[netconf] Benjamin Kaduk's Discuss on draft-ietf-netconf-yang-push-23: (with DISCUSS and COMMENT)

Benjamin Kaduk via Datatracker <noreply@ietf.org> Wed, 01 May 2019 19:08 UTC

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Subject: [netconf] Benjamin Kaduk's Discuss on draft-ietf-netconf-yang-push-23: (with DISCUSS and COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-netconf-yang-push-23: Discuss

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----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------

Note that I reviewed the -22 and the current version is -23.  Briefly
skimming the diff, it seems that some changes touch on points I make in
my review, but there is probably still discussion to have on them.

(pro-forma) I see the GenArt reviewer noted the author count (seven)
already, but I couldn't find a response or note in the ballot or
shephert writeup acknowledging this.  So failing that, I'll put up a
discuss point until the responsible AD says it's fine.

[See also discussion on draft-ietf-netconf-subscribed notifications
about the pre-RFC5378 boilerplate and whether or not it can be removed
from this document]

Section 3.3 states:

            In order for a subscriber to determine whether objects
   support on-change subscriptions, objects are marked accordingly on a
   publisher.  Accordingly, when subscribing, it is the responsibility
   of the subscriber to ensure it is aware of which objects support on-
   change and which do not.  For more on how objects are so marked, see
   Section 3.10.

Chasing the reference, we see that this marking is left for future work
or implementation-specific usage.  I'm not very comfortable with the way
we are describing this situation, as it seems pretty fragile in the face
of different implementations trying to interoperate.


----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

Thank you for the very thoughtful document!  I've lost track of the
number of places where I started writing a comment only to note that my
concern had already been addressed in the following text.  In general
the writing style is great, though I did find some grammar and clarity
nits (noted inline).

Abstract

               Providing such visibility into updates enables new
   capabilities based on the remote mirroring and monitoring of
   configuration and operational state.

This phrasing ("new capabilities based on") is hard for me to follow,
particularly about whether these are protocol-level capabilities and
what actors are granted the new capabilities.

Section 1

   Traditional approaches to providing visibility into managed entities
   from remote have been built on polling.  [...]

nit: "remote" is an adjective and needs a noun to bind to; "from remote
systems", "from remote vantages", or "from afar" would all be fine
wordings here.

   o  Polling incurs significant latency.  This latency prohibits many
      application types.

nit: I'd suggest wording as "types of application", since on my first
reading I thought this was referring to some sort of codepoint.

   o  Polling cycles may be missed, requests may be delayed or get lost,
      often when the network is under stress and the need for the data
      is the greatest.

nit: the grammar is a bit weird, here, as if a comma splice.  I think
replacing the first comma with a colon or em dash would suffice.

   o  Datastore node update: A data item containing the current value of
      a datastore node at the time the datastore node update was
      created, as well as the path to the datastore node.

Is this storing the "new" or the "old" value as the "current value"?

Section 3.1

         +  Dampening period: In an on-change subscription, detected
            [...]
            is included.  The dampening period goes into effect every
            time an update record completes assembly.

Just to double-check: this means that after a long hiatus, the first
change to the monitored subtree(s) triggers an immediate push with just
the single update, and only then does the dampening period kick in and
defer delivery of potential subsequent updates?

Section 3.5.2

This bit about the "create" and "delete" errors from RFC 8072 Section
2.5 not being errors in our usage is a little interesting, process-wise.
In one sense, we are changing the semantics of an already published RFC,
and would need to apply an "Updates:" relationship to indicate that, but
in the other sense we are building a new custom thing that reuses a lot
of the syntax/semantics of YANG patch but is fundamentally a new
(different) thing.  The phrasing we use to talk about it can affect
which case the reader perceives us to be in...

The discussion on the "change-type" enumeration seems to pretty clearly
place us in the latter case, which is good.

Section 3.6

Thank you for the note about the power of XPath expressions and the duty
of the receiver to understand what it's asking for -- that sort of
statement would potentially even be appropriate in the Security
Considerations (but is fine where it is)!

Section 3.7

   Of note in the above example is the 'patch-id' with a value of '0'.
   Per [RFC8072], the 'patch-id' is an arbitrary string.  With YANG
   Push, the publisher SHOULD put into the 'patch-id' a counter starting
   at '0' which increments with every 'push-change-update' generated for
   a subscription.  If used as a counter, this counter MUST be reset to
   '0' anytime a resynchronization occurs (i.e., with the sending of a
   'push-update').  Also if used as a counter, the counter MUST be reset
   to '0' after passing a maximum value of '4294967295' (i.e. maximum
   value that can be represented using uint32 data type).  Such a
   mechanism allows easy identification of lost or out-of-sequence
   update records.

It's not really clear to me how much value there is in this counter
mechanism if the client' can't rely on the server's behavior actually
being to use a counter (the requirement is only "SHOULD").  Can this be
a "MUST" (or maybe "MUST excpet when [...]") instead?

Section 3.9

                                                          Empty "push-
   change-update" messages (in case of an on-change subscription) MUST
   NOT be sent.  This is required to ensure that clients cannot
   surreptitiously monitor objects that they do not have access to via
   carefully crafted selection filters.  By the same token, changes to
   objects that are filtered MUST NOT affect any dampening intervals.

I appreciate this attention to security-relevant detail; thank you!

Section 3.11.1

Do we want to give any guidance for the "incomplete-update" case about
whether a subscriber should wait and give the publisher a chance to
provide a full "push-update" for resynchronization (per Section 4.3.2)
versus perform a normal query for the datastore contents and
effectuating its own resynchronization?

Section 4.2

   o  For on-change subscriptions, assuming any dampening period has
      completed, triggering occurs whenever a change in the subscribed
      information is detected.  On-change subscriptions have more
      complex semantics that is guided by its own set of parameters:

nit: singular/plural mismatch "semantics"/"is"

Section 4.3.2


   A "time-of-update" which represents the time an update record
   snapshot was generated.  A receiver MAY assume that at this point in
   time a publisher's objects have the values that were pushed.

nit: I think "had the values" (past tense) is more appropriate.

Section 4.4.1

   a publisher that cannot serve on-change updates but periodic updates
   might return the following NETCONF response:

nit: "but can serve periodic updates"

Section 4.4.2

   The specific parameters to be returned in as part of the RPC error
   response depend on the specific transport that is used to manage the
   subscription.  For NETCONF, those parameters are specified in
   [I-D.draft-ietf-netconf-netconf-event-notifications].

nit: "in" and "as part of" are redundant.

Section 5

It is slightly interesting to note that (apparently) the
update-policy-modifiable grouping allows for a subscription to switch
between periodic and triggered at runtime (by virtue of wanting a single
grouping to cover all the cases and needing to be able to modify the
parameters for each case).  I would mostly expect implementations to
deny such modification requests due to the needed complexity, but I'm
also not sure that there's a need to mention this explicitly in the
document.

            leaf period {
              type centiseconds;
              mandatory true;
              description
                "Duration of time which should occur between periodic
                 push updates, in one hundredths of a second.";

It would probably be okay to skip "in one hundredths of a second" given
the usage of the centiseconds typedef.

    rc:yang-data resync-subscription-error {
      container resync-subscription-error {
        description
          "If a 'resync-subscription' RPC fails, the subscription is
           not resynced and the RPC error response MUST indicate the
           reason for this failure.  This yang-data MAY be inserted as
           structured data within a subscription's RPC error response
           to indicate the failure reason.";

It's a little weird to have the normative language here constraining the
RPC error response that must be returned for a specific RPC, since this
is not the description of that RPC.  (It's probably also duplicating
langauge elsewhere but I didn't double-check.)

    rc:yang-data establish-subscription-datastore-error-info {
      container establish-subscription-datastore-error-info {
        description
          "If any 'establish-subscription' RPC parameters are
           unsupportable against the datastore, a subscription is not
           created and the RPC error response MUST indicate the reason
           why the subscription failed to be created.  This yang-data
           MAY be inserted as structured data within a subscription's
           RPC error response to indicate the failure reason.  This
           yang-data MUST be inserted if hints are to be provided back
           to the subscriber.";

(ditto)

Contrast to modify-subscription-datastore-error-info, which only has
normative language about the yang-data being described and not the RPCs
that (might) use it.


nit: push-update and push-change-update use different langauge about
"does not constitute a general-purpose notification" and I'm not sure
there's a reason to diverge.
Their "incomplete-update" leaves also have divergent descriptions, but I
think that latter divergence is more reasonable.

    augment "/sn:subscription-modified/sn:target" {
      [...]
      case datastore {
        uses datastore-criteria {
          refine "selection-filter/within-subscription" {
            description
              "Specifies where the selection filter, and where it came
               from within the subscription and then populated within
               this notification.  If the 'selection-filter-ref' is

nit: "where the selection filter" seems like an incomplete clause.