What is the TCP protocol?

TCP (Transmission Control Protocol) is a transport layer protocol in the TCP/IP protocol stack that provides reliable, connection-oriented delivery of a byte stream between applications.

Main features of TCP:

• Reliability: Guarantees delivery of all data segments in the correct order, retransmitting lost or corrupted segments. Uses acknowledgments (ACK) and timeouts.
• Connection-oriented: A logical connection is established between sender and receiver before data transfer (three-way handshake). The connection is maintained throughout the session.
• Flow Control: Regulates the data transfer rate so that the sender does not overwhelm the receiver's buffer. Uses a receive window.
• Congestion Control: Prevents network congestion by dynamically adjusting the data transfer rate based on network conditions. Uses algorithms like Slow Start, Congestion Avoidance.
• Stream-oriented: TCP treats data as a continuous stream of bytes, not separate packets. Ensures data is delivered in the same order it was sent.
• Full duplex: Allows data to be transmitted in both directions simultaneously after the connection is established.

Applications of TCP:

• HTTP/HTTPS (web traffic)
• FTP (file transfer)
• SMTP (email)
• SSH (remote management)

Comparison with UDP:

Characteristic	TCP	UDP
Reliability	High (guaranteed delivery)	Low (no guarantee)
Connection-oriented	Yes (establishment and maintenance)	No (connectionless transfer)
Data ordering	Yes (delivery in send order)	No (possible out-of-order delivery)
Flow control	Yes	No
Congestion control	Yes	No
Speed	Lower (due to overhead)	Higher
Examples	HTTP, FTP, SSH, SMTP	DNS, DHCP, VoIP, online gaming