You may wonder why the default gateway includes eight bytes of the datagram. The eight bytes contain the destination and source port fields. By examining these eight bytes, you can determine what process sent this packet. The ICMP specifications stipulate that the redirecting gateway should forward the original datagram to the appropriate router. ICMP is connectionless because it does not require hosts to handshake before establishing a connection.
Connectionless protocols have advantages and disadvantages.
The main disadvantage of connectionless protocols is that delivery of ICMP messages is not guaranteed. If an ICMP message gets lost in transmission, the communication error must occur again, prompting another ICMP message to be transmitted. Before you can use ICMP to troubleshoot your company's network, you must capture the ICMP traffic on that network. For example, I use post-filtering with Novell's LANalyzer for Windows and ManageWise, but I use prefiltering with Network Associates's Sniffer and Sniffer Pro. After setting up the network analyzer to filter ICMP traffic, you should take a good look at the ICMP traffic that crosses the network.