Re: [Teas-ns-dt] Availability

[Greg Mirsky <[e-mail]>]

Hi Eric,
sorry to be a pain in the neck but the current definition:
      Availability: It a measure of how often a customer defined service
      is lost or degraded to the point of unacceptable performance due
      to any fault in the network.  It is a ratio of time the transport
      slice meets agreed SLO over the total time where the transport
      slice is contracted
, in my reading, is circular. For example, what is the "unacceptable
performance" to the particular SLO? Is that all metrics being out of
profile at the same time or at least one of the metrics? Further, a fault
is, usually, understood as a defect, loss of path continuity. Is that the
only possible cause of a service degradation? If we understand what is
unacceptable we can easily add it to the definition.

Regards,
Greg

On Mon, Jun 1, 2020 at 2:56 PM Eric Gray <[e-mail]> wrote:

> Greg,
>
>
>
>               I agree with Jeff, and would add that this is (again) a
> definitions draft.  We can iron out a more definite description almost any
> time after the draft is adopted.
>
>
>
>               I practically guarantee that this text will change at least
> a few times before we are done with last call.
>
>
>
>               So, unless we feel that we really need to improve on the
> part of the definition that talks about the ratio that is used to determine
> availability before we ask the WG (again) to adopt the draft, we should get
> there first.
>
>
>
> --
>
> Eric
>
>
>
> *From:* Jeff Tantsura <[e-mail]>
> *Sent:* Monday, June 1, 2020 5:24 PM
> *To:* Greg Mirsky <[e-mail]>
> *Cc:* Eric Gray <[e-mail]>om>; [e-mail]; Kiran
> Makhijani <[e-mail]>
> *Subject:* Re: [Teas-ns-dt] Availability
>
>
>
> Greg,
>
> I thought the definition provided was pretty clear and [comprehensible],
> why do we need to rephrase it?
>
>
>
> Cheers,
>
> Jeff
>
> On Jun 1, 2020, 2:00 PM -0700, Greg Mirsky <[e-mail]>om>, wrote:
>
> Hi Jeff,
>
> if we define availability as the ratio of the period all requested in the
> SLO metrics are within an acceptable range to the time since the service
> was handed to the customer (I propose to refer to this metric as
> "availability ratio"), then I think it can be expressed as
>
> [image: \bigcap _{i=1}^{n}A_{i}], where Ai is the time period the
> particular metric remained within its acceptable boundary.
>
>
>
> Regards,
>
> Greg
>
>
>
> On Mon, Jun 1, 2020 at 1:35 PM Jeff Tantsura <[e-mail]>
> wrote:
>
> Mostly agree with Eric/Kiran
>
> It should not be removed, but further clarified.
> Network/service availability is a measurable metric, availability =
> uptime/total_time(uptime+downtime)
> Rule of thumb - a service is deemed available when all the SLO’s
> associated with it are met(TRUE).
> In a complex/multidimensional service, different objects might have
> different availability metrics .
> For simplicity sake - total_availability(normalized metric) =
> Σ(subservice-1..subservice-n), so both, per SLO as well as composite
> metrics can be used.
>
>
>
> Cheers,
>
> Jeff
>
> On Jun 1, 2020, 10:08 AM -0700, Kiran Makhijani <[e-mail]>om>,
> wrote:
>
> Thanks! I support not removing it.
>
> Sticking with individual SLO seems to be a right decision but can be
> deferred to NBI document. we need not state that here.
>
> -Kiran
>
>
>
> *From:* Teas-ns-dt <[e-mail]> *On Behalf Of* Eric Gray
> *Sent:* Monday, June 1, 2020 7:35 AM
> *To:* [e-mail]
> *Subject:* [Teas-ns-dt] Availability
>
>
>
> I agree that the definition needs to be cleaned up, but I disagree that it
> should be omitted.
>
>
>
> A part of what probably should be cleaned up is the part that talks about
> service degradation.  In general, this is an important factor in
> determining availability, but it is a bit vague for the purpose of
> definition.
>
>
>
> I also disagree that availability is not measurable.
>
>
>
> As a proof of concept for measuring , if there are any mandatory
> measurable objectives, then failing to meet any of those objectives makes
> the service measurably unavailable.  That is, if you can determine if
> specific mandatory objectives are being met, then you can determine if they
> are not being met and therefore determine if the service is unavailable.
>
>
>
> Availability is an important aspect of any service, because it is
> understood that the higher the required availability, the more difficult
> (and thus expensive) it is to provide that service.
>
>
>
> Defining availability as a fraction as we have done in the draft, allows
> for services that may experience a certain amount of outages over a service
> period.  A service request may ask for as high an availability as the
> provider and requester have agreed to (under the terms they agreed to) in
> advance.
>
>
>
> Note that this elevates the importance of having (at least mostly)
> measurable objectives, simply because you cannot determine if a
> non-measurable objective is being met – hence you cannot (necessarily)
> determine the availability of any service that depends on that objective.
>
>
>
> It is further interesting to note that the notion of a service depending
> on objectives that it cannot determine are not being met is a non-sequitur.
>
>
>
> Measuring availability in terms of mandatory objectives – as a whole – is
> the simplest approach; one could group one or more mandatory objectives and
> define an availability separately for the group – thus allowing for a
> higher degree of acceptance for failing to meet one set of service
> objectives compared to others.
>
>
>
> If we were going to do that, it would probably be better to define
> availability as a parameter that applies individually to service objectives.
>
>
>
> In my opinion we should at least initially stick to the simple case, where
> availability is defined as a service objective, rather than as a parameter
> of every service objective – but I am willing to go either way.
>
>
>
> --
>
> Eric
>
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