In this programming assignment you will program the simple message relay application shown below.

The sender process accepts user-entered "messages" from the keyboard, and sends these messages to the receiver process via the message-relay process. The receiver process displays a message once the message has been received in its entirety. All communication among the processes is done using TCP sockets.There are two application-level messages defined: a DATA message (containing the text of a message) and a CLOSE message, which instructs the receiver of the CLOSE message to shut down, as discussed below

• Sender process. The sender process accepts user-entered "messages" (e.g., containing characters entered by the user via keyboard) and sends each message to the relay and then waits for another user-entered message. You can use whatever convention you want for user to indicate that an input message is complete. Note that because of the btye-stream semantics of the TCP socket, your sender process will need to define a application-layer DATA message that lets the relay process know how many characters are in the incoming byte stream. When the user indicates that there are no more messages to send (use whatever covention you want to allow the user to indicate this as well), the sender process should send a CLOSE message to the intermediate relay, and then terminate itself gracefully (i.e., releasing any sockets it has been using).
• Intermediate-relay process. After establishing a connection with the sender and receiver processes, this process reads messages sent by the sender process. It forwards DATA messages to the receiver process over a TCP socket.When the intermediate relay receives a CLOSE message from the sender process, it should send a CLOSE message to the receiver process, and then gracefully terminate itself.
• Receiver process. This process reads mesages sent by the intermediate-relay process. It displays the content of each DATA message once that message has been received in its entirety. On receipt of a CLOSE message, the receiver process should terminate itself gracefully.

Since the goal of the programming assignment is for you to learn socket programming (not to build a hardened application), you can hardwire in specific IP addresses where the processes are known to be running (i.e., you need not somehow ask the user for the host names or IP addresses of the process to which messages will be sent). You may also assume that the relay and the receiver will only be receiving messages from a single sender or relay, respectivly. The extra credit option (i.e., not required) for this assignment asks you to program a relay that can handle multiple senders concurrently.

Please read the information posted on the class WWW site about what to hand with your assignment. Note that a short design document is required.Your should program your each process to print an informative statement whenever it takes an action (e.g., sends or receives a message, detects termination of input, etc), so that you can see that your processes are working correctly (or not!). Tis also allows the TA to also determine from this output if your processes are working correctly. You should hand in screen shots (or file content, if your process is writing to a file) of these informative messages.

You may write your programs in Java, C++, or C. Other languages are OK, but ask me first. You are welcome to do this assignment using the WINSOCK API, rather than the Unix API, if you have a Windoze machine. See http://www.lowtek.com/sockets/ for links to tutorials on the WINSOCK API.

Programming the assignment in JAVA

• Recall from our discussions in class that Java encapsulates the concept of a client-side connection-oriented (TCP) socket with the class, Socket.  Details about the socket class, and the other classes noted below, can be found here.
• Java encapsulates the concept of a server-side connection-oriented socket with the class, ServerSocket.  You'll need to use the accept() and close(0) methods of this class, which are also document at this URL.
• If you're interested in a quick Java tutorial targeted specifically at socket programming, check out "Socket Programming in Java: a tutorial," by Q. Mahmoud,  Javaworld, Dec. 1996, http://www.javaworld.com/javaworld/jw-12-1996/jw-12-sockets_p.html. Sun's socket tutorial is at http://java.sun.com/docs/books/tutorial/networking/sockets/index.html. 
  

Programming the assignment in C

If you choose to write your programs in C, you'll need to master the C system calls needed for socket programming.  These include socket(), bind(), listen(), accept(), connect(), and close(). For an audio/html tutorial of socket programming in C, see "Unix Network Programming," by Jim Kurose at http://www-aml.cs.umass.edu/%7Ejsmanic/unetpgm/.  (You need only sessions 2, 3 and 4 to do this assignment; enter any username). For a quick text-only tutorial of socket programming specifically under Linux, see http://www.lowtek.com/sockets/.
 

Programming notes

Here are a few tips/thoughts to help you with the assignment:

• You must chose a server port number greater than 1023 (to be safe, choose a server port number larger than 5000).
• I would strongly suggest that everyone begin by writing the sender and relay processes first, i.e., just getting the two of them to interoperate corrections. Then modify the relay and program the receiver.
• You will  need to know your machine's IP address, when one process connects to another.  You can telnet to your own machine and seeing the dotted decimal address displayed by the telnet program. You can also use the UNIX nslookup command or you can ask your local system administrator for the info. Or, under Windows, see the WINIPCFG utility.
• Many of you will be running the sender, relay, and receiver on the same UNIX machine (e.g., by starting up the receiver and running it in the background, then starting up the relay in the background, and then starting up the sender. This is fine; since your using sockets for communication these processes can run on the same machine or different machines without modificaiton. Recall the use of the ampersand to start a process in the background. If you need to kill a process after you have started it, you can use the UNIX kill command. Use the UNIX ps command to find the process id of your server
• Make sure you close every socket that you use in your program. If you abort your program, the socket may still hang around and the next time you try and bind a new socket to the port ID you previously used (but never closed), you may get an error.   Also, please be aware that port ID's, when bound to sockets, are system-wide values and thus other students may be using the  port number you are trying to use.

OK.  You've got your sender, relay, and receiver processes running.  You want to do more.  For extra credit, rewrite your relay to be able to handle multiple senders concurrently. Your relay still only needs to forward messages to a single receiver (the goal here is to let your program a concurrent server). The relay waits on the welcoming socket and then creates a new thread or process to handle all communication with the incoming sender. You'll need to do some extra reading on your own to figure out how to do this. See the resources listed above.