Transmission Control Protocol. TCP is, along with UDP, is the main transport-layer protocol used in IP networks. TCP is connection-oriented - before two network hosts can communicate using TCP they must first establish a connection. TCP is a byte stream protocol that guarantees delivery. TCP ensures that data packets are transmitted error-free and in the right order, even if the underlying network is unreliable.
TCP uses port numbers 1-65535 to identify application end-points. Examples of well-known TCP applications and port numbers are FTP (21), TELNET (23), SMTP (25), HTTP (80) and POP3 (110).