📘 04-02: TCP vs. UDP¶

📌 The Two Transport Protocols¶

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed, ordered	Best-effort
Handshake	Yes (3-way)	No
Acknowledgments	Yes	No
Sequencing	Yes	No
Overhead / speed	Higher / slower	Lower / faster
Use cases	Web, email, file transfer	VoIP, video, DNS, gaming

📌 TCP Three-Way Handshake¶

Client → SYN     →  Server     (synchronize)
Client ← SYN-ACK ←  Server     (acknowledge + synchronize)
Client → ACK     →  Server     (acknowledge)
   ↳ connection established, data flows

Teardown uses FIN (graceful) or RST (abrupt reset).

📌 Key TCP Flags¶

Flag	Meaning
SYN	Start/synchronize a connection
ACK	Acknowledge received data
FIN	Gracefully close
RST	Abruptly reset
PSH	Push data to the app immediately
URG	Urgent data

📌 How TCP Ensures Reliability¶

Sequence numbers order bytes.
Acknowledgments confirm receipt; unACKed data is retransmitted.
Window size (flow control) limits unacknowledged data so the receiver isn't overwhelmed.

💡 Why UDP for Real-Time?¶

For VoIP/video, a late retransmitted packet is useless — low latency beats guaranteed delivery. UDP simply sends, letting the app handle any loss.

Tip

Key idea — TCP = reliable, ordered, connection-oriented (3-way handshake, ACKs, windowing). UDP = fast, connectionless, best-effort. Match the protocol to the app's needs.

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