TCP vs UDP¶

TCP (Transmission Control Protocol) — reliable, ordered, connection-oriented. Uses 3-way handshake, retransmissions, flow control. Used for: HTTP, FTP, email. UDP (User Datagram Protocol) — unreliable, no ordering, connectionless. Lower latency, no overhead. Used for: DNS, streaming, gaming, VoIP. Choose TCP when data integrity matters, UDP when speed matters.

Key Concepts¶

Deep Dive: TCP vs UDP Comparison

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	Best-effort
Ordering	Maintained	Not guaranteed
Speed	Slower (overhead)	Faster
Header size	20 bytes	8 bytes
Flow control	Yes (sliding window)	No
Congestion control	Yes	No
Use case	Web, email, file transfer	Streaming, DNS, gaming

Deep Dive: TCP 3-Way Handshake

Client              Server
  │                   │
  │── SYN ──────────→│   1. Client requests connection
  │                   │
  │←── SYN-ACK ──────│   2. Server acknowledges + requests
  │                   │
  │── ACK ──────────→│   3. Client acknowledges
  │                   │
  │ Connection Established │

TCP teardown (4-way):

Client ── FIN ──→ Server
Client ←── ACK ── Server
Client ←── FIN ── Server
Client ── ACK ──→ Server

Deep Dive: TCP Reliability Mechanisms

Sequence numbers: Track byte order
Acknowledgments: Confirm receipt
Retransmission: Resend lost packets (timeout)
Flow control: Receiver advertises window size
Congestion control: Slow start, congestion avoidance

Common Interview Questions

What is the difference between TCP and UDP?
Explain the TCP 3-way handshake.
Why is UDP faster than TCP?
When would you use UDP over TCP?
What is flow control? Congestion control?