Re: [yang-doctors] Yangdoctors early review of draft-ietf-dots-telemetry-09

[[e-mail]]

Hi Jan, Martin, 

Thank you for the review. 

Please see inline. 

Cheers,
Med

> -----Message d'origine-----
> De : Martin Björklund [mailto:[e-mail]]
> Envoyé : mardi 30 juin 2020 20:18
> À : [e-mail]; [e-mail]
> Cc : [e-mail]; [e-mail];
> [e-mail]
> Objet : Re: [yang-doctors] Yangdoctors early review of draft-ietf-dots-
> telemetry-09
> 
> Hi,
> 
> Jan Lindblad via Datatracker <[e-mail]> wrote:
> > Reviewer: Jan Lindblad
> > Review result: Not Ready
> >
> > This is the YD early review of draft-ietf-dots-telemetry as requested by
> the
> > DOTS WG.
> >
> > The ietf-dots-telemetry YANG module is not like other YANG modules. It
> > specifically states that it does not pertain to NETCONF/RESTCONF
> management,
> > but is only meant to be mapped to a specific CoAP based protocol
> > "dots-signal-channel" created by the DOTS WG. The draft-ietf-dots-
> telemetry
> > says in section 4.7:
> >
> >    The DOTS telemetry module (Section 9.1) is not intended to be used
> >    via NETCONF/RESTCONF for DOTS server management purposes.  It serves
> >    only to provide a data model and encoding, but not a management data
> >    model.  DOTS servers are allowed to update the non-configurable 'ro'
> >    entities in the responses of DOTS telemetry messages.
> >

[Med] I confirm. We are using YANG for structuring the data that will be exchanged between the client and the server. We may think about this as some sort of service model as per RFC8309.

> > While this approach is highly uncommon, after giving it some thought, I
> believe
> > this approach could be useful to client and server implementors. However,
> I
> > find two fundamental problems in the particular way this has been done in
> this
> > case. I believe the problems are fixable, but may involve updating
> existing
> > DOTS RFCs.
> >
> > #1) YANG data declarations used as message bodies
> >
> > You can define your own message types with whatever content you desire in
> YANG.
> > Just use rpc/action and notification statements.
> 
> You may also want to look at RFC 8791, which defines a new YANG
> extension statement "sx:structure".  This statement can be used to
> define messages that are not intended to be generic rpcs or
> notifications.
> 
> 

[Med] Thanks for the pointer, Martin.  

Importing ietf-yang-structure-ext + s/container dots-signal/sx:structure in ietf-dots-signal-channel (RFC8782) would be sufficient to define the structure as per RFC8791 but we will lose the "tagging" of parts of the structure/schema that applies only in the server to client direction.

We used rw/ro so that we can have a compact module/structure that can be used in both directions of the DOTS communication, rather than defining two structures.  

The good news is that with both the current approach in RFC8782 and an updated one that strictly follows RFC 8791, we will have the same JSON/CBOR mappings that are authoritative for our spec (Section 6 of RFC8782). 

Given that we don't have interoperability issues, would it be OK with you if we update draft-ietf-dots-telemetry to add a reference to inform the reader about RFC8791 and that we are not using it here because we are augmenting a module not using sx:structure? 

If there is any RFC8782bis in the future, making changes to follow RFC8791 will be part of the things to be considered.   

> 
> > This draft does not do that,
> > but instead defines message bodies as if they were top level YANG config
> > elements. This obviously violates the aras of YANG that speak about data
> > stores, accessible data trees, and what not. Apart from confusing
> experienced
> > YANG users, the meaning of statements like "config false" is suddenly
> most
> > unclear when the foundations of YANG are disregarded.
> >
> > In my opinion the top level of the module needs to be rewritten to use
> proper
> > YANG rpc/action and notification statements. The existing message bodies,
> and
> > the way they are augmented, could all work very well. It's just the
> framework
> > around them that isn't sound.
> >
> > Unfortunately the problematic top level structures of draft-ietf-dots-
> telemetry
> > are based on structures imported from ietf-dots-signal-channel.yang,
> which is
> > already published in DOTS RFC 8782. I have not been able to find any YD
> review
> > of this module, so I would suspect none were ever performed. To make the
> DOTS
> > signal-channel framework usable, I would suggest obsoleting RFC8782 and
> write a
> > bis with an improved YANG action/notification based foundation. The
> number of
> > changed lines in the YANG can probably be kept rather small.
> >
> > #2) Missing pieces in YANG to dots-signal-channel protocol mapping
> >

[Med] What is key for the mapping is listed in draft-ietf-dots-telemetry-09#section-10. All the foundations are already specified in RFC8782. Section 3 of that RFC describes the rationale. 

> > The draft-ietf-dots-telemetry references CoAP RFC 7959, which defines a
> number
> > of basic management operations (GET, PUT, POST, ...), and CBOR RFC 7049
> content
> > encoding, with the draft-ietf-core-yang-cbor-12 addendum for how YANG
> data is
> > to be encoded in CBOR. Unfortunately, a protocol specification that
> consists of
> > these three references is not quite complete. Since this is an early
> review, it
> > may be that the draft authors are already planning to pull the protocol
> > specification up a notch, or I may have missed some piece(s) of the
> puzzle. In
> > any case, a protocol specification on par with what is available for
> RESTCONF
> > or NETCONF would be required.
> >
> > The dots-signal-channel specification in RFC 8782 makes an Informative
> > Reference to draft specification for a protocol called CoMI
> > (draft-ietf-core-comi-09), which attempts to take on the YANG to CoAP
> mapping.
> > One way of filling in the blanks for dots-signal-channel might be to
> leverage
> > the CoMI work. Fixing #1 would be required to leverage CoMI.
> >
> > Currently, dots-signal-channel protocol specification (taken as the sum
> of
> > CoAP, CBOR and draft-ietf-core-yang-cbor-12) is missing information about
> > operation semantics, operation encoding, error handling, for example.

[Med] Such details are discussed in the base CoAP spec, RFC8782, and error handling specifics are included in this draft.    

 RFC
> 8040
> > sections 5, 6, 7 or RFC 6241 sections 4, 7, 8 contain the sort of
> information
> > I'm looking for.
> >
> > #3) Other than that, probably good
> >
> > Other than issues #1 and #2 above, the module seems to be in pretty good
> shape.
> > Some content is hard to judge since the intended meaning of config
> true/false

[Med] "config false" is used to tag nodes that must not appear in messages sent from the client to the server.

> > for example is unclear, so a new review will be required after the
> fundamentals
> > have been resolved. The module is both easy to read and conforms with
> IETF
> > standards except as noted above, staying on the clean and simple side.
> >

[Med] Thanks. FWIW, we fixed some issues in the examples. The latest version of the spec is available at: https://github.com/boucadair/draft-dots-telemetry.

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