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Transmission Control Protocol (TCP)
The Transmission Control Protocol (TCP) is an internetwork protocol that corresponds to the OSI Transport layer. TCP provides full-duplex, end-to-end connections. When the overhead of end-to-end communication acknowledgment isn’t required, the User Datagram Protocol (UDP) can be substituted for TCP at the Transport (host-to-host) level. TCP and UDP operate at the same layer.

TCP corresponds to SPX in the NetWare environment. TCP maintains a logical connection between the sending and receiving computer systems. In this way, the integrity of the transmission is maintained. TCP detects any problems in the transmission quickly and takes action to correct them. The trade-off is that TCP isn’t as fast as UDP.

TCP also provides message fragmentation and reassembly and can accept messages of any length from upper-layer protocols. TCP fragments message streams into segments that can be handled by IP. When used with IP, TCP adds connection-oriented service and performs segment synchronization, adding sequence numbers at the byte level.

In addition to message fragmentation, TCP can maintain multiple conversations with upper-layer protocols and can improve use of network bandwidth by combining multiple messages into the same segment. Each virtual-circuit connection is assigned a connection identifier called a port, which identifies the datagrams associated with that connection.