Node Net

Third Laboratory: Systems of Objects; Nodes and Channels

Node net is designed to develop testing and debugging skills. Students also get practice implementing an interface and handling exceptions. The node net program allows the students to build networks out of nodes and channels. By default three types of nodes are available: generator nodes, intermediate nodes, and terminator nodes. Nodes are connected by channels. Generator nodes create "packets" and send them out to channels, intermediate nodes read "packets" from channels and retransmit them, and terminator nodes consume "packets." The students write their own class(s) implementing the NodeBehavior interface, and load it into the program, usually for use as an type of intermediate node.

• View Handout
• Browse Javadoc
• Usage notes
• Screen shots
• Executable jar
• Source jar
• To quickly see it run: java -jar nodenet.jar (using the executable jar)
• Report bugs

Running the Program

There are many ways to run the code in class. You should choose the one that your students are ready for. They are presented in an order that represents an increasing usage of the command line utilities and options.

1.)a) Click here
   b) File > Expand Classpath > (choose jar/dir of package of student code.)   
   c) File > Load Behavior > (enter fully qualified Class name for student code)

      (requires Java WebStart). If you want to put a link on your course 
      web site, you may use a link like this (subject to our terms of use 
      like all materials on the site): 

      <a href="http://www.cs101.org/psets/ballworld/demo/ballworld.jnlp">Click Here</a>

2. a) java -jar nodenet.jar
   b) File > Expand Classpath > (choose jar/dir of package of student code.)   
   c) File > Load Behavior > (enter fully qualified Class name for student code)

3. a) java -cp nodenet.jar nodenet.Main
   b) File > Expand Classpath > (choose jar/dir of package of student code.)
   c) File > Load Behavior > (enter fully qualified Class name for student code)

4. a) java -cp "nodenet.jar:/path/to/student/code" nodenet.Main
   b) File > Load Behavior > (enter fully qualified Class name for student code)

5. a) java -cp "nodenet.jar:/path/to/student/code" nodenet.Main stupkg.StudentBehavior [stupkg.StudentBehavior2]

6. a) jar -xvf nodenet.jar
   b) java nodenet.Main
   c) File > Expand Classpath > (choose jar/dir of package of student code.)
   d) File > Load Behavior > (enter fully qualified Class name for student code)

7 thru 9 same permutations on the unpacked executable jar.

10 - 13 again but start with the source jar, add compilation step(s).

(Line 6b assumes that you have the current directory on your default class path)

Usage Notes

• Students must not create their classes in a package named "nodenet." This is because WebStart requires us to sign our jar, and the JVM gets very upset if 2 classes from the same package have different "signer information." The result is a very ugly SecurityException. Students should use no package name, or their own package name.
• The simulation may be started, stopped, reset, and cleared (erased) using the simulation menu.
• Nodes can be configured with a name, which can be hidden or displayed.
• Channels can be configured with a capacity indicating how many packets they can hold at one time, and a latency indicating how long a packet remains in the channel before it reaches the destination.
• Each node behavior loaded into the program is given a colored icon which can be used to create a network.
• Networks may also be saved as templates, and reloaded (file menu).
• When a template is reloaded the currently loaded node behaviors are assigned to the template positions in the order that they are displayed.
• Right clicking on the node buttons on the left produces a pop-up menu which will allow you to reorder the node behaviors.
• If the template requires more node behaviors than are currently available the first node behavior will be substituted for missing behaviors.

• When loading a NodeBehavior class the program looks for public static final String NODE_BEHAVIOR and uses it's value to label the button. If this field does not appear in the class, the node behavior's fully qualified class name is used to label the button.
• Note that correct student code or even our intermediate nodes may hold packets breifly. See the screen shots below for tips on assessing possible packet loss.
• Node behaviors may be unloaded by clicking on their button while holding down the control key.
• Loading and unloading Node behaviors can eventually cause the program to fail when it runs out of colors to represent nodes with. Currently, 10 total colors are available, which allows students to work with up to 8 classes at a time (2 are reserved for Generator and Terminator). Once a Node behavior is assigned a color it will always be that color until the program is re-started, so if you webstart the program Intermediate node also takes up a color and 7 colors remain for student classes.

Screen Shots

The screenshot below shows the program run with method 1 above, and a user loaded node behavior named Foo. Channels turn red when they are full to capacity. The channel on the right that has turned red has been configured to a capacity of 3 and a latency of 5000 miliseconds causing a traffic jam.

Packets are probably not being lost in this simulation despite the fact that 9 packets on the display and plus 89 at the terminator don't add up to 100 (matching the sum of the generators). There is definately one packet waiting to be transmitted in the upper right Intermediate node, and any one of the other intermediate nodes or the foo node might hold the last packet. To be sure that packets are not being held temporarily by student node behavior code one should disable all generators and let the simulation run until it seems to reach a steady state as shown below. Only after letting the network run without active generators can one be sure that a network is or is not dropping packets.