The most common network protocol suite is TCP/IP, which consists of several protocols over two layers. At the network layer, the Internet Protocol (IP) and Internet Control Message Protocol (ICMP) protocols are employed, while at the transport layer, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) operate.
UDP is an unreliable protocol that provides a datagram (connectionless) service and is suited to simple query and response applications. TCP is reliable and provides stream-oriented connections using data packet sequence numbers to coordinate transmission. Services such as FTP, telnet, email delivery by SMTP and Web services via HTTP are defined at this level, with each service being assigned a specific port number. (A port is an endpoint to a logical connection. Numbers are used to distinguish between different logical channels on the same network interface.)