Subject: [ippm] Feedback on "Packet Delay Variation Applicability Statement" draft

From: Benoit Claise <[email-masked]>
Date: Fri, 01 Dec 2006 15:24:35 +0100

Al, 

[Note: I just realized that I reviewed version 00, which was discussed 
in San Diego, while a new version has just been posted. So some of the 
comments might already be inserted in version 1]

Thanks for your draft: it's a very useful draft!
Before reading the draft, I wrote down what I would expect from such an 
applicability draft.
What I would expect from your document is: reference + guideline:
        - a reference for the industry and all (my) customers
               Different definitions + pointers
               Potentially an entry point for IPPM
       -    a guideline:
               How to measure the DV?
                   Keep all the samples? Depends on IPDV versus PDV
                   Poisson or not (we discussed that already together)
                   Interval, packet size, etc...
                   Clock: NTP, GPS, etc...
                   What if some packets have a long delay (almost lost)?
                    What if there is re-ordering?
                    Etc...
               Which definitions to choose? In which circumstances
               How to evaluate the DV results?
               How to use the DV results? Composite?
              What to use the DV for?
              How to reduce the DV in a network?
              How to implement the DV?
               What to do in case of load-balancing? I.E when there is a 
bimodal distribution.
                       Should we have the 95^th percentile for each mode?
               Etc...          

When reading it, I took a few notes. Let me share them with everybody.

- Since this is document intend is to be informational, I would add a 
section on the different names... in order to educate everybody that the 
right term is the delay variation, and not jitter. We still have a lot 
of people (including customers) who speak about jitter.
Even if this is simply copying RFC 3393, I think it's worth mentioning

    1.1. Terminology

    The variation in packet delay is sometimes called "jitter". This
    term, however, causes confusion because it is used in different ways
    by different groups of people.

    "Jitter" commonly has two meanings: The first meaning is the
    variation of a signal with respect to some clock signal, where the
    arrival time of the signal is expected to coincide with the arrival
    of the clock signal. This meaning is used with reference to
    synchronous signals and might be used to measure the quality of
    circuit emulation, for example. There is also a metric called
    "wander" used in this context.

    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.

    In this document we will avoid the term "jitter" whenever possible
    and stick to delay variation which is more precise.

- You mention IPDV and PDV.
I would stress so more that RFC 3393 is compliant with both definitions.
Based on my bad memory, I would have sweared that the following 
definition contained "consecutive". After checking, well, it doesn't ;)
   "The ipdv of a pair of packets within a stream of packets is defined
   for a selected pair of packets in the stream going from measurement
   point MP1 to measurement point MP2."
IMHO, too much flexibility is sometimes a problem ... but that's a 
different problem ;)

-  Do you want to introduce the notion that "undefined delay" means 
basically packet loss (even if it arrives, but late, after a threshold), 
for which obviously, it doesn't make sense to report the DV.

- PDV. Another series of questions interesting for this document, for 
which I haven't found the answer so far.
    - how is this D(min) calculated? Is it DV(99%) as mentioned by Roman 
in http://www3.ietf.org/proceedings/05nov/slides/ippm-3.pdf?
    - which interval?
    - do we need to keep all the values from the interval, then take the 
minimum? Or do we keep the minimum from previous intervals?
Btw, all this might be described in Y.1541... I haven't checked.

- I would stress in the case of IPDV, there are situation where we only 
care about the positive IPDV. Example: a delay in a VoIP conversation if 
the de-jitter buffer size is not big enough

- Coming back to the guideline, do you try to say that both PDV and IPDV 
metrics should be measured?
Well, at first glance, it would make sense:
    - IPDV and PDV have different goals.
    - no real extra cost in terms of computation, as we have the results 
already
    - PDV is easier to spot bimodal distribution

- As I said, I like the draft as it contains already a lot of 
information. I think however it could be improved by stressing some more 
the guideline. A kind of " ... SHOULD do this in those circumstances"

For information, some less important issues have been sent directly to Al.

Regards, Benoit.

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