Dear Dots Authors, NETMOD WG,

This is my YD LC review of draft-ietf-dots-telemetry-14. This draft contains two YANG modules, ietf-dots-telemetry and module ietf-dots-mapping. Both modules are unusual from a YANG perspective in that they consist of solely typedefs, groupings and sx:structures, and that their purpose is to define message bodies for a domain specific management protocol.

Since my previous review of this module (-09, June 2020) the underpinnings of the modules have changed significantly with a new RFC in the works, addressing the fundamental issues pointed out at that time. Very good & impressive. I still think there are important issues to resolve, so I will call the current state of -14 "Almost Ready".

Links to previous relevant reviews:
https://datatracker.ietf.org/doc/review-ietf-dots-rfc8782-bis-00-yangdoctors-early-aries-2020-09-23/
https://datatracker.ietf.org/doc/review-ietf-dots-telemetry-09-yangdoctors-early-lindblad-2020-06-30/

Because of this hitherto unusual application of YANG, the usual YD review procedures are not really applicable. Whoever reads this should feel free to comment on which perspectives should be included (or not) in an YD review of a YANG module defining a new protocol.

I think the YD review should not be expected to cover the usability/interoperability of the newly defined protocol as a whole. E.g. which messages are sent when, what is the relevant/reasonable content in each message, are there any races or scalability issues, etc. IMO, such an analysis is essential, but too much work for an YD review (and outside my area of expertise). I have placed my review focus on the clear interpretation of the YANG modules, since that will be key to interoperability.

When YANG is mapped to NETCONF or RESTCONF, there are entire RFCs devoted to describing how that mapping is done, from what "mandatory" actually means in the protocol and how keys are sent, all the way to how the XML or JSON payloads are constructed and potential error messages. A lot more of that mapping is now present in this document, but I still find many aspects that are undefined/unclear.

#1 mandatory true

A few elements are marked mandatory true in the model. The meaning of this is not defined in the draft. I would expect this means clients as well as servers MUST provide a value for the mandatory element in each message if the parent element exists. This would be like how mandatory elements in YANG actions/rpcs/notifications work (and different from how mandatory works with configuration data in NETCONF/YANG).

Section 7.1 states: "DOTS telemetry attributes are optionally signaled and therefore MUST NOT be treated as mandatory fields in the DOTS signal channel protocol." This blanket statement makes things less clear for me. Can I trust the YANG mandatory and key statements to indicate what has to be included, and everything else is optional? If so, I would advise to state just that.

#2 key

There are many key statements around these modules. The exact meaning of the key keyword in this dots-signal protocol is not defined. Are key sets always unique in a list? Are keys mandatory? Are there any sequencing requirements on keys, e.g. do keys need to be sent first (as in NETCONF) in a list element? Such a requirement may not be unreasonable for servers with limited resources.

#3) if-feature

The ietf-dots-mapping.yang module defines one YANG feature and a few instances of model statements conditional on it with an if-feature statement. There is a description for how a server advertises support for this specific feature (by including a very particular leaf in one of the messages). From a YANG mapping perspective, it might have been better to have a general mechanism for advertising features. With the current specification, no other features can (ever) be added in a backwards compatible way.

#5) augment or sx:augment-structure

The draft is using augment (but means sx:augment-structure) and sx:augment-structure. Are implementors allowed to use augment in their implementations? If so, how would a client know if an augmented model is in use? Section 6.1.2 states that servers MUST reject messages when "attribute values are not acceptable". Does that have any implications for augment?

#6) deviations

Deviations are never mentioned. Are implementors allowed to deviate from the protocol in any way, if declared using deviation statements? If so, how would a client know if a deviated model is in use?

#7) config true|false

RFC 8791 (which defines sx:structure) expressly states that config true and false statements have no defined meaning and will be ignored in sx:structure. Therefore it's very good that this draft defines behavior for the GET operation with respect to config true and config false elements. Nothing is said about how the config attribute plays with PUT, POST or DELETE however. 

#8) PATCH

Is the PATCH HTTP operation supported in this context? It is never mentioned. If so, it's is unclear how it would work in relation to the protocol HTTP headers, the tsid mechanism and the YANG mandatory concept. If not, that might be good to point out somewhere.

#9) Payload encoding references

As far as I understand, the message payloads are meant to be encoded in JSON or CBOR. This draft has no references to the relevant documents describing these encodings (https://tools.ietf.org/html/rfc7951 and https://tools.ietf.org/html/draft-ietf-core-yang-cbor-13).


And now a few minor comments on particular points or wordings.

#10) The tree diagram in Fig. 30 is not using the correct format (see the other tree diagrams).

#11) Section 6.3.1 states "The overlapped lower numeric 'tsid' MUST be automatically deleted and no longer be available." Since this mechanism is a central part of the protocol, I would think specifying exactly what "overlapped" means would improve interoperability.

#12) Section 6.3.1 states "If no target clause is included in the request, this is an indication that the baseline information applies for the DOTS client domain as a whole." Define "target clause" (= tsid?)

#13) Section 4.5 states "The use of "identities" is thus suboptimal from a message compactness standpoint." Clearly, but they excel in forward extensibility. There may be cases where the identities would be better and even more compact than an alternative solution without them. To rule out all use of identities based on a blanket statement like this does not seem like a good design practice. I suggest to remove this sentence.

#14) In the YANG, there is a choice with a single case "case server-to-client-only". This way of using choice is a little unusual, but it works.

#15) In the YANG, there is a list telemetry without any key. This is allowed for config false lists in NETCONF/YANG, and it could be allowed in this protocol. For a constrained device, however, this situation may not be ideal. Without a key, the only way for a client to read this information is to read it all at once. If that is the only reasonable use case for this information, you kan keep it as is. If this data may be large, a different way of modeling may be in order so that data could be requested a little at a time.

#16) Section 6.1.1 states " 'server-originated-telemetry' under 'max-config-values' to 'true' ('false' is used otherwise).  If 'server-originated-telemetry' is not present in a response, this is equivalent to receiving a request with 'server-originated-telemetry' set to 'false'. " In YANG, this is usually expressed by adding "default false" to the leaf.

#17) There are many leafs in the models that have no default, no mandatory and no description which tells what happens if the leaf is not provided. E.g. what happens if "query-type" is not specified?

#18) Many lists have "unit" as a key. This can work, but is an unusual choice. It opens up for sending the same information several times over with different units.

#19) Section 7.1.1 states "At least one of the attributes 'target-prefix', 'target-fqdn', 'target-uri', 'alias-name', or 'mid-list' MUST be present in the target definition." This may be a good thing to add in a must-statement, or at least in a description statement in the module.

#20) Section 7.1.1 mentions a "optional subattribute" Please define this term, or change to a defined term.

#21) leaf-list alias-name {type string; description "An alias name that points to a resource."; 
Unclear; what is a resource here? What is the format of the string? Could this be a leafref?

#22) There are typedefs called "unit-type" and "unit". I would suggest changing at least one of those names so that readers have a chance at understanding how they differ. Also, some of their content overlaps, so maybe one could incorporate the other?

#23) Similarly for grouping percentile-config and grouping percentile. Names that better reflect their content and usage would be good.

#24) There are still a few instances of augment in the model. They should be sx:augment-stucture.

#25) I didn't go through the entire table in section 11, but I looked at a few random leafs from the model. A couple seem to be missing in section 11, e.g. last-updated and rate-limit. Maybe good to go through the latest version to really check that all relevant elements are listed.

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