TCP
also: Transmission Control Protocol
TCP (Transmission Control Protocol) is a reliable, connection-oriented network protocol that ensures data arrives in order and without errors. It is fundamental to internet communication and works alongside IP to form TCP/IP.
TCP operates at the transport layer (Layer 4) of the OSI model and guarantees that data sent from one computer reaches another in the correct sequence and without corruption. Unlike UDP, TCP establishes a connection before transferring data through a three-way handshake and confirms receipt of each packet.
TCP uses ports (numbered 0–65535) to identify specific services on a host. For example, HTTP traffic typically uses port 80, while HTTPS uses port 443. A TCP connection is uniquely identified by the combination of source IP, source port, destination IP, and destination port.
You can view active TCP connections on your system with ss -t or the older netstat -t command. TCP is essential for applications requiring reliability, such as email (SMTP), file transfer (FTP), and web browsing (HTTP/HTTPS), though it is slower than UDP due to its error-checking overhead.