RE: [ippm] Review of: draft-ietf-ippm-reordering-12.txt

It is still not clear to me how this would work in the case one entity
determines gaps in units of time and the other uses message sequence
numbers. Or, in case message sequence numbers are mandatory anyway, why
does Section 4.5 still refer to units of time? 

Dan

> -----Original Message-----
> From: Al Morton [mailto:acmorton@att.com] 
> Sent: Friday, April 21, 2006 10:05 PM
> To: Wijnen, Bert (Bert); ippm@ietf.org
> Cc: Romascanu, Dan (Dan); Mreview (E-mail)
> Subject: Re: [ippm] Review of: draft-ietf-ippm-reordering-12.txt
> 
> Hi Bert,
> 
> Thanks for your review and questions.
> Please see my take on the answers, below.
> hope this helps,
> Al
> 
> At 05:55 AM 4/21/2006, Wijnen, Bert (Bert) wrote:
> >...
> >Looks pretty good.
> >
> >I have one question that maybe the authors or other WG members can 
> >answer for me and that is:
> >
> >   In section 4.5, it seems to allow for using msg sequence 
> numbers OR
> >   units of time (without even having defined what the unit is).
> >
> >So I wonder how this definitions specifies an exact metric. 
> The metric 
> >would not be comparable from one to the other measurement if one of 
> >them uses msg sequence numbers, while the other uses "units 
> of time". 
> >Even if two of them use "units of time" but different units 
> (e.g. micro 
> >seconds vs
> >milliseconds)
> >even then they would not be comparable.
> 
> I'll tackle the issue of different time resolutions below.
> 
> In the case of the Gap metrics of Section 4.5, all the 
> parameters necessary to compute the Gap (in units of msg 
> numbers) and GapTime (in units of time) are mandatory, 
> inherited from earlier metrics (as 4.5.2 states).
> 
> If one reports a metric from this section, then the Gap 
> metric is mandatory, while the GapTime is optional:
> 
>        "Gaps may also be expressed in time:" (equation follows)
> 
> So, I don't think we should run into a problem when two 
> different testers report metrics that are compliant with 
> section 4.5.  They should be able to compare the Gap (msg 
> number-based) metric, at least.
> 
> 
> >Was it not the goal of IPPM to define EXACT metrics, so that 
> results of 
> >two different tests/measurments could be compared?
> 
> None of the current IPPM metric RFCs mandate a resolution for 
> time stamps.
> The IPPM Framework RFC 2330 treats Clock Issues in Section 
> 10, and simply defines the term "resolution" as "the smallest 
> unit by which the clock's time is updated."
> 
> The One-way Delay RFC 2679 even allows for different 
> resolutions to be used on the Source and Destination clocks 
> (from 3.7.1):
> >    +  The resolution of a clock adds to uncertainty about any time
> >       measured with it.  Thus, if the source clock has a 
> resolution of
> >       10 msec, then this adds 10 msec of uncertainty to any 
> time value
> >       measured with it.  We will denote the resolution of the source
> >       clock and the destination clock as Rsource and Rdest,
> >       respectively.
> 
> So, I believe we have specified the metric definitions 
> "exactly", or without ambiguity.  But the degree to which 
> measurements from different implementations will be 
> comparable depends on the details of each implementation, 
> including the time stamp resolution and other sources of 
> error or uncertainty (such as time accuracy and skew).
> 
> 

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