Use TCP when Accuracy > Speed

• TCP: Guaranteed delivery, ordered packets. (Web, Email, File Transfer).
• UDP: Fire and forget. (Video Streaming, VoIP, Gaming, DNS).

The 3-Way Handshake Connection

Before sending data, Alice and Bob must agree on Sequence Numbers.

sequenceDiagram
    participant C as Client (Alice)
    participant S as Server (Bob)
    
    Note left of C: State: CLOSED
    Note right of S: State: LISTEN
    
    C->>S: SYN (Seq=x)
    Note left of C: State: SYN_SENT
    
    S->>C: SYN (Seq=y), ACK (Ack=x+1)
    Note right of S: State: SYN_RCVD
    
    C->>S: ACK (Ack=y+1)
    Note left of C: State: ESTABLISHED
    Note right of S: State: ESTABLISHED
    
    Note over C,S: Data Transfer Begins...

1. SYN: "Let's talk. My sequence starts at $x$."
2. SYN-ACK: "I hear you ($x+1$). My sequence starts at $y$."
3. ACK: "I hear you ($y+1$)."

Connection Termination (4-Way Wave)

1. Client: FIN "I'm done."
2. Server: ACK "Roger."
3. Server: FIN "I'm done too." (Can happen later).
4. Client: ACK "Bye."

Flow Control vs Congestion Control

These are often confused.

Congestion Algorithms

1. Slow Start: Start fast, double speed every RTT. (1 packet -> 2 -> 4 -> 8...)
2. Packet Loss: Oops, router dropped a packet.
   • Reno/CUBIC (Loss-based): Cut speed in half. "Loss signals congestion."
   • BBR (Model-based): Google's algo. Measures Bandwidth and RTT. "Loss isn't always congestion."

The "Bufferbloat" Problem

Routers have large queues (buffers).

• Packet Loss algo: Fills the buffer -> Latency increases -> Finally drops packet -> Sender slows down.
• Result: High latency (Lag) before speed adjusts.
• BBR Solution: Detects when RTT rises (buffer filling) and slows down before packet loss.

Code Example: Socket Programming (C-style)

This is what happens under the hood of http.Get().

import socket

# 1. Create Socket (IPv4, TCP)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# 2. Connect (Triggers 3-Way Handshake)
# Sends SYN
s.connect(("google.com", 80)) 
# Returns after ACK received (ESTABLISHED)

# 3. Send Data (Push)
msg = "GET / HTTP/1.1\r\nHost: google.com\r\n\r\n"
s.sendall(msg.encode())

# 4. Receive Data
response = b""
while True:
    chunk = s.recv(4096)
    if not chunk: break # Server sent FIN
    response += chunk

s.close() # Sends FIN
print(response.decode())

Interview Tips 💡

• "Why 3-way handshake?" — To prevent old duplicate connection initiations from confusing the server. Both sides must confirm readiness.
• "SYN Flood Attack" — Attacker sends thousands of SYNs but never ACKs. Server memory fills up waiting.
  • Defense: SYN Cookies (Stateless handshake until ACK).
• "Nagel's Algorithm" — Buffers small writes to send full packets. Bad for real-time games (disable with TCP_NODELAY).
• "TIME_WAIT" — Why can't I restart my server immediately? The OS holds the port for 2 mins to ensure stray packets from the old connection die.

Related Concepts

• UDP vs TCP
• HTTP Evolution (Why QUIC replaces TCP)
• OS Networking Stack