Transmission Control Protocol - The highest-level protocol in the TCP/IP (Internet) protocol stack. This protocol, using only the services of the unreliable IP protocol beneath it, adds reliable packet delivery to networks that use TCP/IP. Reliable packet delivery means you are assured that your packet will make it to the destination or that you will be informed if it did not.

This protocol also provides the option of "Connection" or "Session" oriented services to applications that use it. What this means is, --though the network is based on single packets being address and sent along--, the user of the TCP services can set up a session or connection, in which all information sent will be automatically packetized, addressed and sent to a predefined destination system. These "sessions" are usually referred to as STREAMS because the data does not have to be manually broken into chunks and sent to specific addresses. It can instead be sent in a continuous "stream" to the destination address, which was defined when the connection was set up.

The user of TCP also has the option of using the packet services directly without setting up connections (or streams). In this case, a user sends a UDP ("User Datagram Protocol") packet-type to a specified address.