CS 4700/5700 - Fundamentals of Computer Networks

Problem Set 1: Socket Programming

Work to be done in teams of two students.

Due date: 11:59PM EST, January 23, 2012.

Late submissions will result in a 10% penalty per day (e.g., 2.5 days late result in 25% penalty)

Sockets Communication

Write a simple client-server broadcast chat application. The server listens at a specified UDP port, and waits for GREETING messages from remote clients. Once a greeting has been received the client may send a MESSAGE command to the server, which will forward the contents in an INCOMING message to every remote point that has previously sent a GREETING.

The client creates a socket which it will maintain throughout its lifetime, and is capable of receiving and sending packets from/to the server.

The types and format of messages to be exchanged in the application are:

1. GREETING: Greets the server. Client to server only.

2. MESSAGE text-of-msg: Sends some text to the server for further distribution. Client to server only.

3. INCOMING text-of-msg ip port: The server has received some text, and is passing it along with the sender's ip and port. Server to client only.

4. Any other message (e.g. ICMP errors) must be ignored.

You can use either Java or C/C++ to program the application. Here are C sample codes for a udp server and client.

A sample run of your application must work as follows:

• server$ java ChatServer 9090 runs the server on port 9090
  Server Initialized... Server is left running
• user1$ java ChatClient server-ip 9090 runs the client and GREETs the server.
  ] MESSAGE Hello World! Sends a message
  <From w.x.y.z:aa>: Hello World! Every client sees this, where w.x.y.z:aa is the senders ip address and port.
  ]

Note the following:

• The client program keeps waiting for the user to enter new messages and sends them to the server.
• The messages forwarded by the server should be immediately displayed by the client program.
• The messages do not need to be authenticated, confirmed nor encrypted.
• There are no usernames whatsoever.
• The server sends the message to everyone who has GREETED him. There is no mechanism to make the server "forget" or logout clients.
• There is no limit to the number of clients the server supports.

Network Diagnostic

On your own machine, install wireshark, a widely used network protocol analyzer. Familiarize yourself with wireshark capabilities. Run the server chat application you developed in the previous section, on login.ccs.neu.edu, and the client on your own machine.

• Capture and filter the traffic of your application. Show how you did it. (Hint: select the right network interface, filter based on your apppliation port.)
• Capture (1) a GREETING packet, (2) a MESSAGE packet, and (3) an INCOMING packet. Print a screen dump for each of the packets. Describe the protocol stack encapsulation headers for your application packets.

Mechanisms for Reliable Communication

• Describe a mechanisms that makes the delivery of the MESSAGE packet reliable from the clients to the server.
• Dicuss how you could make the delivery of INCOMING packets reliable. How about efficiency and scalability when the number of clients is very large?