Re: comments on draft-dcbench-def-00 (part 1)

Hi David,

Please see inline

On 7/30/13 7:45 PM, David Newman wrote:
> On 7/30/13 4:39 AM, MORTON JR., ALFRED C (AL) wrote:
>
>> the absolute value function used in 4689
>> colapses the positive and negative inter-packet delays together,
>> and sometimes these have different meanings or causes.
>
> Both the RFC 3393 and 4689 definitions use absolute values.
>
> 3393 gives two methods of measuring jitter. One references RFC 2598, the
> other references RFC 1889, and both of these absolute values. RFC 3393
> explicitly mentions the use of absolute values for both methods.
>
> Thus, this is not really a meaningful differentiator between the 3393
> and 4689 jitter definitions.
>
> Indeed, it would be very problematic *not* to use absolute numbers.
> Without them, the mean of a uniform distribution of positive and
> negative values around some midpoint would give us -- zero.
>
> Given Lucien's stated goal of "how to make jitter a simple term for
> datacenter engineers to understand and to measure" (presumably by
> representing it as a single number, the mean), and given his examples of
> 5% and 50% variance, it's unclear how a metric that is typically zero
> would describe that variance in any simple way.

I wanted to just give a representation of the jitter variance, the 
example values [5%,50%] were just meant to show how it is significant 
for a user and illustrate my point, but it's *not* the jitter 
definition. The goal is to first measure it correctly and what it means 
and then have a comprehensive optional chart histogram representing the 
jitter variance, this is usually done by packet size as on figure 10 for 
example here:

http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps11541/ps12581/white_paper_c11-716751.pdf


The *key* to me is to first *explain* simply what jitter means for the 
datacenter engineer which I re-quote here from draft-dcbench-def-00


3.2 Discussion

    Jitter can be measured in different scenarios:
   -packet to packet delay variation
   -delta between min and max packet delay variation for all
    packets sent.

3.3 Measurement

    The jitter MUST be measured when sending packets of the same size.
    Jitter MUST be measured as packet to packet delay variation and delta
    between min and max packet delay variation of all packets sent. A
    histogram MAY be provided as a population of packets measured per
    latency or latency buckets.



>
>> If, for example, they wish to know the overall jitter from a path
>> with ten DUTs in series, we can estimate that value but we have to
>> measure the right metric at the start to do it
>
> Also unclear, given the bmwg's black-box approach to measurement, why
> the number of elements within a single SUT would matter.
>
> Agreed that the key question here is what constitutes the "right metric"
> regardless of whether we measure jitter for one box or lots of boxes as
> a system.
>
> dn
>
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