16_Unit2_Topic08_Socket_Programming.md

title	Unit 2: Socket Programming: Creating Network Applications
id	unit2-topic8
tags	unit2 application-layer socket-programming tcp udp python
aliases	Socket Programming

Unit 2, Topic 8: Socket Programming

We have studied the principles of application-layer protocols, but how are they implemented? The answer lies in socket programming.

[!note] Definition: A socket is a software interface—an Application Programming Interface (API)—that provides a "doorway" for an application process to send and receive messages to/from the network.

When creating a socket, the developer must choose a transport protocol, which determines the type of socket created.

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1. Socket Programming with TCP

TCP is connection-oriented. A connection must be established before data can be sent.

TCP Server Lifecycle

1. socket(): Create a "listening socket".
2. bind(): Assign a port number to the socket.
3. listen(): Tell the socket to listen for incoming connection requests.
4. accept(): Wait for a client to connect. When one does, create a new, dedicated connection socket for that client.
5. recv() / send(): Communicate with the client using this new socket.
6. close(): Close the connection socket.

TCP Client Lifecycle

1. socket(): Create a client socket.
2. connect(): Initiate a TCP connection to the server.
3. send() / recv(): Exchange data.
4. close(): Close the connection.

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2. Socket Programming with UDP

UDP is connectionless. There is no handshake and no dedicated connection.

UDP Server Lifecycle

1. socket(): Create a UDP socket.
2. bind(): Assign a port number to it.
3. recvfrom(): Wait for a datagram to arrive. This call also provides the client's address.
4. sendto(): Send a reply back to the client's address.

UDP Client Lifecycle

1. socket(): Create a UDP socket.
2. sendto(): Create a datagram, attaching the server's IP and port, and send it.
3. recvfrom(): Optionally wait for a reply.

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3. High-Level Python Examples

a. TCP Server Example

This snippet illustrates the TCP server lifecycle, creating a new socket for each client.

import socket
server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket.bind(('', 12000))
server_socket.listen(1)
print("The TCP server is ready to receive")
while True:
    connection_socket, addr = server_socket.accept() # New socket created
    message = connection_socket.recv(1024).decode()
    connection_socket.send(message.upper().encode())
    connection_socket.close()

b. UDP Server & Client Example

These snippets illustrate the connectionless nature of UDP. Note how the server uses a single socket and how addresses are specified in each sendto call.

UDPServer.py

import socket
server_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server_socket.bind(('', 12000))
print("The UDP server is ready to receive")
while True:
    message, client_address = server_socket.recvfrom(2048)
    modified_message = message.decode().upper()
    server_socket.sendto(modified_message.encode(), client_address)

UDPClient.py

import socket
server_name = 'hostname'
server_port = 12000
client_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
message = input('Input lowercase sentence:')
client_socket.sendto(message.encode(), (server_name, server_port)) # Attach destination addr
modified_message, server_address = client_socket.recvfrom(2048)
print(modified_message.decode())
client_socket.close()

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4. Exam Focus

• 2-Mark Questions:
  • What is the difference between a listening socket and a connection socket on a TCP server?
  • Which function call in the TCP socket API triggers the three-way handshake from the client side? (connect()).
  • In UDP socket programming, how does the server know where to send a reply? (The client's address and port are part of the datagram received via recvfrom()).
• 10-Mark Question:
  • "Compare the socket programming steps (API calls) required for a TCP application versus a UDP application. Describe the process from both the client and server perspectives."
    • Tip: Create two columns, one for TCP and one for UDP. List the sequence of socket calls for the server and client. Highlight the key differences, such as the listen()/accept() calls in TCP and the lack of a connect() call in UDP, explaining why these differences exist.