[bmwg] draft-ietf-bmwg-dcbench-terminology

["MORTON, ALFRED C (AL)" <[e-mail]>]

Hi Lucien and Jacob,

Following the bmwg session yesterday there were several
reviews and comments expressed.

I have one section revision to suggest, 
Al
(as a participant)


In the latest draft of Data Center Terminology -01:
- using the term Jitter, should be delay variation now
- still citing RFC 3393 almost exclusively, which is too flexible
- only citing IPDV from RFC 5481, 
- in the past meeting we agreed on RFC 5481 PDV as primary, IPDV only if you
  want to understand if traffic has become more bursty in transfer
- Metric Units section is the place to specify Units of Measure (seconds
  in this case, not much else)
 
I suggest to revise section 3 as follows:
NEW:

3 Delay Variation

3.1 Definition

Delay variation is derived from multiple measurements
of one-way delay, as described in RFC 3393.
The mandatory definition of Delay Variation is the PDV form
from section 4.2 of RFC 5481. When considering a stream of packets,
the delays of all packets are subtracted from the minimum delay
over all packets in the stream. This facilitates assessment of 
the range of delay variation (Max - Min), or a high percentile 
of PDV (99th percentile, for robustness against outliers).

If First-bit to Last-bit timestamps are used for Delay measurement, then
Delay Variation MUST be measured using packets or frames of the 
same size, since the definition of latency includes the serialization
time for each packet. Otherwise if using First-bit to First-bit,
the size restriction does not apply.


3.2 Discussion
   In addition to PDV Range and or a high percentile of PDV,
   Inter-Packet Delay Variation (IPDV) as defined in section 4.1 of
   RFC5481 (differences between two consecutive packets) MAY be used
   for the purpose of determining how packet spacing has changed during
   transfer, for example to see if packet stream has become closely-spaced
   or "bursty". However, the Absolute Value of IPDV SHOULD NOT be used, as 
   this collapses the "busrty" and "dispersed" sides of the IPDV distribution
   together.

3.3 Measurement Units

The measurement of delay variation is expressed in units of seconds.

A PDV histogram MAY be provided for the population of packets measured.

-=-=-=-=-=-=-=-=-=-
OLD:
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.

   Even with the reference to RFC 3393, there are many definitions of
   "jitter" possible. The one selected for Data Center Benchmarking is
   closest to RFC 3393.


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 Units

   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. Inter-Packet Delay Variation as defined
   in RFC5481 (differences between two consecutive packets) MAY be used
   for this purpose.