Introducing the Internet protocols

Transport layer

The TCP/IP transport layer protocols enable communications between processes running on separate machines. Protocols at this level are TCP and UDP.

Transmission Control Protocol (TCP) enables applications to talk to one another via virtual circuits, as if connected by physical circuits. TCP is a connection-oriented, reliable delivery protocol; any data written to a TCP connection will be received by its peer in sequence, or an error indication will be returned.
User Datagram Protocol (UDP) is the alternative protocol available at the transport layer. UDP is a connectionless datagram protocol. Datagrams are groups of information transmitted as a unit to and from the upper layer protocols on sending and receiving hosts. UDP datagrams use port numbers to specify sending and receiving processes. However, no attempt is made to recover from failure or loss; packets may be lost with no error message returned.

Whether TCP or UDP is used depends on the network application invoked by the user. For example, if the user invokes telnet, that application passes the user's request to TCP. If the user's request involves the Domain Name Service (DNS), that application passes the request to UDP.

For information about the TCP and UDP protocols, refer to the tcp(7tcp) and udp(7tcp) manual pages.