ICMPv4 Destination Unreachable Messages
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Interpretation of Destination Unreachable Messages
Finally, it's important to remember that just as IP is best effort, the reporting of unreachable destinations using ICMP is also best effort. For one things, these ICMP messages are themselves carried in IP datagrams. More than that, however, one must remember that there may be problems that prevent a router from detecting failure of delivery of an ICMP message, such as a low-level hardware problem. A router could, theoretically, also be precluded from sending an ICMP message even when failure of delivery is detected, for whatever reason.
For this reason, the sending of Destination Unreachable messages should be considered supplemental. There is no guarantee that every problem sending a datagram will result in a corresponding ICMP message. No device should count on receiving an ICMP Destination Unreachable for a failed delivery any more than it counts on the delivery in the first place. This is why the higher-layer mechanisms mentioned at the start of this topic are still important.
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