TCP (Transmission Control Protocol)

Definition: Learn what the Transmission Control Protocol is and the role it plays in the TCP/IP protocol suite.

TCP (Transmission Control Protocol) is a protocol that works with the Internet Protocol (IP) to send packets of data between computers over the Internet. Together, the TCP and IP protocols establish rules for how information is passed through the Internet.

TCP is known as a connection-oriented protocol, which means that a connection is established and maintained until the application programs at each end have completed exchanging messages. In the Open Systems Interconnection (OSI) communication model, TCP is in Layer 4, the Transport Layer. The transport protocol determines the size of the largest data unit that's supported by any of the involved networks and segments the data into packets accordingly. It also manages flow control and the acknowledgement (ACK) services that ensure all packets arrive safely at their destination.

When an HTML file is sent to a client from a Web server, the TCP program layer in that server divides the file into packets, numbers them and then forwards the packets individually to the IP program layer. The IP program layer encapsulates each TCP packet with a header that includes the source and destination IP addresses as well as the packet's time-to-live and makes a best attempt effort to deliver the packets. Although each packet in the transmission will have the same IP address destination, the packets may be routed in many different directions. The TCP program layer in the client computer waits until all the packets have arrived, re-orders the packets, assembles them into a file and then forwards the file to the receiving application.

TCP, which is designed to be error-free, may experience delays of milliseconds while waiting for out-of-order packets or the retransmission of lost packets. Highly time-sensitive applications such as voice over IP (VoIP) may require a more lightweight transport protocol that places emphasis on reducing latency and jitter instead of on error checking and validation. In cases, the User Datagram Protocol (UDP) or Real-Time Transport Protocol (RTP ) may be used in place of TCP.

The Transmission Control Protocol (TCP) standard is defined in the Request for Comment (RFC) standards document number 793 by the Internet Engineering Task Force (IETF).

In this video tutorial, Pieter De Decker compares the UDP and TCP protocols.

See also: NAK, segmentation and reassembly, checksum

This was first published in August 2014

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