Subject: [OPSAWG] Comments on draft-akhter-opsawg-perfmon-method-02

From: Al Morton <[email-masked]>
Date: Sun, 13 May 2012 13:06:56 -0400

Hi Aamer,

I finally made some time to review one of your drafts:
http://tools.ietf.org/html/draft-akhter-opsawg-perfmon-method-02

and comments follow below.  Not sure exactly which list
will be best, so I CC'd ipfix and opsawg.

hope this helps,
Al

-=-=-=-=-=-=-=-=-=-=-=-=-=-

Main Comments:

1. Packet Loss Calculation Method (4.1.1)
Each metric has a paragraph labeled Calculation Method, but
they contain a useful discussion of the possible issues instead.
Many details are left to the implementor, with the likely outcome
that different implementations will produce different results.
For example, "synchronization" is mentioned many times, but what
does this process entail? Try to minimize interpretation,
perhaps by including a high-level step-by-step procedure for
processing each arriving packet, both during initiation
(synchronization, when the first few packets arrive) and
steady-state processing (which involves using the info from sync).
Since this draft is proposed for the Standards Track, it
really needs to provide definitions that will stand-up to
scrutiny of multiple implementations.

2. Packet Loss "Rate" (4.1.3)
This metric is really a Statistic, or a metric derived from the
fundamental metrics of Packet Loss and Expected Packet (counts)
over a particular measurement interval.  Thus, there's no need
to talk about measurement points, you've already defined these
in the Loss and Expected metrics.
Also, this is more accurately named "Loss Ratio", especially when
expressed as a percentage as you've done.

3. Jitter (4.1.5 and beyond)
This metric will disappoint for use in SLAs or de-jitter buffer
sizing if it is based on the RFCC3550 smoothed jitter calculation.
Regardless of the measurement interval, only the last 16 packets
have any influence over the current value (which is your Average?)

General Editorial:
There are a number of sentences that seem to contain alternate
wordings, as though some deleted text returned somehow. Also,
running a spell-check will help.

Specific comments follow:

3.1.  Quality of Service (QoS) Monitoring

    The network operator needs to be able to gauge the end user's
    satisfaction with the network service.  While there are many
    components of the satisfaction such as pricing, packaging, offering,
    etc., a major component of satisfaction is delivering a consistent
    service.
Consistent Service in what dimensions?  Is Service Performance meant here?

4.  New Information Elements

    The information elements are organized into two main groups:

    Transport Layer:  Metrics that might be calculated from observations
       at higher layers but essentially provide information about the
       network transport of user date.
s/date/data/

4.1.1.  perfPacketLoss

    Name:  perfPacketLoss

In this section, s/loss packet/lost packet/

Also, what is the 5-tuple? SSRC + Src IP and Port + Dst IP and Port?

At the end:
    Measurment Timing  To be able to calculate this metric a continuous
       set of the flow's packets (as each would have an incrementing
       sequence number) needs to be monitored.  Therefore, per-packet
       sampling would prevent this metric from being calculated.
s/per-packet/sub-rate/ ??

4.1.2.  perfPacketExpected

    Name:  perfPacketExpected
...
    Use and Applications  The perfPacketExpected is a mid-calculation
       metric used in the generation of perfPacketLossRate.  It is
       equivilent to the highest received packet sequence number at the
       time of measurement.
True when there is no roll-over in Seq numbers.

                       ... As the value only increments when packets
       are received, packet loss may be occouring at the time of
       measurement but perfPacketExpected remains constant.
This is an issue worth mitigating, some sort of average packet rate
could be used to estimate the number of losses while the Expected count
remains constant.

    Calculation Method:  The subtraction of the last sequence number from
       the first sequence number in monitoring interval yields the
       expected count.  As discussed with perfPacketLost, there might be
       a delay due to synchronization with the flow's sequence numbers
       and in such times the value of the metric should be set to
       0xFFFFFFFE.  Care has to be taken to account for cases where the
       packet's sequence number field wraps.
This is an opportunity for more specifics, as an example
(I didn't check the Appendix 3 reference, no access today...)

4.1.3.  perfPacketLossRate

    Name:  perfPacketLossPercentage
These aren't the same!  Which is it?

4.1.4.  perfPacketLossEvent

    Name:  perfPacketLossEvent
Need "consecutive" in the name, or some other way to distinguish
single losses from "bursts".

...
    Calculation Method:  This data value is a simplified version of the
       Lost Packets metric.  Whereas Lost Packets counts individual
       packet loss, the 'loss event count' metric counts sets of packets
       that are lost.  For example, in the case of a sequence of packets:
       1,3,6,7,10 the packets marked 2,4,5,8 and 9 are lost.  So, a total
How long does the process wait to make the lost determination?

4.1.5.  perfPacketInterArrivalJitterAvg

    Name:  perfPacketInterArrivalJitterAvg

    Description:  This metric measures the absolute deviation of the
       difference in packet spacing at the measurement point compared to
       the packet spacing at the sender.
I don't think that's what 3550 Jitter does...

...
    Use and Applications  The inter arrival jitter data value can be used
       be network operator to determine the network's impact to the
       spacing in between a media stream's packets as they traverse the
       network.
What about Source Jitter?  this can be very significant, more than network.

Again, Min and Max are statistics.

4.2.  User and Application Layer

4.2.1.  perfSessionSetupDelay

    Name:  perfSessionSetupDelay
This seems to be the only two-point metric, worth noting.
and
[I-D.ietf-pmol-sip-perf-metrics]
is an RFC now (which mostly defined metrics, not methods).