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

Hi David and Lucien,

Thanks for the additional reference, David.

The definition in 4689 is like the Interpacket delay variation definition
that can be formulated using 3393, but these definitions both depend on
the sequence of delays. Also, the absolute value function used in 4689
colapses the positive and negative inter-packet delays together,
and sometimes these have different meanings or causes. 

I think the question I'm asking is 
"How with the Jitter results be used by Datacenter Engineers?"

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.

hope this helps, or gets people thinking at least. 
regards,
Al

________________________________________
From: [email protected] [[email protected]] On Behalf Of David Newman [[email protected]]
Sent: Monday, July 29, 2013 5:35 PM
To: [email protected]
Subject: Re: [bmwg] comments on draft-dcbench-def-00 (part 1)

On 7/29/13 9:14 AM, Lucien Avramov (lavramov) wrote:

>> 3 Jitter
>>
>> 3.1 Definition
>>
>>     The definition of Jitter is covered extensively in RFC 3393. This
>>     definition is not meant to replace that definition, but it is meant
>>     to provide guidance of use for data center network devices.
>>
>>     The use of Jitter is in according with the variation delay definition
>>     from RFC 3393:
>>
>>     The second meaning has to do with the variation of a metric (e.g.,
>>     delay) with respect to some reference metric (e.g., average delay or
>>     minimum delay).  This meaning is frequently used by computer
>>     scientists and frequently (but not always) refers to variation in
>>     delay.
>>
>> Comment:  Even with the reference to RFC 3393, there are many definitions
>> of "jitter" possible. Fortunately this is not the first draft to examine
>> the possibilities :-)
>>
>> I find that a comment I made on the Traffic Management draft is
>> applicable here:
>>> ...
>>>    - Packet Delay Variation (PDV): the Packet Delay Variation metric is
>>>    the variation between the timestamp of the received egress port
>>>    packets and specified in RFC 1889.
>>>
>>> Comment: since you have One-way Delay (PD defined above PDV), why
>>> resort to the RFC 1889 definition?
>>> What does this give you?
>>> Suggest: Specify PDV as defined in
>>> http://tools.ietf.org/html/rfc5481#section-4.2 instead, for the many
>>> reasons given in http://tools.ietf.org/html/rfc5481#page-27
>>> (or call this IPDV to avoid confusion).
>>
>> In the Isolated test lab environment, we don't have time clock drift
>> issues that
>> we can't solve, in many cases the same clock is used for ingress and
>> egress timestamps.
>> So, we are free to choose the delay variation metric(s) that serve our
>> audience best.
>>
>>>>>> I think "Jitter" is another topic that we can advance through
>>>>>> face2face discussion.
>>       What do we really want to learn from the "jitter" metric?
>>       How will these results be used? (in network design, application
>> planning, etc.)
>
> For the Data Center benchmarking, we found that the rfc 3393 definition
> was the easiest to consume for its applicability to the data center. we
> wanted to add to this that what is key is to have packets of the same
> size when jitter is measured. It's mentioned in the RFC 3393 but in a
> more complex method as follows:
>
> '‘+ L, a packet length in bits. The packets of a Type P packet stream
> from which the singleton ipdv metric is taken MUST all be of the same
> length.’’
>
> We want to make this simpler to the end data center user by calling it
> packets of the same size.
>
> Also we add to this definition, by explaining the necessity for a
> histogram with latency buckets, this will help understand where there is
> higher jitter for instance.

RFC 4689, approved by this wg a few years ago, provides a simpler
definition of jitter:

http://tools.ietf.org/html/rfc4689#section-3.2.5

|D(i) - D(i-1)|, where D equals the Forwarding Delay
      and i is the order the packets were received.

This paragraph compares the RFC 3393 and 4689 definitions:

"Jitter is related to the IPDV (IP Delay Variation) taking the absolute
value of the ipdv. The two metrics will produce different mean values.
Mean Jitter will produce a positive value, where the mean ipdv is
typically zero. Also, IPDV is undefined when one packet from a pair is
lost."

The RFC 4689 definition may be more applicable here.

dn

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