / data tree.  See section 6.4.1 of RFC 7950.

So the latter path (as verfied by pyang 2.1) is correct.

<tp>
Martin,

Thank you for the clarifications.  Just looking at the first issue, what gave me pause was section 9.9; the path statement is used to identify the referred leaf in the schema tree and 'input' is in the schema tree.  But I can see that 6.4.1 overrides that implication.

Tom Petch



> 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?

Yes, this is correct.  It is not possible to refer to the
corresponding "input" instance data in the XPath expressions for the
"output".


/martin


>
> Thanks for your help
>
> Sergio and Italo (on behalf of co-authors/contributors)
>
> Italo Busi
> Principal Optical Transport Network Research Engineer
>
> [cid:image001.jpg@01D5AC11.9575BB40]
> ____________________________________________________________________
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