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The TCP/IP transport layer protocols are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). These protocols allow application programs to communicate over the IPv4 and IPv6 network protocols. Both TCP and UDP use source and destination port numbers to tell a host how it should process the message.
Higher-level protocols and applications use UDP to make transient datagram connections, and TCP to make long-lived stream connections. The sockets and TLI/XTI programming interfaces provide access to these protocols.
Whether TCP or UDP is used depends on the network application invoked by the user. For example, telnet uses TCP to make a connection to a remote system, and name-to-address resolution via DNS is usually performed using UDP.