Re: [netmod] WG Last Call: draft-ietf-netmod-yang-instance-file-format-06

[Balázs Lengyel <[e-mail]>]

-----Original Message-----
From: netmod <[e-mail]> On Behalf Of Schönwälder, Jürgen
Sent: 2020. január 20., hétfő 15:46
To: Kent Watsen <[e-mail]>
Cc: NETMOD Working Group <[e-mail]>
Subject: Re: [netmod] WG Last Call:
draft-ietf-netmod-yang-instance-file-format-06

On Tue, Jan 07, 2020 at 12:41:23PM +0000, Kent Watsen wrote:
> 
> This begins a two-week Working Group Last Call (WGLC) on
draft-ietf-netmod-yang-instance-file-format-06.  The WGLC ends on Jan 21.
Please send your comments to the working group mailing list.
> 
> Positive comments, e.g., "I've reviewed this document and believe it is
ready for publication", are welcome!  This is useful and important, even
from authors.  Objections, concerns, and suggestions are also welcomed at
this time.
>

I have reviewed draft-ietf-netmod-yang-instance-file-format-06. I believe
this is an important document but not quite ready yet. Most of the points I
am raising below should, however, be easy to resolve, many concern
terminology and writing consolidation and do not affect the technical
solution.

/js

* Abstract

  I think we should avoid referring to some <get> operation. Here is a
  proposal of a rewrite:

OLD

   running server available.  This document specifies a standard file
   format for YANG instance data (which follows the syntax and semantic
   from existing YANG models, re-using the same format as the reply to a
   <get> operation/request) and annotates it with metadata.

NEW

   running server available.  This document specifies a standard file
   format for YANG instance data, which follows the syntax and semantic
   of existing YANG models, and annotates it with metadata.
BALAZS: Other have expressly asked for a reference to "get" but if you want
I can remove it.

* Terminology

  - Add missing dots (full stops) at the end of sentences
BALAZS: OK

  - I fail to see the difference between 'content-schema' and 'content
    defining YANG module(s)'. The 'content-schema' is already a set of
    YANG modules. I suggest to remove 'Content defining YANG module(s)
    as it is not a necessary term. Rewrite all places where the phrase
    'content defining YANG modules' is used.
BALAZS: a schema is a full set of YANG modules needed to define the 
structure and properties of the instance data (+features, deviations).  
A  "content defining YANG module" is an individual YANG module is 
part of the content-schema. So the difference is a set versus one item. 
I updated the description to emphasize this difference.

  - Is "YANG Instance Data" a newly defined term? It's introduction
    does not follow the colon style. I also wonder why we need this
    term. Why is YANG in there? I would prefer to have this defined in
    RFC 7950 terms. Is 'instance data' a collection of instantiated
    'data nodes'? Perhaps then we should do the following and move
    this up to the first definition, so we define instance data first,
    then instance data set, and finally instance data file.

    OLD

   YANG Instance Data, or just instance data for short, is data that
   could be stored in a datastore and whose syntax and semantics is
   defined by YANG models.

    NEW

   Instance Data: A collection of instantiated data nodes.
BALAZS: OK, updated. 

* Introduction

  - It seems UC5 subsumes UC4.
BALAS: OK UC4 and 5 merged

  - One could add UCx: Storing instance data used as test cases but
    then this list of use cases does not need to be exhaustive (means
    I do not care much).
BALAZS: Valid use case, but not added for now. If you say so I can add it.

  - Is it necessary to describe P2 in terms of (presumably) NETCONF
    operations? I would prefer to have the document written in a
    protocol agnostic style. Perhaps simply drop "similar to the
    response of a <get> operation/request".
BALAZS: This is a reference both to NETCONF and RESTCONF. It was explicitly
asked for by other reviewers. 

  - P4: What is 'many'? Or did you want to use 'multiple'?
BALAZS: OK, changed to multiple,

* Instance Data File Format

  - Replace "real data" with instance data

  OLD

    "real data" that we want to document/provide.

  NEW

    instance data that we want to document/provide.
BALAZS: OK

  - I do not understand that text about the default attribute. Section
    4.8.9 defines a query parameter, not an attribute. And I do not
    know how that fits into content data.
BALAZS: https://tools.ietf.org/html/rfc8040#section-4.8.9:
" If the "with-defaults" parameter is set to "report-all-tagged", then
   the server MUST adhere to the default-reporting behavior defined in
   Section 3.4 of [RFC6243].  Metadata is reported by the server as
   specified in Section 5.3.  The XML encoding for the "default"
   attribute sent by the server for default nodes is defined in
   Section 6 of [RFC6243].  The JSON encoding for the "default"
   attribute MUST use the same values, as defined in [RFC6243], but
   encoded according to the rules in [RFC7952].  The module name
   "ietf-netconf-with-defaults" MUST be used for the "default"
   attribute. "
Here the usage of the default ATTRIBUTE is defined.

  - Similarly, I do not understand why implementation specific
    metadata may be included in the content-data. This seems to be the
    wrong place, no? Should metadata not go into the header?
BALAZS: As this might be meta-data about the individual instance 
data nodes (e.g.  metadata following the principles from rfc7952) it belongs
here.

  - Why MUST XML attributes be ignored, why is there no text about
    unknown JSON data, 'attributes' (or annotations)? What should
    implementations generally do about unknown elements, attributes,
    objects, arrays, ...)? Why are we specific about only one specific
    case?
BALAZS:  Generally we want to allow users/creators to decorate the data 
with additional information, that is not standardized. Like YANG extensions
 these may be useful, but at least should not cause problems.
XML attributes are often used as meta-data and I was asked to list them
specifically.

It is not stated what an application should do with additional unknown data 
(XML elements, JSON data) that do not fit the above categories.  Should we
say something about it? 
IMHO no. We don't want to be too restrictive, as there are many potential 
users with different needs. We could state
 "Users of the instance data MAY discard any other unknown data".
However that does not mean much. 

  - References may be helpful in this sentence since <get-data> is not
    part of the original NETCONF specification:

   The content-data part will be very similar to the result returned for
   a NETCONF <get-data> or for a RESTCONF get operation.
BALAZS: OK: will add reference.

    It is unclear what "will be very similar" really means but perhaps
    this is clarified later. If not, this sentence says nothing in
    terms of a technical specification.

  - Does the following sentence imply that any additional data in an
    instance file renders the instance file useless?

   The content-data part MUST conform to the content-schema.
BALAZS: Maybe not useless, but at least partly corrupt.

  - You first write that instance data MUST conform to the schema and
    two paragraphs later you state that instance data MAY be partial,
    i.e., it MAY NOT conform to the content-schema. Perhaps I have an
    idea what you wanted to say but the text that is written here is
    a contradiction.
BALAZS: OK. I will correct it. The content-data part MUST conform to 
the content-schema, while allowing for the   exceptions listed below.

  - The introduction contains several MAYs and MUSTS that are not
    understandable yet and they do not seem to belong into an
    'Introduction' in the first place.
BALAZS: Section 2 Introduction 1 'may'
" further instance data formats may be specified"
I was specifically asked to include this. Why is this not understandable? 
Where should this be if not in the introduction chapter?
Section 2 does not contain the word must. 
Maybe I am not understanding your comment.

  - Why is EXTERNAL in all caps but Inline in capitalized form?  In
    the YANG definitions, EXTERNAL seems to be uri. I think we reduce
    ambiguity by being consistent with how we name things.
BALAZS: OK, EXTERNAL should not be all caps. 
Here external means that the content-schema is defined externally 
to the instance data set, not even a URI is included.

  - What is a 'real-life YANG module'?
BALAZS: OK,  will rephrase it. 

  - 3.1.1 How are the details specified in the anydata? Perhaps a
    forward reference might help. What are 'version labels'?
BALAZS: Added reference to example.
Version/Revision labels are defined in
draft-verdt-netmod-yang-module-versioning; 
added as a reference. I added them here (only as an example) as they are 
highly relevant to specifying module versions even if they are not 
agreed in Netmod yet. The name was changed from version-label to
revision-label lately.

  - 3.1.2 What is a 'list of content'? Which revision is used? What
    about these 'version labels' here?
BALAZS: You cut the sentence in half: List of content defining YANG modules"
The term "Content defining YANG module" is defined in the terminology
section.
In this case there is no possibility for using version/revision labels.
People asked for a simple method.

  - 3.2 I do not understand the example. Has this been validated?  As
    far as I can tell, the ietf-yang-library defines modules-state and
    not module-state. This inconsistency shows up multiple times.
BALAZS: Corrected to modules-state

  - I like to understand why we need several methods to specify the
    schema. Having N solution is always bad for interoperability and
    also for maintainability. Perhaps the WG failed to reach consensus
    on a single solution.  Or there are strong technical reasons - but
    then they should be clearly stated. What are implementations
    expected to support, all methods? Or whatever the implementer
    prefers? How do we achieve interoperability across tools?
BALAZS: Different people in the WG wanted different solutions.
- Some (as I remember you too) asked for a full flexible solution 
which can use multiple modules potentially not even the 
ietf-yang-library to define the schema  (Inline solution)
- some asked for a simple solution listing the content schema modules
- some wanted just to use a reference (If any this is the one, I would
remove)
- some stated that they do not want to define the 
content-schema at all because it is already known 
So we ended up with 4 methods

* Data Life Cycle

  - I am not sure the first paragraph is needed.
BALAZS: OK removed

  - In the second paragraph, I like to see some discussion of snapshot
    consistency.  How much consistency can be expected? Are there
    indicators for the level of consistency? I would remove the
    sentence about "valid values can be retrieved at run-time" as this
    is obvious but then I am not sure why 'valid' values? Perhaps the
    authors meant 'current' values?
BALAZS: OK< Changed to current. I want to keep the second sentence 
as it describes the duality between the original documented values and
 the current values that can be read in run-time.
Consistency is out of scope. No indicators are provided. It is very 
much use-case and implementation specific.

  - How do I implement the "SHOULD be described"? The default is that
    data can change, only in rare cases data is static. But how does a
    tool creating instance data know 'when and how' data changes in the
    future? I suggest to remove the SHOULD. The text saying that instance
    data is a snapshot is in my view sufficient.
BALAZS: We do not want to specify the how the changes should be described, 
But we do want to state that this information should be made available.
Just a few ideas how this could be done. Provide
- some plain text in the description of the instance data set
- some additional metadata e.g. etags, timestamp for the individual data
nodes.
- a change indicator in the content defining yang module itself 

  - This section talks about YANG instance data but it likely should
    talk about YANG instance data sets.
BALAZS: I think both are acceptable terms here. Naturally if the data
changes 
the data set containing it also changes.

* Delivery of Instance Data

  - Why do we need this SHOULD? I do not think we should use RFC 2119
    keywords to define how organizations may use the instance data
    format. My proposal is to delete this entire section.
BALAZS: I will change it to lower case may.
I was asked to and I want to state that we want to use instance data 
both for offline delivery of design time information and for run-time
delivery of other data.

(The first 3 users of this format all want to use this for early delivery of

server capabilities. It is for now the dominant use case for which the  
2119 SHOULD is important.). 

* Backwards Compatibility

  - I do not think 'managed entity' is a YANG term.
BALAZS: What term do you propose for something that is managed like 
an interface or user etc. ? I was told managed entity is a generic term 
that is commonly understood . Would "managed item" or "managed thing" be
better?

  - I think this text is use case specific and the items are kind of
    conflicting with each other (2nd says changing the semantics of a
    list should lead to a change of the key while the 1st suggests
    that changing keys may lead to misinterpretation of something
    being new).

  - My proposal is to simply drop this entire section. If use case
    specific text is needed, add it to the use cases in the appendix.
BALAZS: You don't know how many trouble reports we got in 
multiple use-cases for violating these recommendations. While 
they may not be important for all use-cases, the are important for many.
Actually we met the problem or had to avoid it in all but one of 
the listed use-cases.

* YANG Model

  - How is the inline-content-schema feature used? Which component
    does indicate that inline content-schema is supported? Do all
    implementations have to support simplified-inline? If
    inline-schema is used, how do I find out which schema formats are
    supported? The more formats there are, the more interoperability
    issues will arise.
Balazs:
- case inline { is decorated with "if-feature inline-content-schema"
- feature support is generally indicated as part of the ietf-yang-library
- simplified-inline is mandatory to support. It is relatively simple, so
IMHO not a problem
- what do you mean with schema-formats? The yang schema is not actually
included anywhere.
If the "inline" case is used, instance data corresponding to the 
inline-modules is included, not the schema. 
anydata inline-schema {
             description
               "Instance data corresponding to the YANG modules
                specified in the inline-module nodes defining the set
                of content defining YANG modules for this
                instance-data-set."


* Security Considerations

  - "is designed as a wrapper" - what does this tell me? I suggest to
    rewrite the first paragraph and to remove this phrase or to explain
    what it means.

  - Why is the header part not security sensitive? Almost all data is
    security sensitive in certain situations.
BALAZS: IMHO it is a valid and meaningful statement to differentiate 
between security sensitive data like passwords and non-sensitive data 
like a revision date. RFC8341 states:
"One of the most important aspects of the data model documentation,
and one of the biggest concerns during deployment, is the
identification of security-sensitive content." 
So the differentiation between sensitive and non-sensitive information is
important.
In your opinion which part of the header data is sensitive?

  - I would prefer if the text would not use the phrase "result of a
    <get> operation". As stated before, I like to see things written
    in protocol neutral forms.
BALAZS: OK, I will change to the generic "read" operation as used by 
RFC8341 (although for me <get> is much more clear).

  - Since instance data files may require protection, is there any
    recommendation how to do this, e.g., by wrapping everything into a
    cryptographic message syntax or so? It would be important in
    certain use cases to be able to verify that instance data is
    authentic (i.e., it is signed by the original source). In other
    cases, it may be crucial to protect the instance data itself
    against occasional readers.
BALAZS: File security is an important but really big topic and
 I was instructed by multiple people to avoid a half baked discussion on the
topic.

  - It may be useful to explain that data in instance data sets may
    have been filtered by access control rules like NACM and that data
    in instance data sets itself won't be filtered anymore by access
    control rules like NACM. In other words, if I take snapshots and
    stored them as instance data files, these snapshots may leak
    information that is otherwise protected. Hence it is important
    that NACM rules and file access control rules are consistent.
BALAZS: We do not know if the instance data set was originally 
filtered by NACM or not. We don't know if the users on 
Netconf/Restconf/cli are the same as the users defined in the 
file system., so I fear defining what consistent means would be impossible. 
It is stated that " The same kind of handling should be applied, that would
   be needed for the result of a <get> operation returning the same
   data." IMHO we can't really say more.

-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
Fax:   [phone]         <https://www.jacobs-university.de/>

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