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[netmod] rfc7223bis Backward compatibility in YANG - BAD !!!

Balazs Lengyel <balazs.lengyel@ericsson.com> Thu, 09 November 2017 17:00 UTC

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From: Balazs Lengyel <balazs.lengyel@ericsson.com>
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Subject: [netmod] rfc7223bis Backward compatibility in YANG - BAD !!!
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Hello,
In draft-ietf-netmod-rfc7223bis-00 I read

The "/interfaces-state" subtree with "config false" data nodes is
    deprecated.  ...

    Servers that do not implement NMDA, or that wish to support clients
    that do not implement NMDA, MAY implement the deprecated
    "/interfaces-state" tree.

This means that a server MAY simply remove a big branch 
(interfaces-state). If I have a network management system (NMS) and the 
YANG server upgrades to a new revision of the module suddenly my NMS SW 
breaks.

I find it really strange that, on a one hand YANG has very strict rules 
about updating a module, on the other hand just by deprecating something 
you can remove anything. Are we taking backward compatibility seriously 
or not? It seems not.

Earlier we had discussions about the definition of deprecated and 
obsolete in https://tools.ietf.org/html/rfc7950#section-7.21.2.
 From an NMS point of view the current definition just states: Anything 
can be deprecated/obsoleted anytime. If something has a status different 
from current there is no guarantee its usable or even that it exists.

IMHO these definitions are unusable for an NMS. Ericsson needed to 
introduce its own rules. IETF should do something about this! I have 
proposals if there are people interested.

As a minimum I would propose, that a server that does not implements a 
fully functional /interfaces-state" subtree MUST obsolete it, not just 
deprecate it.

regards Balazs

-- 
Balazs Lengyel                       Ericsson Hungary Ltd.
Senior Specialist
Mobile: +36-70-330-7909              email: Balazs.Lengyel@ericsson.com

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