[e2e] Estimating MS windows RTO equation

Sushant Rewaskar rewaskar at email.unc.edu
Thu Feb 2 05:26:37 PST 2006

It is not that we are interested in only MS windows machines; however, it is
the only one which we have not figured out yet. 

We have created a partial state machine based tool for tracing the state of
the TCP connections. This helps us identify the cause of retransmission of a
packet and by using extra logic we can identify other things like whether
the packet was needed or not. 

Now the problem with creating a general partial state-machine is that there
is a lot of variation in the way TCP is implemented in the different OSes
(windows for e.g. enters fast retransmit after 2 dupack instead of 3 as
suggested in the RFC). There are many other differences across the different
OSes especially in the way the RTO is estimated. We are trying to determine
the RTO equations so that we can (i) exactly tell which packet is triggered
by a timeout and (2) if we get a very good match between the observed
retransmission time and calculated RTO, we would also be able to tell that
the particular connection below to a particular OS.  We have been able to
build almost perfect state machines for Linux, Solaris and FreeBSD (MacOS)
and have been able to do a decent job for windows (including identifying
certain unusual behavior which is peculiar to windows OSes). However, we are
still struggling to get the RTO estimation perfected.  Details about the
tool are available in the following tech report

About the RTT calculation. For the experiments I mentioned in the last mail
we are collecting a tcpdump at the sender (FreeBSD machine) and calculate
RTT using karls Algorithm. The details for RTT estimation are in the
following paper  http://www.cs.unc.edu/~jasleen/papers/imc03.pdf 

Thank you,
Sushant Rewaskar
UNC Chapel Hill

-----Original Message-----
From: detlef.bosau at web.de [mailto:detlef.bosau at web.de] 
Sent: Thursday, February 02, 2006 6:31 AM
To: rewaskar at cs.unc.edu
Cc: end2end-interest at postel.org
Subject: Re: [e2e] Estimating MS windows RTO equation


First of all, its interesting why you are interested in just MS Windows´
RTO esitmator ;-)
Basically, MS is free to implement anything they like as long as their
OS are well behaved, i.e. do not cause trouble to other network
Of course, it might be a good idea to follow the actual RFC ;-)

To your problem.

How did you obtain the rtxtimer samples from which you try to estimate
your parameters?

Presumably, you did not take them from the OS kernel code, because if
that were available for you you could easily look up the RTO equation
that ;-)

Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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