Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

A connection is a link between two or more computer systems that need to exchange messages and data. On a shared network and internetwork, connections are usually virtual, meaning that a connection state is set up in software that tracks the exchange of data across what appears to be a dedicated circuit to the application that is using it. These connections take place in the transport layer and are handled by TCP in the Internet protocol suite. This topic discusses TCP connections.

A connection is a requirement of a reliable data delivery service. It is set up before the actual data exchange takes place. The connection is used to acknowledge the receipt of packets and retransmit those that are lost. The opposite of this is a best-effort service. A file transfer is an example of a service that requires guaranteed delivery services. The delivered file must be an exact copy of the original. See "Reliable Data Delivery Services" and "Best-Effort Delivery."

To reliably exchange data, an application on one network system creates an end-to-end connection with an application on another network system. A single computer may establish and terminate multiple connections at any time. The packets from these connections are multiplexed over a single physical link. Thus, they are virtual connections. In addition, each connection is full duplex, allowing bidirectional packet exchange.