Question

• The goal of this assignment is to implement a simple client-server system. You have to use Python. The basic functionality of the system is a remote calculator. You first will be sending expressions through the client to the server. The server will then calculate the result of the expression and send it back to the client where it is displayed. The server should also send back to the client the string "Socket Programming" as many times as the absolute value of the result from the expression.

  Your server will start in a "passive mode", in other words, it will listen to a specified port for instructions from the client. Separately, the client will be started and will connect to the server on a given host name or IP address plus a port number. The client should take as input from the user the host name or IP address plus the port number and an expression to be calculated. Consider solving an expression that would generate relatively big result, allowing you to test transmission of multiple packets.

• The client, when started, should request the following information:

• • 1. Enter server name or IP address:
    2. Enter port:
    3. Enter expression:
  • 
    
  • 
    
• and should accept a single line for each query.

  The client should make sure that the specified port is valid, in other words, the client should check if the port is in the range from 0 to 65535 and if not should display a message of the format "Invalid port number. Terminating."

  The client, then, will try to connect to the specified server on the specified port and transmit the expression to the server.

  The interaction flow between the client and server is as follows:

  1. The server starts and waits for a connection to be established by the client.
  2. When an expression is received, the server will:
     • Find the result of the expression and store it in a local variable.
     • Write the result and the corresponding number of strings "Socket Programming" to the connection established by the client.
  3. Finally, the client will receive the result from the socket and display it to the user.

Answer 1

We've seen the overview of socket API and how the client and server can communicate let's create our first client and server.

The server will be echo whatever it recieves.

Echo Server

echo-server.py:

Python

#!/usr/bin/env python3

import socket

HOST = '127.0.0.1' # Standard loopback interface address (localhost)

PORT = 65432 # Port to listen on (non-privileged ports are > 1023)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:

s.bind((HOST, PORT))

s.listen()

conn, addr = s.accept()

with conn:

print('Connected by', addr)

while True:

data = conn.recv(1024)

if not data:

break

conn.sendall(data)