Exam 300-101 topic 1 question 804 discussion

A. They are sent when a route to the destination is missing from the routing table: If a router receives a packet and it doesn’t have a route in its routing table for the destination IP address, it will send an ICMP “destination unreachable” message back to the sender. This message indicates that the network specified in the IP address of the packet is not reachable. E. They include a portion of the original data so that the source can identify the process that generated the message: When an ICMP “destination unreachable” message is sent, it includes the IP header and the first 8 bytes of data from the original packet. This information allows the sender to match the ICMP message with the correct process that generated the original packet. For example, if the original packet was a TCP packet, the first 8 bytes would include source and destination port numbers, which could be used to identify the correct application or process at the sender.

"The ICMP destination unreachable message is generated by a router to inform the source host that the destination unicast address is unreachable. The IP header plus the first 8 bytes of the original datagram's data is returned to the sender. This data is used by the host to match the message to the appropriate process. If, according to the information in the gateway's routing tables, the network specified in the internet destination field of a datagram is unreachable, e.g., the distance to the network is infinity, the gateway may send a destination unreachable message to the internet source host of the datagram. If a router cannot forward a packet because it has no routes at all (including no default route) to the destination specified in the packet, then the router MUST generate a Destination Unreachable, Code 0 (Network Unreachable) ICMP message." http://www.networksorcery.com/enp/protocol/icmp/msg3.htm