[netmod] Abusing YANG extensions

Andy Bierman <andy@yumaworks.com> Wed, 02 May 2018 16:19 UTC

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From: Andy Bierman <andy@yumaworks.com>
Date: Wed, 2 May 2018 09:19:45 -0700
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Subject: [netmod] Abusing YANG extensions
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Hi,

The discussion about yang-data is stuck because the NETMOD WG does not
understand or does not agree on what it means to abuse a YANG extension
and use it improperly.

If a tool implementing a standard cannot do so without implementing
certain extensions, then those extensions are not optional,
but rather they are mandatory, and therefore violating YANG 1.1.

The NACM extensions are OK because they can be safely ignored
unless the implementation supports NACM, These extensions
are only implemented by the server, so the client is not impacted.

The <config> element is a use-case where YANG fails completely.
According to YANG, the <config> element should only contain child nodes
if they are defined directly or defined with the augment-stmt.
YANG says absolutely nothing about nested data nodes that contain
top-level data nodes.

The "mount-point" extension in YANG Schema Mount also violates
YANG 1.1 in this way. A plain client that does not support schema-mount
will find container and list elements that contain undefined child nodes.
If the server chooses to support schema-mount, the client
is forced to support it, and this is a violation of YANG 1.1.

However, the yang-data extension cannot possibly interfere with standard
YANG 1.1 statements because it is used to specify data structures
that are outside the plain data nodes, instead of over-riding the
definitions of the plain data nodes.


Andy