Re: [Lsr] [netmod] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg

"Rob Wilton (rwilton)" <rwilton@cisco.com> Mon, 10 June 2019 07:55 UTC

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From: "Rob Wilton (rwilton)" <rwilton@cisco.com>
To: Qin Wu <bill.wu@huawei.com>, Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>, Xufeng Liu <xufeng.liu.ietf@gmail.com>
CC: "lsr@ietf.org" <lsr@ietf.org>, NETMOD WG <netmod@ietf.org>
Thread-Topic: [netmod] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg
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Date: Mon, 10 Jun 2019 07:55:43 +0000
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Subject: Re: [Lsr] [netmod] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg
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Fore hierarchical configuration, generally the YANG default statement only makes sense for the top level, i.e. the least specific, configuration node.

Hence, I think that it would be better if the level-1 and level-2 containers did not use a YANG default statement at all.  Instead, the description should explain (perhaps once at the level-1/level-2 container level, or alternatively for every leaf) that the configuration is hierarchical, and if not explicitly configured for a particular level, it uses whatever value is either explicitly configured, or otherwise the default, associated with the top level container.

There is an example of this (but without a top level default) in appendix C.2 of RFC8342.

Thanks,
Rob


-----Original Message-----
From: netmod <netmod-bounces@ietf.org>; On Behalf Of Qin Wu
Sent: 10 June 2019 03:37
To: Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>;; Xufeng Liu <xufeng.liu.ietf@gmail.com>;
Cc: lsr@ietf.org; NETMOD WG <netmod@ietf.org>;
Subject: Re: [netmod] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg

I think what they are looking for in RFC7950 is generic overridden rule, i.e., a parent node statement can be overridden by its child node substatement.

-Qin
-----邮件原件-----
发件人: netmod [mailto:netmod-bounces@ietf.org] 代表 Juergen Schoenwaelder
发送时间: 2019年6月9日 23:28
收件人: Xufeng Liu <xufeng.liu.ietf@gmail.com>;
抄送: lsr@ietf.org; NETMOD WG <netmod@ietf.org>;
主题: Re: [netmod] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg

Hi,

YANG does not have 'levels'. This seems to be an ISIS specific question you should ask on the ISIS list.

/js

On Sun, Jun 09, 2019 at 10:35:11AM -0400, Xufeng Liu wrote:
> In Section 2.3. and many other locations, the current IS-IS model 
> applies the parameter overriding rule as below:
> 
> [Quote]:
> 
> 2.3 <https://tools.ietf.org/html/draft-ietf-isis-yang-isis-cfg-35#section-2..3>;.
> Per-Level Parameters
> 
> 
>    Some parameters allow a per level configuration.  In this case, the
>    parameter is modeled as a container with three configuration
>    locations:
> 
>    o  a top-level container: corresponds to level-1-2, so the
>       configuration applies to both levels.
> 
>    o  a level-1 container: corresponds to level-1 specific parameters.
> 
>    o  a level-2 container: corresponds to level-2 specific parameters.
> 
>                +--rw priority
>                |  +--rw value?     uint8
>                |  +--rw level-1
>                |  |  +--rw value?   uint8
>                |  +--rw level-2
>                |     +--rw value?   uint8
> 
>    Example:
> 
>            <priority>
>                <value>250</value>
>                <level-1>
>                    <value>100</value>
>                </level-1>
>                <level-2>
>                    <value>200</value>
>                </level-2>
>            </priority>
> 
>    An implementation SHOULD prefer a level specific parameter over a
>    level-all parameter.  As example, if the priority is 100 for the
>    level-1, 200 for the level-2 and 250 for the top-level configuration,
>    the implementation should use 100 for the level-1 and 200 for the
>    level-2.
> 
> [End of Quote]
> 
> 
> In the model, all three value leaves above have a default statement 
> “default 64”, which brings up my question for the following example:
> 
> 
>            <priority>
>                <value>250</value>
>                <level-1>
>                    <value>100</value>
>                </level-1>
>            </priority>
> 
> 
> The user does not provide a configured value for level-2. According to 
> Section 7.6.1. of RFC7950, because the default value is in use, “the 
> server MUST operationally behave as if the leaf was present in the 
> data tree with the default value as its value”. This means the 
> priority value for level-2 will be 64 (the default value), so the 
> value 250 can never take effect as intended in the above quoted Section 2.3.
> 
> 
> Is my understanding correct?
> 
> 
> Since this is a generic question, I am CC’ing NETMOD WG too.
> 
> 
> Thanks,
> 
> - Xufeng

> _______________________________________________
> netmod mailing list
> netmod@ietf.org
> https://www.ietf.org/mailman/listinfo/netmod


-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>

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