Unfortunately there is little information on how to generate this distribution tables based on real measured data. The only reference available is in the iproute2 source code itself and is a little confusing. After some reading and re-reading I think I finally got the magic formula and here I present it so others can benefit of my work.
First download the iproute2 source code:
git clone \
git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git
Then enter the netem subdirectory and compile the maketable and stats utilities:
cd iproute2/netem
gcc -lm -o maketable maketable.c
gcc -lm -o stats stats.c
Next you need to extract some delay statistics of the channel you want to emulate. Suppose you want to emulate the network between a server in Japan and a client PC in Chile. Then simply run a ping between both machines for some period of time. A whole day of ping data would be enough:
ping 192.168.4.111 > mydata.txt
The command above does a ping to the address 192.168.4.111 and stores the output to a file named mydata.txt. The file will contain the round-trip-times between the local machine and the machine with address 192.168.4.111 like shown below:
PING 192.168.4.111 (192.168.4.111) 56(84) bytes of data.
64 bytes from 192.168.4.111: icmp_seq=1 ttl=43 time=306 ms
64 bytes from 192.168.4.111: icmp_seq=2 ttl=43 time=273 ms
64 bytes from 192.168.4.111: icmp_seq=3 ttl=43 time=310 ms
64 bytes from 192.168.4.111: icmp_seq=4 ttl=43 time=300 ms
64 bytes from 192.168.4.111: icmp_seq=5 ttl=43 time=300 ms
64 bytes from 192.168.4.111: icmp_seq=6 ttl=43 time=304 ms
...
The more data you have the better so leave the ping running for some long period of time. Next we need to extract the round-trip-time data only:
cat mydata.txt | grep icmp_seq | cut -d'=' -f4 | cut -d' ' -f1 > myrtt.txt
The above command will extract all the round-trip-times from mydata.txt and put them in a single column in a file named myrtt.txt. Finally we can create a distribution table that Netem understands using the maketable utility:
./maketable myrtt.txt > mydist.dist
This will create a distribution table and store it in the file mydist.dist that must be copied in a special folder (/usr/lib/tc) so Netem can find it:
sudo cp mydist.dist /usr/lib/tc
Additionally we can obtain the mean and standard deviation of the round-trip-time using the stats utility:
./stats myrtt.txt
> mu = 296.636364
> sigma = 12.808378
> rho = -0.103472
This command gives use the mean (mu) the deviation (sigma) and the correlation (rho) of the round-trip-time. And at last we can now use Netem to emulate the channel's round-trip-time using the following command:
sudo tc qdisc add dev eth0 root netem delay 296.636364ms 12.808378ms distribution mydist
Note the the distribution name is the same as that of the file we copied inside the /usr/lib/tc folder but without the .dist extension. Now Netem will provide you with a channel that has similar round-trip-time characteristics as those of the real channel.
