UDP and ICMP

User Datagram Protocol (UDP) is for applications where packet sequencing is not a concern of the application. UDP fits into the IP system much like TCP; however UDP does not split data into multiple packets as TCP does. UDP is connectionless, meaning that UDP does not keep track of what it has sent. UDP is used most often for name lookup services.

Internet Control Message Protocol (ICMP) is used for network health (measurement of availability and reliability) and error messages. For example, if a connection attempt to a host is made, the requesting system might receive an ICMP message saying the intended host is unreachable. The most popular ICMP message is ping, which is used to test reachability of a network host. ICMP is simpler than TCP or UDP; ICMP does not use port numbers. Because all ICMP messages are interpreted by the TCP/IP networking software itself, no port numbers are needed to direct ICMP messages at the receiving host.

NOTE

It is a little known fact that ping is an abreviation for Packet Internet Groper.