Subject: [ippm] Re: Owamp precision

From: "Jeff W. Boote" <[email-masked]>
Date: Tue, 16 Dec 2003 12:49:17 -0700

[email-masked] wrote:
> 
> I'm working in a research project that use owamp infrasctruct and I have some doubts about this protocol.

I am CC'ing the [email-masked] list due to the "protocol" question.

> First I'd like to know how owamp calculate this precision, it gets from ntp estimated error?
> 
> I was reading the owamp draft and saw in section 6.1 that estimated error is result from two variables scale and multiplier. Why this function (estimated error =  multiplier * 2^(-32) * 2^scale) this is used?!?! What are the meaning of these two values?!

Our owamp "implementation" does get this value from the ntp estimated
error. Other implementations might compute this number in other ways.
(In fact, I could change the way this number is computed in the future
for this implementation. It is not entirely clear to me that the
estimated error reported by the FreeBSD ntp kernel call is computed
properly based on the NTP error budget. This is something I want to look
closer at in the future.)

The reason these values are represented with two "variables" (mult,
scale) is to allow the error value to be represented with as much
precision as possible at many different levels of scale. (It is simply a
floating point number.)

jeff

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From: "Jeff W. Boote" <[email-masked]>
Date: Tue, 16 Dec 2003 12:49:17 -0700

jeff

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From: stanislav shalunov <[email-masked]>
Date: 16 Dec 2003 15:25:25 -0500

"Jeff W. Boote" <[email-masked]> writes:

> precision as possible at many different levels of scale.

...in as few bits as possible.  We could have used another timestamp
value to represent precision; it's a tradeoff between space (expensive
space in OWAMP-Test, not cheap space in OWAMP-Control) and precision.
Given that errors can't be known with the same precision as the actual
timestamps, the resolution of error estimate (1.5%) spec seems sufficient.

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

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From: stanislav shalunov <[email-masked]>
Date: 16 Dec 2003 15:25:25 -0500