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

mohamed.boucadair@orange.com Tue, 08 December 2020 14:47 UTC

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From: <mohamed.boucadair@orange.com>
To: "supjps-ietf@jpshallow.com" <supjps-ietf@jpshallow.com>, 'Jan Lindblad' <janl@tail-f.com>, "yang-doctors@ietf.org" <yang-doctors@ietf.org>, "last-call@ietf.org" <last-call@ietf.org>, "draft-ietf-dots-telemetry.all@ietf.org" <draft-ietf-dots-telemetry.all@ietf.org>, "dots@ietf.org" <dots@ietf.org>
Thread-Topic: [Dots] Yangdoctors last call review of draft-ietf-dots-telemetry-14
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Subject: Re: [Dots] Yangdoctors last call review of draft-ietf-dots-telemetry-14
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Hi all, 

FWIW, an updated version of the draft is available online:
https://www.ietf.org/rfcdiff?url1=draft-ietf-dots-telemetry-14&url2=draft-ietf-dots-telemetry-15&difftype=--hwdiff

This version takes into account Jan's review:

* add new text to record why keys are not used for some lists + the meaning of keys when such statement are used.
* add a reminder how mapping works in DOTS
* add a warning against deviations for the signal channel + a reminder how such deviations (and alos features) can be learned for the data channel
* further insist on unit auto-scaling
* rename some groupings
* explain the meaning of "mandatory" use
* update some descriptions and some other edits.  

Cheers,
Med

> -----Message d'origine-----
> De : supjps-ietf@jpshallow.com [mailto:supjps-ietf@jpshallow.com]
> Envoyé : vendredi 4 décembre 2020 18:39
> À : 'Jan Lindblad' <janl@tail-f.com>om>; yang-doctors@ietf.org; last-
> call@ietf.org; draft-ietf-dots-telemetry.all@ietf.org; dots@ietf.org
> Objet : RE: [Dots] Yangdoctors last call review of draft-ietf-dots-
> telemetry-14
> 
> 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.
> 
> Regards
> 
> Jon
> 
> > -----Original Message-----
> > From: Dots [mailto: dots-bounces@ietf.org] On Behalf Of Jan
> Lindblad
> > via Datatracker
> > Sent: 26 November 2020 11:37
> > To: yang-doctors@ietf.org
> > Cc: last-call@ietf.org; draft-ietf-dots-telemetry.all@ietf.org;
> dots@ietf.org
> > Subject: [Dots] Yangdoctors last call review of
> draft-ietf-dots-telemetry-14
> >
> > 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
> 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.
> 
> 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
> 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.
> 
> 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
> 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.
> 
> 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
> 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."
> 
> 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
> always
> > 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
> element?
> 
> Answered by [Med] [4]
> > 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.
> 
> 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
> 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?
> 
> Answered by [Med] [4]
> >
> > #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?
> 
> Answered by [Med] [4]
> >
> > #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.
> 
> 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
> 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.
> 
> Answered by [Med] [4]
> >
> > #9) Payload encoding references
> >
> > As far as I understand, the message payloads are meant to be
> encoded
> > in
> JSON
> > or
> > CBOR.
> 
> 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 (https://tools.ietf.org/html/rfc7951 and
> > https://tools.ietf.org/html/draft-ietf-core-yang-cbor-13).
> 
> 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
> other
> > 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
> a
> > 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,
> this
> > 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
> 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.
> 
> 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
> it
> > works.
> 
> Answered by [Med] [1]
> >
> > #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.
> 
> 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
> 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.
> 
> Answered by [Med] [1]
> >
> > #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?
> 
> 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
> choice.
> > It opens up for sending the same information several times over
> with
> different
> > 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
> term,
> > or change to a defined term.
> 
> Answered by [Med] [1]
> >
> > #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?
> 
> Answered by [Med] [1]
> >
> > #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?
> 
> Answered by [Med] [1]
> >
> > #23) Similarly for grouping percentile-config and grouping
> percentile.
> Names
> > 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
> be
> > sx:augment-stucture.
> 
> Answered by [Med] [1]
> >
> > #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.
> >
> 
> Answered by [Med] [1]
> 
> [1] https://mailarchive.ietf.org/arch/msg/dots/VbVmlhvao-
> kk_Z2GzTkV2QN7Qbw/
> [2] https://mailarchive.ietf.org/arch/msg/dots/JWg4NYsKXGzj_mGu-
> TUsMf9E4kM/
> [3] https://mailarchive.ietf.org/arch/msg/dots/L5vEwPU5B9d-
> srruoV26Wq6mQco/
> [4]
> https://mailarchive.ietf.org/arch/msg/dots/qwO96u377lEdzx6G_HL_QppxE
> Go/
> [5]
> https://mailarchive.ietf.org/arch/msg/dots/1A8z9MiqDOQzPamYdHWqXvwQg
> rk/
> 
> 


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