The simulation window is divided into two main areas: an animation area and a control area. The animation area is the top part of the window. The control area is the bottom part of the window and contains a table listing size information, the radio buttons for the MTU of the first network, the radio buttons for the MTU of the second network, and the buttons to control the action of the simulation.
The animation area shows the protocol stack layers on the source machine, a router, and on the destination machine. The box for each protocol stack layer has the name for that layer inside it. The source machine and the router are connected by network one. The router and the destination machine are connected by network two. Initially nothing else is visible in the animation area. When the Start button is clicked, a packet starts going from the application layer on the source machine to the application layer on the destination machine.
The left section of the control area is a table listing the size (2000 bytes) of the message originating at the application layer of the source machine, and the sizes of the headers that are added at the transport layer, the network layer, and the data link layer.
The bottom of the right section of the control area is a row of buttons: Start, Clear, Help, and Info. The Start button starts a simulation using the current MTU settings. The Clear button clears the images of the copies of the packet from the animation area. The Help button causes a message to appear explaining how to use the applet. The Info button causes a message to appear explaining the points the applet illustrates. Each button is only enabled when it initiating its action would be appropriate.
The top of the right section of the control area is used to select the sizes of the MTUs for the two networks. The default simulation assumes that the maximum transmission unit (MTU) of the first network is 3000 bytes and that the MTU of the second network is also 3000 bytes. Since the corresponding size of the packet being considered is 2040 bytes (2000 bytes for the message plus the 20 bytes for the transport layer header and 20 bytes for the network layer header), the MTU of neither network is a constraint. Thus, no fragmentation of the packet occurs in the default simulation. This was chosen so that the first simulation the user sees is the simplest.
Changing the MTU of the first network to 1500 bytes causes fragmentation to occur before the first network is reached. Changing the MTU of the second network to 1000 bytes causes fragmentation to occur before the second network is reached. Changing both the MTU of the first network to 1500 bytes and the MTU of the second network to 1000 bytes causes fragmentation to occur twice, once before each network is reached.