09 Sockets

Sockets

Basics

• Sockets provide a communication endpoint between programs
• Acts like a bidirectional pipe sending bytes
• Can communicate within same machine or across network

Identification (IP + Port)

• Each socket endpoint identified by (IP address, port number)
• IP address → identifies machine
• Port → identifies program on machine
• BOTH required for unique identification

IP Address

• IPv4 format: x.x.x.x (0–255 each)
• 127.0.0.1 → localhost (same machine)
• 0.0.0.0 → all network interfaces

Port Numbers

• Range: 0–65535
• Ports <1024 are reserved
  • 80 → HTTP
  • 443 → HTTPS

Client vs Server

Server

• bind((IP, port)) → attach socket
• listen() → wait for connections
• accept() → accept client (returns new socket)
• Handles incoming connections

Client

• connect((IP, port)) → initiate connection
• Must know server IP and port

Key Flow

• Server starts first
• Client connects
• Server accepts → new socket created
• Communication becomes bidirectional

Data Transmission

Bytes vs Strings

• Sockets send bytes, NOT strings
• Must convert:
  • str → bytes: .encode()
  • bytes → str: .decode()

Receiving Data

Problem

• recv(n) does NOT guarantee full message
• Data may arrive in chunks

Solution: Protocol

• Define message boundaries (e.g. ‘\n’)

Correct Pattern

data = b''
while b'\n' not in data:
    data += socket.recv(1024)

Socket Methods (Core)

• bind((host, port)) → attach socket
• listen() → wait for clients
• accept() → accept connection
• connect((host, port)) → client connects
• recv(n) → receive bytes
• sendall(bytes) → send all data

Blocking Behavior

• accept() blocks until connection arrives
• recv() blocks until at least 1 byte received

Server Types

Iterative Server

while True:
    accept client
    handle client

• Handles ONE client at a time

Concurrent Server

• Uses threads/processes
• Handles MULTIPLE clients simultaneously

Protocol Design

• Must define:
  • Message format
  • Valid commands
  • Start/end markers

Example Messages

• b’MOVE5\n’
• b’END\n’

Templates

Server

import socket
s = socket.socket()
s.bind(('127.0.0.1', 12345))
s.listen()

conn, addr = s.accept()
conn.sendall(b'Hello\n')
conn.close()

Client

import socket
s = socket.socket()
s.connect(('127.0.0.1', 12345))

print(s.recv(1024))
s.close()