Subject: wire-time and OWD

From: ADAM Yann FTRD/DAC/LAN <[email-masked]>
Date: Mon, 30 Oct 2000 14:11:19 +0100

Hi all,
I have questions related to the framework document (rfc2330) and to the
owe-way delay metric (rfc2679):
- in rfc2330, the notion of wire-time is declined in 'wire arrival time' and
'wire exit time'. The 'wire exit time' is defined as: 'For a given packet P,
the wire exit time of P at H on L is the first time T at which ALL THE BITS
OF P have appeared at H's observational position on L'.
- in rfc2679, the OWD is defined as: 'For a real number dT, >>the
*Type-P-One-way-Delay* from Src to Dst at T is dT<< means that Src SENT THE
FIRST BIT of a Type-P packet to Dst at WIRE-TIME* T and that Dst received
the last bit of that packet at wire-time T+dT'.

I guess for the OWD metric, one should write "... Src sent the first bit of
a ....at WIRE EXIT TIME T .....Dst received the last bit of ..... at WIRE
ARRIVAL TIME T+dT".

MY FIRST QUESTION:
I have the feeling that there is a mistake here: the OWD specifies the Src
sent the FIRST BIT at wire time T, whereas the wire exit time specifies that
ALL THE BITS have appeared.
Has anybody got a explanation ?

MY SECOND QUESTION:
I have read the rfc2679, and I have the feeling that it is assumed that an
IPPM compliant probe should be active, but it is not written in the
document.
Is it possible that a passive probe (which does not introduce its own type-P
packets) should be IPPM-compliant ?

Thank you.
Best regards.

Yann.

---

From: Demichelis Carlo <[email-masked]>
Date: Tue, 31 Oct 2000 12:27:34 +0100

I would like to offer my personal opinion.
1) - Each definition of delay, measured by observing or sending information
between two points has been always given, in standardization bodies (like
ITU-T) by considering the initial time of transmission of the FIRST bit and
the ending time of reception of the LAST bit. In this way the duration of
the information transmitted is included in the delay, that will not be zero,
even if the two Measurement Points coincide. The delay is therefore defined
from the time at which the System Under Test is engaged in the information
transfer, up to the time at which the SUT ends this job.
With this definition the packet length is a measurement paramenter, and all
the characteristics of the network between the two measurement points are
considered omogeneously.
With a definition that considers the time of transmission of the last bit
(all the bits), the packet length is still a parameter due to the reception
time and a possible different behavior of the network, but the transmission
speed (speed of the first link of the SUT) is no more considered in the
measurement (or it is considered in a non omogeneus way), and therefore the
result is not significant. It is therefore an error to consider the time at
which ALL THE BITS have appeared at the first measurement point.
2) - If the SUT is the network between the two points at which we imagine to
measure the wire time (beginning and end of physical links), the measurement
is always "virtual". An Abstract Test Equipment (ATE) should be defined,
both for sending / receiving test packets and for observing packets not
intrusively. Any real test equipment should then be characterized by a value
of precision. Fixed and known differences can be taken into account, while
uncertainties are part of the measurement error.
In my opinion IPPM has defined methodologies for tests, using testing
packets. For non intrusive monitoring, not only an appropriate ATE should be
defined, but probably also other definitions should be revisited. (Packets
are no more under the control of the "entity" performing the measurement, as
well as intervals between packets. Packets are coming from a preceding
"cloud", and it is difficult to put constraints on the characteristics of
the flow assumed to be measured). Always in my opinion, there is still the
need of definitions for non intrusive monitoring.
I have considered this question in the first part of the paper presented in
Birmingham at WTC-ISS 2000 (May 2000)
C. Demichelis - CSELT: "Improvement of the IPDV concept and applications"
Bye
Carlo Demichelis
[email-masked]

> ----------
> From: 	ADAM Yann FTRD/DAC/LAN[SMTP:[email-masked]]
> Sent: 	Monday, October 30, 2000 2:11 PM
> To: 	[email-masked]'
> Subject: 	wire-time and OWD
> 
> Hi all,