What are TCP and UDP? What is the fundamental difference? Where is UDP used?

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are transport protocols that operate over IP, but with different characteristics.

TCP:

• Reliable, connection-oriented.
• Guarantees data delivery in the correct order.
• Uses acknowledgments (ACK), retransmits lost packets.
• Suitable for applications where data integrity is important (HTTP, FTP, SMTP).

UDP:

• Connectionless, unreliable.
• Does not guarantee delivery, order, or absence of duplicates.
• Minimal overhead, low latency.
• Used where speed is crucial and some data loss is acceptable.

Where UDP is used:

• Video conferencing and VoIP — minimal delay is important, packet loss is not critical.
• Online gaming — fast data transfer is more important than complete reliability.
• DNS queries — short requests and quick responses.
• Broadcasting and multimedia protocols (e.g., RTP).

Thus, the choice between TCP and UDP depends on the application's requirements for reliability and data transfer speed.