Subject: [ippm] FW: IETF Packet Delay Variatation

From: "Robert Holley \(rholley\)" <[email-masked]>
Date: Sun, 9 Sep 2007 21:45:01 -0400

All,

Sorry for the delay on these comments....... 

********************************************
Comments;
********************************************

Summary of Comments;
- IPDV appears to favor an engineering perspective wrt queue algorithms,
feature analysis, sw engineering
- PDV appears to favor an operational perspective wrt configurable
parameters, jitter buffers, dashboard metrics
- My perception is that section 6.2.2 "Path change with Loss" may be the
most critical point.
- If I am reading 6.2.2 correctly, it appears that IPDV has limits in
detecting a common network anomaly

Section 4.3 Examples and Initial Comparisons
=============================================
The fact that IPDV histograms are time dependent seems like it would
complicate analysis. In order to determine time independent link
quality, I assume one would have to do a secondary averaging of the
series of IPDV data sets.

Concerning +/- Jitter values;
---------------------------------
It seems like for voip or other real time applications there is no
relatively significant information contained in negative (-) jitter
values. However, for queue occupancy analysis a series of negative
jitter values would indicated shorter queue wait times. Assuming that
queue wait times fluxuate significantly over time (depending on
congestion and queue servicing algorithms) there may be some
statistically valuable information in maintaining both +/- values. For
example, a stats analysis may be able to quantify the effectiveness of
queue service or congestion recovery.

>From an operational perspective, a primary configurable option would be
queue buffer sizes. These would benefit from the positive valued PDV
data set more than from the +/- data set of IPDV. However, from an
engineering perspective (e.g. feature analysis and/or software
engineering), the efficiency of the queue algorithms would be of more
interest. Therefore, its possible that an engineering perspective might
favor the IPDV method and the operations perspective might favor the PDV
method.

Section 5.1 Demichelis' Comparison
===================================

Comments on Demichelis' IPDV Observations:
------------------------------------------
#1 agrees with mine above in different terms. For example, consider my
text on queue occupancy and Demichelis' comments on "the ability of a
network to preserve spacing...".

#2. He observes the distribution is usually symmetric about the origin
with a mean of zero. I would argue in this case that one plane of
information is redundant (disregarding the engineering perspective for
the moment). In that case, many forms of implementation efficiency can
be obtained. For example, less cpu computations and less data to store,
transport, and analyze. Also, the data store for numeric values can be
reduced if only positive values are maintained. This implies that more
bits in the data element can be used for accuracy.

#3. Again, similar to my queue occupancy argument.

#4. Good point about reduced clock dependencies.

Comments on Demichelis' PDV Observations:
------------------------------------------
#4 is probably the strongest argument in these observations. I would
like to understand better how each of the algorithms respond to this
observation.

#5 is addressed by my discussion on "Ops" perspective vs. "Eng"
perspective.

Section 5.2 Ciavattone et al's Comparison
=========================================
Seems pretty consistent with my comments on section 4.3

Section 5.3 IPPM List's Comparison
===================================
No comments

Section 6.1 Packet Loss
===================================
This is another significant point on each algorithm's robustness.

Section 6.2 Path Change
===================================

Section 6.2.1 Lossless Path Change
----------------------------------
In this section I did not see how IPDV reacted to lossless path change.
However, the complications presented for PDV may offset the efficiency
gains speculated above.

Section 6.2.2 Path Change with Loss
-----------------------------------
This is perhaps the most significant point. A path change with loss is
probably the most common characteristics of routing protocol
re-convergence. This section implies that routing protocol reconvergence
would not be detected in most cases by IPDV. This should be studied
more.

Section 6.5 Reporting a Single Number
======================================
The dashboard concept is very powerful and relevant to operations. Not
so much to engineering. The paper argues that PDV is more compatible
with a single value aggregation. Which appears to be a valid point.
However it must also be recognized that the operational perspective is
one part of the whole picture.

Section 7.2.2 Frequent Path Changes
====================================
There seems to be a contradiction (unless it's my mis-understanding).
This section implies that frequent path changes with loss can be
suitably characterized by IPDV. In section 6.2.2 I understood that path
change with loss may not be detected by IPDV.
Need to clarify.

Section 8.2 Measurement Units
====================================
This section appears to be discussing variations on implementation
issues. True?

Regards
Bob
============================================
Robert Holley
US AS Network Infrastructure
Cisco Systems
7025 Kit Creek Road.
Research Triangle Park, NC 27709
Phone:  +1 919-392-2359
Pager:   +1 800-796-7363 pin 100-9187
============================================  

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