V. Making it more interesting

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In this section we are going to define a topology with four nodes in which one node acts as router that forwards the data that two nodes are sending to the fourth node. I will explain find a way to distinguish the data flows from the two nodes from each other, and I will show how a queue can be monitored to see how full it is, and how many packets are being discarded.


V.1. The topology
As always, the first step is to define the topology. You should create a file 'example2.tcl', using the
code from section IV.1 as a template. As I said before, this code will always be similar. You will always have to create a simulator object, you will always have to start the simulation with the same command, and if you want to run nam automatically, you will always have to open a trace file, initialize it, and define a procedure which closes it and starts nam.

Now insert the following lines into the code to create four nodes.

set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]

The following piece of Tcl code creates three duplex links between the nodes.

$ns duplex-link $n0 $n2 1Mb 10ms DropTail
$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link $n3 $n2 1Mb 10ms DropTail

You can save and start the script now. You might notice that the topology looks a bit awkward in nam. You can hit the 're-layout' button to make it look better, but it would be nice to have some more control over the layout. Add the next three lines to your Tcl script and start it again.

$ns duplex-link-op $n0 $n2 orient right-down      
$ns duplex-link-op $n1 $n2 orient right-up 
$ns duplex-link-op $n2 $n3 orient right 
You will probably understand what this code does when you look at the topology in the nam window now. It should look like the picture below.

Nam snap shot

Note that the autolayout related parts of nam are gone, since now you have taken the layout into your own hands. The options for the orientation of a link are right, left, up, down and combinations of these orientations. You can experiment with these settings later, but for now please leave the topology the way it is.


V.2. The events
Now we create two UDP agents with CBR traffic sources and attach them to the nodes n0 and n1. Then we create a Null agent and attach it to node n3.

#Create a UDP agent and attach it to node n0
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0

# Create a CBR traffic source and attach it to udp0
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0

#Create a UDP agent and attach it to node n1
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1

# Create a CBR traffic source and attach it to udp1
set cbr1 [new Application/Traffic/CBR]
$cbr1 set packetSize_ 500
$cbr1 set interval_ 0.005
$cbr1 attach-agent $udp1

set null0 [new Agent/Null] 
$ns attach-agent $n3 $null0 

The two CBR agents have to be connected to the Null agent.

$ns connect $udp0 $null0 
$ns connect $udp1 $null0

We want the first CBR agent to start sending at 0.5 seconds and to stop at 4.5 seconds while the second CBR agent starts at 1.0 seconds and stops at 4.0 seconds.

$ns at 0.5 "$cbr0 start" 
$ns at 1.0 "$cbr1 start"
$ns at 4.0 "$cbr1 stop"
$ns at 4.5 "$cbr0 stop"

When you start the script now with 'ns example2.tcl', you will notice that there is more traffic on the links from n0 to n2 and n1 to n2 than the link from n2 to n3 can carry. A simple calculation confirms this: We are sending 200 packets per second on each of the first two links and the packet size is 500 bytes. This results in a bandwidth of 0.8 megabits per second for the links from n0 to n2 and from n1 to n2. That's a total bandwidth of 1.6Mb/s, but the link between n2 and n3 only has a capacity of 1Mb/s, so obviously some packets are being discarded. But which ones? Both flows are black, so the only way to find out what is happening to the packets is to monitor them in nam by clicking on them. In the next two sections I'm going to show you how to distinguish between different flows and how to see what is actually going on in the queue at the link from n2 to n3.


V.3. Marking flows
Add the following two lines to your CBR agent definitions.

$udp0 set class_ 1
$udp1 set class_ 2

The parameter 'fid_' stands for 'flow id'.

Now add the following piece of code to your Tcl script, preferably at the beginning after the simulator object has been created, since this is a part of the simulator setup.

$ns color 1 Blue
$ns color 2 Red

This code allows you to set different colors for each flow id.

Nam snap shot

Now you can start the script again and one flow should be blue, while the other one is red. Watch the link from node n2 to n3 for a while, and you will notice that after some time the distribution between blue and red packets isn't too fair anymore (at least that's the way it is on my system). In the next section I'll show you how you can look inside this link's queue to find out what is going on there.


V.4. Monitoring a queue
You only have to add the following line to your code to monitor the queue for the link from n2 to n3.

$ns duplex-link-op $n2 $n3 queuePos 0.5
Start ns again and you will see a picture similar to the one below after a few moments.

Nam snap shot

You can see the packets in the queue now, and after a while you can even see how the packets are being dropped, though (at least on my system, I guess it might be different in later or earlier releases) only blue packets are being dropped. But you can't really expect too much 'fairness' from a simple DropTail queue. So let's try to improve the queueing by using a SFQ (stochastic fair queueing) queue for the link from n2 to n3. Change the link definition for the link between n2 and n3 to the following line.

$ns duplex-link $n3 $n2 1Mb 10ms SFQ     
The queueing should be 'fair' now. The same amount of blue and red packets should be dropped.

Nam snap shot

You can download the full example here.


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