Re: [Dots] Yangdoctors last call review of draft-ietf-dots-telemetry-14 Fri, 04 December 2020 17:38 UTC

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Subject: Re: [Dots] Yangdoctors last call review of draft-ietf-dots-telemetry-14
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Hi Jan,

Thanks for the detailed review.

As there has been some responses to different parts, I will try to answer
the parts not responded to, but indicate what has been responded to.

Please see inline.



> -----Original Message-----
> From: Dots [mailto:] On Behalf Of Jan
> Lindblad via Datatracker
> Sent: 26 November 2020 11:37
> To:
> Cc:;;
> Subject: [Dots] Yangdoctors last call review of
> Reviewer: Jan Lindblad
> Review result: Almost Ready
> Dear Dots Authors, NETMOD WG,
> This is my YD LC review of draft-ietf-dots-telemetry-14. This draft
> 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.

Answered by [Andy] [2]
Commented on by [Med] [3]
Answered by [Med] [4]
More clarifications by [Andy] [5]
> Since my previous review of this module (-09, June 2020) the underpinnings
> the modules have changed significantly with a new RFC in the works,
> 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
> state of -14 "Almost Ready".
> Links to previous relevant reviews:
> early-aries-2020-09-23/
> 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
> 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.
> messages are sent when, what is the relevant/reasonable content in each
> message, are there any races or scalability issues, etc. IMO, such an
> 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
> YANG modules, since that will be key to interoperability.

Answered by [Andy] [2]
> 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
> 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.

Answered by [Andy] [2]

[Jon] The goal in the signal channel was to have the payload in CBOR for
packet size optimization, including mapping the CBOR parameter names into
integers to further reduce the data size. This channel is run over CoAP over
(D)TLS.  The CBOR mappings are key here, but YANG is used to describe the
different CBOR entities / mappings as far as possible.  So, this is not
really the classic YANG where other mappings can easily be derived from it.
An agent needs to know how to process a particular CBOR parameter/value and
whether it can be safely ignored if it is not known. 

[Jon] The data channel is essentially RESTONF/YANG over HTTPS where the YANG
in this draft is simply "augment"ing and hence follows the classic YANG
usage.  For this draft, the data channel is only used to exchange vendor
mapping details (which the signal channel can subsequently use with the
information reduced as much as possible in size).
> #1 mandatory true
> A few elements are marked mandatory true in the model. The meaning of this
> not defined in the draft. I would expect this means clients as well as
> MUST provide a value for the mandatory element in each message if the
> element exists. This would be like how mandatory elements in YANG
> actions/rpcs/notifications work (and different from how mandatory works
> 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
> protocol."

Answered by [Med] [4]
> 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.

[Jon] We have found some mandatory usage which is no longer required - this
will be corrected.
> #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
> unique in a list?

Answered by [Med] [4]
> Are keys mandatory?

Answered by [Med] [4]
> Are there any sequencing requirements on
> keys, e.g. do keys need to be sent first (as in NETCONF) in a list

Answered by [Med] [4]
> Such a requirement may not be unreasonable for servers with limited
> #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
> including a very particular leaf in one of the messages). From a YANG
> perspective, it might have been better to have a general mechanism for
> advertising features. With the current specification, no other features
> (ever) be added in a backwards compatible way.

Answered by [Med] [4]
> #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
> use? Section 6.1.2 states that servers MUST reject messages when
> values are not acceptable". Does that have any implications for augment?

Answered by [Med] [4]
> #6) deviations
> Deviations are never mentioned. Are implementors allowed to deviate from
> protocol in any way, if declared using deviation statements? If so, how
would a
> client know if a deviated model is in use?

Answered by [Med] [4]
> #7) config true|false
> RFC 8791 (which defines sx:structure) expressly states that config true
> false statements have no defined meaning and will be ignored in
> Therefore it's very good that this draft defines behavior for the GET
> with respect to config true and config false elements. Nothing is said
> how the config attribute plays with PUT, POST or DELETE however.

Question raised by [Med] [4]
[Jon] For the DOTS data channel, the YANG is augment, and so config
true|false still stand for this.
> #8) PATCH
> Is the PATCH HTTP operation supported in this context? It is never
> 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.

Answered by [Med] [4]
> #9) Payload encoding references
> As far as I understand, the message payloads are meant to be encoded in
> or

Answered by [Med] [4]
[Jon} CBOR is only used for the signal channel.  However, JSON is used for
the payload over the data channel when updating the vendor mappings.

> This draft has no references to the relevant documents describing these
> encodings ( and

Answered by [Med] [4]
> 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
> tree diagrams).

Answered by [Med] [1]
> #11) Section 6.3.1 states "The overlapped lower numeric 'tsid' MUST be
> automatically deleted and no longer be available." Since this mechanism is
> central part of the protocol, I would think specifying exactly what
> "overlapped" means would improve interoperability.

Answered by [Med] [1]
> #12) Section 6.3.1 states "If no target clause is included in the request,
> is an indication that the baseline information applies for the DOTS client
> domain as a whole." Define "target clause" (= tsid?)

Answered by [Med] [1]
> #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
> 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
> design practice. I suggest to remove this sentence.

Answered by [Med] [1]
> #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
> works.

Answered by [Med] [1]
> #15) In the YANG, there is a list telemetry without any key. This is
> 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
> 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,
> kan keep it as is. If this data may be large, a different way of modeling
> be in order so that data could be requested a little at a time.

Answered by [Med] [1]
> #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
> to receiving a request with 'server-originated-telemetry' set to 'false'.
" In
> YANG, this is usually expressed by adding "default false" to the leaf.

Answered by [Med] [1]
> #17) There are many leafs in the models that have no default, no mandatory
> no description which tells what happens if the leaf is not provided. E.g.
> happens if "query-type" is not specified?

In part answered by [Med] [1]
[Jon] If the server does not indicate that it supports any query-type, then
the client will be unable to use any of the potential query-type to reduce
the returned data content from the server.
> #18) Many lists have "unit" as a key. This can work, but is an unusual
> It opens up for sending the same information several times over with
> units.

Answered by [Med] [1]
> #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.

Answered by [Med] [1]
> #20) Section 7.1.1 mentions a "optional subattribute" Please define this
> or change to a defined term.

Answered by [Med] [1]
> #21) leaf-list alias-name {type string; description "An alias name that
> to a resource."; Unclear; what is a resource here? What is the format of
> string? Could this be a leafref?

Answered by [Med] [1]
> #22) There are typedefs called "unit-type" and "unit". I would suggest
> at least one of those names so that readers have a chance at understanding
> they differ. Also, some of their content overlaps, so maybe one could
> incorporate the other?

Answered by [Med] [1]
> #23) Similarly for grouping percentile-config and grouping percentile.
> that better reflect their content and usage would be good.

Answered by [Med] [1]
> #24) There are still a few instances of augment in the model. They should
> sx:augment-stucture.

Answered by [Med] [1]
> #25) I didn't go through the entire table in section 11, but I looked at a
> random leafs from the model. A couple seem to be missing in section 11,
> last-updated and rate-limit. Maybe good to go through the latest version
> really check that all relevant elements are listed.

Answered by [Med] [1]