The Internet Control Message Protocol
From time to time, IP datagrams will fail to get delivered. This may be due to errors in the datagram structure, a general network outage, or a delivery timeout. IP doesn’t really care about these problems, since it never promised delivery in the first place. However, applications care about these problems very much. They would like to be able to react to a failure, either by taking an alternative course of action, or by at least informing the user that a problem has occurred.
IP uses the Internet Control Message Protocol (ICMP) for error-reporting services. When a system needs to report a problem that is preventing delivery from occurring, it generates an ICMP message that describes the general problem, and then sends it back to the original sender of the original datagram. ICMP messages are not sent when a packet is lost in transit or when some other transient error occurs. Rather, ICMP error messages are only sent when there is a detectable problem that is preventing certain packets or datagrams from being delivered due to a specific reason. This indicates that the sending host should probably stop trying to send those kinds of datagrams to this specific destination system, or that a different path should be used.
Even if two IP-enabled systems are able to communicate effectively, there are no guarantees that everything will work, since the data inside the datagrams may be corrupt, or packets may get lost without any ICMP errors being generated. ...
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