netdev - Netem Delay Normal Distribution

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Message-ID: <CAJx7G_xkrVkC7HGQZogivA_6-i2Gas6aJd-sOFNMEbmq8=cS0g@mail.gmail.com>
Date:	Sat, 26 Oct 2013 21:31:03 -0500
From:	anirup dutta <adutta2@...edu>
To:	netdev@...r.kernel.org
Subject: Netem Delay Normal Distribution

Hello,

I try this on my device

sudo tc qdisc del dev eth0 root netem delay 100ms 20ms distribution normal

I use iperf for transmitting UDP packets and I modified its code to
get per packet delay. When I plot the distribution of delays and
analyze the delays they pass the normality tests.

The only problem that I am not able to understand is that the mean of
those delays shift to 125ms. Its not only me. There was another study
and it observed the same thing.

http://www.researchgate.net/publication/224256550_An_Empirical_Study_of_NetEm_Network_Emulation_Functionalities/file/d912f5058c9b1e409b.pdf

Figure 7

I found out that this command is valid

sudo tc qdisc del dev eth0 root netem delay 1ms 20ms distribution normal

So I have a feeling that mean gets shifted from the base delay to
avoid negative delay values.

It would be great if someone can confirm how it is implemented?

Regards,

Anirup
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