Exercise 1: Echo Server

Objective

Build a TCP echo server that receives messages from clients and sends them back.

Requirements

1. Create a TCP server that listens on localhost:8080
2. Accept client connections
3. Receive messages from clients
4. Echo (send back) the received message
5. Handle multiple clients (use threading)
6. Implement proper error handling

Basic Implementation (Single Client)

Starter Code

import socket

def echo_server():
    # TODO: Create TCP socket
    # TODO: Bind to localhost:8080
    # TODO: Listen for connections
    # TODO: Accept connection
    # TODO: Receive and echo messages
    # TODO: Close connections
    pass

if __name__ == '__main__':
    echo_server()

Expected Behavior

Server side:

Server listening on localhost:8080...
Connection from ('127.0.0.1', 54321)
Received: Hello, Server!
Echoed: Hello, Server!
Connection closed

Client side:

Connected to server
Sent: Hello, Server!
Received: Hello, Server!

Intermediate Implementation (Multiple Clients)

Extend the basic server to handle multiple clients simultaneously using threading.

Requirements

• Use threading.Thread for each client
• Server should continue accepting new connections while handling existing ones
• Each client should be handled independently

Advanced Implementation (Enhanced Echo Server)

Add the following features:

1. Connection logging: Log all connections with timestamp
2. Message statistics: Count total messages received
3. Command support: - Send "QUIT" to disconnect - Send "STATS" to get message count - Send "TIME" to get server time
4. Timeout handling: Disconnect idle clients after 30 seconds
5. Graceful shutdown: Handle Ctrl+C and close all connections

Testing

Test 1: Basic Echo

# Terminal 1: Start server
python echo_server.py

# Terminal 2: Test with telnet
telnet localhost 8080
# Type: Hello, Server!
# Should receive: Hello, Server!

Test 2: Multiple Clients

# Terminal 1: Start server
python echo_server.py

# Terminal 2: Client 1
telnet localhost 8080

# Terminal 3: Client 2
telnet localhost 8080

# Both should work simultaneously

Test 3: Python Client

import socket

client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect(('localhost', 8080))

messages = ["Hello", "World", "Test"]
for msg in messages:
    client.sendall(msg.encode())
    response = client.recv(1024)
    print(f"Sent: {msg}, Received: {response.decode()}")

client.close()

Solution Hints

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server.bind(('localhost', 8080))
server.listen(5)

while True:
    data = client.recv(1024)
    if not data:
        break
    client.sendall(data)

import threading

def handle_client(client, address):
    # Handle client here
    pass

while True:
    client, addr = server.accept()
    thread = threading.Thread(target=handle_client, args=(client, addr))
    thread.start()

Success Criteria

• ✅ Server accepts connections
• ✅ Messages are echoed correctly
• ✅ Multiple clients can connect simultaneously
• ✅ Proper error handling (connection closed, etc.)
• ✅ Server can be stopped gracefully (Ctrl+C)

Bonus Challenges

1. Add support for UDP echo server
2. Implement message history (last N messages)
3. Add client authentication (simple password)
4. Implement rate limiting (max messages per second)
5. Add support for binary data (images, files)

Next Steps

After completing this exercise, you should understand: - TCP server creation and binding - Accepting client connections - Sending and receiving data - Handling multiple clients - Basic error handling

Proceed to Exercise 2: Chat Application to build on these skills.