Subject: Re: [ippm] Re: draft-ietf-ippm-owmetric-as-00.txt

From: stanislav shalunov <[email-masked]>
Date: 08 Aug 2002 01:03:28 -0400

Sorry about letting this sit in my queue for so long.

"Henk Uijterwaal (RIPE-NCC)" <[email-masked]> writes:

> > Section 3.4 `Test duration' makes an assertion about desirability of
> > continuous monitoring that on the surface is obvious but in fact
> > involves a subtle trade-off with ability to differentiate between
> > losses and measurement device failures when periods of extended losses
> > are involved.
> 
> I'm not quite sure if I understand this comment.

Suppose you want to conduct a measurement session from A to B.  B
stops receiving packets after a certain time and does not receive them
for an hour (or a minute, or a day).  Were these packets lost or did A
crash?  There's no sure way to tell, of course, but if you have
measurement *sessions* with certain limited duriation then if no
contact can be made with the host when the session ends, you will have
to assume (conservatively) that A crashed.

> > Section 3.5 `Data volumes' appears to fail to explain why the
> > measurement traffic needs to be low volume.  In fact, for some
> > measurements it needs to be low and for some it must be comparable
> > with link capacity.
> 
> Such as?

One-way delay -> low traffic load.

Many `available capacity' measurements -> high traffic load.

Reordering at line rate with a standing queue -> very high traffic load.

(Probably don't want to do the last one on a production network,
unless it's yours; but what if it is yours and it's December 31 and
you only have five nines availability SLAs, no downtime this year, and
great curiousity?  Or what if the network isn't production?  What if
it temporarily has no users for whatever reason?)

> > It should be made clear that specific metrics (such as one-way loss and
> > delay) and specific methodology is discussed. Further, the number 3% is
> > pulled out of the hat: 300Mb/s of active delay and loss measurement
> > traffic on a 10Gb/s network is surely excessive, isn't it?
> 
> Yes and no. The 3% number comes from discussions with providers, who
> typically claim that "using a few percent" of the total capacity to
> monitor the performance of their equipment.  If you have a better
> number, then I'd like to hear it. Also see section 3.1, if we send
> 3% of 10 Gb/s measurement traffic, will we see all interesting
> variations in traffic?

As with any survey:  What was the exact question asked?  Surely, it was
not `I would like to send traffic into your network (I'll also record
some statistics).  I'm prepared to pay your normal transit rates.  How
much can I send?', was it?

Also, our network on many weeks exceeds 3% for active measurement
(because of available capacity measurements).  Are we in violation of
something?

> > Section 5.0 `Reporting the IPDV metric' says that `the distribution
> > generally is not Gaussian and the sigma is not well defined'.  Many
> > non-Gaussian distributions happen to have finite standard deviation.
> > Reporting standard deviation makes sense for more than Gaussian
> > distribution, too.
> 
> Yes.  However, look in our latest PAM paper (or wait until my Yokohama
> presentation) to see some of the typical IPDV distributions.  Reporting
> sigma for these distributions simply doesn't make sense.

I don't disagree with the need to report IPDV stuff in a way that's
different from, e.g., delay.  The justification, however, is worded in
a way that implies a mathematical claim (that the distribution has to
be Gaussian for the standard deviation to be defined).  The IPPM
participants (with the exception of the possibly empty subset that is
not familiar with the definition of standard deviation) understand
what was meant, but an edit might decrease the probability of someone
assuming that the document makes this claim about non-Gaussian
distributions.

-- 
Stanislav Shalunov		http://www.internet2.edu/~shalunov/

A fool's brain digests philosophy into folly, science into superstition,
and art into pedantry.  Hence University education.       -- G. B. Shaw
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