Internet Transport-Level Protocols
The TCP/IP transport-level protocols allow application programs to communicate with other application programs.
User Datagram Protocol (UDP) and the TCP are the basic transport-level protocols for making connections between Internet hosts. Both TCP and UDP allow programs to send messages to and receive messages from applications on other hosts. When an application sends a request to the Transport layer to send a message, UDP and TCP break the information into packets, add a packet header including the destination address, and send the information to the Network layer for further processing. Both TCP and UDP use protocol ports on the host to identify the specific destination of the message.
Higher-level protocols and applications use UDP to make datagram connections and TCP to make stream connections. The operating system sockets interface implements these protocols.