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Assignment 1

Computer Networks (CS 436/636) Winter 2022

Introductory Socket Programming

Work on this assignment is to be completed individually

 

1. Assignment Objective

The goal of this assignment is to gain experience with both TCP and UDP socket programming in a client-server environment (Figure 1). You will use Python or any other programming language to design and implement a client program (client) and a server program (server) to communicate between themselves.

 

 

2. Assignment Specifications
   1. Summary

In this assignment, the client will send requests to the server to reverse strings (taken as a com- mand line input) over the network using sockets.

This assignment uses a two-stage communication process. In the negotiation stage, the client and the server negotiate through a fixed negotiation port (<n_port>) of the server, a random port (<r_port>) for later use. Later in the transaction stage, the client connects to the server through the negotiated random port (<r_port>) for actual data transfer.

1. 2. Signaling

The signaling in this project is done in two stages as shown in Figure 2.

Stage 1. Negotiation using TCP sockets: In this stage, the client creates a TCP connection with the server using <server_address> as the server address and <n_port> as the negotiation port on the server (where the server is listening). The client sends a request to get the random port number from the server where it will send the actual request (i.e., the string to be reversed). To initiate this negotiation, the client sends a request code (<req_code>), an integer (e.g., 13), after creating the TCP connection. If the client fails to send the intended <req_code>, the server closes the TCP connection and continues listening on <n_port> for subsequent client requests. Once

 

the server verifies the <req_code>, it replies back with a random port number <r_port> where it will be listening for the actual request. After receiving this <r_port>, the client closes the TCP connection with the server.

Stage 2. Transaction using UDP sockets: In this stage, the client creates a UDP socket to the server in <r_port> and sends the <msg> containing a string. On the other side, the server re- ceives the string and sends the reversed string back to the client. Once received, the client prints out the reversed string and exits. Note that the server should continue listening on its <n_port> for subsequent client requests. For simplicity, we assume, there will be only one client in the system at a time. So, the server does not need to handle simultaneous client connections.

1. 3. Client Program (client)

You should implement a client program, named client. It will take four command line inputs:

<server_address>, <n_port>, <req_code>, and <msg> in the given order.

1. 4. Server Program (server)

You should also implement a server program, named server. The server will take <n_port> and

<req_code> as a command line parameters.

 

2. 5. Example Execution
      • Run server: python Server.py <n_port> <req_code>
      • Run client: python Client.py <server address> <n_port> <req_code> 'A man, a plan, a canal— Panama!'
3. Hints

Below are some points to remember while coding/debugging to avoid trivial problems.

• You can use and adapt the sample codes of TCP/UDP socket programming in Python slides (last few slides Chapter 2).
• Use port id greater than 1024, since ports 0-1023 are already reserved for different pur- poses (e.g., HTTP @ 80, SMTP @ 25)