Socket Programming

🎯 Learning Objectives

• Create TCP client/server sockets in Python
• Implement UDP communication
• Handle multiple connections with select/poll
• Build a simple HTTP server
• Apply socket options and timeouts

1. TCP Socket

1.1 Server

python
Run Inline
import socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(('127.0.0.1', 8080))
server.listen(5)  # backlog = 5
while True:
    client, addr = server.accept()  # Blocks until connection
    data = client.recv(1024)        # Receive up to 1024 bytes
    client.send(b"HTTP/1.1 200 OK\r\n\r\nHello!")
    client.close()

1.2 Client

python
Run Inline
import socket
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect(('127.0.0.1', 8080))
client.send(b"GET / HTTP/1.1\r\nHost: localhost\r\n\r\n")
response = client.recv(4096)
print(response.decode())
client.close()

2. UDP Socket

python
Run Inline
# Server
server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server.bind(('127.0.0.1', 8081))
data, addr = server.recvfrom(1024)  # Returns (data, address)
server.sendto(b"Response", addr)
# Client
client = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
client.sendto(b"Hello", ('127.0.0.1', 8081))
data, _ = client.recvfrom(1024)

3. Handling Multiple Connections

3.1 Using select

python
Run Inline
import select
server = socket.socket()
server.bind(('0.0.0.0', 8080))
server.listen(5)
read_list = [server]
while True:
    readable, _, _ = select.select(read_list, [], [])
    for s in readable:
        if s is server:
            client, addr = server.accept()
            read_list.append(client)
        else:
            data = s.recv(1024)
            if not data:
                read_list.remove(s)
                s.close()
            else:
                s.send(b"Echo: " + data)

4. Common Pitfalls

Pitfall 1: Blocking on recv

Pitfall 2: UDP Datagram Size

5. Key Concepts Reference

6. 📝 Practice Questions

Q1: Why does recv(1024) return only 200 bytes when the sender sent 2000?

Answer: TCP is a stream protocol — recv returns whatever data is currently available in the receive buffer (up to the requested size). The sender's 2000 bytes may arrive in multiple packets. Always loop on recv until you have the expected amount, or use message framing (send length prefix, then data). Q2: What happens if a TCP server doesn't call accept() quickly enough?

Answer: The TCP listen backlog queue fills up (default 5-128 depending on OS). Once full, new connection SYNs are either dropped (client retries) or RST (connection refused). The backlog parameter in listen() controls this queue depth. Q3: Why is UDP faster than TCP for real-time applications?

Answer: UDP has no connection setup (no 3-way handshake), no ACKs (no retransmission delay), no congestion control (no throttling), no ordering guarantees (no head-of-line blocking). For real-time applications (VoIP, gaming), losing some packets is acceptable; TCP's retransmission delay would be worse.

7. 🔗 Cross-References

• Week 2 - TCP/UDP: Transport layer concepts
• Week 5 - Application Layer: HTTP over sockets
• BSCS4022 (OS): System calls, I/O Join Discord PreviousWireless NetworksNextNetwork Management