Internet Transport-Level Protocols

The TCP/IP transport-level protocols allow application programs to communicate with other application programs.

Figure 1. Transport layer of the TCP/IP Suite of Protocols.
Transport layer of the TCP/IP Suite of Protocols
This illustration shows the various layers of the TCP/IP Suite of Protocols. From the top, the application layer consists of the application. The transport layer contains UDP and TCP. The network layer contains the network (hardware) interface. And finally, the hardware layer contains the physical network.

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.