Subject: RE: [ippm] RFC3393 selection function question

From: "Perser, Jerry" <[email-masked]>
Date: Tue, 8 Apr 2003 13:37:51 -0700

After rereading RFC3393, I agree the intent was between 2 consecutive
transmitted packets.  The question I have is, what's wrong with making the
selective function between 2 consecutive received packets?

I see advantages to this, but no disadvantages.  I'll introduce two
abbreviations for the following discussion. IPDV-TX is per RFC3393.  IPDV-RX
is an alternative selection function based on received packets.

Both metric will return the same results unless there is reordering,
duplicate, or loss.

Under reordering IPDV-TX and IPDV-RX will return the same set of numbers,
just in a different order.  Evaluate the examples in
draft-ietf-ippm-reordering-02.txt, both IPDV-TX and IPDV-RX return the same
set of numbers.

Under duplicate, IPDV-TX only looks at only the first instance, where
IPDV-RX gives a result for each and every pair of received packets.  The
reason for the duplicate is unknown, but at least you can study the effects
of delay with IPDV-RX.

Loss is the most interesting case.  IPDV-TX is blind when there is loss.
Suppose you are measuring IPDV over a high speed link.  During the
measurement it switches to low speed (i.e. delay goes from 1mS to 500mS).
The switch over also introduces loss.  IPDV-TX will never see this variation
of 499mS (500mS - 1mS).

I am looking for disadvantages to IPDV-RX.

Jerry.

> -----Original Message-----
> From: Philip Chimento [mailto:[email-masked]]
> Sent: Tuesday, April 01, 2003 2:07 PM
> To: Al Morton
> Cc: Perser, Jerry; IPPM (E-mail)
> Subject: Re: [ippm] RFC3393 selection function question 
> 
> 
> Hi Al: Yes. That was the intent. Perhaps it should have been clearer. 
> Regards, Phil 
> 
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From: Philip Chimento <[email-masked]>
Date: Mon, 28 Apr 2003 14:59:08 -0400

Hi Jerry: 
Please excuse the lateness of the reply. I have been out of the office 
a lot
in the past couple of weeks. 

> selective function between 2 consecutive received packets?

The basic problem is that you are measuring different things. There can 
be clusters of loss events or periodicities in the queue lengths of 
intermediate queues that will mask differences in delay variation and 
give you false results.
 
> Both metric will return the same results unless there is reordering,

This RFC is based on the one-way delay metric. We implicitly assume 
(and this really should have been stated explicitly) that the packets 
are sequence-numbered. They are in the Surveyor tool and I think also 
in the RIPE active measurment tool (right Henk?). In that case, 
re-ordering doesn't matter. We state in the RFC that in the case of 
duplicates, only the FIRST packet counts (i.e. if one of the packets in 
the packet pair is duplicated, only the first one with that sequence 
number is counted and used in the computation of delay variation).
 
> The switch over also introduces loss.  IPDV-TX will never see this variation
If you are using the IPDV metric to detect a switchover in link speeds, I think that you are doing the wrong thing. It really isn't intended for that. You would do better in this case using one-way delay and look for discontinuities. If a single event in the network is associated with loss, yes, you will miss that event by throwing away the lost pairs. However, that is the right thing to do from a statistical point of view. The point of IPDV is to look at a longer time-scale phenomenon:i.e. how packet delay changes over time. You are really looking at a first derivative.  

Now, to take an example of why counting received packets can mislead. Suppose
that we take the "jitter" definition given in the RFC (i.e. absolute value of samples). Pick your own units.   

Packet number      1  2  3  4  5  6  7  8  9  10  11  12  13  14  15 

One-way delay      3  X  7  5  3  1  3  5  7   9   X   5   3   1   3 

IPDV samples        -  -  2  2  2   2 2  2   2   -   -   2   2   2   

Your proposal         4   2  2  2   2 2  2   2     4     2   2   2 

IPDV average           2 
Your proposal average  2.33

Now this may not seem to be a big difference, but it is a 16 percent relative error. I could have picked the numbers to make the difference arbitrarily large. Now, you can argue that loss is so infrequent that when you average, any error will be miniscule. Perhaps, but that also depends on network behaviour. A couple of consecutive loss events could throw the statistics off. 

>From a theoretical point of view, your proposal does not measure the phenomenon consistently. (It's like doing a survey and including only the 
samples that whose answers you like in your calculation of the results.) I think that throwing out the samples where one or more of the packets is lost is the only correct way to do it. 

Regards, Phil Chimento

-- 
Phil Chimento             Ericsson IPI
Phone: 240-314-3597	  7301 Calhoun Place
[email-masked]   Rockville, Md. 20855
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