TCP vs UDP

Both are Layer 4 (Transport Layer) protocols, but designed for different needs.

TCP (Transmission Control Protocol)

• Connection-oriented — establishes connection before data transfer.
• Reliable — guarantees delivery.
• Ordered — data arrives in sequence.
• Error checking — retransmits lost packets.
• Flow control — prevents overwhelming receiver.
• Congestion control — prevents overwhelming network.
• Slower than UDP due to overhead.

TCP 3-Way Handshake (Connection Establishment)

Client                                Server
  |                                      |
  |------- SYN ------------------------>|  "I want to connect, seq=x"
  |                                      |
  |<------ SYN-ACK ----------------------|  "OK, seq=y, ack=x+1"
  |                                      |
  |------- ACK ------------------------->|  "Got it, ack=y+1"
  |                                      |
  |         Connection Established       |

• SYN — synchronize sequence numbers
• SYN-ACK — server acknowledges and sends its own SYN
• ACK — client acknowledges server's SYN

TCP 4-Way Handshake (Connection Termination)

Client                                Server
  |                                      |
  |------- FIN ------------------------>|  "I'm done sending"
  |                                      |
  |<------ ACK ---------------------------|  "Got it"
  |                                      |
  |<------ FIN ---------------------------|  "I'm done too"
  |                                      |
  |------- ACK ------------------------->|  "Got it, bye!"
  |                                      |

4 steps because each direction closes independently.

TCP Header (20 bytes minimum)

• Source/Destination Port
• Sequence Number — tracks order of bytes
• Acknowledgement Number — confirms received bytes
• Flags — SYN, ACK, FIN, RST, PSH, URG
• Window Size — flow control
• Checksum

UDP (User Datagram Protocol)

• Connectionless — no handshake, just send.
• Unreliable — no guarantee of delivery.
• Unordered — packets may arrive out of order.
• No retransmission — lost packets stay lost.
• No flow/congestion control.
• Faster than TCP — minimal overhead.
• Lightweight — small header (8 bytes only).

UDP Header (8 bytes only)

• Source Port
• Destination Port
• Length
• Checksum

UDP Operation

Client                                Server
  |                                      |
  |------- Data ------------------------>|
  |------- Data ------------------------>|
  |------- Data ------------------------>|
  |                                      |
  |     No handshake, no ACK             |

TCP vs UDP — Side by Side

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Reliable	Unreliable
Ordering	Ordered	Unordered
Speed	Slower	Faster
Header size	20 bytes	8 bytes
Flow control	Yes	No
Congestion control	Yes	No
Retransmission	Yes	No
Handshake	3-way handshake	None

When to Use Which?

Use TCP when: Data must arrive correctly; order matters. Examples: HTTP/HTTPS, FTP, SMTP/IMAP, SSH, Telnet.

Use UDP when: Speed matters more than reliability; small amount of loss is acceptable. Examples: DNS, DHCP, VoIP, Video streaming (YouTube, Netflix), Online gaming, TFTP.

TCP	UDP
Like registered post — confirmed delivery	Regular post — just send it