The notable exception is provided by the user program
ping, the "Packet INternet Groper".
Ping generates one or more ICMP "echo request" packets addressed
to a specified remote host. On receipt of such a packet, the remote
host is required to send a time-stamped "echo reply" packet to the
originating host.
Such a transaction confirms packet delivery, and gives a rough
indication of the presence of unacceptable delays in the network.
In an IP network, ping is the single most important frontline
weapon of the intelligent network manager...